JP5788814B2 - Raman amplifier excitation apparatus and Raman amplifier excitation method - Google Patents

Description

  The present invention relates to a Raman amplifier excitation device and a Raman amplifier excitation method for exciting a Raman amplifier using the Raman effect. In particular, the present invention relates to a Raman amplifier excitation apparatus and a Raman amplifier excitation method for exciting a distributed Raman amplifier amplifier that performs optical amplification in an optical waveguide for transmitting an optical transmission signal over a long distance.

  In designing a high-speed and large-capacity optical transmission system, it is important to reduce SN degradation of a received signal due to transmission path loss. For this reason, various configurations for compensating the transmission line loss by performing optical amplification inside the repeater or in the optical transmission line itself have been studied. In particular, erbium-doped fiber obtained by adding erbium (Er) to quartz fiber is gained. Optical amplifiers used as media have been widely put into practical use due to their simplicity.

  In addition, since a Raman amplifier using the Raman effect can realize a wide gain band, it has been actively attempted to adapt to a wavelength division multiplexing transmission system. In particular, distributed Raman amplification that utilizes an optical fiber transmission line itself as a gain medium has a great advantage that an existing optical fiber can be used as a gain medium, and is expected to be applied to next-generation high-speed and large-capacity optical communication. Yes.

A schematic diagram showing the configuration of a conventional distributed Raman amplifier 100 is shown in FIG. In the distributed Raman amplifier 100 shown in FIG. 8, the signal light propagates through the transmission optical fiber 102 after passing through the optical filter 101. Also, the pumping light emitted from the pumping light source 103 is sent to the transmission optical fiber 102 via the wavelength multiplexing coupler 104, and the transmission optical fiber 102 is pumped. Here, the relationship between the wavelength λ _s of the signal light and the wavelength λ _p of the pumping light may be approximately λ _s −λ _p = 100 nm in the case of an optical fiber formed from silica. Since the signal light is amplified by the Raman effect in the transmission optical fiber 102, power reduction of the transmission signal due to transmission path loss can be suppressed. Then, the excitation light after passing through the transmission optical fiber 102 is blocked by the optical filter 101. 8 shows a so-called backward pumping configuration in which the traveling directions of the signal light and the pumping light are opposite to each other, the so-called forward pumping in which the pumping light propagates in the same direction as the signal light is shown. Alternatively, a bidirectional excitation configuration in which excitation light is input from both directions may be employed.

  By the way, since Raman amplification is an optical effect having polarization dependence, when the pumping light has a single polarization, the gain received by the signal light has polarization dependence. That is, the optical power at the time of output of the optical transmission line changes according to the polarization state at the time when the signal light is incident on the optical transmission line, and the amount of change in this change is represented by the polarization-dependent gain fluctuation (Polarization). It is called Dependent Gain (PDG). Such a PDG appears particularly prominently in the forward excitation configuration. In the configuration of the backward excitation as shown in FIG. 6, the PDG is greatly suppressed (suppressed), but it is difficult to make it completely zero. (For example, refer nonpatent literature 1.).

In order to suppress (suppress) PDG, for example, it is effective to depolarize the excitation light (depolarized). Hereinafter, this depolarization will be described. In the case of coherent excitation light, the polarization state is fixed to one and occupies one point on the Poincare sphere. The polarization state of the excitation light is determined as follows. When the x-axis and y-axis are taken as two orthogonal axes on a plane orthogonal to the traveling direction of the excitation light, and the photoelectric fields of the excitation light in the x-axis and y-axis directions are respectively E _x and E _y , E Each of _x and E _y is a single sine wave. Although the frequency of these sinusoidal is equal to the frequency of the excitation light, the optical phase and amplitude of E _x and E _y are not generally equal, the polarization of the excitation light is determined by the phase difference and amplitude difference of the two sinusoidal It will be. On the other hand, in reality there is no perfectly coherent light. Since the beam width of the laser light is not 0, the frequency of the pumping light has a width, and _Ex and _Ey are not strictly a single sine wave, the polarization of the pumping light is strictly determined uniquely. A certain amount of randomness will occur. Depolarization is to positively change the polarization state of pumping light into a random state. In the depolarized excitation light, E _x and E _y are not consistent sine waves, but are a random addition of a plurality of sine waves, so that the phase difference between them is also random. Therefore, the polarization state is not uniquely determined, and occupies all the areas on the Poincare sphere at random.

  As configuration means for depolarizing the pumping light, for example, (1) a configuration using a depolarizer, (2) a configuration for polarization multiplexing of pumping light, and the like are known. For (1), a configuration in which a polarization beam splitter and a panda fiber are combined (see, for example, Patent Document 1), or the polarization plane of excitation light is inclined 45 degrees with respect to the optical axis of the panda fiber. (For example, refer to Patent Document 2). In Patent Document 2, as a combined structure of (1) and (2), the excitation light is polarized and multiplexed with respect to the optical axis of the panda fiber. A configuration is also described in which the polarization plane of the excitation light is incident at an angle of 45 degrees. The depolarizer described in (1) requires a very long panda fiber. As an example, when the pump light oscillates in multiple modes and the line width of each longitudinal mode is about 10 GHz, a group delay time difference of 100 psec or more (Differential Group Delay) is required in the panda fiber to perform sufficient depolarization. : DGD) (see, for example, Non-Patent Document 2).

  Here, as an example of the depolarizer (1), a configuration in which the polarization plane of the pumping light is inclined by 45 degrees with respect to the optical axis of the panda fiber will be described with reference to FIG. FIG. 9 is a schematic diagram showing the configuration of a conventional depolarizer 200, and the excitation light output as output light from the excitation light source 201 is input to the panda fiber 202 for excitation light source. As shown in FIG. 7, the slow axis of the excitation light source panda fiber 202 is inclined 45 degrees with respect to the x axis and the y axis, and the polarization plane of the excitation light output from the excitation light source 201 is also the x axis and y. It is inclined 45 degrees with respect to the axis. Since the polarization plane of the pump light coincides with the slow axis of the panda fiber 202, the polarization state of the pump light is kept constant in the process of propagating the panda fiber 202 for the pump light source due to the nature of the panda fiber. become. The excitation light output from the excitation light source panda fiber 202 is input to the depolarization panda fiber 203.

The slow axis of the depolarizing panda fiber 203 is parallel to the y-axis. Due to the DGD of the panda fiber, the x component and the y component of the excitation light have different optical path lengths. As the pump light propagates through the depolarizing panda fiber 203, the phase difference between Ex and Ey of the pump light gradually changes due to the accumulation of DGD. As shown schematically in FIG. 9, the polarization of the pump light is It changes variously. However, as described above, since the coherency of the pumping light is finite, if the difference in optical path length due to DGD exceeds the coherence length, the phase difference between E _x and E _y is not uniquely determined, so that the polarization is random Change and depolarization is achieved. Then, the excitation light enters the transmission optical fiber 205 via the wavelength multiplexing coupler 204 and amplifies the signal light as in FIG.

Next, (2) a configuration for polarization multiplexing of pump light will be described. This can be easily realized by providing two pump light sources and a polarization multiplexing coupler, but it should be noted that the power of the two pump lights is kept equal. Two of the excitation light as long as the orthogonality of the polarization plane (hereinafter, lambda ₁ and the lambda ₂₎ is maintained, since equally excited power enters the E _x and E _y, for any polarization state Even Raman gain occurs, and PDG can be minimized.

E. S. Son et al. "Statistics of Polarization Dependent Gain in Fiber Raman Amplifiers", J. et al. Lightwave Technol. Vol. 23, pp. 1219-1226, 2005. Tokura et al., “Efficient Pump Depolarizer Analysis for Distributed Raman Amplifier With Low Polarization Dependence of Gain” J. Lightwave Technol. Vol. 24 pp 3889-3896, 2006.

JP-A-6-51243 JP 2003-224327 A

  However, among the conventional depolarizing means described above, (1) the configuration using the depolarizer is described in Non-Patent Document 2, because the DGD of the panda fiber is about 1 psec per meter. In order to achieve this, an extremely long panda fiber of the order of 100 m has been required. On the other hand, when winding the panda fiber around the bobbin, the bobbin diameter must be at least several centimeters to prevent loss of the fiber and deterioration of polarization maintaining characteristics. The problem of having to expand has arisen.

Also, (2) the configuration for polarization multiplexing of the pumping light can reduce the PDG when the Raman gain medium is isotropic, but the case where the Raman gain medium is anisotropic and has DGD. Cannot suppress PDG. This is because the orthogonality of the _two excitation lights (λ ₁ and λ ₂ ) is impaired by DGD. For example, in the configuration shown in FIG. 8, a transmission optical fiber is assumed as the Raman gain medium, and such a transmission optical fiber is ideally DGD = 0, but due to slight distortion of the core and other reasons. Anisotropy may occur and DGD may occur. When DGD occurs in this way, λ ₁ and λ ₂ cause different polarization fluctuations in the process of propagation of the two pumping lights in the transmission fiber, resulting in partial orthogonality of the pumping light. Will result in PDG. In addition, since the DGD value and the direction of anisotropy of the transmission optical fiber fluctuate due to time and a slight tension applied to the fiber, there is a problem that the size of the PDG fluctuates accordingly.

FIG. 10 shows a PDG actually measured with a transmission optical fiber having a DGD. The pumping light had λ ₁ and λ ₂ of 1468 nm and 1470 nm, respectively, and after polarization multiplexing, backward pumping was performed. The length of the transmission optical fiber is 50 km, and the DGD is 1 to 2 psec. A maximum of 1 dB PDG was observed. The size of the PDG is not uniform as described above, and the size of the PDG varies greatly only by changing the tension of the fiber around the pumping light input section.

  The present invention makes it possible to provide sufficient depolarization to the pumping light for Raman amplification using a relatively short polarization-maintaining optical transmission medium (panda fiber). An object of the present invention is to provide a Raman amplification excitation device and a Raman amplifier excitation method.

The Raman amplifier pumping apparatus of the present invention includes a first pumping light source that outputs first polarized light of single polarization as output light, and a second pump that outputs second pumped light of single polarization as output light. An excitation light source, two input ports, and two output ports, wherein the first excitation light and the second excitation light are input to each of the two input ports, and the first excitation light input and A first polarization maintaining coupler that splits and outputs the second pumping light at the two output ports in a polarization multiplexed state, and is output from the two output ports of the first polarization maintaining coupler. The first polarization-maintaining optical transmission medium that receives one of the polarization-multiplexed pump lights and propagates while maintaining the polarization-multiplexed state, and the two output ports of the first polarization-maintaining coupler enter the other excitation light polarization multiplexing output from, are polarization multiplexed A polarization maintaining optical transmission medium of the second branch that propagates while maintaining the state, two input ports and one output port, and the polarization maintaining optical transmission medium of the first branch and the polarization of the second branch The first branch is input to each of the two input ports so that the polarization planes of the excitation light derived from the same excitation light source out of the polarization multiplexed excitation light output from the wave holding optical transmission medium are orthogonal to each other. The first pumping light propagated through the polarization maintaining optical transmission medium and the first pumping light propagated through the second polarization maintaining optical transmission medium are polarization multiplexed and simultaneously The second pumping light propagated through the polarization maintaining optical transmission medium and the second pumping light propagated through the second branch polarization maintaining optical transmission medium are polarization multiplexed and output from the one output port. A second polarization maintaining coupler, and the second polarization maintaining coupler An excitation light output port for outputting doubly polarization multiplexed excitation light output from an output port of the plastic, and an optical path length of the polarization maintaining optical transmission medium of the first branch and the second branch The optical path lengths of the polarization-maintaining optical transmission media are different from each other so that the double-polarized multiplexed pump light output from the output port of the second polarization-maintaining coupler is depolarized. The difference is longer than the coherence length of the first excitation light and the second excitation light.

  The Raman amplifier pumping apparatus of the present invention includes a first pumping light source that outputs first polarized light of single polarization as output light, and a second pump that outputs second pumped light of single polarization as output light. A pumping light source, two input ports, and two output ports, wherein the first pumping light and the second pumping light are input to the two input ports, respectively, A first polarization maintaining coupler that branches and outputs at the two output ports in a state in which the waves are aligned, and the first polarization that is output from the two output ports of the first polarization maintaining coupler. One of the first pumping light and the second pumping light is input, and the first branching polarization maintaining optical transmission medium that propagates while maintaining the state in which the polarization is aligned, and the first polarization maintaining coupler The first excitation light and the second excitation light output from two output ports and having the same polarization A second branch polarization maintaining optical transmission medium that inputs the other of the light and propagates while maintaining a state of polarization alignment; two input ports; and one output port. Polarization planes of the first pumping light and the second pumping light output from the wave holding optical transmission medium, and the first pumping light and the first pumping output from the polarization maintaining optical transmission medium of the second branch Are input to the two input ports so that the planes of polarization of the two pumping lights are orthogonal to each other, and the two input first pumping light and the second pumping light are polarized and multiplexed. A second polarization maintaining coupler that outputs from one output port, and a pumping light output port that outputs polarization multiplexed pumping light that is output from the output port of the second polarization maintaining coupler, The optical path length of the polarization maintaining optical transmission medium of the first branch and the polarization of the second branch A difference is provided in the optical path length of the optical transmission medium, and the difference in the optical path length is determined so that the pump light output from the output port of the second polarization maintaining coupler is depolarized. It is characterized by being longer than the coherence length of the excitation light output from the second excitation light source.

  The Raman amplifier excitation apparatus according to the present invention is characterized in that polarization multiplexing is performed using a polarization beam combiner (PBC) instead of the second polarization maintaining coupler.

  The Raman amplifier excitation apparatus according to the present invention includes a changing unit that changes wavelengths and powers of the first excitation light source and the second excitation light source.

  In the Raman amplifier excitation device of the present invention, at least one of the first-branch polarization maintaining optical transmission medium and the second-branch polarization maintaining optical transmission medium includes an optical attenuator.

  The Raman amplifier excitation device according to the present invention includes a frequency modulation unit capable of modulating an optical frequency of output light from the first excitation light source and the second excitation light source.

  The Raman amplifier pumping apparatus of the present invention includes a first pumping light source that outputs first polarized light of single polarization as output light, and a second pump that outputs second pumped light of single polarization as output light. A pumping light source, a first frequency modulating means for modulating the optical frequency of the first pumping light, a second frequency modulating means for modulating the optical frequency of the second pumping light, and two input ports And one output port, the first excitation light modulated by the first frequency modulation means is input from one of the two input ports, and the first excitation light modulated by the second frequency modulation means A first polarization maintaining coupler that inputs second pumping light from the other of the two input ports, and outputs the one output port in a state where the two pumping lights that are input are polarization multiplexed; 1 The polarization output from the output port of the polarization maintaining coupler A polarization maintaining optical transmission medium having DGD for inputting multiplexed pumping light, and a pumping light output port for outputting depolarized pumping light output from the polarization maintaining optical transmission medium, The planes of polarization of the first pumping light and the second pumping light output from the output port of the first polarization maintaining coupler are relative to both the fast axis and the slow axis of the polarization maintaining optical transmission medium. It is inclined 45 degrees.

The Raman amplifier excitation device of the present invention is characterized in that wavelengths of the first excitation light and the second excitation light are different.
The Raman amplifier excitation apparatus according to the present invention includes a changing unit that changes wavelengths and powers of the first excitation light source and the second excitation light source.

  The Raman amplifier excitation device of the present invention is characterized in that a panda fiber is used as the polarization maintaining optical transmission medium.

  In the Raman amplifier excitation device of the present invention, the polarization maintaining optical transmission medium is a polarization maintaining optical waveguide formed on a PLC substrate.

The Raman amplifier excitation method of the present invention outputs single-polarized excitation light as output light from the first excitation light source and the second excitation light source, respectively, and the first excitation light source and the second excitation light source. Output light from the first polarization maintaining coupler is input to each of the first polarization maintaining couplers, and the two polarizations of the two output lights that are input are branched from the first polarization maintaining coupler and output from the first polarization maintaining coupler. Then, one of the output lights output from the first polarization maintaining coupler is input to the polarization maintaining optical transmission medium of the first branch, and propagates while maintaining the state where the polarizations are orthogonal to each other. The other of the output light output from the polarization maintaining coupler is input to the polarization maintaining optical transmission medium of the second branch, propagates while maintaining the state where the polarization is orthogonal, and the polarization maintaining light of the first branch There is a difference between the optical path length of the transmission medium and the optical path length of the polarization maintaining optical transmission medium of the second branch. The difference between the optical path lengths is made longer than the coherence length of the excitation light source so that the excitation light output from the output port of the second polarization maintaining coupler is depolarized, and the polarization of the first branch The output light of the holding optical transmission medium and the second branch polarization maintaining optical transmission medium is input to a second polarization maintaining coupler, and each of the two output lights input is polarization multiplexed and the first The second polarization maintaining coupler outputs the double polarization polarized pumping light output from the second polarization maintaining coupler from the pumping light output port.

The Raman amplifier excitation method of the present invention is characterized in that wavelengths of the first excitation light and the second excitation light are different.
The Raman amplifier excitation method of the present invention is characterized in that the wavelength and power of the first excitation light source and the second excitation light source are changed by changing means.

The Raman amplifier excitation method of the present invention is characterized in that at least one of the first-branch polarization-maintaining optical transmission medium and the second-branch polarization-maintaining optical transmission medium includes an optical attenuator.
The Raman amplifier excitation method of the present invention is characterized in that modulation is applied to the frequency of the output light from the first excitation light source and the second excitation light source.
The Raman amplifier excitation method of the present invention is characterized in that a panda fiber is used as the polarization maintaining optical transmission medium.
The Raman amplifier excitation method of the present invention is characterized in that the polarization maintaining optical transmission medium is a polarization maintaining optical waveguide configured on a PLC substrate.

  The above inventions can be combined as much as possible.

  The Raman amplifier excitation apparatus and the Raman amplifier excitation method of the present invention are configured so that the excitation light is split into two while maintaining the polarization, and the polarization is multiplexed with a delay applied to one of them. It is possible to provide sufficient depolarization to the pumping light for Raman amplification using a short polarization maintaining optical transmission medium, and the device can be miniaturized.

  In addition, the Raman amplifier excitation device and the Raman amplifier excitation method of the present invention incline the polarization plane of the output light output from the excitation light source by 45 degrees with respect to both the fast axis and the slow axis of the polarization maintaining optical transmission medium. In addition, the excitation light output from the excitation light source is modulated to degrade the coherence, so that a relatively short polarization-maintaining optical transmission medium is used to provide sufficient excitation light for Raman amplification. It is possible to provide depolarization, and the size of the device can be reduced when the device is made.

It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 1 of this invention. It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 2 of this invention. It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 3 of this invention. It is the figure which showed the result of PDG actually measured using the Raman amplifier excitation apparatus which concerns on Embodiment 3. FIG. It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 4 of this invention. It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 5 of this invention. It is the schematic which showed the structure of the Raman amplifier excitation apparatus which concerns on Embodiment 6 of this invention. It is the schematic which showed the structure of the conventional distributed Raman amplifier. It is the schematic which showed the structure of the conventional depolarizer. It is the figure which showed the result of PDG actually measured.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components. Each embodiment can be combined as much as possible.

(Embodiment 1)
Hereinafter, Embodiment 1 of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram showing the configuration of a Raman amplifier excitation apparatus 1 according to Embodiment 1 of the present invention. A Raman amplifier pumping apparatus 1 shown in FIG. 1 is a first polarization maintaining coupler having a pumping light source 11 that outputs pumping light with a single polarization, one input port 13a, and two output ports 13c and 13d. A first panda coupler 13, a first panda fiber 14 that is a polarization maintaining optical transmission medium of the first branch, a second panda fiber 15 that is a polarization maintaining optical transmission medium of the second branch, and two A basic configuration of a second panda coupler 16 that is a second polarization maintaining coupler having input ports 16a and 16b and one output port 16c, and a pumping light output port 17 for outputting polarization multiplexed pumping light Have as.

  A pumping light source 11 constituting the Raman amplifier pumping apparatus 1 outputs single polarized pumping light as output light. In the present embodiment, the single-polarized pumping light output from the pumping light source 11 is input to the pumping light source panda fiber 12, and the polarization plane of the pumping light input and the pumping light source panda fiber The 12 slow axes are made to coincide. This enables stable power branching and polarization multiplexing, as will be described later.

  The first panda coupler 13 which is the first polarization maintaining coupler connected to the pump light source 11 via the pump light source panda fiber 12 has one input port 13a and two output ports 13c and 13d. Then, the excitation light that is the output light from the excitation light source 11 is input to the input port 13 a via the excitation light source panda fiber 12.

  As described above, since the polarization plane of the excitation light and the slow axis of the panda fiber 12 for excitation light source coincide with each other, the polarization plane of the excitation light input to the input port 13a is always constant. For this reason, the first panda coupler 13 can output the excitation light to the two output ports 13c and 13d with a branching ratio of 1: 1 while maintaining the polarization of the excitation light.

  The first panda fiber 14 that is the first branch polarization maintaining optical transmission medium includes one output port 13c of the two output ports 13c and 13d formed in the first panda coupler 13, and a first panda fiber 14 to be described later. One input port 16a of the two input ports 16a and 16b formed in the second panda coupler 16 which is the second polarization maintaining coupler is connected. The plane of polarization of the pumping light output from the output port 13 c is matched with the slow axis of the first panda fiber 14.

  Similarly, the second panda fiber 15 that is the polarization maintaining optical transmission medium of the second branch includes the other output port 13d of the two output ports 13c and 13d formed in the first panda coupler 13, The other input port 16b of the two input ports 16a and 16b formed in the second panda coupler 16 which is a second polarization maintaining coupler described later is connected. The polarization plane of the pumping light output from the output port 13d is made to coincide with the slow axis of the second panda fiber 15.

  Due to the nature of the panda fiber, in the process of propagating through the first branch panda fiber 14 and the second branch panda fiber 15, the polarization of the pumping light is maintained, and the polarization fluctuation due to DGD does not occur.

  Here, in the Raman amplifier excitation device 1 of the present embodiment, the optical path length of the second branch panda fiber 15 causes a difference in delay time compared to the optical path length of the first branch panda fiber 14. Keep L longer. The length L, which is the difference between the optical path lengths, needs to be longer than the coherence length of the excitation light output from the excitation light source 11.

  As an example, if the pumping light output from the pumping light source 11 oscillates in multimode (multimode), and the line width of each longitudinal mode is, for example, about 10 GHz, the delay of the first branch panda fiber 14 is delayed. The length L is determined so as to give a difference of 100 p seconds (psec) or more between the time and the delay time of the second branch panda fiber 15. Due to this time difference, the optical path length difference between the two exceeds the coherence length of the pumping light. Therefore, the phase difference between the pumping light output from the first branch panda fiber 14 and the pumping light output from the second branch panda fiber 15 is unique. It becomes a state that changes randomly without being fixed.

  The second panda coupler 16 that is a second polarization maintaining coupler has two input ports 16 a and 16 b and one output port 16 c, and includes a first branch panda fiber 14 and a second branch panda fiber 15. The output light is input to the two input ports 16a and 16b, respectively. Here, the slow axes of the first branch panda fiber 14 and the input port 16a are made to coincide with each other, but the slow axes of the second branch panda fiber 15 and the input port 16b are orthogonal to each other. That is, the connection is made so that the slow axis and the fast axis are reversed. With this configuration, two pump lights are output from the output port 16c after being polarization multiplexed. However, since the two pump lights do not have a unique phase difference as described above and change randomly, they are polarization multiplexed. The excited light is depolarized.

  Since the delay time of the fiber is 5 nsec (nsec) for 1 m, if L = 0.1 m, polarization multiplexing can be performed with a delay time difference of about 500 psec (5 nsec × 0.1 m / 1 m). It becomes. On the other hand, in the conventional apparatus 200 shown in FIG. 9, since a delay time difference of several hundreds p seconds (psec) is generated by DGD of a panda fiber, about 100 m of the depolarizing panda fiber 203 is required. In contrast, in this embodiment, if two panda fibers 14 and 15 having a length of several tens of centimeters are used and the difference in length between the two is set to about 0.1 m, sufficient depolarization is achieved. Excitation light can be obtained. In the above description, the length L of the optical path length difference L is described as L = 0.1 m for convenience. However, the excitation light output as output light from the output port 16c is depolarized so as to be depolarized. What is necessary is just to make it longer than the coherence length of the excitation light output from the light source 11, and what is necessary is just to determine suitably that length by the relationship with the specification, design, etc. which the Raman amplifier excitation apparatus 1 is calculated | required.

  Further, as a guideline for sufficient depolarization of the excitation light, for example, as described in Non-Patent Document 2, one standard is that DOP is 5% or less. It is not limited to this, and may be appropriately determined according to the required specifications and design of the Raman amplifier excitation device 1.

  An optical attenuator (not shown) is provided in at least one of the first branch panda fiber 14 and the second branch panda fiber 15 in order to compensate for the difference in loss between the first branch panda fiber 14 and the second branch panda fiber 15. (Polarization-maintaining optical attenuator) is preferably provided. Since the first branch panda fiber 14 and the second branch panda fiber 15 are provided with optical attenuators, the difference in loss between the two is efficiently compensated. The optical attenuator (not shown) is particularly preferably provided in both the first branch panda fiber 14 and the second branch panda fiber 15.

  The Raman amplifier pumping apparatus 1 according to the first embodiment described above branches the pump light into two while maintaining the polarization, and the difference between the optical path lengths of the first branch panda fiber 14 and the second branch panda fiber 15 is different. And a configuration in which polarization multiplexing is performed with a delay applied to one of them, so that it is possible to provide sufficient depolarization to the pumping light for Raman amplification using a relatively short polarization maintaining optical transmission medium. Thus, the Raman amplifier excitation device 1 can be downsized.

  In the above configuration example, a mode in which polarization multiplexing is performed using the second panda coupler 16 is shown. However, instead of the second panda coupler 16, a conventionally known polarization beam combiner (PBC) is used. ) May be used to perform polarization multiplexing of pumping light.

(Embodiment 2)
Hereinafter, Embodiment 2 of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are given to the same structures and the same members as those in the first embodiment, and the detailed description thereof is omitted or simplified.

  FIG. 2 is a schematic diagram illustrating a configuration of a Raman amplifier excitation device 1 according to Embodiment 2 of the present invention. The Raman amplifier excitation apparatus 1 according to Embodiment 2 shown in FIG. 2 uses two excitation light sources 11 (first excitation light source 11a and second excitation light source 11b), and the first panda coupler 13 has two input ports. The first embodiment is different from the first embodiment in that it is changed to one having 13a, 13b and two output ports 13c, 13d. A first pumping light source 11 (first pumping light source 11a and second pumping light source 11b) that outputs pumping light with a single polarization, two input ports 13a, 13b, and two output ports 13c, 13d. A first panda coupler 13 that is one polarization maintaining coupler, a first panda fiber 14 that is a first polarization maintaining optical transmission medium, and a second panda fiber 14 that is a second polarization maintaining optical transmission medium. A two-branch panda fiber 15, a second panda coupler 16, which is a second polarization maintaining coupler having two input ports 16 a and 16 b and one output port 16 c, and the polarization multiplexed pump light are output. And a pumping light output port 17 as a basic configuration.

  The pumping light sources 11a and 11b constituting the Raman amplifier pumping apparatus 1 output pumping light as output light with a single polarization, as in the first embodiment. Two excitation light sources, a light source 11a and a second excitation light source 11b, are provided. The single-polarized pumping light output from the two pumping light sources 11a and 11b is input to the first pumping light source panda fiber 12a and the second pumping light source panda fiber 12b, respectively. The polarization plane of the pumping light output from the pumping light source 11a and input to the first pumping light source panda fiber 12a matches the slow axis of the first pumping light source panda fiber 12a. On the other hand, the polarization plane of the pumping light output from the second pumping light source 11b and input to the second pumping light source panda fiber 12b is made to coincide with the fast axis of the second pumping light source panda fiber 12b.

  The pumping lights output from the first pumping light source panda fiber 12a and the second pumping light source panda fiber 12b are respectively input and input to one input port 13a and 13b of the first panda coupler 13. Further, the two pumping lights kept polarized in the first panda fiber 14 from one output port 13c of the first panda coupler 13 and in the second panda fiber 15 from the other output port 13d. Each branch is output as it is. The branching ratio is 1: 1 for both excitation lights.

  In the first branch panda fiber 14, the pumping light output from the first pumping light source 11a propagates through the slow axis (x-axis), and the pumping light output from the second pumping light source 11b is the fast axis (y-axis). ) Is propagated. The same applies to the second branch panda fiber 15. Due to the nature of the panda fiber, in the process of propagating through the first branch panda fiber 14 and the second branch panda fiber 15, the polarization of the two pump lights is maintained, and the polarization fluctuation due to DGD does not occur.

As in the first embodiment, the optical path length of the second branch panda fiber 15 in this embodiment is longer than the optical path length of the first branch panda fiber 14 by the length L. There is a difference in the delay time between the fiber 14 and the second branch panda fiber 15.
The length L of the optical path length difference is longer than the coherence length of the excitation light output from the excitation light source 11a and the excitation light source 11b.

  The second panda coupler 16 that is a second polarization maintaining coupler has two input ports 16 a and 16 b and one output port 16 c, and includes a first branch panda fiber 14 and a second branch panda fiber 15. The output light is input to the two input ports 16a and 16b, respectively. Here, the slow axes of the first branch panda fiber 14 and the input port 16a are made to coincide with each other, but the slow axes of the second branch panda fiber 15 and the input port 16b are orthogonal to each other. That is, the connection is made so that the slow axis and the fast axis are reversed.

  With this configuration, the output light from the first pumping light source 11a (pumping light) is output from the output port 16c in a polarization multiplexed manner, and the output light (pumping light) from the second pumping light source 11b is also output. Double polarization multiplexing is performed in which polarization multiplexing is performed and output. Since these four pumping lights have a phase difference that is not uniquely determined and changes randomly as in the first embodiment, both the pumping light output from the pumping light source 11a and the pumping light output from the pumping light source 11b are simultaneously depolarized. can do.

  In the present embodiment, there is provided a changing means (not shown) that can arbitrarily adjust the wavelength and power of output light (excitation light) output from the first excitation light source 11a and the second excitation light source 11b. It is preferable. By providing such a changing means, it is possible to finely adjust the Raman gain profile and generate a flatter Raman gain.

  As a changing means that can arbitrarily adjust the wavelength and power (not shown), for example, the oscillation wavelength of the first excitation light source 11a and the second excitation light source 11b may be changed using a temperature controller. Using a wave holding attenuator, it is arranged in an optical path (for example, the first excitation light source panda fiber 12a and the second excitation light source panda fiber 12b) connected to the first excitation light source 11a and the second excitation light source 11b. You may make it install.

  In addition, it is preferable that the difference in the wavelength of the output light (excitation light) output from the two excitation light sources 11a and 11b is, for example, larger than 0 nm and several tens of nm or less. If the wavelengths of the two pump light sources 11a and 11b are completely the same and the coherency is extremely high, the process of propagation is caused by the interaction of the two pump lights propagating through the first branch panda fiber 14 and the second branch panda fiber 15. Therefore, it is difficult to perform polarization multiplexing by the second panda coupler 16. On the other hand, when the wavelength is several tens of nanometers or more, the difference between the gain bands becomes significant, which may affect the flatness of the gain.

  By the way, it is impossible to completely flatten the gain spectrum of Raman amplification by the output light (excitation light) output from the first excitation light source 11a, and there is always a wavelength region in which a gradient occurs. In order to compensate for this gradient, it is preferable to adjust the wavelengths of the two excitation lights so that the gain spectrum of the output light (excitation light) output from the second excitation light source 11b has an opposite gradient. This makes it possible to create a flatter Raman gain. Here, as a level of “flatter Raman gain”, for example, one criterion is that the gain ripple is 0.01 dB or less (out of the nominal 20 dB), or the fluctuation is 0.05% or less. However, it is not limited to this value, and may be determined as appropriate according to the required specifications and design of the Raman amplifier excitation device 1.

  As in the first embodiment, in order to suppress the difference in loss between the first branch panda fiber 14 and the second branch panda fiber 15, at least one of the first branch panda fiber 14 and the second branch panda fiber 15 is used. On the other hand, it is preferable to provide an optical attenuator (polarization maintaining optical attenuator) (not shown), and it is particularly preferable that the optical attenuator is provided to both the first branch panda fiber 14 and the second branch panda fiber 15.

  The Raman amplifier excitation device 1 according to the second embodiment described above enjoys the effects of the Raman amplifier excitation device 1 according to the first embodiment and uses two excitation light sources 11a and 11b, so that it is flatter. Raman gain can be achieved.

  In this embodiment, since polarization multiplexing has already been performed at the time of input to the second panda coupler 16, unlike the first embodiment, it is not possible to use PBC instead of the second panda coupler 16. Can not.

(Embodiment 3)
Hereinafter, Embodiment 3 of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are given to the same structure and the same members as those in the second embodiment, and the detailed description thereof is omitted or simplified.

  FIG. 3 is a schematic diagram showing the configuration of a Raman amplifier excitation device 1 according to Embodiment 3 of the present invention. The Raman amplifier excitation apparatus 1 according to Embodiment 3 shown in FIG. 3 includes a first branch panda fiber 14 that is a polarization maintaining optical transmission medium of the first branch and a second branch that is a polarization maintaining optical transmission medium of the second branch. The second polarization having two input ports 16a and 16b and one output port 16c, with the relative relationship between the polarizations of the two pumping lights propagating through the panda fiber 15 being the same (aligned) without being orthogonal to each other Unlike the second embodiment, the pumping light is output with a single polarization instead of using the PBC 20 having two input ports 20a and 20b and one output port 20c instead of the second panda coupler 16 which is a holding coupler. A first polarization maintaining coupler having two pumping light sources 11 (first pumping light source 11a and second pumping light source 11b), two input ports 13a and 13b, and two output ports 13c and 13d. A first panda coupler 13, a first branch panda fiber 14 that is the first polarization maintaining optical transmission medium, and a second branch panda fiber 15 that is the second polarization maintaining optical transmission medium, The basic configuration includes a PBC 20 having two input ports 20a and 20b and one output port 20c, and a pumping light output port 17 for outputting polarization multiplexed pumping light.

  The pumping light sources 11a and 11b constituting the Raman amplifier pumping apparatus 1 output pumping light with a single polarization as output light, as in the second embodiment. In the present embodiment, the first pumping light is output. Two excitation light sources, a light source 11a and a second excitation light source 11b, are provided. The single-polarized pumping light output from the two pumping light sources 11a and 11b is input to the first pumping light source panda fiber 12a and the second pumping light source panda fiber 12b, respectively. The polarization plane of the pumping light output from the pumping light source 11a and input to the first pumping light source panda fiber 12a matches the slow axis of the first pumping light source panda fiber 12a. The polarization plane of the pump light output from the second pump light source 11b and input to the second pump light source panda fiber 12b is made to coincide with the slow axis of the second pump light source panda fiber 12b.

  The pumping lights output from the first pumping light source panda fiber 12a and the second pumping light source panda fiber 12b are respectively input to one input ports 13a and 13b of the first panda coupler 13, The two input pump lights are polarized from one output port 13c of the first panda coupler 13 to the first branch panda fiber 14 and from the other output port 13d to the second branch panda fiber 15. Each branch is output while being held. The branching ratio is 1: 1 for either excitation light.

  In the first branch panda fiber 14, both the pumping light output from the first pumping light source 11 a and the pumping light output from the second pumping light source 11 b propagate along the slow axis (x-axis). The same applies to the second branch panda fiber 15. Due to the nature of the panda fiber, in the process of propagating through the first branch panda fiber 14 and the second branch panda fiber 15, the polarization of the two pump lights is maintained, and the polarization fluctuation due to DGD does not occur.

  As in the second embodiment, the optical path length of the second branch panda fiber 15 in this embodiment is longer than the optical path length of the first branch panda fiber 14 by the length L, so that the first branch panda There is a difference in the delay time between the fiber 14 and the second branch panda fiber 15. The length of the optical path length difference L can be determined in the same manner as in the second embodiment.

  The PBC 20 has two input ports 20a and 20b and one output port 20c, and inputs output light from the first branch panda fiber 14 and the second branch panda fiber 15 to the two input ports 20a and 20b, respectively. . Here, the slow axes of the first branch panda fiber 14 and the input port 20a are matched, but the slow axes of the second branch panda fiber 15 and the input port 20b are orthogonal. That is, the connection is made so that the slow axis and the fast axis are reversed.

  With this configuration, the output light (pumping light) from the first pumping light source 11a is output by polarization multiplexing from the output port 20c, and the output light (pumping light) from the second pumping light source 11b is also output. Polarization multiplexed and output. Since these four pump lights have a phase difference that is not uniquely determined and changes randomly as in the second embodiment, both the pump light output from the pump light source 11a and the pump light output from the pump light source 11b are simultaneously depolarized. can do. In general, polarization multiplexing by PBC has less dead loss than polarization multiplexing by a polarization maintaining coupler, so that the pumping efficiency can be increased as compared with the second embodiment.

  In the present embodiment, as in the second embodiment, output light (excitation light) output from the first excitation light source 11a and the second excitation light source 11b is transmitted from the two excitation light sources 11a and 11b. It is preferable to provide a changing means (not shown) that can arbitrarily adjust the wavelength and power of output light (excitation light) to be output.

  Note that the difference in the wavelength of the output light (excitation light) output from the two excitation light sources 11a and 11b propagates through the same plane of polarization, and is therefore stricter than that of the second embodiment. It is preferable that it is nm or less. If the wavelengths of the two pump light sources 11a and 11b are 1 nm or less, the two pump lights propagating through the first branch panda fiber 14 and the second branch panda fiber 15 cause mutual interference, resulting in irregular optical power. Since it changes, stable Raman amplification cannot be performed. On the other hand, when the wavelength is several tens of nanometers or more, the difference between the gain bands becomes significant, which may affect the flatness of the gain.

  As in the second embodiment, a flatter Raman gain can be created by adjusting the wavelength of the output light output from the first excitation light source 11a and the second excitation light source 11b.

  As in the second embodiment, in order to suppress the difference in loss between the first branch panda fiber 14 and the second branch panda fiber 15, at least one of the first branch panda fiber 14 and the second branch panda fiber 15 is used. On the other hand, it is preferable to provide an optical attenuator (polarization maintaining optical attenuator) (not shown), and it is particularly preferable that the optical attenuator is provided to both the first branch panda fiber 14 and the second branch panda fiber 15.

  The Raman amplifier excitation device 1 according to the third embodiment described above enjoys the effects of the Raman amplifier excitation device 1 according to the second embodiment and performs polarization multiplexing using PBC. Can be raised.

  FIG. 4 shows the result of actual PDG suppression experiments using this embodiment. The wavelength of the excitation light and the magnitude of the excitation intensity are the same as those in FIG. Compared to FIG. 10, it can be seen that PDG is suppressed in FIG. 4. Although PDG of about 0.4 dB remains, this is due to the polarization dependence of the transmission loss of the optical transmission line and is polarization dependence unrelated to Raman amplification. Unlike the result of FIG. 10, in FIG. 10, even if the tension of the fiber around the excitation light input portion is changed, the size of the PDG is hardly changed. This result shows that the present embodiment is effective for PDG suppression.

(Embodiment 4)
Hereinafter, a fourth embodiment of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are given to the same structures and the same members as those in the first embodiment, the second embodiment, and the third embodiment, and the detailed description thereof is omitted or simplified.

  FIG. 5 is a schematic diagram showing the configuration of the Raman amplifier excitation device 1 according to Embodiment 3 of the present invention. The Raman amplifier excitation apparatus 1 of Embodiment 4 shown in FIG. 5 performs the first frequency modulation on the first excitation light source 11a and the second excitation light source 11b constituting the Raman amplifier excitation apparatus 1 of Embodiment 2. The frequency modulation means 18a is provided in the first excitation light source 11a, and the second frequency modulation means 18b is provided in the second excitation light source 11b.

  As the two frequency modulation means 18a and 18b, for example, when the first excitation light source 11a and the second excitation light source 11b are formed of a laser diode or the like, the output light can be obtained by superimposing an alternating current with a small amplitude on the excitation current. Can be easily frequency-modulated.

  In this way, by modulating the optical frequency of the output light (excitation light) output from the first excitation light source 11a and the second excitation light source 11b, the first excitation light source 11a and the second excitation light source 11b emit light. The coherence length of the output light can be greatly shortened, and the length of the optical path length difference L for achieving sufficient depolarization can be further shortened.

  As shown in FIG. 5, the frequency modulation means 18a and 18b may directly modulate the excitation currents of the first excitation light source 11a and the second excitation light source 11b using, for example, an oscillator. The optical paths connected to the first excitation light source 11a and the second excitation light source 11b using the attached optical frequency modulator (for example, the first excitation light source panda fiber 12a and the second excitation light source panda fiber 12b) ) May be arranged.

  The frequency modulation by the frequency modulators 18a and 18b requires that the dithering speed be increased to some extent in order to make the Raman gain constant. For example, the frequency is set to several MHz or higher. Further, when frequency modulation is performed by modulating the excitation current, it is preferable that the amplitude of the modulation is such that the light source is not reverse-biased and modulated above the emission threshold.

  The Raman amplifier excitation device 1 according to the fourth embodiment described above enjoys the effect produced by the Raman amplifier excitation device 1 according to the second embodiment, and the two excitation light sources 11a and 11b have frequency modulation means 18a and 18b, respectively. Therefore, the coherency of the output light from the excitation light sources 11a and 11b is greatly deteriorated, and the length L of the optical path length difference L for achieving sufficient depolarization can be further shortened.

(Embodiment 5)
Hereinafter, Embodiment 5 of the present invention will be described in detail with reference to the drawings.
FIG. 6 is a schematic diagram illustrating a configuration of a Raman amplifier excitation apparatus 1 according to Embodiment 4 of the present invention. A Raman amplifier pumping apparatus 1 shown in FIG. 6 includes a pumping light source 11 that outputs pumping light having a single polarization, frequency modulation means 18 that modulates the frequency of output light from the pumping light source, and polarization-maintaining optical transmission having a DGD. The basic configuration includes a depolarizing panda fiber 14a, which is a medium, and a pumping light output port 17 that outputs depolarized pumping light.

  The pumping light source 11 constituting the Raman amplifier pumping apparatus 1 outputs pumping light as output light with a single polarization.

  In the present embodiment, the excitation light source 11 described above is configured to include frequency modulation means 18 that performs frequency modulation. As the frequency modulation means 18, for example, when the excitation light source is formed by a laser diode or the like, the output light can be easily frequency-modulated by using means that superimposes an alternating current with a small amplitude on the excitation current. . By modulating the optical frequency of the output light (excitation light) output from the excitation light source 11 in this way, the coherence length of the output light from the excitation light source 11 is greatly reduced, and a depolarization panda having a relatively short length. Sufficient depolarization can also be achieved with the fiber 14a.

  As shown in FIG. 6, the frequency modulation means 18 may directly modulate the excitation current of the excitation light source 11 using, for example, an oscillator, or may be excited using an external optical frequency modulator. You may make it arrange | position in the optical path (for example, 1st panda fiber 12 for excitation light sources, etc.) connected with the light source 11. FIG.

  The frequency modulation by the frequency modulation means 18 requires that the dithering speed be increased to some extent in order to make the Raman gain constant. For example, the frequency is set to several MHz or more. Further, when frequency modulation is performed by modulating the excitation current, it is preferable that the amplitude of the modulation is such that the light source is not reverse-biased and modulated above the emission threshold.

  Excitation light output as output light from the excitation light source 11 is input to the panda fiber 12 for excitation light source. As shown in FIG. 6, the slow axis of the excitation light source panda fiber 12 is inclined 45 degrees with respect to the x-axis and the y-axis. The polarization plane of the excitation light output from the excitation light source 11 is matched with the slow axis of the panda fiber 12. Due to the nature of the panda fiber, the polarization state of the pump light is kept constant in the process of propagating through the panda fiber 12 for the pump light source. The pumping light output from the pumping light source panda fiber 12 is input to the depolarizing panda fiber 14a.

The slow axis of the depolarizing panda fiber 14a is parallel to the y-axis. Due to the DGD of the depolarizing panda fiber 14a, the x component and the y component of the excitation light have different optical path lengths. In the course of the excitation light propagates panda fiber 14a for depolarization, since the optical electric field in the x direction and the y direction of the excitation light due to the accumulation of DGD, the phase difference between the E _x and E _y which varies gradually, illustrating a prior art FIG. As schematically shown in FIG. 9, the polarization of the pumping light changes variously. However, in the present embodiment, since the coherence length of the pumping light is greatly shortened by the frequency modulation means 18, the depolarizing panda fiber 14a has a much shorter length than the conventional example shown in FIG. Depolarization can be achieved.

  Next, an example of the operation of the Raman amplifier excitation device 1 according to the fifth embodiment will be described with reference to FIG.

  The single-polarized pump light output as output light from the pump light source 11 is output in a state where the frequency is modulated by the frequency modulation means 18 provided in the pump light source 11 and the coherency is greatly deteriorated. The output light is input to the depolarization panda fiber 14a via the excitation light source panda fiber 12, and the polarization plane of the output light (excitation light) of the excitation light source 11 is the fast axis and the slow axis of the depolarization panda fiber 14a. It is inclined 45 degrees with respect to both axes. Further, since the depolarizing panda fiber 14a has DGD, the excitation light is depolarized.

  The depolarized output light (excitation light) output from the depolarization panda fiber 14 a is output from the excitation light output port 17.

  The Raman amplifier excitation apparatus 1 according to the fourth embodiment described above uses the depolarization panda fiber 14a that depolarizes and outputs the input excitation light, and also uses the polarization plane of the output light output from the excitation light source 11 as follows. The depolarization panda fiber 14a is inclined 45 degrees with respect to both the fast axis and the slow axis, and the coherence is deteriorated by modulating the excitation light output from the excitation light source 11. It is possible to provide sufficient depolarization to the pump light for Raman amplification using a short polarization maintaining optical transmission medium, and the Raman amplifier excitation device 1 can be downsized.

(Embodiment 6)
Hereinafter, Embodiment 6 of the present invention will be described in detail with reference to the drawings. In the following description, the same structure and the same members as those of the above-described fourth embodiment are denoted by the same reference numerals, and detailed description thereof is omitted or simplified.

  FIG. 7 is a schematic diagram showing a configuration of a Raman amplifier excitation apparatus 1 according to Embodiment 6 of the present invention. The Raman amplifier excitation apparatus 1 shown in FIG. 7 uses two frequency-modulated excitation light sources 11a and 11b, and the first panda coupler 13 preliminarily converts the excitation light output from the two excitation light sources 11a and 11b. Unlike the fourth embodiment, the two polarization planes of the pumping light are input with being inclined by 45 degrees with respect to the optical axis of the depolarizing panda fiber 14a after being wave-multiplexed. Two excitation light sources (first excitation light source 11a and second excitation light source 11b) for outputting excitation light) and first frequency modulation means 18a for modulating the frequency of the output light from the first excitation light source 11a. The first polarization maintaining power having the second frequency modulation means 18b for modulating the frequency of the output light from the second excitation light source 11b, the two input ports 13a and 13b, and the one output port 13c. It has a first expander coupler 13 is La, and depolarized for panda fiber 14a is polarization maintaining optical transmission medium having a DGD, the pumping light output port 17 for outputting excitation light depolarized, as basic structure.

  The pumping light sources 11a and 11b constituting the Raman amplifier pumping apparatus 1 output pumping light as output light with a single polarization as in the fourth embodiment. In the present embodiment, the first pumping light is output. Two excitation light sources, a light source 11a and a second excitation light source 11b, are provided.

  In this embodiment, it is set as the structure provided with the frequency modulation means 18a and 18b which performs optical frequency modulation of the above-mentioned excitation light sources 11a and 11b. As the frequency modulation means 18a and 18b, the same one as in the fourth embodiment can be used.

  Similar to the fourth embodiment, the coherence length of the pumping light is significantly shortened, and sufficient depolarization can be achieved even with a relatively short depolarization panda fiber 14a.

  The single-polarized output light (pump light) output from the two pump light sources 11a and 11b is input to the first pump light source panda fiber 12a and the second pump light source panda fiber 12b, respectively. The polarization plane of the output light (excitation light) output from the first excitation light source 11a and input to the first excitation light source panda fiber 12a and the slow axis of the first excitation light source panda fiber 11a are matched. On the other hand, the plane of polarization of the output light (excitation light) output from the second excitation light source 11b and input to the second excitation light source panda fiber 12b matches the fast axis of the second excitation light source panda fiber 12b. Let me. Since the slow axes of the first excitation light source panda fiber 12a and the second excitation light source panda fiber 12b are inclined by 45 degrees with respect to the x axis and the y axis, the first excitation light source 11a and the second excitation light source The polarization plane of the excitation light output from the light source 11b is inclined 45 degrees with respect to the x axis and the y axis. Due to the nature of the panda fiber, the polarization state of the pumping light is kept constant in the process of propagating through the first pumping light source panda fiber 12a and the second pumping light source panda fiber 12b.

  The first polarization source 11a is connected to the first excitation light source panda fiber 12b, and the second excitation light source 11b is connected to the second excitation light source panda fiber 12b. The first panda coupler 13 which is a wave holding coupler has two input ports 13a and 13b and one output port 13c, and pumps excitation light which is output light from the two excitation light sources 11a and 11b. The light is input to the two input ports 13a and 13b via the panda fibers 12a and 12b for the light source, and is output from the output port 13c while maintaining the state where the polarizations are orthogonal. That is, the plane of polarization of the pump light derived from the first pump light source 11a coincides with the slow axis of the output port 13c, and the plane of polarization of the pump light derived from the second pump light source 11b coincides with the fast axis of the output port 13c. Output in the correct state.

  The depolarization panda fiber 14a, which is a polarization maintaining optical transmission medium, depolarizes and outputs the input pump light, and outputs the frequency modulation means 18a output from the output port 13c of the first panda coupler 13. The pumping light output from the two pumping light sources 11a and 11b modulated by 18b is input while maintaining the state in which the polarizations are orthogonal. The slow axis of the depolarizing panda fiber 14a is parallel to the y-axis. For this reason, as shown in FIG. 7, the polarization planes of the pumping light derived from the pumping light sources 11a and 11b are inclined 45 degrees with respect to both the fast axis and the slow axis of the depolarizing panda fiber 14a. Due to the DGD of the depolarizing panda fiber 14a, the x component and the y component of the pump light output from the pump light source 11a have different optical path lengths. The same applies to the excitation light output from the excitation light source 11b. For this reason, it becomes possible to depolarize the two excitation lights simultaneously with the depolarizing panda fiber 14a having the same length as that of the fourth embodiment.

  Similar to the fifth embodiment, in this embodiment, a Raman gain profile is obtained by providing a function of arbitrarily adjusting the wavelength and power of the output light (pump light) output from the two pump light sources 11a and 11b. It is possible to generate a flatter Raman gain by fine adjustment.

  As the changing means (not shown), for example, the oscillation wavelength of the first pumping light source 11a and the second pumping light source 11b may be changed using a temperature controller. For example, a polarization maintaining attenuator is used. Thus, it may be arranged in an optical path (for example, the first excitation light source panda fiber 12a and the second excitation light source panda fiber 12b) connected to the first excitation light source 11a and the second excitation light source 11b. Good.

  In addition, it is preferable that the difference in the wavelength of the output light (excitation light) output from the two excitation light sources 11a and 11b is, for example, larger than 0 nm and several tens of nm or less. If the wavelengths of the two pumping light sources 11a and 11b are completely the same, there is a possibility that the power of the pumping light fluctuates in the course of propagation due to the mutual interference of the two pumping lights propagating through the depolarizing panda fiber 14a. On the other hand, when the wavelength is several tens of nanometers or more, the difference between the gain bands becomes significant, which may affect the flatness of the gain.

  Next, an example of the operation of the Raman amplifier excitation device 1 according to the sixth embodiment will be described with reference to FIG.

  The single-polarized pumping light output as output light from the first pumping light source 11a is modulated in frequency by the first frequency modulation means 18a provided in the first pumping light source 11a, and the coherency is greatly deteriorated. In the same manner, the single-polarized pumping light that is input to the first pumping light source panda fiber 12a and output from the second pumping light source 11b is the second pumping light source 11b. The frequency is modulated by the frequency modulation means 18b and input to the second excitation light source panda fiber 12b in a state where the coherency is greatly deteriorated.

  The output light (excitation light) output from the first excitation light source 11a passes through the first excitation light source panda fiber 12a, the polarization plane of the output light (excitation light), and the first excitation light source panda fiber. In a state where the slow axes of 12a are matched, the signals are input from the input port 13a to the first panda coupler 13. On the other hand, the output light (excitation light) output from the second excitation light source 11b passes through the second excitation light source panda fiber 12b and the polarization plane of the output light (excitation light) and the second excitation light source The signal is input from the input port 13b to the first panda coupler 13 in a state where it matches the fast axis of the panda fiber 12b.

  The two pumping lights output from the two pumping light sources 11a and 11b modulated by the frequency modulation means 18a and 18b and input to the first panda coupler 13 remain in a state where the polarizations are orthogonal to each other. The signal is output from the output port 13c of the first panda coupler 13 and input to the depolarizing panda fiber 14a. The planes of polarization of the output light from the two excitation light sources 11a and 11b are inclined by 45 degrees with respect to both the fast axis and the slow axis of the depolarizing panda fiber 14a, and the depolarizing panda fiber 14a has a DGD. Therefore, two output lights (excitation light) are simultaneously depolarized for the reason described above. The two pumping lights simultaneously depolarized by the depolarizing panda fiber 14 a are output from the depolarizing panda fiber 14 a and output from the pumping light output port 17.

  The Raman amplifier excitation device 1 according to the sixth embodiment described above enjoys the effects of the Raman amplifier excitation device 1 according to the fourth embodiment and uses two excitation light sources 11a and 11b, so that it is flatter. Raman gain can be achieved. In the above embodiment, the polarization multiplexing is performed using the first panda coupler 13, but the polarization multiplexing may be performed using PBC instead of the panda coupler 13.

(Modification of the embodiment)
In the above-described embodiment, the panda fiber (the first branch panda fiber 14, the second branch panda fiber 15, and the depolarizing panda fiber 14a) is used as the polarization maintaining optical transmission medium. There is no problem even if a polarization-maintaining optical waveguide configured on a PLC (Planar Lightwave Circuit) substrate or other polarization-maintaining optical transmission medium is used. By using a polarization-maintaining optical waveguide configured on a PLC substrate as the polarization-maintaining optical transmission medium, there is an advantage that an optical circuit can be integrated.

  The panda fibers (pumping light source panda fiber 12, first pumping light source panda fiber 12 a, second pumping light source panda fiber 12 b, first branching panda fiber 14, and second branching shown in the above-described embodiment. The slow axis and the fast axis in the panda fiber 15 and the depolarizing panda fiber 14a) may be replaced with each other.

  The present invention can be used as an excitation means of a Raman amplifier that excites a Raman amplifier using the Raman effect, and has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Raman amplifier excitation apparatus 11 Excitation light source 11a 1st excitation light source 11b 2nd excitation light source 12 Panda fiber for excitation light sources 12a 1st panda fiber for excitation light sources 12b 2nd panda fiber for 2nd excitation light sources 13 1st panda coupler (First polarization maintaining coupler)
13a Input port (one input port)
13b The other input port 13c The output port (one output port)
13d Other output port 14 First branch panda fiber (first branch polarization maintaining optical transmission medium)
14a Panda fiber for depolarization (polarization-maintaining optical transmission medium)
15 Second branch panda fiber (second branch polarization maintaining optical transmission medium)
16 Second panda coupler (second polarization maintaining coupler)
16a one input port 16b other input port 16c output port 17 pumping light output port 18 frequency modulation means 18a first frequency modulation means 18b second frequency modulation means 20 PBC
20a One input port 20b The other input port 20c Output port 100 Conventional distributed Raman amplifier 101 Optical filter 102 Optical fiber 103 Excitation light source 104 Wavelength multiplexing coupler 200 Conventional depolarization 201 Excitation light source 202 Excitation light source panda fiber 203 Depolarization panda fiber 204 Wavelength division multiplexing coupler 205 Transmission optical fiber

Claims (14)

A first pumping light source that outputs first polarized light of single polarization as output light; a second pumping light source that outputs second pumped light of single polarization as output light;
The first pumping light and the second pumping light are input to each of the two input ports, and the first pumping light and the second pumping light are input. A first polarization maintaining coupler for branching and outputting light at the two output ports in a state where light is polarization multiplexed;
One of the polarization multiplexed pump lights output from the two output ports of the first polarization maintaining coupler is input, and the first polarization maintaining light that propagates while maintaining the polarization multiplexed state. A transmission medium;
The polarization-maintaining light of the second branch that propagates while maintaining the polarization-multiplexed state by inputting the other of the polarization-multiplexed pumping light output from the two output ports of the first polarization-maintaining coupler A transmission medium;
It has two input ports and one output port, and is the same among the polarization multiplexed pump lights output from the polarization maintaining optical transmission medium of the first branch and the polarization maintaining optical transmission medium of the second branch Are input to the two input ports so that the polarization planes of the excitation light derived from the excitation light source are orthogonal to each other,
Simultaneously performing polarization multiplexing of the first pumping light propagated through the polarization maintaining optical transmission medium of the first branch and the first pumping light propagated through the polarization maintaining optical transmission medium of the second branch, The second pumping light propagated through the polarization maintaining optical transmission medium of the first branch and the second pumping light propagated through the polarization maintaining optical transmission medium of the second branch are polarization multiplexed and the one A second polarization maintaining coupler that outputs from the output port;
A pumping light output port that outputs a double polarization multiplexed pumping light that is output from the output port of the second polarization maintaining coupler;
A difference is provided between the optical path length of the polarization maintaining optical transmission medium of the first branch and the optical path length of the polarization maintaining optical transmission medium of the second branch, and 2 output from the output port of the second polarization maintaining coupler. The Raman amplifier excitation characterized in that the difference in optical path length is made longer than the coherence length of the first excitation light and the second excitation light so that the excitation light multiplexed in a polarization manner is depolarized. apparatus. 2. The Raman amplifier excitation device according to claim 1, wherein wavelengths of the first excitation light and the second excitation light are different. Raman amplifier pumping apparatus according to claim 1 or 2, characterized in that it comprises a changing means for changing the wavelength and power of the first excitation light source and the second excitation light source. Wherein at least one of the first branch of the polarization-maintaining optical transmission medium and the second branch of the polarization-maintaining optical transmission medium, Raman amplifier according to any one of claims 1 to 3, characterized in that it comprises an optical attenuator Excitation device. Raman according to any one of claims 1 to 4, characterized in that it comprises a frequency modulation means capable of applying a modulation to the optical frequency of the output light from the first excitation light source and the second excitation light source Amplifier excitation device. Raman amplifier pumping apparatus according to any one of claims 1-5, characterized by using a panda fiber as the polarization maintaining optical transmission medium. Raman amplifier pumping apparatus according to any one of claims 1 to 6, characterized in that said polarization maintaining optical transmission medium is an optical waveguide having polarization maintaining constructed on the PLC substrate. Single-polarized pumping light is output as output light from the first pumping light source and the second pumping light source, respectively.
Output light from the first excitation light source and the second excitation light source is input to the first polarization maintaining coupler, respectively, and the first output light is orthogonalized with the polarization of the two output lights input thereto. Branch out from the polarization maintaining coupler into two, and output
One of the output lights output from the first polarization maintaining coupler is input to the polarization maintaining optical transmission medium of the first branch, and propagates while maintaining the state where the polarizations are orthogonal,
The other of the output light output from the first polarization maintaining coupler is input to the polarization maintaining optical transmission medium of the second branch, and propagates while maintaining the state where the polarization is orthogonal,
Excitation output from the output port of the second polarization maintaining coupler by providing a difference between the optical path length of the polarization maintaining optical transmission medium of the first branch and the optical path length of the polarization maintaining optical transmission medium of the second branch The difference in the optical path length is made longer than the coherence length of the excitation light source so that the light is depolarized.
Output lights of the polarization maintaining optical transmission medium of the first branch and the polarization maintaining optical transmission medium of the second branch are input to a second polarization maintaining coupler, and each of the input two output lights is input. Polarization multiplexed and output from the second polarization maintaining coupler,
A pumping method for a Raman amplifier, characterized in that the dual polarization multiplexed pumping light output from the second polarization maintaining coupler is output from a pumping light output port.   9. The Raman amplifier excitation method according to claim 8, wherein wavelengths of the first excitation light and the second excitation light are different. 10. The Raman amplifier excitation method according to claim 8, wherein the wavelength and power of the first excitation light source and the second excitation light source are changed by changing means. 11. The Raman amplifier according to claim 8, wherein at least one of the polarization maintaining optical transmission medium of the first branch and the polarization maintaining optical transmission medium of the second branch includes an optical attenuator. Excitation method. The Raman amplifier excitation method according to claim 8, wherein the frequency of output light from the first excitation light source and the second excitation light source is modulated. 13. The Raman amplifier excitation method according to claim 8, wherein a panda fiber is used as the polarization maintaining optical transmission medium. 14. The Raman amplifier excitation method according to claim 8, wherein the polarization maintaining optical transmission medium is a polarization maintaining optical waveguide configured on a PLC substrate.

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