JPH06318755A - Optical amplifier - Google Patents

Abstract

(57) [Summary] [Object] To provide an optical amplifier capable of improving a gain characteristic with a simple configuration. In an optical amplifier for amplifying input signal light by using pumping light from a pumping light source and an optical amplifying medium, a terminal A to a terminal B, a terminal B to which the signal light and the pumping light are inputted.
From the terminal C to the terminal C and from the terminal C to the terminal D, an optical circulator 20 having a forward transfer characteristic, a first optical amplification medium 23 having one end connected to the terminal B, and a first optical amplification medium 23.
A first optical reflector 24 connected to the other end of the first optical amplification medium 23 and returning the signal light and the excitation light emitted from the first optical amplification medium 23 to the first optical amplification medium 23, and one end of which is connected to the terminal C. The second optical amplification medium 25, and the signal light and the pumping light that are connected to the other end of the second optical amplification medium 25 and are emitted from the second optical amplification medium 25 are reflected by the second optical amplification medium 25. The second optical reflector 26 for returning is provided, and the amplified signal light is output from the terminal D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier, and more particularly to an optical amplifier using an optical circulator.

[0002]

2. Description of the Related Art An optical amplifier is a device for amplifying input signal light.

FIG. 8 is a block diagram of a conventional optical amplifier.

In FIG. 1, the optical amplifier includes a front optical isolator 1 to which the signal light Li is input, a front pumping light source 2 and a rear pumping light source 3 which generate pumping light, and a signal light Li from the front optical isolator 1. A front optical multiplexer 4 that multiplexes the pumping light from the front pumping light source 2, one end of the rare-earth-doped optical fiber 5 as an amplification medium connected to the front optical multiplexer 4, and the other end of the rare-earth-doped optical fiber 5. And a rear optical multiplexer 6 connected to the rear pumping light source 3 and a rear optical isolator 7 connected to the rear optical multiplexer 6 and outputting the amplified signal light Lo.

When the signal light Li is input to the front optical isolator 1, the signal light Li enters the rare earth-doped optical fiber 5 via the front optical isolator 1 and the front optical multiplexer 4.

On the other hand, the pumping light emitted from both pumping light sources 2 and 3 is incident on the rare earth-doped optical fiber 5 via both optical multiplexers 4 and 6, respectively. The excitation light is signal light Li due to the stimulated emission phenomenon caused by the population inversion of the energy level of the rare earth element ions added to the rare earth-doped optical fiber 5.
To amplify. The amplified signal light Lo is output via the backward optical multiplexer 6 and the backward optical isolator 7.

Here, in both the optical isolators 1 and 7 provided on the input and output sides, the signal light reflected from the outside and the spontaneous emission light generated in the rare earth-doped optical fiber 5 return to the inside of the amplifier, and the optical amplifier It prevents an unstable state such as oscillation and amplification.

The above-mentioned optical amplifier uses a method of pumping from both the front side (incident side of the signal light Li) and the rear side (exit side of the signal light Lo) of the rare earth-doped optical fiber 5, which is used. It is a pumping method used to enhance noise, gain characteristics, and saturated output characteristics, and sufficiently functions as an amplifier by pumping only forward or backward.

Further, in order to improve the amplification efficiency by one pumping light source, a reflection pumping type optical amplifier as shown in FIG. 9 has also been proposed (Japanese Patent Laid-Open No. 4-29123).
FIG. 9 is a block diagram of another conventional example of the optical amplifier. still,
The same reference numerals are used for the members common to the above-mentioned optical amplifier.

In the figure, the optical amplifier is a three-terminal type optical circulator 8 having a forward transfer characteristic from the terminal A to the terminal B to which the signal light Li is input, and from the terminal B to the terminal C.
The optical multiplexer 9 connected to the terminal B, the excitation light source 10 connected to the optical multiplexer 9, the rare earth-doped optical fiber 5 having one end connected to the optical multiplexer 9, and the other end of the rare earth-doped optical fiber 5. And a light reflector 11 connected to.

The signal light Li incident from the terminal A is
After that, the light is multiplexed with the pumping light from the pumping light source 10 by the optical multiplexer 9 and is incident on the rare earth-doped optical fiber 5. Signal light L
i passes through the rare earth-doped optical fiber 5, is amplified, and then is reflected by the optical reflector 11 and again returns to the rare earth-doped optical fiber 5 to pass and is amplified. The amplified signal light Lo is output from the terminal B to the terminal B. It reaches C and is output.

According to this method, the signal light Li will pass back in the rare earth-doped optical fiber 5 and the pumping light will also pass back in the rare earth-doped optical fiber 5, thus being efficiently amplified. In addition, good noise characteristics and saturated output characteristics can be obtained.

FIG. 10 is a diagram showing the relationship of the amplification gain G with respect to the power Pi of the signal light Li of the above-mentioned optical amplifier. The horizontal axis represents the power Pi of the signal light Li and the vertical axis represents the amplification gain G. Shows.

In the figure, the solid line shows the characteristics of the optical amplifier shown in FIG. It can be seen that the amplification gain G maintains a constant value in a region where the input power Pi of the signal light Li is small, but the amplification gain G decreases with an increase in the input power Pi in a region where the input power Pi is large. Of these, the latter region is caused by the fact that the pump light power is finite and the output is saturated, and improvement can be achieved by increasing the pump light power.

On the other hand, the amplification gain G in the region where the input power Pi is small is limited because the spontaneous emission light generated in the rare earth-doped optical fiber 5 propagates to the signal incident side while being amplified. Receive. Therefore, in such a region, even if an attempt is made to increase the pumping light power to increase the amplification gain G, the spontaneous emission light is also amplified accordingly, and there is a disadvantage that a large improvement cannot be expected.

Therefore, an optical amplifier as shown in FIG. 11 has been proposed.

The optical amplifier shown in the same figure has a configuration similar to that of the optical amplifier shown in FIG. 8, except that the rare earth-doped optical fiber is divided into two and an optical isolator is inserted between them.

With this structure, the spontaneous emission light generated in the rear rare earth-doped optical fiber 5b is
The optical isolator 12 provided between the front and rear rare-earth-doped optical fibers 5a and 5b prevents the optical isolator 12 from propagating to the forward rare-earth-doped optical fiber 5a, and thus cannot propagate to the forward rare-earth-doped optical fiber 5a. . Therefore, the spontaneous emission light reaching the incident side of the signal light Li can be significantly reduced, so that the amplification gain G in the region where the input power Pi of the signal light Li is small can be improved as shown by the broken line in FIG. 7 #. .

[0019]

However, as shown in FIG.
In the method shown in, at least three optical isolators 1,
Since 7 and 12 are required, the optical amplifier becomes large and complicated. Further, since the method shown in FIG. 11 cannot be applied to the reflection pumping type optical amplifier shown in FIG. 9, there is a problem that the upper limit of the amplification gain is limited.

Therefore, an object of the present invention is to solve the above problems and to provide an optical amplifier which can improve the gain characteristic with a simple structure.

[0021]

In order to achieve the above object, the present invention provides an optical amplifier that amplifies input signal light by using pumping light from an optical amplification medium and a pumping light source. An optical circulator having a forward transfer characteristic from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D, a first optical amplification medium whose one end is connected to the terminal B, and A first optical reflector connected to the other end of the first optical amplification medium and reflecting the light emitted from the first optical amplification medium back to the first optical amplification medium; and a second optical reflector having one end connected to the terminal C. And a second optical reflector which is connected to the other end of the second optical amplification medium and which reflects the light emitted from the second optical amplification medium back to the second optical amplification medium. The amplified signal light is output from the terminal D.

Further, according to the present invention, in the optical amplifier for amplifying the input signal light by using the pumping light from the optical amplification medium and the pumping light source, the signal light is input from the terminal A to the terminal B.
An optical circulator having forward transfer characteristics from the terminal B to the terminal C and from the terminal C to the terminal D, a first optical amplification medium having one end connected to the terminal B, and another end connected to the first optical amplification medium. Optical multiplexer, an excitation light source connected to the optical multiplexer and emitting excitation light, a first optical reflector connected to the optical multiplexer and returning light emitted from the optical multiplexer to the optical multiplexer, and one end A second optical amplification medium connected to the terminal C, and a second optical amplification medium connected to the other end of the second optical amplification medium, the light emitted from the second optical amplification medium being reflected back to the second optical amplification medium. The second optical reflector is provided, and the amplified signal light is output from the terminal D.

Furthermore, the present invention is an optical amplifier for amplifying input signal light by using pumping light from an optical amplifying medium and a pumping light source, from terminal A to terminal B to which signal light is input, and terminal B to terminal C. And an optical circulator having forward transfer characteristics from the terminal C to the terminal D, a first optical multiplexer connected to the terminal B, and a first optical amplification medium having one end connected to the first optical multiplexer. A second optical multiplexer connected to the other end of the first optical amplification medium; a pumping light source connected to the second optical multiplexer to emit pumping light; and a second optical multiplexer connected to the second optical multiplexer. A first optical reflector that reflects the light emitted from the second optical multiplexer back to the second optical multiplexer, and a third optical multiplexer connected to the terminal C and the first optical multiplexer , A second optical amplifying medium whose one end is connected to the third optical multiplexer, and the other end of which is connected to the second optical amplifying medium. The light emitted from the second optical amplifying medium and a second second optical reflector reflected back to the optical amplifying medium is intended to output the amplified signal light from the terminal D.

Furthermore, according to the present invention, in an optical amplifier for amplifying input signal light by using pumping light from an optical amplification medium and a pumping light source, the signal light is input from terminal A to terminal B and from terminal B to terminal. C and an optical circulator having a forward transfer characteristic from the terminal C to the terminal D, a first optical multiplexer connected to the terminal B, and a first optical amplifier whose one end is connected to the first optical multiplexer. A medium, a first optical reflector connected to the other end of the first optical amplification medium and reflected from the first optical amplification medium to return to the first optical amplification medium, and connected to a terminal C. A second optical multiplexer, a pumping light source connected to the second optical multiplexer to emit pumping light, a second optical amplification medium having one end connected to the second optical multiplexer, A third optical multiplexer connected to the other end of the optical amplifying medium and the first optical multiplexer, and a third optical multiplexer connected to the third optical multiplexer. By the light emitted from the third optical multiplexer and a third optical multiplexer second light reflectors reflect back to one in which to output the amplified signal light from the terminal D.

The present invention is an optical amplifier for amplifying input signal light by using pumping light from a pumping light source and an optical amplification medium, from the terminal A to which the signal light is input to the terminal B and from the terminal B to the terminal C. An optical circulator having a forward transfer characteristic, an optical multiplexer connected to the terminal B, an excitation light source connected to the optical multiplexer, an optical amplification medium having one end connected to the optical multiplexer, and an optical amplification medium The optical reflector, which is connected to the other end through the lens and is emitted from the optical amplification medium, reflects the signal light and the excitation light to return to the optical amplification medium and returns the light passing through the optical reflector arranged between the optical reflector and the lens. A non-reciprocal polarization rotation element that rotates so that the planes of polarization are orthogonal to each other is provided, and the amplified signal light is output from the terminal C.

The present invention is an optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium, and an optical multiplexer to which the signal light and pumping light from the pumping light source are input, and one end thereof. A first optical amplification medium connected to the optical multiplexer, and a terminal A connected to the other end of the first optical amplification medium
From the terminal B to the terminal B and from the terminal B to the terminal C, a second optical amplification medium having one end connected to the terminal B, and a second optical amplification medium connected to the other end of the second optical amplification medium. And an optical reflector that reflects and returns the signal light and the pumping light emitted from the second optical amplification medium to the second optical amplification medium,
The amplified signal light is output from the terminal C.

The present invention is an optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplification medium, wherein a first optical multiplexer receives the signal light and pumping light from the pumping light source. A first optical amplification medium whose one end is connected to the first optical multiplexer, a second optical multiplexer connected to the other end of the first optical amplification medium, and a second optical multiplexer. An optical circulator having forward transfer characteristics from the connected terminal A to the terminal B and from the terminal B to the terminal C; and the terminal B and the second
And a third optical multiplexer connected to the optical multiplexer of
Second optical amplification medium connected to the second optical amplification medium, and the signal light and the excitation light emitted from the second optical amplification medium connected to the other end of the second optical amplification medium to the second optical amplification medium. An optical reflector for reflecting and returning the signal light is output from the terminal C.

[0028]

According to the above structure, a reflection type optical amplifier is provided by an optical circulator having a forward transfer characteristic from the terminal A to the terminal B for inputting the signal light, the terminal B to the terminal C and the terminal C to the terminal D. Since it is separated into two stages, the spontaneous emission light of the latter stage does not affect the former stage, and the upper limit of the amplification gain is improved. Since the signal light reciprocates in the amplification medium in the front stage and the rear stage of the optical amplifier, a high amplification gain can be obtained with a small pumping light power, and high efficiency can be realized. Since one optical circulator and one pumping light source excite the amplifying medium in the front and rear two stages, three optical isolators can be omitted, and the optical amplifier can be miniaturized and simplified.

[0029]

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram of an embodiment of the optical amplifier of the present invention.

In the figure, reference numeral 20 denotes a four-terminal type optical circulator having forward transfer characteristics from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D to which the signal light Li is input. is doing.

An optical multiplexer 21 is connected to the terminal A of the optical circulator 20 by an optical fiber, and an excitation light source 22 for emitting excitation light is connected to the optical multiplexer 21. An optical fiber is connected to the optical multiplexer 21, and when the signal light Li having the input power Pi is input to this optical fiber, the pump light from the pump light source 22 and the signal light Li are multiplexed. There is.

The terminal B of the optical circulator 20 is connected to one end of a rare earth-doped optical fiber 23 as a first optical amplification medium, and the other end of the rare earth-doped optical fiber 23 is a first optical reflector (for example, a mirror). ) 24.
The terminal C of the optical circulator 20 is connected to one end of a rare earth-doped optical fiber 25 as a second optical amplification medium, and the other end of the rare earth-doped optical fiber 25 is connected to a second optical reflector (eg, mirror) 26. It is connected. The rare earth elements include scandium (Sc), yttrium (Y), and the lanthanoids lanthanum (La) and cerium (C).
e), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),
Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Y
b), lutetium (Lu) and the like.

These optical circulator 20 and optical multiplexer 2
1, an excitation light source 22, rare earth-doped optical fibers 23 and 25, and optical reflectors 24 and 26 constitute an optical amplifier,
The amplified signal light Lo (power Po) is output from the terminal D of the optical circulator 20.

Next, the operation of the embodiment will be described.

In the optical amplifier shown in FIG. 1, the wavelength of the pumping light and the wavelength of the signal light are close to each other, and the optical circulator normally operates at the wavelengths of both lights.

The excitation light emitted from the excitation light source 22 reaches the terminal B from the terminal A of the optical circulator 20 through the optical multiplexer 21, enters the rare earth-doped optical fiber 23, and excites the rare earth-doped optical fiber 23. The excitation light emitted from the rare earth-doped optical fiber 23 is reflected by the light reflector 24 and returns to the rare earth-doped optical fiber 23 to pass therethrough.
From terminal B to terminal C. The excitation light emitted from the terminal C enters the rare earth-doped optical fiber 25 and excites the rare earth-doped optical fiber 25.

On the other hand, the signal light Li input to the optical multiplexer 21
Similarly to the excitation light, the light also goes from the terminal A to the terminal B of the optical circulator 20, is amplified by being passed back through the rare earth-doped optical fiber 23 by the optical reflector 24, and then reaches the terminal C from the terminal B to the optical reflector 26. Is further amplified by being passed back through the rare earth-doped optical fiber 25. The amplified signal light Lo reaches the terminal D from the terminal C and is output from the terminal D.

Here, the optical circulator 20 has a terminal B
Since it has a forward characteristic from the terminal C to the terminal C, it functions as a kind of optical isolator between the terminals B and C.
Therefore, the spontaneous emission light generated in the rare earth-doped optical fiber 25 is blocked by the optical circulator 20 and is not propagated to the rare earth-doped optical fiber 23.

In the above, in the present embodiment, the former stage (on the side of the rare earth-doped optical fiber 23 and the optical reflector 24).
Is not affected by the latter stage composed of the rare earth-doped optical fiber 25 and the optical reflector 26, and the optical amplifier as a whole can obtain a high amplification gain. Besides, in the first stage,
In both the latter stages, the signal light Li is the rare-earth-doped optical fiber 23,
Since it is configured to pass back inside 25, the amplification efficiency is high and a high amplification gain can be obtained with a small input power Pi. Further, conventionally, two pumping light sources 2, 3 and three optical isolators 1, 7, 12 were required as in the optical amplifier shown in FIG. 8 #, whereas the pumping light source is a component. Since only one unit and one optical circulator are required, the number of parts can be greatly reduced and the configuration can be simplified.

FIG. 2 is a block diagram of another embodiment of the optical amplifier of the present invention. The same reference numerals are used for the members common to the members shown in FIG.

The difference from the optical amplifier shown in FIG.
The point is that an excitation light source is provided between the optical reflector of 1) and the first rare earth-doped optical fiber via an optical multiplexer, and the signal light Li is directly input to the terminal A of the optical circulator. Further, in the optical amplifier shown in FIG. 2, the wavelength of the pumping light and the wavelength of the signal light are close to each other as described above,
It is assumed that the optical circulator normally operates at the wavelengths of both lights.

The pumping light emitted from the pumping light source 22 enters the rare earth-doped optical fiber 23 as the first optical amplifying medium through the optical multiplexer 21, and enters the rare earth-doped optical fiber 23.
From the terminal B to the terminal C of the optical circulator 20 to excite the rare earth-doped optical fiber 25 as the second optical amplification medium.

On the other hand, the signal light Li incident from the terminal A of the optical circulator 20 through the optical fiber reaches the terminal B, is amplified through the rare earth-doped optical fiber 23, is then amplified by the optical multiplexer 21, and is then reflected by the optical reflector 24. Is further amplified when it returns to the rare earth-doped optical fiber 23 through the optical multiplexer 21 and passes through again, and enters the terminal B. The amplified signal light travels from the terminal B to the terminal C, passes through the rare earth-doped optical fiber 25, is reflected by the optical reflector 26, is incident on the rare earth-doped optical fiber 25 again, and is further amplified. Incident. The amplified signal light Lo has a terminal C
Is output to the terminal D. In addition, rare-earth doped optical fiber 2
Since the spontaneous emission light generated at 5 is blocked by the optical circulator 20 and does not propagate to the rare earth-doped optical fiber 23, the front stage of the optical amplifier is the rear stage composed of the rare earth-doped optical fiber 25 and the optical reflector 26, as described above. It is possible to obtain a high amplification gain as a whole without being affected by.

FIG. 3 is a block diagram of still another embodiment of the optical amplifier according to the present invention.

The difference from the optical amplifier shown in FIG. 1 is that the terminals B and C of the optical circulator are connected via two optical multiplexers.

As shown in FIG. 3, the signal light Li is input to the terminal A of the optical circulator 20 through the optical fiber,
The first optical multiplexer 27 is connected to the terminal B of the optical circulator 20.
Are connected. One end of the rare earth-doped optical fiber 23 as the first optical amplification medium is connected to the optical multiplexer 27. A second optical multiplexer 28 is connected to the other end of the rare earth-doped optical fiber 23, and the optical multiplexer 28 reflects the signal light Li emitted from the pumping light source 22 and the optical multiplexer 28 to cause the optical multiplexer 28. A first optical reflector 24 returning to the rare earth-doped optical fiber 23 is connected via the. The third optical multiplexer 29 is connected to the terminal C of the optical circulator 20,
The optical multiplexer 29 is connected to the optical multiplexer 27 via an optical fiber and is also connected to one end of a rare earth-doped optical fiber 25 as a second optical amplification medium. The other end of the rare earth-doped optical fiber 25 is connected to a second optical reflector 26 that reflects the signal light emitted from the rare earth-doped optical fiber 25 and returns the signal light to the rare earth-doped optical fiber 25 again. The light Lo enters the terminal C and is output from the terminal D.

In the optical amplifier shown in FIG. 1, the signal light wavelength and the pumping light wavelength have been described as being close to each other, but the two wavelengths are not always close to each other. If the wavelength of the signal light and the wavelength of the pumping light are distant from each other, the optical circulator will not operate properly.

However, in the case of the optical amplifier shown in FIG. 3, the pumping light emitted from the pumping light source 22 passes through the rare-earth-doped optical fiber 23 and then the optical multiplexers 27 and 29 to reach the rare-earth-doped optical fiber 25. To excite. on the other hand,
The signal light Li enters the terminal A and reaches the terminal B,
Optical multiplexer 27, rare earth-doped optical fiber 23, optical multiplexer 2
After passing through 8, the signal is amplified and reflected by the optical reflector 24, again amplified by the optical multiplexer 28 and the rare earth-doped optical fiber 23, and reaches the optical multiplexer 27 and the terminal B. The signal light incident on the terminal B is amplified from the terminal C via the optical multiplexer 29 and the rare earth-doped optical fiber 25, is reflected by the light reflector 26, is incident on the rare earth-doped optical fiber 25 again, is further amplified, and is further input to the terminal C. Reach The amplified signal light Lo enters the terminal C and then is output from the terminal D.

In the above, the optical amplifier shown in FIG. 3 can obtain the same amplifying action as in the case of the optical amplifier shown in FIG. 1, but since the pumping light does not pass through the optical circulator, the wavelength of the signal light and the pumping light are Even if the wavelength is significantly different, it operates without problems.

FIG. 4 is a block diagram of another embodiment of the optical amplifier of the present invention.

Similar to the optical amplifier shown in FIG. 3, two rare earth-doped optical fibers 23 (first optical amplifying medium) and 25 (second optical amplifying medium) and two optical reflectors 24 (first optical amplifying medium). 2 optical reflectors), 26 (first optical reflectors), and 3 optical multiplexers 2
7 (second optical multiplexer), 28 (first optical multiplexer), 29
(Third optical demultiplexer). The excitation light emitted from the excitation light source 22 passes through the optical multiplexer 28 through the rare earth-doped optical fiber 23, passes through the two optical multiplexers 27 and 29, and is incident on the rare earth-doped optical fiber 25. Exciting the fibers 23, 25.

On the other hand, the signal light Li incident on the terminal A of the optical circulator 20 reaches the terminal B from the terminal A, passes through the optical multiplexer 29, passes through the rare earth-doped optical fiber 25, is amplified, and is then reflected by the optical reflector 24. The optical multiplexer 2 after being processed and returned to the rare earth-doped optical fiber 25 and further amplified
It is incident on the terminal B via 9. The signal light incident on the terminal B passes through the optical multiplexer 28, is input to the rare earth-doped optical fiber 23, is amplified, is reflected by the optical reflector 26, and is incident on the rare earth-doped optical fiber 23 again and is amplified. It reaches the terminal C of the optical circulator 20. The amplified signal light Lo is output from the terminal D. Therefore, since the pumping light does not pass through the optical circulator 20, the pumping light operates without problems even if the wavelength of the signal light Li and the wavelength of the pumping light are significantly different.

As described above, according to this embodiment, a reflection type optical circulator having a forward transfer characteristic from the terminal A to the terminal B for inputting the signal light, the terminal B to the terminal C, and the terminal C to the terminal D is used. Since the optical amplifier is separated into the front and rear stages, the spontaneous emission light in the rear stage does not affect the front stage, and the upper limit of the amplification gain is improved. Since the signal light reciprocates in the amplification medium in the front stage and the rear stage of the optical amplifier, a high amplification gain can be obtained with a small pumping light power, and high efficiency can be realized. Since one optical circulator and one pumping light source excite the amplifying medium in the front and rear two stages, three optical isolators can be omitted, and the optical amplifier can be miniaturized and simplified.

By the way, the reflection type optical amplifier described above is characterized in that high amplification efficiency and good noise characteristics can be obtained, but on the other hand, excessive noise due to the use of reflection may occur. is there.

The excess noise will be described with reference to FIG.

The optical components such as the optical multiplexer 9, the optical circulator 8 and the like, which constitute the optical amplifier, cause a slight reflection of light. If there is a reflection point having a small reflectance R at the terminal B of the optical circulator 8, a part of the amplified signal light Lo is reflected at this reflection point and is propagated back in the rare earth-doped optical fiber 5 and amplified again. To be done. Assuming that the gain of the amplifier is G, the reflected light output that has been folded and amplified becomes RGPo. The phase noise of the signal light is converted into the intensity noise due to the interference between the signal light Lo and the reflected light that has been reflected and amplified. Spectral width Δ as signal light
Considering CW (continuous) light having a ν Lorentz type spectrum shape, and the optical path length of the rare earth-doped optical fiber is longer than the coherence length of the signal light source. The noise due to the interference becomes maximum when the polarizations of the two interfering lights coincide with each other, and the spectrum of the relative intensity noise (hereinafter referred to as RIN) at this time is expressed by the formula 1.

[0058]

[Equation 1]

For example, the gain G is 30 dB and the reflectance R is -5.
If 0 dB and the spectral width Δν of the light source are 100 MHz, the RIN at low frequencies reaches -100 dB or more, and not only analog signal transmission that requires a high S / N ratio such as video signals but also digital signal transmission. In this case, too, the transmission quality will be greatly deteriorated. Number 1
As is clear from the above, RIN is proportional to R and G, and there is an advantage that a high gain can be obtained in the reflection type amplifier, but when the gain is high, the above-mentioned interference noise becomes large,
There is a trade-off of degrading the signal S / N ratio.

The optical amplifier shown in FIG. 8 has the characteristics shown in FIG.

FIG. 12 is a graph showing the relationship between the gain and the noise figure with respect to the length of the rare earth-doped optical fiber. The horizontal axis represents the length of the rare earth-doped optical fiber doped with Er, and the vertical axis represents the small signal. Gain and noise figure are shown.

The wavelength of the pumping light source is 1480 nm, and the pumping light power has a certain value. The solid line in the figure is the characteristic curve of the amplifier shown in FIG. The gain increases as the length of the Er-doped rare earth-doped optical fiber 5 increases, but when the length exceeds a certain length, the gain decreases conversely with the length. Therefore, there is a certain optimum length in the length of the rare earth-doped optical fiber 5 that maximizes the gain.

On the other hand, the noise figure shows a tendency to increase monotonically with the length of the rare earth-doped optical fiber, and as it further increases, it converges to a certain noise figure.

Therefore, there is a trade-off relationship between the gain and the noise characteristic, and if the length of the rare earth-doped optical fiber 5 is optimized so as to increase the gain, the noise characteristic is deteriorated. . This means that when the gain increases, the spontaneous emission light generated in the rare earth-doped optical fiber 5 is also amplified as it propagates in the rare earth-doped optical fiber 5. Of the amplified spontaneous emission light, the excitation light is consumed by the spontaneous emission light propagating to the signal incident end side, and the population inversion of the energy levels of the rare earth ions near the signal incident end is relaxed and the noise figure deteriorates. is there.

Therefore, the present inventors have proposed an optical amplifier as shown in FIG.

The feature of this optical amplifier is that it is 4
The point is that a Faraday element of 5 degrees is arranged.

This optical amplifier includes a three-terminal optical circulator 30 having forward transmission from terminal A to terminal B and terminal B to terminal C to which signal light is input, and an optical multiplexer 31 connected to terminal B. , A pumping light source 32 which is connected to the optical multiplexer 31 and generates pumping light, a rare earth-doped optical fiber 33 whose one end is connected to the optical multiplexer 31, and a rare earth-doped optical fiber 3
An optical reflector 35 which is connected to the other end of 3 through a lens 34 and which reflects the signal light and the excitation light emitted from the rare earth-doped optical fiber 33 back to the rare earth-doped optical fiber 33 and returns the optical reflector 35 and the lens 34. And a Faraday element (using the magneto-optical effect) 36 of 45 degrees as a non-reciprocal polarization rotation element that is arranged between and and rotates so that the polarization planes of the light passing back are orthogonal to each other. Signal light L
This is to output o from the terminal C.

Light reflector 35 and rare earth-doped optical fiber 33
Since the lens 34 and the Faraday element 36 are provided between the Faraday element 3 and the Faraday element 3, the light emitted from the rare earth-doped optical fiber 33 is collimated by the lens 34.
After passing through 6, the light is reflected by the light reflector 35, and again enters the rare earth-doped optical fiber 33 through the Faraday element 36 and the lens 34.

When the polarization rotation angle of the Faraday element 36 is set to 45 degrees, since the Faraday element 36 is non-reciprocal, the light emitted from the end of the rare-earth-doped optical fiber 33 passes back through the Faraday element 36. By doing so, the polarized light is rotated by 90 degrees and returns to the rare earth-doped optical fiber 33 again.

With such a configuration, even if a part of the amplified output signal light is reflected by the optical circulator 30 and the like and is again passed through the rare earth-doped optical fiber 33 and is amplified, the aliased light is returned. The amplified reflected light does not interfere with the original output signal because it has a polarization state orthogonal to each other. That is, the output signal light Lo and the reflected light that is reflected and propagates back and propagated are orthogonalized to each other to prevent them from interfering with each other.
It is possible to prevent the generation of excess noise due to the interference as described above.

The present inventors also proposed an optical amplifier as shown in FIG.

The characteristic of this optical amplifier is that the rare earth-doped optical fiber is arranged immediately before the signal incident end of the optical circulator.

This optical amplifier includes an optical multiplexer 31 to which the signal light Li and the pumping light from the pumping light source 32 are input, and a rare-earth-doped light as a first optical amplifying medium whose one end is connected to the optical multiplexer 31. Fiber 33 and rare-earth doped optical fiber 3
3, an optical circulator 30 having forward transfer characteristics from terminal A to terminal B and terminal B to terminal C connected to the other end, and a rare earth addition as a second optical amplification medium having one end connected to terminal B The optical fiber 37 and the optical reflector 35 which is connected to the other end of the rare earth-doped optical fiber 37 and which reflects the signal light and the excitation light emitted from the rare earth-doped optical fiber 37 back to the rare earth-doped optical fiber 37 are amplified. The signal light Lo is output from the terminal C.

In such a structure, it is assumed that the wavelength of the pumping light is close to the wavelength of the signal light Li, and the optical circulator 30 operates correctly at both the wavelengths of the signal light Li and the pumping light. In this case, the excitation light emitted from the excitation light source 32 enters the rare earth-doped optical fiber 33 through the optical multiplexer 31 and excites it, and further reaches the terminal B from the terminal A of the optical circulator 30 to reach the rare earth-doped optical fiber 37. Excite. on the other hand,
The signal light Li is sent from the optical multiplexer 31 to the rare earth-doped optical fiber 3
After being input to the optical circulator 3 and amplified, it reaches the terminal B from the terminal A of the optical circulator 30, passes through the rare earth-doped optical fiber 37, is amplified, is reflected by the optical reflector 35, and passes through the rare earth-doped optical fiber 37 again. Is further amplified. The amplified signal light Lo is output from the terminal B to the terminal C.

Here, all the spontaneous emission light generated in the rare earth-doped optical fiber 37 reaches the terminal C from the terminal B by the operation of the optical circulator 30, and therefore cannot propagate to the rare earth-doped optical fiber 33. Therefore, it is possible to prevent the noise figure from increasing and the gain from decreasing due to the amplified spontaneous emission light propagating to the signal light incident end. Therefore, the characteristics of the reflection type amplifier can be fully utilized, and a high amplification gain can be obtained with a small pumping power without deterioration of noise characteristics. Moreover, only one excitation light source and one optical circulator are required as constituent parts, which can greatly reduce the number of parts and simplify the structure.

The optical amplifier shown in FIG. 6 deals with the case where the wavelength of the signal light and the wavelength of the pumping light are close to each other.
In reality, the two wavelengths are not necessarily close to each other, and in this case, the optical circulator does not operate properly with respect to the wavelength of the excitation light. Another embodiment in which the wavelength of the signal light and the wavelength of the pump light are separated from each other in this way will be described with reference to FIG.

The overall structure is similar to that of FIG. 6, but the optical multiplexer 3 is provided between the terminals A and B of the optical circulator 30.
The difference is that they are connected via 8 and 39.

This optical amplifier has a first optical multiplexer 31 to which the signal light Li and the pumping light from the pumping light source 32 are input.
A rare earth-doped optical fiber 33 as a first optical fiber amplification medium having one end connected to the optical multiplexer 31, a second optical multiplexer 38 connected to the other end of the rare earth-doped optical fiber 33, and an optical multiplexer. An optical circulator 30 having a forward transfer characteristic from the terminal A to the terminal B and a terminal B to the terminal C connected to the wave multiplexer 38; and a third optical multiplexer 39 connected to the terminal B and the optical multiplexer 38. , A rare earth-doped optical fiber 37 having one end connected to the optical multiplexer 39 and serving as a second optical amplification medium, and signal light and excitation light emitted from the rare earth-doped optical fiber 37 connected to the other end of the rare earth-doped optical fiber 37. And a light reflector 35 that reflects the light back to the rare earth-doped optical fiber 37 and outputs the amplified signal light Lo from the terminal C.

With this structure, the excitation light emitted from the excitation light source 32 is supplied with the rare earth-doped optical fiber 33.
To excite this. On the other hand, the same amplification action as that of the optical amplifier shown in FIG. 6 is obtained for the signal light Li.

Therefore, since the pumping light does not have to pass through the optical circulator 30, the optical circulator 30 operates normally even if the wavelength of the signal light Li and the wavelength of the pumping light are largely different.

Therefore, the rare earth-doped optical fiber 33,
Even when the length of 37 is adjusted to obtain a large gain, the noise characteristic is not deteriorated. Also, the maximum gain that can be reached is improved, a high amplification gain can be obtained with a small amount of pumping light power, a highly efficient optical amplifier can be obtained, and the number of small parts is small and the configuration is simple, so downsizing and cost reduction can be achieved. It is possible.

[0082]

In summary, according to the present invention, the following excellent effects are exhibited.

(1) It is possible to realize an optical amplifier capable of improving gain characteristics with a simple configuration.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an optical amplifier of the present invention.

FIG. 2 is a block diagram of another embodiment of the optical amplifier according to the present invention.

FIG. 3 is a block diagram of still another embodiment of the optical amplifier according to the present invention.

FIG. 4 is a block diagram of another embodiment of the optical amplifier of the present invention.

FIG. 5 is a block diagram of another embodiment of the optical amplifier according to the present invention.

FIG. 6 is a block diagram of another embodiment of the optical amplifier according to the present invention.

FIG. 7 is a block diagram of another embodiment of the optical amplifier according to the present invention.

FIG. 8 is a block diagram of a conventional optical amplifier.

FIG. 9 is a block diagram of another conventional example of an optical amplifier.

FIG. 10 is a signal light Li of the optical amplifier shown in FIGS. 8 and 9.
FIG. 5 is a diagram showing the relationship of the amplification gain G with respect to the power Pi of FIG.

FIG. 11 is a block diagram of another conventional example of an optical amplifier.

FIG. 12 is a diagram showing the relationship between the gain and the noise figure with respect to the length of the rare earth-doped optical fiber.

[Explanation of symbols]

 20 optical circulator 21 optical multiplexer 22 pumping light source 23, 25 optical amplification medium (rare earth doped optical fiber) 24, 26 optical reflector

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area H01S 3/094

Claims (7)

[Claims] 1. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
An optical circulator having forward transfer characteristics from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D, to which the signal light and the pumping light are input, and one end thereof are connected to the terminal B. A first optical amplification medium, and a signal light and a pumping light, which are connected to the other end of the first optical amplification medium and are emitted from the first optical amplification medium, are reflected back to the first optical amplification medium. No. 1 optical reflector, a second optical amplification medium having one end connected to the terminal C, and signal light emitted from the second optical amplification medium connected to the other end of the second optical amplification medium. A second optical reflector, which reflects pumping light back to the second optical amplification medium, and outputs amplified signal light from the terminal D. 2. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
An optical circulator having a forward transfer characteristic from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D to which the signal light is input, and a first circulator having one end connected to the terminal B. An optical amplification medium, an optical multiplexer connected to the other end of the first optical amplification medium, a pumping light source connected to the optical multiplexer for emitting pumping light, and an optical multiplexer connected to the optical multiplexer. A first optical reflector that returns the light emitted from the optical device to the first optical amplification medium through the optical multiplexer, a second optical amplification medium whose one end is connected to the terminal C, and the second optical reflector. And a second optical reflector connected to the other end of the optical amplification medium, the signal light emitted from the second optical amplification medium being reflected back to the second optical amplification medium. Is output from the terminal D. 3. An optical amplifier for amplifying input signal light by using pumping light from a pumping light source and an optical amplifying medium,
An optical circulator having a forward transfer characteristic from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D, to which the signal light is input, and a first optical multiplexer connected to the terminal B. A first optical amplifying medium having one end connected to the first optical multiplexer, a second optical multiplexer having the other end connected to the first optical amplifying medium, and the second optical multiplexer. An excitation light source that is connected to the optical multiplexer and emits excitation light, and a second optical multiplexer that reflects the signal light and the excitation light that is connected to the second optical multiplexer and that is emitted from the second optical multiplexer. A first optical reflector for returning to the first optical amplifying medium via a device, a third optical multiplexer connected to the terminal C and the first optical multiplexer, and one end of the third optical multiplexer. A second optical amplification medium connected to the optical multiplexer, and pumping light emitted from the second optical amplification medium connected to the other end of the second optical amplification medium The called signal light and a second light reflector reflecting back to the second optical amplifying medium, an optical amplifier the amplified signal light, characterized in that to the output from the terminal D. 4. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
An optical circulator having a forward transfer characteristic from the terminal A to the terminal B, the terminal B to the terminal C, and the terminal C to the terminal D to which the signal light is input, and a first optical multiplexer connected to the terminal C. A pump, a pumping light source connected to the first optical multiplexer, a first optical amplification medium having one end connected to the first optical multiplexer, and the other end of the first optical amplification medium. Second connected
An optical multiplexer, a first optical reflector connected to the second optical multiplexer, which returns the signal light emitted from the second optical multiplexer to the first optical amplification medium and returns the signal light. A third optical multiplexer connected to the first optical reflector and the terminal B; a second optical amplification medium having one end connected to the third optical multiplexer;
And a second optical reflector connected to the other end of the optical amplification medium to reflect the signal light emitted from the second optical amplification medium and return the signal light to the second optical amplification medium. To the terminal D
An optical amplifier characterized by being output from. 5. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
An optical circulator having a forward transfer characteristic from the terminal A to the terminal B and the terminal B to the terminal C to which the signal light is input, an optical multiplexer connected to the terminal B, and an optical multiplexer connected to the optical multiplexer. Pumping light source, an optical amplifying medium whose one end is connected to the optical multiplexer, and another end of the optical amplifying medium connected through a lens, and the signal light and pumping light emitted from the optical amplifying medium are amplified by the optical amplifying medium. A light reflector that reflects and returns to the medium, and a non-reciprocal polarization rotation element that is arranged between the light reflector and the lens and that rotates so that the polarization planes of the light passing back through are orthogonal to each other, amplifying An optical amplifier which outputs the signal light thus generated from the terminal C. 6. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
An optical multiplexer to which the signal light and the pumping light from the pumping light source are input, a first optical amplification medium having one end connected to the optical multiplexer, and the other end of the first optical amplification medium. An optical circulator having forward transfer characteristics from the terminal A to the terminal B and from the terminal B to the terminal C, and one end of the optical circulator.
A second optical amplifying medium connected to the second optical amplifying medium, and a signal light and a pumping light emitted from the second optical amplifying medium which are connected to the other end of the second optical amplifying medium and reflected to the second optical amplifying medium. And an optical reflector for returning the optical signal, and outputs the amplified signal light from the terminal C. 7. An optical amplifier for amplifying input signal light using pumping light from a pumping light source and an optical amplifying medium,
A first optical multiplexer into which the signal light and the pumping light from the pumping light source are input, a first optical amplification medium having one end connected to the first optical multiplexer, and the first optical amplifier. A second optical multiplexer connected to the other end of the medium; and an optical circulator having forward transfer characteristics from the terminal A to the terminal B and from the terminal B to the terminal C connected to the second optical multiplexer. , The terminal B
And a third optical multiplexer connected to the second optical multiplexer, a second optical amplification medium having one end connected to the third optical multiplexer, and a second optical amplification medium An optical reflector which is connected to the end and which reflects the signal light and the pumping light emitted from the second optical amplification medium back to the second optical amplification medium, and outputs the amplified signal light from the terminal C. An optical amplifier characterized by: