JP2894436B2 - Optical transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter which enables long-distance transmission in an optical fiber communication system.

[0002]

2. Description of the Related Art An optical fiber communication system is a system for transmitting laser light emitted from an optical transmitter to an optical receiver via a transmission line optical fiber. In order to realize long-distance transmission in such an optical fiber communication system,
It is conceivable to increase the power of the signal light transmitted from the optical transmitter. However, as the power of the signal light is increased, the energy density of the propagating signal light becomes extremely large because the core diameter of the optical fiber is small, so that stimulated Brillouin scattering (Stimulated Brillouin S) is performed.
catterring: hereinafter abbreviated as SBS).

[0003] When this SBS occurs, the signal light incident on the optical fiber is backscattered to the incident side in the optical fiber. At the end, no signal light exceeding a certain value appears. In addition, the occurrence of SBS causes distortion, fluctuation (jitter), and the like in the optical waveform of the signal light, thereby causing waveform deterioration. As a result, the bit error rate characteristics of the optical receiver deteriorate,
Eventually, transmission of digital signals will be hindered.

[0004] The SBS is generated when the optical power incident on the optical fiber exceeds a certain threshold. This threshold is determined not only by the value of the optical power but also by the spectral line width of the incident light, the Brillouin band, the core diameter of the optical fiber, the effective length of the optical fiber, and the like. _Assuming that the threshold value of the optical power is P _th , the threshold value P _th is represented by the following Equation 1.

[0005]

(Equation 1) From Equation 1, it can be seen that as the spectral line width Δν _s of the signal light is larger, the threshold value P _th of the incident light power is higher, and SBS is less likely to occur. In an actual intensity-modulated optical transmitter using an external optical modulator, the threshold value P _th of the incident light power is determined by the optical spectrum line width of the semiconductor laser light source. The line width of this optical spectrum is generally as narrow as several MHz to several tens of MHz, and the probability of occurrence of SBS is very high.

For this reason, in the prior art, the signal light spectrum line width is equivalently increased by frequency modulation or the like by utilizing the fact that the light spectrum line width of the semiconductor laser light source is independently changed regardless of the bit rate or the like. , SBS occurrence is suppressed.

FIG. 4 shows a block diagram of a conventional optical transmitter. The optical transmitter shown is a semiconductor laser light source 401.
, An optical intensity modulator 402, and an optical fiber amplifier 403
, DC power supply 404, oscillator 405, mixer 406
It is composed of

The laser light emitted from the semiconductor laser light source 401 is intensity-modulated by an external optical modulator 402 according to a digital signal, optically amplified by an optical fiber amplifier 403, and transmitted to a transmission line optical fiber. On the other hand, the current applied from the DC power supply 404 to the semiconductor laser light source 401 is minutely modulated by the output of the oscillator 405 by the mixer 406, thereby frequency-modulating the laser light emitted from the semiconductor laser light source 401, The line width of the light spectrum of is expanded.

Various prior arts related to the present invention are known. For example, Japanese Patent Application Laid-Open No. Hei 4-293024 (hereinafter referred to as
The prior art 1) discloses an “optical transmission device” in which stimulated Brillouin scattering is suppressed even when the transmission power level is increased, and which can be applied to optical fiber communication over a longer distance than before. In the prior art 1, as in the embodiment shown in FIG. 2, the power of the SBS actually generated in the optical fiber of the transmission line is monitored, and the frequency modulation degree is set accordingly.

Japanese Patent Application Laid-Open No. Hei 6-224503 (hereinafter referred to as Prior Art 2) provides a method for suppressing excitation Brillouin scattering in an existing optical fiber communication system. Prior Art 2 discloses a method for suppressing pump Brillouin scattering in an optical communication system in which an information-encoded optical signal is transmitted from an optical source via an optical fiber to an optical receiver having a fixed sensitivity bandwidth. In order to vibrate, AC waves are used. The frequency of this AC wave is extremely lower than the cutoff frequency of the receiver connected to the optical fiber, but is high enough to suppress the Brillouin gain. The vibration causes the information-encoded optical signal to be frequency-modulated at a frequency outside the sensitivity bandwidth of the optical receiver, thereby increasing the line width of the information-encoded optical signal.

Further, Japanese Patent Application Laid-Open No. 62-1332 (hereinafter referred to as Prior Art 3) discloses an "optical fiber communication device" in which the transmission distance is greatly increased by suppressing the generation of stimulated Brillouin scattered light. Is disclosed. The laser light source for signal light generation is intensity-modulated by an electric pulse train generated from a drive circuit, and emits multi-axis mode laser light. The signal light pulse train emitted from the light source is coupled to the optical fiber by a lens. The optical fiber is connected to the optical receiver by a lens. In the optical fiber communication apparatus, to a light source of the oscillation longitudinal mode number N and the signal light power incident on the transmission path P _in the following relationship. 2
≦ N ≦ (1+ (√ (4T ² − (Δt) ² ) / M (λ ₀ ))
L · Δλ), P _in > P ₀ . Here, P ₀ is a threshold input optical power at which stimulated Brillouin scattered light occurs in the transmission line when single mode light is incident, Δt is a signal light pulse width, T is a light pulse repetition period, and λ ₀ is a multimode light. The center wavelength, L is the transmission distance, and M (λ) is the dispersion at wavelength λ.

In Japanese Patent Publication No. 3-4141 (hereinafter referred to as Prior Art 4), an optical signal having an optical power of one or more narrow beam width is incident from one end of an optical fiber, and the phase angle of the optical signal is adjusted. An "optical transmission method and apparatus" is disclosed that suppresses Brillouin scattering in optical transmission of a dielectric waveguide and transmits a high-power signal by changing with the time for suppressing stimulated Brillouin scattering. In this prior art 4, a device for detecting stimulated Brillouin scattering SBS is provided with a laser light source, a test fiber, and a power or frequency monitoring device. The laser light of a single wavelength from this light source enters the fiber via the variable attenuator. The frequency spectrum of the light passing through the fiber is monitored by the frequency monitor of this device. Then, the phase angle of the optical signal having the optical power of the narrow beam width changes with the time for suppressing the scattered SBS, the scattered SBS is suppressed by the optical transmission of the dielectric waveguide, and the high-power optical signal is transmitted.

[0013]

As described above, if the threshold value _{Pth of the} optical power incident on the optical fiber is increased, transmission over a longer distance becomes possible. For this purpose, the above equation 1
As can be seen from FIG. 6, the realization becomes possible by increasing the spectral line width Δν _s of the signal light.

However, the frequency modulation efficiency of the semiconductor laser light source 401 changes due to a change over time or the like, or the magnitude of the output of the oscillator 405 changes for some reason. In such a case, the magnitude of the optical spectrum line width Δν _s changes. This changes the threshold value P _th of the incident light power. For this reason, depending on this change condition, SBS is generated, causing waveform deterioration such as distortion and fluctuation (jitter) in the optical waveform of the signal light, and impairing the transmission of digital signals due to deterioration of the bit error rate characteristics in the optical receiver. May cause

Further, as disclosed in Prior Art 1,
In the method of setting the frequency modulation degree by monitoring the power of the SBS, the optical spectrum line width becomes too large when the power of the SBS increases. This leads to an increase in jitter on the optical receiver side, which causes deterioration of the bit error rate characteristics.

Accordingly, an object of the present invention is to monitor a frequency modulation degree necessary for increasing the optical spectrum line width of laser light emitted from a semiconductor laser light source in order to suppress the occurrence of SBS, An object of the present invention is to prevent the bit error rate in the optical receiver from deteriorating by controlling the line width to be always constant.

Each of the prior arts 2 to 4 only discloses a technical idea for suppressing excitation Brillouin scattering, but does not always keep the magnitude of the optical spectrum line width constant.

[0018]

According to the present invention, a semiconductor laser light source for emitting laser light, intensity modulation means for intensity-modulating the laser light with a digital signal and outputting modulated laser light, and the modulated laser Transmission line light using light as incident light
Means for sending out to a fiber, means for superimposing an AC signal on the current supplied to the semiconductor laser light source, means for frequency-modulating the laser light, and a part of the incident light is incident,
Optical fiber having a smaller core diameter than the transmission path optical fiber
Including stimulated Brillouin scattering generated in the optical fiber
Detects the power, and detection means for outputting a detection signal indicating the detection result, the semiconductor laser in response to said detection signal
Variable means for varying the optical spectrum line width of the light source ,
The semiconductor laser is detected by the detecting means and the variable means.
So that the optical spectrum line width of the laser light source is constant.
Controlling the optical transmitter.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows the configuration of an optical transmitter according to an embodiment of the present invention. The illustrated optical transmitter includes a semiconductor laser light source 101 that emits CW light, and a CW according to a digital signal.
Light intensity modulator 10 for intensity-modulating W light to generate signal light
2, an optical fiber amplifier 103 for amplifying the signal light and transmitting the amplified signal light to the transmission line optical fiber, and a semiconductor laser light source 101.
A DC power supply 104 for applying a current to the semiconductor laser 101, an oscillator 105 for expanding the optical spectrum line width of the CW light emitted from the semiconductor laser light source 101, and a minute modulation of the current from the DC power supply 104 by the oscillation signal from the oscillator 105. And
And a mixer 106 for applying a modulated current to the semiconductor laser light source 101.

Further, the optical transmitter is an optical fiber amplifier 1
03, an optical branching element 107 for taking out a part of the light incident on the transmission line optical fiber, and an optical fiber 109 having a smaller core diameter than the transmission line optical fiber and more likely to cause SBS.
And the light branched by the light branching element 107
9 and S generated in the optical fiber 109.
An optical coupler 108 for detecting the power of the BS, and a light receiving element 110 for performing optical-electrical conversion of the detected power of the SBS
And a control circuit 111 for changing the oscillation signal of the oscillator 105 based on the output signal of the light receiving element 110 to change the optical spectrum line width of the CW light emitted from the semiconductor laser light source 101. .

That is, according to the present invention, the optical spectrum line width of the light incident on the transmission line optical fiber is monitored, and the value is maintained at a constant value. The size of the width changes,
The generation of SBS in the transmission line optical fiber is suppressed.

The operation of the optical transmitter according to the present embodiment will be described below with reference to FIG.

The CW light emitted from the semiconductor laser light source 101 enters a light intensity modulator 102. The semiconductor laser light source 101 is driven by a DC power supply 104, and the emitted CW light is controlled to have a constant light power as described later. The CW light incident on the light intensity modulator 102 is intensity-modulated by a digital signal. The intensity-modulated light is amplified by the optical fiber amplifier 103 to the optical power required in the transmission system, and sent out to the transmission line optical fiber.

As described above, the threshold at which SBS occurs depends on the magnitude of the optical spectrum line width. In order to suppress the occurrence of SBS in the transmission line optical fiber, an oscillator 10
5, the output current of the DC power supply 104 is
6, the semiconductor laser light source 10
The CW light emitted from No. 1 is frequency-modulated to increase the optical spectrum line width of the CW light to about several hundred MHz.

A part of the output light of the optical fiber amplifier 103 is extracted by an optical branching element 107 and is supplied to an optical fiber 109 having a small core diameter. This optical fiber 1
The tip of 09 is terminated so that no reflected light is generated there.

Since the core diameter of the optical fiber 109 is smaller than that of the transmission line optical fiber, the threshold value of the optical power at which SBS occurs because the effective area _Ae of the optical fiber 109 is small, as can be seen from the above-described formula (1). P _th decreases. For this reason, even if SBS does not occur in the transmission line optical fiber, SBS is emitted in the optical fiber 109 described above.
The power of the SBS generated here is detected by the optical coupler 108, and the detected amount is determined by the semiconductor laser light source 101.
Is proportional to the optical spectrum line width of the CW light emitted from. That is, the spectral line width Δ of the signal light represented by the above-mentioned formula 1
If ν _s changes, the threshold value P _{th of the} optical power also changes. Therefore, if the power of the generated SBS is detected, the optical spectrum line width can be relatively determined.

The DC voltage photo-electrically converted by the light receiving element 110 changes the magnitude of the oscillation signal of the oscillator 105 through the control circuit 111.

[0029] Here, the control circuit 111 controls the magnitude of the oscillation signal of the oscillator 105 so that constant Merare was a certain value the DC voltage. As a result, the line width of the light spectrum incident on the transmission line optical fiber can always be kept constant.

With the above operation, the semiconductor laser light source 10
Even if the optical spectrum line width of the CW light changes due to the deterioration with time of 1 or the output fluctuation of the oscillator 105 and SBS occurs, the control is performed so that the size of the optical spectrum line width of the CW light becomes constant. be able to. As a result, it is possible to prevent the deterioration of the quality of the digital signal and the deterioration of the bit error rate occurring on the optical receiver side.

[0031]

Next, an optical transmitter according to an embodiment of the present invention will be described with reference to FIG. The illustrated optical transmitter includes a semiconductor laser light source 201, an optical intensity modulator 202, a data signal amplifier 203, an optical fiber amplifier 204, an automatic optical power control circuit 205, an oscillator 206, a variable gain amplifier 207, Mixer 208 and optical coupler 209
, An optical coupler 210, an optical fiber 211, a photodiode 212, and a comparison control circuit 213.

The semiconductor laser light source 201 is a semiconductor laser light source that emits CW light having a wavelength of 1.55 μm, and has a built-in light receiving element (not shown) for optical monitoring. The light intensity modulator 202 is an LN (LiNbO ₃ ) light intensity modulator.
The data signal amplifier 203 converts the transmitted digital signal to L
The signal is amplified to an amplitude (about 5 V _pp ) at which the N (LiNbO ₃ ) light intensity modulator 202 can be driven. Optical fiber amplifier 20
Reference numeral 4 denotes an Er optical fiber amplifier that outputs about +15 dBm. The automatic optical power control circuit 205 controls the level of the CW light output from the semiconductor laser light source 201 to be constant based on the output of the light receiving element built in the semiconductor laser light source 201. The oscillator 206 generates an oscillation signal having an oscillation frequency of about 100 kHz, which is much lower than the digital signal to be transmitted. The variable gain amplifier 207 changes the gain of the oscillator 206 to amplify the oscillation signal. The mixer 208 minutely modulates a direct current for driving the semiconductor laser light source 201 with an oscillation signal output from the oscillator 206. The optical coupler 209 is an optical coupler having a branching ratio of 2: 1 for extracting a part of the output light of the Er optical fiber amplifier 204.
The optical fiber 211 is a 1.55 μm zero dispersion fiber having a core diameter of 2 to 4 μm and a length of about 20 km. Optical coupler 21
0 is the SB generated in the 1.55 μm zero dispersion fiber 211
This is an optical coupler having a branching ratio of 20: 1 for detecting the power of S. The photodiode 212 is an InGaAs photodiode for performing photoelectric conversion. The comparison control circuit 213 compares a predetermined voltage with the output voltage of the InGaAs photodiode 212 to generate a control signal.

Next, the operation of this embodiment will be described with reference to FIG. The signal light output from the semiconductor laser light source 210 is output to the LN light intensity modulator 202 by, for example, 10
The intensity is modulated by a digital signal having a bit rate of Gbps. At this time, the digital signal is amplified by the data signal amplifier 203 up to a maximum amplitude of about 5 V _pp and provided to the LN light intensity modulator.

C output from the semiconductor laser light source 201
The power of the W light is feedback-controlled by the automatic light power control circuit 205 so as to be constant.

The intensity-modulated signal light is amplified to about +15 dB by the next-stage Er optical fiber amplifier 204.
The amplified signal light is sent to the transmission line optical fiber, and a part thereof is extracted by the optical coupler 209. The level of the extracted light is about +10.0 dBm,
The light is sent to the 1.55 μm zero-dispersion fiber 211 through the next-stage optical coupler 210. The core diameter of the 1.55 μm zero-dispersion fiber 211 is 2 to 4 μm, which is less than half the core diameter (about 8 μm) of the optical fiber used for the transmission line. Therefore, SBS is generated at an incident light power much lower than that of the transmission line optical fiber.

For example, in the case of optical fibers having the characteristics shown in FIG. 3, it is assumed that CW light (the optical spectrum line width is in a minimum state) is incident on each optical fiber. In this case, in the optical fiber (1.55 μm zero-dispersion fiber) of the transmission line, SBS is generated at about +7 dBm of the incident light power, but in an optical fiber having a small core diameter, SBS is already generated at about +0 dBm. Actually, since the optical spectrum line width is increased to several hundred MHz to suppress SBS, the threshold optical power at which SBS occurs is +15 dBm and +5 dBm, respectively.

In the configuration of FIG. 2, the optical output power on the transmission line side of the optical coupler 209 is about +13.2 dBm, and the optical output power to the opposite side, that is, the optical output power to the optical coupler 210 is about +1.
0.2 dBm. 1.55μm with small core diameter
The optical power incident on the zero dispersion fiber 211 is +9.8 dB because the branching ratio of the optical coupler 210 is 20: 1.
m.

Accordingly, when the optical spectrum line width is about several hundred MHz, the transmission line
The incident light power is +1 with respect to the S generation threshold +15 dBm.
Since it is 3.2 dBm, SBS does not occur. However, a 1.55 μm zero dispersion fiber 211 having a small core diameter is used.
Since the incident light power is +9.8 dBm with respect to the threshold value of SBS generation of +5 dBm, SBS is generated.

The power of the generated SBS is transferred to the optical coupler 2.
10 and the InGaAs photodiode 212
To convert to DC voltage. This DC voltage is equal to the detected SB
It is proportional to the power of S. That is, as the optical spectrum line width increases, the power of the SBS increases, so that the InGa
The output voltage of the As photodiode 212 also increases.
Conversely, when the optical spectrum line width decreases and the power of the SBS also decreases, the InGaAs photodiode 212
Output voltage becomes smaller.

This DC voltage is supplied to a comparison control circuit 213 at the next stage.
to go into. Here, the DC voltage is compared with a predetermined reference voltage, and the InGaAs photodiode 212
The gain of the variable gain amplifier 207 at the next stage is varied so that the optical spectrum line width is controlled so that the output voltage of the next stage is always equal.

That is, when the optical spectrum line width of the laser light decreases and the output voltage of the InGaAs photodiode 21 falls below the reference voltage, the comparison control circuit 213 increases the gain of the variable gain amplifier 207 to increase the semiconductor laser light source. The frequency spectrum 201 is increased to increase the optical spectrum line width.

Conversely, the optical spectrum line width of the laser light increases, and the output voltage of the InGaAs photodiode 21 rises above the reference voltage. The comparison control circuit 213 reduces the gain of the variable gain amplifier 207 to reduce the semiconductor laser light source. By lowering the frequency modulation degree 201, the optical spectrum line width is reduced.

If the reference voltage of the comparison control circuit 213 is set so that the optical spectrum line width does not cause SBS in the transmission line optical fiber, the reference voltage is stabilized at the set optical spectrum line width unless the reference voltage fluctuates. You.

The present invention is not limited to the above-described embodiment, and various changes and modifications can be made without departing from the spirit of the present invention.

[0045]

As described above, according to the present invention, SBS
By monitoring the optical spectrum line width of the laser light emitted from the semiconductor laser light source in order to suppress the occurrence of laser light, and controlling the optical spectrum line width to be always constant, the time of the semiconductor laser light source Frequency modulation due to change, etc.
Even if the efficiency changes, the optical spectrum line width does not change, and it is possible to prevent the deterioration of the quality of the digital signal and the deterioration of the bit error rate in the optical receiver. Further, by using an optical fiber having a small core diameter for detecting the optical spectrum line width, the configuration of the detecting means can be simplified.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an optical transmitter according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating an optical transmitter according to an embodiment of the present invention.

FIG. 3 is a diagram for explaining SBS generation by an optical fiber.

FIG. 4 is a block diagram showing a conventional optical transmitter.

[Explanation of symbols]

 Reference Signs List 101 semiconductor laser light source 102 light intensity modulator 103 optical fiber amplifier 104 DC power supply 105 oscillator 106 mixer 107 optical branching element 108 optical coupler 109 optical fiber with small core diameter 110 light receiving element 111 control circuit 201 semiconductor laser light source 202 light intensity modulator 203 Data signal amplifier 204 Optical fiber amplifier 205 Automatic optical power control circuit 206 Oscillator 207 Variable gain amplifier 208 Mixer 209 Optical coupler (optical branching element) 210 Optical coupler 211 Optical fiber with small core diameter 212 Photodiode (light receiving element) 213 Comparison control circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ identification code FI H04B 10/152 (58) Investigated field (Int.Cl. ⁶ , DB name) H01S 3/133 H01S 3/30 H04B 10/04 H04B 10/06 H04B 10/142 H04B 10/152 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A semiconductor laser light source for emitting a laser beam, and intensity modulation of the laser beam by a digital signal.
Intensity modulation means for outputting laser light, and the modulated laser light
And means for sending the transmission line optical fiber as the incident light, by superimposing an AC signal to the current supplied to the semiconductor laser light source, and means for frequency modulating the laser beam, the incident
A part of the light is incident on the transmission line optical fiber.
Including optical fiber with small diameter,
Induction Brillouin by detecting the scattering power, comprising: a detecting means for outputting a detection signal indicating the detection result, and a varying means for varying the optical spectral line width of the semiconductor laser light source in response to the detection signal, the Detecting means and the variable means;
The light spectrum line of the semiconductor laser light source
An optical transmitter, wherein the width is controlled to be constant .