JP2839593B2 - Optical transmitter - Google Patents

Description

The present invention relates to an optical transmitter suitable for coherent optical communication and the like. The purpose is mainly to stabilize the modulation efficiency.For example, a coherent light source that outputs light that is angle-modulated based on an input modulation signal, and a light having an intensity according to the frequency by discriminating the light from the light source with a frequency. A light frequency discriminating means for outputting, a light receiving means for receiving light from the discriminating means and outputting a signal of a level corresponding to a received light intensity, and a coherent signal from a high frequency component corresponding to a modulation frequency of a signal from the light receiving means. A modulation efficiency stabilizing means for detecting a modulation efficiency of the light source and controlling an amplitude of a modulation signal input to the coherent light source so that the efficiency becomes constant. It is formed.

The present invention relates to an optical transmitter suitable for coherent optical communication and the like.

In order to realize coherent optical communication that can effectively utilize the properties of light waves, a light source with a narrow spectral line width and high frequency stability (in the case of angle modulation, high average frequency stability) An optical transmitter using is required. Regarding the frequency stability, specifically, 10 to several tens
MHz (several tenths of the channel interval when performing frequency division multiplexing transmission) is required, and the wavelength is 1.55 μm
A light source (about 200 THz) requires a stability of the order of 10 ^-8 . When information is transmitted by coherent optical communication, it is necessary to modulate an optical carrier from a light source. In particular, when the light source is a semiconductor laser, the oscillation frequency changes by changing the drive current (bias current). Therefore, the modulation method is a frequency shift keying method (FSK) that directly modulates the semiconductor laser with the drive current. Method) is promising. In the construction of a system to which the FSK method by direct modulation is applied, it is usual that the FM modulation efficiency (frequency shift amount per unit drive current change) is fixed, but FM modulation efficiency is changed due to aging of the semiconductor laser. Since the modulation efficiency may change, FM
There is a demand for an optical transmitter that is not affected by a change in modulation efficiency.

On the other hand, most recently, frequency modulation or phase modulation is performed directly on a semiconductor laser driven to have a substantially constant light output intensity, and this modulated light is converted into intensity-modulated light by passing it through an appropriate optical resonator. However, there has been proposed a transmission system in which this is received by a conventional IM / DD (intensity modulation / direct detection) optical receiver. According to this method, although it takes time to use an optical resonator, it is possible to perform modulation by a small current change under an optical bias current, so that wavelength fluctuation (chirping) during high-speed modulation is less likely to occur, and a long distance Transmission becomes possible. Therefore, even when such a method is implemented, it is required to stabilize the modulation efficiency in angle modulation in order to keep the reception sensitivity constant.

2. Description of the Related Art FIG. 13 shows a block diagram of a general coherent optical communication system to which the FSK scheme is applied. Reference numeral 201 denotes a coherent light source that outputs frequency-modulated light, and a semiconductor laser or the like that is controlled so that a drive current changes based on an input modulation signal is used. For example, light having frequencies f ₁ and f ₂ (f ₁ ≠ f ₂ ) is output in accordance with “1” (mark) and “0” (space) of the input digital signal. In this conventional example, a coherent light source is used to stabilize the frequency.
The light from 201 is received by the receiver 20 via the Fabry-Perot interferometer 202
3 to detect the slow frequency fluctuation of the output light of the coherent light source 201 based on the received light signal,
Feedback is applied to the driving conditions (bias current, temperature, etc.) of the coherent light source 201 so as to cancel this fluctuation. 204 is a control circuit for that.

Light from the coherent light source 201 is transmitted to the receiving side via the optical transmission path 205, and the received light is multiplexed with local light (local oscillation light) from the local light source 206 by the optical coupler 207. When this multiplexed light is made incident on a photodetector 208 having a normal square characteristic, an intermediate frequency signal in a microwave band having a difference frequency between the signal light frequencies (f ₁ , f ₂ ) and the local light frequency (f) Occurs. Therefore, a demodulated output can be obtained by adding the intermediate frequency signal to the post-subtractor 211 through, for example, the dual filters 209a and 209b and the demodulators 210a and 210b. Note that AFC is applied to the local light source 206 so that the intermediate frequency is constant.

FIG. 14 shows the relative relationship between the power spectrum of the intermediate frequency signal and the pass band of the dual filter. 212a is a pass band of one filter 209a, and its center frequency is | f−f ₁
Is set to |. 212b is a pass band of the other filter 109b, and its center frequency is set to | f−f ₂ |. When the frequency of the coherent light source 201 has been stabilized and the FM modulation efficiency has not changed since the beginning of the system construction, the spectrum of the intermediate frequency signal has a pass band of the dual filter as shown in FIG. Is included.
When the frequency of the light from the coherent light source 201 changes,
As shown in FIG. 7B, the spectrum of the intermediate frequency signal deviates from the pass band of the dual filter, and the code error rate increases. The frequency fluctuation of the light source is generally based on temperature fluctuation and the like, and the fluctuation frequency is significantly smaller than the modulation frequency. In this case, the light source is stabilized by the frequency stabilization technique on the transmitting side shown in FIG. The average frequency of light from the light source can be kept constant. Therefore, the reception sensitivity can be kept stable by the frequency stabilization technique on the transmission side or the AFC on the reception side. However, if the FM modulation efficiency changes from that at the beginning of system operation due to aging of the semiconductor laser itself or its optical system, the average frequency of light from the light source is stabilized, for example, as shown in FIG. 12 (c). Despite this, the spectrum of the intermediate frequency signal may deviate from the pass band of the dual filter, and the code error rate may increase.

In the system shown in FIG. 13, the average frequency of the light from the light source is stabilized, but the FM modulation efficiency is not stabilized. As described above, there has not been a concept of monitoring and controlling the modulation efficiency on the transmission side.

Further, as described above, even in a system to which the IM / DD scheme is applied, stabilization of modulation efficiency in frequency modulation or phase modulation may be required.

The present invention has been created in view of such circumstances,
It is intended to stabilize modulation efficiency in an optical transmitter. The purpose other than this purpose will become apparent from the following description.

Means and Action for Solving the Problems FIG. 1 shows a first basic configuration of the present invention. The optical transmitter includes a coherent light source 1 that outputs light that has been subjected to angle modulation (frequency modulation or phase modulation) based on an input modulation signal.
An optical frequency discriminating means 2 for discriminating the frequency of the light from the light source 1 and outputting light having an intensity according to the frequency; and receiving a light from the discriminating means 2 to generate a signal having a level corresponding to the received light intensity. The light receiving means 3 for outputting the light and the modulation efficiency of the coherent light source 1 are detected from the high-frequency component corresponding to the modulation frequency of the signal from the light receiving means 3, and the modulation signal input to the coherent light source 1 is controlled so that the efficiency becomes constant. And modulation efficiency stabilizing means 4 for controlling the amplitude. still,
The modulation efficiency in the case of phase modulation is, for example, a phase shift amount with respect to a unit drive current change of the semiconductor laser.

This configuration differs from the conventional configuration for stabilizing the frequency in that the high frequency component of the signal from the light receiving means 3 is actively used. That is, in the conventional configuration for stabilizing the frequency, it is meaningless for the frequency-modulated coherent light source to perform the feedback control so as to detect and remove the high frequency component of the signal from the light receiving unit. On the other hand, while the low-frequency component is detected and controlled so as to cancel the low-speed frequency fluctuation of the light source, in the configuration of the present invention, the high-frequency component corresponding to the modulation frequency of the signal from the light receiving means 3 is modulated. In consideration of the efficiency, the amplitude of the modulation signal is controlled so that the modulation efficiency becomes constant. The reason why the amplitude of the modulation signal is controlled is that the frequency shift amount (or phase shift amount) of the light source changes according to the amplitude of the input modulation signal. The terms “frequency shift” and “phase shift” are broadly inclusive, one of which includes the other or the other of which includes one. In this specification, these terms may be used in a broad sense.

This first basic configuration is effective in maintaining a high modulation sensitivity by, for example, keeping the modulation index constant in the case of analog modulation, and assuming that the frequency of the coherent light source 1 is stabilized. , FSK is effective in reducing the bit error rate.

However, if the frequency is not stabilized, stabilization with modulation efficiency may not always be effective in reducing the bit error rate. Therefore, a second basic configuration is provided for the purpose of improving the bit error rate by stabilizing the modulation efficiency and the frequency.

FIG. 2 is a diagram showing a second basic configuration of the present invention. This optical transmitter for coherent optical communication, in addition to the first basic configuration, detects a frequency variation of the output light of the coherent light source 1 based on a signal from the light receiving unit 3 and cancels the variation. Is provided with frequency stabilizing means 5 for controlling the driving conditions of.

According to this configuration, it is possible to effectively prevent fluctuations in the frequency and the modulation efficiency, and to prevent a reduction in the bit error rate due to the transmission device.

When the modulation efficiency and the frequency are stabilized by the second basic configuration, the light output (intensity) of the coherent light source 1 may fluctuate as a result of the control. Therefore, a third basic configuration is provided for the purpose of stabilizing the modulation efficiency, the frequency, and the optical output, respectively.

FIG. 3 is a diagram showing a third basic configuration of the present invention. The optical transmitter for coherent optical communication has an optical branching unit 6 for branching light from the coherent light source 1 between the coherent light source 1 and the optical frequency discriminating unit 2 in addition to the second basic configuration.
And another light receiving means 7 for receiving the light split by the light splitting means 6 and outputting a signal of a level corresponding to the received light intensity, and coherently controlling the signal level from the light receiving means 7 to be constant. Light output stabilizing means 8 for controlling the driving conditions of the light source 1 is provided.

When the driving conditions to be controlled by the frequency stabilizing unit 5 and the driving conditions to be controlled by the optical output stabilizing unit 8 are the same, it is difficult to stabilize the frequency and the optical output independent of each other. Since the third basic configuration is difficult to realize, the third basic configuration is suitable when there are a plurality of driving conditions of the coherent light source 1. For example, when the coherent light source 1 includes a semiconductor laser and the angle modulation is frequency modulation by direct modulation of the semiconductor laser, the oscillation frequency and the optical output change depending on the bias current and temperature of the semiconductor laser, and Since the ratio between the change rate of the oscillation frequency and the change rate of the optical output when the bias current changes is different from the change rate of the oscillation frequency and the change rate of the optical output when the temperature changes, the bias current and the temperature are different. One of them is controlled by the frequency stabilizing means 5 and the other is controlled by the light output stabilizing means 8, or the bias current and the temperature are controlled by the frequency stabilizing means 5 and the light output stabilizing means 8. By performing complex control as a control target of, the oscillation frequency and the optical output can be simultaneously stabilized. When the coherent light source 1 includes a multi-electrode type semiconductor laser, the distribution ratio of the bias current applied to each electrode can be selected as one of the driving conditions.

Examples Examples of the present invention will be described below. In addition, when the same name is given the same reference numeral throughout the drawings, the same target is considered to be the same target, but when the same name is differently denoted, the target may be different. .

FIG. 4 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining an embodiment of the second basic configuration.
The coherent light source 1 includes a semiconductor laser 12 and a bias current whose set value is controlled by a control signal.
And a modulation circuit that changes a bias current from its set value based on an input modulation signal.
16 is provided.

The optical frequency discriminating means 2 includes a Fabry-Perot interferometer, a waveguide ring resonator, and other optical interferometers 18 whose temperature is controlled so as to keep the temperature constant.
Specifically, the optical interferometer 18, a temperature sensor 76 for detecting the temperature of the portion of the optical interferometer 18 where the interference light propagates, and a Peltier for changing the temperature of at least the portion of the optical interferometer 18 where the interference light propagates A temperature variable element 78 such as an element, and a temperature stabilizing circuit 80 that drives the temperature variable element 78 so that the temperature detected by the temperature sensor 76 is constant. When the temperature of the optical interferometer 18 is controlled to be constant, the optical path length in the optical interferometer 18 is kept constant, and the resonance frequency becomes constant, so that a predetermined frequency discrimination curve can be obtained with good reproducibility. be able to. If the optical frequency discriminating means 2 includes an optical interferometer, the curve of the output light periodically changes on the frequency axis with respect to the change in the frequency of the input light. The frequency of the light source can be stabilized. As the optical frequency discriminating means 2, an optical interferometer or a device using an atomic or molecular absorption line can be used. In this case, the frequency of the light source can be absolutely stabilized.

The light receiving means 3 includes a first light receiver 20 for photoelectrically converting the light from the optical frequency discriminating means 2. Here, photoelectric conversion refers to generation of a photocurrent corresponding to (in proportion to) the intensity of incident light. Reference numeral 22 denotes an amplifier for amplifying a light receiving signal.

The modulation efficiency stabilizing means 4 includes a high-pass filter 24 for passing a high frequency component of the signal from the light receiving means 3 corresponding to the modulation frequency, a peak detecting circuit 26 for detecting the amplitude of the signal passing through the filter 24, The amplitude control circuit 28 controls the amplitude of the modulation signal input to the modulation circuit 16 so that the amplitude value detected by the circuit 26 becomes constant.

The frequency stabilizing means 5 uses an optical interferometer from the semiconductor laser 12.
A beam splitter 30 for splitting the light incident on the beam splitter 30, a second light receiver 32 for photoelectrically converting the light split by the beam splitter 30, and the light receiving unit 3 corresponding to the frequency fluctuation of the output light of the coherent light source 1. , A low-pass filter 36 that passes the low-frequency component of the signal, a divider 38 that calculates the ratio of the signal from the filter 36 and the signal from the second light receiver 32, and a signal from the divider 38 to a reference power supply 40. And a bias control circuit 44 for sending a control signal to the bias circuit 14 so that the difference signal becomes zero or constant.
Numeral 34 denotes an amplifier for amplifying the signal from the second light receiver 32 and making its level substantially coincide with the level of the signal from the low-pass filter 36.

Reference numeral 46 denotes a mark ratio detection circuit, and 48 denotes a correction circuit. First, operating conditions when these are not added and control modes for stabilizing the frequency and the modulation efficiency will be described.

FIG. 5 shows the relationship between the output of the divider 38 and the frequency of light from the light source. This relationship corresponds to the relationship between the output light intensity of the optical interferometer 18 measured under the condition that the light intensity from the light source is constant and its frequency. By setting the interference optical path length of the optical interferometer properly, because the frequency discrimination curve 101 as peak interval as illustrated of about 10GHz is obtained, the frequency f _s of the light frequency discriminator from a light source The bias current of the semiconductor laser 12 is set so as to be within a frequency range in which an increase (or decrease) in the curve is given (a shoulder portion P on the curve having the largest change rate in the figure)
By setting the corresponding reference voltage V _s, it is possible to stabilize the frequency.

On the other hand, the intensity change component generated on the frequency discrimination curve by the frequency modulation at this time is input to the peak detection circuit 26 after passing through the high-pass filter 24. Peak detection circuit 26
Since the modulation efficiency becomes clear from the amplitude value detected in,
The amplitude of the input modulation signal is controlled so that this becomes constant. As a result, it is possible to prevent deterioration in modulation efficiency due to aging or the like.

By the way, when the modulation signal is given as a digital signal, if the mark ratio fluctuates, control for frequency stabilization may become inappropriate. That is, when the FSK method is applied, and particularly when the coupling between the modulation circuit 16 and the semiconductor laser 12 is an AC coupling, the modulation signal (input data) is changed as shown in FIG. When the mark ratio changes from the high mark state 102 to the low mark rate state 103, the average frequency of light from the semiconductor laser 12 becomes apparently changed, and the output 104 of the comparator 42 decreases as indicated by the solid line. Or it rises as shown by the broken line. In other words, even though the actual oscillation frequency of the semiconductor laser 12 does not fluctuate, the bias current is controlled so as to cancel the fluctuation of the output signal of the comparator 42. It is.

Therefore, in order to prevent improper control of frequency stabilization due to fluctuation of the mark ratio,
An embodiment is provided in which a mark ratio detection circuit 46 and a correction circuit 48 are added.

The mark ratio detection circuit 46 detects a mark ratio of a modulation signal provided as a digital signal. The correction circuit 48 corrects the control signal from the bias control circuit 44 so as to eliminate the fluctuation of the difference signal from the comparator 42 due to the fluctuation of the mark rate.
By additionally providing these circuits 46 and 48, it is possible to improve the bit error rate by eliminating the adverse effect of the mark rate fluctuation described above.

FIG. 7 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining an embodiment of the third basic configuration.
In the configuration shown in this figure, in addition to the configuration shown in FIG. 4, a temperature control circuit 82 to which a signal from the amplifier 34 is input is provided.
And a temperature variable element 84 whose temperature is controlled based on a signal from the temperature control circuit 82. The temperature variable element 84 is provided integrally with the semiconductor laser 12. Since the signal from the amplifier 34 corresponds to the fluctuation of the optical output of the semiconductor laser 12, the temperature control circuit 82 and the temperature variable element 84 control the temperature of the semiconductor laser 12 so that the signal level from the amplifier 34 becomes constant. Is controlled.

When the bias current of the semiconductor laser 12 is controlled by the frequency stabilizing means 5, the optical output of the semiconductor laser 12 may inevitably change. Therefore, stabilizing the optical output of the semiconductor laser 12 by controlling the temperature as in this embodiment is effective in simultaneously stabilizing the oscillation frequency and the optical output of the semiconductor laser 12. If there is a concern that the control does not converge, although not shown, it is possible to input signals from the amplifier 34 and the comparator 42 to an appropriate arithmetic circuit and perform complex control on temperature and bias current. good.

In the case where the FSK method is applied and the mark rate of the input data hardly changes, or in the case where the analog modulation method is applied, the mark rate detection circuit 46 is used.
The correction circuit 48 may not be provided. In addition, these circuits 46, 4
8 is unnecessary even when the modulation circuit 16 and the semiconductor laser 12 and the like are DC-coupled.

FIG. 8 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining another embodiment of the third basic configuration. In this embodiment, the coherent light source 1, optical frequency discriminating means 2, light receiving means 3, and modulation efficiency stabilizing means 4 shown in FIGS. 4 and 7 are used.

The frequency stabilizing means 5 includes a low-frequency oscillator 50 for secondly modulating the frequency of the coherent light source 1 at a frequency much lower than its modulation frequency, and a light receiving means 3 corresponding to the oscillation frequency of the oscillator 50. A low-pass filter 52 that passes the low-frequency component of the signal, a synchronous detector 54 that synchronously detects the signal from the low-pass filter 52 using the signal from the low-frequency oscillator 50 as a reference signal, and a synchronous detection output of the detector 54 is zero. Alternatively, a bias control circuit 56 for sending a control signal to the bias circuit 14 so as to be constant is provided.

If there is a concern about the adverse effect of the mark rate fluctuation, the mark rate detection circuit 46 and a correction circuit 62 for correcting the control signal from the bias control circuit 56 so as to eliminate the fluctuation of the synchronous detection output due to the mark rate fluctuation. Are provided.

In this embodiment, there is no beam splitter 30, second light receiver 32 and amplifier 34 in the frequency stabilizing means 5 shown in FIGS. Therefore, a beam splitter 58, a light receiver 60, and an amplifier 61 are provided in the same manner, and a signal from the amplifier 61 is input to the temperature control circuit 82.

The functions of the modulation efficiency stabilizing means and the light output stabilizing means are the same as those described so far, and the description thereof will be omitted.

The operation of the frequency stabilizing means 5 will be described. In FIG. 9, reference numeral 105 denotes a frequency discriminating curve obtained by the optical frequency discriminating means 2 and light receiving means 3, and reference numeral 106 denotes its first derivative curve, that is, the output characteristic of the synchronous detector 54. It is. As described above, the frequency of the semiconductor laser 12 is modulated by the low-frequency signal and the accompanying change in the received light level is synchronously detected. Can be converted. Generally the first derivative curve 10
6 may be referred to as a frequency discrimination curve, but in this specification, the consistency with the terms used in the description of the embodiment (FIGS. 4 and 7) for performing frequency discrimination without performing synchronous detection will be described. For the purpose of illustration, the one corresponding to the resonance spectrum of the optical interferometer (105) is called a frequency discrimination curve. 1st derivative curve 1
To effectively use the linear portion 06, as the frequency of the output light of the semiconductor laser 12 is in the range in the vicinity of frequency f _p which gives a peak in the frequency discrimination curve 105, the bias current supplied to the semiconductor laser 12 Is set.

In this embodiment, the modulation frequency is high (for example, several GHz).
Is assumed, but if the modulation frequency is low,
It is also possible to omit the low-frequency oscillator 50 and perform synchronous detection using the modulated signal itself as a reference signal.

If the beam splitter 58, the light receiver 60, the amplifier 61, and the temperature control circuit 82 are omitted and, for example, the temperature of the semiconductor laser 12 is controlled to be constant, an embodiment of the second basic configuration is obtained. The mark ratio detection circuit 46 and the correction circuit 62 may not be provided.

The embodiment described with reference to FIGS. 4 and 7 is compared with the embodiment described with reference to FIG.

First, the operation of the modulation efficiency stabilizing means 4 will be compared. 4th
In the embodiment shown in FIG. 7 and FIG. 7, as shown in FIG. 10 (a), the optical output waveform (the temporal change of the frequency of the output light)
Since 107 is set in a frequency range that gives an increase (or decrease) in the frequency discrimination curve 105, the waveform 108 of the signal that has passed through the high-pass filter 24 has a relatively large amplitude and is the same as the period of the optical output waveform 107. Is obtained. On the other hand, in the modulation efficiency stabilizing means 4 shown in FIG. 8, since the optical output waveform 109 is set in a range near the frequency giving the peak in the frequency discrimination curve 105, the signal passing through the high-pass filter 24 Waveform 110
As a result, a signal having a relatively small amplitude and a cycle corresponding to a half of the cycle of the optical input waveform 109 is obtained. Therefore, in consideration of sensitivity characteristics and high-speed response required of the peak detection circuit 26, the modulation efficiency stabilizing means 4 shown in FIGS. 4 and 7 is much easier to realize.

On the other hand, looking at the frequency stabilizing means 5,
In the frequency stabilizing means 5 shown in FIG. 7 and FIG.
Since the bias current control is performed using the difference signal between the signal from the reference signal 38 and the reference signal from the reference power supply 40, it may be difficult to perform high-precision control due to the influence of DC drift. In the frequency stabilizing means 5 shown in FIG.
Since there is no influence of the drift, extremely accurate bias current control is possible.

Therefore, in order to realize the modulation efficiency stabilizing means 4 with a simple circuit configuration or to enable highly accurate control of the modulation efficiency, the embodiment described with reference to FIGS. 4 and 7 is excellent. The embodiment described with reference to FIG. 8 is excellent in performing stabilization with high accuracy. Therefore, the embodiment shown in FIG. 11 is provided mainly for the purpose of extremely accurately stabilizing both the modulation efficiency and the frequency.

FIG. 11 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining still another embodiment of the third basic configuration. The ones used in this embodiment are as follows.

The coherent light source 1 is that shown in FIGS.

The optical frequency discriminating means 2 includes an optical interferometer 64 such as a Fabry-Perot interferometer, and a birefringent optical medium such as TiO ₂ is included on the interference optical path. For example, it is preferable to form a Fabry-Perot interferometer by polishing both end faces of a birefringent optical crystal in parallel. Further, in order to maintain the relative relationship between two frequency discrimination curves described later accurately and constant, a temperature sensor 76, a temperature variable element 78 and a temperature stabilization circuit 80 are provided as in the previous embodiments. .

The light receiving means 3 includes a polarization beam splitter 66 for polarizing and separating the light from the optical interferometer 64 into an ordinary light component and an extraordinary light component of the birefringent optical medium. The light receiving device includes second light receivers 68 and 70 and amplifiers 72 and 74 for amplifying the photoelectrically converted signal.

The modulation efficiency stabilizing means 4 is the one shown in FIGS. 4 and 7, and its high-pass filter 24 has a second photodetector 70.
The signal from is input. The frequency stabilizing means 5 is the one in the embodiment shown in FIG. 8, and a signal from the first light receiver 68 is input to the low-pass filter 52.

If the birefringent optical medium is included on the interference optical path in the optical interferometer 64, the frequencies at which the peaks of the ordinary light and the extraordinary light component differ from each other by a small amount (Δ) as shown in FIG. Of the frequency discrimination curves 111 and 112 are obtained. Therefore, the frequency f of the output light of the semiconductor laser 12
Is in a frequency range that gives an increase (or decrease) in the frequency discrimination curve (111) for one of the ordinary light and the extraordinary light component, and the frequency discrimination curve for the other of the ordinary light and the extraordinary light component By setting various conditions so as to be in the vicinity of the frequency giving the peak in (112), both the modulation efficiency stabilization and the frequency stabilization can be performed with extremely high accuracy. The above conditions are set by changing the structural parameters (refractive index, optical path length, etc.) and temperature of the optical interferometer 64 to a desired Δ
This is performed by setting the bias current of the semiconductor laser 12 after setting such that is obtained.

The embodiment according to the second basic configuration in which the beam splitter 58, the light receiver 60, the amplifier 61, and the temperature control circuit 82 are omitted is also within the scope of the present invention, and the mark ratio detection circuit 46 and the correction circuit
Structures omitting 62 are also within the scope of the invention.

In the embodiment, when the spectral line width of the coherent light source is relatively wide, it is preferable to perform frequency modulation with a large-amplitude modulation signal and perform dual filter detection or the like on the receiving side. If the spectral line width of the coherent light source is sufficiently narrow, frequency modulation (CPFSK method, etc.) with a small amplitude modulation signal or phase modulation such as differential phase modulation is performed, and delay detection is performed on the receiving side. Good to do.

In the above description of the embodiment, it is assumed that the object to be controlled by the optical output stabilizing means 8 in the third basic configuration is the temperature of the semiconductor laser 12, or that the temperature and the bias current of the semiconductor laser 12 are controlled in a complex manner. However, in these cases, instead of controlling the temperature of the semiconductor laser 12, the semiconductor laser 12 may be configured to have a multi-electrode type and the distribution ratio of the bias current may be controlled. Further, in the third basic configuration, the control target of the frequency stabilizing means 5 is the distribution ratio of the bias current, the control target of the optical output stabilizing means 8 is the total bias current, or a composite control of these. May be performed.
The multi-electrode type semiconductor laser has a FBH-DFB type semiconductor laser device with a long resonator length, a shallow grating with a quarter-wave phase shift point at the approximate center in the longitudinal direction, and a common electrode. The other electrode can be configured by, for example, dividing it into three in the longitudinal direction. In this case, the ratio of the bias current supplied to the center electrode to the bias current supplied commonly to the electrodes on both ends is the distribution ratio described above. According to this configuration, the frequency can be changed while keeping the optical output constant, or the optical output can be changed while keeping the frequency constant, so that this configuration is suitable for implementing the invention. When the frequency or the optical output is changed by the bias current distribution ratio, the control response characteristic is better than when the frequency or the optical output is changed by the temperature. By using a multi-electrode semiconductor laser, it is possible to obtain carrier light having a very narrow spectral line width.

Since the first basic configuration can be implemented according to the second or third basic configuration, description of the embodiment will be omitted.

Applied to the IM / DD system by performing frequency modulation or phase modulation by the optical transmitter with the configuration clarified in the embodiments, and passing the modulated light through an optical resonator (optical interferometer) such as a Mach-Zehnder type resonator. A possible optical transmission means is realized. Conventionally, when intensity modulation light is obtained by directly modulating a semiconductor laser, the semiconductor laser is biased near an oscillation threshold and laser oscillation is turned on / off with a large current amplitude, so that modulation is performed satisfactorily. In some cases, wavelength fluctuation (chirping) occurs during high-speed modulation, and the transmission distance is limited through chromatic dispersion in the transmission path. On the other hand, when the present invention is applied and the intensity modulation is performed as described above, the direct modulation of the semiconductor laser is frequency modulation or phase modulation, and modulation of a small current amplitude becomes possible. In the distance transmission system, the relay interval can be increased or the number of repeaters can be reduced, and the reception sensitivity can be kept constant, for example, at a high level through stabilization of the modulation efficiency in frequency modulation or phase modulation. become.

Effects of the Invention The effects of the present invention are as described in the examples, etc., but the main effects are confirmed and clarified as follows. That is, according to the first basic configuration, there is an effect that the modulation efficiency of the coherent light source can be kept constant irrespective of aging or the like.

[Brief description of the drawings]

FIG. 1, FIG. 2, and FIG. 3 show the first, second, and third aspects of the present invention, respectively.
FIG. 4 is a diagram showing a third basic configuration, FIG. 4 is a circuit block diagram of a coherent optical communication optical transmitter for explaining an embodiment of the second basic configuration, FIG. 5 is an explanatory diagram of a frequency discrimination curve and the like, FIG. 6 is a diagram for explaining the influence of mark rate fluctuation, FIG. 7 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining an embodiment of the third basic configuration, and FIG. 3 is a circuit block diagram of an optical transmitter for coherent optical communication for explaining another embodiment of the basic configuration of FIG. 3, FIG. 9 is an explanatory diagram of synchronous detection output, and FIG. 10 is described with reference to FIG. 4 and FIG. Example and Eighth
FIG. 11 is a diagram for comparing the embodiments described with reference to the drawings, FIG. 11 is a circuit block diagram of a coherent optical communication optical transmitter for explaining still another embodiment of the third basic configuration, and FIG. FIG. 13 is an explanatory diagram of a frequency discrimination curve for an extraordinary light component, FIG. 13 is an explanatory diagram of a conventional technique, and FIG. 14 is a diagram illustrating a relationship between an intermediate frequency spectrum and a band of a dual filter. DESCRIPTION OF SYMBOLS 1 ... Coherent light source, 2 ... Optical frequency discriminating means, 3 ... Light receiving means, 4 ... Modulation efficiency stabilizing means, 5 ... Frequency stabilizing means, 6 ... Light branching means, 7 ... .

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/26 10/28

Claims (15)

(57) [Claims] 1. A coherent light source (1) for outputting light that is angle-modulated based on an input modulation signal, and frequency-discriminating light from the light source (1) to output light having an intensity corresponding to the frequency. An optical frequency discriminating means (2); a light receiving means (3) for receiving light from the discriminating means (2) and outputting a signal having a level corresponding to a received light intensity; Modulation efficiency stabilizing means for detecting the modulation efficiency of the coherent light source (1) from a high frequency component corresponding to the modulation frequency and controlling the amplitude of a modulation signal input to the coherent light source (1) so that the efficiency is constant. (4) An optical transmitter, comprising: 2. A configuration according to claim 1, further comprising detecting a frequency variation of output light of said coherent light source based on a signal from said light receiving means, and canceling the variation. An optical transmitter comprising frequency stabilizing means (5) for controlling driving conditions of the coherent light source (1). 3. An optical splitting means (6) for splitting light from said coherent light source (1) between said light source (1) and said optical frequency discriminating means (2). Another light receiving means (7) for receiving the light split by the light splitting means (6) and outputting a signal of a level corresponding to the received light intensity; An optical transmitter, comprising: an optical output stabilizing means (8) for controlling a driving condition of the coherent light source (1) so that a signal level is constant. 4. The optical transmitter according to claim 2, wherein the coherent light source is a semiconductor laser.
And a bias circuit (14) for supplying a bias current whose set value is controlled by a control signal to the semiconductor laser (12).
And a modulation circuit (16) for changing the bias current from its set value based on the input modulation signal. The optical frequency discriminating means (2) controls the temperature so that the temperature becomes constant. A light interferometer (18), and a light receiving means (3) for performing photoelectric conversion of light from the optical frequency discriminating means (2). The modulation efficiency stabilizing means (4) includes a high-pass filter (24) that passes a high-frequency component of a signal from the light-receiving means (3) corresponding to a modulation frequency, and a signal that passes through the filter (24). A peak detection circuit (26) for detecting the amplitude of the obtained signal,
An amplitude control circuit (28) for controlling the amplitude of the modulation signal input to the modulation circuit (16) so that the amplitude value detected by the detection circuit (26) becomes constant; The means (5) comprises: a beam splitter (30) for splitting light from the coherent light source (1);
A second light receiver (32) for photoelectrically converting the light split by the beam splitter (30), and a signal from the light receiving means (3) corresponding to a frequency variation of the output light of the coherent light source (1). A low-pass filter (36) for passing a low-frequency component, a divider (38) for detecting a signal ratio from the filter (36) and the second photodetector (32), and a signal from the divider (38). A comparator (42) for comparing the signal with a reference signal from a reference power supply (40) and outputting a difference signal; and a bias for sending a control signal to the bias circuit (14) so that the difference signal becomes zero or constant. A control circuit (44), wherein the bias current is set so that the frequency of the output light of the coherent light source (1) falls within a frequency range that gives an increase or decrease in a frequency discrimination curve. And an optical transmitter. 5. The optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) is the one described in claim 4, wherein the bias current is set as described in claim 4, and the driving condition to be controlled by the light output stabilizing means (8) is the semiconductor laser ( An optical transmitter having a temperature of 12). 6. An optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) The invention according to claim 4, wherein the bias current is set as described in claim 4, and the driving condition to be controlled by the light output stabilizing means (8) is a multi-electrode type. An optical transmitter characterized by a bias current distribution ratio of the semiconductor laser (12) configured. 7. The optical transmitter according to claim 4, wherein a mark ratio detecting circuit (46) for detecting a mark ratio of a modulation signal provided as a digital signal, and An optical transmitter, comprising: a correction circuit (48) for correcting a control signal from the bias control circuit (44) so as to eliminate a change in a difference signal from the comparator (42). 8. An optical transmitter according to claim 2, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), and modulation efficiency stabilizing means (4).
The frequency stabilizing means (5) controls the coherent light source (1) at a frequency much lower than its modulation frequency.
A low-frequency oscillator (50) for frequency modulation, and a low-pass filter (52) for passing a low-frequency component of the signal from the light receiving means (3) corresponding to the oscillation frequency of the low-frequency oscillator (50). A synchronous detector (54) for synchronously detecting the signal from the low-pass filter (52) using the signal from the low-frequency oscillator (50) as a reference signal; and the synchronous detector (54).
And a bias control circuit (56) for sending a control signal to the bias circuit (14) so that the synchronous detection output of the coherent light source (1) becomes zero or constant. Wherein the bias current is set so as to be in a range near a frequency giving a peak in the optical transmitter. 9. The optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) According to claim 8, wherein the bias current is set as described in claim 8, and the driving condition to be controlled by the light output stabilizing means (8) is the semiconductor laser ( An optical transmitter having a temperature of 12). 10. The optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) The invention according to claim 8, wherein the bias current is set as described in claim 8, and the driving condition to be controlled by the light output stabilizing means (8) is a multi-electrode type. An optical transmitter characterized by a bias current distribution ratio of the semiconductor laser (12) configured. 11. The optical transmitter according to claim 8, wherein a mark ratio detecting circuit (46) for detecting a mark ratio of a modulation signal provided as a digital signal, and wherein the mark ratio is detected by a change in the mark ratio. An optical transmitter, comprising: a correction circuit (62) for correcting a control signal from the bias control circuit (56) so as to eliminate fluctuations in synchronous detection output. 12. The optical transmitter according to claim 2, wherein said coherent light source (1) is as defined in claim 4, and said optical frequency discriminating means (2) is birefringent on the optical path of the interference light. An optical interferometer (64) including a birefringent optical medium, wherein the light receiving means (3) converts the light from the optical interferometer (64) into an ordinary light component and an extraordinary light component of the birefringent optical medium. A polarizing beam splitter (66), and first and second photodetectors (68, 70) for photoelectrically converting polarized components separated by the polarizing beam splitter (66), respectively. 5. The frequency stabilizing means according to claim 4, wherein the modulation efficiency stabilizing means (4) receives a signal from one of the first and second light receivers (68, 70). Means (5) is the other of the first and second light receivers (68, 70). The signal of claim 8, wherein the frequency of the output light of the coherent light source (1) increases or decreases in a frequency discrimination curve for one of the ordinary light and the extraordinary light component. Structural parameters and temperature of the optical interferometer (64) and the bias so as to be in a range of frequencies to be provided and in a range near a frequency giving a peak in a frequency discrimination curve for the other of the ordinary light and the extraordinary light component An optical transmitter, wherein a current is set. 13. An optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) According to claim 12, wherein the structural parameters and temperature of the optical interferometer (64) and the bias current are set as described in claim 12, and the optical output stabilizing means (8) An optical transmitter characterized in that the driving condition to be controlled in ()) is the temperature of the semiconductor laser (12). 14. The optical transmitter according to claim 3, wherein said coherent light source (1), optical frequency discriminating means (2), light receiving means (3), modulation efficiency stabilizing means (4), and frequency stabilizing means. (5) According to claim 12, wherein the structural parameters and temperature of the optical interferometer (64) and the bias current are set as described in claim 12, and the optical output stabilizing means (8) An optical transmitter characterized in that the drive condition to be controlled is a distribution ratio of a bias current of the semiconductor laser (12) configured as a multi-electrode type. 15. An optical transmitter according to claim 12, wherein a mark ratio detecting means (46) for detecting a mark ratio of a modulation signal given as a digital signal; An optical transmitter, comprising: a correction circuit (62) for correcting a control signal from the bias control circuit (56) so as to eliminate fluctuations in synchronous detection output.