JPH0567832A - Frequency stabilizer for semiconductor laser - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide a semiconductor laser wavelength stabilizing circuit which is capable of stably controlling the wavelength of a semiconductor laser with a high loop gain without causing an unstable factor in a feedback loop. [Structure] Light emitted from a semiconductor laser is received by a photodetector through an optical resonator, and its output is amplified by a plurality of amplifiers.
While slightly modulating the resonator length of the optical resonator according to the AC signal of frequency f, the outputs of a plurality of amplifiers are synchronously detected based on the AC signal, and the laser oscillation wavelength is feedback controlled based on the detected output. A filter that is interposed between any of a plurality of amplifiers and that allows a signal of frequency f to pass and cuts off signals of frequency 1 / (2πτ) or less, where τ is the response time of the feedback loop. Is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser wavelength stabilizing circuit for stabilizing the wavelength of semiconductor laser light used for coherent optical transmission.

[0002]

2. Description of the Related Art As is well known, in coherent optical transmission, a wavelength stabilization technique for a light source is extremely important. Conventionally,
In order to stabilize the oscillation wavelength of the semiconductor laser, a method of detecting the wavelength by an external circuit and performing feedback control is well known.

FIG. 6 shows a general structure of a conventional circuit. The forward light of the semiconductor laser 1 is converted into parallel light by the optical lens 2 and then focused on the end of the optical fiber 5 for optical transmission by the optical lens 4 to be used for signal transmission.

In contrast to such a semiconductor laser 1, in the wavelength stabilization circuit, the backward light of the semiconductor laser 1 is converted into parallel light by the lens 8, transmitted through the Fabry-Perot resonator 9, and the optical lens 10 detects the light. Focus on 11.

Here, the Fabry-Perot resonator 9 has reflecting mirrors 91 and 92 arranged perpendicularly to the optical axis, and a crystalline lens 93 between them.
Configured through. A pair of electrodes 14 and 15 are arranged in parallel and opposite to each other on the side surface of the crystalline lens 93. These electrodes 14 and 15 are connected to an AC signal generator 16. That is, the resonator length is minutely modulated by the AC signal generator 16 by utilizing the electro-optical effect of the Fabry-Perot medium (lens 91).

The output of the photodetector 11 is the amplifiers 19, 2
After being amplified by 0, it is supplied to the synchronous detector 17. The AC signal generated by the AC signal generator 16 is also supplied to the synchronous detector 17. The synchronous detector 17 synchronously detects the output signal of the amplifier 20 based on the AC signal.

The output of the synchronous detector 17 is proportional to the first derivative of the Fabry-Perot wavelength transmission characteristic. This signal is supplied to the feedback controller 18 as an error signal. The feedback controller 18 changes the injection current of the semiconductor laser 1 based on the error signal and feedback-controls the oscillation wavelength thereof, thereby stabilizing the emission light frequency of the semiconductor laser.

When the oscillation wavelength of the semiconductor laser is controlled, the injection current of the semiconductor laser is generally controlled as described above, but the change of the injection current varies not only the oscillation wavelength but also the output level. Therefore, the fluctuation amount of the oscillation output level is also amplified by the amplifiers 19 and 20, and the output of the amplifier 19 is saturated or the excessive input is supplied to the synchronous detector 17 in some cases. This makes the feedback loop extremely unstable and causes oscillation when a high loop gain is set. Therefore, in the conventional wavelength stabilization circuit, the loop gain cannot be made high, and there arises a problem that the control error increases only by the proportional control.

Further, when the semiconductor laser is directly modulated, the injection current to the semiconductor laser is changed according to the modulation signal, so that the oscillation wavelength and the oscillation output fluctuate due to the modulation. Therefore, the output level of the photodetector 11 fluctuates, and this fluctuation is also amplified by the amplifiers 19 and 20, which mixes the modulation signal in the feedback loop, saturates the output of the amplifier 19, or causes the synchronous detector to be detected. Excessive input is supplied to 17. Therefore, in the conventional wavelength stabilization circuit, when the semiconductor laser is directly modulated, the modulation signal is mixed in the feedback loop, which causes a problem that the feedback loop becomes extremely unstable.

[0010]

As described above, in the conventional semiconductor laser wavelength stabilization circuit, if the loop gain of the feedback loop is set high, it causes oscillation, and the loop gain cannot be set high. Then, problems such as an increase in control error occurred. Further, when the semiconductor laser is directly modulated, a modulation signal is mixed in the feedback loop, which causes a problem of making the feedback loop extremely unstable.

The present invention has been made in order to solve the above-mentioned problems. The feedback loop has no instability factor, the wavelength of the semiconductor laser can be stably controlled with a high loop gain, and the semiconductor laser is directly modulated. Even in this case, it is an object of the present invention to provide a semiconductor laser wavelength stabilization circuit which can prevent a modulation signal from entering the feedback loop and realize stable operation of the loop.

[0012]

In order to achieve the above object, a semiconductor laser wavelength stabilizing circuit according to the present invention comprises an optical resonator or a gas cell which makes emitted light of a semiconductor laser incident and resonates at a specific wavelength. A photodetector that receives the light emitted from the optical resonator or the gas cell and converts it into an electrical signal,
Amplifying means for amplifying the output of the photodetector through a plurality of amplifiers, an AC signal source for generating an AC signal of frequency f, and a resonator length of the optical resonator based on the output of the AC signal source. Of the semiconductor laser based on the detection output of the synchronous detector and the synchronous detector that synchronously detects the output of the signal amplifying means based on the output of the AC signal source. When a feedback controller for feedback controlling the oscillation wavelength and one of a plurality of amplifiers of the amplifying means are provided and the response time of the feedback loop is τ, the signal of the frequency f is passed and the frequency 1 / The first feature is that the first filter is provided to cut off signals of (2πτ) or less.

Further, if the lowest frequency among the frequency components of the modulation signal that intervenes between any of the plurality of amplifiers of the amplifying means and modulates the oscillation wavelength of the semiconductor laser is F, then the frequency is A second feature is that a second filter is provided, which passes a signal of f and cuts off a signal of frequency F or higher.

Further, instead of the first and second filters, it is interposed between any of a plurality of amplifiers of the amplifying means, the response time of the feedback loop is τ, and the oscillation wavelength of the semiconductor laser is modulated. When the lowest frequency of the frequency components of the modulated signal is F, a filter that allows the signal of the frequency f to pass and cuts off signals of frequency 1 / (2πτ) or less and signals of frequency F or more is provided. This is the third feature.

[0015]

[Function] First, in order to stabilize the wavelength, 1 / (2πτ)
When signals of the following frequency components are applied to the semiconductor laser, the output signal of the photodetector is not limited to the signal centered on the frequency f of the AC signal, but 1
Signals having frequency components of / (2πτ) or less are mixed. Therefore, in the configuration having the first characteristic, the latter signal is removed by the first filter to eliminate the cause of making the loop operation unstable.

Further, in the case where the semiconductor laser is directly modulated, when a signal having a frequency component of F or more is applied to the semiconductor laser for wavelength stabilization, the output signal of the photodetector is further added to the output signal of the semiconductor laser. Signals of frequency F or higher are mixed due to fluctuations in oscillation output due to direct modulation. Therefore, in the configuration having the second characteristic, the signal having the frequency F or higher is removed by the second filter to eliminate the cause of making the loop operation unstable.

In the third characteristic configuration, a signal having a frequency component of 1 / (2πτ) or less due to the oscillation output variation of the semiconductor laser and a frequency component of F or more due to the oscillation output variation due to the direct modulation of the semiconductor laser are provided. The signal which it has is removed by one filter, and the cause of making the loop operation unstable is eliminated. From the output signal of the photodetector, 1
Even if the frequency signal components below / (2πτ) and above F are removed, the feedback control system is not adversely affected.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows the configuration. However, in FIG. 1, the same parts as those in FIG. 6 are designated by the same reference numerals.

In FIG. 1, the light emitted from the left side of the semiconductor laser 1 is focused on the optical fiber 5 by the lenses 2 and 4 and used for the transmission of optical signals. The light emitted to the right is converted into parallel light by the lens 8, passes through the Fabry-Perot resonator 9, and then is condensed by the lens 10 on the photodetector 11. 91, 9
Reference numeral 2 is a reflecting mirror forming the Fabry-Perot resonator 9.
Medium (lens) 93 that forms the Fabry-Perot resonator 9
Has an electro-optical effect. The output of the photodetector 11 is amplified by the amplifier 19 and then amplified by the high-pass filter 30.
Is further amplified by the amplifier 20 of the next stage and input to the synchronous detector 17.

On the other hand, a pair of electrodes 14 and 15 are formed in the Fabry-Perot resonator 9, and the resonator length is set to the AC signal generator 1 by utilizing the electro-optic effect of the Fabry-Perot medium.
It is designed to be modulated by 6. This AC signal is also supplied to the synchronous detector 17, and the signal of the photodetector 11 is used for synchronous detection. The output of the synchronous detector 17 is proportional to the first derivative of the Fabry-Perot wavelength transmission characteristic, and this signal is supplied to the feedback controller 18 as an error signal and used for feedback control of the oscillation wavelength of the semiconductor laser 1. That is, the present invention is to investigate the reason why the conventional wavelength stabilization circuit cannot obtain a high loop gain and take a countermeasure.

Generally, when controlling the oscillation wavelength of a semiconductor laser, the injection current of the semiconductor laser is controlled as described above, but the change of the injection current changes not only the oscillation wavelength but also the oscillation output level. Now, assuming that the response time of the control loop is τ, a signal having a frequency component of 1 / (2πτ) or less is added to the semiconductor laser 1 for control.

Therefore, the output signal of the photodetector 11 is a signal centered on the frequency f of the AC signal generator 16 and 1 / (2π due to fluctuations in the oscillation output of the semiconductor laser 1).
Signals having frequency components equal to or lower than τ) are mixed. The amplifier 2 without the latter signal component being removed
When 0 is input to the synchronous detector 17, it causes instability in the control loop.

Therefore, before the signal is input to the amplifier 20, the high-pass filter 30 blocks a signal having a frequency component of 1 / (2πτ) or less. That is, the high pass filter 30
The cutoff frequency fc of is set so as to satisfy the following equation. 1 / (2πτ) <fc <f By setting the cutoff frequency characteristic of the filter 30 in this way, the signal of the originally required frequency component f is not cut off, and only the component that gives an unstable element is reduced.

Therefore, the semiconductor laser wavelength stabilizing circuit having the above structure compensates the fluctuation component of the oscillation output level due to the change of the injection current and removes the unstable element in the control loop, so that the semiconductor laser with a high loop gain is obtained. The wavelength of can be controlled stably.

2 and 3 show an embodiment in the case of directly modulating a semiconductor laser. 2 and 3, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 2, when the semiconductor laser 1 is directly modulated, the modulation signal is added to the injection current through the adder 6 to modulate the semiconductor laser 1. At this time, the oscillation output level variation and the oscillation wavelength variation in the semiconductor laser 1 occur due to the modulation. Therefore, the output signal of the photodetector 11 includes a signal centered on the frequency f and 1 /
Modulation signals are mixed in addition to signals having frequency components of (2πτ) or less.

Therefore, before the signal is input to the amplifier 20, the low pass filter 31 blocks the modulated signal. That is, the lowest frequency of the frequency components of the modulated signal is F
, The cutoff frequency fc2 of the low-pass filter 31
Is set to satisfy the following equation. f <fc2 <F Also in this embodiment, if the cutoff frequency characteristic of the filter 31 is set in this way, the modulation signal is cut off, and the semiconductor laser can be stably controlled with a high loop gain.

Before the modulated signal is input to the adder 6, a filter 35 may be inserted in the control loop to cut off a frequency component which gives a disturbance to the signal of the frequency component f. That is, the filter 35 is a high-pass filter having a cutoff frequency fc3 that satisfies the following equation. f <fc3 <F Further, the filter 35 may be a band elimination filter having a center frequency f.

By setting the cutoff frequency characteristic of the filter 35 as described above, the modulated signal can be cut off without fail, and the stability of the oscillation wavelength control of the semiconductor laser can be further improved.

The wavelength stabilizing circuit shown in FIG. 3 uses a bandpass filter 32 having a center frequency f in place of the highpass filter 30 and the lowpass filter 31. That is, the low cutoff frequency fc of the bandpass filter 32.
1, the high cutoff frequency fc2 is set to satisfy the following equation. 1 / (2πτ) <fc1 <f <fc2 <F

Also in this embodiment, if the cutoff frequency characteristic of the filter 32 is set as described above, the modulation signal is cut off and the semiconductor laser can be stably controlled with a high loop gain.

The present invention can be applied to systems other than the Fabry-Perot resonator shown in FIG. 4 and 5 show other embodiments according to the present invention. 4 and 5, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

The wavelength stabilizing circuit shown in FIG. 4 uses a normal Fabry-Perot resonator 9 without electrodes, and instead adds the output of the AC signal generator to the injection current through the adder 6, The emitted light of the semiconductor laser 1 is directly modulated by. Also in this example,
By providing the high-pass filter 30 between the amplifiers 19 and 20, the stability at high loop gain can be improved.

The wavelength stabilizing circuit shown in FIG. 5 uses the gas cell 3 instead of the Fabry-Perot resonator 9 of FIG. 2 and stabilizes the wavelength by utilizing a predetermined absorption line of the gas cell 3. Also in this embodiment, by inserting the high-pass filter 30 between the amplifiers 19 and 20, the stability at a high loop gain can be improved.
Besides, the same can be applied to an example using an optical resonator such as a ring resonator or a Mach-Zehnder interferometer.

[0035]

As described in detail above, according to the present invention,
It is possible to provide a semiconductor laser wavelength stabilization circuit that can stably control the wavelength of a semiconductor laser with a high loop gain without causing an unstable factor in the feedback loop.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a semiconductor laser wavelength stabilization circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration for directly modulating a semiconductor laser according to another embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration when a bandpass filter is used in place of the highpass filter and the lowpass filter of FIG. 2 according to another embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration when an injection current to a semiconductor laser is modulated by an AC signal according to another embodiment of the present invention.

5 is a block diagram showing a configuration when a gas cell is used instead of the Fabry-Perot resonator shown in FIG. 2 according to another embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a conventional semiconductor laser wavelength stabilization circuit.

[Explanation of symbols]

1 ... Semiconductor laser, 2, 4, 8, 10 ... Optical lens, 3
... gas cell, 5 ... optical fiber, 6 ... adder, 9 ... Fabry-Perot resonator, 91, 92 ... reflecting mirror, 93 ... crystalline lens,
11 ... Photodetector, 14, 15 ... Electrode, 16 ... AC signal generator, 17 ... Synchronous detector, 18 ... Feedback controller, 19, 20 ... Amplifier, 30 ... High-pass filter, 3
1 ... Low-pass filter, 32 ... Band-pass filter, 3
5 ... Filter.

Claims (9)

[Claims] 1. An optical resonator for making emitted light of a semiconductor laser incident and resonating at a specific wavelength, a photodetector for receiving the emitted light of the optical resonator and converting it into an electric signal, and a photodetector of the photodetector. Amplifying means for amplifying the output through a plurality of amplifiers, an AC signal source for generating an AC signal of frequency f, and a resonator length of the optical resonator is minutely modulated based on the output of the AC signal source. Modulation means, a synchronous detector for synchronously detecting the output of the signal amplifying means based on the output of the AC signal source, and feedback control for feedback controlling the oscillation wavelength of the semiconductor laser based on the detection output of the synchronous detector. If the response time of the feedback loop is τ, which is interposed between any one of the amplifier and the plurality of amplifiers of the amplification means, the signal of the frequency f is passed and the signal of frequency 1 / (2πτ) or less is passed. A semiconductor laser wavelength stabilizing circuit including a first filter for blocking the signal. 2. When F is the lowest frequency component of the frequency components of the modulation signal that intervenes between any of the plurality of amplifiers of the amplification means and modulates the oscillation wavelength of the semiconductor laser, 2. The semiconductor laser wavelength stabilizing circuit according to claim 1, further comprising a second filter that allows the signal of the frequency f to pass and cuts off signals of the frequency F and higher. 3. An optical resonator for making the emitted light of a semiconductor laser incident and resonating at a specific wavelength, a photodetector for receiving the emitted light of this optical resonator and converting it into an electric signal, and a photodetector of this photodetector. Amplifying means for amplifying the output through a plurality of amplifiers, an AC signal source for generating an AC signal of frequency f, and a resonator length of the optical resonator is minutely modulated based on the output of the AC signal source. Modulation means, a synchronous detector for synchronously detecting the output of the signal amplifying means based on the output of the AC signal source, and feedback control for feedback controlling the oscillation wavelength of the semiconductor laser based on the detection output of the synchronous detector. Between the amplifier and the plurality of amplifiers of the amplifying means, the response time of the feedback loop is τ, and the frequency component of the modulation signal that modulates the oscillation wavelength of the semiconductor laser is the maximum. If the lower frequency is F, the through frequency f becomes signal, frequency 1 / semiconductor laser wavelength stabilization circuit comprising a filter that blocks (2.pi..tau) following signal and the frequency F or more signals. 4. The optical resonator is a Fabry-Perot resonator in which a pair of electrodes are arranged opposite to each other in parallel with a transmission optical path,
4. The modulation means is realized by applying the output of the AC signal source to a pair of electrodes of the Fabry-Perot resonator to change the resonator length.
The semiconductor laser wavelength stabilization circuit according to any one of the above. 5. The modulator is equivalent to modulating the resonator length by adding the output of the AC signal source to the amount of current injected into the semiconductor laser to modulate the optical wavelength. 4. The semiconductor laser wavelength stabilizing circuit according to claim 3, which is realized by 6. A gas cell that receives the emitted light of a semiconductor laser and absorbs light of a certain wavelength, a photodetector that receives the emitted light of this gas cell and converts it into an electrical signal, and the output of this photodetector. Amplification means for amplifying through a plurality of amplifiers,
An alternating current signal source for generating an alternating current signal of frequency f; a modulating means for modulating the oscillation wavelength of the semiconductor laser based on the output of the alternating current signal source; and an output of the amplifying means based on the output of the alternating current signal source. Synchronous detector for synchronous detection, a feedback controller for feedback controlling the oscillation wavelength of the semiconductor laser based on the detection output of the synchronous detector, and interposed between any of a plurality of amplifiers of the amplifying means, When the response time of the feedback loop is τ, the signal of the frequency f is passed and the frequency 1 / (2π
A semiconductor laser wavelength stabilization circuit comprising: a first filter for blocking signals equal to or less than τ). 7. If the lowest frequency of the frequency components of the modulation signal that intervenes between any of the plurality of amplifiers of the amplification means and modulates the oscillation wavelength of the semiconductor laser is F, then 7. The semiconductor laser wavelength stabilizing circuit according to claim 6, further comprising a second filter that allows a signal having a frequency f to pass and cuts off a signal having a frequency F or higher. 8. A gas cell that receives the emitted light of a semiconductor laser and absorbs light of a constant wavelength, a photodetector that receives the emitted light of this gas cell and converts it into an electrical signal, and the output of this photodetector. Amplification means for amplifying through a plurality of amplifiers,
An alternating current signal source for generating an alternating current signal of frequency f; a modulating means for modulating the oscillation wavelength of the semiconductor laser based on the output of the alternating current signal source; and an output of the amplifying means based on the output of the alternating current signal source. Synchronous detector for synchronous detection, a feedback controller for feedback controlling the oscillation wavelength of the semiconductor laser based on the detection output of the synchronous detector, and interposed between any of a plurality of amplifiers of the amplifying means, When the response time of the feedback loop is τ and the lowest frequency of the frequency components of the modulation signal that modulates the oscillation wavelength of the semiconductor laser is F, the signal of the frequency f is passed and a signal of 1 / (2πτ) or less and A semiconductor laser wavelength stabilizing circuit comprising a filter for cutting off signals of frequency F or higher. 9. A filter for adding a modulation signal, which modulates an oscillation wavelength of the semiconductor laser, to the semiconductor laser after removing a frequency component which gives a disturbance to the signal of the frequency f. Item 9. A semiconductor laser wavelength stabilization circuit according to any one of items 1, 3, 6 and 8.