JPH03120774A - Light frequency control device - Google Patents

Abstract

PURPOSE:To maintain the operation speed of a phase comparator which is needed when controlling a semiconductor laser diode by offset blocking and enable designing to be easy by providing an oscillation frequency control means for changing the oscillation frequency of the semiconductor diode. CONSTITUTION:An oscillation frequency nu of a semiconductor laser diode 101 to be controlled in the state where locking is being applied to, namely a second intermediate frequency fIF2 is equal to a reference frequency fr due to a second local oscillator 114 can be considered to be available in four ways depending on the relationship of a reference light with a light frequency nur and the relationship between a first intermediate frequency fIF1 defined by fIF1=¦nu-nur¦ and an oscillation frequency f of a first local oscillator 111. It is controlled so that the frequency of input signal to a phase comparator 113 is always equal to fr (260MHz), thus achieving a stable frequency offset locking operation.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is used in optical measurement equipment that uses optical frequency information, such as frequency division multiplex communication in the optical domain and measurement of the frequency response of high-speed light receiving circuits. Optical frequency control that can control the oscillation optical frequency of semiconductor laser diodes that oscillate in a single longitudinal mode, such as DFB (distributed feedback) laser diodes and DBR (distributed reflection) laser diodes, with high precision and over a wide frequency band. It is related to the device.

[Prior art] In recent years, in light wave communication technology that utilizes the properties of light as a wave, research has been developed in the optical domain as a method that effectively utilizes the wide transmittable frequency band of several T degrees possessed by optical fiber. Frequency division multiplexing (optical FDM) is attracting attention.

However, in order to put this optical FDM method into practical use, it is first necessary to develop a variable frequency light source that can freely set the optical frequency within a wide variable range and stably maintain that value. present is required. In addition, such variable frequency light sources can be used in optical measurement equipment that measures the frequency response of high-speed light receiving circuits and other devices that have a wide frequency band exceeding 10 GHz when combined with a reference light source, so their importance is extremely high. expensive.

On the other hand, semiconductor laser diodes have made remarkable progress in recent years.
Laser diodes (LDs) such as distributed feedback (DFB) or distributed reflection (DBR) laser diodes (LDs) oscillate at a single optical frequency (single longitudinal mode) and do not allow temperature, injection current, or control current to be varied. A device that can continuously change the oscillation light frequency by changing the oscillation frequency has also been developed.

For example, the oscillation optical frequency of a DFB LD changes by approximately 10 GHz per 1°C of temperature and several hundred MHz per 1 mA of injection current, and the oscillation optical frequency of a DBRLD changes by approximately 10 GHz per 1°C of temperature and several hundred MHz per 1 mA of injection current, and the oscillating optical frequency of a DBRLD changes by approximately 10 GHz per 1°C of temperature and several hundred MHz per 1 mA of injection current.
Several GHz per 1 mA of control current injected into the DBR) region
known to change. Furthermore, the frequency range that can be changed continuously is from tens of GHz to hundreds of GHz.
has reached.

However, this frequency variable characteristic also becomes a factor that deteriorates the stability of the oscillation optical frequency.

Generally, when these single longitudinal mode laser diodes are used for light wave communication, efforts are made to keep the temperature of the laser diode constant using a temperature control device, but light emission itself is a heat generation process. Fluctuations also occur in the ambient temperature, which usually causes frequency fluctuations of several tens of MHz to several hundred MHz.

Furthermore, a phenomenon in which the oscillation optical frequency of a semiconductor laser diode drifts by several MHz per hour has been observed. Issue pp, 1028
). This state is called "free running," and if left as it is, it would be difficult to realize optical FDM communications at narrow frequency intervals of several hundred MHz, or to realize optical receivers with resolution on the order of MHz. It is difficult to configure a frequency characteristic measuring device such as

Therefore, as a method to achieve high frequency stability of several MHz or less, the oscillation frequency of the semiconductor laser diode can be adjusted using an external frequency reference such as a Fabry-Perot cavity, a Fabry-Perot etalon, or an atomic or molecular absorption line. A method of stabilizing it to a specific value is widely used. In this way, light sources that serve as standards for light frequencies are gradually being realized. Then, by detecting the frequency difference of the light relative to the reference light, the frequency off-sinking method is used as a method to control the oscillation frequency of the semiconductor laser tie auto continuously and with high stability. is proposed. Below, Kuboki, 0
IEEE Journal of Q by htsu
uantum Electronics (Vol, Q
B-23, No, +l, April 1987.
This conventional optical frequency control device will be described in accordance with the report on pp. 388-393).

FIG. 2 is a diagram showing an example of the configuration of an optical frequency control device using this conventional method. The output of the controlled semiconductor laser diode 101 oscillating in a single longitudinal mode I- is guided to an isolator 104 for preventing the oscillation frequency from being affected by reflected light, and then a part of it is sent to the optical coupler 1. Retrieved by .05. This optical coupler 1.0
A part of the output light of No. 5 and the reference light are mixed by an optical coupler 106 so that the polarization states of both lights match, and the mixture is input to a photodiode (photodetector) 107. Here, if the oscillation optical frequency of the semiconductor laser diode 101 is ν, and the optical frequency of the reference light is νr, the respective optical electric fields are E=Acos(2πνt+φ) (
1) Er=Arcos (2yr vrt+φr)
−(2) can be written. However, A, Ar are electric field amplitudes, φ, φ
r is the initial phase. Since the photodiode 107 generates a photocurrent proportional to the power of the incident light, its output is 1 −RI E+Erl −(3
) is given by However, R is the photodiode 107
It is a coefficient determined by In the above formula (3), (1) and (2
), in addition to the DC component,
It can be seen that a beat signal having a frequency flF corresponding to the frequency difference between the two lights is obtained. That is, flF= l v −νrl −・=
(4) The intermediate frequency signal is changed.

Next, this intermediate frequency signal (frequency flF) and a local oscillator 11 that can continuously change the oscillation frequency
4 (for example, a microwave frequency synthesizer) (frequency 1) is input to the phase comparator 113. In this way, a result of comparing the respective phases, that is, an error signal containing information on the phase difference is obtained. By applying this error signal to the oscillation frequency control circuit 28 to change the oscillation frequency of the semiconductor laser diode 101 and controlling the intermediate frequency signal and the output signal of the local oscillator 114 to always maintain a constant phase difference, the frequency can be changed from phase to phase. Because of the relationship that they are differentials of , flF and f can be matched without any steady error. Therefore, the semiconductor laser diode 101
As shown in Figure 3 (a) and (b), the oscillation frequency ν of
(5) is stabilized. Here, Fig. 3(a) and (
Which state b) is stabilized is arbitrarily determined depending on which direction of increase or decrease of the oscillation frequency of the semiconductor laser diode 101 corresponds to the positive or negative phase difference. Note that the oscillation frequency control circuit 28 may include, for example, a semiconductor laser diode 10 depending on the error signal.
A device that changes the injection current of 1 is used.

The above is the principle of frequency offset locking, which is an application of phase-locked loop (PLL) technology, which is widely used to stabilize the oscillation frequency in the electrical frequency (kHz-GHz) domain. It is also called heterotine-type optical PLL technology7. However, in order to realize such frequency offset locking using a semiconductor laser in the optical domain, it is necessary to deal with problems unique to semiconductor laser diodes, and special measures have been taken as described below.

As explained earlier, DFB LDs, DBRLDs, etc. oscillate at a single frequency (single longitudinal mode oscillation) and can continuously change the oscillation frequency, but this is not the same as the frequency in the normal electrical frequency domain. There is a significant difference in the oscillation spectral linewidth when compared to a variable oscillator. With a variable frequency oscillator in the electrical frequency domain, it is relatively easy to obtain a full width at half maximum of the spectrum, whereas with a laser diode that can continuously change the oscillation frequency, the oscillation frequency can be adjusted to several hundred kHz, although there have been significant improvements in recent years. It is several dozen meters long. In other words, these semiconductor laser diodes have extremely large phase noise (or it may be called frequency noise). If the dispersion value of the phase fluctuation caused by this phase noise during the time interval τ is σ2, then the following relationship holds between this value and the spectral linewidth △ν (for example, K, Kikuchiet al, ” Degrad
ation of Bet-Error Rate-
IEEE J, Lightwave Techno
logy, vol, I+T-2, pp1024-10
33.1984).

σ2−2π△shiτ・・・・・・(6
) Therefore, for example, in the case of light with a line width of IQM)Iz,
The standard deviation σ of the phase fluctuation occurring during 1 μs is 2πX
It also becomes 1.26. In other words, even in just 1 μs, 1
This means that there is a phase fluctuation that is equal to or more than the period. For this reason, if you try to use a phase comparator with a comparable phase range of ±π/2 or ±π, which is normally used in PLL circuits in the electrical frequency domain, the speed will be extremely high (
control must be performed at a frequency of several hundred MHz or higher), which is impractical.

Therefore, Kuboki et al. mentioned above stated that ``the frequency division ratio M is 500
or 2000 frequency divider," and a digital phase comparator consisting of "a binary up counter, a binary down counter, a full adder, and a D/A converter." The operating characteristics of this phase comparator are shown in FIG. In this figure, the horizontal axis represents the phase difference between the two human input signals, and the vertical axis represents the error signal output as a result of the comparison on an arbitrary scale. The phase range that can be compared is ±2π×M
It has been expanded to 2 + 1, and the number + M117. Even with a semiconductor laser diode having a linewidth of , stable frequency offset locking can be achieved with relatively slow control on the order of MHz.

The above is an explanation of the frequency offset locking method using a semiconductor laser diode. In the conventional optical frequency control device shown in FIG. 2, if the oscillation frequency f of the local oscillator 114 is continuously changed or swept while frequency offset locking is realized, the above-mentioned relationship of equation (4) is obtained. Semiconductor laser diode 101
The oscillation optical frequency ν can be swept or stabilized at a set value. FIGS. 3(c) and 3(d) show this situation, and by changing the oscillation frequency f by δf, the oscillation optical frequency ν also changes by δf.

In fact, the above-mentioned paper by Kuboki et al. also states that the difference from the reference light is from 80 MHz to 1.3 GHz, that is, 1.
It has been reported that a variable width of optical frequency of 22 GHz has been achieved. Here, f is a frequency obtained from an oscillator in the electrical frequency domain, so the accuracy is extremely high, and when tracking the oscillation optical frequency ν to the reference optical frequency νr (deviated by the frequency f), a PLL is used. Since it is controlled, there is no steady frequency error. Therefore, semiconductor laser diode 1.0 with extremely high precision and stability
1 oscillation light frequency control is realized.

[Problems to be Solved by the Invention] Incidentally, in the conventional optical frequency control device described above, the frequency difference (corresponding to the oscillation frequency r of the local oscillator 114) given to the phase comparator 113 by the above-mentioned equation (4) is Since it is directly manually operated, the variable optical frequency range Δν is limited by the operating speed of the phase comparator 113. As already explained, frequency offset locking (heterodyne type optical P L
In order to realize L) of 1, it is necessary to configure the phase comparator 113 with a relatively complicated digital circuit, and its operating speed is limited to several hundred MHz to several GHz. That is, the tunable frequency range is limited to several GHz at most. Also,
In order to design the phase comparator 1.13 to operate over a wide speed range, it is difficult to set the timing especially during high-speed operation.

On the other hand, if the frequency division ratio M of the frequency divider constituting the phase comparator 113 is increased, only this frequency divider needs to operate at high speed, so it is thought that expansion to more than ten GHz is possible even at present. However, in this case, when the intermediate frequency flF defined by the above-mentioned equation (4) is small, the operating speed of the phase comparator after the frequency divider is less than flP/M, so the phase comparison operation becomes slow. In this case, the offset locking operation may become unstable due to the influence of large phase noise peculiar to semiconductor laser diodes. In addition, the minimum phase difference △φ that can be compared with − times of comparison operations is △φ−2π×M ・・・・・・
(7) Since it is given by be.

This invention has been made in view of the above-mentioned circumstances, and its purpose is to overcome the drawbacks of the prior art described above, and to improve the operation of the phase comparator required when controlling a semiconductor laser diode by offset locking. The speed can be kept constant, design can be done easily, and tens of Gs can be maintained.
The object of the present invention is to provide an excellent optical frequency control device that has a wide tunable frequency range of 1(z or more) and can stabilize its set value with high accuracy.

[Means for Solving the Problems] In order to solve the above problems, the present invention mixes at least a part of the output light of a semiconductor laser diode with a reference light, and then converts the output light into a first intermediate frequency signal by photoelectric conversion. a first local oscillator whose oscillation frequency can be changed continuously; the first intermediate frequency signal;
a mixer that mixes the outputs of the local oscillators and generates a second intermediate frequency signal having a frequency that is the sum or difference of these respective signals; a second local oscillator that oscillates at a constant reference frequency; a phase comparator that detects a phase difference between the output of the local oscillator and the second intermediate frequency signal and outputs an error signal corresponding to the phase difference; The present invention is characterized by comprising oscillation frequency control means for changing the oscillation frequency of the semiconductor laser diode so that the frequency of the signal and the reference frequency are always equal.

[Function] According to the configuration of the two researchers, the output of the first local oscillator with variable frequency for changing the oscillation frequency of the semiconductor laser diode and the first intermediate frequency signal whose frequency changes as a result of control are The oscillation frequency of the semiconductor laser diode is controlled using a frequency offset locking technique so that the frequency of the second intermediate frequency signal input to the mixer and generated by the mixer is always constant. In this state where offset locking is applied, if the oscillation frequency of the variable frequency first local oscillator is changed and swept, the oscillation frequency of the semiconductor laser diode is also changed and swept at the same time, making it possible to control the frequency of light. That is, in this invention, large frequency changes when sweeping the frequency over a wide band are absorbed by the mixer, so that the input (second intermediate frequency signal) to the phase comparator for offset locking always has a constant frequency. I have to. Here, the mixer has a much simpler structure than the digital phase comparator required for frequency offset locking of a semiconductor laser diode, and one that operates over a wide frequency band (~10-odd GHz) can be easily obtained. Therefore, it is possible to widen the frequency variable range without being limited by the operating speed of a phase comparator with a complicated configuration as in the past.

In addition, as an optical frequency control device using a semiconductor laser,
The first intermediate frequency signal (Bee I signal) and the output of the first local oscillator (first synthesizer) are mixed to generate a second intermediate frequency signal, and this second intermediate frequency signal and A configuration in which offset locking is performed by comparing the phase with the output signal of a second local oscillator (second synthesizer) has been reported by K., Kuboki et al. [Optical frequency control device J with multiple feedback loops] 1,988 Conference on Precision Electromagnetic Measurement (CPE)
M'88) Digest PP 24-PP, 25
, K., Kuboki, et al., ,Performa
nce and 1tsevaluation of
optical frequency 5unthes
iserby 51m1 conductor 1aser
In this device, a third local oscillator is also included in a separate feedback loop, and the method of operation of each local oscillator when changing the frequency continuously is also suggested in the above. In the present invention, there is no separate feedback loop including the third local oscillator as seen in the above reported example, and the second feedback loop is not clearly shown.
The above-mentioned effect is achieved by sweeping the frequency using the first local oscillator while keeping the oscillation frequency of the first local oscillator at a constant value.

[Embodiments] 6 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an optical frequency control device according to an embodiment of the present invention. In this figure, as the controlled semiconductor laser diode 1.01, for example, a 15 μm band DBRLD having an active layer region and a distributed reflection layer region is used, and a current of about 80 mA is applied to the active layer region by a current injection device 102. For example, about 30 mA is applied to the distributed reflection layer region by a controlled current injection device 103.
supply the control current. At this time, the laser diode 101 oscillates in a c-single longitudinal mode of approximately 193.5 THz (near 1,535 μm), and has a spectral linewidth of approximately 16 MHz. Furthermore, the control current injection device 1
1,9 over approximately 100 GHz by changing the control current value to the distributed reflective layer region.
Laser diode 101 at a rate of about GHz/mA
The oscillation light frequency can be changed. In this embodiment, the laser diode 101 has l/1
The temperature is stabilized to around 00℃.

The output light of the controlled semiconductor laser diode 101 is transmitted through an isolator 104 to prevent the oscillation frequency from fluctuating due to reflected light, and is then input into an optical fiber. The light used for offset locking is separated.

A part of the output light of the controlled semiconductor laser diode thus obtained and the reference light are mixed by the optical coupler 106 so that the polarization states of both lights match, and the mixed light is sent to the photodiode 1.
07 and detect the beat signal. In this case, the photodiode 107 is, for example, QDEtlCH manufactured by La5ertron, which has a band of 15 GHz or more.
-035-001 is used. This beat signal has a frequency given by "lF, =lcsrl - (8), where ν is the frequency of the controlled light and νr is the optical frequency of the reference light. It is also called the intermediate frequency signal.

Note that the reference light source 108 is a semiconductor laser diode with an external resonator and a spectral linewidth of several tens of kHz.
The optical frequency stability has been improved as well as narrowed to
Narrow spectrum LD light source (for example, Santic N5
L-155) was used, but a single semiconductor laser diode may also be used. Further, in this invention, ordinary single mode optical fiber couplers are used as the optical couplers 105 and 106, and ordinary single mode optical fibers are used for the optical wiring, and both lights mixed by the optical coupler 106 are polarized. After the initial adjustment so that the wave states match, the optical coupler 105 does not particularly control the polarization states of both lights.
and using a polarization-maintaining optical fiber coupler as 106,
Furthermore, if optical wiring is performed using polarization-maintaining optical fiber,
It is possible to minimize the mismatch between the polarization states of both lights and obtain an extremely stable beat signal.

The first intermediate frequency signal obtained from the photodiode 107 is amplified by an amplifier 109 having a band of 16 GH2 or more, and then input to the RF terminal of a mixer 110 (M87C manufactured by Watkins Johnson, RP band approximately 189 GHz). be done. On the other hand, mixer 11
The output (frequency r) of the first local oscillator 111 is input to the Lo terminal of 0, and as a result, the frequency flF given by the following equation is output from the IF terminal of the mixer 110.

A second intermediate frequency signal is output.

flF2-1 flFl-fl...
...(9) Note that the first local oscillator 1.1.1 may, for example, change the frequency over a wide frequency band.
A synthesized sweeper that can sweep (for example, HEWLETT PACKARD HP8340)
A: Frequency variable range 10 MHz ~ 26 MHz, 5 GH
z) is used.

The second intermediate frequency signal thus obtained has a frequency of approximately 1 GHz.
After being amplified by an amplifier 112 having a band of
The signal is input to the phase comparator 113. As a signal whose phase should be compared with this second intermediate frequency signal, the output of the second local oscillator 114 oscillating at a certain reference frequency fr is used. Here, the phase comparator 113 is, for example, a combination of a prescaler divider with a frequency division ratio of M-20, an 8-bit synchronous counter, and a current adder, as described in the conventional example. A digital phase comparator with a phase comparison range is used.

In addition, in this embodiment, the reference frequency fr is set to 2 (3
QMHz was set, and phase comparison was performed at a clock cycle of 4.3 MHz. Note that the second local oscillator 114
By synchronizing the reference clock of the first local oscillator 111 with the reference clock of the first local oscillator 111, it is possible to achieve frequency stability with higher accuracy.

The error signal output from the phase comparator 113 is processed through a loop consisting of a low-pass filter for reducing ripples due to clock cycle components of phase comparison and a second-order lag lead filter for expanding the lock range of the PLL. The control current is fed back to the control current injection device 103 via the filter 115, and the control current to the distributed reflection layer region of the DBRLD is changed. In this way, the oscillation frequency of the controlled semiconductor laser diode 101 is controlled so that the frequency flF of the second intermediate frequency signal is equal to the reference frequency fr, and optical frequency locking is achieved.

First, in order to show that the optical frequency control device according to the embodiment described above can control the oscillation optical frequency of the controlled semiconductor laser diode 101 with high accuracy,
The second
An example will be described in which the conventional device shown in the figure is configured to realize frequency offset locking. The measurement results in this case are shown in FIG. Figure 5(a) shows the spectrum of the beat signal due to the reference light and the output of the controlled semiconductor laser diode, where (a-1) is during control (locking), (a-1) is during control (locking), and (a) is
-2) is the one when not controlled (free running). As is clear from comparing the two, the frequency difference between the two lights fluctuates greatly in the free running state, whereas
During control, the center frequency is stabilized with high precision at 26QMHz.

Further, FIG. 5(b) shows the results of measuring the frequency stability of the beat signal under control (white circles) and under no control (black circles). As a measure of frequency stability, the standard deviation σf(τ) of frequency fluctuation was defined by the following equation and used.

However, τ is the time required for one frequency measurement, N is the number of measurements, and f is the average frequency during τ in the first measurement. This standard deviation corresponds to the value of the center frequency multiplied by the square root of the Allan variance, which is often used as an index of frequency stability. When measuring under control or non-control,
N=100 and N=20, respectively. Figure 5(b)
From this, the frequency fluctuation per second is the IQ when uncontrolled.
It can be seen that while it was above MI(z), it decreased to 20 Hz during control.

Therefore, it can be seen that with these frequency offset locking circuits, the oscillation optical frequency of the controlled semiconductor laser diode 101 is stabilized with extremely high accuracy relative to the optical frequency of the reference light. Further, when the isometric lock range was measured by changing the DC current amount of the control current injection device 103, it was 26 GI (z).

3. The upper limit of the controllable frequency band in such a conventional device is limited by the operating speed of the phase comparator, and is at most 4. It is about OOMHz.

Next, it will be explained that the oscillation optical frequency of the controlled semiconductor laser diode 101 can be controlled over a wide frequency band by the optical frequency control device according to the embodiment of the present invention shown in FIG.

In FIG. 1, the oscillation frequency ν of the controlled semiconductor laser diode 101 in a locked state, that is, in a state in which the second intermediate frequency flF is equal to the reference frequency fr generated by the second local oscillator 114, is the reference frequency ν. Depending on the magnitude relationship between the optical frequency νr of the light and the magnitude relationship between the first intermediate frequency flF defined by the above-mentioned equation (8) and the oscillation frequency f of the first local oscillator 111, four cases can be considered. Figure 6 shows these relationships. In this figure, the oscillation frequency of the controlled semiconductor laser diode 101 depends on the positive/negative of the phase difference detected by the phase comparator 113 to determine whether the state is ■ or ■ or whether the state is ■ or ■. It is determined arbitrarily depending on whether the number is increased or decreased. for example,
In this embodiment, when the phase of the second intermediate frequency signal leads the phase of the output signal of the second local oscillator 114, feedback is provided in the direction of decreasing the oscillation frequency ν of the controlled semiconductor laser diode 101. Since it is configured to apply , it will be locked depending on either ■ or ■. By comparing the first intermediate frequency (beat frequency) flF in the rocking state with the oscillation frequency f of the first local oscillator 111, it is possible to determine whether the relationship is (1) or (2). In other words, ■ ~ in Figure 6
■Which relationship is used for locking is uniquely determined.

Here, among the frequency values shown in FIG. 6, νr and fr are fixed values serving as references. Therefore, by changing the oscillation frequency f of the first local oscillator 111, the oscillation optical frequency ν of the controlled semiconductor laser diode 101 can be changed. On the other hand, even when the oscillation frequency f is changed, the frequency of the human input signal to the phase comparator 113 is always fr (here 260 M)
z), and it is possible to realize stable frequency offset locking operation.

FIGS. 7(a) to 7(e) show the beat spectra (A in FIG. FIG. By comparing the beat frequency and the output frequency of the synthesizer, which is the first local oscillator 111, it can be seen that the relationship shown in (■) in FIG. 6 is established. FIG. 7(f) shows the beat spectrum measured by manually changing the output frequency of the synthesizer continuously using the maximum value tracing function of the spectrum analyzer.

From these measurement results, it can be seen that a wide frequency variable range of 1QGHz or more is realized.

From the above explanation, it will be clear that the improvement effect of the embodiment of the present invention is extremely large compared to the conventional device (frequency variable range of 400 MHz or less) using the same phase comparator 113.

In the above-described embodiment, the mixer 110 is used for frequency downconversion, but it is also possible to construct an optical frequency control device with a similar configuration by using it for upconversion.

In addition, as the controlled semiconductor laser diode 101, D
Although the method of controlling the oscillation light frequency by controlling current to the distributed reflection layer region using a BRLD has been described as an example, the semiconductor LD used in the present invention is not limited to this in any way.

[Effects of the Invention] As detailed above, according to the present invention, it is no longer necessary to require high performance from the phase comparator necessary for offset locking the oscillation frequency of the semiconductor laser diode to the optical frequency of the reference light. , it becomes possible to control the frequency of light over a wide range exceeding 10 Gtlz. In other words, large frequency fluctuations when sweeping the optical frequency can be avoided by using a phase comparator that has a simpler structure and operates over a wider frequency band (~10-odd G) than a phase comparator for frequency offset locking. On the other hand, the stabilization of the oscillation optical frequency of the semiconductor laser diode is achieved by the principle of offset locking using a phase comparator that is always operated with a constant manual frequency, allowing a wide optical frequency range to be absorbed by the easily obtained mixer. It is possible to achieve both variable range and high frequency accuracy.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an optical frequency control device according to an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of an optical frequency control device using a conventional optical frequency offset locking method, and FIG. The figure shows the relationship between optical frequencies in a conventional optical frequency control device, FIG. 4 is a diagram for explaining the operation of a digital phase comparator used in a conventional optical frequency control device, and FIG. A diagram showing measurement results of frequency stability during optical frequency control operation by a frequency control device, FIG. 6 is a diagram for explaining an example of optical frequency control operation according to the present invention, and FIG. 7 is an example of the present invention. It is a figure which shows the measurement result of the optical frequency control operation of the optical frequency control apparatus by. 8 1... Controlled semiconductor laser diode, 3...
... Control current injection device, 4 ... Isolator, 5.1.06 ... Optical coupler, 7 ... Photodiode, 8 ... Reference light source, 9 ... ...Amplifier, 0...Mixer, l...First local oscillator, 2...Amplifier, 3...Phase comparator, 4... Second local oscillator, 5...Loop filter.

Claims (1)

[Claims] A semiconductor laser diode that oscillates in a single longitudinal mode is used as an optical frequency variable oscillator, and both lights are detected from at least a part of the output light of the semiconductor laser diode and at least a part of the reference light. In an optical frequency control device that controls the oscillation light frequency of the semiconductor laser diode based on information regarding the frequency difference of light of the semiconductor laser diode, after mixing at least a part of the output light of the semiconductor laser diode with a reference light, photoelectric conversion is performed. a first local oscillator whose oscillation frequency can be changed continuously; and a means for mixing the first intermediate frequency signal and the output of the first local oscillator. , a mixer that generates a second intermediate frequency signal having a frequency that is the sum or difference of these respective signals, a second local oscillator that oscillates at a constant reference frequency, and an output of the second local oscillator and the second intermediate frequency signal. a phase comparator that detects a phase difference between the second intermediate frequency signal and the reference frequency, and outputs an error signal according to the phase difference; An optical frequency control device comprising: oscillation frequency control means for changing the oscillation frequency of the semiconductor laser diode so that the oscillation frequency is always equal.