JP2612919B2 - Laser oscillation frequency stabilizer - Google Patents

Description

The present invention relates to a laser oscillation frequency stabilization for stabilizing a laser oscillation frequency using a light frequency reference such as a resonator or an absorption spectrum of atoms and molecules. It concerns the device.

(Prior art) Conventionally, the oscillation frequency of a laser is stabilized at an optical frequency value at which the transmittance of the frequency reference becomes maximum or minimum using an absorption spectrum of atoms and molecules, an optical interferometer, etc. as an external frequency reference. In this case, a method of periodically changing the oscillation frequency of the laser to be stabilized, that is, so-called FM modulation, is employed.

FIG. 2 is a block diagram showing a first conventional example of a laser oscillation frequency stabilizing device to which this method is applied (for example, IEICE OQE88-1 Yoshida et al., “Frequency Accuracy of Semiconductor Laser with Frequency Controlled” And stability evaluation ”).

In the first conventional example, the output light of the stabilized semiconductor laser 1 is supplied to the low-frequency signal oscillator 2 by the bias current.
FM modulation is performed by superimposing a very small current from. The output light is split into two by the optical coupler 3,
After one of the split lights is input to the light frequency reference 4, photoelectric conversion is performed by the light receiver 5. As a result, a change in the light frequency is converted into a change in the light intensity due to the frequency dependence of the transmittance of the light frequency reference 4, so that the output signal of the light receiver 5 is used as the output frequency of the low-frequency signal oscillator 2. Then, by performing synchronous detection by the lock-in amplifier 6, an output corresponding to the differential characteristic of the transmittance is obtained.

That is, when the center frequency of the FM-modulated laser coincides with the maximum or minimum of the transmittance, the output is “0”, and when the center frequency is higher or lower, the output is inverted. Using this as an error signal es, a bias current control circuit 7 for controlling a bias current to the stabilized semiconductor laser 1 is used so that the output is always “0”.
By performing so-called feedback control, stabilization of the oscillation frequency of the light to the frequency reference 4 is realized.

In addition, as an optical frequency standard for obtaining high frequency sensitivity, the frequency at which the transmittance is maximum or minimum must be stable for a long period of time. For a semiconductor laser in a short wavelength band, absorption of rubidium vapor and cesium vapor is required. The spectrum is used. In particular, when the saturated absorption spectrum of these alkali metal vapors is used, the absorption spectrum width becomes 10
It is as narrow as MHz or less, and frequency stabilization with higher sensitivity becomes possible.

FIG. 3 is a block diagram showing a second conventional example, in which 0.79
The configuration of a laser oscillation frequency stabilizing device using the saturated absorption spectrum of rubidium vapor at 5 μm is shown (El
ectronics Letters Vol.24 No.13 1988 CGBarwood et
al.). In the figure, 1 is a stabilized semiconductor laser, 2 is a low frequency signal oscillator, 6 is a lock-in amplifier, and 7 is a bias current control circuit, which represents the same components as in FIG.

In the second conventional example, the output light of the stabilized semiconductor laser 1 is split into two by a beam splitter 8, and one of the split lights is further split by a beam splitter 9 into a saturated light sb, a search light hb, It is split into three beams of reference beam rb.
The saturated light sb is reflected by the mirror 10 and the component transmitted through the beam splitter 11 is converted into a cell containing rubidium vapor.
Enter in 12. On the other hand, the search light hb and the reference light rb are reflected by the mirror 13 and enter the cell 12 from the opposite direction to the saturation light sb. At this time, the search light hb is input so as to overlap with the saturated light sb.

The search light sb and the reference light rb transmitted through the cell 12 are reflected by the beam splitter 11 and detected by the light receivers 13a and 13b, respectively. At this time, the saturated absorption spectrum is detected by the output of the photodetector 13a, and the influence of the linear absorption spectrum is simultaneously detected because the search light hb is FM-modulated. Therefore, the next-stage subtractor (or divider)
At 14, the effect is removed using the output of the light receiver 13b. In addition, FM is directly applied to the stabilized semiconductor laser 1.
When applying modulation, AM modulation is also applied at the same time,
In order to prevent the occurrence of so-called offset, which is stabilized at a frequency value shifted from the peak of the absorption spectrum due to the influence, the third harmonic of the low frequency signal oscillator 2 by the third harmonic generation means 15 is used. The lock-in amplifier 6 performs synchronous detection, detects a third-order differential signal of the saturated absorption spectrum, and generates an error signal es. This error signal es
The oscillation frequency of the stabilized semiconductor laser 1 is stabilized by the bias current control circuit 7 based on the input.

Further, in the 1.5 μm wavelength band, as in the absorption spectrum of alkali metal vapor, the amount of absorption is large and there is no frequency reference for light whose frequency can be easily stabilized. A method utilizing a rubidium atomic absorption spectrum of 0.78 μm has been studied (for example, Japanese Patent Application No. 63-137494). FIG. 4 is a configuration diagram showing a third conventional example employing this principle. In the figure, 2 is a low frequency signal oscillator, 5 is a photodetector, 6 is a lock-in amplifier, 7 is a bias current control circuit, which represents the same components as in FIG.

In the third conventional example, output light of a stabilized semiconductor laser 16 oscillating at a wavelength of about 1.56 μm is split into two by a beam splitter 17, and one of the split lights is constituted by a nonlinear optical element. Input to the second harmonic generation means 18 to obtain an optical output of 0.78 μm. This second harmonic generation means 18
Is input to a cell 19 containing rubidium.
Next, the output light transmitted through the cell 19 is detected by the light receiver 5. As a result, the rubidium atomic absorption spectrum is detected, and the output signal of the optical receiver 5 is synchronously detected by the lock-in amplifier 6 using the output frequency of the low frequency signal oscillator 2 to generate an error signal es, as described above. . This gives 1.56
The oscillation frequency of the stabilized semiconductor laser 16 in the μm band can be stabilized.

(Problems to be Solved by the Invention) However, in the first conventional example, FM modulation is performed on the stabilized semiconductor laser 1 to detect and generate an error signal es for control from the optical frequency reference 4. Therefore, a non-modulated optical output cannot be obtained, and a problem may occur when this device is used for optical communication or the like. Also, S / N
In order to detect a good error signal es and realize high frequency stabilization and accuracy, it is necessary to input strong optical power to the optical frequency reference 4, which limits the optical output that can be extracted outside the device. Has problems.

Further, in the second conventional example, in addition to the disadvantage that an unmodulated optical output cannot be obtained, it is necessary to use the reference light rb to remove the influence of the linear absorption spectrum,
The frequency stability and accuracy are reduced due to the complexity of the device and the drift of the accuracy and characteristics of the subtractor 14 or the poor S / N ratio of the third-order differential signal compared to the first-order differential signal. There is a problem of being restricted.

Further, in the third conventional example, in addition to the various disadvantages described above, the frequency stability and accuracy are affected by the conversion efficiency of the second harmonic generation means 18, and when the conversion efficiency is small, the detected error is small. The S / N of the signal es becomes low, making it difficult to obtain high frequency stability and accuracy. Therefore, the second harmonic generation means having a large conversion efficiency is indispensable, and there is a problem that the second harmonic generation means that can be used is limited.

In view of the above problems, it is an object of the present invention to obtain a stabilized optical output without modulation and with a large optical power, and to obtain a stable optical output without being affected by the performance of components. It is an object of the present invention to provide an excellent laser oscillation frequency stabilizing device that can perform the above operation.

(Means for Solving the Problems) To achieve the above object, according to claim (1), in a laser oscillation frequency stabilizing device for stabilizing the oscillation frequency of a laser oscillating near a predetermined frequency, A laser oscillation frequency control unit comprising: a first oscillation frequency control circuit that causes the oscillation frequency of the laser to follow a predetermined frequency reference in response to the input of the error signal; and a first optical branching unit that branches the output light of the laser. A reference laser that oscillates around a predetermined frequency, and a second laser that branches output light of the reference laser.
An optical branching unit, a beat signal detecting unit for multiplexing one branch light by the second optical branching unit and one branch light by the first optical branching unit and detecting a beat signal, A signal oscillator that oscillates a signal, a reference signal oscillator that oscillates a reference signal whose frequency is changed according to the input of an output signal of the signal oscillator, and compares the reference signal and the beat signal to determine the phase difference or frequency of both signals. A comparator for generating a second error signal corresponding to the difference, and inputting the second error signal to cause the oscillation frequency of the reference laser to follow changes in the oscillation frequency of the laser and the output frequency of the reference signal oscillator. A reference laser oscillation frequency control unit comprising a second oscillation frequency control circuit, and a maximum when the frequency of the split light matches a predetermined frequency reference due to the input of another split light by the second optical splitting means. Young An error comprising a frequency difference detecting section for generating a detection signal which is minimized, and a synchronous detecting section for detecting a component synchronized with the output signal of the signal oscillator from the detection signal and outputting the detected signal as the first error signal. A signal generator.

According to a second aspect of the present invention, in the laser oscillation frequency stabilizing device for stabilizing the oscillation frequency of a laser that oscillates near a predetermined frequency, the oscillation frequency of the laser is set to a predetermined value by inputting a first error signal. A first oscillation frequency control circuit for following a frequency reference, a laser oscillation frequency control unit including a first optical splitting unit for splitting the output light of the laser, a reference laser oscillating near a predetermined frequency, A second optical splitter for splitting the output light of the reference laser, and a beat signal detected by multiplexing one split light by the second optical splitter and one split light by the first optical splitter. A beat signal detection unit, a signal oscillator that oscillates a signal having a predetermined period, a reference signal oscillator that oscillates a reference signal whose frequency is changed according to an input of an output signal of the signal oscillator, A comparator for generating a second error signal corresponding to the phase difference or frequency difference between the compared two signals to issue,
A second oscillation frequency control circuit for causing the oscillation frequency of the reference laser to follow changes in the oscillation frequency of the laser and the output frequency of the reference signal oscillator in response to the input of the second error signal A third optical branching unit for branching the other branched light of the first optical branching unit; and an input of another branched light by the second optical branching unit and a branched light by the third optical branching unit. The detection signal which becomes maximum or minimum when the frequency shifted from the predetermined frequency reference by 1/2 of the difference between the frequencies of the two branched lights and the frequency of the branched light by the third optical branching unit match. An error signal generating section comprising: a frequency difference detecting section to be generated; and a synchronous detector for detecting a component synchronized with the output signal of the signal oscillator from the detected signal and outputting the detected signal as the first error signal.

According to claim (3), the N-th harmonic generating means is arranged between one of the branched light output sides of the first optical branching means, the beat signal detecting section and the one of the branched light input sides.

(Operation) According to claim (1), the output light of the laser oscillated at a predetermined frequency is input to the first optical splitting means and split,
The one split light beam is output to the beat signal detection unit of the reference laser oscillation frequency control unit.

Similarly, the output light of the reference laser oscillated at a predetermined frequency is input to the second optical splitting unit and split. One branched light is output to the beat signal detecting unit, and the other branched light is output to the frequency difference detecting unit of the error signal generating unit. The beat signal detection unit detects and outputs a beat signal by multiplexing the respective split lights by the first and second optical splitters. on the other hand,
The reference signal oscillator receives an output signal of the signal oscillator and outputs a reference signal whose oscillation frequency is periodically changed according to the input. Accordingly, the comparator compares the beat signal with the reference signal, generates a second error signal corresponding to the phase difference or the frequency difference between the two signals, and outputs the second error signal to the second oscillation frequency control circuit. The second oscillation frequency control circuit controls the oscillation frequency of the reference laser so as to follow a change in the oscillation frequency of the laser and the output frequency of the reference signal oscillator by inputting the second error signal.

Thus, it is possible to obtain the output light of the reference laser which is synchronized with the oscillation frequency fluctuation of the laser and is FM-modulated. This output light is incident on the second light splitting means and split as described above. One of the branched lights is output to the beat signal detection unit, and is subjected to the same operation as described above.

On the other hand, the other branched light is input to the frequency difference detector of the error signal generator. The frequency difference detection unit generates a detection signal that becomes maximum or minimum when the frequency of the branched light matches a predetermined frequency reference by input of another branched light by the second optical branching unit. Is output to the synchronous detector. The synchronous detection unit detects a component synchronized with the output signal of the signal oscillator from the detection signal, and outputs the detected component to the first oscillation frequency control circuit as a first error signal.

The first oscillation frequency control circuit controls the oscillation frequency of the laser so as to follow a frequency reference in response to the input of the first error signal. Thereby, the oscillation frequency of the laser is stabilized without modulation. The output light of the stabilized laser is input to the first optical splitting means and split as described above, and one split light is output to the beat signal detecting unit, and the same as described above. Be affected. On the other hand, the other branched light is output as the optical output of the device.

According to claim (2), the output light of the laser oscillated at a predetermined frequency is input to the first optical branching unit and branched. One branched light is output to the beat signal detecting unit of the reference laser oscillation frequency control unit, and the other branched light is output to the third optical branching unit.

Similarly, the output light of the reference laser oscillated at a predetermined frequency is input to the second optical splitting unit and split. One branched light is output to the beat signal detecting unit, and the other branched light is output to the frequency difference detecting unit of the error signal generating unit. The beat signal detection unit detects and outputs a beat signal by multiplexing the respective split lights by the first and second optical splitters. on the other hand,
The reference signal oscillator receives an output signal of the signal oscillator and outputs a reference signal whose oscillation frequency is periodically changed according to the input. Accordingly, the comparator compares the beat signal with the reference signal, generates a second error signal corresponding to the phase difference or the frequency difference between the two signals, and outputs the second error signal to the second oscillation frequency control circuit. The second oscillation frequency control circuit controls the oscillation frequency of the reference laser so as to follow a change in the oscillation frequency of the laser and the output frequency of the reference signal oscillator by inputting the second error signal.

Thus, it is possible to obtain the output light of the reference laser which is synchronized with the oscillation frequency fluctuation of the laser and is FM-modulated. This output light is incident on the second light splitting means and split as described above. One of the split lights is output to the beat signal detection unit, and the same operation as described above is performed. The other split light is output to the frequency difference detection unit of the error signal generation unit.

On the other hand, another light split by the first light splitting means is input to the third light splitting means and split. One branched light beam is output to the frequency difference detection unit of the error signal generation unit. The frequency difference detection unit shifts from a predetermined frequency reference by の of the difference between the frequencies of the two split lights due to the input of another split light by the second optical splitting means and the split light by the third optical splitting means. When the obtained frequency matches the frequency of the light split by the third optical splitting means, a detection signal that becomes maximum or minimum is generated, and this detection signal is output to the synchronous detection unit. The synchronous detection unit detects a component synchronized with the output of the signal oscillator from the detection signal, and outputs the detected component to the first oscillation frequency control circuit as a first error signal.

The first oscillation frequency control circuit controls the oscillation frequency of the laser so as to follow a frequency reference in response to the input of the first error signal. Thereby, the oscillation frequency of the laser is stabilized without modulation. The output light of the stabilized laser is input to the first optical splitting means and split as described above, and one split light is output to the beat signal detecting unit, and the same as described above. Be affected. On the other hand, the other branched light is input to the third optical branching unit and branched,
One of the split lights is output to the frequency difference detection unit, subjected to the same operation as described above, and the other split light is output as an optical output of the device.

Further, according to claim (3), the output light of the laser oscillated at a predetermined frequency is input to the first optical splitting means and split, and one split light is the Nth harmonic generation light. Output to the means. The N-th harmonic generation means generates the N-th harmonic from the input branched light and outputs the N-th harmonic to the beat signal detection unit of the reference laser oscillation frequency control unit. Thereafter, the same operation as in the above-described claim (1) or (2) is performed.

(Embodiment) FIG. 1 is a configuration diagram showing a first embodiment of a laser oscillation frequency stabilizing device according to the present invention. In FIG.
Reference numeral 20 denotes a stabilized laser oscillation frequency control unit, reference numeral 30 denotes a reference laser oscillation frequency control unit, and reference numeral 40 denotes an error signal generation unit.

The stabilized laser oscillation frequency control unit 20 includes a stabilized semiconductor laser 21 that oscillates near the frequency ν and a stabilized semiconductor laser 2 based on an input of a first error signal ES1 described later.
1, a bias current control circuit (first oscillation frequency control circuit) 22 for supplying a bias current whose oscillation frequency ν is controlled to follow a frequency reference described later, and an output light LD of the stabilized semiconductor laser 21. And a first optical branching unit for outputting one branched light to the reference laser oscillation frequency control unit 30 and outputting the other branched light as an optical output of the device, for example, an optical coupler 23. Have been.

The reference laser oscillation frequency control unit 30 splits the output light RLD of the reference laser 31 and the reference laser 31 oscillating at the frequency ν _r into two, and outputs one split light to a beat signal detection unit 33 described later, The second for outputting another branched light to the error signal generator 40
Optical branching means, for example, an optical coupler 32, and an optical multiplexing means for multiplexing one branch light of the optical coupler 32 with one branch light of the optical coupler 23, for example, the output light of the optical coupler 33a and the optical coupler 33a. A beat signal detection unit 33 including a photodetector 33b that outputs a photoelectrically converted beat signal BT, and a low-frequency signal oscillator that oscillates a periodic low-frequency signal LS having a frequency of about fm = 1 kHz, for example.
A microwave reference oscillator 35 for oscillating a reference signal BS which is a microwave signal whose output frequency f (center frequency f ₀ , frequency deviation Δf) is periodically changed according to the input of the low frequency signal LS; A phase comparator 36 for generating a second error signal ES2 corresponding to the phase difference between the reference signal BS and the beat signal BT;
Based on the input of the second error signal ES2, the reference semiconductor laser 31 is supplied with the oscillation frequency v _r of the stabilized laser.
And a bias current control circuit (second oscillation frequency control circuit) 37 for supplying a bias current controlled so as to match the sum or difference between the oscillation frequency ν of 21 and the output frequency f of the microwave reference oscillator 35. ing.

The error signal generator 40 is used as a frequency reference of light, receives the transmitted light of the cell 41a and the cell 41a filled with the rubidium vapor input to the other branched light of the optical coupler 32, performs photoelectric conversion, and performs the reference semiconductor laser. A frequency difference detector 41 comprising a photodetector 41b for outputting a detection signal DS corresponding to the frequency difference between the frequency of the output light 31 and the frequency reference;
To the low frequency signal LS (frequency
fm), a first error signal ES1 is generated by synchronous detection, and a synchronous detection unit that outputs the first error signal ES1 to the bias current control circuit 22, for example, a lock-in amplifier 42 is provided.

Next, the operation of the above configuration will be described. It is assumed that the control band of the oscillation frequency of the reference semiconductor laser 31 is set higher than the oscillation frequency fm of the low-frequency signal oscillator 34.

First, the output light LD of the stabilized semiconductor laser 21 oscillated at the frequency ν is input to the optical coupler 23 and split into two, and one of the split lights is output to the optical coupler 33a of the beat signal detection unit 33. You.

Furthermore, output light RLD reference semiconductor laser 31 oscillates at a frequency [nu _r is branched to enter the optical coupler 32. One branched light is output to the optical coupler 33a of the beat signal detecting unit 33, and the other branched light is output to the cell 41a of the error signal generating unit 40. In the beat signal detector 33, the optical coupler 33a
Then, one branch light of the optical coupler 32 is combined with one branch light of the optical coupler 23, the output light thereof is received by the light receiver 33b, and the beat signal BT photoelectrically converted is output to the phase comparator 36. You.

On the other hand, the low-frequency signal oscillator 34
The LS oscillates periodically. Accordingly, the microwave reference oscillator 35 oscillates the reference signal BS periodically changed according to the input of the low frequency signal LS, and outputs the reference signal BS to the phase comparator 36. The phase comparator 36 compares the phase of the input beat signal BT with the phase of the reference signal BS, generates a second error signal ES2 corresponding to the phase difference between the two signals, and outputs the second error signal ES2.
Output to 7. The bias current control circuit 37 changes the oscillation frequency v _r of the reference semiconductor laser 31 from the oscillation frequency v of the stabilized semiconductor laser 21 to the output frequency f of the microwave reference oscillator 35 in accordance with the input of the second error signal ES2. A bias current controlled so as to stabilize at a value deviated by _r = ν ± f is supplied to the reference semiconductor laser 31. As a result, the oscillation frequency v _r of the reference semiconductor laser 31 is stabilized to a value of v ± f (this operation is called frequency offset locking of the semiconductor laser. No. 9 1988, pp. 678-684).

That is, the oscillation frequency v _r of the reference semiconductor laser 31 follows the oscillation frequency fluctuation of the stabilized semiconductor laser 21, and
FM modulation of the modulation frequency fm and the frequency shift Δf is performed.

Next, the output light RLD of the reference semiconductor laser 31 subjected to the FM modulation is split by the optical coupler 32 as described above, and one split light is output to the beat signal detection unit 33 and used for the above-described operation. Is done. On the other hand, another branch light by the optical coupler 32 is input to the cell 41a of the error signal generator 40. This cell 4
The transmitted light 1a detects the frequency difference between the center value ν + f ₀ (or ν−f ₀ ) of the oscillation frequency ν _r of the reference semiconductor laser 31 and the peak frequency ν ₀ of the rubidium absorption spectrum. The detection signal DS received by the light receiver 41b and photoelectrically converted is output to the lock-in amplifier 42. Next, the lock-in amplifier 42 synchronously detects the detection signal DS with a low-frequency signal LS (frequency fm) from the low-frequency signal oscillator 34, and generates a first error signal ES1 corresponding to the frequency difference. , To the bias current control circuit 22.

The bias current control circuit 22 controls the oscillation frequency ν of the stabilized semiconductor laser 21 according to the input of the first error signal ES1.
Is supplied to the stabilized semiconductor laser 21 so that the bias current is controlled to be stabilized at ν ₀ −f ₀ (or ν ₀ + f ₀ ). This makes the stabilized semiconductor laser stable
The output light LD of 21 is split by the optical coupler 23 as described above, and one split light is output to the beat signal detection unit 33 and subjected to the same operation as described above. On the other hand, the other branched light of the optical coupler 23 is output to the outside of the device as a stabilized light output of the device.

FIG. 5 is a diagram theoretically showing an example of the above-described series of frequency stabilizing operations. That is, as shown in FIG. 5 (a), the oscillation frequency ν of the stabilized semiconductor laser device 21 is equal to the frequency difference (ν) between the peak frequency ν ₀ which is the frequency reference and the center frequency f ₀ of the reference signal BS. ₀ −f ₀ ) (ν <
ν ₀ −f ₀ ), a detection signal indicated by DSa in the figure is detected by the frequency difference detection unit 41, and a first error signal based on the detection signal is shown in FIG.
For example, it becomes a minus (-) signal as shown by ES1a. Therefore, the bias current control circuit 22 controls the oscillation frequency ν of the stabilized semiconductor laser 21 to increase.

Further, as shown in FIG. 5B, the oscillation frequency ν
Is the frequency difference between the peak frequency ν ₀ and the center frequency f ₀ (ν
₀₋ f ₀ ) (ν> ν ₀ −f ₀ ),
A detection signal indicated by DSb is detected, and a first error signal based on the detection signal is, for example, a plus (+) signal as indicated by ES1b in FIG. Therefore, the bias current control circuit 22
Controls the oscillation frequency ν of the stabilized semiconductor laser 21 to decrease.

Further, as shown in FIG. 5 (c), the oscillation frequency ν
Is the frequency difference between the peak frequency ν ₀ and the center frequency f ₀ (ν
₀₋ f ₀ ) (ν = ν ₀ −f ₀ ), a detection signal indicated by DSc in the figure is detected, and a first error signal based on the detection signal is “±” as indicated by ES1c in the figure. 0 "signal. Therefore, the bias current control circuit 22
The control is performed so that the oscillation frequency ν of 21 is maintained as it is.

As described above, according to the first embodiment, the stabilized semiconductor laser 21 is used by using the reference semiconductor laser 31 and detecting the beat signal BT and using the beat frequency.
Since the reference semiconductor laser 31 synchronized with the oscillation frequency fluctuation is FM-modulated, the oscillation frequency ν can be stabilized with high accuracy without applying frequency modulation to the stabilized semiconductor laser 21.

Further, the first error signal ES1 is determined based on the frequency reference of light.
Since the power of the reference semiconductor laser 31 is used to detect and generate the signal, when the optical output of the reference semiconductor laser 31 is sufficiently large, it is necessary for the light receiver 33b to detect the beat signal BT. The optical power of the stabilized semiconductor laser 21 may be small, and this has the effect that the stabilized optical output power that can be extracted to the outside can be increased. Furthermore, since the oscillation frequency ν of the stabilized semiconductor laser 21 also depends on the center frequency f ₀ of the output of the microwave reference oscillator 35, by changing this, the value of the stabilized optical frequency can be continuously changed. It also has the effect that it can be changed to.
If the obtained stabilized light output is modulated using an external modulator, it can be used for optical communication as it is.

In the first embodiment, the cell 41a filled with rubidium vapor is used as a light frequency reference. However, the present invention is not limited to this. A Perot interferometer, a ring resonator, a waveguide ring resonator, or the like may be used, or a detection system for a saturated absorption spectrum may be used.

FIG. 6 is a block diagram showing a second embodiment of the laser oscillation frequency stabilizing device according to the present invention. The difference between the second embodiment and the first embodiment is that the output side (stabilized light output side) of the other branched light of the optical coupler 23 in the stabilized laser oscillation frequency control unit 20 is different from the first embodiment. Split the split light into two,
A third optical branching means, for example, an optical coupler 50, which outputs the one branched light to the error signal generating unit 40 and uses the other branched light as an optical output of the device, is arranged. The other branch light (hereinafter, referred to as saturated light SB) of the optical coupler 32 of the cell 41a is transmitted to the frequency difference detection unit 41 of the cell 41a.
a branch light (hereinafter referred to as a search light HB
That. The beam splitter 41c that reflects the light reflected by the light receiving device 41) and inputs the reflected light to the light receiver 41b is provided.

Next, the operation according to the second embodiment will be described only for parts different from the first embodiment.

A part of the output light LD of the stabilized semiconductor laser 21 is input as the search light HB into the cell 41a filled with rubidium vapor by the optical coupler 50. On the other hand, of the output light RLD of the reference semiconductor laser 31, the other split light split by the optical coupler 32 passes through the beam splitter 41c as the saturated light SB and enters the cell 41a from the opposite direction to the search light HB. .

Here there is an optical frequency difference f ₀ during the search light HB saturated light SB, and the search beam HB is unmodulated, since the saturated light SB FM modulation is applied, the optical frequency difference f ₀ rubidium if narrower than the Doppler broadening of the steam absorption spectrum (half width 500MHz about room temperature), the primary differential signal saturated absorption spectrum frequency by ± f _0/2 is shifted is detected. Cell 41a
The search light HB that has passed through is detected by the light receiver 41b via the beam splitter 41c, and the detection signal DS is input to the lock-in amplifier. The lock-in amplifier 42 performs synchronous detection of the detection signal DS with the low-frequency signal LS (frequency fm) from the low-frequency signal oscillator 34, generates a first error signal ES, and outputs the first error signal ES to the bias current control circuit 22.

The bias current control circuit 22 changes the oscillation frequency ν of the stabilized semiconductor laser 21 with the input of the first error signal ES.
ν ₀ -f _0/2 (or ν _{0 +} f _0/2) to supply a bias current which is controlled to stabilize the target-stabilized semiconductor laser 21. This makes the stabilized semiconductor laser stable
The output light LD 21 is output to the outside of the device via the optical couplers 23 and 50 as a stabilized light output of the device.

In the second embodiment, the same effects as in the first embodiment can be obtained. In addition, at the time of detection of the saturated absorption spectrum, the output light RLD of the reference semiconductor laser 31 is used as the saturated light SB, the output light LD of the stabilized semiconductor laser 21 is used as the search light HB, and no search light HB is used. Since no modulation is applied, no linear absorption spectrum is detected, and there is no need to use the reference light as described in the related art (FIG. 3). Therefore, the configuration of the error signal generator 40 is simpler than that in the case of FIG. 3, and the deterioration of the frequency stability and accuracy in this part can be avoided.
Further, the AM generated when the reference semiconductor laser 31 is directly FM-modulated.
The influence of the modulation component only changes the amount of absorption saturation, and has no primary effect when detecting a saturated absorption spectrum from the search light HB. For this reason, there is also an advantage that the first-order differential signal having a better S / N than the third-order differential signal can be used as the first error signal from the frequency reference of light without causing a frequency offset.

If the polarization state of both the saturation and search light is controlled by using a polarization beam splitter as the beam splitter 41c, it is possible to increase the effective use power of both the saturation and search light.

FIG. 7 is a block diagram showing a third embodiment of the laser oscillation frequency stabilizing device according to the present invention. The difference between the third embodiment and the first embodiment is that a nonlinear optical element is provided between one branch light output side of the optical coupler 23 and the one branch light input side of the beat signal detector 33. And a semiconductor laser 21a oscillating in a 1.56 μm band is employed as a stabilized laser, and a semiconductor laser 31a oscillating in a 0.78 μm band is employed as a reference laser. That is.

In this configuration, the output light LDa of the stabilized semiconductor laser 21a in the 1.56 μm band is split by the optical coupler 23, and one of the split lights is incident on the second harmonic generation means 24, and the light (0.78 μm band) (Second harmonic). This second harmonic generation means
As in the first embodiment, a first error signal ES1 is generated by using the output light of 24, and based on this, the oscillation frequency of the 1.56 μm band stabilized semiconductor laser 21 is controlled by the bias current control circuit 22. ν is controlled.

In the third embodiment, the same effects as in the first embodiment can be obtained. In addition, even when the conversion efficiency of the second harmonic generation means 24 in the 1.56 μm band is small,
If the output power of the reference semiconductor laser 31 in the 0.78 μm band is sufficiently large, high frequency stability and accuracy can be obtained. This is a particularly effective advantage at present when only low frequency conversion efficiency can be realized in the harmonic generation means in the optical region.

Further, in the second embodiment described with reference to FIG. 6, the stabilized laser oscillation frequency control section 20 is replaced with the stabilized laser oscillation frequency control section 20 in the third embodiment shown in FIG. Accordingly, a laser oscillation frequency stabilizing device in the 1.56 μm band can be configured, and the same effect as in the second or third embodiment can be obtained.

In each of the first, second and third embodiments, the semiconductor laser is used as the stabilized laser and the reference laser. However, the present invention is not limited to this. In this case, a circuit for controlling the cavity length may be used as the bias current control circuits 22 and 37 in order to control the oscillation frequency.

If a direct synthesis type frequency synthesizer is used as the microwave reference oscillator 35, a microwave reference signal BS having a large frequency shift Δf can be obtained while maintaining its center frequency f ₀ with high accuracy ( For example, Δf = 10MH
z) is effective.

Further, when the second error signal ES2 is generated, the second error signal ES2 is generated in accordance with the phase difference between the beat signal BT and the reference signal BS. The same operation and effect as described above can be obtained.

(Effect of the Invention) As described above, according to claim (1), a part of the output light of the laser whose oscillation frequency is stabilized and a part of the output light of the reference laser are combined and detected. The reference frequency of the reference laser is controlled by comparing the phase difference or the frequency difference between the beat signal and the reference signal generated by the reference signal oscillator whose oscillation frequency periodically changes, and uses the output light of the reference laser. An error signal is generated based on the optical frequency reference, and the oscillation signal is used to control the oscillation frequency of the stabilized laser.Therefore, FM modulation can be applied to the reference laser synchronized with the oscillation frequency fluctuation of the laser. This makes it possible to stabilize the oscillation frequency of the laser with high accuracy, and to obtain a non-modulated and stabilized optical output.

Also, when the first error signal is generated from the light frequency reference, the optical power of the reference laser is used. Therefore, if the optical output of the reference laser is sufficiently large, it is necessary to detect the beat signal. The light power of the laser is small, and the light power of the stabilized light output that can be extracted outside can be increased. Further, since the oscillation frequency of the laser also depends on the center frequency of the output of the reference signal oscillator, by changing this, there is also an effect that the value of the stabilized optical frequency can be continuously changed.

Furthermore, if the obtained stabilized light output is modulated using an external modulator, it has an advantage that it can be used for optical communication as it is.

According to claim (2), in addition to the effect of claim (1), when detecting and generating the first error signal, the output light of the unmodulated laser is used together with the output light of the reference laser. Therefore, a stabilized optical output can be obtained with higher accuracy.

According to claim (3), in addition to the effects of claim (1) or claim (2), the oscillation frequency of a laser in a long wavelength band such as a 1.5 μm band can be stabilized with high accuracy. . Furthermore, even when the conversion efficiency of the N-th harmonic generation means is small, if the output power of the reference laser is sufficiently large, high frequency stability and accuracy can be obtained. This is a particularly effective advantage at the present time when only low frequency conversion efficiency can be realized in the harmonic generation means in the optical region.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a laser oscillation frequency stabilizing device according to the present invention, FIG. 2 is a configuration diagram showing a first conventional example, and FIG. 3 is a configuration showing a second conventional example. FIG. 4, FIG. 4 is a block diagram showing a third conventional example, FIG. 5 is an explanatory diagram of a laser oscillation frequency stabilizing operation example according to the present invention, and FIG. FIG. 7 is a block diagram showing a second embodiment of the laser oscillation frequency stabilizing apparatus according to the present invention. In the figure, 20: stabilized laser oscillation frequency control unit, 21:
Stabilized semiconductor laser, 21a... 1.56 μm band stabilized semiconductor laser, 22... Bias current control circuit (first oscillation frequency control circuit), 23... Optical coupler (first optical branching means), 24. ... Second harmonic generation means, 30... Reference laser oscillation frequency control unit, 31... Reference semiconductor laser, 31a... 0.7
8 μm band reference semiconductor laser, 32... Optical coupler (second optical branching means), 33... Beat signal detector, 33a... Optical coupler, 33b... Light receiver, 34. …
Microwave reference oscillator, 36 phase comparator, 37 bias current control circuit (second oscillation frequency control circuit), 40
... Error signal generator, 41 ... Frequency difference detector, 41a ... Rubidium-filled cell, 41b ... Receiver, 41c ... Beam splitter, 42 ... Lock-in amplifier (synchronous detector), 50 ...
... Optical coupler (third optical branching means).

Claims (3)

(57) [Claims] 1. A laser oscillation frequency stabilizing device for stabilizing an oscillation frequency of a laser oscillating around a predetermined frequency, wherein the laser oscillation frequency follows a predetermined frequency reference by inputting a first error signal. A laser oscillation frequency control unit comprising: an oscillation frequency control circuit; a first optical branching unit for branching the output light of the laser; a reference laser oscillating around a predetermined frequency; and an output light of the reference laser. A second optical branching unit that branches, a beat signal detection unit that combines the one branch light by the second optical branching unit and the one branch light by the first optical branching unit, and detects a beat signal; A signal oscillator that oscillates a signal having a predetermined period, a reference signal oscillator that oscillates a reference signal whose frequency is changed in accordance with the input of an output signal of the signal oscillator, and compares the reference signal with the beat signal. A comparator for generating a second error signal corresponding to the phase difference or frequency difference between the signals, and the input of the second error signal to change the oscillation frequency of the reference laser to the oscillation frequency of the laser and the output of the reference signal oscillator. A reference laser oscillation frequency control unit comprising a second oscillation frequency control circuit for following a change in frequency; and a frequency of the split light and a predetermined frequency reference by input of another split light by the second optical splitting means. A frequency difference detection unit that generates a detection signal that becomes a maximum or a minimum when the two signals coincide with each other, and a synchronous detection unit that detects a component synchronized with the output signal of the signal oscillator from the detection signal and outputs the component as the first error signal. A laser oscillation frequency stabilizing device, comprising: 2. A laser oscillation frequency stabilizing device for stabilizing an oscillation frequency of a laser which oscillates around a predetermined frequency, wherein the laser oscillation frequency follows a predetermined frequency reference by inputting a first error signal. An oscillation frequency control circuit, a laser emission frequency control unit including first light splitting means for splitting the output light of the laser, a reference laser oscillating around a predetermined frequency, and an output light of the reference laser. A second optical branching unit that branches, a beat signal detection unit that combines the one branch light by the second optical branching unit and the one branch light by the first optical branching unit, and detects a beat signal; A signal oscillator that oscillates a signal having a predetermined period, a reference signal oscillator that oscillates a reference signal whose frequency is changed according to an input of an output signal of the signal oscillator, and compares the reference signal with the beat signal to compare A comparator for generating a second error signal corresponding to the phase difference or frequency difference between the signals, and the input of the second error signal to change the oscillation frequency of the reference laser to the oscillation frequency of the laser and the output of the reference signal oscillator. A reference laser oscillation frequency control unit comprising a second oscillation frequency control circuit for following a change in frequency; a third optical splitting unit for splitting another split light beam of the first optical splitting unit; The frequency obtained by shifting the difference between the frequencies of the two split lights by a half from a predetermined frequency reference by the input of the other split light by the optical splitting means and the split light by the third optical splitting means and the third A frequency difference detection unit that generates a detection signal that is maximum or minimum when the frequency of the light split by the optical splitting unit matches, and detects a component synchronized with the output signal of the signal oscillator from the detection signal, and One mistake The laser frequency stabilization device being characterized in that a error signal generator comprising a synchronous detector for outputting a signal. 3. A method according to claim 1, wherein: between the one branch light output side of the first light branch means and the one branch light input side of the beat signal detection section.
3. The laser oscillation frequency stabilizing device according to claim 1, further comprising an N-th harmonic generating means.