JP4608512B2 - Frequency stabilized light source - Google Patents

Description

  The present invention relates to a frequency-stabilized light source, and more particularly to a frequency-stabilized light source that can stabilize the frequency of output light to a reference frequency in optical communication.

  DPSK (Differential Phase-Shift Keying) and DQPSK (Differential Quadrature Phase-Shift Keying) as modulation techniques to achieve higher wavelength density in optical communications Etc. are being considered.

  In these methods, since a delay Mach-Zehnder interferometer is used on the receiving side, high accuracy and stability are required for the optical frequency of the signal light source on the transmitting side.

  When realizing a high-accuracy frequency-stabilized light source that matches the optical frequency on the ITU-T grid, a configuration using two laser light sources and one absorption cell has been proposed as a conventional technique (for example, see Patent Document 1). Yes.

  FIG. 1 is a block diagram showing a configuration example of a frequency stabilized light source using two conventional laser light sources and one absorption cell. The operation principle will be described below.

  The output light of the laser light source 104 is incident on the absorption cell 108 via the optical splitter 106. The temperature control circuit 102 sets the output light frequency of the laser light source 104 in the vicinity of one of the absorption line frequencies of the absorption cell 108 in advance.

The current from the laser drive circuit 112 is minutely modulated at the oscillation frequency f ₁ of the oscillator 114, and as a result, the output light from the laser light source 104 is slightly frequency-modulated.

  Due to the optical frequency dependence of the transmittance of the absorption cell 108 in the vicinity of the absorption line frequency, the light transmitted through the absorption cell 108 is subjected to intensity modulation.

  The output light of the absorption cell 108 is detected by the photodetector 110, and the output is input to the synchronous detector 116. A signal from the oscillator 114 is also input to the synchronous detector 116.

  The synchronous detector 116 sends an error signal corresponding to the difference between the output light frequency of the laser light source 104 and the absorption line frequency of the absorption cell to the laser driving circuit 112.

  The laser drive circuit 112 adjusts the drive current so that the error signal becomes 0, and as a result, the output light frequency of the laser light source 104 is stabilized at the absorption line frequency of the absorption cell 108.

  On the other hand, the output light from the laser light source 154 enters the optical multiplexer 164 via the optical demultiplexer 156 and is combined with the output light from the laser light source 104. The output light frequency of the laser light source 154 is set in the vicinity of the target frequency in advance by the temperature control circuit 152.

  The combined light enters the photodetector 162 and generates a beat signal equal to the frequency difference between the two laser beams.

The oscillator 166 outputs a signal having a frequency f _{2 that} is the difference between the output light frequency of the laser light source 104 and the target frequency of the laser light source 154.

  The output of the oscillator 166 and the output of the photodetector 162 are incident on the frequency comparator 160, and an error signal corresponding to the difference between these two frequencies is sent to the laser drive circuit 158.

The laser drive circuit 158 adjusts the drive current so that the error signal becomes 0, and as a result, the output optical frequency of the laser light source 154 is stabilized at a frequency separated by f ₂ from the absorption line frequency of the absorption cell 108. .

  As described above, the frequency-stabilized light source having the configuration shown in FIG. 1 generates laser light offset from the absorption line frequency of the absorption cell by a certain frequency.

By using an absorption cell filled with acetylene (C ₂ H ₂ ) gas or hydrogen cyanide (HCN) gas, it is possible to realize a frequency-stabilized light source that serves as a reference frequency light source in wavelength division multiplex transmission using C-band wavelengths. It becomes possible.

JP 2000-357840 A SL Gilbert and WC Swann, “Carbon Monoxide Absorption References for 1560 nm to 1630 nm Wavelength Calibration-SRM 2514 (12C160) and SRM 2515 (13C160)”, NIST Special Publication 260-146, Oct. 2002. (http: // ts .nist.gov / MeasurementServices / ReferenceMaterials / upload / SP260-146.PDF)

  As described above, in the conventional frequency stabilized light source, in order to stabilize the laser light at a frequency other than the absorption line frequency of the absorption cell, two laser light sources (104, 154) are prepared. It is necessary to stabilize the laser light from the laser light source 104 at the absorption line frequency of the absorption cell 108 and stabilize the optical frequency of the laser light from the other laser light source 154 to the target value by using the output light.

  Thus, since a circuit for stabilizing each of the two laser light sources is necessary, the configuration of the conventional frequency stabilized light source is complicated.

  Further, in the configuration of the conventional frequency stabilized light source, there is a problem that the line width of the output light is widened because the drive current of the laser light is modulated to modulate the output light frequency.

  The present invention has been made in view of the above-described circumstances, and in a frequency-stabilized light source that stabilizes to an optical frequency other than the absorption line frequency of the absorption cell, a simple configuration using only one laser light source, and a laser. An object of the present invention is to realize a frequency-stabilized light source capable of obtaining output light with a stabilized frequency without placing a modulation signal on the driving current of light.

In order to achieve the above object, the present invention provides a frequency-stabilized light source, a laser light source for generating continuous wave laser light, and a laser light generated by the laser light source. An optical demultiplexer for separating a part of the power of the laser, an optical modulator for modulating the separated laser light to generate a modulation sideband, and a variable for generating an electric signal to be applied to the optical modulator The optical modulation comprising: a frequency generation unit; a low-frequency oscillator that generates an electrical signal for changing an output frequency of the variable frequency generation unit; and light that is provided with modulation output from the optical modulator. An absorption cell having an absorption line frequency near one of the modulation sidebands generated by the detector, a detector for receiving intensity-modulated light output from the absorption cell, an output signal of the detector and the low frequency A synchronous detector that performs synchronous detection from the output signal of the vibrator, and laser light frequency control that controls the output light frequency of the laser light source based on the output signal of the synchronous detector so that the output signal becomes zero And the variable frequency generation means is separated by the same frequency up and down around a frequency corresponding to a difference between a target laser light frequency of the continuous wave laser light and an absorption line frequency of the absorption cell. It is characterized by comprising two fixed frequency oscillators each generating a frequency and a switch for switching the output frequency of the two fixed frequency oscillators by an electric signal from the low frequency oscillator .

The invention according to claim 2 is a frequency-stabilized light source, a laser light source that generates continuous wave laser light, an optical modulator that modulates the laser light to generate a modulation sideband, and the light Variable frequency generating means for generating an electric signal to be applied to the modulator, a low frequency oscillator for generating an electric signal for changing the output frequency of the variable frequency generating means, and a specific light from the output light of the optical modulator An optical frequency separation means for separating frequency components, and an absorption cell having an absorption line frequency in the vicinity of one of modulation sidebands generated by the optical modulator, which receives the separated light of the specific optical frequency component as an input A detector that receives intensity-modulated light output from the absorption cell, a synchronous detector that performs synchronous detection from the output signal of the detector and the output signal of the low-frequency oscillator, and the synchronous detection Based on the output signal, and a laser optical frequency controlling means for controlling the output light frequency of the laser light source such that the output signal becomes 0, the variable frequency generating means, the target of the continuous wave laser beam Centered on a frequency corresponding to the difference between the laser light frequency and the absorption line frequency of the absorption cell, and two fixed frequency oscillators that respectively generate frequencies separated by the same frequency up and down, and the two fixed frequencies The output frequency of the oscillator is switched by a switch that is operated by an electric signal from the low-frequency oscillator .

  According to a third aspect of the present invention, the optical modulator according to the first or second aspect is any one of a phase modulator, a Mach-Zehnder intensity modulator, and a single sideband modulator.

According to a fourth aspect of the invention, the said absorption cell of claim 1 or 2, enclosing one of the interior ^{12 C 16 O, 13 C 16} O, C 2 H 2 and HCN It is characterized by.

According to a fifth aspect of the present invention, the absorption cell according to the first or second aspect includes any one of a temperature sensor that measures a temperature in the cell and a pressure sensor that measures a pressure in the cell. The output frequency of the variable frequency generating means is controlled in accordance with the above signal.

According to a sixth aspect of the present invention, the laser light frequency control means according to the first or second aspect controls the temperature of the laser light source to control the output light frequency, and drives the laser light source. One of a laser driving circuit that controls an output light frequency by controlling current and a resonator length control circuit that controls the output light frequency by controlling the resonator length of the laser light source are provided.

According to a seventh aspect of the present invention, the synchronous detector according to the first or second aspect is any one of a frequency that is the same as the frequency of the low-frequency oscillator and a frequency that is an integral multiple of the frequency of the low-frequency oscillator. The signal component is detected.

According to an eighth aspect of the present invention, the synchronous detector according to the first or second aspect includes two phase detectors that receive the signals of the low-frequency oscillator signals that are shifted by 90 degrees from each other, A long axis direction component of an output signal vector from the phase detector is obtained and a synchronous detection signal is output.

The invention according to claim 9 is characterized in that the absorption cell according to claim 1 or 2 includes a temperature control circuit for controlling the temperature in the cell so as to be kept constant.

  As described above, according to the present invention, a simple configuration using only one laser light source is used to stabilize the optical frequency other than the absorption line frequency of the absorption cell without applying a modulation signal to the laser drive current. It is possible to realize a frequency stabilized light source from which output light can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
2 and 10 show the principle block diagrams of the frequency stabilized light source of the present invention, respectively.

  As shown in FIG. 2, the frequency-stabilized light source of this embodiment includes a laser light source 210 that generates continuous-wave laser light and an optical demultiplexer 220 that separates part of the power of the laser light generated by the laser light source 210. An optical modulator 230 that modulates the laser light separated by the optical demultiplexer 220 to generate a modulation sideband, and a variable frequency generation means 270 that generates an electric signal to be applied to the optical modulator 230, A low frequency oscillator 280 that generates an electrical signal for changing the output frequency of the variable frequency generating means 270 and a light that is provided with the modulation output from the optical modulator 230 and is generated by the optical modulator 230 An absorption cell 240 having an absorption line frequency in the vicinity of one of the modulation sidebands, a detector 250 that receives the intensity-modulated light output from the absorption cell 240, and a detector 2 A synchronous detector 290 for performing synchronous detection from the output signal of 0 and the output signal of the low frequency oscillator 280, and laser light frequency control means 260 for controlling the output light frequency of the laser light source 210 in accordance with the output signal of the synchronous detector 290. With.

  As shown in FIG. 10, the frequency-stabilized light source of the present embodiment generates a modulated sideband by modulating a laser light generated by the laser light source 210 and a laser light source 210 that generates continuous-wave laser light. An optical modulator 230 to be generated, a variable frequency generating means 270 for generating an electric signal to be applied to the optical modulator 230, a low frequency oscillator 280 for generating an electric signal for changing the output frequency of the variable frequency generating means 270, Generated by the optical modulator 230 that receives as input the optical frequency separation means 320 that separates a specific optical frequency component from the output light of the optical modulator 230, and the light of the specific optical frequency component separated by the optical frequency separation means 320 An absorption cell 240 having an absorption line frequency in the vicinity of one of the modulated sidebands, and a test for receiving the intensity-modulated light output from the absorption cell 204. Detector 250, synchronous detector 290 that performs synchronous detection from the output signal of detector 250 and the output signal of low-frequency oscillator 280, and control of the output optical frequency of laser light source 210 according to the output signal of synchronous detector 290 Laser light frequency control means 260.

  As described above, the present invention modulates a part of the laser light generated by the laser light source 210 to generate a modulation sideband, and has an absorption line frequency in the vicinity of one of the generated modulation sidebands. It may be configured to be incident on the cell 240, or the laser light generated by the laser light source 210 is modulated to generate a modulation sideband, and a specific frequency component of the modulated laser light is separated, You may comprise so that it may inject into the absorption cell 240 which has an absorption line frequency in the vicinity of one of the generated modulation sidebands.

  The optical modulator 230 can be, for example, any of a phase modulator, a Mach-Zehnder intensity modulator, and a single sideband (SSB) modulator.

For example, the absorption cell 240 may contain carbon monoxide ( ¹² C ¹⁶ O or ¹³ C ¹⁶ O), acetylene (C ₂ H ₂ ), or hydrogen cyanide (HCN). it can.

  The variable frequency generation means 270 is, for example, separated by the same frequency up and down around a frequency corresponding to the difference between the target laser light frequency of the laser light generated by the laser light source 210 and the absorption line frequency of the absorption cell 240. Two fixed frequency oscillators each generating a frequency and a switch for switching the output frequency of the two fixed frequency oscillators by an electric signal from the low frequency oscillator 280 can be provided.

  Alternatively, the variable frequency generation means 270 can be configured to include a frequency modulator that is driven by an electric signal from the low frequency oscillator 280, for example. The frequency modulator can be configured to include a voltage controlled oscillator.

  The frequency-stabilized light source of this embodiment further includes a temperature sensor that measures the temperature in the absorption cell 240, and is configured such that the output frequency of the variable frequency generating means 270 is controlled in accordance with a signal from the temperature sensor. You can also

  Or the frequency stabilization light source of this embodiment is further provided with the pressure sensor which measures the pressure in the absorption cell 240, and is comprised so that the output frequency of the variable frequency generation means 270 may be controlled according to the signal from a pressure sensor. You can also

  The laser light frequency control means 260 includes, for example, a temperature control circuit that controls the temperature of the laser light source 210 to control the output light frequency and a laser drive circuit that controls the drive current of the laser light source to control the output light frequency. It can be either.

  Alternatively, the laser light frequency control means 260 can be a device that controls the resonator length of the laser light source 210.

  The synchronous detector 290 can be, for example, a device that detects any signal component having the same frequency as the frequency of the low-frequency oscillator 280 or an integer multiple thereof.

  The synchronous detector 290 can be a device that performs synchronous detection by digital processing. Alternatively, the synchronous detector 290 can be a device that performs synchronous detection by analog processing.

  Furthermore, the synchronous detector 290 includes two phase detectors that receive the signals of the low-frequency oscillator 280 while shifting the phases of the signals by 90 degrees from each other, and the major axis direction component of the output signal vector from these phase detectors is obtained. It can be configured to obtain the synchronous detection signal. With this configuration, a two-phase lock-in amplifier can be realized, and a frequency stabilized light source with simplified phase adjustment can be configured.

  The absorption cell 240 may include a temperature control circuit and be configured to control the temperature inside the cell to be constant.

  The optical frequency separation means 320 may be, for example, one in which an optical circulator is connected to a narrow band fiber Bragg grating having a reflection wavelength at the output optical frequency of a stabilized laser light source. With this configuration, the optical frequency separation means 320 with one input and two outputs can be realized, and the output optical frequency component stabilized at the target laser light frequency and the modulation sideband component can be separated. The output optical frequency component stabilized at the target laser light frequency is the output of the frequency stabilized light source of the present embodiment, and the modulation sideband component is incident on the absorption cell 240.

Example 1
A first embodiment of the frequency-stabilized light source of the present invention will be described with reference to FIGS.
FIG. 3 shows a laser light source 210 of the frequency stabilized light source according to the first embodiment shown in FIG. 2 using a distributed feedback laser diode (DFB LD) 211 and an optical demultiplexer 220 of −10 dB. The coupler 221, the optical modulator 230 by the LiNbO ₃ phase modulator 231, the variable frequency generating means 270 by the modulator driver amplifier 271, the oscillator 272, the variable attenuator 273, the oscillator 274, the variable attenuator 275, and the PIN 276, 280 is a rectangular wave oscillator 281, absorption cell 240 is a ¹² C ¹⁶ O absorption cell 241, detector 250 is a photodetector 251, and synchronous detector 290 is a current input preamplifier 291, a low noise preamplifier 292, and a lock-in amplifier 293. , And laser light frequency control hand 260 shows an example configuration in which each configured by a temperature control circuit 261 and the laser driving circuit 262.

The frequency-stabilized light source of this embodiment uses one of the absorption lines of the ¹² C ¹⁶ O absorption cell 241 and outputs a frequency-stabilized light of 189.100000 THz that is offset from the absorption line frequency by a fixed frequency. .

According to Non-Patent Document 1, the absorption line frequency of the nearest ¹² C ¹⁶ O absorption cell with a target laser light frequency of 189.100000 THz is the absorption center frequency of P10 absorption line (189.087460 THz at a pressure of 133 kPa). This value is offset by 12.540 GHz from the target frequency of 189.100000 THz.

In the following description, it is assumed that the absorption center frequency of the P10 absorption line of the ¹² C ¹⁶ O absorption cell is 189.087460 THz).

The operating principle of the frequency stabilized light source of this embodiment will be described below.
CW (continuous wave) laser light (also referred to as CW light or continuous wave laser light in this specification) output from the DFB LD 211 is incident on the LiNbO ₃ phase modulator 231 via the −10 dB coupler 221. .

The temperature control circuit 261 adjusts the output optical frequency of the DFB LD 211 in advance so that the optical frequency of the modulation sideband is close to the absorption center frequency of the P10 absorption line of the ¹² C ¹⁶ O absorption cell 241. Keep it.

In the LiNbO ₃ phase modulator 231, phase modulation is given to the laser light, and a plurality of modulation sidebands are generated.

  Modulation sidebands are generated on the frequency axis at the same interval as the modulation frequency, and the intensity of two modulation sidebands separated by the same frequency with the original CW optical frequency as the center is equal.

  Here, by adjusting the input from the modulator driver amplifier 271 so that the modulation index of the phase modulation is 1.84, the intensity of two modulation sidebands separated from the original CW optical frequency by the modulation frequency can be obtained. An optical spectrum waveform that can be maximized can be obtained.

  FIG. 4 shows the calculation result of the optical spectrum of the output of the phase modulator 231 when the output optical frequency from the DFB LD 211 is 189.100000 THz, the modulation frequency is 12.540 GHz, and the modulation index is 1.84. Black circles in FIG. 4 represent the intensity of each line spectrum. The vertical axis in FIG. 4 represents the relative intensity when the intensity of the original CW laser beam is 0 dB.

  If the excess loss of the phase modulator 231 is ignored, the intensity of these two modulation sideband intensities is 4.70 db smaller than the intensity of the original CW light.

FIG. 4 also shows the center frequencies of the P8, P9, P11, and P12 absorption lines near the ¹² C ¹⁶ O P10 absorption line.

  The modulation sideband generated by the phase modulation exists in a frequency region sufficiently separated from the center frequency of these adjacent absorption lines.

  In the frequency-stabilized light source of this embodiment, the phase modulation frequency is set to a frequency of ± 20 MHz centered on 12.540 GHz, that is, two frequencies of 12.520 GHz and 12.560 GHz are switched at a constant cycle.

  This is realized by inputting two outputs of an oscillator 272 having a frequency of 12.520 GHz and an oscillator 274 having a frequency of 12.560 GHz to the PIN switch 276 and alternately switching the output frequency of the PIN switch 276.

  Here, the PIN switch 276 receives a signal having a frequency of 1 kHz from the rectangular wave oscillator 281 so that the two frequencies of 12.520 GHz and 12.560 GHz are switched at repetitions of 1 kHz.

  The output of the PIN switch 276 is input to the phase modulator 231 via the modulation driver amplifier 271.

  The variable attenuators 273 and 275 adjust the output intensities of the oscillators 272 and 274 so that the modulation indexes of the phase modulation at the two frequencies of 12.520 GHz and 12.560 GHz have the same value near 1.84.

  The phase modulation gives chirp to the CW light but does not change the envelope intensity of the light, so that the light intensity at the phase modulator output is constant over time.

The output light of the phase modulator 231 is incident on the ¹² C ¹⁶ O absorption cell.

FIG. 5 schematically shows the relationship between the output optical frequency and the modulation sideband frequency of the DFB LD 211, the absorption center frequency of the P10 absorption line of the ¹² C ¹⁶ O absorption cell 241, and the transmission spectrum of the absorption line. .

In the present embodiment, v _CO = 189.087460 THz, v _offset = 12.540 GHz, v _dif = 40 MHz, f _m = 1 kHz, and the target frequency of v _LD is 189.100000 THz.

  Two modulation sidebands are generated on the 12.520 GHz lower frequency side and the 12.560 GHz lower frequency side than the original CW optical frequency, and these two are alternately generated at a repetition frequency of 1 kHz.

The frequency interval 40 MHz between the two modulation sidebands is sufficiently smaller than the transmission spectrum width of the P10 absorption line of the ¹² C ¹⁶ O absorption cell 241.

  When the output optical frequency of the DFB LD 211 is exactly 189.100000 THz, the transmittance of the absorption line at the two modulation sideband frequencies shown in FIG. The intensity of the subsequent light is constant over time.

  However, if the output optical frequency of the DFB LD 211 is shifted from the lower frequency side of 189.100000 THz, the loss due to the absorption line becomes larger in the modulation sideband on the 12.520 GHz lower frequency side, and the modulation sideband on the lower side of 12.570 GHz The strength becomes smaller than. As a result, the output light intensity of the absorption cell 241 vibrates at a frequency of 1 kHz.

  When the output optical frequency of the DFB LD 211 shifts from 189.100000 THz to the higher frequency side, the intensity of the modulation sideband on the 12.570 GHz low frequency side becomes smaller, and as a result, the output light intensity of the absorption cell 241 also becomes the frequency. Vibrates at 1kHz.

  Here, the phase of the 1 kHz vibration is shifted by π depending on whether the output optical frequency of the DFB LD 211 is shifted to the high frequency side from 189.100000 THz or to the low frequency side.

  The light output from the absorption cell 241 is received by the photodetector 251, and the output is sent to the lock-in amplifier 293 via the current input amplifier 291 and the low-noise preamplifier 292.

  The 1 kHz signal from the rectangular wave oscillator 281 is also input to the lock-in amplifier 293.

The lock-in amplifier 293 synchronously detects a 1 kHz component in the envelope intensity of the output light of the ¹² C ¹⁶ O absorption cell 241. The intensity of the 1 kHz component that is synchronously detected changes according to the difference between the output optical frequency of the DFB LD 211 and the target frequency of 189.100000 THz.

  FIG. 6 shows the result of actual measurement of the optical frequency dependence of the synchronous detection signal. Here, the horizontal axis of FIG. 6 is a value obtained by subtracting the absorption center frequency from the frequency at the midpoint of the two modulation sidebands in FIG. 5, that is, the target frequency of the output optical frequency of the DFB LD 211. Matches the deviation from.

  The optical frequency dependence of the synchronous detection signal becomes an S curve as shown in FIG. 6, and the synchronous detection signal is obtained when the optical frequency at the midpoint between the two modulation sidebands coincides with the absorption center frequency of the absorption cell. Becomes 0.

  The synchronous detection signal output from the lock-in amplifier 293 is sent to the laser drive circuit 262 as an error signal.

  The laser drive circuit 262 adjusts the drive current so that the error signal becomes 0, and as a result, the output optical frequency of the DFB LD 211 is stabilized at the target frequency 189.100000 THz.

In this embodiment, the P10 absorption line of ¹² C ¹⁶ O is used to output the frequency stabilized light of 189.100000 THz. However, the frequency stabilized light at the optical frequency on the ITU-T grid other than that is used. It is also possible to use other absorption lines to obtain.

For example, the absorption center frequency of the P7 absorption line of ¹² C ¹⁶ O is 189.508355 THz at a pressure of 133 kPa, which is a value offset by 8.355 GHz from the optical frequency of 189.500000 THz on the ITU-T grid. Therefore, by setting the center of the modulation frequency in this embodiment to 8.355 GHz, frequency stabilized light of 189.500000 THz can be obtained.

Similarly, by using a ¹² C ¹⁶ O P12 absorption line (absorption center frequency is 188.791330 THz at a pressure of 133 kPa) and setting the center of the modulation frequency to 8.670 GHz, frequency stabilized light of 189.800000 THz can be obtained. .

  When the optical frequency of the light source is stabilized according to the present invention, the optical frequency accuracy (accuracy) of the resulting light source is: the accuracy of the optical frequency of the absorption peak of the absorption line + the frequency accuracy of the electric oscillator for generating sidebands + It is determined by the accuracy of the control circuit.

The oscillation frequency (v _offset ) of the electric oscillator is usually increased in frequency accuracy by synchronously controlling the built-in crystal oscillator using a phase-locked loop (PLL) technique.

On the other hand, the present invention is characterized in that this transmission frequency is used after FM modulation. However, if the maximum frequency deviation (± v _dif / 2) of FM modulation is increased in order to improve the sensitivity of absorption line peak detection, there arises a problem that the frequency accuracy of the carrier frequency is lowered.

Therefore, as shown in the present embodiment, an oscillator having a transmission frequency of v _offset + v _dif / 2 and v _offset −v _dif / 2 is prepared, and these are alternately switched to be used, thereby achieving a desired operation. Realized. Thereby, it becomes possible to realize an optical frequency stable light source with high frequency accuracy by a relatively simple configuration.

(Example 2)
A second embodiment of the frequency stabilized light source of the present invention will be described with reference to FIG.
FIG. 7 shows the configuration of the second embodiment of the frequency stabilized light source of the present invention. FIG. 3 shows the configuration of FIG. 3 except that a frequency multiplier 294 is inserted between the rectangular wave oscillator 281 and the lock-in amplifier 293. This is the same as the first embodiment shown. Therefore, repeated description is omitted.

  In this embodiment, the frequency multiplier 294 multiplies the frequency of the signal from the rectangular wave oscillator 281 by an integral multiple and inputs it to the lock-in amplifier 293.

With this configuration, a component corresponding to an integral multiple of the frequency of the rectangular wave oscillator is detected from the fluctuation component of the intensity of light output from the ¹² C ¹⁶ O absorption cell 241.

  By detecting a component corresponding to an integral multiple of the frequency of the rectangular wave oscillator, a larger error signal can be obtained, and optical frequency control with higher system can be realized.

( Reference Example 1 )
With reference to FIG. 8, the 1st reference example of the frequency stabilization light source of this invention is demonstrated.
8, the frequency showing the configuration of a first reference example of the stabilization source, except for the configuration of the frequency-stabilized light source of variable frequency generating means 270 according to the first embodiment shown in FIG. 2, FIG. 3 This is the same as the first embodiment. Therefore, repeated description is omitted.

In this reference example , the variable frequency generation means 270 includes a modulator driver amplifier 271, a voltage controlled oscillator 277, and a variable attenuator 278.

That is, in this reference example , a voltage-controlled oscillator 277 whose center frequency is set to 12.540 GHz is used, and two different frequencies near 12.540 GHz are repeatedly switched at 1 kHz and output by a 1 kHz signal from the rectangular wave oscillator 281.

  Here, by adjusting the intensity of the 1 kHz signal input to the voltage controlled oscillator 277, the two frequencies output from the voltage controlled oscillator 277 are oscillated with an amplitude of about several tens of MHz centering around 12.540 GHz. .

  The output of the voltage controlled oscillator 277 is input to the phase modulator 231 via the modulation driver amplifier 271. The variable attenuator 278 adjusts the strength of the electric signal input from the modulation driver amplifier 271 to the phase modulator 231.

  According to the present embodiment, it is possible to realize a frequency-stabilized light source having a simplified configuration as compared with the first embodiment.

(Example 3 )
A third embodiment of the frequency stabilized light source of the present invention will be described with reference to FIG.

Figure 9 shows the configuration of a third embodiment of a frequency-stabilized light source of the present invention, ¹² C ¹⁶ temperature sensor 279 to the O absorption cell 241 is mounted, the oscillator according to the temperature of ¹² C ¹⁶ O absorption cell 241 3 is the same as that of the first embodiment shown in FIG. 3 except that the output frequency of both the 272 and the oscillator 274 is adjusted. Therefore, repeated description is omitted.

The absorption center frequency of the absorption line of the ¹² C ¹⁶ O absorption cell 279 slightly depends on the gas pressure in the cell, and the pressure in the cell depends on the outside temperature.

Therefore, when the temperature of the ¹² C ¹⁶ O absorption cell 241 used changes, the center frequency of the absorption line changes, and as a result, the difference between the target frequency of the output light of the DFB LD 261 and the center frequency of the absorption line (in FIG. 5). v _offset ) changes.

Therefore, the correlation between the temperature of the ¹² C ¹⁶ O absorption cell 241 and the above v _offset is examined in advance, and even if the temperature of the absorption cell changes, the frequency at the intermediate point between the output frequencies of the oscillator 272 and the oscillator 274 is always v _offset. By adjusting each oscillator so as to match, the accuracy of the output optical frequency can be further increased.

In the present embodiment, ¹² temperature sensor 279 to the C ¹⁶ O absorption cell 241 is mounted, it has been monitoring the temperature of ¹² C ¹⁶ O absorption cell 241, by attaching a pressure sensor instead of the temperature sensor ¹² C The pressure in the ¹⁶ O absorption cell 241 may be monitored, and the output frequencies of the oscillator 272 and the oscillator 274 may be adjusted according to the pressure.

  As mentioned above, although the preferred example of the present invention was concretely illustrated and explained, the present invention is not limited to the above-mentioned illustration, and if it is within the scope of the claims, Various modifications such as replacement, change, addition, increase / decrease of the number of components, change of shape design, change of setting parameters, etc. are all included in the present invention.

It is a block diagram which shows the structural example of the prior art of a frequency stabilization light source. It is a block diagram which shows the fundamental structure of the frequency stabilization light source of this invention. It is a figure which shows the 1st Example of the frequency stabilization light source of this invention. It is a figure which shows the calculation result of the optical spectrum at the time of giving the phase modulation of the modulation index 1.84 with respect to CW light. It is a figure which shows typically the relationship between the output optical frequency of DFB LD, the modulation sideband frequency, the absorption center frequency of the P10 absorption line of the ¹² C ¹⁶ O absorption cell, and the transmission spectrum of the absorption line. It is a figure which shows the result of having measured the optical frequency dependence of the synchronous detection signal. It is a figure which shows the 2nd Example of the frequency stabilization light source of this invention. It is a figure which shows the 1st reference example of the frequency stabilization light source of this invention. It is a figure which shows the 3rd Example of the frequency stabilization light source of this invention. It is a block diagram which shows the fundamental structure of the frequency stabilization light source of this invention.

Explanation of symbols

210 Laser light source 211 Distributed feedback laser diode (BFD LD)
220 Optical demultiplexer 221 Fiber coupler 230 Optical modulator 231 LiNbO ₃ phase modulator 240 Absorption cell 241 ¹² C ¹⁶ O absorption cell 250 Photo detector 251 Photo detector 260 Laser light frequency control means 261 Temperature control circuit 262 Laser drive circuit 270 Variable Frequency generator 271 Modulator driver amplifier 272, 274 Oscillator 273, 275 Variable attenuator 276 PIN switch 280 Low frequency oscillator 281 Rectangular wave oscillator 290 Period detector 291 Current input preamplifier 292 Low noise preamplifier 293 Lock-in amplifier 294 Frequency multiplier

Claims (9)

A laser light source for generating continuous wave laser light;
An optical demultiplexer for separating a part of the power of the laser light generated by the laser light source;
An optical modulator that modulates the separated laser light to generate a modulation sideband;
Variable frequency generating means for generating an electrical signal to be applied to the optical modulator;
A low-frequency oscillator for generating an electrical signal for changing the output frequency of the variable frequency generating means;
An absorption cell having an absorption line frequency in the vicinity of one of the modulation sidebands generated by the optical modulator, wherein the modulated light output from the optical modulator is input;
A detector for receiving intensity-modulated light output from the absorption cell;
A synchronous detector that performs synchronous detection from the output signal of the detector and the output signal of the low-frequency oscillator;
Laser light frequency control means for controlling the output light frequency of the laser light source based on the output signal of the synchronous detector so that the output signal becomes 0, and
The variable frequency generating means includes
Two fixed frequency oscillators each generating a frequency separated by the same frequency up and down around a frequency corresponding to a difference between a target laser light frequency of the continuous wave laser light and an absorption line frequency of the absorption cell; ,
A frequency-stabilized light source comprising a switch for switching output frequencies of the two fixed-frequency oscillators by an electric signal from the low-frequency oscillator . A laser light source for generating continuous wave laser light;
An optical modulator that modulates the laser light to generate a modulation sideband;
Variable frequency generating means for generating an electrical signal to be applied to the optical modulator;
A low frequency oscillator for generating an electrical signal for changing the output frequency of the variable frequency generating means;
Optical frequency separation means for separating a specific optical frequency component from the output light of the optical modulator;
An absorption cell having an absorption line frequency in the vicinity of one of the modulation sidebands generated by the optical modulator, wherein the separated light of the specific optical frequency component is input;
A detector for receiving intensity-modulated light output from the absorption cell;
A synchronous detector that performs synchronous detection from the output signal of the detector and the output signal of the low-frequency oscillator; and
Based on the output signal of said synchronous detector, and a laser optical frequency controlling means for controlling the output light frequency of the laser light source such that the output signal becomes 0,
The variable frequency generating means includes
Two fixed frequency oscillators each generating a frequency separated by the same frequency up and down around a frequency corresponding to a difference between a target laser light frequency of the continuous wave laser light and an absorption line frequency of the absorption cell; ,
A frequency-stabilized light source comprising a switch for switching output frequencies of the two fixed-frequency oscillators by an electric signal from the low-frequency oscillator .   The frequency-stabilized light source according to claim 1 or 2, wherein the optical modulator is one of a phase modulator, a Mach-Zehnder intensity modulator, and a single sideband modulator. 3. The frequency-stabilized light source according to claim 1, wherein any one of ¹² C ¹⁶ O, ¹³ C ¹⁶ O, C ₂ H _2, and HCN is sealed in the absorption cell. The absorption cell is
Either a temperature sensor that measures the temperature in the cell and a pressure sensor that measures the pressure in the cell,
The frequency-stabilized light source according to claim 1 or 2, wherein an output frequency of the variable frequency generator is controlled in accordance with a signal from the sensor. The laser light frequency control means includes
A temperature control circuit for controlling the output light frequency by controlling the temperature of the laser light source;
A laser drive circuit that controls the output light frequency by controlling the drive current of the laser light source; and a resonator length control circuit that controls the output light frequency by controlling the resonator length of the laser light source. The frequency-stabilized light source according to claim 1 or 2. The synchronous detector is
3. The frequency-stabilized light source according to claim 1, wherein a signal component having a frequency that is the same as the frequency of the low-frequency oscillator and a frequency that is an integral multiple of the frequency of the low-frequency oscillator is detected. The synchronous detector is
Comprising two phase detectors for receiving the phase of the signal of the low frequency oscillator shifted by 90 degrees from each other;
The frequency-stabilized light source according to claim 1 or 2, wherein a long-axis component of an output signal vector from the phase detector is obtained to output a synchronous detection signal. The absorption cell is
The frequency-stabilized light source according to claim 1, further comprising a temperature control circuit that controls the temperature in the cell so as to keep the temperature constant.

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