Laser radar device

Abstract

The present invention provides a laser radar device comprising: a single reference light source (1) that outputs a laser light; a sideband wave generator (3; 37) that generates two or more sideband waves for the laser light output from the reference light source and outputs the laser light for which the generated sideband waves have been generated; an optical splitter (5) that splits the laser light output from the sideband wave generator or the laser light output from the reference light source to two systems; an optical phase modulator (6) that uses reference signal to apply optical phase modulation to one of two split beams of laser light; a gas cell or etalon (8) through which the laser light to which the optical phase modulation has been applied is transmitted; an optical receiver (9) that converts the transmitted laser light to an electrical signal; a mixer (10) that generates an error signal from the converted electrical signal and the reference signal; and a feedback circuit (11) that uses the generated error signal to lock the wavelength of the laser light from the reference light source to a predetermined wave length.

Description

Laser radar device

[0001] The present disclosure relates to laser radar devices, and more particularly to differential absorption lidar devices.

[0002] In a differential absorption lidar, laser light of two wavelengths, which have different light absorption rates by the target gas molecules, is emitted, and the concentration of the gas molecules is measured based on the ratio of the received intensities of the scattered light from the gas molecules. Since the difference in the wavelength of this laser light has a significant impact on the observation, wavelength control is one of the most important factors.

[0003] Patent Document 1 discloses a technique for stabilizing the wavelengths of laser beams output from a plurality of light sources in a differential absorption lidar device using the plurality of light sources.

[0004] Japanese Patent Application Laid-Open No. 2022-127514

[0005] In a configuration using multiple light sources as described in Patent Document 1, the optical frequency noise characteristics of the laser light emitted from the light sources differ between light sources depending on individual differences between the light sources or the characteristics of the driver, which presents a problem of reduced measurement accuracy of the laser radar device.

[0006] The present disclosure has been made to solve such problems, and aims to construct a laser radar device that can suppress a decrease in measurement accuracy.

[0007] a feedback circuit that uses the generated error signal to lock the wavelength of the laser light from the reference light source to a predetermined wavelength; a light source that outputs a laser beam; a mixer that generates an error signal from the converted electric signal and the reference signal; and a feedback circuit that uses the generated error signal to lock the wavelength of the laser light from the reference light source to a predetermined wavelength.

[0008] According to the laser radar device according to the embodiment of the present disclosure, it is possible to suppress a decrease in measurement accuracy.

[0009] FIG. 4 is a diagram showing an example of the configuration of a laser radar device. FIG. 4 is a waveform diagram of a trigger signal and an optical frequency shifter drive signal. FIG. 4 is a conceptual diagram of an optical spectrum after external modulation. FIG. 4 is a conceptual diagram of an optical spectrum after modulation by an optical phase modulator provided in a wavelength controller. FIG. 4A to FIG. 4D are diagrams showing the operation of an optical switch. FIG. 4A is a diagram showing the intensity of ON light before input to the optical switch. FIG. 4B is a diagram showing the intensity of OFF light before input to the optical switch. FIG. 4C is a diagram showing an optical switch drive signal that drives the optical switch. FIG. 4D is a diagram showing the intensity of laser light output from the optical switch. FIG. 6A to FIG. 6D are diagrams showing the operation of an optical frequency shifter (optical phase modulator). FIG. 6A is a diagram showing the optical frequency shifter drive signal for the ON optical path. FIG. 6B is a diagram showing the optical frequency shifter drive signal for the OFF optical path. FIG. 6C is a diagram showing output light (G12) from the ON optical path. FIG. 6D is a diagram showing output light (G12) from the OFF optical path. Fig. 6E is a diagram showing output light (G14) from the ON optical path of the semiconductor optical amplifier. Fig. 6F is a diagram showing output light (G14) from the OFF optical path of the semiconductor optical amplifier. Fig. 6F is a diagram showing a received signal spectrum. Fig. 6F is a diagram showing an example configuration of a laser radar device. Fig. 6F is a diagram showing input and output spectra of an optical comb generator. Fig. 9A shows the input spectrum to the optical comb generator. Fig. 9B shows the output spectrum of the optical comb generator.

[0010] Various embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Components with the same or similar reference numerals in the drawings have the same or similar configurations or functions, and redundant descriptions of such components will be omitted. Furthermore, in this disclosure, the term "or" is used to mean an inclusive logical OR unless otherwise specified.

[0011] First Embodiment <Configuration> A laser radar device LR1 according to a first embodiment will be described with reference to Fig. 1. Fig. 1 is a configuration diagram showing the configuration of the laser radar device LR1 according to the first embodiment. Here, as an example, the gas to be measured is H ２ O. The explanation will be given assuming that the reference molecule is HCN.

[0012] As shown in FIG. 1, the laser radar device LR1 includes a single reference light source 1, an optical distributor 2, an optical phase modulator 3, an oscillator 4, an optical distributor 5, a wavelength controller WLC, an optical filter 12, a semiconductor optical amplifier 13, an optical switch 14, a control device 15, an optical distributor 16, an optical frequency shifter 17, a pulse generator 18, an optical amplifier 19, an optical circulator 20, an optical antenna 21, an optical multiplexer 22, an optical receiver 23, and an arithmetic unit 24.

[0013] (Reference Light Source) The reference light source 1 has an optical frequency f ０ The laser radar device LR1 oscillates a laser beam having a wavelength of 1000 nm and outputs the oscillated laser beam. As the reference light source 1, for example, a DFB laser capable of continuous oscillation in a single mode can be used. Unlike the prior art which is equipped with multiple light sources, the laser radar device LR1 is equipped with a single reference light source 1. The laser beam output from the reference light source 1 is transmitted to the optical distributor 2 via an optical waveguide G1.

[0014] (Optical Splitter) The optical splitter 2 splits the laser light received via the optical waveguide G1 into two and outputs the split laser light. One of the split laser light beams is transmitted to the optical switch 14 via the optical waveguide G2. In this disclosure, the optical waveguide G2 may be referred to as the OFF optical path 25. The other of the split laser light beams is transmitted to the optical phase modulator 3 via the optical waveguide G3. In this disclosure, the optical waveguide G3 may be referred to as the ON optical path 26.

[0015] (Optical Phase Modulator; Oscillator) The optical phase modulator 3 is an example of a sideband generator. The optical phase modulator 3 receives an optical frequency f ０ The optical phase modulator 3 is a modulator that externally modulates the laser light to generate sidebands and outputs the laser light with the generated sidebands. ３The oscillator 4 generates a modulation signal with a frequency Δf and supplies the generated modulation signal to the optical phase modulator 3. For example, the frequency Δf is 20 GHz or more. When the optical phase modulator 3 receives the optical frequency f ０ By applying phase modulation to the laser light of optical frequency f ０ It is possible to generate sidebands of laser light (modulation + side (high frequency side): f+, modulation - side (low frequency side): f-). A conceptual diagram of the optical spectrum after external modulation is shown in Figure 2. f ０ By appropriately setting and Δf, for example, wavelength locking can be performed to lock the f-sideband to the absorption line of the HCN gas cell. The laser light in which the sideband is generated by the optical phase modulator 3 is transmitted to the optical distributor 5 via the optical waveguide G4.

[0016] (Optical Splitter) The optical splitter 5 splits the laser light output from the optical phase modulator 3, received via the optical waveguide G4, into two and outputs the split laser light. One of the split laser lights is transmitted to the optical phase modulator 6 via the optical waveguide G5. In this disclosure, the optical waveguide G5 may be referred to as a wavelength error detection circuit 27. The other of the split laser lights is transmitted to the optical filter 12 via the optical waveguide G6. In this disclosure, the optical waveguide G6 may be referred to as an ON output optical path 28.

[0017] (Wavelength Controller) The wavelength controller WLC is a wavelength controller that stabilizes the wavelength using a sideband wave as a frequency reference light. To achieve this function, the wavelength controller WLC includes, as an example, an optical phase modulator 6, a reference oscillator 7, a gas cell 8, an optical receiver 9, a mixer 10, and a feedback circuit 11, as shown in FIG.

[0018] [Optical Phase Modulator; Reference Oscillator] The optical phase modulator 6 is a modulator that further superimposes optical phase modulation of δf on the laser light output from the optical phase modulator 3 received via the wavelength error detection circuit 27. The reference oscillator 7 is an oscillator that generates a reference signal of frequency δf to drive the optical phase modulator 3. The reference oscillator 7 also outputs the reference signal to the mixer 10.

[0019] A conceptual diagram of the optical spectrum after modulation by the optical phase modulator 6 is shown in FIG. 3. As shown in FIG. 3, the optical frequency f ０ , f ＋ , and f － The laser light after being superimposed with δf by the optical phase modulator 6 is transmitted to the gas cell 8 via the optical waveguide G7.

[0020] [Gas Cell] The gas cell 8 is a cell in which a light absorbing medium is sealed. ２ If O, H ２ The absorption rate of O is small, so H ２ Hydrogen cyanide (HCN), which has a high absorption rate, is sealed in place of O. The transmitted laser light after passing through the gas cell 8 is transmitted to the optical receiver 9 via a waveguide G8.

[0021] [Optical Receiver 9] The optical receiver 9 detects the transmitted laser light through the waveguide G8 and converts the detected laser light into an electrical signal. The optical receiver 9 transmits the converted electrical signal (received signal) to the mixer 10.

[0022] [Mixer] The mixer 10 mixes the reference signal generated by the reference oscillator 7 with the received signal from the optical receiver 9 to generate a signal containing an error signal component equivalent to δf.

[0023] [Feedback Circuit] The feedback circuit 11 is a circuit that extracts an error signal from the signal generated by the mixer 10 and performs feedback control to adjust the oscillation frequency of the reference light source 1. The error signal is extracted using a low-pass filter (not shown). The feedback circuit 11 performs feedback control to adjust the current or temperature of the reference light source 1 so that the extracted error signal becomes zero.

[0024] In this way, wavelength locking can be performed to lock the f-sideband to the absorption line (predetermined wavelength) of HCN. Since the frequency fluctuation of the f-sideband is due to the frequency fluctuation of the f0 laser light, by locking the f-sideband to the absorption line of HCN, wavelength locking can be performed to indirectly lock the f0 laser light to the absorption line of HCN via the f-sideband.

[0025] (Optical Filter) The optical filter 12 is a filter that suppresses unnecessary laser light from the laser light output from the optical phase modulator 3 and received via the ON output optical path 28, and extracts the f+ sideband. An optical filter such as a DWDM (Dense Wavelength Division Multiplexing) filter can be used as the optical filter 12. The extracted f+ sideband is transmitted to the semiconductor optical amplifier 13 via the optical waveguide G9.

[0026] (Semiconductor Optical Amplifier) ​​The semiconductor optical amplifier 13 is an amplifier that maintains constant the output optical power of the f+ sideband wave received via the optical waveguide G9. The amplified f+ sideband wave is transmitted to the optical switch 14 via the optical waveguide G10.

[0027] (Optical Switch; Control Device) The optical switch 14 is a switch that switches between multiple optical paths and selectively outputs one of ON light and OFF light in accordance with an optical switch drive signal input from the control device 15. More specifically, the optical switch 14 selectively outputs either laser light (ON light) with a wavelength that is absorbed by molecules in the atmosphere or laser light (OFF light) with a wavelength that is not easily absorbed by molecules in the atmosphere by switching between optical paths. The laser light selectively output by the optical switch 14 is transmitted to the optical distributor 16 via the optical waveguide G11.

[0028] (Control Device) The control device 15 is a device that generates an optical switch drive signal and supplies the generated optical switch drive signal to the optical switch 14 .

[0029] (Optical Splitter) The optical splitter 16 splits the laser light received from the optical switch 14 via the optical waveguide G11 into two beams and outputs the split laser light. One of the split laser light beams is transmitted as signal light to the optical frequency shifter 17 via the optical waveguide G12. The optical waveguide G12 may be referred to as a signal light path 30 in this disclosure. The other of the split laser light beams is transmitted as local light to the optical multiplexer 22 via the optical waveguide G13. The optical waveguide G13 may be referred to as a local light path 29 in this disclosure.

[0030] (Optical Frequency Shifter) The optical frequency shifter 17 is driven by a pulsed sine wave output from the optical frequency shifter driving circuit 18-1 at the timing of a trigger signal from the pulse generating device 18, thereby pulsing the signal light input via the signal light path 30 and adding a frequency shift of a preset value to the optical frequency of the signal light at the pulse ON timing. For example, an AOM (Acousto Optical Modulator) can be used as the optical frequency shifter 17. The pulsed light to which the frequency shift has been added by the optical frequency shifter 17 is transmitted to the optical amplifier 19 via the optical waveguide G14.

[0031] (Pulse Generator) The pulse generator 18 generates a trigger signal for pulse-driving the optical frequency shifter 17, and supplies the generated trigger signal to the optical frequency shifter driver circuit 18-1.

[0032] (Optical Frequency Shifter Driving Circuit) The optical frequency shift driving circuit 18-1 synchronizes with the trigger signal output from the pulse generating device 18 and supplies a pulsed sine wave to the optical frequency shifter 17.

[0033] FIG. 1B shows a waveform diagram of the trigger signal generated by the pulse generator 18 and the optical frequency shifter drive signal, which is a pulsed sine wave output by the optical frequency shift drive circuit 18-1.

[0034] (Optical Amplifier) ​​The optical amplifier 19 amplifies the pulsed light output from the optical frequency shifter 17 and input via the optical waveguide G14, and outputs the amplified pulsed light. For example, a fiber amplifier can be used as the optical amplifier 19. The pulsed light amplified by the optical amplifier 19 is transmitted to the optical circulator 20 via the optical waveguide G15.

[0035] (Optical Circulator; Optical Antenna) The optical circulator 20 separates the optical waveguide G16 (sometimes referred to as the "output optical path 31") used for measurement from the optical waveguide G16 (sometimes referred to as the "receiving optical path 32"). The transmission optical system represented by the output optical path 31 irradiates pulsed light output from the optical amplifier 19 into the atmosphere from the optical antenna 21. Backscattered light from aerosols in the atmosphere is received as received light by the optical antenna 21, and the received light is guided to the receiving optical path 32 by the optical circulator 20. The received light is transmitted to the optical multiplexer 22 via the receiving optical path 32.

[0036] (Optical Multiplexer) The optical multiplexer 22 is an optical multiplexer that multiplexes received light input via the receiving optical path 32 and local light input via the optical waveguide G13, and outputs the multiplexed light. The optical multiplexer 22 transmits the multiplexed light to the optical receiver 23 via the optical waveguide G18.

[0037] (Optical Receiver) The optical receiver 23 detects the combined light input via the optical waveguide G18, photoelectrically converts the detected combined light into a received signal, which is an electrical signal, and transmits the converted received signal to the calculation device 24.

[0038] (Calculation Device) The calculation device 24 is a device that calculates the concentration of gas molecules based on the intensity ratio between the intensity of the received signal of the ON light and the intensity of the received signal of the OFF light. ２ In the case of O, the calculation unit 24 ２ The calculation device may include a processor and a memory (not shown). The processor reads and executes a program stored in the memory to calculate the water vapor amount.

[0039] Next, the operation of the laser radar device LR1 will be described.０ ) is distributed by the optical distributor 2 to an OFF optical path 25 and an ON optical path 26 .

[0040] In the ON optical path 26, the oscillator 4 supplies a modulation signal of frequency Δf to the optical phase modulator 3, and the optical phase modulator 3 outputs an optical frequency f ０ By applying phase modulation to the laser light with a modulation signal, sidebands (f+, f-) as shown in FIG. 2 are generated. Here, Δf and f ０ is determined so that the generated sideband (modulation negative side: f-) is near the absorption line of the HCN gas cell. ０ By setting Δf to 195,740.83 GHz and Δf to 25.02 GHz, the f- sidebands are aligned with the R17 absorption line of HCN, and the f+ sidebands are aligned with the H ２ It is possible to match the absorption lines of O.

[0041] Next, the laser light that has generated the sidebands is distributed by the optical distributor 5 to the wavelength error detection circuit 27 and the ON output optical path 28. In the wavelength error detection circuit 27, the optical phase modulator 6 of the wavelength controller WLC further superimposes optical phase modulation of δf. Here, δf is equivalent to the HCN absorption linewidth (about 1 GHz). The wavelength controller WLC performs detection using the general PDH method (Pound-Drever-Hall) in the absorption band of the HCN gas cell. Furthermore, the wavelength controller WLC performs direct detection when optical phase modulation is superimposed at frequencies outside the absorption band. Since the phases of the δf sidebands are inverted, no beat signal is generated during detection. Therefore, a beat signal is detected only in the area affected by the absorption band of the HCN gas cell. f ０ Since the frequency fluctuation of the carrier of f appears in the frequency fluctuation of the sideband of f-, by feeding back the wavelength error signal detected at f- to the current or temperature of the reference light source 1 by the feedback circuit 11, the sideband of f- is wavelength-locked to the absorption line of HCN, and ０ The carriers of are indirectly locked to the absorption line of HCN via the f-sidebands.

[0042] In the ON output optical path 28, H ２Laser light of wavelengths other than f+ used for O measurement is suppressed by the optical filter 12. After that, the semiconductor optical amplifier 13 compensates for the losses occurring in the optical phase modulator 3, the optical distributor 5, and the optical filter 12, and the laser light of the ON output optical path 28 system is amplified to an optical power equivalent to that of the laser light of the OFF optical path 25.

[0043] The ON light (optical frequency f) in which unnecessary light is suppressed by the optical filter 12 and the OFF light (optical frequency f) distributed from the reference light source 1 before modulation are ０ ) are multiplexed by the optical switch 14.

[0044] The operation of the optical switch 14 is shown in Fig. 4. Fig. 4A is a diagram showing the intensity of ON light before it is input to the optical switch 14. Fig. 4B is a diagram showing the intensity of OFF light before it is input to the optical switch 14. Fig. 4C is a diagram showing an optical switch drive signal that drives the optical switch 14. Fig. 4D is a diagram showing the intensity of laser light output from the optical switch 14. As shown in Fig. 4D, ON light and OFF light are output while being switched by the optical switch 14. At this time, the ON light output time and the OFF light output time are the same.

[0045] After the ON light and OFF light are multiplexed by the optical switch 14, the laser light is distributed by the optical distributor 16 into signal light to the signal light path 30 and local light to the local light path 29. In the signal light path 30, the optical frequency shifter 17 is pulse-driven to generate frequency-shifted pulsed light. The pulsed light is then amplified by the optical amplifier 19, and the amplified pulsed light is output into space via the optical circulator 20 and the optical antenna 21.

[0046] The pulsed light emitted into space is scattered by the aerosol. The backscattered light is received by the optical antenna 21 and then guided to the receiving optical path 32 by the optical circulator 20. The local light and the receiving light are combined by the optical combiner 22 and then photoelectrically converted by the optical receiver 23. After the photoelectric conversion, the calculation device 24 calculates H from the ratio of the received intensities of the ON light and the OFF light. ２ The amount of O is calculated.

[0047] As described above, by externally modulating the reference light source, detecting an error signal of the reference gas absorption line using the sidebands generated by the external modulation, and feeding the error signal back to the reference light source, it is possible to generate laser light of multiple wavelengths wavelength-locked to the reference gas absorption line. Because these multiple wavelengths are generated from the same light source, the optical frequency noise is the same, and the distance dependence of the detected spectrum that occurs during coherent detection does not occur. Therefore, the laser radar device LR1 can improve measurement accuracy compared to conventional technology.

[0048] Although the above-described embodiment uses the optical phase modulator 3 as an external modulator and the semiconductor optical amplifier 13 for intensity stabilization, other devices may also be used. For example, an optical intensity modulator may be used instead of the optical phase modulator 3, and an optical fiber amplifier may be used instead of the semiconductor optical amplifier 13.

[0049] In addition, in this embodiment, the absorption line of HCN gas is used as a reference for wavelength locking, but other references such as the absorption line of a gas other than HCN gas may be used. For example, the gas absorption line of acetylene may be used, or an element having wavelength absorption characteristics such as an etalon may be used.

[0050] In addition, the measurement target is H ２ The explanation was given assuming that the measurement target is H ２ It is not limited to O, but may be CO ２ It may also be other molecules such as

[0051] Second Embodiment. <Configuration> A laser radar device LR2 according to the second embodiment will be described with reference to FIG. 5 . The laser radar device LR2 according to the second embodiment differs from the laser radar device LR1 according to the first embodiment in that, unlike the laser radar device LR1 which uses the optical switch 14 to temporally switch between ON and OFF light for measurement, the laser radar device LR2 according to the second embodiment achieves simultaneous measurement of ON and OFF light by varying the frequency that drives the optical frequency shifter 17 for ON and OFF light. To achieve this difference, the laser radar device LR2 according to the second embodiment has a configuration as shown in FIG. 5 . Specifically, the laser radar device LR2 has a wavelength multiplexer 33 instead of the optical switch 14 in the first embodiment. The laser radar device LR2 also has an optical phase modulator in each of the ON and OFF optical paths. As shown in FIG. 5 , the ON optical path is provided with an optical phase modulator 17a, and the OFF optical path is provided with an optical phase modulator 17b. The pulse generators 18 and 34 are operated by the clock signal synchronized by the timing control device 35, and the pulse generators 18 and 34 generate synchronized trigger signals. In synchronization with this trigger signal, two optical frequency shifter driving circuits 18-1 and 34-1 output pulsed sawtooth wave modulation signals of different frequencies. Here, the optical frequency shifter driving circuit 18-1 is fm ＯＮ , the optical frequency shifter driver circuit 34-1 is fm ＯＦＦ The optical phase modulators 17a and 17b are operated in the range of 0 to 2π at a frequency of δf. The relationship between the instantaneous phase change amount and the frequency shift amount is expressed by the formula δf = dφ / (2πdt). By changing the phase in a sawtooth waveform between 0 and 2π, a frequency shift with a sawtooth period is achieved.

[0052] <Operation> Next, the operation of the laser radar device LR2 will be described. The ON light output from the semiconductor optical amplifier 13 and the OFF light of the OFF optical path 25 distributed by the optical distributor 2 are multiplexed by the wavelength multiplexer 33.

[0053] After combining, the combined laser light is distributed to the signal light path and the local light path. In the signal light path 30, the combined laser light is converted into pulsed light by a semiconductor optical amplifier 39 pulse-driven by a pulse generator 38. Figure 6 shows the timing of the pulsed light. As shown in Figures 6C to 6F, both the ON light and the OFF light are converted into pulsed light at the same timing. Here, the ON light has fm ＯＮ , fm for OFF light ＯＦＦ As a result, the output light from the optical frequency shifter 17 (optical phase modulators 17a and 17b) becomes pulsed light with different frequency shifts at the same timing.

[0054] The received signal spectrum after coherent reception by the optical receiver 23 is shown in FIG. ＯＮ , fm for OFF light ＯＦＦ Since the frequency shift of each is added, the received signal by ON light is fm ＯＮ , the received signal by OFF light is fm ＯＦＦ The calculation device 24 detects the difference in the amount of light absorption from the frequency information and intensity information obtained after coherent reception.

[0055] By using the laser radar device LR2 described above, measurements can be made without the need for switching using an optical switch. With a method that uses switching using an optical switch, there is a discrepancy in the measurement timing of the ON light and the OFF light, making it impossible to measure fluctuations when each laser light is OFF. Therefore, there is a problem in that spectrum abnormalities caused by environmental fluctuations occur in only one laser, which can lead to measurement errors. By using the laser radar device LR2 according to this embodiment, environmental fluctuations appear equally in both the ON light and the OFF light, eliminating the problem of being unable to measure fluctuations when each laser light is OFF. Therefore, the laser radar device LR2 enables highly accurate measurements.

[0056] Third Embodiment <Configuration> A laser radar device LR3 according to a third embodiment will be described with reference to Fig. 8. The laser radar device LR3 according to the third embodiment differs from the laser radar device LR1 according to the first embodiment mainly in that an optical comb generator 37 is used as a sideband generator instead of the optical phase modulator 3. As shown in Fig. 8, the optical comb generator 37 is disposed between the optical splitter 5 and the optical filter 12. Use of the optical comb generator 37 makes it possible to achieve wavelength stabilization to any absorption line.

[0057] In relation to such differences, as shown in FIG. 8, the laser radar device LR3 includes an oscillator 36 that is connected to an optical comb generator 37 and generates an RF reference signal that causes the optical comb generator 37 to operate at a frequency Δf.

[0058] In the third embodiment, the laser light output from the reference light source 1 is split into two by the optical splitter 5, and one of the split laser lights is transmitted to the wavelength controller WLC. The other split laser light is transmitted to the optical comb generator 37.

[0059] <Operation> Next, the operation of the laser radar device LR3 will be described. When the ON output light (ON output optical path 28) wavelength-locked to the absorption line of the reference gas cell and an RF reference signal are input to the optical comb generator 37, the optical comb generator 37 generates an optical comb. The input spectrum to the optical comb generator 37 is shown in FIG. 9A, and the output spectrum from the optical comb generator 37 is shown in FIG. 9B. As shown in FIG. 9B, the output light from the optical comb generator 37 has a comb-like spectrum centered at f0 and spaced at Δf intervals.

[0060] The optical spectrum of the comb is generated at equal intervals (Δf), which allows stable measurements to be performed using not only the R17 absorption line of HCN gas described in the first embodiment, but also other absorption lines and absorption lines of other molecules such as acetylene.

[0061] It is possible to combine the embodiments, and to modify or omit each embodiment as appropriate.

[0062] The laser radar device of the present disclosure can be used as a differential absorption lidar device.

[0063] 1 Reference light source, 2 Optical distributor, 3 Optical phase modulator (second optical phase modulator), 4 Oscillator, 5 Optical distributor, 6 Optical phase modulator (first optical phase modulator), 7 Reference oscillator, 8 Gas cell, 9 Optical receiver, 10 Mixer, 11 Feedback circuit, 12 Optical filter, 13 Semiconductor optical amplifier, 14 Optical switch, 15 Control device, 16 Optical distributor, 17 (17a, 17b) Optical frequency shifter (optical phase modulator; third optical phase modulator), 18 Pulse generation device, 18-1 Optical frequency shifter drive circuit, 19 Optical amplifier, 20 Optical circulator, 21 Optical antenna, 22 Optical multiplexer, 23 Optical receiver, 24 Arithmetic unit, 25 OFF optical path, 26 ON optical path, 27 Wavelength error detection circuit, 28 ON output optical path, 29 Local light path, 30 Signal optical path, 31 output optical path, 32 receiving optical path, 33 wavelength multiplexer, 34 pulse generation device, 34-1 optical frequency shifter drive circuit, 35 timing control device, 36 oscillator, 37 optical comb generator, 39 semiconductor optical amplifier, G1 to G18 optical waveguides, LR1 to LR3 laser radar device, WLC wavelength controller.

Claims

1. A laser radar device comprising: a single reference light source that outputs laser light; a sideband generator that generates two or more sidebands from the laser light output from the reference light source and outputs the laser light generated by the sidebands; an optical distributor that distributes the laser light output from the sideband generator or the laser light output from the reference light source into two systems; a first optical phase modulator that performs optical phase modulation on one of the distributed laser lights using a reference signal; a gas cell or etalon through which the optically phase-modulated laser light is transmitted; an optical receiver that converts the transmitted laser light into an electrical signal; a mixer that generates an error signal from the converted electrical signal and the reference signal; and a feedback circuit that uses the generated error signal to lock the wavelength of the laser light of the reference light source to a predetermined wavelength.

2. The laser radar device according to claim 1, further comprising an optical amplifier for stabilizing the power of the other of the laser beams split by the optical splitter.

3. The laser radar device according to claim 1, wherein the sideband generator is a second optical phase modulator.

4. The laser radar device described in claim 1, further comprising: an optical antenna that irradiates the laser light of the reference light source or the high-frequency sideband wave toward the atmosphere and receives backscattered light from water vapor in the atmosphere; and a calculation device that calculates the amount of water vapor from the difference between the received signal of the laser light of the reference light source and the received signal of the high-frequency sideband wave. The two or more sideband waves include a low-frequency sideband wave and a high-frequency sideband wave used for observation with respect to the laser light of the reference light source. The error signal is an error signal from an HCN absorption line generated using the low-frequency sideband wave.

5. The laser radar device according to claim 4, wherein the frequency of the sideband generator is 20 GHz or higher.

6. The laser radar device according to claim 1, further comprising: a frequency shifter which superimposes different amounts of frequency shift on the laser light of the reference light source and the sideband waves used for observation; an optical antenna which irradiates the reference light source laser light and the sideband waves used for observation, which have been given different amounts of frequency shift, toward the atmosphere, and receives backscattered light from water vapor in the atmosphere; and a calculation device which calculates the amount of water vapor from frequency information and intensity information after coherent reception.

7. The laser radar device according to claim 6, wherein the frequency shifter is a third optical phase modulator.

8. The laser radar device according to claim 1, wherein the sideband generator is an optical comb generator, the optical distributor distributes the laser light of the reference light source into two systems, and the first optical phase modulator performs optical phase modulation on one of the laser lights of the reference light source by using the reference signal.

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