Wavelength control device, wavelength control method, and differential absorption lidar device

Abstract

To provide a wavelength control device capable of stably controlling laser light to a desired wavelength.SOLUTION: A wavelength control device 20 includes: a wavelength correction unit 6 that outputs an error signal Serr1 to a laser light source 12 and irradiates with a laser beam L1 having a wavelength based on the error signal Serr1; a sideband wave generation unit 4 that modulates reference light L0 irradiated by a reference laser light source 11 and outputs modulated light L0mod in which one or more sideband waves are generated in the reference light L0; and a photodetector 51 that receives light obtained by combining the modulated light L0mod and the laser light L1, and outputs an electric signal SDF1 containing the same frequency as a frequency difference between the modulated light L0mod and the laser light L1. On the basis of the electric signal SDF1, the wavelength correction unit 6 generates the error signal Serr1 that shifts the wavelength of the laser light L1 on the basis of the electric signal SDF1 such that the frequency difference between the laser light L1 and one sideband wave closest in wavelength to the laser light L1 in the modulated light L0mod becomes a predetermined offset frequency.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a wavelength control device and a wavelength control method for controlling the wavelength of light for a laser, and a differential absorption LIDAR device. [Background technology]

[0002] Lidar is a sensing technology that uses laser light and is suitable for observing gas molecules and fine particles (aerosols) in the atmosphere, and can obtain their concentration, distance from a light source, speed, and the like. There are various Lidar techniques, and for example, Differential Absorption Lidar (DIAL) (for example, Patent Document 1, Non-Patent Documents 1 and 2) and Raman Lidar are known for observing water vapor. In observing water vapor, Raman Lidar has difficulty in daytime observations due to strong background light, whereas Differential Absorption Lidar can observe water vapor at any time of the day or night, and can also use eye-safe infrared rays (wavelength 1.4 μm or more).

[0003] A differential absorption lidar (DIAL) device is equipped with two lasers, and irradiates two wavelengths of laser light with different light absorption by the target gas molecules, etc., and measures the concentration of the gas molecules based on the received intensity ratio of the reflected and scattered light. Since light absorption by gas molecules other than the target also exists with wavelength dependency, one of the most important factors in a DIAL device is to determine the wavelength of the laser light used for observation and to control it to that wavelength. Since the wavelength of the light emitted by a laser can drift or change due to changes in the surrounding environment and deterioration over time, a technology is required to continuously control the oscillation wavelength to a specific wavelength in order to stably measure gas molecules, etc., for a long period of time with a DIAL device.

[0004] One method for controlling the wavelength of laser light is the Pound-Drever-Hall (PDH) method, in which the laser light is phase-modulated and transmitted through a gas cell or an etalon element, a feedback signal is generated based on the detected intensity and phase information of the transmitted light, and the wavelength of the laser light is feedback-controlled by the feedback signal (e.g., Non-Patent Documents 1 and 3). Another method is the offset lock method, in which a laser light (master laser light) controlled to a specific wavelength is used as a reference and the wavelength is controlled to an arbitrary wavelength (e.g., Non-Patent Document 2). In the offset lock method, a master laser light wavelength-controlled to a reference wavelength by the PDH method and a laser light to be wavelength-controlled are optically heterodyned to generate a difference frequency signal, and the difference frequency signal is phase-locked with the reference signal using an optical phase-locked loop (OPLL) technology to control the wavelength of the laser light. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-198536 A [Non-patent literature]

[0006] [Non-Patent Document 1] Shoken Ishii, et al., "Coherent 2 μm differential absorption and wind lidar with conductively cooled laser and two-axis scanning device", Applied Optics Volume 49, Issue 10, pp. 1809-1817, 2010 [Non-Patent Document 2] Shoken Ishii, et al., "Partial CO2 column-averaged dry-air mixing ratio from measurements by coherent 2-μm differential absorption and wind lidar with laser frequency offset locking", Journal of Atmospheric and Oceanic Technology Volume 29, Issue 9, pp. 1169-1181, 2012 [Non-Patent Document 3] Eric D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” American Journal of Physics Volume 69, Issue 1, pp. 79-87, 2001 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the PDH method, the wavelength can only be controlled to one specific wavelength, such as the central wavelength of the absorption line of the gas molecules sealed in the gas cell or the resonant wavelength of the etalon element. In the offset locking method, if the difference frequency between the master laser light of the reference wavelength and the laser light to be controlled is large, the photodetector used for optical heterodyne detection cannot respond, so it becomes difficult to control the wavelength to be used for observation if the difference between the wavelength and the reference wavelength is large. Furthermore, in the OPLL technology, the frequency response characteristics of the feedback circuit have a large effect on the accuracy of wavelength control, so there is a problem that changing the frequency of the difference frequency signal, i.e., changing the offset amount, reduces the control accuracy. For example, the DIAL device can control the laser light to a wavelength suitable for measuring CO2 by equipping it with a gas cell containing CO2, but it is difficult to control it to a wavelength suitable for measuring water vapor.

[0008] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a wavelength control device and wavelength control method that can stably control the wavelength of laser light, and a differential absorption LIDAR device. [Means for solving the problem]

[0009] The wavelength control device of the present invention is configured to include a wavelength correction means for outputting an error signal to a laser light source and irradiating it with laser light having a wavelength based on the error signal, a sideband generating means for modulating a reference laser light irradiated by a reference laser light source and outputting modulated light in which one or more sidebands are generated in the reference laser light, and a photodetector for receiving light in which the modulated light and the laser light are combined and outputting an electrical signal having the same frequency as the frequency difference between the modulated light and the laser light, and the wavelength correction means generates the error signal based on the electrical signal to shift the wavelength of the laser light so that the frequency difference between the laser light and one sideband in the modulated light whose wavelength is closest to that of the laser light becomes a predetermined value.

[0010] In addition, the differential absorption LIDAR device of the present invention is configured to include a first laser light source and a second laser light source that irradiate laser light of different wavelengths as seed light, the wavelength control device, and a reference laser light source that irradiates the reference laser light, and the wavelength control device controls the wavelength of the laser light by providing the photodetector and the wavelength correction means for the first laser light source and the second laser light source, respectively.

[0011] The wavelength control method of the present invention is a method for controlling laser light irradiated by a laser light source to have a wavelength with a predetermined difference from the wavelength of a reference laser light, and includes the following steps: a sideband generation step of modulating the reference laser light irradiated by a reference laser light source to output modulated light in which one or more sidebands are generated in the reference laser light; a light detection step of outputting, from the modulated light and the laser light, an electrical signal having the same frequency as the frequency difference between the modulated light and the laser light; and a wavelength correction step of generating an error signal from the electrical signal and inputting it to the laser light source, and causing the laser light source to adjust the wavelength of the laser light based on the error signal so that the frequency difference between the laser light and one sideband in the modulated light that has the closest wavelength to the laser light becomes a predetermined value. Effect of the Invention

[0012] According to the present invention, there is provided a wavelength control device and wavelength control method capable of stably controlling laser light to a desired wavelength, and a differential absorption lidar device capable of observing desired gas molecules with high accuracy. [Brief description of the drawings]

[0013] [Figure 1] 1 is a block diagram illustrating the structure of a differential absorption LIDAR device according to an embodiment of the present invention. [Diagram 2] 1 is a block diagram illustrating a structure of a wavelength control device according to an embodiment of the present invention. [Diagram 3] 4 is a flowchart illustrating a wavelength control method according to an embodiment of the present invention. [Figure 4A] FIG. 2 is a schematic diagram showing the frequencies of phase-modulated reference laser light and laser light, for explaining a wavelength control method according to an embodiment of the present invention. [Figure 4B] FIG. 2 is a schematic diagram showing the frequencies of phase-modulated reference laser light and laser light, for explaining a wavelength control method according to an embodiment of the present invention. [Diagram 5] This is the spectrum of absorption lines of water vapor and carbon dioxide in the 2.05 μm band. [Figure 6]1 is a schematic diagram showing the frequency of a phase-modulated reference laser light and a target frequency of the laser light, for explaining a method of controlling the wavelength of the laser light in a differential absorption LIDAR device according to an embodiment of the present invention. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A wavelength control device, a wavelength control method, and a differential absorption LIDAR device according to the present invention will now be described with reference to the drawings. Elements having the same structure are designated by the same reference numerals.

[0015] [Differential Absorption LIDAR Device] A DIAL device (differential absorption LIDAR device) 10 according to an embodiment of the present invention is a device that irradiates the atmosphere with laser light to observe gas molecules and fine particles (aerosols), etc. in the atmosphere, and is particularly suitable for observing water vapor. As shown in Fig. 1, the DIAL device 10 according to this embodiment includes two laser light sources (first laser light source, second laser light source) 12, 13 that irradiate laser light L1, L2 of different wavelengths as seed light, a wavelength control device 20 that controls the wavelengths of the laser light L1, L2, and a reference laser light source 11 that irradiates reference light (reference laser light) L0 used to control the wavelengths of the laser light L1, L2.

[0016] The DIAL device 10 further includes a pulsed laser amplifier 7, a telescope (transmitting and receiving optical system) 8, and a signal processing and observation control device 9, and also includes optical elements such as an optical isolator 14, optical splitters 15, 16, and 18, an optical switcher 17, a mirror 19, and a lens (not shown) as necessary. The DIAL device 10 can use devices similar to those installed in a general DIAL device for components other than the wavelength control device 20. In addition, in FIG. 1 and FIG. 2 described later, solid arrows represent electrical signals, and thick dotted or hatched thick arrows represent light. In addition, for the sake of simplicity, light is represented as traveling in a straight line, but the DIAL device 10 can include an optical waveguide such as an optical fiber to configure an optical path of any shape. Each element will be described in detail below.

[0017] (Reference laser light source, laser light source) The reference laser light source 11 and the laser light sources 12, 13 are semiconductor lasers, solid-state lasers, etc., and known devices corresponding to the wavelengths of the laser light to be irradiated can be applied. In addition, the reference laser light source 11 and the laser light sources 12, 13 are provided with, for example, a resonator and a piezoelectric element for adjusting the resonator length so that the oscillation wavelength can be slightly changed by changing the input voltage, etc. The laser light sources L1, L2 irradiated by the laser light sources 12, 13 are selected as seed light for the DIAL device 10, and have wavelengths that are different from each other in the gas molecules to be observed (e.g., water vapor (H2O)) and have sufficiently small light absorption by other gas molecules in the atmosphere (e.g., CO2). In addition, since the laser light L1, L2 is irradiated into the atmosphere, it is preferable that the laser light sources L1, L2 are eye-safe infrared rays (wavelength 1.4 μm or more). For example, when the laser light L1, L2 is an infrared ray with a wavelength of about 2.05 μm, a Tm,Ho:YLF continuous laser can be applied as the laser light sources 12, 13.

[0018] The laser light source 12 irradiates the laser light L1 controlled to a wavelength λ1, and the laser light source 13 irradiates the laser light L2 controlled to a wavelength λ2. The wavelength λ1 of the laser light L1 is the wavelength with the larger amount of light absorption of the gas molecules to be observed, and the wavelength λ2 of the laser light L2 is the wavelength with the smaller amount of light absorption. If the wavelength λ1 is a wavelength with an excessive amount of light absorption, such as the central wavelength of the absorption line of the gas molecules to be observed, the observation distance by the DIAL device 10 becomes short. In addition, in the vicinity of the central wavelength of the absorption line, the amount of light absorption is highly wavelength-dependent, and the amount of light absorption changes due to a small fluctuation in wavelength. At such wavelengths, the amount of light absorption is highly dependent on temperature and atmospheric pressure. Therefore, in order to measure the altitude distribution of the gas molecule concentration, it is preferable to select the wavelength λ1 that is somewhat away from the central wavelength of the absorption line and has a moderate amount of light absorption. On the other hand, it is preferable to select the wavelength λ2 that is sufficiently away from the absorption line or has a sufficiently small amount of light absorption in order to improve the measurement accuracy. In order to improve the measurement accuracy, it is preferable that the wavelengths λ1 and λ2 are selected so that they are less affected by light absorption by other gas molecules in the atmosphere, and that the amount of light absorption is equal to each other. The laser beams L1 and L2 are controlled to the wavelengths λ1 and λ2 thus set by the wavelength control device 20 with reference to the wavelength (reference wavelength) λ0 of the reference light L0, as described later.

[0019] The reference laser light source 11 irradiates a reference light (reference laser light) L0. The reference light L0 is a laser light having a reference wavelength λ0 for controlling the laser lights L1 and L2 to wavelengths λ1 and λ2, respectively. The reference wavelength λ0 can be set to any value that can be stably fixed within a range in which the difference between the wavelengths λ1 and λ2 is not too large, and it is preferable that the difference in frequency is 50 GHz or less. Specifically, as described later, the Pound-Drever-Hall (PDH) method using the wavelength stabilizing device 3 of the wavelength control device 20 can be used to control the central wavelength of the absorption line of the gas molecules sealed in the gas cell 34 (see FIG. 2) of the wavelength stabilizing device 3.

[0020] (Optical elements) The optical isolator 14 is provided in each of the reference laser light source 11 and the laser light sources 12, 13 to prevent disturbance due to return light of the oscillation operation. The optical splitters (beam splitters) 15, 16 split the laser light L1, L2 emitted by the laser light sources 12, 13 to introduce a part of each to the wavelength control device 20. The optical splitters 15, 16 are shown as half mirrors in Figs. 1 and 2, but are, for example, optical couplers, and are connected to the laser light sources 12, 13 via optical fibers, and can have a desired splitting ratio. The optical switch 17 is an optical switch that alternately outputs the laser light L1 and the laser light L2 periodically (for example, for each pulse period output by the pulse laser amplifier 7). The optical splitter 18 can be configured similarly to the optical splitters 15 and 16, and branches the laser light L1 or laser light L2 (referred to as laser light L1 / L2 as appropriate) output by the optical switch 17 for input to the pulse laser amplifier 7 and the signal processing / observation control device 9, respectively.

[0021] (Pulse laser amplifiers, telescopes, signal processing and observation control devices) The pulsed laser amplifier 7 converts the laser beams L1 and L2 emitted as continuous waves from the laser beam sources 12 and 13 into pulsed beams L1 and L2 having a predetermined period and pulse width for observation. pls The telescope 8 is a transmitting and receiving optical system, and the pulsed light L outputted from the pulsed laser amplifier 7 is amplified. pls is irradiated into the atmosphere, and pulsed light L is emitted from the atmosphere. pls The telescope 8 is, for example, an off-axis telescope. The signal processing and observation control device 9 receives the laser beam L1 / L2 input from the optical switch 17 and the light L′ received by the telescope 8. pls From this, electrical signals are obtained by heterodyne detection for each of the light of wavelengths λ1 and λ2, and the concentration of the gas molecules being observed is calculated.

[0022] [Wavelength Control Device] As shown in FIG. 2, the wavelength control device 20 according to the embodiment of the present invention outputs an error signal S err 1 and output the error signal S errand a wavelength correction unit (wavelength correction means) 6 that irradiates a laser beam L1 having a wavelength based on the wavelength correction unit 1, and a modulated light beam L0 that is generated by modulating a reference light beam L0 irradiated by a reference laser light source 11 and generating one or more sidebands in the reference light beam L0. mod A sideband wave generating unit (sideband wave generating means) 4 that outputs modulated light L0 mod The light obtained by combining the laser light L1 and the modulated light L0 mod and the electrical signal S that contains the same frequency as the frequency difference between the laser light L1 and DF The wavelength correction unit 6 includes a photodetector 51 that outputs an electrical signal S DF 1, based on the modulated light L0 mod An error signal S shifts the wavelength of the laser light L1 so that the frequency difference between the laser light L1 and one sideband wave having a wavelength closest to that of the laser light L1 in the err Generates 1.

[0023] The wavelength control device 20 may further include a wavelength stabilizing device 3 that controls the wavelength of the reference light L0 to a reference wavelength λ0 for the reference laser light source 11. As shown in FIG. 1, the DIAL device 10 uses laser light L1 and L2 having different wavelengths as seed light, so the wavelength control device 20 provided in the DIAL device 10 includes photodetectors 51 and 52 and wavelength correction units 6 and 6 to individually control the laser light L1 and L2. The wavelength control device 20 may further include optical elements such as optical splitters 21 and 22, optical multiplexers 23 and 24, and a mirror 29 as necessary, as well as an electrical signal S from the wavelength correction unit 6 along with the oscillation wavelengths of the reference laser light source 11 and the laser light sources 12 and 13. DF 1,S DF The signal processing unit 100 is provided with a notification means (not shown) including a frequency spectrum analyzer or the like for outputting the difference frequency component included in the reference signal 2 and its phase information to an external monitor or the like in real time.

[0024] With this configuration, the wavelength control device 20 can control one of the sidebands of the reference light L0 to a wavelength close to the target wavelength of the laser light L1 to be wavelength-controlled, regardless of the wavelength λ0 of the reference light L0, and obtain a difference frequency signal at the photodetector 51. Therefore, the error signal S based on the difference frequency signal errThe wavelength control device 20 can correct the wavelength of the laser light L1. Furthermore, the differential absorption LIDAR device 10 equipped with such a wavelength control device 20 can stably irradiate the laser light L1, L2 at any wavelength to an observation target by this configuration. The wavelength control device 20 will be described in detail below.

[0025] In the wavelength control device 20, the reference light L0 emitted from the reference laser light source 11 passes through the optical isolator 14, is branched by the optical splitter (beam splitter) 21, and is input to the wavelength stabilizing device 3 and the sideband wave generating unit 4, respectively. Since the reference light L0 is input to the optical modulators 33 and 43 of the wavelength stabilizing device 3 and the sideband wave generating unit 4, respectively, the polarization direction of the reference light L0 is set to correspond to the optical modulators 33 and 43, and the polarization direction is changed by passing through a half-wave plate (HWP) as necessary. The optical splitter 22 is a splitter that splits the reference light L0 into two optical detectors 51 and 52 so that the wavelength control device 20 in the DIAL device 10 can individually control the laser beams L1 and L2, and thus outputs a common modulated light L0 to each of them. mod 1 and 2, the optical demultiplexers 21 and 22 and the optical multiplexers 23 and 24 are shown as half mirrors, but may have the same configuration as the optical demultiplexers 15 and 16, for example, optical couplers, and may further have a desired branching ratio.

[0026] (wavelength stabilization device) The wavelength stabilizing device 3 includes a gas cell 34, and synchronizes the reference laser light L0 with the central wavelength of the absorption line of the gas molecules sealed in the gas cell 34 by the Pound-Drever-Hall (PDH) method. To this end, the wavelength stabilizing device 3 receives the reference light L0 irradiated by the reference laser light source 11 and outputs a feedback signal S fb and feeds it back to the reference laser light source 11. The wavelength stabilizing device 3 can have a configuration similar to that of a known wavelength stabilizing device using the PDH method (see Non-Patent Documents 1 and 3), and as an example, as shown in Fig. 2, in addition to a gas cell 34, further includes a local oscillator 31, a phase shifter 32, an optical modulator 33, a photodetector 35, a frequency mixer 36, a low-pass filter (LPF) 37, and a proportional-integral-derivative (PID) controller 38.

[0027] The gas cell 34 is a resonant system in the wavelength stabilizing device 3, and is a cell that transmits light and is filled with gas molecules. The gas molecules of the gas cell 34 are selected from those that have strong absorption in the wavelength range of the laser beams L1 and L2. For example, when the laser beams L1 and L2 are infrared rays in the 2.05 μm wavelength band, carbon dioxide (CO2) can be applied, and the center (2050.967 nm) of the absorption line R30 of CO2 can be set as the wavelength λ0 of the reference beam L0. When the laser beams L1 and L2 are infrared rays in the 1.53 μm wavelength band, hydrogen cyanide (HCN) can be applied. The wavelength stabilizing device 3 may include an etalon element as a resonant system, and the resonant wavelength based on the optical path length of the etalon element can be set as the wavelength λ0 of the reference beam L0. The etalon element has a structure in which a cavity serving as an optical path is sandwiched between two reflecting mirrors facing each other in parallel, and an example of the etalon element is a Fabry-Perot resonator.

[0028] The local oscillator 31 supplies an electrical signal as a modulation signal to the optical modulator 33. The optical modulator 33 phase-modulates the reference light L0 based on the modulation signal to obtain a frequency f mod0 Therefore, for the frequency ν0 of the reference light L0, the light modulated by the optical modulator 33 is mainly ν0-f mod0 ,ν0,ν0+f mod0 The optical modulator 33 uses a device corresponding to the wavelength band of the reference light L0, and is preferably a known optical DSB-SC modulator that performs double side band suppressed carrier (DSB-SC) modulation on the reference light L0.

[0029] The photodetector 35 is a photodiode that converts the input light into an electrical signal, and is preferably one that corresponds to the wavelength range of the reference light L0 and has high sensitivity. The photodetector 35 further preferably has a high response speed, i.e., a high (large) detectable frequency, and is preferably a frequency of 2f mod0In the wavelength stabilization device 3, the modulated light, which is the reference light L0 with sidebands, is input to the photodetector 35 after passing through the gas cell 34. As described above, the modulated light is mainly ν0-f mod0 ,ν0,ν0+f mod0 However, when passing through the gas cell 34, the light of frequency ν0 is absorbed, and the light of frequency ν0-f mod0 ,ν0+f mod0 Then, by heterodyne detection of the optical beat between the carrier (reference light L0) and each sideband, the photodetector 35 detects the frequency -f mod0 ,+f mod0 , and their difference frequency 2f mod0 The output of the device is an electrical signal including:

[0030] The frequency mixer 36 mixes the electrical signal output by the photodetector 35 with the modulated signal from the local oscillator 31. The phase shifter 32 is provided to compensate for delays in the paths of the two signals input to the frequency mixer 36. The low-pass filter 37 selects the low-frequency signal component of the signal output by the frequency mixer 36 as a feedback signal. The PID controller 38 generates a feedback signal S having a voltage for driving a piezoelectric element for resonator adjustment built into the reference laser light source 11 from the feedback signals generated by the frequency mixer 36 and the low-pass filter 37. fb Calculate.

[0031] (Sideband wave generator) The sideband generating unit 4 includes a local oscillator 41 and an optical modulator 43, and further includes an amplifier 42 as required. The local oscillator 41 supplies an electrical signal as a modulation signal for the optical modulator 43, and the amplifier 42 amplifies this modulation signal. The optical modulator 43 phase-modulates the reference light L0 based on the modulation signal to generate a frequency f mod The sideband generating unit 4 generates sidebands at intervals of ±2nd order or higher with sufficient intensity by supplying the modulated signal amplified by the amplifier 42 to the optical modulator 43. Therefore, the modulated light L0 has a frequency ν0 of the reference light L0. mod is ν0,ν0±f mod ,ν0±2fmod ,ν0±3f mod , .... The optical modulator 43 uses a device corresponding to the wavelength band of the reference light L0, and the same one as the optical modulator 33 of the wavelength stabilization device 3 can be applied, and an optical DSB-SC modulator is preferable. Alternatively, an optical frequency shift keying (FSK) modulator or an optical single side band (SSB) modulator can be used according to the side band used for controlling the wavelengths of the laser lights L1 and L2, as described later.

[0032] (Photodetector) The photodetectors 51 and 52 are photodiodes that convert the input light into an electrical signal. In the wavelength control device 20, the photodetector 51 detects modulated light L0, which is a reference light L0 with a sideband. mod and laser light L1 are superimposed and input, and an electrical signal S containing a difference frequency signal is generated from the optical beat by heterodyne detection. DF Similarly, the photodetector 52 outputs modulated light L0 mod and the laser light L2 are superimposed and input, and an electrical signal S DF 2. Therefore, it is preferable that the photodetectors 51 and 52 correspond to the wavelength regions of the laser beams L1 and L2, respectively, and have high sensitivity. Furthermore, the photodetectors 51 and 52 should have a high response speed, i.e., a high detectable frequency f FD It is preferable that the frequency is high, specifically, on the order of GHz, and more preferably 5 GHz or higher.

[0033] (Wavelength correction section) The wavelength correction unit 6 detects the electrical signal S DF 1, an error signal S is generated to control the laser light L1 to a wavelength λ1. err In detail, the wavelength correction unit 6 generates an electrical signal S DF 1, modulated light L0 mod The frequency difference (f beat ) at a given offset frequency f o1, the wavelength of the laser light L1 is shifted by the error signal S err 1. Such wavelength correction unit 6 can be configured in the same manner as a phase-locked loop (PLL) circuit having an offset lock function (see Non-Patent Document 2), and as an example, includes a local oscillator 61, a frequency divider (DIV) 62, a phase comparator (PFD) 63, a loop filter (LF) 64, and a proportional-integral-derivative (PID) controller 65. Two wavelength correction units 6 are provided in the wavelength control device 20 provided in the DIAL device 10, and one of them is a wavelength correction unit that receives the electrical signal S from the photodetector 51 as described above. DF 1 is input and the error signal S err The other is an electrical signal S 1 generated from the photodetector 52 and output to the laser light source 12. DF 2 is input and the error signal S err 2 and outputs it to the laser light source 13.

[0034] The local oscillator 61 has an offset frequency f o The frequency divider 62 oscillates a reference signal having a frequency corresponding to the frequency of the electrical signal S input from the photodetector 51 and inputs it to the phase comparator 63. DF Since the difference frequency signal included in the electrical signal S1 has a high frequency of several GHz, this is divided to several tens of MHz. DF The phase comparator 63 compares the signal obtained by dividing 1 with a reference signal oscillated by a local oscillator 61, and outputs an error signal. Since this error signal contains unnecessary frequencies and has large phase noise, a loop filter 64 removes the unnecessary frequencies as a low-pass filter. The PID controller 65 derives an error signal S having a voltage for driving a piezoelectric element for adjusting the resonator built into the laser light source 12 from the error signal generated by the phase comparator 63 and loop filter 64. err 1. The PID controller 65 enables early recovery even if the wavelength of the laser light L1 is deviated from the state in which it was locked to λ1.

[0035] [Wavelength Control Method] The wavelength control method according to the embodiment of the present invention is a method for controlling the laser light L1 emitted by the laser light source 12 to have a wavelength with a predetermined difference from the wavelength of the reference light L0. The laser light source 12 and the laser light L1 are appropriately referred to as the control laser light source 12 and the control laser light L1 to distinguish them from the reference laser light source 11 and the reference light (reference laser light) L0. As shown in FIG. 3, the wavelength control method according to the present embodiment modulates the reference light L0 to generate one or more sidebands in the reference light L0, thereby generating modulated light L0. mod and a sideband generating step S31 for outputting modulated light L0 mod and the laser light L1, modulated light L0 mod and the electrical signal S that contains the same frequency as the frequency difference between the laser light L1 and DF 1, and an optical detection step S32 outputting an electrical signal S DF 1 to the error signal S err 1 is generated and input to the laser light source 12, and the laser light L1 is converted into modulated light L0 mod The frequency difference between the laser light L1 and the sideband with the closest wavelength at beat is the offset frequency f o 1, an error signal S err and a wavelength correction step S34 for adjusting the wavelength to a wavelength based on 1. By this procedure, the laser light can be stably controlled to a desired wavelength.

[0036] First, a start-up step S1 is performed to start up the reference laser light source 11 and the laser light source 12 that irradiate the reference light L0. Furthermore, it is preferable to perform a reference laser light stabilization process (S21 to S23) to control the reference light L0 to the reference wavelength λ0, and when the reference light L0 is locked to the reference wavelength λ0 (S24, YES), a sideband wave generation step S31 is executed. Hereinafter, the wavelength control operation of the laser light L1 by the wavelength control device 20 according to the embodiment of the present invention will be described in detail with reference to Figs. 1, 2, 4A and 4B.

[0037] (Standard laser light stabilization process) As described for the wavelength stabilizing device 3, the reference laser light stabilization process can be performed by synchronizing the reference laser light L0 with the central wavelength of the absorption line of the gas molecules sealed in the gas cell 34 of the wavelength stabilizing device 3 by the PDH method. In detail, as shown in FIG. 3, the process includes a sideband wave generating step S21 of modulating the reference light L0 to generate ±1st order sideband waves in the reference light L0 and outputting modulated light, a light detection step S22 of outputting an electrical signal from the light that has been transmitted through the gas cell 34 (resonance system), and a feedback signal S fb is generated and input to the reference laser light source 11, and the reference light L0 is adjusted to the center wavelength of the absorption line of the gas molecules sealed in the gas cell 34 by the feedback signal S fb and a wavelength correction step S23 for adjusting the wavelength to a wavelength based on the above.

[0038] In the sideband generating step S21, the optical modulator 33 modulates the reference light L0 based on a modulation signal input from the local oscillator 31 to generate a sideband having a frequency ν0-f mod0 ,ν0,ν0+f mod0 In the light detection step S22, the modulated light is transmitted through the gas cell 34 and input to the light detector 35, and a difference frequency 2f is detected from the optical beat between the carrier and each sideband. mod0 The wavelength correction step S23 outputs an electrical signal including the difference frequency 2f mod0 A frequency mixer 36 and a low-pass filter 37 generate a feedback signal by comparing the electrical signal output by the photodetector 35 with the modulating signal so that the modulating signal is cancelled out. A PID controller 38 then generates a feedback signal S fb The reference laser light source 11 calculates the feedback signal S fb The wavelength of the reference light L0 is controlled based on the voltage at the reference laser light source 11. This feedback loop makes it possible to lock the oscillation wavelength of the reference laser light source 11 to the central wavelength of the absorption line of the gas molecules in the gas cell .

[0039] (Sideband Generation Step) The reference light L0 is modulated by the sideband generator 4 at a frequency f modOne of the sidebands is set to be close to the target wavelength λ1 of the laser light L1 to be controlled. mod is set according to the difference between the wavelength λ0 of the reference light L0 and the target wavelength λ1 of the laser light L1. Here, as shown in FIG. 4A, the frequency ν0+f mod is set to be near the frequency ν1 of the target wavelength λ1. In this embodiment, the laser light L1 is offset-locked by this +1-order sideband. The sideband used for this offset locking is appropriately referred to as the main sideband. The main sideband is preferably a lower-order sideband with a relatively high intensity, and most preferably a +1-order or -1-order sideband. The modulation frequency f mod With the setting of o 1 is the main sideband frequency ν0+f mod and the difference between ν0 and ν1 (|ν0+f mod -ν1|). The offset frequency f o 1 is the modulation frequency f mod and the detection frequency f FD smaller than (f o 1 <f mod / 2, f o 1 <f FD ), and f mod The offset frequency f is preferably sufficiently small relative to f / 2. o The modulation frequency f mod Set.

[0040] (Light detection step) Modulated light L0 obtained by modulating reference light L0 mod Both the laser light L1 and the modulated light L0 are input to the photodetector 51 via the optical multiplexer 23. mod The modulated light L0 and the modulated light L1 are superimposed by the optical multiplexer 23 and input to the photodetector 51. As a result, an optical beat is generated in the light input to the photodetector 51. mod is ν0,ν0±f mod ,ν0±2f mod ,ν0±3f mod, .... Therefore, the optical beat is a modulated light L0 mod The difference frequencies |ν0-ν1|, |ν0±f mod -ν1|,|ν0±2f mod -ν1|, .... However, the photodetector 51 includes the detection frequency f FD Only the light within the range is converted into an electrical signal S DF It can be converted to 1.

[0041] (wavelength correction step) The frequency (initial frequency) of the laser light L1 before wavelength control is ν1 init As shown in Figure 4A, ν1 init is the modulated light L0 mod Difference between the +1st sideband and f beat (=|ν0+f mod -ν1 init |) is the only frequency f FD Therefore, the electrical signal S DF 1 is the difference frequency f beat The wavelength correction unit 6 corrects the input electrical signal S DF 1 frequency (difference frequency f beat ) is the offset frequency f o 1(=|ν0+f mod As shown by the white arrow in Figure 4A, the laser light L1 is set to a frequency of ν1 init The error signal S that shifts from err When the laser light L1 reaches the frequency ν1, that is, when the laser light L1 reaches the target wavelength λ1 (S35, YES), the electrical signal S DF 1 is the offset frequency f o Since the frequency is locked to ν1 (wavelength λ1), the frequency is continuously stabilized to ν1 (wavelength λ1) by the wavelength correction unit 6.

[0042] In FIG. 4A, the difference frequency detected by the photodetector 51 is the initial frequency ν1 of the laser light L1. init This is either 1 or 0 depending on the modulation frequency f mod is the detection frequency f of the photodetector 51. FDMore than twice as large as (f mod >2f FD ) is set. Thus, the modulation frequency f mod If is set large, the photodetector 51 can detect only one difference frequency.

[0043] Depending on the difference between the wavelength λ0 of the reference light L0 and the target wavelength λ1 of the laser light L1, the modulation frequency f mod is small, and the detection frequency f FD For example, f FD <f mod ≦2f FD In this case, the difference frequency detected by the photodetector 51 is the initial frequency ν1 of the laser light L1. init In FIG. 4B, the difference frequency detected by the photodetector 51 is modulated light L0 mod ν1 in init f is the difference between the +1st order sideband (main sideband) closest to beat (=|ν0+f mod -ν1 init |) and the difference between the +2nd sideband, f beat ´(=|ν0+2f mod -ν1 init |), and therefore the electrical signal S DF The frequency f1 has the difference between these two frequencies. Therefore, the wavelength correction unit 6 is provided with a low-pass filter between the frequency divider 62 and the phase comparator 63 to correct the frequency f beat In order to select only the components of f before division, mod All you need to do is remove frequencies above 1 / 2.

[0044] Whether the laser light L1 is locked to the wavelength λ1 by the wavelength control device 20 (locked state) or not is determined by the wavelength correction unit 6 in response to an electrical signal S DF The difference frequency component and its phase information contained in the wavelength correction unit 6 can be output in real time to the outside. By providing the wavelength correction unit 6 with the PID controller 65, when the laser beam L1 becomes unlocked, the locked state can be restored automatically or manually.

[0045] If the wavelength deviation of the laser light L1 is large at the beginning, the modulated light L0 mod The frequency difference between the main sideband and the detection frequency f FD If the electric signal S DF 1 to difference frequency f beat cannot be detected (S33, NO), and the error signal S err 1 cannot be generated. Or, the modulated light L0 mod In such a case, the laser light source 12 is offset-locked to a sideband or carrier (reference light L0) other than the main sideband in the laser light source 12, or offset-locked to the opposite side of the original offset side (positive side in Figs. 4A and 4B) relative to the main sideband, resulting in a malfunction in which the laser light source 12 is locked to a wavelength other than the target wavelength λ1 (S35, NO). In such a case, a laser light source parameter change step S36 is performed to change the parameters of the laser light source 12 by an external operation or the like, to adjust the oscillation wavelength. Specifically, the parameters of the laser light source 12 are the drive current (current for exciting the laser medium in the case of a solid-state laser, or current supplied to the semiconductor in the case of a semiconductor laser), the housing temperature, and the voltage applied to the piezoelectric element for adjusting the resonator. Note that even if the reference light L0 is not locked to the reference wavelength λ0 (S24, NO), the parameters of the reference laser light source 11 are changed (S25) in the same manner to adjust the oscillation wavelength.

[0046] In FIG. 4A and FIG. 4B, the frequency of the main sideband is (ν1-f o 1), the initial frequency of the laser light L1 is ν1 init is ν1-f o 1<ν1 init <ν1-f o 1+f mod / 2 and ν1 init ≦ν1-f o 1+f FD If the frequency of the main sideband is to be offset-locked to the negative side, the frequency of the main sideband is (ν1+f o 1) Therefore, ν1+f o 1-f mod / 2<ν1 init <ν1+f o 1, and ν1 init≧ν1+f o 1-f FD Therefore, in order to prevent malfunction, the modulation frequency f mod It is preferable that the frequency of the modulated light L0 input to the photodetector 51 is large, and although it depends on the oscillation wavelength accuracy of the laser light source 12, it is preferable that the frequency of the modulated light L0 is 10 GHz or more. mod If the intensity of the main sideband or the laser light L1 in the DF Difference frequency f beat The intensity of the light is too low to be detected (S33, NO). In this case, the branching ratio of the optical splitter 15, the amplitude of the modulation signal of the sideband wave generating unit 4, etc. are adjusted to increase the optical intensity at the photodetector 51.

[0047] [Method for controlling wavelength of laser light in a DIAL device] A wavelength control method for laser beams L1 and L2 by wavelength control device 20 provided in DIAL device 10 will be described with reference to Figs. 5 and 6. In this embodiment, DIAL device 10 observes water vapor (H2O) in the atmosphere. First, target wavelengths λ1 and λ2 of laser beams L1 and L2 as seed beams are set. Here, infrared rays in the 2.05 μm band are used as laser beams L1 and L2.

[0048] (Setting the target wavelength of the laser light) As shown in FIG. 5, in the 2.05 μm band, there is an absorption line R30 of CO2, so a gas cell 34 filled with CO2 is applied to the wavelength stabilization device 3, and the wavelength λ0 of the reference light L0 is set to 2050.967 nm, which is the central wavelength of the absorption line R30. The laser light L1 is controlled to a wavelength that is relatively absorbed by H2O but is not the central wavelength of the absorption line, and is sufficiently absorbed by CO2 that exists in the atmosphere together with H2O. Therefore, the target wavelength λ1 is set to 2050.550 nm, which is shifted to the long wavelength side from the central wavelength of the absorption line of 2050.53 nm. On the other hand, the laser light L2 is controlled to a wavelength that is absorbed by H2O as little as possible and is absorbed by CO2 equivalent to the wavelength λ1. As such a wavelength, 2051.103 nm is set to the target wavelength λ2.

[0049] (Modulation frequency and offset frequency settings) The reference wavelength λ0 and the target wavelengths λ1 and λ2 are converted to frequencies ν0, ν1 and ν2, respectively, and the frequency differences ν1-ν0 and ν2-ν0 from the reference light L0 for the laser lights L1 and L2 are calculated and shown in Table 1. Based on the frequency differences ν1-ν0 and ν2-ν0, the main sideband of the laser light L1 is assumed to be the +2nd order sideband, and the main sideband of the laser light L2 is assumed to be the -1st order sideband, as shown in Figure 6, so that the laser lights L1 and L2 are offset locked with the lower order sidebands. The modulation frequency f mod and offset frequency f o 1,f o Set 2.

[0050] [Table 1]

[0051] For example, f mod When set to =13.1392GHz, f o 1 = |ν1-(ν0+2f mod )|=3.447GHz, f o 2=|ν2-(ν0-f mod )|=3.447 GHz. Therefore, for example, the detection frequency f FD In other words, as assumed, the main sideband of the laser light L1 can be set to the +2nd order sideband, and the main sideband of the laser light L2 can be set to the -1st order sideband. o 1=f o 2, the wavelength correction units 6, 6 provided in the laser light sources 12, 13, respectively, can have the same structure. o 1,f o 2 may be set to a different value. In this case, the wavelength correction unit 6 provided in the laser light source 12 adjusts the offset frequency f o 1, and a wavelength correction unit 6 provided in the laser light source 13 has an offset frequency f oThe device includes a local oscillator 61 that generates a reference signal corresponding to .

[0052] In this way, the wavelength control device 20 controls the common modulated light L0 mod Therefore, the two laser beams L1 and L2 can be stably controlled to the desired wavelengths λ1 and λ2, respectively. Note that the difference between the wavelengths λ1 and λ2 is small, specifically, the frequency difference is the detection frequency f FD is smaller than twice that of |ν1-ν2|<2f FD ) in the case where modulated light L0 mod In addition, one of the wavelengths λ1 and λ2 is close to the reference wavelength λ0 (the frequency difference is the detection frequency f FD In the case where the difference between the reference light L0 and the modulated light L1 is smaller than the reference light L2, one of the laser beams can be offset-locked directly with the reference light L0 as in the conventional offset locking method (see Non-Patent Document 2). mod It is also possible to offset lock the wavelength of the laser light source 12 and the wavelength control device 20 to the carrier in ...

[0053] [Observation method using the DIAL device] When the laser beams L1 and L2 are both locked to the respective target wavelengths λ1 and λ2 (FIG. 3, S35, YES), the DIAL device 10 can start observation with the laser beams L1 and L2. As shown in FIG. 1, in the DIAL device 10, the pulse laser amplifier 7 is started and the laser beams L1 and L2 alternately output from the optical switcher 17 are converted into pulse beams L pls and the pulsed light L pls The pulsed light L is emitted from the telescope 8 into the atmosphere. pls The scattered light from atmospheric molecules and aerosol particles in the atmosphere is reflected by the pulsed light L pls The signal processing and observation control device 9 receives the pulsed light L plsThe DIAL device 10 measures the concentration of water vapor in the atmosphere by heterodyne detecting the pulsed light L′ with the laser light L1 and L2 for each wavelength λ1 and λ2 and comparing the electrical signals from the light of each wavelength. pls The distribution of aerosols in the atmosphere can be measured from the intensity of the electrical signal converted from the '. Also, as a Doppler LIDAR, it can measure the distribution of aerosols in the atmosphere by using pulsed light L pls It is possible to measure the wind speed in the direction of the line of fire.

[0054] Although the wavelength control device and wavelength control method, as well as the embodiments for implementing the differential absorption LIDAR device according to the present invention have been described above, the present invention is not limited to these embodiments, and various modifications are possible within the scope of the claims. [Explanation of symbols]

[0055] 10 DIAL (Differential Absorption Lidar) 11 Reference laser light source 12 Laser light source (first laser light source) 13 Laser light source (second laser light source) 14 Optical isolator 15,16 Optical demultiplexer 17 Optical switch 18 Optical demultiplexer 19. Mirror 20 Wavelength control device 21,22 Optical demultiplexer 23,24 Optical multiplexer 29. Mirror 3 Wavelength stabilizer 31 Local oscillator 32 Phase shifter 33 Optical Modulator 34 Gas Cell 35 Photodetector 36 Frequency Mixer 37 Low frequency filter 38 Proportional Product Derivative (PID) Controller 4. Sideband wave generating section (sideband wave generating means) 41 Local oscillator 42 Amplifier 43 Optical Modulator 51,52 Photodetector 6 Wavelength correction section (wavelength correction means) 61 Local Oscillator 62 Frequency divider 63 Phase comparator 64 Loop Filter 65 Proportional Product Derivative (PID) Controller 7. Pulsed Laser Amplifier 8 Telescope (transmitting and receiving optical system) 9 Signal processing and observation control equipment S21 Sideband Generation Step S22 Light detection step S23 Wavelength correction step S25 Laser source parameter change step S31 Sideband Generation Step S32 Light detection step S34 Wavelength correction step S36 Laser source parameter change step

Claims

1. a wavelength correction means for outputting an error signal to a laser light source and causing the laser light source to irradiate the laser light having a wavelength based on the error signal; a sideband generating means for modulating a reference laser beam emitted by a reference laser light source and outputting modulated light obtained by generating one or more sidebands from the reference laser beam; a photodetector that receives light obtained by combining the modulated light and the laser light and outputs an electrical signal having the same frequency as the frequency difference between the modulated light and the laser light, the sideband generating means generates the sidebands at a frequency interval greater than twice the frequency that the photodetector can detect; The wavelength correction means is a wavelength control device that generates the error signal based on the electrical signal to shift the wavelength of the laser light so that the frequency difference between the laser light and one of the sideband waves in the modulated light whose wavelength is closest to that of the laser light becomes a predetermined value.

2. The reference laser light source further includes a wavelength stabilizing device for controlling the wavelength of the reference laser light, 2. The wavelength control device according to claim 1, wherein the wavelength stabilization device includes a gas cell or an etalon element, and controls the reference laser light to a central wavelength of an absorption line of gas molecules sealed in the gas cell or a resonant wavelength of the etalon element.

3. a first laser light source and a second laser light source which irradiate laser light having different wavelengths as seed light, a wavelength control device according to claim 1 or 2, and a reference laser light source which irradiates the reference laser light, The wavelength control device is a differential absorption LIDAR device in which the photodetector and the wavelength correction means are provided in each of the first laser light source and the second laser light source, and which controls the wavelength of the laser light irradiated by the first laser light source and the second laser light source, respectively.

4. A wavelength control method for controlling a laser beam emitted by a laser light source to have a wavelength having a predetermined difference from the wavelength of a reference laser beam, comprising the steps of: a sideband generating step of modulating the reference laser light irradiated by a reference laser light source to generate one or more sidebands in the reference laser light and outputting modulated light; a light detection step of outputting an electrical signal having a frequency less than half the frequency interval of the sideband waves generated in the sideband wave generating step from the modulated light and the laser light; a wavelength correction step of generating an error signal from the electrical signal, inputting the error signal to the laser light source, and causing the laser light source to adjust the wavelength of the laser light to a wavelength based on the error signal so that a frequency difference between the laser light and one of the sidebands in the modulated light that has a wavelength closest to that of the laser light becomes a predetermined value; A wavelength control method characterized by performing a laser light source parameter changing step in which, when the electrical signal does not contain a frequency equal to the frequency difference between the modulated light and the laser light, a parameter of the laser light source is changed to adjust the wavelength of the laser light.

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