JP2022127514A - Wavelength control device, wavelength control method, and differential absorption lidar device - Google Patents

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

The present invention relates to a wavelength control device and a wavelength control method for controlling the wavelength of light with respect to a laser, and a differential absorption LIDAR device.

Lidar is a sensing technology that uses laser light and is suitable for observing gas molecules and fine particles (aerosols) in the atmosphere. can be done. There are various lidar techniques. For example, differential absorption lidar (DIAL) (for example, Patent Document 1, Non-Patent Documents 1 and 2) and Raman lidar are known for water vapor observation. In water vapor observation, the Raman lidar is difficult to observe during the daytime due to the strong background light, while the differential absorption lidar can be observed regardless of day or night. can be used.

The differential absorption lidar (DIAL) device is equipped with two lasers, and irradiates laser light of two wavelengths with different light absorption amounts by gas molecules to be observed. Measures the concentration of molecules. Since light absorption by gas molecules other than those to be observed also exists with wavelength dependence, determining the wavelength of the laser light used for observation and controlling it to that wavelength are the most important factors in the DIAL device. is one. Lasers can drift and fluctuate in the wavelength of the light they irradiate due to changes in the surrounding environment and deterioration over time. Therefore, a technology for continuous control is required.

One method of controlling the wavelength of laser light is to phase-modulate the laser light and transmit it through a gas cell or etalon element, generate a feedback signal based on the detected intensity and phase information of the transmitted light, and use the feedback signal A Pound-Drever-Hall (PDH) method that feedback-controls the wavelength of laser light is exemplified (for example, Non-Patent Documents 1 and 3). Another method is an offset lock method in which a laser beam controlled to have a specific wavelength (master laser beam) is used as a reference and controlled to have a wavelength shifted by an arbitrary wavelength (for example, Non-Patent Document 2). In the offset lock method, a master laser beam whose wavelength is controlled to a reference wavelength by the PDH method and a laser beam to be wavelength-controlled are subjected to optical heterodyne detection to generate a difference frequency signal, and an optical phase-locked loop (OPLL) is generated. ) technique to control the wavelength of the laser light by phase-locking the difference frequency signal with the reference signal.

JP 2017-198536 A

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 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 Eric D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” American Journal of Physics Volume 69, Issue 1, pp. 79-87, 2001

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 enclosed in the gas cell or the resonant wavelength of the etalon element. In the offset lock 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. On the other hand, if the difference is large, it becomes difficult to control. Furthermore, in the OPLL technique, the frequency response characteristics of the feedback circuit have a large effect on the accuracy of wavelength control. Therefore, changing the frequency of the difference frequency signal, ie, changing the offset amount, causes a problem of lowering the control accuracy. For example, the DIAL device is equipped with a gas cell filled with CO ₂ so that the laser light can be controlled to a wavelength suitable for measuring CO ₂ , but it is difficult to control the wavelength suitable for measuring water vapor. is.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a wavelength control device, a wavelength control method, and a differential absorption lidar device capable of stably controlling the wavelength of laser light. .

The wavelength control device according to the present invention includes wavelength correction means for outputting an error signal to a laser light source and irradiating laser light having a wavelength based on the error signal, and modulating the reference laser light emitted by the reference laser light source. sideband wave generating means for outputting modulated light obtained by generating one or more sideband waves in the reference laser light; a photodetector that outputs an electric signal including a frequency that is the same as the frequency difference from the laser light, and the wavelength correction means corrects the wavelength of the laser light in the modulated light based on the electric signal. The error signal is generated to shift the wavelength of the laser light so that the frequency difference between the closest one sideband and the laser light is a predetermined value.

Further, the differential absorption lidar apparatus according to the present invention includes 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 for irradiating the reference laser light. The wavelength control device includes a laser light source, wherein the photodetector and the wavelength correction means are provided in each of the first laser light source and the second laser light source to determine the wavelength of the laser light. Control.

A wavelength control method according to the present invention is a method for controlling laser light emitted by a laser light source to have a wavelength with a predetermined difference from a wavelength of a reference laser light, wherein the reference laser light emitted by the reference laser light source is a sideband wave generating step of modulating and outputting modulated light generated by generating one or more sideband waves in the reference laser light; and a frequency difference between the modulated light and the laser light based on the modulated light and the laser light. a photodetection step of outputting an electrical signal including the same frequency as that of the laser light, generating an error signal from the electrical signal and inputting it to the laser light source, and converting the laser light into the modulated light having the wavelength closest to the laser light and a wavelength correction step of adjusting the laser light source to a wavelength based on the error signal so that the frequency difference between the two sideband waves becomes a predetermined value.

According to the present invention, a wavelength control device and a wavelength control method capable of stably controlling a laser beam to a desired wavelength, and a differential absorption lidar device capable of observing desired gas molecules with high precision are obtained. be done.

It is a block diagram explaining the structure of the differential absorption LIDAR apparatus which concerns on embodiment of this invention. 1 is a block diagram illustrating the structure of a wavelength control device according to an embodiment of the present invention; FIG. 4 is a flow chart explaining a wavelength control method according to an embodiment of the present invention; FIG. 3 is a schematic diagram showing frequencies of phase-modulated reference laser light and laser light for explaining the wavelength control method according to the embodiment of the present invention. FIG. 3 is a schematic diagram showing frequencies of phase-modulated reference laser light and laser light for explaining the wavelength control method according to the embodiment of the present invention. It is a spectrum of each absorption line of water vapor and carbon dioxide in the 2.05 μm band. FIG. 4 is a schematic diagram showing the frequency of a phase-modulated reference laser beam and the target frequency of the laser beam, for explaining the wavelength control method of the laser beam in the differential absorption lidar device according to the embodiment of the present invention;

Embodiments for implementing a wavelength control device, a wavelength control method, and a differential absorption lidar device according to the present invention will be described with reference to the drawings. Elements having the same structure are denoted by the same reference numerals.

[Differential absorption lidar device]
A DIAL device (differential absorption lidar device) 10 according to an embodiment of the present invention is a device for observing gas molecules and fine particles (aerosols) in the atmosphere by irradiating laser light into the atmosphere. preferred. As shown in FIG. 1, the DIAL device 10 according to the present embodiment includes two laser light sources (a first laser light source and a second laser light source) that emit laser light L1 and L2 having different wavelengths as seed light. 12 and 13, a wavelength control device 20 that controls the wavelengths of the laser lights L1 and L2, a reference laser light source 11 that emits a reference light (reference laser light) L0 used for controlling the wavelengths of the laser lights L1 and L2, Prepare.

The DIAL device 10 further comprises a pulsed laser amplifier 7, a telescope (transmitting and receiving optical system) 8, and a signal processing/observation control device 9, and if necessary, an optical isolator 14 and optical demultiplexers 15, 16, 18. , an optical switch 17, a mirror 19, and optical elements such as a lens (not shown). In the DIAL device 10, components other than the wavelength control device 20 can be the same devices as those mounted in a general DIAL device. In FIG. 1 and FIG. 2 described later, electric signals are represented by solid arrows, and light is represented by thick dotted or hatched arrows. Also, although the light is represented as traveling straight for the sake of simplicity, the DIAL device 10 can comprise an optical waveguide such as an optical fiber to form an optical path of any shape. Each element will be described in detail below.

(reference laser light source, laser light source)
The reference laser light source 11 and the laser light sources 12 and 13 are semiconductor lasers, solid-state lasers, or the like, and known devices corresponding to the wavelengths of laser light to be irradiated can be applied. Further, the reference laser light source 11 and the laser light sources 12 and 13 are, for example, piezoelectric elements for adjusting the resonator and the resonator length so that the oscillation wavelength can be slightly changed by changing the input voltage or the like. Prepare. The laser beams L1 and L2 emitted by the laser light sources 12 and 13 are used as seed beams for the DIAL device 10, and have different optical absorption amounts of gas molecules to be observed (for example, water vapor (H ₂ O)). A wavelength is chosen at which the light absorption of other gas molecules (eg, CO ₂ ) is sufficiently low. In addition, since the laser beams L1 and L2 are emitted into the atmosphere, they are preferably infrared rays (wavelength of 1.4 μm or longer) for eye-safety. For example, when the laser beams L1 and L2 are infrared rays having a wavelength of about 2.05 μm, a Tm, Ho:YLF continuous laser can be applied as the laser light sources 12 and 13 .

The laser light source 12 emits a laser beam L1 controlled to have a wavelength λ1, and the laser light source 13 emits a laser beam L2 controlled to have a wavelength λ2. Let the wavelength λ1 of the laser light L1 be the wavelength λ1 of the laser light L1, and let the wavelength λ2 of the laser light L2 be the wavelength λ2 of the light absorption of the gas molecules to be observed. 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 molecule to be observed, the observation distance of the DIAL device 10 will be shortened. Further, in the vicinity of the central wavelength of the absorption line, the amount of light absorption is highly dependent on the wavelength, and the amount of light absorption varies with minute fluctuations in wavelength. At such wavelengths, the dependence of the amount of light absorption on temperature and atmospheric pressure is high. Therefore, in order to measure the altitude distribution of the gas molecule concentration, it is preferable to select a wavelength λ1 that is somewhat distant from the central wavelength of the absorption line and has an appropriate amount of light absorption. On the other hand, for the wavelength λ2, it is preferable to select a wavelength sufficiently distant from the absorption line or a wavelength with a sufficiently small amount of light absorption, in order to improve measurement accuracy. Also, in order to improve the measurement accuracy, it is preferable that both the wavelengths λ1 and λ2 are selected so that the influence of light absorption by other gas molecules in the atmosphere is small, and that the light absorption amounts are 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 will be described later.

The reference laser light source 11 emits reference light (reference laser light) L0. The reference light L0 is laser light having a reference wavelength λ0 for controlling the laser lights L1 and L2 to have 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 the difference in frequency is preferably 50 GHz or less. . Specifically, as will be described later, the Pound-Drever-Hall (PDH) method using the wavelength stabilizer 3 of the wavelength controller 20 is used to fill the gas cell 34 of the wavelength stabilizer 3 (see FIG. 2). It can be controlled to the central wavelength of absorption lines of molecules.

(optical element)
The optical isolator 14 is provided in each of the reference laser light source 11 and the laser light sources 12 and 13 to prevent disturbance due to return light of the oscillation operation. Optical demultiplexers (beam splitters) 15 and 16 split the laser beams L1 and L2 emitted by the laser light sources 12 and 13 in order to partially introduce them into the wavelength control device 20 . The optical demultiplexers 15 and 16, represented by half mirrors in FIGS. 1 and 2, are, for example, optical couplers, are connected to the laser light sources 12 and 13 via optical fibers, and have a desired branching ratio. can be applied. The optical switch 17 is an optical switch that alternately outputs the laser light L1 and the laser light L2 periodically (for example, every pulse period output by the pulse laser amplifier 7). The optical demultiplexer 18 can have the same configuration as the optical demultiplexers 15 and 16, and the laser light L1 or the laser light L2 output from the optical switch 17 (arbitrarily referred to as laser light L1/L2) is , to be input to the pulse laser amplifier 7 and the signal processing/observation controller 9, respectively.

(Pulse laser amplifier, telescope, signal processing/observation controller)
The pulse laser amplifier 7 converts the laser beams L1 and L2 emitted as continuous waves from the laser light sources 12 and 13 into pulsed light Lpls having a predetermined period and pulse width for observation, and further amplifies the pulsed light _Lpls to a predetermined output. The telescope 8 is a transmission/reception optical system, and irradiates the atmosphere with the pulsed light L _pls output from the pulsed laser amplifier 7 and receives the pulsed light L _pls ' from the atmosphere. For the telescope 8, for example, an off-axis telescope is applied. The signal processing/observation control device 9 obtains electrical signals by heterodyne detection of the light beams of wavelengths λ1 and λ2 from the laser light beams L1/L2 input from the optical switch 17 and the light beam L _pls ' received by the telescope 8. to calculate the concentration of gas molecules to be observed.

[Wavelength controller]
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 to the laser light source 12, and emits a laser beam L1 having a wavelength based on the error signal S _err 1. A wavelength correction unit (wavelength correction means) 6 for irradiating, and a sideband wave generator that modulates the reference light L0 emitted by the reference laser light source 11 and outputs modulated light L0 _mod in which one or more sideband waves are generated in the reference light L0. The light obtained by combining the modulated light L0 _mod and the laser light L1 is input to the section (sideband wave generating means) 4, and an electric signal _SDF containing the same frequency as the frequency difference between the modulated light L0 _mod and the laser light L1 is generated. and a photodetector 51 that outputs 1. Then, based on the electrical signal S _DF 1, the wavelength correction unit 6 adjusts the frequency difference between the laser light L1 and the sideband wave having the wavelength closest to the laser light L1 in the modulated light L0 _mod to a predetermined value. An error signal S _err 1 is generated that shifts the wavelength of the laser light L1.

Furthermore, the wavelength control device 20 may be configured to include a wavelength stabilizing device 3 that controls the wavelength of the reference light L0 to the reference wavelength λ0 for the reference laser light source 11 . As shown in FIG. 1, the DIAL device 10 uses laser beams L1 and L2 having different wavelengths as seed light. Photodetectors 51 and 52 and wavelength correctors 6 and 6 are provided for control. The wavelength control device 20 further includes optical elements such as optical demultiplexers 21 and 22, optical multiplexers 23 and 24, and mirrors 29, as well as each oscillation of the reference laser light source 11 and the laser light sources 12 and 13, as required. _{Notifying means} including a frequency spectrum analyzer (Fig. not shown).

With such a configuration, the wavelength control device 20 controls one of the sideband waves to a wavelength near the target wavelength of the laser light L1, which is the target of wavelength control, regardless of the wavelength λ0 of the reference light L0, thereby generating a difference frequency signal. It can be obtained with the photodetector 51 . Therefore, the wavelength of the laser light L1 can be corrected with the error signal S _err 1 based on the difference frequency signal. Further, the differential absorption lidar apparatus 10 having such a wavelength control device 20 can stabilize the laser beams L1 and L2 at arbitrary wavelengths and irradiate the observation target with such a configuration. The wavelength control device 20 will be described in detail below.

In the wavelength control device 20, the reference light L0 emitted from the reference laser light source 11 is transmitted through the optical isolator 14, branched by the optical demultiplexer (beam splitter) 21, and sent to the wavelength stabilizer 3 and the sideband wave generator. Enter 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 section 4, respectively, the polarization direction is set to correspond to the optical modulators 33 and 43. (HWP) to change the polarization direction. The optical demultiplexer 22 is provided with two photodetectors 51 and 52 so that the wavelength control device 20 in the _DIAL device 10 individually controls the laser beams L1 and L2. is provided for entering The optical demultiplexers 21 and 22 and the optical multiplexers 23 and 24 are represented by half mirrors in FIGS. , and a desired branching ratio can be applied.

(wavelength stabilizer)
The wavelength stabilizer 3 has a gas cell 34 and synchronizes the reference laser light L0 with the center wavelength of the absorption line of the gas molecules enclosed in the gas cell 34 by the Pound-Drever-Hall (PDH) method. Therefore, the wavelength stabilizing device 3 receives the reference light L<b>0 emitted by the reference laser light source 11 and outputs a feedback signal S _fb to feed it back to the reference laser light source 11 . The wavelength stabilizer 3 can have the same configuration as a known wavelength stabilizer based on the PDH method (see Non-Patent Documents 1 and 3). As an example, as shown in FIG. , a local oscillator 31 , a phase shifter 32 , an optical modulator 33 , a photodetector 35 , a frequency mixer 36 , a low frequency filter (LPF) 37 and a proportional integral derivative (PID) controller 38 .

The gas cell 34 is a resonance system in the wavelength stabilizing device 3, and is a light-transmitting cell filled with gas molecules. The gas molecules of the gas cell 34 are selected to have strong absorption in the wavelength regions of the laser beams L1 and L2. For example, when the laser beams L1 and L2 are infrared rays in the wavelength band of 2.05 μm, carbon dioxide (CO ₂ ) is applied, and the center (2050.967 nm) of the absorption line R30 of CO ₂ is the center of the reference light L0. The wavelength can be λ0. Hydrogen cyanide (HCN) can be used when the laser beams L1 and L2 are infrared rays with a wavelength of 1.53 μm. The wavelength stabilizing device 3 may include an etalon element as a resonance system, and the resonance wavelength based on the optical path length of the etalon element can be the wavelength λ0 of the reference light 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 is, for example, a Fabry-Perot resonator.

The local oscillator 31 supplies an electrical signal to the optical modulator 33 as a modulating signal. The optical modulator 33 phase-modulates the reference light L0 based on the modulated signal, and generates ±first-order sideband waves in the reference light L0 at intervals of the frequency f _mod0 of the modulated signal. Therefore, the light modulated by the optical modulator 33 mainly has three frequencies of ν0−f _mod0 , ν0, and ν0+f _mod0 with respect to the frequency ν0 of the reference light L0. The optical modulator 33 uses a device corresponding to the wavelength band of the reference light L0, and a known optical DSB that performs double side band suppressed carrier modulation (DSB-SC) on the reference light L0. - SC modulators are preferred.

The photodetector 35 is a photodiode that converts the input light into an electrical signal, corresponds to the wavelength range of the reference light L0, and preferably has high sensitivity. Further, the photodetector 35 preferably has a high response speed, that is, a high (large) detectable frequency, and can obtain an electrical signal with little noise near the frequency 2f _{mod 0} . In the wavelength stabilizing device 3 , the photodetector 35 receives the modulated light, which is the reference light L 0 accompanied by the sideband wave, transmitted through the gas cell 34 . As described above, the modulated light mainly has three frequencies of ν0−f _mod0 , ν0, and ν0+f _mod0 . Light with frequencies ν0−f _mod0 and ν0+f _mod0 becomes stronger. Then, by heterodyne detection of the optical beats of the carrier (reference light L0) and each sideband, the photodetector 35 outputs an electrical signal containing frequencies -f _mod0 , +f _mod0 and their difference frequency 2f _mod0 .

A frequency mixer 36 mixes the electrical signal output from the photodetector 35 and the modulated signal from the local oscillator 31 . Phase shifter 32 is provided to compensate for delays in the paths of the two signals entering frequency mixer 36 . A low-frequency filter 37 selects a signal component on the low-frequency side of the signal output from the frequency mixer 36 as a feedback signal. The PID controller 38 generates a feedback signal S having a voltage for driving the resonator adjustment piezo element built in the reference laser light source 11 from the feedback signals generated by the frequency mixer 36 and the low-frequency filter 37 . Calculate _fb .

(Sideband wave generator)
The sideband wave generator 4 includes a local oscillator 41 and an optical modulator 43, and further includes an amplifier 42 as necessary. A local oscillator 41 supplies an electrical signal as a modulating signal for the optical modulator 43, and an amplifier 42 amplifies this modulating signal. The optical modulator 43 phase-modulates the reference light L0 based on the modulated signal to generate sideband waves at intervals of the frequency f _mod of the modulated signal. By supplying the modulated signal amplified by the amplifier 42 to the optical modulator 43, the sideband wave generator 4 can generate sideband waves of orders higher than ±2nd order with sufficient intensity. Therefore, the modulated light L0 _mod has a plurality of frequencies of ν0, ν0±f _mod , ν0±2f _mod , ν0±3f _mod , . The optical modulator 43 uses a device corresponding to the wavelength band of the reference light L0, and can be the same as the optical modulator 33 of the wavelength stabilizing device 3, preferably an optical DSB-SC modulator. Alternatively, as described later, an optical frequency shift keying (FSK) modulator or an optical SSB (Single Side Band) modulator is used according to the sidebands used for wavelength control of the laser beams L1 and L2. You can also

(photodetector)
The photodetectors 51 and 52 are photodiodes that convert input light into electrical signals. In the wavelength control device 20, the photodetector 51 receives the modulated light L0 _mod in which the sideband wave is attached to the reference light L0 and the laser light L1 superimposed thereon. Output signal _SDF1 . Similarly, the photodetector 52 receives the superimposed modulated light L0 _mod and the laser light L2 and outputs an electrical signal S _DF 2 . Therefore, it is preferable that the photodetectors 51 and 52 correspond to the wavelength ranges of the laser beams L1 and L2 and have high sensitivity. Furthermore, the photodetectors 51 and 52 preferably have a high response speed, that is, a high detectable frequency _fFD , specifically on the order of GHz, in order to detect optical beats as described later, and are preferably 5 GHz or higher. is more preferable.

(Wavelength corrector)
Based on the electrical signal S _DF 1 output from the photodetector 51, the wavelength corrector 6 generates an error signal S _err 1 for controlling the wavelength λ1 of the laser light L1. Specifically, the wavelength correction unit 6 determines that the frequency difference (f _beat ) between the laser light L1 and one sideband wave having the wavelength closest to the laser light L1 in the modulated light L0 _mod included in the electric signal S _DF 1 is , to generate an error signal S _err 1 that shifts the wavelength of the laser light L1 so as to have a predetermined offset frequency f _o 1 . Such a wavelength correction unit 6 can have the same configuration as a phase-locked loop (PLL) circuit having an offset lock function (see Non-Patent Document 2). It comprises 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, one of which receives the electrical signal S _DF 1 from the photodetector 51 and the error signal S as described above. Generate _err 1 and output it to the laser light source 12 . The other receives the electrical signal S _DF 2 from the photodetector 52 to generate an error signal S _err 2 and outputs it to the laser light source 13 .

The local oscillator 61 oscillates a reference signal having a frequency corresponding to the offset frequency f _o 1 and inputs it to the phase comparator 63 . Since the difference frequency signal contained in the electrical signal _SDF1 input from the photodetector 51 has a frequency as high as several GHz, the frequency divider 62 divides it into several tens of MHz. The phase comparator 63 compares the signal obtained by frequency-dividing the electric signal S _DF 1 with the reference signal oscillated by the local oscillator 61 and outputs an error signal. Since this error signal contains unwanted frequencies and has large phase noise, the loop filter 64 acts as a low-pass filter to remove unwanted frequencies. A PID controller 65 generates an error signal S _err 1 having a voltage for driving a piezoelectric element for resonator adjustment built into the laser light source 12 from the error signals generated by the phase comparator 63 and the loop filter 64 . calculate. Even if the wavelength of the laser light L1 deviates from the state locked to λ1 by the PID controller 65, early recovery is possible.

[Wavelength control method]
The wavelength control method according to the embodiment of the present invention is a method of controlling the wavelength of the laser light L1 emitted by the laser light source 12 to have a predetermined difference in wavelength from the wavelength of the reference light L0. Note that 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 includes a sideband wave generation step S31 of modulating the reference light L0 and outputting the modulated light L0 _mod in which one or more sideband waves are generated in the reference light L0. a light detection step S32 for outputting an electric signal S _DF 1 containing the same frequency as the frequency difference between the modulated light L0 _mod and the laser light L1 from the modulated light L0 _mod and the laser light L1 _; The error signal S _err 1 is generated and input to the laser light source 12 , and the frequency difference f _beat between the laser light L 1 and one sideband wave of the modulated light L 0 _mod whose wavelength is closest to the laser light L 1 is the offset frequency f ₀ 1 . A wavelength correction step S34 is performed to adjust the laser light source 12 to the wavelength based on the error signal S _err 1 so that . By such a procedure, the laser light can be stably controlled to a desired wavelength.

First, a starting step S1 for starting the reference laser light source 11 and the laser light source 12 for emitting the reference light L0 is performed. Further, reference laser light stabilization processing (S21 to S23) is performed 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), the sideband wave generation step S31 is executed. preferably. Hereinafter, with reference to FIGS. 1, 2, 4A and 4B, the wavelength control operation of the wavelength control device 20 according to the embodiment of the present invention for controlling the wavelength of the laser light L1 will be described in detail.

(Reference laser light stabilization treatment)
The reference laser light stabilization process synchronizes the reference laser light L0 with the center wavelength of the absorption line of the gas molecules enclosed in the gas cell 34 of the wavelength stabilization device 3 by the PDH method, as described for the wavelength stabilization device 3. It can be executed by letting More specifically, as shown in FIG. 3, a sideband wave generation step S21 of modulating the reference light L0 and outputting modulated light generated by generating sideband waves of the ±1st order in the reference light L0, and generating the modulated light in the gas cell 34 ( A light detection step S22 of outputting an electric signal from the light transmitted through the resonance system), and a feedback signal S _fb is generated from the electric signal and input to the reference laser light source 11, and the reference light L0 is injected into the gas cell 34. A wavelength correction step S23 is performed to adjust the wavelength based on the feedback signal S _fb so that the central wavelength of the absorption line of the gas molecules is obtained.

In the sideband wave generating step S21, the optical modulator 33 modulates the reference light L0 based on the modulated signal input from the local oscillator 31, and the frequency ν0 of the reference light L0 is ν0-f _mod0 , ν0 , ν0+f _mod0 of ±1st order sidebands. In the photodetection step S22, the modulated light is transmitted through the gas cell 34 and input to the photodetector 35, and an electric signal including the difference frequency 2f _mod0 is output from the optical beat of the carrier and each sideband wave. In the wavelength correction step S23, the frequency mixer 36 and the low frequency filter 37 compare the electrical signal output from the photodetector 35 with the modulated signal so that the difference frequency 2f _mod0 is canceled, thereby generating a feedback signal. Further, the PID controller 38 calculates a feedback signal S _fb from this feedback signal and feeds it back to the reference laser light source 11 . The reference laser light source 11 controls the wavelength of the reference light L0 based on the voltage of the feedback signal _Sfb . This feedback loop allows the oscillation wavelength of the reference laser light source 11 to be locked to the center wavelength of the absorption line of the gas molecules in the gas cell 34 .

(Sideband wave generation step)
The reference light L0 is generated by the sideband wave generator 4 to generate sideband waves at intervals of the frequency f _mod of the modulated signal, and one of the sideband waves is set near the target wavelength λ1 to be controlled of the laser light L1. Therefore, the frequency f _mod of the modulated signal of the sideband wave generator 4 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. The frequency ν0+f _mod of the +1st order sideband wave 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 +1st order sideband. The sideband used for this offset lock is appropriately called the main sideband. The primary sideband is preferably a lower order sideband of relatively high intensity, most preferably a +1st or -1st order sideband. As the modulation frequency f _mod is set, the offset frequency f _o 1 in the wavelength corrector 6 is set to the difference (|ν0+f _mod -ν1|) between the main sideband frequencies ν0+f _mod and ν1. The offset frequency f _o 1 is smaller than half the modulation frequency f _mod and smaller than the detection frequency f _FD of the photodetector 51 (f _o 1<f _mod /2, f _o 1<f _FD ), Furthermore, it is preferably sufficiently small relative to f _mod /2. The modulation frequency f _mod is set such that the offset frequency f _o 1 has such a value.

(Photodetection step)
Both the modulated light L0 _mod obtained by modulating the reference light L0 and the laser light L1 are input to the photodetector 51 via the optical multiplexer 23 . That is, the laser light L1 and the modulated light L0 _mod are superimposed by the optical combiner 23 and input to the photodetector 51 . As a result, an optical beat occurs in the light input to the photodetector 51. As described above, the modulated light L0 _mod is a plurality of ν0, ν0±f _mod , ν0±2f _mod , ν0±3f _mod , . has a frequency of Therefore, the optical beats are the difference frequencies |ν0−ν1|, |ν0±f _mod −ν1|, |ν0±2f _{mod −ν1} |, …including. However, the photodetector 51 can convert only light within the detection frequency f _FD into the electrical signal S _DF 1 due to its detectability.

(Wavelength correction step)
Denoting the frequency (initial frequency) of the laser light L1 before wavelength control as ν1 _init , as shown in FIG. 4A, ν1 _init is the difference f _beat (=| _ν0 +f _mod −ν1 _init |) is below the detection frequency f _FD , so the electrical signal S _DF 1 has a frequency of this one difference frequency f _beat . The wavelength correction unit 6 adjusts the frequency (difference frequency f _beat ) of the input electrical signal S _DF 1 to the offset frequency f _o 1 (=|ν0+f _mod −ν1|), as indicated by the white arrow in FIG. 4A. , an error signal S _err 1 is generated that shifts the laser light L1 from frequency ν1 _init to ν1. When the laser beam L1 reaches the frequency ν1, that is, the target wavelength λ1 (S35, YES), the electric signal _SDF1 is locked to the offset frequency _f01 , so that the wavelength corrector 6 continues to operate at the frequency ν1 (wavelength λ1).

In FIG. 4A, the difference frequency detected by the photodetector 51 is 1 or 0 depending on the initial frequency ν1 _init of the laser light L1. This is because the modulation frequency f _mod of the sideband wave generator 4 is set to be greater than twice the detection frequency f _FD of the photodetector 51 (f _mod >2f _FD ). Thus, if the modulation frequency f _mod is set high, the photodetector 51 can detect only one difference frequency.

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 of the sideband wave generator 4 may be small, and may be less than twice the detection frequency f _FD of the photodetector 51. be. For example, when f _FD <f _mod ≦2f _FD , the difference frequency detected by the photodetector 51 is two or one depending on the initial frequency ν1 _init of the laser light L1. In FIG. 4B, the difference frequency detected by the photodetector 51 is the difference f _beat (=| _ν0 +f _{mod −ν1 init} | ) and the difference f _beat '(=|ν0+2f _mod −ν1 _init |) with the +2nd order sideband, so the electrical signal S _DF 1 has the difference frequency of these two. Therefore, the wavelength correction unit 6 includes a low-frequency filter between the frequency divider 62 and the phase comparator 63 so that only the frequency f _beat component before frequency division can be selected. It is sufficient to remove frequencies above f _mod /2.

Whether or not the laser light L1 is locked (locked state) at the wavelength λ1 by the wavelength control device 20 is determined by the wavelength correcting unit 6 by detecting the difference frequency component and its phase information included in the electric signal S _DF 1 in real time. It can be known by outputting to the outside with Since the wavelength corrector 6 includes the PID controller 65, when the laser beam L1 is unlocked, it can be automatically or manually restored to the locked state.

If the laser light L1 has a large wavelength shift at the initial stage, the frequency difference between the modulated light _{L0 mod} and the main sideband wave exceeds the detection frequency f _FD of the photodetector 51, and the difference frequency f _beat cannot be detected (S33, NO), and the error signal S _err 1 cannot be generated. Alternatively, it is offset-locked by a sideband wave other than the main sideband wave or carrier (reference light L0) in the modulated light L0 _mod , or the side opposite to the original offset side (+ side in FIGS. 4A and 4B) with respect to the main sideband wave. , and an erroneous operation of being locked to a wavelength other than the target wavelength λ1 (S35, NO) occurs. In such a case, a laser light source parameter changing 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 driving current (the current that excites the laser medium in the case of a solid-state laser, and the current that is supplied to the semiconductor in the case of a semiconductor laser), the housing temperature, and the piezoelectric element for adjusting the resonator. It is the applied voltage to the element. Even when the reference light L0 is not locked to the reference wavelength λ0 (S24, NO), the parameters of the reference laser light source 11 are similarly changed (S25) to adjust the oscillation wavelength.

In FIGS. 4A and 4B, since the frequency of the main sideband is (ν1−f _o 1), the initial frequency ν1 _init of the laser light L1 is ν1−f _o 1<ν1 _init <ν1−f _o 1+f _mod /2 and ν1 _init ≦ν1−f _o 1+f _FD . When the main sideband is to be offset-locked on the negative side, since the frequency of the main sideband is (ν1+f _o 1), ν1+f _o 1−f _mod /2<ν1 _init <ν1+f _o 1, and It suffices if ν1 _init ≧ν1+f _o 1−f _FD . Therefore, in order to prevent malfunction, it is preferable that the modulation frequency f _mod of the sideband wave generator 4 is large. If the intensity of the main sideband wave or the laser beam L1 in the modulated light L0 _mod input to the photodetector 51 is too small, the intensity of the difference frequency f _beat of the electric signal S _DF 1 is too small to be detected (S33, NO ). In this case, the optical intensity at the photodetector 51 is increased by adjusting the branching ratio of the optical demultiplexer 15 and the amplitude of the modulated signal of the sideband wave generator 4 .

[Wavelength Control Method of Laser Light of DIAL Device]
A method of controlling the wavelengths of the laser beams L1 and L2 by the wavelength control device 20 provided in the DIAL device 10 will be described with reference to FIGS. 5 and 6. FIG. In this embodiment, the DIAL device 10 observes water vapor (H ₂ O) in the atmosphere. First, the target wavelengths λ1 and λ2 of the laser beams L1 and L2 used as seed beams are set. Here, infrared rays in the 2.05 μm band are applied to the laser beams L1 and L2.

(Setting the target wavelength of laser light)
As shown in FIG. 5, since there is an absorption line R30 of CO ₂ in the 2.05 μm band, the gas cell 34 filled with CO ₂ is applied to the wavelength stabilizer 3, and the central wavelength of the absorption line R30 Let 2050.967 nm be the wavelength λ0 of the reference light L0. The laser beam L1 is controlled to have a relatively large amount of absorption by H ₂ O, but is not at the center wavelength of the absorption line and has a sufficiently small amount of absorption by CO ₂ that exists together with H ₂ O in the atmosphere. Therefore, the target wavelength λ1 is 2050.550 nm, which is shifted to the longer wavelength side with respect to the center wavelength of the absorption line of 2050.53 nm. On the other hand, the laser beam L2 is controlled to have a wavelength that has as little absorption by H ₂ O as possible and an amount of absorption by CO ₂ that is equivalent to the wavelength λ1. As such a wavelength, 2051.103 nm is set as the target wavelength λ2.

(Setting of modulation frequency and offset frequency)
The reference wavelength λ0 and the target wavelengths λ1 and λ2 are converted into frequencies ν0, ν1 and ν2, respectively, and the frequency differences ν1−ν0 and ν2−ν0 from the reference light L0 are calculated for the laser beams L1 and L2. . Based on the frequency differences ν1−ν0 and ν2−ν0, the main sideband of the laser beam L1 is shifted to +2nd order as shown in FIG. It is assumed that the sideband wave, the main sideband wave of the laser beam L2, is the -1st order sideband wave. Then, the modulation frequency f _mod and the offset frequencies f _o 1 and f _o 2 at this time are set.

For example, when f _mod = _13.1392 GHz, f o1=|ν1−(ν0+2f _mod )|=3.447 GHz and f o2=|ν2−( _ν0 −f _mod )|=3.447 GHz. . Therefore, for the photodetectors 51 and 52, general ones having a detection frequency _fFD of about 5 GHz, for example, can be applied. That is, as assumed, the main sideband wave of the laser light L1 can be set to the +2nd order sideband wave, and the main sideband wave of the laser light L2 can be set to the -1st order sideband wave. Furthermore, since f _o 1=f _o 2, the wavelength correctors 6 and 6 provided in the laser light sources 12 and 13 can have the same structure. Note that the offset frequencies f _o 1 and f _o 2 may be set to different values. In this case, the wavelength correction unit 6 provided in the laser light source 12 has a local oscillator 61 that oscillates a reference signal corresponding to the offset frequency f _o 1, and the wavelength correction unit 6 provided in the laser light source 13 A local oscillator 61 that oscillates a reference signal corresponding to the offset frequency f _o 2 is provided.

In this manner, the wavelength control device 20 can stably control the two laser beams L1 and L2 to different desired wavelengths λ1 and λ2 from the common modulated light L0 _mod . When the difference between the wavelengths λ1 and λ2 is small, specifically, when the frequency difference is smaller than twice the detection frequency f _FD (|ν1−ν2|<2f _FD ), the common sideband in the modulated light L0 _mod Waves can be offset locked. Further, when one of the wavelengths λ1 and λ2 is close to the reference wavelength λ0 (the frequency difference is smaller than the detection frequency f _FD ), one of the laser beams is compared with the conventional offset lock method (see Non-Patent Document 2). Similarly, it is possible to offset-lock directly with the reference light L0, or offset-lock with the carrier in the modulated light L0 _mod . Further, the DIAL device 10 may also include the sideband wave generator 4 in each of the laser light sources 12 and 13 to generate sideband waves at individual frequency intervals in the common reference light L0. In other words, the DIAL device 10 may be configured to have one wavelength control device 20 for each of the laser light sources 12 and 13 .

[Observation method by DIAL device]
Once both the laser beams L1 and L2 are locked to their 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 pulsed laser amplifier 7 is activated, and the laser beams L1 and L2 alternately output from the optical switch 17 are _{used as the pulsed beam Lpls .} emitted into the atmosphere from Scattered light from fine particles such as atmospheric molecules and aerosols in the atmosphere caused by the pulsed light L _pls is received by the telescope 8 as pulsed light L _pls '. The signal processing/observation control device 9 heterodyne-detects the pulsed light L _pls ' with the laser light L1, L2 for each wavelength λ1, λ2, and compares the electrical signals from the light of each wavelength to determine the water vapor in the atmosphere. Measure the concentration of In addition, the DIAL device 10 can measure the distribution of aerosols in the atmosphere from the intensity of the electrical signal converted from the received pulsed light L _pls ′ using only the laser light L2. The wind speed in the ray direction of the pulsed light _Lpls can be measured.

The embodiments for implementing the wavelength control device, the wavelength control method, and the differential absorption LIDAR device according to the present invention have been described above, but the present invention is not limited to these embodiments. Various changes are possible within the range shown in .

10 DIAL device (differential absorption lidar device)
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 controller 21, 22 optical demultiplexer 23, 24 optical multiplexer 29 mirror 3 wavelength stabilizer 31 local oscillator 32 shifter phase detector 33 optical modulator 34 gas cell 35 photodetector 36 frequency mixer 37 low frequency filter 38 proportional product derivative (PID) controller 4 sideband wave generator (sideband wave generating means)
41 local oscillator 42 amplifier 43 optical modulator 51, 52 photodetector 6 wavelength corrector (wavelength corrector)
61 local oscillator 62 frequency divider 63 phase comparator 64 loop filter 65 proportional integral derivative (PID) controller 7 pulse laser amplifier 8 telescope (transmitting and receiving optical system)
9 Signal processing/observation control device S21 sideband wave generation step S22 photodetection step S23 wavelength correction step S25 laser light source parameter change step S31 sideband wave generation step S32 photodetection step S34 wavelength correction step S36 laser light source parameter change step

Claims (5)

wavelength correction means for outputting an error signal to a laser light source and irradiating laser light having a wavelength based on the error signal;
sideband wave generating means for modulating a reference laser beam irradiated by a reference laser light source and outputting modulated light in which one or more sideband waves are generated in the reference laser beam;
a photodetector receiving the light obtained by combining the modulated light and the laser light and outputting an electric signal having the same frequency as the frequency difference between the modulated light and the laser light;
Based on the electrical signal, the wavelength correction means adjusts the wavelength of the laser light so that the frequency difference between the laser light and one sideband wave in the modulated light whose wavelength is closest to the laser light is a predetermined value. A wavelength controller that generates the error signal that shifts the 2. The wavelength control device according to claim 1, wherein the sideband wave generating means generates sideband waves at frequency intervals larger than twice the detectable frequency of the photodetector. further comprising a wavelength stabilizing device for controlling the wavelength of the reference laser light with respect to the reference laser light source;
2. The wavelength stabilizing device comprises a gas cell or an etalon element, and controls the reference laser light to a central wavelength of an absorption line of gas molecules enclosed in the gas cell or a resonance wavelength of the etalon element. 3. The wavelength control device according to 2. A first laser light source and a second laser light source for irradiating laser light having different wavelengths as seed light, the wavelength control device according to any one of claims 1 to 3, and the reference laser light for irradiation. with a reference laser source that
The wavelength control device is a differential absorption lidar device in which the photodetector and the wavelength correction means are provided in the first laser light source and the second laser light source, respectively, to control the wavelength of the laser light. A wavelength control method for controlling a laser beam emitted by a laser light source to have a wavelength with a predetermined difference from the wavelength of a reference laser beam,
a sideband wave generating step of modulating the reference laser light irradiated by a reference laser light source to output modulated light in which one or more sideband waves are generated in the reference laser light;
a photodetection step of outputting an electrical signal containing the same frequency as the frequency difference between the modulated light and the laser light from the modulated light and the laser light;
An error signal is generated from the electrical signal and input to the laser light source, and the laser light is adjusted so that the frequency difference between the modulated light and one sideband wave closest in wavelength to the laser light is a predetermined value. and a wavelength correction step of adjusting the laser light source to a wavelength based on the error signal.

Images