JP5340129B2 - Differential absorption lidar device - Google Patents

Description

  The present invention transmits laser light having a plurality of different wavelengths into a space to receive scattered light from a reflector, and determines a gas (for example, methane) to be measured in the space from the ratio of received light intensity for each wavelength. Or the like).

  As a conventional differential absorption lidar apparatus of this type, the following is known (for example, see Patent Document 1). The differential absorption lidar apparatus in this Patent Document 1 applies intensity modulation to laser light of two wavelengths at different frequencies, and then transmits the laser light into space, receives scattered light from the ground as a hard target, and receives the received light. Direct detection. Further, the frequency spectrum of the signal obtained by direct detection is obtained, the two wavelengths are separated from the signal intensities of the two modulation frequency components generated in the frequency spectrum, and the received light is detected for these two wavelengths. Yes.

  At this time, one of the two wavelengths is set to a wavelength having a large light absorption coefficient for the gas to be measured, and the other one wavelength is set to a wavelength having a small light absorption coefficient. By setting in this way, a ratio corresponding to the gas concentration to be measured between the apparatus and the target is generated in the received light quantity, and the gas concentration is measured from this ratio.

  Compared with other differential absorption lidars, this type of differential absorption lidar apparatus described in Patent Document 1 is superior in that it can simultaneously transmit and receive two wavelengths and separate two wavelengths in reception. There is. Specifically, it is effective in improving high accuracy and real-time measurement in gas concentration measurement. This high accuracy and real-time measurement property has been specifically demonstrated with respect to the measurement of carbon dioxide concentration (for example, see Non-Patent Document 1).

  However, the conventional differential absorption lidar apparatus has the following problems when applied to various gas concentration measurements other than carbon dioxide, which is the measurement target in Patent Document 1, in particular.

  In general, the differential absorption lidar apparatus has a strong light absorption coefficient at at least one of the wavelengths to be transmitted and received, and it is necessary for light absorption to occur significantly. Light in the vicinity of a light absorption line having a large light absorption coefficient is required. Need to send and receive. From this point of view, the conventional device disclosed in Patent Document 1 transmits and receives the output wavelength of the light source as it is, and therefore, it is necessary to set the output wavelength of the light source in the vicinity of the light absorption line having a large light absorption coefficient.

  However, the light absorption line is unique to the gas to be measured, and the output wavelength range of the light source that can be used does not necessarily cover the vicinity of the light absorption line of the gas to be measured. For example, in the case of methane gas, it is known that there is a light absorption line having a large light absorption coefficient in the wavelength 3 μm band, but there is currently no light source that can directly output this wavelength.

  On the other hand, as a means for outputting a wavelength in the 3 μm band, a method using a wavelength converter is known, and there is a technique for measuring methane gas using this wavelength converter (for example, see Non-Patent Document 2).

JP 2007-248126 A

S. Kameyama et al., Development of 1.6 micron CW modulation ground-based DIAL system for CO2 monitoring, Proceedings of SPIE, Vol. 7153, 71530L-1, 2008 M. Imaki et al., Infrared frequency upconverter for high-sensitivity imaging of gas plumes, Optics Letters, Vol. 32, 1923-1925, 2007

However, the prior art has the following problems.
The conventional apparatus disclosed in Non-Patent Document 2 is not configured to transmit and receive two wavelengths simultaneously. For this reason, in particular, when laser concentration is transmitted to and received from the surface of the earth while moving at high speed like a satellite or an aircraft, and the gas concentration measurement between the device and the earth is performed, the measurement space between the two wavelengths becomes different, and the measurement accuracy There was a problem that decreased.

  Further, simply incorporating the wavelength conversion function of Non-Patent Document 2 into the configuration of Patent Document 1 has the following problems. The differential absorption lidar apparatus needs to accurately lock the wavelength of the laser beam to be transmitted and received to a predetermined value. In Patent Document 1, this wavelength lock is performed on the wavelength of the light source. However, when the wavelength conversion function is used, it is necessary to lock the wavelength after wavelength conversion, and the conventional apparatus does not have a function of locking the wavelength at the stage after such wavelength conversion.

  The present invention has been made to solve the above-described problems, and maintains the high-accuracy and real-time measurement performance by simultaneous transmission and reception of two wavelengths, and the light absorption line suitable for measurement and the output wavelength range of the light source. It is an object of the present invention to obtain a differential absorption lidar apparatus that can measure a gas whose wavelength does not match.

  The differential absorption lidar apparatus according to the present invention includes a plurality of laser light generating means for generating laser beams having a plurality of different wavelengths, and a plurality of different frequencies for each wavelength component of the laser light having a plurality of wavelengths. A light intensity modulating means for performing intensity modulation with the CW signal, an optical combining means for combining laser light of each wavelength component after intensity modulation by the light intensity modulating means, and a wavelength of the laser light after combining by the optical combining means Wavelength converting means for converting, light distributing means for distributing the light after wavelength conversion by the wavelength converting means to monitor light and transmission light, transmission optical means for transmitting transmission light toward a target existing in space, Receiving light receiving means for receiving scattered light from a target as received light and converting it into an electrical signal as a received signal by direct detection, and a gas-filled cell of the same type as the gas to be measured After passing through the monitor light, the monitoring light receiving means for converting it into an electrical signal as a monitor signal by direct detection and the plurality of frequency components included in the monitor signal are separated into the respective frequency components of the plurality of CW signals. A frequency dividing means for monitoring that calculates the corresponding signal strength and phase and a plurality of frequency components included in the received signal are classified to calculate the signal strength and phase corresponding to each frequency component of the plurality of CW signals. A frequency control means for receiving, a wavelength control means for controlling each wavelength of the laser light outputted from the plurality of laser light generating means based on the signal intensity calculated by the frequency separating means for monitoring, and a frequency sorting for monitoring Based on the signal strength and phase calculated by the means, and the signal strength and phase calculated by the receiving frequency classification means Device - in which a signal processing means for measuring the concentration of a gas to be measured between the target.

  According to the differential absorption lidar apparatus of the present invention, the laser light near the light absorption line of the gas to be measured is generated and transmitted / received simultaneously at two wavelengths, and further, the laser light after wavelength conversion is monitored by 2 Gas that does not match the output wavelength range of the light source and the wavelength of the light absorption line suitable for measurement, while maintaining high accuracy and real-time measurement by simultaneous transmission and reception of two wavelengths by providing a configuration that locks the wavelength to a predetermined wavelength Can be obtained as a measurement target.

It is a schematic diagram which shows the structure of the differential absorption lidar apparatus concerning Embodiment 1 of this invention. It is explanatory drawing which shows the relationship between the light absorption amount and use wavelength in the differential absorption lidar apparatus concerning Embodiment 1 of this invention. It is a figure which shows the structural example of the wavelength converter in Embodiment 1 of this invention. It is a figure which shows the structural example of the frequency separator for a monitor in Embodiment 1 of this invention, or the frequency separator for a reception. It is a schematic diagram which shows the structure of the differential absorption lidar apparatus concerning Embodiment 2 of this invention. It is a figure which shows the structural example of the frequency separator for a monitor in Embodiment 2 of this invention, or the frequency separator for a reception.

  Hereinafter, a preferred embodiment of a differential absorption lidar apparatus of the present invention will be described with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing the configuration of a differential absorption lidar apparatus according to Embodiment 1 of the present invention. The differential absorption lidar apparatus in the first embodiment includes a light source 1, a light intensity modulator 2, a modulation signal generator 3, an optical multiplexer 4, a wavelength converter 5, an optical distributor 6, a gas-filled cell 7, and monitoring light. A receiver 8, a monitor frequency separator 9, a light source control circuit 10, a transmission optical system 11, a reception optical system 12, a reception optical receiver 13, a reception frequency separator 14, and a signal processor 15 are provided. 1 illustrates a case where the light source 1, the light intensity modulator 2, and the modulation signal generator 3 are each composed of two, and suffixes a and b are given.

In FIG. 1, the following constituent elements are all connected by an optical circuit line, for example, an optical fiber.
Between the light source 1a and the light intensity modulator 2a Between the light source 1b and the light intensity modulator 2b Between the light intensity modulator 2a and the optical multiplexer 4 With the light intensity modulator 2b and the optical multiplexer 4 Between the optical multiplexer 4 and the wavelength converter 5 Between the wavelength converter 5 and the optical distributor 6 Between the optical distributor 6 and the gas-filled cell 7 The gas-filled cell 7 and the monitoring light Between the receiver 8. Between the optical distributor 6 and the transmitting optical system 11. Between the receiving optical system 12 and the receiving optical receiver 13.

Moreover, in FIG. 1, all the components shown below are connected by the electric wire cable.
Between the modulation signal generator 3a and the light intensity modulator 2a Between the modulation signal generator 3b and the light intensity modulator 2b Between the monitoring optical receiver 8 and the monitoring frequency separator 9 For monitoring Between the frequency separator 9 and the light source control circuit 10 Between the receiving optical receiver 13 and the receiving frequency separator 14 Between the receiving frequency separator 14 and the signal processor 15 The monitoring frequency separator 9 and the signal processor 15

  The light sources 1a and 1b have a function of transmitting CW laser light, and the light wavelengths to be transmitted differ between the light sources 1a and 1b. These different wavelengths are each converted by the wavelength converter 5. Here, as the wavelength after wavelength conversion, the wavelength after wavelength conversion related to transmission from the light source 1a is λON, and the wavelength after wavelength conversion related to transmission from the light source 1b is λOFF.

  At this time, λON is set to a wavelength having a large light absorption coefficient for the gas to be measured. On the other hand, λOFF is set to a wavelength with a small light absorption coefficient. FIG. 2 is an explanatory diagram showing the relationship between the amount of light absorption and the wavelength used in the differential absorption lidar apparatus according to Embodiment 1 of the present invention. In FIG. 2, λON is set at the center of the light absorption characteristic, but is not particularly limited to the center of the light absorption characteristic as long as the light absorption is larger than λOFF.

  The modulation signal generators 3a and 3b have a function of generating CW signals having two different frequencies fm1 and fm2. The light intensity modulator 2a. 2b has a function of applying intensity modulation to the laser beams from the light sources 1a and 1b using these CW signals.

  The optical multiplexer 4 has a function of combining the intensity-modulated laser beams from the light intensity modulators 2 a and 2 b and sending them to the wavelength converter 5. The wavelength converter 5 has a function of converting the wavelengths of the two laser beams combined from the optical multiplexer 4 and then sending them to the optical distributor 6. The light distributor 6 has a function of distributing the inputted laser light, sending one to the gas filled cell 7 as monitor light, and sending the other to the transmission optical system 11 as transmission light.

  The transmission optical system 11 has a function of simultaneously transmitting two wavelengths of laser light toward a target (a hard target such as the ground surface in FIG. 1) after shaping the input laser light into a predetermined beam shape. Yes.

  The gas-filled cell 7 is filled with a gas of the same type as the gas to be measured, for example, a gas such as methane, and transmits the input light to the gas and then sends it to the monitoring optical receiver 8. have. The monitoring optical receiver 8 has a function of converting the input laser beam into an electrical signal as a monitor signal by direct detection and sending it to the monitoring frequency separator 9. The monitor frequency discriminator 9 has a function of separating and extracting components of the modulation frequencies fm1 and fm2 from the input monitor signal and sending both components of fm1 and fm2 to the light source control circuit 10 and the signal processor 15. .

  The receiving optical system 12 has a function of receiving scattered light from the target as received light, coupling it to an optical circuit (for example, an optical fiber) of the receiving system, and sending this received light to the receiving optical receiver 13. Yes. The receiving optical receiver 13 has a function of converting the inputted laser light into an electric signal as a received signal by direct detection and sending it to the receiving frequency classifier 14. The reception frequency separator 14 has a function of separating and extracting components of the modulation frequencies fm1 and fm2 from the input reception signal and sending both components to the signal processor 15.

  The light source control circuit 10 has a function of controlling the wavelengths of the light sources 1a and 1b based on the input signal strengths of the frequencies fm1 and fm2. The signal processor 15 receives the signal strength and phase of the components of the frequencies fm1 and fm2 in the monitor signal and the received signal input from the monitor frequency separator 9 and the reception frequency separator 14, respectively, between the device and the target. It has a function to calculate the concentration of the gas to be measured.

  Next, a series of operations of the differential absorption lidar apparatus in the first embodiment shown in FIG. 1 will be described. First, CW laser light is transmitted from the light sources 1a and 1b. On the other hand, CW signals having two different frequencies fm1 and fm2 are output from the modulation signal generators 3a and 3b, and sent to the light intensity modulators 2a and 2b, respectively. Each of the light intensity modulators 2a and 2b modulates the intensity of the CW laser light from the light sources 1a and 1b based on the modulation signals from the modulation signal generators 3a and 3b.

  Next, the optical multiplexer 4 combines the laser beams whose intensity is modulated by the optical intensity modulators 2a and 2b. Further, the wavelength converter 5 performs wavelength conversion of two wavelengths simultaneously on each of the laser beams from the light sources 1a and 1b. The laser light after wavelength conversion is sent to the optical distributor 6. Then, the light distributor 6 sends one of the distributed laser lights to the gas-filled cell 7 as monitor light, and sends the other one to the transmission optical system 11 as transmission light.

  First, the monitor light is passed through the gas filled cell 7. Here, the component from the light source 1a and the component from the light source 1b are superimposed on the wavelength of the monitor light. If these wavelength components are in the vicinity of the center of the light absorption line of the gas sealed in the gas-filled cell 7, they are absorbed and attenuated in the gas-filled cell 7, but deviated greatly from the light absorption line. If so, it is output without any attenuation.

  Next, the monitoring optical receiver 8 directly detects the output from the gas filled cell 7 and converts it into an electrical signal as a monitor signal. Since the intensity waveform of the laser beam becomes an electric signal waveform by this direct detection, the monitor signal includes a component of frequency fm1 and a component of fm2, which are modulation frequencies. In other words, the component from the light source 1a and the component from the light source 1b are simultaneously superimposed on the monitor signal, but can be separated because the modulation frequency is different. Therefore, the monitoring frequency separator 9 separates the two superimposed frequencies and sends them to the light source control circuit 10 and the signal processor 15.

  Next, the light source control circuit 10 feedback-controls the wavelengths of the light sources 1a and 1b based on the input signal strengths of the components of the frequencies fm1 and fm2. At this time, the light source control circuit 10 applies feedback control to the light source 1a so as to reduce the signal intensity of the component of the frequency fm1. As a result, the wavelength-converted component of the output from the light source 1a is locked to the wavelength at which the attenuation of the laser light in the gas-filled cell 7 is greatest, that is, the ON wavelength.

  On the other hand, the light source control circuit 10 applies feedback control to the light source 1b so as to increase the signal intensity of the component of the frequency fm2. As a result, the wavelength-converted component of the output from the light source 1b is locked to the wavelength with the smallest attenuation of the laser light in the gas-filled cell 7, that is, the OFF wavelength. By this operation, the two light wavelengths after wavelength conversion are accurately locked to the ON wavelength and the OFF wavelength.

  In such monitor signal processing, the transmission light transmitted to the transmission optical system 11 is formed into a predetermined beam shape, and then simultaneously transmitted to the target for two wavelengths. The transmitted light is scattered by the target, and the scattered light is received simultaneously by the receiving optical system 12 as received light at two wavelengths.

  When there is almost no gas to be measured in the space between the apparatus and the target, there is almost no influence of light absorption on the propagation process in the space of the ON wavelength and the OFF wavelength. Accordingly, the ratio of the intensity of the two wavelength components included in the transmission light from the transmission optical system is substantially the same as the ratio of the intensity of the two wavelength components included in the reception light.

  On the other hand, when the gas to be measured exists in the space between the device and the target, there is an influence of light absorption that the two wavelengths receive in the propagation process in the space, and the ON wavelength component is between the device and the target. Received after being relatively attenuated depending on the concentration of the gas to be measured.

  The received light is coupled to the optical circuit of the receiving system, then directly detected by the receiving optical receiver 13, and converted into an electrical signal as a received signal. Since the intensity waveform of the laser beam becomes an electric signal waveform by such direct detection, the received signal includes a component of frequency fm1 and a component of fm2, which are modulation frequencies, as in the monitor signal. That is, the component from the light source 1a and the component from the light source 1b are simultaneously superimposed on the received signal, but can be separated because the modulation frequency is different. Therefore, the reception frequency separator 14 separates the two superimposed frequencies and sends each to the signal processor 15.

  Next, the signal processor 15 calculates the signal intensity and phase of the components of the frequencies fm1 and fm2 in the monitor signal and the received signal, which are input from the monitor frequency separator 9 and the reception frequency separator 14, from the device to the target. The concentration of the gas to be measured in is calculated.

In calculating the gas concentration, first, the distance between the apparatus and the target is calculated. In this distance calculation, the phase difference between the frequency fm2 component from the monitoring frequency discriminator 9 and the frequency fm2 component from the receiving frequency discriminator 14 is obtained by phase detection of both signals. Specifically, the phase difference is Δφ, the speed of light is c, and n is an integer equal to or greater than 1, and is calculated by the following equation (1).
L = n × c × Δφ / (2π × 2 × fm2) (1)

  In the above equation (1), the integer n can be known as long as there is rough foresight information about the device-target. For example, if 100 kHz is used for the modulation frequency fm2, the round trip distance corresponding to one period of this modulation frequency is 1.5 km. Therefore, it is only necessary to grasp how many times the distance between the apparatus and the target is 1.5 km, and this grasp is possible in the case of normal measurement.

  Next, the amount of light absorption by the gas to be measured between the apparatus and the target is calculated. This light absorption amount calculation is obtained by using the signal intensities of the components of the frequencies fm1 and fm2 from the monitoring frequency separator 9 and the components of the frequencies fm1 and fm2 from the receiving frequency separator 14.

  Specifically, the signal intensities of the frequencies fm1 and fm2 from the monitoring frequency discriminator 9 are Im1 and Im2, respectively, and the signal intensities of the frequencies fm1 and fm2 from the receiving frequency discriminator 14 are Is1 and The light absorption amount is calculated by the following formula (2), where Is2 is T and the transmittance of the ON wavelength in the gas-filled cell 7 is T.

  The ON wavelength transmittance T in the gas-filled cell 7 can be known if the concentration and light absorption characteristics of the gas sealed in the gas-filled cell 7 are known. The reason why the square root is taken in the parentheses in the above equation (2) is that the intensity of the electric signal obtained by direct detection is proportional to the square of the signal intensity of the laser beam. By this calculation, the power ratio between the ON wavelength and the OFF wavelength of the transmission light inside the apparatus is canceled, and the light absorption amount only between the apparatus and the target can be calculated.

  By the procedure described above, the signal processor 15 calculates the distance between the apparatus and the target and the light absorption amount, and the gas concentration between the apparatus and the target is calculated from the distance and the light absorption amount.

  The differential absorption lidar apparatus according to the first embodiment realizes a wavelength conversion function while maintaining a two-wavelength simultaneous transmission / reception function, which was difficult with the conventional differential absorption lidar apparatus described in Patent Document 1, Furthermore, the wavelength of the laser beam after wavelength conversion can be accurately locked to a predetermined value. For this reason, it is possible to measure the concentration of gas that could not be measured in the past while performing simultaneous transmission and reception of two wavelengths, and improvement in measurement accuracy can be realized. Specifically, as shown in Non-Patent Document 2, it is possible to measure methane gas with high accuracy by converting laser light from a light source having a wavelength of 1.5 μm into a 3 μm band.

  In the differential absorption lidar apparatus according to the first embodiment, the ON wavelength has been described as being locked to the center of the light absorption line. However, the wavelength to be locked does not have to be the center as long as it has a light absorption significantly greater than the OFF wavelength near the center of the light absorption line. If the light absorption characteristics of the gas sealed in the gas-filled cell 7 are grasped, the ON wavelength is set near the light absorption line by controlling the light source 1a so that the transmittance in the gas-filled cell 7 becomes a predetermined value. It is possible to lock to a predetermined wavelength.

  Further, the differential absorption lidar apparatus according to the first embodiment requires the wavelength converter 5, and this configuration will be specifically described with reference to the drawings. FIG. 3 is a diagram illustrating a configuration example of the wavelength converter 5 according to the first embodiment of the present invention. The wavelength converter 5 shown in FIG. 3 includes a wavelength conversion light source 51, an optical multiplexer 52, a wavelength conversion element 53, and an optical filter 54.

  The optical multiplexer 52 combines and outputs the laser light from the wavelength conversion light source 51 and each of the two-wavelength laser lights input from the outside. Next, the wavelength conversion element 53 wavelength converts the output signal from the optical multiplexer 52. Then, the optical filter 54 at the final stage removes the wavelength component other than the desired component generated in the wavelength conversion, and then outputs it.

  With the configuration of FIG. 3, for example, the output wavelength of the light sources 1a and 1b is set to 1.5 μm, the wavelength of the wavelength conversion light source 51 is set to 1 μm, and PPMgLN is used for the wavelength conversion element 53. It becomes possible to generate the laser beam.

  Further, in the differential absorption lidar apparatus according to the first embodiment, the monitor frequency separator 9 and the reception frequency separator 14 are necessary. This configuration will be specifically described with reference to the drawings. FIG. 4 is a diagram illustrating a configuration example of the monitoring frequency separator 9 or the receiving frequency separator 14 according to Embodiment 1 of the present invention. Note that the monitoring frequency separator 9 and the receiving frequency separator 14 have the same configuration, and in the following description, the internal configuration of the monitoring frequency separator 9 will be described in detail.

  The monitoring frequency separator 9 shown in FIG. 4 includes an AD conversion unit 91, an FFT calculation unit 92, and a phase / intensity extraction unit 93. The AD conversion unit 91 performs AD conversion on the monitor signal, for example. Next, the FFT operation unit 92 obtains a frequency spectrum with a complex amplitude by performing FFT processing on the signal after AD conversion. This frequency spectrum has peaks at the positions of two modulation frequencies fm1 and fm2 corresponding to two wavelengths. Then, the phase / intensity extraction unit 93 in the final stage can obtain the intensity at this peak from the square value of the complex amplitude, and can obtain the phase at this peak from the phase of the complex amplitude.

  As described above, according to the first embodiment, the laser light in the vicinity of the light absorption line of the gas to be measured is generated, the two wavelengths are simultaneously transmitted and received, and the laser light after wavelength conversion is monitored. In this configuration, the two wavelengths are locked to a predetermined wavelength. As a result, a differential absorption lidar capable of measuring a gas whose output wavelength range of the light source does not match the wavelength of the light absorption line suitable for measurement while maintaining high accuracy and real-time measurement by simultaneous transmission and reception of two wavelengths. A device can be obtained.

Embodiment 2. FIG.
FIG. 5 is a schematic diagram showing the configuration of the differential absorption lidar apparatus according to the second embodiment of the present invention. The differential absorption lidar apparatus according to the second embodiment includes a light source 1, a light intensity modulator 2, a modulation signal generator 3, an optical multiplexer 4, a wavelength converter 5, an optical distributor 6, a gas-filled cell 7, and monitoring light. Receiver 8, monitor frequency separator 9, light source control circuit 10, transmission optical system 11, reception optical system 12, reception optical receiver 13, reception frequency separator 14, signal processor 15, and electrical signal distributor 16 is provided. FIG. 5 illustrates a case where the light source 1, the light intensity modulator 2, the modulation signal generator 3, and the electric signal distributor 16 are each composed of two, and the subscripts a and b are It is attached.

  The configuration of FIG. 5 in the second embodiment is different from the configuration of FIG. 1 in the first embodiment in that it further includes electric signal distributors 16a and 16b. Further, as will be described later with reference to FIG. 6, the internal configurations of the monitoring frequency separator 9 and the receiving frequency separator 14 in the second embodiment are different from the internal configuration shown in the first embodiment. Yes. Regarding the operation other than the frequency separation of the monitor signal and the reception signal, the monitor frequency separator 9 and the reception frequency separator 14 are the same as those in the first embodiment. Therefore, the following description will focus on these different configurations and operations.

  In FIG. 5, the modulation signals from the modulation signal generators 3a and 3b are distributed into three by the electric signal distributors 16a and 16b, respectively. The three signals distributed by the electric signal distributor 16a are connected to the light intensity modulator 2a, the monitor frequency separator 9, and the reception frequency separator 14, respectively. Similarly, the three signals distributed by the electric signal distributor 16b are connected to the light intensity modulator 2b, the monitor frequency separator 9, and the reception frequency separator 14, respectively.

  FIG. 6 is a diagram illustrating a configuration example of the monitoring frequency separator 9 or the receiving frequency separator 14 according to Embodiment 2 of the present invention. Note that the monitoring frequency separator 9 and the receiving frequency separator 14 have the same configuration, and in the following description, the internal configuration of the monitoring frequency separator 9 will be described in detail.

  The monitoring frequency separator 9 shown in FIG. 6 includes three 0 degree electrical signal distributors 94a to 94c, three 90 degree electrical signal distributors 95a to 95c, four mixers 96a to 96d, and four low-pass filters 97a to 97a. 97d, and one phase / intensity extraction unit 93. Here, the three 0 degree electrical signal distributors 94a to 94c, the three 90 degree electrical signal distributors 95a to 95c, the four mixers 96a to 96d, and the four low-pass filters 97a to 97d correspond to a lock-in detection unit. To do.

  In FIG. 6, the monitor signal from the monitoring optical receiver 8 or the reception signal from the receiving optical receiver 13 is divided into two by the 0 degree electrical signal distributor 94a and then further to the 0 degree electrical signal distributor 94b. , 94c. 6, lock-in detection is performed at the frequencies fm1 and fm2, and a complex amplitude signal having two frequencies is extracted as an IQ signal from the monitor signal or the received signal, and is output from the four low-pass filters 97a to 97d. Is done. The phase / intensity extraction unit 93 can obtain the intensities / phases of the components of the frequencies fm1 and fm2 from the IQ signals that are the outputs of the low-pass filters 97a to 97d.

  As described above, the differential absorption lidar apparatus according to the second embodiment performs a complicated digital calculation like the FFT described in the first embodiment by adopting the lock-in detection method for the frequency separation function. There is no need, and a frequency separation function can be realized. Therefore, it becomes possible to control the wavelength of the light source and measure the gas concentration in real time.

  As described above, according to the second embodiment, the laser light in the vicinity of the light absorption line of the gas to be measured is generated, the two wavelengths are simultaneously transmitted and received, and the laser light after wavelength conversion is monitored. In this configuration, the two wavelengths are locked to a predetermined wavelength. As a result, a differential absorption lidar capable of measuring a gas whose output wavelength range of the light source does not match the wavelength of the light absorption line suitable for measurement while maintaining high accuracy and real-time measurement by simultaneous transmission and reception of two wavelengths. A device can be obtained.

  Furthermore, since it is not necessary to perform complicated digital calculation like FFT and a frequency separation function can be realized, it is possible to control the wavelength of the light source and measure the gas concentration in real time.

  1, 1a, 1b Light source, 2, 2a, 2b Light intensity modulator, 3, 3a, 3b Modulation signal generator, 4 Optical multiplexer, 5 Wavelength converter, 6 Optical distributor, 7 Gas-filled cell, 8 For monitor Optical receiver, 9 Monitor frequency separator, 10 Light source control circuit (wavelength control means), 11 Transmit optical system, 12 Receive optical system, 13 Receive optical receiver, 14 Receive frequency separator, 15 Signal processor ( Wavelength control means), 16, 16a, 16b Electric signal distributor, 51 Light source for wavelength conversion, 52 Optical multiplexer, 53 Wavelength converter, 54 Optical filter, 91 AD converter, 92 FFT calculator, 93 Phase / intensity extraction 94a-94c 0 degree electrical signal distributor, 94a-95c 90 degree electrical signal distributor, 96a-96d mixer, 97a-97d low pass filter.

Claims (4)

A plurality of laser beam generating means for generating laser beams each having a plurality of different wavelengths;
A light intensity modulating means for performing intensity modulation with a plurality of CW signals having different frequencies for each wavelength component of the laser light having a plurality of wavelengths;
Optical multiplexing means for multiplexing laser light of each wavelength component after intensity modulation by the light intensity modulation means;
Wavelength converting means for converting the wavelength of the laser light after being combined by the optical combining means;
Light distribution means for distributing the light after wavelength conversion by the wavelength conversion means to monitor light and transmission light;
Transmission optical means for transmitting the transmission light toward a target existing in space;
Receiving light receiving means for receiving scattered light from the target as received light and converting it into an electric signal as a received signal by direct detection; and
A monitoring light receiving means for passing the monitoring light through a gas-filled cell of the same type as the gas to be measured and then converting it into an electrical signal as a monitoring signal by direct detection;
A frequency separation means for monitoring for calculating a signal intensity and a phase corresponding to each frequency component of the plurality of CW signals by separating a plurality of frequency components included in the monitor signal;
Receiving frequency separation means for calculating a signal intensity and a phase corresponding to each frequency component of the plurality of CW signals by separating a plurality of frequency components included in the received signal;
Wavelength control means for controlling each wavelength of laser light output from the plurality of laser light generating means based on the signal intensity calculated by the monitoring frequency sorting means;
Based on the signal intensity and phase calculated by the monitoring frequency classification means and the signal intensity and phase calculated by the reception frequency classification means, the concentration of the gas to be measured between the apparatus and the target is measured. A differential absorption lidar apparatus comprising: a signal processing means. The differential absorption lidar apparatus according to claim 1,
The wavelength converting means is
A light source that outputs laser light for wavelength conversion;
Second optical multiplexing means for combining the laser light after being combined by the optical combining means and the laser light output from the light source;
A wavelength conversion element for wavelength-converting an optical signal after being combined by the second optical combining means;
A differential absorption lidar apparatus comprising: an optical filter that outputs an optical signal after wavelength conversion in which wavelength components other than a desired component generated in the wavelength conversion are removed. In the differential absorption lidar apparatus according to claim 1 or 2,
Each of the monitoring frequency sorting means and the receiving frequency sorting means is:
An AD conversion unit that performs AD conversion on an input electric signal;
An FFT calculation unit that calculates a frequency spectrum as a complex amplitude by performing FFT processing on the signal after AD conversion;
A differential absorption lidar apparatus comprising: a phase / intensity extraction unit that obtains a signal intensity at a peak from the square value of the complex amplitude and obtains a phase at the peak from the phase of the complex amplitude. In the differential absorption lidar apparatus according to claim 1 or 2,
Each of the monitoring frequency sorting means and the receiving frequency sorting means is:
A lock-in detection unit that extracts a complex amplitude of each frequency component by performing lock-in detection with each frequency component of the plurality of CW signals with respect to an input electric signal;
A differential absorption lidar apparatus comprising: a phase / intensity extraction unit that obtains a signal intensity at a peak from the square value of the complex amplitude and obtains a phase at the peak from the phase of the complex amplitude.

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