JP2017198536A - Wavelength controller and difference absorption lidar device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength controller which realizes a precise and stable wavelength lock even when the absorption coefficient of an optical absorption medium is small.SOLUTION: A wavelength controller 10 includes: an optical coupler 12 forming a loop optical path; an optical absorption cell 13; and an optical divider 14. A photodetector 15 detects a branch light output from the optical divider 14 and outputs a detection signal DS. A wavelength estimation unit 40 samples the attenuation waveform of the detection signal DS, and estimates the current wavelength of a laser light pulse based on the sampled attenuation waveform. A control signal generator 46 generates a feedback control signal reducing the difference between the estimated current wavelength and a target wavelength.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a technique for controlling the wavelength of laser light, and more particularly, to a technique for controlling the wavelength of laser light used in a lidar (LIDAR: Light Detection And Ranging) apparatus.

  The lidar device irradiates the target to be measured with laser light, observes the scattered light reflected by the target, and based on the observation results, obtains physical information (for example, concentration, temperature, distance or speed) about the target. Widely used as a means of obtaining. As a kind of this kind of lidar apparatus, a differential absorption lidar (DIAL: Differential Absorption Lidar) apparatus that measures the concentration of molecules such as gas or atoms is known. The differential absorption lidar device basically uses a laser beam having a wavelength where the light absorption amount of the target molecule is large and a laser beam having a wavelength where the light absorption amount of the target is small. The concentration of the target can be measured based on the reception intensity ratio of the corresponding scattered light.

  In such a differential absorption lidar apparatus, in order to calculate the target concentration with high accuracy, at least one of the two types of laser light wavelengths described above is matched with a specific wavelength (that is, locked to a specific wavelength). Circuit configuration is required. In particular, for a laser beam having a wavelength with a larger amount of light absorption by the target, it is desirable to accurately lock the wavelength of the laser beam to the center wavelength of the absorption line of the target light. A differential absorption lidar apparatus having a wavelength lock circuit is disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-248126.

JP 2007-248126 A (for example, FIG. 1 and paragraphs 0017 to 0024)

  As a method of locking the wavelength of the laser beam described above, for example, the phase-modulated laser beam is transmitted through a light absorption cell in which a light absorption medium of the same type as the target (for example, a reference gas such as carbon dioxide gas) is sealed. A method of generating an error signal for feedback control based on the detected intensity of the transmitted laser light and further controlling the output wavelength of the laser light source using this error signal is conceivable. However, if the absorption line intensity of the light absorbing medium with respect to the wavelength is small, it is difficult to obtain a highly accurate error signal because the light absorption coefficient of the light absorbing medium with respect to the wavelength is small. In this case, there is a problem that a stable wavelength lock cannot be realized with high accuracy.

  In view of the above, an object of the present invention is to provide a wavelength controller and a differential absorption lidar apparatus that can realize stable wavelength locking with high accuracy even when the absorption coefficient of the light absorption medium is small.

  A wavelength controller according to an aspect of the present invention has a light input terminal to which a series of laser light pulses are input, and includes a light absorption medium that attenuates each laser light pulse input to the light input terminal. An optical branching device that divides the transmitted light pulse output from the light absorption cell into a first branched light and a second branched light, and the second branched light from the light input terminal to the light absorption cell. An optical coupler to be input to the optical detector, a photodetector that converts the first branched light into an electrical signal and outputs the signal, an attenuation waveform of the output signal of the photodetector is sampled, and the sampled attenuation waveform is used as a basis. A wavelength estimation unit that estimates a current wavelength of the laser light pulse, and a control signal generation unit that generates a feedback control signal that reduces a difference between the estimated current wavelength and a target wavelength, A light absorption cell, the optical branching device and It said optical coupler is characterized in that it constitutes a loop optical path for multiple times cycles through each laser beam pulse input to the optical input end.

  The differential absorption lidar apparatus according to another aspect of the present invention irradiates a series of laser light pulses toward an external target, receives scattered light reflected by the target, and determines the concentration of the target based on the light reception result. A differential absorption lidar unit to be calculated; and the wavelength controller to which the series of laser light pulses are input. The differential absorption lidar unit is configured to perform the series according to a feedback control signal generated by the wavelength controller. The wavelength of the laser light pulse is made to coincide with the target wavelength.

  According to the present invention, the current wavelength of the laser light pulse can be estimated with high accuracy based on the attenuation waveform generated by the circulation of the laser light pulse in the loop optical path. Since the feedback control signal is generated using the estimated current wavelength, stable wavelength locking can be realized with high accuracy even when the light absorption coefficient of the light absorbing medium is small.

It is a figure which shows schematic structure of the differential absorption lidar apparatus which is Embodiment 1 which concerns on this invention. It is a graph which shows roughly an example of light absorption amount distribution to a wavelength. FIG. 2 is a functional block diagram schematically illustrating a configuration of a signal processing unit illustrated in FIG. 1. 6 is a diagram illustrating an example of a waveform of a laser light pulse input to the intensity modulator according to the first embodiment. FIG. It is a figure which shows the example of the waveform of the laser beam pulse shortened. 6A and 6B are graphs schematically showing examples of attenuation waveforms of detection signals. 6 is a graph showing three examples of attenuation curves obtained by the fitting unit in the first embodiment. It is a graph which shows the example of the attenuation | damping curve corresponding to an ON wavelength and an OFF wavelength, respectively. It is a graph which shows the example of an attenuation curve. It is a graph which shows the example of the attenuation | damping curve which has a saturation area | region. It is a graph which shows the example of the attenuation | damping curve which has a saturation area | region. FIG. 6 is a diagram schematically showing a part of a configuration of a first modification of the wavelength controller in the first embodiment. 6 is a diagram schematically showing a part of a configuration of a second modification of the wavelength controller in Embodiment 1. FIG. It is a figure which shows schematic structure of the differential absorption lidar apparatus which is Embodiment 2 which concerns on this invention. It is a figure which shows schematic structure of the differential absorption lidar apparatus which is Embodiment 3 which concerns on this invention.

  Hereinafter, various embodiments according to the present invention will be described in detail with reference to the drawings. In addition, the component to which the same code | symbol was attached | subjected in the whole drawing shall have the same structure and the same function.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a schematic configuration of a differential absorption lidar apparatus 1 according to the first embodiment of the present invention. As shown in FIG. 1, the differential absorption lidar apparatus 1 includes a differential absorption lidar unit 20 that irradiates a series of laser light pulses toward a target in an external space and receives scattered light reflected by the target, and the series. And a wavelength controller 10 for performing feedback control (hereinafter also referred to as “wavelength lock”) for matching the wavelength of the laser light pulse with the target wavelength.

The differential absorption lidar unit 20 is a laser beam having a wavelength λ _off (hereinafter also referred to as “off wavelength λ _off ”) in which a light absorption amount of a target to be measured (for example, a gas molecule such as carbon dioxide, ozone, or water vapor) is small. Is irradiated toward the target, and the target is irradiated with laser light having a wavelength λ _on (hereinafter, also referred to as “on wavelength λ _on ”) having a large light absorption amount of the target. FIG. 2 is a graph schematically showing an example of the light absorption amount distribution with respect to the wavelength. The light absorption amount distribution shown in FIG. 2 forms a maximum peak at the ON wavelength λ _on . On the other hand, there is almost no light absorption at the off-wavelength λ _off . The differential absorption lidar unit 20 can measure the concentration of the target by utilizing a phenomenon in which the light absorption amount by the target is different between the on wavelength λ _on and the off wavelength λ _off .

As shown in FIG. 1, the differential absorption lidar unit 20 includes a first laser light source 21A that outputs continuous wave laser light having an off wavelength λ _off to the optical waveguide G1, and a light source driving unit that drives the first laser light source 21A. 22A, a second laser light source 21B that outputs continuous wave laser light having an on wavelength λ _on to the optical waveguide G2, and a light source driving unit 22B that drives the second laser light source 21B. The light source driver 22B matches the output wavelength λ _on of the second laser light source 21B with a predetermined target wavelength Λ _on according to the wavelength control signal CTR supplied from the wavelength controller 10 (in other words, the output wavelength λ _on It also has a function of locking to the target wavelength Λ _on . The target wavelength Λ _on is set to the center wavelength of the absorption line observed in advance. With this function, the output wavelength λ _on of the second laser light source 21B can be tuned to the target wavelength Λ _on . As the first laser light source 21A and the second laser light source 21B, for example, light sources including semiconductor laser diodes capable of continuous oscillation in a single mode may be used. Hereinafter, the continuous wave laser light is referred to as “CW laser light”.

  1 further includes an optical switch 23, an optical branching device 24, an optical modulator 25, an optical branching device 26, an optical amplifier 27, an optical circulator 28, a transmission / reception optical system 29, and an optical multiplexer 31. A balanced receiver 32, an A / D converter (ADC) 33, and a signal processing unit 34.

The optical switch 23 responds to the switching control signal SW supplied from the signal processing unit 34, and the off-wavelength λ _off laser light input from the optical waveguide G1 and the on-wavelength λ _on laser input from the optical waveguide G2. The light is switched to and output to the optical branching device 24. The optical splitter 24 splits the CW laser light input from the optical switch 23 into two branched laser beams, outputs one branched laser beam to the optical waveguide G3, and outputs the other branched laser beam to the optical waveguide G4. To do.

  The optical modulator 25 shifts the frequency of the branched laser beam input from the optical waveguide G3, and then performs pulse modulation on the branched laser beam shifted in frequency to generate and output a series of laser beam pulses. The pulse width of the laser light pulse is controlled to be in the range of several hundred nanoseconds to several thousand nanoseconds, for example. Such an optical modulator 25 can be configured using, for example, an acousto-optic modulator (AOM: Acoustic-Optic Modulator), but is not limited thereto.

  The optical branching unit 26 divides the laser light pulse output from the optical modulator 25 into two and outputs it to the optical waveguides G5 and G6. The laser light pulse output to the optical waveguide G5 is input to the wavelength controller 10. On the other hand, the laser light pulse output to the optical waveguide G6 is amplified by the optical amplifier 27 and then supplied to the transmission / reception optical system 29 via the optical circulator 28 forming the transmission / reception light separation unit. The optical amplifier 27 can be configured using, for example, an optical fiber amplifier such as an EDFA (Erbium Doped Optical Fiber Amplifier). The transmission / reception optical system 29 shapes the laser light pulse input from the optical circulator 28 through the optical waveguide G7 so as to have a predetermined light intensity, beam size, and beam shape, and irradiates the target in the external space. The transmission / reception optical system 29 can be configured by, for example, an optical telescope.

  The scattered light reflected by the target is received by the transmission / reception optical system 29. The optical circulator 28 outputs the received light input from the transmission / reception optical system 29 to the optical waveguide G8. The optical multiplexer 31 combines the CW laser light input from the optical waveguide G4 and the received light input from the optical waveguide G8 to generate a combined optical signal, and this combined optical signal is sent to the balanced receiver 32. Output.

  The balanced receiver 32 is composed of an optical heterodyne detector. The balanced receiver 32 performs heterodyne detection on the combined optical signal input from the optical multiplexer 31 and supplies an analog electric signal indicating the detection result to the A / D converter 33. The A / D converter 33 converts the input analog electric signal into a digital signal and supplies it to the signal processing unit 34.

The signal processing unit 34 receives a digital signal input according to the irradiation of the laser light pulse with the on wavelength λ _on to the target and a digital signal input according to the irradiation of the laser light pulse with the off wavelength λ _off. Based on the above, the concentration of the target can be calculated. For the concentration calculation, a known concentration measurement algorithm based on DIAL (Differential Absorption Lidar) may be used. The signal processing unit 34 also has a control function for supplying a switching control signal SW to the optical switch 23.

  Next, the configuration of the wavelength controller 10 will be described. As shown in FIG. 1, the wavelength controller 10 includes an intensity modulator 11, an optical coupler 12, an optical absorption cell 13, an optical splitter 14, a photodetector 15, and a signal processing unit 16. . FIG. 3 is a functional block diagram showing a schematic configuration of the signal processing unit 16 shown in FIG.

The intensity modulator 11 has a function of shortening a series of laser light pulses input from the optical waveguide G5 (that is, shortening the pulse width of the laser light pulses). FIG. 4 is a diagram illustrating an example of the waveform of a laser light pulse input to the intensity modulator 11. FIG. 4 is a diagram schematically showing the waveform of a laser light pulse having a pulse period T and a pulse width Δτ ₀ . FIG. 5 is a diagram showing an example of the waveform of the laser light pulse obtained by shortening the laser light pulse shown in FIG. The laser light pulse shown in FIG. 5 has a pulse period T and a pulse width Δτ shorter than the pulse width Δτ ₀ . The intensity modulator 11 can convert, for example, a laser light pulse having a pulse width Δτ ₀ within a range of several hundred nanoseconds to several thousand nanoseconds into a laser light pulse having a pulse width Δτ of about several nanoseconds. it can. The intensity modulator 11 can be configured using, for example, a known LN intensity modulator.

  Referring to FIG. 3, the light absorption cell 13 includes a light input terminal to which a series of laser light pulses are input from the intensity modulator 11 via the optical coupler 12, and transmitted light transmitted through the light absorption cell 13. And an optical output terminal for outputting a pulse. The light absorption cell 13 has a region in which a light absorption medium mainly composed of the same kind of gas molecules as the target is enclosed between the light input end and the light output end. The laser light pulse input to the light absorption cell 13 is attenuated in the process of propagating in the light absorption medium.

  The optical branching device 14 divides the transmitted light pulse input from the light emitting end of the light absorption cell 13 into two branched light pulses. One branch light pulse (first branch light pulse) is output to the optical waveguide G11, and the other branch light pulse (second branch light pulse) is output to the optical waveguide G12. The optical waveguide G12 is disposed so as to connect between the optical output end of the optical splitter 14 and the optical input end of the optical coupler 12. The optical coupler 12 is an optical member that optically couples the light output end of the optical waveguide G12 with the light input end of the light absorption cell 13. Therefore, as shown in FIG. 3, the optical coupler 12, the light absorption cell 13, the optical branching device 14, and the optical waveguide G12 constitute a loop optical path.

  Each time one laser light pulse circulates in the loop optical path once, the optical branching device 14 outputs one branched light pulse. Assuming that one laser light pulse can be circulated 1000 times in the optical path of the loop without extinguishing to the end, the optical branching unit 14 has 1000 branched light pulses whose intensity gradually decreases with the passage of time. Will be output continuously.

  The photodetector 15 converts the branched light pulse input from the optical waveguide G11 into an analog electrical signal (hereinafter referred to as “detection signal”) DS, and outputs the detection signal DS to the signal processing unit 16. Here, the pulse period T (FIG. 5) is set to a time length during which one laser light pulse can circulate a plurality of times in the loop optical path and sufficiently attenuate within the pulse period T. Therefore, the detection signal DS shows an attenuation waveform that decays with time for each pulse period T.

The signal processing unit 16 operates when a detection signal DS corresponding to the on-wavelength λ _on to be controlled is input. As shown in FIG. 3, the signal processing unit 16 samples the attenuation waveform of the detection signal DS and estimates the current wavelength of the laser light pulse based on the sampled attenuation waveform, A control signal generator 46 that generates a feedback control signal that reduces the difference between the estimated current wavelength and the target wavelength Λ _on, and D that converts this feedback control signal into a wavelength control signal CTR that is an analog signal A / A converter (DAC) 47 is included. The wavelength estimation unit 40 includes an A / D converter (ADC) 41, an integration processing unit 42, a normalization unit 43, a fitting unit 44, and a wavelength calculation unit 45.

  The A / D converter 41 is a sampling circuit that converts the detection signal DS into a digital signal with a predetermined time resolution. 6A and 6B are graphs schematically showing an example of an attenuation waveform of the detection signal DS. In these graphs, the vertical axis represents the intensity of the detection signal DS, and the horizontal axis represents the time τ counted for each pulse period T. The attenuation waveform shown in FIG. 6A is an aggregate of a large number of pulse waveforms whose intensities are attenuated over time. In the example of FIG. 6A, since the time resolution of the A / D converter 41 is high, the shape of each pulse waveform can be identified. On the other hand, FIG. 6B is a graph showing an attenuation waveform when the time resolution of the A / D converter 41 is lower than in the case of FIG. 6A. In the example of FIG. 6B, since the time resolution is low, individual pulse waveforms cannot be identified.

  Referring to FIG. 3, the integration processing unit 42 has a function of generating an integrated waveform by integrating a plurality of attenuation waveforms sampled by the A / D converter 41. The A / D converter 41 outputs one attenuation waveform for each pulse period T. The integration processing unit 42 can generate integrated waveform data by integrating a plurality of attenuation waveforms continuously input through a plurality of pulse periods. The normalizing unit 43 generates normalized waveform data by normalizing the integrated waveform with the maximum value of the integrated waveform.

  Then, the fitting unit 44 calculates an attenuation curve by fitting the normalized waveform with an attenuation function prepared in advance. The fitting can be performed using a known least square method. For attenuation of light intensity, the insertion loss of the optical coupler constituting the optical branching device 14, the branching ratio of the optical branching device 14, the light absorption coefficient of the light absorbing medium in the light absorbing cell 13, the molar concentration of the light absorbing medium. Since various parameters such as the propagation length of light in the light absorption cell 13 influence the attenuation function suitable for the fitting may be determined in consideration of these parameters. According to Lambert-Beer's law, when light of a certain wavelength passes through gas molecules, the intensity of the light is known to decay exponentially. An attenuation type exponential function can be used as the attenuation function used. However, the present invention is not limited to the attenuation type exponential function. Other types of attenuation functions such as a polynomial function may be used depending on the assumed usage environment of the differential absorption lidar apparatus 1. The attenuation function can be determined in advance by performing an experiment or a computer simulation.

FIG. 7 is a graph showing three examples of attenuation curves obtained by the fitting unit 44. In this graph, the vertical axis indicates the intensity (normalized intensity) of the attenuation curves D1, D2, and D3, and the horizontal axis indicates the time τ counted in pulse cycle units. The maximum value of the normalized strength is “1”. As shown in FIG. 7, the attenuation curves D1, D2, and D3 are exponentially attenuated with the passage of time starting from τ = 0. As the amount of light absorption increases, the attenuation rate of the attenuation curve increases. Therefore, among the three wavelengths corresponding to the attenuation curves D1, D2, and D3, the wavelength with the largest amount of light absorption is the wavelength corresponding to the attenuation curve D1. For example, focusing on the observation times τ ₁ , τ ₂ , and τ ₃ of the attenuation curves D1, D2, and D3 corresponding to the reference value 1 / e (e: the base of natural logarithm), the observation time becomes longer as the amount of light absorption increases. It can be seen that the time approaches τ = 0. FIG. 8 is a graph showing examples of attenuation curves D _on and D _off corresponding to the on wavelength λ _on and the off wavelength λ _off , respectively. As shown in FIG. 8, when attention is paid to the observation times τ _on and τ _off of the attenuation curves D _on and D _off corresponding to the reference value 1 / e, the observation time τ _on is τ = 0 rather than the observation time τ _off. You can see that it is close to the point of time. Further, the attenuation rate of the attenuation curve _{D on} is greater than the attenuation rate of the attenuation curve _{D off.}

Next, the wavelength calculator 45 shown in FIG. 3 determines a wavelength corresponding to the characteristic value of the attenuation curve obtained by the fitting unit 44 by referring to a conversion table prepared in advance or using a conversion function, The current wavelength of the laser light pulse is estimated as the determined wavelength. Here, the conversion table is a lookup table that defines the correspondence between the characteristic value of the attenuation curve and the wavelength. The conversion function calculates a wavelength corresponding to the input value when a characteristic value of the attenuation function is given as the input value. FIG. 9 is a graph for explaining the characteristic values of the three types of attenuation curves D1, D2, and D3. The determination time τ _p shown in FIG. 9 is a time set in advance so that one laser light pulse output from the intensity modulator 11 can circulate a predetermined number of times or more in the loop optical path. The wavelength calculation unit 45 can use the normalized intensity values I ₁ , I ₂ , I ₃ of the attenuation curves D1, D2, D3 at the determination time τ _p as characteristic values. The wavelength calculator 45 can acquire the wavelength corresponding to the characteristic value using a conversion table or conversion function prepared in advance.

The control signal generation unit 46 generates a feedback control signal that reduces the difference between the current wavelength determined by the wavelength calculation unit 45 and the target wavelength Λ _on . In the example of FIG. 9, the wavelength closest to the target wavelength Λ _on among the three wavelengths respectively corresponding to the attenuation curves D1, D2, and D3 is the wavelength corresponding to the attenuation curve D1.

By the way, a part of the integrated waveform generated by the integration processing unit 42 may be saturated. In this case, even if the normalizing unit 43 normalizes the integrated waveform using the maximum intensity value of the saturated portion of the integrated waveform, it is difficult to obtain a normalized waveform suitable for fitting. For this reason, when a part of the integrated waveform is saturated, it is desirable that the normalization unit 43 generates the normalized waveform using the intensity value at the time τ _N avoiding the saturation region. FIG. 10 is a diagram illustrating an example of a normalized waveform generated using the intensity value at time τ _N avoiding the saturation region. In this case, the wavelength calculating unit 45, as shown in FIG. 11, utilizes a time later than the time tau _N as determined time tau _p. Thereby, even if a part of the integrated waveform is saturated, the current wavelength of the laser light pulse can be estimated with high accuracy.

  Such a hardware configuration of the signal processing unit 16 can be realized by an information processing apparatus having a computer configuration with a built-in CPU (Central Processing Unit) such as a workstation or a mainframe. Alternatively, the hardware configuration of the signal processing unit 16 is an LSI (Large Information System) such as a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), or an FPGA (Field-Programmable Gate Array). May be realized.

  Next, a modification of the wavelength controller 10 in the first embodiment will be described. FIG. 12 is a diagram schematically showing a part of the configuration of the first modification of the wavelength controller 10. This first modified example has an optical amplifier 51 arranged in front of the optical coupler 12, and this optical amplifier 51 is provided between the intensity modulator 11 and the optical coupler 12. .

  Since the optical amplifier 51 increases the intensity of the laser light pulse to be input to the light absorption cell 13, the laser light pulse can be circulated in the loop optical path without disappearing as compared with the case without the optical amplifier 51. The number of times increases. As a result, compared with the case where the optical amplifier 51 is not provided, the decay rate of the normalized waveform is reduced, and the period until the normalized waveform decays to near zero intensity from the time point τ = 0 (hereinafter referred to as “attenuation period”). Is longer). Therefore, the normalization unit 43 can generate a normalized waveform that is more suitable for fitting. As a result, the wavelength calculation unit 45 can estimate the current wavelength of the laser light pulse with higher accuracy than when the optical amplifier 51 is not provided. As the optical amplifier 51, an optical fiber amplifier such as EDFA or a semiconductor optical amplifier (SOA) may be used.

  FIG. 13 is a diagram schematically illustrating a part of the configuration of the second modification of the wavelength controller 10 according to the first embodiment. In this second modification, an optical amplifier 52 is provided in an optical waveguide that connects between the input end of the optical coupler 12 and the output end of the optical branching device 14. That is, the optical input end of the optical amplifier 52 is connected to the optical output end of the optical splitter 14 via the optical waveguide G12a, and the optical output end of the optical amplifier 52 is connected to the light of the optical coupler 12 via the optical waveguide G12b. Connected to the input terminal.

  Since the optical amplifier 52 increases the intensity of the laser light pulse that circulates in the loop optical path, the number of times that the laser light pulse can circulate in the loop optical path without extinguishing as compared with the case without the optical amplifier 52 is increased. Become more. As a result, compared with the case without the optical amplifier 52, the decay rate of the normalized waveform is reduced, and the decay period of the normalized waveform is lengthened. Therefore, the normalization unit 43 can generate a normalized waveform that is more suitable for fitting. As a result, the wavelength calculation unit 45 can estimate the current wavelength of the laser light pulse with higher accuracy than when the optical amplifier 52 is not provided. As the optical amplifier 52, an optical fiber amplifier such as an EDFA or a semiconductor optical amplifier may be used.

  As described above, the wavelength controller 10 of the first embodiment is based on the attenuation waveform generated by the circulation of the laser light pulse in the loop optical path even when the absorption coefficient of the light absorption medium in the light absorption cell 13 is small. Thus, the current wavelength of the laser light pulse can be estimated with high accuracy. Therefore, it is possible to realize a stable wavelength lock with high accuracy.

Embodiment 2. FIG.
Next, the differential absorption lidar apparatus 1A according to the second embodiment of the present invention will be described. FIG. 14 is a diagram showing a schematic configuration of the differential absorption lidar apparatus 1A. As shown in FIG. 14, the differential absorption lidar apparatus 1A includes a wavelength controller 10 having the same configuration as the wavelength controller 10 of the first embodiment and a differential absorption lidar unit 20A.

  The configuration of the differential absorption lidar unit 20A of the present embodiment is the same as the configuration of the differential absorption lidar unit 20 of the first embodiment except that the arrangement of the optical branching device 26 and the optical amplifier 27 is different. That is, in the differential absorption lidar unit 20A of the present embodiment, the optical amplifier 27 is interposed between the optical modulator 25 and the optical branching device 26. The optical branching unit 26 divides the laser light pulse output from the optical amplifier 27 into two and outputs it to the optical waveguides G5 and G6a. The laser light pulse output to the optical waveguide G5 is input to the wavelength controller 10. On the other hand, the laser light pulse output to the optical waveguide G 6 a is supplied to the transmission / reception optical system 29 via the optical circulator 28.

  The optical amplifier 27 increases the intensity of the laser light pulse input to the wavelength controller 10. For this reason, compared with the case where the optical amplifier 27 is not provided, the number of times that the laser light pulse can be circulated in the loop optical path of the wavelength controller 10 without annihilation increases. As a result, compared with the case where the optical amplifier 27 is not provided, the attenuation rate of the normalized waveform generated by the normalizing unit 43 (FIG. 3) is reduced, and the attenuation period of the normalized waveform is increased. Therefore, the normalization unit 43 can generate a normalized waveform that is more suitable for fitting. As a result, the wavelength calculation unit 45 can estimate the current wavelength of the laser light pulse with higher accuracy than when the optical amplifier 52 is not provided.

  As in the first embodiment, in the second embodiment, the configuration of the wavelength controller 10 may be modified to have one or both of the optical amplifiers 51 and 52 shown in FIGS. .

Embodiment 3 FIG.
Next, a third embodiment according to the present invention will be described. FIG. 15 is a diagram illustrating a schematic configuration of the differential absorption lidar apparatus 1B according to the third embodiment. The differential absorption lidar apparatus 1B includes a differential absorption lidar unit 20 having the same configuration as the differential absorption lidar unit 20 of the first embodiment and a wavelength controller 10B. The configuration of the wavelength controller 10B is the same as the configuration of the wavelength controller 10 of the first embodiment, except that the signal processing unit 16B of FIG. 15 is provided instead of the signal processing unit 16 of the first embodiment. . Further, the configuration of the signal processing unit 16B is the same as the configuration of the signal processing unit 16 of the first embodiment except that the wavelength estimation unit 40B of FIG. 15 is provided instead of the wavelength estimation unit 40 of the first embodiment. is there. Furthermore, the configuration of the wavelength estimation unit 40B is the same as that in the above embodiment except that the attenuation intensity detection unit 44B and the wavelength calculation unit 45B in FIG. 15 are provided instead of the fitting unit 44 and the wavelength calculation unit 45 of the first embodiment. 1 is the same as the configuration of the wavelength estimation unit 40.

As described above, the normalization unit 43 generates normalized waveform data by normalizing the integrated waveform generated by the integration processing unit 42 with the maximum value of the integrated waveform. The attenuation intensity detection unit 44B according to the present embodiment normalizes the normalization during a predetermined observation time (for example, a determination time τ _p after the time corresponding to the maximum value of the normalization waveform) within the attenuation period of the normalization waveform. The intensity value of the waveform is detected as the attenuation intensity. As the amount of light absorption in the loop optical path including the light absorption cell 13 increases, the attenuation rate of the normalized waveform increases and the attenuation intensity decreases. The wavelength calculation unit 45B refers to a conversion table prepared in advance or uses a conversion function to acquire a wavelength corresponding to the attenuation intensity, and estimates the current wavelength of the laser light pulse as the acquired wavelength. Can do. Here, the conversion table is a lookup table that defines a correspondence relationship between attenuation intensity and wavelength. The conversion function calculates a wavelength corresponding to the input value when the attenuation intensity is given as the input value. The control signal generator 46 generates a feedback control signal that reduces the difference between the current wavelength estimated by the wavelength calculator 45B and the target wavelength Λ _on .

  As described above, the wavelength controller 10B according to the third embodiment is based on the attenuation intensity of the normalized waveform generated by the normalizing unit 43 even when the absorption coefficient of the light absorption medium in the light absorption cell 13 is small. The current wavelength of the laser light pulse can be estimated with high accuracy. Therefore, it is possible to realize a stable wavelength lock with high accuracy.

  Instead of the differential absorption lidar unit 20 of the third embodiment, the differential absorption lidar unit 20A of the second embodiment may be used. Similarly to the first embodiment, in the third embodiment, the configuration of the wavelength controller 10B may be modified so that one or both of the optical amplifiers 51 and 52 shown in FIGS. 12 and 13 are included. .

  Although various embodiments according to the present invention have been described above with reference to the drawings, these embodiments are examples of the present invention, and various forms other than these embodiments can be adopted. For example, the optical waveguides G1 to G8, G10 to G12, G12a, G12b, and G6a can be configured with optical cables, but are not limited thereto.

Moreover, although the said Embodiment 1, 2, 3 has the structure which locks only the output wavelength (lambda) _on of the 2nd laser light source 21B, it is not limited to this. For example, the output wavelength λ _on of the second laser light source 21B is locked within the period in which the laser light pulse having the on wavelength λ _on is input to the optical waveguide G5, and the laser light pulse having the off wavelength λ _off is input to the optical waveguide G5. The configurations of the first, second, and third embodiments may be modified so that the output wavelength λ _off of the first laser light source 21A is locked within a period.

  In the present invention, within the scope of the invention, the above-described first, second, and third embodiments can be freely combined, any constituent element in each embodiment can be modified, or any constituent element in each embodiment can be omitted. Is possible.

  G1-G8, G10-G12, G6a, G12a, G12b Optical waveguide, 1, 1A, 1B Differential absorption lidar device, 10, 10B Wavelength controller, 11 Intensity modulator, 12 Optical coupler, 13 Optical absorption cell, 14 Light Branching device, 15 photo detector, 16 signal processing unit, 20, 20A differential absorption lidar unit, 21A first laser light source, 21B second laser light source, 22A, 22B light source driving unit, 23 optical switch, 24 optical branching unit, 25 Optical modulator, 26 Optical splitter, 27 Optical amplifier, 28 Optical circulator, 29 Transmission / reception optical system, 31 Optical multiplexer, 32 Balanced receiver, 33 A / D converter (ADC), 34 Signal processing unit, 40, 40B Wavelength estimation unit, 41 A / D converter (ADC), 42 integration processing unit, 43 normalization unit, 44 fitting unit, 45, 45B wavelength Calculation unit, 46 control signal generation unit, 47 D / A converter (DAC), 51 and 52 an optical amplifier.

Claims (13)

A light absorption cell having a light input end to which a series of laser light pulses are input, and including a light absorption medium for attenuating each laser light pulse input to the light input end;
An optical branching device that splits a transmitted light pulse output from the light absorption cell into a first branched light pulse and a second branched light pulse;
An optical coupler for inputting the second branched light pulse from the light input terminal to the light absorption cell;
A photodetector that converts the first branched optical pulse into an electrical signal and outputs the electrical signal;
A wavelength estimation unit that samples an attenuation waveform of an output signal of the photodetector and estimates a current wavelength of the series of laser light pulses based on the sampled attenuation waveform;
A control signal generation unit that generates a feedback control signal that reduces the difference between the estimated current wavelength and the target wavelength;
The wavelength controller, wherein the light absorption cell, the optical branching unit, and the optical coupler constitute a loop optical path that circulates each laser light pulse input to the optical input terminal a plurality of times.   2. The wavelength controller according to claim 1, wherein a pulse period of the series of laser light pulses is longer than a period in which the laser light pulses circulate a plurality of times in the loop optical path to form the attenuation waveform. Characteristic wavelength controller. The wavelength controller according to claim 1 or 2, wherein
The wavelength estimator is
A sampling unit for sampling the attenuated waveform;
An integration processing unit that integrates a plurality of attenuation waveforms sampled by the sampling unit to generate an integrated waveform; and
A wavelength controller that estimates a current wavelength of the series of laser light pulses based on the integrated waveform. 4. The wavelength controller according to claim 3, wherein the wavelength estimation unit further includes a normalization unit that normalizes the integrated waveform to generate a normalized waveform,
The wavelength controller, wherein the wavelength controller estimates a current wavelength of the series of laser light pulses based on the normalized waveform. 5. The wavelength controller according to claim 4, wherein the wavelength estimation unit further includes a fitting unit that calculates an attenuation curve by fitting the normalized waveform with an attenuation function prepared in advance.
The wavelength controller, wherein the wavelength controller estimates a current wavelength of the series of laser light pulses based on the attenuation curve.   6. The wavelength controller according to claim 5, wherein the attenuation curve is an attenuation type exponential function. The wavelength controller according to claim 4, wherein the wavelength estimation unit further includes an attenuation intensity detection unit that detects an attenuation intensity of the normalized waveform,
The wavelength controller, wherein the wavelength controller estimates a current wavelength of the series of laser light pulses based on the attenuation intensity.   8. The wavelength controller according to claim 1, further comprising an intensity modulator that shortens a series of laser light pulses to be input to the optical input terminal. Characteristic wavelength controller.   9. The wavelength controller according to claim 1, further comprising an optical amplifier disposed in front of the optical coupler. 10.   9. The wavelength controller according to claim 1, wherein the loop optical path is disposed between a light emitting end of the optical branching device and a light incident end of the optical coupler. 10. A wavelength controller further comprising an optical amplifier constituting a part of the wavelength controller. A differential absorption lidar unit that irradiates a series of laser light pulses toward an external target, receives scattered light reflected by the target, and calculates the concentration of the target based on the light reception result;
The wavelength controller according to any one of claims 1 to 10, wherein the series of laser light pulses are input.
The differential absorption lidar unit is configured to make the wavelength of the series of laser light pulses coincide with the target wavelength in accordance with a feedback control signal generated by the wavelength controller.   12. The differential absorption lidar apparatus according to claim 11, wherein the differential absorption lidar unit includes an optical amplifier that amplifies a series of laser light pulses to be input to the wavelength controller. . The differential absorption lidar apparatus according to claim 12,
The differential absorption lidar part is:
A laser light source that outputs continuous wave laser light;
An optical modulator that pulses the laser light to generate the series of laser light pulses;
The differential absorption lidar apparatus, wherein the optical amplifier is arranged downstream of the optical modulator.

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