Gas absorption spectrometer

Abstract

This method achieves frequency stabilization and linewidth reduction of the laser beam without locking the laser beam in the resonator used in CRDS measurements. [Solution] The gas absorption spectrometer (1) comprises a light source (10), a first resonator (40), a photodetector (60), a controller (70), and a first frequency stabilization circuit (30). The first frequency stabilization circuit (30) includes a second resonator (80), a frequency adjustment unit (310), and a locking unit (320). The frequency adjustment unit (310) generates a main band and a subband in the laser light, and the locking unit (320) locks the subband to the second resonator (80).

Description

Technical Field

[0001] The present disclosure relates to a gas absorption spectroscopic apparatus for determining the concentration of a target component in a gas using cavity ring-down absorption spectroscopy (CRDS).

Background Art

[0002] As one of the gas absorption spectroscopic methods, cavity ring-down absorption spectroscopy (CRDS) is known. CRDS is a spectroscopic method for highly sensitively determining the concentration of a target component contained in a gas in a resonator by increasing the effective optical path length using a resonator (cavity).

[0003] In CRDS, laser light is input from a light source into the resonator. The laser light input into the resonator is accumulated in the resonator. After the laser light is sufficiently accumulated in the resonator, the input of the laser light into the resonator is blocked. Then, the attenuation of the light leaking from the resonator is measured. The gas absorption spectroscopic apparatus acquires the output signal of the photodetector as a "ring-down signal". The gas absorption spectroscopic apparatus measures the concentration of the target component contained in the gas in the resonator by calculating the attenuation time constant (ring-down time) using the acquired ring-down signal.

[0004] Non-Patent Document 1 discloses a gas absorption spectroscopic apparatus in which a feedback system operating based on the PDH (Pound-Drever-Hall) method is inserted into the optical path from the light source toward the resonator. In Non-Patent Document 1, it is assumed that the laser light is narrowed in line width by locking the laser light to the resonator based on the PDH method.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] Generally, high-finesse resonators are used in CRDS. For this reason, in configurations such as those described in Non-Patent Document 1, it is extremely difficult to lock the laser beam into the resonator, making it difficult to achieve frequency stabilization and linewidth reduction of the laser beam.

[0007] The present invention was made to solve these problems, and its objective is to achieve frequency stabilization and linewidth reduction of laser light without locking the laser light in the resonator used in CRDS measurements. [Means for solving the problem]

[0008] The gas absorption spectrometer according to this disclosure is a gas absorption spectrometer for measuring gas components, comprising: a light source that outputs laser light used for measuring gas components; a first resonator into which the laser light is input; a photodetector that detects the light output from the first resonator; a controller that measures gas components using the output signal of the photodetector; and a first frequency stabilization circuit arranged to form a negative feedback circuit between the light source and the first resonator, wherein the first frequency stabilization circuit includes a second resonator having lower finesse than the first resonator, a frequency adjustment unit that adjusts the frequency of the laser light, and a locking unit that locks the laser light to the second resonator, wherein the frequency adjustment unit generates a main band and a subband relative to the main band in the laser light used for measuring gas components, and the locking unit locks the subband to the second resonator. [Effects of the Invention]

[0009] According to this disclosure, frequency stabilization and linewidth reduction of laser light can be achieved without locking the laser light in the resonator used in CRDS measurements. [Brief explanation of the drawing]

[0010] [Figure 1] This diagram schematically shows the configuration of a gas absorption spectrometer. [Figure 2] This is a conceptual diagram to explain modal frequencies. [Figure 3] This diagram schematically shows the detailed configuration of the frequency stabilization circuit. [Figure 4] This figure shows an example of the oscillation frequency of the measured QCL obtained when the measured QCL is modulated with two different frequencies. [Figure 5] This is a block diagram showing the configuration of the frequency adjustment section and the locking section in a frequency stabilization circuit. [Figure 6] This diagram schematically shows the configuration of a gas absorption spectrometer related to a modified example. [Figure 7] This diagram schematically shows the configuration of a gas absorption spectrometer related to a comparative example. [Modes for carrying out the invention]

[0011] This embodiment will be described in detail below with reference to the drawings. In the following description, the same or corresponding parts in the drawings will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0012] <Configuration of a gas absorption spectrometer> Figure 1 is a schematic diagram showing the configuration of a gas absorption spectrometer 1 according to this embodiment. The gas absorption spectrometer 1 comprises a measurement QCL (Quantum Cascade Laser) 10, an isolator 11, an AOM (Acousto-Optic Modulator) 20, a frequency stabilization circuit 30, a CRDS resonator 40, a photodetector (PD: Photodiode) 60, and a controller 70.

[0013] The measurement QCL10 is an example of a light source that outputs laser light used for measuring gas components. The measurement QCL10 is configured to change the oscillation frequency of the laser light according to commands from the controller 70. Specifically, the measurement QCL10 is a distributed feedback type quantum cascade laser (QCL).

[0014] The AOM20 and frequency stabilization circuit 30 are located in the optical path between the measurement QCL10 and the CRDS resonator 40. The AOM20 is an example of an optical modulator. The AOM20 is an optical switch (switch) that rapidly switches between outputting and blocking laser light from the measurement QCL10 to the CRDS resonator 40. The AOM20 is turned ON when an ON command for outputting light is applied from the controller 70, causing the laser light from the measurement QCL10 to be output to the CRDS resonator 40. The AOM20 is turned OFF when an OFF command for blocking light is applied from the controller 70, causing the laser light from the measurement QCL10 to not be output to the CRDS resonator 40.

[0015] The CRDS resonator 40 is provided in the optical path between the AOM 20 and the photodetector 60. The CRDS resonator 40 is an example of a first resonator. The CRDS resonator 40 is configured to include a container (cell) capable of sealing the sample gas, and has an introduction pipe 44 for introducing the sample gas therein before the start of measurement and a discharge pipe 45 for discharging the sample gas to the outside after the end of measurement. An introduction valve 46 is provided in the introduction pipe 44. A discharge valve 47 is provided in the discharge pipe 45. The controller 70 controls the opening and closing of the introduction valve 46 and the discharge valve 47.

[0016] Inside the CRDS resonator 40, a pair of mirrors 41 and 42 are provided. The mirrors 41 and 42 are arranged to face each other so that light reflects between them inside the CRDS resonator 40. Among the mirrors 41 and 42, at least one of them is concave to make it easier to satisfy the stability conditions of the CRDS resonator 40. Also, the mirrors 41 and 42 are made of a high reflectivity (for example, about 99.9%) so that the light leaking out of the CRDS resonator 40 becomes extremely weak. Note that the number of mirrors arranged inside the CRDS resonator 40 is not limited to two, and may be three or more. That is, a resonator in which mirrors are arranged so that light reflects between them may be used, or a resonator in which mirrors are arranged in a ring shape so that light reflects in one direction may be used.

[0017] A piezo element (piezoelectric element) 43 is arranged on the mirror 42. The piezo element 43 drives the mirror 42 constituting the CRDS resonator 40 according to a command from the controller 70, thereby displacing the mirror 42 in the optical axis direction. As a result, the resonator length of the CRDS resonator 40 changes. Note that the piezo element may be arranged on the mirror 41 instead of the mirror 42, or piezo elements may be arranged on both the mirror 41 and the mirror 42.

[0018] The photodetector 60 is, for example, a photodiode. The photodetector 60 detects the weak light extracted from the mirror 42 of the CRDS resonator 40 as the output light of the CRDS resonator 40, and outputs a detection signal to the controller 70. The photodetector 60 is an example of a photodetector that detects the light output from the first resonator. For the photodetector 60, for example, a liquid nitrogen cooled InSb (indium antimonide) detector can be adopted.

[0019] An optical path between the measurement QCL 10 and the AOM 20 is provided with a beam splitter 112 and a mirror 111. The beam splitter 112 branches the laser light output from the measurement QCL 10 into an optical path toward the AOM 20 and an optical path toward the frequency stabilization circuit 30. The mirror 111 reflects one of the two lights split by the beam splitter 52 in the direction of the AOM 20.

[0020] An isolator 11 is arranged in the optical path between the measurement QCL 10 and the beam splitter 112. The isolator 12 restricts the transmission direction of the laser light to the direction from the measurement QCL 10 toward the beam splitter 112. By arranging the isolator 12 near the measurement QCL 10, it is possible to prevent the frequency of the laser light from becoming unstable due to the return light.

[0021] The controller 70 includes a processor 71 such as a CPU (Central Processing Unit) or an FPGA (Field-Programmable Gate Array), a memory 72 such as a ROM (Read Only Memory) and a RAM (Random Access Memory), and an input / output port (not shown).

[0022] The controller 70 controls each component of the gas absorption spectrometer 1. Specifically, the controller 70 outputs commands to scan the oscillation frequency of the laser light and outputs the aforementioned ON or OFF signals to the AOM 20. The controller 70 also outputs commands to the introduction valve 46 to introduce the sample gas into the CRDS resonator 40 and outputs commands to the discharge valve 47 to discharge the sample gas to the outside of the CRDS resonator 40.

[0023] The controller 70 applies a voltage to the piezoelectric element 43 to displace the mirror 42, thereby adjusting the resonator length of the CRDS resonator 40. The controller 70 performs various data processing operations. These operations include calculating the concentration (absolute concentration) of the target component contained in the sample gas based on the detection signal from the photodetector 60.

[0024] The controller 70 may be configured by dividing it into two or more units for each function. For example, the controller 70 may be divided into a unit that controls each device and a unit that performs various data processing.

[0025] <Measurement principle using cavity ring-down spectroscopy (CRDS)> The measurement principle of cavity ring-down absorption spectroscopy in gas absorption spectrometer 1 will be explained. Generally, resonance occurs in a resonator when the frequency of the light irradiated onto the resonator is a specific frequency. Hereinafter, the frequency of the laser light input to the CRDS resonator 40 will be referred to as the "laser frequency," and the frequency of light in which resonance can occur in the CRDS resonator 40 will be referred to as the "mode frequency."

[0026] Figure 2 is a conceptual diagram illustrating mode frequencies. As shown in Figure 2, multiple mode frequencies exist at predetermined frequency intervals. Hereafter, the interval between two adjacent mode frequencies will be referred to as the "free spectral range" (FSR).

[0027] If the laser frequency does not match any of the mode frequencies, no light power is stored in the CRDS resonator 40. On the other hand, if the laser frequency matches any of the mode frequencies, light power is stored in the CRDS resonator 40. The controller 70 adjusts the resonator length of the CRDS resonator 40 so that the laser frequency and the mode frequency match.

[0028] The controller 70 determines whether sufficient laser light power has been stored in the CRDS resonator 40 based on the output signal of the photodetector 60. If the controller 70 determines that sufficient laser light power has been stored in the CRDS resonator 40, it outputs an off signal to the AOM 20. This blocks the light input to the CRDS resonator 40.

[0029] As a result, the light stored in the CRDS resonator 40 travels back and forth between mirror 41 and mirror 42 many times (usually several thousand to tens of thousands of times). As this light travels back and forth between mirrors 41 and 42, it is gradually attenuated due to losses from reflection leakage between mirrors 41 and 42 and absorption by the target component in the sample gas. Therefore, the output light from the CRDS resonator 40 leaking from mirror 42 is gradually attenuated. In a CRDS, by using the CRDS resonator 40 to increase the distance the light travels through the sample gas (effective optical path length), it is possible to detect light absorption by the target component even if the absorption is extremely small.

[0030] The controller 70 acquires the signal detected by the photodetector 60 as a "ring-down signal" while the light input to the CRDS resonator 40 is blocked. The controller 70 calculates the decay time constant of the acquired ring-down signal as the "ring-down time". From the calculated ring-down time, the controller 70 calculates the concentration of the target component contained in the sample gas.

[0031] <Configuration of the frequency stabilization circuit> Figure 3 is a schematic diagram showing the detailed configuration of the frequency stabilization circuit 30. Figure 4 is a diagram showing an example of the oscillation frequency of the measured QCL obtained when the measured QCL is modulated with two different frequencies. Figure 4 also shows the frequency spectrum resonating in the high-stability resonator 80 and the frequency spectrum resonating in the CRDS resonator 40.

[0032] As shown in Figure 3, the frequency stabilization circuit 30 is arranged such that a negative feedback circuit is formed between the measurement QCL 10 and the CRDS resonator 40. The frequency stabilization circuit 30 includes a photodetector 31, a QCL driver 32, a phase shifter 33, a phase comparator 34, a low-pass filter (LPF) 35, a servo circuit 36, RF (radio frequency) generators 37, 38, an adder 39, a highly stable resonator 80, a polarizing beam splitter 113, and a quarter-wave plate 114.

[0033] The laser beam output from the measurement QCL10 is split at the beam splitter 112 into two directions: one towards the CRDS resonator 40 and the other towards the frequency stabilization circuit 30. The laser beam incident on the frequency stabilization circuit 30 is directed towards the polarizing beam splitter 113.

[0034] The polarizing beam splitter 113 allows a portion of the laser light to pass towards the high-stability resonator 80. The quarter-wave plate 114 changes the polarization state of the laser light incident from the polarizing beam splitter 113 and directs the laser light into the high-stability resonator 80. The quarter-wave plate 114 changes the polarization state of the laser light returning from the high-stability resonator 80 again and returns it to the polarizing beam splitter 113. The polarizing beam splitter 113 reflects the laser light returning from the high-stability resonator 80 via the quarter-wave plate 114 in a direction approximately perpendicular to the beam. The photodetector 31 detects the laser light reflected by the polarizing beam splitter 113. The photodetector 31 outputs an electrical signal corresponding to the intensity of the laser light to the phase comparator 34.

[0035] RF generators 37 and 38 function as oscillators that oscillate a modulated signal. RF generator 37 oscillates a modulated signal at a reference frequency. The reference frequency is, for example, 20 MHz. When the laser light is modulated at the reference frequency, a frequency spectrum of the main band MB appears, with a peak value at the center and an amplitude of 20 MHz, as shown in Figure 4. The peak value of the main band MB is used for measurement by CRDS. As shown in Figure 4, the controller 70 ultimately matches the peak value of the main band MB with the resonant frequency of the CRDS resonator. RF generator 37 is an example of a first generator.

[0036] The RF generator 38 oscillates a modulated signal at the "sweep frequency." The "sweep frequency" indicates the sweep amount of the main band MB. The RF generator 38 oscillates a modulated signal at the sweep frequency according to the command from the controller 70, within a predetermined sweep range. The predetermined sweep range is, for example, 100 to 1000 MHz. When the laser light is modulated at the sweep frequency in addition to the reference frequency, a frequency spectrum of the subband SB, with an amplitude of 20 MHz centered on the peak value, appears to the left and right of the main band MB, as shown in Figure 4. The frequency difference between the peak value of the subband SB and the peak value of the main band MB coincides with the sweep frequency. The RF generator 38 is an example of a second generator.

[0037] The RF generator 37 outputs a modulated signal to the phase shifter 33 and the adder 39. The RF generator 38 outputs a modulated signal to the adder 39 in accordance with the command from the controller 70.

[0038] The phase shifter 33 shifts the phase of the modulated signal output from the RF generator 37 by 180 degrees. The phase comparator 34 calculates a comparison value between the detection signal detected by the photodetector 31 and the modulated signal from the RF generator 37. The phase comparator 34 outputs the comparison value (difference) between the detection signal and the modulated signal from the RF generator 37 to the low-pass filter 35. The comparison value output to the low-pass filter 35 corresponds to the error between the resonant frequency of the highly stable resonator 80 and the frequency of the laser light. The low-pass filter 21 generates an error signal (beat signal) based on the error.

[0039] The QCL driver 32 outputs a DC current signal for modulation to the adder 39. The servo circuit 22 outputs a signal to the adder 39 to adjust the frequency of the laser light to the resonant frequency of the highly stable resonator 16.

[0040] The adder 39 adjusts the frequency of the laser light output from the measurement QCL 10 using the signal received from the servo circuit 36. Furthermore, the adder 39 modulates the frequency of the laser light output from the measurement QCL 10 using the modulation signal received from the RF generator 37 and the modulation signal received from the RF generator 38. The measurement QCL 10 outputs laser light modulated according to the signal output from the adder 39. As a result, the frequency spectrum of the laser light shows a main band MB and a subband SB, as explained with reference to Figure 4.

[0041] The process described above is repeated between the measurement QCL10 and the frequency stabilization circuit 30. As a result, the aforementioned error gradually decreases.

[0042] In this manner, the frequency stabilization circuit 30 outputs the frequency-modulated laser light to the high-stability resonator 80, detects the laser light output from the high-stability resonator 80 with the photodetector 31, and stabilizes the frequency of the laser light by applying feedback based on the detected laser light.

[0043] In other words, the frequency stabilization circuit 30 operates according to the PDH (Pound-Drever-Hall) method, locking the laser beam from the measurement QCL 10 onto the highly stable resonator 80. In other words, the frequency stabilization circuit 30 performs PDH control.

[0044] The high-stability resonator 80 does not have a mechanism for adjusting the resonator length. In other words, the resonator length of the high-stability resonator 80 is fixed. Inside the high-stability resonator 80, a pair of mirrors (not shown) are provided, similar to the CRDS resonator 40. However, the reflectivity of the mirrors placed in the high-stability resonator 80 is lower than that of the mirrors placed in the CRDS resonator 40. For this reason, the "finesse" of the high-stability resonator 80 is lower than that of the CRDS resonator 40.

[0045] Generally, finesse increases as the spectrum of the resonant frequency becomes steeper and the free spectral range (FSR) widens. To improve the accuracy of CRDS measurements, it is necessary to make the resonant linewidth of the CRDS resonator 40 as narrow as possible. For this reason, a high-finesse CRDS resonator 40 is used in the gas absorption spectrometer 1.

[0046] In this embodiment, it is assumed that the coupling between the main band MB and the CRDS resonator 40 will be enhanced by narrowing the linewidth of the main band MB, and the peak value of the narrowed main band MB will be used for measurement. In this assumption, it is conceivable to directly lock the laser beam of the main band MB output from the measurement QCL 10 to the CRDS resonator 40. However, directly locking the laser beam output from the measurement QCL 10 to the high-finesse CRDS resonator 40 is extremely difficult. The reason for this is that the wavelength stability of the laser beam is insufficient for the extremely narrowed resonant linewidth.

[0047] Therefore, in this embodiment, instead of locking the laser beam to the CRDS resonator 40, we propose locking the laser beam to a highly stable resonator 80 with low finesse (broad resonant linewidth). To obtain a finesse sufficient to stably perform the lock, the highly stable resonator 80 employs a mirror with appropriately suppressed reflectivity. In particular, in this embodiment, by employing a resonator with a fixed resonator length as the highly stable resonator 80, fluctuations in the resonator length are prevented. This makes it possible to lock the laser beam even more stably in the highly stable resonator 80. The highly stable resonator 80 is an example of a second resonator with lower finesse than the first resonator.

[0048] Furthermore, in this embodiment, the target of locking in the highly stable resonator 80 is the subband SB, not the main band MB. The reason for this is that if the oscillation peak of the main band MB is locked to the resonant frequency of the highly stable resonator 80 with a fixed resonator length, wavelength sweeping becomes impossible.

[0049] By locking the subband SB laser beam into the highly stable resonator 80, the subband SB laser beam resonates within the highly stable resonator 80. Due to the low finesse of the highly stable resonator 80, the laser beam is locked into the highly stable resonator 80 with a relatively broad, narrow linewidth (see Figure 4). In other words, by using the highly stable resonator 80, the laser beam can be easily locked with relatively loose criteria.

[0050] In this embodiment, the subband SB of the laser beam is controlled so that the resonant frequency of the high-stability resonator 80 matches the subband SB. That is, the frequency stabilization circuit 30 locks the subband SB to the high-stability resonator 80. For example, when the sweep frequency is 100 MHz, the measurement QCL 10 oscillates when the subband SB is locked to the high-stability resonator 80 and the frequency difference between the peak of the main band MB and the peak of the subband SB is 100 MHz.

[0051] The light sources for the main band MB and subband SB are the same. Therefore, by locking the subband SB laser beam into the highly stable resonator 80 and stabilizing its frequency, the frequency of the main band MB can also be stabilized in the CRDS resonator 40.

[0052] To stabilize the frequency of the main band MB in the CRDS resonator 40, the controller 70 locks the laser beam of the subband SB onto the high-stability resonator 80, and then shifts the resonant frequency of the CRDS resonator 40 by an amount corresponding to the sweep frequency. More specifically, the controller 70 adjusts the resonator length of the CRDS resonator 40 so that the peak value of the main band MB matches the resonant frequency of the CRDS resonator 40. As a result, the frequency of the main band MB resonating in the CRDS resonator 40 shifts in the sweep direction. Consequently, the frequency of the main band MB in the CRDS resonator 40 is stabilized, and linewidth reduction is achieved. After that, the controller 70 proceeds to the measurement process.

[0053] As shown in Figure 3, the controller 70 may be included in the personal computer (PC) 90. The personal computer 90 may consist of, for example, an operation receiving unit such as a keyboard and mouse, an information input interface, and a monitor. The personal computer 90 may receive the sweep frequency via the operation receiving unit or the information input interface. In this case, the controller 70 may output a command signal to the RF generator 38 to cause the received sweep frequency to oscillate.

[0054] By locking the subband SB in the highly stable resonator 80 and changing the sweep frequency, the controller 70 can control the frequency difference between the subband SB and the main band MB used for CRDS measurement. Therefore, the controller 70 can freely sweep the main band MB with a narrowed linewidth and acquire a ring-down signal at the required frequency difference. As a result, the controller 70 can measure the spectrum in the same manner as conventional methods.

[0055] According to this embodiment, frequency stabilization and linewidth reduction of the laser beam can be achieved for the main band MB used for measurement without locking the laser beam with the CRDS resonator 40. In this example, the error signal for PDH control is obtained by current modulation of the signal output from the QCL driver 32, but phase modulation may be used instead of current modulation. In this case, an EOM (Electro Optic Modulator) may be used.

[0056] Figure 5 is a block diagram showing the configuration of the frequency adjustment section 310 and the locking section 320 in the frequency stabilization circuit 30. The frequency stabilization circuit 30 described using Figure 3 functionally includes a frequency adjustment section (modulation section) 310 and a locking section (PDH control section) 320. The frequency adjustment section 310 and the locking section 320 are realized by combining the necessary functions from among the photodetector 31, QCL driver 32, phase shifter 33, phase comparator 34, low-pass filter 35, servo circuit 36, RF generators 37, 38, and adder 39.

[0057] The frequency adjustment unit 310 generates a main band MB and a subband SB relative to the main band MB in the laser light, which are used for measuring gas components. More specifically, the frequency adjustment unit 310 generates the main band MB and the subband SB by modulating the laser light. The frequency adjustment unit 310 includes a main band generation unit 311 that generates the main band MB and a subband generation unit 312 that generates the subband SB. The locking unit 320 locks the subband SB to the highly stable resonator 80.

[0058] The frequency adjustment unit 310 includes an RF generator 37 that outputs a modulation signal for generating the main band MB, and an RF generator 38 that outputs a modulation signal for generating the subband SB. The controller 70 changes the oscillation frequency of the RF generator 38 to change the frequency difference between the main band MB and the subband SB. In other words, the controller 70 performs wavelength sweeping by changing the oscillation frequency of the RF generator 38.

[0059] The CRDS resonator 40 has an adjustment mechanism 430 for adjusting the resonator length. The adjustment mechanism 430 is composed of a piezoelectric element 43 as shown in Figure 1. The controller 70 uses the adjustment mechanism 430 to adjust the resonator length of the CRDS resonator 40 so that the CRDS resonator 40 resonates in the main band MB when the subband SB is locked in the high-stability resonator 80. This makes it possible to perform CRDS measurements.

[0060] <Variation> Figure 6 is a schematic diagram showing the configuration of a modified gas absorption spectrometer 1A. Gas absorption spectrometer 1A differs from gas absorption spectrometer 1 in that it includes a frequency stabilization circuit 50, a polarizing beam splitter 115, and a quarter-wave plate 116. Except for the frequency stabilization circuit 50, the polarizing beam splitter 115, and the quarter-wave plate 116, the configuration of gas absorption spectrometer 1A is the same as that of gas absorption spectrometer 1. Note that the polarizing beam splitter 115 and the quarter-wave plate 116 can also be replaced with an isolator with a return light output port. This allows linearly polarized light to be incident on the resonator instead of circularly polarized light.

[0061] The frequency stabilization circuit 50 comprises a photodetector 51, a phase shifter 53, a phase comparator 54, a low-pass filter 55, and a servo circuit 56. The modulation signal from the RF generator 37 is input to the phase shifter 53. The servo circuit 56 controls the oscillation frequency of the RF generator 38.

[0062] The laser light incident on the AOM20 from the measurement QCL10 is directed towards the polarizing beam splitter 115. The polarizing beam splitter 115 allows a portion of the laser light to pass towards the CRDS resonator 40. The quarter-wave plate 116 changes the polarization state of the laser light incident on the polarizing beam splitter 115 and directs the laser light into the CRDS resonator 40. The quarter-wave plate 114 changes the polarization state of the laser light returning from the CRDS resonator 40 again and returns it to the polarizing beam splitter 115. The polarizing beam splitter 115 outputs the laser light returning from the CRDS resonator 40 via the quarter-wave plate 116 towards the frequency stabilization circuit 50. The laser light input to the frequency stabilization circuit 50 is detected by the photodetector 51.

[0063] The frequency stabilization circuit 50 operates according to the PDH method, similar to the frequency stabilization circuit 30, and performs PDH control. The photodetector 51, phase shifter 53, phase comparator 54, low-pass filter 55, and servo circuit 56 are configured to realize PDH control, similar to the photodetector 31, phase shifter 33, phase comparator 34, low-pass filter 35, and servo circuit 36.

[0064] The phase comparator 54 outputs a comparison value between the detection signal detected by the photodetector 51 and the modulated signal from the RF generator 37 to the low-pass filter 55. The comparison value output to the low-pass filter 55 corresponds to the error between the resonant frequency of the CRDS resonator 40 and the main band MB of the laser light. The low-pass filter 55 generates an error signal based on the error. The servo circuit 56 controls the oscillation frequency of the RF generator 38 to adjust the main band MB to the resonant frequency of the CRDS resonator 40.

[0065] In this way, the frequency stabilization circuit 50 locks the main band MB of the laser beam to the CRDS resonator 40. In the frequency stabilization circuit 30, the subband SB of the laser beam is locked to the high-stability resonator 80, so the main band MB with a narrow resonant linewidth can be easily locked to the CRDS resonator 40. The servo circuit 56 sweeps the wavelength of the laser beam output from the measurement QCL 10 by varying the oscillation frequency of the RF generator 38. The servo circuit 56 matches the resonant frequency of the CRDS resonator 40 with the frequency of the laser beam output from the measurement QCL 10 by inputting an error signal to the RF generator 38.

[0066] The gas absorption spectrometer 1 shown in Figure 3 does not have a means for locking the main band MB to the CRDS resonator 40. Therefore, if there is a slight difference in the resonance frequency between the CRDS resonator 40 and the laser light output from the measurement QCL 10, the ring-down signal cannot be obtained. In that case, the user needs to fine-tune the sweep frequency of the RF generator 38 so that the resonance condition is met.

[0067] In contrast, such adjustments are unnecessary with the gas absorption spectrometer 1A. Therefore, the gas absorption spectrometer 1A can always acquire a ring-down signal without considering the difference in resonance frequencies between the CRDS resonator 40 and the laser light output from the measurement QCL 10. With the gas absorption spectrometer 1A, wavelength sweeping can be performed while the CRDS resonator 40 and the laser light output from the measurement QCL 10 are always in resonance. Of course, with the gas absorption spectrometer 1A, the resonance linewidth in the CRDS resonator 40 can be kept narrow.

[0068] Furthermore, since the laser light output from the measurement QCL10 and the CRDS resonator 40 are always in resonance, the speed of measuring the ring-down signal can be increased. This allows the user to perform CRDS measurements in a more stable state. The frequency stabilization circuit 50 is an example of a second frequency stabilization circuit that locks the main band to the first resonator.

[0069] The servo circuit 56 may output a control signal to the controller 70 instead of outputting a control signal to the RF generator 38. In this case, the controller 70 adjusts the oscillation frequency of the RF generator 38, taking into account the control signal from the servo circuit 56. The frequency stabilization circuit 50 controls the modulation frequency of the RF generator 38 to achieve PDH control. However, the frequency stabilization circuit 50 may also control the frequency and phase using an AOM (Acousto-Optic Modulator) or EOM (Electro-Optic Modulator) to achieve PDH control.

[0070] The frequency stabilization circuit 50 controls the frequency of the generator 38. The gas absorption spectrometer 1A includes an AOM 20 positioned between the measurement QCL 10 and the CRDS resonator 40. The frequency stabilization circuit 50 may control the frequency of the AOM 20. The frequency stabilization circuit 50 may also control the resonator length of the CRDS resonator 40.

[0071] <Comparative Example> Figure 7 is a schematic diagram showing the configuration of a gas absorption spectrometer 1000 related to a comparative example. The gas absorption spectrometer 1000 comprises a measurement QCL 10, an isolator 11, an AOM 20, a CRDS resonator 40, a photodetector 60, a controller 70, and frequency stabilization circuits 100,200.

[0072] In the comparative example gas absorption spectrometer 1000, as with gas absorption spectrometer 1, frequency stabilization and linewidth reduction of the laser beam can be achieved without locking the laser beam in the CRDS resonator 40. However, as will be explained below, gas absorption spectrometer 1000 requires more components than gas absorption spectrometer 1.

[0073] The frequency stabilization circuit 100 includes a photodetector 101, a QCL driver 102, a phase comparator 104, a low-pass filter 105, a servo circuit 106, an RF generator 107, an adder 109, a mirror 118, and a beam splitter 119. The RF generator 107, like the RF generator 38, oscillates a modulated signal in the range of 100 to 1000 MHz in response to a command from the controller 70.

[0074] The frequency stabilization circuit 200 includes a photodetector 101, a QCL driver 102, a phase comparator 104, a high-stability resonator 80, a polarizing beam splitter 113, a quarter-wave plate 114, a beam splitter 120, a phase shifter 203, a low-pass filter 205, a servo circuit 206, an RF generator 207, an adder 209, a reference QCL 210, and an isolator 211.

[0075] The frequency stabilization circuit 200 locks the laser beam output from the reference QCL 210 onto the high-stability resonator 80. The phase comparator 104, phase shifter 203, low-pass filter 205, servo circuit 206, RF generator 207, and adder 209 included in the frequency stabilization circuit 200 function as a PDH circuit to lock the laser beam output from the reference QCL 210 onto the high-stability resonator 80 by PDH control.

[0076] The laser beam from the frequency-stabilized reference QCL210 and the laser beam output from the measurement QCL10 are output to the frequency stabilization circuit 100. In the frequency stabilization circuit 100, the laser beam output from the reference QCL210 and the laser beam output from the measurement QCL10 are detected by the photodetector 101. The photodetector 31 outputs an electrical signal corresponding to the intensity of the laser beam to the phase comparator 104. The RF generator 107 outputs the sweep frequency to the phase comparator 104.

[0077] The phase comparator 104 calculates the difference between the laser beam output from the reference QCL210 and the laser beam output from the measurement QCL10. The calculated value corresponds to the error between the frequency of the highly stabilized reference QCL210 laser beam and the frequency of the measurement QCL10 laser beam. The phase comparator 104 further outputs the difference between this error and the sweep frequency to the low-pass filter 105. The low-pass filter 105 generates an error signal based on the difference.

[0078] The QCL driver 102 outputs a DC current signal for modulation to the adder 109. The servo circuit 106 outputs a signal to the adder 109 for adjusting the frequency of the laser beam based on the error signal.

[0079] The adder 109 adjusts the frequency of the laser light output from the measurement QCL 10 using the signal received from the servo circuit 36. As a result, the measurement QCL 10 of the gas absorption spectrometer 1000 outputs a laser light modulated according to the reference frequency and sweep frequency, similar to the measurement QCL 10 of the gas absorption spectrometer 1. This laser light is based on the laser light of the reference QCL 210, which has been highly stabilized and narrowed in the frequency stabilization circuit 200. Therefore, the laser light output from the measurement QCL 10 is also highly stabilized and narrowed in line. Accordingly, the gas absorption spectrometer 1000, which relates to the comparative example, can achieve frequency stabilization and narrowing of the laser light for the main band used for measurement, without locking the laser light in the CRDS resonator 40, similar to the gas absorption spectrometer 1.

[0080] However, the comparative example requires numerous components in addition to the reference QCL210, resulting in a complex configuration. In contrast, the gas absorption spectrometer 1 simplifies the configuration and achieves frequency stabilization and linewidth reduction of the laser beam for the main band used in measurement, without locking the laser beam with the CRDS resonator 40. In particular, compared to the comparative example, the gas absorption spectrometer 1 requires only one laser light source for wavelength stabilization, simplifying the optical elements and control system.

[0081] [Pattern] Those skilled in the art will understand that the embodiments and their modifications described above are specific examples of the following embodiments.

[0082] (Section 1) The gas absorption spectrometer according to the present disclosure is a gas absorption spectrometer for measuring gas components, comprising: a light source that outputs laser light used for measuring gas components; a first resonator into which the laser light is input; a photodetector that detects light output from the first resonator; a controller that measures gas components using the output signal of the photodetector; and a first frequency stabilization circuit arranged such that a negative feedback circuit is formed between the light source and the first resonator, wherein the first frequency stabilization circuit includes a second resonator having lower finesse than the first resonator, a frequency adjustment unit that adjusts the frequency of the laser light, and a locking unit that locks the laser light to the second resonator, wherein the frequency adjustment unit generates a main band and a subband relative to the main band in the laser light used for measuring gas components, and the locking unit locks the subband to the second resonator.

[0083] In the gas absorption spectrometer described in paragraph 1, frequency stabilization and linewidth reduction of the laser beam can be achieved without locking the laser beam in the resonator used for CRDS measurements.

[0084] (Section 2) In the gas absorption spectrometer described in Section 1, the frequency adjustment unit generates a main band and a subband by modulating laser light, and the frequency adjustment unit has a first generator that outputs a modulation signal for generating the main band and a second generator that outputs a modulation signal for generating the subband, and the controller changes the oscillation frequency of the second generator to change the frequency difference between the main band and the subband.

[0085] In the gas absorption spectrometer described in paragraph 2, the main band can be freely swept while the main band's linewidth is narrowed, and a ring-down signal can be acquired at the required frequency difference.

[0086] (Clause 3) In the gas absorption spectrometer described in paragraph 1 or 2, the first resonator has an adjustment mechanism for adjusting the resonator length, the second resonator does not have an adjustment mechanism, and the controller uses the adjustment mechanism to adjust the resonator length of the first resonator so that the first resonator resonates in the main band when the subband is locked in the second resonator.

[0087] In the gas absorption spectrometer described in paragraph 3, the frequency of the main band is stabilized and a narrow linewidth is achieved even in the first resonator.

[0088] (Article 4) The gas absorption spectrometer described in any one of Articles 1 to 3 further comprises a second frequency stabilization circuit, the second frequency stabilization circuit locks the main band to the first resonator.

[0089] In the gas absorption spectrometer described in Section 4, a ring-down signal can always be acquired without considering the difference in resonance frequencies between the first resonator and the laser light.

[0090] (Section 5) The gas absorption spectrometer described in Section 4 further comprises an acousto-optic modulator positioned between the light source and the first resonator, and the second frequency stabilization circuit controls the frequency of the acousto-optic modulator.

[0091] In the gas absorption spectrometer described in Section 5, the frequency of the acousto-optic modulator is controlled by a second frequency stabilization circuit.

[0092] (Section 6) In the gas absorption spectrometer described in Section 4 or 5, the second frequency stabilization circuit (50) controls the resonator length of the first resonator.

[0093] In the gas absorption spectrometer described in Section 6, the resonator length of the first resonator is controlled by the second frequency stabilization circuit.

[0094] (Section 7) The gas absorption spectrometer described in Section 2 further comprises a second frequency stabilization circuit, the second frequency stabilization circuit locks the main band to the first resonator, and the second frequency stabilization circuit controls the frequency of the second generator.

[0095] In the gas absorption spectrometer described in Section 7, the frequency of the second generator is controlled by a second frequency stabilization circuit.

[0096] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0097] 1,1A,1000 Gas absorption spectrometer, 10 Measurement QCL, 11,211 Isolator, 20 AOM, 30,50,100,200 Frequency stabilization circuit, 32,102,202 QCL driver, 33,53,203 Phase shifter, 34,104,204 Phase comparator, 35,105,205 Low-pass filter, 36,106,206 Servo circuit, 37,38,107,207 RF generator, 39,109,209 Adder, 40 CRDS resonator, 41,42,111,118 Mirror, 43 Piezo element, 44 Inlet tube, 45 Outlet tube, 46 Inlet valve, 47 Outlet valve, 60,31,101,201 Photodetector, 70 Controller, 71 Processor, 72 Memory, 80 High-stability resonator, 90 Personal computer, 112, 119, 120 Beam splitter, 113, 115, 117 Polarizing beam splitter, 114, 116 Quarter wave plate, 210 Reference QCL, 310 Frequency adjustment section (modulation section), 320 Lock section (PDH control section), 311 Main band generation section, 312 Subband generation section, 430 Adjustment mechanism, MB Main band, SB Subband.

Claims

1. A gas absorption spectrometer for measuring gas components, A light source that outputs laser light used for measuring the aforementioned gas components, A first resonator into which the laser light is input, A photodetector for detecting the light output from the first resonator, A controller that measures the gas component using the output signal of the photodetector, The system comprises a first frequency stabilization circuit arranged such that a negative feedback circuit is formed between the light source and the first resonator, The first frequency stabilization circuit is, A second resonator with lower finesse than the first resonator, A frequency adjustment unit for adjusting the frequency of the laser light, The second resonator includes a locking unit for locking the laser beam, The frequency adjustment unit generates a main band used for measuring the gas components and a subband relative to the main band in the laser light. The locking mechanism is a gas absorption spectrometer that locks the subband to the second resonator.

2. The frequency adjustment unit generates the main band and the subband by modulating the laser light. The frequency adjustment unit is A first generator that outputs a modulation signal for generating the main band, It has a second generator that outputs a modulation signal for generating the aforementioned subband, The gas absorption spectrometer according to claim 1, wherein the controller changes the oscillation frequency of the second generator and changes the frequency difference between the main band and the subband.

3. The first resonator has an adjustment mechanism for adjusting the resonator length, The second resonator does not have the adjustment mechanism, The gas absorption spectrometer according to claim 1 or 2, wherein the controller adjusts the resonator length of the first resonator using the adjustment mechanism so that the first resonator resonates in the main band when the subband is locked in the second resonator.

4. Further equipped with a second frequency stabilization circuit, The gas absorption spectrometer according to claim 1 or claim 2, wherein the second frequency stabilization circuit locks the main band to the first resonator.

5. The system further comprises an acoustic-optic modulator disposed between the light source and the first resonator, The gas absorption spectrometer according to claim 4, wherein the second frequency stabilization circuit controls the frequency of the acousto-optic modulator.

6. The gas absorption spectrometer according to claim 4, wherein the second frequency stabilization circuit controls the resonator length of the first resonator.

7. Further equipped with a second frequency stabilization circuit, The second frequency stabilization circuit locks the main band to the first resonator, The gas absorption spectrometer according to claim 2, wherein the second frequency stabilization circuit controls the frequency of the second generator.

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