Rapid scanning cavity ring-down spectroscopy implementation system and application

Abstract

The invention discloses a fast scanning cavity ring-down spectroscopy implementation system and application, and the system comprises a laser, a lambda / 2 wave plate, a polarization beam splitter prism, a lambda / 4 wave plate, a reflector I, a reflector II, a lens, an optical resonant cavity and a photoelectric detector which are sequentially arranged along a light path, and also comprises a signal generator and a laser driver, the signal generator outputs a semi-sawtooth wave signal, the semiconductor laser outputs laser under the action of the laser driver, the laser passes through the lambda / 2 wave plate, the lambda / 2 wave plate is rotated, the optical power of the laser passing through the polarization beam splitter prism is made to be maximum, then the laser passes through the lambda / 4 wave plate, and the laser is coupled with the optical resonant cavity after passing through the lens under the action of the two reflectors. The distance between the optical resonant cavity and the laser is controlled to be integral multiple cavity length. According to the invention, the minute-order spectrum measurement time is shortened to hundreds of milliseconds, and the atmospheric flux detection requirement can be met.

Description

Technical Field

[0001] The invention relates to an optical resonant cavity ring-down spectroscopy technology, and in particular to a fast scanning cavity ring-down spectroscopy implementation system and application. Background Art

[0002] The cavity ring-down spectroscopy technology based on the shut-off mode of the acousto-optic modulator requires the use of an optical isolator to isolate the reflected light from the optical resonant cavity, and based on the judgment of the intensity of the transmission signal of the resonant cavity, the acousto-optic modulator is used to actively shut down the laser to stimulate the generation of the ring-down signal. This process requires shutting down each longitudinal mode one by one to obtain a complete spectrum. Therefore, the measurement time of this mechanical ring-down spectroscopy is usually limited to minutes. This response time is unacceptable for some applications that require fast measurement. Summary of the invention

[0003] The object of the present invention is to provide a fast scanning cavity ring-down spectroscopy realization system and application, which is based on optical feedback self-excitation and does not require active judgment and identification, thereby being able to improve the measurement speed.

[0004] To this end, the present invention provides, on one hand, a system for realizing fast scanning cavity ring-down spectrum, comprising a laser, a λ / 2 wave plate, a polarization beam splitter prism, a λ / 4 wave plate, a first reflector, a second reflector, a lens, an optical resonant cavity, and a photoelectric detector arranged in sequence along an optical path, a signal generator and a laser driver, a data acquisition card, a computer, a high-voltage source, and piezoelectric ceramics. The signal generator outputs a half-sawtooth wave signal. Under the action of the laser driver, the semiconductor laser outputs laser light. The laser light passes through a λ / 2 wave plate, and the λ / 2 wave plate is rotated so that the light power passing through the polarization beam splitter prism is maximized. Then, the laser light passes through a λ / 4 wave plate, and under the action of two reflectors, passes through the lens and is coupled to the optical resonant cavity. The distance between the optical resonant cavity and the laser is controlled to be an integer multiple of the cavity length.

[0005] According to another aspect of the present invention, a method for quickly realizing a ring-down spectrum is provided, comprising the following steps: S1, making a signal generator output a half-sawtooth wave signal, and under the action of a laser driver, a semiconductor laser outputs a laser; S2, the laser passes through a λ / 2 wave plate, and the λ / 2 wave plate is rotated to maximize the optical power passing through a polarization beam splitter prism, and then passes through a λ / 4 wave plate, and under the action of two reflectors, passes through a lens, and is coupled to an optical resonant cavity; S3, controlling the distance between the optical resonant cavity and the laser to be an integer multiple of the cavity length, at which point each longitudinal mode where the laser is coupled to the optical resonant cavity has a light shearing process caused by optical feedback, and this process generates a ring-down event, and the ring-down events of continuous longitudinal modes constitute a ring-down spectrum.

[0006] The invention also provides an application of a system for realizing fast scanning cavity ring-down spectroscopy in measuring gas concentration.

[0007] Compared with the traditional shutdown method based on acousto-optic modulators, this technology uses optical feedback to spontaneously realize the excitation of ring-down events. It does not require the use of optical isolators and acousto-optic modulators, reducing system costs. At the same time, it improves the measurement speed of the ring-down spectrum, shortening the spectral measurement time from minutes to hundreds of milliseconds, which can meet the needs of atmospheric flux detection.

[0008] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 It is a structural schematic diagram of a system for realizing fast scanning cavity ring-down spectroscopy of the present invention; Figure 2 The output frequency of the laser in the fast scanning cavity ring-down of the present invention is shown; Figure 3 Shows Figure 2 A partial enlarged view of the output frequency shown; Figure 4 The transmitted light signal under the medium of the present invention is shown; Figure 5 The enlarged result of a certain longitudinal mode is shown; Figure 6 It shows that the signal at the falling edge of the longitudinal mode is the ring-down signal to be fitted; Figure 7 The fit of the ring-down signal is shown; Figure 8 The ring-down spectrum of the entire scan range is shown; Fig. 9 The absorption spectrum obtained by rapid scanning is shown. DETAILED DESCRIPTION

[0010] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0011] like Figure 1 As shown, the system for realizing fast scanning cavity ring-down spectroscopy of the present invention includes a laser, a λ / 2 wave plate, a polarization beam splitter prism, a λ / 4 wave plate, a first reflector, a second reflector, a lens, an optical resonant cavity, and a photoelectric detector arranged in sequence along the optical path. It also includes a signal generator, a laser driver, a data acquisition card, a computer, a high voltage source, and piezoelectric ceramics.

[0012] The signal generator outputs a half-sawtooth wave signal. Under the action of the laser driver, the 1654nm semiconductor laser outputs 6046.0 cm -1 ~ 6047.5 cm -1 The laser passes through a λ / 2 wave plate, which is rotated to maximize the optical power passing through the polarization beam splitter prism. The laser passes through a λ / 4 wave plate, and under the action of two reflectors, passes through a lens and is coupled to the optical resonant cavity. The distance between the incident cavity mirror of the optical resonant cavity and the light output surface of the laser is controlled at 1 times the cavity length.

[0013] The signal generator drives the laser through the driver. The optical detector collects data through the data acquisition card and transmits it to the computer. The computer drives the piezoelectric ceramic through a high-voltage source to fine-tune the position of the second reflector, thereby adjusting the distance between the incident cavity mirror of the optical resonant cavity and the light-emitting surface of the laser in real time to compensate for the slow drift of the laser wavelength over time.

[0014] In the present invention, the distance from the laser to the resonant cavity satisfies an integer multiple of the length of the optical resonant cavity. At this time, the laser is not disturbed by the direct reflected light of the optical resonant cavity, and the coupling of the laser and the continuous longitudinal mode of the optical resonant cavity is realized at the same time; the coupling of each longitudinal mode is accompanied by a rapid decoupling process, which can stimulate the generation of a ring-down event. The ring-down event of the continuous longitudinal mode constitutes a ring-down spectrum, which can be used to measure the concentration of the gas in the optical resonant cavity.

[0015] The method for implementing the fast scanning cavity ring-down spectroscopy system of the present invention comprises the following steps: 1. The reflected light from the optical resonant cavity passes through a λ / 4 wave plate. Rotating the λ / 4 wave plate can change the intensity of the light fed back into the laser.

[0016] 2. According to the resonant cavity transmission signal captured by the photodetector, the λ / 4 wave plate is rotated to control the duty cycle of the transmission signal in the entire scanning signal to be less than 50%.

[0017] 3. Evacuate the optical resonant cavity and use a data acquisition card to collect a complete scanning signal.

[0018] 4. Pour the gas to be tested into the optical resonant cavity and use the data acquisition card to collect a complete scanning signal.

[0019] 5. Extract the falling edge of each longitudinal mode transmission signal and perform exponential fitting on this part of the signal. The fitting formula is: , get the decay time ; respectively get the ring-down time in the cavity state , and the ring-down time under gas ;

[0020]

[0021] In the above formula is the cavity length of the resonant cavity, is the speed of light, is the reflectivity of the optical resonant cavity high reflective mirror, is the absorbance of the molecule to be measured.

[0022] 6. Calculate absorbance , and obtain the ring-down spectrum.

[0023] Compared with the traditional shutdown method based on acousto-optic modulators, this technology uses optical feedback to spontaneously realize the excitation of ring-down events. It does not require the use of optical isolators and acousto-optic modulators, reducing system costs. At the same time, it improves the measurement speed of the ring-down spectrum, shortening the spectral measurement time from minutes to hundreds of milliseconds, which can meet the needs of atmospheric flux detection.

[0024] The theoretical response expression of the laser output wavelength of a semiconductor laser under the action of the reflected light feedback from the optical resonant cavity is as follows:

[0025] In the formula is the output frequency of the laser without optical feedback (free running state), is the output frequency under the action of optical feedback. The meanings of other parameters in the formula are shown in the following table.

[0026]

[0027] Taking the values ​​of the parameters in the above table as an example, the frequency curve of the laser output light wave is as follows: Figure 2 and Figure 3 shown.

[0028] Figure 3 The arrow in the figure indicates the frequency response of the laser during actual operation. It can be seen that the laser has frequency jumps. This process is used to replace the light cutting process of the traditional acousto-optic modulator.

[0029] At each longitudinal mode where the laser is coupled to the optical resonant cavity, there is a light shearing process caused by optical feedback. This process further leads to a ring-down event. The resulting signal can be described by the following formula:

[0030] in is the light intensity at the moment of light cutting, The time elapsed from the moment of light cutting off The light intensity after Decline time The ring-down time can be obtained by fitting the above formula to the ring-down signal. When the optical resonator is in a vacuum state, the ring-down time is Cavity ring-down time , including the ring-down time obtained from the gas state to be measured Absorption ring-down time Based on the above two kinds of ring-down time, the absorbance of the gas absorption spectrum can be obtained: :

[0031] Where c is the speed of light. ), the integrated absorbance A can be calculated by the following formula:

[0032] in For a known optical resonant cavity temperature , pressure P and the absorption line intensity corresponding to the absorption line of the selected absorption spectrum In this case, the concentration of the gas to be measured in the optical resonant cavity can be further obtained. :

[0033] Generally, in the field of atmospheric environment measurement, it is usually necessary to know the concentration of a certain gas, such as atmospheric CO2 concentration, the change of CH4 concentration over time, CO2 flux of vegetation or ocean, etc. The method of the present invention can be applied thereto.

[0034] The above description is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fast scanning cavity ring-down spectroscopy implementation system, characterized in that: It includes a laser, a λ / 2 wave plate, a polarization beam splitter prism, a λ / 4 wave plate, a first reflector, a second reflector, a lens, an optical resonant cavity, and a photoelectric detector arranged in sequence along the optical path, a signal generator and a laser driver, a data acquisition card, a computer, a high-voltage source, and piezoelectric ceramics. The signal generator outputs a half-sawtooth wave signal. Under the action of the laser driver, the semiconductor laser outputs laser light. The laser light passes through a λ / 2 wave plate, and the λ / 2 wave plate is rotated to maximize the optical power passing through the polarization beam splitter prism. The laser light then passes through a λ / 4 wave plate, and under the action of two reflectors, passes through a lens, and is coupled with an optical resonant cavity. The distance between the optical resonant cavity and the laser is controlled to be an integer multiple of the cavity length.

2. The fast scanning cavity ring-down spectroscopy implementation system according to claim 1, characterized in that: The distance between the optical resonant cavity and the laser refers to the distance between the incident cavity mirror of the optical resonant cavity and the light emitting surface of the laser.

3. The fast scanning cavity ring-down spectroscopy implementation system according to claim 1, characterized in that: The laser is a 1654 nm semiconductor laser with an output of 6046.0 cm -1 ~ 6047.5 cm -1 Range of laser.

4. The fast scanning cavity ring-down spectroscopy implementation system according to claim 1, characterized in that: The method for obtaining the ring-down spectrum is as follows: S1. According to the resonant cavity transmission signal captured by the photodetector, the λ / 4 wave plate is rotated to control the duty cycle of the transmission signal in the entire scanning signal to be less than 50%; S2, evacuate the optical resonant cavity and use a data acquisition card to collect a complete scanning signal; S3, flush the gas to be tested into the optical resonant cavity, and use a data acquisition card to collect a complete scanning signal; S4, extract the falling edge part of each longitudinal mode transmission signal, perform exponential fitting on the falling edge part signal, and obtain the ring-down time, thereby obtaining the ring-down time in the cavity state respectively , and the ring-down time under gas ; S5. Calculate absorbance , and obtain the ring-down spectrum, The speed of light.

5. The fast scanning cavity ring-down spectroscopy implementation system according to claim 4, characterized in that: At each longitudinal mode where the laser is coupled to the optical resonant cavity, there is a light shearing process caused by optical feedback. This process further leads to a ring-down event. The resulting signal can be described by the following formula: ， in is the light intensity at the moment of light cutting, The time elapsed from the moment of light cutting off The light intensity after Decline time The ring-down time can be obtained by fitting the above formula to the ring-down signal. .

6. The fast scanning cavity ring-down spectroscopy implementation system according to claim 1, characterized in that: The computer drives the piezoelectric ceramics through a high voltage source to slightly adjust the position of the second reflector to adjust the distance between the incident cavity mirror of the optical resonant cavity and the light-emitting surface of the laser in real time to compensate for the slow drift of the laser wavelength over time.

7. The fast scanning cavity ring-down spectroscopy implementation system according to claim 1, characterized in that: The distance between the optical resonant cavity and the laser is controlled to be 1 times the cavity length.

8. The fast scanning cavity ring-down spectroscopy implementation system according to claim 7, characterized in that: The distance between the laser and the external resonant cavity is 1 m, and the cavity length of the external resonant cavity is 1 m.

9. A method for quickly realizing ring-down spectroscopy, characterized in that: The following steps are involved: S1, make the signal generator output a half-sawtooth wave signal, and under the action of the laser driver, the semiconductor laser outputs laser; S2, the laser passes through a λ / 2 wave plate, rotates the λ / 2 wave plate to maximize the optical power passing through the polarization beam splitter prism, and then passes through a λ / 4 wave plate. Under the action of two reflectors, after passing through the lens, it is coupled with the optical resonant cavity; S3. Control the distance between the optical resonant cavity and the laser to be an integer multiple of the cavity length. At this time, at each longitudinal mode where the laser is coupled to the optical resonant cavity, there is a light shearing process caused by optical feedback. This process produces a ring-down event, and the ring-down events of continuous longitudinal modes constitute a ring-down spectrum.

10. Use of the fast scanning cavity ring-down spectroscopy implementation system according to any one of claims 1 to 8 in measuring gas concentration.

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