A system and application for realizing fast-scanning cavity ring-down spectroscopy

Abstract

The present invention discloses a system for realizing fast-scanning cavity ring-down spectroscopy and its application. The system includes a laser, a λ / 2 wave plate, a polarization beam splitter prism, a λ / 4 wave plate, a first mirror, a second mirror, a lens, an optical resonator, and a photodetector arranged in sequence along the optical path. It also includes a signal generator and a laser driver. The signal generator outputs a semi-sawtooth wave signal. Under the action of the laser driver, the semiconductor laser outputs laser light. The laser light passes through one λ / 2 wave plate, and the λ / 2 wave plate is rotated to maximize the optical power passing through the polarization beam splitter prism. Then it passes through one λ / 4 wave plate. Under the action of the two mirrors, after passing through the lens, it is coupled with the optical resonator. The distance between the optical resonator and the laser is controlled to be an integer multiple of the cavity length. The present invention shortens the spectral measurement time from the minute level to the hundreds of milliseconds level, which can meet the requirements of atmospheric flux detection.

Description

Technical Field

[0001] The present invention relates to optical cavity ring-down spectroscopy technology, and particularly to a system and application for realizing fast-scanning cavity ring-down spectroscopy. Background Art

[0002] For the cavity ring-down spectroscopy technology based on the acousto-optic modulator turn-off method, an optical isolator is required to isolate the reflected light from the optical resonator, and based on the judgment of the intensity of the resonator transmission signal, the acousto-optic modulator is used to actively turn off the laser to stimulate the generation of ring-down signals. This process requires turning off each longitudinal mode one by one to obtain a complete spectrogram. Therefore, the measurement time of this mechanical ring-down spectroscopy is usually limited to the order of minutes. This response time is unacceptable for some applications that require rapid measurement. Summary of the Invention

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

[0004] To this end, on the one hand, the present invention provides a system for realizing fast-scanning cavity ring-down spectroscopy, including a laser, a λ / 2 waveplate, a polarization beam splitter prism, a λ / 4 waveplate, a first mirror, a second mirror, a lens, an optical resonator, and a photodetector arranged in sequence along the optical path. It also includes a signal generator and a laser driver, and also includes a data acquisition card, a computer, a high-voltage source, and a piezoelectric ceramic. The signal generator outputs a half-sawtooth wave signal. Under the action of the laser driver, the semiconductor laser outputs laser. The laser passes through one λ / 2 waveplate, and the λ / 2 waveplate is rotated to make the optical power passing through the polarization beam splitter prism maximum. Then it passes through one λ / 4 waveplate. Under the action of the two mirrors, after passing through the lens, it is coupled with the optical resonator. The distance between the optical resonator 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 ring-down spectroscopy is provided, including the following steps: S1, making 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 one λ / 2 waveplate, and the λ / 2 waveplate is rotated to make the optical power passing through the polarization beam splitter prism maximum. Then it passes through one λ / 4 waveplate. Under the action of the two mirrors, after passing through the lens, it is coupled with the optical resonator; S3, controlling the distance between the optical resonator and the laser to be an integer multiple of the cavity length. At this time, at each longitudinal mode where the laser is coupled with the optical resonator, there is a chopping process due to optical feedback, and this process generates ring-down events. The ring-down events of consecutive longitudinal modes constitute a ring-down spectrum.

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

[0007] Compared with the traditional method of turning off the acousto-optic modulator, the present technology uses optical feedback to spontaneously realize the excitation of the ring-down event, without the need to use an optical isolator and an acousto-optic modulator, reducing the system cost; at the same time, it improves the measurement speed of the ring-down spectrum, shortening the spectral measurement time from the order of minutes to the order of hundreds of milliseconds, which can meet the requirements of atmospheric flux detection.

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

[0009] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0010] Figure 1 is a schematic structural diagram of the implementation system of the fast-scanning cavity ring-down spectrum of the present invention;

[0011] Figure 2 shows the output frequency of the laser in the fast-scanning cavity ring-down of the present invention;

[0012] Figure 3 shows Figure 2 a partial enlarged view of the shown output frequency;

[0013] Figure 4 shows the transmitted optical signal with a medium in the present invention;

[0014] Figure 5 shows the amplification result of a certain longitudinal mode;

[0015] Figure 6 shows that the signal at the falling edge of the longitudinal mode is the ring-down signal to be fitted;

[0016] Figure 7 shows the fitting of the ring-down signal;

[0017] Figure 8 shows the ring-down spectrum over the entire scanning range;

[0018] Figure 9 shows the absorption spectrum obtained by fast scanning. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0020] As Figure 1As shown in the figure, the implementation system of the rapid-scanning cavity ring-down spectroscopy of the present invention includes a laser, a λ / 2 wave plate, a polarization beam splitting prism, a λ / 4 wave plate, a first mirror, a second mirror, a lens, an optical resonator, and a photodetector, which are 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 a piezoelectric ceramic.

[0021] The signal generator outputs a semi-sawtooth wave signal. Under the action of the laser driver, the 1654 nm semiconductor laser outputs a laser in the range of 6046.0 cm -1 ~ 6047.5 cm -1 The laser passes through a λ / 2 wave plate. By rotating the λ / 2 wave plate, the optical power of the light passing through the polarization beam splitting prism can be maximized. The laser passes through a λ / 4 wave plate. Under the action of two mirrors, after passing through the lens, it is coupled with the optical resonator. The distance between the incident mirror of the optical resonator and the light output surface of the laser is controlled within 1 times the cavity length.

[0022] The signal generator drives the laser through the driver. The optical detector transmits the collected data to the computer through the data acquisition card. The computer drives the piezoelectric ceramic through the high-voltage source to finely adjust the position of the second mirror, thereby adjusting in real time the distance between the incident mirror of the optical resonator and the light output surface of the laser to compensate for the slow drift of the laser wavelength over time.

[0023] In the present invention, the distance from the laser to the resonator satisfies an integer multiple of the optical resonator cavity length. At this time, the laser is not interfered by the direct reflected light of the optical resonator, and at the same time, the coupling of the laser with the continuous longitudinal modes of the optical resonator is realized; the coupling of each longitudinal mode is accompanied by a process of rapid decoupling, and this process can stimulate the generation of ring-down events. The ring-down events of the continuous longitudinal modes constitute a ring-down spectrum, and the ring-down spectrum can be used to measure the concentration of the gas in the optical resonator.

[0024] The implementation method of the rapid-scanning cavity ring-down spectroscopy system of the present invention includes the following steps:

[0025] 1. The reflected light of the optical resonator passes through a λ / 4 wave plate. By rotating the λ / 4 wave plate, the light intensity fed back into the laser can be changed.

[0026] 2. According to the resonator transmission signal captured by the photodetector, rotate the λ / 4 wave plate to control the duty cycle of the transmission signal in the entire scanning signal to be less than 50%.

[0027] 3. Evacuate the optical resonator and use the data acquisition card to collect a complete scanning signal.

[0028] 4. Fill the optical resonator with the gas to be measured and use the data acquisition card to collect a complete scanning signal.

[0029] 5. Extract the falling edge part of the transmission signal of each longitudinal mode, and perform exponential fitting on this part of the signal. The fitting formula is: , and obtain the decay time ; respectively obtain the decay time in the cavity state, and the decay time with gas;

[0030]

[0031]

[0032] In the above formula is the cavity length of the resonant cavity, is the speed of light, is the reflectivity of the highly reflective mirror of the optical resonant cavity, is the absorbance of the molecule to be measured.

[0033] 6. Calculate the absorbance , and obtain the decay spectrum.

[0034] Compared with the traditional method based on the off state of the acousto-optic modulator, the present technology uses optical feedback to spontaneously realize the excitation of the decay event, does not require the use of an optical isolator and an acousto-optic modulator, reduces the system cost; at the same time, improves the measurement speed of the decay spectrum, shortens the spectral measurement time of the minute level to the order of hundreds of milliseconds, and can meet the needs of atmospheric flux detection.

[0035] Under the action of optical feedback of the reflected light of the semiconductor laser in the optical resonant cavity, the theoretical expression of the response of the laser output wavelength is as follows:

[0036]

[0037] 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.

[0038]

[0039] Taking the numerical values of the above parameters in the table as an example, the frequency curve of the laser output light wave is as Figure 2 and Figure 3 shown.

[0040] Figure 3 The arrow in

[0041] At each longitudinal mode where the laser is coupled to the optical resonator, there is a light chopping process due to optical feedback, which further leads to the generation of ring-down events. The formed signal can be described by the following formula:

[0042]

[0043] where is the light intensity at the moment of light chopping, is the light intensity after time has passed since the moment of light chopping, is the reciprocal of the ring-down time . Fitting the ring-down signal with the above formula can obtain the ring-down time . When the optical resonator is in a vacuum state, the obtained ring-down time is the ring-down time of the empty cavity , and the ring-down time obtained with the state of the gas to be measured is the ring-down time with absorption . According to the above two ring-down times, the absorbance of the gas absorption spectrum can be obtained:

[0044]

[0045] where c is the speed of light. For the entire absorption peak (i.e., multiple absorbances obtained from continuous longitudinal modes), the integrated absorbance A can be calculated by the following formula:

[0046]

[0047] where . For a known optical resonator temperature , pressure P, and the absorption line strength corresponding to the absorption line of the selected absorption spectrum, the concentration of the gas to be measured in the optical resonator can be further obtained:

[0048]

[0049] Generally, in the field of atmospheric environment measurement, it is usually necessary to know the concentration of a certain gas, such as the variation of atmospheric CO2 concentration, CH4 concentration over time, the CO2 flux of vegetation or the ocean, etc. The method of the present invention can be applied thereto.

[0050] The above are only embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A fast scanning cavity ring-down spectroscopy implementation system, characterized in that: Optical feedback is used to spontaneously realize the excitation of the ring-down event, and no optical isolator and acousto-optic modulator are needed. The system includes a laser, a λ / 2 wave plate, a polarization beam splitter prism, a λ / 4 wave plate, a reflector 1, a reflector 2, a lens, an optical resonant cavity, a photodetector, a signal generator and a laser driver, a data acquisition card, a computer, a high-voltage source, and piezoelectric ceramics arranged in sequence along the optical path. 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. Then, the laser light passes through a λ / 4 wave plate. Under the action of two reflectors, the laser light passes through a lens and is coupled with an optical resonant cavity. The distance between the optical resonant cavity and the laser light is controlled to be an integer multiple of the cavity length. Under the action of the optical feedback of the reflected light of the optical resonant cavity, the semiconductor laser light has a frequency jump. This process is used to replace the light cutting process of a traditional acousto-optic modulator. 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 is 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 is obtained by fitting the ring-down signal using the above formula. , 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.

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 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.

5. 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.

6. The fast scanning cavity ring-down spectroscopy implementation system according to claim 5, 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.

7. A method for quickly realizing ring-down spectroscopy, characterized in that: Using the fast scanning cavity ring-down spectroscopy implementation system according to any one of claims 1 to 6, the method comprises the following steps: 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 a lens, it is coupled with the optical resonant cavity. 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%; 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.

8. Use of the fast scanning cavity ring-down spectroscopy implementation system according to any one of claims 1 to 6 in measuring gas concentration.

Images