Intermediate infrared spectrum cavity ring-down system based on quantum cascade laser

Abstract

The invention belongs to the technical field of laser spectrum, and particularly relates to an intermediate infrared spectrum cavity ring-down system based on a quantum cascade laser. In order to overcome the wavelength drift of the laser and improve the accuracy of gas concentration measurement, a light beam emitted by the quantum cascade laser passes through two isolators and is divided into two light beams through a beam splitter, and the light with relatively low light intensity is reflected by a transmitting mirror, passes through a standard gas pool and a focusing lens and is emitted into a first detector to obtain a direct absorption signal. The light with higher light intensity is reflected by the gold film reflector, enters the optical resonant cavity and is driven into the second detector through the focusing lens, the signal is interrupted through DG535 to obtain a ring-down time signal, and the ring-down time signal is processed through a program to obtain a gas absorption signal. A direct absorption signal of the first detector is collected, a polynomial is used for fitting the absorption signal, each parameter of the fitting polynomial is obtained, and the fitting polynomial is used for fitting the signal obtained by the second detector.

Description

Technical Field

[0001] The invention belongs to the technical field of laser spectroscopy, and in particular relates to a mid-infrared spectroscopy cavity ring-down system based on quantum cascade lasers. Background Art

[0002] The mid-infrared band occupies an important position in the spectrum from visible light to far infrared, and has a wide spectral range. It is an important spectral band that humans can use. At the same time, this band is not only located in the molecular fingerprint area, but also covers the two atmospheric transmission windows of 3-5μm and 8-13μm. Therefore, mid-infrared lasers have great application value in the fields of material processing, biomedicine, spectral diagnosis, laser remote sensing, and national defense security.

[0003] Cavity ring-down spectroscopy is a highly sensitive laser absorption spectroscopy technique that determines the absorption information of the target sample by measuring the attenuation rate of the intracavity optical cavity. The gas to be measured is placed in a high-precision cavity composed of two highly reflective mirrors. When a laser pulse is coupled into the cavity, the light field will be reflected multiple times between the two cavity mirrors. Since the lenses have a certain transmission loss, a portion of the light will be transmitted out of the cavity from the cavity mirrors at each reflection. By measuring the transmitted light intensity of the cavity, the ring-down time constant of the cavity can be determined. By measuring the ring-down time of the cavity with and without a sample, the absorption of the target sample in the cavity can be obtained.

[0004] In cavity ring-down spectroscopy, the accuracy of concentration inversion of the ring-down signal depends on the accuracy of the laser frequency. Since the duration of the ring-down signal generated by the excitation is in the microsecond range, the commonly used wavelength meter based on Fourier transform infrared spectroscopy is difficult to apply. The usual practice is to calibrate the ring-down signal points containing dozens or even hundreds of unknown frequencies to 2 to 3 gas absorption peaks. This wavelength calibration technology has a serious problem of insufficient accuracy because it cannot solve the problem of nonlinear scanning of the laser wavelength. Summary of the invention

[0005] The invention provides a mid-infrared spectral cavity ring-down system based on a quantum cascade laser, which adopts a wavelength calibration technology based on standard gas absorption spectrum fitting, can overcome the problem of laser wavelength drift, ensure the correctness of fitting, and improve the accuracy of gas concentration measurement.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a mid-infrared spectroscopy cavity ring-down system based on a quantum cascade laser, comprising: a laser current controller, a laser temperature controller, a quantum cascade laser, a standard gas pool, a first detector, an FP optical resonant cavity, and a second detector;

[0008] The output end of the laser current controller is connected to the first input end of the quantum cascade laser, and the output end of the laser temperature controller is connected to the second input end of the quantum cascade laser, so as to realize dual tuning of the temperature and current of the quantum cascade laser. The output end of the quantum cascade laser emits a light beam, which passes through the first isolator and the second isolator in sequence, and then is split by the beam splitter. The light beam with weaker light intensity is reflected by the first gold film reflector and enters the standard gas pool, and then enters the first detector through the first focusing lens. The light beam with stronger light intensity enters the FP optical resonant cavity through the second gold film reflector, the third gold film reflector, the first matching lens, and the second matching lens. The transmitted light beam of the FP optical resonant cavity enters the second detector through the second focusing lens.

[0009] Furthermore, it also includes a PZT, which is arranged on the cavity of the FP optical resonant cavity to adjust the cavity length. The FP optical resonant cavity has a cavity mirror on one side and the PZT on the other side, and then the cavity mirror is glued on the PZT, and the PZT is a part of the cavity length.

[0010] To prevent optical feedback, an isolator is used to block the emitted light. One isolator does not provide enough isolation, so two isolators are used.

[0011] Furthermore, the splitting ratio of the beam splitter is 1:9.

[0012] Furthermore, the central wavelength of the quantum cascade laser is in the mid-infrared band, and the wavelength can be adjusted within a certain range by adjusting the laser current and the temperature controller, so what is measured is the concentration of trace gases in the mid-infrared spectral band.

[0013] Furthermore, the signal in the second detector is interrupted by a digital delay and pulse generator to obtain a ring-down time signal, and a gas absorption signal is obtained by program processing;

[0014] The direct absorption signal of the first detector is collected, and a polynomial is used to fit the direct absorption signal to obtain various parameters of the fitting polynomial, and the fitting polynomial is used to fit the gas absorption signal obtained by the second detector to achieve gas concentration measurement.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The system designed by the present invention can improve the accuracy of concentration measurement. Generally, since temperature drift is difficult to completely avoid, the laser wavelength will drift, and the same gas often has multiple absorption lines, which may result in the absorption obtained by the measured data not being the same absorption, thereby causing inaccurate concentration measurement. However, if the system designed by the present invention is not the same absorption, it will not fit. Only the same absorption can satisfy the fitting of the polynomial, thereby obtaining an accurate gas concentration.

[0017] The laser wavelength calibration method proposed in the present invention is different from the traditional method of calibrating first and then using. It can obtain the laser wavelength at the moment of ring-down signal excitation in real time, eliminating errors caused by factors such as nonlinear effects of laser light emission and random coupling between laser and optical cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The schematic diagram of the structure of the mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser;

[0019] Among them, 1 is a laser current controller, 2 is a laser temperature controller, 3 is a quantum cascade laser, 4 and 5 are isolators, 6 is a beam splitter, 7 is a first gold film reflector, 11 is a second gold film reflector, 12 is a third gold film reflector, 8 is a standard gas pool, 9 is a first focusing lens, 16 is a second focusing lens, 10 is a first detector, 13 is a first matching lens, 14 is a second matching lens, 15 is a FP optical resonant cavity, and 17 is a second detector;

[0020] Figure 2 Schematic diagram of the principle of mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser. DETAILED DESCRIPTION

[0021] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0022] like Figure 1 As shown, a mid-infrared spectroscopy cavity ring-down system based on a quantum cascade laser comprises: a laser current controller 1, a laser temperature controller 2, a quantum cascade laser 3, a standard gas pool 8, a first detector 10, an FP optical resonant cavity 15 and a second detector 17;

[0023] The output end of the laser current controller 1 is connected to the first input end of the quantum cascade laser 3, and the output end of the laser temperature controller 2 is connected to the second input end of the quantum cascade laser 3, so as to realize the dual tuning of the temperature and current of the quantum cascade laser 3. The output end of the quantum cascade laser 3 emits a light beam, which passes through the first isolator 4 and the second isolator 5 in sequence, and then is split by the beam splitter 6. The light beam with weaker light intensity is reflected by the first gold film reflector 7 and enters the standard gas pool 8, and then enters the first detector 10 through the first focusing lens 9. The light beam with stronger light intensity enters the FP optical resonant cavity 15 through the second gold film reflector 11, the third gold film reflector 12, the first matching lens 13, and the second matching lens 14. The transmitted light beam of the FP optical resonant cavity 15 enters the second detector 17 through the second focusing lens 16.

[0024] The system further includes a PZT, which is disposed on the cavity of the FP optical resonant cavity 15 to adjust the cavity length.

[0025] The beam splitter 6 of this system has a splitting ratio of 1:9, which divides the light beam into two beams of weaker and stronger intensity.

[0026] The central wavelength of the quantum cascade laser 3 of this system is 4527nm, and the wavelength can be adjusted within a certain range by adjusting the laser current and the temperature controller, which is in the mid-infrared band.

[0027] The signal in the second detector 17 is interrupted by a digital delay and pulse generator (DG535) to obtain a ring-down time signal, and a gas absorption signal is obtained by program processing;

[0028] The direct absorption signal of the first detector 10 is collected, and a polynomial is used to fit the direct absorption signal to obtain various parameters of the fitting polynomial, and the fitting polynomial is used to fit the gas absorption signal obtained by the second detector 17 to achieve more accurate gas concentration measurement.

[0029] Cavity ring-down spectroscopy is an absolute measurement method based on two physical foundations: First, because different molecular chemical bonds have specific vibration and rotation energy levels, molecules have characteristic infrared absorption spectra, that is, gas molecules must have one or more absorption wavelengths that are different from the wavelengths of all other molecules. In this way, we can calculate the gas content by testing the gas's absorption of lasers of this wavelength; second, the laser pulse can complete thousands of back and forth reflections in a high-quality factor ring-down cavity, and the light intensity decays exponentially with time:

[0030] I(t)=I 0 exp(-t / τ)

[0031] Where I is the light intensity, t is the time, I0 is the initial light intensity, and τ is called the ring-down time. When the laser wavelength is the same as the absorption wavelength in the ring-down cavity, the ring-down time τ will be greatly reduced. The greater the concentration of the gas to be measured, the smaller τ. In the actual measurement process, the I(t) curve can be measured, τ can be obtained by fitting, and then the concentration of the gas to be measured can be inferred.

[0032] Compared with traditional laser absorption spectroscopy, cavity ring-down spectroscopy (CRDS) overcomes the influence of light intensity and coupling efficiency fluctuations, measures the time domain variation of light intensity in the cavity, and inverts gas concentration with a sensitivity of up to 10 -10 .

[0033] like Figure 2 As shown, the laser light source passes through the beam splitter prism, and then passes through the absorption cell and the cavity ring-down. The absorption cell is filled with absorption gas with fixed concentration, pressure, temperature and other parameters, and the ring-down cavity is filled with the gas to be measured. For the absorption cell under fixed conditions, the reference frequency of the absorption spectrum obtained is used as the independent variable, which corresponds to the corresponding absorbance one by one. At this time, a polynomial can be used to fit it. After obtaining the fitted polynomial, we can obtain the laser frequency (or wavelength) that strictly corresponds to the ring-down spectrum (discrete points).

[0034] The above shows and describes the main features and advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present invention.

[0035] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A mid-infrared spectroscopy cavity ring-down system based on quantum cascade lasers, characterized in that: include: Laser current controller (1), laser temperature controller (2), quantum cascade laser (3), standard gas pool (8), first detector (10), FP optical resonant cavity (15) and second detector (17); The output end of the laser current controller (1) is connected to the first input end of the quantum cascade laser (3), and the output end of the laser temperature controller (2) is connected to the second input end of the quantum cascade laser (3), so as to realize dual tuning of the temperature and current of the quantum cascade laser (3). The output end of the quantum cascade laser (3) emits a light beam, which passes through a first isolator (4) and a second isolator (5) in sequence, and is then split by a beam splitter (6). A light beam with a relatively low light intensity is reflected by a first gold film reflector (7) and enters a standard gas pool (8), and then enters a first detector (10) through a first focusing lens (9). A light beam with a relatively high light intensity enters an FP optical resonant cavity (15) through a second gold film reflector (11), a third gold film reflector (12), a first matching lens (13), and a second matching lens (14). The light beam transmitted through the FP optical resonant cavity (15) enters a second detector (17) through a second focusing lens (16).

2. A mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser according to claim 1, characterized in that: It also comprises a PZT, which is arranged on the cavity of the FP optical resonant cavity (15) and is used to adjust the cavity length.

3. The mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser according to claim 1, characterized in that: The beam splitter (6) has a splitting ratio of 1:

9.

4. The mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser according to claim 1, characterized in that: The signal in the second detector (17) is interrupted by a digital delay and pulse generator to obtain a ring-down time signal, and a gas absorption signal is obtained by program processing; The direct absorption signal of the first detector (10) is collected, and a polynomial is used to fit the direct absorption signal to obtain various parameters of the fitting polynomial, and the fitting polynomial is used to fit the gas absorption signal obtained by the second detector (17) to achieve gas concentration measurement.

5. The mid-infrared spectroscopy cavity ring-down system based on quantum cascade laser according to claim 1, characterized in that: The central wavelength of the quantum cascade laser (3) is in the mid-infrared band.

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