Optical feedback cavity ring-down spectroscopy system without phase control

Abstract

Disclosed is an optical feedback cavity ring-down spectroscopy system without phase control. The system comprises: a light source module, used for generating a laser of a predetermined wavelength; an optical feedback module, used for feeding back a part of the laser generated by the light source module into the light source module, and generating strong feedback or moderate feedback, so as to narrow the line width of the laser; an optical cavity module, used for increasing the effective action distance between the laser and a target detection substance, and exciting an optical cavity ring-down signal by means of achieving resonance between the laser and an optical cavity; a trigger light-cutting module, used to instantaneously cut off incident light of the optical cavity module to facilitate detection; a detection module, used to detect a transmitted optical signal outputted by the optical cavity module, convert the transmitted optical signal into a voltage signal, to be used as an optical cavity ring-down signal; and a data acquisition module, used to acquire the optical cavity ring-down signal outputted by the detection module, and perform exponential fitting on the optical cavity ring-down signal to obtain a ring-down time.

Description

Optical feedback optical cavity ring-down spectroscopy system without phase control TECHNICAL FIELD

[0001] The present application relates to an optical feedback optical cavity ring-down spectroscopy technology, in particular to an optical feedback optical cavity ring-down spectroscopy technology system without controlling the phase of feedback light, realizing high signal-to-noise ratio optical cavity ring-down spectroscopy measurement, and solving the typical problems faced by traditional optical cavity ring-down spectroscopy technology. BACKGROUND

[0002] Optical cavity ring-down spectroscopy technology obtains the actual loss in the cavity by measuring the decay time of photons in a high-precision optical cavity, and then inversely calculates the information of the sample to be measured in the cavity. In the optical cavity ring-down spectroscopy measurement technology, in order to realize high detection sensitivity, a high reflectivity mirror with reflectivity higher than 99.99% and a longer optical cavity length are generally selected to realize the effective action distance of kilometer level of light and gas, which leads to the longitudinal mode linewidth of the optical cavity generally being in the order of kilohertz (kHz). However, the laser emitted by the semiconductor laser in the free-running state generally has a large frequency noise, and the linewidth is generally in the order of megahertz (MHz), which is much larger than the linewidth of the optical cavity, resulting in a very low efficiency of coupling the laser into the optical cavity, and the transmission intensity of the optical cavity is very weak. At the same time, factors such as scanning laser wavelength, scanning optical cavity length and environmental changes will lead to unstable coupling in the optical cavity, and the transmission signal of the optical cavity is chaotic, which limits the signal-to-noise ratio and stability of the optical cavity ring-down spectroscopy detection.

[0003] Reducing the laser linewidth is the key to solving these problems. Based on a high-precision optical cavity, the Pound-Drever-Hall (PDH) electric locking method can realize laser frequency stabilization and laser linewidth compression. However, this technology requires a complex locking system, including an optical phase modulator and electronic devices for error signal extraction and feedback control. In addition to the PDH technology, some researches have shown that the laser and optical frequency comb can be locked to realize the compression of the laser linewidth, but this method also requires a complex hardware system and locking device.

[0004] Optical feedback is another common method to realize laser frequency locking. A part of the laser output photons is returned to the laser cavity through the optical cavity, and the feedback light is coupled with the cavity light field in the laser, thereby changing the operating state of the laser. If the ratio of the optical power returned to the laser to the laser output power is defined as the optical feedback ratio, for general semiconductor lasers, strong feedback (optical feedback ratio greater than -10dB) or moderate feedback (optical feedback ratio between -30dB and -10dB) can easily make the laser unstable and significantly widen the laser linewidth. Such optical feedback usually needs to be avoided in laser systems. For weak feedback (optical feedback ratio less than -30dB), if the feedback phase is precisely controlled so that the feedback light always resonates with the cavity light field in the laser, the laser linewidth can be narrowed. In the past decade, some researchers have used the transmission light of the optical cavity as feedback light to lock the laser frequency to the resonant mode of the optical cavity to narrow the laser linewidth (such as US8539816B2 and CN102445423A), but this method requires a feedback system to precisely and quickly control the phase of the feedback light.

[0005] The traditional optical cavity ring-down spectroscopy technology has the problems of weak optical cavity transmission signal intensity and signal clutter caused by the laser linewidth being much larger than the longitudinal mode linewidth of the optical cavity.

[0006] SUMMARY

[0007] The present application is directed to the typical problem of the conventional optical cavity ring-down spectroscopy technology, i.e., the problem of weak optical cavity transmission signal intensity and signal clutter caused by the laser linewidth being much larger than the longitudinal mode linewidth of the optical cavity. A new optical feedback optical cavity ring-down spectroscopy system without phase control is proposed. The core of the scheme of the present application is to introduce optical feedback in the conventional optical cavity ring-down spectroscopy technology. When the optical feedback ratio reaches strong feedback or moderate feedback, the laser linewidth is narrowed, and the phase of the feedback light does not need to be controlled, thereby maintaining the simplicity of the system.

[0008] The present application provides an optical feedback optical cavity ring-down spectroscopy system without phase control, which comprises: an optical source module for generating laser of a predetermined wavelength; an optical feedback module for feeding back a part of the laser generated by the optical source module into the optical source module to generate strong feedback or moderate feedback to narrow the laser linewidth; an optical cavity module for increasing the effective action distance of the laser and the target detection substance, and exciting the optical cavity ring-down signal by realizing the resonance of the laser and the optical cavity; a trigger light cutting module for instantaneously cutting off the incident light of the optical cavity module for detection; a detection module for detecting the transmission light signal output by the optical cavity module and converting it into a voltage signal as the optical cavity ring-down signal; and a data acquisition module for acquiring the optical cavity ring-down signal output by the detection module and performing exponential fitting on the optical cavity ring-down signal to obtain the ring-down time.

[0009] Preferably, the light source module comprises: a laser for outputting laser light with a wavelength sufficient to cover the absorption spectral line of the target detection substance, and a laser controller for controlling the operating current and temperature of the laser.

[0010] Preferably, the light feedback module comprises: a light beam splitter for splitting the laser light emitted by the laser into two beams, a polarizer, and a mirror, wherein one of the two beams is reflected by the mirror back into the interior of the laser after passing through the polarizer to achieve light feedback, and when the light feedback ratio reaches a strong feedback or moderate feedback state, the laser light reflected back into the laser narrows the laser linewidth of the emitted laser light; wherein the light feedback ratio is adjusted by rotating the polarizer to change the angle between the transmission axis of the polarizer and the polarization direction of the incident laser light.

[0011] Preferably, the trigger light cutting module comprises: an acousto-optic modulator for outputting first-order diffracted light, an acousto-optic modulator driver, and a voltage comparator; the voltage comparator is used to trigger the acousto-optic modulator driver to cut off the output after receiving the output signal of the detection module, instantaneously cut off the first-order diffracted light output by the acousto-optic modulator, and trigger the data acquisition module to start collecting data.

[0012] Preferably, the optical cavity module comprises: an optical isolator, a mode matching lens group, a front cavity mirror, a rear cavity mirror, a piezoelectric ceramic, a signal generator, and a voltage amplifier; the front cavity mirror and the rear cavity mirror form an optical cavity, and the piezoelectric ceramic is fixed on the rear cavity mirror; the first-order diffracted light output by the acousto-optic modulator is transmitted through the optical isolator; the optical isolator is placed behind the acousto-optic modulator to prevent the light reflected and transmitted by the front cavity mirror from returning to the laser; the mode matching lens group is placed behind the optical isolator to match the laser and the transverse mode of the optical cavity for the light transmitted by the optical isolator, and the laser is coupled into the optical cavity formed by the front cavity mirror and the rear cavity mirror to realize the output of the optical cavity ring-down signal; the signal generator is used to generate a triangular wave signal, which drives the piezoelectric ceramic to expand and contract through the voltage amplifier to scan the length of the optical cavity, realizes the resonance of the laser and the optical cavity, and excites the optical cavity ring-down signal.

[0013] Preferably, the detection module comprises: a photodetector for detecting the intensity of the light transmitted by the optical cavity module and converting it into a voltage signal as the optical cavity ring-down signal output.

[0014] Preferably, the data acquisition module comprises: a power divider and a data acquisition system, the power divider is used for dividing the voltage signal output by the probe module into two signals, which are respectively used as the input signal of the voltage comparator and the input signal of the data acquisition system; the voltage comparator is used for sending a trigger level when the value of its input signal reaches a predetermined threshold, triggering the acousto-optic modulator driver to cut off the driving signal of the acousto-optic modulator, and further cutting off the first-order diffraction light of the acousto-optic modulator; the data acquisition system is used for starting to collect the voltage signal output by the power divider after being triggered by the trigger level output by the voltage comparator, and performing exponential fitting on the collected voltage signal to obtain the decay time.

[0015] Preferably, the light source module comprises a mid-infrared quantum cascade laser or an interband cascade laser.

[0016] Compared with the existing optical cavity ring-down technology, the present application has the following beneficial effects:

[0017] The phase control-free optical feedback optical cavity ring-down spectroscopy system provided by the present application mainly aims to solve the problem that the traditional optical cavity ring-down spectroscopy technology is limited by the weak and unstable intensity of the optical cavity transmission signal. The present application does not need to use a complex and high-cost feedback locking system, and is easy to apply to actual optical cavity ring-down measurement. Moreover, the optical feedback narrow linewidth scheme of the present application has the advantage of being universally applicable to various cavity-enhanced spectroscopy technologies, and can be used to improve the coupling efficiency and stability of the laser and optical cavity. BRIEF DESCRIPTION OF DRAWINGS

[0018] Fig. 1 is a schematic diagram of an experimental device of the phase control-free optical feedback optical cavity ring-down spectroscopy system according to an embodiment of the present application;

[0019] Fig. 2 is a comparison of the optical cavity transmission signals of a laser in a phase control-free optical feedback state and a free-running state according to an embodiment of the present application;

[0020] Fig. 3 is a phase control-free optical feedback optical cavity ring-down signal according to an embodiment of the present application;

[0021] Fig. 4 is an absorption spectrum of N2O with a concentration of 4 ppb measured at one atmosphere by the phase control-free optical feedback optical cavity ring-down spectroscopy system according to an embodiment of the present application. DETAILED DESCRIPTION

[0022] The present application will be further described in detail below in conjunction with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, but not to limit the present application. In addition, it should be noted that, for the convenience of description, only the parts related to the present application are shown in the drawings, but not all the structures.

[0023] The middle infrared quantum cascade laser (Quantum Cascade Laser, QCL) and interband cascade laser (Interband Cascade Laser, ICL) have super-short carrier lifetime and small line width expansion factor, and the laser still has high stability under strong feedback and moderate feedback, therefore, the application proposes to use strong feedback or moderate feedback to narrow the line width of QCL (or ICL) semiconductor laser, without additional phase locking system, to realize the technology of high-sensitivity and high-signal-to-noise ratio optical cavity ring-down spectroscopy measurement based on the QCL and ICL semiconductor laser cavity ring-down spectroscopy system.

[0024] Embodiment

[0025] As shown in Figure 1, the experimental device of the optical feedback optical cavity ring-down spectroscopy system without phase control comprises a laser 1, a laser controller 2, an optical beam splitter 3, a polarizer 4, a mirror 5, an acousto-optic modulator 6, an optical isolator 7, a mode matching lens group 8, a front cavity mirror 9 and a rear cavity mirror 10 constituting a high-precision optical cavity, a piezoelectric ceramic 11, a signal generator 12, a voltage amplifier 13, a photodetector 14, a power divider 15, a data acquisition system 16, a voltage comparator 17, and an acousto-optic modulator driver 18, wherein the first-order diffraction light 19 is generated by the acousto-optic modulator 6.

[0026] An optical feedback optical cavity ring-down spectroscopy system without phase control comprises a light source module, an optical feedback module, a trigger light switching module, an optical cavity module, a detection module, and a data acquisition module.

[0027] The light source module comprises a laser 1 and a laser controller 2, wherein the laser 1 outputs laser with a certain center wavelength under the control of the laser controller 2 on the working current and temperature, the wavelength of the laser output by the laser can cover the absorption spectrum line of the target detection substance, and the laser can realize the narrowing of the line width of the outgoing laser under the action of strong feedback (optical feedback ratio greater than -10 dB) or moderate feedback (optical feedback ratio between -30 dB and -10 dB), and the laser includes quantum cascade laser, interband cascade laser and other semiconductor lasers.

[0028] The optical feedback module comprises an optical beam splitter 3, a polarizer 4 and a mirror 5, wherein the optical beam splitter 3 divides the laser output by the laser 1 into two beams, one of which is reflected by the mirror 5 back to the inside of the laser 1 after passing through the polarizer 4, when the optical feedback ratio reaches the state of strong feedback or moderate feedback, the laser reflected back to the laser 1 narrows the line width of the outgoing laser of the laser 1, realizing the optical feedback narrow laser line width; the optical feedback ratio is adjusted by rotating the polarizer 4 to change the angle between the transmission axis of the polarizer 4 and the polarization direction of the incident laser.

[0029] The trigger light module includes an acousto-optic modulator 6, an acousto-optic modulator driver 18, and a voltage comparator 17. The voltage comparator 17 is triggered by the output signal of the detection module, and sends a level signal to cut off the output of the acousto-optic modulator driver 18, thereby cutting off the first-order diffracted light 19 generated by the acousto-optic modulator 6. At the same time, the level signal sent by the voltage comparator 17 is also used to trigger the data acquisition card to start collecting data.

[0030] The optical cavity module excites the cavity ring-down signal by scanning the length of the optical cavity, and includes an optical isolator 7, a mode matching lens group 8, a front cavity mirror 9, a rear cavity mirror 10, a piezoelectric ceramic 11, a signal generator 12, and a voltage amplifier 13. The optical cavity is composed of the front cavity mirror 9 and the rear cavity mirror 10, which are two high-reflectivity mirrors (the reflectivity is higher than 99.99%), used to increase the effective interaction distance between the laser and the target detection substance, output the optical cavity ring-down signal, and fix the piezoelectric ceramic 11 on the rear cavity mirror 10. The optical isolator 7 is placed behind the acousto-optic modulator 6, used to prevent the light reflected and transmitted by the front cavity mirror 9 of the optical cavity from returning to the laser 1. The first-order diffracted light 19 generated by the acousto-optic modulator 6 is transmitted through the optical isolator 7. The mode matching lens group 8 is placed behind the optical isolator 7, used to match the laser and the transverse mode of the light transmitted by the optical isolator 7, and couple the laser into the optical cavity composed of the front cavity mirror 9 and the rear cavity mirror 10, to realize the output of the optical cavity ring-down signal. The signal generator 12 generates a triangular wave voltage signal, which is driven by the voltage amplifier 13 to drive the piezoelectric ceramic 11 to stretch and contract, so as to scan the length of the optical cavity, realize the resonance of the laser and the optical cavity, and excite the optical cavity ring-down signal.

[0031] The detection module includes a photodetector 14, which detects the light intensity transmitted from the rear cavity mirror 10 and converts it into a voltage signal output.

[0032] The data acquisition module includes a power divider 15 and a data acquisition system 16. The power divider 15 divides the voltage signal output by the photodetector 14 into two signals, which are used as the input signals of the voltage comparator 17 and the data acquisition system 16, respectively. The voltage comparator 17 is used to send a trigger level when the value of its input signal reaches a predetermined threshold, to trigger the acousto-optic modulator driver 18 and cut off the driving signal of the acousto-optic modulator 6, thereby cutting off the first-order diffracted light 19 generated by the acousto-optic modulator 6. At the same time, the data acquisition system 16 is used to start collecting the voltage signal (i.e., the optical cavity ring-down signal) output by the power divider 15 after being triggered by the trigger level output by the voltage comparator 17, and to perform exponential fitting on the collected voltage signal (i.e., the optical cavity ring-down signal) to obtain the ring-down time.

[0033] In the free running state of the laser 1, the voltage comparator 17 is turned off, the acousto-optic modulator 6 continuously outputs the first-order diffracted light 19, which is incident on the optical cavity composed of the front mirror 9 and the back mirror 10, and the data acquisition system 16 continuously acquires the voltage signal from the power divider 15. In the optical feedback state of the laser 1 in the embodiment, the voltage comparator 17 receives the signal from the power divider 15 and sends out a trigger level when the signal value reaches a set threshold value (for example, when the voltage signal output by the detection module is 1 V or higher, the threshold value is generally set to be lower than 1 V to ensure that the ring-down signal can be generated. Preferably, the threshold value is 90% of the voltage signal value output by the detection module.) to trigger the acousto-optic modulator driver 18 to cut off the driving signal of the acousto-optic modulator 6, thereby cutting off the first-order diffracted light 19 generated by the acousto-optic modulator 6, i.e., the incident light of the optical cavity composed of the front mirror 9 and the back mirror 10. At the same time, the trigger level output by the voltage comparator 17 synchronously triggers the data acquisition system 16 to start acquiring the voltage signal from the power divider 15, i.e., the optical cavity ring-down signal. As shown in FIG. 2, in the laser 1 under the appropriate optical feedback ratio (the appropriate optical feedback ratio refers to the feedback ratio that can achieve the required line width requirement according to the actual system, which is within the range of strong feedback or moderate feedback), the transmission signal intensity of the optical cavity is much greater than the transmission signal intensity of the optical cavity in the free running state of the laser 1, and the transmission signal of the optical cavity has the characteristics of stable accumulation and ring-down. As shown in FIG. 3, although the incident light power of the optical cavity composed of the front mirror 9 and the back mirror 10 is only 0.4 mW, the optical cavity ring-down signal acquired by the data acquisition system 16 has a high signal-to-noise ratio of more than 160.

[0034] In order to verify the feasibility of the present application, the spectral signal of N2O with a concentration of 4 ppb was measured by using the experimental device shown in FIG. 1, as shown in FIG. 4. The measured spectral data can be fitted with a Lorentz line, and the fitting residual is less than ±10%.

[0035] Although the content of the present application has been described in detail through the above preferred embodiments, the above description should not be considered as a limitation of the present application. Therefore, the protection scope of the present application should be defined by the appended claims. Those skilled in the art can make various modifications and replacements to the phase-free optical feedback cavity ring-down spectroscopy technology of the present application without departing from the spirit and scope of the present application.

Claims

1. A ring-down spectral system with optical feedback cavity that does not require phase control, characterized in that, include: A light source module for generating laser light of a predetermined wavelength; The optical feedback module is used to feed back a portion of the laser generated by the light source module into the light source module, generating strong or moderate feedback to narrow the laser linewidth. The optical cavity module is used to increase the effective interaction distance between the laser and the target detection material, and to excite the optical cavity ring-down signal by realizing the resonance between the laser and the optical cavity; A trigger beam-cutting module is used to instantly cut off the incident light from the optical cavity module for detection. The detection module is used to detect the transmitted light signal output by the optical cavity module and convert it into a voltage signal as the optical cavity ring-down signal; The data acquisition module is used to acquire the cavity ring-down signal output by the detection module and perform exponential fitting on the cavity ring-down signal to obtain the ring-down time.

2. The optical feedback cavity ring-down spectral system without phase control according to claim 1, characterized in that, The light source module includes: A laser (1) and a laser controller (2), wherein the laser (1) is used to output a laser with a wavelength sufficient to cover the absorption spectral lines of the target detection substance, and the laser controller (2) is used to control the operating current and temperature of the laser (1).

3. The optical feedback cavity ring-down spectral system without phase control according to claim 2, characterized in that, The optical feedback module includes: The laser beam splitter (3), polarizer (4), and reflector (5) are used to split the laser emitted from the laser (1) into two beams. One beam passes through the polarizer (4) and is reflected back into the laser (1) by the reflector (5). In the case of optical feedback, when the optical feedback ratio reaches a strong or moderate feedback state, the laser reflected back to the laser (1) narrows the laser linewidth of the laser emitted by the laser (1); wherein, the optical feedback ratio is adjusted by rotating the polarizer (4) to change the angle between the transmission axis of the polarizer (4) and the polarization direction of the incident laser.

4. The optical feedback cavity ring-down spectral system without phase control according to claim 2, characterized in that, The trigger light-cutting module includes: Acousto-optic modulator (6), acousto-optic modulator driver (18), and voltage comparator (17); The acousto-optic modulator (6) is used to output first-order diffraction light (19); The voltage comparator (17) is used to cut off the output of the acousto-optic modulator driver (18) after being triggered by the output signal of the detection module, instantly cut off the first-order diffraction light (19) output by the acousto-optic modulator (6), and trigger the data acquisition module to start acquiring data.

5. The optical feedback cavity ring-down spectral system without phase control according to claim 4, characterized in that, The optical cavity module includes: an optical isolator (7), a pattern matching lens group (8), a front cavity mirror (9), a rear cavity mirror (10), a piezoelectric ceramic (11), a signal generator (12), and a voltage amplifier (13). The front cavity mirror (9) and the rear cavity mirror (10) form an optical cavity, and the piezoelectric ceramic (11) is fixed on the rear cavity mirror (10). The first-order diffraction light (19) output by the acousto-optic modulator (6) is transmitted through the optical isolator (7); The optical isolator (7) is placed after the acousto-optic modulator (6) to prevent light reflected and transmitted by the front cavity mirror (9) from returning to the laser (1); The pattern-matching lens group (8) is positioned after the optical isolator (7) to... The light transmitted by the optical isolator (7) is used for transverse mode matching between the laser and the optical cavity. The laser is coupled into the optical cavity composed of the front cavity mirror (9) and the rear cavity mirror (10) to realize the output of the optical cavity ring-down signal. The signal generator (12) is used to generate a triangular wave signal, which drives the piezoelectric ceramic (11) to expand and contract to scan the length of the optical cavity through the voltage amplifier (13), thereby achieving resonance between the laser and the optical cavity and exciting the cavity ringing signal.

6. The optical feedback cavity ring-down spectral system without phase control according to claim 1, characterized in that, The detection module includes: The photodetector (14) is used to detect the light intensity transmitted by the optical cavity module and convert it into a voltage signal as the output of the optical cavity ring-down signal.

7. The optical feedback cavity ring-down spectral system without phase control according to claim 4, characterized in that, The data acquisition module includes: The power divider (15) and the data acquisition system (16) are used to divide the voltage signal output by the detection module into two signals, which are respectively used as the input signal of the voltage comparator (17) and the input signal of the data acquisition system (16). The voltage comparator (17) is used to issue a trigger level when the value of its input signal reaches a predetermined threshold, triggering the acousto-optic modulator driver (18) to cut off the drive signal of the acousto-optic modulator (6), thereby cutting off the first-order diffraction light (19) of the acousto-optic modulator (6). The data acquisition system (16) is used to start acquiring the voltage signal output by the power divider (15) after being triggered by the trigger level output by the voltage comparator (17), and to perform exponential fitting on the acquired voltage signal to obtain the oscillation time.

8. The optical feedback cavity ring-down spectral system without phase control according to claim 1, characterized in that, The light source module includes a mid-infrared quantum cascade laser or an interband cascade laser.

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