Ring-down signal noise suppression device and method

Abstract

The invention belongs to the technical field of laser spectrum, and particularly relates to a ring-down signal noise suppression device and method. Aiming at inaccurate measurement and long-term instability of ring-down signals caused by unavoidable errors caused by various noises, the detection sensitivity of the ring-down signals is improved and relative errors are reduced by using a difference method of the ring-down signals of the cavity front mirror and the cavity rear mirror; an acousto-optic modulator is used in a light path for detecting a ring-down signal of a cavity front mirror, the light path is synchronously opened when laser is cut off so as to avoid the overshoot phenomenon of a detector, and interference of direct reflection light of the high-intensity cavity front mirror on the ring-down signal before the laser is cut off is avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of laser spectroscopy, and in particular relates to a device and method for suppressing ring-down signal noise. Background Art

[0002] Laser spectroscopy technology based on high-quality passive optical resonant cavities performs well in the precise analysis of trace gases, and has the advantages of in-situ detection, rapid response, high precision and high accuracy, and is widely used in industries, agriculture, scientific research, safety monitoring, etc. This high-quality resonant cavity is composed of at least two highly reflective mirrors. Through multiple reflections of the laser in the cavity and multi-beam interference, a powerful light field is formed, which effectively increases the absorption path, thereby improving the detection capability and ensuring the accurate measurement of trace gas concentrations.

[0003] In order to evaluate the performance of high-quality passive optical resonators, the resonator ring-down time has become a key indicator. It describes the phenomenon that the intensity of pulsed light or interrupted continuous light in the cavity decays exponentially. When a narrow-band laser pulse or interrupted continuous light enters the resonator, it will be repeatedly reflected in the cavity, and part of the light will be transmitted due to the transmission loss of the cavity mirror, forming a ring-down signal. The time it takes for the ring-down signal to be cut off and stabilized is the ring-down time.

[0004] In order to accurately measure the decay time of an optical resonant cavity, the duration of the laser pulse must be less than the time it takes for the light to travel back and forth in the cavity to avoid the superposition of the light field in the cavity. In contrast, the method of interrupting continuous light requires the laser to be turned off very quickly.

[0005] When measuring the ring-down time, the initial intensity of the ring-down signal and the suppression of noise are crucial. Cavity ring-down spectroscopy technology links the ring-down time to the intracavity loss of the resonant cavity. In the cavity state, the intracavity loss mainly depends on the transmission loss of the lens, that is, the reflectivity of the lens. The higher the coupling efficiency of the cavity and the smaller the transmission loss, the higher the resonant light intensity in the cavity during interruption, and the stronger the ring-down signal. Noise mainly comes from electrical noise, ambient temperature fluctuations and ambient vibrations. Electrical noise includes power supply noise, component noise and RF line transmission noise; ambient temperature fluctuations cause the resonant cavity to expand or contract, change the cavity length, and thus affect the cavity ring-down time; environmental vibrations affect the optical path collimation and the internal stress of the resonant cavity, thereby affecting the coupling efficiency. Therefore, effective noise suppression can greatly improve the accuracy of the ring-down time measurement and enhance long-term stability. Summary of the invention

[0006] In view of the inaccurate measurement and long-term instability of the ring-down signal caused by errors caused by various unavoidable noises, the present invention provides a ring-down signal noise suppression device and method, which uses the ring-down signal of the rear mirror of the cavity and the ring-down signal of the front mirror of the cavity for measurement, and integrates the two signals in the signal processing process to achieve the effect of amplifying the differential mode signal and reducing the common mode noise.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a ring-down signal noise suppression device, comprising a function generator, wherein the function generator outputs a scanning triangle wave, which passes through an electric switch and a laser controller to a laser, wherein the laser emits a laser, and the laser passes through an optical fiber isolator, a first optical fiber collimator, a first isolator, a half-wave plate, a polarization beam splitter, a quarter-wave plate, a first reflector, a matching lens, and a second reflector in sequence to be incident on an FP passive optical cavity, wherein the cavity rear mirror transmission light of the FP passive optical cavity passes through a first focusing lens to converge to a first photodetector, wherein the output end of the first photodetector is respectively connected to a first input end of an acquisition card and an input end of a digital delay trigger. The light transmitted by the front mirror of the FP passive optical cavity returns to the polarization beam splitter prism along the original path. After being reflected by the polarization beam splitter prism, the light transmitted by the front mirror passes through the third reflector, the second focusing lens, the second isolator, the fourth reflector, the fiber coupler, the acousto-optic modulator, the second fiber collimator and the third focusing lens in sequence, and converges to the second photodetector. The output end of the second photodetector is connected to the second input end of the acquisition card. The first output end of the digital delay trigger is connected to the electric switch, the second output end of the digital delay trigger is connected to the third input end of the acquisition card, and the third output end of the digital delay trigger is connected to the acousto-optic modulator.

[0009] Furthermore, the FP passive optical cavity can be replaced by a three-mirror ring cavity.

[0010] The present invention uses a polarization beam splitter prism to separate the ring-down signal of the front mirror of the cavity for collection. The polarization beam splitter prism is an optical element used to separate the horizontal polarization and vertical polarization of light. When the light is incident on the bonding surface coated with a multi-layer film structure at the Brewster angle, the P polarized light is almost completely transmitted because its transmittance is approximately equal to 1, and the transmission direction is not changed; the S polarized light is mostly deflected by the beam splitter prism because its transmittance is less than 1, and the light path is deflected by about 90 degrees.

[0011] In order to allow the transmitted light from the laser to the optical resonant cavity to pass through the polarization beam splitter with minimal loss, a half-wave plate is added in front of the polarization beam splitter to change the polarization direction of the linear polarization light so that the polarization state of the laser output light matches the polarization beam splitter.

[0012] In order to obtain the ring-down signal of the front mirror of the FP passive optical cavity, a quarter wave plate is added in front of the polarization beam splitter prism, and its optical axis angle is rotated so that the incident light is converted into circularly polarized light after passing through the wave plate and enters the cavity. The ring-down light of the front mirror transmitted from the cavity is converted into linearly polarized light perpendicular to the incident light after passing through the wave plate. The ring-down signal transmitted by the front mirror is separated at the beam splitter prism and can be captured by the photodetector.

[0013] In order to realize the cutting off of continuous laser, an electric switch is used to cut off the input of the laser driving electric signal; at the same time, the acousto-optic modulator is turned on so that the first-order diffraction light of the ring-down signal of the front-cavity mirror can be detected by the photodetector.

[0014] The present invention also provides a method for suppressing ring-down signal noise, comprising the following steps:

[0015] Step 1, output a scanning triangle wave through a function generator, and after passing through an electric switch and a laser controller, tune the current of the laser to scan the corresponding wavelength;

[0016] Step 2, the laser output laser passes through the optical fiber isolator to the first optical fiber for collimation, and then passes through the first isolator to be sent to the half wave plate to change the polarization direction of the laser;

[0017] Step 3, sending the laser to the polarization beam splitter prism, wherein the polarization direction of the laser is consistent with the direction in which the polarization beam splitter prism allows the laser to pass through;

[0018] Step 4, after the laser passes through the polarization beam splitter prism, it passes through the quarter wave plate, and the polarization state is changed to circular polarization. Then, it passes through the first reflector, the matching lens, and the second reflector, and is incident on the FP passive optical cavity. The matching lens is used to match the FP passive optical cavity. The first reflector and the second reflector are used to change the direction of the optical path.

[0019] Step 5, the light transmitted by the rear mirror of the FP passive optical cavity is converged to the first photodetector through the first focusing lens, and the optical signal detected by the first photodetector is divided into two parts, one part is input to the trigger port of the digital delay trigger as a trigger signal, and the other part is input to the acquisition card;

[0020] Step 6: After receiving the cavity membrane peak signal exceeding the set threshold, the digital delay trigger inputs a high level to the electric switch to turn it off, inputs a low level to the acquisition card to trigger sampling, and inputs a high level to the drive of the acousto-optic modulator to turn it on;

[0021] Step 7, the light transmitted by the front mirror of the FP passive optical cavity returns along the original path, passes through a quarter wave plate, and the polarization state is changed back to linear polarized light, and the polarization direction is orthogonal to the original laser;

[0022] Step 8: The light transmitted by the cavity front mirror is sent to the polarization beam splitter prism, and its polarization direction is orthogonal to the transmission direction allowed by the polarization beam splitter prism. The light transmitted by the cavity front mirror is reflected by the polarization beam splitter prism and separated from the main light path. Then, after passing through the third reflector, the second focusing lens, the second isolator, and the fourth reflector, the waist spot falls on the end face of the optical fiber coupler and is coupled into the optical fiber.

[0023] Step 9, the light transmitted by the cavity front mirror is emitted to the second fiber collimator after passing through the acousto-optic modulator, and then converged to the second photodetector through the third focusing lens to monitor the cavity front mirror ring-down signal after the FP passive optical cavity is cut off. The cavity front mirror ring-down signal after the cut-off is input into the acquisition card.

[0024] Furthermore, the FP passive optical cavity can be replaced by a three-mirror ring cavity.

[0025] Furthermore, the ring-down signal of the front-cavity mirror and the ring-down signal of the rear-cavity mirror are averaged to achieve the effect of suppressing noise and reducing the drift of the ring-down signal.

[0026] The fitting function of the ring-down signal is:

[0027]

[0028] Among them, I0 is the incident light intensity, t is the time, τ0 is the cavity ring-down time, c is the speed of light, R is the cavity mirror reflectivity, and L is the cavity length.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] (1) This method utilizes the ring-down signal of the front-cavity mirror that is discarded by other methods, and uses the differential method of the ring-down signal of the front-cavity mirror and the rear-cavity mirror to improve the detection sensitivity of the ring-down signal and reduce the relative error;

[0031] (2) The environmental noise of the ring-down signal of the pre-cavity mirror is used to suppress the double-ended noise, thus improving the signal-to-noise ratio;

[0032] (3) An acousto-optic modulator is used in the optical path of the ring-down signal of the cavity front mirror, and the optical path is opened synchronously when the laser is cut off to avoid the overshoot of the detector and the interference of the high-intensity direct reflection light of the cavity front mirror on the ring-down signal before the cut-off.

[0033] (4) No frequency locking and optical feedback are required, and the system is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of a ring-down signal noise suppression device;

[0035] Figure 2 Schematic diagram of the device in which the FP passive optical cavity is replaced by a three-mirror ring cavity;

[0036] Figure 3 Fitting curve for ring-down signal;

[0037] Figure 4 are the ring-down time of the mirror before the cavity (tR), the ring-down time of the mirror after the cavity (tT) and the average after-ring-down time (tA). DETAILED DESCRIPTION

[0038] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0039] like Figure 1 As shown, a ring-down signal noise suppression device of this embodiment includes a function generator 1, The function generator 1 outputs a scanning triangle wave, which passes through an electric switch 2 and a laser controller 3 to a laser 4. The laser 4 emits a laser, which passes through a fiber isolator 5, a first fiber collimator 6, a first isolator 7, a half-wave plate 8, a polarization beam splitter prism 9, a quarter-wave plate 10, a first reflector 11, a matching lens 12, and a second reflector 13 in sequence and is incident on a FP passive optical cavity 14. The cavity rear mirror transmitted light of the FP passive optical cavity 14 passes through a first focusing lens 15 and converges to a first photodetector 16. The output end of the first photodetector 16 is respectively connected to a first input end of an acquisition card 26 and an input end of a digital delay trigger 27. The cavity of the FP passive optical cavity 14 The front mirror transmitted light returns to the polarization beam splitter prism 9 along the original path. After being reflected by the polarization beam splitter prism 9, the cavity front mirror transmitted light passes through the third reflector 17, the second focusing lens 18, the second isolator 19, the fourth reflector 20, the fiber coupler 21, the acousto-optic modulator 22, the second fiber collimator 23 and the third focusing lens 24 in sequence, and converges to the second photodetector 25. The output end of the second photodetector 25 is connected to the second input end of the acquisition card 26; the first output end of the digital delay trigger 27 is connected to the electric switch, the second output end of the digital delay trigger 27 is connected to the third input end of the acquisition card 26, and the third output end of the digital delay trigger 27 is connected to the acousto-optic modulator 22.

[0040] The FP passive optical cavity 14 has a cavity length of 50 cm, a cavity fineness of about 24164.5, a cavity mirror with a broadband cavity mirror of 1600 nm to 1700 nm, two plano-concave mirrors with a curvature radius of 1 meter and a reflectivity of about 99.987%, and a cavity material of Invar;

[0041] The laser 4 uses a butterfly DFB laser with a center frequency of 1658 nm, and the laser controller 3 uses a temperature control of 30° C. and a center current control of 110 mA, and uses a triangle wave voltage with an amplitude of 95 mV for scanning;

[0042] The electric switch 2 has a pass frequency range of DC to 5 GHz and a turn-off time of about 20 ns;

[0043] The frequency shift of the AOM 22 is 100 MHz.

[0044] Based on the above device, a method for suppressing ring-down signal noise in this embodiment includes the following steps:

[0045] Step 1, output a scanning triangle wave through a function generator 1, and after passing through an electric switch 2 and a laser controller 3, tune the current of a laser 4 to scan the corresponding wavelength;

[0046] Step 2, the laser output by the laser 4 passes through the optical fiber isolator 5 to the first optical fiber collimator 6, and then is sent to the half wave plate 8 through the first isolator 7 to change the polarization direction of the laser;

[0047] Step 3, sending the laser to the polarization beam splitter prism 9, wherein the polarization direction of the laser is consistent with the direction in which the polarization beam splitter prism 9 allows the laser to pass through;

[0048] Step 4, after the laser passes through the polarization beam splitter prism 9, it passes through the quarter wave plate 10, and the polarization state is changed to circular polarization, and then passes through the first reflector 11, the matching lens 12 and the second reflector 13, and is incident on the FP passive optical cavity 14, and the matching lens 12 is used to match the FP passive optical cavity 14, and the first reflector 11 and the second reflector 13 are used to change the direction of the optical path;

[0049] Step 5, the light transmitted by the cavity rear mirror of the FP passive optical cavity 14 is converged to the first photodetector 16 through the first focusing lens 15 to detect the optical signal of the FP passive optical cavity 14 (transmitted cavity mode signal or cavity rear mirror ring-down signal after chopped off), and the optical signal detected by the first photodetector 16 is divided into two parts, one part is input to the trigger port of the digital delay trigger 27 as a trigger signal, and the other part is input to the acquisition card 26;

[0050] Step 6, after receiving the cavity membrane peak signal exceeding the set threshold, the digital delay trigger 27 inputs a high level to the electric switch 2 to turn it off, inputs a low level to the acquisition card 26 to trigger sampling, and inputs a high level to the drive of the acousto-optic modulator 22 to turn it on;

[0051] Step 7, the light transmitted by the front mirror of the FP passive optical cavity 14 returns along the original path, passes through the quarter wave plate 10, and the polarization state is changed back to linear polarized light, and the polarization direction is orthogonal to the original laser;

[0052] Step 8, the light transmitted from the cavity front mirror is sent to the polarization beam splitter prism 9, and its polarization direction is orthogonal to the transmission direction allowed by the polarization beam splitter prism 9. The light transmitted from the cavity front mirror is reflected by the polarization beam splitter prism 9 and separated from the main light path, and then passes through the third reflector 17, the second focusing lens 18, the second isolator 19, and the fourth reflector 20, and the waist spot falls on the end face of the optical fiber coupler 21 and is coupled into the optical fiber;

[0053] Step 9, the transmitted light of the cavity front mirror is emitted to the second fiber collimator 23 after passing through the acousto-optic modulator 22, and then converged to the second photodetector 25 through the third focusing lens 24 to monitor the cavity front mirror ring-down signal after the FP passive optical cavity 14 is cut off. The cavity front mirror ring-down signal after the cut-off is input to the acquisition card 26.

[0054] In addition, the FP passive optical cavity 14 of this embodiment can also be replaced by a three-mirror ring cavity (such as Figure 2 As shown, the laser is incident into the three-mirror annular cavity by the plane high-reflection mirror 1. After passing through the plane high-reflection mirror 2, a small part of the laser is transmitted from the cavity and received by the photodetector 4, and most of it is reflected in the cavity. After passing through the high-reflection plano-concave mirror 3, a small part of the laser is transmitted from the cavity and received by the photodetector 5, and most of it is reflected in the cavity and returns to the high-reflection plane mirror 1.

[0055] The ring-down signal of the front mirror and the ring-down signal of the rear mirror are averaged, and the fitting function of the ring-down signal is:

[0056]

[0057] Where I0 is the incident light intensity, t is the time, τ0 is the cavity ring-down time, c is the speed of light, R is the cavity mirror reflectivity, and L is the cavity length. The fitting curve is as follows Figure 3 shown.

[0058] Figure 4 They are the ring-down time of the front-cavity mirror (tR), the ring-down time of the rear-cavity mirror (tT) and the average rear-cavity ring-down time (tA), among which the relative error of the front-cavity mirror is 0.00074, the relative error of the rear-cavity mirror is 0.00070, the average rear relative error is 0.00057, and the relative optimization rate is 20.8%; the standard deviation of the front-cavity mirror is 0.0055, the standard deviation of the rear-cavity mirror is 0.0057, the average rear standard deviation is 0.0045, and the relative optimization rate is 19.6%.

[0059] 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.

[0060] 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 ring-down signal noise suppression device, characterized in that: The invention comprises a function generator (1), wherein the function generator (1) outputs a scanning triangle wave, which passes through an electric switch (2) and a laser controller (3) to a laser (4), wherein the laser (4) emits laser light, which passes through an optical fiber isolator (5), a first optical fiber collimator (6), a first isolator (7), a half-wave plate (8), a polarization beam splitter (9), a quarter-wave plate (10), a first reflector (11), a matching lens (12), and a second reflector (13) in sequence and is incident on an FP passive optical cavity (14), wherein the light transmitted through the cavity rear mirror of the FP passive optical cavity (14) passes through a first focusing lens (15) and converges to a first photodetector (16), wherein the output end of the first photodetector (16) is respectively connected to a first input end of an acquisition card (26) and an input end of a digital delay trigger (27); The light transmitted through the cavity front mirror of the optical cavity (14) returns to the polarization beam splitter prism (9) along the original path. After being reflected by the polarization beam splitter prism (9), the light transmitted through the cavity front mirror passes through the third reflector (17), the second focusing lens (18), the second isolator (19), the fourth reflector (20), the fiber coupler (21), the acousto-optic modulator (22), the second fiber collimator (23) and the third focusing lens (24) in sequence and converges to the second photodetector (25). The output end of the second photodetector (25) is connected to the second input end of the acquisition card (26); the first output end of the digital delay trigger (27) is connected to the electric switch, the second output end of the digital delay trigger (27) is connected to the third input end of the acquisition card (26), and the third output end of the digital delay trigger (27) is connected to the acousto-optic modulator (22).

2. The ring-down signal noise suppression device according to claim 1, characterized in that: The FP passive optical cavity (14) can be replaced by a three-mirror ring cavity.

3. A method for suppressing ring-down signal noise based on the device of claim 1, characterized in that: The following steps are involved: Step 1, outputting a scanning triangle wave through a function generator (1), passing through an electric switch (2) and a laser controller (3), tuning the current of a laser (4) to scan the corresponding wavelength; Step 2, the laser output from the laser (4) passes through the optical fiber isolator (5) to the first optical fiber collimator (6), and then passes through the first isolator (7) to be sent to the half wave plate (8) to change the polarization direction of the laser; Step 3, sending the laser light to the polarization beam splitter prism (9), wherein the polarization direction of the laser light is consistent with the direction in which the polarization beam splitter prism (9) allows the laser light to pass through; Step 4, after the laser passes through the polarization beam splitter prism (9), it passes through the quarter wave plate (10), and the polarization state is changed to circular polarization. Then, the laser passes through the first reflector (11), the matching lens (12) and the second reflector (13), and is incident on the FP passive optical cavity (14). The matching lens (12) is used to match the FP passive optical cavity (14). The first reflector (11) and the second reflector (13) are used to change the direction of the optical path. Step 5, the light transmitted by the rear mirror of the FP passive optical cavity (14) is converged to the first photodetector (16) through the first focusing lens (15), and the optical signal detected by the first photodetector (16) is divided into two parts, one part is input to the trigger port of the digital delay trigger (27) as a trigger signal, and the other part is input to the acquisition card (26); Step 6, after receiving the cavity membrane peak signal exceeding the set threshold, the digital delay trigger (27) inputs a high level to the electric switch (2) to turn it off, and simultaneously inputs a high level to the drive of the acousto-optic modulator (22) to turn it on, and inputs a low level to the acquisition card (26) to trigger sampling and collect the ring-down signal; Step 7, the light transmitted by the front mirror of the FP passive optical cavity (14) returns along the original path, passes through the quarter wave plate (10), and the polarization state is changed back to linear polarized light, and the polarization direction is orthogonal to the original laser; Step 8, the light transmitted by the cavity front mirror is sent to the polarization beam splitter prism (9), and its polarization direction is orthogonal to the transmission direction allowed by the polarization beam splitter prism (9). The light transmitted by the cavity front mirror is reflected by the polarization beam splitter prism (9) and separated from the main light path, and then passes through the third reflector (17), the second focusing lens (18), the second isolator (19), and the fourth reflector (20), and the waist spot falls on the end face of the optical fiber coupler (21) and is coupled into the optical fiber; Step 9, the light transmitted by the cavity mirror passes through the acousto-optic modulator (22) and then is emitted to the second fiber collimator (23), and then converged to the second photodetector (25) through the third focusing lens (24) to monitor the cavity mirror ring-down signal after the FP passive optical cavity (14) is cut off. The cavity mirror ring-down signal after the cut-off is input to the acquisition card (26).

4. A ring-down signal noise suppression method according to claim 3, characterized in that: The FP passive optical cavity (14) can be replaced by a three-mirror ring cavity.

5. The method for suppressing ring-down signal noise according to claim 3, characterized in that: The ring-down signal of the front mirror and the ring-down signal of the rear mirror are averaged, and the fitting function of the ring-down signal is: Among them, I0 is the incident light intensity, t is the time, τ0 is the cavity ring-down time, c is the speed of light, R is the cavity mirror reflectivity, and L is the cavity length.

Images