Ring-down time signal-to-noise ratio improving method based on beat frequency demodulation

Abstract

The invention discloses a ring-down time signal-to-noise ratio improving method based on beat frequency demodulation, and belongs to the technical field of laser spectrum. In order to solve the problems of inaccurate measurement and long-term instability of ring-down time caused by errors caused by various noises in a ring-down signal measurement process, the method disclosed by the invention simultaneously utilizes ring-down signals of a front cavity mirror and a rear cavity mirror, and a beam-combining beat frequency demodulation method is used for improving the detection sensitivity of the ring-down signals and reducing relative errors. Compared with the prior art, the method has the advantages that the cavity front mirror ring-down signal abandoned by other methods is utilized, so that the energy utilization rate is improved; signals are amplified through beat frequency demodulation, so that the sensitivity is improved, and high-frequency noise is filtered through a low-pass filter during demodulation; frequency locking and optical feedback are not needed, and the system is simple.

Description

Technical Field

[0001] This invention relates to the field of laser spectroscopy technology, specifically to a device and method for demodulating the beat frequency of co-cavity ring-down signals based on cavity ring-down spectroscopy, thereby improving the accuracy of ring-down time measurement. Background Technology

[0002] To accurately and quantitatively evaluate the performance of a high-quality passive resonator, the ring-down signal is an important reference, and the ring-down time is a key performance indicator. The ring-down signal refers to the phenomenon where the intensity of a narrowband pulsed laser or an interrupted continuous laser beam within the cavity decays exponentially over time. When a narrowband pulsed laser or an interrupted continuous laser beam enters the resonator, it undergoes multiple reflections within the cavity under the influence of a high-reflectivity cavity mirror. During these reflections, some light is lost due to transmission losses through the cavity mirror, forming a ring-down signal. The time taken for the ring-down signal to decay to its maximum value of 1 / e from the initial interruption is called the ring-down time.

[0003] To accurately obtain the ring-down signal of the optical resonator, the duration of the laser pulse must be less than the time it takes for light to travel one round trip within the cavity, in order to avoid superposition of the light field within the cavity. Conversely, methods that interrupt continuous light require an extremely fast laser shut-off speed.

[0004] Theoretically, the signal intensity, shape, and decay time of the ring-down signal transmitted from the front mirror and the rear mirror of the same resonant cavity should be exactly the same, and the two decaying lights should only have a fixed phase difference related to the cavity length. In reality, due to the different transmission losses of the front and rear mirrors, the initial light intensities of the decaying signals at both ends are also different. Furthermore, the different number and types of elements and optical path lengths on the optical paths from the front and rear mirrors to the detector lead to further changes in the light intensity of the two decaying signals, and the phase difference is no longer solely affected by the cavity length. However, as a noise-immune technique, cavity ring-down spectroscopy should still maintain equal decay times for the signals at both ends.

[0005] Theoretical derivation shows that the combined beat frequency of the pre- and post-cavity end-scope decay signals should be a superposition of a complete exponentially decaying signal and a sinusoidal signal with an exponentially decaying amplitude. After demodulation, this beat frequency signal should be a sinusoidal signal modulated by an exponentially decaying intensity, where the independent variable of the sinusoidal function is the sum of the phase difference between the pre- and post-cavity end-scope decay signals and the phase of the demodulated signal. The result should be a fixed value, so the sinusoidal term is a constant, the demodulated signal is an exponentially decaying function, and the decay time remains unchanged. Summary of the Invention

[0006] To address the inaccurate and long-term instability in fading time measurement caused by various noises during fading signal measurement, this invention aims to propose a fading signal acquisition device and processing method with relatively small errors.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] To address the problems existing in the aforementioned technologies, this invention utilizes both the post-cavity mirror ring-down signal and the often-discarded pre-cavity mirror ring-down signal, and spatially combines and beats both signals. Then, an optimized ring-down signal is obtained through demodulation.

[0009] A method for improving the signal-to-noise ratio of fading time based on beat frequency demodulation is proposed. This method utilizes the fading signals from both the pre-cavity and post-cavity mirrors and employs a combined beat frequency demodulation method to improve the detection sensitivity of the fading signal and reduce the relative error.

[0010] A scanning triangular wave is output by a function generator, and the current of the semiconductor laser is tuned to scan the corresponding wavelength.

[0011] The first fiber acousto-optic modulator is used in the optical path for detecting the decaying signal of the cavity mirror. When it is not decaying, it is turned off to avoid interference from the high-intensity cavity mirror direct reflected light to the decaying signal before it is cut off. When the laser cuts off, the optical path is turned on synchronously to avoid the detector overshoot phenomenon. At the same time, it plays the role of frequency shifting of the cavity mirror decaying signal and meeting the beat frequency condition.

[0012] The decaying signal detected by the photodetector installed in the cavity mirror is divided into two paths. One part is input to the trigger port of the digital delay trigger as the trigger signal for turning off the first fiber acousto-optic modulator and turning on the second fiber acousto-optic modulator. The other part is input to the mixer. The output of the second fiber acousto-optic modulator is connected to the coupler. The output of the coupler is connected to the second fiber acousto-optic modulator for driving. The coupling end is connected to the mixer for demodulation. The demodulated signal is input to the acquisition card after passing through a low-pass filter.

[0013] After receiving a cavity mode peak signal exceeding a set threshold, the digital delay trigger inputs a low level to the acquisition card to trigger sampling, inputs a low level to the first fiber optic acousto-optic modulator to turn it off, and inputs a high level to the second fiber optic acousto-optic modulator to turn it on.

[0014] A ring-down time signal-to-noise ratio (SNR) enhancement device includes a function generator that outputs a scanning triangular wave, which passes through a laser controller to a laser. The laser emits a laser beam, which sequentially passes through an optical fiber isolator, a first acousto-optic modulator, a first optical fiber collimator, a first half-wave plate, an isolator, a polarizing beam splitter, a quarter-wave plate, a first mirror, a matching lens, and a second mirror before entering a passive FP (fiber optic) optical cavity. The transmitted light from the cavity back mirror of the passive FP optical cavity sequentially passes through a third mirror, a beam splitter, and a first focusing lens, converging at... The photodetector's output is connected to the input of a mixer and the input of a digital delay trigger, respectively. The transmitted light from the front mirror of the FP passive optical cavity returns along its original path to the polarizing beam splitter. After being reflected by the polarizing beam splitter, the transmitted light passes sequentially through a fourth mirror, a second focusing lens, a third focusing lens, a fifth mirror, an optical fiber coupler, a second optical fiber acousto-optic modulator, a second optical fiber collimator, a sixth mirror, a seventh mirror, a second half-wave plate, a beam splitter, and a first focusing lens before converging onto the photodetector.

[0015] The first output terminal of the digital delay trigger is connected to the first acoustic-optic modulator driver, and the other end of the first acoustic-optic modulator driver is connected to the first acoustic-optic modulator.

[0016] The second output terminal of the digital delay trigger is connected to the second input terminal of the acquisition card, and the third output terminal of the digital delay trigger is connected to the second acousto-optic modulator driver.

[0017] The second acousto-optic modulator driver is connected to the coupler, which is connected to the mixer and the second fiber optic modulator respectively; the output of the mixer is connected to the low-pass filter; the low-pass filter is connected to the acquisition card.

[0018] Furthermore, the FP passive optical cavity can be replaced with a three-mirror ring cavity.

[0019] This invention uses a polarizing beam splitter to separate the ring-down signal of the cavity mirror from the main optical path for acquisition. A polarizing beam splitter is an optical element used to separate the horizontal and vertical polarization components of light. When light is incident on its cemented surface at Brewster angle, due to the multilayer film structure coated on the surface, P-polarized light is almost completely transmitted because its transmittance is about 1, without changing the transmission direction; S-polarized light is mostly deflected by the beam splitter because its transmittance is less than 1, with the optical path deflected by about 90 degrees.

[0020] Preferably, in order to ensure that the transmitted light from the laser to the optical resonant cavity passes 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 linearly polarized light, so that the polarization state of the laser output light matches that of the polarization beam splitter.

[0021] Preferably, in order to obtain the ring-down signal of the front mirror of the passive optical resonator, a quarter-wave plate is added in front of the beam splitter. The optical axis angle is rotated so that the incident light passes through the wave plate and the linearly polarized light becomes circularly polarized light coupled into the cavity. The ring-down light transmitted from the cavity is converted into linearly polarized light perpendicular to the incident light by the wave plate. The ring-down signal transmitted by the front mirror is separated from the main optical path at the beam splitter, so that it can be captured by the detector.

[0022] Preferably, in order to achieve continuous laser ablation, we use an optical fiber acousto-optic modulator in the electrical section to ablate the input of the laser optical signal; at the same time, the optical fiber acousto-optic modulator is turned on so that the first-order diffracted light of the cavity mirror ring-down signal can be detected by the photodetector. The optical fiber acousto-optic modulator is in the off state when no ring-down event occurs so as to block the direct reflected light from the cavity mirror from entering the detector.

[0023] The fiber optic acousto-optic modulator's drive connection coupler splits the drive signal into two parts with a 1:9 ratio. The high-power radio frequency signal drives the fiber optic acousto-optic modulator, while the low-power signal is fed into a phase shifter, then into a mixer, where it is mixed and demodulated together with the beat frequency signal received by the detector. The demodulated signal is then input to the data acquisition card after passing through a low-pass filter.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) Compared with the prior art, this method utilizes the precavitary mirror ring-down signal that has been discarded by other methods, thereby improving energy utilization;

[0026] (2) The random noise of the cavity mirror ringing signal is neutralized by the phase randomness of the random noise of the cavity mirror ringing signal, thereby reducing the random noise. The signal is amplified by beat frequency demodulation, thereby improving the sensitivity. The high-frequency noise is filtered by the low-pass filter during demodulation.

[0027] (3) No frequency locking and optical feedback are required, making the system simple. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 Schematic diagram of the oscillation signal curve;

[0030] Figure 2 Schematic diagram of a beat frequency demodulation device for time decay;

[0031] Figure 3 Schematic diagram of beat frequency of decaying signal;

[0032] Figure 4 Schematic diagram of beat frequency signal demodulation;

[0033] Figure 5 Schematic diagram of a three-mirror annular cavity.

[0034] 1. Function generator; 2. Laser controller; 3. Laser; 4. Fiber optic isolator; 5. First fiber optic acousto-optic modulator; 6. First fiber optic collimator; 7. First half-wave plate; 8. Isolator; 9. Polarizing beam splitter; 10. Quarter-wave plate; 11. First mirror; 12. Matching lens; 13. Second mirror; 14. FP passive optical cavity; 15. Third mirror; 16. Beam splitter prism; 17. First focusing lens; 18. Photodetector; 19. 20. Fourth reflecting mirror; 21. Second focusing lens; 22. Third focusing lens; 23. Fifth reflecting mirror; 24. Fiber optic coupler; 25. Second fiber optic acousto-optic modulator; 26. Second fiber optic collimator; 27. Sixth reflecting mirror; 28. Seventh reflecting mirror; 29. ​​Second half-wave plate; 30. Digital delay trigger; 31. Second acousto-optic modulator driver; 32. Coupler; 33. Mixer; 34. Low-pass filter; 35. Acquisition card; 36. First acousto-optic modulator driver. Detailed Implementation

[0035] To gain a deeper understanding of this invention, we will provide a comprehensive and detailed description. However, this invention has various implementations and is not limited to the specific examples listed herein. These examples are presented to enhance a full understanding of the disclosure of this invention.

[0036] A ring-down time signal-to-noise ratio (SNR) enhancement device includes a function generator 1, which outputs a scanning triangular wave, which passes through a laser controller 2 to a laser 3. The laser 3 emits a laser beam, which sequentially passes through an optical fiber isolator 4, a first acousto-optic modulator 5, a first optical fiber collimator 6, a first half-wave plate 7, an isolator 8, a polarizing beam splitter prism 9, a quarter-wave plate 10, a first reflecting mirror 11, a matching lens 12, and a second reflecting mirror 13 before entering a passive FP (photonic photodetector) optical cavity 14. The transmitted light from the back mirror of the passive FP optical cavity 14 sequentially passes through a third reflecting mirror 15, a beam splitter prism 16, and a first focusing lens 17 before converging onto a photodetector. 18. The output terminal of the photodetector 18 is connected to the input terminal of the mixer 32 and the input terminal of the digital delay trigger 29, respectively. The transmitted light from the front mirror of the FP passive optical cavity 14 returns to the polarizing beam splitter 9 along the original path. After being reflected by the polarizing beam splitter 9, the transmitted light passes sequentially through the fourth mirror 19, the second focusing lens 20, the third focusing lens 21, the fifth mirror 22, the fiber coupler 23, the second fiber acousto-optic modulator 24, the second fiber collimator 25, the sixth mirror 26, the seventh mirror 27, the second half-wave plate 28, the beam splitter 16, and the first focusing lens 17, and converges to the photodetector 18.

[0037] The first output terminal of the digital delay trigger 29 is connected to the first acoustic-optic modulator driver 35, and the other end of the first acoustic-optic modulator driver 35 is connected to the first acoustic-optic modulator 5.

[0038] The second output terminal of the digital delay trigger 29 is connected to the second input terminal of the acquisition card 34, and the third output terminal of the digital delay trigger 29 is connected to the second acoustic-optical modulator driver 30.

[0039] The second acousto-optic modulator driver 30 is connected to the coupler 31, which is connected to the mixer 32 and the second fiber optic modulator 24 respectively; the output of the mixer 32 is connected to the low-pass filter 33; and the low-pass filter 33 is connected to the acquisition card 34.

[0040] The cavity ring-down signal of the high-quality FP passive optical cavity in the device used in this invention, such as Figure 1 As shown, its amplitude decreases exponentially.

[0041] The beat frequency signal of the ringing signal output from the front and rear endoscopes, such as Figure 3 As shown: In Figure 1 A sine wave is superimposed on the beat frequency of the decaying signal, and the amplitude of the sine wave also decays exponentially with respect to the decaying signal, resulting in a signal that is modulated by the amplitude of the decaying signal while using the decaying signal as a carrier wave.

[0042] The demodulated signal of the beat frequency signal, such as Figure 4As shown, the result is a decaying signal with exponential decay (e), and the decay time is the same as the original cavity decay time. After discarding the unstable oscillation part at the front end, the fitted signal-to-noise ratio is better than the directly measured cavity decaying signal, demonstrating a suppression effect on low-frequency noise such as 1 / f noise.

[0043] Example 2

[0044] The difference from Example 1 is that a three-mirror ring cavity is used instead of the FP passive optical cavity. The structure of the three-mirror ring cavity is as follows: Figure 5 As shown.

[0045] Contents not described in detail in this specification are prior art known to those skilled in the art. Although illustrative specific embodiments of the invention have been described above to facilitate understanding by those skilled in the art, it should be understood that the invention is not limited to the scope of the specific embodiments. Various modifications are readily apparent to those skilled in the art as long as they fall within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of this invention are protected.

Claims

1. A method for improving the signal-to-noise ratio of fading time based on beat frequency demodulation, characterized in that: Simultaneously, by utilizing the oscillation signals from the pre-cavity and post-cavity mirrors, a beam-combining beat frequency demodulation method is employed to improve the detection sensitivity of the oscillation signal and reduce the relative error.

2. The method for improving the signal-to-noise ratio of fading time based on beat frequency demodulation according to claim 1, characterized in that: A scanning triangular wave is output by a function generator, and the current of the semiconductor laser is tuned to scan the corresponding wavelength. The first fiber acousto-optic modulator is used in the optical path for detecting the cavity mirror ringing signal. When not ringing, it is turned off to avoid interference from the high-intensity cavity mirror direct reflected light to the ringing signal before the laser cut. When the laser cuts, the optical path is turned on synchronously to avoid detector overshoot. At the same time, it plays the role of shifting the frequency of the cavity mirror ringing signal to meet the beat frequency condition. The decaying signal detected by the photodetector set in the cavity mirror is divided into two paths. One part is input to the trigger port of the digital delay trigger as the trigger signal for turning off the first fiber acousto-optic modulator and turning on the second fiber acousto-optic modulator. The other part is input to the mixer. The output of the second fiber optic acousto-optic modulator is connected to the coupler. The output of the coupler is connected to the second fiber optic acousto-optic modulator for driving. The coupling end is connected to the mixer for demodulation. The demodulated signal is input to the acquisition card after passing through a low-pass filter. After receiving a cavity mode peak signal exceeding a set threshold, the digital delay trigger inputs a low level to the acquisition card to trigger sampling, inputs a low level to the first fiber optic acousto-optic modulator to turn it off, and inputs a high level to the second fiber optic acousto-optic modulator to turn it on.

3. A decay time signal-to-noise ratio (SNR) enhancement device based on the beat frequency demodulation-based decay time SNR enhancement method according to any one of claims 1 or 2, characterized in that: The system includes a function generator (1), which outputs a scanning triangular wave, which passes through a laser controller (2) to a laser (3). The laser (3) emits a laser beam, which passes sequentially through an optical fiber isolator (4), a first acousto-optic modulator (5), a first optical fiber collimator (6), a first half-wave plate (7), an isolator (8), a polarizing beam splitter (9), a quarter-wave plate (10), a first mirror (11), a matching lens (12), and a second mirror (13) before being incident on an FP passive optical cavity (14). The transmitted light from the back mirror of the FP passive optical cavity (14) passes sequentially through a third mirror (15), a beam splitter (16), and a first focusing lens (17) before converging onto a photodetector (18). The output of the photodetector (18) is connected to the input of the mixer (32) and the input of the digital delay trigger (29), respectively. The transmitted light from the front mirror of the FP passive optical cavity (14) returns to the polarization beam splitter (9) along the original path. After being reflected by the polarization beam splitter (9), the transmitted light passes through the fourth mirror (19), the second focusing lens (20), the third focusing lens (21), the fifth mirror (22), the fiber coupler (23), the second fiber acousto-optic modulator (24), the second fiber collimator (25), the sixth mirror (26), the seventh mirror (27), the second half-wave plate (28), the beam splitter (16), and the first focusing lens (17) in sequence, and converges to the photodetector (18). The first output terminal of the digital delay trigger (29) is connected to the first acoustic-optic modulator driver (35), and the other end of the first acoustic-optic modulator driver (35) is connected to the first acoustic-optic modulator (5). The second output terminal of the digital delay trigger (29) is connected to the second input terminal of the acquisition card (34), and the third output terminal of the digital delay trigger (29) is connected to the second acoustic-optical modulator driver (30). The second acousto-optic modulator driver (30) is connected to the coupler (31), which is connected to the mixer (32) and the second fiber optic modulator (24) respectively; the output of the mixer (32) is connected to the low-pass filter (33); the low-pass filter (33) is connected to the acquisition card (34).

4. The time-of-flight signal-to-noise ratio enhancement device according to claim 3, characterized in that: The FP passive optical cavity (14) can be replaced with a three-mirror annular cavity.

5. The time-of-flight signal-to-noise ratio enhancement device according to claim 3, characterized in that: The coupler divides the drive signal into two parts with a ratio of 1:9.

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