A method for dynamic adjustment of phase of frequency lock feedback with strong interference suppression

Abstract

A method for dynamic adjustment of frequency locking feedback phase in strong interference is disclosed, which comprises the following steps: scanning laser frequency by modulating driving current of semiconductor laser, collecting transmission cavity mode signal of optical resonant cavity by photoelectric detector, obtaining error signal for controlling feedback phase based on asymmetry and amplitude of transmission cavity mode signal, amplifying error signal by high voltage amplifier and then applying it to piezoelectric ceramic material with mirror adhered, controlling expansion and contraction of piezoelectric ceramic, and realizing dynamic adjustment of feedback phase. The method has very wide phase adjustment range based on asymmetry and amplitude of transmission cavity mode signal, and can realize stability of feedback system even in harsh environment.

Description

Technical Field

[0001] This invention belongs to the field of laser spectroscopy technology, and more specifically, relates to a method for adjusting the feedback phase in optical feedback frequency-locked resonator enhancement technology. Background Technology

[0002] Whether in scientific research, food safety, medical testing, or the prevention of major safety accidents, rapid and accurate qualitative and quantitative analysis of multi-component mixed gases has become an urgent need. Examples include fault characteristic gas detection in power transformers (H2, CH4, C2H6, C2H4, C2H2, CO, CO2), fault characteristic gas detection in gas-insulated metal-enclosed switchgear (GIS) (SOF2, CF4, SO2, H2S, HF, CO), thermal runaway gas analysis in lithium iron phosphate energy storage batteries (HCl, HF, H2, CO, CO2), online monitoring of natural gas components (H2, N2, CO2, H2S, CxHy), dissolved gas monitoring in seawater (CO2, CH4, C2H6, C3H8), and food management (O2, CO2, C2H4, N2). Laser spectroscopy is based on the interaction between laser and gas. The sensitivity of laser absorption spectroscopy depends on the absorption length, while the sensitivity of laser Raman spectroscopy is closely related to the laser power. To apply these two spectroscopic methods to the detection of trace gases, resonant cavity enhancement technology has been used. When the laser frequency matches the longitudinal mode frequency of the resonant cavity, the interaction length with the gas and the laser power can be significantly increased, thereby improving the sensitivity of laser spectroscopy.

[0003] US Patent 5432610 proposes a scheme to lock a semiconductor laser into a V-shaped resonant cavity. The narrow linewidth light from the external optical resonant cavity is fed back into the laser diode. Under appropriate feedback phase, the laser diode emits laser light with the same frequency as the external resonant cavity. However, the system cannot control the feedback phase, causing the resonant frequency of the system to drift slowly with changes in the environment, making it difficult to guarantee the system stability. The literature "Optical feedback cavity enhanced absorption spectroscopy with diode lasers" (Gus Hancocket et al., Analyst, 2009, 134, 243–249) discloses a linear resonant cavity enhanced absorption spectroscopy technique, and the literature "Cavity-enhanced Raman spectroscopy with optical feedback frequency-locking for gassensing" (Wang Pinyi et al., Optics Express. 2019. 27(23)) discloses a resonant cavity enhanced Raman spectroscopy technique. Both of the aforementioned papers employ an optical feedback frequency-locking method, using the asymmetry of the cavity mode transmission signal to obtain the feedback phase error control signal. Ultimately, this locks the frequency of the semiconductor laser to the cavity mode frequency of the external resonant cavity, significantly improving the sensitivity of spectral detection. However, for harsh application scenarios, the method of obtaining the phase error control signal solely based on the asymmetry of the cavity mode transmission signal may fail. For example, sudden acoustic waves or mechanical vibrations can cause significant fluctuations in the feedback phase. In such cases, the asymmetry of the cavity mode transmission signal cannot accurately reflect the offset of the feedback phase, thus the system cannot maintain stability.

[0004] Therefore, researching a frequency-locked feedback phase dynamic adjustment method to suppress strong interference is of great value. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a frequency-locked feedback phase dynamic adjustment method for suppressing strong interference. This method, based on both the amplitude and asymmetry of the transmission cavity mode signal, has a very wide phase adjustment range. Addressing the problem in optical feedback frequency-locked resonant cavity technology where large fluctuations in the feedback phase cause traditional phase adjustment algorithms to fail, preventing the laser frequency from effectively locking to the resonant cavity's longitudinal mode frequency, this invention achieves locking between the laser and the resonant cavity even in harsh environments. This allows the optical feedback frequency-locked resonant cavity enhanced gas laser spectroscopy detection device to operate in environments with strong mechanical vibration and acoustic noise.

[0006] The present invention adopts the following technical solution.

[0007] In optical feedback frequency-locked resonator enhancement technology, the error signal is determined by the asymmetry and peak amplitude of the transmission cavity mode signal. The range of feedback phase variation in optical feedback frequency-locked resonator enhancement technology includes the adjustment trap region, positive adjustment region, negative adjustment region, and adjustment target region.

[0008] A frequency-locked feedback phase dynamic adjustment method for suppressing strong interference includes the following steps:

[0009] Step 1: The asymmetry of the cavity membrane is obtained after the transmitted cavity membrane signal is processed by noise reduction, smoothing, differentiation, truncation, integration or summation.

[0010] Step 2: Select different adjustment methods for the feedback phase based on the different asymmetry values ​​and the auxiliary determination of the peak amplitude of the transmitted signal after the cavity.

[0011] Preferably, in step 1, the data extracted is the data between the maximum and minimum values ​​after differentiation.

[0012] Preferably, in step 2, if the feedback phase is in the negative adjustment region, the high-voltage amplifier outputs a negative voltage to negatively adjust the feedback phase until the feedback phase is adjusted to the adjustment target region, at which point the adjustment of the feedback phase is stopped.

[0013] If the feedback phase is in the positive adjustment region, the high-voltage amplifier outputs a positive voltage to positively adjust the feedback phase until the feedback phase is adjusted to the target adjustment region, at which point the adjustment of the feedback phase stops.

[0014] When the asymmetry is between the upper and lower limits of the asymmetry in the target adjustment region, if the peak amplitude of the transmitted signal after the cavity is greater than the threshold, the feedback phase is considered to be in the target adjustment region, and no further adjustment of the feedback phase is made. If the peak amplitude of the transmitted signal after the cavity is less than the threshold, the feedback phase is considered to be in the adjustment trap region. In this case, the feedback phase is adjusted in the positive direction until the feedback phase enters the target adjustment region. At this point, the asymmetry is again between the upper and lower limits of the asymmetry in the target adjustment region.

[0015] The lower limit of the asymmetry of the target region is -0.06, and the upper limit is 0.06.

[0016] The amplitude threshold value of the transmission cavity mode signal is 25% of the global maximum amplitude value of the transmission cavity mode signal.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. The phase adjustment method provided by the present invention is not constrained by the phase fluctuation amplitude, and can still obtain an accurate error signal when the phase fluctuation is large.

[0019] 2. The phase adjustment method provided by this invention allows the optical feedback frequency-locked resonant cavity enhanced gas laser spectroscopy detection device to operate in an environment with strong mechanical vibration and acoustic noise, which can suppress strong interference and has good prospects for field application. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a device for dynamic adjustment of feedback phase in an optical feedback frequency-locked resonant cavity technology.

[0021] Figure 2 This is a schematic diagram showing the variation of the asymmetry of the transmission cavity mode signal and the maximum value of the transmission signal with the feedback phase L0.

[0022] Figure 3 This is a flowchart of the feedback phase adjustment method. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0024] A frequency-locked feedback phase dynamic adjustment method for suppressing strong interference, such as Figure 1 As shown, the signal generator outputs a modulation signal to the laser controller to scan the drive current of the laser controller. The laser controller then drives the semiconductor laser to output laser light. The emitted laser light passes through the feedback rate control module, which includes a half-wave plate, a polarizing beam splitter prism, and a quarter-wave plate. It then passes through the feedback phase adjustment module, which consists of a mirror attached to a piezoelectric ceramic. Finally, it passes through the spatial coupling module, which consists of two cylindrical lenses, and enters the optical resonant cavity. The transmitted light is detected by a photodetector, and the reflected light returns along the original path. The phase and intensity of the reflected light are respectively adjusted by the feedback phase adjustment module and the feedback rate control module before finally returning to the laser, forming optical feedback.

[0025] The signal from the photodetector is sent to the data acquisition card. After being denoised, smoothed, differentiated, truncated, integrated, or summed by LabVIEW, the asymmetry of the transmission cavity mode signal is obtained. At the same time, the magnitude of the error signal is determined based on the maximum value of the transmission signal. Finally, an error signal is output, which is amplified by the high-voltage amplifier module and drives the piezoelectric ceramic to adjust the extension and retraction length of the piezoelectric ceramic, thereby adjusting the feedback phase.

[0026] The modulation signal output by the signal generator is a sawtooth wave with a frequency in the kHz range.

[0027] The feedback rate control module consists of a half-wave plate, two polarizing beam splitters, and a quarter-wave plate. The half-wave plate is used to rotate the laser polarization state so that most of the laser passes through the polarizing beam splitter. The quarter-wave plate is used to rotate the feedback light polarization state. The two polarizing beam splitters can attenuate the feedback light over a very wide range.

[0028] The two cylindrical lenses in the spatial coupling module are used to transform the circular light spot emitted by the semiconductor laser into a circular light spot that is the same as the fundamental mode light spot of the resonant cavity, thereby reducing the spatial coupling loss when the laser is injected into the resonant cavity.

[0029] The resonant cavity is a V-shaped resonant cavity composed of two concave mirrors and one plane mirror. The plane mirror serves as the incident cavity mirror and is at a certain angle to the direction of the incident laser. This effectively avoids the influence of directly reflected light and only allows the resonant light coming out of the resonant cavity to return to the laser.

[0030] The curves showing the asymmetry of the transmission cavity mode signal and the peak amplitude of the transmission signal as a function of the feedback phase L0 are as follows: Figure 2 As shown. In a preferred but non-limiting embodiment of the present invention, the variation range of L0 can be divided into an adjustment trap region, a positive adjustment region, a negative adjustment region, and an adjustment target region. The purpose of the feedback phase adjustment method is to keep the feedback phase within the adjustment target region, because the asymmetry is close to 0 in this region, and the intensity of the back-cavity transmission signal reaches its global maximum. The lower limit of the asymmetry corresponding to the adjustment target region is set to -0.06, and the upper limit is 0.06.

[0031] The feedback phase adjustment method provided by this invention is as follows: Figure 3 As shown, it includes the following steps:

[0032] Step 1: After the transmitted cavity mode signal is processed by noise reduction, smoothing, differentiation, truncation, integration or summation, the cavity mode asymmetry is obtained.

[0033] In step 1, the data between the maximum and minimum values ​​after differentiation is extracted. The extracted data is then integrated or summed. The result of integration or summation is the asymmetry of the cavity mode transmission signal. Its sign reflects the direction of phase deviation, and its absolute value reflects the degree of phase deviation in a certain direction.

[0034] Step 2: Based on different asymmetry values ​​and the auxiliary determination of the peak amplitude of the transmitted signal after the cavity, select different adjustment methods for the feedback phase L0:

[0035] 1. If the asymmetry is greater than the upper limit of the asymmetry in the target adjustment region, the feedback phase L0 is considered to be in the negative adjustment region. At this time, the high-voltage amplifier outputs a negative voltage, and L0 is adjusted negatively, gradually decreasing in size. When the asymmetry is adjusted to between the upper and lower limits of the target adjustment region, L0 is considered to have been adjusted to the target adjustment region, and adjustment of L0 is stopped.

[0036] 2. When the asymmetry is less than the lower limit of the asymmetry in the target adjustment region, the feedback phase L0 is considered to be in the positive adjustment region. At this time, the high-voltage amplifier outputs a positive voltage, and L0 is adjusted in the positive direction, gradually increasing in size. When the asymmetry is adjusted to be between the upper and lower limits of the target adjustment region, the feedback phase is considered to have been adjusted to the target adjustment region, and the adjustment of L0 is stopped.

[0037] 3. When the asymmetry is between the upper and lower limits of the asymmetry in the target adjustment region, it is worth noting that the peak amplitude of the back-channel transmission signal may not be the global maximum value, but may be in the global valley region. Correspondingly, the feedback phase L0 may be in the target adjustment region or the adjustment trap region. The determination method is to preset a threshold value, which is 25% of the global maximum value of the transmission cavity mode signal amplitude. If the peak amplitude of the back-channel transmission signal is greater than this threshold value, L0 is considered to be in the target adjustment region, and L0 is no longer adjusted. If the peak amplitude of the back-channel transmission signal is less than this threshold value, L0 is considered to be in the adjustment trap region. At this time, L0 is positively adjusted, and its value increases and gradually enters the positive adjustment region. The cavity mode asymmetry will gradually become smaller than the lower limit of the asymmetry. Continuing to positively adjust L0 will bring the asymmetry back between the upper and lower limits of the target adjustment region, at which point L0 enters the target adjustment region.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for dynamic adjustment of feedback phase in frequency-locked feedback phase dynamic adjustment method for suppressing strong interference, characterized in that: the error signal in the optical feedback frequency-locked resonant cavity enhancement technology is determined by the asymmetry of the transmitted cavity mode signal and the amplitude peak value; the change range of the feedback phase in the optical feedback frequency-locked resonant cavity enhancement technology includes the adjustment trap region, the positive adjustment region, the negative adjustment region, and the adjustment target region; and the method comprises the following steps: Step 1: the asymmetry of the cavity mode is obtained after the transmitted cavity mode signal is processed by denoising and smoothing, differentiation, interception, integration, or summation; Step 2: different adjustment modes of the feedback phase are selected according to different asymmetry values and the auxiliary judgment of the amplitude peak value of the transmitted signal after the cavity; wherein, in Step 2, if the feedback phase is in the negative adjustment region, the high-voltage amplifier outputs a negative voltage to negatively adjust the feedback phase until the feedback phase is adjusted to the adjustment target region, at which time the adjustment of the feedback phase is stopped; if the feedback phase is in the positive adjustment region, the high-voltage amplifier outputs a positive voltage to positively adjust the feedback phase until the feedback phase is adjusted to the adjustment target region, at which time the adjustment of the feedback phase is stopped; and when the asymmetry is between the upper limit and the lower limit of the asymmetry in the adjustment target region, if the amplitude peak value of the transmitted signal after the cavity is greater than a threshold value, it is considered that the feedback phase is already in the adjustment target region, and the feedback phase is no longer adjusted; if the amplitude peak value of the transmitted signal after the cavity is less than the threshold value, it is considered that the feedback phase is in the adjustment trap region, at which time the feedback phase is positively adjusted until the feedback phase enters the adjustment target region; at this time, the asymmetry is between the upper limit and the lower limit of the asymmetry in the adjustment target region. 2.The method for dynamic adjustment of feedback phase in frequency-locked feedback phase dynamic adjustment method for suppressing strong interference according to claim 1, characterized in that: in Step 1, the intercepted data is between the maximum value and the minimum value after differentiation. 3.The method for dynamic adjustment of feedback phase in frequency-locked feedback phase dynamic adjustment method for suppressing strong interference according to claim 1, characterized in that: the lower limit of the asymmetry in the adjustment target region is -0.06, and the upper limit is 0.

06. 4.The method for dynamic adjustment of feedback phase in frequency-locked feedback phase dynamic adjustment method for suppressing strong interference according to claim 1, characterized in that: the amplitude threshold value of the transmitted cavity mode signal is 25% of the global maximum value of the amplitude of the transmitted cavity mode signal. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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