A method and apparatus for reducing optical interference noise

Abstract

The application provides a method and device for reducing optical interference noise, and belongs to the technical field of laser gas analysis. The method comprises the following steps: a periodic micro-displacement is applied to a TO packaged laser through a piezoelectric ceramic phase modulation structure, and a phase of an interference cavity formed between a laser window and a collimating lens is actively modulated; a long-focus collimating optical design is used to increase the cavity length of the main interference cavity, so that the optical interference noise signal and the gas absorption signal are effectively distinguished; original transmission light intensity signals collected in a plurality of laser scanning periods are subjected to normalization and arithmetic average processing; and finally, the processed signals are subjected to convolution matching filtering, so that the noise is further suppressed. The device integrates a piezoelectric ceramic phase modulation structure and an emission end of a long-focus collimating lens. The application combines active phase modulation with passive optical design, realizes step-by-step deep purification of the optical interference noise, and significantly improves the signal-to-noise ratio, measurement stability and compatibility with the TO packaged laser of the system.

Description

Technical Field

[0001] This invention relates to the technical field of laser gas analysis, and more particularly to a method and apparatus for reducing optical interference noise. Background Technology

[0002] Patent application number CN201210106825.2, publication number CN102654447A, and application date September 5, 2012, discloses a device and method for reducing optical interference in a laser gas analyzer. The main drawbacks of this patent are:

[0003] 1. Insufficient structural stability and rigidity, easily introducing additional errors. The piezoelectric ceramic (PZT) design in this technology uses a "seesaw" or "parallelogram" double-ended suspension design, where the two ends of the two plates are connected only by spring bolts and piezoelectric ceramic (PZT). This structure is a statically indeterminate system under stress, prone to torsion and yaw, resulting in an impure laser trajectory. This causes the laser to be affected by the PZT during actual use. When vibration occurs, although it will move in the desired direction, it will also produce small, uncontrolled lateral movements and angular yaws. This extra degree of freedom introduces unpredictable optical path disturbances and may even generate new, time-varying interference noise, thereby reducing the system's stability and measurement repeatability.

[0004] 2. The motion pattern is not ideal, resulting in a coarse modulation mechanism. This technology fixes the laser to the side of the upper clamp near the PZT. When the PZT extends or retracts, the laser mainly undergoes angular displacement with the spring bolt as the fulcrum. This structural design causes a periodic and significant change in the laser emission angle. Its core noise suppression mechanism is to "randomize" the interference pattern by changing the geometry of the optical path. This is a macroscopic and coarse modulation method, which cannot achieve precise pure phase modulation aimed at modulating the phase of the light wave. Therefore, the noise suppression efficiency and control precision are low.

[0005] 3. Insufficient Integration and Targeted Design. The structure of this technology is a general "vibration platform," without specifying a highly integrated device or apparatus, nor does it describe the applicable lasers. Since the spring and PZT are attached as external components between the two plates, the structure is relatively loose, making it difficult to guarantee ideal compatibility with TO (transistor) packaged lasers widely used in industry. Because the TO package itself is a precision unit, subjecting it to large-amplitude angular vibrations may pose a potential risk of damage to the internal structure. Furthermore, the loose structure is not conducive to device miniaturization.

[0006] 4. Lack of in-depth processing capability for residual noise. This technology effectively perturbs high-frequency, fine interference fringes by changing the laser angle. After suppressing noise through PZT design, there is a lack of effective follow-up methods to specifically process the remaining, difficult-to-eliminate interference noise, preventing further qualitative improvement in system performance.

[0007] Therefore, the technical problems to be solved by the present invention include:

[0008] 1. Solving the problem of modulation inaccuracy and signal distortion caused by "macro-geometric perturbation". The above-mentioned technology drives the laser to make large-scale angular deflection and position movement through piezoelectric ceramics. Its core is a "macro-geometric perturbation" of the optical path, which aims to randomize the interference pattern. This coarse modulation method will simultaneously perturb the useful gas absorption signal baseline and cause signal feature distortion during incoherent averaging. The primary problem to be solved by this invention is how to transform this coarse and destructive "geometric perturbation" into a precise and non-destructive "phase modulation", so as to suppress noise while maximizing the preservation of the fidelity of the gas absorption signal.

[0009] 2. Solving the problem of poor system reliability and performance drift caused by "mechanical instability". The existing "double-ended suspension structure" is a statically indeterminate system with uncontrollable motion trajectory, prone to excessive yaw and torsion, and its mechanical parameters are prone to drift over long-term use. This directly leads to instability in interference suppression effects and the need for frequent instrument calibration. Therefore, this invention aims to solve the problem of designing a highly rigid, pure motion trajectory, and long-term stable mechanical structure to ensure the reliability and consistency of noise suppression performance and meet the needs of long-term online monitoring in industrial settings.

[0010] 3. To address the limitations of existing technologies in terms of single-mode noise suppression. Existing angular perturbation modes have limited effectiveness in suppressing mid-to-low frequency, wide-fringe interference noise. This invention aims to provide a comprehensive solution.

[0011] 4. Addressing the applicability and safety issues of existing technologies when adapting to TO-packaged lasers. Existing vibration schemes are not optimized for widely used TO-packaged lasers, and their macroscopic angular vibrations may pose a risk to the precision structure inside the TO package. Another specific problem this invention aims to solve is how to provide a safe phase modulation scheme that is highly compatible with the characteristics of TO-packaged lasers, enabling its stable and reliable application to this mainstream low-cost, miniaturized light source. Summary of the Invention

[0012] To address the limited noise suppression capabilities of existing single technologies, this invention proposes a method and apparatus for reducing optical interference noise. By combining two innovative approaches—PZT phase modulation and long-focal-length collimating optical design—it achieves deep and progressive purification of the same type of optical noise, ultimately reaching a comprehensive signal-to-noise ratio far exceeding that of any single technology.

[0013] To achieve the above objectives, the technical solution of the present invention is implemented as follows: a method for reducing optical interference noise, comprising the following steps:

[0014] Step 1: Design a piezoelectric ceramic phase modulation structure. Apply periodic micro-displacement to the laser using the piezoelectric ceramic phase modulation structure to modulate the phase of the interference cavity formed between the laser window and the collimating lens.

[0015] Step 2: Design a long focal length for the collimating optical path at the transmitting end to increase the cavity length of the interference cavity;

[0016] Step 3: The transmitted light intensity signals collected during multiple laser scanning cycles are normalized and arithmetic averaged to suppress optical interference noise with random phase, resulting in a preliminary denoised absorption signal.

[0017] Step 4: Perform convolutional matched filtering on the initially denoised absorption signal to further suppress interference noise, thereby achieving higher noise suppression.

[0018] Preferably, the piezoelectric ceramic phase modulation structure includes an upper plate and a lower plate, one end of the upper plate and the lower plate are fixedly connected by a support column, and the other end of the upper plate and the lower plate are fixedly connected by a screw, with a spring between the end of the screw and the upper plate; a laser is fixed in the middle of the upper plate, and a piezoelectric ceramic is fixed in the middle of the lower plate, with the end of the piezoelectric ceramic in contact with the upper plate.

[0019] Preferably, the upper plate has a groove, one end of the spring is disposed in the groove, and the other end of the spring is in contact with the end of the screw.

[0020] Preferably, the laser is a TO-packaged laser, and the piezoelectric ceramic is positioned close to the center of the laser's optical axis.

[0021] Preferably, the upper and lower plates are stabilized by a support column. When a periodic voltage signal is applied to the piezoelectric ceramic, the piezoelectric ceramic undergoes periodic deformation. The deformation, in conjunction with the spring, is transmitted to the laser located on the upper plate, causing the laser to undergo a micrometer-level displacement, thereby changing the distance between the laser and the collimating lens. The piezoelectric ceramic and the spring work together to control the movement trajectory of the upper plate, so that the upper plate only translates along the optical axis.

[0022] Preferably, the method for designing the collimating optical path of the emitting end with a long focal length is as follows: the collimating lens in the optical path of the laser emitting source adopts a long focal length lens, so that the distance between the emitting point of the laser and the focal point of the collimating lens is increased.

[0023] Preferably, the laser beam emitted by the laser undergoes multiple reflections within the interference cavity formed between the laser window and the first surface of the collimating lens, creating a stable interference signal. When the laser scans, the interference signal in the acquired signal will exhibit a periodic sine or cosine form of interference noise that varies with wavelength. When the optical path length from the laser window to the collimating lens is changed periodically by the piezoelectric ceramic, it is equivalent to continuously and periodically changing the length of the interference cavity, resulting in a change in the phase of the interference noise. Synchronized modulation, and: ;

[0024] in, The length of the interference cavity, representing the time-varying length under the action of piezoelectric ceramics. It is a periodic function, where φ0 is the initial phase. λ is the wavelength of the light beam emitted by the laser, and n is the equivalent refractive index in the interference cavity;

[0025] Within a complete piezoelectric ceramic modulation cycle, the phase of the interference noise varies continuously from 0 to 2π.

[0026] Preferably, the intensity of the gas absorption signal is only related to the laser wavelength emitted by the laser and the gas absorption coefficient. As long as the wavelength scan is linear, the waveform of the gas absorption signal is consistent and stable in each acquisition. The phase of the interference noise is randomized under the drive of the piezoelectric ceramic. In multiple consecutive acquisitions, the value of the interference noise phase fluctuates randomly between positive and negative.

[0027] The transmitted light intensity signal acquired within one laser scanning cycle is normalized using the Beer-Lambert law; the arithmetic mean processing is performed by averaging the absorption signals after normalization of the original transmitted signals within multiple laser scanning cycles to obtain the absorption signal after preliminary noise reduction.

[0028] The method for implementing the convolution matched filtering process is to perform a convolution operation on the initially denoised absorption signal and a pre-constructed convolution kernel that matches the absorption line shape of the target gas, thereby locking in and enhancing the features originating from gas absorption from the derivative signal containing interference noise.

[0029] An apparatus for reducing optical interference noise, utilizing the aforementioned method for reducing optical interference noise, includes an emitter that integrates a piezoelectric ceramic phase modulation structure and a long focal length collimating lens.

[0030] Preferably, the optical path of the transmitting end is provided with a gas to be measured and a matching receiving end. The transmitting end and the receiving end are electrically connected through a junction box. The laser of the transmitting end is fixed on a piezoelectric ceramic phase modulation structure. The transmitted light intensity signal collected by the detector of the receiving end is connected to a data processing module. The data processing module includes a normalization unit, an arithmetic averaging unit, and a convolution matched filter unit connected in sequence. The normalization unit normalizes the transmitted light intensity signals collected from multiple different points. The arithmetic averaging unit performs an arithmetic average calculation on the normalized transmitted light intensity signals to obtain a preliminary denoised absorption signal. The convolution matched filter unit performs a convolution operation on the preliminary denoised absorption signal through a convolution kernel to filter out signals that are not related to the convolution kernel, thereby obtaining a noise-filtered absorption signal.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1. Using PZT to design and implement phase shifting of optical noise, and eliminating noise through averaging algorithm, the influence of optical interference noise can be significantly suppressed and the signal-to-noise ratio can be improved;

[0033] 2. This invention has a compact structure and is applicable to TO-packaged lasers;

[0034] 3. By using long focal length collimating optical design to increase the periodic frequency of optical interference fringes and reduce the fringe width, the difference between the fringe width and the absorption line width in the spectrum can be easily distinguished. Then, optical noise can be reduced through separation or filtering algorithms. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is an overall structural diagram of the gas detection system in this invention.

[0037] Figure 2 This is a schematic diagram of the process of the present invention.

[0038] Figure 3 This is a structural diagram of the laser emission system of the transmitter of the present invention.

[0039] Figure 4 for Figure 3 A schematic diagram of the PZT phase modulation structure.

[0040] Figure 5 This is a schematic diagram of gas absorption signals.

[0041] Figure 6 This is a schematic diagram of the measurement signal.

[0042] Figure 7 This is a schematic diagram of interference signals with different phases under PZT modulation.

[0043] Figure 8 This is a schematic diagram of the total signal under different PZT modulation phases.

[0044] Figure 9 This is a schematic diagram of the averaging effect of PZT modulation.

[0045] Figure 10 This is a schematic diagram illustrating the effect of changing the distance L on the PZT modulation results.

[0046] Figure 11 The diagram illustrates the effect of further suppression, where (a) represents lengthening the distance and (b) represents reducing the distance.

[0047] In the diagram, 1 is the upper plate, 2 is the lower plate, 3 is the support column, 4 is the piezoelectric ceramic, 5 is the screw, 6 is the spring, and 7 is the laser. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Example 1

[0050] like Figure 2 As shown, a method for reducing optical interference noise includes: applying periodic micro-displacements to a TO-packaged laser using a piezoelectric ceramic phase modulation structure to actively modulate the phase of the interference cavity formed between the laser window and the collimating lens; simultaneously, employing a long-focal-length collimating optical design to increase the cavity length of the main interference cavity, making the interference fringes denser and narrower within the wavelength scanning range, effectively distinguishing the optical interference noise signal from the gas absorption signal; then, the original transmitted light intensity signals acquired over multiple scanning cycles are normalized and arithmetically averaged to suppress phase-random optical interference noise; finally, the processed signal is subjected to convolutional matched filtering, utilizing the difference between the gas absorption signal and the residual interference noise to further suppress the noise. This invention combines active phase modulation with passive optical design, achieving a progressively deeper purification of optical interference noise, significantly improving the system's signal-to-noise ratio, measurement stability, and compatibility with TO-packaged lasers. The specific implementation steps of this invention are as follows:

[0051] Step 1: Design a piezoelectric ceramic phase modulation structure. Apply periodic micro-displacement to the laser through the piezoelectric ceramic phase modulation structure to modulate the phase of the interference cavity formed between the laser window and the collimating lens.

[0052] This invention provides a low optical interference noise gas detection system based on tunable diode laser spectroscopy (TDLAS) technology, comprising a transmitter and a receiver connected in sequence, wherein the transmitter and receiver are electrically connected through a junction box, as shown below. Figure 1 As shown, the gas under test is positioned in the optical path between the transmitter and receiver. The junction box connects the electrical signals between the transmitter and receiver, ensuring synchronization and stability of laser driving, data acquisition, and signal processing. The gas under test is located between the transmitter and receiver. Figure 1 The receiving end includes a converging lens and a detector. The laser transmitted from the transmitting end passes through the gas to be measured, then through the converging lens, and is focused by the lens before being received by the detector. The detector converts the collected optical signal into an analog signal and transmits it to the data processing module connected to the detector. The data processing module includes a photoelectric conversion module, a data acquisition module, a concentration calculation module, and an input / output module. The photoelectric conversion module converts the optical signal collected by the detector into an electrical signal. The analog signal is amplified by a signal amplification circuit and then converted into a digital signal by the data acquisition module. The concentration calculation module calculates the corresponding target gas concentration value based on the collected spectral digital signal, after averaging and suppressing interference noise, and then combining it with a pre-set spectral concentration calculation program. Finally, the gas concentration calculation result is output through the input / output module. The core improvement of this invention lies in the transmitting end, which contains a laser, a piezoelectric ceramic phase modulation structure, and a collimating lens. A schematic diagram of the structure is shown below. Figure 3 As shown, the laser is fixed on a vertically arranged piezoelectric ceramic phase modulation structure. Both the laser and the piezoelectric ceramic phase modulation structure are connected to the driving circuit. The driving circuit outputs a modulation voltage of a fixed frequency to drive the piezoelectric ceramic to expand and contract at the same frequency, thereby realizing a small-amplitude vibration of the laser position to reduce system noise caused by optical interference. The driving circuit achieves coarse adjustment of the laser wavelength scanning range by adjusting the magnitude of the laser driving current.

[0053] like Figure 4As shown, the piezoelectric ceramic phase modulation structure includes an upper plate 1 and a lower plate 2. One end of the upper plate 1 and the lower plate 2 are fixedly connected by a support column 3, and the other end of the upper plate 1 and the lower plate 2 are connected by a piezoelectric ceramic 4. The upper plate 1 is tightly attached to the piezoelectric ceramic 4 by screws 5, and the piezoelectric ceramic 4 is also attached to the lower plate 2 and fixedly connected to it. This design ensures that the extension and retraction of the piezoelectric ceramic 4 can be directly transmitted to the upper plate 1, driving the micro-displacement of the laser. The laser 7 is fixedly mounted on the upper plate 1, and the support column 3 provides rigid support to prevent the upper plate 1 from swaying or twisting during movement, increasing stability. In motion control, it is ensured that when the piezoelectric ceramic 4 is driven, the laser 7 mainly performs micron-level translation along the optical axis, rather than large-angle deflection, achieving pure phase modulation. Screws 5 are also fixed to the upper plate 1 and the lower plate 2 outside the piezoelectric ceramic 4. A spring 6 is provided on the upper part of the screw 5, and the spring 6 is located on the upper plate 1. The screw 5 compresses the spring 6 by tightening downwards, ensuring that the upper plate 1 and the piezoelectric ceramic 4 are in contact. Screw 5 is used to fix the position of spring 6 and provide preload, ensuring that the spring can effectively transmit force when the piezoelectric ceramic 4 extends and retracts. Spring 6 provides restoring force, which, in conjunction with the extension and retraction of piezoelectric ceramic 4, allows the upper plate to return to its initial position after being driven by piezoelectric ceramic 4, achieving periodic modulation. At the same time, spring 6 buffers vibration and reduces impact. The upper plate 1 has a groove for limiting one end of spring 6. Both ends of spring 6 are limited rather than completely fixed: one end is embedded in the groove of the upper plate 1, and the other end contacts the upper part of screw 5. This design allows spring 6 to deform freely during compression and extension while maintaining a stable position.

[0054] The two plates are stabilized by support column 3. When the driving circuit applies a periodic voltage signal to the piezoelectric ceramic 4, the PZT will undergo periodic deformation. This deformation, combined with spring 6, is transmitted to the laser 7 located on the upper plate 1, causing it to undergo micrometer-level displacement, thereby changing the distance between the laser 7 and the collimating lens. When a periodic voltage is applied, the piezoelectric ceramic 4 will undergo expansion and contraction deformation. Spring 6, as an elastic element, compresses when the piezoelectric ceramic 4 extends and extends when the piezoelectric ceramic 4 contracts. This interaction converts the deformation of the piezoelectric ceramic into micro-displacement of the upper plate 1, ensuring smooth and controllable motion. The presence of support column 3 ensures the stability of the laser 7's displacement when driven by the piezoelectric ceramic 4, preventing unnecessary yaw and torsion. Support column 3 provides rigid support, restricting the rotational degree of freedom of the upper plate 1; the piezoelectric ceramic 4 and spring 6 work together to control the trajectory of the upper plate, causing it to translate only along the optical axis.

[0055] Laser 7 is a TO packaged laser. Piezoelectric ceramic 4 is disposed between upper plate 1 and lower plate 2. The specific position should be close to the center of the optical axis of laser 7 to minimize angular deflection.

[0056] It is worth noting that although the structure experiences slight angular changes during expansion and contraction, due to its compact size and the placement of the piezoelectric ceramic 4 near the center of the upper plate 1, the optical path actually experiences only a tiny shift during operation. However, this shift angle is so small that it can almost be considered as the optical path still propagating in a straight line. The core purpose of this structure design is to ensure that the laser undergoes a constrained motion when driven by the piezoelectric ceramic 4, making it a pure, minute translation along the optical axis. This tiny displacement directly and proportionally changes the optical path difference, thereby precisely and linearly modulating the phase of the interference signal.

[0057] The laser used in this invention is a TO-packaged laser, which is better suited for industrial production and use. The design of the piezoelectric ceramic phase modulation structure is also fully suitable for industrial production and use.

[0058] The core purpose of placing a PZT phase modulation structure in front of the laser in a gas detection system is to eliminate optical noise generated during instrument operation. One of the main sources of interference noise in a gas detection system is the "pseudo-interference cavity" formed between the laser window and the first surface of the collimating lens.

[0059] When a laser emits laser light, a small portion of it is reflected by the first mirror of the collimating lens as it travels from the laser mirror to the collimating lens. This reflection then returns to the laser mirror, and is reflected again by the laser mirror to the collimating lens mirror, repeating this process repeatedly to form an optical interference signal. When the laser wavelength is scanned, this interference signal manifests as a periodic sine or cosine form of interference noise that varies with the wavelength. It can be expressed by the following formula:

[0060]

[0061] Where φ0 is the initial phase, A is the amplitude of the interference noise, n is the equivalent refractive index of the medium inside the interference cavity, and L is the cavity length of the interference cavity. It is the laser wavelength.

[0062] To illustrate with the simplest example, if only one gas absorption signal exists in the scanning band (e.g.) Figure 5 As shown in the figure, due to the introduction of interference noise, the final result obtained when measuring the signal will be as follows. Figure 6 The signal diagram shown is completely obscured by interference noise; it is impossible to distinguish which line represents the gas absorption signal and which represents the interference noise signal. In short, the gas absorption signal is entirely hidden within the interference noise signal.

[0063] Therefore, the main principle of using PZT to suppress interference noise is that when the laser is driven to make a micro-displacement by PZT displacement, it is equivalent to modulating the phase of the interference signal. ,Right now:

[0064]

[0065] in, This represents the length of the interference cavity as it changes over time. Due to the micro-displacement of the PZT-driven laser, the length... It is a periodic function, corresponding to the modulation of the interference cavity length.

[0066] During a complete PZT drive cycle, the interference noise I noise The phase of (t) varies continuously within the range of 0 to 2π (or larger). This means that for the data acquisition system, the interference noise at a fixed wavelength is no longer a fixed value, but a rapidly changing AC signal, such as... Figure 7 As shown. Figure 7 This diagram illustrates the signal variation of the interference signal over multiple cycles at a PZT modulation frequency. As can be seen from the figure, the interference signal values ​​obtained at different times after PZT modulation are different, while the intensity of the useful signal (gas absorption signal) depends only on the instantaneous wavelength of the laser and the gas absorption coefficient. That is, as long as the wavelength scan is linear, the waveform of the gas absorption signal is consistent and stable in each rapid acquisition, unaffected by PZT phase modulation. However, the noise signal (interference noise) will fluctuate randomly between positive and negative values ​​in consecutive rapid acquisitions due to the phase randomization caused by PZT. The measurement signal near the gas absorption signal will be amplified as shown below. Figure 8 As shown, the gas absorption signal is faintly visible, and the interference signal exhibits periodic changes.

[0067] Therefore, an averaging algorithm can be used to eliminate interference noise. Assume that in the i-th scan, at time t... j Transmitted light intensity signal (corresponding to a specific wavelength):

[0068]

[0069] in, Indicates at time The gas absorption signal (corresponding to a specific wavelength) is a stable value. Indicates at time and the Interference noise in the sub-sampling varies randomly with phase.

[0070] The Nth data collection is averaged.

[0071]

[0072] Due to gas absorption signal It is a constant, that is: .

[0073] Due to interference noise It is a phase random signal with a statistical expectation value of ,therefore

[0074] (If N is large enough)

[0075] Therefore, after averaging, the interference noise in the output signal will be greatly suppressed (approximately 80%), while the gas absorption signal will be completely preserved, such as... Figure 9 As shown.

[0076] Step 2: Design a long focal length for the collimating optical path at the transmitting end to increase the cavity length of the interference cavity.

[0077] In the above steps, PZT modulation can accurately extract the gas absorption signal from the interference noise signal and simultaneously suppress the interference noise. However, it can be seen that some interference noise still exists and is difficult to eliminate. Therefore, we can return to analyzing the signal essence of the interference noise. For a sine or cosine signal, the two most important parameters are phase and frequency. Using PZT modulation is essentially a shift in thinking from adjusting the phase of this signal itself. Since we can process it from the phase, we can also consider how to suppress it from the frequency perspective.

[0078] Interference noise is caused by the back-and-forth reflection of the laser beam between two mirrors, equivalent to the laser passing through a Fabry-Perot interferometer. In optics, there exists a physical quantity that represents the wavelength or frequency interval between two peaks in the transmission spectrum of a Fabry-Perot interferometer, known as the free spectral range (FSR). In other words, the interval between the two transmission peaks in the measured signal is the FSR, which is also the period of the sine or cosine signal. Therefore, the period frequency of the interference noise signal can be changed by altering the FSR. The formula for calculating the FSR is shown below:

[0079]

[0080] In the formula, FSR represents the free spectral region; c represents the speed of light; n represents the refractive index; and L represents the cavity length of the interference cavity, i.e., the distance from the laser to the collimating lens.

[0081] It is worth noting that the purpose of the long focal length design is to further process residual optical noise on the basis of the previous PZT optical noise reduction, thereby improving the signal-to-noise ratio. Both the PZT design and the long focal length design can reduce interference noise, but they can only eliminate about 80% of the noise. However, by combining the two designs simultaneously, a noise reduction effect of 96% can be achieved, i.e., Noise * (80% + 20% * 80%) = Noise * 96%.

[0082] According to interference theory, the periodic frequency (or spatial frequency) of interference fringes is related to the precision and length of the interference cavity. In gas detection systems, the frequency of interference fringes (the number of periods within the wavelength scanning range) is inversely proportional to the length of the interference cavity. By using a long focal length lens, the effective optical distance between the laser emission point and the lens principal surface (the distance is fixed after assembly) is essentially increased, thereby increasing the cavity length of the most important interference cavity in the system (laser window-collimating lens). This results in a smaller period for the interference fringes and more, denser, and narrower fringes within the wavelength scanning range. Conversely, reducing the effective optical distance between the laser emission point and the lens principal surface reduces the cavity length, resulting in a larger period for the interference fringes and fewer, sparser, and wider fringes within the wavelength scanning range.

[0083] Therefore, at the current distance, the distance between the laser and the collimating lens can be increased or decreased. For example... Figure 10 As shown, the black line represents the signal after PZT suppression, the red line represents the signal when L is doubled, and the blue line represents the signal when L is halved. It can be seen that when the distance is doubled, the interference noise signal frequency becomes higher and denser; when the distance is halved, the interference noise signal frequency becomes lower and sparser. In this way, the gas absorption signal and interference noise, which originally only differed in amplitude intensity, now have completely different overall signal structures, making signal separation much easier and simpler.

[0084] Both stretched and shrunk signals can suppress noise signals through methods such as convolution or other smoothing filters, for example... Figure 11 As shown. Figure 11 In (a), the red line represents the signal after the distance is increased, and the black line represents the signal after further suppression; in (b), the blue line represents the signal after the distance is reduced, and the black line represents the signal after further suppression.

[0085] Step 3: The transmitted light intensity signals acquired in multiple scanning cycles are normalized and arithmetic averaged to suppress optical interference noise with random phase, resulting in a preliminary denoised absorption signal.

[0086] The transmitter and receiver are collimated and aligned. The laser is emitted from the transmitter, passes through the gas to be measured, and enters the receiver. The detector at the receiver then receives the transmitted light intensity signal. Within one laser scanning cycle, the raw transmitted light intensity signal is acquired. All acquired raw transmitted light intensity signals are normalized to obtain the absorption signal, which is only related to the gas concentration. This absorption signal is then smoothed and filtered.

[0087] The specific implementation is as follows: After obtaining the original absorption signal, a normalization process is first performed. Within one laser scanning cycle, the Beer-Lambert law is used to normalize all acquired original transmitted light intensity signals, resulting in an absorption signal that is only related to the gas concentration. The normalization process itself eliminates common-mode noise such as laser intensity fluctuations. Next, a smoothing and filtering process is performed, which involves arithmetically averaging the normalized absorption signals. Because the phase of interference noise is periodically modulated by PZT, it exhibits high-frequency fluctuations at different acquisition points, while the actual gas absorption signal is stable. Through arithmetic averaging, the phase-randomized interference noise is significantly suppressed. Finally, the averaged signal is smoothed and filtered to further remove residual noise.

[0088] Step 4: Perform convolutional matched filtering on the initially denoised absorption signal to achieve further noise reduction.

[0089] Because the TDLAS system uses direct absorption (DAS) technology, its gas absorption line is a relatively wide and smooth "depression." When the laser wavelength scans linearly across the absorption line over time, this absorption depression appears as a slowly varying broadband signal dominated by low-frequency components in the time domain. Its characteristic width is the gas absorption linewidth. However, after the long focal length design, the interference noise signals result in denser interference fringes. These dense fringes appear as high-frequency, periodic sinusoidal fluctuations in the wavelength scanning signal.

[0090] After PZT modulation and related algorithm processing, subsequent algorithm processing dependent on the long focal length design is performed. The core idea of ​​this processing is to utilize the key difference that the gradient of the interference fringe variation caused by the long focal length design is much greater than the gradient of the gas absorption line variation for separation. That is, the width of the sine wave represented by the interference fringes in the absorption signal within one scan cycle is much smaller than the width of the gas absorption signal.

[0091] This invention builds upon previous signal processing by employing convolution matched filtering, which involves convolving the initially denoised absorption signal with a pre-constructed convolution kernel that matches the absorption line shape of the target gas. The main principle is that, since the convolution kernel mathematically represents the standard differential property of the target gas absorption line, the convolution operation can achieve a "template matching" effect.

[0092] A strong response is generated when the convolution kernel encounters a portion of the absorption signal whose shape and gradient changes are highly similar to its own (i.e., the differential features corresponding to the real gas absorption signal). Meanwhile, signals unrelated to the convolution kernel are greatly suppressed or averaged out during the convolution process. This step can extremely accurately pinpoint and enhance features originating from gas absorption from the derivative signal containing interference noise, thereby achieving a decisive improvement in the signal-to-noise ratio.

[0093] The present invention brings about significant technological progress through the optimization scheme of hardware improvements and algorithmic collaborative processing described above. Its beneficial effects are specifically reflected as follows:

[0094] 1. Regarding the effectiveness of PZT active phase modulation technology: This invention mainly solves the problem of suppressing high-frequency, periodic interference noise by introducing PZT modulation and corresponding algorithms.

[0095] Mechanism of Action: After the PZT is driven, it applies a periodic micro-displacement to the laser. This operation is equivalent to actively and periodically modulating the phase of the interference noise formed by the laser within the optical cavity (laser window and collimating lens). From a signal processing perspective, this makes the fixed-mode interference noise no longer stable in the time series, but rather transform it into an AC signal with periodically changing phase.

[0096] Beneficial effects: In data processing, the absorption signals over multiple scan cycles are arithmetically averaged. Since the true absorption signal, directly related to gas concentration, remains stable across different scan cycles, while the phase of interference noise varies with PZT driving, multiple averagings effectively cancel out and significantly reduce the interference noise due to its phase randomness. Experimental data show that this technique can effectively suppress approximately 80% of this type of high-frequency noise, thereby greatly improving the signal-to-noise ratio and providing a cleaner data foundation for subsequent trace gas concentration inversion.

[0097] If only PZT modulation is used to reduce optical interference noise, the changes in the gas absorption signal and the interference noise signal are as follows: Figure 6 , Figure 9 As shown, where, Figure 6 This is a signal graph of the gas absorption signal and optical interference noise obtained through a normalization algorithm after the system signal was acquired. Figure 9This is the signal spectrum after phase modulation with PZT enabled and then averaging. As can be seen from the graph, enabling PZT and applying the corresponding filtering algorithm significantly reduces the amplitude of interference noise.

[0098] 2. Regarding the effectiveness of long focal length collimating optical design techniques: By changing the optical structure and subsequent algorithms, the problem of separating interference noise, which is similar to the gas absorption characteristics, was mainly solved.

[0099] Mechanism Analysis: The core function of long focal length optical design is to increase the periodic frequency of optical interference fringes, resulting in the characteristic width of the interference fringes being much smaller than the characteristic width of the gas absorption lines in the spectrum. This hardware-level change creates an essential difference in "morphology" between interference noise (narrow width, high gradient) and gas signals (wide width, gently varying gradient), making them distinguishable by algorithms.

[0100] Beneficial effects: Calculations are performed using a convolution kernel matched to the target gas absorption line. This efficiently enhances the true gas absorption characteristics matched to the template, while strongly suppressing amplified interference noise spikes unrelated to the template. Theoretical analysis and experimental verification show that this technique can further suppress 80% of residual noise and effectively avoid distortion of the gas absorption signal during noise filtering, ensuring the accuracy of the concentration inversion results.

[0101] If a long focal length design is used to reduce optical interference noise, the spectra of the gas absorption signal and the optical interference noise signal obtained by the normalization algorithm are as follows: Figure 10 As shown, it is clear that the difference between this method and the method used to reduce noise using the PZT algorithm is that the sinusoidal signal exhibited by the interference noise has a higher and denser frequency, or a lower and sparser frequency, and there is no longer a noise signal with a width not much different from the gas absorption signal. Figure 11 It is a signal spectrum after interference noise is suppressed by convolution and other corresponding algorithms. Figure 11 Figure (a) shows the signal after increasing the distance between the laser and the collimating lens. The red curve represents the suppression result after PZT modulation, and the black curve represents the result after applying convolution and filtering algorithms to the red curve. Figure (b) shows the signal after shortening the distance between the laser and the collimating lens. The blue curve represents the suppression result after PZT modulation, and the black curve represents the result after applying convolution and filtering algorithms to the blue curve. It can be seen that both distances can further suppress about 80% of the noise signal on top of the PZT modulation result. Combining the two suppression methods, the interference noise signal can ultimately be suppressed to about 4% of its original value.

[0102] Example 2

[0103] like Figure 1As shown, a device for reducing optical interference noise includes a transmitter integrating a piezoelectric ceramic phase modulation structure and a long focal length collimating lens. A matching receiver is located in the optical path of the transmitter, and a gas to be measured is placed between the transmitter and receiver. The transmitter and receiver are electrically connected through a junction box. The transmitter contains the piezoelectric ceramic phase modulation structure and the collimating lens. The laser of the transmitter is mounted on the piezoelectric ceramic phase modulation structure. The collimating lens is a long focal length lens. The transmitted light intensity signal obtained by the detector is connected to a data processing module. The data processing module includes a normalization unit, an arithmetic averaging unit, a smoothing filter unit, and a convolution matched filter unit connected in sequence. The normalization unit normalizes the transmitted light intensity signals from different collected points. The arithmetic averaging unit performs an arithmetic average calculation on the normalized transmitted light intensity signals. The smoothing filter unit smooths the arithmetic averaged transmitted light intensity signals to obtain a pre-denoised absorption signal. The convolution matched filter unit performs a convolution operation on the pre-denoised absorption signal using a convolution kernel, filtering out signals unrelated to the convolution kernel to obtain a noise-filtered absorption signal.

[0104] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing optical interference noise, characterized in that, The steps include the following: Step 1: Design a piezoelectric ceramic phase modulation structure. Apply periodic micro-displacement to the laser using the piezoelectric ceramic phase modulation structure to modulate the phase of the interference cavity formed between the laser window and the collimating lens. Step 2: Design a long focal length for the collimating optical path at the transmitting end to increase the cavity length of the interference cavity; Step 3: The transmitted light intensity signals collected during multiple laser scanning cycles are normalized and arithmetic averaged to suppress phase-random optical interference noise and obtain the preliminarily denoised absorption signal. Step 4: Perform convolutional matched filtering on the initially denoised absorption signal to further suppress interference noise, thereby achieving higher noise suppression. The piezoelectric ceramic phase modulation structure includes an upper plate (1) and a lower plate (2). One end of the upper plate (1) and the lower plate (2) are fixedly connected by a support column (3), and the other end of the upper plate (1) and the lower plate (2) are fixedly connected by a screw (5). A spring (6) is provided between the end of the screw (5) and the upper plate (1). A laser (7) is fixed in the middle of the upper plate (1), and a piezoelectric ceramic (4) is fixed in the middle of the lower plate (2). The end of the piezoelectric ceramic (4) is in contact with the upper plate (1). The method for designing a long focal length for the collimating optical path at the transmitting end is as follows: the collimating lens in the optical path of the laser emitting source is a long focal length lens, which increases the distance between the laser's emitting point and the focal point of the collimating lens. The method for implementing the convolution matched filtering process is to perform a convolution operation on the initially denoised absorption signal and a pre-constructed convolution kernel that matches the absorption line shape of the target gas, thereby locking in and enhancing the features originating from gas absorption from the derivative signal containing interference noise.

2. The method for reducing optical interference noise according to claim 1, characterized in that, The upper plate (1) is provided with a groove, one end of the spring (6) is placed in the groove, and the other end of the spring (6) is in contact with the end of the screw (5).

3. The method for reducing optical interference noise according to claim 2, characterized in that, The laser (7) is a TO packaged laser, and the piezoelectric ceramic (4) is located close to the center of the optical axis of the laser (7).

4. The method for reducing optical interference noise according to any one of claims 1-3, characterized in that, The upper plate (1) and the lower plate (2) are stabilized by the support (3). When a periodic voltage signal is applied to the piezoelectric ceramic (4), the piezoelectric ceramic (4) generates periodic deformation. The deformation, in conjunction with the spring (6), is transmitted to the laser (7) located on the upper plate (1), causing the laser (7) to undergo a micrometer-level displacement, thereby changing the distance between the laser (7) and the collimating lens. The piezoelectric ceramic (4) and the spring (6) work together to control the movement trajectory of the upper plate (1), so that the upper plate (1) only translates along the optical axis.

5. The method for reducing optical interference noise according to claim 4, characterized in that, The laser beam emitted by the laser undergoes multiple reflections within the interference cavity formed between the laser window and the first surface of the collimating lens, creating a stable interference signal. When the laser scans, the interference signal appears as a periodic sine or cosine form of interference noise that varies with wavelength. When the optical path length from the laser window to the collimating lens is changed periodically by the piezoelectric ceramic, it is equivalent to continuously and periodically changing the length of the interference cavity, leading to a change in the phase of the interference noise. Synchronized modulation, and: ; in, The length of the interference cavity, representing the time-varying length under the action of piezoelectric ceramics. It is a periodic function, where φ0 is the initial phase. λ is the wavelength of the light beam emitted by the laser, and n is the equivalent refractive index in the interference cavity; Within a complete piezoelectric ceramic modulation cycle, the phase of the interference noise varies continuously from 0 to 2π.

6. The method for reducing optical interference noise according to claim 1 or 5, characterized in that, The intensity of the gas absorption signal is only related to the laser wavelength emitted by the laser and the gas absorption coefficient. As long as the wavelength scan is linear, the waveform of the gas absorption signal is consistent and stable in each acquisition. The phase of the interference noise is randomized under the drive of the piezoelectric ceramic. In multiple consecutive acquisitions, the value of the interference noise phase fluctuates randomly between positive and negative. The transmitted light intensity signal acquired within one laser scanning cycle is normalized using the Beer-Lambert law; the arithmetic averaging process is performed by averaging the absorption signals after normalization of the original transmitted signals within multiple laser scanning cycles to obtain the absorption signal after preliminary noise reduction.

7. A device for reducing optical interference noise, characterized in that, The method for reducing optical interference noise according to any one of claims 1-6 includes an emitter that integrates a piezoelectric ceramic phase modulation structure and a long focal length collimating lens.

8. The apparatus for reducing optical interference noise according to claim 7, characterized in that, The optical path of the transmitter is provided with a gas to be measured and a matching receiver. The transmitter and receiver are electrically connected through a junction box. The laser of the transmitter is fixed on a piezoelectric ceramic phase modulation structure. The transmitted light intensity signal collected by the detector of the receiver is connected to a data processing module. The data processing module includes a normalization unit, an arithmetic averaging unit, and a convolution matched filter unit connected in sequence. The normalization unit normalizes the transmitted light intensity signals collected from multiple different points. The arithmetic averaging unit performs an arithmetic average calculation on the normalized transmitted light intensity signals to obtain a preliminary denoised absorption signal. The convolution matched filter unit performs a convolution operation on the preliminary denoised absorption signal through a convolution kernel to filter out signals that are not related to the convolution kernel, thereby obtaining a noise-filtered absorption signal.

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