Optical system for reducing optical noise

Abstract

The invention provides an optical system for reducing optical noise, which is used in the field of laser gas analyzers, and comprises a doublet collimating lens, the doublet collimating lens is arranged on an emission light path of an infrared laser light source, the doublet collimating lens is used for shaping a laser beam emitted by the infrared laser light source, and the doublet collimating lens is used for outputting the shaped laser beam. And the obtained laser collimated light beam is a nearly flat-topped light beam with uniform energy, is freely propagated in the space and is received by a receiving end. According to the invention, a single-piece doublet lens is used as a core optical element, the free light laser beam of TO packaging is shaped to realize laser beam homogenization and collimation, the introduction of intensity noise caused by non-uniform light intensity distribution of the light beam after long-distance light beam transmission is reduced, and the optical interference noise frequency is increased in combination with the long-focal-length light path design, so that the optical interference efficiency is improved. The influence of optical interference noise is reduced through a filtering algorithm. And the structure is compact, the installation and adjustment are simple and convenient, the environmental adaptability is strong, and the application effect of the laser gas analyzer in the gas detection field is realized.

Description

Technical Field

[0001] This invention relates to the technical field of optical systems, and more particularly to an optical system for reducing optical noise. Background Technology

[0002] Chinese patent application CN202210059759.1 discloses a broadband laser beam shaping system based on aspherical lenses. This system uses a combination of four lenses to achieve achromatic correction and beam shaping of a broadband light source. While it can be used across a wide spectrum, it is only applicable to the visible light band and cannot meet the measurement needs of many gases. Furthermore, the optical system in this patent is a combination of multiple lenses, resulting in stringent assembly and adjustment tolerances. This system is intended for non-imaging optics and illumination applications.

[0003] Patent application CN202311767513.0 discloses an optical system capable of homogenizing and collimating a laser beam. It achieves this by using a combination of a thick lens with double-sided even-order aspherical surfaces and a thin lens with double aspherical surfaces to homogenize and shape a Gaussian beam into a flat-top beam. However, the design physically separates the collimation unit and the shaping unit, which may result in a less compact system. Furthermore, the design of the collimation unit itself (such as focal length and chromatic aberration correction) is not discussed in detail. This system is intended for use in laser processing.

[0004] The drawbacks of applying existing technologies to laser gas analyzers:

[0005] (1) Wide spectrum applicability: Laser gas analyzers operate in the near-infrared 0.7μm ~ 2.4μm band, which can usually cover more than 90% of the measurement needs of industrial gases. Traditional optical materials have a narrower applicable band, so they cannot meet the measurement needs of a variety of near-infrared gases and have poor versatility.

[0006] (2) Special operating conditions: Gas analyzers used in industrial settings must meet explosion-proof requirements, which imposes special constraints on the materials, thickness, and structural design of optical components. Conventional cemented doublet lens designs do not optimize this as a key performance indicator.

[0007] (3) Energy distribution problem: The natural Gaussian beam emitted by the laser has high energy at the center and low energy at the edge. This not only increases the difficulty of beam alignment during on-site installation, but also causes the detector to become saturated at the center or the edge signal to be too weak after long-distance transmission, affecting the accuracy of quantitative detection. Furthermore, when the gas analyzer is used in harsh on-site environments, equipment vibration will introduce more noise. To improve the uniformity of energy distribution, existing technologies use multiple lenses to shape the beam, aiming to convert the Gaussian distribution into a flat-top distribution. However, such solutions inevitably increase the number of optical components, resulting in a complex system structure, stringent optical path assembly tolerances, and significantly increased manufacturing costs. Moreover, because the functions of beam collimation, achromatic correction, and homogenization are separated into different components, it is difficult to balance the aberrations globally, and the entire optical system is difficult to make compact and miniaturized, failing to meet the application requirements of modern industrial testing equipment for high integration, high stability, and low cost.

[0008] (4) Signal-to-noise ratio (SNR) problem: In wavelength scanning-based laser gas analysis systems, interference noise generated by surface reflection of optical components results in very similar signal widths when the free spectral range (FSR) of the interference fringes is comparable to the full width at half maximum (FWHM) of the gas absorption spectral lines. This means that while traditional digital filtering effectively suppresses interference noise, it inevitably filters out some useful gas absorption signals, leading to distortion of the true gas absorption signal and measurement errors. Therefore, relying on subsequent signal processing methods (such as signal filtering algorithms or complex convolution algorithms) is insufficient to fundamentally solve this SNR degradation problem, thereby reducing the system's SNR and detection limit. Summary of the Invention

[0009] To address the technical problems of poor wide-spectrum applicability of optical systems in laser gas analyzers, difficulty in meeting industrial explosion-proof requirements, uneven beam energy distribution leading to debugging difficulties and signal instability, and difficulty in separating optical interference noise within the system resulting in low signal-to-noise ratio, this invention proposes an optical system for reducing optical noise. At the same time, the system has wide-spectrum compatibility, high economy, strong environmental adaptability, high detection stability and excellent system signal-to-noise ratio, and the overall structure is compact and easy to assemble and adjust.

[0010] To achieve the above objectives, the technical solution of the present invention is implemented as follows: 1. An optical system for reducing optical noise, used in a laser gas analyzer, comprising a cemented doublet collimating lens, the cemented doublet collimating lens being disposed in the emission optical path of an infrared laser source, the cemented doublet collimating lens shaping the laser beam emitted by the infrared laser source, resulting in a near-flat-top laser collimated beam with uniform energy, which propagates freely in space and is received by a receiving end. The near-flat-top beam has a stable energy distribution during long-distance transmission, reducing signal instability caused by equipment jitter or optical path offset, and suppressing optical intensity noise in the system noise. 2. An optical system for reducing optical noise, used in a laser gas analyzer, employing a long focal length optical design, increasing the interference cavity length formed by the laser, the main interference device in the optical path, and the cemented doublet collimating lens, thereby increasing the optical interference noise frequency of the system noise, thus distinguishing it from the absorption signal of the gas to be measured, and effectively separating it from the absorption signal of the gas to be measured, and further suppressing the optical interference noise in the system noise through a back-end signal processing method.

[0011] Preferably, the double-cemented collimating lens includes a first lens and a second lens. The first lens is a negative lens made of N-LASF45HT optical glass, and the second lens is a positive lens made of N-BK7 optical glass. The mid-surface between the first lens and the second lens is fixed by cementing, so that the working wavelength of the laser gas analyzer can cover the band of 0.7μm to 2.4μm, which meets the requirements for wide-band use.

[0012] Preferably, the first lens is a meniscus-shaped biconcave lens, with the side near the infrared laser source having a concave surface with a radius of curvature of 127.35 mm, serving as the input surface of the cemented doublet collimating lens, and the side away from the infrared laser source having a concave surface with a radius of curvature of 45.39 mm; the second lens is a biconvex lens, with the side near the first lens having a convex surface with a radius of curvature of 45.39 mm, and fixed to the concave surface of the first lens by air bonding, and the side away from the first lens having a concave surface with a radius of curvature of 41.49 mm, serving as the output surface of the cemented doublet collimating lens.

[0013] Preferably, the center thickness of the double-cemented collimating lens is 10.2 mm.

[0014] Preferably, the infrared laser source is placed near the object-side focal point of the double-cemented collimating lens.

[0015] Preferably, the infrared laser source is a TO-packaged infrared free-beam laser, and the divergence angle of the emitted laser beam is between 20° and 40°.

[0016] The double-cemented collimating lens has a diameter of 25mm and a light-transmitting aperture of 20mm. The cementation of the first lens and the second lens is achieved through an edge-limited cementing process.

[0017] The surface equations of the four optical surfaces of the first and second lenses are all spherical equations.

[0018] Preferably, the cemented doublet collimating lens collimates the input Gaussian beam into a parallel beam and shapes it into a near-flat-top beam. The cemented doublet collimating lens precisely controls spherical aberration characteristics by optimizing the radius of curvature, thickness, and material combination, generating a specific phase modulation function. Based on the spherical aberration formula, starting from optimizing the spherical aberration factors affecting the phase modulation function, the cemented doublet collimating lens generates a spherical aberration coefficient. The beam with spherical aberration propagates a distance in free space. Due to the different phase delays generated by light rays at different radial positions, interference occurs on the target plane. The superposition result cancels out the characteristic of the Gaussian beam being strong at the center and weak at the edges, forming a uniform flat-top distribution.

[0019] With the output surface of the second lens as the origin of the coordinate system and the optical axis as the positive z-axis, the process of the laser beam propagating from the output surface of the second lens to the target plane is described by Fresnel diffraction. Each point on the output surface of the second lens serves as a secondary spherical wave source. The spherical waves coherently superimpose after propagating to the target plane. Due to the phase difference caused by spherical aberration, spherical waves along different paths interfere with each other on the target plane. After optimizing the spherical aberration coefficient and propagation distance, the light intensity distribution at the center and the edge reaches a precise balance, forming a uniform light intensity distribution across the entire beam cross-section.

[0020] Preferably, the double-cemented collimating lens is configured with a spherical aberration coefficient adjustable in the range of 0.8λ to 1.5λ, where λ is the center operating wavelength of the infrared laser source.

[0021] Preferably, the laser beam is incident directly onto the double cemented collimating lens; a parasitic Fabry-Perot interferometer cavity is formed between the emission window of the infrared laser source and the input surface of the first lens;

[0022] The double-cemented collimating lens is designed with a long focal length to increase the cavity length of the Fabry-Perot interferometer, thereby narrowing the free spectral range of the interference noise generated by the Fabry-Perot interferometer and increasing the oscillation frequency.

[0023] Preferably, the receiving end includes a plano-convex converging lens and a photodetector. The plano-convex lens converges the received near-flat-top laser collimated beam, shaped by the double-cemented collimating lens, onto the photosensitive surface of the photodetector. The photodetector is connected to a data processing module, and the data processing module is connected to a host computer.

[0024] The beneficial effects of the present invention are as follows: the double-cemented collimating lens of the present invention can meet the requirements of wide spectrum applicability, while meeting the industrial explosion-proof standard requirements, shaping the Gaussian beam into a flat-top beam with uniform energy to reduce the intensity noise of the optical system, and increasing the optical interference noise frequency through the long focal length design of the optical lens, and reducing the influence of optical interference noise through the filtering algorithm, thereby improving the signal-to-noise ratio of the system.

[0025] This invention combines a TO-packaged infrared free-beam laser with an optimized single cemented doublet lens, resulting in a laser collimated beam with both collimation and near-flat-top distribution characteristics. The flat-top beam exhibits stable energy distribution during long-distance transmission, significantly reducing signal fluctuations caused by equipment jitter or optical path misalignment, suppressing system noise, and enhancing the system's anti-interference capability. The long-focal-length optical design increases the cavity length formed by the infrared laser and the cemented doublet collimating lens, the main interfering devices in the optical path. This raises the frequency of system noise, distinguishing it from the absorption signal of the target gas and effectively separating it from the absorption signal of the target gas, further suppressing system noise and significantly improving the system's signal-to-noise ratio and detection sensitivity.

[0026] This invention achieves laser beam homogenization and collimation through a single-piece double cemented lens, reducing the introduction of intensity noise caused by uneven beam intensity distribution during long-distance beam transmission. Combined with a long focal length optical path design, the noise frequency is increased, and the influence of optical interference noise is reduced through a filtering algorithm. Overall, through the combination of optical design and filtering algorithms, active noise suppression is achieved from an optical principle perspective. Furthermore, this invention has a compact structure and strong environmental adaptability, achieving the application effects of laser gas analyzers in the field of gas detection, and is suitable for industrial laser gas analysis applications requiring high precision and high stability.

[0027] Compared with the prior art, the present invention has the following significant advantages:

[0028] 1. Wide spectral applicability: The combination of N-BK7 and N-LASF45HT materials makes the double cemented collimating lens suitable for a wide spectral range of 0.7μm to 2.4μm. In the near-infrared region, the 0.7μm to 2.4μm range can meet the measurement needs of more than 90% of industrial gases, and the platform has good versatility.

[0029] 2. Excellent economic performance: It uses only one cemented doublet collimating lens, and all surfaces are spherical. The manufacturing process of spherical lenses is mature, the cost is low, and the assembly and adjustment are simple.

[0030] 3. Strong environmental adaptability: The design takes into account both optical performance and explosion-proof requirements, enabling the double cemented collimating lens to be reliably applied in laser gas analysis equipment in harsh industrial environments such as petroleum and chemical industries.

[0031] 4. Good gas detection stability: By shaping the output flat-top beam, the energy accumulation at the center of the Gaussian beam is eliminated, making the energy distribution of the light spot uniform. Even after long-distance transmission, it can still maintain uniformity, ensuring that the detection signal can truly reflect the gas absorption. It can also reduce noise caused by equipment vibration, and at the same time, it makes it easy to align the laser analyzer when it is installed in the field.

[0032] 5. Excellent system signal-to-noise ratio: The innovative long focal length design narrows the width of the interference noise, distinguishing it from the absorption signal. This interference noise can be effectively suppressed by traditional digital filtering, reducing the system's background noise and improving the ability to detect weak absorption signals, thereby lowering the detection limit. Attached Figure Description

[0033] 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.

[0034] Figure 1 This is a schematic diagram of the structure of the present invention.

[0035] Figure 2 for Figure 1 The diagram shows the structure of a cemented doublet collimating lens.

[0036] Figure 3 This is a comparison of the energy distribution and intensity noise-induced signals of a Gaussian beam and a near-flat-top beam.

[0037] Figure 4 This is a schematic diagram showing the signal comparison of the long focal length design of this invention.

[0038] In the figure, 1 is the infrared laser source, 2 is the laser beam, 3 is the double cemented collimating lens, 4 is the laser collimating beam, 5 is the receiver, 31 is the first lens, 32 is the second lens, 11 is the laser driver, 12 is the first display screen, 51 is the plano-convex converging lens, 52 is the photodetector, 53 is the data processing module, and 54 is the second display screen. Detailed Implementation

[0039] 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.

[0040] like Figure 1 As shown, this invention proposes an optical system for reducing optical noise in a laser gas analyzer, comprising a laser emitting device and a double-cemented collimating lens 3 forming the emitting end. The laser emitting device includes an infrared laser source 1, a laser driver 11, and a first display screen 12. A plano-convex converging lens 51, a photodetector 52, a data processing module 53, and a second display screen 54 form the receiving end, which is connected to a host computer 6. The laser driver 11 is connected to the infrared laser source 1, and its function is to provide a stable and modulated driving current and control the operating temperature of the infrared laser, ensuring that the laser outputs a stable wavelength and constant power. The first display screen 12 displays the operating status of the laser gas analyzer's emitting end and real-time measurement parameters: gas concentration, transmittance, operating temperature, and gas pressure. The Gaussian beam generated by the infrared laser source 1 is shaped by the double-cemented collimating lens 3, outputting a collimated, uniformly energetic near-flat-top beam, which is received by the receiving end. The infrared laser source 1 is a TO-packaged infrared free-beam laser. The divergence angle of the emitted laser beam 2 is typically determined by the model of the selected laser. In this invention, the infrared free-beam laser has a full-angle divergence angle between 20° and 40°. The receiver 5 includes a plano-convex converging lens 51 and a photodetector 52. The photodetector 52 converts the optical signal into an electrical signal, which is then analyzed and processed by the data processing module 53. The plano-convex lens 51 focuses the received near-flat-top collimated laser beam onto the photosensitive surface of the photodetector 52, increasing the intensity of the optical signal. The photodetector 52 converts the received converged optical signal into a corresponding electrical signal. The data processing module 53 amplifies, filters, performs analog-to-digital conversion, and concentration inversion on the electrical signal output by the photodetector 52 to finally obtain the measured gas concentration. The receiver 5 also has a second display screen 54. The function of the second display screen 54 is to display the working status of the laser gas analyzer receiver and the real-time measurement parameters consistent with those of the transmitter, such as gas concentration, transmittance, and operating temperature and pressure. The laser gas analyzer consists of a transmitter and a receiver, which are connected to a host computer via a network cable. The detection information is then displayed visually on a web interface on the PC.

[0041] like Figure 2As shown, the core optical element, the cemented doublet collimating lens 3, is specifically designed as follows: it includes a first lens 31 and a second lens 32. The first lens 31 is a negative lens, made of N-LASF45HT optical glass, and the second lens 32 is a positive lens, made of N-BK7 optical glass. The intermediate surfaces of the first lens 31 and the second lens 32 are cemented together. The pairing of N-BK7 and N-LASF45HT optical materials allows the laser gas analyzer to cover a wide wavelength range (0.7μm~2.4μm). In the near-infrared region, the 0.7μm~2.4μm wide spectral band can meet the measurement requirements of over 90% of industrial gases, adapting to the core requirement of various gas absorption spectra. The specific material combination design of the cemented doublet collimating lens 3 achieves the measurement requirements of over 90% of industrial gases in the 0.7μm~2.4μm near-infrared band. The lens materials selected in this invention, specifically the first lens 31 and the second lens 32, both exhibit a transmittance >85% in the wavelength range of 0.7μm to 2.4μm. This makes the cemented doublet collimating lens 3 suitable for a wide spectral range of 0.7μm to 2.4μm, satisfying infrared measurement requirements while ensuring versatility. The first lens 31 is a meniscus negative lens, a biconcave lens with one side having a radius of curvature of 127.35mm and the other side having a radius of curvature of 45.39mm. Specifically, the side of the first lens 31 closest to the infrared laser source 1 is a concave surface with a radius of curvature of 127.35mm, serving as the input surface of the cemented doublet collimating lens 3, while the side furthest from the infrared laser source 1 is a concave surface with a radius of curvature of 45.39mm. The second lens 32 (positive lens) is a biconvex lens with one side having a radius of curvature of 45.39mm and the other side having a radius of curvature of 41.49mm. The second lens 32 is a biconvex positive lens made of N-BK7 material. Its side closest to the first lens 31 is a convex surface with a radius of curvature of 45.39 mm, which is fixed to the concave surface of the first lens 31 by bonding. Its side away from the first lens 31 is a convex surface with a radius of curvature of 41.49 mm, which serves as the output surface of the double-bonded collimating lens 3.

[0042] Considering the explosion-proof requirements in industrial applications, the mechanical structure design of the optical system must ensure sufficient mechanical strength. In this invention, the cemented doublet collimating lens 3, during installation, has its second lens 32, made of N-BK7 material with higher mechanical strength and better chemical stability, facing the gas-generating end. Furthermore, the lens center thickness is designed to be 10.2 mm. These factors combined ensure that the cemented doublet collimating lens 3 itself possesses extremely high structural strength, enabling the system to operate reliably in harsh industrial environments such as petroleum and chemical plants. This represents a comprehensive performance improvement across the entire chain, from optical principles to mechanical environment considerations.

[0043] In long-distance laser gas measurements, signal attenuation is significant. Traditional Gaussian beams, with their bell-shaped energy distribution (highly concentrated at the center and rapidly decaying at the edges), have obvious limitations in applications: firstly, uneven energy distribution leads to low long-distance transmission efficiency; secondly, their distribution characteristics are extremely sensitive to optical path alignment and mechanical vibrations, and even a slight beam deviation (such as...) can cause significant damage. Figure 3 As shown by the red dashed line in the image, the total luminous flux within the fixed detector area fluctuates drastically, forming a pseudo-signal called "intensity noise." This noise signal is easily misinterpreted as a gas absorption signal, causing baseline drift and absorption peak distortion, such as... Figure 3 As shown, this seriously affects the accuracy and stability of the measurement.

[0044] Therefore, this invention uses an optimized double-cemented collimating lens 3 to convert a Gaussian beam into a near-flat-top beam with uniform energy distribution. For example... Figure 2 As shown, the radius of curvature of the near-source surface of the first lens 31 in this lens group is 127.35 mm, the radius of curvature of the surface cemented with the second lens 32 is 45.39 mm, and the radius of curvature of the output surface of the second lens 32 is 41.49 mm. Through synergistic optimization of curvature, thickness, and materials, collimation and achromaticity are achieved while beam shaping from Gaussian to near-flat top is completed.

[0045] like Figure 3 As shown, the shaped light spot distribution exhibits a uniform "flat-top" shape, with minimal intensity difference between the center and edges. This distribution offers significant advantages: high energy efficiency, which is beneficial for maintaining signals over long distances; more importantly, when the beam is deflected by mechanical vibration, the total luminous flux within the detector's receiving area remains essentially constant, thereby greatly suppressing intensity noise. Figure 3 As shown in the comparison curves, the absorption signal (blue curve) corresponding to the near-flat-top beam is less affected by interference, can maintain a near-ideal Lorentz curve, has a clear absorption peak and a stable baseline, ensuring high fidelity in gas detection.

[0046] In terms of mechanical structure, the laser emission point is positioned near the object-side focal point of the cemented doublet collimating lens 3 by adjusting the position of the laser mount. Placing the infrared laser near the object-side focal point of the cemented doublet collimating lens 3 in the optical path structure allows for precise adjustment of the distance between the two, ensuring the collimation of the output beam. The surface equations of the four optical surfaces of the first lens 31 and the second lens 32 are all spherical. During lens fabrication, grinding and polishing are performed according to the designed radius of curvature values. This invention's optical system using a single cemented doublet collimating lens simplifies assembly and adjustment; the spherical design of the cemented doublet collimating lenses improves the economic efficiency of the optical system application.

[0047] The principle of beam propagation collimation and shaping of the cemented doublet collimating lens 3 is described as follows: The cemented doublet collimating lens 3 collimates the input Gaussian beam into a parallel beam and shapes it into a near-flat-top beam. The essence of this beam shaping process is wavefront modulation. The lens group (cemented doublet collimating lens 3) precisely controls spherical aberration characteristics by optimizing the radius of curvature, thickness, and material combination, thereby generating a specific phase modulation function. .

[0048] The complex amplitude distribution of the output surface (surface with a radius of curvature of 41.19 mm) of the cemented doublet collimating lens 3 is as follows:

[0049]

[0050] in, This represents the field distribution of the incident Gaussian beam. The formula for this field distribution is a mathematical function that describes the complex amplitude of the optical field when the laser beam is incident on the input surface of the lens. Indicates the phase modulation factor; This represents the phase modulation function that includes spherical aberration control; in the Cartesian coordinate system of the plane containing the output surface of the cemented doublet lens, Represents the coordinates of a point on a plane.

[0051] The complete phase modulation function considering the effects of spherical aberration is: .in, This represents the radial distance along the lens aperture. It is a radial coordinate in polar coordinates, calculated from rectangular coordinates (x, y). This represents the equivalent focal length of the cemented doublet collimating lens 3. The wave number is represented by k = 2π / λ, where λ represents the laser wavelength. Spherical aberration wave aberration function. Represented as: , The spherical aberration coefficient quantifies the intensity of spherical aberration and is determined by the lens curvature, refractive index, and thickness. The refractive index is determined by the optical properties of the selected material (N-BK7, N-LASF45HT) at a specific working wavelength and can be obtained from the material data sheet. The aperture radius of a lens is a parameter determined during optical design based on the system's requirements for spot size and beam diameter; it is one of the physical dimensions of a lens. Spherical aberration term. By focusing light rays from different annular zones of a cemented doublet collimating lens 3 at different axial positions, an interference pattern with uniform energy distribution is formed over a specific propagation distance. Based on the spherical aberration formula, by optimizing the factors affecting spherical aberration (i.e., optimizing the curvature, thickness, and material of the cemented doublet), a spherical aberration coefficient is generated in the cemented doublet collimating lens. A beam with this specific spherical aberration propagates a specific distance in free space. Subsequently, due to the different phase delays produced by rays at different radial positions, they interfere on the target plane. Their superposition results precisely cancel out the characteristic of the Gaussian beam being strong at the center and weak at the edges, forming a uniform flat-top distribution.

[0052] Typically, the output surface of the second lens 32 is taken as the origin of the coordinate system (z=0), and the optical axis direction is the positive z-axis. The laser beam 2 propagates from the output surface of the second lens 32 to the target plane. The process is described by Fresnel diffraction:

[0053]

[0054] in, This indicates the propagation distance from the exit surface of the cemented doublet lens, i.e., the right side of the second lens 32, to the target plane; Indicates the propagation phase factor; Indicates the amplitude attenuation factor; This represents the integral coordinates of a point on the output surface of the second lens 32; dξdη represents the coordinates on the target plane; dξdη represents the integral area element. This indicates the wavelength of the laser in a vacuum or air. i represents the imaginary unit. 2 = -1. Output each point on the surface. As secondary spherical wave sources, these spherical waves propagate to the target plane and coherently superimpose, forming diffraction nuclei. The propagation of each spherical wave is described, and due to the phase difference caused by spherical aberration, spherical waves along different paths interfere at the target plane. The optimized spherical aberration coefficients are also described. and transmission distance The light intensity distribution at the center and the edge reaches a precise balance, forming a uniform light intensity distribution across the entire beam cross-section.

[0055] Based on the aforementioned optical principles, the key to shaping a Gaussian beam into a flat-top beam lies in the precise control of lens spherical aberration. The magnitude of spherical aberration is influenced by the lens's radius of curvature, thickness, and material properties; therefore, these parameters must be systematically optimized to achieve an ideal beam distribution. Specifically, the spherical aberration coefficient is controlled primarily by adjusting the radius of curvature. By selecting materials with a specific refractive index (approximately 1.5 to 1.9 for selected materials (such as N-BK7 and N-LASF45HT) within the 0.7μm to 2.4μm wavelength range, with the exact value varying with wavelength), spherical aberration can be fine-tuned in localized areas. Simultaneously, the thickness is strictly limited to the safe range required for explosion-proof performance (meeting Exd explosion-proof requirements (edge ​​bonding length > 10mm, center thickness around 8-10mm) as a boundary condition for auxiliary optimization. The spherical aberration coefficient is then synergistically optimized using Zemax simulation software. and transmission distance The original Gaussian beam is shaped into... Figure 3 The near-flat-top beam distribution is shown. Using the Physical Optical Propagation (POP) module in the optical design software Zemax, which employs rigorous wave optics simulation based on the angular spectrum method, the curvature, thickness, and material combination of the cemented doublet collimating lens are synergistically optimized. This simulation method can accurately calculate the wavefront modulation caused by lens spherical aberration and its diffraction propagation in free space, thus verifying the successful shaping of the Gaussian beam. Figure 3 The feasibility of the near-flat-top beam was demonstrated, and the collimation performance and beam uniformity of the system were evaluated. This design effectively meets the optical performance requirements of the laser gas analyzer, ensuring that the beam maintains a uniform energy distribution after long-distance transmission, thereby significantly reducing noise interference caused by equipment vibration and greatly reducing the complexity of on-site optical path adjustment.

[0056] This invention optimizes the curvature, thickness, and material combination of the double-cemented collimating lens 3 through design and simulation, based on the phase modulation function. Spherical aberration wave aberration function W sph The beam shaping logic of (r) adjusts the spherical aberration coefficient A. sph The wavefront modulation method of Gaussian beam to near-flat-top beam is achieved by using Fresnel diffraction, so that the optical system outputs a collimated near-flat-top beam. The near-flat-top beam can reduce the optical intensity noise caused by equipment jitter, and at the same time make it easier to align the laser analyzer when it is installed in the field.

[0057] This invention relates to a compact design for the optical system of a laser gas analyzer. To achieve good collimation, the laser is placed near the focal point of the lens. However, the diffusion of the laser beam 2 between the infrared laser source 1 and the cemented doublet collimating lens 3 leads to stray light (such as reflection from the pool wall and scattering from the lens edge). To meet the collimation requirements, the laser beam 2 is directly opposite the cemented doublet collimating lens 3, resulting in direct transmission and reflection of the outgoing and reflected light, leading to significant interference effects. The optical surfaces of the laser's emission window and the window of the cemented doublet collimating lens 3 (first lens 31) easily form a FP cavity, causing some of the reflected beam to reflect multiple times between the two optical surfaces. Because the optical interference noise generated in this part contributes the most to the optical noise of the entire gas analyzer system, this invention combines optical design with traditional signal processing methods to eliminate this interference noise. When the free spectral width (FSR) of the interference fringes is comparable to the full width at the top of the gas absorption spectrum, their signal widths are very close, making it difficult to distinguish and extract the absorption signal. Figure 4 As shown. The black curve represents the simulated signal calculated based on the gas absorption theoretical model. The black / red curves represent signals that are difficult to distinguish. When simulating short focal lengths (short cavity lengths), the calculated interference noise (red) has a free spectral range (FSR) that is close to the width of the simulated gas absorption spectral lines (black), making it difficult to filter and separate. The black / blue curves represent signals that are easy to distinguish. When simulating long focal lengths (long cavity lengths), the calculated interference noise (blue) has a smaller FSR, a higher oscillation frequency, and appears as denser stripes with narrower width, which is significantly different from the wider gas absorption signal (black), making it easy to separate through digital filtering.

[0058] An unintentional, parasitic FP (Fabry-Perot) interferometer cavity is formed between the cemented doublet collimating lens 3 and the laser. In the wavelength domain, the approximate expression for the free spectral width (FSR) of the FP interferometer is:

[0059]

[0060] in, This refers to the free spectral range (wavelength interval). The center wavelength, Let be the refractive index of the medium inside the FP interferometer cavity (usually 1, which can be ignored). The length of the FP interferometer cavity is its physical length.

[0061] To facilitate the separation of absorbed signals from interference noise, the double-cemented collimating lens with a long focal length design reduces the cavity length. Increasing this directly leads to a decrease in the wavelength spacing of the calculated noise fringes. The size is reduced. Through precise optical design, the cemented doublet 3 achieves a longer equivalent focal length f. This is achieved by using a surface shape with a large radius of curvature, optimizing the lens thickness, and utilizing a combination of positive and negative lenses to achieve the long focal length of the cemented doublet. The interference fringes are denser and narrower, which manifests as an increase in the interference noise oscillation frequency, making it distinguishable from the wider absorption signal. Figure 4 The black gas absorption signal and the blue noise signal can be well distinguished due to their different signal widths. At this time, the high-oscillation frequency interference noise can be filtered out by signal filtering circuit or digital filtering method and separated from the absorption signal, which greatly improves the detection accuracy and sensitivity of the gas.

[0062] Due to the long focal length design of the cemented doublet collimating lens 3, the free spectral width (FSR) formula of the FP interferometer is used to actively control the noise signal morphology by increasing the cavity length (lens focal length), thus narrowing the width of the interference noise and separating it from the absorption signal, thereby reducing the optical interference noise of the optical system. The cavity length L refers to the physical distance between the laser's output window and the input surface of the cemented doublet lens. The long focal length design indirectly increases this physical distance (as part of the FP cavity). Implementation: The laser needs to be placed near the object-side focal point of the lens. The longer the lens focal length f, the farther its focal point is from the lens; therefore, the laser installation position also needs to be farther, thus increasing the physical distance between the laser and the lens (i.e., the cavity length L).

[0063] To improve two key performance aspects of the laser gas analyzer detection system—namely, enhancing the stability of the laser signal during long-distance transmission and improving the system's signal-to-noise ratio—this invention designs an active noise suppression system as the core solution. To achieve this design, two main approaches are taken: First, beam shaping: the Gaussian beam emitted by the infrared laser is shaped into a near-flat-top beam. A flat-top beam has a more uniform energy distribution, reducing signal fluctuations caused by optical path jitter or offset, thereby improving the system's anti-interference capability and transmission stability. Second, combining optimized optical path design with traditional signal processing methods: a long focal length optical design is adopted. By accurately calculating the focal length design distance, the interference cavity length is increased, raising the frequency of system noise. This effectively separates the noise from the absorption signal of the gas being measured in the frequency domain. Digital filtering then removes the noise, further improving the signal-to-noise ratio.

[0064] This invention is a specialized design, reverse-engineered based on the actual application requirements of laser gas analyzers (broad spectrum applicability, industrial explosion-proof, long-distance transmission, high signal-to-noise ratio), rather than simply applying general-purpose optical components. Functional integration: A single cemented doublet lens assembly (cemented doublet collimating lens 3) simultaneously achieves four major functions: achromatic correction, collimation, shaping, and improved signal-to-noise ratio, replacing the effects that traditional solutions might require multiple optical components or subsystems to achieve. Synergistic optimization: The selection of optical materials, curvature design, thickness parameters, and focal length of the cemented doublet collimating lens 3 are not isolated decisions, but rather interconnected and synergistically optimized, collectively contributing to the final system-level performance improvement. Based on the requirements of high transmittance and achromaticity in the working wavelength range (0.7μm~2.4μm), N-BK7 and N-LASF45HT were selected as the matching materials. Using the radii of curvature of the four spherical surfaces and the center thickness of the two lenses as optimization variables, a multi-objective evaluation function was established, incorporating wavefront modulation (i.e., beam shaping), collimation, focal length, and mechanical constraints. An algorithm combining global optimization and damped least squares was used to iteratively optimize the above variables in optical design software, ensuring the spherical aberration coefficient A... sph Precise adjustment within the range of 0.8λ to 1.5λ (λ=1.55μm) achieves efficient shaping of the Gaussian beam into a near-flat-top beam, while simultaneously achieving a focal length f of approximately 80mm to meet long focal length design requirements. Furthermore, the edge thickness of the cemented doublet lens is >10mm, and the center thickness is 8-10mm to meet explosion-proof strength requirements. Ultimately, the synergistic effect of these parameters results in the simultaneous achievement of achromatic and wide-band applicability, beam homogenization and shaping, long focal length collimation, and explosion-proof structural strength on a single cemented doublet collimating lens, meeting the comprehensive requirements of laser gas analyzers for high performance, high reliability, and compactness.

[0065] This invention innovatively employs two specific optical materials, N-BK7 and N-LASF45HT, for wide-spectrum application, resulting in good applicability of the cemented doublet collimating lens 3 across a wide spectral range of 0.7μm to 2.4μm, meeting the measurement requirements for over 90% of industrial gases in the near-infrared range. It is suitable for special industrial environments: the material and thickness design of the cemented doublet collimating lens 3 in this invention meets the explosion-proof requirements for industrial applications. It achieves efficient beam shaping and system simplification from Gaussian beam to near-flat-top beam: through the optimized surface shape and thickness design of the cemented doublet collimating lens 3, the Gaussian beam is shaped into a near-flat-top beam while achieving collimation. This invention achieves beam shape conversion within a single cemented doublet collimating lens 3, greatly simplifying the system structure and reducing size, cost, and assembly difficulty.

[0066] Active noise suppression based on long focal length optical design: A long focal length design is adopted in the doublet collimating lens 3 as an active noise management scheme. By increasing the length of the interference cavity, the width of the interference noise signal is narrowed, so that it is naturally separated from the wider gas absorption signal, thus clearing the way for subsequent low-cost and high-efficiency signal processing methods.

[0067] This invention utilizes materials with a transmittance >85% in the 0.7μm~2.4μm range to meet the gas measurement requirements in this infrared band. Besides the already established combination of N-BK7 and N-LASF45HT materials, the scope of protection should be extended to combinations of other materials that can meet similar transmittance requirements. Many types of glass combinations are available, such as N-KZFS2 & N-PK51.

[0068] The surface of the double-cemented collimating lens 3 of the present invention can be spherical or aspherical. Although a spherical surface is preferred for cost considerations, for comprehensive protection, embodiments including at least one aspherical surface should be covered, which may further optimize performance or reduce system size in certain cases.

[0069] To enhance noise frequency through long focal length design, any optical design that effectively increases the optical path difference within the system should be included. For example, introducing an additional unfocused beam expander (Galilean or Keplerian type) before or after the cemented doublet collimating lens 3 achieves the same long focal length, small divergence angle collimated output and shifts interference noise to higher frequencies. This method of enhancing noise frequency through long focal length design increases the laser beam diameter by a factor of M (M>1). According to diffraction principles, the output beam divergence angle decreases to 1 / M of the original divergence angle, thus achieving collimated output with a small divergence angle. Simultaneously, the introduction of the beam expander significantly increases the effective optical path between the laser and the cemented doublet, increasing the cavity length L of the FP interferometer cavity mentioned above, thereby raising the noise signal frequency and significantly separating it from the gas absorption signal.

[0070] The multiple functions of this invention, which can be integrated into a single cemented doublet collimating lens, can be broken down into several independent, known optical elements. For example, a general-purpose cemented doublet collimating lens can be used for collimation and fundamental chromatic aberration correction, followed by a dedicated homogenizing optical element (such as a microlens array or diffractive optical element) to generate a flat-top beam; finally, high-quality anti-reflective coatings and precise assembly can be used to suppress interference noise as much as possible. By using this system integration approach, each sub-component can be claimed to be prior art, thus circumventing the core innovation point of achieving multiple functions with a single element, all of which are within the scope of protection of this invention.

[0071] 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. An optical system for reducing optical noise, characterized in that, Used in a laser gas analyzer, including a double-cemented collimating lens (3), which is set in the emission path of an infrared laser source (1). The double-cemented collimating lens (3) shapes the laser beam (2) emitted by the infrared laser source (1), and the resulting laser collimated beam (4) is a near-flat-top beam with uniform energy, which propagates freely in space and is received by the receiving end (5).

2. The optical system for reducing optical noise according to claim 1, characterized in that, The double-cemented collimating lens (3) includes a first lens (31) and a second lens (32). The first lens (31) is a negative lens made of N-LASF45HT optical glass, and the second lens (32) is a positive lens made of N-BK7 optical glass. The intermediate surfaces between the first lens (31) and the second lens (32) are fixed by cementing, so that the working wavelength of the laser gas analyzer can cover the band of 0.7μm ~ 2.4μm, which meets the requirements for wide-band use.

3. The optical system for reducing optical noise according to claim 2, characterized in that, The first lens (31) is a crescent-shaped biconcave lens. The side closest to the infrared laser source (1) is a concave surface with a radius of curvature of 127.35 mm, serving as the input surface of the cemented doublet collimating lens (3). The side away from the infrared laser source (1) is a concave surface with a radius of curvature of 45.39 mm. The second lens (32) is a biconvex lens. The side of the second lens (32) closest to the first lens (31) is a convex surface with a radius of curvature of 45.39 mm, and is fixed to the concave surface of the first lens (31) by cementing. The side of the second lens (32) away from the first lens (31) is a convex surface with a radius of curvature of 41.49 mm, serving as the output surface of the cemented doublet collimating lens (3).

4. The optical system for reducing optical noise according to claim 2 or 3, characterized in that, The center thickness of the double cemented collimating lens (3) is 10.2 mm.

5. The optical system for reducing optical noise according to claim 4, characterized in that, The infrared laser source (1) is placed near the object-side focal point of the double cemented collimating lens (3).

6. The optical system for reducing optical noise according to claim 5, characterized in that, The infrared laser source (1) is a TO-packaged infrared free-beam laser, and the divergence angle of the emitted laser beam (2) is between 20° and 40°. The diameter of the double-cemented collimating lens (3) is 25 mm and the light-transmitting aperture is 20 mm. The bonding of the first lens (31) and the second lens (32) is achieved by edge-limited bonding process. The surface equations of the four optical surfaces of the first lens (31) and the second lens (32) are all spherical equations.

7. The optical system for reducing optical noise according to claim 5 or 6, characterized in that, The cemented doublet collimating lens (3) collimates the input Gaussian beam into a parallel beam and shapes it into a near-flat-top beam. The cemented doublet collimating lens (3) precisely controls the spherical aberration characteristics by optimizing the radius of curvature, thickness and material combination, and generates a specific phase modulation function. According to the spherical aberration formula, starting from optimizing the spherical aberration factors affecting the phase modulation function, the cemented doublet collimating lens (3) generates a spherical aberration coefficient. The beam with spherical aberration propagates a distance in free space. Due to the different phase delays generated by the light rays at different radial positions, interference occurs on the target plane. The superposition result cancels out the characteristic of the Gaussian beam being strong at the center and weak at the edge, forming a uniform flat-top distribution. With the output surface of the second lens (32) as the origin of the coordinate system and the optical axis direction as the positive z-axis, the process of the laser beam (2) propagating from the output surface of the second lens (32) to the target plane is described by Fresnel diffraction. Each point on the output surface of the second lens (32) serves as a secondary spherical wave source. After the spherical waves propagate to the target plane, they coherently superimpose. Due to the phase difference caused by spherical aberration, spherical waves from different paths interfere with each other on the target plane. After optimizing the spherical aberration coefficient and propagation distance, the light intensity distribution at the center and the edge reaches a precise balance, forming a uniform light intensity distribution across the entire beam cross section.

8. The optical system for reducing optical noise according to claim 7, characterized in that, The double-cemented collimating lens (3) is configured such that the spherical aberration coefficient is controlled within the range of 0.8λ to 1.5λ, where λ is the center operating wavelength of the infrared laser source (1).

9. The optical system for reducing optical noise according to claim 8, characterized in that, The laser beam (2) is incident directly onto the double cemented collimating lens (3); a parasitic Fabry-Perot interferometer cavity is formed between the emission window of the infrared laser source (1) and the input surface of the first lens (31); The double-cemented collimating lens (3) adopts a long focal length design to increase the cavity length of the Fabry-Perot interferometer, thereby narrowing the free spectral range of the interference noise generated by the Fabry-Perot interferometer and increasing the oscillation frequency.

10. The optical system for reducing optical noise according to any one of claims 4, 5, 8, and 9, characterized in that, The receiving end (5) includes a plano-convex converging lens (51) and a photodetector (52). The plano-convex lens (51) converges the received near-flat-top laser collimated beam, shaped by the double cemented collimating lens (3), onto the photosensitive surface of the photodetector (52). The photodetector (52) is connected to the data processing module (53), and the data processing module (53) is connected to the host computer (6).

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