A gas Raman detection device based on a semiconductor laser self-locking enhanced cavity

Abstract

The present invention discloses a gas Raman detection device based on a semiconductor laser self-locked enhancement cavity, belonging to the field of gas detection. The device comprises a laser unit, a laser linewidth narrowing unit, a self-locked enhancement cavity unit, and a Raman scattered light collection unit. A mode-matching lens is used to tilt the laser beam emitted by the semiconductor laser into an enhanced external cavity. The laser beam is then repeatedly reflected within a confocal spherical mirror cavity formed by a first spherical reflector M1 and a second spherical reflector M2. Part of the laser light is emitted from the external cavity and returns to the semiconductor laser, forming optical feedback, causing the semiconductor laser and the enhanced external cavity to resonate in frequency. The device eliminates the need for enhanced cavity length feedback, resulting in a simple and stable optical path layout, which can simultaneously reduce optical path complexity and device cost. The device boasts an external cavity laser power of tens of watts, and the laser linewidth narrowed by the laser linewidth narrowing unit enables the Raman signal spectrum to have higher resolution and intensity.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas detection, and more specifically, relates to a gas Raman detection device based on a semiconductor laser self-locking enhancement cavity. Background Art

[0002] Gas detection technology is commonly used in areas such as gas composition analysis, environmental monitoring, industrial process control, and early fault diagnosis of power transformers. Spontaneous Raman spectroscopy can directly detect and analyze mixed characteristic gases using a single wavelength laser. It can simultaneously detect multiple gases without component separation or the need for multiple light sources, making it a popular technology for high-concentration gas detection across various industries. However, due to the small Raman scattering cross-section of gases and the weak spontaneous Raman scattering signal, the minimum detection concentration for Raman spectroscopy cannot meet the requirements for trace characteristic gas analysis.

[0003] In order to solve the problem of small gas Raman scattering cross-sectional area and weak spontaneous Raman scattering signal, increasing the laser power is a common and effective method. There are many ways to increase the laser power, among which building a laser resonance enhancement cavity is one method. In the existing technology, most of them use optical feedback and electrical feedback mechanisms to make a cavity length controlled power enhancement cavity instrument to increase the laser power, but the electrical feedback mechanism is relatively cumbersome, and the overall optical path is relatively complex. The total length of the optical path is very long and the vibration resistance is poor. Some technologies also use semiconductor lasers coated with ultra-low reflectivity anti-reflection films. This type of laser is not mass-produced and difficult to obtain, and uses ultra-high reflectivity mirrors. This type of mirror is relatively expensive, and the instrument also has the problem of insufficient spectral resolution. Summary of the Invention

[0004] In response to the defects of the existing technology and the need for improvement, the present invention provides a gas Raman detection device based on a semiconductor laser self-locking enhancement cavity, which aims to simultaneously reduce the complexity of the optical path and the cost of the device to improve the laser power and spectral resolution.

[0005] To achieve the above objectives, the present invention provides a gas Raman detection device based on a semiconductor laser self-locked enhancement cavity, comprising a laser unit arranged along the laser beam, a laser linewidth narrowing unit, and a self-locked enhancement cavity unit, and a Raman scattered light collection unit arranged perpendicular to the laser beam;

[0006] The laser unit includes a semiconductor laser for generating a laser beam;

[0007] The laser linewidth narrowing unit is used to narrow the linewidth of the laser beam;

[0008] The self-locking enhanced cavity unit includes a mode matching lens and an enhanced external cavity;

[0009] The mode matching lens is used to focus the laser beam and then couple it to the enhanced external cavity;

[0010] The enhanced external cavity includes a first spherical reflector M1 and a second spherical reflector M2 forming a confocal spherical mirror cavity, wherein a normal of the first spherical reflector M1 forms an angle with the optical axis, an incident point of a laser beam on the first spherical reflector M1 is the center of a sphere of the second spherical reflector M2, and the laser beam is reflected back and forth between the first spherical reflector M1 and the second spherical reflector M2. The laser beam that reversely transmits the first spherical reflector M1 is emitted from the enhanced external cavity and returns to the semiconductor laser to form optical feedback, thereby causing frequency resonance between the semiconductor laser and the enhanced external cavity;

[0011] The Raman scattered light collecting unit is used to collect Raman scattered signals generated by irradiating a gas sample with the laser beam in the enhanced external cavity.

[0012] Furthermore, the first spherical reflecting mirror M1 and the second spherical reflecting mirror M2 have the same curvature radius, and the first spherical reflecting mirror M1 and the second spherical reflecting mirror M2 form a symmetrical confocal spherical mirror cavity.

[0013] Furthermore, by setting the focal length and position of the mode matching lens, the laser beam is mode-matched with the transverse mode of the enhanced external cavity.

[0014] Furthermore, the laser linewidth narrowing unit includes one of a narrow linewidth interference filter, a birefringence filter or an etalon.

[0015] Furthermore, the narrow linewidth interference filter has a transmission bandwidth of 0.6 nm and a transmittance of greater than or equal to 85%.

[0016] Furthermore, an anti-reflection film is coated on the outer surface of the reflector, and a high-reflection film is coated on the inner surface. The reflectors are the first spherical reflector M1 and the second spherical reflector M2.

[0017] Furthermore, the Raman scattered light collection unit includes a collection lens and a spectrometer;

[0018] The collecting lens is used to collect the Raman scattered light generated by the laser beam in the enhanced external cavity irradiating the gas sample and focus it on the slit of the spectrometer;

[0019] The height direction of the spectrometer slit is parallel to the laser beam in the enhanced external cavity.

[0020] Furthermore, the semiconductor laser is a free-running laser.

[0021] Furthermore, the wavelength of the semiconductor laser is in the blue-violet light band.

[0022] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0023] (1) The gas Raman detection device based on the semiconductor laser self-locking enhancement cavity of the present invention adopts an ordinary semiconductor laser, and couples the laser beam into the enhanced external cavity through a mode matching lens, and repeatedly reflects in the confocal spherical mirror cavity composed of the designed first spherical reflector M1 and the second spherical reflector M2. Part of the laser is emitted from the external cavity and returns to the semiconductor laser to form optical feedback. When the frequency of the semiconductor laser is the same as the natural frequency of the enhanced external cavity, the enhanced external cavity and the laser form a resonance, and the laser is transmitted back and forth in the external cavity to form power accumulation. Experiments have shown that the power can be enhanced by dozens of times, and the semiconductor laser used has a large frequency operating range, which can ensure that the resonant frequency of the enhanced external cavity is selected within it. At the same time, the normal of the first spherical reflector M1 has an angle with the optical axis, which can avoid the direct reflected light passing through the first spherical reflector M1 interfering with the resonance. In this resonance mode of the present invention, the oscillation frequency is mainly determined by the enhanced external cavity, no feedback circuit is required, the optical path structure layout is simple and stable, and it can achieve intracavity laser power enhancement to enhance the Raman scattering signal in combination with an ordinary semiconductor laser, which can simultaneously reduce the complexity of the optical path and the cost of the device. The laser linewidth narrowed by the laser linewidth narrowing unit enables the spectrum of the Raman signal to have higher resolution and higher intensity.

[0024] (2) The present invention uses two ordinary spherical mirrors to form a confocal spherical mirror cavity, eliminating the need for ultra-high reflectivity mirrors; and uses ordinary semiconductor lasers, eliminating the need for semiconductor lasers coated with ultra-low reflectivity anti-reflection films, further reducing device costs.

[0025] (3) Through the confocal spherical mirror external cavity designed by the present invention, when the laser beam is transmitted in the confocal spherical mirror cavity, the laser beam can form a degenerate transverse mode, providing feedback light of a single or limited number of frequencies for the semiconductor laser, greatly improving the efficiency of optical feedback.

[0026] (4) Preferably, when the curvature radius of the first spherical reflector M1 and the second spherical reflector M2 are the same, and the first spherical reflector M1 and the second spherical reflector M2 form a symmetrical confocal spherical mirror cavity, the Raman scattering signal intensity can be further enhanced.

[0027] (5) Preferably, when the laser beam forms a mode match with the transverse mode of the enhanced external cavity, the laser coupling efficiency can be improved.

[0028] (6) Through the research of the present invention, the transmission bandwidth of the narrow linewidth interference filter is 0.6nm, which can effectively improve the spectral resolution of the instrument while retaining sufficient bandwidth. After the laser emitted by the semiconductor laser is filtered by the narrow linewidth interference filter, there are sufficient longitudinal modes, so that the eigenmode of the semiconductor laser and the enhanced external cavity mode can form resonance when they are the same; at the same time, the transmittance of the narrow linewidth interference filter exceeds 85%, and the gain of the laser beam in the enhanced external cavity is high enough. The enhanced external cavity mode can form a competitive advantage compared with other non-matching eigenmodes of the semiconductor laser, and the other non-matching eigenmodes are suppressed and disappear, so that the laser oscillation in the enhanced external cavity can improve the spectral resolution while not affecting the subsequent enhanced external cavity resonance.

[0029] (7) The Raman scattered light collection unit of the present invention includes a collection lens and a spectrometer. Since there are only two enhanced light paths in the enhanced cavity designed by the present invention, when the laser beam in the enhanced cavity is parallel to the height direction of the spectrometer slit, more signal light enters the spectrometer, further enhancing the Raman detection signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The figure is a schematic diagram of the structure of a semiconductor laser self-locking enhancement cavity for gas Raman spectroscopy detection provided by an embodiment of the present invention.

[0031] Figure 2 1 is a light path diagram of the laser beam in the enhanced external cavity in an embodiment of the present invention.

[0032] Figure 3 This is a diagram of the test results of the embodiment of the present invention in a laboratory environment. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0034] In the present invention, the terms "first", "second", etc. in the present invention and the accompanying drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0035] like Figure 1As shown, the semiconductor laser self-locked enhancement cavity gas Raman detection device for gas Raman analysis provided by the present invention includes, in sequence, a laser unit, a laser linewidth narrowing unit, and a self-locking enhancement cavity unit arranged along the laser beam, and a Raman scattered light collection unit arranged perpendicular to the laser beam; wherein, the laser unit, the laser linewidth narrowing unit, and the self-locking enhancement cavity unit together constitute a composite external cavity of the semiconductor laser.

[0036] The laser unit includes a semiconductor laser, which is an ordinary semiconductor laser without anti-reflection coating. Preferably, the semiconductor laser is a free-running laser. The wavelength of the semiconductor laser is in the blue-violet light band. The Raman scattering cross section of the sample to be measured is inversely proportional to the fourth power of the wavelength. The large Raman scattering cross section excited by a short wavelength near 400nm is used to obtain a strong Raman scattering signal. Compared with the commonly used red light semiconductor device, the use of the blue-violet light band can increase the scattering signal generated by the gas by several times.

[0037] The laser linewidth narrowing unit is arranged between the mode matching lens and the enhanced external cavity, and includes any one of a narrow linewidth interference filter, a birefringent filter or an etalon, which is used to narrow the linewidth of the laser beam, improve the resolution of the spectrum of the Raman signal collected by the Raman scattered light collection unit, and at the same time, make the Raman signal peak sharper and have higher intensity, which is conducive to improving the detection range of the instrument.

[0038] A self-locking enhanced cavity unit, comprising a mode matching lens and an enhanced external cavity;

[0039] The mode matching lens is used to focus the laser beam emitted by the semiconductor laser and couple it into the enhanced external cavity. By setting the focal length and position of the mode matching lens, the mode matching between the laser beam and the transverse mode of the enhanced external cavity is achieved, which can improve the laser coupling efficiency.

[0040] The enhanced external cavity includes a first spherical reflector M1 and a second spherical reflector M2. In this embodiment, the first spherical reflector M1 and the second spherical reflector M2 are both concave spherical reflectors, forming a confocal spherical mirror cavity, and the normal of the first spherical reflector M1 has an angle with the optical axis; the incident point of the laser beam passing through the mode matching lens on the first spherical reflector M1 is the center of the second spherical reflector M2, and the laser beam reflects back and forth between the first spherical reflector M1 and the second spherical reflector M2, so that the semiconductor laser and the enhanced external cavity resonate in frequency, so as to increase the laser power in the enhanced external cavity. Specifically, the laser light incident on the second spherical reflector M2 through the center of the second spherical reflector M2 will return to its original path after reflection, and then continue to be reflected by the first spherical reflector M1. With each reflection, a small amount of laser light passes through the first spherical reflector M1 and transmits out of the enhanced external cavity. When the frequency of the incident laser beam resonates with the enhanced external cavity, as the incident laser beam continues to be incident, a coherent superposition of the returning laser beams is formed in the enhanced external cavity, resulting in power enhancement. The laser light transmitted out of the enhanced external cavity through the first spherical reflector M1 returns to the semiconductor laser along its original path, forming feedback to the semiconductor laser, causing the semiconductor laser to oscillate at the resonant frequency of the enhanced external cavity and resonate with the enhanced external cavity. At the same time, because the normal of the first spherical reflector M1 is placed at a certain angle to the optical axis, the light directly reflected from the outer surface of the first spherical reflector M1 when the laser light is incident will be emitted at an angle, which can avoid interfering with the resonance.

[0041] Preferably, the first spherical reflector M1 and the second spherical reflector M2 have the same curvature radius and the inner surface has high reflectivity. When the first spherical reflector M1 and the second spherical reflector M2 form a symmetrical confocal spherical mirror cavity, the Raman scattering signal intensity can be further enhanced.

[0042] A Raman scattered light collection unit is used to collect Raman scattered signals generated by irradiating a gas sample with a laser beam in an enhanced external cavity. In an embodiment of the present invention, the Raman scattered light collection unit includes a collection lens and a spectrometer. The collection lens is used to collect Raman scattered light generated by irradiating a gas sample with a laser beam in an enhanced external cavity and focus it on the slit of the spectrometer. The spectrometer slit is preferably rectangular, with its height direction parallel to the laser beam in the enhanced external cavity. Raman scattered light can enter the slit and the spectrometer to the maximum extent possible, thereby obtaining a strong Raman scattering signal on the spectrometer.

[0043] When the laser linewidth narrowing unit is a narrow linewidth interference filter, the present invention has, through research and experiments, found that the narrow linewidth interference filter has a transmission bandwidth of 0.6 nm and a transmittance greater than or equal to 85%. This filter can improve spectral resolution while not affecting the resonance of the subsequent enhanced external cavity. Specifically, the narrow linewidth interference filter has a transmission bandwidth of 0.6 nm, which can effectively improve the spectral resolution of the instrument while retaining sufficient bandwidth. After being filtered by the narrow linewidth interference filter, the laser light emitted by the semiconductor laser has sufficient longitudinal modes, allowing the eigenmode of the semiconductor laser and the enhanced external cavity mode to resonate when they are the same. At the same time, the transmittance of the narrow linewidth interference filter exceeds 85%, and the laser beam gain in the enhanced external cavity is sufficiently high. The enhanced external cavity mode can form a competitive advantage over other non-matching eigenmodes of the semiconductor laser, and these other non-matching eigenmodes are suppressed and disappear, thereby enhancing the laser oscillation in the enhanced external cavity, which can improve spectral resolution while not affecting the resonance of the subsequent enhanced external cavity.

[0044] Specifically, in the embodiment of the present invention, a 700mw semiconductor laser DL with a wavelength of 410nm is used, and the laser line width is 1.1nm. After the line width is compressed by a narrow line width interference filter, the laser line width is 0.3nm. The semiconductor laser is a multi-longitudinal mode laser.

[0045] In the self-locking enhancement cavity unit, the first spherical reflector M1 and the second spherical reflector M2 both have a curvature radius of 50 mm. The first spherical reflector M1 has an antireflection coating on its outer surface and a high-reflection coating on its inner surface, with a reflectivity of 96.5%. The second spherical reflector M2 also has an antireflection coating on its outer surface and a high-reflection coating on its inner surface, with a reflectivity of 99.5%. The focal length of the mode-matching lens is 4.01 mm, ensuring that the optical waist radius of the laser beam emitted by the semiconductor laser matches the intrinsic transverse film of the enhancement cavity.

[0046] like Figure 2As shown, the first spherical reflector M1 and the second spherical reflector M2 are 50 mm apart, forming a confocal spherical mirror cavity. The normal of the first spherical reflector M1 or the optical axis of the confocal spherical mirror cavity has a certain angle with the incident laser beam. The laser beam L0 emitted by the multi-longitudinal mode laser is incident from the center point A of the spherical surface of the first spherical reflector M1, and point A is also the center of the spherical surface of the second spherical reflector M2. A part of the laser beam L0 passing through point A is directly reflected, and a part is transmitted to the laser beam L1 as the laser beam L1. The laser beam L2 is reflected on the second spherical reflector M2 and intersects with the second spherical reflector M2 at point B. After vertical reflection at point B, it forms laser beam L2 and returns to point A along the same path. Laser beam L2 is reflected again by the first spherical reflector M1 to form laser beam L3, which intersects with the second spherical reflector M2 again at another point C. It is vertically reflected at point C to form laser beam L4 and returns to point A again. The laser beam reflects back and forth between the two reflectors multiple times along the paths of L1, L2, L3, and L4, thus enhancing power. Among them, laser beam L2 forms laser beam L5 after being transmitted through the first spherical reflector M1. Laser beam L5 returns to the semiconductor laser and has a feedback effect on the oscillation frequency of the semiconductor laser. When the longitudinal mode frequency of the laser emitted by the semiconductor laser is the same as that of the enhanced external cavity, the semiconductor laser resonates with the enhanced external cavity. Since multiple modes can resonate with the enhanced external cavity at the same time, no feedback circuit is required. Specifically, the longitudinal mode frequency of the enhanced external cavity is v = qC / 2nL, where q is an integer, c is the speed of light, n is the refractive index of the medium, and L is the length of the enhanced external cavity. Laser beam L6 is the composite light of the laser beam directly reflected by laser beam L0 and the laser beam transmitted by laser beam L4. This is the composite light of the laser beam directly reflected by the first spherical reflector M1 and the laser beam transmitted by the external cavity. Because the normal of the first spherical reflector M1 is at an angle to the optical axis, laser beam L6 can be tilted out of the enhanced cavity to avoid entering the semiconductor laser and interfering with its resonance.

[0047] In this embodiment, the collection lens has a focal length of 50 mm. It enhances the interaction between the laser beam in the external cavity and the gas sample, generating Raman scattered light that is collected by the collection lens and focused at the slit. The minimum slit width is 10 μm, and the slit is rectangular. When the laser beam is parallel to the slit height, the Raman scattered light can pass through the slit to the maximum extent possible and enter the spectrometer, producing a strong Raman scattering signal.

[0048] Experiments have shown that the use of a semiconductor laser self-locking enhancement cavity can enhance the laser by 30 times in the external cavity, and the light intensity in the enhanced cavity is 21W. Figure 3 As shown in the figure, within the 1s integration time, carbon dioxide gas in the air (at 1285cm -1 and 1388cm -1 There is a signal at 3110cm) and the second harmonic of oxygen (at 3110cm -1The gas concentration detection limit of the instrument is finally in the ppm level at one atmosphere.

[0049] The semiconductor laser self-locked enhanced cavity gas Raman detection device for gas Raman analysis of the present invention uses a semiconductor laser. The laser beam is coupled into an enhanced external cavity through a mode matching lens and repeatedly reflected in a confocal spherical mirror cavity formed by a designed first spherical reflector M1 and a second spherical reflector M2. When the semiconductor laser frequency is the same as the natural frequency of the external cavity, the external cavity and the laser resonate, and the laser is transmitted back and forth in the external cavity, forming power accumulation. Experiments have shown that the power can be enhanced by dozens of times. The semiconductor laser used has a large frequency operating range, which can ensure that the resonant frequency of the enhanced external cavity is selected within it. In this resonance mode of the present invention, the oscillation frequency is mainly determined by the enhanced external cavity, and there is no need to implement feedback control of the enhanced cavity length. The optical path structure layout is simple and stable. In combination with an ordinary semiconductor laser, it can achieve intracavity laser power enhancement, thereby enhancing the Raman scattering signal, and can also reduce the complexity of the optical path and the cost of the device.

[0050] At the same time, two ordinary spherical mirrors are used to form a confocal spherical mirror cavity, and there is no need to use ultra-high reflectivity mirrors, which further reduces the cost of the device. In addition, through the confocal spherical mirror cavity designed by the present invention, when the laser beam is transmitted in the confocal spherical mirror cavity, the laser beam can form a degenerate transverse mode, which is conducive to the light beam returning to the semiconductor laser and forming resonance.

[0051] Furthermore, since the enhancement cavity designed in the present invention has only two enhancement light paths, the laser beam in the enhancement cavity can be made parallel to the height direction of the spectrometer slit, so that more signal light enters the spectrometer, further enhancing the Raman detection signal.

[0052] Preferably, an excitation light source with a wavelength of 410 nm is used. The shorter the excitation wavelength, the stronger the Raman signal generated by the excited sample.

[0053] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A gas Raman detection device based on a semiconductor laser self-locked enhancement cavity, characterized in that: It includes a laser unit, a laser line width narrowing unit and a self-locking enhancement cavity unit arranged along the laser beam, and a Raman scattered light collection unit arranged perpendicular to the laser beam; The laser unit includes a semiconductor laser for generating a laser beam; The laser linewidth narrowing unit is used to narrow the linewidth of the laser beam; The self-locking enhanced cavity unit includes a mode matching lens and an enhanced external cavity; The mode matching lens is used to focus the laser beam and then couple it to the enhanced external cavity; The enhanced external cavity includes a first spherical reflector M1 and a second spherical reflector M2 forming a confocal spherical mirror cavity, wherein a normal of the first spherical reflector M1 forms an angle with the optical axis, an incident point of a laser beam on the first spherical reflector M1 is the center of a sphere of the second spherical reflector M2, and the laser beam is reflected back and forth between the first spherical reflector M1 and the second spherical reflector M2. The laser beam that reversely transmits the first spherical reflector M1 is emitted from the enhanced external cavity and returns to the semiconductor laser to form optical feedback, thereby causing frequency resonance between the semiconductor laser and the enhanced external cavity; The Raman scattered light collection unit is used to collect Raman scattered signals generated by irradiating the gas sample with the laser beam in the enhanced external cavity; The first spherical reflecting mirror M1 and the second spherical reflecting mirror M2 have the same curvature radius, and the first spherical reflecting mirror M1 and the second spherical reflecting mirror M2 form a symmetrical confocal spherical mirror cavity.

2. The device according to claim 1, characterized in that The focal length and position of the mode matching lens are set so that the laser beam forms a mode matching with the transverse mode of the enhanced external cavity.

3. The device according to claim 1, characterized in that The laser linewidth narrowing unit includes one of a narrow linewidth interference filter, a birefringence filter or an etalon.

4. The device according to claim 3, characterized in that The narrow linewidth interference filter has a transmission bandwidth of 0.6 nm and a transmittance greater than or equal to 85%.

5. The device according to claim 1, characterized in that The outer surface of the reflector is coated with an anti-reflection film, and the inner surface is coated with a high-reflection film. The reflectors are the first spherical reflector M1 and the second spherical reflector M2.

6. The device according to claim 1, characterized in that The Raman scattered light collection unit includes a collection lens and a spectrometer; The collecting lens is used to collect Raman scattered light generated by irradiating the gas sample with the laser beam in the enhanced external cavity and focus the light on the slit of the spectrometer; The height direction of the slit of the spectrometer is parallel to the laser beam in the enhanced external cavity.

7. The device according to claim 1, characterized in that The semiconductor laser is a free-running laser.

8. The device according to claim 1, characterized in that The wavelength of the semiconductor laser is in the blue-violet light band.

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