A compact optical path folding device

Abstract

This application relates to the field of optical sensing and provides a dense optical path folding device. It employs a plane mirror and a primary concave mirror with aberrations to form a reflecting cavity, and introduces a sub-concave mirror. The sub-concave mirror is positioned above the plane mirror, such that at least one of the reflecting surfaces of the plane mirror and the optical centers of the sub-concave mirror is not on the focal plane of the primary concave mirror. Due to the aberrations of the primary concave mirror, the light signal reflected by the sub-concave mirror is reflected multiple times within the reflecting cavity. When it returns to the sub-concave mirror, its position and angle do not coincide. Furthermore, because the normal directions of different positions of the sub-concave mirror are different, the light signal, upon returning to the sub-concave mirror, is reflected in a different direction than before, deviating from its original trajectory, forming a semi-closed trajectory within the reflecting cavity. After the light signal is output from the output end, the total optical path within the reflecting cavity is increased by more than 10 times compared to existing optical path folding devices, resulting in a higher optical path-to-volume ratio.

Description

Technical Field

[0001] The present application relates to the field of optical sensing, in particular to a dense optical path folding device. Background Art

[0002] An optical device that achieves multiple reflections of optical signals within a limited volume to allow the optical signals to travel a relatively long optical path has important applications in the field of optical sensing, especially in the field of sensing and analysis of special gases.

[0003] At present, Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology and Fourier Transform Infra-Red (FTIR) technology are two mainstream technical routes. The former mainly performs spectral analysis in the near-infrared band based on a tunable laser light source, while the latter adopts a broad-spectrum light source to perform spectral analysis in the mid- and far-infrared bands through Fourier transform.

[0004] To achieve sufficient detection accuracy, both TDLAS technology and FTIR technology require the use of a long optical path gas cell, so that the optical signal can travel a sufficient optical path in the gas to be analyzed to enhance the absorption spectrum of the gas. In order to keep the volume of the detection instrument within an acceptable range, long optical path gas cells usually need to adopt an optical path folding device, so as to reflect the optical signal as many times as possible within the limited volume to obtain a sufficient optical path.

[0005] For TDLAS applications, due to the small divergence angle of laser signals, the Herriott cell structure is generally adopted in the industry, as Figure 1 shown, the Herriott cell 100 adopts two concave reflecting mirrors 103, 104 with the same focal length f to form a reflecting cavity. When the input direction and position of the optical signal input from the input end 101 and the distance d between the two concave reflecting mirrors along the z direction satisfy certain conditions (generally a defocus configuration with 0<d<2f or 2f<d<4f is adopted), the optical signal will reflect back and forth multiple times between the two concave reflecting mirrors, and finally output from the output end 102. Figure 2 shows that on the two concave reflecting mirrors 203 and 204, the reflection points form a circular light spot trajectory 201 on the x-y plane.

[0006] For FTIR applications, since an incoherent broad-spectrum thermal light source is required as the light source, the divergence angle of the optical signal is large, and the performance of the Herriot cell cannot meet the requirements. This is caused by the necessary defocus configuration characteristic of the Herriott cell: after incoherent optical signals are reflected multiple times in a defocus system, the divergence angle cannot converge. Therefore, the traditional White cell structure is generally adopted in the industry, as Figure 3As shown, the White chamber 300 consists of three concave mirrors with the same radius of curvature and focal length f. The primary mirror 301 is located on one side, and two secondary mirrors 302 and 303 are located on the opposite side of the primary mirror. The input optical signal 304 and the output optical signal 305 are located on either side of the primary mirror. The two secondary mirrors 302 and 303 have a certain tilt angle. The distance between the primary mirror 301 and the two secondary mirrors 302 and 303 is set to 2f, causing the optical signal to be reflected multiple times between the primary mirror 301 and the two secondary mirrors 302 and 303, finally outputting from the primary mirror 301. The trajectory of the light spot on the primary mirror 301 is as follows... Figure 4 As shown, the position 404 of the input optical signal is typically offset from the axis 402 of the primary reflector 401, so that the light spot is distributed on two tracks 403 and 406 to obtain the maximum number of reflections. The position 405 of the output optical signal is typically on the other side of the track 406 in the same row as the input optical signal.

[0007] As industry demands higher precision in gas detection, the requirements for long-path gas chambers have also increased. Longer optical paths (over 20 meters, even 100 meters) need to be achieved within a limited volume. Heriot-Lewis and White chambers struggle to achieve more reflections within a given volume. Many improvements have been made to Heriot-Lewis and White chambers, such as Heriot's own proposal to use astigmatic lenses to achieve more reflections. However, astigmatic lenses are difficult to manufacture. Although subsequent attempts have reduced manufacturing precision by rotating the astigmatic lens, the high manufacturing cost remains unresolved. Joel A. Silver et al. proposed using bicylindrical mirrors to achieve a denser light spot distribution, i.e., more reflections. However, due to the non-rotational symmetry of the bicylindrical mirrors, the light signal loses its original characteristics after multiple reflections, making it unsuitable for applications requiring the preservation of light signal characteristics (light signal radius, divergence half-angle, etc.).

[0008] Other design improvements, such as Figure 5The optical path folding device 500 shown includes an input end 501, an output end 502, a concave mirror 503, a principal plane mirror 504, and a tilting mirror 505. The distance 507 from the focal plane 506 of the concave mirror 503 to the concave mirror 503 is the focal length of the concave mirror 503. The origin 509 of the focal plane 506 is the intersection point of the optical axis 508 of the optical system composed of the principal plane mirror 504 and the concave mirror 503 on the focal plane 506. The tilt angle between the normal of the first tilting sub-mirror 505 and the normal of the principal plane mirror 504 is not zero. The light beam is input from the input end 501, and after multiple reflections between the concave mirror 503, the principal plane mirror 504, and the tilting mirror 505, it is output from the output end 502. Compared to the Heriot-100 and White-300 optical path folding devices, the 500 optical path folding device can achieve more reflections, i.e., a longer optical path, and a higher optical path-to-volume ratio. However, its improvement in optical path and optical path-to-volume ratio is still limited and cannot meet the industry's requirements for longer optical paths and greater optical path-to-volume ratio. Summary of the Invention

[0009] One of the objectives of this application is to provide a dense optical path folding device that can achieve a longer optical path and a larger optical path-to-volume ratio compared to existing optical path folding devices.

[0010] This application provides a dense optical path folding device, including:

[0011] The input terminal is used to input optical signals;

[0012] The output terminal is used to output optical signals.

[0013] A principal concave mirror with a focal length of f, a radius of curvature of R, and aberrations;

[0014] A plane mirror, wherein the distance from the reflecting surface of the plane mirror to the optical center of the principal concave mirror is L1 = (1 + x1)f, -1 < x1 < 1;

[0015] A sub-concave reflector with a focal length of f0 and a radius of curvature of R0 is disposed on the plane reflector. The orthographic projection area of ​​the reflector surface of the sub-concave reflector on the reflector surface of the plane reflector is smaller than the area of ​​the reflector surface of the plane reflector. The distance from the optical center of the sub-concave reflector to the optical center of the main concave reflector is L2 = (1 + x2)f, R0 = mR, -1 < x2 < 1, x1 and x2 are not simultaneously 0, and m > 0.

[0016] The input terminal is disposed on the main concave reflector or the plane reflector, and the output terminal is disposed on the main concave reflector, the plane reflector or the sub-concave reflector. The reflecting surface of the main concave reflector is disposed opposite to the reflecting surface of the plane reflector and the reflecting surface of the sub-concave reflector.

[0017] The optical signal is input from the input terminal, and after multiple reflections between the main concave mirror, the plane mirror, and the sub-concave mirror, it is output from the output terminal.

[0018] In one embodiment, the dense optical path folding device includes:

[0019] The two sub-concave mirrors are arranged symmetrically about the optical axis, and the orthographic projections of the reflecting surfaces of the two sub-concave mirrors onto the reflecting surface of the planar mirror do not coincide.

[0020] Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror 0 < x2(1+2mx1-x1x2) < 2m;

[0021] The optical signal is input from the input terminal, and after multiple reflections between the main concave mirror, the plane mirror, and the two sub-concave mirrors, it is output from the output terminal.

[0022] In one embodiment, the orthographic projection of the optical center of the sub-concave mirror onto the reflecting surface of the planar mirror is located at the origin of the optical axis.

[0023] Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror, the origin is the intersection of the optical axis and the reflecting surface of the plane mirror, and 0 < x2(1+2mx1-x1x2) < 2m.

[0024] In one embodiment, x2 > 0;

[0025] And / or, 1≤m≤10, 0.05≤x1≤0.5.

[0026] In one embodiment, the orthographic projection of the reflecting surface of the sub-concave mirror onto the reflecting surface of the planar mirror deviates from the origin of the optical axis.

[0027] Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror, the origin is the intersection of the optical axis and the reflecting surface of the plane mirror, x1=x2=x, 0<x(4x+1 / m)<1.

[0028] In one embodiment, x > 0.

[0029] In one embodiment, 0 < x < 0.1.

[0030] In one embodiment, the primary concave mirror is a spherical mirror with inherent aberrations, or the primary concave mirror is an aspherical mirror designed with aberrations.

[0031] In one embodiment, the input terminal, the output terminal, and the sub-concave reflector are separately disposed, with both the input terminal and the output terminal disposed on the planar reflector;

[0032] Alternatively, the input terminal and the output terminal may overlap to form an input-output terminal, and the input-output terminal and the sub-concave reflector may be separately disposed and disposed on the planar reflector;

[0033] Alternatively, the input end and the output end are separately configured, with the input end located on the plane mirror and the output end being the main concave mirror; wherein, the optical signal is input from the input end, and after multiple reflections between the main concave mirror, the plane mirror, and the sub-concave mirror, it is output from the output end to the first converging lens, and then converged to the receiving end by the first converging lens;

[0034] Alternatively, the input end and the output end are separately configured, with the input end located on the planar reflector and the output end being the sub-concave reflector; wherein, the optical signal is input from the input end, and after multiple reflections between the main concave reflector, the planar reflector and the sub-concave reflector, it is output from the output end to the receiving end.

[0035] In one embodiment, the input terminal, the output terminal, and the two sub-concave mirrors are separately disposed, with both the input terminal and the output terminal disposed on the planar mirror.

[0036] The dense optical path folding device provided in this application uses a plane mirror and a primary concave mirror with aberrations to form a reflecting cavity, and introduces a sub-concave mirror. The sub-concave mirror is placed on the plane mirror, and at least one of the reflecting surfaces of the plane mirror and the optical center of the sub-concave mirror is not on the focal plane of the primary concave mirror. Due to the aberrations of the primary concave mirror, the light signal reflected by the sub-concave mirror is reflected multiple times in the reflecting cavity. When it returns to the sub-concave mirror, the position and angle do not coincide. Furthermore, since the normal directions of different positions of the sub-concave mirror are different, the light signal returns to the sub-concave mirror in a different direction than before, deviating from the original trajectory direction. This forms a semi-closed trajectory in the reflecting cavity. Ultimately, after the light signal is output from the output end, the total optical path in the reflecting cavity can be increased by more than 10 times compared to existing optical path folding devices, thus achieving a higher optical path-to-volume ratio. Attached Figure Description

[0037] To more clearly illustrate the technical applications in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the Heriot-Turkey chamber in the prior art;

[0039] Figure 2 This is a diagram of the reflection point light spot trajectory in the existing Heriot-Lewis chamber;

[0040] Figure 3 This is a schematic diagram of the White chamber in existing technology;

[0041] Figure 4 This is a diagram of the light spot trajectory of the optical signal reflection point in the White chamber in the existing technology;

[0042] Figure 5 This is a schematic diagram of an optical path folding device in the prior art;

[0043] Figure 6 This is a schematic diagram of the first dense optical path folding device provided in the embodiments of this application;

[0044] Figure 7 When m = 6.37, N = 45, x1 = x2 = x, as provided in the embodiments of this application, x and B N A diagram illustrating the correspondence between them;

[0045] Figure 8 This is a schematic diagram of the second type of dense optical path folding device provided in the embodiments of this application;

[0046] Figure 9 This is a schematic diagram of the third type of dense optical path folding device provided in the embodiments of this application;

[0047] Figure 10 This is a schematic diagram of the fourth type of dense optical path folding device provided in the embodiments of this application;

[0048] Figure 11 This is a schematic diagram of the fifth type of dense optical path folding device provided in the embodiments of this application;

[0049] Figure 12 This is a schematic diagram of the sixth type of dense optical path folding device provided in the embodiments of this application;

[0050] Figure 13 This is a schematic diagram of the seventh type of dense optical path folding device provided in the embodiments of this application. Detailed Implementation

[0051] To enable those skilled in the art to better understand this application, the technical applications in the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this application.

[0052] The term "comprising," and any variations thereof, in the specification, claims, and accompanying drawings of this application are intended to cover non-exclusive inclusion. Furthermore, the terms "first," "second," etc., are used to distinguish different objects, not to describe a particular order.

[0053] like Figure 6 As shown, this application provides a dense optical path folding device 600, comprising:

[0054] Input terminal 601 is used for inputting optical signals;

[0055] Output terminal 602 is used to output optical signals;

[0056] The principal concave mirror 603 has a focal length of f, a radius of curvature of R, and exhibits aberrations.

[0057] The distance from the reflecting surface of the plane mirror 604 to the optical center of the principal concave mirror 603 is L1 = (1 + x1)f;

[0058] A sub-concave reflector 605 has a focal length of f0 and a radius of curvature of R0 and is positioned on a plane reflector 604. The orthographic projection area of ​​the reflecting surface of the sub-concave reflector 605 onto the reflecting surface of the plane reflector 604 is smaller than the area of ​​the reflecting surface of the plane reflector 604. The distance from the optical center of the sub-concave reflector 605 to the optical center of the main concave reflector 603 is L2 = (1 + x2)f, and R0 = mR.

[0059] In the optical system composed of the plane mirror 604 and the concave mirror 603, the optical axis 608 is perpendicular to the reflecting surface of the plane mirror 604 and passes through the optical center and focal point of the concave mirror 603. The optical axis 608 has an origin 609, which is the intersection of the optical axis 608 and the reflecting surface of the plane mirror 604.

[0060] The input terminal 601 is disposed on the main concave reflector 603 or the plane reflector 604, and the output terminal 602 can be disposed on the main concave reflector 603, the plane reflector 604, or the sub-concave reflector 605. The orthographic projection of the optical center of the sub-concave reflector 605 onto the reflecting surface of the plane reflector 604 is located at the origin 609. Alternatively, the orthographic projection of the reflecting surface of the sub-concave reflector 605 onto the reflecting surface of the plane reflector 604 is offset from the origin 609. Figure 6 The example shows that the input terminal 601 and the output terminal 602 are separately arranged and both are arranged on the plane mirror 604. The orthographic projection of the optical center of the sub-concave mirror 605 on the reflecting surface of the plane mirror 604 is located at the origin 609.

[0061] The reflecting surface of the main concave mirror 603 is arranged opposite to the reflecting surface of the plane mirror 604 and the reflecting surface of the sub-concave mirror 605;

[0062] The optical signal is input from the input terminal 601, and after multiple reflections between the main concave reflector 603, the plane reflector 604 and the sub-concave reflector 605, it is output from the output terminal 602.

[0063] In application, x1 is the deviation of the reflecting surface of the plane mirror relative to the focal plane of the primary concave mirror, defined as the first defocusing amount. When x1 = 0, L1 = f, and the reflecting surface of the plane mirror is located at the focal plane of the primary concave mirror. x2 is the deviation of the optical center of the sub-concave mirror relative to the focal plane of the primary concave mirror, defined as the second defocusing amount. When x2 = 0, L2 = f, and the optical center of the sub-concave mirror is located at the focal plane of the primary concave mirror. Both x1 and x2 are dimensionless. At least one of the reflecting surface of the plane mirror and the optical center of the sub-concave mirror is not on the focal plane of the primary concave mirror, that is, x1 and x2 are not both 0.

[0064] In application, since R0 = mR, then f0 = mf. Without loss of generality, let f = 1, then we have:

[0065] L1=1+x1; (Formula 1)

[0066] L2 = 1 + x2; (Formula 2)

[0067] f1 = m; (Formula 3)

[0068] Where -1 < x1 < 1, -1 < x2 < 1, m > 0.

[0069] In one embodiment, the planar reflector includes multiple reflective regions, at least one of all reflective regions of the planar reflector is the input end, and the reflectivity of the input end is less than or equal to the reflectivity of the other reflective regions of the planar reflector;

[0070] Alternatively, the concave mirror may include multiple reflecting regions, at least one of the reflecting regions of the concave mirror being the input end, and the reflectivity of the input end being less than or equal to the reflectivity of the other reflecting regions of the concave mirror.

[0071] Alternatively, the input end can be a light-transmitting hole or an opening in the planar reflector.

[0072] In one embodiment, the concave mirror includes multiple reflecting regions, at least one of the reflecting regions of the concave mirror is an output end, and the reflectivity of the output end is less than or equal to the reflectivity of the other reflecting regions of the concave mirror.

[0073] Alternatively, a plane mirror may include multiple reflective regions, at least one of which is an output end, and the reflectivity of the output end is less than or equal to the reflectivity of the other reflective regions of the plane mirror.

[0074] Alternatively, the subconcave mirror may include multiple reflecting regions, at least one of the reflecting regions of the subconcave mirror being an output end, and the reflectivity of the output end being less than or equal to the reflectivity of the other reflecting regions of the subconcave mirror.

[0075] Alternatively, the output end can be a main concave mirror or a sub-concave mirror;

[0076] Alternatively, the output end can be a light-passing hole or an open angle formed in a plane mirror.

[0077] In applications, the reflective area is the location on the reflective surface of a mirror used to reflect light signals. Due to limitations in modern coating technology, the maximum reflectivity R of a mirror's reflective surface can typically reach 0.99999. Further increasing the reflectivity becomes drastically more difficult and costly. Consequently, the minimum transmittance T (i.e., 1-R) of the reflective surface can be 0.00001, meaning T can be as low as 10. -5 The reflectivity of the reflective region used as an input or output terminal is less than or equal to the reflectivity of other reflective regions. In other words, the transmittance of the reflective region used as an input or output terminal is greater than or equal to the transmittance of other reflective regions. The magnitude of T0 of the transmittance of the reflective region used as an input or output terminal can be 10. -5 -10 -3The values ​​are within the order of magnitude, for example, between 0.0009 and 0.005. The position, size, and number of reflective regions used as input or output terminals can be set according to actual needs, as long as the input terminal is set on the main concave mirror or plane mirror, and the output terminal is set on the main concave mirror, plane mirror, or sub-concave mirror. When there are at least two reflective regions used as input terminals, the transmittance of these input terminals can be the same or different; similarly, when there are at least two reflective regions used as output terminals, the transmittance of these output terminals can also be the same or different; when both input and output terminals are reflective regions, the transmittance of the input and output terminals can also be the same or different.

[0078] In applications, different reflective regions with varying transmittance in the primary concave mirror, plane mirror, and sub-concave mirror can be coated using either an integrated coating method or a separate coating method. In the integrated coating process, a mask is used to create different film layers in different regions of the mirror's reflective surface. In the separate coating process, the mirror's reflective surface is separated into independent regions, and each region is coated individually.

[0079] In applications, by making the reflectivity of the input or output end less than that of the remaining reflective regions, the average reflectivity of all reflective regions is slightly reduced, which in turn slightly reduces the effective optical path of the reflective cavity, negatively impacting the signal-to-noise ratio (SNR). However, this technique also significantly increases the optical energy of the output signal. When applied to gas optical sensing devices (e.g., gas absorption spectroscopy detection devices), it can increase the optical energy coupled from the reflective cavity to the receiving end (e.g., photodetector or spectrometer), positively impacting the SNR of the gas absorption spectroscopy detection device. Since the positive impact far outweighs the negative impact, the SNR is also significantly improved. The SNR improvement factor is positively correlated with the number of reflections of the optical signal in the reflective cavity.

[0080] In applications, when the input or output end is a light-passing aperture or an open angle, it is suitable for inputting or outputting incoherent optical signals with a large divergence angle. The optical signal enters the input end from free space and is output to free space from the output end.

[0081] In one embodiment, the input terminal and the output terminal coincide to form an input-output terminal, and the input-output terminal is disposed on a plane mirror or a main concave mirror.

[0082] Alternatively, the input and output terminals can be set separately and both can be located on a plane mirror;

[0083] Alternatively, the input and output terminals can be set separately, with the input terminal set on the plane mirror and the output terminal set on the main concave mirror, or the output terminal set on the main concave mirror.

[0084] Alternatively, the input and output terminals can be set separately, with the input terminal set on a plane mirror and the output terminal set on a sub-concave mirror, or the output terminal being a sub-concave mirror.

[0085] In applications, the input and output terminals can be located simultaneously or separately. When the input and output terminals are located simultaneously, they are considered the same entity, defined as the input and output terminals, which can be located on either a plane mirror or a primary concave mirror. When the input and output terminals are located separately, they can be located on the same mirror (e.g., a plane mirror) or on different mirrors (e.g., the input terminal is located on a plane mirror, and the output terminal is located on the primary concave mirror, or the output terminal is the primary concave mirror itself; the input terminal is located on a plane mirror, and the output terminal is located on a secondary concave mirror, or the output terminal is the secondary concave mirror itself). When the output terminal is the primary concave mirror or the secondary concave mirror itself, the output terminal includes all the reflection areas of the primary concave mirror or the secondary concave mirror.

[0086] In one embodiment, the primary concave mirror is a spherical mirror with inherent aberrations, or the primary concave mirror is an aspherical mirror designed with aberrations.

[0087] In applications, the primary concave mirror can be a spherical mirror, as spherical mirrors inherently possess aberrations and are easy to manufacture. Alternatively, an aspherical mirror can be used. Since aspherical mirrors typically do not possess aberrations, special aspherical mirrors with aberrations can be designed according to actual needs, offering a degree of design flexibility.

[0088] In one embodiment, the dense optical path folding device includes:

[0089] Two sub-concave mirrors are arranged symmetrically about the optical axis, and the orthographic projections of the reflecting surfaces of the two sub-concave mirrors onto the reflecting surface of the plane mirror do not coincide.

[0090] The optical signal is input from the input end, and after multiple reflections through the main concave mirror, the plane mirror, and the two sub-concave mirrors, it is output from the output end.

[0091] In applications, subconcave mirrors can be recessed or protruding relative to the reflecting surface of a plane mirror. For example, a subconcave mirror can be embedded in a plane mirror or attached to the reflecting surface of a plane mirror. When a dense optical path folding device includes two subconcave mirrors, the reflecting surface sizes of the two subconcave mirrors can be the same or different; that is, the orthographic projection areas of the reflecting surfaces of the two subconcave mirrors onto the reflecting surface of the plane mirror can be the same or different. The fact that the orthographic projections of the two subconcave mirrors onto the reflecting surface of the plane mirror do not coincide and deviate from the origin means that the two subconcave mirrors can be arranged adjacent (i.e., in contact) or spaced apart (i.e., not in contact) on the plane mirror without obstructing the origin.

[0092] In one embodiment, the orthographic projection of the optical center of the sub-concave mirror onto the reflecting surface of the plane mirror is located at the origin of the optical axis. Alternatively, in the first case where the dense optical path folding device includes two sub-concave mirrors, for the reflective cavity of the optical system to be stable, the optical system needs to satisfy the first stability condition: 0 < x2(1+2mx1-x1x2) < 2m.

[0093] In the second case where the orthographic projection of the subconcave mirror onto the plane mirror deviates from the origin of the optical axis, the optical system needs to satisfy the second stability condition to make the reflective cavity of the optical system stable: 0 < x(4x+1 / m) < 1.

[0094] In applications, the derivation process of the stability condition of the optical system is as follows:

[0095] Let the transmission matrix of the optical signal during a single cyclic reflection in the reflecting cavity be... It can be proven that det(T) = 1, that is, AD - BC = 1;

[0096] Let the transmission matrix of the optical signal during N cyclic reflections in the reflecting cavity be:

[0097]

[0098] Where θ = acos[(A+D) / 2].

[0099] To ensure that the optical signal does not leak out of the cavity after N cycles of reflection within the cavity, A N B N C N D N It must be a real number and convergent, therefore, it must satisfy -1 < (A+D) / 2 < 1 (Formula 4);

[0100] For the first case, the transmission process of the optical signal being reflected once in the transmitting cavity is defined as follows: the optical signal starts from the reflecting surface of the plane mirror and is transmitted to the reflecting surface of the main concave mirror. After being reflected by the reflecting surface of the main concave mirror, it is transmitted to the reflecting surface of the sub-concave mirror. After being reflected by the reflecting surface of the sub-concave mirror, it is transmitted to the reflecting surface of the main concave mirror again. After being reflected by the reflecting surface of the main concave mirror again, it is transmitted to the reflecting surface of the plane mirror again and is reflected by the reflecting surface of the plane mirror (Definition 1).

[0101] Based on Definition 1, Equations 1 to 3, and the principles of matrix optics, the transmission matrix corresponding to Definition 1 is expressed as:

[0102]

[0103] Where A = D = -(m - x² + x₁x²) 2 -2mx1x2) / m, B=(x1x2-1)(2mx1-x1x2+1) / m, C=x2(2m-x2) / m;

[0104] and The transmission matrix represents the transmission of optical signals in free space;

[0105] This represents the transmission matrix when the principal concave mirror reflects light signals.

[0106] This represents the transmission matrix when the subconcave mirror reflects light signals.

[0107] According to Formula 4, the first stability condition can be obtained as follows:

[0108] -1 < -(m - x2 + x1x2) 2 -2mx1x2) / m<1; (Formula 5)

[0109] Simplifying Equation 5, we obtain the first stability condition as follows:

[0110] 0 < x²(1 + 2mx₁ - x₁x₂) < 2m;

[0111] For the second scenario, the transmission process of the optical signal undergoing one cyclic reflection within the transmitting cavity is defined as:

[0112] The light signal originates from the reflecting surface of the plane mirror and is transmitted to the reflecting surface of the principal concave mirror. After being reflected by the reflecting surface of the principal concave mirror, it is transmitted to the reflecting surface of the plane mirror. After being reflected by the reflecting surface of the plane mirror, it is transmitted to the reflecting surface of the principal concave mirror again. After being reflected by the reflecting surface of the principal concave mirror once more, it is transmitted to the reflecting surface of the plane mirror once more and is reflected by the reflecting surface of the plane mirror.

[0113] The light signal is reflected by the plane mirror and then transmitted to the main concave mirror. After being reflected by the main concave mirror, it is transmitted to the sub-concave mirror. After being reflected by the sub-concave mirror, it is transmitted to the main concave mirror. After being reflected by the main concave mirror, it is transmitted to the plane mirror and then reflected by the plane mirror (Definition 2).

[0114] Let x1 = x2 = x, then according to Definition 1, Equations 1 to 3, and the principles of matrix optics, the transmission matrix corresponding to Definition 2 is expressed as:

[0115]

[0116] Where, A = -(4x 5 +8mx 4 +5x 3 -8mx 2 -x+m) / m, B=-(x 2 -1)(4x 2 -8m 3 -5x 2 +4mx+1) / m,C=-x(4x 3 -8mx 2 -3x+4m) / m, D=(-4x 5 +8mx 4 +7x 3 -8mx 2 -3x+m) / m;

[0117] The third matrix on the right side of the equation is equal to the product of the eighth and ninth matrices, that is...

[0118] The fourth matrix on the right side of the equation can be omitted;

[0119] Omitted terms of third order and above (i.e., those containing x) 3 x 4 x 5 According to Formula 4, the second stability condition is:

[0120] 0 < x(4x+1 / m) < 1.

[0121] In one embodiment, for the first case, x2 > 0 if the optical system satisfies the first stability condition.

[0122] In one embodiment, for the first case, if the optical system satisfies the first stability condition, 1≤m≤10, 0.05≤x1≤0.5.

[0123] In application, for the first case, under the condition that the optical system meets the first stability condition, when x2 is negative, in order to ensure that x2(1+2mx1-x1x2)>0, x1 must also be negative and its absolute value must be greater than 1 / 2m, where m ranges from 1 to 10 and x1 ranges from 0.05 to 0.5. In this case, the optical system is configured with a long focal length, which is not conducive to the distribution of light spots. Therefore, x2 can be set to a positive value, that is, the distance between the optical center of the sub-concave mirror and the reflecting surface of the main concave mirror is greater than the focal length of the main concave mirror.

[0124] In one embodiment, for the second case, x > 0 when the optical system satisfies the second stability condition.

[0125] In applications, when the optical system satisfies the second stability condition, if x is negative, in order to ensure that x(4x+1 / m)>0, the absolute value of x is large. In this case, the optical system is configured with a long focal length, which is not conducive to the distribution of light spots. Therefore, x can be set to a positive value, that is, the distance between the optical center of the sub-concave mirror and the reflecting surface of the plane mirror to the reflecting surface of the main concave mirror are both greater than the focal length of the main concave mirror.

[0126] In applications, further analysis shows that, under the condition that the optical system satisfies the stability condition, after the optical signal undergoes N cyclic reflections in the reflecting cavity, the transmission matrix T... N B in N When the value is equal to or close to 0, the position of the output light spot has the least dependence on the angle of the input light, and the optical system is the most stable.

[0127] In one embodiment, for the second case, 0 < x < 0.1, provided that the optical system satisfies the second stability condition.

[0128] In application, for the second case, such as Figure 7 As shown, an example is given where m = 6.37, N = 45, x1 = x2 = x, and x and B are related. N The correspondence between them is shown, where the horizontal axis represents the value of x and the vertical axis represents B. N The value. From Figure 7 As can be seen from this, when x is less than 0, the reflecting cavity is unstable; as x changes from 0 to 1, B... N Oscillates between -2 and 2 (for different optical system structures, B) N The oscillation ranges are also different. Figure 7 (This is just one example) and has multiple zero crossings. Since an over-defocused optical system is detrimental to spot distribution, the value of x can be set to the range of 0 to 0.1. For example, x can take... Figure 7 Any value among multiple zero-crossing points in the equation.

[0129] like Figure 8 As shown, in one embodiment, the input terminal 601, the output terminal 602 and the sub-concave reflector 605 are separately disposed, and the input terminal 601 and the output terminal 602 are both disposed on the planar reflector 604;

[0130] The orthographic projection of the optical center of the concave mirror 605 onto the reflecting surface of the plane mirror 604 is located at the origin.

[0131] like Figure 9 As shown, in one embodiment, the input terminal 601, the output terminal 602 (not shown in the figure) and the sub-concave reflector 605 are separately disposed, and the input terminal 601 and the output terminal 602 are both disposed on the planar reflector 604;

[0132] The orthographic projection of the optical center of the sub-concave mirror 605 onto the reflecting surface of the plane mirror 604 deviates from the origin.

[0133] In application, Figure 8 and Figure 9 The difference in the structure of the dense optical path folding device shown lies in the different positions of the optical center of the sub-concave mirror projected onto the reflecting surface of the plane mirror. This results in different light spot trajectories formed by the optical signal within the reflecting cavity, increasing design flexibility. Figure 9 Compared to Figure 8 The size of the light spot on the plane mirror changes even less.

[0134] like Figure 10 As shown, in one embodiment, the input terminal 601 and the output terminal 602 overlap to form an input-output terminal, and the input-output terminal and the sub-concave reflector 605 are separately disposed and disposed on the planar reflector 604;

[0135] The orthographic projection of the optical center of the concave mirror 605 onto the reflecting surface of the plane mirror 604 is located at the origin.

[0136] like Figure 11 As shown, in one embodiment, the input terminal 601 and the output terminal 602 are separately disposed, with the input terminal 601 disposed on the planar reflector 604 and the output terminal 602 disposed on the concave reflector 603 itself.

[0137] The orthographic projection of the optical center of the sub-concave mirror 605 onto the reflecting surface of the plane mirror 604 is located at the origin;

[0138] The optical signal is input from the input terminal 601, and after multiple reflections between the main concave reflector 603, the plane reflector 604 and the sub-concave reflector 605, it is output from the output terminal 602 to the first converging lens 610, and then converged to the receiving terminal 611 by the first converging lens 610.

[0139] In application, Figure 11 The structure of the dense optical path folding device shown is an application example based on off-axis integrating cavity technology. Since the entire concave mirror is used as the output end, the cross-sectional areas of the first converging lens and the concave mirror in the direction perpendicular to the optical axis must be equivalent (for example, the cross-sectional area of ​​the first converging lens in the direction perpendicular to the optical axis is greater than or equal to the cross-sectional area of ​​the concave mirror in the direction perpendicular to the optical axis) so that the light signal output through the output end can be completely collected by the first converging lens and converged on the receiving surface of the receiving end.

[0140] like Figure 12 As shown, in one embodiment, the input terminal 601 and the output terminal 602 are separately disposed, with the input terminal 601 disposed on the planar reflector 604 and the output terminal 602 being a sub-concave reflector 605;

[0141] The orthographic projection of the optical center of the sub-concave mirror 605 onto the reflecting surface of the plane mirror 604 is located at the origin;

[0142] The optical signal is input from the input terminal 601, and after multiple reflections between the main concave reflector 603, the plane reflector 604 and the sub-concave reflector 605, it is output from the output terminal 602 to the receiving terminal 611.

[0143] In application, Figure 12 The structure of the dense optical path folding device shown is also an application example based on off-axis integrating cavity technology. Since the cross-sectional area of ​​the sub-concave mirror in the direction perpendicular to the optical axis is small, when the area of ​​the receiving surface at the receiving end is large enough (for example, the cross-sectional area of ​​the sub-concave mirror in the direction perpendicular to the optical axis is less than or equal to the area of ​​the receiving surface at the receiving end), the optical signal output through the output end can be completely received by the receiving surface at the receiving end.

[0144] like Figure 12 As shown, in one embodiment, a second converging lens 612 is provided between the output terminal 602 and the receiving terminal 611;

[0145] The optical signal is input from the input terminal 601, and after multiple reflections between the main concave reflector 603, the plane reflector 604 and the sub-concave reflector 605, it is output from the output terminal 602 to the second converging lens 612, and then converged to the receiving terminal 611 by the second converging lens 612.

[0146] In the application, by Figure 12A second converging lens is placed between the output and receiving ends of the dense optical path folding device shown. It is only required that the cross-sectional areas of the second converging lens and the sub-concave mirror in the direction perpendicular to the optical axis be comparable (for example, the cross-sectional area of ​​the second converging lens in the direction perpendicular to the optical axis is greater than or equal to the cross-sectional area of ​​the sub-concave mirror in the direction perpendicular to the optical axis). This allows the light signal output from the output end to be completely collected by the second converging lens and converged to the receiving surface of the receiving end, making the area of ​​the receiving surface of the receiving end smaller, thereby reducing the volume of the receiving end and thus reducing the overall volume of the optical system. Figure 12 The dense optical path folding device shown uses an entire sub-concave mirror as the output end, and the cross-sectional area of ​​the sub-concave mirror perpendicular to the optical axis is smaller than that of the main concave mirror perpendicular to the optical axis. Compared to... Figure 11 The dense optical path folding device shown requires a second converging lens with a smaller cross-sectional area perpendicular to the optical axis than the first converging lens, thus allowing for a smaller overall size of the optical system.

[0147] In one embodiment, Figure 12 The dense optical path folding device shown can be applied to a Raman spectroscopy detection system, where the input terminal is used to input the excitation optical signal.

[0148] The output terminal is used to output Raman optical signals.

[0149] The reflective surface of the concave mirror is coated with a film layer for reflecting excitation light signals and transmitting Raman light signals;

[0150] The excitation light signal is input from the input end and undergoes multiple reflections between the main concave mirror, the plane mirror, and the sub-concave mirror. During these multiple reflections, the photons of the excitation light signal collide with the molecules of the sample under test in an inelastic manner, generating an enhanced Raman light signal. This enhanced Raman light signal is then output from the output end to the receiving end.

[0151] In applications, the sample to be tested in a Raman spectroscopy detection system can be a gas with Raman characteristic spectra, such as nitrogen, hydrogen, or oxygen, and the receiving end can be a spectrometer.

[0152] like Figure 13 As shown, in one embodiment, the input terminal 601 (not shown in the figure), the output terminal 602 and the two sub-concave reflectors 605 are separately disposed, and the input terminal 601 and the output terminal 602 are both disposed on the plane reflector 604.

[0153] The orthographic projections of the two sub-concave mirrors 605 onto the reflecting surface of the plane mirror 604 are spaced apart and deviated from the origin.

[0154] In application, Figure 13The dense optical path folding device shown uses two sub-concave mirrors arranged symmetrically with respect to the optical axis and offset from the origin, compared to a structure using a single sub-concave mirror (e.g., Figure 9 The dense optical path folding device shown makes the light spot trajectory formed by the optical signal in the reflection cavity different, increasing the design flexibility.

[0155] The dense optical path folding device provided in this application uses a plane mirror and a primary concave mirror with aberrations to form a reflecting cavity, and introduces a sub-concave mirror. The sub-concave mirror is placed on top of the plane mirror, and at least one of the reflecting surfaces of the plane mirror and the optical centers of the sub-concave mirror is not on the focal plane of the primary concave mirror. Due to the aberrations of the primary concave mirror, the light signal reflected by the sub-concave mirror is reflected multiple times within the reflecting cavity, and when it returns to the sub-concave mirror, its position and angle do not coincide. Furthermore, due to the different normals at different positions of the sub-concave mirror... The different directions cause the light signal to return to the concave mirror, but the reflection direction is different from the previous one and deviates from the original trajectory direction. This forms a semi-closed trajectory within the reflection cavity, ultimately increasing the total optical path length within the reflection cavity by more than 10 times compared to existing optical path folding devices. This results in a higher optical path-to-volume ratio, allowing for different light spot trajectories formed within the reflection cavity according to actual needs. This design is highly flexible and can be applied to various gas optical sensing devices, such as gas absorption spectroscopy detection devices and Raman spectroscopy detection systems.

[0156] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A compact optical path-folding device, characterized by, include: The input terminal is used to input optical signals; The output terminal is used to output optical signals. A principal concave mirror with a focal length of f, a radius of curvature of R, and aberrations; A plane mirror, wherein the distance from the reflecting surface of the plane mirror to the optical center of the principal concave mirror is L1 = (1 + x1)f, -1 < x1 < 1; A sub-concave reflector with a focal length of f0 and a radius of curvature of R0 is disposed on the plane reflector. The orthographic projection area of ​​the reflector surface of the sub-concave reflector on the reflector surface of the plane reflector is smaller than the area of ​​the reflector surface of the plane reflector. The distance from the optical center of the sub-concave reflector to the optical center of the main concave reflector is L2 = (1 + x2)f, R0 = mR, -1 < x2 < 1, x1 and x2 are not simultaneously 0, and m > 0. The input terminal is disposed on the main concave reflector or the plane reflector, and the output terminal is disposed on the main concave reflector, the plane reflector or the sub-concave reflector. The reflecting surface of the main concave reflector is disposed opposite to the reflecting surface of the plane reflector and the reflecting surface of the sub-concave reflector. The optical signal is input from the input terminal, and after multiple reflections between the main concave mirror, the plane mirror, and the sub-concave mirror, it is output from the output terminal.

2. The dense optical path folding device as described in claim 1, characterized in that, include: The two sub-concave mirrors are arranged symmetrically about the optical axis, and the orthographic projections of the reflecting surfaces of the two sub-concave mirrors onto the reflecting surface of the planar mirror do not coincide. Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror, and 0 < x2(1+2mx1-x1x2) < 2m; The optical signal is input from the input terminal, and after multiple reflections between the main concave mirror, the plane mirror, and the two sub-concave mirrors, it is output from the output terminal.

3. The dense optical path folding device as described in claim 1, characterized in that, The optical center of the sub-concave mirror is projected onto the reflecting surface of the plane mirror at the origin of the optical axis. Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror, the origin is the intersection of the optical axis and the reflecting surface of the plane mirror, and 0 < x2(1+2mx1-x1x2) < 2m.

4. The dense optical path folding device as described in claim 1, characterized in that, The orthographic projection of the reflecting surface of the sub-concave mirror onto the reflecting surface of the planar mirror deviates from the origin of the optical axis. Wherein, the optical axis is perpendicular to the reflecting surface of the plane mirror and passes through the optical center and focal point of the main concave mirror, the origin is the intersection of the optical axis and the reflecting surface of the plane mirror, x1=x2=x, 0<x(4x+1 / m)<1.

5. The dense optical path folding device as described in claim 4, characterized in that, x＞0。 6. The dense optical path folding device as described in claim 5, characterized in that, 0＜x＜0.1。 7. The dense optical path folding device as described in claim 2 or 3, characterized in that, x2＞0； And / or, 1≤m≤10, 0.05≤x1≤0.

5.

8. The dense optical path folding device according to any one of claims 1 to 6, characterized in that, The concave mirror is a spherical mirror with inherent aberrations, or the concave mirror is an aspherical mirror designed with aberrations.

9. The dense optical path folding device as described in claim 3, characterized in that, The input end, the output end, and the sub-concave reflector are separately disposed, and the input end and the output end are both disposed on the planar reflector; Alternatively, the input terminal and the output terminal may overlap to form an input-output terminal, and the input-output terminal and the sub-concave reflector may be separately disposed and disposed on the planar reflector; Alternatively, the input end and the output end are separately configured, with the input end located on the plane mirror and the output end being the main concave mirror; wherein, the optical signal is input from the input end, and after multiple reflections between the main concave mirror, the plane mirror, and the sub-concave mirror, it is output from the output end to the first converging lens, and then converged to the receiving end by the first converging lens; Alternatively, the input end and the output end are separately configured, with the input end located on the planar reflector and the output end being the sub-concave reflector; wherein, the optical signal is input from the input end, and after multiple reflections between the main concave reflector, the planar reflector and the sub-concave reflector, it is output from the output end to the receiving end.

10. The dense optical path folding device as described in claim 2, characterized in that, The input end, the output end, and the two sub-concave mirrors are separately disposed, with the input end and the output end both disposed on the planar mirror.

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