A method for passive optical resonator mode matching based on an additional external cavity

Abstract

The application discloses a passive optical resonant cavity mode matching method based on an additional external cavity, and comprises the following steps: establishing a resonant cavity theoretical model based on actual requirements of a required resonant cavity; the resonant cavity comprises an incident end cavity mirror and an emission end cavity mirror; the radius of collimated laser of the incident resonant cavity is determined according to the resonant cavity theoretical model; the image sensor is used to observe the light spot center of the transmitted light beam passing through the incident end cavity mirror, and the incident end cavity mirror is adjusted to be coaxial with the incident light beam; a resonant cavity is built by a partial mirror and the incident end cavity mirror to obtain an additional external cavity; the position of the incident end cavity mirror is adjusted by using the additional external cavity to realize the matching of the wavefront of the incident light beam and the surface curvature of the incident end cavity mirror; the image sensor is used to observe the resonant cavity transmission light mode image, and the position of the emission end cavity mirror is adjusted to realize the resonant cavity mode matching. The method can accurately judge the wavefront matching condition of the resonant cavity mirror surface by constructing an additional resonant cavity; the sequence of optical path adjustment is clear, and the mode matching process is simple and efficient.

Description

TECHNICAL FIELD

[0001] The application relates to a method for adjusting an optical resonant cavity, and belongs to the technical field of laser optical path adjustment. BACKGROUND

[0002] An optical resonant cavity is a cavity formed by reflecting mirrors with a certain distance, and light waves of a specific frequency can be reflected back and forth in the cavity to form a closed path and establish a stable mode distribution, so that light frequency and mode selection can be realized, and the optical resonant cavity is widely used in optical devices such as lasers and optical parametric oscillators. Mode matching of a passive optical resonant cavity is of great significance. When the mode of an incident light beam matches the eigenmode of the resonant cavity, the maximum energy coupling efficiency can be obtained, at which time the beam waist of the incident light beam and the beam waist of the eigenmode of the resonant cavity have the same size and position. At the same time, the wavefront curvature radius of the incident light beam at the resonant cavity mirror is consistent with the curvature radius of the mirror reflecting surface. In actual operation, the parameters of the eigenmode of the resonant cavity are difficult to accurately determine, and the incident light beam has more adjustment degrees, which brings technical challenges to efficient mode matching.

[0003] In order to reduce the difficulty of mode matching adjustment, the prior art provides the following scheme: a method for adjusting a mode excitation light path of a passive optical resonant cavity is disclosed in Chinese Patent CN105811232A, which uses an adjustable diaphragm for coarse adjustment and uses an oscilloscope to observe the transmission spectrum for fine adjustment. However, in the process of fine adjustment of the mode matching, the position of the pre-cavity matching lens, the resonant cavity incident end mirror and the transmission end mirror need to be adjusted jointly, and the best positions of the three may need to be determined through multiple repeated adjustments, which increases the debugging difficulty and workload. Therefore, a more efficient and more accurate resonant cavity mode matching method is needed. SUMMARY

[0004] The purpose of the present application is to solve the above problems, and a passive optical resonant cavity mode matching method based on an additional external cavity is provided. First, the best position of the pre-cavity matching lens is obtained by constructing an additional optical resonant cavity, and the wavefront of the incident laser beam is matched with the surface curvature of the resonant cavity incident end mirror. On this basis, the resonant cavity transmission light mode image is observed by using an image sensor to realize the adjustment of the resonant cavity exit end mirror, so that efficient resonant cavity mode matching adjustment is realized.

[0005] The application adopts the following technical scheme:

[0006] A passive optical resonant cavity mode matching method based on an additional external cavity comprises the following steps:

[0007] S1, a theoretical model of a resonant cavity is established based on actual requirements of a required resonant cavity, the resonant cavity comprising an incident end mirror and an exit end mirror, and a radius of collimated laser light incident to the resonant cavity is determined according to the theoretical model of the resonant cavity;

[0008] S2, a center of a light spot of a transmitted light beam passing through the incident end mirror is observed by using an image sensor, and the incident end mirror is adjusted to be coaxial with the incident light beam;

[0009] S3, a resonant cavity is built by using a partial mirror and the incident end mirror to obtain an additional external cavity, and a position of the incident end mirror is adjusted by using the additional external cavity to realize matching of a wave front of the incident light beam and a surface curvature of the incident end mirror;

[0010] S4, a mode image of transmitted light of the resonant cavity is observed by using the image sensor, and a position of the exit end mirror is adjusted to realize mode matching of the resonant cavity.

[0011] Further, the step S1 comprises the following sub-steps:

[0012] (1) a theoretical model of the resonant cavity is established: eigenmode parameters of the resonant cavity are calculated according to geometric parameters of the resonant cavity, a focal length of a matching lens is selected according to actual requirements, and a theoretical position of the matching lens relative to the resonant cavity is determined;

[0013] (2) the radius of the collimated laser light incident to the resonant cavity is determined according to the focal length of the matching lens and the eigenmode parameters of the resonant cavity: a transform lens group satisfying a transform multiple is selected according to a measured beam waist size of an output light beam of a laser to expand or shrink the output light beam of the laser, so that the beam waist size of the incident collimated laser after passing through the matching lens matches the beam waist size of a resonant cavity eigenmode.

[0014] Further, the step S2 comprises the following sub-steps:

[0015] (1) a partial mirror is placed in an optical path, and an angle of the partial mirror is adjusted to make the partial mirror perpendicular to the incident laser light beam;

[0016] (2) a first 45° mirror and a second 45° mirror are placed, the direction of the optical path is adjusted by using the first 45° mirror and the second 45° mirror, the optical axis of the light beam passing through the two mirrors is made parallel to the surface of an optical platform, and the height of the light beam is adjusted;

[0017] (3) the matching lens is adjusted to be coaxial with the incident light beam: first, an image sensor is placed downstream of the optical path to record the position of the light spot center at this time; then, the matching lens is placed at the theoretical position relative to the resonant cavity in the optical path, the matching lens is moved forward and backward along the optical axis direction of the light beam, and the movement of the light spot on the image sensor is observed; the height and angle of the optical axis of the matching lens are adjusted, and when the position of the light spot remains unchanged during the forward and backward movement of the matching lens, the coaxial adjustment of the matching lens and the incident light beam is completed;

[0018] (4) Adjusting the coaxiality of the incident end mirror of the resonant cavity and the incident light beam: first, place the image sensor downstream of the light path and record the position of the light spot at this time; then place the incident end mirror fixed with the piezoelectric driver at the theoretical position of the light path, and the adjustment dimension of the piezoelectric driver is along the light beam optical axis direction; when the insertion of the incident end mirror does not change the center position of the transmitted light beam on the display screen, the coaxial adjustment of the incident end mirror and the incident light beam is completed.

[0019] Further, the step S3 comprises the following sub-steps:

[0020] (1) Build an additional external cavity: the partially reflecting mirror and the reflecting surface of the incident end mirror form a resonant cavity, which is called an additional external cavity;

[0021] (2) Wavefront matching adjustment using the additional external cavity: place the image sensor downstream of the light path, apply a triangular wave voltage on the piezoelectric driver to scan the cavity length of the additional external cavity, observe the transmitted light beam of the additional external cavity through the image sensor, adjust the first 45° mirror and the second 45° mirror until the pattern on the image sensor is a circularly symmetric multi-turn fringe, at this time it indicates that the angle adjustment of the incident light beam meets the requirements; then adjust the position of the matching lens through the first one-dimensional displacement table to adjust the fringe in the direction of reducing the number of turns until it gradually becomes a flickering solid circular light spot, and the wavefront of the incident light beam and the reflecting surface of the incident end mirror are matched.

[0022] Further, the step S4 is specifically:

[0023] Remove the partially reflecting mirror from the light path, install the exit end mirror of the resonant cavity, apply a triangular wave voltage on the piezoelectric driver to scan the cavity length of the resonant cavity, and observe the light pattern of the exit light beam of the resonant cavity with the image sensor; adjust the angle and position of the exit end mirror until the observed light pattern is a flickering circularly symmetric light spot, indicating that the incident light beam is coupled to the fundamental mode of the resonant cavity, and thus the mode matching adjustment of the resonant cavity is completed.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (1) The present application can accurately judge the wavefront matching condition of the resonant cavity mirror surface by constructing an additional resonant cavity, avoiding the problem of blindly adjusting multiple optical elements when directly coupling the light beam into the resonant cavity.

[0026] (2) The order of light path adjustment of the present application is clear, and the mode matching process is simple and efficient. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a schematic diagram of the light path coupling system used in the passive optical resonant cavity mode matching method based on an additional external cavity;

[0028] Wherein:

[0029] 1-laser, 2-laser output beam, 3-transform lens group, 4-collimated laser, 5-partial mirror reflected light, 6-partial mirror, 7-partial mirror reflection surface, 8-additional external cavity internal resonance light beam, 9-first 45° mirror, 10-second 45° mirror, 11-first one-dimensional displacement table, 12-matching lens, 13-piezoelectric driver, 14-distance between matching lens and flat-concave lens concave surface, 15-incoming end cavity mirror, 16-incoming end cavity mirror reflection surface, 17-resonant cavity length, 18-outgoing end cavity mirror reflection surface, 19-outgoing end cavity mirror, 20-second one-dimensional displacement table, 21-image sensor. DETAILED DESCRIPTION

[0030] The application will be described in detail below with reference to the accompanying drawings.

[0031] A passive optical resonant cavity mode matching method based on an additional external cavity, comprising the following steps:

[0032] S1, establishing a resonant cavity model and a laser output beam transformation step, comprising:

[0033] (1) Establishing a resonant cavity theoretical model. The resonant cavity is composed of an incoming end cavity mirror 15 and an outgoing end cavity mirror 19. According to the resonant cavity length 17, the curvature radius of the incoming end cavity mirror reflection surface 16 and the outgoing end cavity mirror reflection surface 18, the eigenmode parameters of the resonant cavity are calculated, the focal length of the matching lens 12 is selected according to the actual demand, and the theoretical position of the matching lens 12 relative to the resonant cavity is determined.

[0034] (2) According to the focal length of the matching lens 12 and the eigenmode parameters of the resonant cavity, the radius of the collimated laser 4 incident into the resonant cavity is determined. According to the measured beam waist size of the laser output beam 2, the transform lens group 3 that meets the transformation multiple is selected to expand or shrink the laser output beam 2, so that the beam waist size of the collimated laser 4 after passing through the matching lens 12 matches the beam waist size of the resonant cavity mode.

[0035] S2, optical element and light beam coaxial adjustment step, comprising:

[0036] (1) Placing a partial mirror 6 in the optical path. The optical center height of the partial mirror 6 is adjusted to be the same as the optical path. The angle of the partial mirror 6 is adjusted so that the partial mirror reflected light 5 reflected by the collimated laser 4 is basically coincided with the spatial distribution of the incident collimated laser 4, which indicates that the partial mirror reflection surface 7 is perpendicular to the optical axis direction of the collimated laser 4. The partial mirror 6 is also the incoming cavity mirror of the additional external cavity.

[0037] (2) Put the first 45° mirror 9 and the second 45° mirror 10, and adjust the direction of the light path with the two mirrors, so that the optical axis of the light beam passing through the mirrors is parallel to the surface of the optical platform, and the height of the light beam is adjusted.

[0038] (3) Adjust the matching lens 12 to be coaxial with the incident light beam. First, place the image sensor 21 downstream of the light path, and mark the position of the center of the light spot at this time. Then install the matching lens 12 on the first one-dimensional displacement table 11, and adjust the dimension of the displacement table along the optical axis of the light beam. Adjust the height of the optical axis of the matching lens 12 to be equal to the height of the optical axis of the light beam, and place it at the theoretical position in the light path relative to the resonant cavity. Then adjust the coaxiality of the matching lens 12. Take the marked position of the center of the light spot on the image sensor 21 without the matching lens 12 as the reference, and adjust the first one-dimensional displacement table 11 to move the matching lens 12 forward and backward along the light path. Adjust the angle of the matching lens 12 until the position of the light spot on the image sensor 21 does not change, indicating that the movement of the matching lens 12 does not change the direction of the incident light beam, and the coaxial adjustment of the matching lens 12 is completed.

[0039] (4) Adjust the incident end cavity mirror 15 to be coaxial with the incident light beam. Place the incident end cavity mirror 15 with the bonded piezoelectric driver 13 in the light path. When the insertion of the incident end cavity mirror 15 does not change the center position of the transmitted light beam on the display screen, the coaxial adjustment is considered complete.

[0040] S3, wavefront matching adjustment step at the incident end cavity mirror reflecting surface 16, including:

[0041] (1) Additional external cavity construction. The partially reflecting mirror reflecting surface 7 and the incident end cavity mirror reflecting surface 16 form a resonant cavity, which is called an additional external cavity. The additional external cavity can be used to evaluate the wavefront matching at the incident end cavity mirror reflecting surface 16. In the additional external cavity, part of the collimated light beam passes through the partially reflecting mirror 6, the first 45° mirror 9, the second 45° mirror 10, the matching lens 12, and the incident end cavity mirror 15, is reflected by the incident end cavity mirror reflecting surface 16, and when the reflected light beam returns along the original path, a large difference between the diameter of the reflected light spot and the diameter of the incident light beam may be observed, indicating that the incident light beam and the wavefront at the incident end cavity mirror reflecting surface 16 are not matched, and the image sensor 21 will also observe a circularly symmetric interference pattern.

[0042] (2) Wavefront matching adjustment using the additional external cavity. The matching lens 12 is a key optical element for focusing the collimated light beam to achieve wavefront matching, and its position directly affects the matching efficiency of the incident light beam and the fundamental mode of the resonant cavity, which is the reason for configuring the first one-dimensional displacement table 11 under the matching lens. First, place the image sensor 21 at a suitable position downstream of the resonant cavity exit, so that the size of the light spot on the display is easy to observe. For example, the size of the light spot on the display is about 1 / 3 of the size of the light spot on the image sensor 21, which is convenient for observation. Figure 1The matching of the wavefront can be judged by observing the additional external cavity transmission light interference pattern through the image sensor 21. During the adjustment, a triangular wave voltage is applied on the piezoelectric driver 13 to scan the cavity length of the additional external cavity, and the transmission light beam of the additional external cavity is observed through the image sensor 21. The first 45° mirror 9 and the second 45° mirror 10 are adjusted until the pattern on the image sensor 21 is a circularly symmetric multi-turn fringe, indicating that the angle adjustment of the incident light beam meets the requirements, but the matching lens 12 is not placed at the optimal position, and the wavefront at the incident end cavity mirror reflecting surface 16 is not matched. Then the position of the matching lens 12 is adjusted through the first one-dimensional displacement table 11, and the fringe is adjusted in the direction of decreasing the number of turns until it gradually becomes a flickering solid circular spot, at which time the wavefront of the incident light beam is matched with the incident end cavity mirror reflecting surface 16.

[0043] S4, resonant cavity exit end cavity mirror 19 adjustment step, comprising:

[0044] (1) After the wavefront matching at the incident end cavity mirror reflecting surface 16 is completed, the partial mirror 6 is removed from the optical path. Another exit end cavity mirror 19 constituting the resonant cavity is placed on the second one-dimensional displacement table 20 to fine-tune the position of the exit end cavity mirror 19. According to the theoretical calculation result of step S1, the exit end cavity mirror 19 is placed at the theoretical position in the optical path, which constitutes a resonant cavity with the incident end cavity mirror 15. During the fine adjustment, a triangular wave voltage is applied on the piezoelectric driver 13 to scan the cavity length of the resonant cavity, and a multi-mode spot image can be observed on the image sensor 21 when the angle or position of the exit end cavity mirror 19 is mismatched. At this time, the angle and position of the exit end cavity mirror 19 can be fine-tuned, and adjusted in the direction of gradually decreasing the light mode order and the mode number until the resonant cavity exit light beam becomes a flickering solid circular spot, indicating that most of the incident light beam is coupled to the fundamental mode of the resonant cavity, and the mode matching adjustment of the resonant cavity is achieved.

Claims

1. A method of passive optical resonator mode matching based on an additional external cavity, characterized in that, The method comprises the following steps: S1, a theoretical model of a resonant cavity is established based on actual requirements of the resonant cavity, the resonant cavity comprising an incident end mirror and an exit end mirror, and a radius of a collimated laser beam of the incident resonant cavity is determined according to the theoretical model of the resonant cavity; S2, a light spot center of a transmitted light beam passing through the incident end mirror is observed by using an image sensor, and the incident end mirror is adjusted to be coaxial with the incident light beam; S3, an additional external cavity is obtained by building a resonant cavity through a partial mirror and the incident end mirror, and a position of the incident end mirror is adjusted by using the additional external cavity to realize matching of a wave front of the incident light beam and a surface curvature of the incident end mirror; S4, a mode image of the resonant cavity is observed by using the image sensor, and a position of the exit end mirror is adjusted to realize mode matching of the resonant cavity; The step S2 comprises the following sub-steps: (1) a partial mirror is placed in an optical path, and an angle of the partial mirror is adjusted to make the partial mirror perpendicular to the incident laser beam; (2) a first 45° mirror and a second 45° mirror are placed, the first 45° mirror and the second 45° mirror are used to adjust a direction of the optical path, the optical axis of the light beam passing through the two mirrors is parallel to a surface of an optical platform, and equal-height adjustment of the light beam is realized; (3) a matching lens is adjusted to be coaxial with the incident light beam: first, the image sensor is placed downstream of the optical path, and a light spot center position at this time is recorded; then, the matching lens is placed at a theoretical position of the resonant cavity in the optical path, the matching lens is moved forward and backward along the optical axis direction of the light beam, and light spot movement on the image sensor is observed; the height and angle of the optical axis of the matching lens are adjusted, and when the light spot position remains unchanged during the forward and backward movement of the matching lens, the coaxial adjustment of the matching lens and the incident light beam is completed; (4) the incident end mirror is adjusted to be coaxial with the incident light beam: first, the image sensor is placed downstream of the optical path, and a light spot position at this time is recorded; then, the incident end mirror fixed with a piezoelectric driver is placed at a theoretical position of the optical path, and the adjustment dimension of the piezoelectric driver is along the optical axis direction of the light beam, and when the insertion of the incident end mirror does not change the center position of the transmitted light beam on the display screen, the coaxial adjustment of the incident end mirror and the incident light beam is completed; The step S3 comprises the following sub-steps: (1) an additional external cavity is built: a reflecting surface of the partial mirror and a reflecting surface of the incident end mirror form a resonant cavity, which is called the additional external cavity; (2) the additional external cavity is used for wave front matching adjustment: the image sensor is placed downstream of the optical path, a triangular wave voltage is applied to the piezoelectric driver to scan the cavity length of the additional external cavity, the transmitted light beam of the additional external cavity is observed by using the image sensor, the first 45° mirror and the second 45° mirror are adjusted, until the image on the image sensor is a circularly symmetric multi-turn fringe, at this time, it is indicated that the angle adjustment of the incident light beam meets the requirements; then, the position of the matching lens is adjusted by using the first one-dimensional displacement table, the fringe is adjusted in the direction of decreasing the number of turns, until it gradually becomes a flashing solid circular light spot, and the wave front of the incident light beam and the reflecting surface of the incident end mirror are matched; The step S4 is specifically: Part of the mirror is removed from the optical path, the exit mirror of the resonant cavity is installed, a triangular wave voltage is applied to the piezoelectric driver for scanning the cavity length of the resonant cavity, and the light mode of the resonant cavity exit light beam is observed by using an image sensor; the angle and position of the exit mirror are adjusted until a flickering circular symmetric light spot is observed, indicating that the incident light beam is coupled to the fundamental mode of the resonant cavity, and thus the mode matching of the resonant cavity is completed.

2. The method of claim 1, wherein the additional external cavity is based on passive optical resonator mode matching. The step S1 comprises the following sub-steps: (1) establishing a theoretical model of the resonant cavity: calculating the eigenmode parameters of the resonant cavity according to the geometric parameters of the resonant cavity, selecting the focal length of the matching lens according to actual requirements, and determining the theoretical position of the matching lens relative to the resonant cavity; (2) determining the radius of the collimated laser incident to the resonant cavity according to the focal length of the matching lens and the eigenmode parameters of the resonant cavity: selecting a transform lens group that satisfies the transformation multiple according to the measured beam waist size of the laser output beam to expand or shrink the laser output beam, so that the beam waist size of the incident collimated laser matches the beam waist size of the resonant cavity after passing through the matching lens.

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