Alignment method for resonant cavity of beam expanding telescope

Abstract

The invention discloses an alignment method for a resonant cavity of a beam-expanding telescope, and relates to the technical field of beam-expanding telescopes. The method comprises the following steps: when no beam expander exists, enlarging the plane mirror to maximize the power of the through hole diaphragm and make the through hole diaphragm vertical to a light beam; and adjusting the third reflector to the CCD to form circular interference to obtain a reference light path. After the beam expander is arranged, the first reflector and the second reflector are finely adjusted to enable the light beam to penetrate through the small hole again, the CCD reproduces the circular ring, and incidence-telescope axis alignment is completed; the first cavity mirror and the second cavity mirror are sequentially added, pitching / yawing fine adjustment is conducted, and circular ring interference is kept, namely resonant cavity alignment is achieved. The difficulty of parallel alignment between the input light beam and the optical axis of the telescope and the difficulty of alignment between the endoscope and the telescope can be effectively reduced.

Description

Technical Field

[0001] This application relates to the field of beam expander telescope technology, and in particular to an alignment method for a beam expander telescope resonant cavity. Background Technology

[0002] To measure the stability of a beam expander telescope using a resonant cavity-based method, the beam expander telescope needs to form a stable resonant cavity. Therefore, aligning the beam expander telescope becomes a challenge. Commonly used two-mirror resonant cavities achieve coarse alignment of the two mirror centers, then adjust the beam translation and direction by adjusting mirrors along the input beam path, and finally precisely align the beam and resonant cavity by observing the spot of the transmitted light. Commonly used multi-mirror resonant cavities employ methods such as calculating coupling efficiency, observing higher-order modes, ensuring the accuracy of the support structure, and adjusting the external beam direction to achieve alignment and mode matching between the input light and the resonant cavity. However, for telescope resonant cavities, these methods cannot determine whether the beam is near the zero-degree field of view of the telescope or whether the input beam is parallel to the telescope's optical axis. Furthermore, because the telescope's assembly and adjustment are not perfect, although the beam direction can be returned to its original state after beam expansion by adjusting the rear cavity mirror, changes in the position and size of the beam waist will cause significant changes in the beam during expansion and contraction, making it impossible to achieve resonance between the input beam and the telescope resonant cavity.

[0003] In existing technologies, alignment of two-mirror straight cavities and multi-mirror folded or annular cavities is commonly used. Common resonator alignment methods include observing the transmitted light spot of the resonator, designing a structure with a high error tolerance, or aligning the mirror positions based on image recognition. However, when the above multi-mirror cavity alignment methods are applied to telescope resonators, alignment between the mirrors and the telescope cannot be achieved.

[0004] Therefore, there is an urgent need for a method for aligning the resonant cavity of a beam expander telescope to reduce the difficulty of parallel alignment between the input beam and the telescope optical axis, as well as the difficulty of alignment between the cavity mirror and the telescope. Summary of the Invention

[0005] Therefore, it is necessary to provide an alignment method for the resonant cavity of a beam expander telescope to address the aforementioned technical problems.

[0006] The present invention adopts the following technical solution: This invention provides a method for aligning the resonant cavity of a beam expander telescope, comprising: Without placing the beam expander, adjust the large plane mirror to maximize the light power of the incident beam through the small aperture stop, and make the large plane mirror perpendicular to the direction of the incident light; adjust the third reflecting mirror until alternating bright and dark circular interference fringes appear on the CCD detector to obtain the aligned optical path. Place the beam expander telescope into the alignment optical path, and adjust the first and second reflecting mirrors to make the incident beam pass through the small aperture aperture. Observe the interference fringes on the CCD detector until the beam expander telescope appears as a circular interference fringe when placed into the alignment optical path, thus completing the alignment of the incident beam with the telescope's optical axis. The first and second cavity mirrors are added to the resonant cavity of the beam expander telescope one after another; the tilt angles of the first and second cavity mirrors in the pitch and yaw directions are adjusted one after another until the CCD maintains the circular interference fringes, thus completing the alignment of the resonant cavity of the beam expander telescope.

[0007] Preferably, a small aperture stop is used to determine the initial position of the incident beam and whether the reflected light from the incident beam on the large plane mirror returns along the original path.

[0008] Preferably, when the optical power passing through the small aperture aperture reaches its maximum, the plane of the large plane mirror is perpendicular to the direction of the incident beam, and the reflected light of the incident beam on the large plane mirror returns along the original path.

[0009] Preferably, the alignment of the incident beam with the telescope's optical axis is determined by setting a reference beam and interfering with the aligned beam, and by observing the shape of the interference fringes on the CCD, specifically including: The output beam of the laser, after being collimated, passes sequentially through a λ / 2 waveplate, a lens, and a first reflecting mirror, so that the polarizing beam splitter has reflected light and transmitted light, namely the reference beam and the aligned beam; The reference beam passes sequentially through a λ / 4 waveplate and a third reflecting mirror before returning to the polarizing beam splitter. The aligned beam passes sequentially through a λ / 4 waveplate, a second reflecting mirror, a small aperture stop, a beam expander, and a large plane mirror before returning to the polarizing beam splitter. The return beams of the reference beam and the alignment beam are combined by a polarizing beam splitter and then pass through a polarizer, displaying interference fringes on the CCD detector surface.

[0010] Preferably, when the alignment beam coincides with the reference beam, the interference fringes displayed on the CCD detection surface are circular fringes.

[0011] Preferably, the beam expander telescope has an off-axis dual-reflector structure, and its lenses include parabolic or hyperboloid aspherical mirrors.

[0012] Preferably, the size of the large plane mirror is larger than the beam cross-section of the expanded beam telescope.

[0013] The above-mentioned at least one technical solution adopted in this invention can achieve the following beneficial effects: In the alignment method of the resonant cavity of a beam expander telescope provided by this invention, the optical power of the incident beam passing through the small aperture stop is maximized by adjusting the large plane mirror, and the third reflecting mirror is adjusted to present circular interference fringes on the CCD detector. An alignment optical path is pre-established, ensuring that the large plane mirror is perpendicular to the incident light direction. The beam expander telescope is pre-assembled and fully adjusted using a wave aberration detection method, avoiding wave aberration interference introduced by the beam expander element when the input beam is aligned with the telescope optical axis. This significantly reduces the complexity of telescope assembly and resonant cavity alignment, and reduces the need for repeated manual adjustments. After the beam expander telescope is placed, the direction of the incident beam is changed by adjusting the first and second reflecting mirrors, allowing the incident beam to pass through the small aperture stop. The CCD interference fringes are observed until the beam expander telescope presents a circular characteristic when placed in the alignment optical path. This directly corrects the beam offset or distortion caused by the telescope's beam expanding function due to the offset of the incident beam direction, effectively reducing the difficulty of aligning the optical axis of the beam expander telescope resonant cavity. This not only achieves precise alignment of the input beam with the telescope optical axis but also improves the stability of the entire system. After adding the first and second cavity mirrors, the circular interference fringes on the CCD are maintained by adjusting their tilt angles in the pitch and yaw directions, respectively. Since wavelet aberration has been pre-corrected during the telescope assembly and adjustment stage, resonant cavity alignment can be completed with only fine-tuning of the cavity mirrors. This method decouples resonant cavity alignment from telescope assembly and adjustment, avoiding the mutual interference between the two in traditional methods, thus significantly reducing the difficulty of alignment.

[0014] In summary, by aligning the telescope resonator after completing the telescope wavefront aberration adjustment, this invention effectively reduces the difficulty of telescope assembly and adjustment and resonator alignment during the telescope resonator alignment process, solves the influence of the telescope beam expansion function, and achieves alignment of the input beam with the telescope optical axis. This effectively reduces the difficulty of parallel alignment between the input beam and the telescope optical axis, as well as the difficulty of alignment between the cavity mirror and the telescope. Attached Figure Description

[0015] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0016] Figure 1 A schematic flowchart illustrating an alignment method for a beam expander telescope resonator provided by the present invention; Figure 2 A schematic diagram of the beam expander telescope resonator structure for an alignment method of the beam expander telescope resonator provided by the present invention; Figure 3 A schematic diagram of beam propagation when the incident beam of a telescope is misaligned, which is part of the alignment method for the resonant cavity of a beam expander provided by the present invention. Figure 4A diagram illustrating the alignment scheme of a telescope resonator, provided by the present invention, for an alignment method of a beam expander telescope resonator. Figure 5 Interference fringes of the alignment beam and the reference beam in an alignment method for a beam expander telescope resonator provided by the present invention. Figure 6 The present invention provides a method for aligning the resonant cavity of a beam expander telescope, including a flowchart of the telescope resonant cavity alignment process. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments in the specification without creative effort are within the scope of protection of this application.

[0018] Existing technologies are commonly used for aligning two-mirror straight cavities and multi-mirror folded or annular cavities. A similar technical solution to this invention focuses on aligning annular cavities. Common alignment methods include observing the transmitted light spot of the resonant cavity, designing structures with high error tolerance, or aligning the mirror positions based on image recognition. However, when these multi-mirror cavity alignment methods are applied to telescope resonant cavities, the following problems arise: ① The mirror curvature radii of commonly used multi-mirror cavities are large, having little impact on Gaussian beam transformation and not involving beam expansion; ② Beam-expanding telescopes often use aspherical structures such as parabolic or hyperboloid surfaces, which significantly affect Gaussian beam transformation; ③ Common multi-mirror cavities can be aligned sequentially, but this cannot be achieved with telescope resonant cavities. The telescope must be aligned first to meet wavefront aberration requirements before the mirrors can be aligned.

[0019] Based on the above problems, the technical solution adopted in this invention is to align the cavity mirror after completing the telescope wave aberration adjustment, which reduces the difficulty of telescope adjustment and resonant cavity alignment during the telescope resonant cavity alignment process, solves the influence of the telescope beam expansion function, and realizes the alignment of the input beam with the telescope optical axis.

[0020] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0021] Figure 1 This is a schematic flowchart of an alignment method for a beam expander telescope resonator according to the present invention, which specifically includes the following steps: S101: The laser emits an alignment laser; S102: The alignment laser sequentially passes through the first rotating half-wave plate, lens, and first reflecting mirror, and is then split into a reference beam and an alignment beam by a polarizing beam splitter. The alignment beam sequentially passes through the second rotating half-wave plate, second reflecting mirror, small aperture stop, and large plane mirror, and then returns to the polarizing beam splitter along its original path. The reference beam sequentially passes through the third rotating half-wave plate and third reflecting mirror, and then returns to the polarizing beam splitter along its original path. The reference beam and the returning beam of the alignment beam are combined by the polarizing beam splitter and then sequentially pass through a polarizer and a CCD detector to form interference fringes.

[0022] Specifically, the alignment of the incident beam with the telescope's optical axis is determined by setting up a reference beam and a alignment beam interference, and observing the shape of the interference fringes on the CCD. This process includes: the laser's output beam, after passing through a collimator, sequentially passes through a λ / 2 waveplate, a lens, and a first reflecting mirror, resulting in a polarizing beam splitter having reflected and transmitted light, i.e., a reference beam and an alignment beam; the reference beam sequentially passes through a λ / 4 waveplate and a third reflecting mirror before returning to the polarizing beam splitter; the alignment beam sequentially passes through a λ / 4 waveplate, a second reflecting mirror, a small aperture stop, a beam expander, and a large plane mirror before returning to the polarizing beam splitter; the returning beams of the reference beam and the alignment beam are combined by the polarizing beam splitter, pass through a polarizer, and display interference fringes on the CCD's detection surface.

[0023] When the aligned beam coincides with the reference beam, the interference fringes displayed on the CCD detector are circular fringes.

[0024] Among them, the beam expander telescope is an off-axis two-reflector structure, and its lenses include parabolic or hyperboloid aspherical mirrors.

[0025] Among them, the size of the large plane mirror is larger than the beam cross-section of the beam expanded by the beam expander telescope.

[0026] Among them, the small aperture stop is used to determine the initial position of the incident beam and whether the reflected light of the incident beam on the large plane mirror returns along the original path.

[0027] Specifically, when the light power passing through the small aperture aperture reaches its maximum, the plane of the large plane mirror is perpendicular to the direction of the incident beam, and the reflected light of the incident beam on the large plane mirror returns along the original path.

[0028] Specifically, the telescope resonant cavity consists of a beam expander telescope and two cavity mirrors (C1, C2), such as Figure 2 As shown. To achieve alignment of the telescope's resonant cavity, it is necessary to ensure the parallelism of the input beam and the telescope's optical axis. For example... Figure 3As shown, taking an off-axis double-reflector beam expander telescope as an example, a large plane mirror is used to assist in aligning the telescope's optical axis with the parallelism of the input beam. First, a large plane mirror is placed at the front end of the expanded beam, with its surface perpendicular to the direction of the incident beam. A small aperture stop is placed along the path of the incident beam to determine its initial position and to check whether the reflected light from the large plane mirror returns along its original path, ensuring the large plane mirror is perpendicular to the direction of the incident light. When the incident beam is parallel to the telescope's optical axis, the expanded beam returns along its original path after reflection by the large plane mirror, as shown by the solid black line. When the incident beam is not parallel to the telescope's optical axis, the large plane mirror is adjusted to be perpendicular to the direction of the incident beam. However, the beam reflected back by the large plane mirror has an angle relative to the direction of the incident beam, and the angle between the returning beam and the optical axis is greater than the angle between the incident beam and the optical axis, as shown by the dashed black line.

[0029] S103: Adjust the large plane mirror so that the optical power of the alignment beam through the small aperture aperture reaches the maximum. At this time, the large plane mirror is perpendicular to the direction of the alignment beam. Adjust the third reflecting mirror so that the reference beam and the alignment beam appear as circular interference fringes on the CCD detector after beam combining.

[0030] S104: Place the beam expander between the large plane mirror and the small aperture stop, adjust the first and second reflecting mirrors to make the alignment beam pass through the small aperture stop until the CCD detector shows circular interference fringes, thus completing the alignment of the alignment beam with the optical axis of the telescope.

[0031] Specifically, the alignment scheme of the telescope resonator is as follows: Figure 4 As shown, an off-axis two-reflector telescope is used as an example. A small aperture stop (SDA) is used for coarse alignment of the incident beam and the telescope's optical axis, maximizing the returned light power. An interference pattern for the reference beam and the alignment beam is designed; the alignment is confirmed by observing the shape of the interference fringes on the CCD, achieving fine alignment between the incident beam and the telescope's optical axis. The laser's output beam, after collimation, passes through a rotating half-wave plate (λ / 2), a lens (Len1), and a plane mirror 1 (PM1), causing the polarizing beam splitter (PBS) to have reflected light (reference beam) and transmitted light (alignment beam). The reference beam returns to the polarizing beam splitter via a λ / 4 wave plate and a plane mirror (PM3), while the alignment beam returns to the polarizing beam splitter via a λ / 4 wave plate, a plane mirror (PM2), the small aperture stop (SDA), the telescope, and a large plane mirror (LPM). The returned beams of the reference beam and the alignment beam are combined by the polarizing beam splitter and then observed as interference fringes on the CCD's detection surface via a polarizer (P1). Figure 5 As shown, when there is an angle between the aligned beam and the telescope optical axis, the returned light appears on the CCD as a non-circular spot, and the interference fringes are semi-circular or straight.

[0032] S105: Add the first cavity mirror and the second cavity mirror to the beam expander telescope resonant cavity in sequence; adjust the tilt angles of the first cavity mirror and the second cavity mirror in the pitch and yaw directions in sequence until the CCD detector continuously presents circular interference fringes, thus completing the alignment of the beam expander telescope resonant cavity.

[0033] Specifically, the alignment process of the telescope resonator is as follows: Figure 6 As shown, before placing the telescope, first adjust the LPM to maximize the optical power of the returned light passing through the SDA. Then adjust the third reflecting mirror PM3 until alternating bright and dark circular fringes are observed on the CCD. Finally, move the CCD back and forth to observe whether the circular fringes change. If alternating bright and dark semi-circular or straight fringes appear, there is an angle between the reference beam and the alignment beam, and the above alignment steps need to be repeated until the alignment beam and the reference beam coincide. Place the telescope in the alignment optical path, align the telescope position, and then coarsely align the telescope so that the returned beam passes through the SDA. Then, achieve fine alignment by adjusting the telescope position. When the alignment beam is not circular, it means that there is a deviation in the direction of the incident beam. Adjust PM1 to adjust the beam direction, and adjust PM2 to maximize the optical power of the incident beam passing through the SDA, thus achieving the beam direction adjustment. Then remove the telescope and readjust the LPM and PM3 to complete the alignment of the alignment beam and the reference beam. Repeat the above steps until the fringes observed on the CCD after connecting the telescope are circular fringes, that is, the alignment of the incident beam and the telescope optical axis is completed. Finally, cavity mirrors C2 and C1 are added in sequence, and the tilt of the two degrees of freedom of the cavity mirrors is adjusted so that the interference fringes on the CCD are in a circular shape to align with the cavity mirrors, thus completing the alignment of the telescope resonant cavity.

[0034] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this invention.

Claims

1. A method for aligning a beam expander telescope resonator, wherein the optical path system used in the method comprises: A laser, a beam expander, a large plane mirror, a polarizing beam splitter, a small aperture stop, a first reflecting mirror, a third reflecting mirror, a second reflecting mirror, a first rotating half-wave plate, a second rotating half-wave plate, a third rotating half-wave plate, a CCD detector, a first cavity mirror, and a second cavity mirror, characterized in that it comprises: The laser emits a targeting laser; The alignment laser beam passes sequentially through a first rotating half-wave plate, a lens, and a first reflecting mirror, and is then split into a reference beam and an alignment beam by a polarizing beam splitter. The alignment beam passes sequentially through a second rotating half-wave plate, a second reflecting mirror, a small aperture stop, and a large plane mirror, and then returns to the polarizing beam splitter along its original path. The reference beam passes sequentially through a third rotating half-wave plate and a third reflecting mirror, and then returns to the polarizing beam splitter along its original path. The return beams of the reference beam and the alignment beam are combined by the polarizing beam splitter and then pass sequentially through a polarizer and a CCD detector to form interference fringes. Adjust the large plane mirror so that the optical power of the alignment beam through the small aperture aperture reaches the maximum. At this time, the large plane mirror is perpendicular to the direction of the alignment beam. Adjust the third reflecting mirror so that the reference beam and the alignment beam appear as circular interference fringes on the CCD detector after beam combining. Place the beam expander between the large plane mirror and the small aperture stop, adjust the first and second reflecting mirrors to make the alignment beam pass through the small aperture stop until the CCD detector shows circular interference fringes, thus completing the alignment of the alignment beam with the optical axis of the telescope. The first and second cavity mirrors are added to the resonant cavity of the beam expander telescope in sequence; the tilt angles of the first and second cavity mirrors in the pitch and yaw directions are adjusted in sequence until the CCD detector continuously presents circular interference fringes, thus completing the alignment of the resonant cavity of the beam expander telescope.

2. The alignment method for a beam expander telescope resonator as described in claim 1, characterized in that, The small aperture stop is used to determine the initial position of the aligned beam and to determine whether the reflected light of the incident beam on the large plane mirror returns along the original path.

3. The alignment method for a beam expander telescope resonator as described in claim 2, characterized in that, When the light power passing through the small aperture aperture reaches its maximum, the plane of the large plane mirror is perpendicular to the direction of the incident beam, making the alignment beam parallel to the optical axis of the telescope, and the reflected light of the alignment beam on the large plane mirror returns along the original path.

4. The alignment method for a beam expander telescope resonator as described in claim 1, characterized in that, When the alignment beam coincides with the reference beam, the interference fringes displayed on the CCD detection surface are circular fringes.

5. The alignment method for a beam expander telescope resonator as described in claim 1, characterized in that, The method further includes: After placing the beam expander between the large plane mirror and the small aperture stop, if the interference fringes displayed on the CCD detection surface by the aligned beam are not circular, it means that there is a deviation in the direction of the output beam emitted by the laser. Adjust the direction of the output beam emitted by the laser by adjusting the first reflector, and adjust the second reflector to maximize the optical power of the aligned beam transmitted through the SDA. Remove the beam expander, readjust the large plane mirror and the third reflecting mirror so that the alignment beam and the reference beam coincide, and then place the beam expander between the large plane mirror and the small aperture stop.

6. The alignment method for a beam expander telescope resonator as described in claim 1, characterized in that, The beam expander telescope is an off-axis dual-reflector structure, and its lenses include parabolic or hyperboloid aspherical mirrors.

7. The alignment method for a beam expander telescope resonator as described in claim 1, characterized in that, The size of the large plane mirror is larger than the beam cross-section of the expanded beam telescope.

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