Ghost-free variable divergence angle laser beam expanding system, parameter confirmation method and application thereof

Abstract

The application relates to a ghost-free variable divergence angle laser beam expanding system and a parameter confirmation method and application thereof, and belongs to the technical field of laser optics. The system comprises a first negative focal length lens, a positive focal length lens and a second negative focal length lens along an optical axis in sequence. The core of the application is that only by precisely translating the single positive lens (the stroke is 0-30mm) can the output laser divergence angle be continuously and monotonously adjusted in the range of 1 times to 50 times. The optical system is designed, ghost foci generated in the whole adjustment process of laser are completely eliminated, the problem of optical element damage caused by ghost focusing of high peak power laser is fundamentally avoided, and the beam quality and system safety are ensured. The system is simple in structure, linear in control and high in reliability, has been successfully applied to the fields of space laser communication and the like, and has undergone long-term on-orbit working verification.

Description

Technical Field

[0001] This invention belongs to the field of laser optics technology, specifically relating to a ghost-free variable divergence angle laser beam expander system and its parameter confirmation method and application, which is particularly suitable for high peak power laser application scenarios, such as space laser communication and lidar. Background Technology

[0002] Laser beam expanders are key components in the laser emission chain used to control the beam divergence angle and match the distance to optical antennas or targets. Traditional variable divergence angle beam expanders often use two or more lens groups to adjust the divergence angle through complex mechanical structures that allow for relative displacement. However, these systems have the following shortcomings in practical applications:

[0003] 1. Complex adjustment mechanism: It requires precise movement of multiple optical components, resulting in a large system size, heavy weight, and reduced reliability, especially poor adaptability in harsh environments such as aerospace.

[0004] 2. Risk of ghosting: During multi-lens adjustment, the laser may reflect multiple times between optical elements, forming ghost images that focus on the surface and interior of the elements. For high peak power lasers (such as Q-switched pulsed lasers), these ghost images may reach extremely high power densities, sufficient to damage optical elements and their films, severely impair beam quality, and even lead to system failure.

[0005] 3. Nonlinearity and calibration difficulties: The divergence angle and the adjustment displacement often change non-monotonicly, which increases the complexity of the control algorithm and makes the calibration process cumbersome.

[0006] Therefore, existing laser beam expanding systems have significant shortcomings in terms of structural simplification, linear adjustment, and ghost image suppression. There is an urgent need for a high-reliability beam expanding and focusing system that is simple in structure, linear in adjustment, free from ghost image risk, and suitable for high peak power lasers. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a system that is compact, linearly adjustable, and free of ghosting intersections. This system can achieve a wide range of continuous adjustment of the divergence angle while ensuring beam quality, and has high reliability and environmental adaptability. It is suitable for fields with extremely high requirements for system stability, such as space laser communication and lidar.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A ghost-free variable divergence angle laser beam expander system, characterized in that it includes a first lens group, a second lens group, and a third lens group arranged sequentially along the optical axis;

[0010] The first lens group includes at least one fixedly mounted first negative power lens;

[0011] The third lens group includes at least one fixedly mounted second negative power lens;

[0012] The second lens group includes at least one positive power lens. The second lens group is mounted on a drive mechanism and is configured to be able to move axially between a first position and an Nth position along the optical axis. Under the premise of keeping the distance between the first to third lens groups unchanged, the distance between the first lens group and the second lens group, as well as between the second lens group and the third lens group, is changed.

[0013] The surface curvature radius, center thickness, and material refractive index of the negative power lens, positive power lens, and negative power lens are synergistically optimized to ensure that within the entire axial displacement range of the second lens group:

[0014] The divergence angle of the system output beam changes monotonically and continuously with the change of the interval;

[0015] The ghost image focus formed by the primary and secondary reflected beams generated by any two optical surfaces within the system is located outside the physical optical element and far from the photosensitive area, or forms a virtual focus.

[0016] Furthermore, the first negative power lens is optical power. A convex-concave negative meniscus lens, wherein the concave surface is positioned facing the incident light direction or the outgoing light direction; the positive power lens is of optical power. The first is a biconvex lens or a plano-convex lens; the second negative power lens is an optical power lens. The concave-convex negative meniscus lens, and and .

[0017] Furthermore, the length D of the axial displacement range satisfies 5mm ≤ D ≤ 30mm; within the axial displacement range, the system's output divergence angle adjustment factor M ranges from 1× to 50×, and the divergence angle... With displacement derivative The sign remains unchanged throughout the entire interval, and there are no extreme points or abrupt changes in the divergence angle.

[0018] Furthermore, the divergence angle adjustment factor M, ,in, To adjust the divergence angle to its maximum value, The minimum value for divergence angle adjustment is set; by changing the interval to change the equivalent focal length, the divergence angle adjustment factor M, i.e., the beam expansion ratio, is adjusted.

[0019] Furthermore, the drive mechanism includes a stepper motor, a precision lead screw or a linear motor, and a grating ruler or encoder for real-time feedback of the position of the second lens group; the second lens group is connected to the lens barrel through a flexible support structure or a low-friction guide rail to eliminate the interference of mechanical stress on the optical surface shape and ensure that the wavefront distortion under high-frequency reciprocating motion is less than λ / 15 (RMS).

[0020] Furthermore, all light-transmitting surfaces of the first lens group, the second lens group, and the third lens group are coated with an anti-reflection film for the working wavelength, and the residual reflectivity R of a single surface is less than 0.2% within the incident angle range of 0°-45°.

[0021] On the other hand, the present invention also provides a laser emitting terminal, characterized in that it includes:

[0022] Laser source, used to generate high-energy laser beams;

[0023] As described above, the input end of the ghost-free variable divergence angle laser beam expander system is coupled to the laser source.

[0024] The control unit, electrically connected to the drive mechanism, is used to calculate the required divergence angle based on the target communication distance or detection field of view, and control the second lens group to move to the corresponding axial position.

[0025] Third, the present invention also provides a spacecraft payload, characterized in that it is equipped with a laser emitting terminal as described above; the spacecraft payload utilizes the ghost-free variable divergence angle laser beam expander system to dynamically adjust the laser divergence angle according to the attenuation of the satellite-to-ground link during on-orbit, and there is no need to worry about the risk of ghost image damage caused by mechanical vibration or staying at a specific position during the entire adjustment process.

[0026] Fourth, a method for determining the parameters of a ghost-free variable divergence angle laser beam expander system as described above, characterized by including the following steps:

[0027] S1: Set the system's operating wavelength, input beam diameter, and target divergence angle adjustment range [θ] min ,θ max ];

[0028] S2: Construct an optical path along the direction of light transmission, consisting of a negative lens, a positive lens, and a negative lens, with the middle positive lens set as the only moving variable;

[0029] S3: Establish the evaluation function F, which includes a beam quality term, a divergence angle linearity term, and a ghost image suppression term;

[0030] The ghost image suppression term is constructed as follows: ghost image ray tracing is enabled in the optical design software to track the primary and secondary reflected rays between all surfaces. If the energy concentration of any reflected ray inside or on the surface of the optical element exceeds a preset safety threshold, a very large penalty value is applied to the structural parameter.

[0031] S4: Run the global optimization algorithm to iteratively adjust the curvature radius, thickness and air gap of each lens until the evaluation function F converges, and obtain the optical parameter combination that satisfies the requirement of no high-risk ghosting focus throughout the entire stroke.

[0032] Furthermore, the parameter determination method described above is characterized by:

[0033] In step S3, the preset safety threshold is set as follows: the ghost image power density does not exceed 10% of the main beam power density. -4 Furthermore, the distance between the focal point of the ghost image and the nearest optical surface is greater than 5mm.

[0034] Compared with the prior art, the present invention has the following significant advantages:

[0035] 1. Extremely simple and reliable structure: Only one lens needs to be moved, which greatly simplifies the mechanical structure, reduces weight and failure rate, and improves the stability of the system under vibration and shock environments.

[0036] 2. Linear adjustment with a wide range: The divergence angle and the adjustment displacement change monotonically, making control simple and precise. A wide range of adjustment from 1 to 50 times can be achieved with a stroke of less than 30 mm.

[0037] 3. Completely eliminate the risk of ghosting: From the source of optical design, ghosting focal points are avoided on the surface and inside of the mirror body, fundamentally solving the problem of optical component damage in high peak power laser systems and ensuring long-term high-quality output of laser beams.

[0038] 4. Verified in the space environment: This invention has been successfully applied to aerospace laser communication systems and has been operating stably in orbit for more than 3 years, proving its extremely high reliability and environmental adaptability. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the optical path of the ghost-free variable divergence angle laser beam expander system of the present invention.

[0040] Figure 2 This is a graph showing the relationship between the laser divergence angle output by the system of this invention and the displacement of the positive focal length lens.

[0041] Figure 3 The diagrams show the optical path diagrams of the system under different divergence angles and the corresponding ghost image optical path analysis diagrams, where a is 50x, b is 22.5x, and c is 1x.

[0042] Figure 4 This is a diagram showing the output laser far-field spot effect after the actual assembly and adjustment of the system of the present invention.

[0043] In the diagram: 1 - First negative power lens, 2 - Positive power lens, 3 - Second negative power lens Detailed Implementation

[0044] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. These embodiments are only for explaining the present invention and are not intended to limit the scope of protection of the present invention.

[0045] like Figure 1 As shown, this embodiment provides a ghost-free variable divergence angle laser beam expander system, which sequentially includes, along the optical axis, a first negative power lens 1, a positive power lens 2, and a second negative power lens 3. The system employs a three-piece Galilean variant structure with a "negative-positive-negative" configuration. The first negative power lens 1 and the second negative power lens 3 are fixed within their respective frames by clamping rings, located at opposite ends of a guide rail, and their positions are locked. The positive power lens 2 is mounted within its corresponding frame in the middle of a linear guide rail and is driven by a stepper motor, allowing for reciprocating translational motion along the optical axis. The entire system has no moving parts other than the positive power lens 2, greatly simplifying the mechanical structure.

[0046] The first negative optical power lens 1 has a focal length of f1 and is used to receive and initially collimate the incident beam from the laser.

[0047] The positive focal length lens 2 is a movable single positive lens with a focal length of f2 (f2>0). It is mounted on a precision guide rail and can move back and forth along the optical axis with a travel distance of less than 30mm.

[0048] The second negative power lens 3, with a focal length of f3, is used to expand the output beam.

[0049] The overall divergence angle of the system can be continuously and monotonically adjusted from 1x to 50x simply by translating the movable monolithic lens.

[0050] By precisely moving the central negative lens, its relative position to the front fixed group is changed, thereby effectively changing the beam expansion ratio M=θmax / θmin of the entire system, where θmax is the maximum value of the divergence angle and θmin is the minimum value of the divergence angle adjustment. Therefore, moving a single lens can linearly adjust the output divergence angle.

[0051] Through rigorous optical path tracing and optical design, the curvature radius, spacing, and coating of all optical components have been optimized to ensure that, throughout the entire adjustment stroke, the residual reflected light path generated by the laser between any two optical surfaces cannot form a solid focal point (i.e., a ghost image) on the optical surface or inside the lens of the component. All ghost images converge into a spot, and its power density is always lower than the threshold of air breakdown or optical film damage.

[0052] Example: For a Q-switched pulsed laser with a wavelength of 532nm (high peak power density)

[0053] First fixed lens 1: A convex-concave negative lens with a focal length f1 = -26.99 mm is used. Its front surface curvature radius has been optimized to receive the initial diverging beam emitted by the laser and to perform preliminary collimation.

[0054] Movable single lens 2: It adopts a single biconvex positive lens with a focal length f2=37.29mm. It is mounted on an electric translation guide rail with a stroke of 15mm.

[0055] The second fixed lens 3 is a meniscus negative lens with a focal length of f3 = -117.99 mm. It is used to expand the beam after the beam passes through the first fixed lens 1 and the movable single lens 2.

[0056] All lens substrates are made of fused silica with a high damage threshold, and all optical surfaces are coated with a high anti-reflection coating (AR<0.05%) in the 532nm band to minimize residual surface reflectivity and reduce the energy basis for ghosting at the source.

[0057] The method for continuously and monotonically adjusting the divergence angle is as follows:

[0058] When the system is working, the 532nm pulsed laser output from the laser (with an inherent divergence angle θ0 = 2.5 mrad) is incident on the first negative optical power lens 1, and after initial beam expansion, it is incident on the positive optical power lens 2, and then expanded again by the second negative optical power lens 3 before being output. The control system drives the positive optical power lens 2 to move along the optical axis, changing the interval d1 between it and the first negative optical power lens 1, thereby changing the equivalent beam expansion ratio of the system and achieving continuous adjustment of the output divergence angle θ.

[0059] In the optical design of this embodiment, when the positive focal power lens 2 moves within a stroke range of 0~15mm, the divergence angle θ of the system output laser is monotonically related to the displacement x of the positive focal power lens 2. Measured data shows (see...) Figure 2With only a tiny displacement of 15mm, it can achieve a continuous coverage of the output divergence angle from 1x (collimated state) to 50x (large divergence angle state). This linear characteristic makes the control algorithm extremely simple, eliminating the need for complex lookup tables or nonlinear compensation, thus improving response speed and control accuracy.

[0060] Depend on Figure 4 The actual assembly and adjustment results show that the far-field beam spot is visible, the beam energy distribution is uniform, and there are no discontinuities. It is evident that there is no component damage at the stopping points within the travel range.

Claims

1. A ghost-free variable divergence angle laser beam expander system, characterized in that, It includes a first lens group, a second lens group, and a third lens group arranged sequentially along the optical axis; The first lens group includes at least one fixedly mounted first negative power lens; The third lens group includes at least one fixedly mounted second negative power lens; The second lens group includes at least one positive power lens. The second lens group is mounted on a drive mechanism and is configured to be able to move axially between a first position and an Nth position along the optical axis. Under the premise of keeping the distance between the first to third lens groups unchanged, the distance between the first lens group and the second lens group, as well as between the second lens group and the third lens group, is changed. The surface curvature radius, center thickness, and material refractive index of the negative power lens, positive power lens, and negative power lens are synergistically optimized to ensure that within the entire axial displacement range of the second lens group: The divergence angle of the system output beam changes monotonically and continuously with the change of the interval; The ghost image focus formed by the primary and secondary reflected beams generated by any two optical surfaces within the system is located outside the physical optical element and far from the photosensitive area, or forms a virtual focus.

2. The ghost-free variable divergence angle laser beam expander system according to claim 1, characterized in that, The first negative power lens is optical power. A convex-concave negative meniscus lens, wherein the concave surface is positioned facing the incident light direction or the outgoing light direction; the positive power lens is of optical power. The first is a biconvex lens or a plano-convex lens; the second negative power lens is an optical power lens. The concave-convex negative meniscus lens, and and .

3. The ghost-free variable divergence angle laser beam expander system according to claim 1, characterized in that, The length D of the axial displacement range satisfies 5mm ≤ D ≤ 30mm; within the axial displacement range, the system's output divergence angle adjustment factor M ranges from 1× to 50×, and the divergence angle... With displacement derivative The sign remains unchanged throughout the entire interval, and there are no extreme points or abrupt changes in the divergence angle.

4. The ghost-free variable divergence angle laser beam expander system according to claim 3, characterized in that, The divergence angle adjustment factor M, ,in, To adjust the divergence angle to its maximum value, The minimum value for divergence angle adjustment is set; by changing the interval to change the equivalent focal length, the divergence angle adjustment factor M, i.e., the beam expansion ratio, is adjusted.

5. The ghost-free variable divergence angle laser beam expander system according to claim 1, characterized in that, The drive mechanism includes a stepper motor, a precision lead screw or a linear motor, and a grating ruler or encoder for real-time feedback of the position of the second lens group; the second lens group is connected to the lens barrel through a flexible support structure or a low-friction guide rail to eliminate the interference of mechanical stress on the optical surface shape and ensure that the wavefront distortion under high-frequency reciprocating motion is less than λ / 15 (RMS).

6. The ghost-free variable divergence angle laser beam expander system according to any one of claims 1-5, characterized in that, All light-transmitting surfaces of the first lens group, the second lens group, and the third lens group are coated with anti-reflection films for the working wavelength, and the residual reflectivity R of a single surface is less than 0.2% within the incident angle range of 0°-45°.

7. A laser emitting terminal, characterized in that, include: Laser source, used to generate high-energy laser beams; The ghost-free variable divergence angle laser beam expander system as described in any one of claims 1-6 has its input end coupled to the laser source; The control unit, electrically connected to the drive mechanism, is used to calculate the required divergence angle based on the target communication distance or detection field of view, and control the second lens group to move to the corresponding axial position.

8. A spacecraft payload, characterized in that, Equipped with the laser emitting terminal as described in claim 7; the spacecraft payload utilizes the ghost-free variable divergence angle laser beam expander system to dynamically adjust the laser divergence angle during on-orbit operation based on the attenuation of the satellite-to-ground link, and there is no need to worry about the risk of ghost image damage caused by mechanical vibration or staying at a specific location during the entire adjustment process.

9. A method for determining parameters of a ghost-free variable divergence angle laser beam expander system as described in claim 1, characterized in that, Includes the following steps: S1: Set the system's operating wavelength, input beam diameter, and target divergence angle adjustment range [θ] min ,θ max ]; S2: Construct an optical path along the direction of light transmission, consisting of a negative lens, a positive lens, and a negative lens, with the middle positive lens set as the only moving variable; S3: Establish the evaluation function F, which includes a beam quality term, a divergence angle linearity term, and a ghost image suppression term; The ghost image suppression term is constructed as follows: ghost image ray tracing is enabled in the optical design software to track the primary and secondary reflected rays between all surfaces. If the energy concentration of any reflected ray inside or on the surface of the optical element exceeds a preset safety threshold, a very large penalty value is applied to the structural parameter. S4: Run the global optimization algorithm to iteratively adjust the curvature radius, thickness and air gap of each lens until the evaluation function F converges, and obtain the optical parameter combination that satisfies the requirement of no high-risk ghosting focus throughout the entire stroke.

10. The parameter determination method according to claim 9, characterized in that: In step S3, the preset safety threshold is set as follows: the ghost image power density does not exceed 10% of the main beam power density. -4 Furthermore, the distance between the focal point of the ghost image and the nearest optical surface is greater than 5mm.

Images