Integrated anastigmatic compound curvature lens for double-axis synchronous collimation of laser diode

Abstract

The invention relates to an integrated anastigmatic compound curvature lens for laser diode double-axis synchronous collimation, and belongs to the technical field of laser optical devices. According to the lens, a cylindrical surface and a spherical surface are fused in a single optical element through the design of a composite curvature surface, so that the lens integrates an independent collimation function in a fast axis direction and a slow axis direction, and the astigmatism aberration of laser diode light beams can be actively eliminated at the same time. Specifically, the curved surface of the lens adopts the combination of a high-curvature cylindrical surface and an aspheric surface structure in the fast axis direction to compress a large divergence angle, adopts an aspheric surface to adapt to a small divergence angle in the slow axis direction, and realizes symmetry and low-aberration output of a biaxial collimated light beam by optimizing curvature combination and a high-order term coefficient. In addition, the lens forms two non-coincident focuses in the meridian direction and the sagittal direction, so that the astigmatism length of the laser diode is compensated, and the anastigmatism collimation effect is achieved. The integrated design of the lens replaces a traditional multi-lens combination system, the size and the assembling complexity of an optical system are remarkably reduced, and the lens is suitable for the fields of high-precision laser communication, laser radar, industrial machining and the like and has the advantages of being high in efficiency, low in cost and high in reliability.

Description

Technical Field

[0001] The present invention relates to an integrated, anti-stigmation, composite curvature lens for dual-axis simultaneous collimation of laser diodes. Specifically, the curvatures of the lens' two surfaces are designed, combining cylindrical and spherical surfaces to achieve different focal lengths in the meridional and sagittal directions. This compensates for the inherent astigmatism of the laser diode, significantly enhancing the single lens's simultaneous collimation capabilities in both the fast and slow axes. This technology belongs to the field of optical design for laser diode beam collimation. Background Art

[0002] Laser diodes (LDs), as high-brightness and high-efficiency semiconductor light sources, are widely used in laser radar, fiber-optic communications, industrial processing, and medical equipment. However, due to the asymmetry of their semiconductor waveguide structures, the beam output by LDs has significant astigmatism and biaxial divergence angle differences. The divergence angle of the fast-axis beam perpendicular to the pn junction is usually as high as 20° to 40°, while the divergence angle of the slow-axis beam parallel to the pn junction is relatively small, about 5° to 15°. In addition, the waist positions of the fast and slow axis beams are separated along the optical axis, forming an astigmatism A with a length ranging from microns to millimeters. s. This astigmatism characteristic causes wavefront distortion, spot asymmetry and energy loss when the light beam is freely transmitted or coupled to the optical fiber, which seriously restricts the direct application of LD in high-precision optical systems. Traditional solutions usually use a multi-lens combination system to collimate the fast and slow axes of the LD separately. For example, a cylindrical lens in the fast axis direction is used to compress the large divergence angle, and then an aspheric single lens or lens group in the slow axis direction is used to perform secondary collimation on the light beam. Finally, a complex optical path design is used to compensate for the astigmatism. However, such solutions have obvious defects: First, the stacking of multiple lenses significantly increases the volume, weight and assembly complexity of the optical system, which makes it difficult to meet the requirements of compact equipment design in scenarios such as vehicle-mounted lidar and miniaturized optical communication modules; second, the step-by-step collimation of the fast and slow axes requires strict assurance that the relative position tolerance between lenses is lower than the micron level, resulting in high production and assembly costs and insufficient long-term stability; third, the surface reflection and aberration superposition of discrete components will further reduce the beam quality, limiting the output power and collimation efficiency. Although recent studies have attempted to achieve dual-axis collimation of LD beams using monolithic optical elements, their designs are mostly limited to simple cylindrical or spherical lens combinations. It is difficult to simultaneously achieve efficient compression of the fast and slow axis divergence angles and active elimination of astigmatism within a single lens, resulting in residual aberrations or asymmetric light spots in the collimated beam. For example, a single cylindrical lens can compress the fast axis divergence angle, but lacks effective control over the slow axis direction; and designs using dual aspheric surfaces or compound curvatures may introduce additional wavefront distortion if the astigmatism length of the LD is not precisely matched. Therefore, how to achieve efficient synchronous collimation of the fast and slow axes, precise compensation of the astigmatism length, and balance beam symmetry and low-aberration output in a single optical element has become a technical bottleneck that urgently needs to be broken through in the field of laser optical devices.

[0003] Furthermore, with the evolution of LiDAR towards solid-state and integrated technology, and the increasing demand for high-power laser beam quality in industrial processing, the market is increasingly demanding miniaturized, low-cost, and highly reliable LD collimation solutions. To address these challenges, this paper proposes an innovative integrated, anti-astigmatism compound curvature lens design. By deeply integrating the fast and slow axis collimation functions with an astigmatism compensation mechanism, high-precision, simultaneous optimization of dual-axis beams is achieved within a single lens. This overcomes the limitations of traditional multi-lens collimation systems and provides core optical device support for innovative applications of laser diodes. Summary of the Invention

[0004] The present invention relates to an integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of a laser diode. The lens utilizes the divergence angles of the fast and slow axes of the laser diode, the size of the active light-emitting area, and the inherent astigmatism length to gradually design and achieve synchronous collimation of the fast and slow axes of the LD beam. The lens compensates for the astigmatism length based on the dual focal lengths in the sagittal and meridional directions. The lens is characterized in that the lens specifically comprises the following steps:

[0005] 1. According to the laser diode factory report, obtain the target laser diode fast axis divergence full angle θ ⊥ The divergence angle of the slow axis is θ, where the divergence angle refers to the full divergence angle when the light intensity drops to 1 / e2 of the maximum value. The astigmatism length of the laser diode is A s , and define the active light-emitting area pn junction surface as the beam waist position in the fast axis direction of the light beam, and the distance from the light-emitting surface A s A certain position inside the active region is the beam waist position in the slow axis direction of the light beam.

[0006] 2. Define the two surfaces of the target lens as surface a and surface b. The emission direction of the laser diode beam is consistent with the direction from surface a to surface b of the lens. That is, during the collimation process, the laser beam first passes through surface a of the lens. The distance from surface a of the lens to the light-emitting surface of the laser diode is defined as L.

[0007] 3. Based on the parameters in steps 1 and 2, determine that one of the two surfaces of the lens is cylindrical, with the direction of the cylindrical curvature aligned with the fast axis of the laser diode, and the remaining surface is aspherical. Preferably, surface a is determined to be cylindrical, and surface b is determined to be aspherical.

[0008] 4. Determine that the meridian direction of the lens is the slow axis direction of the laser diode, the focal length in the meridian direction is f, the sagittal direction of the lens is the fast axis direction of the laser diode, and the focal length in the sagittal direction is f ⊥ The focal length of the lens in the meridian and sagittal directions is different, which is used to compensate for the astigmatism of the laser diode. ⊥ and f satisfy equations (1) to (2).

[0009] f ⊥ =L (1)

[0010] f=L+A s (2)

[0011] 5. Define the radius of curvature of lens surface a in the sagittal direction as The radius of curvature in the meridian direction is R a , the curvature radius of the b-surface in the sagittal direction is The radius of curvature in the meridian direction is R b The refractive index of the lens material at the design wavelength λ is n λ , the refractive index of the material surrounding the lens is n0, and the thickness of the lens is d. Then the curvature radius of the lens in the meridian and sagittal directions can be calculated from the focal length defined in 4. The specific calculation method is shown in formulas (3) to (4).

[0012] R a =0 (3)

[0013]

[0014] Surface a is a cylindrical surface. Its sagittal direction and that of surface b contribute to the alignment of the laser diode along its fast axis. Surface b also aligns the laser diode along its slow axis. Therefore, the curvature of surface a in the meridian direction is zero. Surface b is a rotationally symmetric surface, so its curvature in the meridian and sagittal directions is equal. Preferably, the curvature radius of surface b is controlled to be less than 0.

[0015] 6. Based on the curvature radius parameters obtained in the previous steps, it is necessary to further determine the aspheric parameters of surface b and define the aspheric surface shape to satisfy formula (6).

[0016]

[0017] Where z is the surface sag parallel to the optical axis, that is, the rotational symmetry axis of the surface is the z-axis, the vertex of the surface coincides with the origin of the coordinate system, in the xz or yz plane, the surface radius r is the x or y coordinate, and z is the z-axis coordinate; the curvature of the surface is c, the conic coefficient is k, and α is the coefficient of the higher-order term. Preferably, |k| ≤ 10, α i (i=3,4,5,6,7,8)=0.

[0018] 7. Based on the divergence angle parameters and astigmatism length of the laser diode, first determine the curvature radius of the two surfaces of the lens according to equations (1) to (5). Furthermore, according to equation (6) combined with ray tracing, optimize the aspheric surface shape of surface b so that the difference in divergence angle between the fast axis and slow axis beams after passing through the lens is less than 0.5 mrad.

[0019] Beneficial effects:

[0020] (1) The method described in the present invention deeply integrates the fast and slow axis collimation and astigmatism compensation functions through a single composite curvature lens, replacing the traditional multi-lens stacking solution, significantly reducing the volume of the optical system and the assembly complexity, while avoiding the tolerance sensitivity and optical path misalignment risk between discrete components, providing core support for miniaturized laser modules.

[0021] (2) The one-piece lens structure proposed by the method of the present invention greatly reduces the reflection loss of laser at the interface, improves the utilization rate of light energy and the ability to resist environmental interference; simplifies the manufacturing process and avoids the risk of aging of the traditional multi-lens group, and can be applied on a large scale in the fields of laser radar, industrial processing, etc., taking into account both performance improvement and cost control needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of laser diode astigmatism

[0023] Figure 2 Cap lens structure for dual-axis synchronous collimation of laser diodes

[0024] Figure 3 Assembly diagram of TO package laser diode and cap lens

[0025] Figure 4 The effect diagram of the light spot after the dual-axis synchronous collimation of the lens of the present invention

[0026] Figure 5 Schematic diagram of the assembly of the ball cap lens and the second collimating lens of the present invention

[0027] Figure 6 The light spot collimation effect diagram after the ball cap lens of the present invention is used in combination with the second collimating lens DETAILED DESCRIPTION

[0028] The present invention is further described below with reference to the accompanying drawings and specific embodiments. The methods described are conventional methods unless otherwise specified, and the raw materials described can be obtained from public commercial sources unless otherwise specified.

[0029] (1) This example verifies the effectiveness of the method described in the invention, and specifically compares the divergence angle of the light spot after collimation using the lens of the present invention with the divergence angle of the laser diode light spot collimated by an aspheric lens of equivalent focal length. The light source used in this example is a nano-stacked edge-emitting laser diode with a wavelength of 905nm, an active light-emitting area of 110μm×18μm, 4 vertical stacks, and a spacing of 4μm (peak intensity spacing). The full width at half maximum (FWHM) of the fast axis divergence of the light beam is θ ⊥FWHM =22°, the full angle of divergence in the slow axis direction is θ FWHM =10°, astigmatism length is A s =0.15mm. According to the relationship between the full-angle width defined by FWHM and the full-angle divergence when the light intensity drops to 1 / e2 of the maximum value, we can get The definition of laser diode fast and slow axis divergence angle and astigmatism is as follows Figure 1 shown.

[0030] (2) The laser diode is a typical TO package. The target lens surface a is cylindrical and the surface b is aspherical. The distance L between the lens surface a and the chip is 1.5 mm. The lens is mounted so that the curvature of the lens surface a is parallel to the fast axis of the laser diode (this can be changed to parallel to the slow axis depending on the design).

[0031] (3) Define the lens meridian direction as the slow axis direction of the laser diode, the focal length in the meridian direction is f, the sagittal direction is the fast axis direction of the laser diode, the focal length in the sagittal direction is f ⊥ According to the parameters in (1) to (3), the focal lengths of the lens in the sagittal and meridional directions are f = 1.65 mm, f ⊥ =1.5mm.

[0032] (4) In order to reasonably control the manufacturing cost and manufacturing accuracy, the lens material is selected as a new type of low thermal expansion coefficient plastic K26R, which has a refractive index n at 905nm. λ =1.53, the material around the lens is air, n0=1, the initial thickness of the lens is determined to be 1.5mm, and the curvature radius of each surface of the lens is determined according to formulas (1) to (3) as shown in Table 1.

[0033] R a =0 (1)

[0034]

[0035] Surface a is a cylindrical surface. Its sagittal direction and that of surface b are jointly responsible for collimating the laser diode's fast-axis beam. Surface b also aligns the laser diode's slow-axis beam. Therefore, the curvature of surface a in the meridian direction is zero. Surface b is a rotationally symmetric surface, so its curvature in the meridian and sagittal directions is equal.

[0036] Table 1 Curvature radius of each lens surface

[0037] direction / face a side(mm) b side (mm) Meridian (R) 0 -0.795 <![CDATA[Arc Sagitta (R ⊥ )]]> -3 -0.795

[0038] (5) Based on the surface parameters obtained in the previous steps, further determine the aspheric parameters of surface b, and define the aspheric surface shape to satisfy formula (6).

[0039]

[0040] Where z is the surface sag parallel to the optical axis, that is, the rotational symmetry axis of the surface is the z-axis, the vertex of the surface coincides with the origin of the coordinate system, in the xz or yz plane, the surface radius r is the x or y coordinate, and z is the z-axis coordinate; the curvature of the surface is c, the cone coefficient is k, and α is the coefficient of the higher-order term.

[0041] According to formula (4) combined with ray tracing, the aspheric surface shape of surface b is optimized so that the difference in divergence angle between the fast axis and slow axis beams after passing through the lens is less than 0.5 mrad. The aspheric surface parameters of surface b are finally obtained as shown in Table 2.

[0042] Table 2 Aspheric parameters of each lens surface

[0043] parameter A side b-side Cone coefficient 0 -1.823 <![CDATA[Second-order term (α1)]]> 0 -0.018 <![CDATA[Fourth-order term (α2)]]> 0 -0.033

[0044] (6) The optimized lens structure is as follows Figure 2 As shown, the number 1 is the aspheric surface (b surface), the number 2 is the mounting surface, the number 3 is the meridian direction of the cylindrical surface (a surface), and the number 4 is the sagittal direction of the cylindrical surface. The assembly structure of the lens of the present invention and the TO package laser diode is as shown in FIG. Figure 3 As shown, the numbers 1 to 4 have the same meanings. Figure 2, number 5 is the TO package metal cap, and number 6 is the installation position of the laser diode active light-emitting chip.

[0045] (7) In order to highlight the collimation effectiveness of the lens of the present invention, an aspheric lens with equivalent focal length is designed. The a surface of the lens is flat, the b surface is aspheric, and the focal length of the lens is 1.5 mm. After collimation by the ordinary lens, the spot size at a distance of 6000 mm from the light source is as follows Figure 4 As shown in (right), the fast axis divergence angle after collimation is calculated to be 6.67 mrad, and the slow axis divergence angle is 23.4 mrad. The spot size after collimation by the lens of the present invention is as follows Figure 4 As shown in (left), the divergence angles of the fast and slow axes after collimation are calculated to be 6.67 mrad.

[0046] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0047] Example 2: Design example of a laser radar with a collimating lens and a spherical cap lens of the present invention

[0048] (1) This example expands the application scenario of the method of the present invention based on Example 1. This example involves the design of a pre-collimating lens for the light source of a laser radar emission path. Since the laser radar emission path contains a collimating lens (a second collimating lens with known parameters), the parameters of the spherical cap lens must be changed to achieve the dual-axis synchronous collimation effect of the laser diode. The light source used in this example is a nano-stacked edge-emitting laser diode with a wavelength of 905nm. The size of its active light-emitting area is 110μm×18μm, the number of vertical stacks is 4, and the spacing is 4μm (peak intensity spacing). Its full width at half maximum (FWHM) in the fast axis direction is θ ⊥FWHM =22°, the full angle of divergence in the slow axis direction is θ FWHM =10°, astigmatism length is A s =0.15mm. According to the relationship between the full-angle width defined by FWHM and the full-angle divergence when the light intensity drops to 1 / e2 of the maximum value, we can get

[0049] (2) The parameters of the second collimating lens in this example are shown in Table 3. This lens is an ordinary spherical lens. To ensure the compactness of the overall structure, the design stipulates that the distance between the second lens and the ball cap lens is 14.5 mm.

[0050] Table 3 Optical parameters of the second collimating lens

[0051] Distance ball cap lens focal length Refractive index@905nm Aperture thickness 14.5mm 20mm 1.8174 10mm 2mm

[0052] (3) The laser diode is a typical TO package. The spherical cap lens is designed with a cylindrical surface (a) and an aspheric surface (b). The distance L between the lens surface a and the chip is 0.15 mm. The lens is mounted so that the curvature of the lens surface a is parallel to the fast axis of the laser diode (this can be changed to parallel to the slow axis depending on the design).

[0053] (4) The distance between the two lenses is D = 14.5 mm, the focal length of the spherical cap lens is f1, and the focal length of the second collimating lens is f2. The meridian direction of the lens is defined as the slow axis direction of the laser diode, the sagittal direction is defined as the fast axis direction of the laser diode, and the focal length of the combined lens in the meridian direction is f ZH , the focal length in the sagittal direction is f ZH ⊥ , taking the spherical cap lens in Example 1 as the initial structure, preliminarily determine the equivalent focal lengths of the combined lens in the meridian and sagittal directions are f ZH =5.076mm, f ZH ⊥ =5.203mm, and the meridian and sagittal curvature radii of the spherical cap lens are re-solved according to equations (1) to (4).

[0054] R a =0 (1)

[0055]

[0056]

[0057] (5) According to the above parameters, the parameters of each surface of the spherical cap lens are obtained as shown in Table 4. At this time, the equivalent focal length f of the combined lens is ZH =5.076mm, f ZH ⊥ =5.225mm, focal length difference 0.149mm, compensated for astigmatism length 0.15mm.

[0058] Table 4 Parameters of each surface of the ball cap lens

[0059] parameter A side b-side Radius of curvature -2.569 (meridian) -0.816 Cone coefficient 0 -1.823 2nd-order term 0 -0.018 4th-order term 0 -0.033

[0060] (6) The assembly combination of the laser diode ball cap lens and the second collimating lens is as follows Figure 5 As shown, reference numeral 7 is a spherical cap lens, reference numeral 8 is a second collimating lens, reference numeral 9 is a cylindrical curvature surface of the spherical cap lens, and reference numeral 10 is a plane of the cylindrical surface of the spherical cap lens. The spot of the laser beam after being collimated by the combined lens is shown in FIG. Figure 6 As shown, the light screen is 6000 mm away from the light source, and the divergence angles of the light spot in the fast and slow axis directions are 5 mrad respectively.

[0061] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: The laser diode fast axis divergence full angle is θ ⊥ , the full angle of slow axis divergence is θ, where the divergence angle refers to the time when the light intensity drops to 1 / e of the maximum value 2 The divergence angle is the total angle, and the astigmatism length is A s , and define the active light-emitting area surface as the waist position in the fast axis direction of the light beam, and the distance from the light-emitting surface A s A certain position inside the active region is the beam waist position in the slow axis direction of the light beam.

2. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: The two surfaces of the lens are surface a and surface b. The emission direction of the laser diode light beam is consistent with the direction from surface a of the lens to surface b. The distance between surface a of the lens and the light-emitting surface of the laser diode is L.

3. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: According to the parameters in claim 1 and claim 2, one of the two surfaces of the lens is determined to be a cylindrical surface, and the direction of the cylindrical curvature is consistent with the fast axis direction of the laser diode, and the remaining surface is an aspherical surface.

4. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: Determine that the meridian direction of the lens is the slow axis direction of the laser diode, the focal length in the meridian direction is f, the sagittal direction is the fast axis direction of the laser diode, and the focal length in the sagittal direction is f ⊥ The focal length of the lens in the meridian and sagittal directions is different, which is used to compensate for the astigmatism of the laser diode. ⊥ and f satisfy equations (1) to (2). in ⊥ =L (1) f=L+A s (2) 5. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: The sagittal curvature radius of lens surface a is The radius of curvature in the meridian direction is R a , the curvature radius of the b-surface in the sagittal direction is The radius of curvature in the meridian direction is R b The refractive index of the lens material at the design wavelength λ is n λ , the refractive index of the material surrounding the lens is n0, and the thickness of the lens is d, then the curvature radius of the lens in the meridian and sagittal directions can be obtained using equations (3) to (4). Ra=0 (3) 6. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: The b-surface is an aspherical surface. According to the curvature radius value obtained in claim 5, the aspherical coefficients and the surface high-order coefficients need to be further optimized. The aspherical surface shape satisfies formula (6). Where z is the surface sag parallel to the optical axis, that is, the rotational symmetry axis of the surface is the z-axis, the vertex of the surface coincides with the origin of the coordinate system, in the xz or yz plane, the surface radius r is the x or y coordinate, and z is the z-axis coordinate; the curvature of the surface is c, the cone coefficient is k, and α is the coefficient of the higher-order term.

7. An integrated anti-stigmation compound curvature lens for dual-axis synchronous collimation of laser diodes, characterized in that: According to the divergence angle parameters and astigmatism length of the laser diode, the curvature radii of the two surfaces of the lens are first determined according to equations (1) to (5). Furthermore, the aspheric surface shape of surface b is optimized according to equation (6) combined with ray tracing so that the difference in divergence angles of the light beams in the fast and slow axis directions after passing through the lens is less than 0.5 mrad.

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