A degenerate cavity laser based on thick lens effect

Abstract

The utility model discloses a degenerate cavity laser based on thick lens effect, including the high -power line mirror, laser crystal, first flat convex lens, pinhole diaphragm, second flat convex lens and partial mirror which are sequentially arranged from front to back, the convex surface of first flat convex lens faces laser crystal, the convex surface of second flat convex lens faces partial mirror, the high -power line mirror and partial mirror constitute the end mirror of resonant cavity, laser crystal, first flat convex lens and second flat convex lens constitute focusing telescope system for realizing the self -reproduction imaging of single -pass crossing of high -power line mirror to partial mirror, pinhole diaphragm 4 is placed in fourier spectrum plane for realizing the selection module function. The utility model utilizes the thick lens effect of solid laser crystal itself, and will laser crystal and first flat convex lens constitute combination lens, has reduced the influence of thermal lens under the continuous wave maser to degenerate cavity laser construction, has good practicality.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic technology, specifically relating to a degenerate cavity laser based on the thick lens effect. Background Technology

[0002] Degenerate cavity lasers possess characteristics such as tunable coherence and perfect self-reproduction of transverse mode optical fields, making them widely applicable in research fields such as coherence control, holographic imaging, and optical computing. However, in degenerate cavity lasers, severe thermal effects of the gain medium (such as thermal distortion and thermal delamination) can affect the output laser beam quality and power. On the other hand, thermal effects can broaden the emission spectral lines of the material, significantly reducing the effective emission cross-section and leading to a decrease in laser gain. Taking a cylindrical laser crystal as an example, since the temperature-dependent refractive index change is proportional to the square of the material radius, the crystal exhibits a spherical lensing effect. This effect causes wavefront distortion of the light field after passing through the crystal, resulting in deterioration of beam quality and a decrease in conversion efficiency. If the focal length is too short, this effect can create a solid focal point inside the gain medium, causing laser damage within the material due to excessively high energy density. Most seriously, the thermal lensing effect of the laser crystal disrupts the degeneracy of the cavity, degenerating it from a critical cavity to a stable cavity, and the modes are no longer degenerate, posing difficulties for subsequent research on the dynamics of degenerate cavity lasers.

[0003] To mitigate the effects of thermal lensing on degenerate cavity lasers, common methods include quasi-continuous pumping and the use of thin-film gain media. In quasi-continuous pumping, the material has ample idle time for heat dissipation, reducing thermal effects. In 2013, Nixon et al. from the Weizmann Institute of Science in Israel used quasi-continuous pumping to construct a degenerate cavity laser using a xenon flash lamp with a repetition frequency of 1 Hz and a pulse width of 100 μs as the pump source (Efficient method for controlling the spatial coherence of a laser [J]. Optics Letters, 2013, 38(19): 3858-3861.). While this method can reduce thermal lensing, its low time utilization makes it difficult to promote for practical applications.

[0004] Leveraging the electro-optical conversion properties of semiconductor gain media and the mature TEC mechanism, in 2016, a research team at Yale University replaced the gain medium with a semiconductor quantum well material, realizing the construction of an electrically pumped degenerate cavity laser. Due to the micrometer-scale thickness of the gain medium and the use of TEC cooling, they achieved continuous-wave tunable coherent lasing and used low and high spatial coherence light to image the embryonic heart function of the African Xenopus laevis (Coherence switching of adegenerate VECSEL for multimodality imaging [J]. Optica, 2016, 3(4): 403-406.). However, due to the limitations of semiconductor gain media in material growth, large-area lasing cannot be achieved.

[0005] Because the ratio of heat dissipation area to volume of a thin sheet is very large, using a solid thin sheet as the gain medium can reduce the impact of thermal effects and achieve very high cooling efficiency. In 2023, Gadasi et al. from the Weizmann Institute of Science in Israel used a 3 mm thick Nd:YO4 microsheet as the gain medium to realize a degenerate cavity laser through end-face pumping, and combined it with pump shaping to improve mode locking performance. However, since the pumping method is generally end-face pumping, an external fiber laser is required for pumping in free space, making integration and packaging difficult. Utility Model Content

[0006] The purpose of this invention is to provide a degenerate cavity laser based on the thick lens effect, which aims to reduce the impact of the thermal lens effect under continuous wave lasing on the construction of the degenerate cavity laser.

[0007] This utility model is mainly achieved through the following technical solutions: A degenerate cavity laser based on the thick lens effect includes, from front to back, a high-power line mirror, a laser crystal, a first plano-convex lens, a pinhole aperture, a second plano-convex lens, and a partial mirror; the convex surface of the first plano-convex lens faces the laser crystal, and the convex surface of the second plano-convex lens faces the partial mirror; the high-power line mirror and the partial mirror constitute the end mirror of the resonant cavity; the laser crystal, the first plano-convex lens, and the second plano-convex lens constitute a focusing telescope system for realizing self-reproducing imaging of a single-pass transit from the high-power line mirror to the partial mirror; the pinhole aperture is placed on the Fourier spectrum plane for mode selection.

[0008] To better realize this utility model, further, the front principal distance of the laser crystal is... h 1. Rear principal distance h 2 are respectively: ; ; in: R 1 and R 2 represents the radii of curvature of the front and rear ends of the laser crystal, respectively; n 1 represents the refractive index of the surrounding medium; n 2 represents the refractive index of laser crystal 2; l For length; The effective focal length of the laser crystal f T The calculation formula is: .

[0009] To better realize this utility model, the laser crystal and the first plano-convex lens further constitute a combined lens, and the focal length of the combined lens is... f for: ; in: f 1 represents the effective focal length of the first plano-convex lens; h 2 represents the distance between the rear principal plane and the rear end face of the laser crystal; d 2 represents the distance from the first plano-convex lens to the right end face of the laser crystal.

[0010] To better realize this utility model, further, the length of the aperture stop from the right end face of the first plano-convex lens is... f B for: .

[0011] To better realize this utility model, further, the actual distance between the front focal plane and the front end face of the laser crystal 2 is... d 1 is: ; in: h 1 represents the distance between the front principal plane and the front end face of the laser crystal.

[0012] To better realize this utility model, the effective focal length of the second plano-convex lens is further defined as follows: f 2. The distance between the plano-convex lens and the pinhole aperture is... d 3. The distance between the partially reflecting mirror and the plano-convex lens is... d 4; and d 3= d 4= f 2.

[0013] To better realize this utility model, the laser crystal is further installed between the first plano-convex lens and the pinhole aperture, so that the rear focal plane of the combined lens formed by the thermal lens of the laser crystal and the first plano-convex lens coincides with the front focal plane of the second plano-convex lens.

[0014] To better realize this utility model, the laser crystal is further installed between the pinhole aperture and the second plano-convex lens, so that the front focal plane of the combined lens formed by the thermal lens of the laser crystal and the second plano-convex lens coincides with the rear focal plane of the first plano-convex lens.

[0015] To better realize this utility model, the laser crystal is further installed between the second plano-convex lens and a partial reflector, so that the front focal plane of the combined lens formed by the thermal lens of the laser crystal and the second plano-convex lens coincides with the rear focal plane of the first plano-convex lens.

[0016] To better realize this utility model, the first plano-convex lens and the second plano-convex lens are further replaced with biconvex lenses.

[0017] The beneficial effects of this utility model are as follows: This invention utilizes the inherent thick lens effect of solid-state laser crystals and combines the laser crystal with a first plano-convex lens to form a combined lens, reducing the impact of thermal lensing under continuous-wave lasing on the construction of degenerate cavity lasers, thus demonstrating good practicality. Specifically, compared to previous methods, this invention abandons the traditional approach of using quasi-continuous pumping and large-end-face long-sized crystal rods to reduce thermal lensing effects. Instead, for laser crystals with small-end-face short-sized continuous pumping, it fully utilizes the inherent thick lens effect of solid-state laser crystals, comprehensively considering the optical characteristics of each component, and clearly provides analytical forms of the relative positional relationships and distances between optical components, achieving the construction of a perfect degenerate cavity laser. This invention can be applied to laser coherence modulation, holographic imaging, quantum simulation, laser communication, and optical diffraction neural network calculations. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the laser optical path of this utility model; Figure 2 This is a schematic diagram of a degenerate cavity laser with different pinhole aperture placement positions.

[0019] Wherein: 1-High-power line reflector; 2-Laser crystal; 3-First plano-convex lens; 4-Pinhole aperture; 5-Second plano-convex lens; 6-Partial reflector. Detailed Implementation

[0020] Example 1: A degenerate cavity laser based on the thick lens effect, such as Figure 1As shown, it includes a high-power line reflector 1, a laser crystal 2, a first plano-convex lens 3, a pinhole aperture 4, a second plano-convex lens 5, and a partial reflector 6. The optical elements are arranged sequentially from left to right, with equal height and coaxial alignment.

[0021] The distance between the high-power linear reflector 1 and the left end face of the laser crystal 2 is: d 1. The distance between the first plano-convex lens 3 and the right end face of the laser crystal 2 is _____. d 2. The distance between the pinhole aperture 4 and the right end face of the first plano-convex lens 3 is _ f B The distance between the second plano-convex lens 5 and the pinhole aperture 4 is d 3. The distance between the partial reflecting mirror 6 and the second plano-convex lens 5 is... d 4. The distances between the front principal plane and the front end face, and between the rear principal plane and the rear end face of laser crystal 2 are respectively... h 1 and h 2. To minimize spherical aberration, the lens should be positioned so that the side with the greatest curvature faces the farthest conjugate point. For a plano-convex lens used with an infinite conjugate ratio, this means that the curved surface should face the parallel beam, i.e., the convex surface of the first plano-convex lens 3 faces the laser crystal 2, and the convex surface of the second plano-convex lens 5 faces the partial reflector 6.

[0022] In use, the high-power line reflector 1 and the partial reflector 6 serve as end mirrors of the resonant cavity, causing the light to oscillate back and forth, thus increasing the photon number density within the cavity. The laser crystal 2 provides an energy level structure for stimulated emission and achieves population inversion during operation, generating laser light. The laser crystal 2, the first plano-convex lens 3, and the second plano-convex lens 5 form a focusing telescope system, enabling self-reproducing imaging of a single-pass journey from the high-power line reflector 1 to the partial reflector 6. The pinhole aperture 4 is placed on the Fourier spectrum plane to achieve mode selection.

[0023] The principle of this utility model is as follows: In an ideal 4f degenerate cavity laser, the forward transfer transformation matrix of the system from the high-power line mirror 1 to the partial mirror 6 is... H 1 is: ; in, A 1. B 1. C 1. D The physical meanings of 1 are the lateral magnification, transmission length, optical power, and angular magnification of the system's forward transmission.

[0024] Similarly, the reverse transmission transformation matrix of the system from partial reflector 6 to high-power line reflector 1... H 2 is: ; in,A 2. B 2. C 2. D The physical meanings of 2 are the lateral magnification, transmission length, optical power, and angular magnification of the system's reverse transmission, respectively.

[0025] The system's cyclic matrix H for: ; in, A , B , C , D The physical meanings of these are the system's lateral magnification, transmission length, optical power, and angular magnification, respectively.

[0026] However, due to the heat absorption of laser crystal 2, an uneven temperature gradient distribution is generated, resulting in a thermal lensing effect. Therefore, laser crystal 2 can be considered equivalent to a laser with a focal length of... f T The thin lens, assuming the distance from the high-power linear reflector 1 is... d T With partial reflector 6 as the reference plane, the system's loop matrix is... M for: ; It is evident that, in the presence of the thermal lensing effect, A' = D' ≦1, The system is degenerate into a stable cavity.

[0027] When the focal length of the thermal lens of laser crystal 2 is comparable to the crystal length, laser crystal 2 becomes a thick lens, making the properties of the laser more complex. We consider laser crystal 2 as an equivalent thick lens, assuming the radii of curvature of the end faces at the front and rear ends are respectively... R 1 and R 2. The refractive index of laser crystal 2 is n 2, length is l The refractive index of the surrounding medium is n 1. The front principal distance of the thick lens h 1. Rear principal distance h 2 are respectively: ; ; Let the effective focal length of the thick lens be... f T ,have: ; Using the critical cavity measurement method, we measured the front and rear focal lengths of the thick lens of laser crystal 2 to be... fe The relationship between front focal length, effective focal length, and principal distance: ; The radius of curvature of the end face is obtained by solving. R 1 and R 2. Thus, the effective focal length of the thermal lens is obtained. f T Front principal distance h 1 and rear principal distance h 2.

[0028] Due to the focal length of the lens f T Since the laser is on the order of dm, a combined lens method is used to construct a perfectly degenerate laser. The specific steps are as follows: Let the effective focal length of the first plano-convex lens 3, which is one of the composite lenses, be... f 1. The focal length of the combined lens is f ,but: ; Front focal length is f F : ; The actual distance between the front focal plane and the front end face of laser crystal 2 d 1 is: ; The actual distance between the back focal plane and the first plano-convex lens 3 is the back focal length. f B : ; Let the effective focal length of the second plano-convex lens 5 be... f 2. To achieve the degeneracy condition, we have: ; At this point, the loop matrix of the system from partial reflector 6 to high-power line reflector 1 is... H' 1: ; The reverse transmission transformation matrix of the system from high-power line mirror 1 to partial mirror 6 H' 2: ; The system's cyclic matrix H' T for: ; At this point, the system still satisfies the perfect degeneracy condition. A = D =1.

[0029] The principle of the spectral surface filtering function of pinhole aperture 4 is as follows: The wave entering aperture 4 is expanded according to the angular spectrum, and the amplitude of the wave in a single direction is multiplied by a function characterizing the transmittance of the selector. g ( i The wave is transformed back to its original spatial coordinate function using the inverse Fourier transform. Therefore, the introduction of the selector is equivalent to having a kernel. S ( x The action of the integral operator ; in, Let λ be the wave vector of the laser beam, and λ be the laser wavelength. i It is the angle between the direction of the light ray and the axis of the resonant cavity; g ( i ) is a scene in i The attenuation of the plane wave amplitude as the angle passes through. According to imaging theory, inserting a pinhole aperture 4 into the central spectral plane is equivalent to replacing part of the reflecting mirror 6 with the pinhole aperture 4. When the dimensions of the pinhole aperture 4 are much smaller than the cavity length of the resonant cavity, the field distribution on the degenerate resonant cavity surface... ( x The integral equation of ) has the following form: ; in, L It is the length of the laser cavity. a Let γ be the radius of the aperture stop 4, and γ be the eigenfield distribution. ( x The eigenvalues ​​of ).

[0030] By solving the integral equation and eigenvalues ​​of this resonant cavity with a selector, we can obtain the field distribution.

[0031] As can be seen from the form of the diffraction integral self-reproducibility equation, the consideration of aperture 4 in the studied case leads to the use of an integrator kernel with a selector: ; It replaces the integrator core of a conventional degenerate resonant cavity. This change in the integrator core inevitably alters the modes of the resonant cavity.

[0032] For aperture 4: ; Δ i This refers to the numerical aperture angle of the pinhole aperture 4, which measures the angular range of light that the system can collect.

[0033] Accordingly, ; exist kL ( Dth ) 2 When << 1, it is equivalent to finding the solution to the integral equation: ; Its solution is as follows: .

[0034] and These are the angular and radial wave functions in the long ellipsoidal coordinate system, also known as the oblate spheroid function.

[0035] The energy loss of the field during one round trip within the resonant cavity is determined as follows: .

[0036] It is evident that introducing a pinhole aperture 4 into the degenerate cavity laser increases the mode loss spacing and the loss of higher-order modes, thereby reducing the number of oscillating modes for the same gain. When the angular distribution is formed by multimode oscillation, the addition of pinhole aperture 4 will reduce the beam divergence angle.

[0037] Regarding the number of modes involved in the oscillation: ; in, j It is the ratio of pump power to threshold pump power, and α is the loss coefficient determined by the transmission of partial reflector 6 and the absorption of optical elements. When kαΔ i >>1, meaning when the selector's transmission bandwidth is very wide, by adjusting... j By using α, an angular distribution close to the diffraction-limited beam can be obtained.

[0038] Preferably, the laser crystal 2 can be installed between the first plano-convex lens 3 and the pinhole aperture 4, or between the pinhole aperture 4 and the second plano-convex lens 5, or between the second plano-convex lens 5 and a portion of the reflector 6. The key point is that the front or rear focal plane of the combined lens formed by the thermal lens of the laser crystal 2 and one of the plano-convex lenses coincides with the focal plane of the other plano-convex lens.

[0039] Specifically, such as Figure 2 As shown in (a), the laser crystal 2 is installed between the first plano-convex lens 3 and the pinhole aperture 4. The rear focal plane of the combined lens formed by the thermal lens of the laser crystal 2 and the first plano-convex lens 3 coincides with the front focal plane of the second plano-convex lens 5.

[0040] like Figure 2As shown in (b), the laser crystal 2 is installed between the pinhole aperture 4 and the second plano-convex lens 5. The front focal plane of the combined lens formed by the thermal lens of the laser crystal 2 and the second plano-convex lens 5 coincides with the rear focal plane of the first plano-convex lens 3.

[0041] like Figure 2 As shown in (c), the laser crystal 2 is installed between the second plano-convex lens 5 and the partial reflector 6. The front focal plane of the combined lens formed by the thermal lens of the laser crystal 2 and the second plano-convex lens 5 coincides with the rear focal plane of the first plano-convex lens 3.

[0042] The distances between the aforementioned optical elements can be calculated using the relationship between the thick lens and the combined lens provided above.

[0043] Preferably, the laser crystal 2 can be an Nd:YAG, Nd:YLF, Nd:YO4, or other laser crystals. 3+ The doping concentration is 0.8at%~1.2at%, the diameter is 3~5 mm, the length is 50~70 mm, and the structure is cylindrical.

[0044] Preferably, the high-power line reflector 1 needs to be coated with a high-reflectivity film layer for the laser wavelength, the reflector needs to withstand high-power laser irradiation, the reflectivity is >99.9%, and the diameter is 25.4mm or 50.8mm; Some reflectors 6 can adopt a 7:3, 8:2, 9:1 ratio of reflection to transmission, and need to be coated with reflective and transmission films for the laser wavelength, with a diameter of 25.4 mm or 50.8 mm. The first plano-convex lens 3 and the second plano-convex lens 5 mentioned above can be replaced with biconvex lenses with a focal length of 20cm or 30cm and a diameter of 25.4mm or 50.8mm, and need to be coated with a high transmittance film layer for the laser wavelength.

[0045] The above description is merely a preferred embodiment of the present utility model and is not intended to limit the present utility model in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present utility model shall fall within the protection scope of the present utility model.

Claims

1. A degenerate cavity laser based on the thick lens effect, characterized in that, The system includes a high-power line reflector (1), a laser crystal (2), a first plano-convex lens (3), a pinhole aperture (4), a second plano-convex lens (5), and a partial reflector (6) arranged sequentially from front to back. The convex surface of the first plano-convex lens (3) faces the laser crystal (2), and the convex surface of the second plano-convex lens (5) faces the partial reflector (6). The high-power line reflector (1) and the partial reflector (6) constitute the end mirrors of the resonant cavity. The laser crystal (2), the first plano-convex lens (3), and the second plano-convex lens (5) constitute a focusing telescope system, which is used to realize self-reproducing imaging of a single-pass transit from the high-power line reflector (1) to the partial reflector (6). The pinhole aperture (4) is placed on the Fourier spectrum plane to realize the mode selection function.

2. A degenerate cavity laser based on the thick lens effect according to claim 1, characterized in that, The front principal distance of the laser crystal (2) h 1. Rear principal distance h 2 are respectively: ； ； in: R 1 and R 2 are the radii of curvature of the front and rear ends of the laser crystal (2), respectively; n 1 represents the refractive index of the surrounding medium; n 2 represents the refractive index of the laser crystal (2); l For length; The effective focal length of the laser crystal (2) f T The calculation formula is: 。 3. A degenerate cavity laser based on the thick lens effect according to claim 2, characterized in that, The laser crystal (2) and the first plano-convex lens (3) constitute a combined lens, and the focal length of the combined lens is... f for: ； in: f 1 is the effective focal length of the first plano-convex lens (3); h 2 is the distance between the rear principal plane and the rear end face of the laser crystal (2); d 2 is the distance from the first plano-convex lens (3) to the right end face of the laser crystal (2).

4. A degenerate cavity laser based on the thick lens effect according to claim 3, characterized in that, The distance between the pinhole aperture (4) and the right end face of the first plano-convex lens (3) is... f B for: 。 5. A degenerate cavity laser based on the thick lens effect according to claim 2, characterized in that, The actual distance between the front focal plane and the front end face of the laser crystal (2) d 1 is: ； in: h 1 is the distance between the front principal plane and the front end face of the laser crystal (2).

6. A degenerate cavity laser based on the thick lens effect according to claim 1, characterized in that, The effective focal length of the second plano-convex lens (5) is f 2. The distance between the plano-convex lens and the pinhole aperture (4) is d 3. The distance between the partially reflecting mirror (6) and the plano-convex lens is... d 4; and d 3= d 4= f 2.

7. A degenerate cavity laser based on the thick lens effect according to any one of claims 1-6, characterized in that, The laser crystal (2) is installed between the first plano-convex lens (3) and the pinhole aperture (4). The back focal plane of the combined lens formed by the thermal lens of the laser crystal (2) and the first plano-convex lens (3) coincides with the front focal plane of the second plano-convex lens (5).

8. A degenerate cavity laser based on the thick lens effect according to any one of claims 1-6, characterized in that, The laser crystal (2) is installed between the pinhole aperture (4) and the second plano-convex lens (5). The front focal plane of the combined lens formed by the thermal lens of the laser crystal (2) and the second plano-convex lens (5) coincides with the rear focal plane of the first plano-convex lens (3).

9. A degenerate cavity laser based on the thick lens effect according to any one of claims 1-6, characterized in that, The laser crystal (2) is installed between the second plano-convex lens (5) and the partial reflector (6). The front focal plane of the combined lens formed by the thermal lens of the laser crystal (2) and the second plano-convex lens (5) coincides with the rear focal plane of the first plano-convex lens (3).

10. A degenerate cavity laser based on the thick lens effect according to claim 1, characterized in that, The first plano-convex lens (3) and the second plano-convex lens (5) are respectively replaced with biconvex lenses.

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