CN214069079U

CN214069079U - Cascaded frequency doubling structure of nonlinear crystal

Info

Publication number: CN214069079U
Application number: CN202121728753.6U
Authority: CN
Inventors: 彭艳红; 杨毅
Original assignee: Sichuan Guangtianxia Laser Technology Co ltd
Current assignee: Sichuan Guangtianxia Laser Technology Co ltd
Priority date: 2021-07-28
Filing date: 2021-07-28
Publication date: 2021-08-27
Anticipated expiration: 2031-07-28

Abstract

The utility model provides a cascade frequency doubling structure of nonlinear crystals, which belongs to the field of solid lasers and comprises two heat sinks which are arranged in parallel and a plurality of cascade nonlinear crystals which are positioned between the two heat sinks; when the thermal lens of each nonlinear crystal is a positive lens, each nonlinear crystal is a negative lens; when the thermal lens of each nonlinear crystal is a negative lens, each nonlinear crystal is a positive lens; the fundamental frequency light is parallel to the surface of the heat sink, and the frequency doubling is realized by a plurality of cascaded nonlinear crystals in a unidirectional mode. The utility model discloses the hot lens condition according to nonlinear crystal sets up the incident terminal surface and the outgoing terminal surface of different curvatures for the influence of spontaneous dispersion or self-focusing can be resisted to the light beam in the nonlinear crystal, leads to laser focusing to the influence of doubling of frequency efficiency can not accumulate always because of the fuel factor in the nonlinear crystal, and the light beam is close parallel propagation between adjacent nonlinear crystal, when guaranteeing doubling of frequency efficiency, promotes doubling of frequency light beam quality.

Description

Cascaded frequency doubling structure of nonlinear crystal

Technical Field

The utility model belongs to the solid laser field, concretely relates to cascade doubling of frequency structure of nonlinear crystal.

Background

In order to realize nonlinear frequency doubling change, one mode is to utilize the birefringence characteristic and the dispersion characteristic of a nonlinear crystal and realize birefringence phase matching by selecting the wave vector direction and the polarization direction of the light wave, and the other mode is to adopt a quasi-phase matching technology and compensate the phase matching between interaction waves by periodically modulating the nonlinear coefficient of the crystal so as to realize the phase matching of the light wave with any wavelength in the whole light transmission range of the nonlinear crystal.

The nonlinear crystal can absorb fundamental wave energy in the harmonic generation process, so that the nonlinear crystal is subjected to high-power laser radiation to generate frequency conversion, local temperature rise in the light passing direction of the crystal can be caused, refractive index gradient change is generated in the crystal, laser is diffused towards two sides or focused towards the middle, wave vectors generate phase mismatch, and frequency doubling efficiency and beam quality are affected. The frequency doubling efficiency of nonlinear crystals such as PPLN (periodically poled lithium niobate crystals) is generally in direct proportion to the square of the crystal length, the length of the PPLN crystal used in a nanosecond-to-continuous laser system is generally 10-40 mm during practical use, obvious heat accumulation exists, the thermo-optic coefficient value of the nonlinear crystals is generally large, the change of temperature can seriously affect the refractive index, and the laser generates obvious self-focusing or self-dispersion.

SUMMERY OF THE UTILITY MODEL

The utility model provides a cascade doubling of frequency structure of nonlinear crystal to improve the phenomenon that the light beam quality variation or the doubling of frequency efficiency reduce that nonlinear crystal doubling of frequency in-process thermal effect leads to.

The utility model discloses specific technical scheme as follows:

the cascade frequency doubling structure of the nonlinear crystal is characterized by comprising two heat sinks arranged in parallel and a plurality of cascade nonlinear crystals positioned between the two heat sinks; when the thermal lens of each nonlinear crystal is a positive lens, each nonlinear crystal is a negative lens, and the incident end face and the emergent end face are both sunken towards the interior of the nonlinear crystal; when the thermal lens of each nonlinear crystal is a negative lens, each nonlinear crystal is a positive lens, and the incident end face and the emergent end face both protrude to the outside of the nonlinear crystal.

Further, the fundamental frequency light is parallel to the surface of the heat sink, and the frequency doubling is realized by a plurality of cascaded nonlinear crystals in a unidirectional mode.

Further, the incident end face radius of curvature R of each nonlinear crystal₁And radius of curvature R of exit end face₂The focal length f of the lens of the nonlinear crystal is equal to the equivalent focal length f' of the thermal lens, and the radius of curvature R of the incident end face is determined by the thermal lens effect of the nonlinear crystal₁And radius of curvature R of exit end face₂Satisfies the following conditions:

(ii) a Wherein n is the refractive index of the nonlinear crystal, d is the central thickness of the nonlinear crystal, and the equivalent focal length f' of the thermal lens is measured by experiments.

Furthermore, the heat sink is provided with a groove between adjacent nonlinear crystals to prevent the heat sink from sputtering materials due to laser energy absorption to pollute the incident end face or the emergent end face of the nonlinear crystal.

Furthermore, the incident end face and the emergent end face of each nonlinear crystal are plated with antireflection films.

Further, when the total length of the structure formed by a plurality of cascaded nonlinear crystals is constant, the shorter the length of each nonlinear crystal is, the larger the number of nonlinear crystals is, and the smaller the influence of thermal effect is.

The utility model has the advantages that:

the utility model discloses a plurality of cascaded short period polarization's nonlinear crystal has been set up, and set up the incident terminal surface and the emergent terminal surface of different curvatures according to the thermal lens condition of nonlinear crystal, make the influence of spontaneous dispersion or self-focusing can be resisted to the light beam among the nonlinear crystal, lead to laser focusing to the influence of doubling of frequency efficiency can not accumulate always in the nonlinear crystal because of the fuel factor, the light beam between adjacent nonlinear crystal is close parallel propagation, when guaranteeing doubling of frequency efficiency, promote doubling of frequency light beam quality.

Drawings

Fig. 1 is a schematic perspective view of a cascaded frequency doubling structure of a nonlinear crystal proposed in embodiment 1;

FIG. 2 is a schematic diagram of a cascaded frequency doubling structure of a nonlinear crystal proposed in example 1;

FIG. 3 is a schematic diagram showing a propagation route of fundamental frequency light in a nonlinear crystal in example 1;

the reference numbers are as follows:

1. a nonlinear crystal; 2. a heat sink.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention clearer, the present invention will be further described with reference to the following embodiments and accompanying drawings.

The following non-limiting examples will provide those of ordinary skill in the art with a more complete understanding of the present invention, but are not intended to limit the invention in any way.

Example 1

The embodiment provides a cascade frequency doubling structure of a nonlinear crystal, as shown in fig. 1 and 2, the cascade frequency doubling structure comprises two heat sinks 2 arranged in parallel and 4 cascade nonlinear crystals 1 positioned between the two heat sinks 2, each nonlinear crystal 1 is made of PPLN, the length is 10 mm, the width is 5 mm, and the thickness is 1.5 mm, because the thermo-optic coefficient of the PPLN is positive, and the thermal lens is a positive lens, both the incident end face and the emergent end face are recessed towards the inside of the nonlinear crystal 1, and the nonlinear crystal 1 itself is a negative lens; the 1064 nm fundamental frequency light is parallel to the surface of the heat sink 2, and passes through 4 cascaded nonlinear crystals 1 in a single direction, so that quasi-phase matching frequency multiplication conversion from 1064 nm to 532 nm is realized.

And antireflection films of 1064 nm fundamental frequency light and 532 nm frequency doubling light are plated on the incident end face and the emergent end face of each nonlinear crystal 1.

Two heat sinks 2 are provided with a groove between adjacent nonlinear crystals 1, so that the heat sinks 2 are prevented from sputtering materials due to the absorption of laser energy, and the incident end face or the emergent end face of the nonlinear crystals 1 is prevented from being polluted.

When the nonlinear crystal 1 is used, due to the fact that laser energy is absorbed, the temperature of the middle of the nonlinear crystal 1 is obviously higher than that of the side close to the heat sink 2, the refractive index inside the crystal changes in a gradient mode, the thermal light coefficient of the PPLN is positive, the refractive index of the middle of the nonlinear crystal 1 is higher, and the refractive index close to the side close to the heat sink 2 is lower, and therefore laser is converged to the middle, and therefore wave vector phase mismatch can occur in a traditional nonlinear crystal (a nonlinear crystal with planar incident end faces and planar emergent end faces), and frequency doubling efficiency and light beam quality are affected.

The utility model discloses well nonlinear crystal 1's incident terminal surface and emergent terminal surface all are sunken to nonlinear crystal 1 inside, are equivalent to a negative lens. As shown in fig. 3, since the incident end surface of the nonlinear crystal 1 is recessed toward the inside of the nonlinear crystal 1, the fundamental light should be refracted in the direction a when the incident end surface passes through the fundamental light, but the light is self-focused due to the change of the refractive index of the nonlinear crystal 1 during actual use, and the light path actually propagates in the direction b. By adjusting the curvature radius of the incident end face and the emergent end face, the influence of self-focusing is resisted, and the light beams are not focused seriously. Radius of curvature R of incident end face of nonlinear crystal 1₁And radius of curvature R of exit end face₂By the thermal lens effect decision of nonlinear crystal, record the thermal lens equivalent focal length f ' of traditional nonlinear crystal (incident end face and emergent end face are planar nonlinear crystal) that length is 10 mm through the experiment, the utility model discloses well nonlinear crystal 1's lens focal length f equals thermal lens equivalent focal length f ', and lens focal length f satisfies:

further, the radius of curvature R of the incident end face is obtained₁And radius of curvature R of exit end face₂(ii) a Wherein n is the refractive index of the nonlinear crystal, and d is the center thickness of the nonlinear crystal.

The frequency doubling efficiency is decided by the length of the nonlinear crystal 1 of periodic polarization, because the utility model discloses the structure of constituteing is cascaded by 4 nonlinear crystal 1 that length is 10 mm, and total length is 4 x 10 mm =40 mm, compares in the big nonlinear crystal of the same single of total length, and the heat effect obviously reduces. This is because in the longer single big nonlinear crystal of traditional length, the influence of the light focus that leads to because of the heat effect of crystal to doubling of frequency efficiency can accumulate always, and in the utility model provides a nonlinear crystal 1 cascade structure, a cascade unit (being nonlinear crystal 1) is equivalent to the negative lens, and the self-focusing that the heat effect leads to is rectified, and the light focus also can not be accumulated to next cascade unit to the influence of doubling of frequency efficiency, and then in the structure of cascading by 4 nonlinear crystals 1 and forming, the influence of thermal effect to doubling of frequency efficiency is carefully improved, still can promote doubling of frequency light beam quality.

Because the incident end face and the emergent end face of the nonlinear crystal 1 are curved surfaces, when laser passes through the nonlinear crystal in a single direction, few parts of the laser can be transmitted to the heat sink 2 after being emitted from the nonlinear crystal 1, the heat sink 2 can cause material sputtering due to the fact that the energy of the laser is absorbed, and the grooves can prevent the heat sink material from being sputtered to the incident end face or the emergent end face of the nonlinear crystal, so that the incidence and the emission of the laser are prevented from being influenced. The side surface of the nonlinear crystal is generally plated with a protective film layer, and the heat sink material is sputtered to the surface without influencing the use of the nonlinear crystal.

Claims

1. The cascade frequency doubling structure of the nonlinear crystal is characterized by comprising two heat sinks arranged in parallel and a plurality of cascade nonlinear crystals positioned between the two heat sinks; when the thermal lens of each nonlinear crystal is a positive lens, each nonlinear crystal is a negative lens; when the thermal lens of each nonlinear crystal is a negative lens, each nonlinear crystal is a positive lens.

2. The cascaded frequency doubling structure of claim 1, wherein a lens focal length f of each nonlinear crystal is equal to a thermal lens equivalent focal length f', and an incident end surface radius of curvature R of each nonlinear crystal₁And radius of curvature R of exit end face₂Satisfies the following conditions:

(ii) a Wherein n is the refractive index of the nonlinear crystal, and d is the center thickness of the nonlinear crystal.

3. The cascaded frequency doubling structure of nonlinear crystals according to claim 1, wherein the heat sink is provided with a groove between adjacent nonlinear crystals.

4. The cascaded frequency doubling structure of nonlinear crystals according to claim 1, wherein an antireflection film is coated on both the incident end face and the exit end face of each nonlinear crystal.