CN108107641B - Second harmonic generation method and device based on photovoltaic photorefractive diffusion management twin - Google Patents

Abstract

A second harmonic generation method and device based on photovoltaic photorefractive diffusion management twins. The photovoltaic photorefractive diffusion management twins are formed by symmetrical coupling of two photovoltaic photorefractive crystals of the same material along a common crystal plane. The method and device can overcome the large-angle self-deflection of the spatial soliton beam caused by the carrier diffusion effect existing in a single photovoltaic photorefractive crystal and the large-angle self-deflection of the soliton beam caused by the fundamental frequency light and the two The phase mismatch between subharmonics can also protect the second harmonic generation process from external environment interference and prevent the generated second harmonics from leaking into the air, thereby fully improving the conversion efficiency of second harmonic generation .

Description

Second harmonic generation method and device based on photovoltaic photorefractive diffusion management twin

technical field

The invention relates to the field of optical second harmonic generation, in particular to a second harmonic generation method and device based on photovoltaic photorefractive diffusion management twins.

Background technique

In the field of optics, the generation of the second harmonic is an optical frequency conversion effect, which has been developed into a laser frequency conversion technology widely used in the generation of coherent light sources, and can be used to obtain new laser light sources. The generation of second harmonics can expand the wavelength range of limited light sources to meet the actual needs of coherent light sources with different wavelengths in various application fields such as industry, agriculture, national defense, medical treatment and scientific research. Therefore, in terms of expanding the coverage of laser spectral lines, Especially in promoting the development of short-wavelength laser technology, it plays an important role.

The generation of the second harmonic is a degenerate three-wave mixing process caused by the second-order nonlinear electrical polarization of the medium under the action of the fundamental frequency light wave electric field. Among them, achieving phase matching is a necessary condition for exciting the second harmonic, which determines the conversion efficiency and light wave quality of the second harmonic. At present, the commonly used methods for realizing phase matching in the industry are birefringent phase matching (Birefringent Phase Matched, referred to as BPM) and quasi-phase matching (Quasi-Phase Matched, referred to as QPM). Among them, the birefringence phase matching is to use the birefringence effect and dispersion characteristics of the excellent uniaxial or biaxial nonlinear crystal, and change the relative refractive index of the fundamental frequency light and the second harmonic wave by selecting the wave vector direction and polarization direction of the light wave. The size and make it meet the phase velocity matching, and realize the birefringent phase matching by adjusting the incident angle of the light beam on the incident surface of the crystal. However, when this method is used to achieve phase matching, the fundamental frequency light and the second harmonic will gradually separate during the transmission process, thereby reducing the conversion efficiency, which is also called the walk-off effect. Quasi-phase matching is to form a grating structure with opposite polarization directions by periodically modulating the nonlinear polarizability of the optical crystal in one-dimensional space, and use the period to provide an appropriate additional wave vector for the micron-scale grating to compensate Due to the phase mismatch between the fundamental frequency light and the second harmonic, there is no walk-off effect in quasi-phase matching, and a large conversion efficiency can be obtained. However, due to the complex process of manufacturing periodically polarized crystals and the difficulty of obtaining large-sized polarized crystals, the cost of using quasi-phase matching to excite second harmonics is relatively high and it is difficult to achieve high conversion efficiency.

Since the conversion efficiency of the second harmonic will increase with the increase of the intensity of the fundamental frequency light, if the fundamental frequency light is focused and injected into the crystal, the conversion efficiency of the second harmonic can be improved. However, when using birefringent phase matching and quasi-phase matching to generate second harmonics in bulk materials, if the width of the focused incident beam is narrow, the beam will quickly diverge under the effect of diffraction Broadening, which will cause the phase mismatch between the fundamental frequency light and the second harmonic, which will greatly reduce the conversion efficiency of the second harmonic. This problem can be solved by confining the interacting light waves to propagate in the waveguide. The optical waveguide is a structure formed by a medium with a lower refractive index wrapped around a medium with a higher refractive index, and uses the principle of total reflection of light to confine light to a micron-scale area for transmission. Optical waveguides include cured waveguides and flexible waveguides. Among them, the solidified waveguide is an optical waveguide structure fabricated in optical materials such as optical crystals, glass, semiconductors, and organic polymers by using technologies such as ion implantation, proton exchange, and femtosecond laser writing. The structure of the cured waveguide is fixed, and once it is manufactured, it cannot be changed and erased, which makes the structural parameters of the cured waveguide unable to be adjusted. The flexible waveguide is a light-induced refractive index waveguide structure induced by light beams in photorefractive materials. Since the structure of the light-sensing refractive index waveguide can be adjusted by changing the intensity profile and power of the induced beam, and the light-sensing refractive index waveguide can be repeatedly erased and written, the structural parameters of the flexible waveguide induced by the light beam can be flexibly adjusted .

In practical applications, the light beam used to induce the flexible waveguide should have the characteristics of no diffraction broadening, so the best choice for the light beam used to induce the flexible waveguide is to use photorefractive spatial solitons. The generation of photorefractive spatial solitons is the result of the strict compensation of the broadening caused by the diffraction effect and the constraint effect caused by the nonlinear photorefractive effect. At present, three kinds of steady-state photorefractive solitons have been found: shielding solitons, photovoltaic solitons and shielding photovoltaic solitons; among them, shielding solitons exist in non-photorefractive photorefractive crystals with an external electric field, and photovoltaic solitons exist in non-photorefractive crystals without an external electric field. In photovoltaic photorefractive crystals, shielding photovoltaic solitons exist in photovoltaic photorefractive crystals with an external electric field. A beam of light with a refractive index-sensitive wavelength forms a spatial soliton in the photorefractive crystal, and at the same time a flexible optical waveguide is written inside the crystal, and the waveguide can be used to guide another beam of light with a non-refractive index-sensitive wavelength. The flexible optical waveguide induced by photorefractive spatial solitons can not only guide a beam of strong light with a beam of weak light, but also realize the efficient generation of second harmonics. In practical applications, bright photorefractive spatial solitons are especially suitable for applications that require intense Second harmonic generation of nonlinear optical effects.

The advantage of generating the second harmonic in the waveguide is that the confinement of the waveguide can concentrate the energy of the beam, which will easily excite strong nonlinear effects, so that it is easy to obtain the second harmonic generation with high conversion efficiency. However, the method of using bright photorefractive space solitons to induce flexible waveguides to generate second harmonics is affected by the carrier diffusion effect in photorefractive crystals. The diffusion effect will cause the soliton beam to undergo lateral self-deflection during transmission (the size of the self-deflection is related to the width of the beam, and the narrower the beam is, the greater the self-deflection will be), resulting in the bending of the flexible waveguide induced by the soliton beam. A flexible waveguide that is too bent cannot effectively guide the interacting fundamental frequency light and the second harmonic at the same time, so that not only the conversion efficiency of the second harmonic cannot be enhanced, but the conversion efficiency will be reduced, and even the second harmonic cannot be generated. Therefore, in order to fully improve the conversion efficiency of the second harmonic generated in the flexible waveguide, the self-deflection of the soliton beam caused by the diffusion effect must be effectively suppressed.

It has been found in practice that the direction of self-deflection of bright photorefractive space solitons under the effect of carrier diffusion in the crystal is related to the type of carriers. For crystals in which the effective carriers are electrons (such as KNSBN crystals), the diffusion effect makes the bright photorefractive space solitons have a transverse acceleration opposite to the c-axis of the crystal and deflects the soliton beams to the negative c-plane of the crystal; while for the effective current-carrying For crystals in which the son is a hole (such as BaTiO ₃ crystal), the diffusion effect makes the bright photorefractive space soliton have a transverse acceleration in the same direction as the crystal c-axis and deflects the soliton beam to the positive c-plane of the crystal. In 2010, HZKang et al. used the total reflection of the beam on the surface of the crystal in contact with the air to suppress the self-deflection of the beam caused by the diffusion effect, and formed a monolithic SBN crystal (whose effective carriers are electrons) with an external electric field. The surface bright photorefractive space solitons transmitted along the natural straight path of the negative c surface of the crystal, and the second harmonic with a conversion efficiency of 83.4%/W was obtained by using the flexible waveguide on the crystal surface induced by the surface bright photorefractive space solitons waves are generated.

It is found in practice that although the crystal surface flexible waveguide close to the crystal surface can overcome the adverse effect of the diffusion effect on the second harmonic conversion efficiency, but due to the large contact surface between the crystal surface flexible waveguide and the air, the transmission in the waveguide The light beam is easily affected by the surrounding environment (such as the intensity of ambient light, the humidity of the air, and the cleanliness of the crystal surface), so that the conversion efficiency of the second harmonic is easily affected by the surrounding environment, and even leads to the generated second harmonic Some of the wave's energy leaks out of the waveguide into the air.

Contents of the invention

The embodiment of the present invention discloses a second harmonic generation method and device based on photovoltaic photorefractive diffusion management twins, which can overcome the spatial solitons caused by the carrier diffusion effect existing in a single photovoltaic photorefractive crystal The large-angle self-deflection of the beam and the phase mismatch between the fundamental frequency light and the second harmonic caused by the large-angle self-deflection of the soliton beam can also protect the second harmonic generation process from external environmental interference and prevent the generated The second harmonic leaks into the air, thereby substantially improving the conversion efficiency of the second harmonic generation.

The first aspect of the embodiments of the present invention discloses a second harmonic generation device based on photovoltaic photorefractive diffusion management twins. The photovoltaic photorefractive diffusion management twins are composed of two photovoltaic photorefractive crystals of the same material along a The common crystal plane is mirror-symmetrically coupled, and the device is used to induce an intracrystalline strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin by using bright photovoltaic photorefractive space solitons, Phase matching is achieved by adjusting the propagation constants of the fundamental frequency light and the second harmonic in the waveguide, thereby achieving efficient generation of the second harmonic.

As a preferred implementation mode, in the first aspect of the embodiment of the present invention:

When the effective carriers of the two photovoltaic photorefractive crystals of the same material are electrons, the negative c-sides of the two photovoltaic photorefractive crystals of the same material are closely bonded together after optical polishing to form Electronic photovoltaic photorefractive diffusion management twinning;

When the effective carriers of the two photovoltaic photorefractive crystals of the same material are both holes, the front c-planes of the two photovoltaic photorefractive crystals of the same material are optically polished and tightly bonded together so that Formation of hole-type photovoltaic photorefractive diffusion-managed twins.

As a preferred implementation mode, in the first aspect of the embodiment of the present invention:

The electron-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane;

The hole-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane.

As a preferred implementation, in the first aspect of the embodiment of the present invention, the second harmonic generation device further includes a precision lifting platform, and the precision lifting platform is used to place the photovoltaic photorefractive diffusion management twin , and make the twin planes of the photovoltaic photorefractive diffusion management twins parallel to the horizontal plane, and be used to adjust the vertical height of the photovoltaic photorefractive diffusion management twins.

As a preferred implementation, in the first aspect of the embodiment of the present invention, the second harmonic generation device further includes a fundamental frequency light input optical device group, a second harmonic output optical device group, and a background light bidirectional illumination optical device device group, where:

The fundamental frequency light input optical device group includes a fundamental frequency light source, a first collimating convex lens, a first polarizer and a focusing convex lens;

The second harmonic output optical device group includes a second collimating convex lens and a beam splitter;

The background light two-way illumination optical device group includes a background light source, a third collimating convex lens, a second polarizer, a half mirror, a forward illumination reflector, a total reflection mirror and a back illumination reflector;

The background light source is used to provide an incoherent background light;

The third collimating convex lens is used to collimate the divergent background light into parallel light;

The second polarizer is used to change the collimated background light into linearly polarized light whose polarization direction is parallel to the twin plane of the photovoltaic photorefractive diffusion management twin;

The half-mirror is used to divide the polarized background light into equal-intensity forward illumination light and back illumination light;

the forward-illuminating reflector for reflecting the forward-illuminating light onto the front face of the photovoltaic photorefractive diffusion management twin;

The total reflection mirror is used to change the direction of the backlighting light;

The back-illumination reflector is used to reflect the reversed back-illumination light to the rear end face of the photovoltaic photorefractive diffusion management twin, so that the forward-illumination light and the back-illumination light with equal intensity The photovoltaic photorefractive diffusion management twins are transmitted in parallel and opposite directions, so that the superimposed forward illumination light and back illumination light perform bidirectional and uniform illumination on the photovoltaic photorefractive diffusion management twins;

The fundamental frequency light source is used to provide coherent fundamental frequency light;

The first collimating convex lens is used to collimate the divergent fundamental frequency light into a parallel beam;

The first polarizer is used to change the collimated fundamental frequency light into linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin;

The focusing convex lens is used to focus the polarized fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and distribute the focused light spots symmetrically on both sides of the twin plane;

Wherein, when the output power of the background light source is changed, the intensity of the forward illumination light and the back illumination light is also changed, thereby adjusting the intensity of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin and response time;

Wherein, when the output power of the fundamental frequency light source and the distance between the focusing convex lens and the photovoltaic photorefractive diffusion management twin are changed, the fundamental frequency light will be in the photovoltaic photorefractive diffusion management twin The light intensity and size of the focused light spot on the front face of the crystal are also changed, thereby adjusting the photovoltaic photorefractive nonlinear self-focusing effect that the fundamental frequency light receives in the photovoltaic photorefractive diffusion management twin, until the The self-focusing effect of the fundamental frequency light compensates its diffraction broadening effect, so that the fundamental frequency light can be transmitted without diffraction in the photovoltaic photorefractive diffusion management twin, that is, the fundamental frequency light is transmitted in the photovoltaic photorefractive diffusion management twin Realize the transmission of bright photovoltaic photorefractive spatial solitons, and make the transmission track of the center of the fundamental frequency light beam be a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin;

Wherein, by lifting the precision lifting table, the focus spot center of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin deviates from the twin plane, so that the fundamental frequency light points to the lateral direction of the twin plane The transmission trajectory of the beam center under the action of acceleration evolves into a serpentine curve that periodically passes through the twin plane;

Wherein, the distance between the central position of the focused spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and the twin plane is changed by lifting the precision lifting table, thereby adjusting the fundamental frequency light on the photovoltaic photorefractive diffusion management twin. Photorefractive diffusion manages the bending degree of the serpentine transmission trajectory in the twin, so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and achieve optical The second harmonic is observed at the output end of the device group; wherein, when observing the second harmonic, the second collimating convex lens is used to convert the divergent containing base emitted from the photovoltaic photorefractive diffusion management twin The mixed light beam of frequency light and second harmonic is collimated into a parallel mixed light beam; the beam splitter is used for reflecting the second harmonic in the mixed light beam, and for Transmission is performed to observe the light intensity change of the second harmonic alone;

Wherein, the operating state of the second harmonic generation device is regulated and optimized, so that the mixed light beam composed of the fundamental frequency light and the second harmonic coherently superimposed is formed in the photovoltaic photorefractive diffusion management twin Stable two-color bright photovoltaic photorefractive space soliton, and make the second harmonic generation achieve the largest possible conversion efficiency.

The second aspect of the embodiment of the present invention discloses a second harmonic generation method based on photovoltaic photorefractive diffusion management twins. The photovoltaic photorefractive diffusion management twins are composed of two photovoltaic photorefractive crystals of the same material along a The common crystal surface is mirror-symmetrically coupled, and the method includes:

Using bright photovoltaic photorefractive spatial solitons to induce an intragranular strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin crystal, so that by adjusting the fundamental frequency light and the second harmonic The propagation constant in the waveguide described above is used to achieve phase matching, resulting in efficient generation of the second harmonic.

As a preferred implementation, in the second aspect of the embodiment of the present invention: when the effective carriers of the two photovoltaic photorefractive crystals of the same material are electrons, the photovoltaic photorefractive crystals of the two same materials The negative c-sides of the foldable crystals are optically polished and closely bonded together to form electronic photovoltaic photorefractive diffusion management twins; when the effective carriers of the two photovoltaic photorefractive crystals of the same material are both In the case of holes, the front c-planes of the two photovoltaic photorefractive crystals of the same material are optically polished and closely bonded together to form hole-type photovoltaic photorefractive diffusion management twins.

As a preferred implementation, in the second aspect of the embodiment of the present invention: the electron-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane; the hole Type photovoltaic photorefractive diffusion-managed twins have opposite carrier diffusion effects on both sides of the twin plane.

As a preferred implementation manner, in the second aspect of the embodiment of the present invention, the method further includes:

placing the photovoltaic photorefractive diffusion management twins on a precision lifting platform, making the twin planes of the photovoltaic photorefractive diffusion management twins parallel to the horizontal plane, and adjusting the photovoltaic photorefractive diffusion management twins vertical height.

As a preferred implementation manner, in the second aspect of the embodiment of the present invention, the method further includes:

Use background light sources to provide incoherent background lighting;

Collimating the divergent background light into parallel light through the third collimating convex lens;

changing the collimated background light into linearly polarized light whose polarization direction is parallel to the twin plane of the photovoltaic photorefractive diffusion management twin through the second polarizer;

The polarized background light is evenly divided into forward lighting and back lighting with equal intensity through the half mirror;

reflecting the forward illumination light onto the front face of the photovoltaic photorefractive diffusion management twin via a forward illumination mirror;

Utilizing a total reflection mirror to change the direction of the backlighting light;

The reversed back-illumination light is reflected to the rear end face of the photovoltaic photorefractive diffusion management twin through the back-illumination reflector, so that the forward and back illumination lights with equal intensities The refractive diffusion management twins are parallel to each other and transmitted in opposite directions, so that the superimposed forward illumination light and back illumination light illuminate the photovoltaic photorefractive diffusion management twins bidirectionally and uniformly;

Using a fundamental frequency light source to provide coherent fundamental frequency light;

collimating the divergent fundamental frequency light into a parallel beam through the first collimating convex lens;

Transforming the collimated fundamental frequency light into linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin through the first polarizer;

Focusing the polarized fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin through a focusing convex lens and making the focused light spots symmetrically distributed on both sides of the twin plane;

changing the intensity of the forward illumination light and the back illumination light by changing the output power of the background light source, thereby adjusting the intensity and response time of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin;

Adjusting the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin by changing the output power of the fundamental frequency light source and the distance between the focusing convex lens and the photovoltaic photorefractive diffusion management twin focusing the light intensity and size of the light spot, thereby adjusting the photovoltaic photorefractive nonlinear self-focusing effect suffered by the fundamental frequency light in the photovoltaic photorefractive diffusion management twin, until the self-focusing effect of the fundamental frequency light is compensated Its diffraction broadening effect enables the fundamental frequency light to realize non-diffraction transmission in the photovoltaic photorefractive diffusion management twin, that is, the fundamental frequency light realizes bright photovoltaic photorefractive space in the photovoltaic photorefractive diffusion management twin Soliton transmission, and make the transmission track of the center of the fundamental frequency light beam be a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin;

Wherein, by lifting the precision lifting table, the focus spot center of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin deviates from the twin plane, so that the fundamental frequency light points to the lateral direction of the twin plane The transmission trajectory of the beam center under the action of acceleration evolves into a serpentine curve that periodically passes through the twin plane;

Wherein, the distance between the central position of the focused spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and the twin plane is changed by lifting the precision lifting table, thereby adjusting the fundamental frequency light on the photovoltaic photorefractive diffusion management twin. Photorefractive diffusion manages the bending degree of the serpentine transmission trajectory in the twin, so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and achieve optical The second harmonic is observed at the output end of the device group; wherein, when observing the second harmonic, the second collimating convex lens is used to convert the divergent containing base emitted from the photovoltaic photorefractive diffusion management twin The mixed light beam of frequency light and second harmonic is collimated into a parallel mixed light beam; the beam splitter is used for reflecting the second harmonic in the mixed light beam, and for Transmission is performed to observe the light intensity change of the second harmonic alone;

Wherein, the operating state of the second harmonic generation device is regulated and optimized, so that the mixed light beam composed of the fundamental frequency light and the second harmonic coherently superimposed is formed in the photovoltaic photorefractive diffusion management twin Stable two-color bright photovoltaic photorefractive space soliton, and make the second harmonic generation achieve the largest possible conversion efficiency.

Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

In the embodiment of the present invention, by managing the diffusion effect of photorefractive photorefractive carriers in twin crystals, the large angle of occurrence of bright photovoltaic photorefractive spatial solitons with narrow beam widths under the effect of carrier diffusion is suppressed. Self-deflection, so that bright photovoltaic photorefractive spatial solitons can always be transported inside the photovoltaic photorefractive diffusion management twins without deflecting to the surface of the twins, thereby being able to overcome the existing in monolithic photovoltaic photorefractive crystals The large-angle self-deflection of the spatial soliton beam caused by the carrier diffusion effect and the phase mismatch between the fundamental frequency light and the second harmonic caused by the large-angle self-deflection of the soliton beam can also protect the second harmonic generation process It is free from external environmental interference and prevents the generated second harmonic from leaking into the air, thereby fully improving the conversion efficiency of the second harmonic generation. It has high practical value in the fields of laser, all-optical frequency converter and all-optical communication.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For Those of ordinary skill in the art can also obtain other drawings based on these drawings without making creative efforts.

Figure 1 is a schematic structural diagram of a second harmonic generation device based on photovoltaic photorefractive diffusion management twins disclosed in an embodiment of the present invention;

Figure 2 is a schematic diagram of the structure of electronic photovoltaic photorefractive diffusion management twins;

Figure 3 is a schematic diagram of the structure of a hole-type photovoltaic photorefractive diffusion management twin;

Fig. 4 is a schematic diagram of the transmission track of the beam center of the bright photovoltaic photorefractive spatial soliton disclosed in the embodiment of the present invention in the photovoltaic photorefractive diffusion management twin;

Fig. 5 is a schematic diagram of the principle of second harmonic phase matching of the two-color bright photovoltaic photorefractive space soliton disclosed in the embodiment of the present invention in a strip-shaped flexible optical waveguide with the twin plane as the central axis;

Among them, 1-photovoltaic photorefractive diffusion management twins, 1A-photovoltaic photorefractive crystal, 1B-photovoltaic photorefractive crystal, 2-precision lifting platform, 3-fundamental frequency light source, 4-first collimating convex lens, 5 - the first polarizer, 6 - focusing convex lens, 7 - the second collimating convex lens, 8 - beam splitter, 9 - background light source, 10 - the third collimating convex lens, 11 - the second polarizer, 12 - semi-transparent Mirror, 13-forward lighting reflector, 14-full mirror, 15-backward lighting reflector.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

It should be noted that the terms "comprising" and "having" and any variations thereof in the embodiments of the present invention are intended to cover non-exclusive inclusion, for example, a process, method, system, product or process that includes a series of steps or units. The apparatus is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to the process, method, product or apparatus.

The embodiment of the present invention discloses a second harmonic generation method and device based on photovoltaic photorefractive diffusion management twins. The photovoltaic photorefractive diffusion management twins consist of two photovoltaic photorefractive crystals of the same material along a common The method and device can use bright photovoltaic photorefractive space solitons to induce an intracrystalline strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin crystal, so that By adjusting the propagation constants of the fundamental frequency light and the second harmonic in the intracrystalline strip-shaped flexible optical waveguide to achieve phase matching, the efficient generation of the second harmonic can be achieved, which can overcome the limitations existing in monolithic photovoltaic photorefractive crystals. The large-angle self-deflection of the spatial soliton beam caused by the carrier diffusion effect and the phase mismatch between the fundamental frequency light and the second harmonic caused by the large-angle self-deflection of the soliton beam can also protect the second harmonic generation process It is free from external environmental interference and prevents the generated second harmonic from leaking into the air, thereby fully improving the conversion efficiency of the second harmonic generation. A detailed description is given below in conjunction with the accompanying drawings.

Please refer to FIG. 1 . FIG. 1 is a schematic structural diagram of a second harmonic generation device based on photovoltaic photorefractive diffusion management twins disclosed in an embodiment of the present invention. As shown in Figure 1, the second harmonic generation device based on photovoltaic photorefractive diffusion management twins may include:

The photovoltaic photorefractive diffusion management twin 1, wherein the photovoltaic photorefractive diffusion management twin 1 is formed by symmetrically coupling a photovoltaic photorefractive crystal 1A and a photovoltaic photorefractive crystal 1B along a common crystal plane;

The precision lifting platform 2 is used to place the photovoltaic photorefractive diffusion management twin 1, and make the twin plane of the photovoltaic photorefractive diffusion management twin 1 parallel to the horizontal plane, and to adjust the photovoltaic photorefractive diffusion management twin 1 the vertical height of

As shown in Figure 1, the second harmonic generation device based on photovoltaic photorefractive diffusion management twins also includes a fundamental frequency light input optical device group, a second harmonic output optical device group, and a background light bidirectional illumination optical device group, in:

The fundamental frequency light input optical device group includes a fundamental frequency light source 3, a first collimating convex lens 4, a first polarizer 5 and a focusing convex lens 6;

The second harmonic output optical device group includes a second collimating convex lens 7 and a beam splitter 8;

The background light two-way illumination optical device group includes a background light source 9, a third collimating convex lens 10, a second polarizer 11, a half-transparent mirror 12, a forward illumination reflector 13, a total reflection mirror 14 and a back illumination reflector 15.

Among them, the photovoltaic photorefractive diffusion management twin 1 is the core component to realize the second harmonic generation. ) are mirror-symmetrically coupled. As shown in Figure 2, when the effective carriers of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B are electrons, the negative c-planes of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B can be passed through After optical polishing, they are tightly bonded together to form electronic photovoltaic photorefractive diffusion management twins; or, as shown in Figure 3, when the effective carriers of photovoltaic photorefractive crystal 1A and photovoltaic photorefractive crystal 1B are both When it is a hole, the positive c-sides of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B can be closely bonded together after optical polishing to form a hole-type photovoltaic photorefractive diffusion management twin. It should be pointed out that the electron-type photovoltaic photorefractive diffusion management twins have opposite carrier diffusion effects on both sides of the twin plane; similarly, the hole-type photovoltaic photorefractive diffusion management twins The two sides of the face also have opposite carrier diffusion effects.

In the second harmonic generation device based on photovoltaic photorefractive diffusion management twins shown in Figure 1:

Background light source 9, for providing incoherent background light;

The third collimating convex lens 10 is used to collimate the divergent background light into parallel light;

The second polarizer 11 is used to change the collimated background light into linearly polarized light whose polarization direction is parallel to the twin plane of the photovoltaic photorefractive diffusion management twin 1 (that is, parallel to the Y axis);

Half mirror 12, used to divide the polarized background light into forward lighting light and back lighting light with equal intensity;

A forward lighting reflector 13, used to reflect the forward lighting light to the front face of the photovoltaic photorefractive diffusion management twin 1;

Total mirror 14, used to change the direction of backlighting light;

The back-illumination reflector 15 is used to reflect the reversed back-illumination light to the rear end face of the photovoltaic photorefractive diffusion management twin 1, so that the forward-illumination light and the back-illumination light with equal intensity are in the same direction as the photovoltaic light. The refractive diffusion management twins 1 are parallel to each other and transmitted in opposite directions, so that the superimposed forward illumination light and back illumination light illuminate the photovoltaic photorefractive diffusion management twins 1 bidirectionally and uniformly;

The fundamental frequency light source 3 is used to provide coherent fundamental frequency light;

The first collimating convex lens 4 is used to collimate the divergent fundamental frequency light into a parallel beam;

The first polarizer 5 is used to change the collimated fundamental frequency light into linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin 1 (that is, parallel to the X axis);

The focusing convex lens 6 is used to focus the polarized fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 and distribute the focused light spots symmetrically on both sides of the twin plane;

Wherein, when the output power of the background light source 9 is changed, the intensity of the forward illumination light and the back illumination light is also changed, thereby adjusting the intensity and response of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin 1 time;

Wherein, when the output power of the fundamental frequency light source 3 and the distance between the focusing convex lens 6 and the photovoltaic photorefractive diffusion management twin 1 are changed, the fundamental frequency light is focused on the front face of the photovoltaic photorefractive diffusion management twin 1 The light intensity and size of the light spot are also changed, thereby adjusting the photovoltaic photorefractive nonlinear self-focusing effect suffered by the fundamental frequency light in the photovoltaic photorefractive diffusion management twin 1, until the self-focusing effect of the fundamental frequency light compensates for its Diffraction broadening effect, so that the fundamental frequency light can be transmitted without diffraction in the photovoltaic photorefractive diffusion management twin 1, that is, the fundamental frequency light can realize bright photovoltaic photorefractive spatial soliton transmission in the photovoltaic photorefractive diffusion management twin 1, And make the transmission track of the center of the fundamental frequency light beam be a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin 1;

Among them, by lifting the precision lifting table 2, the center of the focused spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 deviates from the twin plane, so that the fundamental frequency light points to the lateral acceleration of the twin plane The transmission trajectory of the beam center below evolves into a serpentine curve that periodically passes through the twin plane; Transmission track of photovoltaic photorefractive crystal 1A and photovoltaic photorefractive crystal 1B incident on crystal 1;

Among them, the distance between the central position of the focused light spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 and the twin plane is changed by lifting the precision lifting platform 2, thereby adjusting the fundamental frequency light on the photovoltaic photorefractive diffusion management twin. The bending degree of the serpentine transmission track in the variable diffusion management twin 1 (see Fig. 4, the solid line, dotted line and dashed line in Fig. The transmission conditions when incident at three different positions above), so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and reach the output of the second harmonic output optical device group The second harmonic is observed at the end; wherein, when observing the second harmonic, the second collimating convex lens 7 is used to divert the emitted light from the photovoltaic photorefractive diffusion management twin 1 including the fundamental frequency light and the second harmonic The mixed light beam of the wave is collimated into a parallel mixed light beam; the beam splitter 8 is used to reflect the second harmonic in the mixed light beam, and transmit the fundamental frequency light in the mixed light beam to achieve separate observation of the second harmonic light intensity changes;

Wherein, the operating state of the second harmonic generation device is regulated and optimized, so that the mixed light beam composed of the fundamental frequency light and the second harmonic coherently superimposed forms a stable two-color light in the photovoltaic photorefractive diffusion management twin 1 Bright photovoltaic photorefractive space solitons and enables second harmonic generation to achieve the largest possible conversion efficiency.

In addition, based on the second harmonic generation device based on photovoltaic photorefractive diffusion management twins described in Figure 1, the embodiment of the present invention also discloses a second harmonic generation method based on photovoltaic photorefractive diffusion management twins . In this method, the photovoltaic photorefractive diffusion management twin 1 is formed by symmetrically coupling a photovoltaic photorefractive crystal 1A and a photovoltaic photorefractive crystal 1B along a common crystal plane. This method can use bright photovoltaic photorefractive spatial solitons to induce an intracrystalline strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin crystal, so that by adjusting the fundamental frequency light and the second harmonic The propagation constant of the wave in the waveguide is used to achieve phase matching, resulting in efficient generation of the second harmonic. In this method, when the effective carriers of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B are both electrons, the negative c-planes of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B can be optically After polishing, they are tightly bonded together to form electronic photovoltaic photorefractive diffusion management twins; or, when the effective carriers of photovoltaic photorefractive crystal 1A and photovoltaic photorefractive crystal 1B are both holes, the The positive c-planes of the photovoltaic photorefractive crystal 1A and the photovoltaic photorefractive crystal 1B are closely bonded together after optical polishing to form hole-type photovoltaic photorefractive diffusion management twins. The above-mentioned electron-type photovoltaic photorefractive diffusion management twins have opposite carrier diffusion effects on both sides of the twin plane; similarly, the above-mentioned hole-type photovoltaic photorefractive diffusion management twins also have opposite carrier diffusion effects on both sides of the twin plane. The opposite carrier diffusion effect. The method mainly includes the following steps:

Step 1: Place the photovoltaic photorefractive diffusion management twin 1 on the precision lifting platform 2, and make the twin plane of the photovoltaic photorefractive diffusion management twin 1 parallel to the horizontal plane, and adjust the photovoltaic photorefractive diffusion management twin a vertical height of 1;

Step 2: Utilize the background light source 9 to provide incoherent background light; use the third collimating convex lens 10 to collimate the divergent background light into parallel light; pass the second polarizer 11 to change the collimated background light into a polarization direction Linearly polarized light parallel to the twin plane of photovoltaic photorefractive diffusion management twin 1 (that is, parallel to the Y axis); through the half mirror 12, the polarized background light is equally divided into forward illumination light with equal intensity and backlighting light; the forward lighting light is reflected to the front face of the photovoltaic photorefractive diffusion management twin by the forward lighting reflector 13; the direction of the backlighting light is changed by the full reflection mirror 14; The mirror 15 reflects the reversed back illumination light to the rear end face of the photovoltaic photorefractive diffusion management twin 1, so that the forward illumination light and the back illumination light with equal intensity can pass through the photovoltaic photorefractive diffusion management twin 1. The interiors are parallel to each other and transmitted in opposite directions, so that the superimposed forward illumination light and back illumination light can uniformly irradiate the photovoltaic photorefractive diffusion management twin 1 in two directions;

Step 3: Utilize the fundamental frequency light source 3 to provide coherent fundamental frequency light; through the first collimating convex lens 4, the divergent fundamental frequency light is collimated into a parallel beam; through the first polarizer 5, the collimated fundamental frequency light becomes Linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin 1 (that is, parallel to the X axis); through the focusing convex lens 6, the polarized fundamental frequency light is focused on the photovoltaic photorefractive diffusion management twin The front face of the crystal 1 and the focused spot are symmetrically distributed on both sides of the twin plane;

Step 4: Change the intensity of the forward illumination light and the back illumination light by changing the output power of the background light source 9, thereby adjusting the intensity and response time of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin 1;

Step 5: Adjust the focus of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 by changing the output power of the fundamental frequency light source 3 and the distance between the focusing convex lens 6 and the photovoltaic photorefractive diffusion management twin 1 The light intensity and size of the light spot, so as to adjust the photovoltaic photorefractive nonlinear self-focusing effect of the fundamental frequency light in the photovoltaic photorefractive diffusion management twin 1, until the self-focusing effect of the fundamental frequency light compensates for its diffraction broadening effect , so that the fundamental frequency light can be transmitted without diffraction in the photovoltaic photorefractive diffusion management twin 1, that is, the fundamental frequency light can realize bright photovoltaic photorefractive spatial soliton transmission in the photovoltaic photorefractive diffusion management twin 1, and make the fundamental The transmission track of the frequency light beam center is a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin 1;

Step 6: Make the focus spot center of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 deviate from the twin plane by lifting the precision lifting platform 2, so that the fundamental frequency light points to the lateral acceleration of the twin plane in the direction The transmission track of the beam center under the action evolves into a serpentine curve that periodically passes through the twin plane;

Step 7: Change the distance between the central position of the focused light spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin 1 and the twin plane by lifting the precision lifting platform 2, thereby adjusting the distance between the fundamental frequency light and the photovoltaic light Refractive diffusion manages the bending degree of the serpentine transmission trajectory in twin 1, so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and achieve the second harmonic output optical The output end of the device group observes the second harmonic; wherein, when observing the second harmonic, the divergent light emitted from the photovoltaic photorefractive diffusion management twin 1 includes the fundamental frequency light and the second harmonic through the second collimating convex lens 7 The mixed light beam of the subharmonic is collimated into a parallel mixed light beam; then, the second harmonic in the mixed light beam is reflected by the beam splitter 8, and the fundamental frequency light in the mixed light beam is transmitted to achieve separate observation of the two Light intensity variation of sub-harmonic;

Step 8: Repeat steps 4 to 7 to adjust and optimize the operating state of the second harmonic generation device, so that the mixed beam composed of the fundamental frequency light and the second harmonic coherently superimposed on the photovoltaic photorefractive diffusion management twin 1 to form a stable two-color bright photovoltaic photorefractive space soliton, and make the second harmonic generation achieve the largest possible conversion efficiency.

In the embodiment of the present invention, the physical mechanism for generating the second harmonic is as follows: when the bright photovoltaic photorefractive spatial solitons are transmitted in a serpentine curve around and close to the twin planes in the photovoltaic photorefractive diffusion management twin 1 A strip-shaped light-induced refractive index flexible optical waveguide with the twin plane as the central axis plane is induced, which in turn constrains and guides the fundamental frequency light and the second harmonic to transmit and interact in the waveguide; in this The phase matching condition for generating the second harmonic in the waveguide is only to make the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane satisfy β _2ω = 2β _ω (as shown in Figure 5), and It is not necessary for the wave vectors of the fundamental frequency light and the second harmonic to satisfy k _2ω =2k _ω . When adjusting the bending degree of the serpentine transmission track of the bright photovoltaic photorefractive space soliton in the photovoltaic photorefractive diffusion management twin 1, the stripe light-induced refraction induced by the bright photovoltaic photorefractive space soliton is simultaneously adjusted The bending degree of the flexible optical waveguide of the index is adjusted accordingly, and the components of the propagation constants of the fundamental frequency light and the second harmonic in the strip-shaped light-induced refractive index flexible optical waveguide in the direction parallel to the twin plane are adjusted accordingly; when the strip-shaped When the bending degree of the light-sensitive refractive index flexible optical waveguide is appropriate, the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane will meet the phase matching.

In Figure 5, (a) and (b) are schematic diagrams of the phase matching principle when the center of the fundamental frequency light beam is incident from the photovoltaic photorefractive crystal 1A and photovoltaic photorefractive crystal 1B of the photovoltaic photorefractive diffusion management twin 1 . Among them, k _ω and k _2ω are the wave vectors of the fundamental frequency light and the second harmonic wave, respectively, and β _ω and β _2ω are the propagation constants of the fundamental frequency light and the second harmonic wave in the direction parallel to the twin plane, respectively.

The second harmonic generation method and device based on photovoltaic photorefractive diffusion management twins disclosed in the embodiments of the present invention can suppress bright beams with narrow beam widths by managing the carrier diffusion effect in photovoltaic photorefractive diffusion management twins. The large-angle self-deflection of photovoltaic photorefractive space solitons under the action of carrier diffusion effect, so that bright photovoltaic photorefractive space solitons can always be transported inside the photovoltaic photorefractive diffusion management twins without deflecting to The surface of the twin crystal can overcome the large-angle self-deflection of the spatial soliton beam caused by the carrier diffusion effect existing in the monolithic photovoltaic photorefractive crystal and the fundamental frequency light and the two The phase mismatch between the sub-harmonics can also protect the second harmonic generation process from external environmental interference and prevent the generated second harmonics from leaking into the air, thereby fully improving the conversion efficiency of the second harmonic generation . It has high practical value in the fields of laser, all-optical frequency converter and all-optical communication.

The method and device for generating second harmonics based on photovoltaic photorefractive diffusion management twins disclosed in the embodiments of the present invention are described above in detail. In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; meanwhile, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and scope of application. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims (6)

1. A second harmonic generation device based on photovoltaic photorefractive diffusion management twins, characterized in that, the photovoltaic photorefractive diffusion management twins are composed of two photovoltaic photorefractive crystals of the same material along a common crystal Surface-mirror surface symmetric coupling, the device is used to induce an intracrystalline strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin by using bright photovoltaic photorefractive space solitons, so that Phase matching is achieved by adjusting the propagation constants of the fundamental frequency light and the second harmonic in said waveguide, thereby achieving efficient generation of the second harmonic; The second harmonic generation device also includes a precision lifting platform, the precision lifting platform is used to place the photovoltaic photorefractive diffusion management twins, and make the twin planes of the photovoltaic photorefractive diffusion management twins parallel in the horizontal plane, and for adjusting the vertical height of the photovoltaic photorefractive diffusion management twins; The second harmonic generation device also includes a fundamental frequency light input optical device group, a second harmonic output optical device group, and a background light bidirectional illumination optical device group, wherein: The fundamental frequency light input optical device group includes a fundamental frequency light source, a first collimating convex lens, a first polarizer and a focusing convex lens; The second harmonic output optical device group includes a second collimating convex lens and a beam splitter; The background light two-way illumination optical device group includes a background light source, a third collimating convex lens, a second polarizer, a half mirror, a forward illumination reflector, a total reflection mirror and a back illumination reflector; The background light source is used to provide an incoherent background light; The third collimating convex lens is used to collimate the divergent background light into parallel light; The second polarizer is used to change the collimated background light into linearly polarized light whose polarization direction is parallel to the twin plane of the photovoltaic photorefractive diffusion management twin; The half-mirror is used to divide the polarized background light into equal-intensity forward illumination light and back illumination light; the forward-illuminating reflector for reflecting the forward-illuminating light onto the front face of the photovoltaic photorefractive diffusion management twin; The total reflection mirror is used to change the direction of the backlighting light; The back-illumination reflector is used to reflect the reversed back-illumination light to the rear end face of the photovoltaic photorefractive diffusion management twin, so that the forward-illumination light and the back-illumination light with equal intensity The photovoltaic photorefractive diffusion management twins are transmitted in parallel and opposite directions, so that the superimposed forward illumination light and back illumination light perform bidirectional and uniform illumination on the photovoltaic photorefractive diffusion management twins; The fundamental frequency light source is used to provide coherent fundamental frequency light; The first collimating convex lens is used to collimate the divergent fundamental frequency light into a parallel beam; The first polarizer is used to change the collimated fundamental frequency light into linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin; The focusing convex lens is used to focus the polarized fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and distribute the focused light spots symmetrically on both sides of the twin plane; Wherein, when the output power of the background light source is changed, the intensity of the forward illumination light and the back illumination light is also changed, thereby adjusting the intensity of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin and response time; Wherein, when the output power of the fundamental frequency light source and the distance between the focusing convex lens and the photovoltaic photorefractive diffusion management twin are changed, the fundamental frequency light will be in the photovoltaic photorefractive diffusion management twin The light intensity and size of the focused light spot on the front face of the crystal are also changed, thereby adjusting the photovoltaic photorefractive nonlinear self-focusing effect that the fundamental frequency light receives in the photovoltaic photorefractive diffusion management twin, until the The self-focusing effect of the fundamental frequency light compensates its diffraction broadening effect, so that the fundamental frequency light can be transmitted without diffraction in the photovoltaic photorefractive diffusion management twin, that is, the fundamental frequency light is transmitted in the photovoltaic photorefractive diffusion management twin Realize the transmission of bright photovoltaic photorefractive spatial solitons, and make the transmission track of the center of the fundamental frequency light beam be a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin; Wherein, by lifting the precision lifting table, the focus spot center of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin deviates from the twin plane, so that the fundamental frequency light points to the lateral direction of the twin plane The transmission trajectory of the beam center under the action of acceleration evolves into a serpentine curve that periodically passes through the twin plane; Wherein, the distance between the central position of the focused spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and the twin plane is changed by lifting the precision lifting table, thereby adjusting the fundamental frequency light on the photovoltaic photorefractive diffusion management twin. Photorefractive diffusion manages the bending degree of the serpentine transmission trajectory in the twin, so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and the second harmonic output optical The second harmonic is observed at the output end of the device group; wherein, when observing the second harmonic, the second collimating convex lens is used to convert the divergent containing base emitted from the photovoltaic photorefractive diffusion management twin The mixed light beam of frequency light and second harmonic is collimated into a parallel mixed light beam; the beam splitter is used for reflecting the second harmonic in the mixed light beam, and for Transmission is performed to observe the light intensity change of the second harmonic alone; Wherein, the operating state of the second harmonic generation device is regulated and optimized, so that the mixed light beam composed of the fundamental frequency light and the second harmonic coherently superimposed is formed in the photovoltaic photorefractive diffusion management twin Stable two-color bright photovoltaic photorefractive space soliton, and make the second harmonic generation achieve the largest possible conversion efficiency. 2. The second harmonic generation device according to claim 1, characterized in that: When the effective carriers of the two photovoltaic photorefractive crystals of the same material are electrons, the negative c- sides of the two photovoltaic photorefractive crystals of the same material are closely bonded together after optical polishing to form Electronic photovoltaic photorefractive diffusion management twinning; When the effective carriers of the two photovoltaic photorefractive crystals of the same material are both holes, the front c -planes of the two photovoltaic photorefractive crystals of the same material are optically polished and tightly bonded together so that Formation of hole-type photovoltaic photorefractive diffusion-managed twins. 3. The second harmonic generation device according to claim 2, characterized in that: The electron-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane; The hole-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane. 4. A second harmonic generation method based on photovoltaic photorefractive diffusion management twins, characterized in that the photovoltaic photorefractive diffusion management twins are composed of two photovoltaic photorefractive crystals of the same material along a common crystal Surface-mirror surface symmetric coupling, the method includes: Using bright photovoltaic photorefractive spatial solitons to induce an intragranular strip-shaped flexible optical waveguide with the twin plane as the central axis plane at the twin plane in the twin crystal, so that by adjusting the fundamental frequency light and the second harmonic The propagation constant in the waveguide is used to achieve phase matching, so as to realize the efficient generation of the second harmonic; The method also includes: placing the photovoltaic photorefractive diffusion management twins on a precision lifting platform, making the twin planes of the photovoltaic photorefractive diffusion management twins parallel to the horizontal plane, and adjusting the photovoltaic photorefractive diffusion management twins the vertical height of The second harmonic generating device includes a fundamental frequency light input optical device group, a second harmonic output optical device group and a background light bidirectional illumination optical device group, wherein: The fundamental frequency light input optical device group includes a fundamental frequency light source, a first collimating convex lens, a first polarizer and a focusing convex lens; The second harmonic output optical device group includes a second collimating convex lens and a beam splitter; The background light two-way illumination optical device group includes a background light source, a third collimating convex lens, a second polarizer, a half mirror, a forward illumination reflector, a total reflection mirror and a back illumination reflector; The method also includes: Use background light sources to provide incoherent background lighting; Collimating the divergent background light into parallel light through the third collimating convex lens; changing the collimated background light into linearly polarized light whose polarization direction is parallel to the twin plane of the photovoltaic photorefractive diffusion management twin through the second polarizer; The polarized background light is evenly divided into forward lighting and back lighting with equal intensity through the half mirror; reflecting the forward illumination light onto the front face of the photovoltaic photorefractive diffusion management twin via a forward illumination mirror; Utilizing a total reflection mirror to change the direction of the backlighting light; The reversed back-illumination light is reflected to the rear end face of the photovoltaic photorefractive diffusion management twin through the back-illumination reflector, so that the forward and back illumination lights with equal intensities The refractive diffusion management twins are parallel to each other and transmitted in opposite directions, so that the superimposed forward illumination light and back illumination light illuminate the photovoltaic photorefractive diffusion management twins bidirectionally and uniformly; Using a fundamental frequency light source to provide coherent fundamental frequency light; collimating the divergent fundamental frequency light into a parallel beam through the first collimating convex lens; Transforming the collimated fundamental frequency light into linearly polarized light whose polarization direction is perpendicular to the twin plane of the photovoltaic photorefractive diffusion management twin through the first polarizer; Focusing the polarized fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin through a focusing convex lens and making the focused light spots symmetrically distributed on both sides of the twin plane; changing the intensity of the forward illumination light and the back illumination light by changing the output power of the background light source, thereby adjusting the intensity and response time of the photovoltaic photorefractive effect in the photovoltaic photorefractive diffusion management twin; Adjusting the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin by changing the output power of the fundamental frequency light source and the distance between the focusing convex lens and the photovoltaic photorefractive diffusion management twin focusing the light intensity and size of the light spot, thereby adjusting the photovoltaic photorefractive nonlinear self-focusing effect suffered by the fundamental frequency light in the photovoltaic photorefractive diffusion management twin, until the self-focusing effect of the fundamental frequency light is compensated Its diffraction broadening effect enables the fundamental frequency light to realize non-diffraction transmission in the photovoltaic photorefractive diffusion management twin, that is, the fundamental frequency light realizes bright photovoltaic photorefractive space in the photovoltaic photorefractive diffusion management twin Soliton transmission, and make the transmission track of the center of the fundamental frequency light beam be a straight line coincident with the twin plane of the photovoltaic photorefractive diffusion management twin; Wherein, by lifting the precision lifting table, the focus spot center of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin deviates from the twin plane, so that the fundamental frequency light points to the lateral direction of the twin plane The transmission trajectory of the beam center under the action of acceleration evolves into a serpentine curve that periodically passes through the twin plane; Wherein, the distance between the central position of the focused spot of the fundamental frequency light on the front face of the photovoltaic photorefractive diffusion management twin and the twin plane is changed by lifting the precision lifting table, thereby adjusting the fundamental frequency light on the photovoltaic photorefractive diffusion management twin. Photorefractive diffusion manages the bending degree of the serpentine transmission trajectory in the twin, so that the propagation constants of the fundamental frequency light and the second harmonic in the direction parallel to the twin plane meet the phase matching, and achieve optical The second harmonic is observed at the output end of the device group; wherein, when observing the second harmonic, the second collimating convex lens is used to convert the divergent containing base emitted from the photovoltaic photorefractive diffusion management twin The mixed light beam of frequency light and second harmonic is collimated into a parallel mixed light beam; the beam splitter is used for reflecting the second harmonic in the mixed light beam, and for Transmission is performed to observe the light intensity change of the second harmonic alone; Wherein, the operating state of the second harmonic generation device is regulated and optimized, so that the mixed light beam composed of the fundamental frequency light and the second harmonic coherently superimposed is formed in the photovoltaic photorefractive diffusion management twin Stable two-color bright photovoltaic photorefractive space soliton, and make the second harmonic generation achieve the largest possible conversion efficiency. 5. The second harmonic generation method according to claim 4, characterized in that: when the effective carriers of the photovoltaic photorefractive crystals of the two identical materials are electrons, the The negative c- sides of the photovoltaic photorefractive crystals are closely bonded together after optical polishing to form electronic photovoltaic photorefractive diffusion management twins; when the effective carriers of the two photovoltaic photorefractive crystals of the same material When both are holes, the front c -planes of the two photovoltaic photorefractive crystals of the same material are optically polished and closely bonded together to form hole-type photovoltaic photorefractive diffusion management twins. 6. The second harmonic generation method according to claim 5, characterized in that: the electron-type photovoltaic photorefractive diffusion management twin has opposite carrier diffusion effects on both sides of the twin plane; Hole-type photovoltaic photorefractive diffusion-managed twins have opposite carrier diffusion effects on both sides of the twin plane.

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