CN103594918B - A kind of method and apparatus exporting hollow laser beam - Google Patents

Abstract

The invention discloses a method for outputting a hollow beam, comprising: (1) the solid beam generated by a laser is shaped into a hollow annular pump light by a shaping system; (2) the hollow annular pump light is incident along the optical axis To the coupling system, the coupling system transforms the annular pump light into a hollow beam of suitable diameter; (3) the hollow beam is coupled into the resonator from one end face of the laser resonator, and the laser gain medium is pumped, through The phase control makes the gain medium in the resonant cavity absorb the pumping light to generate continuous hollow laser output. The invention also discloses a hollow beam output device. The device and method of the present invention change the incident pump light from the usual solid form to the hollow form, so that there is no gain in the axis of the laser oscillation, and then through phase control, the output of the hollow laser beam is obtained, which has the advantages of simple implementation and reliability. High efficiency, high conversion efficiency and high output power.

Description

A method and device for outputting a hollow laser beam

technical field

The invention belongs to the field of lasers, and in particular relates to a method and a device for outputting a hollow laser beam.

Background technique

For a long time, solid laser beams have been widely used in traditional laser industries such as cutting, cladding, welding and marking. But in recent years, due to the promotion of application technology, various laser beams with zero central intensity—hollow laser beams have been produced one after another, and a new family of so-called hollow beams (also known as dark hollow beams) is being formed. As a laser catheter, optical tweezers (optical tweezers) and optical wrench, the hollow beam has a wide range of applications in the precise, non-contact manipulation and control of microscopic particles (such as microparticles, nanoparticles, free electrons, biological cells and atoms or molecules, etc.) application.

Due to the generation of hollow beams, in turn, it also promotes the further optimization of reflective beam forms in many traditional laser application fields. For example, it has even recently been found that for traditional laser processing fields such as laser shock forming, hollow beams also have certain advantages. In addition to the general parameters of laser beams such as laser frequency, laser power and beam divergence angle, this hollow beam also has some special parameters such as dark spot size, beam width, beam radius and width-to-radius ratio. All kinds of hollow beams have their unique physical properties, such as cylindrical intensity distribution, small dark spot size, no heating effect, propagation invariance, and spin and orbital angular momentum, etc. These properties make hollow beams in the Laser optics, optical information processing, particle waveguide, isotope separation, microelectronics and material science, biotechnology, medicine, atomics, molecular science and other fields have broad application prospects.

So far, there are many methods for generating hollow laser beams in the prior art, such as geometric optics, hollow fiber, π phase plate, optical holography, and computer holography. Among them, the geometric optics method, the hollow fiber method and the π phase plate method to obtain the hollow beam have the advantages of simple structure and easy implementation, but the disadvantages of low conversion efficiency, low purity of the output hollow beam and low beam quality are also obvious; However, computer holography and optical holography obtain hollow beams. Although the output hollow beams have high purity and good beam quality, they are expensive and difficult to manufacture.

Chinese patent document 201210049178.6 discloses a method for obtaining a hollow laser beam by using end pumping to couple the focus of the pump light to the inside or outside of the laser gain medium. The spot of the hollow beam obtained by this method is composed of many small side lobes. This kind of discontinuous annular hollow beam will have certain limitations in the actual application process. For example, in stimulated loss microscopy, due to the side lobes With the existence of the lobes, the fluorescence inhibition effect of the hollow beam on the excitation light is not so obvious, resulting in the generation of sporadic impurity fluorescence, which reduces the resolution of stimulated depletion microscopy.

Contents of the invention

Aiming at the above defects or improvement needs of the prior art, the present invention provides a method and device for outputting a hollow laser beam, the purpose of which is to form a hollow form of gain in the gain medium by using annular pump light through axial end-face pumping area, directly obtain the output of the hollow laser beam, through the precise control of the annular spot size of the annular pump light, the control of the laser mode is realized, and the problem that the center of the hollow beam is not completely dark is solved. At the same time, through the phase control, it is solved The problem of the discontinuity of the hollow ring of the hollow beam is solved.

According to one aspect of the present invention, a method for outputting a hollow beam is provided. After shaping the solid pump light into a hollow pump light, the coupling system is used to couple the pump light into the laser resonator to pump In this way, a hollow gain region is formed in the gain medium, and the direct output of the hollow laser beam is realized through laser oscillation. The specific steps are as follows:

(1) The solid beam generated by the laser is shaped into a hollow annular pump light by the shaping system;

(2) The hollow annular pump light is incident to the coupling system along the optical axis, and the coupling system transforms the annular pump light into a hollow beam with a suitable diameter;

(3) The hollow beam is coupled into the resonator from one end face of the laser resonator to pump the gain medium of the laser, and through phase control, the gain medium in the resonator absorbs the pump light to generate continuous hollow laser output.

As an improvement of the present invention, the continuous hollow laser is realized by continuously changing the phase of the incident light beam, so that only one phase mode in the resonant cavity can continuously oscillate.

As an improvement of the present invention, the continuous change of the phase of the incident light beam is realized by a spiral phase plate arranged in the resonant cavity, and the spiral phase plate is a phase diffractive optical element whose optical thickness is proportional to the rotation azimuth angle.

According to another aspect of the present invention, there is provided a device for outputting a hollow beam, which is used to convert the solid pump light into a continuous hollow pump beam, characterized in that the device includes:

A shaping system, which is arranged after the input solid beam, is used to shape the solid beam into a hollow beam;

A pump light coupling system, which is arranged behind the shaping system along the optical path, and is used to transform the hollow beam into a hollow beam with a suitable diameter;

A laser resonator, which is arranged behind the coupling system along the optical path, and the hollow beam with a suitable diameter is coupled into the resonator from one end face of the laser resonator to pump the laser gain medium, and the phase control makes the The gain medium in the resonator absorbs the pump light to generate continuous hollow-core laser output.

As an improvement of the present invention, the shaping system is composed of an outer conical reflector and an annular hollow inner conical reflector with the same apex angle placed coaxially with it, that is, the emitting surface of the outer conical reflector and the annular hollow inner The reflective surface of the conical reflector is parallel, the optical axis of the incident light coincides with the rotational symmetry axis of the outer conical reflector and the annular hollow inner conical reflector, the light beam is incident from the outer conical reflector, and the reflected light is centered on the optical axis The divergent annular light beam in the center, after being reflected by the annular hollow inner conical mirror, exits in parallel with the incident light beam that first entered the shaping system, that is, the hollow light beam parallel to the optical axis.

As an improvement of the present invention, the shaping system is composed of an inner conical reflector and an outer conical reflector with the same apex angle placed opposite it. ° included angle, the rotational symmetry axis of the outer conical reflector is parallel to the rotational symmetry axis of the inner conical reflector, and the line connecting the vertices of the inner conical reflector and the outer conical reflector is perpendicular to the optical axis of the incident beam.

As an improvement of the present invention, the shaping system is composed of an inner conical lens and an outer conical lens with the same apex angle and the same refractive index placed coaxially with it. Both lenses are plane on one side and conical on the other. The mirror structure of the surface, in which the rotational symmetry axis of the conical surface is perpendicular to the plane and coaxial with the optical axis of the incident beam.

As an improvement of the present invention, the coupling of the end pump is realized through spherical lens combination or cylindrical lens combination, through self-focusing lens or through optical fiber coupling.

As an improvement of the present invention, the laser resonator 4 includes a lens system arranged sequentially along the optical axis, a spiral phase plate, a gain medium, and a cylindrical mirror with a focal length f. The distance between the lens and the two cylindrical mirrors is f, the light beam is incident from the lens, and oscillates back and forth between the lens and the lens, wherein each time the light beam passes through the spiral phase plate, the phase factor will increase so that only the output A single high-order beam in a single phase mode, resulting in a continuous hollow-core beam.

As an improvement of the present invention, the gain medium is a YAG rod.

In the present invention, the incident pump light can be solid. The solid pump light first enters a shaping system, and the output pump light becomes hollow ring-shaped pump light after shaping.

In the present invention, the solid pump light may be a solid laser beam emitted by a semiconductor laser, and the coupling system may be a semiconductor laser coupling system.

Generally speaking, compared with the prior art, the device and method of the present invention change the incident pump light from a solid form to a hollow form, so that there is no gain in the axis of the laser oscillation, and then through phase control, it is obtained Hollow laser beam output has the advantages of simple implementation, high reliability, high conversion efficiency and high output power.

Description of drawings

Fig. 1 is a schematic diagram of the device structure according to an embodiment of the present invention;

Fig. 2 is a structural representation of an embodiment of the shaping system in Fig. 1;

Fig. 3 is a schematic structural view of another embodiment of the shaping system in Fig. 1;

Fig. 4 is the structural representation of another embodiment of shaping system in Fig. 1;

Fig. 5 is a schematic diagram of the coupling system in Fig. 1;

Fig. 6 is a schematic diagram of the laser resonator in Fig. 1;

Fig. 7 is a schematic diagram of the spiral phase plate in Fig. 6;

Fig. 8 is a schematic diagram of a single LG degenerate mode laser beam superposition process;

Figure 9 is an analysis of the phase factor of the LG _0,+l mode beam in the resonant cavity and back and forth;

Figure 10 is an analysis of the phase factor of the LG _0,-l mode beam traveling back and forth in the resonator;

In all the drawings, the same reference numerals represent the same technical features, wherein, 1-pumping light, 2-shaping system, 3-coupling system, 4-laser resonant cavity, 5-hollow output beam.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not constitute a conflict with each other.

The light beam emitted by the semiconductor laser is a solid light beam, but its light beam is an astigmatic light beam in the direction of the fast axis and the slow axis. It can be collimated by an ordinary lens to obtain a circular solid output light beam, as shown in Figure 1 of the embodiment of the present invention. The pump light 1.

The pump light 1 passes through the shaping system 2, so that the solid beam becomes a hollow beam. The shaping system in the present invention is used to shape a solid light beam into a hollow light beam. There are various specific devices and methods. The following are the specific structures of the three preferred shaping systems.

As shown in Figure 2, the shaping system 2 of the present embodiment consists of an outer conical reflector 201 and an annular hollow inner conical reflector 202 with the same apex angle placed coaxially therewith, that is, the outer conical reflector 201 The emission surface is parallel to the reflection surface of the annular hollow inner conical reflector 202, and the optical axis of the incident light coincides with the rotational symmetry axis of the outer conical reflector 201 and the annular hollow inner conical reflector 202, and the light beam passes from the outer conical reflector 201 The reflected light is a divergent annular light beam centered on the optical axis. Since the reflection surface of the annular hollow inner conical reflector 202 is parallel to the reflective surface of the outer conical reflector 201, the light beam is reflected by the annular hollow inner conical surface After being reflected by the mirror 202, it will emerge parallel to the incident beam that initially entered the shaping system, that is, a hollow beam parallel to the optical axis.

As shown in Fig. 3, the shaping system of another embodiment of the present invention is made up of inner conical reflector 203 and the outer conical reflector 204 with the same apex angle placed opposite it, and inner conical reflector 203 rotational symmetry axis Form an included angle of 135° with the optical axis of the incident light beam, the axis of rotational symmetry of the outer conical reflector 204 is parallel to the axis of rotational symmetry of the inner conical reflector 203, and the line connecting the vertices of the inner conical reflector 203 and the outer conical reflector 204 is The optical axis of the incident beam is vertical, and the beam shaping process is the same as 1.

As shown in Fig. 4, the shaping system of another embodiment of the present invention is made of an inner conical lens 205 and an outer conical lens 206 with the same apex angle and the same refractive index placed coaxially therewith, and the two lenses are all on one side. is a mirror structure with a conical surface on one side, wherein the rotational symmetry axis of the conical surface is perpendicular to the plane and coaxial with the optical axis of the incident beam. When the incident light beam is incident from the plane side of the inner conical surface lens 205, and then emerges from the conical surface side, due to the refraction effect, the light beam will be deflected away from the optical axis, and then the light beam is incident from the outer conical surface side of the outer conical surface lens 206 , refraction occurs again, and because the refraction planes of the two refractions are parallel and the refractive index is the same, the outgoing beam will be parallel to the beam incident on the inner conical lens 205, that is, a hollow beam with parallel optical axes.

The hollow laser beam is shaped by the shaping system 2 above to obtain a hollow pump beam, which is then coupled into the laser resonator in an end-pumped manner to pump the gain medium.

The preferred end-pump coupling methods in this embodiment are as follows: 1) Combination lens system for concentrating light: use spherical lens combination or cylindrical lens combination for coupling. 2) Self-focusing lens coupling: the self-focusing lens replaces the combined lens for coupling. The advantage is that the structure is simple, and the size of the collimated spot depends on the numerical aperture of the self-focusing lens. 3) Fiber coupling: refers to the pump coupling with LD with pigtail output, the advantage is that the structure is flexible. Figure 5 is a schematic diagram of a coupling system composed of combined lenses. After the hollow beam with a larger beam radius passes through the lens group, it outputs a thin hollow beam that is easier to pump and improves the pumping efficiency.

The incident hollow pump light excites the gain medium in the laser resonator. Since there is no gain in the axial part of the laser oscillation, the gain of the LG _0,0 mode, which is usually the first to oscillate, is lower than that of the high-order mode and is suppressed. The hollow high-order mode Then it is the first to oscillate and output, and a hollow beam is obtained. Through the precise control of the annular spot size of the annular pump light by the shaping system and the coupling system, a specific high-order LG _{0, ± l} mode oscillation is formed in the resonator. 6 is a schematic diagram of a laser resonator in an embodiment of the present invention. The pump light is incident through the lens 401 and oscillates back and forth between the lens 401 and the lens 405. The gain medium 403 is a YAG rod, and the lens 401 is fully transparent for the pump light. 1064nm total reflection, lens 405 has a certain reflectivity (non-total reflection) for 1064nm light, and the laser beam is output by lens 405.

Since LG _{0 , +l} and LG _{0 , -l} have the same radial distribution, these two phase modes will oscillate together and exist stably in conventional laser resonators, in this case, two hollow ring beams After LG _{0 , +l} and LG _{0 , -l} are superimposed together, the hollow beam formed is in the form of side lobes, as shown in Figure 8. In order to obtain a continuous hollow beam, only one phase mode in the resonator needs to be able to oscillate continuously. The present invention adds a helical phase plate 402 and a cylindrical mirror system in the resonator. Wherein, the spiral phase plate 402 (as shown in FIG. 7 ) is a phase-only diffractive optical element whose optical thickness is proportional to the rotation azimuth angle, and is used to change the phase of the incident light beam. The cylindrical mirror system is composed of cylindrical mirrors 404 and 406 whose focal lengths are f. The distance between the cylindrical mirror 404 and the cylindrical mirror 406 is 2f, and the two cylindrical mirrors are symmetrical to the left and right of the lens 405, that is, the lens 405 and the two cylindrical mirrors The distance is f.

LG _{0 , +l} mode has exp(+ilθ) phase term, LG _{0 , -l} mode has exp(-ilθ) phase term, (+l in Fig. 9 and Fig. 10 represents that the phase factor is exp(+ilθ) LG _{0 , +l} mode beam, -l represents LG _{0 , -l} mode beam with phase factor exp(-ilθ)). Every time the light beam passes through the spiral phase plate 402, the phase factor will increase by 1; when the light beam with the phase factor of ±1 passes through the cylindrical mirror system, the phase factor will be reversed as (The effect of the light beam passing through the cylindrical mirror 404, being reflected by the mirror 405 and then passing through the cylindrical mirror 404 is the same as the effect of passing through the cylindrical mirror 404, the mirror 405, and the cylindrical mirror 406 in turn).

Taking the right side of the gain medium 403 as the starting position of beam oscillation, the phase factor of the LG _{0 ,} + _l mode is still +1 after a round trip in the cavity, which satisfies the self-reproducible phase distribution, and the LG _{0 , +l} mode can Stable oscillation in the resonant cavity, as shown in Figure 9. However, after the LG _{0 , -l} mode goes back and forth in the cavity for one cycle, the phase factor becomes +3l, which does not satisfy the self-reproducible phase distribution, and the LG _{0 , -l} mode will be suppressed in the resonant cavity and cannot continue to oscillate. Therefore, after adding the helical phase plate 402 and the cylindrical mirror system in the resonator, the laser will only output a single high-order beam of a single phase mode, so that the obtained hollow beam is no longer an ordinary side lobe beam, but a continuous The hollow beam is gone.

A Q-switching device can also be added in the resonator to form a pulsed Q-off laser and output a ring-shaped Q-switched pulse beam; modulation can also be added to form a mode-locked oscillation to output a ring-shaped mode-locked pulse; in addition, frequency multiplication or other frequency conversion devices can also be added , output frequency-doubling or other frequency-converted ring beams.

The method for obtaining a hollow laser beam described in the present invention adopts an intracavity device to transform an ordinary solid pump beam into a hollow pump beam. The energy of the hollow beam obtained by this method is more concentrated on the ring, and the output of the hollow mode beam is realized by controlling the annular spot size of the ring pump light and the phase of the oscillation beam in the laser resonator. Compared with the previous method, this method has better output laser beam quality, and at the same time, because the energy concentration is more concentrated on the ring, the slope efficiency of the output hollow beam is higher, and it can provide a feasible method for higher power hollow beam output. At the same time, the device using the method to obtain a hollow laser beam has the characteristics of low cost, simple assembly and easy adjustment.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (10)

1. A method of outputting a hollow beam, by shaping the solid pump light into a hollow pump light, and then using a coupling system to couple the pump light into the laser resonator, and pumping it in the gain medium by axial end pumping A hollow gain region is formed in the middle, and the direct output of the hollow laser beam is realized through laser oscillation. The specific steps are as follows: (1) The solid beam generated by the laser is shaped into a hollow annular pump light by the shaping system; (2) The hollow annular pump light is incident to the coupling system along the optical axis, and the coupling system transforms the annular pump light into a hollow beam with a suitable diameter; (3) The hollow beam is coupled into the resonator from one end face of the laser resonator to pump the gain medium of the laser, and through phase control, the gain medium in the resonator absorbs the pump light to generate continuous hollow laser output. 2. The method for outputting a hollow beam according to claim 1, wherein the continuous hollow laser is realized by continuously changing the phase of the incident beam so that only one phase mode in the resonator can continue to oscillate. 3. The method for outputting a hollow light beam according to claim 2, wherein the continuous change of the phase of the incident light beam is realized by a spiral phase plate arranged in the resonant cavity, and the spiral phase plate is a combination of optical thickness and rotation orientation Angle-proportional phase diffractive optical elements. 4. A device for outputting a hollow beam, used to convert the solid pumping light into a continuous hollow pumping beam, characterized in that, the solid pumping light is a solid laser beam sent by a semiconductor laser, and the device includes: A shaping system (2), which is arranged behind the input solid beam, is used to shape the solid beam into a hollow beam; A pump light coupling system (3), which is arranged behind the shaping system (2) along the optical path, and is used to transform the hollow beam into a hollow beam with a suitable diameter; A laser resonator (4), which is arranged behind the coupling system (3) along the optical path, and the hollow beam with a suitable diameter is coupled into the resonator from one end face of the laser resonator (4) in the form of end pumping , to pump the gain medium of the laser, and through phase control, the gain medium in the resonant cavity absorbs the pump light to generate continuous hollow laser output. 5. The device for outputting a hollow light beam according to claim 4, characterized in that, the shaping system (2) consists of a first outer conical reflector (201) and an annular ring with the same apex angle placed coaxially therewith. The hollow inner conical reflector (202) is formed, that is, the reflective surface of the first outer conical reflector (201) is parallel to the reflective surface of the annular hollow inner conical reflector (202), and the optical axis of the incident light is parallel to the first outer conical reflector. The rotational symmetry axes of the surface reflector (201) and the annular hollow inner conical reflector (202) coincide, the light beam is incident from the first outer conical reflector (201), and the reflected light is a divergent annular light beam centered on the optical axis , which is reflected by the annular hollow inner conical reflector (202), and emerges parallel to the incident light beam initially entering the shaping system, that is, a hollow light beam parallel to the optical axis. 6. The device for outputting a hollow light beam according to claim 4, characterized in that, the shaping system (2) consists of an inner conical mirror (203) and a second outer cone with the same apex angle placed opposite it. surface reflector (204), the rotational symmetry axis of the inner conical reflector (203) forms an included angle of 135° with the optical axis of the incident beam, and the rotational symmetry axis of the second outer conical reflector (204) and the inner conical surface reflect The mirror (203) is parallel to the axis of rotational symmetry, and the line connecting the vertices of the inner conical reflector (203) and the second outer conical reflector (204) is perpendicular to the optical axis of the incident beam. 7. The device for outputting a hollow light beam according to claim 4, characterized in that, the shaping system (2) consists of an inner conical lens (205) and a lens with the same apex angle and the same refractive index placed coaxially therewith. The outer conical surface lens (206) is composed of two lenses with a mirror body structure with one side being a plane and the other side being a conical surface, wherein the axis of rotational symmetry of the conical surface is perpendicular to the plane and coaxial with the optical axis of the incident light beam. 8. The device for outputting a hollow light beam according to any one of claims 5-7, wherein the end pump coupling is coupled through a spherical lens combination or a cylindrical lens combination, coupled by a self-focusing lens or through an optical fiber Coupled implementation. 9. The device for outputting a hollow light beam according to any one of claims 5-7, characterized in that the laser resonator (4) comprises a first lens (401) arranged in sequence along the optical axis, a spiral phase plate (402), the gain medium (403), the cylindrical mirror system formed by the two cylindrical mirrors (404, 406) whose focal length is f and the connection between the two cylindrical mirrors (404, 406) and the two cylindrical mirrors The second lens (405) with a distance f, the light beam is incident from the first lens (401), and oscillates back and forth between the first lens (401) and the second lens (405), wherein the light beam passes through Once the phase plate (402) is spiraled, the phase factor is increased so that only a single high-order beam of a single phase mode is output, thereby forming a continuous hollow beam. 10. The device for outputting a hollow beam according to any one of claims 5-7, characterized in that the gain medium (403) is a YAG rod.