CN103887695B - Laser transmitting radial polarized beams based on conical uniaxial crystal - Google Patents

Abstract

A laser based on a conical uniaxial crystal outputting a radially polarized beam, which consists of: a uniaxial crystal conical mirror, a laser gain medium, a planar output coupling mirror and a pump source, and the laser gain medium is located at the Between the bottom surface of the uniaxial crystal conical mirror and the planar output coupling mirror, the apex angle of the uniaxial crystal conical mirror In the formula, n _o and n _e are the main axis refractive indices of the e light and o light of the uniaxial crystal of the uniaxial crystal conical reflector, respectively, and n _o and n _e of the uniaxial crystal should satisfy the following condition: The laser of the invention has the characteristics of simple device structure, good system stability, high laser efficiency, high output power and good beam quality.

Description

Laser outputting radially polarized beams based on conical uniaxial crystals

technical field

The invention relates to a laser, in particular to a laser outputting a radially polarized light beam based on a conical uniaxial crystal.

Background technique

A radially polarized beam is a vector polarized beam whose intensity and polarization direction are axisymmetric and the polarization direction at any point is along the radial direction. Due to a series of unique properties, radially polarized beams have broad application prospects in high-resolution imaging, laser photography, biomedicine, electron acceleration, and material processing.

There are currently two main methods for generating radially polarized beams. One method is to passively convert linearly polarized light or other forms of polarized light into radially polarized light outside the laser cavity. Such methods mainly include interference superposition method, wave plate spatial transformation delay method, spiral gradient phase retarder or liquid crystal polarization conversion method, etc. However, these extracavity conversion methods generally have defects such as poor beam quality, low conversion efficiency, and complicated devices. Therefore, in recent years, the active method of directly outputting radially polarized beams from lasers has become a hot spot in related international research. The Chinese invention patent application with the authorized announcement number CN101465512B discloses a method that uses the thermally induced birefringence effect in Nd:YAG crystals, adopts a special asymmetric resonant cavity design to suppress the vibration of one of the polarized lights, and realizes radial polarization or Tangentially polarized laser output. However, this kind of laser resonator based on crystal thermal effect mode selection is not stable, and can only achieve mode selection within a specific power range, and the output power of the laser will be greatly limited. The Chinese patent application with the authorized announcement number CN202333432 discloses a method of using an axisymmetric polarization resonator mirror to make a laser output an axisymmetric polarized beam laser. This method of using a grating mirror for mode selection has also been reported in many international reports. However, the manufacturing process of the grating mirror that can realize the mode selection of radially polarized light is relatively complicated and the cost is high, and there is no related localized product at present.

Contents of the invention

The purpose of the present invention is to overcome the disadvantages of the above-mentioned technologies, and provide a laser that outputs radial polarization. The laser has the characteristics of simple device structure, good system stability, high laser efficiency, high output power, and good beam quality.

Technical solution of the present invention is as follows:

A laser based on a conical uniaxial crystal outputting a radially polarized beam is characterized in that its composition includes: a uniaxial crystal conical mirror, a laser gain medium, a planar output coupling mirror and a pump source, and the laser gain medium is located in the Between the bottom surface of the uniaxial crystal conical mirror and the planar output coupling mirror, the apex angle of the uniaxial crystal conical mirror In the formula, n _o and n _e are respectively the main axis refractive indices of the e light and o light of the uniaxial crystal of the uniaxial crystal conical reflector, and n _o and n _e of the uniaxial crystal should satisfy the following condition:

The material used to make the uniaxial crystal conical mirror can be either a negative uniaxial crystal or a positive uniaxial crystal.

The laser gain medium is a gas laser medium, a laser crystal, a laser ceramic, a single-clad optical fiber or a double-clad optical fiber whose core is doped with rare earth active particles.

The planar output coupling mirror is a planar mirror with partial reflection and partial transmission.

The pumping source is a lamp pumping source, a laser pumping source, an electric pumping source or a semiconductor laser pumping source.

Principle of the present invention is as follows:

When a beam of monochromatic light is incident on the interface between the uniaxial crystal and air from the inside of the uniaxial crystal, two beams of linearly polarized light will be reflected, and their vibration directions are perpendicular to each other, which is the phenomenon of double reflection. The two beams of linearly polarized light whose polarization directions are perpendicular to each other are o light (ordinary light) and e light (abnormal light). The propagation path of o light in the uniaxial crystal is consistent with the propagation path of light in isotropic media (such as glass) , while the propagation path of e light will be deflected. For a conical reflector whose vertex angle is α and the optical axis is perpendicular to the bottom surface of the conical mirror processed by uniaxial crystal material, the incident light perpendicular to the bottom surface of the conical reflector will be decomposed into o light and e light, which can be controlled by reasonable control of the cone top The size of the angle α makes the light e return parallel to the direction of the incident light after two total reflections on the conical surface, while the outgoing direction of the light o cannot remain parallel to the incident direction. Therefore, the laser resonator composed of the conical mirror made of specially designed uniaxial crystal material and the planar output coupling mirror can lose the o light and make the e light form laser oscillation, so as to realize the radial polarization Laser output.

The present invention has the following advantages:

1. The laser has no special requirements on the gain medium. It can be made into a gas laser, or a solid or fiber laser.

2. In this laser, the resonant cavity formed by the uniaxial crystal conical reflector and the planar output coupling mirror can be equivalent to a planar cavity structure, which can realize laser output with high beam quality.

3. In this laser, no additional mode selection elements are introduced into the cavity or outside the cavity, the structure is simple and compact, the realizability is strong, and the working efficiency is high.

4. Q-switching or mode-locking components can be easily introduced into the laser to realize high peak power pulse operation.

Description of drawings

FIG. 1 is a schematic structural diagram of a laser outputting a radially polarized beam according to the present invention.

Fig. 2 is a schematic diagram showing the variation of the e-ray refractive index with the angle between the optical axes in the negative uniaxial crystal.

Fig. 3 is a schematic diagram of double reflection of a negative uniaxial crystal conical mirror.

FIG. 4 is a schematic diagram of the polarization distribution of a radially polarized light beam, wherein arrows indicate polarization directions.

detailed description

As shown in Figure 1, a laser based on a conical uniaxial crystal outputting a radially polarized beam consists of: a uniaxial crystal conical mirror 1, a laser gain medium 2, a planar output coupling mirror 3 and a pump source 4, The laser gain medium 2 is located between the bottom surface of the uniaxial crystal conical mirror 1 and the plane output coupling mirror 3, and the apex angle of the uniaxial crystal conical mirror 1 is In the formula, n _o and n _e are respectively the principal axis refractive indices of the e light and o light of the uniaxial crystal that make the uniaxial crystal conical mirror 1, and n _o and n _e of the uniaxial crystal should satisfy The following conditions:

The arrow in the cavity marks the resonant circuit of the laser cavity, and the arrow on the right side of the planar output coupling mirror 3 marks the direction of the tangentially polarized laser beam output by the laser.

If the angle between the normal direction of the e-ray wave and the optical axis of the uniaxial crystal is θ, then the corresponding refractive index of the e-ray propagating in the uniaxial crystal

where n _o and n _e are the two main axis refractive indices of the uniaxial crystal.

Taking a negative uniaxial crystal as an example, the refractive index of e-ray varies with the angle θ between it and the optical axis, as shown in Figure 2.

Also taking the negative uniaxial crystal as an example, the double reflection of the uniaxial crystal conical mirror is shown in Figure 3. in, is the half-vertex angle of the cone, θ ₁ is the common incident angle of e-light and o-light, and θ ₂ is the reflection angle of e-light.

To make the e light return parallel to the direction of the incident light after two total reflections at the conical interface, then

θ ₁ +θ ₂ =90° (ii)

It can be seen from the geometric relationship between the angles in Figure 3 that

According to the law of reflection

n _e (0°)sin(θ ₁ )=n _e (90°)sin(θ ₂ ) (iv)

Combining the above formulas, the apex angle of the cone can be determined

In addition, if the uniaxial crystal conical mirror is to be used as a laser resonator mirror, the e-ray incident perpendicular to the bottom surface of the cone should satisfy the total reflection condition on the cone surface:

Combining (i), (iii), (v) and (vi), we can obtain the conditions that the two principal axis refractive indices n _o and ne _e of the uniaxial crystal should satisfy:

The uniaxial crystal conical mirror 1 is a key device of the present invention, and is used to separate o-light and e-light. The apex angle of the uniaxial crystal conical mirror

The optical axis direction of the uniaxial crystal conical reflector 1 is perpendicular to the bottom surface of the conical reflector. Taking the optical axis negative uniaxial crystal as an example, the working principle of the uniaxial crystal conical mirror is shown in Figure 3.

The material used to make the uniaxial crystal conical mirror 1 can be either a negative uniaxial crystal or a positive uniaxial crystal.

The two principal axis refractive indices no and _ne of the _uniaxial crystal at the wavelength of the oscillating light wave should satisfy

The laser gain medium 2 can be a gas laser medium, a laser crystal, a laser ceramic, a single-clad or double-clad optical fiber whose core is doped with rare earth active particles.

The planar output coupling mirror 3 is a partially reflective and partially transmissive plane mirror, which is used to couple out the generated radially polarized light, and its reflectivity can be coated according to the output requirements of the laser.

The pumping source 4 provides energy for the laser so that the gain medium 2 excites transitions to form laser light. The pumping source can be implemented in various forms, such as lamp pumping, laser pumping, electric pumping, LD (semiconductor laser) pumping, and the like.

Claims (5)

1. A laser for outputting a radially polarized beam based on a conical uniaxial crystal, characterized in that it comprises: unipolar crystal conical reflection mirror (1), laser gain medium (2), plane output coupling mirror (3) and pumping source (4), laser gain medium (2) be located unipolar crystal conical reflection mirror (1) bottom surface and plane output coupling mirror (3) between, the apex angle of unipolar crystal conical reflection mirror (1)In the formula n_oAnd n_eThe main axis refractive indexes of e light and o light of the uniaxial crystal conical reflector (1) are respectively made, and n of the uniaxial crystal_oAnd n_eThe following conditions should be satisfied:

$arctan\frac{n_e}{n_o}>arcsin\frac{1}{n_o}.$

2. a laser for outputting a radially polarized beam according to claim 1, wherein the material used to fabricate said uniaxial crystal conic mirror is a negative uniaxial crystal or a positive uniaxial crystal.

3. The laser of claim 1, wherein said laser gain medium is a gas laser medium, a laser crystal, a laser ceramic, a single clad fiber with rare earth active particles doped in the core, or a double clad fiber.

4. The laser of claim 1, wherein said planar output coupler is a partially reflective and partially transmissive flat mirror.

5. The laser of any one of claims 1 to 4, wherein said pump source is a lamp pump source, a laser pump source or an electric pump source.