CN104505703A - Laser device for outputting flattened beams - Google Patents

Abstract

The invention relates to a laser device for outputting flattened beams. The laser device comprises a reflection cavity mirror, a gain medium, a polarizing film, a phase adjustment mirror, a compensating lens and a Porro prism. The laser device utilizes a cavity to place the phase adjustment mirror, and the phase delay of endovascular laser is modulated radially through the polarizing film, the phase adjustment mirror and the Porro prism, so that the radial variation of coupling output rate of the polarizing film is realized, and the laser emergent through the polarizing film is a flattened beam with a flat middle part. Meanwhile, the compensating lens is adopted to eliminate the lens effect caused by inserting the phase adjustment mirror into the laser cavity. The laser device is simple in structure, less in loss, high in efficiency, and high in filling factors of the outputted beams.

Description

Laser for outputting flat-top beam

Technical Field

The invention belongs to the technical field of laser, and particularly relates to a laser for reshaping and homogenizing spatial distribution of an output light beam in a cavity.

Background

The energy distribution of the output beam of a conventional laser is a non-uniform gaussian spatial distribution. In the technical fields of laser processing, cutting and welding, biomedical engineering and the like, the non-uniform distribution characteristic of laser energy can cause local temperature rise to destroy the material characteristics. Meanwhile, the non-uniformity of the input beam in the high-power laser system can cause non-linear effects such as B integral and the like to cause poor beam quality, and easily cause over-high local power density, thereby causing damage to the laser medium. Moreover, the filling factor of the Gaussian beam is low, so that high energy extraction efficiency cannot be guaranteed, and the efficiency of an amplification and frequency multiplication system is limited. Therefore, in many laser technology applications, the non-uniform nature of the conventional gaussian laser beam profile limits its application.

There are many methods for realizing the spatial uniform distribution of laser beams, mainly including microlens arrays, random phase plates, binary optical elements, birefringent lens groups, aspherical lens groups, liquid crystal spatial light modulators, and the like. These methods use optical devices inserted into the external optical path of the laser to shape the beam, which causes large losses and more complex shaping systems.

Disclosure of Invention

The invention mainly aims to overcome the defect of high loss of shaping outside a cavity, and provides a simple, effective and low-loss laser for outputting a flat-top light beam.

The technical solution of the invention is as follows:

a laser for outputting a flat-top beam is characterized by sequentially comprising a reflecting cavity mirror, a gain medium, a polaroid, a phase adjusting mirror, a compensating lens and a Porro prism which have the same optical axis, wherein the included angle between the polaroid and the optical axis is a Brewster angle. The reflecting cavity mirror, the gain medium, the polaroid, the phase adjusting mirror, the compensating lens and the Porro prism form a laser resonant cavity. The phase adjusting mirror is a plano-convex lens or a plano-concave lens made of birefringent materials. The compensating lens is a plano-concave lens or a plano-convex lens made of isotropic medium or cubic crystal, and the curvature radius of the compensating lens is equal to the curvature radius of the phase adjusting mirror in numerical value and opposite in sign. The reflecting cavity mirror is a total reflection mirror or a Porro prism.

The laser in the cavity sequentially passes through the polaroid, the phase adjusting mirror and the compensating lens, is reflected by the Porro prism, passes through the compensating lens and the phase adjusting mirror again, and is reflected and output by the polaroid. The phase delay of the laser is radially modulated by using the polaroid, the phase adjusting mirror and the Porro prism, so that the radial change of the coupling output rate of the polaroid is realized, and the laser emitted by the polaroid can be shaped into a flat-topped beam with a flat middle part.

The invention has the following specific advantages:

1. and the intracavity light beam shaping is adopted, and compared with an extra-cavity shaping system, the intracavity light beam shaping system has the advantages of low loss, high efficiency and simple structure.

2. The output laser beam is a flat-top beam, the spatial distribution of the beam is relatively uniform, and the filling factor is high.

3. The curvature of the phase adjusting mirror is compensated by the compensating lens, and the lens effect caused by inserting the phase adjusting mirror into the laser cavity is eliminated.

Drawings

FIG. 1 is a schematic diagram of the optical path of a laser outputting a flat-topped beam according to the present invention

FIG. 2 is a schematic diagram of a phase adjusting mirror in a laser according to the present invention

FIG. 3 is a normalized intensity profile of the laser output beam for two different parameters using a phase adjusting mirror of quartz single crystal material

Detailed Description

The invention is further illustrated with reference to the following figures and examples, which should not be construed as limiting the scope of the invention.

As shown in figure 1, the laser for outputting the flat-topped beam comprises a reflecting cavity mirror 1-1, a gain medium 1-2, a polarizing plate 1-3, a phase adjusting mirror 1-4, a compensating lens 1-5 and a Porro prism 1-6 which are arranged in sequence on the same optical axis. The laser resonator is composed of a reflector cavity mirror 1-1, a gain medium 1-2, a polarizer 1-3, a phase adjusting mirror 1-4, a compensating lens 1-5 and a Porro prism 1-6. The included angle between the polaroid 1-3 and the optical axis is a Brewster angle, and the phase adjusting mirror 1-4 is a plano-convex lens or a plano-concave lens made of a birefringent material. The compensating lens 1-5 is a plano-concave lens or a plano-convex lens, an isotropic medium or a cubic crystal with a refractive index close to that of the phase adjusting lens 1-4 is adopted, the curvature radius of the compensating lens is equal to that of the phase adjusting lens 1-4 in numerical value, and the sign of the compensating lens is opposite.

The intracavity laser forms linearly polarized light in the horizontal direction after passing through the polaroid 1-3, sequentially passes through the phase adjusting mirror 1-4 and the compensating lens 1-5, reaches the Porro prism 1-6, is reflected by the Porro prism 1-6, and then passes through the compensating lens 1-5 and the phase adjusting mirror 1-4 again. The phase delay of the laser in the cavity is radially modulated by the polaroids 1-3, the phase adjusting mirrors 1-4 and the Porro prisms 1-6, so that the radial modulation of the reflectivity of the coupled-out polaroids is realized, and the laser emitted by the polaroids 1-3 can be shaped into relatively uniform linearly polarized light with a flat middle part.

As shown in fig. 2, the phase adjusting mirror is a plano-convex lens or a plano-concave lens made of a birefringent material, and the direction of the principal axis of the phase adjusting mirror is perpendicular to the direction of the optical axis of the system. d (r) is the thickness of the phase adjusting mirror, d (r) varies with the radial distance r.The phase retardation of the o light and the e light by the phase adjusting mirror is changed along with the radial distance r in the radial direction after the linearly polarized light passes through the phase adjusting mirror:

<math><mrow> <mi>d</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>=</mo> <msub> <mi>d</mi> <mn>0</mn> </msub> <mo>-</mo> <mfrac> <msup> <mi>r</mi> <mn>2</mn> </msup> <mrow> <mn>2</mn> <mi>&rho;</mi> </mrow> </mfrac> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow></math>

wherein Δ n ═ n₀-n_eThe difference between the refractive indexes of the phase adjusting mirror for o light and e light, λ is the working wavelength of the laser, d₀Rho is the radius of curvature of the convex surface of the phase adjusting mirror for the central thickness of the phase adjusting mirror, and is greater than 0 for a convex lens and less than 0 for a concave lens.

And the included angle between the main axis of the phase adjusting mirror and the polarization direction of the polaroid 1-3p is theta, and the Jones matrix of the phase adjusting mirror is expressed as follows:

$M_{\mathrm{RBE}}=\left[\begin{array}{cc}A& B\\ B& A^{*}\end{array}\right]$

<math><mrow> <mo>=</mo> <mfenced open='[' close=']'> <mtable> <mtr> <mtd> <mi>cos</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mo>+</mo> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mi>cos</mi> <mn>2</mn> <mi>&theta;</mi> </mtd> <mtd> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mi>sin</mi> <mn>2</mn> <mi>&theta;</mi> </mtd> </mtr> <mtr> <mtd> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mi>sin</mi> <mn>2</mn> <mi>&theta;</mi> </mtd> <mtd> <mi>cos</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mo>-</mo> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mi>&alpha;</mi> <mrow> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> <mo>)</mo> </mrow> <mi>cos</mi> <mn>2</mn> <mi>&theta;</mi> </mtd> </mtr> </mtable> </mfenced> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>3</mn> <mo>)</mo> </mrow> </mrow></math>

wherein,

the jones matrix for Porro prisms 1-6 is:

<math><mrow> <msub> <mi>M</mi> <mi>Porro</mi> </msub> <mo>=</mo> <mfenced open='[' close=']'> <mtable> <mtr> <mtd> <mi>C</mi> </mtd> <mtd> <mi>D</mi> </mtd> </mtr> <mtr> <mtd> <mi>D</mi> </mtd> <mtd> <msup> <mi>C</mi> <mo>*</mo> </msup> </mtd> </mtr> </mtable> </mfenced> <mo>=</mo> <mfenced open='[' close=']'> <mtable> <mtr> <mtd> <mi>cos</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mi>cos</mi> <mn>2</mn> <mi>&beta;</mi> </mtd> <mtd> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mi>sin</mi> <mn>2</mn> <mi>&beta;</mi> </mtd> </mtr> <mtr> <mtd> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mi>sin</mi> <mn>2</mn> <mi>&beta;</mi> </mtd> <mtd> <mi>cos</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mo>-</mo> <mi>i</mi> <mi>sin</mi> <mrow> <mo>(</mo> <mfrac> <mi>p</mi> <mn>2</mn> </mfrac> <mo>)</mo> </mrow> <mi>cos</mi> <mn>2</mn> <mi>&beta;</mi> </mtd> </mtr> </mtable> </mfenced> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>4</mn> <mo>)</mo> </mrow> </mrow></math>

wherein,for the phase retardation caused by the Porro prisms 1 to 6, γ is the angle of light incident on the right-angled surface of the Porro prisms 1 to 6 (γ ═ 45 °), n is the refractive index of the material of the Porro prisms 1 to 6, and β is the azimuth angle of the ridge line of the Porro prisms 1 to 6.

Therefore, the intensity distribution function of the laser reflected by the polaroids 1-3 of the laser in the cavity can be simulated and calculated through the Jones matrix method. The laser passes through the polaroids 1-3, the phase adjusting mirrors 1-4 and the compensating lenses 1-5, is reflected by the Porro prisms 1-6, then passes through the compensating lenses 1-5 and the phase adjusting mirrors 1-4 again, and then is polarized and coupled out through the polaroids 1-3.

The Jones vector of the laser after passing through the polaroid 1-3 is $M_{\mathrm{in}}=\left[\begin{array}{c}1\\ 0\end{array}\right],$ The Jones matrix of the polarizers 1-3 is $M_P=\left[\begin{array}{cc}1& 0\\ 0& 0\end{array}\right],$ The jones matrix M for the entire shaping system can be expressed as:

M＝M_P·M_RBE·M_Porro·M_RBE·M_in (5)

the method is simplified to obtain:

$M=\left[\begin{array}{c}{A}^{2}C+2\mathrm{ABD}+B^2{C}^{*}\\ 0\end{array}\right]=\left[\begin{array}{c}a+\mathrm{ib}\\ 0\end{array}\right]---\left(6\right)$

therefore, the reflectance R of the polarizing plates 1 to 3 can be expressed as:

R＝1-(a²+b²) (7)

the light intensity distribution of a conventional laser beam is gaussian, and the normalized light intensity distribution of an incident fundamental mode gaussian beam after passing through the polarizers 1-3 can be expressed as:where w is the spot size of the laser. Therefore, the normalized distribution of the laser light reflected and output by the polarizers 1 to 3 can be expressed as:

<math><mrow> <msub> <mi>I</mi> <mi>out</mi> </msub> <mo>=</mo> <mi>R</mi> <mo>&CenterDot;</mo> <mi>exp</mi> <mrow> <mo>(</mo> <mo>-</mo> <mfrac> <msup> <mrow> <mn>2</mn> <mi>r</mi> </mrow> <mn>2</mn> </msup> <msup> <mi>w</mi> <mn>2</mn> </msup> </mfrac> <mo>)</mo> </mrow> <mo>=</mo> <mo>[</mo> <mn>1</mn> <mo>-</mo> <mrow> <mo>(</mo> <msup> <mi>a</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>b</mi> <mn>2</mn> </msup> <mo>)</mo> </mrow> <mo>]</mo> <mo>&CenterDot;</mo> <mi>exp</mi> <mrow> <mo>(</mo> <mo>-</mo> <mfrac> <msup> <mrow> <mn>2</mn> <mi>r</mi> </mrow> <mn>2</mn> </msup> <msup> <mi>w</mi> <mn>2</mn> </msup> </mfrac> <mo>)</mo> </mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>8</mn> <mo>)</mo> </mrow> </mrow></math>

at this time, the output light intensity distribution function of the laser and θ, d₀Rho and beta, and the homogenization of the output laser beam can be realized by reasonably designing the four parameters.

The functions and advantages that can be achieved by the present invention are illustrated by specific parameters below. Let w equal to 3.5mm, θ equal to 45 °, and β equal to 45 °. The operating wavelength of the laser is 1064 nm. The phase adjusting mirrors 1-4 are made of quartz single crystal materials, and the compensating lenses 1-5 are made of fused quartz materials with the refractive indexes close to those of the phase adjusting mirrors 1-4. Fig. 3 shows normalized light intensity distribution of the output laser of the phase adjusting mirrors 1 to 4 under two different parameters: (a) d₀＝2006.4μm，ρ＝-50.37mm；(b)d₀2008.3 μm, ρ -53.80 mm. It can be seen from fig. 3 that the light intensity distribution of the middle part of the output laser is relatively flat, and the effect of homogenizing the light spots is achieved.

Claims (3)

1. A laser for outputting a flat-top beam is characterized by comprising a reflecting cavity mirror (1-1), a gain medium (1-2), a polarizing plate (1-3), a phase adjusting mirror (1-4), a compensating lens (1-5) and a Porro prism (1-6) which are sequentially arranged along the direction of an optical axis, wherein the included angle between the polarizing plate and the optical axis is a Brewster angle.

2. The laser of claim 1, wherein: the phase adjusting mirror (1-4) is a plano-convex lens or a plano-concave lens processed by adopting a birefringent material, and the birefringent material is a uniaxial crystal or a biaxial crystal.

3. The laser of claim 1, wherein: the compensating lenses (1-5) are plano-concave lenses or plano-convex lenses, the materials of the compensating lenses are isotropic media or cubic crystals, and the curvature radius of the compensating lenses is equal to the curvature radius of the phase adjusting lenses (1-4) in numerical value and opposite in sign.