CN105116664A - Method for simultaneously achieving laser frequency doubling and line aggregation in optical superlattice - Google Patents

Abstract

The invention discloses a method for simultaneously realizing laser frequency doubling and line focusing in an optical superlattice, which includes the following steps: (1) designing a tilted domain structure with the same period in the upper and lower parts in the superlattice; The light direction is axially symmetrical; (2) When the fundamental wave is vertically incident on the superlattice, the upper half of the superlattice will generate a downwardly bent laser frequency doubled wave, and the lower half will generate an upwardly bent laser frequency-doubled wave, and the bending angle and intensity of the laser frequency-doubled wave generated by the upper half and the lower half are the same; thus, similar (3) After the laser enters the superlattice, the reciprocal lattice vector provided by the domain structure of the superlattice can compensate the wavevector mismatch between the fundamental wave and the doubled laser wave, forming a wavevector matching triangles. Through the periodic and symmetrically tilted domain structure in the optical superlattice, the frequency doubling and line focusing of the laser are realized simultaneously.

Description

A Method for Simultaneous Laser Frequency Doubling and Line Focusing in Optical Superlattices

technical field

The invention relates to the intersection field of nonlinear optics and superlattice material technology, in particular to a method for simultaneously realizing laser frequency doubling and line focusing in an optical superlattice.

Background technique

In practical applications such as laser heat treatment and laser cutting, a line-focused beam is often required. In linear optics, axicons are often used to achieve line focusing of beams. The axicon bends the incident beam isotropically, and offsets the diffraction effect through the interference effect of light waves, so that the generated outgoing beam has the characteristics of approximately diffraction-invariant within a certain distance, that is, the line focus of the beam. However, since the axicon is a pyramidal structure, it has disadvantages such as difficult processing and high cost, and the frequency conversion of the laser cannot be realized.

Recently, inspired by the Fresnel lens, someone made a Fresnel axicon. Compared with traditional axicons, Fresnel axicons can be made of plastic sheets, which are light in weight and low in cost, and are convenient for mass production. Although it has the advantage of low cost, due to the inherent defects in the principle, there will be a large number of "void" areas with low light intensity in the interference area of the Fresnel axicon, and the quality of the line-focused beam is not as high as that of the traditional axicon.

On the other hand, in nonlinear optics, the optical superlattice designed based on the principle of quasi-phase matching can simultaneously complete multiple functions such as frequency doubling, sum frequency and difference frequency of laser light in a specific direction. Not long ago, Qin Yiqiang and others from Nanjing University further expanded the function of optical superlattice, realized the point focusing of double frequency wave through the design of domain structure, and realized the point focusing function of linear optical convex lens by nonlinear optics.

Using the principle of quasi-phase matching to design optical superlattice, and realize the line focusing of laser through nonlinear optics, on the one hand, it can overcome the disadvantages of complicated and expensive processing of axicon surface, and can be fabricated at low cost through mature photolithography and polarization technology. Optical superlattice with arbitrary domain structure; on the other hand, it can overcome the shortcomings of Fresnel axicons with diffraction holes, and the quality of line-focused beams is better.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for realizing laser frequency doubling and line focusing simultaneously in an optical superlattice with a high degree of integration and low cost.

In order to solve the above-mentioned technical problems, the technical solution adopted in the present invention is to realize the goal of linear optics with the principle of quasi-phase matching through the design of the domain structure in the optical superlattice, specifically comprising the following steps:

(1) Design a tilted domain structure with the same period in the upper and lower parts in the superlattice, and the distribution is symmetrical with the crystal light transmission direction as the axis;

(2) When the fundamental wave is vertically incident on the superlattice, the upper half of the superlattice will generate a downwardly bent laser frequency doubled wave, and the lower half will generate an upwardly bent laser frequency doubled wave, and the upper half The laser frequency-doubled wave generated by the half part and the laser frequency-doubled wave generated by the lower part have the same bending angle and the same intensity; in this way, a line focus similar to an axicon will be generated in the overlapping area of the upper and lower parts interference pattern;

(3) After the laser enters the superlattice, the reciprocal lattice vector provided by the domain structure of the superlattice can compensate the wavevector mismatch between the fundamental wave and the laser frequency doubled wave, thus forming a wavevector matching triangle.

Through the above-mentioned technical scheme, a tilted domain structure with the same period of the upper and lower parts is designed in the superlattice (the domain structure in the superlattice is a periodically symmetrical and tilted leaf texture shape), and it is symmetrical with the crystal light transmission direction as the axis distribution; when the fundamental wave is vertically incident on the superlattice, since the upper and lower domain structures have reciprocal lattice vectors in different directions, the upper half of the superlattice will generate downwardly bent double frequency waves, and the lower half will generate upwardly bent The folded frequency doubled wave, the bending angle of the two parts of the laser is the same, the intensity is the same, so that a line-focusing interference pattern similar to an axicon will be generated in the area where the two overlap; after the laser enters the superlattice, due to the fundamental wave and There is a wave-vector mismatch between double-frequency waves, which needs to be compensated by the reciprocal lattice vector provided by the domain structure of the superlattice. Since it is non-collinear matching, a wave-vector matching triangle is formed, that is, through the periodic symmetry in the optical superlattice The inclined domain structure matches the wavelength of the laser with the period of the domain structure, and adjusts the length of the line focus with the inclination angle of the domain structure, which can realize the frequency doubling and line focus of the laser at the same time; not only has a high degree of integration, but also can effectively reduce production cost.

A further improvement is that the wave vector matching triangle formed in the step (3) adopts K ₁ and K ₂ to represent the wave vectors of the fundamental wave and the laser frequency-doubled wave respectively, and G represents the inverted wave vector provided by the domain structure of the superlattice. The lattice vector, the wave vector K ₁ of the fundamental wave, the wave vector K ₂ of the laser frequency-doubled wave, and the reciprocal lattice vector G form a closed triangle under the condition of quasi-phase matching.

A further improvement is that the included angles formed by the propagation direction of the fundamental wave, the propagation direction of the laser frequency-doubled wave inside the superlattice, and the propagation direction of the laser frequency-doubled wave outside the superlattice are α and θ, respectively.

A further improvement is that the inclination direction of the domain structure of the superlattice is perpendicular to the direction of the reciprocal lattice vector G provided by the superlattice domain structure, and the inclination direction of the domain structure of the superlattice is in the clip formed by the fundamental wave. The angle is β.

A further improvement is that the periods of the oblique direction and the horizontal direction of the domain structure of the superlattice are Λ ₁ and Λ ₂ respectively, and the wave vector K ₁ of the inverted vector lattice G and the fundamental wave and the wave vector K of the laser frequency doubled wave ₂ 's relationship is:

$\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; =</span>\sqrt{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ;</span>$

The relationship between period Λ ₁ and G is: Λ ₁ = 2π/G;

The relational expression between period Λ ₁ and period Λ ₂ is: Λ ₂ = Λ ₁ / sin β;

The angle between the inclination direction of the domain structure and the fundamental wave is β, and the relationship between the inverted vector lattice G, the wave vector K ₁ of the fundamental wave, and the wave vector K ₂ of the laser frequency doubled wave is:

β+π/2=arccos((4K ₁ ² +G ² -K ₂ ² )/4K ₁ G);

Where n ₂ sinα=n ₁ sinθ, the size of θ can be obtained from the radius of the incident fundamental wave and the length of the line gathering; n ₁ and n ₂ are the refractive indices of the laser doubled wave in air and in the optical superlattice, respectively.

A further improvement is that the material of the superlattice is a trigonal distorted perovskite structure material or an orthorhombic nonlinear optical crystal material.

As a preferred solution of the present invention, the trigonal distorted perovskite structure material is lithium niobate or lithium tantalate, and the orthorhombic nonlinear optical crystal material is potassium titanyl phosphate.

As a preferred solution of the present invention, the wavelength of the fundamental wave is 1.064 μm; when the material of the superlattice is lithium niobate, the matching temperature is 150° C. In order to avoid the photorefractive effect, the matching temperature is selected as 150°C.

Compared with the prior art, the beneficial effects of the present invention are: based on the principle of nonlinear optics, the present invention replaces the manufacture of complex optical curved surfaces with the preparation of superlattice domain structures; The domain structure can achieve laser frequency doubling and line focusing at the same time; not only the degree of integration is high, but also the production cost can be effectively reduced.

Description of drawings

Below in conjunction with accompanying drawing and embodiment of the present invention further describe in detail:

Fig. 1 is a schematic diagram of laser wave frequency line focusing in the optical superlattice of the present invention;

Fig. 2 is a schematic diagram of beam wave vector matching of the present invention;

Fig. 3 is that the present invention takes lithium niobate superlattice as example domain structure partial figure;

Fig. 4 is a diagram of the laser frequency doubled wave propagating outside the superlattice of the present invention.

Detailed ways

The method for simultaneously realizing laser frequency doubling and line focusing in an optical superlattice specifically includes the following steps:

(1) Design a tilted domain structure with the same period in the upper and lower parts in the superlattice, and the distribution is symmetrical with the crystal light transmission direction as the axis;

(2) When the fundamental wave is vertically incident on the superlattice, the upper half of the superlattice will generate a downwardly bent laser frequency doubled wave, and the lower half will generate an upwardly bent laser frequency doubled wave, and the upper half The laser frequency-doubled wave generated by the half part and the laser frequency-doubled wave generated by the lower part have the same bending angle and the same intensity; in this way, a line focus similar to an axicon will be generated in the overlapping area of the upper and lower parts Interference pattern; as shown in Figure 1;

(3) After the laser enters the superlattice, the reciprocal lattice vector provided by the domain structure of the superlattice can compensate the wavevector mismatch between the fundamental wave and the laser frequency doubled wave, thus forming a wavevector matching triangle.

The included angles formed by the propagation direction of the fundamental wave, the propagation direction of the laser frequency-doubling wave in the superlattice, and the propagation direction of the laser frequency-doubling wave outside the superlattice are α and θ respectively; the inclination of the domain structure of the superlattice The direction is perpendicular to the direction of the reciprocal vector G provided by the superlattice domain structure, and the angle formed between the inclination direction of the domain structure of the superlattice and the fundamental wave is β; the inclination direction of the domain structure of the superlattice and the periods in the horizontal direction are Λ ₁ and Λ ₂ respectively, and the relationship between the inverse vector lattice G and the wave vector K ₁ of the fundamental wave and the wave vector K ₂ of the laser frequency doubled wave is:

$\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; =</span>\sqrt{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ;</span>$

The relationship between period Λ ₁ and G is: Λ ₁ = 2π/G;

The relational expression between period Λ ₁ and period Λ ₂ is: Λ ₂ = Λ ₁ / sin β;

The angle between the inclination direction of the domain structure and the fundamental wave is β, and the relationship between the inverted vector lattice G, the wave vector K ₁ of the fundamental wave, and the wave vector K ₂ of the laser frequency doubled wave is:

β+π/2=arccos((4K ₁ ² +G ² -K ₂ ² )/4K ₁ G);

Where n ₂ sinα=n ₁ sinθ, the size of θ can be obtained from the radius of the incident fundamental wave and the length of the line gathering; n ₁ and n ₂ are the refractive indices of the laser doubled wave in air and in the optical superlattice, respectively.

Taking the lithium niobate superlattice as an example, in order to avoid the photorefractive effect, the matching temperature is selected as 150°C; the fundamental wave is a commonly used laser with a wavelength of 1.064 μm; at this time, the refractive index of the fundamental wave in the lithium niobate superlattice is 2.161806, The refractive index of doubled wave is 2.242572, and the refractive index in air is 1; the superlattice length is set to 2mm, the fundamental wave diameter is set to 1mm, and the line focusing distance is set to 20cm; according to Figure 2 and the above formula, the domain structure can be obtained The slope is 32.3059 and the horizontal period is 6.58704 μm. Fig. 3 is a partial pattern of lithium niobate superlattice domain structure designed according to these parameters.

The propagation of frequency doubled waves outside the superlattice simulated by the central difference method is shown in Figure 4. It can be seen that the propagation and interference of light waves in this scheme are very similar to those of axicons, and the laser energy is concentrated on the line at the center of propagation within the shape area. Outside the main focal line, secondary interference fringes similar to axicons also appear. The brightness of the main focused beam is uniform, the quality is high without voids, and the line focus length is long, which meets the application requirements.

The present invention is not limited by the above-mentioned embodiments, as long as the laser wavelength within the light-passing range of the superlattice can be applied; the matching temperature is not limited to 150°C, as long as the photorefractive effect of the superlattice can be eliminated.

The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the present invention is not limited by the above-mentioned embodiments, and what described in the above-mentioned embodiments and the description only illustrates the principles of the present invention, and the present invention will also have other functions without departing from the spirit and scope of the present invention. Variations and improvements are possible, which fall within the scope of the claimed invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims (8)

1. in optical superlattice, realize a method for laser freuqency doubling and line gathering simultaneously, specifically comprise the following steps:

(1) in superlattice, design the domain structure of identical inclination of upper and lower two parts cycle, and with crystal optical direction for axle is symmetrically distributed;

(2) when in first-harmonic vertical incidence superlattice, the first half branch of described superlattice produces the laser freuqency doubling ripple of bending downwards, Lower Half branch produces the laser freuqency doubling ripple upwards bent, and the laser freuqency doubling ripple that the first half produces is identical with the bending angle of the laser freuqency doubling ripple that the latter half produces, intensity is identical; The line focus interference pattern being similar to axle pyramid will be produced like this at the overlapping region of the first half and the latter half;

(3) after laser enters superlattice, the reciprocal lattice vector that the domain structure of superlattice provides can compensate the wave vector mismatch between first-harmonic and laser freuqency doubling ripple, thus forms the triangle of wave vector coupling.

2. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 1, is characterized in that, the triangle of the wave vector coupling formed in described step (3), adopts K ₁and K ₂represent the wave vector of first-harmonic and laser freuqency doubling ripple respectively, G represents the wave vector K of the reciprocal lattice vector that the domain structure of superlattice provides, first-harmonic ₁with the wave vector K of laser freuqency doubling ripple ₂under the condition of quasi-phase matched, a closed triangle is formed with reciprocal lattice vector G three.

3. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 2, it is characterized in that, the angle that described base direction of wave travel and superlattice inner laser frequency multiplication direction of wave travel, the outer laser freuqency doubling direction of wave travel of superlattice are formed is respectively α and θ.

4. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 3, it is characterized in that, the vergence direction of the domain structure of described superlattice is vertical with the direction of the reciprocal lattice vector G that superlattice domain structure provides, and the angle that the vergence direction of the domain structure of described superlattice and first-harmonic are formed is β.

5. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 4, it is characterized in that, the vergence direction of the domain structure of described superlattice and the cycle of horizontal direction are respectively Λ ₁and Λ ₂, the wave vector K of reciprocal-vector lattice G and first-harmonic ₁with the wave vector K of laser freuqency doubling ripple ₂relational expression be:

Periods lambda ₁with the relational expression of G be: Λ ₁=2 π/G;

Periods lambda ₁with periods lambda ₂relational expression be: Λ ₂=Λ ₁/ sin β;

And the angle that the vergence direction of domain structure and first-harmonic are formed is the wave vector K of β and reciprocal-vector lattice G and first-harmonic ₁with the wave vector K of laser freuqency doubling ripple ₂relational expression be:

β+π/2＝arccos((4K ₁ ²+G ²-K ₂ ²)/4K ₁G)；

Wherein n ₂sin α=n ₁the size of sin θ, θ is assembled length by the radius of incident first-harmonic and line and can be obtained; n ₁, n ₂be respectively laser freuqency doubling ripple in atmosphere with the refractive index in optical superlattice.

6. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to any one of claim 1-5, it is characterized in that, the material of described superlattice is the distortion perovskite structural material of trigonal system or the non-linear optical crystal material of orthorhombic system.

7. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 6, it is characterized in that, the distortion perovskite structural material of described trigonal system is lithium niobate or lithium tantalate, and the non-linear optical crystal material of described orthorhombic system is potassium titanium oxide phosphate.

8. the method simultaneously realizing laser freuqency doubling and line gathering in optical superlattice according to claim 7, it is characterized in that, the wavelength of described first-harmonic is 1.064 μm; When the material of described superlattice is lithium niobate, coupling temperature is 150 DEG C.