CN100462747C - Three-step beam splitting grating and manufacturing method thereof - Google Patents

Abstract

The utility model provides a three-step beam splitting grating for divide into a plurality of even light beam with a laser, its characterized in that: the structure of the grating is that 2 periods T are taken as structure change periods (namely, the structure of each 2 period T is the same), wherein the structure of the 1 st period T is a groove structure of one period T of the binary phase beam-splitting grating, the structure height of the 2 nd period T is improved by h, and the groove structure of the 2 nd period T is a groove structure of the binary phase beam-splitting grating opposite to the structure of the 1 st period T.

Description

A kind of three step splitting beam grating and method for making

Technical field

The present invention relates to a kind of beam-splitting optical grating, particularly be applied to beam of laser is divided into a plurality of uniform beams.

Background technology

In information optics, optical communication, swash the hot spot dot matrix that often beam of laser evenly need be divided into the beamlet of multi beam homenergic in the system such as combiner or on lens focal plane, obtain the light intensity unanimity.Utilize the binary phase grating (Darman raster) of particular design, can achieve this end.But when beam splitting increased, its diffraction efficiencies at different levels changed to the lateral error susceptibility very greatly.When at this moment using the binary beam-splitting optical grating since laterally the existence of mismachining tolerance will influence the homogeneity of beam splitting greatly, and beam splitting is many more, uniformity coefficient is poor more, therefore in fact the maximum array number that can make of binary beam-splitting optical grating is limited.

On structure, the unit cycle of binary position phase beam-splitting optical grating is made up of the groove of some different in width, and gash depth is h=λ/2 (n-1), modulates (as shown in Figure 1) mutually corresponding to the π position, and wherein λ is an optical maser wavelength, and n is the material refractive index.Utilize the set { x of the coordinate of all the slot wedge positions in the one-period ₁, x ₂, Λ, x _k, Λ, x _KOptical grating construction can fully be described.For simplicity, 0＜x is satisfied in the whole cycle normalization of all coordinates ₁＜x ₂＜Λ x _k＜x _K＜1.In general, splitting ratio is the grating of 2N+1, and the numerical value of edge coordinate number K is N.

According to the theory of Fourier optics, the diffraction efficiency formula at different levels of this structure fringe are:

$A_0=(1-2\underset{k=1}{\overset{k\→K}{\mathrm{\Σ}}}{(-1)}^k{x}_k)---1)$

$A_n=\left\{[\underset{k=1}{\overset{k\→K}{\mathrm{\Σ}}}{(-1)}^k\left[\cos \left({2\mathrm{\πnx}}_k\right)\right]\right]+i[\underset{k=1}{\overset{k\→K}{\mathrm{\Σ}}}{(-1)}^k\left[\sin \left({2\mathrm{\πnx}}_k\right)\right]\]\},n\≠0---2)$

I _n＝|A _n| ² 3)

Wherein, n is that the order of diffraction is inferior, I _nBe the inferior diffraction efficiency of the corresponding order of diffraction.

For the design splitting ratio is the grating of 2N+1, need find the solution following system of equations,

I ₀＝I _±1＝I _±2＝Λ＝I _±N， 4)

Generally, do not have strict analytic solution, generally need the numerical optimization algorithm to find the solution formula (1～4), obtain one group of { x ₁, x ₂, Λ, x _k, Λ, x _K, its target is 0～± good uniformity in the N order of diffraction time, and diffraction efficiency is high as far as possible.For the ease of the uniformity coefficient of comparative analysis raster beam-splitting, the General Definition uniformity coefficient is estimated the factor and is

This factor is more little, and the uniformity coefficient of beam splitting is high more.General application requirements uniformity coefficient is at least in 10%.

Although can design the high evenness binary phase grating beam splitter of any array size in theory, the actual maximum array number that can make is always limited.This be because: all inevitably there is the mismachining tolerance problem in the preparation of any binary beam-splitting optical grating, and this error will have very big influence to the array uniformity coefficient, and beam splitting is many more, influences big more; Can accept under the uniformity coefficient level, the maximum array number that can make is limited.

Particularly, mismachining tolerance mainly contains depth error (longitudinal error) and width error (lateral error).The former is a groove processing degree of depth off-design value, and this error is relatively very little to the grating quality influence, does not consider here.Width error shows each groove width off-design value (slot wedge position deviation design load in other words), and its main source has the limited exposure resolution ratio, underdevelop or excessive of mismachining tolerance, the lithographic equipment of mask.Formula (1～3) is done the width error analysis, can obtain estimating the approximate formula of diffraction efficiency relative error size.

$\frac{{\mathrm{\&Delta;I}}_n}{I_n}\&ap;\frac{2}{\sqrt{\mathrm{\&eta;}/(2N+1)}}\mathrm{K\&Delta;x}\&ap;\frac{2\sqrt{2}}{\sqrt{\mathrm{\&eta;}}}{\mathrm{<figure-callout\; id="3"\; label="N"\; filenames="C200510130742E00132.png,C200510130742E00141.png"\; state="\{\{state\}\}">N</figure-callout>}}^{3/2}\mathrm{\&Delta;x}---5)$

Wherein, Δ x is normalized lateral error size, and η is the total diffraction efficiency of grating, promptly $\mathrm{\&eta;}=\underset{n=-N}{\overset{N}{\mathrm{\&Sigma;}}}{I}_n.$ As can be seen, when splitting ratio N increase, its diffraction efficiencies at different levels change to the lateral error susceptibility very greatly.Therefore the existence of lateral error has had a strong impact on binary position phase beam-splitting optical grating quality, makes each order of diffraction time efficient off-design value greatly to cause dot matrix homogeneity variation.

Summary of the invention

The problem that the technology of the present invention solves is: overcome the deficiency of traditional binary beam-splitting optical grating, provide a kind of three step position phase beam-splitting optical grating and method for makings, to reduce horizontal mismachining tolerance to the grating Effect on Performance.

A kind of three step splitting beam grating, it is characterized in that: its structure is to be that the structure of each 2 period T is all the same in the structural change cycle with 2 period T, wherein the structure of the 1st period T is the groove structure of binary position phase beam-splitting optical grating one-period T, the structure height of the 2nd period T improves h, and the groove structure of the 2nd period T is the binary position opposite with the 1st a periodic structure beam-splitting optical grating groove structure mutually, wherein h=λ/2 (n-1), λ is an optical maser wavelength, n is the material refractive index.

A kind of method for making of three step splitting beam grating, mainly by following step:

(1) preparation two mask plates: the pattern period of mask plate M1 is 2T, wherein is black pattern in preceding T section, is white pattern in the T section of back, is white pattern in preceding T section perhaps, is black pattern in the T section of back; The cycle of mask plate M2 also is 2T, for dividing the lines of grating structure, is the lines of the branch grating structure of inverse structure in preceding T section in the back T section;

(2) preparing grating: utilize two mask plates difference exposure imaging etching on optical material substrates such as glass, promptly earlier with mask plate M1 exposure imaging etching, use mask plate M2 exposure imaging etching again, or earlier with mask plate M2 exposure imaging etching, use mask plate M1 exposure imaging etching again, etching depth is the π position phase degree of depth.The double exposure process, pattern needs fine registration.

The present invention is because the inverse structure of adjacent periods, its output uniformity error is approximate cancels out each other, homogeneity is subjected to the influence of horizontal mismachining tolerance little, therefore the present invention compares with two step Darman rasters of routine, it is advantageous that the beamlet array strength uniformity coefficient of its beam splitting, low to requirement on machining accuracy, can make the grating of bigger splitting ratio under the same mismachining tolerance.

Description of drawings

Fig. 1 is traditional binary phase grating embossment structure figure;

Fig. 2 is three step splitting beam grating embossment structure figure of the present invention;

Fig. 3 is a three step splitting beam grating manufacturing process synoptic diagram of the present invention;

Fig. 4 is the component structure figure of embodiments of the invention 1-1 * 13 three step splitting beam gratings;

Fig. 5 is the desirable output maps of embodiments of the invention 1-1 * 13 Darman rasters;

When Fig. 6 is the lateral error 0.001 of embodiments of the invention 1,1 * 13 binary tradition Darman raster output map;

When Fig. 7 is the lateral error 0.001 of embodiments of the invention 1, during alignment error 0.002, three step Darman raster output maps;

Fig. 8 is the component structure figure of 1 of embodiments of the invention 2 * 23 3 step gratings;

Fig. 9 is the desirable output map of 1 * 23 Darman raster of embodiments of the invention 2;

When Figure 10 is lateral error 0.001,1 * 23 binary tradition Darman raster output map;

When Figure 11 is lateral error 0.001, during alignment error 0.002,1 * 23 3 step grating output map.

Embodiment

Adjacent cells cycle inner structure of the present invention as shown in Figure 2, traditional relatively binary beam-splitting optical grating (as shown in Figure 1) is at interval with the period T, the structure in the next cycle T is done following processing: highly improve h, optical grating construction negate (as shown in Figure 2).(for simplicity, institute's drawings attached structure in adjacent two cycles in the grating of only having drawn in this instructions, the grating other parts only are the repetitions in these two cycles.)

Manufacturing process of the present invention:

(1) the grating preparation needs two mask plates.The pattern of mask plate M1 is the lines between the black and white homogeneous phase of cycle 2T.Mask plate M2 is the optical grating construction lines, is that adjacent two intraperiod line bar patterns are just in time opposite.See Fig. 3.

(2) preparing grating.Utilize two mask plates, on optical material substrates such as glass, exposure imaging etching twice, etching depth is the π position phase degree of depth.The double exposure process, pattern needs fine registration, but order can be put upside down.See Fig. 3.

(3) can be if make three step gratings of two dimension by one dimension three step gratings at x and y direction quadrature gained.

The present invention compared with prior art has following advantage: beam splitting light intensity uniformity coefficient is subjected to the influence of horizontal mismachining tolerance little, thereby low to requirement on machining accuracy, can make the grating of bigger splitting ratio under the same error.

Be convenient narration, below implementation process, analysis and example one-dimensional grating all is discussed.Because two-dimensional grating is the quadrature gained of one-dimensional grating in x and y direction, its one dimension conclusion can be promoted.

1, utilize optimisation technique (such as minimal gradient method, simulated annealing etc.) design to realize the binary phase grating groove structure { x of a certain size dot matrix ₁, x ₂, Λ, x _k, Λ, x _K.

Position phase modulating function (Fig. 2) corresponding to grating is:

$\mathrm{\&phi;}\left(x\right)=\left\{\begin{array}{c}0,x_i<x<x_{i+1}(i=\mathrm{1,3,5}\mathrm{\&Lambda;K}-1)\\ \mathrm{\&pi;},x_i<x<x_{i+1}(i=\mathrm{2,4,6}\mathrm{\&Lambda;K})\end{array}\right.---6)$

2, design and produce the needed binary mask (see figure 3) of three step splitting beam grating.

Two mask transmittance functions are respectively (see figure 3):

Mask M1 is to be that the light and shade of 2T replaces mask in the cycle, and light transmission part and lightproof part width are T.Getting transmittance function in its 2T cycle can be written as:

$t_{M1}\left(x\right)=\left\{\begin{array}{c}0,0<x<T,\\ 1,T<x<2T\end{array}\right.---7)$

Mask M2 is to be that the light and shade of 2T replaces mask in the cycle, and its corresponding mask one light transmission part and lightproof part have opposite transmitance and distribute.Transmittance function in the 2T cycle is:

$t_{M2}\left(x\right)=\left\{\begin{array}{c}0,x_i<x<x_{i+1}(i=\mathrm{1,3,5}\mathrm{\&Lambda;K}-1)\\ 1,x_i<x<x_{i+1}(i=\mathrm{2,4,6}\mathrm{\&Lambda;K})\end{array}\right.,0<x<T---8)$

$t_{M2}\left(x\right)=\left\{\begin{array}{c}0,x_i<x-T<x_{i+1}(i=\mathrm{1,3,5}\mathrm{\&Lambda;K}-1)\\ 1,x_i<x-T<x_{i+1}(i=\mathrm{2,4,6}\mathrm{\&Lambda;K})\end{array}\right.,T<x<2T---9)$

3, utilize two mask manufactures, three step position phase beam-splitting optical gratings.

4,, utilize two mask manufactures, the three step position phase beam-splitting optical gratings that design by traditional many steps optical element alignment job operation.Concrete manufacturing process is seen Fig. 3.(for simplicity, institute's drawings attached manufacturing process of the structure in adjacent two cycles in the grating of only having drawn in this instructions, the grating other parts only are the repetitions in these two cycles.) utilize mask plate M1 photoetching for the first time, on material base, obtain the groove of T width after development and the etching, etching depth is h=λ/2 (n-1).Photoetching for the second time utilizes mask M2, and needs and pattern alignment exposure last time, and etching depth is similarly h=λ/2 (n-1).Structure function in the adjacent 2T cycle of the grating that finally obtains is:

$h\left(x\right)=\left\{\begin{array}{c}0,x_i<x<x_{i+1}(i=\mathrm{1,3,5}\mathrm{\&Lambda;K}-1)\\ h,x_i<x<x_{i+1}(i=\mathrm{2,4,6}\mathrm{\&Lambda;K})\end{array}\right.,0<x<T---10)$

$h\left(x\right)=\left\{\begin{array}{c}2h,x_i<x-T<x_{i+1}(i=\mathrm{1,3,5}\mathrm{\&Lambda;K}-1)\\ h,x_i<x-T<x_{i+1}(i=\mathrm{2,4,6}\mathrm{\&Lambda;K})\end{array}\right.,T<x<2T---11)$

Because the position has 2 π periodically mutually, the cycle of three step parts has identical position phase modulating function distributed (6) with two step part cycles, so they can realize identical branch beam function.But coarse analysis can learn that it is very little that the homogeneity of the diffraction efficiencies at different levels of the beam splitting of three ledge structure gratings is influenced by lateral error.This moment, lateral error mainly contained from two aspects.The one, when making the grating hachure, line edge change in location Δ x, the 2nd, there is alignment error delta x in the twice etching figure _αThe former is owing to the inverse structure of adjacent periods structure, and its output uniformity error is approximate cancels out each other, and homogeneity mainly is subjected to the influence of alignment error.Utilize formula (1～3) error analysis, can obtain its diffraction efficiency relative changing values at different levels and be approximately:

$\frac{{\mathrm{\&Delta;I}}_n}{I_n}\&ap;\frac{2}{\sqrt{\mathrm{\&eta;}/(2N+1)}}\mathrm{\&Delta;}{x}_a\&ap;\frac{2\sqrt{2}}{\sqrt{\mathrm{\&eta;}}}{\mathrm{<figure-callout\; id="1"\; label="N"\; filenames="C200510130742E00111.png,C200510130742E00112.png"\; state="\{\{state\}\}">N</figure-callout>}}^{1/2}\mathrm{\&Delta;}{x}_a.---12)$

The error analysis that contrasts traditional two step splitting beam gratings is formula (5) as a result, and the relative changing value of diffraction efficiencies at different levels has approximately reduced In general, alignment error delta x _αX is on close level with lines mismachining tolerance Δ.Therefore, three ledge structure phase gratings of this design and processing and traditional two step positions beam-splitting optical grating are mutually compared, and under the same lateral error, the uniformity coefficient of beam splitting array is higher, thereby can make the bigger beam splitter of array number.Can also find out that in addition under the same error level, when the beam splitting number N increased, the advantage of the uniformity coefficient of three step splitting beam grating on anti-error ability was more obvious from formula (12).

Embodiment 1

Design beam splitting array is several 1 * 13, grating cycles 250 μ m, wavelength 0.6328 μ m.Optical material is for melting quartz, and corresponding refractive index is 1.457.The horizontal mismachining tolerance of binary optical processing technology is 0.25 μ m, and the alignment error is 0.5 μ m.

Utilize the mathematical optimization technology to obtain Grating Modulation position phase trip point normalization coordinate to be: { 0.17770.3135 0.4124 0.4985 0.5763 0.7086}.The structure of corresponding three step gratings as shown in Figure 4, gash depth 0.692 μ m.Fig. 5 has provided when not having error, the grating that calculates diffraction efficiencies at different levels, and they are very even.

When Fig. 6 had provided the lateral error of considering in the process, the diffraction efficiencies at different levels of three ledge structure gratings, its uniformity coefficient are 0.028, and were still very even.For ease of comparative analysis, Fig. 7 has provided the diffraction efficiencies at different levels of two steps tradition Darman raster structure output under the same error level, and its uniformity coefficient is variation greatly, only is 0.084.Very obvious, under the same error level, the branch beam quality of three step splitting beam grating is better than two ledge structures.

Embodiment 2

Design beam splitting array is several 1 * 23, grating cycles 250 μ m, wavelength 0.6328 μ m.Optical material is for melting quartz, and corresponding refractive index is 1.457.The horizontal mismachining tolerance of optics processing technology is 0.25 μ m, and the alignment error is 0.5 μ m.

The cycle normalization coordinate that utilizes the mathematical optimization technology to obtain Grating Modulation position phase trip point is: { 0.1044 0.1570 0.2191 0.2565 0.2615 0.2783 0.31710.3654,0.4245 0.5574 0.6526 0.8995 0.9410}.The structure of corresponding three step gratings as shown in Figure 8, gash depth 0.692 μ m.Fig. 9 has provided when not having error, the diffraction efficiencies at different levels of grating, and they are very even.

When Figure 10 had provided the lateral error of considering in the process, the diffraction efficiencies at different levels of three ledge structure gratings, its uniformity coefficient are 0.04, and were still very even.For ease of comparative analysis, Fig. 7 has provided the diffraction efficiencies at different levels of two steps tradition Darman raster structure output under the same error level, and its uniformity coefficient only is 0.24, and diffraction efficiencies at different levels are very inhomogeneous.Very obvious, under the same error level, the branch beam quality of three step splitting beam grating will be better than two ledge structures greatly.

Claims (2)

1, a kind of three step splitting beam grating, it is characterized in that: its structure is the structural change cycle with 2 period T, the structure that is each 2 period T is all the same, wherein the structure of the 1st period T is the groove structure of binary position phase beam-splitting optical grating one-period T, the structure height of the 2nd period T improves h, and the groove structure of the 2nd period T is and the binary position of the 1st period T inverted configuration beam-splitting optical grating groove structure, wherein h=λ/2 (n-1) mutually, λ is an optical maser wavelength, and n is the material refractive index.

2, a kind of method for making of three step splitting beam grating is characterized in that being realized by following step:

(1) preparation two mask plates: the pattern period of mask plate M1 is 2T, wherein is black pattern in preceding T section, is white pattern in the T section of back, is white pattern in preceding T section perhaps, is black pattern in the T section of back; The cycle of mask plate M2 also is 2T, for dividing the lines of grating structure, is the lines of the branch grating structure of inverse structure in preceding T section in the back T section;

(2) preparing grating: utilize two mask plates difference exposure imaging etching on the optical material substrate, promptly earlier with mask plate M1 exposure imaging etching, use mask plate M2 exposure imaging etching again, or earlier with mask plate M2 exposure imaging etching, use mask plate M1 exposure imaging etching again, etching depth is the π position phase degree of depth; The double exposure process, pattern needs fine registration.

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