SU1640663A1 - Semiconductor laser beam correction and collimation system - Google Patents

Description

The invention relates to optical instrumentation and laser technology, and is intended for collimation and correction of the wavefront and shape of the cross section of a semiconductor laser beam.

The purpose of the invention is to provide a correction of the wavefront of a collimated light beam while maintaining the correction of the shape of its cross section.

FIG. Figure 1 shows the optical layout of the device; in fig. 2 - phase statistical transparency with scattering centers of oval form, fragment; in fig. 3 is a diagram of a beam cross section in the plane of a complex conjugate filter, by means of a shift mechanism for correcting the beam cross section shape.

In a system containing a sequentially located semiconductor laser 1, a preliminary lens collimator 2. a system for correcting the beam section shape and a final collimator, the system for correcting the beam shape is made in the form of a phase statistical transformer 3, the scattering centers of which have the shape and orientation of such such that their large size is collinear to the plane of the p – n junction of the semiconductor laser 1, and the final collimator is made in the form of a complex conjugate filter 4 remote from atisticheskogo transparency by a distance Z, satisfying the Fraunhofer diffraction approximation. For this purpose, an additional Fourier lens is located in the system diagram, located between the statistical transparency and the complex conjugate filter, which is located in the rear focal plane of this lens. Thus, in the plane of the complex conjugate filter, 4, the angular spectrum of the laser beam diffracted on the statistical transparency is formed.

The luminescence body of laser 1 is aligned with the front focal plane of the preliminary collimator 2. The dimensions of the scattering centers of the phase static transparency 3 are chosen from the ratio

I ai

-t- + T

O | fk

ABOUT)

A + aji dii fk

where dn, di are the dimensions of the scattering centers in the direction collinear and orthogonal to the p-n-junction plane, respectively;

au, ai are the dimensions of the luminescence body of a semiconductor laser in the p – n-junction plane and orthogonal to its plane, respectively;

fk is the focal length of the preliminary lens collimator 2;

I is the radiation wavelength of a semiconductor laser 1.

The system works as follows.

The radiation of a semiconductor laser 1, having a glow body with size ai in the p - plane. The n-junction and ai in the orthogonal plane (ac ai) are preliminarily collimated by collimator 2, at the output of which a slightly divergent (1-3 °) beam is formed with a cross section elongated along the direction orthogonal to the p – n junction plane. This beam diffracts on the phase statistical transparency 3, the scattering centers of which are made, for example, oval with dimensions dn in the plane of the pn junction and di in the plane orthogonal to it1 (Fig. 2). The averaged cross-section (envelope) in the Fraunhofer diffraction pattern (the plane of the complex conjugate filter 4), without taking into account the size of the speckle inhomogeneities, is determined by the relations

Dn AZ / dii,

(2)

Di AZ / di,

where Dii.Di are the dimensions of the cross section of the diffraction pattern in the p – n junction plane and in the orthogonal plane, respectively. Consideration of the size of speckle inhomogeneities leads to an increase in the cross section of the diffraction pattern by an amount equal to the diameter of the speckle inhomogeneities, if

Since the speckle inhomogeneities are round, the shape of the diffraction pattern is not distorted.

Diffraction of a light beam with an asymmetric cross section, like a semiconductor laser has, distorts the diffraction pattern. This distortion is caused by a change in the shape of the speckle inhomogeneities, which corresponds to the shape of the glowing body of the semiconductor laser 1; the dimensions of the speckle inhomogeneities ai and ai in the p-n-junction plane and the plane orthogonal to it, respectively, are related to the dimensions of the glow body ai and ai by the following relations:

a fi Zaii / fk,

af Zai / fk. (3)

Taking into account the size of the SPECA inhomogeneities (3), the expression for the cross section of the diffraction pattern (2) turns into

- Dit AZ / dn + aii AZ / dii + Zan / fk,

Di AZ / di + ai AZ / di + Zai / fk, (4)

from which it follows that in order to form a symmetric section in the diffraction pattern, the condition DII DI must be fulfilled, which leads to relation (1).

From expression (1) it also follows the possibility of forming a beam with a cross section that is different from the round one, but also different from the cross section of the original beam. For example, if it is necessary to form a beam with a cross section that satisfies the DII / DI / 3 ratio (some given value), expression (1) becomes more general A, ai a / A aA.

cJiI + iT + (d + fkj-®

The mechanism for correcting the shape of the beam section in the plane of the complex-matched filter 4 is illustrated in FIG. 3. Curve 5 describes the diffraction pattern of an axially symmetric beam on the phase statistical transparency with round-shaped scattering centers. Curve 6 shows the deformation of the cross section of the diffraction pattern in violation of the symmetry of the laser beam cross section from the astigmatic source and the retention of the round shape of the scattering centers. Curve 7 shows the recovery of a circular cross section in the diffraction pattern from an astigmatic beam using oval scattering centers, the dimensions of which satisfy condition (1). From FIG. The three curves show that an increase in the laser beam divergence in one plane due to astigmatism is compensated for by an increase in the beam divergence in the orthogonal plane due to an increase in the diffraction angle due to a decrease in the size of the scattering centers of the statistical transparency in this plane.

The final collimation of the laser beam is carried out using a complex conjugate filter 4 made, for example, by a holographic method. The complex conjugate filter has a transmittance that is complex conjugate to the distribution of the field over the cross section of the light beam incident on it. As a result, the amplitude-phase correction of the light beam passing through the complex conjugate filter, which consists in eliminating phase inhomogeneities in the wave front of the beam, occurs. Consequently, at the output of the complex conjugate filter 4, a light beam is formed with a plane wave front and a non-uniform intensity distribution associated with the presence of speckle inhomogeneities. The cross section of this beam is determined by the shape of the initial laser beam and the shape of the scattering centers of the statistical transpoent 3 according to expressions (1) and (5).

The angular spectrum of the corrected beam at the output of the complex conjugate filter 4 is determined by the spatial autocorrelation function of the statistical transparency 3, which is 5-visible.

The table shows data representing an example of a possible specific implementation of the correction system and collimation of a semiconductor laser radiation beam for different values of the size of the glow body in the p – n junction plane.

It is assumed that the size of the scattering centers di is the same, and the compensation of astigmatism is achieved by varying the value of dn in accordance with relation (1). The values of the system parameters are as follows: А 780 nm: fk 10 mm; Z 100 mm; a micron; di 20 microns

The considered example of the implementation of the phase statistical transparency is intended for the formation of round or oval light beams. The use of scattering centers of another shape, for example, rectangular, makes it possible to form beams with a rectangular or square section, respectively.

Claims (2)

1. A system for the correction and collimation of a semiconductor laser beam containing a series of pre-lens a collimator, a system for correcting the beam section shape, and a final collimator, characterized in that, in order to ensure the correction of the wavefront of the collimated light beam while maintaining the correction of its section shape, the system for correcting the beam section shape is made in the form of a phase statistical transparency, the centers of which have an elongated shape and are oriented in such a way that their larger size is collinear to the p – n junction plane of the semiconductor laser, and the final collimator is made as a complex conjugate filter remote from the phase statistical transparency for a distance satisfying the Fraunhofer diffraction approximation. 2. The system according to claim 1. Characterized in that the dimensions of the scattering centers phase statistical transparency are chosen from the ratio A. + li eft fk dn fk where K is the radiation wavelength of a semiconductor laser; di and dit are the sizes of the scattering centers of the phase statistical transporant in the direction collinear and orthogonal to the p-n-path direction of the semiconductor laser, respectively; ats at is the size of the glow body of a semiconductor laser in the p – n junction plane and the plane orthogonal to it, respectively; fk is the focal length of the preliminary lens collimator. Schi 2 Figz