The Aberration Correction of a Diode Laser

Xueqing Liu
SUNY at Stony Brook
Optics Rotation Project 2, Spring 2001

Advisor: Prof. Luis A. Orozco


1. Introduction

Laser diodes have many advantages: they are small and can be directly modulated, and the power requirements are the modest. However, the output directly from the diode is asymmetric. The beam divergence is different in the plane parallel and perpendicular to the emitting junction. The beam needs aberration correction for spherical aberration, coma, astigmatism. The object of this project is to make the output beam from the laser diode LTO24 (wavelength=780nm, working voltage=4.0V) as round as possible by the use of some optical objects.

2. Experimental Set-up

3. Measurements and Corrections

3.1 Circularizing the elliptical beam

Firstly, I made measurements of the beam profiles of the collimated light coming out of the laser diode directly in both the horizontal and vertical axis.

   

The sizes of the spot are asymmetric(0.06 inch for x, 0.175 inch for y). In order to circularize the beam, an anamorphic prism-pair is needed. Anamorphic prisms allow for the magnification of the beam size along one axis while leaving the beam unchanged along the other axis.

The anamorphic magnification is chosen to 3.0. The prism angles are 30.4 degs and 0.1 deg respectively. The prisms are put about 9 inches from the laser. Then I measured the beam profile right after the prism pairs.

   
The sizes in both x and y directions are about the same (0.2 inch) now.

3.2 Checking for astigmatism.

Laser diodes usually suffer from astigmatism, which results in an inability to collimate both the perpendicular and parallel directions simultaneously. In order to check the astigmatism, a lens is put after the prism pairs.

   

We see from the graph that the divergences in x and y direction are different which means the astigmatism exists(x:20 degs, y:12.4 degs). In order to correct for it, the simplest way is to use a tilted lens. The lens is thick lens. The effective focal length is associated with d (distance between the vertices of the two spherical refracting surfaces of the thick lens). When the lens is rotated along its optical axis, the focal length changes in one dimension (d changes) while remains the same in the other dimension (d is the same). By tilting the lens to 32(deg) relative to the vertical direction, the divergences become close (x,y: about 22 degs).

Another way to correct the astigmatism is to use cylindrical lenses. Cylindrical lens can image in one dimension, not two. In order to get symmetric beams, we could choose two such lenses with appropriate focal length. Collimate the beam first in one dimension with the first cylindrical lens, and then collimate the orthogonal dimension with a second cylindrical lens. The ratio of the two focal lengths should be approximately equal to the ratio of the two beam divergences. I haven't checked this way yet. However, after putting several optical objects for aberration correction, the intensity of the laser beam concentrates more in the y direction than x axis, esp. in the far field. There is still more to be corrected in order to get a circularized laser beam.

Acknowledgements

I am very appreciated of Prof. Orozco for his helpful instruction. Thanks to Wade Smith for his kind help in this project.

References:

1. Eugene Hecht, 'Optics', Addison-Wesley Publishing Company, 1990, Second edition.
2. http://www.mellesgriot.com

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