Laser Diode Collimation Optics

It is now well known that certain semi-conductor diodes produce electromagnetic laser radiation when an electrical current is conducted therethrough. A single diode laser does not provide a useful output, but an array of stacked diodes, which can be placed side by side with others, will produce laser intensity of high power. Since the emitted beams tend to diverge greatly and lose their intensity at a distance from the diodes various optical schemes proposed in the past collimate the divergent beam to one of negligable divergence. One such scheme is that shown in U.S. patent application Ser. No. 616,264 for "Semiconductor Laser Array" by J. T. Fulton and I. M. Kaplan which is assigned to the assignee of this application and is now abandoned. In that prior application a stack of laser diodes is located behind a lenticular convex cylindrical lens array, wherein each diode emitting plane is precisely located on the optical axial plane of the cylindrical lens, at the focal plane thereof, to collimate the laser beam in the plane perpendicular to the lenticular lens array, e.g. vertical plane. A single plane convex lens placed in front of the lenticular lens captures substantially all of the laser beams emanating therefrom and collimates the beams in the plane perpendicular to the plane of initial collimation, e.g. the horizontal plane.

The present invention is a modification of the prior invention wherein the width of laser beam is also compressed as well as collimated. The emitting plane of each diode is displaced from the optical plane of its corresponding convex cylindrical lens rather than being in the optical plane. The degree of displacement is a function of the distance of the individual cylindrical lens from the nominal optical axis of the system so that the diode images are focused as a single point on the central optical axis of the system. A negative (concave) cylindrical lens with its longitudinal axis parallel to the longitudinal axis of the convex cylindrical lenses is placed on the optical axis of the system to capture the emission of the diode before reaching the optical axis to collimate the output in the plane perpendicular to the longitudinal axis of the negative lens, e.g. vertically. The output is collimated in the orthogonal (e.g. horizontal) plane by the single plano-convex lens placed in front of the single plano-concave lens as in prior art. Also as in prior art the physical size of the system may be reduced by using telephoto principles where the focal length to the single plano-convex lens is reduced by inserting a negative cylindrical lens between the diodes and the convex lens.

The negative and positive cylindrical output lens may be combined into a single optical element if desired.

For a more complete understanding of the invention reference may be had to the accompanying diagrams in which,

FIG. 1 shows the element of one embodiment of invention, partly in section

FIG. 2 is a top view of the arrangement in FIG. 1, which also shows an alternative arrangement of lens 48 and lens 50, and

FIG. 3 is a detail of one portion of FIG. 1.

In FIG. 1 there is shown a plurality of linear diodes 12, such as gallium arsenide diodes, producing infra red laser radiation at their junction plane upon energization thereof. The diodes 12 are stacked one on another to establish a vertical linear diode array 16. The diode array 16 is positioned at the rear conjugate plane of a lenticular lens 18 comprising a plurality of short focal length cylindrical convex lens elements 20 which cause the radiation from diodes 16 to converge toward a common point 26 on the central optical axis 24 of the system at the forward conjugate of the lenticular lens 18.

FIG. 2, a plan view of FIG. 1, shows the extended source of the output of the diode 12 in the plane perpendicular to the plane of FIG. 1. As well known in prior art the extended source beam may be collimated without excessive divergence by the long focal length (f.sub.2) plano-convex cylindrical lens 46, whose focal plane is at the plane of the diodes 12. One alternative arrangement which may be used in place of lens 46 is the telephoto combination of a negative (concave-plano) cylindrical lens 48 and positive plano-convex cylindrical lens 50, which are shown dotted in FIG. 2. The telephoto combination provides a shorter length system of equivalent focal length for more compact packaging of the device.

FIG. 3 shows the geometry and preferred construction of a portion of the system shown in FIG. 1 in detail. The diode stack 16 includes the diodes 12, 12a, 12b etc. which are obtained from the supplier encased in housings 30, 30a, 30b etc. The radiating junction 14 of diode 12 is positioned at the rear conjugate plane 21 of lens 20, a distance f, behind the front of lens 20 and a distance c above the optical axis 22 of lens 20. Thus, the beam from lens 20 is directed at an angle .theta. with respect to axis 22 where tan .theta.=c/f.sub.1. The optical axis 22 is located at a distance D from the optical axis of the system so that the image of junction 14 is focussed on axis 24 at a point 26 (see FIG. 1) a distance X in front of lens 20, where X=(f.sub.1 /c)D. Similarly, junction 14a, 14b, 14c of diodes 12a, 12b, 12c, are displaced above the optical axes 22a, 22 b, 22c of lenses 20a, 20b, 20c, respectively by distances Ca, Cb, Cc such that the images of junctions 14a, 14b, 14c, are also focused at the same point 26 on axis 24. Accordingly, since f.sub.1 and X are fixed, the distances Ca, Cb, Cd, are determined by the distance Da, Db and Dc of axes 22a, 22b, 22c, above axis 24 as expressed by a transposition of the equation above i.e. C=(f.sub.1 /x)D C=Ca, Cb, Cd and D=Da, Db, Dc etc. It is seen that the spacings between the optical axes 22, 22a, 22b, etc. The spacers 32, 32a, 32b between the housings 30, 30a, 30b, 30c of diodes 12, 12a, 12b, 12c are each specially tailored to make sure that the junctions 14, 14a, 14b and 14c are properly positioned with respect to the optical axes 22, 22a etc.

Referring again to FIG. 1, the energy from diode bank 16, directed as described through lens 18, is captured by the negative (concave-plano) lens 44 which has a cylindrical axis parallel to the cylindrical axis of lens 18. The virtual focal point 26 of lens 44 is at point on axis 24. The laser energy from diode bank 16 is collimated by lens 44.

Thus the energy beam is compressed from the width A at diode stack 16 to width B at the cylindrical lens 44 and the intensity of the beam is increased accordingly. Typically A=1.5B representing a 67% increase in intensity.

It should be understood that a laser diode array of the type shown generates considerable heat and must be cooled to prevent disintegration. Appropriate cooling methods are discussed in the aforementioned U.S. patent application Ser. No. 616,264 but since they form no part of this invention are not discussed herein.

It should be further understood that many changes can be made to the preferred embodiment described without departing from the spirit of the invention. For example the spacing between junctions 14, 14a, 14b, etc., be uniform with lenses 20, 20a, 20b, etc., separated by non-uniform distances to establish the proper displacement of junction 14 from plane 22 to effect the focusing of the image at about point 26. Thus the invention should not be limited by the preferred embodiment described herein but is to be defined by the scope of the appended claims.