Recently, a high-power laser diode having output power of about 1W and made of GaAlAs laser diode, has been put on sale. The optical characteristics of such a high-power laser diode will be explained below, with reference to FIG. 1. Referring to FIG. 1, a laser diode 1 is set in a package (not shown), and a laser beam is emitted from a window of the package. Now, a case where a laser diode has maximum output power of 1W, will be explained, by way of example. In this case, the width a of a light emitting area in the lengthwise direction of an emitting area 15 (hereinafter referred to as "parallel direction") is 200 .mu.m, and the height b of the light emitting area in the direction perpendicular to the lengthwise direction of the emitting area 15 (hereinafter referred to as "perpendicular direction") is 1 .mu.m. Further, when a beam divergence indicative of the size of a far field pattern is given by full width at half maximum (FWHM), a beam divergence .theta..sub.//, in the parallel direction is about 10.degree. and a beam divergence .theta..sub..perp. in the perpendicular direction is about 40.degree.. In other words, in the perpendicular direction, the height b of the emitting area 15 is 1 .mu.m, and thus the emitting area t5 can be almost regarded as a point source. While, in the parallel direction, the width a of the emitting area 15 is 200 .mu.m, and moreover the output beam isn't spatially coherent but it seems to be emitted like a filament. This fact will be a serious obstacle when the output beam of the high-power laser diode is intended to be focused on a small spot.
Two kinds of lens systems have hitherto been known which are used for focusing the output beam of a high-power laser diode. A first one of the conventional lens systems will first be explained, with reference to FIGS. 2A and 2B.
FIG. 2A is a sectional view of the first conventional lens system taken along the perpendicular direction. Referring to FIG. 2A, the emitting area 15 of a laser diode 1 is projected on a focal plane 7 by a collimating lens 20 and a focusing lens 22. FIG. 2B is a sectional view of the first conventional lens system taken along the parallel direction. Referring to FIG. 2B, a far field pattern 16 (FIG. 1) of the laser diode 1 is formed at the focal point 23 of the collimating lens 20, and the above image is projected on the focal plane 7 by a cylindrical convergent lens 21, having refracting power only in the parallel direction, and the focusing lens 22.
Next, a second one of the conventional lens systems will be explained, with reference to FIGS. 3A and 3B. FIG. 3A is a sectional view of the second conventional lens system taken along the perpendicular direction. Referring to FIG. 3A, as in the first conventional lens system, the emitting area 15 of the laser diode 1 is projected on the focal plane 7 by a collimating lens 25 and a focusing lens 27. FIG. 3B is a sectional view of the second conventional lens system taken along the parallel direction. Referring to FIG. 3B, although the emitting area 15 of the laser diode 1 is projected on the focal plane 7 by the collimating lens 25 and the focusing lens 27 as in the perpendicular direction, a beam expander made up of a pair of prisms 26 is disposed between the collimating lens 25 and the focusing lens 27, and thus the projection magnification in the parallel direction is lower then that in the perpendicular direction.
A drawback of the first conventional lens system will first be explained. In the parallel direction, a far field pattern 16 of the laser diode 1 is projected on the focal plane 7. Hence, the size of a beam spot 8 which is formed on the focal plane 7, in the parallel direction is determined by the size of the far field pattern 16 in the parallel direction, that is, by the beam divergence .theta..sub.// in the parallel direction. In general, .theta..sub.// increases as the output power of the laser diode 1 is larger. Accordingly, in a case where the input face of an optical fiber or a pinhole is placed on the focal plane 7, the transmittance of the optical fiber or the pinhole decreases as the output power of the laser diode 1 is made larger.
Incidentally, in the second conventional lens system, the emitting area 15 of the laser diode 1 is projected on the focal plane 7 in both of the parallel and perpendicular directions. Even when the output power of the laser diode 1 is increased, the size of the emitting area 15 at the output face 2 of the laser diode 1 will be kept substantially constant, and thus the size of the beam spot 8 will also be kept substantially constant.
Next, a drawback of the second conventional lens system will be explained. Let us consider a case where the input face of an optical fiber or a pinhole is placed on the focal plane 7, and an additional lens system is disposed behind the output face of the optical fiber or the pinhole. When the laser beam incident on the focal plane 7 has a small covergent angle, the laser beam emerging from the optical fiber or the pinhole has a small divergent angle. Accordingly, a lens system having a small numerical aperture can be used as the additional lens system disposed behind the optical fiber or the pinhole. That is, in many cases, it is advantageous that the laser beam incident on the focal plane 7 has a small convergent angle. In the second conventional lens system, however, the laser beam incident on the focal plane 7 has a large convergent angle in the parallel direction for the following reason.
FIG. 4 is a sectional view of the laser diode 1 taken along the parallel direction. Referring to FIG. 4, contours BC and BG of laser beam emitted from a point B which is deviated from an optical axis AF, are parallel to contours AD and AH of laser beam emitted from a point A on the optical axis AF, respectively, and principal ray BE of the laser beam emitted from the point B is parallel to the optical axis AF. The divergent angle of that one of laser beam incident on the focal plane 7 which is emitted from the point A on the optical axis AF, is determined only by the divergence angle .angle.DAH of the laser beam and the projection magnification of the lens system. Although it is impossible to make the convergent angle of the total laser beam incident on the focal plane 7 smaller than that of the laser beam which is emitted from the point A and is incident on the focal plane 7, the convergent angle of the former laser beam can be reduced to that of the latter laser beam by making the principal ray BE parallel to the optical axis in the focal plane 7, that is, by forming a telescopic optical system of the lens system in the parallel direction. Conversely speaking, when the telescopic optical system is not formed of the lens system, the convergent angle of the laser beam incident on the focal plane 7 becomes large unnecessarily. In order to form a telescopic optical system of the second conventional lens system in the parallel direction shown in FIG. 3B, it is required to dispose the collimating lens 25 and the focusing lens 27 so that these focuses agree with each other. However, each of collimating lens 25 and focusing lens 27 usually has a short focal distance, and moreover the beam expander 26 is disposed between the lenses 25 and 27. Accordingly, it is very difficult to form a telescopic optical system of the lens system shown in FIG. 3B, and thus the laser beam incident on the focal plane 7 is obliged to have a large convergent angle. Although the second conventional lens system in the parallel direction has been explained in the above, the light emitting area in the perpendicular direction can be regarded as a point source. Accordingly, in the perpendicular direction, it is unnecessary to consider the above-mentioned principal ray.