Semiconductor laser module

The invention provides a semiconductor laser module that enhances the coupling efficiency of the semiconductor laser with an optical fiber, and reduces the dispersion of the coupling efficiency due to a fitting error of the semiconductor laser. The structure of the semiconductor laser module according to the invention is such that the outgoing point of beams from the semiconductor laser is dislocated by a distance r.sub.1 from the optical axis of the optical fiber, so that the laser beams fallen on the end face of the core with a refractive index n.sub.1 can penetrate in the direction parallel to the optical axis of the core. Here, the distance r.sub.1 meets the following inequality: EQU 0.6 f sin .theta..sub.2 <r.sub.1 <f sin .theta..sub.2, .theta..sub.2 is an angle made by an optical axis of the outgoing beams from the coupling lens with a focal length f and an optical axis of the optical fiber, and .theta..sub.2 meets, according to the Snell's law, EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi..

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor laser module to guide 
laser beams emitted from a semiconductor laser to an end face of an 
optical fiber core, particularly to a coupling structure of a 
semiconductor laser and an optical fiber in the semiconductor laser module 
in which the end face of the optical fiber core is cut slant in order to 
prevent the laser beams from returning to the laser. 
2. Description of the Related Art 
Generally, this type of a semiconductor laser module is provided with a 
construction as shown in FIG. 1, in which a semiconductor laser 1 emits 
laser beams, and a coupling lens 2 converges the emitted laser beams so as 
to fall on an end face of a core 4 of an optical fiber 3. Further, the end 
face of the core 4 is cut slant so as to prevent the laser beams reflected 
on the end face from returning to the semiconductor laser 1. 
This type of coupling structure is disclosed, for example, in the Japanese 
Patent Publication No. Hei 4-66324. According to this, the structure is 
designed such that the laser beams fallen on the end face 5 of the core 4 
cut obliquely by an angle .phi. from a surface perpendicular to the 
optical axis of the optical fiber 3 penetrate in the direction parallel to 
the optical axis of the core 4. To achieve the foregoing, an angle 
.theta..sub.2 made by an optical axis 6 of the outgoing beams from the 
coupling lens 2 and an optical axis 7 of the optical fiber 3 needs to 
satisfy the following equation, based on the Snell's law, 
EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi. (1) 
here, n.sub.1 : refractive index of the core 4. 
The coupling structure of the foregoing disclosure is made such that the 
outgoing point of beams from the semiconductor laser 1 is dislocated by a 
distance r.sub.1 from the optical axis of the coupling lens 2. Here, if 
the focal length of the coupling lens 2 is f, according to the equation 
(1), the distance r.sub.1 is given as follows. 
EQU r.sub.1 =f sin .theta..sub.2 (2) 
However, generally deviating the position of the optical axis of a light 
source from the optical axis of a lens will increase an aberration 
virtually in proportion to the deviation r.sub.1, as shown in FIG. 2. The 
foregoing conventional example only considers the Snell's law shown in the 
equation (1) and (2), and does not consider this aberration; and 
therefore, the example is worse in the coupling efficiency, which is a 
problem. Further, the coupling efficiency becomes asymmetric, depending on 
a fitting error deviated upward and/or downward from the position of the 
semiconductor laser 1 being dislocated by the distance r.sub.1 from the 
optical axis. Therefore, the dispersion of the coupling efficiency becomes 
large, which is a problem. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the foregoing problems, and 
an object of the invention is to provide a semiconductor laser module in 
which the coupling efficiency of the semiconductor laser with the optical 
fiber can be enhanced and the dispersion of the coupling efficiency due to 
the fitting error of the semiconductor laser can be reduced. 
In order to accomplish the foregoing object, considering that the deviation 
of the optical axis of a semiconductor laser from the optical axis of a 
lens increases an aberration in correspondence with the deviation to 
thereby deteriorate the coupling efficiency, the invention designs the 
semiconductor laser module wherein the semiconductor laser is located in a 
distance shorter than the deviation obtained by the Snell's law. 
Concretely, provided that an angle .theta..sub.2 made by an optical axis 
of the outgoing beams from the coupling lens and an optical axis of the 
optical fiber is represented by 
EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi., 
the outgoing point of beams from the semiconductor laser is dislocated by a 
distance r.sub.1 from the optical axis of the optical fiber, wherein the 
distance r.sub.1 meets the following inequality: 
EQU 0.6 f sin .theta..sub.2 &lt;r.sub.1 &lt;f sin .theta..sub.2. 
The structure being thus designed, the coupling efficiency of the 
semiconductor laser with the optical fiber can be enhanced, and also the 
symmetry of the coupling efficiency can be bettered, even though there is 
an upward and/or downward fitting error of the semiconductor laser, which 
reduces the dispersion of the coupling efficiency due to a fitting error 
of the semiconductor laser.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the semiconductor laser module of the invention, when guiding the laser 
beams emitted from the semiconductor laser into an optical fiber in which 
the end face of the core with a refractive index n.sub.1 is cut obliquely 
by an angle .phi. through a coupling lens with a focal length f, the 
outgoing point of beams from the semiconductor laser is dislocated by a 
distance r.sub.1 from the optical axis of the optical fiber, wherein the 
distance r.sub.1 meets the following inequality: 
EQU 0.6 f sin .theta..sub.2 &lt;r.sub.1 &lt;f sin .theta..sub.2, 
so that the incident laser beams on the end face of the core can penetrate 
in the direction parallel to the optical axis of the core. Here, 
.theta..sub.2 is an angle made by an optical axis of the outgoing beams 
from the coupling lens and an optical axis of the optical fiber, and 
.theta..sub.2 meets the following equation according to the Snell's law: 
EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi.. 
The embodiment of the invention will be described with reference to the 
accompanying drawings. 
FIG. 1 illustrates a construction of a semiconductor laser module, FIG. 2 
is a chart to explain the relation between the deviation of the optical 
axis of the semiconductor laser and the aberration, and FIG. 3 is a chart 
to explain the deterioration of the coupling efficiency in the 
semiconductor laser module and the conventional example. 
As shown in FIG. 1, the end face 5 of the core 4 of the optical fiber 3 is 
cut slant by the angle .phi. from a surface perpendicular to the optical 
axis 7 of the optical fiber 3. And, the semiconductor laser 1 is located 
to deviate by the distance r.sub.1 from the optical axis 7 of the optical 
fiber 3 (and the coupling lens 2). The laser (LD) beams emitted from the 
foregoing laser 1 are converged by the coupling lens 2 with the focal 
length f, and fall on the end face 5 of the core 4 of the optical fiber 3. 
FIG. 2 illustrates the aberration (.lambda./RMS) in correspondence with the 
distance r.sub.1 (.mu.m) , wherein the semiconductor laser 1 emits LD 
beams whose full width at half maximum of the divergent angle is 
30.times.30, the wavelength .lambda.=1310 (mm) to fall on the lens 2 whose 
magnification is 5, the numerical aperture (NA) of incidence is 0.55, and 
the focal length f is 1.37 mm. From FIG. 2, the aberration increases 
virtually proportionally as the distance R.sub.1 increases. Here, in the 
foregoing equations (1) and (2), substituting .phi.=8, n.sub.1 =1.465 
gives r.sub.1 =90 .mu.m. Accordingly, the aberration in this case becomes 
about 0.07 (.lambda./RMS). 
The curve a shown in FIG. 3 illustrates the efficiency deterioration 
against the distance r.sub.1 in the conventional example. If this is 
correct, the efficiency deterioration becomes 0 (dB) at r.sub.1 =90 .mu.m, 
and the efficiency deterioration becomes symmetric as the semiconductor 
laser 1 deviates upward and downward from this position. However, 
practically the aberration increases as the distance r.sub.1 increases, as 
shown by the curve b. Therefore, if the efficiency deterioration by this 
aberration is added on the curve a, the optimum value of the distance 
r.sub.1 becomes smaller than 90 .mu.m; when r.sub.1 is nearly equal to 72 
.mu.m, the efficiency becomes maximum. The curve c in FIG. 3 illustrates 
the measured value of the efficiency, and the curve a +b almost coincides 
with the measured value (curve c). 
Therefore, as clearly indicated in FIG. 3, the structure wherein the laser 
beams fall with the maximum coupling efficiency on the end face 5 of the 
core 4 cut slant by the angle .phi. from a surface perpendicular to the 
optical axis of the optical fiber 3 is, provided that 
EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi. (1), 
a structure wherein the outgoing point of beams from the semiconductor 
laser 1 is located at the distance r.sub.1 from the optical axis of the 
optical fiber 3, wherein the distance r.sub.1 meets the following. 
EQU 0.6 f sin .theta..sub.2 &lt;r.sub.1 &lt;f sin .theta..sub.2 (3) 
As shown in FIG. 3, according to the conventional example, when the 
position of the semiconductor laser 1 moves nearer than that of r.sub.1 
=90 .mu.m, the coupling efficiency becomes better due to the aberration; 
when the position moves farther than that of r.sub.1 =90 .mu.m, the 
coupling efficiency becomes asymmetric and worse extremely. On the other 
hand, according to the embodiment, since the position of the semiconductor 
laser 1 deviates in the .+-. direction from the position at the distance 
r.sub.1 .apprch.72 .mu.m, the coupling efficiency deteriorates 
symmetrically; and therefore, the dispersion due to a fitting error of the 
semiconductor laser 1 can be reduced to a minimum. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.