Laser device, notably for optical pumping, and method of fabricating it

In a laser device, notably for optical pumping, the wafer includes an expansion segment adjacent the exit to prevent degradation of the exit facet of the semiconductor wafer that constitutes the amplifying part of the laser. The other facet is reflective and a Bragg grating is formed in a coupling fiber disposed near the exit facet. Applications include optical pumping of erbium-doped fiber amplifiers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention is in the field of laser devices in which optical 
amplification is effected by a semiconductor wafer. It is more 
particularly concerned with semiconductor lasers having a high optical 
output power. This applies in particular to the lasers used for optical 
pumping of erbium-doped fiber amplifiers. 
2. Description of the Prior Art 
This application requires lasers producing a high optical power output at a 
wavelength of 0.98 .mu.m. Semiconductor lasers fabricated on gallium 
arsenide substrates that are capable of producing this wavelength are 
usually employed. These lasers essentially comprise an optical amplifier 
formed on the substrate, two opposite ends of which are cleaved to form a 
resonant cavity. The laser wave is produced in an active layer of the 
amplifying part. This layer has a generally rectangular cross-section 2 
.mu.m to 4 .mu.m wide and 0.1 .mu.m to 0.2 .mu.m thick. As a result the 
light wave passing through the exit facet of the laser has a high energy 
density. Also, deterioration or even destruction of this facet occurs 
above a particular power level. 
One solution to this problem that has previously been used is to cleave the 
facets in a very hard vacuum and to treat the exit facet by deposition of 
oxide, also in a very hard vacuum. The oxide layer passivates the facet 
which makes it more resistant to high energy densities. This technique is 
complex, however, since it requires expensive equipment (to produce the 
very hard vacuum) and is not suited to mass production. 
The invention proposes another approach which is directed to reducing the 
energy density passing through the facet rather than making the latter 
more resistant. 
Consideration might be given to modifying the conventional structure of a 
semiconductor laser by providing at one end an expansion segment allowing 
the light from the active layer to be diffracted before encountering the 
exit facet. This solution is unsatisfactory, however, since it introduces 
intracavity losses that reduce the effective reflection coefficient of the 
exit facet and reduce the laser yield. Also, to maintain the operation of 
the laser, the length of the expansion segment must be limited, which 
rules out significant reduction of the energy density. 
SUMMARY OF THE INVENTION 
With the aim of solving this problem, the invention consists in a laser 
device comprising: 
an amplifier segment comprising an active layer formed in a semiconductor 
wafer delimited by first and second facets, and 
first and second mirrors disposed in such manner as to form a resonant 
cavity, said first mirror being external to said wafer, 
wherein a coupling fiber has one end disposed near said first facet, said 
first mirror is formed in said coupling fiber and said wafer includes an 
expansion segment adjacent said first facet to enable transverse expansion 
of light emitted by said active layer before it passes through said first 
facet. 
The fact that the first mirror is formed in the coupling fiber enables 
anti-reflection treatment of the exit facet, which eliminates the 
intracavity reflections and the mode jumps that can result from them. 
The above structure can be used for any type of substrate but is of 
particular benefit in the case of gallium arsenide which has a particular 
sensitivity to high energy densities. In this case, the length of the 
expansion segment is advantageously between 10 .mu.m and 50 .mu.m to 
enable significant expansion of the light wave (by a factor in the order 
of 3). 
The efficiency of the laser device of the invention may be considerably 
improved by paying particular attention to the coupling between the fiber 
and the exit facet. To this end, in accordance with one particular aspect 
of the invention, the end of the fiber is terminated in the form of a 
diopter dimensioned for maximum optimum coupling with the facet. 
In accordance with another aspect, the fiber is a photosensitive fiber and 
the mirror is a Bragg grating written in the fiber by optical means. This 
solution has the advantage that it enables a first order grating to be 
produced in all cases. A grating of this kind cannot always be obtained in 
an integrated fashion, especially in the case of gallium arsenide. 
The length of the grating written by optical means is advantageously 
between 1 mm and 5 mm. In one preferred embodiment the semiconductor wafer 
and the end of the fiber are contained in a package through one wall of 
which the fiber passes and by which wall the fiber is held. This 
"packaged" implementation protects the fiber part containing the grating 
from any external mechanical stresses that would otherwise modify the 
specifications of the grating. 
Turning to the fabrication of the device and in particular of the expansion 
segment, one solution could be to etch one end of the wafer after the 
formation of these constituent layers, and then to grow substrate over 
this area. This solution, which requires further epitaxial growth, may 
cause problems because the surface state of the etched planes may degrade 
the optical properties of the device. This problem is particularly 
difficult to solve in the case of a gallium arsenide substrate. 
To solve this problem, there is also proposed a method of fabricating a 
semiconductor wafer to form the expansion segment. The invention therefore 
also consists in a fabrication method including: 
a first step of growing a n-doped alloy layer on a semiconductor substrate, 
a second step of growing an active layer on said n-doped layer, 
a third step of growing a p-doped alloy layer on said active layer, and 
cleaving operations to form first and second facets delimiting said wafer, 
wherein said first growth step is preceded by a step of etching said 
substrate over the whole of the surface of said substrate except for an 
area near one of said facets so as to form in said substrate an expansion 
segment enabling transverse expansion of light emitted by said active 
layer. 
As a consequence of using the above method the active layer extends the 
whole length of the wafer, and is all in one plane except for its part in 
the expansion segment. 
Other aspects and advantages of the invention will emerge from the 
remainder of the description given with reference to the figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The laser device is shown diagrammatically in FIG. 1. It comprises a 
semiconductor wafer P and a coupling fiber FC of which only the end 
portion E is shown. The wafer P includes an amplification segment SA 
containing an active layer CA through which an electrical current can be 
passed by means of a top electrode EH and a bottom electrode EB. The wafer 
is delimited in the longitudinal direction by two cleaved facets F1, F2. 
The facet F2 is reflective and may receive a dielectric treatment to 
increase its reflectivity and to force laser emission via the front facet. 
The facet F1 has an anti-reflection coating. One of the guide ends of the 
active layer CA is separated from the transparent facet F1 by an expansion 
segment SE formed in one end of the wafer P. The tip of the fiber FC is 
located near the facet F1, at a distance enabling optimal optical coupling 
with the wafer. The end portion E includes a Bragg grating BR. If the 
fiber is photosensitive, the grating BR can be written by optical means 
using a known method. The pitch of the grating BR is chosen for first 
order agreement with the wavelength chosen for the device. Thus, in the 
case of a gallium arsenide substrate, the wavelength is 0.98 .mu.m, which 
for a fiber having an effective index around 1.5 requires a pitch of 0.35 
.mu.m. 
The end portion E of the fiber advantageously terminates in a diopter 
having a shape chosen to optimize coupling. In this way a coupling 
coefficient in the order of 0.5 can be obtained. The reflection 
coefficient of the Bragg grating BR can then be in the order of 0.10, the 
reflection coefficient of the facet F1 being negligible. 
FIG. 2 shows in more detail the structure of the water 1 in the vicinity of 
the expansion segment SE when the method of the invention is used. 
In this example, the wafer is fabricated from the gallium arsenide 
substrate 1 which is etched over virtually all of its surface except for 
the area that is to become the expansion segment SE. n-doped AlGaAs 
quaternary alloy is then grown on the substrate. The InGaAs or InGaAsP 
alloy active layer CA is then formed. The p-doped AlGaAs alloy layer 4 is 
finally formed. 
The localized etching of the substrate 1 is such that the active layer CA 
is highly curved (2) in the vertical plane, which eliminates its optical 
guide property starting from this curvature. 
The expansion segment SE is then equivalent to a diffraction area enabling 
spreading of the wave from the active layer. This solution has the 
advantage of avoiding the surface state problem arising from etching the 
AlGaAs layers. 
FIG. 3 shows a packaged implementation of the device of the invention. The 
wafer P is fixed to the interior of a casing B. The fiber FC passes 
through the wall 5 of the casing so that the end portion E is inside the 
casing and located in front of the facet F1 at the height of the active 
layer CA. As shown diagrammatically, the wall 5 is adapted to hold the end 
of the fiber accurately relative to the casing. 
Of course, the figures that have just been described are merely 
diagrammatic representations which, for reasons of clarity, do not 
represent the actual proportions of a real device. For a GaAs laser device 
tuned to a wavelength of 0.98 .mu.m, the orders of magnitude of the 
dimensions may be as follows: 
Length of active layer: 600 .mu.m-1 mm 
Width of active layer: 2 .mu.m-4 .mu.m 
Thickness of active layer: 0.1 .mu.m-0.2 .mu.m 
Length d of expansion segment: 10 .mu.m-50 .mu.m 
Facet F1 to fiber end distance: 5 .mu.m-30 .mu.m 
Length of Bragg grating BR: 1 mm-5 mm 
Effective index of fiber: 1.5 approx. 
Pitch of Bragg grating: 0.35 .mu.m approx.