Semiconductor laser pump source

A laser source for pumping an optical device which requires for its operation a significant amount of light power, the source comprising a semiconductor array (11) for providing a plurality of spaced apart light beams at different wavelengths, an optical assembly (14, 15, 16) for focussing the light beams into an optical waveguide (19), the optical waveguide (19) being coupled to the optical device.

FIELD OF THE INVENTION 
This invention relates to a semiconductor laser source which can produce 
relatively high powers, say of the order of 100 mW, for coupling into 
devices which require such high powers for their operation, such as 
optical amplifiers. 
BACKGROUND ART 
Fibre optical amplifiers comprising Erbium doped single mode optical 
fibres, are seen as vital components for future optical communication 
systems require pumping with high optical powers. Coupling power of the 
order of 100 mW into a single mode optical fibre from a semiconductor 
laser is difficult to achieve. This power level is not readily achievable 
from reliable long lived single lasers operating in monomode, which 
commonly provide only about 10 mW. One solution which has been applied is 
multiplexing two orthogonally polarised lasers with a polarisation beam 
splitter. However this solution only couples the power of two lasers into 
the fibre. Powers of up to 100 mW are required to pump fibre amplifers. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a source of 
semiconductor laser power which can reliably provide relatively high power 
and which is capable of coupling the light into monomode optical fibre. 
The basis of the present invention is to provide several lasers at slightly 
different wavelengths and to couple the laser beam into the optical fibre 
by means of an optical focussing or multiplexing arrangement. Many 
individual lasers with wavelength multiplexers would be cumbersome and 
expensive, but a multi-wavelength source based on a laser array and an 
external dispersive cavity may provide an efficient pump for erbium 
fibres. 
In accordance with the invention, there is provided a laser source for 
pumping an optical device which requires for its operation a significant 
amount of light power, the source comprising a semiconductor means for 
providing a plurality of spaced apart light beams at different 
wavelengths, an optical assembly for focussing the light beams into an 
optical waveguide, the optical waveguide being coupled to said optical 
device. 
In a preferred form, a laser source is employed of the type described and 
claimed in our copending application GB-A-2,202,404, namely an apparatus 
for optical wavelength division multiplexing, apparatus of the type 
comprising: 
an optical assembly for collimating, dispersing, and focusing light; 
one laser, at least, effectively located at or near a focus of this 
assembly; and, 
an optical waveguide, located at or near a common and conjugate focus of 
this assembly and arranged thus to receive light emitted from said one 
laser, wherein, 
the optical waveguide is adapted by the provision of reflection enhancement 
means so as to reflect light emitted by said one laser and to control 
thereby the resonant emission thereof. 
Whilst as described in GB-A-2,202,404, the laser source is primarily 
intended as a form of frequency or wavelength multiplexing for increasing 
the information carrying capability of the source, in the present 
invention, the source is primarily used as a means of increasing the power 
which can be coupled into a monomode fibre arrangement. 
In certain circumstances it may be desired to couple light in accordance 
with the invention into multimode fibre having a wider core. However the 
present invention is particularly intended for use with monomode fibre 
having a narrow core (say 10 .mu.m). 
The optical device requiring pumping may be a fibre laser, fibre switch, 
optical amplifier, or sensor device. In the case of an optical amplifier 
the amplifier may be an integrated optical device or a doped optical 
fibre. As preferred Erbium doped fibre amplifiers are employed, requiring 
pumping at about 100 mW and used as preamplifiers, regenerators, power 
amplifiers. 
A further problem which arises with a laser source employing an external 
laser cavity is that the light from the semiconductor source has 
previously been reflected back to this source from a reflective region 
formed at the end of the optical waveguide (fibre). It is difficult to 
form a reliable and accurate reflective region in this way. 
To overcome this problem, in a second aspect the present invention provides 
a laser source comprising a semiconductor means for providing a plurality 
of spaced apart light beams at different wavelengths, an optical assembly 
for focussing the light beams into an end of an optical waveguide, said 
optical assembly including reflection means for forming a laser cavity 
external of said semiconductor means so that the light emitted by said 
semiconductors means is reflected to control resonant emission, the 
reflection means comprising a mirror means coupled to the optical 
waveguide by coupling means at a point remote from said end of the optical 
waveguide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is described with reference to FIGS. 1 to 3. The laser source 
is an array of lasers 11 which will preferably be fabricated on a single 
substrate. Each laser is defined by a stripe and is insulated from its 
neighbour by a suitable well. The front end of the laser chip has an 
antireflection coating 12 to prevent oscillation and the back end has a 
high reflection coating 13. The laser cavity is formed by the back facet 
of the array 11 and the reflector in the external cavity. A dispersive 
element, which is preferably a diffraction grating 14, is placed in the 
cavity such that the feedback from the reflector to each laser element is 
at a different but well defined wavelength. In such a configuration each 
element of the laser array 11 will oscillate at a different wavelength 
which is determined by the position of the reflector, the diffraction and 
position of the grating 14 and also the position and power of the lens 15. 
The reflector may be a mirror as shown in FIG. 1 or a partially reflecting 
splice as described in the original patent. Alternatively the reflector 
could be formed by a mirror 16 placed on the end of an arm of a fibre 
coupler 17, the output being taken from the coupler's other arm as shown 
in FIG. 2. A preferable solution which avoids the use of reflecting 
surfaces is to use a Sagnac loop mirror 18 as shown in FIG. 3. The loop 
mirror is formed by a fibre coupler with the two output arms spliced 
together. Depending on the coupling ratio a proportion of the laser output 
will be returned to the laser chip as feedback and a proportion will be 
passed to the output port. 
A specific version of this invention which is applicable for pumping fibre 
amplifiers and lasers is described as follows. The primary absorption 
bands of Erbium doped optical fibre which may be pumped by semiconductor 
lasers are at wavelengths of 670 nm, 807 nm, 980 nm and 1490 nm. The 
preferable pump bands are 980 nm and 1490 nm due to their absence of 
excited state absorption and large absorption cross sections. Both of 
these bands have significant pumpable absorption over linewidths of 20 nm. 
The multi-wavelength source described above can be specifically tailored to 
provide simultaneous pumping at many wavelengths in either band. In the 
case of a laser for pumping the 980 nm band the laser chip will preferably 
be a GaInAs/GaAs quantum well laser array. The array will have stripes 
which define the laser regions and at a spacing of 5 to 10 .mu.m. With an 
external cavity consisting of a 10 mm focal length lens and a 1200 
lines/mm diffraction grating a wavelength spacing of approximately 0.4 mm 
will be achieved between stripe channels. An array of 50 laser stripes 
will produce a source which will pump across the 20 nm absorption band. 
In the case of the 1490 laser the chip will preferably by a GaInAsP/InP 
laser array. The longer wavelength requires a diffraction grating of 900 
or 600 lines/mm. An array of 25 laser stripes with a wavelength spacing of 
0.8 nm will cover the absorption band from 1475 nm to 1495 nm. 
There are additional advantages of these sources for fibre amplifier 
pumping. The wavelength dependence of the gain between 1500 nm and 1550 nm 
is known to be dependent on the pump wavelength due to selective 
excitation of certain ion sites. Pumping with a range of wavelengths will 
ensure that all the erbium ions are excited and a uniform gain spectrum is 
produced. Secondly the control of the pump wavelength at 1490 nm is very 
important in order to prevent pump light interfering with the signal. The 
external cavity ensures that the pump wavelengths are kept in the pump 
band and do not drift. A greatly enhanced reliability may be expected from 
the wavelength multiplexed pump source in that the individual elements are 
operating at only relatively low power and failure of even half of the 
elements would still result in a usable pump source.