Hybrid laser

A scalable laser system system providing a single, phase-locked output which comprises a flexible bundle of single mode optical fibers with the optically polished end faces at one end being index matched via matching material to a single, partially transmitting, output mirror. The opposite end of the bundle is divided into a series of smaller bundles whose end faces are index matched to the ends of a series of solid state laser media. These are optically pumped by remotely sited laser diode arrays via additional bundles and mirrors, which are 100% reflective at the laser wave-length.

FIELD OF THE INVENTION 
This invention relates to a system for effectively combining the output 
beams of a group of rod or slab lasers into a single laser beam of 
comparable quality and whose power is the sum of the output powers of the 
individual rod and slab lasers. The invention has application in the 
industrial and defence fields where powerful, scaleable laser beams are 
deployed. 
SUMMARY OF THE PRIOR ART 
Prior art rod lasers have suffered from scaling problems which severely 
limit and eventually prohibit their operation at high power levels. 
Firstly, the fact that the surface of a rod laser system can be more 
effectively cooled than its central region leads to the well known 
"thermal lensing" effect which in turn leads to severe beam distortion. 
Secondly, as the peak power of the laser pulse propogating through a rod 
laser medium increases, it eventually reaches the "self-focusing" 
threshold within the laser medium which results in the self collapse of 
said pulse into a fine filament, destroying the said rod laser medium in 
the process. The only way to avoid laser rod damage due to self-focusing 
is to limit the length of said rod to less than its "self-focusing" length 
at a given power level. Thirdly, many excellent laser media cannot be 
produced in bulk and one is, therefore, limited to small rods of such 
laser media in any case. Where the laser medium can be produced in large 
quantities, such as is the case with neodymiun doped glass, 
self-oscillation problems arise where the parasitic lasing actions deplete 
the stored energy within the excited laser medium before the arrival of 
the laser pulse to be amplified within said excited medium. In general the 
same problems apply to slabs of laser media as apply to rods, although, 
several specific laser slab configurations can avoid laser rod induced 
defects. 
It follows that both prior art rod and slab lasers were very limited as far 
as the generation of laser beam powers were concerned. The fundamental 
defect of both prior art rod and slab lasers could be overcome to some 
extent in applications where the number of laser beams used was not a 
limitation. For example, in laser fusion studies it is possible to use a 
large number of laser beams to irradiate laser fusion pellets. This 
situation has led to the development of large, multi-beam laser systems 
worldwide in an effort to overcome the power restrictions of single beam 
laser systems. However, simply duplicating the individual arms of rod and 
slab lasers to produce more laser power is not a cost effective approach 
to scaling laser systems to high power levels. For example, a single arm 
of a large laser fusion system may cost $10 m. If one needs 100 such arms 
the cost suddenly shoots up to $1B and this prior art approach to scaling 
rod and slab lasers becomes prohibitively expensive. At the other end of 
the scale, excellent quality neodymiun doped yttrium aluminium garnet 
crystals can be grown and selected, but only crystals of very small 
volume. Obviously it is beneficial to combine the outputs of a series of 
such high quality crystalline lasers into a single beam format. However, 
prior art beam combining techniques cannot produce a single beam of 
similar quality whose total power is the sum of the power out of the 
individual crystal lasers. 
The present invention overcomes the defects of prior art beam combining 
systems in that it automatically provides a single, scaleable, laser beam 
of similar, if not superior quality to that of the individual laser beams 
which it replaces. The invention achieves its superiority over prior art 
systems by coupling individual rod or slab laser media into a common 
scaleable output aperture using bundles of passive single mode optical 
fibres. The invention represents a parallel array of laser oscillators 
which have one common output aperture, the laser medium being represented 
by laser rods or slabs with the remainder of the resonant cavity between 
the laser mirrors being filled with coherently packed, passive bundles of 
single mode optical fibre whose cladding thickness should be comparable to 
or less than the fibre core diameter. However, the invention will still 
operate even if the fibre cladding thickness is much greater than the 
fibre core diameter but the laser medium will not be optimally utilized 
under these conditions. 
BACKGROUND OF THE INVENTION 
Soon after the advent of the laser in 1960, one of the inventors (John 
Leonard Hughes) proposed its use in particle physics (Hughes, Nature 
1963). However, the peak powers required to achieve such a goal in 
particle physics are very large (&gt;10.sup.15 watts) and an intensive effort 
has been underway since that time to adapt both solid state slab and fibre 
bundle based lasers for the generation of powerful laser beams (see Hughes 
U.S. Pat. Nos. 4,039,962 (1977) and 4,132,955 (1979)) and Hughes Laser 
Radar Patents (classified U.S., UK and Australia). 
In this invention we have combined the advantages of rod and slab lasers 
with those of passive, phased-locked fibre bundles. The first of these 
hybrid lasers was operated by one of us (JLH) in 1986 in its simplest 
format when one million laser oscillators were fired simultaneously. The 
fibre technology of the invention would be significantly enhanced by the 
use of multicolored optical fibres which effectively reduce the cladding 
thickness relative to the diameter of the fibre cores leading to a higher 
degree of utilization of the rod and slab laser medium of the invention as 
well as a much more compact fibre bundle for a given output power. 
SUMMARY OF THE INVENTION 
It is an object of the invention to combine the output beams of a plurality 
of rod lasers into a single laser beam of comparable quality. 
It is also an object of the invention to combine the output beams of a 
plurality of slab lasers into a single laser beam of comparable quality. 
It is an object of the invention to provide a flexible output end for a 
scaleable solid state laser. 
Another object of the invention is to distribute the electrical and thermal 
loadings in a solid state laser so that they can be effectively managed as 
the laser input power is increased. 
Yet another object of the invention is to allow very high quality rod and 
slab sections of laser media to be used to produce a single powerful laser 
beam which would not otherwise be possible. 
It is also an object of the invention to extract more laser beam energy 
from a given volume of laser rod and slab media at high quality laser beam 
output than would otherwise be possible with prior art rod and slab laser 
media which would operate under multimode conditions at the equivalent 
power levels. 
Yet another object of the invention is to enhance the phase-locking of a 
plurality of laser media by using bundles of optical fibre to optically 
couple said media to a level where mutual phase-locking effect will occur. 
It is an object of the invention to provide bundles of equal lengths of 
fibres whose end faces are polished to a twentieth of the laser wavelength 
or better. 
The invention combines the advantages of rod and slab lasers with the 
scaling properties of fibre optical bundles which also provide a high 
degree of flexible laser oscillator structure which allows for the 
mounting of said invention within industrial workstations without the need 
for multiple mirror articulated arms to guide the laser beam to, for 
example, a workpiece. The optical path lengths within the laser oscillator 
can be equalized via various fibre paths by using index matching media at 
the interfaces between the rod end faces and the fibre bundle end faces as 
well as the interface between the fibre bundle and the output mirror.

DETAILED DESCRIPTION 
In FIG. 1, numeral 1 indicates the laser medium which may be in a rod or a 
slab format. 
Numeral 2 indicates a bundle of single mode optical fibres whilst numerals 
3 and 4 respectively indicate the laser resonator mirrors with mirror 3 
being totally reflecting at the laser wavelength whilst mirror 4 is 
partially transmitting. Numeral 5 indicates an index matching medium 
between the laser medium 1 and the fibre bundle ends 5. 
In FIG. 1, numeral 7 indicates a bundle of optical fibres used to convey 
optical power from the remotely sited diode laser array indicated by 
numeral 8 to excite laser medium 1. 
Numeral 9 indicates the phase-locked output laser beam of the invention. 
In FIG. 2, numeral 10 indicates a single mode fibre bundle which consists 
of one optical polished end composed of all the fibre ends and a series of 
other opposite end faces each composed of fewer fibre ends. A plurality of 
laser oscillators is now formed by matching the multiple ends of said 
fibre bundle to a series of laser media. Numeral 11 indicates a single 
output mirror for the plurality of laser oscillators whilst numeral 12 
indicates the index matching medium. 
Numeral 13 indicates the single phase-locked output coupled from the 
plurality of laser media 1. 
In FIG. 3, numeral 14 indicates the smallest face of a slab laser medium, 
numeral 15 indicates the next largest face of said slab whilst numeral 16 
indicates the largest face of the slab. 
Numeral 17 indicates the direction of propogation of a laser beam through 
the small end face 14 whilst numeral 18 indicates a laser beam of circular 
cross-section that can pass through the slab along direction 17. Numeral 
19 indicates a laser beam of elliptical cross-section that can propogate 
through the slab along direction 15. Numeral 20 indicates the largest 
elliptical cross-section laser beam that can propogate through the slab 
along direction 16. Any of these three slabs laser structures can form the 
active part of the single element of the invention. 
In FIG. 4, numeral 21 indicates additional fibre bundles that can be used 
to enhance the phase-locking of a plurality of laser media 1 of the 
invention, said single mode fibre bundles 21 being interfaced with fibre 
bundles 10 as indicated by numeral 22. 
In FIG. 5, numeral 23 indicates a slab laser media used in the element of 
the invention whilst numeral 24 indicates fibre bundles of rectangular 
cross-section combined into a single aperture of circular cross-section 
indicated by numeral 25 with an output mirror of circular cross-section 
indicated by numeral 26. Numeral 27 indicates 100% reflecting mirror at 
the laser wavelength which is of rectangular cross-section. Numeral 8 
indicates the lead which connects diode arrays 9 to a power supply not 
shown. 
In FIG. 6, numeral 30 indicates a single mode core of a multicolored fibre 
of circular cross-section indicated by numeral 31. 
FIG. 7 shows a multicore optical fibre of rectangular cross-section with 
numeral 32 indicating the single mode core and numeral 33 indicating their 
common cladding. 
The invention has application in the industrial, medical and defence fields 
where a scaleable laser beam output is required. The invention has a 
flexible output section represented by the fibre bundle which for example, 
allows its use of robotic arms without the need for beam deflecting 
mirrors.