As overall power increases in fiber lasers and amplifiers, the amount of optical power which must be dealt
with in order to obtain high core to core and core to cladding isolation also increases. This unwanted light
can represent hundreds of watts and must be managed adequately. By combining a proper termination (end
cap) design and cladding stripping techniques it is possible to obtain a robust output beam delivery
component. The cladding stripping techniques are inspired by previous work done on high power cladding
strippers. All measurement presented here are done with a flat end cap. Both core to core and core to
cladding isolation will be better with an angled end cap. A core-to-core isolation of over 25dB was
measured, while core to cladding was over 30dB. Power handling was characterized by the capability of
the device to handle optical power loss, rather than transmitted power. The component dissipated over 50
watts of optical power due to isolation. The above results show that understanding the mechanisms of
optical loss for forward and backward propagating light in a end cap and the heat load that these losses
generate is the key to deliver kilowatts of optical power and protect the integrity of the system.
The ability to strip cladding light from double clad fiber (DCF) fibers is required for many different reasons, one example is to strip unwanted cladding light in fiber lasers and amplifiers. When removing residual pump light for example, this light is characterized by a large numerical aperture distribution and can reach power levels into the hundreds of watts. By locally changing the numerical aperture (N.A.) of the light to be stripped, it is possible to achieve
significant attenuation even for the low N.A. rays such as escaped core modes in the same device. In order to test the power-handling capability of this device, one hundred watts of pump and signal light is launched from a tapered fusedbundle (TFB) 6+1x1 combiner into a high power-cladding stripper. In this case, the fiber used in the cladding stripper and the output fiber of the TFB was a 20/400 0.06/0.46 N.A. double clad fiber. Attenuation of over 20dB in the cladding was measured without signal loss. By spreading out the heat load generated by the unwanted light that is stripped, the
package remained safely below the maximum operating temperature internally and externally. This is achieved by
uniformly stripping the energy along the length of the fiber within the stripper. Different adhesive and heat sinking
techniques are used to achieve this uniform removal of the light. This suggests that these cladding strippers can be used
to strip hundreds of watts of light in high power fiber lasers and amplifiers.
We present an all-fiber monolithically integrated fiber laser based on a custom tapered fused bundle pump combiner
with 32 inputs ports connected to a double clad gain fiber. The pump combiner is designed to provide high isolation
between signal and pumps fibers providing intrinsic pump protection. This configuration can generate more than 100W
of continuous wave (CW) laser light using single-chip multimode pumps enabling long term reliability.
In order to test power-handling at 1kW, a special splitter component had to be developed to make use of available
sources. A tapered fused-bundle (TFB) 1X7 splitter using a 1.00mm core diameter 0.22NA input fiber coupled to seven
400 micron core 0.22 NA output fibers was tested up to 860W at 976nm. Surface temperature rise was measured to be
less than 15°C with active heat sinking. The above results suggest that understanding the mechanisms of optical loss for
forward and backward propagating light in a TFB and the heat load that these losses generate is the key to producing
multi kW components, and demonstrates that reliable kW-level all fiber devices are possible.
Light absorption in structural adhesives constitutes the main source of heat in tapered fused bundle (TFB) devices.
Efficient heat dissipation solutions were developed by studying these thermal loads. The relative merits of transparent
vs. opaque package designs were established experimentally. In the former, light escapes without being absorbed by the
package walls, whereas in the latter, the spurious optical signal is directly absorbed and dissipated. The fact that heat is
generated directly in the adhesive largely favors the opaque package, which offers more efficient heat extraction. By
using a thermally conductive package, a temperature rise of 1.1°C per Watt of dissipated power was measured. These
numbers demonstrate that passive heat sinking at 20°C is sufficient to allow reliable operation up to 45Watts of
dissipated power (1kW with 0.2dB optical loss) without compromising long-term reliability.