Pulseshaping is important in high energy pulsed fiber MOPA system to mitigate non-linear effects and optimize the
processing of different materials. However, pulseshaping is greatly limited by the spectral features of the semiconductor
seed source commonly used as the master oscillator. Through the appropriate design of an external fiber Bragg grating
(FBG) and adequate current modulation, the spectrum of the fiber-coupled seed laser was broadened to suppress
stimulated Brillouin scattering occurring in the amplifier chain and the central emission wavelength and bandwidth were
controlled. Pulseshaping is also quickly limited by the saturation energy and doping level of standard aluminosilicate
ytterbium doped fibers used in the power amplifier even with large core diameter. Co-doping the fiber with phosphorus
greatly increases the saturation energy of the system, which gives smoother pulseshape and significantly lower
stimulated Raman scattering (SRS). It is shown that going from 1060 nm to longer emission wavelength at 1090 nm with
this fiber increases further the pulseshaping capabilities and reduces SRS. The phosphorus codoping also allows higher
ytterbium doping level without photo-degradation, which decreases nonlinear effects generation during the amplification
while giving more flexible pump wavelength choice and efficiency.
The thermal degradation of double clad optical fiber coatings is known to be the prime limiting factor for the operation
of high power CW fiber lasers. In this paper, we conduct a study of thermal effects in high power CW fiber lasers. A
particular focus is put on heating at the splice points and in the doped fiber due to the quantum defect in 100-W class
CW fiber lasers. A theoretical model and experimental measurements taken with a high resolution IR camera on 125 to
400 μm diameter fibers are presented. Thermal contact resistance between the fiber and its heat sink are considered in
the conduction heat transfer model and measured for different geometries. Proper designs for cooling apparatus are
proposed and optimization of the active fiber is discussed. Some predictions for power scaling and temperature
management of fiber lasers to kW power level are also described.
A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding
fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to eliminate higher order modes. We describe here multimode (MM) high NA ytterbium doped fibers used in single mode output high power laser/amplifier configuration. Efficient single mode amplification is realized in the multimode doped fiber by matching the fundamental mode of the doped fiber to the LP<sub>01</sub> mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP<sub>01</sub> to the higher order modes. We show that negligible mode coupling is realized in the doped fiber, which ensures a stable power output over external perturbation without the use of tapers. Fundamental mode operation is maintained at all time without coiling through the use of FBG written in a single mode fiber. We show that such fiber is inherently more photosensitive and easier to splice than LMA fiber. We demonstrate an efficient 75W singlemode CW fiber laser using this configuration and predict that the power scaling to the kW level can be achieved, the design being more practical and resistant to photodarkening compared to conventional low NA LMA fiber.
We designed a high output power double cladding erbium-ytterbium fibre amplifier that showed no amplified
spontaneous emission (ASE) at 1.0 &mgr;m using a quasi singlemode fibre. The reduction of the amplified stimulated
emission (ASE) at 1.0 &mgr;m was found to be the combination of fibre design and temperature effect in the core. A 10W
output double cladding Er-Yb amplifier with a core/cladding fibre diameter of 10/125 &mgr;m was realized with a seed signal
of 1.4 W at 1563 nm and with counter-propagating pump power of 35 W at 976 nm without any significant ASE
generation at 1.0 &mgr;m. The fibre also exhibits singlemode behaviour with M<sup>2</sup> <1.1 and a high slope efficiency of 30%.
The fibre was designed to minimize ASE at 1.0 &mgr;m by heavily doping the fibre and using the appropriate ratio between
Yb<sup>3+</sup> and Er<sup>3+</sup> ions. By incorporating into our model the core temperature increase coming from the quantum defect of
the Er-Yb system, we can also predict a raise in the absorption cross-section of the ytterbium ions around 1060 nm
yielding to an increase of the 1 &mgr;m ASE threshold from 14 W to 35 W pump power, which allowed us to reach a 10 W
output power at 1563 nm instead of 5 W normally predicted by the theory. These results show potential power scaling of
the output power or double cladding erbium ytterbium amplifier using quasi singlemode core erbium ytterbium fibre
avoiding the need of large core dimension that degrades the beam quality.
A single cladding ytterbium doped fibre amplifier pumped at 980 nm that exhibits negligible amount of photodarkening
over a long period of time is demonstrated. The output power as a function of time decreased by a very small factor
compared to standard single mode ytterbium fibres. To achieve this photodarkening resistant amplifier, a special
ytterbium doped fibre has been developed. Codoping with aluminium or other rare-earth such as erbium is shown to
decrease the multi-excitation of ytterbium clusters and thus lower photodarkening. Photodarkening was characterized by
comparing the amount of excess loss created by core pumping single cladding fibres at high intensity at 980 nm.
Photodarkening was found to be directly proportional to the excitation of the ytterbium ions by comparing different
pumping scheme and pump wavelength. Core pumping of a single cladding ytterbium doped fibre amplifier at 980 nm
represents the worst case scenario for photodarkening. Engineering ytterbium fibres for low photodarkening is therefore
critical in pulsed amplification where short length of fibre with high doping level is required as demonstrated with 6 &mgr;m
core ytterbium fibre amplifier pumped in the core or in the cladding. Photodarkening was correlated to clustering from
cooperative luminescence measurement at 500 nm produced by ytterbium clusters that would emit UV radiation under
We developed a model that accurately predicts the performances of high power double cladding erbium ytterbium fibre
amplifiers. We experimentally validate the model in co- and counter-propagation configuration for different pump
wavelengths and fibre lengths. By adjusting the ytterbium to erbium cross-relaxation rate with a simple amplifier
experiment, we obtained a complete agreement between the model predictions and the experimental data regarding the
output signal. We measured that the McCumber relationship considerably overestimates the ytterbium emission crosssection;
with a correction of this parameter, we obtained an excellent prediction of the ytterbium spontaneous emission at
1.0 μm. The model is valid for high power singlemode amplifiers as we obtained a full agreement for a 4W output power amplifier from a seed signal of 8 mW at 1556nm. The output was diffraction limited with a measured M<sup>2</sup> parameter of
1.03 without doing any mode selection from a slightly multimode fibre with a core diameter of 10 μm and a numerical
aperture of 0.18.
Night vision capability has become an indispensable tool for military and civilian surveillance operations. Low-light- level television (LLLTV) and Forward-Looking-IR (FLIR) devices have long been used for these applications. Nevertheless, both have their shortcomings when the identification of the target is essential for the success of the mission. LLLTV cannot provide god image resolution in ultra low-light level conditions and is very sensitivity to parasitic light. FLIR system have poor resolution when the temperature difference contrast conditions are not met.
Propagation of light in a homogeneous scattering slab is conveniently modelled with a diffusion equation. This approach can be extended to a heterogeneous slab through a perturbation analysis. Within BornÕs approximation, the effect of an inclusion on the transmitted light is described by space-time integrals. Closed-form time integration is possible, which reduces the perturbation expressions to volume integrals over the inclusion. These can be useful to model small inclusions over which the integrand can be considered as constant. In the case of cubic inclusions with sides parallel and perpendicular to the boundaries of the surrounding slab, closed-form volume integration over the inclusion can be performed instead. Only time integrals are left, which reduces the numerical work. Numerical examples are presented. It is shown that inclusions with different volume and contrast with regards to the surrounding medium can produce the same effect on the transmitted light and are thus indistinguishable. The perturbation analysis has been used to assess the possibility of obtaining some longitudinal localization of an inclusion by using source beams and detectors of different sizes. Calculation results are also compared to experimental measurements to illustrate the validity of this analysis in the presence of small perturbations.