Stéphane Chatigny

THERMAL EFFECTS IN HIGH POWER CW FIBER LASERS

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.

Inclusion characterization in a scattering slab with time-resolved transmittance measurements: perturbation analysis

A procedure for the time-domain optical characterization of an inclusion in a scattering slab is investigated theoretically and experimentally. The method relies on the measurement of a contrast function, which is defined as the time-dependent relative change in the transmitted signal resulting from the presence of the inclusion. Analytical expressions for the contrast functions of absorptive and diffusive inclusions are obtained through a perturbation solution of the diffusion equation. This procedure is used successfully to determine the optical properties of absorptive, diffusive, and mixed inclusions located at midplane in a scattering slab by use of time-resolved transmittance measurements.

Hybrid Monte Carlo for photon transport through optically thick scattering media

A Monte Carlo simulation code developed to model time-domain transillumination measurements with small-area detectors through an optically thick scattering slab is presented. A hybrid approach has been implemented to reduce calculation times. Most of the scattering slab is treated stochastically, albeit with variance reduction techniques and the isotropic diffusion similarity rule. The contribution to the output signal per unit area and time of photon packets propagating in a thin slice near the output face of the slab is calculated analytically after each propagation step. This approach drastically reduces the calculation time but produces spikes in the temporal signals.

Enhanced Pulseshaping Capabilities and Reduction of Non-Linear Effects in All-fiber MOPA Pulsed System

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.

Modeling of an active TV system for surveillance operations

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.

Simple Design for Singlemode High Power CW Fiber Laser using Multimode High NA Fiber

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 LP01 mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP01 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.

Time-domain perturbation analysis of a scattering slab

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 BornOs 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.

Dual-spatial integration for longitudinal localization of inclusions in turbid media.

We introduce a technique called dual-spatial integration (DSI) that is used to isolate and enhance inclusions that differ only by their longitudinal placement within a scattering medium. DSI uses three different source-detector configurations to section a scattering medium into three longitudinal zones. This sectioning permits the extraction of structures close to surfaces and the enhancement of those structures located in the central part of the medium. Both the simulation and the experimental results indicate that DSI has potential interest for applications in biomedical imaging such as optical mammography.