André Croteau

Microstructured fiber splicing

We present experimental results on Microstructured Optical Fiber (MOF) splicing with a simple method relying on conventional electric-arc splicers. The results are presented in terms of fusion losses and tensile strength. An electric-arc splicing system is used to demonstrate its effectiveness in splicing MOFs together as well as splicing MOF with a single mode fiber.

Bragg Gratings Made With a Femtosecond Laser in Heavily Doped Er–Yb Phosphate Glass Fiber

Bragg gratings made in heavily doped phosphate glass fibers using ultrafast infrared radiation and a phase mask are reported. Refractive indexes >1.5times10-3 were induced in Er-Yb-doped phosphate fiber in a few seconds of exposure corresponding to reflectivity above 99.99% for a 6-mm-long grating. Annealing test up to 400 degC shows good thermal stability of the grating structure

Yb-doped LMA triple-clad fiber for power amplifiers

High-energy pulsed narrow-linewidth diffraction-limited ytterbium-doped power amplifiers in the 1030 to 1100 nm wavelength range and in the nanosecond regime require large mode area (LMA) fibers to mitigate stimulated Brillouin scattering (SBS). However, typical LMA fibers with mode-field diameters larger than 20 &mgr;m are inherently multimode. To achieve a diffraction-limited output, several techniques are available such as low core numerical aperture, fiber coiling and selective doping. The triple-clad fiber design takes advantage of the three techniques. The first clad located next to the core allows a reduction and a better control of the effective numerical aperture for high ytterbium doping that is difficult to achieve with the standard double-clad fiber design. Also, the thickness of the first clad gives an extra degree of freedom that allows either a nearly bending-insensitive output or mode filtering through bending losses that can be enhanced by a depressed-clad design. Incorporating to the triple-clad design an optimized selective rare-earth doping of the core favors the fundamental mode over higher-order modes by the gain differential. Using the right dopants, it can also favor SBS suppression by reducing the overlap between optical and acoustic field distributions. Ytterbium-doped LMA triple-clad fibers with a large depressed first clad and selective ytterbium doping are tested in a power amplifier configuration. Also, ytterbium-doped polarization-maintaining LMA triple-clad fibers with a thin first clad are tested for SBS.

Yb-doped LMA triple-clad fiber laser

The ytterbium-doped large mode area triple-clad fiber design allows for a high concentration of ytterbium in the fiber core which is difficult to achieve with a standard double-clad design. The novelty of the triple-clad fiber design consists in adding to the double-clad fiber design, a first clad next to its core. This first clad offers a better control of the core effective area. With this design a low numerical aperture is achievable (~0.06) for highly rare earth doped large mode area fiber. A 33-μm core ytterbium doped fiber has been fabricated using MCVD and solution doping processes. Selective doping and optimized first clad thickness have been used in the triple-clad design to obtain a nearly bending insensitive and nearly diffraction-limited fiber output. The fiber has been tested in a free-running laser configuration and its slope efficiency is 84% with a laser threshold of 1.4 W. A maximum output power of 26 W at 1070 nm has been achieved for a launched pump power of 34 W at 976 nm. The mode-field diameter has been measured at 18 μm and the output beam M2 quality factor is below 1.1. Both output power and beam quality were not significantly affected by fiber bending with loops diameter as small as 2.5 cm. The optical performance of the triple-clad fiber design is robust to mechanical stress and well suited for building very compact high power fiber lasers and amplifier sources.

Bending insensitive highly Yb-doped LMA triple-clad fiber for nearly diffraction-limited laser output

The new highly rare-earth doped triple-clad fiber design comprises a first clad next to the core of the well-known double-clad design. The added clad allows to reduce and to better control the core effective numerical aperture for achieving a highly doped large mode area amplifying fiber with a very low numerical aperture (~0.07). The triple-clad design is optimized to obtain a nearly bending insensitive fiber output while keeping excellent beam quality through proper ytterbium doping. The high ytterbium concentration allows for very high gain from a short (~1 m) fiber length which, in many applications, is required to prevent the onset of nonlinear effects such as stimulated Brillouin scattering. A polarization-maintaining 22-μm core Yb-doped triple-clad fiber was first tested. A laser slope efficiency of up to 86% with a polarization extinction ratio exceeding 24 dB and a M2 output beam quality factor below 1.1, for both laser and amplifier configurations, have been measured. Moreover, beam quality and output power were not significantly affected when coiling the fiber down to a 1.2 cm diameter, thus showing the optical robustness of the triple clad fiber design and offering the opportunity to build very compact high power fiber amplifiers and laser sources.

Co-seeded Er 3+ :Yb 3+ single frequency fiber amplifier with 60 W output power and over 90% TEM 00 content

We report on the design and fabrication of an Er(3+):Yb(3+) triple clad fiber and on the power scaling of a single frequency fiber amplifier at 1.5 μm based on that fiber. In addition, we report on mode content measurements in order to reveal the overlap of the amplifier output with the TEM(00) mode. The triple clad design was used to enable high output power levels, a good slope efficiency and an excellent beam quality. A maximum single frequency output power of 61 W at 1.5 μm could be achieved with the aid of the co-seeding method, which was used to suppress parasitic processes at 1.0 μm. With a scanning ring cavity the mode content of the amplifier output was analyzed with respect to the TEM modes. For all output power levels the TEM(00) content was above 90%.

Selecting the appropriate splitter for a reflective optical fiber dosimeter probe

Based on an innovative in-vivo optical dosimeter platform developed by scientists at University Health Network, we miniaturized the optical dosimeter in a tiny probe that fits the tip of an optical fiber. The approach consists in a measure of the absorbance change of a sensitive radiochromic material. The increase in absorbance is measured at a single wavelength and the linearly depends on the ionizing radiation dose. For compactness and design reasons, the proposed probe works in a reflective mode. A significant drawback when working with a reflective configuration is that reflections coming from splitter interfaces add to the signal and cause an apparent deviation from linearity. We studied the back reflections coming from a standard splitter and two custom made bifurcated optical fibers assemblies; 1) 7 fibers and 19 fibers. The 7 fibers connected to a 500 μm plastic optical fiber had the lowest reflection of 0.016% which was 3 times less than the 19 fibers and 100 times less than the standard splitter. An appropriate choice of the splitter was then imperative otherwise an under evaluation of the relative absorbance of −30% will happen.

Real-time optical fiber dosimeter probe

There is a pressing need for a passive optical fiber dosimeter probe for use in real-time monitoring of radiation dose delivered to clinical radiation therapy patients. An optical fiber probe using radiochromic material has been designed and fabricated based on a thin film of the radiochromic material on a dielectric mirror. Measurements of the net optical density vs. time before, during, and after irradiation at a rate of 500cGy/minute to a total dose of 5 Gy were performed. Net optical densities increased from 0.2 to 2.0 for radiochromic thin film thicknesses of 2 to 20 μm, respectively.

Fabrication and Characterization of a Real-Time Optical Fiber Dosimeter Probe

There is a pressing need for a low cost, passive optical fiber dosimeter probe for use in real-time monitoring of radiation dose delivered to clinical radiation therapy patients. An optical fiber probe using radiochromic material has been designed and fabricated based on the deposition of a radiochromic thin film on a dielectric mirror. Measurements of the net optical density vs. time before, during, and after irradiation at a rate of 500 cGy/minute to a total dose of 5 Gy were performed. Net optical densities increased from 0.2 to 2.0 for radiochromic thin film thicknesses of 2 to 20 μm, respectively. An improved optical fiber probe fabrication method is presented.

Low-loss splicing of microstructured fibers with conventional electric-arc splicers

We present experimental results demonstrating the possibility of obtaining low-loss splices of microstructured optical fibers (MOFs) by using conventional electric-arc splicers. We show evidence of the effectiveness of the method by splicing two MOFs together as well as a MOF with a standard single mode fiber (SSMF). The results are presented in terms of fusion losses and tensile strength. Theoretical calculations of the losses attributable to mode mismatch between the MOF and the SMF suggest that the splicing losses could be further reduced by optimizing the MOF design parameters. For the case of a MOF-MOF splicing, the loss that could be due to a possible rotational misalignment that comes with the non-cylindrical symmetry of the modal distribution is also evaluated.