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.
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.
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.
The fabrication of a polarization-maintaining version of a large-mode-area multi-clad fiber design with high Yb
concentrations and a robust output beam represents a significant challenge due to the high risk of cracking of the doped
silica multi-clad next to the core during the drilling procedure. A new preform fabrication approach permitting the
realization of a large first-clad fiber featuring a high birefringence, while preserving the preform integrity is presented.
The birefringence was improved by locating the stress-applying-parts in the first-clad region and by increasing their
boron content. The preform and fiber fabrication will be presented as well as the fiber performances in a pulsed amplifier
Spectroscopic studies of intrinsic defect centers in Yb doped silica fibers are presented. The relationship between
the defect centers and photodarkening in Yb doped silica fibers is investigated. Photoluminescence from nonbridging
oxygen hole center (NBOHC) and oxygen deficiency center (ODC) defects in several ytterbium-doped
silica fibers are presented and analyzed. The photoluminescence spectra and temporal decays are given to determine
if the Yb dopant and associated co-dopants which serve as network modifiers in the silica matrix could provide
pathways whereby infrared photons could be upconverted into the UV, leading to damage in the fiber.
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.
We present current work developed at INO on phosphate glass optical fiber for laser and amplifier applications at 1.54
microns. Core and cladding glasses were fabricated by a multi-components melting process which gave an uniform
refractive index core profile. Rod-in-tube method under Argon atmosphere was used to fabricate optical fibers. The
effect of nitrogen atmosphere on hydroxyl groups OH<sup>-</sup> during glass melting was studied. The absorption coefficient
calculated at 3.42 μm was found to be lower than 0.5 cm<sup>-1</sup> which corresponds to less than 70 ppm OH<sup>-</sup>. Absorption and
emission cross sections were calculated at 1534 nm. Fabrication process allowed us to decrease background losses of
core Er<sup>3+</sup> - Yb<sup>3+</sup> co-doped fiber between 0.02 and 0.04 dB/cm. Laser power was measured at 1563 nm and a 26% slope
efficiency was achieved with a 22 cm-long single-clad fiber co-doped with 1.1 wt% in Er<sup>3+</sup> and 11.1 wt% in Yb<sup>3+</sup>. For
the same fiber, an internal gain was found to be 20 dB at 1536 nm for a 5-cm-long fiber.
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 M<sup>2</sup> 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.
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.
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 (SMF). 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.
Cladding mode coupling loss below 0.1 dB for a 30 dB fiber Bragg grating are reported for a wide range of H<sub>2</sub> treatment pressure up to 1500 psi. Extension of the photosensitive region into the clading was used for reducing the cladding mode coupling. It is shown that to take proper advantage of this, one must use a wider laser scan beam. Otherwise, the cladding mode coupling loss may increase as the H<sub>2</sub> pressure treatment is raised. The benefit of a properly matched photosensitivity obtained by matching the Ge concentration in the core and cladding regions is also highlighted. The fiber was also designed to match a standard single-mode fiber in order to lower the average splicing loss below 0.03 dB and the attenuation to about 0.2 dB/km at 1550 nm.
Cobalt-doped high attenuation fibers were tested in terms of temperature, humidity, and optical power. The maximum attenuation variation recorded was less than 3% for temperatures between -40 to + 65°C under uncontrolled humidity. When the humidity was controlled, the maximum attenuation variation was less than 3% under the worst case: +85°C and 85% R.H. Optical power test were carried out at 1W and 1550 nm over 12 minutes without any recorded damage to the fibers.
Since the beginning of optical fiber communications, many fiber designs, driven by the desire to extend the fiber limited performances, have been proposed. In the last decade, the most innovative concept that came out is probably the HF (Holey Fibre). This new fiber design consist of a pure silica fiber with a periodic array of air holes running along the length of the fiber. Usually, the air holes forming the cladding region are arranged in an hexagonal lattice and the introduction of a defect, absence of a hole, in the center of this periodic structure creates the core of the fiber. Over the past few years, impressive possibilities offered by this new type of fiber have been demonstrated in various fields of optical fiber technology such as single-mode fiber, high optical power guidance, polarization control, dispersion compensation, soliton propagation, continuum generation, fiber lasers and amplifiers, remote sensing, etc. In this paper, we review the technology and present our design, fabrication capability, as well as some results obtained with our HFs.