Concepts of Large Mode Area (LMA) fibers tailored for passive mitigation of Stimulated Brillouin Scattering (SBS) are discussed. The presented designs are based on the management of the overlap between the optical fundamental mode and the acoustic modes by lowering the contrast of acoustic velocity in the region surrounding the fiber core and considering the coiling of the fiber on a support. Analogously to the electromagnetic case, the circular curvature of the optical fiber changes the intensity distribution of the acoustic modes due to the propagation path lengthening with increasing distance from the winding axis. Numerical calculations were performed to determine the acoustic velocity profiles leading to an increased SBS threshold in the optical fiber by assessing the contribution of both guided and leaky acoustic modes.
A polarization-maintaining ytterbium-doped tapered double-cladding fiber was made from an aluminophosphosilicate glass preform, with core/cladding diameters of 17/125 μm and 56/400 µm at the small and large ends of the flared section, respectively. Amplifier gain exceeding 50 dB and average output power beyond 200 W were simultaneously achieved after the 1032-nm seed laser, while amplified spontaneous emission was measured ⪅ 1% of total output power. This fiber also yields an optical efficiency close to 90%, near diffraction-limited output with M2 ⪅ 1.2 and polarization extinction ratio ⪆ 18 dB. The newly developed fiber holds the potential to combine two successive amplifier stages in a single device, with foreseen benefits for ultrafast fiber amplifiers and laser harmonics generation.
An ytterbium-doped large mode area polarization-maintaining fiber with core/cladding diameters of 35/250 µm was fabricated from modified chemical vapor deposition technique and solution doping process. High cladding absorption and low photodarkening were achieved from aluminophosphosilicate core glass with optimal molar composition. The fiber was tested as a power amplifier using a 1064-nm narrow-linewidth laser oscillator with 34 ps pulse duration and 120 MHz pulse repetition frequency. The slope efficiency was seen to exceed 80% while the average output power was scaled beyond 420 W, before the onset of transverse mode instability. The fabricated fiber also yields near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio.
KEYWORDS: Transverse mode instability, Cladding, Spectral density, Design and modelling, Polarization maintaining fibers, Doping, Fiber characterization, High power fiber amplifiers, High power fiber lasers
The optimization of large-mode-area fiber design for the amplification of narrow-linewidth content or short pulses, susceptible to nonlinear effects, while reaching average powers exceeding the kW level is not a simple endeavor. The rapidly decreasing TMI-threshold with increasing core size leaves very little room in the 20 to 30 μm core diameter space for improved performance with respect to nonlinear effects while still delivering significant average power. We present results on a 29 μm core, polarization-maintaining LMA fiber, with a 400 μm cladding for high average power scaling. A carefully designed depressed-clad surrounds the core and enhances the bending losses for the Higher-Order Modes (HOM). Even when the fiber is loosely coiled (25 cm diameter), the filtering is very efficient which is advantageous for spreading out the fiber heat load and minimizing the effective area reduction resulting from the coiling-induced mode deformation. The fiber has been tested in a CW MOPA configuration, seeded with a longitudinally single-mode source emitting at 1064 nm, phase-modulated for Stimulated Brillouin Scattering (SBS) mitigation. The resulting slope efficiency has been measured at 88%, the PER was in the 12-15 dB range. The main feature of this fiber is its highly efficient HOM filtering capability, consequently one can maintain single mode-like operation up to the TMI threshold (slightly below 1 kW) without significant beam quality (BQ) degradation. As soon as coupling occurs between the fundamental mode and the first higher-order mode through the thermo-optic long-period grating, the energy is shed away and is coupled out in the fiber cladding.
A short-pulse Yb-doped fiber laser based on a master oscillator and power amplifier scheme is reported to yield an average power exceeding 500 W and pulse energy over 1 mJ. The final amplifier stage features a polarization-maintaining, large mode area tapered fiber with core/cladding diameters of 35/250 μm and 56/400 μm at each end of the flared section. The latter yields excellent optical conversion efficiency, near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio. The threshold for the onset of stimulated Raman scattering was further investigated using a pulsed seeder with ps-ns digitally programmable waveforms. Besides, no indication for transverse mode instability could be observed below the stimulated Raman scattering threshold, as beam quality M2 was measured < 1.3 and no fluctuations were further detected from photodiode time-traces of near-field laser beam samples.
Optical biopsy of tissue using fiber optic probes has proven to be a powerful tool for non-invasive and minimally invasive diagnostics. However, there are still many challenges to improving diagnostic value and commercial translation of these techniques. Many fiber-based methods are limited by background noise, which impairs sensitivity and specificity. Aspects of quality control, such as adequacy of the target of interest sampled and validation of optical measurements with histopathology can be problematic. Complexity, cost, and disposability or sterilizability are roadblocks to widespread clinical use. Here, we present new approaches to using fibers for optical biopsy aimed at solving these problems. Specifically, the new concepts are designed with the goals of being simple and disposable, to improve control of light delivery and collection from the sample, and to inherently enable better quality control of the biopsy process. A concept-of-operation aimed at nearly zero impact to the work flow of the biopsy and standard pathology procedures will be outlined. Several concepts for fiber implementations will be presented. A trade-off analysis of the concepts used to select a first implementation for testing will be presented. Preliminary experimental validation in phantoms and tissue samples will be presented for the selected configuration.
Theoretical and experimental work is reported on a distributed polarimetric fibre optic sensor designed for application in
capillary gas chromatography. Emphasis is on the improvement of the modeling of the polarization mode coupling
mechanism so as to get not only qualitative (vapor identification), but also quantitative (vapor concentration)
information from the analysis of a time-modulated signal transmitted through a polarizer at the end of a custom capillary
fibre.
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
configuration.
A 225-μJ polarization maintaining ytterbium-doped large-mode-area multiclad fiber was designed and fabricated with an
effective mode area of 450 μm2 and a photodarkening maximum excess loss of ~1 dB/m at 1064 nm. The fiber index
profile is based on a depressed-clad to obtain a diffraction-limited output. Optimization for low photodarkening and high
conversion efficiency while maintaining a good control on the core's refractive index profile has been achieved by
adjusting the ytterbium/phosphorus/aluminum concentrations in the fiber core. Concentration ratios of
phosphorus/aluminum from 0.12 to 1.25 were experimentally investigated in terms of photodarkening rate and excess
loss. Within this range, the photodarkening excess loss was observed to decrease by a factor of 8. The large-mode-area
fiber was used in a 10-ns pulse amplifier at 1064 nm with a repetition rate of 100 kHz and 0.5-nm bandwidth. The
diffraction-limited output has a measured M2 value of 1.04 when the fiber is coiled to a diameter of 12 cm. The fiber
amplifier slope efficiency is 70% with a polarization extinction ratio greater than 23 dB. It is shown how the
phosphorus/aluminum ratio reduces photodarkening, and how a depressed-clad design improves higher-order mode
filtering for reliable, efficient, and compact ytterbium-doped fiber amplifiers.
The relations between dopant concentrations (phosphorus and aluminum) and photodarkening rate, excess loss, and
activation energies in ytterbium-doped silica fibers are experimentally investigated. It is shown that increasing the
concentration of phosphorus from 0.2 to 2.5 mol% in phosphorus/aluminum codoped fiber cores decreases the
photodarkening excess loss by a factor of 8 and the photodarkening rate by a factor of 10. Moreover, the effective
number of ytterbium ions involved in the photodarkening process increases from 4 to more than 6 for tested
phosphorus/aluminum concentration ratios varying from 0.1 to 1 respectively. In contrast, increasing the aluminum
concentration from 2 to 5 mol% for a fixed phosphorus concentration of 0.2 mol% has negligible effect on the initial
photodarkening rate or the effective number of ytterbium ions involved in the process, but still decreases the
photodarkening excess loss by a factor of 5. Those results suggest photodarkening activation energies of 5.2 eV for
ytterbium/aluminum-codoped silica fibers and more than 7.8 eV for ytterbium/phosphorus/aluminum-codoped silica
fibers. The net improvement in photodegradation of fiber amplifiers based on such phosphorus and aluminum codoping
is measured experimentally and numerically simulated. The output power loss of 1064-nm ytterbium-doped LMA fiber
amplifiers with phosphorus/aluminum ratios of 0.1 and 0.6 is reduced after 10 000 hours from 17% to less than 2%,
respectively. Better understanding of the effects of phosphorus and aluminum on photodarkening will help to design
reliable and efficient ytterbium-doped fiber amplifiers.
Based on a depressed-clad index profile design, a PM Yb-doped large mode area (LMA) fiber with an effective mode
area of 450 μm2 was designed and fabricated. The fiber was used to amplify 10-ns pulses at 1064 nm with a repetition
rate of 100 kHz, and an output energy higher than 200 μJ was obtained, within a bandwidth of 0.5 nm. The fiber was
coiled on a 12-cm diameter mandrel to obtain a single-mode output having a measured M2 value of 1.04. The output
polarization extinction ratio was higher than 20 dB. The photodarkening excess loss of the Ytterbium//Phosphorus
Aluminum co-doped fiber was measured to be a factor 5 lower than that of a reference Ytterbium/Aluminum co-doped
fiber. It is shown how a depressed-clad index profile design can improve higher-order mode filtering while keeping the
coiling diameter practical for compact fiber amplifier packaging.
An analysis of the optical signal transmitted by a polarimetric sensor developed for the measurement of velocities of fluids in a capillary optical fiber is presented. It allows one to determine whether a fluid is moving in the vapor or the liquid phase.
Photodarkening and photobleaching processes affect the level of photodegradation of Yb-doped fibers. Characterization
and modeling of each process is crucial to understand how to optimize the operating conditions of fiber amplifiers and
lasers to obtain acceptable output power degradation. We show that photobleaching is a key factor in the modeling and
simulation of a 10-ns pulsed Yb-doped LMA fiber amplifier. Each parameter of the model was separately determined
from induced excess loss measurements under selective pump and wavelength excitations. The model was used to
simulate accurately the measured fiber amplifier degradation. Optimized fiber length and gain were calculated to
improve the output power stability over time and increase the fiber lifetime. Furthermore, eight fibers have been
fabricated with various Yb, Al, and P content using the MCVD process to optimize the core composition. The level of
photodarkening in each fiber was evaluated by measuring separately rate coefficient and excess loss. It was found that all
fibers followed a similar inversion-dependent rate while the maximum excess loss was dependent on the ratios [Al]/[Yb]
and [P]/[Yb]. The proposed model allows for rapid evaluation and optimization of fiber parameters and operation
conditions to assist Yb-doped laser system design in achieving the desired performance with low photodegradation.
Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber
lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of
single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no
comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi-empirical
approach seems at the moment the best way to gain a better understanding of the growth behavior of photo-induced
losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation
describing the activation and deactivation of color centers involving stretched exponential functions has been developed.
For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and
three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been
characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential
functions has been measured separately from the others. The model has then been used to simulate the power decay from
a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior
predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is
general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will
be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb-doped
fiber-based devices affected by photodegradation.
We present a novel microstructured optical fiber (MOF) structure which consists of a 7-missing-holes core surrounded
by a cladding formed by an Archimedean-like lattice of air holes, as opposed to the standard MOF cladding structure
consisting of a triangular lattice. We demonstrate that this new cladding geometry can be achieved through the standard
stack-and-draw MOF fabrication method, the main difference being the fact that the circular capillaries commonly used
in conventional MOF are replaced by hexagonal canes comprising 7 air holes disposed in a centred hexagon (forming
the so-called A7 unit cells). The Archimedean-like lattice MOF (ALMOF) is particularly interesting by the fact that it
features a dodecagonal core, which is a shape much closer to a perfect circle than the hexagonally-shaped core
characteristic of the conventional 7-missing-holes triangular-lattice MOF. As a result, we show that the improved core
circularity of the ALMOF design leads to a significantly more circular fundamental mode profile, which can prove to be
quite interesting for specific applications where the circularity of the mode profile is critical, such as precision laser
micromachining for example.
A liquid-core air-clad microstructured fiber has been developed for determination of ethanol concentration in aqueous
solutions by Raman spectroscopy. The sensor shows a linear response and a low ethanol concentration solution has been
characterized using the calibrated sensor. The configuration used for light and liquid injection is stable and robust,
making the sensor useful for on-line measurements.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.