A new few-mode fiber design taking advantage of a micro-structured core consisting of 19 secondary cores embedded in a pedestal geometry is presented. This design offers the possibility of precisely tailoring the rare earth ion distribution in the core in order to manage the differential modal gain. An optimized configuration of an erbium-doped few-mode fiber supporting 10 modes in the C-band with a theoretical low gain excursion is designed and realized. Preliminary optical characterizations of this fiber are presented.
We report on the first polarization maintaining single-mode fiber that delivers a flat-top intensity profile at 1050 nm. A high quality fundamental flat mode was obtained. We showed that our fiber can be considered as single-mode in practice with low confinement losses. Its birefringence was measured to be 0.6x10-4, and the PER was measured at more than 20 dB even for a 20 m fiber long. Strategies to enhance this birefringence preserving the flat top profile and the singlemode behaviour as well are also discussed.
Compactness, long term stability and no free-space alignment are important advantages of fiber lasers over bulky systems. These fiber lasers have also demonstrated their capability to deliver high-power pulses and are thus suitable for numerous applications. Nevertheless the intensity profile delivered usually has a Gaussian-like shape, which most of the time is sufficient, but it could be interesting, for many applications (laser-biological tissues interactions, heat treatment, industrial laser processing or for seeding large-scale laser facilities like Laser MegaJoule) to obtain a homogeneous intensity profile at the fiber laser output. Moreover several of these applications required a linearly polarized output beam. In order to achieve all these requirements we have developed and realized a new fiber design. This fiber is the first polarization maintaining single-mode fiber delivering a flat top intensity. A high quality flat mode was obtained at 1.05μm through the use of a well-tailored index profile and single-mode behavior was verified by shifting the injection and using the S² imaging. Moreover, boron Stress Applying Parts (SAPs) including in the cladding led to a birefringence of 0.6x10-4 and a measured PER better than 20dB even for a long fiber length (~20 m). Alongside the fabrication, we developed a simulation code, using Comsol Multiphysics®, to take into account the stress dependency induced by the SAPs. Further modeling allows us to present an effectively single-mode fiber design, delivering a top-hat mode profile and exhibiting a polarizing behavior.
We report on the design and the fabrication of a new design of an all-solid Bragg fiber based on the pixelization and heterostructuration of a cladding made of only two high index rings. The thickness of the low index ring as well as the geometry of the heterostructuration (its symmetry and the number of removed pixels) have been chosen to maximize the confinement losses of the Higher Order Modes (HOM) (above 10 dB/m) while keeping the Fundamental Mode (FM) losses low (below 0.1 dB/m). The proposed geometry allows having access to different Mode Field Diameter (MFD) from 54 μm to 60 μm at 1 μm wavelength by drawing the same stack to different fiber (and hence, core) diameters. As a result, a record MFD of 60 μm is reported for a Solid Core Photonic Bandgap Fiber (SC-PBGF) and single-mode behavior is obtained experimentally even for a short fiber length (few tens centimeters) maintained straight.
Characterization of spatial mode content and dispersion properties in fiber laser systems and space-division
multiplexing (SDM) applications is key to understanding fiber properties and system performance. Several
techniques exist for modal characterization but often present limitations in the context in which they can be
used efficiently. In this paper, we present a powerful analysis scheme that removes several of those limitations
and pushes modal content analysis to a new level.
Solid-Core Photonic BandGap Fibers (SC-PBGF) belongs to the family of microstructured optical fibers whereby the cladding is made of high refractive index inclusions as compared to that of the fiber core. In such fibers, light is confined to the core by an anti-resonant mechanism and several high transmission windows separated by high loss regions compose the transmission spectrum. Guiding mechanism is then identical to the one observed in Hollow-Core PBGF (HC-PBGF) except that a solid core can be exploited. Such PBGFs have proven to be good candidates for single-mode high power delivery and for controlling the spectral extension of supercontinuum generation. Mixing different types of resonators in the cladding or mixing PBG with modifed-Total Internal Reection (m-TIR) mechanism also lead to original and more exible fiber designs. Recent developments in the design and realization of Large Mode Area (LMA) and Highly NonLinear (HNL) fibers are presented, including single-mode ring-structured Bragg _bers, LMA fibers exhibiting a fundamental mode with a flat-top profile, and hybrid fibers for supercontinuum generation or frequency conversion.
We present a new passive air/silica microstructured optical fiber designed to be single mode and which delivers a flat-top
intensity profile at 1 μm. By inclusion of a raised index ring surrounding the central core, the refractive index profile of
the fiber flattens the intensity distribution of the fundamental mode. Experimental results clearly demonstrate the
feasibility of all-fibered top-hat beam delivery systems with one spatial mode suitable for many applications.
We report the first experimental demonstration of an optical fiber supporting a fundamental mode with flattened intensity
profile around 1050 nm. The design has been defined through intensive numerical simulations by paying a special
attention to the constraints imposed by the fabrication process. We show that the fabricated fiber presents a single-mode
Fibers used for high power delivery are designed to ensure single-mode operation (in order to guarantee good output beam quality), large effective areas (Aeff) and resistance to bend-induced distortions (in order to avoid non-linear effects). For simple step index fibers, the maximum Aeff of the fundamental mode that can practically be achieved at 1.06μm is ~350μm2. All-solid-silica Bragg fibers with large cores were proposed as an alternative solution for high power delivery through their fundamental core mode. These fibers consist of a low-refractive index core surrounded by a multilayer cladding that acts as a Bragg mirror. The loss spectrum of such fibers consists of a concatenation of several transmission windows separated by high-loss peaks. Here, we simultaneously study, for the first time (at our knowledge), the bending impact on Bragg fibers for the three critical properties required for high power delivery: large Aeff, single-mode propagation and low bend losses for the fundamental mode. Thanks to their specific guiding mechanism, Aeff as large as ~500μm2 at 1.06μm can be achieved in Bragg fibers, while maintaining single-mode operation and bend losses lower than 0.1dB/m. Our numerical results are validated by experimental measurements on a PCVD Bragg fiber with a 40μm diameter core.
Fiber Bragg gratings with strong resonance peaks for both Bragg and cladding modes are made in photonic crystal fiber
modified with a germanium doped core. Experimental results for strain, temperature and refractive index sensitivities of
fiber Bragg gratings are reported. We show the existence of an inner cladding mode that has the largest coupling from
the core mode and that is insensitive to surrounding index changes. The core mode, inner cladding mode and outer
cladding modes all have the same temperature sensitivity. By tracking the cladding mode resonances shifts relative to
the core mode, a temperature and index insensitive strain sensor can be made.
Second harmonic generation was obtained with an interesting efficiency in thermally poled sulfide glass. The best results obtained to date for chalcogenide glasses were on a Ge-Sb-S system thanks to an adapted treatment of thermal poling. The poling parameters like temperature (100-310 °C), applied voltage (2.5-4 kV)and duration (5-60min) were explored. A large NL second-order susceptibility χ(2) of about 10 ± 0.5 pm/V was measured. The nonlinear susceptibility profile as a function of the depth under the anode for Ge25Sb10S65 poled glass was determined using the analyze of remained second harmonic signal during the NaOH etching treatment. In parallel, a study of the concentration variation of elements being able to be involved in the formation of a charge space was achieved by using the secondary ion mass spectroscopy.
We present experimental results of thermal polings performed on Suprasil I samples (Heraeus) under square alternative voltages at various frequencies. We report a large increase (×5 compared to a continuous voltage poling) of the second order non-linear coefficient within a sample poled at 1mHz.
Chalcogenide glasses in the [Ge-Se-S-As] system have been synthesized and studied with respect to their nonlinear optical properties from third and second order. Z-scan and Mach Zehnder interferometry measurements of the nonlinear refractive index (n2) and nonlinear absorption ((beta) ) have been performed at 1064 nm. Some z-scan measurements have been also realized at 1430 nm. The results have been correlated to the structures of the glasses and the figure of merit has been calculated with the purpose of a potential utilization of these glasses in the realization of ultra- fast all-optical switches. Nonlinearities as high as 850 times the nonlinearity of silica glass have been obtained and some glasses exhibit at 1430 nm nonlinear optical characteristics suitable for telecommunication applications. The all-optical poling of a chalcohalogenide glass has been realized with a Q-switch mode-locked Nd:YAG laser at 1064 nm emitting 45 ps pulses at a repetition rate of 10 Hz with frequency doubling at 532 nm. A nonlinear coefficient deff equals 2.8 10-17 m/V similar to that of the reference glass Schott SF 57 has been obtained. The thermal poling of a chalcogenide glass also has been realized and a transient second order nonlinear susceptibility (chi) (2) has been observed.