Optimizing single mode robustness of the distributed modal filtering rod fiber amplifier

Advances in Silica-Based Large Mode Area and Polarization-Maintaining Photonic Crystal Fiber Research

In recent years, photonic crystal fibers (PCFs) have attracted increasing attention. Compared with traditional optical fibers, PCFs exhibit many unique optical properties and superior performance due to their high degree of structural design freedom. Using large-mode area (LMA) fibers with single-mode operation is essential to overcoming emerging problems as the power of fiber lasers scales up, which can effectively reduce the power density and mitigate the influence of nonlinear effects. With a brief introduction of the concept, classification, light transmission mechanism, basic properties, and theoretical analysis methods of PCFs, this paper mainly compiles the worldwide development of large-mode area and polarization-maintaining (PM) PCFs, and finally proposes possible technical routes to realize the single-mode operation of LMA-PCFs and PM-LMA-PCFs. Finally, the future development prospects of the PCFs are discussed.

Wavelet-Based Demodulation of Multimode Etched Fiber Bragg Grating Refractive Index Sensor

Etched fiber Bragg grating (EFBG)-based sensors are used as evanescent field sensors for refractive index detection. When the fiber thickness is thin and the refractive index sensitivity increases, the number of propagating modes increases, resulting in a spectral enlargement that complicates the interrogation of the sensor. In this work, we present a method to analyze the spectrum of a multimode etched fiber Bragg grating (MMEFBG) in the wavelet domain, which analyzes the amount of spectral density independently from the peak reflectivity value. The proposed interrogation method permits defining the integral of the spectral density as a novel and unconventional estimator. With respect to the conventional estimators based on wavelength shift, this estimator can better exploit the larger amount of information given by the spectral enlargement typical of multimode behavior. Results were obtained by etching an MMEFBG in hydrofluoric acid and using water/sucrose mixtures to evaluate the refractive index sensitivity, validating the interrogation method.

Static and dynamic mode coupling in a double-pass rod-type fiber amplifier

This Letter describes an experimental realization of a double-pass amplifier using rod-type fiber. In this device, the gain reaches 26 dB amplifying a 300 mW, 20 ps, 20 MHz seed up to 120 W, with an optical-to-optical efficiency of 50% and excellent beam quality. In addition, by design the output of the amplifier has a polarization extinction ratio of 33 dB. Besides these good performances, we report a marginal degradation of mode quality and degree of polarization followed by the so-called transverse mode instability which occurs at 120 W signal power. The first degradation is static, and by analyzing its two polarizations, we conclude it is caused by a coupling between modes due to the formation of a static thermal long-period grating, which in turn initiates the dynamic instability.

Flexible cross-correlated (C2) imaging method for the modal content characterization in a broad range of wavelengths

We demonstrate a flexible cross-correlated (C²) imaging method in the time domain by application of a tunable and highly flexible light source. An advantage of the flexible C² method is shown by characterization of the step-index fiber (SMF28) over a broad range of wavelengths from 870nm to 1090nm and by the modal analysis of the distributed modal filtering (DMF) rod fiber within a wavelength range from 1050nm to 1090nm. Also, the influence of the spectral shape and bandwidth on the imaging trace is investigated by deliberately adjusting the input spectrum of the light source. The modal intensity as well as the phase distribution are extracted by the alternative method of 2D FT filtering. Being exceptionally tunable the flexible C² method gives an ability to adapt the system's parameters in a desired manner satisfying even measurements of very specific fiber designs opening up new possibilities for advanced modal characterization of fibers over broad range of wavelengths.

Extreme large mode area in single-mode pixelated Bragg fiber

This paper reports the design and the fabrication of an all-solid photonic bandgap fiber with core diameter larger than 100 µm, a record effective mode area of about 3700 µm² at 1035 nm and robust single-mode behavior on propagation length as short as 90 cm. These properties are obtained by using a pixelated Bragg fiber geometry together with an heterostructuration of the cladding and the appropriated generalized half wave stack condition applied to the first three higher order modes. We detail the numerical study that permitted to select the most efficient cladding geometry and present the experimental results that validate our approach.

Thermal modeling of gain competition in Yb-doped large-mode-area photonic-crystal fiber amplifier

We propose a new model for gain competition effects in high-power fiber amplifiers, which accounts for the thermal effects of heat load on the doped core overlap of the propagating light field. The full-vectorial nature of the fiber modes is modeled by an embedded finite-element method modal solver, and the temperature profile is calculated by a simple and efficient radial heat propagation solver. The model is applied to a Yb³⁺-doped LPF45 air-clad photonic-crystal fiber amplifier for coand counter-propagating pumping setups, showing gain competition in conditions of severe heat load.

Large-mode-area hybrid photonic crystal fiber amplifier at 1178 nm

Amplification of 1178 nm light is demonstrated in a large-mode-area single-mode ytterbium-doped hybrid photonic crystal fiber, relying on distributed spectral filtering of spontaneous emission at shorter wavelengths. An output power of 53 W is achieved with 29 dB suppression of parasitic lasing. Further power scaling is limited by parasitic lasing.

Phosphate ytterbium-doped single-mode all-solid photonic crystal fiber with output power of 13.8 W

Single-mode ytterbium-doped phosphate all-solid photonic crystal fiber (AS-PCF) with 13.8 W output power and 32% slope efficiency was reported. By altering the diameter of the rods around the doped core and thus breaking the symmetry of the fiber, a polarization-maintaining AS-PCF with degree of polarization of >85% was also achieved, for the first time to knowledge, in a phosphate PCF.

Thermally resilient Tm-doped large mode area photonic crystal fiber with symmetry-free cladding

The design of a Tm-doped photonic crystal fiber with ∼ 80 μm core diameter and robust single-mode guiding is proposed. State-of-art modal discrimination is obtained through the suppression of the inner cladding C(6ν) symmetry, which fosters the delocalization of the LP(11)-like mode. The effects of thermally-induced refractive index change are investigated by means of a computationally-efficient thermal model, and the possibility to obtain wide-band single-mode propagation and effective area exceeding 2500 μm(2) under a heat load of over 300 W/m is demonstrated.

Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber

Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

We present a semi-analytic numerical model to estimate the transverse modal instability (TMI) threshold for photonic crystal rod amplifiers. The model includes thermally induced waveguide perturbations in the fiber cross section modeled with finite element simulations, and the relative intensity noise (RIN) of the seed laser, which seeds mode coupling between the fundamental and higher order mode. The TMI threshold is predicted to ~370 W - 440 W depending on RIN for the distributed modal filtering rod fiber.

Cross-correlated imaging of single-mode photonic crystal rod fiber with distributed mode filtering

Photonic crystal bandgap fibers employing distributed mode filtering design provide near diffraction-limited light outputs, a critical property of fiber-based high-power lasers. Microstructure of the fibers is tailored to achieve single-mode operation at specific wavelength by resonant mode coupling of higher-order modes. We analyze the modal regimes of the fibers having a mode field diameter of 60 µm by the cross-correlated (C(2)) imaging method in different wavelength ranges and evaluate the sensitivity of the modal content to various input-coupling conditions. As a result, we experimentally identify regimes of resonant coupling between higher-order core modes and cladding band. We demonstrate a passive fiber design in which the higher-order modal content inside the single-mode guiding regime is suppressed by at least 20 dB even for significantly misaligned input-coupling configurations.

Mode area scaling with multi-trench rod-type fibers

We propose a novel all-solid rod-type fiber structure that presents a cylindrical symmetry and low refractive-index contrasts. Effectively single-mode propagation for the fundamental mode is ensured thanks to resonant couplings between Higher Order Modes (HOMs) and cladding modes. Numerical simulations demonstrate the possibility of achieving a fundamental mode effective area as large as 5000 µm² at a wavelength of 1.06 μm in fibers ensuring a high leakage loss ratio (>100) between the HOMs and the fundamental mode while keeping the fundamental mode leakage losses at a level lower than 0.2dB/m. Further scaling to an effective area of 12,200 µm² at 1.06 μm in an effectively single-mode fiber is also presented by exploiting the power delocalization of several HOMs on top of the high-leakage loss filtering.