Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber.

We demonstrate a seven-core erbium-doped fiber amplifier in which all the cores were pumped simultaneously by a side-coupled tapered multimode fiber. The amplifier has multicore (MC) MC inputs and MC outputs, which can be readily spliced to MC transmission fiber for amplifying space division multiplexed signals. Gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band.

Bend compensated large-mode-area fibers: achieving robust single-modedness with transformation optics.

Fibers with symmetric bend compensated claddings are proposed, and demonstrate performance much better than conventional designs. These fibers can simultaneously achieve complete HOM suppression, negligible bend loss, and mode area >1000 square microns. The robust single-modedness of these fibers offers a path to overcoming mode instability limits on high-power amplifiers and lasers. The proposed designs achieve many of the advantages of our previous (asymmetric) bend compensation strategy in the regime of moderately large area, and are much easier to fabricate and utilize.

Low-loss hollow-core fibers with improved single-modedness.

Hollow-core fibers (HCFs) are a revolution in light guidance with enormous potential. They promise lower loss than any other waveguide, but have not yet achieved this potential because of a tradeoff between loss and single-moded operation. This paper demonstrates progress on a strategy to beat this tradeoff: we measure the first hollow-core fiber employing Perturbed Resonance for Improved Single Modedness (PRISM), where unwanted modes are robustly stripped away. The fiber has fundamental-mode loss of 7.5 dB/km, while other modes of the 19-lattice-cell core see loss >3000 dB/km. This level of single-modedness is far better than previous 19-cell or 7-cell HCFs, and even comparable to some commercial solid-core fibers. Modeling indicates this measured loss can be improved. By breaking the connection between core size and single-modedness, this first PRISM demonstration opens a new path towards achieving the low-loss potential of HCFs.

Higher-order mode fiber enables high energy chirped-pulse amplification.

Energy scaling of femtosecond fiber lasers has been constrained by nonlinear impairments and optical fiber damage. Reducing the optical irradiance inside the fiber by increasing mode size lowers these effects. Using an erbium-doped higher-order mode fiber with 6000 µm(2) effective area and output fundamental mode re-conversion, we show a breakthrough in pulse energy from a monolithic fiber chirped pulse amplification system using higher-order mode propagation generating 300 µJ pulses with duration <500 fs (FWHM) and peak power >600 MW at 1.55 µm. The erbium-doped HOM fiber has both a record large effective mode area and excellent mode stability, even when coiled to reasonable diameter. This demonstration proves efficacy of a new path for high energy monolithic fiber-optic femtosecond laser systems.

Long-term stable, sub-femtosecond timing distribution via a 1.2-km polarization-maintaining fiber link: approaching 10(-21) link stability.

Long-term stable, sub-femtosecond timing distribution over a 1.2-km polarization-maintaining (PM) fiber-optic link using balanced optical cross-correlators for link stabilization is demonstrated. Novel dispersion-compensating PM fiber was developed to construct a dispersion-slope-compensated PM link, which eliminated slow timing drifts and jumps previously induced by polarization mode dispersion in standard single-mode fiber. Numerical simulations of nonlinear pulse propagation in the fiber link confirmed potential sub-100-as timing stability for pulse energies below 70 pJ. Link operation for 16 days showed ~0.6 fs RMS timing drift and during a 3-day interval only ~0.13 fs drift, which corresponds to a stability level of 10(-21).

Crosstalk in multicore fibers with randomness: gradual drift vs. short-length variations.

Random perturbations play an important role in the crosstalk of multicore fibers, and can be captured by statistical coupled-mode calculations. In this approach, phase matching contributes a multiplicative factor to the average crosstalk, depending on the perturbation statistics and any intentional heterogeneity of neighboring cores. The impact of perturbations is shown to be qualitatively different depending on whether they are gradually varying, or have short-length (centimeter-scale) variations. This insight implies a novel crosstalk suppression strategy: fast modulation of a bend perturbation by spinning the fiber can disrupt the bend-induced phase matching.

Dispersion-stabilized highly-nonlinear fiber for wideband parametric mixer synthesis.

Conventional highly-nonlinear fiber (HNLF) designs are optimized for high field-confinement but are also inherently susceptible to dispersion fluctuations. The design compromise prevents fiber-optical parametric mixers from possessing high power efficiency and extended operating bandwidth simultaneously. Using a new fiber waveguide design, we have fabricated and tested a new class of HNLF that possesses a significantly lower level of dispersion fluctuations while maintaining a high level of field-confinement comparable to that in conventional HNLFs. The fiber was used to demonstrate an all-fiber parametric oscillator operating in short-wavelength infrared (SWIR) band with a watt-level pump, for the first time.

Large-mode-area multicore fibers in the single-moded regime.

The effectively single-mode regime is analyzed for a class of large mode area multicore fibers. The performance tradeoff between bend loss, single-modedness, and mode area for these fibers is shown to be at best equivalent to step-index fiber.

Large mode area fibers with asymmetric bend compensation.

Fibers with asymmetrical bend compensation offer to completely remove the tradeoff between mode area and single-modedness, with potentially huge impact on high-power amplification. These fibers would be difficult to fabricate, but are the only fundamental-mode strategy that can remove the bend-distortion limitations on mode-area scaling. Here, we show that even imperfect fibers can achieve essentially complete HOM suppression for areas of 2000 square microns or larger. Ultimate performance limits due to finite cladding size and fabrication imperfections are calculated.

Characterization and optimization of Yb-doped photonic-crystal fiber rod amplifiers using spatially resolved spectral interferometry.

Spatially resolved spectral interferometry is used to measure the mode content of a Yb-doped photonic-crystal fiber rod amplifier with a 2300 µm(2) mode area. The technique, known as S(2) imaging, was adapted for the short fiber amplifier at full power and revealed a small amount of a copolarized LP(11) mode. Simulations illustrate the potential for weak mode suppression in this fiber and agree qualitatively with the measurements of S(2) and M(2). Higher-order-mode content depends on the alignment of the input signal at injection and ranged from -18 dB for optimized alignment to -13 dB when the injection alignment was offset along the LP(11) axis by 30% of the 55 µm mode-field diameter.

Statistics of crosstalk in bent multicore fibers.

A statistical theory for crosstalk in multicore fibers is derived from coupled-mode equations including bend-induced perturbations. Bends are shown to play a crucial role in crosstalk, explaining large disagreement between experiments and previous calculations. The average crosstalk of a fiber segment is related to the statistics of the bend radius and orientation, including spinning along the fiber length. This framework allows efficient and accurate estimates of cross-talk for realistic telecommunications links.

Bend-resistant design of conventional and microstructure fibers with very large mode area.

Achieving very large mode area is a key goal in current research on microstructure and solid fibers for high power amplifiers and lasers. One particular design regime of recent interest has effective area over 1000 square microns and has effectively-single-mode operation ensured by bend losses of the higher-order modes. Simulations show that these fibers are extremely prone to bend-induced distortion and reduction in mode area. The calculated area reduction would significantly impact nonlinear impairments for bend radii relevant to any reasonable spooled package, and can be over 50 percent for bend radii tighter than 10cm. The parabolic-profile design has a natural immunity to bend-induced mode distortion and contraction, and shows superior performance in simulated fair comparisons with other fiber families, including microstructure fibers.

Aircore microstructure fibers with suppressed higher-order modes.

A strategy for suppressing higher-order modes in aircore bandgap fibers is proposed. Simulations confirm that significant suppression of unwanted modes is achieved by including index-matched air-guiding structures in the cladding. Suppressing higher-order modes offers to improve the fundamental loss limit in aircore fibers, addressing a key obstacle to the development of this technology.

Distributed fiber filter based on index-matched coupling between core and cladding.

A new type of fiber for distributed filtering is proposed, designed to have resonant coupling between core and cladding at desired wavelengths. Design principles are illustrated with simulations of several fibers. A filter fiber was fabricated following this design strategy. Measured transmission spectra and imaging of mode output confirm the expected resonant coupling between core and cladding near 1100nm.

Polarization maintaining single-mode low-loss hollow-core fibres.

Hollow-core fibre (HCF) is a powerful technology platform offering breakthrough performance improvements in sensing, communications, higher-power pulse delivery and other applications. Free from the usual constraints on what materials can guide light, it promises qualitatively new and ideal operating regimes: precision signals transmitted free of nonlinearities, sensors that guide light directly in the samples they are meant to probe and so on. However, these fibres have not been widely adopted, largely because uncontrolled coupling between transverse and polarization modes overshadows their benefits. To deliver on their promises, HCFs must retain their unique properties while achieving the modal and polarization control that are essential for their most compelling applications. Here we present the first single-moded, polarization-maintaining HCF with large core size needed for loss scaling. Single modedness is achieved using a novel scheme for resonantly coupling out unwanted modes, whereas birefringence is engineered by fabricating an asymmetrical glass web surrounding the core.

Intuitive modeling of bend distortion in large-mode-area fibers.

Bend distortion is emerging as an important consideration in the design of amplifier fibers with a large mode area, in addition to bend loss and mode-coupling effects. Simple, intuitive estimates of distortion sensitivity are presented for two interesting classes of fibers, in agreement with full numerical simulations.

Natural bend-distortion immunity of higher-order-mode large-mode-area fibers.

Bend-induced distortion is an important limitation in the development of fibers with very large mode areas. Simulations demonstrate that higher-order modes, recently proposed for amplification, are naturally immune to bend distortion, despite their extremely large mode areas. These numerical results compliment measured resistance to mode coupling. An interesting indirect coupling between nearly degenerate modes is shown to dominate the distortion.

Bend-compensated design of large-mode-area fibers.

Cone-shaped profiles illustrate the recently proposed bend-compensated strategy for large-mode-area amplifier fiber design. Simulations show that introducing a bend-compensating gradient into the fiber profile simultaneously improves mode area and reduces higher-order-mode impairments in a spooled-fiber amplifier.

Analysis of microstructure optical fibers by radial scattering decomposition.

Radial scattering decomposition, a tool for the modeling and design of microstructure optical fiber, including photonic bandgap fiber, is described. Building on the multipole approach, the method decomposes a fiber structure into radial shells, allowing the confinement of light to be analyzed layer by layer. This approach lends new understanding and suggests a number of new approaches to bandgap fiber design.