The combination of high efficiency, high power, excellent beam quality, and low weight make ytterbium-doped fibre lasers a critical component in high-power laser systems. For high power operation, nonlinear impairments such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) become limiting factors in further increases in output power beyond a certain limit.

We report new Yb-doped gain fibers with approximately 20-μm core diameter and 400-μm cladding. These fibers have mode-field diameter greater than 19 μm, and increased higher-order-mode loss compared to conventional 19-μm modefield diameter fibers. The increased higher-order mode loss allows for high transverse-mode instability thresholds in fibers that also have large MFD and high absorption. A 21-μm fiber with 6.5 m operating length, was free from transverse-modal instabilities at 3.6 kW signal power and had 81% optical-optical efficiency. In narrow-linewidth amplifier experiments, the next-generation fiber with 21 μm fiber achieved 2.74 kW output power at 6 GHz linewidth, limited by pump power. This represents almost 2x increase in the ratio of power to signal linewidth compared to existing commercially available 19.5 μm MFD fibers, achieving 510 kW/GHz.

We present nanosecond-pulse amplification in very-large mode-area amplifiers with varying Er absorptions and effective areas. Diffraction limited, 0.54 mJ pulses with 851 kW peak power in a 10 kHz pulse train at 1560 nm are achieved.

In this paper we review recent developments in multicore optical fibers with continuous gratings suitable for various distributed sensing applications including shape, temperature, strain and acoustic signals. We describe an integrated optical fiber assembly for shape sensing. Our shape sensor module consists of a length (>1m) of twisted multicore optical fiber with fiber Bragg gratings inscribed along its length. Our fiber has a compact 200 micron coated diameter, a twist of 50 turns per meter and grating reflectivities greater than 0.001% per cm of array, suitable for high efficiency scatter measurements over many meters of fiber. Multicore fiber splicing with an OFS Fitel splicer is also demonstrated. A UV transparent coating protects the fiber at near pristine mechanical strength, while allowing for reel to reel processing during grating inscription without stripping the coating. We then demonstrate a shape reconstruction algorithm with the fiber to obtain various shapes using only the core reflected light. Finally, we describe enhanced Rayleigh-like scattering fibers with spatially continuous scattering over a large optical bandwidth, suitable for improved distributed strain, temperature and acoustic sensing using interrogator algorithms that traditionally use fiber Rayleigh backscattering.

We report on improved spatial uniformity of sensor grating arrays in offset and multicore fibers. We show improvement over conventional side writing in such fibers, in which cores offset from the center of the fiber exhibit grating strength variations due to lensing at the fiber surface. Such strength variations can degrade the performance of sensing systems that rely on continuous scattering from offset cores along a fiber. Our improved system uses multicore fibers whose coating is UV transparent and applies index matching materials to mitigate lensing aberrations. We show that it is capable of continuously inscribing gratings over any length of fiber.

We demonstrate the sensing capability of a twisted multicore optical fiber with continuous grating enhanced back scatter. Our fiber allows spatially continuous temperature measurement in a sensor with more than 20dB round trip transmission loss.

In this work we report on a fiber grating fabrication platform suitable for parallel fabrication of Bragg grating arrays over arbitrary lengths of multicore optical fiber. Our system exploits UV transparent coatings and has precision fiber translation that allows for quasi-continuous grating fabrication. Our system is capable of both uniform and chirped fiber grating array spectra that can meet the demands of medical sensors including high speed, accuracy, robustness and small form factor.

In this paper we report on the development of a complete integrated optical fiber assembly suitable for shape sensing. Our shape sensor module consists of a length (>1m) of twisted multicore optical fiber with fiber Bragg gratings inscribed along its length. Our fiber has a compact 180 micron coated diameter, a twist of 50 turns per meter and grating reflectivities greater than 0.01% per cm of array, suitable for high efficiency scatter measurements over many meters of fiber. Single core to multicore fanouts and low reflectivity fiber termination are used to terminate the end of the array.

We report on recent development of multicore fiber amplifiers for applications in space division multiplexed systems. Amplification and noise properties of cladding pumped multicore erbium doped fiber amplifiers employing end- and side-pumping, are presented.

The paper presents monolithic fiber-optic CPA systems - where the mode-locked oscillator, pulse stretcher, pulse shaper, pulse picker, and all amplifier stages are comprised of fusion spliced fiber devices - emitting up to 300 μJ compressed pulse output with duration <;500 fs (FWHM). This is a 6 times higher pulse energy than previously reported for monolithic fiber femtosecond lasers. Higher femtosecond pulse energy enables cutting and drilling through thicker materials, at faster rates, without imposing a heat affected zone (HAZ). Generating this femtosecond beam with a monolithic fiber-optic system provides the most compact and stable form factor, suitable for industrial work cell integration and global factory deployment.

We describe femtosecond laser systems optimized for high value applications in precision industrial micro-machining. Unprecedented system performance and reliability are enabled by novel fiber-optic architectures and integrated autonomous software control systems.

We demonstrate a 7-core erbium doped fiber amplifier employing side pumping using tapered multimode fiber. The amplifier has multicore inputs and outputs which can be readily spliced to multicore transmission fiber for amplifying space division multiplexed signals. Gain over 25dB was obtained in each of the cores over a 40-nm bandwidth covering C-band.

Single-frequency and narrow-linewidth pulse amplification is demonstrated in an erbium-doped higher-order mode fiber with effective area of 6000m2 and output long-period grating for re-conversion of the output beam back to the fundamental mode.

We demonstrate scaling of the effective area of higher-order mode, Er-doped fiber amplifiers. Two Er-doped higher-order mode fibers, one with 3800 μm(2) A(eff) in the LP(0,11) mode, and one with 6000 μm(2) effective area in the LP(0,14) mode, are demonstrated. Output beam profiles show clean higher order modes, and S(2) imaging measurements show low extraneous higher order mode content. CW and pulsed amplifier experiments are reported. Nanosecond pulses are amplified to 0.5 mJ pulse energy with 0.5 MW peak power.

Continuous wave and nanosecond pulse amplification in a higher-order-mode, Er-doped-fiber amplifier with 6000 μm2 effective area is demonstrated. Both 1480nm pump and 1560nm signal propagate in the LP0,14 mode.

We perform detailed measurements of the higher-order-mode content of a low-loss, hollow-core, photonic-bandgap fiber. Mode content is characterized using Spatially and Spectrally resolved (S2) imaging, revealing a variety of phenomena. Discrete mode scattering to core-guided modes are measured at small relative group-delays. At large group delays a continuum of surface modes and core-guided modes can be observed. The LP11 mode is observed to split into four different group delays with different orientations, with the relative orientations preserved as the mode propagates through the fiber. Cutback measurements allow for quantification of the loss of different individual modes. The behavior of the modes in the low loss region of the fiber is compared to that in a high loss region of the fiber. Finally, a new measurement technique is introduced, the sliding-window Fourier transform of high-resolution transmission spectra of hollow-core fibers, which displays the dependence of HOM content on both wavelength and group delay. This measurement is used to illustrate the HOM content as function of coil diameter.

We present for the first time a cascaded Raman fiber laser where the Yb-doped fiber laser and Raman fiber are combined into a single fiber. We achieve 42.6 % slope efficiency at 1236 nm with respect to launched pump power.

We demonstrate parallel fabrication of seven fiber distributed feedback (DFB) lasers in a hexagonally arrayed multicore core Er doped fiber with 40 μm core spacing. DFB grating cavities 8 cm long and operating near 1545 nm were fabricated with a single UV inscription exposure. We observed dual polarization, single longitudinal mode operation with a linewidth below 300 kHz for each laser.

We report the first experimental demonstration of coupled identical super-high Q-factor bottle microresonators formed by periodic nanoscale variation of the optical fiber radius. The Q-factor of the fabricated microresonator series exceeds 107.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text J. W. Nicholson, C. Headley, J. Phillips, A. Desantolo, E. Gonzalos, S. Ghalmi, M. F. Yan, P. W. Wisk, D. Trevor, J. Fleming, E. Monberg, F. Dimarcello, R. S. Windeler, J. M. Fini, D. J. DiGiovanni, and S. Ramachandran, "Higher-Order-Mode Fiber Amplifiers," in Lasers, Sources and Related Photonic Devices, OSA Technical Digest Series (CD) (Optica Publishing Group, 2010), paper LSWD1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article