I-Ning Hu

Single-mode chirally-coupled-core fibers with larger than 50µm diameter cores

In this paper, we report an advance in increasing core size of effective single-mode chirally-coupled-core (CCC) Ge-doped and Yb-doped double-clad fibers into 55 µm to 60 µm range, and experimentally demonstrate their robust single-mode performance. Theoretical and numerical description of CCC fibers structures with multiple side cores and polygon-shaped central core is consistent with experimental results. Detailed experimental characterization of 55 µm-core CCC fibers based on spatially and spectrally resolved broadband measurements (S(2) technique) shows that modal performance of these large core fibers well exceeds that of standard 20 μm core step-index large mode area fibers.

Propagation-length independent SRS threshold in chirally-coupled-core fibers.

Both analytical study and numerical simulations show that the propagation-length independent Stimulated Raman Scattering (SRS) threshold can be achieved by Stokes wave suppression in optical fibers. We propose a specific design based on Chirally-Coupled-Core (CCC) fibers with spectrally-tailored wavelength-selective transmission to suppress the Stokes wave of Raman scattering. Fibers with length-independent nonlinearity threshold could be particularly advantageous for high power lasers and fiber beam delivery for material processing applications.

Coherent pulse stacking amplification using low-finesse Gires-Tournois interferometers

We demonstrate a new technique of coherent pulse stacking (CPS) amplification to overcome limits on achievable pulse energies from optical amplifiers. CPS uses reflecting resonators without active cavity-dumpers to transform a sequence of phase- and amplitude-modulated optical pulses into a single output pulse. Experimental validation with a single reflecting resonator demonstrates a near-theoretical stacked peak-power enhancement factor of ~2.5 with 92% and 97.4% efficiency for amplified nanosecond and femtosecond pulses. We also show theoretically that large numbers of equal-amplitude pulses can be stacked using sequences of multiple reflecting resonators, thus providing a new path for generating very high-energy pulses from ultrashort pulse fiber amplifier systems.

Short-Term and Long-Term Stability in Ytterbium-Doped High-Power Fiber Lasers and Amplifiers

In this paper we discuss recent progress in exploring short-term and long-term stability of high power Yb-doped fiber lasers and amplifiers. Long-term stability is associated with photodarkening effects that can significantly reduce operational lifetime of a high-power laser system. Short-term stability is associated with so-called transverse modal instabilities that degrade output beam quality at average powers above a certain threshold. In this paper we review ongoing studies that provide experimental characterization, and explore physical causes and mitigation strategies of the different physical phenomena involved. Those studies are critical for achieving stable operation of high power fiber lasers.

3C Yb-doped fiber based high energy and power pulsed fiber lasers

3C fiber technology advances the performance frontier of practical, high-pulse-energy fiber lasers by providing very large core fibers with the handling and packaging benefits associated with single mode fibers. First-generation fibers demonstrate scaling to > 240 W average power coincident with 100-kW peak power in 1-mJ, 10-ns pulses while maintaining single-mode beam quality, polarized output, and efficiencies > 70%. Peak powers over 0.5 MW with negligible spectral distortion can be achieved with sub ns, near-transform-limited pulses. In-development second-generation 3C Yb-fiber based on core sizes around 55 μm1 have produced >8 mJ, 13 ns pulses with peak powers exceeding 600 kW.

Single-frequency and single-transverse mode Yb-doped CCC fiber MOPA with robust polarization SBS-free 511W output

We demonstrate 511W single-frequency (Δν << 40MHz), single-transverse mode and robust linear polarization output fiber MOPA based on 37µm diameter Yb-doped Chirally-Coupled-Core air-clad fiber.

Analytical time-dependent theory of thermally induced modal instabilities in high power fiber amplifiers

A time-dependent analytical model is rigorously derived which shows that the thermally induced modal instability in high power rare-earth doped fiber amplifiers is fundamentally a two-wave mixing between fundamental and higher-order modes through a thermally-induced grating imprinted by beating between these modes. We show that previously postulated movement of this grating to phase-match the coupling between the modes naturally occurs due to a finite thermal-response time of a fiber. This theory is consistent with experimental observations in that it accurately predicts the onset-like threshold and temporal instabilities in the kilohertz-frequency range.

Experimental demonstration of SRS suppression in chirally-coupled-core fibers

We experimentally demonstrate suppression of Stimulated Raman Scattering using a spectrally tailored transmission of a large-core effectively-single-mode Chirally-Coupled-Core fiber with a large loss at Stokes wavelengths.

Coherent Pulse Stacking Amplification of Nanosecond and Femtosecond Pulses

Coherent stacking of several pulses into a single output pulse using Gires-Tourmois Interferometer reflecting resonators is demonstrated, enabling a new technique for achieving high energy pulses from short-pulse and ultrashort-pulse fiber amplifier systems.

Characterization of single-mode performance of Chirally-Coupled-Core fibers with cores larger than 50μm

We demonstrate robust single-spatial mode performance in fabricated Ge-doped 50μm -60μm core Chirally-Coupled-Core fibers using spatially and spectrally resolved (S²) measurements.