Michael J. Freeman

Bio: Michael J. Freeman is an academic researcher from University of York. The author has contributed to research in topics: Supercontinuum & Laser. The author has an hindex of 26, co-authored 66 publications receiving 2023 citations. Previous affiliations of Michael J. Freeman include University of Michigan.

Mid-infrared supercontinuum generation to 4.5 μm in ZBLAN fluoride fibers by nanosecond diode pumping

Abstract: A mid-infrared supercontinuum (SC) is generated in ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF...) fluoride fibers from amplified nanosecond laser diode pulses with a continuous spectrum from approximately 0.8 microm to beyond 4.5 microm. The SC has an average power of approximately 23 mW, a pump-to-SC power conversion efficiency exceeding 50%, and a spectral power density of approximately -20 dBm/nm over a large fraction of the spectrum. The SC generation is initiated by the breakup of nanosecond laser diode pulses into femtosecond pulses through modulation instability, and the spectrum is then broadened primarily through fiber nonlinearities in approximately 2-7 m lengths of ZBLAN fiber. The SC long-wavelength edge is consistent with the intrinsic ZBLAN material absorption.

Supercontinuum generation from ~19 to 45 μmin ZBLAN fiber with high average power generation beyond 38 μm using a thulium-doped fiber amplifier

Abstract: A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from ∼1.9 to 4.5 μm is generated with ∼0.7 W time-average power in wavelengths beyond 3.8 μm. The laser outputs a total average power of up to ∼2.6 W from ∼8.5 m length of ZrF4─BaF2─LaF3─AlF3─NaF (ZBLAN) fiber, with an optical conversion efficiency of ∼9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 μm is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 μm by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 μm laser diode pulses with ∼2.5 kW peak power generate a SC extending beyond 2.1 μm in ∼25 m length of standard single-mode fiber (SMF). The ∼2 μm wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to ∼270 nm SC long wavelength edge extension and ∼2.5× higher optical conversion efficiency to wavelengths beyond 3.8 μm are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrodinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN.

10.5 W Time-Averaged Power Mid-IR Supercontinuum Generation Extending Beyond 4 $\mu$ m With Direct Pulse Pattern Modulation

Abstract: A novel, all-fiber-integrated supercontinuum (SC) laser is demonstrated and provides up to 10.5 W time-averaged power with a continuous spectrum from ~0.8 to 4 mum. The SC is generated in a combination of standard single-mode fibers and ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fibers pumped by a laser-diode-based cladding-pumped fiber amplifier system. The output SC pulse pattern can be modulated by directly modulating the seed laser diode. Near-diffraction-limited beam qualities are maintained over the entire SC spectrum. The SC average power is also linearly scalable by varying the input pump power and pulse repetition rate. We further investigate the theoretical limitations on the achievable average power handling and spectral width for the SC generation in ZBLAN fibers. Based on the thermal modeling, the standard ZBLAN fiber can handle a time-averaged power up to ~15 W, which can be further scaled up to ~40 W with a proper thermal coating applied onto the ZBLAN fiber. The SC long-wavelength edge is limited by the nonlinear wavelength generation processes, fiber bend-induced loss, and glass material loss. By using a ZBLAN fiber with a 0.3 numerical aperture, the SC spectrum could extend out to ~4.5 mum, which is then limited by the material loss.

Stand-off detection of solid targets with diffuse reflection spectroscopy using a high-power mid-infrared supercontinuum source

Abstract: We measure the diffuse reflection spectrum of solid samples such as explosives (TNT, RDX, PETN), fertilizers (ammonium nitrate, urea), and paints (automotive and military grade) at a stand-off distance of 5 m using a mid-infrared supercontinuum light source with 3.9 W average output power. The output spectrum extends from 750–4300 nm, and it is generated by nonlinear spectral broadening in a 9 m long fluoride fiber pumped by high peak power pulses from a dual-stage erbium-ytterbium fiber amplifier operating at 1543 nm. The samples are distinguished using unique spectral signatures that are attributed to the molecular vibrations of the constituents. Signal-to-noise ratio (SNR) calculations demonstrate the feasibility of increasing the stand-off distance from 5 to ∼150 m, with a corresponding drop in SNR from 28 to 10 dB.

Chirped period gratings for raman amplification in circulator loop cavities

Abstract: The present invention provides a structure for Raman amplification of signals with counter-propagation of signal (s) and pump (p) and wavelength control while permitting broad bandwidth within each Raman order. The broadband optical amplifier of the invention combines Raman amplification with a circulator loop cavity (10, 20) and/or chirped Bragg gratings (70) to achieve bandwidth performance improvements. The beneficial properties of the circulator loop cavity (10, 20) and/or chirped Bragg gratings (70) can also be combined with noise dampening property of the Sagnac Raman cavity.

Towards high-power mid-infrared emission from a fibre laser

Abstract: Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Mid-infrared supercontinuum covering the 1.4–13.3 μm molecular fingerprint region using ultra-high NA chalcogenide step-index fibre

Abstract: Mid-infrared supercontinuum generation with a record-breaking spectral coverage of 1.4–13.3 µm is demonstrated by launching intense ultra-short pulses into short pieces of ultra-high numerical aperture step-index chalcogenide glass optical fibre consisting of a GaAsSe cladding and an As2Se3 core.

Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs.

Abstract: We report broad bandwidth, mid-IR supercontinuum generation using a sub-cm (8 mm) length of highly nonlinear tellurite microstructured photonic crystal fiber (PCF). We pump the fiber at telecommunication wavelengths by using 1550 nm, 100 fs pulses of energy E=1.9 nJ. When coupled in the PCF, these pulses result in a supercontinuum (SC) bandwidth of 4080 nm extending from 789 to 4870 nm measured at 20 dBm below the peak spectral power. This bandwidth is comparable or in excess of previously reported spectra for other nonlinear glass fiber formulations despite the significantly shorter fiber length. In addition, besides offering a convenient pump wavelength, short fiber lengths enable smoother SC spectra, lower dispersion, and reduced material absorption at longer wavelengths making the use of this PCF particularly interesting.

Ten years of nonlinear optics in photonic crystal fibre

Abstract: The year 2009 marks the tenth anniversary of the first report of white-light supercontinuum generation in photonic crystal fibre. This result had a tremendous impact on the field of nonlinear fibre optics and continues to open up new horizons in photonic science. Here we provide a concise and critical summary of the current state of nonlinear optics in photonic crystal fibre, identifying some of the most important and interesting recent developments in the field. We also discuss several emerging research directions and point out links with other areas of physics that are now becoming apparent.