R. Andrew Sims

High-power widely tunable thulium fiber lasers

Applications requiring long-range atmospheric propagation are driving the development of high-power thulium fiber lasers. We report on the performance of two different laser configurations for high-power tunable thulium fiber lasers: one is a single oscillator utilizing a volume Bragg grating for wavelength stabilization; the other is a master oscillator power amplifier system with the oscillator stabilized and made tunable by a diffraction grating. Each configuration provides >150 W of average power, >50% slope efficiency, narrow output linewidth, and >100 nm tunability in the wavelength range around 2 μm.

1 μJ, sub-500 fs chirped pulse amplification in a Tm-doped fiber system

We demonstrate a Tm-doped fiber laser system producing ~300 fs pulses with 1 μJ energy, corresponding to peak powers greater than 3 MW. Pulses of 150 fs with 30 nm spectral bandwidth and 3 nJ pulse energy are generated in a Raman-soliton self-frequency shift amplifier, then stretched to ~160 ps using a chirped Bragg grating. The 60 MHz oscillator repetition rate is reduced to 100 kHz using an electro-optic modulator. After a single-mode fiber preamplifier and a large-mode-area fiber power amplifier, pulses were compressed using a folded Treacy grating setup to below 500 fs with up to 1 μJ pulse energy. To the best of our knowledge, this is the highest energy yet demonstrated as well as the first demonstration of peak powers exceeding 1 MW from a Tm:fiber laser system.

Lasing in thulium-doped polarizing photonic crystal fiber

We describe lasing of a thulium-doped polarizing photonic crystal fiber. A 4 m long fiber with 50 μm diameter core, 250 μm diameter cladding, and d/Λ ratio of 0.18 was pumped with a 793 nm diode and produced a polarized output with a polarization extinction ratio (PER) of 15 dB and an M(2) of <1.15. An intracavity polarizer and half-wave plate minimally increased the PER to 16 dB. The output power had 35% slope efficiency relative to the absorbed pump power. The maximum cw output power was limited to 4 W due to the quantum defect heating of the fiber.

Q-switched thulium-doped photonic crystal fiber laser

We report a Q-switched thulium doped polarizing photonic crystal fiber producing 435 μJ, 49 ns pulses at 10 kHz. The 8.9 kW peak power is the highest of any nanosecond Tm:fiber oscillator published to date.

Comparison of higher-order mode suppression and Q-switched laser performance in thulium-doped large mode area and photonic crystal fibers

We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at ~2 μm wavelength. S(2) measurements confirm guiding of LP(11) and LP(02) fiber modes in a large mode area (LMA) step-index fiber, whereas a prototype photonic crystal fiber (PCF) provides nearly single-mode performance with a small portion of light in the LP(11) mode. The difference in HOM content leads to a significant difference in Q-switched oscillator performance. In the step-index fiber, the percentage of cladding light increases by 20% to >40% with increasing pulse energy to ~250 µJ. We accredit this degradation to saturation of the gain in the fundamental mode leading to more light generated in the HOMs, which is eventually converted into cladding light. No such degradation is seen in PCF laser system for >400 µJ energies.

Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power

We report on a Tm:fiber master oscillator power amplifier (MOPA) system producing 109 W CW output power, with >15 dB polarization extinction ratio, sub-nm spectral linewidth, and M2 <1.25. The system consists of polarization maintaining (PM) fiber and PM-fiber components including tapered fiber bundle pump combiners, a single-mode to large mode area mode field adapter, and a fiber-coupled isolator. The laser components ultimately determine the system architecture and the limits of laser performance, particularly considering the immature and rapidly developing state of fiber components in the 2 μm wavelength regime.

Spectral narrowing and stabilization of thulium fiber lasers using guided-mode resonance filters

We used guided-mode resonance filters (GMRFs), fabricated using thin-film deposition and chemical etching, as intracavity feedback elements to stabilize and narrow the output spectrum in thulium-doped fiber oscillators operating in the 2 μm wavelength regime, producing linewidths of <700 pm up to 10 W power levels. A Tm fiber-based amplified spontaneous emission source was used to characterize the reflective properties of the GMRFs. Linewidths of 500 pm and a 7.3 dB reduction in transmission were measured on resonances.

High-power spectral beam combining of linearly polarized Tm:fiber lasers

To date, high-power scaling of Tm:fiber lasers has been accomplished by maximizing the power from a single fiber aperture. In this work, we investigate power scaling by spectral beam combination of three linearly polarized Tm:fiber MOPA lasers using dielectric mirrors with a steep transition from highly reflective to highly transmissive that enable a minimum wavelength separation of 6 nm between individual laser channels within the wavelength range from 2030 to 2050 nm. Maximum output power is 253 W with M(2)<2, ultimately limited by thermal lensing in the beam combining elements.

Lasing in Thulium doped Polarizing Photonic Crystal Fibers (PCF)

We describe lasing in polarizing thulium doped PCF fiber for the first time. The ~4 m long fiber had 50/250 μm core/cladding diameters and hole-diameter to pitch ratio of 0.18. CW lasing was achieved by end pumping with a 793 nm diode in an oscillator configuration. Slope efficiencies of ~35 % have been obtained with single mode beam quality (M2 <1.15). The polarization extinction ratio (PER) was >13 dB without any intra-cavity polarizing elements. This fiber, with MFD of ~36 μm and ~5.8 dB/m cladding pump absorption, is an attractive option for high energy pulsed amplifiers in the 2 μm wavelength regime.

Atmospheric propagation testing with a high power, tunable thulium fiber laser system

A tunable master oscillator power amplifier (MOPA) fiber laser system based on thulium doped silica fiber designed for investigation of multi-kilometer propagation through atmospheric transmission windows existing from ~2030 nm to ~2050 nm and from ~2080 nm to beyond 2100 nm is demonstrated. The system includes a master oscillator tunable over >200 nm of bandwidth from 1902 nm to beyond 2106 nm producing up to 10 W of linearly polarized, stable, narrow linewidth output power with near diffraction limited beam quality. Output from the seed laser is amplified in a power amplifier stage designed for operation at up to 200 W CW over a tuning range from 1927 - 2097 nm. Initial field tests of this system at the Innovative Science & Technology Experimental Facility (ISTEF) laser range on Cape Canaveral Air Force Station, Florida will be discussed. Results presented will include investigation of transmission versus wavelength both in and out of atmospheric windows, at a variety of distances. Investigations of beam quality degradation at ranges up to 1 km at a variety of wavelengths both in and out of atmospheric transmission windows will be also presented. Available theoretical models of atmospheric transmission are compared to the experimental results.