Efficient coupling of a quantum cascade laser to a few-mode chalcogenide fiber: erratum.

The authors wish to acknowledge an additional funding source which was absent from our manuscript.

Efficient coupling of a quantum cascade laser to a few-mode chalcogenide fiber.

We report efficient coupling of a quantum cascade laser (QCL) into step-index chalcogenide fibers (As 2S 3). Mechanically robust and low-loss chalcogenide fibers were fabricated using a hybrid, multi-material thermal drawing process. With suitable free-space optics, more than 160 mW of optical power was coupled into the fiber with predominantly single-mode excitation. Antireflection coatings on the fiber facets enabled 88.9% transmission with strong core confinement. By accurately tailoring the core diameter and antireflection-coating thickness, these fibers offer a versatile platform for high-power and low-loss transmission across the infrared spectrum. This work introduces an attractive alternative to the use of hollow-core fibers or multimode solid-core fibers for diffraction-limited infrared beam delivery.

Divided-pulse amplification to the joule level.

Divided-pulse amplification (DPA) has proven to be a valuable tool in scaling the peak power of diode-pumped ytterbium-doped amplifiers to beyond the single-pulse threshold for parasitic nonlinear effects. DPA enables the amplification of picosecond pulses in solid-state amplifiers with limited bandwidth beyond the single-pulse damage threshold. In this Letter, we demonstrate DPA of picosecond pulses in a flashlamp-pumped Nd:YAG amplifier for the first time, to the best of our knowledge, yielding a combined pulse energy of 167 mJ.

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.

High peak-power mid-infrared ZnGeP2 optical parametric oscillator pumped by a Tm:fiber master oscillator power amplifier system.

We report on the utilization of a novel Tm:fiber laser source for mid-IR ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumping. The pump laser is built in a master oscillator power-amplifier configuration delivering up to 3.36 W of polarized, diffraction limited output power with 7 ns pulse duration and 4 kHz repetition rate. This corresponds to a peak power of ∼121 kW and a pulse energy of ∼0.84 mJ. With this source, we generated 27.9 kW of total mid-IR peak power in a doubly resonant oscillator (DRO) configuration. This is, to the best of our knowledge, the highest ever demonstrated mid-IR peak power from a directly Tm:fiber laser pumped ZGP OPO. Moreover, a DRO output with about 284 µJ of total mid-IR pulse energy was demonstrated using 100 ns pump pulses. The wavelength tuning of the idler was extended to 6 µm with lower output power in another OPO experiment.

Highly polarized all-fiber thulium laser with femtosecond-laser-written fiber Bragg gratings.

We demonstrate and characterize a highly linearly polarized (18.8 dB) narrow spectral emission (<80 pm) from an all-fiber Tm laser utilizing femtosecond-laser-written fiber Bragg gratings. Thermally-dependent anisotropic birefringence is observed in the FBG transmission, the effects of which enable both the generation and elimination of highly linearly polarized output. To our knowledge, this is the first detailed study of such thermal anisotropic birefringence in femtosecond-written FBGs.

Amplification of nanosecond pulses to megawatt peak power levels in Tm3+-doped photonic crystal fiber rod.

We report amplification of sub-10-100 ns pulses with repetition rates from 1 to 20 kHz in a rod-type thulium-doped photonic crystal fiber with 80 µm core diameter. The rod is pumped with a 793 nm laser diode and produces the highest peak power at 1 kHz repetition rate with 6.5 ns pulse duration and more than 7 W average output power. This result exemplifies the potential of this fiber design to scale pulse peak powers and pulse energies to the megawatt and multi-millijoule range in the 2 µm wavelength regime.

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.

CW-lasing and amplification in Tm(3+)-doped photonic crystal fiber rod.

We report lasing and amplification in a rod type thulium-doped photonic crystal fiber with 80 µm core diameter. The rod is pumped with a 793 nm laser diode and produces more than 20 W output power at a beam quality M(2)<1.3. The laser/amplifier has a slope efficiency of 27.8%/20.1% relative to absorbed pump power with a lasing threshold at 28.6 W. The output wavelength in the lasing configuration can be tuned over 180 nm from 1810-1990 nm.

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.

Q-switched thulium-doped photonic crystal fiber laser.

We report a novel, Tm-doped photonic crystal fiber (PCF) actively Q-switched oscillator that provides ~8.9 kW peak power with 435 µJ, 49 ns pulses at 10 kHz repetition rate at 2 µm wavelength. This fiber has a mode-field area >1000 µm2, the largest of any flexible PCF providing diffraction-limited beam quality to the best of our knowledge. As an application, the oscillator is used as pump to generate >350 nm broadening in ~50 m of SMF-28 fiber.

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.

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 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.

Laser-induced breakdown spectroscopy of copper with a 2 microm thulium fiber laser.

We report the first implementation of a 2 microm thulium fiber laser in a Laser-Induced Breakdown Spectroscopy system. Emission from plasma on copper samples was analyzed from 200 to 900 nm. The low ablation fluence (<100 J.cm(-2)) and 200 ns pulse duration lead to a plasma with neither continuum emission, nor air emission in the near-infrared region.

Micromachining with a 50 W, 50 muJ, subpicosecond fiber laser system.

A 50 W sub-picosecond fiber chirped pulse amplification system generating 50 muJ pulses at a repetition rate of 1 MHz is demonstrated. As required for precision high speed micro-machining, this system has a practical system configuration enabled by the fiber stretcher and 1780 l/mm dielectric diffraction grating compressor and is capable of ablation rates >0.17 mm3/s metal, ceramic, and glass.

Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate.

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

High energy femtosecond Yb cubicon fiber amplifier.

The generation of cubicon pulses from an Yb fiber chirped pulse amplification system at pulse energies up to 200 microJ is demonstrated. After pulse compression 650 fs pulses with a pulse energy of 100 microJ are obtained, where pulse compression relies on the compensation of third-order dispersion mismatch between the stretcher and compressor via self-phase modulation of the cubicon pulses in the fiber amplifier. Values of self-phase modulation well in excess of pi can be tolerated for cubicon pulses, allowing for the nonlinear compensation of very large levels of dispersion mismatch between pulse stretcher and compressor.