CW hollow-core optically pumped I₂ fiber gas laser

Fabrication and characterization of iodine photonic microcells for sub-Doppler spectroscopy applications

We report on the development of all-fiber stand-alone iodine-filled photonic microcells demonstrating record absorption contrast at room temperature. The microcell's fiber is made of inhibited coupling guiding hollow-core photonic crystal fibers. The fiber-core loading with iodine was undertaken at 10^-1-10^-2mbar vapor pressure using what, to the best of our knowledge, is a novel gas-manifold based on metallic vacuum parts with ceramic coated inner surfaces for corrosion resistance. The fiber is then sealed on the tips and mounted on FC/APC connectors for better integration with standard fiber components. The stand-alone microcells display Doppler lines with contrasts up to 73% in the 633 nm wavelength range, and an off-resonance insertion loss between 3 to 4 dB. Sub-Doppler spectroscopy based on saturable absorption has been carried out to resolve the hyperfine structure of the P(33)6-3 lines at room temperature with a full-width at half maximum of 24 MHz on the b4 component with the help of lock-in amplification. Also, we demonstrate distinguishable hyperfine components on the R(39)6-3 line at room temperature without any recourse to signal-to-noise ratio amplification techniques.

Numerical simulation and observed rotational relaxation in CW and pulsed HBr-filled hollow-core fiber lasers

Gas-filled hollow-core fiber (HCF) lasers have emerged as a promising technology for generating mid-infrared lasers. A four-energy level system laser model is presented to predict the performance of optically pumped HBr-filled HCF lasers under continuous wave (CW) and pulsed excitations. The steady state condition is considered in CW pumping and the characteristics of simulated population density and power distribution along HCF are investigated. The finite-difference time-domain method is employed in pulsed pumping and the simulated evolutions of pump pulse and laser pulse at different positions along the HCF are studied. In addition, the phenomena of rotational relaxation in HBr-filled HCF lasers are investigated experimentally for the first time, to the best of our knowledge, showing that using the absorption lines away from the strongest absorption lines and tuning the pump wavelength deviating from the center of the absorption line makes the rotational relaxation occur easily. The demonstration is conductive to reveal the underlying mechanism of such gas-filled HCF lasers.

Mid-infrared fiber gas amplifier in acetylene-filled hollow-core fiber

We report here the first, to the best of our knowledge, demonstration of a mid-infrared (mid-IR) fiber gas amplifier based on acetylene-filled hollow-core fibers. A quasi-all-fiber structure fiber acetylene laser in a single-pass configuration is used as a seed. The injection of the seed removes the threshold and increases the laser efficiency, which are more pronounced at high pressure. In a 3.1-m HCF filled with 2.5 mbar of acetylene, the fiber gas amplifier shows a conversion efficiency (relative to the coupled pump power) of 22.2% at 3.1 µm, which is increased by 35% compared with that without the seed. Both the seed laser and the amplifier laser have good beam quality with M² < 1.1. It is predictable that such a fiber gas amplifier can achieve a more efficient and higher power mid-IR output for other selected molecular species compared with the single-pass structure, which is beneficial to the development of high-power mid-IR fiber gas lasers.

Mid IR hollow core fiber gas laser emitting at 4.6 μm

Emission at 4.6 μm was observed from an N₂O filled hollow core fiber laser. 8-ns pump pulses at 1.517 μm excited a vibrational overtone resulting in lasing on an R and P branch fundamental transition from the upper pump state. At optimum gas pressure of 80 Torr, photon conversion efficiency of 9% and slope efficiency of 3% were observed from a mirrorless laser. The laser threshold occurred at absorbed pump energy of 150 nJ in a 45-cm long fiber with 85 μm core diameter. The observed dependence of the laser output on gas pressure was shown to be a result of line broadening and relaxation rates.

Efficient mid-infrared cascade Raman source in methane-filled hollow-core fibers operating at 2.8 μm

We report here for the first time, to the best of our knowledge, a novel and efficient cascade Raman laser source operating at 2.8 μm by two stages of methane-filled hollow-core fibers (HCFs). In the first stage, a commercial 1064.6 nm laser is used as the pump source, and an efficient first-order Stokes wave of 1543.9 nm is obtained with a quantum conversion efficiency of ∼87% in 2 m ice-cream HCF filled with 2 bar methane gas. In the second stage, efficient 2.8 μm laser emission is also generated by the first-order stimulated Raman scattering of methane, while the pump source is the Stokes wave at 1543.9 nm. A maximum quantum conversion efficiency of ∼75% is obtained with 2.2 m node-less HCF filled with 11 bar methane gas, resulting in a record total quantum efficiency of ∼65%, which is 1.6 times the previous similar result. This work provides a significant efficient method to obtain a wide wavelength range of mid-infrared, even far-infrared fiber laser sources from conveniently available 1 μm band lasers with proper HCFs and different active gases.

High-power tunable mid-infrared fiber gas laser source by acetylene-filled hollow-core fibers

High-power tunable pulsed and CW mid-infrared fiber gas laser sources in acetylene-filled hollow-core fibers, to the best of our knowledge, are demonstrated for the first time. By precisely tuning the wavelength of the pump source, an amplified tunable 1.5 μm diode laser, to match different absorption lines of acetylene, the laser output is step-tunable in the range of 3.09~3.21 μm with a maximum pulse average power of ~0.3 W (~0.6 μJ pulse energy) and a maximum CW power of ~0.77 W, making this system the first watt-level tunable fiber gas laser operating at mid-infrared range. The output spectral and power characteristics are systemically studied, and the explanations about the change of the ratio of the P over R branch emission lines with the pump power and the gas pressure are given, which is useful for the investigations of mid-infrared fiber gas lasers.

Numerical investigation of pulsed gas amplifiers operating in hollow-core optical fibers

Optically pumped molecular gas amplifiers having a gain medium contained in a hollow-core optical fiber are investigated with numerical modeling to understand the primary physical processes that affect amplifier output and efficiency. A comparison of computational results with experimental measurements of incident pump, absorbed pump, and emitted mid-IR from a pulsed, acetylene-filled, hollow-core fiber amplifier [ Opt. Exp.25, 13351 (2017)] is used to explore the effects of various physical processes on pulsed amplifier operation. Single frequency, one-dimensional, time-dependent models are shown to align with experimentally measured lasing thresholds and ratios of absorbed pump to emitted laser energy but significantly over predict the amount of incident pump energy that is absorbed. A two-dimensional, time-dependent model that assumes Gaussian spectral and radial intensity profiles for the pump is developed and shows an improved ability to capture pump absorption. Results indicate that 1D, time-dependent models have utility in guiding experiments but the significant influence of the pump and laser propagation modes and the pump spectral characteristics on efficiency, threshold, and signal output must be explicitly included in high-fidelity high-power modeling.

0.83 W, single-pass, 1.54 μm gas Raman source generated in a CH4-filled hollow-core fiber operating at atmospheric pressure

We report here the first watt-level efficient single-pass 1.54 μm fiber gas Raman source by methane-filled hollow-core fiber operating at atmospheric pressure. Pumped with a high-power MOPA (master oscillator power amplifier) structure Q-switched 1.06 μm pulsed solid-state laser, efficient 1.54 μm Stokes wave is generated in a single-pass configuration by vibrational stimulated Raman scattering of methane molecules. With an experimentally optimized fiber length of 3.2 m, we get a 1543.9 nm Stokes wave operating at atmospheric pressure with a record average power of ~0.83 W, which is about 12 times higher than the similar experiment previously reported, and the corresponding power conversion efficiency is about 45%. Operating at atmospheric pressure makes it more convenient in future applications.

High photon conversion efficiency continuous wave lasing in an optically pumped I2 hollow fiber gas laser in the visible region

Continuous wave lasing in the visible spectral region from a molecular iodine-filled hollow core photonic crystal fiber is demonstrated. More than an order of magnitude improvement in photon conversion efficiency has been achieved compared to previous nonfiber-based geometries in this spectral region. The laser shows strong coupling of pump and laser polarization.

Mid-infrared 1 W hollow-core fiber gas laser source

We report the characteristics of a 1 W hollow-core fiber gas laser emitting CW in the mid-IR. Our system is based on an acetylene-filled hollow-core optical fiber guiding with low losses at both the pump and laser wavelengths and operating in the single-pass amplified spontaneous emission regime. Through systematic characterization of the pump absorption and output power dependence on gas pressure, fiber length, and pump intensity, we determine that the reduction of pump absorption at high pump flux and the degradation of gain performance at high gas pressure necessitate the use of increased gain fiber length for efficient lasing at higher powers. Low fiber attenuation is therefore key to efficient high-power laser operation. We demonstrate 1.1 W output power at a 3.1 μm wavelength by using a high-power erbium-doped fiber amplifier pump in a single-pass configuration, approximately 400 times higher CW output power than in the ring cavity previously reported.

Near diffraction-limited performance of an OPA pumped acetylene-filled hollow-core fiber laser in the mid-IR

We investigate the mid-IR laser beam characteristics from an acetylene-filled hollow-core optical fiber gas laser (HOFGLAS) system. The laser exhibits near-diffraction limited beam quality in the 3 μm region with M² = 1.15 ± 0.02 measured at high pulse energy, and the highest mid-IR pulse energy from a HOFGLAS system of 1.4 μJ is reported. Furthermore, the effects of output saturation with pump pulse energy are reduced through the use of longer fibers with low loss. Finally, the slope efficiency is shown to be nearly independent of gas pressure over a wide range, which is encouraging for further output power increase.