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

Raman amplifier based on stimulated Raman scattering in a methane-filled hollow core fiber

This article reports on a single pass amplifier based on stimulated Raman scattering in a methane-filled negative curvature hollow core fiber (HCF) to transition 1.06 μm power to 1.54 μm. The researchers measured the highest average Raman power at a single frequency in a methane filled HCF to date of 4.92 W (246 μJ/pulse), with a high average quantum efficiency of 95.9%. A numerical model for the system was developed and shows good agreement with measured thresholds and efficiencies. Model results from a trade space study indicate configuration regimes necessary to maximize 1.54 μm power while avoiding power loss from the secondary shift.

High power mid-infrared fiber amplifier at 3.1 µm by acetylene-filled hollow-core fibers

We characterized high-power continuous-wave (CW) and pulsed mid-infrared (mid-IR) fiber amplifiers at a wavelength of 3.1 µm in acetylene-filled hollow-core fibers (HCFs) with a homemade seed laser. A maximum CW power of 7.9 W was achieved in a 4.2-m HCF filled with 4-mbar acetylene, which was 11% higher than the power without the seed. The maximum average power of the pulsed laser was 8.6 W (pulse energy of 0.86 µJ) at 7-mbar acetylene pressure, a 16% increase over the power without the seed. To the best of our knowledge, backward characteristics are reported for the first time for fiber gas lasers, and the backward power accounted for less than 5% of the forward power. The optimum acetylene pressure and HCF length for the highest mid-IR output are discussed based on theoretical simulations. This study provides significant guidance for high-power mid-infrared (mid-IR) output in gas-filled HCFs.

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.

Fiber laser source of 8 W at 3.1 µm based on acetylene-filled hollow-core silica fibers

We report here the characteristics of a nanosecond high-power mid-infrared (mid-IR) light source based on an anti-resonant hollow-core fiber (AR-HCF) filled with acetylene gas. It is a single-pass configuration with 9.3-m HCFs, pumped by a modulated and amplified diode laser. A maximum average power of approximately 8 W (pulse energy of ∼0.8 µJ and peak power of ∼40 W) at 3.1 µm is achieved with a laser slope efficiency of ∼22.8% at 6 mbar of acetylene, which is, to the best of our knowledge, a record output power for such mid-IR HCF lasers. This work demonstrates the great potential of fiber gas lasers for high-power output in the mid-IR.

Towards high-power mid-IR light source tunable from 3.8 to 4.5 µm by HBr-filled hollow-core silica fibres

Fibre lasers operating at the mid-IR have attracted enormous interest due to the plethora of applications in defence, security, medicine, and so on. However, no continuous-wave (CW) fibre lasers beyond 4 μm based on rare-earth-doped fibres have been demonstrated thus far. Here, we report efficient mid-IR laser emission from HBr-filled silica hollow-core fibres (HCFs) for the first time. By pumping with a self-developed thulium-doped fibre amplifier seeded by several diode lasers over the range of 1940-1983 nm, narrow linewidth mid-IR emission from 3810 to 4496 nm has been achieved with a maximum laser power of about 500 mW and a slope efficiency of approximately 18%. To the best of our knowledge, the wavelength of 4496 nm with strong absorption in silica-based fibres is the longest emission wavelength from a CW fibre laser, and the span of 686 nm is also the largest tuning range achieved to date for any CW fibre laser. By further reducing the HCF transmission loss, increasing the pump power, improving the coupling efficiency, and optimizing the fibre length together with the pressure, the laser efficiency and output power are expected to increase significantly. This work opens new opportunities for broadly tunable high-power mid-IR fibre lasers, especially beyond 4 μm.

Mid-infrared photoacoustic gas monitoring driven by a gas-filled hollow-core fiber laser

Development of novel mid-infrared (MIR) lasers could ultimately boost emerging detection technologies towards innovative spectroscopic and imaging solutions. Photoacoustic (PA) modality has been heralded for years as one of the most powerful detection tools enabling high signal-to-noise ratio analysis. Here, we demonstrate a novel, compact and sensitive MIR-PA system for carbon dioxide (CO₂) monitoring at its strongest absorption band by combining a gas-filled fiber laser and PA technology. Specifically, the PA signals were excited by a custom-made hydrogen (H₂) based MIR Raman fiber laser source with a pulse energy of ⁓ 18 μJ, quantum efficiency of ⁓ 80% and peak power of ⁓ 3.9 kW. A CO₂ detection limit of 605 ppbv was attained from the Allan deviation. This work constitutes an alternative method for advanced high-sensitivity gas detection.

High pulse energy and quantum efficiency mid-infrared gas Raman fiber laser targeting CO2 absorption at 4.2 µm

In this Letter, we demonstrate a high pulse energy and linearly polarized mid-infrared Raman fiber laser targeting the strongest absorption line of ${\rm CO}_2$CO₂ at $\sim{4.2}\;\unicode {x00B5} {\rm m}$∼4.2µm. This laser was generated from a hydrogen (${\rm H}_2$H₂)-filled antiresonant hollow-core fiber, pumped by a custom-made 1532.8 nm Er-doped fiber laser delivering 6.9 ns pulses and 11.6 kW peak power. A quantum efficiency as high as 74% was achieved, to yield 17.6 µJ pulse energy at 4.22 µm. Less than 20 bar ${\rm H}_2$H₂ pressure was required to maximize the pulse energy since the transient Raman regime was efficiently suppressed by the long pump pulses.

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.

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.