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

Buffer gas enhancing of power conversion efficiency of a continuous-wave acetylene-filled fiber gas laser at the 3 μm wavelength

In this paper, we report the use of ammonia as the buffer gas in the acetylene-filled hollow-core fiber gas laser (A-HCFGL) in which the power conversion efficiency of continuous-wave (CW) operation reaches a record of 35.74%, the highest reported so far to the best of our knowledge. The intermolecular collision with ammonia assists the depopulation of lower laser levels of acetylene, which otherwise relies on the non-radiation relaxation by collision with the fiber core only. About 3.9 W CW laser output power is achieved at the 3.1 μm wavelength. A numerical model illustrates the buffer gas enhancement effect and explore the optimization of A-HCFGL for high output power and slope efficiency.

4.8-μm CO-filled hollow-core silica fiber light source

Mid-infrared (MIR) fiber lasers are important for a wide range of applications in sensing, spectroscopy, imaging, defense, and security. Some progress has been made in the research of MIR fiber lasers based on soft glass fibers, however, the emission range of rare-earth ions and the robustness of the host materials are still a major challenge for MIR fiber lasers. The large number of gases provide a variety of optical transitions in the MIR band. When combined with recent advances in low-loss hollow-core fiber (HCF), there is a great opportunity for gas-filled fiber lasers to further extend the radiation to the MIR region. Here, a 4.8-μm CO-filled silica-based HCF laser is reported for the first time. This is enabled by an in-house manufactured broadband low-loss HCF with a measured loss of 1.81 dB/m at 4.8 μm. A maximum MIR output power of 46 mW and a tuning range of 180 nm (from 4644 to 4824 nm) are obtained by using an advanced 2.33-μm narrow-linewidth fiber laser. This demonstration represents the longest-wavelength silica-based fiber laser to date, while the absorption loss of bulk silica at 4824 nm is up to 13, 000 dB/m. Further wavelength expansion could be achieved by changing the pump absorption line and optimizing the laser structure.

High-power hollow-core fiber gas laser at 3.1 µm with a linear-cavity structure

Mid-infrared hollow-core fiber (HCF) gas lasers based on a population inversion regime of gas molecules have made advanced development in recent years, but mostly with single-pass cavity-free structures. Here, we demonstrated a 3.1 µm high-power acetylene-filled HCF continuous wave (CW) laser and a self-Q-switched pulse laser with a linear-cavity structure. This configuration not only facilitates the transformation of amplified spontaneous emission into the laser output but also enhances the coherence of the light source and imparts distinct cavity mode characteristics. Harnessing a homemade high-power 1535 nm single-frequency fiber laser that served as the pump source, a CW laser output of 8.23 W at 3.1 µm was achieved, which is over three orders of magnitude higher than those in reported works so far. The corresponding slope efficiency of 31.8% and beam quality of Mx 2 = 1.18 and My 2 = 1.15 were characterized. When the gas pressure was up to 50 mbar, the laser generated a 3.1 µm self-Q-switched pulse with an output power of 1.98 W as well as a pulse width of 45 ns under the repetition rate of 4.59 MHz. To the best of our knowledge, this is the first time that an HCF gas laser achieves a self-Q-switched pulse. Future studies will aim to further optimize the experimental setup, potentially enabling the direct generation of picosecond pulses in the mid-infrared wavelength band.

21.8 W acetylene-filled hollow-core anti-resonant fiberamplified spontaneous emission source at 3.1 µm

We report a 20-W-level acetylene-filled nested hollow-core anti-resonant fiber (nested HC-ARF) amplified spontaneous emission (ASE) source at 3.1 µm. A 1535 nm hundred-watt wavelength tunable single-frequency fiber laser with a high signal-to-noise ratio and narrow linewidth is built for pumping acetylene molecules. Simultaneously, a homemade 120 µm core diameter eight-tube nested HC-ARF is used as a gas chamber to obtain high pump laser coupling efficiency. The mid-infrared (mid-IR) ASE source output power of 21.8 W is achieved at 3.1 µm through the low-pressure acetylene gas-filled nested HC-ARF, and the slope efficiency is 25.1%. In addition, the ASE source features an excellent beam quality of Mx 2 = 1.16 and My 2 = 1.13. To the best of our knowledge, for the first time, it is a record output power for such mid-infrared ASE sources while maintaining excellent beam quality. This work provides a new way to achieve high-power mid-infrared emission.

4.3 µm high-power amplified spontaneous emission fiber source based on CO2-filled nested hollow-core anti-resonant fiber

We report a 4.3 µm mid-infrared (mid-IR) high-power amplified spontaneous emission (ASE) fiber source based on CO2-filled nested hollow-core anti-resonant fiber (Nested HC-ARF). The pump source is a homemade hundred-watt-level wavelength-tunable 2 µm single-frequency fiber laser. A 5.7 m long 8-tube Nested HC-ARF is used as the gas cell, with a core diameter of 110 µm and cladding diameter of 400 µm, which exhibits transmission loss of 0.1 dB/m at 2 µm and 0.24 dB/m at 4.3 µm respectively. To improve the coupling efficiency of the high-power pump laser and reduce the influence of the thermal effect at the input end of the hollow-core fiber, the fiber is designed for multimode transmission at the pump wavelength. A continuous wave output power of 6.6 W at 4.3 µm is achieved, and the slope efficiency is 17.05%. To the best of our knowledge, it is the highest output power for such gas-filled HC-ARF ASE sources in 4∼5 µm. This work demonstrates the great potential of gas-filled HC-ARF generating high-power mid-IR emission.

Direct performance comparison of antiresonant and Kagome hollow-core fibers in mid-IR wavelength modulation spectroscopy of ethane

In this paper, we experimentally asses the performance of wavelength modulation spectroscopy-based spectrometers incorporating 1.3 m-long gas absorption cells formed by an antiresonant hollow core fiber (ARHCF) and a Kagome hollow core fiber. To evaluate the discrepancies with minimum methodology error, the sensor setup was designed to test both fibers simultaneously, providing comparable measurement conditions. Ethane (C2H6) with a transition located at 2996.88 cm-1 was chosen as the target gas. The experiments showed, that due to better light guidance properties, the ARHCF-based sensor reached a minimum detection limit of 4 ppbv for 85 s integration time, which is more than two times improvement in comparison to the result obtained with the Kagome fiber.

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.

3.1 W mid-infrared fiber laser at 4.16 µm based on HBr-filled hollow-core silica fibers

We present the characteristics of a continuous-wave (CW) mid-infrared fiber laser source based on HBr-filled hollow-core fibers (HCFs) made of silica. The laser source delivers a maximum output power of 3.1 W at 4.16 µm, showing a record value for any reported fiber laser beyond 4 µm. Both ends of the HCF are supported and sealed by especially designed gas cells with water cooling and inclined optical windows, withstanding higher pump power accompanied by accumulated heat. The mid-infrared laser exhibits a near-diffraction-limited beam quality with a measured M² of 1.16. This work paves the way for powerful mid-infrared fiber lasers beyond 4 µm.

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.