Achieving a 1.5 μm fiber gas Raman laser source with about 400 kW of peak power and a 6.3 GHz linewidth

Synthesizing gas-filled anti-resonant hollow-core fiber Raman lines enables access to the molecular fingerprint region

The synthesis of multiple narrow optical spectral lines, precisely and independently tuned across the near- to mid-infrared region, is a pivotal research area that enables selective and real-time detection of trace gas species within complex gas mixtures. However, existing methods for developing such light sources suffer from limited flexibility and very low pulse energy, particularly in the mid-infrared domain. Here, we introduce a concept that is based on the combination of an appropriate design of near-infrared fiber laser pump and cascaded configuration of gas-filled anti-resonant hollow-core fiber technology. This concept enables the synthesis of multiple independently tunable spectral lines, with >1 μJ high pulse energies and a few nanoseconds pulse width in the near- and mid-infrared regions. The number and wavelengths of the generated spectral lines can be dynamically reconfigured. A proof-of-concept laser beam synthesized of two narrow spectral lines at 3.99 µm and 4.25 µm wavelengths is demonstrated and combined with photoacoustic modality for real-time SO2 and CO2 detection. The proposed concept also constitutes a promising way for infrared multispectral microscopic imaging.

Cascaded All-Fiber Gas Raman Laser Oscillator in Deuterium-Filled Hollow-Core Photonic Crystal Fibers

Hollow-core photonic crystal fibers (HC-PCFs) provide an ideal transmission medium and experimental platform for laser-matter interaction. Here, we report a cascaded all-fiber gas Raman laser based on deuterium (D2)-filled HC-PCFs. D2 is sealed into a gas cavity formed by a 49 m-long HC-PCF and solid-core fibers, and two homemade fiber Bragg gratings (FBGs) with the Raman and pump wavelength, respectively, are further introduced. When pumped by a pulsed fiber amplifier at 1540 nm, the pure rotational stimulated Raman scattering of D2 occurs inside the cavity. The first-order Raman laser at 1645 nm can be obtained, realizing a maximum power of ~0.8 W. An all-fiber cascaded gas Raman laser oscillator is achieved by adding another 1645 nm high-reflectivity FBG at the output end of the cavity, reducing the peak power of the cascaded Raman threshold by 11.4%. The maximum cascaded Raman power of ~0.5 W is obtained when the pump source is at its maximum, and the corresponding conversion efficiency inside the cavity is 21.4%, which is 1.8 times that of the previous configuration. Moreover, the characteristics of the second-order Raman lasers at 1695 nm and 1730 nm are also studied thoroughly. This work provides a significant method for realizing all-fiber cascaded gas Raman lasers, which is beneficial for expanding the output wavelength of fiber gas lasers with a good stability and compactivity.

CO2-based hollow-core fiber Raman laser with high-pulse energy at 1.95 µm

In this Letter, we present a high-pulse energy (>10µJ) Raman laser at 1946 nm wavelength directly pumped with a 1533 nm custom-made fiber laser. The Raman laser is based on stimulated Raman scattering (SRS) in an 8 m carbon dioxide (CO₂)-filled nested anti-resonant hollow-core fiber. The low-energy phonon emission combined with the inherent SRS process along the low-loss fiber allows the generation of high-pulse energy up to 15.4 µJ at atmospheric CO₂ pressure. The Raman laser exhibits good long-term stability and low relative intensity noise of less than 4%. We also investigate the pressure-dependent overlap of the Raman laser line with the absorption band of CO₂ at the 2 µm spectral range. Our results constitute a novel, to the best of our knowledge, and promising technology towards high-energy 2 µm lasers.

All-fiber gas Raman laser oscillator

Here, we report the first, to the best of our knowledge, all-fiber gas Raman laser oscillator (AFGRLO), which is formed by fusion splicing solid-core fibers and a hydrogen-filled hollow-core photonic crystal fiber, and further introducing fiber Bragg gratings at a Stokes wavelength. Pumping with a homemade 1.54 µm fiber amplifier seeded by a narrow linewidth diode laser, we obtain the maximum output Stokes power of 1.8 W at 1693 nm by rotational stimulated Raman scattering of hydrogen molecules. Due to the involvement of the resonant cavity, the measured Raman threshold is as low as 0.98 W, which has been reduced nearly 20 times, compared with that of the single-pass structure. Moreover, a numerical model of an AFGRLO is established for the first time, to the best of our knowledge, and the simulations agree well with the experimental results. This Letter is significant for the development of fiber gas Raman lasers (FGRLs), particularly for achieving compact CW FGRLs towards the mid-infrared.

Pulsed fiber laser oscillator at 1.7 µm by stimulated Raman scattering in H2-filled hollow-core photonic crystal fibers

We have reported a pulsed fiber gas Raman laser oscillator at 1.7 µm based on an all-fiber resonant cavity, which is made by splicing solid-core fibers with a 50-meter-long hydrogen-filled hollow-core photonic crystal fiber and further introducing homemade fiber Bragg gratings at the Raman wavelength. Pumping by a homemade pulsed 1540 nm fiber amplifier, a 1693 nm Stokes wave is obtained by pure rotational stimulated Raman scattering of H₂. The maximum optical-to-optical efficiency inside the hollow-core fiber is about 54% with the repetition frequency of 6 MHz, giving an average Raman power of 1.5 W, and the Raman threshold of peak power is as low as 3.6 W, which is more than 10 times lower than that of the single-pass structure. The relationship between pulse characteristics and Raman threshold is systematically studied, and the Raman threshold can be reduced dramatically when the repetition frequency of pulses is consistent with the resonant frequency of the cavity. This work provides good guidance for achieving low-threshold pulsed all-fiber gas Raman lasers, which is significant for development and application.

Delivery of CW laser power up to 300 watts at 1080 nm by an uncooled low-loss anti-resonant hollow-core fiber

In this paper, we report the use of a 3-meter low-loss anti-resonant hollow-core fiber (AR-HCF) to deliver up to 300 W continuous-wave laser power at 1080 nm wavelength from a commercial fiber laser source. A near-diffraction-limited beam is measured at the output of the AR-HCF and no damage to the uncooled AR-HCF is observed for several hours of laser delivery operation. The limit of AR-HCF coupling efficiency and laser-induced thermal effects that were observed in our experiment are also discussed.

Highly Efficient Nanosecond 1.7 μm Fiber Gas Raman Laser by H2-Filled Hollow-Core Photonic Crystal Fibers

We report here a high-power, highly efficient, wavelength-tunable nanosecond pulsed 1.7 μm fiber laser based on hydrogen-filled hollow-core photonic crystal fibers (HC-PCFs) by rotational stimulated Raman scattering. When a 9-meter-long HC-PCF filled with 30 bar hydrogen is pumped by a homemade tunable 1.5 μm pulsed fiber amplifier, the maximum average Stokes power of 3.3 W at 1705 nm is obtained with a slope efficiency of 84%, and the slope efficiency achieves the highest recorded value for 1.7 μm pulsed fiber lasers. When the pump pulse repetition frequency is 1.3 MHz with a pulse width of approximately 15 ns, the average output power is higher than 3 W over the whole wavelength tunable range from 1693 nm to 1705 nm, and the slope efficiency is higher than 80%. A steady-state theoretical model is used to achieve the maximum Stokes power in hydrogen-filled HC-PCFs, and the simulation results accord well with the experiments. This work presents a new opportunity for highly efficient tunable pulsed fiber lasers at the 1.7 μm band.

Pure rotational stimulated Raman scattering in H2-filled hollow-core photonic crystal fibers

We conducted comprehensive theoretical research on rotational stimulated Raman scattering (SRS) of hydrogen molecules in hollow-core fibers. A reliable model for describing the steady-state rotational SRS of hydrogen was established and the influences of various factors was investigated. To verify the theoretical model, a single-pass fiber gas Raman laser (FGRL) based on hydrogen-filled hollow-core photonic crystal fibers pumped by a 1.5 µm nanosecond-pulsed fiber amplifier was constructed. Experimental results were congruent with simulation results. As the output powers and pulse shapes can be well calculated, the model can offer guidance for FGRL investigation, particularly for achieving high-efficiency and high-power FGRLs.

Low-loss coupling from single-mode solid-core fibers to anti-resonant hollow-core fibers by fiber tapering technique

We demonstrate here for the first time, to the best of our knowledge, an effective method to achieve low-loss light coupling from solid-core fibers to anti-resonant hollow-core fibers (AR-HCFs) by fiber tapering technique. We establish the coupling models by beam propagation method (BPM), and the simulation results show that the coupling efficiency can be optimized by choosing a proper waist diameter of the tapered solid-core fiber. Two types of AR-HCFs have been tested experimentally, and the maximum light coupling efficiency is ∼91.4% at 1.06 µm and ∼90.2% at 1.57 µm for the ice-cream AR-HCF, and ∼83.7% at 1.57 µm for the node-less AR-HCF. This work provides a feasible low-loss light coupling scheme for AR-HCFs, which is very useful for implementing all fiber systems.

High-efficiency Raman conversion in SF6- and CF4-filled hollow-core photonic bandgap fibers

We present a comparative experimental investigation of vibrational stimulated Raman scattering in hollow-core photonic bandgap fibers pressurized with sulfur hexafluoride (${{\rm SF}_6}$SF₆) and tetrafluoromethane (${{\rm CF}_4}$CF₄) gases. Nanosecond-duration pulses at a wavelength of 1030 nm are coupled into the gas-filled fiber, and the first and second Stokes orders are measured at the fiber output. We characterize the conversion process as a function of gas, fiber length, and input power. With a 15 m fiber filled with ${{\rm SF}_6}$SF₆, we obtain conversion efficiency to the first Stokes of 55.7% at an input peak power of 0.63 kW. In comparison, with ${{\rm CF}_4}$CF₄, we obtained a higher conversion threshold and maximum conversion efficiency of 45.4%. To the best of our knowledge, this is the first reported conversion experiment with hollow-core fibers filled with ${{\rm SF}_6}$SF₆ gas.

High-efficiency laser wavelength conversion in deuterium-filled hollow-core photonic crystal fiber by rotational stimulated Raman scattering

We report here, to the best of our knowledge, for the first time high-efficiency laser wavelength conversion from 1.5 µm band to 1.7 µm band in deuterium-filled hollow-core photonic crystal fibers by rotational stimulated Raman scattering (SRS). Due to the special transmission properties of this low-loss hollow-core fiber, the ordinary dominant vibrational SRS is suppressed, permitting efficient conversion to the rotational stokes wave in a single-pass configuration pumped by a fiber amplified and modulated tunable 1.55 µm diode laser. Using proper pump pulse energy and gas pressure, the power conversion efficiencies over the whole output laser wavelength range from 1640 nm to 1674 nm are higher than 48%. And the maximum Raman conversion efficiency of 61.2% is achieved with 20 m fiber and 20 bar deuterium pressure pumped at 1540 nm, giving a maximum average power of about 0.8 W (pulse energy of 1.6 µJ). This work points to a new way for engineerable and compact fiber lasers operation at 1.7 µm band, which has significant applications in biological imaging, laser medical treatment, material processing and detecting.

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.

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.

Efficient high power, narrow linewidth 1.9 μm fiber hydrogen Raman amplifier

We report here an efficient, high power, narrow linewidth 1.9 μm gas Raman amplifier by means of a hydrogen-filled hollow-core fiber. A 1.9 μm narrow linewidth continuous wave seed laser is coupled into the hollow-core fiber together with a high power pulsed 1064 nm MOPA laser through a shortpass dichroic mirror, and then amplified by stimulated Raman scattering of hydrogen. With 2 m fiber length and 4.5 bar gas pressure, the maximum average 1908 nm Stokes power of 570 mW is obtained, a record average power level for such experiments. The maximum peak power is about 50 kW, the linewidth is about 1 GHz, and the quantum efficiency is about 51%. This work has demonstrated the potential to get a high average power gas Raman laser in a hollow-core fiber, and it further provides the possibility to achieve a high average power 4 μm midinfrared fiber laser by cascaded gas stimulated Raman scattering.

Watt-level tunable 1.5 μm narrow linewidth fiber ring laser based on a temperature tuning π-phase-shifted fiber Bragg grating

A watt-level tunable 1.5 μm narrow linewidth fiber ring laser using a temperature tuning π-phase-shifted fiber Bragg grating (π-PSFBG) is demonstrated here, to the best of our knowledge, for the first time. The π-PSFBG is employed as both a narrow band filter and a wavelength tuning component, and its central wavelength is thermally tuned by a thermo-electric cooler. The maximum laser power is about 1.1 W with a linewidth of ∼318  MHz (∼2.57  pm) and a power fluctuation of less than 3%. The wavelength tuning range of the laser is about 1.29 nm with a sensitivity of ∼14.33  pm/°C, and the wavelength fluctuation is about 0.2 pm. This work provides important reference for tunable fiber lasers with both high power and narrow linewidth.

Demonstration of a 150-kW-peak-power, 2-GHz-linewidth, 1.9-μm fiber gas Raman source

We report here for the first time, to the best of our knowledge, a 100-kW-peak-power, GHz-linewidth, sub-nanosecond, 1.9-μm laser source by gas stimulated Raman scattering (SRS) in hollow-core fiber. A H₂-filled, anti-resonance, hollow-core fiber is pumped with a sub-nanosecond, high-peak-power, 1064-nm microchip laser, generating a 1908-nm Stokes wave by vibrational SRS of H₂ molecules. A maximum peak power of about 150 kW (average power 55 mW, pulse energy 55 μJ) is achieved with a 1.4-m fiber length and only 3 bar H₂ pressure. The maximum quantum efficiency is about 54%, and the corresponding slope efficiency is about 37%. The linewidth of the Stokes wave is about 2 GHz, which decreases about 1-2 orders compared with the rare-earth-doped fiber lasers of the same peak-power level. Operation close to atmospheric pressure makes it more convenient in future applications. If a tunable pump laser is used, a high-peak-power, narrow-linewidth, broadly tunable, 2-μm fiber laser source can be easily achieved.

Ultra-efficient Raman amplifier in methane-filled hollow-core fiber operating at 1.5 μm

We report on what is, to the best of our knowledge, the first ultra-efficient 1.5 μm Raman amplifier in a methane-filled anti-resonance hollow-core fiber. A 1.5 μm single frequency seed laser is coupled into the hollow-core fiber together with a 1064 nm pulsed pump laser using a shortpass dichromic mirror, and then amplified by stimulated Raman scattering of methane. A maximum optical-to-optical conversion efficiency of 66.4% has been obtained, resulting in a record near quantum-limit efficiency of 96.3% in a 2 m long hollow-core fiber filled with only 2 bar methane gas. This kind of gas filled hollow-core Raman amplifier provides a potential method to obtain high efficiency mid-infrared laser sources with low threshold and narrow linewidth in various applications.