Efficient diode-pumped mid-infrared emission from acetylene-filled hollow-core fiber

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.

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.

Ultralow-loss fusion splicing between antiresonant hollow-core fibers and antireflection-coated single-mode fibers with low return loss

The Fresnel reflection of a splice from the air-silica interface between a hollow-core fiber (HCF) and a solid-core conventional fiber will increase the splicing loss and also cause possible instability of transmission. Here, for the first time, we develop a novel approach to fusion splicing an antireflection-coated (AR-coated) conventional fiber and an antiresonant HCF, which was generally claimed to be impossible because of the heat-induced damage of the coating, and achieve state-of-the-art ultralow fusion splicing loss less than 0.3 dB and a low return loss less than -28 dB by optimizing the splicing procedures and parameters. Our new fusion splicing approach will benefit the wide application of HCFs in telecoms, laser technologies, gyroscopes, and fiber gas cells.

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.

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.

All-Fiber Gas Cavity Based on Anti-Resonant Hollow-Core Fibers Fabricated by Splicing with End Caps

In recent years, fiber gas lasers have obtained a rapid development, however, efficient and stable pump coupling is a key limitation for their applications in the future. Here, we report an all-fiber gas cavity based on anti-resonant hollow-core fibers which have the beneficial properties of adjustable broad transmission bands and potential low transmission attenuation, especially in the mid-infrared. This kind of all-fiber gas cavity is fabricated by directly splicing with end caps at both ends for the first time. The high-power laser transmission characteristics were studied, and the experimental results show that the all-fiber gas cavities have a very stable performance. The maximum input laser power at 1080 nm is about 260 W, and the output power is 203 W, giving a total transmission efficiency of 78.1%. This work opens a new opportunity for the development of high-power all-fiber structured fiber gas lasers.

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.

Mid-infrared transmission by a tellurite hollow core optical fiber

We experimentally demonstrate for the first time a successful fabrication of a tellurite hollow core optical fiber which has a mid-infrared transmission range. The wall thickness of each cladding air-hole is about 2.8 µm and the outer diameter of the full air-hole structure D is approximately 110 µm. The results show that the measured transmission spectrum can expand up to 3.9 µm. In addition, it is expected that the transmission can extend to around 6 µm. When the input light is linearly polarized, it can be maintained after propagating through a 17-cm-long fiber.

A Review of Photothermal Detection Techniques for Gas Sensing Applications

Photothermal spectroscopy (PTS) is a technique used for determining the composition of liquids, solids and gases. In PTS, the sample is illuminated with a radiation source, and the thermal response of the analyte (e.g., refractive index) is analyzed to gain information about its content. Recent advances in this unique method of detecting gaseous samples show that photothermal gas spectroscopy can be an interesting alternative to commonly used absorption techniques. Moreover, if designed properly, sensors using PTS detection technique can not only reach sensitivities comparable with other, more complex techniques, but can significantly simplify the design of the sensor. In this review, recent developments in photothermal spectroscopy of gases will be summarized and discussed.

Second-Order Vector Mode Propagation in Hollow-Core Antiresonant Fibers

Second-order vector modes, possessing doughnut-shaped intensity distribution with unique polarization, are widely utilized in material micromachining, optical tweezers, and high-resolution microscopy. Since the hollow-core fiber can act as a flexible and robust optical waveguide for ultra-short pulse delivery and manipulation, high-order vector modes guided in hollow-core fibers will have huge potential in many advanced applications. We firstly reveal that a second-order vector mode can be well guided in a hollow-core antiresonant fiber with the suppression of the fundamental mode and other second-order vector modes at the red side of transmission band. We interpret our observation through a phase-matched coupling mechanism between core modes and coupled cladding modes. A single second-order vector mode such as TE₀₁, TM₀₁, or HE₂₁ can be guided with low confinement loss at specific wavelengths with appropriate structure parameters. Our proposed hollow-core fibers have a modal engineering function which will open up a new avenue toward the single second-order vector mode propagation and its fiberized applications.

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.

Understanding Dispersion of Revolver-Type Anti-Resonant Hollow Core Fibers

Here, we analyze the dispersion behavior of revolver-type anti-resonant hollow core fibers, revealing that the chromatic dispersion of this type of fiber geometry is dominated by the resonances of the glass annuluses, whereas the actual arrangement of the anti-resonant microstructure has a minor impact. Based on these findings, we show that the dispersion behavior of the fundamental core mode can be approximated by that of a tube-type fiber, allowing us to derive analytic expressions for phase index, group-velocity dispersion and zero-dispersion wavelength. The resulting equations and simulations reveal that the emergence of zero group velocity dispersion in anti-resonant fibers is fundamentally associated with the adjacent annulus resonance which can be adjusted mainly via the glass thickness of the anti-resonant elements. Due to their generality and the straightforward applicability, our findings will find application in all fields addressing controlling and engineering of pulse dispersion in anti-resonant hollow core fibers.

Anti-Resonant Hollow Core Fibers with Modified Shape of the Core for the Better Optical Performance in the Visible Spectral Region-A Numerical Study

In this paper, we present numerical studies of several different structures of anti-resonant, hollow core optical fibers. The cladding of these fibers is based on the Kagomé lattice concept, with some of the core-surrounding lattice cells removed. This modification, by creating additional, glass-free regions around the core, results in a significant improvement of some important optical fiber parameters, such as confinement loss (CL), bending loss (BL), and dispersion parameter (D). According to the conducted simulations (with fused silica glass being the structure's material), CL were reduced from ~0.36 dB/m to ~0.16 dB/m (at 760 nm wavelength) in case of the structure with removed cells, and did not exceed the value of 1 dB/m across the 700⁻850 nm wavelength range. Additionally, proposed structure exhibits a remarkably low value of D-from 1.5 to 2.5 ps/(nm × km) at the 700⁻800 nm wavelength range, while the BL were estimated to be below 0.25 dB/m for bending radius of ~1.5 cm. CL and D were simulated, additionally, for structures made of acrylic glass polymethylmethacrylate, (PMMA), with similarly good results-DPMMA ∊ [2, 4] ps/(nm × km) and CLPMMA ≈ 0.13 dB/m (down from 0.41 dB/m), for the same spectral regions (700⁻800 nm bandwidth for D, and 760 nm wavelength for CL).

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.

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.

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.

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.

Analytic model for the complex effective index of the leaky modes of tube-type anti-resonant hollow core fibers

Due to their promising applications, hollow-core fibers, in particular, their anti-resonant versions, have recently attracted the attention of the photonics community. Here, we introduce a model that approximates, using the reflection of a wave on a single planar film, modal guidance in tube-type anti-resonant waveguides whose core diameters are large compared to the wavelength. The model yields analytic expressions for the real and imaginary parts of the complex effective index of the leaky modes supported, and is valid in all practically relevant situations, excellently matching all the important dispersion and loss parameters. Essential principles such as the fourth power dependence of the modal loss on the core radius at all wavelengths and the geometry-independent transition refractive index, below which modal discrimination favors the fundamental mode are discussed. As application examples, we use our model for understanding higher-order mode suppression in revolver-type fibers and for uncovering the tuning capabilities associated with nonlinear pulse propagation.

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.

Composite material hollow antiresonant fibers

We study novel designs of hollow-core antiresonant fibers comprising multiple materials in their core-boundary membrane. We show that these types of fibers still satisfy an antiresonance condition and compare their properties to those of an ideal single-material fiber with an equivalent thickness and refractive index. As a practical consequence of this concept, we discuss the first realization and characterization of a composite silicon/glass-based hollow antiresonant fiber.

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.

Function of second cladding layer in hollow core tube lattice fibers

Modes attenuation of the tube lattice fiber (TLF) is characterized by D/λ, where D is the core diameter and λ is the wavelength. Hence, the TLF is structured with a large core to ensure a low attenuation loss. A small core, on the other hand, facilitates the gas-filled TLF applications, but at the expense of the increased mode attenuation. We show that adding a second cladding layer to the conventional one layer TLF (1TLF) can resolve the contradicting requirements. The mode attenuation of TLF with two cladding layers (2TLF) is less influenced by the D/λ value as compared to 1TLF, thus realizing a low loss small core TLF. Furthermore, we found that adding the second layer brings another advantage to a bending performance. With a determined core size, D, a 1TLF with smaller capillary hole size, d, experiences less bending loss. However, the reduced d increases the confinement loss that counteracts the bending loss improvement. This confliction is substantially alleviated in 2TLF thanks to the second cladding layer. Theoretical investigations and experimental demonstrations are presented to evidence the important role of the second cladding ring in the TLF, which has been overlooked in prior studies.

Hollow core anti-resonant fiber with split cladding

An improved design for hollow core anti-resonant fibers (HAFs) is presented. A split cladding structure is introduced to reduce the fabrication distortion within design tolerance. We use numerical simulations to compare the Kagome fibers (KFs) and the proposed split cladding fibers (SCFs) over two normalized transmission bands. It reveals that SCFs are able to maintain the desired round shape of silica cladding walls, hence improving the confinement loss (CL) compared to the KF and is comparable to that of the nested antiresonant nodeless fiber (NANF) with the same core size. In addition, the SCF allows stacking multiple layers of cladding rings to control the CL. The influences of the number of cladding layers and the cladding gap width on the CL of the SCFs have been studied. SCF with three cladding rings is fabricated by the stack-and-draw technique. A measured attenuation spectrum matches well with the calculation prediction. The measured near field mode patterns also prove the feasibility of our fiber design.