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Photoacoustic spectroscopys (PAS)-based methane (CH4) detectors have garnered significant attention with various developed systems using near-infrared (NIR) laser sources, which requires high-energy and narrow-linewidth laser sources to achieve high-sensitivity and low-concentration gas detection. The anti-resonant hollow-core fiber (ARHCF) lasers in the NIR and mid-infrared (MIR) spectral domain show a great potential for spectroscopy and high-resolution gas detection. In this work, we demonstrate the generation of a frequency-comb-like Raman laser with high pulse energy spanning from ultraviolet (UV) (328 nm) to NIR (2065 nm wavelength) based on a hydrogen (H2)-filled 7-ring ARHCF. The gas-filled ARHCF fiber is pumped with a custom-laser at 1044 nm with ~100 {\mu}J pulse energy and a few nanoseconds duration. Through stimulated Raman scattering process, we employ the sixth-order Stokes as case example located at ~1650 nm to demonstrate how the developed high-energy and narrow-linewidth laser source can effectively be used to detect CH4 in the NIR-II region using the photoacoustic modality. We report the efficient detection of CH4 with sensitivity as low as ~550 ppb with an integration time of ~40 s. In conclusion, the main goal of this work is to demonstrate and emphasize the potential of the gas-filled ARHCF laser technology for compact next-generation spectroscopy across different spectral regions., Comment: 18 pages, 7 figures

In this work, we develop a high energy laser at 1.4 {\mu}m wavelength through the 1st order vibrational Raman Stokes generation in a nitrogen (N2) filled nodeless anti-resonant hollow core fiber (ARHCF), pumped using a custom-made 1060 nm Yb-doped narrow-linewidth fiber laser with 3.7 ns pulse duration and 98 {\mu}J pulse energy. The maximum Raman energy of 26.5 {\mu}J is achieved at 15 bar N2 pressure, corresponding to a quantum efficiency of 45% and peak power of 8.03 kW with 3.3 ns pulse width. This Raman laser has a narrow linewidth of 0.16 nm, and it is tunable over ~2 nm range by thermally tuning the laser diode seed wavelength of the pump fiber laser. This work provides a promising alternative for developing high energy and narrow-linewidth lasers outside the gain spectral regions of rare earth ion doped fibers., Comment: 6 pages

Dissipative soliton resonance (DSR) is a promising way for high-energy pulse generation typically having a symmetrical square pulse profile. While this method is well known, the impact of third order dispersion (TOD) on DSR is yet to be fully addressed in the literature. In this article, the impact of TOD on DSR is numerically investigated under the frame of the complex cubic-quintic Ginzburg-Landau equation (CQGLE). Our numerical investigations indicate that DSR can stably exist under TOD with nearly the same pulse amplitude, but with a (significantly) different pulse duration. Depending on the value of chromatic dispersion, the pulse duration can be notably longer or shorter due to the presence of TOD. The TOD effect also alters the dependence of pulse duration on the nonlinear gain. Another impact of TOD on DSR is that the DSR exists with an asymmetric pulse profile, leading to steepening of one edge of the DSR pulse, while flattening of the other. Our results indicate that TOD has a critical role for realizing DSR in mode-locked lasers and it should be taken into consideration during design and development of DSR-based lasers.

In this letter, we present a high pulse energy Raman laser at 1946 nm wavelength directly pumped with a 1533 nm custom-made fiber laser. The Raman laser is based on the stimulated Raman scattering (SRS) in an 8-meter carbon dioxide (CO2) filled nested anti-resonant hollow-core fiber (ARHCF). 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 {\mu}J at atmospheric CO2 pressure. The Raman laser exhibits good long-term stability and low relative intensity noise (RIN) of less than 4%. We also investigate the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at 2 {\mu}m spectral range. Our results constitute a novel and promising technology towards high energy 2 {\mu}m lasers., Comment: I would like to withdrawal this paper because a mistake needs to be corrected. I will upload a new version

In this letter, an energetic and highly efficient dispersive wave (DW) generation at 200 nm has been numerically demonstrated by selectively exciting LP$_{02}$-like mode in a 10 bar Ar-filled hollow-core anti-resonant fiber pumping in the anomalous dispersion regime at 1030 nm with pulses of 30 fs duration and 7 $\mu$J energy. Our calculations indicate high conversion efficiency of $>$35\% (2.5 $\mu$J) after propagating 3.6 cm fiber length which is due to the strong shock effect and plasma induced blue-shifted soliton. It is observed that the efficiency of fundamental LP$_{01}$-mode is about 15\% which is much smaller than LP$_{02}$-like mode and also emitted at longer wavelength of 270 nm.

Understanding the impact of the cladding tube structure on the overall guiding performance is crucial for designing single-mode, wide-band, and ultra low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly investigate on how the propagation loss is affected by the nested elements when their geometry is realistic (i.e., non-ideal). Interestingly, it was found that the size rather than the shape of the nested elements, have a dominant role in the final loss performance of the HC-ARFs. We identify a unique 'V-shape' pattern for suppression of higher-order modes loss by optimizing free design parameters of HC-ARF. We find that a 5-tube nested HC-ARF has wider transmission window and better single-mode operation than 6-tube HC-ARF. We show that the propagation loss can be significantly improved by using anisotropic nested anti-resonant tubes elongated in the radial direction. Our simulations indicate that with this novel fiber design, a propagation loss as low as 0.11 dB/km at 1.55 $\mu$m can be achieved. Our results provide design insights towards fully exploiting single-mode, wide-band, and ultra low-loss HC-ARF. In addition, the extraordinary optical properties of the proposed fiber can be beneficial for several applications such as future optical communication system, high energy light transport, extreme non-nonlinear optics and beyond.

In this work, we present a high pulse energy multi-wavelength Raman laser spanning from 1.53 um up to 2.4 um by employing the cascaded rotational stimulated Raman scattering (SRS) effect in a 5-m hydrogen (H2) -filled nested anti-resonant fiber (NARF), pumped by a linearly polarized Er/Yb fiber laser with a peak power of ~13 kW and pulse duration of ~7 ns in the C-band. The developed Raman laser has distinct lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm, with pulse energies as high as 18.25 uJ, 14.4 uJ, 14.1 uJ, and 8.2 uJ, respectively. We demonstrate how the energy in the Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar

In this letter, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging mid-IR gas-filled hollow-core fiber (HCF) Raman laser technology.

Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region. Noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 {\mu}m by pumping an argon-filled hollow-core anti-resonant fiber at a wavelength of 2.45 {\mu}m. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity (RIN) noise of the strongest DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the RIN of the pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.

This article offers an extensive survey of results obtained using hybrid photonic crystal fibers (PCFs) which constitute one of the most active research fields in contemporary fiber optics. The ability to integrate novel and functional materials in solid- and hollow-core PCFs through various post-processing methods has enabled new directions towards understanding fundamental linear and nonlinear phenomena as well as novel application aspects, within the fields of optoelectronics, material and laser science, remote sensing and spectroscopy. Here the recent progress in the field of hybrid PCFs is reviewed from scientific and technological perspectives, focusing on how different fluids, solids and gases can significantly extend the functionality of PCFs. In the first part of this review we discuss the most important efforts by research groups around the globe to develop tunable linear and nonlinear fiber-optic devices using PCFs infiltrated with various liquids, glasses, semiconductors and metals. The second part is concentrated on the most recent and state-of-the-art advances in the field of gas-filled hollow-core PCFs. Extreme ultrafast gas-based nonlinear optics towards light generation in the extreme wavelength regions of vacuum ultraviolet (VUV), pulse propagation and compression dynamics in both atomic and molecular gases, and novel soliton - plasma interactions are reviewed. A discussion of future prospects and directions is also included.

We investigate various methods for extending the simple analytical capillary model to describe the dispersion and loss of anti-resonant hollow-core fibers without the need of detailed finite-element simulations across the desired wavelength range. This poor-man's model can with a single fitting parameter quite accurately mimic dispersion and loss resonances and anti-resonances from full finite-element simulations. Due to the analytical basis of the model it is easy to explore variations in core size and cladding wall thickness, and should therefore provide a valuable tool for numerical simulations of the ultrafast nonlinear dynamics of gas-filled hollow-core fibers., Comment: In preparation

Supercontinuum (SC) generation based on ultrashort pulse compression constitutes one of the most promising technologies towards an ultra-wide bandwidth, high-brightness and spatially coherent light sources for applications such as spectroscopy and microscopy. Here, multi-octave SC generation in a gas-filled hollow-core antiresonant fiber (HC-ARF) is reported spanning from 200 nm in the deep ultraviolet (DUV) to 4000 nm in the mid-infrared (mid-IR). A measured average output power of 5 mW was obtained by pumping at the center wavelength of the first anti-resonance transmission window (2460 nm) with ~100 fs pulses and an injected pulse energy of ~7-8 {\mu}J. The mechanism behind the extreme spectral broadening relies upon intense soliton-plasma nonlinear dynamics which leads to efficient soliton self-compression and phase-matched dispersive wave (DW) emission in the DUV region. The strongest DW is observed at 275 nm having an estimated pulse energy of 1.42 {\mu}J, corresponding to 28.4 % of the total output energy. Furthermore, the effect of changing the pump pulse energy and gas pressure on the nonlinear dynamics and their direct impact on SC generation was investigated. The current work paves a new way towards novel investigations of gas-based ultrafast nonlinear optics in the emerging mid-IR spectral regime.

Detection and identification of lipids are highly coveted for the interrogation of chronic diseases such as atherosclerosis and myocardial infarction. Intravascular photoacoustic imaging (IVPA) and deep tissue imaging are modern techniques, which rely on complex near infrared (NIR) optical parametric oscillators (OPOs) and other high-power solid-state laser systems for the diagnosis, which in turn make the systems bulky and expensive. In this work, we propose a cost-effective directly modulated high pulse energy supercontinuum source (operating in kHz regime) based on a standard optical fiber with pulse energy density as high as ~ 26 nJ/nm. We demonstrate how such supercontinuum source combined with a tunable filter can be highly suitable for vibration-based photoacoustic imaging and spectroscopy of lipids in the molecular overtone band of lipids (1650-1850 nm). We show the successful discrimination of two different lipids (cholesterol and lipid in adipose tissue) and the photoacoustic cross-sectional scan of lipid-rich adipose tissue at three different locations. The proposed high pulse energy supercontinuum laser paves a new direction towards compact, broadband and cost-effective source for multi-spectral spectroscopic photoacoustic imaging., Comment: 6 pages, 5 figures

In this work, we numerically investigate an experimentally feasible design of a tapered Ne-filled hollow-core anti-resonant fiber and we report the generation of multiple dispersive waves (DWs) in the range 90-120 nm, well into the extreme ultraviolet (UV) region. The simulations assume an 800 nm pump pulse with 30 fs 10 $\mu$J pulse energy, launched into a 9 bar Ne-filled fiber with $34~\mu$m initial core diameter that is then tapered to a $10~\mu$m core diameter. The simulations were performed using a new model that provides a realistic description of both loss and dispersion of the resonant and anti-resonant spectral bands of the fiber, and also importantly includes the material loss of silica in the UV. We show that by first generating solitons that emit DWs in the far-UV region in the pre-taper section, optimization of the following taper structure can allow re-collision with the solitons and further up-conversion of the far-UV DWs to the extreme-UV with energies up to 190 nJ in the 90-120 nm range. This process provides a new way to generate light in the extreme-UV spectral range using relatively low gas pressure.

We investigate numerically soliton-plasma interaction in a noble-gas-filled silica hollow-core anti-resonant fiber pumped in the mid-IR at 3.0 {\mu}m. We observe multiple soliton self-compression stages due to distinct stages where either the self-focusing or the self-defocusing nonlinearity dominates. Specifically, the parameters may be tuned so the competing plasma self-defocusing nonlinearity only dominates over the Kerr self-focusing nonlinearity around the soliton self-compression stage, where the increasing peak intensity on the leading pulse edge initiates a competing self-defocusing plasma nonlinearity acting nonlocally on the trailing edge, effectively preventing soliton-formation there. As the plasma switches off after the self-compression stage, self-focusing dominates again, initiating another soliton self-compression stage in the trailing edge. This process is accompanied by supercontinuum generation spanning 1-4 {\mu}m. The technique could be exploited to generate an ultrafast sequence of several few-cycle pulses., Comment: submitted