Enhanced birefringence in conventional and hybrid anti-resonant hollow-core fibers.

A hollow-core anti-resonant fiber (HC-ARF) design based on hybrid silica/silicon cladding is proposed for single-polarization, single-mode and high birefringence. We show that by adding silicon layers in a semi-nested HC-ARF, one of the polarization states can be strongly suppressed while simultaneously maintaining low propagation loss for other polarization states, single-mode and high birefiringence. The optimized HC-ARF design exhibits propagation loss, high birefringence, and polarization-extinction ratio of 0.05 dB/m, 0.5 × 10-4, >300 respectively for y-polarization while the loss of x-polarization is >5 dB/m at 1064 nm. The fiber also has low bend-loss and thus can be coiled to a small bend radii of 5 cm having ≈0.06 dB/m bend loss.

Thermally tunable dispersion modulation in a chalcogenide-based hybrid optical fiber.

A hybrid optical fiber comprising metal electrodes, high performance polymers, and a highly nonlinear glass core is presented in this work as a novel, to the best of our knowledge, platform for mid-infrared nonlinear devices. The fiber allows for electrical tuning of the temperature by joule heating using a set of embedded tungsten wires. Unlike temperature tuning by an external heater, this results in a strong modulation, which introduces alternating signs of its dispersion. Enhanced spectral broadening through supercontinuum generation in the mid-infrared due to this modulation is investigated numerically.

Multi-wavelength high-energy gas-filled fiber Raman laser spanning from 1.53 µm to 2.4 µm.

In this work, we present a high-pulse-energy multi-wavelength Raman laser spanning from 1.53 µm up to 2.4 µm by employing the cascaded rotational stimulated Raman scattering effect in a 5 m hydrogen (H2)-filled nested anti-resonant fiber, pumped by a linearly polarized Er/Yb fiber laser with a peak power of ∼13kW and pulse duration of ∼7ns 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 µJ, 14.4 µJ, 14.1 µJ, and 8.2 µJ, respectively. We demonstrate how the energy in the Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar.

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.

Spectral broadening of ultraviolet dispersive waves in gas-filled hollow-core fiber using pump pulse modulation.

UV supercontinuum laser sources based on resonant dispersive wave (RDW) generation in gas-filled hollow-core (HC) fibers offer an attractive architecture for numerous applications. However, the narrow UV spectral peak inherent to RDW generation limits the suitability for applications that require broad spectral coverage within the UV region such as spectroscopic scatterometry. In this Letter, we demonstrate how the UV spectrum can be shaped by modulating the peak power of the pump pulses driving the RDW generation, thereby creating a broadened and flattened UV spectrum. Using an argon-filled anti-resonant HC fiber, we generate a UV spectrum with a center wavelength of 323.6 nm with an FWHM of 51.7 nm, corresponding to a relative bandwidth of 16.1%.

Low-noise tunable deep-ultraviolet supercontinuum laser.

The realization of a table-top tunable deep-ultraviolet (UV) laser source with excellent noise properties would significantly benefit the scientific community, particularly within imaging and spectroscopic applications, where source noise has a crucial role. Here we provide a thorough characterization of the pulse-to-pulse relative intensity noise (RIN) of such a deep-UV source based on an argon (Ar)-filled anti-resonant hollow-core (AR HC) fiber. Suitable pump pulses are produced using a compact commercially available laser centered at 1030 nm with a pulse duration of 400 fs, followed by a nonlinear compression stage that generates pulses with 30 fs duration, 24.2 µJ energy at 100 kHz repetition rate and a RIN of < 1%. Pump pulses coupled into the AR HC fiber undergo extreme spectral broadening creating a supercontinuum, leading to efficient energy transfer to a phase-matched resonant dispersive wave (RDW) in the deep-UV spectral region. The center wavelength of the RDW could be tuned between 236 and 377 nm by adjusting the Ar pressure in a 140 mm length of fiber. Under optimal pump conditions the RIN properties were demonstrated to be exceptionally good, with a value as low as 1.9% at ~ 282 nm. The RIN is resolved spectrally for the pump pulses, the generated RDW and the broadband supercontinuum. These results constitute the first broadband RIN characterization of such a deep-UV source and provide a significant step forward towards a stable, compact and tunable laser source for applications in the deep-UV spectral region.

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.

Noise and spectral stability of deep-UV gas-filled fiber-based supercontinuum sources driven by ultrafast mid-IR pulses.

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, which can even also extend into the mid-infrared (IR). 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 µm by pumping an argon-filled hollow-core anti-resonant fiber at a mid-IR wavelength of 2.45 µm. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity noise (RIN) of the 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 high RIN of the mid-IR 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.

Deep-UV to Mid-IR Supercontinuum Generation driven by Mid-IR Ultrashort Pulses in a Gas-filled Hollow-core Fiber.

Supercontinuum (SC) generation based on ultrashort pulse compression constitutes one of the most promising technologies towards 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) having an output energy of 5 µJ. This was obtained by pumping at the center wavelength of the first anti-resonant transmission window (2460 nm) with ~100 fs pulses and an injected pulse energy of ~8 µJ. 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 which corresponds to the calculated phase-matching wavelength of the pump. 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. This work represents another step towards gas-filled fiber-based coherent sources, which is set to have a major impact on applications spanning from DUV to mid-IR.