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.
We demonstrate a route to supercontinuum generation in gas-filled hollow-core anti-resonant fibers through the creation of a broad vibrational Raman frequency comb followed by continuous broadening and merging of the comb lines through either rotational Raman scattering or the optical Kerr effect. Our demonstration experiments, utilizing a single pump pulse with 20 ps duration at 532 nm in a nitrogen-filled fiber, produce a supercontinuum spanning from 440 nm to 1200 nm, with an additional deep ultraviolet continuum from 250 nm to 360 nm. Numerical results suggest that this approach can produc…
We report the generation of a purely vibrational Raman comb, extending from the vacuum ultraviolet (184 nm) to the visible (478 nm), in hydrogen-filled kagomé-style photonic crystal fiber pumped at 266 nm. Stimulated Raman scattering and molecular modulation processes are enhanced by higher Raman gain in the ultraviolet. Owing to the pressure-tunable normal dispersion landscape of the fiber-gas system in the ultraviolet, higher-order anti-Stokes bands are generated preferentially in higher-order fiber modes. The results pave the way towards tunable fiber-based sources of deep- and vacuum ultra…
We have developed an analytical model based on the perturbation theory in order to study the optical propagation of two successive intense solitons in hollow-core photonic crystal fibers filled with Raman-active gases. Based on the time delay between the two solitons, we have found that the trailing soliton dynamics can experience unusual nonlinear phenomena such as spectral and temporal soliton oscillations and transport towards the leading soliton. The overall dynamics can lead to a spatiotemporal modulation of the refractive index with a uniform temporal period and a uniform or chirped spat…
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 b…
As shown in the early 1960s, the gain in stimulated Raman scattering (SRS) is drastically suppressed when the rate of creation of phonons (via pump-to-Stokes conversion) is exactly balanced by the rate of phonon annihilation (via pump-to-anti-Stokes conversion). This occurs when the phonon coherence waves synchronized vibrations of a large population of molecules have identical propagation constants for both processes, i.e., they are phase-velocity matched. As recently demonstrated, hydrogen-filled photonic crystal fiber pumped in the vicinity of its zero dispersion wavelength provides an idea…
Optical properties of hollow-core revolver fibers are numerically investigated depending on various parameters: the hollow-core diameter, the capillary wall thickness, the values of the minimum gap between the capillaries, the number of capillaries in the cladding and the type of glass (silica and chalcogenide). Preliminary, similar calculations are made for simple models of hollow-core fibers. Based on the obtained results, the optimal design of the revolver fiber for Raman laser frequency conversion (1560 nm to 4420 nm in H2) was determined. As a result, efficient ns-pulsed 4420 nm Raman las…
Coherent ultraviolet (UV) light has many uses, for example in the study of molecular species relevant in biology and chemistry. Very few if any laser materials offer UV transparency along with damage-free operation at high photon energies and laser power. Here we report efficient generation of deep and vacuum UV light using hydrogen-filled hollow-core photonic crystal fiber (HC-PCF). Pumping above the stimulated Raman threshold at 532 nm, coherent molecular vibrations are excited in the gas, permitting highly efficient thresholdless wavelength conversion in the UV. The system is uniquely press…
In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if the rates of phonon creation and annihilation (by inelastic scattering) exactly balance. This is only possible if the momentum required for each process is identical, i.e., phonon coherence waves created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk gas cells, this can only be achieved over limited interaction lengths at an oblique angle to the pump axis. Here we report a simple system that provides dramatic Raman gain suppression over long collinear path-l…
Broadband-tunable sources of circularly-polarized light are crucial in fields such as laser science, biomedicine and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components, and are limited by the availability of suitably transparent crystals and glasses. Although gas-filled hollow-core photonic crystal fiber provides pressure-tunable dispersion, long well-controlled optical path-lengths, and high Raman conversion efficiency, it is unable to preserve circular polarization state, typically exhibiting weak linear biref…
We propose supercritical xenon in a hollow-core photonic bandgap fiber as a highly nonlinear medium, and demonstrate 200 nm control over the guidance window of the fiber as the xenon goes through its supercritical phase transition. The large optical polarizability and monoatomic nature of xenon are predicted to allow large optical nonlinearity, on the same order as that of fused silica, while maintaining greatly reduced Raman scattering, offering benefits for nonlinear and quantum optics.
Many fields such as bio-spectroscopy and photochemistry often require sources of vacuum ultraviolet (VUV) pulses featuring a narrow linewidth and tunable over a wide frequency range. However, the majority of available VUV light sources do not simultaneously fulfill those two requirements, and few if any are truly compact, cost-effective and easy to use by non-specialists. Here we introduce a novel approach that goes a long way to meeting this challenge. It is based on hydrogen-filled hollow-core photonic crystal fiber pumped simultaneously by two spectrally distant pulses. Stimulated Raman sca…
The unique ring-shaped intensity patterns and helical phase fronts of optical vortices make them useful in many applications. Here we report for the first time efficient Raman frequency conversion between vortex modes in twisted hydrogen-filled single-ring hollow core photonic crystal fiber (SR-PCF). High fidelity transmission of optical vortices in untwisted SR-PCF becomes more and more difficult as the orbital angular momentum (OAM) order increases, due to scattering at structural imperfections in the fiber microstructure. In helically twisted SR-PCF, however, the degeneracy between left- an…
We report a novel source of twin beams based on modulational instability in high-pressure argon-filled hollow-core kagomé-style photonic-crystal fibre. The source is Raman-free and manifests strong photon-number correlations for femtosecond pulses of squeezed vacuum with a record brightness of ~2500 photons per mode. The ultra-broadband (~50 THz) twin beams are frequency tunable and contain one spatial and less than 5 frequency modes.
Backward stimulated Raman scattering in gases provides a promising route to compression and amplification of a Stokes seed-pulse by counter-propagating against a pump-pulse, as has been already demonstrated in various platforms, mainly in free-space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backw…
We report evidence of beam cleanup during stimulated Raman scattering in silicon. An amplified near-diffraction-limited Stokes beam is obtained from a severely aberrated pump beam.
We present the results of an experimental and numerical investigation into temporally non-local coherent interactions between ultrashort pulses, mediated by Raman coherence, in gas-filled kagomé-style hollow-core photonic crystal fiber. A pump pulse first set up the Raman coherence, creating a refractive index grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second probe pulse allows high degree of control over its evolution as it propagates along the fiber, in particular soliton self-compression and dispersive wave (DW) emission. In the ex…
In this paper we consider mid-infrared Raman lasers based on gas-filled hollow-core silica fibers and provide theoretical and experimental analysis of factors that limit the efficiency and output power of these lasers. As a result, we realized an efficient ns-pulsed 4.42 μm Raman laser based on an 1H2-filled revolver silica fiber. Quantum efficiency as high as 36 % is achieved, and output average power as high as 250 mW is demonstrated. The possibilities of further improving the laser efficiency are discussed.
We study coherent anti-Stokes Raman spectroscopy in air-filled anti-resonance hollow-core photonic crystal fiber, otherwise known as 'revolver' fiber. We compare the vibrational coherent anti-Stokes Raman signal of N$_2$, at 2331 cm$^{-1}$, generated in ambient air (no fiber present), with the one generated in a 2.96 cm of a revolver fiber. We show a 170 times enhancement for the signal produced in the fiber, due to an increased interaction path. Remarkably, the N$_2$ signal obtained in the revolver fiber shows near-zero non-resonant background, due to near-zero overlap between the laser field…
Strong mid-infrared light-matter interaction has attracted extensive attention as it opens up new frontiers in nonlinear optics. Here we observe through simulations a novel aspect of mid-infrared pulse dynamics in a high-pressure gas-filled capillary, where a pulse with a power well below the critical power for Kerr self-focusing undergoes an astonishing rise of the peak intensity following an extremely efficient spectral broadening. This intensity enhancement is attributed to the Kerr-induced focusing of the supercontinuum. Our study provides an interesting perspective for controlling the las…
We present the design and anticipated performance of an optically pumped monolithic all-silicon mid-IR (MIR) Raman laser. For the proposed 3.15 um cw laser design pumped at a 2.71 um pump wavelength, we estimate a threshold pump power of < 10 mW and an estimated output laser linewidth of <50 MHz. This design may pave the road toward demonstration of the first mid-IR silicon Raman laser.
The possibility of performing time-resolved spectroscopic studies in the molecular fingerprinting region or extending the cut-off wavelength of high-harmonic generation has recently boosted the development of efficient mid-infrared ultrafast lasers. In particular, fibre lasers based on active media such as thulium or holmium are a very active area of research since they are robust, compact and can operate at high repetition rates. These systems, however, are still complex, are unable to deliver pulses shorter than 100 fs and are not yet as mature as their near-infrared counterparts. Here we re…
We report a study of soliton self-frequency shifting in hydrogen-filled hollow-core fiber. The combination of hydrogen and short 40-fs input pulses underlies clean and efficient generation of Raman solitons between 1080 and 1600 nm. With 240-nJ input pulses, the Raman soliton energy ranges from 110 to 20 nJ over that wavelength range, and the pulse duration is approximately 45 fs. In particular, 70-nJ and 42-fs pulses are generated at 1300 nm. Numerical simulations agree reasonably well with experiments and predict that microjoule-energy tunable pulses should be possible with higher-energy inp…
Based on the interplay between photoionization and Raman effect in gas-filled photonic crystal fibers, we propose a new optical device to control frequency-conversion of ultrashort pulses. By tuning the input-pulse energy, the output spectrum can be either down-converted, up-converted, or even frequency-shift compensated. For low input energies, Raman effect is dominant and leads to a redshift that increases linearly during propagation. For larger pulse energies, photoionization starts to take over the frequency conversion process, and induces a strong blueshift. We have found also that the fi…
Ultrafast broadband ultraviolet radiation is of importance in spectroscopy and photochemistry, since high photon energies enable single-photon excitations and ultrashort pulses allow time-resolved studies. Here we report the use of gas-filled hollow-core photonic crystal fibers (HC-PCFs) for efficient ultrafast nonlinear optics in the ultraviolet. Soliton self-compression of 400 nm pulses of (unprecedentedly low) ~500 nJ energies down to sub-6-fs durations is achieved, as well as resonant emission of tunable dispersive waves from these solitons. In addition, we discuss the generation of a flat…
We demonstrate for the first time a mid infrared silicon Raman amplifier. Amplification of 12 dB is reported for a signal at 3.39 micron wavelength. The active medium was a 2.5 cm long silicon sample that was pumped with 5ns pulses at 2.88 micron. The absence of the nonlinear losses which severely limit the performance of silicon Raman devices in the near infrared combined with unsurpassed crystal quality, high thermal conductivity render silicon a very attractive Raman medium. Such a technology can potentially extend silicon photonics application beyond data communication in the near infrared…
We demonstrate the first low-noise mid-IR frequency comb source using a silicon microresonator. Our observation of strong Raman scattering lines in the generated comb suggests that Raman and four-wave mixing interactions play a role in assisting the transition to the low-noise state. In addition, we characterize, the intracavity comb generation dynamics using an integrated PIN diode, which takes advantage of the inherent three-photon absorption process in silicon.
We report generation of an ultrafast supercontinuum extending into the mid- infrared in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) pumped by 1.7 $μ$m light from an optical parametric amplifier. The simple fiber structure offers shallow dispersion and flat transmission in the near and mid-infrared, enabling the generation of broadband spectra extending from 300 nm to 3.1 $μ$m, with a total energy of a few $μ$J. In addition, we report the emission of ultraviolet dispersive waves whose frequency can be tuned simply by adjusting the pump wavelength. SR-PCF also provides an …
Raman-enhanced spectral compression of high-energy femtosecond laser pulses in molecular gases
Huazhong University of Science and Technology（华中科技大学）；Wuhan National Laboratory for Optoelectronics（武汉光电国家研究中心）；Cornell University（康奈尔大学）；Xi’an Institute of Applied Optics（西安应用光学研究所）
We report a novel scheme for the generation of dense clusters of Raman sidebands. The scheme uses a broadband-guiding hollow-core photonic crystal fiber (HC-PCF) filled with a mixture of H2, D2, and Xe for efficient interaction between the gas mixture and a green laser pump pulse (532 nm, 1 ns) of only 5 uJ energy. This results in the generation from noise of more than 135 ro-vibrational Raman sidebands covering the visible spectral region with an average spacing of only 2 THz. Such a spectrally dense and compact fiber-based source is ideal for applications where closely spaced narrow-band las…
When a laser pump beam of sufficient intensity is incident on a Raman-active medium such as hydrogen gas, a strong Stokes signal, red-shifted by the Raman transition frequency Ω$_R$, is generated. This is accompanied by the creation of a "coherence wave" of synchronized molecular oscillations with wavevector Δβ determined by the optical dispersion. Within its lifetime, this coherence wave can be used to shift by Ω$_R$ the frequency of a third "mixing" signal, provided phase-matching is satisfied, i.e., Δβ is matched. Conventionally this can be arranged using non-collinear beams or higher-order…
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) …
Gas-filled hollow-core photonic crystal fibre (PCF) is being used to generate ever wider supercontinuum spectra, in particular via dispersive wave (DW) emission in the deep and vacuum ultraviolet, with a multitude of applications. DWs are the result of the resonant transfer of energy from a self-compressed soliton, a process which relies crucially on phase matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched DW generation in the mid-infrared (MIR)-something that is forbidden in…
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 μm. 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…
We propose an optical trapping technique in which a fundamental soliton traps an ultrashort small amplitude radiation in a symmetric hollow-core photonic crystal fiber filled with a noble gas, preventing its dispersion. The system is Raman- and plasma-free. Trapping is due to the cross phase modulation effect between the two pulses. The trapped radiation inside the soliton-induced potential will oscillate periodically due to the shock effect, similar to the motion of a mechanical pendulum.
We report on two types of Raman laser sources emitting in the near and middle ultraviolet spectral ranges by the use of a solarization-resilient gas-filled inhibited-coupling (IC) hollow-core photonic-crystal fiber (HCPCF) with record low transmission loss (minimum of 5 dB/km at 480 nm). The first source type emits a Raman comb generated in a hydrogen-filled HCPCF pumped by a 355 nm wavelength microchip nanosecond pulsed laser. The generated comb lines span from 270 nm to the near-infrared region with no less than 20 lines in the 270-400 nm wavelength range. The second type stands for the firs…
We show theoretically that the photoionization process in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas leads to a constant acceleration of solitons in the time domain with a continuous shift to higher frequencies, limited only by ionization loss. This phenomenon is opposite to the well-known Raman self-frequency red-shift of solitons in solid-core glass fibers. We also predict the existence of unconventional long-range non-local soliton interactions leading to spectral and temporal soliton clustering. Furthermore, if the core is filled with a Raman-active molecul…
A unified framework for sensitivity comparison across stimulated Raman, stimulated Raman photothermal, and mid-infrared photothermal microscopy
Institute for Photon Science and Technology, The University of Tokyo（东京大学光子科学研究所）；Department of Advanced Materials Science, The University of Tokyo（东京大学先进材料科学系）；RIKEN Center for Advanced Photonics, RIKEN（理化学研究所先进光子学中心）
We study theoretically the propagation of relatively long pulses with ionizing intensities in a hollow-core photonic crystal fiber filled with a Raman-inactive gas. Due to photoionization, previously unknown types of asymmetric self-phase modulation and `universal' modulational instabilities existing in both normal and anomalous dispersion regions appear. We also show that it is possible to spontaneously generate a plasma-induced continuum of blueshifting solitons, opening up new possibilities for pushing supercontinuum generation towards shorter and shorter wavelengths.
We numerically study the mechanisms of frequency comb generation in the mid-infrared spectral region from cw pumped silicon microring resonators. Coherent soliton comb generation may be obtained even for a pump with zero linear cavity detuning, through suitable control of the effective lifetime of free-carriers from multiphoton absorption, which introduces a nonlinear cavity detuning via free-carrier dispersion. Conditions for optimal octave spanning Raman comb generation are also described.
We present a technique that uses noisy broadband pulse bursts generated by modulational instability to probe nonlinear processes, including infrared-inactive Raman transitions, in molecular gases. These processes imprint correlations between different regions of the noisy spectrum, which can be detected by acquiring single shot spectra and calculating the Pearson correlation coefficient between the different frequency components. Numerical simulations verify the experimental measurements and are used to further understand the system and discuss methods to improve the signal strength and the sp…
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 μm by pumping an argon-filled hollow-core a…
Supercritical Xe at 293 K offers a Kerr nonlinearity that can exceed that of fused silica while being free of Raman scattering. It also has a much higher optical damage threshold and a transparency window that extends from the UV to the infrared. We report the observation of nonlinear phenomena, such as self-phase modulation, in hollow-core photonic crystal fiber filled with supercritical Xe. In the subcritical regime, intermodal four-wave-mixing resulted in the generation of UV light in the HE12 mode. The normal dispersion of the fiber at high pressures means that spectral broadening can clea…
We propose a scheme for the emission of few-cycle dispersive waves in the mid-infrared using hollow-core photonic crystal fibers filled with noble gas. The underlying mechanism is the formation of a plasma cloud by a self-compressed, sub-cycle pump pulse. The resulting free-electron population modifies the fiber dispersion, allowing phase-matched access to dispersive waves at otherwise inaccessible frequencies, well into the mid-IR. Remarkably, the pulses generated turn out to have durations of the order of two optical cycles. In addition, this ultrafast emission, which occurs even in the abse…
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 μJ 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 …
We report on experimental and theoretical demonstration of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerr-type nonlinearity. This combination enabled to transform a conventional quasi-periodic Raman comb into a comb with a single and smaller frequency spacing. This new phenomenon is realized using a hollow-core photonic crystal fiber filled with 40 bars of deuterium, and pumped with a high-power picosecond laser. The resultant comb shows more than 100 spectral lines spanning over 220 THz fr…
Many reports on stimulated Raman scattering in mixtures of Raman-active and noble gases indicate that the addition of a dispersive buffer gas increases the phase-mismatch to higher-order Stokes and antiStokes sidebands, resulting in preferential conversion to the first few Stokes lines, accompanied by a significant reduction in Raman gain due to collisions with buffer gas molecules. Here we report that, provided the dispersion can be precisely controlled, the effective Raman gain in gas-filled hollow-core photonic crystal fiber (PCF) can actually be significantly enhanced when a buffer gas is …
Highly-efficient perturbative Raman shifting by engineering the nonlinear temporal response
School of Applied and Engineering Physics, Cornell University（康奈尔大学应用工程物理学院）；CREOL, The College of Optics and Photonics, University of Central Florida（中佛罗里达大学光学与光子学院CREOL）
Coherent interconversion of signals between optical and mechanical domains is enabled by optomechanical interactions. Extreme light-matter coupling produced by confining light to nanoscale mode volumes can then access single mid-infrared (MIR) photon sensitivity. Here we utilise the infrared absorption and Raman activity of molecular vibrations in plasmonic nanocavities to demonstrate frequency upconversion. We convert λ~10 μm incoming light to visible via surface-enhanced Raman scattering (SERS) in doubly-resonant antennas that enhance upconversion by >10^10. We show >200% amplification of th…
Generation of laser-like narrow bandwidth emissions from nitrogen molecular ions generated in intense near- and mid-infrared femtosecond laser fields has aroused much interest because of the mysterious physics underlying such a phenomenon as well as the potential application of such an effect in atmospheric spectroscopic sensing. Here, we perform a pump-probe measurement on the nonlinear interaction of rotational quantum wavepackets of nitrogen molecular ions generated in mid-infrared (e.g., at a wavelength centered at 1580 nm) femtosecond laser fields with an ultrashort probe pulse whose broa…
We report on the generation of broadband, high-energy femtosecond pulses centered at 1.28 um by stimulated Raman scattering in pressurized hydrogen cell. Stimulated Raman scattering is performed by two chirped and delayed pulses originating from a multi-mJ Ti:Sapphire amplifier. The Stokes pulse carries energy of 4.4 mJ and is recompressed down to 66 fs by reflective grating pair. We characterized the short-wavelength mid-infrared source in view of energy stability, beam profile and conversion efficiency at a repetition rate of 100 Hz and 10 Hz. The demonstrated laser will benefit intense THz …
