Showing papers by "Philip St. J. Russell published in 2021"

Scaling rules for high quality soliton self-compression in hollow-core fibers

Abstract: Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Synthesis and dissociation of soliton molecules in parallel optical-soliton reactors

Abstract: Mode-locked lasers have been widely used to explore interactions between optical solitons, including bound-soliton states that may be regarded as "photonic molecules". Conventional mode-locked lasers normally, however, host at most only a few solitons, which means that stochastic behaviours involving large numbers of solitons cannot easily be studied under controlled experimental conditions. Here we report the use of an optoacoustically mode-locked fibre laser to create hundreds of temporal traps or "reactors" in parallel, within each of which multiple solitons can be isolated and controlled both globally and individually using all-optical methods. We achieve on-demand synthesis and dissociation of soliton molecules within these reactors, in this way unfolding a novel panorama of diverse dynamics in which the statistics of multi-soliton interactions can be studied. The results are of crucial importance in understanding dynamical soliton interactions and may motivate potential applications for all-optical control of ultrafast light fields in optical resonators.

Broadband mid-infrared supercontinuum generation in dispersion-engineered As 2 S 3 -silica nanospike waveguides pumped by 2.8 μm femtosecond laser

Abstract: Broadband mid-infrared (IR) supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region. Here, we report generation of octave-spanning and coherent mid-IR supercontinua in As2S3-silica nanospike hybrid waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The waveguides are formed by pressure-assisted melt-filling of molten As2S3 into silica capillaries, allowing the dispersion and nonlinearity to be precisely tailored. Continuous coherent spectra spanning from 1.1 μm to 4.8 μm (30 dB level) are observed when the waveguide is designed so that 2.8 μm lies in the anomalous dispersion regime. Moreover, linearly tapered millimeter-scale As2S3-silica waveguides are fabricated and investigated for the first time, to the best of our knowledge, showing much broader supercontinua than uniform waveguides, with improved spectral coherence. The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As2S3 by the fused silica sheath. They offer an alternative route to generating broadband mid-IR supercontinua, with applications in frequency metrology and molecular spectroscopy, especially in humid and aqueous environments.

Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction.

Abstract: Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV2+) to the radical XV•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Efficient self-compression of ultrashort near-UV pulses in air-filled hollow-core photonic crystal fibers.

Abstract: We report generation of ultrashort near-UV pulses by soliton self-compression in kagome-style hollow-core photonic crystal fibers filled with ambient air. Pump pulses with the energy of 2.6 µJ and duration of 54 fs at 400 nm were compressed temporally by a factor of 5, to a duration of ∼11 fs. The experimental results are supported by numerical simulations, showing that both Raman and Kerr effects play a role in the compression dynamics. The convenience of using ambient air and the absence of glass windows that would distort the compressed pulses makes the setup highly attractive as the basis of an efficient table-top UV pulse compressor.

Stimulated Brillouin scattering in chiral photonic crystal fiber

Abstract: Stimulated Brillouin scattering (SBS) has many applications, for example, in sensing, microwave photonics and signal processing. Here we report the first experimental study of SBS in chiral photonic crystal fiber (PCF), which displays optical activity and robustly maintains circular polarization states against external perturbations. As a result, circularly polarized pump light is cleanly back-scattered into a Stokes signal with the orthogonal circular polarization state, as is required by angular momentum conservation. By comparison, untwisted PCF generates a Stokes signal with an unpredictable polarization state, owing to its high sensitivity to external perturbations. We use chiral PCF to realize a circularly polarized continuous-wave Brillouin laser. The results pave the way to a new generation of stable circularly polarized SBS systems with applications in quantum manipulation, optical tweezers, optical gyroscopes and fiber sensors.

Tumbling and anomalous alignment of optically levitated anisotropic microparticles in chiral hollow-core photonic crystal fiber.

Abstract: The complex tumbling motion of spinning nonspherical objects is a topic of enduring interest, both in popular culture and in advanced scientific research. Here, we report all-optical control of the spin, precession, and nutation of vaterite microparticles levitated by counterpropagating circularly polarized laser beams guided in chiral hollow-core fiber. The circularly polarized light causes the anisotropic particles to spin about the fiber axis, while, regulated by minimization of free energy, dipole forces tend to align the extraordinary optical axis of positive uniaxial particles into the plane of rotating electric field. The end result is that, accompanied by oscillatory nutation, the optical axis reaches a stable tilt angle with respect to the plane of the electric field. The results reveal new possibilities for manipulating optical alignment through rotational degrees of freedom, with applications in the control of micromotors and microgyroscopes, laser alignment of polyatomic molecules, and study of rotational cell mechanics.

Cross-phase modulational instability of circularly polarized helical Bloch modes carrying optical vortices in a chiral three-core photonic crystal fiber

Abstract: We report the first, to the best of our knowledge, observation of cross-phase modulational instability (XPMI) of circularly polarized helical Bloch modes carrying optical vortices in a twisted photonic crystal fiber with a three-fold symmetric core, formed by spinning the fiber preform during the draw. When the fiber is pumped by a superposition of left-circular polarization (LCP) and right-circular polarization (RCP) modes, a pair of orthogonal circularly polarized sidebands of opposite topological charge is generated. When, on the other hand, a pure LCP (or RCP) mode is launched, the XPMI gain is zero, and no sidebands are seen. This observation has not been seen before in any system and is unique to chiral structures with N-fold rotational symmetry. The polarization state and topological charge of the generated sidebands are measured. By decomposing the helical Bloch modes into their azimuthal harmonics, we are able to deduce the selection rules for the appearance of modulational instability sidebands. We showed that the four waves in the nonlinear mixing process must exhibit the same set of azimuthal harmonic orders.

Deep-UV-enhanced supercontinuum generated in a tapered gas-filled photonic crystal fiber.

Abstract: We present the use of a linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single stage, pumped with pulses from a compact infrared (IR) laser source, to generate a supercontinuum (SC) carrying significant spectral power in the deep ultraviolet (UV) [200–300 nm]. The generated SC extends from the near IR down to ∼213nm with 0.58 mW/nm and down to ∼220nm with 0.83 mW/nm in the deep UV.

Efficient self-compression of ultrashort UV pulses in air-filled hollow-core photonic crystal fiber

Abstract: We report generation of ultrashort UV pulses by soliton self-compression in kagome-style hollow-core photonic crystal fiber filled with ambient air. Pump pulses with energy 2.6 uJ and duration 54 fs at 400 nm were compressed temporally by a factor of 5, to a duration of ~11 fs. The experimental results are supported by numerical simulations, showing that both Raman and Kerr effects play a role in the compression dynamics. The convenience of using ambient air, and the absence of glass windows that would distort the compressed pulses, makes the setup highly attractive as the basis of an efficient table-top UV pulse compressor.

Deep-UV-enhanced supercontinuum generated in tapered gas-filled photonic crystal fiber

Abstract: We present the use of linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single-stage, pumped with pulses from a compact infrared laser source, to generate a supercontinuum carrying significant spectral power in the deep ultraviolet (200 - 300 nm). The generated supercontinuum extends from the near infrared down to around 213 nm with up to 0.83 mW/nm in the deep ultraviolet.

Doppler optical frequency domain reflectometry for remote fiber sensing.

Abstract: Coherent optical frequency domain reflectometry has been widely used to locate static reflectors with high spatial resolution. Here, we present a new type of Doppler optical frequency domain reflectometry that offers simultaneous measurement of the position and speed of moving objects. The system is exploited to track optically levitated “flying” particles inside a hollow-core photonic crystal fiber. As an example, we demonstrate distributed temperature sensing with sub-mm-scale spatial resolution and a standard deviation of ∼10°C up to 200°C.

Doppler optical frequency domain reflectometry for remote fiber sensing: erratum.

Abstract: This erratum corrects a typographical error in Eq. (10) of our paper [Opt. Express29, 14615 (2021)10.1364/OE.421842].

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle

Abstract: We propose a platform that combines the fields of cavity optomagnonics and levitated optomechanics in order to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to the spin oscillations and induce fast rotations of the particle around its anisotropy axis.

UV Extension of Supercontinuum via Tapered Single-ring PCF

Abstract: Short wavelength light sources in the blue and ultraviolet (UV) are extremely useful for applications such as semiconductor metrology, photolithography, molecular spectroscopy and high performance liquid chromatography [1] . Gas-filled single-ring hollow-core photonic crystal fibre (SR-PCF), guiding by anti-resonant-reflection, is highly suitable for efficient generation of broadband supercontinua and tunable narrowband UV light because its nonlinearity and dispersion can be tuned by varying the species and pressure of gas [2] . We report the use of tapered SR-PCF filled with 15 bar krypton, pumped with ultrashort (220 fs) 1030 nm pulses carrying 7.8 μJ of energy from a compact laser source, to extend the UV bandwidth and achieve significant spectral power in the 200-350 nm region. The pulses are launched into the untapered end of a SR-PCF with core diameter of 36 μm and an average core-wall thickness of 386 nm (standard deviation 23 nm). Anti-crossings between the core modes and the resonances in the cladding walls result in loss peaks that interrupt the broad transmission window of the SRPCF. Upon propagation, the pulses break up due to modulational instability, resulting in a broad supercontinuum, the blue side of which forms by dispersive wave (DW) emission. By tapering the SR-PCF to half its original dimensions, the phase-matching wavelength for DW emission shifts to higher frequencies [3] , following the shift in zero dispersion wavelength indicated in Fig. 1(a) by dashed lines. As the thickness of the capillary walls falls, the loss peaks (marked by the shaded grey areas in Fig. 1(b) ) shift further into the blue, as previously reported [4] . As a result, the generated UV band extends down to 220 nm in the tapered fibre, a considerable improvement over untapered fibre, where the high frequency edge ends at 310 nm ( Fig. 1(a) ). The tapers were fabricated by brushing an oxy-butane flame across the fibre while pulling it apart axially, keeping the core at near vacuum pressure (60 mbar) and the cladding capillaries at atmospheric pressure [4] . The insets in Fig. 1(b) show optical micrographs of the untapered (above) and tapered (below) ends of the SR-PCF.

Reconfigurable millimeter-range optical binding of dielectric microparticles in hollow-core photonic crystal fiber.

Abstract: Optical binding of microparticles offers a versatile playground for investigating the optomechanics of levitated multi-particle systems. We report millimeter-range optical binding of polystyrene microparticles in hollow-core photonic crystal fiber. The first particle scatters the incident LP01 mode into several LP0n modes, creating a beat pattern that exerts a position-dependent force on the second particle. Particle binding results from the interplay of the forces created by counterpropagating beams. A femtosecond trapping laser is used so that group velocity walk-off eliminates disturbance caused by higher order modes accidentally excited at the fiber input. The inter-particle distance can be optically switched over 2 orders of magnitude (from 42 µm to 3 mm), and the bound particle pairs can be translated along the fiber by unbalancing the powers in the counterpropagating trapping beams. The frequency response of a bound particle pair is investigated at low gas pressure by driving with an intensity-modulated control beam. The system offers new degrees of freedom for manipulating the dynamics and configurations of optically levitated microparticle arrays.

Position Measurement of Multiple Microparticles in Hollow- Core Photonic Crystal Fiber by Coherent Optical Frequency Domain Reflectometry

Abstract: Flying particle sensors in hollow-core photonic crystal fibers require accurate localization of the optically trapped microparticles. We report position measurement to micrometer- resolution, using optical frequency domain reflectometry, of two 1.65-µm-diameter polystyrene particles.

Gas Mixtures to Suppress Thermal Buildup Effects Caused by High-Repetition-Rate Photoionization of Confined Gases

Abstract: Gas-based systems (e.g. based on hollow-core fibres) are extensively used over a wide spectral range to temporally compress ultrashort pulses. Recent progress in solid-state lasers and compression schemes has enabled the generation of sub-femtosecond pulses at unprecedented energies and average powers [1] – [3] . However, the latest generation of lasers deliver mJ pulses at hundreds-of-kHz repetition rates [4] , which raises new challenges for gas-based pulse compression. At high repetition rates, even weak single-shot energy deposition in the gas (e.g. by ionisation-driven heating) is sufficient to cause a transversely non-uniform gas density depression to build up pulse-by-pulse [5] , leading to thermal instabilities. Using lighter noble gases is often insufficient to mitigate this, despite the weaker nonlinear absorption. This is because sufficient nonlinearity for spectral broadening requires substantially higher pressures, resulting in much slower gas relaxation (the thermal diffusivity scales inversely with gas density).

Position Measurement of Multiple Microparticles in Hollow-Core Photonic Crystal Fiber by Coherent Optical Frequency Domain Reflectometry

Abstract: Flying particle sensors in hollow-core photonic crystal fibers require accurate localization of the optically trapped microparticles. We report position measurement to micrometer-resolution, using optical frequency domain reflectometry, of two 1.65-$\mu$m-diameter polystyrene particles.

Soliton-effect self-compression: limits and high repetition rate scaling

Abstract: Soliton dynamics can be used for compressing optical pulses to few fs durations over a wide spectral range, and can be conveniently scaled in gas-filled hollow-core fibre by suitable design of the fibre structure and choice of gas [1] , [2] . In this fashion, similar patterns of pulse propagation can be obtained at pulse energies ranging from a few tens of nJ up to the mJ-level, and for pulse durations τ 0 up to hundreds of fs. The dynamics are controlled by the soliton order N and the spectral distance of the pulse frequency ν 0 from the zero dispersion frequency ν ZD . Here we report the existence of a region in ( N , ν ZD , τ 0 )-space where soliton-effect self-compression is optimal, provided the transmission loss of the fibre is negligible. We assess the limits set by modulational instability (MI) [3] , self-focusing (SF), and photoionisation (ION) [1] and derive a new limit set by third-order dispersion (TOD). By numerical simulation (not shown) we validate these limits, observing that if they are exceeded the compression quality degrades. Furthermore, we investigate, both theoretically and experimentally, scaling to MHz-level repetition rates, when inter-pulse effects cannot be neglected.

Importance of Topological Charge Preservation in Vectorial Modulational Instability in Chiral Three-Core PCF

Abstract: Circular-symmetric optical fibres are ideal for studying nonlinear optical processes involving optical vortices [1] . Chiral photonic crystal fibres (PCFs), fabricated either by post-processing techniques or by spinning the preform during fibre drawing, have been shown to support circularly polarized helical Bloch modes (HBMs), which are superpositions of multiple vortices with topological charges l ( m ) = l (0) + mN , where N is the number of azimuthal periods and m the order of the m -th azimuthal Bloch harmonic [2] . Here we report on the importance of the preservation of topological charge in polarisation modulational instability (PMI) between circularly polarised modes in a circularly birefringent chiral PCF with a 3-fold rotationally symmetric core.

Mid-IR supercontinua in dispersion-engineered As 2 S 3 -silica nanospike waveguides pumped by fs pulses at 2.8 μm

Abstract: We report generation of coherent octave-spanning mid-infrared supercontinua in As2S3-silica waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The fabricated hybrid waveguides are demonstrated to be long-term stable and water-resistant.

Quantum-Correlation-Preserving Single-Photon Conversion by Molecular Modulation in Gas-filled Hollow-Core Fibres

Abstract: Preservation of the photon statistics of quantum states of light upon frequency conversion is of great importance for modern quantum optics. For example, it holds the key to practical implementation of systems such as quantum networks which often require interfacing of different sub-units that do not operate in the same frequency bands. Although many integrated schemes have been demonstrated [1] , [2] , most offer small frequency shifts, limited tunability, and often suffer from high insertion loss and Raman noise generated in bulk materials.

Optoacoustic mode-locking based on micro-core photonic crystal fibre

Abstract: We review the principles and the implementation of optoacoustic mode-locking based on stimulated Raman-like scattering in PCF. Optically-driven acoustic vibrations in the few-µm-sized PCF-core creates a self-sustained temporal optomechanical lattice in a mode-locked laser cavity, enabling self-stabilized high-harmonic mode-locking as well as various highly-ordered compound pulse-patterns. (50 words).

340 - 40,000 nm coherent light source

Abstract: We present an ultra-high brightness source of carrier-to-envelope phase-controlled light waveforms providing simultaneous spectral coverage across 7 optical octaves. The spectral brightness 2–5 orders of magnitude above synchrotron sources ranging from 340 nm to 40,000 nm

Towards CEP-stable single-cycle pulses with microjoule-level energy at 8 MHz repetition rate

Abstract: Low-noise trains of few-fs pulses are widely used for studying and controlling ultrafast dynamics in matter. Microjoule-level pulse energies are often more than sufficient to trigger and observe these dynamics, and repetition rates in the few MHz range allow reduced acquisition times and improved signal-to-noise ratios—crucial for studies of processes with low-interaction cross-sections. The latest generation of fibre lasers can provide ultrashort pulses with stable carrier-envelope phase (CEP) at unprecedented energies and hundreds of kHz repetition rates [1] , [2] . However, because the generated pulses are hundreds of fs long, multiple compression stages are required to reach the few fs regime, thus increasing the system complexity and reducing the throughput efficiency.