Guided-wave Brillouin scattering in air

Hollow core photonic crystal fibres (HC-PCFs) have promising applications in high power beam delivery, chemical sensing and gas-based nonlinear optics. According to the waveguide mechanisms they based on, HC-PCFs are typically classified into two kinds. One kind guide light via a two dimensional photonic bandgap while the other through anti-resonant reflection. The former one is so-called hollow core photonic bandgap fibres (HC-PBGFs) while the latter is commonly named hollow core anti-resonant fibres (HAFs).Compared to HC-PBGFs, HAFs can have much broader transmission windows and more flexible fibre designs. As a result, HAFs are currently more popular than HC-PBGFs. Hence the topic of this thesis is focused on HAFs. HAFs have already been demonstrated to have potential on the application of gas filled fibre lasers. As early as 2002, the first demonstration of the Raman scattering emission had been observed with an H2 filled HAF. During the past decades, a lot of efforts have been devoted into the gas filled HAF lasers. However, so far there is no demonstration of any all HAF based fibre laser because there exist two restrictions. The first restriction is that hollow core fibres are favoured in long wavelength regions where solid core glass fibres suffer high material loss, besides, a small cavity cross section (i.e., fibre core size) is preferred to reduce the lasing threshold. However, the confinement loss of a HAF (i.e., cavity loss) increases fastly when core size gets smaller or wavelength gets longer. The second restriction is there is no hollow core fibre based fibre coupler to build an all hollow core fibre based laser ring cavity. Therefore, the task of this thesis is to get rid of these two restrictions to make our contributions to all HAF based fibre lasers. Firstly, I develop the hollow core fibre fabrication technique based on our local facilities. Secondly, I design, fabricate, and characterize a HAF which can have relatively low transmission loss in small core and long wavelength conditions. Thirdly, I design, fabricate, and characterize a HAF based fibre coupler whose power splitting ratio is tunable by adjusting the fibre strain. Thirdly, I use the HAF based fibre coupler to build a ring cavity in a fibre laser system, where the tunable feature of the fibre coupler helps to optimize the conversion efficiency. Moreover, the HAF based fibre coupler is experimentally proved to be able to split and deliver the ultra fast laser pulse without degradation.

Over the last two decades the interest in photonic crystal fiber (PCF) has grown considerably, particularly in nonlinear optics where it allows enhanced control over the dispersion landscape. Although silica is the material most commonly used to fabricate PCF, its limited window of transmission and its susceptibility to optical damage at wavelengths below ~350nm is driving the development of fibers made from glasses with transmission windows extending into the deep ultraviolet and the mid-infrared. An alternative is offered by gas-filled hollow-core fiber, in which the light propagates predominantly in the gas. In kagome-style hollow-core PCF filled with noble gas, the weak anomalous dispersion of the empty fiber is balanced by the normal dispersion of the filling gas, resulting in a versatile system whose dispersion landscape can be adjusted in real time [Travers et al., JOSAB 28, A11 (2011)]. Under appropriate conditions the launched pulse undergoes soliton self-compression followed by emission of a band of dispersive radiation in the UV. UV light tunable down to 113 nm has been generated with this technique [Russell et al., Nat. Photon. 8, 278 (2014)]. Solid-core ZBLAN (fluorozirconate) glass PCF is transparent from 0.2 to ~7.8µm. Launching ~1nJ 140fs pulses at 1µm wavelength into a ~1µm diameter core resulted, after 4cm of propagation, in generation of a supercontinuum spectrum extending from ~210nm to beyond 2µm. In strong contrast to silica PCF, the ZBLAN PCF showed no signs of any solarization-related damage, even when operating over many hours [Jiang et al., Nat. Photon. 9, 133 (2015)].

Researchers at the University of Adelaide, Australia, and the Islamic University of Technology, Bangladesh, report the optimum design for a porous photonic crystal fibre capable of terahertz frequency transmission. Photonic crystal fibres (PCFs), also known as micro-structured optical fibres (MOFs), are a new class of optical fibre. The significance of PCFs is their remarkable optical properties, such as providing single mode operation, high birefringence, and controllable chromatic dispersion characteristics, which are not obtainable using conventional optical fibres. As a result, PCFs are used extensively in different medical imaging applications, for example, in Optical Coherence Tomography (OCT). OCT is an imaging technology that allows non-invasive cross-sectional imaging of biological tissue. Bringing such techniques into the terahertz domain will be assisted by the development of PCFs that operate in the terahertz regime. There are different types of PCFs, including solid core PCF, hollow core PCF and porous core PCF. A solid core PCF experiences large material loss, which is a property that makes these PCFs unsuitable for terahertz signal transmission. In a hollow core PCF, the loss is greatly reduced, but such fibres operate over a narrow frequency range. To overcome the described disadvantages of solid core and hollow core fibres, porous core fibres were introduced. A porous core fibre contains a number of micro-structured air holes in the core. The advantage of the porous structure is that the fibre global parameters, including air hole size, pitch distance (centre-to-centre distance between air holes), core diameter and shape of air holes can be tuned as required for different applications. Thus, for porous fibre PCFs, the effective material loss, birefringence, dispersion, confinement loss, numerical aperture and other modal properties can be obtained by design. Nowadays, PCFs are used in the field of sensing, biomedical imaging, time domain spectroscopy, security, detecting DNA hybridisation and bio-sensing, skin cancer and breast cancer detection, and in communication applications. However, there are no previously reported articles on terahertz PCFs with sufficient numerical aperture (NA) to be used for OCT applications. In this issue of Electronics Letters, Saiful Islam and colleagues propose a terahertz PCF with high birefringence, high NA, and near zero dispersion variation. The measured high birefringence is reported to be due to the elliptical-shaped asymmetrical structure of the air holes inside the core. The asymmetrical structure of air holes created a refractive index difference between the polarisation modes, thus generating birefringence. The observation of high NA is reported to be due to the high birefringence and high modal effective area. Moreover, flattened dispersion is reported due to the waveguide structure and chosen materials that the waveguide was formed of. The kagome-based PCF has the advantage that it has a significantly lower confinement loss than other PCF structures. Terahertz time-domain spectroscopy set-up in the National T-Ray lab at the University of Adelaide. Saiful Islam at the University of Adelaide. The reported simulations in this Letter have allowed for rapid design optimisation that can now be experimentally tested. The major challenge of a PCF design is to anticipate difficulties in fabrication. Thus, the fabrication possibilities of a PCF must be addressed. For accuracy, obtaining correct parameters to describe the structural and optical properties of the fibre, such as refractive index, material absorption loss, relative permittivity and relative permeability, is of great importance. S. Islam states that ‘‘Our PCF design was produced with ease of fabrication in mind, allowing future realisation. Thus, applications in THz-OCT, chemical sensing, and multichannel communications are envisaged.’’ S. Islam and colleagues are currently using photonic crystal fibres to design chemical sensors for skin cancer detection. Non-invasive breast cancer detection is possible via detecting the presence of certain protein markers in human saliva. The authors are also designing sensors to detect hazardous chemicals for security applications. ‘‘We are planning to extend our PCF designs with the use of graphene or gold coasting to exploit surface plasmons for enhanced sensing’’, explains S. Islam. In practice, PCFs are limited to reasonably simple geometries due to difficulties in fabrication. However, emerging techniques exploiting developments in 3D printing are promising methods to overcome current limitations. S. Islam predicts that ‘‘It is possible that in the next decade will we begin to see more sophisticated PCFs with remarkable properties that are not deemed practical in today's technology.’’

We report on a highly efficient experimental scheme for the generation of deep-ultraviolet (UV) ultrashort light pulses using four-wave mixing in gas-filled kagomé-style photonic crystal fiber. By pumping with ultrashort, few microjoule pulses centered at 400nm, we generate an idler pulse at 266nm and amplify a seeded signal at 800nm. We achieve remarkably high pump-to-idler energy conversion efficiencies of up to 38%. Although the pump and seed pulse durations are ∼100 fs, the generated UV spectral bandwidths support sub-15fs pulses. These can be further extended to support few-cycle pulses. Four-wave mixing in gas-filled hollow-core fibers can be scaled to high average powers and different spectral regions such as the vacuum UV (100-200nm).

High birefringence rectangular-hole photonic crystal 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.

Resonant radiation phase-matched to a soliton in gas-filled hollow-core fibers is an efficient femtosecond UV source. By tapering the fiber our simulations show that the supercontinuum can be significantly extended into the extreme ultra-violet.

I review the historical observation and subsequent research on optical soliton dynamics in gas-filled hollow-core optical fibres. I include both large-core hollow capillary fibres, and hollow-core photonic-crystal or microstructured fibres with smaller cores, in particular photonic bandgap and antiresonant guiding fibres. I discuss how the optical guidance properties of these different fibre structures influence the soliton dynamics that can be obtained. The dynamics I consider include: soliton propagation at peak power levels ranging from the megawatt to terawatt level, and pulse energies from sub-microjoule to millijoule range; pulse self-compression, leading to sub-cycle and sub-femtosecond pulse duration; soliton self-frequency shifting due to both the Raman effect, and the influence of photoionisation and plasma formation; and resonant dispersive wave emission, leading to the generation of tuneable few-femtosecond pulses across the vacuum and deep ultraviolet, visible, and near-infrared spectral regions.

While single-mode waveguides are commonly used in integrated photonic circuits, emerging applications in nonlinear and quantum optics rely fundamentally on interactions between modes of different order. Here we propose several methods to evaluate the modal composition of both externally and device-internally excited guided waves and discuss a technique for efficient excitation of arbitrary modes. The applicability of these methods is verified in photonic circuits based on aluminum nitride. We control modal excitation through suitably engineered grating couplers and are able to perform a detailed study of waveguide-internal second harmonic generation. Efficient and broadband power conversion between orthogonal polarizations is realized within an asymmetric directional coupler to demonstrate selective excitation of arbitrary higher-order modes. Our approach holds promise for applications in nonlinear optics and frequency up/down-mixing in a chipscale framework.

A simple and compact spectral-broadening system is presented that is based on a single-stage statically pressurized Ar filled hollow core fiber. By optimizing the inner diameter of the hollow core fiber, a bandwidth of 300 nm is obtained. This is the broadest bandwidth known to date with millijoule level energy near the 1-μm wavelength by a single stage gas filled hollow core fiber.

Multidimensional solitary states (MDSS)—self-sustained wavepackets—have attracted renewed interest in many different fields of physics. They are of particular importance in nonlinear optics, especially for the nonlinear propagation of ultrashort pulses in multimode fibres, which contain rich spatiotemporal intermodal interactions and dynamics, albeit often in an unstable manner. Here, we report the observation of the formation of highly stable multidimensional solitary states in a molecular gas-filled large-core hollow-core fibre. We experimentally and numerically demonstrate the creation of MDSS by multimillijoule, subpicosecond near-infrared pulses and the underlying physics. We find that the MDSS have a broadband redshifted spectra with an uncommon negative quadratic spectral phase at the output of the hollow-core fibre, originating from Raman enhancement due to the strong intermodal nonlinear interactions. The spatial and temporal localization of MDSS enables the compression of the broadened pulses at the output to 10.8 fs by simple linear propagation in a piece of fused silica. The high spatiotemporal quality of MDSS is further verified by high-harmonic generation. Our results present new opportunities for studying multimodal spatiotemporal dynamics in the high-energy regime. This work also presents a route toward a new class of compact, tunable and high-energy spatiotemporally engineered coherent light sources based on picosecond ytterbium technology. The formation of multidimensional solitary states through the nonlinear propagation of high-energy pulses in a molecular gas-filled large-core hollow-core fibre is demonstrated, offering new opportunities for studying multimodal spatiotemporal dynamics in the high-energy regime.

In this paper, the authors describe a new class of silicon photonic crystal fibre (SiPCF) that brings together two powerful optical technologies, the photonic crystal fibre (PCF) and the semiconductor optical fibre. The PCF is now a well established fibre paradigm that has proven to be a very versatile waveguide and has found applications in nonlinear optics, fibre lasers, and sensors. The versatility of the PCF is due to its microstructured cladding which enables complex manipulation of the waveguide's characteristics, and also allows for enhanced light interaction with materials that are infiltrated into the cladding voids. The most typical form of semiconductor optical fibre has a fused silica cladding and guides light in the high refractive index semiconductor core. Although semiconductor optical fibres are a nascent technology, practical applications, such as nonlinear pulse shaping and all optical modulation, have begun to emerge in the last couple of years. However, material losses are currently preventing this fibre type from becoming a major disruptive technology and, with this in mind, the initial steps to decouple the functionality of the semiconductor from its material losses were presented . It was achieved by filling the holes of a modified total internal reflection guiding silica PCF with hydrogenated amorphous silicon (a-Si:H) inclusions. The resulting SiPCF light guide in the low loss core via the antiresonant reflecting optical waveguiding (ARROW) mechanism was shown.

Raman spectroscopy is an essential optical tool for tracing gases that exhibit weak or no infrared absorption. Existing Raman spectroscopic methods are deficient for precision sensing applications due to the extremely low Raman cross-section of gas. Herein, we report an approach, named stimulated Raman photothermal spectroscopy (SRPTS), to indirectly probe stimulated Raman scattering (SRS) via detecting the induced photothermal phase modulation in a gas-filled hollow-core fiber (HCF). Photothermal interferometry enables a linear increase of the SRPTS signal with the product of the pump and Stokes power while maintaining low background noise at the idle probe wavelength. The HCF tightly confines the light beams and the gas sample to significantly enhance both the SRS gain and the photothermal phase modulation efficiency. Preliminary experimentation with a 3.9-m-long HCF demonstrates hydrogen detection with a noise equivalent concentration of 6.8 ppm (parts-per-million) under 100 s averaging time and 6 bar gas pressure, indicating the potential for high-precision gas detection in chemical, medical, and energy industries.

Photonic crystal fibers with kagome lattice are a particular kind of micostructured hollow-core fibers whose cross-section is characterized by a web of thin silica struts intersecting in a Star-of-David pattern. Such fibers show unusual properties, such as light confinement in the air-core in absence of a full photonic bandgap. The primary design parameter for such fibers is the strut thickness, which is responsible for the position and the width of the transmission bands. In this paper the guiding properties of hollow-core photonic crystal fibers with kagome lattice are investigated by means of a full-vector modal solver based on the finite element method. The fundamental mode effective index and confinement loss have been evaluated in a wide wavelength range, spanning from 300 nm to 1600 nm, accounting for the influence of the silica strut width on the transmission window. Moreover, the effects of selective alteration of the width and the shape of the silica struts surrounding the core have been analyzed. Simulation results show that the core-surrounding silica ring has the strongest effect on the transmission band, the loss level and the resonance wavelength position and, consequently, it should be carefully controlled during the fiber fabrication.

We report on a hollow-core fiber (HCF) whose fundamental transmission band covers almost the whole visible spectral window, starting at 440 nm. This HCF, in the form of a nodeless structure (NL-HCF), exhibits unprecedented optical performance in terms of low transmission attenuation of 80 dB/km at 532 nm, a broad transmission bandwidth from 440 to 1200 nm, a low bending loss of 0.2 dB/m at 532 nm under 8 cm bending radius, and single-mode profile. When launched to high-power picosecond laser systems at 532 nm, the fiber, exposed to ambient air, could easily deliver an 80 ps, 58 MHz, 32 W average power laser pulse with no damage and a 20 ps, 1 kHz high-energy laser pulse with a damage threshold in excess of 144 μJ at a fiber output. A proof-of-concept experiment on Raman spectroscopy in ambient air shows the significance of this broadband visible guiding HCF for interdisciplinary applications in nonlinear optics, ultrafast optics, lasers, spectroscopy, biophotonics, material processing, etc.