Transformation and control of ultra-short pulses in dispersion-engineered photonic crystal fibres

Critical-Layered MoS2 for the Enhancement of Supercontinuum Generation in Photonic Crystal Fibre

Supercontinuum generation (SCG) from silica-based photonic crystal fibers (PCFs) is of highly technological significance from microscopy to metrology, but has been hindered by silica's relatively low intrinsic optical nonlinearity. The prevailing approaches of filling PCF with nonlinear gases or liquids can endow fibre with enhanced optical nonlinearity and boosted SCG efficiency, yet these hybrids are easily plagued by fusion complexity, environmental incompatibility or transmission mode instability. Here this work presents a strategy of embedding solid-state 2D MoS2 atomic layers into the air-holes of PCF to efficiently enhance SCG. This work demonstrates a 4.8 times enhancement of the nonlinear coefficient and a 70% reduction of the threshold power for SCG with one octave spanning in the MoS2-PCF hybrid. Furthermore, this work finds that the SCG enhancement is highly layer-dependent, which only manifests for a real 2D regime within the thickness of five atomic layers. Theoretical calculations reveal that the critical thickness arises from the trade-off among the layer-dependent enhancement of the nonlinear coefficient, leakage of fundamental mode and redshift of zero-dispersion wavelength. This work provides significant advances toward efficient SCG, and highlights the importance of matching an appropriate atomic layer number in the design of functional 2D material optical fibers.

Resonant Akhmediev breathers

Modulation instability is a phenomenon in which a minor disturbance within a carrier wave gradually amplifies over time, leading to the formation of a series of compressed waves with higher amplitudes. In terms of frequency analysis, this process results in the generation of new frequencies on both sides of the original carrier wave frequency. We study the impact of fourth-order dispersion on this modulation instability in the context of nonlinear optics that lead to the formation of a series of pulses in the form of Akhmediev breather. The Akhmediev breather, a solution to the nonlinear Schrödinger equation, precisely elucidates how modulation instability produces a sequence of periodic pulses. We observe that when weak fourth-order dispersion is present, significant resonant radiation occurs, characterized by two modulation frequencies originating from different spectral bands. As an Akhmediev breather evolves, these modulation frequencies interact, resulting in a resonant amplification of spectral sidebands on either side of the breather. When fourth-order dispersion is of intermediate strength, the spectral bandwidth of the Akhmediev breather diminishes due to less pronounced resonant interactions, while stronger dispersion causes the merging of the two modulation frequency bands into a single band. Throughout these interactions, we witness a complex energy exchange process among the phase-matched frequency components. Moreover, we provide a precise explanation for the disappearance of the Akhmediev breather under weak fourth-order dispersion and its resurgence with stronger values. Our study demonstrates that Akhmediev breathers, under the influence of fourth-order dispersion, possess the capability to generate infinitely many intricate yet coherent patterns in the temporal domain.

Supercontinuum generation in a carbon disulfide core microstructured optical fiber

We demonstrate supercontinuum generation in a liquid-core microstructured optical fiber using carbon disulfide as the core material. The fiber provides a specific dispersion landscape with a zero-dispersion wavelength approaching the telecommunication domain where the corresponding capillary-type counterpart shows unsuitable dispersion properties for soliton fission. The experiments were conducted using two pump lasers with different pulse duration (30 fs and 90 fs) giving rise to different non-instantaneous contributions of carbon disulfide in each case. The presented results demonstrate an extraordinary high conversion efficiency from pump to soliton and to dispersive wave, overall defining a platform that enables studying the impact of non-instantaneous responses on ultrafast soliton dynamics and coherence using straightforward pump lasers and diagnostics.

High-order dispersion mapping of an optical fiber

We report on measurements of high-order dispersion maps of an optical fiber, showing how the ratio between the third and fourth-order dispersion (β₃/β₄) and the zero-dispersion wavelength (λ₀) vary along the length of the fiber. Our method is based on Four-Wave Mixing between short pulses derived from an incoherent pump and a weak laser. We find that the variations in the ratio β₃/β₄ are correlated to those in λ₀. We present also numerical calculations to illustrate the limits on the spatial resolution of the method. Due to the good accuracy in measuring λ₀ and β₃/β₄ (10 ^-3% and 5% relative error, respectively), and its simplicity, the method can be used to identify fiber segments of good uniformity, suitable to build nonlinear optical devices such as parametric amplifiers and frequency comb generators.

Tailoring modulation instabilities and four-wave mixing in dispersion-managed composite liquid-core fibers

We show that the ultrafast nonlinear dynamics in supercontinuum generation can be tailored via mixture-based liquid core fibers. Samples containing mixtures of inorganic solvents allow changing dispersion from anomalous to normal, i.e., shifting zero dispersion across pump laser wavelength. A significant control over modulation instability and four-wave mixing has been demonstrated experimentally in record-long (up to 60 cm) samples in agreement with simulations when using sub-psec pulses at 1.555 µm. The smallest concentration ratio yields indications of soliton-fission based supercontinuum generation at soliton numbers that are beyond the coherence limit. The presented dispersion tuning scheme allows creating unprecedented dispersion landscapes for accessing unexplored nonlinear phenomena and selected laser sources.

High-quality factor, high-confinement microring resonators in 4H-silicon carbide-on-insulator

Silicon carbide (SiC) exhibits promising material properties for nonlinear integrated optics. We report on a SiC-on-insulator platform based on crystalline 4H-SiC and demonstrate high-confinement SiC microring resonators with sub-micron waveguide cross-sectional dimensions. The Q factor of SiC microring resonators in such a sub-micron waveguide dimension is improved by a factor of six after surface roughness reduction by applying a wet oxidation process. We achieve a high Q factor (73,000) for such devices and show engineerable dispersion from normal to anomalous dispersion by controlling the waveguide cross-sectional dimension, which paves the way toward nonlinear applications in SiC microring resonators.

Review of Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) for Gas Sensing

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) is of importance for gas detection in environmental monitoring. This review summarizes the unique properties, development and recent progress of the IBBCEAS technique. Principle of IBBCEAS for gas sensing is described, and the development of IBBCEAS from the perspective of system structure is elaborated, including light source, cavity and detection scheme. Performances of the reported IBBCEAS sensor system in laboratory and field measurements are reported. Potential applications of this technique are discussed.

Experimental measurement of supercontinuum coherence in highly nonlinear soft-glass photonic crystal fibers

We present experimental measurements illustrating the power-dependent coherence evolution for supercontinuum generated in highly nonlinear SF6 photonic crystal fibers. The measurements were performed for fiber lengths close to and much longer than the soliton fission length. Simulations of the spectral evolution were also carried out to accompany the experimental observation. Many parameters were estimated by matching the simulated and the measured evolution. Both the measured and the simulated coherence evolution confirm the association between coherence degradation and soliton fission.

Phase matching as a gate for photon entanglement

Phase matching is shown to provide a tunable gate that helps discriminate entangled states of light generated by four-wave mixing (FWM) in optical fibers against uncorrelated photons originating from Raman scattering. Two types of such gates are discussed. Phase-matching gates of the first type are possible in the normal dispersion regime, where FWM sidebands can be widely tuned by high-order dispersion management, enhancing the ratio of the entangled-photon output to the Raman noise. The photon-entanglement gates of the second type are created by dual-pump cross-phase-modulation-induced FWM sideband generation and can be tuned by group-velocity mismatch of the pump fields.

Spatial-spectral (space-wavenumber) correspondence relationship and Fresnel zone spectra

A correspondence relationship between space and wavenumber for fully spatially coherent uniform monochromatic and polychromatic light in near- and far-field diffraction is fully illustrated, and it is used to study a phenomenon called the Fresnel zone spectra, which is verified experimentally. The spectra can be filtered or manipulated by moving the detection position relative to Fresnel zone plate along the optical axis.

Microstructured optical fiber for multichannel sensing based on Fano resonance of the whispering gallery modes

We present the design and theoretical demonstration of a microstructured optical fiber (MOF) for multichannel sensing applications based on the Fano resonance among the different whispering-gallery modes (WGMs) propagating in the MOF. The proposed MOF consists of a number of capillary channels with different diameters inside a tubular frame. When the phases of the WGMs in the capillary channels and the frame are matched, the Fano resonance will occur and the resonant peaks can be observed in the output spectrum of the tubular frame resonator. Sensing signals from the individual channels can be detected by measuring the central wavelengths of the corresponding Fano resonant peaks. To demonstrate the practicality, we study a dual-channel MOF for bio-sensing applications, i.e., detecting the refractive index variation in biological samples. In the analysis, we have shown that channel 1 and 2 achieve a sensitivity of 29.0557 nm/RIU (refractive index unit) and 22.9160 nm/RIU in the TE mode; and 16.0694 nm/RIU and 13.3181 nm/RIU in the TM mode respectively, when the refractive index of the biological samples varies between 1.330 and 1.345. The new MOF can be a compact, flexible, and low-cost solution for a variety of applications including multichannel bio/chemical sensing, multi-microcavity laser, and tunable photonics devices.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores

Photonic crystal fibers (PCFs) made from ZBLAN glass are of great interest for generating broadband supercontinua extending into the ultraviolet and mid-infrared regions. Precise sub-micrometer structuring makes it possible to adjust the modal dispersion over a wide range, making the generation of new frequencies more efficient. Here we report a novel ZBLAN PCF with six cores, each containing a central nanobore of a diameter ∼330  nm. Each nanobore core supports several guided modes, and the presence of the nanobore significantly modifies the dispersion, strongly influencing the dynamics and the extent of supercontinuum generation. Spectral broadening is observed when a single core is pumped in the fundamental and first higher order core modes with 200 fs long pulses at a wavelength of 1042 nm. Frequency-resolved optical gating is used to characterize the output pulses when pumping in the lowest order mode. The results are verified by numerical simulations.

Large-mode-area single-polarization single-mode photonic crystal fiber: design and analysis

A rectangular core photonic crystal fiber structure has been presented and analyzed for single-polarization single-mode operation. Single-polarization is obtained with asymmetric design and by introducing different loss for x-polarization and y-polarization of fundamental modes. Single-polarization single-mode operation of the proposed photonic crystal fiber is investigated in detail by using a full vector finite element method with an anisotropic perfectly matched layer. The variations of the confinement loss and effective mode area of x-polarization and y-polarization of fundamental modes have been simulated by varying the structural parameters of the proposed photonic crystal fiber. At the optimized parameters, confinement loss and effective mode area is obtained as 0.94 dB/m and 60.67  μm² for y-polarization as well as 26.67 dB/m and 67.23  μm² for x-polarization of fundamental modes, respectively, at 1.55 μm. Therefore simulation results confirmed that, 0.75 m length of fiber will be sufficient to get a y-polarized fundamental mode with an effective mode area as large as 60.67  μm².

Pure-quartic solitons

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we experimentally demonstrate a class of bright soliton arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. We derive the approximate shape of the fundamental pure-quartic soliton and discover that is surprisingly Gaussian, exhibiting excellent agreement with our experimental observations. Our discovery, enabled by precise dispersion engineering, could find applications in communications, frequency combs and ultrafast lasers.

Optical solitons are pulses that propagate undistorted. Here, the authors demonstrate a class of soliton arising from the interaction of self-phase modulation with quartic dispersion, rather than with quadratic dispersion as occurs in conventional solitons.

Numerical calculation of phase-matching properties in photonic crystal fibers with three and four zero-dispersion wavelengths

Photonic crystal fibers with three and four zero-dispersion wavelengths are presented through special design of the structural parameters, in which the closing to zero and ultra-flattened dispersion can be obtained. The unique phase-matching properties of the fibers with three and four zero-dispersion wavelengths are analyzed. Variation of the phase-matching wavelengths with the pump wavelengths, pump powers, dispersion properties, and fiber structural parameters is analyzed. The presence of three and four zero-dispersion wavelengths can realize wavelength conversion of optical soliton between two anomalous dispersion regions, generate six phase-matching sidebands through four-wave mixing and create more new photon pairs, which can be used for the study of supercontinuum generation, optical switches and quantum optics.

Observation of soliton compression in silicon photonic crystals

Solitons are nonlinear waves present in diverse physical systems including plasmas, water surfaces and optics. In silicon, the presence of two photon absorption and accompanying free carriers strongly perturb the canonical dynamics of optical solitons. Here we report the first experimental demonstration of soliton-effect pulse compression of picosecond pulses in silicon, despite two photon absorption and free carriers. Here we achieve compression of 3.7 ps pulses to 1.6 ps with <10 pJ energy. We demonstrate a ~1-ps free-carrier-induced pulse acceleration and show that picosecond input pulses are critical to these observations. These experiments are enabled by a dispersion-engineered slow-light photonic crystal waveguide and an ultra-sensitive frequency-resolved electrical gating technique to detect the ultralow energies in the nanostructured device. Strong agreement with a nonlinear Schrödinger model confirms the measurements. These results further our understanding of nonlinear waves in silicon and open the way to soliton-based functionalities in complementary metal-oxide-semiconductor-compatible platforms.

Midinfrared supercontinuum generation via As2Se3 chalcogenide photonic crystal fibers

Using numerical analysis, we compare the results of optofluidic and rod filling techniques for the broadening of supercontinuum spectra generated by As2Se3 chalcogenide photonic crystal fibers (PCFs). The numerical results show that when air-holes constituting the innermost ring in a PCF made of As2Se3-based chalcogenide glass are filled with rods of As2Se3-based chalcogenide glass, over a wide range of mid-IR wavelengths, an ultra-flattened near-zero dispersion can be obtained, while the total loss is negligible and the PCF nonlinearity is very high. The simulations also show that when a 50 fs input optical pulse of 10 kW peak power and center wavelength of 4.6 μm is launched into a 50 mm long rod-filled chalcogenide PCF, a ripple-free spectral broadening as wide as 3.86 μm can be obtained.

Engineering chromatic dispersion and effective nonlinearity in a dual-slot waveguide

In this paper, we propose a new dual slot based on rib-like structure, which exhibits a flat and near-zero dispersion over a 198 nm wide wavelength range. Chromatic dispersion of dual-slot silicon (Si) waveguide is mainly determined by waveguide dispersion due to the manipulating mode effective area rather than by the material dispersion. Moreover, the nonlinear coefficient and effective mode area of the waveguide are also explored in detail. A nonlinear coefficient of 1460/m/W at 1550 nm is achieved, which is 10 times larger than that of the Si rib waveguide. By changing different waveguide variables, both the dispersion and nonlinear coefficient can be tailored, thus enabling the potential for a highly nonlinear waveguide with uniform dispersion over a wide wavelength range, which could benefit the performance of broadband optical signal systems.

Multi-peak-spectra generation with Cherenkov radiation in a non-uniform single mode fiber

We propose, by means of numerical simulations, a simple method to design a non-uniform standard single mode fiber to generate spectral broadening in the form of "ad-hoc" chosen peaks from dispersive waves. The controlled multi-peak generation is possible by an on/off switch of Cherenkov radiation, achieved by tailoring the fiber dispersion when decreasing the cladding diameter by segments. The interplay between the fiber dispersion and the soliton self-frequency shift results in discrete peaks of efficiently emitted Cherenkov radiation from low order solitons, despite the small amount of energy contained in a pulse. These spectra are useful for applications that demand low power bell-shaped pulses at specific carrier wavelengths.

Generation of tightly compressed solitons with a tunable frequency shift in Raman-free fibers

Optimization of the compression of input N-solitons into robust ultra-narrow fundamental solitons, with a tunable up- or downshifted frequency, is proposed in photonic crystal fibers free of the Raman effect. Due to the absence of the Raman self-frequency shift, these fundamental solitons continue propagation, maintaining the acquired frequency, once separated from the input N soliton's temporal slot. A universal optimal value of the relative strength of the third-order dispersion is found, providing the strongest compression of the fundamental soliton is found. It depends only on the order of the injected N-soliton. The largest compression degree significantly exceeds the analytical prediction supplied by the Satsuma-Yajima formula. The mechanism behind this effect, which remains valid in the presence of the self-steepening, is explained.

Trapping of light in solitonic cavities and its role in the supercontinuum generation

We demonstrate that the fission of higher-order N-solitons with a subsequent ejection of fundamental quasi-solitons creates cavities formed by a pair of solitary waves with dispersive light trapped between them. As a result of multiple reflections of the trapped light from the bounding solitons which act as mirrors, they bend their trajectories and collide. In the spectral domain, the two solitons receive blue and red wavelength shifts, and the spectrum of the trapped light alters as well. This phenomenon strongly affects spectral characteristics of the generated supercontinuum. Consideration of the system's parameters which affect the creation of the cavity reveals possibilities of predicting and controlling soliton-soliton collisions induced by multiple reflections of the trapped light.

Soliton trapping of dispersive waves in photonic crystal fiber with two zero dispersive wavelengths

Based on the generalized nonlinear Schrödinger equation, we present a numerical study of trapping of dispersive waves by solitons during supercontinuum generation in photonic crystal fibers pumped with femtosecond pulses in the anomalous dispersion region. Numerical simulation results show that the generated supercontinuum is bounded by two branches of dispersive waves, namely blue-shifted dispersive waves (B-DWs) and red-shifted dispersive waves (R-DWs). We find a novel phenomenon that not only B-DWs but also R-DWs can be trapped by solitons across the zero-dispersion wavelength when the group-velocity matching between the soliton and the dispersive wave is satisfied, which may led to the generation of new spectral components via mixing of solitons and dispersive waves. Mixing of solitons with dispersive waves has been shown to play an important role in shaping not only the edge of the supercontinuum, but also its central part around the higher zero-dispersion wavelength. Further, we show that the phenomenon of soliton trapping of dispersive waves in photonic crystal fibers with two zero-dispersion wavelengths has a very close relationship with pumping power and the interval between two zero-dispersion wavelengths. In order to clearly display the evolution of soliton trapping of dispersive waves, the spectrogram of output pulses is observed using cross-correlation frequency-resolved optical gating technique (XFROG).

Low-loss hollow-core fibers with improved single-modedness

Hollow-core fibers (HCFs) are a revolution in light guidance with enormous potential. They promise lower loss than any other waveguide, but have not yet achieved this potential because of a tradeoff between loss and single-moded operation. This paper demonstrates progress on a strategy to beat this tradeoff: we measure the first hollow-core fiber employing Perturbed Resonance for Improved Single Modedness (PRISM), where unwanted modes are robustly stripped away. The fiber has fundamental-mode loss of 7.5 dB/km, while other modes of the 19-lattice-cell core see loss >3000 dB/km. This level of single-modedness is far better than previous 19-cell or 7-cell HCFs, and even comparable to some commercial solid-core fibers. Modeling indicates this measured loss can be improved. By breaking the connection between core size and single-modedness, this first PRISM demonstration opens a new path towards achieving the low-loss potential of HCFs.

All-optical control of group velocity dispersion in tellurite photonic crystal fibers

We demonstrate all-optical control of group velocity dispersion (GVD) via optical Kerr effect in highly nonlinear tellurite photonic crystal fibers. The redshift of the zero-dispersion wavelength is over 307 nm, measured by soliton self-frequency shift cancellation, when the pump peak power of a 1.56 μm femtosecond fiber laser is increased to 11.6 kW. The all-optical control of GVD not only offers a new platform for constructing all-optical-control photonic devices but also promises a new class of experiments in nonlinear fiber optics and light-matter interactions.

Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy

We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented.

Silicon waveguide with four zero-dispersion wavelengths and its application in on-chip octave-spanning supercontinuum generation

We propose a novel silicon waveguide that exhibits four zero-dispersion wavelengths for the first time, to the best of our knowledge, with a flattened dispersion over a 670-nm bandwidth. This holds a great potential for exploration of new nonlinear effects and achievement of ultra-broadband signal processing on a silicon chip. As an example, we show that an octave-spanning supercontinuum assisted by dispersive wave generation can be obtained in silicon, over a wavelength range from 1217 to 2451 nm, almost from bandgap wavelength to half-bandgap wavelength. Input pulse is greatly compressed to 10 fs.

Wavelength conversion of nanosecond pulses to the mid-IR in photonic crystal fibers

We investigate degenerate four wave mixing with nanosecond pulses in fused silica photonic crystal fibers. Phase-matching curves are calculated taking into account the material and waveguide dispersion. Experiments with a nanosecond pulsed Nd:YAG pump laser and relatively short fiber lengths show more than an octave spanning conversion to idler and signal wavelengths at 3.105 μm and 0.642 μm, respectively. Conversion efficiency depends on the fiber length and pump intensity and is limited in our experiments by damage of the fiber input facet. Our results represent a new stretch towards the limit of the silica transmission window in the mid-infrared (IR).

Femtosecond Fiber Lasers Based on Dissipative Processes for Nonlinear Microscopy

Recent progress in the development of femtosecond-pulse fiber lasers with parameters appropriate for nonlinear microscopy is reviewed. Pulse-shaping in lasers with only normal-dispersion components is briefly described, and the performance of the resulting lasers is summarized. Fiber lasers based on the formation of dissipative solitons now offer performance competitive with that of solid-state lasers, but with the benefits of the fiber medium. Lasers based on self-similar pulse evolution in the gain section of a laser also offer a combination of short pulse duration and high pulse energy that will be attractive for applications in nonlinear bioimaging.

Nonlinear pulse propagation in chalcogenide As2Se3 glass photonic crystal fiber using RK4IP method

Pulse propagation through chalcogenide As(2)Se(3) glass photonic crystal fiber (PCF) is numerically investigated using fourth-order Runge-Kutta in the interaction picture (RK4IP) method. The fully vectorial effective index method (FVEIM) is used to calculate the variation of effective refractive index, effective area, dispersion, and nonlinear coefficient (γ) in As(2)Se(3) PCF with wavelength for different values of pitch and air hole size. The RK4IP method is used to demonstrate the soliton propagation, self-phase modulation (SPM), soliton collision and cross phase modulation (XPM) in the designed As(2)Se(3) PCF. The numerically obtained value of soliton collision length (L(col)=51.3L(D)) using the RK4IP method is found to be in good agreement with the theoretical value of soliton collision length (L(col)=51.408L(D)) obtained from inverse scattering transform method, thus providing a verification of the RK4IP accuracy in solving generalized nonlinear schrödinger equation (GLNSE). We also evaluate and apply the value of wavelength for distortionless (L(NL)=L(D)) propagation of the soliton pulse.

On-chip two-octave supercontinuum generation by enhancing self-steepening of optical pulses

Dramatic advances in supercontinuum generation have been made recently using photonic crystal fibers, but it is quite challenging to obtain an octave-spanning supercontinuum on a chip, partially because of strong dispersion in high-index-contrast nonlinear integrated waveguides. We show by simulation that extremely flat and low dispersion can be achieved in silicon nitride slot waveguides over a wavelength band of 500 nm. Different from most of previously reported supercontinua that were generated either by higher-order soliton fission in anomalous dispersion regime or by self-phase modulation in normal dispersion regime, a two-octave supercontinuum from 630 to 2650 nm (360 THz in total) can be generated by enhancing self-steepening in pulse propagation in nearly zero dispersion regime, when an optical shock as short as 3 fs is formed.

Dynamics of Raman soliton during supercontinuum generation near the zero-dispersion wavelength of optical fibers

We observe unique dynamics of Raman soliton during supercontinuum process when an input pulse experiences initially normal group-velocity dispersion with a negative dispersion slope. In this situation, the blue components of the spectrum form a Raman soliton that moves faster than the input pulse and eventually decelerates because of Raman-induced frequency downshifting. In the time domain, the soliton trajectory bends and becomes vertical when the Raman shift ceases to occur as the spectrum of Raman soliton approaches the zero dispersion point. Parts of the red components of the pulse spectrum are captured by the Raman soliton through cross-phase modulation and they travel with it. The influence of soliton order, input chirp and dispersion slope on the dynamics of Raman soliton is discussed thoroughly.

Coherent supercontinuum generation in photonic crystal fiber with all-normal group velocity dispersion

We demonstrate supercontinuum generation in a photonic crystal fiber with all-normal group velocity dispersion. Pumping a short section of this fiber with compressed pulses from a compact amplified fiber laser generates a 200 nm bandwidth continuum with typical self-phase-modulation characteristics. We demonstrate that the supercontinuum is compressible to a duration of 26 fs. It therefore has a high degree of coherence between all the frequency components, and is a single pulse in the time domain. A smooth, flat spectrum spanning 800 nm is achieved using a longer piece of fiber.

Generalized retarded response of nonlinear media and its influence on soliton dynamics

We demonstrate by means of numerical simulations of the generalized Nonlinear Schrödinger Equation that the retarded response of a nonlinear medium embedded in a single hole of a photonic crystal fiber crucially affects the spectrum generated by ultrashort laser pulses. By introducing a hypothetic medium with fixed dispersion and nonlinearity and with a variable retarded response, we are able to separate the influence of the retarded response from other effects. We show that the fission length of a launched higher-order soliton dramatically increases if the characteristic time of the retarded response is close to the input pulse duration. Furthermore, we investigate the effect of the retarded response on the soliton self-frequency shift and find that the optimum input pulse duration for maximizing the spectral width has to be shortened for a larger characteristic retarded response time. Our work has important implications on future studies of spatiotemporal solitons in selectively liquid-filled photonic crystal fibers.

Maximizing the bandwidth of supercontinuum generation in As2Se3 chalcogenide fibers

We describe in detail a procedure for maximizing the bandwidth of supercontinuum generation in As(2)Se(3) chalcogenide fibers and the physics behind this procedure. First, we determine the key parameters that govern the design. Second, we find the conditions for the fiber to be endlessly single-mode; the fiber should be endlessly single-mode to maintain high nonlinearity and low coupling loss. We find that supercontinuum generation in As(2)Se(3) fibers proceeds in two stages--an initial stage that is dominated by four-wave mixing and a later stage that is dominated by the Raman-induced soliton self-frequency shift. Third, we determine the conditions to maximize the Stokes wavelength that is generated by four-wave mixing in the initial stage. Finally, we put all these pieces together to maximize the bandwidth. We show that it is possible to generate an optical bandwidth of more than 4 microm with an input pump wavelength of 2.5 microm using an As(2)Se(3) fiber with an air-hole-diameter-to-pitch ratio of 0.4 and a pitch of 3 microm. Obtaining this bandwidth requires a careful choice of the fiber's waveguide parameters and the pulse's peak power and duration, which determine respectively the fiber's dispersion and nonlinearity.

Enhanced degree of temporal coherence through temporal and spatial phase coupling within a focused supercontinuum

In the diffraction of a supercontinuum source, a redistribution of amplitude and phase at the focal region is incurred by the coupling between the supercontinuum and the spatial phase caused by the lens diffraction, making it extremely difficult to predict the focal behaviour. We show that the coupling between the temporal phase of a SC source and the spatial phase from the diffraction by a low numerical aperture (NA) lens causes dramatic alterations in the spectra and the temporal coherence near the focal region, and that this effect is maximized in points of singularity. Furthermore, we show that such an enhancement in temporal coherence can be controlled by the pulse evolution through the photonic crystal fiber, in which nonlinear and disperive effects such as the soliton fission process provides the key phase evolution necessary for dramatically changing the coherence time of the focused electromagnetic wave.

Broadband tunable optical parametric amplification from a single 50 MHz ultrafast fiber laser

We have demonstrated a 0.7 microm - 1.9 microm wavelength-tunable light source based on a single-pass optical parametric amplification (OPA) in a multiperiod magnesium oxide-doped periodically poled lithium niobate crystal. The OPA pump was a frequency-doubled ultrafast ytterbium-doped fiber oscillator, and the residual 1040 nm laser power after frequency doubling was recycled to generate a supercontinuum seeding source. Compared with conventional OPAs, this system is free from timing jitter between the pump laser and the seeding source. Over 50% conversion efficiency was obtained with 10 nJ pump energy. Combined with a 50 MHz repetition rate, this versatile source is ideal for biomedical and spectroscopic applications.

Polarization effects in a highly birefringent nonlinear photonic crystal fiber with two-zero dispersion wavelengths

A theoretical and experimental study is presented on polarized pulsed propagation from a highly birefringent nonlinear photonic crystal fiber with two-zero dispersion wavelengths. Experimental observations show that the input polarization state can maintain its linearity and that the fiber birefringence creates different spectral properties dependent on the input polarization orientation. The most extensive spectra are obtained for a coupling polarization angles aligned with the fast and slow axis, which is created by the high-order dispersion and Kerr nonlinearity.

Stabilized soliton self-frequency shift and 0.1- PHz sideband generation in a photonic-crystal fiber with an air-hole-modified core

Fiber dispersion and nonlinearity management strategy based on a modification of a photonic-crystal fiber (PCF) core with an air hole is shown to facilitate optimization of PCF components for a stable soliton frequency shift and subpetahertz sideband generation through four-wave mixing. Spectral recoil of an optical soliton by a red-shifted dispersive wave, generated through a soliton instability induced by high-order fiber dispersion, is shown to stabilize the soliton self-frequency shift in a highly nonlinear PCF with an air-hole-modified core relative to pump power variations. A fiber with a 2.3-microm-diameter core modified with a 0.9-microm-diameter air hole is used to demonstrate a robust soliton self-frequency shift of unamplified 50-fs Ti: sapphire laser pulses to a central wavelength of about 960 nm, which remains insensitive to variations in the pump pulse energy within the range from 60 to at least 100 pJ. In this regime of frequency shifting, intense high- and low-frequency branches of dispersive wave radiation are simultaneously observed in the spectrum of PCF output. An air-hole-modified-core PCF with appropriate dispersion and nonlinearity parameters is shown to provide efficient four-wave mixing, giving rise to Stokes and anti-Stokes sidebands whose frequency shift relative to the pump wavelength falls within the subpetahertz range, thus offering an attractive source for nonlinear Raman microspectroscopy.

Fabrication of photonic crystals with functional defects by one-step holographic lithography

A one-step introduction of functional defects into a photonic crystal is demonstrated. By using a multi-beam phase-controlled holographic lithography, line-defects in a Bragg structure and embedded waveguides in a two-dimensional photonic crystal are fabricated. Intrinsic defect introduction into a 3-dimensional photonic crystal is also proposed. This technique gives rise to a substantial reduction of the fabrication complexity and a significant improvement on the accuracy of the functional defects in photonic crystals.

Cavity enhanced absorption spectroscopy of multiple trace gas species using a supercontinuum radiation source

Supercontinuum radiation sources are attractive for spectroscopic applications owing to their broad wavelength coverage, which enables spectral signatures of multiple species to be detected simultaneously. Here we report the first use of a supercontinuum radiation source for broadband trace gas detection using a cavity enhanced absorption technique. Spectra were recorded at bandwidths of up to 100 nm, encompassing multiple absorption bands of H(2)O, O(2) and O(2)-O(2). The same instrument was also used to make quantitative measurements of NO(2) and NO(3). For NO(3) a detection limit of 3 parts-per-trillion in 2 s was achieved, which corresponds to an effective 3sigma sensitivity of 2.4 x 10(-9) cm(-1)Hz(-1/2). Our results demonstrate that a conceptually simple and robust instrument is capable of highly sensitive broadband absorption measurements.

Soliton switching and multi-frequency generation in a nonlinear photonic crystal fiber coupler

Soliton switching in nonlinear directional couplers implemented in photonic crystal fibers (PCF) examined here. A vector finite element method (FEM) has been developed to precisely calculate the dispersion along with coupling length of the guided modes. The PCF coupler geometry was carefully designed so that it can support soliton pulses. Soliton switching is demonstrated numerically at 1.55 microm for 100 femto-second (fs) pulses. Our theoretical results explain some of the key spectral features previously observed in the experiment.

Fibre-optic nonlinear optical microscopy and endoscopy

Nonlinear optical microscopy has been an indispensable laboratory tool of high-resolution imaging in thick tissue and live animals. Rapid developments of fibre-optic components in terms of growing functionality and decreasing size provide enormous opportunities for innovations in nonlinear optical microscopy. Fibre-based nonlinear optical endoscopy is the sole instrumentation to permit the cellular imaging within hollow tissue tracts or solid organs that are inaccessible to a conventional optical microscope. This article reviews the current development of fibre-optic nonlinear optical microscopy and endoscopy, which includes crucial technologies for miniaturized nonlinear optical microscopy and their embodiments of endoscopic systems. A particular attention is given to several classes of photonic crystal fibres that have been applied to nonlinear optical microscopy due to their unique properties for ultrashort pulse delivery and signal collection. Furthermore, fibre-optic nonlinear optical imaging systems can be classified into portable microscopes suitable for imaging behaving animals, rigid endoscopes that allow for deep tissue imaging with minimally invasive manners, and flexible endoscopes enabling imaging of internal organs. Fibre-optic nonlinear optical endoscopy is coming of age and a paradigm shift leading to optical microscope tools for early cancer detection and minimally invasive surgery.

Phase fluctuations of linearly chirped solitons in a noisy optical fiber channel with varying dispersion, nonlinearity, and gain

The phase fluctuations of arbitrarily nonlinearity- and dispersion-managed solitons propagating in a noisy fiber channel are studied both analytically and numerically. We begin by discussing the stability problem of such linearly chirped solitons with a full linear stability analysis. It is shown that these sophisticated solitons possess an enhanced stability against perturbations and therefore hold promise for applications in optical telecommunications. We then make an approach to the phase statistics of these solitons, which stems from an inevitable random walk in phase evolutions due to amplified spontaneous emission noise. By using the variational approach together with impulse-response (Green) functions, an elegant closed-form expression for the phase variance is derived based on an unconstrained self-similar soliton ansatz in which the effect of chirp fluctuations has been critically taken into account as well as the dispersive and nonlinear effects. An inspection of the intriguing subtleties of the interplay among these effects reveals that the chirp fluctuations effect does play an important role in the control of nonlinear phase noise via fiber dispersion, independently of whether the input solitons are initially chirped or not. Our analytical result also offers many possibilities of optimally manipulating nonlinear phase noise with engineered fiber parameters that may lead to the steady pulse propagation, broadening, or compression under favorable parametric conditions. Last, we demonstrate our result by several convincible examples and show an excellent agreement between analytical predictions and numerical simulations.

Perturbative analytical treatment of adiabatically moderated soliton self-frequency shift

We provide a perturbative analytical treatment of the soliton self-frequency-shift (SSFS) in optical fibers including the main physical mechanisms limiting the SSFS, such as the high-order dispersion, the wavelength dependence of the effective mode area, and optical loss. We use this approach to estimate the frequency shift of a soliton with adiabatically varying local parameters and compare this estimate with the results of numerical simulations for SSFS in photonic-crystal fibers. This comparison shows that, in many situations of practical interest, the proposed approach can adequately predict important tendencies of SSFS, and allows a fair estimation of characteristic length scales for the mechanisms limiting the SSFS.

Octave-spanning, high-power microstructure-fiber-based optical parametric oscillators

We investigate femtosecond optical parametric oscillators (OPO's) based on short pieces of microstructure fiber that generate sub-picosecond pulses with record average output power (50 mW) and >200 nm of wavelength tunability (yellow to near-IR). Signal and conjugate (idler)fields spanning an octave are also demonstrated. These systems can operate with a wide range of microstructure fibers, pump laser wavelengths and pulse durations, and our analysis shows that in terms of wavelength tunability and output power using short (less than a few cm's) optical fibers leads to performance that is superior to that with longer lengths.

Multifrequency third-harmonic generation by red-shifting solitons in a multimode photonic-crystal fiber

While the standard scenario of third-harmonic generation (THG) by a dispersive-wave pump involves the emission of light with a frequency 3omega, thrice the frequency omega of the input pump field, solitons undergoing a continuous shift of their central frequency omega due to the Raman effect in a multimode optical fiber can generate the third harmonic in a different fashion. In the experiments reported here, we provide the first direct experimental evidence of THG by a continuously red-shifting soliton pump by studying the third-harmonic buildup in relation to the spectral evolution of the soliton pump field in a silica photonic-crystal fiber (PCF). We show that solitons excited in a PCF by unamplified femtosecond pulses of a Cr:forsterite laser sweep through the spectral range from 1.25 to 1.63 microm , scanning through a manifold of THG phase-matching resonances with 3omega dispersive waves in PCF modes. As a result, intense third-harmonic peaks build up in the range of wavelengths from 370 to 550 nm at the output of the fiber, making PCF a convenient fiber-format multifrequency source of short-wavelength radiation. Time-resolved fluorescence measurements with photoexcitation provided by the third-harmonic PCF output are presented, demonstrating the high potential of PCF sources for an ultrafast photoexcitation of fluorescent molecular systems in physics, chemistry, and biology.