Showing papers on "Supercontinuum published in 2007"

Nonlinear optical phenomena in silicon waveguides: Modeling and applications

Abstract: Several kinds of nonlinear optical effects have been observed in recent years using silicon waveguides, and their device applications are attracting considerable attention. In this review, we provide a unified theoretical platform that not only can be used for understanding the underlying physics but should also provide guidance toward new and useful applications. We begin with a description of the third-order nonlinearity of silicon and consider the tensorial nature of both the electronic and Raman contributions. The generation of free carriers through two-photon absorption and their impact on various nonlinear phenomena is included fully within the theory presented here. We derive a general propagation equation in the frequency domain and show how it leads to a generalized nonlinear Schrodinger equation when it is converted to the time domain. We use this equation to study propagation of ultrashort optical pulses in the presence of self-phase modulation and show the possibility of soliton formation and supercontinuum generation. The nonlinear phenomena of cross-phase modulation and stimulated Raman scattering are discussed next with emphasis on the impact of free carriers on Raman amplification and lasing. We also consider the four-wave mixing process for both continuous-wave and pulsed pumping and discuss the conditions under which parametric amplification and wavelength conversion can be realized with net gain in the telecommunication band.

Ultrashort filaments of light in weakly ionized, optically transparent media

Abstract: Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration and peak powers exceeding several terawatt (TW). When such pulses propagate through optically transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid on the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Far-field spectra provide promising diagnostics. Attention is also paid to the multifilamentation instability of broad beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy and the possibility of guiding electric discharges in air are finally addressed on the basis of experimental results.

A Fourth-Order Runge–Kutta in the Interaction Picture Method for Simulating Supercontinuum Generation in Optical Fibers

Abstract: An efficient algorithm, which exhibits a fourth-order global accuracy, for the numerical solution of the normal and generalized nonlinear Schrodinger equations is presented. It has applications for studies of nonlinear pulse propagation and spectral broadening in optical fibers. Simulation of supercontinuum generation processes, in particular, places high demands on numerical accuracy, which makes efficient high-order schemes attractive. The algorithm that is presented here is an adaptation for use in the nonlinear optics field of the fourth-order Runge-Kutta in the Interaction Picture (RK4IP) method, which was originally developed for studies on Bose-Einstein condensates. The performance of the RK4IP method is validated and compared to a number of conventional methods by modeling both the propagation of a second-order soliton and the generation of supercontinuum radiation. It exhibits the expected global fourth-order accuracy for both problems studied and is the most accurate and efficient of the methods tested.

Soliton fission and supercontinuum generation in silicon waveguides

Abstract: We show through numerical simulations that silicon waveguides can be used to create a supercontinuum extending over 400 nm by launching femtosecond pulses as higher-order solitons. The physical process behind continuum generation is related to soliton fission, self-phase modulation, and generation of Cherenkov radiation. In contrast with optical fibers, stimulated Raman scattering plays little role. As low-energy(≈1 pJ) pulses and short waveguides (<1 cm) are sufficient for continuum generation, the proposed scheme should prove useful for practical applications.

Fiber supercontinuum sources (Invited)

Abstract: We review supercontinuum generation in optical fibers for particular cases where the nonlinear spectral broadening is induced by pump radiation from fiber-format sources. Based on numerical simulations, our paper is intended to provide experimental design guidelines tailored ytterbium and erbium-based pumps around 1060 and 1550 nm, respectively. In particular, at 1060 nm, we consider conditions under which the generated spectra are phase and intensity stable, and we address the dependence of the supercontinuum coherence on the input pulse parameters and the fiber length. At 1550 nm, special attention is paid to the case of dispersion-flattened dispersion-decreasing fiber, where we revisit the underlying physics in detail and explicitly examine the use of such fiber for supercontinuum generation with pumps of peak power in the range 200-1200 W and sub-10 m fiber lengths. We show that supercontinuum generation under such conditions can be highly coherent and can be applied to nonlinear pulse compression.

Light trapping in gravity-like potentials and expansion of supercontinuum spectra in photonic-crystal fibres

Abstract: Femtosecond pulses of light propagating along photonic-crystal fibres can generate a broad optical supercontinuum1,2. This striking discovery has applications ranging from spectroscopy and metrology3 to telecommunication4 and medicine5,6. Among the physical principles underlying supercontinuum generation are soliton emission7, a variety of four-wave mixing processes8,9,10,11, Raman-induced soliton self-frequency shift12,13, and dispersive wave generation mediated by solitons7,13,14. Although all of the above effects contribute to supercontinuum generation, none of them can explain the generation of blue and violet light from infrared femtosecond pump pulses. In this work we argue that the most profound role in the shaping of the short-wavelength edge of the continuum is played by the effect of radiation trapping in a gravity-like potential created by accelerating solitons. The underlying physics of this effect has a straightforward analogy with the inertial forces acting on an observer moving with a constant acceleration.

Generation of an extreme ultraviolet supercontinuum in a two-color laser field.

Abstract: We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.

Mid-IR Supercontinuum Generation From Nonsilica Microstructured Optical Fibers

Abstract: In this paper, the properties of nonsilica glasses and the related technology for microstructured fiber fabrication are reviewed. Numerical simulation results are shown using the properties of nonsilica microstructured fibers for mid-infrared (mid-IR) supercontinuum generation when seeding with near-IR, 200-fs pump pulses. In particular, bismuth glass small-core fibers that have two zero-dispersion wavelengths (ZDWs) are investigated, and efficient mid-IR generation is enabled by phase-matching of a 2.0-mum seed across the upper ZDW into the 3-4.5 mum wavelength range. Fiber lengths considered were 40 mm. Simulation results for a range of nonsilica large-mode fibers are also shown for comparison.

Enhanced Kerr nonlinearity in sub-wavelength diameter As2Se3 chalcogenide fiber tapers

Abstract: We experimentally demonstrate enhanced Kerr nonlinear effects in highly nonlinear As2Se3 chalcogenide fiber tapered down to subwavelength waist diameter of 1.2 µm. Based on self phase modulation measurements, we infer an enhanced nonlinearity of 68 W-1m-1. This is 62,000 times larger than in standard silica singlemode fiber, owing to the 500 times larger n2 and almost 125 times smaller effective mode area. We also consider the potential to exploit the modified dispersion in these tapers for ultra-low threshold supercontinuum generation.

Millijoule-level phase-stabilized few-optical-cycle infrared parametric source

Abstract: Ultrabroadband self-phase-stabilized near-IR pulses have been generated by difference-frequency generation of a filament broadened supercontinuum followed by two-stage optical parametric amplification. Pulses with energy up to 1.2 mJ and duration down to 17 fs are demonstrated. These characteristics make such a source suited as a driver for high-order harmonic generation and isolated attosecond pulse production.

Supercontinuum generation in optical fibers

Abstract: In this paper, we studied SC generation in fiber lasers and in optical fibers pumped by different light sources which include fs and ps pulse sources, and continuous-wave (CW) amplified spontaneous emission (ASE) light sources. First, we demonstrated SC generation with a 10dB spectral bandwidth of 430nm in a fiber ring laser with conventional nonlinear fiber. Second, we proposed and demonstrated a new and efficient approach to generation of a CW SC in optical fibers pumped by a CW ASE light. A bandwidth of 268nm (at -15dB level) with an average spectral density of 2.7mW/nm was demonstrated. Various approaches to flattening the spectrum and increasing the spectral width were also studied. The application of this SC source in WDM passive optical access networks (WDM-PONs) was investigated. Third, the approach of SC generation in a fiber combination of standard SMF and nonlinear DSF pumped by an all-fiber fs pulse Master Oscillator Power Amplifier (MOPA) system was developed. A spectral bandwidth of over 1000nm was demonstrated. Finally, the generation of broad comb-like-spectral light based on the pulse compression of 40GHz optical pulses in a new nonlinear dispersion-decreasing fiber with high SBS threshold was studied. A continuum light source with over 125 channels and a channel spacing of 40 GHz was achieved. The use of this continuum light source as WDM source in WDM-PONs was investigated.

Supercontinuum generation in silicon photonic wires

Abstract: We observe spectral broadening of more than 350 nm, i.e., a 3/10-octave span, upon propagation of ultrashort 1.3-μm-wavelength optical pulses in a 4.7-mm-long silicon-photonic-wire waveguide. We measure the wavelength dependence of the spectral features and relate it to waveguide dispersion and input power. The spectral characteristics of the output pulses are shown to be consistent, in part, with higher-order soliton radiative effects.

High bandwidth absorption spectroscopy with a dispersed supercontinuum source

Abstract: An optical gas sensor is presented, making use of a dispersed supercontinuum source, capable of acquiring broad bandwidth spectra at ultrahigh wavelength sweep and repetition rates. Wavelength sweeps from 1100 nm to 1700 nm can be performed in 800 ns at a spectral resolution of 40 pm. This is comparable to line-widths of molecular spectra at atmospheric pressure. Quantitative measurements are presented of CH4 employing 80 nm wide sweeps over the P- Q- and R-branches of the 2ν3 transition near 1665 nm, at rates exceeding 100 kHz. The effective acquisition rate is determined by the amount of averaging required, and the effect of this averaging on observed precision is investigated.

Photonic crystal fibers : properties and applications

Abstract: Preface Acknowledgements Introduction 1 Basics of photonic crystal fibers 11 From conventional optical fibers to PCFs12 Guiding mechanism 121 Modified total internal reflection 122 Photonic bandgap guidance 13 Properties and applications 131 Solid-core fibers 132 Hollow-core fibers 14 Loss mechanisms 141 Intrinsic loss 142 Leakage loss 143 Bending loss 15 Fabrication process 151 Stack-and-draw technique 152 Extrusion fabrication process 153 Microstructured polymer optical fibers 154 OmniGuide fibers 16 Photonic crystal fibers in the market Bibliography 2 Guiding properties 21 Square-lattice PCFs 211 Guidance 212 Cut-off 22 Cut-off of large mode area triangular PCFs 23 Hollow-core modified honeycomb PCFs 231 Guidance and leakage 232 Birefringence Bibliography 3 Dispersion properties 31 PCFs for dispersion compensation 32 Dispersion of square-lattice PCFs 33 Dispersion-flattened triangular PCFs 331 PCFs with modified air-hole rings 332 Triangular-core PCFs Bibliography 4 Nonlinear properties 41 Supercontinuum generation 411 Physics of supercontinuum generation 412 Highly nonlinear PCFs 413 Dispersion properties and pump wavelength 414 Influence of the pump pulse regime 415 Applications 42 Optical parametric amplification 421 Triangular PCFs for OPA Dispersion and nonlinear properties 422 Phase-matching condition in triangular PCFs Optical parametric gain in triangular PCFs 43 Nonlinear coefficient in hollow-core PCFs Bibliography 5 Raman properties 51 Raman effective area and Raman gain coefficient 52 Raman properties of triangular PCFs 521 Silica triangular PCFs 522 Tellurite triangular PCFs 523 Enlarging air-hole triangular PCFs 53 Raman properties of honeycomb PCFs 54 PCF Raman amplifiers 541 Model for PCF Raman amplifiers 542 Triangular PCF Raman amplifiers 55 Impact of background losses on PCF Raman amplifiers 56 Multipump PCF Raman amplifiers Bibliography 6 Erbium-doped fiber amplifiers 61 Model for doped-fiber amplifiers 62 EDFAs based on honeycomb and cobweb PCFs 63 EDFAs based on triangular PCFs Bibliography A Finite Element Method A1 Formulation A2 PCF parameter evaluation Bibliography

Nonlinear envelope equation modeling of sub-cycle dynamics and harmonic generation in nonlinear waveguides

Abstract: We describe generalized nonlinear envelope equation modeling of sub-cycle dynamics on the underlying electric field carrier during one-dimensional propagation in fused silica. Generalized envelope equation simulations are shown to be in excellent quantitative agreement with the numerical integration of Maxwell's equations, even in the presence of shock dynamics and carrier steepening on a sub-50 attosecond timescale. In addition, by separating the effects of self-phase modulation and third harmonic generation, we examine the relative contribution of these effects in supercontinuum generation in fused silica nanowire waveguides.

Octave spanning high-quality supercontinuum generation in all-fiber system

Abstract: Recently, widely broadened supercontinuum (SC) has been generated using ultrashort pulse and highly nonlinear fibers. However, inherent noise and fine structures have been the problem for the application of SC. We demonstrate wideband, low-noise, highly coherent, and ultraflat SC generation using soliton pulse and normal dispersion highly nonlinear fibers. Characteristics of generated SC are experimentally evaluated, and they are compared with those of the conventional SC. Octave spanning high-quality SC is also generated using high-power soliton pulse.

Composite frequency comb spanning 0.4-2.4μm from a phase-controlled femtosecond Ti:sapphire laser and synchronously pumped optical parametric oscillator

Abstract: A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared (0.4-2.4μm) is obtained by phase locking a femtosecond Ti:sapphire laser and a synchronously pumped optical parametric oscillator to a common supercontinuum reference. The locking results have bandwidths lower than 3kHz. By changing the locking frequencies, different relative and absolute offsets of the constituent frequency combs are achievable.

Theory of radiation trapping by the accelerating solitons in optical fibers

Abstract: We present a theory describing trapping of the normally dispersive radiation by the Raman solitons in optical fibers. Frequency of the radiation component is continuously blueshifting, while the soliton is redshifting. Underlying physics of the trapping effect is in the existence of the inertial gravitylike force acting on light in the accelerating frame of reference. We present analytical calculations of the rate of the opposing frequency shifts of the soliton and trapped radiation and find it to be greater than the rate of the redshift of the bare Raman soliton. Our findings are essential for understanding of the continuous shift of the high-frequency edge of the supercontinuum spectra generated in photonic crystal fibers toward higher frequencies.

Single-shot, space- and time-resolved measurement of rotational wavepacket revivals in H(2), D(2), N(2), O(2), and N(2)O.

Abstract: Femtosecond laser-induced alignment and periodic recurrences in hydrogen and deuterium are measured in a single shot for the first time, in a room temperature gas cell. Single-shot Supercontinuum Spectral Interferometry (SSSI) is employed, with measurements also performed in room temperature samples of nitrogen, oxygen, and nitrous oxide. Unlike previous optical techniques for probing molecular alignment in gases or liquids, SSSI quantitatively and directly measures the degree of molecular alignment without reliance on model fits, and it can do so with spatial resolution transverse to the pump beam. In addition, wavepacket collisional dephasing rates can be directly measured in gas samples at useful densities.

Power scalable mid-infrared supercontinuum generation in ZBLAN fluoride fibers with up to 1.3 watts time-averaged power

Abstract: Mid-infrared supercontinuum (SC) extending to ~4.0 mum is generated with 1.3 W time-averaged power, the highest power to our knowledge, in ZBLAN (ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF...) fluoride fiber by using cladding-pumped fiber amplifiers and modulated laser diode pulses. We demonstrate the scalability of the SC average power by varying the pump pulse repetition rate while maintaining the similar peak power. Simulation results obtained by solving the generalized nonlinear Schrodinger equation show that the long wavelength edge of the SC is primarily determined by the peak pump power in the ZBLAN fiber.

Simultaneous dual-band ultra-high resolution optical coherence tomography

Abstract: Ultra-high resolution optical coherence tomography (OCT) imaging is demonstrated simultaneously at 840 nm and 1230 nm central wavelength using an off-the-shelf turn-key supercontinuum light source. Spectral filtering of the light source emission results in a double peak spectrum with average powers exceeding 100 mW and bandwidths exceeding 200 nm for each wavelength band. A free-space OCT setup optimized to support both wavelengths in parallel is introduced. OCT imaging of biological tissue ex vivo and in vivo is demonstrated with axial resolutions measured to be <2 µm and <4 µm at 840 nm and 1230 nm, respectively. This measuring scheme is used to extract spectroscopic features with outstanding spatial resolution enabling enhanced image contrast.

Toward Hyperspectral Lidar: Measurement of Spectral Backscatter Intensity With a Supercontinuum Laser Source

Abstract: We have tested the use of a supercontinuum laser source in laser-based spectral backscatter measurement. The calibration and first results with the prototype instrument are presented with a discussion of improvements and applications in laser-based hyperspectral remote sensing and laboratory measurements. This technique enables the spectral study of the backscatter effects and the calibration and test measurements for the purpose of airborne laser measurement. We also explore the prospect of using a supercontinuum laser source in a broadband (hyperspectral) lidar

Tailoring CW supercontinuum generation in microstructured fibers with two-zero dispersion wavelengths

Abstract: We theoretically study broadband supercontinuum generation in photonic crystal fibers exhibiting two zero dispersion wavelengths and under continuous-wave pumping We show that when the pump wavelength is located in between the zero-dispersion wavelengths, a wide and uniform spectral broadening is achieved through modulation instability, generation of both blue-shifted and red-shifted dispersive waves and subsequently through soliton self-frequency shift This supercontinuum is therefore bounded by these two dispersive waves which allow the control of its bandwidth by a suitable tuning of the fiber dispersion As a relevant example, we predict that broadband (1050-1600 nm) continuous-wave light can be generated in short lengths of microstructured fibers pumped by use of a 10-W Ytterbium fiber laser

Supercontinuum generation in an ultrafast laser inscribed chalcogenide glass waveguide

Abstract: The authors report supercontinuum generation in an ultrafast laser inscribed chalcogenide glass waveguide. The waveguides were fabricated using a Yb:glass cavity-dumped femtosecond oscillator with 600- kHz repetition rate. The waveguides were pumped using an optical parametric amplifier tuned to 1500 nm with a bandwidth of 100 nm. The broadest resulting supercontinuum spanned 600 nm (at -15 dB points) from 1320 to 1920 nm. The supercontinuum was generated in the normal dispersion regime, enhancing stability, and exhibits a smooth spectral shape.

Visible supercontinuum generation controlled by intermodal four-wave mixing in microstructured fiber

Abstract: We present an experimental and numerical study of supercontinuum generation extended in the visible part of the spectrum by using a selective optical coupling of the pump wave in the largely anomalous dispersion regime. The broadband frequency generation is induced by an initial four-wave mixing process that converts the pump wave at 1064 nm into 831 nm anti-Stokes and 1478 nm Stokes wavelengths. Phase matching is ensured on such a large frequency shift thanks to a microstructured multimodal fiber with a specific design. Continuum generation is therefore enhanced around the two generated sidebands.

Sub-10-fs supercontinuum radiation generated by filamentation of few-cycle 800 nm pulses in argon.

Abstract: Focusing 12 fs pulses of 800 nm with moderate energy (0.35 mJ) into atmospheric-pressure argon (Ar) gives rise to filamentation (self-focusing) and a supercontinuum with a very broad pedestal, extending to 250 nm. According to the present understanding, the short wavelengths are produced by self-phase modulation in the self-steepened trailing edge of the pulse. Pulses in this spectral range might thus be intrinsically short. Indeed we demonstrate this by extracting the light near the end of the filament, terminating self-focusing by a pressure gradient at a pinhole, beyond which the Ar is pumped away. We obtain pulses of 9.7 fs in the region of 290 nm without the necessity of compression.

Supercontinuum generation in a microstructured optical fiber by picosecond self Q-switched mode-locked Nd:GdVO4 laser

Abstract: An effective supercontinuum (SC) generation is demonstrated by injecting picosecond self Q-switched mode-locked Nd:GdVO4 laser pulses into a 1-m long microstructured fiber. The laser is operated at wavelength 100-nm away from the longer zero-dispersion of this dual zero-dispersion wavelength microstructured fiber. The phenomena of modulation instability, stimulated Raman effect, and dispersive wave can be sequentially observed from experimental results, leading to spectral broadening as pumping increases.

New Thomson scattering diagnostic on RFX-mod

Abstract: This article describes the completely renovated Thomson scattering (TS) diagnostic employed in the modified Reversed Field eXperiment (RFX-mod) since it restarted operation in 2005. The system measures plasma electron temperature and density profiles along an equatorial diameter, measuring in 84 positions with 7mm spatial resolution. The custom built Nd:YLF laser produces a burst of 10 pulses at 50Hz with energy of 3J, providing ten profile measurements in a plasma discharge of about 300ms duration. An optical delay system accommodates three scattering volumes in each of the 28 interference filter spectrometers. Avalanche photodiodes detect the Thomson scattering signals and allow them to be recorded by means of waveform digitizers. Electron temperature is obtained using an alternative relative calibration method, based on the use of a supercontinuum light source. Rotational Raman scattering in nitrogen has supplied the absolute calibration for the electron density measurements. During RFX-mod experimenta...

Coherent anti-Stokes Raman scattering microscopy using photonic crystal fiber with two closely lying zero dispersion wavelengths.

Abstract: We demonstrate coherent anti-Stokes Raman scattering (CARS) microscopy of lipid-rich structures using a single unamplified femtosecond Ti:sapphire laser and a photonic crystal fiber (PCF) with two closely lying zero dispersion wavelengths (ZDW) for the Stokes source. The primary enabling factor for the fast data acquisition (84 μs per pixel) in the proof-of-principle CARS images, is the low noise supercontinuum (SC) generated in this type of PCF, in contrast to SC generated in a PCF with one ZDW. The dependence of the Stokes pulse on average input power, pump wavelength, pulse duration and polarization is experimentally characterized. We show that it is possible to control the spectral shape of the SC by tuning the pump wavelength of the input pulse and the consequence for CARS microscopy is discussed.

Supercontinuum Generation in Silica Fibers by Amplified Nanosecond Laser Diode Pulses

Abstract: Supercontinuum (SC) with a continuous spectrum from ~0.8-3 mum is generated in a standard single-mode fiber followed by high-nonlinearity fiber. The SC is pumped by 2-ns laser diode (LD) pulses amplified in a multistage fiber amplifier, and the two octave spanning continuum is achieved by optimizing a two-stage process that separates pulse breakup and soliton formation from spectral broadening. We also demonstrate scalability of the average power in the continuum from 27 mW to 5.3 W by increasing the pulse repetition rate from 5 kHz to 1 MHz, while maintaining comparable peak power. We attribute the generated SC spectrum to the ensemble average of multiple solitons and the superposition of their corresponding spectra. The hypothesis is confirmed through simulation results obtained by solving the generalized nonlinear Schrodinger equation (NLSE). Similar SC spectra can also be obtained by using both femtosecond and nanosecond pump pulses. Furthermore, by tailoring the input pulse shape, we propose and simulate the generation of the entire SC spectrum in one single soliton under quasi-continuous-wave (CW) pulse pumping scheme.