Enhanced soliton self-frequency shift of ultrashort light pulses

Photonic-crystal fibers are used to study scenarios of soliton self-frequency shift for laser pulses with initial pulse lengths much less than the Raman-mode period of the fiber material. A typical frequency shift of subnanojoule Ti: sapphire-laser pulses with an initial duration of about 30 fs transmitted through a fiber with a core diameter of about 1.6 μm and a length of about 7 cm exceeds 100 THz. The rate of soliton self-frequency shift is radically increased by reducing the initial pulse width.     

Polarization-sensitive non-3ω third-harmonic generation by femtosecond Cr: Forsterite laser pulses in birefringent microchannel waveguides of photonic-crystal fibers

Femtosecond Cr: forsterite laser pulses coupled into small-diameter birefringent channel waveguides off the central core of a photonic-crystal fiber are shown to generate multiple narrowband spectral peaks within the 380–460 nm wavelength region through multimode-phase-matched third-harmonic generation. Some of these peaks are shifted by tens of terahertz from the tripled frequency of the pump field, dictated by standard energy conservation for third-harmonic generation in monochromatic fields. The spectral contents of the third-harmonic signal generated in such a regime are controlled by changing polarization and the intensity of the input pump field.     

Designing dispersion-compensating photonic-crystal fibers using a genetic algorithm

A genetic algorithm is combined with a fully vectorial finite-element solver to design photonic-crystal fibers (PCFs) for a broadband dispersion compensation in a generic stretcher-compressor system of an ytterbium fiber laser. Two types of PCFs are compared in terms of their dispersion-compensation capability, optical nonlinearity, and confinement loss. Fibers of the first type are standard PCFs where a solid core is surrounded by a triangular uniform lattice of identical air-holes. In PCFs of the second type, the solid core is surrounded by a dual-scale cladding, where the inner part comprises air-holes of different diameters, while the outer cladding consists of large-diameter air-holes. Second-type PCFs are shown to provide a much more accurate dispersion compensation. The influence of fiber-fabrication tolerances on the precision of dispersion compensation in short-pulse fiber laser systems is examined.     

Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber

Microjoule supercontinuum generation is demonstrated using a large-mode-area photonic-crystal fiber (PCF) pumped by an amplified stretched-pulse output of a mode-locked Cr:forsterite laser. A PCF with a mode area of 380 μm² is employed to transform 300-fs Cr:forsterite laser pulses with a peak-power of a few megawatts into a supercontinuum radiation with a spectrum spanning from 700 to 1800 nm and a total energy of 1.15 μJ.     

Spectral broadening of 40-fs Ti:sapphire laser pulses in photonic-molecule modes of a cobweb-microstructure fiber

Photonic-molecule modes of a cobweb-microstructure fiber allow efficient nonlinear optical spectral broadening of nanojoule femtosecond light pulses. Spectral widths of approximately 200 nm are achieved at the output of a 6-cm sample of such a fiber for 40-fs Ti:sapphire laser pulses with an energy of a few nanojoules coupled into photonic-molecule modes. Higher values of the group index and a lower group-velocity dispersion, attainable with higher-order photonic-molecule modes, allow the efficiency of spectral broadening of femtosecond laser pulses to be increased relative to the efficiency of spectral broadening in the fundamental photonic-molecule mode. Received: 9 June 2002 / Revised version: 29 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/939-3959, E-mail: zheltikov@top.phys.msu.su     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Electromagnetic field confined and tailored with a few air holes in a photonic-crystal fiber

Conventional and scanning near-field optical microscopy techniques are cross referenced to femtosecond nonlinear-optical measurements and finite-element numerical simulations to visualize and analyze a strong confinement of electromagnetic radiation in guided modes of a photonic-crystal fiber with only a few air holes surrounding the fiber core. A nonlinear coefficient of about 120 W⁻¹ km⁻¹ is achieved at the wavelength of 670 nm for a fused-silica fiber with a full hexagonal cycle of closely packed air holes around the fiber core. The removal of a single element from this array of air holes is shown to frustrate field confinement in guided modes, leading to mode leakage.     

Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers

Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers. Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications. Received: 23 October 2001 / Accepted: 16 January 2002 / Published online: 14 March 2002     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

Coherent, polarization and temporal properties of self-frequency shifted solitons generated in polarization-maintaining microstructured fibre

Coherence of both self-frequency shifted solitons and short-wavelength non-soliton radiation generated in a micro-structure fibre was investigated experimentally for the first time. Their spectral, temporal and polarization characteristics were studied as well. We found the solitons generated in 30-cm-long fibre pumped with 50-fs pulses at 795 nm and 835 nm from Ti:Sapphire oscillator to be coherent, whereas degree of coherence of blue-shifted radiation was amounted to 0.25–0.57 depending on wavelength.     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Propagation of short soliton pulses through a parabolic index fiber with dispersion decreasing along length

A parabolic index dispersion decreasing fiber (DDF) has been designed and optimized to produce high capacity soliton communication system. Variation of different fiber parameters such as core radius, effective core area and GVD factor along the 25 km of DDF length has been carried out to optimize a best possible DDF which can sustain the propagation of fundamental soliton. The variation of non-linearity with length along with the conventional power and GVD factor variation has been included in the generalized non-linear Schrodinger equation (NLSE). This NLSE has been solved numerically by split step Fourier method for shorter pulse propagation, incorporating the term for third order dispersion and intrapulse Raman scattering. Stable soliton pulses in transmission system have been achieved by our simulation, when a correction factor due to Raman induced soliton mean frequency shift is incorporated to the GVD profile predicted by the fundamental soliton condition. The interaction of neighboring soliton pulse pair through the proposed fiber has also been studied.     

Four-wave mixing of picosecond pulses in hollow fibers: expanding the possibilities of gas-phase analysis

Third-harmonic, difference-frequency, and sum-frequency generation processes in hollow fibers are experimentally studied with 30-ps pulses having an energy of several millijoules. The experimental dependence of the difference-frequency signal on the pressure of the gas filling the fiber agrees well with the results of calculations when the contribution of higher order waveguide modes is taken into consideration, thus indicating the importance of nonlinear-optical processes involving higher order waveguide modes of a hollow fiber. Hollow fibers are also shown to expand the possibilities of nonlinear-optical analysis of gases by allowing the generation of third-harmonic and sum-frequency signals, which vanish in the regime of tight focusing in a medium with normal dispersion. Received: 26 September 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

Enhanced spectral broadening of short laser pulses in high-numerical-aperture holey fibers

The influence of the structure of holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800-nm Ti:sapphire laser radiation transmitted through 2- and 3-μm-pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%. Received: 23 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Differential group delay monitoring using an all-optical signal spectrum-analyser

We demonstrate an all-optical device that monitors differential group delay (DGD) degradation of picosecond optical pulses. This device is based on an ultrafast all-optical signal analyser that uses nonlinear effects (cross-phase modulation) to transfer rapid temporal fluctuations into frequency domain effects that can be measured on an conventional optical spectrum analyser (incorporating a slow-detector). This monitoring scheme will enable rapid dynamic monitoring and compensation of DGD in ultrafast optical networks, at 160 Gb/s data rates and beyond, where electronic monitoring techniques cannot operate. We discuss the required signal polarisation condition.     

Spectral-temporal dynamics of ultrashort Raman solitons and their role in third-harmonic generation in photonic crystal fibers

We use cross-correlation frequency-resolved optical gating to obtain spectral-temporal portraits of ultrashort Raman solitons in photonic crystal fibers at telecommunication wavelengths. Power-dependent Raman frequency shifts of 200 nm in 63 mm of fiber are observed accompanied by spectral broadening and 2.5-times soliton compression. Complete time-frequency dynamics at the fundamental wavelength thus visualized enables us to explain the details of the intermodally phase-matched third harmonic generation by the propagating solitons.This revised version was published online in August 2005 with a corrected cover date.     

All-solid silica-based photonic crystal fibers

An index-guiding all-solid photonic crystal fiber (PCF) composed entirely of silica material is proposed in this paper. The core of this optical fiber is composed of pure silica, and the cladding consists of doped silica rod in the background of pure silica. The dependence of confinement loss on the diameter of the doped rods, the number of doped-rod rings, and the doping level is investigated numerically. In addition, the proposed fiber possesses a shorter cutoff wavelength as compared with the air/silica PCF, which is directly confirmed by the V parameter, and explained based on a scalar approximation method. Furthermore, the bending loss for the fiber is predicted. A low-loss single-mode all-solid silica-based PCF with a large-mode-area is possible by the appropriate selection of configuration parameters.     

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth.     

Design of defect-core in highly birefringent photonic crystal fibers with anisotropic claddings

The influence of defect-core on the birefringence and confinement losses of rectangular-lattice photonic crystal fibers are investigated numerically by applying the multipole method. Numerical results illustrate that the birefringence in such fibers is determined not only by the arrangement of air holes in the cladding but also the shape of the core. It is found that asymmetry of the core represented by its rectangular shape implies a higher effective index of the mode that is parallel with the longer side of the rectangle, whereas the anisotropic rectangular-lattice cladding gives rise to just the opposite effect and thus the resulting birefringence can be controlled by a proper combinations of both mechanisms. In particular, effect of the asymmetry of the core on the birefringence is dominant for shorter wavelength. Increased birefringence and reduced confinement loss can be achieved, if we form the core by the omission of several air holes in a row to reduce its negative effect on the birefringence. On the other hand, when asymmetry is increased in the other direction, a negative birefringence at shorter wavelength can be achieved. This occurs due to the fact that asymmetry of the core at higher frequencies overcomes the effect of the asymmetric cladding. As a result, its possible to achieve zero birefringence in anisotropic cladding photonic crystal fiber with an asymmetric core.     

Soliton control in optical lattices with periodic modulation of nonlinearity coefficient

We address the dynamics of solitons in the optical lattices with periodic modulation of the nonlinearity coefficient. Based on the quasi-particle approach, the properties of fundamental soliton localized in optical lattices are theoretically analyzed and shown its potential application for controllable soliton switching. Moreover, the phenomena of multi-soliton splitting and the single-soliton constituent trapping in the optical lattices are illustrated and discussed.