SPM-induced spectral compression of picosecond pulses in a single-mode Yb-doped fiber amplifier

In a fiber amplifier, spectral compression due to self-phase modulation is demonstrated for ultrashort pulses. We report the generation of near-transform-limited picosecond pulses with peak powers of several kW at a repetition rate of 74 MHz and diffraction-limited beam quality in a Yb-doped fiber amplifier when seeding with a negative chirped pulse. Received: 17 September 2001 / Revised version: 22 November 2001 / Published online: 17 January 2002     

Photorefractive spatial solitons as waveguiding elements for optical telecommunication

Spatial solitons permit optical waveguiding. This holds true for the soliton write beam (i.e. the driving laser beam), as well as for additional probe beams, which may carry optically encoded information. This feature of spatial solitons is of significant interest for applications in optical telecommunication. We present systematic experimental investigations on single and multiple spatial solitons in the infrared spectral regime (i.e. around optical telecommunication wavelengths), applied as controllable all-optical devices. In particular, we present the implementations of a Y-coupler as an optical signal divider, a switchable Y-coupler as an optical add multiplexer, and a novel design for a 1 × 3 optical beam switch, i.e. applied as a router for infrared signal beams. We report large waveguiding efficiencies up to 40% and transmission rates of 90 Tbit/s in our setups. The presented experimental data are confirmed by numerical simulations.     

Incoherent optical switching of semiconductor resonator solitons

We demonstrate experimentally the bistable nature of the bright resonator solitons in a semiconductor microresonator with mixed absorptive/defocusing nonlinearity and show that they can be written and erased by incoherent local optical injection. Received: 22 February 2001 / Published online: 27 April 2001     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Quasi-two-dimensional dark spatial solitons and generation of mixed phase dislocations

This paper presents experimental evidence that orthogonally crossed dark soliton stripes form quasi-two-dimensional spatial solitons with a soliton constant equal to that of singly charged optical vortices. Besides the pairs of oppositely charged optical vortex solitons, the snake instability of the dark formation at moderate saturation is found to lead to generation of steering mixed edge–screw phase dislocations with zero total topological charges. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Integrated all-optical nonlinear device for re- configurable add/drop and wavelength shifting of WDM signals

A novel configuration is proposed for an all-optical device performing two key functionalities in a communication network based on wavelength-division-multiplexing (WDM): reconfigurable add/drop and wavelength shifting of single channels. The device is based on guided second-order nonlinear interactions, such as sum-frequency generation and difference-frequency generation, between the WDM channels and a suitable pump beam. A directional coupler and two parallel waveguides allow the spatial separation between the main WDM signal and the dropped or wavelength-shifted channel for a subsequent routing in the desired path. A first numerical simulation provided a cross-talk level in the dropped channel lower than -41 dB and a wavelength-shifting range of more than 40 nm. Received: 18 May 2001 / Published online: 30 October 2001     

Incoherently coupled Hermite-Gaussian breather and soliton pairs in strongly nonlocal nonlinear media

We theoretically investigate the propagation of incoherently coupled Hermite-Gaussian breather and soliton pairs in strongly nonlocal nonlinear media. It is found that multipole-mode soliton pairs with arbitrary different orders of Hermite-Gaussian shape can exist when the total power of two beams equals the critical power and the ratio of the beam widths for the Gaussian part is inversely proportional to the square root of the ratio of the wave numbers. When the total power does not equal the critical power, the Hermite-Gaussian breather pair exists and their beam widths evolve analogously. For general cases where the ratio of the beam widths is arbitrary, soliton-breather pairs or breather-breather pairs can be formed and their beam widths evolve synchronously in-phase or out-of-phase. Numerical simulations directly based on the nonlocal nonlinear Schrödinger equation are conducted for comparison with our theoretical predictions. The numerical stability analysis shows the higher-order Hermite-Gaussian solitons can not be stable for small nonlocality or for some media like liquid crystals.     

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.     

Multimode to single-mode conversion of a Yb-doped fiber amplifier with a photorefractive Rh:BaTiO3 crystal

We present a technique to convert a multimode-transverse beam obtained from a multimode large core fiber amplifier into a single-mode beam by means of two-wave mixing in an infrared-sensitive Rh:BaTiO₃ crystal. Received: 26 March 2001 / Revised version: 27 March 2001 / Published online: 27 April 2001     

Effect of partial coherence on four-wave mixing in photorefractive materials via reflection grating approximation

We investigate the effect of beam coherence on four-wave mixing via reflection gratings in photorefractive media. For the case of phase conjugation, the results of our theoretical analysis indicate that partial coherence always leads to a drop of signal gain and phase conjugate reflectivity in non-depleted cases. In general, the mutual coherence of the signal beam and the pump beam can be enhanced due to the process of wave mixing. The mutual coherence of the phase conjugate beam and one of the pump beams depends on the beam intensity ratio as well as the optical path difference. This is distinctly different from the four-wave mixing case with a transmission grating. Received: 15 October 1999 / Revised version: 26 June 2000 / Published online: 7 February 2001     

Defect vector gap solitons in photonic lattices

The existence and general properties of different kinds of defect vector gap solitons in one dimensional optically induced photonic defect lattice with focusing saturable nonlinearity in photorefractive crystal are analyzed. The defect is well localized in a single site with two existence forms, namely repulsive and attractive defect. Propagation constants of two beams that compose defect vector gap solitons could be from same gap or from different gaps. We show that some kinds of unstable scalar defect gap solitons could be stabilized by their corresponding vector cases.     

Spatial solitons in a semiconductor microresonator

We show experimentally the existence of bright and dark spatial solitons in a passive quantum-well-semi-conductor resonator of large Fresnel number with mixed absorptive defocusing nonlinearity. Several of the solitons can exist simultaneously as required for applications. Received: 17 November 2000 / Published online: 13 December 2000     

Self-deflection of partially spatially incoherent beams and solitons in photovoltaic photorefractive crystals

By considering the effect of background light and diffusion, the self-deflection process of partially spatially incoherent (PSIC) beams and photovoltaic (PV) solitons in open-circuit PV photorefractive crystals has been investigated by employing numerical method and the perturbation technique, respectively. The results from the two approaches are in good agreement: the center of PSIC PV solitons moves on a parabolic trajectory, which is similar to those of coherent solitons. In addition, we also discuss that the dependence of self-deflection effect on the coherent parameter θ₀ and find it is slight relative to θ₀ for quasi-soliton but decreases monotonously with θ₀ for PSIC beam.     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

Solitary Wave Evolution of Optical Planar Vortices in Self-Defocusing Photorefractive Media

Solitary wave evolution of optical planar vortices in isotropie self-defoeusing photorefractive media is investigated in detail. We demonstrate that the formation of a planar vortex soliton intensively depends on the diameter and maximum intensity of the input vortex Seam. The exact solutions of planar vortex solitons are obtained due to the Petviashvili iteration method. It is found that, with the increasing soliton maximum intensity, the soliton core will be gradually diminished to a minimum value.     

Temporal behavior of dark spatial solitons in closed-circuit photovoltaic media

We show that the time-dependent nonlinear wave equation in closed-circuit photovoltaic media can exhibit quasi-steady-state and steady-state spatial solitons. We demonstrate that the formation time of open-circuit quasi-steady-state and open-circuit steady-state dark solitons decreases with an increase in the intensity ratio of the soliton, which is the ratio between the soliton peak intensity and the dark irradiance. We find that for the time-dependent nonlinear wave equation that exhibits only an open-circuit steady-state dark soliton, changing the electric current density J₀ does not generate quasi-steady-state dark solitons and affects the formation time of steady-state dark solitons and that for the time-dependent nonlinear wave equation that exhibits an open-circuit quasi-steady-state dark soliton, changing J₀ gives rise to three different time evolution regimes of the full width half maximum of the soliton’s intensity. The first regime shows that the formation time of steady-state dark solitons increases with J₀ whereas the formation time of quasi-steady-state dark solitons is independent of J₀. The second regime shows that the formation time of steady-state dark solitons decreases with an increases in J₀ and the formation time of quasi-steady-state dark solitons increases with J₀. The third regime shows that changing J₀ enables only steady-state dark solitons in the time-dependent nonlinear wave equation, of which the formation time increases with J₀.     

Coherent switching of semiconductor resonator solitons

We demonstrate the switching on and off of spatial solitons in a semiconductor microresonator by injection of light coherent with the background illumination. The results give evidence that the formation of the solitons and their switching are not limited by thermal processes. Received: 11 February 2002 / Revised version: 25 April 2002 / Published online: 8 August 2002     

Experimental study of pulse compression in a side-pumped Yb-doped double-clad mode-locked fiber laser

We have developed a side-pumped passively mode-locked Yb-doped double-clad fiber laser emitting sub-picosecond pulses at around 1.05 μm. Mode locking is achieved through a polarization additive-pulse mode-locking technique and compression is obtained with a grating pair inserted in the cavity. We have investigated the compression properties of this laser. High-energy pulses are emitted and different behaviors are observed, such as sideband generation, secant-like or Gaussian-like pulse emission. Received: 7 May 2001 / Revised version: 13 November 2001 / Published online: 17 January 2002     

Dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals

Using the coherent density approach, we study the dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals. We show that when the full width half maximum (FWHM) of the optical beam’s intensity is increased, the odd incoherent dark beam splits into an odd-number sequence of multiple dark stripes, whereas the even incoherent dark beam splits into an even-number sequence of multiple dark stripes. We find that when more incoherent solitons are generated, the separations between adjacent dark stripes become smaller and the stripes far away from the center become less visible and that for a given physical system and for a given splitting, the separations between adjacent dark stripes decrease with an increase in the intensity FWHM of the optical beam. On the other hand, the dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals is the dark incoherent screening soliton splitting when the bulk photovoltaic effect is neglectable and the dark incoherent closed- and open-circuit photovoltaic soliton splitting when the external bias field is absent.     

Two-Dimensional Laguerre--Gaussian Asymmetric Soliton Family in Strongly Nonlocal Media

We demonstrate the existence of a broad class of higher-order Laguerre Gaussian asymmetric spatial optical solitons in strongly nonlocal nonlinear media. Furthermore, we discuss specific values （q = 0） of the modulation depth parameter for different rational values of the topological charge in detail. Our results show that higherorder asymmetry spatial sofiton family can exist in various forms, such as two-dimensional defect haff-solitons, asymmetric single-layer and multi-layer necklace solitons.