Anisotropic photonic lattices and discrete solitons in photorefractive media

We study experimentally two-dimensional periodic photonic lattices optically imprinted in photorefractive nonlinear media, and explore the effect of anisotropy on the induced refractive-index patterns. The orientation anisotropy is demonstrated by comparing square and diamond lattices, while the polarization anisotropy is shown to distinguish ordinarily and extraordinarily polarized light. In particular, the extraordinarily polarized lattice induces much stronger refractive-index modulation for the same conditions. Finally, we exploit the photorefractive anisotropy to generate a quasi-one-dimensional refractive-index pattern for the observation of two-dimensional solitons and corroborate these experiments by numerical simulations. PACS 42.65.Tg; 42.65.Wi     

Soliton complexes and flat-top nonlinear modes in optical lattices

We describe a continuous analog of the quasirectangular flat-top nonlinear modes earlier found for discrete nonlinear models. We show that these novel nonlinear modes can be understood as multi-soliton complexes with either in-phase or out-of-phase neighboring solitons trapped by the periodic potential of the lattice. We demonstrate a link between the flat-top states and the truncated nonlinear Bloch waves, and discuss how these nonlinear localized modes can be monitored experimentally in photonics and Bose–Einstein condensates. PACS 42.65.Tg; 42.65.Jx; 03.75.Lm     

Pulsed optical parametric oscillators with intracavity optical parametric amplification: a critical study

It is known that the idler conversion efficiency of optical parametric oscillators (OPOs) can be increased by adding a second nonlinear crystal in the cavity. This crystal is pumped by the signal and acts as an optical parametric amplifier (OPA) for the idler. However, this technique unavoidably increases the oscillation threshold because of additional losses and increased build-up time due to cavity lengthening. In this paper, we investigate both theoretically and experimentally the benefits and drawbacks of this so called OPO–OPA configuration versus the singly resonant OPO (SRO) configuration. Calculations are found to be in agreement with an experimental study of a SRO and an OPO–OPA operating near 3.4 μm both pumped by a 90-mJ 27-ns Nd:YAG laser. Our study reveals that the OPO–OPA needs to be driven at least two times above threshold to produce more idler energy than the SRO. In addition, near 3 μm the OPO–OPA is particularly efficient given that the difference frequency wave generated in the second crystal is also output coupled. PACS 42.65.Yj; 42.65.Sf     

Pattern formation in a passive incoherent ring resonator system based on nonlinear material with the self-focusing non-instantaneous Kerr response

We have analyzed the pattern formation in a nonlinear medium with self-focusing non-instantaneous Kerr response by employing the passive incoherent ring resonator system. In such a system, coherence time of the light is much shorter than the time of one round trip in the resonator. This delayed response of the nonlinearity can amplify the noise of certain spatial frequencies of the perturbed wave field and thus patterns can form when nonlinear gain (i.e., amplification of the noises) overcomes the loss (i.e., a well defined cavity threshold set by the coherence properties) in a single pass. The expression for the spatial spectral density of the perturbed wave field, which is the characteristic parameters of the pattern formation, have been derived in the case of lowest order approximation. It is found that for a specific value of the spatial frequency of the perturbed wave field, the intensity feedback of the cavity is much effective factor rather than the crystal thickness of the nonlinear media and amplitude of the incoming beam in the cavity for the enhancement of the spatial spectral density of the intensity pattern, which greatly improved the performance and applications of the pattern formation such as information processing, symmetry-breaking, and dynamics in non-equilibrium systems.     

Experimental control of unstable patterns and elimination of spatiotemporal disorder in nonlinear optics

A method based on Fourier filtered control signals for stabilization of unstable patterns in a nonlinear optical single feedback system is experimentally realized. The successful stabilization of the homogeneous solution and different stationary periodic patterns far above the pattern forming threshold as well as the elimination of spatiotemporal disorder above a secondary instability has been achieved. The temporal evolution of the control signal verifies the underlying philosophy of noninvasive control that changes only the linear stability of the systems' solution and not the solution itself.     

A mirror based on stimulated brillouin scattering with a double feedback loop in a laser cavity

The possibility of switching an external mirror based on stimulated Brillouin scattering (SBS) with a double feedback loop to a laser cavity is shown experimentally. A significant increase in the output laser power is obtained with conservation of the diffraction quality of the initial radiation. Experimental comparison with an ordinary SBS mirror, excited in a single pass through a nonlinear medium, is performed and the advantage in the energy parameters of the scheme with a double feedback loop is shown. The threshold characteristics of the SBS mirror under study are calculated and good agreement with the experimental data is obtained.     

Spontaneous and induced motion of optical patterns

The universality of two mechanisms of motion of patterns in nonlinear optics is demonstrated. In the first one, two-dimensional disordered patterns are shown to scroll with constant velocity after transients are discarded. In the second case, pattern motion is induced by background modulations. Spatially periodic patterns lock to maxima or minima of the underlying modulation depending on its wavevector. PACS 42.65.Sf; 05.65.+b; 45.70.Qj     

Experimental investigation of the spectro-temporal dynamics of the light pulses of Q-switched Nd:YAG lasers and nanosecond optical parametric oscillators

We report an experimental investigation of the spectro-temporal dynamics of the pulse formation in Q-switched Nd:YAG lasers and in nanosecond optical parametric oscillators (OPOs). The temporal evolution of the spectral intensity distribution of the light pulses was measured with a 1-m Czerny–Turner spectrometer in combination with a fast streak camera. This detection system allows the analysis of temporal changes in the spectrum of single nanosecond pulses. The measurements were performed for a flashlamp-pumped, Q-switched Nd:YAG laser and for an unseeded as well as for a seeded singly-resonant nanosecond OPO. The laser output spectrum varies strongly from pulse to pulse and even within a single pulse due to mode beating. In an unseeded OPO, individual spectral modes start to oscillate statistically from the parametric noise for pump powers close to the OPO threshold. With increasing pump power a strong modulation in the spectral formation of the pulse is observed, resulting from a strong interaction of parametric conversion and back conversion of signal and idler radiation into pump radiation. By means of injection seeding, the starting condition was controlled for a single mode. Due to the seed radiation, the seeded mode starts sooner than the unseeded modes. These are suppressed completely in the case of sufficient seed power and moderate pump power. The observations are in good agreement with results of corresponding numerical simulations. PACS 42.65.Sf, 42.65.Yj     

Optical phase conjugation and double-exposure phase-conjugate interferometry in dye-doped thin film using He–Ne laser

We have experimentally demonstrated optical phase conjugation and double-exposure phase-conjugate interferometry in a methyl green dye-doped gelatin film via degenerate four-wave mixing using a low-power He–Ne laser for the first time. The origin of phase conjugation associated with this dye-doped film is discussed. A phase-conjugate reflectivity of 0.13% is obtained with a He–Ne laser of total power 35 mW. PACS 42.65.Hw; 42.65.-k; 42.40.-i     

Experimental observation of speckle instability in Kerr random media

In a disordered nonlinear medium the transmitted speckle pattern was predicted to become unstable as a result of the positive feedback between intensity fluctuations and local variations of the refractive index. We show experimental evidence of speckle instability for light transversally scattered in a liquid crystal cell, where a two-dimensional controlled disorder is imprinted by suitable illumination of a photoconductive wall and nonlinearity is obtained through optical reorientation of the liquid crystal molecules. The speckle pattern spontaneously oscillates at discrete frequencies above a critical threshold, whose dependence on the scattering mean free path confirms the crucial role of disorder in the feedback process.     

Improvement of characterization accuracy of the nonlinear photonic crystals using finite elements-iterative method

We investigate nonlinear one- and two-dimensional photonic crystals by applying a finite element-iterative method. Numerical results show the essential influence of nonlinear elements embedded into a quarter-wave stack and the sharp photonic crystal waveguide bend on the spectral characteristics of these structures. We compare our results with those obtained in [21] from the discrete equation method for the case of a sharp waveguide bend. The comparison shows that neglecting the nonuniform field distribution inside the embedded nonlinear elements leads to overestimation of the waveguide bend transmissivity. PACS 42.65.-k; 42.70.Qs     

Hollow-fiber pulse compressor for KrF lasers

Application of the hollow-fiber pulse compression technique to 100 fs DUV pulses at 248 nm is reported. Sub-20 fs pulses with up to 20 μJ energy were obtained by using krypton in a 65 cm long fiber of 138 μm core radius. The physical process of the spectral broadening is modeled by a nonlinear propagation equation. The experimental data is in good agreement with the results of the model calculations. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control

We present an adaptive technique for the formation of multiple co-propagating and stationary filaments in a gaseous medium. Wavefront shaping of the initial beam is performed using a deformable mirror to achieve a complete two-dimensional control of the multi-spot intensity pattern in the laser focus. The spatial organization of these intensity spots yields reliable formation of up to five stable and stationary filaments providing a test bed for fundamental studies on multiple filamentation. PACS 42.65.Jx; 52.38.Hb; 42.65.Sf     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

Comparison of the filamentation and the hollow-core fiber characteristics for pulse compression into the few-cycle regime

The gas-filled hollow-core fiber compression and the optical filamentation technique are compared experimentally in a parameter regime suitable for intense few-cycle pulse generation. In particular, pointing stability, spectral properties, and spatial chirp are investigated. It is found that in the case of filamentation, the critical parameter for pointing stability is gas pressure inside the generation cell whereas for the hollow-core fiber it is alignment that plays this role. The hollow-core fiber technique yields spectra that are better suited for chirped-mirror pulse compression whereas filamentation offers higher throughput and prospects for easy-to-implement self-compression. We present spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements that directly show the transition in the spectral phase of the output continua into the self-compression regime as the gas pressure is increased. PACS 42.65.Re; 42.65.Jx; 42.65.Tg     

Supercontinuum generation using Raman fiber laser

Using a phosphosilicate-doped continuous-wave Raman fiber laser, a 350-m single-mode fiber, and a 50% feedback fiber Bragg grating at 1483.4 nm, we obtained a high-efficiency ultra-broadband (1434–1527 nm) supercontinuum (SC) centered at 1483.4 nm with an average output power of 2.1 W, a spectral intensity of 22.3 mWnm^-1, and a nonlinear conversion efficiency of 94%. Increasing the fiber length in the Raman cavity to 1840 m gave simultaneous SC centered at 1483.4 nm and 1239 nm. An average power of 2.3 W in the bandwidth of 1434–1544 nm and 0.2 W in the bandwidth of 1224–1257 nm is obtained. The temperature dependence of the SC was also investigated.PACS 42.55.Wd; 42.81.-i; 42.65.-k; 42.65.ReThis revised version was published in August 2003 with corrections to the affiliation of Mr. Taniguchi.     

Selecting optical patterns with spatial phase modulation

Optical pattern selection by use of spatial phase modulation is investigated experimentally in a photorefractive feedback system. A feedback mirror with spatially periodic phase modulation is used for selection of different spatial patterns. Local phase modulation is used to create patterns with coexisting spatial symmetries. The experimental results are consistent with numerical simulations based on a model with a cubicly nonlinear medium.     

The generation of intense, transform-limited laser pulses with tunable duration from 6 to 30 fs in a differentially pumped hollow fibre

We have investigated experimentally the energy transmission and spectral broadening of 30-fs, 700-μJ laser pulses in a neon-filled, 250-μm inner diameter hollow fibre. We implement a differentially pumped fibre, where a vacuum is maintained at the fibre entrance, and compare this to a statically filled fibre. We obtain significantly higher transmission and increased spectral broadening in the differentially pumped case due to a reduction of ionisation defocusing at the fibre entrance. This arrangement provides a method for the generation of near-transform- limited pulses with smoothly varying pulse duration whilst maintaining constant pulse energy, by simple adjustment of the gas pressure. Compression of ∼450-μJ pulses from the differentially pumped fibre to a duration of 6.5 fs has been achieved for pulses with spectra spanning 650–900 nm, by use of negatively dispersive chirped mirrors. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Discrete and dipole-mode gap solitons in higher-order nonlinear photonic lattices

We investigate the formation of fundamental discrete solitons and dipole-mode gap solitons in triangular photonic lattices imprinted in photorefractive nonlinear media. These lattices are strongly affected by the photorefractive anisotropy, resulting in orientation-dependent refractive index structures with reduced symmetry. It is demonstrated that two different orientations of the lattice wave enable the formation of fundamental discrete solitons in the total internal reflection gap. Furthermore, it is shown that one lattice orientation additionally supports dipole-mode solitons in the Bragg reflection gap. The experimental results are corroborated by numerical simulations using the full anisotropic model. PACS 42.65.Tg; 42.65.Wi; 42.70.Qs     

Image analysis for the destabilization process of the petal pattern in a liquid crystal light valve with rotational optical feedback

The destabilization process from the static petal pattern to the fluctuating petal pattern in a liquid crystal light valve with optical rotational feedback is investigated experimentally. When a spatial rotation of π/6 is imposed on the feedback, the six-fold static petal pattern appears. At a threshold voltage of the bifurcation, rotationally propagating patches arise on the petals. With the increase of applied voltage, the motion of the patches becomes irregular and fluctuations of the pattern develop. By measuring the time correlation function for the spatial wave numbers along the azimuthal direction in polar coordinates, it is found that a half of the fundamental spatial mode relevant to the six-fold petal pattern plays an important role in the bifurcation process. The development of fluctuations is also analyzed by the Karhunen-Loéve decomposition in detail.