Fast wavefront adaptive holography in Nd:YVO4 for ultrasound optical tomography imaging.

Several approaches exist to perform acousto-optic imaging of multiple-scattering media such as biological samples. Up to now, most of the coherent detection methods use holographic setup based on photorefractive crystals such as BSO or SPS. One of the issue of these techniques is the moderate response time compared to the speckle decorrelation time in biological sample. We introduce a new approach for the holographic detection based on two-wave mixing in a Nd:YVO4 gain medium enabling us to perform a fast wavefront adaption (50 µs) of the speckle field from a multiple-scattering sample.

Slow and stored light by photo-isomerization induced transparency in dye doped chiral nematics.

Decelerating and stopping light is fundamental for optical processing, high performance sensor technologies and digital signal treatment, many of these applications relying on the ability of controlling the amplitude and phase of coherent light pulses. In this context, slow-light has been achieved by various methods, as coupling light into resonant media, Brillouin scattering in optical fibers, beam coupling in photorefractive and liquid crystal media or engineered dispersion in photonic crystals. Here, we present a different mechanism for slowing and storing light, which is based on photo-isomerization induced transparency of azo-dye molecules hosted in a chiral liquid crystal structure. Sharp spectral features of the medium absorption/dispersion, and the long population lifetime of the dye metastable state, enable the storage of light pulses with a significant retrieval after times much longer than the medium response time.

Optical phase conjugation in Nd:YVO4 for acousto-optic detection in scattering media.

Acousto-optic imaging is a technique that maps the optical properties of a thick scattering sample with millimetric resolution. The detection of the acousto-optic signal represents a challenge, because it is very weak among a strong parasitic signal. Various methods based on holography in photorefractive crystals or digital holography have been studied. Here dynamic holography is obtained with the gain medium Nd:YVO(4). We study the experimental feasibility of a detection system based on holography in a gain medium and show acousto-optic results obtained in a 5 mm slice of chicken breast.

Optical pulsing in a resonator with a highly dispersive nonlinearity.

By introducing an intracavity Doppler shift in a resonator with a highly dispersive nonlinear medium, a train of optical pulses is generated whose features are related to the slow/fast-light response of the medium. The cavity transmission is asymmetric and the pulse shape is modified differently depending on the direction of the Doppler shift, hence, on the sign of the group delay provided by the dispersive process.

Sagnac interferometer with adaptive nonlinear detection.

We present a Sagnac interferometer that uses a nonlinear adaptive medium for detection. The nonlinearity of the medium being characterized by a finite response time, the detection has a finite frequency bandwidth determining two operational regimes: at low variation rate of the rotation speed the interferometer operates in the slow-light regime, providing a direct measurement of the angular acceleration. For high variation rate of the rotation speed the amplitude of the alternating component of the Sagnac phase shift is instead detected, with the continuous and low-frequency components filtered out by the adaptive nonlinear process in the medium.

Phase and amplitude control of a multimode LMA fiber beam by use of digital holography.

Amplitude and phase control of the output beam of a multimode LMA fiber supporting 4 modes is demonstrated by digital holography in both continuous and ns pulsed regimes at 1064 nm. Our system allows dynamic compensation of beam pointing instabilities, external perturbations introducing low order aberrations and fluctuations of the relative phase of the modes supported by the fiber.

High energy, single-mode, narrow-linewidth fiber laser source using stimulated Brillouin scattering beam cleanup.

We propose an original nonlinear beam cleaning fiber laser architecture to obtain high energy pulses with a good beam quality and a narrow linewidth. The output beam of a large core Er:Yb co-doped multimode fiber amplifier (M(2) approximately 6, 220 microJ) is converted into a near diffraction limited beam (M(2) = 1.6) through a stimulated Brillouin scattering injection seeded beam cleanup process. We report in this experiment a multimode to single mode conversion efficiency of 50% while preserving the master oscillator linewidth.

Picometer detection by adaptive holographic interferometry in a liquid-crystal light valve.

The large dispersive properties and the narrow frequency bandwidth of the two-wave mixing in a liquid-crystal light valve is used to realize an adaptive holographic interferometer in the Raman-Nath regime. We report experimental observation of picometer periodic displacements and estimate the theoretical signal-to-noise ratio and the minimum quantum-noise-limited detectable displacement.

Slow and fast light in liquid crystal light valves.

We show that fast and slow light results from multiple scatterings in a liquid crystal light valve, where nondegenerate two-wave mixing occurs in the Raman-Nath regime of optical diffraction. The large nonlinear response and dispersive characteristics of the liquid crystals allow us to obtain group velocities as slow as less than 0.2 mm/s, which is attractive for the realization of ultrahigh precision interferometers and metrology measurements.

Coherent fiber combining by digital holography.

We present an original technique for coherent beam combining of an array of fiber amplifiers based on self-adaptive digital holography. In this method, the phase errors between the fibers of the array are compensated by using the diffracted phase-conjugated -1 order of a digital hologram. The proposed method is compatible with a large number of fibers and simply implemented with a CCD detector matrix and a spatial light modulator. This concept is analyzed and experimentally demonstrated with three polarization-maintaining passive fibers at 1.06 microm.

Very compact external cavity diffraction-coupled tapered laser diodes.

To maintain the same beam quality as that of a single emitter and to be close to diffraction limit, we have combined a phase corrected array, emitting at lambda = 975 nm, coherently using the Talbot effect. First, to improve the beam quality of the array, a phase correcting system was added. The FWHM divergences of the array (which is approximately the same as that of the single emitter since the emitters within the array are not optically coupled to each other) were reduced from 34 degrees to 0.17 degrees in the fast axis and from 3.5 degrees to 0.7 degrees in the slow axis at 6 A. Then, to be close to the diffraction limit, we have combined this corrected array coherently using the Talbot effect. We have obtained a quasi-monolobe slow axis far field profile for the in-phase mode with a central peak divergence of only 0.27 degrees at 1.5 A, 315 mW under cw operation, and of only 0.20 degrees at 2.5 A, 787 mW under pulsed operation. To our knowledge, this is the first demonstration of coherent coupling of a corrected tapered laser diode array in an external Talbot cavity.

Self-pumped phase conjugation in a liquid crystal light valve with a tilted feedback mirror.

We report self-pumped phase conjugation in a photorefractive liquid crystal light valve with a tilted feedback mirror. This is a new scheme for the self-generation of a phase-conjugate replica and can in general be applied to thin nonlinear layers, opening the way to distortion-correction applications of thin media.

Spatiotemporal pulses in a liquid crystal optical oscillator.

A nonlinear optical medium results by the collective orientation of liquid crystal molecules tightly coupled to a transparent photoconductive layer. We show that such a medium can give a large gain; thus, if inserted in a ring cavity, it results in an unidirectional optical oscillator. We report new dynamical regimes characterized by the generation of spatiotemporal pulses, localized in three dimensions and arising from the random superposition of many longitudinal and transverse modes with different frequencies.

Enhancement of the two-wave-mixing gain in a stack of thin nonlinear media by use of the Talbot effect.

We show that the Talbot effect can be used to increase the gain of two-wave mixing in a stack of thin nonlinear media. We demonstrate that a large gain enhancement is achieved for a few nonlinear elements, provided that they are placed at distances that satisfy Talbot recurrences.

Beam cleanup in a self-aligned gradient-index Brillouin cavity for high-power multimode fiber amplifiers.

We propose a beam cleanup setup to convert a multimode beam into a single-mode beam by use of the Brillouin effect in a multimode gradient-index (GI) fiber. Phase conjugation and beam cleanup regimes in highly multimode fibers are discussed, and the self-aligned GI fiber Brillouin cavity is presented. We report a preliminary conversion from an M2=6.5 beam into an M2=1.3 beam with 31% efficiency.

Two-wave mixing in an erbium-doped fiber amplifier for modulation depth enhancement of optically carried microwave signals.

We experimentally and theoretically analyze an original method based on two-wave mixing in an erbium-doped fiber amplifier for optical carrier reduction of microwave signals. 75% optical carrier attenuation has been observed, and a 10 dB modulation depth increase of the microwave signal is experimentally demonstrated. Moreover, calculated results are in good agreement with measurements and predict that up to 80% carrier attenuation is easily possible.

Self-induced coherent oscillations with photorefractive Bi(12)SiO(20) amplifier.

We report coherent oscillations due to self-induced moving gratings in photorefractive BSO crystals when recording with an external applied field. Different types of ring and phase-conjugate resonators pumped with low-incident- intensity beams and having short response times are demonstrated. The conditions permitting oscillation and the respective frequencies of the pump and the signal beams in the cavity are analyzed.

Phase selection of synthetic discriminant function filters.

In this paper, we address the problem of correlation peak phase selection for synthetic discriminant function filters. We show that the minimization of the output variance and the optimization of the correlation peak form are problems of the same complexity (analogous to the determination of the ground state of a magnetic disordered system). We propose a general framework and, with examples, we show that, although the variance reduction by a proper selection of the correlation peak phases can be interesting in some situations, the optimization of the sharpness of the correlation peak is often more fruitful.

Phase conjugation in Cr(4+):YAG at 1.06 microm.

Phase conjugation in a Cr(4+):YAG saturable absorber by degenerate four-wave mixing with nanosecond pulses at lambda = 1.06 mu m is demonstrated for the first time to our knowledge. An experimental phase-conjugate energy ref lectivity of ~2% and an overall eff iciency of ~0.4% are achieved. Because of the presence of absorbing dipoles oriented along specif ic directions, it is shown that the phase-conjugated beam can be made orthogonally polarized relative to the signal beam or to both pump beams with almost no reduction of the energy reflectivity.

Transient self-diffraction in Nd:YAG saturable amplifiers.

We investigate self-diffraction and multiwave scattering by a nonsinusoidal gain grating induced in a thick laser medium with pulses of duration much less than the population relaxation time. The energies of the transmitted and of the two first-order self-diffracted beams are calculated. Experimental measurements performed in a flash-lamp-pumped Nd:YAG amplifier at lambda = 1.064 microm show good agreement with the theoretical results. A total energy in the two first-order scattered waves of 1.1 mJ is obtained for a total incident energy of 14 mJ.