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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Diffraction-limited polarized emission from a multimode ytterbium fiber amplifier after a nonlinear beam converter.

The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction-limited, linearly polarized beam by a self-referencing two-wave-mixing process in an infrared-sensitive photorefractive crystal (Rh:BaTiO3). As much as 11.6 W of single-mode output is achieved with a 78% multimode-to-single-mode photorefractive conversion efficiency.

Mid-infrared high-resolution absorption spectroscopy by use of a semimonolithic entangled-cavity optical parametric oscillator.

By recording low-pressure absorption lines of N2O around 3.9 microm, we fully qualify a pulsed entangled-cavity doubly resonant optical parametric oscillator as a power tool for high-resolution spectroscopy. This compact source runs at a high repetition rate (>10 kHz) with a low threshold of oscillation (<8 microJ), is mode-hop-free tunable over 5 cm(-1), and displays single-frequency Fourier-transformed-limited operation (linewidth <0.005 cm(-1)). A high potential for nonlinear spectroscopy is also expected given the high peak power (70 W) and the good quality (M2 < 2) of the output beam.

Efficient single-mode Brillouin fiber laser for low-noise optical carrier reduction of microwave signals.

We experimentally demonstrate efficient optical carrier reduction of microwave signals with a single-mode 1.5-microm wavelength Brillouin all-fiber ring laser. Because of the tunable optical coupling, the lasing threshold of the short-length (20-m) fiber cavity is lower than 5 mW, and high conversion efficiencies (up to 60%) are obtained at any pump power up to approximately 200 mW. Using the single-mode Stokes beam as a seed for the stimulated Brillouin scattering process allows up to 40-dB optical carrier depletion with almost no added noise for an optically carried microwave signal at 6 GHz. In addition, using this resonator, we provide evidence of generation of high-spectral-purity beatnotes.

Life history of flight morph females of Callosobruchus maculatus F.: evidence of a reproductive diapause.

Callosobruchus maculatus (F.) (Coleoptera : Bruchidae) is a tropical beetle that develops in the seeds of Vigna unguiculata. C. maculatus adults show an imaginal polymorphism with differences in morphological, behavioral and reproductive characteristics. Adults of the flight morph that emerge in cowpea storage systems were studied under natural climatic conditions. A large number of the flight morph females were in reproductive diapause and had a long imaginal life. These females did not synthesize vitellogenin, produced a specific diapause protein and possessed significant protein reserves. This suggests that the beetles survived in the tropical ecosystem for a long time and colonized the crops during the cowpea growing and flowering phases. Analysis of reproductive activity in females captured in the V. unguiculata crops indicates that they terminated their reproductive diapause and began to lay eggs as soon as the pods were formed. Few females of the flight morph were sexually active at the beginning of imaginal life. In this paper we discuss the adaptive significance of these two reproductive strategies in females of C. maculatus.