Angular Talbot effect

International audienceWe predict the possibility of observing integer and fractional self-imaging (Talbot) phenomena on the discrete angular spectrum of periodic diffraction gratings illuminated by a suitable spherical wave front. Our predictions are experimentally validated, reporting what we believe to be the first observation of self-imaging effects in the far-field diffraction regime

Coherence properties of modeless lasers

International audienceMost of classical light sources show a close similarity between their first and second order correlation functions (resp. g(1) and g(2)) functions. We present here the original coherence properties of a peculiar type of laser named modeless laser or Frequency Shifted Feedback (FSF) laser where the g(1) and g(2) functions show a different behaviour. We calculate and evidence experimentally the first and second order correlation functions of modeless lasers, through measurements of the homodyne beat signal and interferometric autocorrelation of a dye FSF laser at the output of a Michelson interferometer. Whereas the degree of first-order coherence vanishes beyond the coherence length of the FSF source, the degree of second-order coherence exhibits periodic revivals far beyond the coherence length, with a period equal to the cavity roundtrip time. Our observations are in good agreement with the theoretical treatment of Yatsenko et al. (Opt. Comm. 282 (2009) 300) [1]

Bidirectional frequency-shifting loop for dual-comb spectroscopy

We present a bidirectional recirculating frequency-shifting loop, seeded by a continuous-wave (cw) laser, to perform multi-heterodyne interferometry. This fiber-optic system generates two counter-propagating “acousto-optic” frequency combs with a controllable line spacing. Apart from its simple architecture, coherent averaging allows us to reach acquisition times up to the second scale without resorting to any active stabilization mechanism. We also show that the relative phase between the combs is quadratic and can be easily controlled by adjusting the parameters of the loop. The capability of our scheme to perform molecular spectroscopy is proven by dual-comb measurements of a transition of hydrogen cyanide in the near-infrared region (1550 nm)

Coherent microscopy by laser optical feedback imaging (LOFI) technique

The application of the non conventional imaging technique LOFI (Laser Optical Feedback Imaging) to coherent microscopy is presented. This simple and efficient technique using frequency-shifted optical feedback needs the sample to be scanned in order to obtain an image. The effects on magnitude and phase signals such as vignetting and field curvature occasioned by the scanning with galvanometric mirrors are discussed. A simple monitoring method based on phase images is proposed to find the optimal position of the scanner. Finally, some experimental results illustrating this technique are presented

Deep and optically resolved imaging through scattering media by space-reversed propagation

We propose a novel technique of microscopy to overcome the effects of both scattering and limitation of the accessible depth due to the objective working distance. By combining Laser Optical Feedback Imaging (LOFI) with Acoustic Photon Taging (APT) and Synthetic Aperture (SA) refocusing we demonstrate an ultimate shot noise sensitivity at low power (required to preserve the tissues) and a high resolution beyond the microscope working distance. More precisely, with a laser power of 10mW, we obtain images with a micrometric resolution over ~8 transport mean free paths, corresponding to 1.3 times the microscope working distance. Various applications such as biomedical diagnosis, research and development of new drugs and therapies can benefit from our imaging setup

Effects of laser beam propagation and saturation on the spatial shape of sodium laser guide stars

International audienceThe possibility to produce diffraction-limited images by large telescopes through Adaptive Optics is closely linked to the precision of measurement of the position of the guide star on the wavefront sensor. In the case of laser guide stars, many parameters can lead to a strong distortion on the shape of the LGS spot. Here we study the influence of both the saturation of the sodium layer excited by different types of lasers, the spatial quality of the laser mode at the ground and the influence of the atmospheric turbulence on the upward propagation of the laser beam. Both shape and intensity of the LGS spot are found to depend strongly on these three effects with important consequences on the precision on the wavefront analysis

Control of the differential interference contrast in reinjected bimode laser

International audienceWe have demonstrated, both theoretically and experimentally, that it is possible to control (i.e., to enhance or cancel) the contrast of the interference pattern appearing in the intensity images obtained with a laser optical feedback imaging (LOFI) setup using a bimode laser. The laser is composed of two coupled orthogonally polarized states that interact (i.e., interfere) through the cross saturation laser dynamics. We created the contrast control by choosing the frequency shift (i.e., the beating frequency) between the feedback electric fields and the intracavity electric fields. We have shown that the interference contrast of the output power modulation of the laser total intensity is independent from the frequency shift and is always maximal. On the other hand, the interference contrast of each polarization state is frequency dependent. We obtained the maximal contrast when the frequency shift was equal to one of the resonance frequencies of the bimode dynamics, and was very low (and almost cancels) for an intermediate frequency located at the intersection of the two resonance curves

Heterodyne beatings between frequency-shifted feedback lasers

International audienceFrequency-shifted feedback (FSF) lasers are potential candidates for long distance telemetry due to the appearance of beatings in the noise spectrum at the output of a homodyne interferometer: the frequencies of these beatings vary linearly with the path delay. In this Letter we demonstrate that these beatings also occur in the heterodyne mixing of two identical, but distinct, FSF lasers. This phenomenon is explained by the passive cavity model and is exploited to characterize the time-spectrum properties of FSF lasers. Consequences on telemetry with FSF lasers are presented