Hugues Guillet de Chatellus

Experimental comparison of autodyne and heterodyne laser interferometry using an Nd:YVO 4 microchip laser

Using an Nd:YVO₄ microchip laser with a relaxation frequency in the megahertz range, we have experimentally compared a heterodyne interferometer based on a Michelson configuration with an autodyne interferometer based on the laser optical feedback imaging (LOFI) method regarding their signal-to-noise ratios. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity, while in the autodyne configuration, the wave beating takes place inside the laser cavity, and the relaxation oscillations of the laser intensity then play an important part. For a given laser output power, object under investigation, and detection noise level, we have determined the amplification gain of the LOFI interferometer compared to the heterodyne interferometer. LOFI interferometry is demonstrated to show higher performance than heterodyne interferometry for a wide range of laser powers and detection levels of noise. The experimental results are in good agreement with the theoretical predictions.

Demonstration of a plenoptic microscope based on laser optical feedback imaging.

A new kind of plenoptic imaging system based on Laser Optical Feedback Imaging (LOFI) is presented and is compared to another previously existing device based on microlens array. Improved photometric performances, resolution and depth of field are obtained at the price of a slow point by point scanning. Main properties of plenoptic microscopes such as numerical refocusing on any curved surface or aberrations compensation are both theoretically and experimentally demonstrated with a LOFI-based device.

Synthetic aperture laser optical feedback imaging using a translational scanning with galvanometric mirrors

In this paper we present an experimental setup based on laser optical feedback imaging (LOFI) and on synthetic aperture with translational scanning by galvanometric mirrors for the purpose of making deep and resolved images through scattering media. We provide real two-dimensional optical synthetic aperture image of a fixed scattering target with a moving aperture and an isotropic resolution. We demonstrate theoretically and experimentally that we can keep microscope resolution beyond the working distance. A photometric balance is made, and we show that the number of photons participating in the final image decreases with the square of the reconstruction distance. This degradation is partially compensated by the high sensitivity of LOFI.

Sensitivity of synthetic aperture laser optical feedback imaging.

In this paper, we compare the sensitivity of two imaging configurations, both based on laser optical feedback imaging (LOFI). The first one is direct imaging, which uses conventional optical focalization on target, and the second one is made by a synthetic aperture (SA) laser, which uses numerical focalization. We show that SA configuration allows us to obtain good resolutions with high working distance and that the drawback of SA imagery is that it has a worse photometric balance in comparison to a conventional microscope. This drawback is partially compensated by the important sensitivity of LOFI. Another interest of SA relies on the capacity of getting three-dimensional information in a single x-y scan.

Functional white-laser imaging to study brain oxygen uncoupling/recoupling in songbirds.

Contrary to the intense debate about brain oxygen dynamics and its uncoupling in mammals, very little is known in birds. In zebra finches, picosecond optical tomography with a white laser and a streak camera can measure in vivo oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) concentration changes following physiologic stimulation (familiar calls and songs). Picosecond optical tomography showed sufficient submicromolar sensitivity to resolve the fast changes in the hippocampus and auditory forebrain areas with 250 μm resolution. The time course is composed of (1) an early 2-second-long event with a significant decrease in Hb and HbO2 levels of −0.7 and −0.9 μmol/L, respectively, (2) a subsequent increase in blood oxygen availability with a plateau of HbO2 (+ 0.3 μmol/L), and (3) pronounced vasodilatation events immediately after the end of the stimulus. One of the findings of our study is the direct link between blood oxygen level-dependent signals previously published in birds and our results. Furthermore, the early vasoconstriction event and poststimulus ringing seem to be more pronounced in birds than in mammals. These results in birds, tachymetabolic vertebrates with a long lifespan, can potentially yield new insights, e.g., into brain aging.

Return flux budget of polychromatic laser guide stars

The polychromatic laser guide star (PLGS) is one of the solutions proposed to extend the sky coverage by large telescopes to 100% by enabling a complete knowledge of all perturbation orders of the wavefront. The knowledge of the tip-tilt is deduced from the monitoring of the chromatic components of the PLGS, from 330 nm to the visible or near infrared. Here we study the original scheme to create the PLGS by resonant excitation of the mesospheric sodium by two pulsed lasers (tens of kilohertz repetition rate, tens of watts average power, tens of nanoseconds pulse duration), at 589 and 569 nm, respectively. The efficiency of this process is investigated numerically by means of both Bloch equation and rate equation models. The influence of numerous laser parameters is studied. In the best case, having optimized all laser parameters, the return flux at 330 nm should not exceed 7x10(4) photons/s/m2 for 2x18 W laser average power at the mesosphere. This maximum is obtained for a modeless laser whose spot diameter corresponds to 4 times the diffraction limit. For a diffraction-limited spot, the return flux falls down to 4x10(4)photons/s/m2.

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

The 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.

Suppression of Rayleigh scattering noise in sodium laser guide stars by hyperfine depolarization of fluorescence

We propose what we believe is a novel method for enabling the complete suppression of noise due to Rayleigh scattering in sodium laser guide star systems by means of selective discrimination between Rayleigh and fluorescence signals based on polarization properties. We show that, contrary to the nearly 100% polarized Rayleigh scattering, fluorescence from the D(2) sodium line is strongly depolarized under excitation by a modeless laser. This offers the possibility of completely cancelling the effects of the Rayleigh scattering background while preserving the fluorescence signal to about 40% of its maximal value, leading to an improvement of the signal-to-noise ratio by several orders of magnitude. Both theoretical and experimental data confirm this new proposal.

Polychromatic laser guide star using a single laser at 330 nm

The differential atmospheric tip-tilt can be measured using a Polychromatic Laser Guide Star. A two photon excitation has been proposed. It consists in exciting the 4D5/2 level of mesospheric sodium atoms with two identical lasers operating at 589 nm and 569 nm. With two modeless lasers of 2×15W at the mesosphere level, this scheme will produce a returned flux at 330 nm of about 4×104 photons/s/m2. Thanks to our modeless laser, we propose a new method which consists in exciting directly the 4P3/2 sodium level with one photon excitation, using a single laser operating at 330 nm. This solution was previously rejected probably because of strong saturation problems using single longitudinal mode lasers. We show that 1 W modeless laser at 330 nm can produce the same returned flux at 330 nm. This solution will save at least 400 k€ of equipment. Moreover, our new method is very promising in terms of simplicity but also in terms of flux because the returned flux above will probably be not sufficient for getting a good Strehl ratio. We propose very efficient solid state laser systems for the production of tens of watts at 330 nm.

Coherent multi-heterodyne spectroscopy using acousto-optic frequency combs

We propose and characterize experimentally a new source of optical frequency combs for performing multi-heterodyne spectrometry. This comb modality is based on a frequency-shifting loop seeded with a continuous-wave (CW) monochromatic laser. The comb lines are generated by successive passes of the CW laser through an acousto-optic frequency shifter. We report the generation of frequency combs with more than 1500 mutually coherent lines, without resorting to non-linear broadening phenomena or external electronic modulation. The comb line spacing is easily reconfigurable from tens of MHz down to the kHz region. We first use a single acousto-optic frequency comb to conduct self-heterodyne interferometry with a high frequency resolution (500 kHz). By increasing the line spacing to 80 MHz, we demonstrate molecular spectroscopy on the sub-millisecond time scale. In order to reduce the detection bandwidth, we subsequently implement an acousto-optic dual-comb spectrometer with the aid of two mutually coherent frequency shifting loops. In each architecture, the potentiality of acousto-optic frequency combs for spectroscopy is validated by spectral measurements of hydrogen cyanide in the near-infrared region.