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The early detection of microvascular changes in cancer diagnosis is needed in the clinic. A change in the vascular bifurcation density is a biomarker for the sprouting activity. Here, Optical-Resolution PhotoAcoustic Microscopy is used for quantitative vascular bifurcation mapping in 2D after the creation of Virtual Tubes out of Bifurcations. In stacks of OR-PAM images of the hemoglobin distribution, bifurcations become tubes and are selected by the 3D tubeness filter. These fast analyses will be compared to a classical approach and are easier to implement for functional analysis of the vascular bifurcation density in healthy and diseased tissues.

International audience; An optical-resolution photoacoustic microscope with modulated CW laser diodes allowing multi-channel imaging is presented that can be used for both imaging biological tissues and for targeted photo-dynamic therapy (PDT) varying the optical power and exposure time. The effects of this therapy are immediately monitored in order to optimize the time of irradiation. After the description of the experimental setup, in vitro and in vivo applications are presented on a synthetic sample and on the mouse ear using hemoglobin as endogenous and methylene blue as exogenous dye for imaging and PDT, respectively.

We present how a laser optical feedback imaging (LOFI) setup can be used for the optical detection of ultrasound in photo-acoustic tomography (PAT). A PAT image is reconstructed by an inversion algorithm using surface displacement measurements made at several locations with our LOFI setup and following the optical irradiation with a pulsed Nd:YAG laser of a sample with absorbing inclusions. The width of the reconstructed inclusions and the signal-to-noise ratio (SNR) of the reconstructed images are first studied on the numerical model of a sample with three absorbing inclusions (i.e., with three acoustic punctual sources). Finally, an experimental PAT image of a phantom composed of two polyamide tubes with an internal diameter of 800 μm filled with red ink and submerged at -3.5 mm depth in a tank filled with water is reconstructed. Experimentally, the water surface displacement measurements have been made with our LOFI vibrometer, which provides an amplitude sensitivity of 1 nm (for a single-shot measurement) in a detection bandwidth of roughly 1 MHz adapted to the detection of the polyamide tubes. Under our experimental conditions, the surface energy densities of the LOFI focalized beam for the detection and of the pulsed Nd:YAG laser used for the irradiation, are compatible with the maximum permissive exposure for future biomedical measurements. The SNR and the resolution of the reconstructed PAT images are in good agreement with the theoretical predictions.

Reservoir computing (RC) systems are computational tools for information processing that can be fully implemented in optics. Here, we experimentally and numerically show that an optically pumped laser subject to optical delayed feedback can yield similar results to those obtained for electrically pumped lasers. Unlike with previous implementations, the input data are injected at a time interval that is much larger than the time-delay feedback. These data are directly coupled to the feedback light beam. Our results illustrate possible new avenues for RC implementations for prediction tasks., SCOPUS: ar.j, info:eu-repo/semantics/published

In this article, we study the non-linear coupling between the stationary (i.e. the beating modulation signal) and transient (i.e. the laser quantum noise) dynamics of a laser subjected to frequency shifted optical feedback. We show how the noise power spectrum and more specifically the relaxation oscillation frequency of the laser are modified under different optical feedback condition. Specifically we study the influence of (i) the amount of light returning to the laser cavity and (ii) the initial detuning between the frequency shift and intrinsic relaxation frequency. The present work shows how the relaxation frequency is related to the strength of the beating signal and the shape of the noise power spectrum gives an image of the Transfer Modulation Function (i.e. of the amplification gain) of the nonlinear-laser dynamics.The theoretical predictions, confirmed by numerical resolutions, are in good agreements with the experimental data., in Physical Review, American Physical Society (APS), 2016

International audience; We 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.

International audience; In this article, we study the nonlinear dynamics of a laser subjected to frequency shifted optical reinjection coming back from a vibrating target. More specifically, we study the nonlinear dynamical coupling between the carrier and the vibration signal. The present work shows how the nonlinear amplification of the vibration spectrum is related to the strength of the carrier and how it must be compensated to obtain accurate (i.e., without bias) vibration measurements. The theoretical predictions, confirmed by numerical simulations, are in good agreement with the experimental data. The main motivation of this study is the understanding of the nonlinear response of a laser optical feedback imaging sensor for quantitative phase measurements of small vibrations in the case of strong optical feedback.

International audience; The use of frequency-shifted feedback (FSF) lasers in optical metrology is based on a unique coherence property: the appearance of beats in the noise spectrum at the output of a two-beam interferometer, whose frequencies vary linearly with the path delay of the interferometer. A description of the output of a FSF laser as a moving comb of optical frequencies is generally admitted to explain these specific coherence properties. Here starting from the model of a passive FSF cavity seeded by spontaneous emission we give a rigorous description of the time-spectrum properties of FSF lasers and show that the moving comb exists only in the limit of small frequency shift.

This paper examines the detection of ultrasound vibrations with nanometric amplitude by using a laser optical feedback imaging (LOFI) setup. By means of numerical simulations, we show typical examples of ultrasound vibrations having different temporal shapes (harmonic and transient), extracted from the laser output power modulation induced by the frequency-shifted optical feedback. Considering the laser quantum noise dynamic and the detection noise separately, we show that the simulated vibration noise is in good agreement with the theoretical prediction. Also, we demonstrate that ultra-high frequencies (in the gigahertz range) can be detected by using a usual LOFI setup with a low-power laser (few mW) and a conventional detection with a usual white noise level. Then we show how the noise of a short transient vibration can be reduced by the reconstruction of its wide vibration spectrum by concatenation. Finally, the experimental detection of transient-harmonics ultrasound vibrations propagating in water and detected at the air/water interface is presented.

International audience; The original Talbot (self-imaging) effect is observed in the vicinity of a grating of slits shined with a plane wave, and results in periodic images of the initial diffraction pattern (integer Talbot effect) and the appearance of images with a periodicity reduced by an integer factor (fractional Talbot effect). Most of the studies on Talbot effect so far have focused on the distribution of the intensity of the diffracted light. However the phases of the Talbot images, obtained in both the integer and fractional self-imaging cases, can be calculated in a closed form and display interesting auto-correlation properties. This paper reports what we believe to be the first experimental investigation of the phases of Talbot images beyond the integer self-imaging case. We address the problem of experimental measurement of the phases of the Talbot images in the equivalent frame of the angular Talbot effect, a recently reported manifestation of the Talbot effect in the far field. The phases of the Talbot images are measured by far-field holography and the obtained results are in excellent agreement with theoretical calculations. They also suggest the possibility of using the scheme for a precise " fractional ruler " aimed at distances' measurements.

This paper proposes what we believe is a new method to remove the contribution of parasitic reflections in the images of the laser optical feedback imaging (LOFI) technique. This simple method allows us to extend the LOFI technique to long-distance applications, as imaging through a fog or a smoke. The LOFI technique is an ultrasensitive imaging technique that is interesting for imaging objects through a scattering medium. However, the LOFI sensitivity can be dramatically limited by parasitic optical feedback occurring in the experimental setup. In previous papers [Appl. Opt.48, 64 (2009)10.1364/AO.48.000064APOPAI1559-128X, Opt. Lett.37, 2514 (2012)10.1364/OL.37.002514OPLEDP0146-9592], we already have proposed methods to filter a parasitic optical feedback, but they are not well suited to metric working distances. This new method uses a Doppler frequency shift induced by the moving mirror used to scan the object to be imaged. Using this Doppler frequency shift, we can distinguish the photons reflected by the target and the parasitic photons reflected by the optical components in the experimental setup. In this paper, we demonstrated theoretically and experimentally the possibility to filter the parasitic reflection in LOFI images using the Doppler frequency shift. This method significantly improves the signal-to-noise ratio by a factor 15 and we can obtain a shot noise limited image through a scattering medium of an object at 3 m from the detector.

We present a device for characterizing red blood cells flows in capillaries (speed, hematocrit, aggregation, …). We've used the LOFI technique on a microfluidic system that mimics the human skin and subcutaneous tissues.

The dynamical response of a laser simultaneously subject to frequency-shifted optical feedback and pump modulation is investigated both theoretically and experimentally. We show that the amount of light reinjected in the intra-cavity laser can be determined by the pump modulation contrast, either for a destructive interference condition between the optical feedback and the pump modulation, or near the parametric instability threshold.

International audience; The interference of waves evenly separated in frequency generates a periodic signal, whose intensity fluctuations depend on the phases of the individual waves. A fundamental question in telecommunications and acoustics is the minimization of the peak-to-average-power ratio (PAR): for a given spectrum, how to arrange the phases of the individual frequency components to ensure minimal intensity fluctuations of the resulting signal? For a flat spectrum a near-optimal solution is brought by the so-called Newman phases, commonly used in acoustic and radio waves. Here we transpose this property into the optical domain and prove the possibility to suppress the intensity fluctuations resulting from intermodal beating of a broadband comb of optical frequencies, whose phases are set according to a generalization of the Newman phases. We demonstrate experimentally a broadband laser, whose intensity contrast (defined as the standard deviation-to-mean ratio) is reduced down to 0.54 as compared to 1 when the phases are set randomly.

International audience; For given laser output power, object under investigation and photodiode noise level, we have theoretically compared the signal to noise ratio (SNR) of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the Laser Optical Feedback Imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e. directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared 1D scans obtained by numerical simulations with different lasers dynamical parameters. Finally we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive comparing to a heterodyne interferometer. © 2010 Optical Society of America

International audience; — We present an overview of the performances of a plenoptic microscope which combines the high sensitivity of a laser optical feedback imaging setup , the high resolution of optical synthetic aperture and a shot noise limited signal to noise ratio by using acoustic photon tagging. By using an adapted phase filtering, this microscope allows phase drift correction and numerical aberration compensation (defocusing, coma, astigmatism …). This new kind of microscope seems to be well adapted to make deep imaging through scattering and heterogeneous media.

A small and efficient chemical/biological sensor can be realized by the use of surface plasmon resonance in integrated optics. The attenuation coefficient of the device is modified by the presence of a sensitive material whose index and/or thickness depends on the concentration in certain molecules that are characteristic of it. A modal analysis of the sensor structure is performed and a very simple model is proposed to assist in the structure design and optimization. Finally, some experimental results for gas detection are presented.

Summary form only given. We present an experimental demonstration of a novel technique for the generation of Fourier transform-limited pulses at tuneable and possibly ultrahigh repetition rates, limited only by the spectral bandwidth of the laser. Our proposal is based on a frequency shifted feedback (FSF) laser injected with a single-mode laser. Recall that a FSF laser consists in a laser cavity with an internal frequency shifter (usually an acousto-optics frequency shifter or AOFS): each time a photon makes a roundtrip in the cavity, its frequency is increased by twice the frequency of the acoustic wave in the AOFS (in the case of a linear cavity). When seeded with a single-mode laser, the output of the FSF laser consists in a frequency comb with a mode spacing equal to the shift frequency fs: the frequency of the nth mode is fn = f0 + nfs where f0 is the frequency of the seed laser. However contrary to mode-locked frequency combs where all modes share the same phase, the phases of the modes at the output of the FSF laser are quadratic with n and are given by πn(n+1)fs/fc, where fc is the free spectral range of the FSF cavity [1]. Early numerical simulations predicted the generation of Fourier transform-limited pulses when the frequency shift per roundtrip and the cavity free spectral range are in the ratio of two integers, that is fs/fc = p/q, p and q being coprime integers [2]. In this case the repetition rate is equal to qfs (=pfc). We demonstrated experimentally this property for large values of p and q, by implementing a FSF dye cavity seeded with a monochromatic (dye) laser. The spectral bandwidth of the comb is 150 GHz. By adjusting the length of the FSF laser cavity (i.e. fc), we have demonstrated the generation of 6 ps pulses at a repetition rate tuneable by more than two orders of magnitude, between 240 MHz and 47 GHz, by steps of 80 MHz. This result is interpreted in terms of temporal fractional Talbot effect [3] and described analytically. A very good agreement with the experiment is obtained. In particular we characterize the phases of the optical pulses. This work offers important novel perspectives for the generation of ultrahigh repetition rates (GHz to THz) for various applications in data processing, opt-electronics, THz generation and spectroscopy of metallic nanoparticles.

Summary form only given. Getting resolved images through scattering media is a crucial concern as it is related to important issues. For instance one can cite high resolution biological imaging. Such an imaging system could provide us information about deep tissues without requiring invasive biopsy followed by histological sections.When light propagates through turbid media, the major problem is to find the trade-off between the sensitivity and the resolution. The first possibility is to measure the multi-scattered photons [1,2]; the advantage is the good sensitivity at large depths but at the price of a highly degraded resolution (multi-scattered photons have lost their initial direction). On the contrary one can choose to measure only ballistic photons and thus to get an optically resolved image. The main drawback is that their number decreases exponentially during the propagation, impacting signal to noise ratio and limiting the accessible depth. Laser Optical Feedback Imaging (LOFI) microscope [3] is a setup belonging to the second family, a laser acts both as an emitter and a receiver of photons, and an image is obtained point by point by scanning the target with galvanometric mirrors. Photons are emitted, frequency shifted near the relaxation frequency of the laser, retroreflected by the target and finally reinjected into the laser cavity. This leads to a beating at the frequency shift of the laser mode, which is detected by a photodiode and demodulated in amplitude and in phase. Finally, one has an equivalent detector which benefits from the dynamics of the laser with an ultimate shot noise sensitivity. The counterpart of these advantages is the time of an image acquisition (at least 30 seconds for 512*512 pixels). Another interesting property of this system is the ability to keep the resolution of microscope objective beyond the working distance of the microscope objective. Indeed this setup can be seen as an autodyne interferometer with both amplitude and phase information on the reinjected electric field. As a result it is possible to use Synthetic Aperture (SA) techniques to treat numerically raw defocus images (from fig. 1a to 1b), with a final resolution corresponding to the microscope objective resolution.The major problem with synthetic aperture LOFI is the photometric balance which degrades with the defocus during raw recording (the coupling of the reinjected electric field with the laser cavity acts as a confocal pinhole) limiting the accessible lattitude of numerical refocusing. This problem coupled with parasitic reinjected photons from reflection on all interfaces (lenses, mirrors) before the target prevents the system from reaching the shot noise limit. To handle this problem, a system coupling the LOFI and an the acoustic tagging [4] is proposed. In this setup only photons tagged (by an acoustical transducer) just before the target are taken into account, leading to an elimination of this parasitic echoes (fig. 1c) restoring the LOFI full sensitivity.

Plenoptic photography was first introduced by the French Nobel Prize Gabriel Lippmann in 1908 [1] under the name "integral photography". Unlike conventional imaging systems where only the impact position of photons is recorded, a device based on plenoptic technology also records their initial direction of propagation, all the information about the incident light is therefore accessible. This property allows to obtain images with unusual properties such as the possibility to refocus a blurry photo after the picture has been taken or to have an extended depth of field without reducing the aperture of the lens and thus degrading the photometric balance [2]. The opportunity to correct the objective's aberrations by digital post-processing has also been demonstrated [2].

International audience; 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.

International audience; In autodyne interferometry, the beating between the reference beam and the signal beam takes place inside the laser cavity and therefore the laser fulfills simultaneously the roles of emitter and detector of photons. In these conditions, the laser relaxation oscillations play a leading role, both in the laser quantum noise which determines the signal to noise ratio (SNR) and also in the laser dynamics which determines the response time of the interferometer. In the present study, we have theoretically analyzed the SNR and the response time of a Laser Optical Feedback Imaging (LOFI) setup based on an autodyne interferometer. More precisely, we have compared the images quality of two lasers having the same output power, the same relaxation frequency, but having two different values of the LOFI gain induced by two different values of the laser response time. From this study, we have finally determined the best laser dynamical parameters and the best experimental conditions for high speed imaging at the shot noise limit. Finally, we conclude that a laser diode with a very short response time (in the nanosecond range) seems to be an interesting candidate compared to solid-state microchip laser with a response time of several tens of microsecond. Analytical predictions are confirmed by numerical simulations.

International audience; 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 focalisation on target and the second one is made by Synthetic Aperture (SA) Laser, which uses numerical focalisation. We show that SA configuration allows 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 conventional microscope. This drawback is partially compensated by the important sensitivity of LOFI. Another interest of SA relies on the capacity of getting a 3D information in a single x-y scan.

International audience; Frequency-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.

International audience; We present a photon noise and diffraction limited imaging method combining the imaging laser and ultrasonic waves. The laser optical feedback imaging (LOFI) technique is an ultrasensitive imaging method for imaging objects through or embedded within a scattering medium. However, LOFI performances are dramatically limited by parasitic optical feedback occurring in the experimental setup. In this work, we have tagged the ballistic photons by an acousto-optic effect in order to filter the parasitic feedback effect and to reach the theoretical and ultimate sensitivity of the LOFI technique. We present the principle and the experimental setup of the acousto-optic laser optical feedback imaging (AO-LOFI) technique, and we demonstrate the suppression of the parasitic feedback.

International audience; 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.

International audience; In this paper we present an experimental setup based on Laser Optical Feedback Imaging (LOFI) and on Synthetic Aperture (SA) with translational scanning by galvanometric mirrors for the purpose of making deep and resolved images through scattering media. We provide real 2D 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.

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.

International audience; Using a 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 performances than heterodyne interferometry for a wide range of laser power and detection level of noise. The experimental results are in good agreement with the theoretical predictions.

International audience; 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.

International audience; Bubbles confined between the parallel walls of microchannels experience an increased drag compared to free standing bubbles. We measure and model the additional friction from the walls, which allows the calibration of the drag force as a function of velocity. We then develop a set-up to apply locally acoustic waves, and demonstrate the use of acoustic forces to induce the motion of bubbles. Because of the bubble pulsation, the acoustic forces---called Bjerknes forces---are much higher than for rigid particles. We evaluate these forces from the measurement of bubble drift velocity, and obtain large values of several hundreds of nanoNewtons. Two applications have been developed to explore the potential of these forces: asymmetric bubble break-up to produce very well controlled bi-disperse populations and intelligent switching at a bifurcation.

For synthetic aperture (SA) operation in the optical domain, microchip laser sources seem to be relevant by reasons of their compactness and their high temporal and spatial coherence (laser linewidth less than 1 kHz and laser beam divergence close to the diffraction limit) [1,2]. The main objective of this study is to combine the high resolution of SA experiment (i.e. beyond the diffraction limit) with the high sensitivity and the simplicity of the LOFI (Laser Optical Feedback Imaging) experiment [3] to obtain a very efficient and compact device for 2D imaging.

International audience; In this work, we present a new development of the laser optical feedback imaging technique for imaging biological structures with a high resolution. The first results obtained on human red blood cells and mice cerebral and muscular tissues slices are shown. The performances of the system and its future developments are also discussed.

International audience; The use of laser optical feedback Imaging (LOFI) for scattering-type scanning near-field optical microscopy (sSNOM) is proposed and investigated. We implement this sensitive imaging method by combining a sSNOM with optical heterodyne interferometry and the dynamic properties of a B class laser source which is here used both as source and detector. Compared with previous near field optical heterodyne experiments, this detection scheme provides an optical amplification that is several orders of magnitude higher, while keeping a low noise phase-sensitive detection. Successful demonstration of this complex field imaging technique is done on Silicon on Insulator (SOI) optical waveguides revealing phase singularities and directional leakage.

We have used parametric instabilities to amplify frequency-shifted optical feedback in a microchip laser. We show that this effect is phase sensitive.

The dynamical response of a laser simultaneously subject to frequency-shifted optical feedback and pump modulation is investigated both theoretically and experimentally. A theoretical bifurcation analysis shows that the intensity modulation contrast induced by the frequency-shifted optical feedback can be amplified by modulating the laser pump power near a parametric instability. We show that this amplification is phase sensitive and more significant at a weak optical feedback level. Compared with conventional optical heterodyne detection, this method permits higher sensitive interferometry and hence imaging.

The metallic and concrete structures used in civil Engineering show their defects by the appearance of cracks. We report crack detection laboratory tests on a piece of armed concrete stimulated by acoustic waves. The feasibility of detecting non-emerging (hence invisible to a human observer) cracks is demonstrated. Our results show a good visual correlation between differential speckle interferometry, (a.k.a. shearography) and laser optical feedback imaging (LOFI). The advantages of shearography are its compacity, resolution and rapidity (it is a direct full view technique), but in this application, the information it provides is essentially qualitative. On the contrary the LOFI technique measures the difference of vibration amplitude on each side of the defect. Moreover its good sensitivity enables long-range measurement, but the image resolution and acquisition time depend on the tuning of scanning mirrors. The metallic and concrete structures used in civil Engineering show their defects by the appearance of cracks. We report crack detection laboratory tests on a piece of armed concrete stimulated by acoustic waves. The feasibility of detecting non-emerging (hence invisible to a human observer) cracks is demonstrated. Our results show a good visual correlation between differential speckle interferometry, (a.k.a. shearography) and laser optical feedback imaging (LOFI). The advantages of shearography are its compacity, resolution and rapidity (it is a direct full view technique), but in this application, the information it provides is essentially qualitative. On the contrary the LOFI technique measures the difference of vibration amplitude on each side of the defect. Moreover its good sensitivity enables long-range measurement, but the image resolution and acquisition time depend on the tuning of scanning mirrors.

For further interferometric application on diffusive target, the phase fluctuation of a solid-state laser submitted to frequency shifted optical feedback is analyzed both theoretically and experimentally. As a drawback of the laser high sensitivity to optical feedback, the phase fluctuations induced by a strong phase-amplitude coupling noise are several orders of magnitude higher than the standard interferometric phase noise induced by the laser frequency width (Schawlow-Townes limit). Nevertheless, by sending a few milliwatts output power microchip laser beam on a diffusive target with an effective reflectivity of ${10}^{\ensuremath{-}9}$, a target displacement precision of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}∕\sqrt{\mathrm{Hz}}$ has been experimentally determined.

We propose a new method for reconstitution of submicrometric profile based on the LOFI (Laser Optical Feedback Imaging) technique [E. Lacot, et al., 2002]. We show that this method is equivalent to an optical system with infinite aperture.

We evidence experimentally that the linewidth-enhancement factor $\ensuremath{\alpha}$ can take a rather large value ($\ensuremath{\alpha}\ensuremath{\approx}1$) for a nonsemiconductor laser, here a ${\mathrm{Nd}}^{3+}$: YAG microchip laser. This measure is performed using an original and simple method adapted to this kind of laser and based on the variations of the laser relaxation frequency when the laser is subjected to an optical feedback.

The dynamical response of a laser to a frequency-shifted optical feedback (heterodyne optical feedback) is investigated both theoretically and experimentally. Compared to a conventional heterodyne method, the intensity modulation induced by the coherent interaction between the laser electric field and the frequency-shifted reinjected electric field can be several orders of magnitude higher. This method permits high sensitive interferometry and hence imaging. We call this laser detection technique: Laser optical feedback imaging (LOFI). By adding to the heterodyne feedback a small amount of homodyne optical feedback (i.e., feedback with the same optical frequency as the laser optical frequency), the laser can reach a Hopf instability domain. By working close to this instability, we are able to amplify still further the LOFI modulation induced by the heterodyne optical feedback.

International audience; The poor selectivity of most of the current chemical sensors is a major problem that limits their application fields. A molecular filter disposed ahead of those sensors can enhance this property. We have realised such filter with zeolite and some tests have been conducted with semiconductor and optical chemical sensors.

International audience; In autodyne interferometry, the beating between the reference beam and the signal beam takes place inside the laser cavity and therefore the laser fulfills simultaneously the roles of emitter and detector of photons. In these conditions, the laser relaxation oscillations play a leading role, both in the laser quantum noise which determines the signal to noise ratio (SNR) and also in the laser dynamics which determines the response time of the interferometer. In the present study, we have experimentally analyzed the SNR and the response time of a Laser Optical Feedback Imaging (LOFI) interferometer based on a Nd3+ microchip laser, with a relaxation frequency in the megahertz range. More precisely, we have compared the image quality obtained, when the laser dynamics is free and when it is controlled by a stabilizing electronic feedback loop using a differentiator. From this study, we can conclude that when the laser time response is shorter (i.e. the LOFI gain is lower), the image quality can be better (i.e. the LOFI SNR can be higher) and that the use of an adapted electronic feedback loop allows high speed LOFI with a shot-noise limited sensitivity. Despite the critical stability of the electronic feedback loop, the obtained experimental results are in good agreement with the theoretical predictions.

International audience; We show both theoretically and experimentally that frequency-shifted feedback (FSF) lasers seeded with a single frequency laser can generate Fourier transform-limited pulses with a repetition rate tunable and limited by the spectral bandwidth of the laser. We demonstrate experimentally in a FSF laser with a 150 GHz spectral bandwidth, the generation of 6 ps-duration pulses at repetition rates tunable over more than two orders of magnitude between 0.24 and 37 GHz, by steps of 80 MHz. A simple linear analytical model i.e. ignoring both dynamic and non-linear effects, is sufficient to account for the experimental results. This possibility opens new perspectives for various applications where lasers with ultra-high repetition rates are required, from THz generation to ultrafast data processing systems.

En optique, l'interferometrie est un outil extremement puissant permettant des mesures de distance, de vitesse, de vibration, avec une excellente precision. D'un autre cote, les lasers sont extremement sensibles a la retroaction optique. Nous avons combine ces deux proprietes pour developper au laboratoire de nouvelles methodes d'imagerie basees sur le comportement dynamique des lasers.

In this paper we study the origin and the effect of amplitude and phase noise on Laser Optical Feedback Imaging (LOFI) associated with Synthetic Aperture (SA) imaging system. Amplitude noise corresponds to photon noise and acts as an additive noise, it can be reduced by increasing the global measurement time. Phase noise can be divided in three families: random, sinusoidal and drift phase noise; we show that it acts as a multiplicative noise. We explain how we can reduce phase noise by making oversampling or multiple measurements depending on its type. This work can easily be extended to all SA systems (Radar, Laser or Terahertz), especially when raw holograms are acquired point by point.

We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a synthetic aperture (SA) process. We report a two-dimensional (2D) SA LOFI experiment where the unprocessed image (i.e., the classical LOFI image) is obtained point by point, line after line using full 2D galvanometric scanning. The 2D superresolved image is then obtained by successively computing two angular SA operations while a one-dimensional angular synthesis is preceded by a frequency synthesis to obtain a 2D superresolved image conventionally in the synthetic aperture radar (SAR) method and their corresponding laser method called synthetic aperture ladar. The numerical and experimental results are compared.

We present an optical architecture for the laser optical feedback imaging (LOFI) technique that makes it possible to avoid the effect of the optical parasitic reflections introduced by the optical components located between the laser source and the studied object. These reflections damage phase and amplitude information contained in the images. This phenomenon is a leading problem that strongly limits the LOFI performance for weak feedback detection. Consequently, it is essential to be able to limit or avoid the effect of these parasitic reflections to reach the optimal LOFI performance.

We describe a new one-dimensional synthetic aperture imaging laser radar (ladar) using the resonant sensitivity of a microchip laser to frequency-shifted optical feedback and galvanometric scanning of the target under investigation. In our experiment, the laser is both the source and the detector, providing optical amplification with self-aligned heterodyne detection. By using galvanometric scanning, we achieve an along-track spatial resolution better than the diffraction limit.

Compared with conventional optical heterodyne detection, laser optical feedback imaging (LOFI) allows for a several orders of magnitude higher intensity modulation contrast. The maximum contrast amplification is typically 10(3) for a diode laser in the gigahertz range and 10(6) for a microchip laser in the megahertz range. To take advantage of the wavelength tunability of a laser diode and of the lower resonant detection frequency of a microchip laser, we used LOFI modulation induced by the frequency-shifted optical feedback in a laser diode as a modulated pumping power for a microchip laser for resonant dynamic amplification. In this way, we were able to transfer the optical feedback sensitivity of the laser diode to the megahertz range. Application to telemetry is also reported.