Active polarimetric imaging by orthogonality breaking is an alternative polarimetric imaging method developed at the Institut FOTON, Rennes. By illuminating a sample with a dual-frequency dual-polarization (DFDP) beam whose polarizations are orthogonal, it is possible to characterize its diattenuation and the orientation of the anisotropy in a single acquisition. However, this technique is not sensitive to other polarimetric effects such as birefringence or pure depolarization and requires a detection/demodulation chain that introduces non-linearity effects and does not allow results to be obtained quantitatively. In this paper, after a presentation of the orthogonality-break imaging system, we will detail the calibration/correction protocol which is now implemented to take into account the effects of non-linearities. Then, we will show that it is possible, by adding a polarimetric analysis module, to make this method sensitive to the main polarimetric effects. The results obtained on a simulated operational scene will be presented.
Since long, optical intensity modulation/demodulation techniques have encountered numerous applications in telemetry, free-space communications or optical characterization of scattering media. Upgrading those techniques to a full-field, real-time imaging modality can allow massive multiplexing, an essential asset not only for 3D imaging or optical communications, but also for imaging in turbid media (medical diagnosis, underwater vision, imaging in colloids, or navigational aid for safe transports). In this context, we have recently proposed a new concept of Full-field All-optical Snapshot Technique for QUADrature demodulation imaging (FAST-QUAD), whose capacity in real-time image demodulation have been demonstrated up to frequencies of 500 kHz, without requiring any synchronization between the receiver and the intensity-modulated source(s) in the imaged scene.
This technique relies on an all-optical architecture, at the heart of which is an electro-optical crystal and appropriate polarization optics components, making it possible to spatially multiplex four transmission « gates » in quadrature to each other (0°, 90°, 180°, 270°), addressing four sub-images detected on the same single standard sensor (CCD/CMOS). This setup behaves as a quadrature lock-in detection circuit, well-known in the electronics field, but in the optical domain and in a massively spatially multiplexed way, using the acquisition time of the camera as a low-pass integrator. This optical module can therefore be inserted in front of any camera, and allows the number of electronics components to be minimized. This property provides FAST-QUAD with a major asset, as its operating frequency is fully and continuously tunable in the RF range, which allowed us to establish an experimental proof-of-concept between 0 Hz (DC) and 500 kHz on the first prototype built in the laboratory. We will detail the instrumental conception of this prototype as well as the calibration/processing pipeline developed. Experimental validation results and examples of application of the FAST-QUAD approach will also be presented.
Vertical-external-cavity surface-emitting lasers employing QDs as gain media in comparison to QW-based VECSELs can offer beneficial lasing features, such as, temperature resilience, broadband gain and wider wavelength tunability. We demonstrate the first QD-based VECSEL providing 2 W emission at 1.5 µm and a tuning range of 60 nm. This achievement paves the way to multi-Watt VECSELs with extended wavelength tunability.
We report an in-depth experimental characterization and analysis of an infrared active polarimetric imaging system based on the orthogonality breaking polarization-sensing approach. We first recall the principle of this laser scanning polarimetric imaging technique, based on the illumination of a scene by means of a dual-frequency dual-polarization light source. The experimental design is then described, along with measurements on test scenes with known polarimetric properties used to validate/calibrate the imaging system and to characterize its optical properties (sensitivity and resolution). The noise sources that temporally and spatially affect the quality of the orthogonality breaking data are then investigated. Our results show that the raw temporal signals detected at a given location of the scene are perturbed by Gaussian fluctuations, and the spatial information contained in the images acquired through raster scan of the scene are dominated by speckle noise, which is a common characteristic of active polarimetric imaging systems. Finally, the influence of the source temporal coherence on the images is analyzed experimentally, showing that orthogonality breaking acquisitions can still be performed efficiently with a low-coherence source.
In this work, we report InGaAs based photodiodes integrating liquid crystal (LC) microcells resonant microcavity on their surface. The LC microcavities monolithically integrated on the photodiodes act as a wavelength selective filter for the device. Photodetection measurements performed with a tunable laser operating in the telecom S and C bands demonstrated a wavelength sweep for the photodiode from 1480 nm to 1560 nm limited by the tuning range of the laser. This spectral window is covered with a LC driving voltage of 7V only, corresponding to extremely low power consumption. The average sensitivity over the whole spectral range is 0.4 A/W, slightly lower than 0.6 A/W for similar photodiodes that do not integrate such a LC tunable filter. The quality of the filter integrated onto the surfaces of the photodiodes is constant over a large tuning range (70 nm), showing a FWHM of 1.5 nm.
Fully fibered microwave-optical sources at 1.5 μm are studied experimentally. It is shown that the beat note between two orthogonally polarized modes of a distributed-feedback fiber laser can be efficiently stabilized using an optical phaselocked loop that uses the pump-power induced-birefringence as actuator. Beat notes at 1 GHz and 10 GHz of Erbium doped fiber laser are successfully stabilized to a reference synthesizer, passing from the 3 kHz free-running linewidth to a stabilized sub-Hz linewidth, with a phase noise as low as -75 dBc/Hz at 100 Hz offset from the carrier. An ErbiumYtterbium co-doped fiber laser is also investigated and successfully stabilized. Such dual-frequency stabilized lasers could provide compact integrated components for RF and microwave photonics applications.
Optimization of spin-lasers relies on the proper design of the active medium but also on a thorough understanding of the vectorial dynamics of the electromagnetic field in the laser cavity itself. A vectorial approach based the Jones formalism associated to the resonant condition of the field in the laser cavity is derived in order to draw the main guidelines for developing functional spin-controlled VCSELs. This general modelling framework, which accounts for spin injection effects as a gain circular dichroism in the active medium, shows that any residual phase anisotropy in the laser has a detrimental role on polarization switching. The same framework, is used to propose two solutions enabling to overcome this drawback: either by compensating the phase anisotropy or by preparing the laser cavity so that its eigenstates are circularly polarized. Moreover, unlike in spin-LED, we show that the leverage effect existing in the laser due to eigenmodes coupling makes it possible to switch the laser from a polarized oscillation to the orthogonal one despite the weak spin injection efficiency due to spin decoherence. All these predictions are confirmed using external cavity VCSELs which offer an ideal playing field for experimental investigations. Based on these developments, future trends towards the achievement of efficient and compact spin-lasers will be given.
We report the design of a free-space active infrared polarimetric imaging demonstrator operating at 1.55 μm and based
on a non-conventional approach: the orthogonality breaking sensing technique. Relying on the illumination of a scene
with a specific light source, the imager offers an original tradeoff between image acquisition time (~ 1 s) and
polarimetric consistency in comparison to standard polarimetric imagers such as division of time or division of amplitude
systems. We will illustrate the capability of such an imager to enhance the visibility of hidden objects on homemade
scenes.
We report theoretical and experimental analysis of spin-injected VECSELs. First, we fabricate and characterize an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well VECSEL. The structure is designed to allow the integration of a Metal-Tunnel-Junction ferromagnetic spin-injector for future electrical injection. We report here the control at room temperature of the VECSEL polarization using optical spin injection in the active medium. The switching between two highly circular polarization states had been demonstrated using an M-shaped extended cavity in multi-modes lasing. This first result witnesses an efficient spin-injection in the active medium of the laser. Then, we report birefringence measurements of the VECSEL in oscillating conditions. The proposed technique relies on the measurement in the microwave domain of the beatnote between the oscillating mode and the amplified spontaneous emission of the cross-polarized non-lasing field lying in the following longitudinal mode. This technique is shown to offer extremely high sensitivity and accuracy enabling to track the amount of residual birefringence according to the laser operation conditions. Finally, we discuss the compensation of the residual linear phase anisotropy by controlling the birefringence of an intracavity electro-optical crystal. A 44-fold birefringence reduction is obtained. Besides, we study the modification of the laser polarization eigen states with birefringence compensation: a rotation of the linear polarization state is observed when the total phase anisotropy is reduced. An elliptical polarization eigen state is obtained at the minimum of the birefringence into the laser cavity, more favorable for spin injection.
We show the use of a simplified snapshot polarimetric camera along with an adaptive image processing for optimal detection of a polarized light beacon through fog. The adaptive representation is derived using theoretical noise analysis of the data at hand and is shown to be optimal in the Maximum likelihood sense. We report that the contrast enhancing optimal representation that depends on the background noise correlation differs in general from standard representations like polarimetric difference image or polarization filtered image. Lastly, we discuss a detection strategy to reduce the false positive counts.
We report the experimental validation of a snapshot computational degree of polarization imaging technique, based on local analysis of the statistics of a single speckle image acquisition. The applicability of this imaging technique is demonstrated on various samples, and it precision is analyzed and compared with theoretical predictions. Then, we theoretically study the ability of this approach to discriminate samples with various depolarization degrees while sharing similar reflectance properties. We quantitatively compare the detection performances of this approach with standard with standard polarization imaging strategies and evaluate the increase in spatial resolution required to share similar detection efficiency.
A novel technique is proposed to unambiguously determine the magnitude and orientation of linear dichroism. It relies on the use of a dual-frequency dual-polarization coherent source emitting two orthogonal circularly polarized modes at the output. The interaction of such beam with dichroic media is shown to give rise to a beatnote signal in the radiofrequency range. The amplitude and phase of such beatnote makes it possible to fully determine the magnitude and orientation angle of the diattenuation. We also report the application of this method to polarimetric imaging, with promising perspectives in biomedical imaging. Indeed, it provides a direct characterization of dichroic sample orientation, showing uniform estimated dichroism magnitude, whatever the orientation of the sample.
Optimally enhanced vision of a polarized lightmark in obscured weather conditions (fog, haze, cloud) is reported when imaged over long distances (above 1 km) using a snapshot polarimetric camera. We derive and experimentally validate an optimal adaptive polarimetric representation, whose expression is shown to depend on the correlation of the noise fluctuations in the two orthogonal polarimetric images. We quantitatively compare the gain (experimental and theoretical) in contrast with respect to standard intensity imaging, and standard polarimetric representations. Lastly, we discuss efficient implementation strategies for automated detection in real-time in obscured weather conditions.
We investigate, both experimentally and theoretically, the spectral behavior of the intensity noises as well as the phase noise of the radio frequency (RF) beatnote generated by optical mixing of two orthogonally polarized modes of a dual-frequency VECSEL. To be more speci c, we measure the relative intensity noises (RINs) and the correlation between the intensity noises of the two laser modes for di erent nonlinear coupling strengths between them within frequencies 10 kHz to 50 MHz. Moreover for these frequencies, we explore the spectral behavior of the phase noise of the RF beatnote generated by optical mixing of two laser modes and the dependence of this RF phase noise spectrum on the strength of non-linear coupling between the laser modes. The theoretical model considers pump intensity uctuations as the only source of noise within the considered frequency range. The pump uctuations, entering into the two spatially separated laser modes on the active medium, are measured to be white noises of identical amplitudes, partially correlated, and in phase. To model the RF phase noise, we take into account two di erent physical mechanisms: (i) the coupling of intensity noise with phase noise due to large Henry factor of the semiconductor gain medium and (ii) the thermal uctuations of the refractive index of the semiconductor active medium induced by pump intensity uctuations. For all the results, theory shows very good agreement with the experiment.
KEYWORDS: Terahertz radiation, Signal to noise ratio, Signal detection, Signal attenuation, Photodiodes, Heterodyning, Tunable lasers, Physics, Spectroscopy, Metrology
THz has become a wide field of investigation opening new opportunities in a growing number of domains of physics,
chemistry, and biology. Among the different techniques existing today to generate THz fields, heterodyning two optical
frequencies is a useful approach when tunability is required. Moreover, to address high-resolution spectroscopy or
metrology applications, a key point is the achievement of a narrow linewidth source. To this aim, two-propagation-axis
dual-frequency lasers have been already shown to provide narrow linewidth tunable beat notes up to 2 THz. We report in
this paper the demonstration of a narrow linewidth THz radiation source based upon this laser. Indeed the beat note
provided by the laser is sent into a unitravelling carrier photodiode (UTC-PD), and radiated by a transverseelectromagnetic-
horn antenna (TEM-HA). All components operate at room temperature. The emitted THz signal is
detected by a subharmonic mixer coupled to an electrical spectrum analyzer. The THz signal is observed and analyzed
thanks to a heterodyne detection. The measured dynamic range is 75 dB at 282 GHz, 50 dB at 500 GHz, 35 dB at
700 GHz and decreases to 20 dB at 1 THz. The decrease is due to the UTC-PD efficiency and conversion losses in the
sub-harmonic mixer. The measured linewidth is better than 30 kHz at any frequency from DC to 1 THz.
A novel scheme based on opto-electronic down conversion is proposed and demonstrated to obtain an ultra-high spectral purity and tunable optical beat note in the GHz and millimeter wave range. An in-loop relative frequency stability better than 4 10-12 is reported. This approach opens the way to THz metrology.
We optimize the simultaneous oscillations at two frequencies in a class-A Vertical External Cavity Surface Emitting
Laser (VECSEL). We perform this task by measuring the coupling constant between the two perpendicular polarized
modes for different values of the transverse spatial separation between the two modes.
Low noise-level optical sources are required for numerous applications such as microwave photonics, fiber-optic sensing
and time/frequency references distribution. In this paper, we demonstrate how inserting a SC active medium into a
centimetric high-Q external cavity is a simple way to obtain a shot-noise-limited laser source over a very wide frequency
bandwidth. This approach ensures, with a compact design, a sufficiently long photon lifetime to reach the oscillation-relaxation-
free class-A regime. This concept has been illustrated by inserting a 1/2-VCSEL in an external cavity including
an etalon filter. A -156dB/Hz relative intensity noise level is obtained over the 100 MHz to 18 GHz bandwidth of
interest. This is several orders of magnitude better than the noise, previously observed in VCSELs, belonging to the
class-B regime. The optimization, in terms of noise, is shown to be a trade-off between the cavity length and the laser
mode filtering. The transition between the class-B and class-A dynamical behaviors is directly observed by continuously
controlling the photon lifetime is a sub-millimetric to a centimetric cavity length. It's proven that the transition occurs
progressively, without any discontinuity. Based on the same laser architecture, tunable dual-frequency oscillation is
demonstrated by reducing the polarized eigenstates overlap in the gain medium. The class-A dynamics of such a laser,
free of relaxation oscillations, enables to suppress the electrical phase noise in excess, usually observed in the vicinity of
the beat note. An original technique for jitter reduction in mode-locked VECSELs is also investigated. Such lasers are
needed for photonic analog to digital converters.
We developed a predictive model describing harmonic generation and intermodulation distortions in semiconductor
optical amplifiers (SOAs). This model takes into account the variations of the saturation parameters
along the propagation axis inside the SOA, and uses a rigorous expression of the gain oscillations harmonics.
We derived the spurious-free dynamic range (SFDR) of a slow light delay line based on coherent population
oscillation (CPO) effects, in a frequency range covering radar applications (from 40 kHz up to 30 GHz), and for
a large range of injected currents. The influence of the high order distortions in the input microwave spectrum
is discussed, and in particular, an interpretation of the SFDR improvement of a Mach-Zehnder modulator by
CPOs effects in a SOA is given.
Active imaging systems that illuminate the scene with polarized light and acquire two images in two orthogonal
polarizations yield information about the intensity contrast and the Orthogonal State Contrast (OSC) in the
scene. However, in real systems, the illumination is often spatially or temporally non uniform. We first study
the influence of this non uniformity on estimation performances. We derive the Cramer Rao Lower Bound and
determine a profile likelihood-based estimator. We demonstrate the efficiency of this estimator and compare its
performance with other standard estimators as a function of the degree of non-uniformity of the illumination.
Concerning target detection, illumination non uniformity creates artificial intensity contrasts that can lead to
false alarms. We derive the Generalized Likelihood Ratio Test (GLRT) detectors when intensity information is
taken into account or not, and determine the relevant expressions of the contrast in these two situations. These
results are used to determine in which cases taking intensity information in addition to polarimetric information
is relevant or not.
In this article we address the design and exploitation of a real field laboratory demonstrator combining active
polarimetric and multispectral modes in a single acquisition. Its buildings blocks, including a multi-wavelength
pulsed optical parametric oscillator at emission side, and a hyperspectral imager with polarimetric capability at
reception side, are described. The results obtained with this demonstrator are illustrated on some examples and
discussed.
A compact laboratory demonstrator providing both active polarimetric and multispectral images is designed. Its
buildings blocks include, at emission part, a multi-wavelength optical parametric oscillator and, at the reception part, a
polarimetric hyperspectral imager. Some of the results obtained with this system are illustrated and discussed. In
particular, we show that a multispectral polarimetric image brings additional information on the scene, especially when
interpreted in conjunction with its counterpart intensity image, since these two images are complementary in most cases.
Moreover, although hyperspectral imaging might be mandatory for recognition of small targets, we evidence that the
number of channels can be limited to a set of few wavelengths as far as target detection is considered.
We demonstrate a high-spectral-purity continuous-wave terahertz source, using a diode pumped Yb3+:KGd(WO4)2 dual frequency laser. THz radiation is generated by photomixing the two frequencies in a low temperature grown In:25Ga:75As photoconductor loading a dipole antenna. The frequency difference between the two optical modes is tuneable by step from d.c. to 3.1 THz. A maximum optical output power of 120 mW CW has been obtained with a beatnote-linewidth narrower than 30 kHz. Preliminary measurements show a tunable THz emission with a maximum output power in the order of a few tens of nW.
We propose to review two concepts that can be used for target detection and identification in optronic systems: lidar-radar and multipectral polarimetric active imaging.
The lidar-radar concept uses an optically pre-amplified intensity modulated lidar, where the modulation frequency is in the microwave domain (1-10 GHz). Such a system permits to combine directivity of laser beams with mature radar processing. As an intensity modulated or dual-frequency laser beam is directed onto a target, the backscattered intensity is collected by an optical system, pass through an optical preamplifier, and is detected on a high speed photodiode in a direct detection scheme. A radar type processing permits then to extract range, speed and profile of the target for identification purposes. The association of spatially multimode amplifier and direct detection allows low sensitivity to atmospheric turbulence and large field of view. We present here the analysis of a lidar-radar that uses a radar waveform dedicated to range resolution. Preliminary experimental results are presented and discussed.
For the multispectral polarization active imaging concept, the acquisition, at different wavelengths, of images coded in intensity and in degree of polarization enables to get information about the spectral signature of targets as well as their polarization properties. A theoretical analysis and a experimental validation of this technique are presented. Preliminary experiments, using a monostatic configuration, will be also presented.
Combining active multispectral and polarimetric imaging significantly enhances detection capability of very low contrast targets through the control of both the polarization state and the wavelength of the illumination light. However, increasing the operation range of the imaging system relies on the use of coherent sources, such as lasers and optical parametric oscillators, to illuminate the scene, leading to a dramatic decrease of the image quality due mainly to speckle noise. In order to investigate the benefits and drawbacks brought by coherent illumination, a preliminary laboratory demonstrator of an active multispectral polarimetric imager has been designed to operate with both polarized natural light and coherent sources. The orthogonal state contrast images recorded at different wavelengths in both configurations (coherent and non-coherent) clearly demonstrate the benefits of using active illumination of the scene to discriminate between real and fake targets and also to reveal very low contrast objects. Noise characteristics of polarimetric images under coherent illumination are also investigated. In particular the study of noise statistics of recorded images shows that the actual distribution of noise is log-normal. As a result, the so-called "natural" representation of the polarimetric image offers important advantages in terms of image processing. Indeed, if the intensity image is perturbed with multiplicative noise, the noise in the image with natural representation has uniform variance and is quasi-gaussian. The potential increase of target detection performance brought by properly processing the active polarimetric image is illustrated on a very low contrast scene.
Two-frequency solid-state lasers are shown to provide beat notes at frequencies from dc to the THz range with continuous tunability, high spectral purity and 100% modulation depth. Depending on the desired range, one- and two-axis cavities are built. Applications of such lasers as a local oscillator in radio-over-fiber communication systems or as a quasi-absolute tunable frequency source for highly-dense wavelength division multiplexed networks are emphasized.
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