Spin injection enhancement in CW VECSEL at room temperature using push-pull optical pumping.

We report the enhancement of spin injection efficiency in an external-cavity VCSEL based on a non-resonant pumping coupled with a polarized optical resonant illumination. This double pumping scheme allows both the injection of spin polarized electrons in the conduction band and the selection of the spin orientation for the electron/hole recombination laser process. Experimentally, a flip of the polarization state of the laser is achieved with an ellipticity of +31° (spin down) and -33° (spin up), so an increase of about 50% of the ellipticity is achieved in comparison to an optical non-resonant pumping alone.

Performance and limitations of dual-comb based ranging systems.

Dual-comb LiDARs have the potential to perform high-resolution ranging at high speed. Here, through an implementation involving electro-optic modulators and heterodyne detection, we quantify the ranging systems trade-off between precision and non-ambiguity range (NAR) using a unique performance factor. We highlight the influence of the comb amplitude envelope on the precision with a distance measurement limited by the repetition rate of the optical comb. The influence of the combs repetition rate on the NAR and on the precision is illustrated through a setup allowing distance measurement with a tunable NAR. Finally, we demonstrate the impossibility to resolve different targets, quantify the impact on the measured distance and develop on the conditions in which non-linear effects of the interference make the measurement impossible.

All-optical coherent pulse compression for dynamic laser ranging using an acousto-optic dual comb.

We demonstrate a new and simple dynamic laser ranging platform based on analog all-optical coherent pulse compression of modulated optical waveforms. The technique employs a bidirectional acousto-optic frequency shifting loop, which provides a dual-comb photonic signal with an optical bandwidth in the microwave range. This architecture simply involves a CW laser, standard telecom components and low frequency electronics, both for the dual-comb generation and for the detection. As a laser ranging system, it offers a range resolution of a few millimeters, set by a dual-comb spectral bandwidth of 24 GHz, and a precision of 20 µm for an integration time of 20 ms. The system is also shown to provide dynamic measurements at scanning rates in the acoustic range, including phase-sensitive measurements and Doppler shift velocimetry. In addition, we show that the application of perfect correlation phase sequences to the transmitted waveforms allows the ambiguity range to be extended by a factor of 10 up to ∼20 m. The system generates quasi-continuous waveforms with low peak power, which makes it possible to envision long-range telemetry or reflectometry requiring highly amplified signals.

VSPIN: a new model relying on the vectorial description of the laser field for predicting the polarization dynamics of spin-injected V(e)CSELs.

A new vectorial model (VSPIN) based on the Jones formalism is proposed to describe the polarization dynamics of spin injected V(e)CSELs. This general modelling framework accounts for spin injection effects as a gain circular dichroism in the active medium and provides guidelines for developing functional spin-controlled lasers. We investigate the detrimental role of phase anisotropy on polarization switching and show that it can be overcome by preparing the laser cavity to achieve efficient polarization switching under low effective spin injection. The VSPIN model predictions have been confirmed experimentally and explain the polarization behavior of spin-VCSELs reported in the literature.

Compensation of the residual linear anisotropy of phase in a vertical-external-cavity-surface-emitting laser for spin injection.

We report on the compensation of the linear anisotropy of phase in a vertical-external-cavity surface-emitting laser from 21 to 0.5 mrad with an intracavity PLZT electro-optical ceramic. It allows dynamic and accurate control of the laser linear anisotropy, as well as dynamic control of the laser polarization eigenstates. At the birefringence compensation point, we observe an elliptical polarization state with 41° of ellipticity, rotated from its initial position of 32°. The experimental observations are in close agreement with the theoretical predictions. Finally, we are able to demonstrate control of the polarization state with spin injection.

Reduction of residual excess noise in class-A lasers using two-photon absorption.

We demonstrate experimentally a significant reduction of the remaining excess intensity noise in a class-A semi-conductor laser. This is obtained by inserting into the laser cavity a buffer reservoir mechanism based on two-photon absorption in GaAs. The excess noise peaks at the laser-free spectral range, induced by the beating between the lasing mode and the amplified spontaneous emission in the adjacent non-oscillating modes, is reduced by 20 dB, while preserving the class-A dynamical behavior of the laser cavity.

Measuring the nonlinear refractive index of graphene using the optical Kerr effect method.

By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2=-1.1×10-13 m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping.

Thirty Gigahertz Optoelectronic Mixing in Chemical Vapor Deposited Graphene.

The remarkable properties of graphene, such as broadband optical absorption, high carrier mobility, and short photogenerated carrier lifetime, are particularly attractive for high-frequency optoelectronic devices operating at 1.55 µm telecom wavelength. Moreover, the possibility to transfer graphene on a silicon substrate using a complementary metal-oxide-semiconductor-compatible process opens the ability to integrate electronics and optics on a single cost-effective chip. Here, we report an optoelectronic mixer based on chemical vapor-deposited graphene transferred on an oxidized silicon substrate. Our device consists in a coplanar waveguide that integrates a graphene channel, passivated with an atomic layer-deposited Al2O3 film. With this new structure, 30 GHz optoelectronic mixing in commercially available graphene is demonstrated for the first time. In particular, using a 30 GHz intensity-modulated optical signal and a 29.9 GHz electrical signal, we show frequency downconversion to 100 MHz. These results open promising perspectives in the domain of optoelectronics for radar and radio-communication systems.

Adaptive holographic interferometer at 1.55 µm based on optically addressed spatial light modulator.

We report the realization of an adaptive holographic interferometer based on two-beam coupling in an optically addressed liquid crystal spatial light modulator operating at 1.55-µm. The system allows efficient phase demodulation in noisy environment and behaves as an optical high-pass filter, with a cut-off frequency of approximately 10 Hz, thus filtering slow phase disturbances (due to, for example, temperature variations or low frequency fluctuations) and keeping the detection linear without the need of heterodyne or active stabilization.

Accurate measurement of the residual birefringence in VECSEL: Towards understanding of the polarization behavior under spin-polarized pumping.

In this paper we report birefringence measurements of an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well (MQWs) Vertical External Cavity Surface Emitting Laser (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. The experience fits within the broader framework of polarization selection in spin-injected lasers.

Self-adaptive vibrometry with CMOS-LCOS digital holography.

A self-adaptive interferometer based on digital holography is here reported for applications involving measurements of very small amplitude vibrations. The two-beam coupling gain is optimized through an electronic feedback, while the dynamic character of the hologram allows reaching a high sensitivity of the interferometric measurements even in unstable environments and with strongly distorted wave-fronts. The frequency bandwidth of the adaptive interferometer and its spatial resolution are determined, respectively, by the maximum frame rate and the pixel size of the camera and of the spatial light modulator used to build the digital holographic setup.

Integrable microwave filter based on a photonic crystal delay line.

The availability of a tunable delay line with a chip-size footprint is a crucial step towards the full implementation of integrated microwave photonic signal processors. Achieving a large and tunable group delay on a millimetre-sized chip is not trivial. Slow light concepts are an appropriate solution, if propagation losses are kept acceptable. Here we use a low-loss 1.5 mm-long photonic crystal waveguide to demonstrate both notch and band-pass microwave filters that can be tuned over the 0-50-GHz spectral band. The waveguide is capable of generating a controllable delay with limited signal attenuation (total insertion loss below 10 dB when the delay is below 70 ps) and degradation. Owing to the very small footprint of the delay line, a fully integrated device is feasible, also featuring more complex and elaborate filter functions.

Fully tunable active polarization imager for contrast enhancement and partial polarimetry.

We present the design and the practical implementation of a polarimetric imaging system based on liquid-crystal modulators that allows generation and analysis of any polarization state on the Poincaré sphere. This system is more versatile than standard Mueller imagers that are based on optimized, but limited, sets of illumination and analysis states. Examples of benefits brought by these extra degrees of freedom are illustrated on two different applications: contrast enhancement and extraction of partial polarimetric properties of a scene.

General state contrast imaging: an optimized polarimetric imaging modality insensitive to spatial intensity fluctuations.

In active polarization imaging, one frequently needs to be insensitive to noninformative spatial intensity fluctuations. We investigate a way of solving this issue with general state contrast (GSC) imaging. It consists in acquiring two scalar polarimetric images with optimized illumination and analysis polarization states, then forming a ratio. We propose a method for maximizing the discrimination ability between a target and a background in GSC images by determining the optimal illumination and analysis states. A further advantage of this approach is to provide an objective way of quantifying the performance improvement obtained by increasing the number of degrees of freedom of a GSC imager. The efficiency of this approach is demonstrated on simulated and real-world images.

On the influence of noise statistics on polarimetric contrast optimization.

In active scalar polarimetric imaging systems, the illumination and analysis polarization states are degrees of freedom that can be used to maximize the performance. These optimal states depend on the statistics of the noise that perturbs image acquisition. We investigate the problem of optimization of discrimination ability (contrast) of such imagers in the presence of three different types of noise statistics frequently encountered in optical images (Gaussian, Poisson, and Gamma). To compare these different situations within a common theoretical framework, we use the Bhattacharyya distance and the Fisher ratio as measures of contrast. We show that the optimal states depend on a trade-off between the target/background intensity difference and the average intensity in the acquired image, and that this trade-off depends on the noise statistics. On a few examples, we show that the gain in contrast obtained by implementing the states adapted to the noise statistics actually present in the image can be significant.

Joint contrast optimization and object segmentation in active polarimetric images.

We present a method for automatic target detection based on the iterative interplay between an active polarimetric imager with adaptive capabilities and a snake-based image segmentation algorithm. It successfully addresses the difficult situations where the target and the background differ only by their polarimetric properties. This method illustrates the benefits of integrating digital processing algorithms at the heart of the image acquisition process rather than using them only for postprocessing.

Polarimetric target detection in the presence of spatially fluctuating Mueller matrices.

In polarimetric imaging systems, the main source of perturbations may not be detection noise but fluctuations of the Mueller matrices in the scene. In this case, we propose a method for determining the illumination and analysis polarization states that allow reaching the highest target detection performance. We show with simulations and real-world images that, in practical applications, the statistics of Mueller matrix fluctuations have to be taken into account to optimize polarimetric imagery.

Time delay generation at high frequency using SOA based slow and fast light.

We show how Up-converted Coherent Population Oscillations (UpCPO) enable to get rid of the intrinsic limitation of the carrier lifetime, leading to the generation of time delays at any high frequencies in a single SOA device. The linear dependence of the RF phase shift with respect to the RF frequency is theoretically predicted and experimentally evidenced at 16 and 35 GHz.

Optimal discrimination of multiple regions with an active polarimetric imager.

Until now, most studies about polarimetric contrast optimization have focused on the discrimination of two regions (a target and a background). In this paper, we propose a methodology to determine the set of polarimetric measurements that optimize discrimination of an arbitrary number of regions with different polarimetric properties. We show on real world examples that in some situations, a few number of optimized polarimetric measurements can overcome the performance of full Mueller matrix imaging.

Broadband true time delay for microwave signal processing, using slow light based on stimulated Brillouin scattering in optical fibers.

We experimentally demonstrate a novel technique to process broadband microwave signals, using all-optically tunable true time delay in optical fibers. The configuration to achieve true time delay basically consists of two main stages: photonic RF phase shifter and slow light, based on stimulated Brillouin scattering in fibers. Dispersion properties of fibers are controlled, separately at optical carrier frequency and in the vicinity of microwave signal bandwidth. This way time delay induced within the signal bandwidth can be manipulated to correctly act as true time delay with a proper phase compensation introduced to the optical carrier. We completely analyzed the generated true time delay as a promising solution to feed phased array antenna for radar systems and to develop dynamically reconfigurable microwave photonic filters.