Tunable lasers and photonic integrated circuits are a promising technology to provide compact and high performance solutions for coherent remote sensing applications such as Lidar, and distributed acoustic fiber sensing (DAS). A hybrid tunable laser was fabricated within the EU funded INSPIRE project, based on the micro-transfer printing of a pre-fabricated InP gain section on the IMEC low-loss silicon nitride platform. By simultaneously modulating the laser SOA current and Vernier ring resonators, we demonstrate a 20 GHz chirp amplitude, while maintaining a <5 kHz linewidth. DAS measurement with this laser are presented.
As part of the European Defense Agency-funded PICTURE project, we have investigated the potential of integrated photonics for radar systems, targeting a photonics-based architecture for electronically scanned active array antenna systems, including multifunctional signal generation and detection. In this context, we have developed integrated photonic transmit and receive modules with up- and down-conversion capability, powered by a mode-locked laser for frequency reference. Key elements such as narrowband optical bandpass filters are evaluated for different technology platforms (silicon-on-insulator and silicon nitride). Perspectives and lessons learned are presented, based on the evaluation of system performance with IF-to-RF and RF-to-IF conversion efficiencies.
Mitigation of atmospheric turbulence is a major challenge in optical wireless communication, especially for optical feeder links. In this paper, we present a free space optical (FSO) mode diversity receiver, based on a spatial demultiplexer and a silicon photonic coherent combiner to reduce the atmospheric turbulence deleterious effects. We simulate the spatial light distribution in the ground receiver aperture for a use case consisting of a FSO link from a GEO satellite. We then generate experimentally wavefronts corresponding to the spatial light distribution for that use case with a wavefront emulator, and we compare the collection efficiency of the proposed mode diversity receiver with a FSO single mode fiber (SMF) receiver. The proposed FSO receiver outputs a signal much more stable as the system is resilient to energy redistribution among higher order spatial modes.
To concurrently cope with the scarcity of RF frequency bands, the growing capacity demand and the required lower cost of the ground segment, Very High Throughput Satellites systems must rely on new technical solutions. Optical feeder links are considered as a promising alternative to surpass classical RF technology, offering assets inherent to optical technologies (large bandwidth, no frequency regulation, low beam divergence, components availability). Nevertheless the potential of this technology shall not conceal the remaining challenges to be overcome to make it relevant for operational missions : clouds, turbulence, power generation and high efficiency modulations. VERTIGO (Very High Throughput Satellite Ground Optical Link) is a 3-year H2020 project funded by the European commission and started mid-2019 focusing on the optical link itself regardless of site diversity aspect and aiming at demonstrating in a ground demonstration required technologies to implement very high capacity optical feeder links. In particular, VERTIGO is built on 3 pillars each addressing a key issue for the implementation of optical feerder links: 1) Throughput increase through the use of advanced schemes with high spectral and power efficiency compared to current modulations used in space, as well as RF-over-Fiber approach. 2) High optical power generation to close the demanding link budgets by developing on-board and ground means to raise the transmitted optical power, not only based on amplifier power increase, but also on incoherent/coherent power combining. 3) Opto-mechanical and digital techniques for the mitigation of atmospheric propagation impairments, to make full use of throughput and power increases. Several demonstrations in-flight or on-ground already demonstrated separately key aspects (atmospheric propagation and impairments mitigation techniques, modulation format, high power…), for the implementation of optical (feeder) links. These aspects are closely linked since the solutions to each of them are necessary but not sufficient to allow for high throughput transmissions. VERTIGO concept is to address each key issue with at least one solution and to combine them in an unprecedented manner. To reach these objectives, VERTIGO will lean on a highly skilled consortium composed of : CREONIC, ETH Zürich, Fraunhofer HHI, Gooch and Housego, Leo Space Photonics RD, ONERA, Thales Research and Technology, Thales Alenia Space in France and Switzerland. This paper will present the VERTIGO project and its status.
We report on the first coherent beam combining of 60 fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beams recombination in the far field, this technique appears well suited for the combination of a large number of fiber amplifiers. The 60 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal microlens array. The measured residual errors within the fiber array yields standard deviations of 4.2 μm for the fiber pitch and 3.1 mrad for the beam-to-beam pointing, allowing a combining efficiency of 50 %. The phasing of 60 fiber amplifiers demonstrates both pulse synchronization and phase stabilization with a residual phase error as low as λ/100 RMS.
The XCAN project aims at the coherent combination of 61 fiber amplifiers in the femtosecond regime. An important intermediate step towards this goal is the implementation of a seven fiber test setup, which allows to address key scientific and technical challenges which might occur in the scaled version of 61 fibers. This work includes the design and characterization of a support unit able to hold 61 fibers with the high precision required for an efficient coherent combination in tiled aperture configuration. This configuration, in combination with an interferometric phase measurement and active phase control, is particularly well suited for the coherent combination of a very large number of beams. Our first preliminary results with seven fibers include a combination efficiency of 30 % and a residual phase error between two fibers as low as λ/40 rms. Experiments conducted with three fibers in order to evaluate technical improvements revealed an increase of efficiency to 54 %. The combined beam was temporally compressed to 225 fs, which is Fourier transform limited with respect to the measured spectrum.
The XCAN project, which is a three years project and began in 2015, carried out by Thales and the Ecole Polytechnique aims at developing a laser system based on the coherent combination of laser beams produced through a network of amplifying optical fibers. This technique provides an attractive mean of reaching simultaneously the high peak and high average powers required for various industrial, scientific and defense applications. The architecture has to be compatible with very large number of fibers (1000-10000). The goal of XCAN is to overcome all the key scientific and technological barriers to the design and development of an experimental laser demonstrator. The coherent addition of multiple individual phased beams is aimed to provide tens of Gigawatt peak power at 50 kHz repetition rate.
Coherent beam combining (CBC) of fiber amplifiers involves a master oscillator which is split into N fiber channels and then amplified through series of polarization maintaining fiber pre-amplifiers and amplifiers. In the so-called tiled aperture configuration, the N fibers are arranged in an array and collimated in the near field of the laser output. The N beamlets then interfere constructively in the far field, and give a bright central lobe. CBC techniques with active phase locking involve phase mismatch detection, calculation of the correction and phase compensation of each amplifier by means of phase modulators. Interferometric phase measurement has proven to be particularly well suited to phase-lock a very large number of fibers in continuous regime. A small fraction of the N beamlets is imaged onto a camera. The beamlets interfere separately with a reference beam. The phase mismatch of each beam is then calculated from the interferences’ position. In this presentation, we demonstrate the phase locking of 19 fibers in femtosecond pulse regime with this technique.
In our first experiment, a master oscillator generates pulses of 300 fs (chirped at 200 ps). The beam is split into 19 passive channels. Prior to phase locking, the optical path differences are adjusted down to 10 μm with optical delay lines. Interferograms of the 19 fibers are recorded at 1 kHz with a camera. A dedicated algorithm is developed to measure both the phase and the delay between the fibers on a measurement path. The delay and phase shift are thus calculated collectively from a single image and piezo-electric fiber stretchers are controlled in order to ensure compensation of time-varying phase and delay variations. The residual phase shift error is below λ/60 rms. The coherent beam combining is obtained after propagation and compression. The combined pulse width is measured at 315fs. A second experiment was done to coherently combine two amplified channels of the XCAN demonstrator. A residual phase shift error of λ/30 rms was measured in this case.
Coherent beam combining of fiber amplifiers provides an attractive mean of reaching high power laser. In an interferometric phase measurement the beams issued for each fiber combined are imaged onto a sensor and interfere with a reference plane wave. This registration of interference patterns on a camera allows the measurement of the exact phase error of each fiber beam in a single shot. Therefore, this method is a promising candidate toward very large number of combined fibers. Based on this technique, several architectures can be proposed to coherently combine a high number of fibers. The first one based on digital holography transfers directly the image of the camera to spatial light modulator (SLM). The generated hologram is used to compensate the phase errors induced by the amplifiers. This architecture has therefore a collective phase measurement and correction. Unlike previous digital holography technique, the probe beams measuring the phase errors between the fibers are co-propagating with the phase-locked signal beams. This architecture is compatible with the use of multi-stage isolated amplifying fibers. In that case, only 20 pixels per fiber on the SLM are needed to obtain a residual phase shift error below λ/10rms. The second proposed architecture calculates the correction applied to each fiber channel by tracking the relative position of the interference finges. In this case, a phase modulator is placed on each channel. In that configuration, only 8 pixels per fiber on the camera is required for a stable close loop operation with a residual phase error of λ/20rms, which demonstrates the scalability of this concept.
Slow light systems are particularly attractive for analog signal processing, since their inherent limitation to a delay-bandwidth
product of 1 is less critical for analog systems such as those used in microwave photonics. We present here
the implementation of two basic functions - phase shifting and true time delaying - fully optically controlled using
stimulated Brillouin scattering in optical fibers. The combination of these two functions makes possible the
implementation of true time delays without limitation on the microwave carrier frequency using the separate carrier
tuning technique. This is illustrated by the implementation of the delaying system for the realization of a microwave
tunable notch filter.
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.
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.
High resolution inertialess beam steering systems are required for numerous applications, including laser radar, multitarget
designation or active imaging. We present a 1.55μm operating continuous laser beam steering system based on the
cascading of an electro-optic PMN-PT ceramic optical phased array (OPA) and of two piezoactuated microlenses arrays
(MLA). The function of the single devices and the principle and the operation of the combination of both are explained.
Then we describe the experimental setup which was realized and outline the test results.
The MLA large angle scanner consists of two MLAs, one of which can be moved with respect to the other by piezodrivers.
This setup acts as a blazed grating and thus results in a discrete beam steering. Each steering position
corresponds to a multiple of 2pi phase shift between adjacent beamlets. In order to get a continuous steering, we
combined the MLA scanner with an electro-optic ceramic OPA. The OPA generates the piston distribution that
compensates the phase difference between adjacent beamlets of the MLAs and reconstructs a continuous wavefront.
The PMN-PT OPA consists in 64 phase modulators with 210µm period and a 0.5 fill factor. The maximum required
voltage corresponding to the 2π phase shift is 150 Volts at 1.55µm. The OPA is imaged on the 105μm period MLAs
with a 0.5-magnification telescope. The two MLAs are in a Keplerian telescope with field lenses arrangement. The
steering performances of the MLAs alone are +/-12° scan angle with 28 discrete positions. Using the combined
architecture, we were able to resolve 64 angular directions between each of these 28 positions. We thus experimentally
obtained a continuous steering at 1.55μm over +/-12° with an angular resolution of 0.24mrad, i.e. 1800 resolved
directions, with only 64+1 control voltages.
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 the use of an acousto-optic modulator to enhance the refresh rate and dynamic properties of a liquid-crystal spatial light modulator (SLM). The useful area of the SLM surface is split in several zones which are addressed separately, and read in a sequence by a steered laser beam. This configuration allows to increase the refresh rate by five orders of magnitude. Furthermore, improvements on the nature of the transition between different holograms are experimentally shown. The advantages of this technique are discussed in the particular context of cold atom manipulation with holographic optical tweezers.
We propose an active and adaptative optics device dedicated to programmable femtosecond beam shaping, based on the use of an optically addressed light valve. A theoretical investigation of the system is presented. The experimental set-up incorporating an active beam shaping device, is depicted. Results are then described and discussed.
We present a new beam shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles, as flattop super-Gaussian or square-shaped beams.
We investigate the influence of thermally induced spherical aberrations on the fundamental mode of a rod solid-state laser. Results concerning the additional losses and beam quality degradation are presented and point out that for a large volume fundamental mode resonator, a spherical aberration greater than 0.5(lambda) dramatically deteriorates the laser performances. To control the resonator performances, we present a new phase control technique with an intracavity optically addressed liquid-crystal spatial light modulator. The presented Nd:YAG resonator is able to deliver beams with various spatial profiles, as flattop super-Gaussian or square-shaped beams, and is thus potentially able to compensate for the thermally induced aberrations of the laser medium.
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