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.
Quantum technologies have been identified as breakthrough technologies with a potential high impact on future navigation, sensing and communication systems since the end of the 90’s. In this paper we will review how these technologies can contribute to electromagnetic spectrum dominance through the use of SHB (spectral hole burning) based spectral holography and of NV (nitrogen vacancy) centers in diamond. Quantum technologies, combined with integration techniques, will also improve the performances of navigation systems thanks to ultra-precise compacts atomic clocks, accelerometers and gyros.
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.
In this paper, we present the design of a very precise collimated fiber array that meets requirements for beam combining. Calculations permit to determine the tolerances toward key parameters and specify the components to manufacture. Thus, the collimated fiber array is composed of a high quality commercial microlens array and an especially dedicated fiber holder that we design and realize experimentally. Manufacture techniques for both the microlens and the holder are chosen to be collective and then compatible with a high number of fibers. With the collimated fiber array hence obtained, the individual beam quality was measured to be λ/10 and the pointing accuracy is under 0.6 mrad.
Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of
compactness, reliability, efficiency and beam quality. In order to obtain much higher output power than it is possible
from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new
technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error
measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern
between the beams with phase φ(x,y) issued from the fiber array and a plane reference beam is recorded on a digital
camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This
hologram is read out simultaneously and a phase conjugate beam with phase -φ(x,y) is generated in order -1 of the
diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the
phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital
holography is demonstrated with polarization maintaining fibers operating at 1 μm. Digital holography is realized thanks
to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this
technique could be applied to phase lock a large number of fiber amplifiers.
The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction limited, linearly polarized beam by a self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO3). Up to 11.6W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency.
The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction limited, linearly polarized beam by a self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO3). Up to 11.6W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency.
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.
Experimental and theoretical investigations of self-pumped phase-conjugate (SPPC) resonators with four-wave mixing (FWM) in diode-pumped amplifiers are presented. A model that uses a transient treatment of FWM (in one-pass or two-pass geometries) allows us to analyze the temporal dynamics and the energy characteristics of these resonators in both injected and self-starting configurations. The influence of the input energy in the injected case, and the influence of the output coupler reflectivity and diode pump energy in the self-starting case are analyzed. In the self-starting case, the SPPC resonators demonstrate self-adaptive compensation of phase distortions and produce a TEM00 mode in a single-longitudinal-mode pulse by dynamic gain-grating formation with a reasonable optical-optical efficiency.
Self-pumped phase-conjugate (SPPC) loop resonators using four-wave mixing (FWM) in solid-state gain media are investigated in both injected and self-starting configurations. A model, using a transient treatment of FWM with the appropriate boundary conditions imposed by the loop geometry, allows us to analyze the threshold condition for oscillation, the temporal dynamics and the energy characteristics of such resonators. The influence of the input energy in the injected configuration, and the influence of the different gain gratings and output coupler reflectivity in the self-starting configuration are also analyzed. A SPPC loop resonator using FWM in a flash-lamp- pumped Nd:YAG amplifier is experimentally investigated. In the injected case, a maximum phase-conjugate reflectivity of 42, and a maximum extraction of 47 mJ are obtained. In the self-starting case, a TEM00 mode output of approximately 130 mJ in a 13 ns single-longitudinal-mode pulse is produced up to 30 Hz. Experimental and theoretical results are in good agreement.
We present a method to achieve phase conjugation based on saturable-gain degenerate four- wave mixing in the laser medium itself. Experimental and theoretical investigations show that phase conjugate reflectivities and efficiencies higher than 100% can be achieved with such an interaction. High conjugate-reflectivity at (lambda) equals 1.06 micrometers is demonstrated in conventional flash-pumped Nd:YAG amplifiers and in compact diode-pumped Nd:YVO4 amplifiers.
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