We describe and compare in this paper two common methods to measure the half-wave voltage (Vpi) of an electro-optical modulator versus RF frequency of the driving electronic signal. We detail the required calibration procedures when necessary and demonstrate the methods experimentally from 100MHz up to 40GHz. Guidelines are finally given to select the most appropriate method depending on the application and modulator characteristics.

Optical satellite communications offer a viable path to meet the growing needs in transmission capacity of space communication networks, as well as the saturation of conventional RF bands. Yet long propagation distances and atmospheric turbulence limit coupled power, necessitating high-power amplification. To date, high-speed transmission at such power levels has not been demonstrated in a single system. Here, we report 1 Tbit/s data rates at 100 W optical power with a signal-to-noise ratio (SNR) advantage of up to 3.9 dB over a conventional single-amplifier system. The SNR advantage of 3.0 dB is obtained by mitigating amplifier-induced nonlinearities by an artificial neural network (ANN) equalizer, thereby outperforming conventional approaches by up to 2.3 dB. An additional SNR gain has been enabled by coherent beam combining of two 50 W amplifiers. The impact of beam combining is investigated, and an advantage of up to 0.9 dB SNR is shown. A system-level investigation into the SNR penalty due to high power amplification, the influence of the polarization multiplexing, and the best-case performance of the system is given. These findings provide critical insights for future high-capacity optical satellite links.

In this paper, we present a method based on a modal decomposition to quantify the efficiency of photonic lanterns (PLs) based free space optical (FSO) communication receivers. We fabricate a seven-port PL, and we evaluate numerically the free space to fiber coupling efficiency based on a reconstruction of the fields at the PL FSO multimode port. We validate the numerical approach with an experimental characterization of the PL. Then we compare the PL to a commercial multiplane light converter spatial demultiplexer. The PL shows better coupling efficiency for low-order spatial modes with orders of magnitude of demultiplexer size reduction. Finally, we evaluate the PL receiver with a simulation of received optical wavefront in a FSO communication.

An optical coherent transmission link with 100Watt output power is tested for satellite communications. Modulation formats are tested for transmission of the highest data-rates despite of nonlinear amplifier impairments across a linear, low-SNR free-space link.

We present advancements in free-space optical communication for satellite-ground networks. Using advanced atmospheric-turbulence-mitigation techniques, we tested new modulation formats at low-received optical powers at data rates of up to 1Tbit/s and transmitted output powers of up to 100 Watts across linear, free-space links of up to 53km.

Highly efficient coherent beam combining (CBC) of two very-high-power optical amplifiers (VHPOA) with applications to long-range FSO communications such as ground-to-space feeder links is presented. The CBC setup is designed to minimize the telecom signal degradation, with a polarization beam splitter used to minimize the power fluctuations and to control the output polarization state of the beam. The system delivers 80 W output power and is proven to be compatible with 25 Gb/s telecom signals with a less than 1 dB power penalty.

Free space optical (FSO) links are considered as an essential part of future satellite communications as they benefit from wide optical bands, absence of frequency regulation, and synergy with terrestrial networks [1], [2]. Nevertheless, several challenges remain for enabling very high capacity optical links. For the optical feeder uplink, a major one is the generation of optical carriers strong enough to carry the communication signal from a ground station up to geostationary satellites. The output power of telecom optical amplifiers has been increased over the past decade, notably with the recent demonstration of very high power optical amplifier (VHPOA) capable of delivering up to 50 W [3]. However, the coherent beam combination (CBC) of several VHPOAs is a promising solution to meet the very demanding requirements of future optical ground-based terminals.

High power generation is an enabling technology for high capacity free space optical feeder links. We show the feasibility of optical transmission of WDM OOK / DPSK at high power based on 50 W amplifiers and their coherent combination to deliver up to 97 W power.

We demonstrate a narrow-linewidth tunable laser through micro-transfer printing a prefabricated III-V gain section on imec’s 200-mm Si/SiN platform. Lasing in distinct bands in the C+L band is demonstrated, with linewidth down to 2.87-kHz.

The experimental assessment of a novel all-optical FSO communication receive device based on a spatial mode demultiplexer and a photonic integrated coherent combiner, is reported. The device collects light from the spatial modes coupled into the input multimode fiber and coherently recombines these modes into an output single-mode fiber via a binary-tree integrated photonic circuit. It was evaluated in FSO transmission laboratory experiments featuring an atmospheric propagation channel emulator. The BER performance of a pre-amplified, OOK direct-detection receiver were measured at 10 Gbps, under various disturbance conditions and strengths. Efficient coupling into the receiver input was shown to be maintained resulting in almost constant BER. Low BER floors and limited detection sensitivity penalties were observed. The feasibility of the proposed FSO receiver is proven, together with its ability to provide higher collection efficiency and greater robustness to phase and intensity disturbances than standard SMF receivers.

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.

Multimode receivers based on spatial or modal diversity are promising architectures to mitigate in real time the atmospheric turbulence effects for free space optical (FSO) communications. In this paper, we evaluate and comment on the dynamical communication performances of a FSO mode diversity receiver, based on a spatial demultiplexer and a silicon photonic coherent combiner, for an optical link from a GEO satellite to an optical ground station (OGS). We simulate time series of distorted wavefronts received by the OGS and we show numerically that the coherent combination of spatial modes mitigate the signal fading compared to a conventional single mode fiber (SMF) receiver. We verify this property in a laboratory environment by generating the wavefronts corresponding to the use case with an atmospheric propagation channel emulator. Then we modulate the optical carrier prior to the wavefront emulator with 10G OOK and DPSK data sequences to measure the BER performance of the proposed receiver during the time series emulation. Finally, we study and comment on the influence of the number of modes combined and the wavelength multiplexing on the BER performances. We prove that the mode diversity receiver provide a higher collection efficiency, has better BER performances and much less synchronization losses.

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.

Tiled-aperture Coherent Beam Combination architecture opens the way to digital laser operating in high peak and average power regimes.

Atmospheric turbulence can generate scintillation or beam wandering phenomena that impairs free space optical (FSO) communication. In this paper, we propose and demonstrate a proof-of-concept FSO communication receiver based on a spatial demultiplexer and a photonic integrated circuit coherent combiner. The receiver collects the light from several Hermite Gauss spatial modes and coherently combine on chip the energy from the different modes into a single output. The FSO receiver is characterized with a wavefront emulator bench that generates arbitrary phase and intensity patterns. The multimode receiver presents a strong resilience to wavefront distortions, compared to a monomode FSO receiver. The system is then used to detect an analog modulation of an optical beam through a random wavefront profile to mimic the transmission of a signal on a degraded optical link.

We report on the use of a 61 beamlets coherent beam combination femtosecond fiber amplifiers as a digital laser source to generate high-power orbital angular momentum beams. Such an approach opens the path for higher-order non-symmetrical user-defined far field distributions.

We report on the coherent beam combining of 61 femtosecond fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beam recombination in the far field, this technique appears suitable for the combination of a large number of fiber amplifiers. The 61 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal micro-lens array. The residual phase error between two fibers is as low as λ/90 RMS, while a combining efficiency of ∼50% is achieved.

For the last few years, coherent beam combining (CBC) has been drastically increasing the performances of ytterbium-doped femtosecond fiber amplifiers, up to more than 10 mJ output energy and to the multi-kilowatt level [1,2]. CBC consists in coherently adding the output beams of several independent amplifiers seeded by a common source. This method involves both an efficient combination process along with a phase detection and control technique applied on all the beams to combine. However, for femtosecond pulses to reach the Joule level and address applications such as particles acceleration, several thousands of fibers need to be combined. Thus, highly scalable CBC architectures along with adapted phase measurements techniques need to be investigated.