This paper presents experimental studies on heterodyne Frequency Modulated Continuous Wave (FMCW) signal reception for different optical heterodyne configurations including internal and external mixing between an incoming signal and a local oscillator. Signals and potential noise sources from a fibered FMCW Mach-Zehnder Interferometer (FMCW MZI) are theoretically evaluated. These optical estimations (signal and noise) of various power spectral densities (PSD) are converted into electrical unities to be compared to the measurements.The PSD are validated by using a known alternating voltage with controlled frequency and amplitude. This validation is used to compare the experimental and theoretical detection limits of different FMCW photodetectors, including a Photonic Integrated Circuit (PIC) detector developed and produced at CEA. The detection limit achieved with this PIC module closely matches with the expected theoretical performances. It validates the optical and electronic architecture and the achievements of CEA’s design. The miniaturization of this operational detection module is underway. In the future, it will be located on a single chip alongside two Optical Phased Arrays (OPA), one for emission and the other for reception.
CEA aims at developing a compact 1550 nm Frequency Modulated Continuous Wave (FMCW) LiDAR on chip. In this paper, individual demonstrators, corresponding to three main components of a LiDAR (Light Distance And Ranging) system, are combined in a test bench: a FMCW laser source, an emission and reception Optical Phased Array (OPA) and an optical heterodyne detection module. Each component has been individually tested, but also evaluated in order to derive the system performance of a complete LiDAR. A test platform has been developed to calibrate an OPA fabricated at CEA platform, either in emission or in reception mode. The tested OPA includes 256 channels based on grating antennae, with 1.5 μm pitch and 256 thermo-optic phase shifters. More recently, this platform has been completed with a FMCW interferometer, where the FMCW LiDAR detection can be evaluated through a mixed propagation setup, composed of optical fibers and free space. Then, the OPA may be inserted into this setup. Therefore, the optical fiber FMCW interferometer has been optimized to detect the lowest signal (typically less than one hundred fW) and to estimate the signal-to-noise ratio (up to almost 30 dB) with low noise photodiodes. Performance has been compared to theoretical predictions. Then, our custom OPA is included inside this experimental setup in a free space propagation environment. The performance measurements extracted from the spectral analysis are in agreement with the expectations.
An Optical Phased Array (OPA) is similar to a one dimensional (1D) dynamic diffraction array. The phase law of the emitters is numerically programmable and enables to form a beam, that point towards a targeted direction. OPAs have a high potential for a new generation of LiDAR (Light Detection and Ranging) systems, since they avoid mechanical beam scanning. For the development of such LiDAR, many characterizations are essential to optimize the OPA and to get a full control of their performance. To carry out these tests, CEA-Leti has developed a modular optical bench designed to characterize large scale 1D-OPAs in free space. This bench allows beam-forming calibration at various angles thanks to an optical setup based on far-field imaging in the Fourier plane. This set up directly analyses a field of view of 22° (-11°/+11°) and can rotate in the azimuthal plane of the OPA to cover angles ranging from -50° to +50°. The OPA board is mounted on an additional rotation stage to match the OPA beam output with the beam forming set-up optical axis. For practical use, the optical axis is parallel to the floor (i.e. to the optical table). Moreover, after calibration, additional options allow to switch the setup for practical operations, as the OPA use in real space, e.g. for aiming at a target. A Peltier and a regulation loop allow testing the OPA at various temperatures. Fast photodiodes have been implemented to measure the switching time between distinct angular positions. In this paper, we report data acquired with this setup on a 256 channels OPA operating at @1550 nm, that is based on grating antennas with 1.5 μm pitch and thermo-optic phase shifters.
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