Silicon nitride (SiN) have become an essential material for integrated photonics. It is needed whenever very low optical losses, high optical power, low thermal sensitivity or broad wavelength range is required. SiN waveguides therefore present a great interest for applications as diverse as quantum photonics, data-communication, neuromorphic computing, LiDAR, sensing and microwave optic. We present our latest results on two distinct platforms: An 8” LPCVD SiN platform featuring ultra-low loss (5dB/m) and a 12” Physical Vapor Deposition (PVD) SiN platform allowing low loss in the C-band with a low-temperature deposition.
There is an increasing demand for superconducting nanowire single photon detectors (SNSPDs) which combine high efficiency, low dark counts and fast response time. They play a crucial role in developing strategic application areas such as photon-based quantum computing and quantum key distribution. We demonstrate the fabrication of high crystalline quality NbN-based waveguide-integrated SNSPDs using a completely CMOS-compatible process on 200 mm SOI wafers. We achieve a detection efficiency over 80 % at 100 Hz dark count rate, a short decay time below 4 ns and a maximum count rate of 200 MHz.
In order to produce a powerful, single and low divergence output beam for 3D sensing applications, integrated Optical Phased Arrays (OPA) must have a large number of closely spaced optical antennas. This high density leads to specific constraints in component design compared to devices for optical transceivers. Furthermore, OPA characterization requires significant adaptations compared to traditional photonic wafer level measurement systems. In this presentation, we will focus on some key components used in a large scale OPAs, describing specific challenges and solutions. We will show characterization results of single components as well as active beam-steering with OPA circuits using our modified wafer-scale prober setup.
Quantum information processing brings new protocols to the field of communications, by ensuring absolute security of information transfer thanks to the laws of quantum physics. Furthermore, in the field of computing, quantum processing offers the perspective of performing massively parallel calculations, orders of magnitude faster than with a classical computer. For these two applications, excellent detectors are required with ultimate performances. Superconducting nanowire single photon detectors (SNSPDs) are the best candidate, as they can reach near-unity detection efficiency. We are developing on-chip waveguide integrated SNSPDs on 200 mm SOI wafers, addressing both the material, architecture design and fabrication process challenges.
The last few years have shown the success of silicon nitride platforms for ultra-low loss tightly confining waveguides. In addition to the low optical losses, the high Kerr nonlinearity, the high power handling capability and small bending radii makes the platform ideal for nonlinear photonics. Therefore, the potential for applications is huge: LiDAR, microwave optics, quantum photonics, neuromorphic computing, telecommunication, sensors… Here, we present our 200mm platform based on 800nm-thick LPCVD Si3N4 with optical losses below 5dB/m. It is completed with a set of photonic components: grating couplers, edge couplers, MMI, directional couplers, Y-junction and AWG multiplexer, which constitute the building blocks for advanced applications.
We are developing a non-conventional retinal projector for augmented reality (AR) applications. In our concept, light at λ = 532 nm is guided in silicon-nitride (SiN) photonic integrated circuits (PICs) embedded in the lens of a pair of glasses. We use holographic elements to transmit the emissive points towards the user’s retina without using lenses. Pixels are formed in the eye using the self-focusing effect and the eye lens. The transparency of the device is an absolute requirement for our application. In this work, we present the fabrication and the characterization of our latest SiN PICs on transparent substrate. The device was fabricated by transferring the SiN PICs from a silicon to a glass substrate. We characterized the PICs and the free-space optical transmission properties of our device using in-house goniometers and a Modulation Transfer Function (MTF) setup. We found a 76% transparency at our wavelength and no image alteration. However, we measured significant waveguide propagation losses; solutions are discussed to tackle this problem. Our glass-substrate device is a major step towards a future prototype for our AR retinal projector.
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.
For several years, there has been a diversification of applications addressed by silicon photonics. Historically intended for telecom applications, silicon photonic platforms must now address the needs of transceivers for Data Center Interconnect, 5G backhaul / fronthaul but also those of emerging applications such as circuitry for LiDAR or for high performance computing (Artificial Intelligence, Quantum Computing). In order to meet this growing demand and the diversity of needs accompanying all these applications, CEA LETI has developed a new silicon photonics platform based on 300mm SOI wafers. This development is based in part on the experience acquired over more than 15 years on 200mm technology. Switching to 300mm equipment allows access to more advanced and above all, much more stable manufacturing tools, thus making it possible to envisage the production of complex circuits and large-scale integration of photonic components. For the most critical mask levels, the use of an immersion lithography stepper supported by OPC algorithms dedicated to photonics also opens up new perspectives in the possibilities of component design. In this presentation we will describe this new platform by going through its constituent modules and highlighting application versatility. Characterization results of various components fabricated on this platform will also be presented.
Roughness has always been a key detractor of the optical losses within the silicon photonics devices. With scaling at 300mm wafer, there is an introduction of new tools such immersion lithography scanner, OPC technique that can help to drive furthermore the optical losses reduction. This study will detail the work done on characterizing multiple steps of the process (Lithography, Etch, Annealing) and using roughness tools such LER (Line Edge Roughness), LWR (Line Width Roughness) and finally PSD (Power Spectral Density) to understand the main detractor of the optical losses at each step. These data will be extracted using SEM imaging from VeritySEM 6i.
Over the last decade, Optical Phased Arrays (OPA) have been extensively studied, targeting applications such as Light Detection And Ranging (LiDAR) systems, holographic displays, atmospheric monitoring and free space communications. Leveraging the maturity of the silicon photonics platform, the usual mechanical based beam steering system could be replaced by an integrated OPA; significantly reducing the cost and size of the LiDAR while improving its performance (scanning speed, power efficiency, resolution…) thanks to solid state beam steering. However, the realization of an OPA that meets the specifications of a LiDAR system (low divergence and single output beam) is not trivial. Targeting the realization of a complete LiDAR system, the technical challenges inherent to the development of high performance OPAs have been studied at CEA LETI. In particular, efficient genetic algorithms have been developed for the calibration of high channel count OPAs as well as an advanced measurement setup compatible with wafer-scale OPA characterization.
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