We present a photonic tensor accelerator (PTA) employing nearly all dimensions (complex amplitude, polarization, wavelength and spatial mode) of light for scalable tensor multiplication. In this architecture, multiplications are performed through coherent mixing and accumulations are performed via simultaneous photodetection of multiple degrees of freedom within one dimension of light. We have implemented the PTA on micro-optics and integrated-photonics platforms. Experimental results of the PTA as well as its applications in artificial neural networks and computing will be presented.
Currently, most optical communication links shorter than ∼40 km employ intensity-modulation direct-detection (IMDD), and most longer links employ coherent. Demand for reduction in cost-per-transmitted-bit is relentlessly continuing, forcing IMDD and coherent to higher and higher rates. Coherent has better impairment equalization capability, better sensitivity, and a larger number of bits per transmitted symbol, ideal characteristics for next-generation links. However, coherent requires more complex lasers, more signal processing, more power consumption, and gear-boxing. IMDD, on the other hand, can be directly driven from electrical I/O and from electro-absorption-modulated lasers, making it significantly lower cost and power. We present taking one of the features of coherent, the use of dual polarization, and applying it in a blind way to IMDD. This allows a true and near-term way to continue on the cost-per-transmitted bit reduction path for optical data-center links.
Silicon photonics and coherent communications are a near perfect match. Coherent requires complex transceivers with polarization diversity, multi-element modulators, hybrids, ease of packaging with electronics, etc.; and silicon photonics favors applications that use external lasers because of its indirect bandgap. The commercial success of this marriage has helped generate silicon photonics offspring in other areas, including short-reach non-coherent communications, optical coherent tomography, LiDAR, and deep learning.
In this work, two different integrated-optics based spectrometer designs are presented. The first one is called interleaved arrayed waveguide grating (AWG) spectrometer provides large bandwidth (i.e. 30 nm) and high resolution (i.e. 0.1 nm) for a compact size (i.e. 2.4 cm × 3 cm). The second spectrometer is called ultra-high resolution Fourier transform (FT) spectrometer provides 1 pm of resolution for only 2 cm × 0.5 cm (1 cm2) device size at 1.3 μm. For the interleaved AWG spectrometer, the primary AWG has narrow closely spaced passbands (that equal the final desired channel spacing) that repeat N times in the desired wavelength range, using the frequency-cyclic nature of the AWG. The channel spacing of the secondary AWGs should be equal to the free spectral range (FSR) of the primary AWG. In this configuration, the FSR of the secondary AWGs defines the FSR of the overall configuration whereas the channel spacing (resolution) of the primary AWG defines the overall system resolution. The ultra-high resolution FT spectrometer is formed by sequentially-activated 60 Mach-Zehnder interferometers that are connected to photodetectors through very-low-loss beam combiners based on two-mode interference. The long optical delays are provided by tapping the propagating light out at certain locations on the optical waveguides by using electro-optically-controlled directional couplers. A design example with a spectral resolution of 500 MHz (~1 pm) and bandwidth of 15 GHz is presented for a device size of only 2 cm × 0.5 cm (1 cm2).
Miniaturization and cost reduction of OCT systems are important for enabling many new clinical applications as well as accelerating the development of existing applications. Silicon photonics is an important low-cost, high-volume, multi-functional platform for integrated optics because it can benefit from existing semiconductor fabrication techniques to integrate many advanced optical functions onto a single microchip. We present a miniaturized silicon photonic integrated swept source OCT receiver, measuring 3×4mm2, with advanced functionalities including dual polarization, dual balanced, in-phase and quadrature detection, essentially enabling the detection of the full vector field (amplitude, phase, and polarization) of the optical signal. With this integrated receiver, we demonstrate full-range OCT for complex conjugate artifact suppression, polarization diversity detection for removing polarization fading artifact, and polarization sensitive OCT for tissue birefringence imaging. The silicon photonic integrated receiver is a key advance towards developing a miniaturized, multi-functional swept source OCT system.
We summarize our recent work on integrated silicon photonics for applications in on-chip optical interconnects and
telecommunications, including high performance germanium detectors and a multi-channel receiver, a compact and low
power on-chip photonic link, an integrated diplexer and a coherent receiver.
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