Multispecies Heterodyne Phase Sensitive Dispersion Spectroscopy over 80 nm Using a MEMS-VCSEL

Vertical cavity surface emitting lasers fabricated with movable microelectromechanical mirrors can offer continuous single-mode wavelength tuning up to 100 nm with high efficiency and in a very small package. Wide tunability makes these diode lasers ideally suited for multispecies and high-density gas analysis and the first demonstrations of direct absorption spectroscopy and wavelength modulation spectroscopy have already been published. The performance of these systems could nevertheless be enhanced by the use of the new molecular dispersion spectroscopic methods, as heterodyne phase sensitive dispersion spectroscopy. This technique bases its operation on the detection of the profile of the refractive index of the sample under analysis, in contrast to traditional architectures based on the measurement of optical absorption, and this provides noticeable advantages. First, the method is normalization-free, and therefore, the characteristic issue of the nonmonotonic intensity profile during wavelength tuning of tunable vertical cavity surface emitting lasers is directly overcome. In addition, dispersion spectroscopy also provides an intrinsic linearity with concentration, high suitability for calibration-free operation, and an extended dynamic range that are very desirable features to have in an optical gas analyzer. In this Letter we present the first multispecies spectrometer based on a widely tunable vertical cavity surface emitting laser and heterodyne phase sensitive dispersion spectroscopy that is capable of operating in a tuning range of more of 80 nm for the simultaneous detection of several species.The authors would like to thank the Spanish Ministry of Economy and Competitiveness for supporting the projectunder the TEC-2014-52147-R grant and the Deutsche Forschungsgemeinschaft within the program Graduiertenkolleg TICMO (GRK 1037). The authors also acknowledge J. Cesar, S. Al-Daffaie, and A. Hajo for their contribution to the experiments

Deep Learning with Coherent VCSEL Neural Networks

Deep neural networks (DNNs) are reshaping the field of information processing. With their exponential growth challenging existing electronic hardware, optical neural networks (ONNs) are emerging to process DNN tasks in the optical domain with high clock rates, parallelism and low-loss data transmission. However, to explore the potential of ONNs, it is necessary to investigate the full-system performance incorporating the major DNN elements, including matrix algebra and nonlinear activation. Existing challenges to ONNs are high energy consumption due to low electro-optic (EO) conversion efficiency, low compute density due to large device footprint and channel crosstalk, and long latency due to the lack of inline nonlinearity. Here we experimentally demonstrate an ONN system that simultaneously overcomes all these challenges. We exploit neuron encoding with volume-manufactured micron-scale vertical-cavity surface-emitting laser (VCSEL) transmitter arrays that exhibit high EO conversion (<5 attojoule/symbol with $V_\pi$=4 mV), high operation bandwidth (up to 25 GS/s), and compact footprint (<0.01 mm$^2$ per device). Photoelectric multiplication allows low-energy matrix operations at the shot-noise quantum limit. Homodyne detection-based nonlinearity enables nonlinear activation with instantaneous response. The full-system energy efficiency and compute density reach 7 femtojoules per operation (fJ/OP) and 25 TeraOP/(mm$^2\cdot$ s), both representing a >100-fold improvement over state-of-the-art digital computers, with substantially several more orders of magnitude for future improvement. Beyond neural network inference, its feature of rapid weight updating is crucial for training deep learning models. Our technique opens an avenue to large-scale optoelectronic processors to accelerate machine learning tasks from data centers to decentralized edge devices.Comment: 10 pages, 5 figure

High-β\beta lasing in photonic-defect semiconductor-dielectric hybrid microresonators with embedded InGaAs quantum dots

We report an easy-to-fabricate microcavity design to produce optically pumped high-$\beta$ quantum dot microlasers. Our cavity concept is based on a buried photonic-defect for tight lateral mode confinement in a quasi-planar microcavity system, which includes an upper dielectric distributed Bragg reflector (DBR) as a promising alternative to conventional III-V semiconductor DBRs. Through the integration of a photonic-defect, we achieve low mode volumes as low as 0.28 $\mu$m$^3$, leading to enhanced light-matter interaction, without the additional need for complex lateral nanoprocessing of micropillars. We fabricate semiconductor-dielectric hybrid microcavities, consisting of Al$_{0.9}$Ga$_{0.1}$As/GaAs bottom DBR with 33.5 mirror pairs, dielectric SiO$_{2}$/SiN$_x$ top DBR with 5, 10, 15, and 19 mirror pairs, and photonic-defects with varying lateral size in the range of 1.5 $\mu$m to 2.5 $\mu$m incorporated into a one-$\lambda/n$ GaAs cavity with InGaAs quantum dots as active medium. The cavities show distinct emission features with a characteristic photonic defect size-dependent mode separation and \emph{Q}-factors up to 17000 for 19 upper mirror pairs in excellent agreement with numeric simulations. Comprehensive investigations further reveal lasing operation with a systematic increase (decrease) of the $\beta$-factor (threshold pump power) with the number of mirror pairs in the upper dielectric DBR. Notably, due to the quasi-planar device geometry, the microlasers show high temperature stability, evidenced by the absence of temperature-induced red-shift of emission energy and linewidth broadening typically observed for nano- and microlasers at high excitation powers