Rachel Sampson

Within telecommunications, it is well established that optical preamplified receivers offer better sensitivity than linear-mode (LM) avalanche photodiodes (APDs) for direct-detection measurements. Long-range light detection and ranging (LiDAR), however, has predominantly made use of LM APDs for detection. This discrepancy arose due to the high coupling loss between the multimode return signals in LiDAR and single-mode optical preamplifiers. Recently, few-mode (FM) optical preamplifiers have been experimentally demonstrated, which solve this modal mismatch. We evaluate the applicability of FM preamplified receivers to long-range pulsed LiDAR in comparison with currently deployed LM APDs by considering the two receivers’ noise, gain, and bandwidth characteristics. We show that FM preamplified receivers can offer an improvement in both receiver signal-to-noise ratio and bandwidth.

Non-mode-selective (NMS) multiplexers (muxes) are highly desirable for coherent power combining to produce a high-power beam with a shaped profile (wavefront synthesis) from discrete, phase-locked emitters. We propose a design for a multi-plane light conversion (MPLC)-based NMS mux, which requires only a few phase masks for coherently combining hundreds of discrete input beams into an output beam consisting of hundreds of Hermite–Gaussian (HG) modes. The combination of HG modes as a base can further construct a beam with arbitrary wavefront. The low number of phase masks is attributed to the identical zero-crossing structure of the Hadamard-coded input arrays and of the output HG modes, enabling the practicality of such devices. An NMS mux supporting 256 HG modes is designed using only seven phase masks, and achieves an insertion loss of − 1.6    dB , mode-dependent loss of 4.7 dB, and average total mode crosstalk of − 4.4    dB . Additionally, this design, featuring equal power for all input beams, enables phase-only control in coherent power combining, resulting in significant simplifications and fast convergence compared with phase-and-amplitude control.

We propose and experimentally demonstrate high-speed operation of random-channel cryptography (RCC) in multimode fibers. RCC is a key generation and distribution method based on the random channel state of a multimode fiber and multi-dimension to single-dimension projection. The reciprocal intensity transmittance of the channel shared between the two legitimate users is used to generate and distribute correlated keys. In previous work, RCC's key rate-distance product was limited by the speed of light. In this work, we show that adding a fast modulator at one end of the channel decouples the key rate and distance, resulting in a significant improvement in the key rate-distance product, limited only by the fiber's modal dispersion. Error-free transmission at a key rate-distance product of 64.7 Mbps $ \times $ 12 km, which is seven orders of magnitude higher than the previous demonstration, was achieved. The proposed method's security arises from a fundamental asymmetry between the eavesdroppers’ and legitimate users’ measurement complexity.

We propose a Hermite–Gaussian (HG) mode-demulti-plexing hybrid (MDH) for coherent detection of mode-division multiplexed signals. The MDH, which performs multiple functionalities, including demultiplexing, local oscillator splitting, and optical 90-deg mixing, is realized based on the multi-plane light conversion technique. An isosceles right triangle output layout is employed to reduce the number of phase masks to fewer than the number of modes, significantly simplifying the construction of the MDH. A 10-Hermite–Gaussian (HG) mode MDH with only five phase masks is demonstrated by numerical simulation, achieving an insertion loss (IL) and mode dependent loss as low as − 2.3 and 1.7 dB, respectively. The IL was further reduced to − 1.5 d B through optimization of MDH parameters, such as the beam waists of the input HG modes and the output spots.

We demonstrate the first few-mode DPSK receiver for turbulence-resistant free-space optical communication. Compared to a single-mode DPSK receiver, an 8.2 dB improvement in the power budget was achieved.

An order of magnitude improvement in the signal-to-noise ratio (SNR) of a 1550 nm LiDAR receiver was achieved using a few-mode optical preamplifier as compared to a multi-mode avalanche photodiode (APD). €