Zile Jiang

Free-space optical (FSO) communication in the mid-infrared (mid-IR) region has attracted increasing interest due to its lower atmospheric attenuation compared to near-infrared (near-IR) wavelengths. To further enhance link capacity, mode-division multiplexing (MDM) can be employed to transmit multiple orthogonal spatial channels simultaneously. Previous schemes for mid-IR MDM have converted Gaussian data beams from the near IR into the mid-IR before changing the beams into orthogonal modes. However, such a scheme employs a separate nonlinear wavelength converter for each mode (i.e., N converters for N channels), potentially causing high system complexity. In this paper, we demonstrate using a single wavelength converter from near-IR to achieve a mid-IR mode-division multiplexing (MDM) link. Two orbital angular momentum (OAM) channels with tunable mode orders are generated using spatial light modulators (SLMs) at 1550 nm. The channels are multiplexed and simultaneously converted to 3.4 μm via difference frequency generation (DFG) in a single PPLN crystal. We demonstrate mid-IR intensity modulation/direct detection (IM/DD) and coherent heterodyne communication. A total of 2-Gbit/s on-off keying (OOK) and 4-Gbit/s 16-quadrature amplitude modulation (QAM) MDM links are achieved, all with bit error ratios (BERs) below the 7% forward error correction (FEC) threshold.

Securing data transmission against eavesdropping is a critical requirement in modern communication systems. Beyond traditional encryption methods, physical-layer techniques that exploit the physics of electromagnetic wave propagation have emerged as promising solutions for enhancing security. However, existing physical-layer security approaches typically do not inherently prevent eavesdropping from listeners positioned along the line-of-sight path. Here, we propose, simulate, and experimentally demonstrate a line-of-sight physical-layer security scheme that employs a longitudinally structured artificial-noisecarrying beam co-propagating with the data-carrying signal beam. This noise beam spatially overlaps the signal beam throughout the propagation path but undergoes significant spatial broadening only near the legitimate receiver, resulting in a low-intensity noise region at the receiver's aperture. Consequently, the signal-to-noise ratio (SNR) exhibits a tailored distance dependence, enabling successful data recovery only at the intended receiver's location while significantly degrading reception elsewhere. We demonstrate the feasibility of this approach across diverse electromagnetic carrier frequencies through proof-of-concept wireless experiments: a 120-Gbit/s 16-QAM optical link at 1550 nm and a 20-Gbit/s QPSK sub-terahertz link at 300 GHz. The measured SNR values along the propagation path outside the designed receiver region remain at least 17 dB lower than those at the receiver's aperture.

In FSO communication links employing conventional Gaussian beams, power loss can become an issue due to the divergence nature of a Gaussian beam. This issue becomes more severe when the Rx aperture is relatively small and/or the link length varies. Additionally, atmospheric turbulence can further degrade the link performance by causing beam wandering and scintillation. In this paper, we use a “pin-like” beam to mitigate the adverse effects of limited aperture, variable link length, and turbulence in an FSO communication link. The beam rapidly contracts upon transmission and keeps a small size over a relatively long distance. Experimental results show the pin-like beam’s diameter converges from 7 mm to < 1 mm upon 0.6-m propagation, resulting in average power loss reduction of > 3.6 dB compared to a Gaussian beam at different distances (0.6 m and 0.9 m) and with different Rx aperture diameters (1 mm and 3 mm), in the presence of turbulence effects emulated by a phase plate of r₀ = 1 mm. This improvement translates to reduced BER and outage probability in a 2-Gbit/s OOK link. As an example, under a 3-mm-diameter Rx aperture, using the pin-like beam achieves error-free transmission in 40 000 symbols for 100 random turbulence realizations, while the Gaussian beam has a 3% outage probability (BER > 3.8e-3). We further simulate a 1.5-km FSO link to compare the pin-like beam with the regular Gaussian beam and focused Gaussian beams with different focal lengths. Results reveal that, while a focused Gaussian beam may have low turbulence-induced power loss at its focal point, the pin-like beam delivers consistent performance across the link.

The surge in deployment of Low Earth Orbit (LEO) satellites over the last two decades has resulted in a number of incompatible communication protocols, limiting efficient inter-satellite communication. In response, we present the first reconfigurable silicon photonic link supporting both coherent and intensity modulation with direct detection (IM-DD). Our link design supports transmission and reception of on-off keying (OOK), binary phase-shift keying (BPSK), and quadrature phase-shift keying (QPSK) modulated signals across two polarization states, enabling a single transceiver to accommodate multiple common modulation formats. We demonstrate successful operation of the custom transmitter, polarization multiplexer, and receiver portions of our link, including: 1) transmission of OOK, BPSK, and QPSK at symbol rates up to 10 GBaud with an error vector magnitude (EVM) $< $ 22% in all cases, 2) polarization cross-talk suppression of over 21 dB, and 3) reception of OOK, BPSK, and QPSK at rates up to 5 GBaud with resolved EVM $< $17%. This innovation can address the pressing need for standardized communication within LEO satellite constellations, improving interoperability and efficiency in satellite communications.

Optical ranging systems using temporal information of the light may encounter challenges in turbid underwater environments. Alternatively, one ranging approach leveraging the transverse spatial properties of a light beam has been demonstrated with relatively stable performance under scattering conditions. This scheme utilizes the range-dependent angular rotation of a spatially structured beam comprising 2 Bessel-Gaussian (BG) modes for underwater ranging. In this paper, we enhance the measurement range and resolution of the structured-beam-based ranging by combining the coarse and fine approaches. The longitudinal wavenumber difference of BG modes is tuned to generate structured beams with varying rotation rates. Each rotation rate corresponds to a different measurement range and resolution. We experimentally demonstrate a 1-m combined coarse-and-fine underwater ranging system. The coarse beam (1.0-period rotation) and fine beam (4.5-period rotation) are alternated in each measurement step. Our results show a ∼4X enhanced measurement range and accuracy compared to the fine-only or coarse-only approach, respectively. An average ranging error of 6.2 mm is achieved over 1-m turbid water with an attenuation coefficient of 2.0 m⁻¹. We also simulated a 10-m underwater ranging system with varying scattering strengths. Under an attenuation coefficient of 0.4 m⁻¹, compared with the coarse-only approach, the simulated ranging error is reduced from 11.5 cm to 1.6 cm using the combined coarse-and-fine approach.

Optical communication networks employ various modulation formats depending on the performance capabilities and desired functionalities of their different zones. Different modulation formats may be used in different parts of a network, such that an edge node may aggregate multiple channels of lower-order formats into a single channel of higher-order format. In this work, we experimentally demonstrate the optical data aggregation of two independent channels into a single higher-order format channel using nonlinear wave mixing. By transmitting each data channel with two coherent pilot tones, we apply complex weights to the input data channels and shift the output constellation by a bias vector. These functions are achieved in a periodically poled lithium niobate (PPLN) waveguide. We show that our system is format reconfigurable, allowing for both amplitude- and phase-encoded inputs and that the two inputs can be in different formats. Moreover, we demonstrate the tunability of our approach by performing aggregation at two separate baud rates, 3 and 5 Gbaud. Bit error rate (BER) measurements for our cases show a power penalty is < = 1 dB for the aggregated quadrature phase shift keying (QPSK) output. Overall, we have shown the following format aggregations: (i) two on-off keying (OOK) inputs to one QPSK, (ii) two binary phase shift keying (BPSK) inputs to one QPSK, (iii) one OOK and one BPSK inputs to one QPSK, (iv) two 4-level pulse amplitude modulations (PAM4) inputs to one 16-ary quadrature amplitude modulation (16QAM), (v) two QPSK inputs to one 16QAM, and (vi) one QPSK and one 16QAM inputs to one 64QAM.

Optical signal processing (OSP) has the potential to enable line-rate processing functionalities in optical communication networks through direct operation on optical data and avoidance of optical-to-electrical-to-optical (OEO) conversion. One such functionality is optical pattern recognition, in which the location of target patterns in the transmitted optical data stream is identified; common optical data formats include quadrature-phase-shift-keying (QPSK) data. Whereas prior approaches have performed optical pattern recognition to locate a single pattern in a QPSK optical data stream, it might be advantageous to optically recognize multiple patterns simultaneously. In this paper, we experimentally demonstrate optical recognition of multiple QPSK patterns using two approaches based on nonlinear wave mixing, one using time multiplexing and one using wavelength multiplexing. In the intra-symbol time multiplexing (IS-TM) approach: (i) the multiple target patterns are encoded on the phases of auxiliary patterns at different time instances, and (ii) the nonlinear wave mixing generates a single output representing the multiple pattern occurrences at different instances. Using IS-TM, we recognize (i) 2 patterns with 2 or 3 symbols for 2- and 3-Gbaud rates; and (ii) 3 patterns with 2 or 3 symbols for 1-Gbaud input QPSK signals. In the wavelength multiplexing approach: (i) the multiple target patterns are encoded on the phases of separate continuous wave (CW) pumps at different wavelengths, and (ii) the nonlinear wave mixing generates multiple outputs at different wavelengths corresponding to different pattern recognitions. Using wavelength multiplexing, we recognize (i) 1 pattern with 2 or 3 symbols at a 15-Gbaud rate and (ii) 2 patterns with 2 or 3 symbols at 15-Gbaud and 20-Gbaud input QPSK data. Both approaches have error-free correlation results across 1536 transmitted QPSK symbols.

We experimentally demonstrate a beam shaping technique for simultaneously mitigating crosstalk effects due to turbulence and misalignment in a 50 Gbit/s/channel quadrature phase shift keying (QPSK) modulated two-channel mode-division-multiplexing (MDM) free-space optical (FSO) communication link. Co-propagating probe beams on a different wavelength close to the data channels are used to measure the channel crosstalk caused by turbulence and misalignment. We modify the transmitted beams to be a superposition of different orbital angular momentum (OAM) modes using the inverse modal coupling of the channel, which reduces the crosstalk between the two data channels at the receiver. Our experimental results show that when both turbulence and misalignment are present: (a) crosstalk reduces by > 9 dB between OAM +1 and +2 to <-14 dB when considering both turbulence and misalignment; (b) the proposed approach induces an optical signal-to-noise ratio (OSNR) penalty of ∼2 dB for both data channels compared to the back-to-back (B2B) scenario near the 7% forward error correction (FEC) threshold.