Terahertz (THz, 0.3–10 THz) radar systems have garnered significant attention due to their superior capabilities in high-precision and robust sensing. However, the susceptibility to jamming, along with the sensing precision loss and ranging ambiguity induced by inflexible implementation of the conventional radar signal source, presents major challenges to the practical deployment of THz radars. Herein, a flexible photonic chaotic radar system is proposed at the THz band and investigate the ranging performance in precision and ambiguity. The photonic heterodyne detection scheme facilitates the generation of optoelectronic feedback loop-based THz chaos at 300 GHz, achieving a seamless connection between THz domains and optical domains. The system is experimentally demonstrated its superior performance of sub-centimeter resolution with 0.9345 cm and ranging unambiguity simultaneously. This work bridges the THz gap in the practical deployment of chaos theory and will pave the way for a new regime of THz radar empowered by chaos.

The radio-over-fiber (RoF) system is promising to support broadband transmission and increased flexibility. To boost channel capacity in multi-carrier RoF systems with variable-rate forward error correction, probabilistic shaping and water-filling-based entropy loading outperforms bit-power loading in terms of achievable information rate. However, its reliance on specific channel conditions limits practical use in channel-dynamic RoF systems, highlighting the need for adaptive entropy loading that requires minimal channel state information. This paper presents a deep neural network-based transfer learning model for adaptive entropy prediction in discrete multi-tone signals, addressing frequency-selective responses in RoF systems. Numerical and experimental results confirm capacity-approaching generalized mutual information (GMI) and smoother normalized GMI (NGMI) performances, consistently achieving the 0.83 NGMI threshold across subcarriers. Unlike traditional methods requiring pre-measured signal-to-noise ratios (SNR), this approach simplifies implementation by using only demodulated data and the received SNR, providing a more channel-independent entropy loading option in dynamic RoF systems. 

This research presents a novel approach to 28 (Formula presented.) impulse radio ultra-wideband (IR-UWB) transmission using pulse position modulation (PPM) over an analog radio-over-fiber (ARoF) link, investigating the impact of fiber-based fronthaul on the overall performance of the communication system. In this setup, an arbitrary waveform generator (AWG) is employed for PPM signal generation, while demodulation is performed with a commercial time-to-digital converter (TDC) based on an event timer. To enhance the reliability of transmitted reference PPM (TR-PPM) signals, the transmission system integrates Gray coding and Consultative Committee for Space Data Systems (CCSDS)-standard-compliant Reed-Solomon (RS) error correcting code (ECC). System performance was evaluated by transmitting pseudorandom binary sequences (PRBSs) and measuring the bit error ratio (BER) across a 5-m wireless link between two 20 (Formula presented.) gain horn (Ka-band) antennas, with and without a 20 (Formula presented.) single-mode optical fiber (SMF) link in transmitter side and ECC at the receiver side. The system achieved a BER of less than 8.17 × 10−7, using a time bin duration of 200 (Formula presented.) and a pulse duration of 100 (Formula presented.), demonstrating robust performance and significant potential for space-to-ground telecommunication applications. 

The rapid evolution of high-speed communication technologies has brought serious inter-symbol interference (ISI). To cope with this issue, channel equalization techniques, such as feedforward equalization (FFE) and decision feedback equalization (DFE), are typically used to compensate for channel response to facilitate signal recovery, and forward error correction (FEC) code, such as low-density parity check (LDPC) codes, is widely deployed to improve link budget margin. However, the mutual influence between the equalizer and the decoder has yet to be thoroughly explored. In this paper, the state error decoding (SED) algorithm is proposed to enhance the decoding performance by effectively reducing the error probabilities and surpassing the limitations of traditional signal recovery methods. Experimental results in a photonic terahertz wireless communication system achieve error propagation mitigation and additional 0.2 dB link budget improvement by utilizing the proposed SED method, which confirms the effectiveness of the algorithm in reducing error propagation and improving system performance, paving the way for the development of broadband communications. 

Development of Data Center based computing technology require energy efficient high-speed transmission links. This leads to optical amplification-free intensity modulation and direct detection (IM/DD) systems with low complexity equalization compliant with IEEE standardized electrical interfaces. Switching from on-off keying to multi-level pulse amplitude modulation would allow to reduce lane count for next generation Ethernet interfaces. We characterize 106.25 Gbaud on-off keying, 4-level and 6-level pulse amplitude modulation links using two integrated transmitters: O-band directly modulated laser and C-band externally modulated laser. Simple feed forward or decision feedback equalizer is used. We demonstrate 106.25 Gbaud on-off keying links operating without forward error correction for both transmitters. We also show 106.25 Gbaud 4-level and 6-level pulse amplitude modulation links with performance below 6.25% overhead hard-decision forward error threshold of 4.5×10-3. Furthermore, for EML-based transmitter we achieve 106.25 Gbaud 4-level pulse amplitude modulation performance below KP-FEC threshold of 2.2×10-4. That shows that we can use optics to support (2x)100 Gbps Ethernet on single lambda at expense of simple forward error correction.

We experimentally demonstrate a room-temperature LWIR FSO link with a 9.1-μm directly modulated QCL and an MCT detector. Net bitrate of up to 16.9 Gb/s is achieved at both 15°C and 20°C over a 1-meter distance. 

We demonstrate a record 170 Gbaud on-off keying C-band silicon photonics ring resonator modulator-based transmitter with performance below the 6.7% overhead HD-FEC threshold after optical back-to-back and transmission over 100 meters of single mode fiber. © 2024 IEEE.

Photonic time-delay reservoir computing (TDRC) is an optical neural network structure known for its simple hardware implementation. However, this simplicity reduces information processing speed due to its sequential time multiplexing mechanism, such as the masking operation in practical experiments. To address this, we employ a deep photonic TDRC structure to enhance reservoir dynamics, effectively reducing the mask size to accelerate processing while maintaining high performance. An extended state matrix is proposed to leverage the enriched dynamics without additional hardware costs, combining different nonlinear intensities and memory lengths to augment node states without physically expanding the reservoir. Experimentally validated in a speech recognition task, our approach accelerates processing by 10 times with only a 2.4% decrease in recognition accuracy, compared to a 13.1% accuracy deterioration in the conventional scheme, indicating significant acceleration in TDRC information processing while maintaining performance.

This study investigates the potential of long-wave infrared (LWIR) free-space optical (FSO) transmission using multilevel signals to achieve high spectral efficiency. The FSO transmission system includes a directly modulated-quantum cascade laser (DM-QCL) operating at 9.1 µm and a mercury cadmium telluride (MCT) detector. The laser operated at the temperature settings of 15°C and 20°C. The experiment was conducted over a distance of 1 m and in a lab as a controlled environment. We conduct small-signal characterization of the system, including the DM-QCL chip and MCT detector, evaluating the end-to-end response of both components and all associated electrical elements. For large-signal characterization, we employ a range of modulation formats, including non-return-to-zero on-off keying (NRZ-OOK), 4-level pulse amplitude modulation (PAM4), and 6-level PAM (PAM6), with the objective of optimizing both the bit rate and spectral efficiency of the FSO transmission by applying pre- and post-processing equalization. At 15°C, the studied LWIR FSO system achieves net bitrates of 15 Gbps with an NRZ-OOK signal and 16.9 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps NRZ-OOK below the 2.7% overhead Reed-Solomon RS(528,514) pre-FEC (KR-FEC limit). At 20°C, we obtained net bitrates of 14.1 Gbps with NRZ-OOK, 16.9 Gbps with PAM4, and 16.4 Gbps with PAM6. Furthermore, we evaluate the BER performance as a function of the decision feedback equalization (DFE) tap number to explore the role of equalization in enhancing signal fidelity and reducing errors in FSO transmission. Our findings accentuate the competitive potential of DM-QCL and MCT detector-based FSO transceivers with digital equalization for the next generation of FSO communication systems. 

This paper highlights the potential of photonic-assisted analog fronthaul solutions, particularly analog radio-over-fiber (ARoF) and analog radio-over-free-space-optics (ARoFSO), as prospective alternatives for the development of 6G applications. First, we present (New-Radio) NR/5G conformance testing of ARoF and ARoFSO fronthaul links, including the assessment of the error vector magnitude (EVM) and adjacent channel leakage power ratio (ACLR) to demonstrate compliance with the minimum transmitter requirements outlined by the 3rd Generation Partnership Project (3GPP) standards. Then, with focus on future 6G Distributed-MIMO (D-MIMO) networks, we conduct experimental validations of coherent joint transmissions (CJT) using ARoF and ARoFSO fronthaul links in a 2-transmitter D-MIMO network, demonstrating MIMO gains of up to 5.35 dB and that these links meet the stringent synchronization demands for CJT. These tests represent the first realizations of CJT utilizing ARoF and ARoFSO links. Finally, for consistency, we validate CJT in a 4-transmitter D-MIMO network with ARoF fronthaul links, with MIMO gains up to 9.4 dB and confirming our previous results. This evidence indicates that these technologies hold significant potential for applications in future 6G systems.