Di Che

61 Gbits/s direct-detection optical OFDM based on blockwise signal phase switching with signal-to-signal beat noise cancellation.

We propose a scheme for a direct-detection optical orthogonal frequency-division multiplexing (OFDM) system based on blockwise signal-phase-switching (SPS). Experimental demonstration of 61 Gbits/s SPS direct-detection optical OFDM signal transmission over 80 km standard single-mode-fiber was successfully achieved with single polarization and a single photodetector.

Advanced modulation formats for high-performance short-reach optical interconnects

The explosive growth of the traffic between data centers has led to an urgent demand for high-performance short-reach optical interconnects with data rate beyond 100G per wavelength and transmission distance over hundreds of kilometers. Since direct detection (DD) provides a cost-efficient solution for short-reach interconnects, various advanced modulation formats have been intensively studied to improve the performance of DD for high-performance short-reach optical interconnects. In this paper, we report the recent progress on the advanced DD modulation formats that provide superior electrical spectral efficiency (SE) and transmission reach beyond that of simple direct modulation (DM) based direct detection (DM/DD). We first provide a review of the current advanced modulation formats for high-performance short-reach optical interconnects. Among these formats, Stokes vector direct detection (SV-DD) achieves the highest electrical spectrum efficiency, presenting itself as a promising candidate for future short-reach networks. We then expound some novel algorithms to achieve high-performance SV-DD systems under severe impairments of either polarization mode dispersion (PMD) or polarization dependent loss (PDL).

Stokes Vector Direct Detection for Linear Complex Optical Channels

Data center interconnect has stimulated the research on the short-reach communications with data rate beyond 100G per wavelength and transmission distance of hundreds of kilometers. Aiming at the high-speed short-reach communications, we recently proposed the Stokes vector direct detection (SV-DD) that realizes a linear complex optical channel similar to the coherent detection. In SV-DD, the transmitter places the signal and the carrier onto the orthogonal polarizations, while the receiver achieves the polarization insensitive 3-D detection in the Stokes space with the digital signal processing enabled polarization acquisition. SV-DD achieves 100% spectral efficiency with reference to the single-polarization coherent detection, and simultaneously attains the receiver phase diversity and the cancellation of photo-detection nonlinearity. We experimentally demonstrate the SV-DD signal transmission over 160-km standard single-mode fiber at data rates of both 80 and 160-Gb/s. SV-DD significantly decreases both the system hardware and DSP complexity compared with the polarization multiplexed coherent detection, while increases the system spectrum efficiency compared with the conventional intensity modulation direct detection. Therefore, SV-DD offers a cost-effective solution for the 100G per wavelength and beyond metropolitan area network (MAN). It also owns the potentials to be deployed in the future high-speed passive optical network (PON).

Block-wise phase switching for double-sideband direct detected optical OFDM signals.

We propose phase switching for either main-carrier or subcarriers of two consecutive signal blocks to achieve fading-free double-sideband direct-detection (DD). In our demonstration, 40-Gb/s DD-OOFDM is successfully received over 80-km SSMF with single polarization and single detector.

Measurement of distributed mode coupling in a few-mode fiber using a reconfigurable Brillouin OTDR

We report nondisruptive measurement of distributed mode coupling along a five-mode fiber (5MF) using a reconfigurable Brillouin optical time domain reflectometer (OTDR) based on programmable spatial light modulators (SLMs). Mode coupling between LP01 mode and higher order modes LP11a,b and LP21a,b is successfully characterized with a spatial resolution of 50 m.

Stokes vector direct detection for short-reach optical communication.

We propose the Stokes vector direct detection (SV-DD) algorithm to achieve photo-detection nonlinearity cancellation and phase diversity. A 160-Gb/s single-polarization modulated SV-DD signal is successfully received after 160-km SSMF transmission with 7.76-bit/s/Hz electrical spectrum efficiency.

Signal-carrier interleaved optical OFDM for direct detection optical communication

We propose signal-carrier interleaved (SCI) optical OFDM for direct detected transmission systems. Such a scheme can be considered as a variation of self-coherent detection where the carrier and signal are supplied at the transmitter and extracted at the receiver for coherent-like detection. This provides high OSNR sensitivity while maintaining very low carrier-to-signal power ratio (CSR). Our experiment results show that with 0 dB CSR, 43.2 Gb/s 16 QAM OFDM signal can be successfully delivered over 80 km standard single mode fiber (SSMF) with 24 dB OSNR requirements at 7% FEC limit.

Linearization of Direct Detection Optical Channels Using Self-Coherent Subsystems

Intensity modulation with direct detection (IM-DD) dominates the commercial short-reach optical communications. However, when upgrading the data-rate distance product to 1000 Gb/s·km per wavelength and beyond, IM-DD faces severe performance barrier. Although a linear mapping exists between the intensity of transmitter and receiver in IM-DD back-to-back system, it becomes nonlinear under fiber channel impairments; for instance, the dominant chromatic dispersion. Coherent detection overcomes this fundamental obstacle by recovering a linear replica of the optical field instead of intensity. The local oscillator provides a reference carrier which mixes with the received signal, from which both intensity and phase of the signal can be recovered. Borrowing the idea from coherent detection, in DD system, a carrier can be sent along with the signal so that the receiver utilizes the reference carrier originated from transmitter; namely, the receiver conducts self-coherent (SCOH) detection. In this paper, we review a variety of optical SCOH subsystems, and reveal how they realize linear channels, while maintaining the simple and low-cost DD. SCOH can readily reach a data-rate distance product beyond 10000 Gb/s·km, making it suitable for the future high-speed short- and medium-reach applications, such as the data center interconnect, passive access network, and metropolitan area network.

Beyond amplitude-only detection for digital coherent system using directly modulated laser.

We report the first experiment of digital coherent detection of directly modulated signals that utilizes both phase and amplitude of the signals. A novel algorithm is proposed to take advantage of simultaneous detection of both phase and amplitude of the modulated signal. Using our proposed scheme, the experiment shows 6-dB improvement over the amplitude-only detection scheme.

Spectrally efficient optical transmission based on Stokes vector direct detection

We propose a novel detection scheme called Stokes vector direct detection (SV-DD) to realize high electrical spectral efficiency and cost-effective optical communication for short and medium reach. With SV-DD, the signal is modulated in only one polarization and combined with the carrier in the orthogonal polarization for fiber transmission. At reception, the combined signal is detected in Stokes space by three or four photo-detectors. Compared with conventional DD technique, SV-DD is resilient to both chromatic dispersion and signal-to-signal beat noise. Furthermore, SV-DD does not require polarization tracking or narrow band optical filtering for carrier extraction. In this paper, we present for the first time the numerical analysis and experimental demonstration of single-carrier SV-DD. We report 62.5-Gb/s data rate single-carrier SV-DD transmission over 160-km SSMF using 12.5-Gbaud 32-QAM modulation.