Qian Hu

Few-mode fiber based optical sensors.

Few-mode fiber (FMF) has recently attracted much interest due to the additional freedom in the spatial domain. In this talk, we report our recent progress on FMF based optical sensors. We show two types of FMF sensor, and then demonstrate the potential of multi-parameter sensing.

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).

Characterization of stimulated Brillouin scattering in a circular-core two-mode fiber using optical time-domain analysis

We show characterization of stimulated Brillouin scattering (SBS) in a circular-core two-mode fiber (c-TMF) using Brillouin optical time-domain analysis (BOTDA) with a pulsed pump and a counterpropagating continuous wave probe. By using two free-space mode combiners (FSMCs), we can launch any combination of spatial modes into both ends of the c-TMF. Combined with coherent detection, measurement of distributed Brillouin gain spectra (BGS) is achieved for all possible counter-propagating spatial mode pairs with high spectral resolution and stability. Both intra- and inter-modal SBS are investigated for the c-TMF. The inter-modal SBS between two degenerate LP11 modes (LP11a/LP11b) is demonstrated for the first time. From the Brillouin frequency shift (BFS) measured in each intra-modal SBS, the distributed modal birefringence between non-degenerate modes (LP01/LP11) and degenerate LP11 modes is obtained. The proposed setup can potentially be used as a c-TMF based distributed Brillouin sensor.

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.

Autocorrelation function of channel matrix in few-mode fibers with strong mode coupling.

Channel matrix plays a critical role in receiver design and ultimate channel performance. To fully describe the channel matrix of a few-mode fiber (FMF), we choose the generalized high-dimensional Gell-Mann matrices, an equivalent of the 2-dimensional Pauli matrices used for a single-mode fiber (SMF), as the basis for the channel matrix decomposition. The frequency dependence of channel matrix can be studied in terms of the autocorrelation function (ACF), showing how fast channel changes in frequency domain. In this paper, we derive a canonical stochastic differential equation (SDE) for the FMF channel matrix in the regime of strong coupling. With the SDE, we develop an analytical form for the ACF of FMF channel matrix, from which the channel correlation bandwidth is obtained.

Equalization of two-mode fiber based MIMO signals with larger receiver sets.

We demonstrate channel equalization of two-mode fiber based MIMO system using more receivers than transmitters. Specifically, the equalization performances are compared among three methods including zero-forcing (ZF), minimum-mean-square-error (MMSE), and successive-interference-cancellation (SIC).