Qingming Zhu

We propose and experimentally demonstrate an automated wavelength locking scheme for an O-band 1 x 2 silicon photonic switch based on an add-drop structure with two cascaded resonators. In the scheme, two three-layer neural networks are employed to learn the relationship between the monitored optical powers and the heating voltages of the two coupled ring resonators. Then, the two neural networks can quickly predict the required heating voltages according to the monitored optical powers. Therefore, the wavelength locking can be realized through a training process, which is carried out only once, and a locking process. By using this scheme, the dual-ring switch can be locked to the wavelength of the input light with a constant duration of 860 μs. The maximum locking errors for the two rings are 0.08 nm and 0.09 nm, respectively.

We experimentally demonstrate a 56-Gb/s vestigial sideband (VSB) modified duobinary (MDB) 4-level pulse amplitude modulation (PAM4) signal transmission over an 80-km single-mode fiber (SMF) in an intensity-modulation and direct-detection (IM/DD) system without optical dispersion compensation. Compared to the conventional PAM4 signal without pulse shaping, the MDB PAM4 signal exhibits a higher spectral efficiency (SE) due to the modified duobinary precoding. Moreover, if VSB modulation is employed to mitigate the chromatic dispersion (CD)-induced power fading, the VSB MDB PAM4 signal suffers less image interference resulting from VSB filtering and direct detection (DD), thanks to its lower components at the carrier frequency. A proof-of-concept experiment is performed, in which the performances of the conventional VSB PAM4 signal, the VSB Nyquist PAM4 signal, and the VSB MDB PAM4 signal are investigated and compared comprehensively. The experimental results show that only the VSB MDB PAM4 signal can reach the bit error ratio (BER) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.4×10-2 at the 56-Gb/s rate after the 80-km SMF transmission, due to the mitigated image interference.

We introduce and experimentally demonstrate a modified Kramers-Kronig (KK) receiver with direct detection (DD) to realize an accurate optical field reconstruction at low carrier-to-signal power ratio (CSPR) condition. In our scheme, an exponential operation is applied to a single-sideband (SSB) signal, rather than a continuous-wave tone adding method in the conventional KK scheme. By this means, the minimum phase condition (MPC) is inherently satisfied, thus the optical field can be reconstructed accurately through KK relation at a low CSPR, leading to a high receiver sensitivity in a DD system. A proof-of-concept experiment is conducted, in which an SSB signal is generated based on an 8-GBaud 16-quadrature amplitude modulation (QAM) data stream and then transmitted over an 80-km single mode fiber (SMF). By using the modified KK technique, 1.1- and 0.9-dB sensitivity improvements are achieved in the optical back-to-back case and after the fiber transmission, respectively. The proposed and the conventional KK schemes show similar BER performances at the optimum CSPR value with a high received optical power. In addition, we perform numerical simulations to study the impacts of bandwidth limitation and chromatic dispersion on the proposed minimum phase signal.

We propose and experimentally demonstrate a self-homodyne locking method for a silicon microring resonator (MRR). The device employs a self-homodyne detection structure and consists of a tunable MRR with two directional couplers along the ring for monitoring, two phase shifters to calibrate the phase difference between the two monitored optical signals, and a Y-branch to combine the two signals. A single photodetector is used to detect the output power of the Y-branch. If the MRR is on resonance, a destructive interference occurs in the Y-branch, therefore the monitored photocurrent is minimized. By using such a device structure and the homodyne detection scheme, the MRR with a Q factor of 1.9 × 10⁴ can be accurately locked to the signal wavelength, and the locking process is insensitive to input power variation. The wavelength locking range is larger than one free spectral range (FSR) of 6 nm, and the locking errors are ≤0.015 nm.

Switching can be performed with multiple physical dimensions of an optical signal. Previously optical switching was mainly focused in the wavelength domain. In this paper we discuss the general architecture of integrated silicon photonic switches by exploiting multi-dimensions in wavelength, polarization, and mode. To route a data channel from one input port to an arbitrary output port in a network node, three basic functions are required: de-multiplexing, switching, and multiplexing. The multiplexing and de-multiplexing processes can be realized in any one physical dimension. The capacity of a switch can be effectively scaled by using joint physical dimensions. As two examples, we first present a wavelength switch based on dual-nanobeam cavities with high quality factors, a low power consumption, and a compact footprint. We then propose a design of a mode-polarization-wavelength selective switch by leveraging three physical dimensions, and experimentally demonstrate the building blocks and key functionalities.

We propose and experimentally demonstrate an approach to carrier frequency and phase recovery of a microwave photonic signal based on carrier period measurement (CPM). In the approach, a coarse carrier frequency offset (CFO) estimation is firstly performed, which is realized by measuring the crossing period of the microwave signal with its mean level. Then, an envelope detection method is used to simultaneously estimate the residual CFO and the carrier phase. Through the two steps, wideband and high accuracy carrier recovery can be realized. The proposed approach is validated by an experiment. Effective carrier recovery of a microwave photonic signal is achieved with a carrier frequency varying from 7 GHz to 37 GHz carrying a 2-Gbaud on-off-keying (OOK) or quadrature phase shift keying (QPSK) baseband signal. The CFO estimation range is from -21 GHz to +9 GHz, and the CFO estimation errors are less than 0.1 MHz.

We propose and experimentally demonstrate a multiple input multiple output - artificial neural network (MIMO-ANN) nonlinear equalizer (NLE) to process the complex quadrature amplitude modulation (QAM) signal in a single-sideband (SSB) self-coherent detection (SCD) system. In the proposed scheme, a 2-by-2 MIMO structure with two ANNs is employed to effectively mitigate the signal distortions induced by in-phase and quadrature (IQ) imbalance and fiber nonlinear effects. By using the proposed MIMO-ANN NLE, we successfully transmit a 112-Gb/s SSB 16-QAM signal over a single-span 120-km single mode fiber (SMF) in a direct detection (DD) system with a bit error rate (BER) lower than 3.8 × 10⁻³. We also conduct a comparative study between the proposed MIMO-ANN NLE, a feedforward equalizer (FFE), a NLE consisting of two independent real-valued Volterra filters, and a MIMO-Volterra filter. The proposed MIMO-ANN NLE outperforms other equalizers with the longer fiber length and thus stronger nonlinearities, since it can easily approximate a complicated nonlinear function. To the best of our knowledge, this is the first experimental demonstration of an ANN-based equalizer in an SSB SCD system.

We propose and experimentally demonstrate an automated resonance wavelength alignment scheme for an O-band 4 × 4 thermo-optic optical switch using dual-ring resonators. In this scheme, a new control algorithm based on saddle point searching is employed to align dual-input switching elements (SEs) and therefore the 4 × 4 switch. The proposed scheme is proven effective by demonstrating eight routing configurations for two 10-Gb/s data channels. The insertion losses of the three-stage SEs are ≤6.9 dB, and the crosstalk values are below -13.6 dB. Besides, the tolerance of the switch to the wavelength misalignment between the two optical input signals is measured. Negligible performance degradations are observed when the two channels are spaced by 0.05 nm.

We experimentally demonstrate an on-chip optical switching scheme for mode- and polarization-multiplexed signals. A quadrupled switching capacity of 748 Gb/s/λ is achieved by routing eight 93.5-Gb/s data channels on two modes and two polarizations.