A widely tunable optoelectronic oscillator (OEO) based on a broadband phase modulator and a tunable optical bandpass filter is proposed and experimentally demonstrated. A tunable range from 4.74 to 38.38 GHz is realized by directly tuning the bandwidth of the optical bandpass filter. To the best of our knowledge, this is the widest fundamental frequency tunable range ever achieved by an OEO. The phase noise performance of the generated signal is also investigated. The single-sideband phase noise is below $-120\mathrm{dBc}/\mathrm{Hz}$ at an offset of 10 KHz within the whole tunable range.

A wideband tunable optoelectronic oscillator (OEO) based on the deamplification of stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. A tunable single passband microwave photonic filter (MPF) utilizing phase modulation and SBS deamplification is used to realize the tunability of the OEO. Theoretical analysis of the MPF and phase noise performance of the OEO are presented. The frequency response of the MPF is determined by the + 1st sideband attenuation due to SBS deamplification and phase shift difference between the two sidebands due to chromatic dispersion and SBS. The close-in (< 1 MHz) phase noise of the proposed OEO is shown to be dominated by the laser frequency noise via phase shift of SBS. The conversion of the laser frequency noise to the close-in phase noise of the proposed OEO is effectively reduced compared with the OEO based on amplification by SBS. Tunable 7 to 40 GHz signals are experimentally obtained. The single-sideband (SSB) phase noise at 10 kHz offset is −128 dBc/Hz for 10.30 GHz signal. Compared with the OEO based on SBS amplification, the proposed OEO can achieve a phase noise performance improvement beyond 20 dB at 10 kHz offset. The maximum frequency and power drifts at 10.69 GHz are within 1 ppm and 1.4 dB during 1000 seconds, respectively. To achieve better close-in phase noise performance, lower frequency noise laser and higher pump power are preferred. The experimental results agree well with the theoretical models.

In this work, based on a conventional silicon photonic travelling-wave Mach-Zehnder modulator (MZM) with a 3-dB bandwidth of 22.5GHz, we experimentally demonstrate ultra-high speed optical interconnects with Nyquist shaped pulse amplitude modulation (PAM) signals. For bandwidth-limited systems, a two-tap digital post filter is employed to suppress the equalization-enhanced high frequency noise. The post filter induced inter-symbol interference (ISI) is subsequently eliminated by maximum likelihood sequence detection (MLSD). Enabled by the post filter and MLSD, we achieve 200Gb/s (80Gbaud) PAM-6 signal direct detection (DD) transmission over 1km standard single-mode fiber (SSMF) with a bit-error rate (BER) below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5×10 $^{-2}$ . For PAM-4 format, we successfully transmit 192Gb/s (96Gbaud) and 176Gb/s (88Gbaud) signals over 1km and 2km SSMF, respectively, with BERs lower than the 20% HD-FEC threshold. For PAM-8 format, 192Gb/s (64Gbaud) and 168Gb/s (56Gbaud) signals are generated at back-to-back (BTB) and transmitted over 1km SSMF, respectively. To our best knowledge, we achieve the highest single lane bitrates ever reported for single polarization PAM-4/6/8 signal generation and DD transmission with all-silicon MZM. The flexible receiver-side digital signal processing (DSP) can significantly enhance the performance of silicon MZM, which provides a promising solution for future single lane 200G data-center interconnects.

We propose and demonstrate a novel optical frequency comb (OFC) based microwave photonic filter which is able to realize arbitrary filtering shape with linear phase response. The shape of filter response is software programmable using finite impulse response (FIR) filter design method. By shaping the OFC spectrum using a programmable waveshaper, we can realize designed amplitude of FIR taps. Positive and negative sign of FIR taps are achieved by balanced photo-detection. The double sideband (DSB) modulation and symmetric distribution of filter taps are used to maintain the linear phase condition. In the experiment, we realize a fully programmable filter in the range from DC to 13.88 GHz. Four basic types of filters (lowpass, highpass, bandpass and bandstop) with different bandwidths, cut-off frequencies and central frequencies are generated. Also a triple-passband filter is realized in our experiment. To the best of our knowledge, it is the first demonstration of a programmable multiple passband MPF with linear phase response. The experiment shows good agreement with the theoretical result.

Recently optical amplification technologies for mode-division multiplexing (MDM) transmission systems and networks have attracted great interest. In this paper, we firstly propose a compact structure of few-mode Erbium-doped fiber amplifier (FM-EDFA) for weakly-coupled MDM amplification, which is characterized by weakly-coupled few-mode Erbium-doped fiber (EDF), cascaded mode-selective couplers (MSCs) for spatial mode control of pump lights, and few-mode wavelength multiplexer (FM-WMUX) for pump/signal coupling. Compared to previous studies, the proposed FM-EDFA has a similar structure with commercial single-mode EDFA (SM-EDFA) and can support weakly-coupled MDM transmission without inducing large modal crosstalk. Then related components are designed and fabricated for a 2-mode proposed FM-EDFA. The mode-field mismatching between transmission FMF and EDF under central alignment is investigated by simulation. Thanks to the effective suppression of modal crosstalk for the optical components, the transmission FMF, the EDF and their coupling, we experimentally demonstrate the first weakly-coupled MDM-WDM amplification and transmission over 15-km 2-mode FMF with 10-Gb/s optical on-off keying (OOK) modulation and direct detection. The proposed FM-EDFA can be extended to MDM transmission systems and networks supporting more modes.

Weakly-coupled mode division multiplexing (MDM) has been intensively investigated in recent years. In order to be compatible with conventional intensity modulation and direct detection systems, effective solutions to handle degenerate non-circularly-symmetric linearly-polarized (LP) modes ( $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes, l ≥ 1) of weakly-coupled few-mode fibers (FMFs) are highly desired. In this paper, a novel degenerate-mode-selective coupler (DMSC) consisting of an input FMF and an output two-mode fiber (TMF) is proposed based on coupling mechanisms of a FMF directional coupler. Optical powers in both $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes of the FMF of $\text{LP}_{lm}$ DMSC can be selectively and simultaneously transferred to the LP _11a and LP _11b modes of the TMF. Therefore, the DMSC can demultiplex degenerate non-circularly-symmetric LP modes without any digital signal processing (DSP). Necessary conditions for the DMSC are theoretically investigated. LP ₁₁ and LP ₂₁ DMSCs are numerically simulated to verify the principles and fabricated in the form of fused-type fiber couplers. Compared with regular mode-selective couplers (MSCs), the fabricated DMSCs significantly improve the power stability for demultiplexing randomly-rotated non-circularly-symmetric LP modes. Based on a mode demultiplexer composed of cascaded LP ₀₂ MSC, LP ₂₁ DMSC, LP ₁₁ DMSC and LP ₀₁ MSC, 4-LP-mode MDM transmission over 10-km of weakly-coupled FMF is demonstrated with 10-Gb/s on-off-keying and DSP-free direct detection.