Zhangyuan Chen

DSP-Based Evolution From Conventional TDM-PON to TDM-OFDM-PON

Recently, digital signal processing (DSP) methods have been extensively studied and various DSP-based schemes have been proposed for next-generation passive optical network (PON). In this paper, we combine the time-division-multiplexing (TDM) architecture with advanced DSP methods to achieve high spectrum-efficiency colorless transmission. Orthogonal frequency division multiplexing (OFDM), as the original form of various DSP methods with frequency-domain equalization (FDE), is chosen for both upstream and downstream transmissions. In the TDM-PON based on OFDM (TDM-OFDM-PON) architecture, OFDM symbols are transmitted in different time slots instead of on-off-key (OOK) bits in conventional TDM-PON. This architecture can be extended for single-carrier with FDE easily. Due to the same TDMA scheme, it can achieve maximum compatibility with current TDM-PON. A DSP-based evolution scheme from conventional TDM-PON to TDM-OFDM-PON is proposed and experimentally demonstrated by upgrading the optical line terminal (OLT) to support both OOK and OFDM signals, which can maintain the current optical distribution network (ODN) and minimize the impact for the existing service and equipment.

Spectrally efficient terabit optical transmission with Nyquist 64-QAM half-cycle subcarrier modulation and direct detection

We demonstrate 1.728  Tb/s(16×108  Gb/s) direct-detection wavelength division multiplexing (WDM) transmission over 80 km standard single mode fiber (SSMF) with Nyquist 64-ary quadrature amplitude modulation (64-QAM) and half-cycle subcarrier modulation. Each channel carries single sideband 18 GBaud 64-QAM signal and the channel spacing is 27 GHz. Considering 20% soft-decision forward error correction and frame redundancy, a net spectral efficiency record of 3.25 b/s/Hz is achieved for 100 G single polarization direct-detection WDM transmission.

Tunable DC-60 GHz RF Generation Utilizing a Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

A dual-loop optoelectronic oscillator (OEO) based on stimulated Brillouin scattering (SBS) is experimentally demonstrated. Two lasers are utilized to realize the tunability of the OEO. One acts as the signal laser, the other is employed as the pump laser. By directly tuning the wavelength of the pump laser, a widely tunable range from dc to 60 GHz for the RF signal generation can be obtained. To the best of our knowledge, this is the widest fundamental frequency tunable range which has ever been achieved by an OEO. With dual-loop fiber lengths of 2 and 4 km, the single sideband (SSB) phase noise is measured to be -100 dBc/Hz at 10 kHz offset when the oscillation frequency is chosen as 5, 10, or 20 GHz. The side-mode suppression ratio (SMSR) is 35 dB when the oscillation frequency is 10 GHz. The stability of both frequency and power of the proposed OEO is improved with the dual-loop configuration when compared with the singleloop one. The Allan variances of the frequency fluctuation at 1-s average are 1.2 × 10-7 and 4.9 × 10-11 for the single-loop and dual-loop configurations, respectively. Furthermore, a phase noise model based on control theory to evaluate the SSB phase noise performance of the dual-loop OEO based on SBS is detailed for the first time. The experimental phase noise results agree well with the proposed phase noise model at an offset frequency range from 100 Hz to 100 MHz. Among different phase noise tests, the amplified spontaneous emission (ASE) noise induced by SBS is shown theoretically and experimentally to be the dominant source for the phase noise beyond 100-kHz frequency offset in the proposed OEO.

Experimental demonstration of 400 Gb/s optical PDM-OFDM superchannel multicasting by multiple-pump FWM in HNLF

OFDM superchannel that consists of multiple low speed individually-modulated subbands has been proposed for high speed optical transmission and flexible optical networks with multiple data rate accommodation. In this work, we investigate the feasibility of superchannel multicasting and verify it utilizing multiple-pump FWM in highly nonlinear fiber. 400 Gb/s PDM-OFDM superchannel that consists of ten subbands is successfully delivered from one superchannel to up to seven different superchannels with error free operation. Pump power and signal power are also optimized to achieve the optimal multicasting performance.

Novel MDM-PON scheme utilizing self-homodyne detection for high-speed/capacity access networks

In this paper, we propose a cost-effective, energy-saving mode-division-multiplexing passive optical network (MDM-PON) scheme utilizing self-homodyne detection for high-speed/capacity access network based on low modal-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). In the proposed scheme, one of the spatial modes is used to transmit a portion of signal carrier (namely pilot-tone) as the local oscillator (LO), while the others are used for signal-bearing channels. At the receiver, the pilot-tone and the signal can be separated without strong crosstalk and sent to the receiver for coherent detection. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Meanwhile, the self-homodyne detection scheme can effectively suppress laser phase noise, which relaxes the requirement for the lasers line-width at the optical line terminal or optical network units (OLT/ONUs). The digital signal processing (DSP) at the receiver is also simplified since it removes the need for frequency offset compensation and complex phase correction, which reduces the computational complexity and energy consumption. Polarization division multiplexing (PDM) that offers doubled SE is also supported by the scheme. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. Utilizing the proposed scheme, we demonstrate a proof of concept 4 × 40-Gb/s orthogonal frequency division multiplexing (OFDM) transmission over 55-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. Compared with back to back case, less than 1-dB Q-factor penalty is observed after 55-km FMF of the four channels. Signal power and pilot-tone power are also optimized to achieve the optimal transmission performance.

Frequency-Domain Channel Estimation for Polarization-Division-Multiplexed CO-OFDM/OQAM Systems

Intrinsic imaginary interference (IMI) induced by a multiple-path fading channel is an important impairment for orthogonal frequency division multiplexing/offset-quadrature amplitude modulation (OFDM/OQAM) systems. Therefore, the accurate channel estimation is highly desired for such a system. Recently both the simulation studies and the experimental demonstrations for coherent optical OFDM/OQAM (CO-OFDM/OQAM) have been reported. However, there are no theoretical discussions on the IMI effect and the channel estimation method for polarization-division-multiplexed (PDM) CO-OFDM/OQAM systems so far. In this paper, we systematically analyze the frequency-domain optical fiber channel transmission model for PDM CO-OFDM/OQAM systems with the IMI effect induced by chromatic dispersion (CD) and polarization mode dispersion (PMD). The full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods are discussed to mitigate the IMI effect. The computational complexities and robustness against CD and PMD are also compared for both of the FL and the HL methods. The theoretical analysis is validated by numerical Monte Carlo simulations of PDM CO-OFDM/OQAM systems.

Demonstration of Bidirectional PON Based on Mode Division Multiplexing

Recently, mode division multiplexing (MDM) has been introduced into optical transmission systems and networks for capacity enhancement. In this letter, we propose a bidirectional passive optical network (PON) architecture based on MDM. The bidirectional optical distribution network (ODN) of the MDM-PON consists of few-mode fiber, passive mode multiplexer/demultiplexer and few-mode circulators (FMCs), all of which are characterized by low modal-crosstalk. Signals in different modes are able to be transmitted and received independently for both downstream and upstream transmission. We experimentally demonstrate the bidirectional MDM-PON transmission over 10-km 2-mode optical fiber. 10-Gb/s on-off-keying (OOK) signals can be independently transmitted and directly detected without coherent receiver and multi-input-multiple-output (MIMO) processing. Moreover, nonlinear transmission impairments are experimentally investigated.

Cascaded Mode-Division-Multiplexing and Time-Division-Multiplexing Passive Optical Network Based on Low Mode-Crosstalk FMF and Mode MUX/DEMUX

We propose cascaded mode-division-multiplexing and time-division-multiplexing passive optical network (MDM-TDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), in which optical network units communicate with the optical line terminal utilizing different time slots and specific optical linearly polarized spatial modes. An MDM optical distribution network (ODN) is cascaded with multiple conventional TDM ODNs to effectively extend a larger scale of current commercial PON systems based on TDM. The upgrade from TDM-PON to the cascaded MDM-TDM-PON is simple and cost-effective. No multiple-input-multiple-output (MIMO) digital signal processing is required to eliminate the mode crosstalk. The all-fiber mode MUX/DEMUXs are composed of mode selective couplers, which simultaneously multiplex or demultiplex multiple modes. We experimentally demonstrate MDM-PON transmission over 10 and 55 km two-mode FMFs and cascaded MDM-TDM-PON transmission over a 10-km two-mode FMF and a 10-km standard single-mode fiber with 10-Gb/s optical on-off keying (OOK) signal and direct detection.

Low-noise and broadband optical frequency comb generation based on an optoelectronic oscillator

A novel scheme to generate broadband high-repetition-rate optical frequency combs and low phase noise microwave signals simultaneously is proposed and experimentally demonstrated. By incorporating an optical frequency comb generator in an optoelectronic oscillator loop, more than 200 lines are generated for a 25 GHz optical frequency comb, and the single-sideband phase noise is as low as -122  dBc/Hz at 10 kHz offset for the 25 GHz microwave signal. 10 and 20 GHz optical frequency combs and microwave signals are also generated. Unlike the microwave frequency synthesizer, the phase noise of the microwave signals generated by this new scheme is frequency independent.

Symmetric \(4\times 25\) -Gb/s DSP-Enhanced TWDM-PON With DSB Modulation and RSOA

With the ever-increasing bandwidth demand of the end-customers, the 100-Gb/s access solution has attracted more and more attention. In this letter, we propose a symmetric 100-Gb/s digital signal processing-enhanced time wavelength division multiplexed passive optical networ (TWDM-PON) architecture with four pairs of wavelengths. A 4 × 25 -Gb/s TWDM-PON is experimentally demonstrated over a 26.7-km fiber using double sideband (DSB) orthogonal frequency division multiplexing (OFDM) modulation. The reflective semiconductor optical amplifier is deployed in the optical network unit (ONU) to enhance the receiver sensitivity of downstream OFDM signals, and DSB single-carrier frequency domain equalization modulation is used for upstream transmission to reduce the complexity and cost of the ONU.