Zhaowen Xu

WDM-PON Architectures With a Single Shared Interferometric Filter for Carrier-Reuse Upstream Transmission

We propose two novel wavelength-division-multiplexed passive-optical-network (WDM-PON) architectures where subcarriers are employed to transmit downstream data and optical carriers are reused for upstream transmission. Architecture I is designed for the situation where two short distribution fibers are available between the remote node (RN) and each optical network unit (ONU), whereas Architecture II is devised for the case where there is only one distribution fiber between the RN and each ONU. Both architectures use only one interferometric filter located at the RN to simultaneously separate all downlink optical carriers and subcarriers, leading to a considerable cost reduction in the implementation of the WDM-PONs. Separated optical carriers are then reused and injected into reflective semiconductor optical amplifiers as the uplink light sources, which eliminates the necessity of specific wavelength sources at the ONUs. The downstream subcarrier signals are directly detected using baseband receivers. Two multichannel upstream and downstream transmission experiments are carried out at 1.25 Gb/s using the proposed schemes. The impact of optical carrier-to-subcarrier ratio of downlink signal, Rayleigh-backscattering noise, and wavelength mismatch between laser source and filter on system performance is also investigated.

Cladding-Mode Resonance in Polarization-Maintaining Photonic-Crystal-Fiber-Based Sagnac Interferometer and Its Application for Fiber Sensor

A novel cladding-mode-resonance-based polarization-maintaining photonic-crystal-fiber Sagnac interferometer (PMPCF-SI) and its application for fiber sensor are demonstrated. By introducing a mode field diameter mismatch at a splicing joint of an inbuilt single-mode fiber (SMF)-PMPCF-SMF structure connected in an SI, an intermodal interference is constructed by the excited cladding modes and the polarization modes of the PMPCF. With such a PMPCF-SI, simultaneous strain and temperature measurements are demonstrated. The SI behaves as a sensor and a filter simultaneously. By measuring the wavelength variation and transmissivity difference between two almost symmetrical resonance peaks in the spectral response of the SI, simultaneous strain and temperature measurements are achieved.

Performance Comparison of Using SOA and HNLF as FWM Medium in a Wavelength Multicasting Scheme With Reduced Polarization Sensitivity

In this paper, we propose and demonstrate the practical all-optical wavelength multicasting scheme based on the four-wave mixing (FWM) effect in both a highly nonlinear fiber (HNLF) and a semiconductor optical amplifier (SOA). Then we carry out comprehensive comparisons of the performance differences between using the HNLF and SOA for this proposed multicasting scheme. Seven multicast channels are experimentally demonstrated by using three co-polarized probes and a modulated signal. The merit of the proposed scheme is that the polarization sensitivity is significantly reduced from more than 20 dB to approximately 5 dB using the HNFL and 2.5 dB using the SOA. The low polarization sensitivity leads to a difference of less than 1 dB in the power penalty of the multicast channels at bit error rate . In addition, we compare the multicasting performance of the OOK and differential phase-shift keying (DPSK) signals using the proposed scheme and found that the power penalties of the DPSK multicast channels in the HNLF and the SOA are less than 1.25 and 1.1 dB respectively, and the power penalties of the OOK multicast channels are less than 1.5 dB and 3.1 dB in the HNLF and the SOA, respectively.

High-speed optical switch fabrics with large port count

We report a novel architecture that can be used to construct optical switch fabrics with very high port count and nanoseconds switching speed. It is well known that optical switch fabrics with very fast switching time and high port count are challenging to realize. Currently, one of the most promising solutions is based on a combination of wavelength-tunable lasers and the arrayed waveguide grating router (AWGR). To scale up the number of ports in such switches, a direct method is to use AWGRs with a high channel count. However, such AWGRs introduce very large crosstalk noise due to the close wavelength channel spacing. In this paper, we propose an architecture for realizing a high-port count optical switch fabric using a combination of low-port count AWGRs, optical ON-OFF gates and WDM couplers. Using this new methodology, we constructed a proof-of concept experiment to demonstrate the feasibility of a 256 x 256 optical switch fabric. To our knowledge, this port count is the highest ever reported for switch fabrics of this type.

Characteristics of Subcarrier Modulation and Its Application in WDM-PONs

We mathematically analyze and experimentally investigate the characteristics of subcarrier-modulated (SCM) light. Our investigation shows that the baseband data are also carried on the optical carrier after subcarrier modulation. This residual data introduce crosstalk when the optical carrier is separated from subcarriers and reused for uplink data modulation. The influence of the residual data carried on the optical carrier is studied in three kinds of upstream transmissions based on wavelength-seeded reflective semiconductor optical amplifiers (RSOAs) for carrier-reuse wavelength-division-multiplexed-passive optical networks (WDM-PONs). Results show that the crosstalk can be significantly suppressed with proper selection of the seeding power and operation conditions of downlink Mach-Zehnder modulator (MZM), including driving voltage and bias voltage.

A 448 × 448 optical cross-connect for high-performance computers and multi-terabit/s routers

We experimentally demonstrate the feasibility and dynamic switching performance of a 448 × 448 AWGR-based OXC with nanoseconds switching speed. To our knowledge, such a port-count is unprecedented for such switches.

All-optical modulation-transparent wavelength multicasting in a highly nonlinear fiber Sagnac loop mirror

An all-optical wavelength multicasting scheme using four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) Sagnac loop mirror has been demonstrated. This proposed scheme has the advantage that even when the wavelength of a multicast channel overlaps with the pump-pump generated idler, clear eye diagram can still be observed. Six and ten 10-Gb/s multicast channels, compliant with the ITU grid, are successfully obtained by using two- and three-pump lasers, respectively. Multicasting of on-off shift keying (OOK) and differential phase-shift keying (DPSK) signals are both successfully demonstrated. The maximum power penalty of the multicast channels is less than 3.5 dB. Furthermore, compared with the non-loop configuration, up to 1.2 dB power penalty improvement can be achieved in this proposed Sagnac loop configuration.

Light beam coupling between standard single mode fibers and highly nonlinear photonic crystal fibers based on the fused biconical tapering technique.

We propose and experimentally demonstrate light beam coupling between a single-mode fiber (SMF) and a highly nonlinear photonic crystal fiber (HN-PCF) based on the fused biconical tapering (FBT) technique. In our experiment, a standard SMF is pre-tapered to match its propagation constant to that of a HN-PCF. In order to remove the condensation in the air holes, the temperature is increased gradually to preheat the fibers. An appropriate level of hydrogen flow is administered to avoid the air-hole collapse. As a result, coupling ratio exceeding 90% between the SMF and HN-PCF is achieved. This technique avoids back Fresnel reflection, mode-field diameter (MFD) mismatch and fiber-core misalignment, bubble generation and air-hole collapse in the interface fusion splice.

Wavelength-agile hybrid passive optical networks with dynamic ONU re-grouping functionality

We propose a novel hybrid wavelength-division-multiplexed (WDM)/ time-division-multiplexed (TDM) passive optical network (PON) architecture for next-generation optical access networks. This PON is based on a novel virtual PON (VPON) mechanism that will allow wavelength channels to be flexibly allocated to specific ONUs in a dynamic fashion. Dynamic wavelength allocation (DWA) algorithms and dynamic bandwidth allocation (DBA) algorithms can be combined to optimize the use of bandwidth resources in such a wavelength-agile hybrid WDM/TDM network. We experimentally demonstrate the proposed architecture using a 10-Gb/s downlink and 1.25-Gb/s uplink transmission setup. The experiment results show that the proposed scheme has good transmission performances with dynamic ONU re-grouping functionality.

DC Performance of High-Quantum-Efficiency 1.3-

We present the dc performance of a high-quantum-efficiency GaNAsSb/GaAs p-i-n waveguide photodetector. GaNAsSb with N and Sb contents of 3.3% and 8%, respectively, is sandwiched by AlGaAs/GaAs cladding layers. Two types of device epilayer structures, i.e., with and without AlGaAs cladding layer, show high responsivity values of 0.72 and 0.55 A/W, respectively, at a reverse bias voltage of 10 V and a wavelength of 1.3 μm. These correspond to internal quantum efficiencies of 96.7% and 73.9%, respectively. A linear increase in photocurrent with an increase in optical power up to 8 mW and also a high reverse breakdown voltage of -16.6 V were obtained.