PHOTONIC NETWORK TECHNOLOGY

  • Optical Packet Switching

    In the area of optical packet switching, the research is focused on the implementation of photonic add/drop multiplexers for packet- and circuit-switching based on photonic crystal structures. Our research group also develops a semiconductor-based optical switch using self-imaging properties of multimode interference (MMI) couplers. Each device consists of an input guide, a multiway splitter, an array of individually addressed waveguide phase shifters, a multiway combiner, and an array of output guides. By controlling the current applied to the phase shifters, light from the input guide can be switched to any output guide at high speeds based on the principle of self-imaging in MMI couplers.

    The research aims at reaching the operation speed of 40 Gbit/s and to increasing the channel number.

  • Optical code-division multiplexing access

    The research focuses on optical networks based upon optical-code-division-multiplexing (OCDM) technology, which has originally been proposed by our group as the next-generation high-speed optical network architecture. Comparing to the wavelength-based optical networks, the OCDM-based optical networks enable finer bandwidth granularities, in which distinct lightpaths can be multiplexed on one wavelength by employing optical codes (OCs). Although the features exist in OCDM-based optical networks as well as in OCDMA systems in the optical layer, we put our efforts in the network layer to provide fundamental investigation of OCDM-based optical network regardless of the underlying platform. Our main focus points are switch architecture, QoS, routing and resource (including wavelength and OC) allocation, security, and multi-layer switching.

  • Routing and wavelength assignment

    The research focuses on designing and optimizing the dynamic lightpath establishment with nonlinear transmission impairments awareness in wavelength routed networks. By incorporating the four-wave mixing (FWM) effect into the routing-based constraint, we consider one of the problems in the future transparent high-speed transmission systems. The FWM imposes a severe limitation on the maximum launched power of individual DWDM channels especially at bit rates of 40 Gbit/s and above causing a significant impact on the teletraffic performance. The research target is to model and incorporate nonlinear physical impairments (FWM, XPM, SPM) into the dynamic routing and wavelength assignment (RWA). This will allow assessing a realistic photonic network performance while guaranteeing the quality of signal during the lightpath setup.

  • Optical RAM buffer

    In the current network architectures, processing in nodes is performed electronically, which is expected to become a bottleneck in 40 Gbit/s DWDM systems. Optical processing in the nodes may be a solution to this problem. An indispensable node element is a buffer but until now, no technology has been able to provide an optical equivalent of RAM.

    In recent years, photonic crystals produced with help of nano-technology, have attracted attention by realizing many optical components, including optical RAM. The research is focused on the architecture of all-optical routers employing optical RAM with the use of photonic crystal properties.

  • Radio-over-fiber communication system

    Millimeter-wave-band (mm-wave-band) radio-over-fiber (RoF) networks are foreseen as the next paradigm to provide broadband capability to wireless access networks. In mm-wave-band RoF networks, wavelength division multiplexing (WDM) technique is very attractive to broadly distribute RoF signals to a large number of access points (APs) via a remote node (RN) with the minimum final economical cost, where the AP corresponds to antenna base station (BS) for wireless access networks. We have been developing mm-wave-band optical analog signal generation and transmission techniques, optical frequency up- and down-conversions, and optical-frequency-interleaved dense WDM. Photonic dynamic channel allocation in DWDM mm-wave-band RoD access network is a new hot topic, which allow to overlay RoF system in the digital WDM-PON in the near future.

  • Optical performance monitoring

    The research is focused on developing an optical performance monitoring (OPM) system for phase-modulated signals in metropolitan part of the network. The target is to create a monitoring system, which can continuously supervise the level of impairments (OSNR, chromatic dispersion, and PMD) without disrupting the operation of the entire system. An operating OPM system will replace the electronic monitoring methods by providing both standard (Q and BER), as well as unique analogue (level of impairment) quality measures.

  • Access Networks

    The wavelength division-multiplexed passive optical network (WDM-PON) has been considered as one of the promising solutions for high-speed access network such as fiber-to-the-home (FTTH) by providing a dedicated point-to-point link to each optical network user (ONU). However, the relatively high cost of light sources operating at unique wavelength and difficulty in broadcasting video services are major obstacles to the practical implementation of WDM-PON.

    In our research, we have developed a new technique, which allows transmitting the upstream/downstream data and broadcasting signals to each optical network unit (ONU) over a single wavelength. This technique utilizes a cost-effective broadband light source (BLS) for external injection source and broadcast signal, and a Fabry-Perot laser diode and reflective semiconductor optical amplifier (RSOA) for downstream and upstream modulation, thereby providing cost-effective colorless operation. Thus, this technique can considerably reduce the initial cost of light sources and increase the scalability of WDM-PONs.

    In the area of access networks, we also work on radio-over-fiber (RoF) technologies for fiber-to-the-house solutions. This field aims to provide wireless connectivity over optical networks. Current fiber-to-the-home (FTTH) system is based on time division multiple access (TDMA) passive optical network (PON). In the future, it is expected that the network service demands will increase, therefore upgrading from the current FTTH becomes essential. Optical code division multiple access (OCDMA) is one promising candidate for next-generation broadband multiple access technique to achieve high-speed connectivity. The research focuses on enhancing the spectral efficiency of OCDMA technique.

[TOP]
OPTICAL SIGNAL PROCESSING TECHNOLOGY

  • Optical A/D Conversion

    The research is focused on all-optical ultra-fast A/D converter using transfer function of nonlinear optical loop mirrors (NOLMs). All-optical A/D conversion, in which the sampling, quantization, and coding are performed in optical domain, has a potential to overcome the speed limitation. The potential of sampling speeds of hundreds GS/s is expected for NOLM due to the ultra-fast optical Kerr effect.

  • Advanced modulation formats

    The research analyzes the application of phase-modulated formats to modern, high capacity and long-haul transmission systems. The research is concentrated on the timing and phase jitter.

  • Optical Delay

    Recently, tunable optical delay lines based on slow light in optical fiber have gained attention in the context of optical buffers. The research focuses on an all-optical tunable delay line based on soliton self-frequency shift (SSFS) in an optical fiber. Assuming that the width of the launched pulse is less than 1ps, a shift towards a longer wavelength occurs during transmission in optical fibers. As a result, the pulse is delayed. The amount of the temporal shift depends on the power of the initial pulse, so the amount of the delay can be varied. The combination of the tunable delay line as optical buffer and the wavelength converter is highly effective in preventing packet contention. Further research will aim at adding a noise reduction function to the proposed system.

  • Format Conversion

    In future optical networks, different modulation formats may be selectively used depending on the network size and the bit rate. PSK modulation formats are more suitable for long haul backbone networks and the low cost OOK modulation format is feasible for the metro networks, OOK to PSK conversions can be implemented in the node connecting the two networks. We investigate a novel all-optical modulation format conversion from NRZ-OOK to RZ-M-ary PSK based on optical fiber nonlinearity and using the semiconductor optical amplifier based Mach-Zehnder inteferometric (SOA-MZI) wavelength converter.

[TOP]
SENSOR NETWORK TECHNOLOGY

  • COLORS

    Recently, wireless sensor networks composed of network nodes have gained a lot of attention. In wireless sensor networks, location information on network nodes is often required. For example, estimated locations of customers and visitors are used to analyze their interests in supermarket products or museum exhibits.

    We have proposed a location estimation system based on maximum likelihood estimation using received signal power, called “COLORS” (COllaborative LOcalization with RSSI)". Additionally, we proposed a multihop network protocol robust to the time-varying characteristics of the wireless link and a data collecting method suited for COLORS. Our focus is on improving the various performance characteristics of COLORS elements.

[TOP]