Optical code division multiplexing (OCDM) is the other class of multiplexing techniques than time division multiplexing (TDM), wavelength division multiplexing (WDM) and space division multiplexing (SDM). OCDM has been proposed in mid '70s. It has been long since OCDM remains outside the mainstream of research community of optical communications, however, possible scarcity of the wavelength resource in future photonic networks, the simple access protocol as well as versatility of optical codes motivate recent growth of OCDM research activities. In this paper, first, fundamentals of OCDM concept are presented, highlighting optical encoding and optical time gate detection which realize time spreading/despreading. Next, current research activities of OCDM are reviewed by focusing particularly on the optical implementations and the proof-of-concept experiments. It is devoted to three categories; high bit rate point-to-point transmissions, gigabit multiple access, followed by optical path networks using optical code. Finally, future issues are briefly summarized.

We numerically and experimentally demonstrate for the first time a novel all-optical quantization technique using dense spectral slicing with a specially designed arrayed waveguide grating for orthogonal frequency division multiplexed signals. By using a mode-locked laser diode with low jitter, the quantization technique can be achieved a high-speed and low-jitter operation. Both numerical and experimental results confirm the feasibility of 10 GSample/s, completely linear 3-bit step quantization for photonic analog to digital conversion. This optical quantization technique will be beneficial for ultra-high-speed optical communication using digital signal processing.

Full-duplex transmission of 60.0 GHz and 59.6 GHz millimeter-wave (mm-wave) signals of 155.52-Mbit/s differential phase shift keying (DPSK) data, radio-on-fiber (ROF) signals over 25-km-long standard single-mode fibers (SMFs) is experimentally demonstrated for the first time using a single 2-RF-port electroabsorption transceiver (EAT). The simplification of base stations (BSs) is strongly required to realize cost-effective and high-reliability mm-wave wireless access. This single EAT detects a C-band ROF signal modulated by a mm-wave downlink signal and simultaneously modulates the L-band optical carrier by a mm-wave uplink signal. The BS mainly consists of the EAT, leading to a simple and low-cost BS. Optical pilot tones and optical bandpass filters are used for photonic downconversion and photonic upconversion, to convert frequencies between mm-wave signals and intermediate frequency (IF) signals in the optical domain. With the use of optical conversions, these signals have no significant fading problems. The simultaneous transmission of both up- and downlinks has been achieved with the BER of less than 10-9. Also the fading problems due to the fiber dispersion of photonic conversions are analyzed mathematically in this paper. The single-EAT BS will become a promising candidate for a ROF access system.

Target location estimation is one of many promising applications of wireless sensor networks. However, until now only few studies have examined location estimation performances in real environments. In this paper, we analyze the effect of walking people on target location estimation performance in three experimental locations. The location estimation is based on received signal strength indicator (RSSI) and maximum likelihood (ML) estimation, and the experimental locations are a corridor of a shopping center, a foyer of a conference center and a laboratory room. The results show that walking people have a positive effect on the location estimation performance if the number of RSSI measurements used in the ML estimation is equal or greater than 3, 2 and 2 in the case of the experiments conducted in the corridor, foyer and laboratory room, respectively. The target location estimation accuracy ranged between 2.8 and 2.3 meters, 2.5 and 2.1 meters, and 1.5 and 1.4 meters in the case of the corridor, foyer and laboratory room, respectively.

The newly developed GaAs-pin/SLM, that is structured with a GaAs-pin diode photodetector and a ferroelectric liquid crystal as the light phase modulator, shows the accumulative thresholding characteristic against the optical energy of the write-in pulse train. We experimentally investigate this characteristic and discuss its applications to optical parallel processings.

We propose a coherent optical and radio seamless network concept that allows broadband access without deployment of additional optical fibers within an optical fiber dead zone while enhancing network resilience to disasters. Recently developed radio-over-fiber (RoF) and digital coherent detection technologies can seamlessly convert between optical and radio signals. A millimeter-wave radio with a capacity greater than 10 Gb/s and high-speed digital signal processing is feasible for this purpose. We provide a preliminary demonstration of a high-speed, W-band (75–110 GHz) radio that is seamlessly connected to an optical RoF transmitter using a highly accurate optical modulation technique to stabilize the center frequencies of radio signals. Using a W-band digital receiver with a sensitivity of -37 dBm, we successfully transmitted an 18.6 Gb/s quadrature-phase-shift-keying signal through both air and an optical fiber.

A dynamic routing and wavelength assignment (RWA) algorithm encompassing physical impairment due to Four-Wave Mixing (FWM) is proposed, assuming conventional On-Off-Keying (OOK) modulation format. The FWM effect is one of the most severe physical impairments to be considered for the future photonic networks since the accumulation of FWM crosstalk causes a fatal degradation in the wavelength-routed optical network performance. A novel cost function is introduced based upon an impairment-constraint-based routing (ICBR) approach, taking into account the network utilization resources and the physical impairment due to FWM crosstalk. Simulations results show that the proposed algorithm leads to a more realistic system performance compared to those of related approaches of dynamic RWA that fail to consider physical impairments into the routing scheme.

Fourier holographic image storage and reconstruction using BaTiO3 photorefractive crystal waveguide is investigated. The phase conjugation technique, which compensates image distortion caused by modal phase dispersion, successfully retores images stored in a test BaTiO3 crystal waveguide.

In a centralized localization scenario, the limited throughput of the central node constrains the possible number of target node locations that can be estimated simultaneously. To overcome this limitation, we propose a method which effectively decreases the traffic load associated with target node localization, and therefore increases the possible number of target node locations that can estimated simultaneously in a localization system based on received signal strength indicator (RSSI) and maximum likelihood estimation. Our proposed method utilizes a threshold which limits the amount of forwarded RSSI data to the central node. As the threshold is crucial to the method, we further propose a method to theoretically determine its value. We experimentally verified the proposed method in various environments and the experimental results revealed that the method can reduce the load by 32-64% without significantly affecting the estimation accuracy.

In this paper, we comparatively evaluate two photonic packet switch architectures with WDM-FDL buffers for synchronized variable length packets. The first one is an output buffer type switch, which stores packets in the FDL buffer attached to each output port. Another is a shared buffer type switch, which stores packets in the shared FDL buffer. The performance of a switch is greatly influenced by its architecture and a packet scheduling algorithm. We compare the performances of these two packet switches by applying different packet scheduling algorithms. Through simulation experiments, we show that each architecture has a parameter region for achieving better performance. For the shared buffer type switch, we found that void space introduces unacceptable performance degradation when the traffic load is high. Accordingly, we propose a void space reduction method. Our simulation results show that our proposed method enables to the shared buffer type switch to outperform the output buffer type switch even under high traffic load conditions.

This paper describes an all-optical label swapping for the photonic label switching router (LSR). The optical code routing photonic LSR in which label is mapped onto an optical code is one of the most promising photonic network technologies. It utilizes such unique features of optical code division multiplexing (OCDM) as asynchronous transmission, tell-and-go access protocol, and high degree of scalability. In practical photonic LSRs, all optical code conversion will play an important role. All-optical code conversion of 10 Gbit/s binary phase-shift keying (BPSK) codes by use of cross-phase modulation (XPM) in an optical fiber without wavelength-shift is proposed for the photonic LSR and experimentally demonstrated.

The dense wavelength division multiplexing (DWDM) technique is very attractive for effectively increasing the channel capability, even for access networks. Some DWDM radio-on-fiber (ROF) systems have been studied recently. In those systems, fiber Bragg gratings (FBG) or arrayed waveguide gratings (AWG) were used to demultiplex DWDM ROF signals. In this report, an alternative channel-selection scheme of DWDM millimeter-wave-band ROF signals by optical heterodyne detection with dual-mode local light is newly proposed. Error-free demultiplexing and transmission over a 25-km-long SMF of the DWDM signal, which consists of two 60-GHz-band, 155-Mb/s-DPSK ROF signals, are demonstrated.

Error-free transmission of image fiber-optic two-dimensional (2-D) parallel interconnection using vertical-cavity surface-emitting laser (VCSEL)/photodiode (PD) arrays is demonstrated. Simple constructions of transmitter/receiver modules are proposed. Optical alignment is achieved without power-monitoring. Crosstalk from an adjacent channel was -34 dB. Misalignment tolerance for a BER of less than 10-9 was 85 µm. The results clearly indicate that the interconnection system built around an image fiber and 2-D VCSEL/PD arrays has promise for use in the highly parallel high-density optical interconnects of the future.

All-optical wavelength conversion and modulation format conversion will be needed in the next generation high-speed optical communication networks. We have proposed and successfully demonstrated the error free operation of all-optical modulation format conversion from NRZ-OOK to RZ-BPSK using SOA based MZI wavelength converter. In this paper, we experimentally investigate the wavelength conversion characteristics of the proposed NRZ-OOK/RZ-BPSK modulation format converter. The results show that error free modulation format conversion is possible over the entire C band.