chapter 32, Ultrafast Photonic Processing Applied to Photonic Networks

Excerpt

32.1 Introduction

Ultrafast photonic processing is expected to play a major role in photonic networks and photonic sensing systems. At the bandwidth of 40 GHz or above, electronics imposes severe technology and economic constraints, which ultrafast photonic processing could advantageously remove.

The quasi-instantaneous response of Kerr nonlinearity in fibers makes it the most attractive effect to overcome bandwidth limitations. For simplicity, consider two optical beams of different wavelength copropagating in the same optical fiber; the intensity dependence of the refractive index leads to a large number of interesting nonlinear effects, namely, self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). These ultrafast phenomena could be favorably applied to photonic processing. One application is optical multiplexing, which is used as optical switches, multiplexers, and demultiplexers. Another possible application is wavelength conversion, which is to be used in wavelength division multiplexing (WDM) networks. The other application is supercontinuum (SC) generation, which is a promising technique for various applications in photonic networks and sensing systems.

In present photonic networks, there are two primary techniques for multiplexing data signals: optical time division multiplexing (OTDM) and WDM. Optical code division multiplexing (OCDM) is an alternative method for future options. Optical code division multiple access (OCDMA), encoding and decoding of a signal with a kind of temporal waveform (the so-called optical signature code), allows the selection of a desired signal. Different information bits can share nonerror-producing overlaps of time and wavelength. Simultaneous multiple access can thus be achieved without a complex network protocol to coordinate data transfer among the communicating nodes.

The performance of TDM systems is limited by the time-serial nature of the technology. Each receiver should operate at the total bit rate of the system. The allocation of dedicated time slots does not allow TDM to take advantage of statistical multiplexing gain, which is significant when the data traffic is bursty. In spite of the fact that TDM technologies are well matured and developed at up to some tens of Gbit∕s, another problem for the ultrahigh-speed TDM system is the upper limit of electronic circuit operation speed. Although the basic concepts of ultrahigh-speed TDM have been proposed, optical logics still need to be developed. In the WDM system, the available optical bandwidth is divided into fixed wavelength channels that are used concurrently by different channel signals. The problem using WDM is granularity of wavelength. It is limited in that it can only handle traffic on an optical channel of the wavelength path. This may waste the wavelength resources. One of the main applications of WDM systems will be large-capacity long-haul transmission systems because of the relative ease of the transmission technology. OCDMA offers an interesting alternative because neither time management nor frequency management of all nodes is necessary. OCDMA can operate asynchronously and does not suffer from packet collisions; therefore, very low latencies can be achieved. In contrast to TDM and WDM, where the maximum transmission capacity is determined by the total number of channel slots, OCDMA allows flexible network design because the signal quality depends on the number of active channels. Each multiplexing format has its own merits and application area.

In the first half of this chapter, applications in OTDM∕WDM are discussed. Some experimental demonstrations for OTDM∕WDM networks are reviewed: wavelength-band generation in Section 32.2, tunable wavelength conversion in Section 32.3, multiplexing format conversion in Section 32.4, and wavelength-band conversion in Section 32.5. In the latter half of this chapter, applications in OCDM∕WDM are discussed.