Theoretical and experimental investigations of chaos synchronization and its application to chaotic data transmissions in semiconductor lasers with optical feedback are presented. Two schemes of chaos synchronization-complete and generalized synchronization-are discussed in the delay differential systems. The conditions for chaos synchronization in the systems and the robustness for the parameter mismatches are studied. The possibility of secure communications based on the chaos masking technique in semiconductor lasers with optical feedback is also discussed, and message transmission of a 1.5-GHz sinusoidal signal is demonstrated. The method of bandwidth enhancement of chaotic carriers is proposed for broad-band chaos communications.

Narrow-stripe edge-emitting structure is not the only one for semiconductor lasers. Other than these, various kinds of laser structures of semiconductor lasers have been proposed and some of them are now in practical use. For example, self-pulsating semiconductor lasers are used for light sources of optical mass data storage systems, vertical cavity surface-emitting semiconductor lasers (VCSELs) are expected as the next generation laser light sources for optical communications and optical memory systems, and broad-area semiconductor lasers are promising light sources for high power laser applications. Quantum-dot semiconductor laser is a new laser structure expected as a light source for highly coherent beam emission. The region of light emission from the laser is well confined in a certain spatial point in the active area, namely a quantum dot, and the energy levels related to light emission are perfectly quantized by a quantum-dot structure. Another example of recent semiconductor laser is quantum-cascade laser, which is a THz light source. Though a quantumcascade laser is one of the semiconductor lasers, the laser structure and light emission process are completely different from other semiconductor lasers based on interband optical transitions. Such a laser also shows different dynamics from conventional semiconductor lasers. They have their own unique characteristic properties. Here, we do not discuss the details of each device structure and its characteristics, but we introduce the rate equations for such lasers and present their dynamic properties. These new laser structures have extra degrees of freedom and show instabilities and chaotic dynamics without any introduction of external perturbations. In this chapter, we discuss the dynamics of these new lasers both for solitary oscillations and external perturbations.

Synchronization of chaotic oscillations was observed in mutually injected vertical-cavity surface-emitting lasers (VCSELs) in a low-frequency fluctuation regime. In the experiments, only one of the two polarization modes (x mode) showed synchronized oscillations, and the other polarization components (y mode) were synchronized as a result of the effect of anticorrelated oscillations that is a characteristic feature of VCSELs.

In this paper, synchronization of chaotic oscillations in semiconductor lasers by optical injection in a transmitter-receiver configuration is studied numerically. A chaotic signal is generated from a semiconductor laser with optical feedback in the transmitter and is injected into the receiver laser without optical feedback. We examined the conditions of chaotic synchronization in the system. As a result, we observed complete chaos synchronization within a finite area of very small parameter mismatch between the two laser systems. In addition, chaotic oscillation synchronized by amplification phenomena was observed in the ordinary injectionlocking regime with higher optical injection ratios. We demonstrated that synchronization of chaotic oscillations by amplification results from a kind of injection locking under chaotic light injection and is very tolerant to the parameter mismatch between the two lasers, in contrast to the complete synchronization case.

Bandwidth-enhanced chaos synchronization in strongly injection-locked semiconductor lasers with optical feedback is numerically studied based on laser rate equations. The bandwidth of the chaotic carrier frequency in a semiconductor laser with optical feedback is expanded roughly three times by strong optical injection compared with the bandwidth when there is no optical injection. Using a bandwidth-enhanced semiconductor laser as a chaotic transmitter and receiver, we synchronized transmitter and the receiver lasers in a complete chaos synchronization scheme.