Chaos-based communication has been an extremely hot issue for its potential security due to its broadband spectrum, random and noise-like carrier. With the further investigation of chaotic communication, researchers acquire lots of substantial achievements; even message has been successfully extracted with low to 10-7 bit-error-rate after 120km transmission in commercial fiber communication network. With the development of unidirectional chaos-based communication, bidirectional communication utilizing one pair of transceiver has been becoming a seething topic in chaotic communication. Presently, chaos synchronization, as the indispensable condition of chaos-based communication, has been obtained using mutually coupled lasers with or without individual feedback. These schemes can obtain chaos synchronization mainly containing two types: achronal synchronization and isochronous synchronization. Achronal synchronization usually occurs in mutually coupled lasers without feedback with weak or moderate coupling strength. However, this type of synchronization can not be used in bidirectional communication for that only the laggard laser has chaos pass filter and that the leader role of two lasers is time dependent. Isochronous synchronization means the two lasers’output are synchronous with no time delay; thus when one laser’s output reaches its counterpart, they will not synchronize with each other unless the counterpart’s output delays via a delay line equaling to the transmission distance. Hence, this type of synchronization is also unpractical for communication. Recently, it has been reported that stable achronal synchronization can be acquired by extremely unsymmetrical mutual injections. Nevertheless, this type of chaos synchronization is unwieldy for practical bidirectional communication for that only the injection signal and output are synchronous at the strong injected laser side, which means delay line is also indispensable at the strong injecting laser side. More attentively, when these types of synchronization are adopted in practical bidirectional communication, the total delay line induced by transmission is as twice times as transmission distance which bring discommodity to bidirectional communication. In allusion to the problem of too long delay line, this thesis carries through some work to attempt to avoid the transmission-induced delay line as follows:1. We propose two protocol schemes to exempt the transmission-induced delay line. One is that mutual injection-locking synchronization based on two strongly mutually coupled lasers; and the other is that bidirectional injection-locking synchronization via two unsymmetrical mutual coupling dual-mode lasers.2. We demonstrate the former scheme that using two strongly mutually coupled lasers to avoid delay line. First of all, mutual injection-locking synchronization is numerically carried out. Also, the factors that affect the chaos synchronization quality and parameters mismatch are studied. In addition, by using this type of chaos synchronization, bidirectional communication without delay line is primarily demonstrated. Finally, Mismatch in message amplitudes or rates are also investigated.3. The latter scheme is also demonstrated. We obtain the bidirectional synchronization using two unsymmetrical mutually coupled dual-mode lasers. The effect of parameters and parameters mismatch on the chaos synchronization quality is also analyzed. The synchronization mechanism is discussed. At last, we carry through the examination of bidirectional message transmission without delay line, but we fail to extract the message. We analyze the possible reasons for the failure.4. We sum up our work and give expectation and some advice on bidirectional communication.