We experimentally demonstrate the dual synchronization of chaos in two pairs of Nd:YVO(4) microchip lasers in a one-way coupling configuration over one transmission channel. Dual synchronization is achieved when the optical frequency is matched between the corresponding pairs of lasers by using injection locking. We investigate the influence of optical injection from the two master lasers to one slave laser, and found that the dual synchronization is observed when the injection locking is achieved between either of the master lasers and the slave laser. Under the condition of the injection locking between both of the master lasers and the slave laser, the injection locking is alternately achieved and the accuracy of dual synchronization is degraded. We also confirm these results by numerical calculation.

We demonstrate wavelength division multiplexing with chaotic subcarriers in two pairs of one-way-coupled Nd:YVO4 microchip lasers. Two individual digital messages are encoded on two chaotic carriers with different wavelengths in two transmitters by the chaotic masking method, and the mixed signals are sent to two receivers. Dual synchronization of chaos is used to obtain the original chaotic waveforms in the receivers. Two messages are decoded by filtering the difference of the laser outputs between the transmitter and the receiver by use of a low-pass filter for each pair of lasers. The message recovery can be achieved more easily when messages with small amplitude and low frequency are used.

We have achieved dual synchronization of chaos in two pairs of one-way coupled Nd:YVO4 microchip lasers, using only one transmission channel, by experiment and numerical calculation. We observed the individual synchronization of chaos in each pair of two lasers by adjusting the optical frequencies for injection locking between the corresponding pairs. The achievement of dual synchronization is dependent on the injection-locking condition, which is different from the locking condition for a single pair of lasers because of the presence of an additional injection signal from the master laser of the other pair.

We propose a chaos-synchronization scheme using incoherent feedback to the pumping power in two microchip lasers. The population inversion of the slave laser is controlled for synchronization by using the detected signals of the peak heights of chaotic pulse intensities in the two lasers. Matching of the optical frequencies between the two lasers (i.e., injection locking) is not required for synchronization using this method. We numerically demonstrate the incoherent feedback method and investigate synchronization regions against parameter mismatching between the two lasers. Synchronization is maintained within a mismatching of 1% for all laser parameters, which implies that the difficulty in reproducing the synchronized laser pulses is very useful for applications of secure optical communications.