This letter proposes an improved polarization diversity method for heterodyne/coherent optical transmission systems. It is theoretically clarified that this method can reduce the noise caused by polarization fiuctuations and can receive a binary FSK or PSK modulated signal with high sensitivity.

The wavelength shift of a dynamic-single-mode (DSM) laser directly modulated by sinusoidal current has been investigated using the large-signal analysis. A rate equation which includes carrier diffusion terms is solved numerically. The variation of the carrier density as an origin of the dynamic wavelength shift is calculated. The calculated dynamic wavelength sifts are in good agreement with experiments. At high modulation frequency range, the dynamic wavelength shift is mainly caused by relaxation oscillations and the shift is almost constant to the stripe width of active guide. It shows that the dynamic wavelength shift in pulse modulation is also independent of the stripe width of active guide. An analytical expression of dynamic wavelength shift is given at low modulation frequency range. When the modulation depth is below 30 percent, the small-single analysis is in agreement with large-signal analysis. By considering the effect of spontaneous emission, a modified small-signal analysis presented here is applicable to a modulation depth of 100 percent in the case of low frequency limit.

This article briefly reviews the elementary concepts of the quantum communication theory. Specific topics on some of the optical communications by utilizing quantum nature of light wave will be discussed, especially for applications to information transmission of classes of squeezed state exceeding the present limitations, and for quantum coding theory relevant the photon communications.

Optical communication systems using quantum state control have received much attentions. For this quantum state control communication, however, one of the most serious problems is the effect of transmission loss which degrades the advantage of the quantum state controlled signal. In optical long transmission communications, in general, transmission loss is large so that the advantage using quantum state controlled signal is hardly lost and performance of quantum state control communications is almost equal to that of conventional communications. So it is necessaty for a new application of quantum state control to overcome the standard quantum limitation which is achieved by the conventional coherent communication system. In this paper, we propose a realization of a new optical communication system with received quantum state controller, so called Received Quantum State Control (RQSC) system, in order to cope with transmission loss problem. This system can provide the advantage of quantum state control regardless of amount of transmission loss. Particularly, it is shown that the system can overcome the standard quantum limitation which corresponds to the limitation achieved by conventional BPSK homodyne system using coherent state signal. This corresponds to Quantum Receiver" of the broad sense in Helstrom's receiver theory.

An intermediate-frequency-combining (IF-combining) polarization diversity using frequency conversion is proposed. The proposed diversity requires no phase controller as opposed to the conventional IF-combining diversity. It has been theoretically clarified that this diversity has polarization insensitive bit-error-rate (BER) characteristics. The effectiveness has been confirmed by experiments in which the sensitivity dependence on the polarization is suppressed to within 0.8dB and a stable 101km fiber transmission at 600Mbit/s is achieved.