Abstract: An optical network comprises a central source providing light in a plurality of spaced wavelength bands and including variable-gain optical amplifiers enabling the relative intensity of light in respective wavelength bands to be varied. A wavelength-routed network (preferably but not necessarily passive) receives light in all the wavelength bands from the central source and routes each wavelength band to a respective terminals. The terminals are operable to modulate and return received light in any of the said wavelength bands, and may be simple and identical, requiring no precision light source, as would otherwise be needed to allow multiplexing to use optical fibre efficiently. The operator can adjust the semiconductor optical amplifiers to illuminate or disable individual terminals, as required, and can alter the intensity (and if required type) of light supplied to the terminals to maintain a satisfactory signal level, minimising need for engineer visits.

Abstract: An optical signal transmitter has an optical source for generating broad-band optical radiation, a wavelength division multiplexer optically linked to the optical source and operable to receive and slice spectrally the broad-band optical radiation, and at least one optical signal generator optically linked to the wavelength division multiplexer to receive a spectral slice of the broad-band optical radiation.

Abstract: In a method of communication using quantum cryptography, single-photon signals from a highly attenuated source are modulated and subsequently detected. In the step of detecting the modulated signal, the signal is split between two branches according to its encoded statc. Signals corresponding to different encoded states are detected independently in the respective branches and the rate of detection of coincident signals is determined. This rate is compared with a threshold to detect the presence of an eavesdropper.

Abstract: A method of communication based on quantum cryptography is modified to include an initial step of outputting from, e.g., a transmitter, a single-photon signal, which may be unmodulated. A receiver then randomly selects one of a plurality of encryption alphabets corresponding to different, non-commuting quantum mechanical operators. The receiver modulates the single-photon signal with the selected operator and returns the signal to the transmitter. The transmitter in turn randomly selects a quantum mechanical operator and uses that operator in detecting the returned signal modulated by the receiver. Alternatively, the transmitter may randomly select one of a plurality of encryption alphabets and use that encryption alphabet in modulating the signal. The signal is then further modulated at the receiver using a predetermined encryption alphabet. When the signal is received back at the transmitter, it is detected using the same quantum mechanical operator as was initially used to modulate it.

Abstract: A method of communication uses quantum cryptography to encode and decode a signal. A key is distributed on a quantum channel, and information subsequent communicated between the transmitter and receiver on a public channel to determine whether the key has been intercepted. A common transmission medium is used for the quantum channel and public channel. A calibration signal is transmitted over the public channel on the common transmission medium to calibrate the system for the subsequent transmisison of a key on the quantum channel. The trasmission medium may be an optical fibre and the transmitter may switch between a single-photon output and a multiple-photon output to provide quantum channel and public channel respectively.