Abstract: In a Sagnac effect optical fibre gyroscope, which uses a phase nulling photo detection technique to drive a feedback loop controlling one or more non-reciprocal elements connected in series with the coil, a commutating switch reverses the direction of propagation of oppositely directed beams of light directed through the coil and non-reciprocal elements. The null balance is detected as the absence in the photodetector output of any a.c. component in phase or out of phase with the commutation switching. This removes a source of systematic error occasioned by drift in optical parameters of unbalanced components.

Abstract: In a Sagnac effect optical fibre gyroscope a commutating switch (SW) is included immediately before the Sagnac coil (C). Operation of the switch synthesises turning the coil upside down. This provides the advantages of producing an a.c. signal in response to a constant rotation rate and in certain arrangements remove a source of systematic error occasioned by drift in optical parameters of unbalanced components.

Abstract: In a closed-loop multiple data terminal data transmission system, each message includes in addition to a message portion, a prefix portion having at least the address of the terminal for which the message is intended. When such a message reaches a data terminal its address is checked, and if not for that terminal, the prefix and the message portions are repeated along the loop. If the message is for the terminal only the prefix is repeated along the loop, the message portion being retained at the intended terminal. The prefix with no attached message eventually reaches the originating data terminal where its arrival indicates that the message has reached its destination and that the loop is intact.

Abstract: An optical fibre data transmision system in which some terminals at least act as repeaters for received data. Each terminal is allotted a specified transmission time. Terminals are provided with optical fibre by-pass arrangements in case a repeating terminal fails.