Abstract: The system includes two transmitters (T1, T2) and two receivers (R1, R2). One transmitter (T1) may include a wavelength tunable laser, sending an FSK optical signal (OS1) which has a constant intensity. In a first receiver (R1) this FSK optical signal (OS1) is transformed into an IM signal (OS1') by an optical pass band filter. This IM signal (OS1') is then detected by a first direct detection receiver (DM). In a second direct detection receiver (R2) the FSK optical signal (OS1) is not detected, since it has constant intensity. A second transmitter (T2) includes a Fabry Perot laser or a LED with a broad linewidth, sending an IM optical signal (OS2). This IM optical signal (OS2) is detected by the second direct detection receiver (R2). It is not detected by the first direct detection receiver (DM), because it is strongly attenuated when passing through the narrow bandpass optical filter (TM) before being applied to the first direct detection receiver.

Abstract: A protection arrangement protects an optical transmitter device with n optical transmit circuits or an optical receiver device with n optical receive circuits. It includes fault evaluating circuitry to detect a defective transmit/receive circuit, a spare transmit/receive circuit, and switching circuitry. Upon detection of a defective transmit/receive circuit by the fault evaluating circuitry, the fault evaluation circuitry controls the spare transmit/receive circuit and the switching circuitry to bypass the defective transmit/receive circuit with the spare transmit/receive circuit.