Monitoring of Optical Signals

An apparatus and method for monitoring of optical signals 60 at a node (12, 14, 16) in a WDM telecommunications system 10 comprising the employment of photodiodes (54, 56, 58). Each of the photodiodes (54, 56, 58) has a short response time relative to a number of bit periods of the optical signal to permit measurement of the optical power thereof. Such a photodiode (54, 56, 58) can be used to monitor many nodes (12, 14, 16) within the system 10 and facilitates monitoring of optical signals in nodes which are far apart. The photodiode (54, 56, 58) also permits the Optical Signal to Noise Ratio (OSNR) of the optical signal 60 to be calculated by obtaining values for a maximum optical power P1 and a minimum optical power P0 for a particular optical signal 60.