what is OSNR in optical transmission system and its threshold ?

In a DWDM system, the optical signal to noise ratio (OSNR) accurately reflects the quality of transmitted optical signals. The OSNR value is one of the criteria for determining the communication quality at the receive end. Therefore, the OSNR is a common indicator for measuring the quality of optical signals at the transport layer. As the name suggests, OSNR is a ratio of the optical signal power to the noise power. Assume that the total optical power Ps is and the amplifier spontaneous emission (ASE) noise power within the bandwidth (0.1 nm in most cases) of a signal wavelength is Pn . The OSNR is obtained using the following formula:

OSNR= Ps/Pn

The OSNR can be calculated if the optical signal power and the ASE noise power within 0.1 nm bandwidth are available. Optical power can be obtained using an optical spectrum analyzer (OSA). Therefore, the OSNR is generally measured using an OSA.