Glossary entry

Optical signal-to-noise ratio (OSNR)

The ratio of optical signal power to optical noise power in a specified reference bandwidth, in dB. Standard performance metric for optical communication links.

The optical signal-to-noise ratio in decibels is

\text{OSNR} \;=\; 10 \log_{10}\!\left( \frac{P_\text{signal}}{P_\text{noise}} \right),

both measured in a specified reference optical bandwidth. The reporting convention for telecom is a 0.1 nm reference bandwidth, which corresponds to approximately 12.5 GHz at 1550 nm.

OSNR scales with bandwidth — a wider measurement bandwidth captures more noise but the same signal. Conversion between reference bandwidths:

\text{OSNR}_\text{dB}(B') \;=\; \text{OSNR}_\text{dB}(B) + 10 \log_{10}(B / B').

For an amplified link, OSNR degrades through the cascade of amplifiers. The accumulated noise from $N$ amplifiers with noise factors $F_i$ produces:

\frac{1}{\text{OSNR}_\text{out}} \;\approx\; \sum_{i=1}^{N} \frac{F_i}{P_i / (h\nu \, B_\text{ref})},

where $P_i$ is the per-channel input power to amplifier $i$ and $B_\text{ref}$ is the reference bandwidth. For identical-loss spans with identical EDFAs and per-channel input power $P_\text{in}$ :

\text{OSNR} \;\approx\; \frac{P_\text{in}}{N \cdot h\nu \cdot B_\text{ref} \cdot F},

where $F$ is the linear noise figure.

Typical required OSNR at 0.1 nm reference bandwidth:

Modulation format	Bit rate	Required OSNR (BER = $10^{-12}$ )
NRZ on-off keying	10 Gb/s	15 – 18 dB
NRZ OOK	40 Gb/s	19 – 22 dB
DPSK	40 Gb/s	17 – 20 dB
DP-QPSK coherent	100 Gb/s	13 – 16 dB
DP-16QAM coherent	200 Gb/s	18 – 21 dB
DP-64QAM coherent	600 Gb/s	27 – 30 dB

Higher-order modulation formats require higher OSNR to maintain the same bit error rate.

Measurement: OSA scan with the per-channel signal power read at the channel peak and the noise level interpolated between channels. Polarization-nulling techniques (subtracting orthogonal polarization measurements) provide direct in-band OSNR measurement for polarization-multiplexed signals where simple interpolation fails.