On Off Ratio of Optical Transmitters

The on/off ratio of an optical transmitter, also called the extinction ratio, is the ratio of optical power when the transmitter is on (logic 1) to when it is off (logic 0), typically expressed in dB or as a percentage.

Definition and Formula

The on/off ratio quantifies how effectively an optical transmitter distinguishes between logic levels. It is defined as:

where $P_{\text{on}}$ is the optical power when transmitting a '1' and $P_{\text{off}}$ is the power when transmitting a '0' . In decibels, it is expressed as:

A higher extinction ratio indicates a clearer distinction between logic levels, which reduces bit errors and improves system performance.

Typical Values

For practical optical transmitters using on-off keying (OOK):

• Short-range or data center modules often have ER values around 10 dB .
• High-performance systems may achieve 13 dB or higher, depending on the laser type and modulation scheme .
• In percentage terms, an ER of 10 dB corresponds to an on/off power ratio of about 10:1.

Impact on System Performance

The extinction ratio directly affects the bit-error rate (BER) and the maximum transmission distance. A low ER reduces the signal contrast, making it harder for the receiver to distinguish between '1' and '0', which can increase BER and require additional amplification or regeneration . Conversely, a very high ER improves signal integrity but may increase power consumption or stress the laser.

Measurement

The ER is commonly measured using an eye diagram, which visually represents the optical signal levels over time. The vertical opening of the eye corresponds to the difference between $P_{\text{on}}$ and $P_{\text{off}}$, allowing engineers to calculate the extinction ratio and assess transmitter quality . In summary, the on/off ratio (extinction ratio) is a critical parameter for optical transmitters, typically ranging from 10–13 dB in standard systems, and it plays a key role in ensuring reliable optical communication .