Optical module performance is primarily determined by transmit optical power and receiver sensitivity, which together define the effective signal range and reliability of optical communication links.

Transmit Optical Power (TX Power)

Transmit optical power represents the strength of the optical signal emitted by the module's laser under normal operating conditions, typically measured in dBm (decibels relative to 1 milliwatt) or milliwatts (mW) ( ). It indicates how much light energy is sent into the fiber and is influenced by the data pattern, modulation, and wavelength. Maintaining the correct TX power is critical: too low can cause signal attenuation and errors, while too high can saturate the receiver or induce nonlinear effects in the fiber ( ). For example, IEEE 802.3 standards specify that 1.25G SFP modules should operate between -3 dBm and -9 dBm for 20 km links, while 10G LRM modules typically range from -6.5 dBm to -1 dBm for shorter 220 m links ( ).

Receiver Sensitivity (RX Sensitivity)

Receiver sensitivity is the minimum optical power that the receiver can detect while maintaining a low bit error rate (BER), often defined as BER ≤ 10⁻¹² ( ). It is also measured in dBm. A lower RX sensitivity value indicates better performance, meaning the receiver can detect weaker signals. Sensitivity depends on the receiver type: APD (Avalanche Photodiode) receivers typically offer 2–5 dB better sensitivity than conventional PIN photodiodes, which helps compensate for transmission losses over long distances ( ).

Minimum Receiver Power vs. Received Optical Power

• Minimum Receiver Power: The guaranteed lowest optical power at which the module operates reliably under worst-case conditions, including temperature variations and aging ( ).
• Received Optical Power: The actual optical power range the receiver can handle, with the lower limit being the RX sensitivity and the upper limit being the overload power ( ). Signals below RX sensitivity may not be detected, while signals above overload power can cause bit errors.

Link Budget Considerations

The link budget calculates the total allowable loss in an optical path, considering TX power, RX sensitivity, fiber attenuation, and connector losses ( ). Properly matching TX power and RX sensitivity ensures stable communication. For high-loss links (>15 dB), long-distance modules with APD receivers are recommended to maintain signal integrity ( ).

Extinction Ratio

The extinction ratio measures the ability of the transmitter to distinguish between logical "1" and "0" signals, defined as the ratio of optical power when transmitting "1"s to that when transmitting "0"s ( ). A higher extinction ratio improves signal clarity and reduces bit errors.

Summary

In optical modules, TX power determines how strong the signal is sent, while RX sensitivity defines the minimum detectable signal. Together, they dictate the maximum transmission distance, link reliability, and overall network performance. Understanding these parameters is essential for selecting appropriate modules, designing optical links, and troubleshooting network issues ( ).