What signal does an SFP optical module send

An SFP optical module outputs an optical signal (light) that represents the electrical data from the host device, typically using a laser diode for high-speed transmission.

Overview

An SFP (Small Form-factor Pluggable) module is a hot-pluggable transceiver that converts electrical signals from a switch or router into optical signals for transmission over fiber, and vice versa for reception . The module contains a Transmitter Optical Sub-Assembly (TOSA), which generates the optical signal, and a Receiver Optical Sub-Assembly (ROSA), which converts incoming light back into electrical signals .

Optical Signal Characteristics

• Type of Light Source: Most modern SFP modules use laser diodes (LDs), which produce coherent light with a narrow spectral linewidth, enabling efficient fiber coupling and long-distance transmission . Some low-speed or short-distance modules may use LEDs, but these are less common.
• Wavelength: The optical output wavelength depends on the module type and fiber used. Common wavelengths include 850 nm for multimode fiber and 1310 nm or 1550 nm for single-mode fiber .
• Modulation: The optical signal is modulated to represent digital data, typically using non-return-to-zero (NRZ) or pulse-amplitude modulation (PAM) depending on the data rate and SFP type .
• Power Control: SFP modules often include an Automatic Power Control (APC) circuit to maintain stable optical output power across temperature variations, ensuring consistent signal quality .

Electrical Interface

While the module outputs an optical signal to the fiber, it receives differential electrical signals (TX+/TX-) from the host device and converts them into light. Similarly, incoming optical signals are converted back into differential electrical signals (RX+/RX-) for the host . This ensures seamless integration with network equipment.

Summary

In essence, the SFP module outputs an optical signal over fiber that encodes the original electrical data from the host device. The signal's quality, wavelength, and power are carefully controlled to ensure reliable high-speed communication across the intended fiber type and distance .