SFP's Structure And Principle

- Aug 20, 2019-

The SFP optical transceiver is mainly composed of three parts: the TOSA component, the ROSA component, and the PCBA board.

SFP optical transceiver

Transmission component TOSA

TOSA is the main part of the optical module, wherein the LASER Diode is the core, the LD chip and Monitoring photodiode (MD) and other components which are packaged in the TO coaxial to form the light emitting component TOSA.

The Laser’s classification and Features.

LED laser: wavelength is 850nm/1310nm. Can only be used for transmission around 100m.

FP laser: wavelength is 1310nm/1550nm. Can be used for medium-distance high-speed transmission.

DFB laser: wavelength is 1310~1550nm. With high cost, can provide higher power for long-distance transmission.

VCSEL laser: wavelength is 850nm/1310nm. With low cost, can be used for short-distance transmission.

EML laser: wavelength is 1550nm. It’s expensive and be used for high-speed long-distance transmission.

Receiving component ROSA

The function of the receiving component is to convert the weak optical signal into an electrical signal, and amplify and shape the electrical signal into the original input.

The photodetector and the pre-transistor amplifier and other components are packaged in a compact structure to form the receiving component ROSA. The commonly ROSA includes PIN-TIA and APD-TIA.


Photodetector use photoelectric efect to convert the optical signals into electric signals. Most commonly used of photodetector are PIN and APD.

APD is a high-sensitivity photodetector that multiplies photocurrent by avalanche multiplication effect, which can improve the receiving sensitivity, but needs to provide a high voltage of 30~60V.

Trans-Impedance Amplifier (TIA)

The weak signal current generated by the photodetector is converted by the Pre-Amplifier into a signal voltage output with sufficient amplitude for high-speed applications. There is also an automatic gain control (AGC) function circuit in the TIA to ensure sufficient signal dynamic range.


PCBA consists of a optical-emitting circuit and a receiving circuit. The main chips are a driver chip, a limiting amplifier, and a memory chip.


The emitting circuit includes following parts.

1. Differential Current Switching Circuit - Output Modulation Current to LD

2. Bias Current Generator—Provides DC Bias Current to the LD

3. Automatic Power Control (APC) Circuit—Changes BIAS and maintain power at different temperatures and LD aging

4. Fault alarming, protecting circuit

5. Input shaping circuit (D flip-flop)


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