Response Time

There are several factors that influence the response time of a photodiode and its output circuitry (see figure 7-3).

The most important of these are the thickness of the detector active area and the detector RC time constant. The detector thickness is related to the amount of time required for the electrons generated to flow out of the detector active area. This time is referred to as the electron transit time. The thicker the detector active area, the longer the transit time will be.

Figure 7-3. - A schematic representation of a photodiode. 

The capacitance (C) of the photodiode and the resistance (R) of the load form the RC time constant. The capacitance of the photodetector must be kept small to prevent the RC time constant from limiting the response time. The photodiode capacitance consists mainly of the junction capacitance and any capacitance relating to packaging. The RC time constant is given by t_RC = RC.

Trade-offs between fast transit times and low capacitance are necessary for high-speed response. However, any change in photodiode parameters to optimize the transit time and capacitance can also affect responsivity, dark current, and coupling efficiency. A fast transit time requires a thin detector active area, while low capacitance and high responsivity require a thick active region.

The diameter of the detector active area can also be minimized. This reduces the detector dark current and minimizes junction capacitance. However, a minimum limit on this active area exists to provide for efficient fiber-to-detector coupling.

Q.14 Should the capacitance of the photodetector be kept small or large to prevent the RC time constant from limiting the response time?
Q.15 Trade-offs between competing effects are necessary for high speed response. Which competing effect (fast transit time, low capacitance, or high quantum efficiency) requires a thin active area?

Linearity

Reverse-biased photodetectors are highly linear devices. Detector linearity means that the output electrical current (photocurrent) of the photodiode is linearly proportional to the input optical power. Reverse-biased photodetectors remain linear over an extended range (6 decades or more) of photocurrent before saturation occurs. Output saturation occurs at input optical power levels typically greater than 1 milliwatt (mW). Because fiber optic communications systems operate at low optical power levels, detector saturation is generally not a problem.

Q.16 Why is detector saturation not generally a problem in fiber optic communications systems?