Page 221 - Fiber Optic Communications Fund

202 Fiber Optic Communications

Note that a fast transit time implies a thin detector active region, while low capacitance and high responsivity
require a thicker active region. Thus, there are trade-offs between fast transit times and low capacitance for
high-speed response, high quantum efficiency, low dark current, and good coupling efficiency when used in a
fiber system. For example, a fast transit time requires a thin detector photoactive region, while low capacitance
and high responsivity (or quantum efficiency) require a thick active region. It is very generally favorable to
design the absorption region to be larger than the penetration depth using expression (5.22).
Also, a smaller detector active area leads to lower dark current and smaller junction capacitance, but may
be inefficient for detector coupling to the fiber when used in fiber-coupled systems. Therefore, building on
the above examples, a silicon-based sensor that is optimized for 680-nm detection should be designed to have
the thickness of the semiconductor within 4.5–9 μm.

5.2.6 Linearity

Typically, reverse-biased photodetectors are highly linear devices (Fig. 5.9). 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 (six decades or more) of photocurrent
before saturation occurs. Output saturation occurs at input optical power levels typically greater than 1 mW.
Because fiber-optic communication systems operate at low optical power levels, detector saturation is gener-
ally not a problem.

5.3 Common Types of Photodetectors
As mentioned in Section 5.1, semiconductor photodetectors can be broadly classified into those without inter-
nal gain and those with internal gain. In the first category are pn photodiodes, pin photodetectors, Schottky
barrier photodetectors, and MSM-PDs. In the second category are photoconductors, phototransistors, and
APDs. These second types of photodetector are used to improve the overall sensitivity of the front-end pho-
toreceiver.

I PC (A)

Saturation

Dynamic
range Ideal
response
Total noise

P n P n P I (dBm)

Figure 5.9 Response characteristics of a typical photodetector. Important features of the response characteristics are
indicated in the figure.