Biophotonics_Concepts_to_Applications

referred to as thenoise equivalent poweror NEP, which is designated in units of

W/

ffiffiffiffiffiffi

Hz

p
.
As an example, consider the thermal-noise-limited case for apinphotodiode.
A thermal-limited SNR occurs when the optical signal power is low so that thermal
noise dominates over shot noise. Then the SNR becomes

SNR¼R^2 P^2 =ðÞð4kBTBe=RL 5 : 16 Þ

Tofind the NEP, set the SNR equal to 1 and solve for P to obtain

NEP¼

Pmin

ffiffiffiffiffi

Be

p ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

4kBT=RL

p

=R ð 5 : 17 Þ

Example 5.9Let the responsivityR= 0.90 A/W for an InGaAs photode-

tector operating at 1550 nm. What is the NEP in the thermal-noise-limited

case if the load resistor RL= 1000Ωand T = 300 K?

Solution: From (5.17) the value for NEP is

NEP¼½ 4 ð 1 : 38  10 ^23 J=KÞð300 KÞ= 1000 XŠ^1 =^2 =ð 0 :90 A=WÞ

¼ 4 : 52  10 ^12 W=

ffiffiffiffiffiffi

Hz

p

The parameterdetectivity, or D*, is afigure of merit for a photodetector used to
characterize its performance. The detectivity is equal to the reciprocal of NEP
normalized per unit area A.

D¼A^1 =^2 =NEP ð 5 : 18 Þ

Its units commonly are expressed in cm

ffiffiffiffiffiffi

Hz

p

=W.

5.3.4 Comparisons of Photodiodes

Tables5.1and5.2list some generic operating characteristics for GaP, Si, Ge, and
InGaAspinphotodiodes and avalanche photodiodes, respectively. The values were
derived from various vendor data sheets and from performance numbers reported in
the literature. The performance values are given as guidelines for comparison
purposes. Detailed values on specific devices for particular applications can be
obtained from suppliers of photodetector and receiver modules.

132 5 Fundamentals of Optical Detectors