Optical Fiber Communications Third Edition

8/9/2019 Optical Fiber Communications Third Edition

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To Ching-yun and Nishla









































































































































































































































































































































































































































































































































6.1 PHYSICAL PRINCIPLES OF PHOTODIODES 24 5

Band gap

hv›EgPhoton

Ρ

Conduction band

Valence bandη

Simple energy ban d dia gra m

for a pin photodiode. Photons

wit h an ene rgy grea ter than or

equal to the bandgap energy

Eg can generate free electron

hole pairs which act as photo

current carriers.

FIGURE 62

J —Depletion region—^

fl i t l i it ith l t fl i f i i



flow in an extern al circuit, with one elec tron flowing for every carr ier pair

generated. This current flow is known as the photocurrent.

As th e ch ar ge car ri er s fl ow th ro ug h th e ma te ri al , so me el ec tr on hole pairs

will rec omb ine an d hen ce di sa pp ea r. On th e averag e, th e ch ar ge ca rr ier s mo ve a

distance L„ or Lp for electrons and holes, respectively. This distance is known as

the diffusion length. The time it takes for an electron or hole to recombine is

known as the carrier lifetime and is represented by τ „ and τ ̂ , respectively. The

lifetimes and the diffusiori lengths are related by the expressions

where D„ an d Dp are the electron and hole diffusion coefficients (or constants),respectively, which are expressed in units of centi meters s quare d per seco nd.

Optical radiation is absorbed in the semiconductor material according to the

exponential law

Here, α^(λ) is the absorption coefficient at a wavelength λ, PQ is the incident optical

power level, and P {x) is the optical power absorbed in a distance x.

The dependence of the optical absorption coefficient on wavelength is

shown in Fig. 63 for several photodiode materials.'^ As the curves clearly

show, depends strongly on the wavelength. Thus, a particula r semicond uctor

material can be used only over a limited wavelength range. The upper wavelengthcutoff λ c is determined by the band gap energy Eg of the material. If Eg is

expressed in units of electron v olts (eV), then λ̂ . is given in units of micrometers

(µm) by

Ln = {D „τ „y^ an d Lp = (DpTp)

Pix) = Po(l e ) (61)

λ ‚(µm) =:= =he _ 1.24

Eg Eg(cW)(62)

The cutoff waveleng th is abou t 1 .06µm for Si and 1.6/xm for Ge. F or longe r

wavelengt hs, th e ph ot on ene rgy is no t suffic ient to exc ite an el ect ro n from th e

valence to th e co nd uc ti on ba nd .











































































































































































































































































































































































































































































































































































































































































































































Optical Fiber Communications Third Edition

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