Biophotonics_Concepts_to_Applications

where n is the refractive index of the lasing cavity and L is the cavity length. This
can be related to the frequency spacingΔνthrough the relationshipΔν/ν=Δλ/λto
yield

Dm¼

c

2nL

ð 4 : 10 Þ

Example 4.5A GaAs Fabry-Perot laser operating at 850 nm has a 500-μm

length and a refractive index n=3.7. What are the frequency spacing and the

wavelength spacing?

Solution: From Eq. (4.10) the frequency spacing is

Dm¼

3  108 m=s

23 ðÞ: 7 ðÞ 500  10 ^6 m

¼81 GHz

From Eq. (4.9) the wavelength spacing is

Dk¼

ðÞ 850  10 ^9 m^2

23 ðÞ: 7 ðÞ 500  10 ^6 m

¼ 0 :195 nm

In order to create a single-mode optical output, one of a variety of
double-heterostructure configurations can be used. Two popular versions are the
distributed feedback laserorDFB laserand thevertical cavity surface-emitting
laser(VCSEL), which are similar to the LED structures shown in Figs.4.7and4.6,
respectively. The DFB laser is used widely by the telecom industry and has a high
degree of reliability. The laser materials consist of compound semiconductor
materials such as GaAlAs or InGaAsP alloys with emission wavelengths ranging
from 760 to 2400 nm depending on the particular alloy used. A typical single-mode
DFB output is shown in Fig.4.16for a 1557.3 nm peak wavelength.

Possible

lasing

modes

Arbitrary wavelength

Peak

wavelength

Gaussian

output

profile

Fig. 4.15 Typical emission
spectrum from a Fabry-Perot
laser diode

110 4 Fundamentals of Light Sources