4.6 Summary
The biophotonicsfield covers a wide spectrum ranging from about 190 nm in the
UV to over 10μm in the infrared. The numerous diverse biomedical and life
sciences applications that are used within this spectral range necessitate imple-
mentation of a broad selection of light sources with different sizes, shapes, oper-
ational configurations, light output powers, and modulation capabilities. The source
types described in this chapter include the following:
- Arc lamps create an intense light when an electrical discharge passes through a
high-pressure gas or a vapor such as Hg, Ar, Xe, or Ne. - Light-emitting diodes are highly reliable devices that can be used in the UV,
visible, and near infrared spectral regions depending on the material. - Excimer lasers contain a mixture of a noble gas (such as Ar, Kr, or Xe) and a
reactive gas (such asfluorine or chlorine) and emit in the UV range from 193 to
350 nm. - Gas lasers produce a coherent light beam when an electric current is discharged
through a gas medium (He–Ne, Ar, Kr, or CO 2 ). Gas lasers are employed in a
wide range of life sciences and medical disciplines. - Solid-state crystal lasers use lasing media such as crystals or glasses that are
doped with rare earth ions or transition metal ions. The advantages of these
devices are compact designs, long lifetimes, and very good beam qualities. - Semiconductor lasers are highly reliable devices that include Fabry-Perot lasers,
DFB lasers, VCSELs, and quantum cascade lasers. Depending on the device
material, they are used in all segments of the biophotonics spectrum. - Opticalfiber lasers are being implemented in the 1300-nm range for imaging
applications and in the 1550-nm to 4-μm region for surgical use.
4.7 Problems.
4 :1 Consider a laser that emits a highly collimated beam of light. Suppose the
beam diameter is 2 mm and let the power level or radiantflux, be 100 mW.
Neglecting any divergence of the beam, show that the irradiance is
31.8 W/cm^2.
4 :2 For a certain laser diode consider a modified lambertian approximation to the
emission pattern of the form IðÞ¼h I 0 cosmh. Suppose that for this laser
diode the half-power level occurs at an angle of 15° from the normal to the
emitting surface. Show that the value of m is 20.
4 :3 Analogous to Fig.4.3, write a MatLab program to plot and compare the
emission patterns from a lambertian source and a source with an emission
pattern given by IðÞ¼h I 0 cos^3 h. Assume both sources have the same peak
intensity I 0 , which is normalized to unity in each case.4.6 Summary 115