Biophotonics_Concepts_to_Applications

(b) What is the 3-dB electrical bandwidth of this device?

Solution:

(a) The 3-dB optical bandwidth occurs at the modulation frequency for

which P(ω) = 0.5P 0. Using Eq. (4.4) this condition happens when

1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 þðÞxsi^2

q ¼ 0 : 5

so that 1 + (ωτi)^2 =4 orωτi=√3. Solving this expression for the

frequency f =ω/2πyields

f¼

ffiffiffi

3

p

2 psi

¼

ffiffiffi

3

p

2 pðÞ 5  10 ^9

¼ 55 :1 MHz

(b) The 3-dB electrical bandwidth is f/√ 2 =(55.1 MHz)×0.707 =

39.0 MHz.

4.4 Lasers for Biophotonics

During the past several decades, various categories of lasers have made a significant
impact in biophotonics [ 16 – 21 ]. Among the many application areas are cardiology,
dentistry, dermatology, gastroenterology, gynecology, microscopy, microsurgery,
neurosurgery, ophthalmology, orthopedics, otolaryngology, spectroscopy, and
urology. The advantages of lasers over other light sources are the following:

• Lasers can have monochromatic (single-wavelength) outputs, so that the device
  wavelengths can be selected to match the absorption band of the material to be
  analyzed


• The output beam can be highly collimated, which means that the laser light can
  be directed precisely to a specific spot


• Depending on the laser, the outputs can range from mW to kW


• Short pulse durations are possible (e.g., a few femtoseconds) for use in appli-
  cations such asfluorescence spectroscopy where the pulse width needs to be
  shorter than the desired time-resolution measurement.
  This sectionfirst explains the basic principles of laser construction and operation
  and then gives examples of laser diodes, solid-state lasers, gas lasers, and optical
  fiber lasers.

4.3 Light-Emitting Diodes 105