(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