Biophotonics_Concepts_to_Applications

diode modules include small size and weight (e.g., 1 mm across and weighing a few
grams), low electrical power requirements of typically a few mW, and high
coherent optical output up to tens of mW. The optical output of laser diodes can be
varied directly with electrical pumping, which can be an electrical
information-carrying signal. As is shown in Fig.4.14 for a digital signal, this
operation is carried out in the linear lasing region by switching between two drive
currents i 1 and i 2 and is known as direct modulation. Up to a certain
device-dependent modulation limit the optical output power varies directly with an
electrical drive signal. This limit is reached for pulse rates around 2.5 Gb/s.
External modulation techniques are needed for data rates beyond this limit [ 13 ].
Two basic laser diode types are the Fabry-Perot laser and the double-
heterostructure configuration. In aFabry-Perot laserthe light is generated within a
semiconductor lasing cavity such as that shown in Fig.4.12, which is known as a
Fabry-Perot cavity. The mirror facets are constructed by making two parallel clefts
along natural cleavage planes of the semiconductor crystal. The purpose of the
mirrors is to establish a strong optical feedback in the longitudinal direction. This
feedback mechanism converts the device into an oscillator (and hence a light emitter)
with a gain mechanism that compensates for optical losses in the cavity at certain
resonant optical frequencies. Each resonant frequency results in optical emission at a
certain wavelength, which yields the multimode Fabry-Perot emission pattern shown
in Fig.4.15. The output has a Gaussian shaped envelope. The spacing between the
modes is given by

Dk¼

k^2

2nL

ð 4 : 9 Þ

Drive current pulses

Optical power

Input current

Light pulses

i 1

Lasing region

i 2

Spontaneous

emission

Fig. 4.14 A digital input in
the lasing region produces a
pulsed optical output

4.4 Lasers for Biophotonics 109