Biophotonics_Concepts_to_Applications

lasing condition that is dominated by stimulated emission. The lasing efficiency is
given by

geff¼

dP

dI

ð 4 : 8 Þ

where dP is the change in optical output for a change dI in the diode drive current.
Depending on the device material and structure, the light from a laser can be a
continuous wave(CW) output or the light can be modulated or pulsed. Very
short-duration high-energy pulses can be created with either a solid-state
Q-switched laser or a mode-locked laser, which can be a laser diode, a
solid-state laser, or afiber laser. The quality (or Q) of the laser cavity can be
controlled in aQ-switched laser. For example, an optical element can change the
optical transmission gain of the cavity from very low to very high. This feature acts
like a shutter to allow light to leave the device or not. Pulse durations as low as
nanoseconds (10−^9 s) are possible with a Q-switched laser. In amode-locked laser
the different longitudinal modes (running along the cavity) arelocked in phase. This
process can produce a series of very short pulses with pulse durations ranging from
picoseconds (10−^12 s) to femtoseconds (10−^15 s).

4.4.2 Laser Diodes

Various categories of semiconductor laser diodes are available for biophotonics
uses with wavelengths commonly ranging from 375 nm to about 3.0μm, with
some devices having emission wavelengths up to 10μm. The advantages of laser

Lasing efficiency

eff = dP/dI

Laser regime

LED regime

Threshold current

Fig. 4.13 Relationship
between optical output power
and laser diode drive current

108 4 Fundamentals of Light Sources