A simple laser design consists of the following:
- Anactive medium(orgain medium) in which atoms or molecules can be excited
to higher energies - An electrical or optical energypumpsource
- Two reflectors (high-reflectionmirrors)
The active medium and the two mirrors form aresonatoror alaser cavity,as
shown in Fig.4.12. The lasing action takes place when light builds up in amplitude
as it is reflected by the end mirrors and travels back and forth in the cavity. The
cavity lengthdetermines theemitted light wavelength. Usually the cavity length is
fixed so that the laser emits at a number of specific wavelengths that satisfy the
condition of standing wave patterns as shown on the bottom of Fig.4.12. Each of
these standing waves forms alasing modeof the cavity, which gives a specific
wavelength. In some devices the cavity length can be changed. This structure
results in atunable laser, which emits at a selection of different wavelengths
depending on the cavity length.
Consider the case when the lasing medium is a semiconductor material, which is
the basis of a laser diode. In this case the pumping is carried out by electron
injection by means of an external bias current. The relationship between optical
output power and laser diode drive current is shown in Fig.4.13. At low diode
currents only spontaneous radiation is emitted. Both the spectral range and the
beam width of this emission are broad like that from an LED. A dramatic and
sharply defined increase in the optical power output occurs at a current level Ith
known as thelasing threshold. As this transition point is approached from lower
drive current values, the spectral range and the beam width of the emitted light both
become significantly narrower with increasing drive current. The emission is then a
Active medium
Front mirror
80%<R<98% Pumping process
Rear mirror
R = 100%
Cavity length
Example of allowed modes in the laser cavity
Fig. 4.12 Schematic of a
simple laser design and the
allowed lasing modes
4.4 Lasers for Biophotonics 107