Biophotonics_Concepts_to_Applications

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