Page 270 - Optofluidics Fundamentals, Devices, and Applications

244 Cha pte r T e n

feedback in audio systems, for example, when the output from the
loudspeakers is picked up in the microphone and fed back to the
amplifier. Both the amplitude and frequency of these oscillations are
very sensitive to the feedback conditions, for example, by modify-
ing the amplification or changing the microphone position. In lasers,
the input power, I , can be provided by different means, but most
0
commonly the energy is supplied by an electrical current or by
absorption of incident photons. These types of lasers are referred to
as electrically or optically pumped devices, respectively. The optical
feedback in lasers is generally provided by classical optical compo-
nents like mirrors and gratings (see Fig. 10-2b), while the amplification
relies on quantum physics and the statical properties of photons.
The optical gain and amplification in the laser medium is obtained
through the process of stimulated emission of photons, where exist-
ing photons stimulate emission of further “identical” photons hav-
ing the same frequency and phase properties. Through the optical
feedback in combination with the emission spectrum of the gain
medium, a narrow band photon population with a common phase
is built up in the optical resonator. A fraction of these photons are
extracted from the cavity, for example, through a minute transmit-
tance through one of the mirrors, to form the monochromatic and
coherent laser emission. For a thorough introduction to laser phys-
ics, we refer to Svelto’s classical textbook [5].
How to recognize a laser? Figure 10-2c and 10-2d illustrate the
main characteristics of laser output (solid lines) as compared to con-
ventional light sources (dotted lines). Spectrally, the laser manifests
itself by emitting one color at a well-defined frequency f . This is
0
referred to as monochromaticity, as opposed to multicolor or spec-
trally broad emission from conventional sources. The output power
I of a laser exhibits a characteristic, sudden increase I , when the
out out
input power I reaches a threshold, I .
0 th
The threshold, I , that marks the onset of lasing is reached when
th
the round-trip gain experienced by the photons overcomes the
round-trip cavity losses, leading to a positive net feedback, and hence
oscillations. The slope ∂I /∂I above the lasing threshold, called the
out 0
slope efficiency, describes the efficiency by which pump energy is
converted into laser output energy.

10-3 Dye Lasers
Dye lasers are optically pumped devices using organic dye molecules
as gain medium. When optically pumped, the organic dye molecule
can provide optical gain over a broad frequency range, whereby the
laser emission frequency is determined by the spectral properties of
the laser cavity feedback. A range of different dye molecules with
optical gain spanning the full spectrum from ultraviolet over the vis-
ible to near infrared frequencies are commercially available. Since the