Biophotonics_Concepts_to_Applications

The VCSEL is a single-mode laser, which is made mainly for emissions in the
750 – 980-nm range [ 21 ]. A common peak wavelength is 850 nm from a
GaAs/AlGaAs material. VCSEL devices based on InP alloys are available for
operation at 1300, 1550, and 2360 nm. In a VCSEL the emission occurs perpen-
dicular to the active layer. This structure allows a high density of emitters to be
produced over a small area, and thus enables VCSELs to be configured as closely
spaced arrays of lasers. Individual and arrays of VCSELs have been applied to areas
such as absorption spectroscopy, brain imaging, and cell sorting in microfluidic
channels.
A fourth class of semiconductor laser diodes is thequantum cascade laser
(QCL) that emits in the mid- to far-infrared spectrum [ 22 , 23 ]. The devices can
cover the biophotonics spectral range of 2.75μm to beyond 10μm. The QCL
devices are available in either a broadband Fabry-Perot structure or as a
single-wavelength DFB laser. The QCL operates in a continuous-wave (CW) mode
and can have optical outputs up to 500 mW.

4.4.3 Solid-State Lasers

Solid-state lasers use lasing media such as crystals or glasses that are doped with
rare earth ions or transition metal ions [ 24 – 27 ]. To achieve the population inversion
needed for a lasing condition, solid-state lasers are optically pumped withflash
lamps, arc lamps, or laser diodes. The advantages ofdiode-pumped solid-state
lasers(DPSS lasers) are compact designs, long lifetimes, and very good beam
qualities.
The characteristics of several solid-state lasers that are commonly used in bio-
photonics are listed below.

• Ruby lasersuse a synthetic ruby crystal as the gain medium to produce pulses
  of deep red 694.3-nm wavelength light with a linewidth of 0.53 nm. This laser is

Wa v e l e ngth (nm)

Optical output power (dBm)

Peak: 1557.3 nm

Optical output power (

mW)

1.0

10 -4

10 -3

10 -2

1554 1555 1556 1557 1558 1559 1560 1561

-10^10 -1

-20

-30

-40

-50

-60

Fig. 4.16 Typical DFB laser 0
output spectrum (power
shown in mW and dBm)

4.4 Lasers for Biophotonics 111