4.4.4 Gas Lasers
A wide selection of gas lasers is available for many different types of commercial,
hobby, educational, and medical applications. Generally agas laserproduces a
coherent light beam when an electric current is discharged through a gas medium.
The emitted light typically is a very high quality, single-mode beam with a long
coherence length. Common gas lasers used in biophotonics have helium-neon
mixtures, argon, krypton, or carbon dioxide gases. The gas type used in the laser
determines or influences the emission wavelength, the device efficiency, and the
optical output power. Gas lasers are employed in a wide range of life sciences and
medical disciplines including flow cytometry, cell sorting, microscopy, DNA
sequencing, retinal scanning, ophthalmology, dermatology, hair and tattoo removal,
and surgical and dental procedures.
The generic characteristics of some gas lasers are given below.
- Helium–neon (He–Ne) lasersconsist of a gas cell with a length of around
20 cm and generate output powers from about 0.8–20 mW. The nominal beam
width is a few millimeters. A standard wavelength is 632.8 nm (red light), but a
He–Ne laser can emit at many other wavelengths by adjusting the characteristics
of the lasing cavity. Other common wavelengths are 543.5 nm (green),
594.1 nm (yellow), 611.9 nm (orange), 1152.3 nm, or 3391.2 nm. Biophotonics
applications include confocal microscopy, DNA sequencing,flow cytometry,
and hematology. - Carbon dioxide (CO 2 ) lasersuse a mixture of CO 2 , He, and N 2 for generating
laser radiation mostly at 10.6μm. In general the coated hollow-corefibers
described in Sect.3.8are used to deliver this wavelength of light to a specific
pinpoint location. Depending on the application a CO 2 laser can operate in either
a continuous or a pulsed mode. These lasers can be made to emit up to several
kilowatts, but optical powers of 10–20 W are sufficient for most medical
applications such as surgery, ophthalmology, and cosmetics. - Argon (Ar) ion lasers need a nominally 1-m long tube and provide a
single-frequency narrow light beam (1.5 mm). These lasers emit 13 spectral
lines in the UV through visible region from 351 to 528-nm. The commonly used
wavelengths are 488.0 and 514.5 nm (green light) with output powers of up to
20 W in the green region. Because these two wavelengths are in the visible
region, the light can be transmitted through optical fibers. The emission
wavelengths are absorbed strongly by numerous tissue chromophores and thus
can be used for coagulation applications and for vaporization of pigmented
lesions in the skin and retina. - Krypton (Kr) ion lasersare similar in both structure and beam properties to Ar
ion lasers. These lasers emit at 11 wavelengths, the main ones being 530.9,
568.2, and 676.4 nm with output powers up to 10 W. The applications are
similar to Ar lasers.
4.4 Lasers for Biophotonics 113