Biophotonics_Concepts_to_Applications

Example 4.6In a He–Ne gas laser an electrical discharge pumps helium

atoms to a 20.61-eV excited state. This excited helium level is very close to a

20.66-eV level in neon, so that upon collision of a helium atom and a neon

atom, the energy can be transferred from the He to the Ne atom. From this

excited state, the neon atom then drops to a lower 18.70-eV state thereby

emitting a photon. What is the wavelength of this photon?

Solution: Using the relationship given in Eq. (2.21) it follows that

k¼

hc

E

¼

ðÞ 6 : 626  10 ^34 JsðÞ 2 : 998  108 m=s

ðÞ 20 : 66  18 :70 eVðÞ 1 : 602  10 ^19 J=eV

¼633 nm

4.4.5 Optical Fiber Lasers

In anopticalfiber laser, a length of opticalfiber acts as the resonator lasing cavity
[ 28 – 31 ]. These devices have a single-modefiber core doped with erbium, ytter-
bium, or thulium. First energy from a solid-state optical source is coupled into the
cladding of thefiber. The injected light then moves into thefiber core and pumps
the dopant. Desirable medical wavelength ranges for opticalfiber lasers are the
1300-nm range for imaging applications and the 1550-nm to 4-μm region for
surgical use. Output powers fromfiber lasers can vary from milliwatts to over
100 W. This light can be directly coupled into an opticalfiber transmission line for
delivery to a tissue treatment or diagnosis site.
Thulium-doped and erbium-dopedfiber lasers are two commonly used devices.
Thulium-dopedfiber lasers emit in the 2–4-μm spectrum with output powers up to
100 W. These lasers can be used for soft tissue surgery in otolaryngology, urology
(e.g., prostate treatment), ophthalmology, and cardiology. Erbium-dopedfiber
lasers emit in the 1530–1610-nm spectrum for applications in optical coherence
tomography, microsurgery, and skin resurfacing.

4.5 Superluminescent Diodes

In applications such as optical coherence tomography, it is desirable to have a light
source that emits over a broad spectral band. One such device is asuperluminescent
diode(SLD). This device is an edge-emitting semiconductor light source that has
the high output power and intensity features of a laser diode and the low coherence
characteristics of a light-emitting diode. Typical spectral emission widths range
from 5 nm to over 100 nm with center wavelengths that can be selected in the
general range of 830–1550 nm. Optical output powers can vary from a few mil-
liwatts to several tens of milliwatts.

114 4 Fundamentals of Light Sources