Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 6

Tabl e 1. 1 A summary of lasers and their spectral characteristics

Laser type Wavelength range (nm) Comments

ArC 457, 488, 514.5 Gas, CW or pulsed

He-Ne 632.8 Gas, CW

Nitrogen 337 Gas, pulsed

CO 2 10:6 m Gas, CW or pulsed

HeAg 224 Gas

HeCd 325, 442 Gas, CW

Nd:YAG 1,064, 532 Solid state, CW or pulsed

Nd:YVO4 914, 1,064, 1,342 Solid state

Nd:YLF 1,047, 1,053 Solid state

Ti:sapphire 690-1,000 Solid state, CW or pulsed, Q-switched

Diode laser 300-900 Semiconductor, CW

Fiber laser 514, 780, 1,025-1,600 Pulsed, doped with Er, Nd, Yb, and Tm

Kr C 406, 647, 752 Gas, CW, intermediate

KrCl 222 Excimer

KrF 248 Excimer

XeCl 308 Excimer

XeF 351 Excimer

Ruby 694.3 Solid state, pulsed

are also available, which are working in two ways. In one way, white LED is
generated by mixing the primary color red, green, and blue LEDs. In the other way,
white LED is generated by a second emission from a layer of phosphor material
excited by a blue or UV LED. Now, high-power LEDs are available. Because LED
has a very compact size and very long lifetime, it is expected that LED light source
will find broad applications in biomedical spectroscopy, particularly in absorption
spectroscopy, reflectance spectroscopy, and fluorescence spectroscopy.

1.2.2 Optical Fibers

Although free space light delivery is used in many optical spectroscopy systems,
they are usually inconvenient for use in biomedical applications. Nowadays most
biomedical spectroscopy systems use optical fiber-based light delivery. A cross
section of light transmission in optical fibers is shown in Fig. 1.3 .
The light transmission in fibers is based on the law of total internal reflection.
The acceptance angle of an optical fiber is determined by its numerical aperture
(NA), given by

q n co n cl ;

NA D n i sin max D (1.1)

where n i , n co ,andn cl are the refractive index of the air, core, and cladding
material of the fiber and max is the maximum acceptance angle. When increasing