Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 5

Fig. 1.2 Comparison of a PCF-based supercontinuum light source spectrum with other broadband
light sources. SLED - superluminescent light-emitting diode. The ASE source is referred to as
Thorlabs' C&L band light source (Adapted from Hansen et al. [ 16 ])

media, lasers can be divided into gas, excimer, solid state, dye, semiconductor,
and fiber lasers. A summary of the lasers commercially available for biomedical
spectroscopic applications is listed in Table 1.1 .He-Ne,CO 2 ,Ar C , and nitrogen
lasers are the most used gas lasers. Excimer lasers can be used for UV-excited
fluorescence spectra. Ti:sapphire, Nd:YAG, Nd:YLF, and Nd W YVO 4 are the most
commonly used solid-state lasers. In recent years, the application of (semiconduc-
tor) diode laser gained popularity because of the compact size and stable output.
Fiber lasers are a special type of lasers in which rare-earth ion-doped optical fibers
act as gain media (such as erbium, neodymium, ytterbium, or thulium). Choice of
lasers depends on the availability of the wavelength, power, system size, and cost.

1.2.1.3 LED

Light-emitting diode (LED) is a type of solid-state light source. It consists of a
semiconductor chip doped with impurities to form a p-n junction. Current flows
in the p-n junction from p-side (anode) to n-side (cathode) under different voltages.
Once electrons meet holes, they will fall to lower energy level and release the energy
in the form of light. The wavelength depends on the bandgap of the p-n junction.
With the advancement in material science, now broadband LED light sources have
covered the full wavelength range from UV to visible to near infrared. White LEDs