Optical transmissions light sources

There are two main light sources used in PDT, lamps for topical treatments and lasers when transmission via optical fibres is required. 

Equation 3.30 is called the diffraction equation. The solution with n = 0 (9 = 0) is simple transmission of the light. The solutions for n f 0 give intensity in other directions, and the positions of these additional spots can be used to determine A, if d is known. Thus, optical scientists can use a manufactured diffraction grating with known line separations to measure the wavelength of an unknown light source. 

Until recently optical communications were restricted by the lack of fast monochromatic light sources and sensitive photodetectors. Prospects for optical communications improved considerably about two decades ago when a powerful light source became available with the invention of the laser. After that, the transmission medium was the bottleneck of an optical communication system. At that time an intensive search for a new transmission medium was started, particulary because free space propagation could be ruled out for civil use as a consequence of the relative frequent occurrence of atmospheric disturbances. 

Figure 1. Conventional, transmission-based CD spectrophotometer. S, light source M, monochromator (wavelength selection) L, polarizer (linear) P, electro-optic circular polarizer D, light detection (photomultiplier) LI, lock-in amplifier (tuned amplifier) PC, data procurement and display.

It is ironic to consider the III-V nitrides, the premier materials for short wavelength blue and UV emitters, as sources of infrared light. However, Er-doped GaN is of interest for making electrically pumped, temperature insensitive, broad band and compact optical amplifiers or sources of 1.54 pm light. Applications include long-haul communication systems (amplifiers), local area networks (50/50 splitters) and sources (lasers) for transmission in silica-based optical fibres. 

---