velopment of sin- Cross section Productive coating Multi-mode grade of index fiber Cladding Single mode step index fiber Fig. 1-7. Signal propagation modes for fiber types. 12 TELECOMMUNICATIONS gle-mode fiber through the 1980's is expected to encourage new ap- plications for fiber optic technology and to allow transmission of signals greater distances without repeaters. 1.3.2 Light Source Emitters and Detectors The emitter transforms an electrical signal into a light signal which can be coupled to the fiber and transmitted. The two most popular light sources used are infrared emitting diodes (IRED) and injection laser diodes (ILD). The laser diodes provide a small area, powerful source with high speed capability. They are exceptionally well suited to digital trans- mission systems. They are highly directional emitters and substan- tially more power may be coupled if the injection laser diode is used. Light emitting diodes are ideally suited to analog applications. However, their numerical aperture is high, leading to a large mis- match when coupled into fibers with small cones of acceptance. It is therefore often difficult to achieve good power couplings from most LED sources into small diameter fibers. Fiber optic receivers, or detectors, are photodiodes which convert light to electrical current. Photodiodes are used because they are small, inexpensive, and do not need much power to drive them. There are two types of photodiodes in common use today: the PIN diode and the avalanche diode, or APD. The PIN diode is the most simple and inexpensive type of photodiode, and is adequate for a wide range of applications. A more sensitive, and more expensive, type is the avalanche diode (APD). With APD's the internal gain is amplified to produce a larger current than the PIN diode. Also, noise causes less interference with an APD than with a PIN diode, which allows for higher transmission speeds along the link. The commercial availability of light sources and detectors oper- ating in the longer wavelength regions, togeth