SKEDSOFT

Physics For Engineers - 1

Optical Fiber Communications: The optical fiber found its first large-scale application in telecommunications systems . Beginning with the first LED-based systems , the technology progressed rapidly to longer wavelengths and laser-based systems of repeater lengths over 30 km . 3 6 The first applications were primarily digital , since source nonlinearities precluded multichannel analog applications . Early links were designed for the 800- to 900-nm window of the optical fiber transmission spectrum , consistent with the emission wavelengths of the GaAs-AlGaAs materials system for semiconductor lasers and LEDs . The development of sources and detectors in the 1 . 3- to 1 . 55- m m wavelength range and the further improvement in optical fiber loss over those ranges has directed most applications to either the 1 . 3- m m window (for low dispersion) or the 1 . 55- m m window (for minimum loss) . The

design of dispersion-shifted single-mode fiber along the availability of erbium-doped fiber amplifiers has solidified 1 . 55 m m as the wavelength of choice for high-speed communications .
The largest currently emerging application for optical fibers is in the local area network (LAN) environment for computer data communications , and the local subscriber loop for telephone , video , and data services for homes and small businesses . Both of these applications place a premium on reliability , connectivity , and economy . While existing systems still use point-to-point optical links as building blocks , there is a considerable range of networking components on the market which allow splitting , tapping , and multiplexing of optical components without the need for optical detection and retransmission .

Point-to-Point Links The simplest optical communications system is the single-channel (no optical multiplexing) point-to-point digital link . As illustrated in Fig . 3 , it consists of a diode laser (with associated driver circuitry and temperature control) , optical fiber (with associated splices , connectors , and supporting material) , and a detector (with appropriate electronics for signal processing and regeneration) . The physics and principles of operation of the laser
and detector are covered elsewhere in this collection, but the impact of certain device characteristics on the optical fiber communications link is of some importance . Modulation and Source Characteristics . For information to be accurately transmitted , an appropriate modulation scheme is required . The most common modulation schemes employ direct modulation of the laser drive current , thereby achieving a modulation depth of 80 percent or better . The modulation depth is defined as

where P m i n and P m a x are the minimum and maximum laser power , respectively . The modulation depth is limited by the requirement that the laser always remain above threshold , since modulation near the lasing threshold results in a longer turn-on time , a broader spectrum , and higher source noise .
The transmitting laser contributes noise to the system in a fashion that is , generally speaking , proportional to the peak transmitted laser power . This noise is always evaluated as a fraction of the laser power and is therefore termed relati y e intensity noise (RIN) . The RIN contribution from a laser is specified in dB / Hz , to reflect a spectral density which is