Introducion to Optical Receivers

An opto-electronic device that receives an optical signal and converts it to electrical signal is called Optical receiver. Optical receivers convert the incoming optical signal at the output end of the optical fiber network back to the original electrical signal. An Optical receiver consists of a coupler, a photodetector, and a demodulator.

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The optical signals propagating in the fiber become weakened and distorted because of scattering, absorption, and dispersion. The fiber optic device responsible for converting the weakened and distorted optical signal back to an electrical signal is a fiber optic receiver.

The coupler focuses the received optical signal onto the photodetector. Semiconductor photodiodes are used as photodetectors. Semiconductor photodiodes are compatibile with the whole optical transmission system. The demodulator design depends on the modulation format used by the lightwave transmission system. The use of FSK and PSK formats generally requires heterodyne or homodyne demodulation techniques.

Most lightwave systems employ a scheme referred to as “intensity modulation with direct detection” (IM/DD). Demodulation in this case is done by a decision circuit that identifies bits as 1 or 0, depending on the amplitude of the electric signal. The accuracy of the decision circuit depends on the SNR (Signal to Noise Ratio) of the electrical signal generated at the photodetector.

Optical receiver performance parameters that has influence to the over all transmission system operation are receiver spectral response, sensitivity, Frequency response, and dynamic range. Spectral response is determined by the choice of optical detector materials and structures. Silicon (Si), gallium arsenide (GaAs), and gallium aluminum arsenide (GaAlAs) are generally used materials used for detectors in receiving operation in the 850 nm wavelength region. Germanium (Ge), Indium Phosphide (InP), and Indium Gallium Arsenide (InGaAs) are some of the detector materials used for receiver operation in the 1300-nm and 1550-nm wavelength regions. The receiver sensitivity is the minimum amount of optical power required to achieve a specific receiver performance.

Author: Nimura

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