Photonic crystal fibers

For the construction of optical fibers, plastic materials or glass materials are preferred. In PCS fibers, plastic material is used for cladding construction and silica glass for core construction. The new development of photonic crystal fibers offer a fine microstructure within the fiber and are highly periodic. High periodicity is due to the unique fabrication process. There are two mechanisms for light propagation in photonic crystal fibers against the one, total internal reflection in conventional fiber. Photonic bandgap effect results in trapping of some modes. Photonic crystal fibers brought revolutionary changes in various optical fields such as long period gratings, tunable coupled cavity fiber lasers, wavelength converters, wavelength demultiplexers, multimode interference power splitters, fiber amplifiers, multichannel add/drop filters etc. Transmission losses can be appreciably lowered with the usage of photonic crystal fibers. This is due to the fact that during fabrication, highly purified Silicon is used as the base material.

Wavelength converters

Wavelength converter is an optical device which is employed to translate information on an incoming wavelength directly to a new wavelength without entering the electrical domain. There exist two types of wavelength converters. They are optical grating wavelength converters and wave mixing wavelength converters. During initial stages, semiconductor laser systems, semiconductor optical amplifier systems and nonlinear optical loop mirrors were tried to enable wavelength conversion. A semiconductor optical amplifier in a cross-phase modulation mode yielded positive response. Here in the active region, refractive index is dependent on the carrier density. Data rates of 10 Gb/s and more can be easily handled by optical grating wavelength converters. In wave mixing wavelength converters, wavelength conversion is achieved by nonlinear optical wave mixing. The main advantages of this type of converter are multiwavelength conversion capability and transparency to the modulation format.

Eye diagrams

Eye diagrams are used for the assessment of data handling ability of a digital transmission system. Since this method is best in the evaluation of performance of wire-line systems, we can employ this technique for optical fiber data links. The eye-pattern measurements are conducted in time domain and permit the effects of waveform distortion to be shown soon after the measurements on the monitor screen of standard BER (Bit Error Rate) test equipment. The width of the eye opening gives a measure of time interval over which the received signal can be sampled without error due to interference from adjacent pulses. In this experiment, ideal time to receive and sample the waveform is when the height of the eye opening is largest. The height of the eye opening also gives an idea about the noise level. Sensitivity of the whole system to timing errors is related to the rate of closing of eye. Edge jitter which is also known as timing jitter or phase distortion is a signal distortion occurring in optical fiber system due to the noise present in the receiver and pulse distortion introduced in the optical fiber.