Special Report: Next Generation Internet Applications
2.1.1.2 Tunable Lasers and Filter Structures Optical transmission relies on precise wavelength-sensitive equipment to successfully process and direct optical signals. Lasers are the driving force behind optical communications and operate at various wavelengths. The ability to tune these lasers provides administrators with the ability to replace a failed laser from reserves. Since the tunable lasers can be configured in the field to support any of the wavelengths in a system, only a small number of spare lasers need to be kept in inventory. This feature removes the need to stock spare lasers for each particular wavelength used in the system. Current manually tunable lasers are also the first step towards in-circuit automated tuning.
Automated tuning will provide an enabling technology for the development of all-optical networks. Several different methods for tuning lasers currently exist. Mechanically tuned lasers suffer from long tuning times and are used when the connection is mostly fixed to a particular wavelength. Acousto- optically tuned lasers employ a sound wave to change the refractive index of the laser cavity to modify the wavelength. Acousto-optically tuned lasers have a faster tuning time and are limited only in the number of wavelengths that can be affected. Electro-optically tuned lasers operate by changing the refractive index of the laser cavity by implementing an electric field. Injection current tuned lasers utilize a current-controlled diffraction grating to tune the laser. Injection current tuned lasers provide high tuning speeds that are necessary for photonic networks.[7] The ability to effectively tune lasers will provide optical network designers with the scalability and cost effectiveness to proliferate new optical technologies in their networks.
Tunable filters allow certain wavelengths to be singled out from a multi-wavelength optical signal. Researchers have developed several filter technologies, each with different tuning methods and ranges. A filter common in optical network infrastructures is the Fiber Bragg Grating (FBG). FBGs are, in general, a fixed wavelength filter technology. Slight tuning can be achieved by varying environmental or physical conditions of the filter. These filters modify the index of refraction of a small section of fiber to reflect and filter out a predetermined wavelength, much the same way that a notch filter operates in electronic circuitry. FBGs are useful in the creation of optical add-drop multiplexers as well as to compensate for dispersion. Thin film substrates are another type of optical filter. They consist of a fiber coated with a dielectric material. Thin film substrates pass a single wavelength and block all others, providing the inverse functionality of FBGs.[8] Thin film substrates are equivalent to a narrow band-pass filter in electronic circuits. Additional optical filters include Fabry-Perot filters, modecoupling filters, and liquid crystal tunable filters.[7] Tunable filters are partially responsible for the advancements of optical multiplexers and are a critical component in optical networks.
Previous Next Table
of Content for report: Next Generation Internet Applications Home
