Recent trends in optical networks, such as Reconfigurable Optical Add Drop Multiplexing (ROADM) and optical cross connects, require advanced optical amplifiers based on both Erbium Doped Fibre Amplifiers (EDFAs) and Raman technology. To address the dynamic nature of modern networks, EDFAs should provide broadband variable gain operation, flexible mid-stage access, fast transient response to dynamic events, and advanced spectral monitoring and control to adjust to changing spectral conditions in the network. An important supplement to EDFA technology is the use of Distributed Raman Amplification (DRA) to achieve transmission over multi-span Ultra Long Haul (ULH) links in all optical networks, as well as very high loss repeaterless links.
Fiber-optic ring structures with multiple recirculations are reviewed and their applications for the accurate measurement of a variety of physical measurants are analyzed. The methods are based on the fact that minute changes in the round trip time around the loop, are amplified after many recirculations, thereby making them measurable quantities. Some new possibilities of measurements are proposed. Analog self-oscillations and optical pulse multiple propagation are considered. Influences of length of fiber, parameters of optical radiation and pulse form on characteristics of such devices are observed. . INTRODUCTIONHighly sensitive, wide-range fiber-optic sensors are of interest for various practical applications. Increasing their sensitivity often involves a corresponding increase in the length of the fiber, which senses the measured external action. However, a longer fiber is more lossy, resulting in a lower signal-to-noise ratio. Besides, in many cases, the physical effect to be measured can be applied only to a short piece of fiber. Thus, methods which employ multiple passes of the light through the same fiber length can offer significant advantages. Such methods were used earlier for the determination of the length of an optical fiber1. But this principle of multiple recirculations can be also harnessed to the measurements of a variety of physical quantities. The effect of the measurand on a particular parameter of the recirculating light accumulates from one recirculation to the next, leading to increased sensitivity. Thanks to the high speed of light in optical fibers, a large number of recirculations can take place in a relatively short time, and the external influence can be very accurately measured by counting the number of recirculations in a fixed time.Obviously, the sensitivity of the measurement will increase with the number of recirculations.The recirculating fiber-optic delay line2 is a good base for such devices. However, to ensure a significant number of round trips, the loss and dispersion of the fiber must be compensated for. This can be achieved by proper regeneration. This paper describes these fiber-optic recirculation-type structures (FORTS). Sections 2 and 3, respectively, describe analog and pulsed FORTS. The dependence of the characteristics of FORTS on the of fiber length is described in Sec. 4, while Sections 5 and 6 describe the effects of the pulse and light parameters, respectively. ANALOG FORTSAn analog self -oscillator using a fiber-optic delay line for feedback was first investigated by M. Nakazawa et al3. His FORTS comprised (Fig. 1) a laser diode, an optical fiber, a photoreceiver, a bandpass filter and an electronic amplifier. The phase balance condition required for self-oscillations is obeyed by a series of equally spaced RF frequencies:O-8194-1218-X/93/$6.OO
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