Applications of Fiber Optical Resonators in Measurement and Telecommunications Technology

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P. O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Tuomo von Lerber Name of the dissertation Applications of fiber optical resonators in measurement and telecommunications technology Date of manuscript April 10, 2007 Date of the final manuscript August 27, 2007 Date of the dissertation October 5, 2007 Monograph Article dissertation (summary + original articles) Department Electrical and Communications Engineering Laboratory Micro and Nanosciences Laboratory Field or research Optical technology Opponent Peter Andrekson, Professor Supervisor Hanne Ludvigsen, Docent Abstract The advent of optical fibers has greatly impacted the modern technology landscape. Most notably, existing telecommunications infrastructure relies on optical fiber networks, which have the ability to transmit high-bandwidth data over considerable distances. Optical fibers have also visible foothold in the field of metrology, where they are used as sensors in various applications. An optical resonator is a basic building block of many optical devices, such as lasers, measurement probes, and optical signal processing equipment. In this dissertation we propose new methods and tools for metrology and general telecommunications sciences based on fiber optical resonators. Compared to conventional free-space technology, fiber resonators enable easy connectivity and they are shown to be robust against ambient perturbations. This thesis consists of two parts that concentrate on optical measurement and signal processing technology, respectively. The first part introduces new measurement schemes for fiber loss and minute birefringence quantification. A loss of an optical fiber medium is measured using a so-called fiber cavity ring-down method. In presence of an external source of loss, such as fiber bending or degradation, one may obtain information about the environment of the probe. Another scheme measures birefringence and, thus, the beat length of a short fiber section. We also suggest a computational method for resonator photon lifetime (also called resonator time constant) extraction under noisy signal conditions. We demonstrate that the developed algorithm may yield meaningful results even when conventional methods fail. The second part of the thesis deals with all-optical signal processing and temporal data synchronization. In a proof-ofprinciple experiment we perform an all-optical clock recovery for 21 parallel wavelength channels at two simultaneous data rates. The method relies on a birefringent optical resonator, whose transmission spectrum is used to filter carrier and sideband frequencies of return-to-zero-modulated data for multiple wavelength channels. In another clock recovery experiment we investigate the possibility to use sideband filtered signal, combined with a continuous wave light emitted at the carrier wavelength.