312 / CLEO‘98 / WEDNESDAY AFTERNOON 2. 3. 4. L. Bjerkan, D.R. Hjelme, K. Johannessen, in Proceedings of OFS’ll, 1996, p. 236. Emmanuel C. Ifeachor, Barrie W. Jervis, in Digital Signal Processing: A Practical Ap- proach, (Addison-Wesley, 1993). W. Jin, G. Stewart, W. Philip, B. Culshaw, M.S. Demokan, Appl. Opt. 36, 6251 (1997). CWR2 (invited) 4:45 pm Principles and applications of fiber-optic Fabry-Perot sensors Henry F. Taylor, Electrical Engineering Department, Texas A&M University, College Station, Texas 77843 A versatile technology based upon the fiber Fabry-Perot interferometer (FFPI) is proving its merit in industrial sensing applications. The FFPI consists of two internal mirrors separated by a length L of single-mode optical fiber. A sensor head is designed such that the measur- and of interest (e.g., pressure, strain, accelera- tion, magnetic field, temperature) affects the optical length of the interferometer. A signal conditioning unit that performs multiplexing and signal processing functions is capable of monitoring many sensors with a single distrib- uted feedback laser and digital signal proces- sor. An alternative monitoring scheme using a white light interferometry with a low- coherence light source has also been demon- strated. Applications include monitoring gas pressure in engines, liquid pressure in pumps, strain in civil structures, defects in bearings, and vibration in rotating machinery. In exten- sive field tests, the engine pressure sensor has shown an unprecedented combination of ac- curacy and durability at high temperatures. Other advantages over competing technolo- gies include high sensitivity, immunity from electromagnetic interference, small size, the ability to locate sensors far from monitoring equipment, and the potential for least-cost implementation through multiplexing and networking. CWR3 5:15 pm Bidirectional Brillouin/erbium fiber ring laser and its application to current sensing Dmitrii Yu. Stepanov, Ian M. Bassett, Greg J. Cowle,* Australian Photonics Cooperative Research Centre, 101 National Innovation Centre, Australian Technology Park, Eveleigh, NSW 1430 Australia; E-mail: d.stepanov@oj?c. usyd. ed u. au Brillouin fiber lasers (BFL) have found appli- cations such as gyroscopes1 and current sen- sors2 as they are able to operate bidirectionally without mode competition. Two counter- propagating Brillouin lasers sharing the same fiber ring resonator and experiencing different apparent cavity lengths due to nonreciprocal phase shifts induced by either Sagnac or Fara- day effect oscillate at slightly different optical frequencies and the optical beat frequency is proportional to either the rotation rate or the electric current. A critically coupled fiber reso- CWR3 Fig. 1. Schematic ofbidirectional Bril- louin/erbium fiber laser using polarizing spun Hi Bi fiber (PSHBF). 50 I c 4 0 1 m , i I ! 27.484 27.488 27.492 27.496 27.500 27.504 Frequency, MHz (b) (a) Typical optical beat spec- CWR3 Fig. 2. trum measured using a radio frequency spectrum analyzer; (b) AC current waveform reconstructed from the optical beat spectrum. nator is generally required to construct a BFL because of the small magnitude of the Brillouin gain, and for efficient operation the Brillouin pump frequency should match the cavity reso- nance. In Brillouderbium fiber lasers (BEFL)3 the resonator losses are compensated with the erbium-doped fiber (EDF) gain and there is no need for cavity matching, however, the Brillouin pump should be removed from the laser cavity to avoid spurious injection locking. In Ref. 3 the laser cavity was made nonresonant in the direction of the Brillouin pump injection using a pigtailed bulk isolator, an unacceptable approach for the current sen- sor. We report a novel bidirectional BEFL con- figuration using polarizing spun Hi Bi fiber (PSHBF) and polarization properties of the stimulated Brillouin scattering to make the la- ser ring cavity nonresonant for the Brillouin pump. This all-fiber BEFL is applied to current sensing. The laser arrangement is shown in Fig. 1. Elliptically polarized light from an external narrow linewidth laser is injected into the ring cavity to stimulate Brillouin scattering in both clockwise and counter-clockwise directions. The PSHBF acts as an all-fiber elliptical polar- izer5 to block propagation of the Brillouin pump. In scalar-type backward stimulated Brillouin scattering, elliptically polarized Bril- louin pump is reflected into the Stokes wave as Current, A CWR3 Fig. 3. Amplitude of the recon- structed waveform (WF) vs. electric current. if from a conventional mirror? i.e., with rever- sal of rotation sense and preservation of the ellipse orientation. Hence the generated Bril- louin signal is supported by the PSHBF and will eventually have sufficient gain to reach lasing threshold by virtue of the combination of the Brillouin gain and the EDF gain. Figure 2(a) shows a typical optical beat spectrum of the BEFL output, with 50-Hz AC electric current applied to a copper wire coil wound around the PSHBF. The peaks in the spectrum occurred when the instantaneous optical beat frequency matched the frequency position of the spectrum analyzer. The AC waveform can be reconstructed [Fig. 2(b)] from the spectrum, and the dependence of the waveform amplitude versus mean value of the electric current measured using a conventional current transducer is plotted in Fig. 3. In conclusion, we have developed a novel single elliptical polarization bidirectional ver- sion of the Brillouiderbium fiber laser using polarizing spun Hi Bi fiber and the polariza- tion properties of the stimulated Brillouin scattering. The laser offers the advantages of an all-fiber device and does not need a critically coupled cavity and associated electronic feed- back to track the laser cavity resonance. We have demonstrated the application ofthe BEFL to current sensing. *Optoelectronics Research Centre, University of Southampton, Australia 1. S. Huang, K. Toyama, B.Y. Kim, H.J. Shaw, Opt. Lett. 18,555-557 (1993). 2. A. Kung, P.-A. Nicati, P.A. Robert, IEEE Photonics Technol. Lett. 8, 1680 (1996). 3. G.J. Cowle andD.Yu. Stepanov, Opt. Lett. 21,1250-1252 (1996). 4. B.Ya. Zeldovich, N.E. Pilipetsky, V.V. Shkunov, in Principles of Phase Conjuga- tion (Springer-Verlag, Berlin, 1985). I.G. Clarke, Opt. Lett. 18,158-160 (1993). 5. CWR4 530 pm Optical fiber voltage sensor for 420 kV air-insulated electric power systems K. Bohnert, P. Pequignot, J. Kostovic,* ABB Corporate Research Ltd., CH-5405 Baden, Switzerland Optical fiber voltage sensors are of consider- able interest to the electric power industry, among other reasons due to their significantly smaller size and weight as well as their poten- tially lower costs as compared to conventional voltage transformers. Here, we present an op- tical voltage sensor that determines the voltage