Fiber-Fabry-Perot (FFP) Thermometer For Medical Applications

Abstract

ABSTRACT A laboratory version of a novel temperature sensor based on a Fiber Fabry Perot (FFP) interferometer and the wavelength control of a temperature stabilized laser diode is presented. The FFP with finesse of about 10 is observed in its reflected light at a fixed interference order to which the laser wave­ length is continuously tuned by a corresponding variation of the injection current. The sensor in its present state allows for a resolution better than 0,1°C and a measuring interval of about 20°C. These properties along with the short and thin sensor tip geometry makes it an attractive device for medical applications such as continuous temperature measure­ ments in hyperthermia systems. INTRODUCTION One of the specific properties of fiber optic sensors is their capacity to perform in electromagnetically contaminated environ­ ments. In the biomedical field of hyperthermia, i.e. the con­ trolled heating of biological tissues by microwave irradiation, there is the need for a continuous and high resolution temp­ erature measurement in the microwave field [l]. The temperature sensor signal should be suited for controlling the microwave generator itself in order to follow a well defined temperature versus time curve of the tissue under treatment.Conventional temperature sensors made from metallic or semi­ conductor materials have the disadvantage that the sensor element and the necessary wires heat up in the microwave field and distort the electromagnetic power distribution. This may cause wrong measurements as well as an inhomogeneous tempe­ rature distribution leading to so called hot spots within the ti ssue.In the present paper a temperature sensor is described which has specific characteristics to serve the needs for hyperther­ mia systems:-no microwave field deformation and no self heating;-short and thin sensor element suited for local and invasive measurement;-measuring range of about 35 to 55°C with a resolution of 0,1°C or better.