Method of and apparatus for wavelength detection

Our present invention relates to a method of and to an apparatus for wavelength detection in the determination of temperature by means of a fiber Bragg grating impressed in or written in a glass fiber. More particularly, the invention relates to a method of measuring a temperature in an electrical apparatus, particularly an electric transformer, utilizing a form of wavelength detection. A reflected light from a Bragg grating in an electrical transformer or other apparatus at which temperature is to be measured, is positioned by an optocoupler through a second fiber provided with a second Bragg grating whose reflection wavelength is different from that of the first grating. The nonreflected light intensity is measured by a photodetector and is used to signal the temperature measurement.