Patent ID: 8134714

Claim:
A method of determining the distance between a waveguide and the surface of an object, the method comprising: providing a resonator having a first and a second reflective end surface, said second reflective end surface being the object surface, the distance to which is to be measured; transmitting electromagnetic radiation (λ LS ) in a waveguide, and emitting said electromagnetic radiation into said resonator, wherein said electromagnetic radiation comprises at least one resonance wavelength; forming at least one resonant wave inside said resonator by means of said electromagnetic radiation (λ LS ); transmitting a spectrum of said radiation (S R ), emitted out of said resonator, in a waveguide to a recording unit; recording said emitted spectrum (S R ) by said recording unit, and providing said recorded spectrum to an analyzing unit; estimating, at said analyzing unit, at least one value of the resonator length (L EST ), wherein said estimating of at least one value of the resonator length (L EST ) comprises performing a Fourier transform of at least a portion of said recorded spectrum; selecting, at said analyzing unit, at least one mark wavelength (λ M ) of said spectrum by determining a maximum, a minimum or an inflexion point of said recorded spectrum; computing an estimated interference number (I EST ), based on a ratio (2*L EST /λ M ) of two times said at least one value of the resonator length (L EST ) and said working wavelength (λ M ); determining an actual interference number (I CORR ) as the closest integer or half-integer to said estimated interference number (I EST ); and computing a second value of the resonator length (L CORR ) based on half the product (I CORR *λ M /2) of said actual interference number (I CORR ) and said working wavelength (λ M ).