Patent ID: 6097487
Filing Date: 2000-08-01
Classification: G01J

Abstract:
A device for accurate and repeatable measurements of optical wavelengths, which can be the Bragg wavelengths of Fibre Bragg Gratings (FBG), including at least one interrogation broadband light source and a tuneable optical filter operated in a wavelength scanning mode to cover all wavelengths of interest, including the known wavelength of at least one FBG which acts as a wavelength reference, characterised in that a first part of the light emitted from at least one interrogation broadband source is, in either order, transmitted through the wavelength scanning tuneable filter and reflected from, or transmitted through, at least one FBG with a Bragg wavelength to be determined and said at least one fibre Bragg grating with known reference Bragg wavelength, and directed to at least one detector,while a second part of the light emitted from the at least one interrogation broadband source is, in either order, transmitted through the wavelength scanning tuneable filter and transmitted through, or reflected from, a Fabry-Perot filter with fixed and known free spectral range, optionally combined with an optical bandpass filter to restrict the wave-length range, creating a comb spectrum sampling the interrogation source spectrum with frequency equidistant peaks or dips with frequency separation equal to the known free spectral range to provide an accurate and repeatable frequency/wavelength scale independently of non-linearities, drift and hysteresis in the wavelength tuning response of the tuneable filter, as well as a means for compensating for errors in the wavelength measurements caused by gradients in the interrogation source spectrum, or wavelength dependent variations in the tuneable filter transmission, and directed to at least one detector, where the electrical output signals from the detectors are simultaneously sampled, compared and processed in a signal processing unit to provide accurate and repeatable wavelength measurements.