ANALYSIS AND COMPENSATION OF THE MEASUREMENT ERROR IN A LOOK-IN AMPLIFIER USED FOR WAVELENGTH SHIFT MEASUREMENTS IN OPTICAL SENSING APPLICATION

The phase measurement error in an interferometric wavelength shift mea surement scheme, such as that used in association with fiber Bragg gra ting sensors, has been investigated experimentally with appropriate un derpinning analytical theory. It has been shown that when a lock-in am plifier is used to detect the phase shift generated by the Bragg wavel ength shift, a pseudoperiodical measurement error can be introduced ow ing to the difference between the amplitude of the optical path differ ence ramp and the value of the Bragg wavelength. If the initial ramp d eviation equals 20 nm, the measurement error may be as large as +/-2.5 % of the total measurement range. With a double-phase lock-in amplifie r approach to measure the ac strain, the; measurement error can be dec reased to 0.4% of the total measurement range, With the Peal-time meas ured period that corresponds to the Bragg wavelength with the distorte d carrier signal of the interferometer as the reference period oi a di gital lock-in amplifier the effect of the initial ramp deviation can b e principally avoided, and the measurement error can be kept to an acc eptably low level, about 0.1% of the total measurement range. (C) 1997 Optical Society of America.