High precision measurement of the free spectral range of an etalon

摘要

Methods, systems, apparatus and devices for using a modified PDH technique to measure the FSR of an etalon with one part per 10⁴ precision. An embodiment of the method for measuring the free spectral range of an etalon can include generating a laser light from a laser source, generating a RF source signal, RF modulating the laser light with the RF source signal to produce an RF modulated laser signal, coupling the RF modulated laser signal through a circulator to the etalon, coupling a reflected RF signal from the etalon through the circulator to photo detector, converting the reflected RF signal to an electrical signal at the photo detector, amplifying the electrical signal, mixing the amplified electrical signal with a RF delayed source signal, linearly scanning a frequency of the RF source signal, and monitoring a peak-to-peak mixer voltage V_mixer during the linear scanning of the RF source signal frequency to detect a peak-to-peak minimum voltage when the RF modulation frequency is tuned approximately to a free spectral range of the etalon, the result having a precision greater than one part per 10⁴ without the use of a high resolution optical spectrum analyzer or a tunable laser. This method is especially useful for etalons with small FSR (less than 10 GHz) because this method does not require a high resolution OSA or tuneable laser. As the ITU grid for DWDM becomes denser, this method will have a larger impact on the FSR measurement of etalons.