Methods, systems, apparatus and devices for using a modified PDH technique to measure the FSR of an etalon with one part per l0^4 precision. 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
