An intuitive picture of the physics underlying optical ranging using frequency shifted feedback lasers seeded by a phase-modulated field

Frequency shifted feedback (FSF) lasers have been demonstrated to have interesting and useful features when used for optical ranging. The use of a phase-modulated seed to the FSF laser dramatically improves the signal-to-noise ratio, enabling distance measurements with the accuracy expected of optical interferometry. We present here an intuitively accessible description of the physics that underlies this dramatic enhancement of optical ranging signals. Unlike a free-running FSF laser, each one of the many equidistant frequency components of the seeded FSF laser spectrum (typically >104) has a definite amplitude, and a phase which varies with component number and modulation frequency ΩΩ of the seed radiation. Suitable adjustment of ΩΩ gives all components a common phase; the resulting constructive interference enhances the signal by orders of magnitude.