Laser fiber-optic heterodyne interferometer with frequency display of a phase advance of an optical signal in a fiber waveguide

An analysis is made of spontaneous oscillations in a laser fiber-optic heterodyne interferometer utilizing single-mode fiber waveguides forming a closed ring. A theory is given of steady-state single-frequency lasing in such a system, which represents a characteristic spontaneous oscillator with a delayed optical feedback. The condition for self-excitation and steady-state lasing are found and the frequency and amplitude of steady-state spontaneous oscillations are deduced. It is shown that a high stability of the lasing frequency is possible in a self-oscillatory system with heterodyning of optical oscillations in a reference laser. It is suggested that such a lasing interferometer can be used as a highly sensitive interferometric sensor of external perturbations of a fiber waveguide distinguished by a higher precision of manifestation of such perturbations in the frequency of spontaneous oscillations, i.e., in the quantity which at present can be measured more accurately than any other physical parameter.