Techniques for increasing a quality factor of an optical filter without substantially increasing filter size and applications thereof

A method for reducing phase noise of a first optical signal, the method comprising:

• receiving, from an input/output port of an optical circulator, the first optical signal at a first port of a filter, wherein the first optical signal is externally provided from a laser to an input port of the optical circulator;
• filtering, with the filter, the first optical signal;
• emitting a first filtered optical signal from a second port of the filter;
• reflecting the first filtered optical signal into the second port of the filter;
• emitting a first twice filtered optical signal from the first port of the filter, through the optical circulator, and then from an output port of the optical circulator;
• emitting a portion of the first optical signal from a third port of the filter;
• and
• using the portion of the first optical signal, adjusting at least one of an amplitude and a frequency of the first optical signal.

A resonant fiber optical gyroscope (RFOG) comprising:

• an optical signal source comprising at least one filtered optical signal generator and configured to generate a first twice filtered optical signal and a second twice filtered optical signal, wherein each filtered optical signal generator comprises:
• an optical filter comprising:
• a first optical waveguide comprising a first surface, a first port, and a second port, wherein the first port is configured to receive, through an optical circulator, an input optical signal provided by an optical signal generator;
• a second optical waveguide comprising a second surface, a third port, and a fourth port;
• an optical resonator comprising a first portion of a third surface and a second portion of the third surface;
• wherein the first surface is adjacent and optically coupled to the first portion of the third surface;
• wherein the second surface is adjacent and optically coupled to the second portion of the third surface;
• and
• an optical reflector configured to reflect optical energy emitted from the third port back into the third port;
• the optical circulator comprising an input/output port optically coupled to the first port, an input port, and an output port;
• wherein the input optical signal propagates from the input port, through the optical circulator, and then out of the input/output port;
• wherein the twice filtered optical signal is derived from the input optical signal and propagates from the first port, into the input/output port, and out of the output port;
• the optical signal generator comprising a laser and optically coupled to the input port and configured to emit the input optical signal;
• an optical detector optically coupled to the second port and configured to emit a control signal responsive to a portion of the input optical signal incident on the optical detector;
• and
• control circuitry between the optical detector and the optical signal generator, configured to receive the control signal, and further configured to adjust at least one of an amplitude and a frequency of the input optical signal emitted from the laser;
• a first optical signal separation circuit comprising a first circuit port, a second circuit port, and a third circuit port, and configured to receive, at the first circuit port, the first twice filtered optical signal from the optical signal source;
• a second optical signal separation circuit comprising a fourth circuit port, a fifth circuit port, and a sixth circuit port, and configured to receive, at the fourth circuit port, the second twice filtered optical signal from the optical signal source;
• an optical resonator comprising a rotation axis;
• a four port signal separation circuit comprising two mirrors, a seventh circuit port optically coupled to the third circuit port, an eighth circuit port optically coupled to the sixth circuit port, an nineth circuit port, and a tenth circuit port, wherein the nineth and tenth circuit ports are optically coupled to the optical resonator, wherein the four port signal separation circuit is configured to optically couple a portion of each of the first twice filtered and the second twice filtered optical signals into the optical resonator;
• processing circuitry comprising processor circuitry electrically coupled to memory circuitry;
• a first frequency detector coupled to the second circuit port and the processing circuitry, and configured to determine a frequency of a portion of the portion of the second twice filtered optical signal emitted from the optical resonator;
• wherein the four port signal separation circuit is further configured to optically couple a portion of the portion of the first twice filtered optical signal to the first frequency detector;
• and
• a second frequency detector coupled to the fifth circuit port and the processing circuitry, and configured to determine a frequency of a portion of the portion of the first twice filtered optical signal emitted from the optical resonator;
• wherein the four port signal separation circuit is further configured to optically couple a portion of the portion of the second twice filtered optical signal to the second frequency detector;
• wherein the processing circuitry is configured to determine a rate of rotation around the rotation axis of the optical resonator using the frequency of a portion of the portion of the first twice filtered optical signal and the frequency of a portion of the portion of the second twice filtered optical signal.