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

1 . An apparatus for reducing phase noise of an input optical signal, the apparatus 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, the input optical signal from a laser external to the first optical waveguide, a second optical waveguide, and an optical resonator;

the second optical waveguide comprising a second surface, a third port, and a fourth port;

the 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;

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, an input port, and an output port, wherein the input/output port is optically coupled to the first 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 first port is configured to emit an output optical signal which is derived from the input optical signal and propagates from the input/output port to out of the output port;

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 coupled between the optical detector and the laser, 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.

2 . The apparatus of claim 1 , wherein the optical resonator is an optical ring resonator, an optical disc resonator, or an optical race track resonator.

3 . The apparatus of claim 1 , wherein the optical reflector is a mirror, a Bragg grating, or a loop mirror.

4 . The apparatus of claim 1 , wherein the first and the second optical waveguides and the optical resonator each comprise planar optical waveguide formed on a substrate.

5 . The apparatus of claim 1 , wherein the optical detector comprises a photosensor configured to generate the control signal which is an electrical signal whose amplitude is proportional to an optical power of the portion of the input optical signal incident on the optical detector.

6 . 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.

7 . The method of claim 6 , further comprising:

generating a second twice filtered optical signal;

coupling a portion of the first twice filtered optical signal and a portion of the second twice filtered optical signal into an optical resonator;

propagating, in different directions around the optical resonator, the portion of the first twice filtered optical signal and the portion of the second twice filtered optical signal;

emitting, from the optical resonator, a portion of the portion of the first twice filtered optical signal and a portion of the portion of the second twice filtered optical signal;

determine a frequency of each of the portion of the portion of the first twice filtered optical signal and the portion of the portion of the second twice filtered optical signal; and

using a difference between each determined frequency, determine a rate of rotation around a rotation axis of the optical resonator.

8 . The method of claim 7 , wherein the second twice filtered optical signal is generated from the first twice filtered optical signal.

9 . The method of claim 7 , wherein determining the rate of rotation comprises determining the rate of rotation using the difference between each determined frequency, an average of a wavelength of the first twice filtered optical signal and the second twice filtered optical signal, and an area of a cross-section of the optical resonator.

10 . 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.

11 . The RFOG of claim 10 , wherein the optical resonator is an optical ring resonator, an optical disc resonator, or an optical race track resonator.

12 . The RFOG of claim 10 , wherein the optical reflector is a mirror, a Bragg grating, or a loop mirror.

13 . The RFOG of claim 10 , wherein the optical detector comprises a photosensor configured to generate the control signal which is an electrical signal whose amplitude is proportional to an optical power of the portion of the first twice filtered optical signal incident on the optical detector.