US-12624950-B2 - Passive topologically biased Sagnac interferometer as a rotational sensor capable of sensing magnitude and direction of rotation

US12624950B2US 12624950 B2US12624950 B2US 12624950B2US-12624950-B2

Abstract

Many optical gyroscopes are based on an optical Sagnac interferometer configuration, including various interferometric fiber-optic gyroscopes (IFOGs), to measure the magnitude and direction of rotation. IFOGs require active phase modulation in their fiber coil to decipher the direction of rotation. This patent document discloses a new type of IFOG that utilizes a passive topological (also known as geometric) phase shift to sense the magnitude and direction of rotation without requiring active phase modulation.

Inventors

Farhad Hakimi
Hosain Hakimi

Assignees

Farhad Hakimi
Hosain Hakimi

Dates

Publication Date: 20260512
Application Date: 20230516

Claims (6)

1 . A fiber optic Sagnac interferometer passively biased by a topological phase element comprising: a light source; a photodetector; a linear polarizer; a depolarizer; a topological phase element comprising: a first achromatic quarter-wave plate, optically connected to an achromatic half-wave plate with its optical axis oriented at 45 degrees relative to the axis of the first achromatic quarter-wave plate, further optically connected to a second achromatic quarter-wave plate with its optical axis oriented at 45 degrees relative to the axis of the achromatic half-wave plate; a non-polarization-maintaining single-mode fiber coil; a first non-polarizing beam splitter/circulator; a second non-polarizing beam splitter; the output of said light source optically connected to the input of said first non-polarizing beam splitter/circulator; the reflected output of said first beam splitter/circulator optically connected to said photodetector; the transmitted output of said first beam splitter/circulator optically connected to the input of said linear polarizer; the output of said linear polarizer optically connected to the input of said second non-polarizing beam splitter; the transmitted output of said second non-polarizing beam splitter optically connected to the input of said depolarizer; the output of said depolarizer optically connected to said topological phase bias element; the output of said topological phase bias element optically connected to a first input of said fiber coil; the reflected output of said second non-polarizing beam splitter optically connected to a second input of said fiber coil.
2 . The interferometer of claim 1 , wherein said light source is depolarized.
3 . The interferometer of claim 1 , wherein said light source is linearly polarized with its polarization axis aligned to the transmission axis of said linear polarizer.
4 . A tethered passively biased Sagnac interferometer comprising: a first optoelectronic box containing a light source and a photodetector; the output of said light source optically connected to the input of a first non-polarizing beam splitter/circulator residing in said first box; the reflected output of said first non-polarizing beam splitter/circulator optically connected to said photodetector; the transmitted output of said first non-polarizing beam splitter/circulator optically connected to a first input end of a fiber cable tethered outside of said first box; a second optics box optically connected to a second input end of said tethered fiber cable; residing in said second optics box: a linear polarizer; a second non-polarizing beam splitter; a depolarizer; a topological phase bias element; a non-polarization-maintaining single-mode fiber coil; the output of said linear polarizer optically connected to the input of said second non-polarizing beam splitter; the transmitted output of said second non-polarizing beam splitter optically connected to the input of said depolarizer; the output of said depolarizer optically connected to said topological phase bias element; the output of said topological phase bias element optically connected to a first input of said fiber coil; the reflected output of said second beam splitter optically connected to a second input of said fiber coil; wherein said topological phase bias element comprises: a first achromatic quarter-wave plate, optically connected to an achromatic half-wave plate with its optical axis oriented at 45 degrees relative to the axis of the first achromatic quarter-wave plate, further optically connected to a second achromatic quarter-wave plate with its optical axis oriented at 45 degrees relative to the axis of the achromatic half-wave plate.
5 . The interferometer of claim 4 , wherein said light source emits depolarized light and said fiber cable is a single-mode optical fiber.
6 . The interferometer of claim 4 , wherein said light source emits linearly polarized light; said fiber cable, residing outside of said first optoelectronic box, is a polarization-maintaining single-mode fiber with its principal axis aligned to the polarization axis of said polarized light source; and the polarization axis of said polarization-maintaining single-mode fiber cable is aligned to the transmission axis of said linear polarizer in the second optics box.

Description

FIELD OF THE INVENTION The invention relates to fiber-optic sensors, the Sagnac interferometer, a gyroscope, fiber-optic Sagnac interferometry, and a passively biased fiber-optic gyroscope. BACKGROUND OF INVENTION The so called open loop minimum configuration of the interferometric fiberoptic gyroscope (IFOG) was developed at Stanford University in 1981 and is depicted in FIG. 1. Light from a light source exits a 2×2 or 2×1 source coupler/circulator and, after going through a 1×2 or 2×2 coil coupler by way of a phase modulator, is split into two beams that propagate around the fiberoptic sensing coil of the gyro in opposite directions. The phase difference between the two light beams is in turn proportional to an input rotation rate about the input axis of the gyro's sensing fiber coil. When not rotating, the interferometer optics ensure that both beams traverse the same optical path and thus yield a nominally zero bias for the gyro. In FIG. 1, in order to distinguish between clockwise and counter clockwise rotation in an open loop configuration, a phase modulator driven by an electric sinusoidal signal must be placed to one side of the fiberoptic sensing coil of the gyro. Applying a sinusoidal voltage to the modulator induces phase modulation onto the light traveling through the optical path of the gyro. The gyro's output is then synchronously demodulated at the first harmonic by a lock-in amplifier. Synchronous demodulation at the first harmonic converts the output of the gyro from that of a raised co-sinusoid to a sinusoidal scale factor. The sinusoidal scale factor is desirable here because of the slope through zero, which is anti-symmetric, placing the gyro at its quadrature point of operation for maximum sensitivity and thereby making it possible to tell the direction of rotation. In other words, without an active phase modulator in FIG. 1, the gyro would not be able to distinguish clockwise from counterclockwise rotation. The past decades have seen excellent realizations in practical applications of gyroscopic sensors employing active bias, e.g., piezoelectric phase modulators or integrated optics chips (Y-waveguide junctions). The light polarization axis in the fiber coil in FIG. 1 must be controlled. Any change in the light polarization axis behaves as a false rotation signal. Therefore having a depolarized light source and depolarizers in the fiber coil improves the performance of the IFOG substantially. Therefore an improved version of FIG. 1 embraced by most gyro manufacturers uses all linear high birefringence polarization maintaining (PM) fiber coil and components and thus drop the need for polarization control in FIG. 1. PM fiber and components ensure the state of polarization in the fiber does not change due to environmental conditions which as mentioned before cause erroneous rotation signals. However to function effectively, linear PM fibers and components require precise polarization axis alignment between the fiber and the light polarization axis. In practice alignment is within 0.5 degree which limits performance. Furthermore the polarization maintaining performance of PM fiber drops sharply if the length of the fiber coil exceeds 3 kilometer. Additionally, extinction ratios of PM couplers should be better than 20 dB to be suitable for use in practical gyro applications. Finally, PM fiber and components are expensive and add to the price of gyro units considerably. IFOG with PM fiber and components still require active phase modulation and has several shortcomings and disadvantages: 1. Phase modulators require sinusoidal or square wave signal generator and driver as FIG. 1 indicates. The phase modulator itself is prone to optical bias drifts and requires complex electronic driver countermeasures.2. A practical phase modulator is a piezoelectric transducer with standard or PM fiber wrapped around it. The process is mechanical and results in unwanted stresses on the fiber which reduces its longevity.3. The modulation frequency of piezoelectric transducers is limited. For a fiber coil of 100 meters, the modulation rate is about 1 MHz (˜100 kHz-km). This is near the upper limit of piezoelectric transducers and consequently the length of the fiber coil cannot be chosen shorter if reduction of coil length and cost is desired.4. In a vibration prone environment, the piezoelectric transducer can generate voltages (causing erroneous phase shifts) that obscure the real rotation from fictitious signals.5. Alternatively, lithium niobate integrated optical chip phase modulators can be used. While these modulators do not have the mentioned modulation rate limitations of piezoelectric transducers, they are costly and still subject to optical drift due to radiation, thermal stress, and the acoustic environment.6. In some circumstances, it is desirable to have the gyro's sensing coil separated from its optoelectronics box. The separation also thermally insulates the sensing coil and reduce optical drift