Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
2002-01-03
2004-10-05
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
Reexamination Certificate
active
06801319
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to fiber optic gyroscopes. In particular, the present invention relates to a fiber optic gyroscope incorporating a pair of thermally and mechanically symmetrical polarization-maintaining (PM) fiber segments for reducing polarization errors.
BACKGROUND OF THE INVENTION
Fiber optic gyroscopes (FOG) are increasingly employed in inertial guidance systems due to their ruggedness, compactness, and ability to sense very small rotations in contexts where external navigational cues are unavailable or impracticable in its most basic form. A fiber optic gyroscope system comprises, a “minimum reciprocal configuration” as shown in FIG.
1
.
Briefly, a light source
11
is used in conjunction with a fiber optic coupler
12
, an integrated optic chip (IOC)
16
, a loop
10
, a photodetector
14
, an amplifier
21
, a phase sensitive detector (PSD)
23
, a bias modulation system
20
, and a rotation indicator
26
. IOC
16
typically incorporates a polarizer
15
, a splitter combiner
17
, and a phase modulator
19
. Alternatively, individual components, such as a polarizer, coupler and optical phase modulator may be used in the place of IOC
16
.
The optical portion of the system contains several features within the optical path to assure that the system is reciprocal, i.e., that substantially identical optical paths are traversed by each of the opposite-traveling electromagnetic waves, except for the specific introductions of non-reciprocal phase difference shifts, as will be described below. Loop
10
comprises a long segment of optical fiber coiled about the rotational axis which is to be sensed. The optical fiber is typically 50 meters to 2,000 meters in length, and is part of a closed optical path in which an electromagnetic wave or light wave, is introduced and split into a pair of waves which propagate in clockwise (cw) and counterclockwise (ccw) directions through loop
10
, such that portions of both waves are ultimately diverted by coupler
12
onto a photodetector
14
.
The coiled optical fiber which forms loop
10
may be single mode (SM) fiber, a polarization-maintaining (PM) fiber, or a combination of SM and PM fiber. SM fiber allows the paths of the electromagnetic or light waves to be defined uniquely, and further allows the phase fronts of such a guided wave to also be defined uniquely. In contrast, PM fiber is constructed such that a very significant birefringence is produced in the fiber. As a result, polarization fluctuations introduced by unavoidable mechanical stresses, by the Faraday Effect in magnetic fields, or from other sources, which could lead to varying phase difference shifts between the counter-propagating waves, become relatively insignificant.
Thus, either the high refractive index axis, i.e., the slower propagation axis, or the low index axis, is chosen for propagating the electromagnetic waves depending on the other optical components in the system.
The electromagnetic waves which propagate in opposite directions through loop
10
are provided by light source
11
. This source is typically a broadband light source, for example, a semiconductor super-luminescent diode or a rare-earth doped fiber light source providing electromagnetic waves, typically in the near-infrared part of the spectrum, over a range of wavelengths between about 830 nm to 1550 nm. Source
11
preferably exhibits a short coherence length for emitted light in order to reduce the phase shift difference errors between these waves due to Rayleigh and Fresnel scattering at scattering sites in loop
10
. The broadband source also helps to reduce errors caused by the propagation of light in the wrong state of polarization.
Between light source
11
and fiber optic loop
10
there is shown an optical path arrangement formed by the extension of the ends of the optical fiber forming loop
10
to some optical coupling components which separate the overall optical path into several optical path portions. A portion of optical fiber extends from light source
11
to optical coupler
12
also known as a wave combiner/splitter.
Optical directional coupler
12
has light transmission media therein which extends between four ports, two on each end of the media, which are shown on each end of coupler
12
. One of these ports receives the optical fiber extending from light source
11
. Another port on the sense end of coupler
12
communicates with photodetector
14
, which detects electromagnetic waves, or light waves, impinging thereon, such as through the use of a p-i-n diode. In the case of two nearly coherent light waves, this output generally depends on the cosine of the phase difference between such a pair of substantially coherent light waves.
Optical directional coupler
12
has another optical fiber coupled to a port at the other end thereof which extends to a polarizer
15
provided within IOC
16
. The other port on the same side of coupler
12
typically comprises a non-reflective termination arrangement. Optical directional coupler
12
, upon receiving a light beam at any of its ports, transmits approximately half of the incoming light to each port on the other end of coupler
12
. Conversely, little or no light is transmitted to the port which is on the same end of coupler
12
.
In an SM fiber, light can propagate in two polarization modes. Thus, polarizer
15
within IOC
16
is provided for the purpose of passing light propagating at one polarization state such that clockwise (cw) and counterclockwise (ccw) waves of the same polarization are introduced into loop
10
, and only light of the same polarization for the cw and ccw waves interfere at detector
14
.
Because polarizer
15
does not entirely block the light having an undesirable state of polarization, a small non-reciprocity between the counter-rotating light beams is introduced, causing a non-reciprocal phase shift difference which can vary according to, inter-alia, the environmental conditions. In this regard, the high birefringence in the optical fiber used or the broad bandwidth of the light source used again aids in reducing this resulting phase difference.
Light from polarizer
15
is split by a splitter/combiner
17
provided within IOC
16
such that half of the incoming signal is diverted to one end of loop
10
, and half is diverted to the other end of loop
10
. The counter-propagating beams returning to IOC
16
are then combined by splitter/combiner
17
and sent to photodetector
14
through polarizer
15
and coupler
12
.
Optical modulator
19
provided within IOC
16
is capable of receiving electrical signals and thereby introducing a phase difference in electromagnetic waves transmitted therethrough by either changing the index of refraction or the physical length of the transmission medium, thereby changing the optical path length. Such electrical signals are typically supplied to modulator
19
by the bias modulation signal generator
20
providing either: (1) a sinusoidal voltage output signal at a modulation frequency f
b
that is intended to be equal to C
1
sin(&ohgr;
b
t), where &ohgr;
b
is the radian frequency equivalent of the modulation frequency f
b
, and C
1
is the amplitude of the modulation; or (2) a square wave modulation signal at f
b
. Other suitable periodic waveforms may also be used.
In general, operation of a fiber optic gyroscope is based on the Sagnac Effect, which describes the behavior of two beams of light traveling in opposite directions around a path undergoing rotation. Of the two light beams, the beam moving in the same direction as the loop's rotation will necessarily travel a greater distance than the beam traveling the opposite direction. This difference in path length, while small, will necessarily induce a phase shift in the combined beam. The portion of the resultant beam diverted to photodetector
14
through coupler
12
may be analyzed to yield a precise rotation rate. More particularly, the phase shift induced by rotation of the fiber loop is given by:
Δφ
=
2
π
&i
Kaliszek Andrew
Lange Charles
Szafraniec Bogdan
Honeywell International , Inc.
Schiff & Hardin LLP
Turner Samuel A.
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