Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
2000-02-08
2003-03-25
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
Reexamination Certificate
active
06538745
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
(Not applicable)
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
(Not applicable)
BACKGROUND OF THE INVENTION
This invention relates generally to ring laser gyroscopes and more particularly, to multioscillator ring laser gyroscopes.
The ring laser gyroscope (RLG), in its simplest form, is a device comprising an arrangement of mirrors for directing light beams around a closed path through a gain region comprising a lasing gas and an arrangement of electrodes for creating an electrical discharge in the gas and a means for measuring the frequency difference of light beams thereby generated that are propagated around the closed path in opposite directions. The frequency difference of the light beams is a measure of the rotational rate of the RLG apparatus in the plane of the light beams.
A serious problem with this two-frequency RLG is that rotational rates near zero are difficult to measure because of lock-in—the coupling of the counter-propagating light beams as a result of backscatter arising from non-ideal optics. A commercially-successful two-frequency RLG has evolved that circumvents the lock-in problem by separating the frequencies of the counter propagating light beams at zero rotation rate by creating an artificial rotation rate. This artificial rotation rate is brought about by mechanically dithering either the RLG block or a mirror.
The multioscillator RLG represents a more sophisticated approach to solving the lock-in problem by utilizing a purely optical scheme. The scheme is based on the establishment of four resonant modes for the mirror system by placing, for example, a reciprocal polarization rotator and a nonreciprocal polarization rotator in the light path. The lock-in problem is avoided since the four resonant frequencies associated with the four resonant modes are all different, even at a zero rotation rate.
A typical resonant mirror system for a multioscillator RLG is shown in FIG.
1
. The four mirrors
1
constrain resonant light beams traveling in opposite directions to light path
3
. Circularly-polarized light beams experience reciprocal polarization rotations in reciprocal polarization rotator
5
and non-reciprocal polarization rotations in the Faraday rotator
7
. The magnetic field required by the Faraday rotator
7
is provided by permanent magnets
9
with magnetic fields within the magnets having directions as shown by the arrows.
The four resonant modes are CW/LCP, CCW/LCP, CCW/RCP, and CW/RCP, the acronyms CW and CCW standing respectively for clockwise and counterclockwise propagation around the closed path and LCP and RCP standing respectively for left-circularly-polarized light and right-circularly-polarized light. A measure of the rotation rate is obtained by first taking the differences in the frequencies of the right-circularly-polarized light beams and the frequencies of the left-circularly-polarized light beams and then taking the difference in the differences.
A typical example of a reciprocal polarization rotator is a crystalline-quartz element with its optic axis aligned with one portion of the light-beam path. Another way of achieving reciprocal polarization rotation is by using a non-planar light-beam path geometry. The non-reciprocal polarization rotator is typically a Faraday rotator consisting of a thin glass disc in which there is a magnetic field normal to the disc.
Another characteristic of modem RLGs is the use of some means of focusing the light beams so as to minimize the light-beam dimensions transverse to the light path. The usual focusing approach is to utilize a curved mirror for at least one of the mirrors that direct the light beams around the closed path.
The frequency of each of the four resonant laser modes of a multi-oscillator RLG is such that an integral number of wavelengths will fit exactly within the path length of the resonant cavity. A gas mixture, typically comprised of helium and neon, provides gain for the laser beam. In order to ensure proper laser operation, the cavity length must be tuned in such a way that the gas medium will supply sufficient gain at the cavity's resonant frequencies. The methods of ring laser gyro cavity length control are extensively discussed in the literature.
In a multioscillator RLG, four laser modes with widely separated frequencies must be simultaneously sustained within the cavity. A gas mixture providing gain over a wide range of frequencies is used to ensure sufficient gain for all of the modes when the cavity is properly tuned using, for example, the methods disclosed in U.S. Pat. No. 5,208,653.
Cavity length control is usually accomplished by observing the laser intensity and controlling the voltage applied to a piezo-electric transducer which in turn applies force to a mirror diaphragm. This force causes a very slight motion of the mirror face, thereby changing the cavity length. A servo is used to apply the voltage which maximizes the laser intensity.
It has been observed that under high-shock accelerations, the mirror and piezo-electric forcer assembly also move relative to the cavity body due to their inertia and the finite stiffniess of the assembly and the servo loop. As a result, the instantaneous cavity length can change substantially during the shock. If the length change is large enough (more than a third or half a laser-mode wavelength of 630 nm). some of the laser modes may drop out due to insufficient gain. This radically affects the operation of the gyro and can cause erroneous angle indications.
This invention takes advantage of certain properties of the multioscillator RLG to provide accurate gyro outputs even under large-shock conditions. As shown in
FIG. 2
a
, the multioscillator RLG operates, in the absence of the reciprocal and nonreciprocal polarization rotators
5
and
7
of
FIG. 1
, at a single frequency denoted by the intensity arrow near the maximum of the gain curve. The reciprocal polarization rotator
5
splits this single mode into RCP and LCP modes as shown in
FIG. 2
b
. With this mode splitting the multioscillator RLG becomes two circularly-polarized gyros, an RCP gyro and an LCP gyro, co-existing in the same cavity. The resonant frequencies of the two gyros are sufficiently far apart as to be separable, and the gas mixture is chosen to ensure a broad gain curve which allows both modes to lase simultaneously.
The nonreciprocal polarization rotator
7
splits the RCP and LCP modes into CW and CCW modes as shown in
FIG. 2
c
. The mode frequencies in the absence of any angular rotation of the RLG (
FIG. 2
c
and
FIG. 2
d
) are shifted by an angular rotation of the RLG as shown in
FIG. 2
e
. Each of the two gyros is biased by a magneto-optic Faraday crystal in order to provide a separation between the clockwise and counter-clockwise beams and thereby prevent lock-in. The Faraday bias corresponds to a large angular rotation rate (on the order of 1000 degrees/sec) and is sensitive to temperature, magnet field strength, etc.
The two-gyro configuration of the multioscillator RLG permits common mode cancellation of the Faraday bias while providing an accurate angular rotation measurement. This is accomplished in the following way. The output frequencies f
R
and f
L
of the RCP and LCP gyros is the difference in frequency of the CCW and CW modes. Thus,
f
R
=
f
F
-
1
2
f
Δ
0
(
1
)
f
L
=
f
F
+
1
2
f
Δ
θ
(
2
)
where f
F
is the Faraday bias frequency and f
&Dgr;&thgr;
is proportional to the angular rotation measure of the RLG. For convenience, we will refer to f
&Dgr;&thgr;
as the “angular rotation measure”.
In normal operation, the angular rotation measure is determined by taking the difference of the two gyro output frequencies:
f
&Dgr;&thgr;
=f
L
−f
R
(3)
The Faraday bias frequency can be determined from any of the equations
f
F
=
f
L
-
1
2
f
Δ
θ
(
4
)
f
F
=
f
R
+
1
2
f
Δ
θ
(
5
)
f
F
=
1
2
(
f
L
+
f
R
)
(
6
)
In the eve
Fann Shaw W.
Lottman Brian T.
Mark John G.
Tazartes Daniel A.
Litton Systems Inc.
Malm Robert E.
Turner Samuel A.
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