Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
2000-11-28
2003-11-25
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
Reexamination Certificate
active
06654126
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an angular velocity detecting apparatus, and particularly to an apparatus for detecting rotational angular velocity by the utilization of a ring laser type gyro.
2. Related Background Art
A mechanical gyro having a rotor or a vibrator and an optical gyro are known as gyros for detecting the angular velocity of a moving object. Particularly the optical gyro is capable of starting in a moment and has a wide dynamic range and therefore is bringing about an innovation in the technical field of gyro. Optical gyros include a ring laser type gyro, an optical fiber gyro, a passive type ring resonator gyro, etc. Among these, the ring laser type gyro using a gas laser is the one of which the development has been started earliest, and has already been put into practical use in aircraft, etc. Recently, as a compact and highly accurate ring laser type gyro, there is a semiconductor laser gyro integrated on a semiconductor substrate.
FIG. 12
of the accompanying drawings is a plan view of an example of the optical gyro which can detect not only the magnitude of angular velocity but also a direction of rotation. The reference numeral
10
designates a quartz tube, the reference numeral
11
denotes the asymmetrical tapered portion of a light waveguide, the reference numeral
12
designates a mirror, the reference numeral
13
denotes an anode, the reference numeral
14
designates an electrical terminal, the reference numeral
15
denotes a cathode, the reference numeral
100
designates a counter-clockwise laser beam, and the reference numeral
110
denotes a clockwise laser beam.
It is to be understood here that the wavelength of a first laser beam travelling round clockwisely is &lgr;
1
. Also, it is to be understood that the wavelength of a second laser beam travelling round counter-clockwisely is &lgr;
2
(<&lgr;
1
) When the laser is rotated clockwisely, the oscillation frequency f
1
of the clockwise first laser beam decreases by
Δ
f
1
=
2
S
1
λ
1
L
1
Ω
(
1
)
as compared with the oscillation frequency f
10
, during the non-rotation. Here, S
1
is the closed area surrounded by the optical path of the first laser beam, L
1
is the length of the optical path of the first laser beam, and &OHgr; is the angular velocity of the rotation. On the other hand, the oscillation frequency f
2
of the counter-clockwise second laser beam increased by
Δ
f
2
=
2
S
2
λ
2
L
2
Ω
(
2
)
as compared with the oscillation frequency f
20
during the non-rotation. Here, S
2
is the closed area surrounded by the optical path of the second laser beam, and L
2
is the length of the optical path of the second laser beam. At this time, the first laser beam and the second laser beam coexist in the laser. Accordingly, a beat light having the difference between the oscillation frequencies of the first laser beam and the second laser beam, etc.,
f
2
-
f
1
=
f
20
-
f
10
+
(
Δ
f
2
+
Δ
f
1
)
=
f
20
-
f
10
+
(
2
S
2
λ
2
L
2
Ω
+
2
S
1
λ
1
L
1
Ω
)
(
3
)
is created in the laser. On the other hand, during the counter-clockwise rotation, a beat light having a frequency indicated in the following expression (4) is created.
f
2
-
f
1
=
f
20
-
f
10
-
(
Δ
f
2
+
Δ
f
1
)
=
f
20
-
f
10
-
(
2
S
2
λ
2
L
2
Ω
+
2
S
1
λ
1
L
1
Ω
)
(
4
)
When two or more oscillation modes exist in the laser, the inverted population exhibits a time fluctuation conforming to the difference between the oscillation frequencies of the modes. This phenomenon is known as the pulsation of inverted population. In the case of a laser letting an electric current flow such as a gas laser or a semiconductor laser, there is the correspondence relation of 1 to 1 between the inverted population and the impedance of the laser. When lights interfere with each other in the laser, the inverted population changes in conformity therewith and as the result, the impedance between the electrodes of the laser changes. The manner of this change appears as a change in a terminal current if a constant voltage source is used as a driving power source. Also, if a constant current source is used, the manner of interference between lights can be taken in the form of a signal as a change in a terminal voltage. Of course, any change in the impedance can also be directly measured by an impedance meter.
Accordingly, by providing a terminal for detecting any change in the electric current, voltage or impedance of the laser, a beat signal conforming to rotation can be taken out from this terminal. Further, as shown in expressions (3) and (4), the beat frequency increases or decreases in conformity with the direction of rotation.
Accordingly, by observing an increase or decrease in the beat frequency from during the non-rotation, the direction of rotation can be detected. It is when the difference between the oscillation frequencies satisfies the following expression (5) that the direction of rotation can be detected.
f
2
−f
1
≧0 (5)
If the oscillation wavelengths of the first laser beam and the second laser beam are equal to each other,
f
20
−f
10
=0 (6)
and the beat frequency f
2
−f
1
assumes positive and negative signs. If the absolute values of the beat frequencies are equal to each other, the same signal is outputted from terminals and therefore, in this case, the direction of rotation cannot be detected.
In contrast, if design is made such that the signs of the beat frequencies are always the same (in the description, the sign is taken as positive) and the absolute values thereof change depending on the direction of rotation, the detection of the direction of rotation will become possible.
Now, to change the oscillation threshold values of the laser beams propagating round in opposite directions of rotation in the ring laser, loss can be given to only the light propagating round in one direction of rotation. For example, by providing a tapered portion of an asymmetrical shape on a portion of the light waveguide, the total reflection condition deviates relative to a light incident on this tapered portion. Therefore, mirror loss occurs to the light incident on the tapered portion. The angle of incidence onto the tapered portion differs depending on the direction of rotation of the light and therefore, loss can be made great to the laser beam propagating round in a certain direction and loss can be made small to the light propagating round in the opposite direction. As the result, the oscillation threshold value can be changed for the laser beams propagating round in opposite directions of rotation.
Now, it is known that when two modes coexist, there are relations shown in the following expressions (7) and (8) between the oscillation frequency fi and photon number density Si (i=1, 2).
2
&pgr;f
1
+&PHgr;
1
=&OHgr;
1
+&sgr;
1
−&rgr;
1
S
1
−&tgr;
12
S
2
(7)
2
&pgr;f
2
+&PHgr;
2
=&OHgr;
2
+&sgr;
2
−&rgr;
2
S
2
−&tgr;
21
S
1
(8)
Here, &PHgr;i is a phase, &OHgr;i is an resonance angle frequency, &sgr;i is a coefficient representative of the entrainment of the mode, &rgr;i is a coefficient representative of the self-extrusion of the mode, and &tgr;ij is a coefficient indicative of the mutual extrusion of the modes. Here, i, j=1, 2; i≠j. Since the oscillation threshold value differs, the photon number density Si (i=1, 2) differs. Accordingly, in accordance with expressions (7) and (8), a difference can be given between the oscillation frequencies.
Specifically, for example, in the above-described construction, a quartz block is hollowed out by the use of a drill to thereby form the quartz tube
10
. Thereafter, the mirror
12
is attached to the quartz tube
10
.
Canon Kabushiki Kaisha
Morgan & Finnegan L.L.P.
Turner Samuel A.
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