Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
2000-11-09
2003-10-28
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
C356S460000
Reexamination Certificate
active
06639680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser gyro and its driving method and, in particular, to a method for driving the semiconductor laser gyro for generating a laser oscillation by injecting a current.
2. Related Background Art
In the past, there have been known mechanical gyros having rotators and vibrators or optical gyros for detecting the angular velocity of a moving object. Since the optical gyros, in particular, are capable of starting instantaneously and have wide dynamic ranges, innovation is being brought about in the field of gyro technology. The optical gyros include laser gyros, optical fiber gyros, passive resonator gyros and the like. Among them, a development of a laser gyro employing a gas laser was first undertaken and it has been already put to practical use in aircrafts and the like. Recently, as a small and highly accurate laser gyro, a semiconductor laser gyro integrated on a semiconductor substrate has been proposed, for which known literature is available in the form of Japanese Patent Application Laid-Open No. 5-288556.
In order to increase a detecting sensitivity for the angular velocity, when a length of a ring cavity of a ring laser is taken as L and a closed area surrounded by the optical path as S, S/L is made large, by which a beat frequency accompanied by a rotation may be increased. However, there was a problem in that a driving current becomes large if S/L is made large in the semiconductor laser gyro. This is because the start of a laser oscillation is determined by a carrier concentration in an active layer, and the driving current is increased in proportion to the area of the active layer.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser gyro in which the time average value of the driving current is small, and a method for driving the semiconductor laser gyro.
According to an aspect of the present invention, there is provided a ring laser gyro having means for injecting a current of a first current value not less than an oscillation threshold value to a ring laser gyro, and then injecting a current of a second current value smaller than the first current value to the ring laser gyro. In the ring laser gyro, a beat signal can be detected even in a state wherein the current of the second current value is injected and flowing. In the ring laser gyro, circularly counterpropagating first and second laser beams may be different from each other in the oscillation frequency when the gyro is stationary. Further, the second current value may be lower than the oscillation threshold value. Also, in this embodiment of the ring laser gyro, a beat signal can be detected even in a state wherein the current of the second current value is injected and flowing.
According to another aspect of the present invention, there is provided a method for driving a ring laser gyro, which involves injecting a current of a first current value not less than an oscillation threshold value to a semiconductor laser gyro, and then injecting a current of a second current value smaller than the first current value.
According to still another aspect of the present invention, there is provided a method for driving a ring laser gyro, which involves injecting a current of a first current value not less than an oscillation threshold value to a semiconductor laser gyro, and then injecting a current of a second current value smaller than the first current value, which is taken as one pair of operations, and repeating the pair of operations plural times. The method for driving a ring laser gyro may further comprise injecting a current of the first current value in the next pair of the operations while the laser beams are propagating inside the ring resonator after the injecting step of the current of the second current value, wherein the semiconductor laser gyro starts a laser oscillation by the injected current, and propagates laser beams inside the ring resonator.
In the above methods for driving a ring laser gyro, the current of the second current value may be not less than a current value where a medium becomes transparent.
In the above method for driving a ring laser gyro, the current of the second current value may be not less than the oscillation threshold current.
In the above method for driving a ring laser gyro, a sidewall of the semiconductor laser gyro is a surface with total reflection for oscillated laser beams.
The operation of the above configuration will be described by using an equation. First, the beat frequency &Dgr;f in the laser gyro can be expressed as follows by using a cavity length L of the ring laser, a closed area S surrounded by the optical path, an oscillation wavelength &lgr; and an angular velocity &OHgr;:
Δ
f
=
4
S
λ
L
Ω
(
1
)
This equation demonstrates that S/L should be increased in order to make the beat frequency &Dgr;f large. However, if S/L is made large in the semiconductor laser gyro, a driving current becomes large. This is because the start of the laser oscillation is determined by the carrier concentration in the active layer. Consequently, the driving current is increased in proportion to the area of the active layer. In relation to this, the present invention proposes a driving method in which a time average value of the driving current is small and its structure.
Rate equations concerning a photon density S of the laser beam, a photon density S
SP
of a spontaneous emission and the carrier concentration n can be given by the following equations:
ⅆ
S
ⅆ
t
=
G
(
n
)
S
+
β
s
n
τ
r
-
S
τ
ph
(
2
)
ⅆ
S
sp
ⅆ
t
=
n
τ
r
-
S
sp
τ
ph
′
(
3
)
ⅆ
n
ⅆ
t
=
I
eV
-
G
(
n
)
S
-
n
τ
n
+
G
′
(
n
)
S
sp
(
4
)
wherein G(n) denotes a gain coefficient for the laser beam, &bgr;
s
a spontaneous emission coupling factor, &tgr;
r
a radiative recombination lifetime of the carrier, &tgr;
ph
a photon lifetime for the laser beam, &tgr;
ph
′ a photon lifetime for the spontaneous emission, I an injection current, e an elementary electric charge, V a volume of the active layer, &tgr;
n
a carrier lifetime, and G (n)′ an absorption coefficient for the spontaneous emission.
From the above, a steady state d/dt=0 is considered. Since the spontaneous emission coupling factor &bgr;
s
is in the order of 10
−4
to 10
−6
, if the contribution of &bgr;
s
is ignored in equation (2), the following relation holds:
G
(
n
th
)
=
1
τ
ph
(
5
)
Also, from equation (4), the oscillation threshold current I
th
becomes as follows:
I
th
=
eV
τ
n
n
th
[
1
-
G
′
(
n
th
)
τ
ph
′
]
(
6
)
Here, n
th
is a carrier concentration of the oscillation threshold, and if the contribution of a nonradiative recombination is negligible, the following equation:
&tgr;
r
≅&tgr;
n
is established.
Moreover, from equation (2) and equation (4), the photon density S in I≧I
th
is given by the following equation:
S
=
τ
ph
[
I
eV
-
n
th
τ
n
+
G
′
(
n
)
τ
ph
′
τ
r
n
th
]
(
7
)
Now, when injection current I is once taken as a value I
1
not less than the oscillation threshold current I
th
, the laser oscillation begins. If it is given an in-plane rotation including a ring resonator in this state, the circularly counterpropagating laser beams interfere with each other to generate a beat. By detecting this beat frequency, the angular speed of the rotation is known.
Next, after the laser oscillation, injection current I is lowered to I
2
(<I
1
). The laser beam circularly propagates inside the ring resonator after the laser is oscillated. Particularly, if the sidewall of the element is a total reflective surface, the laser beam is not emitted
Fitzpatrick ,Cella, Harper & Scinto
Turner Samuel A.
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