Biplanar cube unidirectional ring laser gyroscope

A laser gyroscope comprising two ninety degree turning prisms optically connected. A non-reciprocal gain layer structure is grown on the hypotenuse face of one prism.

BACKGROUND OF THE INVENTION 
1. Field of the Invention (Technical Field) 
The present invention relates to laser gyroscope apparatuses and methods of 
making and using same. 
2. Background Art 
Laser (or optical) gyroscopes are useful for missile guidance systems, 
aircraft guidance systems, oil drilling equipment, and robotics 
applications. However, preexisting laser gyroscopes have the disadvantages 
of being bulky and/or relatively complex, and plagued by "lock-in" effects 
or absence of response for low rotation. To obtain a response for all 
rotations, the existing laser gyro has to be given a constant mechanical 
motion or dither. There is a need for small laser gyros not having moving 
parts. 
Existing laser gyroscopes are described in U.S. Pat. Nos. 5,311,538, to 
Thorland; 5,241,555, to Spitzer; No. 5,004,343, to Dorschner et al.; 
4,397,027, to Zampiello et al.; 4,035,081, to Sepp et al.; and 4,299,490, 
to Cahill et al. Existing ring lasers are described in U.S. Pat. Nos. 
5,349,601, to Hohimer et al.; 5,177,764, to Nilsson; 5,115,446, to von 
Borstel et al.; 5,027,367, to Rea, Jr. et al.; and 4,955,034, to Scerbak. 
A laser resonator employing one or more resonant prisms is described in 
U.S. Pat. No. 3,611,436, to Rigrod. 
The present invention has a relatively simple structure and can be 
incorporated into semiconductor materials of quite small size. 
SUMMARY OF THE INVENTION 
Disclosure of the Invention 
The present invention is of a laser gyroscope apparatus and a method for 
generating gyroscopic response comprising: two ninety degree turning 
prisms optically contacted to form a cube. In the preferred embodiment, 
the prisms are GaAs turning prisms, a hypotenuse face of one prisms has a 
non-reciprocal gain layer structure, and the cube has at least two planes 
into which a laser beam may be directed, generating a unidirectional ring 
laser. One of the ring lasers operates clockwise and the other 
counterclockwise, the beat frequency between said two planes exhibits a 
gyroscopic response, and the ring lasers do not experience injection 
lock-in because the weak beam of the ring lasers are ninety degrees out of 
phase with the strong beam of the ring lasers. 
A primary object of the present invention is to provide a compact (about 
one cubic centimeter or less) laser gyroscope. 
A primary advantage of the present invention is that it has no moving parts 
and therefore has a long lifetime. 
Another advantage of the present invention is that it is a semiconductor 
device rather than a conventional gas laser gyroscope. 
An additional advantage of the present invention is that it has no separate 
mirrors because the reflecting surfaces are part of the cavity of the 
invention. 
Yet another advantage of the present invention is that it is not subject to 
"lock-in" effects. 
Other objects, advantages and novel features, and further scope of 
applicability of the present invention will be set forth in part in the 
detailed description to follow, taken in conjunction with the accompanying 
drawings, and in part will become apparent to those skilled in the art 
upon examination of the following, or may be learned by practice of the 
invention. The objects and advantages of the invention may be realized and 
attained by means of the instrumentalities and combinations particularly 
pointed out in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Best Modes for Carrying Out the Invention 
The present invention is of a dual ring laser operating on a small cube of 
semiconductor material and exhibiting gyroscopic response. Each laser is 
preferably a unidirectional ring laser with asymmetric reflection 
properties. 
The unidirectionality is provided as follows. A single quantum well has a 
thickness d much smaller than the wavelength of a given laser. In an 
inverted quantum well, an electric field of amplitude .xi..sub.i incident 
from the left, will, after transmission through the layer, become a 
transmitted field .xi..sub.t =(1+.alpha.).xi..sub.i. The reflected field 
will be .xi..sub.t =.alpha..xi..sub.i. The single amplifying quantum well 
can also be represented by a layer of purely imaginary index of refraction 
n.sub.g =-ik. 
Next consider a thin dielectric later, and the combined reflection from 
both interfaces. In the limit of zero thickness (thickness negligible 
compared to the laser wavelength), for an incident field E.sub.i, the 
reflected field is E.sub.r =irE.sub.i, where r is a real quantity, 
proportional to the discontinuity in index .DELTA.n. This result is 
consistent with the previous one: for a gain medium, .DELTA.n=-ik, and the 
reflected field should be of the form E.sub.r =.alpha.E.sub.i. 
The pairing of a thin gain layer with a dielectric reflecting layer leads 
to a structure that is non-reciprocal. Consider, from left to right, a 
thin gain layer (transmission E.sub.t =(1+.alpha.)E.sub.i ; reflection 
E.sub.r =.alpha.I.sub.i) followed at a distance l=.lambda./8 by a 
dielectric layer (reflection E.sub.r =irE.sub.i ; transmission 
##EQU1## 
The round trip between the two layers corresponds to a phase factor 
exp{2ikl}=i. Ignoring multiple reflections, the combined reflection from 
both interfaces, for a beam incident from the left, is: E.sub.r 
=(.alpha.-r) E.sub.i, which is zero if .alpha.=r. For a beam incident from 
the right, the combined reflection is: E.sub.r =(.alpha.+r)E.sub.i, which 
is finite if .alpha.=r. 
This property can be generalized to more complex multiple quantum well 
structures, such as that described by Table 1. 
TABLE 1 
______________________________________ 
thickness 
index index 
material (nm) (r) (i) comment 
______________________________________ 
air 1.000 0.000 boundary 
Al.sub.0.25 Ga.sub.0.75 As 
126.176 3.400 0.000 stop etch layer 
AlAs 71.500 3.000 0.000 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
140.000 3.400 0.000 reflector 
AlAs 30.000 3.000 0.000 reflector 
Al.sub.0.25 Ga.sub.0.75 As 
99.706 3.400 0.000 spacer r - .alpha. 
AlAs 30.000 3.000 0.000 (2 layers) 
Al.sub.0.25 Ga.sub.0.75 As 
99.706 3.400 0.000 spacer r - .alpha. 
AlAs 30.000 3.000 0.000 (2 layers) 
Al.sub.0.25 Ga.sub.0.75 As 
125.500 3.400 0.000 spacer r - .alpha. 
GaAs 10.000 3.646 -0.014 
Quantum well 
Ga.sub.0.80 Al.sub.0.20 As 
115.000 3.400 0.000 pump layer 
10.000 3.646 -0.014 
Quantum well 
GaAs 115.000 3.400 0.000 pump layer 
Ga.sub.0.80 Al.sub.0.20 As 
. . . . . . . . . 19 pairs total between 
. . . 10.000 3.646 -0.014 
Quantum well 
GaAs 115.000 3.400 0.000 pump layer 
Ga.sub.0.80 Al.sub.0.20 As 
10.000 3.646 -0.014 
Quantum well 
115.000 3.400 0.000 pump layer 
GaAs 
Ga.sub.0.80 Al.sub.0.20 As 
GaAs 10.000 3.646 -0.014 
last Quantum well 
Al.sub.0.25 Ga.sub.0.75 As 
125.500 3.400 0.000 spacer 
AlAs 30.000 3.000 0.000 (1 layer) 
Al.sub.0.25 Ga.sub.0.75 As 
99.706 3.400 0.000 spacer 
AlAs 30.000 3.000 0.000 (1 layer) 
Al.sub.0.25 Ga.sub.0.75 As 
149.300 3.400 0.000 reflector 
AlAs 71.500 3.000 0.000 reflector 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 AR 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 AR 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
63.088 3.400 0.000 AR 
AlAs 71.500 3.000 0.000 AR 
Al.sub.0.25 Ga.sub.0.75 As 
126.176 3.400 0.000 etch layer 
AlAs 10.000 3.000 0.000 1st etch 
GaAs 3.646 0.000 substrate 
______________________________________ 
The above structure has 20 gain layers with .alpha.=2.8.times.10.sup.-3. 
The overall intensity transmission versus wavelength is plotted in FIG. 1. 
The peak at 857.64 nm does not match exactly the minimum reflectivity 
wavelength of 858 nm (see FIG. 2). From the other direction, the minimum 
reflection is 0.024, at an even longer wavelength (854.3 nm) as shown in 
the plot of FIG. 3. These plots do not reflect the bandwidth of the laser 
in operation. As the gain saturates, the values of the transmission 
(gain), and reflection change. As a result, the relative intensities 
between both directions also change. 
The smaller the gain, the smaller the non-reciprocity. This tendency is 
obvious from the formulae for the two layer approximation: the smaller the 
gain, the smaller the reflectivity from right to left i(.alpha.+r). 
A simple unidirectional ring laser 10 is illustrated in FIG. 4. Ti:sapphire 
laser 12 drives the ring, initially reflecting off reflecting element 14 
(such as a parabolic mirror). The elements may be made with GaAlAs 
structures grown on a GaAs wafer, in which case the substrate must be 
etched away because it absorbs laser radiation. Preferably, the elements 
are grown on InGaAs, which is transparent to laser radiation and so would 
not have to be etched away. 
The preferred gyroscope of the invention comprises two GaAs 90 degree 
turning prisms 20 and 22 (see FIG. 5). A non-reciprocal gain layer 
structure is grown on the hypotenuse face 24 of one prism (InGaAs). The 
two prisms are thereafter optically contacted to form a cube 30 (see FIG. 
6). Depending on the location of the pump spot (pump radiation around 900 
nm), the lasing will be clockwise 32 or counterclockwise 34 in the 
cross-section of the cube. With two pump spots at different heights in the 
prisms, lasing can occur in opposite directions in parallel planes. Since 
there is no coupling between the cavities, the beat frequency between the 
two lasing planes exhibit a perfect gyroscopic response. 
For all practical purposes, the ring laser of the invention is 
unidirectional. Any asymmetry is not in transmission, but in (passive) 
reflection. In one direction (+), the intensity increases because 
intensity from the other direction (-) is fed into that direction. There 
may be a large difference between the intensities I.sub.+ and I.sub.-, 
but the weak beam (I.sub.-) will never vanish completely. Some of that 
weak beam is coupled back into the strong beam by the reflection at the 
dielectric-gain interfaces. Such a coupling injection does not lock the 
stronger beam (which would eliminate all possibility of gyroscopic 
response) because the coupling of the weak field into the strong one is of 
the form i(r+.alpha.)E.sub.- and is thus 90 degrees out of phase with the 
strong field. Injection lock-in does not occur for this particular angle 
of coupling. 
FIGS. 7 and 8 show the evolution with time of the two intensities I.sub.- 
and I.sub.+ in the cavity. The time is in units of cavity round-trip 
time, where the relevant parameters are: 
______________________________________ 
.alpha. 0.080 unsaturated gain factor 
.alpha..sub.loss 
0.040 loss/pass 
r 0.038 dielectric reflectivity 
I.sub.sat 
0.100 saturation intensity 
______________________________________ 
Although the invention has been described in detail with particular 
reference to these preferred embodiments, other embodiments can achieve 
the same results. Variations and modifications of the present invention 
will be obvious to those skilled in the art and it is intended to cover in 
the appended claims all such modifications and equivalents. The entire 
disclosures of all references, applications, patents, and publications 
cited above are hereby incorporated by reference.