Path Length Adjuster for Ring Laser Gyro

The apparatus of this invention uses some of the principles recited in U.S. Pat. No. 4,436,423.

A ring laser gyroscope generally includes mirrors for reflecting laser beams along a laser cavity. The laser cavity is frequently triangular or rectangular, and the mirrors are positioned at the intersections of the straight portions of the cavity. Because of irregularities on the surfaces of the mirrors, a beam impinging upon a particular mirror is not perfectly reflected. Rather, some back-scattering occurs. This causes errors in the gyro at high rotation rates and also causes the beat frequency between two beams in the gyroscope to disappear at low inertial rotation input rates. Such disappearance is called lock-in which has been recognized for some time in the prior art and has been solved by driving the gyroscope body in rotational oscillation or dither. The means for accomplishing the dither conventionally includes a piezoelectric actuator connected to a suspension for the gyroscope to oscillate the body. The frequency of oscillation is frequently at the suspension's natural mechanical resonant frequency. The dither is superimposed upon the actual rotation of the gyroscope in inertial space. Various techniques have evolved to recover inertial rotation data free from dither, but such is not part of this invention.

The apparatus of this invention uses a plurality of forcers, usually piezoelectric stacks, for delivering forces to the body of a ring laser. The forces of one set of the forces are delivered directly through the apexes of the ring laser path along a line which bisects the angle between the adjacent laser path branches at the apex. The forces of a second set of the forces are delivered between the apexes and have a components perpendicular to the laser branches. The sets of forces operate in opposition, and they are scaled to keep the branches of the laser path substantially constant under temperature and stress changes. The directions of the second set of forces are not as critical as those of the first set of forces, but their direction must be accounted for in the scaling factor on the driving servo amplifiers. In a preferred embodiment, the forces are perpendicular to the branches and centered between the apexes. The length of the laser path is sensed, and sets of forcers are driven in predetermined scaled relation to maintain the lengths of the ring laser branches.

It is therefore an object of this invention to maintain the lengths of the branches of a ring laser.

It is another object of the invention to maintain the overall length of a ring laser path.

It is also an object of the invention to maintain the scatter coefficients of the various mirrors substantially constant by causing the laser signals to impinge on substantially a constant position on each mirror and to hold the distance substantially constant between adjacent mirrors, where "substantially" means on the order of tens of Angstroms.

Other objects will become apparent from the following description, taken together with the accompanying drawings.

FIG. 1 is a drawing, partly in section, showing a typical three branched ring laser gyro.

FIGS. 2 and 3 are schematic sketches of a three branched ring laser used for explaining the operation of the invention.

FIG. 4 shows a symmetrical three branched ring laser gyro with a typical forcer arrangement and servo according to this invention.

FIG. 5 is a graph of intensity of laser signal as a function of length adjustment for a typical ring laser.

FIG. 6 shows a typical piezoelectric forcer stack.

FIG. 7 shows a symmetrical four branched ring laser gyro with a typical forcer arrangement and servo according to this invention.

FIG. 8 shows an asymmetrical, for example an isosceles shaped laser path, ring laser gyro with driving servo amplifiers according to this invention.

FIG. 1 shows a prior art ring laser used in a ring laser gyro. The laser is typically embedded in a dimensionally stable quartz or glass-ceramic block 10 having a channel 12 therein supporting the ring laser path. The channel 12 is sealed and contains, typically a mixture of neon and helium which is then energized into lasing. To create the laser path, mirrors 14 are positioned at the vertexes or corners of the laser body 10. The apparatus shown is a symmetrical one, and that is a typical and usually preferred embodiment. The energy may be supplied to the laser gas by channeling an electron stream between a cathode 16 and a pair of anodes 18 to produce two counterpropagating laser signals within the ring laser. The entire laser body may be supported upon a post 20 on its supporting vehicle or laboratory bench, and a plurality of flexure springs, shown generally at 22, may optionally be connected between the body 10 and the post 20 to allow the body 10 to be dithered angularly as desired.

The apparatus of this invention is directed to the problem of keeping the lengths of the laser branches substantially constant. To that end, if the laser path is a regular polygon, and if the laser body 10 is substantially symmetrical, once initial adjustments are made, it may be assumed that temperature and stress effects will be substantially the same for each branch, and so long as the forcers of this invention are also symmetrically positioned, the control signals applied to the forcers may be substantially identical. When asymmetries occur, individual scaled amplifiers and forcers may be needed, and a calibration may be needed before the apparatus of the invention operates properly to correct for changes in branch length.

The objective of the invention is to prevent such changes in branch length because changes in branch length would cause the laser signals to impinge at different angles and at different places on the corner mirrors of the laser, thereby varying the scattering and hence affecting the lockin and lockin compensation on the ring laser gyro.

In the symmetrical ring laser gyro shown in FIGS. 2 and 3, note that the two sets of forces from the forcers of this invention are operated in opposition or push pull to distend the laser frame. That is, the forces which are directed through the apexes of the ring laser either are directed outwardly as in FIG. 2 or inwardly as in FIG. 3, and the forces centered on the laser branches are directed perpendicular to the branches and substantially at their center point between the apexes or corners but with a sense that is opposite to that of the forces directed at the apexes. The forces of the forcers are typically manually preloaded so that the forcers operate around a predetermined preloaded force.

FIG. 4 shows an equilateral triangular ring laser path with a set of six forcers in two sets, labeled A and B. Portions of the two counterpropagating ring laser signals are extracted through one of the corner mirrors 14A. The two counterpropagating signals are directed into two photosensors 30,32, and the photosensor signals are summed through summing resistors 34,36 and summing amplifier 38. A modulating alternating voltage source 40 is used as a carrier for signals in the servo amplifiers 42,44 and for detection on the synchronous detector 46. The output of amplifier 38 is connected through detector 46 to the low pass filter 48 which has a pass frequency lower than the frequency of the source 40. The control signals from the filter 48 are a measure of the length of the path of the laser 10.

The forcers 50 and 52 are forced between a ring 54 and the body of the laser 10. In one embodiment wherein servo amplifiers 44 have the same polarity, the forcers 50 and 52 are oppositely poled. In another embodiment, where the forcers are poled in the same direction, one of the amplifiers 42,44 produces an inverted signal.

The control signal from the filter 48 is connected through the resistor 60 to the inputs of the amplifiers 42 and 44. That control signal, as well as the carrier signal from the source 40, are delivered to the forcers 50 and 52 as shown by the symbols A and B in FIG. 4.

A typical forcer is shown in FIG. 6. A plurality of piezoelectrical crystals are stacked as shown at 62. The inner surface 64 is preferably contoured to match the ring 54, and the outer surface 66 is preferably contoured to match the inner surface 68 of the body of the ring laser 10.

FIG. 7 is similar to FIG. 4 except that a square laser path is configured. Eight forcers 70, 72 are used instead of the six of FIG. 4. The amplifiers 42,44 drive the forcers 70,72, and they are properly scaled to do so. Note that no two configurations would have the same scale factor.

FIG. 8 demonstrates the complexity that arises when the ring laser become asymmetric. This is a simple isosceles triangular ring laser path. Because of the different path lengths and because of the different thicknesses of the ring laser body where the forces from the forcers are applied, different forces are applied by different forcers. In a comletely asymmetric situation there would need to be a separate servo amplifier for each forcer, but an isosceles triangle still has some symmetry, whereby only four amplifiers would be needed. The remainder of the servo circuit would typically be the same as that shown in FIGS. 4 and 7 with the outputs of the carrier source 40 and the resistor 60 both delivered to the inputs of each of the servo amplifiers 80,82,84,86. In FIG. 8, the outputs of the various servo amplifiers are marked A,B,C and D to show that they deliver signals to the corresponding forcers likewise so labeled. The forces of forcers 90, 92,96 should be directed outwardly when the forces of forcers 91, 93, 95 are directed inwardly. A bridge structure 98 is placed centrally of the laser so that the forcers have purchase to apply force to the body 10 of the ring laser. In the afore-cited example of non-symmetry it may be required to use a method of theoretical mechanical deformation analysis to design the optimal mechanical compliance of the laser frame such that the forces are effective at distending the frame in the desired manner. The method may be that of finite element modeling as shown in "The finite Element Method in Engineering Science" by O. C. Zienkiewicz, 1971.

FIG. 5 shows typically how the intensity of the laser signal varies with a change in length of the laser path. The servo of FIGS. 4 and 7, and also used in FIG. 8, controls the path length to a substantially fixed length Lo.

In operation, as the source 40 delivers a signal to the servo amplifiers, and hence to the forcers, the lengths of the laser branches are dithered slightly as shown at 100 in FIG. 5. Variations in amplitude are sensed by the detector 46 and delivered through the filter 48 to the servo amplifiers 40,42 to produce a differential force on the body 10 of the laser to keep the lengths of the path branches substantially constant.

Although the apparatus of the invention has been described in detail above, it is not intended that the invention be limited by that description, but only according to the appended claims.