GB2032169A

GB2032169A - Lasers

Info

Publication number: GB2032169A
Application number: GB7934149A
Authority: GB
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1978-10-02
Filing date: 1979-10-02
Publication date: 1980-04-30
Also published as: SE449664B; FR2438357A1; IL57936A; JPS5910078B2; IT1164049B; JPS5821385A; IT7950401A0; CH651157A5; JPS584475B2; JPS5548989A; GB2032169B; DE2939804A1; CA1133615A; FR2438357B1; SE7908117L

Abstract

In an optical resonator arrangement, particularly a ring laser gyroscope, a concave mirror 22 and at least one planar mirror 24, 26, 28 are used to guide a laser beam 14, the plane of one of the mirrors being inclined at an angle of between 1 and 3 arc minutes with respect to the plane of the or each other mirror. The laser is tuned by translating the concave mirror substantially in its own plane (which may be inclined itself in one embodiment) so as to alter the length of the laser path. One of mirrors 26 preferably has an aperture for suppressing off-axis modes of oscillation, and this mirror must be translated in its own plane during tuning to realign the aperture to suppress the off-axis modes. Such a ring laser gyroscope may be made very small: the length of each side of the laser path may be less than an inch in length. <IMAGE>

Description

SPECIFICATION Optical Resonator Arrangement The present invention relates to optical resonator arrangements particularly to ring lasers and is applicable to ring laser gyroscopes. It also relates to a method for tuning optical resonator arrangements.

Optical resonator arrangements are tuned by adjusting the length of the laser beam path, usually by moving one of the mirrors which are used to guide the beam along the beam path. For example, in a ring laser, one of the mirrors is moved inwardly, perhaps by a screw mechanism, to shorten the path until the lasing ampitude peaks. One of the mirrors is often made partially transmitting so that the output of the laser through the mirror can be used in a feedback circuit to servo the position of the tuning mirror.

According to a first aspect of the present invention there is provided an optical resonator arrangement comprising means for generating at least one laser beam and a plurality of mirrors arranged to guide the or each laser beam, one of said plurality of mirrors being concave and the or each othermirror being substantially planar, the plane of one of said plurality of mirrors being inclined at an angle with respect to the plane of the or each other mirror.

According to a preferred embodiment of the first aspect of the present invention support means are provided to support the inclined mirror at a present angle.

The inclinable mirror may be the concave mirror or alternatively may be a planar mirror. It preferably has an aperture which is suitably dimensioned for supressing off-axis modes of oscillation.

According to the preferred embodiment of the invention there are four mirrors, three planar mirrors and one concave mirror, arranged one at each corner of a quadrilateral the periphery of which defines the path, in use, of the laser beam.

Preferably also the edges of the quadrilateral are coplanar and define a square laser beam path.

According to a second aspect of the present invention there is provided a method for adjusting the length of the laser path of an optical resonator arrangement wherein a laser beam is guided by at least a concave and a coplanar mirror along the laser path, the method comprising using a mirror inclined at an angle relative to the or each other mirror, translating the concave mirror substantially in its own plane to tune the laser beam and fixing the position of the concave mirror.

It can be seen that a ring laser gyroscope can be constructed according to the present invention to path being an inch or less in length. With such a small laser the cathode of the laser is preferably the same order of size as the laser block itself. The cathode must be made large enough to produce an adequate amount of current to the lasing medium to cause lasing.

For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a side elevation of an arrangement according to the invention showing a laser block and cathode block attached together and an evacuation stem tipped off.

Figure 2 is a sectional view, taken at 2-2 in Figure 1.

Figure 3 is a top view, taken at 3-3 in Figure 1.

Figure 4 is a schematic ray diagram showing a preferred embodiment of a concave mirror and its opposite planar mirror in the apparatus of Figure 1.

Figure 5 is a schematic diagram showing the relation of the curvature of the concave mirror to the displacement of the lasing plane.

Figure 6 is a schematic ray diagram showing the displacement of the laser rays with displacement of the concave and the tilted planar mirror.

Figure 7 is an enlarged schematic view taken at 7 in Figure 6.

Figure 8 shows the angling of the base of the cathode block into a contour parallel with the laser plane for use as a ring laser gyroscope.

The Figures show a preferred embodiment of the arrangement of this invention when used as a ring laser gyroscope. The ring laser has a laser block 10 and a cathode block 12, preferably made of a glass ceramic. Typical acceptable glass ceramics are known by the trademarks Cervit, Zerodur and Ule. These materials have a substantially zero expansion within a usable range of the laser. The laser block 10 carries the lasing path 14. The cathode block 1 2 carries a cathode 16, and the laser block 10 carries two anodes 52 and 54. A voltage is applied between the cathode and the anodes to ionize lasing gas to deliver energy for lasing. No power supply for applying voltage between the anodes 52, 54 and the cathode 1 6 is shown, but any typical DC power supply may be used.The positive voltage is connected to the anodes 52 and 54 while the negative voltage is connected to the cathode 1 6.

The laser block 10 and the cathode block 12 are held together both by atmospheric pressure and a sealant such as indium solder.

The preferred laser path 14 is a rectangular laser path, and more particularly a substantially square laser path as shown in the figures. The laser block is therefore preferably a square laser block, but unused portions of the block may optionally be cut off to minimize cost and fabrication difficulty so that the resulting block, as shown, is shaped as an octagon. Attached to four sides of the square laser block or to alternate nonadjacent sides of the octagonal laser block, at the intersections of the laser branches, are four mirror blocks 22, 24, 26 and 28 which have mirror surfaces on the interior thereof for reflecting the laser beam.At least one of the mirrors is partically transmissive to allow the laser beam to be emitted, at least one of the mirrors is apertured to prevent the production of off-axis modes of oscillation, and one of the mirrors, mirror 22 in the Figures, is concave and has a suitable radius of curvature to focus the laser beam.

The lasing in the laser path occurs in a heliumneon gas mixture at a very low pressure of 3.2 Torr. The gas mixture may be, for example, 20 parts helium to 1 part neon 20 and 1 part neon 22. To contain the lasing gas in the laser path four substantially co-planar conduits 30a, b, c and d are bored in the laser block 10 to connect the mirrors. The bores are large enough in diameter to allow the plane of the laser beam to be translated and tilted through a small angle, typically of the order of 3 and 5 arc-minutes, without interfering with the laser beam. The conduits could be tilted parallel to the laser beam.

Within the region whose perimeter is defined by the laser path, and preferably in the centre of the laser block 10, is a conduit which is perpendicular to the plane of the conduits 30a, b, c and d. That conduit has two portions 32, leading to the top of the laser block 1 0, and 34 leading to the cavity formed by the cathode surface 1 6. The conduits 32 and 34 are connected with the conduits 30a, b, c and d by a conduit 36 which is typically substantially in the plane of the conduits 30a, b, c, and d.

In the regions of the mirrors 22, 24, 26 and 28 are four chambers 38, 40, 42, and 44 which are terminal regions for the branches 30a, b, c and d of the laser conduits and which are large enough to prevent interference with the laser light Cavity 40 is connected by conduit 36 to the conduits 32 and 34.

The conduit 34 is preferably centred on the hemispherical surface of the cathode 1 6, but it need not be so centred. Further although conduit 34 is shown perpendicular to the plane of the laser path 14a, b, c and d, it may be slanted. The conduit 32 extends to the outer surface of the laser block 10 where it is surrounded by a glass or metal stem which is sealed to the laser block. The conduit 32 is used for evacuating the conduits, and its position in the centre of the laser block is not critical. It is, however, convenient to form the conduits 32 and 34 colinearly by a single pass of an appropriate glass drill. It is further to be noted that conduit 32, although shown perpendicular to the path of laser path 14a, b, c and d, may also be slanted, if desired.

The stem 50 is used to evacuate the system and to refill it with the required gas at a low pressure. When the stem 50 is metal, it may also be used as an anode. Note that the conduit 32 is connected through the conduit 34 to the region within the cathode surface 1 6 and through the conduit 36 to the laser conduits 30a, b, c and d.

An exhaust pump (not shown) may be attached to the stem 50 to remove all air from the system.

Further, a getter (not shown) may either be positioned within the stem 50 or within the region of conduits attached to stem 50 (not shown) adjacent the stem 50. After the system is evacuated and gettered, the required lasing gas is filled into the system at a very low pressure, and the stem is tipped off to seal the system. The cathode block 12 is held onto the laser block 10 both by the resulting low pressure vacuum within the chamber formed by the cathode surface 16 and by a sealing material such as indium solder.

In the region of the compartments 38 and 42 are a pair of anodes 52 and 54 which are metallic conductors and extend from the outside of the laser block 10 inward into the chambers 38 and 42.

With a positive voltage connected to the anodes 52 and 54 and a negative voltage connected to the cathodes 16, electrons and ions commence to drift from cathode to anode and anode to cathode in the path defined by the chamber formed by the cathode 1 6 and the conduits 34 and 36 into the compartment 40. At the compartment 40 the path splits, and a portion of the ion-electron drift is in one direction through a gain bore from the compartment 40 to the compartment 38 and thence to the anode 52. The other part of the drift is from the compartment 40 through the gain bore 30b to the compartment 42 and thence to the anode 54. The motion of the electrons and ions in two directions within the gain bores of the proposed laser path excites the gases therein to a higher energy state from which they drop to a lower energy state to produce light at the frequency to which the laser path is tuned.

Thus, energy is provided to the laser from the source which is connected to the cathode 1 6 and anodes 52 and 54.

The length of the laser cavity is tunable because the curved and apertured mirrors are movable. One of the two movable mirrors is tilted at a pyramid angle so that upward and downward movement of that mirror also moves the mirror inward and outward relative to the laser path to peak the laser signal.

In the illustrated embodiments of this invention, the apertured mirror 26 is tilted inward at a small pyramid angle, typically from 3 to 5 arc minutes, so that moving the concave mirror 22 in a direction normal to the plane of the conduits 30a, b, c and d, lengthens and shortens the laser path. The apertured mirror 26 is then moved to keep the aperture opening in the laser path.

A pyramid angle between two planes is defined as 900 minus the dihedral angle between those planes. A dihedral angle is defined by the "Mathematics Dictionary" third edition by James and James, published by Van Nostrand 8 Company. "The union of a line and two halfplanes which have this line as a common edge.

The line is the edge of the dihedral angle and the union of the line and one of the planes is a face. A plane angle of a dihedral angle is an angle formed by the two rays which are the intersections of the faces of the dihedral angle and a plane perpendicular to the edge. Any two plane angles are congruent. A measure of a dihedral angle is a measure of one of its plane angles." The dihedral angle between the tilted mirror and the plane defined by the conduits 30a, b, c and d and the two parallel planar mirrors on the mirror blocks 24 and 28 is slightly less than but almost 900, differing from 900 by the pyramid angle of the plane of the tilted mirror which typically is very small. The pyramid angle is governed by the lasing wavelength, the gain bore diameter and the lasing mode volume.One must be able to change the cavity length by at least a half wavelength of the laser light without causing the mode volume to be affected by the gain bores in a way to cause a substantial loss in lasing gain. As the concave mirror is moved up and down, the laser path is also moved up and down, thereby striking a different portion of the tilted mirror, so as to form a shorter or longer path for the laser beam. By striking a different portion of the tilted mirror, the on-axis beam would be extinguished by the aperture stop unless the tilted mirror is also moved to re-align the aperture opening with the laser beam.

If all of the planar mirrors 24, 26, and 28 had the planes of their mirrors perpendicular to the same plane defined by the plane of the conduits 30a, b, c and d, motion of the concave mirror 22 up and down would merely move the laser beam up and down without changing the length of the lasing path. However, with the mirror 26 having its planar surface tilted at a pyramid angle, the entire plane of the laser path is tilted through that small pyramid angle so that the plane of the incident and relected laser rays on the tilted mirror is perpendicular to the surface of the mirror. This causes the intersection of the laser beam and the concave portion of the concave mirror 22 to move on that concave surface.If the concave surface is a spherical surface, the amount of displacement is determined by the radius of that concave mirror and the abovementioned pyramid angle tilt of the planar mirror 26.

Alternatively, the block 22 carrying the concave mirror surface could merely have been tilted inward through the small pyramid angle whereby motion of the block 22 not up and down but tilted inward at the small pyramid angle would lengthen and shorten the laser path length and move the intersection of the laser path on the mirrors 24, 26 and 28. The mirror 26 would also need to be moved up and down to align the aperture opening with the new laser path. The conduits carrying the lasing path must be large enough- to -accommodate the variations in position described.

With the structure described, a very small laser gyro may be constructed. The sum of the lengths of the four sides 14a, b, c and d may have, for example, a nominal length of 6.8 centimeters.

The cathode surface 1 6 is preferably made of aluminum, and indium solder may be connected to the aluminum and positioned at 60 and 62 as means for applying a negative voltage to the aluminum cathode 16.

The exhaust and fill stem 50 is preferably made from a glass tube section which is flared at the bottom to accommodate a radio frequency fired getter. Alternatively, the exhaust and fill stem may be smaller than shown. It may also be made of metal whence the exhaust and fill stem may be used as an anode.

The mounting faces of the octagonal laser block 10 are for. example only one centimeter across, and the mirror blocks 22, 24, 26 and 28 may be 0.8 centimeters or smaller in diameter.

The mirror surfaces themselves can be 7.75 millimeters in diameter and have a typical thickness of 4 millimeters. The curved concave mirror surface of block 22 has a very long radius of curvature on the order of 60 centimeters.

Mirror 26 is apertured to ensure that off-axis modes of operation are suppressed while allowing the on-axis TEMPO mode to lase. Lasing can best take place when the aperture of the tilted mirror 26 is aligned so that the ray which is normal to the outer of the aperture on the mirror 26 is also on a radius of the curved mirror 22. The mirror blocks 22, 24, 26 and 28 as well as cathode block 12 are typically sealed with indium-gold metal seals. The body of the laser block 10 is of glass ceramic material which has an extremely low, and preferably zero, coefficient of expansion over the range of temperatures desired.

The reflectivity of the mirror coating is typically of the order of 99.94%. Transmission is typically less than 0.1 % and the scattering losses are typically on the order of 100 parts per million. A typical minimum lasing threshold anode cathode current is on the order of 1/2 to 2-1/2 milliamperes. Tuning may be accomplished by attaching mirror blocks by wax and moving the mirror blocks 22 and 26. Then, after the laser is tuned, the positions of the mirror blocks are measured, the mirrors and wax are removed, and the mirror blocks are accurately soldered in place by indium-gold solder seals.

The resonance frequency is an optical frequency which typically is of the order of 1014 Hz. In normal use it is desired that the resonance frequency of the cavity be tuned to the centre of the gain curve, or as near as possible to the centre, to a fraction of the wavelength. In Figures 4, 5, 6, 7 and 8 the pyramid angle is designated by the symbol . In Figure 6, two laser paths are shown, one defined by the numerals 14a, b, c and d and the other defined by the numerals 14e, f, g, and h. The laser path length is nominally a square, but movement of the laser path down varies the path length from that shown by 14a, b, c and d to that shown by the long path 14e, f, g and h. The relation between the mirror motion and the path length is shown more graphically in Figure 7. Two restraints apply: first, the lasing beam must not be vignetted by the laser bores 30a, b, c and d, and second, the beam must be within the aperture of the mirror on the mirror block 26. A vertical projection of Figure 6 is shown in Figure 4.

To a first approximation if we are to change the cavity length by a distance equal to the wavelengths divided by the nominal path lengths (6x 10-5 centimeters/6.8 centimeters pr approximately 0.001%) a displacement h (Fig. 4) effectively translates the aperture of the tilted mirror 26 inward a distance d=htana. The desired motion in the directions shown in Figure 7 by the "s", is on the order of 1/2 of a wavelength of the lasing light. From Figure 7 it is seen that s=0.707d (a square lasing path is assumed), and d=the wavelength divided by 1.414 which is equal to 6.33 x 10 centimeters.

The lasing beam diameter, d is less than or equal to 0.089 centimeters while the bores 30a, b, c, and d are typically of a diameter dg at least equal to 0.1778 centimeters. The total allowable motion, for d, Ad is 0.0880 centimeters where h=0.0440 centimeters. From this, the angle a may be calculated as the ratio of d to h or 1.017x10-3 radians which is about 3.49 arc minutes.

In Figure 5 the curvature of the surface has been exaggerated, and the radius R is out of proportion. The radius of the concave mirror on block 22 is shown very short, but in actuality it is on the order of 60 centimeters. The beam position on the curved mirror face from dead centre, wherein a equals zero, to a point r from the centre, r=aR=(1.017 x 10-3) (60)=0.061 centimeters. The beam radius is 0.0898 centimeters divided by 2 or 0.0449 centimeters.

With the beam displaced a distance r=0.061 centimeters, the edge of the beam lies at a distance of 0,1059 centimeters from the centre of the curved mirror. The curved mirrortypically has an aperture on the order of 0.4 centimeters in diameter or 0.2 centimeters in radius. Thus, a margin of 0.2-0.11=0.09 centimeters is provided.

This sets the limit on the amount of allowed increase in the angle a to 0.09/60=1.5 x10-3 radians, the allowable -shift of the lasing beam on the face of the curved mirror 22. Further, the bores 30 abcd limit the amount by which the mirrors can be shifted.

When mirror 26 is tilted, the plane of the lasing path also tilts through an angle a. To create a laser usable as a gyroscope the base of the cathode block 12, as shown in Figure 8, needs to be tilted through that same angle so that the base is parallel to the lasing plane and the sensing axis is perpendicular to the plane of the laser path. The base of the cathode block 12 is therefore bevelled and may then be attched into, for example, a guidance system with the knowledge that the angular measure determined from information contributed by the ring laser is an angle and angular rate measured both perpendicular to the laser path and to the mounting face.

Although a square laser path has been shown, it is obvious that the principle set forth herein may be used with other rectangular and quadrilateral paths. It is intended that the concepts of this invention should be applied to such paths by tilting at least one of the mirrors at a pyramid angle.

It is also apparent that a three sided laser path may be used or, alternatively, a multi-sided laser path having more than four branches may be used with at least one of the mirrors having a small pyramid angle so that sliding of the concave mirror shortens or lengthens the laser path, and with the aperture on the concave mirror.

It is also apparent that the apparatus and methods of this invention can be used to tune other optical resonators by tilting one of the resonator mirrors so that movement of the tilted mirror in the direction of tilt shortens or lengthens the resonator path length.

The invention may also be used for tuning straight as well as nonstraight optical resonator paths, and for tuning not only of active resonators but of passive resonators as well, whether the optical resonator is stable or nonstable.

Claims

1. An optical resonator arrangement comprising means for generating at least one laser beam and a plurality of mirrors arranged to guide the or each laser beam, one of said plurality of mirrors being concave and the or each other mirror being substantially planar, the plane of one of said plurality of mirrors being inclined at an angle with respect to the plane of the or each other mirror.

2. An optical resonator arrangement according to claim 1 wherein said inclined mirror is supported to be inclined at a preset angle between 1 and 3 minutes.

3. An optical resonator arrangement according to claim 1 or claim 2 wherein said inclined mirror has an aperture which is suitably dimensioned for suppressing off-axis modes of oscillation.

4. An optical resonator arrangement according to any one of the preceding claims wherein the concave mirror is the inclined mirror.

5. An optical resonator arrangement according to any one of the preceding claims wherein said plurality of mirrors is four and comprises three planar mirrors and one concave mirror arranged one at each corner of a quadrilateral, so that, in use, a laser beam path is defined as the periphery of the quadrilateral.

6. An optical resonator arrangement according to claim 5 wherein the edges of said quadrilateral are coplanar

7. An optical resonator arrangement according to claim 5 or claim 6 wherein two non-adjacent ones of said planar mirrors are parallel to each other.

8. An optical resonator arrangement according to claim 7 wherein said quadrilateral is substantially a square.

9. An optical resonator arrangement according to any one of the preceding claims comprising laser conduits in a laser block for containing a laser gain medium and wherein said means for generating at least one laser beam comprises a cathode and at least one anode arranged in said laser block so that the path between the cathode and the or each anode comprises a portion of a laser conduit.

10. An optical resonator arrangement according to claim 9 comprising a cathode in the form of a substantially hemispherical depression in a cathode block which abuts, in the plane of the opening of the depression, the laser block in which are mounted the mirrors.

11. An optical resonator arrangement according to claim 9 or 10 when appended to claim 8 comprising two anodes, one adjacent a respective one of said parallel planar mirrors, there being a passageway formed in said laser block which links the depression to the region of the inclined mirror and then to said anodes along portions of the laser path in opposing senses round the laser path.

12. An optical resonator arrangement according to claim 11 wherein the cathode block has a base surface which is bevelled by the same angle as the angle at which the inclined mirror is inclined.

1 3. An optical resonator arrangement according to any one of the preceding claims when arranged for use as a ring laser.

14. An optical resonator arrangement according to claim 1 3 arranged for use as a ring laser gyroscope.

1 5. An optical resonator arrangement substantially as hereinbefore described with reference to the accompanying drawings.

1 6. A method for adjusting the length of a laser path of an optical resonator arrangement wherein a laser beam is guided by at least a concave and a coplanar mirror along the laser path, the method comprising using a mirror inclined at an angle relative to the or each other mirror, translating the concave mirror substantially in its own plane to tune the laser beam and fixing the position of the concave mirror.

1 7. A method according to claim 1 6 wherein said concave mirror is used as said inclined mirror.

18. A method according to claim 1 6 comprising using as said planar mirror a planar mirror which has an aperture for suppressing offaxis modes of oscillation, and using said planar mirror as said inclined mirror and translating the inclined planar mirror substantially in its own plane to align the aperture so as to suppress offaxis modes of oscillation, and fixing the translated inclined planar mirror.

19. A method according to any one of claims 16 to 1 8 comprising using a support surface to support the inclined mirror at said angle and to guide translation thereof.

20. A method according to any one of claims 16 to 19 when used for adjusting the length of a laser path of an optical resonator arrangement according to any one of claims 1 to 1 5.

21. A method substantially as herinbefore described with reference to the accompanying drawings.