DE3320345A1 - Ring laser gyroscope - Google Patents

Abstract

A single-mode laser gyroscope having two oppositely directed laser beams is provided at the three or four reflection points with mirrors which can be mechanically vibrated. Each mirror is mounted in such a manner that it can be moved due to the expansion and contraction of stacks of piezoelectric elements which are allocated to each mirror. The mirrors are vibrated or oscillated with the same frequency and phase relationship to one another towards the outside and inside so that the circumferential distance for the laser cavity is kept within a fixed number of wavelengths but the laser beam is moved backward and forward over the surfaces of the mirrors. Due to this technique, the unwanted phenomenon of synchronisation at low rotational speeds is avoided without special optical or magnetic arrangements being required in the path of the laser beam.

Description

- ;; - 33203A5

Dipl.-Ing. A. Wasmeier Dipl.-Ing. H. Graf

Admitted to the European Patent Office · Professional Representatives before the European Patent Office Patent Attorneys P.O. Box 382 8400 Regensburg 1

^the D-8400 REGENSBURG

German Patent Office Greflinger Strasse 7

_ΟΛηΛ ".., _ Telephone (09 41) 547 53

8000 Munich 2 "'. " _U

Telegram Begpatent Rgb. Telex 6 5709 repat d

Your reference your message Our reference tag η ή Your Ref. Your Letter Our Ref. Date ^XJ -

L / p 11,140

Applicant: LITTON SXSTEMS, INC., 360 North Crescent Drive, Beverly Hills, California 90210, USA

Title: "Single-lens gyroscope"

Addition to patent 27 49 157

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Subject of patent 2? 49 157 is a ring laser gyroscope with at least three mirrors that form a closed path build up for one and the other of two rays of light, which progress in opposite directions in an active lasοrhodium of the gyroscope **, and nit individually beta tigbarea vendors, each of whom the mirror is vibrantly conscious of offset, at dea the vibrating movement of the mirrors in i'orra a parallel one that occurs backwards and forwards Displacement takes place in the direction perpendicular to the mirror surface, and the vibratory movement is controllable so that constant i-asera beam path lengths are maintained.

:: M 'the art such ring laser gyros, it is known ieckförmige är ^ or rectangular paths for the laser beams au no cloak, wherein the mirror ar./ at the vertices of the dreiecki'crni ^ on path or at the corners of rechteckförsiigen path " are arranged. During operation, a rotation of the gyroscope in the plane of the laser paths results in a beat frequency z * .visehen the two oppositely directed laser beams, and this beat frequency is used to determine the rotation that is associated with a change in the orientation of the gyroscope. At low speeds of rotation, the two rays have the tendency to synchronize with one another, so that no frequency difference is observed. This ir ^ fc partially the result of a back-scattering, whereby a portion of the energy from each of the laser beams is reflected back along the path of the other beam and tends to synchronize cia two beams.

sjti cind been proposed several ways to eliminate this unwanted Phänoaen that often iiinglaser Gyroscope Witziehen is referred. These possibilities use, inter alia, the vibration of the at an angle

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entire body of the laser gyroscope and non-reciprocal, phase-shifting arrangements, e.g. Faraday ¹ see rotating elements. The method of vibrating the entire body of the laser device is not a perfect solution to the problem of dragging, and the "use of Faraday'sei: rotating elements or other magnetic or optical devices in the laser path results in a more complex one Construction as desired.

In particular, ring laser gyroscopes are known (DE-AS 12 92 899), in which the beam path runs on a platform that mounted on radially arranged leaf springs swinging around an axis perpendicular to the plane of the closed path is. In this case, all deflecting mirrors are thus arranged on a common platform.

Furthermore, single-laser gyroscopes are known (US-PS 3 »533 · which have a stack of piezoelectric elements and in which the position of a mirror is changed to the To change the length of a laser cavity exactly. Such a technique can be used in ring laser gyroscopes of the generic type be used to achieve DC bias, which the alternating current signals having a suitable phase position are superimposed.

Finally, from US Pat. No. 3,581,227 a device is known which has stacks of piezoelectric elements and in which the position of a mirror is shifted in order to change the length of a laser beam space exactly.

The object of the present invention is to solve the problem of being dragged along in ring laser gyroscopes by means of a simpler and more direct one Method than is the case with known arrangements to eliminate.

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This object is achieved according to the invention with the features of the characterizing part of claim 1.

Further features of the invention are the subject of the subclaims.

The present invention is based on the knowledge that a simpler solution is achieved by having at least two Mirrors perform a trembling movement or a mechanical vibration movement while the laser path length is kept constant will. With the present invention, several (at least two) mirrors out of the total number of mirrors with the same Frequency in a direction perpendicular to the surface of the mirror! set in vibratory movements, the vibrating mirrors j have non-zero vibration amplitudes and vibration phases i, so that the total distance around the closed loop in j remains essentially constant. With the invention proposal according to the main patent, however, all mirrors are in trembling motion; or mechanical vibration movement offset. :

The mirrors lead at all reflection points of a ring laser j

Gyroscope a trembling movement or mechanical vibration movement! tion, the phase position of the oscillation of the mirror is offset around the circumference of the laser gyroscope assembly so that the .Phase difference between two adjacent mirrors divided by 360 ° by the number of mirrors, and where this condition> in a constant path length for the oppositely orien-; directed rays, accompanied by a shift of the Point of impact of each laser beam over the surface of each mirror. This will reduce the frequency of the backscattered Light shifted so that the coupling between the counterclockwise rotating laser beams changed periodically becomes, which largely eliminates the pull-along phenomenon.

The displacement of each mirror is very small, approximately equal to + 1/5 or 0.19 times the wavelength of the laser radiation multiplied by the reciprocal of the sine of the

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The angle of incidence of the laser beam on the mirror.

The invention is described below in conjunction with the drawing explained on the basis of an exemplary embodiment. It shows:

Pig. 1 a schematic representation of a ring laser gyroscope: Arrangement, ~

2 shows a detailed view of mirror converter arrangements, of which one at each reflection point of the; Laser gyroscope is arranged, and

Pig. 3 is a schematic diagram showing the movement of a each of the three mirrors after Pig. 1 shows in relation to the others.

In Pig. 1, a laser body 12 is shown schematically, which consists for example of quartz. Three peripheral channels 14, 16 and 18, which form the closed path in the shape of a triangle, are in the form of bores through the quartz body 12 educated. In the channels 14, 16 and 18 there is a gaseous laser medium, e.g. a mixture of gases necessary for the laser operation are suitable, provided. The gas ordered a special case Embodiment made of about 90% helmets and 10% neon and is underneath a pressure of about 3 Torr, corresponding to 3 »9996 mbar.

In accordance with the known laser technique are two cathodes 20 and 22 and two anodes 24 and 26 attached to the quartz \ body 12 so that a gas discharge between the cathode 20 and anode 24 and between cathode 22 and anode 26 in the channels 16 and 18 can take place.

At the three reflection points of the triangular shape shown Mirror transducer assemblies 28, 30 and 32 are attached to a single laser gyroscope assembly.

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Al .-. Le internal elements of the ring laser gyroscope assembly including the mirror, cathodes and anodes are arranged properly sealed in the quartz body 12, so that the gas can be kept in the channels of the quartz body under the desired pressure and free from impurities. The laser effect occurs in a natural oscillation with a frequency of about

14th
5 x 10 Hz. This corresponds to a wavelength of about 0.6 ^ 3 microns, i.e. the resulting luminous appearance is brilliant light of red color.

The construction of one of the mirror transducer assemblies 28, 30, 32 is shown in detail in FIG. In this Fig. 2, the mirror transducer arrangement has 28, a mirror 64, the reflective, partially coated surface 34 of which is exposed to the laser beams faces and reflects light from one of the channels 14, 16, 18 into another channel. The mirror 64 is with the quartz body 12 attached along the edge 36. The mirror is in one annular zone 38 surrounding the mirror on its rear surface immediately within the thicker outer rim 36 extends, formed thinner. With the edge 36, a rigid housing 40 is attached, for example, cylindrical shape Has. This cylindrical housing 40 is provided with a heavy bottom 42.

Between the bottom 42 of the housing and the central part 46 of the mirror is a stack 44 of piezoelectric transducer elements intended. The stack 44 consists of a number of thin, flat piezoelectric plates. These platelets have the property that they become a little thicker or a little thinner when a voltage is applied, depending on the polarity of the voltage. The stack 44 is composed, for example, of fifteen piezoelectric plates, each of which has a thickness of about 0.25 mm owns. The platelets have electrodes on their upper and lower surfaces and are connected "back-to-back" to one another, i.e. that alternating common electrodes, one

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Form set of electrodes, each connected to a control line of opposite polarity. The "back-to-back." connected platelets expand in thickness with each other and contract when oppositely directed electrical Fields are placed on alternate platelets, with considerable pressure exerted on the central part 46 of the mirror will, and cause the thin annular portion 38 of the mirror 64 similar to the displacement of a membrane that is attached along its edge with a frame, is deflected. If desired, the movement of the mirror surface 34 to regulate, the piezoelectric plate, which is closest to the mirror, is not switched in such a way that it is driven, instead two separate wires are connected to its surface, on which a voltage is generated which is a measure of is the position of surface 34.

The stacks of piezoelectric transducer elements can be made from im Commercially available piezoelectric platelets be made. The approximate stress value that corresponds to the displacement value discussed below yields is about 160 volts from peak to peak This gives a shift of the right order of magnitude to achieve the amplitudes that are required in practice, as will be explained below. The piezoelectric plates should in particular be linear and insensitive to temperature.

In Fig. 1, the electrical circuits have the. both food sources 52 and 54 of known design for initiation and maintenance the gas discharge between the anodes and cathodes of the laser device.

The mirror surfaces of the assemblies 28, 30 and 32 are through the converters carried out by a three-phase bias circuit 56 are fed, set in vibratory motion. Each of the piezoelectric transducer stacks, such as stack 44 of Figure 2, will applied so that its vibration is 120 ° out of phase with the others.

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In I ¹ Ig. 2, the laser beams 57 and 58 are shown in such a way that they pass through the partially coated mirror surface 24 and through the openings 59 and 60 in the housing 40. As shown in FIG. 1, they hit the outer mirrors 61 and 62. From these mirrors 61 and 62, the beams are directed onto a measuring device 6 *> which determines the beat frequency between the two oppositely directed laser beams; the beats occur when the whole array is rotated.

In Fig. J the three mirrors of the ring laser gyroscope are shown schematically as mirrors 64 with additional mirrors 65 and 66. The neutral position of each mirror is shown as 64 ¹ -, 65 * and 66 *. As indicated by the arrows 68, each mirror moves in and out toward and away from the center of the laser gyroscope assembly In Figure 3, the solid line 70 represents the path of the laser gyroscope beam with the mirrors in the position shown In particular, in this position the mirror 65 assumes a position which is close to the most retracted position, while each of the mirrors 64 and 66 is offset from the mirror 65 by successive increments of 120 and is therefore further advanced in the positions the arrangement is controlled so that the triangular laser beam paths are kept largely constant as the three mirrors progressively into and out of their phase relation movements of the vibratory movement.

With a ring laser gyroscope arrangement working with four mirrors the four mirrors are controlled by transducers fed by a four-phase supply source, and the successive ones Mirrors are operated to vibrate 90 degrees from that of each adjacent one Mirror is out of phase.

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Basically, the phase shift between adjacent mirrors is equal to 360 divided by the number of mirrors.

With respect to the frequency of the bias circuit 56 of Figure 1, it is desirable to keep that frequency relatively high select, expediently in the order of magnitude of a few 10 kHz to several 100 kHz. However, frequencies down to can be used on 1 or 2 kHz.

When the mirrors are withdrawn in and out of their extremes Position move to their extreme advanced positions, as shown in Figure 3, move the impact points of the laser beams across the mirror surfaces from an extreme position where the corresponding beam paths through the lines 72, 74 and 76 are indicated, in the other extreme position, the paths of which are indicated by lines 78, 80 and 82 and here. As shown in Fig. 3 by triangular path 70, the rays reach the extreme positions indicated by lines 72, 74 and 76, and thereafter the inner boundaries 76, 80 and 82 at different times, whereby the laser beam always has exactly the same length.

In US-PS 3 »533,014 · an arrangement is described and shown, in which the mirrors and other sources of backscattering are parallel to their surfaces rather than perpendicular to their surfaces, as in the case of the present invention, are set in oscillations. It has been found to be extremely difficult is to create arrangements for moving the mirrors of a laser gyroscope in their own planes while at the same time creating the Laser rooms are kept sealed and the other requirements are met.

In contrast, in the case of the present invention, in which a vibratory movement of the mirror perpendicular to their

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Surface is applied, the laser beams across the surfaces the mirror moves back and forth in a manner that can be mathematically analyzed by the technique used in the Case of US-PS 3-533,014 is used. In particular, the Formula according to column 6, lines 53-55 of this patent specification exactly to the amplitude of the used in the case of the present invention Mirror applicable. This surprising result arises due to the fact that the impact points of the Laser beams back and forth across the surfaces of the mirrors at exactly the same distance. to be implemented to the the The mirror can be oscillated inwards and outwards, i.e. towards the center of the arrangement and away from it.

The formula given in US Pat. No. 3,533.01A, referred to above is taken, results in an abbreviated form with

O)

where A is the maximum displacement of any mirror in each ^;

Direction from a neutral position, ι

λ is the wavelength of the laser light,! θ is the angle between the direction of incidence of the laser beams

and a plane perpendicular to the mirror surface, and j ß is any number that is used as the argument of the Bessel-

Function of the zero order taken a value of the Bessel,

Function results, which is Hull. ί

For the triangular embodiment as shown in FIG. 1, θ is equal to 30 ° and Sin θ is equal to 15 °. Corresponding mathematical tables show that β has a value of 2.405 for the Bessel function of the lowest order. If these values are used in equation (1), the result is

_A _ 0.191 λ ₍ 2)

^A - Sin O ^KeL)

A = 0.382 Λ (3)

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Is the neon-helium gas mixture referred to above has been used, the wavelength is 0.633 microns and the displacement amplitude "A" is about 0.24-2 microns,

—4

i.e. 0.242 x 10 cm in each direction from the neutral position the mirror.

For a system with four mirrors, 0 = 4-5, and the corresponding The equivalent value of A is about 0.171 microns in each direction from the neutral position of the mirrors.

From U.S. Patent 3,533,014 it can be seen that preferably, but not there should necessarily be a phase difference in the oscillation of the different mirrors. In equation (3) this document is expressed without proof that the optimal phase difference between successive mirrors for a ring laser comprising four mirrors with the same Branches is 90 °. These phase shift values are used in such a way that at a certain point in time the speeds the mirrors parallel to their reflective surfaces are different, so each mirror with a different one irequence backscattered, which prevents the different backscattered waves reinforce themselves. It is also shown that this optimal phase difference is not critical is, and that the phase difference is greater than or equal to 90 °, 180 ° or the like. If the branches of the laser are different in length, the teaching is here that the "optimal" phase shift of 90 ° is different.

When using the abrasion oscillation according to this publication, especially when using an AT cut crystal according to Fig. 3A, the amplitude of the oscillation parallel to the plane of the crystal is given as 0.16 microns. With a crystal thickness of 1.5 aim is the movement perpendicular to the mirror surface in on the order of 0.000015 microns. The wavelength of the laser light is about 0.633 microns. The movement perpendicular to the However, mirror surface has an amplitude of about 2.5 χ 10

Wavelength, which is negligible.

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In the device according to the invention, the oscillation _; amplitude of each mirror 0.242 microns for a laser j with three mirrors, and 0.171 microns for a laser with four ■ mirrors. Such a movement is a substantial part of a " ^? Ί wavelength of the laser light. If all mirrors are set to oscillate in phase, the length of the laser space is significantly increased or reduced, and the primary laser frequency is significantly disrupted.

To avoid a change in the length of the optical cavity, the vibrations of the mirrors in a laser using

ο '

three mirrors shifted in phase by 120. For a laser with four mirrors the phase shift is 90, for one

Laser with six mirrors it is 60, and one with j

eight mirrors 45 °. The phase position must be maintained. This |

is not just an "optimum", it is crucial. j

The mirrors are preferably driven with pure sinusoids. However, the drive with a complex curve shape is useful, \

if all Fourier components are brought into phase in the same way j

become like / lie basic component.

Preferably the drive devices are matched drivers. If this is not the pail then the drive current or the j The drive voltage must be such that the difference compensates for j is sated.

The mirrors and their drive devices are preferably not driven near their resonance frequency. It is It is also essential that temperature influences are compensated so that the length of the cavity remains essentially constant.

The phase position can be maintained as an open loop, or a sensor (e.g., a wafer of crystal stack 44) can be attached to the mirrors to provide a signal that represents the real-time position of the mirrors. the

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Mirror phasing and amplitude can then be closed with a Circuit in the excitation device 56 can be regulated as indicated by the double arrows on each line.

If the three mirrors vibrate exactly 120 out of phase with each other, there is no change in the instantaneous optical cavity length. Thus, the laser operation is with a frequency position in the middle of the gain curve a minimal length perturbation maintained due to the effect achieved with the invention, the expedient as "micro-tremor" modulation (microdither modulation) referred to as. The micro-tremor amplitudes are in the sizes Order of a sub-wavelength.

The geometric analysis shows that the standing wave field that the triangular configuration according to FIG. 5 is assigned, a Translates, but does not rotate, such that the vertices of the triangle describe circles, as through . the arrows in jig. 3 shown while the impact lines describe on the surfaces of the mirrors as far as the mirrors. move in and out so that each laser beam moves moved back and forth across the mirror surfaces. This shows that the scattering centers with respect to the relocated, i.e. in position shifted standing wave field nodes are shifted, so that the phase shift promotion of the phase modulation erf is filled. This shift also results in the standing wave field being shifted with respect to an opening fixed to the body.

In addition, the distance from one scattering group to the next group on the next mirror changes over time. The vector summation of the optically backscattered light from all scattering surfaces of the laser cavity determines the size of the "dragging" "Effect it as well as the final phasing. Thus, it causes the "micro tremors" of the mirrors, that the usable scatter vector is modulated in time. This effect continues to reduce the drag effect.

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For the sake of completeness, reference is also made to US Pat. No. 3,581,227 Reference which is of interest here in that it is shows a stack of piezoelectric elements and the position of a mirror is shifted by the length of a To change the laser cavity exactly. This is known technique and can be used in the device according to the present invention as Direct bias voltage can be used on which the alternating current signals having a suitable phase position are superimposed can. US-PS 3,581,227 gives no indication of a large number of mirrors, each of which has a piezoelectric control, and the phase vibration, i.e. the oscillation of the mirrors, cannot be inferred from it either. Here you can Transducers are used that are not piezoelectric elements to put the mirrors in the appropriate phase relationship To offset oscillations. For example, magnetostrictive Transducers are used. Additionally, other laser effect materials and laser cavities can be used with a different numbers of mirrors, e.g. four, can be used.

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Claims (4)

Fire claim 1: Aj Hinglasergyroskop with at least three mirrors, which build up a geschlo ssenenPfad for the one and the other of two light beams propagating in opposite directions in an active laser medium of the gyroscope, and individually "actuatable transducers offset each of which one of the mirrors in vibratory motion , the vibrating movement of the mirrors being in the form of a parallel, backward and forward displacement in the direction perpendicular to the mirror surface, and the vibrating movement being controllable so that constant laser beam path lengths are maintained, characterized by a device (12) for forming an optical cavity with a closed loop containing an active laser medium for generating primary, counterclockwise rotating laser light beams, the frequency difference between the light beams being a measure of the speed of rotation experienced by the ring laser, and d ie the cavity forming device a plurality of mirrors (28, 30, 32; 64, 65, 66) for reflecting the light rays, and a device (44) which causes a plurality of these mirrors to vibrate for conversion at the same frequency in a direction exclusively perpendicular to the surface of the mirror, the vibrating mirrors of ITull have different vibration amplitudes and vibration phase positions so that the entire distance around the closed loop remains essentially constant. _BATH COPY May 11, 1983 V / He -.ar-- - L / p 11.140 - Z- claims: - 0eee a device (12) for forming an optical cavity with a closed loop that uses an active LasermepbCum Generation of primary counterclockwise laser light beams contains, whereby the 3 frequency difference between the. Light rays is a measure of the speed of rotation that the I. Ring laser experiences, and wherein the denygbhlraum-forming device j tung a variety of mirrors CgS, 30 > 32; 64, 65, 66) for reflecting the light rays ^ contains, and J a device (44), di ^ a plurality of these mirrors for i Implementation with the same frequency in one direction only perpendicular to the surface of the mirror in vibratio-! The vibrating mirrors used by Bull are different Vibr-af £ i ons have amplitudes and vibration phase positions, so that djsr is the total distance around the closed loop in the we- j lcl » _: j 2. Ring laser gyroscope according to claim 1, characterized in that ' that the vibration amplitudes of the vibrating mirrors (64, 65 »j 66) are essentially the same size. 3. ring laser gyroscope according to claim 2, characterized in that the phase difference between the oscillation of the vibrating Mirror (64, 65, 66) is about 360 divided by the number of vibrating mirrors. 4. Ring laser gyroscope according to claim 1, characterized in that the vibration device (44) has means which have a Sinusoidal displacement of each mirror over time. 5. ring laser gyroscope according to claim 1, characterized in that the vibration device (44) has a transducer. copy May 11, 1983 V / He ~ ^ ~ "" "L / p 11.140 -J- ■ 'ί 6. Ring laser gyroscope according to claim 5> characterized in that the transducer (44) has a stack of piezoelectric elements. 7. ring laser gyroscope according to claim 1, characterized by a ring laser assembly (12) of the single mode type having two counterclockwise rotating laser beams and having at least three mirrors (28, 30, 32), a device (44) for converting at least two mirrors into an oscillation mode with substantially the same frequency only in the direction of a line dividing the rays incident on the vibrating mirror, and a device that phases the movement of all vibrating mirrors so as to have a constant primary laser beam path length is maintained when the vibrating mirrors are displaced. 8. Ring laser gyroscope according to claim 1 or 7 »characterized in that that the magnitude of the translational vibration of each i-th mirror from a neutral position on the order of 4 ir sin ÖT is, where λ is the wavelength of the laser light beam and 0. is the angle of incidence of the light rays on the i-th mirror relative to the perpendicular to the mirror at the point of incidence, and ß is an argument of the Bessel's view of the first type of zero order, which is the function J ( β ) returns to zero. 9-ring laser gyroscope according to claim 8, characterized in that that the magnitude of the translational vibration of each i-th mirror from a neutral position on the order of ^ish ' ^X CX) PY May 11, 1983 W / He "' - # - L / p 11.140 where λ is the wavelength of the laser light beam and Θ- equals relative to the angle of incidence of the light rays on the i-th mirror to the perpendicular from the mirror at the point of impact. 10. Ring laser gyroscope according to claim 9> characterized in that the vibration is essentially sinusoidal. 11. Ring laser gyroscope according to claim 7> characterized in that that all trigonometric lOurier components of vibration are in in the same way as the basic component are brought into phase. 12. Ring laser gyroscope according to claim 7> characterized in that the vibrating mirrors vibrate in an open loop. 13 ring laser gyroscope according to claim 7, characterized in that that all mirrors operate in closed loop servo mode to control their amplitude and phasing. 14. Ring laser gyroscope according to claim 7 »characterized in that the phase positions of the η vibrating mirror of a generalized closed laser path are related to each other by 2 Y / n radians, and that the amplitudes χ of the displacements of the vibrating mirror are given by ßm λ 4 ir Sin ΘΓ sin ω t where ßm is a value of the argument of the Fullor-dnungs-Bessel function of the first kind is, J (ß m) = 0 for m = 1, 2, 3 ···· »^ der Angle of incidence at the i-th vibrating mirror, and << J is the tremor angular frequency which is large compared to typical laser synchronization frequencies.