DE2920429A1 - DEVICE FOR LASER GYROMODIC LOCK REDUCTION - Google Patents

Description

Dipl.-Ing. A. Wasmeier

patent application:. :: *** ... · * ..:

Dipl.-Ing. H. Graf

Patent Attorneys PO Box 382 8400 Regensburg

in the

German Patent Office

8 Munich 2

D-8400 REGENSBURG 1

GREFUNGER STRASSE 7 Telephone (09 41) 54753 Telegram Begpatent Rgb. Telex β 5709 repat d

Your sign You 'Ref.

Your message Your Utter Our sign Our Ref.

L / p 9711

ο ".?. March 16, 1979 W / He

Applicant: UTTOH STSTiMS, IHC, 360 North. Crescent Drive, Bererly Hills, California 90210, TISl

Title: "Device to Reduce Laser Gyromode Locking"

Priority: TJSi. SH-Mr. 909.919 of May 26, 1978

909846/0731

Accounts: Bayerische Vereinsbank (BLZ 750 200 73) 5 839 Postscheck Munich 89369-801

Place of jurisdiction is Regensburg

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16.3.1979 V / He ••• - • -4.- · ·· Vp 9711 %

The invention relates to laser Sing, the pros as gyroscopes b

where the difference between the resonance | frequencies of advancing in the opposite direction Radiant energy or Idchtwaves a measure of the rotation is the order in which the propagating waves travel.

Single-laser gyroscopes, which continue in the opposite direction! Using striding laser beams are known per se. These are used to measure the rotation of the single laser gyroscope by combining parts of the oppositely progressing testicles to create a beat frequency, which indicate the frequency differences between the counter modes. The inadruck "Mode" is interchangeable with the designation "Wave" is used, and means a resonant traveling wave of radiant energy propagating within a single laser cavity. When the single-lens body is rotated about an axis containing a component perpendicular to the single-lens component, the Frequency of waves that propagate in a sighting within the cavity, while the frequency of waves that advance in the opposite sighting, diminishes. This change in frequency between the testicles advancing in the opposite direction results in a change in the beat frequency proportional to the speed of rotation. By monitoring the beat signal, information about the speed of rotation of the single laser is obtained.

However, for a single-laser gyroscope operating at low rotational speeds, frequency locking or the Be overcome with dragging «This phenomenon occurs when two waves traveling in opposite sighting in a resonance cavity with only slightly different frequencies are drawn towards each other and combine into a standing wave with a frequency. It follows that for low rotational speeds of the bin laser, at which the frequency differences between the two opposite testicles are very small, the shafts are drawn together so that the

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The beat frequency does not change and the gyroscope is insensitive to low rotational speeds. You influences of heat drawing are detailed in Laser Applications, Honte Boss, Academic Press, Inc., New York, 1971 la the article "Ihe Laser Gyro" by! Frederick Aronowitz, pages 133-200, 'described ·

It is known that the main cause of drag coupling is the mutual scattering of energy from each of the beams in the sighting of the other is. This mutual scattering or backscattering is detailed in Aronowitz (see above), Pages 148-153 explained. The differential frequency between two in opposite sighting waves advancing in a Bipglaser is given by the equation

γ - a + b · sin Y

given, where f is the instantaneous phase difference between the waves propagating in the opposite direction & is proportional to the speed of rotation of the binoculars, and b_ is proportional to the magnitude of the backscattered energy dragged along. TM to get a gyroscope output that corresponds to the rotation of the single-lens body must be a, greater than b.

One way to eliminate drag is to to set the single-lens body in vibration mechanically. The fact that the laser is made to vibrate or performs trembling movements gives the gyroscope a speed of rotation superimposed in such a way that for most of the time a is greater than b, and the influences of b are extremely small or be eliminated. A gyroscope that makes mechanical tremors is used, is explained in DE-OS 27 49 125.

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Another way to

To hold the ring consists in making a directed trembling motion of the magnetic field of a Faraday cell, the internal

half of a ring laser path is arranged. Inside the ring

laser cavity are linearly polarized laser waves in circular

X converted into polarized light, its vector in the

same direction as the windings in the Faraday! see cell

rotates. The circularly polarized light waves is made by the magnetic field counteracted when the light sources the

■ Traverse Faraday cell and an increase or decrease in the length of the optical path is obtained depending on the direction of the field and the direction in which waves travel. To Leaving the Faraday cell it becomes circularly polarized light converted back into linearly polarized lightBecause the current oscillates in the windings of the Faraday cell, the magnetic field oscillates accordingly and changes the lengths of the optical path of the oppositely progressing Wave in a non-reciprocal manner. This can also be used to make a_ greater than b_ in the above equation, so that the influences of dragging along are kept to a minimum. This magnetic tremor using a Faraday cell is explained in the above cited reference by Aronowitz on pages 157-159.

The counter drag techniques described above are passive, that is, they do not depend on active laser gain media. at These methods have the same and opposite influences on waves traveling in one direction in the laser path the influences on the waves traveling in the opposite direction.

To explain the two opposite resonance modes in a ring laser cavity that are combined and

result in rotation information, referred to as ^N primary ^{modes N.}

The object of the invention is to reduce the drag between primary modes advancing in the opposite direction in one

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Bing laser cavity by keeping it as small as possible by introducing additional testes into the single laser cavity. These additional testes or ⁿ secondary ^{modes R} vibrate at frequencies different from those of the primary modes, and are coupled to the primary modes via the strength medium to create a counter-drag effect.

This is done according to the invention with a Bing laser gyroscope arrangement achieved, which is characterized by a bing glass body with reflective surfaces forming a closed loop optical path, a device for creating and maintaining at least two primary resonance converter modes advancing in opposite directions in the optical path, with frequency differences A device for generating corresponds between the primary modes of the angular movement which the bin glass body experiences and maintaining at least one secondary resonant transducer mode advancing in the path and with at least one of the primary modes is coupled, wherein the effects of drag are reduced or eliminated, and an apparatus for processing of the frequency differences between the primary modes Generate signals that correspond to the angular movement of the single-lens body.

Further features of the invention are the subject of the subclaims

In a special embodiment of the invention, four Sehwingu & gsresonanzmode are generated in the single-laser cavity. These four Codes can be generated by detuning the laser cavity so that the two primary modes operate at a frequency that is slightly off the center of the laser gain curve is, while two weaker secondary modes vibrate at frequencies that are only slightly above that on the gain curve Threshold. The threshold is defined as the area on the gain curve in which there is a resonance mode in the laser gain medium starts to get reinforced. The secondary nodes are coupled via the active gain medium, the

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Both strong testicles produce a tremor effect at γ. This tremor, which is produced at y as a result of the coupling of the weak and strong testicles, counteracts the heat-pulling component of the equation and reduces or eliminates it.

Another embodiment of the invention provides that an interfering oscillation is introduced from an external laser source. Two laser beams can be injected into the ring laser cavity of a ring laser with two testes. The injected testes, each of which migrates in a different direction, experience amplification from the laser medium and are thus coupled with two primary U modes that are generated in the ring laser. This inject | th testicles, the frequencies of which are different from those of the primary modes, are coupled with the primary modes, so that a trembling effect is achieved in the difference frequency. The tremor reduces or eliminates the coupling between the two oppositely advancing primary modes, thereby | accordingly, the heat draw is reduced or eliminated. |

An additional embodiment of the invention consists in that a part of one of the waves of the ring laser traveling in the opposite direction is used as an external source. In this ψ. Case, ty in which two primary modes in the ring laser sufficient Yer- strengthening have to swing, a portion of a mode from the 'ά Pinglaserhohlraum extracted via a partially reflecting mirror "1

The extracted part is shifted by Doppler effect so »A resonance frequency is changed and weakened, and then becomes injected back into the ring laser. This testicle, displaced by the Doppler effect and having a slightly different frequency, becomes with the the original primary mode, resulting in a dither effect | which decreases the hit draw.

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V

Veiter, a device is proposed with the Torliegender invention, with which the cables of the main modes are combined to form signals to generate which correspond to the speed and direction of rotation of the laser gyroscope · Porner sees the invention as a device to monitor and optimize the cavity length of the energy gyroscope, so that the resonance modes with the desired frequency oscillate on the gain curve

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The invention is explained below in conjunction with the drawing on the basis of exemplary embodiments. Show it:

1 shows a first embodiment of the ring laser gyroscope according to the invention, wherein the cavity length control circuit adjusts the cavity length so that two strong primary modes and two weaker secondary modes are generated in the gain medium,

Figs. 2 and 5, how the optical frequency of the resonance cavity so can be adjusted so that the resonance waves in the Kohlraum work at the desired points on the laser gain curve,

Fig. 4 shows how a laser cavity is detuned so that it is weaker

Secondary modes coupled with stronger primary modes pulling along, between two opposite primary modes decrease:

5 shows a second embodiment of the ring laser gyroscope according to the invention, secondary modes being generated by an external laser source and injected into the resonance cavity to couple them with the opposite primary modes, and

6 shows a third embodiment of a ring laser gyroscope according to the invention, wherein a portion of a primary mode is extracted from a ring laser resonant cavity, by Eoppler effect shifted in frequency, and then re-injected into the cavity to couple it to a primary mode.

As stated above, the difference frequency is or Switzerland exercise frequency, combining the two primary, opposite Resonance modes within a ring laser cavity are given by the equation

f-.i

': Y = a + b sin ^ f

determined, where ^ is the instantaneous phase difference between

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the waves traveling in the opposite direction, a is proportional to the speed of rotation of the ring laser gyroscope, and b is proportional to the magnitude of the reflected energy. The term "b sin Y" represents the coupling that results from backscattering. For small rotational speeds a_ is greater than b and Ϋ goes to zero. In this case, the ring laser gyroscope is pulled along and does not produce an output that corresponds to the actual rotation. Thus, the ring \ works at small, finite rotational speeds. laser not good as a gyroscope.

Through physical measures on the ring laser, with the help of which it is achieved becomes that the beat frequency is perturbed sinusoidally, becomes an additional time-changing factor of the above equation added so that the equation then reads

γ = a + b sin y + c.cos tvt

In the new equation, c and represent the amplitude and the frequency the disturbance given up at the differential frequency.

Solving this equation for ψ (t) gives a good approximation the equation

r (t) = at - IJ ₀ (§) cos (at)

If the values of c and oJ are chosen so that J = zero,
this equation reduces to

V (t) = at

and the drag expression of the original difference frequency equation is eliminated. In the following explanation of the invention, such an additional interference effect of the difference frequencies: ·; achieved in that additional modes or frequencies are introduced into the .ftinglaser cavity that with the primary resonance modes are coupled. The influence of this additional

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Interfering waves or secondary modes can be identified by adding the expression ^ c cos üit in the manner explained above "inscribed. By * J Controlling the size and frequency of the secondary modes can turn the off-i | press c and * »can be manipulated in such a way that they are dragged along in the i Reduce the ring laser gyroscope. §

Fig. 1 shows a ring laser gyroscope 2. The laser body consists of 4 "
made of quartz, and a sealed cavity 6 within the laser body is filled with 90% helium and 10% neon. Two anodes 8
and 10 and two cathodes 12 and 14 are connected to the cavity 6. The gas mixture in the spaces of the cavity between the cathode 12 and the anode 8 and the cathode 14 and the anode 10 | is electrically charged so that there is a gas plasma which | as an amplification medium for generating and amplifying the resonance -;?; laser mode within the cavity 6 is used. Three dielectric mirrors 16%, 18 and 20 of the dreieckfönmigen V, resonant cavity 6 are arranged at the three corners. These mirrors have multiple layers of dielectric coatings which are known per se. %

The mirror 20 is a partially reflecting mirror that allows | that a small part of the ring laser waves that hit it j | got through the mirror. SLIE of the border in the opposite direction continued'S primary modes in the cavity 6 along the by | Hike line 22 along the indicated path, get through the mirror
and are combined in a prism-like device in the combining and photodetecting array 2J to form a scattering pattern
arises. This scattering pattern is recorded by photosensitive detectors, and the signals generated therein are transmitted via conductors 24 to a logic data reduction circuit 26 which determines the speed and direction of the rotation. A detailed discussion of the combination of waves advancing in the opposite direction and the processing of the information obtained therefrom can be found in the aforementioned part of the literature, pages 139 to

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The laser beam frequency is determined by changing the cavity length, i.e. the distance the laser modes take to complete one: complete loop around the path 22 covered, steered. It is desirable to adjust or tune the cavity length so that the modes that come into resonance within the cavity in the Center of the intensity distribution curve "(gain curve) for the respective laser gain medium. To adjust the cavity length, the mirror 16 is attached to the laser body 4- so that that he can move inwards and outwards. With the back of the mirror 16 is a stack of piezoelectric elements tied together. The cavity length control is achieved in that the mirror 16 by applying an alternating voltage to the piezoelectric Elements 28 is made to vibrate or tremble. When the mirror 16 vibrates at a certain frequency, changes the intensity signal that is in the photodetector arrangement 23 is generated, accordingly and is via the conductor 30 to a Closed loop cavity length control circuit 32 transfer. This circuit determines where the resonance modes are located in the cavity along the gain curve and sets the nominal cavity length is determined by the DC electrical signal transmitted via conductor 34 to the piezoelectric elements 28 is given, is increased or decreased. A detailed discussion of this type of circuit can be found in NASA Report No. CR-1322'61 "Design and Devlopment of the AA13OOAbO2 Laser Gyro", written by T.J. Podgorski and D.N. Thyme, 1973, pages 10 and 11, can be found.

For the embodiment of the invention of FIG. 1, the tremor the difference frequency between the primary modes advancing in the opposite direction in the cavity by detuning the Cavity length reached. For example, in Fig. 2, the laser gain curve is 44, i.e. the intensity distribution of that in the laser gain plasma emitted light shown as a function of the optical frequency of this emitted light. As known only certain frequencies within the ring laser cavity can come into resonance, i.e. be amplified. The frequency spacing

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between these resonance modes is determined by the speed of light (c) divided by the path length (L), or the distance a wave travels when making a full loop around the laser path executes, definitely.

! In FIG. 2, the lines 36 and 38, the testicles in the ear-clockwise and counter-clockwise, which fall f at a particular frequency are present when the vocal laser cavity at the center of the comparison: strengthening curve is tuned 44th Lines 40 and 42 and lines 46 and 48 represent the closest testes on the optical frequency scale, which may also be present within the cavity, with the exception that no reinforcement medium is provided to reinforce these other testes within the cavity. The intensity level represented by dashed line 50 indicates the threshold or level above which the laser gain medium amplifies the resonant waves within the cavity.

For the embodiment of the invention according to FIG. 1, a detuning of the cavity length is achieved in that the direct current Component of the electrical signal is adjusted to the piezoelectric elements 28, so that the cavity length is detuned is used to move the main modes 36 and 38 off the center of the gain curve. A sufficiently large detuning must be made so that secondary resonance waves that Swing above the threshold value, can be introduced into the resonance cavity 6. Fig. 3 shows how the cavity length is adjusted so that resonance modes 36 and 38 are off center Gain curve 44 be shifted enough so that secondary waves 40 and 42 are slightly above the threshold on the ampl curve can oscillate.

The secondary mode 40, which progresses clockwise in the cavity

$ tet is now coupled with the stronger primary mode 36, the

in cavity 6 above the threshold and in the same sighting

progresses. This causes a trembling effect on the f expression

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2320429

in the difference frequency equation. In the same way, the secondary mode 42 is combined in the counterclockwise direction with the primary mode 38 in order to achieve a dither effect. The effects of the spurious modes 40 and 42 are determined by the expression c cos ¹ ^ t in the above equation. By adjusting the intensity along the gain curve of modes 40 and 42, as well as the frequency at which they oscillate, c_ and ω in the equation can be controlled to reduce the effects of drag, as discussed above.

KLg. 4 is a graph showing how detuning occurs the locking frequency between the primary modes 36 and 38 influenced in a specific example. The upset is defined as tuning the path length at the ring laser in such a way that the optical frequency of the primary modes comes from the center the gain curve is changed. In fig. 4, drag was practically eliminated for a case where the cavity path length was so out of tune that the primary modes 150 MHz from the center the gain curve.

Another embodiment of the invention is shown in FIG. This embodiment shows a ring laser with two modes similar to the ring laser gyroscope of Fig. 1. It is a sealed cavity ^ 2 is provided, which contains 90% helium and 10% neon, which is the case with electrical excitation between the anodes 54 and the cathodes 56 represents the laser gain medium. Parts of the two primary waves traveling in opposite directions in the cavity are introduced via a partially transparent dielectric mirror 58 into a combining and photodetector arrangement 60, where signals and transmitted to a data reduction logic circuit 62. An AC signal that is used in the control circuit 66 generated for the cavity length is inserted into a piezoelectric stack 68 that makes the mirror 70 shake and so that the cavity length of the gyroscope vibrates. Intensity signals from the combining and photodetector arrangement are transmitted via the conductor 64 to the control circuit 66 for the cavity length, changes in the intensity signal due to

809848/0731

> # Ti III

··· III ·· III

• I * ··· I I »I ···

¹ ># ti in

3/16/1979 W / He - 16 - L / p 9711 \

of vibrations of the piezoelectric stack 68 are processed in the cavity length control circuit 66. You direct current, - · .; component of the signal transmitted to the piezoelectric stack along line " 72" is set so that the length of the cavity is optimized for maximum intensity of the waves advancing therein in the opposite direction. In contrast to the example shown in FIG adjusted so that the resonant modes operate in the Hitte the encryption] amplification curve. '\

In the Aasführungsform of FIG. 5% interfering secondary waves at frequencies that are different from those of the primary resonance "modes in the laser gyroscope may be introduced from an external source. § The external source in this case is a linear laser 74 with two testes. Two separate testes generated in the linear laser 74- migrate colinearly to the dispersing element 76. Ser-like dispersing elements are known in the art and may have a grating for breaking down different frequencies into different proportions. After passing through the dispersion element 76, a secondary mode 78 is broken up to the dielectric mirror 80, where it is reflected to the partially transmitting mirror 82. When passing through the mirror 82, the testis in the clockwise direction occurs ι 78 in the ring laser cavity 52 and is coupled with the% effective clockwise primary mode, which is generated in the cavity.

The secondary mode 84 is through the dispersion element 76 to Mirror 86 and then deflected by mirror 82. It enters cavity 52, migrating counterclockwise, and is coupled to the counterclockwise primary mode.

The interference of the secondary modes which is introduced into the cavity is given in the difference frequency equation by the Aasdruck c cos ω t. The difference frequency between the secondary modes 78 and 84 is represented by ti) . Ser amplitude part is c_

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proportional to the size of signals 78 and 84 and the size of the Differential frequencies between the secondary and primary modes in the cavity. They can therefore be influenced in this way that they reduce drag by controlling the permeability of mirror 82 and the frequency and magnitude of the signals generated in linear laser 74.

ELg. 6 shows a third embodiment of the invention. This form contains a triangular single-laser gyroscope similar to the bin lasers according to Figures 1 and 5. The cavity length control circuit sets the piezoelectric stack in this way one, that the intensity of the single-laser gyroscope-Ausgar.ges a MaxL-mum. The two opposite testicles, which are longitudinally in the cavity As the path 22 progresses, frequencies substantially approaching the hats of the laser gain curve 44 of FIGS. 2 and 3 are matched.

In the device according to FIG. 6, an interfering secondary mode is introduced into the single-laser cavity and coupled with the counter-clockwise progressing primary mode. To the secondary fashion To obtain, a rope of the counterclockwise progressing mode in path 22 through the partially transmitting dielectric see mirror 88 led. This transmitted wave 102 then arrives through a sighting isolator 90. Such sighting isolators are are known in the art and operate to change the angle of polarization of the traveling waves passing through them. The mode 102 meets the dielectric mirror 92, which is connected to a piezoelectric stack 94. An alternating voltage is selected at a selected frequency on the piezoelectric stack 94 from the oscillation circuit 104 and brought about that the mirror 92 oscillates. This vibration in turn shifts by Doppler effect, the frequency of the mode 102, so that after its deflection from the dielectric mirror 98 and its renewed Introduction through partially transmitting mirror 83 in the single laser path, the frequency of the oscillation relative to the primary mode from which it was taken is changed. This mode, shifted by the Doppler effect, is activated when the

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Path 22 with the primary mode advancing counterclockwise coupled so that there is a counter-drag-trembling effect on ·} 'in the above-described manner is achieved.

The size of the signal shifted by the Doppler effect «* 102, that is reintroduced into the cavity, in dsr Differenjj » frequency equation represented by c_. The expression c_ can by Controlling the size of 102 can be controlled. Ways to control this size include controlling permeability of the partially transmitting dielectric mirror 88. The iris pressure ω in the difference frequency equation corresponds to the oscillation frequency, which are transferred to the piezoelectric stack 94- will. This expression can be controlled in a simple manner by the frequency of the generated in the circuit 104 Vibration is changed or controlled *. By controlling the magnitude and frequency of the mode 102 oscillation upon re-entry The influences significantly reduce dragging.

A polarizer 96 provided in the mode 102 path allows in an effective way that rays of one sense of polarization pass through, while rays of different polarization "are blocked. The polarizer 96 is adjusted so that that rays pass through. Since the alignment isolator 90 has the sense of polarization of mode 102 has changed, parts of the clockwise progressing primary mode have that through the mirror 88 reach a different polarization, and are locked out by the polarizer 96.

The invention is not restricted to the exemplary embodiments described above; it can, for example, be rectangular Single laser paths are used; instead of the piezoelectric stacks, other devices can be provided to vibrate the dielectric mirrors; furthermore, other cavity length controllers can be used; it can also work entirely without a cavity length control device

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and other precautions may also be taken to prevent primary rays traveling in the opposite direction to combine and process to obtain rotational information.

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Claims (4)

16.3,1979 W / He '"-1 - L / p 9711 Patent claims: / ly single-laser gyroscope arrangement with a one accommodated in a housing Glasses with a closed laser beam path and with laser beams progressing in the opposite direction "or Primary modes, characterized by a single laser body (4) with reflective surfaces that forming a closed loop optical path (22), a device (8, 12; 10, 14) for generating and maintaining at least two primary resonance converter modes (36, 38) advancing in opposite directions in the optical path (22), wherein Frequency differences between the primary modes (36, 38) correspond to the angular movement that the single-lens body (4) experiences, a device (18; 74-86; 88-104) for generating and maintaining at least one secondary resonance converter mode (40, 42; 78, 84) which proceeds in the path (22) and is coupled to at least one of the primary modes (36, 38) is, whereby the influences of drag are reduced or eliminated _t and a device (26; 62) for processing the differences in frequency between the primary modes (36, 38) in order to generate signals which correspond to the angular movement of the single-lens body (4). 2. Single-laser gyroscope arrangement according to claim 1, characterized in that that the device (8, 12; 10, 14) for generating and maintaining primary modes and the device (18; 74-86; 88-104) an electrically charged gas plasma amplification medium for generating and maintaining at least one secondary mode for generating and strengthening the primary and secondary testicles (36, 38; 40, 42; 78, 84) and a device (32; 66) for Control of the length of the optical path (22) for tuning the length of the path in such a way that at least two weaker, Secondary testicles (40, 42) progressing in the opposite direction and at least two stronger primary testicles advancing in the opposite direction Testicles (36, 38) are generated and reinforced in the reinforcement medium, 909849/0731 16.3.1979 ^: '''" ^: ' wherein the secondary modes are coupled to the primary modes to reduce drag between the primary modes. 3. Single-laser gyroscope arrangement according to claim 1, characterized in that that the device (18; 74-86; 83-1CW-) for generating at least one secondary mode (40, 42; 78, 84) a device (74) outside the ring laser body (4) for generating at least a secondary mode (78, 84) having a frequency different from that of the primary modes (36, 38), and a device (82) for introducing the external mode or modes in the optical path (22), to a coupling with at least one primary mode (36, 38) Achieve, with an oscillation that is generated between the secondary and the primary modes, with pulling effects between the Primary modes reduced, has. 4. Bing laser gyroscope arrangement according to claim 1, characterized in that that the device for generating and maintaining at least one secondary mode comprises a device (88) for extracting a part of at least one of the primary modes (36, 38) from the closed path (22), one which modifies the oscillation frequency Device (92, 94, 104), which is arranged in the path (102) of the extracted mode, to determine the frequency of the extracted Modify mode, and means (98) for introducing the frequency-modified mode in the path to it with at least to couple one of the primary nodes, the coupling being an oscillation causes that has reduced drag between the primary modes. 5 Single-laser Qyroscope arrangement according to Claim 1, characterized in that that a device (16, 28, 32) the length of the optical path for tuning the path length for the control of the optical Frequency of the primary modes (36, 38) controls. Q0984S / O731