GB2220098A

GB2220098A - Ring laser gyroscope

Info

Publication number: GB2220098A
Application number: GB8914321A
Authority: GB
Inventors: Tae W Hahn; Salim N Jabr
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1988-06-22
Filing date: 1989-06-22
Publication date: 1989-12-28
Anticipated expiration: 2009-06-22
Also published as: GB8914321D0; FR2633389A1; GB2220098B; CA1320557C; FR2633389B1

Abstract

A radio frequency excited ring laser gyroscope 50 comprises a closed resonant cavity 75 which surrounds a helical coil 72 driven at a high radio frequency (eg 5 to 550 megahertz). The coil surrounds one leg of the gyroscope which is carved out and surrounded by the resonant cavity 75. Such a radio frequency excitation system eliminates the need for high power DC discharge components such as cathodes and anodes, and, for example, problems inherent with properly sealing the cathodes anodes to the monolithic frame of the ring laser gyroscope. The resonator shield prevents r.f. interference. Also detailed is a multioscillator ring laser gyroscope (Fig. 5): a split gain multioscillator ring laser gyroscope (Fig. 6) (a low D.C. signal applied to r.f. coil 112 fine tunes the field from permanent magnet 116); and control circuitry (Fig. 7). <IMAGE>

Description

RING LASER GYROSCOPE This invention relates to optical rotation sensors, and, particularly, to ring laser gyros.

Ring laser gyroscopes are a class of optical rotation sensors that have been developed to provide an alternative form of rotational measurement to the mechanical gyroscope. A ring laser gyroscope employs the Sagnac effect to detect rotation. A basic two mode ring laser gyroscope has two independent counter rotating light beams which propagate in an optical ring cavity. These two light beams propagate.in a closed loop with transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. Besides the planar ring laser gyro, other path geometrieshave been used; for example, a non-planar gyroscope has been disclosed in United States Patent 4,482,248, which teaches an out-of-plane light path that provides the reciptocal splitting of two pairs of counter rotating beams.This out-of-plane gyroscope has been known in the literature as the multioscillator ring laser gyroscope.

These non-planar gyro has been developed to avoid the need for mechanical dithering. Dithering is needed in planar gyroscopes to prevent counter rotating travelling waves from locking at low rotation rates.

Heretofore, ring laser gyros have operated using at least a dome-like configured metallic or glass covered metallic cathode and at least two anodes, which extend outward from a monolithic glass body of the ring laser gyro to excite the gas medium contained within the gyroscope. A DC discharge has been used which excites gas contained in the ring laser gyro pathway between the cathode and each of the anodes.

Figure 1 shows a prior art planar DC-excited ring laser gyro. The ring laser gyroscope 10 is formed from a monolithic glass body 12 such as Zerodur, which is a trademark of the Schott Glass Works Co. of West Germany. A similar glass that may be used as the ring laser gyro body 12 is manufactured under the trademark "CERVIT" sold by Ownes-Illinois. Both materials are mixtures of glass and ceramic that have opposite temperature expansion coefficients, thus providing an overall minimal dimensional change over a wide range of temperatures.

A square optical pathway 14 is defined within the gyroscope 10 by four legs, 16, 18, 20 and 22. Leg portion 18A, 22, and 20A form a segment of the optical pathway which glows due to a DC discharge between a cathode 24 and respective anodes 26 and 28. During manufacture, a gaseous mixture of helium and two isotopes of neon provides an active medium that is excited along the DC discharge path defined by segments 18A, 22, and 20A.

Gas is provided to the cavity during manufacture by a fill stem 30 through the anode 26. The cathode is generally grounded, while the anode potentials are each brought up to 1500 volts, through use of a ballast resistor 32. At each corner of the pathway 14, a mirror is positioned to reflect light around the ring laser gyro. The mirrors 34, 36, 38 and 40 are mounted to the body 12. A more detailed description of the operation of the planar ring laser gyro together with the particular manner of DC excitation is also described in US Patent 4,115,004 and US Patent 4,612,647.

In addition to the high voltage and high current regulation requirements needed for DC excitation, a number of problems have been associated with the manufacture and reliability of DC discharge ring laser gyroscopes. A prime problem is that of Langmuir flow which can cause a bias and therefore inaccuracies in the rotational sensing capabilities of the ring laser gyro, unless the gyroscope is provided with two balanced current discharge paths. A discharge between a single anode and cathode causes the molecules in a gas laser cavity to flow in a preferred direction.

This flow gives rise to a bias or inaccuracy in the rotational sensing capability of a ring laser gyroscope, since each of the clockwise and anticlockwise modes of light beams propagating in the cavity will be influenced differently by this flow phenomena. In a DC discharge excitation mechanism, as illustrated in Figure 1, the only manner of offsetting the bias problem is to exactly balance the currents and lengths of the two discharge legs 18A and 20A in each half of the discharge region. This is a difficult and costly process. The power supplies associated with DC excitation are expensive and bulky. A 3-4,000 volt potential is necessary to start the discharge process, and continued operation of the discharge requires a high voltage source of 1500 volts. The cathodes and anodes themselves have associated problems including leaks at the seals and shortened lifetimes.

Also, a phenomenon known as cathode sputtering arises and limits the lifetime of the discharge system.

Cathode sputtering is characterized by the degradation of a protective oxide coating on the outside of the cathode for a good part of its life. The discharge process eventually eats through the oxide coating, exposing the underlying aluminum of the cathode. Once this aluminum is exposed, cathode life deteriorates very quickly and results in an inoperative or nonusable laser structure. This cathode sputtering is a severe limitation on the life of a gas ring laser gyroscope. Also there are instabilities in the discharge when the DC discharge is initially activated after filling the ring lasers with gas during manufacture.

In certain ranges of current operation, instabilities in the current and voltage discharge arise. These instabilities limit the range of current and gas pressure that can be used with a DC discharge ring laser gyroscope. Also, the DC discharge process is relatively inefficient in providing high energy electrons to pump the gas laser atomic energy level.

Some of the problems associated with DC discharge in a ring laser gyroscope are also described in Laser Applications, edited by Monte Ross, Pages 133-200 (Academic Press, 1971).

In addition to the operation of the planar ring laser gyro through DC discharge as shown in Figure 1, the operation of the multioscillator laser gyroscopes, as described in an article by Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics, vol. QE16, No. 9, Sept. 1980 is discussed in this article. In both the out-of-plane And Zeeman effect multioscillator ring laser gyroscopes, it is preferred that the active medium not interfere with certain axially uniform fields needed for the operation of these types of ring laser gyroscopes. As with the planar ring laser gyroscope, DC discharge methods have created similar problems for multioscillator ring laser gyroscopes.

For all the foregoing reasons, an alternative method of excitation of the gain medium of a ring laser gyroscope is desirable.

In the past, alternative methods of excitation of a laser gas medium have been attempted with varying degrees of success. RF excitation of a helium neon mixture has been reported as early as 1961 in Physical Review Letters, vol. 6, No. 3, pages 106-110, in an article by A. Javan. J.P. Goldsborough has described an RF induction excitation of a continuous wave visible laser in vol. 8, No. 6 of Applied Physics Letters, (15 March 1966), pages 137-139.

US Patent 3,772,611 describes an RF excited ring type capillary tube 11 (Figure 1) which may have utility as a rotation rate sensor. The '611 patent, however, does not teach an efficient design for utilizing the RF excitation. This '611 patent also referred to "A Wave Guide Gas Laser" in an article dated Sept. 1, 1971, in vol. 19, Applied Physics Letters, No. 5, pages 132-134. In this article, a combined RF and DC voltage excited capillary wave guide containing a mix of helium neon gas as described.

These inductive coupled RF excitation methods were by necessity high power and created substantial electrical interference and noise which disturbed other instrumentation associated with rotation sensing.

UK patent application 2185846A discloses a ring laser which is excited by transverse electrical discharge operating at a high frequency alternating voltage. Although this disclosure claims a low voltage excitation range, it operates through capacitive coupling to the gaseous medium in a transverse direction to the passageway between the mirrors. Use of this transverse direction-excited, high frequency, alternating voltage, capacitively coupled to the active medium would result in contamination of the passageways of the ring laser gyroscope cavity due to the constant bombardment of the gaseous media against the walls and the high RF powers needed to drive the discharge. This is counter-productive to a long life operation of a ring laser gyroscope.

Thus, although the prior art referenced have disclosed alternative methods of excitation other than DC discharge for gaseous laser and ring laser gyroscopes, these alternative solutions to the problem of DC discharge have been inadequate.

According to one aspect of the invention there is provided a ring laser gyroscope including a resonant cavity defining a closed optical path having a gain medium therein, and closed cavity resonant means for applying a radio frequency signal for exciting discharge of the gain medium. According to another aspect there is provided a ring laser gyroscope including a resonant closed cavity defining a closed optical path which is filled with a gain medium, including means for exciting the gas medium which comprises a resonant cavity formed around a helical coil for applying a radio frequency signal to excite the discharge of a gain medium. The high frequency signal that is imparted by the resonant cavity may be in the range of 5 Mhz to 550 Mhz.A resonant means for applying a radio frequency signal for exciting a discharge of the gain medium can include a helical coil surrounding a portion of the closed optical path where the helical coil is contained within a closed conductive resonator shield. Along at least one leg of a monolithic dielectric body of which the ring laser gyroscope may be formed, a portion of the monolithic body may be carved out to allow one of a number of tubular bores to be surrounded by the helical coil and enclosed in a conductive resonator shield. An RF oscillator may be coupled and connected to the resonant means to supply a radio frequency signal to the resonant means for excitation of the active gain medium.

Other aspects of the invention are set out, for example in claims 7 to 10.

For a better understanding of the present invention and to show how the same may be carried into effect, reference is made, by way of example, to the accompanying drawings in which: Figure 1 shows a prior art configuration of a DCexcited planar ring laser gyro; Figure 2 shows a top plan view of a planar ring laser gyro that is radio frequency excited; Figure 3 shows a side elevational view of the planar radio frequency excited ring laser gyroscope of Figure 2; Figure 4 shows a perspective view of the radio frequency excited planar ring laser gyroscope of Figure 2; Figure 5 shows an alternate embodiment of a multioscillator ring laser gyro; Figure 6 is a perspective view of a split gain multi-oscillator ring laser gyroscope showing, in partial section, the configuration of a radio frequency excitation mechanism;; Figure 7 is a schematic diagram showing control electronics for a radio frequency excitation drive system for driving resonator means in embodiments of the present invention; and Figures 8A and 8B show experimental test results derived from experiments conducted using a radio frequency excited ring laser gyroscope.

With reference to Figures 2, 3 and 4, a radio frequency excited planar ring laser gyroscope 5 is shown.

The ring laser gyroscope is comprised of a low thermal expansion glass body 52 into which an optical pathway defined by legs 54, 56, 58 and 6. Positioned at each corner are mirrors 62, 64, 66 and 68. During manufacture, gas is introduced to the cavity defined by the legs through the passageway 7B.

At least one of the legs 6 has been carved to accomodate the positioning of a helical coil 72 which is surrounded by a resonant cavity shield 74. The coil 72 is driven by an RF frequency oscillator 76 (Figure 2) operating at a voltage of approximately 28 volts. The radio frequency oscillator 76 may be a modified Colpitts Oscillator.

Generally, a signal in the range of 5 to 550 megahertz is imparted to the coil. By completely enclosing the coil 72 with a resonator cavity shield 74, a closed resonant cavity is formed which serves to amplify the signal produced within the cavity. In the preferred embodiment the resonant shield 74 should be of conductive material with low resistivity.

The helical coil 72 is preferrably formed from copper wire and the resonator shield made from copper tube to form the radio frequency resonant cavity. The shield could be formed from other metals coated on a tubular substrate such as, but not limited to, gold, silver or aluminum. The resonant cavity 75 encloses the portion of the gain medium 80 that is excited by the radio frequency discharge. By operating at the proper radio frequencies, the cavity may be run as a fullwave, halfwave, or quarter wave, length resonator.

Readings of the electro magnetic field outside the resonator reveal that the cavity has localized the electro magnetic field. Any spurious signals have been reduced in order to minimize any undesirable effects of the electro magnetic far field on the operation of the ring laser gyroscope or. related measuring electronic components. Thus, unlike radio frequency induced excitation as disclosed in the prior art, the use of a resonant cavity allows the optimization of a strong excitation with low power and minimal radio frequency interference (RFI).

An enclosed helical resonator, such as that shown in Figures 2-4, may provide high Q. The helical resonator assembly is comprised of a coil 72 within the resonator shield 74, where one end of the coil 72 is solidly connected to the shield 74. The other end of the coil 72 is an open circuit, except for a possible trimming capacitor (not shown) . In this configuration, the resonator assembly resembles an L-C circuit; but, instead of being a lumped-constant device, its operation can be described in terms of distributed inductance, capacitance, and resistance.

The resonator should be properly aligned for optimum performance. According to techniques known in the art (See W. w. MacAlpine, et. al., "Coaxial Resonators with Helical Inner Conductor, Proceedings of the IRE, December, 1959, pp.

2999 - 21(35 at 21to), alignment of the resonator coil can be achieved according to the following equations: (1) Qu = S(3D fo /2 where: Qu = unloaded Q; D = inside diameter of the resonator shield; and, f = Resonant frequency (Mhz).

0 (2) N = 1900/(f D) turns where: N = total number of windings f = Resonant frequency < MHz > ; and, 0 D = inside diameter of the resonator shield.

Additional considerations of parameters needed to achieve optimum RF coil operation have been treated in the conventional arts, as indicated by the MACALPINE Article, SUPRA.

Figure 5 shows an alternative embodiment of the radio frequency excitation system of this invention used in conjunction with a multioscillator ring laser gyroscope 82.

As with the planar ring laser gyroscope, the frame 84 is made from a monolithic dielectric material having a low thermal expansion over a wide temperature range (between -5(3 deg. C 15(3 deg. C). As has been disclosed in the art, this form of multioscillator ring laser gyroscope is positioned in an out-of-plane configuration in order to provide reciprocal splitting between sets of left and right circular polarized beams of light. (A more detailed explanation may be found in the Laser Handbook Vol. 4, edited by M.L. Stitch, published by North Holland, 1985, pages 23(3-332). Heretofor, the multioscillator ring laser gyroscope has had its active medium excited by a DC discharge between a cathode and at least 2 anodes.An alternative form of excitation of the multioscillator ring laser gyroscope is shown in Figure 5.

One of the four legs, positioned between mirrors 86 and 88, of the out-of-plane multioscillator ring laser gyroscope 82 is carved out of the body. The passageway 91 defined by the leg between 86 and 88 is integral with the frame 84 and is preferrably a cylindrical pathway. Wound around this pathway 91 is a Radio Frequency helical coil 94. Surrounding the coil 94 is preferrably a cylindrical radio frequency resonator shield 96. Positioned on the leg opposite 91 between mirrors 9 and 92 is the optical rotator 97 (such as a Faraday rotator) which provides non-reciprocal splitting between clockwise and counter-clockwise beams of light within two sets of left and right circularly polarized beams in the presence of a uniform magnetic field.

It will be noted that the radio frequency resonator shield 96 provides good isolation for all signals generated within the helical coil 94 and the resonator cavity 95. By shielding the resonator cavity 95, the radio frequency resonator shield 96 prevents any radio frequency interference from affecting the detection circuitry or any other electronics.

With reference to Figure 6, a Split Gain Multi-Mode Ring Laser Gyroscope 1(3(3 is shown. The frame 182 is a monolithic dielectric material at a low coefficient of thermal expansion. As was taught in the co-pending US application 115,018, the split gain multi-mode ring laser gyroscope is also configured in an out-of-plane configuration. A strong permanent magnet 116 is used to cause the split gain effect needed to operate this form of ring laser gyroscope. When the split gain gyroscope is operated by use of conventional DC discharge, a permanent magnet (not shown) is positioned on a leg opposite the gain medium and the leg where the cathode is positioned. This is not, however, the most desirable positioning of the permanent magnet in the split gain multi-mode ring laser gyroscope.

Preferrably, the split gain multi-mode ring laser gyroscope operates best when the active medium is contained within the permanent magnet 116. This is difficult to achieve when using a conventional DC discharge manner of medium excitation. The radio frequency excitation system of this invention provides an optimum alternative to the conventional DC discharge.

Again with reference to Figure 6, the active medium discharge pathway 184 is positioned between mirrors 16 and 1(38. Within the pathway 1Q4 gaseous medium (preferrably a helium-neon mixture) is excited by use of a radio frequency helical coil which applies a radio frequency signal from an RF oscillator (not shown to a resonant cavity 11(3 formed between the radio frequency helical coil 112 and the radio frequency resonator shield 114. The radio frequency resonator shield 114 may be a copper shield which surrounds copper wire forming the radio frequency helical coil 112.In order to provide compactness and optimum design, the radio frequency shield 114 may be positioned along the inner diameter of the permanent magnet 116 which also surrounds the medium passageway 11Q.

By configuring the radio frequency excitatiop system within the permanent magnet 116 as shown in Figure 6, one is able to achieve optimum results with regard to both the excitation of the active medium 118 within the passageway 110 as well as the confinement of that medium llB within the permanent magnet length 116, in order to achieve optimum split gain multi- mode ring laser gyroscope operation. Each signal then plays a different role. The high radio frequency signal imparted on the helical coil 112 is used to excite the active medium 118. The low DC signal applied simultaneously to the coil 112 is used to fine tune the magnetic field arising from the permanent magnet 116.

Since the permanent magnet 116 preferably has a cylindrical configuration, the radio frequency resonator shield (114) should also be of a concentric cylindrical shape within the inner of the permanent magnet 116.

In this manner, a radio frequency excited Split Gain Multi-Mode Ring Laser Gyroscope is disclosed which does not need independent DC magnets positioned on a leg away from the active medium 118, as is required in a DC excited split gain multi-mode ring laser gyroscope.

Figure 7 shows the control electronics for the radio frequency drive system. In order for the radio frequency excitation to operate efficiently, the radio frequency resonator and coil system 15 must be regulated so that the excited medium is maintained at a relatively constant power level while the radio frequency resonator signal is maintained at a relatively constant frequency. The combination of electronic components shown generally in power control loop 152 controls the power provided to the radio frequency resonator 15(3, while the frequency control loop 154 controls the frequency of the signal drive in the radio frequency resonator 15(3.

The power control loop 152 receives an input signal through the photo-diode 156, in the form of light intensity of the gain medium 155. This signal is then amplified in the pre-amplifier 158 and provided as an output voltage signal to the differential amplifier 16(3. The differential amplier 16(3 compares the voltage power signal to a reference voltage. If the operating power voltage signal is higher than the reference, or lower than the reference, an output signal resulting from common mode rejection by the differential amplifier 16(3 provides an error signal input to the integrator 162. The integrator 162 then provides an output signal which controls the electronic attenuator 164 to adjust the voltage power supply provided to the radio frequency resonator.

The electronic attenuator 164 provides an output signal to the radio frequency amplifier 166 which couples its output signal to the radio frequency coupler 168. The resulting output at 17(3 will either raise or lower the power provided to the RF frequency coil 172 within the radio frequency resonator 15(3. Additionally, the frequency of the RF resonator 15(3 is monitored by the radio frequency coupler 168 and is detected by an amplitude modulator detector 174.

The output from the detector 174 of the frequency control loop 154 is then amplified by the pre-amplifier 176.

The output of the pre-amplifier 176 is then provided to the phase lock loop 18(3. The phase lock loop 18(3 is comprised of synchronous detector 178, an integrator 179, and a local oscillator 177. (This is a known configuration for an analog phase lock loop). The phase lock loop 18(3 locks in on a frequency determined by the local oscillator 177 and the output of the phase lock loop 18(3 is then provided to the summing amplifier 182. The summing amplifier 182 then adjusts the voltage to the voltage controlled oscillator 184.

The voltage controlled oscillator 184 provides an output adjustment to the electronic attenuator 164 which in turn, as previously described, provides frequency control through the radio frequency coupler 168 to the frequency of the signal applied to the coil 172 of the RF resonator 15(3.

In this manner, both power and frequency.control and consistency are maintained.

If it is desired to operate the ring laser gyroscope at full wave resonance the gain medium 155 may be captured within the coil 172 of the RF resonator 15(3. The positioning of this gain medium 155 may be controlled by the electronics shown in Figure 7; but, this is only one embodiment for accomplishing this goal. It would be known generally that a digital or other hybrid analog digital servo-system may also be used to control power and frequency of the RF resonator 150.

Figures 8A and 8B show the results of radio frequency excitation experiments for a planar ring laser gyroscope.

When radio frequency excitation is applied to the ring laser gyroscope, curves 19B and 192 of Figure 8A show that the optimum power is provided at the center of the radio frequency resonator tuning frequency at 189 and 191 respectively. Curve 19(3 is shown at 29 decibels referred to one milliwatt (dBm) or .75 watts, while curve 192 shows a 3(3 dBm input or 1 watt. In either case it can be shown that as one detunes the resonator from the central frequency only 3 Mhz lower or 6 Mhz higher, there is a significant drop off in excitation. Thus the fine tunability of the radio frequency excited gyroscope is shown in figure 8A. Figure BB shows that the power increases relatively linearly.As input power and voltage is increased from a 27 volt input to a 32 volt input, curves 194 and 196 show that the wattage output of the linear laser increases in a substantially linear fashion.

Curve 194 exhibits the 2(3(3 Mhz signal with 31 turns while curve 196 shows a 112 Mhz signal with 55 turns. It thus can be seen from Figure 8B that there is a linear relationship between output and input power that is smooth and provides a high optimum efficiency for excitation of the gain medium.

While preferred means for implementing a radio frequency excitation system for a variety of embodiments of ring laser gyroscopes have been shown, it is clear that alternative resonator configurations may be used. These configurations could also provide a low power, high efficiency output that is simple and avoids all the disadvantages of the traditional DC discharge medium excitation devices of the prior art.

Claims

CLAIMS:

1. A ring laser gyroscope including a resonant cavity defining a closed optical path having a gain medium therein, and closed cavity resonant means for applying a radio frequency signal for exciting discharge of the gain medium.

2. A ring laser gyroscope according to claim 1, wherein said closed cavity resonant means comprises a helical coil surrounding a portion of said closed optical path and contained within an enclosing conductive resonator shield, whereby discharge of the gain medium of the ring laser gyroscope can be achieved.

3. A ring laser gyroscope according to claim 2, and having a monolithic dielectric material body having a plurality of tubular bores in a polygonal configuration defining said closed optical path, at least one side of said body being recessed to allow at least one of said tubular bores to be surrounded by said helical coil and enclosing conductive resonator shield.

4. A ring laser gyroscope according to claim 2 or 3, wherein an RF oscillator is connected to said resonant means to supply radio frequency signal to said coil.

5. A ring laser gyroscope according to claim 4, wherein the RF oscillator is operable to provide a radio frequency signal within a range from 5 to 550 megahertz.

6. A ring laser gyroscope according to claim 4 or 5 and including means for regulating said RF oscillator to provide a radio frequency signal to said helical coil within a predetermined range, said regulating means including frequency control means comprising frequency detection coupling and amplification means coupled to phase lock loop means, said phase lock loop means being arranged to control the frequency provided by said RF oscillator to said helical coil, and means for controlling the power provided to said resonator means including photodetection means coupled to a signal comparator means, said signal comparator means being operable to adjust an attenuator means with reference to a predetermined optimum power supply level.

7. A ring laser gyroscope including a closed optical path having a gain medium therein, there being resonant means for operation as a full wavelength resonator for applying a radio frequency signal for exciting discharge of the gain medium.

8. A ring laser gyroscope including a closed optical path having a gain medium therein, there being resonant means for operation as a half wavelength resonator for applying a half wave radio frequency signal for exciting discharge of the gain medium.

9. A ring laser gyroscope including: a closed optical path having a gain medium therein; and resonant means for operation as a quarterwave length resonator for applying a quarterwave radio frequency signal for exciting discharge of the gain medium.

10. A split gain multi mode ring laser gyroscope, including a resonant cavity defining a closed out-ofplane optical path having a gain medium therein, comprising: a gyroscope body defining at least four legs of said path, the body being recessed to expose at least one of said legs of said optical path, said exposed leg having a radio frequency helical coil would around it, said radio frequency helical coil being surrounded by a resonant shield forming a resonant cavity around said radio frequency helical coil, and a permanent magnet surrounding said resonator shield, said coil and said resonator shield being concentric and within said permanent magnet, whereby a radio frequency-excited split gain multi-mode ring laser gyroscope is provided where a permanent magnet necessary to achieve split gain within the ring laser gyroscope is positioned along the same axis as the resonator excitation means so that the active medium which arises is confined to the length of said permanent magnet.

11. A ring laser gyroscope substantially as hereinbefore described with reference to the embodiment of Figures 2 to 4, or of Figure 5 or of Figure 6, or any of these embodiments when combined with Figure 7 of the accompanying drawings.