GB2185846A

GB2185846A - Ring laser

Info

Publication number: GB2185846A
Application number: GB08601741A
Authority: GB
Inventors: William Gillies Mcnaught; Iain Edward Ross
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1986-01-24
Filing date: 1986-01-24
Publication date: 1987-07-29
Also published as: GB2185846B; CA1309155C; FR2593650B1; FR2593650A1; JPS62217683A; DE3700844A1

Abstract

A ring laser comprises a block (10) of dielectric material in which are formed a number of passages (11) defining a polygonal path enclosing a gaseous active medium. Two electrodes (16) are arranged external to the passage parallel to and on opposite sides of part or all of at least one of the passages (11). In operation the electrodes (10) are capacitively coupled to the gaseous active medium to produce an electric discharge therein transverse to the direction of the passage by the application to electrodes (16) of a high-frequency alternating voltage. <IMAGE>

Description

SPECIFICATION Ring laser Ring lasers are used as angular rate sensors primarily, though not exclusively in inertial navigation applications. Basically a ring laser comprises two laser beams which travel in opposite directions around the same closed loop path. Commonly the path is generally triangular in form. Rotation rates about an axis perpendicular to the plane of the ring laser resonant cavity are measured by detecting the beat frequency which occurs due to a frequency difference between the counterrotating beams resulting from the rotation.

A commonly-used form of ring laser designed for such a purpose is described in US Patent No. 3,390,606 and its British equivalent No. 1,142,962. The ring laser shown therein comprises a solid block of dielectric material in which three passages have been formed to provide a triangular passage through the block. Mirrors are provided at the points where pairs of passages intersect, and one of the mirrors also forms part of the detection means. The passages are filled with a suitable gas or gas mixture and an electric discharge is produced in the gas between anode and cathode electrodes which extend into the lasing gas.

Such an arrangement for producing an electric discharge in the gas has a number of disadvantages. One of these has been the need to provide an excitation power supply producing a dc running voltage of between 500 and 1,000 volts, with a striking voltage of up to 3,000 volts. Such a power supply has of necessity been large in size. In addition the presence of anode and cathode electrodes causes sputtering and gas flow effects. Other problems also occur.

It is an object of the present invention to provide a ring laser in which the above problems are avoided.

According to the present invention there is provided a ring laser which includes a block of dielectric material having formed therein at least one passage defining a closed polygonal path lying in one plane within the block, a gaseous active medium contained within the said passage, and two electrodes arranged external to the passage parallel to and on opposite sides of at least part of said passage and being, in use, capacitively coupled to the gaseous active medium by the dielectric material, whereby laser action is included in the gaseous active medium to produce an electric discharge therein transverse to the direction of the passage by the application of a high-frequency alternating voltage between said two electrodes.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a simplified plan view of a ring laser according to a first embodiment of the invention; Figure 2 shows a part-sectioned side view of the ring laser of Fig. 1; Figure 3 shows a circuit diagram of a power supply suitable for the ring laser of Fig. 1; and Figure 4 shows a plan view according-to a second embodiment of the invention.

Referring now to Figs. 1 and 2 the ring laser is made from a block 10 of a dielectric material such as a glass-ceramic material having high thermal stability. Conveniently the block 10 may be in the shape of a truncated triangle as shown in Fig. 1. Three passages 11 are bored through the block 10 parallel to the three sides of the triangle. The three passages emerge through the truncated corners of the block. Each of these corners carries a mirror 12 mounted so that laser radiation emerging from one passage is reflected along the next passage so that the radiation follows a closed path through the passages. Two of the mirrors may be fully reflecting whilst the third is partially transmitting and is associated with a detector 13 which produces an electrical output on a cable 14.

As shown in Figs. 1 and 2 two grooves 15 are machined into the block respectively above and below on of the passages 11. At the bottom of each of the grooves 15 is located a metal electrode 16 to which may be applied a high-frequency alternating excitation voltage from a suitable power supply.

In the centre of block 10 may be located a dither mechanism, represented by the reference 17, which is able to oscillate the block through a small angle about an axis 18, this being the axis about which the ring laser is able to sense angular movements. Such dither mechanisms are well-known and need not be described here.

In operation, the application of a suitable high-frequency alternating voltage between the two electrodes 16 causes excitation of the laser gaseous active medium, usually a Helium Neon mixture, contained in the passage 11 located between the electrodes. This excitation is transverse to the direction of the passage 11 and produces an electric discharge in the gas, resulting in laser action taking place.

This produces laser beams travelling in both directions around the closed path defined by the passages 11 and the mirrors 12.

The use of a transverse high-frequency alternating voltage to excite the laser has a number of benefits. It is not necessary to insert electrodes into the passages 11, and hence the problems caused by such electrodes are avoided.

Transverse capacitive coupling of high-frequency alternating power into the discharge through the glass-ceramic walls between the electrodes allows a much reduced drive voltage to be used compared with known longi tudinally excited ring lasers. Typically, for a ring laser having a bore of 3mm the drive voltage may be of the order of 5-10 volts at a frequency of between 10MHZ and 5GHZ.

Capacitive ballasting exists so that no ballast resistors are required and hence the power consumption is much less. The frequency range referred to lies within the main communications band and hence components are readily available to enable small low cost power supplies to be produced. A power supply suitable for the purpose is shown in Fig.

3.

As shown in Fig. 3 the power supply circuit is very simple, comprising a radio-frequency oscillator 30 operable to produce an alternating signal at the required frequency. This signal is amplified by amplifier 31 to provide the necessary voltage level and is coupled to the ring laser shown schematically at 32 by an impedance matching network 33.

The laser illustrated in Fig. 1 has electrodes associated with one of the three passages 11 formed in the block. Excitation may be provided on either or both of the other passages if necessary, depending upon the bore of the passage and gain required for satisfactory operation. Fig. 4 show a plan view of 15 a ring laser having two pairs of electrodes.

The ring laser is not operated as a waveguide laser.

The electrodes need not necessarily be located in grooves in the dielectric material, though this is one way of reducing the thickness of material between the electrodes and the passage. If a material of sufficient strength was available the block could be made sufficiently thin for the electrodes to be formed on its surface. Alternatively the thickness of the material may be reduced by using other configurations.

A number of other advantages arise from the use of transverse high-frequency alternating excitation of the laser. The discharge in the gas occurs substantially between the two electrodes, unlike the dc longitudinally-excited ring laser. This means that the discharge occurs well away from the mirrors avoiding the risk of mirror damage due to ultra-violet radiation or ion bombardment. Construction is simpler than with the dc-excited ring laser leading to a lower manufacturing cost. The only vacuum seals required are for the three mirror ports and a gas fill port. Glass-ceramic materials are particularly suitable as they remain stable, having zero or very small coefficients of thermal expansion, over a very wide temperature range, from -500C to 400"C.

Various examples of such materials are known by the trade names Zerodur, Cervit and ULE.

Claims

1. A ring laser which includes a block of dielectric material having formed therein at least one passage defining a closed polygon path lying in one plane within the block, a gaseous active medium contained within the said passage, and two electrodes arranged external to the passage parallel to and on opposite sides of at least part of said passage and being, in use, capacitively coupled to the gaseous active medium to produce electric discharge therein transverse to the direction of the passage by the application of a high-frequency alternating voltage between said two electrodes.

2. A ring laser as claimed in Claim 1 in which the block of dielectric material contains three passages defining a substantially triangular close path.

3. A ring laser as claimed in Claim 1 in which the said two electrodes are associated with one only of said passages.

4. A ring laser as claimed in any one of Claims 1 to 3 in which one at least of said two electrodes is located in a groove extending from the surface of the block of dielectric material towards the said passage.

5. A ring laser as claimed in any one of Claims 2 to 4 in which the said block carries three mirrors each positioned at the intersection of two of said passages so as to reflect laser radiation emerging from on passage into the adjacent passage.

6. A ring laser as claimed in only one of the preceding claims in which the block of dielectric material is made from a glass-ceramic material.

7. A ring laser as claimed in any one of Claims 1 to 6 in which the gaseous active medium is a mixture of Helium and Neon.

8. A ring laser as claimed in any one of Claims 1 to 7 in which the frequency of the high-frequency alternating voltage is in the range from 10 MHz to 5GHZ.

9. A ring laser substantially as herein described with reference to the accompanying drawings.