GB2143076A - Laser mode control - Google Patents

Abstract

A mirror within a laser is adapted to control the mode of light emitted therefrom. The mirror is formed with a reflective area that eliminates higher order laser modes and reduces the amount of light scattering. Using a masking technique the unmasked reflective surface of the mirror is coated with a light absorbing layer to eliminate the reflection of higher order laser modes and limit light scattering, whilst the masked area constitutes a reflective area with substantially the same shape and dimension as the lowest order laser mode. The reflective area is circular for linear solid state lasers and elliptical for ring laser gyros.

Description

SPECIFICATION Laser mode control This invention relates to laser mode control.

It is well-known in the prior artto utilize a flashing external light or a gas discharge to excite a laser gain medium thus causing the medium to lase so as to emit an intense beam of coherent light It has been found that this beam of coherent light may be generated in more than one mode. The existence of multiple laser modes has been found in all laser oscillators, be they linear in their mechanical configuration or nonlinear.

One application for a nonlinear configured laser is within a ring laser gyro in which clockwise and counterclockwise beams of light are propagated in a closed path. The counterrotating beams interfere with one another to create a beat frequency. The rotation ofthe gyro about an axis perpendicular to the plane formed by the clockwise and counterclockwise propagated beams causes the frequency of the beam travelling in a direction ofthe rotation to exhibit a decrease, and the frequency ofthe beam travelling in a direction opposite the rotation to exhibit an increase. Thus, the degree of gyro rotation may be determined by detecting variations in the beat frequency created bythe changing interference ofthe counterrotating beams.

In addition to the problem of multiple laser modes, another problem encountered within a ring laser gyro is mode locking wherein the two counterrotating light beams appear two have the same frequency at low levels of angular rate rotation due to light scattering.

Light scattering can occur at each reflection of a laser beam within a laser. Therefore, a carefully designed laser mirror within the teachings ofthe present invention can assist in reducing light scattering.

Atypical ring laser gyro is shown in United States Letters Patent No. 4,115,004, which issued on September 1978, byThomas J. Hutchings,etal., entitled "Counterbalanced Oscillating Ring Laser Gyro", assigned to the same assignee as the present invention. This gyro utilizes a rectangular path formed by four passageways within a rectangular block of quartz or other suitable material. At each intersection ofthe rectangular path is mounted a mirrorto encourage the clockwise and counterclockwise rotation ofthe laser beams. The lowest order laser mode is generally propagated along the central axis ofthe passageways, while higher order modes are propagated slightly off axis.

One prior arttechnique for eliminating the higher order modes in ring laser gyros is to reduce the diameter ofthe passageways through which the counterrotating beams are propagated. Another approach restricts the diameter of the passageway at a single location while sizing the remaining passageway diameters art a larger dimension. thus forming an aperture. The aperture is carefullyshaped and polished to eliminate higher order modes. However, the aperture is difficult to machine and repeatability from one laser to the next is very difficultto achieve.

The aperture also increases the light scattering of the lowest order mode it is designed to enhance.

According to the present invention there is pro videda lasercomprisinga lasergain medium and means for exciting the laser gain medium to causethe emission of coherent light, a mirror being provided having a reflective area which is positioned to reflect a lowest order mode of coherent light emission, while reflection of higher order modes of coherent light emission is suppressed or eliminated.

According to another aspect of the presentinven- tion there is provided a method of making a laser mirror comprising: i) coating a substrate with a reflective layer; ii) masking an area of the reflective layerwith a mask having an elliptical centre portion, iii) coating the unmasked area of the reflective layerwith a light absorbing layer; and iv) removing the mask.

For a better understanding of the present invention and to show how it may be carried into effect, reference will will now be made, byway of example, to the accompanying drawings, in which: Figure lisa schematic view of a linear laser; Figure 2 is a sectional view of a ring laser gyro assembly; Figure 3 is a viewtaken along line Ill-Ill of Figure 2 illustrating a feature ofthe prior art; Figure 4 is a schematic representation of the principle of the present invention utilizing a mirror; and Figure 5 shows a mask used to produce the mirror shown in Figure 4.

The linear laser 10 shown in Figure 1 comprises a laser rod, such as a solid state ruby rod 12, whose end surfaces are covered respectively with a partially reflective eiement 14 and a fuily reflective element 16.

A suitable power supply, not shown, is connected to terminals 18for driving a flash lamp20thatstimu- latesthe emission of radiation bythe excitation ofthe atoms within the ruby rod I 2which, in turn, stimulates the emission of a coherent light beam 22.

For purposes of illustration, a lowest order laser mode is shown at 26 and a higher order mode, which could exist if no measures are taken to suppress it, is shown at 28. In practice, if no suppressing measures aretaken there may be several higher order modes which may propagate in boththeydirection, as shown, or the x direction or in a combination thereof.

While a solid state laser is shown in Figure 1, a gas laser, such as a helium neon laser, is shown in Figure 2. The gas laser 30 is typical of a laser that may be utilized in a gyroscope. The laser is formed within a body 32 of material such as quartz, a material known as "U.L.E." titanium silicate manufactured by Corning, or a material known as "Cervit" manufactured by Owens Illinois. The laser body 32 is constructed with four passageways 34 arranged therein to form a rectangular laser path. Atriangular construction may alternatively be used. The passageways 34 are sealed and contain a gas mixture consisting of approximately 90% helium and 10% neon at a pressure of approximately 3 torr, it being understood that atmos pheric pressure is approximately 760 torr.

In accordance with known laser practice, the body 32 is provided with two cathodes 36 and 38 and two anodes 40 and 42that are secured to the body in a mannerwhich is also well-known in the art. A gas discharge is established between cathode 36 and anode 40 in the opposite passageway. Getters 44 and 46 are provided at opposite ends of the body 32 to absorb impuritiesfound within the gases in the passageways 34. Mirrors 48,50,52 and 54 are located atthefourcorners of the optical path formed bythe passageways 34 ofthe ring laser gyro 30. The mirrors 48 and 54 are mounted upon photo-detection output devices 56 and 58, respectively.The photo-detection devices measure the beat frequency ofthe oppositely rotating electromagnetic energy formed by the two counter rotating light beams to indicate the rotation ofthe ring laser gyro 30.

In accordance with the prior art, as shown in Figure 3, one ofthe passageways 34 is formed with a first diameter bydrilling the laser body32from opposite directions to suitable depths to form a collar 60. The collar is then pierced with a smaller drill to form a mode blocking aperture 62.

As illustrated in Figure 1, the typical cross section of the lowest order laser mode from a linear laser is generally circular. The cross section of the laser beam from a nonlinear laser is generally elliptical, see Figure 4. In the prior art embodiment illustrated in Figure 2 and 3, it is necessary to form the elliptical shape of the aperture 62 by hand. In manufacturing the aperture 62, tolerances are measured using a microscope. However, the shape ofthe elliptical aperture is difficult to maintain. Further, uniformity between one aperture within a first ring laser gyro 30 and the next aperture within a second gyro is limited at best According to the present invention, the elliptical aperture 62 is dispensed with and instead a specially adapted mirror is used.In a typical ring laser gyro, two ofthefour mirros 48,50,52,54 are curved while the remaining two are flat. Generally, mirrors 50 and 52 might be curved to assist in focusing the counterrotating light beams; while mirrors 48 and 50 are flat.

In accordance with the present invention, the mirror 48, for example, is coated with a light absorbing layer 64asshown in Figure 4. The mirror48would retain an elliptical reflective area 66 within the absorbing layer 64. The reflective area 66 has approximately the same size and shape as the laser beam formed by the lowest order laser mode 26. The size ofthe area 66 must be large enough to encourage propagation of the lowest order mode yet small enough to eliminate orat least suppress propagation ofthe higher order modes. In the preferred embodiment, the ratio ofthe major axis ofthe elliptically shaped reflective area 66 to the minor axis is 1.2to 1.

As illustrated in Figure 4, the higher order laser modes 28 are not reflected by the mirror 48 but are eliminated by the absorbing layer 64. This elimination takes place sincethe absorbing layer reduces the reflection oflightenergy belowthe level at which optical resonance can occur.The absorbing layer 64 also aids in reducing the amount oflightscattering caused by the lowest order beam within a typical laser, such as a ring laser used within a gyroscope.

The same principle can be applied to the linear laser shown in Figure 1, in which the refelctive area 66 has the approximate shape ofthe generally circular laser beam 22. As seen in Figure 1,the circular area 66 is formed on the mirror 16 by the use of an absorbing layer 64. It will be understood thatthe presence of reflective area 66 eliminates the higher order modes 28 ofthe beam 22 shown in Figure 1.

Amask68 utilized toform the mirror48 isshown in Figure 5. The mask consists of a thin material which may be chemically etched into the configuration shown. The material includes a circular collar 70 having inwardly radiating fingers 72which support an elliptical centre mask 74.

Using the mask 68, the mirror 48 may be coated with the light absorbing layerwhile leaving the reflective area 66 uncoated. The first step in the procedure is to apply a reflective coating to the polished surface of a substrate. After successive layers ofthe reflective coating have been applied to the substrate to enable the surface to become suitably reflective, the mask 68 is placed overthe reflective surface. Thereafter, the absorbing layer 64 is applied tothe surface ofthe mirror 48 and to the mask 68. After a suitable number of absorbing layers have been applied, the mask 68 is removed leaving the elliptical area 66 of the reflective surface of mirror 48. In the preferred embodiment, the fingers 72 of the mask 68 also leave outwardly radiating reflective spokes on the mirror 48.However, it has been found thatthese spokes do not interfere with the mode control established by the mirror.

The losses from the 00,01,10 modes have been measured before and afterthe absoring layer ways applied to the reflective surface ofthe mirror 48. The term 00 refers to the lowest order laser mode. The term 01 refers to the next highest laser mode displaced in they plane, while the term 10 refers to the next highest order mode displaced in the x plane.

The reader will now understand that the higher order mode shown in Figures 1 and 3 is a 01 mode.

Reflection losses of the 00,01,10 modes before and afterthe absorbing layer has added are shown below: Losses (PPM) 00 01 10 Before 617 617 617 After 704 1,949 1,949 In this case, the presence ofthe absorbing layer increased the losses in the 00 mode by 87 parts per million,whilethe losses in the 01 and 10 modes increased by 1332 parts permillion.The losses in the 01 and 10 modes after application ofthe absorbing layer are greater than losses created by a restriction placed within the passageway 34.

The elliptical reflective area 66 ofthe present invention may be used in various nonlinear laser configurations to improve the mode control ofthe laser and reduce its scattering. Similarly, a generally circular aperture may be used in a linear laser. Itwill be understood, however, that other modifications may be used within the present invention which should be limited only by the appended claims.

Claims (15)

1. A laser comprising a lasergain medium and means for exciting the laser gain medium to causethe emission of coherent light, a mirror being provided having a reflective area which is positioned to reflect a lowest order mode of coherent light emission, while reflection of higher order modes of coherent light emission is suppressed or eliminated.

2. A laser as claimed in claim 1, in which the reflective area of the mirror is surrounded byan absorbing layer which suppresses or elimiates reflection ofthe higher order modes.

3. A laser as claimed in claim 1 or 2, in which the reflective area of the mirror has approximately the same size and shape as the lowest order mode.

4. A laser as claimed in any one of claims 1 to 3, which is a linear laser, in which the reflective area is substantially circular.

5. A laser as claimed in any one of claims 1 to 3, which is a non-linear laser, in which the reflective area is substantially elliptical.

6. A laser as claimed in claim 5, which is part of a ring laser gyro, the laser gain medium being contained in a plurality of intersecting passageways in which the mirror is disposed.

7. A laser as claimed in claim 6, in which the intersecting passagewaysform a rectangle.

8. A laser as claimed in claim 6, in which the intersecting passagewaysform atriangle.

9. A laser as claimed in any one of claims 6to 8, in which the mirroris aflat mirrorconstituting one of a plurality of mirrors comprising flat and curved mirrors disposed respectively at the intersections of the passageways.

10. A laser as claimed in any one of claims 5 to 9, in which the elliptical reflective area has a major axis and a minor axis whose ratio is 1.2 to 1.

11. A method of making a laser mirror comprising: i) coating a substrate with a reflective layer, ii) masking an area ofthe reflective layer with a mask having an elliptical centre portion, iii) coating the unmasked area ofthe reflective layerwith a light absorbing layer; and iv) removing the mask.

12. A method as claimed in claim 11, in which the elliptical centre portion ofthe mask matches the shape of a lowest order laser mode and has a major axis to minor axis ratio of 1.2 to 1, the centre portion being supported byfinger elements to a surrounding collarofthe mask.

13. A laser as claimed in claim 1, substantially as described herein with reference to the accompanying drawings.

14. A ring laser gyro including a laser in accord ancewithclaim 13.

15. A method of making a laser mirror substantially as described herein with reference to Figure 5 of the accompanying drawings.