GB2044518A - Low loss apertures for ring laser gyros - Google Patents

Abstract

A gas discharge ring laser formed in a glass-ceramic block has a reduced bore section of diameter d for suppressing unwanted TEM modes. The invention makes use of the fact that the gain available from the excited gas of the laser cavity falls off near the walls of the cavity. Thus, the diameter of a portion of the tube enclosing the excited gas is made sufficiently small so that the gain available to both the TEM00 and TEM01 modes is reduced gradually to zero at the cavity wall. Because of the different light distribution in the two modes, the overall gain for the TEM01 mode is significantly less than for the TEM00 mode, thus preventing the TEM01 mode from oscillating. <IMAGE>

Description

SPECIFICATION Low loss apertures for ring laser gyros This invention relates to ring laser gyroscopes.

More particularly, this invention relates to a low loss aperture means for suppressing offaxis modes in gas lasers.

Background of the Invention The ring laser gyro is a significant departure from prior art angular rate sensor devices.

Conventional angular rate sensors employ a spinning mass to provide a reference direction. These sensors have inherent problems among which are high drift rates, caused by friction and unwanted torques. The ring laser gyro for the most part eliminates the undesireable characteristics of the prior art sensors. Its operation is based entirely upon optical and electronic phenomena wherein angular motion is measured by the massless light waves circulating in a closed path.

The heart of any gas laser, including ring laser gyros, is the optical resonator. This consists of a cavity containing an electrically excited gas, typically helium and neon. The cavity is equipped with high reflectivity mirrors so that a closed path is formed which traverses the excited gas mixture. This path may be of any shape, depending upon the number and attitude of the mirrors employed but is frequently triangular, using three mirrors. Optical resonators are capable of oscillating in a large number of different modes each having a different frequency. It is often essential to operate a laser, particularly a ring laser gyro, in the lowest order mode known as TEM00. The output beam in this case is a single circular spot.The next highest mode, known as TEM01, produces two output beams at a slightly different frequency from the TEM00 mode. This is highly undesireable in ring laser gyros because the two modes produce unwanted beat frequencies and because the higher order mode tends to interact with the lower mode through the common source of energy from the excited gas and this interaction shifts the frequency of the lower mode thus producing an erroneous output from the gyro.

The laser will not operate properly as a gyro if both the TEM00 and the TEMP1 modes are present. The usual way of eliminating the TEMP1 mode is to pass the beam through an aperture which blocks some of the light along the edges of the beam. The aperture will block proportionately more of the TEMP1 light compared to the TEM00 light thus increasing the realtive losses to the point where the TEMP1 mode no longer has sufficient gain to oscillate. When the losses equal or exceed the gain in any given mode, that mode will be extinguished. By careful design of the aperture, we can extinguish the TEMP1 mode while still permitting the TEM00 mode to be present.

Apertures, though, are undesirable because they operate by diffracting and scattering the laser light and this introduces errors such as lock-in and bias shifts to the gyro operation.

The present invention eleiminates this scattered light by reducing the gain instead of by increasing the losses. If we remove the aperture and instead, somehow, provide a low gain region in place of it, we will achieve the same effect as the aperture, that is, the gain will be less than the loss which is the condition for extinguishing the mode. This has to be done very carefully so that the gain is less than the loss for the TEMP1 mode but greater than the loss for the TEM00 mode.

The gain actually comes from the electrically excited neon gas which is always present in the laser. Near the walls of the laser cavity the gain drops to zero because the neon atoms lose their excitation by collisions with the wall. Thus, by properly positioning the walls near the optical path we can reduce the gain in the TEMP1 mode to the point where it will no longer exist. This will also reduce the gain the the TEM00 mode but because its energy is concentrated nearer the optical axis or center line where the gain is high, the TEM00 mode will continue to oscillate.

The main advantage of this approach is that there is no longer any aperture and therefore none of the light in the TEM00 mode is scattered, thus avoiding problems of lock-in and bias shifts normallly produced by scattered light from the aperture.

Brief Description of the Invention The present invention in lieu of the discrete apertures taught by the prior art for suppressing the unwanted TEM modes provides a triangularly shaped supporting member with a low gain region or reduced bore in the path of the monochromatic beams. The invention makes use of the fact that the gain available from the excited gas of the laser cavity falls off near the walls of the cavity. In the invention the diameter of the tube enclosing the excited gas is made sufficiently small so that the gain available to both the TEM00 and the TEMP1 modes is reduced gradually to zero at the wall. Because of the light distribution in the two modes, the overall gain for the TEM01 mode is significantly less than for the TEM00 mode, thus, preventing the TEMP1 mode from oscillating.

Accordingly, it is an object of this invention to provide a means for suppressing off optical axis modes in gas lasers.

This and other objects, features and advantages of the present invention will become apparent from the following descrition taken in conjunction with the accompanying drawings wherein: Figure 1 shows a triangularly shaped laser support having low gain region means in the manner of the invention; Figure 2 shows the cross section of the laser beam comprising both the TEM00 and the TEMP1 modes and also shows how the intensity of the beam falls off in a bell shaped curve from the center line of the TEMP1 mode; Figure 3 is a view similar to Fig. 2 showing light intensity versus distance of the two beams from the optical axis; Figure 4 shows the output beam TEM00 mode of the laser which is roughtly circular with the circular TEMP1 modes imposed thereon;; Figure 5 shows a prior art triangularly shaped laser supporting means having adjustable apertures consisting of knife edges which are moveable towards the optical axis in order to extinguish the TEMP1 mode; Figure 6 is a exploded sectional view of a prior art fixed aperture which consists of a polished necked down section in the gain tube; figure 7 is a cross sectional view of the beam showing how the gain available from the excited gas falls off near the walls of the laser cavity; Figure 8 is a schematic view (not showing the cavity or other details of the laser) of a symmetrically placed prior art aperture; and Figure 9 is a schematic view (not showing the cavity or other details of the laser) of a gain limiting aperture in the manner of the invention.

Referring now to Fig. 1 there is shown a ring laser gyro 10 consisting of a glassceramic triangular block 11 into which the cavity 1 2 is machined. The cavity is defined by two high reflectors 1 3 and 14 and an output reflector 1 5. A plasma discharge between cathode 18 and the two anodes 1 6 and 1 7 is used to provide the necessary gain in the He, Ne (helium and neon) filled cavity When the laser is rotated about a given rotational axis, there will be a difference on the distance traveled by the circulating CW and CCW beams in cavity 12. Means (not shown) can convert this difference in the travel of the two counter-circulating beams into an input angular rate.This function af the ring laser gyro is discussed in more detail in our copending Application No. 7899/78. It is noted that although the discussion herein will relate to ring laser gyros, it is understood by those skilled in the art that the teaching of the invention may be employed in all types of gas lasers.

Optical resonators of gas lasers are capable of oscillation in a large number of different modes each having a different frequency. The present discussion will be limited to the transverse modes which concern the invention. It is often essential to operate a laser, particularly a ring laser gyro, in the lowest order mode known as TEM00. The output beam of the laser in this mode is a singular spot. The next higher mode, known as TEM01, produces two output beams at a slightly different frequency from the TEM00 mode.The presence of these two modes is highly undesireable in ring laser gyros because the two modes produce unwanted beat frequencies and because the higher order mode tends to interact with the lower mode through the common source of energy from the excited gas and this interaction shifts the frequency of the lower mode thus producing an erroneous output from the gyro.

Referring to Figs. 2, 3 and 4 the TEM00 mode 20 has most of its energy concentrated near the optical path 21 between bore walls 22 and 23 while the energy in the TEMP1 mode 24 is spread out. Accordingly, if an aperture 25 is placed in the system, it will absorb more energy from the TEMP1 mode 24 than it will from the TEM00 mode 20. Now, if the gain of the whole system is adjusted properly, for instance, by adjusting the degree of excitation of the lasing gas, the aperture induced losses of TEMP1 mode 24 may be so great that it will not oscillate, leaving only the TEMP1 mode 20 as desired. Higher order modes such as TEMP2 have their energy even further dispersed from the optical axis so that if the TEMP1 mode can be suppressed by an aperture so will all the higher order modes.

Referring to Figs. 5 and 6, there are shown prior art apertures used in gas lasers. Fig. 5 shows an adjustable aperture consisting of a pair of adjustable X and Y knife edges 51 and 52 which can be moved towards the optical axis until the TEMP1 mode is extinguished.

Fig. 6 shows in partial exploded view a fixed aperture 61 which consists of a polished necked down section of the gain tube 60.

While the dimensional tolerance of the fixed aperture are most difficult to meet, it has the advantage of greater mechanical stability than the adjustable aperture.

The discrete apertures shown in Figs. 5 and 6 have a serious drawback in that they cause optical losses by means of a combination of scattering, diffraction and reflection. These losses are needed to suppress the higher order modes, but as illustrated in Figs. 2 and 3, there is also light in the wings of the fundamental TEM00 mode 20 distribution and this light will also be scattered, diffracted and reflected in various ways.

It is essential in certain lasers and in particular ring laser gyros that light travelling in one direction through the aperture must be completely decoupled from light travelling in the opposite direction. The major drawback of discrete aperture designs such as shown in Figs. 2, 5 and 6 is that some of the light intercepted by the aperture is backscattered in the opposite direction, thus coupling clockwise and counter-clockwise beams together. In ring laser gyros, this increases the lock-in phenomena and may also lead to bi ases depending upon the phase shift in the scattering process. In addition, if the aperture is not mechanically stable to microinch tolerance with respect to the optical axis, the amount of coupling will vary depending upon the relative position of the aperture and this in turn will cause undesired changes in the bias.

Turning to Fig. 7, the present invention dispenses with the aperture altogether. Instead, it makes use of the fact that the gain of the beam 20 available from the excited gas falls off near the walls 22 and 23 of the cavity 1 2. This is true in all electrically excited gas lasers. In general, a certain proportion of the atoms in the gas are raised to an excited level by means of an electric discharge. When a photon of light comes sufficiently close to one of these excited atoms, it is stimulated to emit a second photon identical with the first.

This process continues in a chain reaction and sustains the optical oscillation in the cavity.

But the atomic excitation can be lost without giving rise to a photon. This can occur by collision with the walls or with an unexcited atom, Accordingly, the atoms adjacent to the walls of the cavity lose their excitation through wall collision and tend to de-excite the atoms further in. The net result is that the excitation and hence the optical gain falls off near the walls and is zero at the walls themselves.

Referring back to Fig. 1, the present inventin makes use of this fact to suppress the TEMP1 and other higher order modes. As illustrated in Fig. 1, the diameter d of the tube enclosing the excited gas is made sufficiently small so that the gain available to both the TEM00 and the TEMP1 modes is gradually reduced to zero at the wall. Because of the light distribution in the two modes shown in Fig. 3, the overall gain for the TEMP1 mode 24 is significantly less than for the TEM00 mode 20, thus preventing the TEMP1 24 mode from oscillating. In addition, the wings of the TEM00 mode 20 are in effect cut off because of the reduced gain so that there is no light present in this mode near the walls.

There are two major advantages to this form of construction. First, none of the laser light is scattered, diffracted or reflected, thus there is no coupling due to the aperture between oppositely directed beams of light.

The result is that problems of lock-in and bias associated with the aperture are completely eliminated. Second, while the walls of the chamber enclosing the excited gas must be brought reasonably close to the optical axis (the exact position varies from laser to laser and must be determined by experiment) the walls are still considerably further away from the optical axis than in the case of the discrete aperture. Because of this, and because no light is present near the walls, the tolerance on the wall location can be considerably enlarged over the discrete aperture case and the laser will not be affected by small variations between the optical axis and the walls.In order to prevent the He Ne fill gas flowing around the optical path due to electrical effects, the reduced bore sections may be symmetically disposed about the path of the electrical discharge necessary to excite the fill gas.

Turning to Figs. 8 and 9, there is shown a prior art aperture and a gain limiting aperture in the manner of the invention respectively.

Figs. 8 and 9 do not show the full cavity and other details of the laser and are used to decribed the bore size in the improved gain limiting aperture of the invention. Fig. 8 shows a symmetrically placed aperture 80 that is similar to the aperture of Fig. 6. The diameter d has to be chosen such that it is large enough to pass the desired TEM00 mode with reasonable diffraction losses, yet present any transverse modes such as a TEMP1 mode with a much larger loss. In this way only the TEM00 mode can be sustained which is necessary for operation of the ring laser gyroscope.

The main drawback, as stated previously, of the symmetrically placed aperture of Fig. 8 is that a major part of this diffracted light backscatters in the ring laser gyro and this causes a high lock-in. This is undesirable because to compensate for it, noise is added to the output of the ring laser gyro.

The diameter d of the aperture can be calculated: d = 2wm (1) where w is the l/e2 radius of the laser beam, and m is the clearance factor which has been found to be 2.06 in typical ring laser gyro designs. To give some numbers, let us examine an experimental ring laser gyroscope where w = 437 X 10-em in the sagittal plane. With m = 2.06 the diameter of the aperture can be found by using equation (1).

d = 437 x 10-s x 2 x 2.06 = 1.80 x 10-3m The diffraction loss is: L=e-2(m)2 (2) Using the above numbers we can find that the diffraction loss, L is: L = e-2(2.0e,2 917 x 10-6 = 917ppm This is a large amount of light diffracted out of the main beam. It is troublesome because of the fact that the phase of the light is equal across the cross section of the bore. When this diffracted light interacts with the main laser beam, a lock-in behavior results. This lock-in can be strongly temperature dependent. The reason for this is not completely understood but we believe that minute changes occur to the phase of the main beam if it moves with respect to the aperture.

The amount of diffracted light is high as stated above. It should be compared to the 50-100 ppm scattering that can be achieved from a good ring laser mirror. It is plain from what has been said above that there is a great need for an improved method of controlling the mode of operation of a ring laser gyroscope.

As previously stated, the gain loss that occurs close to the walls of the gain bores can be used to control the mode of generation. If bore 90 of the laser of Fig. 9 is made somewhat smaller than usual, no diffraction aperture is needed. We have established that an effective mode control, only permitting TEM00 modes, can be achieved using a gain limiting bore that is 0.05 meter long (a typical diffraction loss aperture is approximately 10 times shorter). The diameter d, we have found, can be chosen to give a clearance factor m equal to about 2.30 which is substantially larger then the factor required in the prior art. This will reduce the diffracted light by a factor of 9 as compared to an ordinary aperture. This virtually eliminates the temperature sensitivity associated with ordinary apertures, as mentioned earlier.

In conclusion, the gain limiting aperture is a significant, new way with which the total amount of stray light inside the rng laser gyro can be minimized.

From the foregoing, a gas laser having means for suppressing off-axis modes has been described. Although only preferred embodiments of the present invention have been described herein, it is not intended that the invention be restricted thereto, but it be limited only by the true spirit and scope of the appended claims.

Claims (8)

1. A ring laser gyroscope comprising: means supporting two counter-rotating beams of monochromatic light along a closed loop said beams having the same TEM mode along an optical axis, a difference in frequency occurring between said beams due to any rotation about a rotation axis, and means in said supporting means for reducing diffractionlessly and scatteringlessly the off optical axis gain of said monochromatic beams whereby undesired TEM modes are suppressed.

2. The ring laser gyroscope of Claim 1 wherein said means for reducing the gain of said monochromatic beams comprises: a reduced bore in said support means, whereby said undesired TEM modes are eliminated through the reduction in optical gain near the walls of said reduced bore.

3. The ring laser gyroscope of Claim 2 wherein the diameter of said reduced bore is calculated by the formula: d = 2wm where w is equal to l/e2 radius of the laser beam, and m is equal to the clearance factor having a range between 2.10 to 2.50.

4. The ring laser gyroscope of Claim 3 comprising: reduced bore regions that are symmetrically disposed about the optical electrical excitation paths of said supporting means.

5. The ring laser gyroscope of Claim 4 comprising: a plurality of reduced bore regions in said supporting means for reducing the gain available to off-axis modes.

6. A gas laser comprising: means supporting two counter-rotating beams of monochromatic light along a closed path said beams operating in the same TEM mode along an optical axis of said supporting means, and a reduced bore in said supporting means for reducing the off optical axis gain available to said monochromatic beams whereby undesired TEM modes are prevented from oscillating.

7. The gas laser of Claim 6 wherein the diameter of said reduced bore is calculated by the formula: d = 2wm where w equals l/e2 radius of the laser beam, and m is equal to the clearance factor having a range between 2.10 to 2.50.

8. A ring laser gyroscope substantially as described and as shown with reference to the accompanying drawings.