CA1145023A - Low loss apertures for ring laser gyros - Google Patents

Abstract

ABSTRACT OF THE INVENTION

A laser supporting member having a reduced bore section for suppressing unwanted TEM modes. The invention makes use of the fact that the gain available from the ex-cited gas of the laser cavity falls off near the walls of the cavity. Thus, the diameter 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.

Description

z:~
-This invention relates to ring laser gyroscopes.
More particularly, this invention relates to a low loss aper~
ture means for suppressing off-axis modes in gas lasers.

Background of the Invention The ring laser gyro is a signlficant 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 ~
10 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 electricaIly 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 fre-quently 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 TEMoo. The output beam in this case is a sinqle circular spot. The next highest mode, known as L~_ TEMol, produces two output beams at a sliyhtly different frequency from the TEMoo mode. This is highly undesireable 5 in ring laser gyros ~ecause the two modes produce unwanted b~
beat frequencies and because the higher order mode tends to interact with the lower mode through the common source of energy from the excited yas 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 TEMoo and the TEMo1 modes are present. The usual way of eliminating the TEMol 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 f ~ J~
the TBMol light compared to the TEMoo light thus increasing ~~~
the relative losses to the point where the TEMol mode no longer has sufficient gain to oscillate. When the losses equal or ex-ceed the gain in any given mode, that mode will be extinguished.
By careful design of the aperture, we can extinguish -the TEM
mode while still permitting the TEMoo mode to ~e present.
Apertures, though, are undesireable 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 eliminates this scattered r .. .... . ... . _ _ . . _ _ 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 5 which is the condition for extinguishing the mode. This has r~
to be done very carefully so that the gain is less than the loss for the TEMol mode but greater than the loss for the TEMoo 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 positior,ing the walls near the optical path we can reduce the gain in the TEMol mode to the point where L___ 15 it will no longer exist. This will also reduce the gain for ~
the TEMoo mode but because its energy is concentrated nearer the optical axis or center line where the gain is high, the TEMoo mode will continue to oscillate. ,-, The main advantage of this approach is that there 2n is no longer any aperture and therefore none of the light in the TEMoo mode is scattered, thus avoidlng problems of lock-in and bias shifts normally produced by scattered light from the aperture. L.__ ~;~

~.

5Q~3 Brief Description of the Invention The present invention in lieu of the discrete ~~~~
apertures taught by the prior art for suppressing the un-wanted TEM modes provides a triangularly shaped supporting 5 member with a low gain region or reduced bore in the path of ~t 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 suffi-ciently small so that the gain available to both the TEMo~ and -the TEMol modes is reduced gradually to zero at the wall. Be-cause of the light distribution in the two modes, the overall gain for the ~EMol mode is significantIy less than for the TEMoo mode, thus preventing the TEMol 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 rom the following description taken in conjunction with the accompanying draw-ings wherein: ~
Fig. 1 shows a triangularly shaped laser support having low gain region means in the manner of the invention;
Fig. 2 shows the cross section of the laser beam comprising both the TEMoo and the TEMo1 mode, and also shows ,~

- 6 - r ~Qæ~

how the intensity of the beam falls off in a bell shaped ~
curve from the center line of the TEMol mode; ~_~_ Fig. 3 is a view similar to Fig. 2 showing light intensity versus distance of the two beams from the optical axis;
Fig. 4 shows the output beam TEMoo mode of the laser which is roughly circular with the circular TEMol modes imposed thereon;
Fig. 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 TEMol mode;
Fig. 6 is a exploded sectional view of a prior art fixed aperture which consists of a polished necked down section in the gain tube;
Fig. 7 is a cross sectional view of the beam show-lng how the gain available from the excited gas falls off near the walls of the laser cavity;
Fig. 8 is a schematic view (not showing the cavity or other details of the laser) of a symmetrically placed prior art aperture; and Fig. 9 is a schematic view (not showing the cavity or other details of the laser) of a gain limiting aperture L_~_ in the manner of the invention.
Referring now to Fig. 1 there is shown a ring laser gyro 10 consisting of a glass-ceramic triangular block 11 into which the cavity 12 is machined. The cavity is defined by ~ aa~.
two high reflectors 13 and 14 and an output reflector 15. A ~ ;
..

plasma discharge between cathode 18 and the two anodes 16 and 17 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 in 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 of the ring laser gyro is discussed in more detail in Canadian Patent No. 1,098,201, issued March 24, 1981, and assigned to the same assignee as the present application.
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, particuarly a ring laser gyro, in the lowest order mode known as TEMoo. The output beam of the laser in this mode is a singular spot. The next higher mode, known as the TEMol, produces two output beams at a slightly different frequency from the TEMoo mode. The presence of these two modes is highly undesireable in ring .r - -laser gyros because the two modes produce unwanted beat frequencies and because the higher order mode tends to in ~' teract with the lower mode -through the common source o energy from the excited gas and this interaction shifts S the frequency of the lower mode thus producing an erroneous output from the gyro.
Referring to Figs. 2, 3 and 4 the TEMoo mode 20 has most of its energy concentrated near the optical path 21 between bore walls 22 and 23 while the energy in the TEM
mode 24 is spread out. Accordingly, if an aperture 25 is placed in the system, it will absorb more energy from the TEM
mode 24 than it will from the TEMoo 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 TEMol mode 24 may be so great that it ~ill not oscillate, leaving only the TEMoo mode 20 as de-sired. Higher order modes such as TEMo2 have their energy ~'~
even further dispersed from the optical axis so that if the TEMol mode can be suppressed by an aperture so will all the higher order modes.
Referring to Figs. 5 and 6, there are shown priorart 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 op-tical axis until ~i;;'~
the TEMol mode is extinguished. Fig. 6 shows in partial ex~
ploded view a fixed aperture 61 which consists of a polished -. .. ..... .... . . ... ... ~

necked down sec-tion of the gain tube 60. While the dimen- p~
sional tolerance of the fixed aperture are more difficult ~__ to meet, it has the advantage of greater mechanical stability than the adjus-table aperture.
The discrete apertures shown in Figs. 5 and 6 have ~L_ a serious drawback in that they cause op-tical losses by means of a combination of scat~ering, 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 c 10 the wings of the fundamental TEMoo mode 20 distribution and ~, this llght will also be scattered, diffracted and reflected in various ways.
It is essential in certain lasers and in particular ring laser qyros that liyht travelling in one direction through _~
15 the aperture must be completely decoupled from light travel- ~ ~
ling 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 directlon, thus coupling clockwise and counter-clockwise beams together. In ring Iaser gyros, this increases the lock-in phenomena and may also lead to biases depending upon the phase shift in the scattering process. In addition, if the aperture is not mechanically stable to microinch tol- L_ erance with respect to the optical axis, the amount of coupling will vary depending upon the relative position of the aperture r _ __~ _ J

and this in turn will cause undesired changes in the bias.
Turning to Fig. 7, the present invention dispenses with the aperture altoqether. Ins-tead, 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 aavi-ty 12. ~a~
This is true in all electrically excited gas lasers. In general, a certain proportion of the atoms ln 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 i~
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 colli- ~r-~-sion 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 invention makes use of this fact to suppress the TEMol 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 TEMoo and the TEMol modes R~ f~
25 is gradually reduced to zero at the wall. Because of the P~-light distribution in the two modes shown in ~ig. 3, the overall gain for the TEMol mode 24 is significantly less than for the TEMoo mode 20, thus preventing the TEMol 24 mode from oscillating. In addition, the winas of the TEMoo mode 20 are in effect cut off because of the reduced gain L~
so that there is no light presen-t in this mode near the walls.
There are two major advantayes to this form of construction. First, none of the laser light is scattered, diffracted or reflected, thus there is no coupling due to c~; ~ , 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 15 from laser to laser and must be determined by experiment) t~
the walls are still considerably further away from the optical a~is than in the case of the discrete aperture. secause 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 afected 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 re~
duced bore sections may be symmetrica]ly disposed about the path of the electrical discharge necessary to excite the fill ~w_~
gas.

?J `' ~ .

Turning to Figs. 8 and 9, there is shown a prior art aperture and a gain limi-ting 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 5 describe the bore size in the improved gain limiting aper- ~s"
tul-e of the invention. Fig. 8 shows a symmetricall,y placed aperture 80 that is similar to the aperture of E'ig. 6. The diameter d has to be chosen such that it is large enough to pass the desired TEMoo mode with reasonable diffraction losses, ~a yet present any transverse modes such as a TEMol mode with a much larger loss. In this way only the TEMoo 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 ~__ 15 of this diffracted light backscatters in the ring laser gyro L~
and this causes a high lock-in. This is undesireable 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 L~
ring laser gyro designs. To give some numbers, let us examine an experimental ring laser gyroscope where 5 w = 437 x 10~6m in the sagittal plane. With m = 2.06 the -diameter of the aperture carl be found by using equation (l). I~r~

d = 437 x lO-6 x 2 x 2.06 - 1.80 x 10~3m The diffraction loss is:

L = e~2(m)2 (2) ~ ~
P--S Using the above numbers we can find that the diffraction loss, L is:

L = e~2 (2.06)2 = 917 x 1o~6 = 917 ppm E~
~' 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 strong- ~ ,~
ly temperature dependent. The reason for this ls not com r~
pletely 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 con-trolling 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 25 mode of generation. lf bore 90 of the laser of Fig. 9 is l;~

made somewhat smaller than usual, no diffraction aperture ~ ~
is needed. We have established that an effective mode ~___ control, only permitting TEMoo modes, can be achieved using a gain limitinc~ bore that is 0.05 meter long (a -typical diffraction loss aperture is approximately 10 tlmes shorter).
The diameter d, we have found, can be chosen to give a clear-ance factor m equal to about 2~30 which is substantially larger than 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 tem-perature 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 ring 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 (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 dif-fractionlessly and scatteringlessly the off optical axis gain of said monochromatic beams whereby undesired TEM modes are suppressed, the means comprising 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, the diameter of said reduced bore being calculated by the formula:
d = 2wm where w is equal to 1/e2 radius of the laser beam, and m is equal to the clearance factor having a range between 2.10 to 2.50.

2. The ring laser gyroscope of claim 1, comprising:
reduced bore regions that are symmetrically disposed about the optical electrical excitation paths of said supporting means.

3. The ring laser gyroscope of claim 2 comprising:
a plurality of reduced bore regions in said supporting means for reducing the gain available to off-axis modes.