Diode-pumped solid-state ring laser gyroscope

Coherent light generators – Particular resonant cavity – Folded cavity

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356350, 372 70, G01C 1966, H01S 3083

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active

059600225

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a diode-pumped solid-state ring laser gyro and, more particularly, to a diode-pumped monolithic solid-state ring laser which uses the total reflection on the boundary surfaces for the deviation of the resonator mode.
A currently wide-spread method for measuring the rotating movement of a moved apparatus--for example, of a vehicle, a ship, an airplane or a satellite--is the use of ring laser gyros (RLK). The gyro is used for the control and stabilization of the movement, the "northing", as well as for the calibration of acceleration sensors in inertial navigation systems. For the wide-area navigation in the airplane, a long-term stability of better than 0.01.degree./h is required; for the ship navigation, even 0.001.degree./h; however, for measurements in combination with other navigation methods, such as the Global Positioning System (GPS) as well as for short-term measurements of rotational movements in the vehicle, the missile and the combat airplane, the requirement is at 10.degree./h to 100.degree./h. The measuring range is normally between 0-10.degree./s. Since the ring laser gyro always detects only one axis of rotation because of its planar construction, three gyros are required which are situated perpendicularly with respect to one another for measurements in all directions in space.
The physical basis of the ring laser gyro is the Sagnac effect which describes the influence of a rotational movement on the propagation of light waves. When a light wave is deviated by 360.degree. by a mirror reflection or in an optical wave guide and is caused to superimpose on itself, ring waves are created. Since both rotating directions are equivalent, a left-rotating as well as a right-rotating ring wave can form simultaneously. When the wave-guiding structure is rotated, the frequency of the moving wave will increase and the frequency of the wave moving in the opposite direction will decrease. Particularly in the ring laser gyro (RLK), the opposed waves in the ring are continuously optically amplified. Simultaneously, a portion of the two waves is coupled out of the ring by means of a dividing mirror and is superimposed on a photo detector for measuring the difference frequency. The difference frequency is proportional to the rate of rotation .OMEGA. and proportional to the area A enclosed by the waves, but inversely proportional to the light path L and the wave length .lambda. in the amplifying medium:
When a laser beam of the wave length .lambda.=0.63 .mu.m is deviated along the sides of a square with a side length of 4 cm, the frequency shift (with the rate of rotation .OMEGA. 15.degree./h) caused by the rotation of the earth is .DELTA..nu.=4.4 Hz. A rotating speed of 500.degree./s, which may occur during a rolling motion of a combat plane, will supply .DELTA..nu.=400 kHz as the gyro signal.
The commercially available laser gyros use HeNe gas lasers. The resonator is designed either as an isosceles triangle with three deviating mirrors or as a square with four deviating mirrors. Two gas discharge tubes along the beam path provide the laser amplification at the wave length 0.633 .mu.m or 1.152 .mu.m. So that the structure remains as mechanically and thermally stable as possible, the laser is usually integrated in a block made of a material with an extremely low coefficient of expansion. The deviating mirrors are mounted on the corners in a vacuum-tight manner. The bores are evacuated and are filled with the HeNe mixture to a pressure of a few torr. The gas discharge is ignited between two electrodes.
The deviating mirrors form the optical resonator. As in the case of a longitudinal resonator, one or two of the mirrors are spherically curved; the others are planar. A deviation by 60.degree. (isosceles triangle) or by 90.degree. (square) in the plane on each mirror provides for the formation of a closed ring wave. However, for this purpose a very precise alignment of the mirrors is required, as in the case of the linear resonator.
One of the devi

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Optics Communications 101 (no month) (1993), pp. 371-376 entitled "Electrooptically fast tunable miniature diode-pumped Nd:YAG ring laser" by I. Freitag et al.
Optics Letters, vol. 17, No. 5, Mar. 1, 1992, pp. 378-380 entitled "Fused-silica monolithic total-internal-reflection resonator" by S. Schiller et al.
Optics Letters, vol. 10, No. 2, Feb. 1985, pp. 65-67 entitled "Monolithic, undirectional single-mode Nd:YAG ring laser" by Thomas J. Kane et al.
Laser und Optoelektronik 2, (no month) 1985, pp. 131-140 entitled "The Laser Gyro Influence of Ring Laser Geometry on Gyro Performance" by R. Rodloff.

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