Piezoelectric transducers for ring laser gyroscopes

Ring laser gyro

Modified ceramic material technology provides capability to create a range of piezoelectric transducers suitable for use in optical gyroscope applications.

Morgan Advanced Materials has developed the capability to design and manufacture piezoelectric transducers for use in ring laser gyroscope (RLG) technologies, used in a variety of high-precision applications.

Morgan is creating the transducers in a range of shapes and geometries from a modified range of hard piezoelectric (PZT) materials, developed from the company’s proprietary range of ceramics. Other piezoelectric formulations are available on request, depending on the temperature range required in the application.

In recent years the industry has witnessed a multitude of developments in the area of optical gyroscopes which deliver highly reliable, solid state performance immune from many of the mechanical effects which restrict the performance of conventional spinning mass gyroscopes.

RLGs are inertial sensors based on the Sagnac rotation effect which causes the frequency of the two counter-propagating beam in the ring cavity to be shifted by a quantity proportional to the angular velocity. This shift (the Sagnac frequency) can be easily measured, letting the two beams beat. By bringing the two frequencies of laser light to interference, a beat frequency can be obtained. The beat frequency is the difference between the two frequencies and can be thought of as an interference pattern in time.

Compared with conventional spinning mass gyroscopes, RLGs offer several advantages: they boast a large dynamic range, high precision, a small footprint, and do not require any moving mechanical parts. Their digital output is linear with angular rotation, while they are also highly sensitive and thermally stable across a wide range of operating temperatures, with quick reaction times, and immunity to most environmental effects. They are also insensitive to translational accelerations. Thanks to these features, RLGs are gaining an increasingly prominent role in many applications, ranging from inertial navigation systems on commercial airliners, weapons guidance systems, ships and spacecraft to geodesy and geophysics, to tests of fundamental physics.

RLGs combine the functions of optical frequency generation and rotation sensing into a laser oscillator within a ring-shaped cavity. Typically, they consist of a solid block – either square or triangular – of glass ceramic material, into which a lasing medium is introduced. The electrodes provide gain for the lasing medium, generally a helium/neon mixture due to its short coherent length and index of refraction of nearly 1.0, which generates two independent beams in opposite directions around the cavity.

Frequency stabilisation is obtained using a piezoelectric transducer to precisely move one or more of the four mirrors located on the perimeter of the cavity. Meanwhile, a change in the length of the ring can occur by thermal expansion, bias in the discharge current on either side of the laser. This will produce a change in the readout which is equivalent to the real wander of a mechanical gyroscope. Both of these can be compensated by using active control of the discharge current through an error detection system coupled with feedback system and active control of the path length by moving one or more of the mirrors using piezoelectric ceramic actuation technology.

As Frédéric Pimparel, Technical Application Manager at Morgan Advanced Materials, explains, a number of Morgan’s cutting edge ceramic materials are particularly suitable for RLGs: “Many of our proprietary PZT materials have qualities that are desirable in RLGs. For example, PZT406 and PZT401 offer a fine compromise between high permittivity, low dielectric losses, high density, high piezoelectric activity and a high mechanical factor.

“This combination enables production of an actuator that is extremely efficient under high driving modes, and maximises the accuracy of the mirror positioning in situ during the compensation sequence. Another ceramic, PZT503, is excellent as a feedback sensor, because it offers high permittivity and excellent sensitivity levels, which means a stronger signal to noise ratio back to the amplifier unit. The actuator and feedback sensor are vital factors in the performance of the RLG.”

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