Integrated Optics for Sensing.

A variety of integrated-optic sensors have been reported in the past ten years. Most integrated-optic sensors developed at the early stage may belong to a so-called waveguide sensor in which the waveguide itself is used as a transducer for sensing of temperature, pressure, humidity, and so on. The waveguide sensors, however, do not seem to have a technical advantage over existing fiber-optic sensors in practical use. On the other hand, very stable sensing becomes possible in freedom from an optical bench by integration of the whole interferometer optics on a chip, where an optical fiber, for instance, is used as a sensing probe. Replacement of a bulk-optic component by the waveguide counterpart also results in drastic improvement of the sensor performance. In most integrated-optic sensors reported so far, LiNbO3 was used as a waveguide material because it is relatively easy to make low-loss single-mode waveguides by Ti indiffution into LiNbO3 without reduction of the electrooptic effect. This paper describes recent progress in integrated-optic sensors in LiNbO3. Particularly, integration of heterodyne interferometer optics used for the measurement of velocity and displacement are discussed in detail. The application of integrated optics to fiber-optic gyroscopes is also described.