SCANNING HETERODYNE OPTICAL INTERFEROMETERS

Compact, high performance, scanning heterodyne optical interferometers are introduced for interferometric phase-based measurement applicatio ns. The novel, in-line, almost common-path optical interferometer desi gn offers robustness to externally induced phase noise via mechanical vibrations, thermal effects, and other environmental effects. Novel in strument designs are introduced for both transmissive and reflective i nterferometry. These instruments use acousto-optic devices or Bragg ce lls to implement rapid (e.g., <50 mu s/scan spot) optical scanning of the test medium. Although the read optical beam scans a given test reg ion, the double Bragg diffraction optical design of the instrument mak es the final interfering output beams stationary on the two high speed photodetectors used for radio frequency signal generation via heterod yne detection. One photodetector acts as the fixed phase reference, wh ile the other fixed photodetector picks up the test medium phase infor mation as the optical beam scans the test region. The transmissive des ign instrument is built in the laboratory using flint glass Bragg cell s. A typical 120 MHz heterodyne detected signal output had a carrier-t o-noise ratio of 108.9 dBc/Hz measured at a +160 kHz offset using a sp ectrum analyzer resolution bandwidth of 30 kHz. The corresponding sing le-sideband phase noise was estimated at -101.57 dBc/Hz at 160 kHz off set. The measured instrument radio frequency dynamic range was similar to 60 dB or an equivalent of 30 dB optical dynamic range, with a 1/10 00 of a fringe cycle phase measurement accuracy. Test medium optical p hase mapping was successfully tested with the instrument using a large area, 6 mu m thick, birefringent-mode nematic liquid crystal cell. Ou r instrument allows the use of high continuous wave or peak power, bro ad spectral linewidth, coherent light sources. The instrument can have a high 50% optical power efficiency. High speed two-dimensional optic al scanning of a test medium is possible with our instrument by using a fixed one-dimensional output high speed detector array, or via the u se of high speed nonmechanical electro-optic deflectors. (C) 1996 Amer ican Institute of Physics.