Scanning heterodyne optical interferometers

Abstract

Compact, high performance, scanning heterodyne optical interferometers are introduced for interferometric phase-based measurement applications. The novel, in-line, almost common-path optical interferometer design offers robustness to externally induced phase noise via mechanical vibrations, thermal effects, and other environmental effects. Novel instrument designs are introduced for both transmissive and reflective interferometry. These instruments use acousto-optic devices or Bragg cells to implement rapid (e.g., <50 μs/scan spot) optical scanning of the test medium. Although the read optical beam scans a given test region, the double Bragg diffraction optical design of the instrument makes the final interfering output beams stationary on the two high speed photodetectors used for radio frequency signal generation via heterodyne detection. One photodetector acts as the fixed phase reference, while the other fixed photodetector picks up the test medium phase information as the optical beam scans the test region. The transmissive design instrument is built in the laboratory using flint glass Bragg cells. A typical 120 MHz heterodyne detected signal output had a carrier-to-noise ratio of 108.9 dBc/Hz measured at a +160 kHz offset using a spectrum analyzer resolution bandwidth of 30 kHz. The corresponding single-sideband phase noise was estimated at −101.57 dBc/Hz at 160 kHz offset. The measured instrument radio frequency dynamic range was ∼60 dB or an equivalent of 30 dB optical dynamic range, with a 1/1000 of a fringe cycle phase measurement accuracy. Test medium optical phase mapping was successfully tested with the instrument using a large area, 6 μm thick, birefringent-mode nematic liquid crystal cell. Our instrument allows the use of high continuous wave or peak power, broad spectral linewidth, coherent light sources. The instrument can have a high 50% optical power efficiency. High speed two-dimensional optical scanning of a test medium is possible with our instrument by using a fixed one-dimensional output high speed detector array, or via the use of high speed nonmechanical electro-optic deflectors.