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Image formation in scanned heterodyne microscope systems

Several new scanning microscopic techniques have recently been developed which rely on modulation of the optical beam to enhance the imaging performance. These systems have the common feature that the image is formed by detection or demodulation of an a.c. signal. Techniques have been developed which are sensitive to both intensity and phase information in the sample. We refer to such microscopes as heterodyne imaging systems (and by analogy single‐frequency microscopes, operating at d.c. only, are referred to as homodyne) although as we will point out this term is only strictly applicable to the interferometric‐based methods. Although the theory for the conventional homodyne scanning optical microscope is well developed there has been no systematic study of heterodyne microscopic techniques, which is the purpose of this paper. Several techniques are discussed and compared and the different ways of extracting amplitude and phase information are considered in detail with reference to experimental systems which have been demonstrated to have good imaging performance.