Optical heterodyne detection

Optical heterodyne detection is a method of extracting information encoded as <a href="/facts/Modulation/aAXPjINv">modulation</a> of the <a href="/facts/Phase_modulation/ws1kz7Y9">phase</a>, <a href="/facts/Frequency_modulation/qIExDdtv">frequency</a> or both of <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> in the <a href="/facts/Wavelength/kcJU0ymz">wavelength</a> band of visible or <a href="/facts/Infrared/44XdJNcc">infrared</a> light. The light signal is compared with standard or reference light from a "<a href="/facts/Local_oscillator/VQ5vQMEH">local oscillator</a>" (LO) that would have a fixed offset in frequency and phase from the signal if the latter carried null information. "Heterodyne" signifies more than one frequency, in contrast to the single frequency employed in <a href="/facts/Homodyne_detection/kJIExmpT">homodyne detection</a>.
The comparison of the two light signals is typically accomplished by combining them in a <a href="/facts/Photodiode/152Rpk06">photodiode</a> detector, which has a response that is <a href="/facts/Linearity/fChtg3KC">linear</a> in <a href="/facts/Energy/wd8PrQM7">energy</a>, and hence <a href="/facts/Quadratic_function/mKxHIC3U">quadratic</a> in <a href="/facts/Amplitude/aEckuXJM">amplitude</a> of <a href="/facts/Electromagnetic_field/MacraS8y">electromagnetic field</a>. Typically, the two light frequencies are similar enough that their difference or <a href="/facts/Beat_(acoustics)/zKhaKnMy">beat frequency</a> produced by the detector is in the radio or microwave band that can be conveniently processed by electronic means.
This technique became widely applicable to <a href="/facts/Topography/SS8Bx4t9">topographical</a> and <a href="/facts/Velocity/Q3o1LVDe">velocity</a>-sensitive <a href="/facts/LIDAR/b9SPUmLM">imaging</a> with the invention in the 1990s of synthetic array heterodyne detection. The light reflected from a target scene is focussed on a relatively inexpensive photodetector consisting of a single large physical pixel, while a different LO frequency is also tightly focussed on each virtual pixel of this detector, resulting in an electrical signal from the detector carrying a mixture of beat frequencies that can be electronically isolated and distributed spatially to present an image of the scene.

Optical heterodyne detection open-in-new

Optical heterodyne detection