Editing Heterodyne (section)

===Optical heterodyning===
[[Optical heterodyne detection]] (an area of active research) is an extension of the heterodyning technique to higher (visible) frequencies. Guerra<ref>{{Cite journal |last=Guerra |first=John M. |date=1995-06-26 |title=Super-resolution through illumination by diffraction-born evanescent waves |url=http://aip.scitation.org/doi/10.1063/1.113814 |journal=Applied Physics Letters |language=en |volume=66 |issue=26 |pages=3555–3557 |doi=10.1063/1.113814 |bibcode=1995ApPhL..66.3555G |issn=0003-6951}}</ref> (1995) first published the results of what he called a "form of optical heterodyning" in which light patterned by a 50 nm pitch grating illuminated a second grating of pitch 50 nm, with the gratings rotated with respect to each other by the angular amount needed to achieve magnification. Although the illuminating wavelength was 650 nm, the 50 nm grating was easily resolved. This showed a nearly 5-fold improvement over the Abbe resolution limit of 232 nm that should have been the smallest obtained for the numerical aperture and wavelength used. This super-resolution microscopic imaging through optical heterodyning later came to be know by many as "structured illumination microscopy".  

In addition to super-resolution optical microscopy, optical heterodyning could greatly improve [[optical modulator]]s, increasing the density of information carried by [[optical fiber]]s. It is also being applied in the creation of more accurate [[atomic clock]]s based on directly measuring the frequency of a laser beam.{{refn|group=notes|See [[NIST]] subtopic 9.07.9-4.R for a description of research on one system to do this.<ref>[http://tsapps.nist.gov/ts_sbir/sbirrss/index.cfm?action=contractdetails&id=78 Contract Details: Robust Nanopopous Ceramic Microsensor Platform<!-- Bot generated title -->]</ref><ref>[http://tsapps.nist.gov/ts_sbir/sbirrss/index.cfm?action=contractdetails&id=147 Contract Details: High Pulsed Power Varactor Multipliers for Imaging<!-- Bot generated title -->]</ref>}}

Since optical frequencies are far beyond the manipulation capacity of any feasible electronic circuit, all visible frequency photon detectors are inherently energy detectors not oscillating electric field detectors. However, since energy detection is inherently "[[square-law detector|square-law]]" detection, it intrinsically mixes any optical frequencies present on the detector. Thus, sensitive detection of specific optical frequencies necessitates optical heterodyne detection, in which two different (close by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to the difference between their frequencies. This allows extremely narrow band detection (much narrower than any possible color filter can achieve) as well as precision measurements of phase and frequency of a light signal relative to a reference light source, as in a [[laser Doppler vibrometer]].

This phase sensitive detection has been applied for Doppler measurements of wind speed, and imaging through dense media. The high sensitivity against background light is especially useful for [[lidar]].

In [[optical Kerr effect]] (OKE) spectroscopy, optical heterodyning of the OKE signal and a small part of the probe signal produces a mixed signal consisting of probe, heterodyne OKE-probe and homodyne OKE signal. The probe and homodyne OKE signals can be filtered out, leaving the heterodyne frequency signal for detection.

Heterodyne detection is often used in [[interferometry]] but usually confined to single point detection rather than widefield interferometry, however, widefield heterodyne interferometry is possible using a special camera.<ref>{{cite journal|last=Patel|first=R.|author2=Achamfuo-Yeboah, S. |author3=Light R.|author4=Clark M.|title=Widefield heterodyne interferometry using a custom CMOS modulated light camera|journal=Optics Express|date=2011|volume=19|issue=24|pages=24546–24556|url=https://www.osapublishing.org/oe/abstract.cfm?uri=oe-19-24-24546|doi=10.1364/oe.19.024546|pmid=22109482|bibcode=2011OExpr..1924546P |doi-access=free}}</ref>   Using this technique which a reference signal extracted from a single pixel it is possible to build a highly stable widefield heterodyne interferometer by removing the piston phase component
caused by [[microphonics]] or vibrations of the optical components or object.<ref>{{cite journal|last=Patel|first=R.|author2=Achamfuo-Yeboah, S. |author3=Light R.|author4=Clark M.|title=Ultrastable heterodyne interferometer system using a CMOS modulated light camera|journal=Optics Express|date=2012|volume=20|issue=16|pages=17722–17733|url=https://www.osapublishing.org/oe/abstract.cfm?uri=oe-20-16-17722|doi=10.1364/oe.20.017722|pmid=23038324|bibcode=2012OExpr..2017722P |doi-access=free}}</ref>