Editing Interferometry (section)

===Homodyne versus heterodyne detection <span class="anchor" id="Heterodyne detection"></span>===
In [[homodyne detection]], the interference occurs between two beams at the same wavelength (or [[carrier frequency]]). The phase difference between the two beams results in a change in the intensity of the light on the detector. The resulting intensity of the light after mixing of these two beams is measured, or the pattern of interference fringes is viewed or recorded.<ref name=Rudiger/> Most of the interferometers discussed in this article fall into this category.

The [[heterodyne]] technique is used for (1) shifting an input signal into a new frequency range as well as (2) amplifying a weak input signal (assuming use of an active [[Electronic mixer|mixer]]). A weak input signal of frequency f<sub>1</sub> is [[Frequency mixer|mixed]] with a strong reference frequency f<sub>2</sub> from a [[local oscillator]] (LO). The nonlinear combination of the input signals creates two new signals, one at the sum f<sub>1</sub>&nbsp;+&nbsp;f<sub>2</sub> of the two frequencies, and the other at the difference f<sub>1</sub>&nbsp;−&nbsp;f<sub>2</sub>. These new frequencies are called '''heterodynes'''. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. The output signal will have an intensity proportional to the product of the amplitudes of the input signals.<ref name=Rudiger/>

The most important and widely used application of the heterodyne technique is in the [[superheterodyne receiver]] (superhet), invented in 1917-18 by U.S. engineer [[Edwin Howard Armstrong]] and French engineer [[Lucien Lévy]]. In this circuit, the incoming [[radio frequency]] signal from the antenna is mixed with a signal from a local oscillator (LO) and converted by the heterodyne technique to a lower fixed frequency signal called the [[intermediate frequency]] (IF). This IF is amplified and filtered, before being applied to a [[detector (radio)|detector]] which extracts the audio signal, which is sent to the loudspeaker.<ref name=superhet>{{cite web |last=Poole |first=Ian |title=The superhet or superheterodyne radio receiver |url=http://www.radio-electronics.com/info/rf-technology-design/superheterodyne-radio-receiver/basics-tutorial.php|publisher=Radio-Electronics.com|access-date=22 June 2012}}</ref>

[[Optical heterodyne detection]] is an extension of the heterodyne technique to higher (visible) frequencies.<ref name=Rudiger>{{cite web |last=Paschotta |first=Rüdiger |title=Optical Heterodyne Detection|url=http://www.rp-photonics.com/optical_heterodyne_detection.html |publisher=RP Photonics Consulting GmbH|access-date=1 April 2012}}</ref>  While optical heterodyne interferometry is usually done at a single point it is also possible to perform this widefield.<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|bibcode=2011OExpr..1924546P|doi=10.1364/OE.19.024546|pmid=22109482|doi-access=free}}</ref>