View source for Ultrastable heterodyne interferometry using a modulated light camera

===Ultrastable heterodyne interferometry using a modulated light camera===

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'''PhD student''': [[Rikesh Patel]]

'''Project Supervisor''': [[Matt Clark]]


'''Project Brief''': 

[[Image:Macult.png|350px|thumb|link=|Widefield ultrastable heterodyne interferometer]] [[Image:Twolas2.png|350px|thumb|link=|Widefield two laser interferometer]]

Interferometry is used in a wide variety of fields for the instrumentation and analysis of subjects and the environment. Captured widefield interference patterns can be used to determine changes in the optical path length of interfering beams across a 2D area. The heterodyne interferometers use light with different optical frequencies to generate a fringe pattern that is modulated at a frequency equal to the optical frequency difference (beat frequency). A widefield heterodyne system is not straightforward to bring into practice, however, it does offer some benefits such as direct phase interpretation and the suppression of low frequency background light in interferograms. 

For this project, a widefield heterodyne interferometer system was developed. A custom prototype modulated light camera (MLC) chip was used to capture both homodyne and heterodyne fringe patterns. The 32x32 pixel camera is capable of continuously demodulating incident modulated light at frequencies between 100 kHz and 17 MHz. With modifications to the system, an ultrastable widefield heterodyne interferometer system was implemented. The intention of this system was to eliminate the contribution of piston phase to a captured interferogram without the need for common path optics. In contrast to the standard heterodyne setup, the reference signal used in the demodulation process was derived from one of the pixels on-board the MLC, rather than from an external source. This new local reference signal tracks the common changes in the temporal phase detected by all the MLC's pixels, eliminating piston phase and substantially reducing the contributions of unwanted vibrations and microphonics from interferograms. As an application of the ultrastable system, a novel interferometer has been developed that captures the widefield fringe patterns generated by interfering two independent light sources, rather than by a single split source. The two separately stabilised HeNe lasers, constructed in the laboratory, produce light with a reasonably stable output frequency. Interfering two of these sources produce a heterodyne interference pattern with an unknown beat frequency. The beat frequency continuously varies because of the variation in the output frequency of each laser, but these stabilised lasers produce a beat frequency that drift by as little as 3 MHz over 30 minutes. As the ultrastable system tracks changes in the temporal phase and instantaneous frequency of an incident fringe pattern, it can be used to track the variations in the modulation frequency generated by the fluctuations in the two separate lasers.

'''Thesis link''': [http://optics.eee.nottingham.ac.uk/w/images/0/06/Thesis_rp_2014.pdf PDF]

'''Publications''':

<bibtex>
@Article{Patel2014,  Title                      = {Widefield two laser interferometry},  Author                     = {Rikesh Patel and Samuel Achamfuo-Yeboah and Roger Light and Matt   Clark},  Journal                  = {Opt. Express},  Year                       = {2014},  Month                    = {Nov},  Number                     = {22},  Pages                    = {27094--27101},  Volume                     = {22},  Abstract                 = {A novel system has  been  developed that can capture the wide-field interference pattern  generated  by interfering two independent and incoherent laser sources.  The  interferograms are captured using a custom CMOS modulated light  camera  (MLC) which is capable of demodulating light in the megahertz  region.  Two stabilised HeNe lasers were constructed in order to keep  the optical  frequency difference (beat frequency) between the beams  within the  operational range of the camera.This system is based on  previously  reported work of an ultrastable heterodyne interferometer   \&\#x0005B;Opt. Express 20, 17722 (2012)\&\#x0005D;. The system   used an electronic feedback system to mix down the heterodyne signal   captured at each pixel on the camera to cancel out the effects of time   varying piston phase changes observed across the array. In this paper, a   similar technique is used to track and negate the effects of beat   frequency variations across the two laser pattern. This technique makes   it possible to capture the full field interferogram caused by   interfering two independent lasers even though the beat frequency is   effectively random.As a demonstration of the system\&\#x02019;s   widefield interferogram capture capability, an image of a phase shifting   object is taken using a very simple two laser interferometer.},  Doi                        = {10.1364/OE.22.027094},  Keywords                  =  {Interferometric imaging ; Interferometric imaging},  Owner                      = {rp},  Publisher                = {OSA},  Timestamp                  = {2014.11.18},  Url                      =   {http://www.opticsexpress.org/abstract.cfm?URI=oe-22-22-27094}, pdf = {   http://optics.eee.nottingham.ac.uk/w/images/4/4d/Pap4.pdf} }
</bibtex>

<bibtex>
@ARTICLE{Patel2012,  author = {Rikesh Patel and  Samuel Achamfuo-Yeboah and Roger Light and Matt	Clark},  title =  {Ultrastable heterodyne interferometer system using a CMOS modulated	 light camera},  journal = {Opt. Express},  year = {2012},  volume =  {20},  pages = {17722--17733},  number = {16},  month = {Jul},  abstract  = {A novel ultrastable widefield interferometer is presented. This uses 	a modulated light camera (MLC) to capture and stabilise the  interferogram	in the widefield heterodyne interferometer. This system  eliminates	the contribution of piston phase to the interferogram without  the	need for common path optics and results in a highly stable  widefield	interferometer.The MLC uses quadrature demodulation circuitry  built	into each pixel to demodulate the light signal and extract phase	 information using an electronic reference signal. In contrast to	the  work previously presented \&\#x0005B;Opt. Express 19, 24546  (2011)\&\#x0005D;,	the reference signal is derived from one of the  pixels on board the	MLC rather than an external source. This local  reference signal tracks	the instantaneous modulation frequency detected  by the other pixels	and eliminates the contribution of piston phase to  the interferogram,	substantially removing the contributions of unwanted  vibrations and	microphonics to the interferogram. Interferograms taken  using the	ultrastable system are presented with one of the  interferometer mirrors	moving at up to 85 mm s\&\#x02212;1 over a  variety of frequencies	from 18 Hz to 20 kHz (giving a variation in  optical path length of	220 \&\#x003BC;m, or 350 wavelengths at 62  Hz). This limit was the	result of complex motion in the mirror mount  rather than the stability	limit of the system. The system is shown to be  insensitive to pure	piston phase variations equivalent to an object  velocity of over	3 m s\&\#x02212;1.},  doi = {10.1364/OE.20.017722},   keywords = {Interferometric imaging ; Interferometric imaging},  owner  = {rp},  publisher = {OSA},  timestamp = {2013.06.03},  url =  {http://www.opticsexpress.org/abstract.cfm?URI=oe-20-16-17722}, pdf =  {http://optics.eee.nottingham.ac.uk/w/images/0/0f/Pap3.pdf}}
</bibtex>

<bibtex>
@ARTICLE{Patel2011a,   author = {Rikesh Patel and Samuel Achamfuo-Yeboah and Roger Light and  Matt	Clark},  title = {Widefield heterodyne interferometry using a  custom CMOS modulated	light camera},  journal = {Opt. Express},  year =  {2011},  volume = {19},  pages = {24546--24556},  number = {24},  month =  {Nov},  abstract = {In this paper a method of taking widefield  heterodyne interferograms	using a prototype modulated light camera is  described. This custom	CMOS modulated light camera (MLC) uses analogue  quadrature demodulation	at each pixel to output the phase and amplitude  of the modulated	light as DC voltages. The heterodyne interference  fringe patterns	are generated using an acousto-optical frequency shifter  (AOFS) in	an arm of a Mach-Zehnder interferometer. Widefield images of  fringe	patterns acquired using the prototype MLC are presented. The  phase	can be measured to an accuracy of {\textpm}6.6{\textdegree}. The	 added value of this method to acquire widefield images are discussed	 along with the advantages.},  doi = {10.1364/OE.19.024546},  owner =  {rp},  publisher = {OSA},  timestamp = {2012.01.13},  url =  {http://www.opticsexpress.org/abstract.cfm?URI=oe-19-24-24546}, pdf =  {http://optics.eee.nottingham.ac.uk/w/images/d/d6/Pap1.pdf}}
</bibtex>