Editing Optical ring resonators (section)

== Applications ==
Due to the nature of the optical ring resonator and how it "filters" certain wavelengths of light passing through, it is possible to create high-order optical filters by cascading many optical ring resonators in series. This would allow for "small size, low losses, and integrability into [existing] optical networks."<ref name = "IlchenkoandMatsko">
{{cite book
 |author=Ilchenko
 |author2=Matsko
 |name-list-style=amp
 |title=Optical Resonators With Whispering-Gallery Modes — Part II: Applications
 |publisher=IEEE Journal of Selected Topics in Quantum Electronics (12.1, January 2006)
}}</ref> Additionally, since the resonance wavelengths can be changed by simply increasing or decreasing the radius of each ring, the filters can be considered tunable. This basic property can be used to create a sort of mechanical sensor. If an optical fiber experiences [[Deformation (mechanics)|mechanical strain]], the dimensions of the fiber will be altered, thus resulting in a change in the resonant wavelength of light emitted. This can be used to monitor fibers or waveguides for changes in their dimensions.<ref>{{cite journal|last1=Westerveld|first1=W.J.|last2=Leinders|first2=S.M.|last3=Muilwijk|first3=P.M.|last4=Pozo|first4=J.|last5=van den Dool|first5=T.C.|last6=Verweij|first6=M.D.|last7=Yousefi|first7=M.|last8=Urbach|first8=H.P.|title=Characterization of Integrated Optical Strain Sensors Based on Silicon Waveguides|journal=IEEE Journal of Selected Topics in Quantum Electronics|date=10 January 2014|volume=20|issue=4|pages=101|doi=10.1109/JSTQE.2013.2289992|bibcode=2014IJSTQ..20..101W|doi-access=free}}</ref>
The tuning process can be affected also by a change of refractive index using various means including thermo-optic,<ref name="osapublishing.org">N. Li, E. Timurdogan, C. V. Poulton, M. Byrd, E. S. Magden, Z. Su, G. Leake, D Coolbaugh, D. Vermeulen, M. R. Watts (2016) [http://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-20-22741 “C-band swept wavelength erbium-doped fiber laser with a high-Q tunable interior-ridge silicon microring cavity”], '' Optics Express'', Vol. 24, Issue 20, pp.22741-22748</ref> electro-optic <ref>{{cite journal|last1=Sadasivan|first1=Viswas|title=QCSE Tuned Embedded Ring Modulator|journal=Journal of Lightwave Technology|date=2014|volume=32|issue=1|pages=107–114|doi=10.1109/JLT.2013.2289324|bibcode=2014JLwT...32..107S|s2cid=11004491 }}</ref> or all-optical <ref>{{cite book|last1=Ibrahim, and|first1=Tarek A.|last2=Grover|first2=Rohit|last3=Kuo|first3=Li-Chiang|last4=Kanakaraju|first4=Subramaniam|last5=Calhoun|first5=Lynn C.|last6=Ho|first6=Ping-Tong |title=Integrated Photonics Research |chapter=All-optical switching using a critically coupled InP micro-racetrack resonator |date=2003|page=ITuE4|doi=10.1364/IPR.2003.ITuE4|isbn=978-1-55752-751-6}}</ref> effects. Electro-optic and all-optical tuning is faster than thermal and mechanical means, and hence find various applications including in optical communication. Optical modulators with a high-Q microring are reported to yield outstandingly small power of modulation at a speed of > 50&nbsp;Gbit/s at cost of a tuning power to match wavelength of the light source. A ring modulator placed in a [[Fabry-Perot laser]] cavity was reported to eliminate the tuning power by automatic matching of the laser wavelength with that of the ring modulator while maintaining high-speed ultralow-power modulation of a Si microring modulator.

Optical ring, cylindrical, and spherical resonators have also been proven useful in the field of [[biosensing]].,<ref>{{cite journal|author1=A. Ksendzov  |author2=Y. Lin |name-list-style=amp |title=Integrated optics ring-resonator sensors for protein detection|journal= Optics Letters|volume=30 | issue = 24|pages=3344–3346|year=2005|doi=10.1364/ol.30.003344|pmid=16389826 |bibcode = 2005OptL...30.3344K }}</ref><ref>{{cite book | chapter-url=https://doi.org/10.1117/12.2005832 | doi=10.1117/12.2005832 | chapter=Label-free silicon photonic biosensors for use in clinical diagnostics | title=Silicon Photonics VIII | date=2013 | editor-last1=Kubby | editor-first1=Joel | last1=Talebi Fard | first1=Sahba | last2=Grist | first2=Samantha M. | last3=Donzella | first3=Valentina | last4=Schmidt | first4=Shon A. | last5=Flueckiger | first5=Jonas | last6=Wang | first6=Xu | last7=Shi | first7=Wei | last8=Millspaugh | first8=Andrew | last9=Webb | first9=Mitchell | last10=Ratner | first10=Daniel M. | last11=Cheung | first11=Karen C. | last12=Chrostowski | first12=Lukas | volume=8629 | page=862909 | s2cid=123382866 | editor-first2=Graham T. | editor-last2=Reed }}</ref><ref>{{cite news|author1=K. D. Vos |author2=I. Bartolozzi |author3=E. Schacht |author4=P. Bienstman |author5=R. Baets  |name-list-style=amp |title=Silicon-on-Insulator microring resonator for sensitive and label-free biosensing|journal=Opt. Express|volume=15 | issue = 12|pages=7610–7615|year=2007}}</ref><ref>{{cite journal | author=Witzens, J. | author2=Hochberg, M.|title =Optical detection of target molecule induced aggregation of nanoparticles by means of high-Q resonators|journal=Optics Express|volume=19| issue=8|pages=7034–7061|year=2011|bibcode = 2011OExpr..19.7034W |doi = 10.1364/OE.19.007034 | pmid=21503017|doi-access=free}}</ref><ref>{{cite journal|author1=Lin S. |author2=K. B. Crozier |title=Trapping-Assisted Sensing of Particles and Proteins Using On-Chip Optical Microcavities|journal=ACS Nano|volume=7 |issue=2 |pages=1725–1730 |year=2013|doi=10.1021/nn305826j|pmid=23311448 }}</ref> and a crucial research focus is the enhancement of biosensing performance <ref>{{cite journal | doi=10.1364/OE.23.004791 | title=Design and fabrication of SOI micro-ring resonators based on sub-wavelength grating waveguides | date=2015 | last1=Donzella | first1=Valentina | last2=Sherwali | first2=Ahmed | last3=Flueckiger | first3=Jonas | last4=Grist | first4=Samantha M. | last5=Fard | first5=Sahba Talebi | last6=Chrostowski | first6=Lukas | journal=Optics Express | volume=23 | issue=4 | pages=4791–5503 | pmid=25836514 | bibcode=2015OExpr..23.4791D | doi-access=free }}</ref><ref>{{cite journal | doi=10.1364/OE.22.014166 | title=Performance of ultra-thin SOI-based resonators for sensing applications | date=2014 | last1=Fard | first1=Sahba Talebi | last2=Donzella | first2=Valentina | last3=Schmidt | first3=Shon A. | last4=Flueckiger | first4=Jonas | last5=Grist | first5=Samantha M. | last6=Talebi Fard | first6=Pouria | last7=Wu | first7=Yichen | last8=Bojko | first8=Rick J. | last9=Kwok | first9=Ezra | last10=Jaeger | first10=Nicolas A. F. | last11=Ratner | first11=Daniel M. | last12=Chrostowski | first12=Lukas | journal=Optics Express | volume=22 | issue=12 | pages=14166–14179 | pmid=24977515 | bibcode=2014OExpr..2214166F | doi-access=free }}</ref><ref>{{cite journal | doi=10.1364/OE.24.015672 | title=Sub-wavelength grating for enhanced ring resonator biosensor | date=2016 | last1=Flueckiger | first1=Jonas | last2=Schmidt | first2=Shon | last3=Donzella | first3=Valentina | last4=Sherwali | first4=Ahmed | last5=Ratner | first5=Daniel M. | last6=Chrostowski | first6=Lukas | last7=Cheung | first7=Karen C. | journal=Optics Express | volume=24 | issue=14 | pages=15672–15686 | pmid=27410840 | bibcode=2016OExpr..2415672F | doi-access=free }}</ref><ref>{{cite journal | doi=10.3390/app10124191 | doi-access=free | title=Enhanced Sensitivity of Microring Resonator-Based Sensors Using the Finite Difference Time Domain Method to Detect Glucose Levels for Diabetes Monitoring | date=2020 | last1=Hasanah | first1=Lilik | last2=Nugroho | first2=Harbi Setyo | last3=Wulandari | first3=Chandra | last4=Mulyanti | first4=Budi | last5=Berhanuddin | first5=Dilla Duryha | last6=Haron | first6=Mohamad Hazwan | last7=Menon | first7=P. Susthitha | last8=Md Zain | first8=Ahmad Rifqi | last9=Hamidah | first9=Ida | last10=Khairurrijal | first10=Khairurrijal | last11=Mamat | first11=Rizalman | journal=Applied Sciences | volume=10 | issue=12 | page=4191 }}</ref> One of the main benefits of using ring resonators in biosensing is the small volume of sample specimen required to obtain a given [[spectroscopy]] results in greatly reduced background Raman and fluorescence signals from the solvent and other impurities. Resonators have also been used to characterize a variety of absorption spectra for the purposes of chemical identification, particularly in the gaseous phase.<ref name = "BlairandChen">
{{cite book
 |author=Blair
 |author2=Chen
 |name-list-style=amp
 |title=Resonant-Enhanced Evanescent-Wave Fluorescence Biosensing with Cylindrical Optical Cavities
 |publisher=Applied Optics (40.4, February 2001)
}}</ref>

Another potential application for optical ring resonators are in the form of whispering gallery mode switches. "[Whispering Gallery Resonator] microdisk lasers are stable and switch reliably and hence, are suitable as switching elements in all-optical networks." An all-optical switch based on a high Quality factor cylindrical resonator has been proposed that allows for fast binary switching at low power.<ref name="IlchenkoandMatsko"/>

Many researchers are interested in creating three-dimensional ring resonators with very high quality factors. These dielectric spheres, also called microsphere resonators, "were proposed as low-loss optical resonators with which to study cavity quantum electrodynamics with laser-cooled atoms or as ultrasensitive detectors for the detection of single trapped atoms.”<ref name = "Gotzingeretal">
{{cite book
 |author1=Götzinger
 |author2=Benson
 |author3=Sandoghdar
 |name-list-style=amp
 |title=Influence of a Sharp Fiber Tip on High-Q Modes of a Microsphere Resonator
 |publisher=Optics Letters (27.2, January 2002)
}}</ref>

Ring resonators have also proved useful as single photon sources for quantum information experiments.<ref>{{cite journal|author1=E. Engin |author2=D. Bonneau |author3=C. Natarajan |author4=A. Clark |author5=M. Tanner |author6=R. Hadfield |author7=S. Dorenbos |author8=V. Zwiller |author9=K. Ohira |author10=N. Suzuki |author11=H. Yoshida |author12=N. Iizuka |author13=M. Ezaki |author14=J. O?Brien |author15=M. Thompson  |name-list-style=amp |title=Photon pair generation in a silicon micro-ring resonator with reverse bias enhancement|journal= Optics Express|volume=21 | issue = 23|pages=27826–27834|year=2013|doi=10.1364/OE.21.027826|pmid=24514299 |arxiv = 1204.4922 |bibcode = 2013OExpr..2127826E |s2cid=10147604 }}</ref> Many materials used to fabricate ring resonator circuits have non-linear responses to light at high enough intensities. This non-linearity allows for frequency modulation processes such as [[four-wave mixing]] and [[Spontaneous parametric down-conversion]] which generate photon pairs. Ring resonators amplify the efficiency of these processes as they allow the light to circulate around the ring.