||This article needs attention from an expert in Physics. (November 2012)|
Optical or photonic computing uses photons produced by lasers or diodes for computation. For decades, photons have promised to allow a higher bandwidth than the electrons used in conventional computers.
Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. This approach appears to offer the best short-term prospects for commercial optical computing, since optical components could be integrated into traditional computers to produce an optical-electronic hybrid. However, optoelectronic devices lose 30% of their energy converting electronic energy into photons and back; this conversion also slows the transmission of messages. All-optical computers eliminate the need for optical-electrical-optical (OEO) conversions, thus lessening the need for electrical power. 
Application-specific devices, such as optical correlators, have been designed to use the principles of optical computing. Such devices can be used, for example, to detect and track objects, and to classify serial time-domain optical data.
Optical components for binary digital computer
The fundamental building block of modern electronic computers is the transistor. To replace electronic components with optical ones, an equivalent optical transistor is required. This is achieved using materials with a non-linear refractive index. In particular, materials exist where the intensity of incoming light affects the intensity of the light transmitted through the material in a similar manner to the current response of a bipolar transistor. Such an 'optical transistor' can be used to create optical logic gates, which in turn are assembled into the higher level components of the computer's CPU. These will be non linear crystals used to manipulate light beams into controlling others.
There are disagreements between researchers about the future capabilities of optical computers: will they be able to compete with semiconductor-based electronic computers on speed, power consumption, cost, and size? Critics note that real-world logic systems require "logic-level restoration, cascadability, fan-out and input–output isolation", all of which are currently provided by electronic transistors at low cost, low power, and high speed. For optical logic to be competitive beyond a few niche applications, major breakthroughs in non-linear optical device technology would be required, or perhaps a change in the nature of computing itself.
Misconceptions, challenges, and prospects
A significant challenge to optical computing is that computation is a nonlinear process in which multiple signals must interact. Light, which is an electromagnetic wave, can only interact with another electromagnetic wave in the presence of electrons in a material, and the strength of this interaction is much weaker for electromagnetic waves, such as light, than for the electronic signals in a conventional computer. This may result in the processing elements for an optical computer requiring more power and larger dimensions than those for a conventional electronic computer using transistors.
A further misconception is that since light can travel much faster than the drift velocity of electrons, and at frequencies measured in THz, optical transistors should be capable of extremely high frequencies. However, any electromagnetic wave must obey the transform limit, and therefore the rate at which an optical transistor can respond to a signal is still limited by its spectral bandwidth. However, in fiber optic communications, practical limits such as dispersion often constrain channels to bandwidths of 10s of GHz, only slightly better than many silicon transistors. Obtaining dramatically faster operation than electronic transistors would therefore require practical methods of transmitting ultrashort pulses down highly dispersive waveguides.
Other approaches currently being investigated include photonic logic at a molecular level, using photoluminescent chemicals. In a recent demonstration, Witlicki et al. performed logical operations using molecules and SERS.
- Nolte, D.D. (2001). Mind at Light Speed: A New Kind of Intelligence. Simon and Schuster. p. 34. ISBN 978-0-7432-0501-6.
- Feitelson, Dror G. (1988). "Chapter 3: Optical Image and Signal Processing". Optical Computing: A Survey for Computer Scientists. Cambridge, MA: MIT Press. ISBN 0-262-06112-0.
- Kim, S. K.; Goda, K.; Fard, A. M.; Jalali, B. (2011). "Optical time-domain analog pattern correlator for high-speed real-time image recognition". Optics Letters. 36 (2): 220. doi:10.1364/ol.36.000220.
- Jain, K.; Pratt, Jr., G. W. (1976). "Optical transistor". Appl. Phys. Lett. 28 (12): 719. doi:10.1063/1.88627.
- Jain, K. and Pratt, Jr., G. W., "Optical transistors and logic circuits embodying the same", U.S. Pat. 4,382,660, issued May 10, 1983.
- Tucker, R.S. (2010). "The role of optics in computing". Nature Photonics. 4: 405. doi:10.1038/nphoton.2010.162.
- Philip R. Wallace (1996). Paradox Lost: Images of the Quantum. ISBN 0387946594.
- Witlicki, Edward H.; Johnsen, Carsten; Hansen, Stinne W.; Silverstein, Daniel W.; Bottomley, Vincent J.; Jeppesen, Jan O.; Wong, Eric W.; Jensen, Lasse; Flood, Amar H. (2011). "Molecular Logic Gates Using Surface-Enhanced Raman-Scattered Light". J. Am. Chem. Soc. 133 (19): 7288–91. doi:10.1021/ja200992x.
- Feitelson, Dror G. (1988). Optical Computing: A Survey for Computer Scientists. Cambridge, MA: MIT Press. ISBN 0-262-06112-0.
- McAulay, Alastair D. (1991). Optical Computer Architectures: The Application of Optical Concepts to Next Generation Computers. New York, NY: John Wiley & Sons. ISBN 0-471-63242-2.
- Ibrahim TA; Amarnath K; Kuo LC; Grover R; Van V; Ho PT (2004). "Photonic logic NOR gate based on two symmetric microring resonators". Opt Lett. 29 (23): 2779–81. doi:10.1364/OL.29.002779. PMID 15605503.
- Biancardo M; Bignozzi C; Doyle H; Redmond G (2005). "A potential and ion switched molecular photonic logic gate". Chem. Commun. (31): 3918–20. doi:10.1039/B507021J.
- Jahns, J.; Lee, S.H., eds. (1993). Optical Computing Hardware: Optical Computing. Elsevier Science. ISBN 978-1-4832-1844-1.
- Barros S; Guan S; Alukaidey T (1997). "An MPP reconfigurable architecture using free-space optical interconnects and Petri net configuring". Journal of System Architecture. 43 (6-7): 391–402. doi:10.1016/S1383-7621(96)00053-7.
- D. Goswami, "Optical Computing", Resonance, June 2003; ibid July 2003. Web Archive of www.iisc.ernet.in/academy/resonance/July2003/July2003p8-21.html
- Main T; Feuerstein RJ; Jordan HF; Heuring VP; Feehrer J; Love CE (1994). "Implementation of a general-purpose stored-program digital optical computer". Applied Optics. 33: 1619–28. doi:10.1364/AO.33.001619. PMID 20862187.
- Guan, T.S.; Barros, S.P.V. (April 1994). "Reconfigurable Multi-Behavioural Architecture using Free-Space Optical Communication". Proceedings of the IEEE International Workshop on Massively Parallel Processing using Optical Interconnections. IEEE. pp. 293–305. doi:10.1109/MPPOI.1994.336615. ISBN 0-8186-5832-0.
- Guan, T.S.; Barros, S.P.V. (August 1994). "Parallel Processor Communications through Free-Space Optics". TENCON '94. IEEE Region 10's Ninth Annual International Conference. Theme: Frontiers of Computer Technology. 2. IEEE. pp. 677–681. doi:10.1109/TENCON.1994.369219. ISBN 0-7803-1862-5.
- Guha A.; Ramnarayan R.; Derstine M. (1987). "Architectural issues in designing symbolic processors in optics". Proceedings of the 14th annual international symposium on Computer architecture (ISCA '87). ACM. pp. 145–151. doi:10.1145/30350.30367. ISBN 0-8186-0776-9.
- K.-H. Brenner, Alan Huang: "Logic and architectures for digital optical computers (A)", J. Opt. Soc. Am., A 3, 62, (1986)
- Brenner, K.-H. (1988). "A programmable optical processor based on symbolic substitution". Appl. Opt. 27 (9): 1687–91. doi:10.1364/AO.27.001687. PMID 20531637.
- Streibl N.; Brenner K.-H.; Huang A.; Jahns J.; Jewell J.L.; Lohmann A.W.; Miller D.A.B.; Murdocca M.J.; Prise M.E.; Sizer II T. (1989). "Digital Optics". Proc. IEEE. 77 (12): 1954–69. doi:10.1109/5.48834.
- NASA scientists working to improve optical computing technology, 2000
- Optical solutions for NP-complete problems
- Dolev, S.; Haist, T.; Mihai Oltean (2008). Optical SuperComputing: First International Workshop, OSC 2008, Vienna, Austria, August 26, 2008, Proceedings. Springer. ISBN 978-3-540-85672-6.
- Dolev, S.; Oltean, M. (2009). Optical Supercomputing: Second International Workshop, OSC 2009, Bertinoro, Italy, November 18–20, 2009, Proceedings. Springer. ISBN 978-3-642-10441-1.
- Dolev, S.; Oltean, M. (2011). Optical Supercomputing: Third International Workshop, OSC 2010, Bertinoro, Italy, November 17–19, 2010, Revised Selected Papers. Springer. ISBN 978-3-642-22493-5.
- Dolev, S.; Oltean, M. (2013). Optical Supercomputing: 4th International Workshop, OSC 2012, in Memory of H. John Caulfield, Bertinoro, Italy, July 19–21, 2012. Revised Selected Papers. Springer. ISBN 978-3-642-38250-5.
- Speed-of-light computing comes a step closer New Scientist
- Caulfield H.; Dolev S. (2010). "Why future supercomputing requires optics". Nature Photonics. 4: 261–263. doi:10.1038/nphoton.2010.94.
- Cohen E.; Dolev S.; Rosenblit M. (2016). "All-optical design for inherently energy-conserving reversible gates and circuits". Nature Communications. 7: 11424. doi:10.1038/ncomms11424.