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|issue = OMI7
|issue = OMI7
|author = W. A. Zortman, A. L. Lentine, M. R. Watts, and D. C. Trotter
|author = W. A. Zortman, A. L. Lentine, M. R. Watts, and D. C. Trotter
}}</ref>, Kotura, NTT, Fujitsu and academic institutes have been attempting to prove this functionality. A prototype 80 km, 12.5 Gbps transmission has been reported using microring silicon devices [[recently]] <ref name="Biberman_Manipatruni_2010">{{cite journal
}}</ref>, Kotura, NTT, Fujitsu and academic institutes have been attempting to prove this functionality. A prototype 80 km, 12.5 Gbit/s transmission has been reported using microring silicon devices [[recently]] <ref name="Biberman_Manipatruni_2010">{{cite journal
|doi = 10.1364/OE.18.015544
|doi = 10.1364/OE.18.015544
|title = First demonstration of long-haul transmission using silicon microring modulators
|title = First demonstration of long-haul transmission using silicon microring modulators

Version vom 14. September 2010, 23:56 Uhr

Silicon photonics is the study and application of photonic systems which use silicon as an optical medium.[1][2][3][4][5] The silicon is usually patterned with sub-micron precision, into microphotonic components.[4] These operate in the infrared, most commonly at the 1.55 micron wavelength used by most fiber optic telecommunication systems.[1] The silicon typically lies on top of a layer of silica in what (by analogy with a similar construction in microelectronics) is known as silicon on insulator (SOI).[4][5]

Silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip.[1] Consequently, silicon photonics is being actively researched by many electronics manufacturers including IBM and Intel, who see it is a means for keeping on track with Moore's Law, by using optical interconnects to provide faster data transfer both between and within microchips.[6][7]

The propagation of light through silicon devices is governed by a range of nonlinear optical phenomena including the Kerr effect, the Raman effect, two photon absorption and interactions between photons and free charge carriers.[8] The presence of nonlinearity is of fundamental importance, as it enables light to interact with light,[9] thus permitting applications such as wavelength conversion and all-optical signal routing, in addition to the passive transmission of light.

Silicon waveguides are also of great academic interest, due to their ability to support exotic nonlinear optical phenomena such as soliton propagation.[10][11][12]

Applications

Optical interconnects

Future progress in computer technology (and the continuation of Moore's Law) is becoming increasingly dependent on ultra-fast data transfer between and within microchips.[13] High speed optical interconnects are seen as a promising way forward, and silicon photonics is seen as particularly useful, due to the ability to integrate electronic and optical components on the same silicon chip.[1][14][15] Intel senior vice president Pat Gelsinger has stated that, "Today, optics is a niche technology. Tomorrow, it's the mainstream of every chip that we build."[7] IBM also has an extensive research program in the field.[6]

Optical interconnects require the integration of a range of technologies. Firstly, an on-chip laser source is required. One such device is the hybrid silicon laser, in which the silicon is bonded to a different semiconductor (such as indium phosphide) which acts as the lasing medium.[16] Another possibility is the all-silicon Raman laser, in which the silicon itself acts as the lasing medium.[17]

There must also be a means to modulate the light, thus causing it to carry data in the form of optical pulses. One such technique is to control the density of free charge carriers, which (as described below) alter the optical properties of the waveguide. Modulators have been constructed where the light passes through the intrinsic region of a PIN diode, into which carriers can be injected or removed by altering the polarity of an applied voltage.[18] By using an optical ring resonator with a built in PIN diode, data transmission rates of 18 Gbit/s have been achieved.[19] By constructing devices where the electrical signal co-moves with the light, data rates of 30 Gbit/s have been achieved.[20] By constructing devices where the multiple wavelengths are used to scale the modulation capacity 50 Gbit/s modulation capacity has been achieved.[21] A prototype optical interconnect with microring modulators integrated with germanium detectors has also been demonstrated [22][23]

After passage through a silicon waveguide to a different chip (or region of the same chip) the light must be detected, converting the data back into electronic form. Detectors based on metal-semiconductor junctions (with germanium as the semiconductor) have been integrated into silicon waveguides.[24] More recently, silicon-germanium avalanche photodiodes capable of operating at 40 Gbit/s have been fabricated.[25][26]

Optical routers and signal processors

Another application of silicon photonics is in signal routers for optical communication. Construction can be greatly simplified by fabricating the optical and electronic parts on the same chip, rather than having them spread across multiple components.[27] A wider aim is all-optical signal processing, whereby tasks which are conventionally performed by manipulating signals in electronic form are done directly in optical form.[3][28] An important example is all-optical switching, whereby the routing of optical signals is directly controlled by other optical signals.[29] Another example is all-optical wavelength conversion.[30]

Long Range Telecommunications using Silicon Photonics

Silicon microphotonics can be applied to long range telecommunications to enable the scaling of the internet bandwidth capacity by using micro-scale, ultra low power devices. The power consumption of datacenters may be significantly reduced if this is successfully achieved. Researchers at [Sandia] [31], Kotura, NTT, Fujitsu and academic institutes have been attempting to prove this functionality. A prototype 80 km, 12.5 Gbit/s transmission has been reported using microring silicon devices recently [32]

Physical properties

Optical guiding and dispersion tailoring

Silicon is transparent to infrared light with wavelengths above about 1.1 microns.[33] Silicon also has a very high refractive index, of about 3.5.[33] The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers.[8] This is substantially less than the wavelength of the light itself, and is analogous to a subwavelength-diameter optical fibre. Single mode propagation can be achieved,[8] thus (like single-mode optical fiber) eliminating the problem of modal dispersion.

The strong dielectric boundary effects that result from this tight confinement substantially alter the optical dispersion relation. By selecting the waveguide geometry, it possible to tailor the dispersion to have desired properties, which is of crucial importance to applications requiring ultrashort pulses.[8] In particular, the group velocity dispersion (that is, the extent to which group velocity varies with wavelength) can be closely controlled. In bulk silicon at 1.55 microns, the group velocity dispersion (GVD) is normal in that pulses with longer wavelengths travel with higher group velocity than those with shorter wavelength. By selecting a suitable waveguide geometry, however, it is possible to reverse this, and achieve anomalous GVD, in which pulses with shorter wavelengths travel faster.[34][35] Anomalous dispersion is significant, as it is a prerequisite for soliton propagation, and modulational instability.[36]

In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer on which they are fabricated, it is necessary to have a layer of intervening material. This is usually silica, which has a much lower refractive index (of about 1.44 in the wavelength region of interest [37]), and thus light at the silicon-silica interface will (like light at the silicon-air interface) undergo total internal reflection, and remain in the silicon. This construct is known as silicon on insulator.[4][5] It is named after the technology of silicon on insulator in electronics, whereby components are built upon a layer of insulator in order to reduce parasitic capacitance and so improve performance.[38]

Kerr nonlinearity

Silicon has a focusing Kerr nonlinearity, in that the refractive index increases with optical intensity.[8] This effect is not especially strong in bulk silicon, but it can be greatly enhanced by using a silicon waveguide to concentrate light into a very small cross-sectional area.[10] This allows nonlinear optical effects to be seen at low powers. The nonlinearity can be enhanced further by using a slot waveguide, in which the high refractive index of the silicon is used to confine light into a central region filled with a strongly nonlinear polymer.[39]

Kerr nonlinearity underlies a wide variety of optical phenomena.[36] One example is four wave mixing, which has been applied in silicon to realise both optical parametric amplification [40] and parametric wavelength conversion.[30] Kerr nonlinearity can also cause modulational instability, in which it reinforces deviations from an optical waveform, leading to the generation of spectral-sidebands and the eventual breakup of the waveform into a train of pulses.[41] Another example (as described below) is soliton propagation.

Two-photon absorption

Silicon exhibits two-photon absorption (TPA), in which a pair of photons can act to excite an electron-hole pair.[8] This process is related to the Kerr effect, and by analogy with complex refractive index, can be thought of as the imaginary-part of a complex Kerr nonlinearity.[8] At the 1.55 micron telecommunication wavelength, this imaginary part is approximately 10% of the real part.[42]

The influence of TPA is highly disruptive, as it both wastes light, and generates unwanted heat.[43] It can be mitigated, however, either by switching to longer wavelengths (at which the TPA to Kerr ratio drops),[44] or by using slot waveguides (in which the internal nonlinear material has a lower TPA to Kerr ratio).[39] Alternatively, the energy lost through TPA can be partially recovered (as is described below) by extracting it from the generated charge carriers.[45]

Free charge carrier interactions

The free charge carriers within silicon can both absorb photons and change its refractive index.[46] This is particularly significant at high intensities and for long durations, due to the carrier concentration being built up by TPA. The influence of free charge carriers is often (but not always) unwanted, and various means have been proposed to remove them. One such scheme is to implant the silicon with helium in order to enhance carrier recombination.[47] A suitable choice of geometry can also be used to reduce the carrier lifetime. Rib waveguides (in which the waveguides consist of thicker regions in a wider layer of silicon) enhance both the carrier recombination at the silica-silicon interface and the diffusion of carriers from the waveguide core.[48]

A more advanced scheme for carrier removal is to integrate the waveguide into the intrinsic region of a PIN diode, which is reverse biased so that the carriers are attracted away from the waveguide core.[49] A more sophisticated scheme still, is to use the diode as part of a circuit in which voltage and current are out of phase, thus allowing power to be extracted from the waveguide.[45] The source of this power is the light lost to two photon absorption, and so by recovering some of it, the net loss (and the rate at which heat is generated) can be reduced.

As is mentioned above, free charge carrier effects can also be used constructively, in order to modulate the light.[18][19][20]

The Raman effect

Silicon exhibits the Raman effect, in which a photon is exchanged for a photon with a slightly different energy, corresponding to an excitation or a relaxation of the material. Silicon's Raman transition is dominated by a single, very narrow frequency peak, which is problematic for broadband phenomena such as Raman amplification, but is beneficial for narrowband devices such as Raman lasers.[8] Consequently, all-silicon Raman lasers have been fabricated.[17]

Solitons

The evolution of light through silicon waveguides can be approximated with a cubic Nonlinear Schrödinger equation,[8] which is notable for admitting sech-like soliton solutions.[50] These optical solitons (which are also known in optical fiber) result from a balance between self phase modulation (which causes the leading edge of the pulse to be redshifted and the trailing edge blueshifted) and anomalous group velocity dispersion.[36] Such solitons have been observed in silicon waveguides, by groups at the universities of Columbia,[10] Rochester,[11] and Bath.[12]



References

  1. ↑ a b c d Michal Lipson: Guiding, Modulating, and Emitting Light on Silicon -- Challenges and Opportunities. In: Journal of Lightwave Technology. 23. Jahrgang, 2005, S. 4222–4238.
  2. ↑ B Jalali and S Fathpour: Silicon photonics. In: Journal of Lightwave Technology. 24. Jahrgang, 2006, S. 4600–4615.
  3. ↑ a b Vilson R. Almeida, Carlos A. Barrios, Roberto R. Panepucci and Michal Lipson: All-optical control of light on a silicon chip. In: Nature. 432. Jahrgang, 2004, S. 1081–1084.
  4. ↑ a b c d Lorenzo Pavesi and David J. Lockwood: Silicon photonics. Springer, 2004, ISBN 3-540-21022-9.
  5. ↑ a b c Graham T. Reed and Andrew P. Knight: Silicon photonics: an introduction. John Wiley and Sons, 2004, ISBN 0-470-87034-6.
  6. ↑ a b Silicon Integrated Nanophotonics. IBM Research, abgerufen am 14. Juli 2009.
  7. ↑ a b Silicon Photonics. Intel, abgerufen am 14. Juli 2009.
  8. ↑ a b c d e f g h i R. Dekker, N. Usechak, M. Först and A. Driessen: Ultrafast nonlinear all-optical processes in silicon-on-insulator waveguides. In: Journal of Physics D. 40. Jahrgang, 2008, S. R249-R271.
  9. ↑ Paul N. Butcher and David Cotter: The elements of nonlinear optics. Cambridge University Press, 1991, ISBN 0-521-42424-0.
  10. ↑ a b c I-Wei Hsieh, Xiaogang Chen, Jerry I. Dadap, Nicolae C. Panoiu and Richard M. Osgood,: Ultrafast-pulse self-phase modulation and third-order dispersion in Si photonic wire-waveguides. In: Optics Express. 14. Jahrgang, 2006, S. 12380–12387.
  11. ↑ a b Jidong Zhang, Qiang Lin, Giovanni Piredda, Robert W. Boyd, Govind P. Agrawal and Philippe M. Fauchet: Optical solitons in a silicon waveguide. In: Optics Express. 15. Jahrgang, 2007, S. 7682–7688.
  12. ↑ a b W. Ding, C. Benton, A. V. Gorbach, W. J. Wadsworth, J.C. Knight, D. V. Skryabin, M. Gnan, M. Sorrel and R. M. De-La-Rue: Solitons and spectral broadening in long silicon-on- insulator photonic wires. In: Optics Express. 16. Jahrgang, 2008, S. 3310–3319.
  13. ↑ J. D. Meindl: Beyond Moore's Law: the interconnect era. In: Computing in Science & Engineering. 5. Jahrgang, 2003, S. 20–24.
  14. ↑ T. Barwicz, H. Byun, F. Gan, C. W. Holzwarth, M. A. Popovic, P. T. Rakich, M. R. Watts, E. P. Ippen, F. X. Kärtner, H. I. Smith, J. S. Orcutt, R. J. Ram, V. Stojanovic, O. O. Olubuyide, J. L. Hoyt, S. Spector, M. Geis, M. Grein, T. Lyszczarz and J. U. Yoon: Silicon photonics for compact, energy-efficient interconnects. In: Journal of Optical Networking. 6. Jahrgang, 2006, S. 63–73.
  15. ↑ Demonstration of an Electronic Photonic Integrated Circuit in a Commercial Scaled Bulk CMOS Process. Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies. 2008.
  16. ↑ Hybrid Silicon Laser - Intel Platform Research. Intel, abgerufen am 14. Juli 2009.
  17. ↑ a b Haisheng Rong, Ansheng Liu, Richard Jones, Oded Cohen, Dani Hak, Remus Nicolaescu, Alexander Fang and Mario Paniccia: An all-silicon Raman laser. In: Nature. 433. Jahrgang, Nr. 7023, 2005, S. 292–294, doi:10.1038/nature03273, PMID 15635371.
  18. ↑ a b C. Angulo Barrios, V. R. Almeida, R. Panepucci and M. Lipson: Electrooptic Modulation of Silicon-on-Insulator Submicrometer-Size Waveguide Devices. In: Journal of Lightwave Technology. 21. Jahrgang, 2003, S. 2332–2339.
  19. ↑ a b Sasikanth Manipatruni, Qianfan Xu, Brad Schmidt, Jagat Shakya and Michal Lipson: High Speed Carrier Injection 18 Gb/s Silicon Micro-ring Electro-optic Modulator. In: [in Proceedings of Lasers and Electro-Optics Society (IEEE, 2007)]. 2007, S. 537–538.
  20. ↑ a b High-speed optical modulation based on carrier depletion in a silicon waveguide. In: Optics Express. 15. Jahrgang, 2007, 660–668
  21. ↑ Sasikanth Manipatruni; Long Chen; Lipson, Michal;: 50 Gbit/s wavelength division multiplexing using silicon microring modulators. In: [Group IV Photonics, 2009. GFP '09. 6th IEEE International Conference on ]. 2009, S. 244 - 246, doi:10.1109/GROUP4.2009.5338375].
  22. ↑ Long Chen, Kyle Preston, Sasikanth Manipatruni, and Michal Lipson,: Integrated GHz silicon photonic interconnect with micrometer-scale modulators and detectors. In: Opt. Express. 17. Jahrgang, 2009, S. 15248–15256..
  23. ↑ Vorlage:Cite article
  24. ↑ Laurent Vivien, Mathieu Rouvière, Jean-Marc Fédéli, Delphine Marris-Morini, Jean François Damlencourt, Juliette Mangeney, Paul Crozat, Loubna El Melhaoui, Eric Cassan, Xavier Le Roux, Daniel Pascal and Suzanne Laval: High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide. In: Optics Express. 15. Jahrgang, 2007, S. 9843–9848.
  25. ↑ Yimin Kang, Han-Din Liu, Mike Morse, Mario J. Paniccia, Moshe Zadka, Stas Litski, Gadi Sarid, Alexandre Pauchard, Ying-Hao Kuo, Hui-Wen Chen, Wissem Sfar Zaoui, John E. Bowers, Andreas Beling, Dion C. McIntosh, Xiaoguang Zheng and Joe C. Campbell: Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product. In: Nature Photonics. 3. Jahrgang, 2008, S. 59–63.
  26. ↑ Vorlage:Cite article
  27. ↑ B. Analui, D. Guckenberger, D. Kucharski and A. Narasimha: A Fully Integrated 20-Gb/s Optoelectronic Transceiver Implemented in a Standard 0.13- μm CMOS SOI Technology. In: IEEE Journal of Solid-State Circuits. 41. Jahrgang, 2006, S. 2945–2955.
  28. ↑ Özdal Boyraz, Prakash Koonath, Varun Raghunathan and Bahram Jalali: All optical switching and continuum generation in silicon waveguides. In: Optics Express. 12. Jahrgang, 2004, S. 4094–4102.
  29. ↑ Y. Vlasov, W. M. J. Green and F. Xia: High-throughput silicon nanophotonic wavelength-insensitive switch for on-chip optical networks. In: Nature Photonics. 2. Jahrgang, 2008, S. 242–246.
  30. ↑ a b Mark A. Foster, Amy C. Turner, Reza Salem, Michal Lipson and Alexander L. Gaeta: Broad-band continuous-wave parametric wavelength conversion in silicon nanowaveguides. In: Optics Express. 15. Jahrgang, 2007, S. 12949–12958.
  31. ↑ W. A. Zortman, A. L. Lentine, M. R. Watts, and D. C. Trotter: Power penalty measurement and frequency chirp extraction in silicon microdisk resonator modulators. In: Proc. Optical Fiber Communication Conference (OFC). OMI7, 2010.
  32. ↑ A. Biberman, S. Manipatruni, N. Ophir, L.Chen, M.Lipson, K.Bergman: First demonstration of long-haul transmission using silicon microring modulators. In: Opt. Express. 18. Jahrgang, Nr. 15, 2010, S. 15544–15552, doi:10.1364/OE.18.015544 (opticsinfobase.org).
  33. ↑ a b Silicon (Si). University of Reading Infrared Multilayer Laboratory, abgerufen am 17. Juli 2009.
  34. ↑ L. H. Yin, Q. Lin, and G. P. Agrawal.: Dispersion tailoring and soliton propagation in silicon waveguides. In: Optics Letters. 31. Jahrgang, 2006, S. 1295–1297.
  35. ↑ Amy C. Turner, Christina Manolatou, Bradley S. Schmidt, Michal Lipson, Mark A. Foster, Jay E. Sharping and Alexander L. Gaeta: Tailored anomalous group-velocity dispersion in silicon channel waveguides. In: Optics Express. 14. Jahrgang, 2006, S. 4357–4362.
  36. ↑ a b c Govind P. Agrawal: Nonlinear fiber optics. 2nd Auflage. Academic Press, 1995, ISBN 0-12-045142-5.
  37. ↑ I. H. Malitson: Interspecimen Comparison of the Refractive Index of Fused Silica. In: Journal of the Optical Society of America. 55. Jahrgang, 1965, S. 1205–1209.
  38. ↑ G. K. Celler and S. Cristoloveanu: Frontiers of silicon-on-insulator. In: Journal of Applied Physics. 93. Jahrgang, 2003, S. 4955.
  39. ↑ a b C. Koos and L. Jacome and C. Poulton and J. Leuthold and W. Freude: Nonlinear silicon-on-insulator waveguides for all-optical signal processing. In: Optics Express. 15. Jahrgang, 2007, S. 5976–5990.
  40. ↑ Mark A. Foster, Amy C. Turner, Jay E. Sharping, Bradley S. Schmidt, Michael Lipson and Alexander L. Gaeta: Broad-band optical parametric gain on a silicon photonic chip. In: Nature. 441. Jahrgang, 2006, S. 04932.
  41. ↑ Nicolae C. Panoiu, Xiaogang Chen and Richard M. Osgood, Jr.: Modulation instability in silicon photonic nanowires. In: Optics Letters. 31. Jahrgang, 2006, S. 3609.
  42. ↑ Lianghong Yin and Govind Agrawal: Impact of two-photon absorption on self-phase modulation in silicon waveguides: Free-carrier effects. In: Optics Letters. 32. Jahrgang, 2006, S. 2031–2033.
  43. ↑ Vorlage:Cite article
  44. ↑ Alan D. Bristow, Nir Rotenberg and Henry M. van Driel: Two-photon absorption and Kerr coefficients of silicon for 850– 2200 nm. In: Applied Physics Letters. 90. Jahrgang, 2007, S. 191104.
  45. ↑ a b Energy Harvesting in Silicon Raman Amplifiers. 3rd IEEE International Conference on Group IV Photonics. 2006.
  46. ↑ R. A. Soref and B. R. Bennett: Electrooptical Effects in Silicon. In: IEEE Journal of Quantum Electronics. 23. Jahrgang, 1987, S. 123–129.
  47. ↑ Y. Liu and H. K. Tsang: Nonlinear absorption and Raman gain in helium-ion-implanted silicon waveguides. In: Optics Letters. 31. Jahrgang, 2006, S. 1714–1716.
  48. ↑ D. Dimitropoulos, R. Jhaveri, R. Claps, J.C.S Woo and B. Jalali: Lifetime of photogenerated carriers in silicon-on-insulator rib waveguides. In: Applied Physics Letters. 86. Jahrgang, 2005, S. 071115.
  49. ↑ Richard Jones, Haisheng Rong, Ansheng Liu, Alexander W. Fang and Mario J. Paniccia: Net continuous wave optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering. In: Optics Express. 13. Jahrgang, 2005, S. 519–525.
  50. ↑ P. G. Drazin and R. S. Johnson: Solitons: an introduction. Cambridge University Press, 1989, ISBN 0-521-33655-4.
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