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Dense Integration of Novel Optical Functionalities Using Photonic Crystals

Dense Integration of Novel Optical Functionalities Using Photonic Crystals. B. Momeni, A. Jafarpour, C. Reinke, J. Hunag, M. Askari, M. Soltani, S. Mohammadi, and A. Adibi Center for Advanced Processing-tools for Electromagnetic/acoustic Xtals (APEX)

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Dense Integration of Novel Optical Functionalities Using Photonic Crystals

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  1. Dense Integration of Novel Optical Functionalities Using Photonic Crystals B. Momeni, A. Jafarpour, C. Reinke, J. Hunag, M. Askari, M. Soltani, S. Mohammadi, and A. Adibi Center for Advanced Processing-tools for Electromagnetic/acoustic Xtals (APEX) School of Electrical and Computer Engineering Georgia Institute of Technology

  2. Outline • Introduction: Applications of Photonic Crystals • Photonic Crystal Structures using Dispersion Engineering • Optimal Waveguides • Optimal Wavelength Demultiplexers • On-Chip Integrable PC Spectrometers • Conclusions

  3. Introduction to Photonic Crystals • Photonic Crystals: Periodic dielectric structures • Photonic Bandgap (PBG): Frequency range with no electromagnetic mode allowed

  4. Photonic Crystal Devices Using Photonic Bandgap Cavity Bends Waveguide Discrete Functionalities: Filters, lasers, guides, delay lines, couplers, combiners One key aspect of this research is the integration of these functionalities into one single substrate.

  5. 6 0.22 0.2 4 0.18 0.16 2 0.14 kya w a/2pc 0 0.12 0.1 -2 0.08 -4 0.06 0.04 -6 -6 -4 -2 0 2 4 6 kxa Photonic Crystal Devices Using Anomalous Dispersion Band Structure Diffraction Control Demultiplexers n1 Functionalities: Wavelength MUX/DEMUX, Pulse Shaping, Frequency to Space mapping, Time to Space (Spectroscopy), Time to frequency mapping (Chirping, Coding)

  6. 2r’ More Complex Functionalities Waveguide Couplers Delay Lines

  7. Distance Nonlinear PC Structures • Selective infiltration of PC holes with nonlinear polymers • Functionalities: Tunable structures (lasers, cavities, filters, delay lines), switching, modulation

  8. Advantages of Photonic Crystals • Photonic bandgap • Dispersion control through geometry • Nonlinearity independent of dispersion • Anomalous dispersion (superprism effect) • Devices can be made by adding defects • Compatibility with electronics substrates

  9. Integrated Photonic Crystal Structure • Other Possible Applications: Ultra-compact optical packet switching, Compact transmitters and receivers for secure communications, Adaptive filters, Optical sensing, Lab-on-a-chip, ...

  10. Mode Dispersion (TM) Frequency, ωa/(2πc) Wavevector, ka/π Dispersion Engineering in Photonic Crystal Waveguides (PCWs) • Conventional PCWs: One row of air holes is removed. • The waveguide has two guided modes in the bandgap. • Single-mode PCWs are essential for practical applications.

  11. a : Lattice Constant r : Radius r’: Modified Radius Design of Single-Mode PCWs • By increasing the size of the air holes next to the guiding region, the odd mode can be pushed out of the PBG. • The guiding bandwidth is limited due to mode flattening. Frequency, ωa/(2πc) Normalized Phase Shift, ka

  12. Design of Biperiodic PCWs • Mode flattening is cause by distributed Bragg reflection (DBR) due to the periodicity in the guiding direction. • Idea:Change the period of the air holes next to the guiding region to modify the DBR frequency.

  13. 0.32 0.31 Frequency, a/λ 1.0 0.30 0.8 0.29 0.28 a’/a=0.7 Transmission 0.6 Increased Group Velocity 2a’ a’/a=0.93 0.27 0.4 No Modegap 0.26 0.2 1.2 1.6 2.0 1.4 1.8 a’/a=1.0 Phase Constant, ka/π 0.28 0.30 0.32 0.26 a Frequency, a/λ Optimization of Guiding Bandwidth in Biperiodic PCWs • Pushing the DBR peak frequency upward [1] • Guiding over the full PBG for a’ < 0.7a • Similar results by increasing a’, guiding over PBG for a’>1.25 a [1] A. Jafarpour et al., Physical Review B, vol. 68, p. 233102 (2003)

  14. L a 2r a’ 2r’ Transmission Properties of the Bi-periodic PCW • Loss for a’/a=0.7, r/a=0.3, r’/a=0.25 is as low as 3 dB/mm over a bandwidth of 60 nm. • Loss of a conventional PCW on the same substrate is 66 dB/mm. [1] A. Jafarpour et al., Applied Physics B, 79, 409, 2004.

  15. 0.24 y 0.22 Superprism effect 4 Negative refraction 0.20 0.18 3 x Self-guiding 0.16 kya 2 0.14 Negative effective index 1 0.12 0.10 0 -4 -3 -2 -1 0 1 2 3 4 0.08 kxa 0.06 0.04 Superprism-Based Photonic Crystal Demultiplexers • Anomalous dispersion effects of PCs outside the bandgap • Goal: Engineering PC dispersion for optimum demultiplexing performance

  16. Cross-talk between adjacent channels 0 PC 0.9 Dqg -20 1.0 1.1 -40 Cross-talk (dB) l 1.5 1 -60 d Dqg/d= 2.0 -80 0 0.5 1 1.5 2 2.5 3 Propagation length (normalized to z0) Conventional Superprism-Based PC Demultiplexers • Collimated input beam at optimal incidence angle • Due to beam diffraction in side PC, device size is large and varies as N4 with N being the number of channels.

  17. 0.24 0.22 0.20 Negative effective index Low 3rd-order diffraction 0.18 4 0.16 3 0.14 kya Strong superprism effect 2 0.12 0.10 1 0.08 0 -4 -3 -2 -1 0 1 2 3 4 0.06 kxa 0.04 l1 l2 Preconditioned PC Demultiplexers • Diffraction compensationand superprism effect inside PC • Using the model for higher-order effective indices, device size varies as N2.5 with N being the number of channels.

  18. l1 l2 l2 l1 Working in Negative Refraction Regime • To eliminate unwanted contributions from stray signals (unwanted polarization or wavelengths not in the operation range)

  19. Fabrication of the PC Demultiplexer on SOI • 70nm SiO2 hard-mask; 220nm Si; 3μm SiO2; on Si substrate • 45°-rotated square lattice PC (length: only 100 μm) • A series of output waveguides for high resolution detection • Integrated version of a geometrical optical setup

  20. Preconditioning region Output waveguides Input waveguides Beam blocks 100mm Fabricated Structure Three unique effects combined • Negative diffraction (focusing) • Superprism effect • Negative refraction

  21. Measurement Setup • Free space end-coupling • Lock-in measurement

  22. Ch#1 Ch#2 Ch#3 Ch#4 Ch#5 1591 nm 1580 nm 1568 nm 1557 nm 1545 nm Measurement Results • Imaging the output waveguides on the camera • 5-channel demultiplexer with >6.5dB isolation and 10 nm spacing.

  23. WG# 24 12 4 1 1540.5nm 1548.0nm 1557.6nm 1565.5nm Spatial Isolation of Unwanted Polarization • Output power distribution: TE-like TM-like • Focusing of desired channels is visible.

  24. 1 0 0.9 -4 0.8 0.7 -8 0.6 Normalized Channel Response Normalized Transmission (dB) 0.5 -12 0.4 0.3 -16 0.2 0.1 -20 0 1520 1540 1560 1580 1470 1480 1490 1500 1510 1520 1530 1540 1550 1560 1570 1580 4-channel demultiplexer, 6.5dB isolation, 8nm resolution Wavelength (nm) Wavelength (nm) Measurement Results • Transmitted power measured at output waveguides

  25. Normalized channel response 1500 1520 1540 1560 1580 1600 1620 1640 Operation band changes by the choice of incident angle Wavelength (nm) Multiband Operation: Incidence at Slightly Different Angles a=13° a=15° a=17°

  26. Demultiplexing performance at two-orders of magnitude smaller size Comparison With Other Reported Implementations

  27. Controlling the Dispersion of Optical Materials • New design possibilities with controlled dispersion properties B. Momeni and A. Adibi, “Demultiplexers harness photonic-crystal dispersion properties,” Laser Focus World, vol. 42, no. 6, pp. 125-128, June 2006

  28. M detectors Input light N channels Optical device Integrated Spectrometers • Basic configuration • Mapping from spectrum to space • Post-processing to extract the spectrum • Requirements for an efficient implementation • Strong dispersive properties • Isolation of stray light

  29. Superprism spectrometer Environmental changes Correlator Input light Locating Spectral Features • Along with frequency selective optical components • Spectral-domain sensing • Correlation of the output spatial distribution for spectral pattern recognition

  30. 45 40 Std dev. = 0.3 nm 35 30 25 Number of events 20 15 10 1560 5 Estimated wavelength 0 Input peak wavelength 1555 -1.5 -1 -0.5 0 0.5 1 1.5 Estimation error (nm) 1550 1545 Wavelength (nm) 1540 1535 1530 1530 1535 1540 1545 1550 1555 1560 Wavelength (nm) Locating Spectral Features • Correlation of detected power levels at the output by calibration data is used to find the location of the peak • Estimation error occurs in presence of detection noise 30 nm operation bandwidth

  31. Applications of Ultra-compact Wavelength Demultiplexers • Chip-scale WDM • Spectroscopy (spatial-spectral mapping) • Sensing: Wavelength separation properties are highly affected by the material inside the air holes • Lab-on-a-chip and integrated photonics circuits

  32. Conclusions • Photonic Crystals are excellent candidates for photonics integrated circuits (for communications, information processing, spectroscopy, sensing, …) due to the possibility of dispersion engineering using geometry. • Ultra-low loss wideband guiding and compact demultiplexing with focusing are possible by combining some of the unique dispersion properties of the photonic crystals. • The possibility of designing electromagnetic modes (dispersion, field profile, density of states,…) is a powerful advantage of PCs, yet not highly utilized.

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