Optical Filter 武倩倩

Outline Introduction to silicon photonics Athermal tunable silicon optical filter Working principle Fabricated device Experiments Conclusions

Introduction to silicon photonics Silicon photonics is the study and application of photonics systems which use silicon as an optical medium. Optical interconnects Optical routers and signal process Long range telecommunications using silicon photonics Artist’ concept of 3D silicon processor chip with optical IO layer featuring on-chip nanophotonic network. Courtesy IBM.

CMOS-Compatible Athermal Tunable Silicon Optical Lattice Filters Liangjun Lu, Linjie Zhou, Xiaomeng Sun, Jingya Xie, Zhi Zou, Xinwan Li, and Jianping Chen in proceeding of OFC, USA, 2013

Outline Introduction to silicon photonics Athermal tunable silicon optical filter Background Working principle Fabricated device Experiments Conclusions

Background Silicon: large thermo-optic (TO) coefficient of ~ 1.86×10 -4 K -1 around 1.55 μm wavelength Regular silicon devices: ~100 pm/K Approaches: Cons ： Increasing fabrication challenge Not compatible with the CMOS Unreliable at high temperature Requiring a complex system Extra power consumption J. Lee et. al, Opt. Express 16, (2008) C. Qiu et. al, Opt. Express 19, 5143 (2011) Polymer compensation (negative TO coefficient) Active compensation (local heating)

MZI Mach-Zehnder interferometer (MZI) structure Figure 1. The Mach-Zehnder interferometer silicon modulator contains two reverse-biased p-n diode phase shifters (a). The splitters are multimode interference (MMI) couplers. The radio-frequency (RF) signal is coupled to the traveling-wave electrode from the optical input side, and termination load is added to the output side. The cross-sectional view in (b) shows a p- n junction waveguide phase shifter on silicon on insulator. The refractive index modulation is based on the depletion width variation in response to the reverse bias voltage caused by the free-carrier plasma dispersion effect in silicon. The coplanar waveguide electrode was designed to match the electrical and optical velocities. Images reprinted with permission of Optics Express

Lattice filters: cascaded MZIs Application: (de)multiplexing, dispersion compensation Advantages: flexible design, large FSRs Compared with a single MZI: narrower passband, faster passband roll- off B. Guha et. al, Opt. Express 18, 1879 (2010)B. Guha et. al, Opt. Lett. 37, (2012) Lattice filters

Transmission spectra 1-stage MZI4-stage MZI 7-stage MZI10-stage MZI

Outline Introduction to silicon photonics Optical filter Athermal tunable silicon optical filter Background Working principle Fabricated device Experiments Conclusions

Working principle InputBar Cross W 2, L 2 Athermal condition: W 1, L 1 Phase difference: Asymmetric MZI: Temperature-induced phase difference:  filter central wavelength

p-i-p tuning structure Si substrate Oxide Si Oxide P+P+ P+P+ Al 2 μm 0.22 μm Cross-sectional schematic of the p-i-p junction-based micro-heater Thermal-optical tuning: the generated heat directly interacts with the optical mode Advantages: higher tuning efficiency a faster temporal response without adding fabrication complexity ~10 20 cm -3

Outline Introduction to silicon photonics Athermal tunable silicon optical filter Background Working principle Fabricated device Experiments Conclusions

Fabricated device Input Bar Cross W 2 = 500 nm, L 2 = 100 μm W 1 = 350 nm, L 1 = μm gap =250 nm Microscope image of the lattice filter 10 cascaded MZIs standard CMOS fabrication process inverse tapers with a tip width of 180 nm

Outline Introduction to silicon photonics Optical filter Athermal tunable silicon optical filter Background Working principle Fabricated device Experiments Conclusions

Active tuning measurement Power is applied to the top arms Filtering band redshifts Tuning efficiency 0.17 nm/mW Wavelength shift vs. Power

Active tuning measurement Power is applied to the bottom arms Filtering band blueshifts Tuning efficiency nm/mW Wavelength shift vs. Power

Thermal sensitivity without tuning Bar-port Cross-port dλ/dT = pm/ ℃ Regular silicon devices: ~100 pm/ o C)

Outline Introduction to silicon photonics Optical filter Athermal tunable silicon optical filter Background Working principle Fabricated device Experiments Conclusions

CMOS-compatible athermal tunable silicon optical lattice filters were proposed, fabricated and experimentally demonstrated. Active tuning experiments show that the filter central wavelength can be be red-/blue-shifted by 13.1/21.3 nm with a power tuning efficiency of 0.17/0.22 nm/mW. Temperature shift measurements show that the thermal-sensitivity of the filter without active tuning is pm/ ℃, improved by almost two order compared to regular designs. When the filter central wavelength is tuned in between 1535 nm to 1550 nm, the measured thermal-sensitivity varies within 30 pm/ ℃.

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