SILICON PHOTONICS WAVEGUIDE AND ITS FIBER INTERCONNECT TECHNOLOGY. Jeong Hwan Song

Transcription

1 SILICON PHOTONICS WAVEGUIDE AND ITS FIBER INTERCONNECT TECHNOLOGY Jeong Hwan Song

2 CONTENTS Introduction of light waveguides Principals Types / materials Si photonics Interface design between optical fiber and Si photonics platform Grating coupler Special grating coupler: polarization independent grating coupler Summary

3 PRINCIPAL OF LIGHT WAVEGUIDE Total internal reflection NA=sinθ core n1 θ 1 cladding n2 From Google image 2 Critical angle 3 waveguide N2 <n1

4 TYPES OF WAVEGUIDES: FIBERS Fiber perform Fiber drawing tower Single mode fibers Multimode fibers Photonic crystal fibers Plastic fibers (polymer) Polarization maintain fiber Dispersion shifted fiber Multicore fiber, etc.

5 TYPES OF WAVEGUIDES: PLANAR WAVEGUIDES Materials Semiconductor Waveguides: GaAs, InP, etc. Electro-Optic Waveguides: mostly LiNbO3. Glass Waveguides: silica (SiO2), SiON. Silicon nitride waveguides Silicon-on-Insulator Technology Polymers Waveguides: Several organic polymers Channel waveguide: Small bending radius Mode converter is necessary for fiber coupling. Rib waveguide: Large width and thickness for single mode operation Large bending radius Si / SiN / SiON waveguides: PECVD SiO2 waveguides: PECVD / FHD

6 SINGLE MODE PLANAR WAVEGUIDE INFORMATION FOR 1.55µm SiO 2 waveguide SiON waveguide SiN Si Core index (cladding index ~1.45) Core size for SM SiO 2 SiON SiN Si (Δ=0.75%) 6.0x x x2.5* 1.0x x0.22 Loss (db/cm) Bending radius (µm) Coupling loss with flat SM (db/point) <100 <5 <0.5 ~8 (5*) ~13.5 ~18 1µm Fiber (Corning SM 28)

7 SILICON PHOTONICS High index contrast: index core (Si) 3.5 and index clad (SiO2) 1.45 Extremely low bending loss R<5um ( dB/90, R=3um, channel), Rib bending loss~0.0035db/90 when R=10um Small foot print CMOS compatiable Typical propagation loss: ~2dB/cm (channel), < 1.5dB/cm Transparent only over 1.1um (high absorption loss for near infrared & visible lights) High birefringence: polarization issue Single mode waveguide size ~500nm (w) x 220nm (h) and 2um BOX (channel) ~500nm (w) x 340nm (h) with 150nm slab-height and 2um BOX Fiber interconnection: special interface design required High mode size mismatch loss Mode size converters: Grating coupler, inverse taper (edge coupling) Thick waveguide platform available e.g. 1.5um & 3um SiO2 Under clad buried oxide layer (BOX) Silicon substrate Channel waveguide SiO2 Under clad buried oxide layer (BOX) Silicon substrate Rib waveguide

8 FIBER INTERCONNECTION: GRATING COUPLER Efficiency determined by directionality (D=Pup/Pup+Pdown) and mode matching between diffracted field profile and fiber mode Maximize directionality: optimization of etch depth, BOX, Si-overlay, WG thickness, and bottom mirror Minimize mode mismatching: apodization Typical SOI based grating coupler structure and E-field plot of the grating coupler

9 (TE)

10 POLARIZATION INDEPENDENT GRATING COUPLER DOI: /OL SONG ET AL. OPT. LETTERS 2015 SEP. Problem description Si photonics waveguide geometry: optimized for single TE mode. Therefore, the grating coupler also designed for TE mode only. The gap between effective index of TE and TM modes is significantly large such that a common grating period is nonexistent within 220-nm waveguide thickness (see the figure). Si waveguide effective index for TE & TM (0.5um X 0.22um) Grating coupler effective index for TE & TM Mean of 12um x 0.22um & 12um x 0. 15um (see inset of figure)

11 POLARIZATION INDEPENDENT GRATING COUPLER Problem description continued (a) Grating coupler designed for TE mode (b) Grating coupler designed for TM mode

12 POLARIZATION INDEPENDENT GRATING COUPLER Prior art 2D grating couplers Fabrication limit Simulation time increases... Approach 1D grating coupler composed by intersection & union of TE and TM grating coupler

13 POLARIZATION INDEPENDENT GRATING COUPLER Parameter optimization and FDTD simulation Grating pitch optimization a) intersection b)union Electric field plots: Intersection (a & b) union (c & d)

14 POLARIZATION INDEPENDENT GRATING COUPLER Parameter optimization and FDTD simulation

15 POLARIZATION INDEPENDENT GRATING COUPLER Fabrication & test result: only intersection grating coupler A fabricated intersection grating coupler has the PDL of less than 0.8 db within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is 18%.

16 POLARIZATION INDEPENDENT GRATING COUPLER Conclusion Novel non-uniform grating coupler structures for polarization-independent fiber coupling have been proposed, optimally designed, and evaluated. A fabricated intersection grating coupler has the PDL of less than 0.8 db within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is 18%. The polarization-independent grating couplers will be a good practical solution for SOI technology.

17 SUMMARY q Review light waveguide technology q Introduce Si photonics q Introduce grating coupler technology for fiber interconnection of Si photonics q A special grating coupler is showed: polarization independent grating coupler