A tunable Si CMOS photonic multiplexer/de-multiplexer OPTICS EXPRESS Published : 25 Feb 2010 MinJae Jung M.I.C.S
Content 1. Introduction 2. CMOS photonic 1x4 Si ring multiplexer Principle of add/drop filter Design of this device 3. Performance testing and results Optical characterization High speed data transmission performance 4. Conclusion
Introduction Silicon photonics : Implementing high performance chip scale interconnection network with low cost by using silicon waveguides. Cf) On-chip electrical interconnect : multiple layers can be used for signal transport
Introduction Wavelength division multiplexing(wdm) : Effectively reduce the number of interconnect waveguides, and improve the integration density HowdoesWDM work? : Think that the prism separates a beam of light into its colors WDMmodulates multiple data channels into optical signals that have different wavelengths Then multiplexes these signals into a single stream of light (different light wavelengths) At the other end, it de-multiplexs the signals and distributes them to their various channels
Introduction Several approaches for WDM on silicon platform (1) AWG(array waveguide grating) (2) Echelle grating (3) MZI based interleaver (4) Cascaded ring add/drop filters To make compact multiplexer Ring resonator based add/drop filters using high index contrast silicon waveguide High order ring resonators with multiple coupled rings improve the passband and channel isolation To make multi-channel multiplexer Multiple add/drop filters were cascaded using rings slightly different in size
Introduction Critical hurdle for ring resonator based WDM filter Center wavelength accuracy Channel spacing for multi-channel Effective index of Si waveguide varies Manufacturing tolerances (silicon layer thickness, waveguide width and etch depth variation etc..) Ambient temperature change Effective index variation : cause significant wavelength shift for ring resonator based WDM filters This paper demonstrate a 4-channel CMOS WDM multiplexer/demultiplexer using cascaded identical single ring resonators with integrated thermal tuner
CMOS photonic 1x4 Si ring multiplexer Basic principles of Si micro-ring resonator
CMOS photonic 1x4 Si ring multiplexer Ring resonator coupled with two bus waveguides Input port Through port Drop port R=radius of the ring,, =coupling coefficient, a=amplitude loss per pass at the ring coupling propagation constant, k= wave number), =effective refractive index amplitude attenuation, round trip loss, G= For high speed data transmission To have low-loss and wide pass-band add/drop filter Controlled by Waveguide loss, negligible coupler loss, waveguides-to-ring coupling coefficient
CMOS photonic 1x4 Si ring multiplexer Various coupling coefficient (a=0.999, =10dB/cm) Higher coupling ratio Lower power loss and wider pass-band but lower channel isolation Higher ring waveguide loss Doesn t affect the pass-band much (Loaded Q of the device is dominated by the coupling ratio)
CMOS photonic 1x4 Si ring multiplexer Various coupling coefficient To design the drop filter correctly Set the coupling from bus waveguide to different size rings For small ring with 10 in radius Coupling ratio could be adjusted by varying the gap (Shown above the Fig)
CMOS photonic 1x4 Si ring multiplexer Fabricated 1x4 multiplexer/de-multiplexer (By cascading 4 ring add/drop filters : k=0.15, gap=325nm) Choose ring radius of 12 to have big enough FSR To accommodate 4 channels at 1.6mm spacing The pass-band is determined by the loaded Q Instead of using rings slightly different in size to achieve different center wavelength Use identical rings with integrated thermal tuning Injecting current to the doped resistors through the tuning pads Heat up the ring waveguide and change the index of the waveguides
Performance testing and results Optical characterization (Single add/drop filter performance) Drop port Input port Through port FSR=8.2nm, 3dB pass band( )=0.4nm The filter insertion loss can be calculated from the following equation From above equation ( : fiber to grating coupler coupling loss, λ : center wavelength), 0.84dB insertion loss
Performance testing and results Optical characterization (4 cascaded add/drop filter performance) 4 rings are designed to be identical Need to tune the filters to align with the selected wavelength channels Heat up the ring and move the filter anywhere within the FSR Fig(right side) shows the center wavelength shift vs. tuning power for all four channels. (Linear tuning response with efficiency of about 90pm/mW)
Performance testing and results Optical characterization (4 cascaded add/drop filter performance) The measured spectrum of the 4-channel multiplexer/de-multiplexer The center wavelengths of the 4-channels are 1554.13nm, 1555.75nm, 1557.36nm, 1558.98nm 3dB pass-band larger than 0.4nm, less than 1dB insertion loss, and better than 16dB channel isolation
Performance testing and results High speed data transmission performance (4 cascaded add/drop filter performance) High speed data transmission through the multiplexer was performed Compared to transmitter/receiver back-to-back measurement, 0.6dB power penalty for bit error rate of Characterize impact of center wavelength misalignment Due to the center wavelength offset negligible power penalty at the receiver Power penalty can be approximated by the signal attenuation
Conclusion A tunable CMOS 1x4 multiplexer/de-multiplexer Use cascaded ring resonator based on add/drop filters with integrated doped-resistor thermal tuner This compact WDM(wavelength division multiplexing) device achieved low insertion loss, good channel isolation and wide enough pass band for high speed data transmission Add/drop filters made of identical rings were aligned with WDM ITU grid wavelengths accurately with 200GHz spacing by using thermal tuning Due to non-ideal filter pass band : 0.6dB power penalty for 10Gbps data transmission Filter s center wavelength offset : attenuates the high speed optical data signal with negligible additional power penalty It can and will play an important role in dense chip scale interconnects
< Paper review > Optical Express (2010) Minjae Jung Minjae3716@yonsei.ac.kr