Min-Hyeong Kim High-Speed Circuits and Systems Laboratory E.E. Engineering at YONSEI UNIVERITY JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 9, SEPTEMBER 2004

[ Contents ] 1.Abstract 2.Structure 3.Optical Devices 4.Electronic Devices 5.Measurement Results 6.Conclusion 2

1. Abstract 3 A parallel-optical interconnect with 12 channels operating at 8.5 Gb/s giving an aggregate data rate of 102 Gb/s is demonstrated. A BER<10^-13 was measured on a single channel after transmission through 100m of multimode fiber at a data rate of 8.5 Gb/s with all 12 channels operating simultaneously. How to demonstrate this structure? [ Optoelectronics(OEs) + Integrated Circuit(IC) of TXRX ]  POSH(Parallel Optics for Super-Highways) package design  Flip-Chip Bonding, Link Budget.  990nm Bottom-Emitting VCSELs / PIN PhotoDetector with Laser Driver / differential TIA

2. Structure 4  POSH(Parallel Optics for Super-Highways) package design RX module illustration [ CME ] (Chip-Mounted Enclosure) : OE + IC block by flip-chip bonding and Wall. [ BGA interface ] (ball-grid-array) : Provides an electrical turn while maintaining a low-loss, high-bandwidth 50Ω signal transmission path. [ AlN stiffener ] : For a high thermal conductivity path to the heat sink isolating the CME block electrically.

2. Structure 5  Flip-Chip Bonding 1.Integrated circuits which pads are metalized are created on the wafer. 2. Solder dots are deposited on each of the pads. 3.Chips are flipped and positioned so that the solder balls are facing the connectors on the external circuitry. 4.Solder balls are then remelted. (typically using hot air reflow) 5.Mounted chip is “underfilled” using an electrically-insulating adhesive  The loop inductance and capacitance are reduced by this bonding.  This will result in performance advantages including larger bandwidth, less overshoot and ringing, and lower EMI.

2. Structure 6  Link Budget Link Budget? A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. Ex) Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

7 3. Optical Devices  [LASER] 990nm Bottom-Emitting VCSELs 850nm → 990nm : Compatible with flip-chip bonding 3.4dB link budget advantage  P cladding : 40 pairs of carbon-doped p-type top AlGaAs DBRs  Active : one-wave cavity with multiple InGaAs quantum wells  N cladding : 27 pairs of Si-doped n-type bottom AlGaAs DBRs  Lapped, Polished, and antireflection (AR) coated. ** DBRs : distributed-Bragg reflectors for VCSELs mirror. 1) lower operating voltage; 2) higher differential gain; 3) lower threshold current; 4) higher speed; 5) improved reliability; 6) lower photon energy; 7) lower thermal resistance.

8 3. Optical Devices  [LASER] 990nm Bottom-Emitting VCSELs Test input : NRZ PRBS 2^7-1 Extinction Ratio(ER) : 6dB  The average threshold current : 434uA  The average threshold voltage : 1.34V  Estimated bond-pad capacitance to be only 35 fF.  The device was biased at an average current of 3 mA and 1.5 mW of optical output power.

9 3. Optical Devices  [PD] PIN Photo Detector P : 0.2um n- InGaAs I : 1.8um InGaAs as the absorber layer N : 1um n+ Si-doped InP  2x12 arrays for the differential input of the RX IC front end  Approximately 180fF capacitance to open 10Gb/s eye diagram.  For a conventional 50Ω RX interface, the RX BW would be about 18GHz. It is enough.

10 3. Optical Devices  MicroOptics  A diffractive vortex element is used to minimize back reflection into the VCSEL ( 0.01%) and to create a restricted-mode launch into the MMF, increasing the bandwidth–distance of the MMF.  A high-efficiency refractive surface is designed to achieve a throughput efficiency of 75%.  The RX geometry is similar to the TX geometry, with the diffractive element closest to the fiber replaced with an another refractive lens. 50MMF [ TX optical system illustration ]

11 4. Electronic Devices  Transmitter & Receiver  Both TX and RX circuits have been designed for a conventional 50Ω electrical interface.  The TX driver and RX input power supplies are separated for low crosstalk penalties.  The transition frequency of 45GHz [ TX ]  the output connected directly to the n-side of the VCSEL.  The 12-channel TX IC dissipates 1.5–2 W.

12 4. Electronic Devices  Transmitter & Receiver [ RX ]  Minimizing jitter requires isolating the channels as well as possible, managing ground planes, and maximizing bandwidth.  The recommended transition frequency is > 75GHz, where performance and power dissipation would both be improved.  A dummy photodiode is connected to the dark side of the input amplifier so that impedances are balanced.

13 5. Measurement Results 1) POSH TX module  Test input : NRZ PRBS 2^7-1  Extinction Ratio : 6.3dB  Average Power : -3.10dBm ~ -4.67dBm  Rise/Fall time : 36/84ps, respectively

14 5. Measurement Results 2) POSH RX module  Test input : NRZ PRBS 2^7-1  Extinction Ratio : 6dB  Channel : through 100m of standard 50-um-core MMF ribbon [ POSH RX eye diagrams at 8.5 Gb/s/ch ] [ One sample of RX eye diagrams at 10Gb/s/ch ] Eye opening Every Channel.Not yet every channel……

15 5. Measurement Results 2) POSH RX module [ Single channel BER comparison between 8.5Gb/s and 10Gb/s ] [ BER comparison between single and all channel operations ] at 8.5Gb/s For 10^-12dB BER, PD sensitivities were  -15.3dBm at 8.5Gb/s  -12.4dBm at 10Gb/s The RX channel crosstalk penalty is about 1.3dBm

6. Conclusion I.To improve more (exactly say, to get an all eye-opening at 10Gb/s), Design a new version of the RX IC in a Si-Ge BiCMOS process with an f_t of 75GHz. II.Demonstrate 12-channel TX parallel-optic modules operating at 10 Gb/s/ch and RX parallel-optic modules operating at 8.5–10 Gb/s/ch. III.At 8.5Gb/s, BER<10^-13 was measured on a single channel with all 12 channels operating simultaneously.\ IV.So, a parallel-optical interconnect with an aggregate data rate of 102 Gb/s (8.5*12) was demonstrated. 16