Design of an Energy-Efficient Silicon Microring Resonator-Based Photonic Transmitter Cheng Li, Chin-Hui Chen, Binhao Wang, Samuel Palermo, Marco Fiorentino, Raymond Beausoleil HP Laboratories HPL-2014-21 Keyword(s): Silicon ring resonator; optical interconnects; pre-emphasis Abstract: Silicon microring resonator-based photonic interconnects offer an attractive substitute to conventional electrical interconnects due to the negligible frequency-dependent channel loss and high bandwidth density offered via wavelength-division multiplexing (WDM). This paper presents silicon photonic transmitters employing ring modulators designed in a 130 nm SOI process wire-bonded with CMOS drivers in a 1V standard 65nm CMOS technology. The transmitter circuits incorporate high-swing (2Vpp and 4Vpp) drivers with non-linear pre-emphasis to bypass the bandwidth limitation of the carrier-injection silicon ring modulator. The 1st generation silicon ring modulator wire-bonded with 4Vpp CMOS driver achieves 12.7dB extinction ratio at 5Gb/s with 4.04mW power consumption, while the 2nd generation ring modulator wirebonded with 2Vpp CMOS driver achieves 9.2dB extinction ratio at 9Gb/s with 4.32mW. Both of these measurements exclude the laser power. External Posting Date: May 6, 2014 [Fulltext] Internal Posting Date: May 6, 2014 [Fulltext] Approved for External Publication Copyright 2014 Hewlett-Packard Development Company, L.P.
Design of an Energy-Efficient Silicon Microring Resonator-Based Photonic Transmitter Cheng Li, Chin-Hui Chen, Binhao Wang, Samuel Palermo, Marco Fiorentino, and Raymond Beausoleil April 16, 2014 Abstract Silicon microring resonator-based photonic interconnects offer an attractive substitute to conventional electrical interconnects due to the negligible frequency-dependent channel loss and high bandwidth density offered via wavelength-division multiplexing (WDM). This paper presents silicon photonic transmitters employing ring modulators designed in a 130 nm SOI process wire-bonded with CMOS drivers in a 1V standard 65nm CMOS technology. The transmitter circuits incorporate high-swing (2V pp and 4V pp ) drivers with non-linear pre-emphasis to bypass the bandwidth limitation of the carrier-injection silicon ring modulator. The 1 st generation silicon ring modulator wire-bonded with 4V pp CMOS driver achieves 12.7dB extinction ratio at 5Gb/s with 4.04mW power consumption, while the 2 nd generation ring modulator wirebonded with 2V pp CMOS driver achieves 9.2dB extinction ratio at 9Gb/s with 4.32mW. Both of these measurements exclude the laser power. Index Terms Silicon ring resonator, optical interconnects, pre-emphasis. I. INTRODUCTION Optical channels are potential candidates to replace conventional electrical channels for efficient inter/intra-chip interconnects due to their attractive properties: flat channel loss over a wide frequency range and strong immunity to crosstalk and electromagnetic noise. An important feature 1
of optical interconnects is the ability to combine multiple data channels on a single waveguide via wavelength-division-multiplexing (WDM) to greatly improve bandwidth density and amortize connector costs over high aggregate bandwidth. In order to take full advantage of these benefits, silicon photonic platforms are being developed that enable tightly integrated optical interconnects and novel photonic network architectures. One promising photonic device is the silicon microring resonator [4,12], which can be configured either as an optical modulator or a WDM drop filter. Silicon ring resonator modulators/filters offer the advantages of small size, relative to Mach-Zehnder modulators, and increased filter functionality, relative to electro-absorption modulators [7]. Silicon microring resonator-based photonic links provide a unique opportunity to deliver distanceindependent connectivity whose pin-bandwidth scales with the degree of wavelength-division multiplexing. As shown in Fig. 1, multiple wavelengths generated by an off-chip continuous-wave (CW) laser are coupled into a silicon waveguide via a grating coupler. This off-chip laser can either be a distributed feedback (DFB) laser bank [5], which consists of an array of DFB laser diodes, or a comb laser [11], which is able to generate multiple wavelengths simultaneously. Implementing a DFB laser bank for dense WDM (DWDM) photonic interconnects (e.g., using 64 wavelengths) is quite challenging due to area and power budget constraints. A possible alternative is a single broad-spectrum comb laser source, such as an InAs/GaAs quantum dot comb laser that can generate a large number of wavelengths in the 1100nm to 1320nm spectral range with typical channel spacing of 50-100GHz and optical power of 0.2-1mW per channel [11]. A system operating near 1310nm wavelength (O-band) incurs slightly higher optical loss in fiber compared to a 1550nm (C-band) system. However, this has negligible impact in short-reach interconnect applications. At the transmitter side, each ring modulator inserts data onto a specific wavelength through electrooptical modulation. The modulated optical signals propagate through the link optical waveguides and arrive at the receiver side where ring filters drop the modulated optical signals of a specific wavelength at a photodetector (PD) that converts the signals back to the electrical domain. This paper presents silicon ring resonator-based photonic transmitter prototypes that address the limited 2
intrinsic bandwidth of the carrier-injection ring modulator, achieving energy-efficient high-speed optical modulation in a compact silicon area suitable for on-chip WDM interconnects. The silicon ring modulator and its model are introduced in Section II. Section III outlines the architecture of the WDM transmitter circuits prototype. Section IV describes transmitters with independent dual-edge pre-emphasis to compensate for the bandwidth limitations of the carrier-injection ring resonators used in this work. Experimental results of the silicon photonics transmitter prototype with a CMOS pre-emphasis driver fabricated in a 65nm CMOS technology wire-bonded to photonic devices fabricated in a 130 nm SOI process, are presented in Section V. Finally, Section VI concludes the paper. II. SILICON RING MODULATOR MODELING A basic silicon ring modulator consists of a straight waveguide coupled with a circular waveguide with diameters in tens of micron meters, as shown in Fig. 2a. At the resonance wavelength most of the input light is coupled into the circular waveguide and only a small amount of light can be observed at the through port. As a result, the through port spectrum displays a notch-shaped characteristic shown in Fig. 2b. This resonance can be shifted by changing the effective refractive index of the waveguide through the free-carrier plasma dispersion effect [8] to implement the optical modulation. For example, the ring modulator exhibits low optical power level at through port when the resonance aligned well with the laser wavelength, while a high optical power level is displayed when the resonance blue-shifts due to the increase of carrier density in the waveguide lowering the waveguide effective refractive index. Two common implementations of silicon ring resonator modulators include carrier-injection devices [12] with an embedded p-i-n junction sidecoupled with the circulate waveguide, operating primarily in forward-bias, and carrier-depletion devices [4] with only p-n junction side-coupled, operating primarily in reverse-bias. Although a depletion ring generally achieves higher modulation speeds relative to a carrier-injection ring 3
due to the ability to rapidly sweep the carriers out of the junction, its modulation depth is limited due to the relatively low doping concentration in the waveguide to avoid excessive optical loss in the waveguide. In contrast, carrier-injection ring modulators can provide large refractive index changes and high modulation depths, but are limited by relatively slow carrier dynamics of forward-biased p-i-n junction. As shown in Fig. 2b, when applying a forward-bias voltage over the p-i-n junction of the carrier-injection ring, the resonance shifts towards to shorter wavelength, called blue shift, due to the accumulated carriers changing the waveguide refractive index. While a reverse-bias voltage extracts the carriers accumulated in the junction during the forward-bias modulation and restore the waveguide refractive index. The ring modulator bandwidth is limited by the slow carrier-injection operation due to the relatively slow carrier dynamics of forward-biased p-i-n junction [12]. Increasing the optical rising transition by simply applying a high modulation swing leads to a slow optical falling transition and cause the inter-symbol interference (ISI), since the over-injected carriers need longer time to be swept out from the ring waveguide. Although pre-emphasis modulation scheme [12] has been proposed to break the tradeoff between the optical rising and falling transitions, a major ring modulator driver design challenge is that there are no accurate models to predict the high-speed optical modulation signal quality under different pre-emphasis duration and voltage levels. In this work, an accurate SPICE model of silicon ring modulator is developed to enable an efficient co-simulation with electrical driver circuits. It includes a large-signal SPICE p-i-n model [9] to predict the carrier distribution in the intrinsic region of the p-i-n junction switched under the modulation signals, and a ring macro-model to catch the dynamic electro-optic effects between the junction carrier density and the silicon ring waveguide refractive index at wavelength 1300nm described by (1) (n) = 6.2 10 22 N + 5.9 10 18 P 0.8 (1) where N(cm 3 ) and P (cm 3 ) are the silicon waveguide refractive index changes due to the electron and hole concentration changes, respectively. (n) of 1.5 10 3 was found at wavelength 4
of 1300nm with injection of 10 18 carriers/cm 3 [8]. Fig. 3 shows device model simulation results of the 1 st generation carrier-injection silicon ring [1] modulators with positive and negative 200ps pulse responses overlaid. The 5µm diameter ring device exhibits a quality factor of 9000. A simple 2V pp NRZ modulation produces the excessively long optical rise time shown in Fig. 3a, mainly because the 1V forward-bias voltage is not high enough to overcome the slow carrier dynamics in the p-i-n junction. Increasing the modulation swing to 4V pp dramatically improves the optical rise time at the expense of high-level ringing and a high steady-state charge value. Unfortunately, this large amount of charge results in a slow optical fall time due to the modulator s series resistance ( 2KΩ) limiting the drift current to extract the excess carriers in the junction. As a result, a deteriorated extinction ratio (Fig. 3b) is observed relative to 2V pp NRZ modulation case. The conflicting requirements for fast rising and falling transitions are addressed through the use of a pre-emphasis modulation technique [12]. During a rising-edge transition the positive voltage overshoots (2V) for a fraction of a bit period to allow for a high initial charge before settling to a lower voltage (1V) corresponding to a reduced steadystate charge. A similarly shaped waveform is used for the falling-edge transition to increase the drift current to extract the carriers. As the rising and falling-edge time constants are different, a non-linear modulation waveform is applied. We adjust the amount of over/under-shoot time of the pre-emphasis waveform for a specific modulator, with the rising-edge pre-emphasis pulse typically wider than the falling-edge. Adjusting the pre-emphasis time, rather than utilizing different voltage levels, allows the optimization of the transient response to be decoupled from the steady-state extinction ratio value. A fast optical rising and falling with the pre-emphasis modulation is shown in Fig. 3c. The major issue of the 1 st generation ring modulator is that the large series contact resistance ( 2KΩ) requires high modulation swing (4V pp ) to compensate the voltage overhead on the large 2KΩ series contact resistance, which also limits the drift current to extract the excess carriers in the junction and lead to a slow optical falling transition. The 2 nd generation ring modulator [2] 5
reduces the series contact resistance down to 200Ω, providing a potential for high-speed and energy efficient optical modulation. Unlike the 4V pp driver which outputs a differential voltage swing with approximately 0V average bias level on the p-i-n diode, the 2V pp single-ended driver provides a 2V pp output swing on the modulator cathode and utilizes a non-linear voltage DAC on the anode with adjustable DC-bias levels for an optimized eye opening. The 9-bit segmented bias DAC consists of a coarse 3-bit non-linear R-string DAC to match the p-i-n I-V characteristics and a fine 6-bit linear R-2R DAC to achieve linear voltage steps on each non-linear voltage segment [3]. In order to overcome the relatively slow carrier dynamics in forward-bias, the anode is biased at a voltage level close to the p-i-n junction threshold voltage. Note that since the resonance wavelength blue-shifts to shorter wavelengths due to the accumulation of free carriers in the ring waveguide, when increasing the resonator p-i-n diode anode voltage, the bias DAC can also be used for bias tuning [3] to compensate the resonance drifts due to the fabrication variation, allowing for both improved tuning power efficiency and speed relative to heater-based tuning [6]. The co-simulation result of 2 nd generation ring prototype based on the proposed ring model with CMOS 2V pp preemphasis driver circuits is show in Fig. 4. An optimum 9Gb/s optical eye has been achieved when the pre-emphasis duration is set to 80ps and anode is biased at 1.45V. III. WDM TRANSMITTER ARCHITECTURE Fig. 5a shows a block diagram of the CMOS WDM photonic transmitter prototype integrating five TX modules in a 1mm 2 65nm CMOS area, with one transmitter being used as forwardedclocking and the other four being used as data transmission in the 4-channel data WDM link. Applying a forwarded-clock architecture in a photonic WDM system offers the potential for improved high frequency jitter tolerance with minimal jitter amplification due to the clock and data signals experiencing the same delay over the common low-dispersive optical channel. Two versions of the CMOS drivers are implemented to modulate the two generation designs of the carrier-injection 6
ring modulators. A differential driver, with approximately 0V average bias level, provides a 4V pp output swing to allow for high-speed operation of the 1 st generation ring modulator with relatively large series contact resistance ( 2KΩ), while a single-ended driver delivers a 2V pp output swing on the 2 nd generation ring modulator cathode and utilizes a non-linear bias-tuning DAC on the anode for an adjustable DC-bias level. A half-rate CML clock is distributed to the 5 CMOS transmitter modules where 8-bit parallel data is multiplexed to the full output data rate by cascade 2:1 mux before being buffered by the modulator drivers. The distributed CML clock is converted to CMOS levels by the local CML-to-CMOS buffer. These CMOS drivers are wire-bonded to carrier-injection silicon ring resonator modulators as shown in Fig. 5b. A continuous wavelength light near 1300nm from a tunable laser is vertically coupled into the photonic device s input port via the grating coupler. The modulated light is then coupled out from the modulator s through port into a multi-mode fiber for routing to the optical oscilloscope for high-speed data recovery and eye measurement. IV. NON-LINEAR PRE-EMPHASIS MODULATOR DRIVER The 4V pp and 2V pp pre-emphasis drivers employ the similar circuits architecture. An on-chip 2 7-1 PRBS source generates eight bits parallel outputs. Serialization of eight bits data is performed in both transmitter versions with three 2:1 multiplexing stages, with the serialization clocks generated from a half-rate CML clock which is distributed to five transmitter modules, converted to CMOS levels, and subsequently divided to switch the mux stages. The serialized data is then transmitted by the modulator drivers, with both output stage versions utilizing a main driver, positive-edge and negative-edge pre-emphasis pulse drivers in parallel (Fig. 6a) to generate the pre-emphasis output waveform. Tunable delay cells, implemented with digitally-adjustable current-starved inverters (Fig. 6b), allow for independent control of the rising and falling-edge pre-emphasis pulse duration over a range of 20-100ps. Finally, pulsed-cascode output stages (Fig. 6c) with only thin-oxide 7
core devices reliably provide a final per-terminal output swing of twice the nominal 1V supply. A capacitive level shifter and parallel logic chain generate the signals IN low, swinging between GND and the nominal VDD, and IN high, level-shifted between VDD and 2 VDD, that drive the final pulsed-cascode output stages. During an output transition from high to low, the IN low input switches MN 2 to drive node mid n to near GND and the IN high input triggers a positive pulse from the level shifted NOR-pulse gate that drives the gate of MN 1 to allow the output to begin discharging at roughly the same time that the MN 1 source is being discharged. Similarly, during an output transition from low to high, the IN high input switches MP 1 to drive node mid p to near 2V and the IN low input triggers a negative pulse from the NAND-pulse gate that drives the gate of MP 2 to allow the output to begin discharging at roughly the same time that the MP 2 source is being charged. This scheme guarantees that the drain-source voltage doesn t stress the output pmos/nmos transistors with the nominal 1V supply. V. EXPERIMENTAL RESULTS The CMOS driver circuits were fabricated in a 65nm CMOS general purpose process. As shown in the photographs of Fig. 5b, a chip-on-board test setup is utilized, with the CMOS driver wirebonded both to silicon ring resonator chips for optical signal characterization. For high-speed optical testing, a continuous-wavelength laser is coupled through a grating coupler to a waveguide connected to a silicon ring resonator through a single-mode fiber probe. The current version of the grating coupler used in this work exhibits 7dB loss due to the simplified structure of the grating that is etched at the same time as the waveguide. In future work, further improvement can be achieved with more sophisticated two-mask gratings which have demonstrated loss down to 2-3dB [10]. The waveguide loss is measured to be 3dB/cm, which is negligible for the 500µm waveguide. Overall, with 1mW optical power from the CW laser source, around 40µW is detected at the ring s through-port output when the ring is on off-resonance. After vertically coupling the 8
modulated light out into a single-mode fiber, the light is observed with an optical oscilloscope. The 4V pp CMOS driver modulates the 1 st generation ring modulator to improve the carrier dynamics in the p-i-n junction and compensate the voltage overhead due to the large series contact resistance. Optimizing the pre-emphasis settings allows for an open eye with a 12.7dB extinction ratio. Here the maximum optical data rate is limited to 5Gb/s due to the unanticipated excess contact resistance ( 2kΩ) of the ring resonator modulator. Device contact resistance of 200Ω has been achieved in the 2 nd generation ring modulator. The cathode is modulated by the energy efficient 2V pp CMOS driver and the anode is biased at an adjustable DC level through a non-linear voltage DAC for preemphasis optimization and bias-based ring resonance wavelength tuning. The measured optical eye diagram of 2 nd generation prototype is show in Fig. 7b. It achieves extinction ration of 9.2dB at modulation speed of 9Gb/s. Both generations of ring modulators exceed the 7dB extinction ratios achieved with the depletion-mode devices of [6]. The modulation efficiency of two generations of prototypes are 808fJ/bit (5Gb/s) and 500fJ/bit (9Gb/s) respectively. Half modulation swing and low junction series resistance enable the 2 nd generation prototype to improve the modulation energy efficiency by 38% and increase the modulation speed by 80% relative to the 1 st generation prototype. This provides strong motivation to leverage this photonic I/O architecture in a WDM system with multiple 10Gb/s channels on a single waveguide. The optical modulation speed is limited up to 9Gb/s, mainly due to two reasons. First, the lack of driver output impedance control and the relatively long bond wires introduce some additional reflection-induced ISI, degrading the signal quality. Second, attenuation in the on-chip global clock distribution path limits the CMOS driver operation speed. Improving electrical driver operation speed and adopting advance CMOS and photonics integration technique are the points of emphasis for future planned prototype. 9
VI. CONCLUSION This paper presented silicon ring modulator-based photonic WDM transmitters which incorporate high-swing non-linear pre-emphasis drivers to overcome the limited bandwidth of carrierinjection ring resonator modulators. These prototypes provide the potential for silicon photonic links that can deliver distance-independent connectivity whose pin-bandwidth scales with the degree of wavelength-division multiplexing. 10
List of Figures Fig. 1. Silicon ring resonator-based wavelength-division-multiplexing (WDM) link. Fig. 2. (a) Top and cross section views of carrier-injection silicon ring resonator modulator, (b) optical spectrum at through port. Fig. 3. Simulated 1 st generation ring resonator modulator response to 200ps data pulses with: (a) 2V pp simple modulation, (b) 4V pp simple modulation, (c) 4V pp modulation with pre-emphasis. Fig. 4. Simulated 2 nd generation ring modulator 9Gb/s optical eye diagram driven by the 2V pp CMOS driver. Fig. 5. (a) WDM transmitter architecture, (b) Optical transmitter circuits prototype bonded for optical testing. Fig. 6. Non-linear pre-emphasis modulator driver transmitters: (a) per-terminal 2V pre-emphasis driver, (b) tunable delay cell, (c) pulsed-cascode output stage. Fig. 7. Measured ring modulator optical eye diagram: (a) 5Gb/s optical eye diagrams of 1 st generation ring modulators driven by the 4V pp pre-emphasis driver; (b) 9Gb/s optical eye diagrams of 2 nd generation ring modulators driven by the 2V pp pre-emphasis driver. 11
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Fig. 1. Silicon ring resonator-based wavelength-division-multiplexing (WDM) link. (b) (a) Fig. 2. (a) Top and cross section views of carrier-injection silicon ring resonator modulator, (b) optical spectrum at through port. (a) (b) (c) Fig. 3. Simulated 1st generation ring resonator modulator response (bottom) to 200ps data pulses (top) with: (a) 2Vpp simple modulation, (b) 4Vpp simple modulation, (c) 4Vpp modulation with pre-emphasis. 14
Fig. 4. Simulated 2 nd generation ring modulator 9Gb/s optical eye diagram driven by the 2V pp CMOS driver. (a) (b) Fig. 5. (a) WDM transmitter architecture, (b) Optical transmitter circuits prototype bonded for optical testing. 15
(a) (b) (c) Fig. 6. Non-linear pre-emphasis modulator driver transmitters: (a) per-terminal 2V pre-emphasis driver, (b) tunable delay cell, (c) pulsed-cascode output stage. (a) (b) Fig. 7. Measured ring modulator optical eye diagram: (a) 5Gb/s optical eye diagrams of 1 st generation ring modulators driven by the 4V pp pre-emphasis driver; (b) 9Gb/s optical eye diagrams of 2 nd generation ring modulators driven by the 2V pp pre-emphasis driver. 16