Fifth International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium ) Nov 10-14th

Kona Hawaii USA

Recent Progress in Silicon PhotonicsMario Paniccia¹, Ling Liao¹, Brian R. Koch¹, Ansheng Liu¹, Alexander W. Fang², Richard Jones¹, Erica Lively²,

Yoel Chetrit³, Juthika Basak¹, Hat Hguyen¹, Doron Rubin³, Di Liang², Ying-Hau Kuo², John E. Bowers² ¹ Intel Corporation, Santa Clara, CA, USA

² University of California, Santa Barbara, CA, USA ³ Numonyx Israel Ltd., Qiryat Gat 82109, Israel

E-mail: Mario.paniccia@intel.com

Abstract

Silicon Photonics has emerged as a promising technology for low cost communication in and around the data canter and around the PC. Integrated transmitters and receivers with 1 Tb/s transmit and receive rates are examples of the high performance devices that are feasible using a silicon photonics platform. An integrated 1 Tb/s transmitter can be generated by using a waveguide based wavelength division multiplexer to combine 25 laser signals at different wavelengths, each encoded with 40 Gb/s data by a silicon modulator. A 1 Tb/s receiver is possible by using a wavelength division demultiplexer connected to an array of 25 photodetectors each receiving 40 Gb/s data. At this point in time, in fact all of these components have been demonstrated individually on a silicon platform that allows for their integration together. Here we present recent results in Silicon Photonics research at Intel Corporation. One of those results is a silicon photonic chip capable of transmitting data at an aggregate rate of 200 Gb/s. It is based on wavelength division multiplexing where an array of eight high speed silicon optical modulators are monolithically integrated with a demultiplexer and a multiplexer. The modulators, each capable of 25 Gb/s data transmission, are based on the carrier depletion effect of pn diodes, and the demultiplexer/multiplexer is based on a cascaded asymmetric Mach Zehnder interferometer design. The second result discussed here is an advance in our hybrid laser technology front involving the use of surface corrugated gratings on silicon to make single wavelength lasers. These lasers have 50 dB side mode suppression ratio and 8 mW output power at room temperature, suitable for wavelength division multiplexing applications and long distance transmission.

200 Gb/s Modulator Array Chip Design and Discrete Component Performance The Si PIC is based on a silicon-on-insulator (SOI) substrate. To target aggregate data transmission of 100 Gb/s and higher, we employ an 8-channel wavelength division multiplexing (WDM) design shown in Fig. 1. It includes a 1:8 DEMUX, 8 high-speed Si Mach-Zehnder modulators (MZMs), and an 8:1 MUX. As Fig. 1 illustrates, an input continuous-wave (CW) multi-wavelength laser beam is first split by the DEMUX. Each wavelength then passes through its corresponding modulator. The CW light on each wavelength channel is amplitude modulated by the MZM encoding a high-speed signal onto the optical beam. After the MUX, all 8 channels are combined in the output waveguide. Note that the DEMUX in Fig. 1 can be replaced in the future by an array of hybrid lasers (3) such as those discussed later, to obtain a fully integrated transmitter chip.

Si MZMs

1:8

Dem

ux

8:1

Muxinput output

1, 2,…, 8 1, 2,…, 8

1

2

8

Fig. 1 – Top down schematic of an integrated Si photonic chip on SOI substrate.

The first key component of the PIC is the high-speed Si MZM, whose operation speed along with the number of wavelength channels determine the total data transmission capacity of the chip. This Si modulator is based on the free carrier plasma dispersion effect, obtained through electric-field-induced carrier depletion of a pn diode embedded inside a rib waveguide (2, 4). Since its operation relies on majority carrier dynamics that are inherently fast, this modulator is capable of bandwidths unprecedented for Si modulation. Fig. 2 (a) shows this modulator, which has an asymmetric Mach-Zehnder interferometer (MZI) design with pn diode phase shifters placed in both of its two arms.

Fig. 2 (b) is a cross-section schematic of the phase shifter, which has 0.6 micron waveguide width and height. It has a horizontal junction where the waveguide slab and part of the rib is doped p-type; the remaining top portion of the rib is doped n-type. In this manner, the p-n junction and built-in depletion region is slightly above the center of the optical mode. This design is aimed at optimizing phase efficiency because when a reverse-bias voltage is applied, the depletion region widens and sweeps across the mode center, allowing for good charge-mode overlap. To electrically contact the n-type region without additional optical loss, a thin Si layer is grown on top of the rib so the metal and high concentration doping for Ohmic contact are located close to the waveguide but do not overlap the optical mode.

p-Si

n-Si

p++p++

n++

Traveling-wave electrodes

oxide

Si substrate

signalground ground

waveguide

p-Si

n-Si

p++p++

n++

Traveling-wave electrodes

oxide

Si substrate

signalground ground

waveguide

(a) (b) Fig. 2 – (a) Top-down view of the Si modulator. It contains a pn diode phase shifter in each arm as the high speed modulating element and 1x2 MMI couplers as the 3-dB splitter and combiner. (b) Schematic of the pn diode waveguide phase shifter.

To achieve high-speed performance, a traveling wave electrode based on coplanar waveguide structure is used. The RF traveling wave coplanar waveguide and modulator optical waveguide are carefully designed so that both electrical and optical signals co-propagate along the length of the phase shifter with similar speeds. At the same time, the RF attenuation is kept as small as possible. Furthermore, as Fig. 2 (a) shows, the output end of each traveling wave electrode is terminated with a load resistor optimized to prevent RF reflections that can compromise signal integrity. This termination resistor is monolithically integrated with the modulator and is made of titanium nitride thin film. This Si MZM with 1 mm long phase shifters has a 3-dB bandwidth of ~30 GHz and can transmit data up to 40 Gb/s. For the Si PIC, we adopt a modulator design similar to that used for the 40 Gb/s demonstration. The only difference is that 1.5 mm long phase shifters are used. This slight increase in length is implemented to improve the obtainable phase shift and therefore data transmission extinction ratio (ER). The second key component of the PIC is the MUX/DEMUX, which is based on a cascaded MZI design (5). It consists of 3 stages of asymmetric MZIs shown in Fig. 3 (a). The third stage has a length difference between the two arms of L, the second stage of 2• L, and the first stage of 4• L. To compensate for MZI phase error that can result from the fabrication process, a thermal heater is added to each MZI. The channel spacing ( ) and L are related according to the following equation

)4(2

20

Lng

(1)

where 0 is the central resonance wavelength and ng is the group refractive index of waveguides. The MUX is designed to have a channel spacing of 3.2 nm (400 GHz) with 4• L = 97.2 μm and 0 = 1550 nm. The packaged chip with RF input and electronic controller pins is shown in Fig. 3 (b).

(a) (b) Fig. 3 – (a) Schematic of the 8 channel DEMUX. (b) Photograph of the photonic chip with electronic packaging.

2x1 MMI

pn diode phase shifters

RF source

input output

Load resistor~

1x2 MMI

~

2x1 MMI

pn diode phase shifters

RF source

input output

Load resistor~

1x2 MMI

~

Input

Stage 1 Stage 2 Stage 3heater

2. L

Stage 1 Stage 2 Stage 3heater

L 2. L4.

200 Gb/s Modulator Array Characterization and Performance To characterize the PIC, the outputs of eight DFB lasers, with wavelengths 3.2 nm apart and comparable output powers, are multiplexed together off-chip using fiber-based couplers. This combined output is used as the optical source for testing and is coupled into the Si integrated device using a lensed polarization maintaining fiber. The optical output of the device is collected using another lensed fiber. After using heater tuning for phase mismatch compensation, channel uniformity is ~1.6 dB as shown in Fig. 4. On-chip loss is approximately 10 dB, of which 6.4 dB is due to the DEMUX and MUX, 2 dB is due to the phase shifters, and the remaining 1.6 dB is likely due to the long passive waveguides used for on-chip optical routing.

-80

-70

-60

-50

-40

-30

-201530 1535 1540 1545 1550 1555

Wavelength (nm)

Opt

ical

Pow

er (d

Bm

)

Fig. 4 – Output spectrum of the integrated device after phase mismatch compensation. Channel uniformity is ~1.6 dB.

To enable high speed testing, the Si chip is bonded to a printed circuit board (PCB) with low loss RF connectors. The PCB is also designed for DC bias control of the MZMs and MUX/DEMUX phase tuning. For high-speed testing, the differential RF signals from a pseudo-random bit sequence (PRBS) generator with [231-1] pattern length are amplified to 3.2 Vpp (6.4 Vpp differential) and combined with 2 Vdc using a bias Tee to ensure reverse bias operation for the entire AC voltage swing. The MZI modulators are biased at quadrature for all the high-speed measurements.

Fig. 5 – Optical eye diagrams of the 8 wavelength channels of the Si PIC. Each is transmitting data at 25 Gb/s with 19 ps rise time, 19 ps fall time, and 2 dB ER.

We tested the high-speed data transmission of the Si chip one channel at a time, with the 25 Gb/s eye diagrams shown in Fig. 5. All channels had similar rise time, fall time, and ER, which are 19 ps, 19 ps, and 2 dB, respectively. These modulators have lower bandwidth that the one used for the 40 Gb/s demonstration because their longer phase

shifter lengths experience more RF loss. Clear open eyes at 25 Gb/s suggest that the Si PIC is capable of transmitting data at an aggregate data rate of 200 Gb/s. Single Wavelength Hybrid Silicon Lasers An essential element of any high bit rate transmitter relying on wavelength division multiplexing is a single wavelength laser. By wafer bonding III-V materials above silicon waveguides, electrically pumped lasers can be made on a silicon wafer without requiring difficult active alignment techniques. All of the silicon processing is performed before wafer bonding and all of the III-V material processing is performed after bonding. Therefore the III-V processing techniques have been carefully designed such that they do not affect the silicon components. In this silicon evanescent technique, current is injected into the III-V material, consisting of a top p contact layer, a separate confined heterostructure layer, a quantum well layer, and an n contact layer. Due to the specific structure of the underlying silicon waveguides, the light generated in the III-V material from current injection is coupled into the silicon waveguides because the optical mode exists in both regions simultaneously. Fig. 6 (a) shows a cross section schematic of a passive silicon waveguide region, while Fig. 6 (b) shows a cross section of a hybrid (gain) region used to form a silicon evanescent laser. The first electrically pumped lasers using the silicon evanescent platform [3] were Fabry-Perot based lasers which emitted multiple wavelengths simultaneously and were incompatible with integration with other waveguide based devices, due to their mirrors being formed by the edges of the silicon chip. Recently single wavelength lasers that can be integrated with other silicon photonics components have been demonstrated on this platform by using silicon based distributed Bragg reflector (DBR) mirrors [6].

Fig. 6- (a) Passive silicon rib and (b) hybrid silicon evanescent waveguide cross sections.

Fig. 7 shows a top down microscope image of the silicon evanescent DBR laser. The basic design consists of a 460 micron gain region with an 80 micron taper on each end. The tapers are formed to allow efficient coupling from the hybrid silicon evanescent waveguide of the gain region into the all silicon waveguides of the mirrors and other passive regions. These taper regions are electrically pumped along with the rest of the gain section. Beyond the tapers are the distributed Bragg reflector mirrors which are formed on the silicon waveguide. The output of the laser is therefore entirely contained in a silicon waveguide. A top down scanning electron micrograph (SEM) of the gratings is shown in the zoom in Fig. 7. The gratings are etched 25 nm into the top of the silicon waveguide with a duty cycle of 75%, providing a grating strength of 80 cm-1. The grating loss was measured to be only ~1 dB/mm, resulting in a loss of less than 0.3 dB even for the long back mirror. The back and front mirrors of the device discussed here were 300 and 100 microns long, providing power reflectivities of 97% and 44%.

Fig. 7- Microscope image of a silicon evanescent DBR laser with a zoom showing an SEM of a silicon grating section.

This device had a threshold current of 65 mA and an output power of 8 mW at 25˚C, and was capable of lasing at temperatures up to 45˚C. The loss attributed to each taper in this device is about 1.2 dB. Reduction of this loss in future devices to 0.5 dB per taper is expected to decrease the lasing threshold by nearly a factor of two. As shown in Fig. 8 (a), the lasing peak was approximately 1597.5 nm, and the side mode suppression ratio was more than 50 dB. Although linewidth measurements were not performed on this particular device, a similar distributed feedback (DFB) silicon evanescent laser was recently shown to have a 3.6 MHz linewidth [7]. We expect that these DBR devices have similar linewidths to the DFB laser’s linewidth. The excellent output characteristics of this device indicate that it could be used for long distance communication links. Next we investigated its direct modulation capabilities. We found that the device could be directly modulated with a 231-1 pseudo-random bit sequence (PRBS) at up to 4 Gb/s, with the modulation bandwidth limited by the cavity design utilized for this particular laser. Fig. 8 (b) shows a 231-1PRBS 2.5 Gb/s eye diagram with an extinction ratio of 8.7 dB, resulting from an RF modulation power of 20 mW.

(a) (b) Fig. 8- (a) Output spectrum from the device, showing over 50 dB side mode suppression. (b) Direct modulation of the laser at 2.5 Gb/s.

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