contributed by R. Krishnamurthy
Silicon photonics is not a new idea; there have been many attempts at commercial photonics devices back in the 90’s. However, despite some considerable R&D investments, commercial products have been few and far between, and to our knowledge, integration of silicon photonics with advanced IC technology on a die has never been launched commercially. Until now. An integrated IC with active photonics has finally made it into a commercial product with a Luxterra device found in a Molex Active Optical Cable (AOC).
AOCs are positioned at the forefront of cloud connectivity (i.e., a server farm), to make socket-to-socket, board-to-board, rack-to-rack connections faster and more reliable. Comprised of a fiber cable with built-in active electronics and embedded transceivers at both ends, they consume relatively low power (about 0.8 to 1 watt per port) and are less prone to error to fulfill these needs. Simply put, digital signals are received and converted into modulation of light; the light is transmitted across the die to the I/O ports and then out through the fibers and, of course, vice versa.
The first big design
In November 2011, Molex purchased Luxtera’s AOC business, including the quad small form factor pluggable (QSFP+) 40 Gbps Ethernet and InfiniBand products. Luxtera is a privately owned, fabless semiconductor company that claims to be the first company to successfully integrate high performance optics directly with silicon electronics on a CMOS chip to enable “fiber to chip” connectivity. Luxtera’s collaboration with Freescale Semiconductor has enabled high volume manufacturing of Si CMOS photonics chips using their HiPerMOS7 130 nm silicon-on-insulator (SOI) CMOS process, on a 200 mm line in Austin, USA. More recently, in March 2012, Luxtera announced its collaboration with STMicroelectronics to enable high volume manufacturing of Si CMOS photonics using STMicroelectronics’ PIC25G 65 nm process on a 300 mm platform in Crolles, France.
We have just completed a full analysis of Luxtera’s PN1000001 silicon CMOS photonics chip, extracted from the QSFP+ connectors of a Molex AOC, and thought a few highlights were in order. The QSFP+connector is a low power (0.78 W per cable, 30% less than the power consumption of competing products) 40 Gbps, AOC assembly featuring four bidirectional full duplex optical transceiver channels per end, each operating at data rates from 1.0 Gbps to 10.3125 Gbps with a cable length of 3.0 meters.
A typical QSFP+ connector attached to the ends of an AOC cable is shown in Figure 1. The QSFP module assembly consists of an optical ribbon fiber cable attached to the Luxtera PN1000001 Si CMOS photonics chip, which is mounted on a printed circuit board (PCB) and placed within an electromagnetic interference (EMI) shielded metal housing, shown in Figure 2. The Luxtera PN1000001 Si CMOS photonics chip is wire bonded directly to the PCB as shown in Figure 3.
The Si CMOS photonics chip was removed from the PCB and the gel over the chip carefully removed in our lab to expose the structure of the module featuring the Si CMOS photonics die. Figure 4 and Figure 5 are tilt and cross-sectional views, respectively, of the module featuring a laser module and optical fiber assembly attached to the top surface of the PN1000001 Si CMOS photonics die. The Si CMOS photonics die is attached to a metal support.
Figure 6 is a detailed cross-sectional SEM image showing the coupling between the optical fiber and Si CMOS chip. The fiber assembly is epoxied to the top of the Si CMOS chip such that each fiber is centered over a grating coupler. The fiber core is located in the middle of the fiber cladding and is not visible in this cross-sectional image.
Figure 7 and Figure 8 show plan and side-view X-ray images, respectively, of the laser module attached to the PN1000001 Si CMOS photonics die. The laser module is similar to laser modules used in typical fiber-to-home applications, but modified by Luxtera to bounce the light originating from the side of the laser die into the Si photonics chip. The laser module consists of a laser die, ball lens, and isolator, all aligned and positioned over a micromachined Si optical bench and covered with a Si cap that is etched to form a cavity over the laser module elements. The underside part of the cap is coated with a reflector, which bounces light from the laser into the Si photonics die.
A die photograph and die markings of the PN1000001 Si CMOS photonics die extracted from the module is shown in Figure 9. The transceiver die uses four transmit (TX) and four receive (RX) channels, a digital core, and eight fiber array I/Os. The die also features a channel transmit section in the lower half of the die and a four-channel receive section in the upper half of the die.
The transmit (TX) channels feature light from the laser module that is coupled into the chip near the lower left edge of the chip, using single polarization grating couplers (SPGC). The coupled light is transported using single mode (SMW) and multiple mode (MMW) waveguides, built into the SOI layer of the chip, into the two arms of a segmented high-speed Mach-Zehnder modulator interferometer (MZI) chain placed near the lower left quarter of the die. Electrical signals coupled into the lower left edge of the chip modulate the optical signal in the MZI arms, by biasing the built-in reverse biased PN junction diode of the MZI, and inducing a phase change between the optical signals in the two arms of the MZI, which results in an amplitude modulated optical signal. Waveguides carry the modulated light from MZI to four grating couplers (SPGC) to couple light from the four TX channels to four optical fibers that carry the light out of the chip. These optical fibers are in the lower four fibers of the eight fibers connected at the right half of the die. On the way towards the output, the optical signal is tapped and monitored.
The receive (RX) channels placed under the upper four fibers of the die couple light into the chip using polarization splitting couplers (PSGC). Four PSGC structures are beneath the four upper optical fibers connected at the right edge of the die. The light coupled into the receive channels travels using SMW and MMW waveguides, from the right to the left edge of the die, to reach the front end of the receiver channels. The front ends of the four channels of the receiver placed near the upper left quarter of the device feature a Ge PIN photodetector, which detects the optical light and converts to an electrical signal. The electrical signal from each RX channel is amplified further by multistage transimpedence and limiting amplifiers.
Among the various photonics devices onboard the Luxtera die, e.g., the grating couplers, MZI modulators, PIN detectors, directional couplers and terminators, we will look at the couplers and light detectors here.
The Single Polarization Grating Coupler (SPGC) shown in Figure 10 has two sections: (a) a connecting waveguide flaring to form a horn, and (b) curved grating trenches within the horn to focus the light into the waveguide. The grating trenches of the SPGC structure uses ribs created by STI etched in the SOI as the scattering centers to couple out-of-plane light into an in-plane waveguide. The grating structure features grating ribs of varying widths, as shown in Figure 11.
The PSGC shown in Figure 12 has two SPGC structures at right angles, overlaid on each other to form a 2-D grating structure.
The RX part of the PN1000001 die uses four PIN detectors as part of four RX channels, with one PIN detector used per RX channel. The PIN detector shown in Figure 13 converts an optical signal in to an electrical signal. A TEM cross section through the PIN detector shown in Figure 14 reveals that the PIN detector is constructed from a stripe of Ge selectively grown over the rib of a waveguide and doped to form a PIN structure.
Overall, we are struck by the elegance and clever engineering that has gone into the Luxtera design that will enable volume scaling and ensue cost reduction. While today this technology lives in server farms, mainstream applications for integrated photonics devices, such as fiber to the home, are inching their way closer to commercial reality.
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