Optics & Photonics News

Driver IC Transmitter chip

25 atoms thick. This oxide layer acts as the glue that fuses the two materials together. When a forward bias is applied to the indium phosphide, it emits photons into the silicon-waveguide resonator, where they accumulate and subsequently pass into the next silicon-photonic device.

We have monolithically integrated four hybrid silicon lasers onto a single silicon substrate with four traveling-wave optical modulators, a multiplexer and an inverted taper that couples light into a fiber. The four lasers were designed so that their wavelengths matched the ITU-T G.694.2 grid wavelengths of 1,291, 1,311, 1,331 and 1,351 nm.

The InP bonding can be done at the wafer level in the back end of the silicon processing flow. Therefore, hundreds or thousands of hybrid silicon lasers could be created from a single bond, enabling a high-volume path to light emission on a silicon photonics chip.

Pluggable fiber
connector
Packaging and assembly elements
for the transmitter module.

Courtesy of Intel Corp.

for transmission. An optical connector that includes a plastic lens and alignment pin couples light into fiber.

Traveling-wave modulators

After passing the output grating and exiting the laser resonator, the light enters a traveling-wave silicon modulator. The modulator shutters the light on and off to encode ones and zeros on the beam at rates up to 12. 5 Gbps. There are two reasons for using these modulators rather than directly modulating the laser. First, at high data rates, modulating the laser directly would consume much more power. Second, direct modulation of a laser introduces chirp, which diminishes performance at high speeds. Since the long-term plan is to produce devices running at 40 Gbps per channel and beyond, this is essential for future scaling.

The silicon modulator relies on the free-carrier plasma-dispersion effect, in which silicon’s refractive index is changed when the density of free carriers (electrons and holes) is varied. This modulation can be very fast, because free carriers can be swept out of the junction in approximately seven picoseconds. The speed is thus limited by the parasitic effects such as RC time constant limits. Accordingly, we adopted a traveling-wave scheme allowing the electrical and optical signal to copropa-gate across the device. To operate the traveling-wave modulator, our driver IC feeds a small RF signal into the transmission line, and the transmission line is terminated with an external resistor. We have shown in previous experiments that such modulators can have a 3-dB bandwidth of about 30 GHz and data transmission capability up to 40 Gbps.

Multiplexer

Light beams from the four modulators are combined into a single output beam with an optical multiplexer. Both the transmitter’s multiplexer and the receiver’s demultiplexer use an integrated optical grating. In the transmitter, this consists of a free-space region inside the chip where light beams reflect off the curved surface and the grating, and they are combined onto a single waveguide. The light then leaves the chip through a taper, a passive device that gradually changes the size of the optical beam to couple more efficiently into the optical fiber

Demultiplexer

The optical signals are carried by a standard optical fiber and coupled into the Rx chip through a second optical connector. Signals travel into the demultiplexer to be separated once again into individual channels. The alignment of the wavelength channels between the hybrid silicon laser, the multiplexer and the demultiplexer is within one nanometer with only passive heat sinking.

PIN Photodetectors

An array of four photodetectors converts the photons back into electrons. We use germanium-on-silicon detectors to receive optical data because germanium is highly compatible with silicon IC manufacturing, and it detects light efficiently at the wavelengths emitted by the lasers. The challenge with these devices is minimizing the dark current due to crystal-lattice differences between germanium and silicon. For the 50 Gbps link, the measured 3-dB bandwidth of the four photodetectors was in the range of 9. 4 to 10. 6 GHz, with a measured responsiveness of approximately 0.9 A/W. As with the modulators, we have previously demonstrated that individual PIN photodetectors based on this technique can operate up to 40 Gbps.

Packaging and assembly

We packaged the Tx and Rx chips onto two different printed circuit boards. The system is designed to operate at 10 Gbps at room temperature with a passive heat sink. The Tx is “ flip-chip” mounted to the substrate along with a silicon driver IC. Flip-chip bonding is the standard method of packaging modern microprocessors; it was developed as a high-speed alternative to wire-bonding. In this type of bonding, the silicon chip is flipped upside-down and placed on an array of solder bumps that form the connection between circuitry and the package.

The integrated receiver is flip-chip attached to a second substrate, which is copackaged with a commercially available four-channel receiver IC. Fiber coupling between the two chips is aligned passively with precise metal alignment pins seated into v-grooves in the silicon substrate. These pins allow the chips to