Intel’s Approach to Integrated Silicon Photonics

With a goal of driving down the cost of high-speed optical interconnects and communications, the Intel photonics team is leveraging SOI to integrate multiple photonic components onto a single die.

In order to build smaller, faster, and less expensive optical components that fulfill the goal of universal, ubiquitous, low-cost, high-volume optical communications and interconnects, Intel is actively pursuing research work in silicon photonics.

SOI wafers are the ideal substrate for photonic applications: the buried oxide (BOX) layer acts as a natural bottom cladding for optical waveguides, keeping the photons confined within the silicon layer, and low absorption of infrared light (in particular at the key telecom wavelengths round 1.3 and 1.55 micrometers) in crystalline Si results in very low optical transmission losses. Such substrates are also well suited to high-volume manufacturing in existing fabrication facilities, and can be processed alongside other CMOS electronic devices.

Low-cost opto-electronic solutions may immediately find applications in telecommunications ranging from long-haul and metro fiber optic networks to fiber-to-the-home (FTTH). Longer term, they should enable board-to-board and chip-to-chip interconnects, and may also be used in optical sensing and biomedical devices.

Building blocks

Intel has tackled the main building blocks needed to realize the full potential of silicon photonics:

Waveguides. Waveguides are at the heart of most every component in integrated optics. The large refractive index contrast between silicon and the BOX makes SOI favorable for fabricating waveguide-based high-density photonic circuits. This contrast enables much tighter waveguide bends as compared to silica-based waveguides, resulting in a footprint reduction by three orders of magnitude (see Figure 1).

Figure 1. A SEM image of a p-i-n diode waveguide used for Raman amplification and lasing experiments. (Courtesy of Intel)

Lasers. As silicon is an indirect band gap material, it has very poor quantum efficiency for light emission. Developing a light source is one of the big challenges for silicon based optoelectronics. In collaboration with University of California at Santa Barbara, Intel developed a silicon hybrid laser in 2006. Such a laser leverages SOI-based waveguides and can be monolithically integrated with other silicon photonic devices.

Modulators. A high-speed silicon modulator is another key component for silicon photonic integrated circuits. Based on SOI waveguides, Intel first demonstrated a modulator with a 3 dB bandwidth larger than 1 GHz in 2004, 50x times faster than previous attempts in silicon. The optical modulator is based on MOS capacitors embedded in an SOI waveguide. In 2005, Intel extended the modulator speed to 10Gb/s. Just recently in July 2007, Intel demonstrated an industry first 40Gb/s silicon modulator. The phase-shifting elements are based on reverse biased pn diodes embedded in an SOI waveguide.

Figure 2. Concept of a future integrated terabit silicon optical transmitter integrated on an SOI substrate, containing 25 hybrid silicon lasers, each emitting at a different wavelength, coupled into 25 silicon modulators, all multiplexed together onto one output fiber. (Courtesy of Intel)

Photodetectors (PD). Waveguide based SiGe photo-detectors are indispensable components for silicon photonic integrated circuits. Intel has demonstrated high-speed, high-responsivity germanium p-i-n PDs based on SOI waveguides at data rates of 10Gb/s and is working to push that performance to 40Gb/s.

Photonic integration. Photonic integrated circuits (PIC) could provide a cost-effective solution for optical communication and future optical interconnects. Monolithic integration of various silicon photonic devices is the next goal. The concept for the terabit integrated optical transceiver, for example, calls for 25 hybrid silicon lasers integrated on an SOI substrate with 25 silicon modulators, each running at 40Gb/s. The result would be 1 terabit per second of optical data transmitting from a single integrated SOI chip (see Figure 2).

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