Andrew Pomerene

Electronic-photonic integrated circuits on the CMOS platform

The optical components industry stands at the threshold of a major expansion that will restructure its business processes and sustain its profitability for the next three decades. This growth will establish a cost effective platform for the partitioning of electronic and photonic functionality to extend the processing power of integrated circuits. BAE Systems, Lucent Technologies, Massachusetts Institute of Technology, and Applied Wave Research are participating in a high payoff research and development program for the Microsystems Technology Office (MTO) of DARPA. The goal of the program is the development of technologies and design tools necessary to fabricate an application-specific, electronicphotonic integrated circuit (AS-EPIC). As part of the development of this demonstration platform we are exploring selected functions normally associated with the front end of mixed signal receivers such as modulation, detection, and filtering. The chip will be fabricated in the BAE Systems CMOS foundry and at MITs Microphotonics Center. We will present the latest results on the performance of multi-layer deposited High Index Contrast Waveguides, CMOS compatible modulators and detectors, and optical filter slices. These advances will be discussed in the context of the Communications Technology Roadmap that was recently released by the MIT Microphotonics Center Industry Consortium.

Impedance matching vertical optical waveguide couplers for dense high index contrast circuits

We designed and demonstrated a compact, high-index contrast (HIC) vertical waveguide coupler for TE single mode operation with the lowest coupling loss of 0.20 dB +/- 0.05 dB at 1550 nm. Our vertical coupler consists of a pair of vertically overlapping inverse taper structures made of SOI and amorphous silicon. The vertical coupler can suppress power oscillation observed in regular directional couplers and guarantees vertical optical impedance matching with great tolerance for fabrication and refractive index variations of the waveguide materials. The coupler furthermore shows excellent broadband coupling efficiencies between 1460 nm and 1570 nm.

Demonstration of a Fourth-Order Pole-Zero Optical Filter Integrated Using CMOS Processes

We demonstrate a compact fully tunable narrowband fourth-order pole-zero optical filter that is fabricated in a silicon complementary-metal-oxide-semiconductor foundry. The filter is implemented using silicon on oxide channel waveguides and consists of a Mach-Zehnder interferometer with two ring resonator all-pass filters (APFs) on each arm. The filter architecture is based on the sum and difference of the APFs responses. The ring resonators introduce a nonlinear phase response in each arm that allows carving narrow frequency bands out of a broad spectrum. In this paper, we demonstrate a 3-dB filter bandwidth of 1.0 GHz with a stopband rejection of better than 25 dB. The filter free spectral range is 16.5 GHz. Thermooptic phase shifters are used to tune the filter. As silicon has a large thermooptic coefficient compared to silica, the demonstrated filter requires a low tuning power of less than 300 mW. In addition, this filter is compact with dimensions 25 times smaller than the same filter would be if it were made using standard silica on silicon waveguides with a 0.8% step index contrast

Process flow innovations for photonic device integration in CMOS

Multilevel thin film processing, global planarization and advanced photolithography enables the ability to integrate complimentary materials and process sequences required for high index contrast photonic components all within a single CMOS process flow. Developing high performance photonic components that can be integrated with electronic circuits at a high level of functionality in silicon CMOS is one of the basic objectives of the EPIC program sponsored by the Microsystems Technology Office (MTO) of DARPA. Our research team consisting of members from: BAE Systems, Alcatel-Lucent, Massachusetts Institute of Technology, Cornell University and Applied Wave Research reports on the latest developments of the technology to fabricate an application specific, electronic-photonic integrated circuit (AS_EPIC). Now in its second phase of the EPIC program, the team has designed, developed and integrated fourth order optical tunable filters, both silicon ring resonator and germanium electro-absorption modulators and germanium pin diode photodetectors using silicon waveguides within a full 150nm CMOS process flow for a broadband RF channelizer application. This presentation will review the latest advances of the passive and active photonic devices developed and the processes used for monolithic integration with CMOS processing. Examples include multilevel waveguides for optical interconnect and germanium epitaxy for active photonic devices such as p-i-n photodiodes and modulators.

CMOS-Compatible Si-Ring-Assisted Mach–Zehnder Interferometer With Internal Bandwidth Equalization

We demonstrate, to the best of our knowledge, the first electrooptic ring-assisted Mach-Zehnder interferometric (RAMZI) modulator in a CMOS-compatible technology. The RAMZI modulator is manufactured on a CMOS-compatible platform and entirely fabricated in a commercial CMOS foundry. We demonstrate a small-signal 3-dB bandwidth >15 GHz in a silicon-based carrier-depletion modulator with a 2-V·cm V¿L product, which is approximately two times smaller than previously reported. We achieved a 10-Gb/s eye diagram with a 2-dB extinction ratio using a 4-Vp-p drive in a modulator with a 680-¿m optic/RF interaction region. In addition, we demonstrate internal bandwidth equalization within the tunable CMOS-compatible RAMZI modulator, and discuss the optical carrier and modulation sideband response, and relaxation characteristics that lead to this behavior within resonant modulators.

Low-Loss Amorphous Silicon Channel Waveguides for Integrated Photonics

Amorphous silicon (a-Si), single-mode, channel waveguides were fabricated and measured with transmission losses as low as 6.5 dB/cm for the TE mode and 4.5 dB/cm for the TM mode. Variations in the PECVD a-Si deposition conditions yielded a-Si materials with bulk losses <1 dB/cm

Internal Bandwidth Equalization in a CMOS-Compatible Si-Ring Modulator

Bandwidth equalization using a simple complementary metal-oxide-semiconductor-compatible tunable silicon-ring modulator is shown. We demonstrate >35-GHz small signal bandwidth and use the resonator to mitigate bandwidth limitations from other measurement system components. Configuring the optical carrier to be off resonance within the ring free-spectral range allows high-frequency enhancement and low-frequency suppression of the S21 parameter to achieve system response equalization. Our results suggest that the carrier and modulation sidebands can have very different transient characteristics within the ring modulator.

Demonstration of a fast-reconfigurable silicon CMOS optical lattice filter

We demonstrate a fully-reconfigurable fourth-order optical lattice filter built by cascading identical unit cells consisting of a Mach-Zehnder interferometer (MZI) and a ring resonator. The filter is fabricated using a commercial silicon complementary metal oxide semiconductor (CMOS) process and reconfigured by current injection into p-i-n diodes with a reconfiguration time of less than 10 ns. The experimental results show full control over the single unit cell pole and zero, switching the unit cell transfer function between a notch filter and a bandpass filter, narrowing the notch width down to 400 MHz, and tuning the center wavelength over the full free spectral range (FSR) of 10 GHz. Theoretical and experimental results show tuning dynamics and associated optical losses in the reconfigurable filters. The full-control of each of the four cascaded single unit cells resulted in demonstrations of a number of fourth-order transfer functions. The multimedia experimental data show live tuning and reconfiguration of optical lattice filters.

Low-loss asymmetric strip-loaded slot waveguides in silicon-on-insulator

We report on low-loss asymmetric strip-loaded slot waveguides in silicon-on-insulator fabricated with 248 nm photolithography. Waveguide losses were 2 dB/cm or less at wavelengths near 1550 nm. A 40 nm strip-loading allows low-resistance electrical contact to be made to the two slot arms. The asymmetric design suppresses the TE1 mode while increasing the wavelength range for which the TE0 mode guides. This type of waveguide is suitable for building low insertion-loss, high-bandwidth, low drive-voltage modulators, when coated with an electro-optic polymer cladding.

Photolithographically fabricated low-loss asymmetric silicon slot waveguides.

We demonstrate low-loss asymmetric slot waveguides in silicon-on-insulator (SOI). 130 and 180 nm wide slots were fabricated with a 248 nm stepper, in 200 nm thick silicon. An asymmetric waveguide design is shown to expand the range in which the TE0 mode is guided and suppress the TE1 mode, while still maintaining a sharp concentration of electric field in the center of the slot. Optical propagation losses of 2 dB/cm or less are shown for asymmetric slot waveguides with 130 nm wide slots and 320 and 100 nm wide arms.