Douglas M. Gill

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

Comparison of modulation formats for 42.7-gb/s single-channel transmission through 1980 km of SSMF

We experimentally compare nonreturn-to-zero and return-to-zero on-off keying with a number of recently proposed modulation formats for 1980-km 42.7-Gb/s single-channel transmission over standard single-mode fiber. Substantial performance improvements are obtained with the new formats.

Monolithic silicon coherent receiver

We realized a monolithic polarization-diversity dual-quadrature coherent receiver with on-chip balanced detection in a silicon photonic integrated circuit with integrated germanium detectors. We tested it using a polarization-division multiplexed quadrature phase-shift keyed signal at 112-Gb/s.

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.

Monolithic demodulator for 40-gb/s DQPSK using a star coupler

A novel device that demodulates both quadratures of the optical differential quadrature phase-shift keying (DQPSK) format in a single interferometer by using a 4 /spl times/ 4 star coupler was demonstrated. Simultaneous measurements of both DQPSK quadratures at 42.7 Gb/s are also presented.

Optical /spl pi//2-DPSK and its tolerance to filtering and polarization-mode dispersion

Optical /spl pi//2 differential phase-shift keying (/spl pi//2-DPSK) is a modulation technique which encodes the binary information in relative phase shifts of /spl pi//2 and -/spl pi//2 between adjacent optical pulses. Here, we experimentally demonstrate /spl pi//2-DPSK at the bit rate of 42.7 Gb/s. It is found that /spl pi//2-DPSK outperforms conventional DPSK in the presence of strong optical filtering and polarization-mode dispersion (PMD).

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.

Ridged LiNbO/sub 3/ modulators fabricated by a novel oxygen-ion implant/wet-etch technique

This paper demonstrates a new ion implantation and wet-etch technique for fabricating high-quality ridged optical waveguides for high-speed LiNbO/sub 3/-based optical modulators. In addition, the paper demonstrates the fabrication of optical waveguide ridges >3 /spl mu/m in height with 90/spl deg/, and even re-entrant sidewall angles for the first time. The modeling used indicates that 90/spl deg/ (and re-entrant) sidewall ridges can reduce the required modulator drive voltage by 10-20% over modulators with conventional trapezoidal ridge profiles fabricated with reactive ion etching. A 40-Gb/s modulator with a 30-GHz bandwidth, 5.1-V switching voltage at 1 GHz, and a 4.8-dB optical insertion loss is fabricated using the ion implantation/wet-etch process. Fabricated devices showed good stability against accelerated aging, indicating that this process could be used for commercial purposes.

42.7-Gb/s cost-effective duobinary optical transmitter using a commercial 10-Gb/s Mach-Zehnder modulator with optical filtering

Optical duobinary transmission at 42.7 Gb/s is demonstrated using a commercially available 10-Gb/s LiNbO/sub 3/ modulator. The intrinsic response of the 10-Gb/s electrooptic Mach-Zehnder modulator (MZM) is used to create a low-pass-filtered (LPF) duobinary signal, which shows a minimum optical signal-to-noise ratio (OSNR) requirement penalty of /spl sim/4 to /spl sim/6 dB as compared to nonreturn-to-zero (NRZ) signals. However, when the MZM-LPF duobinary is filtered with a 50-GHz 3-dB bandwidth first-order Gaussian filter, a significant improvement in required OSNR is seen. In contrast, similar optical filtering of NRZ data creates a significant OSNR penalty such that the NRZ and MZM-LPF duobinary data have nearly identical OSNR requirements. Therefore, the MZM-LPF duobinary technique may be a cost-effective approach to high-spectral-efficiency transmission at 42.7 Gb/s.