One of the key challenges in Silicon based optical interconnect system remains to be the efficient coupling of optical signals from the submicron size on-chip waveguides to standard single mode (SM) fibers with low insertion loss (IL) and relaxed alignment tolerance. Large optical alignment tolerance allows optical connectors to be attached to on-chip waveguides passively using standard semiconductor pick-and-place assembly tools that have placement accuracies of 10- 15μm. To facilitate the assembly, optical fiber coupling elements need to be modular and compact. They have to also have low profile to avoid blocking air flow or mechanical interference with other elements of the package. In this paper we report the development of a two-dimensional (2D) SM optical fiber coupling architecture that consists of Si based photonic lightwave circuit (PLC) substrate and a high-density micro-lensed fiber optic connector. The system is compact, efficient and has large optical alignment tolerance. At 1300nm an insertion loss of 2.4dB and 1.5dB was measured for the PLC module and the fiber optic connector, respectively. When the PLC module and connector was aligned together, a total insertion loss of 7.8dB was demonstrated with x,y alignment tolerance of 40μm for 1dB optical loss. The SM optical coupling architecture presented here is scalable, alignment tolerant and has the potential to be manufactured in high volume. To our knowledge, such a system has not been reported in the literature so far.
Electro-optic (EO) polymer cladding modulators are an option for low-power high-speed optical interconnects on a
silicon platform. EO polymers have inherently high switching speeds and have shown 40 Gb/s operation in EO polymer
clad ring resonator modulators (RRM). In EO polymer clad RRM, the modulator’s area is small enough to be treated as
a lumped capacitor; the capacitance is sufficiently low that the modulation speed is limited by the bandwidth of the
resonator. A high Q resonator is needed for low voltage operation, but this can limit the speed and/or require precise
control of the resonator’s wavelength, necessitating power consuming heaters to maintain optimal performance over a
large temperature range. Mach Zehnder modulators (MZM), on the other hand, are not as sensitive to temperature
fluctuations, but typically are relatively long and must employ power consuming terminated travelling wave electrodes.
In this paper, a novel MZM design is presented using an EO polymer clad device. In this device, the electrodes are
broken into short parallel segments and the waveguide folds around them. The segments of the electrode length are
designed to provide good signal integrity up to 20 GHz without termination. The electrodes are driven by a single drive
voltage and provide push-pull modulation. Modulators were designed and fabricated using silicon nitride waveguides
on bulk silicon wafers and were demonstrated at high speed (20 GHz). A VπL as low as 1.7 Vcm is measured on initial
devices. An optimized device could provide 40 Gb/s performance at 1 V drive voltages, ~100 fF total device
capacitance and less than 2 dB optical insertion loss.
As integrated circuit interconnect dimensions continue to shrink and signaling frequencies increase, interconnect performance degrades. The performance degradation is due to several factors such as power consumption, cross-talk, and signal attenuation. On-chip optical interconnects are a potential solution to these scaling issues because they offer the promise of providing higher bandwidth. In this paper, progress on the major on-chip optical building blocks will be reviewed. It will be shown that significant advances have been made in the design and fabrication of waveguides, detectors, and couplers. However, major challenges in high speed electrical to optical conversion and signaling remain.
We have developed high-speed germanium (Ge) photodetectors using standard complementary metal-oxide-semiconductor (CMOS) process technology. We describe the design considerations that led to the devices reported on here. We have characterized these detectors in terms of the following detector metrics: speed, responsivity, dark current and capacitance. The photodetectors exhibit responsivities greater than 0.2 A/W at both 850 and 1550 nm, making them compatible with both long- and short-haul communication systems. Impulse response measurements at both of the above wavelengths indicate 3 dB cutoff frequencies greater than 10 GHz and open eye diagrams have been measured at 20 Gb/s. Dark currents are on the order of 10 to 1000 μA at a bias of 1 V depending on device size. Capacitances measured were on the order of 0.1-10 fF. The performance of the detectors indicates that they are suitable for high speed on-chip optical links. Device simulation models indicate that the fundamental upper limit on the speed of the devices, based on ideal material properties, is high enough to support a number of process generations. Calibration of the models to our experimental data is presented, and areas for improvement are defined.