Robert T. Schulein

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW-1 at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si+ 1times10 13 cm-2 at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm-1 to 200-100 dBmiddotcm-1 and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation

Effect of carrier lifetime on forward-biased silicon Mach-Zehnder modulators

We present a systematic study of Mach-Zehnder silicon optical modulators based on carrier-injection. Detailed comparisons between modeling and measurement results are made with good agreement obtained for both DC and AC characteristics. A figure of merit, static VpiL, as low as 0.24Vmm is achieved. The effect of carrier lifetime variation with doping concentration is explored and found to be important for the modulator characteristics.

Ultra-wide-range multi-rate DPSK laser communications

We propose and demonstrate a scalable high-sensitivity approach for achieving multi-rate DPSK using a single transmitter and fixed-interferometer-receiver design. Near-theoretical real-time performance is demonstrated over static and fading channels at rates from 2.4Mbps to 2.5Gbps.

LLCD operations using the Lunar Lasercom Ground Terminal

The Lunar Lasercom Ground Terminal (LLGT) is the primary ground terminal for NASA’s Lunar Laser Communication Demonstration (LLCD), which demonstrated for the first time high-rate duplex laser communication between Earth and satellite in orbit around the Moon. The LLGT employed a novel architecture featuring an array of telescopes and employed several novel technologies including a custom PM multimode fiber and high-performance cryogenic photon-counting detector arrays. An overview of the LLGT is presented along with selected results from the recently concluded LLCD.

Photonic Analog-to-Digital Conversion with Electronic-Photonic Integrated Circuits

Photonic Analog-to-Digital Conversion (ADC) has a long history. The premise is that the superior noise performance of femtosecond lasers working at optical frequencies enables us to overcome the bottleneck set by jitter and bandwidth of electronic systems and components. We discuss and demonstrate strategies and devices that enable the implementation of photonic ADC systems with emerging electronic-photonic integrated circuits based on silicon photonics. Devices include 2-GHz repetition rate low noise femtosecond fiber lasers, Si-Modulators with up to 20 GHz modulation speed, 20 channel SiN-filter banks, and Ge-photodetectors. Results towards a 40GSa/sec sampling system with 8bits resolution are presented.

High Speed Analog-to-Digital Conversion with Silicon Photonics

Sampling rates of high-performance electronic analog-to-digital converters (ADC) are fundamentally limited by the timing jitter of the electronic clock. This limit is overcome in photonic ADCs by taking advantage of the ultra-low timing jitter of femtosecond lasers. We have developed designs and strategies for a photonic ADC that is capable of 40 GSa/s at a resolution of 8 bits. This system requires a femtosecond laser with a repetition rate of 2 GHz and timing jitter less than 20 fs. In addition to a femtosecond laser this system calls for the integration of a number of photonic components including: a broadband modulator, optical filter banks, and photodetectors. Using silicon-on-insulator (SOI) as the platform we have fabricated these individual components. The silicon optical modulator is based on a Mach-Zehnder interferometer architecture and achieves a VπL of 2 Vcm. The filter banks comprise 40 second-order microring-resonator filters with a channel spacing of 80 GHz. For the photodetectors we are exploring ion-bombarded silicon waveguide detectors and germanium films epitaxially grown on silicon utilizing a process that minimizes the defect density.

High-speed silicon electro-optical modulator that can be operated in carrier depletion or carrier injection mode

A high-speed silicon optical modulator has been demonstrated which can operate either in forward bias using carrier injection, or in reverse bias using carrier depletion. In forward bias, the device requires less than 10 mW of drive power, but has a low bandwidth of 100 MHz. In reverse bias, the device has a nearly flat response to 18 GHz, but requires 700 mW for large modulation depths.

Compact, Low-Power, High-Speed Silicon Electro-Optic Modulator

A 250 mum long, CMOS-compatible, PIN diode Mach-Zehnder modulator has been fabricated with response extending to 13 GHz. Modeling shows that precompensation enables the fabrication of ultracompact 10 GHz 3 dB-bandwidth, optically broadband modulators.

Integrated Optical Components in Silicon for High Speed Analog-to- Digital Conversion

Advances in femtosecond lasers and laser stabilization have led to the development of sources of ultrafast optical pulse trains that show jitter on the level of a few femtoseconds over tens of milliseconds and over seconds if referenced to atomic frequency standards. These low jitter sources can be used to perform opto-electronic analog to digital conversion that overcomes the bottleneck set by electronic jitter when using purely electronic sampling circuits and techniques. Electronic Photonic Integrated Circuits (EPICs) may enable in the near future to integrate such an opto-electronic analog-to-digital converters (ADCs) completely. This presentation will give an overview of integrated optical devices such as low jitter lasers, electro-optical modulators, Si-based filter banks, and high-speed Si-photodetectors that are compatible with standard CMOS processing and which are necessary for the implementation of EPIC-chips for advanced opto-electronic ADCs.

Overview and On-orbit Performance of the Lunar Laser Communication Demonstration Uplink

We present an implementation overview and demonstrated error-free coded performance over the 400,000-km link between an Earth-based laser communication terminal and the LADEE satellite orbiting the moon at 9.72-Mbps and 19.44-Mbps uplink rates.