Marcus S. Dahlem

Photonic ADC: overcoming the bottleneck of electronic jitter

Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

Maximizing the Thermo-Optic Tuning Range of Silicon Photonic Structures

We demonstrate 20 nm thermo-optic tuning in silicon microring resonators with 16 nm free spectral range (FSR), the largest reported full-FSR thermal tuning, with a tuning efficiency of 28 muW/GHz, enabling telecom microphotonic tunable filters.

Demonstration of an electronic photonic integrated circuit in a commercial scaled bulk CMOS process

We demonstrate the first photonic chip designed in a commercial bulk CMOS process (65 nm node) using standard process layers combined with scalable post-processing, enabling dense photonic integration with high-performance microprocessor electronics.

Quantifying charge carrier concentration in ZnO thin films by Scanning Kelvin Probe Microscopy.

In the last years there has been a renewed interest for zinc oxide semiconductor, mainly triggered by its prospects in optoelectronic applications. In particular, zinc oxide thin films are being widely used for photovoltaic applications, in which the determination of the electrical conductivity is of great importance. Being an intrinsically doped material, the quantification of its doping concentration has always been challenging. Here we show how to probe the charge carrier density of zinc oxide thin films by Scanning Kelvin Probe Microscopy, a technique that allows measuring the contact potential difference between the tip and the sample surface with high spatial resolution. A simple electronic energy model is used for correlating the contact potential difference with the doping concentration in the material. Limitations of this technique are discussed in details and some experimental solutions are proposed. Two-dimensional doping concentration images acquired on radio frequency-sputtered intrinsic zinc oxide thin films with different thickness and deposited under different conditions are reported. We show that results inferred with this technique are in accordance with carrier concentration expected for zinc oxide thin films deposited under different conditions and obtained from resistivity and mobility measurements.

Passive interrogation of low-finesse Fabry-Perot cavities using fiber Bragg gratings

An investigation on the application of fiber Bragg gratings for the interrogation of interferometric low-finesse Fabry-Perot cavities is reported. The proposed scheme is based on the generation of two quadrature phase-shifted signals that allows the recovering of the change in the cavity length. Besides being totally passive, this technique offers a high degree of flexibility and has the potential to be used in the interrogation of very short cavities.

Highly transparent conducting cerium incorporated CdO thin films deposited by a spray pyrolytic technique

In the present work, a spray pyrolysis technique was employed to deposit cerium (Ce) doped cadmium oxide (CdO) thin films with low level doping concentrations (0.25, 0.50, 0.75 and 1.0 wt%). The crystallite size and lattice parameter values were estimated from X-ray diffraction analysis. X-ray diffraction patterns reveal the shift of preferential growth orientation from (111) to (200) planes on incorporating Ce in the CdO matrix. The oxidation state of Ce, Cd and O in the deposited films was determined by X-ray photoelectron spectroscopic (XPS) studies. Surface microstructures of the films were analyzed by atomic force microscopy and their surface nature was studied by field emission scanning electron microscopy (FE-SEM). The electrical properties of the deposited films were determined by Hall measurements in the van der Pauw configuration. The charge carrier concentration of the CdO thin film increased from 1.0 × 1020 cm−3 to 3.85 × 1020 cm−3 on doping 0.50 wt% Ce; whereas, resistivity decreased from 9.32 × 10−4 Ω cm to 3.81 × 10−4 Ω cm. The deposited Ce-doped CdO thin films for various concentrations showed an increase in the average optical transmittance to 85% from that of 72% for the CdO film in the visible and NIR region. The band gap was gradually increased from 2.38 eV to 2.63 eV due to an increase in the Ce doping at various levels. The emission properties of CdO and Ce doped CdO thin films were studied by a photoluminescence spectrum recorded at room temperature. The figure of merit estimated for 0.50 wt% of Ce doped CdO film is 9.18 × 10−3 Ω−1.

Ultrafast all-optical modulator with femtojoule absorbed switching energy in silicon-on-insulator

We demonstrate an all-optical switch based on a waveguide-embedded 1D photonic crystal cavity fabricated in silicon-on-insulator technology. Light at the telecom wavelength is modulated at high-speed by control pulses in the near infrared, harnessing the plasma dispersion effect. The actual absorbed switching power required for a 3 dB modulation depth is measured to be as low as 6 fJ. While the switch-on time is on the order of a few picoseconds, the relaxation time is almost 500 ps and limited by the lifetime of the charge carriers.

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.

Reconfigurable silicon photonic circuits for telecommunication applications

Photonic circuits based on silicon wire waveguides have attracted significant interest in recent years. They allow strong confinement of light with moderately low propagation losses. Moreover, the high thermo-optical coefficient of silicon and the small device size in silicon photonics allow for micro-heaters induced trimming, tuning, and switching with relatively low power. In this paper, we review our recent progress towards telecom-grade reconfigurable optical add-drop multiplexers (ROADMs) based on silicon microring resonators. We discuss waveguide and micro-heater design and fabrication as well as the first demonstration of telecom-grade silicon-microring filters and the first demonstration of transparent wavelength switching. The reported devices can be employed in numerous optical interconnect schemes.

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.