Harnessing optical micro-combs for microwave photonics

In the past decade, optical frequency combs generated by high-Q micro-resonators, or micro-combs, which feature compact device footprints, high energy efficiency, and high-repetition-rates in broad optical bandwidths, have led to a revolution in a wide range of fields including metrology, mode-locked lasers, telecommunications, RF photonics, spectroscopy, sensing, and quantum optics. Among these, an application that has attracted great interest is the use of micro-combs for RF photonics, where they offer enhanced functionalities as well as reduced size and power consumption over other approaches. This article reviews the recent advances in this emerging field. We provide an overview of the main achievements that have been obtained to date, and highlight the strong potential of micro-combs for RF photonics applications. We also discuss some of the open challenges and limitations that need to be met for practical applications.Comment: 32 Pages, 13 Figures, 172 Reference

Low-frequency noise in downscaled silicon transistors: Trends, theory and practice

By the continuing downscaling of sub-micron transistors in the range of few to one deca-nanometers, we focus on the increasing relative level of the low-frequency noise in these devices. Large amount of published data and models are reviewed and summarized, in order to capture the state-of-the-art, and to observe that the 1/area scaling of low-frequency noise holds even for carbon nanotube devices, but the noise becomes too large in order to have fully deterministic devices with area less than 10nm×10nm. The low-frequency noise models are discussed from the point of view that the noise can be both intrinsic and coupled to the charge transport in the devices, which provided a coherent picture, and more interestingly, showed that the models converge each to other, despite the many issues that one can find for the physical origin of each model. Several derivations are made to explain crossovers in noise spectra, variable random telegraph amplitudes, duality between energy and distance of charge traps, behaviors and trends for figures of merit by device downscaling, practical constraints for micropower amplifiers and dependence of phase noise on the harmonics in the oscillation signal, uncertainty and techniques of averaging by noise characterization. We have also shown how the unavoidable statistical variations by fabrication is embedded in the devices as a spatial “frozen noise”, which also follows 1/area scaling law and limits the production yield, from one side, and from other side, the “frozen noise” contributes generically to temporal 1/f noise by randomly probing the embedded variations during device operation, owing to the purely statistical accumulation of variance that follows from cause-consequence principle, and irrespectively of the actual physical process. The accumulation of variance is known as statistics of “innovation variance”, which explains the nearly log-normal distributions in the values for low-frequency noise parameters gathered from different devices, bias and other conditions, thus, the origin of geometric averaging in low-frequency noise characterizations. At present, the many models generally coincide each with other, and what makes the difference, are the values, which, however, scatter prominently in nanodevices. Perhaps, one should make some changes in the approach to the low-frequency noise in electronic devices, to emphasize the “statistics behind the numbers”, because the general physical assumptions in each model always fail at some point by the device downscaling, but irrespectively of that, the statistics works, since the low-frequency noise scales consistently with the 1/area law

Design of VCOs in Deep Sub-micron Technologies

This work will present a more accurate frequency prediction model for single-ended ring oscillators (ROs), a case-study comparing different ROs, and a design method for LC voltage-controlled oscillators (LCVCOs) that uses a MATLAB script based on analytical equations to output a graphical design space showing performance characteristics as a function of design parameters. Using this method, design trade-offs become clear, and the designer can choose which performance characteristics to optimize. These methods were used to design various topologies of ring oscillators and LCVCOs in the GlobalFoundries 28 nm HPP CMOS technology, comparing the performance between different topologies based on simulation results. The results from the MATLAB design script were compared to simulation results as well to show the effectiveness of the design methods. Three varieties of 5 GHz voltage controlled ring oscillators were designed in the GlobalFoundries 28 nm HPP CMOS technology. The first is a low current low dropout regulator (LDO) tuned ring oscillator designed with thin oxide devices and a 0.85 V supply. The second is a high current LDO-tuned ring oscillator designed with medium oxide devices and a 1.5 V supply. The third is varactor-tuned ring oscillator with no LDO, and 0.85 V supply. Performance comparison of these ring oscillator systems are presented, outlining trade-offs between tuning range, phase noise, power dissipation, and area. The varactor-tuned ring oscillator exhibits 8.89 dBc/Hz (with power supply noise) and 16.27 dBc/Hz (without power supply noise) improvement in phase noise over the best-performing LDO-tuned ring oscillator. There are advantages in average power dissipation and area for a minimal tradeoff in tuning range with the varactor-tuned ring oscillator. Four multi-GHz LCVCOs were designed in the GlobalFoundries 28 nm HPP CMOS technology: 15 GHz varactor-tuned NMOS-only, 9 GHz varactor-tuned self-biased CMOS, 14.2 GHz digitally-tuned NMOS-only, and 8.2 GHz digitally-tuned self-biased CMOS. As a design method, analytical ex-pressions describing tuning range, tank amplitude constraint, and startup condition were used in MATLAB to output a graphical view of the design space for both NMOS-only and CMOS LCVCOs, with maximum varactor capacitance on the y-axis and NMOS transistor width on the x-axis. Phase noise was predicted as well. In addition to the standard varactor control voltage tuning method, digitally-tuned implementations of both NMOS and CMOS LCVCOs are presented. The performance aspects of all designed LCVCOs are compared. Both varactor-tuned and digitally-tuned NMOS LCVCOs have lower phase noise, lower power consumption, and higher tuning range than both CMOS topologies. The varactor-tuned NMOS LCVCO has the lowest phase noise of -97 dBc/Hz at 1 MHz offset from 15 GHz center frequency, FOM of -172.20 dBc/Hz, and FOMT of -167.76 dBc/Hz. The digitally-tuned CMOS LCVCO has the greatest tuning range at 10%. Phase noise is improved by 3 dBc/Hz with the digitally-tuned CMOS topology over varactor-tuned CMOS

Novel continuous-wave infrared parametric sources and noise analysis of infrared upconversion detectors

The ability to manipulate frequency of light, through parametric frequency conversion sources based on X(2) nonlinear materials, offers an effective route to spectral regions unapproachable by conventional lasers. Most importantly, three-wave mixing processes provide tunable coherent radiation over a broad spectral range. Among the most important tunable devices, narrow linewidth continuous-wave (cw) infrared (IR) optical parametric oscillators (OPOs) are indispensable excitation sources for many applications in molecular spectroscopy and precision metrology. In order to exploit such applications, the development of cw OPOs deploying different wavelength tuning schemes and novel nonlinear materials is highly dezirable, as presented in this thesis. We demonstrated a rapidly tunable cw OPO based on fan-out grating design periodically-poled KTiOPO4 (PPKTP) crystal at room temperature. This approach allows continuous wavelength tuning by avoiding increased thermal fluctuations at higher operating crystal temperatures. The 532 nm-pumped, output-coupled singly-resonant oscillator (OC-SRO) provides widely tunable near-IR radiation across 741-922 nm and 1258-1884 nm, with total output power of 1.65 W. The use of output coupling for the resonating wave reduces thermal loading and enables 30% enhancement in the OPO extraction efficiency over the pure SRO configuration. Towards the goal of developing a next-generation cw source >4 µm using a new found quasi-phase-matched semiconductor material, orientation-patterned gallium phosphide (OP-GaP), we demonstrated the first realization of a tunable cw mid-IR source based on OP-GaP by exploiting single-pass difference-frequency-generation (DFG) between a Tm-fiber laser at 2010 nm and a home-built OPO based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN) crystal. The DFG source generates up to 43 mW of output power, with >30 mW across 96% of the tuning range 4608-4694 nm, in high beam quality. As the tunable mid-IR sources are making great strides, the availabilityof fast and sensitive mid-IR detectors become equally important. However, the conventional mid-IR detectors demand cryogenic systems for low-noise operation which sets a major drawback as these devices are often bulky and expensive. In this context, the nonlinear frequency upconversion technique has emerged as a promising alternative to the direct detection of mid-IR radiation at room temperature. An upconversion detector (UCD) can be further optimized by identifying and suppressing its noise sources. In order to do so, we experimentally and theoretically investigated noise properties of 1064 nm-pumped single-pass UCD designed for signal detection in telecom and mid-IR range using MgO:PPLN crystals. We studied the dependence of newly discovered SHG (532 nm)-induced spontaneous parametric downconversion (SHG-SPDC) noise intensity on the pump power and crystal temperature, and compared it with the well-known UCD noise source upconverted spontaneous parametric downconversion (USPDC). The measurements deduce that SHG-SPDC must be given a careful consideration since it can act as a dominant noise source under certain operating conditions. However, SHG-SPDC can be avoided by choosing a proper combination of MgO:PPLN grating period,operating temperature, and bandpass filter.La capacidad de manipular la frecuencia de la luz, a través de sintonizables fuentes de conversión de frecuencia paramétrica basadas en materiales no lineales del tipo ¿(2), ofrece un acceso eficaz a las regiones espectrales inaccesible por los láseres convencionales. Entre los dispositivos sintonizables más importantes, los osciladores ópticos paramétricos (OPOs) de onda continua (cw) en el infrarrojo (IR) con ancho de banda estrecho, son fuentes esenciales para muchas aplicaciones en espectroscopia molecular y metrología de precisión. Con el fin de explotar tales aplicaciones, es muy deseable el desarrollo de OPOs de cw mediante diferentes esquemas de sintonización de longitud de onda y nuevos materiales no lineales, como se presentan en esta tesis. Mostramos aquí un OPO de cw rápidamente sintonizable basado en un cristal periodically-poled KTiOPO4 con diseño de red fan-out a temperatura ambiente. Este enfoque permite una sintonización continua de la longitud de onda evitando el aumento de las fluctuaciones térmicas a temperaturas de funcionamiento más altas de los cristales. El OPO resonante con acoplador de salida bombeado a longitud de onda de 532 nm, proporciona radiación sintonizable en el infrarrojo cercano (near-IR) a través de 741-922 nm y 1258-1884nm, con una potencia de salida máxima de 1.65 W. El uso del acoplador de salida para la onda resonante reduce la carga térmica y permite una mejora del 30% en la eficiencia de extracción del OPO sobre la configuración SRO pura. Con el objetivo de desarrollar la nueva generación de fuentes de onda continua con longitud de onda >4 µm utilizando la técnica de quasi-phase-matching con materiales semiconductores, presentamos la primera demostración de una fuente de onda continua sintonizable en el infrarrojo medio (mid-IR) utilizando el patrón de orientación de fosfuro de galio (OP-GaP) y generación de frecuencia diferencia (DFG) entre láser de fibra Tm a longitud de onda de 2010 nm y otro OPO hecho en el laboratorio basado en un cristal MgO-doped periodically-poled LiNbO3 (MgO:PPLN). La fuente DFG genera hasta 43 mW de potencia de salida, con >30 mW a través de un 96% del rango de sintonización 4608-4694 nm, con una alta calidad de haz. A medida que las fuentes de emisión en el mid-IR sintonizables mejoren sus prestaciones, la disponibilidad de detectores de mid-IR rápidos y sensibles se vuelve igualmente importante. Sin embargo, los detectores de mid-IR convencionales requieren sistemas criogénicos para operar con poco ruido, lo que presenta un gran inconveniente ya que estos dispositivos suelen ser voluminosos y caros. En este contexto, la técnica no lineal de conversión ascendente de frecuencia ha surgido como una alternativa prometedora a la detección directa de la radiación de mid-IR a temperatura ambiente. Un detector de conversión ascendente (UCD) se puede optimizar aún más dentificando y suprimiendo sus fuentes de ruido. Para hacerlo, investigamos teórica y experimentalmente las propiedades de ruido de un UCD de un solo paso bombeado a 1064 nm diseñado para la detección de señales en telecomunicaciones y rango mid-IR, utilizando cristales MgO:PPLN. Descubrimos una nueva fuente de ruido llamada ruido espontáneo paramétrico de conversión descendente inducido por generación de segundo harmónico SHG (SHG-SPDC). Estudiamos, también, la dependencia de la intensidad de este ruido (SHG-SPDC) con la potencia de bombeo y la temperatura del cristal. Finalmente, también comparamos su intensidad con el conocido ruido generado por el detector UCD, llamado ruido de conversión ascendente por conversión paramétrica descendente espontánea (USPDC). Los resultados obtenidos nos dicen que se debe considerar cuidadosamente el SHG-SPDC, ya que puede actuar como una fuente de ruido dominante en ciertas condiciones de operación. Sin embargo, el SHG-SPDC se puede evitar al elegir una combinación adecuada de MgO:PPLN, el período de red la temperatura de operación y el filtro de tipo pasa banda.Postprint (published version

Novel continuous-wave infrared parametric sources and noise analysis of infrared upconversion detectors

The ability to manipulate frequency of light, through parametric frequency conversion sources based on X(2) nonlinear materials, offers an effective route to spectral regions unapproachable by conventional lasers. Most importantly, three-wave mixing processes provide tunable coherent radiation over a broad spectral range. Among the most important tunable devices, narrow linewidth continuous-wave (cw) infrared (IR) optical parametric oscillators (OPOs) are indispensable excitation sources for many applications in molecular spectroscopy and precision metrology. In order to exploit such applications, the development of cw OPOs deploying different wavelength tuning schemes and novel nonlinear materials is highly dezirable, as presented in this thesis. We demonstrated a rapidly tunable cw OPO based on fan-out grating design periodically-poled KTiOPO4 (PPKTP) crystal at room temperature. This approach allows continuous wavelength tuning by avoiding increased thermal fluctuations at higher operating crystal temperatures. The 532 nm-pumped, output-coupled singly-resonant oscillator (OC-SRO) provides widely tunable near-IR radiation across 741-922 nm and 1258-1884 nm, with total output power of 1.65 W. The use of output coupling for the resonating wave reduces thermal loading and enables 30% enhancement in the OPO extraction efficiency over the pure SRO configuration. Towards the goal of developing a next-generation cw source >4 µm using a new found quasi-phase-matched semiconductor material, orientation-patterned gallium phosphide (OP-GaP), we demonstrated the first realization of a tunable cw mid-IR source based on OP-GaP by exploiting single-pass difference-frequency-generation (DFG) between a Tm-fiber laser at 2010 nm and a home-built OPO based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN) crystal. The DFG source generates up to 43 mW of output power, with >30 mW across 96% of the tuning range 4608-4694 nm, in high beam quality. As the tunable mid-IR sources are making great strides, the availabilityof fast and sensitive mid-IR detectors become equally important. However, the conventional mid-IR detectors demand cryogenic systems for low-noise operation which sets a major drawback as these devices are often bulky and expensive. In this context, the nonlinear frequency upconversion technique has emerged as a promising alternative to the direct detection of mid-IR radiation at room temperature. An upconversion detector (UCD) can be further optimized by identifying and suppressing its noise sources. In order to do so, we experimentally and theoretically investigated noise properties of 1064 nm-pumped single-pass UCD designed for signal detection in telecom and mid-IR range using MgO:PPLN crystals. We studied the dependence of newly discovered SHG (532 nm)-induced spontaneous parametric downconversion (SHG-SPDC) noise intensity on the pump power and crystal temperature, and compared it with the well-known UCD noise source upconverted spontaneous parametric downconversion (USPDC). The measurements deduce that SHG-SPDC must be given a careful consideration since it can act as a dominant noise source under certain operating conditions. However, SHG-SPDC can be avoided by choosing a proper combination of MgO:PPLN grating period,operating temperature, and bandpass filter.La capacidad de manipular la frecuencia de la luz, a través de sintonizables fuentes de conversión de frecuencia paramétrica basadas en materiales no lineales del tipo ¿(2), ofrece un acceso eficaz a las regiones espectrales inaccesible por los láseres convencionales. Entre los dispositivos sintonizables más importantes, los osciladores ópticos paramétricos (OPOs) de onda continua (cw) en el infrarrojo (IR) con ancho de banda estrecho, son fuentes esenciales para muchas aplicaciones en espectroscopia molecular y metrología de precisión. Con el fin de explotar tales aplicaciones, es muy deseable el desarrollo de OPOs de cw mediante diferentes esquemas de sintonización de longitud de onda y nuevos materiales no lineales, como se presentan en esta tesis. Mostramos aquí un OPO de cw rápidamente sintonizable basado en un cristal periodically-poled KTiOPO4 con diseño de red fan-out a temperatura ambiente. Este enfoque permite una sintonización continua de la longitud de onda evitando el aumento de las fluctuaciones térmicas a temperaturas de funcionamiento más altas de los cristales. El OPO resonante con acoplador de salida bombeado a longitud de onda de 532 nm, proporciona radiación sintonizable en el infrarrojo cercano (near-IR) a través de 741-922 nm y 1258-1884nm, con una potencia de salida máxima de 1.65 W. El uso del acoplador de salida para la onda resonante reduce la carga térmica y permite una mejora del 30% en la eficiencia de extracción del OPO sobre la configuración SRO pura. Con el objetivo de desarrollar la nueva generación de fuentes de onda continua con longitud de onda >4 µm utilizando la técnica de quasi-phase-matching con materiales semiconductores, presentamos la primera demostración de una fuente de onda continua sintonizable en el infrarrojo medio (mid-IR) utilizando el patrón de orientación de fosfuro de galio (OP-GaP) y generación de frecuencia diferencia (DFG) entre láser de fibra Tm a longitud de onda de 2010 nm y otro OPO hecho en el laboratorio basado en un cristal MgO-doped periodically-poled LiNbO3 (MgO:PPLN). La fuente DFG genera hasta 43 mW de potencia de salida, con >30 mW a través de un 96% del rango de sintonización 4608-4694 nm, con una alta calidad de haz. A medida que las fuentes de emisión en el mid-IR sintonizables mejoren sus prestaciones, la disponibilidad de detectores de mid-IR rápidos y sensibles se vuelve igualmente importante. Sin embargo, los detectores de mid-IR convencionales requieren sistemas criogénicos para operar con poco ruido, lo que presenta un gran inconveniente ya que estos dispositivos suelen ser voluminosos y caros. En este contexto, la técnica no lineal de conversión ascendente de frecuencia ha surgido como una alternativa prometedora a la detección directa de la radiación de mid-IR a temperatura ambiente. Un detector de conversión ascendente (UCD) se puede optimizar aún más dentificando y suprimiendo sus fuentes de ruido. Para hacerlo, investigamos teórica y experimentalmente las propiedades de ruido de un UCD de un solo paso bombeado a 1064 nm diseñado para la detección de señales en telecomunicaciones y rango mid-IR, utilizando cristales MgO:PPLN. Descubrimos una nueva fuente de ruido llamada ruido espontáneo paramétrico de conversión descendente inducido por generación de segundo harmónico SHG (SHG-SPDC). Estudiamos, también, la dependencia de la intensidad de este ruido (SHG-SPDC) con la potencia de bombeo y la temperatura del cristal. Finalmente, también comparamos su intensidad con el conocido ruido generado por el detector UCD, llamado ruido de conversión ascendente por conversión paramétrica descendente espontánea (USPDC). Los resultados obtenidos nos dicen que se debe considerar cuidadosamente el SHG-SPDC, ya que puede actuar como una fuente de ruido dominante en ciertas condiciones de operación. Sin embargo, el SHG-SPDC se puede evitar al elegir una combinación adecuada de MgO:PPLN, el período de red la temperatura de operación y el filtro de tipo pasa banda

Monolithic low phase noise oscillators for moderate frequency applications

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (leaves 74-75).Low noise oscillators are critical building blocks in a wide range of commercial electronics. Increased levels of integration have created a strong need for integrated oscillator solutions despite generally inferior noise performance. The development of non-linear noise models that can accurately and efficiently predict noise in ring oscillators aids designers in optimizing noise performance in integrated oscillator solutions. Extending a piecewise constant model of noise in an oscillator and the resulting timing jitter reveals how the noise at the oscillator nodes changes during each portion of the cycle. The model can then be used to examine the effects of changing various process and design parameters such as threshold voltages and the effective stage gain. This analysis tool provides a means for designers to evaluate potential improvements of their oscillator design. In some cases approximate analytic solutions can be found that provide better insight into the timing jitter. A simple differential oscillator design illustrates the use of this analysis. The oscillator achieves an analog tuning range of 259MHz-314MHz (extendable with switched capacitors) with a normalized jitter of 102ppm.by Rafael A. Medina.M.Eng

Solid-state lasers for coherent communication and remote sensing

Semiconductor-diode laser-pumped solid-state lasers have properties that are superior to other lasers for the applications of coherent communication and remote sensing. These properties include efficiency, reliability, stability, and capability to be scaled to higher powers. We have demonstrated that an optical phase-locked loop can be used to lock the frequency of two diode-pumped 1.06 micron Nd:YAG lasers to levels required for coherent communication. Monolithic nonplanar ring oscillators constructed from solid pieces of the laser material provide better than 10 kHz frequency stability over 0.1 sec intervals. We have used active feedback stabilization of the cavity length of these lasers to demonstrate 0.3 Hz frequency stabilization relative to a reference cavity. We have performed experiments and analysis to show that optical parametric oscillators (OPO's) reproduce the frequency stability of the pump laser in outputs that can be tuned to arbitrary wavelengths. Another measurement performed in this program has demonstrated the sub-shot-noise character of correlations of the fluctuations in the twin output of OPO's. Measurements of nonlinear optical coefficients by phase-matched second harmonic generation are helping to resolve inconsistency in these important parameters

Hybrid confinement of optical and mechanical modes in a bullseye optomechanical resonator

Optomechanical cavities have proven to be an exceptional tool to explore fundamental and technological aspects of the interaction between mechanical and optical waves. Such interactions strongly benefit from cavities with large optomechanical coupling, high mechanical and optical quality factors, and mechanical frequencies larger than the optical mode linewidth, the so called resolved sideband limit. Here we demonstrate a novel optomechanical cavity based on a disk with a radial mechanical bandgap. This design confines light and mechanical waves through distinct physical mechanisms which allows for independent control of the mechanical and optical properties. Our device design is not limited by unique material properties and could be easily adapted to allow large optomechanical coupling and high mechanical quality factors with other promising materials. Finally, our demonstration is based on devices fabricated on a commercial silicon photonics facility, demonstrating that our approach can be easily scalable.Comment: 16 pages, 11 figure