Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

/pdf/silicon-optical-modulators-578o7o6kvr.pdf

Silicon optical modulators

Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only ∼20 MHz (refs 10, 11), although it has been predicted theoretically that a ∼1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor

In order to enable optical systems to operate with a high degree of compactness and reliability it is necessary to combine large number of optical functions in small monolithic structures. A development, somewhat reminiscent of that that took place in Integrated Electronics, is now beginning to take place in optics. The initial challenge in this emerging field, known appropriately as "Integrated Optics", is to demonstrate the possibility of performing basic optical functions such as light generation, coupling, modulation, and guiding in Integrated Optical configurations. The talk will review the main theoretical and experimental developments to date in Integrated Optics. Specific topics to be discussed include: Material considerations, guiding mechanisms, modulation, coupling and mode losses. The fabrication and applications of periodic thin film structures will be discussed.

Integrated optics

Silicon photonics could enable a chip-scale platform for monolithic integration of optics and microelectronics for applications of optical interconnects in which high data streams are required in a small footprint. This paper discusses mechanisms in silicon photonics for waveguiding, modulating, light amplification, and emission. These mechanisms, together with recent advances of fabrication techniques, have enabled the demonstration of ultracompact passive and active silicon photonic components with very low loss.

Guiding, modulating, and emitting light on Silicon-challenges and opportunities

We analyze, from a theoretical point of view, a novel silicon optical amplitude-phase modulator integrated into a SOI (silicon on insulator) optical waveguide and based on a three terminal electronic structure which gives rise to definite advantages in comparison with a classical p-i-n diode based modulator. The proposed device utilizes the free carrier dispersion effect to produce the desired refractive index and absorption coefficient variations. The MEDICI two-dimensional (2-D) semiconductor device simulator has been used to analyze the electrical operation, with reference to the free carrier concentration injected into the optical channel, its uniformity and the required current density and electrical power. The optical investigation was carried out by means of FDM (finite difference method), EIM (effective index method), and BPM (beam propagation method) tools, giving rise to a complete evaluation of the properties of our device. We report the results for both the amplitude and phase modulators, paying attention to the static and the dynamic behavior. In particular, an amplitude modulation of 20%, with an injection power of about 126 mW, and a switching time of 5.6 ns can be achieved theoretically, Furthermore, as a phase modulator, the device exhibits a very high figure of merit, predicting an induced phase shift per volt per millimeter of about 215/spl deg/, for a injection power of about 43 mW, and a switching time shorter than 3.5 ns.

Silicon electro-optic modulator based on a three terminal device integrated in a low-loss single-mode SOI waveguide

A quantitative measurement of laser-beam quality can be performed by determination of the presence of multiple transverse modes of the laser oscillator and by calculation of their power content. Along this line of argument, we discuss a new approach that, starting from near-field and far-field intensity measurements, can evaluate the complex excitation coefficients of the transverse modes in a laser beam. The exploitation of near-field measurements sharply improves the performances of the technique in those cases in which only far-field measurements are used. The validity of the method is confirmed by several accurate numerical simulations and by some experimental results relative to a multimode Q-switched Nd:YAG laser.

Transverse mode analysis of a laser beam by near- and far-field intensity measurements.

A high sensitivity, pump–probe optical method to measure the bulk minority-carrier lifetime and surface recombination velocity in silicon is proposed. A pig-tailed 1.55 μm CW laser diode is used to detect the transient of the excess carriers generated by a short Nd:YAG laser pulse. The probe beam is launched parallel to the sample surface while the pump beam uniformly illuminates the sample perpendicular to its surface. Separation between bulk lifetime and surface recombination velocity is achieved by performing the measurement on samples of different thickness. Experimental results indicate that separation of bulk and surface contribution is feasible in a wide range of surface recombination velocities thus making this method suitable to investigate the effects of surface passivation techniques.

An optical technique to measure the bulk lifetime and the surface recombination velocity in silicon samples based on a laser diode probe system

The spectral performances of nonideal rectangular Bragg gratings, integrated in a rib waveguide, are analyzed by a multilayer approach based on the effective-index method. The effects of errors on the photolithographic definition of the grating, that is, period and shape, and of errors on the control of etching depth are investigated. Also the influence of the stitching error, which is unavoidable when the grating is realized by means of electron-beam photolithography, is addressed. A novel analytical approach that extends coupled-mode theory to the analysis of real gratings is also presented.

Effect of fabrication errors in channel waveguide Bragg gratings

In the last decade, in light of their superior mechanical properties advanced polymer matrix composites have been indicated as the most suitable candidates as Smart Materials and Structures. However, their final properties are strongly dependent on the processing stage and key points to improve the quality and the reliability of these materials that have been identified in the cure monitoring and the optimization of the manufacturing process. Based on this line of argument, an integrated fiber optic sensing system for simultaneous refractive index and temperature measurements has been designed and developed in order to monitor the curing process of thermoset based composites. A fiber optic refractometer has been designed by using the free end of the sensing optical fiber. A theoretical model has been developed for converting refractive index changes in detailed information on the extent of the curing. Its validation has been proved by comparison with calorimetric characterization. In addition, integrated fiber Bragg gratings has been used for local temperature measurements. The interrogation of the sensing Bragg grating has been implemented by using a different fiber Bragg grating able to convert the resonance wavelength shift in intensity changes. Preliminary results are presented.

Antonello Cutolo

Papers

Silicon electro-optic modulator based on a three terminal device integrated in a low-loss single-mode SOI waveguide

Transverse mode analysis of a laser beam by near- and far-field intensity measurements.

An optical technique to measure the bulk lifetime and the surface recombination velocity in silicon samples based on a laser diode probe system

Effect of fabrication errors in channel waveguide Bragg gratings

Integrated fiber optic sensing system for in-situ characterization of the curing process of thermoset-based composites