Graphene, the single-atom-thick form of carbon, holds promise for many applications, notably in electronics where it can complement or be integrated with silicon-based devices. Intense efforts have been devoted to develop a key enabling device, a broadband, fast optical modulator with a small device footprint. Now Liu et al. demonstrate an exciting new possibility for graphene in the area of on-chip optical communication: a graphene-based optical modulator integrated with a silicon chip. This new device relies on the electrical tuning of the Fermi level of the graphene sheet, and achieves modulation of guided light at frequencies over 1 gigahertz, together with a broad operating spectrum. At just 25 square micrometres in area, it is one of the smallest of its type. Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects1,2. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties3. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms4,5,6. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances7. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm2, which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

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A graphene-based broadband optical modulator

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Micrometre-scale silicon electro-optic modulator

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.

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Silicon optical modulators

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

The pace of the development of silicon photonics has quickened since 2004 due to investment by industry and government. Commercial state-of-the-art CMOS silicon-on-insulator (SOI) foundries are now being utilized in a crucial test of 1.55-mum monolithic optoelectronic (OE) integration, a test sponsored by the Defense Advanced Research Projects Agency (DARPA). The preliminary results indicate that the silicon photonics are truly CMOS compatible. RD however, lasing has not yet been attained. The new paradigm for the Si-based photonic and optoelectric integrated circuits is that these chip-scale networks, when suitably designed, will operate at a wavelength anywhere within the broad spectral range of 1.2-100 mum, with cryocooling needed in some cases

The Past, Present, and Future of Silicon Photonics

Solid state beam steering devices are key elements in low cost, robust, three-dimensional imaging systems. Here we present a silicon photonic beam steering device based upon an 8 x 8 grating coupler focal plane array approach fed by a thermo-optic Mach Zehnder switching tree. In this device transmission of the light from the grating couplers is made through the back side of the chip using topside mirrors allowing for both high efficiency out coupling and direct flip-chip integration of drive electronics, providing a path to scale to denser focal plane arrays with large numbers of points in the future. -13.8dB fibre-to-fibre transmission was achieved in our preliminary test around 1523 nm for the beam steering chip.

Silicon photonic beam steering module with backside coupling elements towards dense heterogeneous integration with drive electronics

Due to their long pulse lengths (120 ns-150 ns duration), alexandrite lasers are very useful, where both high-power and high-frequency resolution are required However, in order to meet the requirements for frequency stability and linewidth, injection seeding of the alexandrite laser is necessary Such laser systems have been reported in the literature and are commercially available today We analyzed the frequency linewidth of a PAL101 alexandrite laser system from LightAge using a slowly scanning confocal etalon with a length of 50 cm

Linewidth narrowing of a pulsed alexandrite laser by intracavity phase modulation

A device with optical switching between multiple layers of a semiconductor die is disclosed. In one aspect of the present invention, the disclosed apparatus includes a first semiconductor material layer of a semiconductor die. The first semiconductor material layer has a first optical waveguide. A second semiconductor material layer is also included in the semiconductor die. The second semiconductor material layer has a second optical waveguide. An insulating layer is disposed between the first and second semiconductor material layers such that there is an evanescent coupling between the first and second semiconductor material layers. There are modulated charge layers proximate to the insulating layer such that a coupling length of the evanescent coupling is controlled in response to the modulated charge layers.

Three dimensional semiconductor based optical switching device

Summary form only given.Laser sources with energies in the microjoule to millijoule range are used in a multitude of applications. Many applications require narrow linewidth and tunability. The traditional approach employed is to use a low power, narrow linewidth master laser to injection seed a high power slave oscillator. In order for the oscillator to operate at the seeder frequency the cavity of the former has to be resonant with the seeder mode. Two alternative approaches are currently employed to achieve this goal: (1) the minimization of the build-up time or (2) the ramp-fire technique, in which the Q-switch is fired, when a resonance is detected while one cavity mirror is ramped rapidly. An alternative approach is presented. The beam of a low power seed source is gated with an electro-optic gate and then consecutively amplified first by a high gain fiber amplifier (up to 60 dB gain) followed by four-passing through a bulk amplifier. In contrast to a Q-switched fiber laser, our configuration does not form a closed cavity and no resonance condition must be fulfilled.

Generation of narrow linewidth pulses via amplification in a chain of fiber/bulk amplifiers

Accurate 3D imaging is essential for machines to map and interact with the physical world1,2. While numerous 3D imaging technologies exist, each addressing niche applications with varying degrees of success, none have achieved the breadth of applicability and impact that digital image sensors have achieved in the 2D imaging world3-10. A large-scale twodimensional array of coherent detector pixels operating as a light detection and ranging (LIDAR) system could serve as a universal 3D imaging platform. Such a system would other high depth accuracy and immunity to interference from sunlight, as well as the ability to directly measure the velocity of moving objects11. However, due to difficulties in providing electrical and photonic connections to every pixel, previous systems have been restricted to fewer than 20 pixels12-15. Here, we demonstrate the first large-scale coherent detector array consisting of 512 (32×16) pixels, and its operation in a 3D imaging system. Leveraging recent advances in the monolithic integration of photonic and electronic circuits, a dense array of optical heterodyne detectors is combined with an integrated electronic readout architecture, enabling straightforward scaling to arbitrarily large arrays. Meanwhile, two-axis solid-state beam steering eliminates any tradeoff between field of view and range. Operating at the quantum noise limit16,17, our system achieves an accuracy of 3.1 mm at a distance of 75 meters using only 4 mW of light, an order of magnitude more accurate than existing solid-state systems at such ranges. Future reductions of pixel size using state-of-the-art components could yield resolutions in excess of 20 megapixels for arrays the size of a consumer camera sensor. This result paves the way for the development and proliferation of low cost, compact, and high-performance 3D imaging cameras, enabling new applications from robotics and autonomous navigation to augmented reality and healthcare.

Remus Nicolaescu

Papers

Silicon photonic beam steering module with backside coupling elements towards dense heterogeneous integration with drive electronics

Linewidth narrowing of a pulsed alexandrite laser by intracavity phase modulation

Three dimensional semiconductor based optical switching device

Generation of narrow linewidth pulses via amplification in a chain of fiber/bulk amplifiers

3D imaging via silicon-photonics-based LIDAR