Microwave photonics (MWP) is an emerging field in which radio frequency (RF) signals are generated, distributed, processed and analyzed using the strength of photonic techniques. It is a technology that enables various functionalities which are not feasible to achieve only in the microwave domain. A particular aspect that recently gains significant interests is the use of photonic integrated circuit (PIC) technology in the MWP field for enhanced functionalities and robustness as well as the reduction of size, weight, cost and power consumption. This article reviews the recent advances in this emerging field which is dubbed as integrated microwave photonics. Key integrated MWP technologies are reviewed and the prospective of the field is discussed.

Integrated microwave photonics

Photonic signal processing has been considered a solution to overcome the inherent electronic speed limitations. Over the past few years, an impressive range of photonic integrated signal processors have been proposed, but they usually offer limited reconfigurability, a feature highly needed for the implementation of large-scale general-purpose photonic signal processors. Here, we report and experimentally demonstrate a fully reconfigurable photonic integrated signal processor based on an InP–InGaAsP material system. The proposed photonic signal processor is capable of performing reconfigurable signal processing functions including temporal integration, temporal differentiation and Hilbert transformation. The reconfigurability is achieved by controlling the injection currents to the active components of the signal processor. Our demonstration suggests great potential for chip-scale fully programmable all-optical signal processing. Scientists experimentally demonstrate a fully configurable photonic integrated signal processor based on an InP–InGaAs material system by controlling the injection currents to the active components.

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A fully reconfigurable photonic integrated signal processor

Integrated microwave photonics, an emerging technology combining radio frequency (RF) engineering and integrated photonics, has great potential to be adopted for wideband analog processing applications. However, it has been a challenge to provide photonic integrated circuits with equal levels of function flexibility as compared with their electronic counterparts. Here, we introduce a disruptive approach to tackle this need, which is analogous to an electronic field-programmable gate array. We use a grid of tunable Mach–Zehnder couplers interconnected in a two-dimensional mesh network, each working as a photonic processing unit. Such a device is able to be programmed into many different circuit topologies and thereby provide a diversity of functions. This paper provides, to the best of our knowledge, the first ever demonstration of this concept and shows that a programmable chip with a free spectral range of 14 GHz enables RF filters featuring continuous, over-two-octave frequency coverage, i.e., 1.6–6 GHz, and variable passband shaping ranging from a 55 dB extinction notch filter to a 1.6 GHz bandwidth flat-top filter.

https://ris.utwente.nl/ws/files/6955872/optica-2-10-854.pdf

Programmable photonic signal processor chip for radiofrequency applications

Optical analog computing offers high-throughput low-power-consumption operation for specialized computational tasks Traditionally, optical analog computing in the spatial domain uses a bulky system of lenses and filters Recent developments in metamaterials enable the miniaturization of such computing elements down to a subwavelength scale However, the required metamaterial consists of a complex array of meta-atoms, and direct demonstration of image processing is challenging Here, we show that the interference effects associated with surface plasmon excitations at a single metal-dielectric interface can perform spatial differentiation And we experimentally demonstrate edge detection of an image without any Fourier lens This work points to a simple yet powerful mechanism for optical analog computing at the nanoscale

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Plasmonic computing of spatial differentiation.

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r42 = 923 pm V−1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s−1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies. Electro-optic modulators based on epitaxial barium titanate (BTO) integrated on silicon exhibit speeds up to 50 Gbit s–1 while the Pockels coefficient of the BTO film is found to be approaching the bulk value.

Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon

We analyze transmission characteristics of two coupled identical cavities, of either standing-wave (SW) or traveling-wave (TW) type, based on temporal coupled mode theory. Mode splitting is observe ...

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Coupled mode theory analysis of mode-splitting in coupled cavity system.

We propose and experimentally demonstrate a temporal differentiator in optical field based on a silicon microring resonator with a radius of 40 µm. The microring resonator operates near the critical coupling region, and can take the first order derivative of the optical field. It features compact size thus is suitable for integration with silicon-on-insulator (SOI) based optical and electronic devices. The performance of this optical differentiator is tested using signals with typical shapes such as Gaussian, sinusoidal and square-like pulses at data rates of 10 Gb/s and 5 Gb/s.

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Compact optical temporal differentiator based on silicon microring resonator

We propose a silicon-compatible plasmonic electro-optic modulator employing a silicon racetrack ring resonator coupled to a bus waveguide. A silicon-plasmonic hybrid phase shifter clad with electro-optic polymer is introduced to achieve high-speed performance and low energy consumption. Simulations show that the proposed modulator can achieve an extinction ratio of more than 15 dB at 1550-nm wavelength under a 1.2-V bias voltage. The misalignment tolerance and fabrication feasibility of the modulator are also discussed.

Design of an Electro-Optic Modulator Based on a Silicon-Plasmonic Hybrid Phase Shifter

We propose and demonstrate a nested configuration of a silicon microring resonator possessing multiple coupling regimes in one passive device. For different resonance notches of the transmission intensity spectrum, diverse notch depths and bandwidths owing to multiple coupling regimes are designed and experimentally achieved. The phase response of the proposed configuration is also investigated. Experimental observation of fast and slow lights at different resonance wavelengths further verifies the realization of multiple coupling regimes.

Nested Configuration of Silicon Microring Resonator With Multiple Coupling Regimes

Proposed and experimentally demonstrated is photonic generation of ultra-wideband (UWB) monocycle pulses using a silicon microring resonator to perform phase-modulation-to-intensity-modulation conversion. The microring resonator features compact size and thus is cost-effective especially for applications in UWB communication systems that support multiple users. As a microring resonator with a pin diode can function as a modulator, the use of the microring resonator to realise bi-phase modulation is discussed.

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Tao Wang

Papers

Coupled mode theory analysis of mode-splitting in coupled cavity system.

Compact optical temporal differentiator based on silicon microring resonator

Design of an Electro-Optic Modulator Based on a Silicon-Plasmonic Hybrid Phase Shifter

Nested Configuration of Silicon Microring Resonator With Multiple Coupling Regimes

On-chip photonic generation of ultra-wideband monocycle pulses