A fully reconfigurable photonic integrated signal processor Weilin Liu 1‡ , Ming Li 1†‡ , Robert S. Guzzon 2‡ , Erik J. Norberg 2 , John S. Parker 2 , Mingzhi Lu 2 , Larry A. Coldren 2 and Jianping Yao 1 * 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. O ne of the fundamental challenges for digital signal processing (DSP) is the limited speed, largely restricted by the electronic sampling rate. In an optical network, signal processing is implemented based on DSP, which involves electronic sampling, optical-to-electrical (OE) and electrical-to-optical (EO) conver- sions. A solution to achieve power-efficient and high-speed signal processing in an optical network is to implement signal processing directly in the optical domain using a photonic signal processor to avoid the need for electronic sampling, OE and EO conversions 1–3 . Numerous photonic signal processors have so far been reported based on either discrete components or photonic integrated cir- cuits 1–10 . Photonic signal processors based on discrete components usually have decent programming abilities but are more bulky and less power efficient, whereas a photonic integrated signal processor has a much smaller footprint and a higher power efficiency. A photonic signal processor can be used to implement fundamental signal generation and processing functions such as optical pulse shaping and arbitrary waveform generation 1 , optical dispersion compensation 7 , temporal integration 8 , temporal differentiation 9 and Hilbert transformation 10 . These functions are basic building blocks of a general-purpose signal processor for signal generation and fast computing. Fast computing processes such as temporal integration, temporal differentiation and Hilbert transformation have important applications 11–22 . For example, a photonic integrator is a device that is able to perform the time integral of an optical signal, which has applications in dark soliton generation 12 , optical memory 13 and optical analog–digital conversion 14 . One of the most important characteristic parameters of a photonic integrator is the integration time. A long integration time means a better inte- gration capability. An ideal photonic temporal integrator should have an infinite integration time. An on-chip all-optical integrator compatible with complementary metal oxide–semiconductor (CMOS) technology was reported 15 , based on an add-drop ring resonator with an integration time of 800 ps. For many applications, however, an integration time as long as a few nanoseconds is needed. To achieve such a long integration time, the insertion loss must be precisely compensated to obtain a high Q-factor, which is very chal- lenging, particularly for stable operation without causing lasing. In addition, an integrator with a fractional or higher order is also needed, which is more difficult to implement 16 . A photonic tem- poral differentiator 17 is a device that performs temporal differen- tiation of an optical signal, and has applications in all-optical Fourier transform 18,19 , temporal pulse characterization 20 and the demultiplexing of an optical time division multiplexed (OTDM) signal 21 , for example. A photonic Hilbert transformer is a device that derives the analytic representation of a signal 10 , and has been widely used for single-sideband (SSB) modulation. Optical SSB modulation is particularly useful in a radio-over-fibre (ROF) link to avoid dispersion-induced power fading 22 . Although the photonic implementations of these functions have been reported 8–10,15,17 ,a signal processor is usually designed to perform a specific function with no or very limited reconfigurability. For general-purpose signal processing, however, a photonic signal processor should be able to perform multiple functions with high reconfigurability. In this Article, we report the design, fabrication and experimental demonstration of a fully reconfigurable photonic integrated signal processor to perform the three signal processing functions intro- duced above. The photonic signal processor consists of three active microring resonators (R1, R2, and R3) and a bypass wave- guide as a processing unit cell, as shown in Fig. 1a,b. To obtain on-chip reconfigurability, we incorporate nine semiconductor optical amplifiers (SOAs) and twelve current-injection phase modulators (PMs) in the unit cell, as shown in Fig. 1b. The tunable coupling between two neighbouring rings and between the outer ring and the bypass waveguide is realized using four tunable couplers (TCs) with each consisting of two multi-mode interference (MMI) couplers and two PMs, as shown in the inset in Fig. 1b. The coupling ratio in each TC can be tuned by adjusting the injection currents to the two PMs in the TC. Within each ring there are two SOAs used to compensate for the waveguide propa- gation loss and the MMI splitting loss and insertion loss. When an SOA is forward biased it can create an optical gain. On the 1 Microwave Photonics Research Laboratory, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada. 2 Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, California 93116, USA. † Present address: State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. ‡ These authors contributed equally to this work. *e-mail: jpyao@eecs.uottawa.ca ARTICLES PUBLISHED ONLINE: 15 FEBRUARY 2016 | DOI: 10.1038/NPHOTON.2015.281 NATURE PHOTONICS | VOL 10 | MARCH 2016 | www.nature.com/naturephotonics 190 © 2016 Macmillan Publishers Limited. All rights reserved