Photonic-chip-based radio-frequency spectrum analyser with terahertz bandwidth Mark Pelusi 1 * , Feng Luan 1 , TrungD. Vo 1 , Michael R. E. Lamont 1 , Steven J. Madden 2 , Douglas A. Bulla 2 , Duk-Yong Choi 2 , Barry Luther-Davies 2 and Benjamin J. Eggleton 1 Signal processing at terahertz speeds calls for an enormous leap in bandwidth beyond the current capabilities of electronics, for which practical operation is currently limited to tens of giga- hertz 1 . This can be achieved through all-optical schemes making use of the ultrafast response of x (3) nonlinear wave- guides 2 . Towards this objective, we have developed compact planar rib waveguides based on As 2 S 3 glass, providing a virtual ‘lumped’ high nonlinearity in a monolithic platform capable of integrating multiple functions. Here, we apply it to demonstrate, for the first time, a photonic-chip-based, all-optical, radio-frequency spectrum analyser with the per- formance advantages of distortion-free, broad measurement bandwidth (>2.5 THz) and flexible wavelength operation (that is, colourless). The key to this is the waveguide’s high optical non- linearity and dispersion-shifted design. Using the device, we characterize high-bit-rate (320 Gb s 21 ) optical signals impaired by various distortions. The demonstrated ultrafast, broadband capability highlights the potential for integrated chip-based signal processing at bit rates approaching and beyond Tb s 21 . The nonlinear signal propagation underpinning the operation of our set-up arises from intensity-dependent effects, which can take the form of self-phase modulation (SPM), cross-phase modulation (XPM) or four-wave mixing 2 to enable a range of all-optical signal- processing functions, such as regeneration 3,4 , wavelength conver- sion 5–7 , switching 8,9 , phase conjugation 10 and performance monitor- ing 11–14 . These have been reported with various waveguides including fibre 6,7,9,12–14 , silicon 4,5,10 and semiconductor optical ampli- fiers (SOA) (ref. 8). Among these, optical-fibre waveguides exhibiting near-instantaneous x (3) nonlinearity 2 provide unrivalled speed capa- bility. However, the low optical nonlinearity of most silica-based fibres means a long propagation length is necessary, leading to detri- mental effects owing to dispersion. Although compact silicon chip devices have been reported, their free-carrier effects and two- photon absorption 5 can inhibit ultrafast signal processing. The SOA offer compact solutions, but their signal bandwidth can also be limited by free carriers. So, finding an alternative compact, ultra- high nonlinearity waveguide is a compelling requirement for future high-speed optical communication systems. An application highlighting the enormous bandwidth potential of all-optical schemes over electronics is the monitoring of the radio frequency (RF) spectrum of a signal, that is, the power spec- trum of its intensity waveform. Used routinely in telecommunica- tions and microwave photonics for measuring distortions in amplitude or phase 15 and for characterizing components, it typically uses an expensive high-speed photodetector in combination with an electrical spectrum analyser. The electronic bandwidth limits of this approach, however, make it inadequate for measuring signals much faster than 40 Gb s 21 . Here, we report the first demonstration of an all-optical terahertz- bandwidth RF spectrum monitor based on a monolithic x (3) nonlinear waveguide. Our approach makes use of XPM in a compact dispersion- engineered photonic chip fabricated from highly nonlinear As 2 S 3 chal- cogenide (ChG) glass. The principle of the all-optical RF spectrum analyser 16 is summarized in Fig. 1a. The RF spectrum of an optical signal (with electric field E) is the power spectrum of its temporal intensity (jE(t)j 2 ) given by the squared magnitude of its Fourier trans- form, S RF (v) ¼j Ð jE(t)j 2 e ivt dtj 2 , where t and v are time and angular frequency, respectively, and Ð denotes integration from 21 to þ1. This differs from the power spectrum of the electric field itself, given by S o (v) ¼ j Ð E(t)e ivt dtj 2 , which is obtained from an optical spectrum analyser (OSA). The optical signal under test at wavelength l s is launched into a x (3) nonlinear waveguide with a weak continuous- wave (c.w.) probe at wavelength l p . By the optical Kerr effect, the wave- guide refractive index n varies with the signal’s temporal intensity I according to n(I) ¼ n 0 þ n 2 I for linear and nonlinear refractive indexes of n 0 and n 2 , respectively 2 . This induces XPM of the probe on an ultrafast timescale, whereby its phase is modulated in proportion to IgL (ignoring propagation losses), where L is the waveguide length and g the waveguide nonlinearity coefficient 2 given by g ¼ (2p/l s ) (n 2 /A eff ) for an effective mode area A eff . This generates frequency modulation sidebands around the monochromatic probe of frequency v 0 (corresponding to l p ) according to S o 0 (v) / jgLj 2 S RF (v – v 0 ), which enables S RF (v) to be measured on an OSA (ref. 16). This scheme is capable of an enormous measurement bandwidth, greater than 10 THz, which is ultimately limited by the response time of the waveguide nonlinearity. Previous all-optical RF spectrum monitors have reported much narrower bandwidths owing to the requirement for long lengths of highly nonlinear silica fibre (HNF), in which the broadband dispersion characteristics degrade the phase (and group velocity) matching between the signal and probe 16 . To overcome this, our photonic-chip RF spectrum analyser (PC- RFSA) takes advantage of the high n 2  3  10 218 m 2 W 21 of As 2 S 3 (.100 times of silica) and our ability to tailor A eff to very small dimensions. This enhances g to hundreds of times that of HNF, allowing a much shorter waveguide to be used, and also produces strong waveguide dispersion, which compensates the material dis- persion, reducing the net dispersion to near zero at 1,550 nm wave- length. This results in a dramatically reduced footprint and the consequent performance advantages of distortion-free, broad measurement bandwidth (.2.5 THz) and colourless operation, allowing our device to be operated on any desired channel. A schematic and image of the fabricated waveguide 17 are shown in Fig. 1b. Reducing the As 2 S 3 film thickness was key to our design, as it leads to a strong waveguide dispersion that can offset the large material component of 2364 ps nm 21 km 21 (normal dispersion) 18 . This is evident for the transverse-magnetic (TM) mode when its field permeates 1 CUDOS, Institute for Photonic Optical Sciences (IPOS), School of Physics, University of Sydney, New South Wales 2006, Australia, 2 CUDOS, Laser Physics Centre, The Australian National University, Canberra, ACT 0200, Australia; *e-mail: m.pelusi@physics.usyd.edu.au LETTERS PUBLISHED ONLINE: 15 FEBRUARY 2009 | DOI: 10.1038/NPHOTON.2009.001 NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics 1 © 2009 Macmillan Publishers Limited. All rights reserved.