RF arbitrary waveform generation using tunable planar lightwave circuits P. Samadi a, ⁎, L.R. Chen a , C. Callender b , P. Dumais b , S. Jacob b , D. Celo b a Department of Electrical and Computer Engineering, McGill University, Montreal, QC, Canada H3A 2A7 b Photonic Component Technologies Group, Communications Research Center, Ottawa, ON, Canada K2H 8S2 abstract article info Article history: Received 15 December 2010 Received in revised form 26 February 2011 Accepted 27 February 2011 Available online 12 March 2011 Keywords: RF photonics Planar lightwave circuits Optical pulse shaping Arbitrary waveform generation Lattice-form Mach-Zehnder interferometer We demonstrate photonically-assisted generation of RF arbitrary waveforms using planar lightwave circuits (PLCs) fabricated on silica-on-silicon. We exploit thermo-optic effects in silica in order to tune the response of the PLC and hence reconfigure the generated waveform. We demonstrate the generation of pulse trains at 40 GHz and 80 GHz with flat-top, Gaussian, and apodized profiles. These results demonstrate the potential for RF arbitrary waveform generation using chip-scale photonic solutions. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Due to the extensive applications of ultrabroad-bandwidth radio- frequency (RF) pulses in several domains such as radar systems [1], wireless communications [2], and remote sensing [3], it is necessary to find a flexible and low-cost method to generate arbitrary RF waveforms. The bandwidths needed for these applications range from the ultra- wideband domain (3.1–10.6 GHz) to the mm-wave regime (~30– 100 GHz). Due to limitations in digital-to-analog conversion technolo- gies, conventional electronic approaches are not able to generate RF waveforms with very high bandwidths. Photonically-assisted RF waveform generation is a powerful technique to overcome the bandwidth limitations of electronics. Moreover, it has the advantage of direct data transmission in the optical domain to a remote location and flexibility in reconfiguring the synthesized waveforms. The general schematic for photonically-assisted RF arbitrary wave- form generation using optical pulse shapers is shown in Fig. 1. This method is based on generating a temporal optical waveform with desired features from an input pulse using an optical pulse shaper, and then converting the shaped waveform to an electrical signal using a broadband photo-detector. The pulse shaping process can be accomplished using time domain or frequency domain techniques. One well-established technique is using pulse shapers either in bulk or arrayed waveguide gratings for shaping the spectra of a broadband coherent or incoherent source through Fourier synthesis and converting it to the time-domain using frequency-to-time mapping [4–6]. Another bulk-optic approach is the direct space-to-time pulse shaper (DTS) which was demonstrated in [7]. The DST consists of a diffractive optical element (DOE) which splits a single input beam into multiple, nearly equal intensity, spatial spots. Subsequent to the DOE, there is a spatial mask to adjust the period, spacing, position, and amplitude of the spots. The main advantage of bulk- optic methods is the ability to tune precisely the spectral components. However, they require a very short input pulse to have a wideband spectrum and they have the common drawbacks which include strict alignment requirements, low power efficiency, and sensitivity to the environment. Pulse shaping can also be performed using fiber Bragg gratins (FBGs). For example, Shen et al., presented a grating and delay structure for amplitude weighting and time delaying the optical samples [8] while Wang et al. demonstrated optical pulse shaping using a single spatially discrete chirped fiber Bragg grating [9]. Another technique for pulse shaping is synthesizing an optical pulse waveform by coherently superposing a set of properly delayed replicas of the input pulse, e.g., using a conventional multi-arm interferometer [10]. An on-chip integrated pulse shaper is a desirable solution to overcome the limitations commonly associated with conventional bulk optics pulse shapers. In a recent work, a spectral pulse shaper is integrated by cascaded multiple-channel micro-ring resonators on a silicon-on-insulator platform; the temporal waveform is then obtained using frequency-to-time mapping [11]. By this approach, it is possible to completely control the RF waveform, including amplitude, frequency, and phase. However, ring resonators require tight control over the fabrication process; otherwise, errors can degrade significantly their response. Another attempt for integrated photonic microwave devices is the programmable photonic micro- wave filter with tunable inter-ring coupling [12]. This device is fabricated using InP/InGaAsP material and is able to generate 2nd and 3rd order coupled ring filters. The tunablity is achieved by tunable couplers and phase modulators. Cascading several stages of the unit Optics Communications 284 (2011) 3737–3741 ⁎ Corresponding author. E-mail address: payman.samadi@mail.mcgill.ca (P. Samadi). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.02.076 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom