OTh4D.3.pdf OFC/NFOEC Technical Digest © 2013 OSA Photonic Processing Using Integrated Optical Filters C.K. Madsen*, Q. Chen, J. Kim and Y. Zhou Solid-State Electronics, Photonics and Nano-Engineering Laboratory, Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX, 77843-3128 firstname.lastname@example.org* Abstract: We will discuss photonic processing tasks that are enabled or assisted by integrated optical filters. A novel platform incorporating a highly nonlinear glass on an electro-optic substrate will be reviewed, namely arsenic-trisulfide (As2S3)-on-LiNbO3 waveguides. ©2013 Optical Society of America OCIS codes: (230.5750) Resonators; (130.3730) Lithium Niobate; (230.7370) Waveguides 1. Introduction Optical filters play a critical role in fiber-optic communication systems as well as for signal processing in radiofrequency (RF) photonics and sensing applications. While a strong emphasis has been placed on silicon photonics and many advanced integrated optical filters realized, other material systems offer unique capabilities. In this paper, we’ll review work on a novel platform that combines a highly nonlinear glass on an electro-optic waveguide layer, namely arsenic-trisulfide (As2S3)-on-LiNbO3 waveguides. Ring resonators are a fundamental building block for achieving sharp features in the frequency response with only a few stages. The optical waveguide modes must be highly confined to provide low-loss, small-bend-radii rings. Ring resonators with large free-spectral ranges (FSRs) and high quality factors have been demonstrated on numerous material platforms, including silica , III-Vs , silicon-on-insulator  and As2S3 . The higher refractive index of As2S3 allows vertical waveguide integration with LiNbO3 waveguides, which provide electro-optic modulation and high coupling efficiency to standard singlemode optical fibers ,as well as the realization of ring resonators. Important properties of As2S3 include its transparency from the visible to the far infrared (8µm) range and nonlinearity. We will review three photonic processing applications using integrated optical filters realized with this hybrid platform 2. Nonlinear Frequency Modulated Waveform Generator Two characteristics of great importance in laser detection and ranging (LADAR) systems are long range performance and fine range resolution. Waveforms that achieve these characteristics are desired, and many different waveforms have been investigated including linear frequency modulation (LFM) chirped waveforms, pseudorandom phase modulated waveforms, poly-phase (P4) waveforms, and others. Sidelobe reduction can be accomplished by creating a non-linear frequency modulated (NLFM) chirp, which can also avoid carrier-to-noise losses. We have reported a novel method for optically generating an NLFM waveform that approximates a tanhfunction using a ring resonator with a modulator in the feedback path and a standard phase modulator as shown in the inset of Fig. 1 . A simulation of the signal and its frequency chirp are shown in the graph. Through simulation, the maximum sidelobe level of the autocorrelation of an NLFM waveform generated by a series of tunable integrated optical ring resonators is shown to be -20 to -30 dB or lower. A key step in realizing the capabilities of this hybrid device platform is the demonstration of electro-optic tuning, which allows for on-chip reconfigurable optical devices and high-speed modulators. While the As2S3 waveguide is a rib waveguide that is external to the substrate, through different design parameters the mode confinement in the As2S3 waveguide can be controlled. By adjusting the As2S3 waveguide width and thickness, along with the silicon dioxide (SiO2) cladding/buffer layer thickness, a portion of the optical mode can be kept in the electro-optically tunable LiNbO3 substrate. An electro-optically tunable, vertically integrated As2S3 Mach-Zehnder interferometer (MZI) side coupled to a Ti-diffused waveguide on LiNbO3 substrate was recently demonstrated , as shown in Fig. 2. Asymmetric coplanar strip electrodes were placed along the As2S3 path and show strong electrooptic tuning capability, shifting the FSR of the interferometer. Work on tuning ring resonators has also been recently completed . 978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America OTh4D.3.pdf Fig. 1. Simulation of time-varying signal amplitude and instantaneous frequency showing tanh-like chirp. 3. OFC/NFOEC Technical Digest © 2013 OSA Fig. 2. Wavelength response for MZI as DC voltage is applied to electrodes. (inset shows device schematic) Linearized Frequency Discriminator While MZIs represent a baseline device for frequency discriminators, ring-assisted MZIs can enhance the frequency response linearity . Nonlinearity in the frequency response produces unwanted intermodulation distortion (IMD). In particular, suppression of 3rd order IMD (IMD3) is important because the generated frequencies are close to the signal frequency, which makes it difficult to be filtered. A schematic of a ring-assisted discriminator is shown in the inset of Fig. 3. An MZI has a ring resonator on one arm and a phase shifter on the other arm. Unlike most ringassisted MZI filters, for which bandwidth is determined by the MZI’s FSR and the ring assists with its unique phase response to improve MZI’s performance, the discriminator’s bandwidth is solely determined by the ring resonator while the FSR of the MZI provides a constant phase shift between the arms as long as the MZI’s FSR is much wider than the ring’s FSR. Since this device does not require a precise control to match both the ring and MZI’s FSR, fabrication tolerances of an MZI-assisted ring resonator are more relaxed than that of a ring-assisted MZI filters. Simulation results show that linearity of the discriminator is dependent on zero magnitude of the ring resonator, which is determined by the coupling efficiency between the ring and bus waveguide. Simulations are shown for an 89% power coupling ratio along and the subsequent enhanced linearity. Fabrication and experimental results have recently been obtained and will be published shortly. 0.1 1 RING MZI Deviation Intensity MZI Ring Linear Fit 0.5 3dB Coupler 0 Ring Resonator ϕ0 0 3dB Coupler -0.1 0.5 Normalized Frequency (a) 1 0.2 0.3 0.4 0.5 0.6 Normalized Frequency 0.7 0.8 (b) Fig. 3. Simulation result for the discriminator (a) magnitude reponse and (b) its deviation from a linear fit. An ideal ring resonator (no roundtrip loss) is assumed. The zero magnitude of the ring is |z|=3, which corresponds to 89% of coupling between ring-bus waveguide. OTh4D.3.pdf 4. OFC/NFOEC Technical Digest © 2013 OSA Nonlinear Optical Processing For a nonlinear ring resonator device, all-optical switching can be achieved by tuning the refractive index inside the ring waveguide via self or cross-phase modulation using substantially lower optical powers than required for straight waveguides or MZI-based devices. Ultra-high Q, long path ring resonators have been demonstrated using As2S3on-LiNbO3 waveguides . In these results, a ring resonator with 1.2 dB/cm propagation loss and a 1.7-cm round-trip length was realized, resulting in an FSR of 7 GHz. For a nonlinear index change of 0.005, the magnitude response is shifted substantially as shown in Fig. 4. The waveguide materials are scalable to the mid-infrared region, where low loss waveguides have already been demonstrated . We have designed dispersion-engineered waveguides for wavelength converters based on four wave mixing (FWM) that allow efficient conversion between the near- and mid-infrared regions. Simulation results are shown for phase-match efficiency in Fig. 5 for a pump of 2.05Pm and signal and idler wavelengths of 1.55 and 3.03 Pm . FWM phase-matching efficiency 1 -10 0.6 -30 0.4 -40 -50 0.2 1550.05 1550.1 Wavelength (nm) 1550.15 1550.2 Fig. 4. Simulation of all-optical tunable ring magnitude response based on fabrication parameters.             0.8 -20 -60 1550 5. w=1.4Pm h=1.7Pm with 0.18Pm MgF2 w=1.5Pm h=1.685Pm pump off pump on K2 Magnitude response (dB) 0 0 1 2 3 Wavelength(Pm) 4 5 Fig. 5. FWM phase-matching efficiency as a function of signal wavelength References T. Kominato, et al., "Silica-based finesse-variable ring resonator," Photonics Technology Letters, IEEE, vol. 5, pp. 560-562, 1993. D. Rafizadeh, et al., "Waveguide-coupled AlGaAs / GaAs microcavity ring and disk resonators with high f inesse and 21.6-nm f ree spectral range," Opt. Lett., vol. 22, pp. 1244-1246, 1997. Q. Xu, et al., "Micrometre-scale silicon electro-optic modulator," Nature, vol. 435, pp. 325-327, 2005. Y. Zhou, et al., "Two-Stage Taper Enhanced Ultra-High Q As2S3 Ring Resonator on LiNbO3," Photonics Technology Letters, IEEE, vol. 23, pp. 1195-1197, 2011. M. E. Solmaz, et al., "Vertically integrated As2S3 ring resonator on LiNbO3," Opt. Lett., vol. 34, pp. 17351737, 2009. D. B. Adams, et al., "NLFM waveform generation using tunable integrated optical ring resonators: simulation and proof of concept experiment," Opt. Express, vol. 18, pp. 12537-12542, 2010. W. Snider, et al., "Electro-optically Tunable As2S3 on Ti:LiNbO3 Mach-Zehnder Interferometer," Photonics Technology Letters, IEEE, vol. 24, pp. 1415-1417, 2012. W. Snider, "Fabrication of Electro-Optically Tunable Microring Resonators for Non-Linear Frequency Modulated Waveform Generation," PhD, Electrical and Computer Engineering, Texas A&M University, College Station, TX, 2012. C. K. Madsen and J. H. Zhao, Optical Filter Design and Analysis: A Signal Processing Approach. New York, NY: John Wiley, 1999. Y. Zhou, et al., "Ultra-high Q long-path As2S3 ring resonator on LiNbO3," in Lasers and Electro-Optics (CLEO), 2011 Conference on, 2011, pp. 1-2. X. Xia, et al., "Low-loss chalcogenide waveguides on lithium niobate for the mid-infrared," Opt. Lett., vol. 35, pp. 3228-3230, 2010. Q. Chen, et al., "Phase-matching and parametric conversion for the mid-infrared in As2S3 waveguides," Optics and Photonics Journal, accepted for publication 2012.