Application of Nonlinear Optical Mixing to Microwave Photonic Instantaneous Frequency Measurement Lam Anh Bui, 1,* Mark Pelusi, 2 Trung Vo, 2 Niusha Sarkhosh, 1 Hossein Emami, 1 Arnan Mitchell, 1 and Benjamin J. Eggleton 2 1 School of Electrical and Computer Engineering and Centre for Ultra-high bandwidth Devices for Optical Systems (CUDOS), RMIT University, Melbourne, VIC 3001, Australia 2 School of Physics and Centre for Ultra-high bandwidth Devices for Optical Systems (CUDOS), University of Sydney, NSW 2006 , Australia Email: lam.bui@rmit.edu.au Abstract: We demonstrate use of all optical mixing in a highly nonlinear fiber to achieve microwave photonic frequency measurement. The system is simple, compact, predictable and stable with potential applications in next generation radar warning receivers. 2008 Optical Society of America OCIS codes: (060.5625) Radio frequency photonics; (190.4380) Nonlinear optics, four-wave mixing 1. Introduction The availability of highly nonlinear optical fiber (HNLF) has enabled access to a range of optical nonlinearities including self phase modulation (SPM) and four wave mixing (FWM) requiring only modest optical powers that are common in modern communication systems. These HNLF components have been highly successful in all-optical signal processing in the digital domain, enabling processes such as multiplexing and wavelength conversion with data rates approaching THz frequencies[1]. In this paper we demonstrate that the same HNLF components can be effectively utilized for mixing of ultra-broadband analogue signals to achieve microwave photonic functions. An important function that is difficult to achieve in the electronic domain is broadband instantaneous frequency measurement (IFM)[2,3]. An IFM system receives a signal of unknown frequency content and identifies which frequencies are present. The system should be broadband (2-40GHz), must be low latency, must operate continuously and should be simple and relatively inexpensive as banks of these devices are often required to measure several frequencies simultaneously. Recently, IFMs using microwave photonics have been demonstrated. These systems, however, have required expensive broadband photodetectors[4] or ineffective RF cable delays[5]. In this paper we demonstrate a microwave photonic system employing all-optical mixing in HNLF which is capable of measuring frequencies from 2-40GHz. Measurements are exceptionally stable and can be received using low-cost optical power measurement. 2. Method Fig. 1 presents the photonic system used to implement the IFM system. Two optical carriers with different wavelengths were combined using an optical coupler and input to a Mach-Zehnder optical intensity modulator. Both carriers were modulated with the same RF signal to be measured. The two carriers were then reflected from different locations in a cascaded grating, generating a short differential time delay between them. The carriers were amplified using an EDFA and introduced to a highly nonlinear fiber (HNLF) where they mixed via FWM producing two new optical wavelengths. The optical power at one of these new wavelengths was isolated using an optical filter and was detected using a DC photodetector. The optical power received at the output can be related to the input RF frequency if the relative delay is known [5]. To demonstrate the system the carrier wavelengths were 1548.51 and 1551.72nm with a relative delay of 80ps at the cascaded grating. The optical filter had a linewidth of 0.2nm and selected the 1554.93nm FWM product. The frequency was varied between 0.2-40GHz and the DC output voltage was measured. 3. Results and Discussion Fig 2 a) presents the optical spectrum at the output of the HNLF. The two original RF modulated carriers and the two new FWM products are evident. Fig 2b) presents the optical spectrum at the output after optical filtering. Fig 3a) presents the measured DC optical power as a function of frequency. Oscillations with a period of 6.25 GHz are evident as expected. The response attenuates with increasing frequency due to the characteristic response of the MZM and the roll-off of the optical filter at the output. The predicted response of the IFM system incorporating these two factors is also presented in Fig. 3a). Excellent agreement between prediction and measurement is evident. Fig. 3b) presents the interpreted frequencies measured from Fig 3a) by inverting the predicted response[4]. Some deviation is evident at frequencies corresponding to the peaks and nulls of the oscillations in Fig. 3a). a2085_1.pdf CTuZ1.pdf © 2009 OSA/CLEO/IQEC 2009 CTuZ1.pdf 978-1-55752-869-8/09/$25.00 ©2009 IEEE