Spurious free frequency doubler using a dual-parallel optical Mach- Zehnder modulator Tetsuya Kawanishi 1 , Keizo Inagaki 1 , Atsushi Kanno 1 and Hitoshi Kiuchi 2 1 National Institute of Information and Communications Technology 4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan, e-mail: kawanish@nict.go.jp 2 National Astronomical Observatory of Japan 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan, e-mail: hitoshi.kiuchi@nao.ac.jp Abstract We propose a novel photonic-based signal generation technique using a dual parallel Mach-Zehnder modulator. Frequency doubled components without spurious can be generated at a photodetector. This technique is useful for measurement of photodetector nonlinearity. Introduction Radio-on-fiber technology plays important roles in wireless systems [1-3], radio astronomy [4], etc. Lightwaves modulated by high-frequency components are converted into micro- or millimeter-wave signals by using high-speed photodetectors [5, 6]. The quality and intensity of generated radio-waves depend on the performance of lightwave modulation devices and photodetectors. Thus, the characterization of components for RoF would be very important. Optical two-tone signals are useful for calibration of high- speed photodetecotors [7]. The ideal response for a pure optical two-tone signal can be defined by the total optical power input to the photodetector. Thus, the conversion efficiency can be precisely estimated by the ratio between the input optical power and the output radio-wave power. Measurement of linearity of detectors is also very important. Electric two-tone test is commonly useful for electric components. However, if we apply two different frequency components to an optical modulator, it is difficult to distinguish nonlinearity of a photodetector from that of the modulator. By using a pair of optical two-tone signals, we can achieve the two-tone test for photodetectors, where the wavelength separation between the two optical light sources should be much wider than that of the two spectral components in a two-tone signal. Thus, the photodetectors should have flat response against the wavelength of the optical input. However, in general, detectors would have wavelength dependence. For example, detectors with multi- sections or resonant structures would have large wavelength dependence. In this paper, we propose a novel technique for spurious free frequency doubler which can generate an electric two-tone radio-wave signal at a photodetector. Thus, we can estimate nonlinearity of a photodetector at a particular optical wavelength. In addition, this technique would be useful for frequency domain multiplexing of wideband RoF signals without undesired nonlinear effect at modulation. Principle of operation Figure 1 shows the principle of operation. By using a dual parallel Mach-Zehnder modulator (DPMZM) shown in figure 2, two double-sideband suppressed carrier (DSB-SC) signals are generated and combined together, where modulation frequencies are f 1 and f2. The DPMZM has two sub MZMs embedded in a main MZM. Optical phase difference between the two sub MZMs can be controlled by dc-bias voltage applied on the electrode C (bias C). The optical output has four spectrum components. The offset frequencies with respect to the optical frequency of input (f0) are +f1, -f1, +f2, and -f2, so that 2f1, 2f2, f1+f2, f1-f2 components would be generated by feeding the optical signal into a high-speed photodetector for square detection. However, the f1+f2 and f1-f2 components (the cross-term components, henceforth) depend on optical phase difference between the two DSB-SC signals. An f1+f2 component is generated from the upper sideband (USB) of f1 and the lower sideband (LSB) of f2. USB of f2 and LSB of f1 also generate another f1+f2 component. These two would interfere with each other. Thus, we can suppress the intensity of the f1+f2 component by controlling the bias C. Similarly, the f1-f2 component is generated from the optical beat between USBs of f1 and f2, and also from LSBs. Fig. 1 Principle of operation. Fig. 2 Schematic of DPMZM. 191 978-1-4244-5369-6/10/$26.00 ©2010 IEEE