2782 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 15, AUGUST 1, 2008 Instantaneous Visualization of K-Band Electric Near-Fields by a Live Electrooptic Imaging System Based on Double Sideband Suppressed Carrier Modulation Kiyotaka Sasagawa, Atsushi Kanno, and Masahiro Tsuchiya, Member, IEEE Abstract—This paper discusses instantaneous electric near-field imaging in the K-band. The imaging technique is based on the live electrooptic imaging (LEI) scheme, which was originally developed for electric near-field imaging in the microwave range. The measurement frequency of the LEI system is extended by using the second-order harmonic component of modulated light as a photonic local oscillator signal. We demonstrated electric near-field imaging over rather ordinary components such as an SMA connector and a microstrip transmission line at frequencies up to 25 GHz. Index Terms—Electromagnetic field imaging, electrooptic mea- surements, massive parallelism, photonic heterodyne, real-time imaging. I. INTRODUCTION A LIVE electro-optic imaging (LEI) system, we have re- cently developed, accomplishes instantaneous and con- tinuous visualization of the radio-frequency (RF) electric near- field [1]–[3]. The time to acquire an electric field image has been drastically reduced by using the system and even a motion picture can be obtained. Real-time imaging with a resolution of pixels at a frame rate of 30 frames/s has been achieved by means of massive parallelization based on photonics technology. A transition in the electric field distri- bution even within a few seconds can be observed on the basis of the LEI system. This LEI feature is expected to be useful in various application fields. For example, LEI would be a pow- erful tool to improve analysis efficiencies of electromagnetic interference and undesired electromagnetic radiation in wire- less communication equipment because of its capabilities for real-time visualization of electric near-fields under variations of imaging area, frequency, phase, and environment around the de- vice under test (DUT). Also, material imaging with RF waves is promising for efficient analyses of its properties [4]. Manuscript received February 8, 2008; revised June 4, 2008. Current version published October 10, 2008. K. Sasagawa is with the New Generation Network Research Center, National Institute of Information and Communications Technology, Tokyo, 184-8795 Japan and also with the Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, 630-0192 Japan (e-mail: sasagawa@ms.naist. jp). A. Kanno and M. Tsuchiya are with the New Generation Network Research Center, National Institute of Information and Communications Technology, Tokyo, 184-8795 Japan (e-mail: kanno@nict.go.jp; mtsu@nict.go.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2008.927601 The basic principle of LEI is based on the Pockels effect and an electrooptic (EO) crystal is used as an electric field sensor. An electric field induces its refractive index change which is measured by using a laser beam. This technique is useful for observation of signals especially in high-speed electric circuits [5]–[9]. Miniaturized fiber-based EO probes realize ultra low in- vasiveness [10]–[14]. It takes a long time, however, to acquire an image of a sufficient resolution with conventional probes where single channel configurations are common and it is required to scan those probes to obtain an electric field image. On the other hand, the LEI system is based on massive parallelism, which al- lows much shorter time for its image acquisition. One of the issues with the previous LEI system was its operation bandwidth. Wireless communication devices using frequencies higher than that of microwaves such as quasi-mil- limeter and millimeter-waves have recently been actively developed. These frequencies are also useful for measuring material properties. However, the measurement frequency of the LEI system previously reported has been limited to ap- proximately 10 GHz. It is known that EO crystals have a high response bandwidth. In fact, millimeter-wave or terahertz-wave systems of measurement based on the Pockels effect have been demonstrated [15], [16]. The measurement frequency in the previous LEI system was not limited by the EO crystal but the optical modulator for generating the signal of the photonic local oscillator (LO). In this paper, we present the first real-time visualization of near-fields up to the K-band, which corresponds to frequencies up to 26.5 GHz. The generation of the photonic LO signal in this range is accomplished by the method of frequency doubling, which is based on double-sideband suppressed carrier (DSB-SC) modulation by an optical intensity modulator. Successful electric near-field images of microstrip transmission lines at frequencies up to 25 GHz were instantaneously acquired. II. LIVE ELECTROOPTIC IMAGING SYSTEM Fig. 1 is a schematic of the concept underlying the LEI system, by which an electric near-field pattern over a DUT at a specific radio frequency can be displayed live on a screen. Its high-speed imaging feature allows patterns of electric near-fields to be observed under various conditions and the DUT to be diagnosed within a short time. In our previous work, we demonstrated an LEI system for microwave circuits by the parallelization of data channels, which was achieved by using a high-speed image sensor and a digital signal processor [1], [2]. 0733-8724/$25.00 © 2008 IEEE