JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 24, DECEMBER 15, 2009 5749 Interferometric In-Phase and Quadrature Oscilloscope David J. Krause, Member, IEEE, John C. Cartledge, Fellow, IEEE, OSA, Charles Laperle, Member, IEEE, and Kim Roberts, Member, IEEE Abstract—A technique is presented for measuring the in-phase and quadrature components of a modulated optical signal, using standard electrical and optical components and a high-bandwidth equivalent-time sampling oscilloscope. The measurement setup consists of an interferometer, optical mixer, and sampling oscil- loscope with two low-speed sampling modules (50 kHz) and two high-speed optical sampling modules (65 GHz). The simultaneous measurement of four de-skewed signals allows for the separate determination of the phase noise, amplitude modulation, and phase modulation. From these results, the complete trajectory in time of the complex signal is constructed. Index Terms—Optical field measurement, optical modulation. I. INTRODUCTION I N optical communications, the use of phase-modulated sig- nals, such as binary phase-shift keying (PSK) and quadra- ture phase-shift keying (QPSK), is becoming increasingly im- portant. Consequently, there is a need for accurate techniques to measure the complex optical field of high-bit-rate signals. Techniques for obtaining the signal trajectory and constella- tion diagram include linear optical sampling [1]–[3], coherent detection and postprocessing of real-time sampled waveforms [4]–[7], and complex spectral analysis [8]. In this paper, a technique is demonstrated for measuring the in-phase and quadrature components of a modulated op- tical signal, using standard electrical and optical components and a high-bandwidth equivalent-time sampling oscilloscope. The development of the technique was motivated by the fact that equivalent-time sampling oscilloscopes typically have much higher bandwidths than current real-time oscilloscopes. Augmenting an equivalent-time oscilloscope to measure the in-phase and quadrature modulation allows for the visualiza- tion and measurement of the next-generation of high-speed optical modulators. The measurement setup consists of an interferometer, optical mixer, and sampling oscilloscope with two low-speed sampling electrical modules (50 kHz) and two Manuscript received July 31, 2009; revised October 06, 2009. First published October 30, 2009; current version published December 02, 2009. This work was supported by the Natural Sciences and Engineering Research Council of Canada. D. J. Krause was with the Department of Electrical and Computer Engi- neering, Queen’s University, Kingston, ON K7L 3N6, Canada. He is now with Nortel, Ottawa, ON K2H 8E9, Canada (e-mail: davidkr@nortel.ca). J. C. Cartledge is with the Department of Electrical and Computer En- gineering, Queen’s University, Kingston, ON K7L 3N6, Canada (e-mail: john.cartledge@queensu.ca). C. Laperle and K. Roberts are with Nortel, Ottawa, ON K2H 8E9, Canada (e-mail: claperle@nortel.com, krob@nortel.com). 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.2009.2035091 Fig. 1. Block diagram for the IQ oscilloscope. LO, local oscillator; APG, arbi- trary pattern generator; LS-PD, low-speed (50 kHz) photodiode; VOD, variable optical delay, AOWG, arbitrary optical waveform generator. high-speed sampling optical modules (65 GHz). The simulta- neous measurement of four deskewed signals allows for the separate determination of the phase noise, amplitude modula- tion, and phase modulation. From these results, the trajectory in time of the modulated signal is constructed. Laser phase noise and thermal drift in the experimental setup cause rotation of the modulated signal trajectory in the complex plane. Estimates of this rotation are obtained from the signals detected with the low-speed sampling modules, which allows its removal from the signals detected with the high-speed sampling modules. Previously, an interferometric approach with real-time sam- pling has been demonstrated for low-bandwidth signals under the assumption that the relative phase between two interfering signals is stable during the measurement time (100 ns) [9]. The technique presented here is demonstrated for a single polarization (based on the available optical hybrid), but could be extended to a polarization diversity configuration. II. MEASUREMENT PRINCIPLE A block diagram depicting the measurement principle is shown in Fig. 1. The interferometric measurement consists of splitting a continuous wave signal from a laser into two branches, modulating the signal in the lower branch, applying the continuous wave and modulated signals to an optical hybrid and then detecting the two output signals, using a four-channel equivalent-time sampling oscilloscope. Each output signal is detected with a low-speed and high-speed sampling module. The envelope of the optical signal from the laser is (1) where is the amplitude and is the random process that describes the laser phase noise. The output signal from the mod- ulator branch at the input to the optical hybrid is (2) 0733-8724/$26.00 © 2009 IEEE