IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 14, JULY 15, 2011 947 Spectral Efficiency Improvement in Photonic Time-Stretch Analog-to-Digital Converter via Polarization Multiplexing Ali Fard, Student Member, IEEE, Brandon Buckley, and Bahram Jalali, Fellow, IEEE Abstract—Dual-polarization photonic time-stretch technique, which exploits polarization multiplexing to improve the spectral efficiency of the conventional photonic time-stretch technique, is proposed. This technique reduces the demand on optical bandwidth for large record length of the photonic time-stretch analog-to-digital converter. It is shown that this technique can capture high-bandwidth radio-frequency signals ( 10-GHz instantaneous bandwidth). Experimentally, 12.5-Gb/s data eye-diagram measurement using this preprocessor operating in equivalent-time mode is demonstrated. Index Terms—Analog-to-digital converter (ADC), eye diagram, oscilloscope, photonic time-stretch, polarization multiplexing. I. INTRODUCTION T HE rapid growth of internet traffic requires optical com- munication networks with high capacity [1], [2]. Data de- modulation at the receiver of such systems relies on high-band- width real-time digitizers, which are becoming the major bottle- neck. High-bandwidth analog-to-digital converters (ADC) with real-time capabilities facilitate rapid measurement and evalua- tion of signal quality in optical performance monitoring [3] of self-managed and reconfigurable optical switch networks. In de- fense applications, such digitizers are the central tools in radar systems, military receivers, and battlefield airborne communi- cations nodes (BACN) [4]. In biomedical imaging, high-per- formance digitizers are needed to record images continuously over a large time period [5]. Moreover, availability of high-res- olution, high-bandwidth digitizers enables development of ad- vanced laboratory instruments such as real-time oscilloscopes and radio frequency (RF) vector network analyzers. The photonic time-stretch (PTS) preprocessor [6]–[9] is a novel technique to slow down and capture the dynamics of fast repetitive or nonrepetitive signals, and rare events using a low- bandwidth ADC. It stretches the RF signal in time by exploiting a dispersive analog optical link and a broadband chirped op- tical source. It can provide continuous digitization of ultrahigh bandwidth electronic signals [9]–[11] with high-resolution that Manuscript received February 15, 2011; revised April 03, 2011; accepted April 09, 2011. Date of publication April 15, 2011; date of current version June 22, 2011. This work was supported by the National Science Foundation through CIAN ERC and by the DARPA-MTO RADER program. The authors are with the Photonics Laboratory, Department of Electrical En- gineering, University of California Los Angeles, Los Angeles, CA 90095 USA (e-mail: motafakk@ee.ucla.edu; bbuckley@ucla.edu; jalali@ucla.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2142414 cannot be achieved by purely electronic ADCs [12]. The real- time burst sampling technique performed by time-stretch en- hanced recording (TiSER) oscilloscope, which is the single- wavelength channel version of the PTS system, enables capture of bursts of samples, spanning several real-time sample points [13]. When operated in equivalent-time mode, it can generate eye-diagrams of repetitive data streams. A crucial feature of the PTS system is the spectral efficiency, i.e., the optical bandwidth required to capture a certain time aperture of an RF signal over sufficient RF bandwidth. It is nat- urally desirable to maximize the time aperture so that more of the signal is captured. The time aperture of the PTS system is equal to the width of the optical pulse after the first dispersive fiber, given by , where is the optical bandwidth and is the initial dispersion. When double sideband modu- lation (DSB) is employed to modulate the RF signal onto the prechirped pulse, a frequency-fading phenomenon due to dis- persion [9], i.e., dispersion penalty, is inevitable. The overall effect is to limit the effective 3-dB RF bandwidth of the PTS to an expression proportional to the inverse square root of the initial dispersion: , where is the group-velocity dispersion (GVD) parameter. Hence, for a certain desired RF bandwidth, a limit is placed on the max- imum predispersion that can be tolerated, at which point, to in- crease the time aperture, one must use time-stretching pulses with larger optical bandwidth. Alternatively stated, the product of the time aperture and RF bandwidth, a figure of merit termed the time-bandwidth product (TBP), depends linearly on optical bandwidth [11]. In order to meet the increasing RF bandwidth demands of modern applications, the PTS system must therefore employ broadband optical pulses with larger optical bandwidth, straining the capabilities of the supercontinuum (SC) source, and eventually leading to undesired distortions, such as wave- length-dependent loss and optical nonlinearity. Improving the spectral efficiency of the PTS system equates to increasing the TBP independent of optical bandwidth, so that large RF band- width demands can be met feasibly and efficiently. In this letter, we demonstrate the dual-polarization photonic time-stretch preprocessor (DP-PTS), which exploits polariza- tion multiplexing in the PTS system to double the spectral effi- ciency. The DP-PTS maps two consecutive segments of the RF signal onto two orthogonal polarization states and multiplexes them on a single-wavelength channel, thereby doubling the ef- fective time aperture while keeping the optical bandwidth con- stant. This technique can also be used in the TiSER oscilloscope to significantly increase the record length and hence, the sam- pling throughput. 1041-1135/$26.00 © 2011 IEEE