ADVANCED radar systems benefit from the ability to have

1086 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 Coherent PM Optical Link Employing ACP-PPLL Yifei Li, Member, IEEE, and Peter Herczfeld, Fellow, IEEE Abstract This paper concerns the modeling, design, and experimental validation of coherent phase modulated RF/optical link employing an ACP-PPLL linear phase demodulator. From theoretical modeling, the closed-form solutions for link gain, noise figure, and SFDR are derived. Then, two examples of link design are addressed: one with 100-MHz and one with 500-MHz bandwidth. Finally, experimental validation was performed using the 100-MHz design. The experimental results are in good agreement with the theoretical prediction. Index Terms Coherent optical link, high dynamic range, photonic phase-locked loop. I. INTRODUCTION ADVANCED radar systems benefit from the ability to have antennas be remoted from signal processing units. Analog fiber-optic links can provide the required connectivity between these subsystems. Photonic links are attractive due to their high bandwidths, low attenuation, and immunity to electromagnetic interference. Existing microwave fiber-optic links employ optical intensity modulation and have inadequate spurious free dynamic range (SFDR) due to the nonlinear distortions incurred in the optical modulation/demodulation processes. Continued research efforts focus on various linearization schemes [1] [8] for analog fiber-optic links that help achieve a higher dynamic range. However, these linearization techniques only cancel the lower order unwanted nonlinear distortions. After linearization, the higher order nonlinearities emerge as the limiting factor for the link SFDR. This is especially acute for systems demanding wide information bandwidth. For example, the existing linearization techniques can barely push the link SFDR over 70 db for a 500-MHz information bandwidth. However, critical applications, such as channelized EW receivers require an SFDR several orders of magnitude higher. In comparison with optical intensity modulation, optical phase modulation employing the linear electrooptic effect is intrinsically linear. Thus, if a linear optical phase demodulation scheme is devised, a linear phase modulated (PM) fiber-optic link can be constructed. Hybrid optical/digital signal processing [9] and phase-locked-loop type Manuscript received January 30, 2008; revised June 26, 2008. Current version published April 24, 2009. This work was supported in part by DARPA and by ONR under Grant N00014000781. Y. Li is with Department of Electrical and Computer Engineering, University of Massachusetts at Dartmouth, Dartmouth, MA 02748 USA (e-mail: yli2@umassd.edu). P. Herczfeld was with the Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA 19104 USA (e-mail: Herczfeld@ece.drexel.edu). Digital Object Identifier 10.1109/JLT.2008.929415 Fig. 1. PM fiber-optic link with ACP-PPLL phase demodulator/detector. linear phase demodulation is the focus of recent research efforts [10], [11]. Here, we analyze a link that uses a novel photonic phase-locked-loop (PPLL) with an attenuation-counterpropagation (ACP) in-loop phase modulator to perform linear phase demodulation. The concept is illustrated in Fig. 1 In the link transmitter (TX), the RF input is applied to a linear optical phase modulator. In the link receiver, the phaselocked loop forces the optical phase of the optical LO to mirror the phase of the incoming TX optical signal. Therefore, the voltage generated over the in-loop phase modulator should be a strict replica of the RF input. However, in actuality due to finite open-loop gain, there is a small but nonvanishing difference between the two optical phases. This gives rise to nonlinear distortion at the output. To minimize the nonlinear distortion, the phase-locked loop must have large open loop gain in the entire RF bandwidth. The challenge in implementing such a phase-locked loop is to minimize the loop propagation delay to ensure its stability with high gain and high bandwidth. For example, a loop propagation delay of 20 ps or less is required for a PPLL with an information bandwidth of 500 MHz. Controlling such a short loop propagation delay is only achievable by employing a novel attenuation-counterpropagating (ACP) in-loop phase modulator (see Fig. 1), where the optical and microwave fields propagate in the opposite direction and the microwave field is strongly attenuated. This unique configuration eliminates the propagation delay of the in-loop phase modulator at the expense of a tolerable decrease in bandwidth. The pertinent attributes of the ACP modulator were fully explored in an earlier publication [12]. In this paper, we consider theoretical modeling, design, and experimental validation of the PM optical link employing ACP- PPLL. Our discussion is organized as follows. First, the link theoretical model is presented, from which closed-form expressions for link gain, noise figure (NF), and spurious free dynamic range (SFDR) are derived. Then, design of the PM link is discussed. Finally, experimental results are presented. 0733-8724/$25.00 2009 IEEE

LI AND HERCZFELD: COHERENT PM OPTICAL LINK EMPLOYING ACP-PPLL 1087 II. LINK THEORETICAL MODEL In this section, we derive a comprehensive theoretical model for the PM optical link. In Fig. 1, the TX and LO optical signals are expressed as (1a) (1b) where and are the TX and LO optical powers, respectively and is the circular optical frequency. In (1b), an initial phase is assumed for the LO optical signal. The TX and LO optical phase shifts are denoted as and. In the frequency domain, they are given by (2a) (2b) where and are the PM sensitivities of the transmitter phase modulator and the ACP in-loop phase modulator, respectively; and are the RF input and the ACP-PPLL output, respectively. The ACP modulator PM sensitivity is [12] where is the complex propagation constant of the RF field, is the refractive index of the electrooptic medium, is the electrooptical coefficient, is the speed of light, is the device length, and is the effective electrode separation. If is sufficiently large, the exponential term in (3) approaches zero, creating a lumped-element response free of propagation delay. When the two optical signals mix at the balanced photodiodes, a photocurrent carrying their phase difference is produced. Accounting for the photodiode saturation, the photocurrent maybe expressed as where is the photodetector responsivity and and are related to the saturation of the balanced PD: (3) (4) Fig. 2. Linear system model of PPLL employing ACP phase modulator, where (!) is the modulator PM sensitivity. Finally, the photocurrent produces the output voltage of the PPLL receiver. In frequency domain, this is given by where is the PD termination impedance. Assuming the PD bandwidth is RC time limited (nontraveling wave device), the termination impedance is given by where is the ACP electrode load impedance, is the external load impedance, and is the capacitance per PD. The dynamic behavior of the PM link is completely decided by (2) (6). Next, the small signal link model will be addressed from which the closed-form expressions for link gain and noise figure are derived. A. Small Signal Model When the PPLL closely tracks the TX and LO optical phases, i.e.,, the last term in (4) may be neglected: Thus, the entire PM link can be presented using a linear system model as depicted in Fig. 2. In Fig. 2, represents the loop propagation delay caused by signal routing and the balanced photodiode. Using this linear system model and assuming that the TX modulator is 50- terminated, the link voltage gain and power gain are found to be (5) (6) (7) (8a) and (8b) where is the open-loop gain of the ACP-PPLL: is the small signal 1-dB saturation power per photodiode [13]. In deriving (4), we assumed that the photodiode saturation is caused by third order nonlinearity. (9)

1088 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 Fig. 3. Linear system model of ACP-PPLL that includes noise sources. Thus, the link gain is determined by the ratio between the sensitivities of TX and the ACP modulators. The feedback system becomes unstable if the loop propagation delay is too large; therefore, the stability of the ACP-PPLL should be validated by a Nyquist plot of the open loop gain, [14]. B. Noise Analysis Using the small signal model the noise performance of the PM link is analyzed next. For this, the RF input to the TX phase modulator is set to zero and only the thermal background noise enters the TX modulator. In addition, in order to eliminate another thermal noise coming from the termination resistor of the TX modulator, the resistor is assumed be cryogenically cooled. Otherwise, the system noise figure (12) will increase by 1 due to this resistor. We considered five independent noise sources, namely the thermal background noise entering the TX phase modulator, the laser relative intensity noise (RIN), the optical phase noise, the thermal noise coming from the PD termination, and the PD shot noise. Fig. 3 shows a revised linear system model for the PM link that includes these noise sources. In Fig. 3, and are the phase noises of the TX and LO optical signals, respectively. is the power of the thermal background noise entering the TX modulator. Because the TX modulator electrode is 50 -terminated, this noise power produces a noise voltage of as shown in Fig. 3. In addition, is the noise current caused by the laser RIN, is the thermal noise current created by, and is the PD shot noise. The power spectral densities (PSD) of, and are given by (10a) (10b) (10c) (10d) where describes the RIN cancellation due to the balanced photodetector, and are the RIN of the TX and the LO optical signals, respectively; is Boltzmann s constant, is room temperature in Kelvin ( 290 K), and is the charge of an electron. Using these relations and the system represented in Fig. 3, we determine the output noise voltage to be [see (11), shown at the bottom of the page]. The noise figure (NF) of the PM fiber-optic link is given by (12) Employing laser sources with high spectral quality and assuming ideal RIN cancellation by the balanced PD, the ACP-PPLL is shot noise limited and (12) reduces to (13) Thus, in the shot noise limit, the link NF is improved by increasing the TX modulator sensitivity and by raising the optical power level. C. Nonlinear Distortion As shown in (4), the nonlinear distortion of the PM link arises from the sinusoidal response of photocurrent to the phase difference and from the photodiode saturation. The distortion diminishes when tightly tracks. To investigate the distortion level, we consider a two tone RF input: (14) where is the amplitude and and are the frequencies of the two tones. Both and are in close proximity of the RF frequency of interest. This produces the following fundamental output tones [upon substituting (14) into (8)]: (15) (11)

LI AND HERCZFELD: COHERENT PM OPTICAL LINK EMPLOYING ACP-PPLL 1089 The link SFDR is given by db (20) Fig. 4. Linear system model of ACP-PPLL that includes nonlinear distortion. In the shot noise limit, this is Upon substituting (14), (15) into (4), we obtain the third-order nonlinear distortion produced by the balanced PD: (21) The nonlinear distortion input RF frequency (16) contains two tones near the (17a) As shown in (21), a high open-loop gain, is critical for a high SFDR. The open-loop gain is proportional to optical powers. According to (9), (10d), and (21), in absence of PD saturation, doubling the optical power results in an 8-dB SFDR improvement, which is noticeably different from the conventional intensity-modulated optical links ( 2 db). Using the small signal link model, we have derived the link gain and link noise figure. Then, by examining the nonlinearities of the photodetectors, we have obtained a closed-form solution for the link output IP3 and SFDR. Next, the design of the PM fiber-optical link is addressed. In the presence of PPLL feedback as shown in Fig. 4, give rise to the following distortion at the output: (17b) and III. PM FIBER-OPTIC LINK DESIGN The PM link contains a conventional optical phase modulator for the transmitter, and an ACP-PPLL phase demodulator. The difficulty in the link design concerns the ACP-PPLL. Here, we consider two PM link designs. The first design is of a lower bandwidth, but easier to implement. It facilitates experimental validation of the PM link concept. The second design shows the potential of the PM link for achieving the desired bandwidth and SFDR. The specific performance goals and component parameters of both designs are stated in Tables I and II, respectively. (18a) (18b) Comparing (15) and (18), we can determine the third-order intermodulation intercept point (IP3), which is defined to be where the intermodulation tones attain the same amplitude as the fundamental tones. At IP3, the output power is given by (19) A. PM Link With Low Bandwidth This design aims to experimentally validate the concept of the PM link. As shown in Table II, except for the ACP in-loop phase modulator, the components are off-the-shelf. Furthermore, in order to ensure sufficient open loop gain for this experimental study, a 150- external load resistance is used instead of a conventional 50- load. For the design and fabrication of the ACP in-loop phase modulator, we opted for a conventional LiNbO substrate because it is the most mature linear electrooptic medium available. To introduce the desired RF attenuation for the ACP modulator [12], a lossy thin-film CPW waveguide electrode, as shown in Fig. 5, was deposited on the substrate. To enhance the PM sensitivity and still maintain a 100-MHz bandwidth, the thin-film thickness was set to be 1 m, and the entire modulator length is fixed at 6.8 cm. As depicted in Fig. 6, this ACP phase modulator design achieves a PM sensitivity of 1.6 rad/v within a bandwidth of 100 MHz. The high load impedance of the ACP modulator, in shunt with, constitutes the termination impedance

1090 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 TABLE I PM LINK DESIGN GOALS TABLE II LINK COMPONENTS PARAMETERS Fig. 5. Cross-section of lossy thin-film ACP electrode. For 100-MHz design, the thin-film thickness d is 1 m; for 500-MHz design, d is 0.36 m. of the photodiode that provides for high open loop gain (9). As the frequency increases, decreases and gradually approaches the characteristic impedance of the CPW waveguide. B. PM Link With 500-MHz Bandwidth For the 500-MHz link, we selected the same ACP in-loop phase modulator structure as shown in Fig. 5; however, to accommodate the larger bandwidth, the modulator length is reduced to 2 cm while the thin-film thickness is adjusted to 0.36 m to enhance the RF attenuation. These changes decrease the PM sensitivity. Specifically, as shown in Fig. 8(a), the ACP modulator achieves a PM sensitivity of 0.45 rad/v, which corresponds to a of 6.9 V. To meet the SFDR design goal, the loss of the PM sensitivity must be compensated for by escalating the optical power. Our calculations indicate that the photodiode must be able to handle a 300-mA photocurrent, which is demanding but achievable. The calculated link gain and SFDR of this design are plotted against the RF frequency in Fig. 9. The SFDR peak value is 147 db HZ and its 3-dB bandwidth is 500 MHz. The PM optical link gain, which is inversely proportional to the ACP modulator sensitivity (6), increases to be 13 db. The predicted NF is 2.2 db. We have analyzed the stability of the 500 MHz ACP-PPLL by varying the loop propagation delay. The Nyquist plot Fig. 6. 100 MHz ACP phase modulator performance: (a) PM sensitivity and (b) load impedance. Fig. 7. SFDR and noise figure at 100 MHz versus photocurrent of each photodiode. (Fig. 10) reveals that the ACP-PPLL is still stable with a loop delay of 20 ps, corresponding to a distance of 2.8 mm in the LiNbO. This is achievable only because the ACP in-loop phase

LI AND HERCZFELD: COHERENT PM OPTICAL LINK EMPLOYING ACP-PPLL 1091 Fig. 8. 500 MHz ACP phase modulator performance: (a) PM sensitivity and (b) load impedance. Fig. 9. Simulated ACP-PPLL performance: (a) SFDR versus frequency and (b) NF versus frequency. modulator itself does not contribute to the delay, but a tight integration of the photodiodes, the 3-dB optical couples with the ACP modulators is still required. Next, we discuss experimental validation of the PM fiberoptic link employing ACP-PPLL. IV. EXPERIMENTAL VERIFICATION To prove these concepts, we assembled a PM link with the ACP-PPLL detector/demodulator subject to the low bandwidth design discussed above. The custom fabricated Mach Zehnder (MZ) like structure shown in Fig. 11 is fabricated on LiNbO. It is fed, for simplicity, from a single laser providing for both the TX and LO optical signals. The lower arm of this MZ-like configuration is the linear phase modulator representing the link transmitter (TX). This TX phase modulator is implemented using a conventional copropagating traveling wave structure with a measured around 1.95 V. The upper arm is the ACP in-loop phase modulator which, in combination with the balanced photodiode and the feedback path, constitutes the ACP-PPLL. The of the ACP phase modulator was measured to be 2 V. The input impedance of the ACP modulator was also measured using an HP 8510 network Fig. 10. Stability of the second-generation ACP-PPLL. analyzer and shown in Fig. 12, which is in good agreement with the theoretical value (Fig. 6). High ACP modulator electrode input impedance helps by enhancing open-loop gain and reducing RF power loss.

1092 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 Fig. 11. Experimental setup for validating the PM fiber-optic link employing ACP-PPLL phase demodulator. Fig. 13. Feedback path of the ACP-PPLL. Fig. 12. Measured input impedance of the ACP modulator. The optical outputs from the two phase modulators are combined and butt-coupled to a pair of balanced high power photodiodes (CC-UTC devices supplied by J. Campbell, the University of Virginia), which are mounted on an AlN submount for assisting in heat dissipation. Each PD has a 100- m-diameter active area and a bandwidth of around 1 GHz when terminated by 50. Because of the high PD termination impedance here, the frequency response of PD in this experiment is much narrower. The responsivity of the PD is rated around 0.4 A/W. However, due to coupling loss, the ratio between total photocurrent from both PDs over the optical input from the fiber is only around 0.18 A/W. The output from the balanced diodes is then fed back to the ACP in-loop modulator electrode through three gold wire bonds (one for signal and two for ground) and a passive translation circuit. The actual feedback path is depicted in Fig. 13. The PPLL output was extracted to the external testing equipment by a GSG air coplanar probe (Cascade MicroTech). The loop delay caused by the 3-dB output optical coupler, wires, and the translation circuit is around 60 ps. The translation circuit contains a 100- resistor, which is in series with the 50- GSG probe, thereby facilitating the 150- external load impedance as specified in the design (Table II). This simple approach is wideband and free of nonlinear distortion, but lowers the RF power output. Since the system noise is not dominated by the resistor thermal noise, the reduced power level does not affect the dynamic range. To begin characterizing the ACP-PPLL we opened the loop and measured its open-loop gain using a network analyzer. Fig. 14 shows both the measured and the calculated open-loop gain when the incident optical power is 550-mW optical power Fig. 14. ACP-PPLL open-loop gain with 50 ma per PD. ( 48 ma per diode). The open-loop gain measurement starts from 45 MHz (limited by the network analyzer). In this range, we see a good agreement between the measurement and the theoretical value. The measured open-loop gain at 100 MHz is around 18 db. Then, the feedback loop was closed and the feedback stability was confirmed. With no modulation signal applied, we monitored the output of the PPLL while increasing the optical power up to 1 W (90 ma per PD). No sign of instability was detected. Next, a standard two-tone intermodulation test was performed to evaluate the linearity of the device. The two tone RF input as captured by an Agilent 8564 E spectrum analyzer is shown in Fig. 15. Under this two-tone input condition, we varied the optical input power, and measured both the fundamental and the two tone intermodulation distortion (IMD3) versus photocurrent per PD. We observed that the fundamental tone power is insensitive to optical input, except at very low optical inputs, which is unique for the feedback loop phase demodulator. On the other hand, as shown in Fig. 16, the relative IMD3 level diminishes rapidly as we increased the photocurrent since a higher photocurrent led to a larger feedback gain and tighter phase tracking. The distortion level was decreased by 18 db as the optical power doubled, which is in good agreement with the theoretical prediction. Fig. 17 shows a sample of measured

LI AND HERCZFELD: COHERENT PM OPTICAL LINK EMPLOYING ACP-PPLL 1093 Fig. 15. Two tone RF input at 100 MHz, 10 dbm per tone. Fig. 18. Two-tone intermodulation measurement versus input RF power. The optical power is 550 mw (photocurrent 50 ma per diode). Fig. 16. Relative intermodulation distortion versus photocurrent per PD. Fig. 19. Noise measured at probe output. Fig. 17. Output spectrum at the probe output. The RF frequency and power were set at 100 MHz and 10 dbm/per tone, respectively, and optical input is 550 mw. in Fig. 16, the IMD3 level has a slope of 3:1, an indication that the system is limited by the third-order nonlinearity. The intermodulation intercept point (IP3) measured at the GSG probe was found to be 34 dbm. The noise floor was measured at the GSG probe point after being boosted 44 db by two series RF LNAs (Minicircuits ZX60-33LN+, 1 db NF). The measured noise floor, shown in Fig. 17, is 167 dbm/hz, which is 6 db higher than the short noise power at the probe point. This seems to suggest that the PM link was in fact affected by the laser RIN noise due to imperfect PD balancing. From the IP3 and noise floor measurements we calculated the system SFDR and found it to be 134 db Hz at 100 MHz, which is 5 db lower than the theoretical prediction based on shot noise limit ( 138.8 db Hz ). ACP-PPLL output spectrum captured at the probe point with a 550 mw optical power (or 48 ma per diode), where we see a relative IMD3 of 66.5 dbc. In addition, as shown in Fig. 16, the measured IMD3 level is 3 db larger than the theoretical prediction. We attribute this to a small discrepancy between the theoretical and the measured open-loop gain (see Fig. 14). The IMD3 is inversely proportional to the third power of the open-loop gain. Next, the input RF power was swept from 5 to 10 dbm, while the input optical power was held constant at 0.55 W. As shown V. CONCLUSION In this paper, we have reported modeling, design, and experimental validation of a high dynamic range PM optical link employing an ACP-PPLL linear phase demodulator. Through theoretical modeling closed-form expressions for the link gain, NF, and SFDR are derived. Using these theoretical results, two link designs based on conventional LiNbO modulator devices are performed: One with 100 MHz bandwidth and one with 500-MHz bandwidth. The 100-MHz link design using discrete components was experimentally validated. In particular, we

1094 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 9, MAY 1, 2009 have demonstrated an SFDR of 134 db Hz with 100-MHz bandwidth. The 500-MHz bandwidth design demonstrated the potential of the PM link employing ACP-PPLL. It predicts a peak SFDR of 147 db Hz, a link gain of 12.5 db, and an NF 2.2 db, and 3 db of 500-MHz bandwidth. This design puts a demanding (yet achievable) power requirement on the photodiodes. ACKNOWLEDGMENT The authors would like thank Dr. B. Krantz (Boos Allan Hamilton), Dr. A. Rosen (Drexel University), and Dr. W. Jemison (Lafayette University) for useful discussions, and Dr. J. Campbell (the University of Virginia) for providing the CC-UTC photodiodes and Dr. G. Evans (Southern Methodist University) for mounting the photodiodes. REFERENCES [1] J. H. Schaffner and W. B. Bridges, Inter-modulation distortion in high dynamic range microwave fiber-optic links with linearized modulators, J. Lightw. Technol., vol. 11, no. 1, pp. 3 6, Jan. 1993. [2] Y. Chiu, B. Jalali, S. Garner, and W. Steier, Broad-band electronic linearizer for externally modulated analog fiber-optic links, IEEE Photon. Technol. Lett., vol. 11, no. 1, pp. 48 50, Jan. 1999. [3] E. Ackerman and C. H. Cox, Effect of pilot tone-based modulator bias control on external modulation link performance, in Int. Topical Meeting Microwave Photonics Tech. Dig., Sep. 2000, pp. 121 124. [4] G. E. Betts, Linearized modulator for suboctave-bandpass optical analog links, IEEE Trans. Microw. Theory Tech., vol. 42, no. 12, pp. 2642 2649, Dec. 1994. [5] E. I. Ackerman, Broad-band linearization of a Mach Zehnder electrooptic modulators, IEEE Trans. Microw. Theory Tech., vol. 47, pp. 2271 2279, Dec. 1999. [6] L. D. Westbrook, D. G. Moodie, I. F. Lealman, and S. D. Perrin, Method for linearization analogue DFB lasers using an integrated MQW electroabsorption modulator, Electron. Lett., vol. 32, pp. 134 135, Jan. 1996. [7] S. A. Pappert, C. K. Sun, R. J. Orazi, and T. E. Weiner, Microwave fiber-optic links for shipboard antenna applications, in Proc. IEEE Int. Conf. Phased Array Systems Tech., May 2000, pp. 345 348. [8] R. Sadhwani and B. Jalali, Adaptive CMOS predistortion linearizer for fiber-optic links, J. Lightw. Technol., vol. 21, pp. 3180 3193, Dec. 2003. [9] Y. Li et al., Receiver for a coherent fiber-optic link with high dynamic range and low noise figure, in Int. Topical Meeting Microwave Photonics Tech. Dig., Oct. 2005, pp. 273 276. [10] T. R. Clark and M. L. Dennis, DSP-based highly linear microwave photonic link, in Proc. IEEE/MTT-S Int. Microwave Symp., Jun. 2007, pp. 1507 1010. [11] J. E. Bowers, A. Ramaswamy, L. A. Johansson, J. Klamkin, M. N. Sysak, D. Zibar, L. A. Coldren, M. J. Rodwell, L. Lembo, R. Yoshimitsu, D. Scott, R. Davis, and P. Ly, Linear coherent receiver based on a broadband and sampling optical phase-locked loop, presented at the Microwave Photonics 07 (Invited), Victoria, Canada, Oct. 2007. [12] Y. Li and P. R. Herczfeld, Novel attenuation counter-propagating phase modulator for highly fiber-optic links, J. Lightw. Technol., pp. 3709 3718, Oct. 2006. [13] X. Li, N. Li, X. Zheng, S. Demiguel, J. C. Campbell, A. D. Tulchinsky, and K. J. Williams, High-saturation-current InP/InGaAs photodiode with partially depleted absorber, IEEE Photon. Technol. Lett., vol. 15, pp. 1276 1278, Aug. 2003. [14] B. Alain, Phase-locked Loops: Application to Coherent Receiver Design. New York: Wiley, c. 1976. Yifei Li (M 05) received the B.Eng. degree in optoelectronics from Huazhong University of Science and Technology, China, in 1996 and the M.S. and Ph.D. degrees in electrical engineering, both from Drexel University, Philadelphia, PA, in 2001 and 2003, respectively. From 2003 to 2007, he was a research faculty member with the Center for Microwave/Lightwave Engineering, Drexel University. Currently, he is an Assistant Professor in the Electrical and Computer Engineering Department of the University of Massachusetts Dartmouth, where he is also the Director of the RF/Photonics Laboratory. His research interests include high dynamic range RF/photonic links, tunable microchip lasers, hybrid lidar/radar, fiber radio systems, coherent optical communications, and laser nonlinear dynamics. Dr. Li recently received the European Microwave Association (EuMA) Young Scientist Prize (first prize) in the Twelfth Colloquium on Microwave Communications held in Budapest, Hungary, for his original work in hybrid optical/digital processing of microwave signals. Peter Herczfeld (S 66 M 67 SM 89 F 91) was born in Budapest, Hungary, in 1936. He received the M.S. degree in physics and Ph.D. degree in electrical engineering from the University of Minnesota, Minneapolis, in 1963 and 1967, respectively. Since 1967, he has been on the faculty of Drexel University, Philadelphia, PA, where he is currently the Lester Kraus Professor of Electrical and Computer Engineering. He is the Director of the Center for Microwave-Lightwave Engineering, Drexel University. He has authored or coauthored more than 400 papers in solid-state electronics, microwaves, photonics, solar energy, and biomedical engineering. He has also served as a consultant to numerous large and small corporations Dr. Herczfeld is a recipient of the IEEE Millennium Medal and served as the Distinguished Lecturer of the IEEE Microwave Theory and Techniques Society (IEEE MTT-S).

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