Experimental Demonstration of 10-Gb/s Data Format ... - IEEE Xplore

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 2, FEBRUARY 2005

Experimental Demonstration of 10-Gb/s Data Format Conversions Between NRZ and RZ Using SOA-Loop-Mirror Chung Ghiu Lee, Member, IEEE, Yun Jong Kim, Student Member, IEEE, Chul Soo Park, Student Member, IEEE, Hyuek Jae Lee, and Chang-Soo Park, Member, IEEE

Abstract—This paper describes the demonstration of a simple all-optical data format conversion scheme between return-to-zero (RZ) and nonreturn-to-zero (NRZ) that employs a semiconductor optical amplifier (SOA) in a nonlinear optical loop mirror. The format conversion has been performed between the most widely used data formats—NRZ and RZ formats. The format conversion scheme is based on gain variation by an intensity-dependent phase change in an SOA-loop mirror. The input data stream acts as a control signal that induces the phase differences between clockwise- and counterclockwise-propagating data inside an SOA-loop mirror. It is possible to change the data format of the output data stream by controlling the phase differences of the clockwise and counterclockwise pulse in an SOA-loop mirror appropriately. For the converted NRZ data from RZ data, 10-Gb/s error-free transmission up to 78 km over standard single-mode fiber has been obtained. By comparing the conventional NRZ transmission with the Mach–Zehnder modulation scheme, the proposed RZ-to-NRZ conversion shows an improved transmission performance. The NRZ-to-RZ conversion has clear eye openings up to 78 km. On the contrary, the conventional RZ binary data from a mode-locked laser has a nearly closed eye even at 52 km. The converted RZ data has a 2-dB conversion power margin to the injected NRZ data, which indicates an increase in the receiver sensitivity due to the signal format conversion. The improved transmission distance of the converted RZ signal is due to the duobinary coding effect of the SOA-loop mirror. The SOA has the possibility of high-speed operation over 40 Gb/s, and the SOA-loop mirror has the capabilities of format and wavelength conversions. Therefore, the SOA-loop mirror can be a universal building block in future all-optical networks. In addition, the proposed format conversion scheme can serve as an important format converter between the ultrafast optical-time-division-multiplexed networks and the lower line-rate wavelength-division-multiplexed networks. Index Terms—Duobinary, format conversion, nonreturn-to-zero (NRZ), optical communication, optical networks, return-to-zero (RZ), semiconductor optical amplifier (SOA), SOA-loop mirror, transmission experiment.

Manuscript received November 17, 2003; revised August 19, 2004. This work was supported in part by the Brain Korea 21 Project and the Basic Program of the Korea Science and Engineering Foundation under Grant R01-2001-00000327-0). C. G. Lee was with the Department of Information and Communications, Kwangju Institute of Science and Technology (KJIST), Gwangju 500-712, Korea. He is now with the Korea Photonics Technology Institute (KOPTI), Gwangju 500-460, Korea (e-mail: [email protected]). Y. J. Kim, C. S. Park, and C.-S. Park are with the Department of Information and Communications, Kwangju Institute of Science and Technology (KJIST), Gwangju 500-712, Korea (e-mail: [email protected]). H. J. Lee is with the Division of Information and Communication, Kyungnam University, Masan 631-701, Korea. Digital Object Identifier 10.1109/JLT.2004.838851

I. INTRODUCTION

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UTURE all-optical networks are likely to employ both wavelength-division multiplexing (WDM) and optical-time-division multiplexing (OTDM) technologies. Moreover, they may extensively use two standard data formats—return-to-zero (RZ) and nonreturn-to-zero (NRZ). There will be a need for all-optical data format conversion between WDM and OTDM signals [1]. It has been stated that fully functional WDM networks should have the capability of all-optical format conversion between RZ and NRZ format [2]. All-optical conversion methods between the RZ and NRZ data format using semiconductor optical amplifier (SOA) cross-gain compression have been also demonstrated [2], [3]. The bit rates of a time-division-multiplexed (TDM) optical signal from the transport network should be lowered near the access networks to feed the low-bit-rate optical network unit (ONU) in the subscriber side. To interface the low-speed electronics, the NRZ format may be preferred to the RZ format for feeding ONUs because of its narrower spectral bandwidth and higher timing jitter tolerance than the RZ format. From the access nodes to the all-optical transport layers, the OTDM system with the RZ data format has been suggested to increase the total transmission capacity over 40 Gb/s by using the bit-interleaving technique. For this high-speed OTDM transmission, the RZ format is preferred due to its robustness to the nonlinear effect in spite of the dispersion-induced effect. Therefore, format conversion between NRZ and RZ data formats is an essential function in linking and interfacing the ultrafast OTDM networks and the low-speed access networks. All-optical data format conversions have been demonstrated using SOA gain compression [2]–[4], a monolithically integrated Michelson interferometer (MI) employing SOAs [17], an SOA/fiber grating wavelength converter [11], a nonlinear optical loop mirror (NOLM) with an SOA [12], [13], cross-phase modulation (XPM) in an integrated Mach–Zehnder interferometer (MZI) employing SOAs [10], [14], [15], and Fabry–Pérot (FP) laser diode with dual-wavelength injection locking [16]. Data format conversions are classified into two main categories: RZ-to-NRZ conversion and NRZ-to-RZ conversion. Previous reports on all-optical RZ-to-NRZ conversion include a Mach–Zehnder photonic integrated circuit (PIC) [14], SOA/fiber grating filter [5], SOA-XGM [2], and NOLM [19]. None except for [14] reported optical fiber transmission of the

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LEE et al.: EXPERIMENTAL DEMONSTRATION OF 10-Gb/s DATA FORMAT CONVERSIONS

converted NRZ data. Also, NRZ-to-RZ format conversion has been demonstrated by various schemes, which use SOA gain modulation [2], [4], a walk-off balanced nonlinear fiber-loop mirror [18], a dual-wavelength injection locking technique of a an FP laser diode [16], and an MZI wavelength converter [15]. In addition, it has been reported that a modified terahertz optical asymmetric demultiplexer (TOAD) yielded NRZ-to-inverted RZ format conversion [13]. In this paper, based on an SOA in a nonlinear optical loop mirror (hereafter, an SOA-loop mirror), 10-Gb/s format conversions between the most widely used data formats, i.e., NRZ and RZ, are schematically explained and experimentally demonstrated. This operational principle is topologically identical to [14], but the proposed scheme has a simpler and more stable structure than that of [14]. To the best of the authors’ knowledge, this paper is the only one that reports data format conversions between RZ and NRZ with transmission experiments at the bit rate of 10 Gb/s. The paper is organized as follows. In Section II, the principle of data format conversion using the SOA-loop mirror is described. In Section III, 10-Gb/s RZ-to-NRZ format conversion is implemented based on the SOA-loop mirror, and its transmission performance is evaluated with various lengths of standard single-mode fiber (SMF). In Section IV, the possibility of NRZ-to-RZ conversion is shown using the same SOA-loop mirror with its transmission performance. This NRZ-to-RZ format conversion has the effect of dispersion-tolerant transmission by imparting the duobinary phase relationship to the converted RZ signal. Finally, the paper is concluded in Section V.

II. PRINCIPLE OF DATA FORMAT CONVERSIONS USING THE SOA-LOOP MIRROR The SOA-loop mirror, having its origin as a terahertz optical asymmetric demultiplexer (TOAD) [6] or a semiconductor laser amplifier in a loop mirror (SLALOM) [7], is frequently used for various applications such as nonlinear pulse regeneration [8], all-optical regenerative memory [20], and optical demultiplexing [9], [20]. In this section, data format conversion using the switching behavior of the SOA-loop mirror is presented.

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Fig. 1. Schematic diagrams of an SOA-loop-mirror to explain the principle of format conversions. T-LD: tunable laser diode, CW: continuous-wave beam, TDC: Tunable directional coupler.

mirror with a tunable directional coupler (TDC), assuming an ideal 3-dB coupler, the transmitted intensity is given in [7] as

(1) where and are the optical intensities of the input data and the output signal , respectively. is signal for ccw beam, and the SOA gain for cw beam, the phase difference between cw and ccw beams, respectively. Physically, and depend on the optical inand the displacement tensity of the input signal simultaneously. To understand the behavior of the SOA-loop mirror, it is helpful to consider the XGM and XPM effects in the SOA-loop mirror separately. Here, only the XGM effect is considered without losing the generality. Instead, phase variation is reflected in gain shapes of the two beams. With the XGM effect only, the phases are assumed to be con). Thus, (1) becomes stant (

(2) A. SOA-Loop Mirror Fig. 1 shows the basic configuration of the SOA-loop mirror. The SOA is placed in a fiber-loop mirror with the time displacement of from the midpoint of the is the length displacement from the midpoint fiber, where is the of the fiber, is the speed of light in vacuum, and and effective refractive index of the fiber. The terms denote the electric fields of the input and output data signals, is to denote the signal propagation in respectively. The term is used to denote signal propthe clockwise direction while agation in the counterclockwise direction. From the SOA-loop

where denotes the SOA-arrival time difference between cw and ccw beams at the SOA. The switching behavior of the SOA-loop mirror can be analyzed with this expression, because the gain is recovered on the arrival time of the ccw pulse, as long as the following condition is satisfied: (3) where SOA.

is the bit period, and

is the gain recovery time of the

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Fig. 2. Operational principle of the RZ-to-NRZ format conversion. T less than T .


is

Therefore, it is understood that an NRZ format can be achieved from the RZ signal by adjusting to be less than with the gain curves obtained from the switching behavior and will be mentioned later. On the other hand, in NRZ-to-RZ is adjusted to be . conversion, the arrival time delay If the input signal is nearly rectangular, an NRZ pulse is obtained when the cw and ccw beams are combined. In practice, due to the bandwidth limitation, each pulse has relatively slow rising and falling edges. In other words, the gain values near the bit boundary lie between zero and the maximum gain. From (2), the output is more shaped near the boundary and, as a result, the RZ format can be achieved [20]. In the following subsections, how to achieve RZ-to-NRZ or NRZ-to-RZ format conversions is introduced with the operational principles. B. RZ-to-NRZ Format Conversion The formation of the NRZ pulse is possible by the and gain-shaped pulse and its delayed pulse in (2). At the rising time of the input signal , phase change is induced from the refractive-index change and in the SOA active medium. As a result, the output are gain-shaped to balance the relative phase is adjusted to be less than , changes (Fig. 2) [20]. If the output of the SOA-loop mirror shows the NRZ-like pulse pattern. This NRZ pulse has red-chirped components at both the rising and falling edge. With anomalous dispersion in fibers, the initial pulse compression is additionally achieved, and it will be explained with the transmission experiments. C. NRZ-to-RZ Format Conversion is adjusted By contrast, in RZ-to-NRZ conversion, to be . Fig. 3 shows the operational principle of NRZ-to-RZ format conversion. To investigate phase relationship between neighboring bits in the converted RZ signal, phase values are denoted on top of each NRZ pulse. The values such as 0 and indicate the XPM-induced phase shift that the continuous wave (CW) beam experiences by the NRZ input . In the

Fig. 3. Operational principle of the NRZ-to-RZ format conversion. T equal to T .

is

ccw beam, the phase shift appears after . When both and are equal, the output of the SOA-loop mirror becomes zero. If both are different from each other (“1” and “0” or “0” and “1”), the output has a certain amount of power acts as an corresponding to “1”. In other words, the output and its delayed exclusive OR (XOR) logic gate to the input [25]. In this case, the output pulse does not signal have a steep rising and falling edge because of aforementioned imperfect phase cancellation. Instead, smooth rising and falling edges are formed, and as a result, the output pulse has an RZ format. This converted RZ signal has an alternate phase transition, which is a typical characteristic of modified duobinary signals [21], [22]. Experimentally, the duty cycle and pulse shape can be controlled within a certain range by using a polar. ization controller in the SOA-loop mirror or adjusting III. DEMONSTRATION OF RZ-TO-NRZ FORMAT CONVERSION Based on the operating principle, an experimental setup for 10-Gb/s RZ-to-NRZ format conversion is organized. The biterror-rate (BER) curves and eye diagrams are measured with various lengths of SMF fiber and simply compared with the conventional NRZ data transmission results. A. Experimental Setup for RZ-to-NRZ Conversion Fig. 4 shows the experimental setup for the RZ-to-NRZ format conversion using the SOA-loop mirror. The SOA-loop mirror consists of an optical tunable delay line (OTDL), a PC3, an SOA, and a TDC for feeding the continuous-wave (CW) beam, which are connected by the WDM coupler for the . A LiNbO electrooptic modulator and a RZ input signal mode-locked laser (~6 ps full-width at half-maximum (FWHM) at a 10-GHz repetition rate) driven by a pulse pattern generator pseudo-random bit sequence (PPG) generate a 10-Gb/s 1557.13 nm. The (PRBS) data stream of RZ format at RZ input signal enters into the SOA-loop mirror through the WDM coupler. For NRZ data generation, the CW beam with


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Fig. 4. Experimental setup for RZ-to-NRZ format conversion using an SOA-loop mirror. Mod: Modulator; EDFA: erbium-doped fiber amplifier; OA: optical attenuator; PC: polarization controller; OBF: optical bandpass filter; SOA: semiconductor optical amplifier; T-LD: tunable laser diode; OTDL: optical tunable delay line; TDC: tunable directional coupler.

Fig. 5. Eye diagrams of the optical transmission for (a) the proposed RZ-to-NRZ converted signal and (b) the conventional NRZ signal generated by LiNbO MZM.

the intensity of at 1550 nm is injected into the SOA-loop mirror from the tunable laser diode (T-LD). The SOA used in this experiment was 1000 m long and nearly polarization insensitive (typically 0.6 dB). The SOA current was set to 190 mA, while the coupling coefficient of the TDC was adjusted to 0.41. The SOA-arrival time difference was 70 ps. The optical powers at points A, B, and C in set to Fig. 4 were set to 8.7, 5.0, and 8.5 dBm, respectively.

Fig. 6. BER characteristics for the converted NRZ signal by the proposed method and the conventional NRZ signal according to fiber transmission lengths. NRZ BtB means NRZ back-to-back.

B. Transmission Experiment of the Converted NRZ Signal Fig. 5(a) shows the eye diagrams for a 10-Gb/s RZ-to-NRZ converted signal and its optical transmission experiments over 26–78-km dispersive standard SMF (Corning SMF-28 fiber with the dispersion parameter of 17.7 ps/(km.nm) at 1560 nm). For comparison of pulse shapes, the eye diagrams for the conventional NRZ signal generated by LiNbO Mach–Zehnder modulator (MZM) are also shown in Fig. 5(b). It should be

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Fig. 7. Experimental setup for NRZ-to-RZ format conversion. EDFA: Erbium-doped fiber amplifier; OA: optical attenuator; PC: polarization controller; OBF: optical bandpass filter; SOA: semiconductor optical amplifier; T-LD: tunable laser diode; OTDL: optical tunable delay line.

noted that the eye diagrams after transmission for the proposed method are better than those for the conventional NRZ modulation method even though the original RZ-to-NRZ converted signal is somewhat distorted. The steepening at the rising edge after 26 km is well described as follows: as the pulse travels through the fiber in the anomalous dispersion regime, the red-chirped (frequency) components on the rising and falling edges of the NRZ signal travel slower than the unchirped components in the pulse center [14]. On the contrary, the continuously accumulated intersymbol interferences (ISIs) are clearly seen in Fig. 5(b). The BER curves for 26-, 52-, 78-, and 104-km fiber transmissions are also shown in Fig. 6. The hollow symbols are for the RZ transmission by the conventional Mach–Zehnder modulation scheme, and the filled symbols for the proposed scheme. The RZ-to-NRZ conversion penalty was only about 1.5 dB from the RZ input to the converted NRZ BtB (back-to-back), as shown in Fig. 6. The sensitivity improvement of 1 dB at 26-km transmission of the converted NRZ signal comes from the pulse compression effect due to the red-chirped components in Fig. 5(a). The 10-Gb/s error-free fiber transmission up to 78 km for the converted NRZ format data is achieved. Conclusively, the proposed method shows improved transmission performance compared with the conventional NRZ signal transmission using the Mach–Zehnder modulation technique. IV. DEMONSTRATION OF NRZ-TO-RZ FORMAT CONVERSION In this section, a 10-Gb/s NRZ-to-RZ format conversion using the SOA-loop mirror and its transmission performance

are experimentally demonstrated. The converted RZ signal also has duobinary characteristics and, as a result, relaxes the dispersion-induced transmission limitation. A. Experimental Setup for NRZ-to-RZ Conversion The experimental setup for format conversion from NRZ-to-RZ is shown in Fig. 7. The SOA-loop-mirror consists of an OTDL, a PC3, an SOA, and a TDC for feeding the CW beam from the T-LDs, which are connected by the WDM . A LiNbO electrooptic coupler for the RZ input signal NRZ data modulator driven by a PPG generates a 1554.13 nm. The NRZ input sequence at 10 Gb/s with signal enters the SOA-loop mirror through the WDM coupler. at 1543.5 nm The CW beam with the intensity of is generated from the T-LD and is coupled to the SOA-loop 320 ps) in this exmirror. The SOA (a carrier lifetime of periment is a polarization-insensitive type (~0.6 dB) with a low-tensile-bulk separate-confinement heterostructure. The gain and cavity length are 25 dB and 1000 m, respectively. The SOA is driven at an operating bias current of 194 mA. is To implement the NRZ to RZ format conversion, adjusted to . As in the case of RZ-to-NRZ conversion (Fig. 2), is copied to cw(t) due the inverse pattern of the input signal to phase variation. In the same manner, ccw(t), the replica of the cw(t) with delay, is also obtained. The only differences are in the value and the smooth increase and decrease in both edges. Fig. 8 shows the measured eye diagrams of the converted RZ signal from a 10-Gb/s NRZ binary signal with a conventional RZ binary signal generated by a LiNbO MZM.


Fig. 8. Measured eye diagrams at dispersive fiber lengths of 0 to 78 km of (a) a 10 Gb/s NRZ signal and its converted RZ signal and (b) a conventional RZ binary signal.

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Fig. 9. BER characteristics of the converted RZ signal after dispersive fiber lengths of 0, 26, 52, and 78 km.

The eye diagram of the converted RZ signal in Fig. 8(a) shows the smooth rising and falling from imperfect interference. B. Transmission Experiment of the Converted RZ Signal The output of the SOA-loop mirror is transmitted through the SMF-28 fiber with the dispersion parameter of 17.7 ps/(km.nm) at 1560 nm (with lengths of 26–78 km). It is electrically converted to measure BER characteristics. The optical power, before being launched through the SMF, was set to 8.5 dBm. For comparison, the transmission results on a conventional RZ binary coding scheme are also shown with the results on the proposed NRZ-to-RZ conversion scheme, as shown in Fig. 8. Before comparing them, it must be pointed out that the two signals could be different in duty cycle even under phase tuning by polarization control. However, this slight difference does not make any significant difference in explaining its superiority on transmission performance. After 26 km, the pulse was compressed compared with the converted RZ signal. It is understood that with the transmission distance, the pulse becomes more broadened, the overlapped region increases, and as a result, the pulse edges become more steepened. Thus, the ISI can be effectively reduced. In the measured eye diagrams of the converted RZ signal [Fig. 8(a)], the eye opening was clearly observed even after 78 km. On the contrary, for the conventional RZ binary signal, even at 52 km, the eye was almost closed, and it could not be sent further. Without any significant change in pulse shape, the peak decreases gradually with the distance, and the ISI is continuously accumulated as seen from 26 and 52 km of Fig. 8(b). On the other hand, the eye opening of the converted RZ signal after 26 km and pulse steepening mainly comes from the destructive interference ( phase difference) in the overlapping region between neighboring pulses under fiber dispersion.

Fig. 10. Optical spectra for (a) the converted RZ and (b) the conventional RZ signal.

To investigate the transmission performance of the converted RZ signal, the BER curves for back-to-back and various dispersive fiber lengths (26, 52, and 78 km) were measured and plotted in Fig. 9. Noise floor was not observed even after 78 km at BER . The converted RZ signal had a 2-dB power penalty to the injected NRZ data signal. The negative power penalty means the increase in receiver sensitivity, which is due to the signal format conversion. Furthermore, the transmission performance at 26 km was improved by the amount of 1 dB compared with that of back-to-back NRZ transmission, which is the implication of the modified duobinary coded RZ signal. The optical spectra of the proposed RZ and conventional RZ signals were measured as shown in Fig. 10. In conventional RZ signal, the sidebands exist at 10 GHz from the optical carrier wavelength of 1543.5 nm. By contrast, the optical spectrum of the proposed RZ signal

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TABLE I SUMMARY OF THE FORMAT CONVERSIONS BASED ON THE SOA-LOOP MIRROR

has a narrow spectral width as an effect of the modified duobinary coding. Only a different point is that the peak appeared at the center wavelength instead of a dip, which is frequently seen in the typical modified duobinary coded RZ signal [21], [22]. It is due to the dc component being carried on the cw and ccw beams (i.e., baseline of the signals in Fig. 3), resulting in the remnant peak at the center wavelength. The opening eye diagram after 52 km is thought to be mainly due to destructive interference by the -phase difference between neighboring bits. The demonstrations have been done at 10 Gb/s. The proposed scheme can be applied to the high-speed OTDM network with an SOA with fast gain dynamics or with the help of a proper carrier recovery time reduction scheme such as a CW-holding beam in [23] and [24]. For simple comparison between the two format conversions, some parameters are listed in Table I. RZ-to-NRZ conversion mainly comes from the gain shape and its delayed characteristics. On the contrary, NRZ-to-RZ conversion is achieved by imperfect phase cancellation, depending on gain shape. V. CONCLUSION In this paper, all-optical data format conversions between NRZ and RZ using the SOA-loop mirror have been demonstrated. The principle of data format conversion is explained based on the switching behavior of the SOA-loop mirror and is applied to explain the format conversions with the timing diagrams. The RZ-to-NRZ format conversion was demonstrated based on the SOA-loop mirror. The RZ-to-NRZ converted signal showed the improved transmission performances compared with the conventional NRZ signal. A 10-Gb/s error-free fiber transmission for the converted NRZ format signal was achieved up to 78 km. NRZ-to-RZ format conversion was also successfully demonstrated based on the SOA-loop mirror. The 2-dB conversion power margin to the injected NRZ input data could be obtained, which indicates an increase in the receiver sensitivity due to the signal format conversion. The eye diagrams and BER curves of 10-Gb/s transmission up to 78 km for the converted RZ format data were presented. The proposed RZ conversion method shows improved fiber transmission performance compared with the conventional RZ binary signal. Furthermore, it is verified that the improved transmission performance came from the duobinary coding effect of the SOA-loop mirror. The proposed format conversion schemes could serve as an important format converter between the ultrafast OTDM networks and the lower speed WDM networks.

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Chung Ghiu Lee (S’99–A’03–M’04) received the B.S. degree in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejon, Korea, in 1997 and the M.S. and Ph.D. degrees from the Kwangju Institute of Science and Technology (KJIST), Gwangju, Korea, in 1999, and 2003, respectively. He is now with the Korea Photonics Technology Institute (KOPTI), Gwangju, Korea. His current research area includes subscriber-loop communication systems, all-optical data processing for optical communications, microwave photonics, and application of chaotic phenomena to communication systems. Dr. Lee is a Member of the IEEE Lasers & Electro-Optics Society (LEOS) and a Member of the Optical Society of America (OSA).

Yun Jong Kim (S’04) received the B.S. degree from the Inha University, Incheon, Korea, in 2000, the M.S. degree from the Kwangju Institute of Science and Technology (KJIST), Gwangju, Korea, in 2002. He is currently working toward the Ph.D. degree at KJIST. His current research interest focuses on all-optical signal processing, all-optical clock recovery, all-optical regeneration, and ultrashort pulse generation. Mr. Kim is a Student Member of the IEEE Lasers & Electro-Optics Society (LEOS).

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Chul Soo Park (S’98) received the B.S. degree from the Kwangwoon University, Seoul, Korea, in 1997 and the M.S. degree from Kwangju Institute of Science and Technology (KJIST), Gwangju, Korea, in 1999. Currently, he is working toward the Ph.D. degree at the Department of Information and Communications, KJIST. His main research interests include optical burstmode transceivers in passive optical access networks and microwave photonics systems. Mr. Park is a Student Member of the IEEE Lasers & Electro-Optics Society (LEOS) and the IEEE Microwave Theory and Technique Society (MTT-S).

Hyuek Jae Lee received the B.S. in electronics engineering from the Chungnam National University, Daejon, Korea, in 1987 and the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST) in 1991 and 1994, respectively. From 1994 to 1995, he was with the LG Electronics Research Center. He then joined the Electronics and Telecommunications Research Institute (ETRI), Daejon, Korea, where he had developed optical packet switching systems and optical transmission systems from 1995 to 2000. From 2000 to 2001, he conducted research in the field of optical label switching as a postdoctoral fellow at University of California, Davis. From 2001 to 2002, he was Chief Executive Officer and Chief Technology Officer of ROSWIN-USA, Inc., San Jose, CA, where he had developed a burst-mode transceiver for passive optical networks. In 2003, he joined the Information and Communication University, Daejon, Korea, as a Research Professor, and currently, he is a Faculty Member with the Division of Information and Communication, Kyungnam University, Masan, Korea. His current interests are optical packet switching systems, WDM transmission system, cost-effective passive optical network, neural networks, and genetic algorithm for optimization.

Chang-Soo Park (M’98) received the B.S. degree from Hanyang University in 1979, the M.S. degree from the Seoul National University, Seoul, Korea, in 1981, and the Ph.D. degree from Texas A&M University, College Station, in 1990, respectively. From 1982 to 1987, he was a Senior Member of Technical Staff in the Electronics and Telecommunications Research Institute (ETRI), Daejon, Korea. From 1987 to 1990, he was Research Assistant with the Engineering Center, Texas A&M University. From 1991 to 2000, he was Principal Member of Technical Staff at ETRI. He joined the Kwangju Institute of Science and Technology (KJIST). Gwangju, Korea, as an Associate Professor in the Department of Information and Communications, where he is currently a Professor. He is also Director of the Photonics Research Center (PRC), which is sponsored by the Ministry of Commerce, Industry, and Energy, Republic of Korea. His current research areas are in high-speed optical communication, optical internet, and microwave photonics. Prof. Park is a Member of the IEEE Lasers & Electro-Optics Society (LEOS) and of the Optical Society of America (OSA).