Simultaneous amplification and channel equalization using raman ...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 19, NO. 3, MARCH 2001

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Simultaneous Amplification and Channel Equalization Using Raman Amplifier for 30 Channels in 1.3-m Band H. S. Seo, Student Member, IEEE, Student Member, OSA, K. Oh, Member, IEEE, Member, OSA, and U. C. Paek, Senior Member, IEEE, Fellow, OSA

Abstract—A new technique is proposed to simultaneously amplify signals and equalize power unbalance caused by Ramaninduced crosstalk among optical channels in the 1.3- m band. The crosstalk is induced in GeO2 -doped silica fibers due to the Raman-gain coefficient with a positive slope below the peakfrequency shift of 440 cm 1 . Highly doped GeO2 silica fiber, however, shows a negative slope across the band between 440 cm 1 and 490 cm 1 . We propose expansion of the Raman-gain band with a negative slope over the bandwidth of 150 cm 1 using multiple pumps. Utilizing the negative gain slope of the proposed band, simultaneous amplification and power equalization was theoretically demonstrated for 30 channels in the 1.310–1.345 m region without external filters. Index Terms—Channels equalization, germanium doped silica, optical fiber, Raman amplifier, wavelength division multiplexing (WDM).

I. INTRODUCTION

S

INCE wavelength division multiplexing (WDM) techniques have developed as a standard for optical communication, wide-band optical amplifiers have been extensively studied to increase the transmission capacity. With the development of high-power compact semiconductor laser diode pumps, Raman amplifiers have regained attention as a practical-silica–based fiber amplifier due to their flexible gain band control by changing wavelengths and powers of pump light sources. Especially, Raman amplifiers could have a strong potential in WDM applications in the 1.3- m band where efficient rare earth-doped silica fiber amplifier does not exist. Using high germanium-doped fiber, researchers have obtained a Raman-signal gain over 30 dB and broad bandwidth about 120 nm [1], [2]. Recently, Masuda et al. developed hybrid amplifier combined with erbium-doped fiber amplifier (EDFA) and Raman amplifier with a gain bandwidth exceeding 80 nm [3]. Lewis et al. proposed a new type of broadband Raman amplifier with a low noise and a high gain using dispersion compensating fiber (DCF) [4]. They obtained the net gain of 20 dB in addition to the total dispersion of 700 ps/nm out of 11.9 km of the DCF. A gain flattened broadband Raman Manuscript received July 27, 2000; revised October 25, 2000. This work was supported in part by UFON, an ERC program sponsored by KOSEF, and BK21 program, supported by MOE in Korea. The authors are with the Department of Information and Communications, Kwangju Institute of Science and Technology, Kwangju 500-712, South Korea (e-mail: [email protected]). Publisher Item Identifier S 0733-8724(01)01889-8.

Fig. 1. Schematic diagram of channel equalization and amplification using a lumped Raman amplifier with a negative gain slope.

amplifier has been reported using gain flattening filters and two pumping sources [5], [6]. Even though Raman amplifiers have shown efficient control of gain band, stimulated Raman scattering (SRS) induces the crosstalk among WDM channels to result in the channel-power unbalance, especially for high input channel power over 10 dBm. Because each optical channel plays a role as a pumping source, optical powers from the shorter wavelength channels are transferred to the longer wavelength channels by SRS process. This causes a positive tilt of power among channels in the wavelength scale (see Fig. 1). This could occur even in linear amplifiers such as in EDFAs if operated in the automatic power control mode [7]. To reduce the Raman-induced crosstalk, various equalization techniques have been proposed using external filters. Shaulov et al. have calculated the design parameters of channel equalizing filter by measuring Raman-gain slope of GeO -doped silica fiber [8]. Spectral inversion technique has been also proposed [9] that requires a phase conjugator to compensate the positive tilt of the gain. Recently, Takeda et al. experimentally demonstrated active gain-tilt equalization in EDFA using standard single-mode fiber (SMF) and dispersion shifted fiber (DSF) as Raman gain media [10]. In this paper, we theoretically proposed a new- type of Raman amplifier whose gain band is synthesized by multiple pumps to result in a negative gain slope across a wide bandwidth, which can compensate the Raman-induced crosstalk. We demonstrated that WDM channels in the 1.310–1.345 m region could be simultaneously equalized and amplified using a high germanium concentration fiber without equalizing filter, for the first time to the best of our knowledge.

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Fig. 2. Normalized Raman gain distribution in pure silica core fiber [11]. Two peaks are shown at 440 cm and 490 cm . The maximum peak value is 1.21 10 / cm/W at 440 cm , where  is the wavelength of pump.

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concentration, consequently , increases, the peak intensity shifts toward 440 cm . And the gain band can be divided into two bands, one with a positive slope, the band I and the other with a negative slope, the band II, as shown in Fig. 3. Note that the negative slope in the band II increases as germanium concentration increases. The Raman-induced crosstalk of 0.005, thus, could be compensated in an SMF with by the gain in the band II with the negative slope in a high germanium-doped fiber. In this paper, we present a thorough theoretical analysis using the Raman-frequency modeling on how the parameters for highly germanium-doped fibers and pumps would affect both amplification and compensation of Raman crosstalk among channels transmitted through SMFs. In order to calculate the crosstalk and amplified gain of Raman medium for various germanium concentrations at a given pump power, Raman-frequency modeling was used, which was developed initially by Liu et al. [11]. The governing equations are given below

(1)

(2)

(3)

Fig. 3. Raman gain distribution of germanium-doped fiber pumped at 1.24 m pumping source. n is defined by n -n where n and n are the core and the cladding refractive index, respectively [12].

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II. THEORY The principal idea of this study is schematically shown in Fig. 1. WDM channels suffer from Raman-induced crosstalk as they travel through conventional single-mode optical fiber and result in the power unbalance. By appropriately synthesized gain with a negative gain slope in a lumped Raman amplifier, the WDM channels could be amplified to an equalized output power. Thus, the spectral characteristics of Raman-gain medium, choice of pump power, and its spectral position would be important parameters for synthesizing an appropriate gain shape. In a Raman medium, a molecule absorbs a photon at the pumping frequency and emits one at a shifted frequency. Fig. 2 shows the measured Raman-gain spectrum for fused silica at a given pump wavelength, 1.24 m [11]. In this graph, Raman-gain coefficient monotonically grows, showing two peaks, 440 and 490 cm , and then decreases rapidly. Fig. 3 shows the measured Raman-gain spectrum for germanium-doped silica at a given pump wavelength, 1.24 m [12]. Here, the refractive index difference ( ) is assumed to be from the germanium contribution only. As germanium

Through the analysis, we have assumed that the pump wavelength is 1.24 m and signal channels are uniformly distributed from 1.310 to 1.345 m. Here, represents the instantaneous power at a distance , which travels at the group velocity of is the signal power at the frethe propagating optical pulse. quency . The subscript “ ” is the final frequency shift fixed at 1000 cm in the simulation. Parameters with the subscript “0” correspond to variables at the pump frequency. Fig. 4 explains frequency intervals and notations of (1)–(3) [11]. As GeO is doped in the silica core, Rayleigh-scattering loss coefficient, increases. Fig. 5 shows the attenuation of optical fiber at 1.31 m manufactured by modified-chemical-vapor deposition (MCVD) technique as a function of germanium concentrations [13]. g is the Raman gain at the frequency induced by the pump at the frequency [12], defined as

(4) cm

(5)

represent Raman gain cross section that deHere, and pends on GeO concentration, as shown in Fig. 3. are the refractive index of core and cladding, respectively. is defined as ( )/ , C is the spontaneous Raman scattering term reference [11]. g is the maximum value of Raman represents the normalized line shape of the gain and g /g

SEO et al.: SIMULTANEOUS AMPLIFICATION AND CHANNEL EQUALIZATION

Fig. 4. Illustration of frequency interval and notations [11].  Stokes’ signal obtains gain from  , j < i and gives its energy to  , k > i. The frequency interval  was 6.66 cm and  , 1000 cm in this simulation.

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Fig. 5. Optical loss as a function of the index difference between core and cladding at 1.31 m [14].

Raman-gain curve. The effective-core area [14], [15]

is defined as

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shift, respectively. The third and the fourth terms are the energy transfer into th stokes, the noise added by the spontaneous Raman scattering from th stokes. The last term corresponds to the loss induced by the spontaneous Raman scattering at the frequency component . In a Raman medium, the signal at obtains optical power from the lower frequency parts at , , and transfer its energy to the higher frequencies , . Equation (3) is the equation of signal at the final frequency . In this simulation, we divided the frequency range of total 1000 cm into 150 frequency slots and each slot had a uniform-frequency interval, 6.66 cm . Each slot corresponds to about 1-nm spacing for the pump at 1.24 m used in a 1.31- m amplifier. In order to calculate the gain spectrum in terms of Ramanmedium length, pump power, and GeO concentration, the governing equations (1)–(3) were solved by using the fourth-order Runge–Kutta method. For numerical stability, the mesh size was chosen as 50 cm, and maximum of the fiber length Raman-medium length was 100 km. This Raman-frequency model is valid in a nonlinear regime, where SRS is the only dominant-nonlinear effect. Table I shows the range of parameters where the above assumption holds for various index differences and signal powers [16], [17]. For optical signal pulse width greater than 100 ps, as in 10 Gbps data rate, the group velocity dispersion (GVD) effect is negligible since the Raman-medium length is shorter than . The dispersion length is defined as the dispersion length , where is initial pulse width and is GVD [16]. In the same manner, the self-phase modulation (SPM) effect can be also ignored since the Raman-medium length is shorter . The nonlinear length is defined than the nonlinear length , where is given as a function of as GeO concentration [16], [17]. These nonlinear effects will be discussed in Section III. Based on these previously reported experimental data on the Raman gain and optical loss, we solved the governing equations numerically including energy transfer into the second stokes for accurate analysis. More detailed discussion on energy transfer to the second stokes has been reported by the authors [18].

(6)

III. RESULTS AND DISCUSSION

(7)

The WDM optical signals transmitted through SMF could experience a significant channel unbalance due to Raman crosstalk if the input power level of individual channel is above 10 dBm. Fig. 6 shows the calculated spectral distribution of received 32 WDM channel power through 100 km of SMF. Here we have assumed initial-input channels are uniformly distributed from 1.310 m to 1.345 m. The WDM channels with a relatively high initial power, 5 dBm, result in the power difference of 7 dBm between channels at 1.310 m and 1.345 m. This crosstalk among channels is mainly due to the positive slope of Raman gain below 440 cm in SMF = 0.005), as shown in Fig. 3. Note that as indicated in ( was about 625 km Table I, the calculated nonlinear length for 5 dBm input signal, and dispersion length by GVD is about 26 315 km. Since the SMF length 100 km is far less than and , both SPM and GVD effects were ignored. Under this

(8) and are mode field radii at the pump,1.24 m, where and the signal, 1.31 m, respectively. is the core radius of fiber, and number is fixed as 2.0. Equation (1) represents the evolution of pump power along the Raman medium. The first and the second term in the right-hand side of the equation correspond to the Rayleigh-scattering loss and the energy conversion into th frequency shift, respectively. The last term is the losses induced by the spontaneous Raman scattering at the pump frequency. Evolution of the signal powers is presented in (2). The first and the second terms in right-hand side of the equation are the Rayleigh-scattering loss, the Raman gain from th frequency

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TABLE I GROUP VELOCITY DISPERSION , DISPERSION LENGTH L , AND NONLINEAR LENGTH L , IN TERMS OF INDEX DIFFERENCE AND PUMP POWER [16], [17]. THE RAMAN MEDIUM LENGTH L DOES NOT EXCEED L FOR THE GIVEN INDEX DIFFERENCE. INITIAL-PULSE WIDTH IS ASSUMED BY 100 ps FOR CALCULATION OF L . EACH PARAMETER IS DEFINED AND EXPLAINED IN THE TEXT

Fig. 6. Channel power spectrum due to Raman-induced crosstalk among 30 channels after 100 km transmission in SMF.

assumption, evolution of the channel power difference, defined , was plotted as a function of SMF length as in Fig. 7. Raman crosstalk within WDM signals could easily result in the power difference over 2 dB even in moderate input powers. In order to compensate the Raman crosstalk, we have tested the feasibility of the proposed idea using the negative slope in the Raman gain band, in Fig. 3 the band II between 440 cm and 490 cm . The band corresponds to the wavelength range of 1.310–1.322 m for 1.24- m pump. Within the band, 11 WDM channels of initial-input power of 5 dBm were assumed with an equal spacing of 1 nm. After transmission through SMF of 100 km, the WDM channels result in the unbalanced power

Fig. 7. Channel power difference between P and P as a function of SMF length. The channel power difference increases as the fiber length and input power increase.

caused by Raman crosstalk, as shown in Fig. 6. These unbalanced optical channels serve as the input of the proposed Raman amplifier (see Fig. 1). The amplifier consisted of a germanium= 0.015 and the medium length in the doped silica fiber of range of 0.1 to 1.5 km. The power of the pump at 1.24 m was set at 300 mW. The spectral response of WDM channels are shown in Fig. 8 for various Raman fiber lengths. The spectrum for the gain medium length of 0 km represents the input for the amplifiers. As the gain medium length increases, signals are amplified and the signal power in the shorter-wavelength region increases more rapidly than that of longer wavelength due to negative Raman-gain slope. For 0.8 km of gain-medium length,

SEO et al.: SIMULTANEOUS AMPLIFICATION AND CHANNEL EQUALIZATION

Fig. 8. Power equalization of ten channels after the Raman amplifier with a negative slope. The Raman medium has n = 0.015, and was pumped at 1.24 m with 300 mW. The input power distribution is indicated as the spectrum at L = 0.0 km, which results from Raman-induced crosstalk after 100-km transmission in conventional SMF.

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Fig. 10. Variation of optimal Raman medium length versus the index difference of Raman medium. The optimal length for power equalization significantly reduces as the high index difference increases.

Fig. 11. Synthesis of broadband Raman gain with a negative slope using two pump sources. The negative gain slope is obtained in 1.31–1.345 m.

Fig. 9. Power equalization of channels for various initial-channel powers. The Raman medium has n = 0.015, and was pumped at 1.24 m with 300 mW. The optimal Raman medium length increases as the initial-channel power increases.

1

the power of the WDM channels was regulated to 17 dBm within 0.75 dBm without external filtering devices. If the gain medium length further increases, Raman crosstalk is over-compensated with an inverted power slope. The SPM and GVD effects within the amplifier were ignored because of short fiber length, ( 1 km) compared with the nonlinear length and dispersion length. Fig. 9 shows the equalized power spectra of WDM signals for various initial input powers. The length in the parenthesis for each signal power level indicates the optimum fiber length required to equalize the channel power. The higher initial input power induces the greater Raman crosstalk such that the longer fiber length is in need to compensate the power unbalance. Fig. 10 shows the relation between the optimal length and the index difference of the fiber in the amplifier for various initial input signal powers. Due to large negative gain slope in highly doped germanium fibers (see Fig. 3), the optimal length for power equalization significantly reduces as the index difference increases. We thus have theoretically confirmed that a lumped Raman amplifier composed of single pump at 1.24 m

could simultaneously amplify and equalize WDM channel signals within a limited band 1.310–1.322 m. In order to expand the gain bandwidth, a dual pumping scheme was studied. The interactions between two pumps at different wavelengths were included in the analysis using the governing equations. Fig. 11 shows the Raman gain spectra for two pumps. The Raman medium is germanium-doped silica of 0.04. Two pump sources fiber with the index difference are at 1.24 m (0.3 W) and 1.259 m (0.12 W). The synthesized Raman gain spectrum results in the negative slope in the range of 1.310 to 1.345 m. The average negative Raman gain slope is about 2.5 10 cm/ m. Note that more than 30 channels of the equal spacing of about 1 nm could be allocated in the band. Fig. 12 shows the power spectrum of the 30 channels for various Raman fiber lengths up to 300 m. The spectrum for the gain medium length of 0 km represents the input for the amplifiers. The initial input power of 5 dBm traveling through 100 km of SMF was assumed as before in Fig. 6. Prior to the amplifier, the channels were positively tilted from 25 to 18 dBm in the wavelength from 1.310 to 1.345 m. For the Raman amplifier with the fiber length of 175 m, the signal power was regulated to 16.5 dBm within 0.62 dBm variation without external filters. SPM and GVD effects were ignored in the analysis

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Fig. 12. Power equalization of 30 channels using the synthesized Raman gain shown in Fig. 11. The channel power distribution with the maximum difference of 7 dBm was equalized to 16.5 dBm within 1.24-dBm variation.

0

because the nonlinear length and dispersion length, were 3420 and 204.3 km, which were significantly longer than the Raman medium length. Note that the equalized signal level of 16.5 dB after the proposed Raman amplifier is still low compared with the initial input signal level of 5 dBm. In order to further amplify the equalized signals, conventional broadband Raman amplifier [1], [2], may be needed in cascade. IV. CONCLUSION Crosstalk among channels in WDM communication was numerically calculated including the power transfer of the first stokes shift to the second. Using a lumped Raman amplifier = 0.015 with a composed of germanium-doped fiber of single pump at 1.24 m, it was theoretically proved that Raman crosstalk accumulated over SMF of 100 km could be compensated within 0.75 dBm in the wavelengths from 1.310 m to 1.322 m. The Raman gain band with a negative slope was further expanded to 35 nm using a germanium-doped fiber of = 0.04 and two pump sources. Feasibility of simultaneous equalization and amplification of WDM 30 channels with 1-nm spacing from 1.310 to 1.345 m was demonstrated by numerical analysis. Regulation of signal power within 0.62 dBm without external filters was predicted by proper control of pump powers, pump wavelengths, and Raman medium length. Experimental studies are being pursued by the authors. REFERENCES [1] E. M. Dianov, A. A. Abramov, M. M. Bubnov, A. M. Prokhorov, A. V. Shipulin, G. G. Devjatykh, A. N. Guryanov, and V. F. Khopin, “30 dB gain Raman amplifier at 1.3 m in low loss high GeO doped silica fibers,” Electron. Lett., vol. 31, no. 13, pp. 1057–1058, June 1995. [2] S. V. Chernikov, S. A. E. Lewis, and J. R. Taylor, “Broadband Raman amplifiers in the spectral range of 1480–1620 nm,” OFC’99 Tech. Dig., vol. WG6, pp. 117–119. [3] H. Masuda, “Review of wide-band hybrid amplifiers,” OFC 2000 Tech. Dig., vol. TUA1, pp. 2–4. [4] S. A. E. Lewis, F. Koch, S. V. Chernikov, and J. R. Taylor, “Low-noise high gain dispersion compensating broadband Raman amplifier,” OFC 2000 Tech. Dig., vol. TUA2, pp. 5–7.

[5] F. Koch, S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Broadband gain flattened Raman amplifier to extend operation in the third telecommunication window,” OFC 2000 Tech. Dig., vol. FF3, pp. 103–105. [6] S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Rayleigh noise suppression using a gain flattening filter in a broadband Raman amplifier,” OFC 2000 Tech. Dig., vol. FF5, pp. 109–111. [7] H. Hatayama, C. Hirose, K. Koyama, N. Akasaka, and M. Nishimura, “Variable attenuation slope compensator (VASC) using silica-based planar lightwave circuit technology for active gain slope control in EDFAs,” OFC 2000 Tech. Dig., vol. WH7, pp. 142–144. [8] G. Shaulov, V. J. Mazurczyk, and E. A. Golovchenko, “Measurement of Raman gain coefficient for small wavelength shifts,” OFC 2000 Tech. Dig., vol. TUA4, pp. 12–14. [9] A. G. Grandpierre, D. N. Christodoulides, C. M. McIntosh, and J. Toulouse, “Stimulated Raman scattering cancellation in wavelength-division-multiplexed systems via spectral inversion,” OFC 2000 Tech. Dig., vol. TUA5, pp. 15–17. [10] M. Takeda, S. Kinoshita, Y. Sugaya, and T. Tanaka, “Active gain-tilt equalization by preferentially 1.43 m- or 1.48m-pumped Raman amplification,” OAA’99 Tech. Dig., vol. ThA3, pp. 76–79. [11] K. X. Liu and E. Garmire, “Understanding the formation of the SRS stokes spectrum in fused silica fibers,” IEEE J. Quantum Electron., vol. 27, no. 4, pp. 1022–1030, Apr. 1991. [12] S. T. Davey, D. L. Williams, and B. J. Ainslie, “Optical gain spectrum of GeO -SiO Raman fiber amplifiers,” Proc. Inst. Elect. Eng. , vol. 136, no. 6, pp. 301–306, Dec. 1989. [13] M. N. Zervas and R. I. Laming, “Rayleigh scattering effect on the gain efficiency and noise of erbium-doped fiber amplfiers,” J. Lightwave Technol., vol. 31, no. 3, pp. 468–471, Mar. 1995. [14] T. Nakashima, S. Seikai, M. Nakazawa, and Y. Negishi, “Theoretical limit of repeater spacing in an optical transmission line utilizing Raman amplification,” J. Lightwave Technol., vol. LT-4, no. 3, pp. 1267–1272, Aug. 1986. [15] L. B. Jeunhomme, Single-Mode Fiber Optics. New York: Marcel Dekker, 1983, ch. 1, pp. 16–18. [16] G. P. Agrawal, Nonlinear Fiber Optics, 2nd ed. New York: Academic, 1995, ch. 2–4, pp. 28–132. [17] A. Boskovic, S. V. Chernikov, J. R. Taylor, L. Gruner-Nielsen, and O. A. Levring, “Direct continuous-wave measurement of n in various types of telecommunication fiber at 1.55 m,” Opt. Lett., vol. 21, no. 24, pp. 1966–1968, Dec. 1996. [18] H. S. Seo and K. Oh, “Optimization of silica fiber Raman amplifier using the Raman frequency modeling for an arbitrary GeO concentration in the core,” Opt. Commun., vol. 181, pp. 145–151, July 2000.

H. S. Seo (S’98) was born in Taejon, Korea, on November 10, 1972. He received the B.S. degree in electronics engineering from Chungnam National University, Taejon, in 1995, and the M.S. degree of engineering in information and communications from Kwangju Institute of Science and Technology, Kwangju, Korea, in 1997. He is currently working in the area of optical fiber devices for optical communications as a Ph.D. candidate at the Department of Information and Communications at the same institute. Mr. Seo is a student member of IEEE/LEOS and the Optical Society of America (OSA).

K. Oh (M’96) received the B.S. and M.S. degrees in physics from Seoul National University, Seoul, Korea, in 1986 and 1988, respectively. He received the M.S. degree in engineering and the Ph.D. degree in physics from Brown University, Providence, RI, in 1991 and 1994, respectively. He was subsequently appointed as a Postdoctoral Research Associate in the Laboratory for Lightwave Technology. Returning to Korea, he was involved in specialty fiber development as a Senior Researcher in Fiber-optics and Telecommunication Laboratory in LG Cable in 1995. From 1996 to 2000, he was an Assistant Professor in the Department of Information and Communications, Kwangju Institute of Science and Technology, Kwangju, Korea. He became an Associate Professor in 2000. In the summer of 1998, he was appointed as a Visiting Professor of research in the division of engineering at Brown University. From September 2000 to August 2001, he has been appointed as a Visiting Scientist in Bell Labs in Lucent Technologies, Murray Hill, NJ. His research interests are in the areas of specialty fiber design and fabrication for active and passive fiber devices in optical communications. Dr. Oh is a member of IEEE/LEOS, the Optical Society of America (OSA), and IEEK.

SEO et al.: SIMULTANEOUS AMPLIFICATION AND CHANNEL EQUALIZATION

U. C. Paek (M’90–SM’94) was born in Korea. He received the B.S. degree from Korea Merchant Marine Academy, Pusan, Korea, in 1957, and the M.S. and Ph.D. degrees from the University of California, Berkeley, in 1965 and 1969, respectively. From 1969 to 1991, he was with Bell Labs, Lucent Technologies, (then AT&T), where he was a Member of Technical Staff, a Distinguished Member of Technical Staff ,and a Bell Labs Fellow. In 1991, he returned to Korea and became the Executive Vice President of Korea Academy of Industrial Technology. In 1994, he became a Professor of the Information and Communications Department, and the Director of the Research Center for Ultrafast Fiber-Optic Networks, Kwangju Institute of Science and Technology, Kwangju, Korea, where he is now a Chaired Professor. His research interests are in the areas of optical communications, optical fiber technology, and fabrication of optical devices and components. Dr. Paek is a Fellow of the Optical Society of America (OSA), a Fellow of the American Ceramic Society, and a member of Sigma Xi.

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