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Volumn 43, Issue 10, 2007, Pages 910-922

Monolithic semiconductor waveguide device concept for picosecond pulse amplification, isolation, and spectral shaping

Author keywords

Integrated optoelectronics; Optical isolators; Optical pulse amplifiers; Optical pulse shaping; Semiconductor optical amplifiers (SOAs); Ultrafast optics

Indexed keywords

EQUIVALENT LENGTHS; GAIN MATERIALS; INGAASP; INP; MONOLITHIC ARRAYS; NON-LINEAR ELEMENTS; OPTICAL AMPLIFIERS; OPTICAL BAND WIDTHS; OPTICAL ISOLATORS; OPTICAL PULSE AMPLIFIERS; OPTICAL PULSE SHAPING; PICO-SECOND PULSE; POWER REFLECTIONS; SATURABLE ABSORBERS; SEMICONDUCTOR WAVEGUIDES; SIMULATION RESULTS; SPECTRAL SHAPING; TEMPORAL BROADENING; ULTRAFAST OPTICS; WAVEGUIDE DEVICES;

EID: 57649143503     PISSN: 00189197     EISSN: None     Source Type: Journal    
DOI: 10.1109/JQE.2007.903363     Document Type: Article
Times cited : (22)

References (48)
  • 1
    • 21244457199 scopus 로고    scopus 로고
    • Semiconductor mode-locked lasers as pulse sources for high bit rate data transmission
    • L. A. Jiang, E. P. Ippen, and H. Yokoyama, "Semiconductor mode-locked lasers as pulse sources for high bit rate data transmission," J. Opt. Fiber. Commun. Rep., vol. 2, pp. 1-31, 2005.
    • (2005) J. Opt. Fiber. Commun. Rep , vol.2 , pp. 1-31
    • Jiang, L.A.1    Ippen, E.P.2    Yokoyama, H.3
  • 3
    • 0037075570 scopus 로고    scopus 로고
    • Optical frequency metrology
    • Mar
    • T. Udem, R. Holzwarth, and T. W. Hänsch, "Optical frequency metrology," Nature, vol. 416, pp. 233-237, Mar. 2002.
    • (2002) Nature , vol.416 , pp. 233-237
    • Udem, T.1    Holzwarth, R.2    Hänsch, T.W.3
  • 4
    • 2542462329 scopus 로고    scopus 로고
    • Flexible pulse waveform generation using silica-waveguide-based spectrum synthesis circuit
    • K. Takiguchi, K. Okamoto, T. Kominato, H. Takahashi, and T. Shibata, "Flexible pulse waveform generation using silica-waveguide-based spectrum synthesis circuit," Electron. Lett., vol. 40, no. 9, pp. 537-538, 2004.
    • (2004) Electron. Lett , vol.40 , Issue.9 , pp. 537-538
    • Takiguchi, K.1    Okamoto, K.2    Kominato, T.3    Takahashi, H.4    Shibata, T.5
  • 5
    • 0042968720 scopus 로고    scopus 로고
    • Recent developments in compact ultrafast lasers
    • Aug
    • U. Keller, "Recent developments in compact ultrafast lasers," Nature, vol. 424, pp. 831-838, Aug. 2003.
    • (2003) Nature , vol.424 , pp. 831-838
    • Keller, U.1
  • 6
    • 0031177649 scopus 로고    scopus 로고
    • Active harmonic modelocking of an erbium fiber laser with intracavity Fabry-Perot filters
    • Jul
    • J. S. Wey, J. Goldhar, and G. L. Burdge, "Active harmonic modelocking of an erbium fiber laser with intracavity Fabry-Perot filters," J. Lightw. Technol., vol. 15, no. 6, pp. 1171-1180, Jul. 1997.
    • (1997) J. Lightw. Technol , vol.15 , Issue.6 , pp. 1171-1180
    • Wey, J.S.1    Goldhar, J.2    Burdge, G.L.3
  • 7
    • 0037817468 scopus 로고    scopus 로고
    • Smoothly wavelength-tunable picosecond pulse generation using a harmonically modelocked fiber ring laser
    • Apr
    • L. Duan, M. Dagenais, and J. Goldhar, "Smoothly wavelength-tunable picosecond pulse generation using a harmonically modelocked fiber ring laser," J. Lightw. Technol., vol. 21, no. 4, pp. 930-937, Apr. 2003.
    • (2003) J. Lightw. Technol , vol.21 , Issue.4 , pp. 930-937
    • Duan, L.1    Dagenais, M.2    Goldhar, J.3
  • 8
    • 0029237659 scopus 로고
    • Femtosecond optical pulse shaping and processing
    • A. M. Weiner, "Femtosecond optical pulse shaping and processing," Prog. Quantum Elect., vol. 19, pp. 161-237, 1995.
    • (1995) Prog. Quantum Elect , vol.19 , pp. 161-237
    • Weiner, A.M.1
  • 12
    • 84951104659 scopus 로고    scopus 로고
    • Y. Barbarin, E. A. J. M. Bente, M. J. R. Heck, Y. S. Oei, R. Nøtzel, and M. K. Smit, Passively modelocked 20 and 40 GHz bulk InGaAsP lasers, in Proc. Eur. Conf. Opt. Commun. ECOC , Glasgow, U.K., 2005, pp. 673-674, Paper We4.P.086.
    • Y. Barbarin, E. A. J. M. Bente, M. J. R. Heck, Y. S. Oei, R. Nøtzel, and M. K. Smit, "Passively modelocked 20 and 40 GHz bulk InGaAsP lasers," in Proc. Eur. Conf. Opt. Commun. (ECOC , Glasgow, U.K., 2005, pp. 673-674, Paper We4.P.086.
  • 13
    • 33750340958 scopus 로고    scopus 로고
    • Characterization of a 15 GHz integrated bulk InGaAsP passively modelocked ring laser at 1.53 μm
    • Y. Barbarin, E. A. J. M. Bente, M. J. R. Heck, Y. S. Oei, R. Nøtzel, and M. K. Smit, "Characterization of a 15 GHz integrated bulk InGaAsP passively modelocked ring laser at 1.53 μm," Opt. Exp., vol. 14, no. 21, pp. 9716-9727.
    • Opt. Exp , vol.14 , Issue.21 , pp. 9716-9727
    • Barbarin, Y.1    Bente, E.A.J.M.2    Heck, M.J.R.3    Oei, Y.S.4    Nøtzel, R.5    Smit, M.K.6
  • 16
    • 0024771723 scopus 로고
    • Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers
    • Nov
    • G. P. Agrawal and N. A. Olsson, "Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers," IEEE J. Quantum Electron., vol. 25, no. 11, pp. 2297-2306, Nov. 1989.
    • (1989) IEEE J. Quantum Electron , vol.25 , Issue.11 , pp. 2297-2306
    • Agrawal, G.P.1    Olsson, N.A.2
  • 17
    • 0022812033 scopus 로고
    • Regimes of feedback effects in 1.5 μm distributed feedback lasers
    • Nov
    • R. W. Tkach and A. R. Chraplyvy, "Regimes of feedback effects in 1.5 μm distributed feedback lasers," J. Lightw. Technol., vol. 4, no. 11, pp. 1655-1661, Nov. 1986.
    • (1986) J. Lightw. Technol , vol.4 , Issue.11 , pp. 1655-1661
    • Tkach, R.W.1    Chraplyvy, A.R.2
  • 18
    • 65349127503 scopus 로고    scopus 로고
    • Y. Barbarin, E. A. J. M. Bente, T. de Vries, J. H. den Besten, P. J. van Veldhoven, M. J. H. Sander-Jochem, E. Smalbrugge, F. W. M. van Otten, E. J. Geluk, M. J. R. Heck, X. J. M. Leijtens, J. G. M. van der Tol, F. Karouta, Y. S. Oei, R. Nötzel, and M. K. Smit, Butt-joint interfaces in InP-InGaASP waveguides with very low reflectivity and low loss, in Proc. Symp. IEEE/LEOS Benelux Chapter, Mons, 2005, pp. 89-92.
    • Y. Barbarin, E. A. J. M. Bente, T. de Vries, J. H. den Besten, P. J. van Veldhoven, M. J. H. Sander-Jochem, E. Smalbrugge, F. W. M. van Otten, E. J. Geluk, M. J. R. Heck, X. J. M. Leijtens, J. G. M. van der Tol, F. Karouta, Y. S. Oei, R. Nötzel, and M. K. Smit, "Butt-joint interfaces in InP-InGaASP waveguides with very low reflectivity and low loss," in Proc. Symp. IEEE/LEOS Benelux Chapter, Mons, 2005, pp. 89-92.
  • 19
    • 33646704214 scopus 로고    scopus 로고
    • Simulation and design of integrated femtosecond passively mode-locked semiconductor ring lasers including integrated passive pulse shaping components
    • Mar.-Apr
    • M. J. R. Heck, E. A. J. M. Bente, Y. Barbarin, D. Lenstra, and M. K. Smit, "Simulation and design of integrated femtosecond passively mode-locked semiconductor ring lasers including integrated passive pulse shaping components," IEEE J. Sel. Topics Quantum Electron., vol. 12, no. 2, pp. 265-276, Mar.-Apr. 2006.
    • (2006) IEEE J. Sel. Topics Quantum Electron , vol.12 , Issue.2 , pp. 265-276
    • Heck, M.J.R.1    Bente, E.A.J.M.2    Barbarin, Y.3    Lenstra, D.4    Smit, M.K.5
  • 20
    • 55649094287 scopus 로고    scopus 로고
    • N. K. Fontaine, C. Ji, R. G. Broeke, S. W. Seo, S. J. B. Yoo, K. Y. Liou, J. R. Lothian, S. Vatanapradit, S. N. G. Chu, B. Patel, W. S. Hobson, and W. T. Tsang, XFROG characterization of a 10 GHz colliding-pulse mode-locked laser, presented at the CLEO'06, May 2006, Paper CThH6.
    • N. K. Fontaine, C. Ji, R. G. Broeke, S. W. Seo, S. J. B. Yoo, K. Y. Liou, J. R. Lothian, S. Vatanapradit, S. N. G. Chu, B. Patel, W. S. Hobson, and W. T. Tsang, "XFROG characterization of a 10 GHz colliding-pulse mode-locked laser," presented at the CLEO'06, May 2006, Paper CThH6.
  • 21
    • 65349120541 scopus 로고    scopus 로고
    • M. J. R. Heck and E. A. J. M. Bente, Halfgeleider optisch element voor het bewerken van ten minste één lichtpuls, alsmede werkwijze voor het gebruik daarvan, Dutch Patent 1032179, Jul. 17, 2006
    • M. J. R. Heck and E. A. J. M. Bente, "Halfgeleider optisch element voor het bewerken van ten minste één lichtpuls, alsmede werkwijze voor het gebruik daarvan," Dutch Patent 1032179, Jul. 17, 2006.
  • 23
    • 0031269179 scopus 로고    scopus 로고
    • Effect of saturation caused by amplified spontaneous emission on semiconductor optical amplifier performance
    • Nov
    • T. Liu, K. Obermann, K. Petermann, F. Girardin, and G. Guekos, "Effect of saturation caused by amplified spontaneous emission on semiconductor optical amplifier performance," Electron. Lett., vol. 33, pp. 2042-2043, Nov. 1997.
    • (1997) Electron. Lett , vol.33 , pp. 2042-2043
    • Liu, T.1    Obermann, K.2    Petermann, K.3    Girardin, F.4    Guekos, G.5
  • 24
    • 0037250224 scopus 로고    scopus 로고
    • Nonlinear polarization rotation in semiconductor optical amplifiers: Theory and application to all-optical flip-flop memories
    • Jan
    • H. J. S. Dorren, D. Lenstra, Y. Liu, M. T. Hill, and G. D. Khoe, "Nonlinear polarization rotation in semiconductor optical amplifiers: Theory and application to all-optical flip-flop memories," IEEE J. Quantum Electron., vol. 39, no. 1, pp. 141-147, Jan. 2003.
    • (2003) IEEE J. Quantum Electron , vol.39 , Issue.1 , pp. 141-147
    • Dorren, H.J.S.1    Lenstra, D.2    Liu, Y.3    Hill, M.T.4    Khoe, G.D.5
  • 25
    • 0032647243 scopus 로고    scopus 로고
    • Amplification of strong picosecond optical pulses in semiconductor optical amplifiers
    • Feb
    • J. M. Tang and K. A. Shore, "Amplification of strong picosecond optical pulses in semiconductor optical amplifiers," in Proc. IEE Optoelectron., Feb. 1999, vol. 146, pp. 45-50.
    • (1999) Proc. IEE Optoelectron , vol.146 , pp. 45-50
    • Tang, J.M.1    Shore, K.A.2
  • 26
    • 0030195422 scopus 로고    scopus 로고
    • Chirp and stability of mode-locked semiconductor lasers
    • Jul
    • M. Schell, M. Tsuchiya, and T. Kamiya, "Chirp and stability of mode-locked semiconductor lasers," IEEE J. Quantum Electron., vol. 32, no. 7, pp. 1180-1190, Jul. 1996.
    • (1996) IEEE J. Quantum Electron , vol.32 , Issue.7 , pp. 1180-1190
    • Schell, M.1    Tsuchiya, M.2    Kamiya, T.3
  • 27
    • 0001629708 scopus 로고    scopus 로고
    • Modeling and measurement of longitudinal gain dynamics in saturated semiconductor optical amplifiers of different length
    • Dec
    • R. Gutiérrez-Castregón, L. Schares, L. Occhi, and G. Guekos, "Modeling and measurement of longitudinal gain dynamics in saturated semiconductor optical amplifiers of different length," IEEE J. Quantum Electron., vol. 36, no. 12, pp. 1476-1484, Dec. 2000.
    • (2000) IEEE J. Quantum Electron , vol.36 , Issue.12 , pp. 1476-1484
    • Gutiérrez-Castregón, R.1    Schares, L.2    Occhi, L.3    Guekos, G.4
  • 28
    • 0141918473 scopus 로고    scopus 로고
    • Gain dynamics of semiconductor optical amplifiers and three-wavelength devices
    • Oct
    • G. Talli and M. J. Adams, "Gain dynamics of semiconductor optical amplifiers and three-wavelength devices," IEEE J. Quantum. Electron. vol. 39, no. 10, pp. 1305-1313, Oct. 2003.
    • (2003) IEEE J. Quantum. Electron , vol.39 , Issue.10 , pp. 1305-1313
    • Talli, G.1    Adams, M.J.2
  • 30
    • 0030104215 scopus 로고    scopus 로고
    • Theory for passive mode-locking in semiconductor laser structures including the effects of self-phase modulation, dispersion and pulse collisions
    • Mar
    • R. G. M. P. Koumans and R. van Roijen, "Theory for passive mode-locking in semiconductor laser structures including the effects of self-phase modulation, dispersion and pulse collisions," IEEE J. Quantum Electron., vol. 32, no. 3, pp. 478-492, Mar. 1996.
    • (1996) IEEE J. Quantum Electron , vol.32 , Issue.3 , pp. 478-492
    • Koumans, R.G.M.P.1    van Roijen, R.2
  • 31
    • 0032206322 scopus 로고    scopus 로고
    • Effects of carrier cooling and carrier heating in saturation dynamics and pulse propagation through bulk semiconductor absorbers
    • Nov
    • A. V. Uskov, J. R. Karin, J. E. Bowers, J. G. McInerney, and J. Le Bihan, "Effects of carrier cooling and carrier heating in saturation dynamics and pulse propagation through bulk semiconductor absorbers," IEEE J. Quantum Electron., vol. 34, no. 11, pp. 2162-2171, Nov. 1998.
    • (1998) IEEE J. Quantum Electron , vol.34 , Issue.11 , pp. 2162-2171
    • Uskov, A.V.1    Karin, J.R.2    Bowers, J.E.3    McInerney, J.G.4    Le Bihan, J.5
  • 33
    • 0032095315 scopus 로고    scopus 로고
    • Gain recovery of bulk semiconductor optical amplifiers
    • Jun
    • F. Girardin and G. Guekos, "Gain recovery of bulk semiconductor optical amplifiers," IEEE Photon. Technol. Lett., vol. 10, no. 6, pp. 784-786, Jun. 1998.
    • (1998) IEEE Photon. Technol. Lett , vol.10 , Issue.6 , pp. 784-786
    • Girardin, F.1    Guekos, G.2
  • 34
    • 33846249625 scopus 로고    scopus 로고
    • Gain measurements of Fabry-Perot InP-In-GaAsP lasers using an ultrahigh-resolution spectrometer
    • Dec
    • Y. Barbarin, E. A. J. M. Bente, G. Servanton, L. Mussard, Y. S. Oei, R. Nötzel, and M. K. Smit, "Gain measurements of Fabry-Perot InP-In-GaAsP lasers using an ultrahigh-resolution spectrometer," Appl. Opt., vol. 45, no. 35, pp. 9007-9012, Dec. 2006.
    • (2006) Appl. Opt , vol.45 , Issue.35 , pp. 9007-9012
    • Barbarin, Y.1    Bente, E.A.J.M.2    Servanton, G.3    Mussard, L.4    Oei, Y.S.5    Nötzel, R.6    Smit, M.K.7
  • 35
    • 10344229933 scopus 로고    scopus 로고
    • Long-wavelength monolithic mode-locked diode lasers
    • K. A. Williams, M. G. Thompson, and I. H. White, "Long-wavelength monolithic mode-locked diode lasers," New J. Phys., vol. 6, no. 179, pp. 1-30, 2004.
    • (2004) New J. Phys , vol.6 , Issue.179 , pp. 1-30
    • Williams, K.A.1    Thompson, M.G.2    White, I.H.3
  • 37
    • 65349196029 scopus 로고    scopus 로고
    • H. G. Weber, R. Ludwig, C. Schmidt, C. Schubert, J. Berger, E. Hiliger, M. Kroh, V. Marembert, C. Boerner, S. Ferber, and H. J. Ehrke, 160 Gb/s TDM-transmission technology, presented at the ECOC, 2002, Paper 2.1.1.
    • H. G. Weber, R. Ludwig, C. Schmidt, C. Schubert, J. Berger, E. Hiliger, M. Kroh, V. Marembert, C. Boerner, S. Ferber, and H. J. Ehrke, "160 Gb/s TDM-transmission technology," presented at the ECOC, 2002, Paper 2.1.1.
  • 38
    • 1642633912 scopus 로고    scopus 로고
    • Noise and regeneration in semiconductor waveguides with saturable gain and absorption
    • Mar
    • F. Öhman, S. Bischoff, B. Tromborg, and J. Mørk, "Noise and regeneration in semiconductor waveguides with saturable gain and absorption," IEEE J. Quantum Electron., vol. 40, no. 3, pp. 245-255, Mar. 2004.
    • (2004) IEEE J. Quantum Electron , vol.40 , Issue.3 , pp. 245-255
    • Öhman, F.1    Bischoff, S.2    Tromborg, B.3    Mørk, J.4
  • 40
    • 0033871015 scopus 로고    scopus 로고
    • Use of multiple pairs of gain and saturable absorber regions for semiconductor optical-pulse compressor
    • Feb
    • M. Shimizu, Y. Suzuki, S. Mukai, and T. Hasama, "Use of multiple pairs of gain and saturable absorber regions for semiconductor optical-pulse compressor," Jpn. J. Appl. Phys., vol. 39, pp. 475-479, Feb. 2000.
    • (2000) Jpn. J. Appl. Phys , vol.39 , pp. 475-479
    • Shimizu, M.1    Suzuki, Y.2    Mukai, S.3    Hasama, T.4
  • 41
    • 32944455094 scopus 로고    scopus 로고
    • Transverse magnetic mode nonreciprocal propagation in an amplifying Al-GaInAs-InP optical waveguide isolator
    • Feb
    • W. van Parys, B. Moeyersoon, D. van Thourhout, and R. Baets, "Transverse magnetic mode nonreciprocal propagation in an amplifying Al-GaInAs-InP optical waveguide isolator," Appl. Phys. Lett., vol. 88, p. 071115, Feb. 2006.
    • (2006) Appl. Phys. Lett , vol.88 , pp. 071115
    • van Parys, W.1    Moeyersoon, B.2    van Thourhout, D.3    Baets, R.4
  • 42
    • 18844447014 scopus 로고    scopus 로고
    • Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III-V material
    • Mar./Apr
    • S. Bhandare, S. K. Ibrahim, D. Sandel, H. Zhang, F. Wüst, and R. Noé, "Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III-V material," IEEE J. Sel. Topics Quantum Electron., vol. 11, no. 2, pp. 417-421, Mar./Apr. 2005.
    • (2005) IEEE J. Sel. Topics Quantum Electron , vol.11 , Issue.2 , pp. 417-421
    • Bhandare, S.1    Ibrahim, S.K.2    Sandel, D.3    Zhang, H.4    Wüst, F.5    Noé, R.6
  • 43
    • 33646699404 scopus 로고    scopus 로고
    • Simulation and design of integrated pulse shaping components for femtosecond modelocked ring lasers
    • Apr
    • M. J. R. Heck, P. Muñoz, E. A. J. M. Bente, Y. Barbarin, D. Lenstra, and M. K. Smit, "Simulation and design of integrated pulse shaping components for femtosecond modelocked ring lasers," in Proc. ECIO 2005, Apr. 2005, pp. 351-354.
    • (2005) Proc. ECIO 2005 , pp. 351-354
    • Heck, M.J.R.1    Muñoz, P.2    Bente, E.A.J.M.3    Barbarin, Y.4    Lenstra, D.5    Smit, M.K.6
  • 45
    • 0348233399 scopus 로고    scopus 로고
    • A 36-Channel × 10-GHz spectrally sliced pulse source based on supercontinuum generation in normally dispersive highly nonlinear holey fiber
    • Dec
    • Z. Yusoff, P. Petropoulos, K. Furusawa, T. M. Monro, and D. J. Richardson, "A 36-Channel × 10-GHz spectrally sliced pulse source based on supercontinuum generation in normally dispersive highly nonlinear holey fiber," IEEE Photon. Technol. Lett., vol. 15, no. 12, pp. 1689-1691, Dec. 2003.
    • (2003) IEEE Photon. Technol. Lett , vol.15 , Issue.12 , pp. 1689-1691
    • Yusoff, Z.1    Petropoulos, P.2    Furusawa, K.3    Monro, T.M.4    Richardson, D.J.5
  • 46
    • 0031561257 scopus 로고    scopus 로고
    • InP-based 64-channel arrayed waveguide grating with 50 GHz channel spacing and up to -20 dB crosstalk
    • M. Kohtoku, H. Sanjoh, S. Oku, Y. Kadota, Y. Yoshikuni, and Y. Shibata, "InP-based 64-channel arrayed waveguide grating with 50 GHz channel spacing and up to -20 dB crosstalk," Electron. Lett., vol. 33, no. 21, pp. 1786-1787, 1997.
    • (1997) Electron. Lett , vol.33 , Issue.21 , pp. 1786-1787
    • Kohtoku, M.1    Sanjoh, H.2    Oku, S.3    Kadota, Y.4    Yoshikuni, Y.5    Shibata, Y.6
  • 47
    • 0032649493 scopus 로고    scopus 로고
    • Second- and third-order dispersion compensator using a high-resolution arrayed-waveguide grating
    • May
    • H. Tsuda, K. Okamoto, T. Ishii, K. Naganuma, Y. Inoue, H. Takenouchi, and T. Kurokawa, "Second- and third-order dispersion compensator using a high-resolution arrayed-waveguide grating," IEEE Photon. Technol. Lett., vol. 11, no. 5, pp. 569-571, May 1999.
    • (1999) IEEE Photon. Technol. Lett , vol.11 , Issue.5 , pp. 569-571
    • Tsuda, H.1    Okamoto, K.2    Ishii, T.3    Naganuma, K.4    Inoue, Y.5    Takenouchi, H.6    Kurokawa, T.7
  • 48
    • 0346342704 scopus 로고    scopus 로고
    • Phase modeling based on the α-factor in bulk semiconductor optical amplifiers
    • May/Jun
    • L. Occhi, L. Schares, and G. Guekos, "Phase modeling based on the α-factor in bulk semiconductor optical amplifiers," IEEE J. Sel. Topics Quantum Electron., vol. 9, no. 3, pp. 788-797, May/Jun. 2003.
    • (2003) IEEE J. Sel. Topics Quantum Electron , vol.9 , Issue.3 , pp. 788-797
    • Occhi, L.1    Schares, L.2    Guekos, G.3


* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.