摘要
在現今高速的光通信系統中，高速的(>10Gbits/sec) 光電調制器為通信系統中非常關鍵性的一部分。它能夠克服雷射直接調變時所產生的頻率飄移(chirp)，和頻寬不足的缺點，使得光通信系統中超高速的光時域多工和電時域多工變的可能。而光電調制器最主要分為兩類；分別是致電-吸光式調制器 (electro-absorption modulator, EAM) 和電-光折變式調制器(Electro-Optics Modulator, EOM)。其中LiNbO3 based 的EOM 已經商品化，而EAM 在市場上仍未普及。然而EAM 和EOM 相比之下仍具有了許多無法比擬的優點，例如說: 可和半導體雷射單晶或其他光電元件積體化結合[1,2]，較低的操作電壓，較低的極化敏感度，較低的製作成本[3]。而EAM至今無法大規模商品化的最大原因就是在於其介入損耗太大，元件阻抗過低，無法實現真正的行波式光電調制器。在傳統的EAM磊晶層結構中，簡單來說就是一個p-i-n的磊晶層結構，其中i-layer為一MQW的結構用來吸收入射光，外加電場也會侷限在此層中而加以調制。然而此結構最大的問題就是在於其為了降低操作電壓，此i-layer的厚度不能太厚，然而薄的i-layer卻會有相當大的元件電容，如此會造成元件速度表現的劣化，和許許多多非理想的微波傳播效應。我們雖然可以微縮元件尺寸來縮小電容，然而此舉卻會造成光信號耦合的困難和介入損耗的增加。因
I
此，我們提出一種有別於傳統p-i-n磊晶層結構的調制器來解決這個根本上速度和驅動電壓表現上的互相抵觸。我們所使用的磊晶層結構為p-i (MQW)-n+-i (collector)-n。此結構會讓大部分的電場集中在第一層i-layer (MQW)，使得驅動電壓不會隨著第二層i-layer(collector)厚度的增加而增加。此設計除了能解決驅動電壓和速度之間的互相牴觸外，也因為非參雜區厚度大幅加厚的關係大幅增加了光在非參雜區的侷限係數(confinement factor)，也同時降低了光在參雜區因為free-carrier-absorption 所造成的介入損耗。故此研究能大幅提升EAM的各種表現指標。由於此一結構擁有兩層空乏區，故我們將此結構的EAM稱之為雙空乏區致電-吸收光調制器 (Dual-Depletion-Region Electro-Absorption Modulator, DDR EAM )。
實驗結果驗證p-i (MQW)-n+-i (collector)-n結構確實能有效地降低元件電容值( C=0.27pF )且並不會使驅動電壓變大(V20dB=1V)，在穿透係數(S21)的量測中f3dB~13.5GHz，而電-光響應頻寬有10 GHz的表現，整體而言，與相同材料系統的EAM相比(見表1.1 )，S.Z.Zhang其FOM約為4.44 GHz/V [17]，而我們的FOM則為10 GHz/V。

參考文獻
[1] B. Mason, A. Ougazzaden, C. W. Lentz, K. G. Glogovsky, C. L. Reynolds, G.. J.Przybylek, R. E. Leibenguth, T. L. Kercher, J. W. Boardman, M. T. Rader, J. M. Geary, F. S. Walters, L. J. Peticolas, J. M. Freund, S. N. G. Chu, A. Sirenko, R. J. Jurchenko, M. S. Hybertsen, L. J. P. Ketelsen, and G. Raybon, “40-Gb/s Tandem Electroabsorption Modulator,” IEEE Photon. Technol. Lett., vol. 14, pp.27-29, Jan., 2002.
[2] H. Kawanishi, Y. Yamauchi, N. Mineo, Y. Shibuya, H. Murai, K. Yamada, and H. Wada, “EAM-Integrated DFB Laser Modules with More Than 40-GHz Bandwidth,” IEEE Photon. Technol. Lett., vol. 13, pp.954-956, Sep., 2001.
[3] G. L. Li, and P. K. L. Yu, “Optical Intensity Modulators for Digital and Analog Applications,” Journal of Lightwave Technol.,vol. 21, pp. 2010-2030
[4] S. Kodama, T. Yoshimatsu, and H. Ito,“320Gbit/s optical gate monolithically integrating photodiode and electroabsorption modulator,” Electronics Letters, vol. 39, pp.383-385, Feb., 2003.
[5] H.-F. Chou, Y.-J. Chiu, A. Keating, J. E. Bowers, and D. J. Blumenthal,“Photocurrent-Assisted Wavelength (PAW) Conversion With Electrical Monitoring Capability Using a Traveling-Wave Electroabsorption Modulator,” IEEE Photon. Technol. Lett., vol. 16, pp.530-532, Feb., 2004.
64
[6] Y. Matsui, H. Murai, S. Arahira, S. Kutsuzawa, and Y. Ogawa, “30-GHz bandwidth 1.55-μm strain-compensated InGaAlAs-InGaAsP MQW laser,”IEEE Photonics Technol. Lett., vol. 9, pp. 25-27, 1997.
[7] C. H. Henry, “Theory of the linewidth of semiconductor lasers,” IEEE J. Quantum Electron., vol. QE-18, 9, pp. 259-264, 1982.
[8] T. L. Koch and J. Bowers, “Nature of wavelength chirping in directly modulated semiconductor lasers,” IEEE Electron Lett., vol. 20, pp. 1038-1040, 1984.
[9] T. L. Koch and R. A. Linke, “Effect of nonlinear gain reduction on semiconductor laser wavelength chirping,” Appl. Phys. Lett., vol. 48, pp. 613-615, 1986.
[10] M. Osinski and J. Buus, “Linewidth broadening factor in semiconductor lasers-an overview,” IEEE J. Quantum Electron., vol. QE-23, pp. 9-29, 1987.
[11] F. Koyama and K. Iga, “Frequency chirping in external modulators,” J. Lightwave technol., vol. 6, pp. 87-93, 1988.
[12] D. C. Hutchings, M. Sheik-Bahae, D. J. Hagan, and E. W. van Stryland, “Kramers-Kronig relations in nonlinear optics,” Opt. Quantum Electron., vol. 24, pp. 1-30, 1992.
[13] F. Dorgeuille and F. Devaux, “On the transmission performances and the chirp parameter of a multiple-quantum-well electroabsorption modulator,” IEEE J. Quantum Electron., vol. 30, pp. 2565-2572, 1994.
[14] L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Integrated Circuits. New York: John Wiley & Sons, Inc., p. 209, 1995.
65
[15] K. Wakita, K. Yoshino, I. Kotaka, S. Kondo, and Y. Noguchi, “High speed, high efficiency modulator module with polarization insensitivity and very low chirp,” IEEE Electron Lett., vol. 31, pp. 2041-2042, 1995.
[16] K. Yamada, K. Nakamura, Y. Matsui, T. Kunii, and Y. Ogawa, “Negative-chirp electroabsorption modulator using low-wavelength detuning,” IEEE Photonics Technol. Lett., vol. 7, pp. 1157-1158, 1995.
[17] Shengzhong Zhang, “Traveling-wave electroabsorption modulators,” University of California, Santa Barbara, Ph. D. Dissertation, 1999.
[18] J. G. Mendoza-Alvarez, L. A. Coldren, A. Alping, R. H. Yan, T. Hausken, K. Lee, and K. Pedrotti, “Analysis of depletion edge translation lightwave modulators,” J. Lightwave technol., vol. 6, pp. 793-808, 1988.
[19] R. G. Walker, “High-speed III-V semiconductor intensity modulators,” IEEE J. Quantum Electron., vol. 27, pp. 654-667, 1991.
[20] O. Mitomi, K. Noguchi, and H. Miyazawa, “Broadband and low driving-voltage LiNbo3 optical modulators,” IEE Proc., Optoelectron., vol. 145, pp. 360-364, 1998.
[21] D. Chen, D. Bhattacharya, A. Udupa, B. Tsap, H.R. Fetterman, C. Antao, L. Sang-Shin, C. Jinghong, W. H. Steier, and L. R. Dalton, “High-frequency polymer modulators with integrated finline transitions and low Vpi ,” IEEE Photonics Technol. Lett., vol. 11, pp. 54-56, 1999.
66
[22] C. Rolland, “InGaAsP-based Mach-Zehnder modulators for high- speed transmission system,” OSA OFC’98, San Jose, CA, pp. 283-284, 1998.
[23] H. R. Khazaei, E. Berolo, and F. Ghannouchi, “High-speed slow-wave coplanar strip GaAs/AlGaAs electro-optic laser modulator,” Microw. Opt. Technol. Lett., vol. 19, pp. 184-186, 1998.
[24] K. Wakita, I. Kotaka, and H. Asai, “High-speed InGaAlAs/InAlAs multiple quantum well electrooptic phase modulators with bandwidth in excess of 20 GHz,” IEEE Photonics Technol. Lett., vol. 4, pp. 29-31, 1992.
[25] T. Ido, S. Tanaka, M. Suzuki, M. Koizumi, H. Sano, and H. Inoue, “Ultra-High-speed multiple-quantum-well electro-absorption optical modulators with integrate waveguides,” J. Lightwave Technol., vol. 14, pp. 2026-2034, 1996.
[26] F. Devaux, J. C. Harmand, I. F. L. Dias, T. Guettler, O. Krebs, and P. Voisin, “High power saturation, polarization insensitive electroabsorption modulator with spiked shallow wells,” Electron. Lett., vol. 33, pp. 161-163, 1997.
[27] K. Kawano, M. Kohtoku, M. Ueki, T. Ito, S. Kondoh, Y. Noguchi, and Y. Hasumi, “Polarization- insensitive traveling-wave electrode electroabsorption(TW-EA) modulator with bandwidth,” J. Lightwave Technol., vol. 14, pp. 2026-2034, 1996.
[28] S. Z. Zhang, Yi-Jen Chiu, P. Abrabam, and John E. Bowers, “25 GHz polarization-insensitive electroabsorption modulators with traveling-wave electrodes ,” IEEE Photon. Technol. Lett., vol. 11, pp. 191-193, 1999.
67
[29] F. Devaux, S. Chelles, A. Ougazzaden, A. Mircea, and J. C. Harmand, “Electroabsorption modulators for high-bit-rate optical communications: a comparison of strained InGaAs/InAlAs and InGaAsP/ InGaAsP MQW,” Semicond. Sci. Technol., vol. 10, pp. 887-901, 1995.
[30] H. Fukano, T. Yamanaka, M. Tamura, Y. Konodo, and T. Saitoh, “Very lowdriving-voltage InGaAlAs/InAlAs electroabsorption modulators operating at 40 Gbit/s,” Electronic Letters, vol. 41, Feb., 2005.
[31] M. Tamura, T. Yamanaka, H. Fukano, Y. Akage, Y. Konodo, and T. Saitoh,“High-speed electroabsorption modulators buried with ruthenium-doped SI-InP,” IEEE Photon. Technol. Lett., vol. 16, pp.2613-2615, Dec., 2004.
[32] Yi-Jen Chiu, Hsu-Feng Chou, Volkan Kaman, Patric Abrabam, and John E. Bowers, “High extinction ratio and saturation power traveling-wave electroabsorption modulator,” IEEE Photon. Technol. Lett., vol. 14, NO. 6, pp. 792-794, JUNE 2002.
[33] G. L. Li, S.A. Pappert, P. Mages, C. K. Sun, W. S. C. Chang, and P. K. L. Yu, “High-Saturation high-speed traveling-wave InGaAsP-InP electroabsorption modulator,” IEEE Photon. Technol. Lett., vol. 13, NO. 10, pp. 1076-1078, OCTOBER 2001.
[34] S. Irmscher, R. Lewen, and U. Eriksson, “InP-InGaAsP high-speed traveling-wave electroabsorption modulator with integrated termination resistors,” IEEE Photon. Technol. Lett., vol. 14, NO. 7, pp. 923-925, JULY 2002.
68
[35] Takayuki yamanaka, “Ultrafast electroabsorption modulators with traveling-wave electrodes,” ECOC’01, Amsterdam, pp. 328-331, 2001.
[36] Hideki Fukano, Munehisa Tamura, Takayuki Yamanaka, Hiroki Nakajima, Yuichi Akage, Yasuhiro Kondo, and Tadashi Saitoh, “Low driving-voltage (1.1Vpp) electroabsorption modulators operating at 40 Gbit/s,” in 16th International Conf. Indium Phosphide and Related Materials(IPRM’04), Kagoshima, Japan, JUNE 2004, pp. 573-576
[37] Pallab Bhattacharya, “Semiconductor Optoelectronic Devices,” 2nd. , Prentice Hall, pp.79-82, 1997
[38] H. Nie, K. A. Anselm, C. Lenox, P. Yuan, C. Hu, G. Kinsey, B. G. Streetman, and J. C. Campbell, “Resonant-Cavity Separate Absorption, Charge and Multiplication Avalanche Photodiodes With High-Speed and High Gain-Bandwidth Product,” IEEE Photon. Technol. Lett., vol. 10, pp. 409-411, Mar., 1998.
[39] T. Nakata, T. Takeuchi, I. Watanabe, K. Makita, and T. Torikai, “10Gbit/s high sensitivity, low-voltage-operation avalanche photodiodes with thin InAlAs multiplication layer and waveguide structure,” Electron. Lett., vol. 36, pp.2033-2034, Nov., 2000.
[40] R. Lewen, S. Irmscher, U. Westergen, L. Thylen, and U. Eriksson, “Segmented Transmission-Line Electroabsorption Modulators,” IEEE/OSA J. of Lightwave Technol., vol. 22, pp.172-179, Jan., 2004.
69
[41] M. Tamura, T. Yamanaka, H. Fukano, Y. Akage, Y. Konodo, and T. Saitoh,“High-Speed Electroabsorption Modulators Buried With Ruthenium-Doped SI-InP,” IEEE Photon. Technol. Lett., vol. 16, pp.2613-2615, Dec., 2004.
[42] S. Chelles, R. Ferreira, and P. Voisin, “High performance polarization insensitive electroabsorption modulator based on strained GaInAs-AlInAs multiple quantum wells,” Appl. Phys., Lett. 67, Issue 2, pp. 247-249, July 1995.
[43] T. Hatta, T. Miyahara, M. Ishizaki, N. Okada, S. Zaizen, K. Motoshima, and K. Kasahara, “Inductance-Controlled Electroabsorption Modulator Modules Using the Flip-Chip Bonding Technique,” IEEE/OSA J. of Lightwave Technol., vol. 23, pp.582-587, Feb., 2005.
[44] Y. Chen, J. E. Zucker, N. J. Sauer, and T. Y. Chang, “Polarization-Independent Strained InGaAs/InGaAlAs Quantum- Well Phase Modulators,” IEEE Photon. Technol. Lett., Vol. 4, No. 10, pp. 1120-1123, Oct. 1992.