k30pm-2:OOpm (Invited) WW1

COMPARATIVE PERFORMANCE OF LIGHTWAVE SYSTEMS USING ELECTRO-ABSORPTION MODULATORS AND SEMICONDUCTOR

MACH-ZEHNDER MODULATORS

J. C. Cartledgel, Department of Electrical and Computer Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6

B. Christensen, Tele Danmark, Lyngsel All6 2, DK-2970 Helmholm, Denmark S. K. Nielsen, Tele Danmark, Sletvej 30, DK-8320 nanbjerg, Denmark N. K. Elnegaard, Department of Electromagnetic Systems, Technical University of Denmark,

DK-3800 L yngb y, Denmark

Recent transmission experiments have demonstrated that semiconductor Mach-Zehnder modulators and electro-absorption modulators are well-suited to high bit rate, long distance telecommunication systems [1]-[6]. These modulators exhibit large bandwidths with low drive power requirements and offer the potential for integration with other optoelectronic devices [4]-[ll]. The chirp of a Mach-Zehnder modulator is a function of the electro-optic properties of the p-i-n waveguide, the splitting ratios of the two Y-junctions, the differential phase shift between the two arms of the unbiased interferometer, and the format of the modulating voltages applied to each arm of the modulator [12]. This provides considerable flexibility in optimizing device performance for transmission applications. The chirp of an electro-absorption modulator depends on the electro-optic properties of the absorption layer in the p-i-n waveguide and is typically a decreasing function of the bias voltage that either is positive or changes from positive to negative. In the latter case, a negative a-parameter can be obtained at the expense of increased insertion loss [13].

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Figure 1: Characterization data for Mach-Zehnder modulator (left) and electro-absorption modulator (right).

The Mach-Zehnder modulator is characterized by the nonlinear dependence of the ab- sorption and phase constant of the optical field in each arm on the applied voltage. This

170 'On leave with Tele Danmark.

dependence is measured for a straight section of waveguide cut from one arm of a modulator and is illustrated in Fig. 1 for a wavelength of 1557 nm and a waveguide length of 600 pm. The electro-absorption modulator is characterized by the dependence of the absorption and a-parameter on the applied voltage. Measured results are illustrated in Fig. 1 for a bulk electro-absorption modulator integrated with a DFB laser.

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Fiber Length (km) 0 40 80 120 160

Fiber Length (km)

Figure 2: Dependence of receiver sensitivity on fiber length for a Mach-Zehnder modulator (left) and an electro-absorption modulator (right).

The results in Fig. 1 can be used to examine the implications of modulator chirp and transmitted optical power on system performance. We consider 10 Gb/s systems using non- dispersion shifted fiber. The dependence of the receiver sensitivity on fiber length for various values of the transmitted optical power is shown in Fig. 2 for the two modulators. The calculations use measured S21 responses for the modulators and receiver. For the negative chirp Mach-Zehnder modulator, a transmitted power up to 12.5 dBm can be used before self-phase modulation causes a significant penalty for a fiber length of 120 km. For the positive chirp electro-absorption modulator, long distance transmission relies on self-phase modulation [14]. A transmitted power of 15.0 dBm yields 100 km transmission with 0.5 dB penalty. [l] C. Rolland et al., Proc. Con$ Optical Fiber Commun., San Jose, CA, 1993, paper PD-27. [2] H. Sano et al., Proc. Conf. Optical Fiber Commun., San Jose, CA, 1993, paper ThK5. [3] J. Yu et al., IEEE Photonics Technol. Lett., to appear August 1996. [4] K. Morito et al., Electron. Lett., vol. 31, pp. 975-976, 1995. [5] K. Morito et al., IEEE Photonics Technol. Lett., vol. 8, pp. 431-433, 1996. [6] V. Rodrigues et al., Electron. Lett., vol. 32, pp. 909-910, 1996. [7] T. Tanbun-Ek et al., IEEE Photonics Technol. Lett., vol. 7, pp. 1019-1021, 1995. [8] D. M. Adams et al., Electron. Lett., vol. 32, pp. 485-486, 1996. [9] A. Ramdane et al., IEEE Photonics Technol. Lett., vol. 7, pp. 1016-1018, 1995. [lo] H. Yamazaki et al., Electron. Lett., vol. 32, pp. 109-111, 1996. [ll] 0. Sahlkn et al., Electron. Lett., vol. 32, pp. 120-122, 1996. [12] J. C. Cartledge et al., IEEE Photonics Technol. Lett., vol. 6, pp. 282-284, 1994. [13] J. A. J. Fells et al., Electron. Lett., vol. 30, pp. 2066-2067, 1994. [14] E. Hummel et al., European Conf. Optical Commun., Brussels, Belgium, 1995, paper We.B.1.3.

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