in Wireless Optical Communication Networks

I. J. Computer Network and Information Security, 2012, 5, 46-55 Published Online June 2012 in MECS (http://www.mecs-press.org/) DOI: 10.5815/ijcnis.2012.05.06

Optimization Design Parameters of Electro-optic Modulators for Low Loss Wide Bandwidth Capability of Optical Communication Systems Ahmed Nabih Zaki Rashed Electronics and Electrical Communications Engineering Department Faculty of Electronic Engineering, Menouf 32951, Menoufia University, EGYPT [email protected] Abstract— The effects of temperature, and operating signal wavelength on high frequency radio frequency transmission characteristics are deeply investigated against various materials based electro optic modulator devices such as lithium Niobate (LiNbO3), polymer, and semiconductor materials. On the other hand, we have developed the optimization of the electro optic modulator parameters where the effective index plays an essential role in the evaluation of the bandwidth structure. The effects of design parameters on the modulating voltage and optical bandwidth are also investigated for different materials based electro optic modulators by using rigorous transmission modeling techniques. The low loss wide bandwidth capability of optoelectronic systems makes them attractive for the transmission and processing of microwave signals, while the development of high capacity optical communication systems has required the use of modulation techniques in optical transmitters and receivers. This paper has presented the low loss wide bandwidth for different electrooptic modulators based on design of optimization parameters for high speed transmission performance. Index Terms— Wide bandwidth, LiNbO3, Polymer material, Semiconductor material, Low loss, and Optimization parameters

I. INTRODUCTION There are several kinds of modulators, depending on their structure, such as electro optic, acousto-optic, magneto-optic and electro-absorption modulators [2]. Each employs a different physical mechanism and has different applications. The electro-optic modulator is the most important type in optical communication systems. Different configurations have been adopted, such as the Mach Zehnder interferometer (MZI) modulator, and the directional coupler modulator [3]. High speed integrated electro-optic modulators and switches are the basic building blocks of modern wideband optical communications systems and represent the future trend in ultra-fast signal processing technology. As a result, a great deal of research effort has been devoted to developing low-loss, efficient and broadband modulators Copyright © 2012 MECS

in which the RF signal is used to modulate the optical carrier frequency [1]. Most of the work done in the area of designing electrooptic modulators has been strongly focused on using LiNbO3 [4]. Interest in research in this field has arisen as lithium niobate devices have a number of advantages over others, including large electro-optic coefficients, low drive voltage, low bias drift, zero or adjustable frequency chirp, and the facility for broadband modulation with moderate optical and insertion losses and good linearity [5]. Significant progresses have been made for investigating silicon low-loss waveguides, light emitters, lasers, amplifiers, and photodetectors in the past few years, which promise an entry of silicon microphotonics interconnection at the hybrid, multichip level with migration to monolithic on-chip architectures [6]. Nevertheless, the development of all-silicon electrooptic modulators, a key optical interconnection component that encodes electrical information signals onto optical carriers, lags a little behind. Among a few standard approaches to realizing optical modulators, Mach– Zehnder interferometer (MZI), which converts a phase modulation to an amplitude modulation [7], is the most extensively studied type of modulator owing to its superior optical performance, such as high extinction ratio and large bandwidth of the operating wavelength [8]. Nonlinear optical and linear electro-optic materials find use in switching and modulation devices for photonic integrated circuits. For modulators in telecommunications small size and modulation voltages are desired. Both electro-absorption (EA) and electrooptic (EO) modulators are candidates for use in external modulation links in telecommunications [9]. In the present study, LiNbO3, Aluminum gallium arsenide (AlGaAs), and polymer electrooptic modulators (EOMs) have been developed for extensive use in high speed and high transmission performance optical fiber transmission systems. This is because they can offer the advantages of modulation exceeding multi Gbit/sec combined with a low driving voltage, and they can eliminate the dynamic laser wavelength chirping which limits the span-rate system product due to their fiber dispersion characteristics. Modulators fabricated on semiconductor substrates such as (AlGaAs) material is

I.J. Computer Network and Information Security, 2012, 5, 46-55

Optimization Design Parameters of Electro-optic Modulators for Low Loss Wide Bandwidth Capability of Optical Communication Systems

particularly attractive in that these exists the possibility of monolithic integration of these devices with other optoelectronic components.

II. OPTICAL INTENSITY MODULATOR SCHEMATIC VIEW A Mach Zehnder intensity modulator may be constructed in the form of an integrated optical device

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with the electrode modulation length L, and distance between electrodes, d are placed on a substrate in the geometry as shown in Fig. 1. The beam splitters are implemented by the use of waveguide Y’s. The optical input and output may be carried by optical fibers. Commercially available integrated optical modulators generally operate at speeds of a few GHz but modulation speeds exceeding 100 GHz have been achieved [6].

L g

Fig. 1. Schematic view of an integrated optical intensity modulator. Applying a voltage across one arm of the Mach Zehnder modulator shifts the phase of the signal through that arm by an amount proportional to the voltage applied. If the phase shift equates to an integral number of wavelengths, the two beams will combine constructively, and the intensity of the output power will be at its maximum. If the phase shift is a half wavelength out of phase, the two beams will combine destructively and the output power will be at its minimum [7].

III. MATHEMATICAL MODEL ANALYSIS For LiNbO3, the investigation of both the thermal and spectral variations of the waveguide refractive index (n0) require Sellmeier equation. The set of parameters required to completely characterize the temperature dependence of the refractive-index is given below, Sellmeier equation is under the form [10]: n 2  B1  B2 M 

B3  B4 M

B B M  7 8  B102   ( B5  B6 M ) 2  B9 2 2

2

(1)

The set of parameters is dimensionally adjusted as: B1=5.35583, B2=4.629x10-7, B3=0.100473, B4=3.862x108 , B5=0.20692, B6=-0.89x10-8, B7=100, B8=2.657x10-5, B9=11.34927, B10=0.015334, and M= (T-To). (T+570.82). Where T is the ambient temperature, and T0 is the room temperature. Above equation can be simplified to be in the following expression: 2

n  B12 

B34

2  B56 2



B78

2  B9 2

 B10 

(2)

Where B12=B1+(B2M), B34=B3+(B4M); B56=B5+(B6M), and B78=B7+(B8M). Differentiation of empirical equation w. r. t λ yields:   B34 B78 dn   n     B10  d  (2  B562 ) 2 (2  B9 2 ) 2   

Copyright © 2012 MECS

(3)

1  2 n d 

d 2n

    

    

 2 2   A 2  2  A2 56  A78 2    A9  A  ,  34 10   2  2 2   2  A56 2  A92 

(4)

Moreover, the refractive index of polymer material based electrooptic modulator can be expressed as [11]: n2 

B12



2

 B22



B32



2



 B42

B52

(5)

  B62 2

The set of parameters of Sellmeier equation coefficients for polymethyl metha acrylate (PMMA) are recast and mentioned in [12]: Then the first and second differentiation of empirical equation with respect to operating wavelength λ yields:  B3 B42 B5 B62 dn  1  B1B22     2 2 2 d n  2 2 2  B42 2  B62    B2

 







 









  ,  

(6)



 2 1  B B 2 32  B22 B3 B42 32  B42 B5 B62 32  B62  dn     1 2      , 3 3  d   d2 n  2  B 2 3 2  B42 2  B62 2  

d 2n













(7) For AlxGa(1-x)As, the parameters required to characterize the temperature and operating wavelength dependence of the refractive-index, where Sellmeier equation is under the form of [12-14]: n 2  C1 

C2  C42 2  C3

(8)

The set of parameters is recast and dimensionally adjusted as [8]: C1= 10.906-2.92x, C2= 0.97501, C3=c3T2; c3= (0.52886-0.735x/T0)2, for x