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Recent Advances of Wide Band Magneto-Optical Modulators in Advanced High Speed Optical Communications Systems

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Recent Advances of Wide Band Magneto-Optical Modulators in Advanced High Speed Optical Communications Systems Powered By Docstoc
					                                        Volume 1, No.1, March – April 2012
                  International Journal of Advanced Trends in Computer Science and Engineering
                                   Available Online at www.warse.ijatcse.current


                Recent Advances of Wide Band Magneto-Optical Modulators In
                    Advanced High Speed Optical Communication Systems
                                                   Ahmed Nabih Zaki Rashed
                               Electronics and Electrical Communications Engineering Department
                         Faculty of Electronic Engineering, Menouf 32951, Menoufia University, EGYPT
                                                     ahmed_733@yahoo.com

ABSTRACT                                                          polarization sensitive beam deflectors, switches, and
                                                                  isolators. In spite of these benefits, MO device technology is
The technology involves a new type of compact device              still in its infancy and has not yet been fully explored for
based on magneto-optics in a fiber micro modulator. This          integrated high-speed optical technology. Magneto photonic
type of modulator allows the user to inter manipulate and         devices to date have relied on the propagation of light waves
control the propagation of the incoming light. The operation      through the MO material. As the light beam travels through
mechanism uses external magnetic field to manipulate Fe           the MO medium, its plane of polarization is rotated
micro particle in order to cause modulation of an optical         according to the Faraday effect [5], resulting in a
signal that propagates along an optical waveguide. This           polarization modulation. However, the transmission
paper has presented Yttrium Iron Garnet (YIG) and lithium         arrangement presents difficulties associated with the MO
niobate (LiNbO3) which are examined for use as a wideband         material such as optical absorption of the light beam as it
magneto-optic modulator. A wideband YIG modulator has             propagates through the interaction region, as well as inherent
recently been developed which represents a great                  birefringence. To observe a maximum MO effect, it is
improvement over other magneto -optic modulators.                 desirable to have a MO material with large Faraday rotation,
                                                                  negligible birefringence, and very little absorption. Of the
Keywords: Magneto-optic Modulation, Faraday Rotation Effect,      various MO materials available, bismuth-substituted yttrium
LiNbO 3, YIG, Ultra Fast Magnetic Field.                          iron garnet (Bi-YIG) is often chosen for the backbone of a
                                                                  variety of MO devices as it has a very large MO interaction
1. INTRODUCTION                                                   [6].

Over the last several years, the demand for high bandwidth        Optical modulators have become indispensible in many
networking has dramatically increased, encouraging the            applications or optical instruments. Light interacts with
continued development of high-speed electrical and optical        matters in various macroscopic ways. The most commonly
devices. In particular, the realization of high bandwidth, low    known phenomenon include Electro-Optical (EO) effect,
power, efficient modulators plays a fundamental role in the       Acousto-Optical (AO) effect, and Magneto-Optical (MO)
improvement of integrated optical systems and networks.           effect. AO modulators and EO modulators have already
Contemporary optical modulation devices rely primarily on         found many applications, and state-of-the-art modulators are
Electro-Optic (EO) interaction in nonlinear materials. By         already commercially available. However, AO modulator
exploiting the Pockels effect, the electric field associated      inherently suffers from slow modulation speed. Although
with a high-speed electrical signal is employed to modulate       EO modulator is capable of fast modulation [7], it requires a
an optical beam. However, in recent years, there has been an      relatively high accuracy on the alignment in free-space
emergence of a new class of photonic devices based on the         applications. MO effect, on the other hand, finds its
magneto-optic (MO) effect [1][2]. These devices utilize the       application only as an optical isolator so far. Although there
Faraday effect, in which the magnetic field associated with       have been many researches on MO modulators in integrated
an electrical signal modulates an optical beam. Such              optics [8], the possibility of a free-space MO modulator has
magneto-photonic devices offer high bandwidth modulation          not been widely investigated. The MO modulator can not
and switching capabilities comparable to those based on the       only work as an intensity modulator when combined with
EO effect. The recent interest in such technology is              two linear polarizer, but can also work as a polarization
stimulated by current advances in the growth of a variety of      rotator alone.
MO materials, namely rare-earth iron garnets [3]. High-
quality magnetic garnet films are readily available in a          In the present study, magneto-optic materials have unique
variety of forms and can be grown by epitaxial or sputter         physical properties that offer the opportunity of constructing
methods for particular applications. Such films have a large      devices with many special functions not possible from other
Faraday rotation, low absorption in the near infrared, and a      photonic devices. The most significant of these properties
low saturation magnetization. Furthermore, their optical          are that the linear magneto-optic effect can produce circular
properties can be easily manipulated by doping, such as the       birefringence and that, unlike other optical effects in
substitution of bismuth in yttrium iron garnets [4]. Research     dielectric media, it is nonreciprocal. All practical magneto-
into the application of rare-earth iron garnets has resulted in   optic devices exploit one or both of these two properties.
the development of a variety of unique MO photonic                Important applications of these devices include polarization
devices, such as resonant/non resonant modulators,

  © 2012, IJATCSE All Rights Reserved                                                                                      1
     Ahmed Nabih Zaki Rashed, International Journal of Advanced Trends in Computer Science and Engineering, 1 (1), March – April, 1-8

control, optical isolation, optical modulation, and magneto-                        Where V is the Verdet constant for the material. The Verdet
optic recording. The basic principles of magneto-optic                              constant is both temperature and wavelength dependent. The
modulator effects are considered in this study.                                     Faraday Effect is similar to optical activity. The difference is
                                                                                    that optical activity doesn’t require an externally applied
2.MAGNETO-OPTICAL MODULATOR SCHEMATIC                                               magnetic field to rotate the light polarization and it only
VIEW                                                                                depends on the traveling length and material concentration.
                                                                                    The modulation depth, Mp can be expressed in terms of
As the internet and modern communications becomes                                   pulse duration τ in psec, and angle of rotation θ in degree, by
increasingly prevalent across the globe, all-optic networks.                        using MATLAB curve fitting program as the following
In recent years, for the sake of satisfying high speed                              formula [12]:
transmission and exchange of optical-message through fiber-
optic networks, optical switch has been significantly                                M p  0.1021  0.0765 x105 (  )  0.0032 x108   2     (2)
improved as an important element for optical
communication by continuous widely research [9].
                                                                                    The transmittance of MO modulator Tm, can be expressed as
                                                                                    follows:
                                                                                                 Tm  cos 2 0.5                             (3)

                                                                                    The signal to noise ratio (SNR) of MO modulator can be
                                                                                    expressed as the following formula [13]:
                                                                                                                 2  P0 r 
                                                                                                 SNR  10 log               ,                      (4)
                                                                                                                   q BWe
Figure. 1: Basic Construction of MO Modulator under study
                                                                                    Where r is the reflection coefficient, BWe is the electrical
As shown in Figure 1, the proposed structure of MO                                  bandwidth, P0 is the laser power, q is the electron charge,
modulator based on a microwave micro strip line with a                              and  is the its modulator responsivity. The bit error rate
polarization sensitive MO active medium and fiber optic                             (BER) essentially specifies the average probability of
continuous wave (CW) light delivery. As the requirement                             incorrect bit identification. In general. The higher the
from the optical communication technique, it should be                              received SNR, the lower the BER probability will be. The
characterized as low crosstalk, low insert loss, short                              bit error rate (BER) is related to the signal to noise ratio
switching time and low polarization sensitivity together with                       (SNR) as follows [12] [13]:
special requirement on extinction ratio, switch scale and                                                            SNR  
dimension. As the factors that will affect the optical switch                                     BER  0.5 1  erf       ,                    (5)
                                                                                                                     2 2 
parameters, it consists of the quality of separate units                                                                 
technical index (transmissivity, rotation angle etc.) and also
the quality of units final assembly and adjusting1. So far, a                       Moreover the refractive index of the LiNbO3, and YIG are
variety of optical switches have been developed. In                                 cast under the Sellemier equation as the following [14] [15]:
comparison to the various other optical switches, the
magneto-optic switch based on Faraday rotation effect for                                                               B3  B4M         B B M
light consists of special optical route including a type of                                      n2  B1  B2M                          7 8  B102
                                                                                                                    2  (B5  B6M)2      2  B92
high-quality magneto-optic material YIG crystal, a novel
switch of generating pico second-order electrical pulses and                                                                                       (6)
Faraday rotator configuration with ultrafast magnetic field. It
is featured as low insertion loss, low crosstalk, high                              The set of parameters is dimensionally adjusted for LiNbO3
switching speed and small bulky size. Using the polarization                        as:    B1Li=5.35583,     B2Li=4.629x10-7,    B3Li=0.100473,
and Faraday Effect of magneto-optic crystal, the magneto-
                                                                                                   -8
                                                                                    B4Li=3.862x10 , B5Li=0.20692, B6Li=-0.89x10-8, B7Li=100,
optic switch can hold the function of all-optical switching,                        B8Li=2.657x10-5, B9Li=11.34927, B10Li=0.015334, and M=(T-
which is needed in all-optical communication networks [10].                         T0)(T+570.82). While the set of parameters is dimensionally
                                                                                    adjusted for YIG as: B1YIG=0.00987, B2YIG=1.43567x10-11,
3. THEORETICAL MODEL ANALYSIS                                                       B3YIG=0.10765x10-3, B4YIG=0.00532x10-8, B5YIG=0.0541,
                                                                                    B6YIG=10.4329x10-9,     B7YIG=1234,    B8YIG=0.05437x10-7,
When a transparent material is placed in a magnetic field                           B9YIG=14927, B10YIG=0.00384, and M=(T-T0)(T+590.54).
and linearly polarized light is passed through it along the                         Thus the previous equation can be simplified as the
direction of the magnetic field, the emerging light is found to                     following formula:
remain linearly polarized, but with a net rotation θ in degree,                                                  B34     B78
                                                                                                 n2  B12                      B102            (7)
of the plane of polarization that is proportional to both the                                                 2  B 2 2  B 2
                                                                                                                    56       9
thickness d in mm, of the sample and the strength of the
magnetic field B in Tesla, along traveling direction,                               Where B12=B1+(B2M), B34=B3+(B4M); B56=B5+(B6M), and
according to the empirical relation [11]:                                           B78=B7+(B8M). In the general case, if the device operation
                                                                                    efficiency, η  1, the inductance L and internal electrical
               180 B V d
                       , Degree                                     (1)           resistance Re of MO modulator, we have [16]:
                   


     © 2012, IJATCSE All Rights Reserved                                                                                                                 2
     Ahmed Nabih Zaki Rashed, International Journal of Advanced Trends in Computer Science and Engineering, 1 (1), March – April, 1-8


                      2
          L                                                          (8)                    v) As shown in Figs. (11, 12) have assured that
               c 2 Lm  0 n 6                                                                modulator inductance and resistance decrease with
                                                                                             increasing MO modulator length for both materials
                                                                                          based MO modulator devices. As well as we have
          Re                                                         (9)                    observed that YIG MO modulator has presented
                  c Lm  0 n 6
                                                                                             lower resistance and inductance compared to LiNbO3
                                                                                             MO modulator at the same operating conditions.
Where Lm is the modulator length, c is speed of light, μ0 is
the free space permeability, Δλ is the spectral linewidth of                              Table 1: Proposed operating parameters for magneto-optical
the laser diode, and n is the refractive index of the selected                                                        modulators
material based MO modulator.
                                                                                            Parameter                 Definition              Value and unit
4. SIMULATION RESULTS AND PERFORMANCE                                                T=T0                        Ambient                300 K
ANALYSIS                                                                                                         temperature=room
                                                                                                                 temperature
                                                                                     τ                           Pulse duration         100 psec
We have deeply investigated the recent advances of wide
                                                                                     d                           Modulator              5 mm —25 mm
band magneto optical modulators in advanced high speed                                                           thickness
optical communication systems over wide range of the                                 B                           Magnetic field         0.1 web/mm2—1
affecting operating parameters as shown in Table 1.                                                              intensity              web/mm2
                                                                                                                Operating signal       1.3 μm
Based on the model equations analysis of MO modulator,                                                           wavelength
assumed set of the operating parameters, and the set of the                                                      Verdet constant        4.54x10-6 rad/A
                                                                                     V
series of the Figs. (2-12), the following facts are assured:                                                     for LiNbO3
                                                                                                                 Verdet constant        5.65x10-6 rad/A
    i) Figs. (2-4) have assured that angle of rotation                                                           for YIG
                                                                                     q                           Electron charge        1.6x10-19C
       increases with increasing MO modulator thickness
                                                                                     BWe                         Electrical             1 MHz
       and applied magnetic field intensity for both materials                                                   bandwidth
       based MO modulator devices. We have indicated that                            P0                          Laser power            1 mWatt
       YIG MO modulator has presented higher angle of                                                           Modulator              0.4 A/Watt
       rotation than LiNbO3 MO modulator at the same                                                             responsivity
       operating conditions.                                                         r                           Reflection             0.55 at λ=1.3 μm
    ii) Figs. (5-7) have demonstrated that modulation depth                                                      coefficient
       increases with increasing MO modulator thickness                              Δλ                          Spectral linewidth     0.1 nm
       and applied magnetic field intensity for both materials                                                   of optical source
       based MO modulator devices. As well as we have
       observed that YIG MO modulator has presented                                  c                           Speed of light         3x108 m/sec
       higher modulation depth compared to LiNbO3 MO                                 Lm                          Modulator length       50 mm —100 mm
       modulator at the same operating conditions.                                   μ0                          Free space             4πx10-7 H/m
    iii) As shown in Fig. 8 has indicated that modulator
                                                                                                                 permeability
       transmittance increases with increasing MO
       modulator thickness and applied magnetic field                                η                           Device operation       0.9
       intensity for both materials based MO modulator                                                           efficiency
       devices. We have indicated that YIG MO modulator
       has presented higher transmittance than LiNbO3 MO
       modulator under the same considerations.
    iv) Figs. (9, 10) have demonstrated that SNR increases
       and BER decreases with increasing MO modulator
       thickness and applied magnetic field intensity for both
       materials based MO modulator devices. As well as we
       have observed that YIG MO modulator has presented
       higher SNR and lower BER compared to LiNbO3 MO
       modulator at the same operating conditions.




     © 2012, IJATCSE All Rights Reserved                                                                                                                       3
Ahmed Nabih Zaki Rashed, International Journal of Advanced Trends in Computer Science and Engineering, 1 (1), March – April, 1-8




© 2012, IJATCSE All Rights Reserved                                                                                                4
Ahmed Nabih Zaki Rashed, International Journal of Advanced Trends in Computer Science and Engineering, 1 (1), March – April, 1-8




© 2012, IJATCSE All Rights Reserved                                                                                                5
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© 2012, IJATCSE All Rights Reserved                                                                                                6
     Ahmed Nabih Zaki Rashed, International Journal of Advanced Trends in Computer Science and Engineering, 1 (1), March – April, 1-8




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 © 2012, IJATCSE All Rights Reserved                                                                                                        8

				
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