A NOVEL p- PHASE SHIFTER IN INTEGRATED OPTICS

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A NOVEL p- PHASE SHIFTER IN INTEGRATED OPTICS Powered By Docstoc
					        INTERNATIONAL JOURNAL OF Engineering & Technology (IJECET),
   International Journal of Electronics and Communication ELECTRONICS AND
   ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)                                                       IJECET
Volume 4, Issue 3, May – June, 2013, pp. 220-226
© IAEME: www.iaeme.com/ijecet.asp                                          ©IAEME
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)
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           A NOVEL π- PHASE SHIFTER IN INTEGRATED OPTICS

                                  1
                                      Rajini V Honnungar, 2T Srinivas
                           1
                               (Department of ECE, IISc, Bangalore, India)
                               2
                                 (Department of ECE, IISc, Bangalore, India)


   ABSTRACT

          The Integrated Optics has resulted in miniaturization of optical devices. Bent
   waveguides have replaced straight waveguides in optical integrated circuits for such reasons.
   In the proposed work, curved/bent waveguide is basic structure for generating delay and
   hence the phase shift. A differential phase shift is obtained by the differential length change
   between the straight waveguide and a S-bend waveguide. We discuss a 1-bit delay line that
   produces the required phase shift. In the design the length of the straight waveguide is
   2000µm. We obtain a phase shift of π radians for a differential length of 0.3629µm. Ti
   diffused Lithium niobate waveguides are employed. An extra-ordinary refractive index of
   2.13806 is considered for the substrate. This is a phase delay method, in which the optical
   phase shift produced is transferred to RF domain by heterodyning as compared to the true
   time delay methods used previously with larger switching speeds in the order of hundreds of
   pico secs.

   Keywords: delay line, diffused waveguides, integrated optics, heterodyning, S-bends.

   I. INTRODUCTION

           The low loss, low cost, light weight systems, high bandwidth and immunity to
   electromagnetic interference makes optical media an excellent media for various applications.
   There has been a wide use of optical delay lines as phase shifters for major applications such
   as the optical beamforming[1] and modulation in optical communication systems. Beam
   Propagation method(BPM) is the most widely used propagation technique ideal for design
   and modelling of photonic devices and photonic integrated circuits. Delay lines are used to
   create a delay usually a time delay in the path of the incoming signal. They are also used for
   coordination and synchronization of electrical signals in RADAR, feed-forward amplifiers,
   telemetry and other systems. Traditional delay line technology used previously for RF
   requires long, bulky, electrical transmission lines to delay electrical signals for a precise
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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME

period of time. Flexible, coaxial cables are commonly used to construct the delay. The first
kind of optical delay lines include Switched fiber /waveguide delay lines. Initially
Madampoulos and Riza[2] have demonstrated a fiber optic delay line that employs lengths of
optical fiber as delay paths. For several years various other methods of generation of delays
were proposed, such as Fiber Bragg grating[3] , MEMS switches[4], dispersion based fiber
delay lines[5] [6] and ring resonators. In the method we propose a phase shifter based on
phase delay where path length variation is employed for the generation of phase shifts instead
of electro-optic effect. 1-bit delay line has two paths which can be changed using by
directional coupler switches. The phase shift generated can be seen as a intensity variation
using a MZI configuration for the optical delay line.

II THEORY

         From the optical waveguide theory, the delay in the path of the light signal introduces
phase shift. This delay can be generated by increasing the path length. Hence a path length
difference between a straight waveguide and bent waveguide can generate differential phase
shift. This is a passive method as compared to the conventional methods employed
previously.
The differential phase shift experienced by the signal propagating in the waveguide is given
by (1) :

                                      ∆Φ=β dl         (1)
Where
          ∆Φ=differential phase shift
           β= propagation constant =2(π/λ)neff
            ‘λ’= wavelength of operation
        neff = effective index of the waveguide
           dl = differential length(between bent and straight waveguide)
The delay line can be a n-bit delay line generating 2n phase shift values. A Cosine generated
S-bend[7] is employed for generating the path length difference .




              Fig.1. A 1-bit phase shifter             Fig. 2. S-bend waveguide

       In the proposed method, as in the Fig1, 1-bit or n-bit, ‘n’ can take any integer value.
Each bit is composed of a reference phase signal pathway and a delayed phase signal
pathway .When the optical signal goes through the reference phase the phase shift is 0
radians, the other is through the delayed path which is, ‘∆Φ’ radians. Switching between the
pathways is by directional coupler switches having a switching speed of 14ps using electro-
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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME

optic effect represented by 1 and 2 in the Fig1. Path length change is produced passively by
S-bend waveguides. A cosine S-bend shown in Figure 2 is employed; since it gives low
losses. The S-bend parameters have been optimized for low losses. The arc length of the S-
bend is found out mathematically by using Cosine generated S- bend function as in Fig.2
given as :

y(x)= A(1-cos(πx/L))                                (2)

Where,

A – Lateral offset value for the bend (h);
x - length variation in x direction;
L= length of the S-bend.

        The S-bend length ‘l+dl’ is computed mathematically. Selection of a proper dl value
gives a corresponding required phase. An interferometric configuration can be employed for
phase measurement. In this configuration the phase change is translated into amplitude or
intensity change. This is shown in Table 3. From the interferometer theory[8], the change in
intensity is given by :

I’=1/2[1+cos(∆Φ)]                                  (3)

Here, change in phase ∆Φ=((2πneff dl)/λ) from equation (1).
Substituting for ∆Φ we get :

I’=1/2[1+cos((2πneff dl)/λ)]                        (4)

Where, ‘I’’ – normalized Intensity at the output of MZI
 neff – effective index for the z-cut lithium niobate waveguide( at λ = 1.55 µm)
dl –path length change due curved/ bent waveguide.
One of the arms of the Machzehdner interferometer has no path length change while the other
arm has a S-bend structure which provides the necessary phase shift as required.

3. RESULTS AND DISCUSSION

       The design parameters used for simulation and modeling are given in Table 1.Since
Ti:LiNbO3 waveguides are considered, a diffused waveguide structure was considered for
the simulation.

                                          Table 1
               Length of straight waveguide                          2000µm
               Effective index of the waveguide(substrate            2.143
               index=2.13806)
               Wavelength of operation                               1.55 µm
               Length of directional couplers                        2000 µm
               Spacing between directional couplers                   5 µm
               Length of a single S-bend                             1000 µm

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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME

       The neff is a very crucial parameter in the design and hence its tolerance values for
the S-bend waveguide structure has been studied. Figure 3 shows that real part of neff is
constant with varying lateral offsets and the imaginary part indicates the losses. However
with variation of lateral offset ‘h’ , the change in phase is critical ,a 1ο phase change was
observed for every 0.1 µm variation in the original lateral offset value. Figure 4 shows the
change in phase with lateral offset variation.




                          Fig. 3. neff versus lateral offset ‘h’ of S-bend


       Fig.5 and Fig.6 shows the MZI configuration and output for π phase shift in one of the
arms respectively.Values of differential phase shifts ranging from 0 radians(0 deg) to π
radians(180 deg)can be realized. Theoretical phase shifts correspond to different intensities or
amplitudes, which are comparable with the simulated results as shown in Table 2.


                                             Table 2

             Lateral offset(in        Phase             Intensity       Intensity
                   µm)            shift(radians )      (Simulation)   (Theoretical)

                    7.5                 π/4               0.80           0.8484

                   10.5                 π/2               0.50           0.4854

                   15.5                  π                  0                0




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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME




                Fig. 4. variation of lateral offset versus change in phase value




                    Fig. 5. Delay line in the MZI for phase shift measurement


                                           Table 3
                   Slno            Parameter              Dimension(inµm)

                    1          Ls(S-bend length)                 1000
                    2           Linput/Loutput                   2000
                    3          Lsplitter/combiner                1000
                    4               Lstraight                    2000
                    5           Lateral offset ‘h’               15.5




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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME




                    Fig. 6. MZI power output for phase shift of π radians


4. CONCLUSION

        In this work we designed and optimized a delay line using S-bend waveguide in
integrated optic domain which can be used as a phase shifter. Tolerance issues of parameters
in the design were also studied. Changes in Lateral offset values relate to phase shift changes.
The optimization has ben done using analytical and numerical methods. The design is
feasible for practical implementation. The future work involves in implementation of Ring
resonator delay lines and Fiber grating delay lines.

REFERENCES

Journal Papers

[1]   Istvan frigyes, “Optically generated truetime delay in phased-array antennas”, Vol 43,
      1995.
[3]    R.A.Soref “ Fiber grating prism for true time delay beamsteering”(Fiber and
      Integrated Optics 1996)
[4]   Yaping Liang,C.W.Domier,N.CLuhmann,Jr , MEMS Extended Tuning range Varactor
      Based True time delay Technology”,Novel Devices and Components(Nano and
      Quantum devices,Photonic crystals)
[6]   True Time delay optical RF phase shifters in lithium niobate”by E.Voges.
      K.Kuckelhaus and B. Hosselbarth.,Electronics Letters ,Vol 33 No.23 Nov,1997
[7]   Kwang T. Koai and Pao-Lo Liu, “Modeling if Ti: LiNbO3 waveguide devices: Part II –
      S-shaped channel waveguide bends”, Journal of lightwave technology, vol.7,no7,July
      1989

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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME


[8]   M.A.Chan, S.D.Collins and R.L.Smith, “A Micromachined pressure sensor with fiber
      optic interferometric readout”, Sensors and Actuators A, Vol.43, pp.196-201,1994.

Proceedings Papers

[2]   Nicholas Madamopoulos,Nabeel A.Riza,’Switched photonic delay line for phased array
      antenna control using externally modulated microwave fiber optic link’, Proc. SPIE
      3160, Optical Technology for Microwave Applications VIII, 45 (October 23, 1997)
[5]   Sullivan, C.T., Mukherjee, S.D., Hibbs-Brenner, M.K.,Gopinath, A.,and Kalweit, E.:
      ‘Switched time delay elements based on AlGaAs/ GaAs optical waveguide technology
      at 1.32 micron for optically controlled phased array antennas’, Proc. SPIE, 1992,
      pp1703.




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