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A novel p phase shifter in integrated optics
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A novel p phase shifter in integrated optics
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1. 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 220 A NOVEL π- PHASE SHIFTER IN INTEGRATED OPTICS 1 Rajini V Honnungar, 2 T 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 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 INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) ISSN 0976 – 6464(Print) ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June, 2013, pp. 220-226 © IAEME: www.iaeme.com/ijecet.asp Journal Impact Factor (2013): 5.8896 (Calculated by GISI) www.jifactor.com IJECET © I A E M E
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 221 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 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 , MEMS switches, dispersion based fiber delay lines  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 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-
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 222 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, 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 index=2.13806) 2.143 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
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 223 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 µm) Phase shift(radians ) Intensity (Simulation) Intensity (Theoretical) 7.5 π/4 0.80 0.8484 10.5 π/2 0.50 0.4854 15.5 π 0 0
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 224 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
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 225 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  Istvan frigyes, “Optically generated truetime delay in phased-array antennas”, Vol 43, 1995.  R.A.Soref “ Fiber grating prism for true time delay beamsteering”(Fiber and Integrated Optics 1996)  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)  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  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
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 226  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  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)  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.