Implementation of a 1x16 Router Using 2x2 MachZehnder Interferometer Electro-Optic Switch Rabeya Khatun, Raqibul Hossen, Kazi Tanvir Ahmmed Department of Applied Physics, Electronics & Communication Engineering University of Chittagong Chittagong-4331, Bangladesh
[email protected],
[email protected],
[email protected] Abstract—In optical communication signal router is a novel photonic component used to ensure communication between optical networks. Signal routing has a significant commercial demand in dense wavelength division multiplexing. This paper presents the description of designing a 1x16 signal router using Mach-Zehnder electro-optic Switch operating at 1.55 μm. To design the router we used 2x2 Mach-Zehnder electro-optic switch made by titanium diffused lithium niobate (Ti:LiNbO3) which is used as waveguide medium in designing the desired signal router. Enhanced performance of the switch with reduced insertion loss, excess loss and high extinction ratio was evaluated first and then the router was implemented using that optimized switch. 16 switches were required to implement the router. Optical power can be switched to each of the 16 output ports with the application of applied voltage at each central electrode of the 16 MZI switches. The results are observed using finite difference beam propagation method of OptiBPM12.2 simulation software.
analytically demonstrated a 1x4 signal router based on 2x2 electro-optic MZI switch and showed the variation in output power due to change in control signal [17]. Santosh Kumar constructed a 1x8 signal router with the application of control signal [18]. As demand for systems with higher capacity is increasing rapidly, so to adapt and to build a large optical cross-connect (OXC) for wide-area communication network and also to provide high-speed interconnections between a no of processors in a parallel, distributed computing system [19], signal router with high configuration is required. In this paper we will demonstrate a 1x16 signal router based on electro-optic effect. Section II represents basics of electro-optic switch, section III shows the construction of 1x16 signal router and section IV shows the simulation results and section V gives a conclusive study.
Keywords—Mach-Zehnder interferometer; Electro-optics switch; Insertion loss (IL); Extinction ratio (ER); Signal router.
Electro-optic effect is referred to the change in the optical properties of a material with varying electric field. The term includes a number of distinct phenomena, which are, i) the change of the absorption coefficient and ii) the change of the refractive index and permittivity. Second term is referred to Pockels effect i.e. the change in the refractive index which is linearly proportional to the change in electric field. Only certain crystalline solids show the Pockels effect like LiNbO3, KNbO3, NH4H2PO4, (ADP), KH2PO4 (KDP), LiTaO3 and CdTe etc. The relation of refractive index n with applied electric field E is given below [20].
I. INTRODUCTION Mach-Zehnder interferometer is a basic component in photonic lightwave circuit (PLC). It is a classical mirror interferometer which was the most common dual-beaminterferometer for a long time and used in measuring refractive index distributions of transparent objects. It was developed by Mach and Zehnder in 1892 [1]-[2]. Now-a-days it has been used for many applications such as electro-optic switch, modulator, optical sensor, wavelength filters, and multiplexers etc [3]-[7]. Directional coupler, Y-shaped splitter, multimode interference coupler etc are used as power splitter and combiner in Mach-Zehnder interferometer [8]-[10]. Electrooptic based 2x2 MZI optical switch using directional coupler requires materials with high nonlinear characteristics such as LiNbO3, KNbO3 etc [11]-[14]. LiNbO3 shows some advantages of having zero residual birefringence, less complexity, high electro-optic coefficient, high speed of operation but it is subjected to polarization sensitivity [8]. Such switches are suitable for time division signal processing and signal routing. Andy W. Brown and Min Xiao demonstrated an all-optical two-port signal router [15]. Ruiqiang Ji experimentally investigated a spatially non-blocking five-port optical router based on micro-ring resonators, which was tuned through the thermo-optics effect and 12.5 Gbps high-speed signal transmission rate was obtained [16]. S. K. Raghuwanshi
II. BASICS OF ELECTRO-OPTIC SWITCH
n( E ) = n + a1 E +
1 a2 E 2 + " 2
(1)
The change in refractive index and the resulting change is phase can be calculated by equation 2 and equation 3 [17].
n3 ) rE 2 2π n 3 Δϕ = [( ) rE ]L. λ 2 Δn = (
(2) (3)
L describes substantial length required to obtain certain phase change for directing light into desired path, which is for
a 2x2 MZI electro-optic switch 10000 μm annd r represents the electro-optic coefficient and for LiNbO3, it iss 3.66 × 10−10 m⁄V. Now, if we consider that the voltage differrence between two electrodes shown in Fig. 1 is V and d is thee distance between the electrodes, then electric field is V⁄d. So, we w can write
Δϕ =
2π
λ
[(
n3 V ) r ]L. 2 d
(4)
When applied voltage is 0 V, phase channge i.e Δφ become zero and with voltage V the phase changge Δφ is π. This particular voltage is called Vπ and is given byy,
Vπ =
λ 1d 3
n rL
.
(5)
Now let us find power at output ports off the switch. From Fig. 1 we can see that inputs at first directioonal coupler i.e to the power splitter are denoted as A(Z) andd B(Z), which are described by the following equations [21]. A( Ζ) = {[cos(qz ) + j
δ q
k sin(qz)] A(0) − j k sin(qz ) B(0)}exp( − jδ z ) (6) q
k δ B ( Ζ) = {− j sin( qz )] A(0) + [cos( qz ) − j sin( qz )] ) B (0)} exp( jδ z ) (7) q q
Here q represents the number of local peaaks of electric field propagating along the x and y-axis directions and given by,
q = (k 2 + δ 2 )
(8)
⎛ l1 − l 2 ⎞ ⎟ ⎝ λ ⎠
(9)
And also,
δ = 2π ⎜
In equation 9, l1 represents the coupling length l of the upper waveguide, where l2 is the coupling lenggth of the lower waveguide and k is the wave number. Let us consider that the case light is launched into the upper waveeguide of a MZI. Since both arms have the same waveguide structures; s so, δ=0 which results in, A(0)=A0 and B(0)=0. Here the t first coupler,
Fig. 1.
2x2 Mach-Zehnder electro-optic switch.
which functions as power spplitter is a 3-dB coupler with kl=π/4, so the light splitting ratiios can be written as,
A1 = A0 cos((kl ) =
A0 2
(10)
B1 = − jA0 sinn(kl ) = − j
A0 2
(11)
l is the coupling length. Affter light being traveled through the interferometer arms, the ouutput from these arms A2 and B2 will be,
A2 = A1 exp(− jβL) = B2 = B1 exp(− jβL + Jϕ) = − j
A0
exp(− jβL)
(12)
exp(− jβL + jϕ)
(13)
2
A0 2
L is the length of switchiing region and φ represents an excess phase shift at the secondd arm of the interferometer. The second coupler is also a 3-dB B coupler where kl=π/4. So, the outputs are given by [20], 2
ϕ
A3
2
= A0 . sin 2 ( ) 2
B3
2
= A0 . cos 2 ( ) 2
2
ϕ
(14) (15)
If the voltage applied is 0 V, V then φ = 0, the output will be switched to the lower port as│B3│2 ≈│A0│2. With the certain applied voltage for which φ =π π, the output will be switched to the upper port as│A3│2 ≈│A0│2. For a 1x16 signal router the same theory is applied, only diffference is that 16 control signal is to be varied to launch light too the desired port. III. CONSTRUCTION OFF 1X16 SIGNAL ROUTER To design a 1x16 signal router, r 16 individual 2x2 MZI switches are required. No of sw witches could not be minimized, because it will either increasse the width of the device or electrodes will overlap each other. o The device then will not function accurately. Titaniuum diffused lithium niobate waveguide is used to design MZI M switch. The switch is made on Z-cut wafer of lithium niobaate with air surrounding. It is not more than 33000 μm long andd 100 μm wide. The waveguide has a width of 8.0 μm along X-axis. X In 3D wafer properties air is defined as coating material with thickness of 2.0 μm and substrate is defined as lithium niobate with a thickness of 10 μm. Three electrodes created on buffer layer with 0.3 μm thickness and refractive indexx of 1.47. The buffer layer has horizontal and vertical permittivvity of the order of 4. Thickness of each electrode is 4.0 μm. Booth first and third electrodes have a width of 50 μm with zeroo voltage applied. The second electrode is defined with a widdth of 26 μm and voltage of 0 V. The separation of 1-2 and 2-3 is 6 μm. Table I shows the list of simulation parameters.
TABLE I. Reference index Wavelength Polarization Mesh-number of points BPM solver Engine Scheme parameter Propagation step Boundary condition
SIMULATION PARAMETERS. Modal 1.55 μm Transverse magnetic (TM) 2000 Paraxial Finite Difference 0.5 1.3 Transparent Boundary Condition (TBC)
Before constructing 1x16 signal router we optimized the device parameters. Best performance was found for 0.0775 μm titanium strip thickness, 8 μm waveguide width, 9900 μm arm length, 34 μm arm gap, 14.5 μm 3dB coupler gap with outputs at 0 V, 0.000437526 mW and 0.997974 mW and at 8.8 V, 0.997974 mW and 0.000437526 mW for port 1 and 2 respectively. As a result we obtain 33.581 dB extinction ratio and 0.00881 dB insertion loss, 0.0069 dB excess loss for both cross state and bar state [22], where in reference [8] lowest insertion losses were 0.0138 dB and 0.0129 dB & maximum extinction ratios were 30 dB and 29.9 dB for cross sate and bar state respectively obtained with titanium strip thickness of 0.0825 μm. In figure 2, 3, 4, 5, 6 simulation results for optimized electro-optic 2x2 MZI switch are shown. Fig. 2 shows refractive index propagation of the optimized device.
Fig. 4. Electric field at 0V and 8.8 V respectively.
These results are obtained by running the following visual script, Const NumIterations = 11 ParamMgr.SetParam “V2”, 0.0 For x= 1 to NumIterations Param Mgr. Simulate Param Mgr. Set Param “V2”, 0.8* x WGMgr.Sleep (50) Next
Fig. 2. Refractive index propagation of MZI switch (3D view) .
Optical field propagation for both 0V and 8.8 V are shown in Fig. 3. Fig. 4 ensures that light is switched completely from port 1 to port 2 with the application of applied voltage. It takes 11 iterations to switch light from port 1 to port 2. Fig. 5 represents power in output waveguides for a voltage range of 0 V to 8.8 V and from Fig. 6 we can observe power overlap integral characteristics versus distance of propagation of waveguide for both low and high voltage.
Fig. 3. Optical field propagation at 0 V and 8.8 V respectively.
Fig. 5. Power in output waveguides.
Fig. 8. Refractive index propagation of 1x16 signal router.
MZI switches are numberred as 1, 2, 3 and so on. we assumed the second electrodde of each switch as control electrode and they are also num mbered similarly as S1, S2, S3 etc. Fig. 6. Power overlap integral at 0V and 8.8 V respecttively.
IV. SIMULATTION RESULTS Results are observed by OpttiBPM software. From Fig. 7 we can see that there are 16 output ports. To switch power into one of these ports particular combbination of voltages across the central electrodes of these 166 MZI switch is required. The width of this structure is not more m than 700 μm and length is not more than 307600 μm. Taable II shows output power at 16 different ports on the basis of control c signal S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S144, S15, S16 representing voltages across central electrodes of 166 switch respectively. In table II logic 0 represents 0 V and loogic 1 represents 8.8 V and X represents don’t care condition. S1 is kept high to switch upper w lower 8 output ports are to be 8 output ports and with S1 low switched. When S1 is high outpuut goes to MZI 3, with S3 kept at 0 V output goes to MZI 5, aggain with S5 low output goes to MZI 9, with S9 low output goees to MZI 11, consequently with S11, S13, S15 low output goes to the output port 1. Similarly for wn in table II we can find high each of the combinations show output at desired output port. Fig. 9 shows the optical field propagation at different output ports. p
Fig. 7 and Fig. 8 show the layout andd refractive index propagation of 1x16 signal router resppectively. Central electrode of each switch was positioned 5.5 μm μ above from the original axis.
Fig. 7. Layout of 1x16 signal router. TABLE II. Output Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 Port 7 Port 8 Port 9 Port 10 Port 11 Port 12 Port 13 Port 14 Port 15 Port 16
S1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
S2 X X X X X X X X 1 1 1 1 1 1 1 1
S3 0 0 0 0 0 0 1 1 X X X X X X X X
S4 X X X X X X X X 1 0 0 0 0 0 0 0
UTER. DIFFFERENT COMBINATIONS OF CONTROL SIGNALS OF 1X16 SIGNAL ROU
S5 0 0 0 0 0 1 X X X X X X X X X X
S6 X X X X X X X X X 1 0 0 0 0 0 0
S7 X X X X X X 1 0 X X X X X X X X
S8 X X X X X X X X X X 1 0 0 0 0 0
S9 0 0 0 0 1 X X X X X X X X X X X
S10 X X X X X X X X X X X 1 0 0 0 0
S11 0 0 0 1 X X X X X X X X X X X X
S12 X X X X X X X X X X X X 1 0 0 0
S13 0 0 1 X X X X X X X X X X X X X
S14 X X X X X X X X X X X X X 1 0 0
S15 0 1 X X X X X X X X X X X X X X
S16 X X X X X X X X X X X X X X 1 0
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
(m)
(n)
(o)
(p)
Fig. 9. Optical field propagation at (a) port 1, (b) port 2, (c) port 3, (d) port 4, (e) port 5, (f) port 6, (g) port 7, (h) port 8, (i) port 9, (j) port 10, (k) port 11, (l) port 12, (m) port 13, (n) port 14, (o) port 15, (p) port 16.
V. CONCLUSION This paper gives elaborate discussion on a 1x16 signal router. 16, 2x2 electro-optic switches are used in construction, which gives the device a length of 307600 μm and width of
700 μm. At first the fundamental device i.e. MZI switch is optimized and we obtained lowest insertion loss of 0.00881 dB, excess loss of 0.0069 dB and highest extinction ratio of 33.581 dB. Then outputs for different combinations of control signals
are observed and we find power being switched to each one of 16 output ports for each combination.
[19]
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