All optical switches based on multimode interference couplers using nonlinear directional couplers (tt)

Tạp cbl Khaa học và Công nghệ 51 1A 2013 1X3 ALL OPTICAL SWITCHES BASED ON MULTIMODE INTERFERENCE COUPLERS USING NONLINEAR DIRECTIONAL COUPLERS Cao Dung Truong “’, Tuan Anh Tran“, Tru

Tạp cbl Khaa học và Công nghệ 51 (1A) (2013)

1X3 ALL OPTICAL SWITCHES BASED ON MULTIMODE

INTERFERENCE COUPLERS USING NONLINEAR

DIRECTIONAL COUPLERS

Cao Dung Truong “’, Tuan Anh Tran“, Trung Thanh Le?’ and Duc Han Tran’

* Hanoi University of Science and Technology, | Dai Co Viel, Hanoi, Vietnam

? Hanoi University of Natural Resources and Environment, Hanoi, Vietnam

“Email: dungtc.vinl@ynpl.vn ; thank Je@hanre.edu.rn

én Loa soan ngay /10/2012 Chấp nhận đăng ngày 21/02/2013

ABSTRACT

‘Multimode intorferenee in optical wavognide is altractive for all optical switching, In this paper, a novel 1x3 all-optical switch based on 3x3 mullimode interference (MMI) slruclures is

proposed Nonlinear directional couplers in two arms of the structure are used as phase shifters

In this study, we use chalcogenide glass on silica for designing the device structure The switching stales of the device can be controlled by adjusting (he optical control signals at the phase shifters The transfer matrix method and beam propagation method (BPM) are used for desiguing and optimizing the device structure

Keywords: All optical switch, MMI conpler, nonlinear directional coupler, phase shifter

1 IN[RODLCTION

Optical communication networks have evolved inlo the era of all oplical switching Today, various approaches ta realize all optical switches have been proposed, Space-division optical switches provide valnabie scconfigurahle interconnecting functions needed by optical crass- connec, (OXC) and by fiber-oplic subscribe line connechons m oplical communications systems The MEMS switches are the choices for large order switch systems In addition, the thin film based switch, the liquid rystal based switch, the directional coupler based switch [1] and the MMI coupler based switch have been cilher commercially available or found in laboratories In comparison with other optical switches, the MMI based switch has the advantages of low loss, ultra-compact size, high stability, large fabrication tolerance and greater feasibility for integration [2] There arc many implementation methods ta realize optical switching based on MMI structures [3] For switching purposes, MMIs caa either be placed in a

‘Mach-Zehnder interferometer (MZI) as splitter or used as distinct region In recent years, there have heen same optical switches using MMI strictures using thermo-apiie [1], [S] and clcero- lic effects [6], [7] However, figh speed oplical communscalion systems require fnigh speed optical switches Therefore, it is particularly necessuy to achieve all-optical switches

Gao Dung Truong, Tuan Anh Tran, Trung Thanh Le, Due Han Tran

Recently, chalcogenide (Asis) wavegnides have been propased as a new platform for optical signal processing, offering superior performance at ultrahigh bit-rates [8] Additionally, the high nonfinearity enables compact components with the potential for monolithic integration, enving to its large nonlinear coefficient n- and love twa-photan ahsorption (good figure of merit), the ability to tailor material propaties via stoichiometry, as well as its photosensitivity: These properties allow the fabrication of pholowrillen gralings and waveguides [9]

‘The main aim of this paper is to propose a new structure for all-optical switching based on two 3x} MMI couplers using nonlinear dircctional couplers as phase shifters Chalcagenide lass on silica platform is used for our designs In this work, the operating principle of MMI based switches using analysis is presented Nonlinear directional couplers at two outermost arms

in the inter-stage of two 3x3 MMI conplers play the role of phasc shifters Tn arder to realize the phasc shillers using nonlincar dircctional couplars, thc control signal is al an arm of lhc nonlinear directional coupler, and the information signal is øt the other arm that is also the autermost arm of the MZI structure The contral signal must be scparatcd from input signals and snlgrs lhc swilching struclurc from a different single-mode access waveguide allcr Lhe switching operation, The aim is to reduce the powers transferring between control waveguides and information signal wwaveshides Numerical sirmulations arc used to verify the operating principle

af the proposed all-optical switch

2, THEORICAL FUNDAMENTAL 2.1, Analytical expression of the MMI coupler

The cpcration of optical MMI coupler is based on the self-imaging principle [10] Self- imaging is a property of 3 multimade waveauide by which as input ficld is reproduced in single

or mnulliple images al periodic intervals along propagation direction af Ihe waveguide MNIL coupler can be characterized by the tranefer matrix theory [10], [11] Following this theory the relationship between the input vector and ontpnt vector can be abtained, To achieve the required transfer malrix, the positions of the input and gulput ports of the MMI coupler must be sel

exactly

In this study, the MMI waveguide has a width of Wear the access waveguides have the same wicth of W, The positions of tie input and output ports are Tocated at x; [LO]

(ly

where W, is the effective width of the MMT coupler and N is the number of inpat/outpur

In the general interference mechanism, the shortest length of the MMI coupler is set by

‘Where L, is the half-heat length af two lowest-order modes that it can be written as

Pe Bì

Where: n, is the refractive index of the core layer, gis the free space wavelength The transfer matrix of the 3x3 MMI coupler [10] is determined by

1x3 all-optical switches based on multimode interference couplers using nonlinear directional couplers

Where = 8+ (i-i)(6- ji) if iej even

Coupler Figure 1 A 1x3 all optical switching based on a 1x3 MMI and a 3x3 couplers using directional couplers

as phase shifters

2.2 Operation principle of the 1x3 all optical switch

‘The configuration of the proposed all-optical switching is shown in Figure 1 It is consist of two 3x3 general interference MMI couplers having the same size Here, two nonlinear directional couplers are used as two phase shifters We assume that input port of the switch is located at position A of the center line and output ports of the switch are located positions bi, b>, byas show in Figure 1

‘The transfer matrix of 3x3 general interference couplers (GI-MMI) can be expressed as follows {10} [11]

@)

‘The input, ouput complex amplitudes and phase shifters can be expressed by the following matrices

bị,

WHee @, and ọ; are phase shifter angles at two outmost arms caused by directional couplers respectively.

Gao Dung Truong, Tuan Anh Tran, Trung Thanh Le, Due Han Tran

‘We have the following relations

a

@

1! 7

bye ae aye aye |

‘Now we calculate the phase shifters to control input signals to any output ports

Case 1: Output switch ta port b, as well as, =b; =0, from (8}we are obtained

me) Fite

Solve this equations system (9) we get: «py = 1, > =

Hence, if (ay, i-(x3 ]then switch to port b., whils( nm i-(3 x will switeh to port by

4 ˆ 2

Case 2: We find the condition for switching to port b,, this condition is equivalent to

ets le? _ hy

0 Clearly, (9), 93} have the same role inta equations system (10) so we have the rnat =0 subsliluting this rool into (10), we have

4x3 all-optical switches based on multimode interference couplers using nonlinear diractional couplers

7

oR

Hence, (9.0: } is the condition for swilching lo port b,

Tabla J phase shifter states for operation af the 1x3 optical switches

an

In summary, phase shifters required to control the input signal to any output ports can be expressed in Table 1

2.3 Phase shifters using nonlinear directional coupler

As the mentian above, the structure of an all optical switching requires two nonlinear directional couplers [12] as phase shiflers at two outermosl arms of optical device as shown in Figure 1 Originally, the nonlinear directional coupler includes two waveguides that have sinall distance and full coupling takes place between them in anc coupling length, pravidcd thai one or both of them fave non-linear behavior ‘This non-linear behavior can be guaranteed wilh high intensity control field which changes the nonlinear refractive index When the distance of two nonlincar directional couplors is very small and mode ficlds amplides vary slowly in the 2- propagation direction, the interaction of electrical fields in nonlinear directional couplers comply wilh coupled mode equations

“iE eat (BP +25) Jp a2)

‘Where x ig the lincar coupling cacfficient, it is determined by ee

fength, A and B ate field amplitudes of waveguide 1 and 2 of the directional coupler and 73.2 are nonlinear coefficients describing the self-phase modulation (SPM) and cross-phase

modulation (XPM) cffcets Nonlincar cacfficicnt is determined as follow

Gao Dung Truong, Tuan Anh Tran, Trung Thanh Le, Due Han Tran

2m

Arar

dy is wavelength in the vacnnm, nis nonlincar refractive index of the waveguide, Ag, is

the cfleclive modal œoss -scdion arca Undor the effect of scliphasc modulation in the nonlinear directional coupler, the phase in directional coupler will be changed proportionally with the intensity of input clecttical fields of waveguides Nonlinear phase shifts in the directional caupling wavegnide can be definition as follow

where lạ, lạ are field intensities of the controll and control? waveguides respectively I, is field intensity of the signal waveguide at outermost arms In the phase matched cage when the input wavelength and the refractive index of twa waveguides are identical, maxiraum coupling will (ake place

3 SIMULATION RESULTS AND DISCUSSION 3.1 Simulation results

Tn thus study, we use the chalcogenide glass AsrS, for designing the whole device The material used in core layer of the proposed oplical swilching structure is chalcogenide glass As.S; with refractive index n=2.45 and the silica material SiO, used in cladding layer has refractive index n, =I46 AssSs (arsenic trisulfide) is a dirce band-gap, amorphous semiconductor By using a highly controlled deposilion process, a photo-polymerizable film of As;# can be deposited oa standard silica glass substrates Chalcogenide As:S; is chosen due to its advantages For example, it is attractive for high rate photonics integrated circuits, especially attractive for all optical swilches m recent years because of the fasl response lime associaled with the near-instantaneous third order nonlinearity allows flexible ultrafast signal processing 13] In- addition, the chalcogenide glass supports the operation of wavelensths range in the windows [.5$ym, and As;S; malerial bas a high reltaciive index contrast lo allow [or a ugh confinement [14]of light also ultra-compact size Therefore, it is useful and important for large scale integrated cironits The other advantage of the chalcogenide plass is that it has a high

phase angle in the phase shifter of the structure increases proportionally in the nonlinear coefficient and the contral field intensity, so if nonlinear coefficient is high then control field intensity is low when the phase angle is constant This is berter for operation of the prapased switch because a very high intensily of the control beam will overwhelm (he signal, Moreover, since the control beam intensity is mnch higher than the signal beam oag the noulinear directional coupler needs an extreme high isolation: so that it is difficult to design and optimize the proposed structure, Sificon dioxide SiO; is used in cladding iayer because of high reltactive index difference between core and cladding layers that ailows for a high confinement of light and alsa supports a larger mode numbers in MMI region In addition, both As.S; and SiC,

4x3 all-optical switches based on multimode interference couplers using nonlinear diractional couplers

materials are available and cheap also they can implement in the practical fabrication Recently, these malcrials arc very allractive for ullrahigh bit-rate signal processmg applications

‘The device used in our designs is shown on Figure 1 Here, we use the TE (Transverse Plectric) polarization and operating wavelength 1550-nm for analyscs and simulations Tf the uniformity of the lime harmonic of ‘TE-polanzzed waves can be assumed along the x direction of Figure 1, the simulation can be done assumúng it as a 2D structure In order to reduce time

consuming but still have accuracy results a 3D device structure is converted to a 2D structure

using the effective index method (IM) first, then the 2D-BPM method is used for sirmilations [ISE

‘The design parameters of the proposed structure are chosen as follows: the width of each 3x3 MMI coupler Wynn is 244un, the width of access waveguides W, is (jum in order for single more condition can be obtained, the length of the multimode region Tans is sctas T., for the general interference mechanism and it can be calculated by the mode propagation analysis (MPA) method is 1259.8uum,

Parameters of the control waveguides are designed as follows: the width is set as Wa, at the beginning, a straight waveguide has the lenglh of 2059.15 calculated by using the BPM Next, it is connected to a sine waveguide which has the length of 1006um in z propagation điredian and the distance of 9m in x-direction Then it is concatenated to another straight waveguide By using the BPM, the length of the straight waveguide of the nonlinear directional couplers Le is chosen lo be 360m Lo satisfy the eliminating condition of the cross transfer power between contral and structure waveguides Gap g between this straight waveguice and the autermost arin is small (Figure 1} to enable mode coupling Finally, a sine waveguide and a straight waveguide are in lurn connected (as shown on Figure 1) We choose the sine waveguide for two purposes: First, the sine waveguides are used ta connect the straight waveguides together

in which it puts a waveguide near outermost arms which link between MMI regions if o¢der to make a full coupling and a phase shift between nonlinear directional waveguides and the second aim is that light beam power can be conserved when propagated through it Both control beams and input signal beams have the same wavelength, amplinide and polarization state in all of

switching states,

‘Now we optimize the whole device structure Firstly, the length Lyaar is optimized by the 2D-BPM methed to find the optimal value by changing the values of the length around L,, Finally, we find ont the optimal value as 126Qum The optimal gap ¢ herwecn two parallel] waveguides of Ine dircclronal couplars uscd as phasc shifters can be found by using lhe BPM The simulations are shown in Figure 2 We need to find the optimal value g to minimize the cross transferring power between autermost arms and the cantrol waveguides and split the total power cnicring mo one inpul port caually inlo 3 amns a3, a)Bạ, asB5 a5 Pains, Prams, Poses respectively This can be done by introducing power into ports al, a2 and a3 and use 2D-BPM method Due to the symmetry of the proposed structure, we only need to consider the power inserted into conlrol waveguide 1 By changing thc value of g gradually from 9.05pm lo 0.11 yam and monitoring and normalizing the power Py: a3 Well as Pye, We choose the optimal value

of g as 0.1m according to Figure 2

Cao Dung Truong, Tuan Anh Tran, Trung Thanh Le, Duc Han Tran

Figure 2 2D BPM simulation results for the optimal values of the distance between control and structure waveguide in two cases: a) In case of the control power is on and b) In case of the

control is off

Figure 2 2D BPM simulation results for optimal value of the distance between control and

structure wavesuide when: a) the control power is on, the data power off and b) the control

power off, the data power on

Simulation result implemented by 2D-BPM method in Figure 3 also show that at the optimal value of the distance between control and structure waveguides, the coupling power between them is reduced to the minimum value

‘To optimize the operation of the MMI regions in the role of the splitter and combiner as well as minimize the insertion loss and crosstalk effect, linear taper waveguides are used to connect between MMI regions and access waveguides In our design, linear tapers have the length =150jum and the widths from 3;m to Spm are calculated and optimized by BPM

‘simulations

As mentioned before in results are shown on the Table 1, when the input field enters the switch from the input A port, if the phase shift in the first linking arm is 2n/3 radian and the second linking arm is zero radian, it will switch to output b; port

For switching from an input to an output of the stricture, we implement numerical

simulation by 2D-BPM method to find optimal values of field intensities of control waveguides,

‘The simulation has to satisfy two requirements: the first, we find the values of field intensities of control waveguides to produce exactly matched phase shifts for switching operations; then those

Gao Dung Truong, Tuan Anh Tran, Trung Thanh Le, Due Han Tran

values must be optimized so that the transfer power hetween signal waveguides and control waveguides is minimal,

Fable 2, Power amplitude und intensilly vtates Lor opcralion af the 1x3 optical switches

Tnpat Output

A by | 4H83 | 219

WWe assme that he normalized inpul pøwer in optical swïtching đevice is sd as 1

normalizcd unil, inpul ñcld micnsily Ip cquals 1 GWécm’ This valuc is chosen because il can

generate the largest nonlinear phase shift To reach the switching state from port al to poft bỊ, firstly we find the intensity [,, which is introduced into contral waveguide 1 (also sco Figure 1),

by varying the inlcnsity slowly ‘The appropriate resull is aboul 277GW/cmn’ rmatang phase shill

23 radian in comparison with the center access waveguide, Secondly, we can also change the value of the intensity In, which is introduced inta control waveguide 2 The appropriate result is about 450GW/em? making phase shift zero radian im comparison with the center access waveguide, Finally, if we use these results to reproduce the simulation and adjust their values

very slowly around them again, we obtain the optimal values 1;-279 GW/cm' and

1148, SGWiem’, respectively The reason far this is due to the lass when the lisht travels in the

‘MMI region and also because the Icngth of MMI region is too long to be operated as a spliller or

a combiner accurately, Table 2 lists optimal field intensities and states of control waveguides used in twa control waveguides

4, DISCUSSION

Results showed high output power intensity which ensures the qualitative performances of the structure in all aspects of a switch Subsequently, a high-level switch should have the suitable insertion loss, exlinclion ralio, crosstalk, and good tolerance independency agama Lhe wavelength and fabrication Thus indicating the listed parameters is important in manufacturing

an optical switch

‘The calculation formulas for the insertion loss ({ L.) and extinclion ralic (Ex 8} [16] are defined by

P, _—

£ 2,

Tác (đR) =1010g,, |

Where P,,, and P,, are the outpnt and input power of the switch in operation state, Pia and Dow ate Oulput power levels in ON and OFF stales respectively,

1x3 all-optical switches based on multimode interference couplers using nonlinear directional couplers

-2o-ig 0 30 20 20-30 0 1D 29 -z6 -10 0 30 29 a

Input A >Output by InputA->Outputbz Input A Output bs Algure 4 Simulation results implemented by BPM method for all switching states of the 1x3 all optical

switches

§»|

i

&

Ệ

: AM THỊ TU l5Đ lẾU TẾ TẾ Teed ves) TÊN TẾ

Tamiegh em)

*iguue 5 Wavelength dependency of the extinction ratio and crosstalk of the proposed switch

“ấm TH HƠ TRO IS eed a) XẾ MÔ MỚI lên Wemsgh(tn)

Figure 6 Wavelength dependency of the insertion loss in all operation states of the proposed switch