IEEE—ICET 2006

2nd International Conference on Emerging Technologies

Peshawar, Pakistan 13-14 November 2006

1-4244-0502-5/06/$20.00©2006 IEEE 8

Crosstalk Analysis in Planner Waveguide Couplers

Abdul Rauf1, Imran Rashid2, Attiq Ahmed3, Fazal Ahmed

Electrical Engineering Department, Military College of Signals,

National University of Sciences and Technology, Pakistan

abdulrauf@nust.edu.pk1 , irashid-mcs@nust.edu.pk2, attiq-mcs@nust.edu.pk3

Abstract: Directional couplers are very important

devices in integrated optics and could well be the

only device necessary for most integrated optics

chips. When light is launched into an input

waveguide of directional coupler, one would like to

get all the power from one of the two output

waveguides. Whereas it is generally observed that

small amount of power does go to the other guide,

causing undesirable crosstalk. This phenomenon

gets worst in WDM systems as at each wavelength

coupling, length vary and so is the output. Crosstalk

is an important figure of merit for a directional

coupler. The crosstalk is attributed to many reasons

like asymmetry of the waveguides, absorption loss,

bending losses etc. This paper also deals with the

directional couplers with special emphasis on

measurement of their response over a wide

frequency range. It also includes the simulation of

the coupler response and comparisons with the

tested results.

Keywords: Optical waveguides, Couplers, Crosstalk

1. INTRODUCTION

The layout of directional coupler is such that it contains portions like, uncoupled waveguide region, taper region and the interaction region.

Figure 1: A directional coupler diagram showing uncoupled

waveguide regions, taper regions and interaction regions

composed of curved and straight waveguides portions. [1]

Taper regions composed of curved waveguides. The optical fields of these taper regions are partially coupling. Most of the coupling between waveguides occurs in the interaction or coupling region where the two

parallel waveguides are closely positioned. The

power coupling ratio is defined as (if the power input port is 2 see figure 1):

3

3 4

P

P P

= Sin2 ( L) ( = 0 for symmetrical couplers) where is the coupling coefficient. [1, 6] and given by,

22 2 * *0

12 c2 2 1 1 1A

kk ( ) A

2A

n n E E d E E dA

Where k0 is a wave number, is

propagation constant, 2 2

c2( )n n describes the

perturbations to the dielectric constant of waveguide 1, E2 is the evanescent wave and is very small. Additional coupling occurs due to the transition regions (known as taper region) and therefore, it contributes to an increase in the total coupling length by Lend. In this case the power coupling ratio becomes

= Sin2 [ (L+ Lend]

2 end

c

(L+L )Sin

2 L

Where cL

2k

Lc is the coupling length, which is needed for complete coupling of light from one waveguide to the other. But practically complete coupling never occurs due to imperfections. [1, 2]

2. DEVICE UNDER TEST

The coupler chip, which was used for testing, was containing four couplers and two straight waveguides. The four couplers were in the centre and two straight waveguides were on both sides

All the couplers were tested in the laboratory. Specifications of the couplers were as under.

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Coupler 1: c2c = 9 m

Ls = 5500 m

Coupler 2: c2c = 8.5 m

Ls = 3600 m

Coupler 3: c2c = 8.1 m

Ls = 2700 m

Coupler 4: c2c = 7.8 m

Ls = 2500 m

Coupler 3 and coupler 4 were examined in detail and were used for simulation as well.

Er ASE Laser

Source

Polarization

ControllerChip under test Optical Spectrum

Analyzer

He-Ne Laser

Source for

Alignment

Figure 2: Schematic Layout of the laboratory test setup. It

includes Er ASE Laser source, Polarization controller, Chipset,

He-Ne Laser source for alignment only and Optical spectrum

analyser.

3. TESTING PROCEDURE

The chip was placed on the test bench and aligned the fiber cable to the lower straight waveguide on one of its sides, initially by setting input from a Helium-Neon laser. After alignment the source was connected and fiber on the other end. The readings were taken from the optical spectrum analyzer after a tedious alignment procedure with the help of manual as well as electronic alignment equipment.

The same procedure was followed h the rest of the couplers i.e coupler 2, coupler 3 and coupler 4.

Minimum and maximum values of polarization were taken for through and cross legs. This gave four curves for each coupler. The graphs were changed to linear scale and plotted all four curves of each coupler onto one graph for simplicity.

4. COUPLERS RESPONSE

The measurements of all the couplers were converted to the linear scale and integrated four readings of each coupler onto one graph which included reading at minimum and maximum

Figure 3: The individual responses from through and cross ports

(including changes in minimum and maximum polarization) of all

four couplers.

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polarization in the through and cross port. In addition to these four curves, two more curves for each coupler, which show the added effect of the maximum through port reading with minimum cross port reading and maximum cross port reading with minimum through port reading. Before detailed analysis it was required to scale these measurements closer to unity, which could represent more realistic view.

All the readings were required to be scaled for “1” for a latter comparison. The algorithm was mathematically derived and results were converted into the Matlab code and were used for scaling the measurements.

5. CROSSTALK MEASUREMENTS

After the scaling process the crosstalk was investigated in the three couplers, that is, coupler 1, 2 and 3. Coupler 4 was a 3 dB coupler and it was not possible to measure the crosstalk in our test frequency range. The amount of crosstalk in dBs is measured by the formula:

2

2 2

Crosstalk in dBs =

(Power in wrong port)10 log

(Power in wrong port) (Power in correct port)

The acceptable crosstalk level considered is normally around –30 dBs. If the couplers show crosstalk more than this then they are considered not good couplers. In our readings coupler 2 and 3 are quiet good as compared to the reference crosstalk level of –30dBs. Whereas coupler 1 has shown not good results and has also indicated that the coupler is quiet sensitive to the polarisation variations.

The reasons for polarisation dependency are due to following reasons:

The waveguide is not round or circular in nature. We have rectangular waveguides. If the dimensions are perfect then we will not experience geometric birefringence whereas in case the dimensions are not perfect then horizontal and vertical components of the optical signal are treated differently, which creates geometric birefringence and cause of polarization dependency.

Figure 4: Shows all the crosstalk measurement values for coupler

1, 2 and 3.

More the waveguide has asymmetry, more polarization dependent it would be.

The inside surface of the waveguide has perturbations and is not smooth. Due to this horizontal and vertical components of the optical signal are treated differently. It could make the waveguide polarization sensitive.

In the manufacturing process, stresses build up in between the different glass layers. This is also often seen as a common explanation of polarization dependency.

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The test experiments show that coupler1 has the worst parameters and has maximum polarization dependency and it is not good for practical use. Whereas the other three couplers have less cross talk and are less polarization sensitive which make them a reasonable candidate for practical use.

6. SIMULATION

We used OlympiOs software for simulation of the couplers after getting the results from lab. We used Beam Propagation method (BPM) module of OlmpiOs.

The response curves of the simulated couplers are calculated by the overlap of integrals at the end of the output waveguides with the input field. One thing is important here that the input field is a modal field, which assures that a minimum of radiation is lost at the boundaries.

Coupler 3 was made according to the dimensions and adjusted all the required parameters. The simulation was run and the results are shown below. Figure 20 shows the simulated power transfer from through leg to the cross leg of the coupler. Whereas, figure 21 shows the simulated response of the coupler 3 over a wide optical range.

Figure 5: The figure shows the simulation results for coupler 3 depicting the power transfer and

distribution in the two ports of the coupler.

The simulation response of coupler 3 shown at figure 6 is similar to the measured response in figure 3 earlier by laboratory tests. The difference is that simulated results do not show crosstalk and have almost zero crosstalk near 1.48 whereas in measured results minimum occurs at 1507 nm with crosstalk of –38.76 dBs.

Figure 6: The individual responses from through and cross ports

of coupler 3 resulting from simulation process.

Coupler 4 was also made according to its dimension and parameters shown in the chip layout. On running the simulations following results came up which are shown in figures 7 and 8 below.

Figure 7: The figure shows the simulation results for coupler 4

depicting the power transfer and distribution in the two ports of

the coupler.

Figure 8: The individual responses from through and cross ports

of coupler 4 resulting from simulation process.

The simulation response of coupler 4 shown at figure 8 is similar to the measured response in

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figure 3 earlier by laboratory tests. Test results indicated that coupler 4 is a 3 dB coupler at 1555 nm and this response is confirmed by the simulation results. In case of coupler 4, tested results and simulation results are quite similar.

The couplers tested in the lab showed certain amount of crosstalk whereas we could not find crosstalk in our simulation results. The software program takes the coupler as an ideal case whereas practically that is not the case.

To investigate the crosstalk we performed two simulation experiment in which we

Raised the index between the two-coupled waveguides in coupler 3 from background index of 1.445 to 1.445+0.002=1.447. This was done to cater for the diffusion into the gap of the two ports of a coupler, which could affect the behaviour of that coupler.

Changed the width of through port of coupler 3 from 3.8 to 4.0 um at 1507 nm and 1470 nm. This was done to introduce the width imperfections into the coupler and observe the behaviour response with these changes.

In the first case of raised index simulation we could not record any cross talk. Figure 9 shows the simulation profile.

Figure 9: The individual responses from through and cross ports

of coupler 3 resulting from simulation process. The coupling gap

index was raised from 1.445 to 1.445+0.002=1.447

The reasons for this result could be following:

The simulation software takes the step index profile in an ideal manner.

Whereas, practically it is not step rather it is closer to a gaussian profile

As the couplers manufacturing involves processes like etching and diffusion of dopants, which are difficult to control at micron meter level and introduce some variations. On the other hand, the simulation software takes all the values as an Ideal case.

Practical couplers have some asymmetry in its parameters like length, width, index variations etc and it is almost impossible to make an ideal coupler due to complicated manufacturing process.

Testing results have some misalignments, which could result in variations from ideal results also.

In the next step we tried to simulate coupler 3 at the wavelength of 1507 nm. This was the wavelength at which this coupler showed minimum crosstalk of –38.76dBs in our laboratory results. Refer to figure 15 (Coupler 3 graph). In this step we varied the width of the through port from 3.8 to 4.0 um. We found out that the response of the coupler does not go to zero at 3.9 um which indicates that this is ideally not the wavelength for the dimensions, as of coupler 3. The response of this step is shown in figure 10.

Figure 10: The individual responses from through and cross ports

of coupler 3 with width variation of through port from 3.80 um to

4 um. The response is recorded at = 1507 nm from simulation

process.

After getting this response we investigated different wavelengths over this range of width to find out ideal wavelength at which response

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of the coupler goes to zero at 3.9 um. We found that at wavelength of 1470 nm the response goes to zero. This shows that if an ideal coupler is made with all the dimensions of coupler 3 we will get the minimum crosstalk at 1470nm. Figure 11 below shows the response of coupler 3 at 1470 nm.

Figure 11: The individual responses from through and cross ports

of coupler 3 with width variation of through port from 3.80 um to

4 um. The response is recorded at = 1470 nm from simulation

process.

This was not the case in our measurements (figures 3 & 4) where we recorded minimum crosstalk at 1507 nm. This indicates that it is really difficult to make an ideal coupler and all the reasons discussed earlier indicate that practical couplers do have variations introduced in manufacturing, which finally result in some amount of crosstalk.

7. CONCLUSION

The process involved in manufacturing these tiny devices is complicated because it involves techniques like etching and diffusion, which are very difficult to control at such a small size. Due to this complicated process, it is almost impossible to make these devices according to the ideal dimensions.

As these couplers are not ideal in nature, therefore, they introduce crosstalk due to their asymmetric behaviour, which is caused by variation in length, width, refractive index distribution etc.

In our project a set of four couplers were analysed in the laboratory and

their behaviour was recorded. Three couplers gave quite satisfactory results regarding crosstalk measurements. One of the coupler gave quite high crosstalk values and exhibited quiet high polarization sensitivity. Such type of coupler cannot be used in practical system due to its poor response.

After analysing the couplers in the laboratory, the behaviour of two couplers was thoroughly checked by simulation. The simulation results were quite comparable to the experimental results but didn’t show any crosstalk. This is because the simulation program takes each coupler as an ideal case. Whereas practically it is not the case.

One of the coupler was made asymmetric with variation in the width and the variation of the refractive index in the gap of coupling region. The simulation of this coupler showed that the laboratory results and the simulation results have small variations. These variations are caused due to the fact that:

o Software takes the ideal coupler parameters

o Software takes into account the ideal step index profile for the refractive index whereas it is almost gaussian in practical couplers.

o Real couplers have processing variation due to etching, diffusion of dopants etc.

o Test results may vary due to misalignment, dirty fiber ends, scattering etc.

All these reasons indicate that simulation is a good tool to observe the behaviour of a device but this always have small variations with the actual results.

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January 1984.

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