SIMULATION AND OPTIMIZATION OF 2 X 2 MACH-ZEHNDER INTERFEROMETER ELECRO-OPTIC

SWITCH.

By Kenjal Jain

HISTORICAL DEVELOPMENT OR INTRODUCTION In this new millennium, we are seeing dramatic changes in the telecommunications industry that have far-

reaching implications on our lifestyles. There are many drivers for these changes.

Relentless need for more capacity in the network

SDH/SONETPoint-to-point

Wavelength routing

Optical Circuit Switch

Optical Burst Switch

Optical Circuit Switch/Optical Packet Switch

(Optical hybrid switch)

Optical Packet Switch

First Generations Second Generations Third Generations10-9

10-6

10-3

10-1

Late 1980’s and 1990’s Present Day 5-10 years 10+ Years

Tremendous growth of the Internet and the World Wide Web

Bandwidth consuming services like video-on-demand and interactive media

Optical Fiber Network Evolution

NEED FOR OPTICAL SWITCHING

The tremendous growth of the internet during the 1990’s led to extensive deployment of WDM. WDM, in turn, provided a solution to transmission bottleneck, but created the challenge of switching the large number of wavelength channels it enables.

But multi-wavelength data streams have to be terminated at every node, converted to the electrical domain for switching purpose before they are converted back to the optical domain and transmitted to the next node. This type of switching is referred to as O/E/O switching.

O/E Conversion

E/O Conversion

E/O Conversion

E/O Conversion

Electronic

Switching

The main attraction of optical switching is that it enables routing of optical data signals without the need for conversion to electrical signals and, therefore, it is independent of data rate and data protocol. Thus, transfer of the switching function from electronics to optics will result in-

An increase in the switching speed, and thus network throughput, Decrease in the operating power. Elimination of E/O and O/E conversions will result in a major

decrease in the overall system cost which represents huge share of cost in today’s networks.

Optical Switching extends the reach of the virtually unlimited optical bandwidth from transmission systems to switching nodes, thereby elevating the information carrying capacity of the network to levels that are way beyond reach with the electronic switching.

O/E Conversion

Electronic Switching

E/O Conversion

E/O Conversion

E/O Conversion

Figure 1.1 Switching after converting optical signal to electronic signal and after forwarding to next hop switching again to optical signal

It mean with increase in O/E converters with increase in number of wavelengths increases the consumption of power, decreases the switching speed and apart from that, electronic equipment is strongly dependent on the data rate and protocol, and thus, any system upgrade results in the addition or replacement of electronic switching equipment, therefore because of these drawbacks created the need for optical switching in which optical signals could be switched without conversion to electrical form. Optical switching has therefore been positioned as the solutions to these problems and key to switching relief.

WHAT IS AN ELECTRO-OPTIC SWITCH OR OPERATING PRINCIPLE OF ELECTRO-OPTIC SWITCH

An electro-optic switch is a device used in integrated fibre optics. The device is based on Mach-Zehnder interferometer made by Titanium diffusion in Lithium Niobate substrate. The switching between the ports is achieved by an electro-optic effect within such structure. Voltage, applied to the electrodes deposited on the integrated Mach-Zehnder interferometer, creates an electric field distribution within the substrate, which consequently changes its refractive index. If properly designed, the induced change in the refractive index leads to different coupling between individual ports.

MACH-ZEHNDER INTERFEROMETER (MZI) SWITCH

Mach-Zehnder interferometer (MZI) consists of a pair of waveguides which are parallel of each other and separated by a separation distance. Input of optical signal into one of the waveguide of MZI is coupled into one of the output over an evanescent coupling.

Coupling power is proportional to the separation of waveguides and size of the waveguide mode depends on the wavelength used. If two of the waveguides are the same, then full coupling between them occurs over a certain separation which depends on the coupling power.

With locating electrode on the waveguides of the optical switch designed, input optical signal can be coupled into desired output by applying certain voltage over the electrode. The corresponding switching voltage V0 is,

Vo =

Where L0 is the transfer distance, which depends on the efficiency coupling C between each two guides, d is the coupling separation, n is the refractive index of each guide which is equals, r is the appropriate Pockels coefficient; V0 depends on the refractive indices and the geometry of the guides. The coupling efficiency can be controlled by external voltage level applied through the electrode.

The coupling process of the optical switch depends on two parameters, which are the coupling constant, C (depends on the dimension, wavelength and refractive index) and propagation constant difference, is the difference between the refractive index of the waveguides.

The switching operation is done with changing the voltage exercised to the electrodes filed on the integrated Mach-Zehnder interferometer. The light, applied to the (single) input, can be switched from one output to the other by changing this effective refractive index such that the light is reflected or transmitted at the crossing.

DESIGN AND SIMULATION RESULTS

Mach-Zehnder interferometers are constructed in integrated optics and consist of two 3 dB directional couplers interconnected through two interferometric arms of same length .

The switch is created on a z-cut wafer of Lithium Niobate and is surrounded by air cladding.

The device is oriented along the Y–optical axis of the Lithium Niobate. The waveguides of Mach-Zehnder interferometer are created by

diffusion of Titanium in Lithium Niobate substrate. Directional coupler are used to combine and split signals in optical

network .The first coupler splits the input signal in two beams which, when passed through the interferometric arms, experience phase difference caused by voltage variations across electrodes. Finally, both beams with different phases are put together again into a single signal by the second -3dB coupler at the output ports in accordance with constructive or destructive interference.

Firstly, results are obtained by changing the distance between 3 dB coupler from 13µm to 15.5µm at 0v and then compared with conventional switch of which gap is 14.5 µm :-

Extinction Ratio is improved from 8.20 dB to 21.60 dB while at the same time Excess loss and Insertion loss is also reduced.

Fig.3. Variation in Extinction Ratio and Insertion Loss with change in 3dB coupler gap

While at Switching voltage is observed with the change in the gap between 3dB coupler. However, Insertion loss and Excess loss is reduced and Extinction ratio is improved at 15 µm gap as compared to 14.5 µm gap by very small amount.

Fig. 4.Output Power in Port 1 with change in voltage for 3dB coupler gap of 14.5 µm and 15 µm

PARAMETERS OF OPTICAL SWITCH

Figure : 2x2 MZI switch in bar state

•Excess Loss (dB):- This is the ratio of total power output from both the ports to the input port. This is measure in decibels and should be as small as possible.Mathematically Excess Loss is defined as

•Insertion Loss (I.L):- This is the fraction of signal power that is lost because of the switch. This loss is usually measured in decibels and must be as small as possible. In addition, the insertion loss of a switch should be about the same for all input–output connections (loss uniformity).

Mathematically Insertion Loss (I.L) is defined as

•Extinction Ratio (E.R) :-This is the ratio of on-off switch, the output power in the on-state to the output power in the off-state. This ratio should be as large as possible and is particularly important in external modulators. In telecommunications, extinction ratio (re) is the ratio of two optical power levels, of a digital signal generated by an optical source, e.g., a laser diode, where P 1 is the optical power level generated when the light source is "on" and P 2 is the power level generated when the light source is "off" [re= P1 / P2]. The extinction ratio may be expressed as a fraction or in dB. Extinction ratio measurement can be done on an eye diagram also Mathematically Extinction Ratio (ER) is defined as

DESIGN AND SIMULATION RESULTS

COMPARISON OF OPTICAL SWITCHING TECHNOLOGIES

CONCLUSION An optimum performance of 2x2 optical switch based on Mach-

Zehnder interferometer is achieved at 15µm gap between the 3dB coupler instead of 14.5 µm

gap as observed in conventional switches. In the current study the performance is optimized for lower losses and higher extinction ratio. At 0V, optimal performance with extinction ratio of 21.6 dB and excess loss and insertion losses of less than 0.02 dB is observed.

while at switching voltage, extinction ratio of 41.87 dB and insertion losses of less than 0.02 dB is observed for 3dB coupler gap of 15µm.

However, with the change in gap between 3 dB coupler no change in the switching voltage is observed.

The scope of further work lies in design optimization to reduce the switching voltage and further minimization of losses with proper channelling of the signals.

REFERENCES

[1]. Zheng et al., “Design and analysis of a polymer Mach-Zehnder interferometer electro-optic switch over a wide spectrum of 110 nm,” Optical Engineering Vol. 48(5), pp.054601-10, May 2009.

[2]. G.I. Papadimitriou et al., “Optical Switching: Switch Fabrics, Techniques, and Architectures,” J. Lightw. Techno. Vol.21 No.2, pp.384-405, February 2003.

[3]. OptiBPM, Waveguide optics modeling software system, version 8.0, Second edition, Optiwave Inc. 2006.

[4]. G. Singh, V. Janyani, R.P. Yadav, “Modelling of a 2×2 electro-optic Mach–Zehnder Interferometer optical switch with s–bend arms,” Photonics letters of Poland, Vol.3(3), pp.119-121, September 2011 .

[5]. Rajiv Ramaswami, Kumar N. Sivarajan “Optical Networks,” Second edition Elsevier publication.