Dr. Ghanshyam Singh Associate ProfessorDepartment of Electronics and Communication Engineering Malaviya National Institute of Technology Jaipur Rajasthan-India http/www.mnit.ac.in **

Contents:

Introduction: Optical Technologies

Photonic Switching: Basics & Applications

Photonic Switches: Types & Architectures

Large Switch Architectures

Design Examples

Conclusion & References

**

**Requirements for next gen. IP-based applications:

Higher Bandwidth & high-capacity signal processing in the optical networks.

Power consumption seriously limits scaling up the signal transmission and processing capability based on electronic circuits.

Low power consumption is, therefore, a very important driving force to develop power-efficient photonic signal-processing techniques for the applications to the future telecommunication networks and information-processing systems.

Development of novel power-efficient, ultrafast, optoelectronic, as well as all-optical signal-processing devices. Introduction

**(Estimated power consumption by the Internet routers in Japan)

Solid circles are the assumed drive voltage for large-scale integration in routers. The percentages show the proportion to the total power generation in Japan.

[A] estimated if one-third of the world population subscribe internet access with a access rate of 10 Mbs, Then total power consumption necessary for the internet traffic (predominantly consumed by routers), would be equal to 58% of the total electric power being generated throughout the world.

[A] J. Baliga, K. Hinton, and R. S. Tucker, Energy consumption of the Internet, presented at the Joint Int. Conf. Opt. Internet, 2007, and the 32nd COIN-ACOFT 2007.

**

**Benefits High speedhigh bandwidth: Potential bandwidth of one optical fiber exceeds several tens Tbps (note that the bandwidth of copper cable is only a few Mbps).

Immunity: Optical signal is virtually immune to all kinds of interference.

Security: Optical signal is ideal for secure communications because it is very difficult to tap into.

Lower cost: Optical networks, based on the emergence of the optical layer in transport networks, can provide higher capacity and reduced costs for new applications such as the Internet, video and multimedia interaction, and advanced digital services.

The Milestones of Optical Networking Technologies1958: Laser discovered

Mid-60s: Guided wave optics demonstrated

1970: Production of low-loss fibersMade long-distance optical transmission possible!

1970: invention of semiconductor laser diodeMade optical transceivers highly refined!

70s-80s: Use of fiber in telephony: SONET**

Mid-80s: LANs/MANs: broadcast-and-select architectures

1988: First trans-atlantic optical fiber laid

Late-80s: EDFA (optical amplifier) developedGreatly alleviated distance limitations!

Mid/late-90s: DWDM systems explode

Late-90s: Intelligent Optical networks

2000 onwards . O O O processing

**

Recent optical devices created:Optical logic gates, optical switchesOptical interconnections, optical memory

Switching device performanceSpeeds of 10-15 secondsPower requirements one millionth of a watt

The big limit:

Lack of efficient nonlinear material that can respond at low power levels

**

**

**Enables routing of optical data signals without the need of opto-electronic conversion and regeneration steps

Independent of bit rate and protocols with unlimited scalability, which leads to more flexibility in the network.

An increase in the switching speed, and network throughput, reduction in the network equipment, operating power and overall system cost.

Lack of processing at bit level and the lack of efficient buffering in the optical domain.22 switch block with switching states: the "bar" state; and the "cross" state

**Important parameters of a Switch Fabrics:

Insertion loss: fraction of signal power that is lost because of the switch, usually measured in dB, must be as small as possible and loss uniformity should be there.

Extinction ratio: ratio of the output power in the on-state to the output power in the off-state. This ratio should be as large as possible.

Polarization-dependent loss: should be as small as possible.

Crosstalk: ratio of the power at a specific output from the desired input to the power from all other inputs

Also..

Switching timeReliabilityEnergy usageScalabilityTemperature resistance.

Si, SOI, InP, InGaAsP, InGaAs, AlGaInAs, LiNbO3 , Ti: LiNbO3 etc.

Application:

Optical Cross-Connects (OXCs)Protection Switching.Optical Add/Drop Multiplexing.Optical Signal Monitoring.

**

Provisioning of light paths. the switches are used inside OXCs to reconfigure them to support new light paths.

OXCs groom and optimize transmission data paths .

Optical switch requirements for OXCs are :

Scalability, high-portcount switches.High reliability, low loss.Good uniformity of optical signals independent of path length,No disruption in other optical paths.

**

Advantages: Switch data without any conversions to electrical form.Independent of data rate and data protocol.Reductions in cost, size, and complexity.

Disadvantages:

Lack of memory and bit processingDo not allow signal regeneration with retiming and reshaping.

**

Allows the completion of traffic transmission in the event of system or network-level errors.

Requires optical switches with smaller port counts of 1x2 or 2x2.

Requires switches to be extremely reliable, since sometimes these switches are single points of failure in the network .

Relax the requirements on the switching speed.

**

OADMs nodes insert (add) or extract (drop) optical channels (wavelengths) to or from the optical transmission stream.

Using an OADM, channels in a multi-wavelength signal can be added or dropped without any electronic processing.

Switches that function as OADMs are wavelength-selective switches, i.e. can switch the input signals according to their wavelengths**

Used in WDM system, where OSM switch monitors each channels optical spectra for wavelength accuracy, optical power levels, and optical crosstalk.

Size of the optical switch is chosen based on the system wavelength density and the desired monitoring thoroughness. Optical switch employed should have a high extinction ratio (low interference between ports), low insertion loss, and good uniformity.

**

Occurs when new data routes have to be established or existing routes need to be modified.

High-capacity reconfigurable switches that can respond automatically and quickly to service requests can increase network flexibility, and thus bandwidth and profitability.

A network switch should carry out reconfiguration requests over time intervals on the order of a few minutes.

**

**

Mechanical and Non-mechanical

Mechanical switches:

switching can be achieved with the help of moving optic fibers or optic elements using mechanical or electro-magnetic means.

Relatively slow, useful for routing of an optical transmission path, such as routing around a fault

Non mechanical switches:

Switches based on electro-optic, thermo-optic, acousto-optic effects and SOA based switches.

Perform fast switching and are best suited for modern network applications and logic operations.

Switching function is performed by some mechanical means, such as prisms, mirrors, and directional couplers etc.

Mainly used in fiber protection and very-low-port-count wavelength add/drop applications.

Advantage: low insertion losses, low polarization-dependent loss, low crosstalk, and low fabrication cost.

Dis. Adv.: Lack of scalability, Switch configurations are limited to 1 X 2 and 2 X 2 port sizes. e.g. : Micro electro-mechanical Devices (MEMs) Uses tiny reflective surfaces to redirect the light beams to a desired port.

**

Microscopic mirrors arranged in a crossbar configuration.Mirror position is bi-stable, hence switches are digital in nature.

When a mirror is activated, it moves into thepath of the beam and directs the light to oneof the outputs.

Can be used for adding or dropping opticalChannels.

**MEMs switching technology

Uses a directional coupler whose coupling ratio is changed by varying the refractive index of the material (usually LiNbO3) in the coupling region.

The change in the index of refraction manipulates the light through the appropriate waveguide path to the desired port.

Fast and reliable switch.

High insertion loss and possible polarization dependence and higher driving voltage.

**

Switching is performed by variation of the refractive index of a dielectric material, due to temperature variation of the material itself.

Two categories :InterferometricDigital optical switches

Generally small in size and high-driving-power characteristics.

Allows the integration of variable optical attenuators and wavelength selective elements on the same chip with the same technology.

Limited integration density and high-power dissipation.Require forced air cooling for reliable operation.

**

Based on MachZehnder interferometers.

A 3-dB coupler splits the signal into 2 beams,which then travel through two distinct arms ofsame length, and a 2nd 3-dB coupler merges and finally splits the signal again.

Phase difference between the light beams,is varied by heating one arm of the interferometer.

As interference is constructive or destructive, the power on alternate outputs is minimized or maximized.

**

Integrated optical devices generally made of silica on silicon.Light propagates through two interacting waveguide arms.The phase error between the Beams at the two arms determines the output port.

**

Based on the change of polarization state of incident light by a liquid crystal as a result of the application of an electric field over the liquid crystal.

Liquid-crystal switches have no moving parts.

Very reliable and satisfactory optical performance.

Can be affected by extreme temperatures if not properly designed.

**

Based on the interaction between sound and light.

Input signal is split into its two polarized components (TE and TM) and directed to two distinct parallel waveguides.

Through an acousto-optic effect in the material, this forms the equivalent of a moving grating, which can be phase-matched to an optical wave at a selected wavelength.

Phase matched signal is lower and non matched is upper output.

The switching speed of acousto-optic switches is limited by the speed of sound and is in the order of microseconds.

**

**

Versatile devices.

Can be used as an ONOFF switch by varying the bias voltage.

If the bias voltage is reduced, the device absorbs input signals. otherwise it amplifies the input signals.

Capable of achieving very high extinction ratios.

Larger switches can be fabricated by integrating SOAs withpassive couplers.

**

**

CrosstalkPath delayCoupling power lossesBlocking featureRearrangeability On-chip viability

**

Crossbar ArchitectureWide-sense non-blocking. NxN switch requirement: N2

Shortest path length = n

longest = 2n-1

(bad loss uniformity)

Fabricated w/o any crossovers

**

Spanke Architecture Strict-sense non-blocking Only 2 stages: 1xn and nx1 switches used instead of 2x2 Switch cost scales linearly with n Lower insertion loss and equal optical path lengths (2log2N) Requires 2N(N-1), 1x2 switches**

Benes Architecture Rearrangeably non-blocking, An NxN Benes switch requires [(N/2)x( 2log2N -1)] numbers of 2x2 switches, N is power of 2. Efficient in number of 2x2 components Network depth 2logN, requires waveguide crossovers**

Spanke-Benes Architecture Rearrangeably non-blocking, requires n(n-1)/2 switches. Efficient in number of 2x2 components. Shortest path length: N/2 & Longest path Length: N Eliminates waveguide crossovers: n-stage planar**

Banyan Architecture Self-routing capability.Small depth* (log2N).Low loss and low attenuation.Absolute loss uniformity. Butterfly interconnection pattern

1616 optical banyan

* NxN banyan switch consist of Log2N stages, each stage contains N/2, 2x2 switches.

**

**

MZI structureConvert a phase modulation into an intensity modulation, Used for variety of applications such as optical modulators, splitters, switches.

Preferred to realize optical switches with silica, polymers, III-V semiconductors and their composites.

Monolithically integrated SOA MZI structures facilitate better control on switching characteristics due to their compact size, thermal stability and low power operation.

Switching in MZI structure

MZI switch with path delay element Layout of (a) MZI structure and (b) Path delay with unequal interferometric arms.

Electro-optic MZI SwitchFig. 11 EO-MZI switch with equal interferometric arm lengths, ref. [1]First coupler divides the light evenly in two parts, which when passed through the interferometric arms experience a net phase change of .

This phase difference is due to a pushpull effect caused by the field applied in opposite directions through the waveguides under the electrodes [79].

The output intensity is periodic with minima and maxima occurring at odd and even integer multiples of applied voltage.

Recombined power by accumulation of a phase difference:

Performance stability?Study of the effect of high power levels on the characteristic of LN MZI based modulator done experimentally.

Important performance parameters are stable for waveguide powers of 75mW

A. R. Beaumont, C. G. Atkins, R. C. Booth Optically induced drift effects in Lithium Niobate electrooptic waveguide devices operating at a wavelength of 1.51m Electronics Letters, vol. 22 no. 23, pp.12601261, Nov. 6th, 1986.

MZI and Asymmetric Y Junctions based polymer EO switchSwitching voltage: 15 V, branching angle (2): 0.1o 0.4o, evaluated for TM polarized light at 1.3m, CT levels: - 27 to 22dB, I.L.: 2 dB

Optimization of waveguide structure and electrode regions is not done, which could lead to reduction in switch voltage requirements and insertion losses.

W. Y. Hwang, M. C. Oh et al. Polymeric 22 EOswitch consisting of asymmetric Y junctions and MZI IEEE Photonics Tech. Lett., vol. 9, no. 6, pp.761763, June 1997.

Polymer based MZI EO switch using two phase generating (PGC) couplers

C.T. Zheng, C. S. Ma, X. Yan, D.M. Zhang Design of a spectrumexpanded polymer MZI electrooptic switch using twophase generating couplers Applied Physics B, Laser and Optics, pp. 110, SpringerVerlag, 2010.

Sine-type bending waveguides are used to create the path-length difference

Optimization parameters: electrode length, width and the gap

EO region length is 5mm, spectrum 1380 1730 nm, switching voltage required: 0.925 V @ 1.55 m, CT: 30 dB.

However, I.L. is higher 5 dB (introduction of two PGC causes longer waveguide length)

Ti LN MZI optical switchSwitching voltage required: 12 V (1.3 m) and 13.4 V (1.55 m), which goes increasing with increase in separation distance. Optimized separation distance (24 m) with length of electrode (10mm)

The change of wavelength is directly proportional to the switching voltage E. H .M Yahya MachZehnder interferometer Thesis, Faculty of Electrical Engineering, University of Technology Malaysia, April, 2007.

MZI Switch Design Guidelines

Modelling of MZI switch Fig. 13 Sectional diagram of the MZI structure (b) S-bend with transition length (l) and lateral offset (h)The guiding channel (8.0 m) with a bell shaped refractive index in the form of MZI structure is created by Ti indiffused LN.

The resulting refractive index distribution can be further classified into ordinary and extra ordinary type, which strongly depends on polarization and crystal cut [75].

The resultant change in the refractive index is a function of parameters like Tistrip thickness (ts), lateral diffusion length (Dh), vertical diffusion length (Dv) and other process parameters.

MZI structure dimensions = 33mm (length) 0.1mm (width, inclusive of electrode regions)Guiding channel width (Cw) across all sections of the structure= 8 mS-bend waveguidesS1: Transition length (l) = 5750 m, Lateral offset (h) = 12.75 mS2: Transition length (l) = 2500 m, Lateral offset (h) = 8.75 mLength of coupling section (Lc) = 3250 m and interferometric arms (LINF) = 10 mm, Interferometric arms spacing : 24 m

**Loss variation (dB/mm) v/s ts (m) for straight waveguidesLoss variation (dB/mm) v/s ts (m) for curved (s-bend) waveguides

Fig. 18 In and Output definitions of both couplers (3dB splitter and 3dB combiner)The imbalance factor (non-uniformity) and the generated CT levels due to Q1 and Q2 are calculated as follows [5]. Imbalance at the output of 1st 3-dB splitterThe CT levels at the end of interferometric arms, prior to 2nd 3-dB combiner. Q1 and Q2 are power levels at the end of interferometric arms Power imbalance of first stage 3-dB coupler

Calculated CT levels due to variation in power imbalance and variations in ts, switch state: cross

Operating wavelength (m)Optimized Ti-strip thickness (m)Min. req. Switch volt. (V)CT levels (-ve dB)I.L. (dB)E.L. (dB)Bar stateCross stateBar stateCross stateBar stateCross stateTi-indiffused z-cut LN based MZI switch with path delay (tapered) arm 1.30.054Not req.22.5641.730.230.0180.190.0171.550.0825Not req.21.2239.660.490.0060.340.005Ti-indiffused z-cut LN based EO-MZI switch with buffer layer1.30.0547.229.6841.730.0230.0180.0190.0171.550.08258.2532.9939.660.0080.0060.0060.005Ti-indiffused z-cut LN based EO-MZI switch without buffer layer1.3 0.0544.630.9941.730.0230.0180.0190.0171.55 0.0825523.8939.660.0240.0060.0060.005Ti-indiffused x-cut LN based EO-MZI switch with buffer layer1.3 0.05410.529.7717.200.0330.0980.0280.0141.55 0.08251230.8815.620.0110.0120.0070.007Ti-indiffused x-cut LN based EO-MZI switch without buffer layer1.3 0.0544.730.8917.200.0320.0980.0280.0141.55 0.08255.522.3415.620.0320.0120.0070.007

Reconsideration of the architecture of basic switch elements.

The basic switch architecture incorporated and several aspects.

New Designs, simulations with guiding algorithm for higher order switch architectures to come up with results practically viable to the recent demand.

**

Incorporation of grated switches as basic switch elements and their respective performance analysis.Analysis of and improvement in noise resistibility of the particular architectures and the proposed ones.Photonic crystals as a building material for our basic switch element.FDTD simulations to establish working principles for the basic switch elements.

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Directions of development:Same architecture with optical components (realistic)All functions in optical mode (ambitious)Use of Surface Plasmon's to develop Switches to be incorporated with system on-chip (SOC)

Many new possibilitiesHigh performance computing, high speed communications, parallel algorithmsOptics and its implications will have a big affect on the future of computingPossibly big changes to software development

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Optical Switching by G.I Papadimitriou, C. Papazoglon and A.S Pomportsis, Wiley series in Microwave & Optical Engg.

Optical components for communications by Ching-Fuh Lin, Kluwer Academic Publishers.

Photonics by Ralf Menzel, Springer International Edition.

Optical waveguides by M.l Calvo, V.Lakshminarayanana, CRC press, Taylor & Fransis group.

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Thank You All !!!!

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