1. ENG735 – COMUNICAÇÕES ÓPTICAS
CHAVEADORES ÓPTICOS
Prof. Dr. Vitaly F. Rodríguez-Esquerre

2. Purely electronic (present situation)
Switching is the process by which the destination of a individual optical information signal is controlled
Types of Optical Switching
Space Division Switching
Wavelength Division Switching
Time Division Switching
Hybrid of Space, Wavelength and Time
Switch control may be:
Purely electronic (present situation)
Hybrid of optical and electronic (in development)
Purely optical (awaits development of optical logic, memory etc.)

3. Optical Switching Element Technologies
Liquid
High Loss
Crystal
Gel/oil based
Not Scalable
Polarization Dependent
LiNbO 3
Mechanical
Poor Reliability
Indium
Phosphide
Optical
Switching Element Technologies
SiO 2 / Si
SOA
Micro-Optic
Fibre (
acousto
-optic)
(MEMS)
Thermo-
optic
Bubble
Can be configured in two or three
dimensional architectures
Waveguide
Free Space

4. Electro-optic Switch Use a directional coupler
Its coupling ratio is changed by varying the refractive index
Thermo-optic Switch
Liquid-Crystal Switch
Bubble Switch
Acousto-optic Switch

6. Two axis motion
Micro mirror

7. 2D MEMS based Optical Switch Matrix SEM photo of 2D MEMS mirrors
Output fibre
Input fibre
Mirrors have only two possible positions
Light is routed in a 2D plane
For N inputs and N outputs we need N2 mirrors
Loss increases rapidly with N
SEM photo of 2D MEMS mirrors

8. 3D MEMS based Optical Switch Matrix SEM photo of 3D MEMS mirrors
Mirrors require complex closed-loop analog control
But loss increases only as a function of N1/2
Higher port counts possible
SEM photo of 3D MEMS mirrors

9. Total Internal Reflection LC Switch

10. Liquid crystal (Total internal Reflection)
The glass and nematic liquid crystal refractive indices are chosen to be equal in the transmittive state and to satisfy the total reflection condition in the reflective state
Schematic diagram of the total reflection switch: 1- glass prisms; 2- liquid crystal layer; 3-spacers

11. Optomechanical Switch
MEMS
Electro-optic
Switching time
Milliseconds
Nanoseconds
Insertion loss
Low
High
PDL
high
Scalability
Bad
Good

12. Optical Number of ports Electrical Data rate
1024 32 64 16 8
512
256
128
Optical
10 MHz
DS3
100 MHz
OC3
OC12
1 GHz
OC48
OC192
100 GHz
Electrical
10 GHz
Data rate
Electrical Limits
High power consumption: e.g. 1024x1024: 4 kW
Jitter: very large
Large switches
Need OE/EO conversion
Bipolar or GaAs
Provide fast switching speed
No bottleneck due to electronics speed
I/O interface and switching fabric in optics
Switching control and switching fabric in optics
Uses a simple 2x2 switch as a building block

13. Switching control Input interface Output Switching fabric Optical
Electrical control
Optical
input
output
Electrical control
Switching control
Input
interface
Output
Switching
fabric

14. Optical Switches: - A comparison
Characteristic
Traditional Optical Switches
Next Generation Optical Switches
Switching Speed
>1ms
<1µsec
Multicast
Not available
Dynamic power partition between ports
Integrated VOA functionality
High dynamic range VOA
Reliability
~10 Million cycles (Mech.dev.)
~10 Billion cycles (Opto-elect.)
Insertion loss
Low
Cross talk
High
Scalability
Medium-High

15. Optical Switches - Tow-Position Switch
Input
port Ii
Output
ports I1 I2
Control Signal
The input signal can be switched to either of the output ports without having any access to the information carried by the input optical signal
In the ideal case, the switching must be fast and low-loss.
100% of the light should be passed to one port and 0% to
the other port.

16. Two Position Switch - contd.
The two-position switch requires three fibres with collimating lenses and a prism. B A C
Lens
Fibre
Prism
Light arriving at port A needs to be switched to port C. B A C

17. Optical Switches - Applications
Provisioning: Used inside optical cross connects to reconfigure them and set-up new path. [ msecs]
Protection Switching: To switch traffic from a primary fibre onto another fibre in the case of a failure. [1 to 10 usecs]
Packet Switching: 53 byte 10 Gb/s. [1 nsecs]
External Modulation: To switch on-off a laser source at a very high speed. [10 psecs << bit duration]
Network performance monitoring
Reconfiguration and restoration: Fibre networks

18. Optical Switching - Technologies
Slow Switches (msecs)
Free space
Mechanical
Solid state
Fast Switches (nsecs)
LiNbO
Non-linear
InP

19. Optical Switches - Criteria
Maximum Throughput:
Total number of bits/sec switched through.
To increase throughput:
Increase the number of I/O ports
Bit rate of each line
Maximum Switching Speed
Important:
Packet switched
Time division multiplexed
Minimum Number of Crosspoints
As the size of the switch increases, so does the number of crosspoints, thus high cost
Multistage switching architecture are used to reduce the number of crosspoints.

20. Criteria - contd.
Minimum Blocking Probability: Important in circuit switching
External blocking: when the incoming call request an output port that is blocked.
Subject to external traffic conditions
Internal blocking: when no input port is available.
Subject to the switch design
Minimum Delay and Loss Probability
Important in packet switching, where buffering is used, which will introduce additional delay.
Scalability
Replacing an old switch with a new larger switch is costly.
Incrementally increasing the size of the existing switching as traffice grows is desirable
Broadcasting and Multicasting
To provide conferencing and multimedia applications

21. Optical Switches - Types
Waveguide
Electro-optic effect
Semiconductor optical amplifier
LiNbO
- InP
Thermo-optic effect
- SiO2 / Si
- Polymer
Free Space
- Liquid crystal
- Mechanical / fibre
- Micro-optics (MEM’s)
- Fast
- Complex
- Maturing
- Lossy
- Slow
- Maturity
- Reliable
- Slow
- Low loss & crosstalk
- Inherently scalable

22. Optical Switches - Thermo-Optic Effect
Some materials have strong thermo-optics effect that could be used to guide light in a waveguide.
The thermo-optic coefficient is:
Silica glass dn/dt = 1 x 10-5 K-1
Polymer dn/dt = -1 x 10-5 K-1
Difference thermo-optic effect results in different switch design.
+ v
Electrodes

23. Thermo-Optic Switch - Silica
Mach – Zehnder Configuration
Analogue I1 I2
Outputs
Input Ii
Heater
Directional coupler

24. Thermo-Optic Switch - PolymerIi I1 I2
PH1
PH2
Y – Junction Configuration
Digital
If PH1 = PH2 = 0, then I1 = I2 = Ii /2
If PH1 = Pon & PH2 = 0, then I1 = 0, and I2 = Ii
If PH1 = 0 & PH2 = Pon, then I1 = Ii, and I2 = 0

25. Thermo-Optic Switch - Characteristics15
S/W power (W) ~4 ~
S/W time (ms) 13
Crosstalk 18
Insertion Loss (dB)
256
No. of S/W
16 x 16 Si
8 x 8
Si Poly.
2 x 2
Si Poly.
Switch Size
Parameters

26. Mechanical Switches 1st Generation – Mid. 1980’s
Loss Low (0.2 – 0.3 dB)
Speed slow (msecs)
Size Large
Reliability Has moving part
Applications: - Instrumentation
- Telecom (a few)
Size: 8 X 8
Loss: 3 dB
Crosstalk: 55 dB
Switching time: 10 msecs

27. Micro Electro Mechanical Switches
Input fibres
Output fibres
Lens
Flat mirror
Raised mirror
Made using micro-machining
Free-space: polarisation independent
Independent of:
Bit-rate
Wavelength
Protocol
Speed: 1 10 ms
4 x 4 Cross point
switch

28. Micro Electro Mechanical Switches
This tiny electronically tiltable mirror
is a building block in devices such
as all-optical cross-connects and new types of computer data projectors.
Lightwave

29. Switching Fabric – contd.
...
Optical transport system
(1.55 mm WDM)
1.3 mm intra-office
Optical
Crossconnect
(OXC)
Transponders
Terminating equipment |
SONET, ATM, IP...

30. Space Division Switching
Crossbar
Clos
Benes
Spank - Benes
Spanke

31. Crossbar Architectures
Each sample takes a different path through the switch, depending on its destination
Crossbar:
Simplest possible space-division switch
Wide- sense blocking: When a connection is made it can exclude the possibility of certain other connections being made
Crosspoints
can be turned on or off
Input
ports
Output ports 1 2 3 4
Sessions: (1,4) (2,2) (3,1) (4,3)

32. Crossbar Architectures - Blocking1 2 3 4
Input channels
Output channels - Bars
Output channels - Cross
N X N matrix S/W
M inputs x N outputs
Switch configuration: “set of input-output pairs simultaneously connected” that define the state of the switch
For X crosspoints, each point is either ON or Off, so at most 2X different configurations are supported by the switch.
Case 1:
- (3,2) ok
- (4,3) blocked
Optical
switching
element

33. Crossbar Architecture - Wide-Sense Non-blocking
Rule: To connect ith input to the jth output, the algorithm sets the
switch in the ith row and jth
column at the “BAR” state and
sets all other switches on its
left and below at the “CROSS”
state. 1 2 3 4
Input channels
Output channels
Case 2:
- (2,4) ok
(3,2) ok
(4,3) ok

34. Crossbar Architectures – 2 Layer
Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81
Penalty is loss of connectivity
3x3 2 5
Inputs
Outputs 1 4 N

35. Crossbar Architectures - 3 Layer1 1 2 2 3 3 4 4 5
Output ports
Input port 5 6 6 7 7 8 8 9 9
Blocking still possible

36. Crossbar Architectures - 3 Layer
Blocking 1 1 2 2 * 3 3
The first four connections have made it impossible for 3rd input to be connected to 7th output 4 4 5 5 6 6 7 7 * 8 8 9 9
The 3rd input can only reach the bottom middle switch
The 7th output line can only be reached from the top output switch.

37. Crossbar Architecture - Features
Architecture: Wide Sense Non-blocking
Switch element: N2 (based on 2 x 2)
Switch drive: N2
Switch loss: (2N-1).Lse +2Lfs
SNR: XT – 10log10(N-1)
Where XT: Crosstalk (dB),
Lse: Loss/switch element
Lfs: Fibre-switch loss

38. Crossbar Architecture - Properties
Advantages:
simple to implement
simple control
strict sense non-blocking
Low crosstalk: Waveguides do not cross each other
Disadvantages
number of crosspoints = N2
large VLSI space
vulnerable to single faults
the overall insertion loss is different for each input-output pair: Each path goes through a different number of switches

39. Time-Space Switching Arch.
MUX 1 2 3 4
2 1
3 4
TSI
4 3
time 1
Each input trunk in a crossbar is preceded with a TSI
Delay samples so that they arrive at the right time for the space division switch’s schedule
Note: No. of Crosspoints: 4 (not 16)

40. Time-Space Switching Arch.
Can flip samples both on input and output trunk
Gives more flexibility => lowers call blocking probability
TSI
Complex in terms of:
- Number of cross points
- Size of buffers
-Speed of the switch bus (internal speed)

41. Clos Architecture kxk nxp pxn Stage 1 Stage 3 Stage 2 Stage 1 (nxp)32 64 33
N= 1024
993 n
It is a 3-stage network
- 1st & 2nd stages are fully
connected
- 2nd & 3rd stages are fully
- 1st & 3rd stages are not
directly connected
Defined by: (n, k, p, k, n)
e.g. (32, 3, 3, 3, 32)
(3, 3, 5, 2, 2,)
Widely used
Stage 1 (nxp)
Stage 2(kxk)
Stage 3 (pxn)

42. Clos Architecture In this 3-stage configuration N x N switch has:
2pN + pk2 crosspoints (note N = nk)
(compared to N2 for a 1-stage crossbar)
If n = k, then the total number of crosspoints = 3pN, which is < N2 if 3p < N.
Problems:
Internal blocking
Larger number of crossovers when p is large.

43. Clos Architecture – Blocking
If p < 2n-1, blocking can occur as follows:
Suppose input 1 want to connect to output 1 (these could be any fixed input and outputs.
There are n-1 other inputs at k-switch (stage 1). Suppose they each go to a different switch at stage 2.
Similarly, suppose the n-1 outputs in the first switch other than output 1 at the third stage are all busy again using n-1 different switches at stage 2.
If p <  n-1 + n-1 +1 = 2n-1 then there will be no line that input 1 can use to connect to output 1.

44. Clos Architecture – Blocking
If p = 2n -1, then
Total Switch Element: 2kn(2n-1) + (2n -1)k2
Since k = N/n, therefore
the number of switch elements is minimised when
n ~(N/2) 0.5.
Thus the number switch elements =
4 (2)0.5 N3/2 – 4N,
which is less than N2 for the crossbar switch

45. Clos Architecture – Non-blocking
If p  2n -1,
the Clos network is strict sense non-blocking (i.e. there will free line that can be used to connect input 1 to output 1)
If p  n,
then the Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches)

46. Clos Architecture – Example
If N and n are equal to 100 and 10, respectively, then
the number of switches at the 1st & 3rd stages are N/n = 1000/10 = 100.
at the 1st stage, they are 10 x p switches
at the 3rd stage they are p x 10 switches.
the 2nd stage will have p switches of size 100 x 100.
If p = 2n-1=19, then the resulting switch will be non-blocking.
If p < 19, then blocking occurs.
For p =19, the number of crosspoints are given as follow:-

47. Clos Architecture – Example contd.
In the case of a full 1000 x 1000 crossbar switch, no blocking occurs, requiring 106 crosspoints.
For n = 10 and p = 19,
each switch at stage 1 is a 10 x 19 crossbar requiring 190 crosspoints and there are 100 such switches. Same for the third stage. So the 1st & 3rd stages use 2x190x100 = 38,000 crosspoints altogether.
The 2nd stage consists of p = 19 crossbars each of size 100 x 100, because N/n = 100. So stage 2 uses 190,000 crosspoints.
Altogether, the Clos construction uses 228,000 crosspoints Vs. the 106 points used by the complete crossbar.

48. Clos Architecture – Example contd.
Since k = N/n, therefore the number of switch elements is minimised when = n ~(N/2)0.5 = (500) 0.5 =~ 32
We would then use 44 switches in the 1st & 3rd stages and
p = 2n-1= 2x23 – 1 = 45.
Since n = 23 does not divide 1000 evenly, thus we actually have 12 extra inputs and outputs that we could switch with this configuration ( 23x44=1012 and = 12).
So we use
2x23x44x45=91,080 crosspoints in the 1st & 3rd stages
and an additional 44x44x45=87,120 crosspoints in the 2nd stage.
Thus the total number of crosspoints in the best Clos construction involves fewer than 180,000 crosspoints for a non-blocking switch as compared with the 1,000,000 for the complete crossbar and about 190,000 for n = 10. This is a factor of over 11 less equipment needed to switch 1000 customers!

49. Benes Architecture
2  2
N/2  N/2
Benes N
NxN switch (N is power of 2) RNB built recursively from Clos network:
1st step Clos(2, N/2, 2, N/2, 2)
Rearrangably non-blocking (RNB)

50. Benes Architecture - contd.
Number of stages = 2.log2N - 1
Number of 2x2 switches /each stage = N/2
Total number of crosspoints ~N.(log2N -1)/2
For large N, total number of crosspoint = N.log2N
Benes network is RNB (not SNB) and so may need re-routing:
Modular switch design
Multicast switches can be built in a modular fashion by including a copy module in front of the point-to-point switch

51. Benes Architecture - contd.1 2 3 4 5 6 7 8 X
e.g. Connection sequence
4 to 2 Fails
2 to 1
1 to 5
3 to 3
Note there is no way 4 to 2 connection could be made

52. Benes Architecture –Non-blocking contd.
Now use different connections
e.g.
4 to 2 OK
2 to 1
1 to 5
3 to 3