1. Optical Components/Devices
Chapter 3
Optical Components/Devices

2. OPTICAL FIBER PASSIVE DEVICES: COUPLERS, ATTENUATORS, ISOLATORS, CIRCULATORS, BRAGG GRATINGS AND ATTENUATORS

3. Optical Passive Devices
Passive Components
i. Couplers
ii. Isolators
iii. Circulators
iv. Fiber Bragg Gratings
v. Attenuator

4. Optical Couplers Couplers can perform both combining and splitting.
The devices are widely used in optical LAN and broadcasting networks

5. Optical Couplers
Couplers are bi-directional, they can carry light in either direction.
Used to split and combine the signals.
A coupler with single fiber at one end and two at the other end would be referred to as 1 x 2 Coupler ( read as one by two).
Although 1 x 2 and 2 x 2, are the most common sizes they can be obtained in wide range types up to 32 x 32.

6. Optical Couplers
An optical coupler is a passive (unpowered) device that diverges (1:N) or converges (N:1) optical signals from one fibre or optical signal path to more than one (or vice versa.)
Configurations: Splitters, taps, combiners, directional couplers

7. Optical Couplers Splitting Ratio (Coupling Ratio):-
The proportion of the input power at each output is called the splitting ratio or coupling ratio.
In a 1 x 2 coupler, the input signal can split between the two outputs in any desired ratio. In practice however, the common ones are 90:10 and 50:50. These are also written as 9:1 and 1:1.
In the cases where the splitting ratio is not 1:1, the port which carries the higher power is sometimes called the throughput port and the other is called the tap port.
2X2 Optical coupler

8. Optical Couplers Coupling Tolerance:
Even when the splitting ratio is quoted as 1:1, it is very unlikely, due to manufacturing tolerances that the input power is actually shared equally between two outputs. The acceptable error of between 1% and 5% is called coupling or splitting tolerance.

9. Optical Couplers Losses:-
a. Excess loss
(The Ratio of the input power to the total output power). The light energy has been scattered or absorbed within the coupler and is not available at the output.
b. Crosstalk or directivity
When we apply power to 1, we expect it to come out of port 2 and 3 but not out of port 4, the other input port. Because of backscatter within the coupler, some energy is reflected back and appear at port 4. This backscatter is very slight and is called directionality loss or crosstalk.

10. Optical Couplers c. Insertion loss
Refers to the loss for a particular port-to-port path. For example, for the path from input port i to output port j. This looks at a single output power compared with the input power. There are two possibilities, the power coming out of port 2 and compare it with the input power at port 1 or port 3 compared with input power port 1.

11. Optical Couplers: Characteristics
COUPLING RATIO

12. Optical Couplers EXCESS LOSS
Ratio of the input power to the total output power

13. Optical Couplers
INSERTION LOSS

14. Optical Couplers
DIRECTIVITY

15. OPTICAL COUPLER SPECIFICATION Standard Type The device is capable of branching or combining an optical power having a single wavelength in a designated ratio.
Standard specification
Number of ports
2x2
Wavelength
1310nm, 1480nm, 1550nm
Excess loss
0.2dB or less
Split ratio
50:50 to 95:5 in (%)
Directivity
50dB or better
Fiber type
0.25mm coated fiber, 0.9mm loose-tube fiber, 2.0mm fiber cord
Ambient temperature
-40 to +75 deg.C, or -20 to +60 deg.C (2.0mm fiber coad)
Applicable connector, etc
Length
0.5m, 1.0m, 1.5m, 2.0m
Connector
FC, SC, SC2, MU, None
End-face polishing
Flat, PC, Super-PC

16. WDM Coupler WDM (wavelength division multiplexing) coupler is an optical device capable of wavelength dividing two wavelengths on a single optical fiber into two, or vice versa; i.e., combining two wavelengths on two optical fibers into one.
Standard specification
Number of ports
2x2
Wavelength
980nm/1550nm, 1310nm/1550nm
Insertion loss
Pass port: 0.2dB or less
Cut-off port: 20dB or more
Split ratio
50:50 to 95:5 in (%)
Directivity
50dB or better
Fiber type
0.25mm coated fiber, 0.9mm loose-tube fiber, 2.0mm fiber cord
Ambient temperature
-40 to +75 deg.C, or -20 to +60 deg.C (2.0mm fiber coat)
Applicable connector, etc
Length
0.5m, 1.0m, 1.5m, 2.0m
Connector
FC, SC, SC2, MU, None
End-face polishing
Flat, PC, Super-PC

17. Optical Fibre Connectors

18. Optical Fibre Connectors

19. Calculate the output power at port 3?
Example: 1
Calculate the output power at port 3?
Sol:

20. Example 2:
A product sheet for a 2x2 single-mode biconical tapered coupler with a 40/60 splitting ratio states that the insertion losses are 2.7 dB for the 40-percent channel.
If the input power Po =200 µw , compute P1 and P2
Evaluate the excess loss
From the calculated values of P1 and P2, verify that the splitting ratio is 40/60.
Sol:

21. ISOLATORS A B To allow light to propagate in one direction only P0 P2

22. ISOLATORS P0 P3 P2 P1 A B

23. ISOLATOR SPECIFICATION

24. CIRCULATORS Optical circulators redirects light sequentially
from port-to-port in a unidirectional path 1 2 3
Same characteristics as isolators
by looking port port 2-3
To extract the desired wavelength, a circulator is used in conjunction with the rating

25. CIRCULATORS: WORKING PRINCIPLE

26. CIRCULATORS Characteristics: high isolation low insertion loss
can have more than 3 ports
Applications:
Optical Amplifier
Add-Drop Multiplexer
Bi-directional transmission
To monitor back-reflection
from devices or optical
subsystems

27. CIRCULATORS: APPLICATION

28. Fiber Bragg Gratings
A grating is a periodic structure or perturbation in a material
that creates a property of reflecting or transmitting light in a certain
direction depending on the wavelength.
External writing technique using UV light l2

29. Fiber Bragg Gratings l2 l1 l3
Reflection
Transmission

30. Fiber Bragg Gratings

31. Fiber Bragg Gratings
                                                  
Figure 2: FBGs reflected power as a function of wavelength
The reflected wavelength (λB), called the Bragg wavelength, is defined by the relationship,
              ,
where n is the average refractive index of the grating and Λ is the grating period.

32. Fiber Bragg Gratings Characteristics:
high reflectivity to be used as a filter
low insertion loss
low cost/simple packaging

33. Fiber Bragg Gratings
Transmission spectrum
band-rejection filter l2

34. Fiber Bragg Gratings
Reflection spectrum
reflective filter

35. FBG APPLICATIONS

36. FBG APPLICATIONS
Gain flattening filter + =

37. FBG APPLICATIONS
Laser diode wavelength stabilizer

38. FIBER BRAGG GRATING SPECS.

39. ATTENUATORS Function: To decrease light intensity (power)
Working Principles
Fiber displacement
Rotating an absorption disk

40. ATTENUATORS Programmable attenuator Set @ 60 dB Insertion loss = 2 dB
Pin = 0 dBm
Insertion loss = 2 dB
Pout = = -22 dBm
Programmable attenuator

41. ATTENUATORS Characteristics: low insertion loss
dynamic attenuation range
wide range of operating wavelength
high return loss
Applications:
adjust optical power to the dynamic range of receivers
equalize power between different WDM signals
To avoid receiver saturation

42. ATTENUATORS Mechanical attenuator - by adjusting a screw
Waveguide attenuator - by adjusting biasing current