Chapter 3Optical Components/Devices

OPTICAL FIBER PASSIVE DEVICES:

COUPLERS, ATTENUATORS, ISOLATORS, CIRCULATORS, BRAGG GRATINGS AND

ATTENUATORS

Optical Passive Devices

Passive Components

i. Couplers

ii. Isolators

iii. Circulators

iv. Fiber Bragg Gratings

v. Attenuator

Optical Couplers

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

networks

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.

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

Optical CouplersSplitting 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

Optical CouplersCoupling 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.

Optical CouplersLosses:- 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.

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.

Optical Couplers: Characteristics

• COUPLING RATIO

Optical Couplers

• EXCESS LOSS– Ratio of the input power to the total output power

Optical Couplers

• INSERTION LOSS

Optical Couplers

• DIRECTIVITY

OPTICAL COUPLERSPECIFICATION

Standard TypeThe 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

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 lessCut-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

Optical Fibre Connectors

Type Typical Insertion Loss (dB) Fibre TypeBICONIC 0.6 - 1.0 SM, MM

D4 0.2 - 0.5 SM, MMEC/RACE 0.1 - 0.3 SMESCON 0.2 - 0.7 MM

FC 0.5 - 1.0 SM, MMFDDI 0.2 - 0.7 SM, MM

HMS-10 0.1 - 0.3 SMSC 0.2 - 0.4 SM, MM

SC DUPLEX 0.2 - 0.4 SM, MMSMA 0.4 - 0.8 MMST 0.4 (SM) 0.5(MM) SM, MM

Optical Fibre Connectors

Example: 1

Calculate the output power at port 3?

Sol:

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.a)If the input power Po =200 µw , compute P1 and P2b)Evaluate the excess lossc)From the calculated values of P1 and P2, verify that the splitting ratio is 40/60.

Sol:

P0

P2

P1

A B

•To allow light to propagate in one direction only

ISOLATORS

3

2log10P

PIsolation

1

0log10lossInsertion P

P

P0

P3P2

P1

A B

ISOLATORS

ISOLATOR SPECIFICATION

CIRCULATORS

1

2

3

Optical circulators redirects light sequentiallyfrom port-to-port in a unidirectional path

Same characteristics as isolatorsby looking port 1-2 @ port 2-3

To extract the desired wavelength, a circulator is used in conjunction with the

rating

CIRCULATORS: WORKING PRINCIPLE

Applications:

•Optical Amplifier•Add-Drop Multiplexer•Bi-directional transmission •To monitor back-reflection from devices or optical subsystems

Characteristics:

•high isolation•low insertion loss•can have more than 3 ports

CIRCULATORS

CIRCULATORS: APPLICATION

Fiber Bragg GratingsA grating is a periodic structure or perturbation in a materialthat creates a property of reflecting or transmitting light in a certaindirection depending on the wavelength.

2

External writing technique using UV light

21 3

2

1 3

Reflection

Transmission

effn22

Fiber Bragg Gratings

Fiber Bragg Gratings

Fiber Bragg Gratings

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.

                                                  

 

Figure 2: FBGs reflected power as a function of wavelength

Characteristics:

•high reflectivity to be used as a filter

•low insertion loss

•low cost/simple packaging

Fiber Bragg Gratings

2

Transmission spectrum•band-rejection filter

Fiber Bragg Gratings

Reflection spectrum•reflective filter

Fiber Bragg Gratings

FBG APPLICATIONS

Gain flattening filter

-20

-15

-10

-5

0

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Insert

ion lo

ss (dB

)

-35

-30

-25

-20

-15

-10

-5

0

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Powe

r (dB

m)

-35

-30

-25

-20

-15

-10

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Powe

r (dBm

) + =

FBG APPLICATIONS

Laser diode wavelength stabilizer

FBG APPLICATIONS

FIBER BRAGG GRATING SPECS.

ATTENUATORSFunction: To decrease light intensity (power)

Working Principles

Fiber displacement Rotating an absorption disk

Set @ 60 dB

Pin = 0 dBm

Insertion loss = 2 dB

Pout = 0 - 20 - 2 = -22 dBm

Programmable attenuator

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

ATTENUATORS

Mechanical attenuator - by adjusting a screw

Waveguide attenuator - by adjusting biasing current

ATTENUATORS