Optical Components/Devices Chapter 3 Optical 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 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

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.

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.

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 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

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

Optical Fibre Connectors

Optical Fibre Connectors

Calculate the output power at port 3? 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. 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:

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

ISOLATORS P0 P3 P2 P1 A B

ISOLATOR SPECIFICATION

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

CIRCULATORS: WORKING PRINCIPLE

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

CIRCULATORS: APPLICATION

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

Fiber Bragg Gratings l2 l1 l3 Reflection Transmission

Fiber Bragg Gratings

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.

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

Fiber Bragg Gratings Transmission spectrum band-rejection filter l2

Fiber Bragg Gratings Reflection spectrum reflective filter

FBG APPLICATIONS

FBG APPLICATIONS Gain flattening filter + =

FBG APPLICATIONS Laser diode wavelength stabilizer

FIBER BRAGG GRATING SPECS.

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

ATTENUATORS Programmable attenuator Set @ 60 dB Insertion loss = 2 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