1. Fiber-Optic Communications
James N. Downing

2. Chapter 7
Fiber-Optic Devices

3. 7.1 Optical Amplifiers Repeaters and Regenerators Repeater Regenerator
An optical receiver converts the light to an electrical signal.
An amplifier increases the signal.
The transmitter converts the electrical signal back to an optical signal.
Regenerator
Removes the noise from the digital signal and regenerates the clean signal for transmission

4. 7.1 Optical Amplifiers Erbium-Doped Fiber Amplifier Consists of:
Coupling device
Fiber: Highly doped with erbium
Two isolators: Suppresses reflection at the ends of the fiber
Pump laser: Excites the erbium ions so they can be stimulated by incoming signal photons

5. 7.1 Optical Amplifiers Erbium-Doped Fiber Amplifier Advantages
Simultaneously amplifies a wide wavelength region with high output powers
Gain is relatively flat across the spectrum
Power transfer efficiency of about 50%
Large dynamic range
Low noise figure
Polarization independent

6. 7.1 Optical Amplifiers Erbium-Doped Fiber Amplifier Disadvantages
Long fiber lengths make them difficult to integrate with other devices
Pump laser creates spontaneous noise even without light
Crosstalk
Gain saturation

7. 7.1 Optical Amplifiers Semiconductor Optical Amplifier
Amplification is achieved by inserting a diode between two fibers
Advantages
Ability to be integrated with other semiconductors
Wide spectral range
Disadvantages
Higher noise figure due to coupling
Changing light intensity causes gain changes

8. 7.1 Optical Amplifiers Raman Amplifier
Based on principle of nonlinear Raman scattering
Discrete
Packaged in a box with a pump laser.
Actual transmission fiber becomes the amplifier.
Amplifier is coupled to the receiver end directed in the opposite direction of the signal. The pump transfers energy to the weak incoming signal.
Signal is amplified as it decays due to fiber losses.

9. 7.1 Optical Amplifiers Raman Amplifier Advantages Disadvantages
Increases transmission length by a factor of four
Lower power signal can be transmitted
Improvement in noise performance
Denser channel counts
Faster transmission speeds
Disadvantages
High power and long fiber lengths required
Thermal controls and safety issues

10. 7.2 Couplers
Types
Tree coupler: Distributes incoming light evenly between the output ports
Star coupler: Many input ports coupled to many output ports
Tee couplers: Three ports—input, output, and monitoring

11. 7.2 Couplers Manufacturing Methods Fused biconical tapered coupler
Used for star, tee, and general coupling
Four-port directional coupler
Two bare fibers are pulled and melted together

12. 7.2 Couplers Loss Insertion loss Excess loss
Directional loss (splitting)

13. 7.3 Modulators Direct Modulation
The amount of drive current can be controlled by simply turning it on and off—pulses.
Small signal modulation or pulse code modulation is more practical for communications.
Limited response time
Large wavelength chirp
High bias currents

14. 7.3 Modulators Indirect Modulation
Devices are inserted into the optical path of the source to implement modulation optically.
Major Devices
Electro-optic—process by which the refractive index of a material is changed through the application of an electric field. May be amplitude, phase, or frequency types.

15. 7.3 Modulators Indirect Modulation Major Devices
Electro-Absorption Modulators are efficient with low chirp and small drive voltage.
Operate at frequencies greater than 40 GHz
Can be integrated on the same chip as a laser diode and other transmitters
Future modulator of choice

16. 7.4 Multiplexers and Demultiplexers
Combine optical signals by wavelength division
Add-drop multiplexers may use gratings or filters
Channel spacing can be widened to limit loss
Demultiplexers
Single wavelengths can be picked off without demultiplexing whole signal

17. 7.4 Multiplexers and Demultiplexers
Optical Filters
Allow certain light frequencies to pass
May transmit or reflect wide range of wavelengths
Interference filters
Used for multiple channel separation
Wavelength locker
Tunes a wavelength through a narrow passband

18. 7.4 Multiplexers and Demultiplexers
Optical Filters
Mach-Zehnder filter
Separates wavelengths channels by using interference of two beams traveling different pathlengths
Used as an interleaver to separate odd and even optical channels
Fiber Bragg gratings
Allow wider channel bandwidth
Used as add-drop multiplexers

19. 7.4 Multiplexers and Demultiplexers
Optical Add-Drop Multiplexers (OADM)
Several different optical devices used together to allow single wavelengths to be retrieved or added to the multiplexed signal.
Regenerative OADM
Performs the electrical-to-optical conversion required for regeneration
Reconfigurable OADM
Can be electronically reconfigured to add or drop specific wavelengths

20. 7.5 Switches
Near Future
Optical networks will be mesh-based WDM nodes with multi-wavelength switching capabilities.
Optical cross connects
ROADMs to establish fast reconfiguration
Transport all types of communications protocols

21. 7.5 Switches Optical Cross Connects (OXC)
Switch data from any input port to any output port
Types of optical functionality
Transparent: entirely optic
Opaque: part electronic, part optic
Electronic: all electronics

22. 7.5 Switches MEMS Switching Micro-electromechanical systems
Miniature devices that contain mirrors that have one or two dimensional motion
Mirrors are controlled digitally to move into or out of the light beam to redirect the channel.

23. 7.6 Integrated Optical Devices
Placement of optical communication devices on a single chip
Will reduce cost
Will improve system performance
Will provide versatile modules
Two methods of connectorization of components
Free space
Planar