Fiber-Optic Communications

1. Fiber-Optic Communications James N. Downing

2. Chapter 7 Fiber-Optic Devices

3. 7.1 Optical Amplifiers • Repeaters and Regenerators • Repeater • 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 • 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 • Multiplexers • 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