Fiber-Optic Communications James N. Downing

Chapter 7 Fiber-Optic Devices

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

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

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

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

7.1 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

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.

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

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

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

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

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

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.

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

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

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

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

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

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

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

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.

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