FiberOptic Communications James N Downing Chapter 7 FiberOptic

Fiber-Optic Communications James N. Downing

Chapter 7 Fiber-Optic Devices

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

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 • • • 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 • 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

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