Lecture10-228a

Lecture10-228a - Lecture 10 -Transmitters ECE 228A Fall...

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ECE 228A Fall 2011 Daniel J. Blumenthal 10.1 Lecture 10 -Transmitters
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ECE 228A Fall 2011 Daniel J. Blumenthal 10.2 Semiconductor Optical Sources The optical sources practically used in optical communications are based on semiconductor devices The generated optical signal is to be ef±ciently coupled to the output optical ±ber Other kinds of sources (non-semiconductor) and/or free space coupling is sometimes done in R&D labs, but only for advanced and prototypal research All commercial sources comes in very compact packages, and are ±ber pigtailed in the factory Pigtailing and packaging is one of the most critical and expensive issues for these devices Fiber Pigtail Laser Package
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ECE 228A Fall 2011 Daniel J. Blumenthal 10.3 Semiconductor Optical Sources Compact integrated devices used to convert a modulated electrical signal to a modulated optical signal that is then ef±ciently coupled to the optical ±ber. Characteristic Description Number of longitudnal modes Number of optical frequencies laser emits. Plays a key role in both laser cost and how fiber dispersion will limit link bit rate. Side Mode Suppression Ratio (SMSR) A measure of how good a single mode laser is. Threshold current The minimum current required to turn on the laser. Low values are key to decrease transmitter power dissipation Laser Noise A measure of how random the optical laser output is. This characteristic can determine the ultimate performance of a link. Linewidth A measure of how noisy the laser. Plays a key role in how dispersion and crosstalk limits the transmission bit rate and capacity. Wavelength Determines the dispersion and loss operating points in the fiber and other network components. Modulation Bandwidth Determines the bit rate that can be attained by current modulation. Chirp A measure of how the optical output frequency changes with current modulation. Impacts transmission bit rate. Linearity Ability to reproduce and analog signal with low distortion. Fiber Output Power Power launched into fiber to achieve high signal-to-noise ratio. Wavelength Tunability The ability to tune the output wavelength over a wide range. Long Term Stability In terms of wavelength, output power and other key factors.
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ECE 228A Fall 2011 Daniel J. Blumenthal 10.4 Laser Diode Issues Direct modulation vs. External modulation Lower cost vs. less chirp and less pattern effect InGaAsP active region vs. InGaAlAs No aluminum issues vs. higher temp operation Bulk vs quantum well active region Less complexity vs. lower threshold Single transverse mode (vertical direction) Essential for single mode ±ber. Not an issue. Single lateral mode (horizontal direction) Essential for single mode ±ber. Narrow waveguide required.
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This note was uploaded on 11/24/2011 for the course ECE 228a taught by Professor Bowers,j during the Fall '08 term at UCSB.

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Lecture10-228a - Lecture 10 -Transmitters ECE 228A Fall...

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