8776CDC0d01 - Chirped Mirrors DOUBLE CHIRPED MIRRORS FOR...

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Chirped Mirrors © 2007 Michael Wiemer, All rights reserved Page 1 D OUBLE C HIRPED M IRRORS FOR O PTICAL P ULSE C OMPRESSION By Michael Wiemer O BJECTIVE In this paper, the design of a double chirped, AR coated mirror will be explored for dispersion compensation in mode-locked lasers. The various design parameters (including material choice) will be explored and their effect on the dispersion characteristics will be cataloged. It will be determined that the major problem in chirped mirror design is impedance matching the ambient medium to the stack. Non-ideal impedance matching results in undesired oscillations in the dispersion vs. wavelength curve of the mirror. A transfer matrix formalism will be used for calculations. M OTIVATION Mode-locked lasers are lasers that put out short pulses of optical energy. These pulses can be very short (down to several femto-seconds). These trains of pulses are comprised of many optical wavelengths with a set phase relationship. One can see this by imagining two optical frequencies spaced by some f propagating collinearly. When this dual frequency beam is shown on a detector, the current out of the detector will oscillate sinusoidal at the beat frequency ( f) of the two optical carrier frequencies. As more wavelengths spaced by f are added to the multi-wavelength beam, the oscillations at the detector output will become sharper until we only see spikes of current between which no current flows. This is the principle of mode-locking (overlap of many wavelengths in time with a set phase between them to create a pulse train out of the laser). The repetition rate of a mode-locked laser is controlled by the length of the cavity (essentially, the round trip time of a photon inside the cavity). If a higher repetition rate is desired, a shorter cavity is needed. The pulses can be made to be as narrow as possible if the phase relation ship between all the wavelengths is a constant. One can express this requirement in another way by saying that the round trip time of a photon in the cavity must be independent of wavelength. In order to achieve this constant phase relationship (constant round trip time) between all wavelengths inside the laser despite material dispersion (and other dispersions inside the cavity), dispersion compensation devices must be placed inside the laser cavity. Properly designed double chirped mirrors can be used as dispersion compensation devices. The competing dispersion compensation devices to chirped mirrors are grating pairs or prism pairs. These devices are able to provide large amounts of tunable negative dispersion to compensate the dispersion inside the cavity thereby reducing the width of the pulses to their Fourier transform limit.
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This note was uploaded on 03/13/2011 for the course ELECTRICAL 605 taught by Professor Mugu during the Spring '08 term at University of Wisconsin - Fond du Lac.

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8776CDC0d01 - Chirped Mirrors DOUBLE CHIRPED MIRRORS FOR...

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