Chirped Mirrors

The mirrors described below aid in creating “ultrashort optical pulses,” which are “capable of taking ‘snapshots’ of the state of matter and hence of following the evolution of ultrafast processes at the microscopic level,” such as the forming and breaking of chemical bonds, etc. (17). If you’re not an expert in this branch of physics, read these sentences aloud and treat them as poetry, not as science, and watch what happens to their “sense.” If you ARE an expert in physics, try putting that knowledge aside for a moment (a suspension of belief) and listen to these sentences as if you were a child again....

“The idea of chirped (aperiodic) refractive-index-modulated distributed-feedback (Bragg) structures dates back to the early 1970s, when Hill proposed the use of chirped Bragg reflectors for dispersion compensation in optical fiber systems. The operation principle of these devices is fairly straightforward: different spectral components penetrate to different depths before reflected back from appropriate resonant sections of the chirped distributed-feedback structure. This introduces a frequency-dependent group delay, i.e. dispersion, which can be engineered....”

[I can hear Hill now: “hey, I know how we can compensate for dispersion! Use those feedback structures Bragg bragged about, and set them up in a staggered array, the way birds start singing in the morning at different times....”]

“These findings are directly applicable to an important special class of Bragg reflectors, namely multilayer dielectric mirrors. ...Varying the layer thicknesses during the evaporation process provides a simple means of modulating the period of the multilayer structure” (19).

Robert Szipöcs, Andreas Stingl, Christian Spielmann, and Ferenc Krausz, “Pushing the Limits of Femtosecond Technology: Chirped Dielectric Mirrors,” Optics and Photonics News 6.6 (June 1995): 16-20, 59.
re the above:
It seems as if the Array itself were being described....

And do not neglect to consider the consonantal cracklings of the authors’ names, the synapses in Szipöcs, the crispness of Stingl, that mann Spielmann’s spiel, and the ruffle of Ferenc’s krausz.

Other resonant, even chirping words from the article’s word-hoard are below and to the right & left:

optical pulse propagation
through a dispersive medium
such as fibre optics

cheep cheep!














Defining Chirping Data not in physics/engineering but in astronomy and cosmology, from the Seti@Home folks:

It's quite unlikely that an alien planet will be at rest with respect to our Earth. You may remember that humankind is whizzing along on a rotating planet which is revolving around the Sun, which itself is orbiting the center of our Milky Way galaxy. We can assume that our extra-terrestrial friends are likewise situated.

There is an interesting effect that all this motion will have on a signal emitted from a moving source and/or received on a moving planet. This is the doppler effect. You are undoubtably familiar with this if you've heard a car honking its horn as it passes you. The frequency, or pitch, of the sound changes as the car passes. You can go out and try this yourself. Stand at the side of the road and listen as a friend drives by with the horn blasting. You could also drive by a stationary car honking its horn and you will also hear the pitch change. It's the relative velocity that's important.

Although our remote friends aren't honking their horns at us, they are sending waves (electromagnetic waves) at us. Their signal will be distorted by the mutual motions of our two systems in much the same way that the car horns are distorted. To disentangle this the SETI@home screensaver analyzes the data many times over trying a great variety of possible doppler accelerations. Actually, the screensaver first takes the raw data and mathematically "undoes" a specific doppler acceleration or "chirp". It then feeds the resulting "de-accelerated" data to the FFT (Fast Fourier Transform) routines. This is called "De-chirping" the data. SETI@home tries to do this at many points between -10 Hz/sec to +10 Hz/sec....









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