physics

# ggjc: Littrow configuration

We covered “Optical charactarization of ultrahigh diffraction efficiency gratings” by Bunkowski et al. with Josh leading the way.

This discussed primarily an application of diffraction gratings.  Wikipedia seems to be a little weak on diffraction gratings, so if I get the chance, I’ll probably try to improve it.  Specifically, the definition of Littrow Configuration was hard to come by.  Josh furnished the answer, and I eventually found more about it in the diffraction handbook, specifically chapter 2.  Jenkins and White didn’t have much of anything on it (though they did say essentially what it was), and the index pointed to the wrong page.

Anyway, the Littrow Configuration is when you have$a = b$ in the grating equation

$m\lambda= d\left(\sin a + \sin b \right)$

that is, the output of the grating is pointed exactly back along the input.  The diffraction order$m=1$, and$d$ is the grating spacing.  Of course,$\lambda$ is the wavelength of light.

then we went into a discussion of “forward thinking IFO design concepts”:

• concept (mitigates)
• cryogenic (thermal noise)
• in a spherical cave (seismic noise, gravity gradient or newton noise)
• all reflective surfaces (thermal noise)
• squeezed light (shot noise, radiation pressure)
• crazy beams (thermal noise)
• in space (seismic isolation, frequency sensitivity)
• white light ifo, or multi-wavelength (multiple tuned sensitivities)
• mass beams
• alternative geometries (triangular, tetrahedral)

This list is not exhaustive, but only what we thought of off the tops of our heads. (here, IFO has meant interferometer, which is standard LIGO shorthand.)