Scatter ShifterScatter Shifter

Motivation/ goalsMotivation/ goals

Getting rid of scattered light reflected back Getting rid of scattered light reflected back into the interferometer from auxilliary portsinto the interferometer from auxilliary ports

Have a versatile device that can easily be Have a versatile device that can easily be inserted into beams without exchanging inserted into beams without exchanging optical components for testing whether optical components for testing whether back-scattering causes observed noise.back-scattering causes observed noise.

To be used in large diameter beamsTo be used in large diameter beams

MirrorScattersource

Interferometerbeam

r= √(1-2)α

and the resulting in the spectrum:

the field in the interferometer

the field of the scattered beam

the resulting interfered beam

the power on a photodiode:

MirrorScattersource

Interferometerbeam

r= √(1-2)α

Modulating the field with Ωm gives the resulting field nside the interferometer

or expressed by Bessel Functions:

This gives the intensity on the photodiode:

resulting in the spectrum:

We can also write this as:

If we compare this to the unmodulated case we see it is just shifted by l*Ω and weighed

by the Bessel functions of the first kind:

Bessel functions Bessel functions of the first kindof the first kind

J0

J1 J2J3 J4

In the LiteratureIn the Literature

Man C N Man C N et al, 1978, Suppression of Optical Feedback et al, 1978, Suppression of Optical Feedback Effects on Saturated Absorption Signals by Effects on Saturated Absorption Signals by Phase Phase Modulation of the Reflected Light, Modulation of the Reflected Light, J.Phys E : Scientific J.Phys E : Scientific Instr. Instr. 2121 19-21 19-21

R. Schilling R. Schilling et al 1981 A method to blot out scattered et al 1981 A method to blot out scattered light effects and its application to a light effects and its application to a gravitational wave gravitational wave detector, detector, J. Phys. E: Scientific Instr. J. Phys. E: Scientific Instr. 1414 65-70 65-70

P. Beyersdorf 2001 The polarization Sagnac P. Beyersdorf 2001 The polarization Sagnac Interferometer for gravitational Wave Detection, Interferometer for gravitational Wave Detection, PhD PhD thesisthesis

http://www.ligo.caltech.edu/docs/P/P010005-00.pdfhttp://www.ligo.caltech.edu/docs/P/P010005-00.pdf

Optical arrangementOptical arrangement

Mechanical mode of the Mechanical mode of the substratesubstrate

24 kHz

Scatterer simulationsScatterer simulations

Movement of the Movement of the scatter sourcescatter source

Resulting noise Resulting noise spectrumspectrum

Shifting the scattered light Shifting the scattered light to higher frequenciesto higher frequencies

Higher harmonics follow Higher harmonics follow the Bessel functionsthe Bessel functions

Deviation from optimal Deviation from optimal modulation indexmodulation index

Optical arrangementOptical arrangement

Reducing the effect Reducing the effect of scattered lightof scattered light