Monitoring Interferometer Strain and Modulated Waveforms

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Monitoring Interferometer Strain and Modulated
Waveforms

• Ramon Armen
• REU 2007
• Advisor Keith Riles

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The LIGO Setup

• Michelson interferometer
• Each arm is optical cavity
• Gravitational waves ? changing arm length
• Strain ?L / L

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StrainWatch

• Online program to monitor strain in real time
• Modification of previous offline program
• Takes in raw interferometer data
• Applies Infinite Impulse Response filters
• Gives strain in the time domain h(t)
• Previously had only Fourier domain online
• Can take Fourier spectrum for comparison

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StrainWatch - Results
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StrainWatch - Results
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Project 2 - Phase Camera

• Light in arms is phase modulated
• Dynamics of cavity yield spatial asymmetries
• Want camera to see asymmetries in real-time
• Can autonomously find and fix problems

Ideal waveform
Tilted mirror
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Phase Camera - Hardware

• Array of photodiodes to see asymmetries
• Transimpedance Amplifier converts current to
  voltage
• Must convert from analog to digital signal
• Send array of photodiode signals to computer for
  processing

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Phase Camera - Software

• Demodulate signal
• Determine dynamics of cavity causing
  asymmetries
• Ultimately can be used to control cavity dynamics

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Phase Camera - Testing

• A) Laser light
• B) Phase modulation
• C) Amplitude modulator unused
• D) Beam Splitter
• E) Error signal
• F) PZT / front mirror
• G) Back mirror

Pound-Drever-Hall Technique
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Pound-Drever-Hall Technique

• Used to lock optical cavity to modulated laser
  light
• Lock cavity kept on resonance

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Error Signal

• Small deviation from resonance yields error
  signal
• Error signal filtered and sent to PZT
• PZT changes cavity length
• Cavity back on resonance
• The cavity thus keeps itself resonating

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Alas, Reality Intervenes
Error Signal
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Questions?