Suspended diffractive cavity investigations

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Suspended diffractive cavity investigations. Bryan Barr Institute for Gravitational Research. Overview. Diffractive Optics Overview of diffractive cavity basics Experimental options The Glasgow JIF laboratory Commissioning the diffractive cavity Length sensing and control
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Suspended diffractive cavity investigationsBryan BarrInstitute for Gravitational ResearchOverview
  • Diffractive Optics
  • Overview of diffractive cavity basics
  • Experimental options
  • The Glasgow JIF laboratory
  • Commissioning the diffractive cavity
  • Length sensing and control
  • Getting the most from the system
  • Results and discussion
  • Expected and unexpected – but all interesting
  • Plans
  • What’s next?
  • All Reflective Optics
  • Why use diffraction gratings?
  • Conventional interferometers use partially transmitting optics (mirrors + beam-splitters) to split laser light along different paths with known phase shifts
  • Diffraction gratings can also split light beams with known phase shifts
  • Using reflection gratings the light need not pass through the mirror substrate
  • Advantages over conventional systems
  • Non-transmissive materials can be used – improved thermal noise
  • Thermal lensing of substrates not an issue.
  • What’s the catch?
  • More complex geometry
  • Complicated phase + length sensing relations
  • Traditional Optical Cavities
  • Optical cavities are the simple building-blocks of gravitational wave interferometers
  • Conventional instruments use highly reflective mirrors to achieve high finesse arm cavities
  • This allows high power build up in the arms
  • To use diffractive optics we must be able to produce a cavity with high finesse using an all-reflective input coupler…
  • Option 1:1st order Littrow mountInput light enters via the 1st diffracted order pathGrating = 2 port deviceRequires high efficiency, low loss grating Option 2:2nd order Littrow mountInput light enters via the 2nd diffracted order path1st order is normal to the grating surfaceGrating = 3 port deviceRequires low 1st order efficiency, low loss gratingDiffractive Cavity OptionsConventional2nd Order LittrowConventional vs. Diffractive Couplersrt=,reflection,transmissiondiffractionConventionalDiffractiveConventional vs. Diffractive CavitiesAdditional Diffractive Properties
  • Translational effects
  • The phase of a diffracted field shifts if the beam translates across the grating : 360 degrees shift per grating period d
  • Well known effect
  • e.g. acousto-optic modulators
  • Angular effects
  • In a conventional cavity misaligning the input coupler simply misaligns the cavity
  • In a diffractive cavity misaligning the input coupler also misaligns the effective input pointing
  • JIF Laboratory – Simple Layout
  • Basic layout of the Glasgow laboratory
  • The aim: build and operate a fully suspended diffractive cavity on a prototype scale
  • Want finesse comparable to a conventional cavity
  • Investigate length sensing and control signals
  • Investigate translational effects
  • Investigate alignment effects on a suspended instrument
  • Reconfiguring The Optical Layout
  • Tasks (mechanical/optical):
  • Add beam-splitter (dual suspension)
  • Move corner steering mirror (dual)
  • Add final steering mirror (dual)
  • Replace silica optic with diffractive coupler (triple suspension)
  • Swap power recycling mirror (not shown) for blank lens to preserve mode-matching
  • Adjust mode-matching for diffractive cavity
  • We run a multi-purpose lab. Should be able to swap between experiments with minimum effort
  • Reconfiguring The Electronics
  • Local controls
  • The JIF suspension alignment system is based on the GEO600 local control set-up
  • 7 new rack-mounted modules were commissioned to control the new cavity
  • An additional module was added to the M1D controller to provide roll alignment control
  • Inside The Diffractive Tank
  • The diffractive optic is a 1 inch optic with a 1x1cm over-coated grating. It is mounted in a jig attached to a standard ‘dummy’ mass.
  • Length Sensing and Control
  • Once the diffractive cavity is in place and aligned…
  • …we need to implement a global control scheme
  • Traditional cavities use the PDH approach
  • Can we implement the same thing for diffractive cavities?
  • Extracting More Information
  • Conventional non-resonant sideband approach
  • Ports c1 and c3 give good information
  • Sidebands do not reach the photodetector at port c2t
  • We can only probe 2 of the 3 ports
  • Partially resonant sideband approach
  • Choose a sideband frequency close to the cavity FSR
  • Some sideband power leaks through to c2t
  • Interesting Diffractive Cavity Feature
  • Unbalanced signals
  • Created diffractive cavity model from the earlier equations – ideal grating
  • Chose reasonable value for eta_1
  • Chose mid-range value for eta_2 to emphasise unbalanced effect
  • Conventional cavity reflected signal
  • Diffractive cavity port c1 + c3 signals
  • Measured and Modelled – Forward Port
  • Comparison of the demodulated signals at port c3
  • Note: differences between the plots are due to the additional partially resonant sidebands in the measured signal – only one set of sidebands used in the model
  • Measured and Modelled – Reflected Port
  • Comparison of the demodulated signals at port c1
  • Note: the signals at the back reflected port behave very much like amplitude modulation signals – with a bit of phase modulation apparent near the minimum
  • Measured and Modelled - Throughput
  • Comparison of the demodulated signals at port c2t
  • Symmetrical -> zero crossing corresponds to centre of resonance
  • So we have good qualitative agreement between measured and modelled, but what about the relative signal sizes…
  • RF Demodulated Signal Responses
  • Model adjusted for losses in steering optics, pick-offs etc
  • DC power measurements for the resonant condition were made at each port
  • Model grating parameters were carefully adjusted within specification errors
  • Once the measured and modelled DC levels match the slopes of the demodulated signals can be compared
  • The relative gradients of the slopes – normalised to Through signal
  • Translational Effects
  • With these signals we can lock the cavity…
  • Next step is to investigate the performance of the locked cavity
  • In particular, what happens if we scan the grating from side to side?
  • We bonded a magnet to the side of the aluminium holder and set up a coil…
  • As mentioned before, the theory states that moving a grating from side to side will modulate the phase of the diffracted beam
  • For a small modulation (i.e. motion << one grating period) the forward signal should have an f response to side displacement – Birmingham/ Hannover
  • Larger displacements can be more complicated due to cavity resonance
  • Won’t go into mathematical detail here
  • Side to Side Displacement
  • The side coil provided side movement, and the global control coils were used to cancel longitudinal twisting…
  • Lock cavity, inject peak into side, minimise longitudinal signal using rear coils
  • An independent measurement of the side to side motion indicates a clear 1/f^2 slope – exactly what one would expect from a freely suspended mass
  • The demodulated forward response of the signal will thus be expected to produce a 1/f response to side-coil driving voltage…
  • Success!!! …or is it?What’s Happening?
  • Given the measured displacement we can predict how large the demodulated forward signal should be… our measurement was approx 700 times too large!
  • The problem lies with the alignment of the input coupler and the variation of the input pointing
  • If the input coupler rotates, then the mode resonating inside the cavity translates across the coupler
  • Thus the phase of the field coupling out of the cavity will be shifted by a different amount than the side-motion induced input coupling shift
  • There is also no suppression of this effect by the cavity resonance
  • Rotational Twisting Only - Success!!!Conclusion
  • Demonstrated that the coupling relations for diffractive cavities are well understood
  • Shown that a suspended diffractive cavity can be locked quite simply using existing length sensing and control techniques
  • Experimental evidence that translational motion of the beam across the input coupler can be detected on the length sensing and control signals for a grating cavity
  • Also, we have indications that alignment and cavity geometry may be even more important that we thought for diffractive systems
  • Future Plans
  • Finalise the translational work
  • Need more understanding of the Through and Reflected responses
  • Alignment control
  • Currently have a project student implementing an auto-alignment sensing system for the diffractive cavity
  • Wavefront sensing
  • Polarisation
  • Gratings have different properties for different polarisations
  • How are the control signals affected?
  • Thanks!
  • Glasgow:
  • B. Barr, M. Edgar, M. Plissi, J. Nelson, K. Strain
  • Hannover
  • O. Burmeister, M. Britzger, K. Danzmann, R. Schnabel
  • Jena
  • T. Clausnitzer, F. Brückner, E. Kley, A. Tünnermann
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