Transcript Higher order LG modes - University of Glasgow

Experimental test of higher-order LG
modes in the 10m Glasgow prototype
interferometer
B. Sorazu, P. Fulda, B. Barr, A. Bell, C. Bond, L. Carbone,
A. Freise, S. Hild, S. Huttner, J. Macarthur, K. Strain
Introduction – Overview

Motivation of the current work.

Presenting Glasgow interferometric GW detector 10 m
prototype.
LG33 mode generation on the prototype’s input laser
bench.


Preliminary results.

Conclusions.
B. Sorazu
GEO-ISC, August 2011
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Motivations
 Higher-order
LG mode beams are being considered
for implementation on future generations of GW
interferometers.
 LG
modes are expected to considerably reduce
thermal noise (both mirror coating and substrate
Brownian noise) in comparison with TEM00
beams [1,2,3].
 Results
from simulation work and benchtop
experiments [3,4] on higher order LG mode
interferometry encourage us to take the next step;
testing in an environment similar to an
interferometric GW detector.
[1] B. Mours et al. Thermal noise reduction in interferometric gravitational wave antennas: using high order TEM modes, CQG 23, 5777, 2006
[2] J.-Y. Vinet On special optical modes and thermal issues in advanced gravitational wave interferometric detectors, Living Revs. Rel. 12(5), 2009
[3] S. Chelkowski et al. Prospects of higher-order Laguerre- Gauss modes in future gravitational wave detectors, Physical Review D 79,122002, 2009
[4] P. Fulda et al. Experimental demonstration of higher-order Laguerre-Gauss mode interferometry, Physical Review D 82, 012002, 2010
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The Glasgow 10m Prototype Interferometer

GEO-like infrastructure:
 Similar
vacuum systems
 Triple
stage GEOsuspensions
 Same
local control
 Ideal
test bed for advanced
interferometry concepts
(e.g. Signal recycling,
optical spring).
 Fast
turn around for rapid,
small-scale tests
 Timely
validation of various
innovative technologies
(e.g. higher order LG
modes, diffractive
interferometry)
 Excellent
training for
students and Postdocs
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The Glasgow 10m Prototype Interferometer
Maximise potential of the Glasgow prototype
by carrying out several strands of
experiments in parallel:
Direct measurement of thermal noise
Thermal Noise
Reduction
Cavity
3-mirror coupled cavity systems:
Control strategies
 S.Huttner et al. Novel sensing and control schemes for
a three-mirror coupled cavity, CQG 24, 3825, 2007.
 B. Barr et al. Optical modulation techniques for length
sensing and control of optical cavities, Appl. Opt. 46,
2007.
 S. H. Huttner et al. Techniques in the optimization of
length sensing and control systems for a three-mirror
coupled cavity, CQG 25, 235003, 2008.
Radiation pressure experiments
Frequency reference for TN experiment
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The Glasgow 10m Prototype Interferometer
Thermal noise reduction cavity systems:
Diffractive grating as cavity incoupler
 M.Edgar et al. Experimental demonstration of a
suspended diffractively coupled optical cavity, Opt. Lett.
34, pp. 3184-3186, 2009.
 M.Edgar et al. Experimental demonstration of a
suspended, diffractively coupled Fabry–Perot cavity,
CQG 27, 084029, 2010.
 B. Barr et al.Translational, rotational and vibrational
coupling into phase in diffractively- coupled optical
cavities, accepted for publication Optics Letters.
Waveguide mirror
Thermal Noise
Waveguide
Reduction
Cavity
Cavity
(First realization of a waveguide suspended cavity
with a finesse of 800)
 D. Friedrich et al. Waveguide grating mirror in a fully
suspended 10 m cavity, accepted for pub. Optics Exp.
http://arxiv.org/abs/1104.2780
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The Glasgow 10m Prototype Interferometer
Thermal noise reduction cavity systems:
Diffractive grating as cavity incoupler
 M.Edgar et al. Experimental demonstration of a
suspended diffractively coupled optical cavity, Opt. Lett.
34, pp. 3184-3186, 2009.
 M.Edgar et al. Experimental demonstration of a
suspended, diffractively coupled Fabry–Perot cavity
 CQG 27, 084029, 2010.
 B. Barr et al.Translational, rotational and vibrational
coupling into phase in diffractively- coupled optical
cavities, accepted for publication Optics Letters.
Thermal Noise
Reduction
Cavity
Waveguide mirror
(First realization of a waveguide suspended cavity
with a finesse of 800)
 D. Friedrich et al. Waveguide grating mirror in a fully
suspended 10 m cavity, accepted for pub. Optics Exp.
http://arxiv.org/abs/1104.2780
Higher order LG modes
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GEO-ISC, August 2011
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LG33 in the Glasgow 10m GW detector prototype
 Outcome
of the successful ongoing collaboration with the
University of Birmingham.
 Main
aims of the experiment:
 Gain
some experience in how to handle high order LG modes in a
prototype environment.
 Study
of interferometer control signals with high order LG modes.
 Locking
a single 10m cavity on high order LG modes.
 Investigating effects from mode-degeneracy (see talk by C. Bond at
Advanced det. Technologies session Amaldi 9 (2011), R. Adhikari at GWADW 2010
at Kyoto)
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Some notes about mode - degeneracy

Different mode orders are separated in cavities because the round trip Gouy
phase for a mode = (order + 1) * Gouy phase

Modes of the same order have the same round trip Gouy phase, therefore
aren't separated in cavities.
 Coupling
into other modes of the same order will lead, potentially, to different
combinations of modes in the different arm cavities.


In real cavities the modes are not perfectly
degenerated, they have slightly different
resonant frequencies due to experiencing a
different average mirror curvature or cavity
length.
This leads to error signals with multiple zero
crossings making it difficult to lock stably to
one of this ‘pseudo’ degenerated modes.
Circulating Power
PDH Error Signal
 Here
we show an example of such ‘pseudo’
degeneracy on a 3 mirror mode cleaner with
astigmatism.
Cavity tuning [deg]
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LG33 generation – Input Laserbench
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LG33 generation – Input Laserbench
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LG33 generation – Input Laserbench

LG33 generation optics have four
purposes:
 Generation
of a LG33 mode from a laser
emitted TEM00 mode. This is done with a
diffractive optical element (DOE).
 Purification
of the generated LG33 mode.
By means of a linear modecleaner
(LMC); 23 cm long planoconcave cavity
with a finesse of 172 and 714MHz FSR.
 Adding
phase modulation onto the beam
for cavity lock. Through an EOM.
 Ensuring
a good alignment of the
prototype’s 10m cavity for the LG33
beam. For this we have a pick off path of
the TEM00 that avoids the DOE and
passes the LMC. TEM00 and LG33 share
same eigenmode inside the LMC.
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GEO-ISC, August 2011
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LG33 degradation through the Glasgow prototype
After 1st MM lens
After LMC
Max. residual 0.18
Max. residual 0.30
Cavity misaligned End
Mirror - transmission
Max. residual 0.50
B. Sorazu
After 4 EOM crystals
and FI
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Max. residual 0.33
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Results – Locking onto high order LG modes

Good cavity alignment as shown by the
visibility of TEM00 mode resonance with
respect to higher order one.
1st order
0th order

Still see reasonable resonances but
so far cavity power buildup is a
factor 5 less.
 However,
still the desired modeorder is dominant.
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Examples of modes we were able to lock to
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Results – High order modes resonance freq. split

B. Sorazu
The resonance peak of the TEM00
mode shows a single peak...as
expected.
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Results – High order modes resonance freq. split
 Scanning
B. Sorazu
several FSR for LG33 mode
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Results – High order modes resonance freq. split

Control signals (errorsignal, feedback signal and transmitted power) and
cavity mode shape when locked onto the highest peak.
 During
the video we intentionally gave longitudinal kicks to the mirrors to lock
to different modes within the peak.
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GEO-ISC, August 2011
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Plans for investigation of modes resonance freq. split

Recording the mode shape of the transmitted beam with a high speed
camera during the transition between the peaks of the frequency split modes.
 HS
IR cameras are expensive so
we built our own
 A commercial
Casio Exilim... was
chosen
 The
camera was completely
disassembled in order to remove
the IR filter on the CCD
 The
camera was assembled back
and tested that it worked fine
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GEO-ISC, August 2011
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Plans for investigation of modes resonance freq. split
 The
HS camera provided a nice view of the multipeak resonance features. In
these results we had cavity aligned to the order 7 mode, but freq. split
resonance still there.
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Plans for investigation of modes resonance freq. split
 The
previous video was accompanied with a synchronized high resolution
trace of the transmission PD DC level.
 We
zoom onto one of the freq. split resonance features and attach video
screenshots associated to each of the freq. splits:
 The
video seems to confirm that the observed freq. splitting of the resonant
peak is due to pseudo-degenerate modes.
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GEO-ISC, August 2011
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Plans for investigation of modes resonance freq. split
 Use
a second auxiliary laser at freq. 1 FSR away from main laser. Freq.
difference between laser stabilized with PLL servo.
 Lock
cavity to aux. laser and modulate PLL LO to scan LG33 freq. relative to
the locking laser.
Technique suggested by Koji Arai et al. Precise measurement of long. and transv. mode spacings of an optical cavity using an auxiliary laser,
G080467, 2008.
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Conclusions


Set up an experiment to test LG33 mode in a suspended 10m cavity featuring a
nominal finesse of 600 (close to Advanced Detector config).
Found it relatively easy to lock to order 9 modes.

After some preliminary experimental test it may seem that the same setup that
was adequate for stable locking to the TEM00 mode was not found to be adequate
for the locking of the cavity to the LG33 mode. Further measurements will be
carried out to allow confirmation. => So far no quantitative statement can be
made about the expected mode degeneracy and associated problems

Observation of multiples peaks around the order-9 resonance. So far
unexplained. Simulations with real mirror imperfections and further
measurements in progress.

Outlook: Different reflectivity mirrors are available to investigate how unexplained
effects scale with different cavity finesse.
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