JTh4J.3.pdf
CLEO:2014 © 2014 OSA
Metrology and Coatings for the 40 kg LIGO Optics
Rana X Adhikari
LIGO Laboratory
California Institute of Technology
Division of Physics, Math and Astronomy
1200 East California Boulevard, Pasadena, CA 91125, USA
rana@caltech.edu
Abstract:
The 4 km LIGO interferometers seek to measure the gravitational radiation from
cosmic explosions. In order to do so, their massive mirrors must meet several demanding
specifications which are sometimes conflicting. I will described why the job is so challenging
and how the challenges may be met.
© 2014 Optical Society of America
OCIS codes:
120.0120, 000.2780
1. Introduction
The search for cosmic gravitational waves started in nearly 50 years ago and has enjoyed several enhancements over
the years due to the tremendous leaps in the quality of lasers and optics. The imminent activation of a world wide
network of kilometer scale laser interferometers is expected to open a new window onto the universe by allowing,
for the first time, the observation of the bulk motions of massive stars, black holes, supernovae, pulsars, and perhaps
a vast panoply of unexpected astrophysical and cosmological phenomena. The dark side of the universe is about to
be revealed. In order to achieve astrophysically interesting strain sensitivities (
'
10
21
10
23
), these large laser
interferometers require mirrors which push the limits of modern fabrication and metrology.
Fig. 1. (left) Pictures of an Advanced LIGO mirror supported by a pendulum suspension. (right)
phase map of a polished, but uncoated mirror.
© Optical Society of America
JTh4J.3.pdf
CLEO:2014 © 2014 OSA
2. The LIGO Mirrors
After an initial phase of commissioning and observation, the initial LIGO interferometers [1] are being upgraded in
multifarious ways, with the new Observatory referred to as Advanced LIGO [2]. The new interferometer will be fed
with a 200 W CW Nd:YAG laser [3] which will be resonantly built up to produce a circulating power of
'
800 kW
in the 4 km Fabry-Perot cavities which comprise the Michelson interferometer arms. The Fabry-Perot cavity mirrors
are made of low absorption fused silica (Heraeus Suprasil 311 for the end mirrors and ultra-low absorption Heraeus
Suprasil 3001). These 40 kg mirrors have a diameter of 34.5 cm and a thickness of 20 cm.
3. Absorption
The absorption in the bulk substrates and on the high reflection coatings of the cavity mirrors is small enough to not be a
significant contributor to the overall power budget of the interferometers. Nonetheless, these sources of absorption lead
to deleterious wavefront distortions on the light through thermal expansion of the cavities’ high reflection faces and
thermo-refraction (dn/dT of fused silica) in the bulk of the input coupling mirrors. These thermal distortions reduce the
contrast achievable at the Michelson dark port [4] and disturb the error signals in the interferometric global alignment
control system [5]. The Suprasil 3001 substrates have an absorption of
<
0
.
5 ppm/cm and the high reflection coatings
(applied by the Laboratory for Advanced Materials in Lyon) have an absorption of 0
.
4
±
0
.
2 ppm per reflection.
4. Scattered Light
Light which is scattered out of the fundamental TEM00 mode of the Fabry-Perot cavities reduces the sensitivity in
many ways: the reduced buildup reduced the overall signal strength, the scattered light may scatter back into the
interferometer after interacting with the vacuum system and introduce phase noise on the light, and the losses de-
grade the entanglement of photons in non-classical states of light which can be used to enhance the interferometer
performance [6].
References
1.
B. P. Abbott, R. Abbott, R. Adhikari, and et al., “LIGO: the laser interferometer Gravitational-Wave observa-
tory,” Reports on Progress in Physics
72
, 076,901+ (2009).
2.
G. M. Harry, “Advanced LIGO: the next generation of gravitational wave detectors,” Classical and Quantum
Gravity
27
, 084,006 (2010).
3.
B. Willke, K. Danzmann, M. Frede, P. King, D. Kracht, P. Kwee, O. Puncken, R. L. Savage, Jr., B. Schulz,
F. Seifert, C. Veltkamp, S. Wagner, P. Weßels, and L. Winkelmann, “Stabilized lasers for advanced gravitational
wave detectors,” Classical and Quantum Gravity
25
, 114,040 (2008).
4.
T. T. Fricke, N. D. Smith-Lefebvre, R. Abbott, R. Adhikari, K. L. Dooley, M. Evans, P. Fritschel, V. V. Frolov,
K. Kawabe, J. S. Kissel, B. J. J. Slagmolen, and S. J. Waldman, “DC readout experiment in Enhanced LIGO,”
Classical and Quantum Gravity
29
, 065,005 (2012).
5.
K. L. Dooley, L. Barsotti, R. X. Adhikari, M. Evans, T. T. Fricke, P. Fritschel, V. Frolov, K. Kawabe, and
N. Smith-Lefebvre, “Angular control of optical cavities in a radiation-pressure-dominated regime: the Enhanced
LIGO case,” Journal of the Optical Society of America A
30
, 2618 (2013).
6.
H. Miao, H. Yang, R. X. Adhikari, and Y. Chen, “Quantum limits of interferometer topologies for gravitational
radiation detection,” arXiv preprint arXiv:1305.3957 (2013).