Download J.Marque: Mode matching of the Virgo FP cavities

Mode Matching of the
Fabry-Perrot cavities
Julien Marque
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Contents
1. Mode matching with the old IB
o Tools
o Issues
o Results
2. Mode matching with the new IB
o New telescope
o New tool
3. Conclusion
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Mode matching principle
o The mode matching consists in adjusting the input
telescope in order to match the mode parameters of
the input beam (waist size, distance to waist) to the
cavity eigenmodes (radius of curvature of the mirrors
of the cavity, length of the cavity).
o This action can be performed by adjusting the position
of the various optical elements of the telescope.
o The input bench telescope has been designed to match
the input beam to the FP cavities and minimize the
aberrations introduced by the telescope itself
(astigmatism, spherical aberration).
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Mode matching discussion
Goal : optimize power coupled into the FP cavities…
and not only?
Improvement of error signals for locking and
alignment?
Does “Mode Matching noise” exist and is that a
concern (phase shift, spurious cavities, …)?
Future issue: thermal lensing effect
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Tool 1 : Optocad
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Tool 1 : Optocad
Astigmatism is introduced by the Mode Matching
Telescope : depends on the incidence angle of the beam
on M5 and M6.
These angles depend obviously on the position of M4,
M5 and M6 and on the centering of the beam on M5 and
M6.
=> the angles have been estimated with Optocad taking
into account rough estimation of the miscentering.
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Tool 2 : Finesse
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Tool 2 : Finesse
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Problems
1. Diaphragm on the input bench too small => beam
shaper effect, impossible to measure the beam size
respect to the telescope adjustment.
Diaphragm replaced in March 2004.
2. Need to translate last mirror of the telescope by a few
mm while keeping the beam aligned in the cavity.
Created dedicated servo loop.
3. Dynamic range / picomotors behaviour
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Results
All the following results have been computed with a Finesse
simulation and verified experimentally (relative DC measurement
+ amplitude of 01 mode resonant in the cavity)
Power coupled into the TEM00 mode:
- Before the matching activity = 76%
- First step
= 92% (dynamic range limitated)
- Second step (M6 substituted) = 95% (astigmatism limitation)
A rearragement of the optics on the bench needed to remove
astigmatism => we could reach 98% (limitation is the tuning of
the distance between PR and the 2 mirrors)
12/12/2005
WG1 Hannover
European Gravitational Observatory
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New IB / parabolic telescope
12/12/2005
WG1 Hannover
European Gravitational Observatory
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New tool : Zemax
Interesting features:
o Tolerancing (by defining a function of merit
= defining the target for beam size and waist
location) => identify critical parameters with
Monte-Carlo method
o Optimization (by defining compensators) =>
good help to now how to proceed!
12/12/2005
WG1 Hannover
European Gravitational Observatory
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New tool : Zemax
No way to define a FP cavity with Zemax …
How to compute the coupling?
C = 4/( (w0/win+win/w0)2 + ((w0/win)*(Deltaz/b))2 )
with:
w0 = cavity waist
b = cavity Rayleigh lenght
win = waist of the input beam
Deltaz = distance between w and win
(Virgo physics book)
12/12/2005
WG1 Hannover
European Gravitational Observatory
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New tool : Zemax
New telescope is compact
=>
Very fine tuning requested
Need for many actuators (closedloop picomotors for M6,
translation stages for M5, openloop picomotors for M5, M3 and
M4)
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Conclusion
o Hope to get good results soon! Goal is a coupling
better than 99% for FP cavities.
o Mode matching of the IMC to be done soon (coupling
is currently 82%). Simulation already done with
Finesse. Results show that astigmatism is not critical
for such a short cavity. Goal is 98%.
o Mode matching of OMC was limitated by astigmatism
(94%). Goal is > 99% now.
12/12/2005
WG1 Hannover
European Gravitational Observatory
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Tuning of Finesse
All numbers, in the Finesse input file, are nominal except
for (nominal numbers between parenthesis):
- DC power of the input beam = 1 W
- Waist of the beam after the IMC = 4.8 mm (4.9)
- Incident angle on M5 = -3.3 degrees (?)
- Incident angle on M6 = 5.4 degrees (?)
- M6 Rc = 3.162 m (3.16), 3.138 m for the old M6
- NE Rc = 3530 m (3580)
12/12/2005
WG1 Hannover
European Gravitational Observatory
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