J' Alnis, A' Matveev, N' Kolachevsky, Th' Udem, T' Hnsch

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High precision spectroscopy of atomic hydrogen

• J. Alnis, A. Matveev, N. Kolachevsky, Th. Udem,
  T. Hänsch

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Energy levels Of H atoms
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Hänsch curve Precision optical spectroscopy of
H atoms
H against Cs frequency measurements with 14
digits of precision Searching for the possible
drift of fundamental constants (global warming
analogy)
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2003 Standard dye laser system for H 1S-2S
spectroscopy
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243 nm semiconductor laser system 40 H
excitation efficiency compared to the dye laser
system
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Optical beat note between Diode Dye laser
systems simultaneously at 486 nm and at 243 nm
The narrow central peak has a broad pedestal
typical for diode lasers.
Optimal feedback too weak feedback
too strong feedback
2 SHG and one two-photon processes are necessary
to get from 972 nm to 121 nm.
(Icentral /Itotal)444 (Icentral
/Itotal)64 99 power in carrior at 972 nm ? 50
percent power in carrior at H excitation .
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Vertical FP cavity for diode laser locking at
972 nm. Finesse 410000. FSR 1.931(1) GHz Line
width lt 10 Hz at 972 nm. Drift 0.2 Hz/s. Compact
table-top setup 40 x 40 cm.
Cavity length 7 cm. Made from single piece of
Ultra Low Expansion glass (manuf.
Corning). Cavity stands on 3 teflon posts and on
a zerodur ring. Optically contacted
super-mirrors on ULE substrates ( Advanced
ThinFilms, Colorado, USA).
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Compact optical assembly 40 x 40 cm active
vibration isolation stage. Optics for a) fiber
noise compensation (AOM), b) intensity
stabilisation (same AOM), c) Pound-Drewer-Hall
lock, Sound proof box appears not to be
necessary.
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ULE thermal expansion cross-over estimation
using optical beat note between two laser systems
ULE has a temperature Tc where thermal
expansion coefficient becomes zero
We estimate Tc for our ULE material to be at
7 ºC. Due to practical reasons we stabilise
cavity at 31 ºC.
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NEW IDEA!Cooling of the ULE Fabry-Perot cavity
to the zero-temperature-expansion point by
Peltier cooler in vacuum
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Optical beatnote so slow that observable with
an eye
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Drift of FP stabilised laser 0.1 Hz/s
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Fast lock-box for diode laser current feedback
5MHz flat gain, two integrators, phase advance
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Summary

• 243 nm diode laser successful but more broadband
  noise than dye laser. TiSa laser instead of
  diode laser?
• Mid-plane mounted vertical laser stabilisation FP
  cavity very successful
• Proton detection instead of photon detection
• New precision measurement planned autumn 2007