Polarisation Spectroscopy

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Polarization Spectroscopy

• brief history

- experimental requirements
- typical signals for Rb
- advantages
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Laser spectroscopy
Laser invented in 1960s
FM spectroscopy in 1980s
- these techniques are all very new and still
under development
- often if you have a problem the answer is not
known - share the knowledge
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The blurb
The experiments themselves - saturation
spectroscopy, polarization spectroscopy - were
magnificent in their design and execution
J. S. Rigden, Hydrogen The Essential Element
(Harvard University Press, 2003), pp. 209.
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Principle
- to induce a birefringence in the atomic medium
Pump with circularly polarized light - creates
an anisotropy of mF populations
Probe with linear polarized light - observe
rotation of polarization
C Wieman and T W Hänsch. Phys. Rev. Lett. 36 1170
(1976) W Demtröder Laser Spectroscopy 2nd edn
(Berlin Springer) 1998 Polarization
spectroscopy of a closed atomic transition
applications to laser frequency lockingC. P.
Pearman, et al. J. Phys. B 35, 5141 (2002)
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Optical set-up
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Optical set-up
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Optical set-up

• pumpprobe spectroscopy
• very similar to normal saturation absorption
  spectroscopy

- Pump beam circularly polarized to drive s
transitions.
- Probe beam is aligned at p/4 so that in absence
of pump equal intensities are measured at both
photodiodes.
- Approximate intensities of Isat for probe and
3Isat for pump
NOTE
- windows on vapor cells are birefringent due to
stress - feedback from other elements (including
pump beam) can be polarization dependent
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Model
Strong pump
Weak probe
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Kramers Kronig relation
? ?r ??i

• Differential absorption
• different refractive indices experienced by s
  components
• Medium is birefringent

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Signal
Separating probe into horizontal and vertical
components
Da0 maximum difference in absorption between
polarisation components at line centre Dn
difference in refractive indices of circularly
polarised components of probe. G absorption
linewidth w0 resonant frequency L
length of cell
C. P. Pearman, et al. J. Phys. B 35, 5141
(2002) ML Harris, et al. Phys Rev A 73, 062509
(2006)
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Test for an anisotropy in medium
Interrogate circular components of probe beam
- put a ?/4 plate before final PBS - this
converts LH and RH circular components into linear
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Signals
Small angle (5 mrad) between pump and probe beams
Deliberately Doppler broadened signals (72 mrad
between beams)
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Locking a laser
- Error signal with only a simple op-amp
difference circuit and no filtering or
de-modulation
-550 MHz
- Huge re-capture range if laser is perturbed
while maintaining Doppler-free resolution.
Zero crossings -920 MHz, 220 MHz
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Improvements Mu-metal
Caveats - care with alignment as mu-metal can
create huge magnetic field gradients - with use
of good shield and a long solenoid to apply
uniform bias field then very clean signals can be
obtained.
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Laser Linewidth
2 lasers locked to 85Rb F3 ? F4 using only I
lock to piezo
FWHM 625 kHz

• Individual linewidths of 300 kHz
• Assuming un-correlated, equal in magnitude laser
  linewidths. Correlated drifts e.g. due to
  temperatures would not be evident with this
  method of measuring linewidths.

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Comparisons for locking to Rb
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Advantages
Modulation free locking
Common mode rejection of laser intensity due to
subtraction of signals
Low cost particularly compared to FM saturation
absorption spectroscopy