1. A. Bramati M. Romanelli E. Giacobino
Optimal intensity noise reduction by polarization-selective attenuation
Single mode VCSEL by optical feedback
Laboratoire Kastler Brossel, Paris
A. Bramati
M. Romanelli
E. Giacobino
VISTA meeting, Florence, april 2004

2. Outline Polarization-selective attenuation
Basic idea
Quantum correlation criterion
Experimental set-up and results
Single mode VCSEL by optical feedback
Polarized feedback: single polarization
Polarization and frequency selective feedack: single transverse mode

3. Noise properties of Vcsels
Orthogonal polarization modes with anticorrelated fluctuations:
high noise on each polarization
low total intensity noise
How to fully exploit the anticorrelations?

4. The basic idea:polarization selective attenuation

5. Quantum correlation criterion
To have squeezing we need:
Fixed by laser characteristics
Accessible by P-S technique

6. Quantum correlation criterion
Optimal attenuation condition: the intensity noises of the two polarizations are made equal
Minimal value needed for anticorrelations in order to have squeezing

7. Experimental set-up Wavelenght : 815 – 850 nm
Low threshold : down to 500 µA
High quantum efficiency : up to 50%
Modal behaviour: single longitudinal mode, orthogonally polarized transverse modes
Pump noise suppression
Thermal stabilization by Peltier
Shot noise calibration with halogen lamp

8. Experimental results
Excess noises of the two beams are equal S1= S2
Noise spectral density (V2/Hz)
To take advantage of the P-S attenuation scheme, we choose a vcsel which exhibits different absolute noises for the orthogonal polarizations. Measured value of C=-0.999

9. Experimental results Noise of the unfiltered beam 21.2 dB
One polarization
Unfiltered beam
Minimal noise
Noise of the unfiltered beam 21.2 dB
Minimal intensity noise 14.4 dB for Topt=0.91
Power of improved beam: 97% of the total beam

10. Best noise reduction To conclude:
P-S attenuation is a simple technique allowing to fully exploit strong anticorrelations between orthogonal polarizations
Experimental evidence of about 7dB of noise reduction with only 3% of attenuation on the total beam
M. Romanelli et al., to be published in Opt. Lett.

11. Single mode Vcsel by optical feedback
Motivations:
Applications (fiber coupling, spectroscopy)
Avoid strong unwanted increase of noise when the beam passes through a polarizing element (polarization anticorrelations)
Definition of single mode
The study of average properties of the emission is not enough (sub-threshold modes are important)
Noise features have to be investigated

12. Working point and free running laser behavior
For a driving current of 1.60 mA TEM00 and TEM01 orthogonally polarized are above threshold
TEM00
TEM01
Optical spectrum for the free running laser

13. Polarized feedback: the set-up
Feedback from a mirror trough a polarizing beam splitter: selective in polarization but not in frequency
TEM00
Laser with polarized feedback: two transverse modes with the same polarization
TEM01

14. Polarized feedback: test of single polarization mode
Polarization sensitive attenuation
For a single mode beam:
S(T)= 1 + Tv
Free running laser
Polarized feedback

15. Polarized and frequency selective feedback: the set-up
Feedback from a grating in Littman configuration and PBS
TEM00
Only the TEM00 mode is lasing.
26 dB of suppression measured for the TEM01 on the same polarization

16. Feedback from grating: test of single polarization, single transverse mode
Transverse profile sensitive attenuation
Razor blade
Free running
Littman configuration

17. Conclusion Polarization selective attenuation Feedback
Simple technique to fully exploit the anticorrelations
Quantum correlation criterion
Experimental evidence of noise reduction with P-S technique
Feedback
Polarized feedback: single polarization operation achieved
Littman configuration: single polarization, single transverse mode operation achieved