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, 16-17 april 2004

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

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?

The basic idea:polarization selective attenuation

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

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

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

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

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

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.

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

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

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

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

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

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

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