Coherent Backscattering Experiments 


Coherent backscattering is a phenomenon where photons scattered at angles very near to 180 degrees self-interfere constructively leading to an observed intensity cone in the exact backscattering direction. The intensity cone has been related to various transfer coefficients of the scattering media, and the mean free path which light takes within the media. The goals for the following experiment was to determine the optimal alignment for a Coherent Backscattering Experiment, and to try and to try and observe the backscattered intensity cones in samples of milk. Experiments were done to determine the optimal distances between optical components, distances between the milk samples and detection mechanism, and optimal camera settings for the CCD Camera used for detection.


Experimental Setup

Here is a diagram of the setup that I am using to observe CBS, and an actual photograph of the setup. The sample holder is cut off at the top, and the beam dump is not visible, but the main components are all there (click on the pictures for larger versions).

More pictures of the setup can be found here.

      *The laser is a 10mW Linearly Polarized HeNe
      *L1, L2 and L3 are all convex lenses with focal lengths 2.54, 15, and 20 cm (respectively)
      *A1 and A2 are Apertures
      *BS is a AR-coated non-polarizing Cube Beam-Splitter for wavelengths near HeNe
      *BD is the Beam-Dump
      *LP is a linear polarizer held orthogonal to the polarization of the laser
      *The CCD camera is the Electrim EDC-1000N


Samples of whole and skim milk were observed using the setup described above. The results are displayed in the figure below. The results were obtained by exporting the images produced by the CCD camera into Scion Imaging Software, and averaging vertical and horizontal cross-sections of the cone.

The blue curve and red curve represent cones observed from 40 mL samples of commercially available whole milk and skim milk, respectively. The fluctuations in the data could be attributed to background light and/or varying pixel sensitivities in the CCD Camera. Also, the artifacts could be attributed to reflections and scattered light from the beamsplitter and lenses. According to theory, the widths of the cone are inversely proportional to the mean free path that photons take within the scattering samples. The shape of the cones have also been related to different optical transfer coefficients.

Maanit Desai
Laser Teaching Center