New Thin Film Polarizing Beamsplitter

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In the paper*,
    Li Li and J.A. Dobrowolski, "New developments in thin film polarizing beamsplitters", in Optical Interference Coatings, Vol. 9, OSA Technical Digest Series, (Optical Society of America, Washington DC, 1998), pp. 158-160.

given at the Optical Interference Coatings conference in June 1998, the authors discuss a new design for a high-performance polarizing beamsplitter based on frustrated total internal reflection. This polarizer has the following advantages: non-absorbing, broad band, wide-angle, and high extinction ratios (Ts/Tp and Rp/Rs). The diagram below illustrates the operation of this polarizer.

Diagram

The coating is sandwiched between two prisms. Note that S polarized light is transmitted, which is the opposite of other designs. Also, note that light strikes the coating at a very high angle.

Although the authors do not give the exact details of the coating design, it is possible to use TFCalc to create such a polarizer. Here we design a polarizer for 400-700 nm to operate for the angles 65-75 degrees in the prism. This design requires a high-index glass prism. An index of 1.85 is used in this design. This means that the range of angles in air is ±9.28, which is considered wide-angle. We use the following 22 continuous optimization targets:

  • S Transmittance > 99.9% for wavelengths 400-700 nm at angles 65,66,...,75
  • P Optical Density > 3.0 for wavelengths 400-700 nm at angles 65,66,...,75
There are many ways to design this coating. One simple method is to start with a stack such as (0.5H L 0.5H)^n that reflects the P polarization for the wavelengths and angles given above, and then use optimization to force the coating to transmit S polarization. Here H and L represent quarter-wave layers of index 2.35 and 1.45, respectively. Some tests show that if we start with the 21-layer design created by setting n=10, then the optimization targets given above will be met. The performance is shown below, with the left scale for the optical density of P reflectance and the right scale for S transmittance. The extinction ratio is greater than 1000 for all design angles and wavelengths.

Plot of beamsplitter design

Here is the design, with the first layer closest to the substrate and thickness given in nm: 

    H     11.16
    L     48.87
    H     35.94
    L     68.58
    H     39.50
    L     79.77
    H     46.20
    L     96.61
    H     49.71
    L    102.74
    H     50.49
    L    102.74
    H     49.71
    L     96.61
    H     46.20
    L     79.77
    H     39.50
    L     68.58
    H     35.94
    L     48.87
    H     11.16

Note, curiously, that optimization preserved the symmetry of the original design (i.e., layers 1 and 21, 2 and 20, 3 and 19, etc., have the same thickness).

The authors are commended for this ingenious polarizer, which is the subject of a patent application. However, it seems that this polarizer has two drawbacks: (1) the dimensions of the prisms may prevent its use in some applications and (2) the prisms seem to require a high-index glass, which presents other problems.

*Recently, a paper was published that gives much more detail about this polarizer. See

    Li Li and J.A. Dobrowolski, "High-performance thin-film polarizing beam splitter operating at angles greater than the critical angle", Applied Optics, Vol. 39, No. 16, pp. 2754-71.
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