The function of the polarization conversion system or PCS is to convert all of the light from the lamp assembly from unpolarized to linearly polarized light. Several of the current spatial light modulators like the LCD and LCoS modulators must have linearly polarized light to work properly. Mercury and Xenon light sources output unpolarized light and if only the light which was polarized in one direction was used, too much of the light would be wasted.

The PCS was invented by engineers at Epson for use with LCD panels and it has also found use elsewhere.

A PCS works by taking the beam of focused light and splitting the light into two different polarizations, S and P. The P vector is denoted as an arrow and stands for light incident with its electric field vector parallel to the plane of incidence. The plane of incidence is defined as the plane which contains the surface normal vector and the incident ray propagation vector at the point where the ray hits the surface. In Figure 1 below, this is the plane of the paper or screen.

In the figure below, the S vector is shown as a letter O denoting the tip of the vector out of or into the page or screen. S stands for the German word for perpendicular which is senkrecht. In a PCS array there are two rhombs with a polarizing beamsplitter coating between them. The entry and exit faces of both rhombs are coated with broadband antireflection coatings.

The polarizing beamsplitter coating has a high reflectivity for S polarization and a high transmission for P polarization. In the above figure we can see that the unpolarized light is incident on this pair of rhombs and when it hits the interface of the two rhombs it encounters the PBS coating on the interface between the rhombs. This coating reflects the S polarization downward and transmits the P polarization straight through the interface. This means that the two beams exiting the upper and lower rhombs have linearly polarized light but it is oriented at 90 degrees from each other.

Now, a half-wave plate is introduced to one of the beams, in this case the lower one:

The half wave plate's function is to rotate the S polarization that transmits through the rhomb interface into P polarization. With the addition of the half-wave plate to the rhombs with S polarization we now have all light exiting the PCS array as P polarization, which is what we need for operation with the LCD and LCoS panels. Note of course the waveplate could have been introduced into the upper channel instead, so that the output would be S polarized

Modeling the PCS in ZEMAX is relatively simple. The rhomb may be modeled using a polygon object or a rectangular volume object. Here I choose to use the polygon object.

The polarizing beamsplitter coating may be designed in a thin-film package and exported into ZEMAX. ZEMAX can read the prescription data of any thin-film coating and accurately model transmission, reflection, absorption, retardance, diattenuation etc. However coating manufacturers are often reluctant to give out prescription information, and so ZEMAX supports a TABLE coating, which is a tabulated listing of coating peformance, and an IDEAL coating which simply lets us tell ZEMAX what we want it to do. In this example, I will use two IDEAL coatings.

The first will be for the broadband anti-reflection coating that is used to maximize transmission of the optics. This will be an ideal, 100% transmissive coating. The second will be the polarizing beamsplitter coating, which will be set to reflect the S polarization with 100% efficiency and to transmit the P polarization with 100% efficiency. Remember that ZEMAX is capable of modeling the real coating prescription, and therefore of giving a much more accurate and detailed treatment of coating performance than I will give here.

The following lines are added to the coating file used by ZEMAX:

! Define some idealized coatings for Polarization Conversion System
! First define a 100% transmissive coating
! Format: IDEAL <TRANSMITTED intensity> <REFLECTED intensity>

IDEAL transmit 1.0 0.0

! Now get 100% reflection, but only for the S
! The P should see 100% transmission 
! Use IDEAL2 coating. Defines reflected and transmitted 
! complex amplitude for S and P, rest is assumed to be absorbed
! Format: IDEAL2 s_rr s_ri s_tr s_ti p_rr p_ri p_tr p_ti no_pi_flag

IDEAL2 S_Reflect 1 0 0 0 0 0 1 0 0

The S_Reflect coating is then placed on the interface between the two prisms. How is this done? Remember that when two objects touch, as these two polygon objects do, the optical properties of the interface are set by whichever object is last in the Non-Sequential Component Editor. I then put the transmit coating on all other faces of the optics. I set a source object to have Jx = 1, Jy =1 so that there are equal amounts of light polarized in x and y, and then trace rays accounting for polarization and splitting:

The two beams have equal energy (the lower one has traveled slightly further and so has expanded more, hence its irradiance is lower). Now, Detector objects have a Polarization property that allws them to detect only polarized light. Setting this property to 0 means the detector detects all light, irrespective of polarization, as in the screenshot above. Setting it to 1 means the detector sees only the x-polarization, and 2 only the y-polarization. With the polarization control set to 2, so that only y-polarized light is detected, we get:

Adding a Jones Matrix object and setting it to be a half-wave plate in y gives:

and we now have 100% energy transmitted and a 100% P-polarized beam. The Jones matrix object is shown as a red circle in the screenshot, for clarity. The real half-waveplate is a rectangle which is the same size as the exit face of the bottom rhomb. A user-defined aperture can be added to the object to make it the desired shape.

Digital projectors often use fly's eyes integrators to provide homogenization of the source lamp's output (see my article on this here). This allows a very elegant integration of the PCS with the homogenizer optics. The PCS optics can be mounted on the field array of the integrator like so:

Because the PCS is mounted at the field array, the small lateral offset introduced does not matter, as the condensor lens produces a uniform illumination on the LCD or LCoS panel:

We can see a layout of the fly’s eye array with the PCS array on the back surface of the second array and then a single element condenser lens above.  The function of the condenser lens is to overlap all of the individual channels, in the fly’s eye array of channels, on top of each other.  This chopping up of the non-uniform illumination from the lamp assembly and overlapping each channel on top of each other is what enable the fly’s eye array to provide uniform illumination at the spatial light modulator panels, plus convert all the unwanted polarization into the desired polarization.