Rotating Linear Polarization
It is sometimes necessary to alter the existing polarization of an optical system. For example, lasers are typically horizontally polarized. If the system calls for laser light to reflect off a metallic surface, then this can be a problem because mirrors work best with vertically polarized light. What is the solution? A λ/2 waveplate with its axes oriented at 45° will rotate the polarization to vertical.
Another example is when it is desirable to adjust the polarization axis to any other orientation. Rotating the waveplate axis an angle of θ from the incident polarization will rotate the exiting polarization by 2θ. Since waveplates are highly parallel, inserting or rotating a λ/2 waveplate can reconfigure an entire optical setup with no realignment.
Transforming between Linear and Circular Polarization
Linearly polarized light can be transformed into circularly polarized light, and vice versa, by orienting a linear polarizer and λ/4 waveplate in a certain way. For example, a λ/4 waveplate with its axes oriented at 45° to linear polarization produces circular polarization. Circular polarization, which is of indeterminate orientation, passing through a λ/4 waveplate produces linear polarization at 45° to the waveplate’s axis. Additionally, if linearly polarized light enters a λ/4 waveplate at any angle besides 45°, it becomes elliptically polarized.
Optical Isolation with a Linear Polarizer
A linear polarizer plus a λ/4 waveplate creates an optical isolation system where light polarized by the linear polarizer passes through the λ/4 waveplate without attenuation but is transformed to circular polarization. If reflected from a mirror, the circularly polarized light encounters the waveplate again and is returned to linear polarization, but rotated 90°. Note: Two passes of a λ/4 are equivalent to one pass of a λ/2. The reoriented light is rejected by the linear polarizer. This system uses a double pass technique to remove feedback.
Optical Isolation with a Beamsplitter: Efficient Routing
A polarizing beamsplitter can be substituted for the linear polarizer in the optical isolation. This redirects the returning light into an alternate path without attenuation. By contrast, double pass through a non-polarizing beamsplitter only returns a theoretical maximum of 25% into the desired path and 25% into the other path.
A clever routing application used in articulated beam delivery arms takes advantage of the orientation insensitivity of circular polarization: each joint consists of a pair of λ/4 waveplates in fixed orientation before and after a mirror. The first λ/4 waveplate converts the light to S-polarization for efficient reflection from the mirror with no phase shifts. The second waveplate converts the light back to circular polarization, ready for the next joint at arbitrary orientation.
Waveplates are ideal for controlling and analyzing the polarization state of light. They are offered in three main types – zero order, multiple order, and achromatic – each containing unique benefits depending upon the application at hand. A strong understanding of key terminologies and fabrication methods helps in choosing the right waveplate, no matter how simple or complex the optical system.