Polarization Overview - Part 2: Waveplates & Retarders (Advanced)

Hi, I'm Katie, an Engineer here at Edmund Optics and I’m going to discuss some advanced polarization topics. I am going to assume you already have a basic understanding of polarized light. If you do not, you may want to watch our Polarization Basics video first. Polarization can often be both a problem and a solution in an optical system. We have already seen how to filter polarization states to reduce glare, so let’s take it a step further. In addition to controlling polarization state, we can control the phase of interacting light beams. Phase is how we describe the interaction of two or more interacting beams. Let’s consider two light waves, one polarized in the YZ plane and the other in the XY plane. The beams are considered in phase if the peaks and valleys are synchronous in time. By performing a bit of vector algebra, we see their net result leads to a single wave, linear polarized at 45 degrees to the original propagation axis. We can achieve different resulting vectors by changing the degree of separation between peaks. Let’s take a look at a case where the two waves are separated by 90 degrees. This shift puts the zero point of one wave at the other’s extrema. The vector sum then results in a wave that is oscillating in a circle as it propagates. We call this circularly polarized. By manipulating phase, we can alter polarization state, steer and isolate beams of light. A wave plate, or retarder, is a birefringent material used to shift phase. The amount of phase shift, or retardation, depends upon both the thickness of the material and the wavelength of light. Retarders are typically specified by the degree of phase shift given in units of waves, where 1 wave equals 360 degrees. In other words, a half waveplate creates a 180 degree or half wave phase shift and a quarter wave creates a 90 degree or quarter wave phase shift. Many times, we need to alter the polarization state in our system to maximize performance for particular optical components. Half waveplates can be used to rotate the polarization states. Let’s use an example to illustrate this concept. Here we have a set up with a linear polarizer that is passing vertically polarized light. Then we shift the phase by using a half waveplate. Similarly, we can use a quarter waveplate to convert light between linearly and circularly polarized states. If incident light is linearly polarized vertically, a quarter waveplate at 45 degrees will transform the passing light to left hand circular polarized light. We can use polarization control to create an optical isolator. An optical isolator allows us to prevent unwanted light from traveling back through the system. Here I have an example setup. We start with a randomly polarized light source which passes through a linear polarizer to become linearly polarized, through a quarter waveplate to become circular polarized, hits the mirror, switches handedness, comes back through the quarter waveplate and becomes polarized, but this time isn't able to pass back through our linear polarizer. We don't have back reflections in our system. Optical isolators are commonly found in laser and fiber optic applications where back reflections are particularly troublesome. There are several options when choosing a waveplate: zero-order, multi-order, and achromatic. Recalling from our knowledge of wave mechanics, angles are periodic in nature. That is, a shift of 2 pi is equivalent to a shift of 4 pi. We can use this relationship to create variable thickness designs. Zero-order waveplates are designed for the first period. This makes them extremely thin. Due to the thickness these are typically mounted on a transparent substrate. Zero-order waveplates have little performance change as temperature changes and have some flexibility to be used outside of the design wavelength. However, they are very fragile and typically, higher cost. Multiple order waveplates are designed for later periods so they can be thicker and easier to manufacture. This also makes them more sensitive to wavelength and incident angle, so they are recommended to be used with monochromatic light and in a lab environment. However, they are often a more cost-effective solution. Achromatic waveplates consist of two different materials that nearly eliminate chromatic dispersion. Standard achromatic lenses are made from two types of glass, which are chosen to achieve a desired focal length while minimizing or removing chromatic aberration. Achromatic waveplates operate on the same basic principle to achieve nearly constant retardation across the broad spectral band. The constant phase shift, regardless of wavelength, makes an achromatic wave plate ideal for use with tunable lasers, multiple laser-line systems, and other broadband spectrum sources. I hope this video helped you develop your understanding of phase control and its applications. If you have any further questions or would like to discuss your application in detail, please feel free to contact our Technical Support team.