Polarization Control Continued: Applications of Control Devices

Polarization control devices work to optimize optical performance in many types of systems. Courtesy of Wikipedia.

Introduction to Polarization Control Applications

A recent post presented the basics of polarization and control devices. This week’s article seeks to continue our analysis of this property and delve into polarization control applications. Polarization control devices optimize light transmission without compromising the system’s output power. They are valuable additions to experiments in almost all branches of optics. We learned that a few of the most common devices include waveplates, fiber optic polarizers, and polarization dependent isolators.  Today, we look into how these devices are used in research and industry.   

Different waveplates work best in different applications depending on environment and bandwidth. Courtesy of Newport Corporation.

Waveplates

Waveplates are one of the most versatile and commonly used pieces of optical equipment.  Researchers use waveplates to shift polarization from linear to circular, from circular to linear, or to rotate linear polarization angle. These devices are often used in combination with other components to optimize optical setups. Light will often shift phase as it reflects and moves throughout an optical system. Waveplates are therefore vital in “optical cleanup.” Below we cover how to choose the correct wave plate for your application based on the desired phase correction. 

Multiple order plates

Multiple order wave plates create a large phase difference that is several times the light’s wavelength. These devices provide either quarter-wave or half-wave retardant.  They are the least expensive of the plates, but it is important to note that multi-order plates are extremely sensitive to changes in wavelength and temperature.  Researchers often use multi-order plates with monochromatic light (such as a laser) . For example, multi-order plates are extremely popular in laser cutting end etching, which greatly benefits from circular polarization. 

Zero order plates

As the name implies, zero-order plates cause a much smaller phase shift- one that is ideally less than a fraction of a wavelength. Retardation for these devices is much more constant in temperature and wavelength changes, so they can be used in a wider range of applications than the multi-order.  They are especially useful in experiments that cannot be executed in temperature controlled environments.   

Achromatic plate

Achromatic waveplates consist of two materials that effectively eliminate chromatic dispersion. These polarization control devices attain almost constant retardation across a wide wavelength range.  This makes achromatic waveplates ideal in applications requiring a broad bandwidth such as spectroscopy or mode-locking.

Fiber Optic Polarizers

Fiber optic polarizers improve performance within fiber cables. They allow transmission of only one type of polarization. Courtesy of DK Photonics.

Polarization-Maintaining Fiber

Optical fibers are an extremely efficient light transmission devices, but they always exhibit some degree of birefringence. This causes changes in the polarization of transmitted light along the optical path. Researchers can utilize polarizer devices within these systems to create a polarization-maintaining fiber. These are specialty fibers with a strong built-in birefringence such that usual fiber disturbances do not shift polarization. Researchers  frequently use the fibers in optical amplification, interferometry, and certain fiber lasers.

Optical Isolator

Optical isolators utilize polarization properties to block unwanted back-reflection during transmission. Courtesy of Deltronic Crystal.

Interferometers

Fiber optic interferometry is a technique in measuring sample properties and detecting changes in the system. This method uses superimposed waves to create an interference pattern and determine measurements of displacement, surface irregularities, and changes in refractive index. Interferometry is common in fields of astronomy, metrology, spectroscopy, and many branches of chemistry.  Fiber optic interferometers often incorporate fiber optic isolators to ensure that light only travel in one direction and to prevent back-reflection.  This allows the polarization devices maximum accuracy and precision.

Laser Resonator

Optical isolators can be useful components within a laser cavity. Laser quality and performance can often decay over time due to back-reflections into the laser cavity.  Installing an optical isolator into the system allows a signal beam to streamline forward while keeping reflection from damaging the resonator. Isolators have shown to produce a more stable, more power efficient, and longer lasting laser. The devices are largely beneficial in systems for laser communications, transmission, and amplification.