Articles/Application Notes

The ability to accurately rotate the polarization of incident light while minimizing any losses in polarization purity has applications in optical switching, polarimetry, and microscopy. Polarization rotators utilizing tunable birefringent plates, such as liquid crystal (LC) devices, have the advantage of non-mechanically tuning the devices’ retardance. However, these devices properly work with incident light within a very specific wavelength range. Ferroelectric liquid crystal (FLC) devices can switch between two orthogonal states of linear polarization, and other response times much faster than their nematic liquid crystal cell counterparts. An achromatic polarization rotator can be constructed with an FLC cell between two half-wave plates that have been constructed to produce a half-wave retardance at a certain design wavelength. This results in a device that others fast response times and high polarization purity over a broader wavelength range.

The Fabry-Perot is an optically resonant cavity formed by two partically reflecting mirrors that ideally are non-absorbing and therefore transmit the light tht is not reflected.

While the liquid crystal industry is primarily driven by the display industry, increasingly important applications in science and engineering have emerged such as beam steering, wavefront modulation and polarization switching and control. We will discuss some of the differences in construction techniques needed to produce a precision optical device rather than a flat panel display along with development work being carried out at Meadowlark Optics in some of the above areas. These include polarization switches capable of greater than 5000:1 contrast and high efficiency beam steering for precision interferometer gauges.

This application note describes how two Liquid-Crystal Variable Retarders (LCVRs) can be used to build a Stokes Polarimeter to accurately characterize the polarization state of a beam of light.

This application note details factors that effect the response time for Nematic Liquid Crystal optics. The response time of a liquid-crystal variable retarder depends on several factors, including the LC layer thickness, viscosity, temperature and surface treatment as well as the driving waveform. The response time is also sensitive to the direction of the retardance change as well as the absolute value of the LC retardance.

Since the human eye is insensitive to polarization, there is a large amount of information in many situations, which is not readily utilized. Measuring the polarization state of light is useful in many research fields including biology, chemistry, astronomy and remote sensing. The first portion of the paper discusses the simple application of accurately measuring the retardance value and fast axis position of an unknown waveplate. We will mention some of the many polarimetry applications especially in the context of non-mechanical, liquid crystal based polarimeter experimental technique. Some of these examples are from biology showing tissue birefringence changes, astronomy for solar imaging, polarimetric visualization and landmine detection.

Meadowlark Optics first introduced nematic liquid crystal (LC) devices for precision optical applications more than 25 years ago. These devices are variable retarders, sometimes called LCVRs. They provide electrical control of retardance at low voltages, usually 20 volts (2 kHz square wave AC) or less. Figure 1 shows an example of the relationship of retardation to voltage for one of these devices. In many applications they have supplanted their much higher voltage cousin, the Pockels cell.

Beamsteering using liquid crystals can be achieved with refractive or diffractive implementations. The common thread in these various structures is that the liquid crystal is employed as an optical phase modulator.

By spectral phase shaping of both the pump and probe pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy, we demonstrate the extraction of the frequencies of vibrational lines using an unamplified oscillator.

JPL is developing an innovative compact, low mass, Electro-Optic Imaging Fourier Transform Spectrometer (E-O IFTS) for hyperspectral imaging applications.

Novel tunable polarization interference filters (PIF) employing active liquid crystal devices are presented, and the principles of operation are described.

Cholesteric liquid crystals were the first liquid-crystalline phase observed.

Meadowlark Optics manufactures four types of non-mechanical, vibration free, shutters. Three of these require polarized light input, preferably linearly polarized. They are used in conjunction with an exit polarizer. The extinction ratio between the open and closed states depends strongly on the polarization purity of the input beam or input polarizer and on the quality of the exit polarizer.

Liquid crystal tunable filters are gaining wide acceptance in such diverse areas as optical fiber communications, astronomy, remote sensing, pollution monitoring, color generation for display and medical diagnostics.