Optical characterisation at wavelength of 1549 nm of switchable POLICRYPS diffraction gratings

Measurements on diffraction gratings made of strips of polymer and nematic liquid crystals (POLICRYPS) performed at wavelength of 1549 nm will be presented. A sample cell with thickness of 6.6 micron was prepared by using a mixture, initially homogeneous of UV curable prepolymer NOA61 and 30% concentration in weight of nematic liquid crystal 5CB. A transmission Bragg diffraction grating was written by using an Ar ion laser at wavelength of 352 nm. The grating period is 1.34 um. The cell glass substrate were ITO coated in order to apply an electric field to the sample. At room temperature the diffraction grating is active with a diffraction efficiency of about 70% which can be decreased by applying an electric field to the cell by reorienting liquid crystal molecules. In particular the first order diffracted beam can be switched off by applying a square waveform to the cell of about 25 V amplitude. Such switching behavior is potentially useful to make switchable optical filters and demultiplexers for fiber optic communication systems. Switching performance were measured by using an experimental set-up which included both an He-Ne and an infrared laser probe. Infrared laser was a single mode fiber pigtailed DFB laser emitting at a wavelength of 1549 nm and thermally stabilised by a thermal electric cooling system. A collimator with a FC/PC connector was used to extract a collimated laser beam from a single mode optical fiber. The laser beam in free space impinged onto the surface of the sample which was positioned on a precision rotating stage driven by a computer. An InGaAs photodetector connected to an oscilloscope was used to measure the intensity of the first diffracted beam. In particular angular selectivity, extinction ratio versus applied voltage and switching time were measured at room temperature of about 25 °C both at wavelengths of 632.8 nm and 1549 nm. Transmission characteristics both at visible and infrared wavelengths of the diffraction grating are similar showing a broad angular spectrum due to cell thickness non uniformity. An extinction ratio of over 20 dB at 632.8 nm and 15 dB at 1549 nm was measured with minimum transmission when a 25 V peak voltage was applied. A switching time of less than 5 ms makes these diffraction gratings interesting and promising for photonic switching applications.