Self-action cw-pulsed lasers and glares attenuators

Stray reflections and intense light sources [e.g. sun glares] present severe problems to sensors in various ranging, atmospheric and remote sensing systems. A device capable of spatial frequency discrimination and intensity dependent transmission is highly desirable to remove these unwanted 'signals', while maximally transmitting the scenery under surveillance is highly desirable. Furthermore, because of the temporal range spanning nanosecond laser pulses to cw light sources, the device should possess response time and responses capable of self-action in such time scales. We report a patented nonlinear fiber array [US Patent #5,589,101-Khoo] faceplate device and several newly developed materials that are capable of such operations [1-3]. One example of such material is dye or nano-particulate doped nematic liquid crystal [LC] film in a twisted alignment. In conjunction with crossed polarizers, the LC film will transmit low light level scenery maximally, i.e. provide maximal coverage and monitoring of the environment. On the other hand, an intense light will 'untwist' the nematic liquid crystal alignment, and therefore self-attenuate. The series of photographs below show how an intense 'spot' will self-diminish in millisecond or shorter time while the surrounding images get transmitted. In particular, for input laser spot powers as high as 200 milliwatts, the transmitted output can be clamped to below a few micro Watt's. With the use of electronic nonlinear multi-photon absorption processes in liquids or liquid crystals, these self-action processes can occur in nanosecond or shorter time scales, thereby providing a means of selectively removing unwanted short laser pulses. We will present theoretical modeling of device operational principle and experimental demonstration results for cw and pulsed lasers. In particular, for a neat two-photon absorbing liquids [4], with a molecular transition schemes as shown in Fig. 2a, the calculated transmission curve for nanosecond pulses show that limiting action can be achieved for input laser energy spanning a very wide dynamic ranges [over 5 orders of magnitude]. In this calculation, a nominal two-photon absorption constant β ∼ 4 cm/GW is used. Besides the ground state two-photon absorption property, the excited state absorption dynamics also play a crucial role in yielding such large dynamic range has also been demonstrated in previous studies [4]. With the use of neat liquids with higher two-photon absorption constants, even larger dynamic range can be achieved, thus making the fiber array self-action optical attenuator a versatile protection device for the eye and sensors.