Passive Optical Limiting

Summary The growth of laser applications in a large number of areas, the emergence of optical information processing, and the prospect of optical computing have lead to a need for various types of optical limiters and switches. One such device, the OPL, is the opticalequivalent of the Zener diode. We have demonstrated such devices with picosecond responsetimes using various liquids and liquid crystals ash the nonlinear optical medium. We havealso done studies on optical limiting in solids.8' '1 The continued development of mate- rials, such as liquid crystals, with large, fast, broad band nonlinearities is needed tolower the threshold power at which such devices limit or switch off. This development, onthe other hand, is paced by research efforts which are directed toward developing a more SPIE Vol. 540 Southwest Conference on Optics (1985) / 3 where n^ is the nonlinear refractive index in esu, A is the laser wavelength, and c is the speed of light in vacuum. Note that the limiting power scales as the wavelength sguared for constant n2.By choosing a material with the appropriate n2 one can adjust the limiting power to the desired value. The fact that liquids having different values of n2 may be mixed to adjust Pc is a distinct advantage of liquid based optical limiters.3 An additional advantage is that if optical breakdown of the material occurs, liquids are self-healing. In general it is a simple matter to adjust PC to a value higher than that shown in Figure 1 since CS2 has a relatively large n2 and there are many solvents (e.g., CCl^ and ethanol) which have very small n^'s. It is quite difficult to decrease the limiting power below that achieved with CS2. While we and other workers have studied materials which exhibit larger nonlinearities than CS2 for selected wavelengths and pulsewidths, to our knowledge no other material has the combined large n2, (1.3 x 10"11 esu) fast response time (2 ps), and broad band response observed in CS2. In fact CS2 has a transmission window in the 10.6 ym region and we have recently demonstrated pulsed-power limiting and cw limiting at this wavelength. The pulsed limiting experiments are reported in detail elsewhere in these proceedings and cw results are shown in Figure 2.The mechanism for the limiting shown in Figure 2 is thermal defocusing. The residual absorption in CS2 at 10 ym is approximately 0.1 cm"1. In this demonstration the beam was focused into a 1 cm thick cell with a 25 cm focal length lens used at f/10. A more optimum geometry, e.g., a tighter focus into the cell, should result in limiting at a lower input than the 0.1 W shown in Figure 2.Another advantage of using liquids for limiter applications is that they can be used as solvents for other materials that can alter their optical properties.-* For example, chlorobenzene is a solvent which exhibits a relatively large, fast optical Kerr effect as shown in Figure 3. In Figure 3 the laser wavelength is 530 nm, the beam is circularly polarized, and the pulsewidths are 30 and 100 ps (FWHM). In this case limiting occurs at approximately 35 kW, which may be reasonable for some ps applications, but would be of little use for cw beams. However, a slight amount of mode-locking dye dissolved in the chlorobenzene results in a small amount of broad band absorption. This absorption, in turn, results in thermal defocusing and the resulting limiting is shown in Figure 4. In this case the laser source is a 545 nm, linearly polarized, cw Argon ion laser beam. Limiting is initiated at 3-5 mW and is maintained up to the 2 W maximum output of the laser source used in this experiment. No attempt was made to optimize the thermal defocusing which produced the limiting shown in Figure 4. For example, higher dye concentrations and a tighter beam focus could significantly reduce the limiting power.Much of our effort is directed toward finding materials which exhibit large nonlinear absorption and/or nonlinear refraction. Three classes of materials which we are currently examining in detail are liquid organics, liquid crystals and intermediate band-gap semicon­ ductors (such as the II-VI materials).8 ' 9 fl(^ Effective limiting has been demonstrated in the liquid crystal MEBBA, (4-methyl benzylidene 4l -n-butylaniline) for 0.53 urn, ps pulses.11 The onset of nonlinear transmission in this material occurs at a considerably lower inut power than observed for CS2 for similar input parameters. The limiting observed in MEBBA and other liquid crystals at 0.53 ym does not have a sharp cutoff power as does CS^f rather there is a region of continuously changing transmission prior to limiting. This behavior is probably due to nonlinear absorption prior to the onset of catastrophic self-focusing. In fact a two-photon absorption coefficient of 0.6±0.2 cm/GW has been measured for this material.11The mechanism for the observed nonlinearity in MEBBA is not yet understood. We are presently conducting more extensive studies of this material, other Shiff base liquid crystals, and liquid crystal esters. More refined characterization techniques are needed in order to more completely characterize the nonlinear properties of materials for which both nonlinear absorption and nonlinear refraction play an important role. Techniques presently used in our laboratories are described in greater detail elsewhere. ' 9fl^SummaryThe growth of laser applications in a large number of areas, the emergence of optical information processing, and the prospect of optical computing have lead to a need for various types of optical limiters and switches. One such device, the OPL, is the optical equivalent of the Zener diode. We have demonstrated such devices with picosecond response times using various liquids and liquid crystals as the nonlinear optical medium. We have also done studies on optical limiting in solids. ffl The continued development of mate­ rials, such as liquid crystals, with large, fast, broad band nonlinearities is needed to lower the threshold power at which such devices limit or switch off. This development, on the other hand, is paced by research efforts which are directed toward developing a more