Electro-Optic Power Limiter: Broadband, Self-Actuating Optical Limiter for Visible and Infrared Radiation
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polarizer transmits most of the incident light, with some losses due to reflections, scattering, and absorption. If, however, the intensity of the incident light increases above some threshold level, the photoconductivity of the crystal begin to limit the amount of light transmitted through EOPL. At high intensities, the photoionized charge carriers start to move away from the light region and aggregate in the dark regions surrounding the light area. This charge separation induces a local field that opposes the externally applied field, with the net effect being decreased field strength inside the light area. The process is similar to the charge transport that occurs in photorefractive materials, and photorefractive crystals are certainly good candidates for EOPL. In an ideal situation, the induced field cancels out the applied field exactly and the beam experiences no polarization rotation. Then, all the light will be rejected by the analyzer since it is oriented orthogonal to the polarizer, thereby limiting the beam completely. However, in practice, there is always some leakage of light coming through the device due to a number of factors, including imperfections in polarizers, induced birefringence in the crystal, and inhomogeneity in the
crystal.
Low Intensity
[
"Light
I
Polarizer
Analyzer Photoconductive E-O Crystal
High Intensity Light Analyzer
Polarizer
Fig. 1 Illustration of the principle of operation of electro-optic power limiter. 3. EOPL MATERIALS ISSUES A more detailed description of what happens inside the crystal can be given using the energy band diagram of an EOPL crystal, shown in Fig. 2. When a charge (an electron, in this example) is ionized into the conduction band from its deep trap site in the light region, it is driven to another location by the processes of diffusion and drift and is recombined with an empty trap. Because photoionized charges are generated only in the light region, the charges tend to aggregate in the dark region and set up an induced field that opposes the applied electric field. If the transport mechanism is drift-dominated, then the magnitude of the induced field will increase until it cancels out the applied electric field exactly, as long as there are enough charges to build up such a field. The process described above is similar to the well-known photorefractive effect. The main difference is that the charges in the EOPL crystals must move the entire aperture of the incident light, whereas in a typical photorefractive application the charges move one-half of the 62
fringe spacing. The photon energy required for photoionization must be at least the difference between the bottom of the conduction band and the trap energy level. Obviously, the photon energy can be greater as long as it does not exceed the band gap energy. This range, then, is what allows EOPL to have frequency-agility or wide-band wavelength responsivity. For example, the range can be - 200 nm in BSO for the visible wavelengths and - 400 nm in CdTe and GaAs for NIR. Another important crystal
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