Optical beam deflection is indispensable for a wide range of applications. So far, mechanical deflectors are the popular systems used to deflect an optical beam, but typically, the response speed is limited to a maximum of approximately 10 kHz. Alternatively, deflectors using an electro-optic effect can deflect an optical beam at much higher frequency. Specifically, deflectors using the electro-optic Kerr effect on a potassium tantalate niobate (KTa1-xNbxO3, KTN) are quite attractive since it can make a substantial change in the refraction index, which is proportional to the square of the electric field. KTN can also store electrons injected from the cathode inside the crystal, which form a refractive index distribution. If the density of the injected electrons and the permittivity are spatially uniform, the refractive index distribution is a convex upward parabolic function of the distance from the cathode. Thus, the crystal functions as a gradient-index lens. When a voltage is applied to the crystal, the center of the lens shifts towards the cathode, and an optical beam passing through the crystal bends towards the vertex of the refractive index, hence the crystal also functions as a deflector.
However, previous research has shown that the electron density tends to be larger near the cathode than near the anode, and does not become uniform within the crystal since electrons are injected from the cathode side only. Consequently, when an AC voltage superimposed on a DC voltage is applied to a KTN crystal, electrons are injected only from one electrode, and an optical beam deflected to the cathode goes through the region in which the electron density is larger than that near the anode. In this case, because the focal length of the lens depends on the electron density, the deflected optical beam cannot be collimated with lenses. Therefore, these fluctuations in one cycle are a big problem when attempting to use the device in various applications. To address this, NTT Corporation researchers in Japan: Dr. Takashi Sakamoto, Dr. Tadayuki Imai, Dr. Yuichi Akage, Dr. Masahiro Ueno, Dr. Sohan Kawamura, and Dr. Soichi Oka developed a new technique to suppress beam-divergence fluctuations in a DC-biased KTN optical beam deflector. Their work is currently published in the research journal, Applied Physics Express.
In their new configuration, they introduced a temperature gradient in the direction of the electric field applied to the KTN crystal, to produce a permittivity gradient in the direction required to cancel out the effect of the asymmetry of the electron injection. Interestingly, the researchers demonstrated experimentally that this temperature gradient successfully reduced the beam-divergence fluctuation.
In summary, the study demonstrated a novel technique and succeeded in reducing the instantaneous voltage dependence of beam divergence of the KTN optical deflector by providing a temperature gradient between the electrodes. In a statement to Advances in Engineering, the authors said that they believe their technique to be indispensable for optimizing beam characteristics for KTN deflectors.
Takashi Sakamoto, Tadayuki Imai, Yuichi Akage, Masahiro Ueno, Sohan Kawamura, Soichi Oka. Reduction of beam-divergence fluctuation for KTN optical beam deflector. Applied Physics Express; volume 13, 062007 (2020).