Heated microprisms deflect beam with no moving parts

John Wallace

Useful for optical switching and other applications, beam deflectors of nonmechanical design can be simpler and more reliable than their counterparts that rely on mechanical motions. For example, electro-optic deflectors operate with no moving parts; however, they must be driven at voltages of at least 1000 V. Researchers at Radiant Research Inc. and The University of Texas at Austin (both in Austin, Texas) have developed an alternative that relies on the thermo-optic effect, in which a temperature change alters the refractive index. The resulting device has a slab waveguide structure, important for optical switching.

The switching structure is fabricated on silicon and consists of alternating thin-film microprisms of silica and polymer clad with silica underneath and polymer on top (see figure). The prisms are 600-µm equilateral triangles with a thickness of 5 µm. The whole is temperature-controlled by a heating electrode. The silica is doped to have the same refractive index as the polymer at 20°C. The silica has a thermo-optic coefficient of +1 × 10-5/°C, while the corresponding figure for the polymer is -1.4 × 10-4/°C; the resulting effect for the design geometry is a temperature-dependent beam deflection of 0.06°/°C. Stable to 250°C, the polymer has an optical loss of 0.2 dB/cm at 633 nm and 0.3 dB/cm at 1550 nm (silica has virtually zero loss at these scales).

Tested using 633-nm light launched into the waveguide by a spherical and a cylindrical lens, the device achieved a maximum beam deflection of 3.7° when subjected to a temperature change of 60°C. Theory predicts six resolvable spots for such a device; the actual three resolvable spots were likely due to a Gaussian beam waist too small by a factor of two. Predicted power consumption of the heating element is 10 mW. For more information, contact Suning Tang at suning@radiantr.com.

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