Deep Inversion Offers Fast Modulation

Jörg Schwartz

Electro-optic modulators need to have a fast response, high bandwidth, a low drive voltage, low coupling loss and high stability, and they need to be small and capable of being integrated. These individual requirements may be standard, but they're not simple to achieve all in one product. However, engineers at the Fraunhofer Institut für Physikalische Messtechnik have developed a technology that combines all these features.

Typical laser modulators are based on an electro-optic effect, inducing a change in refractive index that depends on the strength of the electric field applied. Mach-Zehnder devices, with apertures of just a few microns, require only a few volts, but they work well only with fiber coupling, and a laser beam is difficult to thread. Pockels cells, on the other hand, can be made with larger apertures but have bandwidth limitations.

The new technology still requires that kilovolts be applied, but only once as part of the fabrication process. The locally applied electric field induces deep-domain inversion in ferroelectric crystals, enabling modulation of the refractive index with as little as 30 to 300 V applied to surface electrodes. The inverted areas behave differently because different electro-optic coefficients become effective.

Beam manipulation

Vladimir Hinkov and his group have developed a range of switchable micro-optic components using this principle of beam manipulation. The devices rely mainly on standard optical arrangements; a variable beam deflector, for example, uses about 20 prism pairs "written" into the crystal. However, the components can be highly miniaturized and avoid all precision optical steps such as prism cutting and polishing, Hinkov said.

The basic structure of a beam deflector includes an array of domain-inverted prisms in a 500-µm-thick layer of LiNbO₃. The inverted regions are made visible by etching.

The technology allows rise times down to the nanosecond range, equivalent to megahertz bandwidths. A Bragg modulator demonstrates the speed capabilities using a 6- to 10-µm grating structure of inverted and noninverted line domains. Beams incident under the Bragg angle are deflected and intensity modulated with 98 percent efficiency. The theoretical bandwidth limit of 500 MHz allows 2-ns pulse generation with just a 30-V drive.

Applications such as optical lithography, industrial robotics, optical storage and laser TV projection need miniaturized, integrated, fast devices for modulation, deflection and beam manipulation. But a major prerequisite for commercial success is the capability for mass production, which makes the domain-inversion technology more advantageous: Similar to microelectronics, all functions are based on planar technology, and even complex systems can be integrated onto a single chip.