Fully fiber-optic intensity modulator for large-core, high-NA multi-mode fibers

Abstract

Intensity modulation of broadband optical signals transmitted through large-core fiber-optic cables often resorts to opto-mechanical choppers, usually in the form of bulky rotating wheels. We demonstrate an alternative fiber-optic intensity modulator with a compact footprint of 55.6 × 12.8 × 4.6 mm3 that can be integrated in-line in an optical system. The design comprises two optical fiber cantilevers separated by a small air gap. The optical modulation is achieved by controlling the transverse offset between the cores of the input and output fibers using a piezoelectric actuator. The demonstrated implementation can be used as a quasi-static attenuator down to 0 Hz or an arbitrary waveform modulator up to 400 Hz, providing 30-dB modulation contrast across the complete fiber transmission range. The feasibility of this device is demonstrated in a fiber-based remote pyrometry sensor using lock-in detection in the 2 to 2.5-μm infrared band, where the thermal radiation collected by a multi-mode optical fiber with a 360 μm core is modulated at 390 Hz. The resulting noise-equivalent temperature difference is demonstrated to be better than 0.6°C for object temperatures above 35°C.