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Fiber-optic probe for electro-optic sampling Chandani, Sameer M.

Abstract

In-circuit probing of high-speed circuits is becoming a common need as circuit complexity and operating frequencies increase. Electrical methods are not generally capable of probing internal points that are not matched to 50 Ω. Electro-optic sampling is a proven technique for noninvasive in-circuit probing of high-speed circuits, but remains a laboratory tool used by a few experts. This work is aimed at designing and developing a fiber optic based electro-optic sampling system that can be used by test engineers to measure the voltage waveform at internal points. The use of a fiber based pulsed laser and confinement of the optical sampling pulses to fiber optic cables and components results in a compact probe station style system that is easy to use. A novel fiber optic based sampling tip has been designed, built, tested and incorporated into a custom designed microwave probe station. The sampling tip has been made by attaching AlGaAs, an electro-optic material to the end of a fiber optic cable and thinned to a thickness of 1.6 μm using selective wet etching. A theoretical model has been developed to describe the characteristics of the tip when exposed to an external electric field. Measurements of the electric field over a coplanar waveguide structure with 60 μm center and 30 μm gaps have been successfully made. The measurements are consistent with results obtained from an electrostatic simulation of the coplanar waveguide. The electro-optic sampling tip has a V[sub π] of 2200 kV when used as a sensor over the coplanar waveguide at a height of 6 μm above the surface of the conductor. The tip can also be accurately placed over a test point by monitoring the power reflected from the surface of the circuit. A phase locked loop (PLL) has been implemented to synchronize the optical sampling pulses to the signal driving the circuit. The PLL successfully locks the optical pulses and signal driving the circuit to a reference oscillator without degrading the phase noise of the optical sampling pulses.

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