Electro-optic probing of microwave fields using a wavelength-tunable modulation depth

Abstract

A design for an electro-optic microwave-field-mapping system that eliminates the need for the polarizer, analyzer and quarter-wave retarder in its optical-probe-beam path, while also exhibiting less laser-induced noise and an enhanced signal-to-noise ratio as compared to conventional single-or double-pass electro-optic intensity modulators, is conceived and demonstrated. The measurement approach utilizes a high-reflection, resonant-microcavity probe, an entirely fiber-enclosed optical-beam path and a photonic heterodyne-mixing technique. A model for the reflective resonant probe is proposed and then employed to calculate electro-optic phase retardation and a modulation-efficiency slope, the latter of which is found to indicate that system sensitivity can be optimized via tuning of the input laser wavelength. This concept is experimentally verified using thin lithium tantalate electro-optic probe crystals configured as balanced resonators, and then an entirely fiber-coupled measurement is demonstrated in a variety of near-field, spatial scans over an X-band patch antenna.

Recent Advances in the Design of Electro-Optic Sensors for Minimally Destructive Microwave Field Probing

“…To enhance the interaction length of the crystal, there have been numerous investigations that utilize a resonance effect [10,22–28]. Building an optical cavity around the crystal, the probing light can be contained for much longer than in the single-round-trip case, and thus resonantly enhanced cumulative phase components can contribute to an improvement in sensitivity.…”

“…In practical respects, the reflective scheme is generally preferred, with a sensor tip as the terminal of the optical sensor and the light being modulated at the sensor terminal when it is exposed to an electric field. The reflected beams to be detected are efficiently returned from the sensor using a number of common methods: a mirror [ 7 – 9 , 13 – 15 ]; total internal reflection [ 11 – 13 ]; highly reflective surfaces from the device under tests [ 16 ]; or through fabrication of a resonator on the sensor plate [ 10 ].…”

“…For the bulk probes, interference with the signals to be measured or the operation of the device under test can be substantially decreased, and scanning mobility and spatial resolution can be significantly improved, by mounting the sensor crystals on the output facet of an optical fiber [shown in Figure 1(a) ]. Various types of tip-on-fiber EO sensors have been reported [ 7 – 10 , 17 , 18 ] and we review and compare five distinct types of EO-sensor designs [shown in Figure 1(b) ].…”

“…In the first three types of probes, not only does the conventional type-I case use just a single round-trip phase modulation (based on the double pass scheme of the probes), but so too, essentially, do the other two interference-based probes. To enhance the interaction length of the crystal, there have been numerous investigations that utilize a resonance effect [ 10 , 22 – 28 ]. Building an optical cavity around the crystal, the probing light can be contained for much longer than in the single-round-trip case, and thus resonantly enhanced cumulative phase components can contribute to an improvement in sensitivity.…”

“…It is apparent that the design and implementation of EO sensors are crucial to achieving non-destructive microwave sensing applications with suitable sensitivity. One of the most widely used configurations is the mounting of a tiny EO-sensitive crystal tip onto a fiber facet, as this allows the development of all-dielectric embodiments with reasonably small size that minimize distortion of the electric fields to be sensed [ 7 – 10 ].…”