Using three different optical fiber designs to study humidity effect on the air refractive index

Design and operation of three different optical designs based on the light intensity modulation for investigation of the humidity effects on the air index of refraction are described. The measurement variable is the transmitted power, which depends on the refractive index of the medium in the path of a fiber-to-fiber optical design. Three different probes (probe #1, #2, and #3) are tested in which probe #1 and probe #2 can also check the presence of water or any other transparent liquid in the gap between the two axial fibers. Performances of the new systems are tested as a refractive index monitoring mean and experimental results are given. The variations of the modulated powers as functions of the air humidity change in the light path are determined for the reported optical designs. For probe #1 for a power variation of 255–315 nW, the relative humidity changes from 32% to 41%. For probe #2 for 5 mm gap distance, for a power change of 2470–2754 nW, the relative humidity changes from 54.95% to 90.39%. For probe #3 for the 10 mm fiber gap, a power change 929.5–972.9 nW is noted for the relative humidity change of 33.0–76.75%. Power variations as a function of the axial fiber gap distances are also given and the optimum condition for the highest sensitivity is obtained. Probe #3 shows a higher sensitivity compared to probe #1 and it is superior to probe #2 because it operates in a fog-free condition. Finally, the calibration curve for each probe is given that can be used for the operation of the probe as a humidity sensor.

► For probe #1 power variations in the air and water are given. ► For all probes the calibration curves are obtained. ► Sensitivity is increased by increasing the fiber gap. ► Sensitivity for probe # 3 (1.00 RH%/nW) is higher than that of the probe #1 (0.15 RH%/nW). ► Probe #3 is superior to probe #2 because it performs in a fog-free condition.