Thinly coated hollow core fiber for improved thermal phase-stability performance

Abstract

The optical phase accumulated when light propagates through an optical fiber changes with temperature. It has been shown by various authors that this thermal phase sensitivity is significantly smaller in hollow core fibers (HCFs) than in standard single-mode fibers (SSMFs). However, there have been considerable differences in the level of sensitivity reduction claimed, with factors in the range × 3 to × 20 improvement for HCFs relative to SSMFs reported. Here we show experimentally that this large variation is likely attributable to the influence of fiber coating, which is exacerbated in HCFs with a relatively thin silica glass outer wall (e.g., the wall thickness is typically just 20 µm in a 125 µm diameter HCF). Further, we show that the coating also causes the optical phase stability to suffer from relaxation effects, which have not been previously discussed in the HCF literature, to the best of our knowledge. In addition to demonstrating these relaxation effects experimentally, we analyze them through numerical simulations. Our results strongly suggest that they originate from the viscoelastic properties of the coating. To minimize the adverse effects of the coating, we have fabricated a HCF with a relatively thick wall ( ∼ 50 µ m ) and a very thin coating (10 µm). This results in an almost 30-fold reduction in HCF thermal phase sensitivity relative to SSMFs – a significantly lower sensitivity than in previous reports. Moreover, our thinly coated HCF exhibits no discernable relaxation effects while maintaining good mechanical properties.

Temperature Insensitive Delay-Line Fiber Interferometer Operating at Room Temperature

“…To avoid this, we fabricated the fibers with a very thin coating d (about 10 µm coating thickness). We have demonstrated earlier that this thickness significantly reduces the hysteresis in the HCF [20]. To distinguish a generic SSMF with our thinly coated SSMF, we refer to it as T-SSMF with 'T' for "thin coating".…”

“…As the pigtails in both interferometer arms were cut to the same length, the delay in these two interferometers was determined by the 1.6-m T-SSMF and 40-m of T-HCF only. We measured the thermal sensitivity 𝑆 𝜑 of 1.57 rad/m/K for the T-HCF [20] and 39.3 rad/m/K for T-SSMF at 28°C. We give more details about these measurements later.…”

“…We refer to this as the static measurement. In the literature, both methods are used, but they can lead to different results due to the visco-elastic properties of the fiber coating, which causes a delay between the temperature change and the optical phase change [20]. This is highly undesired in the compensated delay-line interferometer, as this delay may be different in the two arms, possibly causing poor compensation of the phase change when the temperature is changing.…”

“…This is highly undesired in the compensated delay-line interferometer, as this delay may be different in the two arms, possibly causing poor compensation of the phase change when the temperature is changing. This delay can be strongly suppressed when fibers with a thin coating are used [20], justifying our choice of thinlycoated HCF and SSMF.…”

“…The calculated 𝑆 𝜑 (𝜆) (Eq. ( 5)) of SSMF and HCF considering this dependence, 𝑛 𝑒𝑓𝑓 (𝜆) of both fibers calculated in COMSOL Multiphysics, and 𝑑𝐿 𝑑𝑇 = 0.3 ppm/K [20], is shown in Fig. 7.…”