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.
A novel optical injection locking amplifier with acousto-optic modulator based phase modulation and a coherent detection scheme for optical frequency transfer applications is experimentally demonstrated in this study. A commercial distributed feedback diode laser is injection-locked to the resonant frequency of the optical signal with an optical fiber path length of hundreds of kilometers. This provides approximately 59 dB gain and ensures that the input carrier frequency fractional stability can be as good as 10−20 at 1000 s. The amplifier was tested for the transfer of a commercial narrow-linewidth laser in a 180 km fiber link to a remote site with only a single amplification step. The transferred frequency at the remote end reached 10−20 at 20000 s, which is suitable for optical frequency distribution and remote comparison between optical atomic clocks.
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