Fig. 1. (a) Apparatus schematic. Probe and drive laser beams counterpropagate with independent polarization control. A beam splitter (BS) separates the probe into out-of-loop and in-loop detection. Each path has a half-wave plate (λ/2) to set the detection basis, followed by a polarizing beam splitter (PBS) and a balanced photodiode pair. The out-of-loop detection signal goes to a spectrum analyzer, while the in-loop signal splits parts, one to the spectrum analyzer, and the other is amplified, filtered (≈100  kHz wide centered on resonance), and then goes to a control unit that slightly rotates the drive polarization closing the feedback loop. (b) ONF schematic with two effective polarization axes: ordinary and extraordinary indices of refraction, aligned with the optical axes but at an angle Δθ with the input light polarization. The fiber is clamped (not shown) in the unmodified section.

Fig. 2. Mode temperature, calculated from the integral of Sδθ(ω) from out-of-loop measurements, as a function of the drive laser power. The continuous line is a fit to Eq. (8). The inset shows the evolution of Sδθ(ω) as the drive power is raised.

Fig. 3. Mode temperature from the measured statistical distribution of the mean-square angular fluctuation ⟨δθ2⟩ as a function of drive laser power. The continuous line is a fit to Eq. (8). The inset shows representative measured distributions p(θ2) and their fits to Eq. (11). The fits would appear linear on semilogarithmic axes, but making the horizontal axis logarithmic as well facilitates the visualization of the reduction of ⟨δθ2⟩.