Observation of fundamental thermal noise in optical fibers down to infrasonic frequencies

Abstract

The intrinsic thermal noise in optical fibers represents the ultimate limit for fiber-based systems. However, at infrasonic frequencies, the spectral behavior of the intrinsic thermal noise is still unclear. In this letter, we present measurements of the fundamental thermal noise in optical fibers that are obtained using a balanced fiber Michelson interferometer. When an ultra-stable laser is used as the laser source and other noise sources are carefully controlled, the 1/f spectral density of the thermal noise is observed down to infrasonic frequencies, and the measured magnitude is consistent with the results of theoretical predictions at frequencies over the range from 0.2 Hz to 20 kHz. Moreover, as observed experimentally, the level of the 1/f thermal noise can be reduced by changing the coatings of the optical fibers. This therefore indicates one possible way to reduce thermal noise in optical fibers at low Fourier frequencies. Finally, the inconsistency between the experimental data and the existing theory for thermomechanical noise is discussed.

Gravitationally induced phase shift on a single photon

“…The used fiber parameters are: thermal conductivity k = 1.37 W mK −1 , refractive index temperature coefficient =´n T d d 9.52 10 6 K −1 , effective refractive index n=1.468, coefficient of linear expansion a =´-5 −1 , thermal diffusivity =´-D 0.82 10 6 m 2 s −1 , mode-field radius w m = 5.2 m 0 and fiber outer radius a f =62.5 μm. The Wanser theory is in excellent agreement with experimentally observed noise figures for Fourier frequencies above 1 kHz[20,23].…”

“…The measured power spectral density (PSD) for typical fibers used in optical fiber sensing (see e.g. [20,23]), has a f 1 dependence in the low frequency regime, which shows good agreement with theories for mechanical dissipation in optical fibers [24]. For frequencies over 1 kHz, the observed PSD curves show excellent agreement with another theory for thermal phase noise by Wanser [22].…”

“…The rootmean-square amplitude of phase noise fluctuations depends strongly on the geometry and material of the fiber. Using the fiber parameters for Corning's standard single mode fiber SMF-28 [23], the noise contribution from intrinsic thermal phase noise for an interferometer with total length = l 2 200 kmcan be 7 estimated to be around 10 −6 rad Hz −1/2 at 100 kHz according to Wanser's theory ( figure 4). A better estimation for this bound can only be given by measuring the crucial parameters for the fibers actually used in this experiment, although the order of magnitude should not change for commercially available optical fibers.…”

Laser stabilization to a cryogenic fiber ring resonator

“…In spite of recent alternative approaches [8], the best results have been obtained with lasers locked to ultra-stable external frequency references. Well-studied examples for such system include spectral holes in rareearth doped crystals [9,10], fiber-optical delay lines [11][12][13], as well as whispering-gallery-mode [14] and Fabry-Perot resonators [15][16][17]. The latter have demonstrated an impressive performance down to a relative frequency stability of 4 • 10 −17 [18].…”

Single photon detection and manipulation in photonic integrated circuits

“…The most likely reasons for the phase noise in our experiments were thermal fluctuations affecting optical fibers [264] and electronic noise associated with single-photon counting unit [265]. The noise spectral density shown in Fig.…”