Demonstration of an Exposed-Core Fiber Platform for Two-Photon Rubidium Spectroscopy

We demonstrate a promising fiber architecture for generating strong photon-photon interactions. Exposed-core silica optical fibers possess low-loss guidance between 400 and 1700 nm crucial for quantum-logic applications. The potential of this fiber is demonstrated by exciting a two-photon transition within a rubidium vapor using an exposed-core silica optical fiber. Transit-time broadened spectral features enable measurement of the evanescent-field scale length of $(120\pm 20)\mathrm{nm}$ which shows excellent agreement with the characteristics of the modeled fiber mode $(118\pm 2)\mathrm{nm}$. We observe a two-photon absorption coefficient of $8.3{\mathrm{cm}}^{-1}$ for one optical mode in response to a transmitted power of 1.3 mW in the second mode. A clear pathway to an exposed-core fiber exhibiting substantial absorption mediated by a single photon is identified.

Figure 1

(a) A scanning electron microscope image of the exposed-core fiber. (b) Cutback loss measurement of ECF showing $<0.2\mathrm{dB}{\mathrm{m}}^{-1}$ loss from 450 to 1650 nm. An ${\mathrm{OH}}^{-}$ absorption is visible at 1360 nm. Dashed lines indicate the operating wavelength and loss. (c) Finite element model of the power distribution of the fundamental mode. (d) Cross section through the optical mode at $x=0$, indicated by the dashed line in (c). The horizontal line indicates the edge of the silica core. The scale bar shows color scaling for (c)–(e). (e) Experimental near-field mode image observed from the ECF.

Figure 2

(a) Energy-level diagram of the two-photon transition used. Excitation lasers (solid lines) and relevant decay paths (dashed) are indicated. (b) Experimental setup: PBS, polarization beam splitter; HWP, half-wave plate; ECF, exposed-core fiber; PMT, photomultiplier tube.

Figure 3

Fluorescence spectra from the ECF (red line) with fit of the form in Eq. (5) (dashed blue line), the reference beam (green line) as a function of 780-nm detuning from the ${}^{85}\mathrm{Rb}$ $F_g=3\to F^{\prime }=4$ $D_2$ transition. The ECF spectra show transit-time broadening, whereas the reference is a Voigt profile, both expected from Eq. (6). A frequency shift between the two spectra is subtracted, a result of spectra acquired at different times.

Figure 4

Fluorescence from the ECF detected at the PMT showing the presence of copropagating fluorescence (indicated by arrows) alongside counterpropagating fluorescence for difference intermediate-state detunings. The frequency axis is relative to the ${}^{85}\mathrm{Rb}$ $D_2$ $F_g=3\to F^{\prime }=4$ transition.

Figure 5

Transit-time measurements for ${}^{85}\mathrm{Rb}$ (green) and ${}^{87}\mathrm{Rb}$ (red) for different intermediate-state detunings ${\mathrm{\Delta }}_i$. Fluorescence amplitude detected at the PMT for the ${}^{85}\mathrm{Rb}$ $F_g=3\to F^{\prime }$ transition manifold as a function of intermediate-state detuning ${\mathrm{\Delta }}_i$, for fixed laser intensities $I_{780}$ and $I_{776}$ and ${\mathrm{\Delta }}_e=0$. Highlighted points are excluded from the Gaussian fit; see the text.

Figure 6

Fluorescence power detected at the PMT as a function of the product of transmitted laser beam powers.