Slow-Light Fourier Transform Interferometer

We describe a new type of Fourier transform (FT) interferometer in which the tunable optical delay between the two arms is realized by using a continuously variable slow-light medium instead of a moving arm as in a conventional setup. The spectral resolution of such a FT interferometer exceeds that of a conventional setup of comparable size by a factor equal to the maximum group index of the slow-light medium. The scheme is experimentally demonstrated by using a rubidium atomic vapor cell as the tunable slow-light medium, and the spectral resolution is enhanced by a factor of approximately 100.

Figure 1

Schematic diagrams of (a) a conventional FT interferometer with one moving arm and one fixed arm; (b) a FT interferometer with a tunable slow-light medium in one of the two fixed arms.

Figure 2

Experimental setup of the FT interferometer using a rubidium vapor cell as the slow-light medium.

Figure 3

(a) Transmission, (b) refractive index $n$, and (c) group index $n_g$ of the 10-cm-long rubidium vapor cell at the temperature of approximately $100°\mathrm{C}$ as functions of detuning ${\nu }^{\prime }$. The thick curve is the measured transmission, and the thin curves are the fitted theory using Eq. (9). The inset shows the energy levels of the ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ $D_2$ transitions, and the gray region is the frequency region over which the spectral measurement are performed.

Figure 4

Output intensity of the slow-light FT interferometer as a function of the group delay ${\tau }_g$ for an input field of two sharp spectral lines separated by 80 MHz.

Figure 5

Retrieved spectrum of the input field using experimental data (solid line) and simulated data (dashed line) and the actual spectrum of the input field (dotted line). The resolution of a conventional FT interferometer of the same size would be approximately 100 times worse.