Saturated Optical Absorption by Slow Molecules in Hollow-Core Photonic Band-Gap Fibers

We report on saturated absorption in a hollow-core photonic band-gap fiber filled with ${}^{12}\mathrm{C}_2{\mathrm{H}}_2$ molecules. We find that slow molecules provide a major contribution to the signal in the limit of low optical power and low pressure where the signal deviates significantly from the usual Lorentzian line shape. In particular, we observe a linewidth reduction of about 3 times as compared to the transit-time limited linewidth.

Figure 1

Schematic diagram of the main components in the setup.

Figure 2

(a) Saturated absorption signal. Pump power: $100\mu \mathrm{W}$ ($\theta =0.07$). Acetylene pressure: 3 Pa ($\eta =0.01$). Number of averaged scans: 280. (b) Residual after a Lorentzian fit. (c) Residual after a double Lorentzian fit. The residuals are displaced vertically for clarity.

Figure 3

Filled circles: Measured HWHM linewidth as a function of $\theta$. Acetylene pressure: 3 Pa ($\eta =0.01$). Solid line: Empirical quadratic fit for extrapolation to $\theta =0$. Open squares (inset): Theoretical data from 10, Fig. 9, for $\eta =0.04$. Filled squares (inset): Measurements at 9 Pa ($\eta =0.03$) with a linear extrapolation. The data in the inset have the same units as the main plot.

Figure 4

Measured HWHM linewidth extrapolated to $\theta =0$ as a function of pressure. The solid line shows the theoretical linewidths [7].

Figure 5

Open circles: Standard deviation $\sigma$ of the residual after a Double Lorentzian fit to the saturated absorption signal. Solid squares: Calculated slopes $\alpha$ at the center of the first derivatives of the Double Lorentzian fits. $\alpha$ and $\sigma$ are normalized to the amplitude of the saturated absorption signal. Solid lines: Empirical fits to $\alpha$ and $\sigma$ data. Dashed line: Ratio between $\alpha$ and $\sigma$ fits. All data correspond to the 3 Pa data in Fig. 3.