Cavity-enhanced multiplexed comb spectroscopy down to the photon shot noise

We demonstrate quantum-noise-limited frequency-comb cavity-enhanced absorption spectroscopy with spectrally multiplexed detection for acquisition times ranging from 10 ms up to 10 min, where a record absorption sensitivity of $7\times {10}^{-13}$ ${\mathrm{cm}}^{-1}$ per spectral element is attained with nW power levels. For this, a widely tuneable frequency-doubled free-running fs Ti:sapphire laser oscillator is coupled to a cavity of extreme finesse (32 000) whose length is modulated and whose output is spectrally dispersed over a linear charge coupled device array. This scheme is robust, as it does not require tight frequency locking or controlling the comb frequency offset, and works with spectrally broad combs, as it is not limited by cavity dispersion.

Figure 1

Experimental scheme based on a Ti:sapphire fs oscillator, a beta barium borate (BBO) doubling crystal, and mode matching optics to inject a high finesse cavity whose length is finely controlled by a translation stage (TS) and modulated by a piezoelectric actuator (PZT) controlled by tracking electronics. An acousto-optic deflector (AOM) allows laser interruption for ring-down measurements. An avalanche photodiode (APD) shares the cavity output beam with a CCD high resolution spectrometer.

Figure 2

Top left: normalized IO spectrum for one 12 ms CCD exposure. Top right: spectrum of NO${}_2$ from averaging 41 000 CCD exposures. Spectral fits using a linear combination of literature cross sections for IO and NO${}_2$ yield the displayed molecular concentrations. Fit residuals have rms values corresponding to the expected level of photon shot noise (read discussion in the text): At the bottom is a plot of the rms versus the averaging number.