1 Dual Etalon Frequency Comb Spectrometer David W. Chandler and Kevin E. Strecker Sandia National Laboratories – Biological and Energy Sciences Division Combustion Chemistry Department Livermore, California

2 What is the Dual Etalon Frequency Comb Spectrometer? Broadband (frequency comb) cavity ring-down spectroscopy Extremely high frequency resolution, ~1MHz (10 -5 cm -1 ) Configurable to operate from the UV to mid-IR Straight forward and low cost Does not require high-resolution or special laser(s) Does not even require a laser Works in both absorption and fluorescence modes

3 Confocal Resonator Laser Laser Bandwidth High finesse confocal cavity How cavity ring-down is NOT done Mode structure from etalon boundary conditions Mode beating on the detector adds noise to the ring-down signal Modes beat at the free-spectral range (or cavity spacing) of the etalon.

4 Confocal Resonator Laser Laser Bandwidth High finesse confocal cavity Mode structure from etalon boundary conditions How cavity ring-down is NOT done Solution? Get rid of the mode structure.

5 High finesse confocal cavity Add a second cavity, with a slightly different free-spectral range and overlap the output of both cavities onto a single photo-detector. FSR1 FSR1+  Laser Laser Bandwidth How dual etalon cavity ring-down is done Make use of the cavity mode structure The different frequency modes will interfere. The heterodyne signal of each nearest-neighbor frequency pair will have a unique radio-frequency. This allows us to extract high resolution spectra from under the bandwidth of a single broadband laser pulse.

6 Standard cavity ring-down signal but with mode beating Single laser shot data:

7 The Fourier transform of the cavity ring-down signal reveals the frequencies

8 However, a non-transform limited laser has phase/frequency structure Each laser shot has different mode structure under the bandwidth of the laser. This makes direct extraction of the spectrum impossible.

9 The FT can be done as a function of time giving the CRD signal for each frequency mode.

10 Broad bandwidth spectra are obtained in a single laser shot with high (<0.01cm -1 ) resolution spectra. We obtain, single shot spectrum. Each point has a width of 250kHz spaced by 300MHz over the laser bandwidth

ps Pulse amplified ASE, with a 1nm FWHM Band-pass filter at 632nm Dual Etalon cavity ring-down signal FFT 750MHz A coherent light source is not necessary. The boundary conditions of the etalons dictated the phase. The cavities are separated by ~ 250KHz. We can resolve features with ~5MHz There is ~ 3nm or 75cm -1 of spectrum. With ~ 1cm -1 resolution.

12 Summary We have devolved a new technique for performing broadband cavity ring- down frequency comb spectroscopy We can resolve small pieces of spectrum with very high resolution and large pieces of spectrum with standard ~0.1 to 1 cm -1 resolution We have extracted spectra from single laser ns-pulses. And demonstrated the technique using broadband ASE. By monitoring the frequencies as a function of time during the ring-down we can compensate for mode structure, or in the absence of structure, we can monitor transient absorptions. Currently building the next generation spectrometer for use in the mid-infrared Funding: DOE Office of Basic Energy Sciences