Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell, R.D van Zee, D.F. Plusquellic National Institute of Standards and Technology, Gaithersburg, MD joseph.hodges@nist.gov 250 spectra in 0.7 s 68th Ohio State University International Symposium on Molecular Spectroscopy June 17-21, 2013, Columbus OH

Cavity ring-down spectroscopy (CRDS) Single-mode excitation with locked cavity recirculating field Fabry-Pérot resonator low-loss dielectric mirror detector incident laser beam Attributes: compact volume insensitive to atmospheric absorption and laser intensity noise long effective pathlength, leff = lcav(Finesse/) potentially high spectral resolution & negligible instrumental broadening readily modeled from first principles spectra based on observation of time and frequency

multi-mode cavity ring-down spectroscopy (CRDS) A little history … multi-mode cavity ring-down spectroscopy (CRDS) signal with pulsed excitation transform-limited pulse transverse mode beats Signals are dominated by transverse and longitudinal mode beating effects, resulting in suboptimal statistics and severely compromised frequency resolution.

CRDS with continuous wave lasers empty-cavity absorbing medium single-mode decay signals Excitation bandwidth << free-spectral range (FSR) cavity mode spectrum

RD signal amp; acq. rate (Hz) frequency detuning meas. cw-CRDS scanning methods technique RD signal amp; acq. rate (Hz) frequency detuning meas. frequency res. other dither cavity length, step tune laser via current, pzt or temp low; 10 - 100 external etalon, l-meter laser bandwith, >> cavity linewidth std. approach, slow scan dither laser frequency through FSR at fixed cavity length, step tune laser via cavity mode spacing, >> cavity linewidth slow scan, no cav. pzt req’d rapidly sweep laser frequency via current tuning low; ~5 kHz mode spacing cavity line width RD signal distortion optical feedback lock of laser to cavity, scan cavity to drag laser frequency high; ~5 kHz pzt tuning of cavity mirror can’t use 2-mirror cavity, non-linear tuning axis

Frequency-stabilized Cavity Ring-Down Spectroscopy (FS-CRDS) Enables high-fidelity and high-sensitivity measurements of transition areas, widths & shapes, positions and pressure shifts

High-spectral fidelity of FS-CRDS Saturation dip spectroscopy of blended H2O spectra Systematic errors arise from overly simplistic line shapes Voigt Profile Galatry Profile Line shape effects in O2

The problem of slow frequency tuning To record a spectrum in FS-CRDS you typically tune the laser frequency by using a grating, pzt-actuated mirror or by temperature tuning These approaches limit the spectrum acquisition rate to ~5 s/jump optical frequency

Rapid Step Scanning of Laser Frequency

Frequency-agile, rapid scanning (FARS) spectroscopy Method: Use waveguide electro-optic phase-modulator (PM) to generate tunable sidebands Drive PM with a rapidly-switchable microwave (MW) source Fix carrier and use ring-down cavity to filter out all but one selected side band MW source phase modulator cw laser ring-down cavity side-band spectrum Detector gas analyte Advantages: Overcomes slow mechanical and thermal scanning Links optical detuning axis link to RF and microwave standards Wide frequency tuning range (> 90 GHz = 3 cm-1)

FARS measurement principle cavity resonances FSR frequency scanning nC+d nC+d+FSR nC+d+2FSR

How well does the cavity filter out sidebands? FSR Lowest order of a spurious sideband close to a cavity mode is 1- N where, N = Round(R=FSR/ d) d carrier selected sideband

Cavity filtering (fixed TEM) In general for unwanted sideband orders, local detuning/cavity linewidth = e*finesse/N where e = R – N (non-integer remainder) In the absence of dispersion, this level of discrimination does not change as the modulation frequency is stepped in increments of the FSR If there is a spurious overlap, one can readily change carrier detuning to avoid this situation

Sideband filtering for our spectrometer R = 203.076 (MHz)/13 (MHz) = 15.621 so that N = 16 and epsilon = 0.3788, meaning that the m = -15 sideband would be the first one to come near a resonance of the cavity. For our finesse of 20,000, the local detuning would be about 485 times the cavity line width, showing that we have nearly perfect frequency discrimination (assuming perfect mode matching into TEM00). We have never observed any evidence of spurious coupling into other sidebands.

Independent methods for characterizing frequency axis of PDH-locked FARS-CRDS setup 1. Measure frequency, f, of probe laser with optical frequency comb and count change in mode order, q. Gives absolute frequencies and cavity free spectral range (FSR). 2. Measure FSR from differences in microwave frequencies corresponding to transmission resonance peaks. 3. Measure FSR with dual sideband method of Devoe & Brewer. Methods 2 & 3 give agreement in FSR at 2 Hz level and yield dispersion Absolute frequencies are ~5 kHz and are limited by 10 kHz stability of I2-stabilized HeNe reference laser

Dual-sideband FSR measurement scheme q-1 q q+1 D D-d D+d w0 –(w1+w2) w0 -w1 w0 –(w1-w2) w0 -w2 w0 +w2 w0 w0 +(w1-w2) w0 +w1+w2 w0 +w1 Two sets of sidebands: w1 at FSR= s w2 for PDH lock = w0 – qs d = w1 – s Demodulation of heterodyne beat at w1 - w2 gives dispersion signal g(d) centered about d=0, where d = w1 – s. Devoe & Brewer, PRA 30, 2827 (1984).

Accuracy of FARS-CRDS frequency axis cavity dispersion gDD  40 fs2

Measuring losses in terms of cavity line width Due to the quality of our frequency axis we can record the shape and width of individual cavity resonances The width of the resonances provides an equivalent measure of the absorption in the frequency domain, α = Δω1/2/c ~130 Hz relative laser linewidth Uncertainty of the fitted resonance frequency ~1 Hz Uncertainty of the fitted width of the resonances ~0.04%

Effect of beam extinction ratio on ring-down time measurement statistics

Extinction ratio = 10 log( Id/Il) Il = “leakage” intensity Id = “decay” intensity Id Il cavity decay signal = Idexp(-t/t) cw leakage signal = Il Ideal case (infinite extinction ratio): Il = 0,  exponential decay Actual case: leakage intensity interferes with decay signal to yield noisier and/or non-exponential decay

Measured FARS-CRDS decay signals Noise in residuals is insensitive to extinction ratio (phase-locked case) Systematic deviations become important for extinction ratios < 50 dB

Effect of extinction ratio on measurement precision This work Huang & Lehmann, Appl. Phys. B 94, 355 (2009) With DFB laser leakage intensity introduces excess noise in ring-down signal / = 8x10-5 phase locked case, small amount of excess noise

FARS-CRDS has been demonstrated with: waveguide electro-optic phase modulator distributed feedback diode laser (DFB) single-mode fiber laser external cavity diode laser (ECDL) with high-bandwidth Pound-Drever-Hall lock