1 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. PlusquellicNational Institute of Standards and Technology,Gaithersburg, MD250 spectra in 0.7 s68th Ohio State University InternationalSymposium on Molecular SpectroscopyJune 17-21, 2013, Columbus OH
2 Cavity ring-down spectroscopy (CRDS) Single-mode excitation with locked cavityrecirculating fieldFabry-Pérot resonatorlow-loss dielectric mirrordetectorincident laser beamAttributes:compact volumeinsensitive to atmospheric absorption and laser intensity noiselong effective pathlength, leff = lcav(Finesse/)potentially high spectral resolution & negligible instrumental broadeningreadily modeled from first principlesspectra based on observation of time and frequency
3 multi-mode cavity ring-down spectroscopy (CRDS) A little history …multi-mode cavity ring-down spectroscopy (CRDS)signal with pulsed excitationtransform-limited pulsetransverse mode beatsSignals are dominated by transverse and longitudinal mode beating effects, resulting in suboptimal statistics and severely compromised frequency resolution.
4 CRDS with continuous wave lasers empty-cavityabsorbing mediumsingle-mode decay signalsExcitation bandwidth << free-spectral range (FSR)cavity mode spectrum
5 RD signal amp; acq. rate (Hz) frequency detuning meas. cw-CRDS scanning methodstechniqueRD signal amp; acq. rate (Hz)frequency detuning meas.frequency res.otherdither cavity length,step tune laser viacurrent, pzt or templow;externaletalon, l-meterlaser bandwith, >> cavity linewidthstd. approach,slow scandither laser frequency through FSR at fixed cavity length, step tune laser viacavity mode spacing,>> cavity linewidthslow scan,no cav. pzt req’drapidly sweep laser frequency via current tuninglow; ~5 kHzmode spacingcavity line widthRD signal distortionoptical feedback lock oflaser to cavity, scan cavity to drag laser frequencyhigh; ~5 kHzpzt tuning ofcavity mirrorcan’t use 2-mirror cavity, non-linear tuning axis
6 Frequency-stabilized Cavity Ring-Down Spectroscopy (FS-CRDS) Enables high-fidelity and high-sensitivity measurements of transition areas,widths & shapes, positions and pressure shifts
7 High-spectral fidelity of FS-CRDS Saturation dip spectroscopyof blended H2O spectraSystematic errors arisefrom overly simplistic line shapesVoigt ProfileGalatry ProfileLine shape effects in O2
8 The problem of slow frequency tuning To record a spectrum in FS-CRDS you typically tune the laser frequency byusing a grating, pzt-actuated mirror or by temperature tuningThese approaches limit the spectrum acquisition rate to ~5 s/jumpoptical frequency
10 Frequency-agile, rapid scanning (FARS) spectroscopy Method:Use waveguide electro-optic phase-modulator (PM) to generate tunable sidebandsDrive PM with a rapidly-switchable microwave (MW) sourceFix carrier and use ring-down cavity to filter out all but one selected side bandMW sourcephase modulatorcw laserring-down cavityside-band spectrumDetectorgas analyteAdvantages:Overcomes slow mechanical and thermal scanningLinks optical detuning axis link to RF and microwave standardsWide frequency tuning range (> 90 GHz = 3 cm-1)
11 FARS measurement principle cavityresonancesFSRfrequencyscanningnC+dnC+d+FSRnC+d+2FSR
12 How well does the cavity filter out sidebands? FSRLowest order of a spurious sideband close to a cavity mode is1- N where,N = Round(R=FSR/ d)dcarrierselected sideband
13 Cavity filtering (fixed TEM) In general for unwanted sideband orders,local detuning/cavity linewidth = e*finesse/Nwhere 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 FSRIf there is a spurious overlap, one can readily change carrier detuning to avoid this situation
14 Sideband filtering for our spectrometer R = (MHz)/13 (MHz) = so that N = 16 andepsilon = , meaning that the m = -15 sideband would bethe first one to come near a resonance of the cavity.For our finesse of 20,000, the local detuning would be about485 times the cavity line width, showing that we havenearly perfect frequency discrimination (assuming perfect modematching into TEM00).We have never observed any evidence of spuriouscoupling into other sidebands.
15 Independent methods for characterizing frequency axis of PDH-locked FARS-CRDS setup1. Measure frequency, f, of probe laser with optical frequency comb andcount change in mode order, q.Gives absolute frequencies and cavity free spectral range (FSR).2. Measure FSR from differences in microwave frequenciescorresponding to transmission resonance peaks.3. Measure FSR with dual sideband method of Devoe & Brewer.Methods 2 & 3 give agreement in FSR at 2 Hz leveland yield dispersionAbsolute frequencies are ~5 kHz and are limited by10 kHz stability of I2-stabilized HeNe reference laser
16 Dual-sideband FSR measurement scheme q-1qq+1DD-dD+dw0 –(w1+w2)w0 -w1w0 –(w1-w2)w0 -w2w0 +w2w0w0 +(w1-w2)w0 +w1+w2w0 +w1Two sets of sidebands:w1 at FSR= sw2 for PDH lock= w0 – qsd = w1 – sDemodulation of heterodyne beat at w1 - w2 givesdispersion signal g(d) centered about d=0, whered = w1 – s.Devoe & Brewer, PRA 30, 2827 (1984).
17 Accuracy of FARS-CRDS frequency axis cavity dispersiongDD 40 fs2
18 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 resonancesThe width of the resonances provides an equivalent measure of the absorption in the frequency domain,α = Δω1/2/c~130 Hz relative laser linewidthUncertainty of the fitted resonance frequency ~1 HzUncertainty of the fitted width of the resonances ~0.04%
19 Effect of beam extinction ratio on ring-down time measurement statistics
20 Extinction ratio = 10 log( Id/Il) Il = “leakage” intensityId = “decay” intensityIdIlcavity decay signal = Idexp(-t/t)cw leakage signal = IlIdeal case (infinite extinction ratio):Il = 0, exponential decayActual case:leakage intensity interferes with decay signalto yield noisier and/or non-exponential decay
21 Measured FARS-CRDS decay signals Noise in residuals is insensitive toextinction ratio (phase-locked case)Systematic deviations become importantfor extinction ratios < 50 dB
22 Effect of extinction ratio on measurement precision This workHuang & Lehmann, Appl. Phys. B 94,355 (2009)With DFB laser leakage intensity introducesexcess noise in ring-down signal/ = 8x10-5phase locked case, small amount of excess noise