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Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

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Presentation on theme: "Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,"— Presentation transcript:

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. Plusquellic National Institute of Standards and Technology, Gaithersburg, MD 68 th Ohio State University International Symposium on Molecular Spectroscopy June 17-21, 2013, Columbus OH 250 spectra in 0.7 s

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

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

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

5 cw-CRDS scanning methods techniqueRD signal amp; acq. rate (Hz) frequency detuning meas. frequency res. other dither cavity length, step tune laser via current, pzt or temp low; external etalon, - meter laser bandwith, >> cavity linewidth std. approach, slow scan dither laser frequency through FSR at fixed cavity length, step tune laser via current, pzt or temp low; external etalon, - meter cavity mode spacing, >> cavity linewidth slow scan, no cav. pzt reqd rapidly sweep laser frequency via current tuning low; ~ 5 kHzmode spacing cavity line width RD signal distortion optical feedback lock of laser to cavity, scan cavity to drag laser frequency high; ~ 5 kHzpzt tuning of cavity mirror cavity line width cant 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 spectroscopy of blended H 2 O spectra Systematic errors arise from overly simplistic line shapes Voigt Profile Galatry Profile Line shape effects in O 2

8 The problem of slow frequency tuning optical frequency 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

9 Rapid Step Scanning of Laser Frequency

10 Frequency-agile, rapid scanning (FARS) spectroscopy 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 ) 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

11 C + +2FSR C + +FSR C + cavity resonances frequency scanning FSR FARS measurement principle

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

13 In general for unwanted sideband orders, local detuning/cavity linewidth = *finesse/N where = 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 Cavity filtering (fixed TEM)

14 R = (MHz)/13 (MHz) = so that N = 16 and epsilon = , 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 TEM 00 ). We have never observed any evidence of spurious coupling into other sidebands. Sideband filtering for our spectrometer

15 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 I 2 -stabilized HeNe reference laser

16 Devoe & Brewer, PRA 30, 2827 (1984). Dual-sideband FSR measurement scheme 0 –( ) –( ) ( ) qq-1q Two sets of sidebands: 1 at FSR= 2 for PDH lock 0 – q = 1 – Demodulation of heterodyne beat at gives dispersion signal g( ) centered about, where = 1 –

17 Accuracy of FARS-CRDS frequency axis cavity dispersion g DD 40 fs 2

18 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 = Δω 1/2 /c ~130 Hz relative laser linewidth Uncertainty of the fitted resonance frequency ~1 Hz Uncertainty of the fitted width of the resonances ~0.04% Measuring losses in terms of cavity line width

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

20 Extinction ratio = 10 log( I d /I l ) t 0 I l = leakage intensity I d = decay intensity IdId IlIl cavity decay signal = I d exp(-t/ ) cw leakage signal = I l Ideal case (infinite extinction ratio): I l = 0, exponential decay Actual case: leakage intensity interferes with decay signal to yield noisier and/or non-exponential decay

21 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

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

23 FARS-CRDS has been demonstrated with: 1)distributed feedback diode laser (DFB) 2)single-mode fiber laser 3)external cavity diode laser (ECDL) with high-bandwidth Pound-Drever-Hall lock waveguide electro-optic phase modulator


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