cavity-enhanced, frequency-agile rapid scanning spectroscopy: measurement principles j.t. hodges,...
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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, R.D van Zee, D.F. Plusquellic
National Institute of Standards and Technology, Gaithersburg, MD
68th Ohio State University InternationalSymposium on Molecular SpectroscopyJune 17-21, 2013, Columbus OH
250 spectra in 0.7 s
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 noiselong effective pathlength, leff = lcav(Finesse/)potentially high spectral resolution & negligible instrumental broadeningreadily modeled from first principlesspectra based on observation of time and frequency
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
Excitation bandwidth << free-spectral range (FSR)
cavity mode spectrum
0 20 40 60 80tim e (s)
-0.006
0
0.006-0.006
0
0.006
0
0.4
0.8
1.2
Rin
g-do
wn
sign
al (
V) 300 shot average:
= 3 .7097(53) s
= 6 .3783(82) s
Re
sid
ua
ls (
V)
empty-cavity
absorbing medium
single-mode decay signals
CRDS with continuous wave lasers
cw-CRDS scanning methodstechnique RD signal
amp; acq. rate (Hz)
frequency detuning
meas.
frequency res.
other
dither cavity length,step tune laser viacurrent, pzt or temp
low; 10 - 100 externaletalon, l-meter
laser bandwith, >> cavity linewidth
std. approach,slow scan
dither laser frequency through FSR at fixed cavity length, step tune laser viacurrent, pzt or temp
low; 10 - 100 externaletalon, l-meter
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 oflaser to cavity, scan cavity to drag laser frequency
high; ~5 kHz pzt tuning ofcavity mirror
cavity line width
can’t use 2-mirror cavity, non-linear tuning axis
Frequency-stabilized Cavity Ring-Down Spectroscopy (FS-CRDS)
frequency-stabilizedreference laser
cw probe laser
cavity stabilization servo
pztoptical resonator
decay signal
(a)
(b)
time
stabilized comb of resonant frequenciesFSR = 200 MHz
absorption spectrum
frequency
Enables high-fidelity and high-sensitivity measurements of transition areas, widths & shapes, positions and pressure shifts
High-spectral fidelity of FS-CRDS
Saturation dip spectroscopyof 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
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
Rapid Step Scanning of Laser Frequency
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 laserring-down cavity
side-band spectrum
Detector
gas analyte
nC+d+2FSR
nC+d+FSR
nC+d
cavityresonances
frequencyscanning
FSR
FARS measurement principle
d
carrierselected sideband
FSR
Lowest order of a spurious sideband close to a cavity mode is1- N where,
N = Round(R=FSR/ d)
How well does the cavity filter out sidebands?
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
Cavity filtering (fixed TEM)
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.
Sideband filtering for our spectrometer
Independent methods for characterizing frequency axisof 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
Devoe & Brewer, PRA 30, 2827 (1984).
Dual-sideband FSR measurement scheme
w0 –(w1+w2)w0 -w1
w0 –(w1-w2) w0 -w2 w0 +w2
w0w0 +(w1-w2) w0 +w1+w2
w0 +w1
qq-1 q+1
D-d D D+d
Two sets of sidebands:w1 at FSR= sw2 for PDH lock D = w0 – qsd = w1 – s
Demodulation of heterodyne beat at w1 - w2 givesdispersion signal g(d) centered about =0d , whered = w1 – .s
Accuracy of FARS-CRDS frequency axis
cavity dispersion
gDD 40 fs2
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%
Measuring losses in terms of cavity line width
Effect of beam extinction ratio on ring-down time measurement statistics
Extinction ratio = 10 log( Id/Il)
t0
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 signalto 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
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
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