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
250 spectra in 0.7 s
68th Ohio State University International
Symposium on Molecular Spectroscopy
June 17-21, 2013, Columbus OH

2. 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

3. 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.

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

5. 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;
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

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 H2O spectra
Systematic errors arise
from overly simplistic line shapes
Voigt Profile
Galatry Profile
Line shape effects in O2

8. 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

9. Rapid Step Scanning of Laser Frequency

10. 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)

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

12. 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

13. 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

14. Sideband filtering for our spectrometer
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 TEM00).
We have never observed any evidence of spurious
coupling into other sidebands.

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 I2-stabilized HeNe reference laser

16. 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).

17. Accuracy of FARS-CRDS frequency axis
cavity dispersion
gDD  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 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%

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

20. 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

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. 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

23. 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