1. FREQUENCY-AGILE DIFFERENTIAL CAVITY RING-DOWN SPECTROSCOPY
Z.D Reed, J.T. Hodges
National Institute of Standards and Technology

2. Frequency Stabilized Cavity Ringdown Spectroscopy
I = I0 exp-(t/t) + const
time frequency
1 𝑐τ =α0 +α(n)

3. Limitations of CRDS Signal Drift
Variations in experimental environment
(air pressure, mechanical vibration, temperature, etc)
Cavity losses vary with optical alignment
Polarization dependent losses
Optical feedback
Limits averaging time and SNR

4. Coupled cavity effects
Letalon
ring-down cavity
external optic
Phase of etalon shifts with temperature and pressure
Leads to variations in cavity base loss
Courtois, Bielska and Hodges, Differential CRDS, JOSA B, 30, (2013).

5. Differential CRDS DLq,q+Dq = L(nq+Dq) – L(nq) DLq = L(nq) – L(n0)
fixed reference mode1 q
DLq = L(nq) – L(n0)
Reference frequency
fixed frequency difference2
DLq,q+Dq = L(nq+Dq) – L(nq)
Reduce drift and increase optimal averaging time
Reduce etalon amplitudes
Removes baseline shifts (from changes in alignment or cavity length)
1Huang & Lehmann, “Long-term stability in continuous wave cavity CRDS experiments”, Appl. Opt. 49, (2010).
2Courtois, Bielska and Hodges, “Differential cavity ring-down spectroscopy,” JOSA B 30, (2013).

6. Fixed Frequency Difference
DLq,q+1 = L(nq+1) – L(nq)
Δq=1

7. Fixed Frequency Difference
DLq,q+2 = L(nq+2) – L(nq)
Δq=2

8. Fixed Reference Mode
DLq = L(nq) – L(n0) q

9. Differential CRDS fixed reference mode1 fixed frequency difference2 q
Reference frequency
fixed frequency difference2
Can be fit in same fashion as traditional spectrum
Increases optimal averaging time
Removes typical baseline and baseline drift
Reduces etalon amplitudes

10. Differential CRDS method schematic
Courtois, Bielska and Hodges, Differential CRDS, JOSA B, 30, (2013).

11. Differential-CRDS Results
SNR = 170,000:1
SNR = 68,000:1
Conventional spectrum with no etalon fitting/subtraction
Differential spectrum showing reduction in etalon amplitude
Spectrum residuals

12. Frequency-agile, rapid scanning (FARS) spectroscopy
Limitations A solution
Thermal or mechanical tuning
Slow and often non-linear
Discrete frequencies required
Laser not narrowed, ringdown events acquired as laser momentarily becomes resonant with cavity modes
Limited Δq due to AOM bandwidth
Use electro-optic modulator (EOM) to set laser frequency
microwave level accuracy
Pound Drever Hall (PDH) lock to narrow laser
Rapid tuning, acquisition rate limited by τ, wide and variable Δq
Frequency-agile, rapid scanning (FARS) spectroscopy
MW source
side-band spectrum
ring-down cavity
Detector
cw laser
gas analyte
EOM

13. Frequency Agile Differential Spectroscopy (FADS-CRDS)
PD2
PBS
PBS
ring-down cavity
PD1
s-pol
p-pol
PDH servo
signal
acquisition
EOM 1
ECDL
lock leg
EOM 2
probe leg
2f lock
TTL trigger
switch
PDH lock beam
RF source
(0 – 70 GHz)
2f PDH error signal
Cavity modes
D. A. Long et al., Appl. Phys. B, (2013)

14. Averaging Statistics: FADS-CRDS vs FARS-CRDS
Averaging Statistics: FARS-CRDS vs FADS-CRDS
st/t = 0.014%
NEA = 9x10-12 cm-1 Hz-1/2
NEA = 4x10-12 cm-1 Hz-1/2
st/t = 0.023%
30 s
Optimal averaging time increased by three orders of magnitude!

15. FADS-CRDS etalons

16. FADS-CRDS and FARS-CRDS spectra
SNR = 12,500:1
SNR = 9,800:1
SDNGP fit, no etalons fit
Total measurement time 5.6 s
SDNGP fit, one etalon and baseline fit
Total measurement time 4.8 s
SNR limited by structure in baseline for both cases

17. Higher point density spectra
Lock to local mode
Acquire comb of differential points
Shift entire comb with AOM
Laser tracks comb, and differential removes baseline shift
Acquire new comb of differential points

18. FARS
FADS
SNR = 12,500:1
SNR = 3,600:1
2500 ppm CO in N SDNGP profile

19. Scanned Cavity Differential CRDS
profile: SDNGP

20. Future Direction
Further reduce etalons to remove limiting factor of SNR (>106:1 possible in 5 s spectrum)
Utilize scanned cavity tuning with D-CRDS and FADS
High SNR with high point density provides stringent tests for line profiles
Further investigate effects of polarization drift and cross polarization on losses

21. Thanks to J.T Hodges, D.A. Long, A.J. Fleisher Guest Researchers
K. Bielska, H. Lin, V. Sironneau, G.W. Truong
Funding: NIST Greenhouse Gas Measurements and Climate Research Program