1. Spectroscopy with comb-referenced diode lasers
Matt Cich, Gary V. Lopez, Philip M. Johnson, Trevor J. Sears and Chris P. McRaven
65th OSU International Symposium on Molecular Spectroscopy. Talk FD-11
Friday June 25th, 2010
Good morning everyone, and thank you for staying around to the end of the meeting.
Funding: Division of Chemical Sciences, Geosciences and Biosciences, US Department of Energy and the American Chemical Society, Petroleum Research Fund.

2. Aims and Opportunities
Detection and characterization of molecular species involved in combustion. We wish to understand their properties, investigate dynamics and reactivity, and calibrate theory.
Development of non-intrusive, sensitive, and specific experimental techniques is vital.
New technology presents opportunities for greatly enhanced precision and sensitivity for line positions and shapes and, potentially, for high sensitivity broad band measurements.
Here is a brief outline of what I’m planning to cover. This is an exciting time in high resolution spectroscopy as new technology is about to open up new opportunities for vastly improved precision and sensitivity. Our aims are centered on the detection and characterization of molecular free radicals involved in combustion chemistry. In addition to the usual measurement of spectral line positions, and therefore energies, we want to get information on wavefunctions and make precise measurements of line shapes that provide new insights into collisionally mediated dynamics. I’ll illustrate these ideas by examples from some new work on my favorite molecule, CH2.
Part of the work we particularly enjoy revolves around the design of new experimental techniques, and with a view to taking advantage of new ultra precise laser sources, we have expended considerable effort during the past few years on sub-Doppler measurement techniques and we’ll touch on some results in this area before finishing up with where we’re planning to go in the future using frequency comb based spectroscopy.
(80) GHz

3. Increasing the measurement precision
7MHz FWHM
CH2 near 820 nm
Ring or extended cavity diode laser-based FM absorption: we wish to measure spectral splittings and line shapes to 0.1 MHz or better, but laser frequency drifts as the data are acquired can be 10x this.
Plus, the measurement of the absolute frequency of the transitions is only good to the quality of the wavelength measurement, i.e. about 1 part in 108 or about 30 MHz.
We can use frequency comb-based techniques to address these issues.

4. Simplest Experiment ωopt= n ωrep + ω0
Lock external laser to one comb component so its frequency is stable and precisely known.
Tune laser by slightly varying the comb repetition frequency so that each optical frequency tunes by n(Δωrep) and record an absorption spectrum.

5. Block Diagram PC EOM 1550nm LASER CW Beat Detection COMB Wavemeter GPS
l/4 Plate
Fabry-Perot Cavity
Optical
Isolator
Beam Splitter
1550nm LASER
CW Beat Detection
COMB
Wavemeter
Fiber
GPS
194 MHz
PDH detector
194 MHz Oscillator
Coupler
Delay
Mixer
Amplifier
Low pass filter
Diagnostics PC
Photodiode detector
Laser
PDH feedback
Acetylene
Optical Alignment
Cable lines
Piezo
RF power
Lockbox

6. First measurements
Extended cavity diode laser locked to a scanning comb component. 1.5 parts in absolute accuracy. Absorption in confocal cavity-type cell with cavity mode locked to scanning laser.
Sub-Doppler absorption of a combination band vibration-rotation transition in C2H2.
Previously these transitions in acetylene were used as secondary frequency standards near 1.5 microns.
(80) GHz

7. Future
Improve stability of cavity locking and modulate the laser to permit lock-in detection and improved signal to noise.
Investigation of potential analytical spectroscopy using overtone and combination bands in small hydrocarbon molecules. Incorporate collision cooled cell?
Sub-Doppler spectra of radicals. (CH2, PbF)
New experiments to take advantage of the multiplex nature of comb sources, FT-comb designs for lower resolution studies.

8. Acknowledgements Greg Hall,
Chih-Hsaun Chang, Mike Hause, experiments at BNL
Gary Lopez, Matt Cich (Stony Brook), Chris McRaven (Oklahoma), frequency comb experiment.
Thank you!

10. Frequency Combs ωopt= n ωrep + ω0
A source that gives multiple, precisely known, optical frequencies. It starts with a mode-locked ultrafast laser running at high repetition rate.
The Fourier transform of the laser output is a comb of frequencies extending across the bandwidth of a pulse.
The comb spacing is exactly the repetition rate of the laser, and that may be locked to a stable source.
ωopt= n ωrep + ω0

11. Offset frequency ωopt= n ωrep + ω0
ω0 would normally not be stable, and we need a trick to fix it to make optical clocks a reality.
Use a structured fiber to expand the bandwidth so the frequency range covers an octave.
Take part of the red end of the spectrum, double it, and mix the result with the blue end. Beat note is ω0. Self-referenced comb.
ωopt= n ωrep + ω0
Hall and Hansch Nobel prize 2005