Spectroscopy with comb-referenced diode lasers

Matt Cich, Gary V. Lopez, Philip M. Johnson, Trevor J. Sears and Chris P. McRaven65th OSU International Symposium on Molecular Spectroscopy. Talk FD-11

Friday June 25th, 2010

Funding: Division of Chemical Sciences, Geosciences and Biosciences, US Department of Energy and the American Chemical Society, Petroleum Research Fund.

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.

195739.6495135(80) GHz

Increasing the measurement precision

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.

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.

7MHz FWHM

CH2 near 820 nm

Simplest ExperimentLock 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.

ωopt= n ωrep + ω0

Block Diagram

EOM

/4 Plate Fabry-Perot Cavity

OpticalIsolator

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

Photodiode detector

Laser PDH feedback

Acetylene

Optical AlignmentCable lines

Piezo

RF powerAmplifier

Lockbox

Amplifier

First measurements

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.

Extended cavity diode laser locked to a scanning comb component. 1.5 parts in 1012 absolute accuracy. Absorption in confocal cavity-type cell with cavity mode locked to scanning laser.

195739.6495135(80) GHz

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?

Future

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.

Greg Hall,Chih-Hsaun Chang, Mike Hause, experiments at BNL

Acknowledgements

Gary Lopez, Matt Cich (Stony Brook), Chris McRaven (Oklahoma), frequency comb experiment.

Thank you!