Comb Spectroscopy without Comb Locking using AOTFs for Adaptive Chem/Bio Detection
Abstract
Dual-comb spectroscopy is emerging as an exciting alternative to conventional Fourier-transform spectroscopy using moving mirrors since it requires no moving mirror and exploits the high brightness of the comb laser source as compared to conventional incoherent broadband sources. Two laser combs producing femtosecond pulses at slightly different rates yield overlapping pulses that drift through each other at the frequency difference of the comb repetition rates. This requires that the repetition rates of the lasers be offset locked to each other with appropriate servos and actuators (laser cavity stretchers), as well as carrier-envelope offset locking the combs to each other with additional servos and actuators, and finally everything must be fully stabilized with respect to ultra-stable locking lasers. We are developing techniques to allow dual comb spectroscopy without the complexity, cost, and size of having to lock two comb-laser sources to each other. This will dramatically simplify the application of chip-based comb sources for dual-comb spectroscopy applications since the complexity of tightly repetition-rate offset locking, carrier offset frequency locking, and locking to a stabilized-reference CW laser would no longer be required to utilize multiple comb source. These multiple locks are required to coherently detect and then coherently integrate multiple spectral scans in order to increase the dual-comb spectrometer sensitivity to the level necessary for trace gas detection of Chem/Bio agents of interest, especially over remote paths. We utilize an acousto-optic tunable filter (AOTF) to produce a repetition-rate shifted comb much as a very high speed moving mirror would, but in addition the AOTF can control the femtosecond pulse shape and the spectral band of interest under electronic control. The traveling-wave character of the AOTF can be considered to be the moving mirror of a Fourier-transform spectrometer, and results in a Doppler shift of an incident laser beam, but when modulating broadband laser light with a broadband acoustic wave is more correctly considered to be a relativistic compression of a diffracted ultra-broadband comb source resulting in a temporal comb compression and spectral comb expansion, thereby yielding a new comb repetition rate. Alternatively, the Bragg-matched diffraction from the traveling-wave acoustic grating can be thought of as producing a Doppler-shifted, spectrally-filtered, and polarization-switched output from each applied RF frequency, so a broadband RF signal (such as a short RF pulse or RF chirp) will Doppler shift the different portions of the comb in proportion to the optical frequency, yielding a frequency comb with a slightly shifted spacing. The orthogonally polarized diffracted output of the AOTF is passed through a sample and interfered with the non-diffracted comb on a high-speed photodetector, and this interferogram is Fourier transformed to measure the spectra and thereby demonstrate dual comb spectroscopy with just one comb laser source. Since these two beams are fully mutually coherent they can be coherently averaged to reach the desired sensitivity goals for trace gas detection. We will explore the benefits of adaptively modifying the pulse shaping and the spectral content to improve the system performance and to enhance the detection and discrimination ability of the spectrometer.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 20, 2016
- Source ID
- W31P4Q1510014
Entities
People
- Kelvin Wagner
Organizations
- Army Contracting Command
- Defense Advanced Research Projects Agency
- University of Colorado Boulder