High-precision molecular spectroscopy with optical frequency combs

Spectroscopy is the science that studies the interaction between elec- tromagnetic radiation and matter. By electromagnetic radiation is meant the whole spectrum from radio frequency (RF) to X-ray and γ-ray. In this enormous range the near-infrared and mid-infrared region al- though covers from 1 μm to 20 μm, or 300 THz to 15 THz, is of extremely importance because all the most important gas molecules have specific ab- sorption features here. The uniqueness of these features from gas to gas has led the community to label this spectral range as the finger print region. High-precision spectroscopic measurements find applications in many fields. Airport security can be enforced with a spectrometer detecting dangerous and potentially explosive gases. Lung cancer and lung pathologies can be diagnosed with an analysis of the exhaled breath, this technique is called breath analysis. Another application that gained particular attention involves the determina- tion of fundamental physical constants and their variation with time. There are molecules in which some roto-vibrational transitions have line centers that are directly related to fundamental constants, like the fine structure α or the proton to electron mass ratio β. By measuring these line center fre- quencies with extreme high accuracy, variations of 1 part in 1016 can be detected and theories predicting variations of those constants can be proven (or disproven). Optical frequency combs (OFCs) are nowdays the best tool for frequency measurements thanks to their extreme accuracy, broad spectral coverage and high purity. They were originally developed for the purpose of linking I the optical region with the (RF) domain but they have eventually spread in other research filed like optical waveform generation, light detection and ranging (LIDAR), low phase noise RF waveform generation, optical com- munication and high precision spectroscopy. High precision spectroscopy techniques can be divided into two major groups: highly accurate narrow band and fast broad band. In the former, called "comb assisted spectroscopy", an optical frequency comb is employed in its measuring tape fashion, i.e. an optical ruler with which the wavelength of an additional single frequency laser is measured. A single wavelength quantum cascade laser at 8.6 μm is phase locked against a mid-IR fre- quency comb. A relative accuracy of 10−10 is achieved when characterising the system performances with the N2O molecule. A measure of the CHF3 molecule, which is important for the determination of the β ratio, is also performed. The latter, called "direct comb spectroscopy", the frequency comb is di- rectly employed as the probe laser. A novel technique based on the speckle pattern forming at the output of a multimode fiber is employed to resolve the OFC teeth with better than 1 pm resolution and a SNR of 150. A 2.5 MHz accuracy on the absorption line centers is achieved. Thanks to the all-fiber technology it may lead to the development of new class of high resolution spectrometers. The so called dual comb spectroscopy is also explored in the condition of free running laser source and a feed-forward technique to cancel the teeth phase noise employing only one additional single wavelength (CW) laser. An improvement of 29 dB with 1 s averaging time is achieved with respect to the single shot case, bringing a 98 MHz final accuracy and a 3.3 GHz resolution. Being simple without requiring sophisticated phase locked loop and only one CW laser it can be employed when a fast and accurate mea- surement is desirable.