In this paper, a multiheterodyne architecture for molecular dispersion spectroscopy based on a coherent dual-comb source generated using a single continuous wave laser and electro-optic modulators is presented and validated. The phase-sensitive scheme greatly simplifies previous dual-comb implementations by the use of an electro-optic dual comb and by phase-locking all the signal generators of the setup eliminating, in this way, the necessity of any reference optical path currently mandatory in absorption-based instruments. The architecture is immune to the classical baseline and normalization problems of absorption-based analyzers and provides an output linearly dependent on the gas concentration. In addition, the simultaneous parallel multi-wavelength measurement approach has the ability to deliver an improved output bandwidth (measurement speed) over gas analyzers based on tunable lasers.

In this paper, a new approach to dual comb generation based on well-known optical techniques (Gain-Switching and Optical Injection Locking) is presented. The architecture can be implemented using virtually every kind of continuous-wave semiconductor laser source (DFB, VCSEL, QCL) and without the necessity of electro-optic modulators. This way, a frequency-agile and adaptive dual-comb architecture is provided with potential implementation capabilities from mid-infrared to near ultraviolet. With a RF comb comprising around 70 teeth, the system is validated in the 1.5 μm region measuring the absorption feature of H¹³CN at 1538.523 nm with a minimum integration time of 10 μs.

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Electro-optic dual-comb spectrometers have proved to be a promising technology for sensitive, high-resolution and rapid spectral measurements. Electro-optic combs possess very attractive features like simplicity, reliability, bright optical teeth, and typically moderate but quickly tunable optical spans. Furthermore, in a dual-comb arrangement, narrowband electro-optic combs are generated with a level of mutual coherence that is sufficiently high to enable optical multiheterodyning without inter-comb stabilization or signal processing systems. However, this valuable tool still presents several limitations; for instance, on most systems, absolute frequency accuracy and long-term stability cannot be guaranteed; likewise, interferometer-induced phase noise restricts coherence time and limits the attainable signal-to-noise ratio. In this paper, we address these drawbacks and demonstrate a cost-efficient absolute electro-optic dual-comb instrument based on a frequency stabilization mechanism and a novel adaptive interferogram acquisition approach devised for electro-optic dual-combs capable of operating in real-time. The spectrometer, completely built from commercial components, provides sub-ppm frequency uncertainties and enables a signal-to-noise ratio of 10000 (intensity noise) in 30 seconds of integration time.

In this work, the generation of dual optical frequency combs based on gain-switching and optical injection locking is experimentally examined. The study reveals that an effective process of optical injection can lead to optimized RF combs in terms of span and signal-to-noise ratio. The system also minimizes the overlap of lines and reduces the number of optical components involved, eliminating the need for any external modulator (electro-optic, acousto-optic). The validation of the system was performed as a dual-comb spectrometer, which allowed for determination of the absorption and dispersion profiles of the molecular transition of H¹³CN at 1538.523 nm.

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Crowded with the fundamental signatures of many popular molecules, the mid-infrared range of the electromagnetic spectrum is particularly attractive for applications ranging from identification of transient phenomena to sensing of trace gases. Dual-comb spectroscopy is a technique that unveils the potential to access this region with a pair of phase-locked optical frequency combs on a high-resolution, real-time basis without the mechanical limitations of traditional spectrometers. As the ideal characteristics of an optical frequency comb are strongly influenced by the target application, electro-optic dual-comb systems are one of the most promising solutions with full capabilities to neatly fit to the application of interest beyond laboratory environments. Parameters such as resolution, measurement speed, or central wavelength are easily adjustable by means of compact, low-cost arrangements based on commercial off-the-shelf components. To fully exploit their potential for molecular spectroscopy, we present here a modular instrument designed to perform ultrafast dual-comb spectroscopy in the mid-infrared region. The architecture comprises a fiberized near-infrared electro-optic dual-comb scheme and a single-crystal difference frequency generation module to generate mid-infrared combs, thus significantly alleviating the complexity of the free-space setup while preserving absolute independence between the instrument and the sample of study. The feasibility of the instrument is successfully validated by recovering the absorption profile of methane at 2896.98 cm −1 within tens of microseconds.

Our first and very recent results on a new approach to obtain compact and practical, multi-octave spanning, absolute frequency and high accuracy THz dual-comb spectrometers based on Electro-Optic modulators (EOMs) are reported. Two different schemes are described and evaluated with the final objective of obtaining a practical solution for THz spectroscopy and other applications in such frequency range.