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We describe an ultrafast optics laboratory comprising a mode-locked erbium fiber laser, autocorrelation measurements, and a free-space parallel grating dispersion compensation apparatus. The gain spectrum of Er fiber provides a broad bandwidth capable of supporting sub-100 fs pulses centered near a wavelength of 1550 nm. The fiber laser design used here produces a train of pulses at a repetition rate of 55 MHz with pulse duration as short as 108 fs. The pulse duration is measured with a homebuilt autocorrelator using a simple Michelson interferometer that takes advantage of the two-photon nonlinear response of a common silicon photodiode. To compensate for temporal stretching of the short pulse due to group velocity dispersion in the fiber, an apparatus based on a pair of parallel gratings is used for pulse compression. A detailed part that lists in the supplementary material includes previously owned and common parts used by the telecommunications industry, which helps decrease costs of the laboratory. This provides a cost-effective way to introduce the principles of ultrafast optics to undergraduate laboratories.

Broadband and high-resolution absorption spectra of molecular cerium oxide (CeO) are obtained in a laser-produced plasma using dual-comb spectroscopy. Simultaneous measurements of Ce and CeO are used to probe time-resolved dynamics of the system. A spectral resolution of 1.24 GHz (2.4 pm) over a bandwidth of 378.7–383.7 THz (781.1–791.5 nm) allows simultaneous detection of hundreds of closely spaced rotational transitions in complex CeO bands.

We demonstrate the use of dual-comb spectroscopy to perform time-resolved absorption measurements on cerium monoxide molecules formed in a laser-produced plasma.

Using the supercontinuum and second harmonic from a dual-comb spectrometer, simultaneous time-resolved absorption measurements of rubidium atomic transitions originating from ground and highly-excited states in the NIR (1529 nm) are obtained following a laser-produced plasma.

Dual-comb spectroscopy (DCS) represents a novel method of using absorption spectroscopy as a diagnostic tool for multispecies analysis of excitation temperatures and column densities in laser-produced plasmas (LPPs). DCS was performed on a LPP generated by ablating a multielement alloy containing Nd, Gd, and Fe. Transitions from all three elements were observed in absorption spectra measured from 530.08 to 535.19 nm at seven time-delays from 31 to 250 μs after ablation. The spectra were fit using a nonlinear regression algorithm to determine peak areas, and excitation temperatures and column densities were determined for the three atomic species separately using Boltzmann plots. The measured excitation temperatures of Nd I and Gd I showed good agreement at all time-delays, whereas the Fe I temperature was found to be higher, and the ratios between the column densities varied with delay. The observations are understood via effects of LPP spatial averaging, elemental fractionation, and molecular formation and are compared and contextualized with previous work studying LPPs using other spectroscopic techniques. A brief discussion of the precision and accuracy of the determined excitation temperatures and column densities is also presented.

We introduce a new, to the best of our knowledge, modality of dual-comb spectroscopy (DCS) that enables a simplified and powerful new approach for time-resolved measurements with increased acquisition rates. This “burst mode” form of DCS relies on the multiplexing of each probe pulse into a short train of pulses. With this approach we demonstrate a time-resolved series of absorption-based spectroscopic measurements of a laser-induced plasma using only a single laser ablation shot and identify 22 Nd lines not previously reported in the literature. The transmission spectra spanned 3.1 THz and were acquired at an effective acquisition rate of 25 kHz with 40 µs time resolution. This simple modification to ∼ 100 M H z level dual-comb systems provides a flexible approach for studying transient and low-duty-cycle events such as laser-induced plasmas, combustion, and explosive reactions.

We demonstrate a technique for improving performance in dual-comb spectroscopy when utilizing sources based on difference frequency generation without stabilization of the offset frequency by utilizing an etalon to recalibrate time-domain data in post-processing.

We performed simultaneous measurement of absorption, emission, and laser-induced fluorescence spectroscopic signatures for determining femtosecond laser-produced plasma’s physical properties throughout its lifecycle.

Dispersive dual-comb simulation and the expanded view of the labeled spectrum

A laser-induced plasma plume containing multiple atomic species is probed using dual-comb spectroscopy, and quantitative analysis is performed by fitting modelled absorption spectra to experimental spectra to obtain number densities and excitation temperatures.

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH3OH), nitrous oxide (N2O), and ammonia (NH3). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm−1 (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

We present measurements of the time-evolution of multiple transitions present in a laser- induced plasma in a single-shot using dual-comb spectroscopy. The plasma is probed at multiple times using eight delayed pulses in the probe comb.

We present an all-fiber coherent dual-comb system suitable to perform time-resolved spectroscopy measurements in the long-wave infrared.

We introduce a pulse burst mode form of dual-comb spectroscopy to enable time-resolved measurements of singular events. The time-resolved spectrum of iron following a single ablation shot of stainless steel is demonstrated.

We characterize performance of a standoff detection system based on a swept-wavelength external cavity quantum cascade laser (ECQCL) tuned from 955-1195 cm-1 (8.37-10.47 pm) at 400 Hz for detection of transient chemical plumes.

The oscillator strengths of 43 transitions of neutral gadolinium from 530 to 535 nm were determined based on high-resolution absorption spectra measured by probing a laser-produced plasma using time-resolved dual-comb spectroscopy. Absorption spectra were measured at various time delays, ranging from 33 to 252 μs after the onset of the plasma formation, over which the temperature and number density varied as the ablation plume evolved. A Boltzmann analysis was used to determine the excitation temperature and column density of Gd in the ablation plume, based on measured absorption from 20 Gd I spectral lines with known oscillator strengths. Oscillator strengths were then determined for additional dipole-allowed Gd I transitions identified in the absorption spectra, 19 of which were previously unreported. Oscillator strengths of all measured transitions were compared with literature values when available and show good agreement in most cases. Time-resolved measurement of atomic absorption in the cooling plasma provides access to a range of excitation temperatures and optical densities for the atoms in the ablation plume, allowing repeated measurements under different conditions, thereby improving confidence in the results. Our results also highlight that the high spectral bandwidth and resolution capabilities of dual-comb spectroscopy make it well-suited to measuring the dense optical spectrum of Gd, and the technique is applicable to other elements, especially lanthanides and actinides.

A vertical external-cavity surface-emitting laser is presented that emits 2.6 W of continuous-wave radiation centered at 1015 nm. The emission consists of a two-color spectrum with a difference frequency in the terahertz regime. The stability of the dual-wavelength operation is studied, utilizing a cavity design with multiple folds onto the gain medium. It is shown that spatial hole burning effects significantly help to improve the gain competition dynamics of the emitted wavelengths.

We present a novel cavity design for vertical external cavity surface emitting lasers (VECSELs) enabling multiple interactions with the gain structure under different angles in a single round trip. This allows for a low round-trip group delay dispersion (GDD) despite using high-gain resonant VECSEL structures possessing pronounced resonances in their reflective GDD profile. Femtosecond-regime pulses with an average output power of 1.14 W and record peak intensities for mode-locked VECSELs of 6.3 kW are presented with simulations demonstrating the GDD compensating mechanism employed in this scheme.

We demonstrate a carrier-envelope offset-free frequency comb in the mid-wavelength infrared (MWIR) based on a passively mode-locked vertical external cavity surface emitting laser (VECSEL) operating at a 1.6 GHz repetition rate. The 290 mW output spanning 3.0–3.5 μm is generated through difference frequency generation (DFG) in periodically poled lithium niobate. The VECSEL pulse train is centered at 1030 nm and amplified up to 11 W in a Yb fiber amplifier system. The output is split to generate a second pulse train at 1560 nm through nonlinear broadening in a Si3N4 waveguide followed by amplification in an Er gain fiber. DFG between the 1030 and 1560 nm pulse trains results in a coherent and offset-free MWIR frequency comb, verified with optical heterodyne beat note measurements. Active stabilization of the VECSEL repetition rate provides a fully stabilized high repetition rate frequency comb in the MWIR, uniquely suited for applications in molecular spectroscopy.

The mid-infrared (MIR) region above 3 microns is of great interest for spectroscopic applications. Because it is difficult to produce mode-locked laser sources that emit natively in this region, difference frequency generation (DFG) is a popular method to produce mid-IR output using more traditional laser oscillators. Previous examples include fiber based DFG sources and OPOs, which are typically limited to repetition rates on the order of tens to hundreds of MHz. VECSELs allow access to higher repetition rates, while the use of highly nonlinear waveguides enables the requisite spectral broadening despite the lower pulse energy. In this work we present a VECSEL-based frequency comb that uses DFG to produce output in the 3-4 micron range. This system is based on a mode-locked VECSEL emitting at a 1030 nm wavelength with a 1.6 GHz repetition rate. A Yb fiber amplification system is used to increase the power to over 10W and compress the pulses to sub-90 fs. Coherent spectral broadening out to 1560 nm is achieved with a nonlinear waveguide. By combining the 1030 nm and 1560 nm beams in a PPLN DFG crystal, 290 mW of mid IR output between 3.0 and 3.5 microns is produced. Since the DFG light is produced by two wavelengths from the same oscillator, the carrier envelope offset frequency is cancelled, producing an offset free comb requiring stabilization of only a single degree of freedom. We characterize this VECSEL based frequency comb and discuss the advantages it provides for spectroscopic applications.

A swept-wavelength external cavity quantum cascade laser (ECQCL) is used to perform standoff detection of combustion gases in a plume generated from an outdoor high-explosive (HE) open detonation. The swept-ECQCL system was located at a standoff distance of 830 m from a 41 kg charge of LX-14 (polymer-bonded high explosive) and was used to measure the infrared transmission/absorption through the post-detonation plume as it propagated through the beam path. The swept-ECQCL was operated continuously to record broadband absorption spectra at a 200 Hz rate over a spectral range from 2050 to 2230 cm−1 (4.48–4.88 μm). Fitting of measured spectra was used to determine time-resolved column densities of CO, CO2, H2O, and N2O. Analysis of visible video imagery was used to provide timing correlations and to estimate plume dimensions, from which gas mixing ratios were estimated. Measured emission factors and modified combustion efficiency show good agreement with previously reported values.

We present a novel, to the best of our knowledge, system for high-resolution, time-resolved spectroscopy in the mid-wave infrared based on a modelocked vertical external cavity surface emitting laser (VECSEL) frequency comb coupled to a virtually imaged phased array (VIPA) spectrometer. The GHz level repetition rate of VECSEL-based systems coupled to VIPA spectrometers enables comb tooth resolved spectra without the use of additional filter cavities often required to increase comb tooth spacing. We demonstrate absorption spectroscopy on a methane ( C H 4 ) gas mixture at 2900 c m − 1 (3.4 µm) with over 35 c m − 1 spectral bandwidth in a single image. Rapid time-resolved measurements were made using a 300 µs exposure time with an acquisition rate limited to 125 Hz by the available camera. High-resolution absolute frequency measurements were performed by scanning the repetition rate of the VECSEL frequency comb.

We demonstrate an offset-free 1.6 GHz frequency comb in the MIR spectral region (3.0-3.5 microns) based on a mode-locked VECSEL laser. The MIR comb utilizes a SiN waveguide for spectral broadening and DFG in PPLN crystal for down-conversion.

Dual-comb spectroscopy has become a powerful spectroscopic technique in applications that rely on its broad spectral coverage combined with high frequency resolution capabilities. Experiments to date have primarily focused on detection and analysis of multiple gas species under semi-static conditions, with applications ranging from environmental monitoring of greenhouse gases to high-resolution molecular spectroscopy. Here, we utilize dual-comb spectroscopy to demonstrate broadband, high-resolution, and time-resolved measurements in a laser-induced plasma. As a demonstration, we simultaneously detect trace amounts of Rb and K in solid samples with a single laser ablation shot, with transitions separated by over 6 THz (13 nm) and spectral resolution sufficient to resolve isotopic and ground state hyperfine splittings of the Rb D2 line. This new spectroscopic approach offers the broad spectral coverage found in the powerful techniques of laser-induced breakdown spectroscopy (LIBS) while providing the high-resolution and accuracy of cw laser-based spectroscopies., Dual-comb spectroscopy has become a valuable tool for broadband high-resolution measurements. Here Bergevin et al. apply this technique to a laser-induced plasma detecting different species in a solid sample with a spectral resolution sufficient to resolve hyperfine splitting of the Rb D2 line.

We present preliminary results showing the potential of VECSEL technology for the generation of high power coherent supercontinuum. Among these results, we demonstrate a stable output power of 16 W with a pulse duration of 71 fs and a repetition rate of 1.7 GHz from a VECSEL oscillator and Ytterbium fiber amplifier. This system was used to generate a coherent supercontinuum averaging 3 W of power using a highly nonlinear photonic crystal fiber. In addition, we discuss the possible methods for the detection and stabilization of the carrier offset frequency. The beatnote between a VECSEL seeded supercontinuum and an external CW laser reveals a relatively stable signal, well above the detection noise. A discussion about system design considerations for noise reduction and increased offset frequency stability is also included.

A dual-comb Yb laser system is used to measure the time-resolved evolution of absorption spectra from multiple atoms/ions following laser ablation of metal alloys. These results will be extended into the VUV utilizing high-harmonic generation.

We demonstrate time-resolved dual-comb spectroscopy to track the evolution of multiple atomic and ionic species following laser ablation of solids. The approach promises improved plasma characterization and detection confidence in the analysis of solid materials.

We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.

We utilize time-resolved dual-comb spectroscopy to measure the temporal evolution of the population number densities and absorption excitation temperature of Fe in a laser-induced plasma. The spectra of three excited-state transitions of Fe around 533 nm are simultaneously measured at different time delays following laser ablation of a stainless steel sample. This Letter probes late-time behaviors of laser-induced ablation plumes during plasma cooling. The high spectral resolution and broad spectral coverage of the dual-comb technique, combined with the time-resolved measurement capability shown here, will aid in the characterization of laser induced plasmas, including species identification and molecule and particle formation that can occur at later times in the plasma evolution.

While Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) have been successfully used as ultrafast laser sources with pulse durations in the hundreds of femtosecond regime, the dynamics within the semiconductor gain structure are not yet completely understood. With the high carrier densities inside the semiconductor, nonequilibrium effects such as kinetic-hole burning are expected to play a major role in pulse formation dynamics. Moreover, the nonlinear phase change by the intense light field can induce a complex dispersion, which may potentially limit the achievable pulse durations. To shed light on such nonequilibrium dynamics, we perform in-situ characterization of mode-locked VECSELs. We probe the gain media as well as the intracavity absorber with a femtosecond fiber laser source. For measuring temporal characteristics, we employ an asynchronous optical sampling technique by phase-locking the repetition rate of the VECSEL to a multiple of the probe laser with an adjustable offset frequency. This allows for probing dynamics from femtosecond to nanosecond time scales with scan rates up to hundreds of Hertz without compromise of measurement precision which can be introduced by mechanical delays covering such large temporal windows. With a resolution in the femtosecond range, we characterize gain depletion by the intracavity pulse as well as the gain recovery timescales for different power levels and operation regimes.

We show two high power 100 fs frequency combs can be coupled to the same enhancement cavity for performing time-resolved measurements of optical nonlinearities and for simplifying dual-comb spectroscopy in the XUV.

Intracavity high-harmonic generation has extended the frequency comb into the XUV spectral region. We demonstrate a dual-comb system enabling detection of individual comb teeth for a more robust approach to precision spectroscopy in the VUV/XUV.

Numerical simulations of a frequency comb resonant with an enhancement cavity are performed to study nonlinear dynamics resulting from intracavity ionization. Results highlight the critical role played by transverse spatial effects, previously excluded in simulations.

Intracavity high harmonic generation has enable the extension of the fs frequency comb to the XUV. We review the technological development of these sources, their fundamental limitations, and their potential impact on science and technology.

A high power fiber-based dual-comb system generates a train of coherent pulse pairs in the VUV with continuously tunable delay based on HHG in a single femtosecond enhancement cavity. An intracavity cross-correlation diagnostic is demonstrated.

We have actively stabilized the comb of frequencies from a mode-locked femtosecond laser using a Fabry-Perot reference cavity. This technique offers the ability to synthesize a comb of highly stable radio frequencies directly from optical transitions. The measured fractional frequency instability of the components of the frequency comb relative to the reference cavity was $l5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ in 0.1 sec. The variation of the optical mode spacing versus frequency of the reference cavity was also directly measured using this technique.

We report a simple femtosecond fiber laser system based on a carbon nanotube saturable absorber that is capable of generating 14-fs pulses (containing less than four optical cycles) with optical spectrum extending from 1000 nm to around 1500 nm.