The quantitative measurements of the soot volume fraction can deliver important validation data for detailed models of soot inception, growth, and oxidation in hydrocarbon-fueled flames – a process still not fully understood. Therefore, the sensitive and non-intrusive optical measurement of soot levels in fuel-rich steady flames is highly desirable. This work presents the first in situ Cavity Ring-Down Extinction (CRDE) setup for monitoring soot concentration levels in the low-ppb range in premixed ethylene/air stagnation flames stabilized on a flat flame burner at elevated pressure. The high-reflectivity CRD mirrors were mounted in nitrogen-flushed chambers attached to and separated from the flame area by small diameter apertures. CFD simulations supported the dimensioning of the purge flows and visualized possible perturbation of the cold- and hot-gas (from the flame) flows inside the burner housing. Within the uncertainties of the present experimental conditions and based on results from other labs for similar flame conditions, the pressure dependence of the evaluated soot volume fraction fV was fitted to a power-law ansatz with an exponent n between 2.1 and 2.6 depending on the equivalence ratio. A global thermal rate constant for surface growth, k_SG≈ 49± 20 s^-1 , was found for a measured flame temperature of 1750 ± 100 K in an atmospheric pressure flame, in gross agreement with results in the literature, which demonstrates the potential of the current setup for soot diagnostics in laminar premixed elevated pressure flames. A detection limit for fV of above 40 ppt has been determined from a comparative measurement of CO2 having a small absorption coefficient at the laser wavelength of 1064 nm.

Meeting the demands of sustainable energy economy requires diagnostics of the chemical processes surrounding future fuels and contemporary combustion applications. Pioneered in 1970, Intracavity Laser Absorption Spectroscopy (ICAS) has evolved to be a powerful instrument in the toolbox of combustion diagnostics. It owes its ultra-high sensitivity to the enhancement of the effective absorption pathlength by placing the absorber inside the cavity of a broadband laser. In this review we introduce the complementary strengths of ICAS to other methods: ultra-high sensitivity to narrowband absorption alongside the immunity to broadband losses, multiplexed detection and (µs-scale)-temporal resolution. We outline the basic concepts and features of ICAS, focusing on the laser dynamics regime where an absorbing sample in the laser resonator yields the well-known Lambert-Beer law. We chart the progress made over the years in visible (dye-jet laser) and near infrared (fiber laser) ICAS speciation in flames, by highlighting case studies where species like long considered ''hard-to detect'' 1CH2 and HCO radicals, along with O-atoms, C2, NH2, HNO, CN, and HCN were measured, as well as thermometry and speciation applications demonstrated in shock tubes, flow-cells and flames based on (stationary or time-resolved) measurements of multicomponent spectral matrices containing lines of CH4, C2H2, CO2, CO, OH and H2O. We highlight the contributions of ICAS in gas-phase nanomaterial synthesis, exemplified in prototypical iron-doped flames and discuss prospective applications in spray-flame pyrolysis and metal-powder combustion. Finally, we present advances in the development of lasing media based on Cr2+ and Fe2+-doped chalcogenide crystals and fluoride crystals doped with trivalent lanthanides, that meet the (ICAS-specific) requirement associated with the necessity to have a gain media lasing directly in the desired wavelength range, and therefore to expand this technique into the important mid-infrared and ultraviolet spectral ranges.

The enormous sensitivity of intracavity absorption spectroscopy (ICAS), as well as its unique ability to tolerate high broadband losses caused by, e.g., optical windows and light scattering, is being exploited by only a few research groups worldwide. The reason seems to be the lack of comprehensive literature, such that the field remains difficult to access for non-experts, in particular for engineers and chemists, who might derive the most benefits from applying ICAS. In particular, the missing connection to this target audience appears to be two-fold: (i) the seeming complexity of the theory, and (ii) the necessity to setup homemade laser systems. However, once some basic understanding and knowledge is obtained, both aspects appear to be of similar complexity as with other spectroscopic techniques. Therefore, the current work is aiming at (i) providing a comprehensive review of the theoretical basics of ICAS, and (ii) describing the most important practical aspects that need to be considered for a successful realization of ICAS measurements. To ensure maximum clarity, illustrative practical examples of recent work are used throughout the paper.

The results on intensity noise and repetition frequency stability measurements for a passively mode-locked Cr:ZnSe laser pumped by a thulium-doped fiber laser at a wavelength of 1.94 pm are presented. The stability parameters and intensity noise are compared for three different generation regimes of the Cr:ZnSe laser. The laser operated at a repetition rate of 129.5 MHz and a central wavelength of 2.45 pm.

Recent progress in the fabrication of high-purity chalcogenide glasses [1] opened new possibilities for the development of novel mid-infrared rare-earth doped glass lasers (e.g., based on Pr 3+ , Tb 3+ , and Ce 3+ ) emitting in the spectral range of $4-6\ \ \mu \mathrm{m}$ . The best results up to now have been obtained in high-purity Ce 3+ -doped $\text{Ge}_{20}\text{Sb}_{10}\text{Ga}_{5}\text{Se}_{65}$ glass rods [2]–[5].

This paper provides an overview of mid-infrared lasers based on rare-earth-ion-doped selenide glasses. Laser action was demonstrated at the transitions between the first excited and the ground levels of Ce3+, Pr3+, Nd3+ and Tb3+ ions. The highest output parameters for bulk glass lasers (over 40 mJ of output energy) and wavelength tuning in the range of 4.6–5.6 microns were obtained with Ce3+-doped glass. The highest output parameters for fiber lasers (150 mW at 5.1–5.3 μm under continuous pumping) were demonstrated with Tb3+ ions. The longest lasing wavelengths for any glass laser and tunability within the 5.56–6.01 µm spectral band were shown with Nd3+ ions in a Tb3+-Nd3+ co-doped system.

The reaction between Fe atoms and O2 in a mixture of iron-pentacarbonyl (IPC, 2 ppm) and oxygen (100 ppm), diluted in argon, has been studied in a shock tube behind reflected shock waves over the temperature and pressure ranges of 1050-3400 K and 0.7-2.0 bar. Time-resolved measurements of Fe and FeO have been performed simultaneously using a combination of atomic resonance absorption spectroscopy (ARAS) and intracavity laser absorption spectroscopy (ICAS) with a custom-made broadband dye laser, respectively. For ICAS, absorption features in the spectral range from 16,316 to 16,353 cm-1 have been evaluated, and the oscillator strengths for all 41 assigned ro-vibronic transitions have been expressed. For most of the experimental cases, the measured Fe and FeO traces agreed well with the mechanism reported in this work. The quantitative and highly-sensitive measurements revealed the presence of FeO at temperatures below 1400 K, leading to a reconsideration of rate coefficients for different Fe oxidation channels.

We report on our recent progress in advancing intracavity absorption spectroscopy into the mid-infrared spectral range by using novel Cr 2+ - and Fe 2+ -doped chalcogenide-crystal lasers. Broadband and highly sensitive measurements of various species will be presented.

Chemically sensitive diagnostics are indispensable to unravel reactive processes in combustion and beyond, to understand their development in time and space and to monitor the reaction progress under the conditions of interest. A multitude of techniques is available that may provide species composition together with other process-controlling variables as a function of the reaction environment. Analytical tools have been developed that range from one-of-a-kind large-facility instrumentation to robust sensors for use in technical systems and in the field. In this chapter, needs and developments for the near and midterm future are addressed combining individual contributions from selected perspectives and intertwining thoughts and ideas from different fields and expertise. Major advances can be expected from combinations of instrumentation and digital processes, with beneficial uses for a multitude of processes in carbon-reduced and carbon-neutral environments.

Room-temperature laser systems consisting of a 4.6-m pulsed Fe:ZnSe pump laser and different Ce 3+ :Ge 20 Sb 10 Ga 5 Se 65 bulk glasses are demonstrated. The slope efficiency of a Ce 3+ :glass laser reaches 25%, with a maximum output energy of 45 mJ.

The influence of rare earth dopant concentration on selenide laser glass quality was investigated. A problem to be solved was identified – an increase in rare earth doping level leads to optical losses due to light scattering by heterogeneous inclusions and to the decrease of the optical damage threshold. The room temperature laser system consisting of 4.6 μ m pulsed Fe:ZnSe pump laser and Ce 3+ :Ge 20 Sb 10 Ga 5 Se 65 bulk glass was demonstrated. The slope efficiency of Ce 3+ :glass laser with respect to the absorbed pump energy reached 25% with a maximum output energy of 45 mJ.

The first, to the best of our knowledge, mid-infrared Q-switched Ce3+-doped glass laser is demonstrated. As saturable absorbers, Fe2+:CdSe and Fe2+:CdTe are used for the first time. When Q-switched by Fe:CdSe, the laser operates in a multi-pulse regime with an individual pulse width of 110 ns, centered at λ = 5.20 µm. With Fe:CdTe as saturable absorber, 1-3 giant pulses of 30 ns pulse width are generated at λ = 5.13 µm.

We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around λ = 3.1-3.4 µm. A detection scheme based on an integrated recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of Leff ≈ 850 m. Exemplary measurements of atmospheric H2O and CH4, and additionally introduced gas-phase HCl, C2H4, or C2H6 are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 104 are discussed.

In this work, we demonstrate a Ce-doped chalco-genide glass laser pumped by a Fe:ZnSe pulsed laser at room tem-perature. The laser performance of the Ce-doped active element under different pump wavelengths is realized and compared. The thermal effects that occurred in the Ce-doped active element are discussed.

We report on measurements highlighting the spectral dependence of the principal axes’ angular orientation that is present within three hollow-core fibers samples of various geometries. Hollow-core anti-resonant fibers (ARFs) with six, five, and four capillaries are investigated in several transmission windows. It is shown that the six-capillary fiber structure demonstrates a much smaller shift in the principal axes’ orientation, in comparison with five- and four-capillary structures. The four-capillary structure has a 90° shift of its principal axes’ orientation inside even- and odd-numbered transmission windows. These results corroborate the suggestion that the six-capillary ARF structure is a great candidate for polarization-maintaining operation over a wide spectral bandwidth.

We report on experimental observation of four phase-locked bound soliton formation in a SESAM mode-locked Cr2+:ZnSe laser with an output power of 20 mW, a temporal pulse-to-pulse separation of 18.2 ps and single pulse duration of 1.4 ps. We demonstrate that birefringence can be used as an efficient mechanism to achieve bound state generation in solid-state lasers as an alternative method to Kerr-lens and SESAM saturation by a pump power increase or a total net cavity dispersion variation. In this work, we consider two different setup configurations to demonstrate the birefringence influence on bound soliton formation. The suggested technique could be effectively used in the realization of mode-locked lasers based on solid-state active media operating in bound-state or multi-bound regimes.

In high purity Ce3+-doped selenide glass pumped by a 4.08 µm Fe:ZnSe laser, 5.1-5.5 µm laser oscillations were observed. This is the first evidence of laser action corresponding to the 2F7/2→2F5/2 transition of Ce3+ ions.

High-energy mid-infrared (MIR) tunable lasers are of great interest for a variety of scientific, industrial and medical applications. Currently, intensive research is being conducted on new iron-doped telluride crystals, aimed at developing laser materials for the spectral range of 5–7 μm. Here we present our recent results obtained with single-crystal Fe:CdTe lasers. In particular, laser operation upon different pumping schemes and operation temperatures are investigated. The developed laser systems are characterized regarding efficiency, output energy and wavelength tunability. Furthermore, first applications of a Fe:CdTe laser for absorption spectroscopy will be discussed.

First observations of mid-infrared lasing of a Ce3+-doped material are reported. When pumped by a 300-µs pulsed Fe:ZnSe laser emitting 250-mJ at 4.08 µm, the Ce-glass laser emits about 0.5 mJ, centered at 5.3 μm.

We demonstrate an intracavity absorption spectroscopy system based on a broadband single-crystal pulsed Fe:ZnSe laser. The laser operates at room-temperature and is continuously tunable in the spectral range of 3.76–5.29 µm. The long-wavelength emission up to 5.29 µm is a record achievement for Fe:ZnSe lasers, to the best of our knowledge. The developed laser system is applied for measurements of gaseous absorption inside the laser resonator. We demonstrate sensitive detection of (i) CO2 isotopes in the atmosphere and in human breath, (ii) CO in breath (after cigarette smoking) and in the smoke of a smoldering paper, and (iii) N2O in a gas flow. The achieved detection limits are: 0.1 ppm for 12CO2 and 13CO2, 3 ppm for CO, and 1 ppm for N2O. The sensitivity of the current system is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 107 are discussed.