Mid-infrared Laser Research Articles

NH3 line broadening coefficients and intensities measurement and impurities determination in emerging applications: CCUS, Biomethane and H2

A mid-infrared quantum cascade laser (Mid-IR QCL) coupled with a Single Pass Cell and a Multi Pass Cell, was utilized to measure ammonia (NH3) absorption spectroscopic parameters and determine NH3 impurities toward three emerging applications. We for the first time measured the pressure broadening coefficients perturbed by Air, O2, N2, He, CO2, CH4, and H2 and the line intensities of six NH3 transition lines near 1084.6 cm−1. The measured NH3-He, NH3-Air, and NH3-CO2 broadening coefficients align with HITRAN database, while NH3-H2 coefficients exhibit a maximum discrepancy of 46 %. Deviations between the measured line intensities and HITRAN database are minimal. Nevertheless, the uncertainties of line intensities have been significantly reduced from 20 % in HITRAN to below 3 %. The newly measured line parameters are utilized to address NH3 impurity requirements outlined in CCUS (ISO 27913:2016), Biomethane (EN 16723:2016), and H2 (ISO 14687:2019) standards. Based on the concept of optical gas standard (OGS), the NH3 impurity detection requirements in all three standards have been fulfilled with an uncertainty of 1.35 %. The precision of the NH3-OGS is 800 part per trillion (ppt) with an integration time of 100 s. The repeatability of the NH3-OGS is 130 ppt for a continuous measurement time of 48 min. Notably, the NH3-OGS effectively addresses the highly nonlinear adsorption–desorption dynamics, underscoring the potential of OGS as a calibration-free and SI-traceable metrological gas analysis instrument.

2-W Ho:LaF3 laser intracavity pumped with a Tm:YLF laser

In this paper, we explore the laser properties of a 2.1 µm Ho:LaF3 laser intracavity pumped by a LD-pumped Tm:YLF laser. Ho:LaF3 crystals, with a doping concentration of 2%, have been verified as a novel type of laser gain material, whose luminescence lifetime of the 5I7 level was up to 25.7 ms. The Ho:LaF3 laser achieved an average output power of 2.07 W with a slope efficiency of 16.2%. The dual-wavelength output centered at 2089 nm and 2093 nm and exhibited excellent spot quality with M2x and M2y values of 1.19 and 1.23, respectively. Our results effectively demonstrate the impressive capability of Ho:LaF3 crystals for generating high-power 2.1 µm mid-infrared lasers.

2.1–2.2 μm wavelength-tunable cascade gain modulation Raman fiber laser

We present a theoretical and experimental analysis of a mid-infrared wavelength-tunable cascade gain modulation Raman fiber laser. Our simulation model incorporates the Generalized Nonlinear Schrödinger Equation with the fiber Bragg grating transmission functions. Utilizing the pulse tracking method, we investigate and analyze the impact of pump pulse width and Raman gain fiber length on the first and second order Raman laser thresholds. Additionally, we exhibit the evolution process of the pulse and optical spectrum at a constant pump pulse energy. Furthermore, we present the output pulse width and energy, and the Raman laser center wavelength as a function of the pump pulse energy. Subsequently, using the optimal theoretical values of pump pulse width and Raman gain fiber length, we experimentally achieve the gain modulation Raman fiber laser. With a 1990 nm laser pumping, the Raman laser can produce a 2172 nm laser with a pulse width of 1.25 µs and a single pulse energy of approximately 400 nJ. When tuning the pump laser wavelength from 1950 nm to 2030 nm, the gain modulation Raman laser wavelength can vary from 2130 nm to 2215 nm. Importantly, our experimental results closely align with the theoretical predictions. This study provides valuable insights for the investigation of gain modulation Raman fiber lasers.

Advancing mid-infrared microdisk laser emission with Tm:YAG

Advancing mid-infrared microdisk laser emission with Tm:YAG

Millimeter-wave generation with a room-temperature nonlinear quantum cascade laser

Millimeter-wave difference frequency generation is reported for a dual-wavelength mid-infrared quantum cascade laser operating at room temperature. To overcome a low mid-infrared-to-terahertz conversion efficiency below 1 THz, a long-wavelength, high-performance mid-infrared quantum cascade laser structure with higher nonlinear susceptivity is adopted. By designing the efficient allocation of mid-infrared pumps to two sections of fabricated distributed feedback grating, a closely separated dual-wavelength (λ1 ∼ 13.53 μm and λ2 ∼ 13.39 μm) laser oscillation was obtained. Consequently, a millimeter-wave emission at a frequency of 231 GHz was successfully observed at room temperature.

High Water-Resistant and Thermal Stable Fluoride Fibers for Mid-Infrared Laser

High Water-Resistant and Thermal Stable Fluoride Fibers for Mid-Infrared Laser

Temporal variations in NO, N2O, and NO2 generation in filament-induced atmospheric plasmas

Ultra-short pulse lasers generate filaments in air, inducing changes in molecular concentration and the formation of new molecules. However, our understanding of the specific chemical reactions triggered by these filaments remains limited. This study aimed to investigate the NxOy species produced by femtosecond laser filaments in a sealed chamber. We employed mid-infrared laser spectroscopy to analyze the resulting products over the reaction time. The research revealed that filament plasma generates NO, N2O, and NO2. Notably, N2O was detected for the first time in filament plasmas generated in the air. The production of NxOy species depends on the initial pressure and is influenced by factors such as plasma properties and molecular collisions. We measured the equilibrium concentrations of NO, N2O, and NO2 under atmospheric conditions, finding them to be 67, 38, and 518 ppm, respectively. Furthermore, comparative experiments conducted in zero air illustrated significantly higher concentrations of NO and NO2 under identical pressure conditions, indicating a significant negative impact of other air molecules on the generation of these species. These findings provide valuable insight into the understanding of filament-induced atmospheric chemical reactions and the generation of NxOy species.

Off-plane quartz-enhanced photoacoustic spectroscopy.

In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam went neither through the prong spacing of the quartz tuning fork (QTF) nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by the radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of the acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as the excitation source, the noise level was suppressed by ∼2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.

Characterizing mid-infrared micro-ring resonator with frequency conversion

Due to the high cost, low-performance lasers and detectors in the mid-infrared (MIR) band, the development of MIR-integrated devices is very slow. Here, we demonstrate an effective method to characterize the parameters of MIR devices by using frequency conversion technology. We designed and fabricated rib waveguides and the micro-ring resonators (MRRs) on a silicon-on-sapphire platform. The MIR laser for the test is generated by difference frequency generation, and the transmission spectrum of the MIR-MRRs is detected by sum frequency generation. The experimental results show that the waveguide transmission loss is 4.5 dB/cm and the quality factor of the micro-ring reaches 38000, which is in good agreement with the numerical simulations. This work provides a useful method to characterize MIR integrated devices based on the frequency conversion technique, which can boost the development of MIR integrated optics in the future.

Broadband chaos of an interband cascade laser with a 6-GHz bandwidth.

Near-infrared semiconductor lasers subject to optical feedback usually produce chaos with a broad bandwidth of a few GHz. However, the reported mid-infrared interband cascade lasers (ICLs) only show chaos with a limited bandwidth below 1 GHz. Here we show that an ICL with optical feedback is able to generate broadband chaos as well. The mid-infrared chaos exhibits a remarkable bandwidth of about 6 GHz, which is comparable to that of the near-infrared counterpart. In addition, the spectral coverage in the electrical domain reaches as high as 17.7 GHz. It is found that the chaos bandwidth generally broadens with increasing feedback ratio and/or increasing pump current of the laser, while it is insensitive to the feedback length.

Mid-infrared spectral properties and laser performance of bulk Tb-doped selenide glass

Mid-infrared spectral properties and laser performance of bulk Tb-doped selenide glass

Sensitive Discrete Frequency Mid-Infrared Absorption Spectroscopy Using Digitally Referenced Detection.

Broadly tunable mid-infrared (IR) lasers, including quantum cascade lasers (QCL), are an asset for vibrational spectroscopy wherein high-intensity, coherent illumination can target specific spectral bands for rapid, direct chemical detection with microscopic localization. These emerging spectrometers are capable of high measurement throughputs with large detector signals from the high-intensity lasers and fast detection speeds as short as a single laser pulse, challenging the decades old benchmarks of Fourier transform infrared spectroscopy. However, noise in QCL emissions limits the feasible acquisition time for high signal-to-noise ratio (SNR) data. Here, we present an implementation that is broadly compatible with many laser-based spectrometer and microscope designs to address these limitations by leveraging high-speed digitizers and dual detectors to digitally reference each pulse individually. Digitally referenced detection (DRD) is shown to improve measurement sensitivity, with broad spectral indifference, regardless of imbalance due to dissimilarities among system designs or component manufacturers. We incorporated DRD into existing instruments and demonstrated its generalizability: a spectrometer with a 10-fold reduction in spectral noise, a microscope with reduced pixel dwell times to as low as 1 pulse while maintaining SNR normally achieved when operating 8-fold slower, and finally, a spectrometer to measure vibrational circular dichroism (VCD) with a ∼ 4-fold reduction in scan times. The approach not only proves versatile and effective but can also be tailored for specific applications with minimal hardware changes, positioning it as a simple and promising module for spectrometer designs using lasers.

High-output ∼3 μ m MIR pulsed laser enabled by surface state regulation in PtTe2 optical switch

Mid-infrared (MIR) pulsed lasers operating in the ∼3 μm region play a crucial role in various applications, including molecular spectroscopy, ultrafast molecular imaging, and laser-assisted surgery. Despite recent advancements in MIR gain platforms, a notable technological challenge remains in the absence of an effective optical Q-switch. Here, a remarkable optical Q-switch in the 3 μm region based on a Dirac semimetal PtTe2 saturable absorber is realized. By modulating the surface state of PtTe2, the pulsed laser exhibited an increase in average power, escalating from 521 to 588 mW, accompanied by a significant decrease in pulse width from 368 to 187 ns. Nondegenerate pump–probe measurements showed that the recombination rate of the photocarrier in thinner PtTe2 nanoplates was effectively accelerated, primarily attributed to the substantial increase in surface state density, leading to better saturable absorption performance. As the thickness of the PtTe2 nanoplates decreases, the nonsaturable loss decreases from 12% to 3%, while the modulation depth increases from 6% to 12%. The enhanced ultrafast nonlinear absorption enables flexible modulation of saturation absorption parameters, which endows high-performance MIR pulsed laser generation.

Integrated graphene-based semimetal/metallic-like MSe2 (M = Pt, Re) heterostructures for mid-infrared ultrafast photonics

Mid-infrared (MIR) ultrafast photonics has received significant interest due to the development of compact and robust MIR laser sources capable of emitting wavelengths beyond 2 μm. Transition metal selenides (TMSes), with the properties of feasibly tunable bandgaps, desirable photocarrier transmission property, and broadband nonlinear optical response, have become a research focus on ultrafast photonic applications. However, the relatively large bandgap (around 1 eV) of the TMSes greatly restricts their applications in the MIR region. Here, the synergetic interaction between multilayer graphene (Gr) and semimetal or metallic-like (PtSe2 and ReSe2) is employed to tune the optical properties and to further optimize the optical performance of TMSes in the MIR region. Benefiting from the ultrafast photocarrier relaxation time, appropriate modulation depth, and almost zero non-saturated loss of Gr-PtSe2 and Gr-ReSe2 heterostructures, stable Q-switched laser operation at 1.9 μm has been achieved in an integrated design based on waveguide laser platform. The results demonstrate that combining the semimetal or metallic-like TMSes with van der Waals (vdW) heterostructures is a promising strategy to extend the photonic applications of TMSes into the MIR region, indicating the potential applications of graphene-based TMSes vdW heterostructures for MIR integrated photonics.

5 µm CW Ce3+-doped chalcogenide glass fiber laser with 17% slope efficiency.

Efficient room temperature mid-infrared laser action in a Ce3+-doped chalcogenide fiber was demonstrated. The fiber had a doped selenide glass core in an undoped sulfide glass cladding. The pump source was a CW Fe2+:ZnSe laser emitting at 4.14 µm. The optimized fiber length allowed obtaining up to 7 mW of 5.06 µm output with 17% slope efficiency at room temperature.

KTA-OPO for 1742 nm laser generation driven by a composite Nd:YVO4-based self-Raman laser

In this work, a double-end diffusion bonded Nd:YVO4 self-Raman laser was designed to drive an intracavity, noncritically-phase-matched KTiOAsO4 (KTA) optical parametric oscillator (OPO). Both conversion efficiency and output power at 1.7 µm (the wavelength of the OPO signal field) were improved by effectively reducing the thermal lens effect and increasing the effective length of self-Raman medium. At an incident pump power of 15.4 W, the output power for 1742 nm output laser reached 2.16 W with a conversion efficiency of 14%, and the output having a pulse width of 10.5 ns and a pulse repetition frequency of 90 kHz. The competition between the OPO and cascaded Raman laser was observed when the incident pump power was above 12.4 W. The results highlight that in order to improve output power at 1742 nm, it is critical that both the cascaded, second-Stokes field at 1313 nm and the signal field generated at 1534 nm from the 1064 nm field driving the KTA-OPO be minimized, if not completely suppressed. This laser system combining the processes of stimulated Raman scattering and optical parametric oscillation for the generation of laser emission at 1742 nm may find significant application across a broad range of fields including biological engineering, laser therapy, optical coherence tomography and for the generation of mid-infrared laser wavelengths.

Simultaneous measurement of NH3 and NO by mid-infrared tunable diode laser absorption spectroscopy based on machine-learning algorithms

Accurate measurements of NH3 and NO are essential for controlling NOx emissions from coal-fired power plants, reducing ammonia slip, and improving the efficiency of selective catalytic reduction (SCR) operation. We propose a method for measuring NH3 and NO concentrations based on machine learning algorithms to address the challenges posed by overlapping interference of CO2 and H2O spectral lines in flue gas. Our approach introduces a novel method for acquiring mixture spectra for the model. Initially, we measure the spectra of individual components under different concentrations and temperatures. Subsequently, mixed spectral samples are generated by combining the measured spectra of the individual components. This approach simplifies the spectral measurement process while preserving accuracy. The particle swarm optimization support vector machine (PSO-SVM) algorithm is leveraged, providing a reliable foundation for the continuous and synchronous measurement of NH3 and NO. Upon testing, the PSO-SVM demonstrates average relative errors of 4.0 % and 0.8 % for NH3 and NO concentrations, respectively. The corresponding measurement precision is 0.05 ppm for NH3 and 0.42 ppm for NO, better than the conventional integral absorbance method. The minimum detection limit (MDL) for NH3 is 16.1 ppb at an average time of 50 s, while for NO, it is 38.4 ppb at an average time of 71 s. The methodology of this paper is expected to play an important role in reducing the influence of interfering components and improving the accuracy of field measurements.

Unzipping hBN with ultrashort mid-infrared pulses.

Manipulating the nanostructure of materials is critical for numerous applications in electronics, magnetics, and photonics. However, conventional methods such as lithography and laser writing require cleanroom facilities or leave residue. We describe an approach to creating atomically sharp line defects in hexagonal boron nitride (hBN) at room temperature by direct optical phonon excitation with a mid-infrared pulsed laser from free space. We term this phenomenon "unzipping" to describe the rapid formation and growth of a crack tens of nanometers wide from a point within the laser-driven region. Formation of these features is attributed to the large atomic displacement and high local bond strain produced by strongly driving the crystal at a natural resonance. This process occurs only via coherent phonon excitation and is highly sensitive to the relative orientation of the crystal axes and the laser polarization. Its cleanliness, directionality, and sharpness enable applications such as polariton cavities, phonon-wave coupling, and in situ flake cleaving.

Growth, thermal, spectroscopy and ∼2.7 μm multiwavelength laser output of Er:GYAP crystal

We demonstrate the growth, thermal, spectroscopy and laser performance of a Er3+ doped Gd0.1Y0.9AlO3 (Er:GYAP) disorder crystal grown by Czochralski method. The crystal with space group Pbnm and lattice parameters of =0.5185 nm, =0.5327 nm, =0.7378 nm, α=β=γ=90° and =0.203784 nm3 are obtained by fitting powder X-ray diffraction data. The Mohr’s hardnesses along three axes are 6.94, 7.27, and 7.44. The thermal expansion coefficient of axis is 4.20 × 10–6 K–1 and the density is 5.88 g/cm3. The thermal conductivities are characterized as =6.24, =5.57, and =6.83 W/(m·K). Meanwhile, the refractive index, absorption and emission spectra in triaxial directions are determined. Besides, level lifetimes of 4I11/2 and 4I13/2 are 0.86 and 2.83 ms. Finally, the ∼2.7 μm multiwavelength laser outputs are observed with maximum average power of 628 mW and beam quality factors / of 1.42/1.45. The results prove that the Er:GYAP crystal is a promising gain medium for generating mid-infrared lasers.

Determination of Air–Fuel Ratio at 1 kHz via Mid-Infrared Laser Absorption and Fast Flame Ionization Detector Measurements in Engine-Out Vehicle Exhaust

<div>Measurements of air–fuel ratio (AFR) and <i>λ</i> (AFR<sub>actual</sub>/AFR<sub>stoich</sub>) are crucial for understanding internal combustion engine (ICE) performance. However, current <i>λ</i> sensors suffer from long light-off times (on the order of seconds following a cold start) and limited time resolution. In this study, a four-color mid-infrared laser absorption spectroscopy (LAS) sensor was developed to provide 5 kHz measurements of temperature, CO, CO<sub>2</sub>, and NO in engine-out exhaust. This LAS sensor was then combined with 1 kHz hydrocarbon (HC) measurements from a flame ionization detector (FID), and the Spindt exhaust gas analysis method to provide 1 kHz measurements of <i>λ</i>. To the authors’ knowledge, this is the first time-resolved measurement of <i>λ</i> during engine cold starts using the full Spindt method. Three tests with various engine AFR calibrations were conducted and analyzed: (1) 10% lean, (2) stoichiometric, and (3) 10% rich. The measurements were acquired in the exhaust of a light-duty truck with an 8-cylinder gasoline engine. The LAS-FID-based <i>λ</i> sensor results were compared with those obtained from a universal exhaust gas oxygen (UEGO) sensor. The LAS-FID method provided robust <i>λ</i> measurements from the first combustion exhaust event (avoiding the light-off time associated with traditional <i>λ</i> sensors) in addition to enhanced temporal resolution (on the order of 100× increase compared to traditional diffusion-based <i>λ</i> sensors). The insight gained from this novel method could be used to benefit crank, cold start, and open- or closed-loop air–fuel ratio control strategies in gasoline engines for reduced emissions.</div>