Mid-infrared Laser Research Articles

Mid-infrared vortex array generation with a tunable singularity in an Er:YAP laser

Optical vortex arrays are structured light fields with multiple phase singularities that have attracted considerable attention in recent years. Here, we demonstrate the direct generation of an optical vortex array with tunable singularity from a mid-infrared Er: yttrium aluminum perovskite laser under the annular-pumping regime. An axicon combined with a plano–convex lens is used to reshape the pumping beam into an annular one to match the different orders of the Laguerre–Gaussian mode. By adjusting the pump power and the lateral displacement of the gain medium, a tunable vortex array with one to four singularities is obtained in the ∼3-μm mid-infrared wavelength region. The lasing characteristics of the mid-infrared vortex arrays are evaluated and discussed. The experimental results can shed some light on the generation of mid-infrared optical vortex laser arrays and their applications.

Numerical modeling of multi-point side-pumped mid-infrared erbium-doped fluoride fiber lasers.

We investigate the power scaling and thermal management of multi-point side-pumped 2.825 µm heavily-erbium-doped fluoride fiber lasers by numerical simulation. The 4-point (or 6-point) erbium-doped fluoride fiber laser with polished erbium-doped fluoride fiber-based side-pump couplers delivers an output laser power of over 100 W at each launched 981 nm pump power of 100 W (or 75 W). Meanwhile, the core temperature increases of the gain fiber tips are below 1 K, making it possible for a highly reflective fiber Bragg grating to work stably in high-power operation. Once the preparation processes of these erbium-doped fluoride fiber-based side-pump couplers and endcaps with effective coatings are mature, the proposed multi-point side-pumped erbium-doped fluoride fiber lasers with some feasibility may theoretically pave the way for the development of hundred-watt mid-infrared fiber lasers with effective thermal management.

Quantum wave function reconstruction by free-electron spectral shearing interferometry.

The quantum wave function measurement of a free electron remains challenging in quantum mechanics and is subject to disputes about ψ-ontic/epistemic interpretations of the wave function. Here, we theoretically propose a realistic spectral method for reconstructing quantum wave function of an electron pulse, free-electron spectral shearing interferometry (FESSI). We use a Wien filter to generate two time-delayed replicas of the electron wave packet and then shift one replica in energy using a light-electron modulator driven by a mid-infrared laser. As a direct demonstration, we numerically reconstruct a pulsed electron wave function with a kinetic energy of 10 keV. FESSI is experimentally feasible and enables us to fully determine distinct orders of spectral phases and their physical implications in quantum foundations and quantum technologies, providing a universal approach to characterize ultrashort electron pulses.

Realization of broadband ∼1.8 μm luminescence by level population inversion in Tm3+ doped tellurite glass for laser applications

This study aims to investigate the characteristics of Tm3+ doped TeO2–B2O3–Al2O3Na2O glass. The structures that exist in host glass were obtained from the Raman spectrum. Due to the introduction of sodium and aluminum, the TeOTe bond is broken, and the amount of bridge oxygen is reduced, causing the transformation of [TeO4] structural units to [TeO3+1] and [TeO3] structural units. The spectrum characteristics of Tm3+ in the samples were predicted by theoretical calculations. Prepared Tm3+ doped glass has large Ω2 and small Ω6, which is connected to the ligand field's asymmetry and the stiffness of the network. Furthermore, under the excitation of 808 nm laser diode, broadband ∼1.8 μm fluorescence was observed and the effective fluorescent bandwidth Δλeff of 1830 nm emission in the glass is about 234 nm. For the sample with 0.5 mol% Tm2O3, the large values of emission cross-section product (9.08 × 10−21 cm2) and σemi × Δλeff product (21.25 × 10−26 cm3) indicate a good gain property as well as a good gain bandwidth. Eventually, the energy migration process and cross-relaxation process between thulium ions were analyzed. The micro-parameter of the CR process is 4.049 × 10−40 cm6/s. Hence, the results indicate that the prepared Tm3+ doped glass could be a potential candidate for ∼2 μm mid-infrared laser devices.

High-efficiency broad-linewidth widely-tunable mid-infrared OPO based on fan-out PPLT

We reported a high-efficiency, broad-linewidth, wide-tuning-range singly-resonated optical parametric oscillator (OPO) based on a fan-out PPLT crystal. When the PPLT crystal was operated at 120 °C and pumped by Q-switched 1064 nm laser at 14.17 W with a circular pump profile, the overall output power of OPO was 7.22 W (η = 50.4%) for both the signal and idler waves. The central wavelength and linewidth of the mid-infrared (MIR) laser were 3770 nm and ∼40 nm. A wavelength tuning range of 3538–3941 nm was accomplished by adjusting the crystal period through simple mechanical movement. Due to the larger coverage of gratings with an elliptical pump profile, the output linewidth of the MIR laser was further broadened to ∼100 nm, which is ten times more than other non-fan-out PPLT OPOs. Both pumping methods with different profiles were operated at 30 kHz, 49.2 ns for the 1064 nm laser, the power fluctuation of the obtained MIR laser was within 3.5%.

Quantifying acetylene mole fraction in rich flat laminar premixed C2H4/air flames using mid-infrared polarization spectroscopy

Mid-infrared laser polarization spectroscopy (IRPS) has been applied to measure the mole fraction of acetylene in rich premixed laminar C2H4/Air flat flames at equivalence ratios (Φ) of 1.7, 2.1, and 2.3, and under atmospheric pressure. The detection was conducted by probing the ro-vibrational P(19) transition at ~ 3.1 μm. The total collisional broadening coefficient of C2H2 was approximately 0.074 cm−1 atm−1 and varied within a range of 0.5% under different flame conditions, which made the effect of linewidth not obvious in the CH4/air flame. The calculated mole fraction of C2H2, using the Chemkin model, at Φ = 1.3 and 1.5 was used to calibrate the recorded IRPS signal intensities at different Height Above Burner (HAB). A single scaling factor was then used to quantify the measured C2H2 at highly sooting conditions, Φ = 1.7, 2.1, and 2.3, with a Limit of Detection (LoD) of 35 ± 5 ppm. The first observed C2H2 mole fraction appeared at HAB of 3 mm and measured as 2003 ppm, 2217 ppm, and 2495 ppm, for Φ = 1.7, 2.1, and 2.3, respectively. The mole fraction increased as the HAB increased to reach the maximum value of 2296 ppm, 2807 ppm, and 3478 ppm, for Φ = 1.7, 2.1, and 2.3, respectively, up to HAB of 5 mm. It was observed that the C2H2 mole fraction reaches a plateau region at HAB of ~ 8 mm. The production of C2H2 has been observed to be subject to a critical gas temperature of 1400 ± 30 K. The critical soot inception temperature, where the first incepted soot particles are observed, is the same as the gas temperature where {chi }_{{{mathrm{C}}_{2}mathrm{H}}_{2}}^{mathrm{max}} was detected, namely at 1500 ± 30 K. These measurements and calibration procedure demonstrate a plausible technique to probe other flames and to better understand soot inception and its correlation with C2H2.

Open-path sensor for atmospheric NO detection based on mid-infrared laser absorption spectroscopy

Nitric oxide (NO) is one of the primary air pollutants, and open optical path detection of NO holds immense significance in safeguarding the ecological environment and human health. In this paper, a high precision and high sensitivity NO open optical path sensor based on mid-infrared laser absorption spectroscopy technology is developed. The average gas concentration inversion of the NO path is achieved by collecting the backscattered light from the cooperative target. The gas measurement deploys a wavelength modulation method to remove the effects from light intensity variations. Furthermore, a Kalman filter and a temperature–pressure sensor optimization software algorithm were utilized to reduce the effects of temperature–pressure variations during the measurement process, leading to a 33% improvement in detection accuracy. The performance of the sensor was evaluated in the laboratory environment in terms of accuracy, sensitivity, and detection limit. The atmospheric NO concentration was also measured under varying time periods and weather conditions in the field environment, thus laying a strong foundation for studying the spatial and temporal evolution of NO in the atmosphere.

Diatomic molecule in a strong infrared laser field: level-shifts and bond-length change due to laser-dressed Morse potential

We present a general mathematical procedure to handle interactions described by a Morse potential in the presence of a strong harmonic excitation. We account for permanent and field-induced terms and their gradients in the dipole moment function, and we derive analytic formulae for the bond-length change and for the shifted energy eigenvalues of the vibrations, by using the Kramers–Henneberger frame. We apply these results to the important cases of H2 and LiH, driven by a near- or mid-infrared laser in the 1013 W cm−2 intensity range.

SnS2 as a Saturable Absorber for Mid-Infrared Q-Switched Er:SrF2 Laser.

Two-dimensional (2D) materials own unique band structures and excellent optoelectronic properties and have attracted wide attention in photonics. Tin disulfide (SnS2), a member of group IV-VI transition metal dichalcogenides (TMDs), possesses good environmental optimization, oxidation resistance, and thermal stability, making it more competitive in application. By using the intensity-dependent transmission experiment, the saturable absorption properties of the SnS2 nanosheet nearly at 3 μm waveband were characterized by a high modulation depth of 32.26%. Therefore, a few-layer SnS2 was used as a saturable absorber (SA) for a bulk Er:SrF2 laser to research its optical properties. When the average output power was 140 mW, the passively Q-switched laser achieved the shortest pulse width at 480 ns, the optimal single pulse energy at 3.78 µJ, and the highest peak power at 7.88 W. The results of the passively Q-switched laser revealed that few-layer SnS2 had an admirable non-linear optical response at near 3 μm mid-infrared solid-state laser.

Mid-infrared photoacoustic spectroscopy based on ultrasound detection for blood component analysis.

For the non-invasive measurement of biological tissue, a piezoelectric photoacoustic spectroscopy (PZT-PAS) system that detects a single frequency of ultrasound induced by the irradiation of pulse-modulated mid-infrared laser light was developed. PA spectra of the optical phantom and biological samples were obtained, and the relationship between the PA signal intensity and optical absorbance in the fingerprint region (930-1,200 cm-1) was analyzed to estimate the optical absorbance. The resonance vibration of the induced ultrasound was utilized to further increase the signal strength for biological tissue measurement. Consequently, PA spectrum reflecting the absorption of components in biological tissues was obtained.

Ultrafast Fiber Laser Emitting at 2.8 µm Based on a SESAM and a Broadband FBG

Ultrafast mid-infrared fiber lasers have been intensely studied in the last decade for the generation of high harmonics, molecular spectroscopy, material processing and remote sensing. Different designs have been investigated but most of them lacked the ease of use and reliability needed for their democratization. In this paper, we demonstrate a self-starting mode-locked mid-IR erbium-doped fiber laser based on a SESAM and a broadband uniform FBG that produces pulses as short as 15 ps. Different laser cavities were tested with varying FBG peak reflectance, spectral bandwidth and active fiber length. In addition, one cavity uses a pump combiner instead of injecting free-space the pump power through the fiber tip. The results of this study confirm that the FBG spectral bandwidth can efficiently control the duration of the almost Fourier-transform-limited pulses up to a limit seemingly dictated by the presence of water vapor in the laser cavity acting as narrow spectral filters. To a lower effect, the active fiber length influences the pulse duration. Finally, the use of an all-fiber pump combiner allows for a more compact and rugged design without altering the laser performances. This study represents a step towards the development of robust mid-infrared ultrafast all-fiber lasers.

Enhanced ∼2.85 μm mid-infrared emission of Na5Lu9F32: Pr3+/Ho3+ single crystal by introducing of non active Gd3+ ions

A series of novel Na5Lu9F32 fluoride single crystals by keeping the concentrations of Ho3+ (0.5 mol%) and Pr3+ (0.1 mol%) fixed and varying the concentrations of Gd3+ ions (i.e., 0, 0.5, 1.0, 1.2, 1.5 mol%) were prepared successfully by a reformative Bridgman technique for application of 2.85 μm mid-infrared. The impact of the non active Gd3+ ion on the crystallographic structure and optical properties of the single crystal were explored by the aid of the X-ray diffraction, Rietveld refinement, spectra of absorption and emission and decay curves of fluorescence. By leading Gd3+ (1.2 mol%) into Ho3+(0.5 mol%)/Pr3+(0.1 mol%) incorporated Na5Lu9F32 single crystal, the emission intensity at ∼ 2.85 μm was fulfilled significantly about two times enhancement. The destruction of [Ho3+-Ho3+] ion quench clusters and release more luminescent ions are in charge of the enhancement of the emission at 2.85 μm by leading Gd3+ into the fluoride single crystal. The maximum emission cross-section, ∼5.2 × 10−20 cm2, at ∼2.85 μm was estimated on the basis of the emission spectra. Thus, the grown Pr3+/Ho3+/Gd3+ incorporated Na5Lu9F32 fluoride single crystals could be an excellent candidate for source of ∼2.85 μm mid-infrared solid-state laser device.

Self-consistent simulations of intracavity terahertz comb difference frequency generation by mid-infrared quantum cascade lasers

Portable terahertz (THz) frequency comb sources are highly desired for applications in rotational molecular spectroscopy and sensing. To date, direct THz quantum cascade laser (QCL) frequency comb generation is not achievable at room temperature. However, THz comb generation based on intracavity difference frequency generation (DFG) in mid-infrared (mid-IR) QCLs is a promising alternative. Here, we present a numerical study of THz DFG-QCL comb formation in mid-IR QCLs based on a self-consistent multi-domain simulation approach. The dynamical simulations are performed using our open-source software tool mbsolve, which provides a flexible and efficient codebase for solving the generalized full-wave Maxwell–Bloch equations. Here, DFG in the active region of a dual-wavelength mid-IR QCL is considered for the generation of THz radiation. The mixing process and, thus, THz generation require a high second-order intersubband nonlinear susceptibility in the QCL active region and can be obtained by targeted quantum engineering. The associated nonlinear effects are included in the Hamiltonian of our Maxwell–Bloch simulation approach. All necessary input parameters for the description of the quantum system are determined self-consistently using our in-house ensemble Monte Carlo software tool for stationary carrier transport simulations. Notably, such simulations require a full-wave Maxwell–Bloch solver that does not employ the common rotating wave approximation, as a broadband optical field extending from the THz to the mid-IR region is investigated. Our modeling approach and the obtained simulation results for two THz DFG-QCL comb setups are validated against experimental data, showing reasonable agreement. Furthermore, we obtain a locked frequency modulated comb state for mid-IR and THz regimes.

Mid-infrared emission properties of erbium-doped fluorite-type crystals

We report on a comparative study of the spectroscopic properties and mid-infrared laser performance of five 5 at.% Er3+-doped fluorite-type crystals MF2, including parent compounds CaF2, SrF2, BaF2, and solid-solution (“mixed”) ones (Ca,Sr)F2 and (Sr,Ba)F2. In the M = Ca → Sr → Ba series, the host matrix phonon energy decreases, the absorption and mid-infrared emission spectra of Er3+ become narrower and more structured, and the luminescence lifetimes of the 4I11/2 and 4I13/2 Er3+ manifolds increase. The Er3+ transition probabilities were calculated using the Judd-Ofelt theory. In the “mixed” compounds, the Er3+ ions tend to reside in the larger / heavier cation environment. The low-temperature (12 K) spectroscopy evidences the presence of a single type of clusters at this doping level; the crystal-field splitting for Er3+ ions in clusters was determined. Continuous-wave low-threshold laser operation at ∼2.8 µm (the 4I11/2 → 4I13/2 transition) was achieved with all five Er3+:MF2 crystals. The maximum achieved laser slope efficiency was 37.9% (Er3+:CaF2), 23.5% (Er3+:SrF2) and 17.2% (Er3+:BaF2).

Quantum state-resolved methane scattering from Ni(111) and NiO(111) by bolometer infrared laser tagging: The effect of surface oxidation.

We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule-surface collisional energy exchange induced by oxidation of the nickel surface.

A mid-infrared laser microscope for the time-resolved study of light-induced protein conformational changes.

We have developed a confocal laser microscope operating in the mid-infrared range for the study of light-sensitive proteins, such as rhodopsins. The microscope features a co-aligned infrared and visible illumination path for the selective excitation and probing of proteins located in the IR focus only. An external-cavity tunable quantum cascade laser provides a wavelength tuning range (5.80-6.35 µm or 1570-1724cm-1) suitable for studying protein conformational changes as a function of time delay after visible light excitation with a pulsed LED. Using cryogen-free detectors, the relative changes in the infrared absorption of rhodopsin thin films around 10-4 have been observed with a time resolution down to 30ms. The measured full-width at half maximum of the Airy disk at λ = 6.08 µm in transmission mode with a confocal arrangement of apertures is 6.6 µm or 1.1λ. Dark-adapted sample replacement at the beginning of each photocycle is then enabled by exchanging the illuminated thin-film location with the microscope mapping stage synchronized to data acquisition and LED excitation and by averaging hundreds of time traces acquired in different nearby locations within a homogeneous film area. We demonstrate that this instrument provides crucial advantages for time-resolved IR studies of rhodopsin thin films with a slow photocycle. Time-resolved studies of inhomogeneous samples may also be possible with the presented instrument.

Label-free nanoscale mapping of intracellular organelle chemistry

The ability to image cell chemistry at the nanoscale is key for understanding cell biology, but many optical microscopies are restricted by the ~(200–250)nm diffraction limit. Electron microscopy and super-resolution fluorescence techniques beat this limit, but rely on staining and specialised labelling to generate image contrast. It is challenging, therefore, to obtain information about the functional chemistry of intracellular components. Here we demonstrate a technique for intracellular label-free chemical mapping with nanoscale (~30 nm) resolution. We use a probe-based optical microscope illuminated with a mid-infrared laser whose wavelengths excite vibrational modes of functional groups occurring within biological molecules. As a demonstration, we chemically map intracellular structures in human multiple myeloma cells and compare the morphologies with electron micrographs of the same cell line. We also demonstrate label-free mapping at wavelengths chosen to target the chemical signatures of proteins and nucleic acids, in a way that can be used to identify biochemical markers in the study of disease and pharmacology.

Optical property and Judd-Ofelt analysis of Dy3+, Na+:PbGa2S4 crystal grown by Bridgman method

In this research, the high quality Dy3+, Na+:PbGa2S4 single crystal was grown by Bridgman method. The crystal devices with high transmittance were successfully processed. The crystal structure and crystallization property were characterized by the XRD and XRC patterns. The transmission spectrum, temperature-dependent emission spectrum from 77 K to 350 K and decay curves at room temperature of crystal were measured. The strong absorption at 1315 nm of 6H15/2 → 6F11/2+6H9/2 transition could match well with high power 1310 nm semiconductor laser. The spectroscopic parameters were calculated by Judd-Ofelt theory based on the absorption spectra. The fluorescence lifetime of 6H11/2 energy level was fitted to be 2.09 ms and the corresponding fluorescence quantum efficiency η was calculated to be 77%. The emission cross-section at 4325 nm of 6H11/2 → 6H13/2 transition was also calculated to be 0.9 × 10−20 cm2. The experiment results show the advantage of Dy3+, Na+:PbGa2S4 crystal in emitting mid-infrared lasers.

High–Power 792 nm Fiber–Coupled Semiconductor Laser

The pumping of Tm-doped crystal or fiber by a 792 nm semiconductor laser is an important way to generate a mid-infrared laser, which is widely used in various fields. In this paper, a high–power 792 nm fiber–coupled semiconductor laser module was successfully fabricated with the output power of 232 W at a 10 A continuous current and the electro-optic conversion efficiency of 48.6%. The laser module is coupled with 24 chips into a fiber by spatial multiplexing and polarization combination technology. For a single emitting laser chip, the continuous wave (CW) output power and threshold current are 10.45 W at 10 A and 1.55 A, respectively. A polarization as high as 94% can also be realized, which is more suitable for laser spatial beam combining. The laser module was aged for more than 4000 h at 12 A and 25 °C without obvious power degradation.

Towards real-time active imaging of greenhouse gases using tunable mid-infrared all-fiber lasers.

We report a tunable all-fiber laser emitting a maximum output power of 2.55W around 3240nm. The fiber laser cavity based on a fluoride fiber doped with dysprosium ions yields an efficiency of 42% according to the in-band launched pump power at 2825nm. Due to a custom piezoelectric fiber Bragg grating (FBG) package, mechanical strains applied to the narrowband FBG used as the input cavity coupler allowed for fast tuning of the emission wavelength over a spectral range of 1.5nm. This laser was deployed in the field in northern Québec (Canada) to assess its performances for remote sensing of methane in the presence of a significant amount of water vapor, i.e.,over a hydroelectric reservoir. The preliminary results acquired during this field campaign confirm the great potential of the proposed approach for the development of a real-time active imaging system of greenhouse gases.