Mid-infrared Laser Research Articles

Effect of Different Doping Concentrations on the Properties of Fe2+:ZnSe and Fe2+, Cr2+:ZnSe Crystals Grown by the Chemical Vapor Transport Method.

The Fe2+:ZnSe crystal is an important material for 3-5 μm mid-infrared lasers. In this work, we have grown different doping concentrations of Fe2+:ZnSe and Fe2+, Cr2+:ZnSe by the chemical vapor transport method. The doped crystals have the same structure. The lattice constant increases slightly with a higher Fe2+ concentration, while the doped Cr2+ has a great influence on the lattice constant. Besides, with a higher Fe2+ concentration, the TO mode has no change and the LO mode presents a small blue shift. The doped ZnSe crystals all have a wide absorption peak in the range of 2.3-4.5 μm, and these absorption peaks widen with the increase of the Fe2+ doping concentration. Besides, in the range of 5-20 μm, the crystals have good transparency. The concentrations of 3, 2, and 1% Fe2+ samples are calculated to be 4.15 × 1019, 3.27 × 1019, and 3.02 × 1019 cm-3, respectively, and the Fe2+ concentration of the Fe2+, Cr2+:ZnSe sample is 3.71 × 1019 cm-3. The band gap decreases from 2.52 to 2.27 eV with a higher Fe2+ doping concentration. Therefore, we can modulate the absorption bandwidth and band gap by doping the ion concentration, which is more suitable for developing mid-infrared gain medium applications.

Mid-infrared pulsed fiber laser source at 4.3 µm based on a CO2-filled anti-resonant hollow-core silica fiber

Fiber lasers in the mid-infrared (mid-IR) band are of great interest due to their wide range of applications such as manufacturing, defense, spectroscopy, and free-space communication. Due to the immaturity of the soft glass fiber fabrication technology and the limitation of the type of doped rare earth, laser power scaling and wavelength expansion above 4 µm are greatly limited. Lasers based on gas-filled hollow-core fibers (HCFs) have proved to be an effective way of generating mid-IR lasers. We demonstrate a pulsed 4.3 µm laser source based on a CO2-filled HCF for the first time. The pulse energy characteristics and output spectrum of the mid-IR laser have been investigated. The maximum pulse energy of the mid-IR laser is 236 nJ. The maximum average power of the mid-IR laser is 297.8 mW with a slope efficiency of 17.3%. A step-tunable mid-IR output is achieved from 4293.718 nm to 4392.085 nm including 8 emission lines. Furthermore, the time-domain and frequency-domain properties of the mid-IR laser have been studied to understand laser operation better. This work has an important reference value for the development of pulsed mid-IR fiber gas laser sources.

Watt-scale near-diffraction-limited mid-infrared laser delivery by a chalcogenide anti-resonant fiber

Watt-scale near-diffraction-limited mid-infrared laser delivery by a chalcogenide anti-resonant fiber

NbSe2/PtTe2 heterostructures as saturable absorbers for mid-infrared pulsed solid-state lasers

NbSe2/PtTe2 heterostructures as saturable absorbers for mid-infrared pulsed solid-state lasers

InAs/GaSb superlattice SESAM mode-locked fluoride fiber laser at 2.8 µm.

We demonstrate a mode-locked fluoride fiber laser operating at 2.8 µm using an InAs/GaSb superlattice (SL) semiconductor saturable absorber mirror (SESAM). Z-scan measurements show that the SL saturable absorber layer possesses off-bandgap nonlinearity near 2.8 µm, with a saturation fluence of 15.9 µJ/cm2, a single-pass modulation depth of 2.4%, and a damage threshold of 10.4 mJ/cm2. Using this high-damage-threshold SESAM, we achieved a maximum average output power of 1.16 W from the 2.8 µm mode-locked Er:ZBLAN fiber laser, with a pulse duration of 28.1 ps. Continuous mode-locking operation at an average power of 224 mW was sustained for 36 h without interruption, demonstrating the reliability of the InAs/GaSb SL SESAM for the development of mid-infrared mode-locked fluoride fiber lasers.

Infrared Laser Ablation and Capture of Formalin-Fixed Paraffin-Embedded Tissue.

Formalin-fixed paraffin-embedded (FFPE) tissue is a ubiquitous and invaluable resource for biomedical research and clinical applications. However, FFPE tissue proteomics is challenging due to protein cross-linking and chemical modification. Laser ablation sampling allows precise removal of material from tissue sections with high spatial control and reproducibility for offline proteomics by liquid chromatography coupled with tandem mass spectrometry. In this work, we used a pulsed mid-infrared laser for microsampling of rat liver tissue for subsequent identification and quantification of proteins. It was found that more proteins were identified by FFPE tissue laser ablation sampling compared to fresh frozen (FF) tissue laser ablation sampling and that more proteins were identified by laser ablation than by manual dissection of FFPE tissue. In contrast to previous studies, no loss of hydrophilic proteins due to residual cross-linking was observed. The efficient capture of proteins by laser ablation microsampling is attributed to efficient laser breakup of the tissue which facilitates downstream processing of the proteins.

Fluoride and chalcogenide glass fiber components for mid-infrared lasers and amplifiers: Breakthroughs, challenges, and future perspective

Since the early 1990s, when researchers began to explore rare-earth-doped mid-infrared glass fibers, fiber laser systems have emerged as promising high-brightness light sources with wavelengths beyond 2.5 μm for applications in spectroscopy and sensing, optical communications and ranging, and processing of complex materials and bio-tissues, to name a few. Despite a substantial research effort over the years, mid-infrared fiber lasers and amplifiers have yet to reach the maturity required for widespread and/or industrial use. The well-known advantages of fiber lasers over their bulk counterparts, namely superior stability and beam quality, compactness, cost-efficiency, flexibility, and maintenance-free operation, can only be fully harnessed in the mid-infrared wavelength range with the development of non-existent yet essential fiber-based components made of advanced fluoride or chalcogenide-glass materials. This Perspective reports on the recent significant achievements that have been made in the design and fabrication of in-fiber and fiber-pigtailed components for fully integrated mid-infrared fiber laser systems. Building upon a comprehensive overview of the mechanical, thermodynamic, and optical properties of fluoride and chalcogenide glass fibers, as well as their interaction with light, we aim to highlight current challenges and opportunities and provide an informed forecast of future advancements in mid-infrared all-fiber laser research.

Mid-Infrared Lasers and Supercontinuum Sources Based on Stimulated Raman Scattering in Gas-Filled Hollow-Core Fibers

Mid-Infrared Lasers and Supercontinuum Sources Based on Stimulated Raman Scattering in Gas-Filled Hollow-Core Fibers

Measurement of temporal evolution of antiferromagnetic domain change in nickel oxide driven by 2TO phonon mode excitation via pump-probe experiment

The temporal evolution of antiferromagnetic domain pattern changes under resonant excitation of the second-order transverse optical phonon mode in nickel oxide was investigated using mid-infrared free electron laser (MIR-FEL) pulses and visible nanosecond probe laser pulses at room temperature. We visualized the domain patterns through magneto-optical polarized microscopy in transmission and observed their temporal variation after the phonon excitation via MIR-FEL. We evaluated the differences throughout the timeline using the structural similarity index measure technique from domain images with and without MIR-FEL irradiation. We found that the MIR-FEL irradiation induces significant structural changes in the domain patterns. The maximum difference was observed at the timing of the MIR-FEL irradiation and exponentially recovered with the time constant of 1.4 ± 0.2 ms. This rather long-time constant could be owing to the spin relaxation, whilst further investigation is needed to confirm the underlying mechanism.

Characterization and mid-infrared laser operation in Er:CaF2 single crystal fiber grown by the laser heated pedestal growth method

Er:CaF2 crystals, characterized by low doping and high efficiency, are suitable materials for single-crystal fibers. 3 at.% Er:CaF2 single-crystal fibers (SCF) were grown using the laser heated pedestal growth (LHPG) method. Significant oxidation-induced whitening phenomena were observed during growth. Increasing the growth rate helped mitigate further deterioration due to oxidation. This is likely because higher growth speeds allow the SCF to quickly move away from temperature ranges conducive to oxidation. As oxidation progressed, the phonon energy of Er:CaF2 SCF increased, causing the emission intensity at ∼ 1 μm to decrease from 77 % in the initial source rod to 6 % in the fully whitened state. Additionally, the lifetime of the 4I11/2 level decreased by approximately 10 times, from 8.988 ms to 0.822 ms. Continuous laser output at ∼ 2.8 μm was achieved using the transparent portion of 3 at.% Er:CaF2 SCF. With an output mirror transmission of 2 %, a maximum output power of 251 mW and a slope efficiency of 15.9 % were obtained. Laser experiments demonstrated the potential application of LHPG-prepared Er:CaF2 SCF in ∼ 2.8 μm lasers, but further performance enhancement requires additional strategies to address oxidation issues. This work provides valuable insights into the preparation of Er:CaF2 SCF using the LHPG method.

High-peak-power narrow-pulsed linearly polarized laser at ∼3 µm

A hundred-watt-level peak-power linearly polarized Ho,Pr:GdScO3 laser with narrow pulses was first realized at ∼3 µm through a combination of theoretical simulation and experiment. This is the narrowest pulse width, and the highest peak power has been achieved in a passively pulsed Ho,Pr co-doped laser to date. We realized a linearly polarized narrow-pulsed laser at ∼3 µm, with a maximum peak power of 185 W and shortest pulse width of 42 ns. A further theoretical model was built by simulating the dynamic process of the mid-infrared (MIR) pulsed Ho,Pr:GdScO3 laser using coupled rate equations. The numerical simulation results were fundamentally in agreement with the experimental results, which verified the potential of Ho,Pr:GdScO3 crystals to produce sub-50-ns hundred-watt peak power MIR lasers. The results presented an effective way to achieve high-peak-power, narrow-pulse, and linearly-polarized lasers, which have significant research potential and promising applications in the MIR band.

Monolithic dispersion engineered mid-infrared quantum cascade laser frequency comb

The high-power quantum cascade laser (QCL) frequency comb capable of room temperature operation is of great interest to high-precision measurement and low-noise molecular spectroscopy. While a significant amount of research is devoted to the longwave spectral range, shortwave 3–5 μm QCL combs are still relatively underdeveloped due to the excessive material dispersion. In this work, we propose a monolithic integrated multimode waveguide scheme for effective dispersion engineering and high-power-efficiency operation. Over watt-level output power at room temperature with a wall plug efficiency of 7% and robust dispersion reduction is achieved from a quantum cascade laser frequency comb at a wavelength approximately 4.6 μm. Narrow beatnote linewidth less than 1 kHz and clear dual-comb multiheterodyne comb lines manifest the coherent phase relation among the comb modes which is crucial to fast molecular spectroscopy. This monolithic dispersion engineered waveguide design is also compatible to an efficient active–passive optical coupling scheme and would open up a new research playground for ring comb and on-chip dual-comb spectroscopy.

Phase-locked high-brightness interband cascade laser array with multimode interferometer couplers.

We report on the design, fabrication, and characterization of an interband cascade laser (ICL) array incorporating multimode interference (MMI) couplers for phase-locking emitters. The ICL array, emitting at 3-4 µm, was developed to address the challenges of heat dissipation and in-phase operation in mid-infrared lasers. The array features a 7.5-µm-wide ridge design for fundamental transverse mode propagation and employs MMI structures to realize an in-phase operation of multiple emitters. The far-field patterns, characterized by periodic and symmetrical interference fringes, confirm the coherent operation of the array and the efficacy of MMI couplers in achieving phase-locking. The single-ridge side of the array exhibits a single-lobe far-field profile, with higher-order transverse modes effectively suppressed, showcasing a nearly diffraction-limited beam quality (M2 ≈ 1.31) at high output powers (390 mW from the 1 × 4 array). The robust performance and scalable design of the ICL array, validated by experimental results and theoretical simulations, indicate its potential for high-power mid-infrared applications and as an optical phased array.

4.8-μm CO-filled hollow-core silica fiber light source

Mid-infrared (MIR) fiber lasers are important for a wide range of applications in sensing, spectroscopy, imaging, defense, and security. Some progress has been made in the research of MIR fiber lasers based on soft glass fibers, however, the emission range of rare-earth ions and the robustness of the host materials are still a major challenge for MIR fiber lasers. The large number of gases provide a variety of optical transitions in the MIR band. When combined with recent advances in low-loss hollow-core fiber (HCF), there is a great opportunity for gas-filled fiber lasers to further extend the radiation to the MIR region. Here, a 4.8-μm CO-filled silica-based HCF laser is reported for the first time. This is enabled by an in-house manufactured broadband low-loss HCF with a measured loss of 1.81 dB/m at 4.8 μm. A maximum MIR output power of 46 mW and a tuning range of 180 nm (from 4644 to 4824 nm) are obtained by using an advanced 2.33-μm narrow-linewidth fiber laser. This demonstration represents the longest-wavelength silica-based fiber laser to date, while the absorption loss of bulk silica at 4824 nm is up to 13, 000 dB/m. Further wavelength expansion could be achieved by changing the pump absorption line and optimizing the laser structure.

High Peak Power Mini-array Quantum Cascade Lasers Operating in Pulsed Mode

Abstract Broad area quantum cascade lasers (BA QCLs) have significant applications in many areas, but suffer from demanding pulse operating conditions and poor beam quality due to heat accumulation and generation of high order modes. A structure of mini-array is adopted to improve the heat dissipation capacity and beam quality of BA QCLs. The active region is etched to form a multi emitter and the channels are filled with InP: Fe, which acts as a lateral heat dissipation channel to improve the lateral heat dissipation efficiency. A device with λ~4.8 μm, a peak output power of 122 W at 1.2% duty cycle with a pulse of 1.5 μs is obtained in room temperature, with far-field single-lobed distribution. This result allows BA QCLs to obtain high peak power at wider pump pulse widths and higher duty cycle conditions, promotes the application of the mid-infrared laser operating in pulsed mode in the field of standoff photoacoustic chemical detection, space optical communication and so on.

Mid-infrared silicon photonic lasers based on GeSn slab waveguide on silicon

GeSn alloy has emerged as an attractive active material for Si-based mid-infrared (MIR) lasers due to its direct bandgap nature at higher Sn concentrations. Here, we report on an optically-pumped GeSn MIR lasers based on planar slab waveguide with a top Si ridge structure. The inclusion of 10% Sn transforms the GeSn active layer into a direct bandgap material. The Si ridge structure ensures appropriate optical confinements with reduced scattering loss from the waveguide sidewall. Lasing action was achieved under optical pumping with a low threshold of 60.85 kW/cm2 and an emission wavelength of 2238 nm at T = 40 K. Lasing action was also observed up to T = 90 K with a threshold of 170 kW/cm2.

Enhanced 3 μm emission in Ho3+/Yb3+ co-doped bismuth-fluoride glasses by Gd2O3

In this work, (100-b)(45Bi2O3-40GeO2-15BaF2)-bGd2O3-2Yb2O3-0.5Ho2O3 glass was prepared to use a high-temperature melting method(where b= 0, 2.5, 5, 7.5, 10). The absorption and fluorescence spectra indicate that the optimal Gd2O3 content is 7.5 mol% (BGBG7.5HY), with an Ω2 value of 6.20×10−20 cm2 for the main glass, which is superior to that of BGBG0HY glass. This suggests that the incorporation of Gd2O3 enhances the mid-infrared emission capacity. Raman spectroscopy studies demonstrate that the introduction of Gd₂O₃ results in a substantial influx of oxygen atoms into the glass network, accompanied by a transition of [BiO₆] to [BiO₃] within the glass mesh. This is evidenced by a notable redshift of the characteristic peaks at 509 cm−1 and 677 cm−1 in comparison to BGB glass. The introduction of Gd2O3 also expands the distance between Ho3+/Yb3+ ions, inhibits rare earth ion clusters, improves the Ho3+ luminescence center environment, and thus enhances the 3 μm emission performance of bismuth fluoride glass. The maximum emission cross section is 1.82 × 10−20 cm2 and the maximum gain coefficient is 3.71 cm−1. The results show that the bismuth fluoride glass prepared in this paper is an excellent gain material for mid-infrared fiber lasers.

Fabrication of optimized partial-reflection coatings for mid-infrared quantum cascade lasers

This study presents both numerical modeling and experimental fabrication of three different partial-reflection (PR) coatings each optimized for quantum cascade lasers (QCLs) that emit radiation in the mid-infrared range. A novel double-layer PR coating comprising silicon dioxide (SiO2) and silicon nitride (Si3N4) was proposed as a potential solution for compatibility with QCLs fabrication processes. Subsequently, the PR coating was compared with two well-known PR coatings: a single layer of aluminum oxide (Al2O3) and a single layer of yttrium oxide (Y2O3). The coatings were designed to reduce the reflectivity of the front laser mirror from 30% to approximately 13%. The thickness of the dielectric layers was optimized for lasers emitting at 4.4 μm, with applicability in the 2.5–6 μm range. The proposed double-layer coating achieved the desired reflectivity while reducing the total coating thickness by 120 nm. By using the presented coatings it will be possible to increase the optical power of Mid-Infrared QCLs.

Research on mid-infrared quantum cascade lasers spectral beam combining

Quantum cascade lasers (QCLs) in the mid-infrared (MIR) hold significant potential for widespread applications in both military and civilian contexts. However, the utility of present single-chip QCLs is hampered by issues such as low output power and subpar beam quality. This study addresses these limitations by employing spectral beam combining (SBC) based on a diffraction grating, with an aim to enhance both power and beam quality of MIR QCLs. Coaxial power synthesis of three single-chip QCLs (around 4.75 μm) is achieved experimentally, importantly, the beam quality did not decrease after combining, essentially maintaining the same quality as before combining. It is proposed that there are four main factors affecting the combining efficiency, namely operating optical power (or driving current) of the chips, individual differences in QCL chips, diffraction grating, and reflectivity of feedback mirror, and their effects on the combining efficiency are discussed separately. This study confirms that SBC is an effective way to obtain high-power and high-beam quality MIR QCL sources, and lays a research foundation for more beam spectral combining.

The influence of resonant light pulses on high harmonic generation in solids

Abstract We have theoretically studied the high harmonic generation (HHG) in solids driven simultaneously by a mid-infrared (MIR) laser and a high-order harmonic pulse with energy around the band gap between the valence band and conduction band. By adding this resonant harmonic light pulse with the relative intensity ratio of 4\%, the high-order harmonic emission from the crystal is enhanced by 1-2 orders of magnitude. The yield of HHG in solid increases monotonically with the relative strength of the resonant harmonic pulses. In addition, we also found that HHG dynamics from the $k$ channel around the boundary of the Brillouin zone can be selectively enhanced by adjusting the frequency of the resonant high-order harmonic pulse. The resonance-enhanced HHG and $k$ channel selection effect in solids is also investigated by using the three-band semi-conductor Bloch equation for HHG in ZnO. We also find that the harmonic in the plateau region driven by adding a resonant light field to the strong MIR driving field has less red-shifted compared with the case driven by the MIR driving field alone.