Laser Gain Research Articles

Nondissipative non-Hermitian dynamics and exceptional points in coupled optical parametric oscillators

Engineered non-Hermitian systems featuring exceptional points (EPs) can lead to a host of extraordinary phenomena in diverse fields ranging from photonics, acoustics, opto-mechanics, and electronics to atomic physics. In optics, non-Hermitian dynamics are typically realized using dissipation and phase-insensitive gain accompanied by unavoidable fluctuations. Here, we introduce non-Hermitian dynamics of coupled optical parametric oscillators (OPOs) arising from phase-sensitive amplification and de-amplification, and show their distinct advantages over conventional non-Hermitian systems relying on laser gain and loss. OPO-based non-Hermitian systems can benefit from the instantaneous nature of the parametric gain, noiseless phase-sensitive amplification, and rich quantum and classical nonlinear dynamics. We show that two coupled OPOs can exhibit spectral anti-parity-time (anti-PT) symmetry and a EP between its degenerate and nondegenerate operation regimes. To demonstrate the distinct potentials of the coupled OPO system compared to conventional non-Hermitian systems, we present higher-order EPs with two OPOs, tunable Floquet EPs in a reconfigurable dynamic non-Hermitian system, and the generation of a squeezed vacuum around EPs, all of which are not easy to realize in other non-Hermitian platforms. We believe our results show that coupled OPOs are an outstanding non-Hermitian setting with unprecedented opportunities to realize nonlinear dynamical systems for enhanced sensing and quantum information processing.

Spectral Evolution in Fiber Lasers With Soliton Self-Frequency Shift Effect

The generation and evolution process of red shifting in a ultrafast fiber laser with the soliton self-frequency shift are investigated by numerical simulation. And the effect of different parameters including the pump strength, the length of dispersion shift fiber, the modulation depth of saturable absorber and the initial field on the red shifting are analyzed for the first time to our knowledge. By optimizing these parameters, a soliton with the maximum red shifting beyond the laser's gain bandwidth is obtained. In addition, within a narrow parameter range, we have also achieved a novel red-shift period-doubling bifurcated soliton with a periodic switching central wavelength.

Pre-assessments of optical transition, gain performance and temperature sensing of Er3+ in NaLn(MoO4)2 (Ln = Y, La, Gd and Lu) single crystals by using their powder-formed samples derived from traditional solid state reaction

Rare earth doped molybdate single crystals are excellent laser working media. The growths of molybdate single crystals are money-cost and time-consuming. It would be preferable if the spectroscopic properties and laser performance of the laser crystals could be pre-assessed before their growths. In this work, we proposed a route to pre-assess the spectroscopic properties, gain performance and temperature sensing effects for the rare earth doped molybdate single crystals by using the powder-formed rare earth doped molybdate phosphors. The route we proposed was applied for assessing the Er3+ doped NaLn(MoO4)2 (Ln = Y, La, Gd and Lu) crystals by using the corresponding molybdate phosphors which were prepared via a solid state reaction. The optical transition properties of Er3+ in these molybdates were studied in the framework of Judd-Ofelt theory by using the diffuse-diffraction spectra. The radiative transition rates, fluorescence branching ratios and radiative lifetimes were further confirmed and compared for all studied Er3+ doped molybdates. Furthermore, the absorption cross sections, emission cross sections and optical gain coefficient spectra for the interested transitions of Er3+ in the molybdates were also determined via Füchtbauer-Ladenburg formula and McCumber theory. Meanwhile, the laser gain performance for the studied molybdate laser crystals was also assessed and compared with each other. A route for advanced heat management for the laser devices was proposed as well, and the temperature detection performance of the molybdates was also evaluated. This study proves that the pre-assessments for the rare earths doped laser single crystals by using their corresponding easy-prepared phosphors are workable.

Photonic-crystal lasers with two-dimensionally arranged gain and loss sections for high-peak-power short-pulse operation

Realizing high-peak-power (tens to hundreds of watts or higher) short-pulse (tens of picoseconds or less) operation in semiconductor lasers is crucial for state-of-the-art applications including eye-safe high-resolution remote sensing and non-thermal ultrafine material processing. However, it has been challenging to introduce mechanisms that enable stable high-peak-power short-pulse operation in conventional semiconductor lasers. Here, we propose photonic crystal lasers that have two-dimensionally arranged gain and loss sections to enable high-peak-power short-pulse operation in the fundamental mode while suppressing lasing in higher-order modes to avoid laser instability. On the basis of this concept, we experimentally realize a high peak power of ~20 W and a short pulse width of ~35 ps with an injection current of only 3-4 A using a 400-μm-diameter device and theoretically predict that even higher peak power (>300 W) can be achieved in a 1-mm-diameter device. Our results will contribute to the realization of next-generation laser sources for the aforementioned applications. By using engineered gain and loss sections in a photonic crystal laser, pulses with a peak power of ~20 W and pulse width of ~35 ps have been experimentally demonstrated and even higher peak power operation (>300 W) has been theoretically predicted.

The role of applied magnetic field in Co-doped ZnS thin films fabricated by pulsed laser deposition

In recent years, chalcogenide glasses such as ZnS or ZnSe have attracted much attention due to their excellent optical properties. The structure of chalcogenide materials is part of the important factors affecting their optical properties. In this paper, the magnetic environment was set to control the growth structure of nano film in the process of deposition, which could be affected the optical properties of Co-doped ZnS (ZnS:Co) nano-film. All films were prepared by pulsed laser deposition (PLD). The results of XRD and Raman proved that the structure of the film was changed by the magnetic field. A broader band fluorescence characteristic in the film was shown by PL spectrum, which revealed a substantially improvement of the optical properties for this material. Under the condition of the magnetic field, consistency between FDTD (Finite difference time domain method, FDTD) simulation and experimental results further confirmed the improvement of the optical properties. The findings warrant that the optimized ZnS:Co thin films could be applied to develop efficient optical waveguide devices and laser gain media in the infrared field.

Nd3+/Yb3+ co-doped mid-infrared luminescence fluorobromide glass with energy transfer and zero-thermal-quenching IR emission

Nd3+/Yb3+ co-doped fluorobromide glass samples were prepared by melt quenching. The mid-infrared (MIR) luminescence of the Nd3+/Yb3+ co-doped fluorobromide glass was investigated by Br-doping reduces the phonon state density of the matrix. The 3.9 μm MIR luminescence of the samples excited at 793 and 980 nm pump excitation was investigated in detail. There is an effective mutual energy transfer process between Nd3+ and Yb3+. It is proved under 793 nm excitation that the luminescence of Nd3+ at 3.9 μm is reduced by effective energy transfer from Nd3+:2H11/2 → Yb3+:2F5/2. At the same time, it is proved that the effective energy transfer from Yb3+:2F5/2 → Nd3+:2H11/2 under the excitation of 980 nm enhances the luminescence of Nd3+ at 3.9 μm. In addition, it is found that the samples still have good infrared (IR) luminescent properties when the temperature changes. The emission cross-sectional area and the absorption cross-sectional area are σem (3.87 × 10−20 cm2) and σabs (4.25 × 10−20 cm2). The fluorescence decay characteristics of the sample at 3.9 μm at the 2H11/2 level were investigated and the fluorescence lifetime was calculated. The gain performance of the sample was calculated and analyzed, which can reach 4.25 × 10−20 cm2. Those results prove that Nd3+/Yb3+ co-doped fluorobromide glass is the potential mid-infrared laser gain material.

Generation of wideband tunable femtosecond laser based on nonlinear propagation of power-scaled mode-locked femtosecond laser pulses in photonic crystal fiber* *Project supported by the National Natural Science Foundation of China (Grant No. 61805274), the Major Program of the National Natural Science Foundation of China (Grant No. 12034020), and Research Foundation of Inner Mongolia University of China (Grant No. 21200-5215108).

We implement an experimental study for the generation of wideband tunable femtosecond laser with a home-made power-scaled mode-locked fiber oscillator as the pump source. By coupling the sub-100 fs mode-locked pulses into a nonlinear photonic crystal fiber (NL-PCF), the exited spectra have significant nonlinear broadening and cover a spectra range of hundreds of nm. In experiment, by reasonably optimizing the structure parameters of NL-PCF and regulating the power of the incident pulses, femtosecond laser with tuning range of 900–1290 nm is realized. The research approach promotes the development of femtosecond lasers with center wavelengths out of the traditional laser gain media toward the direction of simplicity and ease of implementation.

Nanolasers Incorporating CoxGa0.6-xZnSe0.4 Nanoparticle Arrays with Wavelength Tunability at Room Temperature.

Semiconductor nanolaser has important research value and wide applications in many fields. However, it is still a challenge to obtain a nanolaser with tunability and high intensity at the nanoscale. Here, we report on lasers with two modes of emission wavelengths operating in near-infrared of nanohole filled with CoxGa0.6-xZnSe0.4 nanoparticle arrays at room temperature. The nanohole arrays are drawn on the photoresist by using the method of three-beam laser interferometric etching. Graphene with graphite which is coated on nanohole arrays is conducted by pulsed laser deposition (PLD) to construct the cavity. The CoxGa0.6-xZnSe0.4 nanoparticles are filled into the nanohole acting laser gain medium via the magnetic traction nanofilling technology. The results show that the laser at 868 and 903 nm is radiated, which can be tuned by changing the concentration and position of the filled nanoparticles in terms of wavelength and intensity. The nanolasers based on this approach represent an advantageous alternative to other design and fabrication methods. This nanoparticle nanolasers can be used in a micronano light source of an intelligent photonic chip.

Reality or fantasy—Perovskite semiconductor laser diodes

AbstractPerovskite semiconductor has emerged as a promising laser gain medium; however, it is still a challenge to fabricate electrically pumped perovskite lasers due to the insufficient electrical‐to‐optical conversion efficiency. Here, the current progress on the lasing performance of optically pumped perovskite lasers is reviewed. The advancement in the control of carrier transport and recombination properties of perovskite light‐emitting diode architectures is also studied. Hence, the obstacles preventing the fabrication of perovskite laser diodes are investigated. More importantly, a strategy toward electrically driven perovskite lasers is proposed base on the successful development of organic semiconductor laser diodes.image

Optical properties of transparent polycrystalline ruby (Cr:Al2O3) fabricated by high-pressure spark plasma sintering

Doped transparent ceramics with high optical quality can serve as materials for photonic applications such as laser gain media. In that regard, transparent polycrystalline alumina has potential for high-power applications due to its excellent physical and chemical properties, combined with unique doping possibilities. However, optical birefringence of Al2O3 crystals make achieving sufficiently high optical transmittance a processing challenge. In the present study, we demonstrated fabrication of highly transparent 0.5 at.% Cr:Al2O3 ceramics by high-pressure spark plasma sintering (HPSPS). The optical properties of these polycrystalline ruby ceramics were analyzed in order to assess possible laser operation (at 694.3 nm). The obtained ceramics exhibit high in-line transmittance (∼72.5 % at 700 nm), equivalent to a scattering coefficient of 2.15 cm−1, and characteristic ruby photoluminescence. The theoretically estimated lasing threshold and percentage of absorbed pump power indicate that such ruby ceramic lasers could operate at reasonable thresholds of 80−225 mW with short lengths of 0.5−5 mm. Thus, HPSPS is a promising method for producing laser-quality doped transparent ceramics for compact laser systems.

Growth, spectroscopic properties and up-conversion of Yb, Pr co-doped CaF2 crystals

Yb,Pr:CaF2 crystals have been grown successfully by the temperature gradient technique(TGT). The absorption spectra, fluorescence spectra, fluorescence decay lifetime of Pr3+ ions in CaF2 crystal sensitized by different Yb3+ ions concentration have been measured. The effective J-O intensity parameters were fitted to be Ω2=0.36 × 10−20cm2, Ω4=2.89 × 10−20cm2, Ω6=6.69 × 10−20cm2 for 0.3 at.%Pr,1.5 at.%Yb:CaF2. And the emission cross sections of 3P1→3H5 (535 nm), and 3P0→3F4 (750 nm) are calculated to be 2.02 × 10−20cm2, and 2.50 × 10−20cm2 with the FWHM of 17.2 nm, and 10.5 nm, respectively. The fluorescence lifetime of 3P0 has achieved great increasing by sensitizing of Yb3+ ion. Not only the up-conversion luminescence can be achieved, but also the broad band emission from 1D2 energy level can be observed. The results show that Yb,Pr:CaF2 could be a potential laser gain medium for visible and near-IR laser operation.

Optimized Growth and Laser Application of Yb:LuAG Single-Crystal Fibers by Micro-Pulling-Down Technique

Single-crystal fibers (SCFs) have a great application potential in high-power lasers due to their excellent performance. In this work, high-quality and crack-free Yb3+:Lu3Al5O12 (Yb:LuAG) SCFs were successfully fabricated by the micro-pulling-down (μ-PD) technology. Based on the laser micrometer and the X-ray Laue diffraction results, these Yb:LuAG SCFs have a less than 5% diameter fluctuation and good crystallinity along the axial direction. More importantly, the distribution of Yb ions is proved to be uniform by electron probe microanalysis (EPMA) and the scanning electron microscope (SEM). In the laser experiment, the continuous-wave (CW) output power using a 1 mm diameter Yb:LuAG single-crystal fiber is determined to be 1.96 W, at the central wavelength of 1047 nm, corresponding to a slope efficiency of 13.55%. Meanwhile, by applying a 3 mm diameter Yb:LuAG SCF, we obtain a 4.7 W CW laser output at 1049 nm with the slope efficiency of 22.17%. The beam quality factor M2 is less than 1.1 in both conditions, indicating a good optical quality of the grown fiber. Our results show that the Yb:LuAG SCF is a potential solid-state laser gain medium for 1 μm high-power lasers.

A numerical fitting routine for frequency-domain thermoreflectance measurements of nanoscale material systems having arbitrary geometries

In this work, we develop a numerical fitting routine to extract multiple thermal parameters using frequency-domain thermoreflectance (FDTR) for materials having non-standard, non-semi-infinite geometries. The numerical fitting routine is predicated on either a 2D or 3D finite element analysis that permits the inclusion of non-semi-infinite boundary conditions, which cannot be considered in the analytical solution to the heat diffusion equation in the frequency domain. We validate the fitting routine by comparing it with the analytical solution to the heat diffusion equation used within the wider literature for FDTR and known values of thermal conductivity for semi-infinite substrates (SiO2, Al2O3, and Si). We then demonstrate its capacity to extract the thermal properties of Si when etched into micropillars that have radii on the order of the pump beam. Experimental measurements of Si micropillars with circular and square cross sections are provided and fit using the numerical fitting routine established as part of this work. Likewise, we show that the analytical solution is unsuitable for the extraction of thermal properties when the geometry deviates significantly from the standard semi-infinite case. This work is critical for measuring the thermal properties of materials having arbitrary geometries, including ultra-drawn glass fibers and laser gain media.

Temperature and doping dependence of fluorescence lifetime in Yb:YLF (role of impurities)

We have investigated temperature and doping dependence of fluorescence lifetime in Yb:YLF crystals in the 78–300 K range, for samples with Yb-doping levels of 0.5%, 1% and 25%. Radiation-trapping prolonged fluorescence lifetimes are first measured to understand the pros and cons of this inevitable process on fractional thermal load and laser gain. Later, pinhole method was used to acquire ideally trapping free fluoresce lifetimes. For the lowly Yb-doped samples, fluorescence lifetime was measured to be 1.97 ms and 2.1 ms at 78 K and 300 K, respectively. For the 25% Yb-doped sample, strong upconversion emission due to the presence of Er, Tm and Ho impurities were observed. A double-exponential decay behavior, combined with excitation intensity dependent decay profile was also revealed for this highly doped sample. Even with the pinhole method, the 300 K fluorescence lifetime was measured as 4.5 ms, showing the difficulty to acquire radiation-trapping free signals at high doping levels. Time dynamics of upconverted emission in different spectral regions were also recorded, which provides hints on the energy transfer mechanisms between the Yb ion and the other rare-earth impurities. • Temperature and doping dependence of fluorescence lifetime is investigated in Yb:YLF. • Radiation-trapping extends effective fluorescence lifetime. • Increased doping and crystal temperature boost up radiation-trapping effect. • Amplified spontaneous emission and upconversion influence fluorescence decay curves.

Detailed investigation of absorption, emission and gain in Yb:YLF in the 78–300 K range

We present spectroscopic measurements focusing on a detailed investigation of temperature dependence of absorption, emission and gain in the uniaxial Yb:YLF laser gain medium. Measurements are carried out in the 78–300 K range, but we especially targeted our attention to the 78–150 K interval, which is the desired working range of liquid nitrogen cooled cryogenic Yb:YLF lasers/amplifiers. A tunable (770–1110 nm) Cr:LiSAF laser with around 100 mW continuous-wave output power and sub-0.2 nm bandwidth is used as an excitation source. The average power of the Cr:LiSAF laser is low enough to prevent heating of the sample, and its spectral flux (W/nm) is high enough to enable large signal-to-noise ratio measurements. Measured absorption data is used to cross-check the validity of the emission measurements, while the measured temperature dependent small-signal gain profile provided a second independent confirmation. The acquired absorption cross section curves match the previous literature quite well, whereas the measured strength of c-axis emission is stronger than some of the earlier reports. Direct measurements of small signal gain confirmed the emission cross section data, where single pass gain values above 50 have been measured for the 995 nm transition of E//c axis at 78 K. We further provide simple analytic formulas for the measured temperature dependence of absorption and emission cross section. We hope the presented results to be useful for the development of next generation of cryogenic Yb:YLF laser and amplifier systems.

Spectroscopic investigations on 1.53 μm NIR emission of Er3+ doped multicomponent borosilicate glasses for telecommunication and lasing applications

The multicomponent borosilicate (MBSER) glasses activated with different concentrations of Er3+ ions (varies from 0.1 to 2 mol%) were prepared via conventional melting and rapid quench technique and studied their thermal, structural and spectroscopic characteristics by using DSC, XRD, FTIR, optical absorption, excitation and emission measurements. Judd- Ofelt analysis in correlation with the absorption and emission spectral profiles was performed for the evaluation of several radiative parameters of the as-prepared glasses. Visible and NIR luminescence were recorded at 379 nm excitation and a special consideration is given to 1536 nm emission corresponding to the 4I13/2 → 4I15/2 transition. McCumber theory (MC) is used to determine the stimulated emission cross-section from the calculated absorption cross-section and is found well matched with the emission cross-section obtained from Fuchtbauer-Ladenburg (FL) equation. Better radiative parameters (stimulated emission cross-section (σemi) = 15.78 × 10−21cm2, effective bandwidth (Δλeff) = 67 nm), laser (gain bandwidth (σ(λp) x Δλeff = 1.05 × 10−25 cm3), optical gain (σ(λp) x τR) = 42.5 × 10−25 cm2s)) and gain spectra characteristics suggests the MBSER glasses for optical amplifier and laser applications since they offer a broadened emission spectra in the NIR region.

Non-resonant power-efficient directional Nd:YAG ceramic laser using a scattering cavity

Non-resonant lasers exhibit the potential for stable and consistent narrowband light sources. Furthermore, non-resonant lasers do not require well-defined optics, and thus has considerably diversified the available types of laser gain materials including powders, films, and turbid ceramics. Despite these intrinsic advantages, the practical applications of non-resonant lasers have been limited so far, mainly because of their low power efficiency and omnidirectional emission. To overcome these limitations, here we propose a light trap design for non-resonant lasers based on a spherical scattering cavity with a small entrance. Using a porous Nd3+:YAG ceramic, directional laser emission could be observed with significant enhancements in the slope efficiency and linewidth (down to 32 pm). A theoretical model is also developed to describe and predict the operation characteristics of proposed non-resonant laser.

Laser Gain for Inhomogeneous Boundary Conditions

The method proposed by the authors for solving the Helmholtz equation with homogeneous boundary conditions was verified for an elliptic cross section. The method is generalized to the Helmholtz equation with inhomogeneous boundary conditions. The generalization has been verified for circular and elliptical cross sections.

基于金刚石拉曼转换的光束亮度增强研究进展

High brightness laser sources with different wavelengths play an important role in the fields such as defense, industrial, and life sciences etc. However, due to the intrinsic spectral and thermophysical properties of current available laser gain materials, it is difficult to take into account the wavelength and output power of the traditional inversion lasers, which even leads to the decrease of beam brightness. To overcome this problem, beam cleanup by using nonlinear optical technology has been carried out in recent years, which is directly transferring the low beam quality generated from inversion lasers into the high through the effects such as stimulated Raman or Brillouin scattering. Among them, with excellent properties such as high Raman gain coefficient, high thermal conductivity and wide spectral transmission range, diamond exhibits excellent beam brightness enhancement characteristics while realizing high efficiency Raman conversion, which provides a new technical path to generate high power and high brightness laser beam. Here, the development of brightness enhancement based on first-order and cascaded Raman conversion of diamond was reviewed, and its future applications were discussed.

Kilowatt-Level, Narrow Linewidth, Polarization-Maintained All-Fiber Amplifiers Based on Multi-Phase Coded Signal Modulation and Laser Gain Competition

Kilowatt-Level, Narrow Linewidth, Polarization-Maintained All-Fiber Amplifiers Based on Multi-Phase Coded Signal Modulation and Laser Gain Competition