Laser Medium Research Articles

Experiment-based model of an Er/Yb gain medium for fiber amplifiers and lasers

A model of an Er3+/Yb3+ co-doped glass gain medium is proposed. The system of rate equations taking into account a minimum set of processes required to describe experimental data was built. As a result, a moderate number of fitting parameters is used, and thereby the model reliability is increased. For the populations of the energy levels of Er3+ and Yb3+ions, analytic expressions were obtained, as well as for the gain factor of the medium. The model was validated by a series of measurements of gain in Er3+/Yb3+ fiber amplifiers of different lengths at varying pump power. Such parameters of the model were found to fit the experimental data with an account of the spatial inhomogeneity of the pump power along the fiber. The maximum inversion of erbium ion population at high pump powers was analytically expressed, which describes the “bottleneck” effect. The power required to pump the fiber along the entire length was found.

THz Microscopy With Josephson Cantilevers for Characterization of Additive Manufactured Spiral Phase Plates

Josephson cantilevers from high-temperature superconductors are versatile sensors to measure frequency and power of microwave and terahertz radiation in the range of 1 GHz to about 5 THz. They consist of at least one Josephson junction, which is fabricated from the high-temperature superconductor yttrium barium copper oxide (YBCO). The Josephson cantilever is mounted on a positioning system inside a vacuum chamber to perform three-dimensional scanning measurements in the THz microscope. The frequency and the power of external radiation can be determined from the recorded data by evaluating the occurring Shapiro steps. In this work, spiral phase plates (SPPs) optimized for the terahertz regime were additively manufactured with a fused filament fabrication 3D printer. Multiple SPPs were investigated and the results of two of them are shown, each made of a different thermoplastic. The SPPs are designed to transform Gaussian light beams into twisted light beams with an orbital angular momentum. An optically pumped molecular gas far-infrared laser system with difluoromethane as lasing medium can be employed as radiation source to generate frequencies between 1 THz and 1.6 THz with powers up to 125 mW. The beam is coupled into the THz microscope via quasi-optical windows and an evacuated optical path. The SPP is placed near the far-infrared laser to measure the far-field distribution. In this work, measurement results based on superconducting Josephson cantilevers are obtained with our unique THz microscope setup to characterize SPPs. The results present an important contribution for the evaluation of the design and the fabrication process of SPPs and showcase one of the multiple measurement modes of the THz microscope.

Growth, structure and spectroscopic properties of Er:YSr3(PO4)3 disordered crystal for mid-infrared laser applications

Mid-infrared lasers in the ⁓ 3 µm region activated by Er3+ ion has attracted considerable attention due to their important applications. Herein, a heavy concentration (30 mol%) Er3+ ion doped YSr3(PO4)3 (Er:YSP) single crystal with a high crystalline quality was grown through the Czochralski pulling method. The structure of single crystal was ascertained to be cubic system with the point group 4¯3m using the powder X−ray diffraction and Rietveld structural refinement methods. Furthermore, the band structure and density of states of the crystal were analyzed by the first principal calculations. The absorption and fluorescence spectra were also investigated. An intense absorption band located at 974 nm with a full width half-maximum of 18 nm, where the absorption cross section was obtained to be 3.72 × 10−21 cm2. The full width half-maximum value of emission band centered at about 3090 nm is up to be 140 nm, which is very suitable for realizing ultrashort pulses and tunable lasers. In addition, the fluorescence lifetime of the upper 4I11/2 and lower 4I13/2 levels were obtained to be 0.65 ms and 5.37 ms, respectively. All the results show that the disordered structure crystal Er:YSP could potentially serve as a laser medium for the ⁓ 3µm region.

Tunable Narrowband Carbon Quantum Dots Laser Based on Self‐Assembled Microstructure

AbstractPhotonic integrated circuits (PICs) are coupled with the laser to create active devices for optical communication, optical interconnection, and optical computing. Consequently, the light source remains a focal point of extensive research and development. Currently, the utilization of carbon quantum dots as a gain medium in photonic cavity lasers is accomplished and suggested as a viable approach for achieving active PICs. Nevertheless, challenges persist in achieving narrowband lasing beam output in one direction that is real‐time tunable while minimizing energy losses. These issues represent an active area of ongoing research. This article presents a self‐assembled microstructure and proposes a method for obtaining tunable, narrowband, unidirectional lasing beams. The objective is to stimulate further development in integration of PICs with lasers, extending beyond these applications, to realize substantial performance gains, eco‐friendly solutions, and efficient mass production.

Bismuth-Germanate Glasses: Synthesis, Structure, Luminescence, and Crystallization

Bismuth-germanate glasses, which are well known as a promising active medium for broadband near-infrared spectral range fiber lasers and as an initial matrix for nonlinear optical glass ceramics, have been synthesized in a 5–50 mol% Bi2O3 wide concentration range. Their structural and physical characteristics were studied by Raman and FT-IR spectroscopy, differential scanning calorimetry, X-ray diffraction, optical, and luminescence methods. It has been found that the main structural units of glasses are [BiO6] and [GeO4]. The growth in bismuth oxide content resulted in an increase in density and refractive index. The spectral and luminescent properties of glasses strongly depended on the amount of bismuth active centers. The maximum intensity of IR luminescence has been achieved for the 5Bi2O3-95GeO2 sample. The heat treatment of glasses resulted in the formation of several crystalline phases, the structure and amount of which depended on the initial glass composition. The main phases were non-linear Bi2GeO5 and scintillating Bi4Ge3O12. Comparing with the previous papers dealing with bismuth and germanium oxide-based glasses, we enlarge the range of Bi2O3 concentration up to 50 mol% and decrease the synthesis temperature from 1300 to 1100 °C.

Preparation, spectral properties and Judd-Ofelt analysis of Ho3+:YPO4 crystal

The Ho3+:YPO4 crystal was grown by the high-temperature solution method successfully. The preparation process, crystal structures and spectroscopic properties, including polarized absorption spectra, FT-IR and Raman spectra, polarized fluorescence spectra and lifetime were characterized and analyzed in detail. The absorption cross-sections at around 643 nm were calculated to be 6.93 × 10−20 cm2 for π-polarization and 2.30 × 10−20 cm2 for σ-polarization, respectively. And the spectroscopic quality factor (Ω4/Ω6) was determined to be 1.18. The full width at half the maximum (FWHM) of emission bands at the peak of 2070 nm can reach 102.43 nm for π-polarization and 83.83 nm for σ-polarization, respectively. The results show that Ho3+:YPO4 crystal is a promising laser medium which is expected to be used in around 2.0 μm.

Transparent hexagonal Yb: S-FAP ceramics: Investigation of HP sintering kinetics on densification process and microstructural evolution

Given the downsizing and high integration of laser diodes in recent decades, Yb: Sr5(PO4)3F (S-FAP) crystal materials are predicted to be employed as an all-solid laser medium to produce more energy for inertial nuclear fusion. The effects of various sintering conditions on the microstructure evolution and properties of 2 at. % Yb: S-FAP transparent ceramics by hot pressing sintering were systematically investigated in this work, which including sintering temperature, sintering pressure, heating rate, and holding time. When other sintering variables are held constant, the results reveal that a sintering temperature of up to 1050 °C results in visible grain development, substantial optical scattering, and even a significant carburizing impact at 1100 °C. Excessive sintering dwell time will result in grain development, and a high sintering holding time will result in the emergence of intergranular pores, lowering the optical quality of ceramics. The optimal heating rate range for as-synthesized ceramic is found to be 5–15 °C/min. Moreover, the grain development process of Yb: S-FAP transparent ceramics is determined to be grain boundary diffusion, with an activation energy of 239.4 kJ/mol during hot pressing sintering. Meanwhile, the as-prepared sample has excellent mechanical properties such as Vickers hardness of 7.47 ± 0.045 GPa, which are highly competitive when compared to other Yb-doped optical ceramic media such as Y2O3, YAG, Lu2O3, and so on. Finally, the transmittance was interpreted based on the microstructures of the as-synthesized ceramic samples and the Rayleigh-Gans-Debye scattering theory.

High Brightness Ce:Nd:YAG Solar Laser Pumping Approach with 22.9 W/m2 TEM00-Mode Collection Efficiency

A compact side-pumped solar laser design using a Ce:Nd:YAG laser medium is here proposed to improve the TEM00-mode solar laser output performance, more specifically the beam brightness. The solar laser performance of the Ce:Nd:YAG laser head was numerically studied by both ZEMAX® v13 and LASCADTM v1 software. Maximum multimode laser power of 99.5 W was computed for the 4.1 mm diameter, 34 mm length grooved rod, corresponding to a collection efficiency of 33.2 W/m2. To extract TEM00-mode solar laser, symmetric and asymmetric optical resonators were investigated. For the 4.1 mm diameter, 34 mm length grooved rod, maximum TEM00-mode solar laser collection efficiency of 22.9 W/m2 and brightness figure of merit of 62.4 W were computed using the symmetric optical resonator. While, for the asymmetric optical resonator, the maximum fundamental mode solar laser collection efficiency of 16.1 W/m2 and brightness figure of merit of 37.3 W were numerically achieved. The asymmetric resonator offered a TEM00-mode laser power lower than the one obtained using the symmetric resonator; however, a collimated laser beam was extracted from the asymmetric resonator, unlike the divergent TEM00-mode laser beam provided by the symmetric resonator. Nevertheless, using both optical resonators, the TEM00-mode Ce:Nd:YAG solar laser power and beam brightness figure of merit were significantly higher than the numerical values obtained by the previous Nd:YAG solar laser considering the same primary concentrator.

Amplification and ellipticity enhancement of high-order harmonics in a neonlike x-ray laser dressed by an IR field

In Khairulin et al. [Sci. Rep. 12, 6204 (2022)] we proposed a method for amplifying a train of sub-femtosecond pulses of circularly or elliptically polarized extreme ultraviolet radiation, composed by high-order harmonics of an infrared (IR) laser field, in a neonlike active medium of a plasma-based x-ray laser, additionally irradiated with a replica of a fundamental frequency laser field used to generate harmonics. Here we present the analytical theory and numerical study of an amplification process of an individual harmonic with either preservation of its polarization state or ellipticity enhancement. We also analyze in detail the spectral-temporal and polarization properties of a sub-femtosecond pulse train formed by a set of elliptically or circularly polarized harmonics during the propagation through the medium. We discuss also the possibility of an experimental implementation of the suggested technique in an active medium of an x-ray laser based on neonlike ${\mathrm{Ti}}^{12+}$ ions irradiated by an IR laser field with a wavelength of 3.2 \textmu{}m.

Random lasing in composites of CdSe/CdZnS colloidal quantum wells and silica nanoparticles

Colloidal quantum wells (CQWs) are a new type of atomic flat semiconductor nanocrystals, which present excellent opportunities as optical gain materials owing to their ultranarrow emission linewidth, tunable emission wavelength, suppressed Auger effect, and large gain cross-section. Here we synthesized monodispersed silica nanospheres and CdSe(4ML)/CdZnS core/shell CQWs. We observed random lasing by optically pumping the composites of CdSe(4ML)/CdZnS core/shell CQWs and silica nanoparticle scattering layer. The random lasing ranges from 615 nm to 625 nm with a threshold of 0.55 mJ cm−2. Fourier analysis reveals that sufficient Rayleigh scattering is the primary reason for the low threshold in the spatial frequency spectrum of the random laser. Our findings suggest that CQWs can serve as an efficient optical gain medium for random lasing, enabling various applications such as speckle-free laser imaging, random number generation for secure communication, chemical and biological sensing, high brightness and energy-efficient lighting, and label-free bioimaging.

Thickness Dependence on Wavelength Range of Random Laser in Ultrathin ZnO Crystals Grown by Mist-CVD on C-plane Sapphire Substrate

The ultrathin ZnO crystals were grown by mist chemical vapor deposition technique with potentially low-cost and low-environmental impact on commercially available c-plane sapphire substrates. The photoluminescence spectra of the ultrathin ZnO crystals were evaluated, and we observed the random lasing behaviors. In addition, the range of random laser wavelengths of photoluminescence can be shifted with a constant trend by changing the structural parameters of the ultrathin ZnO crystals. Here, the structural parameter was the thickness of the ultrathin ZnO crystals. The results indicate that the oscillation wavelength range of the random laser may be potentially fine-tuned by changing the structural parameters of the random laser medium.

Growth and spectroscopic properties of a novel Tm3+-doped YSr3(PO4)3 disordered crystal

A novel Tm3+-doped YSr3(PO4)3 (Tm:YSP) crystal with a disordered crystal structure was grown by using the Czochralski pulling method. The single crystal growth, structure, absorption and emission spectra were systematically analyzed. The absorption bandwidth at ∼792 nm was found to be 21 nm, showing that the Tm:YSP crystal was very suitable for commercial laser diodes pumping of AlGaAs. Furthermore, the maximum emission cross section centering at 1835 nm was determined to be 0.86 × 10−20 cm2 with an emission bandwidth of 202 nm, which indicates the promising applications in the fields of ultrafast and tunable lasers. In addition, Spectroscopic parameters, containing Judd – Ofelt intensity, line strengths, radiative transition probabilities, fluorescence branching ratio and radiative lifetime, were analyzed by using the Judd – Ofelt theory. All above results showed that the Tm:YSP crystal with a disordered structure was a potential laser medium in the ⁓ 2 μm region.

Experimental Simulation for Two Optically Filtered Modulation Weights in Laser Diode as a Self-Learning Layer

In this study, the response of a nonlinear laser medium is experimentally studied. In the study, a hybrid version of the input layer that multiplies optically and accumulates electrically is compared with a wholly optical version that multiplies and accumulates optically. This medium is subjected to two different paths of optically filtered and attenuated feedback. With such a system, the variation of feedback weight in one of them is tested in correspondence to the second one. Observations for frequency spectra are carried out to simulate the resultant response with an input layer for a neural network based on chaotic carriers. Chaotic laser emission was observed as a function of several control parameters, which are D.C. bias voltage, branch optical attenuation, and feedback strengths based on filtration with fiber Bragg grating. This learning rule is linear in the difference between each input and output of a neuron. This is an enhancing/inhibiting rule. The thresholds are adjusted in such a way that the output of the neuron is either pushed in the same direction as the input (enhancing) or pushed in the opposite direction (inhibiting).

Assessment of possible performances of compact solar-pumped lasers using transparent Cr3+, Nd3+-co-doped YAG ceramics micro rods

Employing a commercially available off-axis parabolic mirror of ⌀50.8 or ⌀76. 2 mm caliber and ⌀1 or 1 × 1 × 10 mm Cr3+, Nd3+-co-doped YAG ceramic micro laser medium (LM), our compact solar-pumped lasers (μSPL) could oscillate continuously without any forced air cooling from 11:00 a.m. beyond 5:30 p.m. tracking the sun outdoors. However, their energy conversion efficiencies ηpower have not exceeded so far 1%. Numerical assessments of μSPL performance using a fundamental expression of laser oscillation based on experimentally obtained parameters were performed. Dependences of ηpower on transmittance of an output coupler, mode-matching efficiency ηmode, length of LM, distributed loss constant in LM, and Cr content as well as the necessity of the concentration ratio as high as 10,000 were elucidated. The values of ηmode between 0.3 and 0.5 coming from the present confocal resonator roughly reproduced ηpower around 1%. Based on the assessments, possible approaches for further improvement of μSPL performance were evaluated including employment of composite LM to increase ηmode.

Optofluidic laser sensor for the detection of dopamine

Dopamine (DA) is a biomarker of many psychiatric diseases, such as schizophrenia and Parkinson's disease. Most current DA detection research adopts electrochemical method, which suffers interferences caused by many analogs with similar structures or redox potentials. To realize highly sensitive and selective DA sensing, herein we propose an optofluidic-laser-based DA sensor. Via the competitive adsorption mechanism, rhodamine 6G (Rh6G) is fixed on graphene oxide (GO) and replaced by DA in the sample, with the re-excited Rh6G serving as the gain medium of the optofluidic laser. As the DA concentration increases, more Rh6G is released into the sample, which induces an increase in the relative slope efficiency of the optofluidic laser and a reduction in the laser threshold. Both the relative slope efficiency and threshold attain a suitable linear relationship with the DA concentration within the 5–800 μM range. Under ideal circumstances, the detection limit of the biosensor is 0.3 µM. Moreover, the optofluidic laser-based biosensor achieves favorable selectivity for DA detection, proving the advantages of using optical laser sensors for DA detection in complex samples.

Direct laser writing structural color for reversible encryption and decryption in different mediums.

Structural color (SC) has enormous potential for improving the visualization and identification of functional micro/nano structures for information encryption and intelligent sensing. Nevertheless, achieving the direct writing of SCs at the micro/nano scale and the change of color in response to external stimuli simultaneously is rather challenging. To this end, we directly printed woodpile structures (WSs) utilizing femtosecond laser two-photon polymerization (fs-TPP), which demonstrated obvious SCs under an optical microscope. After that, we achieved the change of SCs by transferring WSs between different mediums. Furthermore, the influence of laser power, structural parameters, and mediums on the SCs was systematically investigated, and the mechanism of SCs using the finite-difference time-domain (FDTD) method was further explored. Finally, we realized the reversible encryption and decryption of certain information. This finding holds broad application prospects in smart sensing, anti-counterfeiting tags, and advanced photonic devices.

On the Possibility of Converting Linearly Polarized Attosecond Pulses of High Harmonics into Circularly Polarized Pulses with an Increase in the Energy in an Optically Modulated Neon-Like Active Medium of an X-Ray Plasma Laser

A method is proposed for converting linearly polarized radiation of a single harmonic or a combination of high-order harmonics of an optical field, which form a train of subfemto-/attosecond pulses, into elliptically and, in particular, circularly polarized radiation in an optically modulated neon-like active medium of an X‑ray plasma laser. It is shown that this method can provide a high energy efficiency of radiation conversion due to the amplification of the harmonic field, and is also insensitive to changes in the number of high harmonics that form the amplified pulses. The possibility of experimental implementation of the method is considered on the example of an active plasma of neon-like Ti12+ ions with an unperturbed inverted transition wavelength of 32.6 nm.

Radiation and Emission Phenomena in Combination

Radiation and Emission Phenomena in Combination

Modulation of carbon dots hybrids lasers for high security flexible multi-level anti-counterfeiting

Anti-counterfeiting materials development has been raised for a long time to improve the level of security against counterfeiting. Increasing counterfeiting difficulties as well as reducing difficulties of materials preparation are two major concerns in the development of anti-counterfeiting materials. Herein, carbon dots (CDs) and triethyl 1,3,5-benzenetricarboxylate (Et3BTC), both of which show two random lasing peaks located around 325 nm and 450 nm at 265 nm excitation are chosen as two lasing mediums to form hybrids. Moreover, lasing intensity ratio of the two lasing bands from the hybrids can be tuned by changing the working temperature, excitation area, and even bending angle of substrate. As a result, a strategy of dual random lasing medium modulation based high security flexible multi-level anti-counterfeiting technique is developed. Thus, it is suitable for anti-counterfeiting of high-value items due to its high security and excellent waterproofness. This work will provide a perspective to boost the development of high security anti-counterfeiting techniques.

Efficient 2075-nm laser emission from Ho3+-doped fluorotellurite glass in a compact all-fiber structure.

In this Letter, we report an Ho3+-doped fluorotellurite glass all-fiber laser at 2075 nm. The gain fiber is pumped in-band with a 1976-nm fiber laser and connected by fusion splicing. A high-quality fusion splicing point with a loss of < 0.1 dB was obtained by finely adjusting the splicing power and offset. In addition, by optimizing the writing parameters, a third-order fiber Bragg grating (FBG) with a reflectivity of 98% was achieved at 2075 nm using the femtosecond laser direct-writing method. Using the FBG as the laser cavity mirror and a relatively short 28-cm-long home-made Ho3+-doped fluorotellurite fiber as the laser medium, a laser with a maximum unsaturated output power of 7.33 W was obtained, and the corresponding slope efficiency was as high as 93.4%. The first, to the best of our knowledge, demonstration of the fluorotellurite glass all-fiber ∼2.1-µm laser presented in this work may pave the way for a high-power 2.1-µm fiber laser with a compact structure.