Gain Cross Section Research Articles

Yb: CaYAl3O7 crystal: a promising candidate for an ultrashort pulse laser with a disordered structure

We report on the crystal growth, structure, thermal, spectral properties, and laser performance of a novel disordered crystal, Yb3+-doped CaYAl3O7 (Yb: CYAM). The complete set of anisotropic thermal properties were systematically studied. Due to the random distributions of Y3+ and Ca2+ ions, the internal disordered structure determines a significant broadening of the absorption (FWHM=25 nm) and emission (FWHM=60 nm) bands. Polarized absorption, emission and gain cross-section spectra are determined. A maximum σem at 1011 nm for π polarization was calculated to be 0.57 × 10−20 cm2. CW laser operation at 1.06 µm with the c-cut sample has been demonstrated. A maximum power of 505 mW was achieved at 1026.5 nm, corresponding to the slope efficiency and optical conversion efficiency being 12.5% and 6.7%, respectively. Yb: CYAM crystal reveals its superiority and potentiality in the ultrashort pulse laser field.

Design of High-Gain and Small-Aperture Endfire Antenna Using a Phase-Reversal Technique

A phase-reversal technique is proposed in this article to overcome the gain-aperture tradeoff of endfire antennas. Such a tradeoff fundamentally forbids simultaneous high gain and small cross-section aperture. The phase-reversal technique breaks this tradeoff and hence allows one to design a high-gain endfire antenna with a small cross-section aperture. To illustrate the proposed phase-reversal technique, a crossover stripline structure is implemented. The antenna radiates into the endfire direction within 9.5-10.5 GHz and it achieves 13 dBi endfire radiation gain with only 0.047λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.022λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> cross-section aperture size. These results successfully validate the proposed phase-reversal technique. Such a high-gain and small-aperture endfire antenna is potentially useful for microwave imaging in lossy medium.

Rotman Lens-Based Wide Angular Coverage and High-Gain Semipassive Architecture for Ultralong Range mm-Wave RFIDs

In this letter, the authors demonstrate, for the first time, the implementation of a fully-flexible, energy autonomous Rotman lens-based backscattering RFID tag operating in the 28 GHz 5G band. The Rotman lens front end was designed, simulated, fabricated on a thin flexible substrate and its gain and differential radar cross section (RCS) tested. The structure displays a simultaneous high gain and wide angular coverage, resulting in a remarkable RCS, measured in both planar and severe bending conditions. A low power oscillator was then added to the retrodirective array to test its detectability as a backscatterer at an unprecedented range of up to 64 m while displaying a power consumption as low as 2.64 $\mu$ W, effortlessly supplied by its flexible solar cell in normal office indoor conditions. Enabled by the implementation of a high degree of focalizing retrodirectivity, this unique Rotman lens-based system could enable the birth of RFIDs with practical reading ranges exceeding 1.8 km.

Light-enhanced incoherence of electronic transport in quantum cascade lasers

Since their invention in the middle of the 1990s, quantum cascade lasers (QCLs) attract increasing theoretical interest stimulated by their widening applications. One of the key theoretical issues is the optimization of electronic transport which in most of these devices is governed by the injection barrier of QCL heterostructure. In the paper, the nonequilibrium Green’s function formalism is used to study electronic transition through the injection barrier as a function of laser field in the cavity; for the increasing field, a crossover is observed from the strong coupling regime, in which electronic transport through the barrier is coherent, to the weak coupling regime, in which electronic transport gets incoherent. This crossover is characterized by gain recovery time, τrec, which takes sub-picosecond values for mid-IR QCLs operating at room temperature. This time is also important for the performance of devices under steady-state conditions; the maximum output power is obtained when the figure of merit, FOM = (g(0)/gth − 1)/gcτrec [g(0) is the linear response gain, gth is the threshold gain needed to compensate all losses, gc is the gain cross-section], reaches maximum. It is shown that the use of this optimization criterion can result in the structures essentially different from those which can be obtained when the optimized quantity is the linear response gain, g(0).

Enhanced visible green and 1.5 μm radiative emission of Er3+ ions in Li2O-PbO-Al2O3-B2O3 glasses for photonic applications

A series of Er3+ ions doped lithium lead alumino borate (LiPbAlB) glasses were synthesized via melt quench method and their structural, physical and optical properties were studied. Judd-Ofelt theory in correlation with the emission and absorption profiles was performed for the measurement of various radiative parameters for different transitions of Er3+ doped glasses. The as-prepared glasses show deep green (550 nm) as well as NIR (1532 nm) emission at 380 and 980 nm excitation wavelengths, respectively. The intensity of emission spectra increases with Er3+ ions concentration up to 1.5 mol% and beyond quenching is observed. The Dexter theory applied to the emission profiles reveals the interaction between Er3+ ions as dipole-dipole in nature. CIE color coordinates are found to fall in the green region of the visible spectrum. It is observed that 1.5 mol% of Er3+ ions concentration is optimum to exhibit the maximum quantum efficiency, branching ratios, gain bandwidth, emission cross-section, gain cross-section and aptly suitable for visible and NIR photonic applications.

Growth and spectroscopic properties of Yb0.1Gd1.8La0·1SiO5 crystal: A promising new laser material for ultrashort laser

Recently, in the field of femtosecond lasers, an intense interest has been aroused for Yb3+-doped laser crystals which are able to be pumped directly by semiconductor laser. In this work, a novel Yb3+-doped mixed laser crystal Yb0.1Gd1.8La0·1SiO5 (Yb:GLSO) crystal with high-quality was grown successfully by Czochralski method. Raman spectrum confirmed that the Yb:GLSO has highly disordered crystal structure. The large absorption cross section (0.70 × 10−20 cm2) of Yb:GLSO crystal at around 975 nm indicates the crystal can be pumped efficiently by the commercialized InGaAs laser diode. The stimulated emission cross section at 1030 nm was calculated to be 0.7 × 10−20 cm2 and the bandwidth of the emission peak was determined as high as 175 nm. Positive gain cross sections were appeared at 1030, 1045 and 1088 nm when inversion ratio higher than 0.2. All these results suggest that Yb:GLSO crystal is a very promising novel laser material for realizing tunable solid-state and ultra-short pulsed lasers with LD pumping.

Broadband flat near-infrared emission from tellurite glass doped with Tm3+, Er3+ and Ag NPs

Tellurite glasses doped with trivalent Tm3+, Er3+ ions and metal Ag nanoparticles (NPs) were synthesized by single-step melt-quenching technique, and the near-infrared photoluminescence property was investigated. Differential scanning calorimeter (DSC) curve, X-ray diffraction (XRD) pattern, transmission electron microscopy (TEM) image, absorption spectrum, emission spectrum and fluorescence decay curve were measured. The good thermal stability and amorphous state nature of the as-prepared tellurite glasses were demonstrated by the DSC curve and XRD pattern, respectively. Absorption spectrum disclosed the electronic transitions in the ultraviolet to near-infrared spectral range and was further used to evaluate the potential in near-infrared band luminescence by the calculations of spontaneous radiative transition parameters and gain cross-sections of Tm3+(Er3+) ions. Under the excitation of 808 nm laser diode (LD), a broadband flat near-infrared emission with FWHM of approximately 160 nm was obtained in the Tm3+/Er3+ doped tellurite glass. This broadband flat near-infrared emission, originating from the spectral overlapping of Tm3+:3H4→3F4 transition (~1.47 µm) and Er3+:4I13/2 → 4I15/2 transition (~1.53 µm), was located in the 1350–1650 nm region and covered the E + S + C + L-bands simultaneously. With the precipitation of Ag NPs having diameter in the 10–20 nm range evidenced by TEM image in the Tm3+/Er3+ doped tellurite glass, the peak luminescent intensity increased by about 45%, which is attributed to the increased local electric field and the possible energy transfer from the Ag species to Tm3+(Er3+) ions. The obtained results demonstrate that Tm3+/Er3+/Ag NPs doped tellurite glass is prospective for broadband optical amplifiers, broadband lighting sources and so on.

Growth and mid-infrared luminescence property of Ho3+ doped CeF3 single crystal

A Ho3+-doped CeF3 crystal was successfully grown for the first time using the Bridgman method, and its spectral properties were studied. By analyzing the absorption and emission measurements of the Ho:CeF3 crystal with the Judd–Ofelt theory, the intensity parameters Ω2,4,6, emission cross-sections, gain cross-sections, and excited state lifetimes were calculated. It was found that the peak absorption cross-section around 447 nm is 0.4339 × 10−20 cm2, which matched to the commercially available blue laser diode lasers. The 2.0 μm excitation emission of the Ho:CeF3 crystal corresponding to the stimulated emission Ho3+:5I7→5I8 transition has an emission cross section of 0.24 × 10−20 cm2 with FWHM of 146 nm.

Effect of Gd2O3 on luminescence properties of RE ions in germanium-tellurite glasses

The system of Ho3+/Yb3+ co-doped Ge-Te-Ga-Gd glasses has been successfully prepared by traditional melting method. The thermal stability, Raman spectroscopy and mid-infrared luminescence were studied and analyzed systematically. The effectively enhanced thermal stability of the sample was obtained with the increasing introduced Gd2O3, which was confirmed by the positive ΔT values. The improved robustness and structural stability of the glass network were analyzed through Raman spectroscopy. Enhanced 2–3 μm emissions have been obtained by certain content Gd2O3 with an optimal choice of 15 mol% additive amount under 980 nm pumping. Furthermore, luminous parameters including J-O values, absorption, emission and gain cross sections have been presented together with energy transfer mechanism. Hence, this paper presents a new choice of Ho3+/Yb3+ co-doped Ge-Te-Ga-Gd glass as potential mid-infrared laser materials.

Deep Blue Light Amplification from a Novel Triphenylamine Functionalized Fluorene Thin Film.

The development of high performance optically pumped organic lasers operating in the deep blue still remains a big challenge. In this paper, we have investigated the photophysics and the optical gain characteristics of a novel fluorene oligomer functionalized by four triphenylamine (TPA) groups. By ultrafast spectroscopy we found a large gain spectral region from 420 to 500 nm with a maximum gain cross-section of 1.5 × 10−16 cm2 which makes this molecule a good candidate for photonic applications. Amplified Spontaneous Emission measurements (ASE) under 150 fs and 3 ns pump pulses have revealed a narrow emission at 450 nm with a threshold of 5.5 μJcm−2 and 21 μJcm−2 respectively. Our results evidence that this new fluorene molecule is an interesting material for photonic applications, indeed the inclusion of TPA as a lateral substituent leads to a high gain and consequently to a low threshold blue organic ASE.

Energy-transfer upconversion and excited-state absorption in KGdxLuyEr1-x-y(WO4)2 waveguide amplifiers

We perform a systematic spectroscopic study in channel waveguides of potassium gadolinium lutetium double tungstate doped with different Er3+ concentrations. Transition cross sections of ground-state absorption (GSA) and excited-state absorption (ESA), as well as stimulated emission (SE) at the pump wavelength around 980 nm are determined. ESA is directly measured by the pump-probe technique. Evaluation of GSA and ESA spectra indicates that ESA may be diminished by an appropriate choice of pump wavelength near 980 nm. Besides, GSA and SE at the signal wavelength around 1.5 µm are measured and the wavelength-dependent gain cross section as a function of excitation density is determined. Non-exponential luminescence decay curves from the 4I13/2 and 4I11/2 levels are analyzed and the probabilities of the energy-transfer-upconversion (ETU) processes (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and (4I11/2, 4I11/2) → (4I15/2, 4F7/2) are quantified. Despite the large interionic distance between neighboring rare-earth sites in potassium double tungstates, the probability of ETU is comparatively large because of the large cross sections of the involved transitions. A rate-equation analysis of the influence of ETU and ESA on gain at ∼1.5 µm is performed, revealing that ETU from the 4I13/2 amplifier level strongly limits the gain when the doping concentration increases above ∼6at.%. The calculated maximum achievable internal net gain per unit length amounts to ∼15 dB/cm for an optimized Er3+ concentration of ∼4 × 1020 cm−3 and a launched pump power of 300 mW at a pump wavelength of 984.5 nm, in reasonable agreement with recent experimental results.

Power scaling of the self-frequency-doubled quasi-two-level Yb:YCOB laser with a 30% slope efficiency.

The lab-on-chip integration of photonic devices has been attracting increasing attention recently. Multifunctional materials provide natural platforms for the desirable performance by the coupling of different functionalities. The insufficient coupling efficiency of the laser and nonlinear processes in self-frequency-doubled (SFD) lasers is the limiting factor for the output power and further practical applications. Here we demonstrate a SFD Yb3+-doped calcium yttrium oxoborate (Yb:YCOB) crystal laser with an unprecedented slope efficiency of 30% and output power of 6.2W at 513nm. The successful realization of this laser operating in a quasi-two-level configuration is based on enhanced coupling of the laser and frequency-doubling processes using a monolithic configuration, benefiting from an ultimately small laser quantum defect, the anisotropic gain cross sections, and the high effective nonlinearity of the monoclinic YCOB outside the principal planes. Solid-state lasers in the spectral range around 510nm are scarce, and the results not only present a significant advancement in the field of SFD lasers, but also pave the way for future applications of such green lasers, especially in areas such as medical treatment, daily life, and scientific investigations.

Improvement of spectroscopic properties and luminescence of Er3+ions in phospho-tellurite glass ceramics by formation of ErPO4 nanocrystals

Er3+ doped phospho-tellurite glass was prepared by melt-quenching method. A limited partial crystallization of the glass samples was controlled by thermal annealing process in order to obtain a transparent glass-ceramic. The X-ray diffraction (XRD) patterns indicate the precipitation of nanocrystalline phases after heat treatment. A particular interest is devoted to the formation of ErPO4 nanoparticles (NPs) dispersed in the glassy host. The optical band gap (Eg) has been found to decrease from 3.37 to 2.75eV as the sample changes from glass to glass-ceramic. The Judd–Ofelt (JO) phenomenological intensity parameters (Ω2, Ω4, Ω6) have been estimated by means of the absorption spectrum. Radiative parameters of Er3+ ions have been predictedusing JO intensity parameters. Significant improvements of both the near infrared emission relative to the 4I13/2 → 4I15/2 transition and the corresponding luminescence lifetime were obtained after heat treatment. We show that an efficient excitation transfer occurs from the formed ErPO4 NPs to Er3+ ions. The Mc-Cumber method is used to calculate the absorption cross-section σa, the emission cross-section σe and the gain cross-section G(λ) relative to the 4I13/2 → 4I15/2 transition.Compared with other glass hosts, the results indicate that erbium doped phospho-tellurite glass-ceramic has good prospect as a gain medium applied for 1.53 μm band broad and high-gain erbium-doped fiber amplifiers (EDFA).

Optical properties of peralkaline aluminosilicate glasses doped with Sm3+

Enhancing the fluorescence and thermo-mechanical properties of rare-earth doped aluminosilicate glasses (AS) by modifying the chemical composition with different alkaline earth oxides opens new opportunities for their application as optical devices. This paper provides a study on 1 × 1020 ions/cm3 Sm3+ doped aluminosilicate glasses, with the network modifier oxides BaO, SrO, CaO, MgO (35 mol%), Al2O3 (10 mol%) and SiO2 (55 mol%), denoted as BaAS, SrAS, CaAS and MgAS, prepared by the melt quench technique. Based on the measured UV–visible and NIR absorption spectra, the Judd-Ofelt parameters Ωλ, (λ = 2, 4, 6) were evaluated to predict the radiative properties of Sm3+ ions such as radiative transition probabilities (Arad), radiative lifetimes (τrad) and branching ratios (βR). By using the Füchtbauer-Ladenburg (FL) formula, the gain cross sections were calculated from the stimulated emission cross sections. The glass BaAS has the highest value of Ω2 which corresponds to a high asymmetry at the rare earth sites and the glass MgAS has the highest values of Ω4 and Ω6 which corresponds to a high rigidity and a low optical basicity of the glass. The same glass has the highest quantum efficiency and the highest gain cross section values of the investigated glasses. On the other hand, the glass BaAS has the smallest efficiency and gain cross section, however, it provides the lowest glass transition temperatures which would be beneficial for the production of high-quality glass. Raman spectra of the AS glasses show a broader variation of Qn species and an increasing depolymerization for glasses with network modifier ions of larger ionic radii.

Growth, spectroscopy and first laser operation of monoclinic Ho3+:MgWO4 crystal

A monoclinic 0.86 at.% Ho3+:MgWO4 crystal is grown by the Top-Seeded-Solution Growth method. Its spectroscopic properties are studied with polarized light for E || a, b, c. The Ho3+ ion transition probabilities are determined within the modified Judd-Ofelt theory (mJ-O) accounting for the configuration interaction. The intensity parameters are Ω2 = 21.09, Ω4 = 4.42, Ω6 = 2.28 [10–20 cm2] and α = 0.053 [10-4 cm]. The calculated radiative lifetime of the 5I7 state is 6.18 ms. The Stark splitting of the 5I7 and 5I8 multiplets is determined with low-temperature spectroscopy. The absorption, stimulated-emission (SE) and gain cross-sections for the 5I8 ↔ 5I7 transition are derived. Ho3+:MgWO4 features a large Stark splitting of the ground-state (380 cm-1), high maximum σSE of 1.82 × 10–20 cm2 at 2.083 μm, broad gain spectra and high luminescence quantum yield making it suitable for efficient continuous-wave and mode-locked lasers at ∼2.1 μm. First laser operation of Ho3+:MgWO4 crystal is demonstrated at 2.104 μm reaching a slope efficiency of 72%.

Growth and spectroscopic properties of Er:Lu2O3 crystal grown by floating zone method

Er3+ doped Lu2O3 single crystal with high optical quality was successfully grown by Floating zone (Fz) method. The spectral properties and Judd-Ofelt (JO) analysis were reported. The absorption cross section at 971 nm was 1.5 × 10–21 cm2. The emission cross-section at 1558 and 2742 nm was calculated to be 2.20 × 10–20 cm2 and 3.22 × 10–20 cm2, respectively. The fluorescence lifetime of 4I11/2 and 4I13/2 energy level was fitted to be 1.59 ms and 12.43 ms. Meanwhile, the positive gain cross-section was achieved in 1.5 μm and 2.8 μm with inversion ratio P = 0.5. The low inversion ratios indicate a great potential of Er:Lu2O3 crystal with good gain performance.

Er,Yb:Ca3RE2(BO3)4 (RE=Y, Gd) – Novel 1.5 μm Laser Crystals

The search for new crystalline host materials for the usage in lasers emitting in the eye-safe spectral range of 1.5–1.6 µm is an important task. The aim of this work was to study the growth technique, spectroscopic properties and laser characteristics of new active media – crystals Er3+,Yb3+:Ca2RE2(BO3)4(RE=Y, Gd).Calcium-yttrium Er3+,Yb3+:Ca3Y2(BO3)4(CYB) and calcium-gadolinium Er3+,Yb3+: Ca2Gd2(BO3)4 (CGB) oxoborate crystals co-doped with erbium and ytterbium ions were investigated. Polarized absorption and emission cross-section spectra were determined. The lifetimes of4I11/2and4I13/2energy levels of Er3+ions were measured and ytterbium-erbium energy transfer efficiencies were estimated. The calculation of the gain cross-section spectra was performed. By using of Er3+,Yb3+: Ca2RE2(BO3)4 (RE=Y, Gd) crystals the laser performance was realized, for the first time to the best of our knowledge. The laser characteristics were studied in a quasi-CW (QCW) laser operation.The wide band with a peak at the wavelength of 976 nm is observed in the absorption spectra of both crystals. This peak coincides with the emission wavelength of the pump laser diodes for Yb-doped active media. The maximum value of absorption cross-section was 1.7 × 10–20cm2for polarizationE//bfor both crystals. The lifetimes of the upper laser level4I13/2of Er3+ions were 580 ± 30 μs and 550 ± 30 μs for Er,Yb:CYB and Er,Yb:CGB crystals, respectively. The energy transfer efficiencies from ytterbium to erbium ions for an Er,Yb:CYB and Er,Yb:CGB crystals were 94 % and 96 %, respectively. According to gain spectrum of the Er,Yb:CYB crystal the gain band peak is centered at the wavelength of 1530 nm. The maximum QCW output power was 0.5 W with slope efficiency of 13 % regarding to absorbed pump power for an Er,Yb: CYB crystal. The laser beam parameter M2did not exceed &lt; 1.5.Based on the obtained results, it can be concluded that these crystals are promising active media for lasers emitting in the spectral range of 1.5–1.6 μm for the usage in laser rangefinder and laser-induced breakdown spectroscopy systems, and LIDARs.

Structural, optical and NIR studies of Er3+ ions doped bismuth boro tellurite glasses for luminescence materials applications

Abstract Bismuth Boro Tellurite (BiBT) glasses doped with Er3+ ions (i.e., Er3+ = 0.1, 0.5, 1.0, 1.5, 2.0, 2.5 mol %) were prepared by using melt quenching technique and their structural, optical absorption, emission and up-conversion properties were investigated using XRD, Raman, absorption, emission and decay spectral studies. The oscillator strengths measured from the absorption spectral features were subjected to Judd-Ofelt theory to understand the radiative properties possessed by the as prepared glasses. The titled glasses excited under 379 nm exhibited green luminescence at 547 nm (4S3/2 → 4I15/2). Upon pumping with a laser diode of 980 nm (NIR), a broad NIR emission at around 1572 nm (4I13/2 → 4I15/2) was observed for which stimulated emission cross-section (σse), effective band width (Δλp), gain band width (σse × Δλp) and optical gain (σse × τR) parameters were evaluated. The up-conversion luminescence shown by these glasses has been analysed to understand the utility of these glasses as up-conversion lasers. Among all the as prepared glasses, 1 mol% of Er3+ ions doped BiBT glass exhibits better radiative properties compared to other glasses. The gain cross-section of the 1 mol% of Er3+ ions doped BiBT glass was derived using absorption cross-section (σa) and emission cross-section (σe) for different values of population inversion (γ). The Decay spectral features recorded under 980 nm excitation were used to measure the experimental lifetimes (τexp) and quantum efficiencies. All the aforementioned studies revealed that the Er3+ ions doped BiBT glasses are quite suitable to fabricate optoelectronic devices in visible green and NIR regions.

Fabrication and Laser Parameters of Yb:YAG Transparent Ceramics with High Optical Quality

Yb:YAG transparent ceramics with high optical quality were fabricated by solid-state reaction and vacuum sintering method (1750 ℃×30 h) using high-purity Y2O3, α -Al2O3 and Yb2O3 powders as raw materials. The measured concentration of Yb 3+in 5.0at% Yb:YAG ceramics is 6.41×10 20 cm -3, and the cell density is 4.65 g/cm 3. The microstructures, spectral characteristics and laser performance parameters of Yb:YAG ceramics were studied in this work. FESEM results show that Yb:YAG ceramics have uniform and compact structure, clean and straight grain boundaries, and the average grain size is about (19±3) μm. The in-line transmittance of Yb:YAG ceramics with thickness of 4.0 mm reaches 82.5% at 400 nm and 85.2% at 1100 nm. The minimum pump saturation light intensity occurs at 940 nm, and the pump threshold power at 1030 nm is the lowest. The quality factor of 1030 nm laser pumped at 940 nm is 1.02×10 -22 cm·s. By calculating the gain cross section, it is indicated that Yb:YAG ceramics can be tuned broadband, and are ideal laser gain media.

Gain anticipation of Ho3+ in ion-exchangeable germanate waveguide glasses

Efficient infrared emissions at ~ 1.2 and ~ 2.0 µm were recorded in Ho3+-single-doped and Ho3+/Yb3+-co-doped aluminum germanate glasses (NMAG), respectively. The maximum stimulated emission cross-sections for the ~ 1.2- and ~ 2.0 µm emissions were derived to be 2.3 × 10−21 and 5.8 × 10−21 cm2, respectively; then the gain cross-sections were further evaluated and the effective gains have been anticipated. In addition, the channel waveguide fabricated by K+–Na+ ion-exchanged method exhibited a complete single mode at 1.55 µm and the field diameters were identified to be horizontally 10.6 µm and vertically 6.7 µm. Effective amplified spontaneous emission at ~ 2.0 µm was recorded under 980 nm laser pumping. Broad bandwidth, large emission cross-section and perfect thermal ion-exchangeability indicate that Ho3+- and Yb3+-doped NMAG glasses are promising for the development of optical amplifier, tunable laser and light source operating at ~ 1.2 and ~ 2.0 µm.