Increase In Slope Efficiency Research Articles

Development of High‐Reflectivity and Antireflection Dielectric Multilayer Mirrors for AlGaN‐Based Ultraviolet‐B Laser Diodes and their Device Applications

Fabrication techniques for high‐reflectivity (HR) and antireflection (AR) dielectric multilayer mirrors for AlGaN‐based ultraviolet‐B (UV‐B) laser diodes are developed. After depositing several dielectric materials and evaluating their complex refractive indices via ellipsometry, it is determined that SiO2 as a low‐refractive‐index material and Ta2O5 as a high‐refractive‐index material are appropriate material combinations in the UV‐B region at a light wavelength of ≈300 nm due to their low extinction coefficients and large refractive index difference. Based on these results, HR mirror with a reflectance of >99% in the UV‐B region at a center wavelength of 310 nm and an AR mirror with a reflectance of ≈8% in the same wavelength range are demonstrated; a mirror with reflectance that is almost equal to the designed value is demonstrated. Furthermore, these mirrors are coated on the respective edge surfaces of the UV‐B laser diodes. A comparison of the characteristics of the same device before and after edge coating reveals a reduction in the threshold current density of laser oscillation, whereas, simultaneously, an increase in slope efficiency and external differential quantum efficiency is observed. The improvement of these device characteristics, estimated from the above reflectance values, is confirmed to be almost theoretically explainable.

Hydrogel Lasers Via Supramolecular Host–Guest Complexation

AbstractSupramolecular host–guest complexes are dynamic systems composed of typically macrocyclic host molecules encapsulating smaller guest molecules. One of their applications is the selective encapsulation of fluorescent dye molecules to tackle quenching and photobleaching in solution. When extended to solid matrices, the efficiency of dyes can be especially poor, limiting their use in fluorescence‐based applications. Here it is demonstrated that supramolecular host–guest complexation in a hydrogel network allows readily tunable hydrogel‐based lasers. Poly(N‐isopropylacrylamide)‐based copolymer hydrogel is used with covalently linked Rhodamine B molecules. By controlled doping of cucurbit[7]uril as a host molecule in the hydrogel, it is shown that the fluorescent quantum yield increases from 17% to 51%, accompanied by 15‐fold enhancement in photostability. As proof of concept, a thin‐film distributed feedback hydrogel laser with wavelength tunability is fabricated. The results reveal a 30% increase in slope efficiency in the presence of host molecules. These results, combined with the inherent stimuli‐response of hydrogels, enable new possibilities for affordable and small‐scale photonic devices capable of safely interacting with the environment in, for example, sensing applications.

Amplified spontaneous emission from all-inorganic perovskite on a flexible substrate with silk fibroin

Stretchable microcavity lasers reveal potential application in flexible displays, biomedicine, and wearable devices in the near future. In this work, we investigated the characteristic of amplified spontaneous emission (ASE) from all inorganic CsPbBr3 QDs on a flexible PET substrate with the assistance of biocompatible silk fibroin (SF) film. In comparison with the sample on PET directly, the ASE of all-inorganic perovskite film revealed a lower threshold of 32.7 μJ/cm2, higher slope efficiency, and a larger gain coefficient of around 100.0 cm−1 owing to the better stack and good arrangement of the CsPbBr3 QDs on top of the SF film. For the temperature-dependent ASE measurement, the larger characteristic temperature of around 277 K is obtained from CsPbBr3 QD/SF film, and the emission peak reveals a slight shift with temperature variation, which indicates its temperature-insensitive property. As the curvature of flexible substrate increases under the mechanical bending, the lasing threshold of CsPbBr3 QD/SF film was reduced along with the increase in slope efficiency owing to the enhancement in the index guiding effect.

Nanosecond parametric Raman anti-Stokes SrWO4 laser at 507 nm with collinear phase matching

Collinear phase matching of the Stokes ↔ anti-Stokes interaction for Raman-active crystals with different birefringence was studied theoretically as well as experimentally. It was shown that collinear phase matching of the Stokes ↔ anti-Stokes interaction in low-birefringent crystals can be insensitive to angular mismatch if a phase matching angle is higher than 60°. We have developed and experimentally realized an extracavity parametric Raman anti-Stokes laser based on a low-birefringent SrWO4. Cyan 507-nm anti-Stokes conversion from green (532 nm) pump radiation of a 5-ns, 1-mJ second harmonic Nd:YAG laser has been obtained. Laser setup with a single beam excitation made it possible to use an output face of the SrWO4 crystal as an output coupler because of wide (6°) angular tolerance of collinear phase matching that resulted in an increase of slope efficiency of anti-Stokes generation higher than 3% at the anti-Stokes energy output of a 10-µJ level.

High power broad-area lasers with buried implantation for current confinement

Broad area lasers emitting near 915 nm are fabricated using a 2-step epitaxial growth process, with an intermediate implantation of silicon or oxygen ions. This approach allows for the introduction of buried lateral current confinement layers at moderate cost in terms of process complexity. The effectiveness of this strategy with respect to different implantation conditions is tested, obtaining up to ≈12% reduction of threshold current and ≈15% increase of slope efficiency with respect to standard lasers. Also a significant improvement of the beam quality-which is relevant to coupling efficiency-has been obtained in terms of reduction of the lateral beam product parameter, from 3.8 mm × mrad at 5A for standard lasers to 2.2 mm × mrad for implanted and regrown lasers, but at the expense of energy efficiency.

Mitigation of Efficiency Droop in an Asymmetric GaN-Based High-Power Laser Diode With Sandwiched GaN/InAlN/GaN Lower Quantum Barrier

The phenomenon of efficiency droop is comprehensively investigated in an asymmetric GaN-based laser diode (LD). Numerical simulations and experiments are both conducted. It is found that with the introduction of a sandwiched GaN/InAlN/GaN lower quantum barrier (LQB) instead of the bulk GaN LQB in an asymmetric GaN-based high-power blue LD, the efficiency droop is effectively mitigated to only 9.7% under continuous-wave operation with no degradation in the crystal quality. Compared with the conventional asymmetric GaN-based LD, the wall-plug efficiency is successfully increased by 24.7% from 15.8% to 19.7% at high injection current of 2000 mA, promoting the increase of slope efficiency and output power, while the threshold current is reduced by 15.2%. These improvements are primarily attributed to the inserted high-quality InAlN thin barrier layer, which could form an extra potential barrier beneath the first InGaN quantum well to remarkably alleviate the hole overflow from the active region and enhance the radiative recombination rate.

Suppression the Leakage of Optical Field and Carriers in GaN-based Laser Diodes by using InGaN Barrier Layers

GaN-based laser diodes (LDs) with different barrier layers are investigated by using simulation and experimental methods. it is found that the absorption loss always decreases with the increasing indium content of the InGaN barrier layer in InGaN/InGaN multiquantum well (MQW) LDs, It leads to an increase of slope efficiency and output power of LDs due to the better optical confinement effect of laser beam. However, when the barrier layer changes from InGaN to AlGaN, both threshold current and slope efficiency deteriorate seriously. Except for the increase of the optical absorption loss resulting from the leakage of the optical field, a large leakage of carriers due to the increased piezoelectric polarization field in InGaN/AlGaN MQW region may also be responsible for this deterioration. Therefore, suppressing the leakage of the optical field and carriers by using relatively higher In content InGaN barrier layer is benefit for obtaining high-performance LDs with low threshold current and high output power.

Study on DFB semiconductor laser array integrated with grating reflector based on reconstruction-equivalent-chirp technique.

A 4-channel distributed feedback (DFB) semiconductor laser array with incorporation of a grating reflector utilizing reconstruction-equivalent-chirp technique is theoretically studied and experimentally demonstrated. By integrating with a grating reflector, 40% increase of slope efficiency, about 10mA decrease of threshold current and 7dB increase of side mode suppression ratio (SMSR) are achieved with a deviation of wavelength spacing being less than 0.07nm. The SMSRs of all the lasers are higher than 60dB.

Highly efficient 10 cm long fiber laser.

We investigate an innovative multipass-pumping technique in a short photonic crystal fiber laser. The technique is based on simple optical elements and shows, in a power oscillator configuration, highly efficient CW and Q-switched operation of an extremely short fiber laser. We report significantly higher absorption of the pump light, compared to a single-pass configuration, which results in an impressive fourfold increase in slope efficiency, up to 60%. Moreover, we show pulse energy of 1.22mJ and pulse duration shorter than 12ns, corresponding to a peak power of more than 100kW.

Efficient laser performance of a cryogenic Yb:YAG laser pumped by fiber coupled 940 and 969 nm laser diodes

Laser performance of Yb:YAG at different cryogenic temperatures pumped by a fiber coupled diode laser emitting at 940 and 969 nm were presented. The pump laser diode bandwidth, absorption bandwidth as well as absorption of the laser material at cryogenic temperatures play a vital role on laser performance. The laser threshold decreases and the output power and slope efficiency increase when cooled to cryogenic temperatures.

Short pulsed gain-switched fiber laser with improved efficiency utilizing unabsorbed pump recovery.

A simple solution for increasing the slope efficiency of a gain-switched fiber laser based on Yb-doped active fiber is presented. By adding a fiber amplifier stage, which recovers the unabsorbed pump light from the gain-switched oscillator, a significant increase in slope efficiency is achieved. The pulses at 1030-nm wavelength have an FWHM of 28 ns and a peak power of 2.3 kW.

Improved laser efficiency by direct diode laser pumping of the radiation-resistant Nd:gadolinium–scandium–gallium garnet

Spectroscopic investigation of the radiation-resistant Nd:GSGG crystals and ceramics reported in this paper evidences that the absorption line at 883 nm corresponding to the unusual situation of quasi-degenerate absorption transitions 4I9/2(2) →4F3/2(1) and 4I9/2(3) →4F3/2(2) can be used for efficient direct diode laser pumping of this material, with stable absorption over an extended temperature range. It is inferred that the reduction of the quantum defect at this wavelength of pump compared to the traditional 807 nm pumping could improve the laser parameters and reduce drastically the heat generation, leading to a considerable extension of the power scalability. This possibility is demonstrated for the first time in the case of the 1061 nm laser emission in a continuous-wave and in the repetitive active acousto-optic and passive Q-switched laser emission. In all regimes this manifests in the reduction of the laser threshold, increase of slope efficiency and extension of the average power range. Additionally, in the active Q-switching this enables increased pulse energy and reduced pulse duration. The direct pumping could revitalize the utilization of Nd:GSGG for construction of lasers with storage of population inversion or working in an ionizing radiation environment.

Homogeneous in Distribution of InGaN/GaN Quantum Wells in High Performance GaN-Based Light-Emitting Devices

We have investigated the effect of well growth rate on threshold current and slope efficiency of GaN-based laser diodes (LDs). The lasing performance was significantly dependent on optical and crystal quality of In0.08Ga0.92N/ GaN QWs with different well growth rates. The InGaN QWs grown with lower growth rate represented better interface quality and had low surface defects in the InGaN/GaN QW region. In addition, InGaN QWs grown with lower growth rate exhibited the higher optical properties such as the higher PL intensity and the smaller blueshift with increasing excitation power density. The present results suggest that optical and crystal qualities of InGaN/InGaN MQW are significantly improved by lowering well growth rate, resulting in the increase of slope efficiency and the decrease of threshold current density in GaN-based LDs.

Same-wavelength cascaded-transition quantum cascade laser

Conventional Quantum Cascade (QC) lasers are based on intersubband transitions in repeated stages of precisely engineered quantum wells, with one electron emitting at most one photon in each stage. Here, we report on a same-wavelength cascaded-transition (SWCT) QC structure, with two subsequent optical transitions emitting at the same wavelength (λ ∼ 14.2 μm) in each stage. This SWCT QC gain medium was inserted between two conventional QC stacks, also at ∼14.2 μm. The measured laser power shows a quasi-saturation region with a 46% increase in slope efficiency after it. This quasi-saturation region is attributed to different current injection mechanisms.

Photoluminescence and laser behavior of Yb:YAG ceramic

Yb:YAG transparent ceramics with different doping concentration were fabricated by the traditional solid-state diffusion route, and their fluorescence properties and laser behavior were investigated. It is found that both the fluorescence intensity and lifetime depend deeply on both the doping concentration of Yb ions and the annealing treatment. The continuous wavelength (C.W.) laser performance for 10at.% Yb:YAG ceramic suggests that both the threshold pump power and slope efficiency increase with increasing transmittance of the output couple (Toc). For Toc=15%, the threshold pump power is about 6.1W, the slope efficiency is 25%, and the maximum output power of 2.2W is achieved when pumped by 14.9W.

Efficient quasi-three-level Nd:YAG laser at 946 nm pumped by a tunable external cavity tapered diode laser

Using a tunable external cavity tapered diode laser (ECDL) pumped quasi-three-level Nd:YAG laser, a fivefold reduction in threshold and twofold increase in slope efficiency is demonstrated when compared to a traditional broad area diode laser pump source. A TEM 00 power of 800 mW with 65% slope efficiency is obtained, the highest reported TEM 00 power from any 946 nm Nd:YAG laser pumped by a single emitter diode laser pump source. A quantum efficiency of 0.85 has been estimated from experimental data using a simple quasi-three-level model. The reported value is in good agreement with published values, suggesting that the model is adequate. Improvement of the 946 nm laser due to the ECDL's narrow spectrum proves to be less significant when compared to its spatial quality, inferring a broad spectrum tapered diode laser pump source may be most practical. Experimental confirmation of such setup is given.

High-power quantum-dot-based semiconductor disk laser

We demonstrate multiwatt cw output power from an optically pumped quantum-dot semiconductor disk laser. Continuous-wave output power of 4.35 W with 22% slope efficiency was demonstrated at a center wavelength of 1,032 nm. This represents an increase in power of 15 times and an increase in slope efficiency of 10 times from the previously published results using Stranski-Krastanow grown quantum dots. An intracavity diamond heat spreader was used for thermal management. The maximum output power was limited by the available pump power, and no sign of thermal rollover was observed.

Solid State Dye Lasers Based on Coumarin 440 and Pyrromethene 567 Codoped Polymethyl Methacrylate

Laser dye coumarin 440(C440) is codoped with pyrromethene 567 (PM567) into polymethyl methacrylate (PMMA). The effects of C440 concentration on the performance of the solid state dye medium, including spectra property, slope efficiency and photostability, are studied. When C440 is codoped with PM567 at the same concentration 1 × 10−4 mol/L, the highest efficiency and photostability can be obtained. Compared with the medium based on pure PM567 doped PMMA, about 50% increase in slope efficiency and at least five-fold enhancement in the photostability are observed.

Continuous-wave high-power multi-pass pumped thin-disc Nd:GdVO4 laser

A thin-disc Nd:GdVO4 laser in multi-pass pumping scheme was developed. Continuous-wave output power of 13.9W at 1.06μm for an absorbed power at 808nm of 22W was demonstrated from a 250-μm thick, 0.5-at.% Nd:GdVO4 in a 4-pass pumping; the slope efficiency in absorbed power was 0.65, or 0.47 in input power. Output performances were also investigated under diode laser pumping at 879nm, directly into the emitting 4F3/2 level: maximum power of 3.6W was obtained at 6.2W of absorbed power with 0.69 slope efficiency. Compared with pumping at 808nm, into the highly absorbing 4F5/2 level, improvements of laser parameter in absorbed power (increase of slope efficiency, decrease of threshold) were obtained, showing the advantages of the pumping into the emitting level. However, the laser performances expressed vs. the incident power were modest owing to the low absorption efficiency at 879nm. Thus, increased number of passes of the medium would be necessary in order to match the performances in input power obtained under 808-nm pumping.

Enhanced emission from liquid-crystal lasers

The performance of a photonic band-edge laser fabricated from a low molar mass dye-doped chiral nematic liquid crystal is found to have a strong thermal dependence. At each temperature the performance of the laser has been characterized by the slope efficiency which was calculated from a plot of the emission energy as a function of excitation energy. This slope efficiency was found to increase by 36% when the dye-doped chiral nematic liquid crystal was cooled from 53to43°C. The increase in slope efficiency is considered to be due to a change in the lasing conditions, in particular, changes in the emission efficiency of the dye and possibly the quality factor of the liquid-crystal resonator, which is dependent upon the linewidth of the resonant mode. The wavelength dependency of the spontaneous emission intensity and the quantum efficiency of the dye were not found to influence the lasing conditions in this case. The order parameters relating to the dye-doped chiral nematic liquid crystal were considered to be the principle factors responsible for the increase in performance at the lower operating temperature of 43°C.