Laser Gain Research Articles

Fabrication of heavily doped Nd:YAG transparent ceramics and their thin disc solid state laser performance

Compared to the single crystal counterparts, the laser ceramic materials can achieve higher ion doping concentrations and thus have a considerable application potential in thin disc lasers as laser gain media. In this study, heavily doped Nd:YAG transparent ceramics with excellent optical quality were fabricated, and a watt level thin disc laser output with high efficiency without saturation was realised in 3.0 at.% Nd:YAG ceramic (500 μm thickness). However, the trends in transmission and laser performance of Nd:YAG transparent ceramics were inconsistent with the Nd3+ doping concentration owing to the sintering additive effect of the introduced Nd2O3, and the laser performance of a 2.5 at.% Nd:YAG transparent ceramics with 1.0 mm thickness was almost identical to that of 1.0 at.% samples. This study demonstrates that the ceramic laser gain medium is of great significance for miniaturising and integrating solid state lasers.

Fluorescence spectroscopy based characterisation method for aggregation behaviour of rhodamine B (RhB) in water, ethanol, and propanol

The aggregation behaviour of rhodamine B (RhB) dye has been studied with the change in the concentration of the RhB in water, ethanol and propanol using absorption and laser induced fluorescence spectroscopy. The dimer and monomer fluorescence emissions were observed simultaneously in all the solvents. The monomer to dimer fluorescence intensity ratio , where IM = Monomer fluorescence intensity, ID = Dimer fluorescence intensity, has been calculated and found that at a certain concentration of RhB the magnitude of the ratio is different in different solvents. The ratio becomes equivalent to one for 1.5 gm l−1, 3 gm l−1 and 4.2 gm l−1, (gm l−1: gram litre−1) concentration of RhB in water, ethanol and propanol respectively. This concentration of RhB at which the ratio become unity is proposed as ‘critical concentration’. Above this critical concentration the dimeric fluorescence dominates and below which the monomeric fluorescence dominates. The parameter ‘critical concentration’ provides a condition of equilibrium between the monomer and their aggregates. Moreover, in the solvent for which the magnitude of critical concentration is less the fluorophore molecule aggregates more easily and vice versa. The ratio and critical concentration quantify the aggregation process of the fluorophore in the solvent. Hence, the ‘critical concentration’ value of the fluorophore (RhB) could be assumed as the characteristic parameter to study the aggregation behaviour of the fluorophore in solvents. The described method has the merit of use in the study of lasing ability and the wavelength tunability of the dye laser gain media as well as in the heavy metal sensing technology in water. This method may also be extended for other fluorophores and solvents.

Fast-sweeping continuous wave quantum cascade laser operating in an external cavity with polygon mirror.

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm-1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm-1. The linewidth of the emitting modes is less than ∼0.05 cm-1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

Temporal quality of post-compressed pulses at large compression factors

Post-compression of ultra-short laser pulses via self-phase modulation is routinely employed for the generation of laser pulses with optical bandwidths reaching far beyond the laser gain limitations. Although high compression factors can be routinely achieved, the compressed pulses typically suffer from temporal quality degradation. We numerically and experimentally analyze the deterioration of different measures of temporal quality with increasing compression factor and show how appropriate dispersion management and cascading of the post-compression process can be employed to limit the impact of this effect. The demonstrated saturation of pulse quality degradation at large compression factors puts novel femtosecond laser architectures based on post-compressed picosecond or even nanosecond laser systems in sight.

Low Divergence Coherently Coupled Vertical Cavity Surface Emitting Laser Ring Arrays

We report the theory and experimental verification of a method to reduce beam divergence from coherently-coupled ring arrays of photonic crystal vertical cavity surface emitting lasers (VCSELs). The photonic crystal etched into the top distributed Bragg reflector defines the optically coupled array elements, each of which has independent current injection and thus independent gain. Using a 2-dimensional complex waveguide model we simulate the supermodes of a 6-element ring array of VCSELs. The influence of the photonic crystal, free carrier loss, laser gain, and carrier-induced index suppression arising from the spatially defined gain regions are included in our analysis. Our simulations show the latter enables preferred anti-guided supermodes with reduced beam divergence. Optical characterization of the spectral and beam characteristics of 6-ring photonic crystal implanted VCSEL arrays shows that when the arrays are current-tuned to quasi-single mode operation, we find the beam divergence angle approaches 2° consistent with our simulations. This method of reduced beam divergence may have advantages for high-brightness applications.

Refined modelling of anisotropy influence on the optical gain in Mid-IR quantum cascade lasers

Refined modelling of anisotropy influence on the optical gain in Mid-IR quantum cascade lasers

Spectral investigations of Nd3+:Ba(PO3)2+La2O3 glasses for infrared laser gain media applications

The investigation of the concentration dependent spectroscopic characteristics of Nd3+ ion in Ba(PO3)2+La2O3 (PBaLa) glasses have been carried out. Based on prominent Judd-Ofelt (JO) formalism, the three JO phenomenological quantitative intensity parameters (Ωλ = 2,4,6) were determined by using absorption levels. The very high Ω6 intensity parameter value reveals greater degree of structural distortion of the Nd3+ sites in PBaLa glass. From Ωλ, required radiative characteristics such as radiative emission probability, lifetime (τr), branching ratio (βr) and emission cross-section (σ(λ)) for excited fluorescent levels have been predicted to assess the potentiality of the glasses. The de-excitation (radiative emission) spectra observed for these glasses yield three prominent levels of 4F3/2 → 4I13/2,11/2,9/2 and their related effective bandwidths and de-excitation cross-sections have been quantified. The lifetimes of the 4F3/2 fluorescent state in Nd3+:PBLa for different concentrations (0.1–2.0 mol %) of Nd3+ ion have been determined by exciting with diode laser (808 nm). The quenching of lifetime could be attributed to the enhanced energy transfer phenomenon via cross-relaxation between pairs of the Nd3+ ions. The crucial spectroscopic characteristics that include Ωλ parameters, radiative properties, effective bandwidths, stimulated emission cross-section, gain bandwidth, figure of merit and optical gain of 4F3/2 → 4I11/2 transitions of PBaLaNd1.0 glass are determined which are also compared with other reported 85 Nd3+: glasses which include 63 Nd3+:phosphate glasses. These systematic analysis and critical comparison with similar results reveal that Nd3+:PBaLa glasses with a wide bandwidth of fluorescence can also be useful for optical amplifiers or laser gain media applications.

Preparation of the Flexible Green Body of YAG Ceramic Fiber by Melt Spinning.

YAG ceramic fiber, with its high thermal conductivity and easy to achieve limit size, provides design flexibility as a laser gain medium. Its mainstream forming method was mainly high-pressure extrusion, but there were disadvantages, such as lack of flexibility. In this work, the flexible green body of YAG ceramic fiber was prepared by melt spinning. The melting characteristics of TPU with four different Shore hardnesses were systematically investigated. The microstructure, element homogeneity of the surface and fracture SEM images of the prepared ceramic fiber were also analyzed in detail. The optimized process parameters of YAG ceramic fiber preparation were as follows: the melting temperature was 220 °C, the screw feed rate of the double-cone screw extruder was F = 15.0 mm/min and the TPU-95A# was used. The ceramic fiber with the mass ratio of TPU-95A# to ceramic powder = 4:6 had the best microstructure quality. It had good flexibility and could be knotted with a bending radius of about 2.5 mm, and the tensile strength reached approximately 20 MPa. These results are crucial for advancing YAG ceramic fiber applications.

Effect of Cr co-doping on the optical absorption and emission characteristics of Yb:YVO4 single crystals grown by OFZ technique

Cr co-doped Yb:YVO4 (Cr:Yb:YVO4) single crystals with concentrations of 1.0 and 1.5 at.% of Cr and 3.0 at.% of Yb were grown by the optical floating zone technique. X-ray diffraction analysis on the grown crystals revealed that the lattice parameters of Cr:Yb:YVO4 crystals do not depend on the Cr doping concentration. The dislocation etch pit density estimated from the etching study was found to be 103/cm2. The observed fringes of nearly uniform thickness and spacing in birefringence interferometry for [100] plate and conoscopy pattern for [001] plate indicate the compositional and optical homogeneity of the crystal. Further, the influence of the Cr doping concentration on the 2A1→2B2 transition (at ∼1100 nm) and 2A1→2E transition (at ∼640 nm) due to Cr5+ ion were analysed using two orthogonal polarized light with electric field perpendicular (π-polarization) and parallel (σ-polarization) to [001] axis. These absorption peaks of Cr ion were found to be sensitive to the polarization of incident light. The absorption transition 2A1→2B2 around 1100 nm are permitted for π-polarization and on the other hand, the absorption transition 2A1→2E at 640 nm are permitted only for σ-polarization. The characteristic broader absorption band around ∼1100 nm is of importance as it is useful for self-Q-switching in a laser gain medium. The absorption coefficient corresponding to Cr5+ ions at 1100 nm increases with the increase in Cr doping concentration. The band-gap for σ-polarized light is slightly higher than π-polarized light and decreases with Cr doping. The increase in the estimated Urbach energy (50–122 meV) with the increase in the Cr doping concentration confirms that the defect states close to the band edge increase thus decrease the band gap. Further the photoluminescence intensity is in-line with the absorption data and decreases with increase in the Cr concentration from 0 to 1.5 at.%.

Bridgman growth and characterization of Dy:YCa4O(BO3)3 crystal

A 1-inch Dy3+:YCa4O(BO3)3 single crystal has been grown by the Bridgman method. The segregation coefficient of Dy3+ is 0.784. The structures, thermal and optical properties of Dy3+:YCOB crystal was studied. The results show that the Dy3+:YCOB crystal has high transmittance, specific heat and thermal diffusion coefficient. The Y-axis-oriented Dy3+:YCOB crystal has the largest absorption cross-section σabs(0.85×10−21cm2@444nm) and emission cross-section σem(2.54×10–21cm2@585nm). In addition, the J-O intensity parameters Ωt(t=2,4,6) of the Dy3+:YCOB crystal were calculated, where Ω2, Ω4, Ω6 are 5.01×10−20cm2, 0.33×10−20cm2, 0.52×10−20cm2 respectively. The fluorescence branching ratio β is 78.7%, and the radiative lifetime τrad is 1.588 ms. The fluorescence lifetime τf is 384.47 μs at 585 nm. The above data confirms that the Y-axis-oriented Dy3+:YCOB crystal is a good yellow laser gain material, which has potential application value in the field of yellow laser operation.

RETRACTED: A continuous-wave alexandrite laser and its degenerated optical parametric generator

• To the best of our knowledge, this is the first report of the CW degenerated optical parametric generator at 1503.4 nm for the alexandrite crystal, which is both used as laser gain and DOPG nonlinear medium. At present, there are no papers about a degenerated optical parametric generator of the alexandrite laser. • The spectral width of DOPG is about 7.16 nm and the frequency spacing between adjacent longitudinal modes at 1503.4 nm reduces to 1.72 × 10 11 Hz. • The DOPG central wavelength tunes from 1502.77 to 1505.08 nm when the crystal temperature increases from 20 to 70 °C. • Our investigation provides further insight into studying the complex nonlinear effects of the biaxial alexandrite crystal. We present a continuous-wave (CW) alexandrite laser at 751.7 nm end-pumped by a red laser diode (LD) at 636 nm. A laser output power of 1160 mW with an optical-to-optical efficiency of 11.9% and a slope efficiency of 16.7% is obtained at the pump power of 9.72 W. The spectral width of the alexandrite laser is about 3.58 nm, and the frequency spacing between adjacent longitudinal modes is 3.44 × 10 11 Hz. The first CW degenerated optical parametric generator (DOPG) at 1503.4 nm of the alexandrite crystal is reported. The spectral width of DOPG is about 7.16 nm, and the frequency spacing between adjacent longitudinal modes at 1503.4 nm reduces to 1.72 × 10 11 Hz. The DOPG central wavelength tunes from 1502.77 to 1505.08 nm when the crystal temperature increases from 20 to 70 °C.

Investigation of Eu3+ Doped Oxy-Fluoride Phosphate Glass for Red Laser Gain Medium Application

The oxyfluoride phosphate glass doped with europium oxide was synthesized by melt quenching technique. The structural and spectroscopic analysis has been carried out by Raman, absorption, photoluminescence, phonon side band and lifetime analysis. The absorption spectrum revealed the characteristic transitions between the 7F0 ground state and 7F1 first excited state to the upper-lying electronic states. The several J-O parameters: oscillator strength, radiative transition probability, stimulated emission cross-section and branching ratio. The local structure around the Eu3+ ions and the phonon energy of oxyfluoride phosphate glass have been confirmed on the basis of PSB associated with the 7F0→5D2 transition. The highest excitation spectrum showed at 395 nm for transition 7F0→5L6. The emission spectrum showed two major peaks corresponding to 591 nm orang emission (5D0→7F1) and 613 nm red emission (5D0→7F2). This work confirmed that 1 mol% Eu2O3 doped oxyfluoride phosphate glass performs a high potential for using as efficient luminescence materials for solid-state lighting application, especially for reddish-orange light-emitting glass.

High-Power Dual-Wavelength Laser Based on Coaxial End-Pumping Scheme With Combination of Gain Media

A high-power dual-wavelength laser based on coaxial end-pumping scheme with composite Nd:YAG and Nd:YAP crystals was demonstrated. A theoretical model was built to analyze the thermal lens and accurately calculate the thermal focal length of laser gain medium pumped by a divergent beam. The experiment was carried out with a plane-convex cavity and a V-shape cavity for high-power output. The Q-switched laser produced 9.1 W pulsed dual-wavelength output (4.7 W at 1064 nm and 4.4 W at 1079 nm) at 25-kHz pulse repetition rate with synchronized pulse duration of 35 ns, corresponding to the optical-to-optical efficiency of 30.3%. The continuous-wave (CW) output power reached 15.5 W at laser diode (LD) pump power of 40 W, corresponding to a slope efficiency of 40.9% and an optical-to-optical conversion efficiency of 38.9%. The powers at two wavelengths were almost equivalent and the fluctuations were both less than 2.3%. To our best knowledge, it was the highest output power for stable CW dual-wavelength lasers based on diode-end-pumped configuration.

Spectroscopic properties of stoichiometric CaTbAlO4 crystal as a visible laser gain medium

A CaTbAlO4 crystal was grown by the Czochralski method and the polarized spectroscopic properties of the crystal were investigated at room temperature. The peak absorption cross sections at 484.6 nm for σ and π polarizations are 2.97 × 10−22 and 0.68 × 10−22 cm2, respectively. Based on the Judd-Ofelt theory, the spontaneous transition probabilities, fluorescence branching ratios, and radiative lifetime of the 5D4 multiplet of Tb3+ in the crystal were obtained. For σ and π polarizations, the peak emission cross sections at 545 nm are 9.55 × 10−22 and 2.48 × 10−22 cm2, as well as the full widths at half maximum of the green fluorescence bands are about 7 and 10 nm, respectively. The fluorescence lifetime and fluorescence quantum efficiency of the 5D4 multiplet of Tb3+ are 1.84 ms and 79%, respectively. Above results indicate that the CaTbAlO4 crystal is a promising gain medium for visible laser.

12.3-W output power and 271-nm wavelength tunability of diode-double-end-pumped Tm:CALYO laser

We report on the recent progress on high-power and broadband wavelength tunability of diode-pumped Tm:CaYAlO4 (Tm:CALYO) lasers. Using a double-end pumping geometry, we have achieved a maximum output power of 12.36 W at 1938 nm, which is believed to be the highest power that have ever achieved for Tm:CALYO lasers. Moreover, we have operated 1861 nm, 1982 nm and 2032 nm lasers with maximum output powers of 8.73 W, 5.36 W and 0.73 W, respectively, by using differently coated end-face cavity mirrors. Furthermore, using a MgF2 birefringent filter, we have tuned the lasing wavelengths from 1807 nm to 2078 nm. The 271 nm wavelength range is also believed to be the broadest tunability for Tm:CALYO lasers. Passively Q-switched Tm:CALYO laser has also been demonstrated with a maximum average output power of 3.81 W. The shortest pulse width reached 25.9 ns at a repetition rate of 10.1 kHz, which led to a pulse energy of 377.22 μJ and a peak power of 14.56 kW. These results indicate clearly that the Tm:CALYO crystal is excellent as a high-power and broadband laser gain medium operating at the important 2 μm wavelength region.

Quasi 2D perovskite single-mode vertical-cavity lasers through large-area film transfer

Quasi two-dimensional (2D) perovskites are a new class of laser gain medium; however, the thickness of the solution-processed perovskite layer is usually too thin to support an optical mode due to the low solubilities of precursors. Here, we report a flexible large-area film transfer method to lengthen the cavity according to the resonant condition. Then a single-mode vertical-cavity quasi 2D (PEA)2Csn-1PbnX3n+1 (X:Br, Cl) perovskite pure green laser is demonstrated, which has a low threshold of 39.3 μJ/cm−2, a high-quality factor of 1159, and a highly linear polarization degree of 92.5% under nanosecond pulsed optical pumping. This is a significant step toward low-cost practical perovskite lasers.

Optically active Fe2+-doped ZnSe particles in a chalcogenide glass matrix

High brightness light sources in the mid-infrared are of particular interest for a host of different applications. However, the selection and availability of laser gain media in this spectral region are severely lacking. In this manuscript, we demonstrate broadband optical emission in the 3520-5200 nm region from Fe2+-doped ZnSe microparticles dispersed in a chalcogenide glass matrix which is amenable to fiber drawing. A conformal alumina shell was applied to the Fe:ZnSe particles which prevented ZnSe dissolution and preserved normal Fe2+ optical absorption and emission properties during the composite material manufacturing process. The broadband emission properties of the Fe:ZnSe-doped chalcogenide glass material observed here are promising for the future development of fiber lasers operating at wavelengths > 4 µm.

Simultaneously Wavelength- and Temperature-Insensitive Mid-Infrared Optical Parametric Amplification with LiGaS2 Crystal

Ultrafast mid-infrared (mid-IR) lasers with a high pulse repetition rate are in great demand in various fields, including attosecond science and strong-field physics. Due to the lack of suitable mid-IR laser gain medium, optical parametric amplifiers (OPAs) are used to generate an ultrafast mid-IR laser. However, the efficiency of OPA is sensitive to phase mismatches induced by wavelength and temperature deviations from the preset points, which thus limits the pulse duration and the average power of the mid-IR laser. Here, we exploited a noncollinear phase-matching configuration to achieve simultaneously wavelength- and temperature-insensitive mid-IR OPA with a LiGaS2 crystal. The noncollinearity can cancel the first-order dependence of phase matching on both wavelength and temperature. Benefitting from the thermal property of the LiGaS2 crystal, some collinear phase-matching solutions derived from the first-order and even third-order wavelength insensitivity have comparatively large temperature bandwidths and can be regarded as approximate solutions with simultaneous wavelength and temperature insensitivity. These simultaneously wavelength- and temperature-insensitive phase-matching designs are verified through numerical simulations in order to generate few-cycle, high-power mid-IR pulses.

Ultra‐Low Threshold Titanium‐Doped Sapphire Whispering‐Gallery Laser

AbstractTitanium doped sapphire (Ti:sapphire) is a laser gain material with broad gain bandwidth benefiting from the material stability of sapphire. These favorable characteristics of Ti:sapphire have given rise to femtosecond lasers and optical frequency combs. Shaping a single Ti:sapphire crystal into a millimeter sized high quality (Q) whispering gallery mode resonator (Q ≈ 108) reduces the lasing threshold to 14.2 mW and increases the laser slope efficiency to 34%. The observed lasing can be both multi‐mode and single‐mode. This is the first demonstration of a Ti:sapphire whispering‐gallery laser. Furthermore, a novel method of evaluating the gain in Ti:sapphire in the near infrared region is demonstrated by introducing a probe laser with a central wavelength of 795 nm. This method results in decreasing linewidth of the modes excited with the probe laser, consequently increasing their Q. These findings open avenues for the usage of whispering gallery mode resonators as cavities for the implementation of compact Ti:sapphire lasers. Moreover, Ti:sapphire whispering‐gallery laser can also be utilized as an amplifier inside its gain bandwidth by implementing a pump–probe configuration.

Enhanced Performance of Two-Photon Excited Amplified Spontaneous Emission by Cd-Alloyed CsPbBr3 Nanocrystals

It has been demonstrated that the alloyed perovskite nanocrystals (NCs) with a small amount of Cd element may passivate the inherent halide vacancies in perovskite NCs and improve their stability. However, the study of the optical properties of such alloyed perovskite NCs still remains essentially untouched, which will seriously hinder relevant applications. Herein, using different amounts of CdBr2 as an alloyed metal precursor, a series of CsPbxCd1-xBr3 NCs (x = 1, 0.93, and 0.88) were synthesized. Compared with bare CsPbBr3 NCs, the Cd-alloyed NCs exhibited a near-unity photoluminescence quantum yield (99%), efficiently improved stability, and enhanced electron-phonon coupling strength. As the Cd-alloyed amount was increased, their hot-carrier cooling time became faster and the exciton-biexciton interaction decreased, causing a decreased threshold of two-photon excited amplified spontaneous emission (ASE) from 1.58 to 1.23 mJ cm-2. In addition, the Cd-alloying method can also improve the photostability of ASE, resulting in the initial ASE intensity remaining at 90% even after 7.5 × 105 pulse shots. This work implies that the Cd-alloyed CsPbBr3 NCs will be promising for application in a laser gain medium.