This PDF file contains the front matter associated with SPIE Proceedings Volume 8599, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

High optical quality rare-earth-doped LiYF₄ (YLF) epitaxial layers were grown on pure YLF substrates by liquid phase epitaxy (LPE). Thulium, praseodymium and ytterbium YLF crystalline waveguides co-doped with gadolinium and/or lutetium were obtained. Spectroscopic and optical characterization of these rare-earth doped waveguides are reported. Internal propagation losses as low as 0.11 dB/cm were measured on the Tm:YLF waveguide and the overall spectroscopic characteristics of the epitaxial layers were found to be comparable to bulk crystals. Laser operation was achieved at 1.87 μm in the Tm³⁺ doped YLF planar waveguide with a very good efficiency of 76% with respect to the pump power. Lasing was also demonstrated in a Pr³⁺ doped YLF waveguide in the red and orange regions and in a Yb³⁺:YLF planar waveguide at 1020 nm and 994 nm.

Laser generation in the 2 μm spectral range is interesting for applications in mid-IR fingerprint applications, including remote sensing, gas detection, high resolution molecular spectroscopy, and medicine. Here, we review the different Tm³⁺-based waveguide lasers with emission at around 2 μm recently developed in monoclinic potassium double tungstates, including a CW mirrorless and the first Q-switched waveguide laser based on this ion. We also investigated the influence on the waveguide laser performance of the crystallographic plane on which the epitaxial active layer has been grown.

In this work, we demonstrate 3-level laser operation in a Yb,Gd,Lu:KYW waveguide laser fabricated by combination of liquid phase epitaxy and Ar⁺ ion beam milling. Laser emission was observed at 981 nm with an absorbed threshold power of 23 mW and a slope efficiency of 58% without the use of any mirrors. With an HR/6%T cavity, the threshold was reduced to 13 mW. The output was single mode with beam radii of 4.8 μm and 3 μm in the in-plane and out-of-plane direction respectively. Laser emission was also observed at 999.8 nm with a threshold of 8 mW by using mirrors favouring the 999.8 nm transition and forming an HR/5%T cavity.

We report laser-written chip lasers with potential to be a platform planar technology versatile enough to cover the visible through to the mid-infrared spectral region. By femtosecond laser direct-writing a thulium doped fluoride based glass host (ZBLAN), we have demonstrated a 151% quantum efficiency λ=1.9 μm laser with a close to diffraction limited beam quality (M²~ 1.12 ± 0.08) with 225 nm of continuous tunability in a device that can be rapidly fabricated by singlestep optical processing. The 9 mm long planar chip developed for concept demonstration contains fifteen large modearea waveguides that can operate in semi-monolithic or external cavity laser configurations. This chip laser has achieved the highest quantum efficiency from a planar glass waveguide laser. The depressed cladding geometry supports the largest fundamental modes reported for a rare-earth doped waveguide laser thereby favouring high peak-power operation which is demonstrated by achieving 1.9 kW peak-power pulses when Q-switched.

New devices are required to provide effective tools for DIAL or LIDAR measurements from space, which will enable improved mapping of the concentration and distribution of CO₂ in our atmosphere. Here we present characteristics of Er-doped thin film waveguides, with an extended gain bandwidth, which are applicable to planar waveguide power amplifiers for wavelengths around the 1572 nm CO₂ absorption peaks. Planar waveguide films have been fabricated by sputtering of fluorophosphate and tellurite based glasses onto oxidised silicon wafers, and their properties characterized. The deposition parameters for undoped and Er,Yb-doped films have been assessed and studied, achieving losses of <1.5 dB/cm at 633 nm for the as deposited waveguides. A comparison between the two host materials is made and the potential performance discussed.

We report on diode-pumped distributed-feedback (DFB) and distributed-Bragg-reflector (DBR) channel waveguide lasers in Er-doped and Yb-doped Al₂O₃ on standard thermally oxidized silicon substrates. Uniform surface-relief Bragg gratings were patterned by laser-interference lithography and etched into the SiO₂ top cladding. The maximum grating reflectivity exceeded 99%. Monolithic DFB and DBR cavities with Q-factors of up to 1.35×10⁶ were realized. The Erdoped DFB laser delivered 3 mW of output power with a slope efficiency of 41% versus absorbed pump power. Singlelongitudinal- mode operation at a wavelength of 1545.2 nm was achieved with an emission line width of 1.70 0.58 kHz, corresponding to a laser Q-factor of 1.14×10¹¹. Yb-doped DFB and DBR lasers were demonstrated at wavelengths near 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. An Yb-doped dualwavelength laser was achieved based on the optical resonances induced by two local phase shifts in the DFB structure. A stable microwave signal at ~15 GHz with a –3-dB width of 9 kHz and a long-term frequency stability of ± 2.5 MHz was created via the heterodyne photo-detection of the two laser wavelengths. By measuring changes in the microwave beat signal as the intra-cavity evanescent laser field interacts with micro-particles on the waveguide surface, we achieved real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 μm and 20 μm, which represents the typical size of many fungal and bacterial pathogens. A limit of detection of ~500 nm was deduced.

To the best of our knowledge this is the first demonstration of a 2-μm, Ho:YLF laser source based on a mode-locked oscillator/regenerative amplifier combination and a subsequent single-pass amplifier stage to produce picosecond pulse trains at 1-10 kHz rate with up to ~11 mJ of pulse energy. Further pulse energy scaling can be achieved through addition of single-pass amplifier(s). Availability of 2-μm, kHz-rate lasers providing 10-100-mJ, picosecond-scale pulses is critical for the development of short-pulse mid-IR nonlinear optical devices, in general, and for construction of femtosecond OPCPAs for high-harmonic generation and terahertz generation, in particular.

We have achieved ≥ 840 mW continuous-wave (CW) output power from iron-doped zinc selenide (Fe:ZnSe).1 The beam quality was measured to be M2 ≤ 1.2. The laser exhibited a slope efficiency of 47% with no thermal roll-off at maximum output power. Various dichroic mirrors and other spectral filters were inserted into the cavity to discretely tune the output of the laser from 3843 nm to 4337 nm. Demonstration of arbitrary discrete tuning shows that, in principle, Fe:ZnSe is capable of efficient continuously-tunable CW lasing over nearly 500 nm of bandwidth.

In this paper, we report on building and testing a Cr:ZnSe gain-switched amplifier pumped by a Q-switched Ho:YAG laser and seeded by a continuous wave (CW) tunable Cr:ZnSe laser. A 0.5%-doped, Brewster-cut Ho:YAG rod in an actively Q-switched, folded cavity produced 250 μJ pump pulses at 2.09 μm with pulse widths on the order of 400 ns. The seeded single-pass Cr:ZnSe amplifier exhibited output pulse energy as high as 3.8 μJ at 2.45 μm while pumped at a 10 kHz repetition rate. The gain-switched process showed a peak gain of 380 and an extraction efficiency of 1.5%. The system was tunable from 2160 nm to 2560 nm and had gain of 200 over a 400 nm range.

The goal of this work was to design and investigate a Fe:ZnSe laser operating at room and cryogenic (down to liquid nitrogen) temperature. Pumping was provided by a Q-switched Er:YAG laser at the wavelength of 2.94 μm, the output energy 15 mJ, pulse duration 120 ns, and the repetition rate 1 Hz. Q-switched operation was achieved by the Brewster angle cut LiNbO₃ Pockels cell placed between the rear mirror and the Er:YAG laser active medium. The pump radiation was directed into the Fe:ZnSe crystal placed in the vacuum chamber cooled by liquid nitrogen. The resonator was formed by a dichroic pumping mirror (T = 78 % at 2.94 μm and R = 100 % at 4.5 μm), and a concave output coupler (R = 95 % at 4.5 μm, r = 500 mm). Fluorescence spectra and lifetime of the bulk Bridgman-grown Fe:ZnSe crystal in the range from room temperature down to liquid nitrogen temperature were measured as well as the output characteristics of the Fe:ZnSe laser. The shift of the generated spectral line maximum of ~ 400 nm towards the shorter wavelengths was found for the change of temperature from room to the liquid nitrogen. Also the increase of lifetime was measured from 300 ns at the room temperature up to 100 μs at the temperature of 130 K. Maximum of generated output radiation at 130 K was 150 μJ with the central emission wavelength of 4.1 μm. At the room temperature the central emission wavelength of 4.45 μm was measured with the spectral line-width of ~100 nm. The generated output energy was 1.3 mJ. The comparison of results obtained for Fe:ZnSe active material with the new bulk Fe,Cr:ZnMgSe crystal was also made. The results obtained for Fe:ZnSe active material were compared with the investigation of new bulk crystal Fe,Cr:ZnMgSe.

Eye safe laser operation at 1645.55 nm (6077 cm^-1) of resonantly pumped Er:YAG laser systems is demonstrated in cw and Q-switched operation. High brightness diode laser modules emitting at 1532 nm have been utilized as pump sources providing an absorption efficiency of up to 96%. This leads to an overall efficiency of the Er:YAG laser of 30%. For cw operation, 9 W output power is possible at pump power of 30 W while Q-switching results in generation of more than 7 mJ pulses with duration of 60 ns and repetition rate of 500 Hz. The Er:YAG laser systems have been applied for methane detection measurements demonstrating their feasibility for CH₄-DIAL applications

We report on a single-frequency laser oscillator based on a new Er:YLuAG laser crystal which is spectrally suitable for application as a transmitter in differential absorption lidar measurements of atmospheric CH₄. The laser emits singlefrequency laser pulses with 2.3 mJ of energy and 90 ns duration at a repetition rate of 100 Hz. It is resonantly pumped by two linearly polarized single-mode cw fiber lasers at 1532 nm. A scan of the CH_4-absorption line at 1645.1 nm was performed and the shape of the line with its substructure was reproduced as theoretically predicted. A 2.5-dimensional performance model was developed, in which pump absorption saturation and laser reabsorption is included. Also the spectral output of the laser oscillator longitudinal multimode operation could be predicted by the laser model.

We had demonstrated the use of Coherent Polarization Locking (CPL) to mitigate the intra-cavity optical damage in a Q-switched Ho:YAG laser. By splitting the available pump power into two individual Ho:YAG laser rods, we had passively coherent combined two orthogonal polarized lasers with output pulse energy of 9.13mJ, pulse width of 14ns, operating at 800Hz pulse repetition rate. In contrast, when all the pump power was pumped on a single Ho:YAG laser rod built with the same cavity configuration, severe optical damage was observed. It was occurred at the surface coating of the laser rod when operating at < 2kHz pulse repetition rate, thus limiting the output pulse energy to < 5mJ. We also demonstrated, first to our knowledge, that by performing Q-switching only in one of the laser arms, it resulted in pulse operation for the entire CPL laser cavity.

In this paper, we present our recent results in the development of Ho³⁺ doped sesquioxides for eye-safe solid state lasers. We have synthesized optical quality Lu₂O₃ nanopowders doped with concentrations of 0.1, 1.0, 2.0, and 5% Ho³⁺. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nanopowders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.

We demonstrate mid-infrared (mid-IR) supercontinuum generation with bandwidth from 2 to 2.8 μm at 20 dB below the peak in nonlinear step-index chalcogenide fiber using femtosecond mid-IR pulses directly from the oscillator. We compare the results with a supercontinuum generated in a silica-based high germanium content fiber. Supercontinuum generation occurs at 90 mW of launched average pump power that is equal to the 0.9 nJ pulse energy. The distinctive feature of the obtained supercontinuum is its stability and coherence due to the deterministic supercontinuum generation by the femtosecond pump pulses

In this work, a detailed analysis and redesign of a tunable UV laser is presented. The laser is part of measurement system of “IEK 8, Forschungszentrum Jülich” for airborne LIF analysis of the OH-radical concentration. The design concept of the laser comprises a frequency doubled Nd:YAG laser as pump source, a dye as active medium to emit light at 616 nm, and a NLO crystal as intracavity frequency doubler. The output wavelength is tunable by a combination of dispersion prisms and an etalon. During measurement campaigns, the laser is mounted on top of Zeppelin NT and therefore is exposed to temperatures ranging from 10 to 40 °C and ambient pressures from 800 to 1000 hPa. In former flights the output power of an existing laser decreased rapidly and the wavelength was unstable during the flights and therefore hinders continuous measurements. The analysis of the existing laser combines a theoretical study of tolerance requirements with experimental testing of opto-mechanical components and of the entire laser system in a climatic test chamber. The performance of the laser is measured over the expected temperature range. It is shown that changing the baseplate temperature by a few Kelvin stops laser emission completely. The optical mounts that are used in the laser and worthwhile alternatives were tested separately in the climatic chamber. The stability of the best mounts exceeds those currently used by a factor of 50. A new laser has been built based on the results of the analysis and further experiments for an optical redesign. This laser was on a field campaign for several weeks and worked reliably.

Fibertek has designed and is building two space-based lasers for NASA’s CATS-ISS mission. This space-based lidar system requires lasers capable of provide 4-5 kHz output at 1064 nm, 532 nm and 355 nm with each wavelength having ~2-2.5 mJ pulse energy. The lasers will be based on the ISS for a mission lasting up to 3 years.

Fibertek has developed an environmentally hardened Technology Readiness Level-6 laser transmitter system for the NASA Ice, Cloud and land Elevation Satellite-2 (ICESat-2). The laser transmitter generates over 9 W of 532 nm output with a pulse repetition rate of 10kHz and a FWHM pulse width of < 1.5 ns with an expected lifetime of > 1 trillion shots. This paper presents the results of the Structural, Thermal and Optical analysis, details on the NASA General Environmental Verification Specification testing requirements, and the success of the laser transmitter performance through vibration and thermal vacuum testing.

We have developed and successfully flown a 16-beam, non-scanning laser altimeter instrument with a swath width of 80 m and spatial resolution of 5 m. The Airborne Lidar Surface Topography Simulator (ALISTS) instrument was developed to demonstrate key technologies and a measurement approach achieving the efficiency required for the Lidar Surface Topography (LIST) mission. The approach employs a 10 kHz, near-infrared, microchip laser transmitter, beam splitting optics and waveform capture using a photon-sensitive, linear-mode detector array. In this paper we will present the instrument development effort and access the performance achieved during our two airborne campaigns.

A diode-pumped Nd:YLF laser system that produces nanosecond pulses at fundamental (1053-nm), second (527-nm), and third (351-nm) harmonic wavelengths with the energies of hundreds of millijoules depending on pulse width/shape and wavelength has been demonstrated. These pulses can be arbitrarily shaped in the 1- to 10-ns temporal range. Excellent energy and Gaussian beam far-field pointing stability have been demonstrated. Extensive temporal, energy, and beam profile output diagnostics are provided.

Self-Q-switching (SQS) of lasers enable the generation of Q-switched pulses from simple laser cavities without using any additional saturable absorbers or active modulators. Earlier studies have reported SQS in ruby, Nd:YAG, and Cr:LiSAF lasers. However, these systems were mostly flashlamp pumped and required cooling of the crystal and/or misalignment of the laser cavity for the observation of SQS. In this presentation, for the first time to our knowledge, we report SQS operation of a Cr:LiCAF laser. SQS was achieved in an astigmatically compensated x-cavity containing only a Cr:LiCAF crystal that was end-pumped with a 140-mW continuous-wave (cw) diode at 660 nm. During regular cw operation, the laser produced a diffraction-limited beam with 50 mW of output power and had a spectral width of 0.5 nm near 795 nm. SQS operation of the Cr:LiCAF laser could be initiated by fine adjustment of the separation between the curved mirrors of the cavity and was observed at several mirror separations within the stability range of the resonator. During SQS operation, the laser generated saw-tooth-shaped pulses with 20-30 microsecond duration in the 780-800 nm wavelength range, at repetition rates between 10 and 30 kHz. SQS operation was further accompanied with a decrease in the output power to the 30-45 mW range. In this regime, the output beam became multimode and spectral broadening up to 12.5 nm (FWHM) was observed.

Multiple individually-controllable Q-switched laser outputs from a single diode-pumped Nd:YAG module are presented by using an electrostatic MEMS scanning micromirror array as cavity end-mirror. The gold coated, 700 μm diameter and 25 μm thick, single-crystal silicon micromirrors possess resonant tilt frequencies of ~8 kHz with optical scan angles of up to 78°. Dual laser output resulting from the actuation of two neighboring mirrors was observed resulting in a combined average output power of 125 mW and pulse durations of 30 ns with resulting pulse energies of 7.9 μJ and 7.1 μJ. The output power was limited by thermal effects on the gold-coated mirror surface. Dielectric coatings with increased reflectivity and therefore lower thermal stresses are required to power-scale this technique. An initial SiO₂/Nb₂O₅ test coating was applied to a multi-mirror array with individual optical scan angles of 14° at a resonant tilt frequency of 10.4 kHz. The use of this dielectric coated array inside a 3-mirror Nd:YAG laser cavity led to a single mirror output with average Q-switched output power of 750 mW and pulse durations of 295 ns resulting in pulse energies of 36 μJ.

Solid state pulsed laser systems are of interest for industrial applications. Flashlamps are an effective method for pumping solid state pulsed laser systems. Flashlamp lifetime is hard to quantify past the specification provided by the manufacture and is of concern for applications that are not used or tested on a frequent basis. The flashlamp lifetime can be shortened by three main failure modes: manufacturing quality escapes, shipping and handling damage, and shelf life. Manufacturing and shipping failure modes will be the focus of this research. Manufacturing and shipping failure modes are hard to detect, beyond the obvious non-functioning flashlamp, without testing to failure, which is not a feasible option. A method is being proposed that can estimate the lifetime of flashlamps as well as other key characteristics of the flashlamp while a relatively low number of shots are taken with the flashlamp. Fill pressure and fill gas will be determined by monitoring the input voltage, current, and output spectrum with comparison to the arc length, bore diameter, wall thickness and electrode configuration. Flashlamp lifetime estimations will be determined by monitoring the current, wavelength shift, and output intensity. Experimental results will be discussed focusing on the characteristics and lifetime estimations of flashlamps.

Composite, core-doped, and ceramic crystals are used to reduce the thermal lensing effect in laser crystals. An accurate simulation of laser resonators is needed to nd optimal doping structures for a required laser. Also simulation helps apply these crystals efficiently. In this work, results of simulations performed on resonators containing core-doped Nd:YAG ceramic crystals with radial dependent doping concentration, and composite crystals with different layers of doping concentration are presented. The absorbed pump light in crystals is simulated using the ray tracing algorithm. The thermal lensing effect has been computed by the nite element analysis of the temperature and deformation. The dynamic mode analysis (DMA) was used to compute the laser beam quality and output power. The DMA solves rate equations separately for each Gauss mode. It also calculates population inversion by solving rate equations on nite volume grids. This approach yields a few thousands of rate equations for population inversion on the nite volume grid. The advantage of this method for both mentioned crystals is that different decay rates can be associated to regions with different doping concentration. Simulation results have shown that sophisticated laser crystals are able to reduce thermal lensing effect and increase beam quality.

The use of optical quality ceramics for laser applications is expanding, and with this expansion there is an increasing need for diagnostics to assess the quality of these materials. Ceramic material with flaws and contaminants yields significantly less efficient performance as laser gain media and can generate excessive amounts of waste heat. This is a concern that is especially relevant in high power laser applications where thermally induced damage can be catastrophic. In order to assess a set of ceramic and crystalline samples we induce and measure thermal lensing in order to produce a relative ranking based on the extent of the induced thermal lens. In these experiments thermal lensing is induced in a set of nine 10% Yb:YAG ceramic and single-crystal samples using a high power 940 nm diode, and their thermal response is measured using a Shack-Hartmann wavefront sensor. The materials are also ranked by their transmission in the visible region. Discrepancies between the two ranking methods reveal that transmission in the visible region alone is not adequate for an assessment of the overall quality of ceramic samples. The thermal lensing diagnostic technique proves to be a reliable and quick over-all assessment method of doped ceramic materials without requiring any a priori knowledge of material properties.

The refractive indices of Yb:YAG crystals for 0, 1, 3, 5, 7, and 10 at. % Yb were measured from 0.4-5.2 microns. The coefficients for the Sellmeier fit are reported. The absorption at room temperature was measured from .185-3.3 microns. Using the index and spectroscopic data, the oscillator strengths for the samples were calculated, and their dependence on dopant concentration examined.

We experimentally examined our proposed structure for realizing the control of resonant and lasing properties even in random structures, which was composed of size-mono-dispersive scatterers and intentionally introduced defect regions. In the experiments, by intentionally introducing polymer nanoparticles as point defects into a mono-dispersive zinc oxide nanoparticle film, we succeeded that lasing properties at the defect region were drastically modified, comparing with those of typical random lasers; suppression of the number of lasing modes, decrease in the thresholds, and limiting the lasing position at the defect. These results suggest the possibility that we can realize single-mode random lasers with well-controlled modal properties even in random structures.

In the design of mode-locked lasers, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) have emerged as important alternatives to semiconductor saturable absorber mirrors (SESAMs) due to their favorable optical characteristics, low cost, and relatively simple fabrication scheme. Therefore, it is of great interest to explore the limits of energy scaling in solid-state lasers mode-locked with SWCNT-SAs. Due to their unique wavelength range for biomedical applications, a room-temperature Cr⁴⁺:forsterite laser operating near 1.3 μm was used in the mode-locking experiments. The laser was end-pumped with a continuous-wave Yb-fiber laser at 1064 nm. Furthermore, a q-preserving multipass-cavity (MPC) was added to the short resonator to lower the pulse repetition rate to 4.51 MHz and to scale up the output pulse energy at low average power. The SWCNT-SA was fabricated with SWCNTs grown by the highpressure CO conversion (HiPCO) technique. With dispersion compensation optics, the net group delay dispersion of the resonator was estimated to be around -4440 fs². When mode-locked with the SWCNT-SA, the resonator produced 10-nJ, 121-fs pulses at 1247 nm with a spectral bandwidth of 16 nm, corresponding to a time-bandwidth product of 0.37. To our knowledge, this represents the highest peak power (84 kW) generated to date from a bulk femtosecond solid-state laser, mode-locked by using a SWCNT-SA. The results also suggest that the peak power achieved in our experiments was limited only by the self-focusing in the Cr⁴⁺:forsterite gain medium and further increase in output energy should in principle be possible in other gain media mode-locked with SWCNT-SAs.

We demonstrate a femtosecond Yb:YAG InnoSlab laser amplifier producing 450-μJ pulse energy at 500kHz pulse repetition rate. Ultrafast operation at high energy and high average power could be obtained without the use of chirped pulse amplification on the power amplifier end. The laser setup consists of a compact and robust femtosecond fiber based seed laser producing up to 4μJ pulses with pulse durations down to 260fs. Pulses from the seed source are directly amplified to high pulse energy and average power in an optimized Innoslab amplifier. This laser source is ideally suited for large scale industrial applications requiring high average power ultrashort pulses for high throughput and productivity.

A room-temperature Kerr-Lens modelocked (KLM) Cr:ZnS laser generates <70 fs pulses duration (about eight optical cycles) with 5.6 nJ pulse energy and over 100 nm FWHM spectral width at 105-157 MHz repetition rates. The laser produces 1 W average output power at 20% optical efficiency, limited by the available Er:fiber pump. For further pulse energy scaling we also realized the chirped-pulse regime, with 0.8-2 ps pulse durations. The demonstrated applications of such mid-IR source range from extra- and intra-cavity spectroscopy to subharmonic OPO pumping. For environmentally-protected delivery we suggest and realize duration-preserving soliton delivery in a ZBLAN fiber. Further bandwidth increase is demonstrated by 2.0-2.8 μm supercontinuum generation in a chalcogenide fiber.

A repetitive petawatt-class Ti:sapphire laser system operating with high spatial and temporal beam quality is demonstrated. Maximum pulse energy of 30 J is obtained via five multi-pass amplification stages. Closed-loop feedback control systems in the temporal and spatial domains are used to yield Fourier-transform-limited pulse duration (33.7 fs), and diffraction-limited focal spot sizes (with several different tight focusing optics). The laser parameters have been fully characterized at high-power, and are monitored in real-time, to ensure that they meet the experimental requirements for laser-wakefield electron acceleration and x-ray generation.

Simple stable laser resonators with a single Yb:YAG thin disk module have been designed and demonstrated to produce up to 5 kW CW laser output at 1030 nm with M² factor of 7. Pumped with 940 nm diodes, the optical-to-optical efficiencies were >50 % at full power. Simple I and V-shaped resonators consisting of only two and three optical elements were implemented, including the 16 mm diameter Yb doped thin disk acting as an active mirror. No additional adaptive optics for aberration or mode control was used; instead the results were achieved with laser cavity designs that take into account the changing radius of curvature of the pumped thin disk. The designs ensured the laser always operated well within the stable cavity zone and with an optimised and relatively large fundamental laser mode size on the thin disk. The low optical aberrations and effective thermal management of the thin disk, mounted on a diamond cooled heat sink, together with the above cavity design approach, enabled the realization of such high power and good beam quality thin disk laser in a simple single disk laser oscillator.

A Tm:YLF laser in INNOSLAB design is reported. It produces 200 W of output power at an optical efficiency of 24 % and a slope efficiency of 27 % with respect to incident pump power. The laser crystal is partially end-pumped in a tophat line focus with a width of 12 mm and a height of about 1 mm. It is placed in a stable, spherical laser resonator, which results in a highly elliptical output beam. The beam is near diffraction limit and Gaussian in shape in one axis and contains very high order transversal modes and is Top-Hat-like in shape in the other axis. The beam shape is ideal for pumping a Ho:YLF laser crystal in INNOSLAB design for a pulsed amplifier.

We demonstrate experimental results on the angular dependence of the optical axis in biaxial materials from 430 nm to 1580 nm. Using pure KGd(WO₄)₂ (KGW) and Neodymium doped KGW crystals of around 1 cm in length. The variation of the angle is about 2.4° over the above wavelength range. Within the investigated spectral domain, a rotation of the index ellipsoid along the crystallographic b-axis is not observed, the variation occurs only in one plane. Moreover, no significant influence of doping of the crystal was observed. Comparison with literature refractive index data results in a difference of up to 0.5° with our data based on an arbitrary reference chosen at 560 nm, showing the importance of this measurement for conical refraction applications.

This paper highlights recent developments in resonant optical devices for infrared (IR) and mid-infrared (mid- IR) lasers. Sub-wavelength grating based resonant optical filters are introduced and their application in 2 μm thulium fiber laser and amplifier systems has been discussed. The paper focuses on applying such filtering techniques to 2.8 μm mid-IR fiber laser systems. A narrowband mid-IR Guided-Mode Resonance Filter (GMRF) was designed and fabricated using Hafnium(IV) Oxide film/quartz wafer material system. The fabricated GMRF was then integrated into an Erbium (Er)-doped Zr-Ba-La-Al-Na (ZBLAN) fluoride glass fiber laser as a wavelength selective feedback element. The laser operated at 2782 nm with a linewidth less than 2 nm demonstrating the viability of GMRF’s for wavelength selection in the mid-IR. Furthermore, a GMRF of narrower linewidth based on Aluminum Oxide/quartz wafer material system is fabricated and tested in the same setup. The potentials and challenges with GMRFs will be discussed and summarized.

We designed a cost effective, highly efficient diode-pumped and Q-switched 532nm laser with large laser output power range and fast laser output power switching from zero Watts to any user defined operating power by adjusting the pump diode current. It is well known that pump diodes used in the solid-state Nd:YAG lasers have center wavelength around 808nm or 885nm and will shift their center wavelength while changing its drive current. The pump diode wavelength shift can be large enough to move outside of the laser gain medium absorption band. This results in lower pump absorption efficiency and hence in lower overall system optical to optical conversion efficiency for some portion of the laser output power range. A few typical ways to minimize this wavelength-shift-caused lower pump absorption effect with their trade-offs have been discussed. We report the unique pump diode wavelength of 879nm and single Nd:YAG rod design for this laser system. This system has about 38% optical to optical (879nm to 532nm) conversion efficiency. The pump current can be adjusted to tune the laser output power to anywhere within the range of 20W to 300W. The laser output power responds within 2 seconds to this input current change, and does not result in Nd:YAG rod damage. Furthermore, a high pump light absorption efficiency is maintained.

High power, kW-class, 808 nm pump modules based on the vertical-cavity surface-emitting laser (VCSEL) technology were developed for side-pumping of solid-state lasers. Two 1.2 kW VCSEL pump modules were implemented in a dual side-pumped Q-switched Nd:YAG laser operating at 946 nm. The laser output was frequency doubled in a BBO crystal to produce pulsed blue light. With 125 μs pump pulses at a 300 Hz repetition rate 6.1 W QCW 946 nm laser power was produced. The laser power was limited by thermal lensing in the Nd:YAG rod.

A compact UV pulsed laser pumped by high-power two-dimensional arrays of vertical-cavity surface-emitting lasers (VCSELs) was presented. A passively Q-switched 1064-nm Nd:YAG laser was side-pumped by kW-class 808-nm VCSEL pump modules and the output pulses were frequency quadrupled to 266-nm. 10-ns, 0.68-mJ UV pulsed output was obtained at 1.33k-Hz repetition rate. The average UV power is < 0.9-W. This VCSEL pumped high power UV laser source provides a reliable, low-cost and low-profile solution for military and commercial applications including remote sensing, laser processing and spectroscopy.

A light source based on blue laser and phosphor wheel is developed for portable projection displays. The phosphor wheel has multiple phosphor segments that can convert the blue laser to different color light. When the phosphor wheel is rotating, sequential color light will be generated. A color filter set is integrated with the phosphor wheel; therefore the color coordinates of each color light can be controlled to fulfill the requirements such as Rec. 709 standard. Thermal quenching of the phosphor is studied and improved to enhance the energy efficacy of this light source. Engineering samples were made to do performance, lifetime and reliability tests.

We have fabricated prototype frequency tripled Nd:YAG lasers using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping. The passively Q-switched Nd:YAG laser generated 15mJ pulses with a duration of 2-4 ns. Used as a source for third harmonic generation, the laser produced in excess of 2mJ at 355nm. Of particular concern was the impact of temperature variation on conversion efficiency, which included effects for both the source laser and non-linear crystals. Various solutions to the temperature effects were explored to enable operation of the frequency tripled laser over a wide temperature range.

A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.

A femtosecond thin-disk Yb:YAG oscillator in both the anomalous and the normal dispersion regime is demonstrated. Both regimes are realized with practically the same resonator configuration. The power scaling potential of the anomalous and normal dispersion regimes is analyzed both theoretically and experimentally. The recipe to obtain Kerr-lens mode-locking (KLM) in the thin-disk configuration is presented here and oscillator characteristics as well as start-up difficulties are described. The oscillator stability in terms of output power, beam pointing and sensitivity to back reflections is measured and corresponds to the level of commercial systems.

We report on initial testing of an edge-pumped erbium-based disk laser operating at 1.53 micron. The laser uses a single laser disk having a composite glass construction with erbium-ytterbium co-doped center and undoped perimetral edge designed to channel pump light. Erbium is pumped to a laser transition by 940-nm diode radiation, which is first absorbed by ytterbium with subsequent energy transfer to erbium. This work presents results of initial testing of the laser with resonator optics configured for power extraction with two passes through the disk.

It is well–known that amplified spontaneous emission (ASE) can be a major source of upper laser level loss in high gain pulsed or steady–state solid state lasers. This paper briefly reviews the theory of ASE and, using a simple rate equation model of the upper laser level, a geometric, radiative transport equation to describe the ASE intensity, and the perturbation method of multiple time scales, demonstrates that the loss rate of the upper laser level due to ASE adiabatically follows the spontaneous emission source term. This result which includes gain saturation is applicable to both quasi–three level and four level lasers and rigorously justifies formally using the steady–state expression derived heuristically by Lowenthal and Eggleston¹ to model ASE loss in pulsed laser media. Then, it is shown that the frequency integral occurring in the ASE loss term can be evaluated analytically for both a broad “flat–top” and a Lorentzian stimulated emission lineshape but must be evaluated numerically or using an approximation due to Tommasini and Balmer² for a Gaussian stimulated emission lineshape. It is shown that at high gain loss due to ASE is mitigated by ASE line narrowing. For a thin disk laser an approximate expression for the rate of ASE loss (or ASE lifetime) can be obtained by evaluating the remaining volume integral using either the method of Speiser³ or of Vretenar et al⁴. A new approximate expression for the ASE loss rate is obtained which, unlike Speiser’s³ expression, accounts for ASE line narrowing and, unlike Vretenar et al’s⁴ expression, correctly scales with the cylindrical volume of the disk. Application to both 1D and 3D laser modeling is briefly discussed.

We are developing high-energy thin-disk regenerative amplifier operating at kilohertz repetition rate delivering 1-2 ps long pulses for science and industry applications. We have developed a prototype Yb:YAG thin disk regenerative amplifier which can operate up to 10-kHz repetition rate pumped by the 940-nm laser diode. The pulse energy was achieved to 5-mJ at 10-kHz operation and to 29.5-mJ at 1-kHz operation. We found that the pulsed pumping method can improve the optical-to-optical efficiency. We demonstrated the zero-phonon-line-pumped (968.825-nm¹) regenerative amplifier at 1-kHz, and obtained the output energy of 32-mJ with nearly diffraction limited beam profile.

We report about the development of all-solid-state laser source of narrowband UV-B light for medical applications. The device is based on a gain-switched Ti: Sapphire laser with volume Bragg grating, pumped at 532 nm and operating at 931.8 nm, followed by a third harmonic generator and a fiber optic beam homogenizer. The maximum available pulse energy exceeded 5 mJ at 310.6 nm, with a pulse repetition rates of 50 Hz. The output characteristics satisfy the medical requirements for psoriasis and vitiligo treatment. A new optical scheme for third harmonic generation enhancement at moderate levels of input intensities is proposed and investigated. As a result, 40% harmonic efficiency was obtained, when input pulse power was only 300 kW.

We report efficient continuous-wave laser operation of laser-diode pumped Pr:YAlO₃ crystal in the power-scaled resonator arrangement. Using two pumping GaN-laser-diodes emitting at 448 nm wavelengths with a maximum output power of 1 W each, 290 mW of Pr:YAlO₃ output power in the near-infrared spectral region (747 nm) with oscillation threshold of about 500 mW has been reached. The maximum absorbed pump power was approximately 1.5 W resulting in the slope efficiency of 28 %. Moreover, switchable laser operation at two wavelengths (747 nm and 720 nm) has been demonstrated employing the broadband laser resonator mirrors in connection with the tuning element (Lyot filter).

Ytterbium doped phosphate glass (5 mol% of Yb₂O₃, thickness 1 and 2 mm, uncoated), developed for fibre laser, was tested in bulk form as a laser active medium. For Yb:glass pumping a fibre coupled (fibre core diameter 100 μm, NA = 0.22) laser diode with emission at wavelength 975nm was used. The laser diode was operating in pulsed regime (pulse length 1.5 ms, repetition rate 5 Hz, maximum pulse energy 22 mJ) with low duty cycle to reduce a heat accumulation inside the active medium (Yb:glass was only air-cooled). Longitudinally pumped Yb:glass samples were placed inside the 145mm long semi-hemispherical resonator formed by a flat pumping mirror (HR @ 1.0 − 1.1 μm, HT @ 0.97 μm) and by curved output coupler (r = 150 mm). Set of output couplers with reflectivity 91 – 97% @ 1.0 − 1.1 μm was used. Tuning of the Yb:glass laser was accomplished by using a birefringent filter (single 1.5mm thick quartz plate) placed inside the optical resonator at the Brewster angle between the output coupler (reflectivity 97%) and laser active medium. In untuned regime the energy up to 2.4mJ was obtained at wavelength 1050 nm. Slope efficiency up to 17% in respect to absorbed pumping was reached with the 2mm sample. In tuned regime the smooth laser tuning curve, limited by used Lyot filter, extended from 1005nm up to 1080nm (FWHM 48 nm). The maximum output energy of 1.65mJ was obtained at 1060nm for the absorbed energy 16 mJ.

The temporal intensity profile of pulse(s) from passively Q-switched and passively Q-switched mode locked (QSML) solid-state lasers is known to be dependent on cavity length. In this work, the pulse width, modulation depth, and beat frequencies of a Nd:Cr:GSGG laser using a Cr⁺⁴:YAG passive Q-switch are investigated as function cavity length. Measured temporal widths are linearly correlated with cavity length but generally 3-5 ns larger than theoretical predictions. Some cavity lengths exhibit pulse profiles with no modulation while other lengths exhibit complete amplitude modulation. The observed beat frequencies at certain cavity lengths cannot be accounted for with passively QSML models in which the pulse train repetition rate is τ_RT^-1, τ_RT= round-trip time. They can be explained, however, by including coupled cavity mode-locking effects. A theoretical model developed for a two section coupled cavity semiconductor laser is adapted to a solid-state laser to interpret measured beat frequencies. We also numerically evaluate the temporal criterion required to achieve temporally smooth Q-switched pulses, versus cavity length and pump rate. We show that in flash lamp pumped systems, the difference in buildup time between longitudinal modes is largely dependent on the pump rate. In applications where short pulse delay is important, the pumping rate may limit the ability to achieve temporally smooth pulses in passively Q-switched lasers. Simulations support trends in experimental data. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The goal of our research is a compact Raman laser emitting short pulses with high energy and peak power in “eye-safe" region around wavelength 1.5 μm. We utilize intracavity conversion of giant pulses at wavelength 1.34 μm in a BaWO₄ Raman crystal (18 mm long, AR coated). Required high energy and peak power was reached using a flash-lamp pumped Nd:YAG laser (rod 100 mm long, diameter 4 mm), Q-switched by V:YAG solid-state saturable absorber (initial transmission 37% @ 1.34 μm). The L-shaped oscillator for 1.34 μm radiation consisted of a concave mirror (r = 0.5 m, HR @ 1.3 μm, HT @ 1.06 μm), flat polarizing intracavity mirror, and output coupler (r = 1 m, HR @ 1.3 μm, R = 39 % @ 1.5 μm). The polarizing mirror ensured stable linearly polarized laser emission and prevented parasitic oscillations at 1.06 μm. The Raman laser oscillator was formed by the output coupler and another intracavity mirror (r = 0.5 m, HR @ 1.5 μm, HT @ 1.3 μm), inserted between BaWO₄ and the polarizing mirror. For pumping energy of 28.2 J stable vertically polarized generation of the 1st Stokes radiation at 1528 nm was reached. In multimode operation the output energy was 20 mJ in 2.25 ns pulses. Single mode operation was possible by inserting a 1.5 mm aperture between Nd:YAG and V:YAG crystal. The output energy dropped to 9.7 mJ (even for higher pump power of 30.7 W) and output pulses were shortened to 1.87 ns.

This paper will get to the bottom of the mechanism of a superior inject seeding technology, and take it even further, from the solid state laser into the fiber laser configuration. This low jitter, single frequency Q-switched solid state laser with precisely controllable firing time was realized, developed and reported previously, in which the oscillator can output energy of near 100 mJ and the master oscillator power amplifier (MOPA) reaches the output energy of 300 mJ, operating at a wavelength of 1064 nm, with a pulse width of 10 ns and in near single transversal mode. Comparing two existing commercial techniques, ramp-and-fire and pulse-to-pulse buildup time reduction, this report presents a full understanding of using a CW transformed injection seeding method in which this technique is immune to mechanical vibration or thermal expansion, and it is able to precisely control the high peak energy launching time within a nanosecond jitter and achieve single frequency operation at the same time. It is carefully observed that the CW seeding mechanism is similar but not equivalent to a pulsed seeding with pulse width shorter or equal to the ring cavity length. The advantage of the realized regime is that in stable laser operation there is no need to adjust the slave cavity length to match the seeded light longitudinal mode. Therefore, the extremely strict mechanical requirement can be relaxed. It is found that the slave laser frequency follows exactly to the injected seeded laser’s frequency which can also provide frequency tuning, control and locking.

This report presents the result of the laser beam profile progression in target-in-the-loop (TIL) system. This simulation experiment is to verify whether it is possible to form a tight hot spot similar to a single transversal mode in an extended laser cavity. Therefore, it is very important to observe the progression of the laser profile at a laser cavity mirror when a seeded high energy laser pulse is injected into the TIL system. The extended laser cavity is formed with a high reflectivity mirror on one end and an optical phase conjugated mirror as the second mirror, with potential disturbance media inside. The laser oscillation occurs only when it is triggered with a single frequency high energy laser pulse to overcome the threshold condition. With a laser cavity length of around 11 meters and a seeded laser pulse of 10 ns, we have been able to acquire and distinguish the laser beam profiles of each round-trip. Inserting a scattering media and other distortion elements can simulate atmospheric effects.

We employ an intracavity self-mode-locked laser to systematically measure the group refractive indices and the thermo-optic coefficients for Nd:GdVO4, Nd:YVO4 and Nd:LuVO4 crystals at 1064 nm. We further make a detailed comparison between the present results and those currently reported in the literature. All the experimental results can be found to be fairly consistent with the recent reported data

The aim of the presented work is to demonstrate operation of a Cr:ZnMgSe laser pumped by a Er:YLF laser. Laser output characteristics are compared with a Cr:ZnSe laser operated under similar conditions. Pumping 1.73 μm Er:YLF laser (Er:YLF rod 80 mm long, flashlamp-pumped) radiation was focused into the Cr:ZnMgSe or Cr:ZnSe active crystal (thickness 4.9 mm and 2.2 mm, respectively; absorption coeficient 4.5 and 10.9 cm^-1 @ 1.73 μm, respectively). The active crystal was inserted into the stable non-selective optical resonator or into the resonator with a wavelength-selective element. The 65 mm long non-selective resonator was formed by a flat dichroic pumping mirror (HT @ 1.74 μm, HR @ 2.4 μm), and a concave output coupler (R = 82 % @ 2.4 μm, r = 500 mm). The maximal output pulse energy and efficiency with respect to the absorbed pumping energy were measured 5.8 mJ and 36 % for the Cr:ZnMgSe laser, and 10.2 mJ and 76 % for the Cr:ZnSe laser. Central emission wavelength was 2.46 μm and 2.4 μm for the Cr:ZnMgSe and Cr:ZnSe laser, respectively. The spectral line-width was about 55 nm in both cases. In the case of 80 mm long output wavelength selective resonator the MgF₂ Lyot filter was inserted between the active crystal and the output coupler. Output wavelength tuning was obtained in the range 2.35 – 2.55 μm and 2.29 - 2.53 μm for the Cr:ZnMgSe and Cr:ZnSe laser, respectively. Maximum output energy was ~ 4 mJ in both cases.

Development of laser systems capable of surviving extreme conditions experienced in military applications requires mounts and components that are able to survive these conditions. The characterization of mounted and/or bonded optical assemblies in harsh environments is critical for the development of laser and optical systems for functionality in these extreme conditions. Customized mounts, bonding assemblies and packaging strategies are utilized to develop and field reliable and robust optical subassemblies. Thin film polarizers operating at 45° and polarizing beam splitter cubes were chosen for initial testing based on past experiences, advancements in optical coating and construction technologies and material properties. Shock, vibration, shear strength, tensile strength and temperature testing are performed on mounted polarizing beam splitter cubes and thin film polarizers from two manufacturers.
Previous testing showed that polarizing beam splitter cubes constructed using epoxy would become damaged in the laser resonator. The cubes being tested in this report are constructed using epoxy- free direct optical contact bonding. Thin film polarizers operating at 45° are chosen opposed to Brewster’s angle thin film polarizers to reduce the size and simplify design and construction since an optical wedge is not required. The components and mounts are each environmentally tested beyond the manufacturers’ specifications for shock, vibration, and temperature. Component functionality is monitored during and after the environmental testing. Experimental results from the testing will be discussed as will the impact on future laser resonator designs.

This paper reports a high power diode pump 5th harmonic generation of Nd: YAG laser system, which could generate up to 300 mW TEM00 mode output with 100 Hz repetition rate at 213 nm. A diode pump module was specially designed for a high efficiency and good beam quality at the fundamental wavelength 1064 nm. An amplifier was set up out of the cavity to boost up the energy level of fundamental wavelength. In order to get high efficiency of the 5th harmonic generation, the cavity of the fundamental wavelength is EOM Q-switched which could generate very high peak power of the fundamental wavelength laser for extra cavity harmonic generations. Finally, 14% conversion efficiency from IR to UV was achieved, which is the highest efficiency in the market to the best of our knowledge right now. 213 nm is a very good substitute wavelength of 193 nm for different UV applications, the system of which is more compact, higher energy, less maintenance and better beam quality than the system of 193 nm.

Diode end-pumped solid-state lasers have the potential to yield high quality laser beams with high efficiency for laser range finding and warning receiver applications as well as day and night military laser designation systems. This paper presents theoretical calculations using Advanced Dynamics Professional LASCAD software and experimental studies for a high power pigtailed fiber diode laser module of 8 W operating at 808 nm with a specially designed high efficiency cooling system, end pumped high-efficiency Nd:YVO₄ laser of 3×3×10 mm rod and overall cavity length of 44 mm. An optical to optical slope efficiency of around 76%, the final conversion efficiency of 60% and a maximum average output power of 4.11 W at 1064 nm laser were investigated with a high single-mode beam quality factor M² of < 1.1, and the lowest threshold pumping power of 800 mW was measured, which was in agreement with the theoretical calculations of 776 mW. A high power second harmonic Nd:YVO₄ CW laser at 532 nm (green laser) was achieved with an optical to optical slope efficiency of 66% and a final conversion efficiency of 56% using a 5×5×8 mm KTP crystal placed intracavity of the same 1064 nm CW laser with water cooling system. The green laser average output power of 3.38 W at incident diode laser power of 6 W was measured. To the best of our knowledge a self Q-switching effects was generated in Nd:YVO₄ laser by changing the cavity dimensions and the position of the intracavity KTP crystal at certain regime of operation for the first time, in which the cavity length is reduced to be 30 mm and the distance between Nd:YVO₄ rod and KTP crystal is only 1mm. Self Qswitched laser pulse at 532 nm with high peak power of 96 W, pulse width of 88 ns at FWHM and repetition rate of 400 kHz was achieved. Experimental studies of a passive Q-switched Nd:YVO₄ laser using Cr:YAG crystal with three different transmissions of 30%, 40% and 70% were investigated. Passive Q-switched laser pulse at 1064 nm and narrow line width of less than 1.5 nm with highest peak power of nearly 18 kW, short pulse width of less than 4 ns at FWHM and higher repetition rate of 45 kHz using Cr:YAG with transmission of 30% was achieved for the first time. The main application for the investigated effect is generation of an efficient and high repetitive high peak power self Q-switched second harmonic Nd:YVO₄ laser system that can be realized without the need of using any of passive Q-switched semiconductor saturable absorber mirror SESAM.

We are developing a high-repetition and high-average-power Nd:YAG laser amplifier pumped by fiber coupled LDs in order to apply to laser machining of carbon composite materials such as CFRP (Carbon Fiber Reinforced Plastic). Final target is 1.5 kW output at wavelength of 1064 nm by 8 kW LD pumping. The amplifier consists of several Nd:YAG ceramic thin discs on a non-doped ceramic YAG block. At first, we measured wave-front distortion and small-signal gain of a prototype amplifier whose target is more than 500 W output.

We report on the spectroscopic characteristics and upconversion emission in Er³⁺ doped Yttria (Y₂O₃) transparent ceramics prepared by a modified two-step sintering method. The near-infrared (1.5 μm) emission properties were evaluated as a function of Er³⁺ concentration. Judd-Ofelt intensity parameters, radiative rates, branching ratios, and emission lifetimes were calculated and compared with results reported for Er³⁺ doped Y₂O₃ single crystal and nanocrystals. Following pumping at 1.532 μm, weak blue (~0.41 μm, ²H_9/2→⁴I_15/2), strong green (~0.56 μm, ²H_11/2, ⁴S_3/2→⁴I_15/2) and red (~0.67 μm, ⁴F_9/2→⁴I_15/2) emission bands were observed as well as weak near-infrared emissions at 0.8 μm (⁴I_9/2→⁴I_15/2) and 0.85 μm (⁴S_3/2→⁴I_13/2) at room temperature. The upconversion emission properties under ~1.5 μm pumping were further investigated through pump power dependence and decay time studies. Sequential two-photon absorption leads to the ⁴I_9/2 upconversion emission while energy transfer upconversion is responsible for the emission from the higher excited states ²H_9/2, ²H_11/2, ⁴S_3/2, and ⁴F_9/2. The enhanced red emission with increasing Er³⁺ concentration most likely occurred via the cross-relaxation process between (⁴F_7/2 → ⁴F_9/2) and (⁴I_11/2 → ⁴F_9/2) transitions, which increased the population of the ⁴F_9/2 level.