Evaluation of absorption and emission properties of Yb3+doped crystals for laser applications

The emission and absorption properties of numerous host crystals doped with Yb³⁺ ions have been studied. The hosts which have been selected include LiYF₄, LaF₃, SrF₂, BaF₂, KCaF₃, KY₃F₁₀, Rb₂NaYF₆, BaY₂ F₈, Y₂SiO₅, Y₃Al₅ O₁₂, YAlO₃, LuPO₄, Ca₅(PO₄)₃F, LiYO₂, and ScBO ₃. Spectral determinations have been made of the resonant absorption and emission cross sections between 850 and 1100 nm, and the emission decay times of the upper laser level have been measured. The emission cross sections have been evaluated using the absorption cross section and principle of reciprocity, and again using the Fuchtbauer-Ladenberg formula. Agreement between the two methods is within 20% for most materials. The results are discussed in the framework of requirements for an effective diode-pumped Yb³⁺ laser system. Ca₅(PO₄)₃F:Yb is predicted to exhibit the most useful laser properties and is expected to be far superior to Y₃Al₅O₁₂:Yb in many key microscopic parameter values     

Tuning of a Tm3+:Ho3+: silica fiber laser at2 μm

Thermal tuning of a Tm³⁺:Ho³⁺:SiO₂ glass fiber laser was investigated. As a function of fiber length and temperature, the emission wavelength can be varied between 1960 and 2032 nm. The bandwidth of the laser emission is about 2 mm. Besides the wavelength also the change of threshold and slope efficiency with temperature and fiber length and the fiber absorption were measured     

Gain measurement and upconversion analysis in Tm3+, Ho3+ co-doped alumino-zirco-fluoride glass

The small signal gain coefficients were measured in Tm³⁺,Ho³⁺ co-doped alumino-zirco-fluoride glass. A gain of 15%/cm at 2.05 μm was obtained for pump power density of 42 kW/cm². The temperature increase of the glass was found to be more than 150 K with this pump power, which was estimated from a comparison between fluorescence intensities of Tm^{3+ 3} F₄-³H₆ and Ho^{3+ 5} I₇-⁵I₈. An upconversion rate constant of 12.5×10^-17 cm³ sec^-1 from a coupled (Tm^{3+ 3}F₄, Ho^{3+ 5}I₇) level to a coupled (Tm^{3+ 3}H₅, Ho^{3+ 5}I₆) level was determined by fitting the experimentally obtained gain coefficients to the calculated one which takes into consideration any temperature increase     

LiCaAlF6:Cr3+: a promising new solid-statelaser material

LiCaAlF₆:Cr³⁺ (Cr³⁺:LiCAF) exhibits an intrinsic (extrapolated maximum) slope efficiency of 67%. For comparison, the intrinsic slope efficiencies of BeAl₂O ₄:Cr³⁺ (alexandrite), Na₃Ga₂Li₃F₁₂:Cr³⁺ and ScBO₃:Cr³⁺ were found to be 65, 28, and 26%, respectively. The tuning range of LiCaAlF₆:Cr³⁺ was determined to be at least 720-840 nm. The conventional spectroscopic properties, such as the absorption, emission, and emission lifetimes as a function of temperature, are reported as well     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Continuous-wave laser action in Er3+:YLF at 3.41 μm

Continuous-wave laser emission at 3.41 μm from an erbium-doped LiYF₄ crystal (Er³⁺:YLF) at 77 K is demonstrated. Operation of this four-level laser is based on the Er^{3+ 4}F_9/2-⁴I_9/2 transition. An output power of 12 mW and a slope efficiency of 2% have been achieved     

Improved gain performance in Yb3+-sensitized Er3+-doped alumina (Al2O3) channel opticalwaveguide amplifiers

Small-signal amplification in short, Yb³⁺-sensitized, Er³⁺-doped alumina (Al₂O₃) channel optical waveguides with high Er³⁺ concentrations is analyzed. Taking into account uniform up conversion, excited state absorption (ESA) from the Er³⁺ metastable level (⁴I_13/2 ), and Yb³⁺→Er³⁺ energy transfer by cross relaxation, the obtainable gain improvements compared to Yb³⁺ -free Er³⁺-doped Al₂O₃ optical waveguides are investigated. The amplifier model is based on propagation and population rate equations and is solved numerically by combining finite elements and the Runge-Kutta algorithm. The analysis predicts that 5-cm long Yb³⁺/Er³⁺ co-doped Al₂O ₃ waveguides show 13-dB net signal gain for 100 mW pump power at λ_p=980 nm     

Temperature dependence of pulsed laser threshold in YAG:Er3+, Tm3+, Ho +

The variation of pulsed laser threshold with temperature is graphically presented for YAG: Er³⁺, Tm³⁺, Ho³⁺over the range from 90 to 300°K. The active ion is Ho³⁺and threshold for the 2.13-micron line varies between 4 and 60 joules over this temperature range.     

Crystal growth, frequency doubling, and infrared laser performanceof Yb3+:BaCaBO3F

Yb:BaCaBO₃F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb³⁺ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1×10^-20 cm² with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3×10^-20 cm² with a transition bandwidth of 24 nm. The measured emission lifetime of Yb³⁺ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD ⁺P for a relative measure of the second-harmonic generating potential, yielding d_eff(BCBF)~0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 μm light; the predicted angle is 37° from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported     

Anomalous dispersion in Er3+ and Yb3+-dopedfibers

In experimental and theoretical study of anomalous dispersion in Er³⁺and Er³⁺-Yb³⁺-doped fibers has been developed. Anomalous time delay caused by both absorption and emission at 1.535 μm has been theoretically calculated and experimentally measured. A pump power dependence of anomalous time delay in rare-earth-doped fibers has been theoretically calculated and experimentally investigated. It has been shown that pump power fluctuations lead to propagation time jitter in Er³⁺-doped fiber amplifiers. The pulse interaction due to refractive index change caused by gain saturation is predicted. It has been shown that for Er ³⁺-doped fibers with SiO₂-GeO₂ core composition, the anomalous dispersion per 1-dB gain is twice that of fibers with SiO₂-Al₂O₃ core, which is caused by gain curve form difference. A scheme of mutual compensation of intrinsic fiber dispersion and anomalous dispersion caused by Er³⁺ in the region 1.532-1.537 μm has been suggested     

Absorption and emission cross section of Er3+ dopedsilica fibers

A detailed set of measurements is presented on determining the emission and absorption cross sections of Er³⁺ doped fibers for the ⁴I_13/2→⁴I _15/2 transition. Two techniques are employed: the Fuchtbauer-Ladenberg analysis, based on spectroscopic data, and a more direct technique involving optical saturation of the transition. The cross sections, and in particular their ratio, are significantly different for the two techniques. Possible reasons for this are discussed, and it is concluded that the Fuchtbauer-Ladenberg approach is inappropriate in this situation     

Laser action from Ho3+, Er3+, and Tm3+in YAlO3

Laser action has been observed for the following rare-earth ions in YAlO3:Ho³⁺(sensitized with Er³⁺and Tm³⁺), Er³⁺, and Tm³⁺(sensitized with Er³⁺) at wavelengths of 2.123, 0.851, and 1.861 μm, respectively. Measurements of spectroscopic properties, fluorescence kinetics, and laser performance of these ions in YAlO3are reported.     

Tunable Cr4+:YSO Q-switched Cr:LiCAF laser

Tunable passive Q-switching (781 nm to 806 nm at 300 K) of a flash-lamp pumped Cr³⁺:LiCaAlF₆ (Cr:LiCAF) laser with a Cr⁴⁺:Y₂SiO₅ (Cr⁴⁺:YSO) broad-band solid-state saturable absorber has been realized. Typical pulse widths of the Q-switched laser output ranged from 40 ns to 80 ns, depending on the lasing wavelength. Spectral narrowing and reduced beam diameter with the use of the saturable absorber were observed. The ground state and the excited state absorption cross sections of the Cr⁴⁺:YSO absorber were found by bleaching experiments to be (7.0±1.4)×10^-19 cm² and (2.3±0.5)×10^-19 cm² at 694 nm, respectively. Numerical simulation was utilized to simulate the Cr:LiCAF passive Q-switching with Cr⁴⁺ :YSO solid-state saturable absorber     

Fluorescence dynamics of Er3+ and Ho3+ ionsand energy transfer in some fluoride glasses

A detailed study at room temperature of the fluorescence dynamics of Er³⁺ and Ho³⁺ ions and energy transfer processes Er³⁺→Ho³⁺ in the ABLA, BATY, and BIZYT fluoride glasses is reported. Numerous concentrations and different selective laser excitations in the visible and infrared spectral ranges are used. The best concentrations and the more suitable glasses for laser applications at 2 μm are determined     

Ytterbium-Doped P$_2$O$_5$-TeO $_2$ Glass for Laser Applications

Ytterbium-doped sodium phospho-tellurite glasses are made with different P₂O₅:TeO₂ ratio, and Yb³⁺ concentrations. Physical properties of the new Yb hosts are favorable for laser applications. The glasses show high absorption and emission cross sections and higher lifetime of Yb³⁺:²F_5/2rarr²F_7/2 transition. The emission cross sections are calculated using two different methods and compared. The laser parameters of these Yb³⁺ -doped glasses are better than many reported glasses and crystals making them potential to fabricate high power laser and broadband optical amplifier in the wavelength region around ~1 mum     

Ho3+laser action in YAlO3at 2.119 µ

Pulsed laser action at 2.119 μ has been observed from Ho³⁺in a crystal of YAlO3sensitized with Er³⁺and Tm³⁺. A linearly polarized output of 1.2 J was obtained at 77°K. Measurements of the anisotropy of the stimulated-emission cross sections are reported.     

Cr4+:YAG as passive Q-switch and Brewster plate in apulsed Nd:YAG laser

We demonstrate the performance of a Nd:YAG laser, passively Q-switched with a Cr⁴⁺:YAG plate, which plays the double role of a passive Q-switch and a Brewster plate. The Brewster plate configuration contributes an intracavity loss of approximately 3.2-10 ^-3 cm^-1 along the cavity length. Losses contributed by the active Cr⁴⁺ ions in the plate relate to their excited state absorption. A freshly measured transmission saturation curve of Cr⁴⁺:YAG suggests a ground state absorption cross section σ_gs=(8.7±0.8)-10^-19 cm², and an excited state absorption cross section σ_es=(2.2±0.2)-10^-19 cm² of the Cr⁴⁺ ions at λ=1064 nm     

Spectroscopic Properties and Laser Performance of Er3+ and Yb3+ Co-Doped GdAl3(BO3)4 Crystal

An Er:Yb:GdAl₃(BO₃)₄ crystal was grown and room-temperature polarized absorption, emission, and gain spectra were investigated. Fluorescence decay curves of Er³⁺ at 1530 nm and Yb³⁺ at 1040 nm in the crystal were measured. Efficient laser operation of Er:Yb:GdAl₃(BO₃)₄ crystal at 1.5-1.6 mum was realized. Quasi-continuous-wave output powers of 1.8 W with slope efficiency of 19% and 0.78 W with slope efficiency of 14% were achieved in diode-pumped c-cut and c-cut and a-cut crystals, respectively. The output spectrum and polarization of Er:Yb:GdAl₃(BO₃)₄ laser were also investigated.     

8.8 W diode-cladding-pumped Tm3+,Ho3+-dopedfluoride fibre laser

A maximum output power of 8.8 W was generated from a diode pumped Tm³⁺,Ho³⁺-doped ZBLAN glass fibre laser. The laser operated with a maximum slope efficiency of 36% with respect to the incident pump power. The wavelength was measured to be 2056 nm and some instability was observed in the output     

Dependence of the stimulated emission cross section of Yb3+on host glass composition

The stimulated emission cross section for the²F5/2→²F7/2transition of Yb³⁺has been determined from absorption and emission measurements of 41 different oxide, fluoride, and oxyfluoride glasses at 293 K. The effective peak cross sections for transitions to Stark levels above the ground state range from approximately 0.3 to 0.8 pm². The largest values occur in borate and phosphate glasses; the smallest values occur in silicate and low-refractive-index fluoride glasses. Radiative lifetimes calculated from integrated absorption spectra are also reported and range from 0.6 to 2.7 ms. Systematic variations in cross sections with changes in modifier ions can be used to tailor stimulated emission cross sections and fluorescence lifetimes.