Analysis of competing mechanisms in transitions between excitedstates in Er-doped integrated waveguides

A new procedure for determining the prevalence of either excited-state absorption (ESA) or nonradiative cooperative energy transfer between erbium ions in transitions between excited states in Er-doped integrated waveguides is presented. These transitions are currently very attractive for the development of upconversion lasers. The procedure is based on the analysis of the dependence on the transition-originating mechanisms of the modulation transfer from the pump to the excited levels' population. The accuracy and validity range of the method are studied numerically using ordinary integrated structures. By using this procedure, the ratio of the contributions of the two competing mechanisms to the ⁴I_13/2 ⇒ ⁴I_9/2 transition is determined from fluorescence measurements on a Er,Ti:LiNbO₃ sample excited by a 1480-nm pump. Moreover, new values of the excited-state pump-absorption cross section from level ⁴I_13/2, σ₂₄ (≈1480 nm) = 0.8 × 10^-26 m², and of the parameter associated with nonradiative cooperative energy transfer between Er³⁺ ions, C₂₂ = 3 × 10^-24 m³ s·^-1, are reported     

Strong room temperature photoluminescence from erbium-doped siliconmonoxide

Evaporated films of SiO:Er on silicon substrates are shown to exhibit strong photoluminescence after annealing at 600°C for 30 min in argon. With an erbium concentration of ~2.7×10²⁰ cm ^-3, the ⁴I_13/2-⁴I_15/2 transition is found to decay with two exponential components     

Emission Regimes of a Green Er:YLiF4 Laser

We present an analysis of the emission regimes of an Er:YLiF₄ laser on the transition ⁴S_3/2 rarr ⁴I_15/2, under direct pumping and various upconversion pumping mechanisms. The analysis is based on the study of the steady-state solutions of a rate equation system. The thresholds of the various emission regimes are calculated. The influence of the pump mechanism and the excited-state absorption losses on the emission regime is discussed.     

Er:GSGG 晶体的光谱性质分析及激光特性模拟研究

测试和分析了Er:GSGG的吸收光谱和荧光光谱。应用Judd-Ofelt理论计算了Er3+的强度参数、自发辐射跃迁几率、能级寿命、荧光分支比和吸收截面。结果表明,Er3+在4I13/2和4I11/2能级有较长的能级寿命,在966nm和790nm处有较大的吸收截面,在2.79µm处有较大的积分发射截面值,数值模拟了在966nm泵浦下激光输出特性,在泵浦速率达到一定值时,有较高的量子效率。结果表明Er:GSGG有望成为2.79µm波段的理想激光晶体。     

Room-temperature CW laser operation at ~1.55 μm (eye-safe range)of Yb:Er and Yb:Er:Ce:Ca2Al2SiO7crystals

Room-temperature CW laser operation at 1.55 μm of Yb:Er:Ca₂Al₂SiO₇ (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, Ce³⁺, decreases the Er ³⁺⁴I_11/2 excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of ~5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented     

Q-switch regime of 3-μm Er:YAG lasers

A mathematical model, based exclusively on spectroscopic data concerning radiative, nonradiative, and energy transfer processes, is proposed and used to simulate the Q-switch regime of a 3-μm Er:YAG laser. The connection between the main energy transfer mechanisms that make possible generation on the self-saturated transition ⁴I _11/2→⁴I_13/2 (upconversion from ⁴I_13/2 and ⁴I_11/2 and cross relaxation from ⁴S_3/2) and the giant pulse characteristics are discussed. The radiative as well as nonradiative losses during optical pumping and giant pulse generation are defined and evaluated. A particular attention is given to the frustrated total internal reflection (FTIR) Q-switch which demonstrated real qualities for 3-μm erbium lasers. The reasons responsible for experimental performances of Q-switched Er:YAG lasers inferior to those predicted by the mathematical modeling are analyzed     

Ferroelectric integrated optics: recent developments

Advances in rare-earth-doped waveguide lasers in LiNbO₃, waveguided second harmonic generation through quasi-phase-matching in LiNbO₃ and LiTaO₃, and pulsed laser epitaxial growth of LiNbO₃ on Sapphire are reported. Efficient lasers operating in CW, Q-switch, and mode-lock modes were fabricated in Nd:LiNbO₃. In order to assess the potential of bulk-doped Er:LiNbO₃ samples, spectroscopic measurements have been carried out, the main limiting mechanism identified, the corresponding up-conversion coefficient evaluated, and a comprehensive gain model developed. First lasing action in this material, when pumped at 1.48 μm, is described. Quasi-phase-matched generation of blue light with conversion efficiencies up to 150%/Wcm² is also demonstrated, and finally we present the fabrication of low-loss LiNbO₃ single-crystal waveguiding layers     

Intracavity 1.549-μm pumped 1.634-μm Er:YAG lasers at 300 K

A novel method of generating 1.634-μm laser action from Er:YAG crystals pumped intracavity by an Er:glass laser emitting at 1.549 μm is described. Operation of the Er:glass laser at 1.549 μm (red shifted from the standard 1.532 μm, but with comparable output) at 500 K was obtained using mirrors with tailored spectral reflectivities. Several Er:YAG crystals ranging in concentration from 0.3% to 2% and in length from 1 cm to 8 cm were lased in the intracavity pumping arrangement. All the Er:YAG crystals lased in the ⁴I_13/2 :Y1(6544 cm^-1)-⁴I_15/2:Z6(424 cm ^-1) 1.634-μm transition at 300 K     

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     

Erbium-doped single- and double-pass Ti:LiNbO3 waveguideamplifiers

Single-pass and double-pass Er-diffused Z- and X-cut Ti:LiNbO₃ waveguide amplifiers, optically pumped at λ _p≈1484 nm, have been investigated. With a 48 mm long Z-cut amplifier device, Er-diffusion doped at 1100°C, 6.7 dB (coupled pump power P_p,c=170 mW) and 14.7 dB (P_p,c=90 mW) net small-signal gain have been achieved with a single-pass and a double-pass configuration, respectively, at the signal wavelength λ_s=1531 nm. A Z-cut sample doped at 1135°C showed a considerably improved behavior. 11.3 dB single-pass net small-signal gain has been obtained (P_p,c=170 mW; sample length 5.7 cm). Theoretical calculations predict gain figures up to 20 dB in single-pass and 40 dB in double-pass Er:Ti:LiNbO₃ amplifiers with increased (realistic) lengths of 10 cm     

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.     

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     

Gain and saturation intensity of the green Ho:ZBLAN upconversionfiber amplifier and laser

The gain and saturation intensity of the green Ho-doped fluorozirconate (ZBLAN) glass fiber amplifier and laser, pumped in the red (643 ⩽ λ_p ⩽ 649 nm; ⁵F₅ ← ⁵I₈), have been measured. For a 2.4-μm core diameter fiber 45 cm in length, the single-pass gain at 543.4 nm exceeds 12 dB for 90 mW of pump power at 643.5 nm. The saturation power for the ⁵F₄, ⁵S₂ → ⁵I₈ lasing transition was determined from gain measurements to be 970 ± 175 μW, which corresponds to a saturation intensity of 19.8 ± 3.5 kW · cm^-2 , and a stimulated emission cross section approximately one order of magnitude larger than theoretical estimates     

Cascaded Two-Wavelength Lasers and Their Effects on C-Band Amplification Performance for Er3+-Doped Fluoride Fiber

In this paper, we report cascaded two-wavelength 853-nm (⁴ S_3/2rarr⁴I_13/2 transition) and 1533-nm (⁴I_13/2rarr⁴I_15/2 transition) lasing from Er³⁺-doped fluoride fiber pumped at 974 nm. The cavity for cascaded two-wavelength lasing is composed of two fiber ends with 4% Fresnel reflection. Its optical-to-optical efficiency is up to 26.6%. Its effects on C-band fiber amplifiers and green upconversion fiber lasers are discussed. A new way to get high efficiency and low noise C-band amplifier is suggested, i.e., a fluoride-based Er³⁺-doped fiber amplifier including 853-nm lasing cavity. Our simulated results show that such a new amplifier can enhance the signal gain greatly and break the limit of the saturated gain intensity for a normal amplifier     

Photon avalanche upconversion in Er3+:YAlO3

The results of an investigation of photon avalanche upconversion pumping in Er³⁺:TiAlO₃ are reported. Five pump wavelengths corresponding to transitions from the metastable ⁴I_13/2 state to the ²H_11/2 state generated upconversion laser emission at 549.8 nm. The dependence of the laser output power on pump power near laser threshold is discussed in terms of a four-level kinetics model and is shown to reflect the threshold power requirement for photon avalanche. The maximum output power at 7 K was 33 mW, giving an optical conversion efficiency of 3.5% and a conversion efficiency of 28% based on absorbed power. Pumping Er:YALO by cross relaxation energy transfer produced 166 mW of laser output with an optical conversion efficiency of 17%     

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     

Theoretical modeling of optical amplification in Er-doped Ti:LiNbO3 waveguides

An accurate theoretical analysis is presented describing optical amplification in Er-diffused Ti:LiNbO₃ channel waveguides. It follows as far as possible the theory already developed for Er-doped fibers. As optical pumping around λ_p≈1.48 μm is considered, a quasi-two-level model for the Er³⁺ ions is used with wavelength-dependent cross sections. The optical gain in the 1.53 μm<λ<1.64-μm wavelength range is evaluated. The characteristic parameters, as Er concentration profile, cross sections, pump, and signal mode distributions and waveguide (scattering) losses are taken from experiments. Examples of numerically calculated pump-, small-signal-gain-, and ASE-evolutions are presented. The model has been tested by comparing computed and experimentally observed gain characteristics for Xˆ- and Yˆ-cut LiNbO₃; an almost quantitative agreement has been obtained     

A theoretical study of a Ti-diffused Er:LiNbO3 waveguidelaser

A detailed theoretical procedure of determining the mode sizes of fundamental mode field distribution in a coaxially pumped Ti-diffused Er:LiNbO₃ waveguide laser has been described. The mode sizes, effective refractive indexes, effective pump area, and coupling efficiency as a function of the initial Ti stripe width W were numerically calculated for the waveguide with 95- and 100-nm initial Ti stripe thicknesses, respectively. The results indicated that the threshold pump power is severely affected by the stripe width, while slope efficiency is hardly changed as W; both show little difference between 95- and 100-nm stripe thickness. In addition, the stimulated emission cross section of Er³⁺ in Er:LiNbO₃ channel waveguide versus the wavelength were calculated directly from its fluorescence spectra using the β-τ method. Subsequently, threshold pump power and slope efficiency were evaluated     

Electrooptically Q-switched 2.79 μm YSGG:Cr:Er laserwith an intracavity polarizer

Single Q-switched pulses have been generated in YSGG:Cr:Er with a 360-ns-risetime LiNbO₃ electrooptical modulator. It is shown that birefringence losses can be avoided and spiking emission eliminated by placing an additional polarizing LiNbO ₃ prism inside the laser resonator. Reproducible single Q -switched pulses of 200-ns duration have been generated     

Photon avalanche in Er:ZBLAN fibre pumped at 690 nm

A new photon avalanche mechanism has been observed in Er:ZBLAN fibre when pumped at 700 nm; this pump scheme corresponds to the excited state absorptions from ⁴I_13/2 and ⁴I _11/2. Experimental results of pump power dependence and build-up process are reported