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     

Laser diode-pumped holmium-doped fluorozirconate glass fiber laserin the green (λ~544-549 nm): power conversion efficiency, pumpacceptance bandwidth, and excited-state kinetics

The green (544-549 nm) Ho-doped fluorozirconate (ZBLAN) glass fiber laser, pumped in the red (λ~6;15 nm) by a high-power (~30 mW) InGaAlP laser diode or a ring dye-laser, has been characterized with regard to power conversion efficiency, fiber core-diameter and length, cavity output coupling, and pump acceptance bandwidth. Fibers doped with ~1200 ppm (by weight) of Ho and having core diameters of 1.7, 3, and 11 μm, and lengths ranging from 12.5 to 86 cm, have been studied in Fabry-Perot resonators having output couplings ranging from 1.545 to 96%. For a 1.7-μm core-diameter fiber, 21 cm in length, the threshold-launched pump power for the diode-pumped fiber laser is 1.9 and 3.5 mW for cavity output couplings of 1.5% and 24%, respectively. These values are the lowest for any upconversion-pumped fiber laser reported to date. Also, the noise and threshold-pumping power properties of the diode-pumped fiber laser are superior to those for its dye-laser-pumped counterpart. The highest laser slope efficiency (>22% with respect to launched pump power) was measured for a 3-μm core-diameter fiber and a cavity output coupling of 24%. The spectral interval over which the launched threshold pump power for this laser is <10 mW is almost 20 nm (637-656 nm). Studies of the fiber laser waveform as a function of pump power reveal competition for population between the ⁵S² and ⁵F₄ states and among the Stark sublevels of the ⁵F₄ manifold. Also, measurements of the output power on individual laser lines of the ⁵F₄, ⁵S₂→ ⁵I₈ (ground) transitions of Ho³⁺:ZBLAN as a function of pump power demonstrate the existence of a loss mechanism at the fiber laser wavelength, presumably due to absorption from ground or the ⁵I_y, ⁶S₂ or ⁵F₄ excited states of the ion     

Fluorescence and superfluorescence line narrowing and tunability ofNd3+ doped fibers

The inhomogeneous behavior of the 1.06 μm-neodymium transitions in doped optical fibers have been investigated, using the fluorescence line narrowing technique, pumping on the ⁴F_3/2 and ⁴F_5/2 sublevels at 4 K. Each observed transition has been identified. As the pump wavelength varies, the shift of the main fluorescence line is 40 nm, with the two pumping levels. We have studied the spectral behavior of the superfluorescence as a function of the pump wavelength, the temperature, and the absorbed power. The spectral evolution depends on λ_p with 19 nm-tuning range at low temperature. At 300 K, the quasihomogeneous behavior of the transition decreases the tunability to 14 nm. Based on these results, we present a simple technique permitting precise prediction of gain and spectral line shapes of superfluorescent Nd-doped fiber sources     

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     

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%     

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。     

Reliable Low-Cost Fabrication of Low-Loss $hbox{Al}_{2}hbox{O} _{3}{:}hbox{Er}^{3+}$ Waveguides With 5.4-dB Optical Gain

A reliable and reproducible deposition process for the fabrication of Al₂O₃ waveguides with losses as low as 0.1 dB/cm has been developed. The thin films are grown at ~ 5 nm/min deposition rate and exhibit excellent thickness uniformity within 1% over 50times50 mm² area and no detectable OH^- incorporation. For applications of the Al₂O₃ films in compact, integrated optical devices, a high-quality channel waveguide fabrication process is utilized. Planar and channel propagation losses as low as 0.1 and 0.2 dB/cm, respectively, are demonstrated. For the development of active integrated optical functions, the implementation of rare-earth-ion doping is investigated by cosputtering of erbium during the Al₂O₃ layer growth. Dopant levels between 0.2-5times10²⁰ cm^-3 are studied. At Er³⁺ concentrations of interest for optical amplification, a lifetime of the ⁴I_13/2 level as long as 7 ms is measured. Gain measurements over 6.4-cm propagation length in a 700-nm-thick Al₂O₃:Er³⁺ channel waveguide result in net optical gain over a 41-nm-wide wavelength range between 1526-1567 nm with a maximum of 5.4 dB at 1533 nm.     

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     

Pressure-dependent Sellmeier coefficients and material dispersionsfor silica fiber glass

The pressure-dependent Sellmeier coefficients are essential to characterize the optical design parameters for the optical fiber communication systems under deep sea environmental conditions. These coefficients are calculated for densified silica glass for the first time to compute the pressure dependence of material dispersion at any wavelength from the ultraviolet (UV) to 1.71 μm. The zero dispersion wavelength λ₀ (1.2725 μm at 0.1 10⁶ N m ^-2) varies linearly with pressure, and dλ₀/dP is 0.0027 nm/(10⁶ N m^-2). The calculated value is approximately one-third of the experimental value of 0.0076 nm/(10⁶ N m^-2) for a germanium-doped dispersion shifted fiber having λ₀=1.5484 μm and -0.0070 nm/(10⁶ N m^-2) for a pure silica-core fiber cable having λ₀ =1.2860 μm. Since, the refractive indexes are increased with pressure, the negative value of shift of the zero-dispersion wavelength is erroneous. The explanations are due to Ge-doping in silica glass, a possible temperature fluctuation of 0.16°C in the pressure-dependent measurement system of the zero dispersion wavelength and different experimental conditions of the silica glass and the optical fibers. This anomaly can also be attributed to the internal strain development at the core-cladding and fiber-jacketing boundaries due to pressure, which shows a larger experimental value. It accounts for the experimental values satisfactorily     

Excitation spectra of the green Ho:fluorozirconate glass fiberlaser

Laser and fluorescence excitation spectra of the green Ho:ZBLAN glass fiber laser have been measured at room temperature by pumping 20-90-cm-long fibers in the 640-653-nm spectral region with a CW dye laser. Pronounced structure in the laser excitation spectrum appears to arise from Stark sublevels of the Ho³⁺⁵F₅ and ⁵I₈ states rather than excited state absorption. Because the overall gain profile is inhomogeneously broadened, the laser excitation spectrum width increases with pump power and is ~5.7-nm FWHM when the pump power is a factor of 2.3 above threshold. For a 22-cm fiber, the threshold pump power is 128±5 mW and, with 5% output coupling, more than 12 mW has been extracted in the green for 318 mW of 646.7 nm input power. The slope efficiency for this oscillator (accounting for the pump launch efficiency) exceeds 14%     

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     

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     

The effect of pair-induced energy transfer on the performance ofsilica waveguide amplifiers with high Er3+/Yb3+concentrations

High-concentration Er³⁺/Yb³⁺ co-doped silica waveguide amplifiers are numerically analyzed. With optimized rare-earth concentrations the effect of Er³⁺/Er³⁺ ion-pairs can be neglected and each Er³⁺ ion can be assumed to be paired only to the surrounding Yb³⁺ ions. The rate-equations model includes uniform upconversion mechanisms from ⁴I_13/2 and ⁴I_11/2 erbium levels and an Yb³⁺ to Er³⁺ pair-induced energy transfer process. Numerical results demonstrate the possibility of fabricating short- and high-gain integrated optical amplifiers; it is shown that net gain as high as 3 dB/cm can be obtained     

Tuning, temporal, and spectral characteristics of the green(λ~549 nm), holmium-doped fluorozirconate glass fiber laser

The output power, tuning and temporal behavior of the green (539-550 nm), Ho-doped fluorozirconate glass (ZBLAN) fiber laser pumped in the red (λ~643 nm) has been examined in detail. Fibers ranging in length from ~20-86 cm have been studied, and more than 38 mW of output power has been obtained from a 21 cm long fiber for 30% output coupling and 280 mW of absorbed pump power. By inserting two prisms into the optical cavity, the oscillator has been tuned over ~11 mn, and the emission spectrum of the free-running laser has been recorded as a function of pump power at both room temperature and 77 K. The self-pulsing (spiking)behavior of the laser and the influence of depletion of the Ho³⁺ ground state (⁵I₈) population on the emission spectrum and efficiency of this system are discussed     

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     

An Erbium Fiber Fabricated by the “Core-Suction” Technique

An erbium doped fiber preform having a lead germanate erbium glass core with silica cladding was fabricated by a newly developed technique named “Core-Suction”. This preform was then drawn into fiber and fiber cross-section, loss spectrum and refractive index profile measured. A 30 cm piece of the manufactured fiber was spliced to a standard silica fiber using a commercial available splicer. This spliced fiber was then used to setup an erbium doped fiber amplifier (EDFA) and gain spectrum of the amplifier measured. Distributed gain of the manufactured erbium fiber was measured using an optical frequency domain reflectometry (OFDR) based optical backscatter reflectometer (OBR). It is demonstrated that the “Core-Suction” technique can be used to make a high-gain amplifier that is compatible with conventional silica fibers.      

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     

Design of Hybrid Optical Amplifiers for High Capacity Optical Transmission

This paper describes our design of a hybrid amplifier composed of a distributed Raman amplifier and erbium‐doped fiber amplifiers for C‐ and L‐bands. We characterize the distributed Raman amplifier by numerical simulation based on the experimentally measured Raman gain coefficient of an ordinary single mode fiber transmission line. In single channel amplification, the crosstalk caused by double Rayleigh scattering was independent of signal input power and simply given as a function of the Raman gain. The double Rayleigh scattering induced power penalty was less than 0.1 dB after 1000 km if the on‐off Raman gain was below 21 dB. For multiple channel amplification, using commercially available pump laser diodes and fiber components, we determined and optimized the conditions of three‐wavelength Raman pumping for an amplification bandwidth of 32 nm for C‐band and 34 nm for L‐band. After analyzing the conventional erbium‐doped fiber amplifier analysis in C‐band, we estimated the performance of the hybrid amplifier for long haul optical transmission. Compared with erbium‐doped fiber amplifiers, the optical signal‐to‐noise ratio was calculated to be higher by more than 3 dB in the optical link using the designed hybrid amplifier.     

Anisotropic two-photon transitions in GaAs/AlGaAs multiple quantumwell waveguides

The two-photon absorption coefficient and induced nonlinear refractive index in both TE and TM modes of GaAs/AlGaAs multiple-quantum-well (MQW) strip-loaded channel waveguides are measured in the wavelength range from 1490 nm to 1660 nm. The two-photon absorption coefficient ranges from almost zero to 12 cm/GW, and the nonlinear refractive index is always on the order of 10^-13 cm ²/W in the wavelength window investigated. Strong nonlinear anisotropy is observed. The values of the two-photon absorption coefficient are obtained by calibrating the inverse transmission measurements. Those of the nonlinear refractive index are obtained by monitoring the signal spectrum broadening due to self-phase modulation. The wavelength dependences of the measured two-photon absorption coefficient and nonlinear refractive index are reasonably consistent with the theoretical predictions