I review the fast growing progress in thulium doped fiber amplifiers (TDFA) for S-band optical communication systems. The main results reported on pump schemes and amplifiers efficiencies are given. In particular, the characterization of TDFAs regarding distributed gain optimization using the coherent optical frequency domain reflectometry is described, from which the optimum gain-length product for a given configuration can be inferred in a non-invasive and non-destructive way.
A new theoretical approach for modeling the saturated single pass gain in a three-level fiber amplifier is presented, relevant to the behavior of rare-earth-doped silica fibers. A basic approximation considering the stimulated emission rate Ws(z,ν) as a dimensionless parameter S, independent of the spatial and frequency variables z and ν, allows to obtain analytic expressions for input and output pump, ASE and signal powers inside the fiber core. We show that these expressions only depend on the S parameter, which is determined by solving the photon balance equation, and S is shown to be fully representative of the saturation in the medium. The main result of the model is that the pump repartition P(z) takes a simple analytical form, which can be separated into two parts, below and above the saturated absorption length L0, which is a function of S. The first part 0〈z〈L0 is the saturated absorption where the pump distribution is linear and the second part L0〈z〈L is the non saturated absorption region with an exponentially decreasing pump distribution. Compared to other analytical models with the assumption of an averaged inversion population 〈N2(z)〉, we obtain a good description of the difference between the co- and counter-propagating ASE behaviors accounting for the fiber length and the pumping level. The model, which is well suited to longitudinal pumping, can also describe a side pumped fiber amplifier by simple adjustment of some of the model parameters.
In this work, pulsed laser deposition has been used to produce Er3+ doped lead-niobium-germanate thin films from Er3+ doped 50Geo2-25PbO-25Nb2O5 transparent glasses with an Er2O3 content in the 0.5-3 weight % range. In all cases, by pumping the 4I11/2 level of Er3+, the as deposited films are optically active showing at room temperature the 4I13/2 →4I15/2 emission centered around 1530 nm. The luminescence intensity increases as the Er3+ content in the targets increases, whereas the opposite behavior is observed for the luminescence lifetime. The emission intensity and lifetimes of the 1.5 μm emission increase upon annealing.
SiO2-TiO2:Er3+-Yb3+ waveguides were prepared by rf-sputtering technique. The active films were deposited on silica-on-silicon and v-SiO2 substrates. The parameters of preparation were chosen in order to optimize the waveguides for operation in the NIR region with particular attention to the minimization of losses. The thickness of the waveguides and the refractive index at 632.8 and 543.5 nm were measured by an m-line apparatus. The losses, for the TE0 mode, were evaluated at 632.8 and 1300 nm. Roughness measurements were carried out by means of a stylus profilometer. The structural properties were investigated with several techniques such as Energy Dispersive Spectroscopy and Raman Spectroscopy. All waveguides were single-mode at 1550 nm. An attenuation coefficient equal or lower than 0.2 dB/cm was measured both at 632.8 nm and 1300 nm. The emission 4I13/2 →4I15/2 of Er3+ ion transition with a 40 nm bandwidth was observed upon excitation in the TE0 mode at 981 and 514.5 nm. Back energy transfer from Er3+ to Yb3+ was demonstrated by measurement of Yb3+ emission upon Er3+ excitation at 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the effective excitation efficiency of Er3+ ions by co-doping with Yb3+ ions. Channel waveguides in rib configuration were obtained by etching the active film by a wet etching process. Scanning Electron Microscopy was used to analyze the morphology of the waveguides.
One of the trends that persist in the telecom industry in all market conditions is a continuous push towards lower cost and higher performance optical components. Unlike today’s networks, a more cost efficient network of tomorrow will contain many components utilizing Planar Lightwave Circuits (PLC) technology. PLC technology is a platform for optical integration that could dramatically lower cost-per-function in many optical networks. However, integration may result in degraded optical performance due to higher insertion losses as compared to “standard” fiber-based solutions. A solution to the loss problems is an optical amplifier that can be integrated on the same PLC platform and used to restore optical signals as needed. Inplane Photonics is developing Er-doped waveguide amplifier (EDWA) technology, which is fully compatible with a glass-on silicon PLC platform. We identify that an EDWA is a necessary building block to achieve the full potential of optical integration. In this paper we will present recent EDWA performance that approaches that of an EDFA. Furthermore, we will demonstrate several examples of practical integration between passive and active building blocks on a single PLC chip.
The partial polarization of luminescence in phosphate and silicate erbium glasses in the range of 1.5 μm under the stimulation by the linearly polarized laser light (532 nm, 790-990 nm) has been detected. The degree of polarization depends both on a spectral range of excitation and on a spectral range of registration, reaching maximum value ~1%. The concentration of depolarization has been studied.
We prepared a sol-gel hybrid material which has low loss near 1.5 μm wavelength using methyltriethoxysilane, vinyltriethoxysilane, and phenyltrimethoxysilane as precursors. Several erbium precursors were used in order to incorporate into the hybrid matrix. Among several Er-doped system, the sol-gel hybrid films doped with ErQ showed a clear photoluminescence at 1.5μm when they were pumped by 477nm light, not a resonant wavelength of Er3+ ions, which is expected to absorb the pump energy and transfer to adjacent Er3+ ions. Indirect excitation mechanism was investigated by photoluminescence excitation measurement using various wavelengths of Ar+ laser. The sol-gel hybrid films doped with ErQ show an efficient indirect excitation of Er3+ luminescence through ligand sensitization.
Properties of a new rare-earth doped heavy metal oxide containing silicate glass are presented. The glass has potential for fabrication of ultra-short wideband fiber and planar waveguide amplifiers. We report specific results for a fiber amplifier geometry, discussing achieved improvements in device compactness (Giles gain g* = 210 dB/m allowing up to 100 times shorter fiber) and amplification bandwidth (50% more bandwidth in C-/L-band) compared to the conventional EDFA. We also access the potential of this material for fabrication of active planar integrated waveguide devices.
Er3+ doped tellurite glasses have recently gained interest because of broad emission band in 1.5 μm telecom window. In this paper we present results of our research aiming at the fabrication of surface waveguides in these glasses. Tungsten-tellurite and zinc-tellurite glasses doped with various Er2O3 percentages have been prepared and the glass stability has been assessed based on difference between Tg and Tc (and Tm) when changing glass modifiers. Broad emission and absorption bands corresponding to transition between 4I13/2↔4I15/2 were observed as expected. Lifetimes of the 4I13/2 level were also measured. Ag+-Na+ ion-exchange was performed with a molten salt composition based on AgNO3, KNO3 and NaNO3. Surface quality of the processed samples was analyzed in order to assess the homogeneity and chemical durability of the surface waveguides. Waveguiding was successfully obtained in both types of glasses and the process was characterized with the prism coupling technique. Several propagation modes at different wavelengths were detected and the diffusion process was characterized for the different concentrations of erbium ions.
This paper reports the fabrication of Bi2O3-based glass planar and channel waveguides using two techniques, respectively hot-dip spin-coating, and direct 244 nm UV-writing into the bulk glass. In the former, a 5 μm core glass film was achieved, which indicates a practical potential for realizing single mode operation channel guides. In the later, the laser written structures obtained showed a positive refractive index change, estimated at 4×10-4 at 633 nm, and a loss of less than 4 dB/cm.
Erbium activated SiO2-HfO2 planar waveguides, doped with Er3+ concentrations ranging from 0.01 to 4 mol%, were prepared by sol-gel method. The films were deposited on v-SiO2 and silica-on-silicon substrates using dip-coating technique. The waveguides show high densification degree, effective intermingling of the two film components, and uniform surface morphology. The waveguide deposited on silica-on-silicon substrates shows one single propagation mode at 1.5μm, with a confinement coefficient of 0.81 and an attenuation coefficient of 0.8 dB/cm at 632.8nm. Emission in the C-telecommunication band was observed at room temperature for all the samples upon continuous-wave excitation at 980 nm or 514.5 nm. The shape of the emission band corresponding to the 4I13/2→4I15/2 transition is found to be almost independent both on erbium content and excitation wavelength, with a FWHM between 44 and 48 nm. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime ranging between 1.1-6.6 ms, depending on the erbium concentration. Infrared to visible upconversion luminescence upon continuous-wave excitation at 980 nm was observed for all the samples. Channel waveguide in rib configuration was obtained by etching the active film in order to have a well confined mode at 1.5 μm.
In this work the feasibility of the atomic layer deposition (ALD) in producing erbium-doped waveguides is studied. Two microns thick erbium-doped aluminum oxide layers were grown with ALD on silica-coated silicon wafers. The waveguides were patterned using photolithography and wet etching. Resulted single-mode ridge-type waveguides were measured to obtain absorption, emission, fluorescence lifetime, and gain characteristics. Optical pumping was done using the 980 nm wavelength. The material showed broad emission spectrum with FWHM of 52 nm and maximum absorption of 6.2 dB/cm at 1530 nm. Maximum signal enhancement of 2.6 dB/cm was measured at 1530 nm for the 20 dBm signal power.
A fiber-taper-coupled L-band microsphere laser was proposed and demonstrated. Er3+-doped tellurite glass was used to fabricate the microspheres. The microspheres were made by a novel spin method. The pump and emission light were coupled in and out of the microsphere modes with a tapered fiber. 975nm pumped L-band single/multi-mode microsphere lasers were demonstrated. The laser can be tuned in L-band. We call Er3+-doped microsphere laser as EDML.
By using Yb3+-doped double-clad fiber with rectangular inner-cladding, and a set of back-cavity mirrors with different transmission ratio, the double-clad fiber lasers are constructed. It is found that the back-cavity mirror with a higher transmission ratio is better; a back-cavity mirror can control laser wavelength; the relationship between slope efficiency and transmission ratio is in accordance with exponential function, the maximum slope efficiency is about 60%.
Gain/loss spectra in ytterbium-erbium doped bulk glass samples have been investigated within spectral range of 1.5 μm. Experimental methods for determination of the population of excited metastable level, gain/loss spectra, and their variations versus pump power have been developed. A comparison of the experimental and calculated gain/loss spectra has been done.
Comparative study of spectroscopic properties of Er3+/Tm3+-codoped and Er3+/Tm3+/Yb3+ codoped tellurite glasses under a single wavelength pumping at 980nm were investigated. In the Er3+/Tm3+-codoped tellurite glasses, we have found that there exists fluorescence emissions at both 1.53 and 1.74 μm, which correspond to the transitions of the Er3+:4I13/2→4I15/2 and Tm3+:3F4→3H6, respectively. Tm3+ act as an efficient co-dopant for depopulation of the Er3+:4I13/2 level, and thus reducing the optical amplification efficiency. Meanwhile, the addition of a small amount of Tm3+ to the Er3+ doped tellurite glass causes the quenching of the green up-conversion fluorescence around 524 and 546nm resulting in red visual. In the Er3+/Tm3+/Yb3+ codoped tellurite glasses, fluorescence emissions at both 1.53 and 1.63 μm communication windows were firstly observed. What’ more, The FWHM (fluorescence width at half maximum) at 1.53 and 1.63 μm are 55nm and 50nm, respectively. This novel three earth ions co-doping method could be applied to other low photon energy glasses, which would be possibly used for potential dual wavelength fiber-optic amplifiers to broaden the communication windows.
Spectroscopic properties of Er3+ doped various glasses are investigated experimentally on the basis of Judd-Ofelt theory, McCumber theory, and lifetime measurements. A strong correlation is observed between glass composition and spectroscopic parameters such as Judd-Ofelt intensity parameters Ωt (t=2, 4, 6), emission spectra, and lifetime of 4I13/2 level of Er3+. The emission parameters of Er3+ doped various glasses are discussed from the viewpoints of emission and gain characteristics at the 1.55μm bands. The results show that tellurite glass is an indeed promising material for Er3+ doped to realize broadband and high gain amplification
Glasses based on rare earth oxide-aluminum oxide and containing high concentrations of Er2O3, Tm2O3, Yb2O3, or Ho2O3 were synthesized. The host glass is strong, hard, highly resistant to chemical attack, and stable to temperatures ~1000°C. Addition of up to 20 mole % silica markedly increased glass formability while maintaining infrared transmission to ~ 5000 nm in sections up to a few mm. The fluorescence lifetime of excited states in the dopant ions was measured as a function of dopant concentration, pump power and host composition. The absorption cross section and fluorescence line shape were measured for selected compositions. We present details of the glass synthesis and properties, and results of the optical measurements in the context of developing glass-based optical devices.
Germanium-based glasses containing heavy metal oxides (Sb2O3) have been investigated. These materials are good candidates for near infrared (IR) applications due to their mid-wave IR cut-off wavelengths (5 ≈ 7 μm). Among inorganic glasses, sulfide materials exhibit the largest third-order optical nonlinear susceptibility and good IR transparency but suffer from low thermal-mechanical stability and photo-induced degradation upon exposure to near-bandgap radiation. The preparation of oxysulfide materials for optical applications offers a unique trade off between the superb chemical stability of the oxide and the attractive optical properties of sulfide. In this presentation we describe a new chemical route for the preparation of oxysulfide glasses for optical applications. The evidence of glass network structural modification is confirmed using infrared and Raman spectroscopy. Film deposition based on sputtering techniques compatible with synthesis of such materials has been performed. The initial characterization of the resulting films has been performed and findings are described.
We report the use of amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) overlays to fabricate low-loss distributed Bragg reflectors (DBRs) for active and passive glass waveguide devices. Overlay material issues, overlay DBR design procedures, and fabrication details are presented. We apply the technology to implement a multiple-wavelength source using an array of overlay DBR waveguide lasers on a single Er/Yb doped, ion exchanged, glass substrate. The lasing wavelengths of the laser array are linearly related to the width of the ion-exchange mask openings. One laser with a 8.5 mm long gain section and a 1.5 mm long overlay DBR had a launched pump power threshold of 29 mW and a 8.5% slope efficiency. We also fabricated a Mach-Zehnder-based, ion exchanged, glass waveguide, optical add-drop multiplexer (OADM) using an a-Si overlay DBR. The 3 cm long OADM exhibited a 24 dB transmission dip with a 3 dB bandwidth of 0.5 nm at the drop wavelength. The throughput loss of the OADM, excluding input and output coupling losses, was approximately 1.9 dB. Theory and measurement results are in good agreement for both the laser array and the OADM.