We report on the observations of novel phenomena in rare- earth-doped glasses induced by an 800 nm femtosecond pulsed laser. 3D long-lasting phosphorescence in various colors was observed in calcium aluminosilicate glasses doped with Ce³⁺, Tb³⁺, and Pr³⁺. After irradiation by the femtosecond laser, the focused part of the laser in the glasses emits bright and long-lasting phosphorescence able to be clearly seen with the naked eye in the dark even one hour after the removal of the activating laser. We also observed permanent photoreduction of Sm³⁺ to Sm²⁺ inside a transparent and colorless Sm³⁺- doped sodium aluminoborate glass. After irradiation by the femtosecond laser, the focused part of the laser in the glass became orange in color. Absorption and confocal photoluminescence spectra showed that a part of Sm³⁺ was reduced to Sm²⁺ after the laser irradiation. Photochemical hole burning phenomenon has also been confirmed in the femtosecond laser photoreduced part after the further irradiation by a DCM dye laser. Electron spin resonance spectra of the glasses before and after the laser irradiation have been measured to clarify the mechanisms of these phenomena. Microscopic modifications using a femtosecond pulsed laser is promising to open new field in science and technology of rare-earth-doped materials.

Cooperative upconversion and Yb³⁺-Er³⁺ energy transfer in newly developed phosphate glasses were studied in this paper. The cooperative upconversion coefficients of ⁴I_13/2 level for different Er³⁺ concentrations were deduced from the pump intensity- dependent luminescence decay curves. Yb³⁺-Er³⁺ energy transfer efficiencies for different Yb³⁺ concentrations codoped with Er³⁺ (approximately 2 X 10²⁰ ions/cm³) were estimated from the lifetime measurements of ²F_5/2 level of Yb³⁺ ions. The small cooperative upconversion coefficients and high Yb³⁺-Er³⁺ energy transfer efficiencies indicate these newly developed Er³⁺ and Yb³⁺/Er³⁺ doped phosphate glasses are excellent for amplifier and laser applications.

Rare earth ions are widely used in lasers and amplifiers. Depending on the device, there are different demands on the properties of the active ions and on the host glass. Thus, an extensive characterization and comparison of both host matrix properties and spectroscopic properties of rare earth ions for various glass types is of special interest. In this study, fluoride and phosphate based glasses on the one side and heavy metal oxide containing glasses such as Bi-borate, Pb-Bi-gallate and K-tantalate-gallate glasses on the other side were investigated in order to figure out the advantages of these different glass systems. UV-VIS and IR cut-off, phonon energy and refractive index were studied as characterization of the host matrix. Judd-Ofelt parameters, spontaneous emission probabilities and stimulated emission cross sections were calculated for Nd³⁺ and Er³⁺ ions. Peak wavelength, shape and linewidth of Nd³⁺ and Er³⁺ bands were determined from absorption and emission spectra. The effect of the different glass types on the properties of rare earth ions is considered in detail on the absorption and emission transitions of Nd³⁺ and Er³⁺ in the infrared region.

The chemical compositions, characteristic temperatures and optical transmission spectra of fluoride and chalcogenide glasses are presented. The various techniques used in the preparation of preforms, optical fibers, and planar waveguides are reviewed. Details are given on the application of rare-earth-doped fluoride glass fiber lasers, which operate at discrete wavelengths from the UV to the mid-IR. The performances of fiber amplifiers for telecommunication are reviewed as well. Because of a broad transmission range in the mid-IR, chalcogenide glasses are suitable for various passive applications. Experimental results on laser power delivery, radiometry and remote chemical analysis by means of chalcogenide fibers are reported.

Compositions which vitrify by means of a conventional glass melting method, containing a large amount of erbium ion were investigated in aluminosilicate, phosphate, borate, borosilicate, telluride and fluoride systems. As a result of melting test, we obtain some glasses which contain higher than 20 mol% erbium. In transparent materials containing erbium an anomalous optical absorption phenomena have been reported by Maeda et al. With increasing in an incident laser light intensity, the transmitted one through the transparency such as the glass decreases, that is called negative nonlinear absorption (NNA) effect. With a particular modulation degree of an incident laser light, the transmitted waveform were reversed in phase to an incident one. The NNA effect in these glasses was observed over a wide band of the wavelength, while in erbium doped yttrium aluminum garnet and erbium doped lutetium aluminum garnet crystals were observed at a narrow band. The discrepancy of the NNA effect between the glasses and the crystals might be explained in the term of absorption spectrum associated with erbium. Namely, the absorption spectra of erbium doped yttrium aluminum garnet crystal have a narrow absorption band of 788 nm, while that of the glasses have broad absorption band of about 800 nm.

Er$3+) doped tellurite glasses with various Na₂O/Li₂O, Na₂O/K₂O and K₂O/Li₂O ratios were prepared. Absorption and emission spectra, glass transition temperatures, crystallization temperatures, densities, and refractive index of these glass samples were characterized. For all studied glass samples, the highest effective linewidth was obtained with mixed alkali molar ratio of Na₂O/Li₂O equals 1, Na₂O/K₂O equals 1 and K₂O/Li₂O equals 1. The studied glasses have a (T_x- T_g) exceeding 100 degree(s)C, indicating these glass samples are stable against devitrification. In addition, Judd-Ofelt analysis of several glasses was also performed.

The optical properties of four electron trapping materials are studied, such as photoluminesce excitation spectra and emission spectra, writing/stimulating/readout spectra, temperature dependence of the relative writing/stimulating efficiencies. According to the experimental results, the optical storage mechanisms of these materials are given.

The effect of concentration of Er³⁺ on the upconversion luminescence of the Er:Yb:glass excited by InGaAs laser diode is reported. With different concentration of Er³⁺, the upconversion luminescence intensity, the intensity ration of green and red lights, and the near infrared lights are different. The detailed mechanisms of upconversion luminescence are analyzed.

Erbium-doped silica glasses were made by sol-gel process. Intensive photoluminescence (PL) spectra from the Er-doped silica glasses at room temperature were measured. A broadband peak at 1535 nm, corresponding to the ⁴I_13/2-⁴I_15/2 transition, its full width at half- maximum (FWHM) of 10 nm, and a shoulder at 1546 nm in the PL spectra were observed. At lower temperatures, main line of 1535 nm and another line of 1552 nm instead of 1546 nm appear. So two types of luminescence centers must exist in the samples at different temperature. The intensity of main line does not decrease obviously with increasing temperature. By varying the Er ion concentration in the range of 0.2 wt% - 5 wt%, the highest photoluminescence intensity was obtained at 0.2 wt% erbium doped concentration. Luminescence intensity decreases with increasing erbium concentration. Cooperative upconversion was used to explain the concentration quenching of luminescence from silica glass with high erbium concentration. Extended x-ray absorption fine structure measurements were carried out. It was found that the majority of the erbium impurities in the glasses have a local structure of eight first neighbor oxygen atoms at a mean distance of 0.255 nm, which is consistent with the typical coordination structure of rare earth ion.

In this work we review the most recent results obtained by our group on miniaturized diode-pumped Er-Yb:glass lasers of potential interest for optical communication applications. In single frequency operation we achieved output powers larger than 25 mW using a 200-micrometers length microchip laser and in excess of 100 mW, with a large tunability over the wavelength range between 1528 nm and 1564 nm, using a 5-mm length resonator containing an intracavity 50-micrometers thick Polarcor etalon. The Er-Yb microlaser has been locked and frequency stabilized at a level better than 1 X 10^-9 to C₂H₂ and ¹³C₂H₂ absorption lines in the wavelength range from 1530 nm to 1550 nm, to built a narrow-linewidth absolute frequency reference in this spectral region suitable for WDM applications. A 30-dB suppression of the intensity noise peak at the relaxation oscillation frequency has been obtained and a relative intensity noise level of less than -160 dB/Hz at frequencies higher than 20 MHz is expected. In pulsed operation regime, picosecond pulse trains with repetition rate up to 10 GHz have been generated by either the FM mode-locking and a novel technique based on frequency modulation operation of the laser followed by external spectral filtering of the optical field, able to produce transform-limited single and dual-wavelength pulse trains particularly suitable for soliton transmission.

Er-diffusion doped LiNbO₃ is an excellent electrooptic material for active integrated optics. With low loss Ti- diffused waveguide structures and electrooptic intracavity phase- or amplitude modulators modelocked waveguide lasers with pulse repetition rates beyond 10 GHz and Q-switched lasers with pulse peak power levels in the kW-range have been realized. Both types of lasers have been diode-pumped and fully packaged. Harmonic modelocking allows to combine high pulse repetition rates with high peak power levels. Such a laser has been successfully used as key component of a transmitter for nonlinear optical RZ- (soliton-) type data transmission at 10 Gbit/s. Concepts for the stabilization of a single supermode emission as prerequisite for pulses of high amplitude stability and low timing jitter and hence for low error rate transmission are discussed in detail. Results of Q-switched laser operation with folded Mach-Zehnder type switch of high extinction ratio are presented and potential applications of such a source emitting in the eye-save wavelength range discussed.

Erbium doped BaTiO₃ thin films for optical waveguide applications were investigated. Characteristic 4f emission at 1560 nm is observed for Er concentrations ranging from 10¹⁸ to 10²⁰ cm^-3. Factors which determined the luminescence efficiency were investigated and a model for efficiency was developed. The luminescence intensity depended predominantly upon two factors: the concentration of radiative Er centers and the de-excitation efficiency of the excited 4f electron state. At a growth temperature of 725 degree(s)C, the concentration of radiative Er ions was independent of Er dopant concentration. Annealing the thin films in an oxidizing ambient resulted in increased luminescence efficiency whereas annealing under reducing conditions quenched the luminescence.

Erbium (Er) doped semiconductors are of interest for light- emitting device applications operating at around 1.55 micrometers and for the potential integration with other semiconductor devices. However, the optical emission of Er³⁺ ions in semiconductors has not been as efficient as in dielectric materials, particularly at room temperature. This may be because ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er³⁺ energy levels than are covalent bonds, which are present in most III-V semiconductors. In this paper, we report 1.55 micrometers emission from an Er-doped GaN LED. We also discuss effect of the measurement temperature on the emission spectrum as well as the effect of sample annealing on the emission spectrum.

Optoelectronic regenerators have been used in traditional opticai communication systems to convert signals from the optical to electrical and then back to the optical domain. Since its first report in 1987 11,21, the erbium-doped fiber amplifier (EDFA) has revolutionized optical communications. This optical amplifier does not need high-speed electronic circuitry and is transparent to data rate and format, which dramatically reduces cost. EDFAS also provide high gain, high power, and low noise figure. More importantly, all the optical signal channels can be amplified simultaneously within the EDFA in one fiber, thus enabling wavelength-thvision-mWtiplexing (WDM) technology. Due to the recent exponential growth in data communication and the Internet, there is an urgent demand on high capacity communication networks. To increase the total capacity, one can work on one or both of the two things, 1) high speed, which is currently limited by high speed electronics and fiber dispersion and nonlinear effects, and 2) large numbers of channels which is in turn determined by the available bandwidth and channel spacing. Since signal channel spacing is limited by filtering technology, modulation speed and format and nonlinear effects, much attention has been paid to increasing the optical amplifier bandwidth in the recent years.

Silica based planar waveguides co-doped with Er³⁺, TiO₂ and Al₂O₃ have been fabricated on SOS (silica on silicon) by a sol-gel process using multiple spin-coating and rapid thermal processing (RTP). Their characteristics, such as refractive index, thickness photoluminescence (PL), FTIR (Furior Transform infrared spectrum), XRD (X-ray diffraction), and surface roughness are investigated. The relatively strong PL emitted from planar waveguide has been got using the recipe of 93SiO₂:7TiO₂:10Al₂O₃:0.5Er₂O₃ (mole ratio). The presence of O₂ during annealing in RTP is indispensable for sol-gel waveguide to guide light. Both the average refractive index and thickness of multilayer film increase as the layer number increases.

A new strip-loaded structure for ion-exchanged waveguide amplifiers in Er³⁺-doped glass is suggested. The fabrication of these waveguides lacks steps such as mask deposition and burial process that can be critical for some glasses. A simple numerical modeling shows that similar, or even higher, gain values as in ion-exchanged buried waveguides can be achieved. Preliminary experimental data for the fabrication of the strip-loaded structure are also reported.

Among the various possible production techniques of silica- on-silicon integrated optical devices, sol-gel is the one which combines low cost with a great flexibility and the ease of doping the silica matrix with nonlinear and active compounds. In the frame of an European project, we have investigated the application of the sol-gel technique to the realization of an erbium-doped optical amplifier, operating in the third telecommunication window. Here, in particular, we refer to the development of an optimum fabrication strategy for the guiding structure. A strip-loaded configuration was chosen. Design optimization was carried out by means of a MATLAB software code, mainly based on the Effective Index Method. For what concerns the technical side, two different routes were followed: that of the Low Index Load and that of the High Index Load. Pros and cons of both structures were carefully evaluated through numerical simulations as well as experimental analysis, in order to choose the best performing one. Results of the design procedure and the characterization of the fabricated waveguides are described here.

The polarization dependent performance of EDFA has been explained in terms of polarization hole burning, considering that the erbium sites in the silica matrix preserve its random nature. Although typical EDFAs are built with an erbium fiber longer than 10 m (hence the active fiber is necessarily wound) the influence of the bending has not been included in the investigation of this effect. To verify the relevance of bending induced birefringence, a helically- wound active fiber has been used to build an EDFA. The characterization of this helically-wound optical amplifier has shown that the polarization dependent performance was enhanced. The stronger link between the states of polarization of the signal wave, the pump wave and the active fiber birefringence axes was used to show that a selective excitation of the erbium ions by a polarized pump and a selective de-excitation of those ions by a polarized signal are the origin of the polarization dependent performance of EDFAs.

The absorption and fluorescence spectra of Er-doped phosphate and silicate laser glasses were investigated in a temperature range of 20 - 300 degree(s)C. On heating, the differential spectra demonstrated variations of spectral profiles. The effects of heating for silicate and phosphate glasses are similar. The contours of the absorption and luminescence bands varied nonmonotonically with the temperature. These variations were explained by a redistribution of the populations of the Stark sublevels of the ⁴I_15/2 ground and the ⁴I_13/2 excited terms. The heating resulted in 5 - 10% decrease of oscillator strengths of spectral bands. On the base of temperature variations of spectral profiles we proposed an energy levels model, which allowed to explain the thermal variation of spectrum of Er-doped laser at 1.54 micrometers . According to this model, the transitions in two range of lasing (1534 and 1542 nm) come from two Stark sublevels of the ⁴I_13/2 excited manifold to two sublevels of the ⁴I_15/2 ground manifold. The effective energy gaps between neighboring Stark components are equal to 50 cm^-1 and 100 cm^-1 for the ⁴I_13/2 and the ⁴I_15/2 states, respectively.

An investigation on optical amplification in Ti waveguides in LiNbO₃ doped with Er ions by thermal diffusion of thin metallic stripes is presented. The possibility of fabricating efficient optical amplifiers in LiNbO₃ substrates realized by localization of the dopant on surface areas of the crystals was theoretically evaluated and the feasibility of fabricating efficient amplifiers in such doped structures was experimentally verified. It was concluded that the localized doping technique allows optimization of amplifier performance through adjustment of the active region geometry to the mode intensity profile.

Since the pump power is absorbed in a small region in the fiber core, a high concentration of the Rare Earth Ions is required. As a result boding to Er⁺³ ions is difficult and an inhomogeneity in ion concentration results which causes a clustering of these ions in the host lattice. Clustering depopulates the upper laser level. A solution to this problem is to confine erbium ions in the fiber core center. The Vapor Axial Deposition technique provides this requirement and as a result we have an increase in the amplifier gain. Our aim in this work is to provide a model that considers a symmetrical distribution for erbium ions which obeys a gaussian profile. We then solve the rate and propagation equations analytically. Our results show that the overlap integral between the dopant and optical mode ((Gamma) _k) is independent of the pump power over the (b/w < 1) range, where b is the equivalent radius of the doped region and w is the minimum spot size of the incident radiation. We use this result to find an analytic gain equation which agrees well with the result of other models obtained by standard numerical integration techniques.

The intensity parameters are calculated according to the absorption spectra, from which we can know that the host characteristics are not the same for the two types of glasses. Some important parameters such as absorption cross section and integrated emission cross-section are also calculated. Fluorescence spectra near 1540 nm are measured too. We have designed the resonator carefully, with a 969 nm LD to pump the two types of 1.5 mm-thick-Er:Yb:glasses. More than 5.0 mW laser output is obtained and the central wavelength is 1536 nm, with spectra range of about 30 nm. The experiments show that the new type of glass has a better thermal characteristic, and no obvious saturation phenomena appears.

Optical absorption and luminescence spectra of the Sm³⁺-doped fluorophosphate glasses 75NaPO₃- 24CaF₂-1SmF₃ and 75NaPO₃-24BaF₂-1SmF₃, the phosphate glass 75NaPO₃-24.5ZnO-0.5Sm₂O and the fluoride glass ZBLAN:Sm³⁺ have been recorded. The dipole strengths of the transitions in the absorption spectrum are parameterized in terms of three phenomenological Judd-Ofelt intensity parameters (Omega) _(lambda ) ((lambda) equals 2, 4 and 6). The determination of intensity parameters for Sm³⁺ has inherent difficulties due to very large number of energy levels lying close to each other. In this case, we can take advantage of the property that both the dipole strengths and the squared reduced matrix elements of overlapping transitions are additive. The relation between the spectral intensities and the glass composition is discussed.

Sol-gel materials have been widely used for the fabrication of micro-optical devices during the last decade. Motivation for the sol-gel processing has been the low cost and a large variety of fabrication techniques and material properties. One of the interesting possibilities has been to use sol-gel glasses as an erbium host for integrated optics amplifiers. Several research results have been published, but sol-gel processing has not yet proven its potential for efficient devices in this important and rapidly advancing field. There are still some problems to be solved, such as the removal of residual water from sol-gel matrices and the process reliability and reproducibility. In this paper, we review the present status of the sol-gel materials and discuss their potential and challenges for the erbium-doped waveguide amplifiers.