Erbium-doped planar optical amplifiers can find numerous applications in photonic integrated circuits operating at 1.5 micrometers . The challenge is to fabricate these devices with high gain, operating at low pump power, and having small overall size. In this paper a review is given of our recent work in the area of Er-doped waveguide materials and amplifiers based on three materials classes: oxide films (Al₂O₃, Y₂O₃, SiO₂), polymers, and silicon.

The integration on the same glass substrate of laser sources and passive devices made by low cost techniques such as the ion exchange has already been demonstrated. On the other hand, we have already reported the synthesis of a passive, photosensitive and ion-exchangeable glass, called SGBN. In this paper we present the synthesis of the first (to our knowledge) active, photosensitive and ion-exchangeable glass family, obtained by proper rare-earth ion doping of the SGBN glass. Examples of the typical results of the physical and optical characterization of these glasses (characteristic temperatures, refractive index variation, absorption and emission spectra, cross sections) are given.

Er³⁺ emission and laser properties at 1.5 micrometers have been studied in a fluoride phosphate glass (FP20) and in different meta- and ultraphosphate glasses. Moreover, the commercial Kigre QW phosphate glass has been investigated. The effect of glass composition, oxygen bubbling during melting procedure, and sodium content on Judd-Ofelt parameters, on linewidth, and emission cross section at 1.5 micrometers has been determined. Emission decay behavior and lifetimes at 1.5 micrometers have been examined with respect to OH content, Er³⁺ concentration, and calculated emission lifetime. Gain spectra were calculated for FP20 and QX glass for estimation of laser output and tuning range. Laser performance has been investigated for FP20, QX and ultraphosphate glasses. The laser properties are discussed in terms of gain spectra characteristics, emission cross sections, OH content, and optical quality of the samples. Results demonstrate that the FP20 glass is a very attractive host material for broadband amplifiers and ultraphosphate glasses have a potential for application in high peak power lasers. Furthermore, it is shown that the use of high sodium oxide content required for fabrication of ion-exchanged waveguides does not deteriorate the Er³⁺ emission properties.

We demonstrate a sol-gel method for preparing Eu³⁺ and Sm²⁺ ions-doped glasses exhibiting persistent spectral hole burning at high temperature. Glasses having the composition of Eu³⁺ or Sm²⁺-doped Al₂O₃-SiO₂ have been prepared by the hydrolysis of metal alkoxides and heating at 800 degree(s)C in air or hydrogen gas atmosphere, respectively. The persistent spectral hole burning is measured within the ⁷F₀-⁵D₀ transition band. For the glasses containing OH bonds, the hole is formed by the photoinduced rearrangement of the OH bonds surrounding the Eu³⁺ or Sm²⁺ ions, and is thermally refilled and erased above approximately 200 K. On the other hand, the glasses heated in hydrogen gas show the hole spectra above approximately 200 K. The hole depth is independent of the temperature and is approximately 7 and 15% of the total intensity at room temperature for the Eu³⁺ and Sm²⁺ ions-doped glasses, respectively. The proposed mechanism is the electron transfer between the rare-earth ions and the defect centers related with Al³⁺ ions in glass network.

In the present paper, we report the preparation of Er3+ doped SiO (formula available in paper) planar waveguides deposited on SOS Silica on silicon substrates using sol-gel process combining with spin-coating and rapid thermal annealing. The recipes used for preparing the solutions by sol-gel process are (formula available in paper). The fluorescence spectra and lifetimes of the planar waveguides were measured under the excitation of 514 nm wavelength from an Ar⁺ laser. The lifetimes of 3.80 ms, 3.07 ms and 1.59 ms were obtained for 0.5 mole, 1.0 mole and 2.0 mole of Er³⁺ content planar waveguides respectively. The decay mechanism and the factors that influence the lifetime of the sol-gel planar waveguides have been analyzed.

Two erbium doped germanate glass samples prepared by sol-gel process and melting and casting method, respectively, have been characterized. A fluorescence lifetime of 2.5 ms at the ⁴I_13/2 level and a very broad bandwidth (FWHM: 53.7 nm) around 1.55 micrometers have been observed for the sol-gel sample. The fluorescence lifetime and the spectrum bandwidth for 1.535 micrometers transition for the sample fabricated by the melting and casting technique were measured to be 10 ms, and 19 nm (FWHM). The cooperative up-conversion coefficient is measured to be 1.3 X 10¹⁸ cm³/s for the sample with an erbium concentration of 2.69 X 10²⁰ ions/cm³.

We report the preparation and photoluminescence spectral analysis of Tb³⁺ and Eu³⁺ doped Zn₂SiO₄ powders in sol-gel sol-gel process. On exposure to an UV source, these rare earth ions doped zinc silicate materials have shown prominent luminescent green and red colors, due to the electronic transitions of ⁵D₄ yields ⁷F₅ (Tb³⁺) and ⁵D₀ yields ⁷F₂ (Eu³⁺) respectively. The green emitting transition of Tb³⁺ material has been a magnetic dipole in nature and the red emitting transition of Eu³⁺ material has been due to an electric dipole in nature with a hypersensitivity in its luminescence. Systematic characterization has been carried out to understand the fluorescence efficiency of these two materials by measuring their excitation, photoluminescence, time-resolved luminescence spectra and lifetimes of the emission transitions (⁵D₀ yields ⁷F_0,1,2,3,4&5) of Eu³⁺ at two different excitation wavelengths of 226 nm and 395 nm. However, for the emission transitions (⁵D₄ yields ⁷F_6,5,4,3,2&1) of Tb³⁺ doped material, one excitation wavelength of 229 nm was used. Structural characteristics of these materials was carried out with an XRD and TEM measurements. The computed color coordinates suggest that both the phosphors have all desirable features for CRT screen applications.

The surface morphology and the room temperature 1.54 micrometers photoluminescence (PL) intensity have been investigated as a function of Er flux in GaN:Er grown by gas source molecular beam epitaxy using below bandgap excitation energy. Unlike AlN, GaN:Er showed improved surface smoothness as evidenced by atomic force microscopy and scanning electron microscopy, with RMS roughness values improving from 18.1 to 2.0 nm as the Er concentration is increased. Similarly the PL emission increased with increasing Er concentration and showed no evidence of saturation or concentration quenching. Even further improvements in 1.54 micrometers PL intensity from GaN:Er have been obtained through the introduction via ion implantation of carbon or oxygen, with carbon appearing to produce the greatest increase. Increasing the C concentration through the use of CBr₄ during growth initially improved the surface smoothness, with RMS roughness improving by a factor of seven over undoped GaN. The PL also improved dramatically. However, the highest amounts of C investigated produced a decrease in the PL as well as a roughening of the film surface. These effects indicate that the GaN:Er had reached its C solubility limit, producing an increased amount of defect induced nonradiative recombination.

The nanostructured matrix of porous silicon makes the material an ideal host for erbium because its very large surface area allows easy infiltration of the ions into the matrix and it readily oxidizes obtaining large concentrations of oxygen necessary for erbium emission. Erbium is infiltrated in the pores (<EQ 10^-19 cm^-3) by cathodic electrochemical migration of the ions followed by high temperature annealing (950 - 1100 degree(s)C). Electrochemical doping of porous silicon by erbium is simpler and of lower cost when compared to conventional techniques like ion implantation, epitaxial growth, and chemical vapor deposition used to fabricate erbium-doped c-Si structures. We demonstrate stable room- temperature electroluminescence at 1.54 micrometers from erbium- doped porous silicon devices under both forward and reverse bias conditions.

Si nanocrystals (nc-Si) embedded in silica have recently attracted a lot of attention as a potential optoelectronic material due to their light emission at approximately 1.7 eV. Er³⁺ is attractive because its 1.53 micrometers emission coincides with the low attenuation region of silica optical fibers. In this paper, we report the experimental investigation of energy transfer between nc-Si and Er³⁺ in ion implanted material which may relax requirements on the Er³⁺ pump source and lead to broad-band pumped optical devices.

Up-conversion by Yb³⁺ sensitization in Er3+ doped YVO4 crystals has been studied under excitation with 976 nm laser diode. Up-converted (available in paper) emission bands are observed at 490, 547, 554, 660 and 670 nm at room temperature, together with down-converted Yb³⁺ emission at 1010 nm and Er³⁺ emission at 1470 - 1630 nm. Excited state absorption from the ⁴I_11/2 state of Er³⁺ to the ⁴F_7/2 state is observed in the Yb³⁺ emission spectrum. It is confirmed, from quadratic dependence on pump intensity, the up-conversion is induced by two-photon excitation process.

The use of rare earth oxysulfide-type phosphor screens is common place within x-ray scanning instrumentation either in detection or imaging mode. The x-ray phosphors based on praseodymium-doped gadolinium oxysulfide (Gd₂O₂S:Pr) have short decay times to 10% intensity compared to the more well known europium and terbium analogues. The prompt emission of 511 nm luminescence from the Pr-ion results in excellent capability for fast scanning x-ray applications in the 20 - 100 microsecond(s) time frame.

Raman and Brillouin spectroscopy in planar waveguides can be performed by coupling the laser beam to the waveguide with a prism and by collecting the scattered light from the front surface. Brillouin spectra, which are different for excitation in the different modes of the waveguides, can be accounted for by a model that considers the space distribution of the exciting field. This allows measuring the sound velocity in the film. This technique has been applied to a silica-titania planar waveguide, obtained by r.f. sputtering. The same experimental setup is used for collecting the Raman spectra of the glassy waveguides. In graded index waveguides, as those produced by ion-exchange in glass, different guided modes propagate in layers with different thickness. By comparison of the Raman spectra, taken by waveguide excitation in different modes, it is possible to characterize the guide at different depths. The method has been applied to waveguides obtained by ion- exchange of silver in a soda-lime substrate. Raman scattering from the optical vibrations of the glass and from the acoustic vibrations of silver nanoclusters depends on the silver concentration, providing different spectra for excitation in different modes of the waveguide.

This paper focuses on the preparation and optical characterization of rare-earth-doped fluoride glass channel waveguides. Fluoride glasses are materials with unique optical properties. Because of low phonon energies of the host, emissions of rare-earth ions show higher quantum efficiencies in fluoride glasses as compared to silicates. Waveguides can be prepared by two methods. First, fluorine-- chlorine anionic exchange at the surface of fluorozirconate glass, which results in gradient-index waveguides. Second, physical vapor deposition of Pb-Zn-Ga fluoride glass, which allows the preparation of step-index waveguides. Both systems can be doped with rare-earth ions. The spectroscopy of Nd³⁺ and Er³⁺ ions in both waveguide configurations is investigated and the most recent results on optical amplification around 1.05 micrometers and 1.54 micrometers are presented.

Soda-lime silicate glasses doped with different percentages of Er- and Yb-oxides were produced by melting, and both planar and channel waveguides were fabricated by diluted silver ion exchange. Their optical and spectroscopic properties have been investigated, and some results are reported here. The optical parameters such as refractive index, diffusion depth and number of modes were measured by m-line technique. Absorption and fluorescence spectra were also measured; particular attention was focused on upconversion phenomena. The upconversion luminescence spectra of an Er/Yb-activated waveguide were obtained by continuous-wave excitation at 514.5 nm, as a function of the excitation power. The weak upconversion bands are assigned to the ²H_9/2 yields ⁴I_15/2 and ⁴G_11/2 yields ⁴I_15/2 transitions of the Er³⁺ ion. The upconversion mechanism is found to be a two-photon process. Preliminary results of gain measurements in channel waveguides showed signal enhancement at 1.5 micrometers , for high input signal intensities, upon pumping at 0.98 micrometers .

The erbium-lithium ion exchange is presented as a method for the erbium local doping of lithium niobate crystals. Ion exchange process is performed immersing the LiNbO₃ substrates in a liquid melt, containing erbium ions; due to their high mobility, the lithium ions migrate from the crystal to the melt, and are replaced by erbium ions. A systematic analysis of the doping process is performed, and the influence of the process parameters is investigated: exchange time and temperature, crystal cut direction, composition and chemical reactivity of the Er ions liquid source. By structural (X-Ray Diffraction and Rutherford Backscattering Spectrometry), compositional (Secondary Ion Mass Spectrometry) and spectroscopic techniques (optical spectroscopy and micro-luminescence), the formation of lithium deficient phases and the incorporation of the Er ions into the LiNbO₃ matrix is studied.

We encapsulated rare earth ions such as europium and neodymium with organic ligands to incorporate them into polymer matrices. Their spectroscopic properties in polymer hosts and organic solutions were studied. Infrared emission has been observed from a neodymium-doped perdeuterated core graded index polymer optical fiber.

Broadband erbium-doped fiber amplifiers at 1.5 micrometers band, an important communication band, are generating great interest in order to increase the transmission capacity of wavelength-division-multiplexing transmission networks. The host materials for Er³⁺ doping are very important for obtaining an intrinsically broad amplifier bandwidth. This paper reports the spectral properties of the Er³⁺ doped lead halotellurite glasses, PbX₂-TeO₂ (X equals F, Cl, Br). The measured absorption and emission spectra were analyzed by Judd-Ofelt and McCumber theories. It was found that Er³⁺ doped lead halotellurite glasses have a good glass stability, high refractive indices, high absorption and stimulated emission cross sections, and a very broad bandwidth for 1.5 micrometers absorption and emission. The 1.5 micrometers band, which has a substantial magnetic dipole transition component, is a special band whose bandwidth increases significantly with the refractive index of the host material. It is expected that the halotellurite glasses will be the promising host materials for 1.5 micrometers broadband amplification.

Intense upconversion of yellow-to-blue, in Pr³⁺- doped, and green-to-blue in Er³⁺-doped aluminum fluoride glasses is reported. Pulsed excitations inside the ³H₄ yields ¹D₂ absorption band of Pr³⁺ results in blue emission which is ascribed to the ³P₀ yields ³H₄ transition. We found that the temporal evolution of the upconverted fluorescence is characterized by rise and decay times depending on Pr³⁺ concentration. The upconversion mechanism is a two step process due to the energy transfer involving a pair of Pr³⁺ ions. The data indicate a high solubility of the Pr³⁺ ions without clustering at the doping level between 0.1 - 1% mol Pr³⁺. Measurements performed in Er³⁺ doped glasses indicate a good quantum yield of the upconversion emission confirming the high solubility of Er³⁺ ions in the aluminum fluoride glasses.

A steady-state method was used for characterization of up- conversion quenching in Er-doped glasses. This method is based on measurement of pumping dependencies of quantum yield and the population of the ⁴I_13/2 metastable manifold. The population was determined from absorption spectra variation under the pump of semiconductor laser. The feature of the method consists in measurement of absorption spectra variation in short-wave spectral range of 300 - 520 nm. In this spectral region, a hypersensitive excited state absorption (ESA) band ((lambda) equals 478 nm) for ⁴I_13/2 yields ⁴G_9/2 transition was detected. The following parameters characterizing absolute intensity of ESA band were determined: line strength, integral cross- section, cross-section in maximum, and oscillator strength. This band was used for evaluation of the population of the ⁴I_13/2 metastable manifold and up-conversion coefficients. The up-conversion coefficient, C_up equals 1.7 X 10^-17 cm³s^-1, was obtained for an Er³⁺ concentration of 6.8 X 10²⁰ cm^-3.

High-power compact ultrafast pulse sources based on optical fiber lasers are discussed. The optical efficiency of fiber- based ultrafast pulse sources is optimized by the implementation of double-clad Yb-doped fibers. The spatial limitations of ultrafast single-mode fiber lasers and amplifiers are overcome by the implementation of diffraction-limited multi-mode rare-earth-doped fibers, allowing for the generation of ultrafast pulses with large peak powers. In the temporal domain, a further increase in obtainable peak powers is made possible by the use of chirped pulse amplification or the amplification of pulses with parabolic temporal profiles. Parabolic pulses are generated in the asymptotic limit in high gain fiber amplifiers operating in the positive dispersion regime.

We report an experimental study of passive mode-locking of a flash-pumped Nd:YAG laser using for the first time a non- linear polarization rotation effect in a type II crystal cut for second harmonic generation. The mode-locking regime is self-starting and produces pulses of 20 ps duration with more than 100 uj per pulse. We developed a numerical model to calculate laser pulse steady-state temporal profile as a function of group velocity mismatch between fundamental and second harmonic pulses and group velocity dispersions in the nonlinear crystal. It takes also into account on laser medium and resonator characteristics insuring perfect second order dispersion compensation during one round trip in the cavity.

Wavelength routing and reconfigurable cross connects are emerging concepts for optical multiwavelength telecommunication networks. They provide more efficient usage of the network resources, as individual wavelength channels can be added or dropped from the wavelength multiplex. However, problems arise with the erbium-doped fiber amplifiers (EDFAs), whose gain is dependent on the input power level. If the gain of the EDFA is not by some means controlled, transient effects will occur due to the EDFA's slow gain dynamics. In this paper a simple device for controlling the gain of the EDFA is studied. The gain- controlling scheme is based on a fast pump laser control. Part of the total input power to the amplifier is detected by the gain controlling circuitry, which then compensates for the changing gain by adjusting the pump laser power. In the study, transient effects due to changing number of channels in an EDFA are measured. The response time and the transient suppression of the gain-controlling device are verified through measurements. The effect of the amplifier gain tilt is also studied. A comparison against other proposed gain-controlling schemes is done.

All-optical automatic gain control in loop erbium-doped fiber amplifiers is investigated using single Bragg reflecting fiber grating. Reflecting back part of the filtered amplifier spontaneous emission to the loop mirror generates a lasing signal. The lasing wavelength is selected at 1525 nm, which is outside the amplifier bandwidth, to avoid interference between the signal and laser oscillation, and away from the amplifier surveillance wavelength at 1510 nm. This technique avoids tapping part of the amplifier output signal for generating the lasing signal. The loop amplifier has 20 nm flat gain spectrum with peak to peak gain variation less than 0.5 dB. Adjusting the lasing threshold using variable optical attenuator placed before the fiber Bragg grating controls the achieved gain clamping. An input signal consists from 8 channels picked up on the ITU frequency grid in the range from 1540 nm to 1560 nm and two laser diode pumps at 980 nm each with 60 mW pumping power are used in the experiments. The power excursion when adding or dropping any number of the 8 channels with -25 dBm input power per channel is less than 0.3 dB. The transient response time of the surviving channel(s) is measured to less than 200 microsecond(s) .

The values of measured and calculated spectroscopic quantities of lithium niobate doped with rare earth and transition metal ions, such as polarized emission and absorption cross sections, variation of fluorescence life time with temperature and concentration of the dopant, Judd- Ofelt coefficients, non-radiative transition probabilities and energy levels are presented. Wherever published data is available, comparison with measured or calculated data presented in this work is carried out. The theories utilized in the interpretation of the experimental results, such as Judd-Ofelt theory, Fuchtbauer-Lademburg relation and McCumber theory are summarily presented.

Here, we use molecular dynamics simulation to reconstruct a silica-titania glass with a Ti/Si atomic ratio of 8.5% activated by 0.7 at% of erbium. These quantities are chosen because they give both refractive index and optically ions concentration suitable for applications. We use a modified Born-Mayer-Huggins potential taking into account a three- body interaction. The distribution of TiO₄ and SiO₄ units as well as the bridging to non-bridging oxygen ratios are evaluated. The local environment of rare-earth ions is also analyzed. In particular, the clustering of erbium is discussed. From the simulated structure, the crystal-field strength is computed and discussed according to the Er³⁺ local environment. Finally, results are compared with information obtained by Raman and photoluminescence spectra.

Modeling of laser oscillation at 0.9 micrometers in Ti waveguides in LiNbO₃ doped with Nd ions is presented. Laser emission at 0.9 micrometers in Ti waveguides in Nd:LiNbO₃ crystals was recently demonstrated. However, lasing was reported as unstable and lasting only a few seconds, with parasitic lasing at the higher gain transition at 1.08 micrometers shown to be a problem. In this work the possibility of obtaining efficient and stable laser oscillation at 0.9 micrometers in Ti:LiNbO₃ waveguides, fabricated in substrates doped with Nd ions by thermal diffusion of thin metallic stripes or planar thin films, was theoretically evaluated. It was concluded that emission at 0.9 micrometers , with complete suppression of the parasitic emission at 1.08 micrometers , should be possible by selective increase of the losses at 1.08 micrometers , through optimization of waveguide and laser cavity, spatia localizations of the Nd ions and the use of the dependence on polarization of the emission cross sections at 0.9 and 1.08 micrometers .

An approach based on the theory of McCumber is described to calculate the emission cross sections of a transition of the rare-earth ions in glass. It overcomes the drawback of the theory that the knowledge of the electronic structures of the rare-earth ions is usually demanded to determine the emission cross section. Instead, only the measurement of the absorption spectra of the ions is necessary. The Judd-Ofelt theory is used to determine the energy parameters. The emission cross sections of Tm³⁺, Er³⁺ and Yb³⁺ in fluoride glasses are calculated. The results are consistent with those reported before, which indicates the method is simple and quite effective.

We have investigated the fast power transient effect of the cascaded erbium-doped fiber amplifiers (EDFA) in wavelength routed optical networks. The BER degradation of surviving channel was measured at the different adding/dropping frequencies of the disturbed channels. The result show that, for low channel-adding/dropping frequency closed to EDFA transient rate, the transmission performances of the surviving channels are impaired severely.

We propose a novel structure of Er-doped fiber ring laser, in which a fiber Bragg grating and an all-fiber M-Z interferometer are used to select wavelength. Output laser with line-width narrower than 0.1 nm at 1.53 micrometers waveband is obtained.

The Judd-Ofelt intensity parameters for f-f transitions of Nd³⁺ ions in PMMA-PAAc copolymer samples were determined from optical absorption measurements, and their dependence on the neodymium content investigated within the range of 0.1 to 3.0% mol. The densities, refractive indices and emission spectra have been measured. For our Nd³⁺-doped PMMA-PAAc samples an effective energy transfer between the copolymer host and the neodymium ions is observed and discussed.

Decay curves of 1.5 micrometers (⁴I_13/2) and 980 nm (⁴I_11/2) emissions from Er³⁺ ions in Ga₂S₃-GeS₂-La₂S₃ glasses were measured by the excitation of 1.5 micrometers laser diode pump. The decay curves were simulated using rate equations for a model consisting of four levels (⁴I_15/2, ⁴I_13/2, ⁴I_11/2, and ⁴I_9/2) of Er³⁺. A pair of unknown parameters, i.e., an excited state absorption coefficients for ⁴I_13/2 yields ⁴I_9/2, (sigma) _esa, and a cooperative upconversion coefficient for (⁴I_13/2, ⁴I_13/2) yields (⁴I_9/2, ⁴I_15/2), C, were included in the rate equations. They were estimated by fitting the simulated curves to the measured ones. The obtained values were 3 X 10^-21 cm² and 1 or 3 X 10^-21 cm³/s for the (sigma) _esa and C, respectively.

We suggested a new synthesis process, the sol-gel method, to incorporate high concentration of Er³⁺ ions into the silicate film on silicon substrate. The coating solutions with uniformly dispersed metal ions were prepared from Er(NO)₃ solution, Si(OC₂H₅)₄ and ethanol. Spin coating was carried out into a treated mono-crystalline silicon (100) substrate, at a speed of about 4000-rpm for 8s. The precursor films were annealed in air at 500 degree(s)C for 2 hours. Er-related approximately 1.54 micrometers photoluminescence was observed at room temperature. The optimal doped concentrations of Er³⁺ in SiO₂ film was estimated at about approximately 10²¹/cm³. Through characterizations of AFM, FTIR, DTA, TG and EXAFS, we found that: the synthesized film is compact and smooth in microscope; 500 degree(s)C annealing is enough to perform the structure change of gel to glass; and, the Er-O formed complex with coordinate number of 9. Because of the formation of Er-O complex, Er-O-Si can form the next structure in the synthesized film. The configuration could not only increase the optical activity of ions, but also improve the homogeneous of Er³⁺ ions in SiO₂. Thus we demonstrated the sol-gel process is an efficient way to overcome the quench effect of concentration and increase the distribution quality.

This paper reports an interesting `characteristic' saturation phenomenon of upconversion luminescence of TmYb co-doped ZBLAN glass when excited by 966 nm-diode laser. That is the log-log plot of upconversion fluorescence intensity upon laser power is a straight line basically, however the slope of the log-log curve of two-photon and three-photon upconversion fluorescence intensity upon laser power is smaller than 2 and 3 very evidently respectively in laser focus point. It is very interesting that the slope would be enlarge clearly if the facula of pumping laser was increased. It could be believed that this phenomenon is not a simple `typical' saturation phenomenon that is caused by population exhausting in ground state, which F-P log-log curve is gradually bending. The only reason is just the energy diffusion. That is said, because of the resonant energy transfer among Yb³⁺ ions, Yb³⁺ ions which are within the laser facula and absorbs photon energy of pumping laser would diffuse energy to those Yb³⁺ ions which are out the laser facula. This process would cause energy density waste. And the slope would reduce also. Especially the proportion of wasting energy density is independent to pumping laser power, so the slope would be a straight line basically. However if laser facula was bigger than energy diffusion length, the energy density waste caused by energy diffusion would have be zero basically. Under this case, the slopes of log-log variation F-P curve are quite near the normal multi-photon relation. The similar mechanism has not been reported internationally up to now.

This paper reports a quite interesting 474 nm up-conversion fluorescence when ErYb co-doped oxyfluoride glass is excited by 966 nm-diode laser. It was found that the 474 nm fluorescence is two-photon fluorescence. It can be found also that 474 nm fluorescence is posited between 490.5 nm ⁴F_7/2 yields ⁴I_15/2 and 454.0 nm ⁴F_5/2 yields ⁴I_15/2 fluorescence transition, and cannot be the transition from high excited state to first excited state of Er³⁺ ion also. In brief, the 474 nm fluorescence must be an extraordinary up-conversion fluorescence of ErYb co-doped oxyfluoride glass. In fact, it is reasonable entirely to believe that this is cooperative radiation fluorescence oriented from a kind of Couple State of two Yb³⁺ ions' cluster. The up-conversion processes are as follows: First each isolating Yb³⁺ ion absorbs pumping photon energy and is excited to ²F_5/2 state from ²F_7/2 ground state. Then two Yb³⁺ ions, which are both in ²F_5/2 excited state, form a kind of Couple State of cluster (formula available in paper). And then the 474 nm fluorescence would be radiated from excited state (formula available in paper) of cluster's Couple State to its ground state (formula available in paper). The similar mechanism has not been reported internationally up to now.

In this paper the fluorescence quantum efficiencies and branch ratios of ytterbium doped oxide glasses are firstly calculated with equation derived from the reciprocity principle. It is shown that using absorption spectra of ytterbium doped glasses including borate, phosphate, niobosilicate, telluorogermanate and tellurite glasses, the fluorescence quantum efficiencies are obtained and varies from 80% to 99%, increasing with decreasing phonon energy of glass hosts. The branch ratios change slightly as glass hosts with different phonon energy and are about 40%, 30%, 30%, 0% around 970 nm, 1000 nm, 1020 nm, 1050 nm wavelength, respectively.

The stark energy level split of (²F_7/2, ²F_5/2) of ytterbium ion in glasses are derived from room temperature absorption and emission spectra. It is shown that the stark energy split increases as the base varies from phosphate to tellurite glasses, and the first and second stark energy levels lay in range of 150 - 250 cm^-1, 350 - 450 cm^-1 over ground, the first over ground has smaller energy and thus larger Boltzmann heat effect and is difficult in lasing at this terminal level. The second level is 350 cm^-1 over ground and can be considered as the terminal level whose lasing wavelength varies from 1000 nm to 1020 nm. The partition function ratio of the upper and lower levels of ytterbium ion is calculated and more accurate reciprocity equation by which emission cross section is determined from absorption spectra is obtained from stark energy levels.

Advances in optical fiber amplifier for optical network are reviewed in this paper. Considerable progress has been made in optical amplifier technology in recent years. The bandwidth of amplifiers has increased several times and flat gain amplifiers with more than 80 nm of bandwidth have been demonstrated. With the advent of Raman fiber amplifiers, more wider bandwidth is obtained. Progress has also been made in the understanding of amplifier gain dynamics. Several control schemes have been successfully demonstrated to mitigate the signal impairments due to fast power transients in a chain of amplifiers and will be implemented in optical network design. Terrestrial optical systems have ben increasing in transmission capacity. In this review, we focus on the recent progress in some important aspects of several optical fiber amplifier technology.

A mixed crystal Sm²⁺-doped Mg_0.5Sr_0.5FCl_0.5Br_0.5 has a high potential in the application for a 3D optical memory. We have observed one-photon persistent spectral hole burning in this material at room temperature. The powder sample of (formula available in paper) was synthesized by firing at 1000 degrees C for 2 hours in a reducing stream of hydrogen. The product was examined with an X-ray diffractometer for the crystalline structure. Fluorescence excitation and emission spectra were recorded with a spectrofluorometer.