MXPA00002285A - GLASS FOR HIGH AND FLAT GAIN 1.55&mgr;m OPTICAL AMPLIFIERS - Google Patents

Abstract

The invention relates to a family of erbium-doped fluorophosphate glasses for use in optical signal amplification. The composition, based on 100 parts by weight, is constituted by:P2O5 15-40, A12O3 0-5, MgO 0-9, CaO 0-9, SrO 0-9, BaO 0-45, AlF3 5-25, MgF2 0-10, CaF2 0-25, SrF2 0-25, BaF2 0-20, KHF2 0-2, K2TiF6 0-2, with up to 10 parts by weight of erbium oxide. The glasses according to the present invention exhibit a high gain and a very flat spectrum over the 1550 nm bandwidth, as compared to the glasses of the figure. These glass compositions are particularly well suited for use in fiber or planar optical amplification in WDM and similar applications.

Description

CRYSTAL FOR OPTICAL AMPLIFIERS OF 1.55 um HIGH AND FLAT GAIN FIELD OF THE INVENTION The present invention relates generally to the field of optical signal amplifiers and in particular, to the use of fluorophosphate crystal compositions in optical signal amplifiers operating at wavelengths of approximately 1.55 μm.

BACKGROUND OF THE INVENTION Optical signal amplifiers have quickly found use in optical telecommunication networks, particularly in those networks that use optical fiber over long distances. Although modern silica-based optical fibers generally show a relatively low window loss of 1.55 μm, they have losses to some extent and loss accumulate with distance. To reduce this decrease, optoelectronic devices have been used to increase the strength of the signal. These devices require that the optical signal be converted to an electronic signal. The electronic signal is then amplified using commonly known amplification techniques and converted back to an optical signal for a retransmission.

The optical signal amplifiers amplify the optical signals without requiring an optoelectronic conversion of the signal. In optical amplifiers, the weak light signal is directed through a section of an amplification means that has been doped with ions of a rare earth element. Light from an external source of light, usually a semiconductor laser, is poured into the amplifying medium by stimulating the rare earth atoms to a higher energy level. The light that enters the amplifying medium at the wavelength of the signal also stimulates the excited ions of the rare earths to emit their excess photon energy in the form of light at the wavelength of the signal in phase with the pulsations of the signal, thus amplifying the light signal. One type of optical amplifier uses a length of optic fiber impregnated with erbium. Erbium-doped fiber amplifiers (EDFA) are commonly contaminated in the order of 100-500 ppm erbium ion. The overall lengths of the EDFA fiber are in the order of 10-30 meters, depending on the final gain requirements needed for a particular application. In some applications, it is not practical to use a fiber length of 10-30 meters. Flat-type optical amplifiers have been developed for use in confined spaces. Generally, the useful length of a flat amplification device is not more than 10 centimeters. To achieve the same amplification levels as an EDFA of 10-30 meters in length, a flat amplifier requires a means of amplification with a higher concentration of erbium ions, in the order of up to 4 to 7% by weight. However, in known types of optical amplification means, several loss-of-gain mechanisms occur at high levels of erbium ion concentration, including ion clustering and cooperative homogeneous overconversion (concentration suppression). Because the erbium ions do not dissolve well in a silica matrix, the erbium ions will clump together, allowing the transfer of energy in the pooled region. In addition, in high concentrations of erbium, the ion-to-ion interaction becomes more important. The resulting overconversion of energy suppresses the inverted population. The energy of erbium is used in the grouping and suppression procedures and therefore, it is not available for the phononic amplification procedure required. As a result, the quantum efficiency of the amplification medium decreases rapidly with a higher concentration of erbium ion, with a corresponding decrease in amplifier gain. Furthermore, the known erbium-based silica-based amplifiers have a clear gain spectral inequality. The lack of a flat gain spectrum over a wide bandwidth causes several problems. For example, extremely short optical pulses have a relatively broad energy spectrum and are not amplified accurately if the gain spectrum is not flat. In addition, in more extensive bandwidth applications, such as wavelength division multiplexing (WDM), the fiber receives optical signals modulated by data from several optical transmitters, each using a different optical carrier sequence. If the gain spectrum from the optical amplifier is not flat during the operating wavelength, the carrier frequencies at the maximum gain points could be saturated, while the carrier frequencies in the skirts and valleys may not be sufficiently amplified. Previous efforts to channel gain flattening have basically depended on passive or active filtering of the high gain characteristics of the gain spectrum. However, this requires a close union of the particular amplifier and the filter, and must account for temporary variations in the gain spectrum.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a family of crystals that find particular utility in the production of optical signal amplifiers; these crystals are contaminated with high concentrations (up to 10% by weight) of erbium oxide while exhibiting a weak concentration suppression behavior. These crystals also provide higher fluorescence efficiency and more uniform gain characteristics than the known silicate and fluorozirconate crystal media. These crystals provide high and flat gain characteristics that are particularly useful for optical amplification in the 1.55μm optical wavelength window, and are particularly suitable for use in wavelength division multiplexing (WDM) systems. One aspect of the present invention is directed to a family of crystals, in particular, fluorophosphate crystals, which are especially suitable for high concentration levels of rare earth ions. It is an object of the present invention to provide a fluorophosphate crystal medium doped with erbium oxide ions for use in an optical amplifier to provide a high and flat gain in the optical wavelength window of approximately 1.55μm. The fluorophosphate crystals of the present invention consist of high concentrations of erbium ions (ie, about 10% by weight) and provide a spectrally uniform gain, similar to ZBLAN < - > , and are substantially better than traditional silicate and phosphate crystal compositions. The present invention is directed to a crystal family for optical amplification consisting of a silica-free fluorophosphate crystal medium, doped for 100 parts by weight consisting of: fluorinated crystals (ie fluorinated crystals for optical fibers containing fluorides ZrF4, BaF2, LaF3, AIF3, and NaF) > P2O5 15-40 MgF2 0-10 Al203 0-5 CaF2 0-25 MgO 0-9 SrF2 0-25 CaO 0-9 BaF2 0-20 SrO 0-9 KHF2 0-2 BaO 0-45 K2TiF6 0-2 AIF3 5 -25 up to 10 parts by weight of erbium oxide, preferably between 0.01 and 10. Preferably, the fluorophosphate crystal according to the present invention has a composition consisting, in parts by weight, of: P2O5 16.9-24.0 MgF2 0-7.5 AI2O3 1.6-3.2 CaF2 0-18.7 MgO 0-5.0 SrF2 0-19.7 CaO 0-5.1 BaF2 1.5-11.3 SrO 0-8.5 KHF2 0-1.3 BaO 2.7-43.2 K2TiF6 0-0.6 AIF3 9.5 -19.3 The flurophosphate crystal according to the present invention can also be co-purified with up to 15 parts by weight of YD2O3 as a sensitizer to increase pump efficiency at about 980 nm. Preferably, the fluorophosphate crystal according to the invention has a refractive index of between 1.48 and 1.58. In relation to another aspect, the present invention is directed to an erbium-doped optical amplifier for a band of wavelength of about 1.55 μm, which has a means to Optical amplification consisting of a silica-free fluorophosphate crystal composition further comprising 100 parts by weight of other components, about 0.01 to 10 parts by weight of Er2O3. In accordance with the present invention, the optical amplifier can be a flat type optical amplifier or a simple fiber type optical amplifier. The optical amplifier according to the present invention includes a fluorophosphate crystal consisting of 100 parts by weight constituted by: P205 15-40 MgF2 0-10 AI2O3 0-5 CaF2 0-25 MgO 0-9 SrF2 0-25 CaO 0-9 BaF2 0-20 SrO 0-9 KHF2 0-2 BaO 0-45 K2TiF6 0-2 AIF3 5 -25 up to 10 and preferably between 0.01 and 10 parts by weight of erbium oxide. The fluorophosphate crystal used for the optical amplifier according to the present invention can also be doped with up to 15 parts by weight of Yb 2 O 3 as a sensitizer to increase pump efficiency at about 980 nm and preferably, has a refractive index of between 1.48 and 1.58. Optical amplifiers according to the present invention are particularly useful in wavelength multiplexing (WDM) systems.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will be seen from the following detailed description and in relation to the accompanying drawings, given by way of example only, and in which: Figures 1 and 2 are graphs illustrating the typical behavior of suppression of life concentration and fluorescence efficiency of binary silicate crystals. Figures 3 and 4 are graphs illustrating the life and efficiency of fluorescence of fluorophosphate crystals according to the present invention. Figure 5 is a graph illustrating the gain versus wavelength form for a traditional silicate-based crystal. Figure 6 is a graph illustrating the form of gain versus wavelength for a traditional ZBLAN crystal. Figures 7 to 9 are graphs illustrating the gain versus wavelength form for fluorophosphate crystals according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a family of crystals that have utility particularly in lighting, optical and electronic applications. These crystals have unique characteristics that offer them a particular utility in the production of optical signal amplifiers. A characteristic of these crystals is their important independence from Si02. The erbium ions do not dissolve well in a silica matrix, thus generating the clustering and degradation of the efficiency of the increase, so that the elimination of the silica matrix allows the clustering of ions to be avoided, and the energy of photons and excess ions for amplification. The crystals according to the present invention contain a relatively high concentration of P2O3. The present invention is directed to a family of crystals for optical amplification consisting of a silica-free fluorophosphate crystal medium, doped by 100 parts by weight of other components, up to 10 parts by weight of erbium oxide. Table 1 sets out the essential composition scales for the fluorophosphate crystal according to the present invention.

TABLE 1 (Parts by weight) p205 15-40 MgF2 0-10 AI2O3 0-5 CaF2 0-25 MgO 0-9 SrF2 0-25 CaO 0-9 BaF2 0-20 SrO 0-9 KHF2 0-2 BaO 0-45 K2TiF6 0-2 AIF3 5 The family of erbium-doped crystals according to the invention can also consist of about .01 to 15 parts by weight of Yb203 to be used as sensitizers to increase the pumping efficiency by about 980 nm. Table 2 defines narrower preferred scales of oxide components of these crystals. Within these narrow scales, optimal properties for optical signal amplifiers and their production are obtained.

TABLE 2 (Parts by weight) P2O5 16.9-24.0 MgF2 0-7.5 AI2O3 1.6-3.2 CaF2 0-18.7 MgO 0-5.0 SrF2 0-19.7 CaO 0-5.1 BaF2 1.5-1 1.3 SrO 0-8.5 KHF2 0-1.3 BaO 2.7-43.2 K2TiF6 0-0.6 AIF3 9.5-19.3 Another feature of the crystals according to the present invention is their ability to be contaminated with relatively high concentrations of erbium oxide (Er2O3). In the absence of a silica-based crystal, the high concentrations of Er2O3 doping provide excellent fluorescence effects that are important for the amplification of optical signals through laser pumping due to the reduction of ion clustering and the suppression of overconversion. This property provides an excellent means of amplification for use in optical amplifiers for the 1550 nm wavelength. In relation to another aspect, the present invention is directed to an erbium doped optical amplifier consisting of a means for optical amplification comprising a fluorophosphate crystal composition. Preferably, the fluorophosphate crystal composition is doped by 100 parts by weight consisting of: TABLE 3 (Parts by weight) P2O5 15-40 MgF2 0-10 AI2O3 0-5 CaF2 0-25 MgO 0-9 SrF2 0-25 CaO 0-9 BaF2 0-20 SrO 0-9 KHF2 0-2 BaO 0-45 K2TiF6 0-2 AIF3 5 -25 with about 0.01 to 10 parts by weight of Er2O3. According to another embodiment of the present invention, the optical amplifier means doped with erbium, with respect to components other than erbium oxide, consists of 100 parts by weight constituted as shown in table 4 below.

TABLE 4 P2O5 16.9-24.0 MgF2 0-7.5 AI2O3 1.6-3.2 CaF2 0-18.7 MgO 0-5.0 SrF2 0-19.7 CaO 0-5.1 BaF2 1.5-11.3 SrO 0-8.5 KHF2 0-1.3 BaO 2.7-43.2 K2TiF6 0-0.6 AIF3 9.5 -19.3 The erbium-doped optical amplifier according to the invention may further consist of about .01 to 15 parts by weight of Yb2O3, to be used as a sensitizer to increase the pumping efficiency at about 980 nm. The optical amplifier according to the present invention can take any number of forms, as long as the medium is capable of being doped with erbium ions. The optical amplifier can be a simple fiber type optical amplifier. Alternatively, the optical amplifier could also be a flat type optical amplifier. The effects of suppression of concentration in binary crystals based on silica are illustrated in figure 1. At low levels of Er2O3 concentration, less than 5E19 ions / cc, (the equivalent of less than 0.5 parts by weight), the life of Fluorescence is constant. Above this level of concentration, the fluorescence life decreases rapidly as the concentration increases. Two characteristics can be defined in order to differentiate the crystals. The concentration Cqb corresponds to the beginning of the concentration suppression. The Cq concentration corresponds to the concentration level where the fluorescence life is divided by two. As illustrated in Figure 1, concentration suppression initiates in traditional silicate binary crystals when CqD = 7E19 ions / cc (or approximately 0.9 parts by weight). At this point, the fluorescence life is approximately 13 ms. When Cq = 3E20 ions / cc (or about 3 parts by weight), the fluorescence life is about 7.5 ms. Figure 2 is a graph illustrating the effects of concentration suppression on the fluorescence efficiency of traditional silica-based crystals. The fluorescence efficiency is defined as the fluorescence of 1.55 μm per Er ion against the Er concentration. For the levels of interest of ion concentrations, that is, between 3 to 5 E20 Er ions / cc (approximately 4 to 7 parts by weight), the fluorescence efficiency of the silica-based crystals is between .5 to 2 E-19nW / ion. Figure 3 is a graph illustrating the effects of concentration suppression on the fluorescence life of three types of fluorophosphate based crystals according to the present invention. Figure 4 is a graph illustrating the effects of concentration suppression on the fluorescence efficiency of 3 types of fluorophostat-based crystals according to the present invention. As shown in Figures 3 and 4, both Cqb and Cq are an order of magnitude greater than the values corresponding to the silica-based crystals. This indicates that the behavior of the suppression of concentration at high levels of Er ion concentration is relatively weak in the fluorophosphate crystals according to the invention, and that these crystals are very good candidates for optical signal amplifiers of short length and high gain. Table 5 gives a comparison of composition with the measured and calculated related properties of 3 types of fluorophosphate crystals according to the present invention, a traditional glass based on borosilicate and ZBLAN. The compositions in Table 5 are expressed as the loading amount. The actual ingredients of the cargo may consist of any type of raw material, oxides, fluorides or phosphates, which when combined, are converted into desired oxides and fluorides in the proper proportions. Examples of raw materials (not detailed) are: Ca (PO3) 2, Ba2P207) AI4 (P207) 3) AI (PO3) 3, NaPO3, K2TiF6, X20y, XFy where X is a metal ion of valence y. The values given in Table 5 (like the values given elsewhere in this text) represent the theoretical quantities of the different components in the final crystal, in accordance with normal practice in this field. In the case of oxides, the theoretical quantities are very close to the natural quantities (that is, the "charge yield" is very close to 100% for the oxides). In the case of fluorides, which are more stable, the actual values are slightly lower than the theoretical values (the load efficiency is approximately 90 to 95%).

TABLE 5 Key Example Example Example Crystal of ZBLAN2 1 2 3 boron silicate parts by weight% molar SiO2 66.6 B2O3 11.6 P205 16.9 24.0 30.9 AI2O3 3.2 2.7 1.6 MgF2 5.8 7.5 0.0 CaF2 18.7 0.5 0.0 SrF2 19.7 17.9 0.0 BaF2 1 1.3 14.4 1.5 22 AIF3 19.3 11.3 9.5 4 ZrF4 48 lnF3 LaF3 3.2 NaF 22 KHF2 1.3 0.0 0.0 K2TiF6 0.6 0.5 0.0 Na2O 0.5 0.0 0.0 K2O CaO 0.0 5.1 0.0 SrO 0.0 2.4 8.5 BaO 2.7 13.7 43.2 MgO 0.0 0.0 4.9 ErF3 0.8 Er2O3 6.0 4.0 1 .5 2 Er2O3 5.84 + 20 4.64 + 20 1.6 + 20 1.6E + 20 1.5E + 20 (ions / cm3 Index 1.49 1.54 1.59 1.52 Density 3.62 3.83 3.976 2.552 Key Example 1 Example 2 Example 3 Crystal of ZBLAN2 boron silicate Parts by weight% molar Fluorescence life 6.8 7.6 7 6.3 Fluorescence life in a 9.5 8 7 16 low Er content (ms) EC (%) 68 95 100 39 Fluorescence efficiency 2.3 2.7 3.2 1.3 (nW / Er ¡on) * 1 E-19 Cross sections (cm2) * 1 E-21 Absorption pump 975 nm 1.9 2.3 (980 nm) 0.8 2.6 1480 nm 3.3 3.9 1.2 4.7 Absorption signal (sab (?)) 1533 nm 4.9 5.9 (1527 5.6 nm) FWHM (nm) 65 64 15 Emission signal (sem (?)) 1522 nm 5.3 6.5 (1537 7.2 6 nm) FWHM (nm) 51 49 17 Emission / absorption 1.1 1.1 1.3 1.1 Radiation life (ms) 10 8 16 8 The filler ingredients are mixed together to provide homogeneity, placed inside a platinum crucible, and heated by Joule effect at approximately 1000 ° C. When the fusion is finished, the temperature rises between 1050 and 1350 ° C to obtain homogeneity and purity of the crystal. The melt is then cooled and simultaneously molded into the desired shape, and finally transferred to an annealing oven operating at approximately 400 ° C. An alternative melting process consists of forming the crystal from the filler ingredients and remelting this crystal together with the desired ratio of Er and / or feedstock of Yb. This procedure may in some cases increase the homogeneity of the crystal. As shown in Table 5, the Quantum Efficiency t0bs / Trad is on the scale of 70 to 100% for the fluorophosphate crystals according to the present invention at the desired levels of Er concentration, while the Quantum Efficiency for the Silica-based crystals in the same concentration levels are in the range of 20 to 35%. One limitation for the total use of bandwidth in WDM systems is the spectral inequality of the gain shown in silica-based EDFA crystals. Another important feature of the high concentration of fluorophosphate compositions doped with Er according to the present invention, as compared to silica-based crystals, is that the fluorophosphate crystals show a very flat gain spectrum, on a scale of 28 to 30 nm in the bandwidth of 1550 nm. This is comparable to ZBLAN crystal fibers doped with Er. To obtain this gain surface equality between 1528 and 1563 nm, the glass medium according to the present invention preferably has a fluorine content of at least 18 parts by weight. A good spectral gain representation can be obtained against the wavelength, using the following formula: g (cm-1) =. { will be (?) * N2 - s ab (?) * N?]} where: se (?) is the cross section of emission in cm2; s ab (?) * is the cross section of absorption in cm2; N2 is the highest level (41 13/2) of ion population (averaged with length); Nor is the ground state (4I? S / 2) of the ion population (averaged with length); and Nt is the total concentration of Er ions (in ions per cm3). If the investment percentage is defined as D = (N2-N?) Nt, then equation (1) can be rewritten as: G (dB / cm) 2.15 * Nt *. { sem (?) * (1 + D) - sab (?) * 1-D)} (2) where: D + -1: is the% of investment; and D ++ 1: it is 100% investment. Equation 2 was used to calculate the gain form against the wavelength of the different crystal compositions, of which the results are shown in Figures 5 to 9. Figure 5 illustrates the gain shape of a traditional borosilicate glass used in optical signal amplifiers. It is clear that the gain spectrum around the 1550 nm bandwidth used in WDM is not uniform in character. The amplification between 1535 nm and 1565 nm, a traditional scale used in WDM, is unequal. The variation between the maximum and minimum gain can reach 250%. Figure 6 illustrates the gain shape of a ZBLAN crystal for use in an optical signal amplifier. Compared with borosilicate glass, ZBLAN offers a flat gain shape on a wavelength scale of approximately 30 nm in width. Figure 7 illustrates the gain form for a first fluorophosphate crystal according to the present invention. This crystal, identified as Example 1 has an Er concentration of 7 parts by weight per 100 parts by weight of other components, yet it has a substantially flat gain form over a 28 nm spectrum in the 1530-1560 nm band. Figure 8 illustrates the gain form for a second type of fluorophosphate crystal according to the present invention. This crystal, identified as Example 2, has an Er concentration of 4 parts by weight per 100 parts by weight of other components and has a flat gain shape over a spectrum of about 26 nm. Figure 9 illustrates the gain shape of a phosphate-based crystal. This crystal, identified as Example 3, has an Er concentration of slightly less than 3 parts by weight per 100 parts by weight of other components, and has 2 relatively flat gain areas.; the first being approximately 10 nm wide and the second approximately 9 nm wide. Another aspect of the present invention is the ability to efficiently pump the amplification medium at 980 nm while maintaining relatively low noise levels. Optical amplification requires stimulation of the erbium ions in the crystal medium at a higher energy level, and then the relaxation of the ions. This procedure causes the emission of photons as the erbium ions relax to the fundamental level. The photons emitted during this procedure are at such a wavelength to amplify the optical signals at the same wavelength. Considering the first three energy levels of erbium, a useful emission occurs between level 2 (the metastable level) at level 1 (the fundamental level). To have investment of the population (the population in level 2 is greater or equal to 50%) and, therefore, profit, the means of profit must be pumped with an external source. Generally, with the amplification of optical signals, the gain medium is pumped with a diode laser of 980 or 1480 nm. When a 980 nm diode laser is used, the electrons move to a third level (4ln /? 2) and then relax to the second level and then to the fundamental level emitting a 1.55 μm photon. When using a 1480 nm diode laser the electrons move directly to the permanent level (2) and then to the fundamental level emitting a 1.55 μm photon. The most efficient and reliable pumping for optical amplification is 980 nm. However, because the 980 nm pumping procedure moves the electrons to the third level, the life at the level (3) should be very short, preferably in the order of the microseconds, or on the contrary, the electrons can be excited to higher levels and thus decrease the pumping efficiency. This happens in fact with a glass medium similar to ZBLAN when it is pumped with a 980 nm pump because of its relatively long life in level (3) (around 9 ms). Accordingly, a ZBLAN-like amplification medium can not be pumped as efficiently as the innovative fluorophosphate crystal medium with a 980 diode pump. The fluorofosphate crystal doped with Er according to the present invention, as an amplification medium. with a pump of 980 nm, it has advantages over fluorides doped with Er. Compositions of ZBLAN type (100% non-oxygenated fluoride), lose pumping efficiency due to the long fluorescence life (9 ms) at the pumping level of 4ln /? 2. Therefore, compositions of the ZBLAN type are generally pumped at 1480 nm. However, there are drawbacks to pumping at wavelengths of this height. For example, the ion population can not be completely inverted at this level and the noise in the amplifier increases. In contrast, the fluorophosphate crystal medium according to the present invention can not be efficiently pumped at 980 nm since the lifetime of 4ln /? 2 is in the range of 10 to 70 μs. Figures 5 to 9 illustrate that the fluorophosphate crystals according to the present invention show equal surface gain characteristics using a 980 nm pump similar to ZBLAN, and significantly improved compared to silicates and phosphates. The glass compositions, according to the invention, provide high and flat gain characteristics in short optical amplifiers, and which can then be used for the manufacture of flat amplifiers and / or single short-length fibers having the same ZBLAN type gain useful in WDM and other similar applications.

Claims (13)

NOVELTY OF THE INVENTION CLAIMS

1. - A fluorophosphate crystal for use in optical amplifiers of 1.55μm of high and flat gain, characterized in that it consists, for 100 parts in weight constituted by: 15-40 of P2O5, 0-5 of AI O3, 0-9 of MgO, 0-9 of CaO, 0-9 of SrO, 0-45 of BaO, 5-25 of AIF3, 0-10 of MgF2, 0-25 of CaF2, 0-25 of SrF2, 0-20 of BaF2, 0- 2 of KHF2, 0-2 of K2TiF6, from 0.01 to 10 parts by weight of Er2O3.

2. A fluorophosphate crystal according to claim 1, further characterized in that it also comprises 0.01 to 15 parts by weight of Yb2O3.

3. A fluorophosphate crystal according to claim 1 or 2, further characterized by having a chemical composition, which consists of parts by weight: 16.9-24.0 of P2O5, 1.6-3.2 of AI2? 3, 0-5.0 of MgO , 0-5.1 of CaO, 0-8.5 of SrO, 2.7-43.2 of BaO, 9.5-19.3 of AIF3, 0-7.5 of MgF2, 0-18.7 of CaF2, 0-19.7 of SrF2, 1.5-11.3 of BaF2, 0 -1.3 of KHF2, 0-0.6 of K2T¡F6.

4. A fluorophosphate crystal according to any of claims 1 to 3, further characterized in that its fluoride content is in the range of 7 to 88 parts by weight.

5. - A fluorophosphate crystal according to claim 4, further characterized in that its fluoride content is greater than or equal to 18 parts by weight.

6. An erbium-doped optical amplifier consisting of a means for optical amplification, characterized in that said means comprises a fluorophosphate crystal composition comprising, for 100 parts by weight consisting of: 15-40 P2Os, 0-5 AI2O3, 0-9 MgO, 0-9 CaO, 0-9 SrO, 0-45 BaO, 5-25 AIF3, 0-10 MgF2, 0-25 CaF2, 0-25 SrF2, 0-20 of BaF2, 0-2 of KHF2, 0-2 of K2TiF6, from 0.01 to 10 parts by weight of Er2O3.

7. An erbium-doped optical amplifier according to claim 6, further characterized in that said medium has a chemical composition consisting of parts by weight: 16.9-24.0 of P2Os, 1.6-3.2 of AI2O3, 0-5.0 of MgO, 0-5.1 of CaO, 0-8.5 of SrO, 2.7-43.2 of BaO, 9.5-19.3 of AIF3, 0-7.5 of MgF2, 0-18.7 of CaF2, 0-19.7 of SrF2, 1.5-11.3 of BaF2, 0- 1.3 of KHF2, 0-0.6 of K2T¡F6.

8. An erbium-doped optical amplifier according to claim 6 or 7, further characterized in that it comprises 0.01 to 15 parts by weight of Yb2? 3.

9. An erbium doped optical amplifier according to claim 6, 7 or 8, further characterized in that said optical amplifier is a flat type optical amplifier.

10. - An erbium doped optical amplifier according to claim 6, 7 or 8, further characterized in that said optical amplifier is an optical amplifier of simple fiber type.

11. An erbium doped optical amplifier according to any of claims 6 to 10, further characterized in that its fluoride content is in the range of 7 to 88 parts by weight.

12. An erbium-doped optical amplifier according to claim 11, further characterized in that its fluoride content is greater than or equal to 18 parts by weight.

13. An optical amplifier characterized in that it consists of: an active optical means having an input and an output, said active optical means being contaminated with a fluorescent doping device, said optical means receiving said optical signal at its input having wavelengths on the scale from 1525 to 1570 nm approximately; and a pumping source that supplies pumping light energy at a wavelength of approximately 980 nm to the active optical medium, the pumping light being adopted to stimulate said fluorescent dopant to emit photons that amplify said optical signals in a scale of lengths of wavelengths from about 20 to 30 nm in width with a substantially flat gain spectrum of less than 13% gain variation approximately on the spectral scale of about 1525 to 1565 nm, wherein the quantum efficiency of said active optical medium exceeds about 65%, said quantum efficiency being the ratio between the fluorescence life of said active optical medium and the life of the radiation. APPENDIX SHEET SUMMARY OF THE INVENTION The invention relates to a family of erbium-doped fluorophosphate crystals for use in amplification of optical signals; the composition, based on 100 parts by weight, is constituted by: 15-40 of P2O5, 0-5 of AI2O3, 0-9 of MgO, 0-9 of CaO, 0-9 of SrO, 0-45 of BaO, 5-25 of AIF3, 0-10 of MgF2, 0-25 of CaF2, 0-25 of SrF2, 0-20 of BaF2, 0-2 of KHF2, 0-2 of K2T¡F6, up to 10 parts by weight of erbium oxide; the crystals according to the present invention have a high gain and a very flat spectrum over the bandwidth of 1550 nm, compared with the crystals in the figure; these crystal compositions are particularly suitable for use in flat or fiber optic amplification in WDM and similar applications. P00 / 33F