AMPLIFIER AND PROCESS Of AMPLIFICATION Of an Optical signal

Amplifier and method of amplifying an optical signal.

A amplifier and a method for amplifying a useable optical signal essentially in the area of the optical fibre transmission. The aim of the invention is to provide optical fiber amplifiers which the flatness of the gain is more controlled.

Known is essentially, industrially, two types of optical fiber amplification usable to operate.

A first type said "silica glass fiber" includes a core erbium-doped silica which, in one example, has a diameter of 5 micrometers. Doping is normally performed on a thick bar, before drawing the fiber, MCVD according to a process known as vapor deposition. The core is surrounded by a first and a second sheath, made silica, in an example have both after drawing a thickness of 62.5 micrometers. The manufacture of such fibers is achieved by stretching, through an orifice, of a heated preform. In the preform, the three material layers are present but with thicker [...] thicknesses.

Known is a other type of fiber, said "fluorinated fiber". The fibers comprise fluorinated for their manufacture, a preform in which a mixture, in a preferred example, fluoride Zirconium, with Barium, of [...], Aluminium Sodium of and. Fluorinated The fibers are also said "ZBLAN" the name of their constituents. The doping erbium in the latter case can be performed at the mixture itself.

The behaviors of these fibers doped silica or fluorinated, at the time of the amplification, are different. Indeed, their platitudes gain as a function of wavelength (or frequency) of the optical signals to be amplified are different. Typically, between the wavelengths most promoted and the wavelengths the least promoted, over a range of about 30 nm, amplifications in a silica fiber are offsets the gain of 15%, while the difference is that of 4% in the case of ZBLAN fiber. Having a promoting certain wavelengths (in particular the center wavelengths) has a drawback that if amplifiers made according to this technology are installed in optical fiber networks to long distance. Indeed, such amplifiers used as repeaters then cause successive end by an excess of amplification of the center frequencies to the detriment of side frequencies. It is then corrected by this excess promoting filtering. Is therefore desirable to make the fibers ZBLAN in order to limit this promoting disturbance since the gain is more flat in the band.

ZBLAN The fibers and silica have behavior different in effect is due to non-radiative acoustic phonons. Indeed, because of the nature of the material of the fiber, between two particular levels of excitation energy of the atoms of erbium fiber silica, one can find [...] phonon whose energy is a submultiple, low order (typically 2,3 or 4), the energy difference between these two particular levels. This results to have a high probability that such acoustic phonons can develop in the fiber material silica. However, the type fibre ZBLAN, the sub-multiple priority much higher. In these conditions, the probability that the phonon type therein is much lower. The result is that the population of an energy level is depopulates, by non-radiative effect, much more quickly in the case of a silica fiber that in the case of a fiber ZBLAN. The life of the atoms in the excited state is therefore lower in the case of silica fibres that, in the case of fluorinated fibers. One will manifest in the following the adverse consequences which can occur because of the extended life.

Appended Figure 1, shows both of the prior art and for the invention, and for both silica fibers and the fibers ZBLAN, the overall phenomenon of the amplification to constraints and its limitations. Very schematically, during a pump 1, with a laser source emitting radiation at 1480 nm wavelength, a photon laser produced by the laser source is excite the electrons of the fundamental layer 115/2 of certain atoms for the erbium into the layer of energy 113/2. In these conditions, a photon signal hv, whose wavelength is useful in a preferable example in a band in the range of 1530 nm to 1630 nm, is de-energize the electrons from the 113/2 layer to produce, by radiative effect, photons hv at the same frequency, but in greater number. These signal photons hv themselves may de-energize further into the optical fiber further electrons 113/2 of the layer so that the amplification phenomenon interest may reach high gains (for example 25 dB). In one example arbitrary hv the signal has a wavelength of 1550 nm.

Unfortunately, the signal photons hv may also act as a pump, and also of transferring erbium atoms an excitation state of the layer to the layer 113/2 115/2. A phenomenon is an ion absorption phenomenon which degrades the noise factor of the amplifier. To avoid this ion absorption phenomenon, to organize a total population inversion between the layers and 115/2 113/2, it is suitable to ensure that there are no atoms Erbium to the ground state 115/2. Been easily than if the pump has been to remove all the electrons in the layer 115/2, it will not be ion absorption.

Further some drop randomly excited atoms in the excited state by providing a signal hv spontaneous. The drawback of this spontaneous hv signal is that it is not at all related to the signal to be amplified. Spontaneous hv The signal, and the ionic absorption, thus contribute to increasing the noise, and to reduce the useful signal. The most adverse phenomenon is populating the 115/2 layer which itself generates the ion absorption.

Shown at d 113/2,' a symbolically, Stark the enlargement. Watch The enlargement degeneration of power from the energy level. In practice, by pumping atoms at 1480 nm, the erbium atoms excited state 113/2 are distributed, due to the statistical Boltzmann, on all degenerate layers of that level 113/2. To simplify, excited atoms is created with an energy level 1480 nm relative to the ground state, while are used at 1550 nm excited atoms. This occurs easily since, according to the statistics of Boltzmann, the level is 1550 nm repopulates automatically from the 1480 nm level. Figure 1 shows by a diagram D that the population distribution_{type e >/ [...][...]}^{, f t is}_{in fact} substantially constant at 113/2.

Finally, the problem of 1480 nm pumping is that it results in a stimulated emission. Indeed, the pump signal itself is viewed as a signal to be amplified and results in the de-excitation of excited atoms at certain 113/2. Consequences This to three, two and conveniently a detrimental. First, in that the atoms relax produces photons to 1480, which, themselves, may be used to drive other photons. Second, signals propagated to 1480 nm are not disturbing. They do not disturb the signal-to-noise ratio since they are located outside the useful band: 1530-1630 nm.

In contrast, stimulated with the disadvantage that the drop-out to re-populate the energy level 115/2 and disadvantages ion absorption seen above.

To avoid this phenomenon, instead of pumping to 1480 nm, be pumped at 980 nm with an excitation 2 produced by a laser source. Excitation at 980 nm The has the effect that the electrons of the erbium atoms from the ground state of the layer 115/2 to the arousal state of the layer 11 1/2. By non-radiative Then effect, due to the influence of the phonons, 111/2 depopulates the layer is to the benefit of the layer 113/2 populating. The layer 113/2 can no longer then be depopulated by stimulated emission since the frequency range is mismatched. The wavelength of 980 nm no longer corresponds to the wavelength of deexcitation of atoms from the layer to the layer 115/2 113/2. Is formed in this case, a population inversion perfect (or nearly perfect) between the layer and the layer 115/2 It 3/2.

There appears substantially different behavior between the fibers and the fibers ZBLAN silica. In the silica fibers, due to the high probability of phonon have non-radiative de-excitation at the level 11 1/2 It 3/2, the life of the excited atoms to the state 111/2 is low. May be considered that they fall back immediately 113/2 wherein, during service (same low) takes advantage is to cause amplification useful.

Thus this principle works well for the silica fibers, by simplifying, because the level 111/2 is emptied before another photon at 980 nm it is returning to the ground state.

For against, the-type fibers ZBLAN, because phonon energy is smaller and because their statistic is less favorable, the de-excitation between the level and the level 113/2 11 1/2 is slower. The life of the excited erbium atoms at 111/2 is large. The rate of non-radiative de-excitation is then insufficient to rapidly deplete the level 111/2. Therefore, the level 11 1/2 in the fibers is ZBLAN depopulates pumped laser by stimulated emission to repopulate the fundamental level 115/2 (which will help to produce the absorption effect adverse ion). Or, by exciting even more, the excited erbium atoms in the state 111/2 will populate the energy level S3/2 (or of a higher level) from which also, there is a continuum of the energy level. Therefore, much of excited atoms to the state 111/2 are additionally excited by the pump signal at 980 nm to reach the level S 3/2. These atoms are then lost for amplification and the efficiency of the amplifier is not good.

Briefly, the 980 nm pumping works well in the silica fibres because of the high efficiency of the non-radiative in the fibers. However, the gain with the fibers is not flat. Opposite, this process not to work well with the fibers for which by ZBLAN against, the gain is flat.

Figure 1 also shows an other known pump the energy in the erbium atoms of a silica fiber. For this purpose, there is employed a co-doping ytterbium. The optical fiber has then the atoms of the two dopants finely mixed. Pumping can then by excitation 3, with a laser having a wavelength ranging between 970 nm and 990 nm, to populate the level F5/2 ytterbium from the ground level F7/2 of that atom. Regardless the pump wavelength, the ytterbium atoms are distributed, again due to the statistical Boltzmann, in a widening of the ytterbium atom Stark at F7/2 whose width is typically 100 nm. 4 For a synergistic effect, the ytterbium atoms yield then their energy to atoms of erbium, so that the layers F5/2 ytterbium and erbium 11 1/2 are populated equivalently. The synergistic effect does not have the same consequences that the excitation 2 because the exchange of energy does not propagate.

This solution of co-doping is very attractive because the layer 11 1/2 erbium has an enlargement Stark low, typically 20 nm, and that it is difficult to adjust the pumping a wavelength that corresponds exactly to the energy level difference 115/2-111/2 for the erbium atom. A method of co-doping thus relax the stress on the pump wavelength.

With type fibers, ZBLAN, despite their advantages, this latter technique cannot be better used since the atoms of the layer 11 1/2 erbium does not relax quickly enough level 113/2. In in this technique is not used.

The aim of the invention is to overcome this disadvantage, i.e. allow the use of a-type fiber ZBLAN because this fiber has gain amplification a flatter than the silica fibers, without having to tolerate the harmful effects of repopulating 115/2 the layer from the layer 111/2.

In the invention, to avoid that the distribution of excited atoms at 111/2, is only converted into excited atoms 115/2 levels, or S3/2 or higher (neutralized atoms for amplification), is recommended to select in the pump wavelengths (typically between 970 and 1070 nm) those for which transitions to energy levels or 115/2, S3/2 other are inhibited, i.e. for which no docking energy level exists. For example, the pump wavelength of 980 nm will be markedly different. Typically, can be used very potent lasers, especially the laser said "Yag" whose emission wavelength is 1064 nm.

The object is to propose a method of amplifying an optical signal which is excited in an optical fiber of the fluorine type erbium doped with a laser radiation, characterized in that

-is doped fiber with a co-dopant, and

-the fiber is excited with radiation having a wavelength corresponding to a transition of energy levels of the co-dopant while being outside a range, said gap, transition energy levels of erbium.

The invention also relates to an optical amplifier having an optical fiber of the fluorine type erbium doped and a source of laser radiation, characterized in that it comprises a co-dopant in the fiber and a wavelength of the laser radiation that corresponds to a transition of energy levels of the co-dopant while being outside a range, said gap, of transitions of energy levels of the erbium atom.

The invention be included at the reading of the description that follows and with the examination of the accompanying Figures. These are passed only as [...] and in no way limiting of the invention. Figures show:

-Figure 1: the representation already partially comment from the distribution of the energy levels and means for exciting the atoms to produce improved amplification,

-Figure 2:a schematic representation of the general mode of the amplification in which a light excitation, produced by a laser source, is exciting the material of an optical fiber in conjunction with an optical signal amplifying hv therein.

It is known by the document "Low-noise and gain flattened fluoride based Er3 + doped fiber amplifying [...] by 970 nm diode laser" due to m. [...] and Al and published in the 24 April 1997 Electronics Letters volume 33, no. 9, pages 809 and 810, results in an amplifier for fibers of ZBLAN fluorinated type that combines the noise performance silica fibers and the flatness of the gain of these fluorinated ZBLAN-type fibers. The amplifier includes a first stage containing a silica fiber pumped at 980 nm. The first stage is followed by a second stage with a fluorinated fiber pumped at 1480 nm. This solution is not sufficiently efficient because it requires, expensive in one embodiment, to use two doped fiber of different types and that the effect of the noise characteristic of the second stage on the characteristic of the total noise amplifier cannot be completely masked by the characteristics of the first stage.

The idea of the invention is, on the contrary, first add a co-dopant in a ZBLAN fiber of the type that makes it possible to change the nature of the energy levels in the fiber. Yb3 + The ytterbium ion is from this point of view a good co-dopant that since the absorption spectrum to the ground state has a center wavelength, near 980 nm, but with a spectral bandwidth large enough to allow pump wavelengths as high as 1064 nm, that of the YAG laser. The ytterbium ion thus has good possibilities to population by synergistic effect of the level of the erbium atom 111/2 by transfer of energy from the level of the ytterbium ion F5/2 Yb3 +.

According to the invention, secondly, selecting a wavelength of a pump 5 which is outside the absorption spectrum of the level of the erbium 111/2 atom. The absorption spectrum, which is part of the prohibited range according to the invention, is labelled with cross in Figure 1. Therefore, the erbium atom will not be de-energized by the energy at the pump wavelength to return to the ground state. This will have the effect of promoting the formation of a population inversion favorable amplification. Radiative For effect and [...], 113/2 the level of the erbium atom will be populated in turn from the level 111/2 of that atom. The life of the erbium atoms at 111/2 will not be shortened, but, because there is no pump wavelength de-excitation which corresponds, re-population of 115/2 level n ' is not organized.

II is described in the document " Illumination color Up-conversion in Er3 + + and Yb3 doped fluorozirconate Glasses^1' due to LA. [...] and AL, published in the review Optical Fibre Technology 1,331-334 (1995), the study of a ytterbium-doped ZBLAN fiber. However, the document prompt pumping the fiber with a lower wavelength of 540-590 nm so as, ultimately, to populate the level 11 3/2 of the erbium atom by radiative effect from excited atoms in the state S3/2, or even higher.

It has been measured in the invention that the efficiency is not good ultimately. Opposite, with the method of the invention, by increasing the wavelength a noise factor improved 1 dB (20%) factor by noise amplifiers conventional type fibre ZBLAN. This allows the use of such amplifiers can be made longer because of 20% the distance between repeaters on optical fiber links to long distance. This accordingly makes these links less expensive.

In practice, the pump wavelength of 1064 nm has been preferred but could be any between 950 nm and 1070 nm, to avoid the condition portion of the prohibited range there which is 970 nm to 990 nm. It is intended to impose further that, due to the continuum or quasi-continuum of energy levels above the level S3/2, the excitation wavelength is external to the wavelengths corresponding to transition energies of the erbium atom, the energy level of its levels S3/2 111/2 to or higher. This can be easily achieved by using for example a titanium-sapphire laser source having a wavelength characteristic is of 1020 nm, but which can be adjusted between 950 nm and 1100 nm.

For the nature of the doping and the co-doping, the proportions of erbium and ytterbium have been respectively in a preferred embodiment of 500 PPM (parts per million) and 15000 PPM, is a preferred ratio of 30. The proportion of ytterbium can be between 10000 PPM and 50000 PPM.