WO2004114477A1

WO2004114477A1 - Optical amplifier system

Info

Publication number: WO2004114477A1
Application number: PCT/EP2004/005823
Authority: WO
Inventors: Reinhard Caspary
Original assignee: Technische Universität Braunschweig
Priority date: 2003-06-20
Filing date: 2004-05-28
Publication date: 2004-12-29
Also published as: DE10327947A1

Abstract

The invention relates to an optical amplifier system for amplifying signal radiation, comprising a waveguide section (5), which is doped with triple positively charged thulium ions, and comprising a pumping laser (7) that is connected to the waveguide section (5) and pumps the thulium ions from the base energy level to energy level 3, from which the thulium ions, within the scope of stimulated emission, change to energy level 1 by interacting with the signal radiation. The system also comprises a device for emptying energy level 1, which is characterized in that it has appropriate optical elements (11, 13, 15, 17, 18) that are arranged in such a manner that radiation of certain wavelengths, which is emitted by thulium ions contained in the waveguide section (5) during the transition from energy level 1 to the base energy level, is returned to the ions whereby energy level 1, within the scope of stimulated laser emission, is emptied until the base energy level is reached.

Description

Optical amplifier system

description

The invention relates to an optical amplifier system for amplifying signal radiation propagating in optical fibers or other optical waveguides.

Such optical amplifier systems are used in recent years worldwide in data networks in which glass fibers, preferably one-mode silicate glass fibers are used as the transmission medium, which ensure a high data flow. Although such glass fibers have a low attenuation, it is forced after about 70 - 150 km to amplify the optical signals again. Previously, this was done by way of electrical amplifiers, today almost exclusively uses optical fiber amplifiers, which can amplify large areas in the wavelength band at once, regardless of whether one or more channels are present at different wavelengths and which data rates or encodings have these channels ,

For a long time, almost exclusively the C-band in the range of 1,530 - 1,570 nm was used around the attenuation minimum of quartz glass fibers 103 for data transmission. For amplification of signals in this wavelength range almost exclusively erbium-doped fiber amplifiers (EDFA) are used. Such a fiber amplifier system 101 is shown in FIG. 1 and consists of a glass fiber 105 whose core is doped with optically active erbium ions. The glass fiber 105 is in this case designed as an annular, non-closed loop, wherein a pump laser 107 of suitable wavelength, in the case of erbium usually 980 nm, via a so-called wavelength division multiplex coupler (WDM coupler) 109 is connected to the glass fiber 105. The wavelength division multiplexer 109 combines the radiation in its two input fibers onto a common output fiber. The radiation from the pump laser 107 is absorbed in the fiber 105 and the erbium ions are placed in energetic excited states. If a signal photon now encounters an excited erbium ion, there is a chance that the erbium ion will relax into the ground state by emitting a new photon of the same wavelength, direction and phase (stimulated emission). Signal and pump radiation run together in the fiber core, so that in addition to the stimulated emission and absorption of signal photons by Erbium ions in the ground state takes place. To exclude this effect as much as possible and thus achieve an actual net gain, a certain pump power threshold must be exceeded, so that a significant majority of erbium ions is in an energetically excited state (population inversion).

Since the possible data rates of optical signal transmission seemed almost inexhaustible until just a few years ago, the Internet boom of recent years has ensured that the limits of current technology are in the focus of research. It can be expected that in a few years the first transmission links will reach physical limits. Although a transmission link can have a maximum bandwidth of 10 Tbit / s, which seems gigantic at first glance, this data rate is already almost reached on the continental backbone routes on which traffic is concentrated. In order to avoid having to lay new glass fibers, therefore, novel fiber amplifiers have been developed, which open up new transmission bands and can be connected in parallel to the existing EDFAs. The location of the gain spectra of various rare-earth doped fiber amplifiers as well as some important optical transmission bands is shown in FIG. 2 plotted over the wavelength in nm.

For example, it has been known for many years that thulium-doped fiber amplifiers (TDFA) in the range of 1450-1490 nm, so the S ⁺ band, can be used. The energy level scheme of triply ionized thulium, Tm ³⁺ , is shown in Fig. 3 with the energy plotted upwards in 1000 cm ^-1 from which it can be seen that the amplifier transition occurs between energy levels 3 and 1. The lower Amplifier level is therefore not the ground state, ie there is a 4-level system before pumping with 790 nm initially in the upper amplifier level 3, from which the ion mainly by stimulated emission changes to state 1. This state is very durable, so that the process would terminate without further precautions at this point.To prevent this, is pumped additionally with a second pump laser, eg at 1420 nm, this radiation is not absorbed by the ground state, but by energy level 1, whereby the in Level 1 thulium ions are raised to the energy level 3. This second pump is the quantum from the much more important, and the ions n rarely reach the ground state, so that, so to speak, a quasi-3-level system is present. The basic structure of a conventional TDFA, shown in Fig. 4, therefore differs only slightly from that of the EDFA in Fig. 1. It only has a second pump laser 108 with matching WDM coupler 115th be supplemented.

In addition to the 790 and 1420 nm pumping scheme just presented, a large number of other pumping schemes with different pump wavelengths have been investigated in recent years. All have in common that two pump transitions are used, one for emptying the ground state and one for emptying the average energy level 1. In publications of such fiber amplifier systems, the pumping scheme 1050/800 nm is first described in ASL Gomes, ML Sundheimer, MT Carvalho, JF Martins- Filho, CJA Bastos-Filho, W. Margulis: "Novel dual wavelength (1050nm + 800nm) pumping scheme for thulium doped fiber amplifiers", in Proc. OFC 2002, Anaheim CA, USA, 2002, paper FB2. The pumping scheme 1400/800 nm is first described in F. Roy, A. Le Sauze, P. Baniel, D. Bayart: "O.δμm + 1.4μm pumping for gain-shifted TDFA with power conversion efficiency exceeding 50%", in Proc. OAA 2001, Stresa, Italy, 2001, paper PD4.

A disadvantage of such fiber amplifier systems is that the use of two pump lasers represents a significant cost factor, with the adjustment and tuning of the components is very complicated. At pump wavelengths that are suitable both for emptying the ground state and for emptying the middle energy level 1, one could theoretically make do with a pump laser, which is inevitably very inefficient due to the predetermined position of the energy levels of Tm ³⁺ .

The present invention is therefore based on the object to provide an optical amplifier system based on optically active waveguides, which amplifies wavelengths in a range of 1450-1530 nm, as simple and inexpensive as possible and yet provides an extremely efficient amplification of the signal radiation.

This object is achieved by an optical amplifier system having the features of claim 1.

Such an optical amplifier system for amplifying signal radiation has at least one waveguide section which is doped with threefold positively charged thulium ions, and at least one pump laser which is connected to the at least one waveguide section and which pumps thulium ions from the basic energy level into the energy level 3 from which the thulium ions change to energy level 1 in the context of stimulated emission through interaction with the signal radiation, and has min. at least one device for emptying the energy level 1 on. The system is characterized in that the device for emptying the energy level 1 comprises suitable optical elements arranged to emit radiation of particular wavelengths emitted by thulium ions contained in the waveguide section at the transition from the energy level 1 to the basic energy level, is returned to the ions, whereby the energy level 1 in the context of stimulated laser emission is emptied to the basic energy level. Thus, it is possible, even without the use of a second pump laser to create an efficient and cost-effective amplifier system, which also consumes less energy and less susceptible to interference.

The dependent claims form advantageous embodiments of the amplifier system described in claim 1.

Further details, features and advantages will become apparent from the following description with reference to the drawings. It shows:

FIG. 1 shows a prior art Erbium-doped optical amplifier system. FIG.

2 shows gain spectra of various fiber amplifiers and the location of some important optical transmission bands of S ⁺ - L ⁺ ,

3 shows the energy level scheme of threefold positively charged thulium ions with the relevant transitions,

4 shows an optical thulium-doped fiber amplifier system according to the prior art,

5 shows a first embodiment of an optical amplifier system according to the invention,

6 shows a second embodiment of an optical amplifier system according to the invention,

7 shows a representation of the absorption and emission curve of an optical transmission gangs, and

8 shows a third embodiment of an optical amplifier system according to the invention.

FIG. 5 shows a first embodiment of an optical amplifier system 1 according to the invention. The signal is introduced into the system via single-mode silicate glass fibers (quartz glass fibers) 3 and enters an unclosed annular loop of a glass fiber 5 whose core is doped with threonium ionized thulium ions (Tm ³⁺ ). The glass material is preferably made of fluoride glass, but fibers of quartz glass, tellurite glass or chalcogenide glass may also be used. The location of thulium energy levels is largely identical in all materials because it is a rare earth element.

In front of or behind the annular glass fiber loop 5 of the amplifier system, a pump laser 7 is arranged, which sends pump radiation into the annular glass fiber loop 5. The pump laser can be chosen arbitrarily, in practice, single or parallel laser diodes have proven. Usually, the pump laser causes the direct transition from the ground state of the thulium ions to the energy level 3, in the case of Tm ³⁺ ions it emits radiation in a wavelength range of about 790 nm. It can also be a laser for the transition of the ions from the elemental energy level be used in the energy level 4 or a higher energy level, whereupon the thulium ions relax in the energy level 3. As a result, lower noise figures can be obtained. For such a transition, an external fiber laser as a pump laser can be a better alternative. It is explicitly pointed out that in the general term "pumping into energy level 3" according to the subject-specific terminology, the above-described transient pumping into an even higher energy level with subsequent relaxation of the ion into energy level 3 is also included. Because signal and pump radiation must travel together in the fiber core, the radiation delivered by the pump laser 7 is coupled via a wavelength division multiplex coupler (WDM coupler) 9 in the signal fiber 5, wherein the WDM coupler, the radiation in its two input fibers to a common output fiber summarizes. With the combination of signal and pump wavelengths for which the WDM coupler is designed, only minimal losses occur. Instead of the usual second pump laser at about 1400 nm, the mean energy level 1 is now emptied differently, namely by allowing a fiber laser to run in the amplifier fiber parallel to the amplification process on the transition energy level 1 → ground state. For this purpose, in each case a mirror 11, 13 is arranged in front of and behind the annular loop 5. Each mirror is connected to the optical fiber 5 via a wavelength-division multiplex coupler 15, 17. The fluorescence of the transition energy level 1 → ground state at about 1.9 μm is selectively coupled out via these WDM couplers 15, 17. Together with the two mirrors 11, 13 at the fiber ends results in a very simple fiber laser (Fabry-Perot fiber laser, FP), which automatically seeks the most efficient wavelength in the spectral range of fluorescence. The mirrors in the FP design can be dielectric or metallic mirrors which are pressed, glued or vapor-deposited directly onto the fiber end. Even bare, straight-cut fiber ends are suitable as a mirror for the fiber laser, because the refractive index difference between fiber and air causes a Fresnel reflection of about 4%, which is often sufficient. Also suitable are so-called fiber Bragg gratings (FBG), which are exposed directly into a fiber. The latter also offer the possibility of merging the WDM couplers 9 and 15 and writing an FBG in the fiber between WDM coupler 9 and pump laser 7.

Behind the second WDM coupler 17, the amplified signal is then transported again in quartz glass fibers 3.

The amplifier system now operates such that an incoming signal of about 1470 nm is introduced into the thulium ion-doped annular glass fiber loop 5 and at the same time the first pump laser 7 sends pump radiation in the range of 790 nm into the loop. As a result, the thulium ions are raised to a high energy level, here the energy level 3, from which they relax on stimulation by the signal photons with emission of a new photon of the same wavelength, direction and phase to the energy level 1 (stimulated emission). At the same time, the energy level 1 is emptied by the following process: the small proportion of thulium ions which, due to spontaneous emission, change from the energy level 1 to the ground state and emit a photon of corresponding wavelength (approximately 1.9 μm), becomes one of the two WDM coupler 15, 17 coupled and passed back through the respective mirror 11, 13 back into the system. The returned radiation then stimulates a transition to the ground state with emission of corresponding photons in the case of the remaining thulium ions which are in energy level 1 due to stimulated emission. So it will be Principle of a simple fiber laser used to empty the energy level 1, so that the entire amplification process does not terminate in the long-lived energy level 1. In this way, the energy level 3 is constantly heavily occupied by the use of the pump laser 7 and the process described above, so that the signal photons often generate by stimulated emission more signal photons, which eventually cause the signal amplification.

A second embodiment of the present invention is shown in FIG. The structure is substantially identical to the first embodiment, but no mirrors are used, but the two open fiber ends of the WDM couplers 15, 17 are connected such that a ring resonator 18 results. In the ring resonator 18, an optical isolator 19 is preferably included. The insulator 19 is not absolutely necessary, but it is used in the ring resonator 18 to set the direction of rotation of the laser oscillation. Without the insulator 19, two opposing waves would form which interfere with each other and thus only selectively space-discharge the excited states in the doped fiber. The installation direction of the insulator 19, however, is arbitrary. Again, the two WDM couplers 15, 17 act as wavelength-selective elements, so that only wavelengths in the range of 1, 9 microns are transmitted.

Since the resulting 1.9 μm fiber laser is a 3-level laser, it can not completely empty the energy level 1. In order to empty the level as much as possible, the fiber laser should start at the largest possible wavelength, which can be achieved either by a suitable design of the wavelength characteristic of the WDM coupler 15, 17 or by the use of a filter 21 in the resonator 18. In the structure according to FIG. 5, the spectral reflection of the mirrors 11, 13 can be suitably designed for this purpose (dielectric mirrors).

The amplifier according to the previous description is optimized to work in the spectral range around the maximum of the Tm fluorescence, ie in the S ⁺ band. The absorption peak at approximately 1390 nm shown in dashed lines in FIG. 2 extends with its long-wave tail into the S ⁺ band. This absorption is due to OH ^" ions in the glass matrix, which are eliminated in modern glass fibers but still play a role in many fiber lines already laid, which is why network operators prefer S-band amplifiers at 1490-1530 nm directly to the C-band, where one can take advantage of the fact that, in spectrum is always shifted toward longer wavelengths with respect to the absorption spectrum, as shown in Fig. 7, wherein the absorption line 30 and the emission line 32 of an optical transition are respectively plotted across the wavelength in arbitrary units. This effect can be explained by the Bolzmann distribution.

In a conventional three-level system, the ground state is very heavily occupied in an only weakly pumped amplifier fiber, and therefore the absorption of the signal radiation plays a major role. In contrast to the heavily pumped amplifier with a depleted ground state, where the largest amplification occurs at the maximum of the emission spectrum, these signal components are strongly absorbed (and thus recycled) by the weakly pumped amplifier. The net gain, on the other hand, occurs at longer wavelengths where the absorption spectrum has already decayed.

In the case of the newly developed amplifier system, the situation is somewhat more complicated because the lower amplifier level is not the ground state but the first excited state. For defined displacement of the amplification band so the occupation of the energy level 1 must be controlled. Although the exact setting of the emission wavelength of the fiber laser is not fundamentally necessary, it nevertheless leads to better results. This can be achieved by suitable design of the WDM couplers 15, 17, by fixed or variable filters 21 in the laser resonator 18 or by a suitable design of the mirror 11, 13 in the FP fiber laser.

It is more advantageous, however, to run the fiber laser at a fixed, as large as possible wavelength and to switch a second Tm fiber loop 23 without 1, 9 micron fiber laser behind the first amplifier stage, as shown in Fig. 8. It is irrelevant whether the first amplifier stage works with mirrors or a ring resonator. The second doped fiber 23 is transparent to the signal wavelength as long as the associated pump laser 25 coupled via a WDM coupler 27 is turned off. Even with low pumping power can occupy the energy level 1, which then ensures that the short-wave signal components are absorbed. Since this absorption occurs at energy level 3, the power is not lost, but contributes to amplification at longer wavelengths.

Deviating from these basic ideas, many other modifications are conceivable. Thus, additional additional pump lasers with associated WDM couplers can be practically any desired distributed over the entire structure, and pump the various doped fiber segments from the front and / or from the rear. It is also possible to combine the emptying of the energy level 1 by means of a fiber laser with conventional pumping schemes, for example to increase their power efficiency. Wherever a thulium-doped fiber is mentioned in the above examples, in practice also several thulium-doped fibers may be used. In front of and behind these fibers optical filters and / or optical isolators can be installed.

Wherever in the above examples of glass fibers is mentioned, any optical waveguide can alternatively be used. This applies in particular to the optically active doped glass fibers, which can be designed as doped waveguides of any design.

A WDM coupler is usefully realized as a fiber-melt coupler, but also structures with waveguide couplers in planar optics are conceivable. Then, for example, in the structure in FIG. 5, one could combine the two couplers 9, 15 and the pump laser 7 and the mirror 11 on a substrate.

The present invention thus provides a system which can be used in addition to existing EDFAs and provides a simple way efficient amplification of signal photons.

Claims

Expectations

1. Optical amplifier system for amplifying signal radiation,

with at least one waveguide section (5) doped with triple positively charged thulium ions,

with at least one pump laser (7), which is connected to the at least one waveguide section (5) and pumps the thulium ions from the basic energy level to energy level 3, from which the thulium ions as part of stimulated emission by interaction with the signal radiation into the energy level 1 change, and

with at least one device for emptying energy level 1,

characterized in that

the device for emptying the energy level 1 has suitable optical elements (11, 13, 15, 17, 18) which are arranged in such a way that radiation of specific wavelengths emitted by the thulium ions contained in the waveguide section (5) during the transition from the energy level 1 is emitted into the basic energy level, is returned to the ions, whereby the energy level 1 is emptied towards the basic energy level as part of stimulated laser emission.

2. Amplifier system according to claim 1, characterized in that the waveguide section (5) is designed as at least one doped glass fiber.

3. Amplifier system according to claim 1 or 2, characterized in that the pump laser (7) via a wavelength division multiplex coupler (9) is connected to the waveguide section (5).

4. Amplifier system according to one of the preceding claims, characterized in that the optical elements include at least two wavelength division multiplex couplers (15, 17).

5. Amplifier system according to one of the preceding claims, characterized in that the optical elements contain at least two mirrors (11, 13).

6. Amplifier system according to claim 5, characterized in that the mirrors (11, 13) have a predetermined spectral reflection.

7. Amplifier system according to one of claims 1 to 4, characterized in that the optical elements include a ring resonator (18).

8. An amplifier system according to claim 7, characterized in that the ring resonator (18) has at least one optical isolator (19).

9. Amplifier system according to claim 7 or 8, characterized in that the ring resonator (18) has at least one optical filter (21).

10. Amplifier system according to one of the preceding claims, characterized in that it has at least one variable filter which is suitable for adjusting the radiation density returned to the thulium ions.

11. Amplifier system according to one of claims 1 to 10, characterized in that after the first waveguide section (5), the first pump laser (7) and the device for emptying the energy level 1, a second waveguide section (23) and a second pump laser (25) are arranged.