WO2001001529A1

WO2001001529A1 - A wide band optical amplifier

Info

Publication number: WO2001001529A1
Application number: PCT/GB2000/002432
Authority: WO
Inventors: Shaoxiong Shen; Animesh Jha; Mira Naftaly
Original assignee: University of Leeds
Current assignee: University of Leeds
Priority date: 1999-06-23
Filing date: 2000-06-23
Publication date: 2001-01-04
Anticipated expiration: 2001-12-23
Also published as: GB2367684A; AU5551400A; GB9914549D0; GB0200448D0; GB2367684B

Abstract

A wide band optical amplifier comprising first and second optical glass amplifying sections, the first optical glass amplifying section comprising (a) a TeO2 host glass; (b) an effective quantity of erbium dopant; (c) a network modifying metal oxide; and, (d) further ingredients wherein the amounts of (a), (b), (c) and (d) total 100 %, the second optical glass amplifying section comprising (e) a TeO2 host glass; (f) an effective quantity of thulium dopant; (g) a network modifying metal oxide; and, (h) further ingredients wherein the amounts of (e), (f), (g) and (h) total 100 %.

Description

A WIDE BAND OPTICAL

AMPLIFIER

The present invention relates to a wide band optical amplifier. More particularly, but not exclusively, the present invention relates to a wide band optical amplifier having first and second tellurite optical glass amplifying sections, the first section being doped with erbium and the second section doped with thulium.

Most modern optical transmission systems are so called wavelength division multiplexing systems (WDM) . Such systems can capable of simultaneously transmitting several streams of information through the system, each on a different wavelength channel. This information is typically transmitted along an optical fibre. In systems in which only short lengths of optical fibre are used the number of channels (and hence the transmission capacity) is limited by the width of the transmission window of the fibre. In a water free fibre the transmission window extends from 1260nm to 1680nm.

All fibre optic cables absorb a small fraction of the light passing along their length. If one wishes to transmit optical information over long distances one must therefore include optical amplifiers along the length of the fibre optic cable. These amplifiers boost the signal strength in order to overcome this absorption. However, if one includes an optical amplifier in an optical transmission system the number of channels is limited by the amplifier gain bandwidth. Amplifiers having a broad gain bandwidth are therefore to be preferred.

Doped silica glass is widely used as an amplifying element in optical amplifiers due to its excellent properties and ease of fabrication. Erbium, thulium and neodymium are also known as glass dopants in optical amplifiers. However, erbium and neodymium doped silica glass optical amplifiers have very narrow gain bandwidths. Thulium doped silica glass optical amplifiers are not possible due to the high phonon energy of the silica glass.

The use of fluoride glass (particularly ZBLAN) in optical fibre amplifiers is also known. Both erbium and thulium doped fluoride glass optical amplifiers have relatively broad gain bandwidths. However, in fluoride glass it is necessary to pump erbium at 1480nm which is close to the gain bandwidth of thulium doped optical glass. One cannot have optical gain close to a pump wavelength. An optical amplifier comprising a combination of these two types of glasses therefore has a gain profile comprising two separate bands having a gap centred at 1480nm.

An optical amplifier comprising a combination of an erbium doped tellurite glass and a thulium doped fluoride glass is also known (M Yamafa el al, IEEE Photonics Technology Letters, Vol 10, No 9, ppl244-1246) . This device has a gain profile consisting of two separate bands, 1443-1484 nm and 1532-1608 nm. This device also fails to provide a continuous gain over a broad range of wavelengths.

Accordingly, in a first aspect, the present invention provides a wide band optical amplifier comprising first and second optical glass amplifying sections, the first optical glass amplifying section comprising

(a) a Te0₂ host glass;

(b) an effective quantity of erbium dopant;

(c) a network modifying metal oxide; and,

(d) further ingredients wherein the amounts of (a) , (b) , (c) and (d) total 100% the second optical glass amplifying section comprising

(e) a Te0₂ host glass;

(f) an effective quantity of thulium dopant;

(g) a network modifying metal oxide; and,

(h) further ingredients wherein the amounts of (e) , (f), (g) and (h) total 100%.

The optical amplifier according to the invention has a wide band continuous gain from approximately 1400 nm to approximately 1600 nm.

Preferably, the wide band optical amplifier of the invention further comprises a third optical glass amplifying section, the third optical glass amplifying section comprising

(i) a Te0₂ host glass;

(j) an effective quantity of neodymium dopant;

(k) a network modifying metal oxide; and,

(1) further ingredients wherein the amounts of (i) , (j), (k) and (1) total 100%. Such an optical amplifier has a continuous gain from approximately 1330 nm to 1600 nm.

The concentration of Te0₂ of at least one of the optical sections can be in the range 50 to 90 mol %, more preferably 70-80 mol %. This results in a stable glass.

The network modifying metal oxide of at least one of the optical glass amplifying sections can comprise an oxide of at least one of barium, bismuth, lead, zinc, gallium, lanthanum, niobium, tungsten, tantalum, vanadium and mixtures thereof, the concentration of the network modifying metal oxide preferably being in the range 10 to 45%, more preferably in the range 15 to 35%, more preferably in the range 20 to 30%. Such metal oxides are particularly effective as network modifiers so producing a broad gain profile. In addition, including such oxides in the glass amplifying sections increases the refractive index of the glass to a value of the order 1.7 (or higher) at the 589nm sodium line. This in turn gives rise to a relatively large emission cross section which is important in the production of short fibre amplifiers.

Preferably, the oxide of a metal comprises at least one selected from the group BaO, Bi₂0₃, PbO, ZnO, Ga₂0₃, La₂0₃, Li₂0, Nb₂0₅, W0₃, Ta₂0₅ and V₂0₅. Such oxides are particularly effective at broadening the gain profile of the glass.

At least one of the optical glass amplifying sections can further comprise at least one of Na₂0 or K₂0 and mixtures thereof, the concentration of which preferably being in the range trace to 20 mol %.

Preferably, at least one of the optical glass amplifying sections further comprises a metal halide, preferably selected from the group comprising BaCl₂, PbF₃, LaF₃, ZnF₂, BaF₂, NaF, NaCl, LiF and mixtures thereof. This further broadens the gain profile of the amplifier.

The concentration of the metal halide can be in the range trace to 20mol %.

Preferably the concentration of the erbium dopant is in the range 0.01 to 5 mol %.

Preferably the concentration of the thulium dopant is in the range 0.01 to 5 mol %.

Preferably, the concentration of the neodymium dopant is in the range 0.01 to 5 mol %. The present invention will now be described by way of example only, and not in any limitative sense, with reference to the accompanying tables and drawings in which

figure 1 shows a partial energy level diagram of Er³⁺;

figure 2 shows a partial energy level diagram of Tm³⁺;

figure 3 shows a partial energy level diagram of Nd³⁺;

figure 4 shows normalised emission cross section spectra of optical glass amplifying sections of a wide band optical amplifier according to the invention;

table 1 lists some examples of erbium doped telurite glass compositions for use as optical glass amplifying sections of a wide band optical amplifier according to the invention;

table 2 lists some examples of thulium doped telurite glass compositions for use as optical glass amplifying sections of a wide band optical amplifier according to the invention;

table 3 lists some examples of neodymium doped telurite glass compositions for use as optical glass amplifying sections of a wide band optical amplifier according to the invention.

Shown in figure 1 is a partial energy level diagram of ErX Erbium-doped fibre amplifiers utilize the ⁴1₁₃₂-⁴1_15/2 transition of Er³⁺ to obtain amplification at 1550 nm. Two pumping schemes are available. The 980 nm pump promotes the ions to the I_u/2 level, from which they relax nonradiatively to the lasing ⁴I₁₃₂ level. Alternatively, direct in-band pumping of the ⁴Iι₃₂ level is possible using a 1480 nm pump. In-band pumping at 1480 nm gives rise to increased amplifier noise, and also prevents gain in the short-wavelength part of the emission spectrum. For these reasons pumping at 980 nm is preferred. However, for pumping at 980 nm to be efficient, the lifetime of the I_n/2 level must be short compared with the pumping rate, which requires a glass host with a relatively high phonon energy.

Shown in figure 2 is a partial energy level diagram of Tm³⁺. Thulium-doped fibre amplifiers utilize the ³H₄-³F₄ transition of Tm³⁺ to obtain amplification at 1470 nm. The required pump wavelength is 800 nm, directly into the ³H₄ lasing level. Alternatively, a 680 nm pump can be used to promote ions to the ³F₃ level, from where they relax nonradiatively to the ³H₄ lasing level. Since the energy difference between the lasing ³H₄ level and the underlying ³H₅ level is only about 4500 cm^"1, a glass host with a relatively low phonon energy is required for efficient amplification at 1470 nm. Another problem is posed by the long lifetime of the ³F₄ lower lasing level. Since population inversion is required for amplification to take place, the lower level must be emptied at a rate that is fast compared with the pumping rate. Solutions include allowing lasing to occur at 1850 nm (³F₄→³H₆) , and depleting the ³F₄ level by energy transfer to a co-dopant such as Ho³⁺.

In implementing erbium-doped and thulium-doped amplifiers, tellurite glass has the important advantage that its phonon energy is low enough to enable an efficient thulium-doped amplifier, yet high enough to allow the erbium-doped amplifier to be pumped at 980 nm. Pumping at 980 nm has an important role in enabling continuous gain to be achieved from combined erbium and thulium-doped modules.

Shown in figure 3 is a partial energy level diagram of Nd³⁺ . Nedymium-doped fibre amplifiers utilize the F_3/2- I_13/2 transition of Nd³⁺ to obtain amplification at 1330 nm. The required pump wavelength is 800 nm, which promotes ions to the ⁴F₅₂ level, from where they relax nonradiatively to the ⁴I₁₃₂ lasing level. A serious problem in obtaining an efficient amplifier at 1330 nm is posed by the amplified spontaneous emission (ASE) at 1060 nm (⁴F_3/2^⁴I_n/2) , which has a branching ratio approximately five times larger than the 1330 nm emission. Possible solutions include using a distributed Bragg grating to filter out the 1060 nm radiation, and co-doping with selective absorbers such as Yb³⁺.

The emission wavelengths, profiles and cross-sections of dopant ions such as erbium, thulium and neodymium are strongly influenced by the host glass as disclosed in 'Rare Earth Doped Fibre Lasers and Amplifiers' ed MJF Digonnet, Marcel Dekker 1993. The amplifying transition takes place between two energy level manifolds consisting of several Stark sub-levels. The emission and gain profile combine contributions of all the transitions between the sub-levels. The emission peak and profile are determined by the Stark splitting of the two levels and the oscillator strengths of the individual transitions.

Both the Stark splittings and the oscillator strengths are strongly affected by the ligand field of the ion environment. The ligand field is the local electromagnetic field as experienced by the dopant ion, and is determined by the site geometry and the chemical nature and bonding strength of the host material. The emission and gain profiles of the dopant ions therefore depend on the ligand field at the dopant site. In relatively covalent high refractive index glasses, such as tellurite, the ligand field shifts the emission to longer wavelengths. This is the nephelauxetic effect, well known in lanthanide ions. Higher energy levels are generally affected more strongly than lower energy ones; for this reason neodymium emission shifts more than thulium which shifts more than erbium. The value of the emission cross-section also increases in high refractive index glasses, reflecting the relationship between the oscillator strength of the transition and the ligand field at the ion site.

When the host glass, such as tellurite, offers a multiplicity of dopant sites, the emission spectrum from each type of site is slightly different, and the combined emission spectrum from all sites is broader than in a single-site glass such as silica. Accordingly, the role of network modifiers is twofold: to break up the glass network so as to create numerous different sites for the erbium, thulium and neodymium dopants; and to increase the refractive index of the tellurite glass. In addition, network modifiers provide strong ionic bonding at ion sites, thus ensuring high ion solubility. The tellurite glass of the invention is a high refractive index glass which provides a multiplicity of different dopant sites to erbium, thulium and neodymium ions, and therefore gives rise to broad emission with a large cross-section and emission peaks shifted to longer wavelengths. As a consequence, the gain bands of erbium-, thulium- and neodymium-doped tellurite glass amplifier modules may be combined to achieve continuous gain from 1330 nm to 1660 nm, as shown in figure 4.

Listed in tables 1 to 3 are examples of Er, Tm and Nd doped optical glasses for use as amplifying sections in a wide band optical amplifier according to the invention. Any of the examples of erbium doped glasses can be used in combination with any of the examples of Tm doped glasses and, optionally, any of the examples of Nd doped glasses. Table 1

Table 1 continued

Table 2

Table 3

Claims

Clai s

1. A wide band optical amplifier comprising first and second optical glass amplifying sections, the first optical glass amplifying section comprising

(a) a Te0₂ host glass;

(b) an effective quantity of erbium dopant;

(c) a network modifying metal oxide; and,

(e) a Te0₂ host glass;

(f) an effective quantity of thulium dopant;

(g) a network modifying metal oxide; and,

2. A wide band optical amplifier as claimed in claim 1, further comprising a third optical glass amplifying section, the third optical glass amplifying section comprising (i) a Te0₂ host glass;

(j) an effective quantity of neodymium dopant; (k) a network modifying metal oxide; and, (1) further ingredients wherein the amounts of (i) , (j), (k) and (1) total 100%.

3. A wide band optical amplifier as claimed in either of claims 1 or 2, wherein the concentration of Te0₂ of at least one of the optical sections is in the range 50 to 90 mol %, more preferably 70-80 mol %.

4. A wide band optical amplifier as claimed in any one of claims 1 to 3, wherein the network modifying metal oxide of at least one of the optical glass amplifying sections comprises an oxide of at least one of barium, bismuth, lead, zinc, gallium, lanthanum, niobium, tungsten, tantalum, vanadium and mixtures thereof, the concentration of the network modifying metal oxide preferably being in the range 10 to 45%, more preferably in the range 15 to 35%, more preferably in the range 20 to 30%.

5. A wide band optical amplifier as claimed in claim 4, wherein the oxide of a metal comprises at least one selected from the group BaO, Bi₂0₃,PbO, ZnO, Ga₂0₃, La₂0₃, Li₂0, Nb₂0₅, W0₃, Ta₂0₅, V₂0₅.

6. A wide band optical amplifier as claimed in any one of claims 1 to 5, at least one of the optical glass amplifying sections further comprising at least one of Na₂0 or K₂0 and mixtures thereof, the concentration of which preferably being in the range trace to 20 mol %.

7. A wide band optical amplifier as claimed in any one of claims 1 to 6, at least one of the optical glass amplifying sections further comprising a metal halide, preferably selected from the group comprising BaCl₂, PbF₃, LaF₃, ZnF₂, BaF₂, NaF, NaCl, LiF and mixtures thereof.

8. A wide band optical amplifier as claimed in claim 7, wherein the concentration of the metal halide is in the range trace to 20 mol%.

9. A wide band optical amplifier as claimed in any one of claims 1 to 8, wherein the concentration of erbium dopant is in the range 0.01 to 5 mol%.

10. A wide band optical amplifier as claimed in any one of claims 1 to 9, wherein the concentration of the thulium dopant is in the range 0.01 to 5 mol %.

11. A wide band optical amplifier as claimed in claim 2, wherein the concentration of the neodymium dopant is in the range 0.01 to 5 mol%.

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