WO2008117249A1

WO2008117249A1 - Integrated optical waveguide amplifier or laser with rare earth ions and sensitizer elements co-doped core and related optical pumping method

Info

Publication number: WO2008117249A1
Application number: PCT/IB2008/051126
Authority: WO
Inventors: Fabrizio Di Pasquale; Stefano Faralli; Lorenzo Pavesi
Original assignee: Scuola Superiore Di Studi Universitari E Di Perfezionamento Sant'anna; Teem Photonics S.A.
Priority date: 2007-03-26
Filing date: 2008-03-26
Publication date: 2008-10-02

Abstract

An integrated waveguide device for optical signal amplification employing a low-cost and high-power type source of pump light selected from the group composed of broad band LEDs and broad area laser diodes. The integrated device has an active core (3) made of silica, silicon based dielectric material, or multicomponent glass,' containing at least a uniformly dispersed sensitizer element selected out of silicon nanoclusters and Ytterbium ions and ions of a rare- earth able for producing a stimulated emission within a certain wavelength band. The device comprises: a substrate (1) of a material suitable for defining thereon integrated planar waveguide structures; an optical signal waveguide (2) on the substrate for single mode guiding one or more optical signals, including an active core of the above composition with a first refractive index n 1 and at least an inner cladding (4) of dielectric material having, a second refractive index n 2 lower than n l;a waveguide adapted for multimode guiding the pump light extending adjacently to the single mode optical waveguide; means for coupling pump light from the source (5) to the multimode optical waveguide.

Description

INTEGRATED OPTICAL WAVEGUIDE AMPLIFIER OR LASER WITH RARE EARTH IONS AND SENSITIZER ELEMENTS CO-DOPED CORE AND RELATED OPTICAL

PUMPING METHOD

BACKGROUND OF THE INVENTION Technical field

The invention relates to an integrated waveguide amplifier or laser using substrate materials suitable for the fabrication of planar structures, with a core of mult i- component glass, or dielectric silicon oxide (i.e. silica), doped with ions of at least a rare earth element with internal transitions from metastable levels of long lifetime and used for laser transitions and where at least one kind of sensitizer elements is included with high capacity of energy absorption at the wavelength band of the pump light and able to transfer the absorbed energy to the rare earth ions. In particular the invention relates to a waveguide amplifier or laser that can be efficiently pumped by low cost light sources like low cost high power and broad band LED or by broad area and high power multimode laser diode. Moreover the invention relates to an optical pumping method of such amplifier. State of art Among the rare earth elements (Erbium, Thulium, Praseodymium, Ytterbium and Neodymium) commonly used as solute in a dielectric matrix of solid solvent, the Erbium, in ionic form Er³⁺, has an internal transition that can emit at around 1550 nm and is widely exploited for a laser transition very useful in the optical signal transmission field. Optical fiber lasers and amplifiers have been produced exploiting the transition in such optical band. In such optical systems the Er³⁺ ions excitation is usually obtained by direct absorption of laser radiation finely tuned at a particular internal transition of the ion.

Silica co-doped by Ytterbium and Erbium ions or other rare earth ions has been proposed, because excitation of the ions used to produce useful stimulated emission, for example of the Erbium ions, is achieved with higher efficiency in presence of Ytterbium ions that possess relatively much broader and intense pump light absorption cross-sections, especially in the 900-1000 nm band, and are able to transfer absorbed energy to neighbouring Erbium ions through the physical phenomenon of cross-relaxation. In this way, the gain of the device can be sensibly increased. The energy level scheme and the main physical interactions between Erbium and Ytterbium ions are shown in Figure 1. The Ytterbium ions absorb effectively at wavelength between 900 nm and 1000 nm and at such wavelengths multimode powerful and low cost broad area lasers are commercially available. The Yb³⁺ ions are excited from the level a to the level b and decay at the level a transferring energy to the Er ⁺ ions in the level 1 that are raised to the level 3. From the level 3 the Er³⁺ ions rapidly decay to the level 2 by a non-radiative process, and then stimulated emission and signal amplification is possible at around 1550 nm.

The cross-relaxation phenomenon that permits the energy transfer from Yb³⁺ to Er³⁺ ions is described by the parameter C_cr in Fig. 1.

The interaction between the nearest Er³⁺ ions, called up-conversion and described by the parameter C_up in Fig. 1, concerns the unwanted phenomenon of interaction among Er³⁺ ions, called up-conversion process, that can reduce the amplification efficiency for high Erbium concentration in the dielectric material.

Recently rare-earth doped silica or silica based materials have been widely used for making fiber and waveguide amplifiers [E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, New York: Wiley, 1994; Y. C. Yan et al., Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 mm, Appl. Phys. Lett. 71 (20), 17 November 1997]. In such amplifiers the rare-earth elements are directly excited by pump light with low absorption cross-section (order of 10^"25 m²) and narrow absorption band. These features require the use of costly single mode pump lasers which are longitudinally coupled to the fiber/waveguide by end fired techniques. In order to overcome the use of expensive pump lasers, also double-cladding fibers have been proposed in which the pump light from relatively unexpensive, broad area pump lasers is longitudinally guided by total internal reflection at the interface between inner and outer claddings, pumping then the single mode rare-earth doped fiber [F. Di Pasquale et al., 23 dBm output power Er/Yb co- doped fiber amplifier for WDM signals in the 1575-1605 nm wavelength region, Optical Fiber Communication Conference, 1999]. This approach reduces the pump cost but unfortunately it can not be effectively applied when integration is required. In fact, in compact waveguide amplifiers (length of the order of a few centimetres) the relatively low pump absorption cross-section of the silica doped by common techniques for growth/deposition of dielectrics that include the presence of dissociable compounds with rare earths (the rare earths are merged in the dielectric matrix during the growth or deposition process from vapour phase in accordance with the limits of dissolution) would not provide sufficient gain per unit length. Moreover, it's well known that when the silicon has a structure with nanometric dimension (diameter of 1-5 nm) its electronic property changes. In particular, the absorption threshold of nano-dimensional silicon is shifted to high energy and the emission efficiency increases of some orders of magnitude with respect to the crystalline silicon. The dimensions and the crystal characteristics of the silicon can be tuned controlling the fabrication process parameters of the nano-structured silicon. The nanostructured silicon that is not crystalline is usually named nanocluster, and the crystalline one is usually named nanocrystal. This material has been produced for the first time at the beginning of the nineties and since then it has been object of great interest. At the middle of nineties it has been demonstrated that by merging the Er³⁺ ions and the silicon nanocluster (or nanocrystal) in the silica matrix, it's possible to have an ion excitation by an indirect process that involves the radiation absorption of the nanocluster or nanocrystal and the transfer of excitation from the nanocluster-nanocrystal to the Er ⁺ ion. If the stimulated rare earth ions are associated to the silicon nanoclusters, the Er³⁺ ion indirect excitation is higher than the association with nanocrystal; therefore the silicon nanoclusters are proved to be good sensitizers for the Er³⁺ ions.

The energy levels scheme of the coupled Silicon nanocluster (Si-NC) /Er³⁺ ion and their main physical interactions are shown in Figure 2.

The silicon nanoclusters (Si-NC) are excited from the level a to the level b by the absorption of pump photon with energy corresponding to the energy gap between the level a and b (Si-NC has a very broad absorption spectrum and can be effectively pumped between 400 nm and 700 nm). Si-NCs decay from the level b to the level a and transfer energy to the Er ⁺ ions in the level 1; so the Er ⁺ ions are excited to a higher level and spontaneously decay to the level 2 (level 2 is a metastable level with a long lifetime) by non-radiative processes. The Er ⁺ ions in the level 2 enable the optical amplification of signal photons at wavelength corresponding to the energy gap between the level 2 and level 1 of the Er³⁺ ions (signal photons induce stimulated emission with a electronic transition from the level 2 to the level 1 and emission of photons in phase with the incident photons).

In particular, for the silica (SiO₂), the system Er³⁺ ion - silicon nanocluster has some advantages with respect to the system with only Er ions:

• the efficiency of indirect excitation of the Er³⁺ ions is some order of magnitude higher than the direct excitation and this is due to the higher effective absorption cross section of nanoclusters respect to the Er³⁺ ions;

• the nanocluster absorption band is very wide, and this makes the excitation system easy to implement and at the same time cost-effective; in fact it is not necessary a fine tuning of the excitation at the internal atomic transitions of the Er ⁺ ions. Costly single mode laser diodes at 980 nm can be replaced with much cheaper LED or high power broad area lasers, available on the market for example at 660 nm, at 800 nm and between 900 and 1000 nm; • the nanoclusters are able to conduct electrical current and provide then the possibility to pump Er³⁺ ions electrically;

• the nanoclusters locally increase the refractive index, and make then possible the fabrication of optical structure able to guide the light.

For silica core co-doped with silicon nanoclusters and rare earth elements, like Er³⁺ ions that can provide optical gain at wavelength around 1.55 μm, the silicon nanocrystals absorb the pump radiation in the visible region and by an energy transfer mechanism (like electron-hole recombination or dipole-dipole coupling) excite the rare earth elements [M. Fujii et al, 1.54 μm photoluminescence of Er ⁺ doped into SiO ₂ films containing Si nanocrystals: Evidence for energy transfer from Si nanocrystals to Er³⁺ , Applied Physics Letters, September 1, 1997, Volume 71, Issue 9, pp. 1198-1200; N. Daldosso et al., Absorption cross section and signal enhancement in Er-doped Si nanocluster rib-loaded waveguides, Appl. Phys. Lett. 86, 261103 (2005)].

As the effective excitation cross section of the silicon nanoclusters (order of 10^~21 m²) is approximately four order of magnitude larger than in case of direct rare-earth excitation (order of 10^~25 m²), the pump light is strongly absorbed resulting in unpractical single mode longitudinal pumping. In fact, with a typical silicon nancluster concentration of 10²⁵ m^"3, the absorption length would be of the order of 100 μm. [C. Dufour et. al., Propagation in erbium and silicon codoped silica slab waveguides : analysis of gain, J. Phys.: Condens. Matter 16, (2004) 6627-6638]. In order to overcome this limitation, top pumping schemes by low-priced wide band light sources such as LEDs have been recently proposed. [Jung H. Shin et al., Si Nanocluster Sensitization of Er-Doped Silica for Optical Amplet Using Top-Pumping Visible LEDs, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 12, No. 4, Juy/ August 2006.] In this approach, contrary to the end- fired scheme in which almost all light from a pump source can be effectively coupled into the waveguide through an optical fiber, most of the pump light from the wide band light sources is lost when trying to focus on the active waveguide.

Although several techniques have been proposed to focus light from a top pumping source into the active waveguide [US Patent 7,075,708 B2, JuI. 11, 2006, Top pumped waveguide amplifier;"; PCT WO 2004/066346 A2, 5 August 2004, Doped semiconductor nanocrystal layers, doped semiconductor powders and photonic devices exploiting such layers or powders], due to the large beam spot size of the wide band light sources (order of 1 mm) and small active core dimensions (few μm) most of the pump light is lost. Moreover, due to limited active waveguide depth (approximately 1 μm to ensure single mode operation at the signal wavelengths) not all the focused pump light will be absorbed by the active material.

The patent US 7,075,708 describes several structures for top pumping by LED array. In order to overcome difficulty due to the LED light focusing on the active waveguide, integrated lenses and reflectors of the pump light are used in several configurations. The main technical difficulty of this approach is the poor efficiency of the top pumping by LED, and even though lenses and reflectors are used the technique is not able to effectively exploit the pump radiation. In fact the emission area of the LED (around 1 mm) is very wide with respect to the transversal dimensions (width) of the waveguide active core (few micron) and hence the most of the pump power is lost for the limited focusing on the active zone. The maximum power density of commercial LEDs available in the spectral region of interest (visible light) is limited to less than 20-30 W/cm².

At the state of art, there is a need to have a high power and low cost pump source which does not require complex focusing structures, and which can be effectively used for pumping integrated optical waveguide amplifiers or lasers on silicon chips (or other substrates like silica based glass, compatible with silicon microelectronics technology) and to achieve an high level of efficiency in terms of pump power.

These difficulties are due to the dimensional scale of such integrated structures. Their longitudinal extension is limited to few centimetres for the integration on a chip of silicon or silica based glass; the reduced dimension is obtained by codoping the silicon oxide (silica) in the waveguide core with silicon nanoclusters and rare earth ions, in order to guarantee a suitable gain per unit length in the integrated optical waveguide amplifier, through the strong silicon nanocluster absorption of pump radiation and consequent rare earth ions excitation. This peculiarity has made difficult to find a solution of an effective pump radiation transfer from incoherent light sources like LEDs or broad area lasers to the active waveguide core along the whole longitudinal extension of the waveguide.

OBJECTIVES AND SUMMARY OF THE INVENTION

A main object of the present invention is to provide an effective solution in order to guarantee an efficient pump light transfer to the active waveguide core along the whole longitudinal extent of the waveguide using low cost and high power source, like LEDs or broad area lasers.

The above discussed limitations, drawbacks and inefficiencies of the known techniques have been found to be practically overcome by using a waveguide structure of light amplification wherein one or more active single mode waveguides may be effectively pumped by low cost high power sources like broad band LEDs or broad area laser diodes. In accordance with the effective pumping arrangement of the present invention, the pump light, that for silicon nanocluster sensitizers can be absorbed in the visible light spectrum and in case of rare earths sensitizers, like Ytterbium ions, can be absorbed between 900 nm and 1000 nm, is provided by low cost and high power LEDs in the visible or by multimode high power and broad area laser diodes, and is longitudinally guided in a multi mode waveguide spread along one or more single mode active waveguide cores; the pump light is gradually and periodically absorbed along the active core waveguides either in case of double cladding multi mode waveguide structures, or in case of multi mode waveguide structures in which coupling is performed by evanescent coupling of the pump light into the active cores or by the partial overlap of the pump light in the active cores. The pump light is transferred and guided in single or multimode condition in one or more active waveguides that operate in single mode condition at the signal wavelengths (at around 1.55 μm). The proposed schemes according to the invention overcome the intrinsic limitation of the end- fired longitudinal single mode pumping of the waveguide active core layer, which would be expensive and moreover not effective in case of extremely high pump absorption cross-section (for example silicon nanocluster like sensitizers would almost totally absorb the pump light in a few tens of microns along the propagation direction). On the other hand, if compared to the known top pumping arrangements practiced when using low cost high power pump light sources, the pumping arrangement of the invention provide better pumping efficiency because the pump light from wide band LEDs or broad area lasers is guided with outstandingly low loss by a multimode structure extended along the single mode active waveguides in the light propagation direction, allowing for a gradual pump light transfer to the active cores and pump light absorption from the sensitizers elements, which in turn transfer excitation to the rare-earth ions, along the single mode waveguide amplifiers; the length of the integrated waveguide amplifiers and lasers realized following such schemes can be limited to few centimetres allowing a more advantageous and effective integration.

A further advantage of the pumping method of this invention is given by the possibility to pump with a single pump source more than one single mode active core of the integrated waveguide device due to the larger transversal mode size of the multimode waveguide with respect to the size of the active core waveguide.

Moreover the pumping method of the invention permits to effectively control the power density of the pump light into the active core by a coupling optimization between the multimode waveguide of the pump and the single mode waveguide of the signal; this characteristics permit to effectively overcome amplifier performance limitations at high pump power densities, due for example to confined carrier absorption of the silicon nanoclusters at the signal wavelength, and to fast Auger recombination processes.

The proposed amplifier structure, if compared to known waveguide amplifiers (EDWAs) based on silica doped with rare-earth elements, is advantageous in terms of pump cost reduction. In fact wide band LEDs or broad area lasers in the visible could not be effectively used in standard EDWAs due to low absorption cross-section of rare-earth ions in the silica host. The optimization of multimode pump light transfer into the active core along the signal propagation direction, as obtained by the schemes described in details later, permits to overcome such limitations and to give an effective pumping scheme either based on sensitizers such as silicon nanoclusters or rare earth ions like Ytterbium. Note that, besides Erbium, generally preferred because it provides optical gain at wavelength of minimum attenuation for the silica optical fibers used in telecommunications systems, other rare-earth ions can be considered for co-doping the active core providing gain in other spectral regions or to broaden the amplifier bandwidth; for example Tullium, Praseodymium, Neodymium and Ytterbium ions can be considered for achieving gain respectively at around 1.48 μm, 1.3 μm and 1 μm. Also co-doping with more that one kind of rare-earth ions can potentially provide gain over a larger single bandwidth.

The pump light provided by an array of LEDs or broad area lasers can also be coupled into a multimode waveguide in transverse direction with respect to the signal propagation direction. In this case the active core, co-doped for example with sensitizers and rare-earth elements, is pumped along the whole amplifier length (in the order of few centimetres) with surprisingly high efficiency due to the pump light confinement in the multimode waveguide. The main features of the integrated waveguide amplifier or the waveguide laser according to the present invention and the relative pumping method are respectively defined in the claims 1 and 19. Important embodiments are addressed in the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantageous of integrated waveguide signal amplifiers and lasers according to the present invention will be better understood from the following exemplifying and non-limiting description of some embodiments thereof with reference to the attached drawings, wherein:

Figure 1 shows, as described before, the energy levels scheme and the main physical interaction between Er and Yb ions in the co-doped dielectric active core;

Figure 2, as described before, shows the energy levels scheme of the Silicon nanocluster Si-NC and the Er ion and their main physical interactions.

Figure 3 shows the schematic structure of a double-cladding Si-NC and Er doped waveguide amplifier according to the invention;

Figure 4 shows the input pump and signal spatial distributions for a double-cladding waveguide structure with 1 μm width active core and 10 μm width inner cladding; Figure 5 a shows the transversal single mode signal distribution of the electric field for different position along the waveguide propagation direction;

Figure 5b shows the transversal multi-mode pump distribution of the electric field for different positions along the waveguide propagation direction;

Figure 6 shows the small signal gain versus pump power for a 3 cm long double- cladding Si-NC Er³⁺ co-doped waveguide (N_Er=2xl0²⁶ ions/m³, N_SINC=3X10²⁵ /m³)

Figure 7 shows the small signal gain versus length for double-cladding Si-NC Er³⁺ co- doped waveguides (Pp=600 mW, N_Er=2xl0²⁶ ions/m³, N_SINC=3X10²⁵ /m³)

Figure 8 shows the schematic structure of a double-cladding Si-NC Er³⁺ co-doped waveguide amplifier with multi-core structures sharing the same pump beam; Figure 9 shows the schematic structure of Si-NC Er³⁺ co-doped waveguide amplifier with multimode pump light coupled to the active core by evanescent field mechanism;

Figure 10 shows pump power transfer in a lossless waveguide (as described in Fig. 9, with d=0, active core width: 1 μm, and 7 μm width of upper cladding);

Figure 11 shows the input pump and signal spatial distributions of the electric field for a waveguide structure as described in Fig. 9, with d=0, 1 μm width active core and 7 μm width upper cladding;

Figure 12a shows the transversal signal distribution of the electric field along the waveguide propagation direction for different longitudinal positions (the signal at 1530 nm propagates in single mode condition within the active core)

Figure 12b shows the transversal multi-mode pump distribution of the electric field along the waveguide propagation direction with evanescent coupling to the active core for different longitudinal positions (the pump at 477 nm is multimode in the active core); Figure 13 shows the small signal gain versus pump power for a 3 cm long Si-NC Er³⁺ co-doped waveguide

ions/m³, N_SINC=3X10²⁵ /m³); the pump light is coupled to the active core by evanescent field mechanism;

Figure 14 shows the small signal gain versus length for a Si-NC Er³⁺ co-doped waveguide (Pp=600 mW, N_EΓ ⁼2X10²⁶ ions/m³, N_SINC=3X10²⁵ /m³); the pump light is coupled to the active core by evanescent field mechanism;

Figure 15 shows the schematic structure of a Si-NC Er ⁺ co-doped waveguide amplifier with multi-core structures sharing the same pump beam (with direct wafer bonding, d=0 in Fig. 9);

Figure 16 shows the schematic structure of Si-NC Er³⁺ co-doped waveguide amplifier with multimode pump light coupled to two active cores by evanescent field mechanism;

Figure 17 shows the schematic structure of a Si-NC Er³⁺ co-doped waveguide amplifier with multi-core structures on both sides, sharing the same pump beam (with direct wafer bonding, d=0 in Fig. 9);

Figure 18 shows the schematic structure of two Si-NC Er³⁺ co-doped waveguide amplifiers (signal) laterally coupled to a multi-core structure;

Figure 19 shows the schematic structure (top view) of a waveguide amplifier structure with transversal multimode pumping by a lateral LED array;

Figure 20 shows the lateral view of the waveguide structure shown of figure 19, based on transversal multimode pumping with double cladding configuration Figure 21 shows a possible integrated laser structure according to the present invention;

Figure 22 shows the transversal section of the proposed structure of integrated waveguide amplifier, with a Er³⁺ and Yb³⁺ co-doped core of multicomponent glass;

Figure 23 shows a pumping scheme of the structure described in figure 22; Figure 24 shows the pump light transfer to the inner active core of the figure 22; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the active core;

Figure 25 shows the signal power fraction versus the longitudinal position outside and inside the active core of the structure in figure 22; Figure 26 shows the single mode spatial distribution of the signal electric field at the output of the optical amplifier shown in figure 22;

Figure 27 shows the multimode spatial distribution of the pump electric field at the output of the optical amplifier shown in figure 22; Figure 28 shows the transversal section of an alternative embodiment of an integrated waveguide amplifier, with a Er³⁺ and Yb³⁺ co-doped core of multicomponent glass;

Figure 29 shows the pump light transfer to the active core of the figure 28; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the active core; Figure 30 shows the signal power fraction versus the longitudinal position outside and inside the active core of the structure in figure 28;

Figure 31 shows the single mode spatial distribution of the signal electric field at the output of the optical amplifier shown in figure 28;

Figure 32 shows the multimode spatial distribution of the pump electric field at the output of the optical amplifier shown in figure 28;

Figure 33 shows the transversal section of another alternative embodiment of an integrated waveguide amplifier, with a Er³⁺ and Yb³⁺ co-doped core of multicomponent glass;

Figure 34 shows the pump light transfer to the active core of the figure 33; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the active core;

Figure 35 shows the signal power fraction versus the longitudinal position outside and inside the active core of the structure shown in figure 28.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION In a first embodiment of the present invention (see Fig. 3) showing a longitudinal section thereof the waveguide signal amplifier includes: a substrate 1; an optical waveguide 2 including: a core layer 3 (with refractive index ni) made of silica or silicon based dielectric materials co-doped with silicon nanoclusters and rare-earth elements (i.e. Er³⁺ ions), an inner cladding 4 which surrounds the core layer (with refractive index n₂<ni) and an outer cladding 5 (with refractive index n₃<n₂) which surrounds the inner cladding; a light source S, spaced apart from the waveguide, for optical pumping the waveguide.

The signals (one or more at different wavelengths) are coupled into the core and guided in single mode condition, while the pump light is fed into the inner cladding 4 and is guided in multimode condition by total internal reflection at the interface between inner 4 and outer 5 cladding, and is progressively absorbed by the active core exciting rare-earth elements through the absorption of silicon nanoclusters, potentially providing gain for the signals.

Pump light in the visible can be conveniently provided by low cost wide band and high power LEDs or broad area lasers.

As co-doping with Si-NC increases the refractive index, the Er doped core can easily have a refractive index ni >n₂ as well as ni>n3. The core thickness can be within the range 0.5-2 μm and ni within the range 1.5 - 1.9, depending on silicon excess. The inner cladding 4 and outer cladding 5 can be composed of doped silica (for example SiO₂ZP₂OsZAl₂Os) as well as silicon nitride S13N₄ based material, with suitable concentrations in order to ensure suitable refractive indexes n₂ and n3.

The inner cladding thickness can be for example within the range 5-20 μm, while the outer cladding 5 within the range 10-40 μm. The refractive index step between active core 3 and inner cladding 4 can be for example Δn=ni-n₂=0.25±0.2 (depending on silicon excess); the refractive index step between inner and outer claddings depends on the used material which could be silica based as well as silicon nitride based, providing in this case high index contrast and consequently scaled down device sizes.

In the analyzed structure, as schematically shown in Fig. 3, the active core can be co- doped with 3x10 Si-NcZm and with an Er concentration of 2x10 ionsZm . For simplicity, slab waveguides with active core thickness of 1 μm and a first cladding width of 10 μm (the core refractive index is ni=1.6, while the inner and outer cladding refractive indexes are n₂= 1.457 and n3=1.45) have been considered.

Similar conclusions can be achieved by considering two dimensional waveguides in transverse directions and different material refractive indexes for the inner and outer claddings.

The inner waveguide is single mode at 1.53 μm, while the pump light at 477 nm (where there is not direct Er³⁺ absorption) is guided in multimode condition by total internal reflection at the interface between inner and outer cladding. Up-conversion from the Er³⁺ metastable level limits the maximum usable rare earth ions concentration. Fig. 4 shows the input pump and signal spatial distributions of the electric field, corresponding to the excitation of the fundamental modes respectively of the 1 μm and 10 μm wide waveguides.

Fig. 5a and 5b describe the pump and signal evolutions along a 3 cm long waveguide, with input pump power of 600 mW coupled into the multimode waveguide. Note that the pump power is evaluated assuming a transverse waveguide dimension of 10 μm. For slab waveguides, due to the invariance along one transverse dimension, the pump and signal powers could be effectively described using linear power densities in W/m; from these values, absolute power values can be estimated assuming a finite transverse dimension.

As clearly shown in fig.5a, the signal light remains single mode and is amplified during propagation along the waveguide. On the other hand, as shown in fig.5b, the pump, which is strongly absorbed in the active core, is characterized by two lobes within the multimode waveguide, which gradually transfer energy into the active material along the waveguide. Note that the pump light at 477 nm propagates in multimode conditions within the active core.

Fig. 6 shows the small signal gain in dB at 1532 nm versus pump power; optical gain up to 4 dB/cm can be achieved due to the high Si-NC absorption cross-section and efficient energy transfer from Si-NC to Er³⁺ ions. The proposed structure provides relatively low pump intensity along the active waveguides, avoiding free carrier absorption from the Si-NC at signal wavelength and fast Auger de-excitation of the Si-NCs; these processes can be relevant for high pump power density and so degrade the amplifier performance.

Fig. 7 reports the small signal gain at 1532 nm versus waveguide length with input pump power of 600 mW. It can be noted an optimum length above which the gain decreases; this is due to the fact that the pump power is gradually absorbed along the waveguide and becomes too low to provide population inversion for the Er³⁺ ions.

Note that multiple waveguides can be formed in parallel configuration and share a single multimode pump beam as shown in Fig. 8. Coupling of the single mode signal and multimode pump, respectively to the active waveguide and multimode waveguide, can be realized using micro-optics couplers, as described for example in USA patent 6,359,728 Bl, Mar 19, 2002.

According to an alternative embodiment (shown in Fig. 9), a waveguide amplifier of the present invention includes: a substrate; two optical waveguides including a single mode waveguide (with core refractive index ni and cladding n₂, being ni>n₂) made of silica or silicon based dielectrics materials, co-doped with silicon nanoclusters and rare-earth elements (i.e. Er³⁺ ions) within the active core, and a multimode waveguide structure (with core refractive index n3 and cladding refractive index n₂, being ni_≥n3>n₂) grown on the top (or bottom) of the single mode waveguide; a light source Sl, spaced apart from the waveguide, for optical pumping the waveguide.

The signals (one or more at different wavelengths) are coupled into the core and guided in single mode operation, while the pump light is fed into the second multimode waveguide and gradually coupled to the active core during propagation along the longitudinal waveguides direction through evanescent coupling. The pump light is then progressively absorbed by the core, exciting rare-earth elements through the silicon nanoclusters absorption and energy transfer, providing then gain for the signals.

As co-doping with Si-NC increases the refractive index, the active Er doped core can easily have a refractive index ni>ri3>n₂. The core thickness can be within the range 0.5 - 2 μm and ni within the range 1.5 - 1.9, depending on silicon excess. The multimode waveguide, made for example of doped silica (for example SiO₂ZP₂OsZAl₂Os), as well as silicon nitride S13N4 based material, with suitable concentrations in order to ensure suitable refractive indexes n₂ and n^, can have a multimode core thickness within the range 5-10 μm. The multimode waveguide can be directly bonded on the single mode active waveguide (d=0 in Fig. 9) or slightly separated by a thin dielectric layer (thickness d≠O in Fig. 9). The waveguide geometry must be optimized in order to ensure a suitable transfer of pump light from the multimode core to the active region. In particular this coupling must be low enough to avoid strong pump absorption by Si-NC over a short longitudinal distance, and high enough to effectively pump the active material along the waveguide length (order of several cm).

Similar arguments also apply for alternative schemes where the coupling is not vertical but lateral, i.e. when the multimode waveguide and the active signal waveguide are placed nearby on the substrate. The lateral separation between the waveguides is represented by the coupling distance d in Fig. 9.

A suitable power transfer mechanism is shown in Fig. 10, where a lossless slab waveguide has been considered just to show the coupling properties of the guiding structure (the single mode core is 1 μm width and the multimode slab is 7 μm with d=0). It has to be noted that the beat length, corresponding to a periodical power transfer between the waveguides, is of the order of 1 mm and the maximum coupled pump power into the active core is less than 6% of the total pump power. This means that coupling 500 mW pump power at 477 nm at the multimode waveguide input will provide maximum 30 mW into the active region for exciting the Si-NC and then Er ⁺ ions, avoiding confined carrier absorption and fast Auger de-excitation of the Si-NCs, that can be relevant and degrade the amplifier performance for high pump power density. The essential feature of the novel structure is that the coupling of the pump light from the multimode core to the active region takes place gradually along the waveguide length, ensuring effective pumping all along the amplifier

Fig. 11 reports the input transversal distributions of the pump and signal electric fields which correspond to the fundamental mode of two slab waveguides (ni=1.6, n₂=1.45, n3=1.457). Figs. 12a and 12b show the longitudinal pump and signal variations of the electric fields across the waveguide section considering an active core co-doped with 3x10²⁵ Si-

Nc/m³ and with an Erbium concentration of 2x10²⁶ ions/m³ (the pump power at the multimode waveguide input is 600 mW).

It is evident that the multimode pump light, periodically coupled into the active core along the waveguide length, allows the amplifier to be effectively pumped. The pump light at 477 nm is coupled into the active core and propagates in multimode condition. The signal light at 1532 nm is amplified in the active core along the propagation direction and propagates in single mode condition.

Figs. 13 and 14 show the small signal gain at 1532 nm respectively versus pump power (with L= 3 cm) and versus length (with fixed pump power of 0.6 W).

It has to be noted that pump power has been evaluated considering a longitudinal multimode waveguide transverse dimension of 10 μm. The Er³⁺ concentration is 2x10²⁶ ions/m³ and SiNC content is 3xlO²⁵ /m³; these concentrations are optimized considering up- conversion from the Er³⁺ metastable level. It is worth noting that multiple waveguides can be formed in parallel configuration and share a single multimode pump beam as shown in Fig. 15.

Coupling of the single mode signal and multimode pump light respectively to the active core and multimode waveguide can be made as described for examples in US

6,996,139. Typical parameters of an integrated waveguide amplifier device according to the invention with an active core of silica material co-doped with silicon nanoclusters and Er³⁺ ions, are shown in the following Table I.

Table I: Typical parameters of silica co-doped with silicon nanoclusters and Er ions (from:[l] C. Dufour et. al., "Propagation in erbium and silicon codoped silica slab waveguides : analysis of gain", J. Phys.: Condens. Matter 16, (2004) 6627-6638 [2] Jung H. Shin et al., "Si Nanocluster Sensitization of Er-Doped Silica for Optical Amplet Using Top- Pumping Visible LEDs ", IEEE Journal of Seleted Topics in Quantum Electronics, Vol. 12, No. 4, Juy/August 2006. )

In the two embodiments of integrated waveguide signal amplifier structures shown in Figs. 3 and 9, pumping can also be counter-propagating or bidirectional along the waveguide length.

The structure in Fig. 9 can be easily extended growing or bonding active waveguides, single mode at signal wavelength, on both sides of the multimode waveguide where the pump propagates with low losses and in multimode conditions. This is schematically shown in Figs. 16 and 17. In such structures a single multimode pump light can effectively excite different active waveguides in parallel configurations by coupling pump light from the multimode core to the single mode active cores; the pump light is progressively absorbed in the signal waveguide cores.

The figure 18 shows how the pump light propagating in a multimode waveguide can be coupled to active waveguides by both sides of the multimode waveguide. The shown longitudinal pumping scheme including a vertical coupling ( by stacking ) can alternatively be made by transversal coupling scheme, if such scheme is imposed or favourite for different choice of general layout. This alternative scheme is shown in Figure

19.

In accordance with the example of figure 19, a waveguide amplifier of the present invention includes: a substrate, an array of parallel single mode waveguides (with core refractive index ni and cladding n₂, being ni>n₂) made of silica or silicon based dielectric materials, co-doped with silicon nanoclusters and rare-earth elements (i.e. Er³⁺ ions) within the active core, and a few hundred microns long multimode waveguide structure, which can either be based on double cladding structure or grown on the top (or bottom) of the single mode waveguides, an array of high power LEDs which couple the visible light in lateral direction with respect to the signal propagation direction. The signals (one or more at different wavelengths) are coupled into the active cores and guided in single mode operation, while the pump light, laterally fed into the multimode waveguide, transfers pump light into the active cores either by propagation in a double-cladding structure or by evanescent field coupling. The pump light excites rare-earth elements into the active cores through the silicon nanoclusters, providing then gain for the signals. It is worth noting that a mirror or grating or reflectors can be introduced to reflect back the pump light, improving the amplifier efficiency. Fig 19 shows a top view of the amplifier structure based on this transversal multi mode pumping with pump reflectors. The same happens if a double mirror configuration is assembled such that a resonant cavity structure is realized. Similar configurations can be designed exploiting excitation of the active cores by coupling in a double-cladding structure or by an evanescent pump coupling.

As example the figure 20 shows a lateral view of a guiding structure based on this transversal multimode pumping in case of double-cladding configuration with pump reflectors. The pumping mechanism is similar to the one described in the first two embodiments of the present invention, because the pump light is guided in multimode condition in a low loss waveguide and is then coupled and strongly absorbed into the active cores, where the gain is provided by excitation of rare-earth elements through the silicon nanoclusters. The main difference is that now an array of LED is required in order to efficiently pump the active waveguides all along their longitudinal propagation direction. As the active waveguide length is of the order of centimetres, tens of LEDs should be introduced in the pumping array in order to effectively pump the amplifiers all along their length.

Moreover using an active waveguide pumped by one of the schemes described above and inserting suitable reflectors by the sides of the waveguide, a laser device can be made. In a laser structure an input signal is not included, but only pump light, that generates population inversion in the active medium placed in the cavity and hence laser emission, if the provided gain is higher than the cavity loss. The reflectors, for example, made by grating or dielectric coating, operate as a selective mirrors of wavelength and fix the laser emission wavelength.

The structure shown as example in figure 21 is a double cladding structure where the pump light (provided by LED emitting at visible wavelength or by broad area laser) is coupled in the multimode waveguide where the pump propagates in longitudinal direction and excite the silicon nanoclusters that then excite the Erbium ions. The gain provided by the active medium in case of population inversion condition can exceed the cavity loss and guarantee the necessary conditions of laser emission. The laser cavity is a Fabry-Perot cavity with two mirrors (for example dielectric coating devices), placed at the input and output of the waveguide, that are selective and have a high reflectivity in a narrow spectral range of the gain spectral bandwidth of the Er ions (1530-1560 nm). For example the input mirror can have reflectivity around 100%, while the output mirror can have a reflectivity between 70 and 90%, such as pump light can be coupled outside the waveguide.

Other known geometries of laser implementation for waveguide amplifier structure can be used, such as distributed feedback or Bragg cavity structures. Similar structures of integrated laser can be implemented by the same schemes described for the integrated amplifiers (schemes using evanescent coupling or transversal pumping).

According to an alternative embodiment of the invention, the dielectric material of the core can be a multicomponent glass, for example phosphate glass, containing P2O5 in addition to silica, or an aluminate glass, containing AI₂O₃ or another glass containing germanium oxide (GeO₂), sodium oxide (Na₂O)or also other different oxides that increase the solubility of the rare earth ions in the composite glassy matrix. In this way certain technological limitations can be overcome in order to satisfy the necessity to maximize the rare earth ion concentration in the dielectric material (glassy matrix) of the core, for clear reasons of optical amplification, and, on the other hand, to guarantee the maximum uniformity of doping ions distribution in order to limit as possible doping ions clusters, and consequent processes of concentration quenching and cooperative up-conversion, that limit the amplifier optical gain. [WJ. Miniscalco, Erbium-Doped Glasses for fiber Amplifier at 1500 nm, Journal of Lightwave Technology 9 (2), February 1991].

In fact it is well known that a pure silica can include a limited quantity of ions without making clusters, while multicomponent glasses permit to uniformly distribute in the dielectric material an higher concentration of rare earth ions, more soluble in the composite glassy matrix.

The multicomponent glasses are glasses that contain also compounds different from silicon oxide, like Ge, P, and Al. The presence of these additive elements in the silica modifies the optical, chemical, and mechanical properties of the material. These types of glass are often used to obtain rare earths doped glass.

For example, the aluminate or phosphate glasses doped with Erbium ions suffer less than other glassy materials from the ions interactions due to concentration quenching processes, that limit the maximum suitable Erbium concentration; the use of these glasses permits to increase the concentration of the doping elements.

The presence of the Ge, Al, or P oxides increase the solubility of the rare earth elements in the glass, for example the compounds AI2O3, P2O5 or GeO₂ well dissolve in SiO₂, and at the same time are good solvents for the rare earths; for these reasons chemical groups, like AI2O3, P2O5 , Na2θ and Geθ2 permit to make site in the silica of high solvation for the rare earths, that hence can be incorporated for higher concentrations in the solid structure or matrix of the glass.

Moreover, the different chemical-physical properties of the multicomponent glass depend on different compound components, that modify the optical characteristics of the rare earths doped glass used for optical amplification, for example: the lifetime of the metastable excited states, the emission and absorption cross sections, the absorption cross section at the pump light wavelength, quenching concentration phenomena, etc.

Some embodiments of amplification devices in accordance with the invention will be described below, and are made, rather than with dielectric layers of Siθ2, with multicomponent glasses that can be fabricated using planar definition technique on different substrate and afterwards bonded (by a technique of wafer bonding) in order to make an integrated waveguide device.

Similar structures can however be fabricated also by deposition processes (RF sputtering and PECVD) and etching procedures.

One of the alternative embodiment of the invention is shown in figure 22. This waveguide optical amplifier includes:

1) a structure 10 made with a silica based layer with refractive index n3 and a passive core 13 with refractive index n2 >n3, able to guide a pump light in multimode condition. The core 13 can have a 100 μm transversal dimension and a 2-5 μm vertical dimension in order to effectively couple the light from a broad area and high power pump laser; 2) a structure 11 made with a layer of multicomponent glass (for example phosphate glass, or aluminate glass) co-doped with rare earths (Er ⁺/Yb ⁺) with refractive index n4<n2 and a core 14 with refractive index nl>n4. The layer of the co-doped multicomponent glass is the active zone of the amplifier and is 3-4 μm thick made by milling techniques like laser milling or ion beam milling starting from a glassy slide co-doped with Er³⁺ and Yb³⁺ ions. The core with refractive index n3 can have for example a 2x3 μm² dimension, and is able to guide a signal light in single mode condition.

3) a structure 12 made with a silica based layer with a refractive index and a passive core 15 with a refractive index n2 >n3 able to guide the pump light in multi mode condition. The core 15 can have a 100 μm transversal dimension and a 2-5 μm vertical dimension, in order to effectively couple the light of a broad area and high power pump laser.

In the structure 10 and 12 the transversal dimension of the cores 15 and 13 can be reduced up to 30-40 μm by using a structure able to couple large area of broad area laser emission to the cores that guide the pump light. The structures 10 and 12 can be placed below and above the structure 11 by a direct bonding as schematically shown in figure 22.

The cores 13 and 15 in the respective structure 10 and 12 are obtained by a technique of ion exchange made in a glassy slide with refractive index n3; the core 14 in the structure 11 is obtained by a technique of ion exchange made in a slide of co-doped multicomponent glass with refractive index n4.

The signal light propagates into the core 14 with refractive index nl. The core 14 can also have a serpentine or spiral-type shape in order to increase the length of the amplifier zone while keeping short dimension. The pump light, propagating in the two cores 13 and 15 with refractive index n2, can be gradually transferred to the core 14 in the structure 11 with refractive index nl by a pump light overlap in the active zone and consequently absorption during the propagation along the longitudinal direction of the waveguide.

The figure 23 shows a possible scheme of pumping for the optical amplifier: the pump light is coupled in the waveguide of the structure 10 and 12 laterally respect to the amplifier, on the two opposite side above and below the active zone of the structure 11, using two different broad area laser emitting for example at 980 nm.

In order to provide examples, the amplifier has been studied by a three-dimensional numerical model. Typical parameters of an integrated waveguide device in accordance with the invention, with an active core of silica material co-doped with Yb³⁺ and Er³⁺ ions, are shown in the following Table II.

Table II: Typical parameters of silica co-doped with Yb and Er ions (F. Di Pasquale, S. Faralli, V. Toccafondo, "Er³VYb³⁺ co-doped silica waveguide amplifiers longitudinally pumped by broad area lasers", IEEE Photonics Technology Letters, Vol.19, No. 24, December 15, 2007, pp. 1967-1969)

The passive cores 13 and 15 of the multimode waveguide for pump light propagation have a 30x5 μm² dimension and a refractive index n2=1.54, the structure 11 of multicomponent glass with a refractive index n4=1.53 is 2 μm thick and the core 14 of the single mode waveguide for the signal light propagation have a 3x2 μm² dimension and a refractive index nl=1.56, while n3=1.51.

Figure 24 shows the pump light transfer to the active core; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the active core, as a function of the longitudinal position, considering that total input power is 2 W and the input signal power is -30 dBm. The figure 25 shows the signal power fraction (with respect to the total input signal power) versus the longitudinal position outside and inside the active core of the structure; the computed optical gain for a 3 cm long waveguide amplifier is 8.8 dB.

Figure 26 describes the single mode spatial distribution of the signal electric field at the output of the optical amplifier; Figure 27 shows the multimode spatial distribution of the pump electric field at the output of the optical amplifier.

In accordance with the invention another alternative embodiment of the amplifier made with multicomponent glass is shown in figure 28. This waveguide optical amplifier includes: 1) a structure 20 with a silica based layer, refractive index n3 and a passive core 23 with refractive index n2 >n3, able to guide the pump light in multimode condition. The core 23 can have a 100 μm transversal dimension and a 2-5 μm vertical dimension in order to effectively couple the light of a broad area and high power pump laser. The transversal dimension of the core can be reduced up to 30-40 μm² by using a structure able to effectively couple light from broad area lasers into the core that guides the pump light.

2) a structure 21 made with a layer of multicomponent glass (for example phosphate glass, or aluminate glass) co-doped with rare earths (Er³⁺/Yb³⁺) with refractive index n4<n2 and an active core 24 with refractive index nl>n4 and nl>n2. The layer of the co-doped multicomponent glass is the active zone of the amplifier and the core 24 with refractive index nl can have for example a 2x3 μm² dimension, and is able to guide a signal light in single mode condition.

The structure 21 is placed above the structure 20 by a direct bonding technique between the two structures as schematically shown in figure 28.

The passive core 23 in the structure 20 is made by an ion exchange technique in a silica based layer with refractive index n3. The active core 24 in the structure 21 is made by an ion exchange technique in a silica based layer with refractive index n4.

The signal light propagates in the core 24 with refractive index nl. The pump light propagates in the core 23 with refractive index n2 and can be gradually absorbed in core 24 of the structure 21 with refractive index nl, for the spatial overlap of the pump mode in the active zone.

As an example, the amplifier has been studied by a three-dimensional numerical model.

The passive cores of the multimode waveguide for pump light propagation have a 30x5 μm² dimension and a refractive index n2=1.54, the active core of the single mode waveguide for the signal light propagation has a 3x2 μm² dimension and a refractive index nl=1.56, while n3=1.51 and n4=1.53.

Figure 29 shows the pump light transfer to the active core; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the active core, considering that total input power is IW and the input signal power is -30 dBm. Figure 30 shows the signal power fraction (with respect to the total input signal power) versus the longitudinal position outside and inside the active core of the structure; the computed optical gain for a 3 cm long waveguide amplifier is 11.37 dB.

Figure 31 describes the single mode spatial distribution of the signal electric field at the output of the optical amplifier; Figure 32 shows the multimode spatial distribution of the pump electric field at the output of the optical amplifier.

A third embodiment of the amplifier made with multicomponent glass in accordance with the invention is shown in figure 33. This waveguide optical amplifier includes:

1) a structure 30 made with a silica based layer with refractive index n4 and a passive core 33 with refractive index n2 >n4, able to guide the pump light in multi mode condition. The core 33 can have a 100 μm transversal dimension and a 2-5 μm vertical dimension in order to effectively couple the light of a broad area and high power pump laser. The transversal dimension of the core can be reduced up to 30-40 μm² by using a structure able to effectively couple light from broad area lasers into the core that guides the pump light;

2) a structure 31 made with a layer of multicomponent glass (for example phosphate glass, or aluminate glass) co-doped with rare earths with refractive index nl>n2. The layer of the co-doped multicomponent glass is the active zone of the amplifier and is 3-4 μm thick made by milling techniques like laser milling or ion beam milling starting from a glassy slide co-doped with Er³⁺ and Yb³⁺ ions;

3) a structure 32 made with a silica based layer with a refractive index n4 and having a core 35 with refractive index n2 and n2<n3<nl. The core 35 with refractive index n3 can have a 2x3 μm² dimension.

The structures 30 and 32 can be placed below and above the structure 31 by a direct bonding as shown in figure 33.

The core 33 in the structure 30 and the core 35 in the structure 32 are obtained by a technique of ion exchange made in a glassy slides with refractive index n4.

The amplified signal light propagates in the structure 31 with refractive index nl in the zone where the electromagnetic field confinement is due to the core 35 with refractive index n3. The pump light, propagating in the core 33 with refractive index n2, can be gradually transferred to the structure 31 for the spatial overlap of the pump mode in the active zone during the propagation along the longitudinal direction of the waveguides.

As an example, the amplifier has been studied by a three-dimensional numerical model. The passive core of the multimode waveguide for pump light propagation has a 30x5 μm² dimension and a refractive index n2=1.535, the structure 31 is 2 μm thick and has a refractive index nl=1.55; the core 35 in the structure 32 has 2x3 μm² dimension and a refractive index n3=1.53, while n4=1.51.

Figure 34 shows the pump light transfer to the active region 31; in particular the figure shows the pump power percentage versus the longitudinal position inside and outside the structure 31, considering that total input power is IW and the input signal power is -30 dBm. Figure 35 shows the signal power fraction (with respect to the total input signal power) versus the longitudinal position outside and inside the active structure 31; the computed optical gain for a 3 cm long waveguide amplifier is 8.08 dB.

The present invention, as shown in the previous description for some different embodiments, provides waveguide amplifier structures characterized by low cost and high efficiency due to longitudinal multimode pumping by low cost, wide band light sources. The proposed structures, based on sensitizers of rare earths through silicon nanoclusters, have an high CMOS (Complementary Metal Oxide Semiconductor) compatibility; this enables the integration of the optical amplifiers and lasers within opto-electronic devices made by CMOS technology with important application for inter- and intra-chip optical interconnects. On the other hand the proposed structures, based on sensitizers of rare earths through Yb³⁺ ions in multicomponent glass, even though not properly CMOS compatible, offer interesting solutions for fabrication of low cost and high power integrated optical amplifiers and lasers, with important applications in metro and access networks.

Further variations and modifications can be made to the integrated waveguide amplifiers and lasers with active core doped with rare earths or with rare earths and silicon nanoclusters, in accordance with the present invention, without departing from the scope of this invention as set forth in the following claims.

Claims

1. An integrated waveguide device for optical signal amplification employing a low- cost and high-power type source of pump light selected from the group composed of broad band LEDs and broad area laser diodes, the integrated device having at least an active core made of silica, silicon based dielectric material, or multicomponent glass, containing at least a uniformly dispersed sensitizer element selected from the group composed of silicon nanoclusters and Ytterbium ions for enhancing absorption of pump light, and ions of a rare- earth able of being excited to a certain state and stimulated to decay to a de-excited state for producing a stimulated emission within a certain wavelength band, comprising: a) at least a substrate of a material suitable for planarly defining thereon integrated waveguide structures; b) at least an optical signal waveguide on said substrate for single mode guiding one or more optical signals, including at least an active core of said composition with a first refractive index ni and at least an inner cladding of dielectric material having a second refractive index n₂ lower than ni; c) at least a waveguide adapted for multimode guiding said pump light extending adjacently to said single mode optical waveguide; d) means for coupling pump light from said source to said multimode optical waveguide for progressively transferring into said active core along the longitudinal direction of propagation of said optical signals, multimode guided pump light to be absorbed therein by said dispersed sensitizer element for enhancing excitation of said rare-earth ions producing said stimulated emission.

2. The integrated waveguide device according to claim 1, wherein said waveguides extending adjacently are parts of an integrated double-clad optical waveguide structure adapted for introducing pump light into a single mode active core along the extension length of the integrated structure, said pump light being injected and multi-modally guided in an inner cladding layer of said double-clad waveguide.

3. The integrated waveguide device according to claim 2, wherein the pump light from an external source is coupled to an end surface of the inner cladding layer having a refractive index higher than that of an outer cladding layer and lower than that of the single mode active core.

4. The integrated waveguide device according to claim 1, wherein said waveguides extending adjacently are defined in a stack of planarly defined layers of coordinated different refractive indexes on said substrate, whereby pump light coupled to a multimode guiding layer extending along with said single mode active core or cores transfers pump light to the active core or cores by evanescent field coupling mechanism.

5. The integrated waveguide device according to claim 1, wherein said waveguides extending adjacently are defined in a stack of planarly defined layers of coordinated different refractive indexes on said substrate, whereby pump light coupled to a multimode guiding layer extending along with said single mode active core or cores transfers pump light to the active core or cores by repeated incidence of the multi-mode guided pump light on said active core or cores.

6. The integrated wavelength device according to claim 1, wherein said single mode optical waveguide comprises a plurality of active cores extending parallel and spaced apart in the optical dielectric material of an inner cladding.

7. The integrated wavelength device according to claim 5, wherein said stack comprises a plurality of single mode optical signal guiding cores.

8. The integrated wavelength device according to claim 1, wherein said coupling means are adapted to couple pump light generated by said source to the multimode waveguide from one end.

9. The integrated waveguide device according to claim 1, wherein said coupling means are adapted to couple pump light from said source to the multimode waveguide from both ends.

10. The integrated waveguide device according to claim 1, wherein said coupling means are adapted to couple pump light from said source to the multimode waveguide from one side.

11. The integrated waveguide device according to claim 1, further comprising end- firing coupling means for an optical signal to be injected in an inlet end of said optical signal single mode waveguide, and to be made available once amplified at an outlet end of the waveguide.

12. The integrated waveguide device according to any one of the claims 1 to 9, further comprising selective mirror at an inlet end and at an outlet end respectively, of said single mode optical waveguide to form an integrated laser functioning in the gain band of said rare- earth ions.

13. The integrated waveguide device according to claim 12, wherein said selective mirrors are dielectric coating layers formed on inlet and outlet surfaces of said integrated single mode optical signal waveguide, the selective mirror layer on the outlet surface of the single mode waveguide core having reflectivity comprised between 70 and 90%.

14. The integrated waveguide device according to claim 1, wherein said sensitizer element consists of silicon nanocluster.

15. The integrated waveguide device according to claim 1, wherein said stimulated emission ions are Er ions.

16. The integrated waveguide device according to claim 1, wherein said sensitizer element consists of Yb ions.

17. The integrated waveguide device according to claim 16, wherein said core is made of a multi component glass including one or more oxides selected from the group composed of SiO₂, P2O5, AI2O3, Na₂O, and GeO₂, codoped with Yb and Er ions.

18. The integrated waveguide device according to claim 17, wherein said core is planar Iy defined in a substrate of multicomponent glass composed of one or more oxides selected from the group composed of SiO₂, P2O5, AI2O3, Na₂O and GeO₂ , codoped with Yb and Er ions by a ion exchange technique.

19. A method of pumping with a low-cost and high-power type source of pump light selected from the group composed of broad band LEDs and broad area laser diodes, an integrated optical signal waveguide amplifier, comprising at least an optical waveguide for single mode guiding one or more optical signals to be amplified, said waveguide having at least an active core of multi-component glass, or silica or other silicon base dielectric material with a certain refractive index and containing uniformly dispersed therein at least a sensitizer element, selected from the group composed of silicon nanoclusters and Yb ions, and ions of a rare-earth able to be excited to a certain state and stimulated to decay to a de- excited state for producing stimulated emission in certain wavelength band, characterized in that it comprises the steps of a) forming along with said single mode waveguide at least a second waveguide adapted to guide pump light coupled thereto in a multimode fashion; b) coupling pump light from said external source to said second multimode optical waveguide; c) transferring progressively along the longitudinal direction of propagation of the singe mode optical signal waveguide, pump light from said second waveguide to the active core of said single mode optical signal waveguide by evanescent field coupling mechanism or repeated incidence of the multi-mode guided pump light on the active core, to be absorbed by said dispersed sensitizer element for enhancing excitation of said the rare-earth ions to produce said stimulated emission.