WO2003017440A2

WO2003017440A2 - Optical amplification system

Info

Publication number: WO2003017440A2
Application number: PCT/IL2002/000663
Authority: WO
Inventors: Ram Oron; Doron Nevo
Original assignee: Ram Oron; Doron Nevo
Priority date: 2001-08-13
Filing date: 2002-08-12
Publication date: 2003-02-27
Also published as: WO2003017440A3

Abstract

The invention provides an optical amplification system, including a pump waveguide (4), connectable at at least one of its ends to a pump laser, and a plurality of amplifying waveguides (6) coupled to and extending along the outer surface of the pump waveguide.

Description

OPTICAL AMPLIFICATION SYSTEM Field of the Invention

The present invention relates to optical waveguide amplifiers, and more specifically, to optical amplifiers operable by laser pumping of multiple guidewaves. Background of the Invention

Many optical systems use rare earth-doped fiber amplifiers to boost up the power of the transmitted laser signal. The most common fiber amplifiers are Erbium-doped fiber amplifiers (EDFAs), generally having diode laser pump lightwaves of 980 nm or 1480 nm wavelengths, whereas the amplified signal is near 1550 nm. EDFAs are mainly used in lightwave communication systems. Other common fiber amplifiers and lasers are doped with Neodymium or Ytterbium.

Most of the existing designs use fiber end pump configurations, where the pump lightwave is injected into either one or two fiber ends of the laser or amplifier structure and travels inside the fiber. In these end-pumped amplifiers, the pump power is maximal at the fiber input end and decays as it proceeds into the fiber amplifier. Some designs exploit double or multiple cladding, wherein the pump lightwave travels in an outer cladding and the amplified signal travels in a narrower, inner core. Designs having multiple cores inside a common cladding have also been proposed.

Some fiber amplifiers or laser arrangements have transverse pumping schemes, wherein the pump lightwave does not propagate inside the fiber, but is injected from its side. Such arrangements include a fiber wound around a gas-filled flash lamp pump or side-pumped fiber geometries.

In order to enhance the reliability of the semiconductor laser diode pumped fiber gain medium, the outputs of several laser diodes may be optically coupled to a fiber gain medium. Thus, if one laser diode malfunctions or becomes inoperative, the remaining laser diodes can continue to function in the pumping task. The object of the present invention is to provide a multiple waveguide amplifier having easy connectivity at its ends and high reliability, due to the use of multiple pump diodes. Summary of the Invention

The present invention provides an amplifier for a plurality of spatially separated waveguides. The pump lightwave is produced either by a single diode laser or, preferably, by a plurality of diode lasers. The pump lightwave is coupled into a plurality of optical waveguides, namely, the amplifying waveguides. The rare earth doped waveguides, i.e., the amplifying waveguides, are located on the circumference of the pump waveguide and are rigidly connected to it in a closely packed manner.

In order to obtain amplification, the pump lightwave should be coupled out of the pump waveguide and into the amplifying waveguide. This is performed either by scattering and/or by mechanical contact. In the case of scattering, the pump waveguide has a highly scattering region, which region may be inside its core, its circumference, or in a limited region inside the waveguide. The pump lightwave is scattered out of the pump waveguide, and is absorbed in the doped cores of the amplifying waveguides. The highly scattering region in the pump waveguide is so designed as to provide a nearly uniform pump distribution along the amplifying medium, leading to nearly uniform gain both along the amplifier length and between the different doped waveguides. In order to prevent the loss of the non-absorbed pump lightwaves, the outer part of the rare earth-doped waveguide is highly reflective, by means of a high diffusion or other reflective material.

The present invention discloses two embodiments for obtaining mechanical contact between waveguides. In the first case, portions of the pump waveguide have no cladding, so that the pump lightwave simply passes to the amplifying waveguides. Here, most of the reflections inside the waveguide occur due to the phenomenon of total internal reflection. A highly diffusive or other reflective material may be added at the outer circumference of the waveguide assembly, in order to reduce scattering losses from the waveguide. The second embodiment utilizes an optical coupler, which couples the pump light into the cladding of the amplifying waveguides without interfering with the signals propagating inside their cores. This optical coupler can couple a large number of amplifying waveguides with one or more pump waveguides and may be fabricated by using waveguide fusion techniques. For example, the claddings of the various waveguides can be fused in order to obtain good coupling, while the cores of the amplifying waveguides remain untouched. Using this method, the mechanical or physical contact between the pump waveguide and the amplifying waveguides is effected only at the optical coupler, whereas the rest of the waveguides themselves remain separated.

The entire assembly is tightly packed at the coupling region, and has terminals for the pump and input-output of the rare earth-doped waveguide. The many transverse paths are responsible for final absorption in the rare earth ion. Each one of the rear earth-doped waveguides is terminated on both of its sides in such a way as to accept the input signal and transmit the amplified signal. The fibers may be connected by a waveguide connector, fusion splicing, or free space optics. The entire assembly can be used as a number of individual, separate, rare earth-doped amplifiers, as multi-stage amplifiers, or as a combination of these. Moreover, the assembly can be used as a waveguide laser source when mirrors or Bragg gratings are placed at its ends. Thus, the geometry disclosed herein is efficient in light coupling, amplifier material utilization, and amplified spontaneous emission suppression.

Using a plurality of diode lasers, including some spares, as a pump source, the amplifier lifetime, the mean time between failures (MTBF) and reliability of the system are increased. These are very important properties, for example, in lightwave communication systems. Moreover, since the pumping diodes in optical waveguide systems are relatively unreliable, the large number of diode lasers obtains significant redundancy and lifetime enhancement. The spare diode lasers may also compensate for decay in the average power of a diode below its desired value, or if one or more of the plurality of laser emitters malfunctions or is inoperative.

A scattering pump waveguide can be obtained by embedding scatterers inside the waveguide core or by roughening the fiber core circumference. The amount of scattering is then controlled by the density of scatterers. The amplifying waveguides surrounding the pump waveguide are then attached to the pump waveguide by sol-gel, glass fusion methods through a hot aperture, or adhesives. The external reflective surfaces may be created using highly reflective ceramic materials or a diffuse reflecting powder. This reflective material may be deposited on the amplifying waveguides by immersion or insertion of the amplifying waveguide assembly into a shrinkable tube having a reflective internal surface and shrinking the tube to fit the waveguide dimensions.

In accordance with the present invention, there is therefore provided an optical amplification system, comprising a pump waveguide, connectable at at least one of its ends to a pump laser, and a plurality of amplifying waveguides coupled to and extending along the outer surface of said pump waveguide. Brief Description of the Drawings

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawings: Fig. 1 is a cross-sectional view of a first embodiment of a multiple optical amplifier according to the present invention; Fig. 2 is a cross-sectional view of a modification of the embodiment of Fig. 1 ; Figs. 3 to 5 are cross-sectional views of further embodiments of the multiple optical amplifier according to the invention; Fig. 6 is a cross-sectional view of a waveguide assembly, illustrating pump lightwaves coupling to amplifying waveguides by scattering and reflections; Fig. 7 is a longitudinal cross-sectional view of a waveguide assembly wherein the pump lightwave is coupled to the amplifying waveguide by mechanical contact; Fig. 8 is a schematic illustration of a multiple optical amplifier system according to the present invention; Fig. 9 is a schematic illustration of a laser assembly according to the invention, and Figs. 10 and 11 are schematic side and cross-sectional views, respectively, of a coupler utilizable with the system of the present invention. Detailed Description of Preferred Embodiments

Fig. 1 illustrates a multiple optical amplifier (MO A) system 2 according to the present invention, consisting of an inner pump waveguide 4, e.g., an optical fiber, and surrounding amplifying waveguides 6, e.g., optical fibers. .

Optionally, the pump waveguide 4 and the amplifying waveguides 6 are all enclosed by a reflective surface 8, e.g., a reflective tube. Also, in order to enhance the scattering of light propagation through the pump lightguide at different angles, the outer surface thereof can be roughened or, alternatively, the inner surface can be suitably treated as is per se known. Similarly, the inner surface of the reflecting surface may be suitably treated, e.g., by roughening, to cause the scattering of light and enhanced light absorption by the amplifying waveguides.

Advantageously, the amplifying waveguides 6 are doped waveguides, as is per se known. Pump lightwaves are injected into the central waveguide 4, acting as a lightguide and coupled into the plurality of amplifying waveguides 6 surrounding it. In this embodiment, the outer diameters of the pump waveguide and the amplifying waveguides are identical or similar.

Fig. 2 illustrates a MOA system 2 wherein the diameter of the pump waveguide 4 is larger than the diameters of the amplifying waveguide 6, allowing for even higher pump power and more amplification lines. In both the embodiments of Fig. 1 and Fig. 2, it is desired that the amplifying waveguides be arranged in closely packed configurations.

Hence, there are two basic options: either to provide amplifying waveguides with substantially total internal reflection (TIR), in which case the couplings between the amplifying waveguides themselves and between themselves and the pump waveguide can be constituted merely by mechanical means, or else to provide amplifying waveguides which are not of the TIR type, wherein the coupling between the pump waveguide and the amplifying waveguides is an optical one.

Fig. 3 illustrates another embodiment of a MOA system 2, in which the highly reflective surrounding enclosure is not a tube, but rather is a highly reflective coating or material 10 applied on the outer surfaces of the amplifying waveguides 6.

In Fig. 4, there is shown a MOA system 2 wherein the highly reflective surrounding surface is constituted by the inner surface 12 of a solid tube 14; in Fig. 5, the highly reflective surrounding surface is a tube 16 fit onto the waveguide assembly by shrinking.

Referring now to Fig. 6, there is shown a longitudinal cross-sectional view of a MOA system 2 wherein the pump lightwave travels inside the pump waveguide 4, which has scatterers inside the waveguides, so that after a certain distance, the pump lightwave is scattered out of the pump fiber to the surrounding amplifying waveguides. Scattered pump light which is not absorbed in the doped cores of the amplifying waveguides is reflected by the reflective surface 8. Commonly, after a certain number of reflections, the pump light is abosrbed in one of the cores of the amplifying waveguide 6.

In the embodiment of Fig. 7, the pump lightwave which travels inside the pump waveguide 4 is mechanically connected to the surrounding amplifying waveguides 6. Thus, the pump lightwave travels in either the pump waveguide 4 or in the surrounding, amplifying waveguides 6. The pump lightwave is mostly reflected and remains inside the waveguides by means of total internal reflection. An additional, external reflecting surface may be added to reduce scattering losses. Typically, after a certain number of reflections, the pump lightwave is absorbed in one of the cores of the amplifying waveguides.

The embodiment of Fig. 8 comprises an MOA system 2, in which lightwaves produced by one or more multiple semiconductor laser source 18, made, e.g., of bars and stacks of 980 nm or 1480 nm, are transmitted through a pump waveguide 20 having a scattering portion only inside the amplifier, for scattering lightwaves into the amplifying waveguides. The scattered light passes through the doped amplifying waveguides and excites the rare earth ions after a few passes. The amplifier waveguides 22 are single-mode doped waveguides, and their ends are coupled at 24 to an undoped single mode waveguide 26. A light ray input at one end 28 of the doped waveguide exits the waveguide at the output as an amplified ray. The pump laser is introduced either through one end of the waveguide, in which case a mirror 30 is placed at its other end, or through both ends, allowing for counter-propagation pumping.

A configuration wherein system 2 acts as a multiple laser source is illustrated in Fig. 9. One of the ends of each amplifying doped waveguide is fitted with a mirror or Bragg grating 32, and the other end is fitted with a partially reflecting element 34, which could be constituted by a partially reflecting mirror, a Bragg grating or other reflectors, thus creating a laser cavity. The amplified output is obtained through waveguide 36 coupled to partial reflector 34. The doped waveguides in this embodiment can be all the same, or may have different doping and properties, as desired.

Shown in Figs. 10 and 11 is an optical coupler 40 made, for example, of glass or optical quality plastic, to be utilized with the system of the present invention. Shown are five amplifying waveguides 6 connected to a pump waveguide 4. Each of the amplifying waveguides 6 is composed of an inner core portion 42, surrounded by an outer cladding portion 44. The connection between the waveguides is achieved by fusing the surface of pump waveguide 4 to the surface of cladding portions 44 of the amplifying waveguides 6, as indicated by the dotted lines 48. The core portions 42 are not directly coupled to pump waveguide 4, and thus, the signals propagating inside core portions 42 are not affected. Hence, the light rays emerging from laser source 18 reach coupler 40 and are efficiently coupled into cladding portions 44, traversing these portions and reaching the signal-bearing core portions 42.

While in Fig. 11 there is illustrated a central pump waveguide portion and peripherally connected amplifying waveguide portions, embodiments are envisioned in which, e.g., the pump waveguide portion is not central, but rather is located on one side of the coupler and the amplifying waveguide portions are connected to each other on one side of the pump waveguide portion.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. An optical amplification system, comprising: a pump waveguide, connectable at at least one of its ends to a pump laser, and a plurality of amplifying waveguides coupled to and extending along the outer surface of said pump waveguide.

2. The optical amplification system as claimed in claim 1, further comprising a reflecting surface at least partly surrounding said pump waveguide and amplifying waveguides.

3. The optical amplification system as claimed in claim 1, wherein said amplifying waveguides at least indirectly contact said pump waveguide.

4. The optical amplification system as claimed in claim 1, wherein said amplifying waveguides surround said pump waveguide in a closely packed fashion.

5. The optical amplification system as claimed in claim 1, wherein at least some of said amplifying waveguides are doped with at least one rare earth active ion species.

6. The optical amplification system as claimed in claim 5, wherein different amplifying waveguides are doped differently, so as to be excited and/or amplified by lightwaves of different wavelengths.

7. The optical amplification system as claimed in claim 1, wherein at least some of said waveguides are fibers.

8. The optical amplification system as claimed in claim 1, wherein said waveguides have a circular cross-section.

9. The optical amplification system as claimed in claim 1, wherein the cross-sectional areas of the pump and amplifying waveguides are substantially the same.

10. The optical amplification system as claimed in claim 1, wherein the cross-sectional area of said pump waveguide is larger than the cross-sectional areas of said amplifying waveguides.

11. The optical amplification system as claimed in claim 1, wherein said reflecting surface is a tube having an inner, light-reflecting surface.

12. The optical amplification system as claimed in claim 1, wherein said reflecting surface is applied to the outer surfaces of said amplifying waveguides.

13. The optical amplification system as claimed in claim 1, wherein said pump waveguide is coupled at one of its ends to a pump laser and, at its other end, to a reflecting surface, thereby establishing a resonant laser cavity.

14. The optical amplification system as claimed in claim 1, wherein said pump lightguides are provided with means for enhancing the scattering, at different angles, of the light propagating through said lightguides.

15. The optical amplification system as claimed in claim 1 , wherein said reflecting surface is provided with means for the scattering of light at different angles.

16. The optical amplification system as claimed in. claim 1 , further comprising an optical coupler composed of a pump waveguide portion and. connected amplifying waveguide portions, said amplifying waveguide portions having an inner core and an outer cladding.

17. The optical amplification system as claimed in claim 16, wherein said optical coupler is composed of a central pump waveguide portion and peripherally connected amplifying waveguide portion.

18. The optical amplification system as claimed in claim 16, wherein said amplifying waveguide portions are connected to said pump waveguide portion by fusion.