Abstract:
An optical amplifier includes a substrate having disposed thereon an optical waveguide, the optical waveguide having an active region with Si nanocrystals dispersed in an erbium doped glass matrix. A light source is arranged to irradiate the optical waveguide with pump electromagnetic radiation. The amplifier also includes means for suitably shaping an input electromagnetic field to be amplified into the active region of the optical waveguide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from British Patent Application No. 0415711.1, filed on Jul. 14, 2004. 
     The present invention relates to an improved low cost optical amplifier. More specifically the present invention relates to an LED pumped Silicon (Si) nanocluster erbium doped optical amplifier. 
     BRIEF DESCRIPTION OF RELATED DEVELOPMENTS 
     Erbium doped optical amplifiers have been used for many years in optical telecommunications networks to boost the strength of an optical signal as it is transmitted along a fiber. While there are many different types of these amplifiers, they all typically rely on relatively expensive single wavelength lasers as the pumping sources. It has been recently shown that in some specially prepared erbium doped optical waveguides, such as silica layers containing Si nanocrystals or nanoclusters dispersed in the erbium doped glass matrix, that the erbium can be efficiently excited using light sources which emit over a broad wavelength range in the visible portion of the electromagnetic spectrum. Light emitting diodes (LEDs) have been used as a low cost pumping source for these types of amplifiers as there is no longer a strict requirement on the pumping wavelength as is the case in standard erbium doped glass. Such an arrangement has been discussed in a paper by Jinku Lee and Jung H. Shun entitled “Optical gain at 1.5 microns in nanocrystal Si sensitized erbium doped silica waveguide using top-pumping 470 nm LEDs” presented at the 2004 OFC meeting. Here an array of LEDs was used to pump a section of erbium doped silica waveguide, which had been sensitized with Si nanocrystals. 
     However, as in the case above, the much lower radiance of the LED, compared to a laser, places a limitation on the efficiency in which the pump power can be transferred to the active medium of the waveguide and consequently on the output power of the amplifier. Thus for a given length of fiber in a communications network, more LED based amplifiers would be required thereby negating any cost savings over laser based amplifiers. 
     SUMMARY OF THE DISCLOSED EMBODIMENTS 
     The present invention overcomes the above-mentioned limitations by providing an optical amplifier with increased output power by enabling an almost complete transfer of optical power from the LED source to the active medium in the waveguide. 
     This is achieved according to the present invention by providing an optical amplifier comprising a substrate having disposed thereon an optical waveguide, the optical waveguide having an active region, a light source arranged to irradiate the optical waveguide with pump electromagnetic radiation, and means for suitably shaping an input electromagnetic signal to be amplified into the active region of the optical waveguide, wherein the irradiation of the optical waveguide is orthogonal to the plane of propagation of the signal. 
     Advantageously the present invention offers a compact low cost optical amplifier for use in telecommunications networks. In addition, the optical amplifier of the present invention is arranged in a lateral pumping configuration, which provides more flexibility in the arrangement of the pump sources. Furthermore, the amplifier of the present invention maintains the single mode characteristics of the amplified signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the principal features and advantages of the present invention have been described above, a more detailed understanding can be obtained by referring to the figures and detailed description which follows where: 
         FIGS. 1   a  and  1   b  show a known laterally laser pumped optical amplifier, 
         FIGS. 2   a  and  2   b  show a known LED pumped optical amplifier, 
         FIG. 3  shows a first embodiment of the present invention, 
         FIG. 4  shows a second embodiment of the present invention, and 
         FIG. 5  shows a side view of the embodiments shown in  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIGS. 1   a  and  1   b  schematics are shown of a laterally laser-pumped optical amplifier  10 , based on Si nanocluster erbium doped active waveguide  12 . The waveguide is disposed on substrate  14 . Due to the extremely high absorption coefficient of the doped waveguide, longitudinal pumping is not practical, as only few micrometers of the waveguide would be effectively pumped. Therefore, a complex, anamorphic optical system  16  is required to transform the output beam from the laser  18  into a narrow, straight line  19 , to achieve a sufficient and stable coupling efficiency to the active waveguide. 
     In  FIGS. 2   a  and  2   b , where parts also appearing in  FIGS. 1   a  and  1   b  bear identical numerical designation, an LED-pumped optical amplifier  11  known from the literature is represented. Here there is no need of complex optics, but the fraction of pump power captured by the doped single mode waveguide is very small, thus strongly limiting both the pumping efficiency and the output power attainable by the amplifier, due to the low LED  17  radiance. 
     In  FIG. 3  a schematic is shown according to the present invention of a lateral optical pumping configuration for an expanded-beam optical amplifier  30  having silicon nanocrystals dispersed in an erbium doped silica waveguide. An optical slab waveguide  31  is disposed on substrate  32 . The waveguide has a lower cladding layer  34  and an upper cladding layer  44 , which function to contain optical radiation traveling within the waveguide. Towards the centre of the slab waveguide is an active region  33  of silicon nanocrystals, which has typically been doped with erbium. Input  35  and output  36  waveguides are disposed at either end of the slab waveguide. The input waveguide functions to guide an input optical signal, typically at 1.53 microns, from an optical fiber (not shown) towards the slab waveguide. Once the signal reaches the slab waveguide it expands in a lateral direction due to the physical properties of the slab and input waveguides. The input waveguide can be tapered (i.e. having a gradually increasing width) in order to control the angle of expansion of the signal wave into the slab waveguide. The expanded beam  37  then traverses the active region  33  of the waveguide. A suitably shaped reflective element  39  is disposed at the edge of the slab waveguide. In this embodiment the suitably shaped reflective element is an elliptical mirror. The elliptical mirror refocuses the expanding beam  37  towards output waveguide  36 . An array of LEDs  70  is disposed above the active region and irradiates the active region in a plane orthogonal to the plane of propagation of the signal, thereby amplifying the signal in a manner well known for erbium doped optical amplifiers. The LED array typically emits blue radiation at, for example 470 nm, however the wavelength can be selected according to the nature of the material disposed in the active region. In this configuration the size of the active region to be optically pumped can be matched by selecting the appropriate sized LED array as the pumping source, thus further improving the overall transfer efficiency of optical power from the relatively low power LED source to the active region. In the embodiment shown in  FIG. 3  the input waveguide  35  is arranged to intersect the slab waveguide at the first focal point  39   a  of the elliptical mirror and the output waveguide  36  intersects at the second focal point  39   b . The output waveguide can also be tapered, in order to maximize the coupling between the slab and output waveguides. 
     In  FIG. 4 , where parts also appearing in  FIG. 3  bear identical numerical designation, an alternative embodiment of the present invention  40  is shown where parabolic mirrors  48  and  49  are used as the suitably shaped reflective elements instead of the elliptical mirror. Here, the input and output waveguides  45  and  46  intersect the slab waveguide  31  at the focal points  48   a  and  49   b  respectively of the parabolic mirrors. In this embodiment the expanding input signal beam is collimated by the first parabolic mirror  48  before traversing the active region  37  as a collimated signal beam  47  and is refocused on to the output waveguide  46  by the second parabolic mirror  49 . 
     Advantageously in the first embodiment of the present invention the variable width signal beam  37  only intersects a reflective surface once thus limiting the losses associated with reflection off a mirror. Whereas the second embodiment suffers  2  mirror reflection losses but offers the advantage of a constant signal beam width in the active region. In both embodiments the reflective surfaces can be fabricated by etching a step to a suitable depth into the slab waveguide layer. Reflection can occur due to total internal reflection off this edge. Alternatively the vertical surface of the step can be coated with a suitably reflective material or multilayer reflecting structure. 
     In  FIG. 5  the LED array  70  is shown disposed above the active region  33  of the waveguide. Here both the lower  34  and upper  44  cladding layers can been seen. In addition an optical diffuser  52  can be disposed between the LED array and the active region to improve the power radiation intensity distribution by diffusing the radiation from the LED. 
     It is not intended that the present invention be limited to the above embodiments and other modifications and variations are envisioned within the scopes of the claims.