Abstract:
A waveguide optical amplifier is disclosed. The optical amplifier includes a substrate and a cladding layer disposed on the substrate. The waveguide optical amplifier also includes an amplifying core disposed within the cladding layer and a secondary core disposed within the cladding layer proximate the amplifying core. The secondary core is adapted to absorb at least a portion of a light signal being transmitted through the amplifying core. A feedback loop for dynamically changing the amount of light being absorbed and a method for dynamically controlling light signal absorption are also provided.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to optical amplifiers with incorporated gain flattening filters.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical communication systems based on optical fibers allow communication signals to be transmitted not only over long distances with low attenuation, but also at extremely high data rates, or bandwidth capacity. This capability arises from the propagation of a single mode optical signal in the low-loss windows located at the near-infrared wavelength of 1550 nm. Since the introduction of erbium-doped fiber amplifiers (EDFAs), the last decade has witnessed the emergence of single-mode optical fibers as the standard data transmission medium for wide area networks (WANs), especially in terrestrial and transoceanic communication backbones. In addition, the bandwidth performance of single-mode optical fiber has been vastly enhanced by the development of dense wavelength division multiplexing (DWDM), which can couple up to 40 channels of different wavelengths of light into a single fiber, with each channel carrying up to 10 gigabits of data per second. Moreover, recently, a signal transmission of 10 terabit (10 13  bits) per second has been achieved over a single 100 kilometer fiber on a 120-channel DWDM system. Bandwidth capacities are increasing at rates of as much as an order of magnitude per year.  
           [0003]    The success of the single-mode optical fiber in long-haul communication backbones has given rise to the new technology of optical networking. The universal objective is to integrate voice video, and data streams over all-optical systems as communication signals make their way from WANs down to smaller local area networks (LANs) of Metro and Access networks, fiber to the curb (FTTC), fiber to the home (FTTH), and finally arriving to the end user by fiber to the desktop (FTTD). Examples are the recent explosion of the Internet and use of the World Wide Web, which are demanding vastly higher bandwidth performance in short- and medium-distance applications. Yet, as the optical network nears the end user starting at the LAN stage, the network is characterized by numerous splittings of the input signal into many channels. This feature represents a fundamental problem for optical networks. Each time the input signal is split, the signal strength per channel is naturally reduced.  
           [0004]    EDFA&#39;s are used to amplify signal lights in optical telecommunications systems. In the C band range (between approximately 1525 nm and 1565 nm), EDFA&#39;s provide non-uniform amplification over the bandwidth. A diagram of a typical spectral shape of a C band EDFA is shown in FIG. 1. This non-uniform amplification becomes problematic for wavelength division multiplexing systems since some wavelengths, especially those around 1535 nm, experience significantly more gain than other wavelengths, resulting in accumulation of gain non-uniformity in the system. Long period fiber Bragg gratings are already known for gain flattening. However, these gratings must be inserted between amplifier stages, limiting integration capacity of the system.  
           [0005]    One current solution is to provide a twin core erbium doped fiber, in which two cores extend through a fiber cladding, separated by a generally constant distance. The first core, doped with erbium, amplifies a signal light through the fiber. The proximity of the first core to the second core provides a coupling effect, in which, at predetermined wavelengths, some of the signal light from the first core transfers to the second core, flattening some of the gain realized while transmitting the signal light through the first, or erbium doped, core. A drawback to this approach is that, due to manufacturing constraints, both cores extend the entire length of the fiber, reducing the ability to regulate the amount of the signal light transferred between cores. An additional drawback to a twin core erbium doped fiber is that, since the spacing between each core is generally constant, the coupling efficiency of the fiber is not adjustable, and only wavelengths within a predetermined bandwidth can be flattened.  
           [0006]    Another solution is to provide a wavelength division multiplexer (WDM) in a planar waveguide in which a signal line is optically coupled with a signal flattening line. A portion of the light in the signal line transfers to the signal flattening line, thereby attenuating the signal in the signal line. The amount of attenuation can be predetermined by the length of the coupling between the signal line and the signal flattening line. However, the signal attenuation is fixed, and cannot be readily adjusted.  
           [0007]    It would be beneficial to provide a gain flattening filter in an optical amplifier which can be adjusted to flatten an adjustably desired bandwidth by an adjustably desired amount, thus manipulating the gain shape of the amplifier.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    Briefly, the present invention provides a waveguide optical amplifier comprising a substrate and a cladding layer disposed on the substrate. The waveguide optical amplifier also comprises an amplifying core disposed within the cladding layer and a secondary core disposed within the cladding layer proximate the amplifying core. The secondary core is adapted to absorb at least a portion of a light signal being transmitted through the amplifying core.  
           [0009]    Additionally, the present invention provides a dynamic gain flattening waveguide optical amplifier comprising a substrate and a cladding layer disposed on the substrate. The waveguide optical amplifier also comprises an amplifying core disposed within the cladding layer, the amplifying core having an output and a secondary core disposed within the cladding layer proximate the amplifying core. The waveguide optical amplifier further comprises a feedback loop including a tap optically connected to the output, a gain flattening controller optically connected to the tap, the gain flattening controller including a voltage generator, and an electrical conductor electrically connecting the voltage generator to a heater, the heater being disposed proximate to the secondary core.  
           [0010]    Further, the present invention provides a method of dynamically flattening gain in a waveguide optical amplifier. The method comprises providing a dynamic gain flattening waveguide optical amplifier. The waveguide optical amplifier includes a substrate, a cladding layer disposed on the substrate, an amplifying core disposed within the cladding layer, the amplifying core having an output. The waveguide optical amplifier also includes a secondary core disposed within the cladding layer proximate the amplifying core and a feedback loop. The feedback loop includes a tap optically connected to the output, a gain flattening controller optically connected to the tap, the gain flattening controller including a voltage generator, and an electrical conductor electrically connecting the voltage generator to a heater, the heater being disposed proximate to the secondary core. The method further comprises transmitting an optical signal through the amplifying core, the amplifying core amplifying the optical signal and the secondary core attenuating the amplification of the optical signal over a selected bandwidth; tapping a portion of the amplified optical signal, generating a tapped signal; transmitting the tapped signal to a gain flattening controller; generating a voltage in the amplifier controller based on the value of the tapped signal; and transmitting the voltage to the heater, wherein the voltage changes the temperature of the heater, wherein the change in temperature changes the refractive index of the secondary core, and wherein the change in the refractive index changes the gain flattening of the amplified optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings:  
         [0012]    [0012]FIG. 1 is a graph showing typical amplification gain in a C band amplifier.  
         [0013]    [0013]FIG. 2 is a perspective view of a planar waveguide optical amplifier with a gain flattening filter according to a first preferred embodiment of the present invention.  
         [0014]    [0014]FIG. 3 is a sectional view of the planar waveguide optical amplifier, taken along section lines  3 - 3  of FIG. 2.  
         [0015]    [0015]FIG. 4 is a schematic drawing of an amplifier module incorporating the planar waveguide optical amplifier according to the first preferred embodiment of the present invention.  
         [0016]    [0016]FIG. 5 is a graph showing approximate gain flattened amplification in the C band range.  
         [0017]    [0017]FIG. 6 is a perspective view of a planar waveguide optical amplifier with a dynamic gain flattening filter according to a second preferred embodiment of the present invention.  
         [0018]    [0018]FIG. 7 is a sectional view of the planar waveguide optical amplifier, taken along section lines  7 - 7  of FIG. 6.  
         [0019]    [0019]FIG. 8 is a schematic drawing of an amplifier module incorporating the planar waveguide optical amplifier according to the second preferred embodiment of the present invention.  
         [0020]    FIGS.  9 A- 9 K are planar views of alternative versions of the planar waveguide amplifier according to either of the first or second preferred embodiments of the present invention.  
         [0021]    FIGS.  10 A- 10 C are specific examples of alternative versions shown in FIGS. 9H, 9A, and  9 F, respectively.  
         [0022]    [0022]FIG. 11 is a graph showing calculated loss for wavelengths between 1520 and 1600 nm for the specific examples shown in FIGS.  10 A- 10 C. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention takes advantage of wavelength dependence on coupling efficiency between closely spaced cores in a waveguide optical amplifier to flatten the gain of optical signals as the optical signals are amplified by the waveguide optical amplifier. In the drawings, like numerals indicate like elements throughout.  
         [0024]    A first embodiment of the present invention includes a planar optical waveguide amplifier  100 , as shown in FIGS. 2 and 3. The amplifier  100  has an input end  102  and an output end  104 . The amplifier  100  includes a substrate  110  and a lower cladding  120  disposed on the substrate  110 . Preferably, the substrate  110  is constricted from a polymer, although those skilled in the art will recognize that the substrate  110  may be constructed from other materials, such as glass.  
         [0025]    A plurality of cores  130 ,  132 ,  134  are disposed on the lower cladding  120  in generally straight, parallel lines. The core  130  is an amplifying core which extends from the input end  102  to the output end  104  and transmits a signal light λ S  through the amplifier  100 . The cores  132 ,  134  are secondary, or gain flattening cores, which are proximate the amplifying core  130  and are separated from the amplifying core  130  by distances d 1  and d 2 , respectively. Those skilled in the art will recognize that d 1  and d 2  can be the same or different distances and that the cores  132 ,  134  can be coplanar with the amplifying core  130 , or non-coplanar. Preferably, the distances d 1  and d 2  are between 1.5 and 7.5 microns, although those skilled in the art will recognize that the distances d 1  and d 2  can be less than 1.5 microns and/or greater than 7.5 microns. Those skilled in the art will also recognize that one of the cores  132 ,  134  can be omitted, or that additional cores, not shown, can be disposed about the amplifying core  130 . These additional cores can be coplanar with the cores  130 ,  132 ,  134  or non-coplanar.  
         [0026]    Additionally, although FIG. 2 shows the cores  132 ,  134  having approximately the same length, those skilled in the art will recognize that the cores  132 ,  134  can have different lengths than each other and the amplifying core  130 . Additionally, although FIG. 3 shows the cores  132 ,  134  to have approximately the same cross-sectional sizes, those skilled in the art will recognize that the cores  132 ,  134  can have different cross-sectional sizes. The alternatives for the cores  132 ,  134  as described above can be selected depending on the desired flattening characteristics of the amplifier  100 .  
         [0027]    The core  130  and the cores  132 ,  134  are preferably constructed from a polymer, such as a halogenated polymer, and preferably the same polymer, doped with a rare earth element. A preferred polymer is disclosed in U.S. Pat. No. 6,292,292 and U.S. patent application Ser. Nos. 09/722,821, filed Nov. 28, 2000 and 09/722,282, filed Nov. 28, 2000, which are all owned by the assignee of the present invention and are incorporated herein by reference in their entireties.  
         [0028]    Those skilled in the art will recognize that the cores  130 ,  132 ,  134  can be applied to the lower cladding  120  by processes known to those skilled in the art, such as by spincoating, and then formed by other known process, such as reactive ion etching with photomasks.  
         [0029]    An upper cladding  140  is disposed over the cores  130 ,  132 ,  134  and the portion of the lower cladding  120  not covered by the cores  130 ,  132 ,  134 . Preferably, ends of the amplifying core  130  at the input  102  and the output  104  are not covered by the upper cladding  140  while the cores  132 ,  134  are preferably shorter than the amplifying core  130  and are completely covered by the upper cladding  140 . Preferably, both the lower cladding  120  and the upper cladding  140  are constructed from a polymer, and more preferably, from the same polymer. Also preferably, the refractive indices of the lower and upper claddings  120 ,  140  are sufficiently close to the refractive index of the core  130  to allow for single mode optical signal propagation, as is well known by those skilled in the art.  
         [0030]    In an optical amplifier module using the amplifier  100 , as shown schematically in FIG. 4, a pump laser  150  is optically connected along a signal line  101  to the input  102  of the amplifier  100  through a coupler, preferably a wavelength division multiplexer (WDM)  152 . The pump laser  150  provides a pump light λ P  to amplify a signal light λ S  which is transmitted along the signal line  101 . Preferably, the signal light λ S  is within a bandwidth of approximately between 1525 nm and 1565 nm, although those skilled in the art will recognize that the bandwidth can be larger or smaller, and can be in a different range, such as a range encompassing less than 1525 nm or greater than 1565 nm.  
         [0031]    In operation, the signal light λ S  is transmitted along the signal line  101  toward the amplifier  100 . The pump laser  150  generates the pump light λ P , which combines with the signal light λ S  at the WDM  152 . The combined signals λ S , λ P  enter the amplifier  100  at the input  102  and travel through the amplifying core  130 . In the amplifying core  130 , the pump light λ P  excites the rare earth elements in the amplifying core  130 , which in turn amplify the signal light λ S . However, as is well known in the art, different wavelengths of the signal light λ S  are amplified different amounts, as was previously described in reference to FIG. 1. The gain flattening cores  132 ,  134  are shaped and disposed relative to the amplifying core  130  to couple predetermined wavelengths of the signal light λ S  from the amplifying core  130  into the gain flattening cores  132 ,  134 , thus absorbing some of the signal light λ S . The effect of such coupling is to reduce the amplification of the predetermined wavelengths to provide an amplification spectrum as shown approximately in FIG. 5.  
         [0032]    A second embodiment of the present invention is a planar waveguide amplifier  200  as shown in FIGS. 6 and 7. The amplifier  200  incorporates a dynamic gain flattening feature to dynamically adjust gain flattening of the amplifier  200  based on output of the amplifier  200 .  
         [0033]    The amplifier  200  includes an input end  202  and an output end  204 . The amplifier  200  includes a substrate  210  and a lower cladding  220  disposed on the substrate  210 . A plurality of cores  230 ,  232 ,  234  are disposed on the lower cladding  220  in generally straight, parallel lines. The core  230  is an amplifying core which extends from the input end  202  to the output end  204  and transmits a signal light λ S  through the amplifier  200 . The cores  232 ,  234  are secondary, or gain flattening cores, which are separated from the amplifying core  230  by distances d 3  and d 4 , respectively.  
         [0034]    An upper cladding  240  is disposed over the cores  230 ,  232 ,  234  and the portion of the lower cladding  220  not covered by the cores  230 ,  232 ,  234 . Preferably, ends of the amplifying core  230  at the input  202  and the output  204  are not covered by the upper cladding  240  while the cores  232 ,  234  are preferably shorter than the amplifying core  230  and are completely covered by the upper cladding  240 . Preferably, both the lower cladding  220  and the upper cladding  240  are constructed from a polymer, and more preferably, from the same polymer. Also preferably, the refractive indices of the lower and upper claddings  220 ,  240  are sufficiently close to the refractive index of the core  230  to allow for single mode optical signal propagation, as is well known by those skilled in the art.  
         [0035]    A tap  254  is optically connected to the output  204  of the amplifier  200 . The tap  254  is optically connected to feedback loop comprised of a gain flattening controller  260 . The gain flattening controller  260  includes a voltage generator  262  that is opto-electronically connected to the tap  254 . Heaters  270 ,  272  are electrically connected via electrical conductors  274 ,  276  to the voltage generator  262  and are each disposed in the amplifier  200  proximate to a secondary core  232 ,  234 .  
         [0036]    In an optical amplifier module using the amplifier  200 , as shown schematically in FIG. 8, a pump laser  250  is optically connected along a signal line  201  to the input  202  of the amplifier  200  through a coupler, preferably a wavelength division multiplexer (WDM)  252 . The pump laser  250  provides a pump light λ P  to amplify a signal light λ S  which is transmitted along the signal line  201 . Preferably, the signal light λ S  is within a bandwidth of approximately between 1525 nm and 1565 nm, although those skilled in the art will recognize that the bandwidth can be larger or smaller, and can be in a different range, such as a range encompassing less than 1525 nm or greater than 1565 nm.  
         [0037]    In operation, the signal light λ S , generally having a bandwidth between approximately 1525 nm to 1565 nm, is transmitted to the input  202  of the amplifier  200 . The signal light λ S  travels from the input  202  through the amplifying core  230 , where the signal light λ S  is amplified by the pump light λ P  transmitted by the pump laser  250  as described above with reference to the first embodiment. A portion of the amplified signal light λ S  is directed into the secondary cores  232 ,  234 , attenuating predetermined wavelengths of the signal light λ S . As the amplified signal light λ S  exits the output  204  of the amplifier  200 , a portion of the amplified signal light λ S , preferably approximately 1% of the amplified signal light λ S , is tapped by the tap  254  to form a tapped signal λ T , which is sent to the gain flattening controller  260 .  
         [0038]    Preferably, the gain flattening controller  260  has been preprogrammed to compare the tapped signal λ T  to a predetermined value. If the tapped signal λ T  coincides with the predetermined value, the gain flattening controller  260  does not adjust the gain flattening of the amplifier  200 . However, if the tapped signal λ T  does not coincide with the predetermined value, the gain flattening controller  260 , through the voltage generator  262 , generates and transmits a voltage based on the value of the tapped signal λ T  to the heaters  270 ,  272 . The heaters  270 ,  272  change the temperature of the waveguide  200  proximate the secondary cores  232 ,  234 , which changes the refractive index of the secondary cores  232 ,  234 . This change in the refractive index of the secondary cores  232 ,  234  alters the coupling properties of the secondary cores  232 ,  234 , which in turn alters the gain flattening characteristics of the cores  232 ,  234 . By altering the gain flattening characteristics of the cores  232 ,  234 , the gain shape of the amplifier  200  is changed and predetermined wavelengths of the signal light λ S  can be attenuated. The gain shape of the amplifier  200  can be changed to match the predetermined value in the gain flattening controller  260 .  
         [0039]    The present invention takes advantage of wavelength dependence of coupling efficiency between closely spaced multiple cores; dynamical change of the core properties as a function of temperature, which gives rise to proportional changes in coupling properties; and fabrication flexibility of such structures in a waveguide form. The coupling efficiency of the cores  132 ,  134 ,  232 ,  234  is affected by multiple factors, including the refractive indices of the cores  132 ,  134 ,  232 ,  234  and the claddings  120 ,  140 ,  220 ,  240 ; the change in refractive index as a function of temperature (dn/dT); the shape of the cores  130 ,  132 ,  134 ,  230 ,  232 ,  234 , the distances d 1 , d 2 , d 3 , d 4  between the cores  130 ,  132 ,  134  and  230 ,  232 ,  234 ; the diameters of the cores  130 ,  132 ,  134 ,  230 ,  232 ,  234 ; and the materials comprising the cores  132 ,  134 ,  232 ,  234  and the claddings  120 ,  140 ,  220 ,  240 .  
         [0040]    Possible configurations of the cores  130 ,  132 ,  134 ,  230 ,  232 ,  234  of the first and second embodiments of the amplifiers  100 ,  200  are shown in FIGS. 9A through 9K. Although eleven configurations are shown in FIGS. 9A through 9K, the configurations shown are representative of optional designs and are not meant to be limiting in any way. For example, those skilled in the art will recognize that the configurations in FIGS. 9F and 9I can be combined to provide a waveguide with a straight gain flattening core on one side of the amplifying core, and a curved gain flattening core juxtaposed from the straight gain flattening core across the amplifying core.  
         [0041]    While the cores  130 ,  132 ,  134 ,  230 ,  232 ,  234  disclosed in FIGS. 9A through 9K are generally straight line channels, those skilled in the art will recognize that other shapes can be used, such as the curved waveguide shape disclosed in U.S. patent application Ser. No. 09/877,871, filed Jun. 8, 2001, which is owned by the assignee of the present invention and is incorporated herein by reference in its entirety.  
         [0042]    [0042]FIGS. 10A through 10C show dimensions of specific examples of the general configurations shown in FIGS. 9H, 9A, and  9 F, respectively, with calculated loss measurements using BPV software shown in the graph of FIG. 11. Those skilled in the art will recognize that different combinations of cores  132 ,  134 ,  232 ,  234  can be used to obtain different loss shapes as desired for particular applications.  
         [0043]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.