Abstract:
An apparatus for optical amplification that imparts to a narrow band optical amplifier, specifically an Erbium dope fiber amplifier (EDFA), gain properties found in wide band optical amplifiers. In one preferred embodiment, the apparatus comprises an EDFA and an externally attachable (to the EDFA) module that includes a single un-pumped Erbium doped fiber (EDF) connected to a gain flattening filter (GFF). The GFF may be internal or external to the module. In another embodiment, the apparatus comprises an EDFA and an externally attachable module having a plurality of un-doped EDFs with different lengths, switchably connectable to the GFF and the EDFA to provide the required wide-band gain properties.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a Continuation in Part of U.S. patent application Ser. No. 10/893,882, dated 20 Jul. 2004 now abandoned. 

   FIELD AND BACKGROUND OF THE INVENTION 
   The present invention relates to optical communication systems, and more particularly to amplifiers, specifically Erbium Doped Fiber Amplifiers (EDFA), used in Wavelength Division Multiplexing (WDM) optical communication systems. 
   EDFAs are used in WDM optical communication systems for amplifying many wavelength channels simultaneously in the Erbium doped fiber Gain band. There are two types of EDFAs for WDM: Fixed Gain (FG). optical amplifiers, which can be operated gain flattened and with low Noise Figure only at a pre-determined gain, as described for example in U.S. Pat. No. 5,225,922 to A. R. Charplyvy et. al; and Variable Gain (VG) amplifiers, which can be operated with low noise figure and gain flattened over wide range of gains, as described for example in U.S. Pat. No. 5,812,710 to Y. Seguya et al, U.S. Pat. No. 6,049,413 to M. G. Taylor et al., and U.S. Pat. No. 6,611,641 to U. Ghera et al. 
   Variable gain amplifiers are usually preferred in long distance systems that normally comprise a plurality of amplifiers, because usage of such an amplifier allows the user to operate the system with an overall low noise figure (NF). The drawback of these amplifiers is that they are much more complicated in their optical design, as discussed in the U.S. patents to Seguya, Taylor and Ghera above. Moreover, VG amplifiers are much more expensive than FG amplifiers. On the other hand, FG amplifiers, while inexpensive, suffer from a number of limitations, chiefly the fixed gain which causes non-optimal performance in a WDM systems based on a multiplicity of wavelengths. 
   There is thus a widely recognized need for, and it would be highly advantageous to have, WDM amplifiers and amplification systems that are inexpensive and simple like FG amplifiers, yet capable to endow the WDM system with the advantages of a VG amplifier, e.g. gain flattening over a wide range of gains. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided an apparatus that includes a new type of amplifier for WDM optical communication systems. The new amplifier is based on the design and elements of a narrow band amplifier, specifically an EDFA, but comprises an additional component, a module that imparts to the amplifier gain properties found in wide band amplifiers. The module may be “passive” in the sense that it does not have an internal switching mechanism or “active” in the sense that it has such a mechanism. A passive module includes preferably an un-pumped Erbium doped fiber (EDF) connected to a gain-flattening filter (GFF). In one embodiment, the GFF is included in the passive module. 
   In another embodiment, the GFF is external to the passive module and incorporated in the EDFA. 
   In yet another embodiment, the module is “active” in that it includes a plurality of un-pumped EDFs with different lengths and a switching mechanism to connect one or more of the EDFs to the GFF and the EDFA to provide required wide-band gain properties. 
   In yet another embodiment, the module is “active” in that it includes a plurality of un-pumped EDFs with different lengths, each EDF directly connected in series to an associated optional GFF, and a switching mechanism used to select one or more EDFs and their associated optional GFFs for connection to another optional GFF and the EDFA to provide required wide-band gain properties. 
   In yet another embodiment, the module is connected in series with one or more additional gain stages to achieve a higher output power for the entire amplifier. 
   According to the present invention there is provided a method for operating a narrow band Erbium doped fiber amplifier (EDFA) as a wide band optical amplifier comprising the steps of connecting to the EDFA a module that includes at least one un-pumped EDF coupled to a GFF, and cooperatively operating the EDFA and the module to obtain wide band amplification with substantially flat gain. 
   According to one feature in the method of the present invention, the step of providing a module includes providing a passive module that includes one EDF. 
   According to another feature in the method of the present invention, the step of providing a module includes providing an active module that includes a plurality of EDFs of different lengths and a switching mechanism to select at least one of the EDFs for connection to the EDFA. 
   According to yet another feature in the method of the present invention, the step of providing a module includes providing an active module that includes a plurality of EDFs of different lengths, each EDF connected in series to an optional associated GFF, and a switching mechanism to select at least one of the pairs of EDF and their optional associated GFF for connection to the EDFA. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIG. 1  shows schematically a variable gain narrow band amplifier; 
       FIG. 2  shows details of a basic, standard Erbium doped fiber amplifier; 
       FIG. 3   a  describes a preferred embodiment of an amplifier according to the present invention; 
       FIG. 3   b  describes another embodiment of an amplifier according to the present invention; 
       FIG. 4  shows a preferred embodiment of a passive module according to the present invention, comprising a gain flattening filter and an un-pumped Erbium fiber; 
       FIG. 5  shows an embodiment of an amplifier with a GFF designed for minimal gain incorporated in the EDFA; 
       FIG. 6  shows a passive module embodiment that includes only an Erbium doped fiber of a given length, chosen to match a required gain when added to the amplifier of  FIG. 5 ; 
       FIG. 7  shows exemplary performances of an amplifier according to the present invention, using a passive module with different un-pumped Erbium fiber lengths; 
       FIG. 8   a  shows an embodiment of an amplifier according to the present invention that uses an active module with a plurality of un-pumped EDFs of different lengths; 
       FIG. 8   b  shows an alternative embodiment of the active module used for the amplifier of  FIG. 8   a;    
       FIG. 9  shows yet another embodiment of an amplifier according to the present inventions, where the active or passive module is followed by one or more additional gain stages to achieve a higher output power. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is of a new type of amplifier for WDM systems. This amplifier can be operated either as a variable gain amplifier with large dynamic range at a very narrow band spectrum (typically 1–3 nm), or as a fixed gain wide band amplifier (typically 40 nm). The transformation from a narrow band amplifier to a wide band one is done preferably by adding a passive module to the narrow band amplifier. An amplifier as disclosed herein allows an almost costless upgrading of a narrow band optical communications system to a wide-band optical communication system. It also reduces dramatically the number of amplifiers needed in the network. 
   The principles and operation of the combined amplifier according to the present invention may be better understood with reference to the drawings and the accompanying description. 
   Referring now to the drawings,  FIG. 1  shows schematically a VG narrow band amplifier configured with a passive module  112  to perform as a fixed gain WDM amplifier  100 ′ (left side), or shown in its regular configuration as a NB amplifier  100 ″ on the right side. Arrows numbered 1 to 4 symbolize in this diagram fiber pigtails, where 1 and 4 are input and output pigtails, and 2 and 3 are pigtails that are short circuited as in  100 ″, or pigtails connected to the passive module as in  100 ′. The adjustment from one setting ( 100 ″—without passive module) to another ( 100 ′—including passive module) may optionally be accomplished using an optical switching mechanism. Normally, amplifier  100 ″ is a variable gain amplifier with a very narrow band; by adding to amplifier  100 ″ a passive module as described in more detail below, amplifier  100 ″ can be modified into a fixed gain amplifier at a requested gain. In a long distance WDM communication system, narrow band amplifiers can be located at amplification points/closets, and according to system topology an adequate passive module can be added to one or all such NB amplifiers so that each amplifier is gain flattened and has low noise at the exactly required gain. 
   The amplifier disclosed herein is comparable in building blocks simplicity to a fixed gain amplifier. However, system-wise it can deliver a noise figure comparable to a variable gain amplifier. It can work as a narrow band optical amplifier, and be reconfigured at any time to a fixed gain wide-band amplifier just by adding a passive module, without changing amplifier software/hardware. The availability of such an amplifier has at least two major advantages: a) it reduces the types of amplifiers that system vendors have to keep, because one such amplifier fits all scenarios, and a system vendor needs therefore to stock only passive module boxes, and b) systems can be built with simple narrow band amplifiers in periods that traffic flow is low, and upgraded at a later stage as needs arise to wide band amplifiers. 
   Reference is now made to  FIG. 2 , which shows details of a basic, standard EDFA. A basic EDFA comprises an Erbium doped fiber  202  pumped with either a 980 nm or a 1480 nm laser diode pump  204  via a pump wavelength/signal band WDM coupler  206 . Isolators  208  (input) and  210  (output) are located on both sides of fiber  202 , to prevent the optical amplifier from becoming a laser. An optical tap  212  and a detector  214  are usually located before the input isolator, i.e. between the input isolator and an “input” to the EDFA, and another tap  216  and another detector  218  are usually located after the output isolator, between the isolator and an “output” of the EDFA. With the knowledge of the level of input and output power it is possible to have information on the amplifier gain (Output power/input power), and it is possible to set the gain by varying the pump power, as described for example in “Erbium Doped Fiber amplifiers” by Emanuel Desurvire, J. Wiley and Sons, 1 st  edition, 1994, chapter 5.2, pages 319–336. 
   The gain curve of a typical amplifier is wavelength dependent, and thus if it is required to operate the amplifier at a wide band (for example in the C band from 1527–1565 nm) a GFF is required, as described in U.S. Pat. No. 5,225,922 to A. R. Charplyvy et. al. A GFF is a pass-band filter with a transfer function complimentary to the gain curve of the EDF at a certain gain (population inversion), so superposition of both spectra results in constant gain for all wavelength channels passing through the amplifier. If the amplifier is operated at a narrow band (typically 1–3 nm) inside the Erbium gain band, wavelength gain variation becomes a non-issue and the amplifier can be operated over a very wide gain range (typically 10–20 dB). 
   Reference is made now to  FIG. 3   a , which describes a preferred embodiment of an amplifier or “amplifier box”  300  (also referred to herein as an apparatus for optical signal amplification) according to the present invention.  FIG. 3   a  shows all the elements of the basic EDFA of  FIG. 2 , and in addition a pair of optical cables  302  and  304  that connect the EDFA as shown to a passive module  112 , which is described in more detail in  FIG. 4 . When amplifier  300  is required to be operated as a narrow-band amplifier, optical cables  302  and  304  are connected to each other (short circuited), and the passive module is not included in the optical path of the amplifier. When the amplifier is required to be operated as a wide band gain amplifier, optical cables  302  and  304  are connected to passive module  112 , thus including the module within the optical path, and the amplifier can be operated gain flattened over a wide spectral band at the required gain. 
     FIG. 3   b  shows another embodiment of an amplifier box  300 ′, which is identical to amplifier box  300  except for optical switches  306  and  303 , and cable  310  inserted between the switches. When no passive module is connected to cables  302  and  304 , switches  306  and  308  are configured to be connected to each other via cable  310 . When a passive module is connected to cables  302  and  304 , the switches can be reconfigured so as to include passive module  112  in the optical path of the amplifier. The use of a switching mechanism allows inclusion of passive module  112  within the optical path of the amplifier without communication traffic interruption. Note that switch  306  may be replaced with a passive optical splitter (not shown), or alternatively switch  308  may be replaced by a passive optical combiner (not shown). In either of these two latter cases, the switching mechanism is implemented using only a single optical switch. 
   As shown in more detail in  FIG. 4 , the passive module preferably comprises a GFF  402  designed for the lowest gain that the single stage amplifier can be operated in, and an (un-pumped) EDF  404  whose length is dependent on the gain the amplifier has to be operated in. The function of the additional EDF is this embodiment is to absorb energy in the amplified spectrum in the blue band and to transfer it to the red band, thereby improving gain flattening. Alternatively, in another embodiment shown in  FIG. 5 , an amplifier  500  includes a GFF  502  for lowest gain incorporated in the EDFA itself. In this embodiment, a passive module  504  preferably includes only an un-pumped EDF (but no GFF), as shown in  FIG. 6 . The length of this un-pumped EDF may be changed according to the required gain (a change in length implying the use of a different passive module). In other words, different “EDF only” passive modules  504  may be prepared for different required gains, each such passive module having a fiber with a predetermined length that matches a required gain, and these modules may be changed and attached to the EDFA as needed. Amplifier  500  may be modified to include a switching mechanism in the form of one or more optical switches in the same manner as shown in  FIG. 3   b . This allows the addition of the passive module  504  without communication traffic interruption, as discussed in the context of  FIG. 3   b  above. 
   The amplifier gain is related to a minimum gain through the length of this fiber; to obtain a higher amplifier gain in comparison with the minimal amplifier gain, one needs a longer EDF. For the C Band (1527–1565 nm), which is the most common spectral band used in optical communications, the un-pumped EDF has stronger absorption at shorter wavelengths, and operating the amplifier at higher gains than the GFF was designed for results in higher gains for the longer wavelengths. For this case, the passive module contains only an un-pumped EDF (and no GFF), as shown in  FIG. 6 . An example for the use and the performance of an amplifier according to the present invention is shown in  FIG. 7 . 
   In  FIG. 7 , graph  7 . 1  shows the output spectrum of an EDFA comprising a 980 nm pump, a 10 meter long EDF and isolators and taps. A passive module is added and connected through cables as in the embodiment of  FIG. 3  so that the amplifier is gain flattened for Gain=23 dB. In this case, the passive module comprises only a GFF (no un-pumped fiber, i.e. similar to the embodiment of  FIG. 4 , but with zero length of EDF  404 )) and the result is shown in graph  7 . 2 . In graph  7 . 3 , the same amplifier is operated at Gain=26 dB. A passive module with the same GFF and additional 2 meters of an un-pumped EDF (as shown in  FIG. 4 ) is added and connected through the same cables as above. The result is shown in graph  7 . 4 . In graph  7 . 5 , the same amplifier as in  7 . 1  is operated in Gain=29 dB. The same GFF as in graph  7 . 2  is used, but this time the passive module (comprising an internal GFF as in  FIG. 4 ) includes 4.2 meters of EDF. The EDFA can thus be operated as a narrow band amplifier (1–3 nm) as described in graphs  7 . 1 ,  7 . 3  and  7 . 5 . For narrow band operation, gain flattening is better than ±0.75 dB. If a wide band operation is required, the passive module is added (different for each gain) and the results are described in graphs  7 . 2 ,  7 . 4  and  7 . 6 . 
     FIG. 8   a  shows an embodiment of an amplifier  800  according to the present invention that uses an “active module” with a plurality of un-pumped EDFs of different lengths. Amplifier  800  may be connected to an “active” module  810  that has a few pieces of EDFs with different lengths and two optical switches  802  and  804 , which constitute a switching mechanism used to choose between the fibers, instead of adding manually an “EDF only” passive module as done in  FIG. 5 . The two optical switches are used to choose between different lengths of an EDF such as  806   a ,  806   b    806   c  or  806   d . A module that includes switches and different lengths of EDFs can be either incorporated in the amplifier or added as a separate entity to the amplifier. This addition transforms a narrow band amplifier ( 100 ′ to  100 ″ in  FIG. 1 ) to a wideband VG EDFA. Gain variation is achieved by using an optical switch to choose the adequate length of EDF that will deliver gain flattening at the requested Gain. 
     FIG. 8   b  shows an alternative embodiment  810 ′ of an active module. In this embodiment, an additional switch  808  may be used to configure the module for different combinations of EDF lengths, such as  806   a ′+ 806   c ′,  806   a ′+ 806   d ′,  806   b ′+ 806   c ′, or  806   b ′+ 806   d ′. Additional switches and additional lengths of EDF may be used to achieve additional combinations. 
   In either of the embodiments of the active module ( 810  or  810 ′), an optional GFF (not shown) may be placed in series with each length of EDF. This may be used to achieve improved gain flatness for each possible configuration of the active module. 
   In either of the embodiments of the active module ( 810  or  810 ′), switch  802  may be replaced by a passive optical splitter (not shown), or switch  804  may be replaced by a passive optical combiner (not shown). In this way the switching mechanism may be achieved with one less active switch than shown in active module  810  or  810 ′. 
     FIG. 9  shows yet another embodiment of an amplifier according to the present invention, where the active module is followed by at least one additional gain stage. Amplifier  900  comprises a first gain stage  902  that includes some of the elements of  FIG. 2 , e.g. a Pump wavelength/Signal Band WDM coupler  206 , an EDF  202  of a certain length and an isolator  210 , and is connected to an active module  910 . One or more additional gain stages  904 , similar to first gain stage  902  can be used to compensate potential losses of the active module and GFF  502  and thus to reach a higher output power of the amplifier. The important aspect here is that module  910  is inserted between two gain stages that may be further connected to other amplifier elements. Alternative embodiments of the active module, as discussed in the context of  FIG. 8  above, may be used instead of module  910 . 
   All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. 
   While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.