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 â€œpassiveâ€ in the sense that it does not have an internal switching capability, or â€œactiveâ€ in the sense that it has such a capability. 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 â€œactiveâ€ in that it includes a plurality of un-doped EDFs with different lengths, switchably connectable to the GFF and the EDFA to provide required wide-band gain properties.

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 two switches to switch between the different length EDFs.

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 describes a preferred 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 shows an embodiment of an amplifier according to the present invention that uses an â€œactive moduleâ€ with a plurality of un-pumped EDFs of different lengths.

FIG. 9 shows yet another embodiment of multi-gain stage optical amplifier with an attached â€œactiveâ€ module.

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 100â€² (left side), or shown in its regular configuration as a NB amplifier 100â€³ 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 100â€³ or pigtails connected to the passive module as in 100â€². Normally, amplifier 100â€³ is a variable gain amplifier with a very narrow-band; by adding to amplifier 100â€³ a passive module as described in more detail below, amplifier 100â€³ 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 â€œinputâ€ to the EDFA, and another tap 216 and another detector 218 are usually located after the output isolator, between the isolator and an â€œoutputâ€ 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 â€œErbium Doped Fiber amplifiersâ€ by Emanuel Desurvire, J. Wiley and sonsâ€ , 1^stedition, 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, which describes a preferred embodiment of an amplifier or â€œamplifier boxâ€ 300 (also referred to herein as an apparatus for optical signal amplification) according to the present invention. The figure 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. When the amplifier is required to be operated as a wide-band gain amplifier, passive module 112 is added, and the amplifier can be operated gain flattened over a wide spectral band at the required gain.

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 â€œEDF onlyâ€ 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.

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 Â±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 shows an embodiment of an amplifier 800 according to the present invention that uses an â€œactive moduleâ€ with a plurality of un-pumped EDFs of different lengths. Amplifier 800 may be connected to an â€œactiveâ€ module 810 that has a few pieces of EDFs with different lengths and two optical switches 802 and 804 to choose between the fibers, instead of adding manually an â€œEDF onlyâ€ passive module as done in FIG. 5. The two optical switches are used to choose between different lengths of an EDF such as 806a, 806b 806c or 806d. 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 (100â€² to 100â€³ 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. 9 shows yet another embodiment of multi-gain stage optical amplifier 900 with an attached â€œactiveâ€ module. 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 losses of the active module and GFF 502 and thus to reach an 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.

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.