Abstract:
An optical amplifier contains two amplifier paths. An optical user signal (α, β) is in each case supplied to an optical amplifier (EDF 1 , EDF 2 ) through direction separators (RT 1 , RT 2 ). Each amplifier is followed by a wavelength multiplexer (WDM 1 , WDM 2 ) via which a pump signal ( γ1, γ2 ) is fed in, which, via the direction separator (RT 2 , RT 1 ) which is connected to its output, pumps the amplifier (EDF 2 , EDF 1 ) in the other amplifier path in the direction of the user signal (α, β).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a bidirectional optical amplifier having a first and second direction separator with a first and second amplifier path and feed devices for inputting pump signals.  
           [0003]    2. Description of the Related Art  
           [0004]    Optical amplifiers are required for optical transmission systems in which data signals are transmitted bidirectionally via one fiber.  
           [0005]    Various bidirectional optical fiber amplifiers are described in “Erbium-Doped Fiber Amplifiers” by Emmanuel Disurvire, Verlag John Wiley &amp; Sons, Inc. NY, 1994, pages 462 to 465. In order to pump a fiber amplifier in the direction of the data signal (FIG. 6.7), the fiber amplifier is preceded by a selective-wavelength coupler. However, this results in the noise response becoming worse due to attenuation of the user signal. In another variant (page 465, FIG. 6.8), pumping is carried out in the opposite direction to the signal direction, which once again fundamentally leads to poorer signal-to-noise ratios since a pump signal which has been attenuated by the amplifier fiber is present at the amplifier input.  
         SUMMARY OF THE INVENTION  
         [0006]    The object of the invention is thus to specify a bidirectional amplifier with an improved signal-to-noise ratio.  
           [0007]    This object is achieved by a bidirectional amplifier comprising a first direction separator having a first, second, and third connection, to whose first connection a first fiber section is connected; a second direction separator having a first, second, and third connection, to whose first connection a second fiber section of the transmission fibers is connected; a first fiber amplifier; a second fiber amplifier; a first amplifier path, comprising the first fiber amplifier arranged between the second connection of the first direction separator and the third connection of the second direction separator, the first fiber amplifier amplifying a first optical user signal which is supplied to the first connection of the first direction separator and which is then fed in via the first connection of the second direction separator to the second fiber section; and a first feed device for inputting pump signals that follow the first fiber amplifier, the first feed device having an output that is connected to the third connection of the second direction separator, the second fiber amplifier being pumped in a signal direction via the second direction separator that is connected to the first feed device output, the bidirectional amplifier further comprising a second amplifier path, comprising the second fiber amplifier arranged between the second connection of the second direction separator and the third connection of the first direction separator, the second fiber amplifier amplifying a second optical user signal, which is supplied to the first connection of the second direction separator and is then fed to the first fiber section via the first direction separator; and a second feed device for inputting pump signals that follow the second fiber amplifier, the second feed device having an output that is connected to the third connection of the first direction separator; the first fiber amplifier being pumped in a signal direction via the first direction separator that is connected to the second feed device output; and the second fiber amplifier being pumped in a signal direction via the second direction separator that is connected to the first feed device output.  
           [0008]    In this embodiment, the first and second direction separators may be configured as filters such that each respective pump signal, rather than amplified optical user signals, are fed into the second and the first amplifier path respectively. Also, the feed devices may be wavelength multiplexers.  
           [0009]    In another embodiment of the invention, the bidirectional amplifier may comprise a first four-port direction separator having a first, second, third, and fourth connection, the first connection being connected to a first fiber section; a second four-port direction separator having a first, second, third, and fourth connection, the first connection being connected to a second fiber section; a first fiber amplifier, which is arranged between the second connection of the first four-port direction separator and the third connection of the second four-port direction separator and amplifies a first optical user signal which is supplied to the first connection of the first four-port direction separator and is then fed via the first connection of the second four-port direction separator to the second fiber section; a second fiber amplifier, which is arranged between the second connection of the second four-port direction separator and the third connection of the first four-port direction separator and amplifies a second optical user signal which is supplied to the first connection of the second four-port direction separator and is then fed via the first connection of the first four-port direction separator to the first fiber section; a first pump signal being fed into the fourth connection of the first four-port direction separator, the first pump signal being coupled to the first fiber amplifier in a direction of the first optical user signal and transmitted in a signal direction; and a second pump signal being fed into the fourth connection of the second four-port direction separator, the second pump signal being coupled to the second fiber amplifier in a direction of the second optical user signal and transmitted in a signal direction.  
           [0010]    In this other embodiment, the first and second four-port direction separators may be based on beam splitter principles and the pump signals in each case pump the respective fiber amplifiers in a direction of each respective optical data signal. In either embodiment, the fiber amplifiers may also be pumped in an opposite direction to a transmission direction of respective optical user signals.  
           [0011]    An improvement in the noise response is achieved if the optical user signal and pump signal are propagated in the same direction and there are no feed devices for the respective pump signal to cause additional attenuation of the input user signals to the fiber amplifiers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Exemplary embodiments are explained in more detail with reference to figures, in which:  
         [0013]    [0013]FIG. 1 is a schematic circuit diagram of a bidirectional amplifier with an optical direction separator;  
         [0014]    [0014]FIG. 2 is a schematic diagram showing the construction of a first direction separator;  
         [0015]    [0015]FIG. 3 is a graph showing the pass bands and cut-off bands of this direction separator;  
         [0016]    [0016]FIG. 4 is a schematic diagram showing the construction of a second direction separator;  
         [0017]    [0017]FIG. 5 is a graph showing the pass bands and cut-off bands of this direction separator;  
         [0018]    [0018]FIG. 6 is a schematic circuit diagrams of a variant of a bipolar amplifier.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The bidirectional amplifier illustrated in FIG. 1 has two connections A and B. A first fiber section Fl of the transmission fiber is connected to the first connection A, and a second section F 2  is connected to the second connection B. The connection A corresponds to a first connection of a first direction separator RT 1 , which is in the form of a three-port device. This connection is fed with a first optical user signal α, which is emitted at the second connection  12  and is amplified in a first amplifier path, which comprises a first fiber amplifier EDF 1  with a downstream wavelength multiplexer (feed device) WDM 1 .  
         [0020]    The amplified user signal is supplied to a connection  23  of a second direction separator RT 2 , and is emitted via its first connection B. A second optical signal β is fed in via this connection, is supplied at the second connection  22  of the direction separator RT 2  to the series circuit comprising a second fiber amplifier EDF 2  and a second wavelength multiplex WDM 2 , and is emitted via a third connection  13  of the first direction separator RT 1  at the connection A.  
         [0021]    Filter arrangements are used as direction separators. Reflection filters may be used, provided different wavelength bands are used for the two transmission directions. The pump signals γ 1  and γ 2 , which may originate from one or more sources, are fed in via the wavelength multiplexers. The first pump signal γ 1  is fed in via the second wavelength multiplexer WDM 2 , and the first direction separator RT 1  in the same direction as the transmission direction of the optical signal α. In this arrangement, the optical signal α does not have to pass through any wavelength multiplexers, so that the user signal is not attenuated by such multiplexers, and the signal-to-noise ratio does not become poorer. In the same way, the second fiber amplifier EDF 2  is pumped by the pump signal y 2  via the first wavelength multiplexer WDM 1  and the second direction separator RT 2 . If the direction separators are compared with circulators, then they have a different direction sense for the pump signal than for the user signals. In principle, optical couplers may also be used instead of the wavelength multiplexers, but they have greater attenuation.  
         [0022]    [0022]FIG. 2 shows the fundamental design of the first direction separator RT 1  as a reflection filter. The optical signal α applied to connection A is not reflected, but is emitted at the output  12 . In contrast, the user signal β supplied to the connection  13  is reflected by the reflection filter and is emitted at the connection A, while the pump signal γ 1  is not reflected, it is emitted together with the optical user signal at the connection  12 , and is supplied to the fiber amplifier EDF 1 .  
         [0023]    [0023]FIG. 3 shows the transfer function of the filter as a function of the wavelength λ. For illustrative purposes, the attenuation or reflection (RE) is shown in addition to the transfer function TR (transmission). The signals α, β and γ 1  are represented in the form of bars.  
         [0024]    [0024]FIG. 4 shows the fundamental design of the second direction separator RT 2  as a reflection filter, and FIG. 5 shows the associated transfer function.  
         [0025]    [0025]FIG. 6 shows a variant of the bidirectional amplifier, which does not contain any wavelength multiplexers in the signal path. Four-port direction separators RT 3  and RT 4  are used instead of the selected-wavelength direction separators in the form of three-port devices. The optical user signals α, β are passed on in the illustrated manner from the four-port direction separators, as in the exemplary embodiment in FIG. 1, without any major attenuation.  
         [0026]    Each four-port direction separator has a fourth connection  34  or  44 , respectively. The amplifiers EDF 1  and EDF 2  can be pumped via these connections without any additional feed devices (wavelength multiplexers) being required in the signal path for this purpose. This avoids any attenuation losses in the user signals. The four-port directional couplers may, for example, operate on the beam splitter principle. This principle is known from “Optisch Nachrichtentechnik—eine Einführung” [Optical information technology—an introduction] by G. Grau, W. Freude, 3rd Edition, pages 282 and 283.  
         [0027]    Elements that are required for circuitry reasons, such as optical isolators and so on, are not shown in the outline circuit diagram. In multi-stage amplifiers, at least the first amplifier stage should also be pumped in the direction of the optical user signal.  
         [0028]    The above-described bidirectional amplifiers are illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those skilled in this art without departing from the spirit and scope of the present invention.