Abstract:
A bandstop filter method and system for suppressing ASE noise is disclosed. In one embodiment of the bandstop filter system, there is provided a first switch, having an input port for receiving an optical signal, and two output ports; a bandstop filter coupled to a first of the two output ports of the first switch; and a second switch, having two input ports and one output port, wherein a first of the two input ports of the second switch is coupled to the bandstop filter and a second of the two input ports of the second switch is coupled to a second of the two output ports of the first switch.

Description:
FIELD OF THE INVENTION 
   Embodiments of the present invention relate to optical telecommunication systems and, in particular, to a method and system for suppressing ASE noise using a bandstop filter. 
   BACKGROUND OF THE INVENTION 
   Optical amplifiers such as erbium doped fiber amplifiers (EDFAs) or semiconductor optical amplifiers (SOAs) are employed in optical transmission systems to amplify optical signals along the optical transmission path. In one application, optical amplifiers are used in optical receivers prior to photo detection. While an optical signal is being amplified, unwanted Amplified Spontaneous Emission (ASE) noise may be generated. In many cases, the ASE noise will degrade receiver performance. 
   For example, if an optical amplifier has a wide amplification bandwidth and ASE noise is not sufficiently optically filtered after amplification, ASE noise may present a problem. For those cases, additional filtering of the ASE noise is necessary to achieve good sensitivity of the pre-amplified receiver. The total ASE power is one figure of merit for the sensitivity that can be achieved for this pre-amplified receiver. 
   Known noise filtering is generally performed using one of gain flattening filters, fixed wavelength band pass filters with the signal centered on the band pass filter, band-edge filters for ASE filtering, or continuously tunable band pass filters. These known noise filters have certain disadvantages. 
   For example, a gain flattening filter is used to achieve some degree of ASE suppression. But the total ASE power seen at the output is still unacceptably high for large gain/wide bandwidth amplifiers. Fixed-wavelength filters do not work if the signal wavelength changes and the receiver is wavelength independent over a large wavelength range (e.g., WDM systems). Like fixed wavelength filters, a band-edge filter limits the achievable wave length range. In this case, it is limited to some range outside the gain peak and thus the ASE peak of the optical amplifier. Regarding continuously tunable band pass filters, although technically sound, this approach is costly and bulky in deployed systems. 
   In the laboratory, one would use an optical amplifier and a general filter to remove, or at least minimize, ASE noise. For system implementation, one would use gain flattening filters to remove some, if not all, of the ASE noise between the stages of a multistage optical amplifier. Here, again, one only can attenuate the ASE noise to some degree using these types of filters. One cannot remove all of the ASE noise. 
   Thus, there is a need for improved ASE noise suppression in broadband optical telecommunication systems. 
   SUMMARY 
   Various deficiencies of the prior art are addressed by embodiments of the present invention of a bandstop filter system for ASE suppression. 
   In accordance with an embodiment of the present invention, there is provided a bandstop filter system for suppressing ASE noise, comprising a first switch, having an input port for receiving an optical signal, and two output ports; a bandstop filter coupled to a first of the two output ports of the first switch; and a second switch, having two input ports and one output port, wherein a first of the two input ports of the second switch is coupled to the bandstop filter and a second of the two input ports of the second switch is coupled to a second of the two output ports of the first switch. 
   In accordance with another embodiment of the present invention, there is provided a bandstop filter system for suppressing ASE noise, comprising an optical coupler, having an input port for receiving an optical signal, and two output ports; a bandstop filter coupled to a first of the two output ports of the optical coupler; and a switch, having two input ports and one output port, wherein a first of the two input ports of the switch is coupled to the bandstop filter and a second of the two input ports of the switch is coupled to a second of the two output ports of the coupler. 
   In accordance with yet another embodiment of the present invention, there is provided a free space optics device for suppressing ASE noise, comprising a fiber segment for transmitting an optical signal, the fiber segment having a first portion and a second portion; first and second lenses coupled between the first and second portions of the fiber segment; and a movable bandstop filter, adapted to be positioned between the first and second lenses. 
   In accordance with a further embodiment of the present invention, there is provided a method for suppressing ASE noise in a single channel optical amplifier; comprising receiving an optical signal for transmission to an input of the optical amplifier; determining the wavelength region in which the received optical signal falls; and coupling a bandstop filter to the output of the optical amplifier if the determined wavelength region of the optical signal is outside the gain peak of the optical amplifier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a bandstop filter system in accordance with an embodiment of the present invention; 
       FIG. 2  depicts a bandstop filter system in accordance with another embodiment of the present invention; 
       FIG. 3  depicts a bandstop filter system in accordance with yet another embodiment of the present invention; 
       FIG. 4  depicts a graph of a simulated optical power spectrum for each input wavelength at the optical amplifier output; 
       FIG. 5  depicts a graph of the total output power, signal output power and ASE output power of an XL-band optical amplifier with a −12 dBm input power and 170 mW total pump power; 
       FIG. 6  depicts a graph of an optical power spectrum for each input wavelength at the optical amplifier output in accordance with an embodiment of the present invention; and 
       FIG. 7  depicts a graph of the total output power, signal output power and ASE output power of an XL-band optical amplifier with −12 dBm input power and 250 mW total pump power in accordance with an embodiment of the present invention. 
   

   To facilitate understanding, identical or similar reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a segment of a long haul optical telecommunication segment  100 . The segment is shown at a point in an optical transmission system after a multi-signal optical transmission has been demultiplexed into individual wavelengths or signals. In this embodiment, the segment  100  includes an optical amplifier  102  adapted to receive any individual signal from a demultiplexed broadband optical transmission (single channel). The system further includes a transmission line  110  for coupling the optical amplifier  102  to the input of an optical switch  104 . The switch  104  includes one input and two outputs, although alternative switching configurations are contemplated by the present invention. Coupled to the two outputs of the switch  104  are a first optical path  112  and a second optical path  114 . The first optical path  112  couples the upper output of the switch  104  to a bandstop filter  106 . The segment  100  further includes a second optical switch  108  having two inputs and one output, although alternative switching configurations are contemplated by the present invention. The two inputs of the second optical switch  108  are coupled to the output of the bandstop filter  106  and the optical path  114 , respectively. 
   An incoming signal will be directed through either one of the two optical paths shown in this embodiment depending on whether the incoming signal is in a first or second region. That is, the optical amplifier  102  is capable of operating in single-channel operation mode. In this configuration, there are two distinguishable wavelength regions. First, there is a region in and around the gain peak of the optical amplifier  102 . Second, there is a region outside the gain peak (less than the lower wavelength range and greater than the higher wavelength range) of the optical amplifier  102 , where the gain of the optical amplifier is substantially lower than the peak gain. 
   For example, a signal may be directed through an optical path of the optical segment  100  with the switches  104  and  108  in their upper positions (i.e., the top output and input of switches  104  and  108 , respectively). Here, a one wavelength signal would propagate through optical amplifier  102 , optical path  110 , switch  104  in its upper position, optical path  112 , bandstop filter  106 , and switch  108  in its upper position. 
   A signal may be directed through a second optical path of the segment  100  with the switches  104  and  108  in their lower position (i.e., the bottom output and input of switches  104  and  108 , respectively). Here, a one wavelength signal would propagate through the optical amplifier  102 , optical path  110 , switch  104  in its lower position, optical path  114 , which does not include the bandstop filter, and switch  108  in its lower position. 
   Thus, if an incoming signal is within the first region, then the gain of the optical amplifier  102  is relatively high, and the power generated from the ASE noise from the receiver pre-amplifier is relatively low. Therefore, no additional filtering is necessarily required. In this case, the signal would travel the lower path—optical amplifier  102 , optical path  110 , switch  104  where the switch is in the lower position, then optical path  114  to the second switch  108 , where that switch  108  is in the lower position. 
   If, on the other hand, a signal falls in the lower-gain region, the ASE power generated mainly around the amplifier gain peak will be high at the output if filtering is not performed. Not filtering would result in performance penalties. As most of the ASE noise is concentrated in the gain peak region and the signal is outside of this region, a bandstop filter can be inserted in the optical path in this case, improving the ASE characteristics while leaving the signal unchanged. In this regard, the path would be as described above: optical amplifier  102 , optical path  110 , switch  104  in a top position, optical path  112 , bandstop filter  106  and second switch  108  in the top position. For inserting and removing the bandstop filter, several techniques can be used; for example, two 1:2 switches (namely switch  104  and switch  108 ) may be employed as shown in  FIG. 1 . Other embodiments are shown in  FIGS. 2 and 3 . Yet further alternative embodiments are contemplated by the present invention in which a bandstop filter is adapted to be coupled in and out of an optical path, depending on the signal being amplified by the optical amplifier. 
   If the input signal is within the bandstop characteristics of the filter and the lower path along optical path  114  is selected, the signal passing through the amplifier  102  is amplified and there is no need for additional filtering. In this case the bandstop filter  106  is set to the same wavelength as the gain peak of the amplifier  102 . In this mode, there will not be a high ASE noise level to filter out. Therefore, no filtering is necessarily required. 
   On the other hand, if the input signal is outside of the gain peak of the amplifier  102 , then there will be ample ASE noise around the gain peak and one would be best to filter that out. In this case, the switches are moved to select the upper path, i.e., optical path  112 . In this position, the signal is amplified by the amplifier  102 , passes the bandstop filter  106 , and is not affected by the filter. However, the ASE around the gain peak of the particular amplifier is filtered out by the filter  106 . The operation and control of the switches may be through the use of the internal controller of the optical amplifier such as an EDFA or SOA. 
     FIG. 2  depicts an alternative embodiment where the segment  200  may comprise an optical amplifier  202 , an optical path  210  that couples the optical amplifier  202  to the input of an optical coupler  204 , a first optical path  212  that couples an output of the optical coupler  204  with a bandstop filter  206 , and the upper position input of a switch  208 . The signal would travel through the second path by way of the optical amplifier  202 , the optical path  210 , the coupler  204 , and a second path  214  to the lower position input of the switch  208 . The optical path chosen in this embodiment is similarly determined as with respect to the segment  100  discussed with respect to  FIG. 1 . The coupler  204  may be a 3 dB coupler, which parses the incoming optical signal into two parts. 
   The output signal from the segment  200  is similar to the output signal of the segment  100  of  FIG. 1 . The components used are somewhat different. For example, instead of a first switch, there is provided the coupler  204 . Here, an input signal is sent from the amplifier  202  output along path  210 . Then, the signal is separated into two signals by the coupler stage  204  into an optical path  212  and an optical path  214 . In this scenario, the signal passes through the filter  206  and through the lower path  214 . By using the switch  208 , the end user can determine which of the paths is necessary. The control of the switch may be performed, as discussed above, by way of the internal controller of the optical amplifier. So, if the signal is within the band, the switch  208  switches to the lower path and one does not have to use the bandstop filter  206 . On the other hand, if the signal is outside of the band, then the switch  208  switches to the upper path  212  in order to filter out the ASE noise. The present invention contemplates a multistage amplifier set up as well. For example, there may be another stage of amplifiers after the switch  208 . 
   In an alternate embodiment, as depicted in  FIG. 3 , a similar segment  300  is achieved through free space optics combined with a mechanical insertion/removal of a filter. In  FIG. 3 , there is provided an optical amplifier  302 . Regarding  FIG. 3 , in operation, it is the same concept as that used in  FIGS. 1 and 2  but instead of using one or two switches to select a path, the system uses a micro optic setup. In this regard, there is the optical amplifier  302  and the micro-optic setup  304 . 
   The micro-optic setup  304  includes fiber portions  306 , lenses  308  and a filter  310  therebetween. In this embodiment of the present invention, the fiber  306  and the lenses  308  are positioned such that the signal travels from the first fiber portion  306 , through the first lens  308  to the second lens  308  and the second fiber  306 . If desired, the filter  310  can be inserted into the optical path or can be removed from the optical path. It is a mechanical movement of the filter  310 , which may be, for example, a thin film filter. Alternatively, the filter could be a multi-stage Fabry-Perot filter. 
   In this embodiment, when the filter  310  is in-line with the signal, the filter acts as a bandstop filter. Therefore, if the signal is within the bandstop filter characteristics of the bandstop filter, the signal can travel through the lenses without having to travel through the filter  310 . If, on the other hand, the signal is outside of the gain characteristics of the bandstop filter and the user needs to remove the ASE, the filter  310  is mechanically moved into the optical path. 
   In  FIGS. 2 and 3 , both outputs can be used simultaneously for multi-channel applications. In  FIG. 2 , both paths  212  and  214  would be received. In the case of  FIG. 3 , the system would use a reflective part of the signal. 
   The above embodiments are typically used at the receive side of an optical transmission system. These are independent of the optical network. The optical transmission may use DWDM. As such, the segments discussed above, would be placed after a demultiplexer in order to receive a single channel at a time. 
   Several techniques can be used to accomplish the bandstop function. For example, a thin film filter such as a bandstop filter used for Coarse Wavelength-Division Multiplexing (CWDM) applications can be employed. In the alternative, a multi-stage Fabry-Perot type filter may be used. 
     FIGS. 4-5  are graphs of a simulation depicting the use of a gain flattening filter (GFF).  FIGS. 6-7  depict graphs of the results of signals traveling in one of the embodiments discussed above. 
   The output spectrum of an L-band optical Erbium Doped Fiber Amplifier (EDFA) using a conventional GFF is depicted in  FIG. 4 . Fifty-five different input wavelengths are simulated. All different output spectra are shown in the same plot. The signals are simulated one wavelength at a time but they are overlaid in this diagram.  FIG. 4  depicts a simulation of the signal passing through an amplifier as described above. Depending on the properties of the amplifier, the output includes the amplified signal and the ASE noise signal. The signal peaks shown in  FIG. 4  are the individual signals. The third curve is the ASE noise signal. The gain peak is around 1565 nanometers (nm) in this particular simulation. 
     FIG. 5  depicts a graph showing the corresponding total ASE output power (dashed line), signal output power (solid line) and the total output power (dotted line). Here, only the integral sums of each of the curves from the plot in  FIG. 4  are shown. The X-axis represents the signal wavelength (in nm).  FIG. 5  shows total output power, which is the output curve. Here, the amplifier is designed to have a fixed total output power of about 10 dBm for any input signal wavelength. The signal power is highest around the gain peak and it decreases at the edges of the bandwidth of the optical amplifier. So, here the ASE power comes up and the signal power drops on the edges. Thus,  FIGS. 4 and 5  depict a simulation using a GFF. In contrast,  FIGS. 6 and 7 , as discussed herein, depict graphs of embodiments using the bandstop filter system of the present invention. 
   The same two graphs for an EDFA, for example, using the improved filter system in accordance with embodiments of the present invention are depicted in  FIGS. 6 and 7 , respectively. The distance between total ASE power and signal power is much larger in the latter case using the improved bandstop filter system of the present invention. In  FIGS. 6 and 7 , a bandstop filter is used. However, depending on the signal wavelength, the bandstop filter is inserted after the optical amplifier or is removed from the optical path. Then, if the signal is outside of the bandstop filter characteristic, there is a different behavior. That is, the signals inside the bandstop filter yield similar behavior as in  FIG. 4 . 
   A range of wavelengths, for example, would be between about 1560 nm (lowest wavelength) to about 1583 (highest wavelength). For every signal lower than about 1559 nm and/or higher than about 1584 nm, the bandstop filter is inserted into the optical path.  FIGS. 6 and 7  depict graphs showing an input signal in the above regions as it passes through any one of the segments discussed with respect to  FIG. 1 ,  2  or  3 . The ASE noise drops between about 1560 to 1580 nm for the reasons discussed herein. Outside of those bands, i.e., about 1559 nm and 1581 nm, the ASE noise drops considerably. 
   The wavelength passing through any one of the segments is known because a microcontroller circuit (not shown) is used and the received wavelength is provisioned by a network operator or the circuit detects the signal wavelength when it is being received as discussed above with respect to the optical amplifier&#39;s internal controller. In these types of high bit rate optical transmission systems, the signal has to be amplified with a very high gain amplifier. This is one reason ASE noise is significant enough to cause a problem. As the bit rate increases, it gets more and more likely that there will be such a noise problem. 
   In an alternative embodiment, the segments shown in  FIGS. 1 through 3  can be part of a multi-stage optical amplifier system. Thus, there may be another stage of amplifiers after the switch  108  or  208 . There may also be another stage of amplifiers after the fiber module  304 . One of ordinary skill in the art would recognize that if a single-stage amplifier is discussed, it could equally be used in a multi-stage amplifier setup after the second switch. 
   Combining the advantages of a fixed wavelength filter with the application scope of a continuously tunable filter in the context of optically pre-amplified receivers is an advantageous result of the embodiments of the present invention. 
   In addition, use of the internal controller of the EDFA to control the switching (if required by the received wavelength) is also advantageous. Using an EDFA or other optical amplifier with gain peak substantially of the same signal wavelength is novel. Exploiting the restriction to a single channel amplifier to modify the gain profile of the amplifier drastically thereby improving the performance is also novel. The sensitivity of the optically pre-amplified system is improved over using a standard low-cost design approach (e.g., using GFFs inside an EDFA). 
   While the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the present invention is to be determined according to the claims, which follow.