Abstract:
The effects of optical impairments on optical signal transmission are substantially reduced in a lightwave transmission system by positioning optical amplifiers and network elements in respective upstream-downstream combinations. By placing an optical amplifier at a position upstream from its corresponding network element, sufficient amplification can be provided by the optical amplifier to compensate for losses introduced by its corresponding network element. Advantageously, the corresponding downstream network element provides sufficient attenuation of the forward travelling lightwave signals so that power-dependent nonlinear effects in the optical fiber do not significantly distort the lightwave signals. Moreover, because of the downstream location of the network element in relation to its corresponding network element, the network element substantially suppresses backward travelling optical signal components such as those caused by Rayleigh backscattering, Stimulated Brillioun Scattering (SBS), and the like. As such, the network element prevents unwanted back reflections and back scattered signals from affecting operation of its corresponding optical amplifier.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of lightwave communication systems and, more particularly, to overcoming optical impairments in short haul lightwave communication systems employing optical amplifiers. 
     BACKGROUND OF THE INVENTION 
     To meet the increasing demands for more bandwidth and higher data rates, wavelength division multiplexing (WDM) is being used extensively in long haul optical transmission systems and is being contemplated for use in ,short haul applications, such as metropolitan area networks and the like. As is well known, WDM combines many optical channels of different wavelengths for simultaneous transmission as a composite optical signal in an optical fiber. 
     Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. Although rare earth-doped fiber optical amplifiers, e.g., erbium-doped fiber amplifiers, are commonly used in WDM systems, semiconductor optical amplifiers are being contemplated for use in WDM short haul applications like metropolitan area networks and so on. In particular, semiconductor optical amplifiers appear to be a viable alternative to the more costly erbium-doped fiber amplifiers. A typical short- haul WDM network includes a plurality of network elements interconnected by optical fiber. It is common to include optical amplifiers in the network elements in order to boost the lightwave signal power of signals traversing the optical fiber between network elements. However, the amount of amplification must be properly controlled because of optical impairments that are dependent upon signal power. In addition to affecting the transmission of optical signals, these optical impairments can also adversely affect the operation of optical components in the transmission path. 
     For example, Rayleigh backscattering is a well-known problem in which unwanted reflections are produced as an optical signal propagates through an optical fiber. In Rayleigh backscattering, the power level of the backscattered signals can be especially detrimental to the operation of optical amplifiers, such as semiconductor optical amplifiers, causing instabilities in operation, adding noise, and so on. 
     Non-linear effects can also cause problems in optical transmission. For example, Stimulated Brillioun Scattering (SBS) is a known phenomena which occurs when the power level of optical signals exceeds a certain threshold referred to as the SBS threshold. Briefly, SBS is a stimulated scattering process which converts a forward travelling optical signal into a backward travelling component of the optical signal which is also shifted in frequency. Among other problems, SBS results in increased backward coupling into optical components in the optical fiber path, which can affect operation of the components. For example, a backward travelling component can cause instabilities in optical amplifier operation. Other fiber non-linearities, e.g., four wave mixing, cross-phase modulation, self-phase modulation, Raman effect, and so on, are also well-known and can also be problematic in optical signal transmission. The network environment and topology are significant factors in determining when and to what extent the aforementioned problems will arise. Accordingly, proper design of a system is required in order to operate in the presence of such conditions. For example, long haul optical line systems typically have fiber spans of 80-120 kilometers between optical amplifiers without any intervening network elements. In these systems, optical isolators are typically employed to block unwanted back reflections of optical signals that would otherwise enter back through the output of the optical amplifiers. Moreover, the length of fiber spans serves to attenuate both forward propagating signals as well as backward travelling components, thereby reducing the occurrences of the aforementioned optical impairments. 
     By contrast, short haul optical systems, such as metropolitan area networks, have much shorter fiber spans. These short haul networks therefore cannot rely solely on the length of fiber spans to provide the necessary attenuation. Furthermore, these short haul networks typically have a higher density of network elements in a more geographically confined area, e.g., more closely-spaced optical amplifiers and network elements. Placing optical isolators at each network element location is very costly and highly undesirable in the cost-sensitive, short haul environment. 
     The challenges associated with operating optically amplified short haul networks in the presence of the aforementioned optical impairments are further complicated by the dynamic nature of short haul networks. For example, short haul WDM networks generally include a plurality of network elements capable of adding/dropping, routing, and cross-connecting optical signals. Losses introduced by these network elements can be significant. However, boosting gain of optical amplifiers to compensate for these losses can cause the system to be even more susceptible to the aforementioned optical impairments, e.g., exceeding SBS thresholds, causing higher intensity back reflections, and so on. 
     SUMMARY OF THE INVENTION 
     The effects of optical impairments on optical signal transmission in a lightwave transmission system are substantially reduced according to the principles of the invention by positioning optical amplifiers and network elements in respective upstream-downstream combinations. More specifically, by placing an optical amplifier at a position upstream from its corresponding network element, sufficient amplification can be provided by the optical amplifier to compensate for losses introduced by its corresponding network element. Advantageously, the corresponding downstream network element provides sufficient attenuation of the forward travelling lightwave signals so that power-dependent nonlinear effects in the optical fiber do not significantly distort the lightwave signals. Moreover, because of the downstream location of the network element in relation to its corresponding network element, the network element substantially suppresses backward travelling optical signal components such as those caused by Rayleigh backscattering, Stimulated Brillioun Scattering (SBS), and the like. As such, the network element prevents unwanted back reflections and back scattered signals from affecting operation of its corresponding optical amplifier. 
     Costs associated with installing and operating lightwave transmission systems are also substantially reduced according to the principles of the invention. In particular, costly optical isolator components are no longer needed at the output of every optical amplifier because the respective downstream network elements effectively perform an isolation function. Less expensive semiconductor optical amplifiers can also be used to further reduce system cost. These cost savings can be especially advantageous in the cost-sensitive short haul WDM network environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, with like elements referenced with like reference numerals, in which: 
     FIG. 1 is a simplified block diagram showing a short haul lightwave transmission system configured as an optical ring in which the principles of the invention may be practiced, 
     FIG. 2 is a simplified block diagram of one illustrative embodiment of the invention; and 
     FIG. 3 is a simplified block diagram of an exemplary system configuration used to verify the principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the illustrative embodiments described herein are particularly well-suited for a short haul wavelength division multiplexed (WDM) network having a plurality of network elements, and shall be described in this exemplary context, those skilled in the art will understand from the teachings herein that the principles of the invention may also be employed in conjunction with other types of lightwave communication systems and networks. Accordingly, the embodiments shown and described herein are only meant to be illustrative and not limiting. 
     FIG. 1 shows a simplified ring network  100  comprising a plurality of network elements  102 - 107  interconnected by optical fiber  101 . Ring network  100  could be used, for example, in short haul network environments such as metropolitan optical networks, local area networks, and so on. Ring network  100  can also support single wavelength optical communications or multi-wavelength optical communications employing WDM techniques. For simplicity of explanation, the embodiments of the invention will be described in the context of WDM transmission. As such, optical fiber  101  of ring network  100  carries a composite WDM optical signal comprising a plurality of individual optical channels of different wavelengths. 
     As is well-known, network elements  102 - 107  may be configured to perform one or more different functions such as, for example, adding and dropping optical signals, cross-connecting optical signals, and so on. Consequently, each of network elements  102 - 107  may not necessarily be equivalent in function or structure. As shown in FIG. 2, it is assumed for purposes of describing the embodiments of the invention that network element  102  has the functionality of an optical add/drop device capable of adding and dropping individual optical channels from a composite WDM signal. 
     FIG. 2 illustrates the principles of the invention in one exemplary embodiment. In particular, FIG. 2 illustrates the upstream-downstream combination of optical amplifiers and network elements in lightwave communication system  200 . More specifically, the first upstream-downstream combination  220  includes optical amplifier  201  and network element  102  and the second combination  250  includes optical amplifier  251  and network element  103 . For simplicity of illustration and explanation, only two combinations are shown in FIG. 2, however, any number of combinations may be utilized. Optical fiber  255  represents the span length between the output of network element  102  and the input of optical amplifier  251 . In a short haul metropolitan optical network, for example, the span length could be on the order of approximately 5-20 kilometers. 
     As shown, optical amplifier  201  is coupled along optical fiber  101  at a location upstream from network element  102 . Optical amplifier  201  can be any type of optical amplifier including, but not limited to rare earth-doped fiber optical amplifiers, such as erbium-doped fiber amplifiers, semiconductor optical amplifiers, and so on. The embodiments shown and described herein will refer to semiconductor optical amplifiers to illustrate another aspect of the invention, i.e., a low cost solution for short haul lightwave communication systems. The operation of optical amplifiers, including semiconductor optical amplifiers, is well-known and will not be described in detail herein. 
     As shown in FIG. 2, network element  102  receives an amplified input WDM signal, drops one or more selected optical channels  202  via optical demultiplexer unit  210 , adds one or more selected optical channels  203  via optical multiplexer unit  211  and transmits an output WDM signal in a downstream direction for the next network element  103  in the system. Various well-known devices can be used for optical demultiplexer unit  210  and optical multiplexer unit  211  such as, for example, waveguide grating routers, thin film filters, fiber Bragg gratings in conjunction with optical circulators or directional couplers, and so on. As such, the detailed structure and operation of optical demultiplexer unit  210  and optical multiplexer unit  211  will not be described in detail herein. 
     Regardless of whether network element  102  is an add/drop node as depicted in FIG. 2 or any other type of network element, e.g., cross-connect, it will be appreciated by those skilled in the art that network elements of any type typically introduce losses to the optical signals being processed therethrough. For example, these losses may be insertion losses from optical demultiplexer unit  210  and optical multiplexer unit  211  as well as signal losses associated with the adding and dropping of channels, and so on. As will be described in more detail below, losses introduced by network element  102  are utilized in an advantageous manner according to the principles of the invention. 
     Using prior arrangements for comparison, an optical amplifier is typically used to boost the optical signal power being output from a network element to compensate for losses introduced by the network element as well as to compensate for losses that are expected in the optical fiber path between network elements. As previously described, these schemes fall short in at least two respects. First, optical isolators are typically required at the output of each optical amplifier to prevent unwanted back reflections and back scattered signals from adversely affecting operation of the optical amplifiers. Given the density of network elements and optical amplifiers in a short haul network, optical isolators can raise system costs considerably. Second, the amount of output power launched into the optical fiber from the optical amplifier must be carefully controlled in order to avoid the aforementioned problems such as SBS, fiber non-linearities, and so on. 
     By contrast, a system according to the principles of the invention overcomes these limitations. As shown and described herein, placing semiconductor optical amplifier  201  at a location upstream from network element  102  has several benefits. First, the amount of gain and output power provided by semiconductor optical amplifier  201  can be selected to compensate for the loss that will be subsequently contributed by its corresponding downstream network element  102  as well as the loss expected in the fiber span between network element  102  and optical amplifier  251 . Consequently, the amount of gain and output power provided by semiconductor optical amplifier  201  is a matter of design choice in view of considerations such as the amount of loss contributed by network element  102 , the span length of optical fiber between network element  102  and semiconductor optical amplifier  251 , and the operating parameters of semiconductor optical amplifier  201 . Obviously, less lossy network elements could offset longer span lengths and visa versa. 
     Second, the loss introduced by network element  102  will ensure that the power levels of the amplified signals propagating forward do not exceed thresholds associated with the aforementioned fiber non-linearities. More specifically, the power of optical signals launched into optical fiber  255  will be reduced so that the aforementioned thresholds are not exceeded and so that the adverse effects associated with the fiber non-linearities are not triggered. In prior arrangements, this is typically accomplished using separate power control schemes, dithering signals, or attenuating output power, each of which adds cost (e.g., more components) and complexity to the system. 
     Third, any unwanted back reflections (e.g., from Rayleigh backscattering, SBS, and so on) will be substantially suppressed, e.g., effectively blocked, by network element  102  prior to entering back into semiconductor optical amplifier  201 . 
     So, in operation, the fiber span losses of system  200 , the network element losses, operating parameters of optical amplifiers  201  and  251 , thresholds for non-linear effects (e.g., SBS), and so on would be known design variables. Using this data, optical amplifier gain and output power of optical amplifier  201  can then be selected accordingly to compensate adequately (e.g., maintain an acceptable optical signal to noise ratio, etc.) for: 1) the fiber span loss up to the next upstream-downstream combination  250 , 2) the loss introduced by network element  102 , 3) the thresholds associated with non-linear effects (e.g., SBS threshold), and so on. Determination of the SBS threshold itself will depend on several factors including, but not limited to, type of optical fiber being used. Accordingly, the gain of optical amplifier  201  is a matter of design choice in view of the above factors. 
     The signal launched from optical amplifier  201  into optical fiber  101  will have relatively high power based on the selected gain and output power of optical amplifier  201 . Network element  102  receives this signal and introduces losses as previously described. As such, the signal now launched from network element  102  into optical fiber  255  is attenuated by network element  102  (e.g., reduced in magnitude to have a lower power level) and then further attenuated along optical fiber  255  so that is lower than the SBS thresholds and the like. However, the gain and output power is selected so that the optical signals still have enough power to arrive at optical amplifier  251  with sufficient optical signal to noise ratio and so on. Any back reflected optical signals, whether from Rayleigh back scattering, SBS, or otherwise, will be substantially suppressed (e.g., blocked) by network element  102 . As such, the operation of optical amplifier  201  will not be adversely affected by these back reflections. 
     The following example describes an experiment performed to demonstrate the principles of the invention described above. In general, parameters were selected to be representative of a metropolitan area network. However, it should be noted that the various arrangements, devices, materials, dimensions, parameters, operating conditions, etc., are provided by way of illustration only and are not intended to limit the scope of the invention. 
     EXAMPLE 
     More specifically, FIG. 3 shows an experimental system configuration for confirming the principles of the invention. In this exemplary configuration and experiment, system  300  includes a first semiconductor optical amplifier  301 , a variable attenuator element  302  for simulating a network element, and a second semiconductor optical amplifier  351 . As shown, optical fiber span  355  couples an output of variable attenuator element  302  to an input of semiconductor optical amplifier  351 . For purposes of this experiment, semiconductor optical amplifiers  301  and  351  comprised commercially available, single stage, non-gain clamped amplifiers and optical fiber span  355  comprised commercially available single mode fiber, such as AllWave™ transmission fiber from Lucent Technologies. 
     In the configuration shown in FIG. 3, optical fiber span  355  had a span length of approximately 42 kilometers. Therefore, at approximately 0.2 dB loss per kilometer, the total loss of optical fiber  355  was approximately between 8 dB to 9 dB. Variable attenuator element  302  was used to simulate the loss introduced by a network element of approximately between 6 dB to 7 dB. For satisfactory operation of system  300 , the input power to semiconductor optical amplifier  301  was approximately −3 dBm and the gain was set at approximately 15 dB (e.g., approximately 5 dB below the small-signal gain). As such, the output power launched from semiconductor optical amplifier  301  was approximately 12 dBm. After accounting for loss introduced by network element  302 , the signal power launched into optical fiber  355  was approximately 5 dB to 6 dB, which is well below the typical SBS thresholds for single mode optical fibers. As is well-known, the SBS threshold is typically the lowest of all the thresholds associated with optical impairments. After accounting for loss introduced by optical fiber span  355 , the signal power at the input of semiconductor optical amplifier  351  was approximately −3 dBm. 
     So, by utilizing the upstream-downstream combination of optical amplifier  301  and network element  302  according to the principles of the invention, the output power launched into optical fiber span  355  was maintained below the thresholds for triggering the aforementioned non-linear effects and unwanted back reflections were effectively blocked by network element  302  from entering back into optical amplifier  301 . 
     Accordingly, a system designed and operated according to the principles of the invention does not require expensive optical isolators to protect against back reflections and back scattering. The use of semiconductor optical amplifiers further reduces system costs as compared to doped fiber amplifiers. More specifically, the unique upstream-downstream combination of semiconductor optical amplifiers and network elements utilizes the passive loss characteristics of the network elements instead of using separate optical components to introduce post-amplification loss. As such, the principles of the invention can be applied with the existing system architecture. 
     The foregoing is merely illustrative of the principles of the invention. Those skilled in the art will be able to devise numerous arrangements, which, although not explicitly shown or described herein, nevertheless embody those principles that are within the spirit and scope of the invention. For example, although the illustrative embodiments were described in the context of WDM ring networks utilizing semiconductors optical amplifiers, the principles of the invention may be employed with any type of lightwave transmission system that would benefit from a system architecture in which costly components can either be removed or replaced with less costly components, thereby reducing overall system costs. Accordingly, the invention is only limited by the claims that follow.