Abstract:
A four-port Wavelength-Division-Multiplexing (WDM) element is disclosed that can be employed for various purposes in optical communication networks, particularly in bi-directional fiber optic lines. In one embodiment, one four-port WDM element is used with one optical amplifier to provide bi-directional propagation on the same fiber. In another embodiment, the four-port element is used in conjunction with optical attenuators to provide for gain equalization of channels and the flexibility to add or remove channels. In yet another embodiment, input and output ports are added to current-art elements thereby enabling bi-directional operation to be achieved with equipment that does not provide such capabilities.

Description:
This invention pertains to fiber communication systems and more particularly to the transmission and routing of several multiplexed wavelengths on fiber carrying light simultaneously in two directions of propagation. 
     BACKGROUND OF THE INVENTION 
     Fiber optic networks include a variety of optical components such as transmitters, receivers, optical amplifiers and Wavelength Division Mulitplexers (WDMs). The amount of data that can be transmitted by these networks depends on the equipment bandwidth and the optimization of all the parts to support this bandwidth. Recently, service providers have had to double their system&#39;s capacity approximately every three years due to the increases in communication traffic. A common method of increasing network capacity and system bandwidth is to upgrade the Time Division Multiplexed (TDM) equipment along the transmission path. For example, 10 Gbit/sec systems are being installed today for high-speed networks, replacing slower speed channels with data rates such as 622 Mbit/sec. Another cost effective means for increasing system bandwidth is to install equipment for wavelength division multiplexing (WDM). 
     In a WDM system, multiple wavelengths ki (where “i” is an integer index) are transmitted on the same fiber, and each wavelength is used as an optical carrier for a TDM channel, such as a 2.5 or 10 Gbit/sec channel. All channels pass through similar optical components, such as fibers, WDMs and inline amplifiers. One important parameter in optical networks is the optical gain and loss experienced by individual channels as they propagate from the transmitting to the receiving station. For amplifiers, this parameter is termed Amplifier Gain Flatness. An asymmetry in net gain or loss between these channels leads to a reduction in overall system performance. Hence it is important to equalize and reduce the total loss of these channels as well as reduce the asymmetry in overall amplifier gain when all channels are present simultaneously, including when certain channels arbitrarily disappear due to equipment failure. 
     Channel routing is often performed in WDM networks. A common routing method shown in FIG. 1A employs three-port WDM elements, each with three ports P 1 , P 2 , P 3  and an internal filter F 1 . These elements are available commercially from companies such as E-Tek of San Jose, Calif. and DiCon Fiberoptics of Berkley, Calif. If more than one channel is to be routed, then several of these elements are cascaded serially. However, channels that are routed first pass through fewer components and hence have less loss than those routed later. Hence, channel losses are asymmetric, when reduces overall system performance, particularly if channels are added later to the network. FIG. 1B shows how several three-port WDM elements are integrated into a single multi-channel element for routing four wavelengths k 1 , k 2 , k 3 , k 4  through six pots P 1 -P 6 . Such elements have the same disadvantages as the three-port element of FIG.  1 A. Multi-channel elements of this type are available from companies such as Optical Corporation of America of Marlborough, Mass. 
     It is generally accepted that network configurations that allow bi-directional transmission on the same fiber offer several advantages. Referring to FIG. 2, there is shown a prior art, bi-directional amplifier system, disclosed in U.S. Pat. No. 5,604,627 (Kohn), that employs two three-port WDM elements. In this system, wavelength bands k 1  and k 2  are defined for each direction of propagation and the three-port WDM elements WDMi are configured to deflect a respective wavelength band ki from a common input port to the appropriate optical amplifier Ai. The disadvantage of this model is that it requires the use of two amplifiers and two WDM elements for bi-directional operation on the same fiber, which in-turn increases system complexity and cost. 
     Referring to FIG. 3, there is shown a less-costly system configuration that is employed by BOSCH in its ONS-100 Optical Networking System. In this system, one amplifier and four three-port WDM elements are used in such a manner that the optical signal is transmitted through the amplifier in the same direction, regardless of the direction of transmission in the network, i.e. east to west or west to east. This configuration allows for bi-directional propagation on the same fiber using only one amplifier. However, the BOSCH system also increases the number of required WDM elements from two to four. The elimination of an expensive amplifier element reduces system cost, but doubling the number of WDM elements pushes system cost back up again. 
     Advantage of bi-directional amplifiers and a specific embodiment of a bi-directional amplifier configuration are also disclosed in U.S. Pat. No. 5,633,741 (Giles). Giles teaches the use of two four-port optical circulators C 1  and C 2  in conjunction with fiber gratings G 1  and G 2  and amplifiers A 1  and A 2  to achieve a bi-directional amplifier system as shown in FIG.  4 . The disadvantage in such a method is that optical circulators, which are commercially available from companies such as JDS Fitel of Nepean, Canada, and The Kaifa Group of Sunnyvale, Calif., are complex and costly optical elements. The complexity of optical circulators and fiber gratings, in addition to the doubling in required number of components for bi-directional operation leads to an increase in overall system complexity and cost. Fiber gratings are specialized optical components and are commercially available from companies such as Lucent Technologies of Allentown, Pa. 
     The drawback of the devices shown in the prior art is their failure to provide a low-cost system (e.g., by minimizing the number of parts) or to enable the control of multi-channel gain and loss equalization for bi-directional operation on the same fiber. The objectives of this invention are therefore: (1) to reduce overall system cost by reducing the number of deployed amplifier and WDM elements and simplifying their manufacturability, (2) to equalize the losses incurred by all channels using alternate configurations, regardless of which channels are added or changed in the future, and (3) to provide a provision to flatten net amplifier gain for all wavelengths by incorporating other optical elements such as attenuators. It is a further objective of this invention to remedy the general drawbacks described in the background section. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention is a four-port WDM element and a number of variations thereof that can be used as a principal component of a bidirectional optical amplifier. 
     In particular, the four-port WDM elements of the present invention can be used to add/remove any arbitrary channel or number of channels to/from a WDM communications network while maintaining similar insertion losses for all propagating channels. In one embodiment, attenuators and/of filters are employed by the WDM element to flatten the gain profile of any amplifier in which the WDM is incorporated. A four-port WDM element can be combined with a single amplifier element to form a bi-directional optical amplifier. Advantages of such an optical amplifier include: reduced cost due to simplified design, equalized losses of all channels, and flattened amplifier gain across all wavelengths. 
     An internal configuration of a basic embodiment of a four-port WDM element is shown in FIG.  5 A. This WDM element resembles an “X”, where each optical port Pi lies along a respective arm of the “X” and a filter F is inserted at the intersection of the arms. Light launched into a first port (e.g., P 1 ) is directed to the filter F, which selectively reflects certain channels or wavelengths to a receiving second port (e.g., P 2 ). A third port (e.g., P 3 ) receives the channels in the light launched into the first port P 1  that are transmitted by the filter F. Similarly, light that is launched into the third port P 3  is partially transmitted to the first port P 1  by the filter F. The other channels in the light launched into the third port P 3  are reflected by the filter F and are collected by a fourth port (e.g., P 4 ). The resulting four-port WDM is symmetrical, meaning that light launched into the second and fourth ports behaves analogously to light launched into the first and third ports, respectively. 
     The present invention also includes various embodiments of bi-directional optical amplifiers that can be formed using the WDM elements of the present invention. In a basic bi-directional optical amplifier embodiment, shown in FIG. 7, the second and third ports of the WDM are coupled to an amplifier element A 1 . The ports P 1  and P 4  receive communication signals traveling in opposite directions. Each input signal can include multiple channels at different wavelengths. The filter characteristics are selected so the light input to the first port P 1  is reflected to the second port P 2  and then amplified by the amplifier element A 1 . The light input to the fourth port P 4  is transmitted by the filter to the second port P 2  and then passes through the amplifier A 1  in the same direction as the light input to the first port P 1 . Thus, the four port WDM of the present invention enables the implementation of a bi-directional amplifier that requires only one WDM and one amplifier element. 
     Other disclosed embodiments of bi-directional amplifiers can be used to attenuate or add selected channels and amplify any number of channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which: 
     FIG. 1A shows a prior art WDM element that includes three ports P 1 , P 2 , P 3  and a filter F; 
     FIG. 1B shows a multi-port WDM element built using the three-port WDM element of FIG. 1A; 
     FIG. 2 shows a prior art bi-directional amplifier built using two three-port WDM elements WDM 1  and WDM 2  and two amplifiers A 1  and A 1 ; 
     FIG. 3 shows a prior art bi-directional amplifier using four three-port WDM elements WDM 1 -WDM 4 , and one amplifier A 1 ; 
     FIG. 4 shows a prior art bi-directional amplifier using two four-port optical circulators C 1  and C 2  and two amplifiers A 1  and A 2  with fiber gratings G 1  and G 2  for gain flattening; 
     FIG. 5A shows a preferred embodiment of a four-port WDM element (with ports P 1  to P 4  and a filter F) in accordance with the present invention; 
     FIG. 5B shows a simplified configuration of the four-port WDM of FIG. 5A; 
     FIGS. 6A and 6B show alternative embodiments of the present invention shown in FIGS. 5A and 5B, where variable optical attenuators R 1  and R 2  and ports P 5  and P 6  are added to selectively attenuate channels for gain flattening applications of optical amplifiers; 
     FIG. 6C illustrates an embodiment of the present invention where elements  140  from FIG. 6A are serially connected; 
     FIG. 7 shows an alternative embodiment of the present invention, a bi-directional amplifier; 
     FIG. 8A shows an alternative embodiment of the present invention, employing a single amplifier and three four-port WDM elements that are connected together to achieve bi-directional operation for a number of multiplexed channels; 
     FIG. 8B shows an exemplary transmission schematic for the filter used in the four-port WDM element WDM 7  of FIG. 8A; and 
     FIG. 9 shows an alternative embodiment of the present invention which operates as a bi-directional and gain flattened amplifier and employs one amplifier, four four-port elements and optical attenuators. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5A shows a schematic of a preferred embodiment of a four-port WDM element  100  that includes four fiber optic collimators  102 . 1 ,  102 . 2 ,  102 . 3 ,  102 . 4 ; a filter F and four ports P 1 , P 2 , P 3 , P 4 . Each collimator  102  includes a collimating lens  104  and a fiber  106 , which can be single- or multi-mode fiber. The collimators  102  are all coupled to the filter F. The transmission characteristics of the filter F determine whether it transmits or reflects a particular channel or channels received from the collimators  102 . Typically, the filter&#39;s transmission characteristics are selected so that particular wavelengths are routed through the WDM in a particular direction. For example, the transmission band of the filter F can be defined so that signals of wavelength k 1  are reflected and signals of wavelength k 2  are transmitted. Assuming such a filter F and signals at the respective wavelengths k 1  and k 2  input to the port P 1 , the signals at k 1  would be reflected by the filter F to the port P 2  and the signals at k 2  would be transmitted by the filter F to the port P 3 . 
     More generally, light out of the collimator  102 . 1  that is reflected by the optical filter F is received by the collimator  102 . 2 . Light from the collimator  102 . 1  that is transmitted through the filter F is received by the collimator  102 . 3 . In addition, light from the collimator  102 . 2  that is transmitted through the filter F is received by the fourth collimator  102 . 4 . Light signals input at the ports P 4  and P 3  behave in the WDM element  100  analogously to light input at the first and second ports P 1  and P 2 , respectively. 
     FIG. 5B shows an alternative configuration  120  of the invention shown in FIG.  5 A. This configuration uses two collimating lenses  122 . 1  and  122 . 2  and four fibers  124  bundled into holders at each end of the lenses  122  to form the four ports P 1 , P 2 , P 3 , P 4 . In this configuration, the angle  126  between ports P 1  and P 2  and between the ports P 3  and P 4  are highly reduced. This configuration offers ease of manufacturing, cost reduction, and size reduction, reduction of thermal sensitivity and reduction of incidence angle onto the filter, which in turn reduced polarization dependent losses (PDL). PDL is an important parameter that is always desired to be small for proper network operation. 
     FIGS. 6A and 6B show alternative embodiments that use variable attenuator Ri and selective filters Fi to attenuate signals of a particular wavelength ki. Each of these embodiments, and subsequent embodiments, shows a four-port WDM configured as shown in FIG. 5A; however, any of the embodiments disclosed herein can also employ the four-port WDM configuration of FIG. 5B or an equivalent of either configuration. 
     In particular, FIG. 6A shows an embodiment  140  of a four-port WDM element that can be used to control the power of a specific wavelength, for example k 2 . The embodiment  140  includes four ports P 1 -P 4 , a filter F 2  and a variable attenuator R 1  connected between the ports P 3  and P 4 . For exemplary purposes, assume that the signals input to the port P 1  are at a plurality of wavelengths, including k 1 , k 2  and k 3 . In the illustrated embodiment, the filter F 2  is configured to transmit only the wavelength k 2  and reflect all other wavelengths. In a WDM element so configured all wavelengths ki received by the port P 1  are routed to the port P 2  except for the wavelength k 2 , which is routed to the port P 3 , where it encounters the attenuator R 1 . After it is attenuated, the wavelength k 2  is routed through the port P 4 , filter F 2  and port P 2  to join the rest of the wavelength stream as the attenuated wavelength k 2 _att. 
     The attenuation of multiple wavelengths can be controlled by serially connecting a number of the elements  140  shown in FIG.  6 A. For example, referring to FIG. 6C, if three elements  140 - 1 ,  140 - 2 ,  140 - 3  are connected in series, then port P 2  of the first element would be connected to P 1  of the second element, and port P 2  of the second element would be connected to port P 1  of the third element. Assume that attenuators R 101  to R 103 , and filters F 101  to F 103  are used in these elements, respectively. All wavelengths enter the system at the first port P 1  of the first element and exit at P 2  of the third element. Assuming that filters F 101  to F 103  are configured to transmit wavelengths k 1  to k 3  and reflect everything else, then as these wavelengths are routed through the system they encounter their respective attenuators in a manner similar to that described for FIG.  6 A. Each wavelength is therefore selectively attenuated by its corresponding attenuator making it possible to selectively attenuate each wavelength arbitrarily without affecting the other wavelengths. 
     Although the schematic of FIG. 5A consists of four collimators and an optical filter, the four-port element that is disclosed in this invention is not limited to such a configuration. For example, FIG. 6B shows a six-port WDM  160  that adds additional ports P 5 , P 6  and a filter F 3  to the configuration  140  of FIG.  6 A. This six-port WDM element  160  enables one to independently control transmission losses for the wavelengths k 2  and k 3 , for example. In this case the filter F 3  is configured to transmit the wavelength k 3  and reflect all other wavelengths. Any wavelength (other than the wavelengths k 2  and k 3 ) input to the port P 1  is deflected by the filters F 2  and F 3  to port P 6 . The wavelengths k 2  and k 3  are separately routed to the attenuators R 1  and R 2 , respectively. In particularly, the wavelength k 1  is routed as follows: P 1 -F 2 -F 3 -P 6  (meaning port  1  to filter  2  to filter  3  to port  6 ). The wavelength k 2  is routed as follows: P 1 -F 2 -P 5 -R 1 -P 4 -F 2 -F 3 -P 6 . The wavelength k 3  is routed as follows: P 1 -F 2 -F 3 -P 3 -R 2 -P 2 -F 3 -P 6 . 
     The four-port WDM elements of FIG. 5A may also be used to build a bi-directional amplifier  200 , shown in FIG. 7, which, in addition to the four ports P 1 -P 4  and filter F 4 , includes an amplifier A 1  connected to ports P 2  and P 3 . Assume for illustrative purposes that the filter F 4  reflects red light and transmits blue light. Assume also that in the network in which the element  200  is employed blue light  202  propagates east to west, entering the element  200  at the port P 4  and red light  204  propagates in the opposite direction, entering the element  200  at the port P 1 . Consequently, the blue light  200  that enters at the port P 4  is transmitted through the filter F 4  to the port P 2 , passes through the amplifier A 1  to the port P 3 , is transmitted through the filter F 4 , exits at the port P 1  and continues its east-west propagation. On the other hand, the red light  204  that enters at the port P 1  is reflected by the filter F 4  to the port P 2 , passes through the amplifier A 1  to the port P 3 , is reflected by F 4  to port P 4  and continues on its west-east propagation. 
     Note that the filter F 4  in FIG. 7 is chosen such that the wavelengths that are reflected and transmitted both reach the input end of the amplifier A 1 . The reverse filter, reflecting blue light and transmitting red, could be used if the input/output orientation of the amplifier A 1  were reversed. The four-port WDM element of the present invention therefore offers the flexibility of implementing the filter F 4  using transmission or reflection filters. Comparing the present invention to the bi-directional amplifiers of FIGS. 2-4 and  7 , note that the system of FIG. 7 minimizes the parts used. This offers a major advantage in cost and complexity reduction in networks. 
     The WDM elements shown in FIGS. 5A and 5B are building blocks that can be connected together in any number so that any combination of channel wavelengths, or channel wavelength bands, regardless of their direction of propagation, can be added or eliminated from networks. Referring to FIG. 8, there is shown an alternative embodiment  220  of the current invention, where three four-port elements WDM 5 , WDM 6 , WDM 7  are used with one amplifier A 1  and three filters F 6 , F 8 , F 10  to form an expandable, bi-directional amplifier configuration. In the illustrated embodiment the filters F 6 , F 8  and F 10  in elements WDM 5 , WDM 6 , WDM 7  are respectively configured to transmit even-labeled wavelengths (e.g., k 6 , k 8 , k 10 ) and reflect everything else. For example, referring to FIG. 8B, which shows the reflection and transmission spectra of the filter F 10 , the transmission spectra is high only for the wavelength k 10 . Note that the choice of labeling of wavelengths is arbitrary and is not necessary for proper operation of this amplifier block. The routing in the amplifier  220  of the various input channels k 5 -k 10  is now described. 
     The odd-labeled, west-to-east propagating channels k 5 , k 7  and k 9  enter the amplifier  220  at the port P 1  of FIG.  8 A. Each of these channels is routed as follows: P 1 -P 2 -P 5 -P 6 -P 9 -P 10 -A 1 -P 11 -P 12 -P 7 -P 8 -P 3 -P 4 . All channels k 5 , k 7 , k 9  exit at the port P 4  and continue the eastward path. The channel k 6  enters at the port P 4  is routed as follows: P 4 -P 2 -P 5 -P 6 -P 9 -P 10 -A 1 -P 11 -P 12 -P 7 -P 8 -P 3 -P 1 , and continues on its westward path. Similarly, the channel k 8  enters at the port P 4  and is routed as follows: P 4 -P 3 -P 8 -P 6 -P 9 -P 10 -A 1 -P 11 -P 12 -P 7 -P 5 -P 2 -P 1 , and continues on its westward path. The routing of the channel k 10  follow from the previous descriptions. Note that all channels k 5 -k 10  are routed through the amplifier A 1  in the same direction. Channels other than k 6 , k 8  and k 10  entering the port P 4  are blocked by the amplifier A 1 . 
     The configuration of FIG. 8A may be upgraded if an additional channel wavelength or bands of channel wavelengths are added to the network at a later time. This update can be effected by simply splicing-in (e.g., at the indicated “AA”-“BB” marks) an additional four-port WDM element with an internal filter element configured to transmit the new channel wavelength or band. Furthermore, any channel can be reversed in its direction of propagation by reversing the orientation of the corresponding four-port WDM element. This capability to add/reduce and reverse direction enhances the flexibility of network architectures. 
     Note that, in the disclosed embodiments, all channels that are propagating in the same direction are reflected and transmitted the same number of times. If the four-port elements of FIG. 5B are designed to have similar insertion loss specifications for all channels, it follows that all channels will experience similar insertion losses, which eliminates problems of asymmetry between wavelengths as they pass through different set of network elements. In addition, if insertion loss specifications for the channels that are transmitted through the filters of these four-port elements are similar to those of reflected channels, then it follows that east-west channels and west-east channels would have similar insertion losses. This further decreases the asymmetry to all channels. 
     Furthermore, when additional channels are added to the network by splicing-in additional four-port elements, it follows that the total insertion loss for all channels will be effected in a similar manner, without preferentially improving or degrading particular channels. This flexibility in circuit connectivity is beneficial, particularly in WDM networks that use bi-directional propagation on the same fiber. There are numerous applications of four-port WDM elements. 
     Referring to FIG. 9, there is shown an alternate configuration  240  of the current invention. This configuration employs four four-port elements WDM 5 -WDM 8 , two optical attenuators R 3 , R 4  and one optical amplifier A 1  to form a bi-directional, gain flattened amplifier. In this configuration, odd-labeled channels (k 5 , k 7  and k 9 ) are transmitted west to east, and even channels (k 6 , k 8  and k 10 ) are transmitted east to west. The elements WDM 5 -WDM 7  are identical to those shown in FIG. 8A, having similar port labeling and internal filters F 6 , F 8  and F 10  that respectively transmit the wavelengths k 6 , k 8  and k 10  while reflecting everything else. The WDM 8  has four ports P 13 , P 14 , P 15 , P 16  and a filter F 11  that transmits all odd labeled channels and reflects everything else. The routing in the amplifier  240  of the various input channels k 5 -k 10  is now described. 
     The west-to-east propagating channels k 5 , k 7  and k 9  are routed as follows: P 13 -P 15 -P 10 -P 9 -P 6 -P 5 -P 2 -P 1 -A 1 -P 14 -P 16 -P 4 -P 3 -R 3 -P 8 -P 7 -R 4 -P 12 -P 11  and continue on their eastward path. The channel k 6  is routed as follows: P 11 -P 12 -R 4 -P 7 -P 8 -R 3 -P 3 -P 1 -A 1 -P 14 -P 13  and continues westward. The channel k 8  is routed as follows: P 11 -P 12 -R 4 -P 7 -P 5 -P 2 -P 1 -A 1 -P 14 -P 13  and continues westward. The channel k 10  is routed as follows: P 11 -P 9 -P 6 -P 5 -P 2 -P 1 -A 1 -P 14 -P 13  and continues westward. By comparing these routings note that the odd channels k 6  encounter an additional loss provided by the attenuators R 3 +R 4 , while the channel k 8  experiences an additional loss provided by the attenuator R 4  only and the channel k 10  experiences no additional loss. Such asymmetrical loss configurations allow for example to compensate for uneven gain that is experienced by channels passing through a network. 
     Another advantage offered by the present invention is that of upgrading single-direction networks to bi-directional operation. This can be accomplished with relatively little expense and additional complexity by inserting four-port WDMs designed in accordance with the present invention at the inputs and outputs of each amplifier stage, routing point, or other junction. 
     Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims.