throughput and they are expected to find widespread application in upgrading existing networks as well as future photonic net- works [l]. The star topology is ...
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 3, MARCH 1991
Distributed Optical Passive Star Couplers Mansour Irshid and Mohsen Kavehrad Abstract-In this letter, we present a technique by which a single centralized passive star coupler in an optical star network is replaced by a distributed one. This is done by partitioning the star coupler into numerous pieces and placing them at the network nodes. Such a placement results in a great saving in the number of fibers required to build the star network. In addition to this saving, the problems associated with constructing a large single N x N star coupler are bypossed.
I. INTRODUCTION
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PTICAL star networks using wavelength division multiplexing (WDM) have the potential to provide very large throughput and they are expected to find widespread application in upgrading existing networks as well as future photonic networks [l]. The star topology is preferred in these networks because the link attenuation between users (in decibels) grows logarithmically with the number of users, while that for bus and ring topologies is proportional to the number of users. However, the main disadvantage of the star topology is that the number of fibers required to implement the network is very large compared to that of other topologies [2], [3]. A key component in most of these network architectures is the optical passive star coupler. Two types of star couplers are used in optical star networks; the transmissive N x N star coupler and the reflective N-star coupler [4]-[6]. In [6], it is found that the number of fibers and also the number of components needed to implement a network can be reduced to a half by using a reflective N-star coupler instead of the conventional transmissive N x N star coupler. In this scheme, each user is connected to the reflective N-star coupler by one fiber only. In this letter, a further reduction in the number of fibers is achieved, using a technique by which the centralized transmissive N x N star coupler or the reflective N-star coupler is replaced by a distributed version. The distributed couplers are nothing more than the original single unit couplers divided into smaller parts and distributed at the nodes of the network. In Section 11, the distributed transmissive N x N star coupler is discussed. The distributed reflective N-star coupler is discussed in Section 111. Conclusions are presented in Section IV.
II. DISTRIBUTED TRANSMISSIVE N x N STARCOUPLERS In a basic star network, the users are connected to the passive transmissive N x N star coupler by two fibers, one fiber for transmission to the coupler and the other for reception from the coupler. An ideal N x N star coupler uniformly distributes the signal power transmitted by each user across its N output fibers resulting in a loss of 10 Log N dB in the power received by the other users. In this centralized wiring topology, the number of Manuscript received November 15, 1990. This work was supported by the Telecommunications Research Institute of Ontario (Photonic Networks and Systems Thrust). M. Irshid is with the Department of Electrical Engineering, Jordan University of Science and Technology, Irbid, Jordan. M. Kavehrad is with the Department of Electrical Engineering, University of Ottawa, Ottawa, Ont. KIN 6N5, Canada. IEEE Log Number 9143145.
fibers required to connect the N users to the star coupler is 2 N fibers, large compared to other topologies such as ring and bus. However, in many practical networks, users are geographically distributed in clusters such as several floors in a building, several buildings on a campus, etc. In this case, it is possible to partition the N x N star coupler into smaller parts and distribute them at the network nodes. It is found that the decentralization of the star coupler results in a tremendous saving in the number of fibers required to build the network. Except for the physical separation between its components, the distributed star is identical to the centralized one and therefore will exhibit the same performance. Let us start with the following example: 16 users are located in two buildings on a campus with eight users in each building and they are separated by a distance L km. It is required to interconnect these users, using an optical passive star network. In a centralized star network, each user is connected to the 16 x 16 star coupler by two fibers, thus 32 fibers of length 16 L km are needed to construct the network. Since the most commonly used N x N star couplers are constructed from basic couplers such as the 2 x 2 coupler using a hierarchical cascading algorithm [4], [ 5 ] , it is possible to partition the N x N coupler into smaller parts. In our example, the 16 x 16 star coupler consists of eight 4 x 4 couplers connected as shown in Fig. 1. This 16 x 16 star coupler can be divided into two halves and can be located at the two nodes of the network as shown in Fig. 2. By examining Fig. 2, the number of fibers needed to construct the network is eight with a total length of 8 L Km compared to 16 L * Km for that of the centralized coupler. The loss in the received power by a user in one location on a signal transmitted from another user location is 10 Log N uL dB and it is 10 Log N * dB from a colocated user where U is the fiber loss in dB/km. In a centralized network, the loss is 10 Log N uL * dB for all users, assuming that the N x N star coupler is located half-way between the two locations. If the 16 users in our example are clustered in four locations, the transmissive 16 x 16 coupler can be distributed over these locations as shown in Fig. 3. The number of fibers required to interconnect the star network is 12 as opposed to 32 for the centralized case. To compare the total length of the fibers required to construct each of the two networks, the four locations are assumed to be sited at the comers of a square with a diagonal length of L km. It is found that the total length of the fibers required to construct the distributed network is 9.66 L * km as opposed to 16 L km for the centralized one. The procedure can be generalized easily when the number of users is a perfect square; N = m2. The transmissive N x N star coupler can be distributed over m locations with m users in each location. Each location contains two transmissive m x m star couplers. Two fibers are needed to interconnect each two locations in the network, one for transmission and one for reception. Therefore, the number of fibers needed to interconnect the m locations is m(m - 1) as opposed to 2m2 for the centralized network case. In the above discussion, our prime concern was the saving in the number of fibers; however, there are other
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Fig. 4. A reflective 16-st; . coupler. Fig. 1 . A transmissive 16 x 16 star coupler.
Fig. 5. A reflective 16-star coupler distributed over two locations.
Fig. 2. A transmissive 16 x 16 star coupler distributed over two locations.
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Fig. 6. A reflective 16-star coupler distributed over four locations.
their transmitted and received signals using diplexers at their own remote locations. The main advantage in using a reflective #. . $ star coupler is that the number of fibers and components reFig. 3. A transmissive 16 x 16 star coupler distributed over quired to implement the network is half of that for transmissive four locations. star couplers [6]. A further reduction in the required number of fibers can be achieved by distributing the reflective star coupler over the important advantages to having a distributed coupler. First, the network nodes, similar to what is done to a transmissive star difficulties associated with constructing a large single star cou- coupler as described in Section II. For comparison purposes, we pler for networks with a large number of users is alleviated. will continue with the example discussed in Section 11, which is Second, any failure occurring at a large single N x N star on a star network with 16 users. A reflective 16-star coupler is coupler may result in a complete failure of the network, while shown in Fig. 4. The coupler is built from four transmissive for a distributed coupler this is not the case. 4 x 4 star couplers and four mirrors, i.e., half of that for the transmissive 16 x 16 star coupler [6]. The 16-star coupler of ID.DISTRI~UTED REFLECTIVEN-STARCOUPLERS Fig. 4 can be easily distributed over two locations as shown in A reflective N-star coupler is an N-port passive device where Fig. 5. The number of fibers needed to interconnect the two the power entering any one of the N ports gets divided equally locations is four with a total length of 4 L km where L again and is reflected back out of all the ports. When this is used as a is the distance between the two locations. This number is half of central node in a passive star network, each user would be that for a centralized reflective star coupler and one fourth that connected to the coupler by a single fiber for both transmission of a centralized transmissive star coupler. The reflective star coupler can be distributed over four locaand reception of information. Various users would then separate . . . 2' . c
IRSHID A N D KAVEHRAD: OPTICAL PASSIVE STAR COUPLERS
tions as shown in Fig. 6. The number of fibers needed to interconnect the four locations is six compared to 16 for the case of centralized reflective star coupler. This procedure can be generalized to any reflective N-star coupler, given N as a perfect square number N = m2. The maximum number of locations among which the coupler can be distributed is m with m users at each location. The distributed reflective N-star coupler consists of m transmissive m x m star couplers and m mirrors. One of the outputs of the transmissive m x m star coupler is terminated by a mirror and the remaining ( m - 1) outputs of each of the transmissive m x m couplers are connected to the outputs of the couplers in the other ( m - 1) locations by ( m - 1) fibers. Therefore, the number of fibers needed to interconnect the m locations is m(m - 1)/2 compared to m 2 in centralized reflective star coupler case.
IV . CONCLUSIONS In this letter we proposed a technique by which a transmissive N x N star coupler or a reflective N-star coupler located at the
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hub of an optical star network can be distributed over the nodes of the network. It is found that networks with distributed couplers require less number of fibers than those with centralized couplers. Besides the saving in the required number of fibers, the distributed couplers are more reliable, more flexible for future expansion, and less expensive to construct. REFERENCES
C. A. Bracken, “Dense wavelength division multiplexing networks: Principle and application,” ZEEE J . Select. Areas Commun.,vol. SAC-8, no. 6, pp. 948-964, Aug. 1990. Y. M. Lin, D. R. Spears, and M. Yin, “Fiber-based local access network architectures,” ZEEE Commun. Mag., pp. 64-73, Oct. 1989. P. S . Henry, “High-capacity lightwave local area networks,” ZEEE Commun. Mag., pp. 20-26, Oct. 1989. M. E. Marhic, “Hierarchic and combinatorial star couplers,” Opt. Lett., vol. 9, no. 8, pp. 368-370, Aug. 1984. D. B. Mortimore, “Low-loss 8 x 8 single-mode star coupler,” Efectron. Lett., vol. 21, no. 11, pp. 502-504, May 23, 1985. A. M. Saleh and H. Kogelnik, “Reflective single-mode fiber-optic passive star couplers,” J . Lightwave Technof., vol. LT-6, no. 3, pp. 392-397, Mar. 1988.
