WO2007086777A1

WO2007086777A1 - Passive fibre-optic network

Info

Publication number: WO2007086777A1
Application number: PCT/RU2006/000710
Authority: WO
Inventors: Alexander Gennadievich Popov
Original assignee: Alexander Gennadievich Popov
Priority date: 2006-01-27
Filing date: 2006-12-28
Publication date: 2007-08-02
Also published as: RU2310278C1

Abstract

The inventive passive fibre-optic network comprises a unidirectional fibre optic loop (1), N directional couplers (2, 3,...6) positioned at different loop points , a focal point (7) and a plurality of subscriber points (8, 9,...12). The transmitter (13) of the focal point (7) is optically connected to the beginning (15) of the fibre optic loop (1) and the receiver (14) is connected to the end (16) thereof. The transmitter (17) and receiver (18) of each subscriber point (8, 9,...12) is optically connected to the fibre optic loop (1) through the corresponding coupler (2, 3,...6). Said directional couplers (2, 3,...6) are embodied sequentially in order around the loop from the beginning (15) to the end (16) thereof in such a way that tapping factors increase according to the sequence 1/N, 1/(N-1),..., 1/2, 1 on the emission wavelength of the transmitter (13) of the focal point (7) and decrease according to the sequence 1, 1/2,...,1/(N-1), 1/N on the emission wavelength of the transmitter (17) of any subscriber point (8, 9,...12). The transmission through the network is carried out in two flows : from the focal point (7) to the subscriber points (8, 9,...12) and therefrom to the focal point (7). Said invention makes it possible to increase the number of subscriber points serviced by the network.

Description

Passive fiber optic network.

The invention relates to the field of telecommunications, to passive optical networks with a loop architecture. It can be used in broadcast telecommunication networks, as well as in local data exchange networks.

In the field of telecommunications, passive optical networks are understood to mean networks in which the transmission of an optical signal between a central node and a plurality of subscriber nodes is carried out by passive static components, without amplification, regeneration / relay, etc. active components, regardless of the architecture (or topology) of the network: bus (linear), loop or tree-like.

A typical passive optical network with a tree architecture contains a central node, a plurality of subscriber nodes and a transmission channel including a trunk optical fiber, a 1xN splitter (star type) and N optical fiber taps from a star. The transmitter and receiver of the central node are optically coupled to the trunk fiber through a bi-directional spectral multiplexer / splitter (WDM multiplexer). The transmitter and receiver of the subscriber unit are optically coupled through a WDM multiplexer of the subscriber unit to one of the taps of the “star”. [L]

Transmission from the central node to the subscriber nodes is carried out at the same wavelength (downstream). Transmission from all subscriber nodes to the central node is carried out at a different wavelength (upstream). The downstream is broadcast and organized by the method of temporary compression / separation of signals intended for different subscriber nodes. Upstream is organized according to the network protocol

SUBSTITUTE SHEET (RULE 26) access with temporary compression / separation of signals from different subscriber nodes, when each subscriber node is allocated a certain time interval for transmission. Separation of oncoming streams is carried out in network nodes by WDM-multiplexers.

It is believed that a network with a tree architecture has an advantage over other networks in saving optical fiber [2, p. 479]. However, in the case of a significant removal of subscriber nodes from the “star” and a large number of subscriber nodes (and the current level of technological development already allows you to include up to 64 nodes at gigabit transfer speeds [2]) - this network is less expensive than fiber networks with a unidirectional optical loop. In fact, if for the radius r of the loop we take the average length of the branch from the splitter I xN to the subscriber unit, the perimeter of the loop will be 2 π g, and the total length of the branches Nхr. From where, from the inequality 2 π g <Nхr, it follows that already for the number of subscribers N> 6, a network with a tree-like architecture (without taking into account the length of the main fiber) is inferior to a network with a loop in saving optical fiber. However, the known passive optical networks with a loop (or ring) architecture have a significant drawback: the limitation on the number of nodes. In a passive network with a star, the power of the optical signal at the receiver of the subscriber unit is proportional to N ^"1 (the splitter IxN divides the power into N parts). In a passive network with an optical loop and M nodes, the power of the optical signal at the receiver of the subscriber unit is proportional to M ^{" 2} [3 ,four]. Thus, with equal budgets, transmission speeds and allowable error rates BER, a passive network with a “loop” in VN loses by the number of network nodes with a “tree”. We emphasize that the last statement is true only for fully passive networks with the transmission of information at the same operating wavelength. In the same work [4]

SUBSTITUTE SHEET (RULE 26) this limitation can be circumvented by endowing the subscriber nodes with the function of repeaters and using dynamic couplers with a controlled branch coefficient in the loop, i.e. actually including active components in the transmission line.

Known passive fiber optic network containing a unidirectional fiber optic loop and N directional couplers located at different points of the loop. The network contains a central node and a plurality of subscriber nodes. The transmitter of the central node is optically connected to the beginning of the fiber optic loop, and the receiver to its end. The transmitter and receiver of each subscriber unit are optically coupled to the fiber optic loop through an appropriate directional coupler. [5]

The network implements the transmission of information in two streams: from the central node to the subscriber nodes and from the subscriber nodes to the central node in one direction in the loop. This technical solution is taken as a prototype. The prototype has the same drawback as passive networks - analogues with a unidirectional optical loop: a limitation on the number of subscriber nodes in the network. If the α-coefficient of the branches is the same in both transmission streams, then the attenuation introduced into the optical signal only by the couplers leads to the following expression for the transmission coefficient k in the loop: k = α (l - α) ^{N 1} . (1) in expression (1), the worst cases (in terms of insertion loss) of the propagation of the optical signal in the loop are taken into account: from the transmitter of the central node through (N - I) taps and the branch to the receiver of the last subscriber node in the loop; or from the transmitter of the first subscriber unit to the receiver of the central node. For the transmission coefficient k, the following estimate is true:

SUBSTITUTE SHEET (RULE 26) k = α (l - α) ^{N 1} <α e ^"(N - ^{1) α} . (2)

In turn, the right-hand side of inequality (2) has a maximum at α = l / (N-l). Thus:

5 k <e ^" V (N -I). (3)

Now let Pj be the transmitter power (more precisely, the power introduced into the optical fiber from the transmitter laser), and P _{2 the} minimum detectable receiver power (for a given level of BER error coefficient); then from the equality P _{2 =} k Pi and the inequality

Yu (3) follows:

N <e - ¹ [P, / P ₂ ] + l. (four)

The Pi / P ₂ ratio in inequality (4) is known as the network budget or the energy potential of the network. The integer part of [Pi / P ₂ ] is equal to the limit number of subscriber nodes, theoretically possible in

15 passive networks. From inequality (4) it follows that the prototype is almost three times inferior to the theoretical limit in the number of subscriber nodes.

The proposed invention solves the problem of increasing the number of subscriber nodes in the network and expanding the arsenal of technical means in

20 areas of telecommunications.

To achieve this technical result, a passive fiber-optic network contains a unidirectional fiber-optic loop and N directional couplers located at different points of the loop, a central node and many subscriber

25 knots. The transmitter of the central node is optically connected to the beginning of the fiber optic loop, and the receiver to its end. The transmitter and receiver of each subscriber unit are optically coupled to the fiber optic loop through an appropriate directional coupler. Each coupler with one pair of input and output ports

^{Jυ is} included in the fiber optic loop, and another pair from the input and

SUBSTITUTE SHEET (RULE 26) output ports is designed to connect, respectively, with the transmitter and receiver of the subscriber unit. Unlike the prototype, directional couplers from the first to the last, in the order of their arrangement from the beginning to the end of the fiber optic loop, are made with increasing branch coefficients at the radiation wavelength of the transmitter of the central node and with decreasing branch coefficients at the radiation wavelength of the transmitter of any subscriber node . According to a special case of execution, directional couplers are made with increasing branch coefficients in the sequence: 1 / N, 1 / (NI), ..., 1/3, 1/2, 1 at the radiation wavelength of the transmitter of the central node and with decreasing branch coefficients in the sequence: 1, 1/2, 1/3, ..., 1 / (N- 1), 1 / N at the radiation wavelength of the transmitter of any subscriber unit.

According to a special case of execution, each directional coupler is made on two connected optical waveguides, the ends of which form two pairs of input and output ports. According to a special case of execution, the receiver of each subscriber unit is equipped with a selective filter for one or another radiation wavelength of the transmitter of the central unit, and the receiver of the central unit is equipped with a blocking filter for the same wavelengths. The invention is illustrated in the drawings of Figures 1-5.

Figure l shows a block diagram of a passive fiber optic network according to the invention.

Figure 2 shows a functional diagram of a spectrally dependent directional coupler. Fig. 3 shows a schematic representation of directional

SUBSTITUTE SHEET (RULE 26) coupler on two connected optical waveguides.

Figure 4 shows graphs of the beat of power along homogeneous coupled waveguides for two wavelengths. Figure 5 shows a block diagram of a network with a backup transmission line.

Passive fiber optic network Fig. 1 contains a unidirectional fiber optic loop 1 with one optical fiber, a central node 7 and a plurality of subscriber nodes 8, 9, ..., 12. At different points of the loop 1 are N directional couplers 2, 3, ..., 6. The transmitter 13 of the central node 7 is optically connected to the beginning 15 of the loop 1, and the receiver 14 is connected to the end 16 of the loop 1. The transmitter 17 and the receiver 18 of each subscriber unit 8, 9, ..., 12 are optically connected to the fiber - optical loop 1, respectively, through couplers 2, 3, ..., 6. Directional couplers 2, 3, ..., 6 one a pair of input 19 and output 20 ports are included in the breaks of the optical fiber loop 1 at different points in the loop. Another pair of input 21 and output 22 ports couplers 2, 3, ..., 6 are connected by two-fiber communication lines 23 respectively with a transmitter 17 and a receiver 18 of the subscriber nodes 8, 9, ..., 12. Receivers 18 of the subscriber nodes 8, 9 , ..., 12 are equipped with selective filters 24, 25, ..., 28 for the radiation wavelength of the transmitter 13 of the central node 7. The transmitter 13 contains a laser (not shown) with a fixed or tunable radiation wavelength. Filters 24, 25, ..., 28 are tuned either to the same wavelength I ₁ or to different wavelengths λ _{l b} λ _{] 2} , ..., λiN of the laser radiation of the transmitter. The receiver 14 of the central unit 7 is equipped with a blocking filter 25 for the radiation wavelengths of the transmitter 13. The directional couplers 2, 3, ..., 6, the functional diagram of which is shown in FIG. 2 have spectrally dependent

SUBSTITUTE SHEET (RULE 26) branch coefficient ss (X), where i is the serial number of the location of the coupler in loop 1 when looping in the direction of transmission from beginning 15 to end 16 (Fig. 1). The branch coefficients a, (λ) increase with the number of the coupler i at the radiation wavelength λi of the transmitter 13 and decrease at the radiation wavelength X ₂ of the transmitter 17 of any subscriber unit 8, 9, ..., 12.

Table l shows the optimal branching coefficients a \ for taps 2, 3, ..., 6, which allow connecting the maximum possible number of subscriber nodes to fiber-optic loop 1.

Table.l

Pos. on

2 3 4 5 6 Fig. L i 1 2 3 N - I N

OCi (λ 0 1 / N 1 / (N-I) l / (N-2) 1/2 1

CXi (λ ₂ ) 1 1/2 1/3 1 / (NI) 1 / N

Moreover, the number N is equal to the theoretical limit determined by the network budget: N = [Pi / P ₂ ]. (5)

Below we prove the last statement.

An optical signal at a wavelength λi of the transmitter 13 is sequentially removed from loop 1 to the receivers 18 of the subscriber units 8, 9, ..., 12. In accordance with Table l, the first coupler 2 in loop 1 transfers the power Pi / N. The signal to the second coupler 3 is attenuated (1 - 1 / N) Pi and the coupler 3 diverts power equal to (1 / (N -I)) (I -

Pi / N, etc. T.O. a chain of taps 2, 3, ..., 6, included in loop 1, selects 18 subscriber units 8, 9, ..., 12 to the receivers exactly in terms of Pj / N optical power. Thereby, the theoretical limit (5) is achieved in the first transmission stream: from the central node to the subscriber nodes. Optical signal at a wavelength of λ ₂ from

SUBSTITUTE SHEET (RULE 26) transmitters 17 of the subscriber units 8, 9, ..., 12 are introduced by taps 2, 3, ..., 6 into loop 1. In accordance with Table l, the i-th taps introduces power Pi / i into line 1. The optical signal propagating in loop 1 is sequentially attenuated by couplers with numbers (i + l), (i + 2), ..., (N -I) ₅ N and arrives at the receiver 14 of the central node 7 weakened by N times, independently from the number i of the sending party: (P ₁ Zi) (I - l / (i + l)) (l - l / (i + 2)) ... (l - IZN) = P ₁ ZN. T.O. and in the second transmission stream (from subscriber nodes to the central node), the number of subscriber nodes also reaches the theoretical limit (5).

Among the known directional couplers that meet the requirements of the invention, we should single out the couplers arranged according to the principle of coupled waveguides. A diagram of such a coupler is shown in FIG. Optical waveguides 31, 32 located parallel to each other interact with each other by decreasing external fields. The interaction of the waveguides leads to the fact that the power of the mode of one waveguide is partially transferred to the mode of another waveguide. The power transmitted to the waveguide 32 mode has the form: [6, pp. 231-236] F ₂ = F ₁ [c ² Z (c ² + Δβ ² )] sip ² [z (c ² + Δβ ² ) ^{1 / 2} ] (6) where Fi is the power at the input port 33 of the waveguide 31;

βi, β ₂ are the phase propagation constants of the waveguide modes Z 1, 32, respectively; c is the coupling coefficient between the waveguide modes.

The coupling coefficient c has an inverse exponential dependence on the distance d between the waveguides and inversely proportional to the wavelength λ.

Figure 4 presents graphs of the runout of power F ₂ along the waveguide for two wavelengths λi = 1550 nm and λ ₂ = 1310 nm for

SUBSTITUTE SHEET (RULE 26) normalized power Fi = 1. The beat lengths Ij _, I ₂ at wavelengths λ _b λ _{2 are} correlated as follows: 1]: I ₂ "λj: X ₂ = 1550/1310" 1.28. The graphs clearly show that by varying the values of c, Δβ, and the bond length L (Fig. H), it is always possible to achieve branch coefficients oti (λi) and αj (λ ₂ ) corresponding to Table.l. Note that the couplers considered are broadband devices, and ratios close to those indicated in Table l will be fulfilled for two bands: 1550 ± 40 nm and 1310 ± 40 nm. Spectrally dependent couplers have found application in frequency multiplexing / separation devices (WDM multiplexers), for which they tend to have branch coefficients α (I ₁ ) = 1, α (I ₂ ) = 0. As follows from expression (6) and graphs, this it is possible only if phase synchronism is observed Δβ = 0. The latter is difficult to practice in practice, and therefore WDM multiplexers always have excessive losses that reduce the level of the optical signal at receivers of networks using WDM multiplexing. And, as a result, the number of subscriber nodes is reduced in proportion to the losses introduced. Thus, the use of α couplers in the analogue of [1]

(λi) = 1/2 leads, at a minimum, to halving the number of subscriber nodes. At the same time, the indicated drawback of the couplers does not significantly affect the total number of subscriber nodes in the invention, since it leads to the reduction of only two extreme subscriber nodes in the loop (Fig. 1) (see Tab. 1).

The operation of the claimed network Figure 1 is based on the same principle as the operation of networks known as PON (Network Ortisal Networks) [2, p. 469- 478]

The OLT central unit (Ortisal Lipi Thermal) 7 receives data from the backbone networks via SNI connection interfaces

SUBSTITUTE SHEET (RULE 26) (Servis Node Iperfacs) and forms the first stream (similar to the downstream in PON) to ONU subscriber nodes (Ortisal Network Upit) 8, 9, ..., 12 by loop 1. In this case, any of the known methods of forming the stream is used in PON technologies , for example, synchronously transmitting with time division TDM (Time Divisiop Multiplekhipg) signals intended for different subscriber units 8, 9, ..., 12. The transmitter 13 in this case operates at a fixed wavelength λi = 1550 nm, and all filters 24 , 25, ..., 28 are set to the wavelength λ _s second flow (similar to the upward flow in PON) of subscriber nodes 8, 9, ..., 12 to the receiver 14 of the central node 7 is formed by the method of synchronous access with time division TDMA (Time Divisiop Multifunctional Assess) signals from the transmitters 18 subscriber nodes 8, 9, ..., 12. In this case, the transmission conducted at a wavelength of λ ₂ = 1310 nm and each subscriber node 8,9, ..., 12 sets an individual schedule taking into account the distance of the node from the receiver 14. Selective filters 24, 25, ..., 28 at a wavelength of λj and a barrier filter 29 for the same wavelength λi perform the function of separating the flows propagating in the loop 1 in one direction (in FIG. 1 by the clock).

In a network without filters 24, 25, ..., 28 and 29, transmission is carried out by alternating time cycles of the first and second flows (low-speed network operation mode). In the claimed network, it is also possible to transmit frequency-time packets of signals [7, p. 250]. In the first stream, the temporary positions of the signals intended for different subscriber units 8, 9, ..., 12 are transmitted at different wavelengths λc _, λig _{, ...,} λi _{N of the} tunable transmitter laser 13. Filters 24, 25, ..., 28 tuned to wavelengths λc _s λi _{2j ...;} λш, respectively. In the second stream, transmission from subscriber

SUBSTITUTE SHEET (RULE 26) nodes 8, 9, ..., 12 to the receiver 14 of the central node 7 is conducted at the same wavelength λ ₂ by the method of synchronous access with time division TDMA. It is known that a loopback (ring) network architecture with a backup transmission line provides reliable traffic protection. The solution to the backup task in the claimed network can be implemented in different ways. The simplest solution is to completely duplicate the main network with a backup segment with the opposite direction of transmission in the loop. Duplication of only the passive loop is also possible, or, at the same time, partial duplication of active network components, for example, transmitters in nodes, is possible.

Such a solution is schematically represented in FIG. 5. The passive fiber optic network comprises a main loop 51 and a backup loop 52 with opposite transmission directions in the loop. The central node 53 comprises a transmitter 57 and a receiver 58 connected to the main loop 51; and a transmitter 59 and a receiver 60 associated with the backup loop 52. Each subscriber unit 54, 55, 56 includes a first transmitter 67, a second transmitter 68, and a receiver 69. The transmitter 67 is optically coupled to a main transmission line through a corresponding coupler 61, 62, or 63 51. The transmitter 68 through an appropriate coupler 64, 65 or 66 is optically coupled to the backup line 52. The receiver 69 of each subscriber unit 54, 55, 56 is optically coupled simultaneously with the primary and backup loops 51.52.

The taps 61, 62, 63 of the main loop 51 and the taps 64, 65, 66 of the backup loop 52 have spectrally dependent branch coefficients in accordance with Table 1.

If the optical fiber is broken in the main loop 51, transmission for the duration of the repair is performed through the backup loop 52. In the case of

SUBSTITUTE SHEET (RULE 26) simultaneous breakage of optical fibers in loops 51, 52 (in Fig. 5 at points 70.71), traffic is completely restored by transmission in the remaining “xvostax” loops, as in two-fiber linear buses. Transmitters 67 of the subscriber units work in the left “x”, the transmitters 68 in the right “x” (in Fig. 5, the transmission route is indicated by a dotted line).

The claimed network, despite the loopback architecture, can be considered as a variant of the PON network. That is, it is fully compatible with hardware and software products released by the industry for PON technology.

On the other hand, the invention has an advantage over the known

PON networks with a tree-like architecture in saving optical fiber, ease of monitoring during network operation and availability (traffic protection). Therefore, the industrial application of the invention will be a cost-effective alternative to known networks.

Used sources.

1. Patent US 5.574 584, CL 359-125, 1996 publ. 2. Freeman P. Fiber-optic communication systems. 2nd additional edition: Transl. From English. edited by N.N. Slepov - M .:

Technosphere, 2004

3. Patent RU 2 264692. HO4B 10/12, 2005 publ.

4. GB 2,154,091. HO4B 9/00, 1985 publ. 5. Patent US 5.576.875, cl. 359-125, 1996 publ.

6. H.-G. Unger Planar and fiber optical waveguides / Transl. From English. V.V. Shevchenko - M .: Mir, 1980.

Telecommunication systems and networks: Textbook in 3 vols. Volume 8. - Multiservice networks; Edited by Prof. V.P. Shuvalov. - M.: Hotline - Telecom, 2005.

SUBSTITUTE SHEET (RULE 26)

Claims

Claim:

1. A passive fiber optic network containing a unidirectional fiber optic loop and N directional couplers located at different points of the loop, a central node and a plurality of subscriber nodes, the transmitter of the central node is optically connected to the beginning of the fiber optic loop, and the receiver to its end , the transmitter and receiver of each subscriber unit are optically connected to the fiber optic loop through a corresponding directional coupler, each coupler is included in one pair of input and output ports in a loop-optical loop, and another pair of input and output ports is designed to connect, respectively, to the transmitter and receiver of the subscriber unit, characterized in that the directional couplers from the first to the last, in the order of their arrangement from the beginning to the end of the fiber-optic loop, performed with increasing branch coefficients at the radiation wavelength of the transmitter of the central node and with decreasing branch coefficients at the radiation wavelength of the transmitter of any subscriber unit.

2. The network according to claim 1, characterized in that the directional couplers are made with increasing branch coefficients in the sequence: 1 / N, 1 / (NI), ..., 1/3, 1/2, 1 at the radiation wavelength of the central transmitter node and with decreasing branch coefficients in the sequence: 1, 1/2, 1/3, ..., 1 / (NI), 1 / N at the radiation wavelength of the transmitter of any subscriber unit.

C. Network according to claim 1 or 2, characterized in that each directional coupler is made on two connected optical waveguides, the ends of which form two pairs of input and output ports.

4. The network according to claim 1, characterized in that the receiver of each subscriber unit is equipped with a selective filter for one or another radiation wavelength of the transmitter of the central node, and the receiver of the central node is equipped with a blocking filter for the same wavelengths.

SUBSTITUTE SHEET (RULE 26)