FR2707066A1 - Interconnection loop with time-division and wavelength multiplexing - Google Patents

Abstract

Each station comprises a wavelength-tunable light source (SLi) which sends out a multicoloured sequence (Seq) formed by information at different, time-multiplexed wavelengths. Application to optical telecommunications and to computer networks.

Description

TIME MULTIPLEXED INTERCONNECTION LOOP

WAVELENGTH

DESCRIPTION

  Technical Field The subject of the present invention is an interconnection loop with multiplexing in time and in

  wave length. It finds an application in tele-

  optical communications and in computer networks

  ques. State of the prior art It is known practice to link several stations together, using optical equipment

  uses the wavelength multiplexing technique.

  Figure 1 attached illustrates, in a simplified manner

  connected, a loop interconnecting three stations Si, S2, S3. The information flows on a single-mode optical fiber F0, which can optionally be doubled by a second fiber F0 'where the information flows in the opposite direction. Each station includes: - an optical coupler (respectively C01, C02, C03) disposed on the optical fiber (s) F0, F0 ', - transmission means (ME1, ME2,

  ME3) connected to the optical coupler specific to the links

  sounds to be established between said station Si and the au-

  very Sj stations; these means include two laser sources, that is to say for the station Si,

  two sources referenced SL12 and SL13; these sour-

  these emit respectively at wavelengths X12 (for the link from Si to S2) and X13 (for the link from Si to S3); reception means MRi (respectively MR1, MR2, MR3) connected to the optical coupler COi, these means being able to receive and detect the two wavelengths specific to the two links established between the two other stations and the station considered; in the illustrated variant, the station SI thus comprises a filter F21 centered

  on the wavelength X21 used in the link

  sound from S2 to Si, and a filter F31 centered on the wavelength 131 used in the link of

  S3 to Si. These two filters F21 and F31 are re-

  linked to two photodetectors PD21 and PD31.

  In such a loop, each link of a sta-

  tion If to a Sj station requires a length

  particular wave (? ij), knowing that the link in-

  pours from Sj to If requires another, XJi, different

  first annuity. For a loop interconnecting N stations, it is therefore necessary to have N (N-1) different wavelengths. In the example in Figure 1, it would take 3x2 = 6 wavelengths to interconnect the

three stations.

  Although satisfactory in certain respects, these loops have a double drawback. Of a

  apart, they require a multitude of light sources

  in each station (N-1 if there are N stations at

  interconnect). On the other hand, they impose con-

  severe restrictions on the choice of wavelengths. Indeed, each wavelength must be precise to the nearest 0.3 nm, and the difference between the lengths

  wave is 2 nm. The result is a considerable extent

  maple occupied by the global spectrum.

  The article by A.F. ELREFAIE et al., Entitled "Fiber Amplifiers in ClosedRing WDM Networds", published in Electronics Letters, vol. 28, no 25, 1992, pp. 2340-2341, also describes a loop (which is not exactly closed on itself like that of the

  Figure 1, but which is connected to a trans-

  mission, which does not change anything with regard to the spreading of the spectrum) which uses wavelengths ranging between 1538 and 1566 nm, every 2 nanometers (either on

  thirty nanometers for 15 channels). The cou-

  optical weepers are capable diffraction gratings

  to extract five wavelengths from the fiber. For example-

  ple, for the second station, the network extracts the

  wavelengths at 1558, 1560, 1564 and 1566 nm.

  The article by M.I. IRSHID et al., Entitled "A Fully Transparent FiberOptic Ring Architecture for WDM Networks", published in Journal of Lightwave Technology, vol. 10, n 1, January 1992, pp. 101-108 still describes such a system, but with a filter

acousto-optic as coupler.

  The purpose of the present invention is to

remedy these drawbacks.

  Disclosure of the invention According to the invention, each station comprises a wavelength tunable semiconductor laser (or, optionally, several tunable lasers). This laser is controlled so that a specific wavelength is assigned to various groups of data intended for various stations. For each group, the

  light beam (emitted at the particular wavelength

  related to this group) is modulated by data from the

  group. We therefore obtain, at the laser output, a

  multicolored quence of data groups.

  If you want to transmit from a station i, data to a station j, you will control the laser at the wavelength Iij

  (the high index designating the transmitting station and the information

  dice down the receiving station). If we want to trans-

  always put from the station of rank i, data destined for another station of rank k, we will control the laser at the wavelength Xik and we will organize the two types of data into groups Gij and Gik, respectively, and lron will multiplex over time

these groups.

  The lasers which are well suited to the invention are

  semiconductor lasers of the DFB or DBR mul-

  tielectrodes. Such lasers are known. They are described, for example, in the article by M.J. CHAWKI et al., Entitled "M3NET Demonstration of HD WDM Optical Network Using Two electrode DFB Laser Filter as Tunable

  Narrowband FM Receiver ", published in" Electronics Let-

  ters ", vol. 28, n 2, 1992, pp. 147-148.

  The article by M.J. CHAWKI et al., Entitled "Demonstration of a High Speed Optical Drop Function Using a Tunable DFB Laser", published in Electronics Letters, January 21, 1993, vol. 29, n 2, pp. 193-195, describes, moreover, the extraction of a byte from

  two on a 1.7 Gbit / s transmission line.

  These tunable lasers have two main advantages.

  Pale: switching speed and a range of

  continuous cord of the emission wavelength. The best result obtained by the inventors related to a distributed feedback laser with a length of 200 Am and two electrodes. The tuning range at 1.5 µm was 200 GHz (about 1.5 nm); the line width was 25 MHz maximum. A switching time of 550 ps was measured in an experiment of

mode switching.

  These performances of the bielectrode DFB laser are described in the article by MJ CHAWKI et al., Entitled "2 Gbit / s Direct Detection Transmission Using Optical Mode Switching in a Two-Electrode DFB Laser", published in "Journal of Optical Communications", 1992,

pp.82-84.

  Another advantage of tunable sources is that they allow dense multiplexing with a distance between channels which is typically 0.3 to 0.5 nm. The spectral range used is then less than 2 na for 4 channels whereas it was more than 8 nm in the prior art. More specifically, the present invention therefore relates to an interconnection loop of the kind of

  that of Figure 1 but which is double multi-

  plexing, that is to say both in time and in wavelength. To this end, the loop of the invention is characterized in that:

  a) the transmission means of each station includes

nent:

  * means for receiving the data to be trans-

  put to the other stations, to organize this data into time multiplexed groups,

  each group being destined for another sta-

  tion, and to issue a control signal,

  * at least one semiconductor laser source ac-

  Waveable in wavelength, this source receives

  the data groups and the signal

  command and thus transmitting a multi-sequence

  colors signals at different wavelengths

  annuities specific to the connection to be established between the-

said station and other stations,

  b) the reception means of each station comprising

  include: * filters centered on the wavelengths specific to the links between the other stations and said station, * as many photodetectors connected to the filters, * a demultiplexer having inputs connected to the photodetectors and outputs delivering the

  data from other stations.

  According to an advantageous variant, the groups of

  data can be organized into bytes.

  According to another variant, the data groups can be separated from each other by at least one

binary element.

  In a particular embodiment, each station includes two tunable laser sources which

  cooperate with two optical fibers in the direction of propagation

tion inverses.

Brief description of the drawings

  - Figure 1, already described, shows an information loop

  terconnection according to the prior art, - Figure 2 illustrates, schematically, the structure of a station in a loop according to the invention, - Figure 3 shows the particular case of an interconnection loop with three stations , - Figure 4 illustrates a variant with two sources tunable per station and two optical fibers, - Figure 5 shows the distribution of wavelengths in the previous variant, for five stations. Detailed description of embodiments Figure 2 shows the general structure of a

  station belonging to a loop according to the invention.

  This figure in no way prejudges the number of stations to

  interconnect. The means represented are therefore affected

  t of literal indices i and j not specified, but can

  before taking N-1 integer values, if N is the number

  of stations (with i different from j).

  The station represented is of rank i, and is referenced

  referred to Si. It is supposed to communicate with any

  which other station of rank j (iÉj), denoted Sj.

  The transmission means MEi of the station Si represented

  then include: * MPXi means for receiving the Dij data at

  transmit to the other stations Sj, to organize

  ser this data in (N-1) multiplexed Gij groups

  over time, and to deliver a signal

  mande Scij, * a SLi semiconductor laser source tunable in

  wavelength, this source receiving the

  pes of data Gij and the control signals SCij and thus emitting a multicolored sequence

  Seq of Lij signals at different wavelengths

Xij annuities.

  As for the MRi reception means, they include

  nent: * (Ni) filters FJi centered on the (N-1) wavelengths Iii specific to the links between the (N-1) other stations Sj and said station Si, * (N-1) photodetectors PDj connected to ( N-1) filters FJ i, * a DMPXi demultiplexer having (N-1) inputs connected to (N-1) PDJi photodetectors and (N-1) outputs delivering the Dii data coming from (N-1) to-

very Sj stations.

  In FIG. 3, an example of a loop according to the invention is seen, in the particular case where there are three stations Si, S2 and S3 to be interconnected. With the convention used so far, namely that the high index designates the transmitting station and the low index designates the receiving station, it can be seen that the station Si, for example, includes a laser source SL1, switchable in length wave, a multiplexer MPX1 receiving data D12 intended for station S2 and data D13 intended for station S3. The multiplexer MPX1 organizes this data into groups G12, G13 and delivers wavelength control signals SC12 and SC13. The SL1 laser emits a two-color sequence where alternating

  wavelengths X12, X13 and the data L12, L13.

  On the reception side, there are two filters F21 and F31, the first filtering the information coming from the station S2 and the second that coming from the station S3. There are still two photodetectors PD21 and PD31 and, finally, a DMPX1 demultiplexer which delivers

  data D21 from S2 and D31 from S3.

  The same means equip the stations S2 and S3, the references being easily obtained by circular permutation of the indices i and j (i becomes equal to 2, then

  to 3; j becomes 3 and 1, then 1 and 2).

  Some considerations will now be made.

  loped on the link budgets for a loop

  three stations according to figure 3. The letter f represents

  feel the length of an arc of the loop between two stations and the letter at the linear loss

  (a = 0.25 dB / km). The quantity ae represents the weakening

  fiber between two stations.

  The following table represents the link budget according to the diagram in FIG. 3 for each wavelength Iij present in the loop: 2 successive stations 2 distant stations

(131; X12; X23) (X32; 121; X13)

  af + 17.5 dB 2a + 22 dB The couplers are assumed to be symmetrical. The link budget corresponds to an experiment with the components currently available and in particular optical discriminative frequency filters of the FABRY-PEROT type (insertion losses 5 dB). The highest attenuation is 22 dB + 2ae. With a detection threshold of -37 dBm at 311 Mbit / s of frequency frequency of the optical sources, we obtain a loop length of 60 km if we transmit at -5 dBm and 90 km if

  we emit at O dBm. The link budget can be opti-

  leveraged by the installation of asymmetrical couplers

  before the receiving optical filters.

  The limitation in size of the loops according to the invention is mainly linked to the number of channels

  usable in the field of optical amplification.

  The current state of the art of DFB multi-electrode laser sources enables fast time switching and the allocation of four channels per source. We can therefore install five stations on the same loop (one station is connected to the other four) which requires five lasers. The classic solution, shown in Figure 1, would involve

the installation of twenty sources.

  The following table indicates the number of sources installed according to the architecture of FIG. 1 (solution of the prior art) and that of FIG. 2

(solution of the invention).

  Number of sta- Number of Total number of sources wavelengths per station prior art invention

3 2 6 3

4 3 12 4

4 20 5

6 5 30 6

8 7 56 8

  The principle which has just been described is applicable in particular to computer networks where several stations are directly connected at speeds of 2 Mbit / s for example. In this case, sources that are less efficient in switching time and with a large tunable range (DBR laser diodes) can be used. This makes it possible to carry out 17 time switches, therefore to assign 17 channels per source (as described in "Electronics Letters" of October 10, 1991, pages 1969-1971) by DELORME), which corresponds to a network

of 18 stations.

  In the above description, the station is

  assumed to include only a single laser source

  devil. However, the invention is not limited to this case.

  As the loop can comprise two directions of propagation (bidirectional loop) on two (or more than two) optical fibers of connection, each station can include two (or more than two) laser sources 1! tunable, each source cooperating with a particular fiber. This is represented by FIG. 4 o the station Si comprises a first tunable source (SLi) 1 coupled to a first fiber (FO) 1 by a first coupler (COi) 1 and a second tunable source (SLi) 2 coupled to a second fiber (FO) 2 by a second optical coupler (COi) 2. The other means (multiplexer, filters, photodetectors, demultiplexers) are not shown for simplicity. Each optical fiber can be doubled by

a spare fiber (dashed).

  The first source (SLi) 1 establishes the link with a first group of other stations referenced by the index p, and this, through the first optical fiber (FO) 1. It therefore emits at Xip wavelengths. The second source (SLi) 2 establishes the link with a second group of stations (all the others) referenced by the index k and this, through the second optical fiber

  (FO) 2. It therefore emits at Xik wavelengths.

  All the stations are thus linked together either by one fiber or by the other, that is to say either

  in one direction or the other: the loop is bidirec-

tional.

  FIG. 5 illustrates such a bidirectional loop

  nelle in the case of five stations Si, S2, ..., S5.

  It shows the distribution and circulation of the various

  its wavelengths. The station Si, for example, is in connection with the stations S4 and S2 by the fiber (FO) 1, and in connection with the stations S5 and S3 by the fiber (FO) 2. She is therefore in good contact with

four other stations.

  This station Si therefore comprises a first tunable source emitting on the two wavelengths X14 and 212 specific to the links of Si to S4 and of Si to S2 and a second tunable source emitting on the

two wavelengths% 15 and -13.

  As for the wavelength X54, which also leaves Si via the fiber (FO) 1, it comes from the station S5 and it is intended for S4. Likewise, the wavelength 145, which leaves Si by (FO) 2, comes from S4 and

is for S5.

  In such a bidirectional loop (FIG. 5), a tunable source per station and per fiber is needed, ie two sources per station, therefore ten sources for the entire loop. With the technique of the prior art, two sources per station and per fiber are required, i.e. four sources per station, therefore twenty

sources in total.

  To make the best use of the current fast switching performance of DFB laser diodes (four channels), they can be installed on a high-speed bidirectional loop comprising nine stations; there are two sources per station and each source generates four channels. According to the invention, a bidirectional loop with nine stations requires the installation of eighteen sources while this same loop equipped according to the prior art, would entail the installation of eight sources per station, ie 72

  sources for the entire loop.

  Such a bidirectional loop with fixed laser sources is described in the article by A.F. ELREFAIE, entitled "Multiwavelength Survivable Ring Network Architectures", published in the Accounts of the

ICC Conference, May 1993.

Claims (6)

  1. Multiplexing interconnection loop in the   time and wavelength, this interconnected loop   as many N stations (Si, S2, ...) using at least one optical fiber (FO, FO), any station (Si) of rank i, comprising:   - an optical coupler (COi) placed on the op fiber   tick (FO, FO ') and having an input and an output, - emission means (MEi) connected to the input of the optical coupler (COi), these means (MEi) being   able to emit N-1 wavelengths (? ij) pro-   close to the N-1 links to be established between said station   tion (Si) and the N-1 other stations (Sj), - reception means (MRi) connected to the output of the optical coupler (COi), these means (MRi) being able to receive and detect the N-1 wavelengths (XJi) specific to the N-1 links established between the (N-1) other stations (Sj) and said station (Si), this loop being characterized by the fact that: a) the transmission means ( MEi) of each station (Si) include: * means (MPXi) for receiving the data (Dij) to be transmitted to the other stations (Sj), for organizing this data into time-multiplexed groups (Gij), each group (Gij ) being intended for another station (Sj), and for delivering a control signal (SCij), * at least one semiconductor laser source (SLi) tunable in wavelength, this source receiving the data groups (Gij) and the control signals (SCij) and thus emitting a multicolored sequence (Seq) of signals (Lij) at different wavelengths (Xij) specific to the link to be established between said station (Si) and other stations (Sj), b) the reception means (MRi) of each station (Si) include: * filters (FJi) centered on the lengths   wave (XJi) specific to the connections between other   very stations (Sj) and said station (Si), * as many photodetectors (PDJi) connected to the filters (FJi),   * a demultiplexer (DMPXi) having inputs   linked to photodetectors (PDJi) and outputs   delivering the data (DJi) coming from the very stations (Sj).   2. Loop according to claim 1, characterized in that the data groups (Gij) are bytes.   3. Buckle according to any one of the claims.   1 and 2, characterized in that the data groups (Gij) can be separated from each other   others by at least one binary element.   4. Loop according to claim 1, characterized in that the tunable semiconductor lasers are multi-electrode lasers.   5. Buckle according to any one of the claims.   1 to 4, characterized in that it includes   two optical fibers with reverse propagation directions.   6. Buckle according to claim 5, characterized   by the fact that each station (Si) comprises a pre-   first and second semiconductor laser sources   (SLi, SLi '), the first of the two sources (SLi) is   blocking a link between said station (Si) and a first group of other stations (Sp) through the first optical fiber (FO) in a first direction, the second source (SLil) establishing a link between   said station (Si) and a second group of other stations   tions (Sk) through the second optical fiber (FOI ') in a second sense.