Arrangement for amplifying and combining optical signals, and method for upstream transmission realised therewith

Abstract: The optical amplifier splitter arrangement is used in a tree-like optical network (APON) consisting of the cascade connection of dedicated branches, of the arrangement and of a common branch. The arrangement is coupled between a plurality of optical network users and an optical line terminator, via the dedicated branches and the common branch respectively. The network enables upstream transmission of information signals from the optical network users to the optical line terminator. The optical amplifier splitter arrangement includes for each branch of the dedicated branches : an optical amplifier to amplify an information signal with a gain value and to thereby generate an amplified information signal with a predetermined power level; an optical switch on/off coupled between the optical amplifier and an optical splitter to pass the amplified information signal when the information signal is present and to interrupt the branch when the information signal is not present. The optical splitter is included in the arrangement to combine all amplified information signals according to a multiple access technique and to thereby generate an outgoing optical signal for application to the optical line terminator.

Description: The present invention relates to an optical amplifier splitter arrangement as described in the preamble of claim 1 and a method for upstream transmission realised thereby, used in a tree-like optical network to perform upstream transmission as described in the preamble of claim 8. Such an optical amplifier splitter and such a method are already known in the art, e.g. from the article 'Network Design for the Information Superhighway' written by P.W. France, J.H. Mackenzie and others, published by The European Institute for Communications and Networks in the book Proceedings volume 1 Papers on Broadband Superhighway wherein passive optical networks are described which use such a splitter and which realise such a method. These passive optical network systems are developed to meet the emerging superhighway requirements such as the capability to provide both existing narrow-band and new broadband distributive and interactive services to serve both business and residential customers. However serving business and residential customers requires a much higher splitting factor than provided by these known networks. A main problem in the realisation of high splitting factors occurs during the upstream transmission. Indeed, due to this high splitting factor a high power-budget must be overcome. The demanding optical power-budget for transmission is supported by the implementation of optical amplifiers in the dedicated branches. This is for instance known from the European Patent Application with publication number EP-A-0 499065 with filing date 23.01.92 and title 'Optisches Nachrichtenübertragunssystem für den Teilnehmeranschlussbereich mit optischen Verstärkern'. This document describes, as shown in Figure 1, an optical amplifier splitter arrangement (A) coupled between a plurality of network users and an optical line terminator. The tree like optical network enables upstream transmission of information signals (col. 10, line 48-56). However, due to the multiple access technique used at the optical splitter which combines the information signals, all these optical amplifiers contribute to an accumulation of ASE (amplified spontaneous emission) noise, even when the optical amplifier does not carry an information signal and as a result thereof, the ASE noise can reach such an amplitude that the transmitted signals are degraded. An optical amplifier splitter arrangement and a method such as the above known ones but which is suited for networks with high splitting factors, i.e. which ensure that the transmitted signals can not be degraded by the relatively high ASE noise is known from the European patent application with publication number 0534 433 A2 and filing date 24.09.92 and title 'Optical regenerative-repeater system. This document shows an optical branching system that tackles the problem of reducing noise. The applied solution is to interrupt the branches that carry no information so that these can not contribute to the accumulation of noise (see col. 7 lines 5 to 27 and Figure 3).Since the branches which carry no information signal, are interrupted with the optical switches on/off, these branches can not contribute to the accumulation of ASE noise at the optical splitter. A possible implementation of the control of the optical amplifier splitter arrangement is that for each branch the gain value of the optical amplifier and the optical switch on/off are controlled respectively by means of a first and second electrical control signal. This implementation is described in this document. Indeed, the gain is controlled in that it is determined by the amount of supply current (col. 5 lines 10 to 21 and col. 6 lines 19 to 34) and the optical latch circuit is acting as a switch. The first electrical control signal is composed of gain setting data and gain tuning data. This gain tuning data is introduced to compensate deviations of the optical amplification due to e.g. sensitiveness with respect to temperature and time of the optical amplifier. An additional feature in this document is that the switch on/off is controlled by means of the second electrical control signal that results from the detection of the presence of an information signal on the according branch. Indeed, a power detection means, detecting the peak value of a supplied signal, is incorporated (See col. 7, lines 5 to 14) and is used to detect if there is a signal or not. Furthermore the determination of whether there is a signal or not is applied when a timing signal is applied. A timing tank supplies a timing signal having a frequency synchronised with a rate of transmission data (col. 5 line 57 to col. 6 line 1). An object of the present invention is to provide an optical amplifier splitter arrangement and a method such as the above known ones but with an alternative way of controlling the switch on/off. According to the present invention, this object is achieved by means of the optical amplifier splitter arrangement as described in claim 1, realising the method as described in claim 8. Indeed, the method according to the present invention uses, in an optical network using for upstream transmission a multiple access technique, the detection of special grant information out of downstream information. The use of such special grant information to realise multiple access is well known in the art, e.g. from the published European patent application with publication number EP-0 544 975 wherein a time slot management system is described which allows upstream transmission. The stream of station identities used therein is similar to the special grant information. In claim 1 the second electrical control signal is determined by the detection of special grant information in downstream information. An optical amplifier splitter arrangement according to the present invention that combines the amplified information signals according to a time division multiple access technique is described in claim 2. Yet a further characteristic feature of the present invention is that, as described in claim 3, the gain setting data is determined by measuring the power of the information signal. Another characteristic feature of the present invention is that, as described in claim 4, the gain tuning data is determined by measuring the power of the amplified information signal. An alternative way to determine the gain tuning data is described in claim 5 and consists in using a memory means to memorise the value of the last measured power of an amplified signal and to use this value to control the gain tuning data for a following transmitted information signal. Another way to determine the gain tuning data is described in claim 6, which introduces a power level unit to measure the power level of the outgoing optical signal to determine the gain tuning data for the branches carrying an information signal. In this way, instead of measuring the power for each dedicated branch, only one power measurement has to be performed which is less power consuming. Still another characteristic feature is the integration of the optical splitter with all optical switches on/off in one optical switch, as described in claim 7. The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawing. First, the optical amplifier splitter arrangement OAS will be explained by means of a functional description of the blocks shown in the figure. Based on this description, implementation of the functional blocks in the figure will be obvious to a person skilled in the art. The optical amplifier splitter arrangement OAS is used in a tree-like optical network consisting of the cascade connection of dedicated branches B1, B2, ..., Bi,...Bn and of the optical amplifier splitter arrangement OAS and of a common branch. The optical amplifier splitter arrangement OAS is coupled between a plurality of optical network users ONU1, ONU2, ..., ONUi, ..., ONUn and an optical line terminator OLT via the above mentioned dedicated branches and common branch respectively. The optical amplifier splitter arrangement OAS includes for each branch e.g. Bi, of the dedicated branches B1, B2,..., Bi, ..., Bn two basic parts : an optical amplifier OAi coupled to the respective netwotk user ONUi of the optical network users ONU1, ONU2, ..., ONUi, ..., ONUn; and an optical switch on/off SW ON/OFFi coupled between the optical amplifier OAi and an optical splitter OS. The optical splitter OS is coupled between all optical switches on/off SW ON/OFF1, SW ON/OFF2, ..., SW ON/OFFi, ..., SW ON/OFFn which are simular to the optical switch on/off SW ON/OFFi and the optical line terminator OLT. The optical amplifier splitter arrangement OAS enables upstream transmission in the optical network of information signals IN1, IN2, ..., INi, ... , INn from the optical network users ONU1, ONU2, ..., ONUi, ..., ONUn to the optical line terminator OLT. The principle working of the optical amplifier splitter OAS will be explained in the following paragraph with respect to a signal INi transmitted over a branch Bi, the working with respect to the other branches being identical. The optical amplifier OAi amplifies INi with a gain value Gi (not shown) and generates thereby an amplified information signal A-INi with a predetermined power level. The optical switch on/off SW ON/OFFi passes the amplified information signal A-INi when it is present, but interrupts the branch Bi when the amplified information signal A-INi is not present. The optical splitter OS combines all amplified information signals A-IN1, A-IN2, ..., A-INi, ..., A-INn simular to the amplified information signal A-INi according to a multiple access technique and generates thereby an outgoing optical signal OUT for application to the optical line terminator OLT. In this embodiment the working of the optical amplifier OAi and the optical switch on/off SW ON/OFFi of each branch Bi is controlled by means of control means CTRLi. The control means CTRLi is coupled to the OAi and the optical switch on/off SW ON/OFFi and controls : the gain value Gi by means of a first electrical control signal CTRL-Ai composed of gain setting data GSi (not shown) and gain tuning data GTi (not shown); and the switch on/off SW ON/OFFi by means of a second electrical control signal CTRL-Bi. The gain setting data GSi, the gain tuning data GTi and the second electrical control signal CTRL-Bi can be determined in different ways. A few particular implementations to determine them are described in a functional way in the following paragraphs, but they are not shown in the figure to avoid overloading thereof. To determine the second electrical control signal CTRL-Bi, for each branch Bi, the control means CTRLi further includes power detection means which via an optical tap draws off the information signal INi a small power part. The power detection means determines whether the information signal INi is present or is not present and determines the second electrical control signal CTRL-Bi as a result thereof. To determine the gain setting data GSi, for each branch Bi, the control means CTRLi further includes power measuring means. The same optical tap as above is used and the power measuring means measures the power of the information signal INi and determines the gain setting data GSi based thereon. It has to be noted here, that to ensure that the power measuring means has enough time to fulfil its function, an optical delay line (not shown) is coupled between the optical tap and the optical amplifier OAi. The optical delay line delays the information signal INi untill the optical amplifier OAi is adjusted. To determine the gain tuning data GTi, for each branch Bi, the control means CTRLi further includes additional power measuring means. Via an additional optical tap, coupled between the optical switch on/off SW ON/OFFi and the optical splitter OS a small power part is drawn off the amplified information signal A-INi and the additional power measuring means measures the power of the amplified information signal A-INi and determines the gain tuning data GTi based thereon. Furthermore it is noted that although for the optical amplifier OAi an erbium doped fibre amplifier is used, semiconductor optical amplifiers are very good candidates to use in this implementation because of their low switch-on time. It is also remarked that these semiconductor optical amplifiers can be used to integrate the functions of the optical amplifier OAi and of the optical switch on/off SW ON/OFFi. An alternative implementation to determine the second electrical control signal CTRL-Bi is realised for optical networks where the optical splitter OS combines the amplified information signals A-IN1, A-IN2, ..., A-INi, ...,, A-INn according to a time division multiple access technique. For such an optical network, the optical amplifier splitter arrangement OAS includes an optical network terminator NTOAM (not shown) for performing operation and maintenance functions. The NTOAM is coupled to the optical line terminator OLT, by means of also an additional optical tap, and to the control means CTRLi. The additional optical tap draws off the downstream signals which are sent from the optical line terminator OLT to the optical network terminators ONU1, ONU2, ..., ONUi, ..., ONUn a small powerpart and provides this power fraction to the optical network terminator NTOAM which captures special grant information out of these downstream signals. According to the remark in the introduction, the use of special grant information to realize time division multiple access is well known in the art. With this special grant information, the optical network terminator NTOAM knows exactly when an information signal INi will be sent by an optical network user ONUi and when the information signal INi will be applied to the optical amplifier OAi. The special grant information is applied to the control means CTRLi which is now able to determine whether the information signal INi will be present or willl not be present within a predetermined time interval and determines the second electrical control signal CTRL-Bi as a result thereof. Still another alternative implementation to determine the gain tuning data GTi, for each branch Bi, is realised by including in the control means CTRLi, additional to measuring means to measure the power of an amplified information signal A-INi, also memory means to memorize the value of the measured power of the amplified signal. This value is used to determine the gain tuning data GTi for a following transmitted information signal INi. Finally, it has to be remarked that an alternative way to determine the gain tuning data GTi, for each branch Bi, is realized with the implementation of one power level unit. The power level unit, included in the optical amplifier splitter arrangement OAS, is coupled between a control output of the optical splitter OS and the control means CTRLi associated with each branch Bi. The power level unit measures the power level of the outgoing optical signal OUT and provides the result thereof by means of electrical power level data to the control means CTRLi. The control means CTRLi is now able to determine the gain tuning data GTi if an information signal is present. It has to be remarked that the optical splitter OS and all optical switches on/off SW ON/OFF1, SW ON/OFF2, ..., SW ON/OFFi, ..., SW ON/OFFn simular to the optical switch on/off SW ON/OFFi can be integrated in one optical switch. Since it is obvious for a person skilled in the art how to realize the above alternative implementations, based on their functional description, these realizations are not described in further details. It has also to be remarked that the outgoing optical signal OUT of the present optical amplifier splitter arrangement OAS has to be amplified in order to overcome a longe distance to the optical line terminator OLT. With an efficient working of the different optical amplifiers OA1, OA2, ..., OAi, ..., OAn simular to the optical amplifier OAi in the optical amplifier splitter arrangement OAS and an appropriate access protocol that assures continuous upstream transmission for the outgoing signal OUT, the outgoing optical signal OUT has no longer a bursty character like the information signals IN1, IN2, ..., INi, ..., INn have. In this way, a fast gain setting optical amplifier is no longer required for the amplification of the optical outgoing signal OUT and a standard optical amplifiers can be used. While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

Claims: Optical amplifier splitter arrangement (OAS) in a tree-like optical network (APON) consisting of the cascade connection of dedicated branches (B1, B2,..., Bi, ..., Bn), of said arrangement (OAS) and of a common branch, said arrangement being coupled between a plurality of optical network users (ONU1, ONU2, ..., ONUi, ..., ONUn) and an optical line terminator (OLT) via said dedicated branches and said common branch respectively, said network enabling upstream transmission of information signals (IN1, IN2, ..., INi, ...,INn) from said optical network users (ONU1, ONU2, ..., ONUi, ..., ONUn) to said optical line terminator (OLT), said optical amplifier splitter arrangement (OAS) includes for each branch (Bi) of said dedicated branches (B1,B2, ..., Bi, ..., Bn): an optical amplifier (OAi) to amplify a respective one (INi) of said information signals (IN1, IN2,..., INi, ...,INn) transmitted over said branch (Bi), with a gain value (Gi) and to thereby generate an amplified information signal (A-INi) with a predetermined power level; an optical switch on/off (SW ON/OFFi) coupled between said optical amplifier (OAi) and an optical splitter (OS) which is included in said arrangement to combine all amplified information signals (A-IN1, A-IN2, ..., A-INi, ..., A-INn) similar to said amplified information signal (A-INi) according to a multiple access technique and to thereby generate an outgoing optical signal (OUT) for application to said optical line terminator (OLT), said optical switch on/off (SW ON/OFFi) being provided to pass said amplified information signal (A-INi) when it is present and to interrupt said branch (Bi) when it is not present, said arrangement (OAS) also includes for each said branch (Bi) a control means (CTRLi) coupled to said optical amplifier (OAi) and to said optical switch on/off (SW ON/OFFi), said control means (CTRLi) being adapted to control said gain value (Gi) by means of a first electrical control signal (CTRL-Ai) composed of gain setting data (GSi) and gain tuning data (GTi), and to control said switch on/off (SW ON/OFFi) by means of a second electrical control signal (CTRL-Bi), characterised in that said optical splitter (OS) is provided to combine said amplified information signals (A-IN1, A-IN2, ..., A-INi, ..., A-INn) according to a multiple access technique, and in that said arrangement (OAS) also includes an optical network terminator (NTOAM) for performing operation and maintenance functions and which is coupled to said optical line terminator (OLT) and said control means (CTRLi), said optical network terminator (NTOAM) being included to capture special grant information out of downstream signals sent from said optical line terminator (OLT) to said optical network users (ONU1, ONU2, ..., ONUi, ..., ONUn) and to apply said special grant information to said control means (CTRLi) to thereby enable said control means (CTRLi) to determine whether said information signal (INi) will be present or not present within a predetermined time interval and to determine said second electrical control signal (CTRL-Bi) as a result thereof. Optical amplifier splitter arrangement (OAS) according to claim 1, characterised in that said optical splitter (OS) is provided to combine said amplified information signals (A-IN1, A-IN2, ..., A-INi, ..., A-INn) according to a time division multiple access (TDMA) technique. Optical amplifier splitter arrangement (OAS) according to claim 1 characterised in that for each said branch (Bi) said control means (CTRLi) further includes power measuring means provided to measure the power of said one information signal (INi) and to determine said gain setting data (GSi) based thereon. Optical amplifier splitter arrangement (OAS) according to claim 1 characterised in that , for each said branch (Bi), said control means (CTRLi) further includes power measuring means provided to measure the power of said amplified information signal (A-INi) and to determine said gain tuning data (GTi) based thereon. Optical amplifier splitter arrangement (OAS) according to claim 1 characterised in that , for each said branch (Bi), said control means (CTRLi) further includes power measuring means provided to measure the power of said amplified information signal (A-INi) and to provide a value of measured power of said amplified information signal (A-INi) and further includes memory means to memorize said value, said value being used to determine said gain tuning data (GTi) for a following transmitted information signal (INi). Optical amplifier splitter arrangement (OAS) according to claim 1 characterised in that said arrangement (OAS) also includes a power level unit (P-opt) coupled between a control output of said optical splitter (OS) and said control means (CTRLi) associated with each branch (Bi), said power level unit being (P-opt) provided to measure the power level of said outgoing optical signal (OUT) and to supply as a result thereof electrical power level data (POUT) to said control means (CTRLi) to thereby enable said control means (CTRLi) to determine said gain tuning data (GTi) when said information signal (A-INi) is present. Optical amplifier splitter arrangement (OAS) according to claim 1 characterised in that said optical splitter (OS) and all optical switches on/off (SW ON/OFF1, SW ON/OFF2, ..., SW ON/OFFi, ..., SW ON/OFFn) simular to said optical switch on/off (SW ON/OFFi) are integrated in an optical switch (OSW). A method used in a tree-like optical network (APON) to perform upstream transmission of information signals (IN1, IN2,..., INi, ..., INn) from a plurality of optical network users (ONU1, ONU2, ..., ONUi, ..., ONUn) to an optical line terminator (OLT) via dedicated branches (B1, B2,..., Bi, ..., Bn) and a common branch respectively, said method comprises for each one (INi) of said information signals (IN1, IN2, ..., INi, ..., INn) the steps of: a. amplifying said information signal (INi) and thereby generating an amplified information signal (A-INi); b. passing said amplified information signal (A-INi) when said information signal (INi) is present and interrupting said branch (Bi) when said information signal (INi) is not present; and further includes the step of: c. combining all amplified information signals (A-IN1, A-IN2, ..., A-INi, ..., A-INn) simular to said amplified information signal (A-INi) according to a multiple access technique thereby generating an outgoing optical signal (OUT) which is transmitted to said optical line terminator (OLT), characterised in that said method further comprises capturing special grant information out of downstream signals sent from said optical line terminator (OLT) to said optical network users (ONU1, ONU2, ..., ONUi, ..., ONUn) and therewith determining whether said information signal (Ini) will be present or not present within a predetermined time interval in order to enable execution of said step b.

Inventors: VAN DE VOORDE INGRID ZULMA BEN, VAN DER PLAS GERT, VAN DE VOORDE, INGRID ZULMA BENOIT, VAN DER PLAS, GERT
Applicants: CIT ALCATEL, ALCATEL
Application year: 1995
Main classification: H04B10
IPC codes: H04B10, H04Q11, H01S3