Dispersion compensation device

Abstract: The compensation device is used in an optical network where signals are transmitted from a main station (M) to one or a plurality of user stations (US1 to US3). It compensates for analogue optical signal distortion caused by laser chirp of a laser in the main station (M) which generates said analogue optical signal and fiber dispersion of the fiber link over which the optical signal is transmitted. The device includes a distance measuring device (R) generating a result signal (MS) indicative of the distance between the main station and the user stations and which is possibly already available in the network to perform ranging when the latter network is also used to transmit digital signals from the user stations to the main station using Time Division Multiple Access. It also includes a processing circuit (PC) which in response to a given laser chirp and the result signal, provides a tuning signal (V) to tune the bias voltage of a varactor diode (VD) which is part of the compensation circuit inserted in the electrical signal path. In an optimal implementation the measuring device (R) and the processing circuit (PC) are located in the main station (M), whilst the varactor diode (VD) is located in each user station.

Description: The present invention relates to a compensation device coupled to an optical fiber network and including at least one tunable varactor diode which is tuned so as to compensate for the distortion an analogue optical signal undergoes when being generated by a laser and transmitted over a distance through said fiber network, said distortion being caused by laser chirp of said laser and by fiber dispersion due to said transmission, and being dependent on said distance. Such a compensation device is already known in the art, e.g. from the article "Electrical predistortion to compensate for combined effect of laser chirp and fibre dispersion" by H. Gysel and M. Ramachandran and published in Electronic Letters of 28th February 1991, Vol. 27, No. 5, pp. 421-423. Therein, the optical signal is transmitted from a sending station to a receiving station, and the varactor diode is tuned to compensate optimally for a predetermined distance between the sending and the receiving station. It is clear that whenever the transmission distance between these stations changes this distance has to be measured again and the tuning of the varactor diode by means of its bias voltage has to be adjusted to achieve optimum compensation. An object of the present invention is to provide a compensation device of the above type where the tuning of the varactor diode and the adaption thereof in case of distance change or a change of laser chirp, is performed automatically and not by hand. According to the invention, this object is achieved due to the fact that said compensation device moreover includes an automatic distance measuring device providing a result signal indicative of said distance, and a processing circuit which in response to said result signal and the value of said laser chirp provides a tuning signal for said varactor diode. Another characteristic feature of the compensation device according to the present invention is that said fiber network is a passive optical network where a main station is connected to a plurality of user stations via a common link and a respective user link, said network being additionally used to transmit digital signals from said user stations to said main station and including a ranging device which determines for each of said user stations its distance over said network from said main station in order to schedule the access of said user stations to said network for said transmission of said digital signals, and that said ranging device comprises said distance measuring device. In this way the ranging device which is anyway needed in the above network to be able to schedule, as mentioned, the access of the user stations to the network, can be used as distance measuring device, and, consequently, to realize the mentioned tuning in an automatic way in a network as the above one, the only additionally needed hardware is the processing circuit and the varactor diode. To be noted that in such a network, the ranging is performed periodically, so that a change in the mentioned distance is always detected by the ranging device and can then be transmitted to the processing device for adjustment of the tuning signal when the change is important enough to require such an adjustment. Thereby the above mentioned automatic adjustment is provided. Another characteristic feature of the compensation device according to the invention is that said processing circuit uses at least one table to convert the value of a predetermined characteristic of said result signal indicative of said distance into a value of said tuning signal, based on a predetermined value of said laser chirp. As a consequence, given the length of the fiber over which the optical signal is transmitted, the tuning signal, usually a voltage signal, can be determined by the processing circuit for each possible value of the chirp. The value of this voltage signal is consequently automatically adapted whenever for instance a laser is replaced by another one with a different chirp value or when the length of the connecting fiber cable changes. In an optimal embodiment of the compensation device the automatic distance measuring device and the processing circuit are located in the main station and the varactor diode is part of the user station. The measuring device then provides a result signal for each user station and the corresponding tuning signal is sent to the respective user station. However other configurations are possible. A processing circuit can be located in each user station together with a varactor diode, whilst the automatic distance measuring device is located in the main station. This implies that the result signal and the laser chirp value have to be transmitted to the respective user station and that the processing device has to be multiplicated. Also, the complete compensation device can either be located in each user station or in the main station. However, in the former case this implies multiplication as well of the processing circuit as of the distance measuring device. In the latter case the varactor diode can only perform optimally for user stations for which the distance to the main station lays within a predetermined distance range, called distance window, for which that diode can be tuned to perform optimally. A plurality of varactor diodes or one diode tuned by a plurality of possible bias voltages has then to be used in the main station, i.e. one diode or one bias voltage for each distance window, to be able to send an optimally compensated signal to all the user stations. In such an embodiment, a selection circuit is needed to select the correct diode or the correct bias voltage, since the choice of the used varactor diode or of the correct bias voltage then depends on the destination user station. To be noted that the last mentioned possible implementation is only applicable when the optical signals are not sent simultaneously to all users. As a consequence this implementation is not suited for transmission of CATV signals. 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 drawings wherein : Fig. 1 shows an optical network wherein a compensation device (C, VD) according to the invention is used; and Fig. 2 represents the control circuit C of M of Fig. 1 in more detail. Fig. 1 shows a passive optical network connecting a main station M to three user stations US1 to US3 via a common optical fiber link lc, an optical splitter SP and respective user optical fiber links l1 to l3. M includes a transmitter Tc having a laser (not shown) for sending CATV signals in optical format to US1, US2 and US3, as well as a digital transmitter Td and a digital receiver Rd for transmitting and receiving digital signals to and from US1 to US3 respectively. The digital signals are transmitted from US1, US2 and US3 to M using a Time Division Multiplexing Access (TDMA) method, as described in the European Patent Application EP-A-0 544 975, published on 09.06.93. Both the digital and analogue optical signals are transmitted over the same link lc using wavelength multiplexing. Tc and ,Td and Rd are coupled to a wavelength multiplexer WM via an optical amplifier OA and an optical coupler CO respectively and WM is connected to the optical splitter SP by lc. M additionally includes a control circuit C with an input ARS, at which a like named signal is received, and to which Rd is connected, and with an output TS connected to Td. US1 to US3 are identical user stations of which only US1 is represented in more detail. It includes a wavelength demultiplexer WDM with an input connected to 11 and with an output connected on the one hand, via a link IN on which like named signals are received, to a receiver Rc for receiving the CATV signals, and on the other hand via a coupler CO1 with a digital receiver Rd1 and a digital transmitter Td1. Via a link V on which like named signals are transmitted, Rd1 is connected with Rc and more specifically with a varactor diode VD included therein. RD1 is also connected to Td1. To be noted that the network elements Tc, Td, Rd, CO, OA, WM, SP, WDM, Rc, Rd1, Td1 and CO1 are well known in the art. Therefore they are not described in detail. The control circuit C of M is represented in more detail in Fig. 2. It includes a distance measuring device or more specifically a ranging circuit R which generates a signal RRS and to which a signal ARS is applied. R measures the distances between M and US1 to US3 by means of the time delay between signals ARS and RRS and in the way described in the European Patent Application EP-A-0 521 197, published on 07.01.93. It supplies a result signal MS, indicative of the distance between the main station and a considered user station, to a processing circuit PC which generates a tuning signal V. V and RRS are transmitted to the user stations US1 via TS, Td, CO, WM, lc, SP and l1 as shown in Fig. 1. US2 and US3 receive the tuning signal and the ranging signal in a similar way. PC is not described in further detail since its realization is obvious for a person skilled in the art, thanks to the following description of the working of the compensation device. Due to chirp of the laser in Tc, an optical CATV signal generated by Tc has slightly varying wavelengths as a result of the varying amplitude of the corresponding electric signal applied to that laser. Due to fiber dispersion these varying wavelengths introduce varying delays of the optical signal after transmission through the fiber which are related to the mentioned varying amplitude. As a consequence, the signal is distorted after transmission. More details about such distortion caused by laser chirp and fiber dispersion and about its compensation by using varactor diodes may be found in the earlier mentioned article and in the article from Leonard J. Cimini jr. et al. "Optical Equalisation to Combat the Effects of Laser Chirp and Fiber Dispersion" published in the Journal of Lightwave Technology, vol. 8, No 5 of May 1990, pp. 649-659. The distortion of CATV signals sent by the main station M to the user stations US1 to US3 is compensated for by the varactor diodes included in the user stations, as described in the above article. These diodes are tuned by the signal V as described hereafter, using as references Figs. 1 and 2. This tuning signal V is, as mentioned earlier, dependent on the distance between M and US1 to US3 and on the chirp value of the laser used in M. How this tuning is automatically performed is described for US1, this description being equally applicable to US2 and US3. As described in detail in the second mentioned European Patent Application, the result signal MS which is indicative of the distance between M and US1, is determined by sending the digital signal RRS from Td to Rd1 of US1 via CO, WM, SP, WDM and CO1 and determining how long it takes for the response signal ARS generated by Td1 of US1 to reach Rd in M. MS is passed to the processing circuit PC which converts it to the value of a tuning voltage V using a (not shown) table which, when addressed by the above distance MS and a predetermined laser chirp value CV of the used laser Tc in M, provides a voltage value. The signal V which has a voltage characteristic indicative of the latter value is then transmitted to US1 where it is used to tune VD. To be noted that the above table can for instance be obtained in an empirical way or can be generated by a software program of the processing circuit which determines the value of a voltage function having as parameters the mentioned distance and laser chirp, and this for each considered value of MS and CV. Whenever either the laser chirp or the length of the fiber between M and US1 changes, new values of CV and/or of MS, as determined by R which periodically performs its ranging operation, are used in PC as input of the earlier mentioned table, and V is adapted accordingly. Thus the tuning of the varactor diode VD is adapted dynamicaly and automatically. As long as there is no change in the length of the fiber or in the laser chirp, there is no need to activate the use of the above mentioned table since the previously determined tuning signal can then still be used to tune the diode. Therefore the value thereof is stored in a not shown memory module. This value is retrieved and converted in a tuning signal each time a signal distortion is to be compensated for. Whenever the fiber length or the laser chirp changes, a new tuning signal is generated by PC using the table and the old value is overwritten in the memory module. The realization of the above storage and retrieval is not shown, nor described in detail since it is considered to be obvious for a person skilled in the art. It has to be noted that there is no need for adaption of the tuning in case of small distance variations since VD works optimally for a predetermined window of distances, i.e. for user stations located within a predetermined distance range the same tuning voltage is used to tune their respective varactor diodes. Also to be noted that as mentioned earlier, the complete device could be located either in the main station or in each user station, or that a processor circuit could, together with the varactor diode, be located in each user station. However, these configurations have some important, earlier mentioned, disadvantages with respect to the described one. 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: Compensation device coupled to an optical fiber network and including at least one tunable varactor diode (VD) which is tuned so as to compensate for the distortion an analogue optical signal undergoes when being generated by a laser and transmitted over a distance through said fiber network, said distortion being caused by laser chirp of said laser and by fiber dispersion due to said transmission, and being dependent on said distance, characterized in that said compensation device moreover includes an automatic distance measuring device (R) providing a result signal (MS) indicative of said distance, and a processing circuit (PC) which in response to said result signal and the value of said laser chirp provides a tuning signal (V) for said varactor diode. Compensation device according to claim 1, characterized in that said fiber network is a passive optical network where a main station (M) is connected to a plurality of user stations (US1, ..., US3) via a common link (lc) and a respective user link (l1; ...; l3) , said network being additionally used to transmit digital signals from said user stations to said main station and including a ranging device which determines for each of said user stations its distance over said network from said main station in order to schedule the access of said user stations to said network for said transmission of said digital signals, and that said ranging device comprises said distance measuring device (R). Compensation device according to claim 2, characterized in that said processing circuit (PC) uses at least one table to convert the value of a predetermined characteristic of said result signal (MS) indicative of said distance into said tuning signal (V), based on a predetermined value of said laser chirp. Compensation device according to claim 2, characterized in that said optical signal is transmitted from said main station to each of said user stations. Compensation device according to claim 4, characterized in that at least part (VD) of said compensation device (C,VD) is located in each of said user stations. Compensation device according to claim 5, characterized in that said distance measuring device (R) and said processing circuit (PC) are located in said main station, whilst each of said user station includes one of said varactor diodes,said tuning signal (V) being transmitted to each of said user stations over said network. Compensation device according to claim 5, characterized in that said distance measuring device (R) is located in said main station,whilst said processing circuit includes a plurality of processing subcircuits each of which is located in a corresponding one of said user stations which additionally includes one of said varactor diodes, said result signal (MS) being transmitted to each of said user stations over said network. Compensation device according to claim 2, characterized in that it is constituted by a plurality of compensation circuits each of which is located in said main station and compensates for the distortion of an optical signal sent to a user station belonging to an associated set of said user stations for each of which its transmission distance from said main station lays within a predetermined range, each of said circuits including a dedicated varactor diode and said first mentioned processing circuit being common to at least part of said compensation circuits. Compensation device according to claim 8, characterized in that it includes a selection circuit which indicates to said common processing circuit the result signal to be taken into account and selects one of said dedicated varactor diodes depending on the set the user station to which said optical signal has to be transmitted belongs to. Compensation device according to claim 2 characterized in that each of said user stations belongs to an associated set of said user stations for each of which its transmission distance from said main station lays within a predetermined range, that said compensation device is completely located in said main station and that it includes a selection circuit which selects the result signal to be taken into account by the processing circuit depending on the set the user station to which said optical signal has to be transmitted belongs to. Compensation device according to claim 3, characterized in that said processing circuit includes a memory module wherein the value of said tuning signal is stored, said value being retrieved for said tuning each time one of said analogue optical signals is transmitted and said conversion being activated to provide a new tuning signal at least when either said distance or said laser chirp changes, said stored value then being overwritten by the value of said new tuning signal. Compensation device according to claim 4, characterized in that said optical signal is a CATV signal.

Inventors: BERGER WERNER, HARNISCH STEPHEN, MESTDAGH DENIS JULIEN GILLES, SIERENS CHRISTIAAN HENDRIK JAN, STEIGENBERGER ULRICH, VAN DER PLAS GERT, BERGER, WERNER, HARNISCH, STEPHEN, MESTDAGH, DENIS JULIEN GILLES, SIERENS, CHRISTIAAN HENDRIK JAN, STEIGENBERGER, ULRICH, VAN DER PLAS, GERT
Applicants: ALSTHOM CGE ALCATEL, ALCATEL
Application year: 1993
Main classification: H04B10
IPC codes: H04B10, H01S3, G02F1