Optical repeater and output level control method

Abstract: An optical repeater, in which an output level of each optical signal of WDM optical signals does not change even when the number of wavelengths of inputted WDM optical signals changes, which can be applied to a system in which intervals among WDM optical signals are relatively narrow, is provided. The optical signal repeater provides an optical signal amplifying means (2) for amplifying WDM optical signals transmitted from an optical transmitter via an optical fiber transmission line (1), a pumping light source (7) which outputs pumping light using for amplifying the WDM optical signals, an optical signal multiplexer (3) which transmits the pumping light to the optical signal amplifying means, an optical splitter (4) which splits the amplified WDM optical signals into two parts, a wavelength selection type reflection means (5) which reflects only an optical signal having a specified wavelength in one part of the amplified WDM optical signals split at the optical splitter, and an automatic output level controller (6) to which the optical signal having the specified wavelength is inputted and which controls the pumping light outputting from the pumping light source so that the power level of the optical signal having the specified wavelength reflected from the wavelength selection type reflection means becomes a designated level.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical signal repeater for a wavelength division multiplex (WDM) system, in particular, which can control the output level of a WDM optical signal to be outputted to a designated value, and to an output level control method used in this repeater. Japanese Patent Application Laid-Open No. HEI 9-326770 discloses a linear repeater for a WDM system. In this application, at an output stage of an optical amplifier in which WDM optical signals to be transmitted are amplified, a part of the amplified optical signals is split at an optical splitter. These WDM optical signals split at the optical splitter are inputted to a variable wavelength optical filter that can variably control transmitting wavelength to be transmitted in each optical signal. An optical signal transmitted through this variable wavelength optical filter is inputted to an automatic gain control (AGC) section, and the gain of each optical signal is controlled to a designated value by monitoring the power level of the optical signal inputted to the AGC section. Fig. 1 is a block diagram showing such a known optical signal repeater for a WDM system. This known optical signal repeater consists of an optical fiber transmission line 11 in which WDM optical signals are transmitted, an optical amplifier 12 which amplifies the transmitted WDM optical signals, an optical multiplexer 13, an optical splitter 14 which splits the amplified WDM optical signals outputted from the optical amplifier 12 into two parts, a pumping laser diode (LD) 17 being a pumping light source, a variable wavelength optical filter 18 which can variably control transmitting wavelength of the WDM optical signals to be transmitted in each optical signal, and an AGC section 19. Fig. 2 is a block diagram showing gain control operation of the known optical signal repeater shown in Fig. 1. In Fig. 2, a broken line from the pumping LD 17 shows a flow of pumping light and the other broken lines show flows of optical signals. Spectra of the optical signals are also shown in Fig. 2. In these spectra, the vertical axis shows the optical power level and the horizontal axis shows the optical wavelength. At this known optical signal repeater, the output level of each optical signal of the transmitted WDM optical signals is controlled to a designated level by using the following method. That is, a part of the WDM optical signals outputted from the optical amplifier 12 is split at the optical splitter 14, and each optical signal of the split WDM optical signals is transmitted through the variable wavelength optical filter 18. The transmitted optical signal is inputted to the AGC section 19, and the AGC section 19 controls the output from the pumping LD 17 so that the power level of each optical signal inputted to the AGC section 19 is kept in a designated level. At this kind of optical signal output level control method, the AGC section 19 controls the output of the pumping LD 17 so that the output level of each optical signal of the WDM optical signals becomes a designated level, without any relation with the power level of the WDM optical signals inputted to the optical signal repeater. Therefore, each optical signal passing through the optical fiber transmission line 11 is amplified to a designated level without any relation with the power levels of the inputted WDM optical signals at the optical fiber transmission line 11. Consequently, it is possible to realize a WDM optical signal transmission system in which the fluctuation of the output optical power level of each optical signal is reduced. However, at the known optical signal repeater mentioned above, there are several problems. First, at the known optical signal repeater, a part of the optical signals is transmitted through the variable wavelength optical filter 18 and inputted to the AGC section 19. The variable wavelength optical filter 18 provides a driving unit (not shown) which varies the transmitting wavelength; however, there is a problem that this driving unit does not have high reliability. Secondly, a circuit which controls the transmitted wavelength through the variable wavelength optical filter 18 is additionally required, as a result, the structure of the optical signal repeater becomes complex. Thirdly, at the variable wavelength optical filter 18, it is difficult to make the transmitted wavelength band narrow; therefore optical signals adjacent to the optical signal to be transmitted leak and the leaked adjacent optical signals are inputted to the AGC section 19. As a result, it is difficult to monitor the input level of each optical signal to the AGC section 19 precisely. Moreover, in order to avoid influence from the adjacent optical signals input to the AGC section 19, it is required that the adjacent wavelengths are positioned at positions far from the optical signal to be transmitted. Therefore, there is a problem that the number of the wavelengths to be multiplexed is limited in the amplifying wavelength band. In a paper by Motoshima, K., et al., Automatic Gain Control of Erbium-Doped Fiber Amplifiers for WDM Transmission Systems, IEICE Transactions on Communications, Japan, Institute of Electronics Information and Communications Engineering, Tokyo, vol. EBO-B, no. 9, September 1997, pages 1311-1320, an optical signal repeater for a WDM system is described with reference to Fig. 4(a). In this repeater, WDM optical signals in an optical fiber transmission line are split at an optical coupler acting as a splitter. These WDM optical signals split at the optical splitter are inputted to an interference filter. The output of the interference filter is applied to an optical-to-electrical converter, through an amplifier, and then to an electrical-to-optical converter. The output of this constitutes pump light which is input back into the optical fiber transmission line upstream of the splitter. US Patent 5,504,609 describes a WDM optical communication system with demodulators. In Fig. 4 a receiver/demultiplexer is shown using a reflection-type optical filter. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided an optical signal repeater, for use in an optical fiber transmission system which includes an optical fiber transmission line, an optical transmitter, and an optical receiver, the optical signal repeater comprising an optical signal amplifying means coupled in an optical fiber transmission line for amplifying wavelength division multiplexed (WDM) optical signals transmitted via the optical fiber transmission line, a pumping light source for outputting pumping light used for amplifying the WDM optical signals in the optical fiber transmission line, an optical signal multiplexer coupled to the output of the pumping light source for transmitting the pumping light to the optical fiber transmission line, an optical splitter which splits the amplified WDM optical signals in the transmission line into two parts, a wavelength selection means coupled to the optical splitter for selecting only an optical signal having a specified wavelength in one part of the amplified WDM optical signals split at the optical splitter, and an automatic output level controller to which the optical signal selected by the wavelength selection means is inputted for controlling the pumping light output from the pumping light source, so that the power level of the optical signal having the specified wavelength becomes a designated level, characterised in that the wavelength selection means comprises a reflection-type wavelength selection means of a type that reflects the optical signal having the specified wavelength, and in which the optical signal having the specified wavelength and which has been reflected at the wavelength selection means is transmitted directly from the wavelength selection means and the optical splitter to the automatic output level controller. Preferably the optical signal having the specified wavelength and which has been reflected at the wavelength reflection means is transmitted to the level controller through the optical splitter. Preferably the wavelength selection type reflection means is an optical fiber grating filter. According to a second aspect of the present invention, there is provided an optical signal output level control method, comprising the steps of inputting WDM optical signals transmitted from an optical transmitter via an optical fiber transmission line to an optical signal amplifying means, inputting pumping light to the optical fiber transmission line, amplifying the WDM optical signals in the optical fiber transmission line, splitting the amplified WDM optical signals in the transmission line into two parts, outputting a main part of the split amplified WDM optical signals to an optical receiver via the optical fiber transmission line, inputting one part of the split amplified WDM optical signals to a wavelength selection means, selecting only an optical signal having a specified wavelength in the said one part of the split amplified WDM optical signals at the wavelength selection means, inputting the optical signal having the specified wavelength to an automatic output level controller, and controlling the pumping light so that the power level of the optical signal having the specified wavelength becomes a designated level at the automatic output level controller, characterised in that the wavelength selection means comprises a reflection-type wavelength selection means of a type that reflects the optical signal having the specified wavelength, and in which the optical signal having the specified wavelength and which has been reflected at the wavelength selection means is transmitted directly from the wavelength selection means and the optical splitter to the automatic output level controller. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: Fig. 1 (described above) is a block diagram showing a known optical signal repeater for a WDM system; Fig. 2 (described above) is a block diagram showing gain control operation of the known optical sign al repeater shown in Fig. 1; Fig. 3 is a block diagram showing a first embodiment of an optical signal repeater for a WDM system of the present invention; Fig. 4 is a block diagram showing operation of the first embodiment of the optical signal repeater of the present invention shown in Fig. 3; Fig. 5 is a block diagram showing a second embodiment of the optical signal repeater for a WDM system of the present invention; and Fig. 6 is a block diagram showing a third embodiment of the optical signal repeater for a WDM system of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, embodiments of the present invention are explained in detail. Fig. 3 is a block diagram showing a first embodiment of an optical signal repeater for a WDM system of the present invention. This optical signal repeater is used for an optical fiber transmission system that provides an optical fiber transmission line, an optical transmitter that transmits WDM optical signals to the optical fiber transmission line, and an optical receiver that receives the transmitted optical signals. As shown in Fig. 3, the optical signal repeater of the first embodiment consists of an optical fiber transmission line 1, an optical amplifier 2, an optical multiplexer 3, an optical splitter 4, an optical fiber grating filter (reflection-type wavelength selection means) 5, an automatic output level controller (ALC) 6, and a pumping laser diode (LD) 7 being a pumping light source. The optical amplifier 2 amplifies WDM optical signals, whose wavelengths are different from one another, passing through the optical fiber transmission line 1. This optical amplifier 2 is an optical amplifier such as an erbium doped optical fiber. The optical multiplexer 3 is provided at the input side of the optical amplifier 2, and inputs pumping light from the pumping LD 7 to the optical amplifier 2 in which the pumping light is used for amplifying the inputted WDM optical signals. The optical splitter 4 is provided at the output side of the optical amplifier 2, and splits the WDM optical signals amplified at the optical amplifier 2 into two parts. That is, the main part split at the optical splitter 4 is outputted to the optical receiver (not shown) via the optical fiber transmission line 1, and a part of the amplified WDM optical signals split at the optical splitter 4 is outputted to the optical fiber grating filter 5. The optical fiber grating filter 5 reflects only an optical signal having a specified wavelength in the optical signals split at the optical splitter 4. The ALC 6 to which the optical signal having the specified wavelength is inputted monitors the power level of this inputted optical signal, and controls the power level of the inputted optical signal to a designated level by controlling the pumping light output from the pumping LD 7. The optical signal having the specified wavelength reflected at the optical fiber grating filter 5 is inputted to the ALC 6 via the optical splitter 4. The pumping LD 7 is positioned between the ALC 6 and the optical multiplexer 3. Fig. 4 is a block diagram illustrating the operation of the first embodiment of the optical signal repeater of the present invention shown in Fig. 3. Referring to Fig. 4, the operation of the first embodiment of the present invention is now explained. In Fig. 4, a broken line from the pumping LD 7 shows a flow of pumping light, and the other broken lines show flows of optical signals. Spectra of the optical signals are also shown in Fig. 4. In these spectra, the vertical axis shows the optical power level and the horizontal axis shows the optical wavelength. First, when WDM optical signals transmitted from the optical transmitter (not shown) via the optical fiber transmission line 1 are inputted to the optical signal repeater, the inputted WDM optical signals are amplified at the optical amplifier 2 and outputted. The gain of the optical amplifier 2 is controlled by changing the power of the pumping light inputted to the optical amplifier 2. Next, the optical splitter 4 splits the WDM optical signals amplified at the optical amplifier 2 into two parts. The main part of the amplified WDM optical signals is outputted to the optical receiver (not shown) via the optical fiber transmission line 1. A part of the amplified WDM optical signals is inputted to the optical fiber grating filter 5. The optical fiber grating filter 5 reflects only an optical signal having a specified wavelength in the WDM optical signals inputted to the optical fiber grating filter 5. The optical signal having the specified wavelength reflected at the optical fiber grating filter 5 is inputted to the ALC 6 via the optical splitter 4. The ALC 6 controls the output from the pumping LD 7 so that the power level of the optical signal input to the ALC 6 becomes always a designated level. As mentioned above, by controlling the output level of an optical signal having a specified wavelength to a designated level, even when the number of the wavelengths of the inputted WDM optical signals changes, the optical signal having the specified wavelength reflected at the optical fiber grating filter 5 is included in the WDM optical signals to be amplified. Therefore, the output level of each optical signal of the WDM optical signals does not change. Even when the number of the WDM optical signals does not change and the input level changes, the output level is controlled to a designated level. At the first embodiment of the optical signal repeater of the present invention, only one optical signal having one wavelength is selected and extracted from the WDM optical signals and the output level of the selected and extracted single optical signal is controlled to a designated level. Therefore, even when the number of the wavelengths of the inputted WDM optical signals changes, the output level at each wavelength can be kept in a designated level. Even when the number of the wavelengths does not change and the input level changes, the output level can be kept in a designated level. The wavelength to be selected and extracted is fixed; therefore a control means to vary the wavelength to be selected and extracted is not needed and the structure can be simple. Moreover, by using an optical fiber grating filter which can make the reflecting wavelength band very narrow as a means for extracting a specified optical signal, it is possible to extract only a desired optical signal precisely. Therefore, without any influence from adjacent optical signals, the output level can be controlled precisely. As a means for selecting and extracting a wavelength, an optical fiber grating filter being a fiber component is used, therefore the present system can achieve high reliability compared with a system using a device such as lenses. The optical splitter 4 has two functions. That is, one function is to input the split optical signal to the optical fiber grating filter 5, and the other function is to transmit the optical signal having the specified wavelength reflected at the optical fiber grating filter 5 to the ALC 6. Therefore, the circuit structure can be simplified and the assembly time is shortened; consequently the optical signal repeater can be provided in a low cost. At the first embodiment of the optical signal repeater of the present invention, as a reflection-type wavelength selection means which makes only one optical signal having a specified wavelength reflect in the optical signals split at the optical splitter 4, the optical fiber grating filter 5 is used, consequently a variable wavelength optical filter used in the known optical signal repeater is not needed. Therefore, even when the number of the inputted WDM optical signals changes, the output level of each optical signal of the WDM optical signals does not change. Consequently, high reliability can be achieved and the circuit structure can be simplified, and the optical signal repeater can be applied to a system in which the intervals between optical signals are relatively narrow. Next, a second embodiment of the optical signal repeater of the present invention is explained. Fig. 5 is a block diagram showing the second embodiment of the optical signal repeater for a WDM system of the present invention. As shown in Fig. 5, at the second embodiment, the pumping light outputted from the pumping LD 7 is inputted to the back side (output) of the optical amplifier 2. The optical splitter 4 which splits the amplified optical signals into two parts is disposed after the optical multiplexer 3 which transmits the pumping light to the optical amplifier 2. This second embodiment achieves the same effect as the first embodiment achieves. Fig. 6 is a block diagram showing a third embodiment of the optical signal repeater for a WDM system of the present invention. Referring to Fig. 6, the third embodiment is explained. At the third embodiment, as the same as the second embodiment, the pumping light outputted from the pumping LD 7 is inputted to the back side (output) of the optical amplifier 2. The optical splitter 4 which splits the amplified optical signals into two parts is disposed ahead of the optical multiplexer 3 which transmits the pumping light to the optical amplifier 2. This third embodiment achieves the same effect as the first embodiment achieves. As mentioned above, the optical signal repeater of the present invention is used at an optical fiber transmission system which includes an optical fiber transmission line, an optical transmitter, and an optical receiver. The optical signal repeater provides an optical signal amplifying means for amplifying wavelength division multiplexed (WDM) optical signals transmitted from the optical transmitter via the optical fiber transmission line, a pumping light source which outputs pumping light using for amplifying the WDM optical signals, an optical signal multiplexer which transmits the pumping light to the optical fibre transmission line, an optical splitter which splits the amplified WDM optical signals into two parts, a reflection-type wavelength selection means which reflects only an optical signal having a specified wavelength in one part of the amplified WDM optical signals split at the optical splitter, and an automatic output level controller to which the optical signal having the specified wavelength is inputted and which controls the pumping light output from the pumping light source so that the power level of the optical signal having the specified wavelength reflected from the reflection-type wavelength selection means becomes a designated level, and in which the optical signal having the specified wavelength reflected at the reflection-type wavelength selection means is transmitted to the automatic output level controller. As mentioned above, the optical signal repeater selects and extracts one optical signal having a specified wavelength and controls the output level of the selected and extracted optical signal having the specified wavelength, therefore even when the number of wavelengths of the inputted WDM optical signals changes, the output level of each optical signal can be kept in a designated level. Even when the number of the wavelengths does not change and the input level changes, the output level can be kept in a designated level. The wavelength to be selected and extracted is fixed, therefore a control means to vary the wavelength to be selected and extracted is not needed and the structure can be simple. The optical signal repeater employs a reflection-type wavelength selection means which can reflect only an optical signal having a specified wavelength. Therefore, the variable wavelength optical filter used in the known optical signal repeater is not needed, and even when the number of wavelengths of the inputted WDM optical signals changes, the output level of each optical signal of the inputted WDM optical signals does not change, the optical repeater is able to have high reliability, and its structure can be simplified. Moreover, the present invention is applicable to a system in which the intervals between WDM optical signals are relatively narrow. As the reflection-type wavelength selection means, an optical fiber grating filter which can make the reflecting wavelength band very narrow is used as a means for extracting a specified optical signal. Consequently, it is possible to extract only a desired optical signal precisely, and without any influence from adjacent optical signals, and the output level can be controlled precisely. As a means for selecting and extracting a wavelength, an optical fiber grating filter is used; therefore the present system can achieve high reliability compared with a system using a device such as lenses. The optical splitter has two functions. That is, one function is to input the split optical signal to the reflection-type wavelength selection means, and the other function is to transmit the optical signal having the specified wavelength reflected at the reflection-type wavelength selection means to the automatic output level controller. Therefore, the circuit structure can be simplified and the assembly time is shortened; consequently the optical signal repeater can be provided at a low cost. While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope of the present invention.

Claims: An optical signal repeater, for use in an optical fiber transmission system which includes an optical fiber transmission line, an optical transmitter, and an optical receiver, the optical signal repeater comprising: an optical signal amplifying means (2) coupled in an optical fiber transmission line (1) for amplifying wavelength division multiplexed (WDM) optical signals transmitted via the optical fiber transmission line; a pumping light source (7) for outputting pumping light used for amplifying the WDM optical signals in the optical fiber transmission line; an optical signal multiplexer (3) coupled to the output of the pumping light source for transmitting the pumping light to the optical fiber transmission line; an optical splitter (4) which splits the amplified WDM optical signals in the transmission line into two parts; a wavelength selection means (5) coupled to the optical splitter for selecting only an optical signal having a specified wavelength in one part of the amplified WDM optical signals split at the optical splitter; and an automatic output level controller (6) to which the optical signal selected by the wavelength selection means is inputted for controlling the pumping light output from the pumping light source, so that the power level of the optical signal having the specified wavelength becomes a designated level; characterised in that the wavelength selection means comprises a reflection-type wavelength selection means (5) of a type that reflects the optical signal having the specified wavelength, and in which the optical signal having the specified wavelength and which has been reflected at the wavelength selection means is transmitted directly from the wavelength selection means and the optical splitter to the automatic output level controller (6). An optical signal repeater in accordance with claim 1, wherein the optical signal having the specified wavelength and which has been reflected at the wavelength selection means (5) is transmitted to the automatic output level controller through the optical splitter (4). An optical signal repeater in accordance with claim 1 or 2, wherein the wavelength selection means (5) is an optical fiber grating filter. An optical signal output level control method, comprising the steps of: inputting wavelength division multiplexed (WDM) optical signals transmitted from an optical transmitter via an optical fiber transmission line to an optical signal amplifying means; inputting pumping light to the optical fiber transmission line; amplifying the WDM optical signals in the optical fiber transmission line; splitting the amplified WDM optical signals in the transmission line into two parts; outputting a main part of the split amplified WDM optical signals to an optical receiver via the optical fiber transmission line; inputting one part of the split amplified WDM optical signals to a wavelength selection means (5); selecting only an optical signal having a specified wavelength in the said one part of the split amplified WDM optical signals at the wavelength selection means; inputting the optical signal having the specified wavelength to an automatic output level controller; and controlling the pumping light so that the power level of the optical signal having the specified wavelength becomes a designated level at the automatic output level controller; characterised in that the wavelength selection means comprises a reflection-type wavelength selection means (5) of a type that reflects the optical signal having the specified wavelength, and in which the optical signal having the specified wavelength and which has been reflected at the wavelength selection means is transmitted directly from the wavelength selection means and the optical splitter to the automatic output level controller (6).

Inventors: YOKOYAMA RYU, YOKOYAMA, RYU, Yokoyama, Ryu, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 2000
Main classification: H04B10
IPC codes: H04B10, H01S3, H04J14