Optical branching device and method of optical transmission

Abstract: In an optical transmission system, an optical branching device (3a, 3b, 3c) is in use, the number of original transmission lights decreases or level of transmission lights deteriorates for some cause or other. Since output of optical amplifier (4) is predetermined, when the number of transmission lights reduces, level of remaining transmission light is changed, thus departing from optimum operation condition. There is provided an optical branching device (3a, 3b, 3c) comprising two optical circulators (1,2) and a plurality of fiber gratings (8,9) which are arranged in series. The gratings are located between the two optical circulators. Each grating reflects light with a wave length different from transmission light, at the time of malfunction such as disconnection, it causes a light with a wave length different from the transmission light to transmit from terminal station (5, 6) or branching station (7a, 7b), thus maintaining the level of the transmission light to prescribed level. When the level of the transmission lights deteriorates, it causes the level of transmission light from the branching station (7a, 7b) to be adjusted. As a result, the level of respective transmission lights is maintained to the original level so that optimum operation condition is maintained, thus the level among the respective transmission lights is capable of being adjusted.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical branching device and a method of optical transmission. More to particularly this invention relates to an optical branching device for branching and inserting signal light which is subjected to wavelength multiplexing and to a method of optical transmission for transmitting signal light to transmission line in which the optical branching device is located. Description of the Prior Art Formerly, it is well known that an optical branching device comprises an optical circulator and a fiber grating as shown in Fig. 1. In Fig. 1, the optical branching device 3a is constituted such that two pieces of three-terminal optical circulators 1, and 2 are allocated thereto, and a fiber grating 5 for reflecting specific transmission light with specific wavelength λi within a plurality of transmission lights is allocated therebetween. The transmission light is inputted to a first terminal of a first three-terminal optical circulator 1, and outputted from a third terminal of a second three-terminal optical circulator 2. A fiber grating 8 is allocated between a second terminal of the three-terminal optical circulator 1 and a second terminal of the three-terminal optical circulator 2. Operation of the optical branching device is described as follows: N pieces of transmission lights with wavelength, of λ1 - λn which are subjected to wavelength , multiplexing at a terminal station 5 to be transmitted. The transmission lights are inputted to a first terminal of the first three-terminal optical circulator 1. from an input path of optical branching device 3a via optical amplifier 4. The transmission lights λ1 - λn arrive at the fiber grating 8 via the second terminal. Here, only the light of specific wavelength λi in the transmission lights is reflected by the fiber grating 8, thus being transmitted to a branching station 7a from the third terminal. The transmission lights having wavelength with the exception of specific wavelength λi are permeated through the fiber grating 8, thus being transmitted from the third terminal via the second terminal of the second three-terminal optical circulator 2. While the light with wavelength λi is transmitted from the branching station 7a. The light with wavelength λi is inputted to the first terminal of the second three-terminal optical circulator 2, befdre being transmitted from the second terminal to be reflected by the fiber grating 8, thus being transmitted from the third terminal by way of the second terminal again. Namely, n pieces of transmission lights with the wavelength of λ1 - λn are transmitted to the terminal station 6 from the third terminal of the second three-terminal optical circulator 2. When it causes signal light to transmit using such the optical branching device, it is to be desired that output level of the light with the wavelength λi which is subjected to branch-insertion is in agreement with output level of (n-1) pieces of respective transmission lights with the wavelengths λ1 - λn (exception for the light of the wavelength λi) which are subjected to no branch-insertion. If the output level differs from each other, signal-to-noise ratio namely S/N ratio of the light of the wavelength λi and the light of the another wavelength differ from each other, thereby, there occurs ill-influence in transmission characteristic. The optical amplifier used in this optical transmission system is controlled such that output level thereof comes to be constant even if output of the optical amplifier of transmission path or front stage is changed. For example, when the number of the transmission lights inputted to the optical amplifier increases from two to four, the level of the transmission light in every one piece comes to be half. Namely the level of the respective transmission lights is changed depending on increasing and/or decreasing of the number of the transmission lights which is set beforehand. Consequently, in Fig. 1, in order to adjust the level of the respective transmission lights from the optical branching device 3a, there should be used the optical amplifier with different output level in one optical amplifier allocated at primary transmission path and in the other optical amplifier allocated at transmission path from the optical branching device to the branching station. When it causes the number of the transmission lights for transmitting to be reduced for some reason or other, after constituting transmitting system while causing n pieces of signal lights to be subjected to wavelength multiplexing initially, or when disconnection of the transmission path occurs or other trouble, the number of the transmission lights for transmitting might be reduced. In this occasion, since the level of the respective transmission lights fluctuates from the original value, operating margin decreases because the optical transmission system departs from the optimum operating condition. When the level of the transmission lights of the primary transmission path deteriorates, the level of the light transmitted from the branching station does not agree with the level of the transmission light of the primary transmission path, thus transmission characteristic deteriorates. In two conventional add/drop multiplexers (EP 730 172 A1 and a paper by M.J. Chawki et al., "Evaluation of an Optical Boosted Add/Drop Multiplexer OBADM including circulators and fiber grating filters", ECOC"95 Brussels, pp. 47-50, a plurality of optical signals of different wavelengths are transmitted through two optical circulators and a series of gratings, each grating being able to reflect one of the wavelengths of the optical signals. Thus the signal corresponding to that wavelength may be dropped from the plurality of signals or added thereto. In a further paper by Takashi Yamamoto et al., "High speed optical path routing by using four-wave mixing and a wavelength router with fiber gratings and optical circulators", Optics Communications 120 (1995) 245-248, there is described an arrangement of a plurality of optical branching devices arranged in series for branching out a specific wavelength of an optical signal received from a signal source. SUMMARY OF THE INVENTION In view of the foregoing, it is an object of at least the preferred embodiments of the present invention to provide an optical branching device which is in use for optical transmission system, and a method of optical transmission, wherein when the number of transmission light or the level thereof is changed due to disconnection or other trouble, it causes level of the transmission lights of the primary transmission path to maintain at prescribed level, or it causes the level of the transmission light from the branch office to be adjusted, in order to maintain the optimum operating condition of the system. It is another such object, in the above-described system, that when the number of transmission lights being subjected to wavelength multiplexing is reduced from the original number intentionally, it causes the optimum operating condition of the system to be maintained. In accordance with one aspect of the present invention, there is provided an optical transmission system comprising means configured to generate transmission light signals of a plurality of different wavelengths; means for transmitting the plurality of transmission light signals; and an optical branching device (3a), adapted to receive said plurality of transmission light signals, comprising: two optical circulators (1,2); and an optical grating (8,9) located between said two optical circulators (1,2); wherein said optical grating (8,9) consists of a plurality of grating elements arranged in a series, each of the grating elements being adapted to reflect light of a different wavelength, characterised in that at least one of the grating elements (8) is adapted to reflect light of a wavelength contained in said plurality of transmission light signals, at least one other of the grating elements (9) being adapted to reflect light of a wavelength which is different from any of the wavelengths of the transmission light signals which the generating means is configured to generate. In accordance with a further aspect the present invention provides a method of transmitting light comprising transmitting transmission light signals of a plurality of different wavelengths towards an optical branching device (3a) comprising two optical circulators (1, 2) and an optical grating (8, 9) located between said two optical circulators (1, 2), said optical grating (8, 9) consisting of a plurality of grating elements arranged in a series, each of the grating elements being adapted to reflect light of a different wavelength; characterised by reflecting, by means of at least one of the grating elements (8), a transmission light signal of a wavelength contained in said plurality of transmission light signals; and reflecting, by means of at least one other of the grating elements (9), a further light signal received by the optical branching device and being of a wavelength which is different from any of the wavelengths of said plurality of transmission light signals. The above and further objects and novel features of the invention will be more fully understood from the following detailed description when the same is read in connection with the accompanying drawings. It should be expressly understood, however, that the description and drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a view showing a constitution example and operation example of optical transmission system including conventional optical branching device; Fig.2 is a view showing constitution example and operation example of optical transmission system including optical branching device according to the present invention; Fig.3 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig.2; Fig.4 is a view showing level of respective wavelength when the transmission light is four-wavelength in Fig.3; Fig.5 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig.2; Fig.6 is a view showing level of respective wavelengths when the transmission light is four-wavelength in Fig.5; Fig.7 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig.2; Fig. 8 is a view showing a constitution example and an operation example of an optical transmission system including two optical branching devices; Fig. 9 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig. 8; Fig. 10 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig. 8; Fig. 11 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig. 8; Fig. 12 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig. 8; and Fig. 13 is a view for explaining operation when malfunction occurs in the optical transmission system of Fig. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will now be described in detail referring to the accompanying drawings. Fig. 2 is a view showing constitution example and operation example of optical transmission system including optical branching device according to the present invention. A constitution example of an optical branching device of the present invention will be described referring to Fig. 2. There are allocated two three-terminal optical circulators 1, 2, and two fiber gratings 8, 9 which reflect specific wavelengths λi and λx respectively in a plurality of transmission lights in between. The specific wavelength. λi is the same wavelength as one wavelength in the plurality of the transmission lights. The specific wavelength λx differs from any wavelength of the plurality of transmission lights, and which wavelength λx is of within the wavelength capable of being amplified by optical amplifier (also λy, λz). In Figs. 2 to 12, description of λi and λx in the optical branching device denotes that the optical circulator which reflects these wavelengths λi and λx is allocated therein and symbols 1 ○, 2 ○, 3 ○ denote the number of terminal of the three-terminal optical circulator. At the time of normal operation, the optical transmission system of Fig. 2 operates as follows: when n pieces of transmission lights with the wavelengths λ1 to λn are transmitted from the terminal station 5 (terminal station A), these transmission lights enter into the optical branching device 3a, before only the transmission light of the specific wavelength λi is transmitted from the terminal 3 to the branching station 7a (branching station C) by the fiber grating 8. (n-1) transmission lights with the wavelengths λ1 to λn (except for wavelength λi) pass through fiber gratings 8, 9, then enter the next optical circulator 2 via its second terminal and are then transmitted from the third terminal of optical circulator 2. The above transmission light with the wavelength λi (having a nominal level of "1") is transmitted, from the branching station 7a, together with light (λz) with a level of (n-1) times the level of one transmission light, which light has a wavelength λz which is different from any of the wavelengths λ 1 to λn of the transmission lights. Hereinafter, in the specification and drawings, such a transmission light is represented as λi + (n-1)λz. The λi + (n-1)λz is input to the terminal 1 of the optical circulator 2. The light with the wavelength of λi thereof is reflected by the fiber grating 8, thus being transmitted together with (n-1) pieces of transmission lights with the wavelengths of λ1 to λn (except for wavelength λi) from the optical circulator 1 from the optical branching device 3a. Consequently, n pieces of transmission lights with the wavelengths of λ1 to λn transmitted from the terminal station 5 arrive at the terminal station 6, subject to fluctuation of the level. The light with the wavelength of (n-i)λz transmitted from the branching station 7a returns to the branching station 7a from the branch path while permeating the optical branching device 3a. Next, when the number of transmission lights is reduced intentionally by k pieces at the time to transmit n pieces of transmission lights on the primary transmission path, the level of respective transmission lights of the primary transmission path comes to be n/k times in comparison with normal operation because the output level of the optical amplifier is fixed. In this case, light (kλy) of level of k times the level of a respective transmission light is transmitted from the terminal station 5 with a wavelength λy which is different from any of the wavelengths λ1 to λn of the transmission lights. Due to this transmission, it comes to be the same state as the case where n pieces of transmission lights are transmitted. Namely level of transmitted transmission lights is maintained in the same level as the case where the transmission lights are transmitted initially. Next, operation at the time when malfunction occurs in the optical transmission system of Fig. 2 will be described referring to Figs. 3 to 7. Fig. 3 shows an example of the case where a branch path and insertion path between the branching device 3a and a branching station 7a are interrupted. The terminal station 5 recognizes for some way or other that a route to the branching station 7a is disconnected, thus transmitting (n-1) transmission lights with the wavelengths of λ1 to λn (except for λi) and light with the wavelength λy, which is different from any of the wavelengths λ1 to λn. All of these lights reach the terminal station 6 because none of their wavelengths is reflected at the branching device 3a. Since the number of the transmission lights is n, and the number is not changed when compared with normal operation, the level of respective transmission lights remains. In the case of Fig.3, when four pieces of transmission lights exist therein, operation will be explained referring to Fig. 4. In times of normal state (there is no disconnection), the same level of four pieces of transmission lights are subjected to wavelength multiplexing to be transmitted from the terminal station A. Only the light with the wavelength λi in four pieces of transmission lights is transmitted toward the branching station 7a. The branching station 7a transmits the light with the wavelength λi, simultaneously, the branching station 7a transmits the light with the wavelength λz which is different from any of the wavelengths of the transmission lights, with three times the level of a respective transmission light. The light with the wave length λz is transmitted to the branching station 7a from the branch path of the branching device 3a. The light with the wave length λi is reflected by the fiber grating 8, thus the output of branching device 3a comes to equal to output of terminal station 5. At the time when the line is disconnected, although four pieces transmission lights with the same level are subjected to wavelength multiplexing to be transmitted from the terminal station 5, the transmission light with the wavelength λi is reflected by the fiber grating 8, thus being transmitted to branching path. With the result that the output from the optical branching device 3a comes to be three pieces of transmission lights. When these three pieces of transmission lights permeate the optical amplifier 4, the level in every one of three pieces of transmission lights comes to be 4/3 times initial level. In the present invention, at the time when the line is disconnected, it causes the light with the wavelength λy to transmit from the terminal station 5 which wavelength λy differs from the transmission light instead of the light with the wavelength λi. These three pieces of transmission lights and the light with the wavelength λy are permeated through the branching device 3a to be inputted to the optical amplifier 4. However, since the number of the transmission lights are four pieces, the level of respective transmission lights remains even if respective transmission lights permeate via the optical amplifier 4. Fig. 5 shows the cases where disconnection occurs in between the terminal station 5 and the optical branching device 3a. It causes the light with the wavelength λi of the transmission light and the light with the wavelength λx which differs from the wavelength of the transmission lights to transmit from the branching station 7a. The light with the wavelength λx is taken to be the level of (n-1) level of respective transmission lights. These lights with the wavelengths λi + (n-1)λx are reflected entirely by the fiber gratings 8, 9 in the optical branching device 3a. The total lights with the wavelengths λi + (n-1)λx are the same level as that of n pieces transmission lights, therefore, the level of the light with the wavelength λi remains if the light permeates via the optical amplifier 4. In the case of Fig. 5, when four pieces of transmission lights exist therein, operation will be explained referring to Fig. 6. In times of normal state (there is no disconnection), the same level of four pieces of transmission lights are subjected to wavelength multiplexing to be transmitted from the terminal station 5 (terminal station A). Only the light with wavelength λi in four pieces of transmission lights is transmitted toward the branching station 7a (branching station C). The branching station 7a transmits the light with the wavelength λi, simultaneously, the branching station 7a transmits the light with the wavelength λz which differs from wavelength of the transmission lights, with three times level of respective transmission light. The light with the wavelength λz is transmitted to the branching station 7a from the branch path of the branching device 3a. The light with the wavelength λi is reflected by the fiber grating 8, thus output of branching device 3a comes to equal to output light of terminal station 5. At the time when the line is disconnected, although the same level of four pieces of transmission lights are subjected to wavelength multiplexing to be transmitted from the terminal station A, these transmission lights do not reach the optical branching device 3a. At this time when one of the transmission light with the wavelength λi and the light with the above wavelength λz are transmitted from the optical branching device 3a with the level of three times respective transmission lights, only the light with the wavelength λi is transmitted from the optical branching device 3a, thus being amplified to the level of four times the initial state by the optical amplifier 4. In the present invention, at the time when the line is disconnected, it causes the light of the wavelength λx with the level of three times respective transmission lights and the light of the wavelength λi to transmit which λx differs from the transmission light from the branch office 7a. These lights are reflected by the branching device 3a, thus being inputted to the optical amplifier 4. However, since the number of the transmission lights is four pieces, the level of the light of the wavelength λi remains if these lights permeate through the optical amplifier 4. Fig. 7 shows that disconnection occurs in between the optical branching device 3a and the terminal station 6. There are transmitted n pieces of the transmission lights with the wavelengths λ1 to λn from the terminal office 5. The light with the wavelength λi in the transmission lights is reflected to be transmitted to the branch path. The branching station 7a transmits the light with the wavelength λi and the light with the wavelength λz which differs from the wavelength of respective transmission lights with the level of (n-1) times respective transmission lights. The lights with the wavelengths (n-1) λz in the lights of the wavelengths λi + (n-1) λz permeates the optical branching device 3a to be transmitted to the branch path. Consequently, the transmission light with the wavelength λi is inputted to the branch office 7a with the initial level. Next, operation of the optical transmission system in which two optical branching devices are connected will be explained referring to Figs. 8 to 13. Indications of (λi, λx) in the optical branching device of these drawings denote that the fiber gratings which reflect the lights of these wavelength are allocated, Fig. 8 shows a fundamental constitution of the optical transmission system in which the optical branching devices 3b, 3c are allocated in series to primary transmission path. There are transmitted n pieces transmission lights with the wavelengths λ1 to λn from the terminal station 5. The transmission light of the wavelength λi is reflected at the optical branching device 3b to be transmitted to the branching station 7a. The branching station 7a transmits the light with the wavelengths λi + (n-1)λz. The light with the wavelengths (n-1)λz in the lights with the wavelengths λi + (n-1)λz are permeated through the optical branching device 3b to be transmitted to the branch path, while the light with the wavelength λi is transmitted to next optical branching device 3c. The lights with the wavelengths λ1 to λn are inputted to the optical branching device 3c. The transmission light with the wavelength λj of the lights with the wavelengths λ1 to λn is reflected thereby, thus being transmitted to the branching station 7b. The branching station 7b transmits the light with the wavelengths λj + (n-1)λz. The lights with the wavelengths (n-1)λz of the lights with the wavelengths λj + (n-1)λz are permeated through the optical branching device 3c to be transmitted to the branch path, while the light with the wavelength λj together with the other transmission lights are transmitted from the optical branching device 3c. Namely, n pieces of transmission lights with the wavelengths λ1 to λn which are the same light with the wavelengths λ1 to λn as that transmitted from the terminal station 5 from the optical branching device 3c. Further, the lights of the wavelengths λi, λj are transmitted normally to the branching stations 7a, 7b. Fig. 9 shows an example of the case where disconnection occurs in between the optical branching device 3b and the branching station 7a in the constitution of Fig. 8. There are transmitted (n-1) pieces of the transmission light with the wavelengths λ1 to λn exception for the light with the wavelength λi and the light with wavelength λy which differs from the wavelength of respective transmission light from the terminal station 5. These lights with the wavelengths λ1 to λn exception for the light with the wavelength λi and the light with the wavelength λy are permeated through the optical branching device 3b. The light with the wavelength λj is reflected by the next optical branching device 3c to be transmitted to the optical branching station 7b. The transmission lights transmitted from the terminal station 5 exception for the light with the wavelength λj and the light of the wavelength λy are permeated through the optical branching device 3c. The optical branching station 7b transmits the lights with the wavelengths λj + (n-1)λz for the optical branching device 3c, the light with the wave-. length λj thereof is reflected by the optical branching device 3c to be transmitted to the terminal station 6. While the lights with the wavelengths (n-1) λz is permeated the optical branching device 3c to be transmitted to the optical branching station 7b. As described above, the transmission light outputted from the optical branching device 3c are the lights with the wavelengths λ1 to λn exception for the light with the wavelength λi and the light with the wavelength λy which are the same light as the light transmitted by the terminal station 5. The light with the wavelength to the branching station is the same level as the initial level. For this reason, the level of respective transmission lights remains. Fig. 10 shows an example of the case that disconnection occurs in between the optical branching device 3c and the branching station 7b in the constitution of Fig. 8. There are transmitted (n-1) pieces of the transmission light with the wavelengths λ1 to λn exception for the light with the wavelength λj and the light with wavelength λy which differs from the wavelength of respective transmission light from the terminal station 5. The light with the wavelength, λi is reflected at the optical branching device 3b to be transmitted to the optical branching station 7a. The optical branching station 7a transmits the light with the wavelengths λi + (n-1) λz to the optical branching device 3b. The light with the wavelength λi thereof is reflected by the optical branching device 3b, thus being transmitted to the next optical branching device 3c. The transmission lights transmitted from the terminal station 5 exception for the light with the wavelength λi and the light with the wavelength λy are permeated through the optical branching device 3b. The optical branching station 7a transmits the light with the wavelengths λi + (n-1)λz to the optical branching device 3b. The light with the wavelength λi thereof is reflected by the optical branching device 3b, thus being transmitted to the next optical branching device 3c. While the lights with the wavelengths (n-1) λz is permeated the optical branching device 3b to be transmitted to the optical branching office 7a. Thus the same lights as the lights with the wavelengths λ1 to λn exception for the light with the wavelength λi transmitted from the terminal station 5 are transmitted from the optical branching device 3b, thus permeating the optical branching device 3c as they are. Consequently, the level of respective transmission lights remains. Fig. 11 shows an example of the case that disconnection occurs in between the terminal station 5 and the optical branching device 3b in the constitution of Fig. 8. The transmission lights are not inputted to the optical branching device 3b. The light with the wavelength λi and the light with the wavelength λx with the level of (n-1) times one piece of transmission light are transmitted from the branching station 7a to the optical branching device 3b. These lights with the wavelengths λi + (n-1)λx are reflected by the optical branching device 3b to be entered into the optical branching device 3c. The light with the wavelengths (n-1)λx are transmitted to the branching station 7b, while the light with the wavelength λi permeates. The branching station 7b transmits the transmission light with the wavelength λj, (n-2) pieces of the lights with the wavelength λx, and one piece of the light with the wavelength λy to the optical branching device 3c. The light with the wavelength (n-2)λx and the light with the wavelength λj thereof are transmitted from the optical branching device 3c to the terminal station 6. The light with the wavelength λy is transmitted to the branching station 7b while permeating the optical branching device 3c. Namely, the transmission lights with the wavelengths λi, λj are transmitted normally to the terminal station 6 with unchangeable level because the lights corresponding to n pieces of the lights of the level are transmitted from the optical branching device 3c. The lights corresponding to n pieces of lights go and come back between the optical branching device 3c and the branching station 7b. Fig. 12 shows an example of the case that disconnection occurs in between the optical branching device 3b and the optical branching device 3c in the constitution of Fig. 8. The n pieces of transmission lights with the wavelengths λ1 to λn are transmitted from the terminal station 5. The light with the wavelength λi thereof is transmitted from the optical branching device 3b to the branching station 7a. The branching station 7a transmits the lights with the wavelengths λi + (n-1)λz for the optical branching device 3b. The lights with the wavelengths (n-1)λz are transmitted to the branching station 7a while permeating the optical branching device 3b. While the lights with the wavelengths λj + (n-1)λx are transmitted from the branching station 7b to the optical branching device 3c. These lights are entirely reflected by the optical branching device thus transmitting to the terminal station 6. Consequently, the light with the wavelength λi is normally transmitted from the terminal station 5 to the branching station 7a, while the light with the wavelength λj is normally transmitted from the branching station 7b to the terminal station 6. Fig. 13 shows an example of the case that disconnection occurs in between the optical branching device 3c and the terminal station 6 in the constitution of Fig. 8. The n pieces of transmission lights with thewavelengths λ1 to λn are transmitted from the terminal station 5. The light with the wavelength λi thereof is transmitted from the optical branching device 3b to the branching station 7a. The branching station 7a transmits the lights with the wavelengths λi + (n-1)λz for the optical branching device 3b. The lights with the wavelengths (n-1)λz thereof are transmitted to the branching station 7a while permeating the optical branching device 3b. The light with the wavelength λi is directed to the next optical branching device 3c while reflecting from the optical branching device 3b. Consequently, output light from the optical branching device 3b to the next optical branching device 3c is the same light as the light transmitted from the terminal station. In regard to the optical branching device 3c, the light with the wavelength. λj is transmitted to the branching station 7b. The branching station 7b transmits the light with the wavelengths λj + (n-1)λz for the optical branching device 3c. The light with the wavelengths (n-1)λz thereof are transmitted to the branching station 7b while permeating the optical branching device 3c. Consequently, the light with the wavelength λi is normally transmitted from the terminal station 5 to the branching station 7a, while the light with the wavelength λj is normally transmitted from the terminal station 5 to the branching station 7b. As described above, there is explained the cases where it causes the transmission light by one wavelength to diverge to be inserted. It is capable of diverging to be inserted more than two wavelengths of the transmission lights. Further there is explained the cases where it causes one fiber grating to provide therewith which fiber grating reflects the light of the wavelength which differs from that of the transmission lights in the optical branching device. It is capable of being provided therewith more than two pieces of the fiber gratings. In the above described optical transmission system, there is described that simultaneous disconnection of branch path and insertion path occurs in between the optical branching device and the branching station, however, when either the branch path or the insertion path is disconnected, the present invention comes to be effective in either case. In the above description, the case of disconnection in the transmission path is explained. However, for example in Fig. 2, when the level of the transmission lights with the wavelength λ1 to λn inputted to the optical branching device 3a deteriorates for some cause or other, it causes the light with the wavelength λi transmitted from the branching station to the optical branching device 3a to meet to the respective lights of the deteriorated level. At this time, it causes the level of the light with the wavelengths (n-1)λz transmitted from the branching station 7a to adjust (this case making it larger) so that it is capable of adjusting the level of the light with the wavelength λi being transmitted therewith. Due to this operation, it is capable of transmitting normally the transmission lights with the wavelengths λ1 to λn of the primary transmission path to the terminal station. The transmission lights which are subjected to wavelength multiplexing to be transmitted are set that for example the wavelength is of interval 0.8 n. sec with 1.5585 µm as the center. The fiber grating is of the well known element. The fiber grating is formed such that a plurality of cores and layers with different refractive index are placed one upon another with prescribed intervals. As described above, although the optical circulator is utilized as the element for reflecting specific wavelength, it is capable of constituting the same function as that of the optical circulator due to combination of an optical branching filter, a band pass filter and an edge filter. As described above, in accordance with the present invention, when disconnection occurs at a part of the transmission path, or when it causes the number of the transmission lights to reduce deliberately, optimum operating condition or operating margin of the optical transmission system is capable of being maintained because it causes the level of the transmission light to maintain at the prescribed level while transmitting the light of the wavelength which differs from the transmission lights, from the terminal station or the branching station. Under the condition that the level of the light transmitted from the branching station does not agree with the level of the transmission light of the primary transmission path, deterioration of the transmission characteristic is capable of being prevented while adjusting these levels. Further, in the above described system, the optical amplifier with same characteristic is capable of being utilized. While the present invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made to the invention without departing from its scope as defined by the appended claims. Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. The text of the abstract filed herewith is repeated here as part of the specification. In an optical transmission system, an optical branching device 3a, 3b, 3c is in use, the number of original transmission lights decreases or level of transmission lights deteriorates for some cause or other. Since output of optical amplifier 4 is predetermined, when the number of transmission lights reduces, level of remaining transmission light is changed, thus departing from optimum operation condition. There is provided an optical branching device 3a, 3b, 3c including a fiber grating 8, 9 for reflecting light with a wavelength different from transmission light, at the time of malfunction such as disconnection, it causes a light with a wavelength different from the transmission light to transmit from terminal station 5, 6 or branching station 7a, 7b, thus maintaining the level of the transmission light to prescribed level. When the level of the transmission lights deteriorates, it causes the level of transmission light from the branching station 7a, 7b to be adjusted. As a result, the level of respective transmission lights is maintained to the original level so that optimum operation condition is maintained, thus the level among the respective transmission lights is capable of being adjusted.

Claims: An optical transmission system comprising means configured to generate transmission light signals of a plurality of different wavelengths; means for transmitting the plurality of transmission light signals; and an optical branching device (3a), adapted to receive said plurality of transmission light signals, comprising: two optical circulators (1,2); and an optical grating (8,9) located between said two optical circulators (1,2); wherein said optical grating (8,9) consists of a plurality of grating elements arranged in a series, each of the grating elements being adapted to reflect light of a different wavelength, characterised in that at least one of the grating elements (8) is adapted to reflect light of a wavelength contained in said plurality of transmission light signals, at least one other of the grating elements (9) being adapted to reflect light of a wavelength which is different from any of the wavelengths of the transmission light signals which the generating means is configured to generate. A method of transmitting light comprising transmitting transmission light signals of a plurality of different wavelengths towards an optical branching device (3a) comprising two optical circulators (1, 2) and an optical grating (8, 9) located between said two optical circulators (1, 2), said optical grating (8, 9) consisting of a plurality of grating elements arranged in a series, each of the grating elements being adapted to reflect light of a different wavelength; characterised by reflecting, by means of at least one of the grating elements (8), a transmission light signal of a wavelength contained in said plurality of transmission light signals; and reflecting, by means of at least one other of the grating elements (9), a further light signal received by the optical branching device and being of a wavelength which is different from any of the wavelengths of said plurality of transmission light signals. A method according to Claim 2, comprising maintaining at a prescribed level the intensity of light of individual transmission light signals transmitted towards the optical branching device (3a). A method according to Claim 2 or Claim 3, further comprising transmitting the further light signal or part thereof from the optimal branching device (3a). A method according to any one of Claims 2 to 4, wherein the further light signal or part thereof is transmitted to and/or received from at least one branching station (7a). A method according to Claim 5, wherein light of more than one wavelength is transmitted from said branching station (7a,7b). A method according to any one of claims 2 to 6, further comprising transmitting light from the branching device (3a) to a terminal station (6), wherein said light transmitted from the branching device (3a) to the terminal station (6) includes light of a wavelength which is different from any of the wavelengths of said plurality of transmission light signals, but light of a particular wavelength included in the light transmitted from the branching device (3a) to the terminal station (6) and whose wavelength is different from any of the wavelengths of said plurality of transmission light signals has the same intensity as a said transmission light signal. A method according to Claim 5, wherein light of a first wavelength received from said branching station (7a,7b) is of the same intensity as a said transmission light signal of a second wavelength. A method according to Claim 5, wherein light of a first wavelength received from said branching station (7a,7b) is of greater intensity than a said transmission light signal of a second wavelength. A method according to Claim 2, wherein, when not all of said transmission light signals are received by the optical branching device (3a), their intensities are adjusted by the optical branching device (3a) using the further light signal.

Inventors: KANEKO TOMOYUKI, WATANABE SEIJI, KANEKO, TOMOYUKI, WATANABE, SEIJI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1998
Main classification: G02B6
IPC codes: H04B10, H04J14, G02B6