Method for monitoring an optical waveguide, monitoring system and monitoring unit for said method

Abstract: The invention concerns a Method for monitoring an optical waveguide between a monitoring terminal point (5) and a monitoring unit (3), in which an optical data signal (10) from a transmitter (1), and for amplification, pump light from a pump laser (3), is emitted into the optical waveguide, wherein a monitoring signal (51) transmitted by the monitoring unit (3) is returned via the same waveguide to the monitoring unit (3) by a reflection device located at the monitoring terminal point (5), in particular a Bragg grating placed in the optical waveguide, and wherein the reflection device is tuned to a reflection that is exactly the wavelength of the monitoring signal (51), as well as a monitoring system and a monitoring unit (3) for said Method.

Description: The invention concerns a Method for monitoring an optical waveguide according to the preamble of Claim 1, as well as a monitoring system according to the preamble of Claim 4 and a monitoring unit according to the preamble of Claim 5. In modern optical transmission systems, optical waveguides are being increasingly employed over long routes without opto-electrical conversion and intermediate electrical amplification of the transmitted data signal. In that case the only active system components are optical amplifiers which compensate only the attenuation of the optical waveguide. For example, these optical amplifiers can be realised by means of erbium-doped fibre amplifiers or EDFAs in which the amplification is achieved by stimulated emission when the incident light passes through the amplifying medium. Because of a continuous supply of energy, this emission occurs in the form of higher-energy light, that is to say light that is of a shorter wavelength compared to the data signal; this Method is referred to as "optical pumping". Here the so-called pump light that is produced by a so-called pump laser can be fed to the amplifier via a separate optical waveguide or via the same optical waveguide that is also carrying the data signal. In this case the pump laser can be placed a few kilometres away from the amplifier, which is located at a point under water, for example. As an alternative to a fibre amplifier, amplification can be achieved by means of the so-called Raman effect, in which amplification is effected by controlled, stimulated scatter in the optical waveguide. For this, pump light is also generated by means of a pump laser, which, however, unlike in the case of the fibre amplifier, is not specifically fed to an optical amplifier, but where the amplification of the data signal is achieved in the optical waveguide itself. In this case the pumping power of the so-called pump laser can amount to a few watts. Due to the very small diameter of the optical waveguide, this means an enormous concentration of light energy which, if it enters the human eye can cause irreparable damage to eyesight. For safety reasons, in the case of fracture of the optical waveguide, for example damage due to civil engineering works, care must therefore be taken to minimise the risk of personal injury. To this end, a fracture in the optical waveguide in hazardous areas should be reliably detected so that the corresponding pump laser can be immediately shut down. A known accepted possible solution for detecting a fracture in an optical waveguide consists in installing an optical splitter, constructed as a so-called taper for example, at a specific point in the optical waveguide, via which a small portion of the light energy supplied by the pump laser can be tapped off and fed back to the pump laser via a further dedicated optical waveguide. Despite the supply of light energy, as soon as no energy is fed back a break in the optical waveguide is assumed to have occurred anywhere between the specific point and the pump laser and the pump laser is immediately shut down. However, the implementation of this solution has the disadvantage that, compared to an unmonitored transmission system, an additional optical waveguide is required. WO 9705713 discloses a system for testing an optical waveguide system by a reflectometry method. A laser emits light at a number of different wavelengths. Different terminal locations at different branches of the system are provided each at least with one Bragg grating that reflects light at one of said wavelengths. With this method, reflections that are caused by a break in the line can be distinguished from reflections by said terminals. However, this method only solves the problem to detect a break or fracture of a fiber with significant reflections to be measured in a monitoring (upstream) unit. The object of the invention is to demonstrate a solution in which reliable fracture detection is effected with the minimum amount of additional equipment, in particular without the need for an additional optical waveguide along the monitored route. This object is achieved by a Method according to the teaching of Claim 1, a monitoring system according to the teaching of Claim 4, as well as a monitoring unit according to the teaching of Claim 5. In the following text it is always assumed that the pump laser serves at the same time as the monitoring unit, that is to say the detection of a break in the optical waveguide detects a specific section and effects the shut-down of the pump light. In principle, however, these functions can also be separated, whereby for example, the monitoring unit handles the monitoring of a specific laser path and on detection of a break transmits a shut-down signal to the corresponding pump laser. The basic idea of the invention is that in addition to the pump light, the pump laser emits two monitoring signals with different wavelengths to monitor an optical waveguide between a point up to which the optical waveguide is to be monitored, here also referred to as the monitoring terminal point, and said laser system. A first monitoring signal with a first wavelength is to be reflected to the monitoring unit by said reflection device located at the monitoring terminal point that is tuned to exactly reflect the wavelength of the first monitoring signal, as long as no fracture of the fiber occurs. A second monitoring signal with a second wavelength not to be reflected at said monitoring terminal point. The he monitoring unit evaluates the reflections of both monitoring signals to detect fractures between said monitoring unit and said monitoring terminal point. In the case of a planar cleave this can actually result in total reflection of the signals transmitted by the pump laser. The following different conclusions can be arrived at in the pump laser: if no monitoring signals or both monitoring signals are reflected, then it is concluded that there is a break in the optical waveguide and the pump laser is shut down. If only the first monitoring signal is reflected, then it is assumed that the optical waveguide is intact. As soon as there is a fracture is detected, the pump laser stops emitting the pump light. A further advantageous development consists in placing two or more different reflection devices, which reflect different wavelengths, at different points of the optical waveguide. The pump laser emits two or more monitoring signals at different wavelengths, one monitoring signal being reflected at one reflection device, respectively. In the event of a break in the optical waveguide at a specific point, all the monitoring signals whose reflection devices are located after the break are no longer reflected. By evaluating the received monitoring signals, the fracture point can then be accurately located section-by-section. Further developments of the invention are disclosed in the dependent Claims and in the following description. The invention is explained in more detail with the aid of the attached drawings, of which: Fig. 1 shows an arrangement of an optical transmission system with a monitoring device according to the accepted prior art; Fig. 2 shows an arrangement of an optical transmission system with a monitoring device according to the invention, and Fig. 3 shows schematically a development of the monitoring device according to the invention shown in Fig. 2. Fig. 1 shows an optical transmission system with an (optical) transmitter 1, an (optical) splitter 2, a pump laser 3 and an (optical) receiver 4. The pump laser 3 has a line port 31, a terminal port 32 and a control port 33. The transmitter 1 is connected to the line port 31 of the pump laser 3 by means of an optical waveguide via the splitter 2. An additional optical waveguide runs from the splitter 2 to the control port 33 of the pump laser 3. The terminal port 32 of the pump laser 3 is connected to the receiver 4. A data signal 10 from the transmitter 1 passes along the optical waveguide via the splitter 2 and via the pump laser 3 to the receiver 4. The pump laser 3 emits pump light 20 from the line port 31 towards the transmitter 1 and a monitoring signal 21 passes from the splitter 2 via the additional optical waveguide back to the control port 33 of the pump laser 3. In this case it is assumed, for example that the distance from the transmitter 1 to the splitter 2, as well as the distance from the splitter 2 to the pump laser 3 is relatively long, for example 50 km or more, the route of the optical waveguide from the transmitter 1 to the splitter 2 representing a path running under water which is not hazardous to personnel, while the route from the splitter 2 to the pump laser 3 represents the optical waveguide routed over land which in the event of damage, caused by civil engineering works for example, is hazardous to personnel. It is assumed in this case that the pump laser 3 and the receiver 4 are arranged close together and this section is therefore not taken into further consideration here. When an optical signal is transmitted over a long distance via an optical waveguide, deformation of the optical signal pulse occurs as a result of dispersion and attenuation effects as the transmission path increases. Usually, the optical signal therefore has to be regenerated at specific distances. If for attenuation compensation only amplification of the signal takes place, then this is referred to as 1-R regeneration. If for dispersion compensation the optical signal is additionally reshaped and re-synchronised, then this is also referred to as 3-R regeneration. While today the amplification of the optical signal is purely optical, purely optical 3-R regeneration is still not possible, at least in commercial systems. Dispersion can of course be countered by a so-called soliton transmission in which the light pulses are transmitted with a soliton pulse waveform, which do not change their shape over long routes. Without restricting the invention, it is to be assumed here that only one optical amplification occurs, where, as described at the outset, the amplification can be effected by a fibre amplifier, for example an erbium fibre amplifier or by Raman amplification in the optical waveguide itself. The fibre amplifier can for example be located under water between the transmitter 1 and the splitter 2. In both cases the feed for the amplification is effected by the pump laser 3 that generates high-energy pump light 20. For monitoring the optical waveguide path between the splitter 2 and the pump laser 3, the pump light 20 is tapped off at the splitter 2 and a small portion fed back as the monitoring signal 21 to the pump laser 3 via the additional optical waveguide. In the pump laser 3 a decision logic circuit checks whether a corresponding monitoring signal 21 returns when the pump light 20 is emitted. In the case where no corresponding monitoring signal 21 is received, the pump laser ceases the emission of the pump light 20. In this case it is highly probable that the optical waveguide is broken between the splitter 2 and the pump laser 3. Consequently there is no longer any danger to any personnel located at the point of fracture. The disadvantage of this arrangement compared to an unmonitored optical waveguide is that in order to feed back the monitoring signal 21, an additional optical waveguide has to be laid from the splitter 2 to the pump laser 3. Fig. 2 now shows an arrangement with a Method for fracture detection according to the invention, which dispenses with the aforesaid additional optical waveguide. Like Fig. 1, Fig. 2 shows the transmitter 1, the pump laser 3 and the receiver 4. Instead of the splitter 2, a Bragg grating 5 is installed at the same point. The pump laser 3 has a modified line port 31' and a terminal port 32. The transmitter 1 is connected via the Bragg grating 5 to the modified line port 31' by means of the optical waveguide shown in Fig. 1. The terminal port 32 of the pump laser 3 is connected to the receiver 4. As in Fig. 1, the pump laser 3 sends pump light 20 from the modified line port 31' towards the transmitter 1. Additionally from this port a monitoring signal 51 is transmitted to the transmitter 1, which is reflected at the Bragg grating 5 and returns as the reflected monitoring signal 52 via the optical waveguide to the modified line port 31' of the pump laser 3. Bragg gratings that are located in the optical waveguide itself are preferably used today to obtain a reflection device 5 for reflecting light of a specific wavelength 51. Other options for producing such a type of reflection device are also conceivable, in which a signal is first tapped off by means of a splitter, for example, then filtered and finally again injected into the optical waveguide. In order to produce a Bragg grating in an optical waveguide, periodic non-uniformities which cause modulation of the fibre core's refractive index are produced in the doped fibre core in the longitudinal direction of the optical waveguide by irradiation with ultraviolet light. The selectivity of the Bragg grating increases proportionally with the length of the grating; that is to say the reflection spectrum around the reflected wavelength is more sharply developed with increasing grating length. Present-day methods allow the production of Bragg gratings with virtually complete reflection. Parallel to the pump light 20, the pump laser 3 sends a monitoring signal 51 into the optical waveguide, this monitoring signal having a homogenous wavelength which causes reflection of the monitoring signal 51 at the Bragg grating 5, and results in the largest part of the monitoring signal 52 being fed back to the pump laser 3 as the reflected monitoring signal 52, as long as the optical waveguide between the Bragg grating 5 and the pump laser 3 is optically conducting. As soon as a break occurs in the optical waveguide, the monitoring signal 51 can no longer be reflected by the reflection device 5 to the pump laser 3; the pump laser 3 then immediately shuts down the pump light 20. An advantageous development of the invention consists in the pump laser 3 sending a further monitoring signal which, compared to the monitoring signal 51 described in the preceding section, has a different wavelength and consequently is not reflected at the Bragg grating 5 and is thus not normally received at the pump laser 3. In the rare case of a planar cleave, however, this can result in total reflection of all emitted monitoring signal at the point of fracture. Such total reflection would not be detected by the Method described in the previous section. With total reflection, both monitoring signals are now reflected back to the pump laser 3 and classifies this case as an additional defect in the optical waveguide and, like the case where neither of the two monitoring signals is reflected, ends the emission of the pump light 20. Fig. 3 shows schematically a further advantageous development of the invention which facilitates a more accurate, section-by-section location of the fracture point. The optical waveguide illustrated here represents a section of Fig. 2 between the pump laser 3 and the transmitter 1. A first Bragg grating 10 and a second Bragg grating 11, respectively, is placed at two different points in the optical waveguide. A first monitoring signal 101 and a second monitoring signal 102 are sent in parallel into the optical waveguide. The first Bragg grating 10 sends back a first reflected signal 101 and the second Bragg grating sends back a second reflected signal 111. The Bragg gratings 10 and 11 reflect different wavelengths, the first monitoring signal 100 being exclusively reflected at the first Bragg grating 10 and the second monitoring signal 110 being exclusively reflected at the second Bragg grating 11. If the optical waveguide between the pump laser 3 and the second Bragg grating 11 now breaks, then no reflected monitoring signal 101 or 111 is received at the pump laser; if the optical waveguide breaks between the second Bragg grating 11 and the first Bragg grating 10, then usually only the second reflected monitoring signal 111 is received. Depending on whether the pump laser 3 receives both reflected monitoring signals 101 and 111, or receives only the second reflected monitoring signal 111 or generally no reflected monitoring signals, it can be concluded that the optical waveguide is operational, or is fractured between the pump laser 3 and the second Bragg grating 11, or between the second Bragg grating 11 and the first Bragg grating 10. Several such sections can be obtained by means of an arrangement of several Bragg gratings, thus allowing more precise location of a fracture point.

Claims: Method for monitoring an optical waveguide between a monitoring terminal point (5) and a monitoring unit (3) for fractures, wherein an optical data signal (10) from a transmitter (1), and for amplification, pump light from a pump laser (3), is emitted into the optical waveguide, wherein a first monitoring signal (51) with a first wavelength is transmitted by the monitoring unit (3) to be reflected via the same optical waveguide to the monitoring unit (3) by a reflection device located at the monitoring terminal point(5) that is tuned to reflect the wavelength of the first monitoring signal (51), wherein additionally a second monitoring signal (52) with a second wavelength not to be reflected at any monitoring terminal point(5) is transmitted by the monitoring unit (3), and wherein the monitoring unit (3) evaluates the reflections of both monitoring signals (51, 52) to detect fractures between said monitoring unit (3) and said monitoring terminal point(5) and the pump laser is shut down as soon as either no monitoring signals or both monitoring signals are reflected. Method according to Claim 1, characterised in that the monitoring terminal point(5) consists in an in-fibre Bragg grating. Method according to Claim 1, characterised in that for section-by-section location of a cleave in the optical waveguide, the monitoring unit (3), instead of the first monitoring signal (51), sends two or more monitoring signals (100 and 110) at different wavelengths to be reflected by different monitoring units (10 and 11), wherein each reflection unit (10 or 11) reflects precisely one monitoring signal (101 or 111). Monitoring system for monitoring an optical waveguide, consisting of a monitoring unit (3) and a reflection device (5) that are located at the start and end, respectively, of the monitoring path, wherein the monitoring unit (3) contains transmitting means for transmitting a first monitoring signal (51) of a specific wavelength via the optical waveguide, receiving means for receiving the reflected monitoring signal (52) via the optical waveguide and a reflection device (5) that is tuned to reflect the wavelength of the monitoring signal (51), the monitoring unit (3) comprises additional transmitting and receiving means for transmitting or receiving a second monitoring signal (52) at another wavelength that is not reflected at the reflection device (5) and further comprises evaluation means to detect fractures between said monitoring unit (3) and said monitoring terminal point(5) on the base of an evaluation of the reflections of both monitoring signals (51, 52) and control means for correspondingly initiating a shutdown of a pump laser(3). Monitoring unit (3) for monitoring an optical waveguide between a reflection device (5) and the monitoring unit (3), comprising transmitting means for transmitting a first monitoring signal (51) with a specific wavelength and receiving means for receiving the reflected monitoring signal (52), where further transmitting means for transmitting a second monitoring signal (52) of another wavelength, a switching logic, is realised such, that a fracture is detected if none of monitoring signals (51, 52) or both of said monitoring signals (51, 52) are received and generating means for correspondingly generating a shutdown signal for a pump laser(3) are comprised. Monitoring unit (3) according to Claim 6, characterised in that the monitoring unit (3) is a constituent part of the pump laser (3).

Inventors: BLONDEL JEAN-PIERRE, BRADON ERIC, LE ROUX PATRICE, TOULLIER DENIS, BLONDEL, JEAN-PIERRE, BRADON, ERIC, LE ROUX, PATRICE, TOULLIER, DENIS
Applicants: CIT ALCATEL, ALCATEL
Application year: 2000
Main classification: H04B10
IPC codes: G08C15, H04B17, G01B11, H04B10, G08C23