FI106678B - Monitoring of optical telecommunications connection - Google Patents

Description

106678

Optical communication monitoring

Field of the Invention

The invention relates generally to the control of an optical transmission link. More particularly, the invention relates to monitoring an optical link, in particular an optical link using Wavelength Division Multiplexing (WDM), to monitor the status and characteristics of the signal passing through the link.

BACKGROUND OF THE INVENTION In optical transmission systems, an optical signal is modulated by a data stream to be transmitted, and the modulated optical signal is supplied to an optical fiber. In order to increase the capacity of the system, the bandwidth of the data stream may be increased or additional wavelengths may be introduced, each modulated by its own data stream. The latter option is called wavelength-15 multiplexing.

Wavelength multiplexing is an effective way to multiply the capacity of an optical fiber. In wavelength multiplexing, multiple independent transceiver pairs use the same fiber. Figures 1a and 1b illustrate the principle of wavelength multiplexing using, as an example, a system with four parallel transceiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which produces light at different wavelengths (λ., ... λ4). 1a, the modulation bandwidth of each source is smaller than the wavelength range, so that the spectra of the modulated signals do not overlap. The signals produced by the transmitters are combined with the same optic • · · l, '.' of the fiber in the WDM multiplexer WDM1, which is a fully optical (and often passive) component. At the opposite end of the fiber, the WDM demultiplexer WDM2, which is also a fully optical (and often passive) * · '· * component, separates the various spectral components of the combined signal.

. v '30 Each of these signals is detected by its own receiver. For each; 1; thus, the signal is provided with a narrow wavelength window at a given wavelength ♦ · *. tuusalueella. A typical practical example would be a system in which the signals are in the 1550 nm wavelength range, e.g. such that the first signal is at 1544 nm, the second signal at 1548 nm, the * 3 35 signal at 1552 nm and the fourth signal at 2 106678 1556 nm. At present, the de facto spacing between wavelengths is becoming a multiple of 100 GHz (about 0.8 nm).

A spectrum analyzer is currently used to monitor the spectrum of an optical signal passing through a fiber. However, since one of the 5 important goals in building new networks is cost-effectiveness of the network, spectrum analyzers will be a bad option in the future as they are expensive, "general-purpose" metering devices that make it impossible to integrate cost-effectively into WDM devices or nodes. Thus, spectrum analyzers are not capable of providing a solution that provides the ability to monitor on a large scale e.g. the operation of network transmitters and / or amplifiers.

Summary of the Invention

It is an object of the invention to overcome the drawbacks described above and to provide a solution which can effectively and economically monitor the optical fiber link spectrum (e.g., transmitter and / or amplifier operation) or other optical signal spectrum characteristics.

This object is achieved by the solution defined in the independent claims.

t

The basic idea of the invention is to perform a frequency shift on a WDM signal or a part thereof i * · .. so that the wavelengths are transmitted to a wavelength range ·: ··: whereby the optical signal can produce: an image that can be visually viewed by the human eye.

In accordance with a very preferred embodiment of the invention, the frequency transfer is performed by performing harmonic generation using some non-linear material suitable for this purpose. Rat-. . the advantage of the roller is its simplicity, because the generation (frequency transfer) takes place passively • · ·. material. (Non-linearity in this context simply means that there is no linear relationship between the (external) electric field entering the material in question and the (internal) electric field inside the material.) .. · According to the invention The solution enables the control mechanism to be integrated • in a very cost effective manner into the optical network. Because the monitoring mechanism: ·· * 35 can be implemented using known and economically advantageous components and because of its small space, 106678 can be used to monitor the operation of all transmitters and / or amplifiers in the network. Further, the monitoring mechanism can easily obtain additional information related to the signal, e.g., information on the exact wavelengths or intensity differences of the spectral components.

5

List of figures

In the following, the invention and preferred embodiments thereof will be described in more detail with reference to Figures 2 ... 5 in the accompanying drawings, in which Figs. 1a and 1b Fig. 4 illustrates the basic structure of a monitoring device, and Fig. 5 illustrates a possible view for a supervisor.

Detailed Description of the Invention «

The optical spectrum is generally considered to be that part of the electromagnetic spews which extends from 1 nanometer (1 nm) to 100 micrometres (100 µm), i.e., · ·: ··: from the shortest ultraviolet wavelengths to the longest infrared wavelengths.

. · .I: The visible light region visible to the human eye is the region of this optical spectrum which extends from about 400 nanometers to a wavelength (violet light) of about 800 nanometers • · · *. «·. wavelength (red light).

• · · * Optical transmission systems, on the other hand, operate at wavelengths that are larger than the wavelengths resolved to the human eye. The wavelength ranges currently in use are · 1300 nm and 1500 nm.

* 30 However, in order to monitor the operation of the optical link (transmitter and fiber condition) simply and economically using inexpensive equipment, the composite signal passing through the fiber is subjected to frequency shifting according to the invention so that all wavelengths contained in the WDM signal change. "wavelengths within the visible light range.

Figure 2 illustrates the principle of monitoring according to the invention. From the optical fiber link OL formed between the optical multiplexer 4 106678 OM and the optical demultiplexer OD, a sample of the WDM signal passing through the fiber is sampled at either the transmission or the receiving end (assuming sampling in the receiving end). Sampling can be done, for example, by means of a beam splitter BS. The beam splitter is a known optical device which divides the incoming beam into two or more beams. At its simplest, such a device may consist of a very thin sheet of glass set at a desired angle to the incident beam. A portion of the total power of the WDM signal (e.g., about 1%) is then reflected in the MB of the monitoring path, and the remainder continues its journey along the actual fiber link. 10 Sampling thus means that information on the fiber spectrum is obtained for monitoring purposes. Instead of a beam splitter, for example, an optical tap may be used, which is a device consisting of interconnected light channels in which the desired amount of light is coupled to the sample branch.

In the monitoring branch MB, the incoming signal is subjected to a frequency-15 shift such that the wavelengths used on the optical link are converted to wavelengths in the wavelength range that can be distinguished by the human eye. This frequency transfer is advantageously accomplished by harmonic generation in non-linear material. As is known, in the generation of harmonics, an electromagnetic radiation having a specific frequency is converted to a radiation having a multiple frequency relative to the original frequency. Such a phenomenon can occur when a light beam passes through a non-linear optical medium. Such a medium is utilized in the practical implementation of the device of the invention, as described below described. In the figure, the frequency transfer is generally illustrated by a frequency transfer element FS, which can act, for example, as a frequency multiplier (generator of • · · l. '.').

The signals at the modified wavelengths are then passed to the wavelength separator WS, which separates the different wavelengths spatially from one another to a desired focal plane, where

: 30 be eg a shade SC can be formed by the image of the operator OP

. * · *: Can be viewed. The wavelength separator may be e.g. a lattice or a prism.

Figures 3a and 3b illustrate frequency transfer in a power branch in accordance with the invention. Fig. 3a shows the spectrum from the • "(beam splitter) assuming that the link carries four wavelengths (λ, ... λ4). Fig. 3b, in turn, shows the spectrum after the frequency shift when the frequency shift is implemented by generating harmonics.

5 106678

As a result of harmonics generation, each incoming wavelength is divided by an integer N, so that after the frequency shift, the wavelengths λ / Ν, Xj / N, λ3 / Ν and λ4 / Ν are obtained. In practice, N gets either a value of 2 (second harmonic) or a value of 3 (third harmonic) so that the wavelengths after the frequency shift are 5 such that they are in the visible wavelength range VR. For example, assuming that the wavelengths used are between 1545 nm and 1560 nm, the third harmonic is utilized in the generation of the harmonics, resulting in the resulting wavelengths between 515 and 520 nm (which the human eye sees as greenish light). A sufficient spatial resolution is obtained for different wavelengths when a sufficiently high spectral resolution is used in the separation device WS.

Fig. 4 illustrates a practical structure of a monitoring branch MB according to the invention. The optical fiber OFm of the monitoring branch coming from the beam splitter BS is first coupled to the collimating optic CO, by means of which a first collimated beam is formed from the incoming optical signal. This collimating optic can be implemented, for example, with the aid of the known Gradient Index Lens (GRIN).

The collimated beam is led to a nonlinear element NLE, which acts as a waveguide and generates harmonics. This element may be, for example, a lithium niobate crystal (LiNbO 3) that is wide enough to receive the beam. There are other types of materials that produce harmonics, but lithium niobate is perhaps the most commonly used. Some of these other materials may have higher harmonic efficiency than lithium niobate. The length of the waveguide element depends on the materiality of the material. The dimensioning is done in a manner known per se so as to maximize the generation of harmonics, which occurs in a situation where the damping of the mixing result in the material and the output of the harmonics are in equilibrium. In practice, the length of the element can be, for example, about 10 mm. The element generates practically any harmonic frequencies other than the desired second or third. : 30 harmonic, but they do not interfere with monitoring and do not need to be filtered.

. ***; The beam whose wavelengths have changed in the element NLE is then led to a diffraction element DE, which folds at different wavelengths at different angles. The diffraction element may be, for example, a lattice or a prism. From the diffraction element i ', the beam CB (which is a substantially collimated beam) is led to the focusing optics • · · 35 FO, which produces the image at the desired focal plane. Figure 5 illustrates a possible image visible to the operator: the image may contain 6,106,678 e.g. lines of dots, one for each wavelength. For the human eye (HE), these dots look the same color because the wavelength differences are so small that the human eye cannot distinguish color differences.

The focus plane may have a screen which is directly viewed by 5 human eyes or alternatively the focus plane may be viewed by any suitable eyepiece EP as shown in Figure 4. In practice, it is advantageous to use a magnifying device because the wavelengths are very close to each other. The magnifying device may be e.g. a microscope eyepiece. The device may have, for example, a hair grid (CL, Fig. 5) 10, which provides more accurate wavelength information.

As discussed above, the signal monitoring device may be assembled from components known per se. With such a visual monitoring arrangement, the spectrum of the signal passing through the fiber can be continuously monitored for various link problems. In addition to the absence of one or more wavelengths, the method may provide, by appropriate calibration, e.g., accurate wavelength information. In addition, information on intensity differences of different wavelengths can be obtained with the naked eye alone. This is because the wavelengths (as a result of the transformation) are quite close to each other in the visible region, whereby the intensity differences detected by the human eye correspond to the actual intensity differences.

Since the device according to the invention is obtained in a small space, it can be i \. in principle, to integrate with any optical transmitter or amplifier in question. to control the operation of the element.

Generation of the desired visible wavelengths can also occur. 25, for example, as a frequency shift by means of mixing in a non-linear element, eg by four-wave mixing. In this case, * a separate pump laser can be used, for example, to supply a pump signal of the desired frequency to a non-linear element. If the frequency of the actual signal from the beam splitter '· * ·' is f1 and the pump laser operates at frequency f3, various mixing results are obtained at the output of the nonlinear element, e.g.

. ·· *. frequency 2xf3-f 1, which could in practice be the desired frequency (visible wavelength ···). Also, the sum frequency of the frequencies to be mixed could in practice form the desired frequency. The non-linear element to which the pump signal is fed may be, for example, a single-mode fiber or a semiconductor piece which * · · 35 acts as a single-mode waveguide. It is also possible to use a semiconductor laser amplifier which simultaneously functions as both a pump laser and a nonlinear 7 106678 element, whereby mixing takes place in the laser amplifier. Thus, the frequency transfer need not necessarily be in the form of harmonics generation, although such passive generation is the most economical and simplest way to perform monitoring.

While the invention has been described above with reference to the examples in the accompanying drawings, it is clear that the invention is not limited thereto but can be modified within the scope of the inventive idea set forth in the appended claims. Thus, the exact structure of the viewing device of the invention may vary and different (known) components may be used to achieve similar functionality. The detector used can also be electronic, e.g. a camera, but then some of the advantages of the invention are lost. The principle of the invention can also be applied to a link with only one wavelength. Thus, when the appended claims refer to a plurality of signals at different wavelengths, it is possible that such a set 15 contains only one signal (one wavelength).

«« «« «· II1« «« «i« »i« »i« »i« i «» i «i« i «i« i «i« i «i« i «i« i «i« i «i« i ««. · · 1 1 1 1 ♦ 1 1 1 1 1 1 · 1 1 1 1 1 1 1 1 1

Claims (7)

A method for monitoring the condition of an optical transmission connection, the transmission connection comprising at least one optical fiber (OF), according to which method - a number of signals having different path countries are transmitted to the fiber, and along the fiber passing the signals to a separate monitoring branch (MB) for monitoring the signal's capability, the signal wattage - the path lengths of the signals in the monitoring branch are changed such that they are within a human-visible path length range, and - the signals are created after the change for visual monitoring of the signals. 15 Method according to claim 1, characterized in that the change of the path lengths is carried out passively by harmonic generation in a non-linear material. Method according to claim 1, characterized in that the created image is observed by means of an enlarging observation device (EP). : 4. Monitoring arrangement for an optical transmission connection, Γ \, which transmission connection comprises at least one optical fiber (OF) and which monitoring arrangement comprises branching means (BS) for branching; a test signal representing the signals passing along the fiber, from the fiber to a separate monitoring branch (MB), characterized in that the monitoring arrangement further comprises * - frequency change means (FM; NLE) for the monitoring branch. change the path lengths of the signals such that they are within a path range visible to human eyes, and 30. means for creating image (DE, FO) to create an image of the signals; changed path lengths for visual monitoring of the signals. • ·. ···. Monitoring arrangement according to claim 4, characterized in that the frequency change means include an optical guide (NLE) which is of a non-linear material. 3567678 Monitoring arrangement according to claim 4, characterized in that the image-forming means include a diffraction element (DE) for spatially separating the different path lengths from each other. Monitoring arrangement according to claims 4, 5, characterized in that it further comprises an magnifying observation device (EP) for visual observation of the image. «Tt 1% <« iti «III« · «<« <«1« I «« • · • 1 · • · · • · · · · · · · · · ♦ »» · • · • · • · · • · · • · • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·