WO2000021225A2

WO2000021225A2 - Monitoring of an optical transmission connection

Info

Publication number: WO2000021225A2
Application number: PCT/FI1999/000768
Authority: WO
Inventors: Markku Oksanen
Original assignee: Nokia Networks Oy
Priority date: 1998-10-07
Filing date: 1999-09-20
Publication date: 2000-04-13
Also published as: FI982176A0; FI982176A; AU5864899A; FI106678B

Abstract

The invention relates to monitoring the status of an optical transmission connection. A group of signals of different wavelengths is transmitted along an optical fiber (OF), and a sample of the signal traveling along the fiber is diverted to a special monitoring branch to verify the acceptability of the signals. To ensure that the monitoring system can be cost-effectively integrated even into an extensive optical network, the signal wavelengths are converted, in the monitoring branch, into a wavelength range visible to the human eye and, following the conversion, the signals are used to create an image for visual monitoring of the signals.

Description

Monitoring of an optical transmission connection

Field of the invention

Generally, the present invention relates to monitoring an optical transmission connection. More specifically, the invention relates to the monitoring of an optical link, particularly an optical link that uses Wavelength

Division Multiplexing (WDM) to control the status and properties of the signal passing via the link.

Background of the invention

In optical transmission systems, an optical signal is modulated with an outbound data stream, and the modulated optical signal is applied to optical fiber. In order to increase the capacity of the system, the bandwidth of the data stream can be increased or more wavelengths can be introduced, each of which is modulated with a discrete data stream. The latter method is termed wavelength division multiplexing.

Wavelength division multiplexing (WDM) is an efficient way of multiplying the capacity of optical fiber. In wavelength division multiplexing, several independent transmitter-receiver pairs use the same fiber. Figures 1a and 1b illustrate the principle of wavelength division multiplexing, using as an example a system having four parallel transmitter-receiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which generates light at a different wavelength (λ ..λ₄). As will be seen from Figure 1a, the modulation bandwidth of each source is smaller than the distance between the wavelengths, and thus the spectra of the modulated signals do not overlap. The signals generated by the transmitters are combined onto the same optical fiber OF in a WDM multiplexer WDM1 , which is a fully optical (and often passive) component. At the opposite end of the fiber, a WDM demultiplexer WDM2, which is also a fully optical (and often passive) component, separates the different spectral components of the combined signal from one another. Each of these signals is detected at a discrete receiver. Hence, a narrow wavelength window is assigned for the use of each signal in a given wavelength range. A typical practical example might be a system where the signals are in the 1550 nm wavelength range for example in such a way that the first signal is at wavelength 1544 nm, the second signal at wavelength 1548 nm, the third signal at wavelength 1552 nm and the fourth signal at wavelength 1556 nm. Nowadays a multiple of 100 GHz (approx. 0.8 nm) is becoming the de facto standard for the distance between wavelengths.

To monitor the spectrum of the optical signal traveling in the fiber, a spectrum analyzer is currently used. Because one of the major considerations in building new networks is cost-effectiveness, spectrum analyzers are a poor choice in the future because they are expensive "general-purpose" measuring devices that are impossible to integrate cost-effectively into the WDM equipment or nodes. Thus, spectrum analyzers cannot provide solutions that would make it possible to comprehensively monitor the operation of the transmitters and/or receivers in the network.

Summary of the invention

The purpose of the invention is to overcome the said drawbacks and provide a solution that makes it possible to efficiently and economically monitor the spectrum of an optical fiber link (e.g. the operation of the transmitter and/or amplifier) or other properties relating to the spectrum of an optical signal.

This objective is achieved by means of the solution defined in the independent patent claims. The basic idea of the invention is to carry out a frequency shift on the

WDM signal or part of it so that wavelengths move to a range visible to the human eye, which makes it possible to create an image of the signal that is visible to the human eye.

According to one highly preferred embodiment of the invention, the frequency shift is carried out by performing harmonic generation using some non-linear material suitable for this purpose. This solution offers the advantage of being very simple because generation (frequency shift) takes place passively in the material involved. (Here, non-linearity simply means that there is no linear dependence between the (external) incoming electric field and the (internal) electric field within the material.)

Using the solution provided by the invention, it is possible to integrate the monitoring mechanism into the network at a low cost. Because the monitoring system can be implemented using known and low-price components and because the system can be squeezed into a compact size, it can be used for monitoring the operation of all transmitters and/or amplifiers in the network. Moreover, the monitoring mechanism can be used to obtain additional information relating to the signal, such as the exact wavelengths of the spectral components or their intensity differences.

Brief Description of the Drawings The inventions and its preferred embodiments are explained in greater detail with reference to the examples given in drawings 2 though 5 where

Figures 1a and 1b show an optical transmission system that uses wavelength division multiplexing; Figure 2 illustrates the principle of monitoring in accordance with the invention;

Figures 3a and 3b illustrate harmonic generation in accordance with the invention;

Figure 4 illustrates the basic structure of one monitoring device; and Figure 5 illustrates a potential image visible to the monitoring staff.

Detailed Description of the Invention

Usually, the optical spectrum is understood as the part of the electromagnetic spectrum that extends from 1 nanometer (1 nm) to 100 micrometers (100 μm), i.e., from the shortest ultraviolet wavelength to the longest infrared wavelength. Within this optical spectrum, the wavelengths visible to the human eye start from around 400 nanometers (violet) and extend to around 800 nanometers (red light).

By contrast, optical transmission systems operate on wavelengths that are higher than the wavelengths visible to the human eye. The wavelengths currently in use are the 1300 nm range and the 1500 nm range.

To make it possible to monitor the operation of the optical link (state of the transmitter and fiber) simply and economically using low-price equipment, the aggregate signal traveling in the fiber is subjected to a frequency shift, so that all the wavelengths contained in the WDM signal are converted into the wavelengths of visible light.

Figure 2 illustrates the principle of monitoring in accordance with the invention. On the optical fiber link OL formed between the optical multiplexer

OM and the optical demultiplexer OD, a sample is taken of the WDM signal traveling along the link at the transmission or reception end (here sampling is assumed to take place at the reception end). For example, sampling can be carried out by means of a beam splitter BS. A beam splitter is a known optical device that splits the incoming beam into two or more beams. In its simplest form, this device consists of an extremely thin glass plate that is placed at the desired angle to the incoming beam. A certain percentage of the total power of the WDM signal (such as 1%) is reflected to the monitoring branch MB while the rest keeps traveling along the actual optical link. Sampling means that information on the fiber spectrum is extracted for monitoring purposes. Instead of the beam splitter, an optical tap may be used, which is a device consisting of inter-connected light channels where the desired percentage of the light is diverted to the sampling branch.

In the monitoring branch MB, the incoming signal undergoes a frequency shift in which the wavelengths used on the optical link are converted to wavelengths that fall within the wavelength range visible to the human eye. Preferably, the frequency shift is carried out by means of harmonic generation in a non-liner material. As is known, electromagnetic radiation of a certain frequency is, in harmonic generation, converted into a radiation with a frequency that is a multiple of the original frequency. This type of phenomenon may occur when a light beam passes through a non-liner optical medium. Such a medium is utilized in the practical implementation of a device based on the invention, as will be explained below. In the figure, the frequency shift is generally illustrated by the frequency shift element FS that can serve as a frequency multiplier (harmonic generator).

Then, the signals on converted wavelengths are led to the wavelength separator WS that separates the different wavelengths spatially such that an image can be created on the desired focusing plane, possibly fitted with the screen SC, for viewing by the operator OP. The device used for separating wavelengths may be a grating or a prism.

Figures 3a and 3b illustrate the frequency shift to be carried out in the monitoring branch in accordance with the invention. Figure 3a shows the spectrum from the link (beam splitter) assuming that the link is used for transmitting four wavelengths (λ₁...λ₄). Figure 3b, in turn, shows the spectrum after the frequency shift performed using harmonic generation. In harmonic generation, each incoming wavelength is divided by an integer N, which, after the frequency shift, gives the wavelengths λ.,/N, λ₂/N, λ₃/N and λ₄/N. In reality, N will take the value of either 2 (second harmonic) or 3 (third harmonic) to ensure that the wavelengths after the frequency shift are such that they fall within the wavelength range VR that is visible to the human eye. Assuming, for example, that the wavelengths used are within the range of 1545...1560 nm and the third harmonic is used for harmonic generation, the resulting wavelengths fall within the range of 515...520 nm (which the human eye will detect as greenish light). Sufficient spatial separation is achieved for different wavelengths when the wavelength separator WS uses a sufficiently high spectral resolution.

Figure 4 illustrates the construction of one monitoring branch MB in accordance with the invention. The optical fiber OFm from the beam splitter BS in the monitoring branch is first connected to collimating optics CO, which is used first to create a collimated beam out of the incoming optical signal. This collimating optics can be built by using, for example, the known GRIN lens (Gradient Index Lens).

The collimated beam is led to a non-linear element NL E which serves as a waveguide and in which harmonic generation takes place. This element may be a lithium niobate crystal (LiNb0₃) that is sufficiently wide to receive the beam. Other materials suitable for harmonic generation exist but lithium niobate is probably the most common. Some of these other materials may even be more effective in terms of harmonic generation than lithium niobate. The length of the element serving as the waveguide is determined by the non- linearity of the material. The length is determined using a known technique by maximizing harmonic generation, which takes place in a situation where the attenuation of the mixing result and harmonic generation are in a state of equilibrium. In reality, the element may be around 10 mm long. While the element generates other harmonic frequencies in addition to the desired second or third harmonic frequency, they do not interfere with monitoring and do not have to be filtered.

The beam, whose wavelengths were changed in the element NLE, is then routed to the diffraction element DE that diffracts the various wavelengths at different angles. The diffraction element may consist of a grating or a prism. From the diffraction element, the beam CB (being essentially a collimated beam) passes to focusing optics FO that creates the image in the desired focusing plane. Figure 5 illustrates one possible image displayed to the operator: the image may, for example, contain a row of points, one for each wavelength. To the human eye (HE), these points appear to be of the same color because the differences in wavelengths are too small for the human eye to detect the differences in color.

The focusing plane may include a screen that is viewed directly with the human eye or, alternately, the focusing plane can be viewed through a suitable eyepiece EP, as shown in Figure 4. In practice, it is advisable to use a magnifying device because the wavelengths are so close to one another. For example, such a magnifying device could be a microscope eyepiece. The device could feature cross lines (CL, Figure 5) to provide more accurate wavelength data. As explained above, the device monitoring the signal can be assembled from known components. Using this type of visual monitoring, the spectrum of the signal traveling along the fiber can be continuously monitored for any link problems. As well as indicating the absence of one or several wavelengths, the system can, when appropriately calibrated, be used for obtaining accurate wavelength data. Even the naked eye is accurate enough to determine differences in the intensities of individual wavelengths. This is because the wavelengths (as a result of conversion) are quite close to one another in the visible light range, ensuring that the differences in intensity detected by the human eye are equivalent to actual intensity differences. Because the device in accordance with the invention can be squeezed into a compact space, it can, in principle, be integrated with any optical transmitter or amplifier to monitor the operation of the same.

Also, harmonic generation of the selected visible wavelengths can be carried out by means of a frequency shift performed as four-wave mixing in the non-linear material. Then, a special pump laser can be used to feed a pump signal of desired frequency into the non-linear element. If the frequency of the actual signal received from the beam splitter is f1 and the pump laser operates at the frequency f3, various mixing results, such as the frequency 2χf3-f1 , are obtained at the non-liner element output, which could be the desired frequency (visible wavelength). Similarly, the sum of the frequencies to be mixed could also serve as the desired frequency. The non-linear element, to which the pump signal is fed, may consist of a length of single-mode fiber or a semiconductor chip that serves as a single-mode waveguide. Another possibility is to use a semi-conductor laser amplifier that serves the dual function of a pump laser and a non-linear element, in which case mixing takes place in the laser amplifier. Thus, the frequency shift need not necessarily be carried out by means of harmonic generation, even if such passive generation is the most economical and simplest form of monitoring.

Although the invention has above been explained with reference to the examples given the enclosed drawings, it is clear that the invention is not limited to the said examples but can be varied within the idea of the invention presented in the enclosed patent claims. Consequently, the exact construction of the eyepiece in accordance with the invention may vary and various (known) components can be used to achieve similar functionality. The detector used can also be electronic, such as a camera, but then some of the advantages offered by the invention are lost. The principle of the invention can also be applied to a link with only a single wavelength traveling through it. Thus, when a reference is made to a group of signals with different wavelengths in the enclosed patent claims, such a group may contain only one signal (single wavelength).

Claims

Patent claims:

1. A method for monitoring the status of an optical transmission connection, the said transmission connection comprising at least one optical fiber (OF), the method comprising the steps of - transmitting a group of signals of different wavelengths to the fiber, and

- diverting a sample of the signals traveling along the fiber to a separate monitoring branch (MB) for monitoring the validity of the signal, characterized by - converting the signal wavelengths, in the monitoring branch, into a wavelength range that is visible to the human eye, and

- following the converting step, creating an image of the signals for visual monitoring of the signals.

2. A method in accordance with patent claim 1, characterized in that conversion of the wavelengths is effected passively by means of harmonic generation taking place in non-linear material.

3. A method in accordance with patent claim 1, characterized in that the image is viewed with a magnifying monitoring device (EP).

4. A monitoring arrangement for an optical transmission connection, the transmission connection comprising at least one optical fiber (OF) and branching means (BS) for diverting a sample of the signals traveling along the fiber to a separate monitoring branch (MB), characterized in that the monitoring arrangement further includes - frequency conversion means (FM; NLE) for converting the signal wavelengths in the monitoring branch into a wavelength range visible to the human eye, and

- image-forming means (DE, FO) for creating an image out of the signals on the converted wavelengths for visual monitoring of the signals.

5. A monitoring arrangement in accordance with patent claim 4, characterized in that the frequency conversion means comprise an optical waveguide (NLE) of non-linear material.

6. A monitoring arrangement in accordance with patent claim 4, characterized in that the image-forming means comprise a diffraction element (DE) for spatial separation of individual wavelengths.

7. A monitoring arrangement in accordance with patent claim 4 characterized in that it comprises a magnifying monitoring device (EP) for visual monitoring.