Diagnostic method and diagnosis chip for waveband determination of optical filaments

ES 2 397 533 T3

DESCRIPTION

Diagnostic procedure and diagnostic chip to determine the optical fiber bandwidth.

The invention relates in general to the optical transmission of data and in particular to a method and a diagnostic device for optical fibers.

For trunk cabling and in connection with fast Ethernet and gigabit Ethernet, cabling based on optical cables is becoming increasingly interesting. The transmission of signals in optical fibers, also called light wave conductors (LWL), is effected unidirectionally by means of light pulses, that is, only in one direction, which is why at least two optical fibers are typically used for each cable . The light pulses are generally coupled to the fiber by means of a laser diode or a light emitting diode.

The advantages of optical data transmission lie in the high achievable transmission rate and long range, as well as insensitivity to electromagnetic radiation, safety against eavesdropping and stability against heat and weather influences. Optical cables can also be routed flexibly through the thin inner conductors.

Apart from pure glass fibers, polymer optical fibers have found widespread use due to lower costs. Polymer optical fibers are called POF fibers (Plastic Optical Fiber) and are pure plastic fibers that consist of a transparent core and sheath, the sheath having a lower refractive index than the core material. Polymers such as polymethylmethacrylate or polycarbonate are used as the core material. Apart from purely polymeric fibers, polymeric optical fibers also include hybrid fibers that have a combination of glass fiber and plastic sheath, such as, for example, HCS fiber (Hard Polymer Cladded Silica - Hard Polymer Clad Silica ).

The distance that can be bridged directly with light wave conductors is limited by different scattering and scattering effects. It depends on the bandwidth to be transmitted and is therefore typically indicated as the bandwidth-length product. Compared to glass fibers, polymer fibers, such as POF or HCS, have significantly higher attenuation values. POF or HCS fiber systems are operated with path lengths of 50 or 100 m and data rates of 100 Mbits / s or 125 MBd, already at the edge of their technical possibilities. To ensure safe data transmission, a diagnosis of the optical fibers in the transmission path is now a common procedure.

A method for diagnosing the optical paths of light wave conductors, especially for optical diagnostics on the Interbus, is known, for example, from DE 42 17 899 C2. The procedure described there serves to optimize the LWL transmission path system during commissioning and provides for a variation of the optical transmission power of a transmission receiver until the optical signal received on the opposite side satisfies the system requirements. .

From document EP 1 227 604 A2, a method is known in which its current level reserve is determined for an optical transmission path up to the sensitivity limit, that is, between the current emission power of the transmitter and the sensitivity limit. current receiver.

The optical diagnosis according to this state of the art is directed to the objective of diagnosing the attenuation of optical connection paths in order to deduce a safe transmission from it. This is sufficient also in low bit rate systems. However, at data rates from 100 Mbits / s on polymer fibers (POF) and (HCS) the limiting factor for fault-free transmission is no longer cable attenuation, but cable bandwidth. However, this bandwidth cannot be checked automatically using currently known procedures. Another disadvantage of known systems resides in that the type of fiber used cannot be automatically established, such as, for example, POF or HCS, which is capable of operating on the same interface. This also has a disadvantageous effect on the attenuation diagnostics, since different fiber types also exhibit different attenuation behavior and the diagnostic data therefore have to be evaluated differently. Typically, the fiber type nowadays has to be indicated manually in the diagnostic software.

The German patent application 10 2005 016 522.2, filed on April 18, 2005 by the same applicant of the present invention, with respect to which the present invention constitutes a further development, was based on the problem of indicating a mode referring to how the diagnosis of optical fibers or cables can be improved and / or simplified. In particular, the problem consisted in indicating a method and a device for the simple and inexpensive determination of the bandwidth of optical fibers, as well as indicating a mode of optimization of the system of optical data transmission paths.

To this end, a method was proposed to determine the bandwidth of at least one optical fiber with the steps of coupling light of a first optical power and a first modulation frequency with the optical fiber, measuring a first signal level as a function of of the optical power of the light of the first modulation frequency conducted through the optical fiber, coupling light of a second optical power and a second modulation frequency with the optical fiber, measure a second signal level as a function of the optical power of the light from the

ES 2 397 533 T3 second modulation frequency conducted through the optical fiber, and obtain the bandwidth of the optical fiber as a function of the first and second coupled optical powers and / or the first and second optical levels measured using a predetermined specification that describes the attenuation behavior of optical fiber as a function of frequency.

Unless otherwise indicated in the following, the term frequency is used in the sense of a modulation frequency of an optical signal.

The principle of the method according to German patent application 10 2005 016 522.2 essentially envisaged determining the attenuation of two optical signals of different frequency and obtaining from it the bandwidth of the optical fiber with the help of a known characteristic frequency response. of the attenuation of said fiber.

By attenuation we have to understand the losses of the light that circulates through the optical fiber. Attenuation is typically expressed as ten times the logarithm of the ratio of the optical power at the input and output of the optical fiber:

p (1) A = 10 log ^ -L [_dB]

P

Ausgang where Eingang means entry and Ausgang means exit.

Attenuation is mainly caused by the physical processes of absorption and dispersion, as well as by mechanical deflection.

In optical fibers exhibiting a low-pass characteristic, the bandwidth corresponds to the modulation frequency at which the light power, compared to the value at zero frequency, has optically dropped by 50% or 3 dB.

To increase the accuracy of the method according to German patent application 10 2005 016 522.2, the signals of the first and second frequencies advantageously had a strongly different frequency-dependent attenuation.

Advantageously, it was foreseen that the first frequency would be smaller, especially at least by the factor 10, especially at least by the factor 100, than the bandwidth of the optical fiber and that the attenuation of the optical fiber as a function of the frequency would be substantially zero at the first frequency, since in this case the calculation specification to obtain the bandwidth is clearly simplified.

The second frequency was advantageously in the domain of the optical fiber bandwidth, the second frequency being conveniently in the domain of the ideally expected bandwidth of the optical fiber, and therefore typically above the actual bandwidth, of so that the signal of the second frequency experiences a stronger attenuation than the signal of the first frequency. In particular, the optical fiber was expected to exhibit a frequency-dependent attenuation for circulating light of the second frequency, which would be between 1 and 5 dB, especially between 2 and 4 dB, more especially around 3 dB, for above the attenuation as a function of frequency for circulating light of the first frequency.

Therefore, the basic idea of ​​the invention according to the German patent application 10 2005 016 522.2 was to obtain the bandwidth of an optical fiber by means of two measurement values, a first measurement value that served as a reference and was not substantially influenced by the bandwidth limitation of the optical fiber, and a second measurement value as a function of the bandwidth. Obtaining the bandwidth from these two measurement values ​​is here made possible by the use of a predetermined specification that describes the attenuation behavior of the optical fiber as a function of frequency.

The problem of measuring the bandwidth of an optical connection between two subscribers is solved by the German patent application 10 2005 016 522.2 in such a way that the measurements of the light power typically necessary for the determination of the bandwidth are carried out at two Different frequencies or data rates, since a measurement at a lower frequency is intended as a reference measurement. These solutions described in German patent application 10 2005 016 522.2 for POF and HCS fibers are typically 10 and 100 Mbit / s. Thus, a measurement of the bandwidth with the aid of a standard Ethernet device has already been advantageously made possible in a particularly simple way.

The switching of the data rate is always necessarily linked to a link interruption, which leads to an at least short-term failure of the data communication, so that the procedure described in German patent application 10 2005 016 522.2 it cannot typically be performed online during the ongoing communication operation of an LWL interface.

Therefore, the present invention is based on the problem of further developing the subject of German patent application 10 2005 016 522.2 in such a way that the diagnosis of fibers or fibers can be further simplified.

ES 2 397 533 T3 optical cables. In particular, the problem consists in indicating a method and a device for the simple and inexpensive determination of the bandwidth of optical fibers, by means of which an online bandwidth measurement can also be carried out without refluxing towards the communication.

Another diagnostic method for determining the bandwidth of optical fibers is known from EP-A-1018642.

The problem is solved by a method according to claim 1 and an electronic component according to claim 25. Advantageous and / or preferred embodiments and improvements are the subject of the respective dependent claims.

Therefore, it is provided according to the invention that the first and second modulation frequencies have substantially the same value, so that the first and second signal levels are obtained at substantially the same modulation frequency. Accordingly, according to the present invention, provision has been made to obtain the measurement value that serves as a reference at the same modulation frequency at which the bandwidth-dependent measurement value is also obtained.

Advantageously, the first optical power of the coupled light of the first modulation frequency and the second optical power of the coupled light of the second modulation frequency again have predetermined values ​​which, in particular, are preferably substantially the same.

The inventors have surprisingly discovered that at the same input power of the coupled light of the first and second modulation frequencies and with a predetermined specification to describe the attenuation behavior of the fiber as a function of the frequency, the width of band from the ratio of the respective measured first and second signal levels even though the first and second modulation frequencies are substantially the same. Thus, it can advantageously be provided according to the invention that the coupled light of the first and second modulation frequencies is identical and that the width measurement is based on the different nature of the measured signal levels.

As an alternative, it is again advantageously provided to detect the exceeding of predetermined threshold values. Accordingly, the first optical power of the coupled light of the first modulation frequency is advantageously varied until the first measured signal level reaches a first predetermined threshold value, and the second optical power of the coupled light of the second frequency is varied. modulation until the second measured signal level reaches a second predetermined threshold value.

Advantageously, the first threshold value at the first modulation frequency and the second threshold value at the second modulation frequency are equal, so that the fiber bandwidth can be easily obtained from the respective optical powers of the fiber. input or magnitudes proportional to them.

The at least one optical fiber preferably comprises a polymer and / or a glass and is especially configured as a POF or HCS fiber. However, pure glass fibers also fall within the scope of the invention.

An advantageous specification that describes the attenuation behavior as a function of frequency of an optical fiber, especially a POF or HCS fiber, is a Gaussian low-pass function that is defined by the equation

<img file="ES2397533T3_D0001.tif" />

In an especially preferred embodiment of the invention, the measurement of the first signal level comprises the determination of an average or averaged light power (AVG). This measured value is substantially independent of bandwidth influences and therefore serves as a reference value.

The measurement of the second signal level particularly preferably comprises the determination of the amplitude of the modulated optical signal according to the OMA method (Optical Modulated Amplitude), since the amplitude of the modulated optical signal is influenced by the width fiber optic bandwidth.

The use of the different AVG and OMA methods for measuring light power makes it possible to determine the bandwidth of the optical fiber with the aid of measurement values ​​that are obtained at only one modulation frequency or data rate.

The first and second modulation frequencies, which are thus substantially the same, are preferably located approximately in the bandwidth of the optical fiber. Advantageously, they can also be situated in the domain of the ideally expected bandwidth of the optical fiber and, therefore, typically above the real bandwidth.

Therefore, this preferred embodiment of the present invention is based on the knowledge that, when the theoretical behavior of the bandwidth of an LWL connection or an optical fiber is known and they accept

ES 2 397 533 T3 a data rate or modulation frequency fixes the peak-to-peak value or OMA value of the light power LW and also the average light power (AVG; average), the bandwidth of 3 dB can be obtained even at just one data rate or modulation frequency. Therefore, the method according to the invention can also be carried out in a non-backflow line towards the communication of an optical communication connection.

This lies substantially in the fact that the average light power is independent of the influences of the bandwidth on the optical signals through the cable or the optical fiber, while the peak-to-peak value, that is, the amplitude of the Modulated optical signal is highly influenced by the cable bandwidth. Expressed in a simplified way, the AVG averaged light power measurement has the function of a reference measurement similar to the measurement of the light power at the lowest modulation frequency according to the procedure already described in German patent application 10 2005 016 522.2.

Advantageously, the measurement of the first and / or the second signal levels is carried out by means of an optical transceiver that has a DMI interface (Diagnostic Monitoring Interface) according to SFF8472, since it is advantageously designed to measure power light mean (AVG) and the amplitude of the modulated optical signal (OMA).

The DMI (Diagnostic Monitoring Interface) is an extended serial interface that makes it possible to access certain operating parameters of an optical transceiver for diagnostic purposes. These operating parameters include, for example, the temperature of the transceiver, the output power and especially the optical power received by the transceiver.

Advantageously, an optical transceiver can also be provided on the receiver side, with which the received optical power cannot be directly obtained, but only threshold values ​​can be detected for a given light power. This optical transceiver can be of simpler construction and therefore cheaper. The detection of threshold values ​​for a given light output is implemented, for example, in PSI-MOS devices from PSM.

In the embodiment based on threshold value detection the determination of the bandwidth is carried out in almost the same way as in the embodiment based on the exact measurement of the received optical light power. For this purpose, the optical emission power of the light of the first and second modulation frequencies is varied as precisely as possible, while on the receiving side the achievement of a predetermined threshold value of the received optical light power is monitored. This can be implemented in many cases in a technically simpler way than making an exact measurement of the light power. Preferably, the measurement of the received optical light power comprises the measurement of the light power AVG and the light power OMA, where respective different threshold values ​​may be provided for the light power AVG and the light power OMA.

To determine the bandwidth, at a preset data rate or modulation frequency, which corresponds at the same time to the first and second modulation frequencies, the light power of the emitter is adapted until the predetermined threshold values ​​are reached in the receiver.

The described embodiment, based on the influence of the emission power, is also simpler in technical implementation since there is almost always a fairly exact linear correlation between the emission power and the emitter current in LEDs and lasers and it is technically easier to precisely adjust currents in the range of approximately 1 to 100 mA than to measure light powers at the receiver, as these measurements are typically based on evaluating diode currents in the range of pA or less.

In a particularly preferred manner, the measurement of the first and second signal levels, for example with the aid of the DMI interface of a transceiver, is carried out for coupled signals having a continuous 10-bit pattern, such as that available, for For example, with the IDLE (FREE) signal of a network, such as 10Base-FL or 100Base-FX.

The method according to the invention also advantageously comprises the steps of obtaining the length of the optical fiber by means of a propagation time measurement and determining the bandwidth-length product of the optical fiber from the obtained bandwidth and the length obtained from the optical fiber.

Depending on the product of bandwidth-length, the type of optical fiber can then be obtained in a particularly advantageous manner by means of stored value ranges of the product of bandwidth-length for different types of fibers.

The invention also provides a method for establishing a data communication connection between a first and a second terminal that are connected to each other by means of an optical data transmission path provided with at least one optical fiber, which method advantageously provides for a initialization phase within which the bandwidth of the optical fiber is obtained according to the procedure described above for determining the bandwidth of an optical fiber.

ES 2 397 533 T3

Advantageously, the optical data transmission path comprises a first optical transceiver at the first end of the path and a second optical transceiver at the second end of the path, which are connected to each other through a first and a second optical fiber to the two transmission directions. The method preferably foresees for this arrangement obtaining the bandwidth of the first and second optical fibers. Obtaining the bandwidth of the first and second optical fibers can be carried out successively or in parallel.

Obtaining the bandwidth is especially useful for diagnosing existing bandwidth limitations that have an influence on the maximum possible data transmission rate. Consequently, the method advantageously provides for the step of automatically adapting the data communication transmission rate as a function of the bandwidth obtained from the at least one optical fiber.

Since a determination of the bandwidth without switching the data rate is made possible in a particularly advantageous manner by the present invention, a method of data communication between a first and a second terminal which are connected to one another is also provided. another by means of an optical data transmission path equipped with at least one optical fiber, in which procedure the bandwidth is obtained in the manner described above, specifically in a communication connection between the first and second subscribers.

An electronic component for use in an optical data transmission path with at least one optical fiber, which is especially configured as an integrated control circuit and is suitable for carrying out the above-described method of obtaining the bandwidth of an optical fiber , It comprises, according to the invention, means for calculating a value for the bandwidth of an optical fiber as a function of a first and a second signal level using an algorithm based on a predetermined specification, the prescription describing the attenuation behavior of optical fiber as a function of frequency, and is also configured to measure the average light power (AVG) of an optical signal and the amplitude of a modulated optical signal (OMA; Optical Modulated Amplitude).

The first or second signal level is preferably proportional to a first or a second measurement magnitude of light power that is obtained, respectively, by a first or a second method of measurement of the luminous power of light that is coupled with the fiber. optical at a preset modulation frequency and is received after circulating through the optical fiber, the first method of measuring the light power comprising the determination of the averaged light power of the received light and the second method of measuring the light power comprising the determination of the amplitude of the received modulated optical signal.

The invention is described in more detail below with the aid of preferred embodiments and with reference to the accompanying drawings. Like reference symbols designate the same or like parts in the drawings.

They show:

Figure 1, schematically, an optical transmission path with two optical transceivers that are connected to each other through two optical fibers,

Figure 2, a schematic representation of a preferred embodiment of an electronic component according to the invention,

Figure 3, a schematic diagram of the attenuation behavior of a POF fiber as a function of frequency, and

Figure 4, schematically, the time evolution of an optical signal in a light wave conductor (LWL) or in an optical fiber to illustrate a preferred embodiment of the present invention.

Figure 1 shows an optical data transmission path between a first terminal 10 and a second terminal 20. Terminals 10 and 20 are equipped, respectively, with a first optical transceiver 12 and a second optical transceiver 22 that are connected to each other. through a duplex LWL line with a first optical fiber 30 and a second optical fiber 40 for the two respective data directions.

In this exemplary embodiment, POF fibers are used as optical fibers and the bandwidth test according to the invention is carried out prior to connection establishment as interface initialization. Since a determination of the bandwidth without switching the data rate is made possible in a particularly advantageous manner with the present invention, the bandwidth test according to the invention can alternatively also be carried out after the connection establishment. Furthermore, it can be advantageously provided to repeat the bandwidth test according to the invention at predetermined time intervals.

The following describes the principle development of the bandwidth determination according to the German patent application 10 2005 016 522.2. First, the transceiver 12 sends a signal with a light output

ES 2 397 533 T3 predetermined and with a first frequency that in this embodiment is typically below 10 MHz, and the transceiver 22 measures the light power OMA of the received signal. Next, the transceiver 12 sends a signal of equal light power, but with a second frequency, which in this embodiment is typically between 60 and 100 MHz, and the transceiver 22 measures again the light power OMA of the received signal. At the same time, the second fiber of the duplex LWL line is measured in the same way, swapping the roles of transceivers 12 and 22. Depending on the length of the cable, the coupling of the light (NA coupling) with the fiber and eventually the deflections or fiber damage, the received light power of the second signal is smaller than that of the first signal. When this difference for a fast Ethernet system is greater than 3 dB, then it must be assumed that there are transmission disturbances as a result of bandwidth limitations. In this case, a corresponding alarm message can be generated or the data transmission rate can be adapted automatically.

As an alternative to the exact measurement of the received light power, it is also possible to vary, as described above, the emission power at first and second frequencies until a predetermined threshold value for the received light power is detected on the receiving side. .

In addition, the length of the optical transmission path between terminals 10 and 20 can be obtained by measuring the propagation time, in the case of Profinet, for example, by measuring the transmission time for clock synchronization. Therefore, the obtained bandwidth can still be related to the cable length below and this relationship can be evaluated. Automatic determination of the fiber type is thus possible, since, for example, a POF fiber has a different bandwidth-length product from that of an HCS fiber.

The method according to the invention can also be used for Gbit transceivers and multimode glass fibers (50/135 pm, 62.5 / 125 pm), since also in this case the possible transmission distance between two transceivers. The frequencies can be adapted there to the conditions prevailing in the glass fibers.

Figure 2 shows a schematic representation of a preferred embodiment of an electronic component 60 according to the invention that is configured as a separate diagnostic chip. Diagnostic chip 60 is connected in the data path between a PHY transceiver 50 assigned to the physical layer of a network and an optical LWL transceiver 12. For data communication, a first and a second interface 641 and 642 are provided in component 60, via which the PECL interface 124 of the LWL transceiver 12 is connected to the PECL interface 54 of the PHY transceiver 50. Thus, in In the case of normal Ethernet communication, component 60 configured as a separate diagnostic chip is transparent to Ethernet data signals.

The component 60 also comprises, especially to query the received light power, a first serial interface 662 for connection to a DMI interface 126 of the optical transceiver 12. Furthermore, a second serial interface 661 is provided for connection to a serial interface 56 of the PHY transceiver. fifty.

In this embodiment, interfaces 126 and 662 use the transmission protocol I²C and interfaces 56 and 661 use the SPI transmission protocol. For the conversion between these two transmission protocols, a corresponding converter, not shown, is integrated in the component 60.

The invention can be used advantageously in all optical interfaces for fast Ethernet devices with POF and HCS fiber interfaces, as well as for fiberglass interfaces, which are of interest especially for systems with data transmission rates of 10 Gbits / s. .

Especially in Profinet interfaces, the procedure offers additional advantages over conventional diagnostic systems, as it is not based directly on the LWL transceiver used, but is a digital procedure. The only precondition is that the LWL transceiver used has a DMI interface according to SFF-8472 and measures the AC portion of the light power according to the OMA (Optical Modulated Amplitude) procedure described there and the averaged light power according to the AVG procedure described there . To calculate the bandwidth from the values ​​obtained by means of a transceiver mastering the two different procedures OMA and AVG for measuring the light power, an evaluation unit is advantageously used which is configured, for example, as a processor of an Ethernet device.

The method according to German patent application 10 2005 016 522.2 is based in principle on carrying out, for example with the aid of a DMI interface of an optical transceiver, two attenuation measurements at two different frequencies. Signals of different frequencies can be represented particularly simply by means of a 10-bit continuous pattern with a corresponding frequency. The IDLE (FREE) signal of a network can be used with particular advantage for this.

In the following, a specific example of obtaining the bandwidth according to German patent application 10 2005 016 522.2 is described with reference to FIG. 3. The signals of first and second frequencies are here formed by means of an IDLE signal according to 10Base-FL for the first frequency and by means of an IDLE signal according to 100Base-FX for the second frequency. The IDLE signal according to 10Base-FL has a frequency of

ES 2 397 533 T3

0.5 MHz resulting from changing light on and light off signals of 1 ps duration each. The IDLE signal according to 100Base-FX has a frequency of 62.5 MHz which is derived from the data rate of 100

Mbits / s, 4B / 5B encoding, and a change in the NRZI code for each bit time in the IDLE signal.

The first measurement is made corresponding to a frequency of 0.5 MHz which is well below the bandwidth 84 of approximately 90 MHz of the POF fiber used in this exemplary embodiment. To this end, the emitter of the first transceiver is excited with this frequency and the averaged light power reaching the receiver of said second transceiver is obtained through the DMI interface of the second transceiver. At this frequency, no frequency-dependent dimming takes place and the received light power has the level designated by reference symbol 72 in FIG. 3.

Next, a second measurement is made with the frequency of 62.5 MHz, provided with the reference symbol 82 in Figure 3, which is within the order of magnitude of the 90 MHz bandwidth 84 of the fiber to be measured, in which the received light power 76 has dropped by half compared to the zero frequency.

Due to the low-pass characteristic of the fiber, this second obtained light output 74 is smaller than the first 72. Starting from the known low-pass characteristic of the LWL fiber used, the bandwidth of the connection can now be deduced according to the invention and also , through an automatic measurement of the length of the path, the type of fiber.

To this end, as a specification that describes the attenuation behavior of the fiber as a function of frequency, a Gaussian low-pass function corresponding to the equation

<img file="ES2397533T3_D0002.tif" />

In this exemplary embodiment, the light power received through the DMI interface is measured as the electrical signal level and the following measurement values ​​result:

U (0.5 MHz) = 452 mV = U_or

U (62.5 MHz) = 324 mV

<img file="ES2397533T3_D0003.tif" />

Since the signal level obtained by the OMA procedure and supplied by the DMI interface is proportional to the received light power, it results from equation (2) above

<img file="ES2397533T3_D0004.tif" />

<img file="ES2397533T3_D0005.tif" />

From this, the fiber optic bandwidth can now be calculated as = -j- ln (0.5) fo = 90.8 MHz

Similarly, the bandwidth of the optical fiber can be calculated when the transmission power is varied until the received optical power reaches a predetermined threshold value, the threshold value being equal for both frequencies.

It turns out for this variant

ES 2 397 533 T3

<img file="ES2397533T3_D0006.tif" />

P0; f1: Optical emission power to reach the threshold value at the first frequency and P0; f2: Optical emission power to reach the threshold value at the second frequency.

In the case of a linear correlation between the emission current I of the LED used for emission or of the laser used for emission and the coupled light power, the result is

<img file="ES2397533T3_D0007.tif" />

where ¡0; f1: Emitter current to reach the threshold value at the first frequency and f Emitter current to reach the threshold value at the second frequency.

In the following, the way in which, according to a particularly preferred embodiment of the present invention, the bandwidth of an LWL connection with a known physical behavior of the LWL fiber used can be calculated with reference to FIG. from the mean light power (AVG) and the peak-to-peak light power, i.e. the amplitude of the modulated signal (OMA), In this exemplary embodiment, polymeric fibers are provided as LWL fibers or optical fibers. The terms cable and fiber are used substantially synonymously in the following and generally designate an optical fiber.

Figure 4 represents the time evolution of an optical signal in a light wave conductor (LWL), the light power Popt having been recorded as a function of time t. The signal has a high proportion of DC with a small amplitude of the modulated signal OMA 92, obtaining the value for the amplitude of the modulated signal OMA 92 from the difference of the maximum light power Pmax and the minimum light power P_min. Such relationships are typical in the domain of limiting the bandwidth of an optical transmission path.

It is known for a polymeric fiber that the fiber behaves like a Gaussian low pass. Thus, the signal amplitude 92 can be described by the following relationship:

<img file="ES2397533T3_D0008.tif" />

A0 (l) is the original amplitude of the optical signal at the beginning of the fiber, which is further reduced, after a fiber length l, by a measure equal to the normal attenuation of the cable. Therefore, it turns out

<img file="ES2397533T3_D0009.tif" />

A0 is here the really coupled amplitude and α is the absorption coefficient of the fiber.

The average light output AVG 94 turns out to be as follows according to figure 4:

<img file="ES2397533T3_D0010.tif" />

In this case, Pmin (l, f) is the minimum light output as a function of the length l of the cable and the transmission frequency used f. This value depends on the attenuation α of the cable over the entire length of the cable and also depends on the behavior of the bandwidth of the fiber used at the entire frequency f and over the entire length l of the cable.

ES 2 397 533 T3

However, the average value AVG 94 of the light power now depends only on the attenuation of the cable, since the influence of the bandwidth does not lead to an absorption of energy, but only to a redistribution of the optical energy of the light portion modulated (CA portion) to the continuous light portion (CC portion). Therefore, this consideration is justified by the principle of the conservation of the energy of the optical light power.

So this mathematically means the following:

<img file="ES2397533T3_D0011.tif" />

Equations (7) and (8) now describe a system of equations with two unknowns, namely the values ​​Aü (l) and the frequency f to be obtained, which describes the behavior of the fiber bandwidth. The AVG 94 and OMA 92 values ​​result from the measurement by the LWL transceiver used and Pmin, 0 (l) is known by the transmitter used in the transceiver. Advantageously, the emitter used in the transceiver is so fast that the light initially coupled to the fiber used actually comes out completely within a bit time. In this case, Pmin, 0 = 0 and therefore also Pmin, 0 (l) = 0. f is here the frequency at which the measurements are made. For a fast Ethernet system with a 125 MBd symbol rate this frequency is typically 62.5 MHz.

If equations (7) and (8) are now inserted one inside the other and everything is transposed according to the frequency fe to be established, then we obtain:

F

<img file="ES2397533T3_D0012.tif" />

or for the case that Pmin, 0 (l) ® 0 f

<img file="ES2397533T3_D0013.tif" />

Usually, the frequency f0 is indicated as the bandwidth of a fiber, but the frequency at which the amplitude has been reduced by 3 dB due to the influence of the bandwidth. This frequency is referred to as f3dB in what follows. The correlation can be easily calculated from the Gaussian low-pass and provides:

<img file="ES2397533T3_D0014.tif" />

It thus becomes clear that the bandwidth of an online LWL connection during ongoing communication operation, for example in fast Ethernet applications with polymer fibers, can be achieved by a simple measurement of the average light output AVG and the modulated signal amplitude OMA at the end of a cable path. In this case, it is no longer necessary to switch between at least two different frequencies, as was still proposed in German patent application 10 2005 016 522.2. Therefore, this procedure once again represents a clear simplification of bandwidth measurement.