RU2663179C2 - Method of measuring optical signal-to-noise ratio with four-wave mixing in fiber-optical transmission systems with frequency separation of signals - Google Patents

Abstract

FIELD: optics; communication equipment.

SUBSTANCE: invention relates to an optical communication technique and can be used in fiber-optic transmission systems with frequency-separated channels. For this, a four-channel signal is proposed for the measurement. Frequency spacing between channels is uneven: 3Δf, 4Δf, 5Δf, where Δf is the frequency interval between the channels for the M-channel system. Amplification factor of the optical amplifiers is adjusted in such a way that the fiber maintains an optical power level corresponding to the transmission of the M-channel optical signal. Receiving side measures the power level in one of the four "active" channels and the noise power level in one of the "passive" channels, into which the combinational frequencies of the four-wave mixing fall. Optical signal-to-noise ratio for the four-channel signal is calculated using the obtained value, the ratio of the optical signal to noise for the transmission system with the M-channels is calculated.

EFFECT: technical result consists in solving the technical problem of predicting the noise of four-wave mixing in a channel by means of transmission over an optical fiber.

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Description

The invention relates to optical communication technology and can be used in fiber-optic transmission systems with frequency-divided channels as a method of measuring the ratio of optical signal to noise.

There is a method of increasing the throughput of fiber-optic transmission systems (FOTS) by the method of frequency division of channels (FDM - Frequency Division Multiplexing). The dense placement of channels in the frequency spectrum can further increase spectral efficiency. One of the advanced technologies is the frequency separation of orthogonal channels. In this technology, the optical frequency range is divided between channels, the number of which can reach several hundreds.

,

, где

,

,

- центральные частоты каналов. Для М-канальной системы передачи индексы i, j, k принимают любые значения от 1 до М, что приводит к генерации (М³-М²)/2 частотных продуктов ЧВС. Попадая в каналы частотные продукты ЧВС ухудшают (уменьшают) оптическое отношение сигнал-шум (OSNR - Optical Signal to Noise Ratio) в канале. В реальных условиях внутриканальная оценка влияния частотных продуктов ЧВС на OSNR затруднена. Это связано с тем, что в полосе пропускания канала невозможно отделить (отличить) сигнал от нежелательной помехи и раздельно измерить их уровни мощности. Поэтому разработка способа измерения OSNR является актуальной задачей как для существующих и проектируемых сетей, так и для создания измерительной аппаратуры.To increase the throughput of FOSP-FDM, it is required to increase the number of channels in the spectral range while reducing the frequency interval between them. However, with an increase in the number of channels, the total power level in the optical fiber also increases, which leads to the appearance of nonlinear phenomena in the fiber, for example, four-wave mixing (FWM). The signals of three frequencies during propagation along the fiber interact with each other, as a result of which new frequency combination components arise. Some newly appearing combination frequencies (frequency products of the FWM) fall into the ranges of the channels generating them, which leads to crosstalk between the channels. For FOSP-FDM, frequency products of the form

,

where

,

,

- center frequencies of the channels. For the M-channel transmission system, the indices i, j, k take any values from 1 to M, which leads to the generation of (M ³ -M ² ) / 2 frequency products of the FWM. Once in the channels, the frequency products of the FWM degrade (decrease) the optical signal-to-noise ratio (OSNR) in the channel. In real conditions, the on-channel estimation of the influence of frequency products of FWM on OSNR is difficult. This is due to the fact that in the channel bandwidth it is impossible to separate (distinguish) the signal from unwanted interference and separately measure their power levels. Therefore, the development of an OSNR measurement method is an urgent task both for existing and designed networks, and for the creation of measuring equipment.

A number of technical solutions are known to minimize the influence of the PMC products on the quality of transmission of channel signals in FOSP-FDM. One of these decisions, taken as an analogue [1], allows using uneven distribution in the channel spectrum to minimize the influence of frequency products of the FWM on the channels and increase the transmission system throughput. The method of spectral distribution of channels in the analog is based on the Golomb rule [2], and the location of the channels in the spectrum is calculated using integer (linear) programming (ILP - Integer Linear Programming). In [1], an example is given of uneven distribution of a spectrum of 10 channels along the spectrum and a comparison with the case when the channels are evenly distributed in the spectrum. It was shown in [1] that if the value of the frequency interval between any channels is not repeated, then no frequency products of the FWM fall into the frequency ranges of the channels.

However, such technical solutions along with the positive properties - the ability to exclude the influence of the products of the PMAC on the quality of the transmission channels, have the following main disadvantages:

1. Inefficient use of the optical fiber bandwidth in the transmission system, since when the channels are unevenly distributed (compared to uniform), the spectral band of the multi-channel signal increases.

2. To increase the throughput of such a transmission system, it is required to add and place all channels according to a certain optimal algorithm (unevenly), which can lead to reconfiguration of the entire frequency plan of the current transmission system.

The closest the claimed invention in technical essence is the analogue [3]. He is taken as a prototype of the invention. [3] describes a method for determining the power introduced into an optical fiber for a transmission system with wavelength-division multiplexing taking into account amplified spontaneous emission noise and four-wave mixing noise. This accounting is carried out on the receiving side, where noise levels are compared with predetermined thresholds, both for spontaneous emission and for four-wave mixing. Based on the comparison results, control signals are generated to adjust the gain of the optical amplifiers.

The main disadvantages [3] are:

1. In [3], it was not shown how the ratio of the optical signal to the noise of four-wave mixing is measured at the receiving side. In real conditions, the measurement of four-wave mixing noise affecting the signal is technically difficult or impossible, since these noises fall into the channel bandwidth, and there is no way to separately measure the signal power and unwanted noise.

2. The prototype [3] does not indicate the channels or group of channels in which the measurement and comparison of the noise level with threshold values;

3. The prototype [3] indicates an iterative method for repeatedly adjusting the amplification factors of optical amplifiers depending on the required ratio of the optical signal to noise at the receiving side, which can lead to certain time delays.

4. The method proposed in [3] for measuring the signal-to-noise ratio will not be effective when creating measuring equipment, since when measuring the signal-to-noise ratio, all channels existing in the transmission system should be “active”. The larger their number, the more difficult the technical implementation - the dimensions, weight and cost of the measuring equipment increase.

The aim of the claimed method is to create an effective and universal method for measuring the ratio of the optical signal to the noise of four-wave mixing for a transmission system with frequency division of channels.

This goal is achieved by the fact that instead of transmitting all channel signals (as in the prototype [3]), only four channel signals are transmitted. The frequency interval between these four channels is unified and selected as follows: 3Δf, 4Δf, 5Δf, where Δf is the frequency interval between channels for a multi-channel (more than 13) transmission system. The frequency interval between any pair of four channels is not repeated and satisfies the Golomb rule [2]. As in the prototype [3] on all the spans of the transmission system, where optical amplifiers are present, gain factors are adjusted. Assuming that optical amplifiers completely compensate for signal loss in spans, in the proposed method, the gain of optical amplifiers increases once by M / 4 times, where M is the number of transmission system channels. This is necessary so that the optical power level corresponding to the transmission of the M-channel optical signal is maintained in the fiber. Note that the prototype [3] does not present the exact algorithm for adjusting the gain of optical amplifiers. In the proposed measurement method, on the receiving side, the signal power level in one of the 4 “active” channels and the noise power level in one of the “passive” channels, into which the frequency products of the FWM fall, are first measured once. The ratio of the optical signal to noise for a 4-channel signal is calculated. Using the obtained value, the ratio of the optical signal to noise is calculated (predicted) for a transmission system with M channels. The invention is illustrated by drawings:

- in FIG. 1 - Block diagram of the flight VOSP-FDM, which measures the ratio of the optical signal to noise;

,

,

,

;- in FIG. 2 - Spectrum of Raman frequency combination products during the transmission of 4 channel signals at frequencies

,

,

,

;

- in FIG. 3 - OSNR dependence on the power level of one channel for 4-channel signal with a different number of channels M.

A detailed description of the invention.

VOSP-FDM consists of several spans, in which the optical signal loss fully compensates for optical amplifiers with a controlled gain. In FIG. 1 shows a block diagram of one FOSP-FDM span in which OSNR is to be measured. Channels (kn.) Are formed in the transmitting optical modules 1 and, after multiplexing in the multiplexer 2, a group multi-channel signal, consisting of the sum of frequency-separated M channels, is amplified in the optical amplifier 3. Further, the multi-channel signal is transmitted through the optical fiber 4. At a distance L is located measuring equipment, an integral part of which is an optical demultiplexer 5, dividing the channels. The power in each spectral channel can be measured with a spectrum analyzer 6 with an integrated power meter.

On the transmitting side, it is necessary to select four channels in the frequency range of the M-channel system so that no Raman combinational frequencies fall into the passband of these four channels, and the spectra of the PMV products do not overlap. That is, more than one Raman frequency product should not fall into the “passive” channels. The following non-uniform frequency interval between channels is proposed: 3Δf, 4Δf, 5Δf, where Δf is the frequency interval between channels in a system with M≥13.

,

,

,

. Как видим, продукты ЧВС в эти каналы не попадают. А в другие, «пассивные» каналы попадает по крайней мере один продукт вида

или один продукт вида

.In FIG. Figure 2 shows the spectrum of all Raman combinational frequency products arising from the transmission of 4 channel signals at frequencies along the fiber

,

,

,

. As you can see, the products of the PMA do not get into these channels. And in other, “passive” channels, at least one product of the form

or one kind product

.

, рассчитывают отношение оптической мощности сигнала к уровню мощности помехи OSNR₄.It is necessary to adjust the gain of the optical amplifier 3 (Fig. 1) so that at the beginning of each span at the fiber input, the total optical power is equal to the power level of the M-channel optical signal. At the end of the span, having measured spectrum analyzer 6, the optical power level in one of the four channels, for example, in the channel with index 4 (kn. 4) and the interference power level in the channel, for example with index 5 (kn. 5), where there is interference kind of

, calculate the ratio of the optical signal power to the interference power level OSNR ₄ .

Using the obtained OSNR ₄ value for the 4-channel transmission system, one can find the OSNR _M for the M-channel transmission system.

The choice of four test channels for measuring the ratio of the optical signal to the noise of the FWM is based on the following criteria:

1. From the technical and economic feasibility, it is necessary to select the smallest number of test channels for measuring the ratio of the optical signal to the noise of the CVM.

2. The phenomenon of FWM manifests itself when channel signals of three frequencies during propagation along the fiber interact with each other, as a result of which new (combination) frequency products of FWM arise. With regard to transmission systems, it must be borne in mind that the frequency products of FWMs fall not only into the ranges of the channels generating them, but also affect neighboring channels. Therefore, for a complete description of this phenomenon in multichannel transmission systems, it is necessary to use at least 4 channels.

3. Developers and manufacturers of equipment for transmission systems produce equipment with an even number of channels, which is mainly associated with the needs of the telecommunications market. By purchasing such equipment today, and in the future by retrofitting or upgrading it, you can easily increase throughput by two, four or more times. In the proposed method for measurements, it is necessary to increase the gain of the optical amplifier so that the optical power level corresponding to the transmission of the M-channel optical signal is maintained in the fiber, this gain of the gain should be M / n, where n is the number of test channels for FOTS FDM, M≥4. Therefore, in order for the gain gain M / n to take a certain numerical value, the number of test channels n must be even, otherwise the M / n ratio can take an infinite value. Since the number of test channels should be minimal, and the phenomenon of FWM is manifested when the number of channels is 3 or more, the number of channels is equal to 4. These conditions, therefore, when using a test 4-channel signal, it is necessary to increase the gain of optical amplifiers by M / 4 times .

The ratio of the power of the channel signal to the power of one product of the FWM, measured at the receiving side when a 4-channel signal (OSNR ₄ ) is applied to the fiber input, will be a value characterizing the complex effect of the nonlinearity of the optical fiber on the quality of signal transmission. By measuring OSNR ₄ at a certain value of the total optical signal power at the fiber input, it is possible to predict the ratio of signal power to the total power of the products of OSNR _M nonlinearities falling into the channel bandwidth for a given value of the number of channels M (more than 13).

For an M-channel system, the power of one product of the FWM is determined

[four]:

where (P _i ) m is the power in one channel i,

η _ijk is the efficiency of the generation of combination frequencies,

или

),d is the degeneracy coefficient (d = 3 or 6 for products of the form

or

),

γ is the nonlinear coefficient of the optical fiber,

- эффективная длина,

effective length

α is the attenuation coefficient of the optical fiber,

L is the length of the optical fiber.

, где суммарная мощность продуктов ЧВС, попадающих в канал. С учетом (1) и затухания сигнала в волокне:For the ith channel, OSNR _M is defined as,

, where is the total power of the PMC products falling into the channel. Given (1) and signal attenuation in the fiber:

where (N (m)) _M is the number of PMC products whose frequency coincides with the frequency of the m-th channel, with the total number of channels in the system M.

Suppose that on the transmitting side, the power level in each channel is set and the same for this configuration of the transmission system. From the condition of constant total optical power in the fiber, both with the number of channels M and with the number of channels 4, we have:

Substitute (3) in (2):

:We use the analytical expressions (N (m)) _M for the case of transmission of channels uniformly placed in the spectrum [5]. For product type

:

:For product type

:

-

стремительно растет с увеличением числа каналов М и преобладает над продуктами ЧВС вида

и если М→∞ последние можно не учитывать. Более того, максимальная концентрация этих продуктов наблюдается в середине частотного диапазона размещения каналов, где

[5]. Тогда, при очень большом количестве каналов (М → ∞) число продуктов ЧВС вида

стремится к предельному значениюComparing (5) and (6) we see that the number of products of the PMC type

-

grows rapidly with an increase in the number of channels M and prevails over products of the PMC type

and if M → ∞, the latter can be ignored. Moreover, the maximum concentration of these products is observed in the middle of the frequency range of the channel distribution, where

[5]. Then, with a very large number of channels (M → ∞), the number of FWM products of the form

tends to the limit

или М:For the extreme channels located in the spectrum, where

or M:

. Подставив (8) в (4) можно убедиться, что для крайних каналов (первого и последнего) ухудшение OSNR₄ составит не более 6 дБ, т.е.

.Thus, substituting (7) into (4) for channels located in the middle of the frequency range of channel placement, we note the deterioration of the signal-to-noise ratio during transmission instead of 4 large numbers (M → ∞) of channels. Given a constant average optical power level in the fiber, this ratio will leave no more than 7.8 dB, i.e.

. Substituting (8) into (4), we can make sure that for the outermost channels (first and last), the OSNR ₄ degradation will be no more than 6 dB, i.e.

.

Thus, by measuring OSNR ₄ using the formula (4), the ratio of the optical signal to noise of the PMN OSNR _M during the transmission of the M-channel signal is calculated.

An example implementation of the invention is as follows:

,

,

,

. Остальные каналы пассивны. Частотные интервалы между этими четырьмя каналами выбраны следующим образом: 3Δf, 4Δf, 5Δf, где Δf - частотный интервал между каналами в М-канальной системе передачи, при этом М≥13(фиг. 2).1) On the transmitting side, only the signals of 4 transmitting optical modules 1 (Fig. 1) remain, the center frequencies of which correspond to the frequencies

,

,

,

. The remaining channels are passive. The frequency intervals between these four channels are selected as follows: 3Δf, 4Δf, 5Δf, where Δf is the frequency interval between the channels in the M-channel transmission system, with M≥13 (Fig. 2).

2) The gain of the optical amplifier 3 increases by M / 4 times from the originally set operating value (Fig. 1). This increase is due to the need to maintain the same level of optical power in the fiber as when transmitting an M-channel optical signal. This increase in gain is implemented by software and hardware.

3) At the end of the span, at the output of the demultiplexer port 5 (Fig. 1), corresponding to channel No. 4 (pr. 4), the signal power level in the channel is measured.

4) At the output port of the demultiplexer 5 (Fig. 1), corresponding to channel No. 5 (kn. 5), the interference power level in the channel is measured.

5) Calculate OSNR ₄ , then OSNR _M using formula (4).

In FIG. Figure 3 shows an example of the dependence of OSNR ₄ on the power level of one channel of a 4-channel signal obtained experimentally by the measurement procedure described above. Channel signals are transmitted over a standard single-mode fiber (SSMF-Standard Single-Mode Fiber) to a distance of L = 100 km in the wavelength range of 1550 nm.

In FIG. Figure 3 shows the family of OSNR _M curves for FOTS-FDM using M-channel signal transmission over the same distance (L = 100 km), assuming that the channel frequency distribution is uniform and the channel spacing is 6.25 GHz. Referring to FIG. 3 OSNR _M curves are constructed according to formula (4) for a different number of M channels and 0SNR ₄ values obtained by measurements. As can be seen from FIG. 3, as the number of channels M (from 4 to 64) increases, OSNR _M tends to the limit value (OSNR ₄ - 7.8) dB. Note that at M = 4, the OSNR curve _M is below the OSNR ₄ curve by 3 dB. This is due to the fact that when measuring OSNR _{4 of} 4 test channels that are unevenly distributed in the spectrum, only one frequency product of the FWM is taken into account, while the distribution of 4 channels is even, the number of products of the FWM doubles. With a large number of channels M> 64, we can assume that OSNR _M is independent of the number of channels and, with a known value of OSNR ₄ , it is determined with great accuracy as OSNR _M ≈ (OSNR ₄ -7.8) dB.

Thus, by transmitting a 4-channel signal along the optical fiber, and having performed the OSNR ₄ measurement, the ratio of the optical signal to the noise of the FWM OSNR _{M is} calculated when transmitting the M-channel signal.

The application of this invention in the field of telecommunications is of great practical importance, since the magnitude of the signal-to-noise ratio is one of the main criteria for evaluating the quality of signal transmission along the fiber optic path. Using this method of measuring the signal-to-noise ratio of a FVG allows using available methods to diagnose existing transmission systems, as well as design FOSP-FDM with the required signal transmission quality.

Based on the proposed measurement method, it is possible to create a techno-economical efficient measuring equipment, since only 4-channel signals are used, and the optical power is measured in certain channels.

In addition, using the proposed method for measuring the signal-to-noise ratio of four-wave mixing, the prototype can be improved [3], which eliminates the uncertainty of the technical feasibility of measuring the noise of four-wave mixing at the receiving side. However, these measurements are possible at the initial stage of commissioning of the prototype [3] or during routine maintenance of the prototype [3].

The inventive method for measuring the signal-to-noise ratio of a FWM in VOSP-FDM satisfies the novelty criteria, since no technical solutions have been identified that have features that match the distinguishing features of this invention, and the influence of the distinctive features on the specified technical result is not known.

Information sources

1. US patent No. 5546210, IPC H04J 14/02. Published on August 13, 1996.

2. J.P. Robinson Optimum Golumb Rulers, IEEE Transaction on Computers, vol. C-28, No. 12, Dec. 1979, pp. 943-944.

3. US patent No. 2002/0044340 A1, IPC H01S 3/00. Published April 18, 2002. Patent Application Publication. (Prototype).

4. Tomas Shneider. Nonlinear Optic in Telecommunications. Springer, 2004, p. 415.

5. Vardanyan V.A. A study of the distribution of products of four-wave mixing in VOSP with CRK // Vestnik SibGUTI. 2016. No2. S. 78-84.

Claims (4)

The method of measuring the ratio of the optical signal to noise during four-wave mixing in fiber-optic transmission systems with frequency division multiplexing by multiplexing M channels from different transmitting optical modules, where M≥13 is the number of channels in the transmission system, forming a group M-channel signal, feeding a group M-channel signal to the input of an optical amplifier with a controlled gain, transmitting the amplified signal through an optical fiber, the output of which is a measuring apparatus ra with an optical demultiplexer separating channels and a spectrum analyzer with the ability to measure the power level in the channels, characterized in that on the transmitting side only signals in four channels are left on - in the first, fourth, eighth and thirteenth, and in the rest (M - 4 ) channels, including in the analyzed fifth channel, turn off the signal transmission, increase the gain of the optical amplifier by M / 4 times, on the receiving side using a spectrum analyzer measure the power level in one of the four ex channels and measure the level of noise power in the fifth channel, which lists the frequency products of four wave mixing, calculated optical signal-power-to-noise ratio OSNR ₄ when included signals in four channels, in case of transmission of M-channel signal using the obtained value OSNR ₄ , using the calculation, they predict the ratio of the optical signal to noise in any channel with the number m (m = 1, 2, 3, ... M), using the expression

, where the designation (N (m)) _M is the number of four-wave mixing products falling into the channel with number m, calculated using the expression

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