METHOD AND ARRANGEMENT FOR IN SERVICE RAMAN GAIN MEASUREMENT AND MONITORING

Abstract

The invention refers a method and an arrangement for in service Raman gain measurements and monitoring of a wavelength division multiplex system. By measuring the power level values of a transmitted WDM signal (WMS_TX) and an optical supervisory signal (OSS_TX) at the transmitter and the power level values of the received signals (WMS_RX; OSS_RX) the Raman gain can be calculated for the different channels.

Description

FIELD OF THE INVENTION

The invention refers a method and an arrangement for in service Raman gain measurements and monitoring of a wavelength division multiplex system.

BACKGROUND OF THE INVENTION

In optical transport networks a plurality of high speed data signals is transmitted as a (dense) wavelength division signal via optical fibers. Optical fiber amplifiers (EDFAs) or semiconductor amplifiers and in addition Raman amplifiers are used for signal amplification. Raman amplification is induced in the transmission fiber by one or more Raman pump signals injected into the transmission fiber usually against the signal propagating direction. The Raman amplification depends on the frequency difference between the Raman pump signal and the signal or channel frequency. The exact Raman gain obtained in the network is usually different from an predicted gain.

Several methods for measuring Raman gain are described so far:

US 2004/0120664 discloses a method and an arrangement for measuring Raman gain based on means of optical time domain reflectometry which is not suited for measurements with live traffic.

EP 1 508 985 proposes a solution based on a low frequency intensity modulation of a Raman pump signal. A corresponding intensity modulation of the optical signal is then measured. The Raman on-off gain is then determined from the intensity modulation in the optical signal.

EP 1 248 334 describes a method and a system for automatic in service gain control based on a spectrum analyser. The method and system permit in service measurements, permits derivation of the Raman gain and dynamic adjustments of system parameters, especially a power profile of received signals.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and an arrangement for in service Raman gain measurement with reduced complexity.

The measurement can be used for monitoring or controlling purposes.

The object is achieved by the features recited in the method claims 1 and in the arrangement claim 7.

According to the present invention there is provided a method for Raman gain spectrum measurement of an optical WDM (wavelength division multiplex) signal implying a plurality of channels transmitted via an optical fiber, comprising the steps of

• • at a transmitter side generating an optical supervisory signal;
  • transmitting said supervisory signal together with the WDM signal via the fiber;

determining a transmitter power level value of the WDM signal and a transmitter power level value of the optical supervisory signal;

• • transmitting said transmitter power level values;
  • at the receiver side measuring a the power level of the received WDM signal and a power level of the received optical supervisory signal; and
  • calculating a Raman amplification factor from received power level values and measured power level values, and
  • calculating Raman gain values of the WDM channels considering a Raman gain coefficient.

And according to the present invention there is provided an arrangement for Raman gain spectrum measurement of an optical WDM (wavelength division multiplex) signal implying a plurality of channels transmitted via an optical fiber, comprising

• • at a transmitter side a supervisory transmitter designed for generating an optical supervisory signal;
  • means for determining a transmitter power level value of the WDM signal and a transmitter power level value of the optical supervisory signal;
  • an optical coupler for combining said supervisory signal with the WDM signal and inserting both signals into the fiber;
  • means for transmitting said transmitter power level values of the WDM signal and the optical supervisory signal;
  • at a receiver side an optical splitter for coupling out an received optical supervisory signal and a measurement signal of an received WDM signal;

a supervisory receiver designed for receiving said supervisory signal;

• • means designed for measuring the power level of the received optical supervisory signal and the power level (P_RXT) of the received WDM signal; and
  • calculating a Raman amplification factor from received power level values and measured power values, and
  • calculating Raman gain values of the WDM channels considering a Raman gain coefficient.

The recent invention relies on simple power measurement and exploits existing control mechanism within a WDM system.

The Raman gain of the channels is calculated from power values of the transmitted and the received signals.

The Invention can be used for measurement, configuration or control purposes.

Further advantageous features are described in the pending claims.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a presently preferred embodiment is described below with reference to an accompanying drawing, where

FIG. 1 shows a simplified diagram of a gain measurement and monitoring arrangement and

FIG. 2 shows a Raman gain diagram versus frequency difference.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an optical fiber link or span. At a transmitter a wavelength division multiplex signal WMS_TX comprising a plurality of optical signals with different wavelengths is amplified by a first optical amplifier 1 and input into a transmission fiber 4. Said optical signals are usually also referred to as channels, which are indicated by different wavelengths A. In addition an optical supervisory signal OSS_TX is generated by an OSS transmitter (TX_OSS) 2 and combined with the WDM signal WMS via an optical coupler 3 (dichroic or fused coupler or even the same wavelength division multiplexer combining the single channels and outputting the WDM signal) and transmitted via the fiber link in a separate OSS channel with a wavelength different from the wavelengths of the WDM channels.

Transmitter power level values P_TX (OCFLAT_TX) and P_OSS—TX the transmitted signals WMS_TX and OSS_TX are measured (not shown) and transmitted via a telemetry channel, e.g. via the OSS channel itself to a receiver.

At the receiving end a Raman pump generator 5 generates a Raman pump signal RAP with power P_RAP and a higher frequency than the frequencies of the WDM channels. The Raman pump signal RAP is input via a second coupler 6 into the transmission fiber 4 against the transmission direction of the WDM signal as Raman pump signal RAF with an attenuated power P_AF. The transmitted WDM signal WMS_TX and the optical supervisory signal OSS_TX sustain impairments like attenuation from the transmission fiber and are amplified by the Raman pump signal.

At the receiving end of the transmission fiber a received WDM signal WMS_RX and a received optical supervisory signal OSS_RX are split by a measurement splitter 7. The received WDM signal WMS_RX is fed to an optical receiver 9, which in most cases comprises a fiber or semiconductor amplifier, and branched of parts of received signals WMS-RX and OSS_RX are fed to a supervisory receiver (RX_OSS) 8. Here the received signals WMS_RX and OSS_RX are separated and their powers are measured (instead of the splitter 7 a wavelength division demultiplexer may be used already separating WMS_RX and OSS_RX; measurement means may be integrated in the RX-OSS or arranged externally).

Because the Raman gain depends on the wavelength difference between an optical signal (channel; OSS_TX) and the pump signal the all channels and the supervisory signal are amplified differently. Because the power level values of the WDM signal and the supervisory signal at the transmitter side and the receiver side are measured and the fiber length is also known, the Raman gain spectrum—the Raman gain for the single channels (channel is here used instead of signals) of the WDM signal—can be derived considering a Raman gain coefficient. For Raman gain control the power P_RAP of the pump signal RAP can be also measured.

The following equations are derived for a single pump source and a single optical signal/channel. They are also valid for multiple Raman pumps when interaction—the exchange of energy between the Raman pump signals—can be neglected. Multiple Raman pumps can be considered by superposition.

The power budget for single signal/channel in the span is given by

P _RX(λ)=P_TX(λ)−α(λ)*L−IL_DISCRET+G_RAMAN(λ)   (1)

wherein P_TX(λ)—transmitter power level value at the output of the transmitter/beginning of the span;

P_RX(λ)—power level value of WMS_RX at the input of the receiver at the end of the span;

α(λ)—fiber attenuation; L—fiber (span) length;

IL_DISCRET—discrete insertion loss of filter/splitter/couplers and connector/splice loss in the fiber span;

G_RAMAN(λ)—Raman amplification for a channel with wavelength λ; all values are logarithmic values, e.g. in decibel.

For the supervisory channel the equation (1) is rewritten in logarithmic form

P _{OSS — RX} =P _{OSS — TX}(λ_OSS)−α(λ_OSS)*L−IL_DISCRETE+G_RAMAN(λ_OSS)   (2)

wherein

P_OSS—RX=P_OSS—RX(λ_OSS)—receiver power level value of the supervisory signal with λ_OSS at the input of the OSS receiver;

P_{OSS TX}(λ_OSS)—transmitter power level value of OSS at the output of the OSS transmitter; IL_DISCRETE—insertion loss of discrete elements; G_RAMAN(λ_OSS)—Raman gain of the OSS.

Equations (1) and (2) are rewritten in exponential form

$\begin{array}{cc}{P}_{\mathrm{RX}}\left(\lambda \right)=P_{\mathrm{TX}}\left(\lambda \right)*10\exp \left(\frac{-\alpha \left(\lambda \right)L-{\mathrm{IL}}_{\mathrm{DISCRETE}}+G_{\mathrm{RAMAN}}\left(\lambda \right)}{10}\right)& \left(3\right)\end{array}$

and

$\begin{array}{cc}{P}_{\mathrm{OSS\_RX}}=P_{\mathrm{OSS\_TX}}*10\exp \left(\frac{-\alpha \left({\lambda }_{\mathrm{OSS}}\right)L-{\mathrm{IL}}_{\mathrm{DISCRETE}}+G_{\mathrm{RAMAN}}\left({\lambda }_{\mathrm{OSS}}\right)}{10}\right)& \left(4\right)\end{array}$

with P_OSS—RX=P_OSS—RX(λ_OSS); P_OSS—TX=P_OSS—TX(λ_OSS).

For a plurality of channels we get for the total power P_RXT at the receiver a sum):

$\begin{array}{cc}{P}_{\mathrm{RXT}}=\sum _{m=1}^{\mathrm{Nch}}P_{\mathrm{TX}}\left({\lambda }_m\right)*10\exp \left(\frac{-\alpha \left({\lambda }_m\right)L-{\mathrm{IL}}_{\mathrm{DISCRETE}}+G_{\mathrm{RAMAN}}\left({\lambda }_m\right)}{10}\right)& \left(5\right)\end{array}$

with m=1−Nch, number of channels;

Assuming a flat spectrum for the channels at the output of the first amplifier (booster) 1:

$\begin{array}{cc}{P}_{\mathrm{TXT}}=\sum _{m=1}^{\mathrm{Nch}}P_{\mathrm{CFLAT\_TX}}=P_{\mathrm{CFLAT\_TX}}*\mathrm{Nch}& \left(6\right)\end{array}$

wherein P_CFLAT—TX is a transmitter (calculated) power level of a single channel at TX assuming the spectrum is flat.

The equation (5) can be rewritten:

$\begin{array}{cc}{P}_{\mathrm{RXT}}=P_{\mathrm{CFLAT\_TX}}\sum _{m=1}^{\mathrm{Nch}}10\exp \left(\frac{-\alpha \left({\lambda }_m\right)L-{\mathrm{IL}}_{\mathrm{DISCRETE}}+G_{\mathrm{RAMAN}}\left({\lambda }_m\right)}{10}\right)& \left(7\right)\end{array}$

Writing equation (7) over equation (4) and multiplying with P_OSS—TX/P_CFLAT—TX we get:

$\begin{array}{cc}\frac{P_{\mathrm{RXT}}*P_{\mathrm{OSS\_TX}}}{P_{\mathrm{OSS\_RX}}*P_{\mathrm{CFLAT\_TX}}}=\sum _{m=1}^{\mathrm{Nch}}10\exp \left(\frac{\left[\alpha \left({\lambda }_{\mathrm{OSS}}\right)-\alpha \left({\lambda }_m\right)\right]L+G_{\mathrm{RAMAN}}\left({\lambda }_m\right)-G_{\mathrm{RAMAN}}\left({\lambda }_{\mathrm{OSS}}\right)}{10}\right)& \left(8\right)\end{array}$

wherein

[α(λ_OSS)−α(λ)]L is the linear attenuation difference of the fiber at the OSS wavelength and a wavelength e.g. in the C-band. This can be extrapolated from the increase of fiber attenuation with lower wavelengths due to Rayleigh scattering. Equal IL_DISCRETE for WMS and OSS is assumed.

The Raman gain G_RAMAN(λ) is a function of the Raman gain coefficient g_R(λ) and a factor κ. κ is a function of the Raman pump power and can be derived according

G _RAMAN(λ)=κ*g_R(λ)   (9)

where g_R(λ)=g_R(λ−λ_RAM) is the Raman gain coefficient as a function of the wavelength difference between λ and the wavelength λ of the Raman pump, assumed to be known.

FIG. 2 illustrates an example of the Raman gain coefficient g_R for a pure silica fiber as a function of a frequency difference Δf between the Raman pump signal and another optical signal. And κ is a Raman amplification factor, assumed to be unknown, as a function of the Raman pump power.

The equation (8) can be rewritten with equation (9)

$\begin{array}{cc}\frac{P_{\mathrm{RXT}}*P_{\mathrm{OSS\_TX}}}{P_{\mathrm{OSS\_RX}}*P_{\mathrm{CFLAT\_TX}}}=\sum _{m=1}^{\mathrm{Nch}}10\exp \left(\frac{\left[\alpha \left({\lambda }_{\mathrm{OSS}}\right)-\alpha \left({\lambda }_m\right)\right]L+\left[g_R\left({\lambda }_m\right)-g_R\left({\lambda }_{\mathrm{OSS}}\right)\right]\kappa }{10}\right)& \left(10\right)\end{array}$

or

$\begin{array}{cc}\frac{P_{\mathrm{RXT}}*P_{\mathrm{OSS\_TX}}}{P_{\mathrm{TXT}}*P_{\mathrm{OSS\_RX}}}=\frac{1}{\mathrm{Nch}}\sum _{m=1}^{\mathrm{Nch}}10\exp \left(\frac{\left[\alpha \left({\lambda }_{\mathrm{OSS}}\right)-\alpha \left({\lambda }_m\right)\right]L+\left[g_R\left({\lambda }_m\right)-g_R\left({\lambda }_{\mathrm{OSS}}\right)\right]\kappa }{10}\right)& \left(10b\right)\end{array}$

substituting the transmitter power level P_CFLAT—TX by the WMS_TX transmitter power level P_TXT.

It is possible to extend equation (10) for a multiple Raman pump by measuring the Raman pump power for each Raman wavelength, to retrieve the total Raman gain experienced by the signal. The equation (9) is then extended in the following way. Instead of having:

[g_R(λ−λ_PUMP)−g_R(R_OSS−λ_PUMP)]*κ₀*P_RAP

we now must consider several pump wavelengths:

Σ[g_R(λ−λ_RAP(i))−g_R(λ_OSS−λ_RAP(i))]*κ₀*P_RAP(i)   (11)

wherein i index variable of the pump sources and according to

κ=κ_o*P_RAP   (12)

κ is substituted by κ_o*P_RAP(i).

the only unknown value in equation (10) is the Raman amplification factor κ, or substituting κ by κ_o*P_RAP, the only unknown value in equation 10 or in an extended equation is a Raman amplification coefficient κ₀. And we can proceed as explained above to derive κ₀ from equation (10).

$\begin{array}{cc}\begin{array}{c}{G}_{\mathrm{RAMAN}}\left(\lambda \right)=4.34\left[L_{\mathrm{EFF}}\left({\lambda }_{\mathrm{pump}}\right)/\left(2A_{\mathrm{EFF}}\right)\right]*P_{\mathrm{RAF}}*g_R\left(\lambda \right)\\ =\kappa *g_R\left(\lambda \right)\end{array}& \left(13\right)\end{array}$

and with P_RAF=IL_RAT*P_RAE,

κ=4.34 [L_EFF(2A_EFF)*IL_RAP*P_RAP=κ₀*P_RAP   (14)

wherein

A_EFF—effective area of the fiber;

L_EFF—effective fiber length for the Raman pump wavelength;

P_RAF—Raman pump power sent into the fiber and thus is equal to IL_PAP*P_RAP, where IL_PUMP represent the discrete insertion loss (connector/filter/splitter loss) between the Raman pump output (where the Raman pump power P_RAP is measured) and the fiber.

From (10) we can derive κ or κ₀ and therefore calculate G_RAMAN(λ) for each channel according equation (9) or an extended equation

G _RAMAN(λ)=κ₀*P_RAP*g_R(λ)   (15)

which is advantageously used for Raman gain control.

The invention is e.g. applied when a transmission system is configured. First the transmitter power values P_TXT(P_CFLAT—TX), P_OSS—TX and the receiver power values P_RXT, P_OSS—RX are measured and the Raman gain coefficient is retrieved. Then e.g. formula 10b is used to calculate back the Raman amplification factor K. This is simply done by calculating the right term of equation 10bis for several values of K and then interpolate to the value that gives the closest match to the left hand-side value.

Then the Raman gain is deduced through equation (13).

The invention may not be only used for determining the Raman gain but also for controlling the fiber link. Therefore FIG. 1 comprises also a control unit 10 receiving all necessary measured values and calculating the Raman gain for all channels. The calculated values are then compared with target (reference) values G_RAMTA and the power of the (multiple) Raman pump is controlled (the power level of the Raman pump is forwarded (dashed line) via the RX_—OSS to the control unit) accordingly to an adequate target value P_RAMTA or to “i” target values G_RAMTA(i) for a multiple Raman pump, e.g. to maintain the predetermined Raman gain spectrum. The invention may be combined with further control circuits. Possible aims are e.g. equal power or equal signal to noise ratios of all channels.

The present invention is not limited to the details of the above described principle. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. This applies also to variably points of the system suited for measuring the power values of the transmitted and received signals e.g. before and behind a splitter. The invention may be also adapted for Raman pumps pumping—also in addition—in the transmission direction of the WDM signal.

REFERENCE SIGNS AND ABBRIVATIONS

• • 1 transmitter
  • 2 OSS transmitter (TX_OSS)
  • 3 (dichroic) coupler
  • 4 transmission fiber
  • 5 Raman pump
  • 6 second coupler
  • 7 splitter
  • 8 supervisory receiver (RX_—OSS)
  • 9 receiver
  • 10 control unit
  • OSS supervisory signal
  • OSS_TX transmitted supervisory signal
  • P_OSS—TX— power of transmitted OSS_TX
  • WMS_TX WDM signal
  • P_TX (λ) power of WDM channel λ at TX
  • P_CFLAT—TX channel power for single channel of a flat WDM signal
  • RAP Raman pump signal
  • RAF Raman pump signal inserted into the fiber
  • P_RAP power of generated Raman pump signal
  • P_RAF power of the Raman pump signal inserted into the fiber
  • P_RxT total power of the received WDM signal
  • OSS_RX received supervisory signal
  • P_{OSS RX} power of received OSS_RX
  • G_RAMTA(λ) Raman gain target value
  • P_RAPTA Raman pump power target value
  • g_R(λ) Raman gain coefficient
  • κ_o Raman amplification coefficient
  • κ Raman amplification factor
  • Δf frequency difference

Claims

1-12. (canceled)

13. A method for Raman gain spectrum measurement of an optical WDM (wavelength division multiplex) signal implying a plurality of channels transmitted via an optical fiber, the method which comprises: at a transmitter side, generating an optical supervisory signal (OSS_TX);transmitting the supervisory signal (OSS_TX) together with the WDM signal via the optical fiber;determining a transmitter power level value (PCFLAT—TX/PTXT) of a transmitted WDM signal (WMS_TX) and a transmitter power level value (POSS—TX) of the transmitted optical supervisory signal (OSS_TX);transmitting the transmitter power level values (PCFLAT—TX/PTXT; POSS—TX);at a receiver side, measuring a receiver power level value (PRTX) of a received WDM signal (WMS_RX) and a receiver power level value (POSS—RX) of a received supervisory signal (OSS_RX);calculating a Raman amplification factor from transmitter power level values (POSS—TX, PTXT) and receiver power level values (POSS—RX, PRXT), andcalculating Raman gain values (GRAMAN(λ)) of the WDM channels considering a Raman gain coefficient.

14. The method according to claim 13, which comprises: calculating a Raman amplification factor from:

15. The method according to claim 14, which further comprises the steps of: measuring a Raman pump signal power and substituting the Raman amplification factor by a Raman amplification coefficient and the Raman pump signal power according to κ=κo*PRAP,where:κ is the Raman amplification factor;κo is the Raman amplification coefficient; andPRAP is the Raman pump signal power.

16. . The method according to claim 15, which further comprises comparing the calculated Raman gain values (GRAMAN(λ)) with associated target values (GRAMTA(λ)) and controlling a Raman pump accordingly.

17. The method according to claim 15, which further comprises the steps of: comparing the calculated Raman gain values (GRAMAN(A)) with associated target values and controlling the power of a Raman pump signal according to GRAMTA(λ)=κ gR(λ)=κKo*PRAPTA*gR(λ),whereinGRAMTA(λ) is a Raman gain target value;PRAPTA is a Raman pump power target value for GRAMTA(λ); andκo is the Raman amplification coefficient.

18. The method according to claim 13, which comprises transmitting the transmitter power levels (PCFLA—TX/PTXT; POSS—TX) of the WDM signal (WMS_TX) and the supervisory signal (OSS_TX) by the supervisory signal (OSS_TX) itself.

19. A configuration for Raman gain spectrum measurement of an optical WDM (wavelength division multiplex) signal implying a plurality of channels transmitted via an optical fiber, the configuration comprising: a supervisory transmitter disposed at a transmitter side and configured for generating an optical supervisory signal (OSS_TX);means for determining a transmitter power level value (PCFLAT—TX/PTXT) of a transmitted WDM signal (WMS_TX) and a transmitter power level value (POSS—TX) of the transmitted optical supervisory signal (OSS_TX);an optical coupler for receiving the supervisory signal (OSS_TX) and injecting the supervisory signal (OSS_TX) into the fiber together with the WDM signal (WMS_TX) and transmitting the signals via the fiber;means for transmitting the transmitter power level values (PCFLAT—TX/PTXT; POSS—TX) of the WDM signal (WMS_TX) and the optical supervisory signal (OSS_TX);an optical splitter disposed at a receiver side for coupling out a measurement signal of a received WDM signal (WMS_RX) and a received optical supervisory signal (OSS_RX);a supervisory receiver configured for receiving the supervisory signal;means for measuring the receiver power level value (POSS—RX) of the received optical supervisory signal (OSS_RX) and the receiver power level value (PRXT) of the received WDM signal (WMS_RX), for calculating a Raman amplification factor (κ) from the transmitter power level values (POSS—TX) PTXT/PCFLAT—TX) and the receiver power level values (POSS—RX, PRXT), and for calculating Raman gain values of the WDM channels considering a Raman gain coefficient.

20. The configuration according to claim 19, which is further configured for: calculating a Raman amplification factor from

21. The configuration according to claim 20, which is further configured for: measuring and substituting the Raman amplification factor by a Raman amplification coefficient and the Raman pump signal power according to κ=κo*PRAP,where: κ is the Raman amplification factor;κo is the Raman amplification coefficient; andPRAP is a Raman pump power.

22. The configuration according to claim 21, which is further configured for comparing calculated Raman gain values with target values and controlling a Raman pump accordingly.

23. The configuration according to claim 21, which is further configured for: comparing the calculated Raman gain values with the target values and controlling the power of a Raman pump signal according to GRAMTA(λ)=κ gR(λ)=κo*PRAPTA*gR(λ),wherein GRAMTA(λ) is a Raman gain target value;PRAPTA is a Raman pump power target value for GRAMTA(λ); andκo is the Raman amplification coefficient derived from κ.

24. The configuration according to claim 19, wherein said OSS transmitter is configured for modulating the transmitter power level values (PCFLAT—TX/PTXT; POSS—TX) of the WDM signal (WMS_TX) and the optical supervisory signal (OSS_TX) onto the supervisory signal (OSS_TX).