Method for Raman gain spectrum measurement of an optical WDM (wavelength division multiplex) signal implying a plurality of channels (λ) transmitted via an optical fiber, comprising the steps of - at a transmitter side generating an optical supervisory signal (OSS_TX); - transmitting said supervisory signal (OSS_TX) together with the WDM signal via the fiber; - determining a transmitter power level value (P CFLAT_TX /P TXT) of a transmitted WDM signal (WMS_TX) and a transmitter power level value (P OSS_TX) of the transmitted optical supervisory signal (OSS_TX); - transmitting said transmitter power level values (P CFLAT_TX /P TXT; P OSS_TX); - at a receiver side measuring a receiver power level value (P RXT) of a received WDM signal (WMS_RX) and a receiver power level value (P OSS_RX) of a received supervisory signal (OSS_RX); - calculating a Raman amplification factor (κ) from transmitter power level values (P OSS_TXT, P TXT) and receiver power level values (P OSS_RX, P RXT), and - calculating Raman gain values (G RAMAN (λ)) of the WDM channels (λ) considering a Raman gain coefficient (g R), wherein said Raman amplification factor (κ) is calculated from either P RXT * P OSS_TX P OSS_RX * P CFLAT_TX = ∑ m = 1 Nch 10 exp α λ OSS − α λ m L + g R λ m − λ RAP − g R λ OSS − λ RAP κ 10 or P RXT * P OSS_TX P TXT * P OSS_RX = 1 Nch ∑ m = 1 Nch 10 exp α λ OSS − α λ m L + g R λ m − λ RAP − g R λ OSS − λ RAP κ 10, in case of a single pump wavelength λ RAP, or P RXT * P OSS_TX P OSS_RX * P CFLAT_TX = ∑ m = 1 Nch 10 exp α λ OSS − α λ m L + ∑ i = 1 N PUMP g R λ m − λ RAP i − g R λ OSS − λ RAP i κ 0 * P RAP i 10 or P RXT * P OSS_TX P TXT * P OSS_RX = 1 Nch ∑ m = 1 Nch 10 exp α λ OSS − α λ m L + ∑ i = 1 N PUMP g R λ m − λ RAP i − g R λ OSS − λ RAP i κ 0 * P RAP i 10 in case of plural pump wavelengths λ RAP (i), wherein P OSS_TX (λ OSC) is a transmitter power level value of OSS_TX at the output of the OSS transmitter; P TXT is a transmitter power level value of the WDM signal WMS_TX; α(λ OSS) is the attenuation of the fiber at the wavelength λ OSS of the supervisory signal; α(λ m) is the attenuation of the fiber at the wavelength λ m of a channel "m"; L is the length of the fiber; P CFLAT_TX is a transmitter power level value of a single channel at the transmitter for a flat WMS_TX spectrum; Nch is the number of channels (λ m); P RXT is a receiver power level value of WMS_RX at receiver; P OSS_RX is a receiver power level value of OSS_RX at the receiver input; G RAMAN (λ OSS) is a Raman gain of the OSS; g R (λ-λ RAP) is a known Raman gain coefficient; κ is the Raman amplification factor; κ 0 is a Raman amplification coefficient; P RAP (i) is the power of the i-th pump source; N PUMP is the number of pump sources; and λ RAP (i) is the wavelength of the Raman i-th pump. - at a transmitter side generating an optical supervisory signal (OSS_TX); - transmitting said supervisory signal (OSS_TX) together with the WDM signal via the fiber; - determining a transmitter power level value (P CFLAT_TX /P TXT) of a transmitted WDM signal (WMS_TX) and a transmitter power level value (P OSS_TX) of the transmitted optical supervisory signal (OSS_TX); - transmitting said transmitter power level values (P CFLAT_TX /P TXT; P OSS_TX); - at a receiver side measuring a receiver power level value (P RXT) of a received WDM signal (WMS_RX) and a receiver power level value (P OSS_RX) of a received supervisory signal (OSS_RX); - calculating a Raman amplification factor (κ) from transmitter power level values (P OSS_TXT, P TXT) and receiver power level values (P OSS_RX, P RXT), and - calculating Raman gain values (G RAMAN (λ)) of the WDM channels (λ) considering a Raman gain coefficient (g R), P OSS_TX (λ OSC) is a transmitter power level value of OSS_TX at the output of the OSS transmitter; P TXT is a transmitter power level value of the WDM signal WMS_TX; α(λ OSS) is the attenuation of the fiber at the wavelength λ OSS of the supervisory signal; α(λ m) is the attenuation of the fiber at the wavelength λ m of a channel "m"; L is the length of the fiber; P CFLAT_TX is a transmitter power level value of a single channel at the transmitter for a flat WMS_TX spectrum; Nch is the number of channels (λ m); P RXT is a receiver power level value of WMS_RX at receiver; P OSS_RX is a receiver power level value of OSS_RX at the receiver input; G RAMAN (λ OSS) is a Raman gain of the OSS; g R (λ-λ RAP) is a known Raman gain coefficient; κ is the Raman amplification factor; κ 0 is a Raman amplification coefficient; P RAP (i) is the power of the i-th pump source; N PUMP is the number of pump sources; and λ RAP (i) is the wavelength of the Raman i-th pump.