A method for controlling a difference between a first center frequency of a first optical subcarrier and a second center frequency of a second optical subcarrier of an optical super-channel signal in an optical network, the method comprising: modulating the first optical subcarrier at a first optical side component frequency with a first side modulation frequency; modulating the second optical subcarrier at a second optical side component frequency with a second side modulation frequency, a difference between the first optical side component frequency and the second optical side component frequency being smaller than a difference between the first center frequency and the second center frequency; wherein the first and the second side component frequency are in the range between 191 and 196 THz, wherein the first and the second modulation component frequency are in the range of tens of MHz, and wherein a spacing between the first and the second side component frequency is in the range of hundreds of MHz; coupling together the modulated first and second optical subcarriers to obtain a modified optical signal; and monitoring a variation of the difference between the first center frequency and the second center frequency by detecting a radio-frequency power at a modulated beat frequency tone in the modified optical signal, wherein the monitoring a variation of the difference between the center frequencies of the first and the second optical subcarriers further comprises: converting the modified optical signal to an electrical signal; filtering the electrical signal by a first electronic filter to obtain a filtered electrical signal within a first bandwidth corresponding to the difference between the first optical side component frequency and the second optical side component frequency; squaring the filtered electrical signal; filtering again the filtered and squared electrical signal by a second electronic filter to obtain an RF signal at the modulated beat frequency tone; and measuring the RF power of the RF signal at the modulated beat frequency tone. modulating the first optical subcarrier at a first optical side component frequency with a first side modulation frequency; modulating the second optical subcarrier at a second optical side component frequency with a second side modulation frequency, a difference between the first optical side component frequency and the second optical side component frequency being smaller than a difference between the first center frequency and the second center frequency; wherein the first and the second side component frequency are in the range between 191 and 196 THz, wherein the first and the second modulation component frequency are in the range of tens of MHz, and wherein a spacing between the first and the second side component frequency is in the range of hundreds of MHz; coupling together the modulated first and second optical subcarriers to obtain a modified optical signal; and monitoring a variation of the difference between the first center frequency and the second center frequency by detecting a radio-frequency power at a modulated beat frequency tone in the modified optical signal, wherein the monitoring a variation of the difference between the center frequencies of the first and the second optical subcarriers further comprises: converting the modified optical signal to an electrical signal; filtering the electrical signal by a first electronic filter to obtain a filtered electrical signal within a first bandwidth corresponding to the difference between the first optical side component frequency and the second optical side component frequency; squaring the filtered electrical signal; filtering again the filtered and squared electrical signal by a second electronic filter to obtain an RF signal at the modulated beat frequency tone; and measuring the RF power of the RF signal at the modulated beat frequency tone. converting the modified optical signal to an electrical signal; filtering the electrical signal by a first electronic filter to obtain a filtered electrical signal within a first bandwidth corresponding to the difference between the first optical side component frequency and the second optical side component frequency; squaring the filtered electrical signal; filtering again the filtered and squared electrical signal by a second electronic filter to obtain an RF signal at the modulated beat frequency tone; and measuring the RF power of the RF signal at the modulated beat frequency tone.