Apparatus for monitoring frequency of WDM signals using pilot tone

An apparatus for monitoring frequency of an optical signal, said optical signal including a plurality of wavelength-division-multiplexed (WDM) channels, each of said WDM channels having a corresponding pilot tone, wherein each pilot tone is a low-frequency sinusoidal signal with a distinct frequency, said apparatus comprising:

• splitting means for splitting said optical signal into two optical path signals, each of said optical path signals including said corresponding pilot tone;
• optical filtering means connected to said splitting means, said optical filtering means for filtering each pilot tone of said two optical path signals with particular passbands and outputting two filtered pilot tones;
• optical detecting means connected to said optical filtering means, said optical detecting means for converting said two filtered pilot tones into two electrical signals;
• and
• frequency detecting means comprising a pilot tone detecting means and an optical frequency calculating means,
• wherein said pilot tone detecting means detects the intensities of said two filtered pilot tones based on said electrical signals, and said optical frequency calculating means calculates the optical frequency of each WDM channel based on intensities of said two filtered pilot tones detected by said pilot tone detecting means for each WDM channel.