System and method for pilot tone modulation by data bias

Abstract

Embodiments are provided for applying pilot tone modulations to optical signals by introducing bias bits to data frames carried on the optical signals. Since the modulation is done by modifying data, the modulation depth is accurate, and there is no need for calibration or feedback control, which can improve power monitoring accuracy and simplify implementation. In an embodiment, a transmitter determines a period of a pilot tone modulation for tracking or identifying an optical channel. The transmitter inserts a sequence of bias bits, periodically according to the determined period, in a plurality of frames comprising original data bits. The amplitudes of the optical signals carrying the frames are modulated at a higher frequency than the pilot tone modulation. The optical signals are then transmitted including the bias bits within the frames.

What is claimed is: 
     
       1. A method by a transmitter for low optical frequency modulation, the method comprising:
 determining a period of a pilot tone modulation for tracking an optical channel; 
 inserting a sequence of bias bits, periodically according to the determined period, in a plurality of frames comprising original data bits; 
 modulating amplitudes of optical signals carrying the frames at a higher frequency than the pilot tone modulation; and 
 transmitting, in an optical fiber, the optical signals including the bias bits within the frames, the location of the bias bits within the frames indicating a modulation frequency of the optical signals. 
 
     
     
       2. The method of  claim 1  further comprising overclocking transmission of the frames at a higher rate than a desired baud rate for transmitting the original data bits. 
     
     
       3. The method of  claim 2 , wherein the higher rate compensates for a reduction in transmission rate of original data bits due to inserting the bias bits in the frames. 
     
     
       4. The method of  claim 2 , wherein the higher rate is equal to a product of the baud rate and a factor related to the period of the sequence of the bias bits. 
     
     
       5. The method of  claim 2 , wherein the bias bits are separated by a bit-to-bit spacing equal to an integer number of bits, and wherein the period of the sequence of bias bits is equal to a total number of bias bits in the sequence and the bit-to-bit spacing. 
     
     
       6. The method of  claim 5 , wherein the pilot tone modulation has a frequency equal to the baud rate divided by a product of twice the total number of bias bits in the sequence and the bit-to-bit spacing. 
     
     
       7. The method of  claim 1 , wherein a ratio of a total number of the bias bits to a total number of the original data bits determines a modulation depth for the pilot tone modulation. 
     
     
       8. The method of  claim 1 , wherein the inserted bias bits indicate a wavelength of the optical channel and other channel information. 
     
     
       9. The method of  claim 1  further comprising:
 determining a second period of a second pilot tone modulation for tracking a second optical channel; 
 inserting a sequence of second bias bits, periodically according to the second period, in a plurality of second frames comprising second original data bits; 
 modulating amplitudes of second optical signals carrying the second frames at a higher frequency than the second pilot tone modulation; and 
 transmitting, in the optical fiber, the second optical signals including the second bias bits within the second frames. 
 
     
     
       10. A method by a receiver for receiving frames including bias bits for pilot tone modulation, the method comprising:
 receiving a plurality of frames carried on optical signals in an optical fiber, wherein each of the optical signals has a respective low frequency according to a pilot tone modulation, and wherein the frames include bias bits providing the pilot tone modulation; 
 removing the bias bits from the frames, wherein the bias bits are removed after alignment of the received frames; and 
 detecting original data bits in the frames after removing the bias bits. 
 
     
     
       11. The method of  claim 10 , wherein the frames including the bias bits are received at a higher rate than a baud rate for the original data bits, and wherein the higher rate allows detecting the original data bits after removing the bias bits at the baud rate. 
     
     
       12. The method of  claim 11 , wherein the bias bits in the frames are separated by a bit-to-bit spacing equal to an integer number of bits, and wherein a period of a repeated sequence of the bias bits in the frames is equal to a total number of bias bits in the sequence and the bit-to-bit spacing. 
     
     
       13. The method of  claim 12 , wherein the higher rate exceeds the baud rate by a factor proportional to the bit-to-bit spacing of the bias bits in the sequence. 
     
     
       14. The method of  claim 12 , wherein the pilot tone modulation is proportional to the baud rate and inversely proportional to the period of the repeated sequence of the bias bits. 
     
     
       15. A transmitter for low optical frequency modulation, the transmitter comprising:
 at least one processor; and 
 a non-transitory computer readable storage medium storing programming for execution by the at least one processor, the programming including instructions to:
 determine a period for a pilot tone modulation for tracking an optical channel; 
 insert a sequence of bias bits, periodically according to the determined period, in a plurality of frames comprising original data bits; 
 modulate amplitudes of optical signals carrying the frames at a higher frequency than the pilot tone modulation; and 
 transmit, in an optical fiber, the optical signals including the bias bits within the frames, the location of the bias bits within the frames indicating a modulation frequency of the optical signals. 
 
 
     
     
       16. The transmitter of  claim 15 , wherein a ratio between the bias bits and original data bits in the frames indicates a modulation depth for the optical signals. 
     
     
       17. The transmitter of  claim 15 , wherein the programming includes further instructions to overclock transmission of the frames at a higher rate than a desired baud rate for transmitting the original data bits, and wherein the higher rate compensates for a reduction in transmission rate of original data bits due to inserting the bias bits in the frames. 
     
     
       18. The transmitter of  claim 16 , wherein the bias bits are separated by a bit-to-bit spacing equal to an integer number of bits, and wherein the period of the sequence of bias bits is equal to a total number of bias bits in the sequence and the bit-to-bit spacing. 
     
     
       19. The transmitter of  claim 16 , wherein the optical signals are wavelength division multiplexing (WDM) signals, and wherein the frames are optical transport network (OTN) or optical channel transport unit (OTU) frames. 
     
     
       20. A receiver for receiving frames including bias bits for pilot tone modulation, the receiver comprising:
 at least one processor; and 
 a non-transitory computer readable storage medium storing programming for execution by the at least one processor, the programming including instructions to:
 receive a plurality of frames carried on optical signals in an optical fiber, wherein each of the optical signals has a respective low frequency according to a pilot tone modulation, and wherein the frames include bias bits providing the pilot tone modulation; 
 remove the bias bits from the frames, wherein the bias bits are removed after alignment of the received frames; and 
 detect original data bits in the frames after removing the bias bits. 
 
 
     
     
       21. The receiver of  claim 20 , wherein the frames are received at a higher rate than a baud rate for the original data bits, and wherein the higher rate allows detecting the original data bits after removing the bias bits at the baud rate. 
     
     
       22. A method for receiving data, the method comprising:
 receiving a plurality of frames carried on optical signals in an optical fiber; 
 determining a modulation frequency of the optical signals in accordance with a location of bias bits in the frames; and 
 decoding the optical signals in accordance with the modulation information. 
 
     
     
       23. The method of  claim 22 , further comprising:
 determining a modulation depth of the optical signals based on a ratio between the bias bits and original data bits in the frames.