Comb-based wavelength division multiplexing
An optical transmitter capable of colorless WDM includes a source of optical frequency comb (OFC) light having a plurality of separate optical frequency tones, and a plurality of optical modulators connected in parallel to modulate different parts of the OFC light with corresponding modulating signals, each of the parts including the plurality of separate optical frequency tones. An optical combiner combines the different parts of the OFC light to obtain an output optical signal of the optical transmitter. MIMO processing may be used to recover the modulating signals at an optical receiver using, or to configure the modulating signals at the transmitter so that each of the frequency tones is modulated with a corresponding target data signal.
1 . An apparatus comprising:
an optical transmitter comprising:
an optical frequency comb (OFC) source to provide OFC light comprising a plurality of separate optical frequency tones;
an optical splitter to split the OFC light to propagate along at least three different paths, each of the paths comprising an optical modulator to modulate received light with a respective modulating signal, wherein the received light of each of the optical modulators comprises the plurality of separate optical frequency tones; and
an optical combiner to combine light received from the at least three different paths to obtain an output optical signal of the optical transmitter,
wherein the at least three paths have different optical path lengths to impose different delays to light propagating along different ones of the paths.
2 . The apparatus of claim 1 , wherein two of the delays modulo T OFC differ from each other by about or more than two thirds of a beat period T OFC of the OFC light, the beat period being the inverse of a frequency spacing Δf of the optical frequency tones, T OFC =1/Δf.
3 . The apparatus of claim 1 , wherein the at least three different paths comprise N≥3 paths to impose N of the delays such that every two consecutive ones of the delays modulo T OFC differ from each other by approximately T OFC /N, T OFC /N being T OFC divided by N, T OFC being a beat period of the OFC light, the beat period being the inverse of a frequency spacing Δf of the optical frequency tones, T OFC =1/Δf.
4 . The apparatus of claim 1 wherein the plurality of separate optical frequency tones comprises at least three different frequency tones of approximately equal power.
5 . The apparatus of claim 1 wherein the optical transmitter comprises a digital signal processor (DSP) configured to perform multi-input multi-output (MIMO) processing on a plurality of target modulation signals to obtain the respective modulating signals.
6 . The apparatus of claim 5 wherein the MIMO processing is configured so that different ones of the optical frequency tones in the output optical signal are modulated by respective ones of the target modulation signals.
7 . The apparatus of claim 5 wherein the at least three different paths comprise N≥3 of the paths configured such that every two consecutive ones of the different delays modulo T OFC differ from each other by approximately (T OFC /N), the Torc being the inverse of a frequency spacing Δf of the optical frequency tones.
8 . The apparatus of claim 5 , wherein the DSP is configured to perform the MIMO processing based in part on frequency responses of the optical modulators.
9 . The apparatus of claim 1 , further comprising an optical receiver for receiving the output optical signal of the optical transmitter, the optical receiver comprising a plurality of optical-to-electrical (OE) converters configured to detect modulation of corresponding ones of the optical frequency tones, and to output a plurality of detected modulation signals,
wherein the optical receiver comprises a digital signal processor (DSP) configured to perform multi-input multi-output (MIMO) processing on the plurality of detected modulation signals to separately estimate the modulating signals.
10 . The apparatus of claim 1 , wherein at least some of the optical modulators comprise IQ modulators.
11 . The apparatus of claim 1 , wherein the optical modulators comprise dual-polarization modulators.
12 . A method for optically transmitting a plurality of data signals, the method comprising:
splitting light comprising a comb of separate optical frequency tones to propagate along at least three optical paths, the at least three optical paths having different optical path lengths to impose different delays upon the light propagating along the different paths;
modulating parts of the light propagating along the at least three optical paths with respective modulating signals, each of the parts comprising the comb of separate optical frequency tones; and
combining the at least three different parts of the light to obtain an output optical signal, wherein the different ones of the parts are combined in the output optical signal with the different delays.
13 . The method of claim 12 wherein two of the different delays modulo T OFC differ from each other by about or more than two thirds of a beat period T OFC of the comb, the beat period being the inverse of a frequency spacing Δf of the optical frequency tones, T OFC =1/Δf.
14 . The method of claim 12 wherein the at least three optical paths comprise N≥3 paths, and wherein every two consecutive ones of the different delays modulo Torc differ from each other by approximately T OFC /N, T OFC /N being T OFC divided by N, T OFC being a beat period of the OFC light, the beat period T OFC being the inverse of a frequency spacing Δf of the optical frequency tones.
15 . The method of claim 14 , comprising performing multi-input multi-output (MIMO) processing of the plurality of data signals to obtain the modulating signals.
16 . The method of claim 15 comprising performing the MIMO processing based at least in part on differences between the delays modulo T OFC .
17 . The method of claim 16 , comprising configuring the MIMO processing such that different ones of the optical frequency tones in the output optical signal are modulated with respective ones of the data signals.
18 . The method of claim 15 , including calibrating the MIMO processing using a set of training modulating signals and a MIMO equalizer of a multi-channel optical receiver.
19 . A system comprising:
an optical transmitter comprising:
an optical frequency comb (OFC) source to provide OFC light comprising a plurality of separate optical frequency tones;
a plurality of optical modulators connected to modulate different parts of the OFC light with respective modulating signals, each of the parts comprising the plurality of separate optical frequency tones; and
an optical combiner to combine the different parts of the OFC light with different respective delays to obtain an output optical signal; and
an optical receiver comprising:
a plurality of optical-to-electrical (OE) converters to detect modulation of respective ones of the optical frequency tones in an optical signal received from the optical transmitter, and to output a plurality of detected modulation signals; and
a digital signal processor configured to perform multi-input multi-output (MIMO) processing on the plurality of detected modulation signals to separately estimate the modulating signals of the optical transmitter.
20 . The system of claim 19 , wherein the plurality of optical modulators comprise N≥2 optical modulators to modulate N of the different parts of the OFC light, the N of the different parts of the OFC light being combined in the output optical signal with the respective delays, and wherein every two consecutive ones of the respective delays modulo T OFC differ from each other by approximately (T OFC /N), the Torc being the inverse of a frequency spacing Δf of the optical frequency tones, T OFC =1/Δf.