Analog modulation of optical signals
An optically modulated signal is generated for use in transporting data by generating a so-called sub-carrier modulated optical signal and, then, vector modulating the sub-carrier modulated optical signal to yield the desired modulated optical signal for transmission. The vector modulation includes a phase component and an amplitude component. In one specific embodiment of the invention, an apparatus for use in generating a modulated optical signal includes a generator to generate a sub-carrier modulated optical signal including an optical carrier and at least one sub-carrier and an analog vector modulator coupled to receive both the sub-carrier modulated optical signal from the generator and a data signal. The analog vector modulator generates an output optical signal by phase modulating and/or amplitude modulating the sub-carrier of the received optical signal in response to the data signal.
1. Apparatus for use in generating a modulated optical signal comprising:
a generator to generate a sub-carrier modulated optical signal including an optical carrier and at least one sub-carrier; and
an analog vector modulator coupled to receive both said sub-carrier modulated optical signal from the generator and a data signal and configured to generate an output optical signal by phase modulating and/or amplitude modulating the sub-carrier of the received optical signal in response to said data signal, said analog vector modulator including an optical power splitter, a plurality of optical branches, and an optical power combiner, the splitter being configured to split said optical sub-carrier signal into a plurality of optical branch signals and to supply each optical branch signal to a corresponding one of said optical branches, each of said optical branches including an optical phase modulator for phase shifting its supplied optical branch signal.
2. The apparatus as defined in claim 1 wherein said generator of said sub-carrier modulated optical signal includes a source of the optical carrier signal, an optical amplitude modulator and a device to supply a baseband signal to said optical amplitude modulator, the modulator amplitude modulating said optical carrier signal to generate said sub-carrier modulated optical signal in response to receiving said baseband signal.
3. The apparatus as defined in claim 1 wherein said generator of said optical sub-carrier modulated optical signal includes at least a first tunable source of an optical carrier signal, a second source of an optical carrier signal source, the first source capable of producing the first optical carrier signal with a frequency that is offset relative to a frequency of said second optical signal, and an optical coupler for mixing said first optical carrier signal and said second optical carrier signal to yield said sub-carrier modulated optical signal.
4. The apparatus as defined in claim 1 wherein said analog vector modulator further includes an optical delay for delaying the corresponding optical branch signal, the power combiner being configured to combine said plurality of delayed optical phase shifted signals to generate said output optical carrier signal.
5. The apparatus as defined in claim 4 wherein a delay interval produced by a Mth branch is (M−1), where M is the number of branches and M=1, 2 . . . M.
6. The apparatus as defined in claim 4 wherein said branches are grouped in a plurality of sets including at least two branches each and each branch in a set has a same delay interval.
7. The apparatus as defined in claim 6 wherein a delay interval of a Yth set of branches is (Y−1), where Y is the number of sets and Y=1, 2 . . . Y.
8. The apparatus as defined in claim 1 wherein said splitter and said combiner are each a multimode interference coupler.
9. The apparatus as defined in claim 1 further including a precoder to apply control voltages to said plurality of optical phase modulators in response to receiving said data signal.
10. The apparatus as defined in claim 9 wherein said data signal is a digital signal.
11. A method for use in generating a modulated optical signal comprising:
receiving a sub-carrier modulated optical signal including an optical carrier and at least one sub-carrier; and
analog vector modulating said sub-carrier modulated optical signal with a data signal to generate an output optical signal in which said one sub-carrier has been phase modulated and/or amplitude modulated responsive to said data signal, said analog vector modulating including optically power splitting said sub-carrier modulated optical signal into a plurality of optical branch signals, supplying each optical branch signal to a corresponding one of a plurality of optical branches, optically phase modulating each of said optical branch signals to phase shift it.
12. The method as defined in claim 11 further including the step of generating said sub-carrier modulated optical signal by generating an optical carrier signal and optically amplitude modulating said optical carrier with a prescribed signal to generate said sub-carrier modulated optical signal.
13. The method as defined in claim 11 further including the step of generating said sub-carrier modulated optical signal by generating a first optical carrier signal, generating a second optical carrier signal, and mixing said first optical carrier signal and said second optical carrier signal to yield said sub-carrier modulated optical signal, a frequency of said first optical carrier signal being offset relative to a frequency of said second optical signal.
14. The method as defined in claim 11 wherein said step of analog vector modulating further includes delaying each of said phase modulated optical branch signals by a prescribed delay interval and an optically combining said plurality of delayed optical phase shifted signals to generate said output optical carrier signal.
15. The method as defined in claim 14 wherein a delay interval of a Mth branch is (M−1) where M is the number of branches and M=1, 2 . . . M.
16. The method as defined in claim 14 wherein said branches are grouped in a plurality of sets including at least two branches each and each branch in a set has a same delay interval.
17. The method as defined in claim 16 wherein a delay interval of a Yth set of branches is (Y−1), where Y is the number of sets and Y=1, 2 . . . Y.
18. The method as defined in claim 17 wherein said data signal is a digital signal.
19. The method as defined in claim 14 wherein said splitting and said combining are each realized by employing a multimode interference coupler.
20. The method as defined in claim 11 further including a step of precoding a supplied data signal to generate control voltages that cause said plurality of optically phase modulators to generate said phase shifted optical branch signals.