PDM-(M) Ask Optical Systems And Methods For Metro Network Applications
An apparatus, e.g. an optical communication system, includes an optical transmitter and an optical receiver. The transmitter includes a laser configured to provide an optical signal amplitude-modulated among M different levels, e.g. in two polarizations. The receiver is configured to demodulate the optical signal to produce a received symbol constellation including a plurality of symbol rings in a complex I-Q space.
1 . An apparatus, comprising:
an optical transmitter including a laser configured to provide an optical signal amplitude-modulated among M different levels; and
an optical receiver configured to demodulate the optical signal to produce a received symbol constellation including a plurality of concentric symbol rings in a complex I-Q space.
2 . The apparatus of claim 1 , wherein the laser is a vertical cavity surface-emitting laser (VCSEL).
3 . The apparatus of claim 1 , wherein M equals four.
4 . The apparatus of claim 1 , wherein the optical signal is a first optical signal having a first polarization, and the optical transmitter includes:
a second laser configured to provide a second optical signal amplitude-modulated among N different levels;
a polarization rotator configured to impart a second polarization on the second optical signal; and
a polarization beam combiner configured to combine the first and second optical signals.
5 . The apparatus of claim 1 , wherein the optical receiver includes a local oscillator and optical hybrid configured to determine in-phase and quadrature components of the optical signal.
6 . The apparatus of claim 1 , wherein at least one of said symbol rings is a closed curve.
7 . The apparatus of claim 1 , wherein the optical receiver includes an optical 120-degree hybrid configured to determine in-phase and quadrature components of the optical signal.
8 . The apparatus of claim 1 , wherein the optical receiver includes a polarization beam splitter configured to separate first and second polarizations of the optical signal.
9 . The apparatus of claim 1 , wherein the symbol constellation includes M symbol rings.
10 . The apparatus of claim 1 , wherein the symbol constellation includes M−1 symbol rings and a symbol point located at about the origin of the M−1 symbol rings.
11 . The apparatus of claim 1 , further comprising an optical filter configured to reduce the intensity of a proper subset of the plurality of symbol rings.
12 . The apparatus of claim 1 , wherein the optical transmitter includes an electro-absorption modulator configured to modulate light from the laser into the M different levels.
12 . The apparatus of claim 1 , wherein the optical transmitter includes an MZM configured to modulate light from the laser into the M different levels.
13 . A method, comprising:
configuring an optical source to produce an optical signal amplitude-modulated among M different levels; and
configuring an optical receiver to receive the signal and determine a received symbol constellation including a plurality of symbol rings in a complex I-Q space.
14 . The method of claim 12 , wherein the optical source is a vertical cavity surface-emitting laser (VCSEL).
15 . The method of claim 12 , wherein M equals four.
16 . The method of claim 12 , wherein the optical signal includes a first polarization amplitude-modulated among the M different levels, and further comprising:
configuring the optical source to produce a second polarization of the optical signal amplitude-modulated among N different levels.
17 . The method of claim 13 , further comprising configuring a local oscillator and an optical hybrid to determine in-phase and quadrature components of the optical signal.
18 . The method of claim 13 , wherein the receiving includes determining in-phase and quadrature components of the optical signal.
19 . The method of claim 17 , wherein the optical hybrid includes a 120-degree optical hybrid
20 . The method of claim 16 , further comprising separating the first and second polarizations of the optical signal and determining received symbol constellations for each of the first and second polarizations.
21 . A method, comprising:
generating an optical signal amplitude-modulated among M different levels; and
receiving the signal and determining a received symbol constellation including a plurality of symbol rings in a complex I-Q space.
22 . The method of claim 21 , wherein the laser includes a vertical cavity surface-emitting laser (VCSEL).
23 . The method of claim 21 , wherein M equals four.
24 . The method of claim 21 , wherein the optical signal includes a first polarization amplitude-modulated among the M different levels, and further comprising:
producing a second polarization of the optical signal amplitude-modulated among N different levels, M≠N.
25 . The method of claim 21 , wherein the receiving includes a local oscillator and an optical hybrid configured to determine in-phase and quadrature components of the optical signal.
26 . The method of claim 21 , wherein the receiving includes determining in-phase and quadrature components of the optical signal.
27 . The method of claim 26 , wherein the determining includes employing an optical 120-degree hybrid to determine the in-phase and quadrature components.
28 . The method of claim 24 , further comprising separating the first and second polarizations of the optical signal and determining received symbol constellations for each of the first and second polarizations.