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Optical distribution of analog and digital signals using optical modulators with complementary outputs
| Details |
Inventors: Nazarathy, Moshe; Berger, Josef;
Assignee: Harmonic Lightwaves, Inc. (Santa Clara, CA)
Primary Examiner: Lee; John D.
Assistant Examiner: Barns; Stephen W.
Attorney, Agent or Firm: Caserza; Steven F.
A novel optical system is taught including an optical source, an optical modulator having at least one optical input port for receiving the optical signal from the optical source, a modulation port for receiving the optical signal from the optical source, a modulation port for receiving a modulating signal, and complementary output port signals. The complementary modulated output signals are applied to one or more optical receivers at the other end of an optical transmission link. In one embodiment, the two complementary signals are provided to two separate optical receivers, thereby allowing distribution of optically modulated signals, for example, in a cable television or other type distribution system. In another embodiment, the two complementary modulated signals are applied to a single balanced receiver, thereby providing information content to the complementary receiver twice the power level available on either one channel alone. Furthermore, utilizing a balanced receiver, common mode error signals cancel, thereby increasing the signal to noise ratio of the system. In one embodiment of this invention, a single optical transmission channel is used, and the two optically modulated signals are separated in polarization so as to be capable of being separately detected on the receiving end of the link. In one embodiment feedback means is used to insure proper polarization states for the two complementary signals. |
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DETAILED DESCRIPTION This specification discloses the application of integrated-optic devices, and the exploitation of the peculiar tree-and-branch architecture of video distribution, to effectively eliminate the intrinsic waste of 50% of optical power in external optical modulators that are biased in their linear region of operation.
This specification also discloses a new architecture to reduce intensity noise, and to reduce the even orders of modulating signal distortion for imperfectly biased modulating devices.
Complementary Output Modulation In prior art external modulation techniques for fiber-optic AM transmission, half the optical power is dissipated in the external modulating device because of the need to bias the modulator in quadrature, half-way between its on and off states, in order to attain the maximum degree of linearity.
One major advantage of this invention is the ability to utilize substantially all the optical power, including the 50% which is wasted in prior art modulators.
The power previously wasted in prior art modulators is utilized in accordance with the teachings of this invention to accomplish transmission to either a different receiver, as required in situations involving distribution of signals to multiple points as in cable television, for example, or to the same receiver, in which case a novel signal processing technique is used to recombine the two signals.
When the previously wasted power is routed to the same receiver, an important benefit results: partial cancellation of RIN and even orders of distortion.
Complementary Output Devices In accordance with the teachings of this invention, modulators with pairs of complementary outputs are used, such that optical energy is not wasted but is rather transferred from one output to the other in accordance with the modulating signal.
Unlike prior art integrated optic modulators having multiple ouput ports, in accordance with the teachings of this invention, such an optical modulator having multiple ouput ports is used to provide multiple output signals which are simultaneously routed to a plurality of optical receivers (FIG
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