Patent number 8634718 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "This invention relates generally to optical transmission systems and, more particularly, to polarization multiplexed optical transmission systems.
"In optical polarization multiplexed transmission systems, two optical signals each having a common wavelength, but one of two orthogonal polarizations, may be independently modulated, and then multiplexed together for transmission over a network infrastructure, over a fiber optical cable, for example. Due to their mutual orthogonality, the optical signals can be differentiated from each other at an optical receiver, located at a terminal node where a client signal interfaces to a network infrastructure, for example. Polarization multiplexing, therefore, potentially doubles the transmission capacity for each wavelength channel.
"One requirement for polarization demultiplexing the two optical signals, with acceptable tolerance to noise, is for the two polarizations of the two optical signals to be perfectly orthogonal at the point of multiplexing, for example in an optical transmitter. If the two polarizations are not orthogonal, then a portion of each of the two optical signals will overlap in both frequency and polarization, and may not be distinguished from each other at the receiver or terminal point. The intermixing of one optical signal on top of a second leads to noise on the second channel, and hence data bit errors. The polarization states associated with optical signals propagating in a transmission system are preferably well maintained from the point of origin, e.g. an output of a light source, through one or more optical elements, to a polarization beam combiner. Such one or more optical elements may include, but are not limited to, modulation elements, wavelength multiplexing elements, as well as passive elements such as the various waveguide structures which transport the optical signals from one element to another, as part of a photonic integrated circuit (PIC) for example. However, waveguide attributes such as waveguide imperfections and deviations in the fabrication process which may impart some randomness in the fabrication structure, waveguide surface roughness scattering, waveguide junctions, and waveguide bends may serve to re-orient the polarization state as the optical signal propagates in a photonic integrated circuit (PIC).
"There is a need to maintain, or mitigate changes in, the polarization state of an optical signal propagating in a photonic integrated circuit. Further, there is a need to maintain, or mitigate changes in, the polarization state of an optical signal propagating from one photonic integrated circuit to another photonic integrated circuit."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "The present invention provides a system, apparatus and method to maintain the polarization state of an optical signal propagating within a photonic integrated circuit, or from a first photonic integrated circuit to a second photonic integrated circuit. According to various embodiments of the invention, an optical circuit is provided which includes an optical coupler configured to accept a first optical signal on a first input and a second optical signal on a second input, the second optical signal having a polarization state. The optical coupler combines the first and second optical signal into an optical output signal. A principle axis of a first end of an optical fiber is configured to align with the polarization state of the second optical signal. In a first aspect of the present invention the first optical signal is a first of a plurality of first optical signals, such as a WDM signal for example. Each of the plurality of first optical signals may include s corresponding one of a plurality of first wavelengths and the second optical signal including a second wavelength, the second wavelength being different than each of the plurality of first wavelengths. In other aspects of the present invention the polarization state is a TM polarization state or a TE polarization state, or another polarization state.
"According to other various embodiments of the invention an optical circuit is provided which includes a plurality of signal channels configured to receive data and provide a corresponding one of a plurality of first modulated outputs and a corresponding one of a plurality of second modulated outputs in response to the received data. The optical circuit may include pluralities of first and second waveguides each having inputs and outputs, each of the first waveguides may be configured to receive a corresponding one of the plurality of first modulated outputs and each of the second waveguides may be configured to receive a corresponding one of the plurality of second modulated outputs. The optical circuit may further include first and second multiplexers each having a plurality of inputs and an output. The output of each of the plurality of first waveguides may be coupled to a corresponding one of the plurality of inputs of the first multiplexer, such that the first multiplexer combines the first modulated output from each of the plurality of signal channels into a first multiplexed output provided at the output of the first multiplexer. The output of each of the plurality of second waveguides may be coupled to a corresponding one of the plurality of inputs of the second multiplexer, such that the second multiplexer combines the second modulated output from each of the plurality of signal channels into a second multiplexed output provided at the output of the second multiplexer. The optical circuit may include third and fourth waveguides, the third coupled to receive the first multiplexed output and the fourth waveguide coupled to receive the second multiplexed output. In certain aspects of the invention the optical circuit includes a light source configured to provide an alignment optical signal on a fifth waveguide. The optical circuit may further include an optical combiner having first and second inputs and an output, the first multiplexed output provided to the first input and the alignment optical signal provided to the second input. According to certain aspects of the invention, the optical combiner is configured to combine the first multiplexed output with the alignment optical signal and provide a output optical signal at the output of the combiner. In other aspects of the present invention, one or more of the optical waveguides are birefringent waveguides. In still other aspects of the present invention, the first and second multiplexers are birefringent wavelength selective couplers, such as a birefringent arrayed waveguide grating for example. In another aspect of the present invention each of the plurality of signal channels includes a light source and a modulator. The light source may be a DBR or a DFB laser for example. The modulator may include a Mach-Zehnder interferometer for example. In still other aspects of the present invention, the optical circuit may include a polarization beam combiner which is configured to received the first and second multiplexed outputs and combined them into a combined polarization output signal provided at an output of the polarization beam combiner, the first multiplexed output having a first polarization state and the second multiplexed output having a second polarization state, the second polarization state being different from the first polarization state. According to other aspects of the invention, the optical circuit may include one or more polarization rotators which may act to rotate one or more optical signals such that the second polarization state of the second multiplexed output is different than the first polarization state of the first multiplexed output. The first and second polarization states may be a TM polarization state, a TE polarization state, or another polarization state.
"It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. Other objects, features and advantages of the invention will be apparent from the drawings, and from the detailed description that follows below."
URL and more information on this patent, see: Evans, Peter W.; Studenkov, Pavel V.; Fisher, Matthew; Joyner, Charles H.. Polarization Control in a Photonic Integrated Circuit. U.S. Patent Number 8634718, filed
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