News Column

Patent Issued for Wavelength Division Multiplexed Optical Communication System Having Variable Channel Spacings

July 16, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- Infinera Corporation (Sunnyvale, CA) has been issued patent number 8768177, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Wu, Kuang-Tsan (Kanata, CA); McNicol, John D. (Ottawa, CA); Welch, David F. (Atherton, CA); Grubb, Stephen G. (Reisterstown, MD); Mertz, Pierre (Baltimore, MD).

This patent was filed on October 5, 2010 and was published online on July 1, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals or channels, each having a different wavelength, are combined onto an optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the optical signal output from the laser, and an optical combiner to combine each of the modulated optical signals. Such components are typically provided at a transmit end of the WDM optical communication system to transmit the optical signals onto the optical fiber. At a receive end of the WDM optical communication system, the optical signals are often separated and converted to corresponding electrical signals that are then processed further.

"Preferably, the information carrying capacity of an optical communication system should be optimized to carry a maximum amount of data over a maximum length of optical fiber. In optimizing the capacity, however, certain trade-offs are often made. For example, certain modulation formats may be employed to modulate the optical signals to carry data at higher rates. Such higher rate modulation formats, however, are typically more susceptible to noise, and, therefore, may not be used in transmission of optical signals over relatively long distances.

"Capacity may be further increased by transmitting a relatively large number of channels over the optical fiber. Trade-offs, however, are encountered here as well. For example, when increased numbers of channels are provided, each channel is typically provided spectrally close to each other, thereby increasing error rates due to cross-talk, as well as non-linear effects, such as cross-phase modulation (XPM). Moreover, the susceptibility of channel to cross-talk, non-linear effects, and noise are often wavelength dependent. Thus, a channel at one wavelength may have more or fewer errors due to XPM or other non-linear effects compared to a channel at another wavelengths. Accordingly, a maximum capacity may be achieved by optimizing the above noted parameters, such as modulation format, distance, and channel spacing. Such optimized capacity may require non-uniformly spaced channels, for example.

"Optical demultiplexers are often employed to separate or demultiplex the combined optical signals. Typically, such optical demultiplexers include optical components that have a fixed bandwidth to select optical signals having a particular wavelength. Accordingly, since different WDM optical communication systems extend over different lengths of fiber, include different types of fiber, and may have other differing characteristics, optical demultiplexers must be tailored for each WDM optical communication system if each such system is to have optimized capacity. As a result, such tailored optical demultiplexers are typically expensive.

"Moreover, the wavelengths associated with each optical signal are often uniformly spaced from each other so as to conform to a so-called standardized 'grid.' In one such wavelength grid, standardized by the International Telecommunications Union (ITU), wavelengths are spectrally spaced from one another by 50 GHz. Such 50 GHz spaced wavelengths or grid wavelengths include 1569.18 nm, 1568.36 nm, 1567.54 nm, etc. Typically, systems that transmit optical signals having wavelengths conforming to the ITU grid do not transmit optical signals having wavelengths between the grid wavelengths. Thus, such systems may not have a channel spacing or other optimized parameters to provide maximum capacity.

"An optical communication system is therefore needed that has flexible channel spacing and bandwidth so that the capacity of such a system can be optimized for a given fiber type and distance, as well as other system parameters."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "Consistent with an aspect of the present disclosure, an apparatus is provided that comprises a plurality of optical transmitters, each of which being configured to supply a corresponding one of a plurality of first optical signals. Each of the plurality of first optical signals has a corresponding one of a plurality of wavelengths and carries a corresponding one of a plurality of data streams. The apparatus also includes an optical combiner configured to combine the plurality of first optical signals onto an optical path, and a photodiode configured to receive a portion of each of the plurality of first optical signals and supply a first electrical signal. A local oscillator laser is also provided, such that at least a portion of a second optical signal output from the local oscillator is also supplied to the photodiode. In addition, circuitry is provided that is circuitry configured to receive the first electrical signal and supply a second electrical signal in response to the first electrical signal. The circuitry includes an electronic filter having a variable bandwidth, and the second electrical signal carries one of the plurality of data streams.

"Consistent with an additional aspect of the present disclosure, an apparatus is provided that comprises a digital signal processor circuit configured to receive input data. The digital signal processor is configured to sample or receive the input data at a first sampling or baud rate, and spectrally shape the input data to supply spectrally shaped data as a first plurality of data samples at the first sampling rate or baud rate. An interpolation circuit is also provided that receives the first plurality of data samples at the first sampling rate and outputs the spectrally shaped data as a second plurality of data samples at a second sampling rate greater than the first sampling rate. A digital-to-analog converter circuit is also provided that is configured to receive the second plurality of data samples and generate an analog signal. The apparatus further includes a modulator circuit and a laser configured to supply light to the modulator circuit, the modulator circuit being configured to modulate the light to supply a modulated optical signal in response to the analog signal.

"Moreover, consistent with a further aspect of the present disclosure, an apparatus is provided that comprises a photodiode that receives a portion of each of a plurality of optical signals, each of which being modulated in accordance with a corresponding one of a plurality of data streams, and each having a corresponding one of a plurality of wavelengths. The photodiode supplies an electrical output, such that each of the plurality of optical signals is supplied by a corresponding one of a plurality of transmitters. In addition, a low-pass filter is provided that supplies a filtered output in response to the electrical output, and an analog-to-digital converter is provided that is configured to sample the filtered output at a first sampling rate to generate a plurality of first data samples. In addition, an interpolation circuit is provided that is configured to receive the plurality of first data samples and supply a plurality of second data samples at a second sampling rate less the first sampling rate. Further, a digital signal processor circuit is provided that is configured to receive the plurality of second data samples.

"Consistent with an additional aspect of the present disclosure, a system is provided that comprises a first transmitter including a first digital signal processor circuit configured to receive input data. The digital signal processor is configured to sample the input data at a first sampling rate, and spectrally shape the input data to supply spectrally shaped data as a first plurality of data samples at the first sampling rate. The transmitter also includes a first interpolation circuit that receives the first plurality of data samples at the first sampling rate and outputs the spectrally shaped data as a second plurality of data samples at a second sampling rate greater than the first sampling rate. In addition, the transmitter includes a digital-to-analog converter circuit configured to receive the second plurality of data samples and generate an analog signal. Further, a modulator circuit, and a laser is provided that is configured to supply light to the modulator circuit. The modulator circuit is configured to modulate the light to supply a first modulated optical signal in response to the analog signal. The apparatus also includes a second transmitter that supplies a second modulated optical signal. The apparatus also includes a combiner configured to combine the first modulated optical signal with the second modulated optical signal onto an optical communication path. Further, the system includes a receiver coupled to the optical communication path. The receiver includes a photodiode that receives portions the first and second modulated optical signals. The photodiode supplies an electrical output to a low-pass filter, which, in turn, supplies a filtered output in response to the electrical output. The receiver also includes an analog-to-digital converter configured to sample the filtered output at the second sampling rate to generate a plurality of third data samples, and a second interpolation circuit configured to receive the plurality of third data samples and supply a plurality of fourth data samples at a third sampling rate less than the second sampling rate. The receiver also includes a second digital signal processor circuit configured to receive the plurality of fourth data samples.

"Consistent with a further aspect of the present disclosure, an apparatus is provided that comprises a digital signal processor circuit configured to receive input data. The digital signal processor is configured to sample the input data at a first sampling rate, and spectrally shape the input data to supply spectrally shaped data as a first plurality of data samples at the first sampling rate. An interpolation circuit is also provided that receives the first plurality of data samples at the first sampling rate and outputs the spectrally shaped data as a second plurality of data samples at a second sampling rate greater than the first sampling rate. In addition, a digital-to-analog converter circuit is provided that is configured to receive the second plurality of data samples and generate an analog signal. Further, a low-pass filter is provided, such that the analog signal is supplied to the low pass filter, and the low-pass filter outputs a filtered signal in response to the analog signal. The low pass filter has an associated roll-off factor, the roll-off factor being adjusted in response to a control input. In addition, a modulator circuit and a laser are provided, such that the laser supplies light to the modulator circuit, and the modulator circuit is configured to modulate the light to supply a modulated optical signal in response to the filtered signal.

"Consistent with a further aspect of the present disclosure, an apparatus is provided that comprises a forward error correction (FEC) encoder circuit configured to receive input data, such that, in response to a first control input, the FEC encoder circuit generates first error correcting bits, and, in response to a second control input, the FEC encoder generates second error correcting bits, a number of the second error correcting bits being greater than a number of the first error correcting bits. The apparatus further includes a plurality of optical transmitters, each of which being configured to supply a corresponding one of a plurality of first optical signals. Each of the plurality of first optical signals has a corresponding one of a plurality of wavelengths and carries a corresponding one of a plurality of data streams, one of the plurality of data streams including one of the first error correcting bits and the second error correcting bits. An optical combiner is also provided that is configured to combine the plurality of first optical signals onto an optical path. In addition, a photodiode is provided that is configured to receive a portion of each of the plurality of first optical signals and supply a first electrical signal. Further, a local oscillator laser is provided, such that at least a portion of a second optical signal output from the local oscillator is supplied to the photodiode. Moreover, circuitry is provided that is configured to receive the first electrical signal and supply a second electrical signal in response to the first electrical signal. The circuitry includes an electronic filter having a first bandwidth when the first error correcting bits are included in said one of the plurality of data streams and a second bandwidth when the second error correcting bits are included in said one of the plurality of data streams. The second bandwidth is spectrally wider than the first bandwidth, and the second electrical signal carries one of the plurality of data streams.

"Consistent with an additional aspect of the present disclosure, an apparatus is provided that includes a substrate and an optical splitter provided on the substrate. The optical splitter has an input and a plurality of outputs, such that the optical splitter receives a wavelength division multiplexed (WDM) optical signal including plurality of optical signals, each having a corresponding one of a plurality of wavelengths. Each of the plurality of outputs of the optical splitter supplies a corresponding one of a plurality of first portions of the WDM optical signal. A plurality of local oscillator lasers are also provided on the substrate, as well as a plurality of optical hybrid circuits. Each of the plurality of optical hybrid circuits has a first input coupled to a corresponding one of the plurality of outputs of the optical splitter and a second input configured to receive a first portion of light supplied by a corresponding one of the plurality of local oscillator lasers. Each of the plurality of optical hybrid circuits has an output that supplies a corresponding one of a plurality of second portions of the WDM optical signal and a second portion of the light supplied by a corresponding one of the plurality of local oscillator lasers, each of the plurality of second portions of the WDM optical signal including light at each of the plurality of wavelengths. Further, a plurality of photodiodes is provided on the substrate. Each of the plurality of photodiodes receives a respective one of the plurality of second portions of the WDM optical signal and the second portion of the light supplied by a corresponding one of the plurality of local oscillator circuits. Each of the plurality of photodiodes supplies a corresponding one of a plurality of electrical signals.

"Consistent with an additional aspect of the present disclosure, an apparatus is provided that includes a wavelength selective switch having an input that receives a plurality of optical signals, each of which having a corresponding one of a plurality of wavelengths. The wavelength selective switch has a plurality of outputs, each of which supplying a corresponding one of a plurality of groups of the plurality of optical signals. The apparatus also includes a plurality of photonic integrated circuits, each of which being configured to receive a corresponding one of the plurality of band of the plurality of optical signals. One of plurality of photonic integrated circuits includes a substrate and an optical splitter provided on the substrate. The optical splitter has an input and a plurality of outputs, such that the optical splitter receives one of the plurality of groups, which includes a subset of the plurality of optical signals. Each optical signal within the subset of the plurality of optical signals having respective one of a subset of the plurality of wavelengths, each of the plurality of outputs of the optical splitter supplying a corresponding one of a plurality of first portions of said one of the plurality of groups. A plurality of local oscillator lasers are also provided on the substrate, as well as a plurality of optical hybrid circuits. Each of the plurality of optical hybrid circuits has a first input coupled to a corresponding one of the plurality of outputs of the optical splitter and a second input configured to receive a first portion of light supplied by a corresponding one of the plurality of local oscillator lasers. Each of the plurality of optical hybrid circuits has an output that supplies a corresponding one of a plurality of second portions of said one of the plurality of groups and a second portion of the light supplied by a corresponding one of the plurality of local oscillator lasers. Each of the plurality of second portions of said one of the plurality of groups includes light at each of the subset of the plurality of wavelengths. In addition, a plurality of photodiodes is provided on the substrate, each of which receiving a respective one of the plurality of second portions of said one of the plurality of band and the second portion of the light supplied by a corresponding one of the plurality of local oscillator circuits. Each of the plurality of photodiodes supplies a corresponding one of a plurality of electrical signals.

"Consistent with an additional aspect of the present disclosure, an optical de-interleaver is provided that has an input for receiving a plurality of optical signals, each of which having a corresponding one of a plurality of wavelengths. The optical de-interleaver has a plurality of outputs, each of which supplying a corresponding one of a plurality of groups of the plurality of optical signals. The apparatus also includes a plurality of photonic integrated circuits, each of which being configured to receive a corresponding one of the plurality of band of the plurality of optical signals. One of plurality of photonic integrated circuits includes a substrate and an optical splitter provided on the substrate. The optical splitter has an input and a plurality of outputs, such that the optical splitter receives one of the plurality of groups, which includes a subset of the plurality of optical signals. Each optical signal within the subset of the plurality of optical signals having respective one of a subset of the plurality of wavelengths, each of the plurality of outputs of the optical splitter supplying a corresponding one of a plurality of first portions of said one of the plurality of groups. A plurality of local oscillator lasers are also provided on the substrate, as well as a plurality of optical hybrid circuits. Each of the plurality of optical hybrid circuits has a first input coupled to a corresponding one of the plurality of outputs of the optical splitter and a second input configured to receive a first portion of light supplied by a corresponding one of the plurality of local oscillator lasers. Each of the plurality of optical hybrid circuits has an output that supplies a corresponding one of a plurality of second portions of said one of the plurality of groups and a second portion of the light supplied by a corresponding one of the plurality of local oscillator lasers. Each of the plurality of second portions of said one of the plurality of groups includes light at each of the subset of the plurality of wavelengths. In addition, a plurality of photodiodes is provided on the substrate, each of which receiving a respective one of the plurality of second portions of said one of the plurality of band and the second portion of the light supplied by a corresponding one of the plurality of local oscillator circuits. Each of the plurality of photodiodes supplies a corresponding one of a plurality of electrical signals.

"Consistent with an additional aspect of the present disclosure, an apparatus is provided that includes a first optical transmitter configured to supply a first optical signal that is modulated in accordance with a first modulation format, the first optical signal having a first wavelength and carrying a first information stream. A second optical transmitter is provided which is configured to supply a second optical signal modulated in accordance with a second modulation format and carrying a second information stream. An optical combiner is also provided that is configured to combine the first and second optical signals onto an optical path. In addition, a first photodiode is provided that is configured to receive a first portion of each of the first and second optical signals and supply a first electrical signal. Further, a second photodiode is provided that is configured to receive a second portion of each of the first and second optical signals and supply a second electrical signal. Moreover, a local oscillator laser is provided, such that portions of a third optical signal output from the local oscillator are supplied to the first and second photodiodes. First circuitry is included to receive the first electrical signal and supply a third electrical signal in response to the first electrical signal. The first circuitry includes a first electronic filter having a variable bandwidth. The third electrical signal carries data associated with the first information stream. In addition, second circuitry is provided that is configured to receive the second electrical signal and supply a fourth electrical signal in response to the first electrical signal. The second circuitry includes a second electronic filter having a variable bandwidth, and the fourth electrical signal carries data associated with the second information stream.

"Consistent with an additional aspect of the present disclosure, an apparatus is provided that comprises a plurality of optical transmitters. A first one of the plurality of optical transmitter is configured to supply a first optical signal modulated in accordance with a first modulation format in response to a first control signal supplied to the first one of the plurality of optical transmitters. The first one of the plurality of optical transmitters is also configured to supply a second optical signal modulated in accordance with a second modulation format in response to a second control signal supplied to the first one of the plurality of optical transmitters. Also, a second one of the plurality of optical transmitters is configured to supply a third optical signal modulated in accordance with the first modulation format in response to a third control signal supplied to the second one of the plurality of optical transmitters. The second one of the plurality of optical transmitters also being configured to supply a fourth optical signal modulated in accordance with the second modulation format in response to a fourth control signal supplied to the second one of the plurality of optical transmitters. Also, an optical combiner is provided that is configured to combine one of the first and second optical signals and one of the third and fourth optical signals onto an optical path. Each of the plurality of optical transmitters supplies a corresponding one of a plurality of optical signals, each of which has a corresponding one of a plurality of wavelengths. One of the first and second optical signals is a first one of the plurality of optical signals, and one of the third and fourth optical signals is a second one of the plurality of optical signals. First and second adjacent ones of the plurality of wavelengths are separated from each other by a first spacing, and second and third adjacent ones of the plurality of wavelengths are separated from each other by a second spacing different than the first spacing.

"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.

"The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one (several) embodiment(s) of the invention and together with the description, serve to explain the principles of the invention."

For the URL and additional information on this patent, see: Wu, Kuang-Tsan; McNicol, John D.; Welch, David F.; Grubb, Stephen G.; Mertz, Pierre. Wavelength Division Multiplexed Optical Communication System Having Variable Channel Spacings. U.S. Patent Number 8768177, filed October 5, 2010, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8768177.PN.&OS=PN/8768177RS=PN/8768177

Keywords for this news article include: Electronics, Digital To Analog, Infinera Corporation, Digital Signal Process.

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