Granted Patent B2

US 8,923,706 · App. 13/556,635 · Granted Dec 30, 2014

Abstract

Claims (58)

1. An apparatus, comprising:

a front-end circuit configured to convert one or more electrical digital signals into a modulated optical signal having a first carrier frequency and a modulation symbol rate; and

a digital signal processor configured to generate said one or more electrical digital signals based on an input data stream and use of a frequency-dependent spectral-correction function in a manner that causes the modulated optical signal to have a spectrum whose intensity variation is no greater than about 3 dB across a spectral range that is centered on the first carrier frequency and has a width of the modulation symbol rate, wherein:

the front-end circuit comprises a Mach-Zehnder modulator having a frequency-dependent signal-transfer characteristic; and

the frequency-dependent spectral-correction function is based on said frequency-dependent signal-transfer characteristic.

2. The apparatus of claim 1 , wherein the digital signal processor is configured to generate said one or more electrical digital signals in a manner that causes the spectrum of the modulated optical signal to have an intensity variation across said spectral range of no greater than 2 dB or no greater than 0.5 dB.

3. The apparatus of claim 1 , wherein the digital signal processor is configured to:

apply constellation mapping to a first data stream corresponding to the input data stream to generate a first stream of constellation symbols; and

apply frequency-dependent spectral correction to the first stream of constellation symbols to generate a first electrical digital signal and a second electrical digital signal, wherein:

the first electrical digital signal has digital values representing in-phase components of the constellation symbols in the first stream constellation symbols; and

the second electrical digital signal has digital values representing quadrature components of the constellation symbols in the first stream of constellation symbols.

4. The apparatus of claim 3 , wherein the digital signal processor is further configured to:

partition the first stream of constellation symbols into a plurality of segments;

apply discrete Fourier transformation to said segments to generate a corresponding plurality of discrete spectra;

convolve said discrete spectra with the frequency-dependent spectral-correction function to generate a corresponding plurality of corrected discrete spectra;

apply inverse discrete Fourier transformation to said corrected discrete spectra to generate a corresponding plurality of sequences of corrected constellation symbols; and

apply overlap-and-add processing to said plurality of the sequences to generate the first and second electrical digital signals.

5. The apparatus of claim 4 , wherein, to apply the overlap-and-add processing to said plurality of the sequences, the digital signal processor is configured to:

truncate the sequences to remove time overlap between them; and

concatenate resulting truncated sequences to generate the first and second electrical digital signals.

6. The apparatus of claim 3 , wherein:

the digital signal processor is further configured to:

apply constellation mapping to a second data stream corresponding to the input data stream to generate a second stream of constellation symbols;

apply frequency-dependent spectral correction to the second stream of constellation symbols to generate a third electrical digital signal and a fourth electrical digital signal, wherein:

the third electrical digital signal has digital values representing in-phase components of the constellation symbols in the second data stream of constellation symbols; and

the fourth electrical digital signal has digital values representing quadrature components of the constellation symbols in the second stream of constellation symbols; and

the front-end circuit is configured to:

convert the first and second electrical digital signals into a first polarization component of the modulated optical signal; and

convert the third and fourth electrical digital signals into a second polarization component of the modulated optical signal orthogonal to the first polarization component.

7. The apparatus of claim 1 , wherein a squared absolute value of the frequency-dependent spectral-correction function is represented by a first line segment having a first slope and a second line segment having a second slope different from the first slope, said first and second line segments being connected to one another at a frequency that is smaller than 6 GHz.

8. The apparatus of claim 1 , wherein a squared absolute value of the frequency-dependent spectral-correction function is represented by a first line segment having a first slope and a second line segment having a second slope different from the first slope, said first and second line segments being connected to one another at a frequency that is smaller than one quarter of the modulation symbol rate.

9. The apparatus of claim 7 , wherein the squared absolute value of the frequency-dependent spectral-correction function is further represented by a third line segment connected to the second line segment, said third line segment having a third slope different from the first slope and the second slope.

10. The apparatus of claim 9 , wherein the third slope is substantially zero.

11. The apparatus of claim 9 , wherein the third slope has substantially the same absolute value as the first slope, but an opposite sign.

12. The apparatus of claim 8 , wherein:

the first line segment is located at lower frequencies than the second line segment; and

the first slope is greater than the second slope.

13. The apparatus of claim 1 , wherein the frequency-dependent spectral-correction function is configured to at least partially compensate a frequency roll-off caused by said frequency-dependent signal-transfer characteristic in a manner that causes said spectrum within said spectral range to be flatter than a corresponding spectrum without the use of the frequency-dependent spectral-correction function.

14. The apparatus of claim 1 , wherein:

the front-end circuit is further configured to convert said one or more electrical digital signals into a second modulated optical signal having a second carrier frequency different from the first carrier frequency; and

the frequency-dependent spectral-correction function further includes an additional component to perform pulse shaping based on raised-cosine filtering.

15. An apparatus, comprising:

a front-end circuit configured to convert one or more electrical digital signals into a modulated optical signal having a modulation symbol rate; and

a digital signal processor configured to generate said one or more electrical digital signals based on an input data stream and application of a frequency-dependent spectral-correction function whose squared absolute value is represented by a first line segment having a first slope and a second line segment having a second slope different from the first slope, said first and second line segments being connected to one another at a frequency that is smaller than one half of the modulation symbol rate, wherein:

the front-end circuit comprises a Mach-Zehnder modulator having a frequency-dependent signal-transfer characteristic; and

the frequency-dependent spectral-correction function is based on said frequency-dependent signal-transfer characteristic.

16. The apparatus of claim 15 , wherein the frequency at which said first and second line segments are connected to one another is smaller than quarter of the modulation symbol rate or smaller than 6 GHz.

17. The apparatus of claim 15 , wherein the squared absolute value of the frequency-dependent spectral-correction function is further represented by a third line segment connected to the second line segment, said third line segment having a third slope different from the first slope and the second slope.

18. The apparatus of claim 17 , wherein the third slope the third slope has substantially the same absolute value as the first slope, but an opposite sign.

19. The apparatus of claim 15 , wherein:

the first line segment is located at lower frequencies than the second line segment; and

the first slope is greater than the second slope.

20. An apparatus, comprising:

a front-end circuit configured to convert one or more electrical digital signals into a modulated optical signal having a first carrier frequency and a modulation symbol rate; and

a digital signal processor configured to generate said one or more electrical digital signals based on an input data stream and use of a frequency-dependent spectral-correction function in a manner that causes the modulated optical signal to have a spectrum whose intensity variation is no greater than about 3 dB across a spectral range that is centered on the first carrier frequency and has a width of the modulation symbol rate;

wherein a squared absolute value of the frequency-dependent spectral-correction function is represented by a first line segment having a first slope and a second line segment having a second slope different from the first slope, said first and second line segments being connected to one another at a frequency that is smaller than one quarter of the modulation symbol rate;

wherein the squared absolute value of the frequency-dependent spectral-correction function is further represented by a third line segment connected to the second line segment, said third line segment having a third slope different from the first slope and the second slope; and

wherein the third slope is substantially zero.

Assignments (9)