OPTICAL PULSE CHARACTERIZATION USING PHASE MODULATION

• US 20080114556A1
• Filed: 11/13/2006
• Published: 05/15/2008
• Est. Priority Date: 11/13/2006
• Status: Active Grant

• Current Assignee: WSOU Investments, LLC (WSOU Holdings, LLC)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Kang, Inuk
• Granted Patent: US 7,873,272 B2
• Time in Patent Office: Days
• US Class Current: 702/76
• CPC Class Codes: G01J 11/00 Measuring the characteristics of individual optical pulses or of optical pulse trains

Abstract

An optical pulse monitor (OPM) that determines an optical phase profile for a pulse train by (i) modulating the optical phase of pulses in the pulse train using a periodic waveform and (ii) generating a derivative of a spectrum of the resulting modulated signal with respect to the waveform'"'"'s amplitude. In one embodiment, an OPM has a phase modulator that modulates the optical phase of pulses in a received pulse train using a periodic waveform supplied by a configurable drive circuit. The drive circuit temporally aligns the waveform with the pulse train to serially produce each of four selected temporal alignments. An optical signal analyzer measures a spectrum of the modulated signal generated by the modulator for each of these four temporal alignments and provides the four measured spectra to a processor. The processor uses the measured spectra to determine two spectrum derivatives with respect to the waveform'"'"'s amplitude. The processor samples each of the spectrum derivatives at frequency points separated from one another by the frequency of the periodic waveform and uses these samples to obtain a series of complex values. The processor then calculates the optical phase profile for the pulse train based on the phase of a complex value obtained by summing the series.

Claims

1. A method of optical signal monitoring, comprising the steps of:

modulating the optical phase of pulses in a pulse train using a periodic waveform having an amplitude to generate a modulated signal; and

generating a derivative of a spectrum of the modulated signal with respect to said amplitude to determine an optical phase profile for the pulses in the pulse train.

2. The invention of claim 1, further comprising the step of frequency-resolving the modulated signal to generate said spectrum.

3. The invention of claim 1, wherein:

the periodic waveform has a fixed phase relative to the envelope of the pulse train; and

the spectrum of the modulated signal is a function of the amplitude and the fixed phase.

4. The invention of claim 1, wherein the periodic waveform is a sinusoid.

5. The invention of claim 1, wherein the step of generating comprises:

for each of four selected temporal alignments between the pulse train and the periodic waveform, measuring a respective spectrum of the modulated signal, wherein relative phase offsets for the periodic waveforms corresponding to three of the four selected temporal alignments with respect to the periodic waveform corresponding to the remaining one of the four selected temporal alignment are substantially π

/2, π

, and 3π

/2.

6. The invention of claim 5, wherein the step of generating comprises:

measuring a spectrum of the modulated signal for a first temporal alignment;

shifting the phase of the periodic waveform to produce a second temporal alignment;

measuring a spectrum of the modulated signal for the second temporal alignment;

further shifting the phase of the periodic waveform to produce a third temporal alignment;

measuring a spectrum of the modulated signal for the third temporal alignment;

further shifting the phase of the periodic waveform to produce a fourth temporal alignment; and

measuring the spectrum of the modulated signal for the fourth temporal alignment.

7. The invention of claim 5, wherein:

the step of modulating comprises;

generating two or more copies of the pulse train; and

modulating the optical phase of pulses in each copy using a respective periodic waveform to generate a respective modulated signal, wherein the temporal alignment between a pulse-train copy and the respective periodic waveform differs from the temporal alignment between at least one other pulse-train copy and that other copy'"'"'s respective periodic waveform; and

the step of generating comprises measuring in parallel spectra of the two or more modulated signals corresponding to said two or more copies.

8. The invention of claim 5, wherein:

the step of modulating comprises oscillating the relative phase offset between two selected phase offset values at an oscillation frequency; and

the step of generating comprises synchronously detecting a spectral component of the modulated signal at said oscillation frequency.

9. The invention of claim 8, wherein the step of generating further comprises changing the wavelength of the spectral component being synchronously detected.

10. The invention of claim 1, comprising the steps of:

generating two derivatives of the spectrum of the modulated signal with respect to said amplitude;

sampling each of the spectrum derivatives at frequency points separated from one another by a frequency of the periodic waveform;

obtaining a series of complex values, wherein each complex value has a sample of one of the spectrum derivatives as its real part and a corresponding sample of the other spectrum derivative as its imaginary part; and

calculating the optical phase profile in the frequency domain for the pulses in the pulse train based on the phase of a complex value obtained by summing the series.

11. The invention of claim 10, further comprising the step of:

calculating electric field of the pulse train in the time domain based on (i) the optical phase profile in the frequency domain and (ii) spectral intensity of the modulated signal.

12. The invention of claim 11, further comprising the step of:

calculating the optical phase profile in the time domain based on the calculated electric field in the time domain.

13. Apparatus for optical signal monitoring, comprising:

a modulator adapted to modulate the optical phase of pulses in a pulse train using a periodic waveform having an amplitude to generate a modulated signal;

a frequency-resolving device adapted to spectrally resolve the modulated signal; and

a processor adapted to (i) generate a derivative of a spectrum of the modulated signal with respect to said amplitude based on the spectrally resolved modulated signal and (ii) determine an optical phase profile for the pulses in the pulse train based on said derivative.

14. The invention of claim 13, wherein:

the periodic waveform has a fixed phase relative to the envelope of the pulse train; and

the spectrum of the modulated signal is a function of the amplitude and the fixed phase.

15. The invention of claim 13, wherein the periodic waveform is a sinusoid.

16. The invention of claim 13, further comprising:

a configurable drive circuit adapted to (i) generate the periodic waveform and (ii) shift the phase of said periodic waveform to achieve a selected temporal alignment between the pulse train and the periodic waveform in response to a command from the processor.

17. The invention of claim 16, wherein the drive circuit is adapted to lock the phase of the periodic waveform relative to the envelope of the pulse train.

18. The invention of claim 16, wherein:

the drive circuit is adapted to oscillate the phase of said periodic waveform between two selected phase offset values at an oscillation frequency;

the processor comprises a phase sensitive detector adapted to synchronously detect a spectral component of the modulated signal at said oscillation frequency; and

the frequency resolving device comprises an etalon adapted to change the wavelength of the spectral component being synchronously detected by the phase sensitive detector in response to a command from the processor.

19. The invention of claim 13, comprising:

a splitter adapted to generate two or more copies of the pulse train; and

two or more instances of the modulator, each adapted to modulate the optical phase of a respective pulse-train copy using a respective periodic waveform to generate a respective modulated signal, wherein temporal alignment between the pulse-train copy and the periodic waveform differs from temporal alignment between at least one other pulse-train copy and that other copy'"'"'s respective periodic waveform; and

two or more instances of the frequency-resolving device adapted to measure in parallel spectra of the two or more modulated signals corresponding to said two or more copies.

20. The invention of claim 13, wherein the processor is adapted to:

generate two derivatives of the spectrum of the modulated signal with respect to said amplitude;

sample each of the spectrum derivatives at frequency points separated from one another by a frequency of the periodic waveform;

obtain a series of complex values, wherein each complex value has a sample of one of the spectrum derivatives as its real part and a corresponding sample of the other spectrum derivative as its imaginary part; and

calculate the optical phase profile in the frequency domain for the pulses in the pulse train based on the phase of a complex value obtained by summing the series.

21. The invention of claim 20, wherein the processor is adapted to calculate electric field of the pulse train in the time domain based on (i) the optical phase profile in the frequency domain and (ii) spectral intensity of the modulated signal.

22. The invention of claim 21, wherein the processor is adapted to calculate the optical phase profile in the time domain based on the calculated electric field in the time domain.

23. Apparatus for optical signal monitoring, comprising:

means for modulating the optical phase of a pulse train using a periodic waveform having an amplitude to generate a modulated signal; and

means for generating a derivative of a spectrum of the modulated signal with respect to said amplitude to determine an optical phase profile for the pulse train.

24. The invention of claim 23, further comprising means for frequency-resolving the modulated signal to generate said spectrum.