QUADRATURE MODULATOR WITH FEEDBACK CONTROL AND OPTICAL COMMUNICATIONS SYSTEM USING THE SAME

• US 20070133918A1
• Filed: 02/27/2007
• Published: 06/14/2007
• Est. Priority Date: 07/02/2003
• Status: Active Grant

• Current Assignee: CeLight, Inc.
• Original Assignee: CeLight, Inc.
• Inventors: Shpantzer, Isaac, Khurgin, Jacob, Cho, Pak Shing
• Granted Patent: US 8,050,351 B2
• Time in Patent Office: Days
• US Class Current: 385/1
• CPC Class Codes: G02F 1/225 in an optical waveguide structure
• CPC Class Codes: G02F 2201/126 push-pull
• CPC Class Codes: G02F 2203/06 Polarisation independent
• CPC Class Codes: H04B 10/2543 due to fibre non-linearities, e.g. Kerr effect
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5053 using a parallel, i.e. shunt, combination of modulators
• CPC Class Codes: H04B 10/5057 using a feedback signal generated by analysing the optical output
• CPC Class Codes: H04B 10/50577 to control the phase of the modulating signal
• CPC Class Codes: H04B 10/508 Pulse generation, e.g. generation of solitons
• CPC Class Codes: H04B 10/532 Polarisation modulation
• CPC Class Codes: H04B 10/541 Digital intensity or amplitude modulation
• CPC Class Codes: H04B 10/5561 Digital phase modulation
• CPC Class Codes: H04B 10/60 Receivers
• CPC Class Codes: H04J 14/005 Optical Code Multiplex
• CPC Class Codes: H04J 14/02 Wavelength-division multiplex systems
• CPC Class Codes: H04J 14/08 Time-division multiplex systems
• CPC Class Codes: H04L 7/0075 with photonic or optical means

Abstract

The method and system are disclosed for automatic feedback control of integrated optical quadrature modulator for generation of optical quaternary phase-shift-keyed signal in coherent optical communications. The method comprises the steps of detecting at least a part of an output optical signal from the QPSK modulator, extracting of a particular portion of the output signal in frequency domain, and processing the signal in frequency domain to optimize the transmission of an optical link. The system and method of optical communications in fiber or free space are disclosed that implement the quadrature data modulator with automatic feedback control.

Claims

1. The method for automatic feedback control of an integrated optical quadrature (QPSK) modulator comprising the steps of:

detecting at least a part of an output optical signal from the QPSK modulator,

extracting of a particular portion of the output signal in RF frequency domain and

minimizing the portion of the output signal in RF frequency domain by dithering a voltage applied to a phase shifter between I and Q components of the QPSK modulator.

2. The method of claim 1, further comprising:

detecting a power of the output optical signal,

dithering a first voltage applied to a first bias of the QPSK modulator and a second voltage applied to a second bias of the QPSK modulator, and

minimizing the power of the output optical signal.

3. The method of claim 1, further comprising:

detecting a power of the output optical signal,

dithering a first voltage applied to a first bias of the QPSK modulator and a second voltage applied to a second bias of the QPSK modulator, and

maximizing the power of the output optical signal.

4. The method of claim 1, wherein

dithering of the voltage applied to the phase shifter between I and Q components of the QPSK modulator is performed continuously during the QPSK modulator operation.

5. The method of claim 2 or 3, wherein

dithering the first voltage applied to the first bias of the QPSK modulator and

dithering the second voltage applied to the second bias of the QPSK modulator are performed continuously during the QPSK modulator operation.

6. A modulator for QPSK modulation of an optical signal with an automatic feedback control, comprising:

an input;

a splitter to split the optical signal coming through the input into a first beam and a second beam;

a first Mach Zehnder modulator (MZM) having a first bias input, the first MZM receiving the first beam from the splitter, the first MZM producing a first output signal;

a second Mach Zehnder modulator (MZM) having a second bias input, the second MZM receiving the second beam from the splitter;

the second MZM producing a second output signal;

a phase shifter producing a phase shift between the first and the second output signals;

first means for applying a first dither signal to the phase shifter;

an optical combiner to combine the first an the second output signals and to produce a third output signal;

a low speed optical detector to detect at least a portion of the third signal;

a spectral power detector to detect a power of a RF spectral component of the third signal; and

a digital signal processing (DSP) unit receiving the signal from the spectral power detector, the DSP unit connected to the means for applying the first dither signal to the phase shifter, the DSP unit controlling the first dither signal to minimize the signal from the spectral power detector.

7. The modulator of claim 6, wherein the low speed optical detector is a single photon absorption detector.

8. The modulator of claim 6, wherein the splitter, the first and the second MZMs, the phase shifter and the combiner are integrated on a single chip.

9. The modulator of claim 6, further comprising:

second means for applying a second dither signal to the first bias input;

third means for applying a third dither signal to the second bias input; and

the DSP unit being connected to the second means and the third means, the DSP unit controlling the second and the third dither signals to minimize the signal from the low speed optical detector.

10. The modulator of claim 6, further comprising:

second means for applying a second dither signal to the first bias input;

third means for applying a third dither signal to the second bias input; and

the DSP unit being connected to the second means and the third means, the DSP unit controlling the second and the third dither signals to maximize the signal from the low speed optical detector.

11. An optical communications system for transmitting phase shift keying modulated data, comprising:

a light source;

a first modulator for modulation of an optical signal, the modulator having a first automatic feedback control, the modulator receiving an optical signal from the light source and producing a first QPSK optical signal;

a first coherent optical receiver, the first coherent optical receiver mixing the first QPSK optical signal with a first reference signal from a local oscillator, the coherent received demodulating data encoded in the first QPSK optical signal;

wherein the first automatic feedback control including

a first low speed optical detector to detect at least a portion of the first QPSK optical signal;

a first spectral power detector to detect a power of a first RF spectral component of the first QPSK optical signal; and

a first digital signal procession (DSP) unit receiving the signal from the first spectral power detector, the first DSP unit controlling a first phase shifter of the first modulator to minimize the signal from the first spectral power detector.

12. The optical communications system of claim 11, wherein the first DSP unit controlling a first and a second bias of the first modulator to maximize the signal from the first low speed optical detector.

13. The optical communications system of claim 11, wherein the first DSP unit controlling a first and a second bias of the first modulator to minimize the signal from the first low speed optical detector.

14. The optical communications system of claim 11, wherein

the first modulator is an integrated modulator comprising a first Mach Zehnder modulator, a second Mach Zehnder modulaator and the first phase shifter to introduce a 90-degrees shift between outputs of the first and the second mach Zehnder modulators;

the first Mach Zehnder modulator, the second Mach Zehnder modulator, and the first phase shifter formed on a single chip.

15. The optical communications system of claim 11, wherein

the first coherent optical receiver is an integrated device including a first 90-degrees optical hybrid, and

at least two photodetectors, wherein the photodiodes are spaced at a distance from 0.01 to 1 mm.

16. The optical communications system of claim 11 adapted to operating in two polarization states of light for transmitting phase shift keying modulated data, comprising:

a second modulator for modulation of an optical signal with a second automatic feedback control, the modulator receiving an optical signal from the light source having a second polarization state being orthogonal to the first polarization state and producing a second QPSK optical signal;

a polarization combiner to combine the first and the second QPSK optical signals for transmitting over a communication link,

a second coherent optical receiver, the coherent optical receiver mixing the second QPSK optical signal with a second signal from a local oscillator having the same polarization state as the second QPSK optical signal;

wherein

the second feedback control including

a second low speed optical detector to detect at least a portion of the second QPSK optical signal;

a second spectral power detector to detect a power of a second RF spectral component of the second QPSK optical signal;

a second digital signal processing (DSP) unit receiving the signal from the second spectral power detector, the DSP unit controlling a second phase shifter of the second modulator to minimize the signal from the second spectral power detector.

17. The optical communication system of claim 16, wherein

the first DSP unit controlling a first and a second bias of the first modulator to minimize the signal from the first low speed optical detector and

the second DSP unit controlling a first and a second bias of the second modulator to minimize the signal from the second low speed optical detector.

18. The optical communication system of claim 16, wherein the first modulator, the second modulator, and the polarization combiner are formed on a single chip.

19. The optical communication system of claim 16, wherein

the second coherent optical receiver including

a second 90-degrees optical hybrid, and

at least two photodiodes, wherein the photodiodes are spaced at a distance from 0.01 to 1 mm.

20. The optical communication system of claim 19, wherein the second coherent optical receiver further comprising:

an alignment system for precise alignment of the second 90-degrees optical hybrid relative to the photodiodes during fabrication, the alignment system comprising at least an alignment waveguide and an alignment photodiode.