Apparatus and method of pulsed frequency modulation for analog optical communication

• US 6,804,471 B1
• Filed: 01/05/2000
• Issued: 10/12/2004
• Est. Priority Date: 01/05/2000
• Status: Expired due to Fees

• Current Assignee: HRL Laboratories LLC (The Boeing Co.)
• Original Assignee: HRL Laboratories LLC (The Boeing Co.)
• Inventors: Hayes, Robert R., Ionov, Stanislav I.
• Primary Examiner(s): SEDIGHIAN, REZA
• Application Number:
  US09/475,571
• Time in Patent Office: 1,742 Days
• Field of Search: 359/154, 359/174, 359/182, 359/184, 359/188, 359/195, 359/124, 359/181, 398/188, 398/189, 398/183, 398/187
• US Class Current: 398/187
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/508 Pulse generation, e.g. generation of solitons

Abstract

A method and apparatus for pulse frequency modulation for analog optical communication. A train of optical pulses is generated. The spectrum of the optical pulses in the train of optical pulses can be broadened to provide a train of broad spectrum optical pulses. The broadening can be provided by self-phase modulation. Alternatively, broad spectrum optical pulses can be provided by merely having the optical pulses be less than 1 ps duration. A desired optical frequency slice from the train of spectrum broadened optical pulses is selected by a tunable Fabry-Perot filter. A desired optical frequency slice from the broad spectrum optical pulses is selected by a tunable Fabry-Perot filter. The tunable Fabry-Perot filter has a pair of Distributed Bragg Reflectors separated by an electro-refractive section. The electro-refractive section has tuning electrodes for applying transverse electric fields to the electro-refractive section, corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.

Claims

1. A method of pulsed frequency modulation transmission for analog optical communication, comprising the steps of:

generating a train of broad spectrum optical pulses; and

selecting a desired optical frequency slice from the train of broad spectrum optical pulses by a tunable Fabry-Perot filter, the tunable Fabry-Perot filter having a pair of Distributed Bragg Reflectors separated by an electro-refractive section having tuning electrodes for applying transverse electric fields to the electro-refractive section corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.

2. The method of pulsed frequency modulation transmission for analog optical communication of claim 1 wherein the step of generating a train of broad spectrum optical pulses includes the step of broadening spectrum of the optical pulses in a train of optical pulses by self-phase modulation to provide the train of broad spectrum optical pulses.

3. The method of pulsed frequency modulation transmission for analog optical communication of claim 1 wherein the step of generating a train of broad spectrum optical pulses includes the step of providing an optical pulse having a optical pulse duration shorter than 1 ps.

4. The method of pulsed frequency modulation transmission for analog optical communication of claim 1, further comprising the step of recovering the analog waveform from the pulse-frequency modulated train of optical pulses by:

splitting the pulse-frequency modulated train of optical pulses into a first optical beam and a second optical beam;

providing a first photodetector, the first photodetector providing a first current responsive to the first optical beam input thereon, the first photodetector having a first photodetector spectral response and being biased such that the first current is in a first direction;

providing a second photodetector, the second photodetector providing a second current responsive to the second optical beam input thereon, the second photodetector having a second photodetector spectral response and being biased such that the second current is in the first direction;

coupling an input of a transimpedance amplifier to an output of the first photodetector and to an input of the second photodetector to provide an output of the transimpedance amplifier proportional to the difference between the first current and the second current; and

providing a first optical filter to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter to receive the second optical beam prior to incidence upon the second photodetector,

wherein the first photodetector spectral response and the second photodetector spectral response are each broader than respective passbands of the first optical filter and the second optical filter to provide photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter.

5. The method of pulsed frequency modulation transmission for analog optical communication of claim 4, wherein the step of splitting includes the step of amplifying and limiting the pulse-frequency modulated train of optical pulses prior to splitting.

6. The method of pulsed frequency modulation transmission for analog optical communication of claim 1, wherein the electro-refractive section is a Franz-Keldysh semiconductor waveguide.

7. A pulsed frequency modulation transmission system for analog optical communication, comprising:

a generator for generating a first train of optical pulses;

a self-phase modulator coupled to the generator for broadening spectrum of the optical pulses in the first train of optical pulses to provide a second train of spectrum broadened optical pulses; and

a tunable Fabry-Perot filter coupled to the self-phase modulator for selecting a desired optical frequency slice from the second train of spectrum broadened optical pulses by, the tunable Fabry-Perot filter having a pair of Distributed Bragg Reflectors separated by an electro-refractive section having tuning electrodes for applying transverse electric fields to the electro-refractive section corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.

8. The pulsed frequency modulation transmission system for analog optical communication of claim 7, further comprising a converter for converting a pulse-frequency modulated train of optical pulses into an electrical signal that is proportional to instantaneous frequency deviations of the pulse-frequency modulated train of optical pulses, comprising:

an optical splitter for splitting the pulse-frequency modulated train of optical pulses into a first optical beam and a second optical beam;

a first photodetector providing a first current responsive to the first optical beam input thereon from the optical splitter, the first photodetector having a first spectral response and being biased such that the first current is in a first direction;

a second photodetector, the second photodetector providing a second current responsive to the second optical beam input thereon from the optical splitter, the second photodetector having a second spectral response and being biased such that the second current is in the first direction;

a transimpedance amplifier having an input and an output, the input being coupled to an output of the first photodetector and to an input of the second photodetector to provide at the output of the transimpedance amplifier a current proportional to the difference between the first current and the second current; and

a first optical filter located to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter located to receive the second optical beam prior to incidence upon the second photodetector, wherein the first photodetector spectral response and the second photodetector spectral response are each broader than respective passbands of the first optical filter and the second optical filter to provide photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter.

9. The pulsed frequency modulation transmission system for analog optical communication of claim 8, further comprising an optical amplifier and a limiter serially coupled to the input of the optical splitter.

10. The pulsed frequency modulation transmission system for analog optical communication of claim 7, wherein the electro-refractive section is a Franz-Keldysh semiconductor waveguide.

11. A pulsed frequency modulation transmission system for analog optical communication, comprising:

a generator of optical pulses having a pulse duration of shorter than 1 ps to provide a train of broad spectrum optical pulses; and

a tunable Fabry-Perot filter coupled to the coupled to the generator for selecting a desired optical frequency slice from the train of broad spectrum optical pulses, the tunable Fabry-Perot filter having a pair of Distributed Bragg Reflectors separated by an electro-refractive section having tuning electrodes for applying transverse electric fields to the electro-refractive section corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.

12. The pulsed frequency modulation transmission system for analog optical communication of claim 11, further comprising a converter for converting a pulse-frequency modulated train of optical pulses into an electrical signal that is proportional to instantaneous frequency deviations of the pulse-frequency modulated train of optical pulses, comprising:

an optical splitter for splitting the pulse-frequency modulated train of optical pulses into a first optical beam and a second optical beam;

a first photodetector providing a first current responsive to the first optical beam input thereon from the optical splitter, the first photodetector having a first spectral response and being biased such that the first current is in a first direction;

a second photodetector, the second photodetector providing a second current responsive to the second optical beam input thereon from the optical splitter, the second photodetector having a second spectral response and being biased such that the second current is in the first direction;

a transimpedance amplifier having an input and an output, the input being coupled to an output of the first photodetector and to an input of the second photodetector to provide at the output of the transimpedance amplifier a current proportional to the difference between the first current and the second current; and

a first optical filter located to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter located to receive the second optical beam prior to incidence upon the second photodetector, wherein the first photodetector spectral response and the second photodetector spectral response are each broader than respective passbands of the first optical filter and the second optical filter to provide photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter.

13. The pulsed frequency modulation transmission system for analog optical communication of claim 12, further comprising an optical amplifier and a limiter serially coupled to the input of the optical splitter.

14. The pulsed frequency modulation transmission system for analog optical communication of claim 11, wherein the electro-refractive section is a Franz-Keldysh semiconductor waveguide.

15. A method of optical frequency modulation for an analog electrical signal comprising the steps of:

generating a stream of broad spectrum optical pulses, each optical pulse in said stream of broad spectrum optical pulses having a broad frequency spectrum;

coupling said pulses into a tunable optical filter; and

controlling said tunable optical filter with said analog electrical signal to pass a narrow frequency spectrum from said broad frequency spectrum of each optical pulse.

16. The method of claim 15 wherein said tunable optical filter comprises a Fabry-Perot filter having two distributed Bragg reflector sections.

17. The method of claim 15 wherein the step of generating a stream of optical pulses comprises the steps of:

generating a stream of equally spaced optical pulses with a mode-locked laser; and

directing the stream of equally spaced optical pulses into an optical fiber having a small normal dispersion.

18. An optical modulator for frequency modulation of an analog electrical signal onto optical pulses comprising:

an optical pulse generator generating a stream of broad spectrum optical pulses, each optical pulse in said stream of optical pulses having a broad frequency spectrum; and

a tunable narrowband optical filter coupled to said optical pulse generator, said tunable narrowband optical filter being controlled by said analog electrical signal.

19. The optical modulator of claim 18 wherein said tunable narrowband optical filter comprises a Fabry-Perot filter having two distributed Bragg reflector sections.

20. The optical modulator of claim 18 wherein the optical pulse generator comprises:

a mode-locked laser generating a stream of equally-spaced optical pulses; and

a self-phase modulator coupled to the mode-locked laser, said self-phase modulator producing said stream of broad spectrum optical pulses.