Quadrature processed LIDAR system

• US 6,608,669 B2
• Filed: 09/20/2001
• Issued: 08/19/2003
• Est. Priority Date: 09/22/2000
• Status: Active Grant

• Current Assignee: Virginia Tech Intellectual Properties, Inc.
• Original Assignee: Virginia Tech Intellectual Properties, Inc.
• Inventors: Holton, Carvel E.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US09/956,634
• Publication Number:
  US 20020105632A1
• Time in Patent Office: 698 Days
• Field of Search: 356/28.5, 356/4.01, 356/5.09, 356/5.15, 342/84, 342/99, 342/104
• US Class Current: 356/28.5
• CPC Class Codes: G01S 17/58 Velocity or trajectory determination systems; Sense-of-movement determination systems
• CPC Class Codes: G01S 7/4818 using optical fibres
• CPC Class Codes: G01S 7/491 Details of non-pulse systems
• CPC Class Codes: G01S 7/4917 superposing optical signals in a photodetector, e.g. optical heterodyne detection
• CPC Class Codes: H04B 10/63 Homodyne , i.e. coherent receivers where the local oscillator is locked in frequency and phase to the carrier signal
• CPC Class Codes: H04B 10/64 Heterodyne , i.e. coherent receivers where, after the opto-electronic conversion, an electrical signal at an intermediate frequency [IF] is obtained
• CPC Class Codes: H04L 27/1525 using quadrature demodulation
• CPC Class Codes: H04L 27/206 using a pair of orthogonal carriers, e.g. quadrature carriers
• CPC Class Codes: H04L 27/26 Systems using multi-frequency codes H04L27/32 takes precedence

Abstract

A method of generating in-quadrature signals is disclosed. The method comprises phase shifting a Doppler frequency-shifted signal; phase shifting a local oscillator signal; mixing the phase shifted Doppler frequency-shifted signal and the phase-shifted local oscillator signal generating thereby a signal which includes the phase-shifted Doppler frequency-shifted signal and a further phase-shifted local oscillator signal; and mixing the unphase-shifted Doppler frequency-shifted signal and the unphase-shifted local oscillator signal generating thereby a signal which includes the unphase-shifted local oscillator signal and a further phase-shifted Doppler frequency-shifted signal. A method of determining the velocity of an object is also disclosed. The method comprises receiving a Doppler frequency-shifted signal reflected or backscattered from the object; generating a local oscillator signal; based upon the received Doppler frequency-shifted signal and the local oscillator signal, generating an in-phase signal; based upon the received Doppler frequency-shifted signal and the local oscillator signal generating an in-quadrature signal; summing the in-phase signal and the in-quadrature signal; and transforming the summation of the in-phase signal and the in-quadrature signal. A lidar is disclosed comprising an optical system for transmitting an output signal to an object and receiving thereby a Doppler frequency-shifted signal reflected or backscattered from the object; a signal mixing assembly receptive of the Doppler frequency-shifted signal and a local oscillator signal generating thereby an in-phase signal and an in-quadrature signal; and a signal transformer for transforming the in-phase signal and an in-quadrature signals. A signal mixing system is disclosed comprising an array of signal couplers receptive of a Doppler frequency-shifted signal and a local oscillator signal generating thereby an in-phase signal which includes the unphase-shifted local oscillator signal and a phase-shifted Doppler frequency-shifted signal and an in-quadrature signal which includes the phase-shifted Doppler frequency-shifted signal and a further phase-shifted local oscillator signal; and a plurality of signal detectors receptive of the in-phase and in-quadrature signals.

Claims

1. A method of generating in-quadrature signals, the method comprising:

receiving a Doppler frequency-shifted signal;

phase shifting the Doppler frequency-shifted signal generating thereby a phase-shifted Doppler frequency-shifted signal and an unphase-shifted Doppler frequency-shifted signal;

receiving a local oscillator signal;

phase shifting the local oscillator signal generating thereby a phase-shifted local oscillator signal and an unphase-shifted local oscillator signal;

further phase shifting the local oscillator signal;

mixing the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal generating thereby a signal that includes the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal; and

mixing the unphase-shifted Doppler frequency-shifted signal and the unphase-shifted local oscillator signal generating thereby a signal that includes the unphase-shifted local oscillator signal and the phase-shifted Doppler frequency shifted signal.

2. A method of determining the sign of the velocity vector associated with the velocity of an object, the method comprising:

receiving a Doppler frequency-shifted signal reflected or backscattered from the object;

generating a local oscillator signal;

based upon the received Doppler frequency-shifted signal and the local oscillator signal, generating an in-phase signal;

based upon the received Doppler frequency-shifted signal and the local oscillator signal, generating an in-quadrature signal;

summing the in-phase signal and the in-quadrature signal;

Fourier transforming the summation of the in-phase signal and the in-quadrature signal; and

yielding a power spectral density from the Fourier transform representative of the sign of the velocity vector.

3. A lidar comprising:

an optical system for transmitting an output signal to an object and receiving thereby a Doppler frequency-shifted signal reflected or backscattered from the object;

wherein the optical system comprises;

a laser generating a laser beam at a prescribed wavelength,

a signal coupler receptive of the laser beam and adapted to divide the laser beam into a local oscillator signal and a coupler output signal,

a circulator receptive of the coupler output signal and adapted to provide a circulator output signal to the object, to receive the Doppler frequency-shifted signal from the object, and to output the Doppler frequency-shifted signal, and

a telescope receptive of the circulator output signal and adapted to direct the circulator output signal to the object and to receive the Doppler frequency-shifted signal from the object,

wherein the circulator is adapted to receive the Doppler frequency-shifted signal from the telescope;

a signal mixing assembly including;

a plurality of signal phase shifting devices being adapted to receive the Doppler frequency-shifted signal from the circulator and a local oscillator signal from the signal coupler, and being adapted to generate an in-phase signal, said in-phase signal including an unphase-shifted local oscillator signal, and an in-quadrature signal, said in-quadrature signal including a phase-shifted Doppler frequency-shifted signal and a further phase-shifted local oscillator signal,

the signal mixing assembly including a waveguide system adapted to guide the laser beam, the local oscillator signal, the various output signals and the Doppler frequency-shifted signal therealong; and

a signal processing system adapted to Fourier transform the in-phase and in-quadrature signals.

4. The signal mixing assembly as set forth in claim 3 wherein the plurality of signal phase shifting devices comprise a plurality of single mode directional couplers in signal communication by a plurality of waveguides.

5. The signal mixing assembly as set forth in claim 4 wherein the plurality of directional couplers comprise first, second, third, fourth and fifth directional couplers, and wherein;

the first directional coupler is adapted to receive the local oscillator signal and to generate an unphase-shifted local oscillator signal and a first phase-shifted local oscillator signal;

the second directional coupler is adapted to receive the first phase-shifted local oscillator signal and to generate a further phase-shifted local oscillator;

the third directional coupler is adapted to receive the Doppler frequency-shifted signal and to generate an unphase-shifted Doppler frequency-shifted signal and a phase-shifted Doppler frequency-shifted signal;

the fourth directional coupler is adapted to receive the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal and to generate the in-quadrature signal; and

the fifth directional coupler is adapted to receive the unphase-shifted Doppler frequency-shifted signal and the unphase-shifted local oscillator signal and to generate the in-phase signal.

6. The signal mixing assembly as set forth in claim 3 further comprising a plurality of signal detectors adapted to receive the in-phase and in-quadrature signals and adapted to generate output electrical signals indicative of the intensities of the in-phase and in-quadrature signals.

7. The lidar as set forth in claim 3 wherein the signal transformer comprises a Fourier transformer receptive of the in-phase signal and in-quadrature signal and operative thereby to Fourier transform the complex sum of the in-phase signal and in-quadrature signal.

8. The lidar as set forth in claim 3 further comprising a signal modulator adapted to receive the local oscillator signal and to shift the local oscillator signal to a prescribed frequency.

9. The lidar as set forth in claim 8 wherein the modulator comprises an acousto-optic modulator.

10. A signal mixing system comprising:

a plurality of signal couplers in signal communication by a plurality of waveguides being adapted to receive a Doppler frequency-shifted signal, a local oscillator signal, a phase-shifted Doppler frequency-shifted signal, a phase-shifted local oscillator signal, and a further phase-shifted local oscillator signal, and being adapted to generate an in-phase signal that includes an unphase-shifted local oscillator signal and the phase-shifted Doppler frequency-shifted signals, and an in-quadrature signal that includes the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal; and

a plurality of signal detectors adapted to receive the in-phase and in-quadrature signals.

11. A coherent optical system comprising:

an optical system for transmitting an output signal to an object and receiving thereby a Doppler frequency-shifted signal reflected or backscattered from the object;

a signal mixing assembly receptive of the Doppler frequency-shifted signal and a local oscillator signal generating thereby an in-phase signal and an in-quadrature signal, the signal mixing assembly comprising;

a plurality of signal couplers adapted to receive a Doppler frequency-shifted signal, a local oscillator signal, a phase-shifted Doppler frequency-shifted signal, a phase-shifted local oscillator signal, and a further phase-shifted local oscillator signal, and adapted to generate the in-phase signal, said in-phase signal including an unphase-shifted local oscillator signal and the phase-shifted Doppler frequency-shifted signal, and the in-quadrature signal, said in-quadrature signal including the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal, and

a plurality of signal detectors adapted to receive the in-phase and in-quadrature signals; and

a signal processing system adapted to Fourier transform the in-phase and in-quadrature signals.

12. A signal mixing system comprising:

an array of signal phase shifting devices adapted to receive a Doppler frequency-shifted signal and a local oscillator signal, and adapted to generate thereby;

an in-phase signal that includes an unphase shifted local oscillator signal and a phase shifted Doppler frequency-shifted signal; and

an in-quadrature signal that includes the phase shifted Doppler frequency-shifted signal and a further phase-shifted local oscillator signal;

wherein the array of phase shifting devices comprise a plurality of single mode directional couplers in signal communication by a plurality of waveguides.

13. The signal mixing system as set forth in claim 12 wherein the array of signal phase shifting devices comprise a plurality of single mode directional couplers in signal communication by a plurality of waveguides.

14. The signal mixing system as set forth in claim 13 wherein the plurality of directional couplers comprise first, second, third, fourth and fifth directional couplers, and wherein:

the first directional coupler is adapted to receive the local oscillator signal and to generate an unphase-shifted local oscillator signal and a first phase-shifted local oscillator signal;

the second directional coupler is adapted to receive the first phase-shifted local oscillator signal and to generate a further phase-shifted local oscillator;

the third directional coupler is adapted to receive the Doppler frequency-shifted signal and to generate an unphase-shifted Doppler frequency-shifted signal and a phase-shifted Doppler frequency-shifted signal;

the fourth directional coupler is adapted to receive the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal and to generate the in-quadrature signal; and

the fifth directional coupler is adapted to receive the unphase-shifted Doppler frequency-shifted signal and the unphase-shifted local oscillator signal and to generate the in-phase signal.

15. The signal mixing system as set forth in claim 12 further comprising a plurality of signal detectors adapted to receive the in-phase and in-quadrature signals and adapted to generate output electrical signals indicative of the intensities of the in-phase and in-quadrature signals.

16. The signal mixing system as set forth in claim 14 wherein the phase-shifted local oscillator signal and the unphase-shifted local oscillator signal have an amplitude ratio of 2:

1.

17. The signal mixing system as set forth in claim 14 wherein the second directional coupler comprises a quarter wave retarder.

18. The signal mixing system as set forth in claim 14 further comprising:

a first amplifier receptive of the Doppler frequency-shifted signal for amplification thereof;

a second amplifier receptive of the in-phase signal for the amplification thereof; and

a third amplifier receptive of the in-quadrature signal for amplification thereof.

19. The signal mixing system as set forth in claim 18 wherein the first second and third amplifiers comprise rare earth doped fiber amplifers.

20. The signal mixing system as set forth in claim 14 further comprising:

a first isolator receptive of the Doppler frequency-shifted signal for isolation thereof;

a second isolator receptive of the in-phase signal for the isolation thereof; and

a third isolator receptive of the in-quadrature signal for isolation thereof.

21. A method of generating in-phase and in-quadrature lidar signals, the method comprising:

phase-shifting the Doppler frequency-shifted signal to generate a phase-shifted Doppler frequency-shifted signal and an unphase-shifted Doppler frequency-shifted signal;

phase-shifting the local oscillator signal to generate a phase-shifted local oscillator signal and an unphase-shifted local oscillator signal;

further phase-shifting the phase-shifted local oscillator signal to generate a further phase-shifted local oscillator signal;

mixing the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal generating thereby an in-quadrature signal that includes the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal; and

mixing the unphase-shifted Doppler frequency-shifted signal and the unphase-shifted local oscillator signal generating thereby an in-phase signal that includes the unphase-shifted local oscillator signal and the unphase-shifted Doppler frequency-shifted signal.

22. The method as set forth in claim 21 wherein phase shifting the Doppler frequency-shifted signal comprises phase shifting the Doppler frequency-shifted signal by ninety degrees.

23. The method as set forth in claim 21 wherein;

phase-shifting the local oscillator signal comprises phase-shifting the local oscillator signal by ninety degrees; and

further phase-shifting the local oscillator signal comprises phase-shifting the local oscillator signal by one hundred and eighty degrees.

24. A method of determining the velocity of an object, the method comprising:

generating an output signal directed at the object;

receiving a Doppler frequency-shifted signal reflected or backscattered from the object;

generating a local oscillator signal;

based upon the received Doppler frequency-shifted signal and the local oscillator signal, generating an in-phase signal;

based upon the received Doppler frequency-shifted signal and the local oscillator signal, generating an in-quadrature signal;

summing the in-phase signal and the in-quadrature signal;

Fourier transforming the summation of the in-phase signal and the in-quadrature signal; and

yielding a computational result from the Fourier transform representative of both the velocity magnitude and the velocity direction.

25. The method as set forth in claim 24 wherein Fourier transforming the summation of the in-phase signal and the in-quadrature signal comprises Fourier transforming the summation of the in-phase signal and the in-quadrature signal generating thereby a delta function in the frequency domain located at a radian Doppler shifted frequency of +ω

_d for an object moving toward an observer and −

ω

_d for an object moving away from an observer, wherein ω

_d is the Doppler shifted frequency.

26. The method as set forth in claim 25 wherein the velocity of the object is determined based upon the equation ω

_d=−

4π

V/λ

_s, wherein ω

_d is the Doppler shifted frequency, V is the velocity of the object, and λ

_s is the wavelength of the laser source.

27. The method as set forth in claim 24 wherein generating an in-phase signal comprises mixing the Doppler frequency-shifted signal and the local oscillator signal.

28. The method as set forth in claim 24 wherein generating an in-quadrature signal comprises:

phase-shifting the Doppler frequency-shifted signal;

phase-shifting the local oscillator signal;

further phase-shifting the local oscillator signal;

mixing the phase-shifted Doppler frequency-shifted signal and the further phase-shifted local oscillator signal generating thereby an in-quadrature signal that is ninety degrees out of phase with the in-phase signal.

29. The method as set forth in claim 28 wherein phase shifting the Doppler frequency-shifted signal comprises phase shifting the Doppler frequency-shifted signal by ninety degrees.

30. The method as set forth in claim 28 wherein further phase-shifting the local oscillator signal comprises phase-shifting the local oscillator signal by one hundred and eighty degrees.