Solid state pulsed coherent laser radar for short range velocimetry applications

• US 5,317,376 A
• Filed: 12/03/1992
• Issued: 05/31/1994
• Est. Priority Date: 12/03/1992
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated
• Inventors: Margalit, Eli, Amzajerdian, Farzin
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US07/984,794
• Time in Patent Office: 544 Days
• Field of Search: 356/5, 356/28.5
• US Class Current: 356/28.5
• CPC Class Codes: G01P 5/26 by measuring the direct influence of the streaming fluid on the properties of a detecting optical wave
• CPC Class Codes: G01S 17/58 Velocity or trajectory determination systems; Sense-of-movement determination systems

Abstract

The invention presents a method and apparatus for utilizing a single frequency, self seeding laser pulse signal transmitted to create a target beam to be projected toward an atmospheric target whereby a portion will be reflected back upon itself, and a local oscillator beam constructed to a shape approximating a low level elongated square wave pulse to be heterodyned with the returning target beam. A radio frequency voltage drives an acousto-optic modulator and amplitude modulates the local oscillator beam to form an optical output that is a flat top pulse resembling a square wave function. The local oscillator beam and returning target beam are polarization adjusted to match and are then combined and focused onto detector. The system is effective for use as an aircraft velocimeter.

Claims

1. A single frequency laser system, wherein a high energy pulse signal is generated as an analysis beam and a lower energy flat top pulse is generated as a reference beam, both being generated from a laser source single frequency pulse signal, said laser system capable of providing interferometric type analysis of the two beams when heterodyned, which comprises:

a laser source which generates a single frequency, high energy pulse signal beam plane polarized in a preselected plane;

means for splitting said pulse signal beam simultaneously into an analysis beam and a reference beam;

means for directing said analysis beam to an external target whereby part of said analysis beam energy will become a reflected beam returning back upon itself;

means for reforming said reference beam into a lower energy, flat top signal of a preset time period set to extend for a minimum time period which covers the transit time of said analysis beam to a target medium and back to said laser system;

means for receiving said reflected beam and adjusting its polarization to match the polarization of said reference beam;

means for combining said reference beam and said reflected beam to allow them to be heterodyned together interferometrically;

means for detecting; and

means for measuring preselected properties of said heterodyned beam.

2. A single frequency laser system as presented in claim 1 which further comprises:

means for processing and displaying said measured properties of said heterodyned beam.

3. A single frequency laser system as presented in claim 1 wherein the means for measuring preselected properties measures the Doppler shift between the frequencies of the analysis and reference beams.

4. A single frequency laser system as presented in claim 1 wherein said laser source is a solid state holmium laser.

5. A single frequency laser system as presented in claim 1 wherein said laser source is a solid state thulium, holmium:

YSGG laser.

6. A single frequency laser system, wherein a high energy pulse signal is generated as an analysis beam and a lower energy flat top pulse is generated as a reference beam, both being generated from a laser source single frequency pulse signal, said laser system capable of providing interferometric type analysis of the two beams when heterodyned, which comprises:

a laser source which generates a single frequency, plane polarized, high energy pulse signal beam;

means for splitting said pulse signal beam simultaneously into an analysis beam and a reference beam, said analysis and reference beams holding the plane polarization of said laser source beam;

means for converting said analysis beam to circular polarization and directing it to a partially reflective external target medium whereby part of said analysis beam becomes a reflected beam returning back upon itself,

means for reforming said local oscillator beam pulse into a lower energy, flat top signal of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

means for receiving said reflected beam and adjusting its polarization to match the polarization of said reference beam;

means for combining said reference beam and said reflected beam to allow them to be heterodyned together interferometrically;

means for detecting; and

means for measuring preselected properties of said heterodyned beam.

7. A single frequency laser system as presented in claim 6 which further comprises:

means for processing and displaying said measured properties of said heterodyned beam.

8. A single frequency laser system as presented in claim 6 wherein the means for measuring preselected properties measures the Doppler shift between the frequencies of the analysis and reference beams.

9. A laser radar system which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillator'"'"'s original frequency and of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overtapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically; and

detecting means.

10. A laser radar system according to claim 9 which further comprises:

means for analyzing a signal from said detector.

11. A laser radar system according to claim 10 which further comprises:

means for analyzing a signal from said detector for a Doppler shift between the heterodyned beams.

12. A laser radar system for measuring the air speed of an aircraft which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, atmospheric aerosol medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said local oscillator beam pulse into a lower energy, flat top signal of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically;

detecting means; and

means connected to said detecting means for determining and displaying the relative airspeed of the aircraft.

13. A single frequency laser system as presented in claim 12 wherein said laser source is a solid state holmium laser.

14. A single frequency laser system as presented in claim 12 wherein said laser source is a solid state thulium, holmium:

YSGG laser.

15. A laser radar system for measuring the air speed of an aircraft which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, atmospheric aerosol medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillato'"'"'s original frequency and of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically;

detecting means; and

means for determining and displaying the relative airspeed of the aircraft.

16. A single frequency laser system as presented in claim 15 wherein said laser source is a solid state holmium laser.

17. A single frequency laser system as presented in claim 15 wherein said laser source is a solid state thulium, holmium:

YSGG laser.

18. A laser radar system which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse with pulse width τ

_P ;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said reference beam into a lower energy, flat top signal of a preset time period equal to C τ

_P and set to extend for a time period which covers the transit time of said analysis beam to a target medium and back to said laser system;

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically; and

detecting means.

19. A laser radar system according to claim 18 which further comprises:

means for analyzing a signal from said detector.

20. A laser radar system according to claim 18 which further comprises:

means for analyzing a signal from said detector for a Doppler shift between the heterodyned beams.

21. A laser radar system which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse with pulse width τ

_P ;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillator'"'"'s original frequency and of a preset time period equal to Cτ

_P, said time period set to extend for a time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically; and

detecting means.

22. A laser radar system according to claim 21 which further comprises:

means for analyzing a signal from said detector.

23. A laser radar system according to claim 21 which further comprises:

means for analyzing a signal from said detector for a Doppler shift between the heterodyned beams.

24. A laser radar system which comprises:

a laser source which emits a single frequency, plane polarized, high energy pulse with pulse width τ

_P and an envelope approximating P(t)=P₀ e^-(t/τ

.spsb.P.sup.).spsp.2 where P₀ is the laser peak power and t is time;

means for splitting said high energy pulse emitted by said laser source into a target beam pulse and a local oscillator beam pulse;

means for controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

means for reforming said local oscillator beam pulse into a lower energy, flat top signal approximating the function P_AOM (t)≈

η

[V_RF (t-t_d)]² rP(t) where η

is the AOM deflection efficiency, V_RF is the applied voltage, t_d is the time delay associated with said reforming means, and r is the fraction of the laser energy directed toward said reforming means, and with a frequency shifted a preselected amount from said local oscillator'"'"'s original frequency, and of a preset time period equal to Cτ

_P, said time period set to extend for a time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together,

means for receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

means for combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically; and

detecting means.

25. A laser radar system according to claim 24 which further comprises:

means for analyzing a signal from said detector.

26. A laser radar system according to claim 24 which further comprises:

means for analyzing a signal from said detector for a Doppler shift between the heterodyned beams.

27. A method for a laser radar system which comprises:

generating a single frequency, plane polarized, high energy laser pulse with pulse width τ

_P ;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said reference beam into a lower energy, flat top signal of a preset time period equal to Cτ

_P and set to extend for a time period which covers the transit time of said analysis beam to a target medium and back to said laser system;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse; and

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically.

28. A method as presented in claim 27 which further comprises the step of:

detecting, analyzing and displaying preselected properties of said heterodyned beam.

29. A method for a laser radar system which comprises:

generating a single frequency, plane polarized, high energy laser pulse with pulse width τ

_P ;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said tar-get beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillato'"'"'s original frequency and of a preset time period equal to Cτ

_P, said time period set to extend for a time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse; and

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically.

30. A method as presented in claim 29 which further comprises the step of:

detecting, analyzing and displaying preselected properties of said heterodyned beam.

38. A method as presented in claim 29 which further comprises the step of:

detecting, analyzing and displaying preselected properties of said heterodyned beam.

31. A method for a laser radar system for measuring the air speed of an aircraft which comprises:

generating a single frequency, plane polarized, high energy laser pulse;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, atmospheric aerosol medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said local oscillator beam pulse into a lower energy, flat top signal of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically; and

detecting, determining and displaying the relative airspeed of the aircraft from the heterodyned beam.

32. A method for a laser radar system for measuring the air speed of an aircraft which comprises:

generating a single frequency, plane polarized, high energy laser pulse;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, atmospheric aerosol medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillator'"'"'s original frequency and of a preset time period, said time period set to extend for a minimum time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse;

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically;

detecting, determining and displaying the relative airspeed of the aircraft from the heterodyned beam.

33. A method for laser radar system which comprises:

generating a single frequency, plane polarized, high energy laser pulse with pulse width τ

_P and an envelope approximating P(t)=P₀ e^-(t/τ

.spsb.P.sup.).spsp.2 where P₀ is the laser peak power and t is time;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said local oscillator beam pulse into a lower energy, flat top signal approximating the function P_AOM (t)≈

η

[V_RF (t-t_d)]² rP(t) where η

is the AOM deflection efficiency, V_RF is the applied voltage, t_d is the time delay associated with said reforming means, and r is the fraction of the laser energy directed toward said reforming means, and with a frequency shifted a preselected amount from said local oscillator'"'"'s original frequency, and of a preset time period equal to Cτ

_P, said time period set to extend for a time which ensures that said return beam pulse will be overtapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse; and

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically.

34. A method as presented in claim 33 which further comprises the step of:

detecting, analyzing and displaying preselected properties of said heterodyned beam.

35. A method for a self seeding laser radar system which comprises:

generating a single frequency, plane polarized, high energy laser pulse width pulse width τ

_P ;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said reference beam into a lower energy, flat top signal of a preset time period equal to Cτ

_P and set to extend for a time period which covers the transit time of said analysis beam to a target medium and back to said laser system;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse; and

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically.

36. A method as presented in claim 35 which further comprises the step of:

detecting, analyzing and displaying preselected properties of said heterodyned beam.

37. A method for a self seeding laser radar system which comprises:

generating a single frequency, plane polarized, high energy laser pulse with pulse width τ

_P ;

splitting said high energy laser pulse into a target beam pulse and a local oscillator beam pulse;

controlling the polarization of said target beam pulse and directing it to an external, partially reflective medium whereby part of the energy of said target beam pulse becomes a reflected beam to return to said laser radar system;

reforming said local oscillator beam pulse into a lower energy, flat top signal of a frequency shifted a preselected amount from said local oscillato'"'"'s original frequency and of a preset time period equal to Cτ

_P, said time period set to extend for a time which ensures that said return beam pulse will be overlapped by the reformed local oscillator beam pulse in order that said beams can be heterodyned together;

receiving said reflected beam pulse and adjusting its polarization to match said reformed local oscillator beam pulse; and

combining said local oscillator beam and said reflected beam to allow them to be heterodyned together interferometrically.