Pointing control with fiber nutation

• US 10,298,327 B1
• Filed: 06/08/2018
• Issued: 05/21/2019
• Est. Priority Date: 06/08/2018
• Status: Active Grant

• Current Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Original Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Inventors: Carlson, Robert T, Caseley, Clifford D, Haessig, Jr., David A.
• Primary Examiner(s): Wolf, Darren E
• Application Number:
  US16/003,690
• Time in Patent Office: 347 Days
• Field of Search: None
• CPC Class Codes: H04B 10/11 Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
• CPC Class Codes: H04B 10/112 Line-of-sight transmission over an extended range
• CPC Class Codes: H04B 10/118 specially adapted for satellite communication
• CPC Class Codes: H04B 10/25 Arrangements specific to fibre transmission
• CPC Class Codes: H04B 10/29 Repeaters
• CPC Class Codes: H04B 10/40 Transceivers
• CPC Class Codes: H04B 10/50 Transmitters
• CPC Class Codes: H04B 10/503 Laser transmitters
• CPC Class Codes: H04B 10/60 Receivers

Abstract

A nutation controller and methods of use correct for the small, fixed offset inherent in laser communication systems referred to as pointing bias error, thereby removing the pointing bias error and allowing for more robust and rapid laser communications between distant platforms.

Claims

1. A laser communications system, the system comprising:

a laser-signal receiving module comprising an optical fiber having a proximal end configured to receive laser-light and a distal end configured to transmit laser-light;

a power-in-fiber light sensor in optical communication with the distal end of said optical fiber;

a nutator configured to nutate incoming light onto the proximal end of said optical fiber; and

at least one nutation controller in operative communication with said nutator,

wherein said nutation controller is configured to receive at least two inputs, a nutator displacement angle (ϕ

_N) and an output signal from the power-in-fiber light sensor that corresponds to the strength of a laser signal incident on the proximal end of said optical fiber, and

wherein said nutation controller is configured to cause said nutator to nutate incoming light onto the proximal end of said optical fiber or to nutate the proximal end of said optical fiber itself to maximize the strength of a laser communications signal incident thereon by correcting for pointing bias errors.

2. The system of claim 1 wherein said nutation controller is used to control nutation in the on-elevation axis.

3. The system of claim 1 wherein separate and independent nutation controllers are utilized on each axis on which unwanted motion is present.

4. The system of claim 1 wherein a first nutation controller is used to control nutation in the elevation axis and a second nutation controller is used to control nutation in the cross-elevation axis.

5. The system of claim 1 wherein said nutator is a fast steering mirror.

6. The system of claim 5 wherein the fast steering mirror comprises a fast steering mirror control system that receives a commanded mirror angle.

7. The system of claim 6 wherein the commanded mirror angle is a blended control signal derived from inputs from a base inertial controller and said nutation controller.

8. The system of claim 7 wherein said base inertial controller is an angular rate sensor.

9. The system of claim 1, wherein said nutation controller also produces an output (u_N) that drives a fast steering mirror such that the angle of the mirror relative to the base cancels the base angle (θ

_B), resulting in a line-of-sight (LOS) angle (θ

_L) of zero.

10. The system of claim 9 wherein:

11. The system of claim 9 wherein:

12. The system of claim 9 further comprising a nonlinear filter configured to process the output signal from the power-in-fiber light sensor to derive (θ

_B).

13. The system of claim 12 wherein said non-linear filter is configured to demodulate the power-in-fiber signal using a mixing signal equal to the nutation angular motion, bringing the signal around the nutation frequency to zero, and, subsequently, filter the resulting signal to remove noise from the sensor output and generate an estimate of the base angle state (θ

_B).

14. The system of claim 12 wherein said non-linear filter is selected from the group consisting of Unscented Kalman Filters, Particle Filters, and State Dependent Differential Riccati Equation Filters, wherein said non-linear filter is configured to capture motion corresponding to a base input and model power-in-fiber light sensor output voltage as a function of angle.

15. The system of claim 12 wherein said non-linear filter is an Extended Kalman Filter configured to capture motion corresponding to a base input and model power-in-fiber light sensor output voltage as a function of angle.

16. The system of claim 12 wherein said non-linear filter is configured to capture motion corresponding to a base input and model power-in-fiber light sensor output voltage as a function of angle.

17. The system of claim 16 wherein equations used by the non-linear filter comprise:

{dot over (P)}=AP+PA′

−

PH′

V<sup>−

1</sup>HP+W

K=PH′

V<sup>−

1</sup>

{circumflex over ({dot over (x)})}=A{circumflex over (x)}+K(x−

h({circumflex over (x)},t))

with

18. The system of claim 1 wherein said nutation controller is configured to cause said nutator to move the proximal end of said optical fiber in a circular pattern that causes the nutation displacement angle on each axis to vary sinusoidally in accordance with the following:

ϕ

_NE(t)=A_N cos(2π

f_Nt) on the elevation axis

ϕ

_NX(t)=A_N sin(2π

f_Nt) on the cross-elevation axis.

19. A laser communications system, the system comprising:

a laser-signal receiving module comprising an optical fiber having a proximal end configured to receive laser-light and a distal end configured to transmit laser-light;

a power-in-fiber light sensor in optical communication with the distal end of said optical fiber;

a nutator configured to nutate incoming light onto the proximal end of said optical fiber;

a nutation controller in operative communication with said nutator; and

a nonlinear filter configured to process an output signal from the power-in-fiber light sensor,

wherein said nutation controller is configured to receive at least two inputs, a nutator displacement angle (ϕ

_N) and an output signal from the power-in-fiber light sensor that corresponds to the strength of a laser signal incident on the proximal end of said optical fiber,

wherein said nutation controller is configured to cause said nutator to nutate incoming light onto the proximal end of said optical fiber or to nutate the proximal end of said optical fiber itself to maximize the strength of a laser communications signal incident thereon by correcting for pointing bias errors, and

wherein said nutation controller is configured to utilize the nutator displacement angle (ϕ

_N) as a persistent external excitation.