Passive phasing of fiber amplifiers

• US 20060132903A1
• Filed: 12/20/2004
• Published: 06/22/2006
• Est. Priority Date: 12/20/2004
• Status: Active Grant

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Culver, William Robert, Hendow, Sami T., Nelson, Burke E., Shakir, Sami A.
• Granted Patent: US 7,130,113 B2
• Time in Patent Office: Days
• US Class Current: 359/333
• CPC Class Codes: G02B 27/108 for sampling a portion of a beam or combining a small beam in a larger one, e.g. wherein the area ratio or power ratio of the divided beams significantly differs from unity, without spectral selectivity
• CPC Class Codes: G02B 27/144 using partially transparent surfaces without spectral selectivity G02B27/147 takes precedence
• CPC Class Codes: G02B 6/2817 using reflective elements to split or combine optical signals
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B
• CPC Class Codes: H01S 3/0064 Anti-reflection devices, e.g. optical isolaters absorbing layers for marking or protecting purposes in laser working B23K26/50; magneto-optical non-reciprocal devices G02F1/093, G02F1/0955
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/06758 Tandem amplifiers
• CPC Class Codes: H01S 3/10092 Coherent seed, e.g. injection locking
• CPC Class Codes: H01S 3/1302 by all-optical means, e.g. gain-clamping
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/2383 Parallel arrangements

Abstract

A method passively locks and phases an array of fiber amplifiers in a fiber amplifier system that emits a beam, such as a laser beam. The method locks the fiber amplifiers so that the fiber amplifiers operate at same or similar frequencies. The method samples a small portion of the emitted beam in a far-field around a central lobe on an optical axis and then couples this portion of emitted beam back into the array of fiber amplifiers. The fiber amplifiers may be phased so that the emitted beam concentrates its energy around the central lobe in the far-field. Phasing may be achieved by using an aperture, for example, to restrict the portion of the emitted beam to be coupled back to a restricted region around the optical axis.

Claims

1. A method for passive phasing of fiber amplifiers, the method comprising:

placing an optical device at a far-field of an array of fiber amplifiers;

emitting a beam from the array of fiber amplifiers; and

coupling a first portion of the emitted beam back into the array of fiber amplifiers through a coupling hole, wherein only the first portion of the emitted beam propagates through the coupling hole.

2. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the coupling step includes coupling two to three percent of the emitted beam back into the array of fiber amplifiers.

3. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the coupling step includes coupling four percent of the emitted beam back into the array of fiber amplifiers.

4. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, and wherein the emitting step locks the fiber amplifiers so that the fiber amplifiers operate at similar frequencies.

5. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the array of fiber amplifiers includes a plurality of fiber amplifiers, wherein the emitting step comprises using an array of microlenses connected to the array the fiber amplifiers to emit the beam, and wherein the array of microlenses includes a plurality of microlenses, each microlens corresponding to a single fiber amplifier.

6. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising receiving the first portion of the emitted beam using an array of microlenses connected to the array of fiber amplifiers.

7. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising receiving the first portion of the emitted beam using a fiber splitter connected to the array of fiber amplifiers, wherein the fiber splitter is capable of branching one fiber amplifier into a number of fiber amplifiers.

8. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising using an isolator to transmit the first portion of the emitted beam in one direction, wherein the isolator blocks amplified spontaneous emission (ASE) and stimulated Brillouin scattering (SBS).

9. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising using fiber Bragg grating (FBG) reflectors to form double-pass amplifiers.

10. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a mirror in the far-field, and wherein the method further comprises:

forming a telescope with a second mirror; and

outputting a second portion of the emitted beam as a collimated output beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers.

11. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a collimating mirror, and wherein the method further comprises placing a transmitting end and a receiving end of the array of fiber amplifiers toward each other to form a ring configuration.

12. The method of claim 11, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising:

coupling the first portion of the emitted beam to an array of pre-amplifiers, wherein the array of pre-amplifiers amplifies the first portion of the emitted beam before coupling the first portion of the emitted beam to the array of fiber amplifiers; and

outputting a second portion of the emitted beam as a collimated output beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers.

13. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a beamsplitter, and wherein the method further comprises:

separating a transmitting end and a receiving end of the array of fiber amplifiers to form a ring configuration; and

redirecting the first portion of the emitted beam using a tuning grating, wherein the tuning grating is capable of tuning frequencies.

14. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a beamsplitter enclosed in an output head, and wherein the method further comprises:

using a second harmonic generator (SHG) crystal to double a frequency of a second portion of the emitted beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers; and

using a collimator to receive the first portion of the emitted beam to be coupled back into the array of fiber amplifiers.

15. The method of claim 14, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising using a single-mode fiber to transmit the first portion of the emitted beam to a tuning grating, wherein the tuning grating is capable of tuning frequencies.

16. The method of claim 1, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a beamsplitter enclosed in an output head, and wherein the method further comprises:

using a SHG crystal to double a frequency of a second portion of the emitted beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers;

redirecting the first portion of the emitted beam using a tuning grating, wherein the tuning grating is capable of tuning frequencies; and

using a collimator to receive the first portion of the emitted beam to be coupled back into the array of fiber amplifiers.

17. The method of claim 16, wherein the placing step includes placing the optical device around a central lobe on an optical axis of the array of fiber amplifiers, the method further comprising using a single-mode fiber to transmit the first portion of the emitted beam to fiber splitter enclosed in a receiving end, wherein the fiber splitter is capable of branching one fiber amplifier into a number of fiber amplifiers.

18. A system for passive phasing of fiber amplifiers, comprising:

an array of fiber amplifiers including a plurality of fiber amplifiers, the array of fiber amplifiers emitting a beam; and

an optical device placed at a far-field of the array of fiber amplifiers, wherein the optical device couples a first portion of the emitted beam back into the array of fiber amplifiers through a coupling hole, and wherein only the first portion of the emitted beam propagates through the coupling hole.

19. The system of claim 18, wherein the optical device placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the optical device couples two to three percent of the emitted beam back into the array of fiber amplifiers.

20. The system of claim 18, wherein the optical device placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the optical device couples four percent of the emitted beam back into the array of fiber amplifiers.

21. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an isolator that transmits the first portion of the emitted beam in one direction, wherein the isolator blocks amplified spontaneous emission (ASE) and stimulated Brillouin scattering (SBS).

22. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an array of microlenses connected to the array of fiber amplifiers, wherein the array of microlenses includes a plurality of microlenses, each microlens corresponding to a single fiber amplifier.

23. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an array of microlenses that is connected to the array of fiber amplifiers and receives the first portion of the emitted beam

24. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising a fiber splitter that is connected to the array of fiber amplifiers and receives the first portion of the emitted beam, wherein the fiber splitter is capable of branching one fiber amplifier into a number of fiber amplifiers.

25. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising fiber Bragg grating (FBG) reflectors connected to the array of fiber amplifiers to form double-pass amplifiers.

26. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a mirror in the far-field, wherein the system further comprises a second mirror to form a telescope with the mirror in the far-field, wherein the telescope outputs a second portion of the emitted beam as a collimated output beam, and wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers.

27. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a collimating mirror, and wherein the array of fiber amplifiers are configured as a ring with a transmitting end and a receiving end of the array of fiber amplifiers being positioned toward each other to form the ring.

28. The system of claim 27, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an array of pre-amplifiers that amplifies the first portion of the emitted beam before coupling the first portion of the emitted beam to the array of fiber amplifiers.

29. The system of claim 27, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the collimating mirror outputs a second portion of the emitted beam as a collimated output beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers.

30. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a beamsplitter, wherein the array of fiber amplifiers are configured as a ring with a transmitting end and a receiving end of the array of fiber amplifiers being separated from each other to form the ring, and wherein the system further comprises:

a tuning grating that redirects the first portion of the emitted beam, wherein the tuning grating is capable of tuning frequencies.

31. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device is a beamsplitter enclosed in an output head, and wherein the system further comprises:

a second harmonic generator (SHG) crystal capable of doubling a frequency of a second portion of the emitted beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers; and

a collimator that receives the first portion of the emitted beam to be coupled back into the array of fiber amplifiers.

32. The system of claim 31, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising a single-mode fiber that transmits the first portion of the emitted beam to a tuning grating, wherein the tuning grating is capable of tuning frequencies.

33. The system of claim 18, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the optical device includes a beamsplitter enclosed in an output head, and wherein the system further comprises:

a SHG crystal capable of doubling a frequency of a second portion of the emitted beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers;

a tuning grating that redirects the first portion of the emitted beam, wherein the tuning grating is capable of tuning frequencies; and

a collimator that receives the first portion of the emitted beam to be coupled back into the array of fiber amplifiers.

34. The system of claim 33, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising a fiber splitter enclosed in a receiving end, wherein the fiber splitter is capable of branching one fiber amplifier into a number of fiber amplifiers.

35. The system of claim 34, wherein the optical device is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising a single-mode fiber that transmits the first portion of the emitted beam to the fiber splitter.

36. A system for passive phasing of fiber amplifiers, comprising:

an array of fiber amplifiers including a plurality of fiber amplifiers, the array of fiber amplifiers emitting a beam and configured as a ring with a transmitting end and a receiving end of the array of fiber amplifiers being positioned toward each other to form the ring;

a collimating mirror placed at a far-field of the array of fiber amplifiers, wherein the optical device couples a first portion of the emitted beam back into the array of fiber amplifiers through a coupling hole, and wherein only the first portion of the emitted beam propagates through the coupling hole; and

an array of pre-amplifiers that amplifies the first portion of the emitted beam before coupling the first portion of the emitted beam to the array of fiber amplifiers.

37. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein the collimating mirror outputs a second portion of the emitted beam as a collimated output beam, wherein the second portion of the emitted beam is not coupled back into the array of fiber amplifiers.

38. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the collimating mirror couples two to three percent of the emitted beam back into the array of fiber amplifiers.

39. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, wherein only the first portion of the emitted beam that is in-phase propagates through the coupling hole, and wherein the collimating mirror couples four percent of the emitted beam back into the array of fiber amplifiers.

40. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an isolator that transmits the first portion of the emitted beam in one direction, wherein the isolator blocks amplified spontaneous emission (ASE) and stimulated Brillouin scattering (SBS).

41. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising an array of microlenses connected to the array of fiber amplifiers, wherein the array of microlenses includes a plurality of microlenses, each microlens corresponding to a single fiber amplifier.

42. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising a fiber splitter that is connected to the array of fiber amplifiers and receives the first portion of the emitted beam, wherein the fiber splitter is capable of branching one fiber amplifier into a number of fiber amplifiers.

43. The system of claim 36, wherein the collimating mirror is placed around a central lobe on an optical axis of the array of fiber amplifiers, the system further comprising fiber Bragg grating (FBG) reflectors connected to the array of fiber amplifiers to form double-pass amplifiers.