Fiber active path length synchronization

• US 8,503,070 B1
• Filed: 05/24/2011
• Issued: 08/06/2013
• Est. Priority Date: 05/24/2011
• Status: Active Grant

• Current Assignee: US Department of The Air Force (U.S. Department Of Defense)
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Henry, Leanne J., Shay, Thomas M.
• Primary Examiner(s): BOLDA, ERIC L
• Application Number:
  US13/114,374
• Time in Patent Office: 805 Days
• Field of Search: 359/337, 359/341.1, 359/349
• US Class Current: 359/341.1
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/2383 Parallel arrangements

Abstract

A method of implementing a high-power coherent laser beam combining system in which the output of a master oscillator laser having a linewidth broader than the Stimulated Brillouin Scattering linewidth of the laser signal is split into N signals and fed into an array of N optical fibers. This is a modification of the self-synchronous LOCSET and self-referenced LOCSET phase matching systems in which the optical path length of each optical fiber is matched to less than the signal coherence length of the master oscillator by using a path length matching signal processor to modulate temperature controlled segments of each optical fiber.

Claims

1. A method of implementing a coherent laser beam combining system based on a Self-Synchronous LOCSET system in which the output of a master oscillator laser having a linewidth broader than the Stimulated Brillouin Scattering linewidth is split into N signals and fed into an array of N optical fibers, each being an amplifier leg, that are adjusted to minimize their optical phase and path length differences, the method comprising:

a. feeding said N signals into N temperature controlled spools of passive fiber, wherein the temperature of each of the fiber spools is adjusted to enable expansion and contraction of the optical fiber thereby controlling the length of each passive fiber;

b. phase modulating each of said N signals at a unique RF frequency, wherein the modulation frequencies are selected such that a phase error signal for each of the N array elements can be uniquely isolated;

c. feeding said N array element signals to N optical phase adjusters to enable adjustment of the phase to an array mean approaching zero;

d. optically amplifying the output of said N phase adjusters;

e. feeding said N amplified signals to alignment optics;

f. feeding the N output signals of said alignment optics to a beam sampler, wherein a small portion of the alignment optics output signals is split off and imaged onto a photodetector with the remainder of the alignment optics output signals being passed through the beam sampler to become the final output;

g. directing a portion of an output signal of said photodetector to a signal processor to isolate and extract N unique optical phase error signal amplitudes;

h. directing said N optical phase error signals to said N optical phase adjusters to provide stable negative feedback to said N optical phase adjusters to minimize phase errors between the various amplifier legs; and

i. directing a portion of an output signal of said photodetector to a path length matching signal processor which uses a harmonic of the RF modulation frequencies for each of the N signals to provide an error correction for each of the N temperature controlled fiber spools, whereby the N optical path lengths are matched to a mean array path length producing a high-powered optically-coherent final output signal.

2. The method of implementing a coherent laser beam combining system based on a Self-Referenced LOCSET system in which the output of a master oscillator laser having a linewidth broader than the Stimulated Brillouin Scattering linewidth is split into N−

1 signals and fed into an array of N−

1 optical fibers, each being an amplifier leg, and one signal designated a reference amplifier leg, the method comprising;

a. feeding said N signals into temperature-controlled spools of passive fiber on N−

1 slave legs and one spool of passive fiber which is not temperature controlled on a reference leg wherein the N−

1 temperature controlled fiber spools are adjusted to enable expansion and contraction of the optical fiber thereby controlling the length of each N−

1 passive fibers;

b. phase modulating each of said N−

1 slave signals at a unique RF frequency, wherein the modulation frequencies are selected such that a phase error signal for each of the N−

1 array elements can be uniquely isolated;

c. feeding said N−

1 slave array element signals each having a unique RF frequency to N−

1 optical phase adjusters to enable adjustment of the optical phases to that of the reference;

d. optically amplifying the output of said N−

1 slave phase adjusters on the non-reference legs as well as the output of the non-adjusted reference leg;

e. feeding said N amplified signals to alignment optics;

f. feeding the N output signals of said alignment optics to a beam sampler, wherein a small portion of the alignment optics output signals is split off and imaged onto a photodetector and the remainder of the alignment optics output signals are passed through the beam sampler and become the final output;

g. directing a portion of the an output signal of said photodetector to a signal processor to isolate and extract N−

1 unique optical phase error signal amplitudes for the non reference legs;

h. directing said N−

1 optical phase error signals to said N−

1 optical phase adjusters on said slave legs to provide stable negative feedback to said N−

1 optical phase adjusters to enable optical phase matching between the slave legs and the reference leg; and

i. directing a portion of said output signal of said photodetector to a path length matching signal processor which uses a harmonic of the N−

1 uniquely RF modulated signals on the non-reference legs to drive each of the N−

1 modulated temperature controlled fiber spools, whereby the N−

1 optical path lengths of the non-reference legs are matched to that of the reference leg to produce a high-powered optically-coherent final output signal.

3. The method of implementing a coherent laser beam combining system based on a hybrid Self-Synchronous Self-Referenced LOCSET system in which the output of a master oscillator laser having a linewidth broader than the Stimulated Brillouin Scattering linewidth is split into N signals and fed into an array of N optical fibers, each being an amplifier leg, and one signal designated a reference amplifier leg, the method comprising:

a. feeding said N signals into N−

1 temperature controlled spools of passive fiber designated slave legs and one reference spool of passive fiber that is not temperature controlled designated the reference leg, wherein the N−

1 temperature controlled fiber spools are adjusted to enable expansion and contraction of the optical fiber thereby controlling the length of each N−

1 passive fibers;

b. phase modulating each of said N signals at a unique RF frequency, wherein the modulation frequencies are selected such that a phase error signal for each of the N array elements can be uniquely isolated;

c. feeding said N array element signals to N optical phase adjusters to enable the optical phases to be adjusted to an array mean approaching zero;

d. optically amplifying the output of said N phase adjusters;

e. feeding said N amplified signals to alignment optics;

f. feeding the N output signals of said alignment optics to a beam sampler, wherein a small portion of the alignment optics output signals is split off and imaged onto a photodetector and the remainder of the alignment optics output signals are passed through the beam sampler and become the final output;

g. directing a portion of an output signal of said photodetector to a signal processor to isolate and extract N unique optical phase error signal amplitudes for each of the N sampled signals;

h. directing said N optical phase error signals to said N optical phase adjusters to provide stable negative feedback to said N optical phase adjusters to enable minimization of optical phase errors between array legs; and

i. directing a portion of the output signal of said photodetector to a path length matching signal processor using a harmonic of the N−

1 uniquely RF modulated signals to drive each of the N−

1 modulated temperature controlled fiber spools, whereby the N−

1 optical path lengths of the non-reference legs are matched to that of the reference leg to produce a high-powered optically-coherent final output signal.