Abstract

We demonstrate a novel closed-loop approach for high-precision co-alignment of laser beams in an actively phase-locked, coherently combined fiber laser array. The approach ensures interferometric precision by optically transducing beam-to-beam pointing errors into phase errors on a single detector, which are subsequently nulled by duplication of closed-loop phasing controls. Using this approach, beams from five coherent fiber tips were simultaneously phase-locked and position-locked with sub-micron accuracy. Spatial filtering of the sensed light is shown to extend the control range over multiple beam diameters by recovering spatial coherence despite the lack of far-field beam overlap.

© 2012 OSA

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References

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  1. C. X. Yu and T. Y. Fan, “Beam combining,” in High Power Laser Handbook, H. Injeyan and G. D. Goodno, eds (McGraw Hill, 2011), pp. 533–571.
  2. G. D. Goodno, C. C. Shih, and J. E. Rothenberg, “Perturbative analysis of coherent combining efficiency with mismatched lasers,” Opt. Express 18(24), 25403–25414 (2010).
    [CrossRef] [PubMed]
  3. S. M. Redmond, T. Y. Fan, D. Ripin, P. Thielen, J. Rothenberg, and G. Goodno, “Diffractive beam combining of a 2.5-kW fiber laser array,” in Lasers, Sources, and Related Photonic Devices, OSA Technical Digest (CD) (Optical Society of America, 2012), paper AM3A.1.
  4. T. R. O’Meara, “The multidither principle in adaptive optics,” J. Opt. Soc. Am. 67(3), 306–314 (1977).
    [CrossRef]
  5. T. M. Shay, “Theory of electronically phased coherent beam combination without a reference beam,” Opt. Express 14(25), 12188–12195 (2006).
    [CrossRef] [PubMed]
  6. M. A. Vorontsov and V. P. Sivokon, “Stochastic parallel-gradient-descent technique for high-resolution wave-front phase-distortion correction,” J. Opt. Soc. Am. A 15(10), 2745–2758 (1998).
    [CrossRef]
  7. M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).
  8. S. B. Weiss, M. E. Weber, and G. D. Goodno, “Group delay locking of coherently combined broadband lasers,” Opt. Lett. 37(4), 455–457 (2012).
    [CrossRef] [PubMed]
  9. R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
    [CrossRef]
  10. B. N. Pulford, “LOCSET phase locking: operation, diagnostics, and applications,” Ph.D. dissertation (Univ. of New Mexico, 2011).

2012 (1)

2010 (2)

G. D. Goodno, C. C. Shih, and J. E. Rothenberg, “Perturbative analysis of coherent combining efficiency with mismatched lasers,” Opt. Express 18(24), 25403–25414 (2010).
[CrossRef] [PubMed]

R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
[CrossRef]

2009 (1)

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

2006 (1)

1998 (1)

1977 (1)

Aschenbach, K.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

Beresnev, L. A.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

Bratcher, A.

R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
[CrossRef]

Carhart, G. W.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

Goodno, G. D.

Liu, L.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

O’Meara, T. R.

Rothenberg, J. E.

Shay, T. M.

Shih, C. C.

Sivokon, V. P.

Tiemann, B. G.

R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
[CrossRef]

Uberna, R.

R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
[CrossRef]

Vorontsov, M. A.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

M. A. Vorontsov and V. P. Sivokon, “Stochastic parallel-gradient-descent technique for high-resolution wave-front phase-distortion correction,” J. Opt. Soc. Am. A 15(10), 2745–2758 (1998).
[CrossRef]

Weber, M. E.

Weiss, S. B.

Weyrauch, T.

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

IEEE J. Quantum Electron. (1)

R. Uberna, A. Bratcher, and B. G. Tiemann, “Coherent polarization beam combination,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010).
[CrossRef]

IEEE J. Sel. Topics Quantum Electron. (1)

M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, “Adaptive array of phase-locked fiber collimators: analysis and experimental demonstration,” IEEE J. Sel. Topics Quantum Electron. 15, 269–280 (2009).

J. Opt. Soc. Am. (1)

J. Opt. Soc. Am. A (1)

Opt. Express (2)

Opt. Lett. (1)

Other (3)

B. N. Pulford, “LOCSET phase locking: operation, diagnostics, and applications,” Ph.D. dissertation (Univ. of New Mexico, 2011).

C. X. Yu and T. Y. Fan, “Beam combining,” in High Power Laser Handbook, H. Injeyan and G. D. Goodno, eds (McGraw Hill, 2011), pp. 533–571.

S. M. Redmond, T. Y. Fan, D. Ripin, P. Thielen, J. Rothenberg, and G. Goodno, “Diffractive beam combining of a 2.5-kW fiber laser array,” in Lasers, Sources, and Related Photonic Devices, OSA Technical Digest (CD) (Optical Society of America, 2012), paper AM3A.1.

Supplementary Material (2)

» Media 1: AVI (1477 KB)     
» Media 2: AVI (958 KB)     

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Figures (7)

Fig. 1
Fig. 1

Principle of operation for transducing spatial alignment errors into piston phase errors. The principle applies equivalently for misalignments in either the near field (beam overlap) or far field (beam pointing).

Fig. 2
Fig. 2

Experimental schematic for 5-fiber active positioning control demonstration.

Fig. 5
Fig. 5

Individual and coherently combined far fields, before and after engaging closed-loop tip control. The coherently combined 5-beam far field images are single-frame excerpts from a video recording over the time sequence shown in Fig. 4 (Media 1).

Fig. 3
Fig. 3

Position error signals derived from the X and Y photodetectors upon misaligning a single beam (labeled J2) in the Y-axis.The insets show the spatially filtered near-field beam profiles incident on the detectors. The solid curve in (a) is –4 × the average of the three aligned error signals.

Fig. 4
Fig. 4

Closed loop demonstration of active phase and fiber tip position control. (a) Piston detector signal vs. time, which is proportional to the total CBC power. (b) Extracted error signals for all 8 actuators under control. (c) Fiber tip alignment errors inferred from the calibrated drive commands to the position actuators. The beams are labeled J1…J5, with beam J3 serving as the reference and hence not plotted.

Fig. 6
Fig. 6

Combined 5-beam near-field intensity profiles on the X-detector with varying levels of tip positioning errors between the beams. (a) Well-aligned beams. (b) Beams misaligned as shown in Fig. 5 prior to engaging closed loop. (c) Beams misaligned by up to 5 MFDs (>35 μm) as shown in Fig. 7.

Fig. 7
Fig. 7

Coherently combined 5-beam far fields (a) before and (b) after engaging closed-loop tip control, starting with misalignments corresponding to the near fields shown in Fig. 6(c). Images are single-frame excerpts from a video recording of the loop engagement sequence (Media 2), with intensities renormalized between (a) and (b).

Equations (10)

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ϕ n ( x )= 2π s n x λf
V( φ( x ) )= A | k=1 N I( x ) exp[ i ϕ k ( x ) ] | 2 dx = A I( x ) j,k=1 N exp( i[ ϕ j ( x ) ϕ k ( x ) ] ) dx
V( φ( x )+δφ( t ) )= A I( x ) j,k=1 N exp( i[ ϕ j ( x ) ϕ k ( x ) ]+i[ δ ϕ j ( t )δ ϕ k ( t ) ] ) dx
V( φ( x )+δφ( t ) )= A I( x ) j,k=1 N ( cos[ ϕ j ( x ) ϕ k ( x ) ][ δ ϕ j ( t )δ ϕ k ( t ) ]sin[ ϕ j ( x ) ϕ k ( x ) ] ) dx
lim T [ 1 T 0 T δ ϕ j ( t )dt ]=0
lim T [ 1 T 0 T δ ϕ j ( t )δ ϕ k ( t )dt ]= δϕ 2 δ jk
ε n = lim T [ 1 T 0 T δ ϕ n ( t ) δϕ V( φ( x )+δφ( t ) )dt ]
ε n = lim T [ 1 T 0 T δ ϕ n ( t ) δϕ j,k=1 N [ δ ϕ j ( t )δ ϕ k ( t ) ]( A I( x )sin[ ϕ j ( x ) ϕ k ( x ) ]dx ) dt ]
ε n =2 δϕ A I( x ) k=1 N sin[ 2πx λf ( s n s k ) ] dx
ε nm =2 δϕ A I( x )sin( 2πx λf s m )dx ε m =2( N1 ) δϕ A I( x )sin( 2πx λf s m )dx

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