Abstract

We report laboratory experiments demonstrating a phase-compensating 70-mm-diameter aperture transceiver that comprises a hexagonally close-packed array of seven 23-mm-diameter fiber collimator subapertures. Other than the collimators, the transceiver uses only fiber optics, connected as a master oscillator–multiple amplifier. The master oscillator is a fiber-coupled 1.5-µm diode laser, which is split and fed to 1-W fiber amplifiers before it exits the collimators. To obtain a phase-coherent far field we control each subaperture’s phase by adjusting the current to its amplifier’s pump diodes in a multidither arrangement, maximizing the signal at the receiver. We achieve a diffraction-limited coherent beam combination in the far field that produces 1.4 W of power in the main lobe, in agreement with theory.

© 2005 Optical Society of America

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References

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  1. T. Y. Fan, "Laser beam combining: techniques and prospects," CLEO/IQEC and PhAST Technical Digest on CD-ROM (Optical Society of America, Washington, D.C., 2004), presentation CFL1, Tutorial.
  2. M. Minden, "Coherent coupling of a fiber amplifier array," in Thirteenth Annual Solid State and Diode Laser Technology Review , SSDLTR 2000 Tech. Digest (Air Force Research Laboratory, Albuquerque, N.M., 2000).
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  4. IPG Photonics, Inc., www.ipgphotonics.com.
  5. EG&G Princeton Applied Research, Inc., www. princetonappliedresearch.com.
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    [CrossRef]
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    [CrossRef]
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    [CrossRef] [PubMed]
  10. J. E. Craig and W. C. Rose, "Laser propagation from airborne platforms: a review of aero-optics scaling," paper AIAA-85-1628, presented at the AIAA 18th Fluid Dynamics and Plasmadynamics and Lasers Conference, Cincinnati, Ohio, 16-18 July, 1985 (American Institute for Aeronautics and Astronautics, Reston, Va., 1985), pp. 1-11.
  11. D. Kelsall and R. D'Amato, "AFWL/OPTICS optical degradation by aerodynamic boundary layers," Rep. ESD-TR-78-243, MIT Lincoln Laboratory, Cambridge, Mass., September 30, 1977.
  12. Keopsys, Inc., www.keopsys.com.
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  14. Indigo, Inc., www.indigosystems.com.
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    [CrossRef]
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    [CrossRef]
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1998 (1)

1987 (1)

1985 (1)

J. E. Craig and C. Allen, "Aero-optical turbulent boundary layer/shear layer experiment on the KC-135 aircraft revisited," Opt. Eng. 24, 446-454 (1985).
[CrossRef]

1977 (1)

1973 (1)

Allen, C.

J. E. Craig and C. Allen, "Aero-optical turbulent boundary layer/shear layer experiment on the KC-135 aircraft revisited," Opt. Eng. 24, 446-454 (1985).
[CrossRef]

Arkwright, J. W.

Atkins, G. R.

Craig , J. E.

J. E. Craig and C. Allen, "Aero-optical turbulent boundary layer/shear layer experiment on the KC-135 aircraft revisited," Opt. Eng. 24, 446-454 (1985).
[CrossRef]

Digonnet, M. J. F.

Elango, P.

Kelsall, D.

O'Meara, T. R.

Smith, E. W.

Whitbread, T.

Appl. Opt. (1)

J. Lightwave Technol. (1)

J. Opt. Soc. Am. (2)

Opt. Eng. (1)

J. E. Craig and C. Allen, "Aero-optical turbulent boundary layer/shear layer experiment on the KC-135 aircraft revisited," Opt. Eng. 24, 446-454 (1985).
[CrossRef]

Other (12)

J. E. Craig and W. C. Rose, "Laser propagation from airborne platforms: a review of aero-optics scaling," paper AIAA-85-1628, presented at the AIAA 18th Fluid Dynamics and Plasmadynamics and Lasers Conference, Cincinnati, Ohio, 16-18 July, 1985 (American Institute for Aeronautics and Astronautics, Reston, Va., 1985), pp. 1-11.

D. Kelsall and R. D'Amato, "AFWL/OPTICS optical degradation by aerodynamic boundary layers," Rep. ESD-TR-78-243, MIT Lincoln Laboratory, Cambridge, Mass., September 30, 1977.

Keopsys, Inc., www.keopsys.com.

LightPath Technologies, Inc., www.lightpath.com.

Indigo, Inc., www.indigosystems.com.

L. J. Otten, A. L. Pavel, W. E. Finley, and W. C. Rose, "A survey of recent atmospheric turbulence measurements from a subsonic aircraft," paper AIAA-81-0298, presented at the AIAA 19th Aerospace Sciences Meeting, St. Louis, MO, 12-15 January, 1981 (American Institute of Aeronautics and Astronautics, Reston, Va., 1981), pp. 1-11.

A. E. Siegman, Lasers (University Science, Mill Valley, Calif., 1986), p. 664.

T. Y. Fan, "Laser beam combining: techniques and prospects," CLEO/IQEC and PhAST Technical Digest on CD-ROM (Optical Society of America, Washington, D.C., 2004), presentation CFL1, Tutorial.

M. Minden, "Coherent coupling of a fiber amplifier array," in Thirteenth Annual Solid State and Diode Laser Technology Review , SSDLTR 2000 Tech. Digest (Air Force Research Laboratory, Albuquerque, N.M., 2000).

J. D. Jackson, Classical Electrodynamics , 2nd ed. (Wiley, New York, 1975), p. 311.

IPG Photonics, Inc., www.ipgphotonics.com.

EG&G Princeton Applied Research, Inc., www. princetonappliedresearch.com.

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Figures (8)

Fig. 1
Fig. 1

Atmospheric turbulence effects: 1, near the transmitter; 2, along the path; and 3, near the receiver. Typical values for the transverse coherence and the time constant are indicated. Issues are described in text.

Fig. 2
Fig. 2

Measured frequency spectrum of the atmospheric-induced intensity fluctuations that occurred for a 1.5-µm signal beam with 6.25-mm waist size (radius) transmitted over the 400-m range at the HRL facility in Malibu, California. The Fourier transforms were calculated by an oscilloscope. At low frequencies the signal was sampled 50 times per second; the average Fourier transform of one hundred 1000-s scans is plotted. For the higher-frequency data the signal was sampled 100,000 times a second, and again one hundred Fourier transforms were averaged.

Fig. 3
Fig. 3

Experimental layout. The inset image is of the near-field pattern of the transmitter array obtained with an infrared CCD camera. It represents the transmitted beam, which consists of near-Gaussian subbeams with 6.25-mm waists, and these subapertures have 23-mm nearest-neighbor spacing as shown. WDMs, wavelength-division modulators.

Fig. 4
Fig. 4

Far-field intensity profiles obtained with an infrared CCD. White curves, intensity transections scanned along the dashed lines. (a) Snapshot of the intensity profile when the phase control is off; the pattern is continuously changing. (b) Phase control on; the pattern remains steady.

Fig. 5
Fig. 5

Oscilloscope traces comparing the signal on the slow detector when the feedback loop was open and closed. The lower, fluctuating signal was recorded with the feedback loop disconnected; the upper signal, with the loop closed. All seven amplifiers were operating.

Fig. 6
Fig. 6

Measurements and theory for integrated far-field intensity pattern as a function of distance from the pattern’s center. Filled circles, measured power transmitted through apertures of the indicated angular sizes (i.e., true power in the bucket); no additional power was measured for apertures even as large as 2.5 cm, corresponding to ∼0.5 mrad. The dashed curve was calculated by numerical integration of the far-field intensity pattern measured by the CCD camera. The solid curve was calculated by integration of the far-field pattern of an array of Gaussians in a analytical simulation of the actual configuration. The knee at approximately 25% power corresponds to the central lobe.

Fig. 7
Fig. 7

Way in which the peak intensity, as measured by the slow photodiode, is enhanced coherently as the number of phased-up beams is increased. The four images at the top are the far-field patterns, with phase control on, when the indicated number of beams were turned on. The filled diamonds are the measured intensity when the indicated number of fiber amplifiers, with phase control, were turned on. The filled circles are the incoherent and coherent sums of the measured photodiode voltages when the amplifiers were turned on individually. The incoherent sum, which is a simple sum of the voltages, is fitted by a straight dashed line, and the coherent sum is fitted by a parabola.

Fig. 8
Fig. 8

Slow photodiode’s output as a function of azimuth angle of the detector. The pictures are far-field patterns grabbed by the CCD when the slow diode was translated horizontally. The circles marked in the pictures indicate the locations of the detector. (Note that the actual size of the detector is much smaller than that of the optical lobes.)

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