Abstract

Coherent Optical Adaptive Techniques (coat) offer promise for overcoming the deleterious effects of phase distortions experienced by optical beams propagating in a turbulent and absorbing atmosphere. An 18-element, visible wavelength, multidither coat system is described. The all solid-state servosystem design was based on the results of an extensive computer simulation. The optical system uses a 0.488-μm argon laser and an array of beam splitters, phase shifters, and beam combiners (termed a phasor matrix) to form the output array. To date, 6- and 8-element linear arrays and an 18-element axisymmetric array have been investigated. The system has demonstrated a convergence time of 1.2 msec and can form the array with a strehl ratio of 0.67. Moving glint tracking, multiple glint discrimination, and offset pointing from a fixed reference have been demonstrated. Good agreement has been observed between measured system results and theoretical predictions.

© 1976 Optical Society of America

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References

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  1. J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.
  2. W. B. Bridges, P. T. Brunner, S. P. Lazzara, T. A. Nussmeier, T. R. O’Meara, J. A. Sanguinet, W. P. Brown, Appl. Opt. 13, 291 (1974).
    [CrossRef] [PubMed]
  3. W. B. Bridges, J. E. Pearson, Appl. Phys. Lett. 26, 539 (1975).
    [CrossRef]
  4. T. R. O’Meara, “Coherent Optical Adaptive Techniques,” Hughes Research Laboratories, Malibu, Calif., Final Tech. Rep. on RADC contract F30602-70-C-0265 (December1970), available through NTIS.
  5. T. R. O’Meara, Hughes Research Laboratories; unpublished.
  6. R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).
  7. W. T. Cathey, C. L. Hayes, W. C. Davis, Appl. Opt. 9, 701 (1970).
    [CrossRef] [PubMed]
  8. W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.
  9. The signal-to-noise ratio, SNR, is defined as the ratio of the dc photomultiplier output voltage to the rms shot noise voltage.
  10. Conversely, if Δf can be increased, the gain can be increased thus reducing the convergence time.
  11. One can reasonably ask why the system does not choose to miss the target altogether in the black-hole tracking mode. In the situation illustrated in Fig. 8(d) the system cannot make such a choice since the white area of the target is larger than the area illuminated by each element individually. The elements themselves cannot steer off the target, so the only minimizing strategy is to phase in such a way as to maximize the intensity on the lowest reflectivity spot available, namely, the black hole. This situation is the true inverse of the bright-glint-against-a-dark-background. However, if the target consisted of a black spot on a white area that was smaller than the region illuminated by the elements, all against a dark background, the system might choose to miss the target altogether and hit the surrounding background. This situation is the inverse of a bright glint or a dark target against a highly reflective background, an unlikely situation that can, indeed, confuse the system. Incidently, if the black-hole mode is used against a simple glint against a dark background, the system will simply phase the array to place a null at the position of the glint.
  12. The strehl ratio is defined as the ratio of the observed peak irradiance to the diffraction-limited peak irradiance.
  13. This simulation includes array and target simulations and vacuum electromagnetic propagation in addition to the servo simulation.
  14. The use of this concept in coat systems was first proposed by T. R. O’Meara.

1975 (1)

W. B. Bridges, J. E. Pearson, Appl. Phys. Lett. 26, 539 (1975).
[CrossRef]

1974 (1)

1971 (1)

R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).

1970 (1)

Bridges, W. B.

W. B. Bridges, J. E. Pearson, Appl. Phys. Lett. 26, 539 (1975).
[CrossRef]

W. B. Bridges, P. T. Brunner, S. P. Lazzara, T. A. Nussmeier, T. R. O’Meara, J. A. Sanguinet, W. P. Brown, Appl. Opt. 13, 291 (1974).
[CrossRef] [PubMed]

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Brown, W. P.

Brunner, P. T.

Cathey, W. T.

Davis, W. C.

Futrelle, R. P.

R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).

Hayes, C. L.

R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).

W. T. Cathey, C. L. Hayes, W. C. Davis, Appl. Opt. 9, 701 (1970).
[CrossRef] [PubMed]

Horwitz, L. S.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

Kubo, R. M.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Lazzara, S. P.

W. B. Bridges, P. T. Brunner, S. P. Lazzara, T. A. Nussmeier, T. R. O’Meara, J. A. Sanguinet, W. P. Brown, Appl. Opt. 13, 291 (1974).
[CrossRef] [PubMed]

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Mevers, G. E.

R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).

Nussmeier, T. A.

O’Meara, T. R.

W. B. Bridges, P. T. Brunner, S. P. Lazzara, T. A. Nussmeier, T. R. O’Meara, J. A. Sanguinet, W. P. Brown, Appl. Opt. 13, 291 (1974).
[CrossRef] [PubMed]

T. R. O’Meara, “Coherent Optical Adaptive Techniques,” Hughes Research Laboratories, Malibu, Calif., Final Tech. Rep. on RADC contract F30602-70-C-0265 (December1970), available through NTIS.

T. R. O’Meara, Hughes Research Laboratories; unpublished.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Ogrodnik, R. F.

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

Pearson, J. E.

W. B. Bridges, J. E. Pearson, Appl. Phys. Lett. 26, 539 (1975).
[CrossRef]

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Sanguinet, J. A.

Walsh, T. J.

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

Appl. Opt. (2)

Appl. Phys. Lett. (1)

W. B. Bridges, J. E. Pearson, Appl. Phys. Lett. 26, 539 (1975).
[CrossRef]

ARPA contract F30602-70-C-0298 (1)

R. P. Futrelle, G. E. Mevers, C. L. Hayes, “Coherent Optical Adaptive Techniques,” ARPA contract F30602-70-C-0298 (April1971).

Other (10)

J. E. Pearson, W. B. Bridges, L. S. Horwitz, T. J. Walsh, R. F. Ogrodnik, in OSA Topical Meeting Digest: Optical Propagation Through Turbulence (Optical Society of America, Washington, D.C., 1974), paper ThB5.

T. R. O’Meara, “Coherent Optical Adaptive Techniques,” Hughes Research Laboratories, Malibu, Calif., Final Tech. Rep. on RADC contract F30602-70-C-0265 (December1970), available through NTIS.

T. R. O’Meara, Hughes Research Laboratories; unpublished.

W. B. Bridges, J. E. Pearson, L. S. Horwitz, R. M. Kubo, S. P. Lazzara, T. R. O’Meara, T. J. Walsh, “Coherent Optical Adaptive Techniques,” RADC Technical Reports RADC-TR-73-384, (July1973); RADC-TR-74-38 (October1973); RADC-TR-74-108 (January1974); RADC-TR-74-187 (April1974); RADC-TR-75-46 (January1975), available through NTIS.

The signal-to-noise ratio, SNR, is defined as the ratio of the dc photomultiplier output voltage to the rms shot noise voltage.

Conversely, if Δf can be increased, the gain can be increased thus reducing the convergence time.

One can reasonably ask why the system does not choose to miss the target altogether in the black-hole tracking mode. In the situation illustrated in Fig. 8(d) the system cannot make such a choice since the white area of the target is larger than the area illuminated by each element individually. The elements themselves cannot steer off the target, so the only minimizing strategy is to phase in such a way as to maximize the intensity on the lowest reflectivity spot available, namely, the black hole. This situation is the true inverse of the bright-glint-against-a-dark-background. However, if the target consisted of a black spot on a white area that was smaller than the region illuminated by the elements, all against a dark background, the system might choose to miss the target altogether and hit the surrounding background. This situation is the inverse of a bright glint or a dark target against a highly reflective background, an unlikely situation that can, indeed, confuse the system. Incidently, if the black-hole mode is used against a simple glint against a dark background, the system will simply phase the array to place a null at the position of the glint.

The strehl ratio is defined as the ratio of the observed peak irradiance to the diffraction-limited peak irradiance.

This simulation includes array and target simulations and vacuum electromagnetic propagation in addition to the servo simulation.

The use of this concept in coat systems was first proposed by T. R. O’Meara.

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Figures (12)

Fig. 1
Fig. 1

Multidither coat system block diagram. The inset shows the two-ring, 18-element array pattern used in most of the experiments.

Fig. 2
Fig. 2

Illustration of how an elemental pattern is removed from the input laser beam, phase shifted, and then returned to the output array by the phasor matrix.

Fig. 3
Fig. 3

Near-field phasor matrix outputs showing the assembled array output (right) and the input beam after the array pattern and alignment marks are removed (left): (a) 1 × 8 linear array; (b) 0–6–12, 18-element array.

Fig. 4
Fig. 4

Typical 18-element system convergence performance in the presence of noise: (a) servo simulation; (b) experimental observation.

Fig. 5
Fig. 5

Multidither coat spectral components. The received signal spectrum contains the propagation phase disturbance spectrum superimposed at zero frequency and on each dither frequency. Also shown are the phase shifter responses and the high- and low-pass filter characteristics.

Fig. 6
Fig. 6

Open-loop and closed-loop coat servo gain and phase. The solid lines are theoretical, the circles are measured open-loop values, and the dots are measured closed-loop values.

Fig. 7
Fig. 7

Final maximum convergence values for an 18-channel multidither servo as a function of dither modulation amplitude and SNR. The error bars indicate fluctuations of the glint power around the average converged level. The points are computer simulation values; the theoretical curve is found from Eq. (2).

Fig. 8
Fig. 8

Beam formation for 18-element transmitter array: (a) coat loop inoperative; (b) coat in glint-tracking mode (glint on central canopy of a plastic model); (c) edge-tracking mode (no glint is present, but beam forms on light/dark boundary); (d) black-hole-tracking mode—control signal phase is changed by 180° from (b).

Fig. 9
Fig. 9

coat-formed beam for a single glint off the boresight axis; (a) theoretical; the different irradiance contours are 1.5 dB apart; (b) experimental.

Fig. 10
Fig. 10

Experimental data showing the effect of open-loop gain on convergence time. The G·τc = 50 curve is a best fit to the data; the constant-product theoretical curve is also predicted by computer simulation.

Fig. 11
Fig. 11

Offset pointing performance with 1 × 6 linear array: (a) mechanical offset using microslewing galvanometer mirrors, sample time = 100 msec; (b) electronic offset using programmed biases on element phase shifters, sample time = 10 msec. Holding time is 400 msec in each case.

Fig. 12
Fig. 12

Moving-glint tracking performance for a 1 × 6 linear array and two different glint speeds. A pinhole detector is placed behind the glint, which moves across the beam.

Tables (1)

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Table 1 Optimum coat Servo Parameters

Equations (2)

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Φ T ( x , y ) = M = 1 N A m F m ( x , y ) ,
maximum coveraged level = N - 1 N J 0 2 ( 2 π A 360 ° ) + 1 N ,

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