Conclusions

We have demonstrated in a large volume a general active imaging method based on modulated mixings of many illumination modes that significantly reduces coherent effects (such as speckle). It also eliminates the need for the strategic target orientation typical of many demonstrations of active imaging. The resultant images have many of the favorable characteristics of passive images, but with orders of magnitude greater brightness temperatures.

More specifically we have shown:

1. To achieve a significant reduction in the speckle contrast, the target must be illuminated from many modes, with the speckle reduction factor being $N^{1/2}$, where $N$ is the number of independent modes.
2. Large values of $N (>100)$ can be obtained with simple mode mixing strategies.
3. This approach is made practical by the high brightness temperatures (10¹⁴ - 10¹⁸ K) that can be attained with narrow band electronic illuminators.
4. Because of this high brightness, this approach can be implemented by scattering from surfaces of large volumes (e. g. ~ 100 m in arenas or atriums) or for larger (~ 1 km) and outdoor applications by more directed illumination strategies.
5. The scattering illumination is much more efficient for use with focal plane arrays because the entire target collects a much larger fraction (x 10⁴) of the photons from a scattering surface than does a single pixel.
6. For room temperature operation, the required detector sensitivity is best obtained in narrow bandwidth, heterodyne receivers. The higher powers associated with vacuum electronic sources are capable of supplying the required local oscillator power to large arrays.
7. For the longer ranges and to broaden the range of scenarios by reducing the requirements on appropriate scattering surfaces, the powers associated with vacuum electronics are very useful.