Phase Retrieval for Near-field Imaging

PR imaging of engraved metal plates with a scanningTx/Rx 94GHz system.

Shows two examples of 94GHz Phase Retrieval imaging of engraved metal plates, the first with a depth of 1mm that is quite easy to retrieve, and the second for an engraved profile of 4mm that is a little more challenging for Phase Retrieval. Traps in the second case were finally removed by nudging the image phase with a linear tilt. In this case the image of James Clerk Maxwell is engraved onto a 20cm by 20cm plate, scanned at a distance of 15cm from a 94GHz transmitter/receiver horn pair. The simulated measurement is repeated at a second distance of 18cm for the 1mm engraved plate and 20cm for the 4mm engraved plate.

94GHz scanning Tx/Rx example.

In this simulation of a scanning TX/Rx system, near-field Phase Retrieval is used to recover a complex image from two near-field intensity patterns, at distances of 15cm and 20cm from a 30cm square metal plate. The plate has been engraved to a maximum depth of 4mm with a profile that is taken from the grey scale of a photograph. The 94GHz transmitter and receiver are scanned together so the resolution is double. The advantage of such as system would combine the penetration at 94GHz with the equivalent resolution at 186GHz, although the price to be paid is the time taken to scan.

130GHz fixed illumination example.

In this simulation at 130GHz we illuminate a metallic object and record the scattered field intensity at two planes separated by 20cm. The video demonstrates the user begins with a large constraint without making assumptions about the object size. As nothing happens the user reduces the constraint radius. As the object detail appears the user can select a square constraint and reduce its size in accordance with the phase-retrieved image that he is seeing. PTP3/HIO Phase Retrieval algorithm achieves phase lock in this case remarkable easily, given that the original intensity data sets appear nothing like the object. In fact the object turns out to be a gun!

130GHz fixed illumination example.

In this simulation at 130GHz we illuminate a metallic object and record the scattered field intensity at two planes. The video demonstrates the PTP3/HIO Phase Retrieval algorithm achieving phase lock after the user decreases the aperture constraint. In fact the object turns out to be a head!

The above simulation was for a SNR of 60dB, which is probably far to good for a practical system. If the plane separation is increased then the SNR can be reduced to only 30dB, resulting in the following image.

PTP3/HIO simulation for 30dB SNR at 130GHz

Those of you who counted the wavefronts on the phase images above will realize that something strange has happened, since at 130GHz we would expect much more phase over a normal sized metal head. In fact the head model was compressed along the axial direction to just 10% of its physical size. For this case nearly all of the image is in focus.

The following image shows the Phase Retrieval result for a 50% axial compression, where we see that not all of the image is in focus at any one particular plane. Even so, Phase Retrieval does a pretty good job of recovering the phase. How the image is represented is another story.

Phase Retrieval of a 50% compressed Metalic head illuminated by a 130GHz plane wave.

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