The first harmonic as a known source for wavefront correction
It is demonstrated that distortion of a non-linearly generated first harmonic transmit beam due to a near-field aberrator is reduced as transmit pressure is increased. The first harmonic transmit beam is then used as a source for correction of aberration. In the first experiment, pieces of Lucite 11 mm and 24 mm thick were used as near-field aberrators. Beam plots of the fundamental and first harmonic were measured in a water tank with and without the aberrators present at multiple transmit voltages. The Lucite aberrator was then removed and an electronic aberrator with RMS delay error of 138 ns was applied to the transmit and receive apertures. The first harmonic reflected from the tip of a hydrophone was measured, and correcting delays were determined using a multi-lag least-means-squares cross-correlation method. Corrections were applied to an imaging beam transmitted at twice the frequency of the fundamental beam, the same frequency as the generated first-harmonic. Results from the Lucite experiments showed a -6 dB beam width improvement of 1.8 degrees when transmit voltage was increased from 20 volts to 80 volts. Results from first harmonic based correction of the electronic aberrator resulted in significant improvement in beam width and showed an average improvement of 16.8 dB in transmit beam signal level and 31.9 dB improvement in transmit-receive beam signal level. © 2009 SPIE.
