Photoacoustic coded excitation using pulse position modulation

Solid state laser systems are commonly employed for photoacoustic imaging, but cheap and handy pulsed laser diodes are an attractive alternative. They emit low pulse energies, but fast averaging is possible due to high achievable repetition rates in order to improve the Signal to Noise ratio (SNR). While averaging is limited by the time of flight of the acoustic signal, photoacoustic coded excitation (PACE) can be used to overcome this limitation. Here, we examine the performance of these PACE codes based on pulse position modulation (PPM). PPM codes rely on varying time distances between successive laser pulses. By varying time differences, the autocorrelation sidelobes of the PPM code, which are a measure for the amount of distortion caused by the coding, can be kept low. For short codes or high sampling rates, low maximum autocorrelation side lobe amplitudes equal to the single pulse amplitude are possible. The gain in SNR was calculated theoretically and compared to previously published sequences. The theoretical results were verified in experiments. PPM codes achieve a coding gain that exceeds that of previously reported codes. For long codes the performance drops. Because the reconstruction is not perfect, side lobes induced by the code can degrade image quality.