Coded excitation for photoacoustic imaging using a high-speed diode laser.

A Q-switched Nd:YAG laser providing nanosecond pulse durations and millijoule pulse energies is suitable for typical biomedical PA applications. However, such lasers are both bulky and expensive. An alternative method is to use a diode laser, which can achieve a higher pulse repetition frequency. Although the energy from a diode laser is generally too low for effective PA generation, this can be remedied by using high-speed coded laser pulses, with the signal intensity of the received signal being enhanced by pulse compression. In this study we tested a version of this method that employs coded excitation. A 20-MHz PA transducer was used for backward-mode PA detection. A frequency-coded PA signal was generated by tuning the interval between two adjacent laser pulses. The experimental results showed that this methodology improved the signal-to-noise ratio of the decoded PA signal by up to 19.3 dB, although high range side lobes were also present. These side lobes could be reduced by optimizing the compression filter. In contrast to the Golay codes proposed in the literature, the proposed coded excitation requires only a single stimulus.