Photoacoustic generation using coded excitation

Photoacoustic (PA) imaging has been used to image soft tissue due to its high contrast and high spatial resolution. The generation of PA signal is based on the object's absorption characteristic to the emitted electromagnetic energy. Typically, a Q-switched Nd:YAG laser providing mJ pulse energy is suitable for biomedical PA applications. However, such laser is relatively bulky and expensive. An alternative way is to use a diode laser. A diode laser can generate laser pulse at much higher pulse repetition frequency (PRF). However, the output power of the diode laser is too low for effective PA generation. One method to overcome this problem is to increase the transmission energy using coded excitation. The coded laser signals can be transmitted by a diode laser with high PRF and the signal intensity of the received signal can be enhanced using pulse compression. In this study, we proposed a chirp coded excitation algorithm for a diode laser. Compared to Golay coded excitation seen in the literature, the proposed chirp coded excitation requires only a single transmission. Chirp-coded PA signal was generated by tuning the pulse duration of individual laser pulses in time domain. Result shows that the PA signal intensity can be enhanced after matched filtering. However, high range side-lobes are still present. The compression filter is an important tool to reduce the range side-lobes, which is subject to further investigation.