Generating photoacoustic signals using high-peak power pulsed laser diodes

Photoacoustic signals are usually generated using bulky and expensive Q-switched Nd:YAG lasers, with limited scope for varying the pulse repetition frequency, wavelength and pulse width. An alternative would be to use laser diodes as excitation sources; these devices are compact, relatively inexpensive, and available in a wide variety of NIR wavelengths. Their pulse duration and repetition rates can also be varied arbitrarily enabling a wide range of time and frequency domain excitation methods to be employed. The main difficulty to overcome when using laser diodes for pulsed photoacoustic excitation is their low peak power compared to Q-switched lasers. However, the much higher repetition rate of laser diodes (∼ kHz) compared to many Q-switched laser systems (∼ tens of Hz) enables a correspondingly greater number of events to be acquired and signal averaged over a fixed time period. This offers the prospect of significantly increasing the signal-to-noise ratio (SNR) of the detected photoacoustic signal. Choosing the wavelength of the laser diode to be lower than that of the water absorption peak at 940nm, may also provide a significant advantage over a system lasing at 1064nm for measurements in tissue. If the output of a number of laser diodes is combined it then becomes possible, in principle, to obtain a SNR approaching that achievable with a Q-switched laser. It is also suggested that optimising the pulse duration of the laser diode may reduce the effects of frequency-dependent acoustic attenuation in tissue on the photoacoustic signal. To investigate this, a numerical model based on the Poisson solution to the wave equation was developed. To validate the model, a high peak power pulsed laser diode system was built. It was composed of a 905nm stacked array laser diode coupled to an optical fibre and driven by a high current laser diode driver. Measurements of the SNR of photoacoustic signals generated in a purely absorbing medium (ink) were made as a function of pulse duration. This preliminary study shows the potential for using laser diodes as excitation sources for photoacoustic applications in the biomedical field.