Optimization of the Duty Factor οf Semiconductor Light Sources Used in Photoacoustics

There are two most common approaches to the light intensity modulation that can be found in the papers describing photoacoustic experiments. In the first one, photoacoustic signal is produced by short light pulses of relatively high power, obtained usually from pulse mode lasers. In the other, the light intensity is square or sine modulated with the duty factor of approximately 50%. However, in the case of semiconductor light sources, like LED or laser diodes, it should be taken into account that duty factor of the modulation signal can be (at least in some range) exchanged with the optical output power. The paper presents discussion of the problem, including simplified theoretical analysis and experimental results. The analysis shows that in some cases, if the duty factor of a LED diode is optimized toward maximum amplitude of the induced photoacoustic signal, the resulting signal gain can be greater than 50%. Optimal duty factor depends on characteristics of the particular light source, but at least in the case of LED diodes it will be usually less than the typically used 50%.