Diode laser reliability in dynamic laser speckle application: Stability and signal to noise ratio

The biospeckle, or dynamic laser speckle, is used as a potential tool to monitor activity in many biological and non-biological materials from agriculture to medicine, and it is usually based on the use of He-Ne and diode lasers, the latter of which has great potential to be embedded in portable equipment. Some queries about the stability of the diode laser were raised, such as the real influence of the mode hopping phenomenon as a drawback in solid-state devices in comparison to the well-known He-Ne lasers, and thus we decided to test it . In addition, we present an alternative way to enhance the stability and the signal-to-noise ratio of the information using a relative index rather than the absolute and single data traditionally provided and analysed. We compared the He-Ne and diode lasers using a power light sensor, and the usual dynamic laser speckle indexes (DLSI). We tested both cases using a clean and inert surface, as well as a drying paint process. To test the relative index, we used two different temporal series of speckle patterns and created a signal-to-noise ratio in dB, using a drying paint process, and in a raw data of a drying paint process associated to a drop of alcohol. The results show that the stability of the diode laser is greater than that of the He-Ne laser in all cases, breaking the paradigm of the stability of He-Ne devices. The signal-to-noise ration showed reliable results when the named dynamic laser speckle relative index (DLSRI) in dB was adopted.