A novel concept for a low cost and very efficient system to detect sulphur using near infrared bands is proposed. This concept use customized thick holographic gratings, also referred as Volume Bragg Grating (VBG), as spectral filtering elements. Acting like a wavelength selective mirror, each VBG redirects a chosen spectral band into the corresponding photodiode. The signal integration is performed using built-in custom electronics that amplify and integrate photodiodes output signals. Temporal synchronization of the experiment is performed using a trigger photodiode which receives the non diffracted optical signal (plasma light minus the fraction of each band). The FPGA (field-programmable gated array) of the built-in custom electronics allows a temporal resolution of 10 ns, making this integrated trigger an autonomous, simple and efficient device for time-resolved measurements necessary for the LIBS plasma. This new concept has been preliminary studied for the detection of sulphur in order to evaluate its performances. It is known that the detection of sulphur presents generally some difficulties by optical emission spectroscopy because the strongest lines are in the VUV below 185 nm and are self-absorbed in the air. In our conditions, we used the near infrared lines at 921.287 nm (sulphur signal) and the background at 900 nm.

The prototype uses an entrance slit of 1.25 mm height by 125 μm width, and exhibits an FWHM of 0.25 nm on each band. The diffraction efficiency per band is above 80%, and the spectral band isolation for the prototype is better than 20 dB. Preliminary results taken with an 1/3 Hz, 10 mJ Kigre Nd:YAG laser showed a significant improvement of the LOD of sulphur obtained on compressed avicel/lactose powder samples containing between 1% and 5% of sulphur by comparison to a conventional spectrometer/detector. Direct collection of plasma light using a fiberoptic bundle was used.

This first LIBS multiband sensor dedicated to an application is completely configurable from 400 nm to 2500 nm. Multiple VBS’s with their corresponding photodiode could be added to increase the number of spectral bands for multi elemental analysis. Fiberoptic bundle or free space collection systems could also be customized and coupled to the sensor, making this multiband sensor a sensitive and useful instrument for optical emission spectroscopy.



See figure 1

[1] 1/3 Hz, 10 mJ Kigre Nd: YAG laser
[2] Standard uncoated 250 mm focal lens
[3] Manual X-Y-Z linear stage
[4] Laptop using USB link to the sensor
[5] Prototyped multiband sensor



Comparative measurement taken with a PharmaLIBS 250 having Czerny-Turner configuration and interline readout scientific grade CCD on the same bands shows a LOD of 0.2%. [] See figure 2.