Another requirement is that the laser output should not be absorbed by the sample to allow obtaining information not only from the surface, but also from below the surface. In the case of meat, two main absorbers dominate the VIS-NIR absorption spectra, the heme pigment and water. In the VIS-NIR region, the minimum in the absorption spectrum is situated between 650 nm and 800 nm. Therefore, a wavelength of 671 nm was selected for this work. The benefit of this wavelength is that the full Raman range up to 4000 cm-1 is covered with high quantum efficiency of silicon-based detectors.
Overcoming the Problem
To overcome these problems, a micro-system based external cavity laser (ECL) with reflection Bragg grating was designed. The side-view scheme of the set-up is given in Figure 2. The whole device was mounted on an AlN micro-optical bench with a length of 13 mm and a width of 4 mm.
As gain medium a broad area diode laser with a stripe width of 30 µm and a length of 2 mm was used. The semiconductor material is comparable to the above-referred lasers, but for the ECL the front facet had a reflectivity of 1% whereas the rear side was antireflection coated with a reflectivity below 5x10-4. This device showed amplified spontaneous emission between 660 nm and 685 nm.
The adjustment of the optics and the grating was performed actively. The optical elements were adjusted with a 6-axes precision alignment system. This is necessary for efficient feedback. A change in the angle position Δα of the RBG in the described laser system of only 0.044° causes a decrease in the optical output power of 50%. After the alignment, the optical components were fixed using UV-curing glue. This technology is based on a space qualified mounted technology developed at the FBH. For our experiments the micro-bench is soldered on a standardized conduction cooled heatsink with a footprint of 25 mm x 25 mm.
The threshold current is /th = 335 mA, the slope efficiency is S = 0.9 W/A. The efficiency is comparable to those of standard broad area lasers and shows that the inset of the micro-optics does not deteriorate the efficiency of the devices. The targeted output power of 200 mW is reached at an excitation current of /200mW = 560 mA and a voltage U200mW = 2.3 V. The electrical power to reach 200 mW is herewith Pel-200mW 1.3 W.
Spectra were recorded with a resolution better than 9 pm. The peak wavelength λ = 671.0 nm is constant within the experimental resolution from the laser threshold up to P = 200 mW. Throughout the entire output power range, 95% of the emitted power lies within a spectral window of 0.1 nm. The suitability of the laser source was proven in Raman spectroscopic experiments for well-known substances such as polyethylene. It was possible to resolve Raman-lines with a line width of 6 cm-¹. Due to the low power consumption, the micro-bench ECL can be implemented into the mobile Raman sensor system.
For more information, visit: How Safe is Your Meat? Spectroscopy Knows
2 B. Sumpf, M. Zorn, M. Maiwald, R. Staske, J. Fricke, P. Ressel, G. Erbert, M. Weyers, G. Tränkle
“5.6 W broad area lasers with a vertical far field angle of 31° emitting at 670 nm”
Photonics Technology Letters 20, 575-577 (2008)
3 L. Glebov
“Fluorinated silicate glass for conventional and holographic optical elements”
Window and Dome Technologies and Materials X, Editor: R. W. Tustison,
Proceedings of SPIE Vol. 6545, 654507, 2007.