Experimental Analysis of the Spectral Reflectivity of Metallic Blazed Diffraction Gratings in the THz Range for Space Instrumentation

The core of spectrometers for deep space exploration in the far-infrared spectral range is a diffraction grating optimized for a defined range of wavelengths. This contribution presents an in-depth analysis of the fabrication, morphological characterization, and spectral efficiency verification of this type of gratings operating in the THz range. Two different manufacturing techniques were used: the first one was laser ablation and microstructuring with a five-axis femtosecond laser system, and the second one was a traditional micromachining technique using milling tools. The gratings have a blazed geometry with saw-tooth profiles that enhances the efficiency of the diffracted order of interest, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m=1$</tex-math></inline-formula>, at the TM polarization mode, and within a spectral range between 70 and 114 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m. The morphological features of the fabricated gratings were measured by confocal microscopy and analyzed using topographic parameters. The measured averaged profiles were used to compute the diffraction efficiency of the fabricated gratings and to compare the actual manufactured profiles against the experimental results. Our measurement setup fixes the wavelength of the illuminating source to six values between 60 and 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m (2.5 and 4.7 THz). At each of these spectral lines, we have scanned the angle of incidence between 20<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> and 75<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>. This angular range includes the nominal value of the angle of incidence, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta _\mathrm{inc}=57^\circ$</tex-math></inline-formula>. The experimental values of efficiency can be easily compared with those resulting from computation, where the efficiency is calculated for each one of the available wavelengths as a function of the angle of incidence. This approach has allowed us to validate the design and conclude that gratings fabricated using femtosecond laser ablation perform better than those obtained through micromachining processes. In any case, both manufacturing techniques generate gratings above the validation threshold for diffraction efficiency, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\eta &gt; 0.65$</tex-math></inline-formula>.