Herriott cell spot imaging increases the performance of tunable laser spectrometers

With the availability of high-power (milliwatts) single-mode tunable laser sources that operate at room temperature across the infrared (IR) region, tunable laser spectrometers have seen an explosion of growth in applications that include commercial, Earth and planetary science, and medical and industrial sensing. While the laser sources themselves have shown steady improvement, the detection architecture of using a single-element detector at one end of a multipass cell has remained unchanged over the last few decades. We present here an innovative new approach using a detector array coupled to an IR-transmissive mirror to image all or part of the multipass spot pattern of the far mirror and record spectra for each pixel. This novel approach offers improved sensitivity, increased dynamic range, laser power normalization, contaminant subtraction, resilience to misalignment, and reduces the instrument power requirement by avoiding the need for “fringe-wash” heaters. With many tens of pixels representing each spot during the laser spectral scan, intensity and optical fringe amplitude and phase information are recorded. This allows selection and manipulation (e.g., co-addition, subtraction) of the pixel output spectra to minimize optical interference fringes thereby increasing sensitivity. We demonstrate a factor of ∼ 20 sensitivity improvement over traditional single-element detection. Dynamic range increase of a factor of ∼ 100 is also demonstrated through spot selection representing different pathlengths. Additionally, subtracting the spectrum of the first spot from that of the higher pass normalizes the laser power and removes the contribution of contaminant gas and fringes in the fore-optics region. These initial results show that this imaging method is particularly advantageous for multi-channel laser spectrometers, and, once the image field is analyzed, pixel selection can be used to minimize data rate and volume collection requirements. This technique could be beneficial to enhanced-cavity detection schemes.