The measurements are performed with a cryogenic Martin-Pupplet Fourier Transform Spectrometer (FTS) with a 10cm travel. The signal is acquired by a bolometer cooled down to 300mK allowing us to get high resolution and low signal interferograms. Our optical samples are placed in the optical path between the FTS and the bolometer. For each spectrum showed below, many interferograms have been acquired with the sample in the path and another batch of measurements has been made without the sample, for reference. Therefore, we can eliminate the spectral signature of the test bench environment and retrieve the absolute spectral efficiency of our sample.
Thanks to a measurement of the single silicon sheet we use for the Bragg mirrors, we have been able to know precisely its thickness and thus feed the theoretical model with the spectral response of the two Bragg mirrors that we measured on the FTS at 77K Fig. 3.
The measured spectra of those mirrors are really close to the simulation, they have high reflectivity (more than 95%) on a wavelength range wider than 100µm. The numerical simulation plotted in this article are calculated on the model basics of the thin-films theory developed by Abelès [4].
We then measured the response of the scanning FP with those two Bragg mirrors and saved the spectra for four different sizes of cavity in order to get four different peaks of transmission at 310µm, 320µ, 330µm and 340µm. To begin with, this first series of measurements was carried out with the Fabry-Perot kept at room temperature in order to simplify mirror-paralleling operations. The rest of the optical bench was kept at cryogenic temperature. Figure 4 below shows the four spectra obtained with the scanning FP. By changing the cavity size of a known distance, we shift the position of the transmitted peak in accordance with the model.
With the same model used for the Bragg mirrors, we can compare the performance of our interferometer with the theoretical simulation. We see on the Fig. 5 that the experimental measurements fit perfectly with the theory for the four positions.
Therefore, we clearly see that the efficiencies of the transmitted wavelengths are lower than the ones simulated. We summarize the efficiency and the resolution of each peak of FP in the table below, and compare them with the expected performance:
As shown on Fig. 6, the performance of the transmitted peaks are lower than expected. This can be explained by several factors. The most probable here, in addition to the possible small parallelism defects, is that the Bragg stack are a little bit deformed by the way they are assembled on their mounts (some microns of wedge). Even those small deformations have a big impact on the shape of the peak. Here, we cannot precisely fit those peaks with Lorentzian curves, which confirms our hypothesis. Their shape are closer to the case of FP peak with a non-zero angle of incidence.
The next step is to optimize the assembly of the Bragg mirrors in order to reduce the deformations down to less than 1µm of wedge and for the next cold measurements of this Fabry-Perot, the parallelism procedure should be fine-tuned.