Measurements

As a demonstration for the feasibility of the concept and the performance of Fourier gratings, we present a measurement made at a frequency of 490 GHz with the 2-4-2 beams grating mentioned in section IV. We used a low-noise SIS heterodyne receiver [9] whose antenna pattern was reimaged to form a $w=15$ mm beam waist in the center of a reflection grating of a projected size of 60$\times$60 mm. The grating consists of 2$\times$2 unit cells of the type shown in Fig. 5. The reflected and diffracted beam was scanned in the far field with a small chopped liquid nitrogen cooled absorber (Fig. 6).

The 2-4-2 pattern of identical beams is very well reproduced in the measurement, and all other diffraction orders are strongly suppressed. The grating efficiency has been estimated to 0.84$\pm$0.03, by comparing the sum of the signal strengths in the 8 beams to the signal measured with a single beam reflected by a plane mirror at the position of the grating. It agrees very well with the theoretical value. The RMS scatter in the measured peak intensities of the 8 beams is 8%. Most of this scatter is due to an imbalance between the four beams in the middle column and the four outer beams, which indicates a slight alignment error in the measurement setup.

Figure 6: Measured intensity distribution in a 490 GHz beam diffracted by the structure shown in Fig. 5. Contour levels are from 5 to 95% of the peak intensity in steps of 10%. From the extremely low side lobe levels it is obvious that the diffraction efficiency is very high. The white crosses mark the nominal beam grid positions, the black crosses mark the interleaved beam positions of the complementary 2-4-2 subarray, required to produce a combined 4$\times$4 beam array.