81 supra-THz beams generated by a Fourier grating and a quantum cascade laser

Abstract

J-R. (2019). 81 supra-THz beams generated by a Fourier grating and a quantum cascade laser. Optics Express, 27(23), 34193-34204. https://doi.Abstract: Large heterodyne receiver arrays (~100 pixel) allow astronomical instrumentations to map more area within limited space mission lifetime. One challenge is to generate multiple local oscillator (LO) beams. Here, we succeeded in generating 81 beams at 3.86 THz by combining a reflective, metallic Fourier grating with an unidirectional antenna coupled 3rdorder distributed feedback (DFB) quantum cascade laser (QCL). We have measured the diffracted 81 beams by scanning a single pyroelectric detector at a plane, which is in the far field for the diffraction beams. The measured output beam pattern agrees well with a simulated result from COMSOL Multiphysics, with respect to the angular distribution and power distribution among the 81 beams. We also derived the diffraction efficiency to be 94 ± 3%, which is very close to what was simulated for a manufactured Fourier grating (97%). For an array of equal superconducting hot electron bolometer mixers, 64 out of 81 beams can pump the HEB mixers with similar power, resulting in receiver sensitivities within 10%. Such a combination of a Fourier grating and a QCL can create an LO with 100 beams or more, enabling a new generation of large heterodyne arrays for astronomical instrumentation.

A Tunable Unidirectional Source for GUSTO’s Local Oscillator at 4.74 THz

“…The [OI] array for GUSTO includes 2×4 superconducting hot-electron bolometer (HEB) receivers. The required performance metrics for the local oscillator at 4.7 THz were calculated by researchers at SRON and the University of Arizona to ensure the proper performance of HEB receiver array and phase grating integrated with the optical set-up and explained in detail in [13][14][15]. By taking into account the optical loss due to the non-Gaussian beam pattern of THz QCLs, phase grating, and the available cooling power, the overall performance metrics were calculated and listed in table I.…”

3.9 THz spatial filter based on a back-to-back Si-lens system

“…Thus, the efficiency of the spatial filter is enhanced. One can use a lens and an iris aperture to achieve a quasi-Gaussian beam, as our group did in [21], but the efficiency is very low (∼10%). Two off-axis parabolic mirrors combined with a pinhole can also be used as a beam filter with an efficiency of 22% [22], but the system is bulky and the alignment of the whole system is challenging.…”

“…for the Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) and the Stratospheric Observatory for Infrared Astronomy (SOFIA) [18,19]. For heterodyne mixer arrays, phase gratings are used as the multiplexer, which diffracts a single QCL beam into multiple beams as an array LO [20] and recently reported up to 81 beams [21]. To avoid overlap between the individual diffracted beams, a Gaussian input beam to the grating is required [22,23].…”

Terahertz spatial filter based on enhanced optical transmission in the subwavelength aperture

“…Although it has a certain beam shaping effect, the transmissivity of the filtering system is only 22%. One can use a lens and an iris aperture to achieve a quasi-Gaussian beam [4], but the efficiency is very low (~10%). Another terahertz spatial filter is a back-to-back corrugated horn structure, whose principle is to eliminate the high-order Gaussian modes and random fluctuations of a source at microwave frequencies by using the mode conversion characteristic of corrugated horn and antenna reciprocity theorem [5].…”