Heterodyne Detection of Arcturus at 10.6 µm

Abstract

Heterodyne detection is the best technique for aperture synthesis in the infrared range. In the optical range the technique seems very hard and becomes of great interest only in the infrared, the limitation being the photon noise of the local oscillator^1–3. We have achieved the heterodyne detection of Arcturus (α Bootis) at 10.6 µm in April 1973 during observations at St Michel l'Observatoire (France). The experimental arrangement is shown in Fig 1. It is based on the superimposition of coherent and incoherent beams which is achieved using a servomechanism working at 2.2 µm. The incoherent beam coming from the star is divided by the beam splitter No. 1 into two parts, the first with λ<3 µm goes to the servomechanism, and the second with λ> µm is detected by heterodyning. The position of the star image is controlled by a PbS cell working at 2.2 µm, and the correction signal is sent to the piezo-electric quartz shown in Fig. 1. The mixing of coherent and incoherent beams is by the beam splitter No. 2. This beam splitter is coated so as to reflect 95% of the star beam and to transmit 5% of the laser beam.