Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

Abstract

Extension of very long baseline interferometry (VLBI) to observing wavelengths shorter than 1.3mm provides exceptional angular resolution (~20 μas) and access to new spectral regimes for the study of astrophysical phenomena. To maintain phase coherence across a global VLBI array at these wavelengths requires that ultrastable frequency references be used for the heterodyne receivers at all participating telescopes. Hydrogen masers have traditionally been used as VLBI references, but atmospheric turbulence typically limits (sub) millimeter VLBI coherence times to ~1-30 s. Cryogenic Sapphire Oscillators (CSO) have better stability than Hydrogen masers on these time scale and are potential alternatives to masers as VLBI references. Here, We describe the design, implementation and tests of a system to produce a 10 MHz VLBI frequency standard from the microwave (11.2 GHz) output of a CSO. To improve long-term stability of the new reference, the CSO was locked to the timing signal from the Global Positioning System satellites and corrected for the oscillator aging. The long-term performance of the CSO was measured by comparison against a hydrogen maser in the same laboratory. The superb short-term performance, along with the improved long-term performance achieved by conditioning, makes the CSO a suitable reference for VLBI at wavelengths less than 1.3mm.

Gravitational lensing size scales for quasars

“…The predicted angular sizes of these black-hole shadows for SgrA* and M87 based on their measured black-hole masses are ∼ 50 μas and ∼ 35 μas, respectively. Direct imaging has been employed to image the accretion disk of SgrA* with submm Very Long Baseline Interferometry (Doeleman et al 2008(Doeleman et al , 2011.…”

1.5 μm Lasers with Sub-10 mHz Linewidth

“…Oscillators with coherence times of tens to hundreds of seconds will allow for investigations of extremely small energy shifts in the clock transition, caused by sources such as interactions amongst atoms [6,7]. Ultrastable oscillators beyond the state of the art will find useful applications in sub-mm very long baseline interferometry (VLBI) [8], atom interferometry and future atombased gravitational wave detection [9][10][11], novel radar applications [12], the search for dark matter [13], and deep space navigation [14]. Consequently, large effort has been put into the development of extremely coherent sources based on highly stable optical Fabry-Pérot resonators [15][16][17][18].…”

The Impact of Frequency Standards on Coherence in Vlbi at the Highest Frequencies

“…Figure 9 shows the relative impact estimated for improved weather conditions as a result of using WVRs, for the more precise frequency standard, and for the combined effect of WVR and CSO in observations at 350 GHz. We estimate ∼10%-20% improved performance (i.e., reduced coherence loss) for observations under G weather conditions in a range of integration times from ten to a few tens of seconds, the typical coherence times found in VLBI observations at 1.3 mm with H-masers and no WVR corrections (Doeleman et al 2011). Note that longer coherence times are expected as a result of the WVR atmospheric phase compensation and the ultrastable CSO, which result in an increase of the sensitivity.…”