Iodine Frequency Reference on a Sounding Rocket

Abstract

Frequency-stable laser systems are a key technology in precision experiments and have recently become applicable in space. Optical frequency references based on cavities, atoms, or molecules allow for precise laser frequency stabilization and thus enable, for example, high-precision laser ranging in space missions. Examples are the gravity recovery and climate experiment-follow-on (GRACEFO) mission [1,2] that was recently put in operation, the planned laser interferometer gravitational wave observatory LISA [3], and future global satellite navigation systems based on optical clocks [4,5]. In the context of these and other missions, frequency references based on optical cavities [6,7] and unequal-arm-length interferometers [8,9] have been developed as compact, ruggedized instruments for application in space, featuring good short-term instability. One way to also realize low long-term instability is the stabilization to atomic or molecular transitions, providing an advantage in accuracy over relative frequency references. Together with optical frequency combs, such absolute references can further be operated as optical clocks with instability relevant for global navigation satellite systems (GNSS). Several such systems are under *klaus.doeringshoff@physik.hu-berlin.de investigation today at various wavelengths, based on thermal calcium beams [10–12] or hot vapor cells using single- [13] or two-photon transitions in rubidium [14,15]. Another, already matured frequency reference is based on saturation spectroscopy of molecular iodine using the second harmonic of the narrow linewidth Nd:yttriumaluminum- garnet (YAG) laser at 1064 nm. These systems rely on the modulation-transfer spectroscopy (MTS) technique applied to the rovibronic transition R(56)32- 0, featuring narrow transitions with a natural linewidth of 300 kHz [16]. The hyperfine spectrum of this transition was studied in detail [17], the absolute frequency was accurately measured with an uncertainty of 1.1 kHz by Nevsky et al. [18], and Ye et al. demonstrated operation of a molecular iodine optical clock over the course of a full year [19]. In the past decade, several groups realized compact and portable iodine references with fractional frequency instability in the low 10−15 regime [20,21] and subjected their setups to environmental tests [22]. We believe that, using available technology at 1064 nm, developed in the context of the LISA and the GRACE-FO mission and established in satellite laser communication terminals, such systems can be developed for spaceborne instruments on relatively short time scales. Here, we present a stand-alone iodine frequency reference at 1064 nm, named JOKARUS, that is based on a microintegrated extended-cavity diode laser (ECDL) 2331-7019/19/11(5)/054068(9) 054068-1 © 2019 American Physical Society KLAUS DÖRINGSHOFF et al. PHYS. REV. APPLIED 11, 054068 (2019) [23,24] in a master oscillator plus power amplifier (MOPA) configuration. JOKARUS was built to demonstrate the maturity of our technology and its applicability in space missions. To this end, we operate JOKARUS on a sounding rocket mission and thereby prove the autonomous operation of an optical iodine frequency reference.We compare the optical frequency to a chip-scale atomic clock (CSAC) via a self-referenced frequency comb on the ground and in space. This paper is organized as follows. Section II describes the optical iodine frequency reference and its autonomous operation, as well as the optical frequency comb used to verify the frequency instability of the iodine reference aboard the sounding rocket. Section III presents results on the characterization of the frequency instability of the iodine frequency reference obtained on the ground and from the operation in space. Finally, in Sec. IV, we summarize the results and give a conclusion.