iodine clocks

An Ultra-Stable Frequency Reference for Space Applications Based on Molecular Iodine

Several future space missions require optical frequency references with stability in the 10 -15 domain at long integration times. One of the most auspicious methods to obtain space compatibility in a relatively short time is a setup based on Doppler-free spectroscopy using molecular iodine. We built up and tested an assembly on elegant breadboard (EBB) level which provides a frequency stability of 3 10-15 at an integration time of 200 s. Currently, we design a compact, ruggedized and lightweight setup on engineering model (EM) level utilizing a very compact multipass gas cell and plan as well as perform environmental tests to ensure space compatibility.

Introduction

Future space missions related e.g. to fundamental science, navigation, earth observation require ultrastable frequency references, especially in the optical domain. Lasers stabilized to atomic or molecular transitions offer an absolute frequency reference with high long-term frequency stability. Appropriate setups based on Doppler-free spectroscopy offer frequency stability in the 10-15 domain at longer integration times and have the potential to be developed space qualified in a relatively short period of time.

Optical frequency reference on EBB-level

We realized an ultra-stable optical frequency reference on elegant breadboard (EBB) level, which utilizes modulation-transfer spectroscopy of molecular iodine near 532 nm. With respect to the development of a space qualified iodine frequency reference, the setup on EBB level was built up on a base-plate made of
Clearceram-Z HS, a glass ceramics with an ultra-low coefficient of thermal expansion (CTE) of 2 10-8 K-1.
All components including the triple-pass iodine cell with a length of 30cm, the mirrors, polarizers, beamsplitters and fiber-collimators are either directly, or by the use of special Invar mounts, adhesive bonded to the base-plate. This setup allows for high long-term frequency stability due to enhanced pointing stability by using exclusively components with very low CTE and stable, quasi monolithic, mountings. Figure 1 shows a schematic of the spectroscopy module of the frequency reference on elegant breadboard level. All optical elements are fixed to the base-plate using adhesive bonding technology. The dimensions of the Clearceram base-plate are 550mm x 250mm x 50mm. Figure 2 shows a corresponding photograph where the
interaction of laser beam and molecular iodine is visible.

Pump and probe beam are fiber-coupled to the board using polarization maintaining single-mode fibers with pigtailed fiber-collimators (provided by OZ Optics Inc.). Both collimators are attached to ultra-stable Invar mounts which enable the tilt adjustment using shims so that the parallelism of output beam and baseplate can be assured. The output beam diameter is 3\,mm. Polarizers after the fiber output guarantee a clearly defined polarization. Since all optic elements are directly joint to the base-plate we placed four wedged AR-coated glass plates in probe as well as pump beam to create a possibility for adjustment of the
two counter propagating laser beams in the iodine cell. These wedges which are mounted in commercial rotation mounts allow an independent adjustment of both beams in two perpendicular directions. Half wave plates and thin film polarizers (TFP) are used to control the optical power on the detectors (NC reference beam, RAM detector). A reference beam is led to the NC using a thin film polarizer. The other parts of the beams pass half wave plates again so that they can be led to glass plates which split of another part for intensity stabilization. Subsequently, both beams pass the iodine gas cell thrice. The probe beam is out-coupled at a TFP towards a noise-canceling detector for generating the error signal whereas the pump beam is dumped. In order to focus the laser beams onto the photo diodes of the detectors we adhesive bonded fused silica substrates to the base-plate which are provided with lenses.
First frequency stability measurements by beating with an ULE cavity (results given in Allan deviation in Figure 3) were performed by using the laser system and the electronics of the laboratory setup at the Humboldt-University Berlin (HUB).

Outlook

The frequency stability of the EBB setup is currently optimized in a beat measurement setup using a ULE cavity setup at the HUB and limiting factors are analyzed.
A further current activity is the planning, assembling and environmentally testing of a test board where cuboids of different materials (Invar, aluminum, stainless steel and fused silica) are adhesive bonded to a Clearceram base-plate.
Additionally, we advance the cooling system (including Peltier element, heat-pipe and hermetic sealing) especially with regard to mechanical stability, dissipation of thermal losses and mounting concept. We design a setup on engineering model (EM) level, which is further developed with respect to compactness and stability. With this setup, environmental tests (thermal cycling, vibration tests) will be performed.
This work is supported by the German Space Agency DLR with funds provided by the Federal Ministry of Economics and Technology (BMWi) under grant numbers 50QT1201 and 50QT1102.

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