Scientists from the Humboldt-Universität zu Berlin (HU Berlin) and Ferdinand-Braun-Institut (FBH), also in Berlin, Germany, have developed a frequency-stable laser system for use in space. In the JOKARUS experiment (German acronym for iodine comb resonator under weightlessness), an active optical frequency reference based on molecular iodine was qualified for the first time in space.
The laser system is based on a micro-integrated diode laser module built in a master-oscillator power-amplifier (MO-PA) laser architecture, with an extended cavity diode laser (ECDL) as MO and a ridge-waveguide amplifier as PA. The laser light emitted at a wavelength of 1064 nm is delivered via a single-mode, polarization-maintaining fiber to the rest of the setup. A fiber-coupled system of fiber splitter, acousto-optic components, electro-optic components and frequency doublers prepare the laser beam for Doppler-free spectroscopy of ro-vibronic transitions in molecular iodine inside a vapor cell, so as to stabilize the frequency of the laser light.
The diode laser module
“The diode laser module, the centerpiece of the JOKARUS payload, is the first of its kind to be deployed in space,” says Dr Andreas Wicht, head of the Joint Lab Laser Metrology, Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik. The ECDL-MOPA delivers more than 570 mW output power out of the optical fiber and simultaneously exhibits a free-running linewidth of 26 kHz (FWHM, 1 ms measurement time). The laser module comes as a robust and compact package with dimensions 125 x 75 x 22.5 mm and a total mass of 750 g. “This diode laser module is an enabler for future high-precision experiments in space, thanks to its combination of utmost optical performance, an incomparably small size, weight and power (SWaP) budget, and thermo-mechanical stability suitable for space applications,” Wicht says, adding that, furthermore, the spectroscopy module is realized in a highly robust, thermally insensitive optical assembly. “The novel diode laser system, together with dedicated electronics and flight software, contributes to making JOKARUS a fully autonomous optical reference which can operate in harsh environments, even after the stress of a rocket launch.”
First demonstrated of a frequency reference based on molecular iodine in space
“We have demonstrated for the first time a frequency reference based on molecular iodine that can operate in space environment,” confirms Wicht, explaining that iodine-based frequency references are well characterized systems, featuring high frequency stability with comparably low complexity. They can be used as instruments in space missions that require laser-interferometric ranging between satellites or as a local oscillator of a frequency reference deployed in space, for instance, as a replacement for the Galileo microwave clocks with improved performance in orbit. What is more, narrower lines in molecular iodine, for instance at 508 nm, offer the potential for an even more stable iodine frequency reference. The laser wavelength of 1016 nm required in this case can be easily accessed by FBH’s diode laser technology, the scientists say.
Mission accomplished — in harsh environment
The processing of the mission data is currently underway, and it is therefore still too early to provide results, according to the scientists. However, they report that based on first estimations, the autonomous operation of the frequency reference was confirmed: The laser scanned to search for a preselected transition in molecular iodine with a linewidth of 1 MHz in a tuning range of 5 GHz. “In addition, the FBH team can speak about a successful mission where the diode laser module delivered the expected performance under the harsh environment of the sounding rocket flight,” Wicht adds. “Moreover, the frequency comparison with a frequency comb stabilized to a stable microwave oscillator that was flown as part of the FOKUS-II experiment on the same sounding rocket mission will deliver information on the frequency stability.”
A breakthrough in commercial laser-interferometric ranging applications between satellites
What the team from Germany has accomplished for the first time in the JOKARUS experiment could positively impact the future of interferometric distance measurements based on optical technologies. “The diode laser system flown in JOKARUS provides a highly attractive SWaP budget and production costs that are lower than those for competing technologies,” Wicht says. “It has therefore the potential to bring a breakthrough in future commercial laser-interferometric ranging applications between satellites.”
Before the team could report their successful first-time accomplishment of developing and deploying the new diode laser system, the team needed to overcome a number of taxing issues: “A first huge challenge was the development of the diode laser module, more precisely, the materialization of the idea behind the concept of this laser,” Wicht shares. “Most of the components inside this laser module did not exist and needed to be custom designed and fabricated. After that, new techniques for positioning and for bonding of the components had to be developed in order to fulfill the extremely tight positional tolerance — down to 10 nm — required.”
Their next challenge was to find a rugged optical assembly for Doppler-free spectroscopy. “What can usually be achieved by a couple of standard mirror mounts is not applicable on a rocket or in a space mission, due to great demands regarding small size and high mechanical and thermal stability. Colleagues from the University of Bremen found the solution by using an adhesive bonding technology where optical components are directly bonded to a stable glass plate in order to provide the necessary thermo-mechanical stability. Moreover, the autonomy concept required precise control of the laser to stabilize the laser to the selected optical iodine transition. HU Berlin developed an algorithm to automatically “lock” the laser to the correct transition and keep the electronic feedback control system within its working parameters.
The next step
“The next logical step in this particular endeavor would be to deploy such a frequency reference on a satellite for a mission duration ranging in years,” Wicht projects. He says further efforts are required in order to achieve sufficiently long integration times so as to demonstrate the performance of optical frequency references on timescales relevant to global navigation systems or earth observation. “Mission studies for such a scenario already exist,” the expert says. “However, the team of the Joint Lab Laser Metrology is already building diode laser modules and systems for two missions involving Bose-Einstein condensation and atom interferometry on rubidium and potassium atoms on a sounding rocket (MAIUS-2) and on the International Space Station ISS (BECCAL), respectively. In total, more than 50 diode laser modules at wavelengths 760 nm, 767 nm, 780 nm and 1064 nm will be incorporated inside the laser systems for the two missions.”
A joint effort
The JOKARUS payload was developed and implemented under the direction of the Humboldt-Universität zu Berlin (HU Berlin). The Joint Lab Laser Metrology at Ferdinand-Braun-Institut (FBH) contributed the development of the diode laser system. The Joint Lab Laser Metrology is collectively operated by FBH and HU Berlin and combines the know-how of both institutions in the field of diode laser systems for space applications. A quasi-monolithic spectroscopy module was provided by the University of Bremen, the operating electronics came from Menlo Systems.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today