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German Researchers Make Semiconductor Laser To Test Relativity

Diode laser travels on FOKUS rocket to see whether clocks run differently in space

The Ferdinand-Braun-Institut (FBH) in Berlin, Germany, has developed an ultra-precise, compact semiconductor laser  for measuring relativity in space.

On April 23, the micro-integrated Extended Cavity Diode Laser (ECDL) was used for tests on board the German Aerospace Centre (DLR)-funded FOKUS research rocket. The aim of FOKUS is to see whether clocks run differently under space conditions like Einstein claimed by sending various types of clocks into space and back again.

Einstein tells us that clocks run slower the deeper they are in the gravitational potential well of a mass - the closer they are to a planet or star for example. This effect is described by General Relativity as the gravitational red shift - it is detectable in spectral lines that shift toward the red end of the spectrum.

General Relativity Theory also predicts that the rates of all clocks are equally influenced by gravitation independent of how these clocks are physically or technically constructed. However, more recent theories of gravitation allow for the possibility that the type of clock indeed influences the degree of gravitational red shift.

A team of scientists launched an extremely stable quartz oscillator into space that ticks like a modern wrist- watch - but at a very high frequency - together FBH's micro-integrated semiconductor module. The integration of entire laser system took place at Humboldt-Universität zu Berlin (HU Berlin).

The frequency of the semiconductor laser was stabilised by locking it to a specific electron transition of a rubidium atom in an advanced module developed by Universität Hamburg. These rubidium atoms in conjunction with the lasers provide an 'optical atomic clock' that works according to a different physical principle that the quartz clock and 'ticks' about ten million times faster than the quartz unit. To compare how the two clocks run, the company leading the project, Menlo Systems, is employing an optical frequency comb they developed.

The scientists demonstrated with the tests for the first time that these types of 'optical atomic clocks' and the laser systems required for them can be employed in space for testing gravitational red shift and other precision measurements.

The laser module was built at the Ferdinand-Braun-Institut through the Joint Lab Laser Metrology together with the Optical Metrology research group at Humboldt-University (HU Berlin).

FBH's Joint Lab has been investigating and developing ultra-precise and extremely compact semiconductor laser modules for use in space for some time. Their centerpiece is a DFB (distributed feedback) laser that emits light in an extremely narrow wavelength or frequency region.

The narrow spectral band characteristic is one of the main requirements for the laser module needed for spectroscopy of the rubidium atoms and the associated precision measurements. With the help of a hybrid micro-integration technique, the diode laser chip is assembled together with electronic and optical components into a compact, space-certified package. The palm-sized modules have to operate under the harsh conditions of space including the heavy mechanical loading during liftoff when the acceleration rises to eight times' that of Earth's gravity.

"Our integration techniques can even withstand up to 30-times' Earth's gravity," says Andreas Wicht, head of the Laser Metrology Group at FBH. "In addition, we are working on even narrower bandwidth lasers with a hybrid integrated optical amplifier that is highly suited to even more complex experiments."

The demonstration of the technology also allowed them to lay the technical foundations for examining Einstein's equivalence principle using potassium and rubidium atomic interferometers under the MAIUS project. MAIUS is part of the QUANTUS mission funded by DLR in which new technologies involving quantum physics will be developed for cooling, entangling, and manipulating atoms.

It should also advance miniaturising the laser modules and testing a fully automated quantum sensor in space, says FHB. The long-term objective in this case is to examine Einstein's equivalence principle by which the acceleration of a body by a gravitational field is independent of the nature of the body - i. e. all objects subjected to the same gravity "fall at the same speed".

Countless drop-tower experiments at the Centre of Applied Space Technology and Microgravity (ZARM), University of Bremen were used to prepare for the experiment in space. 

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