laser interferometry

Interferometer setup. The optical components are applied to the base plate 200mm x 200mm via adhesive bonding technology.

Introduction

To support and complement the formulation of various space missions, we ‘ve developed and validated experimentally selected key technologies in the wide field of optical metrology, which is more and more needed for high precision measurements in space. All these  activities have been carry out in  collaboration with EADS Astrium in Friedrichshafen, the HTWG Konstanz and the Humboldt-University Berlin.

In this context, a heterodyne laser interferometer has been developed , which reaches now a verifiable end-to-end noise floor of 2 pm Hz-1/2  in translation measurement and utilizing the method of differential wavefront sensing  a 5 nrad Hz-1/2 in tilt measurement. For the integration of the optical setup of the interferometer a good knowledge of optic, mechanical design and application techniques is required.

Beside the optical setup fast photodiodes are needed in order to detect the laser beams. These quadrant photodiodes detectors have been developed in the LET and they are now being continuously improved and tested. Another enabling technology is the so-called phasemeter. This fast measurement system is based on FPGA (field programmable gate array) technology. It allows rapid digitization of signals coming from the quadrant photodiode detectors and to calculate the essential information.

 

Heterodyne Interferometer

In Astrium’s  `Laboratory for Enabling Technologies' (LET), a heterodyne laser interferometer was developed as technical demonstrator of concepts for the optical readout of the LISA mission. It is mounted on an aluminum breadboard and features a highly symmetrical design a beam height of only 20mm. This first generation heterodyne interferometer enabled measurements within an accuracy of a few picometer/Hz-1/2 (translation) and a few nanoradian/Hz-1/2 (tilt) in a sub-Hertz frequency measurement band.

In the last two years an advanced, second generation (cf. figure IFO) was integrated and tested which incorporates some major improvements. It is now a compact setup on a Zerodur baseplate for optimized thermo-mechanical stability where all optical components are integrated with a specially developed adhesive bonding technology. The interferometer now achieves a contrast higher than 90% on the QPDs, which is more than the needed 80%.

 

Schematic sketch of the beam path. The laser beams are split on a symmetric energy separator cubes (ESC) in two beam pairs. The beams at the frequency f1 passes through a polarizing beam splitter (PBS) and are reflected on the test object and reference mirror. After passing twice the quarter waveplate both beams are reflected by the PBS down to a non-polarzing beamsplitter (BS). Here they are superimposed with the laser beams of the frequency f2 . The heterodyne signals gets detect by two photodetectors.

Quadrant Photodiode Detectors

The developed ultra-low noise quadrant photodiode detector electronic has a bandwidth from DC up to 20MHz. They can be assembled with different types of photodiodes. In 2011 different candidates (made of different semiconductor materials) have been investigated. Especially the spatially resolved sensitivity (cf. figure photodiode) of the quadrant photodiode was investigated and characterized. Therefore a laser measurement setup was designed and set up for spatially resolved sensitivity and response time measurements. It turned out, that some candidates are not able to fulfill the strong  mission requirements, other can and will be part in further investigations in the future.

The picture shows the spatially resolved sensitivity of the quadrant photodiode. The edge effects were expected. The grid structure resulted from the production process of the diode which will now be revised by the manufacturer.

FPGA Phasemeter

A phasemeter is a measurement setup which can measure the phase of a sinusoidal signal. Usually  such a phasemeter is build up by analogue electronics but for space applications it is very attractive to have a digital realization.
The phasemeter developed in the LET is based on FPGA (field programmable gate array) development boards. This can be programmed by use of VHDL (VHSIC hardware description language). The program code is implemented on the FPGA chip hardware -- a reprogramming is possible at any time.
An important aspect of the phasemeter investigating is the interface between analog signal and the FPGA chip, the analog-to-digital converter (ADC). It turned out that the ADC has uncertainties in the sampling time, which affected the performance significantly. This problem can be solved by an additional feed-in of a reference signal (pilot tone). The reference signal is used to detect the uncertainties of the sampling time of the ADC. In post processing the measured phase values can be corrected. After these corrections, the phasemeter fulfills the requirements - it can determine the phase of a sinusoidal signal with a noise level smaller than one part per million of 2ϖ.

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