This paper describes the design of a unit called Tx Attenuating Photoreceiver (TAP), dedicated to the measuring and dumping of a 2 W beam with 300 mm diameter. This unit is a part of an instrumentation called Stray Light Optical Ground Segment Equipment for the test of the Interferometric Measurement System (IMS) of the LISA mission. The geometry of the instrument is studied to reduce the size of the beam, focus it on a photodiode, dump its power and minimize scattering and reflections back to the IMS. An off-axis parabolic absorber with anti-reflection coating will stop the beam, reflecting only 0.2 % towards a pinhole and a photodiode. Most of the materials and components are commercially available, but the design and size of the parabola are specific.
This work describes a method used to calculate the amount of stray light able to couple to the detectors and create disturbing interference on the measurements in an arbitrarily complex optical system. The method is able to take into account the physical properties (diffraction) of the propagating beams and to translate them into geometrical considerations to calculate the stray light directions that arrive to influence the measurements. The system is in this work applied to the telescope of LISA instrument, that will be used to intercept gravitational waves in space. The method is a fast alternative to optical design softwares to have stray light estimates.
The impact on an optical surface by a micrometeoroid gives rise to a specific type of stray light inherent only in the space optical instruments. This causes a double source of light scattering: the impact crater and the ejected contamination. We propose a method of stray light estimation and apply it to the case of the Laser Interferometer Space Antenna telescope. We estimate the backscattering fraction for nominal (4 years) and extended (10 years) mission durations.
We describe a setup based on Michelson interferometry for coherent measurements of the backscattered light from a low roughness optical surface under test. Special data processing was developed for the extraction of the useful signal from the various stray contributions to the coherent signal. We achieve coherent detection of light scattered by a mirror down to -130 dB in optical power. We characterize the dependence of the backscattered light with spot position and incidence angle. Results of cross-polarization scattering coherent measurements and preliminary results of dust deposition experiment are presented here. This work represents the first step in the experimental evaluation of the coherent perturbation induced by the scattered light in the space gravitational wave detector of the LISA mission.
We present a high performance, low cost, simple setup for long term temperature stabilization of a 2 m optical fiber ring cavity for laser frequency stabilization applications thanks to birefringence of the fiber and its dependence on temperature. The fiber temperature is controlled, at millisecond time scale by LED (light emissive diode) illumination. This allows reaching a temperature stability of 0.1 μK at 100 seconds for the 2 m long PM ring fiber cavity. This is a reduction of the fiber temperature by a factor of 2×105 (from 20 mK to 0.1 μK) and 5×105 (from 300 mK to 0.6 μK), at 100 seconds and at 105 seconds, respectively, with respect to the ambient temperature variations.
A novel laser ranging method is described that uses a two-mode laser source, and detection of the phase of the return beam. The design eliminates the cyclic error usually associated with phase measurements and provides unambiguous, absolute distance determination. Measurements of an ≈ 8m path are obtained at a beat frequency of 13 GHz. We analyse the ≈1 μm stability of the data obtained with this preliminary implementation, and expect that an improved version will allow accuracies well below 1 μm, for the kilometer-scale distances involved in satellite formation flight.
We present a laser ranging system, under development, that uses a high frequency modulated beam to achieve sub-nm resolution by the combined use of interferometric and time-of-flight measurements. We first describe how the absolute distance is extracted from a two-mode interference signal. In particular we show that the signal, which presents both optical and synthetic wavelength scales, is essential to achieve nm-scale accuracy, despite the significant long-term phase drifts in the 20 GHz detection chains. Then we present results obtained with the telemeter implemented on an optical table, for a distance of about four meters, implemented by folding the laser beam path to the target. The challenge here is to achieve a phase and amplitude measurement of two 20 GHz signals with a resolution well below 10-4 cycle and 10-4, respectively, despite the fact that the signal undergoes very strong (×3 ) amplitude changes.
We present two different laser ranging systems under development, both based on the use of a high frequency modulated beam. The first range meter makes no use of interferometry: only the phase of the return beam is detected, in a way that rejects cyclic errors due to optical and electronic crosstalk. An Allan deviation slightly better than 10nm has been obtained with this simple system. The other range meter should provide better resolution, at the expense of a somewhat more sophisticated procedure, as it involves both time-of-flight and interferometry measurements.
Distributed feedback (DFB) diode lasers are convenient, small footprint and robust single mode laser sources. DFB lasers have an emission linewidth in the MHz to several MHz range, which may be too large for some applications, such as cold atom physics, optical clocks, laser ranging, lidar or gas sensing... Control of the diode forward current allows for the control the frequency of the emitted laser beam.
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