transmission-multiple-signals-through-an-optical-fiber-using

Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the ...

TU Dresden

Doppler global velocimetry (DGV) is considered to be a useful optical measurement tool for acquiring flow velocity fields. Often near-wall measurements are required, which is still challenging due to errors resulting from background scattering and multiple-particle scattering. Since the magnitudes of both errors are unknown so far, they are investigated by scattering simulations and experiments. Multiple-particle scattering mainly causes a stochastic error, which can be reduced by averaging. Contrary to this, background scattering results in a relative systematic error, which is directly proportional to the ratio of the background scattered light power to the total scattered light power. After applying a correction method and optimizing the measurement arrangement, a subsonic flat plate boundary layer was successfully measured achieving a minimum wall distance of 100 μm with a maximum relative error of 6%. The investigations reveal the current capabilities and perspectives of DGV for near-wall measurements.

Scattering effects at near-wall flow measurements using Doppler global velocimetry.

To reduce the noise of machines such as aircraft engines, the development and propagation of sound has to be investigated. Since the applicability of microphones is limited due to their intrusiveness, contactless measurement techniques are required. For this reason, the present study describes an optical method based on the Doppler effect and its application for acoustic particle velocity (APV) measurements. While former APV measurements with Doppler techniques are point measurements, the applied system is capable of simultaneous measurements at multiple points. In its current state, the system provides linear array measurements of one component of the APV demonstrated by multi-tone experiments with tones up to 17 kHz for the first time.

Optical multi-point measurements of the acoustic particle velocity with frequency modulated Doppler global velocimetry.

Spectroscopic methods are established tools for nonintrusive measurements of flow velocity. However, those methods are either restricted by measuring pointwise or with low measurement rates of several hertz. To investigate fast unsteady phenomena, e.g., in sprays, volumetric (3D) measurement techniques with kHz rate are required. For this purpose, a spectroscopic technique is realized with a power amplified, frequency modulated laser and an Mfps high-speed camera. This allows fast continuous planar measurements of the velocity. Volumetric data is finally obtained by slewing the laser light sheet in depth with an oscillating microelectromechanical systems (MEMS) scanner. As a result, volumetric velocity measurements are obtained for 256×128×25 voxels over 14.4  mm×7.2  mm×6.5  mm with a repetition rate of 1 kHz, which allows the investigation of unsteady phenomena in sprays such as transients and local velocity oscillations. The respective measurement capabilities are demonstrated by experiments. Hence, a significant progress regarding the data rate was achieved in spectroscopy by using the Mfps high-speed camera, which enables new application fields such as the analysis of fast unsteady phenomena.

Lock-in spectroscopy employing a high-speed camera and a micro-scanner for volumetric investigations of unsteady flows.

Multimode fibers are attractive for a variety of applications such as communication engineering and biophotonics. However, a major hurdle for the optical transmission through multimode fibers is the inherent mode mixing. Although an image transmission was successfully accomplished using wavefront shaping, the image information was not transmitted individually for each of the independent pixels. We demonstrate a transmission of independent signals using individually shaped wavefronts employing a single segmented spatial light modulator for optical phase conjugation regarding each light signal. Our findings pave the way towards transferring independent signals through strongly scattering media.

Article	Year
Scattering effects at near-wall flow measurements using Doppler global velocimetry. Applied optics\| PubMed ID: 21772394	2011
Optical multi-point measurements of the acoustic particle velocity with frequency modulated Doppler global velocimetry. The Journal of the Acoustical Society of America\| PubMed ID: 23927110	2013
Lock-in spectroscopy employing a high-speed camera and a micro-scanner for volumetric investigations of unsteady flows. Optics letters\| PubMed ID: 25166079	2014
Transmission of independent signals through a multimode fiber using digital optical phase conjugation. Optics express\| PubMed ID: 27410664	2016

Daniel Haufe

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