Clear air turbulence (CAT) repeatedly leads to unexpected turbulence encounters during flights with sometimes severe injuries of passengers. Several flights reportedly experienced CAT encounters leading to injuries and even one death in 2024. In the years since satellite measurement began, an increase of about 50 % in CAT events has been observed from 1979 to 2020 [1] with prospects rising even more [2]. Considering there currently is no direct way to measure CAT during flight it is apparent why control systems capable of alleviating the loads imposed on the aircraft fuselage by CAT increase in relevance. This also includes laser systems with a tailored parameter set.

Tunable diode-laser absorption spectroscopy (TDLAS) sensors have shown to be applicable to e.g. temperature and pressure measurements in gases. These parameters are indispensable in modern avionics. Even though these systems performed well in laboratory or closed environments, the harsh conditions of avionic flight introduce sources of error. To cope with these challenges, altered variants of the classical direct TDLAS may be taken into consideration. Here, we investigate the differences between an all fiber direct TDLAS and a Mach-Zehnder based amplitude modulated TDLAS variant. We are able to demonstrate the increased noise immunity of the amplitude modulated system as well as the use of the oxygen A-band for the use as an optical pressure detector.

Optical air data sensors, which can serve as alternatives or supplements to traditional air data systems, have been a subject of ongoing research. These remote sensing optical sensors offer the advantage of measuring primary air data parameters in undisturbed airflow. As active sensors, they can self-diagnose, associating each measurement with an uncertainty and accounting for signal loss due to factors such as icing or light blocking, thus avoiding silent false measurements. In this paper, we discuss a recent flight test campaign of optical measurement techniques for potential development into air data sensors for future aircraft. We used laser Doppler anemometry, which relies on aerosol scattering of laser light, to observe the Doppler shift of scattered light from multiple directions. This enables the reconstruction of the full wind vector and the retrieval of true air speed, angle of attack, and angle of sideslip. A second instrument used tunable diode laser absorption spectroscopy, which measures an individual transition of molecular oxygen in the A band, in order to extract the static pressure from the observed collisional broadening. The techniques were implemented as airborne research instruments on the DLR’s Dassault Falcon 20 aircraft and tested in two campaigns in 2022 under different atmospheric conditions and dynamic maneuvers. We present results from these campaigns, focusing on general sensor performance, measurement rates, accuracy, and experimental challenges that need to be addressed for future applications.

Using an airborne vector laser Doppler anemometer (LDA) to measure the air flow outside of the boundary layer of an airplane is a promising optical technique. Measurement of the primary flight data like true airspeed, the angle of attack, and the angle of sideslip can be directly derived from the measured wind vector. We developed an experimental system with interchangeable telescopes to study the change in the LDA sensitivity and signal rate, depending on the focusing of the measurement beam. The system has a real-time-capable field programmable gate array data acquisition system, which also can record full data dumps for off-line analysis. This paper presents the first results of an airborne measurement campaign. The true airspeed is measured with an residual error of 1% compared to the five-hole flow sensor. The angle of sideslip and the angle of attack show a standard deviation of the residual error of 0.6∘ for the angle of sideslip and 0.2∘ for the angle of attack.

We present a parameter optimization of an interleaved streamed fast Fourier transform power spectral trigger engine for laser Doppler anemometry. A moving average filter and the Fourier transform length is optimized for particle signals.

We present a modified version of the two-arm, two-color, single second harmonic generation heterodyne dispersion interferometer, as introduced by Irby et. al. [Rev. Sci. Instrum.70, 699 (1999) 10.1063/1.1149489]. The amount of optical elements is reduced and digital in-phase and quadrature demodulation is used to retrieve the phase shift from a single photodetector signal. The intrinsic system noise and drift for this device are analyzed by measuring the Allan deviation. We investigate the use of this device for relative atmospheric pressure measurement. Relative pressure measurements are performed in a pressure chamber and referenced against a piezoresistive pressure transceiver to demonstrate the concept. It was found that the deviation was less than 150 Pa and an error estimation has been derived.

The phase stability and robustness against vibrations of a heterodyne two-arm dispersion interferometer with real-time digital demodulation is investigated. We assess the feasibility of this technique as a contact-free variometer for aerospace applications.

Multi-classification using a convolutional neural network (CNN) is proposed as a denoising method for coherent Doppler wind lidar (CDWL) data. The method is intended to enhance the usable range of a CDWL beyond the atmospheric boundary layer (ABL). The method is implemented and tested in an all-fiber pulsed CWDL system operating at 1550 nm wavelength with 20 kHz repetition rate, 300 ns pulse length and 180 µJ of laser energy. Real-time pre-processing using a field programmable gate array (FPGA) is implemented producing averaged lidar spectrograms. Real-world measurement data is labeled using conventional frequency estimators and mixed with simulated spectrograms for training of the CNN. First results of this methods show that the CNN can outperform conventional frequency estimations substantially in terms of maximum range and delivers reasonable output in very low signal-to-noise (SNR) situations while still delivering accurate results in the high-SNR regime. Comparing the CNN output with radiosonde data shows the feasibility of the proposed method.

We want to apply O 2 tunable diode laser absorption spectroscopy (TDLAS) to the harsh environment around an aircraft as an optical air-data sensor. To optimize stability, we evaluate amplitude modulated TDLAS against the original method.

We have conceptualized a compact UV source with pulse parameters optimized for airborne clear air turbulence detection. An amplified single frequency ns-Nd:YAG source is frequency-tripled to 355 nm, > 2.5W at 3 kHz pulse repetition frequency.

We report on an analytic model of a laser-controlled thermally deformable mirror. The model shows the spatial low pass behavior of such a mirror system regarding the intensity distribution that controls the temperature distribution and the optical phase difference distribution of the deformable mirror. The model is validated using the data of measurements described by Schmid and Mahnke [J. Opt. Soc. Am. B35, 2661 (2018)JOBPDE0740-322410.1364/JOSAB.35.002661].

This paper presents a laser Doppler anemometer with FPGA-based real-time wind vector retrieval using four measurement channels and design considerations for the application as an aircraft-integrated airspeed sensor are discussed.

This paper presents the determination of atmospheric pressure by hmable diode laser spectroscopy of absorption lines in the oxygen A-band. As a major parameter the linewidth of single absorption lines is evaluated.

Laser Doppler anemometry (LDA), sometimes also referred to as cw Doppler lidar, is a promising optical technique for the measurement of primary flight data such as true air speed, angle of attack, and angle of sideslip. Multiple flight campaigns have shown the potential of this technique for future air data systems. However, LDA remains challenging in low aerosol environments at cruise altitude, and the high raw-data bandwidth of LDA sensors impedes real-time operation. In this work, we present a real-time preprocessing scheme, which is optimized to handle the signal structure of single-particle scattering events. We discuss how this scheme is implemented on system-on-chip field-programmable-gate-array hardware and how it is used with a four channel laser Doppler anemometer.

We report on a full characterization of a laser Doppler anemometer to evaluate an end-to-end model. This model is used to assess the design and predict the performance of an airborne airspeed detector. © 2019 The Author(s)

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text P. Mahnke, "Optical Parametric Oscillator with intra cavity sum frequency mixing of alternating signal and idler radiation for generating tunable UV ns impulses," in Laser Congress 2019 (ASSL, LAC, LS&C), OSA Technical Digest (Optica Publishing Group, 2019), paper JW2A.16. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We report on a scalable data processing scheme of a multi-channel laser Doppler anemometer for the application as an optical air data sensor. The pipelined continuous processing enables dead time free measurements. © 2019 The Author(s)

In this paper we present a full characterization of a fiber-coupled laser Doppler anemometer, a model describing the different influences of the optical setup, and measurement evaluation on the signal-to-noise ratio model capable of describing the statistical nature of such a measurement system. The characterization is done using a novel method to measure an instrument function of the laser Doppler anemometer: this is performed by reproducibly moving well-defined scattering particles on controlled trajectories through the detection system.

We report on a thermally deformable mirror to compensate for aberrations in high-power laser systems, suitable for intracavity and extracavity implementation in laser oscillators and laser amplifiers. The adaptive mirror consists of a thin polished edge filter which is mounted on a heat sink. Local deformations of the adaptive mirror are yielded by absorbing the emission of a so-called " heating laser" in the rear-side cooled mirror substrate. Thus, imaging suitable intensity profiles on the mirror enable us to perform laterally highly resolved profiles of optical path differences (OPD) to compensate systematically for phase aberrations. For this, a digital light processing (DLP) micromirror array of a commercial projector, irradiated by a top-hat shaped " heating beam," is mapped on the adaptive mirror. Achievable deformations of the adaptive mirror have been measured using a Shack Hartmann wavefront sensor. A high lateral resolution of 0.1 mu m/mm radius and a maximal OPD of 0.569 mu m could be demonstrated successfully. (c) 2018 Optical Society of America.