Speckle Noise Reduction by Fiber Scrambling for Improving the Measurement Precision of an Airborne Wind Lidar System

Lux, Oliver; Lemmerz, Christian; Weiler, Fabian; Marksteiner, Uwe; Witschas, Benjamin; Nagel, Engelbert; Reitebuch, Oliver

doi:10.1109/cleoe-eqec.2019.8872955

Cited by 7 publications

(7 citation statements)

References 3 publications

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“…Slow changes in the intensity distribution of the internal reference signal related to variations in laser frequency, polarization, or (ambient) fibre temperature altered the Mie and Rayleigh internal reference response and, in turn, significantly increased the random error. Consequently, a fibre scrambler was implemented into the A2D in preparation of the Cal/Val campaigns to overcome the detrimental speckle noise effect (Lux et al, 2019). As a result, the frequency variations of the internal reference signal were lowered by a factor of 5 (Mie) and 2 (Rayleigh), respectively, while intensity fluctuations were decreased by 55 % (Mie) and 22 % (Rayleigh).…”

Section: Avatarimentioning

confidence: 99%

Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

et al. 2022

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Abstract. The realization of the European Space Agency's Aeolus mission was supported by the long-standing development and field deployment of the Atmospheric LAser Doppler INstrument (ALADIN) Airborne Demonstrator (A2D) which, since the launch of the Aeolus satellite in 2018, has been serving as a key instrument for the validation of ALADIN, the first-ever Doppler wind lidar (DWL) in space. However, the validation capabilities of the A2D are compromised by deficiencies of the dual-channel receiver which, like its spaceborne counterpart, consists of a Rayleigh and a complementary Mie spectrometer for sensing the wind speed from both molecular and particulate backscatter signals, respectively. Whereas the accuracy and precision of the Rayleigh channel is limited by the spectrometer's high alignment sensitivity, especially in the near field of the instrument, large systematic Mie wind errors are caused by aberrations of the interferometer in combination with the temporal overlap of adjacent range gates during signal readout. The two error sources are mitigated by modifications of the A2D wind retrieval algorithm. A novel quality control scheme was implemented, which ensures that only backscatter return signals within a small angular range are further processed. Moreover, Mie wind results with large bias of opposing sign in adjacent range bins are vertically averaged. The resulting improvement of the A2D performance was evaluated in the context of two Aeolus airborne validation campaigns that were conducted between May and September 2019. Comparison of the A2D wind data against a high-accuracy, coherent DWL that was deployed in parallel on board the same aircraft shows that the retrieval refinements considerably decrease the random errors of the A2D line-of-sight (LOS) Rayleigh and Mie winds from about 2.0 to about 1.5 m s−1, demonstrating the capability of such a direct detection DWL. Furthermore, the measurement range of the Rayleigh channel could be largely extended by up to 2 km in the instrument's near field close to the aircraft. The Rayleigh and Mie systematic errors are below 0.5 m s−1 (LOS), hence allowing for an accurate assessment of the Aeolus wind errors during the September campaign. The latter revealed different biases of the Level 2B (L2B) Rayleigh-clear and Mie-cloudy horizontal LOS (HLOS) winds for ascending and descending orbits, as well as random errors of about 3 m s−1 (HLOS) for the Mie and close to 6 m s−1 (HLOS) for the Rayleigh winds, respectively. In addition to the Aeolus error evaluation, the present study discusses the applicability of the developed A2D algorithm modifications to the Aeolus processor, thereby offering prospects for improving the Aeolus wind data quality.

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Section: Avatarimentioning

confidence: 99%

Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

et al. 2022

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“…Moreover, the selected calibration showed the smallest residuals of the fifth-order polynomial fit applied to Rayleigh response curve, thus ensuring the lowest random wind error which may result from discrepancies between the calibration fit function and the actual frequency dependence of the spectrometer response. For the Mie channel, the four calibration results were very consistent which can be traced back to the integration of the fiber scrambler that considerably reduced the speckle noise of the internal reference signal (Lux et al, 2019). Hence, since there were no additional arguments in favour or against a certain calibration, the one with the lowest temperature variability of the Fizeau interferometer was selected for the Mie wind retrieval.…”

Section: Response Calibrationsmentioning

confidence: 68%

“…Utilization of the multimode fiber introduces detrimental speckle noise affecting the precision of the internal reference frequency determination, as explained in Lux et al (2018). Hence, a fiber scrambler was recently integrated between the laser transmitter and the front optics in order to reduce the speckle noise and, in turn, to significantly improve the stability of the internal reference frequency and signal intensity (Lux et al, 2019).…”

Section: The A2d Direct-detection Wind Lidar Systemmentioning

confidence: 99%

Intercomparison of wind observations from ESA’s satellite mission Aeolus and the ALADIN Airborne Demonstrator

Lux¹,

Lemmerz²,

Weiler³

et al. 2020

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Abstract. Shortly after the successful launch of ESA’s wind mission Aeolus, carried out by the European Space Agency, collocated airborne wind lidar observations were performed in Central Europe, employing the prototype of the satellite instrument, the ALADIN Airborne Demonstrator (A2D). Like the direct-detection Doppler wind lidar on-board Aeolus, the A2D is composed of a frequency-stabilised ultra-violet laser, a Cassegrain telescope and a dual-channel receiver to measure line-of-sight (LOS) wind speeds by analysing both Mie and Rayleigh backscatter signals. In the frame of the first airborne validation campaign after the launch still during the commissioning phase of the mission, four coordinated flights along the satellite swath were conducted in late autumn of 2018, yielding wind data in the troposphere with high coverage of the Rayleigh channel. Owing to the different measurement grids and viewing directions of the satellite and airborne instrument, intercomparison with the Aeolus wind product requires adequate averaging as well as conversion of the measured A2D LOS wind speeds to the satellite LOS. The statistical comparison of the two instruments with model wind data from the ECMWF shows biases of the A2D and Aeolus LOS wind speeds of −0.9 m s−1 and +1.6 m s−1, respectively, while the random errors are around 2.5 m s−1. The paper also discusses the influence of different threshold parameters implemented in the comparison algorithm as well as optimization of the A2D vertical sampling to be used in forthcoming validation campaigns.

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“…It is mainly limited by the shot-noise-limited signal-to-noise ratio (SNR) of the Mie signal and the full width at half maximum (FWHM) of the Fizeau interference fringe (≈1.3 pixels). This could be demonstrated with A2D laboratory measurements comparing the spectrometer response from the ALADIN pre-development model Fizeau interferometer and parallel wavemeter measurements using the A2D UV laser (Lux et al, 2019). Hence, frequency fluctuations of the internal path signal can be measured as variations in the calculated Mie response on a pulse-to-pulse basis which allows to assess the frequency stability of the Aeolus laser transmitter over short-and long-term time scales.…”

Section: Utilization Of the Mie Channel As Wavelength Meter In Orbitmentioning

confidence: 98%

ALADIN laser frequency stability and its impact on the Aeolus wind error

Lux

Lemmerz

Weiler

et al. 2021

Preprint

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Abstract. The acquisition of atmospheric wind profiles on a global scale was realized by the launch of the Aeolus satellite, carrying the unique Atmospheric LAser Doppler INstrument (ALADIN), the first Doppler wind lidar in space. One major component of ALADIN is its high-power, ultraviolet (UV) laser transmitter which is based on an injection-seeded, frequency-tripled Nd:YAG laser and fulfills a set of demanding requirements in terms of pulse energy, pulse length, repetition rate as well as spatial and spectral beam properties. In particular, the frequency stability of the laser emission is an essential parameter which determines the performance of the lidar instrument, as the Doppler frequency shifts to be detected are on the order of 108 smaller than the frequency of the emitted UV light. This article reports the assessment of the ALADIN laser frequency stability and its influence on the quality of the Aeolus wind data. Excellent frequency stability with pulse-to-pulse variations of about 10 MHz (root mean square) is evident for over more than two years of operations in space despite the permanent occurrence of short periods with significantly enhanced frequency noise (> 30 MHz). The latter were found to coincide with specific rotation speeds of the satellite's reaction wheels, suggesting that the root cause are micro-vibrations that deteriorate the laser stability on time scales of a few tens of seconds. Analysis of the Aeolus wind error with respect to ECMWF model winds shows that the temporally degraded frequency stability of the ALADIN laser transmitter has only minor influence on the wind data quality on a global scale, which is primarily due to the small percentage of wind measurements for which the frequency fluctuations are considerably enhanced. Hence, although the Mie wind bias is increased by 0.3 m·s−1 at times when the frequency stability is worse than 20 MHz, the small contribution of 4 % from all wind results renders this effect insignificant (

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Speckle Noise Reduction by Fiber Scrambling for Improving the Measurement Precision of an Airborne Wind Lidar System

Cited by 7 publications

References 3 publications

Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

Retrieval improvements for the ALADIN Airborne Demonstrator in support of the Aeolus wind product validation

Intercomparison of wind observations from ESA’s satellite mission Aeolus and the ALADIN Airborne Demonstrator

ALADIN laser frequency stability and its impact on the Aeolus wind error

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