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The advanced hyperspectral imager (AHSI) is one of the sensors aboard the Chinese Gaofen-5 (GF-5) satellite, possessing characteristics of high spatial and spectral resolution, as well as width swath. To better understand the radiometric performance of GF-5/AHIS after its launch, this article presents an on-orbit radiometric calibration approach for AHSI visible and near-infrared (VNIR) and shortwave infrared (SWIR) sensors from field automatic observations with a field spectrometer in the absence of SWIR measurements. A spectrum extension method was proposed to extend the retrieved surface hyperspectral reflectance in the VNIR spectral ranges to SWIR by incorporating the historical hyperspectral reflectance library. The radiometric calibration coefficients of GF-5/AHSI were calculated by linear fitting of the observed digital number (DN) values with GF-5/AHSI and predicted at-sensor radiances with MODTRAN 5 based on extended hyperspectral surface reflectance. Comparisons with onboard calibration results were also performed, and the averaged relative differences were within 5% with 1δ standard deviations less than 10% for most bands, except for those in the atmospheric absorption and low signal-to-noise ratio bands. The comparison results indicate that the on-site radiometric calibration results are consistent with the onboard results, and the operational on-orbit radiometric calibration approach is reliable in the case that there are no measurements in the SWIR spectra range. The on-orbit radiometric performance of GF-5/AHSI rapidly degraded during the first several months after its launch and then tended to be relatively stable.

We demonstrate a widely tunable and passively mode-locked Tm:Y

We report on a Kerr-lens mode-locked Yb:YAlO

Funding Information: This work was supported by the National Natural Science Foundation of China (62175133), the Natural Science Foundation of Shandong Province (ZR2020MF115), and the SDUST Research Fund (skr21-3-049, 2019TDJH103). Publisher Copyright: © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature. Here, we demonstrate a broadband tunable (Lu0.0541Tm0.0548Gd0.8911)3Ga3O12 (Tm:LuGGG) laser in-band pumped by a homemade 1645-nm Er:YAG laser. A maximum average output power of 0.7 W at 1997 nm is obtained with a slope efficiency of 46.3%. We also demonstrate a continuously tuning output ranging from1854 to 2066 nm with a tuning coverage of 212 nm. The results indicate that the Tm:LuGGG crystal is an excellent laser medium for high-efficient broadband light generation, which may help to explore potential applications in ultrafast lasers.

The Sentinel-2B offers high spatial resolution optical imagery from 13 bands in the visible, near infrared, and short-wave infrared range of the electromagnetic spectrum, providing enhanced continuity to monitoring global terrestrial surfaces and coastal waters. In this article, its radiometry calibration is evaluated using 37 clear-sky observations in 2018 from the four sites of radiometric calibration network for assuring data quality. However, since the single calibration results acquired under different surface and atmospheric conditions have different biases and different uncertainties, it is difficult to determine which calibration sample is much more trustable. In view of this, by assuming that the calibration samples are independent of each other, a state-of-the-art reference value is derived by combining 37 calibration samples using a weighted average method, which has much lower uncertainty and approaches the “true” value. The reference value also could be used to compare each calibration result. The result shows that the reference value of the relative difference between the simulated and observed top-of-atmosphere reflectance is 4.29%, 4.95%, 4.54%, 5.34%, respectively, for bands 2, 3, 4, and 8, and the corresponding uncertainty is 1.09%, 1.10%, 1.10%, and 1.12%, respectively; the degree of equivalence for each sample is calculated by comparing each calibration result with the reference value. It is worth noting that the degrees of equivalence are lower than 5%, and the four samples on July 9, July 13, October 4, and October 11 perform worse than the other sample.

International audience; Mixed” calcium rare-earth aluminate laser host crystals Ca(Gd,Lu)AlO4 (CALGLO) with up to 10.8 at.% Lu codoped with Tm3+ and Ho3+ ions are grown by the Czochralski method along the [001] direction. The segregation of rare-earth ions is studied. The crystal structure is refined by the Rietveld method. Tm,Ho:Ca(Gd,Lu)AlO4 crystallizes in the tetragonal system (sp. gr. I4/mmm) exhibiting a K2NiF4 type structure. The lattice constants are a = 3.6585(6) Å and c = 11.9660(9) Å for a crystal with a composition of CaGd0.8947Lu0.0551Tm0.0448Ho0.0054AlO4. The stability of Ca(Gd,Lu)AlO4 solid-solutions is discussed. The polarized Raman spectra are measured, revealing a most intense mode at 311 cm−1 and a maximum phonon frequency of ∼650 cm−1. The polarized absorption spectra are measured. The transition intensities for the Ho3+ ion are analyzed using the modified Judd-Ofelt theory accounting for configuration interaction.

Here, we demonstrate a broadband tunable (Lu0.0541Tm0.0548Gd0.8911)3Ga3O12 (Tm:LuGGG) laser in-band pumped by a home-made 1645-nm Er:YAG laser. A maximum average output power of 0.7 W at 1997 nm is obtained with a slope efficiency of 46.3%. We also demonstrate a continuously tuning output ranging from1854 to 2066 nm with a tuning coverage of 212 nm. The results indicate that the Tm:LuGGG crystal is an excellent laser medium for high-efficient broadband light generation, which may help to explore potential applications in ultrafast lasers.

We report on the continuous-wave (CW) and mode-locked (ML) laser performance of an Yb

We demonstrate the first sub-40 fs soliton pulse generation from a diode-pumped Yb:Sr

We report on a soliton mode-locked Yb:Ca

Optical satellite sensor generally suffers from drifts and biases relative to their prelaunch calibration, caused by launch and the space environment. Radiometric cross-calibration is valuable for calibrating sensors without coincident surface measurements through transferring radiometric values between satellite sensors via a stable target. The data quality of visible and near-infrared (VNIR) sensor onboard Gaofen-4 (GF-4) satellite seriously depends on vicarious calibration, which is performed once every year, thus, a cross-calibration approach is proposed for monitoring its radiometric performance much more frequently. In this study, the GF-4/VNIR is cross-calibrated by Landsat8/OLI, Sentinel-2/MSI, and Terra/MODIS with the aid of the tandem scenes during 2017 and 2018 after correcting the discrepancy on viewing geometries and spectrum. The results show that there is test site dependency and sensor dependency, and cross-calibration coefficients in the four bands roughly have similar variation trend as a sinusoidal function during 2017 and 2018, varying from 0.00023 to 0.00079. Furthermore, uncertainty analysis of the cross-calibration is carried out to analyze the uncertainty contributions affecting the cross-calibration accuracy. Note that the calibration accuracy of referenced sensors, the uncertainty of the MODTRAN model, BRDF uncertainty are the three main factors affecting the cross-calibration accuracy, the total uncertainty is approximately 3.9%–6.64%.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho3+ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.

International audience; We report on sub-30 fs pulse generation from a semiconductor saturable absorber mirror mode-locked Yb:YAP laser. Pumping by a spatially single-mode Yb fiber laser at 979 nm, soliton pulses as short as 29 fs were generated at 1091 nm with an average output power of 156 mW and a pulse repetition rate of 85.1 MHz. The maximum output power of the mode-locked Yb:YAP laser amounted to 320 mW for slightly longer pulses (32 fs) at an incident pump power of 1.52 W, corresponding to a peak power of 103 kW and an optical efficiency of 20.5%. To the best of our knowledge, this result represents the shortest pulses ever achieved from any solid-state Yb laser mode-locked by a slow, i.e., physical saturable absorber.

We report on a Kerr-lens mode-locked Yb:YAlO3 laser generating 24 fs pulses at 1085 nm with an average output power of 186 mW at a pulse repetition rate of ~87.5 MHz.

We report on a detailed spectroscopic study of a Tm3+-doped transparent sesquioxide ceramic based on a solid-solution (lutetia – yttria, LuYO3) composition. The ceramic was fabricated using commercial oxide powders by hot isostatic pressing at 1600°C for 3 h at 190 MPa argon pressure. The most intense Raman peak in Tm:LuYO3 at 385.4 cm-1 takes an intermediate position between those for the parent compounds and is notably broadened (linewidth: 12.8 cm-1). The transition intensities of Tm3+ ions were calculated using the Judd-Ofelt theory; the intensity parameters are Ω2 = 2.537, Ω4 = 1.156 and Ω6 = 0.939 [1020 cm2]. For the 3F4 → 3H6 transition, the stimulated-emission cross-section amounts to 0.27 × 10−20 cm2 at 2059nm and the reabsorption-free luminescence lifetime is 3.47 ms (the 3F4 radiative lifetime is 3.85 ± 0.1 ms). The Tm3+ ions in the ceramic exhibit long-wave multiphonon-assisted emission extending up to at least 2.35 µm; a phonon sideband at 2.23 µm is observed and explained by coupling between electronic transitions and the dominant Raman mode of the sesquioxides. Low temperature (12 K) spectroscopy reveals a significant inhomogeneous spectral broadening confirming formation of a substitutional solid-solution. The mixed ceramic is promising for ultrashort pulse generation at >2 µm.

The generation of Fourier-transform-limited pulses as short as 75 fs at ~2.06 μm for an average output power of 0.26 W at 86.3-MHz repetition rate is demonstrated with a SESAM mode-locked Tm:Y2O3 ceramic laser.