RESUMEN
Observations of the Moon provide a primary technique for the on-orbit cross calibration of Earth remote sensing instruments. Monthly lunar observations are major components of the on-orbit calibration strategies of SeaWiFS and MODIS. SeaWiFS has collected more than 132 low phase angle and 59 high phase angle lunar observations over 12 years, Terra MODIS has collected more than 82 scheduled and 297 unscheduled lunar observations over nine years, and Aqua MODIS has collected more than 61 scheduled and 171 unscheduled lunar observations over seven years. The NASA Ocean Biology Processing Group Calibration and Validation Team and the NASA MODIS Characterization Support Team use the USGS RObotic Lunar Observatory (ROLO) photometric model of the Moon to compare these time series of lunar observations over time and varying observing geometries. The cross-calibration results show that Terra MODIS and Aqua MODIS agree, band to band, at the 1%-3% level, while SeaWiFS and either MODIS instrument agree at the 3%-8% level. The combined uncertainties of these comparisons are 1.3% for Terra and Aqua MODIS, 1.4% for SeaWiFS and Terra MODIS, and 1.3% for SeaWiFS and Aqua MODIS. Any residual phase dependence in the ROLO model, based on these observations, is less than 1.7% over the phase angle range of -80° to -6° and +5° to +82°. The lunar cross calibration of SeaWiFS, Terra MODIS, and Aqua MODIS is consistent with the vicarious calibration of ocean color products for these instruments, with the vicarious gains mitigating the calibration biases for the ocean color bands.
RESUMEN
Wavelength conversion exploiting cascaded second harmonic and difference frequency generation (c(SHG/DFG)) in periodically polarized LiNbO3 (PPLN) waveguides was experimentally researched. While wavelength converter was pumped with a pulsed wave, the pump pulse can be used to carry the information and wavelength conversion occurs between the pump wave and converted wave, thus wavelength conversion transferring the information from the pump wave to the converted waves includes two processes of second order nonlinear reaction: the first wavelength conversion from pump wave to SH wave occurs with SHG process, and the second wavelength conversion from SH wave to converted wave occurs with DFG process. In the first process the group velocities mismatching (GVM) for pulses at different wavelengths due to material property load the temporal walk-off between pump pulse and SH pulse located in the 1.5 microm band and in the 0.8 microm band, respectively, so that SH pulse slowly propagates compared with pump pulse, and SH pulse width is broadened along propagation length. As a result, in the second process the converted DF pulse generates waveform distortion owing to the broadening of SH pulse in the first process. Both the waveform and the spectrum of converted pulse in our experimental results testify to the fact that SH pulse possesses a narrow spectral width, which is consistent with a long SH pulse, and the spectral width of converted DF pulse is compressed but its temporal width is broadened correspondingly. Therefore the influence of walk-off between pulses demonstrates that the pulsed pumping wavelength conversion is disadvantageous to the transparence of the data format. However, pulsed pumping wavelength conversion also presents great potential that can be applied in future optical networks. Tunable wavelength conversion can be easily implemented by changing the wavelength of control CW, and single-to-multiple channel wavelength conversion can be realized by increasing the number of the CW lasing pump channels. This is very important and it enhances the flexibility in the management of the multi-channel WDM network. Finally, a tunable and single-to-dual channel wavelength converter based on the scheme of pulsed pumping wavelength conversion achieved by our experiment setup, and two channel converted pulses simultaneously replicate the bit rate carried on pump pulses. It is pointed out that the quality such the signal-to-noise ratio of converted pulse is affected by spectral width of control CW.