A ground-to-space laser communications experiment was conducted to verify the optical interfaces between a laser communications terminal in an optical ground station and an optical payload onboard a geostationary satellite 38 000 km away. The end-to-end optical characteristics such as intensity, sensitivity, wavelength, polarization, and the modulation scheme of optical signals as well as acquisition sequences of the terminals were tested under fairly good atmospheric conditions. The downlink's bit error rate was on the order of 10 10 in spite of atmospheric turbulence. Atmospheric turbulence-induced signal fading increased the uplink bit error rate, the best value of which was 2.5 10 5 because the turbulent layer near the earth surface affects the uplink signal more than it does the downlink one. The far-field optical antenna patterns were measured through the ground-to-satellite laser links. The long-term statistics of the optical signal data is in good agreement with the calculated joint probability density function due to atmospheric turbulence and pointing jitter error effects, which means the stationary stochastic process can be applied to not only the static link analysis but also the dynamic link performance of the optical communications link. The equivalent broadened optical beam pattern should be used for the fading analysis even though the atmospheric coherence length is larger than the antenna diameter or the optical beam diameter of the transmitter. From these results, a more accurate dynamic link design of the optical communications link can be performed that would be useful for system designers, especially for designers of commercial systems.
Optical devices in free-space laser communication systems are affected by their environment, particularly in relation to the effects of temperature while in orbit. The mutual alignment error between the transmitted and received optical axes is caused by deformation of the optics due to temperature variation in spite of the common optics used for transmission and reception of the optical beams. When a Gaussian beam wave for transmission is aligned at the center of a received plane wave, 3rd-order Coma aberrations have the most influence on the mutual alignment error, which is an inevitable open pointing error under only the Tip/Tilt tracking control. As an example, a mutual alignment error of less than 0.2 microrad is predicted for a laser communication terminal in orbit using the results from space chamber thermal vacuum tests. The relative power penalty due to aberration is estimated to be about 0.4 dB. The results will mitigate surface quality in an optical antenna and contribute to the design of free-space laser communication systems.
We present the SPICA Coronagraphic Instrument (SCI), which has been designed
for a concentrated study of extra-solar planets (exoplanets). SPICA mission
provides us with a unique opportunity to make high contrast observations
because of its large telescope aperture, the simple pupil shape, and the
capability for making infrared observations from space. The primary objectives
for the SCI are the direct coronagraphic detection and spectroscopy of Jovian
exoplanets in infrared, while the monitoring of transiting planets is another
important target. The specification and an overview of the design of the
instrument are shown. In the SCI, coronagraphic and non-coronagraphic modes are
applicable for both an imaging and a spectroscopy. The core wavelength range
and the goal contrast of the coronagraphic mode are 3.5--27$\mu$m, and
10$^{-6}$, respectively. Two complemental designs of binary shaped pupil mask
coronagraph are presented. The SCI has capability of simultaneous observations
of one target using two channels, a short channel with an InSb detector and a
long wavelength channel with a Si:As detector. We also give a report on the
current progress in the development of key technologies for the SCI.Comment: 22 pages, 10 figure
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