Free-Space Optical Data Receivers with Avalanche Detectors for Satellite Downlinks Regarding Background Light.

Data receiving frontends using avalanche photodiodes are used in optical free-space communications for their effective sensitivity, large detection area, and uncomplex operation. Precise control of the high voltage necessary to trigger the avalanche effect inside the photodiode depends on the semiconductor's excess noise factor, temperature, received signal power, background light, and also the subsequent thermal noise behavior of the transimpedance amplifier. Several prerequisites must be regarded and are explained in this document. We focus on the application of using avalanche photodiodes as data receivers for the on/off-keying of modulated bit streams with a 50% duty cycle. Also, experimental verification of the performance of the receiver with background light is demonstrated.

Downlink communication experiments with OSIRISv1 laser terminal onboard Flying Laptop satellite.

Downlink measurement campaigns from the optical downlink terminal OSIRISv1 onboard the LEO satellite Flying Laptop were carried out with the French Observatoire de la Côte d'Azur and with two Optical Ground Stations of the German Aerospace Center. On/off keyed data at 39 Mb/s were modulated on the laser signal, and according telecom reception was performed by the ground stations. The pointing of the laser terminal was achieved by open-loop body pointing of the satellite orientation, with its star sensor as attitude control signal. We report here on the measurements and investigations of the downlink signal and the data transmission.

Sensitivity modeling of binary optical receivers.

The sensitivity characteristics of optical receiver frontends for high-speed data communications depend on modulation format, detector type, and specific operational constraints. A general mathematical model of the receiver sensitivity that fits to analytical as well as measured data is required to compare different receiver implementations and assess the reliability of data links under varying received power as common in free-space optical communication links. In this paper, a new approach based on Q-factor modeling is presented, compared with analytical receiver models, and applied to a multitude of exemplary receiver implementations. A methodology is introduced to generally apply the model to ideal or practical binary optical receiver frontends.

Estimation of channel parameters and background irradiance for free-space optical link.

Free-space optical communication can experience severe fading due to optical scintillation in long-range links. Channel estimation is also corrupted by background and electrical noise. Accurate estimation of channel parameters and scintillation index (SI) depends on perfect removal of background irradiance. In this paper, we propose three different methods, the minimum-value (MV), mean-power (MP), and maximum-likelihood (ML) based methods, to remove the background irradiance from channel samples. The MV and MP methods do not require knowledge of the scintillation distribution. While the ML-based method assumes gamma-gamma scintillation, it can be easily modified to accommodate other distributions. Each estimator's performance is compared using simulation data as well as experimental measurements. The estimators' performance are evaluated from low- to high-SI areas using simulation data as well as experimental trials. The MV and MP methods have much lower complexity than the ML-based method. However, the ML-based method shows better SI and background-irradiance estimation performance.

One-way radio frequency dissemination through the atmosphere using two optical carriers.

A method of transferring an RF reference frequency through the turbulent atmosphere is presented. Using two optical wavelengths close to each other can compensate for the influence of the atmospheric piston error. The influence of the atmosphere on the phase of the optical signal is calculated together with the remaining error by transferring two carriers. The system was implemented in a laboratory test-bed, and stability measurements are shown.

Experimental verification of the limits of optical channel intensity reciprocity.

Optical data links through the atmosphere suffer from turbulence-induced signal scintillation. In a coaxially-symmetric bidirectional link scenario, the variations of the axial intensities at both ends are correlated. This relation can be used as an inherent feedback mechanism, with negligible delay, to enhance the capacity of the transmission system. By experiment, we show the correlation coefficient of both received signals can reach values close to one over long atmospheric distances, provided the receiver apertures are smaller than specific intensity speckle structures, while the correlation reduces gradually with larger apertures. This allows transmission capacity to be optimized with adaptive transceiver systems that take into account the degree of correlation.

Scintillation correlation between forward and return spherical waves.

According to the point-source point-receiver (PSPR) reciprocity, the received field remains equal when the positions of a point source and point receiver are interchanged. We extend the PSPR scenario to a finite receiver that spatially averages scintillation over its aperture. By use of weak-fluctuation theory, an analytical expression for the correlation coefficient between the received powers at both link ends is provided. The effects of turbulence profile, receiver aperture size, and central obscuration on the correlation are assessed. Because correlation is obtained to the detriment of antenna gain and aperture averaging, the net benefit of the channel reciprocity is highly scenario dependent.

Deriving an estimate for the Fried parameter in mobile optical transmission scenarios.

Measuring the Fried parameter r(0) (atmospheric optical coherence length) in optical link scenarios is crucial to estimate a receiver's telescope performance or to dimension atmospheric mitigation techniques, such as in adaptive optics. The task of measuring r(0) is aggravated in mobile scenarios, when the receiver itself is prone to mechanical vibrations (e.g., when mounted on a moving platform) or when the receiver telescope has to track a fast-moving signal source, such as, in our case, a laser transmitter on board a satellite or aircraft. We have derived a method for estimating r(0) that avoids the influence of angle-of-arrival errors by only using short-term tilt-removed focal intensity speckle patterns.