Klaus H. Kudielka

Experimental demonstration of an optical phased array antenna for laser space communications.

The feasibility of an optical phased array antenna applicable for spaceborne laser communications was experimentally demonstrated. Heterodyne optical phase-locked loops provide for a defined phase relationship between the collimated output beams of three single-mode fibers. In the far field the beams interfere with a measured efficiency of 99%. The main lobe of the interference pattern can be moved by phase shifting the subaperture output beams. The setup permitted agile beam steering within an angular range of 1 mr and a response time of 0.7 ms. We propose an operational optical phased array antenna fed by seven lasers, featuring high transmit power and redundance.

Aperture dependence of the mixing efficiency, the signal-to-noise ratio, and the speckle number in coherent lidar receivers.

With the aid of the van Cittert-Zernike theorem we develop an analytical expression for the ensemble-averaged heterodyne mixing efficiency in coherent lidar receivers that are looking at a diffuse target that is in the receivers far field. Our extremely simple and straightforward analysis shows that the dependence of the mixing efficiency on the receive aperture size dR first follows a parabolic decrease and later approaches a (dR)(-2) function. As a consequence, the signal-to-noise ratio does not increase proportionally to the aperture area but saturates. For the system model chosen, the heterodyne mixing efficiency exhibits the same functional dependence on the lidar geometry as the reciprocal of the number of speckle cells within the receive aperture.

Optical homodyne PSK receiver: phase synchronization by maximizing baseband signal power

Optical homodyne receivers are very suitable for free-space laser communications, owing to their high sensitivity and frequency selectivity. Phase synchronization by maximizing baseband signal power is less complex than previously proposed strategies, while still offering the key advantages of a 180 degrees hybrid AC-coupled front-end.

Optical terminals for microsatellite swarms

We studied terminal architectures and configurations for optical cross-links within microsatellite swarms and assessed the applicability of available technologies. Typical applications for microsatellite swarms are phased array telescopes, interferometric missions, and space-based radar. Key drivers for an optical terminal are well-developed technology and ruggedness. The terminal should do without automatic tracking or fine pointing, coarse pointing should be simple. As an example we cover a scenario where four microsatellites form a planar, square formation of 1 km side length, where the data rate is 100 kbit/s, and where an active double-pass lidar between each of the satellites provides a ranging accuracy of better than 10 m. The terminal transmit power is some 160 mW at a wavelength of 980 nm, the receive apertures have a diameter of 5 mm, and the size, weight, and power requirement of one terminal is estimated to be 60X80X70 mm3, 900 g, and 5 W, respectively.

Experimental demonstration of a self-tracking 16-aperture receiver telescope array for laser intersatellite communications

An adaptive receive telescope array with 16 apertures has been designed and breadboarded. With respect to size and performance, such a telescope array is well suited for use as receive antenna in a coherent interorbit laser link. The laboratory demonstrator, designed to operate at a wavelength of (lambda) equals 1.064 micrometers, is completely independent of any subsequent receiver and of the data modulation format employed. The telescope array is self-phasing, i.e. the main lobe of the antenna pattern automatically follows the direction of the incident wave. It thus performs non- mechanical fine tracking. Our experimental setup comprises a subtelescope array and a digital control unit employing digital signal processors. Besides inertia-free tracking, the control unit also checks and, if necessary, restores parallel alignment of the subtelescope axes at regular intervals. Space-worthy concepts have been applied wherever possible, although experiments have been performed only in the laboratory. Automatic fine-tracking is achieved within a single subtelescopes field of view (30 (mu) rad) in the frequency range up to 730 Hz.

Design and breadboarding of a phased telescope array for free-space laser communications

The design of a 18-aperture receive telescope array demonstrator for optical space communications is presented. Preliminary experimental results and computer simulations show that a telescope array with such dimensions and performance could be used as a receive terminal in a coherent optical interorbit link.

Nulling interferometry for spectroscopic investigation of exoplanets: a statistical analysis of imperfections

In ESAs Infrared Space Interferometry mission, a multi-aperture interferometer fed by telescopes will serve to analyse exoplanets orbiting bright stars. Spectroscopy of the planets radiation could give hints on the possibility of the existence of life. However, for a Sun/Earth-like constellation, a star light rejection ratio of some 80 dB is required. This is the factor by which the star light is suppressed, when comparing the interferometer with a standard, wide-field-of-view telescope. We investigate the nulling capability of space-based interferometers, realized either in fiber or bulk optics, in the presence of imperfections of the structure and of optical components. Mismatch of amplitude, optical path length, and polarization among the interferometer arms is taken into account, as well as multiple reflections and telescope imperfections. The parameters describing the interferometers receive characteristic, which are actively controlled or influenced by environmental disturbances, are modeled stochastically. We analyse Sun/Earth-like constellations by numerical simulation for a wavelength range of 6 to 18 microns. The expected value of the star light rejection ratio is calculated for several interferometer configurations. The exemplary numerical results confirm the extreme requirements for interferometer uniformity and give a quantitative insight into the dependence of the attainable rejection ratio on individual and/or combined interferometer imperfections.

OPTICAL CROSS-LINKS FOR MICROSATELLITE FLEETS *

In a study for the European Space Agency we explored architectures and terminal configurations for optical cross-links within fleets of microsatellites and assessed the applicability of available technologies. As an example we cover a scenario where four microsatellites form a planar, square formation of 1 km side length, where the data rate is 100 kbit/s, and where an active doublepass lidar between each of the satellites provides a ranging accuracy of better than 10 m. The terminals operate at a wavelength of 980 nm, the receive apertures have a diameter of 5 mm, and the size, mass, and prime power requirement of one terminal is estimated to be 60x80x70 mm 3 , 900 g, and 5 W, respectively.

Optical phased array antennas for free space laser communications

The principle of phased array microwave antennas can be applied at optical frequencies. The far-field antenna pattern is found by spatial Fourier transform of the optical field distribution across the subaperture plane. Inertia-free antenna pattern steering can be accomplished by proper phasing of the subaperture waves. A tolerance analysis shows that the required accuracy of phase relationship and subantenna alignment can be obtained in practice when implementing control loops. We develop schemes for both transmit and receive array antennas. Experiments carried out at (lambda) equals 1.06 micrometers demonstrated both modes of operation. Optical array antennas may be applied advantageously for fine pointing in intersatellite data links and in space lidar systems.

Analog optical intersatellite links

We have adopted the phase modulation technique from the digital Syncbit communication system, and developed an analog transmission system which allows to transmit a set of analog channels between standard bent - pipe satellites. This new analog coherent modulation communication system provides excellent receiver sensitivity which in turn allows to keep the optical output power of the transmitter to a minimum. As a result power consumption from the host spacecraft -- a valuable resource -- can be kept to a minimum. The analog communication system design is such, that it uses all the electro-optical building blocks of the digital communication system. Transmitter laser, modulator, booster amplifier and optical receiver frontend are nearly identical for the analog and the digital terminals. The subsystem is contained in two standard electronics module frames and fits into the modular electronics unit of the terminal.