Abstract:
After 30 years of development coherent satellite laser terminals will verified in orbit in 2006. General design constraints for space communication optical terminals are reviewed and the design and programmatics of the beacon-less coherent laser crosslink terminals will be detailed.

Figure 1: Receiver front end

General Design Considerations for Satellite Laser Crosslinks
Since 40 years microwave technology dominates with tremendous success all space communications with an excellent in-orbit heritage. The drivers for the utilization of optical carriers for communication links between satellites and to the ground are the growing data rate demand as well as the microwave spectrum limitations. In the very conservative area of space communications technology the trade-off for the application of optical terminals versus microwave technologies is dominated by reliability, environment immunity and compatibility figures. The initial advantages of higher carrier frequency and antenna gain are contrasted by the disadvantage of the low heritage and high complexity. Due to the nature of optical terminals this is a challenging target for typical required design life times of 15 years. The six key factors for the optimization of the optical terminal design with a typical performance of 10 Gbps duplex data link at 10000 km distance with a BER of 10-9 are:
• Link Reliability target above 0,9 for a 15 year design life time at 100% operations
• Full immunity to sun, Doppler and satellite mechanical perturbations in any operational mode
• Power consumption in the area of 100W
• Low complexity and sensitivity of all optics, still a key issue in production and application acceptance
• Sufficient margins on the sensitive area of tracking budget
• Inherent capability of scalability of power
Coherent optical terminals yield the potential for the highest communication and tracking sensitivity combined with the transfer of optical complexity to electrical circuit design.

Figure 2: Acquisition and communication receive path

Specific design considerations for coherent satellite laser crosslinks
Coherent homodyne detection with the implementation of an optical phased locked loop of the local receive oscillator and the received signal of the counter terminal transmit oscillator have been well described and verified in literature. The German efforts started with a carbon dioxide laser terminal 30 years ago, continued with Nd:YAG lasers in the early 1990s with the SOLACOS program and followed by the DLR-LCT, KTV and LCTSX program until today, all funded by the German Space Center, DLR. The performance of a homodyne BPSK modulation scheme was verified with space qualified components and all operational modes of a laser communication terminal were verified under environmental conditions in a system test bed, starting with spatial acquisition, handing-over to heterodyne tracking, followed by frequency acquisition adjusting the local oscillator’s frequency to the one of the signal, locking the phases and ending up with homodyne tracking and communication based on homodyne BPSK modulation. The theoretical limit of sensitivity has been achieved even under ground applied in-orbit conditions, e.g. under sun and venus illumination and under the satellite’s on-orbit vibrations. In the following we detail the design of the 5.5Gbps LCTSX terminal with a telescope diameter of 127mm and an optical output power of 1Watt.

Transmit Chain
The reliability of the laser is dominated by the reliability of the pump laser diodes. A reliability of 0.9998 over ten years operation has been achieved by using a pump module with built-in cold and hot redundancy. The pump light is focused into a multimode fiber connecting the pump module to the Nd:YAG laser oscillator. The ring oscillator copes absolute frequency setting stability (250MHz) and tunability (14GHz) requirements. The light emitted from the laser is guided by a polarization maintaining single-mode fiber to the phase modulator driven by the data electronics. The modulator is designed for data rates up to 10 Gbps. By a polarization maintaining single mode fibre the phase modulator is coupled to an optical power amplifier which amplifies the transmit laser signal to the required output power level. A further polarization maintaining single mode fibre couples the optical amplifier to the collimator.

Receive Chain
The light emitted from the local oscillator is guided by a polarization maintaining single-mode fiber to the space qualified receiver front end shown in Fig. 1. The signal’s optical input is a polarization dependent beam splitter. The receiver front end demodulates data signals up to 10 Gbps and generates the control signals for spatial acquisition, heterodyne tracking, frequency acquisition adjusting the frequency of the local oscillator to the one of the signal, phase locking, and homodyne tracking. The receiver front end is rugged and small with an the miniaturized optical monolithical bench with outer dimensions of only 20 x 20 x 10 mm.
Communication and tracking sensitivity have been verified during the qualification of the current flight hardware with 40 photons per bit.

Pointing , Acquisition and Tracking
For acquisition, both terminals use the communication beam. At the beginning of the first acquisition phase with large pointing errors the two terminals acquire each other according a master-slave procedure. The position of the counter terminal (slave) is known with an uncertainty cone. The (master) terminal scans the uncertainty cone with its communication beam, e.g. in a spiral scan. As the master’s beam hits the slave terminal, the slave terminal detects a short light pulse and – pulse by pulse – adjusts itself to the master’s optical wave front. After a defined time duration the former slave starts scanning, since it is better adjusted it scans a smaller cone, and the former master is waiting for light pulses to adjust itself. A second acquisition phase (fine acquisition) is started for small angles in which both, master and slave align themselves quickly, by scanning simultaneously until tracking. The coarse pointing has a full hemispheric range.

Optical Complexity
The coherent terminal with beacon-less acquisition is a single wavelength terminal. The tracking is performed coherently. Consequently the coating and filter requirements are drastically reduced. Sun immunity for communication and tracking can be achieved without any sun filtering. All sensors are monolithically integrated in the receiver front end, self calibration of Rx- and Tx-optical path can be performed. The number of optical surfaces, optical coatings per surface and optical axis are reduced dramatically. A standard Cassegrain Telescope is used. In all operations all mirrors have fixed angle incidence (except the fine steering mirrors for mrads).

Figure 3: The terminal mounted on the TerraSAR-X satellite

Terminal Intelligence
Optical terminals need processor (and software) control in order to steer the complex functional modes from wakeup, acquisition to communication. On one side this increases complexity, on the other side autonomous power and link parameter optimization is performed, as well as detailed in-situ documentation of all terminal health and link parameters. The software can be upgraded in-orbit.

Figure 4: The LCTSX terminal

Mechanical and thermal design
Compatibility requirements to the satellite interfaces and the typical satellite integration and test procedures lead to a simplistic “bolted” mechanical interface and to the utilization of flexible heatpipes for transferring the dissipated terminal power to the satellite heatpipes as depicted in Figure 4.

Coherent optical terminal programmatic
The two 5.5 Gbps flight terminals under the DLR LCTSX program are planned to be completed in 2005. One of them will be integrated on the DLR TerraSAR satellite to be launched in 2006 as depicted in figure 3. The first verification will be a satellite to ground link at full data rate with a ground station in Calar Alto, Spain with an average contact time of 2,5 minutes. The atmospheric impact on coherent ground links will be measured in 2005 with a measurement campaign on the Canarian islands, Spain. The primary laser ISL verification from TerraSAR to another LEO Satellite is foreseen in 2007.

Outlook for further developments for coherent optical terminals
A new receiver design with 40% reduced optical complexity will be verified begin of 2006. Further on development of the extension of beacon-less acquisition for LEO-GEO links and output powers of 10 W has been started. The suitability of coherent laser communication at 532nm for lunar downlinks to the earth is a promising solution. Reliability as a key factor for the competitiveness against microwave solutions will remain a challenge. In the long term phased array configurations potentially lead to the replacement of the fine steering mechanism. If hemispherical pointing range is not mandatory, this can lead to attractive new terminal designs. The reduction of the power needs for the phase computation and steering might take another decade.
The funding support of the German Space Center DLR is gratefully acknowledged. Work reported in this article was funded by the DLR under contract number 50 YH-0202.