We present first, promising experiments with a novel, compact and simple Nd:YVO<sub>4</sub> slab laser with 12 W of 1.06 μm optical output power and a beam quality factor M<sup>2</sup> ∼ 2.5. The laser is made of a diffusion-bonded YVO<sub>4</sub>/Nd:YVO<sub>4</sub> composite crystal that exhibits two unique features. First, it ensures a one-dimensional heat removal from the laser crystal, which leads to a temperature profile without detrimental influence on the laser beam. Thus, the induced thermo-optical aberrations to the laser field are low, allowing power scaling with good beam quality. Second, the composite crystal itself acts as a waveguide for the 809 nm pump-light that is supplied from a diode laser bar. Pump-light shaping optics, e.g. fast- or slow-axis collimators can be omitted, reducing the complexity of the system. Pump-light redundancy can be easily achieved. Eventually, the investigated slab laser might be suitable for distortion-free high gain amplification of weak optical signals.
A 5.6 Gbps optical communication link has been verified in-orbit. The intersatellite link uses homodyne BPSK (binary phase shift keying) and allows to transmit data with a duplex data rate of 5.6 Gbps and a bit error rate better than 10-9 between two LEO satellites, NFIRE (U.S.) and TerraSAR-X (Germany). We report on the terminal design and the link performance during the measurement campaign. As an outlook we report on the flight units adapted to LEO-to-GEO intersatellite links that TESAT currently builds and on plans to study GEO-to-ground links.
The Receiver Front End (RFE) is the optical receiver of ESA's Semiconductor Laser Intersatellite Link Experiment (SILEX). Optical free space communication between satellites is characterized by narrow beams and therefore by demanding requirements for pointing accuracy. This applies for the steering of the laser beam in transmission, for the pointing of the receiver's field of view (FOV), and for the alignment between transmitted and received beams. The RFE housing, the optical system, the lens and detector's mounting have to be designed to meet the stringent requirements for angular stability. This paper concentrates on the mechanical and thermal aspects which strongly influence the performance. Thermal expansion effects are of major concern when keeping the optical axis stable. All materials have been matched to the thermal expansion characteristics of the hybrid circuit which contains the detector. Assuming only homogeneous temperature changes during life, no stress or angular deviations have to be expected. The relative changes of dimensions in any direction stays equal at different temperatures. The verification of opto-mechanical performance requires sophisticated measurement tools. Measurements have to be performed in order to determine the lateral stability of lens and detector. A dedicated autocollimator was developed which measures the angular stability of the optical axis after vibration, thermo-vacuum test and under temperature changes. It also serves as a test transmitter. Measurement accuracies of 5 (mu) rad have been achieved. For the measurements the RFE is mounted onto a test fixture. A reference mirror on the fixture is the stable reference which has to be more stable than the equipment itself.
The present optical terminal design for a high data rate interorbit communications link with 500 Mbit/sec (or higher) transmission rate is based on high-power laser diode transmitters with 1 W peak power at 800 nm, in conjunction with a direct-detection receiver; two-channel wavelength-division multiplexing, together with the high transmission capability of the off-axis telescope used, contribute to the achievement of the requisite data rates. In order to minimize system mass, the only rotatable device is the telescope.
For the ESA Semiconductor Intersatellite Link Experiment (SILEX), elements of the communication chain have been breadboarded. The electrooptical converter, called the laser diode transmitter package (LDTP), is described here. The requirements on the LDTP optical quality are deduced from the overall system requirements. The tolerable wavefront errors (WFE) and the stability of beam direction are most critical. Four breadboards have been assembled and tested. The very stringent requirements on WFE were surpassed, with a resulting rms value of 1/40 waves. In order to achieve this wavefront quality, the typical astigmatism of index-guided laser diodes (1-10 microns) had to be compensated by adjustable cylindrical lenses.
This paper discusses the characteristics of a new concept involving high-power laser diode (HPLD) arrays which are able to emit up to 1000 mW of optical output power but which suffer from poor far-field pattern and poor optical quality, related to the gain guiding mechanism used in such devices. Methods are proposed for improving the optical quality of HPLDs. These include the use of apertured AM scheme for HPLDs and the use of a graded-index lens and a stripe mirror to stabilize the HPLD. A HPLD optical transmitter is designed which will have the capability of operating the HPLD in the 180 deg phase alternating mode under modulation.