A.

Optical Bench

The optical bench is the main optical device of OSIRIS. The optical bench hosts the tracking sensor as well as the three transmission collimators. Based on a 4-Quadrant-Detector, the tracking sensor enables the attitude control of the satellite to precisely align to the optical ground station. The tracking sensor is equipped with a lens of 30 mm in diameter and a focal length of 100 mm. Together with the lens, a filter with 8 nm width and a transmission of 95% in the pass band is mounted to suppress background light. Tracking sensor, lens and filter are mounted in a carbon fiber tube to overcome defocussing problems in the tracking sensor which would influence the tracking behavior of the attitude control system.

For transmission, three different collimators are used. The three collimators are adjusted to three divergence angles and data rates:

Figure 2 shows a CAD drawing of the optical bench with the mounted tracking sensor and the three transmission collimators. The challenge for designing the optical bench is to perfectly align four optical axes: the three collimators in reference to the tracking sensor. Based on the minimum divergence of 200 μrad, the 6s precision of the alignment of the optical bench is 33 μrad. This is the value that has to be measured during calibration and has to be verified after vibration and thermal-vacuum test.

Figure 2:

Optical bench of the OSIRIS payload

Figure 3:

OSIRIS optical bench Qualification Model

The optical bench is further equipped with an alignment cube (yellow cube in Figure 2). The alignment cube gives the reference for the alignment of the optical axis in the lab and the verification measurements after the qualification tests. Furthermore, the alignment cube gives the reference of the optical bench coordinate system to the attitude control coordinate system. Chapter 4, calibration of the optical bench, will explain the process of measuring and aligning the optical axis in more detail.

B.

Laser sources

The basic design of OSIRIS provides a standard housing and electronics that can be used for long range free-space optical communications. The housing is equipped with different FSO technologies: a laser module combined with an Erbium doped fiber amplifier (EDFA) and a high-power semiconductor laser diode (HPLD). An integrated power supply circuit using DC-DC converter technology is used to supply both of the laser sources.

Both technologies have their advantages and disadvantages. The HPLD is small in size and weight compared to the laser module combined with an EDFA, but provides less optical power than the EDFA and lower data rates. Therefore the EDFA can provide higher data rates and a higher output power. At the moment, the size and weight advantage of the HPLD is not used for the final design of OSIRIS since both technologies are integrated inside the same housing and share the same electronics. For future missions, this advantage could be used if power consumption, weight and size become more critical. The HPLD is more robust against cosmic radiation compared to the EDFA due to the doped optical fibers used in the EDFA. This kind of fiber is especially sensitive to radiation and suffers from degradation effects. These effects have been evaluated in a radiation test, explained in detail in chapter 4.

As a laser source for the EDFA, a fiber-coupled laser module commonly used in terrestrial fiber communications is used. These modules can provide data rates up to 2.5 Gbit/s, however in this setup the data rate is limited to 1 Gbit/s by the transmitter capabilities. Gigabit data to be transmitted is connected to the housing by SMA-connectors. The data path uses PECL differential signaling and has redundant cable connections. The active connection can be chosen by an external control signal, which switches between the data sources, using high-frequency-relays. In addition, it is possible to limit the EDFAs output power to an eye safe level for lab operations.

The wavelength of the laser source can be chosen from the ITU C-band DWDM grid at time of integration. The signal modulated by the laser source is fed to the optical amplifier by a single-mode fiber connection. The EDFA is used to amplify the incoming light to a maximum optical power of 30 dBm. The amplified signal is connected to an x-coupler, which splits the power 50:50 into two fibers for the transmission collimators with different divergence angles, as described in chapter 2.

The HPLD is a directly modulated semiconductor laser diode driven by a specialized driver circuit, which converts the data signals into a current, modulating the laser diode. The data connection is redundant and uses LVDS signaling over twisted pair cables due to the lower data rate. It can provide a maximum data rate of 78 Mbit/s and an optical output power of 17 dBm per collimator. A passive cooling system is used for heat dissipation. The optical output of the HPLD is joined to the second input port of the x-coupler and connected to the collimators in a similar way as the EDFA.

A.

Calibration of optical bench

During calibration of the optical bench, the four optical axes of the tracking sensor and transmission collimators need to be aligned. In chapter II the required accuracy of 33 μrad was given, that will be verified during the calibration process.

For the calibration of the optical bench, the “Kameramessplatz” (KMPL) in the optics lab clean room of DLR’s Institute of Optical Sensor Systems was used. The laboratory is equipped with a measurement collimator as source and a manipulator for positioning the Device Under Test (DUT). The manipulator is able to align payloads with a maximum mass up to 100 kg with a precision of 8 μrad. The precision of the manipulator is higher by a factor of 4 compared to the required accuracy for evolved calibration.

For the calibration of the OSIRIS optical bench, a calibration process has been developed. Figure 6 shows the calibration setup. The optical bench is mounted on the manipulator to enable a precise alignment. In the next step, the measurement collimator is used as light source, illuminating the tracking sensor of OSIRIS. Therefore, the measurement collimator is equipped with the OGSE beacon signal behind a 100 μm pinhole that is interferometrically positioned. Subsequently, the optical bench is tipped and tilted to the center of the tracking sensor. This position is the basis for all further measurements.

Figure 6:

The collimator works as transmitter and illuminates the tracking sensor

For the alignment of the transmission collimators, the OGSE source is replaced by an InGaAs-Photodiode detector behind the pinhole. One after the other, the OSIRIS collimators are connected to the OSIRIS laser sources. In the basic position of the manipulator, the adjustable transmission collimators are aligned to the tracking sensor. Due to mechanically limitations, the resulting mispointing is compensated by the manipulator. The angle between the basic position of the manipulator (equal to the optical axis of the tracking sensor) and the compensated position for every transmission collimator is saved and is the basis of the verification of the alignment before and after the vibration test.

The optical bench is further equipped with an alignment cube. After the compensation of the resulting mispointing with the manipulator, the angular error to the alignment cube is measured with two theodolites so that all four optical axes can be represented as an angular offset to the alignment cube. Due to that fact, the alignment cube is chosen as the basis of the OSIRIS coordinate system that will be referenced to the coordinate system of the satellite’s attitude control and will be used as reference for the verification measurements after the space qualification tests.

B.

Vibration- and Thermal-Vacuum-Test

The vibration test is mostly connected with the launching phase of the satellite. Here the expected accelerations in x, y and z axis are simulated. Afterwards the payload is inspected for potential failures and the optical alignment is verified on the KMPL. In order to verify the integrity of the payload, the resonance frequencies of the DUT can be used. The test includes a sinusoidal-sweep and a random vibration test for every axis. The sinusoidal-sweep stimulation gives generally an indication about the behavior in single frequency systems. Here instead, the passing through a wide frequency range covers potential resonant frequencies inside the payload.

The random vibration is used to represent the expected environment [2] during launch. Even though, a random vibration test gives more realistic results, both methods are foreseen in the applied ECSS standard [3]. After each run, the resonance frequencies are cross checked with an initial reference measurement. Figure 7 shows exemplarily the measured frequency response of the optical bench in z-axis before and after the vibration test. It is visible, that the resonance frequencies remained stable. The qualification model of the OSIRIS payload passed the test within all tolerances.

Figure 7:

Resonance survey of the optical Bench in z-axis

Testing the payload under operating conditions includes the pressure and thermal environment. For this, a thermal-vacuum-test was carried out. Only the combination of both physical values allows conclusions for the actual operation on the satellite.

Two major effects have to be considered during the test. On the one hand the payload is heating up due to its power dissipation. On the other hand it is possible that the payload is heated by an external source like the sun or the satellite bus. Both possibilities are considered in the test. The test-temperature range reaches from -20°C to +60°C. This temperature is introduced by the test adapter where OSIRIS is mounted on in order to optimize the heat transfer. The pressure is reduced to 5·10^-6 mbar. The payload has to pass eight temperature cycles for the qualification test. During each cycle OSIRIS is turned into operational state two times (at -20°C and +60°C, steady state). Like this the payload is tested at its defined operating boundaries and internal as well as external heating effects are considered.

The results of the thermal-vacuum-test show, that the EDFA works according to its specifications. The HPLD suffers from its temperature dependency. The measurements which are performed in the high temperature phases result in a lower output power. This behavior is explainable by the characteristic of the laser diode itself and can be accepted due to the thermally stabilized satellite bus. Also the 4QD is tested and the measurements, which are presented in Figure 8, indicate that it is working stable in the operating environment. Further it can be verified, that there is no change in the relative position due to thermal effects. The OSIRIS payload passed the vibration-as well as the thermal-vacuum-test within the specifications.

Figure 8:

Channel measurement of 4 quadrant tracker during thermal vacuum test

C.

Radiation Test of laser sources

To complete the space qualification tests, a Total Ionizing Dose (TID) test with a Cobalt-60 source was performed at the Hahn-Meitner-Institute in Berlin.

The critical components to be identified are the electronic equipment, which could get degraded or even destroyed under radiation influence, and the optical fibers and components used in the EDFA and HPLD laser sources. Optical fibers - especially Erbium doped fibers - are known to degrade under gamma radiation and suffer from increased attenuation. This effect is intensified by additional dopants like aluminum and germanium, which are used for gain flattening and to increase the index of refraction.

The purpose of this test was to determine the amount of fiber degradation and the reliability of the electronic components in a radiation environment. The test was performed with the radiation parameters in the following table:

Table 2:

TID test parameters

The components were operated during the test in the following way:

Both Laser sources (HPLD and EDFA) were subject to the radiation test. The EDFA was operated in Automatic Current Control (ACC) mode to be able to monitor the degradation process. During the test the EDFA was switched on continuously at a low pump current. Each 15 minutes, the EDFA was switched to a higher pump current for 1 minute. Afterwards, the EDFA is completely switched off for 1 minute while the HPLD is powered. The optical output power was measured with the internal monitor diode of the EDFA module and an external optical power meter at the fiber output port.

Figure 9 shows the output power of O4B-EDFA. The blue line depicts the measured power at the output fiber whereas green is the EDFA output power measured by the internal monitor diode. It can be seen, that the measurement within the EDFA is correct and gives the same values as the external measurement device at the fiber output port. Figure 9 also shows the test procedure: first, the EDFA is running with low output power in standby for 15 minutes (1 krad). After 15 minutes, the EDFA is switched to a higher output power level for 1 minute (first half of the first peak). After 1 minute, the EDFA is switched off completely and the HPLD is switched on with maximum power (second half of the first peak). After 1 minute, the HPLD is switched off and the EDFA goes to standby power level.

Figure 9:

Optical Output Power over TID

It can be seen, that the EDFA output power decreases from 24 dBm to 23 dBm within the first year. Over 5 years, the EDFA loses 5 dB of output power. In addition, a drift between internal measurement and external measurement for low power (standby mode) can be seen. The internal monitor diode is not able to detect power levels lower than a threshold of approximately -10 dBm – this is not relevant for a use in space because the EDFA will be used at higher power levels.

Furthermore, the EDFA will be operated in Automatic Power Control (APC) mode, which will automatically compensate for the increased attenuation. At some level, though, the internal gain of the EDFA will be too small to reach the desired output power, but this is to be expected after the satellites’ end of life.

The output of the HPLD didn’t show a significant decrease of output power. This leads to the conclusion that only Erbium-doped fibers, which are mainly used in EDFAs suffer from increased attenuation caused by gamma radiation.

The electronic devices used in O4B-EDFA didn’t show any degradation or failure during the test. The DC/DC converters providing the supply voltage for the EDFA showed an increase of the output voltage by 3% after a TID of 10 krad, which is within the specification of the EDFA and can be tolerated.