A.

Laser Design

The key performance requirements are listed in Table 1 below:

Table 1

Laser Performance Requirements.

The MOPA laser is based on a product line of multi-Watt MOPA fiber based lasers, ruggedized to meet demanding field applications and is shown in Fig. 1. A semiconductor DFB seed laser was directly modulated and followed by a two stage fiber amplifier. Telcordia certified components were required to the extent possible along with hermetically sealed laser diodes. Monitor and control was performed through the RS422 interface with hardware A/D ports for monitoring output power of each stage and temperatures of the diode lasers as well.

Fig. 1

Laser Transmitter showing interface and cooling fins on the base.

B.

Environmental Requirements

Since the laser was based on a commercial design and operated in a forced air-cooled pressure vessel, vacuum compatibility and additional thermal management were not required. This relaxed the environmental requirements which are listed in Table 2 below and were obtained from a combination of JPL workmanship levels and the vendor input. The product line from which the delivered unit was derived has undergone significant reliability testing, similar to the specifications in Mil Std-810E, and gave confidence that the environmental requirements could be met. This testing is shown as the heritage column of Table 2.

Table 2

Environmental Requirements

A.

Thermal Analysis

Thermal analysis was done on the unit up to a maximum ambient temperature of 45°C when the laser was operated at full power of 2.5 W. An example of the simulation is show in Fig. 2 where the temperature rise is limited to 70°C at the pump diode mounting location. The individual pump diodes have been tested to 85°C separately so this should not be an issue. The forced airflow would mitigate any further temperature rise.

Fig. 2

Laser Transmitter thermal simulation with 45°C ambient temperature at full power.

B.

Unit Ground Testing

Unit testing by the vendor was limited to the vibration, shock and temperature cycling tests specified in Table 2. Before and after testing showed no degradation in the power or output spectra with post-testing representative spectra shown in Fig. 3.

Fig. 3.

(a) Average power before and after testing at various PRFs, and (b) wavelength spectra post testing for laser under fixed PRF.

Temperature cycling from 0 to 50°C was performed while the device was operated at full power with 5 cycles at 30 MHz and 5 cycles at 50 MHz. The output power over the several hours of testing is shown in Fig 4 and shows very good stability.

Fig. 4

Laser Transmitter output power (tapped power in dBm) stability as a function of time. Up to 12:10 pm the laser was operated at fixed current to give 2.4 W output and up to 18:00 it was operated at fixed power of 2.5 W.

C.

System Level Ground Testing

The laser was first integrated to the component tray assembly, itself mounted on wire rope isolators that provide vibrational attenuation of the sensitive components. The sealed container element was completely integrated prior to initiation of the system environmental test regime. Once integrated into the pressure vessel, Fig 5, the instrument was subjected to further system level environmental tests. Tests that were performed and were given to affect the performance of the laser were three axis system vibe and a five day thermal vacuum test. Functional testing of the system was performed at key points during the vibrational tests. The results are shown in Table 3. All measurements are assumed to include a 1 dB insertion loss of the fiber connectors. No degradation in laser performance was observed nor of the optical path for the laser output.

Fig. 5

Laser Transmitter mounted in pressure vessel below circulating fans.

Table 3

System environmental test results.

For the full flight system thermal vacuum test, the payload was run to hot and cold temperatures at the proto-qualification level above the allowable flight temperatures to demonstrate survivability and functionality at the extremes of the expected temperature. The allowable flight temperatures were 10 to 45°C for operations and 5 to 50°C for non-op and the proto-qualification temperature ranges were 5 to 50°C and -10 to +65°C for operation or non-op, respectively. Laser measurements were taken at the collimator output with temperature measurements from externally mounted thermocouples as well as the internal laser monitor points during laser operation. For test success, hot qualification had to operate the laser unit at the hot op proto-qualification value, but without emitting light. The cold qualification was successful if the laser unit was operated at the cold proto-qualification temperature but while emitting light at full power. Hot and cold non-op dwells were also performed, but the laser unit was not powered at this time. All tests were completed successfully.

D.

On-Orbit Operations

The OPALS instrument was launched April 18^th on an ISS resupply mission in the trunk of the SpaceX Falcon 9 Dragon capsule and commissioning activities began once it was installed on the ELC rack of the ISS on May 10^th. The laser was powered on successfully and has operated for over 15 link demonstrations. Each link opportunity typically lasts for 2.5 min for the LEO orbiting ISS over the ground station with the laser powered on to 2.5 W of full power for approximately 10 min before the pass for stabilization. Sample monitor data from the laser telemetry are shown below for a single link on-orbit and show no degradation in performance from the initial ground testing, also shown in the graph for the pre-vibe system testing, taken a year and a half earlier.

Fig. 6

Test data during operations compared to pre-vibe ground test data (a) monitor power, (b) monitor temperatures. Similar time frames are shown for each test, taken 1.5 years apart.

During laser operations the seed temperature is kept at approximately 29°C and the amplifier pump diode temperature is on the order of 37° +/- 0.5°C.

The successful operation on-orbit of the commercial laser transmitter points to the robustness of the component and assembly level technology that is derived from Mil-Std and Telcordia design and test practices used in the fiber telecommunications industry. Long term reliability still needs to be investigated for operational space based optical communication systems. This will be addressed under a program requiring full space qualification of each assembly which is ongoing. However, given the successful performance of the current system, the present work provides a good baseline for the part selection of this future program. It should also be noted that some space programs require a rapid delivery response for instruments that might not be compatible with the test and development times of a fully space qualified system. Integrating COTS parts in an air-cooled pressure vessel could be envisioned as a way to provide the capability without the long lead times of a fully space qualified design and test program.