Laser Pickoff System

Laser Pickoff System :

The Laser Pickoff System couples laser light into optical detectors throughout the GLAS instrument for on-orbit calibrations. As the 1.18-inch (30-mm) square laser beam leaves the GLAS instrument for the earth's surface, the Laser Pickoff System routes 2% of the outgoing laser energy into six fiber optics that perform these functions:

• One is routed to a single fiber optic delay line that acts as a yardstick for comparison to the laser pulse time of flight.
• One is routed to an energy monitor for checking laser health.
• Two are routed to the altimetry detectors to carry the start pulses.
• A 3.3-micron diameter single-mode fiber optic is used as a collimator reference system because its tiny size simulates a point source.
• The sixth fiber is used for tracking purposes in the etalon filter in the GLAS LiDAR system.
• The seventh barrel is for mounting an input test fiber.

This system survived thermal cycling from -40 °F (-40 °C) to 115 °F (46 °C) and vibration testing. The GLAS instrument with the Laser Pickoff system launched on January 12, 2003 from Vandenberg Air Force Base. The system has worked without problems since launch.

Challenges:

This project was our first long-term, critical path project. If this system failed, the spacecraft would not have a start pulse and therefore no data could be taken. The entire system was built and tested without a full engineering model. The design went straight from computer to fabrication, then to testing, and finally to integration with the space flight instrument without significant rebuilds or redesigns.

Only one component did not perform as expected during the initial design. The original design for the laser pickoff optic mount did not provide adequate support and the optic was damaged during a component-level vibration test. The contact pads were too small and located too close to the edge of the optic. Two new designs were created: a bonded approach and an improved clamped design. The bonded design survived vibration and thermal testing and remained aligned to less than the measuring equipment resolution of 3 arcseconds. This was the simpler redesign so the clamped design was never built.

This project taught the importance of understanding environmental testing requirements and what the test is trying to simulate. We learned how vibration testing attempts to simulate launch environments, and we pushed for more realistic testing criteria to be developed to prevent damaging the hardware. This approach allowed the team to build one assembly, test it to appropriate levels, and deliver a flight Laser Pick-off System to the GLAS instrument without costly engineering units.