By Patricia Daukantas
In the June issue of OPN, I wrote about lidar experiments that have flown into space. On Tuesday of the CLEO/IQEC conference, two NASA employees described the rigorous prep work that goes into preparing lasers and optical systems for space travel.
Remember the old television commercials in which a fussy visitor applies the “white-glove test” to see if a home is clean and dust-free? That visual cliché doesn’t hold a candle to the space agency’s procedures.
Anne-Marie Novo-Gradac, now of NASA headquarters (Washington, D.C., U.S.A.), said that for optical systems, contamination levels need to be monitored more closely than the average “clean room.” Ideally, workers should enter the clean room only in teams of three: the “clean hands” person who actually touches the instrument, the “not so dirty hands” person who hands tools to the first person, and the “dirty hands” worker who meticulously documents everything being done to the laser system. (Yes, it’s a bit like brain surgery.) Even supposedly “clean” paper sheds tiny particles into the air when someone writes on it.
“As you probably know, building a laser is like building a violin -- no two are the same,” Novo-Gradac said. This is especially true of “one-off” laser systems built only once, for a specific and expensive spacecraft.
Thermal vacuum testing must be done in an impeccably clean chamber. “Most vacuum chambers are not clean enough for your purposes,” Novo-Gradac warned.
Cleanliness isn’t the only consideration in building lasers for space. The design must be mechanically robust and stable; ideally, the laser should be able to tolerate minor misalignments due to thermal cycling. The builder must consider whether the laser will experience intermittent spikes in fluence levels or will have “hot spots” in its beam profile -- both of which could burn out optics. Unlike the Hubble Space Telescope, most lasers flying in space won’t ever be touched by repair-astronauts ever again.
If any of the optics require bonding agents, the laser scientists need to work with the vendor to space-qualify the bonding agent and the method of application. According to Novo-Gradac, the laser team for GLAS (one of the lidar missions mentioned in the OPN article) worked with the manufacturer of the beam splitter cube to eliminate trapped air between the prisms, which would have caused the optical component to rupture in the vacuum of space.
Vendors of pump laser diodes are often (sometimes monthly) changing or discontinuing their product lines, which makes life difficult for the space scientists trying to qualify products for the great beyond. The NASA team bought enough diodes from one fabrication lot that they could test them all at once and cherry-pick the best diodes for future projects.
Another NASA scientist, John Cavanaugh, discussed some of the non-optical considerations involved in building lasers to fly in space. He suggested engaging an experienced mechanical technician to work closely with the laser assembly team and performing vibrational and acoustic tests on the laser system as early as possible in the development workflow. The design team needs to verify the laser’s performance and alignment after every test along the way.
Finally, according to Cavanaugh, the space-based laser team has to build in safeguards to protect spacecraft assembly workers from accidental injury by the laser (not all of these workers will be familiar with laser eye protection, for example).
Before becoming a program executive in the astrophysics division of NASA headquarters, Novo-Gradac led the laser design teams for the Mercury Laser Altimeter (MLA) aboard the MESSENGER spacecraft and the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO). Cavanaugh, based at Goddard Space Flight Center (Greenbelt, Md., U.S.A.) is the system engineer for LOLA and also worked on the Shuttle Laser Altimeter (SLA) and Mars Orbiter Laser Altimeter (MOLA).
I’m also “tweeting” from CLEO/IQEC -- follow me @OPNmagazine on Twitter.com.