Summary: This in-depth article on space-qualified lasers explains
- what functions laser devices may serve in space applications,
- which special demands on operation characteristics, reliability and side effects typically occur,
- what measures may be taken to meet such demands with sufficiently high reliability,
- what kinds of testing procedures are typically applied in the context of space qualification,
- that laser manufacturers may develop special space-qualified laser models or apply such qualification to more widely used models, and what business implications that may have.
Lasers find a wide range of applications in space missions, where they often play a pivotal role. Some examples:
- Free-space optical communications, e.g. between satellites or from ground to satellites and back again, offers significant benefits over traditional radio wave transmission, including higher bandwidth and reduced requirements for space and volume.
- LIDAR systems are instrumental in navigation, aiding in critical docking and rendezvous operations of spacecraft. This technology is also used for remote surface monitoring for scientific purposes or for mission control, such as identifying a suitable landing site for a moon probe.
- Some pulsed lasers are integral to analytical techniques such as laser-induced breakdown spectroscopy, aiding planetary exploration by determining the composition of celestial bodies.
- Various other kinds of scientific instrumentation can also be based on laser devices, e.g. for laser spectroscopy and for realizing optical tweezers in microgravity experiments.
The vast array of applications necessitates a diverse range of laser devices, each with its unique performance specifications. These include variations in output power, continuous or pulsed operation, linewidth, etc.
In space missions, laser devices are launched (typically as part of some system) into some Earth orbit or some other trajectory with a rocket. To perform effectively in the vacuum of space, amid extreme temperature fluctuations, and under the bombardment of cosmic radiation, lasers must be rigorously space-qualified. This stringent process involves comprehensive testing and documentation against exacting standards, making it likely that each individual laser unit will perform satisfactorily.
Demands on Space-qualified Lasers
While the details vary greatly from mission to mission and application to application, here are some typical requirements for space-qualified lasers:
- Such lasers must reliably meet various performance specifications, which can relate to different aspects such as:
- All of this must be achieved under a variety of potentially detrimental influences:
- Rocket launch is usually associated with extreme vibrations, which may break parts or at least cause some misalignment of sensitive components.
- During launch and possibly during operation (or non-operational periods), extreme temperatures can occur, especially if no temperature stabilization can be provided. This can lead to a variety of problems, such as component breakage due to excessive mechanical stress, misalignment due to thermal expansion, chemical changes e.g. in adhesives, etc.
- The system will also be exposed to significant doses of hard radiation, which can only be mitigated to some quite limited extent by shielding.
- Depending on the system, the lack of gravity and operation in a vacuum can also have some detrimental effects. For example, a vacuum may cause moving parts to malfunction because of changes in surface properties. Ultrahigh vacuum conditions may affect the density and performance of dielectric coatings, and may also affect nonlinear crystal materials, e.g. through loss of interstitial water (hydration). Such dehydration, similar to the loss of oxygen, may affect phase matching or electrical properties in the case of optical modulators. Outgassing, e.g. from adhesives, may affect nearby optical surfaces, making them more susceptible to laser-induced damage. Such mechanism can be substantially stronger under ultrahigh vacuum conditions.
- In addition, it is often highly desirable to minimize weight, despite potentially stringent mechanical stability requirements. The available volume may also be quite limited.
- Finally, it must be ensured that the laser system does not adversely affect other components beyond certain acceptable limits. This may apply to power consumption, heating effects, outgassing, and other aspects.
- It is essential to have a laser design that is not only found to work, but also thoroughly understood in great detail – especially in aspects related to known failure modes. This usually involves developing simulation models for various aspects (related to optics, laser physics, mechanics, thermal behavior, etc.) that accurately describe device operation and can predict the effects of various external influences or non-ideal component properties.
- A wide range of failure modes, including many that occur specifically under space conditions, must be considered. This requires a deep knowledge of failure mechanisms, often at the molecular level. Based on this, special materials can be used and certain other materials or components known to cause problems in space, such as outgassing or failure under certain conditions such as extreme temperatures, high vacuum, or exposure to harsh radiation, can be avoided.
- The quality of all parts used must be carefully optimized. For consistent results, the supply chain must be monitored to ensure that there are no changes in the raw materials used or in the details of the manufacturing processes (regarding aspects such as cleaning, inspection, assembly, soldering, etc.) that could affect performance or lifetime, e.g. by triggering known failure modes. Used parts must be carefully inspected so that individual parts with problematic defects can be removed. For example, optical surfaces must be inspected for microscopic defects that can grow over time-before they are integrated into the device, and possibly again after operational testing. To do this, the laser design may need to improve component accessibility and, in some cases, replaceability. Ideally, thorough inspection of optical components should be possible without affecting system alignment and without introducing new contamination.
- The lasers must be assembled with well-defined tools and procedures, leaving as little room as possible for any relevant random influences.
- For bulk lasers, the resonator design must be developed particularly carefully, taking into account aspects such as the alignment sensitivity of optical resonators, details of thermal lensing, and others.
- Mechanical fixation of parts, and especially the realization of mechanically moveable parts, often requires sophisticated special measures, since operation in a vacuum introduces significant deviations from the normal behavior of parts.
- For maximum reliability, components are often derated. For example, laser diodes are operated at a drive current that is well below the normally allowed level.
- Laser devices can be made more reliable by introducing a degree of redundancy. For example, a solid-state laser may be optically pumped by several laser diodes, which together may provide more pump power than is needed. It must then be checked whether, for example, the sudden failure of one pump diode can be compensated sufficiently well by activating or ramping up another one, and whether this could lead to negative performance aspects such as changes in the beam direction.
- Special attention must also be paid to any required electronics, which may also be sensitive to extreme temperatures, radiation, and possibly vacuum, e.g. due to lack of heat transfer by air.
- All documentation must be much more detailed than that required for less critical applications.
Test Procedures Applied in Space Qualification
Some typical test procedures are as follows:
- Optical inspection: All optical surfaces are microscopically inspected for defects that may affect performance or lifetime. Other searches may target possible defects in mechanical parts, contamination, and other aspects. Such tests, as well as various performance tests, are typically performed by the manufacturer before the laser undergoes the actual space qualification procedures. After that, it may be considered as a “black box” that only has to meet various criteria without taking into account its internal details.
- Vacuum and thermal testing: All relevant aspects of laser performance are tested under the influence of vacuum (where it may be important which vacuum regime is applied in terms of residual pressure) and possibly temperature changes similar to space conditions. Perfect simulation of such environmental effects may be difficult, and some degree of variability must be allowed for by applying sufficiently large safety margins. There are operational tests, where the laser performance must be ensured during the application of the environmental influences, and survival tests, where the laser only has to work well after the exposure.
- Mechanical testing: The equipment is subjected to vibration and/or mechanical shock as it is expected to be exposed to during the mission.
- Testing in zero gravity is more difficult, especially if it is required for more than a short period of time. In some cases, it may be necessary to perform some weightlessness parabolic flights.
- Radiation testing: The equipment is exposed to ionizing radiation that simulates the radiation experienced during the space mission. Radiation can cause various types of degradation, such as loss of transparency of optical components, including optical fibers. Even radiation-resistant fibers have a limited, though greatly improved, tolerance to radiation.
Note that not all aspects can be considered separately. For example, intense radiation and extreme temperatures can have significant effects on vacuum performance. Time may also be a significant factor; certain failure modes may take considerable time to develop.
The development of test procedures is often a major undertaking in itself. A great deal of experience must be applied to achieve sufficiently reliable test results.
Typically, space lasers must undergo extensive preliminary testing before formal space qualification. This can reduce the overall cost and development time, and avoid the need to redesign and re-test after a failed space qualification.
Tests can be performed on a larger number of devices over a longer period of time to provide reliable life testing results. This provides statistical information on expected failure rates.
Qualification testing can be organized in several ways. Tests can be performed at the laser manufacturer's facility if the test facility is set up to the space agency's standards. Alternatively, the space agency's own test facilities may be used, or third-party test facilities, or a combination of these. Sometimes external certification bodies are involved to independently confirm that all tests have been successfully passed. In any case, the space agency, which is ultimately responsible for mission success, will typically require a high level of control over the entire system of processes.
In space applications, failures can be extremely damaging and frustrating. Therefore, such tests must be performed in a very systematic and careful way, trying to eliminate any possible errors (e.g. made by operators), overlooked problems, etc. As a result, detailed test protocols are developed and must be executed very accurately, including systematic documentation of all results and careful statistical analysis. This can be very costly and time consuming. Various space agencies have developed sophisticated standards with rules on how to define and perform the required tests.
Space-qualified Laser Models
Laser manufacturers may need to design certain lasers specifically for space applications, taking into account many aspects of laser design. In addition, other special extras may need to be developed, such as additional electronic control systems. This process typically results in significantly higher prices, also considering that sales volumes for such applications are often particularly low.
In some cases, simpler laser models, typically used in less demanding terrestrial applications, are also space qualified to open up an additional application area. They can then be sold for high-volume, low-cost applications as well as for space. This can be done with laser diodes, for example, while this approach is less likely to make sense for more complex laser systems.
The significant additional cost of developing and qualifying lasers for use in space can in principle be covered simply by the high sales prices of such products, but can also be financed by special contracts with space agencies. For the laser manufacturer, beyond the direct financial result, there may be significant additional positive spin-offs in other business areas, such as increased expertise in producing robust and reliable laser equipment, gains in reputation, and increased modifications for future projects. Therefore, space qualification does not necessarily have to pay for itself.
Space-qualified Optical Parts
Not only entire lasers or laser systems, but also specific smaller parts can be space qualified. This applies for example to optical components like mirrors, lenses and prisms or radiation resistant fibers. One can also qualify assemblies such as objectives, laser scanners, and optical resonators. Other examples include certain optical modulators and integrated photonic circuits. Some of these may be used in space-qualified lasers, making their development somewhat easier.
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