Date Announced: 09 Jan 2019
On September 15th 2018, NASA launched a satellite into space using the last of the Delta II rockets. This satellite, ICESat-2, is on an all-important mission; to analyze the Earth’s changing surface heights in closer detail than ever before. ICESat-2 is carrying on where the original ICESat left off when it ceased it's mission in 2009. This time, the satellite is equipped with a powerful new instrument – the ATLAS. On-board the ATLAS is an RTP Q-Switch, manufactured by Raicol.
The ATLAS is a sophisticated laser system that allows for highly accurate measurements of ice sheet height, land topography, and vegetation canopies across the globe. Standing for ‘Advanced Topographic Laser Altimeter System’, the ATLAS was developed especially for ICESat-2, built and tested in the Goddard Space Flight Center. It works using a method called ‘lidar’. Pulse laser beams are fired at the Earth’s surface, and the photons that return to the satellite are measured by a sensor. This data can then be used to form 3D images and record accurate measurements of the Earth’s surface.
The laser module of the ATLAS was developed by Fibertek Inc. Where ICESat-2 outdoes its predecessor, ICESat, is with the use of a micro-pulse, multi-beam approach. As the satellite orbits, Fibertek’s single green laser is split into six beams, in three pairs. The laser fires 10,000 times per second, sending out over 300 trillion green photons of light. On-board mechanisms record the precise time taken for photons to return to the receiving sensor after reflecting off the ground. This allows for highly comprehensive coverage, measuring height changes to within an inch in place, and taking measurements every 70 centimeters along its path. When you think about the size of the satellite, and its distance from Earth, this is an incredible accomplishment. Thanks to ICESat-2, scientists can now create a clear and detailed picture of ongoing changes on the Earth’s surface, even on rough terrain and sloped areas. So, how does this multi-pulse, multi-beam laser work so fast?
At the heart of the system is none other than Raicol’s RTP Q-Switch.
Crystals in Space
Raicol’s Q-Switch allows Fibertek’s laser to produce the pulsed output beam central to the satellite’s functioning. The two crystals in the Q-Switch are made from RTP, which is a compound of Rubidium, Titanium and Phosphate. Crystals made from RTP are becoming the benchmark for use in Q-Switches, allowing higher laser repetition rates, extremely fast rise and fall time, and a short pulse width. RTP crystals are also extremely reliable, with a high damage threshold, making them perfect for high-power electro-optical applications. These crystals can easily meet the high demands of a space mission, as they are able to withstand extreme conditions, such as high pressure and temperature variations. This promises great performance for ICESat-2.
In the ATLAS, the Q-Switch is connected to a changing power supply. This power supply generates two different voltages, to change and control the crystal’s polarization. The first voltage allows Fibertek’s laser beam to pass through the crystal, and the second voltage blocks the laser beam completely. While
blocked, the laser beam’s energy accumulates. When the voltage is switched, all of the accumulated energy is released in one go, passing through the crystal at an incredibly high speed. This creates the rapid, high intensity pulse required for the ATLAS to fulfil its task.
From a University Lab to Outer Space
The Q-Switch is a critical component for mission success Manufacturing and preparing a critical component for space travel involves meeting a strict set of requirements and carrying out a complex array of exhaustive tests. As the global pioneers of RTP Q-Switch crystals, Raicol were fit for the challenge.
When Raicol was first established by Dr. Nahum Angert, in 1995, it was a small company working out of a laboratory in Ariel University, Israel. In the years that followed, Raicol continually invested in the development of new procedures for crystal growth and assembly.
Dr. Angert, who sadly passed away in 2017, spent over fifty years involved in the research, development and production of non-linear and electro-optic crystals for laser applications. His expertise formed a basis through which Raicol was able to develop and succeed. By the end of his lifetime, Raicol had become a world-renowned leader in the field. Now, the company is considered a one-stop-shop, providing a comprehensive solution from crystal growth through to coating and EO cell assembly, creating crystals for many different optical applications.
Raicol’s state-of-the-art, brand-new manufacturing facility is equipped with the latest technologies. These technologies include proprietary growth systems, cutting equipment, polishing machines, X-ray measurement systems, clean rooms, an optical shop and a coating facility. Having an in-house coating facility is rare, and this unique capability allows Raicol to carry out the entire coating process on-site, without sending crystals to a third party. Many types of crystals are produced at the Raicol facility – including KTP, RTP, LBO and BBO crystals.
High-end internal testing capabilities at the facility include spectrophotometry, assessment of LDT (laser damage thresholds), wave-front distortion, and absorption. The advanced equipment, together with knowledge and expertise, enables Raicol to achieve maximum quality and reliability for each and every product manufactured. The growth and assembly process are tailored to each crystal and its specific application. Customer needs are always met.
Mr. Yehiel Plaut, Raicol's Sales & Marketing VP, is enthusiastic about Raicol’s involvement in producing the Q-Switch for ICESat-2: “Until now, we have been selling our products to the medical, industrial and military markets. This project allows us the opportunity to penetrate the aerospace market as well."
Built for Launch
To produce the RTP Q-Switch used in ICESat-2, the team at Raicol grew two completely identical crystals in proprietary growth chambers, for approximately three months, under strictly controlled conditions, including three-dimensional temperature control. These growth chambers are especially customized for growing RTP crystals.
After growth, the crystals were removed for optical fabrication. It is during this process that the two crystals were cut, shaped and polished to perfection, by a team of over a dozen expert engineers. After this process was completed, the crystals were taken to the coating facility. Here, engineers applied a high quality anti-reflective optical coating, minimizing reflection for maximum performance.
When the coating process was complete, the crystals were sent for an extremely thorough inspection. This inspection was carried out by a team of top physicists at the Raicol facility. Each crystal was inspected individually and completely, including a full visual inspection. The inspection ensured that the crystals were both free from imperfection, and that they matched each other perfectly. This inspection took place in a highly controlled clean room to prevent contamination.
The two matched crystals were then assembled to form the Q-Switch. As part of this process, the crystals were joined together in a special double-crystal configuration. This configuration ensures that the two crystals are able to provide thermal compensation for each other when faced with temperature changes during operation. Thanks to this feature, the Q-Switch can remain functional throughout a huge temperature range, from -50 to +70°C.
After assembly, it was time for the Q-Switch to be tested in all areas of performance. Testing is necessary to ensure that the product is rugged and reliable, made to withstand the harsh conditions that would be experienced during launch and throughout ICESat-2’s orbit. The Q-Switch, and all other components, must continue to function flawlessly for the duration of the mission – which NASA plans to continue for at least three years.
Mr. Plaut says: "To meet launch and space system requirements, the Q-Switch was tested for resistance to extreme temperatures, vibrations, and radiation. A team of engineers exposed the Q-Switch to such conditions in controlled simulations. This involved subjecting it to loud sounds, high heat, fierce cold, strong vibrations and more.”
With these tests passed, the Q-Switch was ready for use in Fibertek’s laser, forming part of the ATLAS on-board ICESat-2. Once all the satellite’s components were assembled and tested, NASA confirmed that the satellite was suitable, and ready, for launch.
After ICESat-2 satellite was launched, the team waited two weeks before firing the ATLAS lasers for the first time. This was to prevent contaminants from Earth from damaging the optics. On September 30th, ICESat-2 laser fired for the first time over Antarctica, returning its first height measurements on October 3rd. This was an exciting time for all involved, seeing the lasers turn on and the first photons return, a clear sign that the ATLAS, and the Raicol Q-Switch at its heart, were functioning correctly. In the days following, the satellite continued flying over the globe, recording elevation measurements close to the South Pole and across the Antarctic ice sheets.
Over the course of the mission, NASA hopes that the ATLAS onboard ICESat-2 will continue to reveal new and important information about our changing planet
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