COOL? The Compact Optical clOck Laser
In brief
The COOL Project (Compact Optical cOckl Laser) goal was to develop a new stabilized laser source for Strontium optical atomic clocks at 689nm.
In-depth
The need for smaller and more precise lasers is a universal demand across almost every market where lasers are a key technology. The key parameter for stable lasers is the linewidth or, simply, the precision of the color of the laser. The smaller the linewidth the more precise the color of laser. This is extremely important for sensitive sensing applications such as advance timekeeping and LIDAR. There are very limited commercial options for precise lasers (linewidths < 100 kHz) at a range of wave lengths.
ESA has extensively invested in next generation optical atomic clock technology based on strontium atoms for use on future space missions. Ultra precise optical atomic clocks rely on lasers for control of the system as well as telling time. The laser systems are critical components, but unfortunately, they are currently very large in size, are heavy and have a high power consumption. In addition to the space segment applications of optical atomic clocks and LIDAR, there are range of ground segment markets that require compact and precise laser systems. There is an active ground segment for optical atomic clocks and well as for sensing applications.

The coherent LIDAR market is currently over 1 Billion USD and growing rapidly. Future ESA missions will rely on optical clocks. One of the most promising candidates for a future space based optical atomic clock is the Strontium neutral atom lattice clock. But the laser systems are currently to large to make space flight practical. The COOL Project (Compact Optical cOckl Laser) goal was to develop a new stabilized laser source for Strontium optical atomic clocks at 689nm.
An optical microresonator from MicroR systems was a novel solution, serving as a reference filter for the discrete mode laser light at the target wavelength. MicroR Systems extended their technology to 689nm from the near infrared to enable around a 25 times reduction in the size over current technology. These crystalline-base optical microresonators are miniature circular crystals that only allow very precise optical frequencies to circulate inside. Electronic feedback is applied to the laser to correct the laser frequency to keep it in resonance, synchronized with the microresonators. The electronic feedback portion is an established technology. But, the use of microresonators to fine tune the laser frequency is the novel solution to a long standing laser problem. The project will yield a 100-fold or more improvement over existing laser technology by using an extremely precise microresonator to improve the performance of a discrete mode laser.