The Koheras laser is a class of compact and robust inherent single frequency fiber lasers with particularly low noise properties and a high spectral stability. It is based on a Distributed Feedback (DFB) fiber laser design consisting of an active fiber, a Bragg grating, and a semiconductor pump diode. Both Erbium and Ytterbium doped fibers are used as a gain medium. The Bragg grating defines the mirror of the laser cavity and is imprinted in the fiber laser by ultra-violet illumination through a phase mask. Finally, a pump laser generates population inversion in the active fiber and so supplies energy to drive the laser.
The key to the fiber laser’s low noise is its gain medium and fundamental structure, with the high Q-value fiber Bragg grating ensuring very stable emission and high frequency stability. The laser packaging is designed to reduce the lasers’ susceptibility to acoustic pickup and vibrations – a feature which is critical for stable laser operation in harsh environments and many industrial applications.
Different grades of laser phase noise are available, and course and fine wavelength tuning is possible whereby the laser wavelength can be tuned to specific wavelengths, e.g. transition lines, and stabilized to fixed references for even higher frequency stability than provided by the free running laser.
The Koheras lasers are typically installed in coherent sensing systems for long range fence security, pipeline integrity monitoring, oil and gas exploration, down the well sensing, high precision vibrometers and wind LIDARs. Any standstill period caused by a component failure in these systems can be very costly for the operator and therefore require unproblematic long term operation of laser sources. The main optical sub components are Telcordia qualified originally designed for the telecommunication industry, and the fiber laser tested to high standards to meet the requirement for many years of stable and reliable operation in the field.
Koheras lasers are also important tools in metrology for atomic clocks, quantum information, and high resolution spectroscopy, and they have contributed to and enabled important research results in different areas of fundamental science. They play a vital role in fusion energy, radio the Atacama Large Millimeter Array (ALMA) radio astronomy telescope, and ESA’s SWARM mission.