| Abstract [eng] |
Optical isolator is a crucial component in laser systems, primarily because it protects these systems from unwanted reflections. Reflections can lead to issues such as power instability, amplitude noise or even damage to sensitive optical components or the laser source itself. Optical isolator allows light to travel only in one direction from the laser source towards the optical system by blocking or redirecting light that attempts to travel in the opposite direction. In small laser systems, there is a need to reduce the size of their components, including the optical isolator. This need arises not only from the requirement for a uniform size but also due to the demands for system integration, compactness, and convenience. Traditional optical isolators are often larger due to easier construction using bigger magneto-optical crystals and magnets, posing challenges when creating integrated and compact isolators. The goals of this study include designing and experimentally testing compact optical isolators optimized for continuous wave lasers at 633 nm and 785 nm wavelengths. The evaluation will involve various magneto-optical materials suitable for these wavelengths, with a particular focus on the Verdet constant. This study also aims to create an optimized geometric design for the optical path and other components to ensure high performance while maintaining compactness, and to test the designed isolator to measure its transmission, isolation, and Verdet constant. The Faraday effect, a fundamental principle in many optical isolators, describes how the polarization plane of a linearly polarized light beam rotates as it passes through a material influenced by a magnetic field parallel to the light propagation direction. A unique aspect of the Faraday effect is its independence from the direction of light travel. This means that if light travels back through the same material in the opposite direction, the polarization plane rotates in the same direction, effectively doubling the rotation angle compared to the initial light beam. This rotation is defined by the Verdet constant values, which are material-dependent. The experiment measured the strength of the Faraday effect in TGG (Terbium Gallium Garnet) and TSAG (Terbium Scandium Aluminum Garnet) crystals. Linearly polarized light was obtained from a laser with a wavelength of either 633 nm or 785 nm, and its collimated beam was transmitted through a fixed polarization cube. A neodymium magnet created a sufficiently strong magnetic field, allowing the measurement of Faraday rotation. Comparing measured Verdet constants of all used crystals, those with best results were selected for optical isolator construction. Transmission of the selected crystals for optical isolator was also measured, which is the ratio of output optical power to input optical power, expressed as a percentage. It indicates how much light passes through the optical isolator without absorption or scattering. The experimental setup involved measuring losses by comparing power passed only through the magnet aperture, through the crystal without the magnet, and through the crystal inside the magnet. These measurements allowed the assessment of optical isolator losses and their origins. Other important optical isolator parameter is isolation which was also experimentally tested. Isolation shows how effectively the isolator blocks backward-traveling light compared to forward-traveling light and is measured in decibels (dB). To calculate isolation, the setup included returning the laser beam through the isolator with a mirror and directing the beam that passed through the isolator a second time into a power meter to evaluate the returning signal. After all experiments it was concluded that selected magneto-optical material was suitable and optical isolator setup met the parameters and was compact enough for integrating it into 633 nm and 785 nm wavelength lasers. |
