Three differents configurations based on cylindrical lenses for efficient coupling of laser diodes into multimode and single mode optical fibers...1. INTRODUCTION
When coupling laser diodes to the optical fibers, the important parameters to consider are the size of the laser diode window, fast and slow axis divergences of the laser, the modal beau diameter and the numerical aperture of the fiber. It is also important to remember that the light coupling is from a nonrotationally-symétrie source into a rotationally-symmetric optical fiber. Refocusing the light from the laser diode inside the modal diameter of the optical fiber and reduction of the incidence angles of the light within fiber's numerical aperture is instrumental for efficient coupling. In single-mode coupling removing the astigmatism of the laser diode is very important. Residual astigmatism can hamper the ability of lenses to focus light to a very small modal diameter. All these requirements could be met by cylindrical lenses. The coupling schemes are similar to those of the laser diode end-pumping. In laser pumping the goal is to contain the beam within the doped region by forming the image of the laser diode strip in the middle of the crystal. In optical fiber coupling, the image of the laser diode window is formed at the fiber end and the light cone does not exceed the numerical aperture of the fiber. Three possible approaches to the design of the fiber coupled optical system that use only cylindrical lenses are presented.
2. SINGLE CYLINDRICAL LENS (for multimode fibers only)
A lens with a diameter slightly larger than the modal diameter of the optical fiber collimates the fast axis of the laser diode and leaves the slow axis divergence unchanged. This is a simple and compact approach that might require tight tolerancing due to the very short focal length of the lens. For example, the 60 um diameter lens has focal length of 41 um and working distance of 11 um. For the laser diode with fast axis divergence of 60°, the beau width in fast axis direction after collimation is only 41 um. Due to the proximity of the laser diode and the fiber end, the beam spread in the slow axis direction only slightly exceeds the length of the laser strip. If the modal diameter is larger than the laser strip, the coupling efficiency will be high.
3. TWO CYLINDRICAL LENSES (for multimode fibers only)
The first lens collimates the fast axis of the laser diode while the second lens projects the image of the laser diode strip onto the fiber end. The second lens simply makes the slow axis beam to converge so that its waist is centered at the fiber-end. If the laser diode window is too long, this lens could be used to demagnify its size to fit inside the fiber's modal diameter. This should be used with caution since the beam divergence or numéral aperture will increase accordingly.
4. THREE CYLINDRICAL LENSES (for singlemode and multimode fibers)
If the beam after the fast axis collimation is larger than the diameter of the fiber, the tir cylindrical lens is added to refocus the beam to smaller size. Longer focal length of this lens reduces the beam divergence and helps match the numerical aperture of the fiber. The second lens is used to image the length of the diode strip, with necessary demagnification, on the fiber end. In this approach the system can be scaled to any size and thanks to extraordinary quality of our gradient cylindrical lenses it can be used for singlemode or multimode coupling. Larger lens diameters increase the system size but relax the positioning tolerances.