Laser-fiber coupling refers to the process of efficiently directing a laser beam into an optical fiber. This technique is crucial in many applications, including telecommunications, medical devices, industrial manufacturing, and scientific research, where lasers are used for transmitting signals or energy through optical fibers.
In Zemax fiber coupling involves designing and optimizing an optical system that efficiently directs light from a source, such as a laser, into an optical fiber. This requires careful attention to parameters like numerical aperture (NA), alignment, and beam focusing to maximize coupling efficiency. The parameters of the fiber include core diameter, which is the size of the core where light is coupled. Typical diameters range from a few micrometers (for single-mode fibers) to hundreds of micrometers (for multimode fibers). Another important fiber parameter is NA, which defines the range of angles over which the fiber can accept light. The acceptance angle of the fiber is related to the NA by the formula below:
The figure below presents a fiber coupling scheme. The beam waist is consistently positioned relative to the first surface, which, in this case, coincides with the object surface. As a result, a Gaussian waist radius is located at the source fiber position, from where it propagates through the optical system.
The lens data are as below. From Paraxial Gaussian Beam Parameters, the parameters of both surface 3 and surface 4 are over 0.06 mm, while the semi-diameter of these two surfaces are 0.12 mm.
It is important to note that the beam is not perfectly focused on the image surface: its size is larger than 4.6 µm, assuming symmetry. To improve this symmetry, we will optimize the thickness of Surface 1, which also adjusts the thickness of Surface 5 through a pick-up solve.
The only variable in Lens Data Editor is the thickness in surface 1, i.e. the distance from the input laser waist to the entrance of the collimation lens.
The Merit Function operand GBPS denotes paraxial Gaussian beam size in the optical space following the specified surface. The following parameters are to be defined to yield constraint of the Gaussian beam size. UseX, when it is 0 in this instance, the computation is for y-direction. W0 denotes the input beam waist size in lens units. S1toW denotes the distance from Surface 1 to the waist location in lens units. The M2 Factor in this instance is set to 1.
After execution of this command, the beam size at surface 6 is optimized to the waist size 4.6 µm.
From Fiber Coupling tab, System Efficiency, Receiver Efficiency and their product Coupling Efficiency are listed.
The system efficiency is the fraction of the energy in the source beam that exits the optical system. This value is determined by the input na, entrance pupil size and position, apodization, transmission of the optics, and vignetting.
The receiver efficiency is the fraction of the transmitted energy that couples from the exit pupil to the receiving fiber. This value is determined by aberrations and the na of the receiving fiber.
The coupling efficiency is the fraction of energy radiated by the source that couples into the receiver. This is the product of the system and receiver efficiencies.
In the figure of Enclosed Energy of surface 4 (the tip of input fiber), it is demonstrated that almost all energy is inside of 120 um circle, visually verifying a high coupling efficiency.