Fiber Coupling of a Laser Diode

Accurately coupling an ND: YAG or another powerful laser into a fiber delivery system can be a challenging task. Here is an example of how the process can be made easier using a beam profiler.
Figure 1 shows the beam profile of a laser diode being used as an optical input to a fiber. The beam was collimated with a lens to bring it to a relatively uniform round profile, rather than the usual rectangular beam profile emitted by laser diodes. The focused spot of the laser diode was then coupled into an optical fiber. This coupling is critical in all three axes, X, Y, and Z.

Figure 2 shows the fiber output when the diode is coupled properly in the X and Y direction, but not in Z. In this case the focus is behind the surface of the fiber, so much of the beam energy is being coupled into the cladding, rather than the center of the fiber. Thus the output of the fiber appears as in Figure 2 with a significant amount of energy not in the central beam. The energy that is not in the central beam is diverging at a much higher rate than the central lobe, and would be easily lost in the application of this fiber. Figure 3 shows much better Z axis alignment of the diode into the fiber. In this case much more of the energy is in the central lobe.

Figures 4 and 5 show the same in the 2D representation. An aperture has been drawn to determine how much energy is in the central lobe. In the poorly aligned coupling in Figure 4, we see that only 31% of the energy is within the central lobe. This means that 69% of the energy is being wasted into the highly divergent part of the beam. In the case of well aligned coupling in Figure 5, roughly 66% of the energy is in the central 1mm spot.

You might also like to read: The NanoScan Beam Profiler For aligning fiber optic components