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The feasibility, efficiency, and final beam quality of coupling a laser beam into an optical fiber all depend on the characteristics of the original laser and the type of optical fiber.
For TEM00 beams, efficient coupling is relatively easy to achieve with both single-mode and multimode fiber. A short focal length ordinary lens or gradient refractive index (GRIN) lens must be precisely mounted one focal length from the fiber core (assuming the input beam is parallel). The pre-assembled fiber coupler permanently pre-aligns this lens. Then, precise alignment of the coupler with the laser is required, controlling four degrees of freedom—X-axis, Y-axis, horizontal rotation, and pitch. If the beam is not parallel, the distance between the fiber tip and the lens (Z-axis) also needs to be adjustable.
For single-mode fiber, the quality of the output beam is similar to that of the input beam, but it diverges and requires collimation with a lens. However, the collimation effect is limited by diffraction and is therefore very good, similar to that of the original laser.
To maximize coupling efficiency, the mode shape of the beam entering the fiber must match the mode shape of the fiber. Simply put, any given fiber has a numerical aperture (NA) specification. The beam entering the fiber should be matched to this specification for optimal coupling. Narrow beams couple less effectively than wide beams because a wide beam can fill the cone defined by the fiber's NA. Therefore, the lens of the fiber coupler also needs to match the diameter of the input beam.
Alignment with single-mode fiber can be challenging because there are often virtual or phantom reflections inside the coupler, which can introduce coupling power. If one of these reflections is detected, the adjustments made to achieve a local maximum will result in power several orders of magnitude lower than the power the main beam could potentially reach.
The simplest way to perform initial alignment and minimize the possibility of reflection is to reverse the beam through the coupler. (Standard fiber optic coupling assemblies are available and can be connected to inexpensive helium-neon laser heads suitable for this purpose.) The result is a collimated beam exiting from the lens. Alignment is close enough when this beam is precisely aligned with the beam emitted by the laser to be coupled into the fiber (at both the fiber coupler and the laser). At this point, the helium-neon laser used for alignment can be removed and replaced with a laser power meter or spectrometer. The alignment can then be optimized by monitoring the optical output power of the fiber.
For multimode fiber, the output beam will be multimode, and its numerical aperture (NA) is similar to that of the fiber. The core diameter limits the collimation capability according to the focal length of the collimating lens. Bending or twisting the fiber will significantly affect the mode pattern of the output beam.
For other types of lasers, the characteristics of the original beam will determine the feasibility of these two coupling methods (coupled with single-mode or multimode fiber).
Let's talk about something interesting, a common question from beginners: "How to couple light from a light bulb into an optical fiber?" The simple answer is: "Unless you use a very large diameter optical fiber, or a highly concentrated light source like a laser, it's practically impossible, or at least impossible to achieve any efficiency."
Another frequently considered idea is, “What if we thinned the fiber? That way, the light entering from one end could be compressed into a smaller diameter at the other.” Unfortunately, this doesn’t work. If the light is coupled to its maximum, the pattern shape of the input beam matches the fiber’s angle of reception (NA). As the fiber diameter decreases, the internal reflection angle increases. However, the internal reflection angle is already at its maximum, determined by the fiber’s NA, so light leakage occurs. If the fiber (well, actually not a regular fiber, but a light guide) has a mirrored boundary, the light is trapped, but due to the larger reflection angle, the beam diverges more from the other end, and the brightness doesn’t increase. This principle—called a lens guide—is used for beam shaping in high-power laser diodes.
Any passive spatial filter—that is, any device that focuses an initially incoherent beam of light through an optical fiber, a pinhole, or any similar optics—works to “improve the spatial coherence of light” by filtering out a large number of spatial modes (or angularly distributed plane waves, etc.) in the beam, allowing only a small fraction of the original light to pass through.
If light from any thermal light source, incandescent lamp (or fluorescent lamp, etc.), is filtered strongly enough to achieve true spatial coherence (e.g., by passing the light through a single-mode fiber), then practically no useful light is retained. Light from a source with a radiation temperature of 6000°K produces approximately one photon per second per hertz bandwidth in each individual spatial mode.
If the filter is a multimode fiber that propagates or transmits N spatial (or lateral) propagation modes, then the aforementioned number of photons must be multiplied by N. With this system, you cannot couple more spatial modes (equivalent to "spatial information").
Fiber optic coupling may seem simple, but the actual operation process requires attention to many complex points. We recommend the BU-LASER Pigtail output laser, a hassle-free solution.
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