:: Laser Diode to Fiber Coupling Techniques
The brute-force approach of simply butting an optical fiber almost against the laser diode allows for only about 10-20% coupling efficiency, or a 7 to 10dB coupling loss. The coupling approach is show below.
This is partially because that the laser diode emission pattern is somewhat elliptical. Analytical expressions for such patterns have been used for both LED and LD (laser diode).
So other coupling techniques were developed using various lenses to reduce the coupling loss. This is shown in the following illustration.
Some of these techniques use lenses that are cylindrical instead of spherical to compensate for laser diode’s elliptical emission pattern. Other factors in selecting the coupling technique include small package size, optimized alignment, etc.
The coupling efficiency is very sensitive to the alignment. This sensitivity is shown in the following figure. This figure shows the sensitivity to lateral displacement between the diode and the lenses or fiber. Longitudinal displacement is another factor. Generally speaking, the lower the coupling loss, the more sensitive the optimum coupling is to displacement.
:: Reflection of Light Back to the Diode
You should also consider the impact of reflection of the emitted light back into the diode. Planar surfaces perpendicular to the optical axis are particularly bad in this respect.
This problem can be corrected by having such surfaces at an angle from the perpendicular, and this angle is typically less than 10º because of the small angular spread of a single mode beam.
The other technique to solve the reflection problem is to use curved surfaces. The radius of the curvature must be small because of the small mode field diameter (MFD) of the fiber or components.
You can also use index matching gel between the offending surfaces or thin-film anti-reflective coatings to reduce the severity of back reflection.
:: Stability of the Coupling Structure
The mechanical stability of the coupling technique is also very important. It must have stability against mechanical vibrations and temperature fluctuation.
That is why the most common laser diode to fiber coupling approach is via a lensed and often tapered fiber.
The tapered fiber tips change the mode field diameter (MFD) of the fiber. Tapered fiber can be manufactured by pulling on a short length of fiber – this approach reduces the diameters of both the core and cladding. In this approach, the MFD is enlarged. This approach is shown in the following figure.
Tapered fiber can also be made by etching a stub of fiber, so that part of the cladding is removed. In this case, the core diameter and MFD keeps the same. This approach is shown below.
In both cases, a lens at the taper tip is fabricated to reduce the MFD for better coupling efficiency. Such lenses can be formed on the fiber end by the following methods.
- Heating by a flame
- Heating by a electric arc
- Heating by a laser
- Dipping into molten glass and fusing
:: Fiber Lens Shapes
Since the laser diode emits at a elliptical pattern, a simple hemispherical lens surface is not enough for better coupling. That is why more complex lens shapes were developed.
A hyperbolic lens is often used. This lens shape has about the same transverse offset loss as in a hemispheric lens, but there was less sensitivity to tilt and more sensitivity to longitudinal translation.
With more complex lens shapes, the sensitivity is also more complicated. The best coupling efficiency is about 90% or 0.45dB coupling loss.
Another technique is using a wedged-shaped fiber end as shown below. This is done by grinding the tip of an un-tapered fiber. Perpendicular to this plane, the fiber and laser modes are closely matched.