Fiber Launch Systems

Fiber launching begins with carefully fixing the fiber end in the launch system or attaching the fiber connector. Ideally, one should check with a fiber microscope whether the cleaved fiber end is intact and clean.

Working with angle-cleaved fibers is more difficult. One should be aware that optimal launching then requires an input beam direction which deviates from the fiber axis, and any rotation around the fiber axis will of course change the required input beam direction. In difficult cases, it may be helpful first to launch some visible light from the other end in order to align the angled end such that the mentioned visible light overlaps well with the input beam.

Next, the collimated input beam should be prepared with an appropriate beam radius and roughly aligned, possibly using auxiliary parts such as beam apertures. Preparing a beam with appropriate beam radius is particularly important for single-mode fibers. If a high launch efficiency is required, one should calculate that beam radius from the fiber mode radius and the focal length of the used lens. Alternatively, one may measure the radius of a collimated beam as obtained when sending light through the fiber and the used lens, this time used as collimation lens.

During the alignment operation, one should avoid excessive optical input powers because those could lead to damage e.g. of a fiber coating.

Further, a suitable photodetector should be attached to the fiber end so that one can monitor the injected optical power. If the fiber is long enough, one should normally have only properly launched light getting to the detector rather than e.g. the fiber cladding; usually, cladding modes have substantial propagation losses. For the initial alignment phase, it can be helpful to have a detector with high sensitivity, possibly using a much higher signal amplification than in the final phase. Obviously, one should take care that no stray light can get to the detector, simulating transmission through the fiber.

In case of an active fiber (rare-earth doped fiber), one may alternatively monitor fluorescence light at the side when launching pump light. The latter technique also helps to ensure that light is injected into the doped core rather than into the fiber cladding – particularly in case of double-clad fibers.

It is helpful if the detector reacts relatively quickly – faster than for example a thermal power meter. A convenient solution can be to use a photodiode in conjunction with an oscilloscope.

It may be necessary to measure the absolute launch efficiency in order to judge whether the launch was fully successful.

One can then play with the alignment controls until some launched light can be detected, and then systematically optimize the transmission by fine adjustment of the controls. Instead of optimizing one control after the other, one often uses a pair of control screws – for example, those for the transverse position and the beam angle in a horizontal plane. Here, one subsequently sets different positions of one screw and checks how much transmission is achieved by optimizing the other one.

If no satisfactory launch efficiency can be achieved, one should check various details, for example whether the beam quality has been deteriorated e.g. due to dirty optics and whether the fiber end is of high quality and clean.

When repeatedly using such a set up for launching, it is of course advisable to keep records on the achievable detector signal for a certain incident optical power. Also, I can be helpful to mount beam apertures at appropriate locations in order to more easily find the right input beam direction.

There are fiber launch solutions which can automatically optimize the alignment for efficient launching with at least two degrees of freedom, and also provide long-term stability of the launch efficiency.

For example, such a system may have an electromechanical system for rapidly rotating the position of the focusing lens around its nominal position. At the same time, the light throughput is monitored with a detector. From the observed installation of the detector signal and its phase relative to the drive signal for the lens mount, the electronics can evaluate any positioning errors and subsequently correct them.

Automatic launch systems can be fairly helpful, although they of course increase the cost and require additional preparations such as connecting a suitable detector. Also, of course they work only as long as continuous input light is available.

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