Loss analysis of laser-fiber coupling and fiber combiner, and its application to wavelength division multiplexing.
The combination of index-guided laser diodes and wavelength-independent multiplexers is shown to be the best choice for wavelength-division-multiplexing (WDM) systems. Such a transmitter end of a four-channel WDM system will only cause an insertion loss of 1.5 dB or less (laser-fiber coupling plus multiplexer), which is hard to obtain with wavelength-selective multiplexing. In the case of gain-guided lasers it will depend on the number of channels and wavelength separation whether or not wavelength-independent multiplexing is preferable. To include all types of laser diode we had to extend laser-fiber coupling theory to astigmatic laser diodes. The surprisingly simple exact relations based on Gaussian beam analysis show that the usual gain-guided lasers (astigmatism of ~20 microm) are not able with symmetric lenses to launch more than ~20% of their power into a single-mode fiber, whereas an index-guided laser with the same far-field divergence couples ~85%.
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