Fibre optic beam delivery. Launching efficiency measurement by dual-detector cladding power monitoring

In the design and operation of high-power fibre optic beam delivery systems, a primary consideration is the efficiency with which power can be launched from the laser source into the core of the fibre. Techniques for the measurement of launching efficiency, based on monitoring power guided by the fibre cladding, are discussed here, and advantages over direct measurement methods are indicated. An extension to the cladding power monitoring technique is introduced, which involves the use of two cladding power detectors separated by a controlled bend in the fibre. It is shown that it is hence possible to normalize the cladding power measurement by the core power, and thus to determine the launch efficiency independent of the laser power.