Alvaro A. P. Boechat

Bend loss in large core multimode optical fiber beam delivery systems

We present results of an experimental investigation of the optical losses produced by bending large core optical fibers, typical of those used in power beam delivery systems. Experiments have been conducted over a range of core diameters for both plastic clad silica and all-silica fibers as a function of bend radius. A theoretical model has been developed for predicting the magnitude of the bend loss, and agreement was obtained with the experimental results. The study thus yields design information for fiber beam delivery systems.

Beam delivery by large-core fibers: effect of launching conditions on near-field output profile

For large-core optical fibers of a few meters length, which are typical of those used in beam delivery systems for high-power Nd:YAG lasers, it is shown that the near-field profile of the output beam is a strong function of the launching conditions. The output profile depends on both the input spot size and its alignment relative to the fiber axis. A simple theoretical model is developed for step-index fiber that shows that the output profile depends on the distribution of guided power between meridional modes and groups of skew modes. A relationship is hence derived between the launching conditions and the output profile. The predictions of the theoretical model are consistent with experiment.

Dependence of output near-field beam profile on launching conditions in graded-index fibers used in delivery systems for Nd:YAG lasers

For graded-index optical fibers of a few meters in length and of large core diameter, which are typical of those used in beam-delivery systems for high-power Nd:YAG lasers, it is shown that the near-field profile of the output beam is a strong function of the launch conditions and laser-beam characteristics. The output profile depends on the input spot size and its alignment relative to the fiber axis as well as on the dependence of the beam divergence on the position within the input spot. A theoretical model has been developed to demonstrate how the output profile depends on the distribution of guided power between meridional modes and groups of skew modes, which are excited by a particular launch condition. The predictions of the theoretical model are consistent with the experiment.

Bi-directional cladding power monitor for fibre optic beam delivery systems

A technique is described for the measurement of the power propagating in either the forward or the reverse direction in the cladding of an optical-fibre beam delivery system. The technique is based on the use of a novel fibre component which is analogous to a directional coupler, except that it transfers power from the cladding of the system fibre (rather than its core) to the monitor fibre. Applications are described for the use of the bi-directional cladding power monitor for determining the efficiency with which light is coupled into the beam delivery system, for monitoring its integrity, and when used in material processing applications, for monitoring the power back-coupled into the fibre from the workpiece.

Curvature induced mode coupling in large core optical fibres with step refractive index profiles

Abstract A simple ray model is developed to approximate the output beam angle increase caused by a symmetric bend with variable curvature in a large core optical fibre beam delivery system of the type commonly used in laser material processing. The results are compared with experiments using curved fibres in the shape of half-cycle cosines with different rates of curvature change. The theoretical model was supported by experiment, and is sufficiently simple for application in the design of an optical fibre beam delivery system.

Monitoring of power coupling efficiency into fibre-optic beam delivery systems: cladding power measurement techniques

In high-power fibre-optic beam delivery systems using Nd:YAG lasers, it is vital that power is coupled into the fibre core with very high efficiency, in order to avoid damage to the fibre, its cable and the beam delivery optics. The authors have shown that by measuring the optical power propagating in the cladding of the fibre, it is possible to monitor the coupling efficiency with much greater sensitivity than is available when the core-guided power is measured. They have developed a theory describing the power in the cladding of the fibre as a function of the launching conditions for a number of types of beam misalignment, and hence as a function of coupling efficiency. They have constructed and evaluated an optical device for monitoring cladding power, and conducted experiments which have validated their theoretical model. They have noted the implications of the cladding power monitoring device in the measurement of coupling efficiency, and have indicated other potential applications for fibre-optic beam delivery systems.

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, and advantages over direct measurement methods are indicated. An extension of 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.

Dependence of output beam profile on launching conditions in fiber-optic beam delivery systems for Nd:YAG lasers

For large-core optical fibers of a few meters length, which are typical of those used in beam delivery systems for high power Nd:YAG lasers, it is shown that the near-field profile of the output beam is a strong function of the launching conditions. The output profile depends on both the input spot size and its alignment relative to the fiber axis. A theoretical model has been developed for step index fiber which shows that the output profile depends on the distribution of guided power between meridional modes and groups of skew modes. It is also shown that the modal distribution is a function of the launching conditions. A relationship is hence derived between the launching conditions and the output profile. The predictions of the theoretical model are consistent with experiment.

Bend loss in large core multimode optical fiber beam-delivery systems

We present results of an experimental investigation of the optical losses produced by bending large core optical fibres, typical of those used in power beam delivery systems. Experiments have been conducted over a range of core diameters for both plastic clad silica and all silica fibres, as a function of bend radius. A theoretical model has been developed for predicting the magnitude of the bend loss, and agreement was obtained with the experimental results. The study thus yields design information for fibre beam delivery systems.