Daoning Su

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.

Fiber optic high‐quality Nd:YAG beam delivery for materials processing

Optical fibers of 50-µm core diameter are investigated for high-quality beam delivery of moderate power (30 W cw) Nd:YAG laser light. The fiber output near-field profile is important for materials processing applications, but is dependent on the modal distribution within the fiber, together with any intermodal interference. Launch optics design, mode-coupling, and modulation techniques are investigated as methods of controlling this beam profile, together with any associated beam quality (M2) changes. As an example application, high-precision, small kerfwidth cutting of stainless steel is demonstrated.

Thermal effects in a hollow waveguide beam launch for CO 2 laser power delivery

Thermal effects caused by launching conditions in a CO(2) laser beam delivery that uses metallic hollow waveguides is investigated. It is found that front-end clipping is the main cause of thermal loading and generates a steep temperature gradient at the fiber front end while the continuous beam attenuation produces a temperature distribution declining slowly along the waveguide.

OPTICAL SENSOR TO MONITOR AND CONTROL TEMPERATURE AND BUILD HEIGHT OF THE LASER DIRECT-CASTING PROCESS

A nonintrusive optical sensor system is applied to real-time process control of the recently developed laser direct-casting process, in which a stream of metal powder is introduced into the beam of a high-power (500-W) cw laser to fabricate complex three-dimensional structures. The sensor system allows two critical parameters, temperature and build height, associated with this process to be monitored and controlled continuously. We achieved a height-sensing resolution of ?0.25 mm and temperature control with a resolution of ?10 degrees C at a typical working temperature of 1500 degrees C with an evident improvement in process quality, especially for complex workpieces comprising relatively high, thin walls at which the conductive heat transfer varies substantially as the process proceeds.

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.

In-situ laser material process monitoring using a cladding power detection technique

Abstract Progress in laser material processing may require real-time monitoring and process control for consistent quality and productivity. We report a method of in-situ monitoring of laser metal cutting and drilling using cladding power monitoring of an optical fibre beam delivery system—a technique which detects the light reflected or scattered from the workpiece. The light signal carries information about the quality of the process. Experiments involving drilling and cutting of two samples, a thin aluminum foil and a 2-mm thick stainless steel plate, confirmed the effectiveness of this method.

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.