Frank Haran

A strain-isolated fibre Bragg grating sensor for temperature compensation of fibre Bragg grating strain sensors

This paper describes a method of isolating a fibre Bragg grating (FBG) from strain. This strain-isolated fibre Bragg grating (SIFBG) was then used to make strain-independent temperature measurements. The SIFBG can then be used to compensate for the temperature cross sensitivity of a FBG strain sensor. Initial results using this compensation scheme are presented, demonstrating proof of the principle.

Real-time focus control in laser welding

We describe a focus control system for Nd:YAG laser welding based on a non-intrusive optical sensor incorporated into the fibre delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands. Simple electronic processing gives an error signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different materials of varying thicknesses and at various welding speeds. The system was demonstrated with both pulsed and CW laser welding.

Optical focus control system for laser welding and direct casting

Abstract A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.

Optical signal oscillations in laser keyhole welding and potential application to lap welding

We describe an optical sensor for process monitoring of Nd:YAG laser welding. This sensor detects the broadband radiation produced by the welding process, dividing it into broad spectral bands (designated UV/visible and IR). Fourier analysis is used to investigate an oscillatory intensity modulation of the optical signals, which is believed to arise from a combination of keyhole and weld-pool oscillations. The spectral content of the oscillations may be used to detect a fully open welding keyhole and to determine the work-piece thickness under this welding regime. These oscillations have also been utilized in the development of a tracking technique which detects the misalignment from an overlap welding seam and excessive gaps in the lap joint.

Fiber Bragg grating strain gauge rosette with temperature compensation

We will describe the construction of a fiber optic strain gauge rosette (FOSGR) using four in-line fiber Bragg gratings (FBGs). These FBGs are formed into a triangular- shaped loop structure. The first three FBGs are orientated at 0 degrees, 60 degrees, and 120 degrees. The final grating, which is at the end of the loop, is isolated from strain and acts as a temperature sensor. Using this independent temperature sensor, the temperature cross sensitivity of the strain sensing gratings can be factored out.Moreover, the inclusion of temperature sensing allows this optical strain gauge to be used on any structural material with arbitrary thermal properties. This FOSGR has been attached techniques. This paper describes experimental results obtained from a FOSGR that has been mounted on an aluminium test piece, which has been subjected to both compressive and tensile loads. Also shown are the temperature characteristics of a FOSGR.

Determination of monomode fiber buffer properties from bend-loss measurements

Bend loss in monomode optical fibers exhibits oscillations as a function of wavelength due to a whispering gallery mode. The period of these oscillations has been used to measure the mean refractive index and thickness of the buffer material on two optical fibers in situ. The mean refractive index of the buffer material was measured to an accuracy of ±0.0025, which is sufficient to distinguish two nominally identical fibers with buffer surface cures of 86% and 96%.

A fibre-optic-based sensor for optimization and evaluation of the laser percussion drilling process

A non-intrusive inter-process measurement technique to determine the diameter of laser-drilled holes is demonstrated. This allows holes to be analysed immediately after processing, before drilling additional holes in the same part. Remedial action can therefore be taken if necessary. The sensor also measures the position of the workpiece relative to the focal point of the drilling laser to ensure optimal processing.

Origin of oscillations in optical signals for Nd:YAG laser welding

Intense radiation is generated by the laser welding process over a broad range of wavelengths, from UV to IR. The core of the delivery optical fibre has previously been used to collect this light from which it is possible to detect various weld faults including focal errors and shield gas interruptions. We have designated this arrangement for the detection of process generated light as the core power monitor (PM). The detected light has a characteristic intensity modulation, postulated to be due to oscillations of the welding keyhole. In this paper, we describe imaging the welding process using a high speed camera, confirming the existence of keyhole oscillations and demonstrating that the light collected with the core PM is predominantly generated within the keyhole.Intense radiation is generated by the laser welding process over a broad range of wavelengths, from UV to IR. The core of the delivery optical fibre has previously been used to collect this light from which it is possible to detect various weld faults including focal errors and shield gas interruptions. We have designated this arrangement for the detection of process generated light as the core power monitor (PM). The detected light has a characteristic intensity modulation, postulated to be due to oscillations of the welding keyhole. In this paper, we describe imaging the welding process using a high speed camera, confirming the existence of keyhole oscillations and demonstrating that the light collected with the core PM is predominantly generated within the keyhole.

Bend loss oscillations in single mode optical fibre: higher order recoupling

Abstract Bend loss in buffered single mode optical fibre is an oscillatory function of wavelength. In previous work, these oscillations have been described by a two-beam interference model between a core guided mode, and a whispering gallery mode propagating in the cladding and buffer. It is shown that higher order recoupling occurs between the two modes, such that the shape of the oscillations is not given by a simple raised sinusoid. It is demonstrated theoretically and experimentally that the shape of the bend loss oscillations is given by a superposition of the harmonics of a sinusoid, and that the distribution of power amongst these harmonics depends on the bend length.

Full penetration detection in Nd:YAG laser welding by analysis of oscillatory optical signals: application to overlap weld-seam tracking

We describe a non-intrusive optical sensor for process monitoring of Nd:YAG laser welding, using light returned through the core of the power delivery optical fiber. This sensor is referred to as the core power monitor (core PM), and uses the delivery fiber to collect the broadband light generated in the process, which is then divided into spectral bands (designated as UV/visible and IR). These optical signals exhibit a characteristic oscillatory intensity modulation within the frequency range 2 - 5 kHz, which is believed to arise from a combination of keyhole, and weld pool oscillations. The frequency content may be related to the size and shape of the welding keyhole, and an alarm system for overlap weeding has been developed based on this principle. This can detect both misalignment of the focused laser spot off the seam, and any excessive gap between the plates.