_Appl. Sci._**2015**, \(5\), 544-554; doi:10.3390/app5030544  
_Applied sciences_  
**ISSN 2076-3417**  
www.mdpi.com/journal/applsci  
_Article_  
Optical Cutting Interruption Sensor for Fiber Lasers  
**[PERSON] 1,*, [PERSON] 1, [PERSON] 2, [PERSON] 2 and [PERSON] 1**  
Applied laser and photonics group, University of Applied Science Aschaffenburg, Wuerzburger  
Strasse 45, D-63743 Aschaffenburg, Germany; E-Mails: [PERSON](a)h-ab.de (M.S. );  
[PERSON](b)hab.de (R.H.)  
A.L.L. Lasersysteme GmbH, Westendstrasse 123, 80339 Munich, Germany;  
E-Mails: b.neumeier(a)all-lasersysteme.de (B.N. ); info(a)all-lasersysteme.de (E.W.) Author to whom correspondence should be addressed; E-Mail: benedikt.adelmann(a)h-ab.de;  
Tel. : +49-6021-4206-840; Fax: +49-6021-4206-801. Academic Editor: [PERSON]  
_Received: 15 July 2015 / Accepted: 1 September 2015 / Published: 9 September 2015_  
**Abstract:** We report on an optical sensor system attached to a 4 kW fiber laser cutting machine to detect cutting interruptions. The sensor records the thermal radiation from the process zone with a modified ring mirror and optical filter arrangement, which is placed between the cutting head and the collimator. The process radiation is sensed by a Si and InGaAs diode combination with the detected signals being digitalized with 20 kHz. To demonstrate the function of the sensor, signals arising during fusion cutting of 1 mm stainless steel and mild steel with and without cutting interruptions are evaluated and typical signatures derived. In the recorded signals the piercing process, the laser switch on and switch off point and waiting period are clearly resolved. To identify the cutting interruption, the signals of both Si and InGaAs diodes are high pass filtered and the signal fluctuation ranges being subsequently calculated. Introducing a correction factor, we identify that only in case of a cutting interruption the fluctuation range of the Si diode exceeds the InGaAs diode. This characteristic signature was successfully used to detect 80 cutting interruptions of 83 incomplete cuts (alpha error 3.6%) and system recorded no cutting interruption from 110 faultless cuts (beta error of 0). This particularly high detection rate in combination with the easy integration of the sensor, highlight its potential for cutting interruption detection in industrial applications. In addition to cameras, which require a high evaluation time and efforts, non-camera-based optical sensors are also applied. [PERSON] _et al._[17] employed a cutting head with three attached optical fibers to detect the nozzle to workpiece distance and the radiation from the process zone, the latter being used to calculate the temperature in the process zone, thereby monitoring the process. However, detailed experiments to prove the ability to detect cutting interruptions were not performed. [PERSON] _et al._[18] assembled a fiber-coupled optical sensor at a 30\({}^{\circ}\) angle at the cutting head, measuring the thermal radiation of the process zone. During steel cutting (thickness 3 to 10 mm) with a 1.5 kW CO\({}_{2}\) laser, the thermal radiation is recorded with an InGaAs and a Si photodiode. The results show varying temperatures at different cutting velocities, which can be used to indicate a cutting interruption. However, lateral off-set sensors must be retraced to watch the cut kerf, which is very extensive, and in small contour cuts with high velocities the retrace is not fast enough. In contrast to other mostly-optical-based sensors, [PERSON] and [PERSON] [19] analyzed the acoustic emissions generated during the laser cutting process to evaluate the cut quality during laser machining. Cutting with good quality emits a continuous sound while burr formation emits acoustic bursts, which are assigned to the fracture of the oxide film during the solidification of the burr. However, in practical use, these bursts are difficult to detect because the sound is disturbed by the noise of the gas jet, the sound of the drives, and the exhaustion system.