A novel non-intrusive sampling technique for laser on-line beam monitoring utilising a silicon mirror
Introduction
Silicon is a frequently used substrate for the reflection of high power laser radiation, due to its light weight, relatively high thermal conductivity [1], and ease of coating. By modifying the design of the optic it can be made into a low cost single element beam sampling solution, capable of propagating small fractions of the incident laser energy (0.01%). Coating design has been optimised to give no rotation of phase on reflection. As with all mirror systems there is a small variation in the reflected energy, S being around 0.05% higher than P. Manufactures quoted variation in coating thickness is between 0.33% and 0.66%. These fluctuations have negligible effects on the optic performance. Fine tuning the design of the optic and coating gives a semi-transmissive mirror which may be used on a range of high power lasers. There are alternative options to using Si as the mirror substrate, e.g. GaAs or ZnSe, these are however considerably more expensive (around for ZnSe and for GaAs) and do not have such high thermal conductivity. Once the beam has been sampled there are many excellent commercial devices capable of examining laser beams [2], [3], e.g. Spiricon Pyrocams or Coherent Mode Master, but as yet all fail to provide a cost effective robust solution; as a whole all push for improvements in accuracy, repeatability and functionality. It is the intention of this work not to make a high end commercial device, but to investigate the potential of the optic as a single point sampler and re-examine some of the more traditional imaging techniques for low cost real-time imaging.
Section snippets
Experimental arrangement
This work outlines results from three generations of optic and tests. The laser sources were Rofin RS 6000 (high power testing), Ferranti LE3000, max 2.5 kW (initial profile transmission trials), and a Rofin DC010 (diagnostic systems). Fig. 1 shows a typical experimental arrangement on the DC010. The beam is passed through a zoom telescope giving variation in beam diameter of –. The beam then goes through a periscope and onto the sampling mirror. The bulk of the energy may then be
Experimental results
The first generation optics were 100 mm in diameter, 12 mm thick and edge water cooled. These were used for online beam diagnostics and automatic control of beam position and diameter, while processing, for power levels up to 6.6 kW [7]. During these trials no degradation of either the optic or the reflected beam could be seen, within the available measuring tolerances. This is not really surprising considering silicon is frequently used as a cavity optic, where it is subjected to much higher
Surface reflection
The front surface of the mirror was examined with a microbolometer camera. It had been anticipated that some heating of the front face would be seen, possibly giving some representation of the beam profile. Nothing could be observed except some specular reflection. When the laser was turned off the image was seen to immediately extinguish, no evidence of residual heating within the mirror being observable.
Transmitted radiation
The simplest method was to image the beam directly into a Pyrocam, a typical profile being given in Fig. 3(a). All images from the Pyrocam had a characteristic energy ripple. This was shown to be an inherent error generated by the camera, and not the sample by manipulation of individual optics (and observation with alternative devices). On advice from the manufacturer it is suspected that this may be a result of the chopper or secondary reflection from the protective window.
Measurement of
Low cost imaging—IR view plate
The simplest, low cost, method to view a laser in real time is to convert the radiation to the visible and then observe using a standard camera. Image conversion applied was via a Macken Instruments thermal imaging plate, observed with a simple webcam (a technique previously employed by Cignoli [8] to view Nd:YAG). This is not the ideal plate for the analysis, specifications giving both limited resolution (16 lines/in) and a slow response rate (1 s), but the optic has been designed for direct
IR plate thermal response
Observation of the plate using the IR160 showed the response of the plate when the laser first strikes to be in the 1s region, as quoted by the manufacturer. Complete cooling was observed after 15 s.
Long term observation of the plate did not show any measurable variation in beam diameter or mode profile due to heating.
Numerical analysis
The system described above was subjected to further numerical testing to evaluate the feasibility and highlight potential sources for measurement error. At first a thermal simulation was performed to evaluate the inhomogeneous temperature field due to surface absorption and transmission of the laser beam. This temperature data was then used in a subsequent stress analysis to evaluate maximum displacement of the mirror surface. Once it was established that this does only introduce an
Conclusion
The potential for using a silicon based optic as a direct sampling device has been shown. The optic is an adaptation of a commonly used configuration, and as such should not be problematic for introduction into a commercial arena. Power transmission has been shown to be linear, and the sampled beam representative of the incident. Analysis of the transmitted data can be achieved either using commercial high quality systems, like the Spiricon Pyrocam, or, if such high performance is not needed,
Acknowledgements
The authors would like to gratefully acknowledge the technical assistance of Dr. N. Ellis (ULO Optics) and funding from the Engineering and Physical Sciences Research Council (EPSRC) GR/R64711/02 and EP/E001769/1.
References (16)
Refractive index of silicon and germanium and its wavelength and temperature derivatives
J Phys Chem Ref Data
(1980)- Roundy CB. Current technology of laser beam profile measurements. Retrieved from the Spiricon web site. Now available...
- et al.
Simple is best for real-time beam analysis
Opto&Laser Europe
(1999) - et al.
On-line beam measurement of critical laser beam properties
(2003) - et al.
Beam monitoring benefits for shop floor laser applications
(2005) In process laser beam diagnostics
(2002)Automatic laser beam positioning for high-power laser beams using high-reflective transmissive optics
Proceedings of SPIE
(2003)- et al.
A webcam as a light probe beam profiler
J Appl Spectrosc
(2004)
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