Abstract:
A laser processing machine head, a laser processing machine head monitoring system, and a method of monitoring an optical element of a laser processing machine feature a light-transmissive optical element and an optical element holder defining a cavity in which the optical element is retained against rotation. A light source mounted to the holder directs a beam of light into the optical element through a peripheral surface of the optical element. A light receiver is responsive to light from the light source reflected through the optical element. Monitoring a signal from the light receiver allows a status of the optical element to be assessed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional and claims priority under 35 U.S.C. §120 to U.S. Ser. No. 11/695,312, filed on Apr. 2, 2007, which is a continuation of and claims priority under 35 U.S.C. §120 to PCT/EP2005/010613, filed on Oct. 1, 2005, and designating the U.S., which claims priority under 35 U.S.C. §119 to European Patent Application No. EP20040023524.4, filed on Oct. 2, 2004. The contents of all the prior applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to a laser processing machine head, a laser processing machine head monitoring system, and a method of monitoring an optical element of a laser processing machine. 
     BACKGROUND 
     Optical elements such as lenses, e.g. meniscus lenses, are normally circularly ground on the outside diameter. The lenses used in CO 2  lasers are additionally usually decentered. The centering error for a 7.5″ lens is typically &lt;0.1 mm after adjustment. If a lens is removed for cleaning, the position of the optical element relative to the holder is usually rotated after re-insertion due to the circular peripheral surface. The optical element must then be realigned (readjusted), since a centering from focus to nozzle better than 0.05 mm is required. 
     When workpieces are processed using a machine for thermal welding or cutting, in particular a laser processing machine, the cutting lenses can become contaminated. Impurities, deposits, or damage to the optics surface can result in increased absorption of the laser radiation. Consequently, the thermal loading of the optical element is increased. This ultimately results in a perceptible reduction in the laser power available in the processing region. In cases of severe contamination, in particular by spattering, the increased absorption of the laser radiation can lead to destruction of the optical element. 
     WO 99/59762 describes a device for checking the condition of a glass plate in a laser machining system with regard to contamination by dirt particles. The device comprises a holder for the glass plate. A temperature sensor in contact with the holder detects an increase in temperature of the holder. The increase in temperature is caused by increased absorption of radiation by the glass plate as a result of the dirt particles. 
     SUMMARY 
     Disclosed herein is a holder that simplifies insertion and alignment and reduces rotation of an optical element in a laser processing machine head, while also allowing monitoring of a status of the optical element. 
     In some embodiments, photocurrent from a light beam transmitted through the optical element may be assessed by a photodiode. This photodiode can subsequently be used to detect radiation intensity or radiation heat which leads to thermal loading of the optical element. The optical element may thus be monitored in a particularly advantageous manner. The reflected light beam may also be used for testing a photodiode before laser machining. 
     In an embodiment, a holder of an optical element includes a radiation measuring device with a light source positioned at a first position of a peripheral surface of the optical element for emitting a light beam and a sensor element positioned at a second position of the peripheral surface of the optical element for receiving the light beam reflected by a reflecting surface. An increased intensity of reflected light at the sensor element indicates an increased thermal loading of the optical element. In an embodiment, the light source is a light-emitting diode and the sensor element is a photodiode, allowing particularly simple and cost-effective monitoring of the reflected radiation. 
     In some embodiments, a delineated surface segment is formed on the peripheral surface of the optical element proximate a light-emitting diode. Light from the light-emitting diode can thereby be coupled more efficiently into the optical element. 
     In a preferred embodiment, the delineated surface segment has a ground or polished surface. The nature of the delineated surface segment is chosen to suit the intended use of the photodiode. 
     In some embodiments, the optical element holder includes a spring-mounted clamping body with a pressure surface for applying pressure in the radial direction to the delineated surface segment of the optical element. In particular, a contour of the pressure surface may be complementary to a contour of the delineated surface segment, allowing the optical element to be installed in the optical element holder with a mated fit. 
     In an embodiment, the clamping body includes a temperature sensor for measuring the temperature of the optical element. The clamping body may be in direct contact with the delineated surface segment via the pressure surface, thus allowing contact temperature measurement. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a laser machining system; 
         FIG. 2  is a perspective view of a holder of an optical element of the laser machining head of a laser machining system; 
         FIG. 3  is a perspective view of the assembly of the optical element holder; 
         FIG. 4  is a side view of an optical element; 
         FIG. 5  is a plan view of an optical element including a monitoring device; 
         FIG. 6  is a plan view of another optical element including a monitoring device. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows the structure of a laser machining system  1  for laser cutting using a CO 2  laser  2 , a control device  3 , a laser machining head  4  (nozzle  4   a ) and a workpiece support  5 . A generated laser beam  6  is guided to the laser machining head using deflecting mirrors and is directed onto the workpiece  8  by means of a focusing lens  7 . 
     Before a continuous flat joint is produced, the laser beam  6  must pass through the workpiece  8 . Workpiece  8  may be, for instance, sheet metal. Workpiece  8  may be fused or oxidized at one location in a point. The plunge-cutting process may be effected rapidly (for instance, at full laser power) or slowly (via a so-called “ramp”). 
     During slow plunge-cutting using a ramp, the laser power may be gradually increased, reduced, and kept constant for a certain period of time until the plunge-cut hole has been formed. Both the plunge-cutting and the laser cutting are assisted by adding a gas. Oxygen, nitrogen, compressed air and/or gases specific to the application may be used as cutting gases  9 . A gas may be chosen based on materials to be cut and desired quality of the cut. 
     When cutting using oxygen, a maximum gas pressure of 6 bar may be used. At the point where the laser beam  6  impinges upon the workpiece  8 , the material is fused and largely oxidized. The melt formed may be blown out together with the iron oxides. Particles and gases formed may be extracted from an extraction chamber  11  by means of an extraction device  10 . During the oxidation process (an exothermic reaction), additional energy is released, facilitating the cutting process. For a given material thickness and laser power, significantly higher cutting speeds may be achieved with oxygen gas than with high pressure nitrogen. Alternatively, a thicker material may be cut with oxygen as the cutting gas than would be possible with nitrogen as the cutting gas. 
     The holder  12  of the optical element  7  may be a cylindrical receptacle  13  and a cylindrical retaining device  14  as shown in the embodiment depicted in  FIG. 2 . 
     The assembly of a holder  12  is shown in  FIG. 3 . The receptacle  13  may have a concentric recess at one end in which a retaining ring  15  can be inserted and the optical element  7  can rest thereon. The optical element is circular, except for a delineated, planar surface segment  16  bounded by circular portions of the peripheral surface  16 . A shoulder  17  of a clamping body  18  can be secured to the retaining device  14  and engage the delineated surface segment  16 , thereby inhibiting rotation of optical element  7 . This is shown more clearly in  FIG. 4 . 
     In some embodiments, optical element  7  may be a lens, as shown in  FIG. 5 . The optical element  7  has a reflecting surface  19 . Reflecting surface  19  may be a delineated surface segment on the peripheral surface  20  of the optical element  7 . A portion of reflecting surface  19  may be planar. The planar portion may be ground or polished. In some embodiments, reflecting surface  19  is substantially parallel to a beam axis of a laser aligned with respect to optical element  7 . 
     In certain embodiments, a clamping body may engage reflecting surface  19  with a pressure surface to align the optical element  7  such that rotation of the optical element with respect to the optical element holder is inhibited. In some embodiments, a light-emitting diode (LED)  22  may be coupled to the optical element holder at a first position  21  of the peripheral surface  20  of the optical element  7 . The LED  22  may emit a light beam  23  continuously or non-continuously (for instance, at specific time intervals) before or during operation of the laser. Light from the LED may enter volume  7 ′ and may be reflected at the reflecting surface  19 . After reflection, the light beam  23  impinges upon a photodiode  24  which is secured at a second position  25  of the peripheral surface  20  on the holder of the optical element  7 . The status of the photodiode  24  may be monitored. The photodiode  24  may be used to detect the radiation heat or radiation intensity absorbed by the optical element  7  as a result of the contamination of the optical element during the laser machining. As the radiation intensity increases, the photocurrent increases. The change in the radiation intensity (change in the photocurrent compared with a reference photocurrent) may thus be used to deduce the change in the optical element  7  related to decreased performance of the optical element, 
     The LED  22  and the photodiode  24  may be part of a radiation measuring device integrated in the holder of the optical element. In some embodiments, the temperature of the optical element  7  may be measured by a temperature sensor positioned in the clamping body of the holder of the optical element. 
     It is not necessary for the delineated surface segment to be a planar surface. The delineated surface segment may be a profiled or shaped surface, such as a notch. The surface of the delineated surface segment may be ground, polished, or machined in another manner. 
     According to the embodiment depicted in  FIG. 6 , an optical element  37  has a delineated surface segment with a face  41 . The delineated surface segment may facilitate orientation and installation of the optical element  37 . In some embodiments, the delineated surface segment may facilitate efficient coupling of light from the LED  38  to the optical element. In some embodiments, a photodiode  39  is located on the optical element  37  for monitoring the process (laser) light. Photodiode  40  may be provided to monitor light from the LED  38 . 
     In addition to the embodiments of the delineated surface segment on the peripheral surface of an optical element described herein, other embodiments are also feasible in which the delineated surface segment is formed on the top or bottom (that is, on a broad surface) of an optical element. The delineated surface segment may include one or more angled faces. The angled faces may be arranged at any location on the outer peripheral surface of the optical element. In addition to inhibiting rotation of an optical element in a holder, a delineated surface segment may be shaped to facilitate proper insertion of the element into the holder. For instance, a delineated surface segment with an angled face may require proper insertion of the element, such that the optical element is not inadvertently inserted upside down with respect to the incident process light. 
     It is to be understood that while the invention has been described in conjunction with the detailed description of multiple examples, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.