High power laser beam delivery monitoring for laser safety

ELSEVIER

Optics and Lasers in Engineering 27 (1997) 479-492 0 1997 Elsevier Science Limited

All rights reserved. Printed in Northern Ireland 0143-8166/97/$17.00

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High Power Laser Beam Delivery Monitoring for Laser Safety

D. A. Cordera*, D. R. Evans”, J. R. Tyrer’, C. M. Freeland’ & J. K. Myler’

“Department of Electrical and Electronic Engineering, Loughborough University, Loughborough, Leicestershire, UK, LEll 3TU

‘Department of Mechanical Engineering, Loughborough University, Loughborough, Leicestershire, UK, LEll 3TU

‘Precision Optical Engineering Ltd, Wilbury Way, Hitchin, Hertfordshire, UK, SG4 OTP

(Received 7 May 1996; accepted 16 August 1996)

ABSTRACT The output of high power lasers used for material processing presents extreme radiation hazards. In normal operation this hazard is removed by the use of local shielding to prevent accidental exposure and system design to ensure eficient coupling of radiation into the workpiece. Faults in laser beam delivery or utilization can give rise to hazardous levels of laser radiation. A passive hazard control strategy requires that the laser system be enclosed such that the full laser power cannot burn through the housing under fault conditions. Usually this approach is too restrictive. Instead, active control strategies can be used in which a fault condition is detected and the laser cut off. This reduces the requirements for protective housing. In this work a distinction is drawn between reactive and proactive strategies. Reactive strategies rely on detecting the effects of an errant laser beam, whereas proactive strategies can anticipate as well as detect fault conditions. This can avoid the need for a hazardous situation to exist. A proactive strategy in which the laser beam is sampled at the final turning mirror is described in this work. Two control systems have been demonstrated; the first checks that beam power is within preset limits, the second monitors incoming beam power and position, and the radiation reflected back from the cutting head. In addition to their safety functions the accurate monitoring of power provides an additional benefit to the laser user. 0 1997 Elsevier Science Ltd.

1 INTRODUCTION

High power lasers are used for a variety of material processing applications. Typically carbon dioxide (CO,) or Neodymium-YAG lasers with

* Corresponding author.

479

480 D. A. Corder et al.

powers in excess of 100 W are required for cutting, drilling, welding and heat treatment of materials. Appropriate measures are required to control the radiation hazards of such lasers. Industrial applications usually require that the laser installation is a Class 1 Laser Product as defined by BSEN 60825.’ The standard requires that the classification be performed under both normal operating conditions and reasonably foreseeable single fault conditions. Fault conditions imply that the laser radiation is not reaching the desired target; instead it is likely to be incident on a protective housing. If the housing has been designed only to withstand radiation levels associated with normal operation, then enclosure burn-through and escape of laser radiation is likely to happen very rapidly. It is not sufficient to rely upon human intervention to cut off the laser under fault conditions.

For many applications laser machining must compete with other technologies. It is important that the additional safety measures required for the use of lasers do not make them uneconomical. The effective cost of the safety measures can be reduced it they are designed from the start to be an integral part of the installation, and if they bring additional functional benefits to the system.

Hazard control measures must fundamentally be effective, but they must not be so over-engineered as to be inflexible or restrictive. Current approaches may be divided into passive or active control measures. A passive approach, also termed the fortress approach, relies on the enclosure to resist the thermal or optical effects of the laser radiation and contain the laser beam during fault conditions. An active approach detects the fault condition and responds to remove the hazard by cutting off the laser.

A passive approach will usually be in conflict with the desire for flexible operation and the rapid flow of workpieces through the system. Active approaches can permit more flexible operation with minimal guarding for normal operation, whilst still maintaining acceptable safety levels.’ This paper examines active control measures and proposes an active system which enhances the functionality of the system in addition to providing fault detection.

Active hazard control is a generally accepted term. It does not distinguish between the two differing approaches to active control measures, consequently two new terms are applied in this work. The first approach, in which the system responds to the effect of an errant laser beam, has been termed reactive. Systems which detect a developing fault and cut the laser off before an errant laser beam occurs are termed proactive. A proactive system removes the hazard of an errant beam and avoids the need for the enclosure to be damaged.

High power laser beam delivery 481

2 CONTROLLING LASER RADIATION HAZARDS

A laser processing installation will consist of a laser, a beam delivery system and some means of controlling the utilization of the laser beam. A simple system with a single laser and single processing point is usual, but it is possible to multiplex the beam delivery system with one laser delivering power to several processing points sequentially. The most simple processing system using a fixed head in the Z-axis and a movable X-Y table onto which the workpiece is placed was considered in this work. More complex, multi-axis systems are possible, for example lasers with robotic arms for beam delivery. Uncertainty exists as to whether such systems can be operated safely with active fault detection.2

For the purposes of assessing enclosure requirements the laser system can be divided into three sections: laser beam generation, delivery and utilization, The primary hazard at the point of laser beam generation is usually electrical rather than optical so only a thin enclosure is required around the laser itself.

Beam delivery requires that high power laser beams be transferred from the laser output to the point of utilization. Open, or free space delivery is possible but extremely hazardous in most situations. Hollow metal tubes (flight tubes) prevent access to the beam path, providing an additional advantage of isolating optical components from atmospheric contamination. Alternatively, for Nd:YAG lasers fibre optic beam delivery may be used.

Generally, at the point of utilization the majority of laser power is absorbed by the workpiece.” Local guarding ensures that any scattered radiation, or radiation generated by the laser-material interaction, is attenuated to below Class 1.

The beam delivery system presents the greatest number of potential fault conditions and accordingly the highest probability of an errant laser beam occurrence (ELBO).” Failure modes include misalignment of the flight tubes or optics arising from gradual displacement by vibration or sudden displacement caused by impact, omission of components after incorrect servicing, or contamination of the optics. Fibre optic beam delivery systems fail if inadvertently cut or bent through an excessively small radius.

Edwards and Bandle4 identified three failure routes leading to an ELBO, two of which affect the beam delivery and utilization. One relates to external events, the second to failure of internal components. The third failure route leading to an ELBO relates to accidental specular reflections from the workpiece. From the results given4 it is apparent that the total


probability of a fault occurring in the delivery system is at least an order of magnitude higher than for one related to the workpiece. A typical workpiece fault condition would be incorrect alignment causing a specular reflection of the incident laser beam. In this case the reflected radiation is likely to be divergent, reducing the distance at which it would be hazardous.

A passive approach to controlling hazards from faults in beam delivery or utilization would be a thick walled enclosure around the entire installation. The system could be situated in an interlocked room to which human access was not possible whilst the laser was operating. Alterna- tively flight tubes capable of withstanding the full laser power could be used to deliver the beam with the processing operation being performed within an interlocked cabinet. Neither approach is suitable for high volume material processing. An active approach to hazard control allows the thickness of enclosures to be reduced. The thickness need only be sufficient to withstand incident laser radiation for the response time of the fault detection equipment (plus a margin for safety).

As explained earlier, it is necessary to subdivide active control measures into reactive and proactive methods. Proactive methods are preferable because a potential fault condition may be detected before a hazardous escape of laser radiation occurs. Two reactive control methods are described by Taylor et aZ.’ and Green”, in the latter example the laser processing system is surrounded by a double walled enclosure. An errant laser beam would burn through the inner, sacrificial wall and cause the release of a pressurized fluid or gas from between the walls. The laser would then be cut off before the outer wall was damaged. Such a design could be applied to the flight tubes and the cabinet surrounding the processing area. Unfortunately the system is complex, cumbersome and very expensive to install and maintain.

If the laser is installed in a room or similar enclosure then the temperature rise caused by an errant laser beam incident on the enclosure can be detected and used to cut off the laser. The system described by Taylor et al.’ used a scanning detector to examine the temperature profile of the entire room. Any change, indicating an errant beam, caused the laser to be cut off. Care must be taken to ensure that the entire interior of the enclosure can be monitored. Since the unit scans the whole room it does not have a sufficiently fast response time to protect any person; the room must be empty during laser use. For example the Maximum Permissible Exposure would be exceeded in less than 1 ms for most material processing lasers, the worst case response time of the scanning system would be in excess of 0.1 s (10 Hz scan rate).

The two previous reactive examples have the advantage that they respond to an ELBO regardless of whether it originates from the delivery


system or point of interaction. Both are complex, require additional enclosures for their operation and operate only as safety systems with no other benefits. Given the significantly higher probability of a fault developing in the delivery system it is reasonable to concentrate the application of hazard control measures to this area. A fault condition in the beam delivery system will result in overheating of the flight tubes or optical components and a loss of power at the point of interaction. Fault conditions can therefore be detected by measuring the laser power as close to the point of interaction as possible. Usually this would be the final turning mirror in the beam delivery system. This power is compared to the output power of the laser, either in real time or by comparison to an initial calibration under correct operation. If the two values are in agreement then the system is operating correctly, otherwise a fault exists. A single fixed detector may be used to measure the power, this reduces the system cost and complexity. Suitable choice of detector can give the system a rapid response limited only by detector and interface electronics frequency response. By placing upper and lower bounds on the acceptable power, an errant laser beam need not occur-a gradual decrease in delivery efficiency can be signalled before the process is affected or any hazard exists. Such a system provides proactive fault detection. Monitor- ing for beam over- and underpower reduces the probability of a failure in the fault detection system, preventing a genuine fault from being detected. A range of faults, for example an open circuit detector, would give an output indicating a high measured power. If the measured power only had to exceed a threshold for the laser to be classed as operating correctly then such a fault in the monitoring equipment could conceal an errant laser beam condition.

The high cost of laser energy combined with resolution and response speed limitations of detectors capable of dissipating high powers require that laser power must be measured non-invasively. A means of sampling the main beam is required. Beam samplers may be transmissive or reflective. Generally reflective beam samplers are to be preferred because a reflective sampler may be cooled more easily than a transmissive one. Consequently reflective beam samplers can be expected to be more reliable with high power laser beams. Hole matrix mirrors and diffractive mirrors reflect the main beam. The operation of the hole matrix mirror has been described by O’Key et al.“; a sample of the main beam is taken through the mirror using an array of small holes. Sample ratio is determined physically by the ratio of hole diameter to hole spacing. A disadvantage is that a lens is required to reconstruct the sample beam. A diffractive mirror uses a standard mirror (copper for CO2 lasers) with a diffraction grating diamond turned on the surface. The diffraction grating


is designed to reflect a known fraction of the main beam at a specific angle at a given wavelength with a very low insertion loss. Typically a sampling ratio of 0.05% would be used, careful design ensures that the sample ratio is accurately known. An alternative sampling method uses the leakage through a standard high reflectivity dielectric mirror. This provides a sampling ratio of approximately O*l%“, but has the disadvantage of being non-repeatable between mirrors, sensitive to angle of incidence and temperature. Hole matrix mirrors and diffractive mirrors give a sample ratio determined by their physical construction. The diffractive mirror is to be preferred since it does not require an additional lens for operation, consequently the unit can be made compact and can directly replace an existing turning mirror.

Taking a small fraction of the main beam for power measurement to permit continuous monitoring of the system integrity has additional benefits. Since the detector only needs to dissipate low power, it is physically smaller and can have a shorter response time. Calibration of low power detectors is more straightforward. The sampling method provides a known ratio of the main beam so an accurate, fast responding measurement of the main beam power is possible. For high power lasers this is a significant advantage over the more usual slow responding, less accurate high power meters. Improving safety therefore provides additional benefits to the user.

3 PRACTICAL IMPLEMENTATION

3.1 Basic implementation for CO, laser beam power monitoring

Figure 1 shows schematically the arrangement for using the diffractive mirror. The diffractive mirror replaces the final turning mirror in the delivery system, ensuring the integrity of the whole system. A detector is placed to intercept the first order of the diffraction pattern. A thin film thermopile (Dexter 2MC) was used. This has a sensitive area considerably smaller than the sampled beam. It does not therefore measure only the total beam power. Instead beam power, position and mode affect the detector output. For a pure power measurement application this would be a disadvantage. In this case it is regarded as an advantage because a change in any of these parameters could signify a developing fault in the laser or delivery system which could have safety or process quality


From Interlock chain Initial

, over-ride

Set upper limit

‘I Detector

?I; --51’ J4

Preamplifier

1 ,/” *’

“-,-Set lower limit

To ihotter COMPARATORS

Fig. 1. Basic implementation of beam delivery monitor.

implications. The system therefore acts as a ‘deviation from ideal’ detector. If an absolute measure of beam power is required then a larger detector or a collecting lens could be used.

A preamplifier is situated adjacent to the thermopile to minimize noise pickup. The gain is adjustable, during calibration it is set to give an output of 1 mV/W of delivered laser power. A first-order low-pass filter with corner frequency 15 Hz is included in the preamplifier to prevent unwanted noise from being amplified. This frequency response matches that of the thermopile. Together these limit the electronic response time to approximately looms. There is no advantage to be gained by faster operation because the mechanical shutter response time limits the overall response time. If the circuit was used to directly control the power supply to the laser then a faster electronic response would be essential. Some improvements in response time with a thermopile detector could be obtained using an amplifier with complementary frequency response to that of the thermopile (i.e. a gain increasing at 20dB/decade from 15 Hz). Ultimately noise would limit the application of this technique, and a faster, but more expensive detector such as HdCdTe would be required.

A separate unit is used to process the output of the preamplifier. The signal is used directly to provide a display of laser power, the accuracy of this display as a measure of true laser power is subject to the sensitivities to mode and position described earlier. Two comparators are used to detect the beam under and over power conditions. Their threshold voltages are set by potentiometers supplied from a voltage reference.


When adjusting the threshold voltage the display is switched automatically to show the current level. If either of the comparator outputs change state then the relay opens and remains open. The relay is the last in the chain of interlocks and controls which govern shutter operation. The shutter is internal to the laser enclosure and, when closed, prevents any radiation entering the delivery system. Shutter closure is ensured by a gravity assisted spring. A signal is taken from the interlock circuit to the relay control circuit. This overrides the comparator outputs for approximately O-5 s to allow the measured power to exceed the lower threshold, a side effect of this is that a genuine fault condition is over-ridden briefly on every attempted start of processing.

The simple system described above has been installed on a 500 W Coherent 525 Everlase system. The accurate measure of power provided has meant that it is regarded as a beneficial addition to the system rather than a restrictive safety measure.

3.2 Advanced uses of the diffractive mirror

A development of the power measurement application is to use a position sensitive detector to indicate movements in the sampled beam, and by implication changes in the angle of incidence or position of the main beam. Such a system monitors the stability of both beam power and position. Deviations from the ideal alignment can provide substantial prior warning of misalignment before either the process is affected, or any hazard exists. As such it implements proactive hazard control based on two laser beam parameters. By monitoring two parameters the probability of a fault in the monitoring system obscuring a genuine ELBO can be further reduced. Both detector elements (or in the case of a quadrant, all four elements) must be giving a satisfactory signal before the laser can operate. If desired each element output could be compared to a threshold and all would have to be within limits before the laser could operate. Combining these completely independent channels with the need for their sum and differences to be within limits would add substantial redundancy into the monitoring system. The prototype system monitored only the sum and difference of the detector element outputs as power and position. This is still a very significant check on the correct operation of the laser and monitoring system, but does not fully utilize the independence of the detector outputs.

An additional advantage of such position sensing is that the process of aligning the delivery system can be simplified. If the diffractive mirror is mounted in a precision machined 45” mount then it can act as a reference. Having approximately aligned the delivery system using a visible coaxial


DIRECTION OF MINIMUM SENSITIVITY TO BEAM POSITION

DIRECTION OF MAXIMUM SENSITIVITY TO BEAM POSITION

SENSITIVE AREAS

Fig. 2. Direction of minimum and maximum beam displacement sensitivity.

laser the diffractive mirror provides guidance on the final adjustment. Since beam alignment is an important aspect to laser safety,3 a simplifica- tion of the process is valuable.

A quadrant thermopile with sufficient sensitivity was not available, instead a twin element thermopile was used for the investigation (Dexter DR34). Having only two elements results in a position sensing ability which is dependent on the direction of the displacement (Fig. 2). For alignment purposes this is not a significant problem as the detector may be rotated through 90”, ideal alignment being the case where the output of the two elements remains equal throughout the rotation.

Figure 3 shows a block diagram of the system components. The preamplifier and low pass filters were similar to those used for the power measurement application. To investigate the operation of the beam position monitor the output of each element was fed via an analogue to

Main Beam

BACK < REFLECTION

POSITION

Fig. 3. Arrangement for monitoring beam power, position and back-reflection.


digital converter to a computer. Here the necessary calculations were performed:

beam power m output A + output B

beam position cc output A - output B

output A + output B

In a practical system these could easily be implemented electronically using analogue adders, subtractors and dividers.

The sensitive area of the DR46 is substantially less than that of the sampled beam. Sensitive areas were separated by approximately 1 mm. Positional sensitivity could be increased by focusing the sample beam onto the detector, but if the detector was placed in the focal plane then a misleading central null in the outputs would occur. To minimize cost and complexity a lens was not used. This will give satisfactory results with all but very large diameter beams with very slowly changing intensity profiles.

The bi-directional nature of the diffractive mirror enabled a second advanced use to be investigated. It was possible to use the sample in the forward direction (main beam) for position sensing and the sample in the reverse direction to measure the amount of back-reflected radiation. By measuring the back-reflected power some aspects of the delivery system beyond the diffractive mirror could be supervised. For example a misaligned nozzle would generate back-reflection as the beam was clipped, a missing lens would give significant back-reflection from around the nozzle and a damaged lens would generate increased back-reflection from the optical imperfections. It had also been considered that the material being processed might generate sufficient back-reflection to detect the case of workpiece not being correctly aligned. However, it was found that this prototype system had insufficient resolution to reliably detect the difference between presence and absence of the workpiece.

Before installation on the laser system the diffractive mirror was tested using the arrangement in Fig. 4. The laser provided a power of approximately 10 W in a near-Gaussian beam with diameter specified as 8 mm. A linear stage was used to displace the blank copper mirror along the axis of the incoming laser beam. This caused an equal linear displacement on the diffractive mirror. Figure 5 shows the individual outputs of the thermopile elements (normalized to a peak value of unity) and the result of the position calculation. The thermopile elements were aligned so as to give a maximum response to the displacement. Positional information was only recorded over the central region where both detectors were providing significant outputs. The increase in detector B output at the end of the scan was attributed to the edge of the main beam


Diffractive

-

I MAIN BEAM

\

Sample beam

/ topper mirror

on linear

L translation stage

‘Gvin element thermopile

’ 1

POWER METER

Fig. 4. Arrangement of intial test of beam sampling mirror and dual thermopile.

2j--- 1

0 5 10 15 20 25 30

Displacement /mm

Fig. 5. Results of initial test. (a) channel A output; (+) channel B output; (t) derived beam position.


0.25

0.20

0.15

0.10

0.05

0.00

-0.05

-0.10

-0.15

-0.20

-0.25

Y----J I

.., _. ‘.. :

i .

I I I I I I /

0 20 40 60 60 100 120 140

Time/s

Fig. 6. Response of system to beam displacements when installed on CO, laser.

reaching the detector. It can be seen that the thermopile outputs are two near identical scans across the beam profile. A linear displacement of approximately 1 mm between the thermopile outputs corresponds the linear separation of the thermopile elements. The beam width indicated by the scan corresponds to the expected value of 8 mm.

Figure 6 shows the results of a test run using the diffractive mirror installed as the final mirror in the delivery system. The laser was a Coherent Everlase operating at approximately 200 W cutting medium density fibreboard. A nozzle diameter of O-8 mm was used. Beam positional errors were artificially introduced by adjusting the positioning screws on the preceding mirror. A rotation of half a turn in each direction of the screw was used, but it was not possible to relate these changes to a simple positional or angular change in beam position. An approximate step change in beam position was generated by turning the screw rapidly. This ensured that the beam position change was easy to distinguish from any background beam wander.

The increase in back-reflection at the start and end of the results corresponded to the opening and closing of the shutter. Contributions to the measured value would come from reflection from the lens, from the nozzle and from the workpiece as well as the diffuse component of the diffractive mirror. Whilst the shutter was open the position sensing information was valid, at other times it was suppressed because it showed extreme random variations caused by small changes in the small detector


outputs. The response of the system can be clearly seen, variations in beam position are recorded in the correct sense and time. Minor variations in indicated position result from the cumulative effect of beam wander, lack of rigidity in the mirror position whilst the adjustment screws were being used, and noise in the electronics. Back-reflection increased once the beam position deviated from a central position. This was caused by the edge of the focused laser beam clipping the nozzle. A reference measurement was taken with the alignment unchanged. It was found that the peak-to-peak noise in back-reflection was approximately 4% of the value when the beam was misaligned. The peak to peak noise in position was approximately 20% of the misaligned value. Noise is more significant in position measurement because the normalisation process uses the difference in two small values. Long term (60 s or longer) averaging would greatly reduce the noise in the position measurement.

4 CONCLUSIONS

To avoid excessively restrictive guarding on high power laser systems, it is possible in some cases to operate with minimal guarding and use an active fault detection system. In the event of a fault which could cause the hazardous escape of laser radiation the laser is shut down. Previous fault detection methods have concentrated on so-called reactive approaches which detect the heating effect of the laser radiation once it has escaped. In this work an alternative proactive approach was considered. An errant laser beam is most likely to be caused by a fault in the beam delivery system. By ensuring that the laser radiation reaches the end of the delivery system the operation of the delivery system is verified. Any fault condition in the delivery system reduces the power at the end of the delivery system and the laser is cut off in response. Since this fault detection method does not rely upon an escape of laser radiation it can be considered as proactive rather than reactive. As such it brings significant safety advantages.

The power at the end of the delivery system is measured using a diamond turned diffractive mirror in place of the final mirror. A small sample of the main beam is provided, the ratio of sample to main beam is accurately determined by the physical design of the diffraction grating. If a small sample is taken then only a low power detector is required. These are easier to calibrate and have a faster response than a detector for measuring the full beam power. In addition to enhancing the safety of the laser machining installation, the implementation of the proposed system provides a valuable extra function of accurate power measurement.


A development of the basic system uses a twin element detector. Changes in beam position can then be detected, thereby indicating that the delivery system is becoming misaligned before the process or safety are affected. The diffractive mirror used for this work provided samples of the main beam and back-reflected radiation. Monitoring back-reflected radiation permits the integrity of the beam delivery system beyond the final mirror to be monitored.

REFERENCES

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2.

3.

4.

5.

British Standards Institution Safety of Laser Products, equipment classification, requirements and users’ guide. BS EN 60825:1, 1994. Taylor, A. F. D. S., Edwards, S. A., Arwel Barett, J. & Bandle, A. M., Detection of Errant Laser Beams, SPIE Vol. 1276 (Lasers and Applications II), 1990. Green, J. M., High power laser safety. Opt. Laser Technol., 21(4) (1989) 244-248. Edwards, S. A. & Bandle, A. M., Risk analysis as applied to high average power industrial laser systems. Proc. 4th. Int. Con5 Lasers in Manufacturing, 1987, pp. 139-52. O’Key, M. A., Osborne, M. R. & Hilton, P. A., Monitoring of high power laser beam profiles. In Laser Materials Processing: Industrial and Micro- electronics Applications, Vol. 2207, Chap. 85, 1994, pp. 135-45.

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High power laser beam delivery monitoring for laser safety