FIELD OF THE INVENTION

The invention describes a method of monitoring the performance of a semiconductor laser diode arrangement. The invention also describes a system for generating laser radiation, a driver unit for use in such a system, and a workpiece processing apparatus.

BACKGROUND OF THE INVENTION

Laser radiation can be used in a number of applications requiring induced heat in which a malleable workpiece can be moulded or shaped when heated. An example of such a procedure is the known "bottle-blowing" procedure, in which small polyethylene terephthalate (PET) "preforms" are evenly heated and expanded or "blown" in a thermo forming process until the desired shape and size is obtained. In conventional bottle-blowing ovens, halogen lamps are arranged along the length of the oven to act as a source of thermal radiation. To ensure a constant quality of the end-product, the level of radiation output by the lamps must be held essentially constant. For this reason, the light output is regularly monitored and any defective lamps are replaced as necessary. Because the halogen light output is in the visible range of the spectrum, such an inspection can be carried out in a straightforward manner, since defective lamps can easily be identified.

Recently, developments in the field of laser diodes have led to their use as a source of thermal radiation. Particularly surface-emitting laser diodes such as vertical cavity surface-emitting laser (VCSEL) and vertical extended cavity surface-emitting laser (VECSEL) diodes are being considered, since these are relatively economical to manufacture, and can be manufactured easily in large numbers.

In an application such as a bottle-blowing oven, arrays of laser diodes could be used to provide the necessary even heating of the preforms. However, the maintenance of the overall quality is made difficult by the fact that a defect in an individual laser diode cannot easily be identified. A laser diode may fail for a number of reasons, for example owing to growth of defects in the crystal structure of the diode, degradation of the thermal contact between the laser diode and a heat sink, or degradation of electrical contacts of the diode. Such defects can hardly be detected, since laser diodes are generally mounted or bonded on a heat sink and cannot be accessed to be physically examined. An alternative method of quality control is required. For example, the individual laser diodes could be scanned with a photosensor, and the light output of each laser diode could be measured and analysed. Furthermore, laser diode heat sources generally comprise arrays of laser diodes, and a single defective laser diode cannot easily be identified in the array of otherwise functional laser diodes. Also, a laser diode emitting heat radiation in the invisible part of the light spectrum cannot be visually examined as described above, but would require dedicated infrared detectors such as photo sensors in order to determine if the laser diode is satisfactorily functioning. Evidently, such techniques are complicated and time-consuming, and may mean that the apparatus (for example a bottle-blowing oven) cannot be used for its intended purpose as long as the measurement is being carried out.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide an economical and straightforward way of identifying defective semiconductor laser diodes in a heating apparatus.

The object of the invention is achieved by a method according to claim 1 of monitoring the performance of a semiconductor laser diode arrangement and by a system according to claim 8 for monitoring the performance of a semiconductor laser diode arrangement, by a driver unit according to claim 13.

The invention describes a method of monitoring the performance of a semiconductor laser diode arrangement comprising at least one semiconductor laser diode module, in turn comprising a plurality of semiconductor laser diodes, which method comprises the steps of driving the semiconductor laser diode module at a specific current value such that the semiconductor laser diodes of the semiconductor laser diode module are electrically stimulated, monitoring the operating voltage of the semiconductor laser diode module, and analysing the operating voltage to estimate the radiation output of the semiconductor laser diode module. The estimation of the radiation output of the semiconductor laser diode module may be used for identifying defective semiconductor laser diodes.

The term 'estimate' is to be understood as follows: in one approach, the method according to the invention allows the actual radiation output of the semiconductor laser diode module to be directly related to the monitored operating voltage, when the semiconductor laser diode module is driven at a sufficiently high current value so that the semiconductor laser diodes emit. In another approach, the method according to the invention allows the radiation output of the semiconductor laser diode module to be predicted, based on the monitored operating voltage. In other words, the semiconductor laser diode module can be driven at a specific current value below the lasing threshold so that the semiconductor laser diodes are electrically stimulated but do not emit, and the monitored operating voltage is used to determine the radiation output that would be obtained if the semiconductor laser diodes were to be driven to above the lasing threshold.

Here, a semiconductor laser diode module can comprise a number of semiconductor laser diodes, for example an array of semiconductor laser diodes. The semiconductor laser diodes of a semiconductor laser diode module can all be mounted on a shared heat sink, and each semiconductor laser diode module can be driven independently of other semiconductor laser diode modules. A semiconductor laser diode arrangement can comprise a plurality of such semiconductor laser diode modules, and each semiconductor laser diode module can be replaced or serviced separately, without influencing other semiconductor laser diode modules of the arrangement. Generally, to obtain an even heat output, the semiconductor laser diodes are usually mounted in close proximity on the shared heat sink, and neighbouring semiconductor laser diode modules are also usually positioned close together, for example on a printed circuit board.

The term 'radiation output' is to be understood to mean the optical output power radiated as electromagnetic radiation by the semiconductor laser diode. The energy applied to a semiconductor laser diode in order to drive it is partially converted into optical output power, and partially dissipated as heat. A 'healthy', i.e. non-defective semiconductor laser diode converts a relatively large proportion of the input energy into optical output, and a relatively small proportion is dissipated as heat. A defective semiconductor laser diode, on the other hand, dissipates more of the input energy as heat, since it is unable to convert it to optical output radiation. In other words, less useful optical radiation is output by a defective laser diode.

An advantage of the method according to the invention is that a good indication relating to the optical output performance of the semiconductor laser diode module can very easily be obtained, without having to monitor the actual radiation output. Since a semiconductor laser diode module must in any case be electrically connected to a driver unit, additional circuitry or additional measuring devices are not necessary, so that the method according to the invention allows a particularly cost- effective fault-detection.

The system according to the invention for generating laser radiation, which is used to induce heat as explained above, comprises at least one semiconductor laser diode module comprising a plurality of semiconductor laser diodes, a driver unit for driving the semiconductor laser diode module at a specific current value, a voltage monitoring means for monitoring the operating voltage of the semiconductor laser diode module, and an analysis means for analyzing the operating voltage to estimate the radiation output of the semiconductor laser diode module.

A driver unit according to the invention for use in a system for generating laser radiation comprises an electric driving means for driving a semiconductor laser diode module of a semiconductor laser diode arrangement at a specific current value, a voltage monitoring means for monitoring the operating voltage of the semiconductor laser diode module, an analysis means for analyzing the operating voltage to estimate the radiation output of the semiconductor laser diode module, and a feedback generator for generating feedback pertaining to the performance of the system and based on an output of the analysis means.

The generated feedback can comprise, for instance, a message indicating that a particular module should be replaced or serviced. Such a driver unit can easily be obtained by carrying out minor modifications, if necessary, to an existing driver unit of a semiconductor laser diode arrangement. The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.

Any suitable type of semiconductor laser diode could be used in the semiconductor laser diode arrangement described herein. However, for the sake of simplicity, it is assumed in the following that vertical cavity surface emitting laser diodes (VCSELs) or vertical extended cavity surface emitting laser diodes (VECSELs) are used, without in any way restricting the invention. Also, in the following, for the sake of simplicity, the terms 'semiconductor laser diode arrangement', 'semiconductor laser diode module', and 'semiconductor laser diode' may be abbreviated to 'arrangement', 'module', and 'laser diode', respectively.

The efficiency of a laser diode is generally expressed as 'wall-plug efficiency' (WPE), which is an indication of the total electrical input to optical output power efficiency. The WPE is calculated based on the electric power delivered to the laser diodes and takes into account any losses in the power supply and any additional power required for a cooling system. State-of-the-art VCSELs can reach a WPE of above 40 %.

As already mentioned above, to obtain an indication of the overall radiation output performance of a semiconductor laser diode module, its operating voltage is analysed. The operating voltage of such a module can be measured while the semiconductor laser diode arrangement is in operation. However, heat is generated by the laser diodes during operation, and, for a defective laser diode, more energy is dissipated as heat than as optical power. Because the laser diodes of a module are generally positioned close together, the heat generated by a defective 'hot' laser diode can spread ("thermal crosstalk") to its neighbouring diodes. Since the electrical conductivity of a laser diode is to a certain extent dependent on its temperature, a defective diode can influence the operating voltage of its neighbouring diodes. Therefore, even though the voltage measurements could be made during operation, when the laser diodes are emitting, it may be preferable to identify any defective modules at a more convenient time, for example before the laser diodes are actually driven to emit radiation. Therefore, in a particularly preferred embodiment of the invention, the operating voltage of a semiconductor laser diode module is monitored while driving the semiconductor laser diode module at a current below a lasing threshold current level. Such a low current level allows the laser diode to 'operate' without actually emitting. Such measurements can be carried out at any convenient time.

Experiments carried out with semiconductor laser diodes in the method according to the invention have shown that the level of optical output power delivered by a laser diode is related to the operating voltage of the diode. As already explained above, when a laser diode becomes defective, it delivers less optical output power than it does in its non-defective or 'healthy' state at the same level of current. Not only does the optical output power decrease for a defective semiconductor laser diode, its operating voltage also drops slightly. This drop in operating voltage can be used as an indication that the semiconductor laser diode is no longer healthy. Experiments carried out while developing the method according to the invention have shown that a laser diode with a degree of defectiveness is characterized by a lower than expected operating voltage value. Furthermore, a greater degree of defectiveness is associated with a lower operating voltage level. Therefore, in a further embodiment of the invention, the step of analysing the operating voltage of a semiconductor laser diode module comprises comparing the operating voltage value to a predefined threshold voltage value and generating feedback according to the comparison result.

As mentioned above, a semiconductor laser diode arrangement can comprise a plurality of semiconductor laser diode modules, each of which in turn can comprise an array of multiple laser diodes. Usually, the laser diodes are all of the same type. Each semiconductor laser diode module can have the same number of laser diodes in the same arrangement. It can be expected that the modules exhibit essentially similar electrical characteristics. Therefore, in a further preferred embodiment of the invention, the method comprises an initial calibration step in which performance-related measurements, which are representative of the performance of a semiconductor laser diode module, are collected and analysed. "Representative of the performance" means that the measurements can be carried out for each semiconductor laser diode module of an arrangement and can be applied to that module or to any module of that type. For example, for an arrangement in which the modules are all constructed in an identical manner, using essentially identical components, performance values collected for one module can be representative of the performance of any module of that type. Equally, measurement values can be obtained from several modules of the same type and combined in an appropriate manner. If values of operating voltage and optical output power are collected for 'healthy' semiconductor laser diodes during operation, and if such values are also collected for defective laser diodes, it can be determined in an advance calibration step which values of the operating voltage are associated with unacceptable output power levels, and therefore with low optical output power. Such values therefore characterize a defective laser diode.

Measurements carried out for healthy and defective semiconductor laser diodes have shown that a linear relationship can be established between operating voltage and optical output power when a laser diode is driven at an essentially constant current level. Therefore, in a further particularly preferred embodiment of the invention, the performance-related measurements of the semiconductor laser diode module comprise operating voltage values and associated radiation output power values for the semiconductor laser diode module (or "representative" for the module), obtained when the module is driven at a specific current level, and the initial calibration step comprises determining parameters describing the linear relationship between the operating voltage and the radiation output power of the laser diodes of the module. As the skilled person will know, a suitable algorithm can be applied using the collected performance-related values or data to obtain a linear fit for those values. Parameters describing this linear relationship or linear fit can be stored, for example, in the memory module of a driver unit, and measured values of operating voltage for a semiconductor laser diode module can then be 'translated' into a corresponding level of optical output power. A user of the system could be informed, by a suitable feedback, of the optical output power estimate to be delivered by the laser diodes of a specific module.

It may be convenient to analyse operating voltage values for a laser diode module obtained at not just one single specific current value. Therefore, in a particularly preferred embodiment of the invention, performance-related measurements are obtained for a module by driving the module successively at a number of different specific current values. For example, depending on the semiconductor laser diode type, performance- related measurements can be obtained for current values of 100 mA, 150 mA, 200 mA, etc.

Generally, to determine whether the laser diodes of a module are still operating on the whole to a satisfactory level, it may be sufficient to simply compare the observed operating voltage level with a predetermined threshold voltage level. This threshold voltage level can have been previously determined, for example, in the prior calibration step such as that described above. Therefore, in another preferred embodiment of the invention, the analysis means comprises a comparator for comparing an operating voltage value to a predefined threshold voltage value, which predefined threshold voltage value is related to a minimum acceptable level of optical output power for that type of module.

As mentioned in the introduction, semiconductor laser diodes such as VCSELs can be used in applications requiring a source of heat radiation. Heat radiation can be generated by a laser diode in the lower region of the light spectrum, i.e. at frequencies lower than that of visible light. In a preferred embodiment of the system according to the invention, the semiconductor laser diodes of a semiconductor laser diode module are realized to emit optical radiation in the invisible region of the light spectrum, for example in the near infrared, or IR-A, region of the spectrum, which covers frequencies from 750 nm to 1400 nm. In a particularly preferred embodiment of the system according to the invention, the laser diodes are realized to emit infrared radiation at a wavelength in the range between 980 nm and 1130 nm.

It has already been mentioned above that the purpose of the fault detection measurements made by the system according to the invention is to determine whether a semiconductor laser module is healthy or defective. Evidently, such useful information should be communicated to a user of the system. To this end, a further preferred embodiment of the system according to the invention comprises a feedback generator for generating feedback pertaining to the performance of the system, and an output means for outputting the feedback to a user of the system, so that the user can be informed when a semiconductor laser diode module is determined to be defective. Feedback might include a notice of a replacement or an indication that servicing is required for a semiconductor laser diode module. The feedback might indicate the actual output optical power being delivered by a module when its diodes are being driven at a level above the threshold current level. The actual output power can be derived using the linear fit described above. Alternatively, when the operating voltage is being monitored while driving the diodes of the module at a current level below the threshold level, the feedback might predict or give an estimation of the optical output power that can be expected when the diodes of that module are driven to emit. Evidently, in systems comprising large numbers of modules, it would be useful for the user to be able to determine which module is to be replaced or serviced. Therefore, the feedback may include an identifier such as a module number so that the user can easily locate the semiconductor laser diode module in the overall arrangement, so that the user of the system can decide whether the module can remain in the system for another while or whether it should be replaced immediately.

The system for monitoring generating laser radiation can be used in a number of applications, for example any application requiring a compact source of radiation for inducing heat at a specific location. Such applications might be, for example, epilation, laser projection, food-processing, etc. A more probable use of the system according to the invention would be in a bottle-blowing apparatus for applying heat radiation to PET preforms. Such an apparatus can comprise an oven lined with semiconductor laser diode modules of the type described herein. The preforms are passed through the oven while being subject to heat radiation emitted by the laser diodes. The heat radiation serves to soften the PET material of the preforms, which can then be inflated or otherwise shaped until the desired overall form is obtained. Using the system according to the invention, the semiconductor laser diode modules can easily and conveniently be monitored so that any defective module is quickly identified. Defective modules can be replaced as necessary, thus ensuring an essentially constant level of heat radiation, so that the quality of the end product can be assured. Monitoring can be carried out at any convenient time, and, since the performance-related measurements can be obtained while driving the diodes at a level of current below the threshold level, the measurements can be obtained without the oven being fully operational, for instance in a run-up or warm-up phase shortly after turning on the apparatus, or just before turning it off.

The voltage measurements for the modules in a system can be obtained at predefined intervals, such as every hour, every half-hour, or as often as required. Generally, the measurements only require a few seconds or less, and therefore do not adversely affect the overall operation of the apparatus. Furthermore, the measurements can be made before the system is operational, so that any replacement can be carried out without having to wait for the apparatus to complete its task, or without having to interrupt the apparatus. Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purpose of illustration and not as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic illustration of a VCSEL diode and a heat sink; Fig. 2a shows a schematic plan view of a semiconductor laser diode array arrangement; Fig. 2b shows a schematic side view of a semiconductor laser diode array arrangement; Fig. 3 shows a graph of output power and wall-plug efficiency for a

VCSEL; Fig. 4 shows a graph of required cooling power and optical output for the VCSEL of Fig. 3; Fig. 5 shows graphs of voltage and output power against current for a number of identical VCSEL diodes; Fig. 6 shows a graph of voltage against optical output power for an array of VCSELs, and a linear fit relating voltage and optical output power and derived using the method according to the invention; Fig. 7 shows a schematic plan view of a bottle-blowing apparatus with a system for generating laser radiation according to an embodiment of the invention; Fig. 8 shows a block diagram of a driver unit according to an embodiment of the invention.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 presents a very simplified cross-section of a state-of-the-art VCSEL D showing the relevant levels in its construction. Electrical contacts 20, 20' are mounted on a diode structure comprising a substrate 24 and a quantum well layer 22 sandwiched between upper and lower Bragg reflectors 21, 23. This semiconductor laser D is pumped, electrically or optically, to generate invisible infrared heat radiation R which exits the diode structure in a direction essentially perpendicular to the surface. The exit area on the surface of the diode D corresponds in this realisation to an opening in the electrical contact 20. The optical output power that can be delivered by such a laser diode D is generally related to the exit area, so that a laser diode is often defined in terms of the diameter of the exit area. For example, a '30-micron VCSEL' has an emitting or exit area with a diameter of 30 μm. To operate the laser diode D, a voltage is applied across the electrical contacts 20, 20'. The functioning of such a diode D will be known to the skilled person, and need not be explained in greater detail here. The diode structure D is mounted onto a heat sink 16, which acts to absorb heat generated by the diode D during operation. The heat sink 16 in turn can be cooled by an external cooling arrangement, not shown in the diagram, for example a cooling-water arrangement. The diagram only shows a single diode D. However, this type of laser diode can easily be manufactured in large numbers on a single wafer. In reality, therefore, a semiconductor laser diode module can comprise a group of neighbouring laser diodes, arranged, for example, in a regular array.

Fig. 2a shows a semiconductor laser diode arrangement 10, in which VCSELs D are arranged in modules 11 on a printed circuit board 12. The modules 11 are electrically connected by means of leads 13 so that each module 11 can be driven by a suitable electrical or electronic driving means, not shown in the diagram.

Fig. 2b shows a side view of the semiconductor laser diode arrangement of Fig. 2a. In this diagram, a heat sink 16 can be seen. The heat sink 16 serves to absorb the heat generated by the semiconductor laser diodes D in the arrangement 10. In this example the heat sink 16 is further cooled by means of a cooling water system. The diagram shows a cooling water inlet 15 for supplying cooling water to the heat sink 16, which water is heated and leaves the arrangement 10 by means of a warm water outlet (not shown in the diagram). Such a cooling arrangement is shown here for the sake of completeness, but need not be explained in further detail since its operation will be known to the skilled person.

Fig. 3 shows a graph of wall-plug efficiency WPE (dimensionless, usually expressed as a value between 0 and 1), indicated by the dotted line, and optical output power P (mW) against current I (mA) for a 30-micron VCSEL diode. As the graph shows, as the current I increases, the wall-plug efficiency WPE of the VCSEL rapidly rises, and reaches its maximum well before the maximum optical output power is reached, i.e. before the VCSEL is emitting at its most intense level. For this reason, a laser diode is generally operated at a 'compromise' level of current, for example at a level of current corresponding to the intersection of the efficiency and output power curves.

Fig. 4 shows the relationship between the optical output power, shown by the solid line, that can be achieved by a laser diode and the heat generated by that laser diode. The optical output power and the heat to be removed are both expressed in mW. The diagram shows that, as current I increases, the optical output power of the laser diode also increases to a maximum and thereafter begins to decrease again. This is referred to as the "thermal rollover" of the laser diode. Further current increase leads to overheating of the laser diode and to reduction of the optical output power. Degradation of the thermal contact between the laser diode and its heat sink results in thermal rollover at lower currents. As the current increases, the amount of heat generated by the diode also increases. The rate of increase in heat becomes steeper as the optical output power declines after its maximum. In other words, a diode that has begun to deteriorate is characterized by a steeper increase in heat output.

Fig. 5 shows a set of curves for operating voltage (the parabolic curves) and output power P (the maxima curves) for a number of identical VCSELs in differing stages of deterioration. Each VCSEL was driven in the same manner, and voltage U (in Volt) and output power P _opt (in milliwatt) were measured for various current values. The resulting graphs show that a 'healthy' device delivers higher output power Pi at a correspondingly high voltage level U as depicted in curve Vi. Progressively more defective devices deliver less output power (P ₂ to P ₄ ), and the voltage measured across these devices is also lower as depicted in curves V ₂ to V ₄ . A greater degree of defectiveness is therefore associated with a lower output power and operating voltage.

Fig. 6 illustrates the relationship between optical output power and the operating voltage of a laser diode. The graph shows values of operating voltage, measured in Volt, indicated by the dots, plotted against optical output power, measured in milliwatt (mW). The values were obtained for a module comprising 120 micron VCSELs driven at a constant current of 400 mA. For the reason already given, namely that the electrical conductivity of a laser diode is strongly temperature dependent, any physical deterioration of a laser diode resulting in a drop in optical output power is accompanied by a subsequent drop in operating voltage. As the graph clearly shows, favourable higher values of optical output power are characterized by higher levels of operating voltage. Calibration values obtained in this way are analysed using the method according to the invention to obtain a linear approximation of the relationship. The graph shows a linear fit, indicated by the line 60. This linear fit can be given by the well-known equation y = a + mx (1) so that, in this case, substituting the observed voltage value for y and the observed optical output power for x, the equation describing the linear fit can be expressed as where P _opt is expressed in Watt.

Rewriting this equation gives

With the linear fit given by equation (2), the optical output power of the laser diodes of a module can be monitored by using observed values of voltage when the module is driven at the corresponding current level, in this case at 400 mA. At this current, a drop in voltage, which would not be observed for a 'healthy' diode, indicates that the diode is probably damaged, and that its optical output power will most likely be less than satisfactory. For example, a functioning laser diode has an optical output power of about 150 mW and its operating voltage is about 2.85 V when driven at 400 mA. When the diode begins to deteriorate, its optical output performance decreases, and a corresponding drop in voltage is observed. For example, the operating voltage is observed to drop to about 2.80 V. By using equation (3) above, the optical output power is calculated to be 110 mW. It may be concluded that the diode is beginning to deteriorate.

Fig. 7 shows a system 1 for generating laser radiation used as part of a bottle-blowing apparatus, comprising an oven 2 through which polyethylene terephthalate (PET) preforms P can be moved in the direction of motion M. A plurality of semiconductor laser diode modules 11 are arranged along the sides of the oven 2. Each semiconductor laser diode module 11 (see Fig. 2a) includes a plurality of semiconductor laser diodes (not shown) such as the VCSEL described in Fig. 1, which emit invisible infrared radiation R directed at the preforms P so that these are heated to obtain their desired shape and size. An electrical or electronic driving means 8 of a driver unit 3 drives the semiconductor laser diode modules 11 at the required constant current. The driver unit 3 can also control the amount of cooling required. To this end, the driver unit 3 comprises a cooling water controller 60 for issuing a control signal 61 for controlling a cooling water module 6 for cooling the semiconductor laser diode arrangement 10. The cooling water module 6 can control a cooling water inlet (not shown in the diagram) for regulating the cooling water flow 63 (indicated only by a dotted line) towards the semiconductor laser diode arrangement 10, and can also monitor the temperature of the water leaving the semiconductor laser diode arrangement 10, and can provide the cooling water controller 60 with an appropriate temperature information signal 62.

The system 1 for generating laser radiation also comprises a voltage measurement module 30 for monitoring the voltage of the semiconductor laser diode modules 11. The diagram only indicates a simplified electrical connection 31 originating from the semiconductor laser diode arrangement 10. However, the electrical connection 31 can in fact comprise a plurality of connectors or leads, one for each of the semiconductor laser diode modules 11. A voltage measurement unit 30 thus provides an observed voltage value 34 (see Fig. 8) for the module being monitored. As the skilled person will know, the voltage measurement unit 30 can easily be realized as part of the driving means 8 and need not be a separate module as shown here.

The driver unit 3 analyses the performance of one or more modules 11 of the arrangement 10 and causes a feedback signal 33 to be generated and forwarded to an output means 4, which can then inform a user of the system 1 that a semiconductor laser diode module 11 is probably defective, and can indicate which module 11 should be replaced or serviced. The semiconductor laser diode modules 11 can each be identified, for example, by an identification number or by a specific location in the oven 2. The output means 4 can be a display, for example a computer monitor, upon which the details concerning the relevant module 11 are displayed.

The relevant modules of the driver unit 3 will now be explained in more detail with the aid of Fig. 8. Other necessary units and components required for generating and supplying power to the semiconductor laser diode arrangement are, for the sake of clarity, not shown in the diagram. For example, a driving means of the driver may comprise a converter to generate a DC current from the AC mains power supply, and may apply a modulator to obtain a pulse-width modulated (PWM) laser-diode current. Such circuitry will be known to the skilled person and need not be elaborated upon here. The cooling water system is also not shown.

In the driver unit 3, the measured input voltage value 34 is compared, in an analysis unit 5, to a previously obtained threshold voltage value 32 corresponding, for example, to a minimum voltage level for that type of module 11 and the specific current value at which the module 11 is being driven. Such values are stored in a memory 51 of the analysis unit 5. The measured voltage value 34 and the threshold voltage value 32 are compared in a comparator 50. If the measured voltage value 34 is less than the threshold voltage value 32, or less than the threshold voltage value 32 by a certain amount, this can be taken as an indication that one or more laser diodes of that module 11 have begun to deteriorate. The result 52 of the comparison is forwarded from the analysis unit 5 to a feedback generator 7, which then issues feedback 33 giving, for example, an identifier for the module 11 , the measured voltage for that module, the deviation from the previously determined threshold for that module, etc. With this information, a user of the system can decide whether the module 11 should immediately be replaced or serviced. In the diagram, the comparator 50 and memory 51 are shown as part of an analysis unit 5, but the functions carried out by these modules could equally well be carried out in existing suitable modules of a driver unit, for example by a suitable software algorithm which can be stored in a memory and run on a processor of the driver.

Although the present invention has been disclosed in the form of a number of preferred embodiments, it is to be understood that additional modifications or variations could be made to the described embodiments without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. A "unit" or "module" can comprise a number of units or modules, unless otherwise stated.