Laser processing apparatus and method

Abstract

There is described a method of processing a target with a laser processing apparatus, the method comprising, directing a processing radiation beam towards the target to process a first portion of the target, the processing radiation beam comprising radiation of a first wavelength, receiving measurement radiation, the measurement radiation comprising radiation of a second wavelength, different to the first wavelength, the measurement radiation being generated by an interaction of the processing radiation beam with the target, directing the measurement radiation towards a distance measurement system and generating, Based upon the measurement radiation, first distance data indicative of a distance between the target and a portion of the laser processing apparatus.

Description

[0001] LASER PROCESSING APPARATUS AND METHOD

[0002] The present disclosure relates to a laser processing apparatus and method.

[0003] Laser processing apparatus are used to processing targets with a processing radiation beam. Typical examples of processing include marking, etching, welding, cutting, and UV curing. Such laser processing apparatus can operate over a wide range of wavelengths, depending on process, but typically operate in the UV-C to mid-IR range. In order to achieve high quality processing results, it is beneficial to take into account a distance between the target and the laser processing apparatus.

[0004] In a first aspect there is described a method of processing a target with a laser processing apparatus. The method comprises directing a processing radiation beam towards the target to process a first portion of the target, the processing radiation beam comprising radiation of a first wavelength, receiving measurement radiation, the measurement radiation comprising radiation of a second wavelength, different to the first wavelength, the measurement radiation being generated by an interaction of the processing radiation beam with the target, directing the measurement radiation towards a distance measurement system, and generating, based upon the measurement radiation, first distance data indicative of a distance between the target and a portion of the laser processing apparatus.

[0005] By monitoring radiation generated by an interaction of the processing radiation beam with the target (e.g. a plasma plume generated during marking of the target surface) it is possible to measure a distance between the laser processing apparatus and the target surface during processing, without having to rely on external measurement devices. In this way, one or more process parameters can be continuously (or at least frequently) adapted so as to ensure appropriate marking conditions (e.g. focal length).

[0006] It will be understood that a plasma plume may emit radiation at a range of different wavelengths and intensities (i.e. there may be a complex radiation spectrum). The measurement device may be configured to substantially ignore (or reject) radiation except for radiation at the second wavelength. This may be achieved using a suitable band-pass filter.

[0007] 69947607-1Directing the processing radiation beam towards the target may be performed by a beam steering apparatus. The beam steering apparatus may be configured to direct the processing radiation beam towards a location on the target specified by a target coordinate (e.g. an (x, y) coordinate).

[0008] The beam steering apparatus may also direct the measurement radiation from the target surface towards the distance measurement system.

[0009] In this way, the processing radiation beam and the measurement radiation may share an optical path (albeit when travelling in opposite directions).

[0010] The distance between the target and a portion of the laser processing apparatus may comprise a distance between the first portion of the target (e.g. a first portion of a surface of the target), and a representative part of the laser processing apparatus, such as, for example a lens. The lens may comprise a focusing lens configured to focus the processing radiation beam onto the target during processing. Knowing the distance between the focusing lens and the first portion of the target allows the processing to be controlled (e.g. the distance can adjusted to bring the target to a desired focal position, or the focal length can be adjusted to bring the focal position to the target).

[0011] The target may comprise a target surface. For example, the target may comprise a surface of a product, or a surface of an item of packaging for a product. The surface may be generally planar, or may comprise a curved, or complex shape.

[0012] Processing may comprise marking. Alternatively, processing may comprise etching, welding, cutting, or otherwise processing the target with the processing radiation beam.

[0013] The laser processing apparatus may comprise a laser marking apparatus. Processing a portion of the target may comprise causing a mark to be created at the portion of the target.

[0014] The radiation may comprise electromagnetic radiation.

[0015] The first wavelength may comprise 1064 nm.

[0016] 69947607-1The second wavelength may comprise 905 nm.

[0017] The generating of the first distance data may be carried out by the distance measurement system.

[0018] Generating the first distance data may comprise generating data indicative of an origin of the measurement radiation, and generating the first distance data based upon the data indicative of an origin of the measurement radiation.

[0019] When monitoring a plasma plume generated during processing, the plasma plume may be positioned slightly above a surface of the target. For example, a peak intensity of the plasma plume may be a few mm above a surface of the target. As such, when generating the first distance data an apparent location (as indicated by the detected plasma plume) may be adjusted slightly, in order to provide a more accurate indication of an actual surface position.

[0020] It will further be understood that a plasma plume may be distributed across a volume of space adjacent to the target surface, and there may not, therefore, be a single “origin” of the measurement radiation. However, the distance measurement system may be configured to measure an average, or apparent, origin. Such a position may correspond to a position of peak plasma intensity.

[0021] Generating the first distance data may be further based upon offset data indicative of a distance between the origin and the target.

[0022] It will be understood, of course, that the distance between the actual origin of the measurement radiation and the target may comprise a plurality of distances. However, for the purposes a simplicity of measurement, a single indicative distance (e.g. offset data) may be determined.

[0023] While a plasma plume offset may vary depending upon many processing conditions (e.g. substrate being processed, wavelength and intensity of processing radiation, surface treatments / coating, etc.), a plasma will generally be consistent for a particular

[0024] 69947607-1combination of process conditions, and will therefore generally be consistent during a single marking operation.

[0025] The method may further comprise, prior to directing the processing radiation beam towards the target to process the first portion of the target, directing a measurement radiation beam towards a second portion of the target, the measurement radiation beam comprising radiation of the second wavelength, receiving reflected measurement radiation, the reflected measurement radiation comprising a portion of the measurement radiation beam reflected by the second portion of the target, directing the reflected measurement radiation towards the distance measurement system, and generating, based upon the reflected measurement radiation, second distance data indicative of a distance between the target and a portion of the laser processing apparatus.

[0026] Prior to performing a measurement during processing (e.g. marking), an initial (second) distance measurement may be conducted by using a measurement laser beam that is directed at the target. This allows the initial distance measurement to be made before any processing radiation is emitted, or any plasma plume is generated, thereby avoiding the risk of interfering with the distance measurement.

[0027] Generating the first distance data may be further based upon the second distance data.

[0028] Generating the first distance data indicative of a distance between the target and a portion of the laser processing apparatus may comprise generating data indicative of an origin of the measurement radiation and adjusting the indicated position based on the second distance data. That is, the second distance data allows the actual initial position of the target to be established, prior to the generation of a plasma, with subsequent measurements being plasma based measurements.

[0029] The method may further comprise, after directing the measurement radiation beam towards the second portion of the target, and prior to directing the processing radiation beam towards the target to process the first portion of the target, directing the processing radiation beam towards the target to process the second portion of the target, receiving measurement radiation comprising radiation of the second wavelength, the measurement radiation being generated by an interaction of the

[0030] 69947607-1processing radiation beam with the second portion of the target, directing the measurement radiation towards the distance measurement system, generating, based upon the received measurement radiation, data indicative of an origin of the measurement radiation, and determining a difference between the data indicative of an origin of the measurement radiation and the second distance data.

[0031] The difference may comprise the offset data.

[0032] By combining an initial (second) distance measurement (by using a measurement laser), and a plasma plume derived measurement once processing has commenced, it is possible to determine an offset between the actual position of the target and an apparent position of the target (as indicated by the apparent origin of plasma radiation). Such an offset can then be applied to subsequent plasma plume derived measurements in real-time, allowing a target position relative to the laser processing apparatus to be tracked during processing, without reliance on an external measurement device, and without interrupting processing.

[0033] Directing the processing radiation beam towards the target to process the first portion of the target, and the directing the measurement radiation towards the distance measurement system may be performed at the same time.

[0034] In this way, the distance measurement can be performed during processing, and does not interfere with the processing operations. Direction the processing radiation beam and the measurement radiation may be, at least partially, performed by the same components, e.g. a beam steering apparatus.

[0035] At least one characteristic of the processing radiation beam during the processing of the first portion may be controlled based upon the first distance data.

[0036] The at least one characteristic of the processing radiation beam that is controlled based upon the first distance data may be a focal length.

[0037] The at least one characteristic of the processing radiation beam that is controlled based upon the first distance data may be a power of the processing radiation beam.

[0038] 69947607-1The at least one characteristic of the processing radiation beam that is controlled based upon the first distance data may be a distance to the target. That is, target may be brought closer to, or further away from, the focusing lens of the laser processing apparatus. In this way, the length of the processing radiation beam is shortened.

[0039] The laser processing apparatus may comprise a focusing system comprising an adjustable optical path length device configured to adjust a focal length of the processing radiation beam.

[0040] The method may further comprise outputting an alert based upon the first distance data.

[0041] For example, outputting the alert based upon the first distance data may comprise comparing the first data distance to a threshold and outputting the alert based upon the comparing. For example, the alert may be output if the first distance data indicates that the distance between the target and the portion of the laser processing apparatus exceeds a threshold. For example, if it is determined that the distance is beyond a maximum predefined distance, the laser processing apparatus, or any other device coupled to the laser processing apparatus, may output the alert in order to notify a user. The alert may take any suitable form, such as an audio alert output by a speaker system and / or a visual alert output by a computer display. In this way, if the target is incorrectly placed such that it is outside of a processing field of the laser processing apparatus, the user is alerted and can remedy the situation. Outputting of the alert may be accompanied by the automatic halting of the laser processing apparatus.

[0042] The method may further comprise, after directing the processing radiation beam towards the target to process the first portion of the target, directing the processing radiation beam towards the target to process a third portion of the target, wherein at least one characteristic of the processing radiation beam directed to the third portion of the target is controlled based upon the first distance data.

[0043] That is, it is possible to control (e.g. adjust if necessary, or maintain in an existing configuration if appropriate) the processing radiation beam to perform a further processing operation (e.g. processing a third portion of the target) based upon the distance measurement performed while processing the first portion of the target. In this

[0044] 69947607-1way, it is possible to adapt the processing to track changes in the relative position of the target and the processing apparatus (e.g. due to changes in position, due to changes in focal length at different beam angles, or due to complex surface shapes).

[0045] The third portion of the target may comprise a region of the target having a different set of coordinates to the first portion.

[0046] Processing the first portion of the target and the third portion of the target may comprise a continuous process.

[0047] For example, the first and third portions may comprise different parts of a single mark made on the target by the laser processing (e.g. marking) apparatus.

[0048] The second, first and third portions may comprise different parts of a single mark made on the target by the laser processing (e.g. marking) apparatus.

[0049] The method may further comprise, after directing the processing radiation beam towards the target to process the first portion of the target, directing the processing radiation beam towards the target to process a plurality of further portions of the target, wherein during processing of each of the plurality of further portions of the target at least one characteristic of the processing radiation beam is controlled based upon distance data indicative of a distance between the target and a portion of the laser processing apparatus generated during an earlier processing of a portion of the target.

[0050] That is, during processing operations (which may comprises a plurality of continuous processing operations), the processing radiation beam may be controlled (e.g. a focal length controlled and / or adjusted) based upon distance data obtained during one or more previous parts of the processing operation. In this way, it is possible to adapt the processing in real-time.

[0051] The method may further comprise, during directing the processing radiation beam towards the target to process the plurality of further portions of the target, receiving further measurement radiation, the further measurement radiation comprising radiation of the second wavelength, the further measurement radiation being generated by interactions of the processing radiation beam with the further portions of the target,

[0052] 69947607-1directing the further measurement radiation towards the distance measurement device, and generating, based upon the further measurement radiation, data indicative of a distance between each of the plurality of further portions of the target and a portion of the laser processing apparatus.

[0053] The data indicative of a distance between each of the plurality of further portions of the target and a portion of the laser processing apparatus may comprise a plurality of data items, each one of the plurality of data items corresponding to a respective one of the plurality of further portions of the target.

[0054] Generating each of the plurality of data values may comprise adjusting an apparent position, or distance value, by an offset. That is, offset data generated during processing the second portion of the target can be used, in real-time, during a plurality of subsequent processing operations to compensate for the difference between the plasma generation position, and an actual surface position.

[0055] In a second aspect there is provided a method of creating a mark on a target with a laser marking apparatus, the mark having a start location and an end location, and defining a marking path extending from the start location to the end location, wherein creating the mark comprises: i) performing an initial distance measurement at the start location by directing a measurement radiation beam towards the start location, receiving a reflected portion of the measurement radiation beam by a distance measurement system, and generating initial distance data indicative of a distance between the target and a portion of the laser marking apparatus based upon the reflected portion the measurement radiation; ii) directing a processing radiation beam towards the start location to create a first portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the initial distance data; iii) receiving measurement radiation, the measurement radiation being generated by an interaction of the processing radiation beam with the target at the start location, directing the measurement radiation towards the distance measurement system, and generating first apparent distance data based upon the measurement radiation; iv) directing the processing radiation beam towards a second location along the marking path to create a second portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the initial distance data; v) receiving measurement radiation, the measurement radiation being generated by an

[0056] 69947607-1interaction of the processing radiation beam with the target at the second location, directing the measurement radiation towards the distance measurement system, and generating second apparent distance data based upon the measurement radiation; vi) determining second distance data based on the second apparent distance data and a difference between the initial distance data and the first apparent distance data; and vii) directing the processing radiation beam towards a third location along the marking path to create a third portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the second distance data.

[0057] Steps v, vi, and vii may be repeated as necessary until the mark reaches the end location, at which point the marking is terminated.

[0058] The measurement process may be repeated periodically during the creation of the mark. That is, while a marking process may be substantially continuous, the measurement process may be discontinuous.

[0059] The at least one characteristic of the processing radiation beam that is controlled based upon the distance data may be a focal length.

[0060] The method may comprise determining offset data indicative of a difference between the initial distance data and the first apparent distance data.

[0061] Determining the second distance data may comprise adding the offset data to the second apparent distance data.

[0062] It will be understood, of course, that distance calculation may be performed in any convenient way. For example for each subsequent measurement following an initial (i.e. non-plasma based) measurement, the initial measurement, the first apparent distance data (which corresponds to the same location as the initial measurement) and the subsequent apparent distance may be processed to generate the subsequent distance data.

[0063] The method may further comprise creating a plurality of marks on the target, each of the plurality of marks having a respective start location and a respective end location, and defining a respective marking path extending from the respective start location to

[0064] 69947607-1the respective end location, wherein creating each of the plurality of marks comprises steps i) to vii).

[0065] The processing radiation may be generated by a laser. The laser may be a fiber laser. The fiber laser may have a wavelength of 1064 mm. The laser may be a UV laser. The UV laser may have a wavelength of 355 nm. The laser may be a CO2 laser.

[0066] The interaction between the processing radiation beam and the target (e.g. a surface of the target) may generate a plasma comprising radiation at a plurality of wavelengths, including at least a measurement wavelength.

[0067] Wavelengths other than the measurement wavelength may be prevented from reaching the distance measurement system. This may be achieved using a suitable band pass filter.

[0068] In a third aspect there is described a laser processing apparatus comprising: a distance measurement system configured to receive measurement radiation; an optical element configured to direct a processing radiation beam towards a target; a processor; a memory storing computer readable instructions which when executed by the processor, cause the processor to carry out the method of any preceding aspect.

[0069] The laser processing apparatus may comprise a processing beam radiation source. The processing beam radiation source may comprise a laser. The laser processing apparatus may be configured to connect to an optical fiber carrying the processing beam.

[0070] The distance measurement system may comprise a measurement radiation source configured to generate a measurement radiation beam.

[0071] The optical element may comprise a dichroic mirror.

[0072] The optical element may be further configured to receive the measurement radiation; and, redirect one of the processing radiation beam and the measurement radiation to form a junction between a shared optical path, a processing radiation beam optical path and a measurement radiation optical path.

[0073] 69947607-1The measurement radiation may comprise a portion of a measurement radiation beam. The measurement radiation beam may be generated and directed at the target, and the reflected measurement radiation beam may be measurement radiation. The measurement radiation may comprise radiation emitted by a plasma plume.

[0074] The processing radiation beam and the measurement radiation beam may have different wavelengths. The optical element may direct the measurement radiation towards the distance measurement system via the measurement radiation optical path.

[0075] The processing radiation beam and the measurement radiation beam may pass along at least a portion of a shared optical path. For example, when performing the distance measurement the measurement radiation beam may pass along at least a portion of the shared optical path from the target towards the laser processing apparatus. When performing the laser processing operation, the processing radiation beam may pass along at least a portion of the shared optical path from the laser processing apparatus towards the target.

[0076] The laser processing apparatus may comprise a focusing lens configured to receive the processing radiation beam.

[0077] The processing radiation beam may comprise a first wavelength and the measurement radiation may comprise a second wavelength that is different to the first wavelength.

[0078] The distance measurement system may generate the distance data based at least partly based on the measurement radiation.

[0079] The laser processing apparatus may comprise an adjustable focus system configured to adjust a focal length of the optical system in at least partial dependence upon the distance data.

[0080] The distance measurement system may comprise a measurement radiation source configured to generate a measurement radiation beam.

[0081] 69947607-1The distance measurement system may be configured to detect at least a portion of the measurement radiation beam that reflects from the target. That is, the portion of the measurement radiation beam that reflects from the target may be measurement radiation.

[0082] The adjustable focus system may comprise a shared module located on the shared optical path configured to adjust focal lengths of the processing radiation beam and the measurement radiation beam in at least partial dependence upon distance data.

[0083] The distance data may be any of the distance data, e.g. the first, second or third distance data.

[0084] The shared module may comprise a movable lens. The movable lens may be movable along the shared optical path.

[0085] The shared module may comprise a variable optical path length assembly configured to define an optical path from an input to an output. The variable optical path length assembly may comprise a rotatable path length adjuster.

[0086] The shared module may be configured to set the focal lengths experienced by the processing radiation beam and the measurement radiation beam to be substantially equal to the distance indicated by any of the distance data.

[0087] The laser processing apparatus may further comprise a beam steering apparatus configured to direct the processing radiation beam about the target.

[0088] The beam steering apparatus may be located on the shared optical path.

[0089] The beam steering apparatus may further be configured to direct the measurement radiation beam about the target.

[0090] The distance measurement system may comprise a first sensor region configured to generate a first signal that is proportional to an amount of measurement radiation incident on the first sensor region, a second sensor region configured to generate a second signal that is proportional to an amount of measurement radiation incident on

[0091] 69947607-1the second sensor region, a sensor lens configured to receive measurement radiation from the target and illuminate the first and second sensor regions with the measurement radiation, and an optical arrangement configured to introduce an unequal distribution of measurement radiation between the first and second sensor regions that varies with a distance between the sensor lens and the target in accordance with a mathematical relationship.

[0092] The processor may be configured to determine the distance data based at least partly based on, the first and second signals; and, the mathematical relationship.

[0093] The laser processing apparatus may comprise an I / O interface. The I / O may be configured to couple to one or more I / O devices, such as keyboards, mice, speaker systems, computer displays, user interfaces, and / or touchscreens.

[0094] Any feature described in the context of one aspect of the present disclosure can be applied to other aspects of the present disclosure. For example, the optional features of the first aspect may be used with the second aspect.

[0095] Embodiments of the present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

[0096] Figure 1 schematically depicts a laser processing apparatus in accordance with the present disclosure;

[0097] Figure 2 schematically depicts a distance measurement system in accordance with the present disclosure;

[0098] Figure 3 schematically depicts a ray diagram showing four target locations and corresponding image formation locations formed by the lens shown in Figure 2;

[0099] Figure 4A schematically depicts a ray diagram corresponding to that of Fig. 3 with a sensor plane present;

[0100] 69947607-1Figure 4B schematically depicts a view of the front of the sensor plane showing four cross-sectional areas of measurement radiation associated with the four target locations of Figure 3;

[0101] Figure 5 schematically depicts a front view of the first and second sensor regions of the distance measurement system of Figure 2 along with the cross-sectional areas of measurement radiation of Figures 4B;

[0102] Figure 6 shows a graph of a normalized difference between cross-sectional areas of measurement radiation incident on the first and second sensor regions with a varying distance between the target and the sensor lens for different values of focal length of the sensor lens;

[0103] Figure 7 is a schematic diagram showing the operation of a laser processing apparatus at a first time;

[0104] Figure 8 is a schematic diagram showing the operation of a laser processing apparatus at a second time;

[0105] Figure 9 is a schematic diagram showing the operation of a laser processing apparatus at a third time;

[0106] Figure 10 is a schematic diagram showing the operation of a laser processing apparatus at a fourth time;

[0107] Figure 11 is a schematic diagram showing the operation of a laser processing apparatus at a fifth time;

[0108] Figure 12 is a schematic diagram showing the operation of a laser processing apparatus at a sixth time;

[0109] Figure 13 is a flow diagram according to a method according to the present disclosure;

[0110] Figure 14 is a flow diagram according to a method according to the present disclosure;

[0111] 69947607-1Figure 15 is a schematic illustration of a mark being applied to a target; and

[0112] Figure 16 is a schematic illustration of a processing system.

[0113] Figure 1 schematically depicts an optical system 300 comprising a focusing lens 302 configured to receive a processing radiation beam 304. The optical system 300 may comprise a processing radiation source (not shown) configured to generate the processing radiation beam 304. Alternatively, the optical system 300 may be configured to receive the processing radiation beam 304 from an external source via, for example, an optical fiber (not shown). The processing radiation beam 304 comprises a first wavelength. The first wavelength may, for example, be about 1064 nm. In the example of Fig. 1, the optical system 300 forms part of a laser processing apparatus, such a laser marking apparatus configured to mark a target 312. The laser marking apparatus comprises a beam steering mechanism 322, 324 (also referred to herein as a beam steering apparatus) configured to steer the processing radiation beam 304 about a marking field 326 of the laser marking system. In the example of Fig. 1, the beam steering mechanism comprises a pair of galvanometer-driven rotatable mirrors 322, 324. One of the mirrors 322, 324 is configured to steer the processing radiation beam 304 about a first axis and the other of the mirrors is configured to steer the processing radiation beam 304 about a second axis that is perpendicular to the first axis. As such, the beam steering mechanism 322, 324 acts to steer the processing radiation beam 304 to mark different locations about a two dimensional marking field 326. An example marking path 328 is shown in Fig. 1.

[0114] It is important that the processing radiation beam 304 is in focus at the desired processing location for desired processing of the target to occur. For example, marking errors may occur if a laser marking beam is not in focus at the desired location on a target. The total distance between the focusing lens 302 and the target 312 (e.g. a substrate) at least partly depends upon on a location of the processing radiation beam spot within the marking field 326. For example, for a substantially flat target arranged substantially perpendicularly to the laser processing apparatus, when the target is at the centre of the marking field 326, the distance between the focusing lens 302 and the target is at a minimum. When the target is at a peripheral corner of the marking field 326, the distance between the focusing lens 302 and the target 312 is at a maximum. Consequently, the processing radiation beam 304 is precisely in-focus only at the

[0115] 69947607-1center of the marking field 326 (or on a concentric ring around the center of the marking field 326). At all other areas of the marking field 326, the processing radiation beam spot size on the target increases, which, in the example of laser marking, may cause different line widths when scribing and / or fading of marking content and / or even no substrate reaction at all. Any slight installation flaw in the laser processing apparatus may cause an undesired change in the distance between the focusing lens 302 and the target 312 or an undesired tilt angle between the target and the marking field 326. In addition, when laser processing more complex three-dimensional shapes such as beverage bottles, household cosmetic containers, etc., the distance between the focusing lens 302 and different parts of the target 312 varies.

[0116] The optical system 300 comprises a distance measurement system 306. The distance measurement system 306 is configured to receive measurement radiation 308 comprising a second wavelength that is different to the first wavelength. The second wavelength may be, for example, about 905 nm. The distance measurement system 306 is configured to generate a signal 342 at least partly based on the measurement radiation 308 that is indicative of a distance between a representative portion of the laser processing apparatus, such as the focusing lens 302, and a target 312 to be processed by the processing radiation beam 304. The signal 342 is also referred to herein as distance data. An example of a distance measurement system 306 is shown and described in connection with Figs. 2-4. Other distance measurement systems may be used such as, for example, a confocal pinhole distance measurement system. Alternatively, other distance measurement systems may utilise time of flight (ToF), LIDAR, or interferometry to obtain distance data.

[0117] The optical system 300 comprises an optical element 314 configured to receive the processing radiation beam 304 and receive the measurement radiation 308. The optical element 314 is configured to redirect one of the processing radiation beam 304 and the measurement radiation 308 to form a junction between a shared optical path 316, a processing radiation beam optical path 318 and a measurement radiation optical path 320. In the example of Fig. 1, the optical element 314 comprises a dichroic mirror. The dichroic mirror has different reflection and transmission properties at two different wavelengths (i.e. the first wavelength of the processing radiation beam 304 and the second wavelength of the measurement radiation 308). The dichroic mirror 314 is configured to transmit substantially all of the processing radiation beam 304 and reflect

[0118] 69947607-1substantially all of the measurement radiation 308. Other types of optical elements may be used such as, for example, a polarization-based optical element configured to have different reflection and transmission properties at two different polarizations, a coupling prism formed from wavelength-sensitive material that has different reflection and transmission properties at two different wavelengths, a metasurface, etc. The optical element 314 may act as a beam combiner to precisely set the outgoing processing radiation beam 304 and the incoming measurement radiation 308 coaxial to each other. By forming a shared optical path 316, the optical element 314 allows the distance measurement system 306 to measure the distance to the location on the target 213 that the processing radiation beam 304 is due to process. This increases an accuracy of the optical system 300 compared to known optical systems. In the example of Fig. 1, the optical element 314 is located such that the junction between the processing radiation beam optical path 318 and the measurement radiation optical path 320 is located proximate the beam steering mechanism 322, 324. Other arrangements may be used. The optical system 300 may comprise one or more wavelength filters (not shown) on the measurement radiation optical path 320 configured to block wavelengths other than the second wavelength to protect components of the distance measurement system 306, e.g. from unwanted back reflections or other unwanted sources of light entering the optical system 300.

[0119] The optical system 300 comprises an adjustable focus system 330 configured to adjust a focal length of the optical system 300 in at least partial dependence upon the signal 342 generated by the distance measurement system 306. In the example of Fig. 1, the adjustable focus system 330 comprises a movable lens 331 and the focusing lens 302. The movable lens 331 is movable along an optical axis of the focusing lens 302. The movable lens 331 may be movable relative to the focusing lens 302. The movable lens 331 is configured to expand the processing radiation beam 304 before the processing radiation beam 304 is incident upon the focusing lens 302. By moving the movable lens 331, the processing radiation beam 304 is able to expand either more or less before being incident upon the focusing lens 302, thereby changing a focal length of the optical system 300. The adjustable focus system 330 may take other forms. For example, the adjustable focus system 330 may comprise a variable optical path length assembly configured to define an optical path from an input to an output. The variable optical path length assembly may comprise a rotatable path length adjuster. In the example of Fig. 1, the optical system 300 comprises a controller 340 configured to

[0120] 69947607-1receive the signal 342 generated by distance measurement system 306 and control the adjustable focus system 330 in at least partial dependence upon the signal 342. The adjustable focus system 330 may be configured to set the focal length of the optical system 300 to be substantially equal to the distance indicated by the signal 342. In the example of Fig. 1, the adjustable focus system comprises a processing radiation beam module located on the processing radiation beam optical path 318 configured to adjust a focal length experienced by the processing radiation beam 304 in at least partial dependence upon the signal 342. For example, if the distance measurement system 306 determines that the distance between the focusing lens 302 and the target 312 has increased (e.g. due to steering by the beam steering mechanism 322, 324 and / or due to movement of and / or three-dimensional shape variation of the target), the controller 340 may control the adjustable focus system 330 to move the movable lens 331 such that the focal length experienced by the processing radiation beam 304 substantially matches the distance measured by the distance measurement system 306. In this way, the processing radiation beam 304 can be kept in-focus despite distance changes, thereby ensuring that desired laser processing can occur at the target 312.

[0121] A relationship between the distance measured by the distance measurement system 306 and a corresponding required position of the movable lens 331 to bring the processing radiation beam 304 into focus could be derived analytically and / or a calibration measurement may be performed to provide a database (e.g. a lookup table) of adjustable focus system setting values and / or some coefficients for a higher order equation that describes the behaviour of the adjustable focus system. The database of movable lens 331 positions and / or the set of coefficients may be stored in memory to be accessed by the controller 340. The database of movable lens 331 positions and / or the set of coefficients may be unique to a given optical system 300 (e.g. a laser marking head).

[0122] A focal depth of the processing radiation beam 304, once adjusted to the correct focal distance by the adjustable focus system 330, may be limited. For example, a beam diameter of about 250pm at a working distance from the focusing lens 302 of about 150mm would substantially increase for shorter or longer distances (i.e. for a mismatch between the required focal length and a focal length set by the adjustable focus system 330). Accurately setting a desired focal length by, for example, moving a movable lens 311, changing optical path lengths, or other means, may not pose the greatest

[0123] 69947607-1problem. A more complex problem involves measuring the distance between the focusing lens 302 and the target 312 with a required accuracy for at least some laser processing applications, such as laser marking. The required accuracy of the distance measurement may be derived from the Rayleigh length of the processing radiation beam 304 (i.e. the distance from the beam waist in the propagation direction where the beam radius is increased by a factor of the square root of two). In practice, the minimum achievable beam diameter at least partly depends on the wavelength of the processing radiation beam 304 and its beam quality.

[0124] For laser processing applications, such as laser marking, it is important to maintain a sufficiently small processing radiation beam 304 diameter at the target 312 to maintain sufficient power density for desired processing to take place. As the focal length experienced by the processing radiation beam 304 moves away from an ideal focal length, the beam diameter of the processing radiation beam 304 increases in a substantially linear relationship with the distance from the ideal focal length. Problematically, if the processing radiation beam diameter increases by a factor of x then the power density of the processing radiation beam 304 would vastly decrease in accordance with an inverse square relationship (i.e. ^). In an area close to the ideal focal length however, the beam diameter increases at a significantly slower rate with increasing distance from the ideal focal length. This area extends from a negative value of the Rayleigh length to a positive value of the Rayleigh length on both sides of the optimal focus (both sides of the beam waist). This distance ZR is named Rayleigh length. As such, in order to allow for some margin of error, the distance measurement device preferably provides an accuracy that is equal to or better than the Rayleigh length of the processing radiation beam 304.

[0125] The Rayleigh length increases as the focal length experienced by the processing radiation beam 304 increases. As such, a required accuracy of the distance measurement performed by the distance measurement system 306 decreases as the focal length of the processing radiation beam 304 increases. That is, less accurate distance measurements are required at greater working distances. Providing a minimum absolute accuracy across the entire range of working distances of a laser processing system that would be sufficient for the smallest focal length would provide far more distance measurement accuracy (and hence complexity, cost and size) than what is actually needed at longer focal lengths (i.e. the majority of the working

[0126] 69947607-1distances). Alternatively, providing a specified relative accuracy, which may be a constant percentage over the entire range of working distances, would also be significantly more accurate than what is actually required across greater focal lengths. The distance measurement system 306 described and depicted in connection with Figs. 2-4 advantageously provides a higher accuracy at smaller focal lengths and a lower (but still acceptable) accuracy at greater focal lengths, thereby satisfying the Rayleigh length requirement whilst avoiding complexity, costand bulkiness.

[0127] Fig. 2 schematically depicts the distance measurement system 306 of Fig. 1 in greater detail. The distance measurement system 306 comprises first and second sensor regions 201, 202. The first sensor region 201 is configured to generate a first signal that is proportional to an illuminated area of the first sensor region 201. The second sensor region 202 is configured to generate a second signal that is proportional to an illuminated area of the second sensor region 202. The distance measurement system 306 comprises a sensor lens 204 configured to receive measurement radiation 206d from the target 312 and illuminate the first and second sensor regions 201 , 202 with the measurement radiation 206d. The distance measurement system 306 comprises an optical arrangement configured to introduce an unequal distribution of measurement radiation 206d between the first and second sensor regions 201, 202 that varies with a distance 208 between the sensor lens 204 and the target 312 in accordance with a mathematical relationship. In the example of Fig. 2, the optical arrangement comprises an offset between an optical axis 220 of the sensor lens 204 and a midpoint between the first and second sensor regions 201, 202. The distance measurement system 306 comprises a processor 210 configured to determine a distance between the distance measurement system 306 and the target 312 at least partly based on the first and second signals and the mathematical relationship. It will be appreciated that the distance may be converted or modified to align with another part of the laser processing apparatus, such as the focusing lens 302, or an output surface (e.g. exit window) of the laser processing apparatus. In the example of Fig. 2, the processor 210 is configured to determine the distance 208 between the sensor lens 204 and the target 312 at least partly based on the first and second signals and the mathematical relationship. The processor 210 may be configured to determine the distance between the target 312 and any fixed component of the distance measurement system 306. For example, the processor 210 may be configured to determine the distance between the target 312 and any one of the first and second sensor regions 201, 202, the polarizing

[0128] 69947607-1beamsplitter 218, etc. The positions of fixed components of the optical system 300, including the distance measurement system 306, relative to each other may be known. As such, determining the distance between the target 312 and one of the fixed components of the distance measurement system 306 would allow for the distance between the target 312 and all other fixed components of the optical system 300 to be determined (e.g. by adding or subtracting a known distance offset as necessary). For example, the distance between the sensor lens 204 and the target 312 is indicative of the distance between the focusing lens 302 of Fig. 1 and the target 312 given that the relative positions of the sensor lens 204 and the focusing lens 302 are known.

[0129] In the example of Fig. 2, the distance measurement system 306 comprises a measurement radiation source 212 configured to generate outgoing measurement radiation 206a. The measurement radiation source 212 may comprise a laser. The laser may comprise, for example, a laser diode or a diode-pumped solid state laser. The measurement radiation source 212 may be configured to generate pulsed electromagnetic radiation. The measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength that is typically used for LiDAR applications. For example, the measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of between about 1500nm and about 2000nm such as those used for meteorology and / or Doppler scientific LiDAR applications. As another example, the measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of between about 850nm and about 940nm such as those used for terrestrial mapping LiDAR applications. As a further example, the measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of between about 500nm and about 750nm such as those used for bathymetry LiDAR applications. As a yet further example, the measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of about 250nm such as those used for meteorology LiDAR applications. The measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of about 250nm or more. The measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of about 2000 nm or less. The measurement radiation source 212 may be configured to generate electromagnetic radiation having a wavelength of, for example, about 905 nm. In general, measurement radiation source 212 is configured to generated electromagnetic

[0130] 69947607-1radiation having a wavelength that is different to the wavelength of the processing radiation beam. The wavelength of electromagnetic radiation generated by the measurement radiation source 212 may therefore vary depending on the type of lase processing that is to be performed. The distance measurement system 306 comprises a second lens 214 configured to receive the outgoing measurement radiation 206a generated by the measurement radiation source 212 and focus the outgoing measurement radiation 206a. In the example of Fig. 2, the distance measurement system 306 comprises a mirror 216 configured to receive the outgoing measurement radiation 206a from the second lens 214 and reflect the outgoing measurement radiation 206a towards a polarizing beamsplitter 218. In the example of Fig. 2, the measurement radiation source 212 is configured to generate polarized outgoing measurement radiation 206a. The polarizing beamsplitter 218 is oriented relative to a polarization direction of the outgoing measurement radiation 206a such that the polarizing beamsplitter 218 reflects substantially all of the outgoing measurement radiation 206a along an optical axis 220 of the sensor lens 204.

[0131] The outgoing measurement radiation 206a is incident upon the target 312 and undergoes diffuse reflection from the target 312. A portion of the reflected measurement radiation 206b is reflected back towards the distance measurement system 306 along the same path as the outgoing measurement radiation 206a. That is, in the example of Fig. 2, a portion of the reflected measurement radiation 206b travels back towards the distance measurement system 306 along the optical axis 220 of the sensor lens 204. It will be appreciated that whilst a portion of the reflected measurement radiation 206b is coaxial with the outgoing measurement radiation 206a, said portion of reflected measurement radiation 206b may include individual light rays that are not substantially parallel to said axis. That is, the reflected measurement radiation 206b may comprise a divergent beam having its origin at the target 312. However, even if not substantially parallel, said light rays that are incident on the sensor lens 204 will form part of the first and second signals generated by the first and second sensor regions 201, 202. The outgoing measurement radiation 206a and the portion of the reflected measurement radiation 206b are co-axial. The portion of the reflected measurement radiation 206b is incident upon the polarizing beamsplitter 218. Due to undergoing diffuse reflection at the target 312, the portion of the reflected measurement radiation 206b lacks polarization. As such, the polarizing beamsplitter 218 reflects about half of the portion of the reflected measurement radiation 206c

[0132] 69947607-1towards the mirror 216 (i.e. off the optical axis 220 of the sensor lens 204) and transmits about half of the portion of the reflected measurement radiation 206d towards the sensor lens 204. The sensor lens 204 focuses the transmitted half of the portion of the reflected measurement radiation 206d to the first and second sensor regions 201, 202.

[0133] In the example of Fig. 2, the measurement radiation source 212 and the first and second sensor regions 201, 202 are located within a single module 222. In other embodiments, the measurement radiation source 212 and the first and second sensor regions 201, 202 may be housed in separate modules.

[0134] Fig. 3 schematically depicts a ray diagram showing four target locations and corresponding image formation locations formed by the sensor lens 204 of Fig. 2. The sensor lens 204 has a focal length 228 and a radius 230. The lens equation may be used to determine a distance between a lens and an image formed by the lens as follows:

[0135] 1 _ 1 1

[0136] =D+d

[0137] where f is a focal length of the lens, D is a distance between an object and the lens, and d is a distance between the lens and an image of the object formed by the lens. The lens equation may be rearranged to the following equation:

[0138]

[0139] The above equations are derived using a thin lens approximation. That is, the above equations assume a thin lens (i.e. a lens having a thickness that is negligible relative to a radii of curvature of the lens’ surfaces) having a point light source placed at a distance D from the lens along an optical axis of the lens. In practice, the point source may correspond to an illuminated area of a target to be processed or a plasma formed on the target to be processed. The thin lens approximation allows optical effects due to the thickness of the lens to be excluded, thereby simplifying the ray diagram and associated calculations. It will be appreciated that strict adherence to the thin lens approximation may be not be necessary or possible in practice. Practical considerations such as, for example, a non-negligible thickness of the sensor lens 204 may result in optical behaviour that does not strictly adhere to the equations provided

[0140] 69947607-1above. Nevertheless, it will be appreciated that such equations may be useful, but not necessarily essential, for understanding and implementing the invention.

[0141] Four different target locations 232, 234, 236, 238 are shown along the optical axis 220 of the sensor lens 204. A first target location 232 is a furthest distance away from the sensor lens 204. A second target location 234 is closer to the sensor lens 204 than the first target location 232. A third target location 236 is closer to the sensor lens 204 than the second target location 234. A fourth target location 238 is closest to the sensor lens 204. A first image 242 formed by the sensor lens 204 of the target located at the first target location 232 is closest to the sensor lens 204. A second image 244 formed by the sensor lens 204 of the target located at the second target location 234 is further away from the sensor lens 204 than the first image 242. A third image 246 formed by the sensor lens 204 of the target located at the third target location 236 is further away from the sensor lens 204 than the second image 244. A fourth image 244 formed by the sensor lens 204 of the target located at the fourth target location 234 is furthest away from the sensor lens 204 (and is not visible in Fig. 3). As demonstrated by Fig. 3, the closer a target is to the sensor lens 204, the further away the image of the target is formed along the optical axis 220 of the sensor lens 204. As such, for relatively close targets, detecting the in-focus image directly would require a sensor to be located relatively far away from the sensor lens 204, resulting in a bulky distance measurement system. The distance measurement system of the present disclosure overcomes this problem through use of the optical arrangement and mathematical relationship.

[0142] Fig. 4A schematically depicts a ray diagram corresponding to that of Fig. 3 with a sensor plane 203 present. Fig. 4B schematically depicts a view of the front of the sensor plane 203 showing four cross-sectional areas of measurement radiation associated with the four target locations of Fig. 3. In the example of Fig. 4A, a separation 241 between the sensor lens 204 and the sensor plane 203 is less than the focal length 228 of the sensor lens 204. This arrangement may advantageously contribute to the mathematical relationship comprising a monotonic function (i.e. the mathematical relationship being entirely non-decreasing or entirely non-increasing) between a size of the cross-sectional area of the measurement radiation at the sensor plane 203 and the distance to the target, such that each cross-sectional area is uniquely coupled to a corresponding distance to the target. The separation 241 between the sensor lens 204 and the sensor plane 203 may, for example, be about

[0143] 69947607-120mm or more. The separation 241 between the sensor lens 204 and the sensor plane 203 may, for example, be about 30mm or less. The sensor plane 203 and is arranged substantially perpendicularly to the optical axis 220 of the sensor lens 204. An area of the sensor plane 203 may be greater than or equal to an area of a circle formed by the radius 230 of the sensor lens 204. For example, in the case of a square or rectangular sensor 202, the width and the height of the sensor plane 203 may both be greater than or equal to a diameter of the sensor lens 204. A first cross-sectional area of measurement radiation 252 at the sensor plane 203 corresponds to the first target location 232. A second cross-sectional area of measurement radiation 254 at the sensor plane 203 corresponds to the second target location 234. A third cross-sectional area of measurement radiation 256 at the sensor plane 203 corresponds to the third target location 236. A fourth cross-sectional area of measurement radiation 258 at the sensor plane 203 corresponds to the fourth target location 238. Each distance between the sensor lens 204 and a target corresponds to an illuminated area on the sensor plane 203. As demonstrated by Figs. 4A and 4B, the closer a target is to the sensor lens 204, the larger the cross-sectional area of the sensor plane 203 that is occupied by measurement radiation. The geometry of the ray diagram may be expressed by the following equation:

[0144] R r

[0145] d d — S

[0146] where R is the radius 230 of the sensor lens 204, d is the distance between the sensor lens 204 and an image 242-248 formed by the sensor lens 204, r is a radius of the illuminated area 252-258 of the sensor plane 203, and S is the separation 228 between the sensor lens 204 and the sensor plane 203. This equation can be solved for the image distance d as follows:

[0147]

[0148] Combining the above equation with the lens equation provides the following:

[0149] RS > Df

[0150] R — r D — f

[0151] which, when solved for the distance between the sensor lens 204 and the target 312, provides the following equation:

[0152]

[0153] Given that the radius of the lens R, the separation between the lens and the sensor plane S, and the focal length of the lens f are known, a distance between the lens and

[0154] 69947607-1the target may be calculated by determining the radius of the illuminated area 252-258 of the sensor plane 203. The distance measurement system of the present disclosure achieves this through use of the optical arrangement and mathematical relationship.

[0155] Fig. 5 schematically depicts a front view of the first and second sensor regions 201, 202 of Fig. 2 along with the cross-sectional areas of measurement radiation 252-258 of Fig. 4B. In the example of Figs. 2 and 5, the optical arrangement comprises an offset 260 between an optical axis 220 of the sensor lens (not shown in Fig. 5) and a midpoint 262 between the first and second sensor regions 201, 202. The first sensor region 201 is configured to generate a first signal that is proportional to an amount of measurement radiation incident on the first sensor region 201, and the second sensor region 202 is configured to generate a second signal that is proportional to an amount of measurement radiation incident on the second sensor region 202. The first and / or second sensor regions 201, 202 may have an area of about 3X3mm. A dividing line extends between the first and second sensor regions 201 , 202 through the midpoint 262. The measurement radiation incident on the sensor regions 201, 202 may be produced by diffuse reflection from the target or by emission from a plasma generated at the target, and therefore has a substantially constant energy density throughout its cross-section. As such, the first signal generated by the first sensor region 201 is proportional to the cross-sectional area of measurement radiation that is above the dividing line, and the second signal generated by the second sensor region 202 is proportional to the cross-sectional area of measurement radiation that is below the dividing line. This may be expressed by the following equations:

[0156] a = cA

[0157] b = cB

[0158] where a is the first signal generated by the first sensor region 201, A is the cross-sectional area of measurement radiation that is incident on the first sensor region 201, b is the second signal generated by the second sensor region 202, B is the cross-sectional area of measurement radiation that is incident on the second sensor region 202, and c is a proportionality constant which may depend on, for example, one or more of a reflectivity of the target, an angle between the target surface and the measurement radiation beam, a distance to the target, etc.

[0159] The offset 260 introduces an unequal distribution of measurement radiation between the first and second sensor regions 201, 202 that varies with a distance between the

[0160] 69947607-1lens and the target in accordance with a mathematical relationship. For the first cross-sectional area of measurement radiation 252 associated with the first target location 232, a first pair of cross-sectional areas A4, B4 of measurement radiation are incident on the sensor regions 201, 202. For the second cross-sectional area of measurement radiation 254 associated with the second target location 234, a second pair of cross-sectional areas A3, B3 of measurement radiation are incident on the sensor regions 201, 202. For the third cross-sectional area of measurement radiation 256 associated with the third target location 236, a third pair of cross-sectional areas A2, B2 of measurement radiation are incident on the sensor regions 201, 202. For the fourth cross-sectional area of measurement radiation 258 associated with the fourth target location 238, a fourth pair of cross-sectional areas A1, Bi of measurement radiation are incident on the sensor regions 201, 202. In the example of Fig. 5, the offset 260 ensures that the cross-sectional area of measurement radiation incident on the first sensor region A4-1 is less than the cross-sectional area of measurement radiation incident on the second sensor region B4-1. As such, the offset 260 ensures that the second signal produced by the second sensor region 202 is always greater than the first signal generated by the first sensor region 201 for any given radius of measurement radiation r4-i. The circular segments of measurement radiation incident on each sensor region 201 , 202 may be calculated as follows:

[0161]

[0162] where r is the radius of the cross-sectional area of measurement radiation incident on the first and second sensor regions 201, 202 and h is the offset 260. It will be appreciated that the radius of the cross-sectional area of the measurement radiation incident on the first and second sensor regions 201 , 202 must be greater than or equal to the offset 260 for both sensor regions 201 , 202 to produce a signal. As such, it may be beneficial in some applications for a suitably small offset 260 to be used for larger target distances (and thus smaller cross-sectional areas of measurement radiation at the sensor regions) to be measured. However, in general, increasing the offset 260 provides greater differentiation between the first and second signals generated by the first and second sensor regions 201, 202, thereby improving measurement resolution and accuracy. As such, a balance may be struck for a given application based on desired working distance range and desired measurement accuracy and resolution. The size of the offset 260 may, in general, depend upon the size of the optical system

[0163] 69947607-1and / or its components, such as the sensor lens. For example, for a sensor lens having a diameter of about 3mm, the offset 260 may be about 0.2mm or more. For example, for a sensor lens having a diameter of about 3mm, the offset 260 may be about 1 ,5mm or less. Larger offset may be used for larger sensor lenses.

[0164] It will be appreciated that other variables, such as the focal length of the sensor lens, may influence the distance measurement range. The focal length of the sensor lens may, for example, be about 10mm or more. The focal length of the sensor lens may, for example, be about 30mm or less. A normalised difference between the cross-sectional areas of measurement radiation incident on each sensor region 201, 202 may be calculated as follows:

[0165] (B -A)

[0166] W =

[0167] (B + A)

[0168] Given that the cross-sectional areas of measurement radiation incident on the sensor regions are proportional to the signals generated by the sensor regions, the normalized difference between the signals may be calculated as follows:

[0169]

[0170] where Si is the first signal generated by the first sensor region 201 and S2is the second signal generated by the second sensor region 202. Using a normalised difference advantageously cancels out various laser and / or target material variables that may vary between distance measurements. Fig. 6 shows a graph of the normalized difference W with a varying distance D between the target and the sensor lens for different values of focal length f of the sensor lens. In the example of Fig. 6, the distance between the target and the sensor lens varies from 0mm to 2000mm, and the focal length of the sensor lens varies from 10mm to 30mm. As shown by Fig. 6, smaller focal lengths result in smaller gradients and distance measurement ranges, whereas greater focal lengths result in greater gradient and greater distance measurement ranges. For example, for a focal length of 10mm, the normalized difference W varies from a minimum at a target distance of 0mm to a maximum at a target distance of about 650mm. For a focal length of 30mm, the normalized difference W varies from a minimum at a target distance of 0mm to a maximum at a target distance of about 2000mm. As such, the focal length of the sensor lens may be selected for a given application based on a desired working distance range and desired measurement accuracy and resolution.

[0171] 69947607-1A shown by the example of Fig. 6, and with reference to Fig. 2, the normalised difference W between the first and second signals (which corresponds to the unequal distribution of measurement radiation 206d between the first and second sensor regions 201, 202) varies with the distance 208 between the sensor lens 204 and the target 312 in accordance with a mathematical relationship. That is, a mathematical relationship, such as a function and / or equation, may be used to convert the first and second signals generated by the first and second sensor regions 201, 202 into a measurement of the distance 208 between the sensor lens 204 and the target 312. The mathematical relationship may be stored in memory for access by the processor 210 of the distance measurement system 306. The mathematical relationship may comprise an equation. The equation may be known via knowledge of the optical arrangement (e.g. the offset 260). For example, knowledge of optical principles (such as the lens equation previously described) and a geometry of the optical arrangement (such as the circular segments of measurement radiation described in relation to Fig. 5) may be used to determine an equation that converts the first and second signals to the distance 208 between the sensor lens 204 and the target 312. Alternatively or additionally, the mathematical relationship may comprise a database, such as a lookup table. The database may comprise an input set of first and second signal values and a corresponding output set of distances 208 between the sensor lens 204 and the target 312. The database may be determined during manufacture of the distance measurement system 306 and stored in memory for access by the processor 210. Alternatively or additionally, the database may be determined in-situ via calibration measurements performed by a user of the distance measurement system 306. For example, the data shown in Fig. 6 may be used by the processor 210 to convert the first and second signals into distance measurements for a given distance measurement system 306.

[0172] The mathematical relationship may comprise a monotonic function. That is, the mathematical relationship may be entirely non-decreasing or entirely non-increasing. For example, as shown in Fig. 6, the normalised difference W between the first and second signals never decreases with increasing distance 208 between the sensor lens 204 and the target 312. The optical arrangement may be such that the illuminated areas A, B of the first and second sensor regions 201, 202 do not increase as the distance between the sensor lens 204 and the target 312 increases. For example, as

[0173] 69947607-1shown in Fig. 5, the cross-sectional areas A4-1, B4-1 of the measurement radiation on the sensor regions 201 , 202 never increase as the distance between the target and the sensor lens increases.

[0174] The mathematical relationship may comprise a bijective function. That is, each input pair of first and second sensor signals may be associated with a unique output value of the distance 208 between the sensor lens 204 and the target 312. The mathematical relationship may comprise a substantially continuous bijective function. An inverse of the mathematical function may comprise a continuous function. The mathematical relationship may comprise a homeomorphism or a bicontinuous function. For example, as shown in Fig. 6, each normalised difference W between the first and second signals corresponds to a unique measurement of distance D between the sensor lens 204 and the target 312.

[0175] In general, the mathematical relationship allows different values of first and second signals to be converted to different distances 208 between the sensor lens 204 and the target 312. It will be appreciated that strict adherence to the mathematical relationships described above may be not be necessary or possible in practice. Practical considerations such as, for example, a limited resolution of the distance measurement system 306 may result in input and output values that do not strictly adhere to the mathematical relationships described above. Nevertheless, it will be appreciated that such mathematical relationships may be useful, but not necessarily essential, for understanding and implementing the invention.

[0176] It will be appreciated that other optical arrangements may be used in addition to, or instead of, the use of an offset 260 between an optical axis 220 of the sensor lens 204 and a midpoint 262 between the first and second sensor regions 201, 202. For example, the first and second sensor regions 201, 202 may have different geometries (e.g. surface areas and / or shapes) which introduces an unequal distribution of measurement radiation between the first and second sensor regions that varies with a distance between the lens and the target. Another example, the optical arrangement may comprises a substantially opaque structure configured to obstruct part of at least one of the first and second sensor regions 201 , 202, and thereby introduce an unequal distribution of measurement radiation between the first and second sensor regions that varies with a distance between the lens and the target.

[0177] 69947607-1When processing a target with a processing radiation beam, such as that generated by the laser processing apparatus described above with respect to Figure 1 , it is beneficial to ensure that the radiation beam is properly configured to carry out the desired processing task. For example, when marking a target, it is beneficial if the radiation beam’s focal point coincides, or at least closely aligns, with the surface of the target to be marked in order to ensure high quality. As such, it is beneficial to obtain a distance between the laser processing apparatus and the target. The distance may be a distance between the target (e.g. a first portion of a surface of the target), and a representative part of the laser processing apparatus, such as, for example the focusing lens 302 configured to focus the radiation beam onto the target during processing. The obtained distance can then be used to configure the processing radiation beam. For example, the distance can be used to set the processing radiation beam’s focal length, such that the focal point is located at the desired location.

[0178] In addition to setting the focal length, the power of the processing radiation beam may be set based on the distance. This may be advantageous in cases where a given distance leads to a relatively large focal spot size, leading to a lower power density at that focal spot. In such cases, the power of the processing radiation beam may be increased to compensate.

[0179] As described above, targets that are to be marked may take a variety of different shapes and sizes. For example, agricultural products such as eggs, fruits and vegetables have differing shapes and sizes relative to one another. Similarly, flexible targets such as paper packages or packets may deform during production such that they exhibit different shapes when presented to a laser processing apparatus to be marked. Consequently, targets do not always present a uniform planar surface on which to apply a mark. Changes in the three dimensional shape of a target across a location to be marked mean that each point on the location to be marked may be at a different distance to the laser processing apparatus. Even when a target does present a uniform planar surface, the distance between, for example, the focusing lens of the laser processing apparatus and the location at which a mark is applied will change as the processing radiation beam is moved across the surface of the target. As such, configuring the radiation beam’s focal point to coincide, or generally align, with the target location to be marked can be challenging. While an entire scan of a target’s

[0180] 69947607-1surface could be carried out to determine each individual target’s exact shape, this would be computational expensive and unpractical in most marking applications, where high speed and throughput is desired.

[0181] When a processing radiation beam is directed onto a target to process the target, the radiation beam physically interacts with the target causing extremely high temperate localised at a point at which a surface of the target coincides with the focal point of the radiation beam. This physical interaction causes the generation of electromagnetic radiation. For example, the interaction between the radiation beam and the target can cause ionization, or the partial ionization, of the matter of the surface of the target, causing a localised plasma plume to form. Such a plasma plume will emit electromagnetic radiation over a wide range of wavelengths, dependent on the material forming the target, the energy and wavelength of the radiation beam, and on components of the surrounding atmosphere (e.g. air).

[0182] When a plasma plume forms, it can be challenging to use a distance measurement system that emits a measuring radiation beam to be reflected from the target, since the plasma plume can obscure the surface of the target. That is, the measuring radiation beam may not be able to penetrate the plasma plume and thus cannot be impinged on the target surface, and / or the reflected beam may be obscured. However, it has been found by the inventors that the interaction between the processing radiation beam and the target during processing of the target can be used to determine relevant distance data, such as the distance between the target and the focusing lens of the laser processing apparatus

[0183] With reference to Figures 7 to 12, Figure 15, and to the flow diagram shown in Figure 13, there is now described a method 600 of creating a mark on a target 502 with a laser processing apparatus 500, the mark having a start location and an end location, and defining a marking path extending from the start location to the end location. The laser processing apparatus 500 may comprise the optical system described above with respect to Figure 1, and comprises a distance measurement system. The distance measurement system may be the distance measurement system 306 of Figure 2. It will however be appreciated that other distance measurement systems may be used.

[0184] 69947607-1When configured to create a mark on a target, the laser processing apparatus 500 may be referred to as a laser marking apparatus. The following example is described in the context of applying a mark, but it will be appreciated that the following example could be equally applicable to other processing operations, such as etching, welding, cutting, and UV curing.

[0185] The mark may be an individual stroke that forms part of a wider mark, such as part of a character, such as an alpha-numeric character, part of code, such as a barcode, or part of an image. When applying an individual stroke, sometimes referred to as vector polygon, the laser processing apparatus 500 will direct the processing radiation beam onto the target and, using a beam steering apparatus, such as beam steering mechanism 322, 324 described above, will sweep the processing radiation beam across the target to form the mark. Once the individual stroke is applied the processing radiation beam is turned off, and the beam steering apparatus moves such that the processing radiation beam’s path aligns with the start location of the next stroke. A schematic illustration of a collection of individual strokes is shown in Figure 15, where the wider mark to be applied is the word “TEXT”. The word “Text” is formed of several individual strokes, shown as solid lines having arrows showing the direction of application (e.g. the direction of sweep of the processing radiation beam). The dotted lines in Figure 15 connect individual strokes and show how the beam path sweeps across the target when the laser is powered down between strokes.

[0186] The examples described with respect to Figures 7 to 12 will be described in the context of applying a mark corresponding to stroke 800 as shown in Figure 15, which corresponds to the vertical line of the letter “T”. It will of course be appreciated that that the mark applied may be any suitable mark.

[0187] Figure 7 shows a schematic illustration of a laser processing apparatus 500 being used to process the target 502. In the example shown, the target 502 is shown as a planar surface, but it will be appreciated that the target 502 may have any shape. That is, the surface on which the mark is to be applied may be curved, rather than planar.

[0188] At step 602, and as shown in Figure 7, the laser processing apparatus 500 performs an initial distance measurement at a start location 506 of the target 502. The start location 506 is shown in Figure 15 as the start of the vertical line of the letter “T”. As can be

[0189] 69947607-1seen in Figure 7, the laser processing apparatus 500 directs a measurement radiation beam 504 towards the start location 506. The measurement radiation beam 504 may be generated as described above with respect to Figures 2 or 3. For example, directing the measurement radiation beam 504 towards the target 502 may be performed by the beam steering mechanism 322, 324 described above, where the beam steering mechanism is configured to direct the measurement radiation beam 504 towards a location on the target specified by a target coordinate (e.g. an (x, y) coordinate)

[0190] As illustrated in Figure 8, when the measurement radiation beam 504 impinges the target 502 at the start location 506, a reflected portion 508 of the measurement radiation beam 504 is reflected from the start location 506 as measurement radiation. This measurement radiation (i.e. the reflected portion 508 of the measurement radiation beam 504) is received by the distance measurement system. For example, the measurement radiation 508 may be directed to the distance measurement system as described above with respect to Figures 1 and 2.

[0191] An initial distance data indicative of a distance DO between the target 502 and a representative portion of the laser processing apparatus 500 is determined based upon the reflected portion 508 of the measurement radiation beam 504. The initial distance data may be indicative of the distance as measured along a portion of the shared optical path 316. The distance DO between the target 502 and the representative portion of the laser processing apparatus may be the distance between the start location 506 of the target 502 (e.g. a first portion of a surface of the target), and the focusing lens 302 configured to focus the processing radiation beam onto the target during processing, as described above. Alternatively, the representative portion of the laser processing apparatus 500 may be to an external surface of the laser processing apparatus 500, such as an end of a marking head from which the processing radiation beam may emerge. As noted above, any distance determined by the distance measurement system may be converted to any other suitable distance. For example, if the distance measurement system determines a distance between the target 502 and a lens of the distance measurement system, such as sensor lens 204, this distance may be converted to align with another portion of the laser processing apparatus by adding, or subtracting the relevant difference. For example, the distance can be modified to align with the focusing lens 302.

[0192] 69947607-1The initial distance data can be used to adjust characteristics of the processing radiation beam. For example, the characteristics may include the focal length of the processing radiation beam, and / or power of the processing radiation beam. In this way, desired characteristics may be adjusted based on the initial distance data to ensure a good quality mark. Similarly, in other processing applications besides marking, the desired characteristics of the processing radiation beam may be adjusted based on the initial distance data to ensure a good quality, such as a good quality cut, or weld. In some cases, if there are restrictions with focal adjustment such that the processing can only be carried out “out of focus”, the power of the processing radiation beam may be adjusted to compensate. For example, if the power density of the processing beam at the focal point is too low to achieve the processing effect, the power of the processing radiation may be increased to increase the power density at the focal point. The characteristic may also include the distance between the target 502 and the laser processing apparatus 500. That is, the target 502 may be moved relative to the laser processing apparatus 500 based on the initial distance data.

[0193] At step 604, and as shown in Figure 9, the laser processing apparatus 500 generates a processing radiation beam 510 and directs the processing radiation beam 510 onto the start location 506 to create a first portion of the mark 800. At least one characteristic of the processing radiation beam 510 is controlled based upon the initial distance data. Directing the processing radiation beam 504 towards the target 502 may be performed using the beam steering apparatus described above.

[0194] The at least one characteristic of the processing radiation beam 510 that is controlled based upon the initial distance data may be a focal length. For example, the focal length of the processing radiation beam 510 may be set based on the initial distance data such that the focal point coincides with the start location 506. The focal length may be changed using focus system 330, described with respect to Figure 1. As noted above, the at least one characteristic of the processing radiation beam may additionally, or alternatively, be the power of the processing radiation beam 510. The at least one characteristic of the processing radiation beam 510 may additionally, or alternatively, be a distance to the target. For example, the target 502 may be moved, rather than changing the focal length of the processing radiation beam 510. For example, the target 502 may be installed in a frame configured to move relative to the laser processing apparatus 500 to achieve a desired distance.

[0195] 69947607-1With reference to Figure 10, and as described above, the interaction between the processing radiation beam 510 and a point on the target 502 at which the processing radiation beam 510 impinges causes a plasma to form, which is ejected from the surface of the target 502 as a plasma plume 512. The plasma plume 512 emits electromagnetic radiation across a range of wavelengths, which generally appear to emanate from an origin 513. While shown in the figures as a single origin, it will of course be appreciated that the origin 513 may comprise a volume, rather than a point source. That is, the plasma plume 512 may be distributed across a volume of space adjacent to the target surface, and there may not, therefore, be a single point source “origin”.

[0196] At step 606, a portion of the emitted electromagnetic radiation from the plasma plume 512, referred to herein as measurement radiation 514, is received by the laser processing apparatus 500 and directed towards the distance measurement system 302. For example, the measurement radiation 508 may be directed to the distance measurement system as described above with respect to Figures 1 and 2. When using the optical system 300, the measurement radiation 514 travels along a portion of the shared optical path 316 towards the laser processing apparatus 500. The measurement radiation 514 has a different wavelength to that of the processing radiation beam. However, it is beneficial if the measurement radiation 514 has a wavelength that is similar to the wavelength of the processing radiation beam. This is because the optical system used in the laser processing apparatus 500 will be configured for handling the processing radiation beam 510 at its specified wavelength. As such, using a similar wavelength for the measurement radiation 514 means that the measurement radiation 514 will more easily propagate through the components along the optical path of the processing radiation beam 510. For example, if the processing radiation beam 510 has a wavelength of 1064 mm, then the portion directed to the distance measurement system may have a wavelength within +- 200 nm of 1064 mm. It will of course be appreciated that the specific range will be application dependent, depending on such things as the material of the components that make up the laser processing apparatus such as the lens, mirrors, and the composition of any coatings on the lens or mirrors. In a specific example, the measurement radiation 514 may be 905 mm. It will of course be appreciated that a range of wavelengths may be received from the measurement radiation 514, but that only the portion is used for subsequent

[0197] 69947607-1processing. The portion of the measurement radiation 514 may be selected using a suitable band-pass filter. That is, where the emitted radiation from the plasma plume comprises a range of wavelengths of electromagnetic radiation, only those wavelengths able to be transmitted through the band-pass filter will be received and processed by the distance measurement system.

[0198] Steps 604 and 606 may be carried out concurrently. That is, while the laser processing apparatus 500 is directing the processing radiation beam 510 onto the target 502, the received measurement radiation 514 may be directed towards the distance measurement system.

[0199] First apparent distance data is generated based upon the measurement radiation 514. For example, the first apparent distance data indicates a distance AD1 between the origin 513 of the measurement radiation 514 generated by the plasma plume 512 and the representative portion of the laser processing apparatus 500 (e.g. the focusing lens). As noted above, the plasma plume 512 may be distributed across a volume of space adjacent to the target surface, and there may not, therefore, be a single “origin” of the measurement radiation. However, the distance measurement system may be configured to measure an average, or apparent, origin. Such a position may correspond to a position of peak plasma intensity.

[0200] The first apparent distance data gives an estimate of the distance between the portion of the laser processing apparatus 500 and the target 502. The first apparent distance data may be indicative of the distance as measured along a portion of the shared optical path 316. That is, the first apparent distance data may be indicative of the distance that the measurement radiation travels along a portion of the path taken by the processing radiation beam 510. The estimate will be shorter than the distance indicated by the initial distance data given that the origin 513 will typically extend above the surface of the target 502. In order to account for such differences, a distance offset d (also referred to herein as offset data) may be determined based on the initial distance data and the first apparent distance data. For example, the value of the distance offset d may be determined by performing a calculation as follows:

[0201] d (offset) = DO (initial distance data) - AD1 (first apparent distance data).

[0202] 69947607-1That is the value of the distance DO indicated by the initial distance data may be subtracted from the value of the apparent distance AD1 indicated by first apparent distance data. The offset distance d therefore indicates the distance between the target 502 and the origin 513. The offset distance d may be the distance as measured along the shared optical path (e.g. the optical path of the processing radiation beam 510).

[0203] As will be described, the offset distance can be used to obtain an improved estimate on subsequent measurements of distance.

[0204] At step 608, and with reference to Figure 11, a processing radiation beam 518 is directed towards a second location 520 along the marking path to create a second portion of the mark. The processing radiation beam 518 may be the same processing radiation beam 510 shown in Figure 9. That is, the processing radiation beam 518 may be a continuous beam that is moved along the surface of the target to the second location 520. For example, Figure 15 shows that the second location 520 is part of the vertical line of the letter “T”, and is vertically displaced from the first location 506.

[0205] At least one characteristic of the processing radiation beam 518 is controlled based upon the initial distance data. For example, as described above, the focal length of the processing radiation beam 518 may be set based on the initial distance data such that the focal point coincides with the second location 520.

[0206] With reference to Figure 12, and as described above, the interaction between the processing radiation beam 518 and the target 502 causes a plasma to form, which is ejected from the surface of the target 502 as a second plasma plume 522. The second plasma plume 522 emits electromagnetic radiation 524 from an origin 523 as described above with respect to Figure 10. At step 610, and with reference to Figure 13, a portion of this emitted electromagnetic radiation 524, referred to herein as measurement radiation 524, is received and directed towards the distance measurement system in the same way as described with respect to Figure 10.

[0207] Steps 608 and 610 may be carried out concurrently. That is, while the laser processing apparatus 500 is directing the processing radiation beam 518 onto the target 502, the received measurement radiation 524 may be directed towards the distance measurement system.

[0208] 69947607-1At step 612, second apparent distance data is generated based upon the measurement radiation 524. For example, the second apparent distance data indicates a distance AD2 between the origin 523 of the measurement radiation 524 generated by the plasma plume 522 and the portion of the laser processing apparatus 500 (e.g. the focusing lens). The second apparent distance data may be indicative of the distance as measured along a portion of the shared optical path 316. That is, the second apparent distance data may be indicative of the distance that the measurement radiation travels along a portion of the path taken by the processing radiation beam 518.. Using the second apparent distance data, second distance data is determined. The second distance data indicates a distance D2 between the target 502 and the portion of the laser processing apparatus 500. For example, the second distance data may be generated based on the second apparent distance data and a difference between the initial distance data and the first apparent distance data (e.g. the offset d). The value of the distance D2 indicated by the second distance data may be calculated as follows:

[0209] D2 (actual) = AD2 (apparent / plasma) + d

[0210] Or

[0211] D2 (actual) = AD2 (apparent / plasma) + DO (initial measurement) - D1 (plasma / apparent) .

[0212] In this way, an estimate of the distance D2 between the second location 520 and the laser processing apparatus 500 may be determined without requiring the use of a separate measuring radiation beam to be directed onto the second location 520. The estimated distance D2 may be used to control the processing radiation beam as described above.

[0213] At step 614, the processing radiation beam is directed towards a third location 530 along the marking path to create a third portion of the mark 800, where at least one characteristic of the processing radiation beam is controlled based upon the second distance data. For example, the focal length of the processing radiation beam may be adjusted to take based on the second distance data. For example, Figure 15 shows that the third location 530 is part of the vertical line of the letter “T”, and is vertically displaced from the second location 520.

[0214] 69947607-1The method 600 provides a way in which the distance between the target 502 and a representative portion of the laser processing apparatus 500 can be accurately measured whenever a processing radiation beam is directed onto the target 502. Based on the measured distance, one or more actions may be performed. For example, the properties of the processing radiation beam may be continually updated whilst performing a processing operation (e.g. marking a target) in real time to take into account any changes in the distance along the marking path. Alternatively, the measured distance may be monitored and an alert output if the measured distance exceeds one or more thresholds. For example, if it is determined that the distance is beyond a maximum predefined distance, the laser processing apparatus 500 may output an alert. The alert may take any suitable form, such as an audio alert output by a speaker system and / or a visual alert output by a computer display.

[0215] The method 600 may be repeated to create a plurality of marks on the target.

[0216] The frequency at which distance data is determined may be any suitable value. For example, the frequency may be linked to a system clock of the processing electronics of the laser processing apparatus 500. For example, a main processor of the laser processing apparatus 500 may operate at 1GHz. A “system clock” will typically operate at a fraction of the main processor clock, such as at 100KHz. The system clock is the frequency at which the various electronics within the laser processing apparatus can control the various control loops that occur. For example, control loops may include a galvanometer positioning algorithm (as used in the beam steering apparatus), and / or the switching on and off of the processing radiation beam. The same system clock frequency may then also be used to determine the distance data.

[0217] While method 600 describes determining an offset distance d, it will be appreciated that it may not be required to calculate the offset distance d as described. For example, a predefined value of offset distance d may be used. The predefined offset d may be based on the material that forms the target. For example, it may be empirically determined what the offset d would be for a given material when subjected to a particular processing radiation beam of a particular power.

[0218] 69947607-1With reference to Figure 14, there is described a method 700 of processing a target with a laser processing apparatus. The laser processing apparatus may be that described above.

[0219] At step 702 a processing radiation beam is directed towards the target to process a first portion of the target, the processing radiation beam comprising radiation of a first wavelength. For example, the processing radiation beam may comprise a wavelength of 1064 nm, and may be used to mark the surface of the target.

[0220] At step 704, measurement radiation is received, the measurement radiation comprising radiation of a second wavelength, different to the first wavelength, the measurement radiation being generated by an interaction of the processing radiation beam with the target. For example, the measurement radiation may be electromagnetic radiation emitted by a plasma plume, the plasma plume being generated due to the interaction between the processing laser beam and the target.

[0221] At step 706, the measurement radiation is directed towards a distance measurement system. The distance measurement system may be that described above.

[0222] At step 708, first distance data indicative of a distance between the target and a portion of the laser processing apparatus is generated based upon the measurement radiation. For example, the first distance data may be the same as the second distance data described above. That is, the first distance data indicative of a distance between the target and a portion of the laser processing apparatus may comprise generating data indicative of an origin of the measurement radiation and adjusting the indicated position based on an offset distance as described above.

[0223] Any suitable processing system may be used to implement the described methods. The processing system may be incorporated into, or otherwise coupled to, the laser processing apparatus described above.

[0224] Figure 16 schematically illustrates an example processing system 1800. The processing system 1800 comprises the processor 1802 which is configured to read and execute instructions stored in either (or both) a volatile memory 1803a which may take the form of a random access memory and a non-volatile memory 1803b. The

[0225] 69947607-1memories 1803a, 1803b may store instructions for execution by the processor 1802 and data used by those instructions. For example, the instructions may include the instructions for causing the laser processing apparatus to carry out any method described herein.

[0226] The processing system 1800 may further comprises an I / O interface 1806 to which peripheral devices may be connected. For example, a display 1813 may be configured so as to display output from the processing system 1800. Input devices may also be connected to the I / O interface 1806. Such input devices may include a keyboard 1811 and a mouse 1812 which allow user interaction with the processing system 1800. Of course, it will be appreciated that the input devices and display may be combined in a touch screen arrangement. The processing system 1800 may comprise a network interface 1810, allowing the processing system 1800 to be connected to appropriate computer networks, such as the Internet or an intranet, and so as to be able to send and receive from and to other computing devices. The processor 1802, volatile memory 1803a, the storage device 1803b, I / O interface 1806, and network interface 1810, are connected together by a bus 1814. It will be appreciated that the arrangement of components illustrated in Figure 16 is merely exemplary, and that the processing system 1800 may comprise additional or fewer components than those illustrated in Figure 16.

[0227] Although specific embodiments have been described above, it will be appreciated that various modifications can be made to the described embodiments without departing from the scope of the claims. That is, the described embodiments are to be considered in all respects exemplary. In particular, where a particular form has been described for particular processing, it will be appreciated that such processing may be carried out in any suitable form arranged to provide suitable output data.

[0228] 69947607-1

Claims

43CLAIMS:

1. A method of processing a target with a laser processing apparatus, the method comprising:directing a processing radiation beam towards the target to process a first portion of the target, the processing radiation beam comprising radiation of a first wavelength;receiving measurement radiation, the measurement radiation comprising radiation of a second wavelength, different to the first wavelength, the measurement radiation being generated by an interaction of the processing radiation beam with the target,directing the measurement radiation towards a distance measurement system; generating, based upon the measurement radiation, first distance data indicative of a distance between the target and a portion of the laser processing apparatus.

2. The method of claim 1, wherein, generating the first distance data comprises generating data indicative of an origin of the measurement radiation, and generating the first distance data based upon the data indicative of an origin of the measurement radiation.

3. The method of claim 2, wherein, generating the first distance data is further based upon offset data indicative of a distance between the origin and the target.

4. The method of any preceding claim, further comprising, prior to directing the processing radiation beam towards the target to process the first portion of the target;directing a measurement radiation beam towards a second portion of the target, the measurement radiation beam comprising radiation of the second wavelength;receiving reflected measurement radiation, the reflected measurement radiation comprising a portion of the measurement radiation beam reflected by the second portion of the target,directing the reflected measurement radiation towards the distance measurement system; and69947607-144generating, based upon the reflected measurement radiation, second distance data indicative of a distance between the target and a portion of the laser processing apparatus.

5. The method of claim 4 as dependent upon claim 2 or 3, wherein generating the first distance data is further based upon the second distance data.

6. The method of claim 5, further comprising, after directing the measurement radiation beam towards the second portion of the target, and prior to directing the processing radiation beam towards the target to process the first portion of the target:directing the processing radiation beam towards the target to process the second portion of the target;receiving measurement radiation comprising radiation of the second wavelength, the measurement radiation being generated by an interaction of the processing radiation beam with the second portion of the target,directing the measurement radiation towards the distance measurement system;generating, based upon the received measurement radiation, data indicative of an origin of the measurement radiation; anddetermining a difference between the data indicative of an origin of the measurement radiation and the second distance data.

7. The method of any preceding claim, wherein the directing the processing radiation beam towards the target to process the first portion of the target, and the directing the measurement radiation towards the distance measurement system are performed at the same time.

8. The method of any preceding claim, wherein at least one characteristic of the processing radiation beam during the processing of the first portion is controlled based upon the first distance data.

9. The method of any preceding claim, further comprising outputting an alert based upon the first distance data.

10. The method of any preceding claim, further comprising:69947607-145after directing the processing radiation beam towards the target to process the first portion of the target, directing the processing radiation beam towards the target to process a third portion of the target;wherein at least one characteristic of the processing radiation beam directed to the third portion of the target is controlled based upon the first distance data.

11. The method of claim 10, wherein processing the first portion of the target and the third portion of the target comprises a continuous process.

12. The method of claim 10 or 11, further comprising:after directing the processing radiation beam towards the target to process the first portion of the target, directing the processing radiation beam towards the target to process a plurality of further portions of the target;wherein during processing of each of the plurality of further portions of the target at least one characteristic of the processing radiation beam is controlled based upon distance data indicative of a distance between the target and a portion of the laser processing apparatus generated during an earlier processing of a portion of the target.

13. The method of claim 12, further comprising, during directing the processing radiation beam towards the target to process the plurality of further portions of the target:receiving further measurement radiation, the further measurement radiation comprising radiation of the second wavelength, the further measurement radiation being generated by interactions of the processing radiation beam with the further portions of the target,directing the further measurement radiation towards the distance measurement device; andgenerating, based upon the further measurement radiation, data indicative of a distance between each of the plurality of further portions of the target and a portion of the laser processing apparatus.

14. A method of creating a mark on a target with a laser marking apparatus, the mark having a start location and an end location, and defining a marking path69947607-1extending from the start location to the end location, wherein creating the mark comprises:i) performing an initial distance measurement at the start location by directing a measurement radiation beam towards the start location, receiving a reflected portion of the measurement radiation beam by a distance measurement system, and generating initial distance data indicative of a distance between the target and a portion of the laser marking apparatus based upon the reflected portion the measurement radiation;ii) directing a processing radiation beam towards the start location to create a first portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the initial distance data;iii) receiving measurement radiation, the measurement radiation being generated by an interaction of the processing radiation beam with the target at the start location, directing the measurement radiation towards the distance measurement system, and generating first apparent distance data based upon the measurement radiation;iv) directing the processing radiation beam towards a second location along the marking path to create a second portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the initial distance data;v) receiving measurement radiation, the measurement radiation being generated by an interaction of the processing radiation beam with the target at the second location, directing the measurement radiation towards the distance measurement system, and generating second apparent distance data based upon the measurement radiation;vi) determining second distance data based on the second apparent distance data and a difference between the initial distance data and the first apparent distance data; andvii) directing the processing radiation beam towards a third location along the marking path to create a third portion of the mark, at least one characteristic of the processing radiation beam being controlled based upon the second distance data.

15. The method of any claim 14, comprising creating a plurality of marks on the target, each of the plurality of marks having a respective start location and a respective end location, and defining a respective marking path extending from the respective start location to the respective end location, wherein creating each of the plurality of marks comprises steps i) to vii).69947607-116. A laser processing apparatus comprising:a distance measurement system configured to receive measurement radiation; an optical element configured to direct a processing radiation beam towards a target;a processor;a memory storing computer readable instructions which when executed by the processor, cause the processor to carry out the method of any preceding claim.

17. The laser processing apparatus of claim 16, wherein the optical element is further configured to:receive the measurement radiation; and,redirect one of the processing radiation beam and the measurement radiation to form a junction between a shared optical path, a processing radiation beam optical path and a measurement radiation optical path.

18. The laser processing apparatus of claim 16 or 17, wherein the processing radiation beam comprising a first wavelength and the measurement radiation comprising a second wavelength that is different to the first wavelength.

19. The laser processing apparatus of any of claims 16 to 18, comprising an adjustable focus system configured to adjust a focal length of the optical system in at least partial dependence upon distance data.

20. The laser processing apparatus of any of claims 16 to 19, wherein the distance measurement system comprises a measurement radiation source configured to generate a measurement radiation beam.

21. The laser processing apparatus of any of claims 16 to 20, further comprising a beam steering apparatus configured to direct the processing radiation beam about the target.69947607-1

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