Patent Publication Number: US-2023155339-A1

Title: Double-pulse laser system

Description:
FIELD 
     The present disclosure relates generally to a double-pulse laser system and to the use of such a system in various fields, including in optics and atomic emission spectroscopy. 
     BACKGROUND 
     Optical pulses are used throughout optics and in scientific analysis. Optical pulses are characterised by a rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value. Optical pulses can be pulses of any type of electromagnetic radiation including, for example, visible light or invisible electromagnetic radiation. 
     When multiple (e.g. two or more) laser pulses are required in quick succession, they are often generated using complex and expensive electronics or by using two different pulsed lasers. 
     Two pulses can be provided by using a single-pulsed laser with modified electronics to control the Q-switch and emit two pulses with a predetermined time delay. In this case, only one laser is necessary, and the two pulses may be emitted collinearly. However, there are high costs associated with the modification of the electronics and there is little flexibility in choosing different time delays (with delays typically being on the order of ˜100 μs). In particular, the duration of the delay strongly affects the energies of the pulses and it can be difficult to provide two pulses of comparable energies. 
     Alternatively, two pulses can be provided by causing the beams of two different pulsed lasers to travel along a common path, where an external trigger causes each laser to emit a pulse with a time delay between the first and second pulses. This requires two lasers, which leads to a doubling in the cost of the system and an increase in the size of the system. Moreover, there is a requirement for precise adjustment of the spatial superimposition of the two pulses. 
     A scenario in which laser pulses are utilised is laser-induced breakdown spectroscopy (LIBS). LIBS has been used for chemical analytical purposes since the 1970s. However, a transfer to quantitative analysis applications was prevented because of insufficient performance compared with other optical emission spectrometric (OES) methods, such as spark-OES or inductively-coupled-plasma-OES. 
     In LIBS, a laser pulse is used to excite a sample. Crucial factors in LIBS are the laser parameters and the interaction of the laser with the material to be analysed. For quantitative analysis, the line emission strength depends on: 1) the amount of material ablated and 2) the temperature of the plasma in the plume. Single pulses, i.e. one laser pulse per pump pulse, are the conventional approach to ablate and vaporise material and to induce the plasma. Nevertheless, if single pulses are used, then ablation and plasma excitation cannot be optimised separately. As the plasma expands from the surface, it begins to absorb the tail of the incoming radiation, increasing the plume temperature, but limiting the amount of light reaching the sample surface and thus limiting the total amount of material ablated. Shorter pulses can be used to ablate more matter at the expenses of the plume temperature and, conversely, longer pulses can be used to increase the plume temperature, but result in line emission saturation arising from the plasma absorption. Similarly, higher single pulse energies do not lead to stronger emission lines precisely because of this saturation effect. 
     To optimise line emission strengths, double-pulse LIBS systems have been proposed. These utilise a train of two laser pulses, which are separated in time by several 10s of nanoseconds up to several microseconds. In a collinear geometry setup, i.e. when the two pulses are directed toward the surface following the same optical path, the first pulse reaches the sample and creates a corresponding first, expanding plasma plume. As this expands, the pressure of the plume decreases and so does its temperature. After a predefined time delay, the second pulse reaches the sample through the plasma plume generated by the first pulse. As the plasma plume density created by the first pulse is strongly decreased by supersonic expansion, this second pulse is partially transmitted and impacts the sample surface, where it generates a new plasma plume. In addition, the energy component absorbed by the first plasma plume causes its temperature to increase. The overall effect is increased material ablation and temperature, which leads to stronger line emissions. Comparative studies of Single and Double pulse systems show a line emission strength increase of typically 10-50 times and up to 100 times for certain elements. 
     To date, double-pulse LIBS systems have typically employed complex Q-switching circuitry or the use of multiple lasers, and so such double-pulse LIBS systems suffer from the previously-noted drawbacks of these approaches. As both approaches cause difficulties for industrial implementations in terms of cost and complexity, existing double-pulse LIBS systems are expensive and complex. 
     It is an object of this disclosure to address these and other problems with prior art LIBS systems and with prior art double-pulse laser systems in general. 
     SUMMARY 
     Against this background and in accordance with a first aspect, there is provided a double-pulse laser system according to claim  1 . A double-pulse laser-induced breakdown spectrometer according to claim  35  is also provided. 
     The present disclosure relates to the use of a multipass cell in a double-pulse laser system to provide a delay between first and second laser pulses. By directing one pulse into a multipass cell, that pulse may be delayed with respect to another pulse that does not enter the cell, by virtue of the multipass cell providing a longer optical path length for the pulse in the cell than the pulse that does not enter the cell. The use of a multipass cell for this purpose provides a way of delaying a laser pulse without requiring the use of complex electronics. 
     The pulses may be generated from a single pulse, for instance by splitting a single pulse. This means that a single laser can be used to provide two (e.g. only two, or in some cases at least two) coherent pulses, avoiding the need to use two lasers. Moreover, the disclosure provides a means for dividing a pulse into two pulses. By providing an aperture in a reflective surface and directing a pulse at the edge of the aperture, a portion of a pulse can be caused to pass through the aperture and a portion of the pulse can be reflected, thereby splitting the pulse. This is an efficient and reliable mechanism for splitting laser pulses and can be integrated easily with a multipass cell (e.g. by attaching the reflective surface having an aperture to the exterior of the cell). Arrangements of conventional beamsplitters can be used additionally and/or alternatively. 
     In preferred embodiments, in which two pulses are generated from a single pulse, the energy of the single pulse can be divided substantially equally between the two pulses. For instance, in general terms, each of first and second pulses can have equal energy (e.g. 50% of the energy of the energy of the original single pulse). This can be achieved using the arrangements described herein, including using conventional beamsplitters arranged as described herein, or using the mechanical beam splitting techniques (e.g. using a reflective surface having an aperture) described herein. 
     As the first and second laser pulses of the present disclosure can be generated from a single laser pulse, the first and second laser pulses preferably have the same frequency. Thus, pulses with substantially equal energy, frequency, intensity and/or size may be provided. Additionally, using the splitting techniques described herein, the splitting of a laser pulse can occur independently of the frequency of the laser pulse. Moreover, in some systems of the present disclosure, splitting can occur independently of the polarisation of the laser pulse. 
     Some specific examples of multipass cells described in this disclosure have particular advantages for use in double-pulse laser systems, as they are highly stable and relatively inexpensive to manufacture. For instance, such cells can provide an optical path length of up to or greater than 50 or 100 metres. The disclosure provides an optical structure that can be fabricated using inexpensive, commercially available components and which exhibits remarkable mechanical tolerances that make it suitable to withstand vibrations and simplify mechanical alignment in industrial implementations. 
     Some of the multipass cells of the disclosure are based on the combination of two prism mirrors and a concave (e.g. spherical) mirror, which respectively serve as two ends of a multipass cell. The prisms define a first end and the concave mirror defines a second opposing end. Light can enter through one end of the cell (typically between the prisms) and bounce repeatedly between the first and second ends of the cell. The optical properties of the combination of two prisms leads to enhanced stability compared to existing multipass cells. For instance, because the prisms are arranged to have perpendicular surfaces, light that is reflected by the concave mirror towards the prisms is at least partially retroreflected by the prisms. Therefore, the spreading of light as it repeatedly traverses the cell can be reduced. Although in principle, divergence of light could occur due to slight misalignment of the optical system, imperfections in the surface of the prisms and/or imperfections in the waveform of the light that enters the cell, in the presently described multipass cells, the partially retroreflective end of the cell is less sensitive to these imperfections and so their effects are reduced. 
     The advantage of improved stability due to reduced spreading of light can also be achieved using three mutually perpendicular reflective surfaces (e.g. a corner reflector). The use of a partially (or fully) retroreflective end of the cell is particularly advantageous in combination with a concave (e.g. a focusing) reflector at the other end of the cell. 
     The multipass cells of the disclosure provide additional benefits. For instance, whilst perpendicular reflective surfaces can be provided using, for example, two mirrors, the combination of two prisms (and especially two prims whose cross sections are right-angled isosceles triangles) is particularly advantageous. Two right-angled isosceles triangular prisms can be positioned side-by side (resting on the face defined by the hypotenuse of the cross section, with the axes of the prisms parallel) such that they define a pair of perpendicular surfaces. Moreover, by positioning the prisms with a small slit between their edges (the edges that are parallel with the axes of the prisms), an aperture for allowing light to pass between the prisms can easily be provided. Triangular prisms are widely available optical components that are easy to arrange precisely (e.g. using a mounting structure) to provide the above-noted advantages and which provide a larger surface area for mounting within an optical arrangement, improving stability of the reflective surface. Therefore, prisms provide an efficient and reliable means for manufacturing a pair of perpendicular reflective surfaces. 
     The enhanced stability provided by the reflector arrangements of the disclosure allow the cells to provide extremely long optical path lengths (and hence also long durations of time during which light is within the cell) for any given separation between the reflectors. For instance, the separation between the ends of the cell can be adjusted and the angle at which light enters the cell can be adjusted. By changing these properties of the cell&#39;s geometry, the total path traversed by light within the cell can be adjusted from less than 1 m up to tens of metres or even greater than 100 m. This can provide relatively long path lengths for providing temporal delays between pulses in double-pulse laser systems. In general terms, greater separations between the ends of the cell lead to greater path lengths and increased path lengths can also be achieved by increasing the angle at which the light enters the cell (i.e. by entering the light at greater angle from the longitudinal axis of the cell). The described multipass cells are particularly tolerant to receipt of light at an angle (compared to prior art multipass cells, used in other fields) and so are particularly beneficial when integrated into a double-pulse laser system. 
     The double-pulse systems of the disclosure are particularly advantageous in the context of double-pulse laser induced breakdown spectroscopy, in which first and second pulses impact a sample and cause the sample to emit light. It is advantageous to provide relatively long temporal delays between pulses, without the need for complex electronics or multiple lasers, by using new combinations of widely-available optical components such as prisms and mirrors. 
    
    
     
       LISTING OF FIGURES 
       Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG.  1    shows schematically a double-pulse laser system; 
         FIG.  2    shows schematically a double-pulse laser system comprising an optical arrangement for splitting light; 
         FIGS.  3 A to  3 D  show schematically a multipass cell; 
         FIGS.  4 A to  4 C  show stability analysis of the multipass cell; 
         FIG.  5    shows a standing mode of the multipass cell in an aligned state; 
         FIG.  6    shows standing modes of the multipass cell when subjected to misalignments; 
         FIG.  7    shows schematically a multipass cell; 
         FIG.  8    shows schematically an alternative first reflector arrangement for the multipass cells of  FIGS.  3 A to  3 D and  7   ; 
         FIGS.  9 A and  9 B  shows schematically mounting structures for the multipass cells described herein; 
         FIG.  10    shows the principle of splitting light; 
         FIGS.  11 A to  11 D  shows schematically a double-pulse laser system utilising the multipass cells described herein; 
         FIGS.  12 A and  12 B  shows a comparison of different types of mechanical beam splitting; and 
         FIG.  13    shows schematically a double-pulse laser-induced breakdown spectrometer utilising the multipass cells described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG.  1   , there is shown a generalised double-pulse laser system for generating first and second laser pulses. The system comprises a multipass cell  100  arranged to delay the second laser pulse with respect to the first laser pulse. The laser system additionally comprises a laser  110  for providing a single laser pulse, which is directed towards the multipass cell  100  along the direction  101 . The multipass cell  100  receives the single laser pulse and causes two pulses to travel in the direction  108  with a temporal separation. The use of a multipass cell for introducing a delay between the first and second laser pulses advantageously requires less space and costs less than systems that use a plurality of lasers for generating multiple laser pulses. Moreover, the use of the multipass cell to introduce the delay between laser pulses eliminates the need for complex electronics for controlling Q-switching. In  FIG.  1   , the multipass cell  100  may itself be capable of splitting a single laser pulse into first and second laser pulses, or optical splitting elements (which be positioned between the laser  110  and the cell  100 , for example) may perform this function. 
       FIG.  2    shows examples of multipass cells  200  and optical arrangements  212  for dividing laser pulses. In  FIGS.  2 ( i ) , ( ii ) and ( iii ), single laser pulses  201  are depicted incident upon optical arrangements  212 , which comprise beamsplitters  212   a - e , for guiding light into multipass cells  200  and towards a sample.  FIG.  2 ( i )  depicts an optical arrangement  212  that generates first and second laser pulses but which fails to direct both pulses towards a desired destination.  FIGS.  2   ( ii ) and  2 ( iii ) depict optical arrangements  212  that successfully generate first and second laser pulses having a relative time delay. In  FIG.  2   ( ii ), 75% of the total energy incident in the single laser pulse is eventually directed to the sample. In  FIG.  2   ( iii ), 100% of the total energy incident in the single laser pulse is eventually directed to the sample. 
     The optical arrangements  212  of  FIG.  2    utilise beamsplitters. Beamsplitters can be unpolarising (sometimes described as non-polarising) or polarising. Polarising beamsplitters split light into two beams of orthogonal polarisation states. In addition to beamsplitters, the optical arrangements  212  also comprise reflecting elements (e.g. mirrors) for directing pulses towards the appropriate beamsplitters. Types of beamsplitter include: half-silvered mirrors; pairs of triangular prisms adhered together; Wollaston prisms; and dichroic mirrored prism assemblies (which use dichroic optical coatings). 
     In  FIG.  2 ( i ) , a single unpolarising beamsplitter  212   a  is depicted. If a laser pulse  201  passes through one non-polarising beamsplitter, as shown in  FIG.  2 ( i ) , then 50% is transmitted (toward the sample) and 50% is reflected. In  FIG.  2 ( i ) , this is shown as being at a 90° clockwise angle with respect to the propagation axis of the incoming pulse. The reflected part of the light passes into the multipass cell  200  and once the pulse exits the cell  200  along the direction of the exiting light  208 , it is incident upon the same beamsplitter  212   a  again. There, 50% of this pulse (that is, 25% of the total initial pulse energy) will be reflected back towards the laser source along the direction of the incoming light  201  (which is dangerous due to potentially damaging the source) and 50% will pass straight through the beamsplitter  212   a  and will not reach the sample. 
       FIG.  2   ( ii ) depicts an optical arrangement  212  comprising two unpolarising beamsplitters  212   b  and  212   c .  FIG.  2   ( ii ) improves upon configuration (i) by adding a second beamsplitter  212   c  rotated by 180° with respect to the first beamsplitter  212   b . The first beamsplitter  212   a  splits a single laser pulse into first and second laser pulses. The first laser pulse passes straight through to the second beamsplitter  212   c , which the first laser pulse also passes straight through. The first laser pulse therefore travels in the direction of a sample. The second laser pulse (i.e. the delayed pulse) passes into the multipass cell  200 , traverses the cell one or more times, and emerges along the direction of the exiting light  208 , before being guided to the second beamsplitter  212   c.  50% of the second laser pulse passes straight through the second beamsplitter  212   c  and 50% of the second laser pulse is directed towards the sample, in a collinear direction to the first laser pulse. In this way, back-reflection to the laser source is avoided and 75% of the original laser energy reaches the sample, with 25% of the original laser pulse energy being the second laser pulse having a temporal delay with respect to the first laser pulse. This is not an optimum scenario due to the loss of 25% of the laser energy. 
       FIG.  2   ( iii ) depicts an optical arrangement  212  comprising two polarising beamsplitters  212   d  and  212   e . The first beamsplitter  212   e  splits the pulse according to its polarisation. Therefore, if circularly polarised light hits the beamsplitter  212   e , the horizontal and the vertical components are separated. Each component corresponds to 50% of the pulse energy as the original pulse is circularly polarised. Hence, 50% of the pulse is transmitted toward the sample and 50% is reflected to the multipass cell  200 . To avoid the same scenario as in Figure (i), a second polarised beamsplitter  212   e  (rotated by 180° with respect to the first polarised beamsplitter  212   d ) causes the two pulses to be targeted toward the sample. The advantage of this scenario is that 100% of the incident laser light is conserved, leading to increased efficiency with respect to  FIG.  2   ( ii ). 
     Hence, in generalised terms, the present disclosure provides embodiments in which an optical arrangement is configured to direct the second laser pulse into the multipass cell (e.g. so as to delay the second pulse with respect to the first pulse). The optical arrangement is preferably configured to generate the first and second laser pulses from a single laser pulse (e.g. by splitting a single pulse into two). The optical arrangement may be configured to split one pulse into only two pulses. Such pulses may have substantially equal energy (i.e. 50% of the energy of the pulse used to generate the pulses). The disclosure provides arrangements for generating first and second laser pulses with a temporal delay, with the degree of the temporal delay depending upon and being controllable by the characteristics (e.g. the optical path length) of the multipass cell that is used. The optical arrangements of the present disclosure may comprise one or a plurality of unpolarising beamsplitters. Additionally or alternatively, the optical arrangement may comprise one or a plurality of polarising beamsplitters. The light may be polarised or unpolarised depending on the combination of beamsplitters that is employed. Such arrangements are advantageous in that they do not require particularly strict alignment between the laser and the optical cavity. Moreover, they can be fabricated efficiently and effectively. 
     The multipass cell  200  may be any type of existing multipass cell, such as a White or Herriott cell. Multipass cells, such as the White or Herriott cell, are generally used as spectroscopic absorption cells. However, the present disclosure also encompasses novel multipass cell geometries that allow surprisingly long optical delays to be achieved using remarkably mechanically stable cells. Examples of such multipass cells are depicted in  FIGS.  3 A to  3 D and  7   , which are discussed in greater detail below. 
     In contrast to existing multipass cells, the novel multipass cells for use in the laser systems of the present disclosure may comprise, in generalised terms: a first reflector arrangement; and a second reflector arrangement; wherein the first reflector arrangement is configured such that light incident on the first reflector arrangement is at least partially retroreflected towards the second reflector arrangement. Advantageously, the use of a reflector arrangement that is at least partially retroreflective provides the effect of improved mechanical stability, because a partially retroreflective surface inhibits scattering of light incident thereon and so light is reflected back to its source with reduced or minimum scattering. In this case, light is reflected from the first reflector arrangement towards the second reflector arrangement, which allows the multipass cell of this disclosure to tolerate more mechanical misalignment than prior art devices, which cannot tolerate significant misalignment. 
     The first reflector arrangement of the present disclosure may be defined in alternative terms based on its structure rather than its partial retroreflectivity. For example, the first reflector arrangement may be defined as having two perpendicular (or substantially perpendicular so as to provide partial retroreflectivity) reflective surfaces or three mutually perpendicular (or substantially perpendicular so as to provide retroreflectivity) reflective surfaces. A planar mirror reflects light incident thereon back to its source only when the light is exactly perpendicular to the mirror, having a zero angle of incidence. Whilst laser light exhibits a low degree of beam (or pulse) divergence, no laser beam is perfectly collimated. Moreover, no mirror is perfectly planar. Therefore, for real light sources, some scattering from a planar mirror typically occurs. Thus, in the context of this disclosure, a planar mirror is not considered to be partially retroreflective. Rather, in the context of this disclosure, a reflector arrangement is at least partially retroreflective if it provides a retroreflective action for light across a range (i.e. a plurality) of angles of incidence (unlike a perfectly planar mirror, which can only retroreflect light incident at a single angle of incidence). 
     Retroreflectivity can be obtained using a corner reflector, which comprises three perpendicular planar reflectors that cause any light incident into the corner reflector to be retroreflected to its source. Partial retroreflectivity can also be achieved using only two perpendicular planar mirrors and in this case, light incident from a range of directions will be retroreflected. However, the lack of a third reflective surface means that light having a component in the direction defined by the line of intersection of the two planes will not be perfectly retroreflected to its source. Rather, two planar perpendicular mirrors are retroreflective for light that is perpendicular to the direction defined by the intersection of the two planes. 
     A multipass cell  300  for use in the laser systems of the present disclosure is depicted in  FIGS.  3 A,  3 B,  3 C and  3 D , which show schematically the multipass cell  300  in four different configurations. 
     The multipass cell  300  comprises a housing  302 . Light  301 , which is typically coherent light (e.g. light generated by a laser), enters the housing  302  through an optical window  304 , which is transparent to the selected wavelength of the light source. The optical window  304  may simply be an aperture in the housing  302 . The light  301  can be a second laser pulse that has been split from a single laser pulse by a beam splitter arrangement as shown in  FIG.  2   . The light  301  is directed at an incoming entry angle Θ with respect to the normal to the window  304 . The angle Θ is also the angle between the direction of the light  301  and the longitudinal axis  300   z  of the cell. The longitudinal axis  300   z  is shown in  FIG.  3 A  but is omitted from  FIGS.  3 B,  3 C and  3 D  for simplicity. The angle Θ is typically from 2° to 10° (although other ranges of angles can be used). 
     The multipass cell  300  comprises first and second reflector arrangements  305  and  307 . The reflector arrangements  305  and  307  are arranged such that light entering the multipass cell  300  is repeatedly reflected between the two arrangements (without being reflected from any surfaces other than the surfaces of the two reflector arrangements) and the reflector arrangements  305  and  307  define an optical cavity  315 . 
     The first reflector arrangement  305  comprises two prism mirrors  305 A,  305 B positioned such that a small slit  306 , which is typically 2 to 10 mm wide, is defined between the prisms  305 A and  305 B. The first reflector arrangement comprises two surfaces (faces of the two prisms) that are substantially perpendicular. The slit  306  is aligned with the window  304  and serves as an aperture through which a beam or pulse of light can enter and exit an optical cavity  315  defined within the multipass cell  300 . 
     The second reflector arrangement  307  of this cell  300  is a spherical, circular mirror, which is positioned at a distance d from the prism mirrors  305 A and  305 B. In this cell  300 , the second reflector arrangement  307  does not have an aperture and so light cannot pass through the second reflector arrangement. The second reflector arrangement  307  faces the prisms  305 A and  305 B of the first reflector arrangement. 
     In use, light  301  enters the cell through the optical window  304  and the slit  306  between the prisms  305 A and  305 B. The light then reflects from the spherical mirror  307 , which reflects and focuses the light back towards the first reflector arrangement  305 . The light reflects from one of prisms  305 A and  305 B to the other of the prisms  305 A and  305 B and, because the prisms  305 A and  305 B are positioned such that their faces are perpendicular, the light is retroreflected by the combination of the two prisms back towards the spherical mirror  307 . The symmetry of the reflector arrangements  305  and  307  causes the light to follow a specific path within the cell  300  and this path is remarkably stable with respect to misalignment. After a number of reflections within the optical cavity  315 , the path of the light is eventually incident upon the slit  306  between the prisms and so the light  308  emerges from the cell  300 . When the optical cavity  315  is viewed in cross-section (in the plane perpendicular to the axes of the prisms  305 A and  305 B; or equivalently in the plane whose normal vector is the line of intersection of the planar reflecting surfaces of prisms  305 A and  305 B), the angle Θ at which the light  308  emerges from the cell  300  is equal (but in the opposite direction) to the angle at which the light  301  enters the cell  300 . 
     Hence, the combination of the two prism mirrors  305 A and  305 B and the spherical mirror  307  defines a set of standing modes that can trap light within the cell  300  for a number of reflections before exiting the cavity  315  along the exit direction of the light  308 . The number of reflections and consequently the total achievable optical path length within the multipass cell  300  depends on a number of factors including: the surface areas of the prism mirrors  305 A,  305 B; the radius of curvature of the spherical mirror  307 ; the angle at which the light  301  enters the cavity  315 ; and the distance, d, between the prism mirrors  305 A,  305 B and the spherical mirror  307 . Thus, the optical path length depends on the geometrical characteristics of the setup. However, the optical path length is not affected by the physical characteristics of the light (including wavelength, beam energy per unit area, or whether the light  301  is pulsed or continuous-wave). 
     The effects of the geometry on the optical path length are shown in  FIGS.  3 A to  3 D , which depict simulated ray traces for different configurations. In  FIG.  3 A , the separation between the first and second reflector arrangements  305  and  307  is d=150 mm. This is the distance between the centre of the aperture between the two prism mirrors  305 A and  305 B and the spherical mirror  307 . This arrangement leads to 8 reflections and a total optical path length of 1.2 m. In  FIG.  3 B , the distance d is increased to 485 mm, leading to 66 reflections and a total optical path length of 31.9 m. In  FIG.  3 C , the distance d has been further increased to 525 mm, leading to 88 reflections and a total optical path length of 46.3 m. The angles of incidence in  FIGS.  3 B and  3 C  are the same as in  FIG.  3 A . 
       FIG.  3 D  shows a special case for the multipass cell  300  in which the distance d is equal to exactly half of the focal length of the second reflector arrangement  307  (which in this case is a circular mirror). It can be seen that in this arrangement, the incident light  301  passes through the first reflector arrangement  305  and strikes the second reflector arrangement  307 , before being reflected back towards the first reflector arrangement  305 . The first reflector arrangement  305  then partially retroreflects the light back towards the second reflector arrangements and, due to the high degree of symmetry of this configuration, the light returns to the centre of the first reflector arrangement where it emerges from the optical cavity  315  along the direction of the exiting light  308 . Ensuring that the first and second reflector arrangements  305  and  307  are separated by half the focal length of the second reflector arrangement  307  causes the light to traverse the length of the cell  300  exactly four times. 
       FIG.  3 D  is simplified and omits the housing of the  FIGS.  3 A,  3 B and  3 C . However,  FIG.  3 D  further illustrates an optical arrangement  312  for guiding the light  308  emerging from the cell  300  to a desired destination (e.g. to a sample for analysis in a LIBS system). In this case, the optical arrangement  312  comprises a mirror and a lens, but various combinations of optical elements may be used to direct light to a desired destination. 
     The multipass cell  300  of  FIGS.  3 A to  3 D  therefore provides a novel architecture based on the combination of two prism mirrors  305 A and  305 B and a concave spherical mirror  307 . It can be seen from these figures that a wide range of optical path lengths are achievable. This architecture may be used to provide relatively long optical delays between laser pulses in LIBS and may provide an optical path length of up to or greater than 50 metres (equivalent to a temporal delay of approximately 167 ns). 
       FIGS.  4 A,  4 B and  4 C  depict simulations of the multipass cell  300  of  FIGS.  3 A to  3 D  when slightly misaligned. As noted previously, an advantage provided by embodiments of this disclosure is the increased stability when up to 4° of misalignment between the reflector arrangements is present. This can be demonstrated by studying the effects of controlled misalignment on the optical path traced by a coherent light beam. 
     Each of  FIGS.  4 A,  4 B and  4 C  is composed of 3 subfigures outlining a different misalignment scenario.  FIG.  4 A  shows a stability study of the multipass cell for the geometry presented in  FIG.  3 A , with a separation between the reflector arrangements  305  and  307  of d=150 mm.  FIG.  4 B  is a stability study of the multipass cell for the geometry presented in  FIG.  3 B , with a separation between the reflector arrangements  305  and  307  of d=485 mm.  FIG.  4 C  is a stability study of the multipass cell for the geometry presented in  FIG.  3 C , with a separation between the reflector arrangements  305  and  307  of d=525 mm. 
     In each case, the central subfigure corresponds to a well-aligned laser beam that follows an optical path on a single plane by creating standing modes between the prism mirrors  305 A and  305 B and the spherical mirror  307 . For the stability analysis depicted in  FIGS.  4 A to  4 C , the exiting beam is collected onto a detection system  309 . 
     When the beam is misaligned in the x-dimension from −2° (left subfigure) to +2° (right subfigure), the optical path is no longer confined to a single plane and can span the entire volume between the prism mirrors  305 A and  305 B and the spherical mirror  307 . The geometry proposed in the multipass cell  300  allows the integrity of the standing modes to be maintained under misalignment, which means the beam may successfully exit the cell  300  even under severe misalignment conditions. In each of  FIGS.  4 A,  4 B and  4 C , a beam with an incoming misalignment angle of up to 4° in the x-dimension −2° to +2°) is shown. This results in the optical path being tilted with respect to the aligned case, where all reflections lie on a single plane. Within these boundaries, the beam is nevertheless able to create standing modes within the multipass cell and successfully exit for detection at a detection system  309 . 
       FIGS.  5  and  6    show a further study of the stability of the geometry of the multipass cell  300 , in which a misalignment is applied to the spherical mirror  307  in the x-dimension, as shown in  FIG.  6   , and in which the spherical mirror is perfectly aligned, in  FIG.  5   . The mirror  307  is considered to be aligned if its centre lies on the same segment originating from the source of the light  301  and passing across the centre of the slit  306  (between the prisms  305 A and  305 B). The mirror  307  is moved away from this segment by 10 mm in the positive direction and then by 10 mm in the negative direction. The stability of the system is demonstrated by simulating the impact location of the light on the prism mirror  305 A as a function of these misalignments. The behaviour on the prism mirror  305 B is analogous. 
       FIG.  5    corresponds to the case in which no misalignment occurs. In this case, the standing modes within the cavity  315  are located onto a single line over the prism mirror  305 A. When negative ( FIG.  6 ( a ) ) or positive ( FIG.  6 ( b ) ) misalignments of 10 mm occur, the standing modes move from a single line and form a set of two parabolas. The light traverses the two parabolas in sequence, one after another. This is important and allows the entry point and the exit point of the light to coincide, which is important for the stability of the cell  300 . 
     An advantage of providing a highly stable multipass cell  300  is that the optical path length traversed by light in the cell  300  is easily adjustable by changing the distance d between the spherical mirror  307  and the two prism mirrors  305 A and  305 B. The benefits of increased optical path length include the ability to provide long optical delays between laser pulses. Thus, it can be seen from  FIGS.  4 A to  4 C ,  FIG.  5    and  FIG.  6    that the multipass cell  300  of  FIGS.  3 A to  3 D  provides a stable system that can provide long optical path lengths even in the presence of misalignment between the optical components. However, a number of features of the multipass cell of  FIGS.  3 A to  3 D  may be omitted or modified whilst retaining these advantages. 
     For example, it will be appreciated that the housing  302  and the optical window  304  may be omitted entirely. Moreover, the advantage of improved stability can be achieved using two planar mirrors that are substantially perpendicular, rather than prisms  305 A and  305 B. Such an arrangement would provide the same effect of being partially retroreflective for light incident thereon. Furthermore, the aperture  306  through which light enters the cavity  315  can be placed in the second reflector arrangement rather than the first reflector arrangement. Additionally, the spherical mirror  307  need not be spherical and could have various other forms whilst benefiting from the partially retroreflective prisms  305 A and  305 B. Thus, it can be seen that the multipass cell  300  is one specific example of an advantageous arrangement but that various alterations and variations may be made. 
     Hence, returning to the generalised terms used previously, the first reflector arrangement of this disclosure preferably comprises first and second surfaces that are reflective. The first reflector arrangement may be configured such that light incident thereon is reflected from the first surface to the second surface, and to the second reflector arrangement. Light reflected from the second surface may be incident on a third surface of the first reflector arrangement before being reflected to the second reflector arrangement, or the light reflected from the second surface may be reflected directly to the second reflector arrangement without being reflected by any further surfaces. 
     The first and second surfaces are preferably substantially perpendicular. The first and second surfaces are preferably substantially planar. This arrangement can be used to provide a retroreflective action on light to improve the mechanical stability of the multipass cell. Perfectly planar, perpendicular surfaces will exhibit full retroreflectivity but some deviations from perfectly planar, perpendicular surfaces may be tolerated. For instance, the surfaces may deviate from being perfectly planar and/or perfectly perpendicular, provided that the effect of (at least) partial retroreflectivity is still achieved. When light possesses some components non-normal to the surface of the second reflector arrangement (e.g. a spherical mirror), then this will enter in the cavity and can form a set of standing wave-like patterns, as shown in  FIGS.  4 A to  4 C ,  FIG.  5    and  FIG.  6   . 
     Furthermore, there is no requirement for the entire first or second surface to be entirely planar. For instance, one or both of the surfaces may have a curved portion (e.g. at the edge or edges) in addition to a planar portion. In this case, provided that the substantially planar portions of the first and second surfaces are substantially perpendicular to one another, they can still work together to partially or fully retroreflect light incident thereon. 
     Thus, the disclosure provides a multipass cell comprising: a first reflector arrangement; and a second reflector arrangement; wherein the first reflector arrangement comprises first and second surfaces that are reflective, wherein the first and second surfaces are substantially perpendicular and/or substantially planar. 
     The planes of the first and second surfaces may define a common axis and the first reflector arrangement may be retroreflective for light incident perpendicular to the common axis. In the context of planar surfaces, the common axis is the line of intersection defined by the planes containing the planar surfaces. Any two non-parallel planes define a line of intersection. Therefore, even if two planar surfaces do not actually intersect, the planes in which the surfaces lie will define an axis of intersection. The axis of intersection may be considered to be the line along which the planar surfaces would intersect if the planes had infinite spatial extent. 
     Preferably, the first reflector arrangement comprises first and second prisms and the first and second surfaces are faces of the first and second prisms respectively. Prism mirrors are widely available optical components that allow the advantageous embodiments described previously to be manufactured accurately and easily. For example, the prism mirrors may have a cross-section that is a right-angled isosceles triangle (i.e. with interior angles of 90°, 45° and 45°). In this case, by placing two such prisms adjacent one another, with both prisms resting on their shorter (non-hypotenuse) face, a partially retroreflective surface (defined by the two surfaces of the prisms that will be perpendicular in this arrangement) can be fabricated easily. Thus, the multipass cells of this disclosure advantageously use inexpensive, commercially available components to provide a cost-effective and reliable method for manufacturing a stable multipass cell. 
     The second reflector arrangement is preferably configured such that light incident thereon is reflected towards the first reflector arrangement. For example, the second reflector arrangement may be configured such that light received from the first reflector arrangement is reflected to the first reflector arrangement and, because the first reflector arrangement is at least partially retroreflective, light may be made to repeatedly bounce between the first and second reflector arrangements. This may be achieved by ensuring that the first and second reflector arrangements face one another. For example, the first reflector arrangement is at least partially retroreflective and is therefore retroreflective for light received from a range of directions. Accordingly, the second reflector arrangement may be positioned within the range of directions for which the first reflector arrangement is retroreflective. When the second reflector arrangement has a concave face, this face may be facing the at least partially retroreflective portion of the first reflector arrangement. In this way, the first and second reflector arrangement can define a stable optical cavity. 
     The second reflector arrangement is preferably configured such that light incident thereon is focused towards the first reflector arrangement. The focusing action of the second reflector arrangement works together with the retroreflective action of the first reflector arrangement to inhibit the spreading of light and improve stability. The relationship between the spacing of the reflector arrangements and the focal length of the second reflector arrangement will influence the number of passes traversed by light within the cell. 
     The second reflector arrangement may comprise a concave surface that is reflective. The concave surface may be an ellipsoidal surface, a spheroidal surface, or a spherical surface. For example, an ellipsoidal reflector having one elongate axis parallel to the line of intersection defined by two reflective planar surfaces could be used. In such a case, the elongated axis would affect the mechanical tolerances as the useful surface to compensate for misalignment would be elongated in one direction and shortened in the other direction. Thus, surfaces with a higher degree of spatial symmetry provide improved stability and consequently, a spherical surface (i.e. a portion of the surface of a sphere with an opening for allowing light in) is most preferred. Minor deviations from spherical may be tolerated. The combination of two plane prism mirrors with a spherical (i.e. centrally symmetrical) mirror provides most improved stability as it means that a slight misalignment of the spherical mirror will not be further amplified, and the light path will still lie in between the volume within the mirrors of the cavity. 
     Advantageously, in this disclosure, the separation between the first and second reflector arrangements is adjustable. Hence, the multipass cell is configured such that the optical path length traversed by light is adjustable. Whilst not shown in  FIGS.  3 A to  3 D  for the purposes of simplicity, the first and second reflector arrangements  305  and  307  are relatively moveable (e.g. by moving one or both). This allows the separation to be controlled and hence the optical path length to be adjusted. The relative motion may be provided by, for example, actuating one or both of the reflector arrangements. The optical path length may be adjustable by changing the number of times light traverses the multipass cell. For instance, increasing the separation may lead to an increase in the distance traversed by light within a single pass, but it may also cause the light to traverse a different number of passes within the cell, further increasing the optical path length. The improved stability of the disclosure allows relatively long optical path lengths to be obtained whilst providing control over the path length. 
     Using the cells of the present disclosure, the optical path length is adjustable to: greater than or equal to 30 cm (and preferably no more than 1 m, 5 m, 15 m, 25 m, 40 m, 50 m, or 100 m); greater than or equal to 1 m (and preferably no more than 5 m, 15 m, 25 m, 40 m, 50 m, or 100 m); greater than or equal to 5 m (and preferably no more than 15 m, 25 m, 40 m, 50 m, or 100 m); greater than or equal to 15 m (and preferably no more than 25 m, 40 m, 50 m, or 100 m); greater than or equal to 25 m (and preferably no more than 40 m, 50 m, or 100 m); greater than or equal to 40 m (and preferably no more than 50 m, or 100 m); greater than or equal to 50 m (and preferably no more than 100 m); or greater than or equal to 100 m (and preferably no more than 150 m). These may be converted into equivalent temporal values by noting that the speed of light is approximately 3×10 8  ms −1 . 
     The described embodiments exhibit unexpectedly high mechanical tolerances to provide a multipass cell that is suitable to withstand vibrations and simplify mechanical alignment in industrial implementations. The advantages of this disclosure compared to previous multipass cells are numerous and include the increased stability up to 4° (approximately 70 milliradians) of misalignment, long optical path lengths that can be adjusted easily, and an architecture that is simple to manufacture reliably and efficiently. 
     In the multipass cell  300  of  FIGS.  3 A to  3 D ,  FIGS.  4 A to  4 C ,  FIG.  5    and  FIG.  6   , the aperture  306  through which light enters the optical cavity  315  is positioned between the two prisms  305 A and  305 B of the first reflector arrangement  305 . However,  FIG.  7    depicts an alternative multipass cell  700  in which many of the advantages described previously are achievable by providing an aperture  706  in a second reflector arrangement  707 , rather than between the prisms  705 A and  705 B. 
     The multipass cell  700  of  FIG.  7    comprises a first reflector arrangement  705  that comprises two prism reflectors  705 A and  705 B, which are positioned such that two faces of the prisms  705 A and  705 B are perpendicular and provide a partially retroreflective surface. A second reflector arrangement in the form of a spherical mirror  707  is provided facing the prisms  705 A and  705 B. The spherical mirror  707  comprises a central aperture  706  for allowing light into and out of the optical cavity  715  of the multipass cell  700 . Light entering  701  the cell  700 , such as a second laser pulse that has been split from a single laser pulse by a first beam splitter arrangement as shown in  FIG.  2   , is repeatedly reflected between the first  705  and second  707  reflector arrangements before exiting the cavity  715  via the aperture  706  along the direction of the exiting light  708 . Due to the high degree of geometric similarity, the standing modes provided by the first  705  and second  707  reflector arrangements are similar to the arrangements  305  and  307  of the multipass cell  300  of  FIGS.  3 A to  3 D . Light emerging from the cell is then directed to its destination via an optical arrangement  712 , which is shown as comprising a mirror and a lens in  FIG.  7   . For example, the light (e.g. second laser pulse) may be directed to a second beam splitter, from where it is directed to a sample (e.g. in a collinear direction to a first laser pulse) as shown in  FIG.  2   . The multipass cell  700  of  FIG.  7    provides the benefits of improved stability and adjustability as the cell  300  of  FIGS.  3 A to  3 D . 
     Turning next to  FIG.  8   , there is depicted a reflector arrangement  805  that comprises three planar reflective surfaces  805 A,  805 B and  805 C that are mutually perpendicular. The three surfaces  805 A,  805 B and  805 C define a corner reflector that is retroreflective. An aperture  806  is provided at the corner of the corner reflector  805  to allow light to pass through the corner reflector. Light  801  passing through the rear side of the corner reflector  805  is depicted. 
     The reflector arrangement  805  of  FIG.  8    can be used in multipass cells such as those of  FIGS.  3 A to  3 D and  7   , in place of prisms  305 A and  305 B, or in place of prisms  705 A and  705 B. If the reflector arrangement  805  of  FIG.  8    is used in the multipass cell  700  of  FIG.  7   , then the aperture  806  may be omitted. The reflector arrangement  805  again provides improved mechanical stability due to the use of a retroreflector to inhibit the spreading of light in an optical cavity. 
     Hence, returning to the generalised language used previously, in the multipass cells of the present disclosure, the first reflector arrangement may further comprise a third surface that is reflective, wherein the first, second and third surfaces are substantially mutually perpendicular. Thus, a corner reflector can be provided to improve mechanical stability. 
     The first and second reflector arrangements may define an optical cavity, and at least one of the first and second reflector arrangements preferably comprises an aperture for allowing light to enter and/or exit the optical cavity. The size of the aperture may be adjustable to provide control over the size of the light beam or pulse that enters the cavity. The aperture can take many forms. 
     When the first reflector arrangement comprises first and second prisms, a slit between the edges of the first and second prisms may define an aperture. A particular advantage of this arrangement is that it is simple to provide an aperture between two prisms by mounting the prisms such that there is a slit between them, without needing to create an aperture in a reflector (e.g. by making an aperture in a spherical reflector or a corner reflector, which could cause damage or mirror imperfections). Thus, this arrangement is easy to make accurately and without risking damage to delicate optical components. The size of the aperture may be adjusted by actuating the prisms to be closer together or further apart. The prisms may be relatively moveable to provide such adjustment. 
     When the first reflector arrangement comprises first, second and third surfaces, an opening at a corner of the first, second and third surfaces may define an aperture (e.g. the point at which the planes of the three surfaces intersect). Similarly, an opening at the centre (e.g. a point on the second reflector surface that is substantially aligned with the longitudinal axis of the cell) of the second reflector arrangement may define an aperture. This could be a small hole in the centre of a concave reflective surface, for example. Such apertures allow light to enter and/or exit the optical cavity in arrangements that are mechanically stable. In such cases, the size of the aperture may be adjusted by partially covering the aperture with an opaque material (which may be moveable). 
     Turning next to  FIGS.  9 A and  9 B , two mounting structures  913   a  and  913   b  are depicted for a reflector arrangement  905  comprising two prisms  905 A and  905 B. The prisms  905 A and  905 B could be the prisms  305 A,  305 B or  705 A,  705 B of the multipass cells  300  or  700  respectively. The mounting structures  913   a  and  913   b  can therefore be used in the multipass cells  300  and  700  of  FIGS.  3 A to  3 D and  7   . 
     The mounting structure  913   a  of  FIG.  9 A  is a frame that is configured to hold the prisms  905 A and  905 B. The mounting structure  913   a  in  FIG.  9 A  is shown from one end of the pair of prisms  905 A and  905 B. The mounting structure may extend along the long edges of the prisms (into the page, along the prism axes) and the opposite end of the mounting structure  913   a  holds the opposite end of the prisms  905 A and  905 B in the same way. The mounting structure  913   a  is dimensioned such that it can hold the non-reflecting edges of the prisms  905 A and  905 B so as to hold the prisms  905 A and  905 B securely in position. A minor portion of the mounting structure covers the reflecting surfaces (i.e. the hypotenuse of the prisms  905 A and  905 B) but the majority of the reflecting surface is exposed so as to allow the prisms  905 A and  905 B to reflect light within the cell. 
     The mounting structure  913   a  may have a friction coating (e.g. rubber) to ensure that the prisms  905 A and  905 B are held firmly in position. The prisms  905 A and  905 B may fit within the mounting structure  913   a  using an interference fit. Alternatively, the prisms  905 A and  905 B may be held to the mounting structure  913   a  with an adhesive. In any case, the mounting structure ensures that the reflecting surfaces of the prisms  905 A and  905 B are substantially perpendicular so as to combine to provide a partially retroreflective surface. 
       FIG.  9 B  shows a further mounting structure  913   b  that may be used in addition to or instead of the mounting structure  913   a  of  FIG.  9 A . The mounting structure  913   b  of  FIG.  9 B  may serve as the base of the mounting structure  913   a  of  FIG.  9 A  or the mounting structure  913   b  may itself be a standalone component. The mounting structure  913   b  of  FIG.  9 B  comprises a flat portion of material to which prisms  905 A and  905 B may be attached. The mounting structure  913   b  comprises a slit  906  for allowing light to pass through. The prisms  905 A and  905 B may be mounted either side of the slit  906  such that the faces of the prisms  905 A and  905 B are substantially perpendicular. Thus, a partially retroreflective reflector arrangement can easily be provided using a single sheet of material with a slit in it, and two prisms  905 A and  905 B, which are standard optical components. 
     The mounting structures  913   a  and  913   b  of  FIGS.  9 A and  9 B  may be used to ensure that the relative angle between the two prism mirrors  905 A and  905 B is zero or substantially zero (e.g. close enough to zero to ensure that at least partial retroreflectivity is obtained). In such a case, the two mirrors can together rotate by up to +/−1° approximately and still provide a stable multipass pattern when used with the previously-described multipass cells. However, if the relative angle between the two prism mirrors is larger than 0.1°, then the pattern may be negatively affected. The use of such a mounting structure can ensure that the relative angle between the prisms  905 A and  905 B is zero or close enough to zero to provide good performance. The mounting structures  913   a  and  913   b  of  FIGS.  9 A and  9 B  may be formed from various materials (e.g. metal such as aluminium) and using various construction techniques (e.g. welding, moulding or 3D printing). 
     Hence, in the generalised language used previously, the first reflector arrangement preferably comprises a mounting structure configured to mount the first and second prisms such that the first and second surfaces are substantially perpendicular. The use of a mounting structure can help to ensure that the surfaces are positioned correctly to within an acceptable degree of misalignment. 
     In  FIG.  10   , the principle of mechanical beam splitting is depicted. The top graph represents a one-dimensional spatial section of a Gaussian laser pulse at an instant in time. The bottom graph displays the temporal profiles of two pulses formed from splitting the top pulse, which are separated by a time delay. The present disclosure proposes the use of a reflective surface to mechanically split a single pulse generated by a pulsed laser into a double (preferably collinear) set of two pulses and to be introduced a delay using a multipass cell. The transmitted portion of the beam (i.e. the left portion of the pulse depicted in the top graph of  FIG.  10   ) is not subjected to any delay and is therefore positioned to the left along the temporal axis of the lower graph of  FIG.  10   . A reflected beam or pulse (i.e. the rightmost portion of the pulse depicted in the top graph) is subjected to a delay and so is positioned to the right on the temporal axis in the lower graph of  FIG.  10   . Thus, it can be seen that a time delay Δt can be introduced between two laser pulses generated by mechanically splitting a single laser pulse. Therefore, a double-pulse laser architecture can be provided. 
       FIGS.  11 A to  11 D  demonstrate how the mechanical beam splitting principle of  FIG.  10    can be applied in combination with the multipass cells of this disclosure, as an alternative to the beam splitting using the beamsplitter arrangements of  FIG.  2   . For instance, in  FIGS.  11 A,  11 B,  11 C and  11 D , there are depicted four configurations of a double-pulse laser system for generating first and second laser pulses. Because the multipass cell provides a relatively long optical path length when compared with existing multipass cells, the cell effectively functions as a delay line that introduces a relatively long time delay between two laser pulses. Moreover, the geometry of the cell ensures that the light  1108  emerging from the cell is collinear with the light reflected from the exterior surface  1114  of the cell. 
     The double-pulse laser system of  FIGS.  11 A to  11 D  is similar to the previously-described systems in that it comprises a multipass cell having two prisms  1105 A and  1105 B and a spherical reflector  1107  that define an optical cavity  1115 . Light  1101  enters the cell at a slight angle, as described previously. The double-pulse laser system also comprises an optical arrangement  1112  for guiding the light  1108  emerging from the cell towards a target destination  1116 , which could be a sample. The optical arrangement comprises a mirror  1112   b . An important difference between the double-pulse laser system and the previously-described multipass cells is that the exterior surface of the prisms  1105 A and  11058  is reflective and comprises a small aperture (aligned with the slit between the prisms  1105 A and  1105 B) for allowing light  1101  into the cell. This reflective surface with an aperture acts as an optical splitting device  1112   a  for splitting light and forms part of the optical arrangement  1112 . 
     More specifically, in the schematic setup of the double-pulse system of  FIGS.  11 A to  11 D , a collimated and pulsed laser beam  1101  is directed towards a planar mirror  1112   a  on the exterior (rear surface) of the prisms  1105 A and  11058 . The pulsed laser beam path is represented in  FIGS.  11 A to  11 D  as solid continuous lines, although these lines should not be mistaken for a continuous wave laser emission. The angle of the pulsed beam  1101  is slightly tilted with respect to the normal of the mirror  1112   a  and is typically 2-6°. The normal of the mirror  1112   a  is parallel to the axis of the cell, (i.e. the longitudinal axis extending between the slit between the prisms  1105 A and  11058  and the centre of the spherical mirror  1107 ). 
     The mirror  1112   a  comprises a central, circular aperture of 1 mm diameter, allowing part of the laser pulse  1101  to be sampled through it and part of the laser pulse  1101  to be reflected from it along the path  1108 . Similarly to the previously-described embodiments, the angle of light  1108  emerging from the cell (relative to the normal of the aperture) is the same magnitude but the opposite direction to the angle of the incoming light  1101 , which arises due to the geometry of the cell. 
     The aperture of the optical splitting device  1112   a  is dimensioned so that an incoming light pulse  1101  is split (e.g. divided into two distinct pulses), with approximately half of the light being reflected from the exterior surface  1114  towards the optical arrangement  1112   b  and half of the light entering the cell, where it is reflected multiple times before ultimately leaving the cell and reaching the optical arrangement  1112   b . Whilst the aperture is 1 mm in diameter in  FIGS.  11 A to  11 D , other widths (e.g. diameters of 0.5 mm, 1.5 mm, 2 mm, 2.5 mm and so on) may be used depending on the width of the laser beam used. In the specific systems depicted in  FIGS.  11 A to  11 D , the pulsed laser beam  1101  possesses a Full-Width-At-Half-Maximum (FWHM) of 1 mm. 
     The system is configured such that the pulse  1101  is centred on the edge of the aperture of mirror  1112   a , and the mirror  1112   a  has a radius of 25 mm (i.e. of a similar size to the prisms  1105 A and  1105 B). Various optical elements could be used to direct the pulse  1101  to the mirror  1112   a  in this way. Half of the pulse is reflected by the surface of the mirror  1112   a  while the other half passes through the aperture. The reflected pulse is directed towards the planar mirror  1112   b  and then towards the surface of a sample  1116 . The transmitted pulse is directed towards the spherical, concave mirror  1107  of the cell, which has a radius of curvature r=1000 mm and a diameter of 50 mm. This mirror  1107  reflects and focuses the pulse back towards the two right-angle prism mirrors  1105 A and  1105 B, as shown in  FIGS.  11 A to  11 D . The two right-angle prism mirrors  1105 A and  1105 B have a segment size of 25 mm. In this context, the segment size is the length of the two sides of the right-angled triangle that meet at right angles (i.e. the length of the non-hypotenuse lengths of the triangular cross-section of the prisms  1105 A and  1105 B). The combination of the mirrors  1105 A,  1105 B and  1107  forms a cavity  1115  system, where the pulse that enters through mirror  1112   a  is reflected back and forth for a number of times before eventually exiting from the aperture of the mirror  1112   a.    
     The total optical path length difference (OPD) provided by the systems of  FIGS.  11 A to  11 D  is defined as the difference between: a) the distance covered by the pulse that passes through mirror  1107  and which is reflected inside the cavity  1115  before exiting from the central aperture of mirror  1112   a  and reaching the sample  1116 ; and b) the distance travelled by the part of the pulse that is reflected at mirror  1112   a  before reaching the sample  1116 . Advantageously, the OPD can be easily tuned by adjusting the distance d between: the first reflector arrangement  1105 , comprising (right-angled) prism mirrors  1105 A and  1105 B; and the second reflector arrangement, which in this case is mirror  1107 . The OPD can be controlled by adjusting just the separation d whilst leaving the geometry of the other components unchanged. By adjusting the OPD, the temporal delay Δt between the first pulse (reflected by mirror  1112   a ) and the second pulse (transmitted through mirror  1112   a ) can be adjusted. 
     The system of  FIG.  11 A  can be adjusted to various configurations, as shown in  FIGS.  11 B,  11 C and  11 D , and can be simulated to investigate the OPDs and temporal delays that are attainable. In the simulations, the laser pulse is taken to be Gaussian, collimated, unpolarised, with a wavelength of 532 nm, and composed by a number of rays equal to 10 4  to achieve statistical significance. In  FIG.  11 A , a distance d=150 mm causes the transmitted pulse to be reflected 4 times and this leads to an OPD of 1.13 m and a corresponding Δt=3.8 ns. In  FIG.  11 B , a distance d=300 mm causes the transmitted pulse to be reflected 21 times and leads to an OPD of 6.75 m and a corresponding Δt=22.5 ns. In  FIG.  11 C , a distance d=400 mm causes the transmitted pulse to be reflected 28 times and leads to an OPD of 12.46 m and Δt=41.5 ns. 
     As the distance d increases and as the number of reflections increase, the tolerances required for the mechanical alignment of the optical system become more demanding. This is of the order of ˜1.5 mm and ˜2° rotation angle (x,y) for the layout displayed in  FIG.  11 A , ˜1 mm and ˜1° angle for the layout displayed in  FIG.  11 B  and ˜0.5 mm and ˜0.5° angle for the layout displayed in  FIG.  11 C . The required alignment limits the OPD that can be achieved. Nevertheless, such alignments are readily attainable using the systems of the present disclosure and temporal delays Δt on the order of 50 ns can therefore be achieved. The temporal delays achieved using the multipass cell therefore can be 1 ns or greater (for example up to 10 ns, up to 50 ns, up to 80 ns, up to 100 ns, up to 150 ns, or greater than 150 ns), or 5 ns or greater (for example up to 10 ns, up to 50 ns, up to 80 ns, up to 100 ns, up to 150 ns, or greater than 150 ns), or 10 ns or greater (for example up to up to 50 ns, up to 80 ns, up to 100 ns, up to 150 ns, or greater than 150 ns), or 50 ns or greater (for example up to 80 ns, up to 100 ns, up to 150 ns, or greater than 150 ns), or 80 ns or greater (for example up to 100 ns, up to 150 ns, or greater than 150 ns), or 100 ns or greater (for example up to 150 ns, or greater than 150 ns). Shorter delays, for example, 0.1 ns or greater can also be obtained depending on the cell design parameters. 
     To reduce the stringency of the alignment requirements, it is possible to increase the size of the right angle mirrors  1105 A and  1105 B such that their segment size (the non-hypotenuse dimension) is 50 mm and to increase the size of the spherical mirror  1107  to a diameter of 75 mm. This relaxes the mechanical tolerance requirements and allows higher OPDs to be obtained with comparable distances d. An example of such layout is depicted in  FIG.  11 D , where a distance d=400 mm causes the transmitted pulse to be reflected 31 times and leads to an OPD of 25.30 m and a corresponding Δt=84.3 ns. The tolerances of this layout are ˜1 mm and 1° (x,y), approximately. Hence, temporal delays on the order of 100 ns (and higher) are readily attainable. 
     In many applications (e.g. in double-pulse LIBS experiments), it is important that two pulses are incident on the same position (for instance, on a sample&#39;s surface). In order to verify the effectiveness of the double-pulse systems, a beam profile study of the example displayed in  FIGS.  11 A to  11 D  can be performed, the results of which are displayed in  FIG.  12 A . The relative beam irradiance is displayed using a colour palette, spanning red (high irradiance) to blue (low irradiance). The results show an excellent circular Gaussian profile in both the x-, and y-dimensions, as shown in  FIG.  12 A . The profile in  FIG.  12 A  is for a circular aperture.  FIG.  12 B  shows an equivalent beam profile study for a system that is identical to the system used for  FIG.  12 A  except in that the splitting of the laser pulse is performed using parallel mirrors rather than a circular aperture. It can be seen from  FIG.  12 B  that a non-circular aperture leads to a deterioration in the quality of the superimposed pulses. Hence, a circular aperture for splitting a single laser pulse is preferred. In each of  FIGS.  12 A and  12 B , in order to aid visualisation, the graphs display the two pulses superimposed on one another, irrespective of the time required for each of them to hit the target. 
     In  FIG.  13   , there is depicted a double-pulse laser-induced breakdown spectrometry system that operates according to the principles described previously. The LIBS system of  FIG.  13    uses a double-pulse laser system such as that depicted in  FIGS.  11 A to  11 D . The system of  FIG.  13    comprises a multipass cell  1300 , which may be any multipass cell described previously, and which comprises a first reflector arrangement  1305  comprising two prisms  1305 A and  1305 B and a spherical mirror  1307  defining a cavity  1315 . 
     The system comprises a laser source  1310 , which is capable of emitting a single laser pulse  1314 . The system also comprises an optical arrangement  1312 , which comprises a number of optical elements  1312   b - d  for guiding light to the cell  1300  and then from the cell  1300  to a sample  1316 . The optical arrangement is also configured to generate first and second laser pulses from the single laser pulse  1314  by virtue of an optical splitting device  1312   a , which is a reflective surface having an aperture on the first reflector arrangement  1305  of the cell  1300 . The optical splitting device  1312   a  is integrally formed with the first reflector arrangement  1305  of the cell  1300 . The optical arrangement  1312  also comprises rotatable mirrors  1312   c  and lens  1312   d  for guiding and focusing laser pulses from the cell  1300  to a sample  1316 . 
     It can be seen in  FIG.  13    that when a laser pulse  1314  is emitted by the laser source  1310 , it is guided by a mirror of the optical arrangement  1312   b  to the cell  1300 . As described previously, part of the single pulse  1314  is reflected by the optical splitting device  1312   a  to form a first laser pulse and part of the single pulse  1314  passes into the cavity  1315  of the cell  1300  to be delayed with respect to the first pulse, thereby forming a second laser pulse. 
     Once the first and second pulses are respectively reflected from the splitting device  1312   a  and emerge from the cell  1300 , they are guided by further mirrors of the optical arrangement  1312   b  to rotatable mirrors  1312   c , which can fine-tune the direction of the pulses such that they are directed to the lens  1312   d . The lens  1312   d  then focuses the pulses such that they impact a point on the sample  1316 . 
     The first laser pulse impacts the sample  1316  and generates a plasma  1317  from the surface of the sample  1316  and the second laser pulse then impacts the plasma  1317  to increase its temperature and additionally impacts the surface to generate further plasma. The first and second pulses thus cause the generation of the plasma  1317  and the subsequent emission of plasma light  1318  from the plasma  1317 . The plasma light is reflected by a mirror  1312   e  that is positioned near the sample. The mirror  1312   e  guides the plasma light to a detector (e.g. a spectrograph)  1319  for analysis of the emissions. The mirror  1317  may be considered to be part of the optical arrangement  1312  or may be a separate optical arrangement. 
     In the specific example depicted in  FIG.  13   , the mirrors  1312   c  are Galvo mirrors (e.g. of a motorised dual-axis galvo system that allows scanning of the position of the laser pulses across the surface in two dimensions, for example to enable surface mapping of the sample) and the lens  1312   d  is an f-theta lens. However, other types of adjustable mirror and focussing elements may be used. Furthermore, any number of mirrors and/or lenses can be used within the optical arrangement  1312  and additional plasma mirrors  1312   e  may be used (e.g. to direct emitted light  1318  from the plasma to one or more further detectors, which may be a different type to the detector  1319 ). Moreover, the optical splitting device  1312   a  of the optical arrangement  1312  may be replaced by a beamsplitter arrangement, such as the arrangement depicted in  FIG.  2   . 
     Hence, it can be seen that first and second laser pulses can be generated using beamsplitters or by mechanical splitting. Therefore, in generalised terms, the optical arrangements of the disclosure may comprise an optical splitting device (e.g. a mechanical beamsplitter rather than a conventional beamsplitter) for generating the first and second laser pulses by splitting a single laser pulse. The optical splitting device may be attached to or integral with the multipass cell. For example, the optical splitting device may be on an exterior surface of the multipass cell. The multipass cell may comprise first and second reflector arrangements defining an optical cavity, and the optical splitting device may be on an exterior surface of one of the first and second reflector arrangements. 
     The optical splitting device may comprise a reflective surface having an aperture through which at least a portion of a laser pulse can pass. The reflective surface of the optical splitting device may be substantially planar. When prisms are used for the first reflector arrangement, it is straightforward to affix a reflective surface to the rear side, facilitating easy manufacturing of the advantageous devices disclosed herein. The aperture of the optical splitting device may be positioned centrally or substantially centrally (e.g. closer to the centre than the edge) on the reflective surface. The centre of the reflective surface may coincide with the aperture. Thus, when prisms are used, the splitting device may allow half of the light through a slit between the prisms and into the cell, whilst diverting the other half of the light away from the cell. The aperture of the optical splitting device is preferably circular. Circular apertures allow the subsequent laser pulses to exhibit a high degree of spatial coherence. The aperture of the optical splitting device is preferably aligned with an aperture of the multipass cell (e.g. the aperture for allowing light into the cell). 
     Hence, in generalised terms, the optical arrangement is preferably configured to direct a single laser pulse towards the aperture of the optical splitting device such that a portion of the single laser pulse passes through the aperture of the optical splitting device and into the multipass cell, thereby generating the second laser pulse, and a portion of the single laser pulse is reflected by the reflective surface of the optical splitting device, thereby generating the first laser pulse. This allows a high proportion of the energy of the light to be conserved, as minimal energy is lost when light reflects from a reflective surface or when light passes through an aperture. Thus, such an arrangement is highly efficient. Moreover, two pulses are generated and a temporal delay between the pulses can readily be applied (which may be adjustable) to the pulse that enters the cell. The optical arrangement may be configured to direct the single laser pulse towards the edge of the aperture such that half of the light passes therethrough. 
     The optical arrangements of the disclosure may comprise: an optical splitting device for generating the first and second laser pulses by splitting a single laser pulse; and/or one or a plurality of unpolarising beamsplitters for generating the first and second laser pulses; and/or one or a plurality of polarising beamsplitters for generating the first and second laser pulses. 
     As noted previously, the angle at which light enters the multipass cells of this disclosure can be used to control the number of times light traverses the reflector arrangements of the cell and hence the optical path length. Thus, in the described embodiments, the light source may be capable of changing the direction at which light enters the cell (e.g. by being rotatable or by being rotatably mounted). Alternatively, further optical elements (e.g. adjustable mirrors) may be provided to allow the angle of light entering the cell to be varied. The optical arrangement may be configured to guide the first and second laser pulses to the sample along collinear paths. The detector can be any type of detector, including a spectrograph, a photodiode, a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS) camera, an intensified charge-coupled device (ICCD), an electron multiplying CCD, or one or more microchannel plate detectors. The detector preferably allows detection of light as a function of its wavelengths. 
     Hence, in generalised terms, the systems of the present disclosure may also comprise: any of the multipass cells described previously; wherein the optical arrangement is configured such that the angle at which light is directed into the multipass cell is adjustable. The angle may be defined relative to an axis defined by the multipass cell (e.g. a longitudinal axis, such as the axis extending between the centres/midpoints of the first and second reflector arrangements). The angle between the direction in which light is directed into the multipass cell and the longitudinal axis defined by the multipass cell may be: from 0° to 20°; from 1° to 15°; or from 2° to 10°. If an aperture of the cell are perpendicular to the axis defined by the cell, then the angle at which light enters the cell may instead be expressed as relative to the normal to the aperture, because in such a case the normal to the aperture would be parallel to the longitudinal axis of the cell. 
     The provision of an adjustable angle allows the optical path length to be controlled whilst retaining a stable configuration and the advantages associated with such stability. Such optical arrangements can be provided independently of a detector or a light source. In other words, the optical arrangements and the multipass cells of the disclosure can be provided together, for use with any detector and/or light source. The optical elements may be affixed to the multipass cell (e.g. attached to the outside of the cell) or formed integrally with the cell housing. 
     As noted previously, various light sources can be used with the optical systems disclosed herein. In generalised terms, the present disclosure provides an optical system for generating first and second light components from spatially coherent light (e.g. from light from a coherent light source), comprising a multipass cell arranged to delay the second light component with respect to the first light component, wherein the multipass cell comprises first and second reflector arrangements defining an optical cavity in which the delayed second light component is reflected back and forth multiple times between the first and second reflector arrangements to provide a temporal delay between the first and second light components of 1 ns or greater. The multipass cell can be any of the cells described herein. The first and second light components can be generated using any of the beamsplitting techniques described herein, and the first and second light components may be, for example, pulses of light. Examples of coherent light sources suitable for use with such optical systems include lasers, or partially coherent light sources such as LED light or certain X-ray beams. In some cases, it is also possible to create coherent light by passing light (e.g. monochromatic light from an emission line of a mercury-vapour lamp) through a pinhole spatial filter. 
     The present disclosure also provides methods for manufacturing the systems, devices, multipass cells and optical arrangements described herein. For instance, a method for manufacturing a multipass cell may comprise providing: a first reflector arrangement; and a second reflector arrangement; wherein the first reflector arrangement is configured such that light incident on the first reflector arrangement is at least partially retroreflected towards the second reflector arrangement. The method of manufacture may further comprise providing any of the features of the multipass cell (e.g. any structural features) described herein. Methods for manufacturing the systems and devices may comprise providing any structural features described herein. 
     The principle of splitting a beam or pulse of light using an aperture is advantageous independently of its use in the double-pulse systems described herein. Such systems do not cause significant absorption of the energy of the light to be split, as noted previously. 
     The following numbered clauses provide illustrative examples of optical systems comprising such mechanical beam splitters. The light in the numbered clauses may be continuous light (e.g. a beam) or it may pulsed light (e.g. a laser pulse). 
     1. An optical system for splitting light into first and second light components, the optical system comprising:
         an optical splitting device comprising a reflective surface having an aperture through which light can pass; and   an optical arrangement that is configured to direct light towards the aperture of the optical splitting device such that a portion of the light passes through the aperture, thereby generating the second light component, and a portion of the light is reflected by the reflective surface, thereby generating the first light component.
 
2 The optical system of clause 1, wherein the reflective surface is substantially planar.
 
3 The optical system of clause 1 or clause 2, wherein the aperture is positioned substantially centrally on the reflective surface.
 
4. The optical system of any preceding clause, wherein the aperture is circular.
 
5. The optical system of any preceding clause, wherein the optical arrangement comprises one or more reflective surfaces for guiding light towards the optical splitting device.
 
6. The optical system of any preceding clause, wherein the optical arrangement comprises one or more focusing elements for focusing light towards the optical splitting device.
 
7. The optical system of any preceding clause, wherein the optical arrangement is configured to direct the light towards the edge of the aperture such that half of the light passes therethrough.
 
8. The optical system of any preceding clause, wherein the optical arrangement is configured such that the angle at which light is directed towards the aperture is adjustable.
 
9. The optical system of any preceding clause, wherein the size of the aperture is adjustable.
 
10. The optical system of any preceding clause, further comprising a light source configured to direct light towards the optical arrangement.
 
11. The optical system of clause 10, wherein the light source is a laser.
 
12. The optical system of clause 11, wherein the laser is a pulsed laser.
       

     The optical system for splitting light into first and second light components may be as described in, for example,  FIGS.  11 A to  11 D ,  FIGS.  12 A and  12 B  and  FIG.  13   . 
     It will be appreciated that many variations may be made to the above apparatus and methods whilst retaining the advantages noted previously. For example, whilst the above embodiments have been described mainly with reference to planar reflective surfaces in the context of providing retroreflective or partially retroreflective surfaces, it will be understood that any material exhibiting retroreflectivity may be used. Moreover, any reflecting surface in this disclosure may be fully reflective or partially reflective. 
     The disclosure has been described with reference to generic lasers and it will be appreciated that any laser can be used with the systems and cells described herein. For instance, whilst a tuneable diode laser is preferred, any solid-state, gas, liquid, chemical, metal-vapour, dye or semiconductor laser may be used. Other preferred examples include Nd:YAG lasers, CO2 lasers, Excimer lasers and Ruby lasers. 
     It will also be understood that although the disclosure has been described with reference to particular types of devices and applications, and whilst the disclosure provides particular advantages in such cases, as discussed herein the disclosure may be applied to other types of devices and applications. For instance, the multipass cells of this disclosure may be employed in any scenario in which precise control over the optical path length of light is required. 
     Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and, where the context allows, vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as “a” or “an” (such as a laser pulse or a reflector) means “one or more” (for instance, one or more laser pulses, or one or more reflectors). Throughout the description and claims of this disclosure, the words “comprise”, “including”, “having” and “contain” and variations of the words, for example “comprising” and “comprises” or similar, mean “including but not limited to”, and are not intended to (and do not) exclude other components. 
     The use of any and all examples, or exemplary language (“for instance”, “such as”, “for example” and like language) provided herein, is intended merely to better illustrate the disclosure and does not indicate a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure. 
     Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise. Moreover, where a step is described as being performed after a step, this does not preclude intervening steps being performed. For instance, if a laser pulse is described as being reflected from a first surface to a second surface, this does not preclude the laser pulse being reflected by additional surfaces before reaching the second surface. 
     All of the aspects and/or features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the disclosure are applicable to all aspects and embodiments of the disclosure and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).