Self-mixing interferometry

Abstract

A self-mixing interferometer configured to monitor particulate material within a monitored region of space comprises a laser cavity assembly 1A and an optical assembly 1B configured to bathe the monitored region with laser light of the interferometer possessing optical wavefronts comprising one or more rapid or discontinuous changes in optical phase within the monitored region. A laser monitoring unit 1C is configured to acquire an interferometric signal 10 generated by the interferometer in response to light returned to the laser cavity assembly from said wavefronts by said particulate material. A processing module 1D is configured to determine a property of the particulate material within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal.

Description

BACKGROUND The self-mixing effect is a form of laser feedback interferometry. It occurs within a laser resonator cavity due to the deliberate mixing of the intracavity electromagnetic wave with an electromagnetic wave that has been emitted from the resonator cavity and subsequently reinjected into the resonator cavity after interaction in an “external cavity”. The external cavity is simply the region of space through which the outward-and-retum path of the emitted electromagnetic wave passes before re-entering the resonator cavity. The laser resonator cavity plus the external cavity collectively define an interferometer, and the external cavity serves as an interferometer arm of the interferometer. Changes in the optical path length of this interferometer arm reveal themselves as modulations in the optical power within the laser resonator cavity, or as modulations in the voltage across the drive terminals of the laser cavity. This phenomenon is typically referred to as self-mixing interferometry (SMI), but is also referred to by other names, such as: laser feedback interferometry, back-scatter-modulation, induced-modulation, self-coupling, optical feedback, external feedback, and auto-dyning. Here we refer to the phenomenon as self-mixing interferometry (SMI), and it is to be understood that this includes a reference to the other names given to this phenomenon as identified above. This phenomenon may occur in lasers regardless of their type, and may be implemented using gas lasers, in-plane semiconductor diode lasers, vertical-cavity surface-emitting lasers (VCSEL), mid-infrared and terahertz quantum cascade lasers (QCLs), inter-band cascade lasers, fibre lasers and fibre ring lasers, solid-state lasers, micro-ring lasers, and quantum dot lasers, for example. The effect is observable when typically as little as 0.1%, or less, of emitted radiation is reflected back into a laser resonator cavity from an external object some distance from the laser resonator cavity. Using a photosensitive detector to collect radiation emitted from one partially transmissive mirror of the laser resonator cavity, displacement of the object in the external cavity may be sensed using radiation reinjected into the laser resonator cavity through the partially transmissive second mirror (i.e., the laser output mirror) after reflection from the object. The phenomenon has also been proposed for use in measuring physical parameters that are capable of altering an optical path length in the external cavity of the self-mixing interferometer. These parameters include physical size and velocity measurement of reflecting objects within the external cavity. The present invention has been devised in light of the above considerations. SUMMARY An SMI system operates on the following principle schematically illustrated in Fig. 1A and Fig. IB. Laser light 2 is emitted from the laser resonator cavity of a laser 1 and is transmitted as an electromagnetic wave 3 to an external target object 4 from which it is partially reflected, or back-scattered. A portion of the reflected or back-scattered light is transmitted back to the laser as a returned electromagnetic wave (5 A or 5B) where a portion of it 6 re-enters the resonator cavity of the laser 1. Inside the resonator cavity of the laser, the re-entered light mixes with resident light of the resonator cavity that exists in one or more of the resonant modes of the laser resonator cavity (see Mixing Modulation 7A, 7B). As a result of the mixing of light within the laser resonator cavity, the re-entered light perturbs the electromagnetic field within the resonator cavity. This perturbation becomes measurable as consequential perturbations (7A, 7B) to the operating parameters of the laser. Consequently, the branch of the laser light path extending from the laser resonator cavity to the target object may be regarded as a first arm (an external arm) of an interferometer for light output from (and returned to) the cavity, and the laser resonator cavity may be regarded as a second arm (an internal arm) of the interferometer for light remaining within the cavity. Light waves (3, 5A, 5B) propagating along these two arms of the interferometer are brought together within the laser resonator cavity where they mix and cause constructive or destructive interferences generating perturbations (e.g., mixing modulations 7A, 7B) in operating parameters of the laser. Perturbations in operating parameters influencing the gain of the laser result in measurable perturbations in the optical power of the laser and the voltage at the drive terminals of the resonator cavity. Variations in optical power may be monitored using a photodetector (‘PD’; Fig. IC) to indirectly measure the optical power of light sampled from the optical resonator cavity of the laser, or variations in the voltage of drive terminals of the resonator cavity can be monitored directly. Indirect measurement of optical power may be achieved either by use of a laser having an optical resonator cavity in which both mirrors of the cavity are partially transmissive thereby to release, from each end of the resonator cavity, a defined proportion of the light within resonator cavity. Of course, the output laser beam, 2, is the result of the front mirror of the optical cavity releasing a defined first proportion of the light from within resonator cavity. In addition, the back mirror of the resonator cavity may also release a defined second proportion of the light from within resonator cavity resulting in a second output of laser light (2B; Fig. IC) which is received and monitored by the photodetector. J1 + a2 Referring to Fig. IC, in a so-called “three-mirror model” of SMI, the laser resonator cavity 1A is represented as the “internal” cavity with length Lint, refractive index nint. Light 2A within this internal cavity has a round-trip propagation time Tlnt. Light 2 leaves the internal cavity through the partially transmissive front mirror M2, of reflectivity R2, and traverses the external cavity of length Lext where it reflects from the external mirror M3, of reflectivity R. This external light has a round-trip propagation time Text. A portion 6 of this light re-enters the laser resonator cavity 1A through the front mirror M2 and mixes with the electromagnetic field inside the laser resonator cavity. A small portion 2B of the intra-cavity light exits the laser resonator cavity 1A through the back mirror Mi and is detected by a monitoring photodetector (PD). When a single reflection from M3 is considered, the rate at which re-injected light 6 is coupled into the laser cavity 1A given by: 1 koc(1-7?2)-- ^int The “feedback level” (C) within the resonator cavity is given by: ., __ ext C — / € ' ^int Here, the term a is known in the art as the “linewidth enhancement factor”. It is known in the art that the optical coupling within the resonator cavity of the laser results in the so-called “excess phase equation”: <Pfb = <Ps + C sm((pFB + tan-1 a) The term Vpb represents the phase accumulated by the electromagnetic field of the laser on feedback (FB) transmission through the external cavity. The term ips corresponds to the phase accumulated by transmission through the external cavity if the laser were not experiencing optical feedback. The feedback level C determines the degree of nonlinear coupling within the laser resonator cavity. In the following discussions, we will consider the “weak feedback” regime in which C 0 such that no nonlinear coupling exist. The observable quantities are either a variation in laser power or a variation in voltage across the laser terminals. The dependence of these quantities on the phase term <pFB can be found as follows. Vfb = Vs + C sin(<pFB + tan-1 a) Phaser = A + B cos[<ps + C sin(<pFB + tan-1 cr)] However, when there is only a weak feedback intensity in the light returned to the laser cavity from the mirror M3 the coefficient C becomes negligible, and one may write: Phaser = A+B COS (ps Here, the amplitude of the modulation B kk, and the resulting SMI signal, for a given laser resonator cavity, depends the reflectivity of the external mirror Ms and the nature of the accumulated phase (ps. Returning to Fig. 1 A, there is schematically shown a situation where the wavelength of the laser light is A and the accumulated phase (ps within the returned light wave 5 A corresponds to an even integer multiple, n, of half-wavelengths, and for simplicity we have set A = B, such that constructive interference modulates the laser power signal 7A, such that: Phaser = A + B COS(Tm) = A + B = 2A Returning to Fig. IB, there is schematically shown a situation where the scattering target object 4 has moved away from the laser resonator cavity 1 so as to increase the length of the external arm of the interferometer by A / 4 corresponding to one quarter of one wavelength of the laser light, noting that the wavelength of the laser light is A. As a result of this movement, the accumulated phase (ps within the returned light wave 5B corresponds to an odd-multiple, n + 1, of these halfwavelengths, such that destructive interference modulates the laser power signal 7B, such that: PLaser = A + B cos(n[n + 1]) = A — B = 0 It is to be understood that the schematic representations shown in Fig. 1A and Fig. IB are simplified representations provided to aid understanding of the principles of self-mixing interferometry. The invention employs self-mixing interferometry for detecting particles (particulate material) and properties thereof. In the following, several aspects of the invention are disclosed, and many desirable or preferred features are described. However, it is to be understood that the invention includes the combination of any one or more of the desirable or preferred features described below with any one or more of the aspects of the invention described below, except where such a combination is clearly impermissible or expressly avoided. In one aspect the invention proposes a particulate matter sensor based on the principle of selfmixing interferometry employing an interferometric laser providing electromagnetic wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region. The inventors have realised that this rapid or discontinuous changes in optical phase generates a consequential structure within an interferometric signal of a self-mixing interferometer. The inventors have realised that this structure may be exploited to reveal estimates of properties of the particulate matter causing the structure. These properties include, but are not limited to: the speed of the particulate matter within the monitored region relative to the interferometric laser; the direction of motion of the particulate matter within the monitored region relative to the interferometric laser; the concentration (e.g., number density) of particulate matter within the monitored region. In any aspect of the invention, the wavelength (A) of laser light of the self-mixing interferometer may be in the visible light range or in the near-infrared (NIR) range. For example, the wavelength (A) of laser light of the self-mixing interferometer may be in the range: 380nm <A <2.0jim. In other examples, the wavelength (A) of laser light of the self-mixing interferometer may outside of this range. The wavelength of the laser light of the self-mixing interferometer may be selected in preference to the range of particle that the apparatus is intended to preferentially detect. This is because the mechanism for the scattering of light from a particle is influenced by the ratio (a / X) of the wavelength (A) of scattering light and the radius (a) of the particle from which the light scatters. For example, if this ratio is much smaller than one (1) then Rayleigh scattering mechanisms dominate. "Mie scattering" mechanisms dominate in situations where the size of the scattering particles is comparable to the wavelength of the light, such that the ratio is comparable to one (1), rather than much smaller or much larger. However, geometrical optical mechanisms (GO) dominate when the ratio is much larger than one (1). By an appropriate choice of laser light wavelength, the user may select the most appropriate scattering mechanism to take place in respect of the particle size range to be sensed. The invention disclosed herein may be configured to monitor particulate material according to a ratio (a / X) of the wavelength (A) of the self-mixing interferometer and the radius (a) of the particles to be monitored which is, for example and without limitation, in the range: 0.1 <a / X <100. In a first aspect, the invention may provide a self-mixing interferometer configured to monitor particulate material within a monitored region of space comprising: a laser cavity assembly; an optical assembly configured to bathe the monitored region with laser light of the interferometer possessing one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region; a laser monitoring unit configured to acquire an interferometric signal generated by the interferometer in response to light returned to the laser cavity assembly from the optical wavefronts by the particulate material; a processing module configured to determine a property of the particulate material within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal. In this way, the inventors have realised that by providing one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase it is possible to generate one or more rapid or discontinuous changes in the phase of a waveform of the interferometric signal. From this, one is able to glean more information about particle properties efficiently. The bathing of the monitored region with laser light may comprise illuminating the monitored region continuously (e.g., with a continuous wave (CW) laser beam or beams) throughout a monitoring time period or interval, or may comprise illuminating the monitored region intermittently or periodically (e.g., with a succession of laser light pulses) throughout a monitoring time period or interval. A pre-specified shape or structure to the spatial distribution or pattern of rapid or discontinuous optical phase changes in the monitored region of space may be provided, if wanted. However, a pre-specification (i.e., a pre-knowledge) of that shape or structure may be unnecessary. The inventors have realised that is merely desirable to provide that the monitored region of space contains any suitable spatial pattern of rapid or discontinuous optical phase changes (“phase pattern”). This is because it is found to be desirable merely that the “phase pattern” is fixed and temporally unchanging within the monitored region of space, and is a suitable pattern in the sense that it causes one or more rapid or discontinuous changes in the phase of a waveform within the interferometric signal when particulate material traverses the “phase pattern” within the monitored region. In this way, one need not necessarily pre-specify the spatial structure of the “phase pattern” and, instead, may be agnostic about the details of that spatial structure and yet still provide a “phase pattern” that satisfies the condition of being able to reliably and reproducibly induce the required, monitorable one or more rapid or discontinuous changes in the phase of a waveform within the interferometric signal when particulate material traverses the “phase pattern”. The processing module may be configured to detect a sequence of phase changes in the phase of a waveform within the interferometric signal and to identify a property of the particulate material within the monitored region according to the detected sequence. In this way, a property of the particulate material may be associated with, or correlated to, a particular particle trajectory and speed through the “phase pattern”, both in terms of the direction of the trajectory and the speed of motion of the particle along it. A consequential sequence of phase changes in the phase of a waveform within the interferometric signal may be correspondingly unique to that particular particle trajectory and speed. The inventors have realised that this unique correspondence between the sequence of optical phase changes and interferometric signal phase changes may be used to uniquely identify a particle trajectory and speed within the monitored region of space in terms of the direction of the trajectory within that region of space and / or the speed of motion of the particle along it. The processing module may comprise a calibration data set comprising data describing a plurality of pre-defined sequences of changes in the phase of a waveform within the interferometric signal, and / or data describing a frequency spectrum thereof and / or a scalogram thereof. Data describing a frequency spectrum and / or a scalogram of a waveform of the interferometric signal comprising such phase changes thereby also comprises data describing the plurality of pre-defined sequences of changes in terms of corresponding changes (i.e., associated detectable structures) in the frequency spectrum or scalogram. Each pre-defined sequence may be associated with a unique respective particle parameter, or set of respective particle parameters, associated with (e.g., derived from) a particle traversing the “phase pattern” according to a particular respective trajectory and speed. The processing module may be configured to determine a property of the particulate material within the monitored region by identifying when a sequences of changes in the phase of a current waveform within the interferometric signal is sufficiently similar to a pre-defined sequence. A pre-defined sequence may thereby be known to be associated with specific, pre-calibrated particle parameters, and the processing module may be configured to determine that a particle associated with a detected sequences of changes in the phase of a current interferometric signal waveform are the same as the pre-calibrated particle parameters. The optical wavefronts of the optical wave may comprise successive optical wavefront sections, portions or segments, each extending in length in a direction transverse to the direction of optical propagation of the laser light, either substantially linearly (e.g., a flat wavefront) or according to a local curvature (e.g., an average curvature, or one uniform curvature along the wavefront section in question) which may be common at least to neighbouring successive such sections of optical wavefronts of the optical wave. Successive optical wavefront sections, portions or segments may be separated by intermediate regions that are each shorter in length (typically much shorter) than any of the sections, portions or segments they separate, and which each accommodate a respective one of the one or more rapid or discontinuous changes in optical phase. Put in other words, the shortest distance between an end of one optical wavefront section and the adjacent end of the nearest adjacent section of that wavefront, may be considered to be an intermediate region. If one were to notionally travel along the length of any given one of the optical wavefront sections, portions or segments, one would experience an unchanging optical phase (i.e., the definition of a wavefront). However, upon reaching an end of the given wavefront section, portion or segment, if one were to extrapolate beyond that end one would experience the aforementioned rapid or discontinuous change in the optical phase of the optical wave there. However, if one were instead to immediately shift in space fully through the intermediate region to the end (e.g., beginning) of the nearest adjacent section of the wavefront, then one would continue to experience the same unchanging optical phase as experienced when travelling along the previous section of the wavefront. In this way, each section, portion or segment of a given optical wavefront is shifted in space relative to a neighbouring section, portion or segment of the optical wavefront, such that the optical wavefront overall may comprise a succession of spatial shifts (e.g., jumps, steps, discontinuities or edges) along its overall length. A given rapid or discontinuous change in optical phase of the optical wave (e.g., as experienced by extrapolating beyond the end of a wavefront section in a continuing direction, rather than jumping to the next wavefront section) may be a change of between about tt / 4 radians and about n radians, such as a change corresponding to an advancement or retardation of an optical wavefront locally by a propagation distance of between one eighth and one half of the wavelength of the optical wave, respectively. The change in optical phase may be a change of between about n / 2 radians and about n radians and is preferably about n radians. The shortest distance (e.g., a linear distance, dL, as shown in Fig.4) between a given end of one optical wavefront section and the adjacent start of the nearest neighbouring section of that wavefront (e.g., the length of an intermediate region) may be substantially equal to the proportion (F) of the wavelength of the optical wave that corresponds to the value of the aforementioned change in optical phase of the optical wave at the given end (i.e., a change experienced by extrapolating beyond the given end of the wavefront section in a continuing direction to a position behind (or ahead of) and parallel to the adjacent wavefront section). The intermediate region, dL, may be considered to be the region of space across which the spatial shift, jump or step occurs and which is necessary to traverse in order that the optical phase is preserved in the sense that the optical phase at one end of intermediate region is the same as the optical phase at the other end of the intermediate region. For example, for an optical phase change of n radians, the shortest distance, or transition region, may be substantially equal to one half of the wavelength (i e., dL = V = 2 / 2) of the optical wave. More generally, the shortest distance may be a value in a range from the value of the aforementioned proportion, F, to about one quarter of the length (X) of either of the two sections of wavefront either side of the intermediate region (i.e., Y <dL <X / 4). For example, a rapid or discontinuous change in optical phase may be in respect of a change in the phase as between two points in the region of space where the phase change occurs (e.g., two end points of a linear region along which the full phase change takes place). For example, a change in optical phase may be considered as being rapid if a rate of change of optical phase with respect to a change in spatial position, x, (e.g., a spatial differential, d(p / dx) within the monitored region exceeds a suitable minimum threshold value (e.g., d(p / dx = 4tt / ,Y) One may quantify the maximum size of the transition region, dL, within the meaning of a “rapid” change in spatial position of a given wavefront when transitioning from one wavefront section to the next section as being such that: X / n <dL <aX / n, where a is a positive real number between 1.0 and 5.0, or between 1.0 and 4.0, or between 1.0 and 3.0, or between 1.0 and 2.0, or between 1.0 and 1.5. A discontinuous change in the optical phase of an optical wave is to be understood to include a reference to a change that is effectively discontinuous practically speaking. For example, when a has a value of 1.0, or something very close to 1.0, it may be considered to define a discontinuous change, practically speaking. A rapid or discontinuous change in the phase of a waveform within the interferometric signal may be in respect of a change in the phase as measured at two points in time in the region of the signal where the phase change occurs (e.g., two end points of the region required to complete the phase change). For example, a change in the phase of a waveform within the interferometric signal may be considered as being rapid if a rate of change of the signal phase value with respect to a change in temporal position (e.g., a time differential) within / along the signal waveform exceeds a suitable minimum threshold value sufficient to render the changes detectable within the interferometric signal (e.g., as producing a detectable artefact within a frequency spectrum, or wavelet scalogram, of the signal). A discontinuous change in the phase of a waveform within the interferometric is to be understood to include a reference to a change that is effectively discontinuous practically speaking. Accordingly, the interferometer may bathe particulate material with light possessing wavefronts having one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase which reflects, scatters or diffracts from the particulate material whilst that material is being bathed by the light, so as to cause returned light that mixes with native light in the resonator cavity of the interferometer to cause an interferometric signal waveform having one or more rapid or discontinuous changes in the phase of the waveform within at least a part of the interferometric signal. The discontinuous changes in the phase of the waveform may be expressed as a difference in phase as between immediately contiguous parts of (e.g., at contiguous, successive time intervals within) the waveform, such that the value of a phase of the waveform may change discontinuously or very rapidly, in an interval of time At, from a first phase value of 01 to a second phase value of 02 = 01 + A0 in which A0 » QAt where fl is the angular frequency of the interferometric signal waveform immediately prior to the occurrence of the discontinuity in the phase of that waveform. For example, the interferometric signal waveform may change from having a first phase value of 01 = 0(t = tl) a time t = tl, to having a second phase value of 02 = 0(t = t2) = 01± A0 a time t2 = tl + At, in which A0 » flAt where fl is the angular frequency of the interferometric signal waveform at time t = tl. The interval of time At during which the rapid or discontinuous change in phase takes place is preferably much shorter than the interval of time, AT, required for the same amount of change to take place in the phase of the interferometric signal waveform in the absence of the discontinuity. In other words, At « AT. The optical assembly may comprise one or more optical elements configured to receive light from the laser cavity and to create therefrom the laser light possessing the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region. The one or more optical elements may comprise a metalens, or metasurface, configured to manipulate the light received from the laser cavity so as to form the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase. Lim, Soon Wei Daniel &Park, Joon-Suh &Meretska, Maryna &Dorrah, Ahmed &Capasso, Federico. (2021). “Engineering Phase and Polarization Singularity Sheets’". Nature Communications. 12. 4190. The Supplementary Information accompanying this publication provides a method for designing a metalens, or metasurface capable of providing optical wavefronts comprising one or more rapid or discontinuous changes in optical phase. This method discloses a method for designing one continuous phase singularity shape positioned at a location in space. For example, Lim et al. chose to make a heart-shaped phase singularity centred in a region of space. The distribution of optical phases in the space around that heart shape also comprises wavefronts comprising one or more rapid or discontinuous changes in optical phase. This distribution of optical phases may be provided in the monitored region of space if such a metalens, or metasurface, is employed in the invention. The inventors have realised that it is not necessary to precisely design a metalens or metasurface in order to generate a precisely pre-defined or specified shape or structure to the pattern of rapid or discontinuous optical phase changes in the monitored region of space. It is merely desirable to provide that the monitored region of space contains rapid or discontinuous optical phase changes (“phase pattern”) at all. This is because it is desirable merely that the “phase pattern” is fixed and temporally unchanging within the monitored region of space. The phase pattern may be highly asymmetric such that a sequence of optical phase changes ‘seen’ by a particle passing through the monitored region of space may be unique to that particular particle trajectory, both in terms of the direction of the trajectory and the speed of motion of the particle along it. In this way, a corresponding (consequential) sequence of phase changes in the phase of a waveform within the interferometric signal may be correspondingly unique to that particular particle trajectory and speed. The inventors have realised that this unique correspondence between the sequence of optical phase changes and interferometric signal phase changes may be used to identify a particle trajectory within the monitored region of space in terms of the direction of the trajectory within that region of space and / or the speed of motion of the particle along it. In examples, a static (temporally unchanging) “phase pattern” may be provided in the monitored region of space. The “phase pattern” may be a calibrated “phase pattern” in the sense that the self-mixing interferometer may be configured to detect, in use, when particular phase changes in a newly-generated interferometric signal correspond to known interferometric signal phase changes in a previously-generated interferometric signal known to be associated with specific, pre-calibrated particle parameters. Calibration of the “phase pattern” may be performed by passing a suitably large number of different calibration particles of specified / controlled particle size, and each individually of a specified / controlled speed and trajectory (i.e., particle parameters) through the “phase pattern” and, for each such individual set of particle parameters, recording the SMI signal (e.g., and / or frequency spectrum or wavelet scalogram) generated by the self-mixing interferometer as one respective calibration SMI signal (e.g., and / or calibration frequency spectrum or calibration wavelet scalogram) amongst a plurality of calibration signals each associated with a unique set of respective calibration particle parameters. Every calibration SMI signal (and / or calibration frequency spectrum or calibration scalogram) may then be associated with a unique set of respective particle parameters to form a calibration data set. The processing module may be configured with either: (a) An algorithm configured to compare a newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) with calibration SMI signals (and / or calibration frequency spectra or calibration scalograms), to select from amongst them a candidate calibration SMI signal (and / or calibration frequency spectra or calibration scalograms) that is found to most closely resemble the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof), and to generate an output identifying that the particle parameters associated with the particle responsible for the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely approximate the particle parameters of the candidate calibration SMI signal (and / or calibration frequency spectra or calibration scalograms). The processing module may output those particle parameters, or store them for use. (b) Machine learning (ML) algorithm(s) that has / have been trained, using the calibration data set, to identify when a newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely resembles a calibration SMI signal (and / or calibration frequency spectrum or calibration scalogram) and to generate an output identifying that the particle parameters associated with the particle responsible for the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely approximate the particle parameters of the calibration SMI signal (and / or calibration frequency spectra or calibration scalograms) in question. The processing module may output those particle parameters, or store them for use. Devices other than metalenses, or metasurfaces can be used to form the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase. For example, a spiral phase plate may be used to generate light, by reflection of incident light, having so-called “Optical Angular Momentum” (0AM). In particular, in a quantum theory, individual photons may have integer values of 0AM which, in a beam of light, corresponds to optical waves having a helical wavefront with an optical vortex in the centre at the beam axis. 0AM states of arbitrary integer values of 0AM may be generated from an incident beam of light by transmitting or reflecting the incident beam of light from a spiral phase plate, a spatial light modulator or a “q-plate”, as would be readily apparent to the person of ordinary skill in the art. For example, a spiral phase plate may comprise a glass or plastic plate having a thickness configured to increase in a spiral pattern in order to imprint a phase gradient on light passing through it. 0AM states of arbitrary integer values of 0AM may be generated from an incident beam of light by diffracting the beam of light via a diffraction grating. The effect is to modify the phase of the light. A diffraction grating consisting of parallel lines would produce diffracted light having zero 0 AM. However, diffraction grating consisting of parallel grating lines containing a "fork" dislocation, with the number of grating lines above the start of the dislocation being one larger than below the number of lines below it, will produce a diffracted beam having a non-zero 0 AM state. Examples include, but are not limited to, the following: Beijersbergen, M. W., Coerwinkel, R.P.C.; Kristensen, M.; Woerdman, J.P. (1994). "Helical-wavefront laser beams produced with a spiral phase plate”. Optics Communications. 112 (5--6): 321. Soskin, M.; Gorshkov, V.; Vasnetsov, M.; Malos, J.; Heckenberg, N. (1997). "Topological charge and angular momentum of light beams carrying optical vortices". Phys. Rev. A. 56 (5): 4064. Karimi, E.; Piccirillo, Bruno; Nagali, Eleonora; Marrucci, Lorenzo; Santamato, Enrico (2009). "Efficient generation and sorting of orbital angular momentum eigenmodes of light by thermally tunedq-plates". Applied Physics Letters. 94 (23): 231124. Du, L., Man, Z., Zhang, Y. et al. "Manipulating orbital angular momentum of light with tailored in-plane polarization states'. Sci Rep 7, 41001 (2017). The optical assembly may be configured to bathe the monitored region with laser light of the interferometer possessing optical wavefronts having different directions at different respective locations within the monitored region. This relative difference in directions may present itself in the form of electromagnetic wavefronts that are curved in space (i.e., relative to a fixed coordinate system of the interferometric laser) and therefore are curved relative to a linear path of particular matter through the monitored region. Alternatively, or in addition, this relative difference in directions may present itself in the form of electromagnetic wavefronts that are themselves substantially flat in space (i.e., relative to a fixed coordinate system of the interferometric laser) and therefore are curved relative to a curved path of particular matter through the monitored region (i.e., as seen in the moving spatial reference frame of the particulate matter, the flat electromagnetic wavefronts appear to be curved). Alternatively, or in addition, this relative difference in directions may present itself in the form of electromagnetic wavefronts that are themselves substantially flat or curved in space (i.e., relative to a fixed coordinate system of the interferometric laser) but are emitted by the electromagnetic wave source (i.e., a laser) in different relative directions in the monitored region and therefore move in different directions at different regions of a monitored region of space relative to the path of the particulate matter to be sensed. The inventors have realised that this difference in directions, as between wavefront motion and particulate matter motion, generates a consequential structure within an interferometric signal of a self-mixing interferometer. The inventors have realised that this structure may be exploited to reveal estimates of properties of the particulate matter causing the structure. These properties include, but are not limited to: the speed of the particulate matter within the monitored region relative to the interferometric laser; the direction of motion of the particulate matter within the monitored region relative to the interferometric laser; the concentration (e.g., number density) of particulate matter within the monitored region. The processing module maybe configured to determine a property of the particulate material within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal. The processing module may be configured to determine a property of the particulate material within the monitored region according to a continuous change in the frequency of the waveform. Desirably, in any aspect of the invention, the waveform within at least a part of the interferometric signal comprises a chirped waveform. A chirped waveform may possess a continuously changing frequency during at one or more respective finite time intervals within the waveform, or throughout substantially the entire duration of the waveform. The continuous change may comprise a continuous increase or a continuous decrease in the frequency of the waveform, or a mixture of both during different respective intervals of time within the waveform. It has been found that the frequency of the waveform and the manner in which the frequency changes, contains useful information about properties of a detected item of particulate material. The use of laser light within the monitored region possessing wavefronts having different directions at different respective locations within the monitored region greatly assists in obtaining this information. The optical assembly may be configured to bathe the monitored region with a static divergent and / or convergent beam of said laser light possessing a curved wavefront defined by an optical wave comprising the one or more rapid or discontinuous changes in optical phase (i.e., curved at locations other than where the phase discontinuity resides), in which the monitored region comprises regions other than the focal region of the laser light. The optical assembly may be configured to bathe the monitored region with a beam of the laser light possessing a substantially flat wavefront defined by an optical wave comprising said one or more rapid or discontinuous changes in optical phase (i.e., flat at locations other than where the phase discontinuity resides), and to move the flat wavefront across the monitored region to a plurality of different directions. The processing module may be configured to determine a property of the particulate material within the monitored region according to a wavelet transformation of the interferometric signal. The interferometer may be configured to generate the interferometric signal comprising a voltage signal to be acquired by the laser monitoring unit, wherein the voltage signal corresponds to a voltage across electrical drive terminals of a laser cavity of the laser cavity assembly and comprises a voltage signal waveform in response to movement of the particulate material along a path within the monitored region of space. The interferometer may be configured to generate the interferometric signal comprising an optical output power signal to be acquired by the laser monitoring unit, wherein the optical output power signal corresponds to an optical output power of a laser cavity of the laser cavity assembly and comprises an optical output power signal waveform in response to movement of the particulate material along a path within the monitored region of space. The property of the particulate material may comprise a property of the path thereof within the monitored region. The property of the path may comprise one or more of a distance to the particulate material relative to the interferometer; a speed of the particulate material relative to the interferometer; a direction of the particulate material relative to the interferometer. The speed of the particulate material may comprise a magnitude of a two-dimensional velocity vector, or a magnitude of a three-dimensional velocity vector describing a velocity of the particulate material in two dimensions of space, or in three dimensions of space. In any aspect of the invention, the processing module may be configured to determine a concentration of the particulate material within the region of space. The optical assembly may be configured to bathe the monitored region with a static divergent and / or convergent beam of said laser light possessing a curved wavefront defined by the optical wave comprising one or more optical phase discontinuities (i.e., curved at locations other than where the phase discontinuity resides), in which the monitored region comprises regions other than the / a focal region of the laser light. The optical assembly may be configured to bathe the monitored region with a laser beam possessing an angle of divergence (or convergence) not less than 5 degrees, or preferably not less than 10 degrees, or preferably not less than 15 degrees, or preferably not less than 20 degrees, or preferably not less than 25 degrees, or preferably not less than 30 degrees. The optical assembly may be configured to bathe the monitored region with a laser beam possessing an angle of divergence (or convergence) not greater than 50 degrees, or preferably not greater than 45 degrees, or preferably not greater than 40 degrees. The optical assembly may be configured to bathe the monitored region with a beam of the laser light possessing a substantially flat wavefront defined by the optical wave comprising one or more optical phase discontinuities (i.e., flat at locations other than where the phase discontinuity resides), moved across the monitored region to a plurality of different directions. The beam may scanned through a pre-set range of angular directions as a continuously moving beam thereby directing the wavefronts of the beam in different directions over each scan. Alternatively, or in addition, the beam may be directed successively, for a given finite duration of time (e.g., a dwell time) in each one of a succession of different static angular directions selected from a pre-set range of angular directions thereby directing the wavefronts of the beam in different directions during each finite duration of time. In a second aspect, the invention may provide a portable electronic device comprising the selfmixing interferometer as described above. In a third aspect the invention may provide an air purification device comprising the self-mixing interferometer described above. For example, the portable electronic device may comprise an air purification device. An air purification device may be any apparatus configured to purify or clean air by any means, such as any means readily available to the skilled person. For example, such a device may be configured to remove contaminants from the air in an environment (e.g., a room, etc.) to improve air quality in domestic, medical, industrial, or commercial areas and industries. Air purification may be performed by methods known in the art, such as (for example but without limitation): - by air filter purification whereby air is forced through a filter and particles are physically captured by the filter, or - by polarized-media electronic air cleaning which applies a voltage to establish the polarizing electric field for filtering particulate material, or - by ionisation purification methods whereby electrically charged air or gas ions are generated to attach to airborne particles which are then electrostatically attracted to a charged collector plate. Of course, other air purification methods and means are readily available to the skilled person. The self-mixing interferometer according to any aspect of the present invention may be used alone, or used within or conjunction with an air purification device, in order to determine properties or airborne particulate matter which may be used as parameters to determine any desired or appropriate measure of air purity or air quality. For example, the self-mixing interferometer may be configured within or upon the portable electronic device to monitor or detect one or more of: a concentration of particulate matter; a speed or velocity of particulate matter; a position or distance of particulate matter; direction of motion of particular matter. The self-mixing interferometer, or a portable electronic device, may be configured to use any one or more of these detected quantities as parameters in a determination, calculation or estimation of any desired or appropriate measure, quantification or definition of air purity or air quality. An appropriate measure, quantification or definition of air purity or air quality may include, for example (but without limitation): a concentration value (e.g., number of particles per unit volume); a number of particles within a monitored region (e.g., during a pre-set time interval); rate of motion (e.g., speed) of particulate material within a monitored region. The air purification device may comprise a self-mixing interferometer disposed at any one of: an air inlet of the air purification device for receiving air to be subject to a purification process; an air outlet of the air purification device for outputting air that has been subject to a purification process; an internal surface of the air purification device other than an at an air input or an air output, for monitoring air while it is being subjected to a purification process; an external surface of the air purification device other than an at an air input or an air output, for monitoring ambient air. In this way, the self-mixing interferometer may be employed to monitor not only the purity of ambient air, but also to monitor the efficacy of an air purification process (e.g., to control the operation, e.g., use and duration of use, of the air purification device accordingly). The self-mixing interferometer may be comprised within a non-portable air purification unit (e.g., for mounting to a wall, to a ceiling or a floor-standing device e.g., weighing more than, e.g., 20kg) such as for commercial or domestic areas and uses, or within a non-fixed moveable air purification unit (e.g., weighing less than, e.g., 20kg, or less than 10kg in weight). In a fourth aspect, the invention may provide wearable electronic device comprising the portable electronic device described above. In this way, the self-mixing interferometer may be employed by individuals to monitor the purity or quality of ambient air in their immediate environment, which may change as the user moves from place to place, or as time passes in one place. Thus, the self-mixing interferometer may be comprised within or upon a portable unit (e.g., weighing less than, e.g., 5kg, or less than 1kg, or less than 0.5kg, or less than 0.25kg, or less than 0.1kg in weight). Examples include a wrist-mounted electronic device, a smartphone device, a tablet device, a laptop computer device, or a bespoke air quality monitoring device. The portable unit may comprise parts (e.g., one or more straps, clips etc.) configured for attaching or mounting the portable unit upon the body of a person, or upon / within clothing. This permits ease of use such as for personal and individual uses. In a fifth aspect, the invention may provide a method for monitoring particulate material within a monitored region of space using self-mixing interferometry comprising: providing an interferometer comprising a laser cavity assembly and an optical assembly; by the optical assembly, bathing the monitored region with laser light of the interferometer possessing one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region; acquiring an interferometric signal generated by the interferometer in response to light returned to the laser cavity assembly from the optical wavefronts by the particulate material; by a processing module, determining a property of the particulate material within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal. The method may comprise, by the optical assembly, receiving light from the laser cavity and creating therefrom the laser light possessing the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region. The method may comprise detecting a sequence of phase changes in the phase of a waveform within the interferometric signal and identifying a property of the particulate material within the monitored region according to the detected sequence. A property of the particulate material may be associated with, or correlated to, a particular particle trajectory through the “phase pattern”, both in terms of the direction of the trajectory and the speed of motion of the particle along it. The method may include providing a calibration data set comprising data describing a plurality of pre-defined sequences of changes in the phase of a waveform within the interferometric signal, and / or data describing a frequency spectrum thereof and / or a scalogram thereof. The method may include determining a property of the particulate material within the monitored region by identifying when a sequences of changes in the phase of a current waveform within the interferometric signal is sufficiently similar to a pre-defined sequence. The method may comprise determining that a particle associated with a detected sequences of changes in the phase of a current interferometric signal waveform are the same as pre-set particle parameters associated with the sufficiently similar pre-defined sequence. A method may comprise providing the optical assembly with a metalens and therewith manipulating the light received from the laser cavity so as to form the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase. The method may comprise providing the optical assembly with a spiral phase plate, a spatial light modulator, a “q-plate”, or a diffraction grating and therewith manipulating the light received from the laser cavity so as to form the one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase. The method may comprise providing an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region as a static (temporally unchanging) “phase pattern”. The method may comprise detecting when particular phase changes in a newly-generated interferometric signal correspond to known interferometric signal phase changes in a previously-generated interferometric signal known to be associated with specific particle parameters. The method may comprise providing the “phase pattern” as a calibrated “phase pattern”. The calibrated “phase pattern” may be generated by a method comprising passing a number of different calibration particles of specified / controlled particle size, and each individually of a specified / controlled speed and trajectory (i.e., particle parameters) through the “phase pattern” and, for each such individual set of particle parameters, recording the SMI signal (e.g., and / or frequency spectrum or wavelet scalogram) generated by the self-mixing interferometer as one respective calibration SMI signal (e.g., and / or calibration frequency spectrum or calibration wavelet scalogram) amongst a plurality of calibration signals each associated with a unique set of respective calibration particle parameters. Every calibration SMI signal (and / or calibration frequency spectrum or calibration scalogram) may then be associated with a unique set of respective particle parameters to form a calibration data set. The determining of a property of the particulate material within the monitored region, by the processing module, may comprise either: (a) Comparing a newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) with calibration SMI signals (and / or calibration frequency spectra or calibration scalograms), selecting from amongst them a candidate calibration SMI signal (and / or calibration frequency spectra or calibration scalograms) that is found to most closely resemble the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof), and generating an output identifying that the particle parameters associated with the particle responsible for the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely approximate the particle parameters of the candidate calibration SMI signal (and / or calibration frequency spectra or calibration scalograms). Those particle parameters may be output for use or stored for use. (b) Using a machine learning (ML) algorithm(s) that has / have been trained, using the calibration data set, to identify when a newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely resembles a calibration SMI signal (and / or calibration frequency spectrum or calibration scalogram) and to generate an output identifying that the particle parameters associated with the particle responsible for the newly generated SMI signal (e.g., or frequency spectrum or wavelet scalogram thereof) sufficiently closely approximate the particle parameters of the calibration SMI signal (and / or calibration frequency spectra or calibration scalograms) in question. The processing module may output those particle parameters, or store them for use. The method may comprise, by the optical assembly, bathing the monitored region with laser light of the interferometer possessing optical wavefronts having different directions at different respective locations within the monitored region. The method may comprise, by the processing module, determining a property of the particulate material within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal. The method may comprise, by the processing module, determining a property of the particulate material within the monitored region according to a continuous change in the frequency of said waveform. The method may comprise, by the optical assembly, bathing the monitored region with a static divergent and / or convergent beam of the laser light possessing a curved wavefront defined by an optical wave comprising the one or more rapid or discontinuous changes in optical phase (i.e., curved at locations other than where the phase discontinuity resides), in which the monitored region comprises regions other than the focal region of the laser light. The method may comprise, by the optical assembly, bathing the monitored region with a beam of the laser light possessing a substantially flat wavefront defined by an optical wave comprising the one or more rapid or discontinuous changes in optical phase (i.e., flat at locations other than where the phase discontinuity resides), and to move the flat wavefront across the monitored region to a plurality of different directions. The method may comprise, by the processing module, determining a property of the particulate material within the monitored region according to a wavelet transformation of the interferometric signal. The method may comprise, by the interferometer, generating the interferometric signal comprising a voltage signal to be acquired by the laser monitoring unit, wherein the voltage signal corresponds to a voltage across electrical drive terminals of a laser cavity of the laser cavity assembly and comprises a voltage signal waveform in response to movement of the particulate material along a path within the monitored region of space. The method may comprise, by the interferometer, generating the interferometric signal comprising an optical output power signal to be acquired by the laser monitoring unit, wherein the optical output power signal corresponds to an optical output power of a laser cavity of the laser cavity assembly and comprises an optical output power signal waveform in response to movement of the particulate material along a path within the monitored region of space. According to the method, the property of the particulate material may comprise a property of the path thereof within the monitored region. The property of the path may comprise one or more of: a distance to said particulate material relative to the interferometer; a speed of said particulate material relative to the interferometer; a direction of said particulate material relative to the interferometer. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A and IB show schematic representations of a self-mixing interferometer. Figure IC shows a schematic representation of a self-mixing interferometer. Figures 2A, 2B and 2C each show a schematic representation of a self-mixing interferometer. Figures 3A and 3B each show a schematic representation of a series of optical wavefronts in an optical wave possessing phase discontinuities. Figure 4 shows a schematic representation of a curved wavefront of an optical wave at either side of a phase discontinuity of the optical wave. Figure 5 shows a schematic representation of steps in a method of use of a self-mixing interferometer according to any of figures 2A to 2C. Figures 6A and 6B show: (6A) a schematic representations of a light source and metalens of a self-mixing interferometer; and (6B) optical phase profiles within the light wave produced thereby. Figure 7 shows a schematic representation of a metalens of a self-mixing interferometer, and optical phase profiles within the optical wave produced thereby. Figures 8A and 8B show examples of a particle path passing through a laser beam comprising an optical wave possessing optical phase discontinuities. Figures 9A and 9B show: (9A) an example of a self-mixing interferometric signal associated with a particle traversing an optical wave output by the self-mixing interferometer, the optical wave possessing a succession of continuously curved optical wavefront sections separated by optical phase discontinuities; (9B) a wavelet scalogram thereof. Figures 10A and 10B show examples of: (10A) a self-mixing interferometric signal associated with a particle passing through a laser beam comprising an optical wave possessing continuously curved optical wavefronts; and (10B) a wavelet scalogram thereof. Figures 11A, 11B and 11C show examples of: (HA) a sequence of discontinuous changes in optical phase (n phase change) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed; (1 IB) a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave possessing a “phase pattern” of discontinuous changes in optical phase (tt phase change); and (1 IC) a wavelet scalogram of the waveform of Figure 1 IB. Figures 12A, 12B and 12C show examples of: (12A) a sequence of discontinuous changes in optical phase (n phase change) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed; (12B) a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave possessing a “phase pattern” of discontinuous changes in optical phase (n phase change); and (12C) a wavelet scalogram of the waveform of Figure 12B. Figures 13A, 13B and 13C show examples of: (13A) a sequence of discontinuous changes in optical phase (tt / 2 phase change) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed; (13B) a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave possessing a “phase pattern” of discontinuous changes in optical phase (tt / 2 phase change); and (13C) a wavelet scalogram of the waveform of Figure 13B. Figures 14A, 14B and 14C show examples of: (14A) a sequence of discontinuous changes in optical phase (tt / 2 phase change) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed; (14B) a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave possessing a “phase pattern” of discontinuous changes in optical phase (tt / 2 phase change); and (14C) a wavelet scalogram of the waveform of Figure 14B. Figures 15A, 15B and 15C show examples of: (15 A) a sequence of discontinuous changes in optical phase (n phase change) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed; (15B) a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave possessing a “phase pattern” of discontinuous changes in optical phase (tt phase change); and (15C) a wavelet scalogram of the waveform of Figure 15B. Here the particle trajectory comprises a particle speed that is twice the speed of the particle possessing the trajectory associated with Figures 11A to 1 IC. DETAILED DESCRIPTION Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Figure 2A schematically illustrates a self-mixing interferometer device 1 for detecting particulate material, according to an embodiment of the invention. The device is configured to monitor particulate material, such as a particle of material 4, within a monitored region of space. It comprises a laser cavity assembly 1A and an optical assembly IB configured to bathe the monitored region with laser light 2 of the interferometer possessing wavefronts 10 having different directions at different respective locations within the monitored region. The laser light 2 forms a cone of light having a cone angle T] as measured from an edge 8a of the light cone to a central cone axis 8b. Light 2 output by the self-mixing interferometer device 1 may be scattered from a particle of material 4 within the cone of light such that some of the scattered light 6 is returned to the laser cavity assembly 1A of the self-mixing interferometer device to interact therewith and form an interferometric signal. The motion of the particle 4 along a trajectory or path 7 passing through the light cone, causes detectable changes in the interferometric signal, as is discussed in more detail below. In Fig. 2A, one particle 4 is shown in three successive positions along its path through the cone of the laser beam 2. The self-mixing interferometer includes a laser monitoring unit IC configured to acquire an interferometric signal generated by the interferometer in response to light returned to the laser cavity assembly 1A from the wavefronts by the particulate material. The self-mixing interferometer also includes a processing module ID configured to determine a property of the particulate material within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal. In particular, Fig. 2B, and Fig. 2C show arrangements consistent with aspects of the invention. The schematic diagram of Fig. 2B corresponds to the arrangement shown in Fig. 2A. Here it can be seen that the optical assembly is configured to structure the laser beam such that it possesses curved wavefronts 10. The optical path length between the optical resonator cavity 1A and any point on a wavefront 10 is constant. This means that any spacing A, in a direction perpendicular to a given wavefront 10, between a point on that wavefront and a position of the particle at a particular instant of time along its part 7 through the laser beam, is equivalent to an optical path difference between light at the optical wavefront and light having reached the particle and about to be reflected from it back to the laser resonator cavity. Because the speed of light is so very much greater than the speed of the particle, the position of a given wavefront 10 when the particle enters the laser beam may be used as a reference wavefront with which to determine how the value of the spacing, A, changes as the particle progresses along its path. In other words, a wavefront corresponding to a particular phase of the electromagnetic wave of the laser light can be considered to be always present at the position of the reference wavefront. As shown in Fig. 2B, the value of the spacing, A, changes continuously (A —> Ai —> A2 —> A3 ... etc.) as the linear path of the particle 7 first coincides with a reference wavefront upon entering the laser beam, then moves further away from the reference wavefront as the particle approaches the central axis of the laser beam, due to the outward curvature of the reference wavefront. Subsequently, the linear path of the particle 7 moves towards the reference wavefront as the particle moves away from the central axis of the laser beam until it finally coincides with the reference wavefront upon exiting the laser beam. Fig. 2C shows a schematic example of the use of a laser possessing flat wavefronts (e.g., a collimated laser beam), but in which the path 7a of the particle is linear. The spacing, A, between the linear path 7a of the particle and a flat reference wavefront once more changes continuously as the linear path 7a of the particle progresses through the laser beam. The apparently linear path 7a of the particle may be not simply linear, but instead may be a path 7b that fluctuates around an average linear path 7c. This may occur, for example, if the particle is within a highly turbulent gas (e.g., air) and / or if the particle is sufficiently small (e.g., a pollen particle) that its path 7b is significantly buffeted or deviated by gas molecules in the manner of a Brownian motion or the like. In this case, the average speed, (v), of the particle, along its average linear path 7c, may be estimated as being the ratio of the linear distance between the points of entry into, and exit from, the laser beam at sides 8a of the beam, and the time interval between these events. The spacing, A, between the non-linear path 7b of the particle and a reference flat wavefront once more changes continuously as the non-linear path 7b of the particle progresses through the laser beam. In the example shown in Fig. 2C, the interferometer device 1 possesses an optical assembly IB configured to bathe the monitored region with laser light of the interferometer possessing wavefronts having the same directions (i.e., substantially flat wavefronts) at different respective locations within the monitored region. In addition, in the examples shown in Fig. 2B and Fig. 2C, the spacing, A, between the path (7c) of the particle and a reference wavefront changes continuously and non-linearly over time as the particle progresses through the laser beam. As will be discussed below, a consequence of this is that the frequency of a waveform in the interferometric signal of the self-mixing interferometer 1 is caused to change over time in at least a part of the interferometric signal, often appearing in the form of a ‘chirp’ or the like, in some part of the signal, sometimes in much of the signal, and sometimes in substantially all of the signal depending upon circumstances. This changing frequency of waveform in the interferometric signal can be used to provide information about properties of the particle causing the signal. The inventors have also realised that additional structures can be created in the interferometric signal by imposing rapid or discontinuous changes in optical phase of the optical wave defining the reference wavefronts 10 which result in rapid or discontinuous spatial changes in the position of a given optical wavefront. Examples of such rapid or discontinuous spatial changes in the position of a given optical wavefront are schematically shown in Fig. 3A-B and Fig. 4 and are discussed in more detail below. It is to be understood that the curved wavefronts 10 described herein with reference to Fig. 2A, Fig. 2B and Fig. 9A are shown on a macroscopic scale, showing a curvature (e.g., circular or spherical curvature) on a locally large scale (i.e., on a scale greater than the wavelength of light forming the optical wave). However, it is to be understood that these wavefronts also possess rapid or discontinuous local changes in spatial position of successive sections of that curved wavefront on a small, or microscopic, scale (i.e., on a scale comparable to the wavelength of light forming the optical wave), such as shown schematically by Fig. 3A-B and Fig. 4 and Fig. 9A. Figure 3 A shows a schematic representation of a series of optical wavefronts 10 in an optical wave 2 output by the optical assembly IB of the self-mixing interferometer 1, according to an embodiment of the invention. The optical wave possesses rapid local phase changes or discontinuities. Each one of the wavefronts 10 is designed to form a succession of smooth, extended wavefront sections, portions or segments each of length X, and separated by short, rapid spatial changes, steps or discontinuities, AL, in wavefront position in a direction of wave propagation. This structure forms within the optical wave a laterally arranged series of neighbouring lateral sections 200 of the optical wave that meet at a longitudinal phase boundary extending in the direction of optical propagation. The optical phase at any point immediately adjacent one side of the phase boundary, within a given section 200, differs by a finite amount Atp (e.g., A(p = k) from the optical phase at another point immediately adjacent the other side of the same phase boundary in the lateral direction perpendicular to the phase boundary, within a neighbouring lateral section 200 or the optical wave. Figure 3B schematically illustrates this distribution of relative optical phase difference in terms of an optical wave having a flat wavefront, for simplicity of illustration, but it is to be understood that the same principle applies to optical waves having curved wavefronts as in Figure 3 A. Figure 3B shows a greyscale heatmap illustrating the relative optical phase differences of the optical wave along individual lateral sections 200 of the optical wave. Optical phase difference (modulo 2k) is represented by a greyscale level. To be clear, Figure 3B shows a map of relative optical phase difference but does not aim to show actual phase values of the optical wave at a point in time. Rather, the map allows one to visualise how much the optical phase at any one spatial position in the optical wave 2 will differ from the optical phase at any other spatial position in the optical wave 2 at the same instant in time. These rapid changes occur along a length 8s of a given wavefront that is small relative to the length X of either lateral section 200 of wavefront at either side of the change (or phase boundary between lateral sections) in question. By providing one or more optical wavefronts comprising one or more rapid or discontinuous changes, AL, in spatial position, it is possible to generate one or more rapid or discontinuous changes in the phase of a waveform of the interferometric signal. From a point upon a wavefront at the location of a discontinuity marking the end of a given wavefront section 10 at the phase boundary of a given lateral section 200, by extrapolating in a direction perpendicular to the phase boundary that continues in the same direction of the discontinued wavefront of the lateral section, one finds that the optical phase along the extrapolated direction changes rapidly to (p + Atp (e.g., Atp = k), and maintains this optical phase along a curved optical wavefront (20A, see Fig.4) that is parallel to the shifted wavefront 22 located ahead of it by a distance equal to the change, AL, in spatial position. This rapid change occurs along a length 8s that defines a spatial region of rapid or discontinuous change, A(p, in the optical phase of the optical wave when measured along the extrapolation. From this, the processing module ID is configured to obtain information about properties of the particle 4. The light possessing these phase changes / discontinuities reflects, scatters or diffracts from the particulate material 4 whilst that material is being bathed by the light, so as to cause returned light 6 that mixes with native light in the resonator cavity of the interferometer to create an interferometric signal waveform having one or more rapid or discontinuous changes the phase within at least a part of the interferometric signal. The spatially discontinuous changes in the optical wavefronts 10 may be expressed as a difference, AL, in a spatial position as between immediately contiguous parts of a given wavefront, such that at a given instant or snap-shot in time, the distance of the optical wavefront from the optical assembly IB changes by an amount AL discontinuously (e.g., very rapidly over a very small distance along the wavefront) at a point on the wavefront. The value of the distance of the wavefront of individual segments 10 of any one overall wavefront, from the optical assembly IB as measured from any one of two consecutive wavefront sections either side of a discontinuity, is continuous and smoothly persists along an extended interval, X. of the wavefront section. By definition, the optical wavefront of an optical wave is the locus of points in space which all share the same optical phase value. This is the case for each one of the continuous sections 200 of wavefront spatially separated by rapid or discontinuous step-changes, AL, in wavefront positionlO generated by the optical assembly IB. This means that the difference, AL, in the in distance to the optical assembly IB as measured from neighbouring sections 200 of the wavefront, either side of a local spatial discontinuity, is related to the optical wave by the relation: AL = A<p(c / m) where m is the angular frequency of the laser light, and c is the speed of light in the medium in which the laser light propagates. This relation arises because the constancy of optical phase along the spatially discontinuous wavefront 10. Consider the electric field, E, of an optical wave of the laser light emitted output by the optical assembly IB and described by a travelling sinusoidal oscillation of amplitude Eo: E = Eq sin(m[t — Z / c]) At a time t, the parts of the wavefront defined by this optical wave at one side of a spatial discontinuity are at a distance from the optical assembly IB, whereas the parts of the wavefront at the opposite side of a spatial discontinuity are at a distance Z2 = + AL from the optical assembly. Since both parts of the wavefront share a common optical phase, and both were emitted from the optical assembly IB at the same time, this means that: "[t - h / c] = w[t - Z2 / c] + Consequently, this condition required that there exists a discontinuity in phase as follows: In other words, the spatial discontinuity of the wavefront corresponds to a phase discontinuity, A(p, in the optical wave defining that wavefront discontinuity as between the parts of the optical wave either side of the spatial discontinuity. In the following examples and discussions, where a reference is made to a wavefront, it is to be understood that the wavefront in question possesses one or more spatial discontinuities, AL, corresponding to rapid or discontinuous changes, A<p, in the optical phase of the optical wave defining the wavefronts (unless the contrary is stated). This is the case even when no explicit reference is made to the wavefront possessing such spatial discontinuities (unless the contrary is stated). Furthermore, in the following examples and discussions, where a drawing of the figures indicates a wavefront the wavefront is to be understood to one or more spatial discontinuities, AL, corresponding to rapid or discontinuous changes, A(p, in the optical phase of the optical wave defining the wavefronts (unless the contrary is stated). Figure 4 shows a schematic representation of two successive, neighbouring sections 200 of a curved wavefront 10 of an optical wave of Fig. 3A-B, at either side of a rapid positional (phase) change of the wavefront corresponding to a rapid change or discontinuity of phase of the optical wave of the laser beam 2. Consider this as being a representative part of a wavefront produced by the optical assembly IB. It comprises a first section 20 of length “X” with a radius of curvature It has a second section 22 also of length “X” and also with a radius of curvature The first section and the second section are in phase. The second section is ‘jumped forward’ relative to the first section by a distance of, for example, half a wavelength. A transition zone 24 joins the two sections and starts at a start point 32 and finished at an end point 30. The line, of length dL, joining the start / end points subtends an angle (0) to the radius of curvature of the second wavefront section at that point. A particle 4 follows a linear path 7 that crosses the first section 20 of wavefront at point A and crosses the second section 22 of wavefront at point B. At each crossing point the particle “sees” a wavefront with the same phase. Now consider a notional wavefront 26 with the same radius of curvature (R, item 36) as the radius of curvature of first and second sections (20, 22). If that notional wavefront 26 was the only wavefront present, and the particle 4 passed this notional wavefront at points A and B, then when crossing this notional wavefront at each of the foresaid crossing points (A, B) the particle once more “sees” a wavefront 26 with exactly the same phase - the same phase it “sees” when crossing the first wavefront section 20 and the second wavefront section 22. This means that the existence of the transition region 24 between the first wavefront section 20 and the second wavefront section 22 produces a rapid ‘step’ which creates a wavefront that has the same effect as the spherical radius of curvature of the notional wavefront 26 relative to the geometry of the particle path 7. The condition needed to allow the effects of the rapid or discontinuous (e.g., ‘stepped’) wavefront position change created by the optical assembly IB is that the “length” of the transition zone 24 must be less than the linear path length along the trajectory 7 of the particle between the crossing points A and B (i.e., where the particle 4 ‘sees’ the wavefront). This condition ensures that the ‘speed’ with which the particle ‘sees’ the two successive and identical phase values in the first wavefront section 20 and the second wavefront section 22 is more rapid than the speed at which it could otherwise ‘see’ that effect by merely crossing a curved wavefront of the same radius of curvature (R) lacking any such positional changes (e.g., notional wavefront 26). In more quantitative terms, the chord length, S, as measured along the notional curved wavefront 26 extending between the two crossing points (A, B) of the linear path 7 of the particle 4, is related to its radius of curvature, R, and the angle, 0, subtended by the chord is as follows: S = «^2(1 - cos(0)) Thus, the rapid change in the spatial position of a wavefront in question preferably at least satisfies the condition that dL <S, or: dL <«72(1 - cos(0)) More preferably: dL « ^2(1 — cos(0)) By Pythagoras: Here, n is a real number divisor of the optical wavelength. In the present example n = 2.0 for a phase change of tt radians but could be another value consistent with the optical phase change in question (e.g., n = 4.0, for a phase change of tr / 2 radians). Thus: cos(0) = 1 — +1 dL <«72(1 - cos(0)) More preferably: dL « «72(1 - cos(0)) This definition of cos(0) allows us to quantify the maximum size of the transition region, dL, within the meaning of a “rapid” change in spatial position of a given wavefront when transitioning from one wavefront section to the next section. The definition is in terms of the wavelength, A, of the light wave defining the wavefront, the radius of curvature, R, of the wavefront section being transitioned / stepped from and its length, X, the angular subtended size, 6, of the wavefront section, and the subtended angular size, 0, of the transition zone / step. In other words, for a desired spatial position change of length A / n in the wavefront to be achieved by that transition zone, one may select a size, defined by length dL and angular size 0. of a transition zone / step to apply to a wavefront of an optical wave of wavelength A, in order to create wavefront sections of length, X, and angular subtended size, 0, for a given the radius of curvature R. The quantity cos(0) may be calculated, and compared against the condition: dL <«^2(1 - cos(0)) More preferably: dL « R^ / 2(1 — cos(0)) Alternatively, one may quantify the maximum size of the transition region, dL, within the meaning of a “rapid” change in spatial position of a given wavefront when transitioning from one wavefront section to the next section as being such that: A / n <dL <aXfn, where a is a positive real number between 1.0 and 5.0, or between 1.0 and 4.0, or between 1.0 and 3.0, or between 1.0 and 2.0, or between 1.0 and 1.5. For example, when a has a value of 1.0, or something very close to 1.0, it may be considered to define a discontinuous change, practically speaking. If this condition is not met, then one may select a different size, defined by length dL and angular size 0, of a transition zone / step that does satisfy this condition. Once the condition is satisfied, the resulting transition zone / step geometry will be sufficient to achieve associated “rapid” changes in the waveform of the self-mixing interferometric signal (and / or data describing its frequency-space scalogram) that could not otherwise arise from the pre-existing curvature of wavefronts present in the optical wave of the self-mixing interferometer in the absence of such a transition zone / step. Of course, if the optical wavefronts of the optical wave are substantially flat (i.e., no curvature; R = oo) then the condition becomes simply: dL <S2 (or preferably, dL « S2) where S2 is simply the linear path length of the particle trajectory 7 between the two crossing points (A, B). Figure 5 shows a schematic representation of steps in a method of use of a self-mixing interferometer according to any of figures 2A to 2C. This sequence of steps is performed by the self-mixing interferometer 1 to determine a property of the particulate material 4 within the monitored region. This method corresponds to those aspects of the invention in which the monitored region is bathed with laser light of the interferometer possessing rapid or discontinuous spatial changes in the position of a given optical wavefront (due to rapid or discontinuous spatial changes in optical phase of the optical wave). The wavefronts may also have different directions at different respective locations within the monitored region (e.g., due to large-scale wavefront curvature, or spatial scanning of flat wavefronts on a large scale; i.e., a macroscopic scale much larger than the optical wavelength), or may have substantially flat wavefronts on a large scale (except for the rapid or discontinuous spatial changes in the position of a given optical wavefront). The property of the particulate material is determined according to changes in the frequency of a waveform within at least a part of the interferometric signal. The method comprises the following steps: Step SI: By the laser resonator cavity 1A and optical assembly IB of the interferometer 1, bathing the monitored region with laser light of the interferometer possessing wavefronts having rapid or discontinuous spatial changes in the position at different respective locations within the monitored region. Step S2: By the monitoring unit IC, acquiring an interferometric signal generated by the laser resonator cavity 1A of the interferometer 1 in response to light returned to the laser cavity assembly from wavefronts of the laser beam by the particulate material 4. Step S3: By a processing module IC, receiving the acquired interferometric signal from the monitoring unit IC and therewith determining a property of the particulate material 4 within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal. The self-mixing interferometer 1 may be configured to implement this method in, for example but without limitation to, any one of the arrangements schematically illustrated in Fig. 2A, Fig. 2B, Fig. 2C, and Fig. 6A. As will be discussed in more detail below, this methodology promotes the appearance of a waveform within at least a part of the interferometric signal that changes in frequency. The inventors have found that these changes may be used to determine properties of the particle within the laser beam. As will be discussed in more detail below, at least a part of the interferometric signal may possess rapid and localised changes in phase. The inventors have found that the position or pattern of these rapid and localised changes may create structure in data describing the interferometric signal in a frequency-space transformation (e.g., a wavelet scalogram), and this frequency-space structure may be used to determine properties of the particle within the laser beam. Fig. 6A schematically illustrates an example of the optical assembly IB within the self-mixing interferometer device 1 shown in Fig. 2A, Fig. 2B or Fig. 2C. The optical assembly IB comprises a metalens 1 lb configured to receive light from the laser cavity assembly Ila and to pass the received light through the metalens such that, as a consequence of being transmitted through the metalens, the laser light 2 acquires a plurality of rapid or discontinuous changes, in optical phase within the monitored region 100. These rapid or discontinuous changes, A(p, in optical phase manifest themselves as a corresponding plurality of rapid or discontinuous changes in the spatial position (along a “Z-axis” propagation direction indicated in Fig. 6A) of optical wavefronts 10 in the optical wave (i.e., a wavefront being the locus of point sharing a common single optical phase). This light 2 then propagates outwardly, along a “Z-axis”, from the optical assembly IB to bathe the monitored region 100. The laser monitoring unit IC (Fig. 2A) is configured to acquire an interferometric signal generated by the interferometer in response to light 6 returned to the laser cavity assembly from the optical wavefronts 10 by the particulate material (4a, 4b, 4c). The processing module ID (Fig. 2A) is configured to determine a property of the particulate material (4a, 4b, 4c) within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal, and / or according to rapid changes in data describing a frequency transform of the waveform of the interferometric signal as discussed in more detail below with reference to figures 10B to 15C. Referring further to Fig. 6A in conjunction with Fig. 6B, the optical wave bathes the monitored region 100 with an electromagnetic field possessing a pre-set optical phase structure extending in three dimensions (X, Y, Z). Note that the optical wave is presented as a substantially nondiverging laser beam 2, and the optical wavefronts 10 within it are shown as lacking macroscopic-scale curvature as a result (i.e., no beam divergence). This corresponds to the arrangement shown in Fig. 2C, but it is to be understood that the following discussion applies equally to the arrangement shown in Fig. 2B, possessing a divergent laser beam 2 with wavefronts curved on a macroscopic scale. The optical wave propagates in along a notional “Z-axis” direction. Consider now a series of 2-dimesions (2D) profiles, or slices, of the optical wave defined by a planar cross-section of the laser beam taken in the X-Y plane (i.e., containing both the “X-axis” and the “Y-axis”) perpendicular to the “Z-axis”. Now consider viewing the phase of the optical wave at all points upon the plane of the cross-sectional slice. Fig. 6A shows four such notional cross-sectional slices (12a, 12b, 12c, 12d) of optical phase. Each shows the spatial profile of the optical phase of the optical wave as distributed across that slice. Fig. 6B shows an exploded view of these four notional cross-sectional profiles of optical phase. In a first phase profile, 12a, positioned closest to the metalens 1 lb, three separate circular regions (14a, 16a, 18a) exist which each defines a circular region of space sharing same optical phase as any of the other circular regions within that phase profile. All other regions within the first profile also share a common optical phase, which is different to the optical phase shared by the three circular regions. For example, the optical phase of the optical wave at the three separate circular regions (14a, 16a, 18a) may differ from the optical phase at all other regions of the first phase profile by a value of n radians. A particle 4 passing through the monitored region 100 along a trajectory 7 within the plane of the first phase profile will experience succession of rapid changes or discontinuous jumps in the optical phase of the laser beam 2 as it passes between, and through, the three separate circular regions (14a, 16a, 18a) in succession. This results in a corresponding succession of rapid changes or discontinuous jumps in the optical phase of the interferometric signal generated by the self-mixing interferometer 1 in response to scattered light returned to it by the particle 4 from these successive circular regions. In a second phase profile, 12b, positioned further along the “Z-axis” the three separate circular regions of the first phase profile have evolved / developed into altered 2-dimensional shapes -e.g., ellipses (14b, 16b, 18b) - naturally due to the propagation of the optical wave. For example, the optical phase of the optical wave at the three separate elliptical regions (14b, 16b, 18b) may differ from the optical phase at all other regions of the second phase profile by a value of n radians. A particle 4 passing through the monitored region 100 along a trajectory 7 within the plane of the second phase profile will experience succession of rapid changes or discontinuous jumps in the optical phase of the laser beam 2 as it passes between, and through, the three separate elliptical regions (14b, 16b, 18b) in succession. This once more results in a corresponding succession of rapid changes or discontinuous jumps in the optical phase of the interferometric signal generated by the self-mixing interferometer 1 in response to scattered light returned to it by the particle 4 from these successive circular regions. However, due to its altered phase profile (spatial pattern), the pattern of changes or discontinuous jumps in the optical phase of the interferometric signal arising from the second phase profile will differ from the pattern arising from the first phase profile. A third phase profile 12c and a fourth phase profile 12d are positioned at successive positions further along the “Z-axis”, one after the other. In each one of these two further phase profiles, the separate regions of the first phase profile and the second phase profile have evolved / developed yet further into more altered 2-dimensional shapes - e.g., ellipses (14c, 16c, 18c) - one or more of which have begun to merge into another naturally due to the propagation of the optical wave along the “Z-axis”. The regions (14c, 16c, 18c; 14d, 16d, 18d) within each of these third and fourth phase profiles differs from the optical phase at all other regions of the phase profile in question by a value of tr radians. Once more, these evolutions of the phase profiles result in further evolution / change in the succession of rapid changes or discontinuous jumps in the optical phase of the interferometric signal generated by the self-mixing interferometer 1. This differences in the pattern of changes or discontinuous jumps in the optical phase of the interferometric signal arising from the different phase profiles located at different positions along the “Z-axis” is diagnostic of the position of the particle within the monitored region 100. In other words, each phase profile differs from each of the other phase profiles in the spatial pattern of the phase changes / discontinuities within it, relative to the (X,Y,Z) coordinates of the monitored region. As a result, the pattern of changes or discontinuous jumps in the optical phase of the interferometric signal arising from each phase profile is unique to that profile and its presence indicates the presence of the particle path 7 within the plane of that phase profile. Indeed, it is to be understood that each one of the regions (14a, 16a, 18a) forms one part of a continuous elongated volume, or projection, of the evolving region along the “Z-axis”, which is connected to each of the other regions (14b,c,d; 16b,c,d; 18b,c,d) and has the common optical phase associated with those regions at all points along its length. This means that a phase profile taken at any point along the “Z-axis” in the monitored region 100 may provide a unique phase profile enabling a unique detection of the presence of the particle path 7 within the plane of that phase profile. Figure 7 shows a schematic representation of a metalens 1 lb of a self-mixing interferometer, and optical phase profiles (25a, 25b, 25c) within the optical wave 2 produced thereby. Each phase profile comprises a highly asymmetrical pattern of zones of substantially constant optical phase that differs significantly from the optical phase of an immediate neighbouring zone. A first phase profile 25a, closest to the metalens 1 lb comprises six zones of constant respective optical phase (42a, 42b, 44a, 44b, 46a, 46b) in which the optical phase differs from that of a neighbouring elliptical zone by a value of n radians. A second phase profile 25b, further from the metalens 1 lb comprises six concentric elliptical zones of constant respective optical phase in which the optical phase differs from that of a neighbouring elliptical zone by a value of tt radians. The six elliptical zones of the second phase profile have evolved naturally from the propagation of the optical wave from the position of the first phase profile. A third phase profile 25c, further from the metalens 1 lb comprises six concentric elliptical zones of constant respective optical phase in which the optical phase differs from that of a neighbouring elliptical zone by a value of n radians. The six elliptical zones of the third phase profile have evolved naturally from the propagation of the optical wave from the position of the second phase profile. Of course, a continuum of intermediate phase profiles exist between the first, second and third phase profiles with intermediate states of evolution of respective phase profiles. The path 7 of a particle 4 within the plane of each phase profile experiences a sequence of optical phase discontinuities along its path, and these result in a corresponding sequence of phase discontinuities within the interferometric signal of the self-mixing interferometer 1. The lack of radial symmetry of the neighbouring phase zones allows results in different sequences of phase discontinuities within the interferometric signal of the self-mixing interferometer 1 according to the orientation of the particle path relative to the major axis of the elliptical zones, thereby allowing the orientation of the trajectory to be identified. In aspects of the invention, the processing module of the self-mixing interferometer may be configured to determine a property of particulate material, e.g., as disclosed herein. As a first example, consider the methodology of Fig. 5 as applied to the circumstances illustrated schematically in Fig. 2C. Here, the optical assembly IB of the self-mixing interferometer 1 is configured to provide a collimated laser beam 8a of laser light 2 possessing substantially flat wavefronts 10. Consider a particle 4 passing through the laser beam along a linear path 7a or along a non-linear path 7b within the laser beam which fluctuates around a “time-averaged path” 7c which is linear and corresponds to average linear velocity (v) of the particle 4. In this sense, the time-averaged path is equivalent to a notional linear particle path 7a of the particle 4 with a notional linear velocity v = (v). This notional linear velocity has a direction subtending an angle 0 relative to the axis of the laser beam. Both the subtended angle 0 and the notional linear velocity (v) may be determined as follows. The angular frequency, of the interferometric signal corresponds to the rate of change of the accumulated phase <ps within the light wave of the laser light (6, Fig. 1 A) re-injected into the optical cavity 1A of the laser by reflection or back-scattering from the particle 4. Noting that a change d<ps in the accumulated phase (ps arises because of a change dl in the difference between the position of the particle 4 and a reference wavefront of the laser beam (as discussed in detail above), we may write: = 4n dl 4n{v) -TTt = — Given that the duration AT of the transit of the particle 4 across the laser beam of width l¥ is: W AT =----- vsin< / > One may combine these two equations to yield: 4nW AmsAT. 1 (v) = — \ / 4n 1 + WA 4nW I2 Aa>sAT The quantities W and A are properties of the laser system and are known. The quantities AT and may be accurately estimated by applying a wavelet transformation to the interferometric signal. As a second example, consider the methodology of Fig. 5 as applied to the circumstances illustrated schematically in Fig. 2A, of Fig. 2B. In aspects of the invention, the processing module of the self-mixing interferometer may be configured to determine a property of particulate material, e.g., as disclosed herein. Here, the optical assembly IB of the self-mixing interferometer 1 is configured to provide a laser beam 8a possessing wavefronts 10 having different directions at different respective locations within the monitored region. Fig. 8 A shows a diverging cone 8a of laser light emanating from the self-mixing interferometer 1. The cone of light is centred upon a cone axis 8b. The angle of divergence of the laser beam relative to the cone axis is rj. Consider a particle 4 following a linear path (or a “time-averaged” linear path) 7 passing through the cone of laser light at speed v (or average speed (v)) the direction of which subtends an angle <p relative to the cone axis 8b. One can see that the following relations exist in relation to a change dl in the distance between the particle 4 and a reference wavefront 10 of the laser beam (8a, 8b), measured in a direction perpendicular to the reference wavefront, as determined from two different points on the path 7 of the particle 4 separated by a linear distance ds. Given that the speed of the particle 4 is v, one may write: ds = vdt Referring to Fig. 8B, the following relations exist between the geometry of the conical laser beam of divergence angle 77, the angle 0 subtended by the path 7 of the particle relative to the central axis 8b of the conical laser beam, the times Gand t3 of entering and exiting, respectively, the conical laser beam, the intermediate time t2 of reaching a tangential position (“X”) at which the part of 7 of the particle is instantaneously parallel to the reference wavefront such that the rate of change dl / dt of the distance between the particle 4 and a reference wavefront 10 of the laser beam momentarily vanishes: G - G1 _ sin^ + sin^ “ sin(2?]) Here, s' is the part of the path 7 of the particle 4 extending between the point of entry of the particle into the conical laser beam, when at a distance from the laser resonator cavity 1 A, and the tangential position. The quantity s", illustrated in Fig. 8B, is the part of the path 7 of the particle 4 extending between the point of exit of the particle from the conical laser beam, when at a distance G from the laser resonator cavity 1 A, and the tangential position. Consequently, the quantity s is the full linear path length of the particle within the conical laser beam, such that: One can see that: COS(0 + T] — -y) sin(0 — / / ) More generally, for a given instant in time, t, we may identify the angular position of the particle, relative to the central beam axis 8b as: Consequently, we may conclude that the rate of change of the distance between the particle 4 and a reference wavefront 10 of the laser beam (8a, 8b), measured in a direction perpendicular to the reference wavefront, is: dl .........................— sin dt Using the following known trigonometric relation: X B sinfx + tan (A / BY = -cos(r) + , sin(x) YA2 + B2 YA2 + B2 And defining tan KA / B} = ^~ ¢ Gives 71 tan(— — 0) = / ? Therefore dl — = v sin dt v = cos( tan-1{y}) + -^=^= sin(tan-1{y}) 2 / 1 _L z?2 Here r t2 — 11 L............7........... tan?] ^2 tan rj tan(0 — tt / 2) Given the known trigonometric relation: 1 Y cos(tan-1{y}) =.............==. ; sin(tan-1{y}) =.............== 71 + y2 71 + 72 We may write that: dl 1 v y + y dt ^1 + p2 1 + y2 ^ / 1 + p2 1 + y2 1 + fl2y] 1 + y2 As discussed above, the response of the laser cavity of the interferometer can be expressed in the following terms: Phaser A A B COS (pFB (pFB = (ps + C sin(<pFB + tan-1 a) Phaser =A + B cos[^s + C sm((pFB + tan-1 a)] However, when there is only a weak intensity in the light returned to the laser cavity from the particle the coefficient C becomes negligible, and one may write: Phaser — A 4~ B COS (ps We may also define the phase <ps as: Vs — t + Vo Therefore, the laser interferometric signal becomes: Phaser A T B COS t + <Po = A + B cos(mst + cpo) In other words, the rate of change of the phase <ps is equivalent to an instantaneous value of the frequency of the laser interferometric signal: s It can be seen that this frequency, a)s, is proportional to the speed, v, of the particle, as well as aspects of the geometry, r], of the laser beam and the orientation, 0, of the path of the particle relative to the axis of the laser beam. Noting again that: An dl 4nv P + Y a)„ =--=---- , A dt A [71+^2^1 + y2 we may use the above known trigonometric relation to express the term in square brackets as follows: B + y ......................................................................................................-.......................................................................= sin(0 + tan Hl / y]) VT+^ / T+T7 Consequently, the signal frequency may be alternatively express as succinctly as follows: 4nv O)s = —— Sin(0 + tan-1[l / y]) Here: For conciseness, we may write the term y as a simple algebraic function of time, t, as follows: c3 + c4t In this way, the term y is a simple polynomial function of time. Here, the terms q, c2, c3 and c4 take the following form: t2 _ h ct = t2 ; c2 = —1 ; c3 =--1- (3t1 ; c4 = — tan T] In the situation where the particle crosses the laser beam axis in a direction perpendicular to the axis, then 0 = n / 2 and ft = 0, such that y -* y± = (q + c2t) / c3, and: 4ttv yx 1 ^i +ri t2-t' ; y = ------ tan n 1¾ - In the situation where the particle crosses the laser beam in a direction which forms a tangent to a wavefront of the laser beam at a point in time t = t2 whilst within the laser beam, then at that point y± = 0, and a)s(t = t2) = 0. In the situation where the particle progresses directly along the laser beam axis in a direction parallel to the axis, then 0 = 0 such that y = 0, and: 4nv 4nv ——sin 4nv 4nv — cos® = — Particles crossing the laser beam axis in directions between these two extremes will produce instantaneous values of the frequency of the laser interferometric signal between these two extremes: 0 <ms(t) < 4nv In the situation where the particle crosses the laser beam axis in a direction which forms a tangent to a wavefront of the laser beam at a point in time t = t2 whilst within the laser beam, then at that point y = 0, and 0 <(p <n; (p n / 2: 4nv B ws(t = = —-- A [Ji + p 4nv ~r cos 0 Fig. 9A shows an example of this waveform in the interferometric signal, PLaser, of the laser: Phaser = A + B COS^st + ( / ¼) Here, 4nv P + y 1 [VT+^VT+P Cl + c? • t ; y =--------^a + b- t + c- t C3 + c^t Thus, 4nv p + y (a + b • t + c • t2 + ) Here, a, b, and c are constants for a given particle trajectory within a given laser beam cone. In aspects of the invention, the processing module of the self-mixing interferometer may be configured to determine a property of particulate material using an interferometric signal of this form, e.g., as disclosed herein. In this example, we have expanded the term y as a polynomial function, assuming y2 « 1, of time variable, t, including terms of order no higher than the second order. Fig. 9B shows this waveform in which a = 3 or 4, b = —6 and c = 2. The different values of the parameter a (i.e., a = 3, and a = 4) occur at different time intervals within the waveform. Note that the two different values of a (i.e., a = 3, and a = 4) may be considered to correspond to the interferometric signals associated with respective neighbouring lateral sections 200 of an optical wave 2 separated by a rapid or discontinuous change on wavefront position due to a rapid / discontinuous change in optical phase there. This is because the parameter a is a constant phase term (i.e., time-independent) and instantaneous changes in its value correspond to instantaneous changes in phase of the interferometric signal waveform. For example, the value a = 3 may be associated with a first optical phase and the value a = 4 may be associated with the discontinuously / instantaneously changed value of that optical phase (e.g., changed by n radians). Fig. 9B shows this waveform in conjunction with a schematic diagram, Fig. 9A, of the optical wave comprising phase discontinuities manifest as discontinuities, AL, in wavefront position as between neighbouring lateral sections 200 of the optical wave 2. In Fig. 10A, the waveform is shown with the parameters A = 3 and B = 1.5. In this way, the value of B represents an amplitude of a modulation of the interferometric signal, PLaser, of the laser. The different values of a, b and c represent differences in the terms c1( c2, c3 and c4 defined above which, in turn are determined by the known divergence angle of the laser beam, the measurable times tt, t0 of the entry of the particle into the laser beam and the time at which the tangential position is momentarily achieved, and the angle (p of the particle path 7 relative to the central axis of the laser beam. As described above, the time at which the tangential position is momentarily achieved corresponds to a moment at which the frequency of the interferometric signal, Phaser, becomes zero. This is indicated by the turning point 31 of the waveform 30 of Fig. 9B and Fig. 10A. The shape and structure of the waveform, is the result of the values of a, b and c which, in turn are the result of differences in the speed v of the particle 4 within the laser beam and the angle <p its path 7 subtends to the central axis 8b of the laser beam. Figure 9A schematically illustrates a magnified view of the particle path 7 across the curved wavefronts of laser light output by the optical assembly IB of the self-mixing interferometer device 1. The curved wavefronts of laser light are shown in more detail to make clear the spatial discontinuities that exist along those wavefronts, resulting from discontinuities on the optical phase of that laser light. In particular, each wavefront possesses a plurality of spatial discontinuities, AL, corresponding to rapid or discontinuous changes, A<p, in the optical phase of the optical wave defining the wavefront at that location. When the path 7 of the particle intercepts a given segment of the optical wavefront that is advanced (or retarded) in space by a distance corresponding to an associated spatial discontinuity, AL, the effect is to change the optical phase of the laser light that is returned by that segment of optical wavefront and received at the optical interferometer. This, in turn, changes the phase of the waveform of the interferometer signal 30 for an interval of time corresponding to the time while the particle 4 is interacting with (i.e., reflecting, scattering or diffracting) light from that lateral section or segment 200 of optical wave. This is explained in more detail with reference to Fig. 10A. This figure shows an example of the waveform of the interferometer signal, 30 having the general form: Phaser = A + B cos(a)st + <p0) « A + Bcos(a + b ■ t + c ■ t2) The discontinuous changes in the phase of the waveform show as a difference in phase as between immediately contiguous parts of the waveform, such that the value of a phase of the waveform changes discontinuously or very rapidly, in an interval of time At, from a first phase value of 01 = a + b • t + c ■ t2, with a = 4, to a second phase value of 02 = 01 — A0 = d + b ■ t + c ■ t2, with d = 3, in which A0 = a — d = —1. Thus, the central portion of the interferometric signal waveform 30 shown in Figure 10A has the form: Phaser ~ + Bcos(a + b ■ t + C ■ t2) Here, A = 3, B = 1.5, a = 4, b = —6, and c = 2. However, the outer wings of the interferometric signal waveform have the form: Phaser ~ + Bcos(d + b ■ t + C ■ t2) Here, A = 3, B = 1.5, a = 3, b = —6, and c = 2. Note that A0 » LlAt where fl is the angular frequency of the interferometric signal waveform immediately prior to the occurrence of the discontinuity in the phase of that waveform. As can be seen, the interferometric signal waveform changes from having a first phase value of 01 = a + b • t + c • t2 for which a = 4 at time t = tl, to having a second phase value of 02 = d + b ■ t + c ■ t2 for which d = a — 1 = 3 at time t = t2, where t2 = tl + At, in which A0 = a — d » flAt where fl is the angular frequency of the interferometric signal waveform at time t = tl. The interval of time At during which the rapid or discontinuous change in phase takes place is much shorter than the interval of time, AT, required for the same amount of change to take place in the phase of the interferometric signal waveform in the absence of the discontinuity. In other words, At « AT. Referring again to Figures 9B and 10 A, the particle path 7 causes the generation of the interferometric signal waveform 300A and 300B when interacting with sections of wavefront that do not possess a spatial discontinuity, AL. However, it causes the generation of the interferometric signal waveform 300C when interacting with sections of wavefront that do possess a spatial discontinuity, AL. The transition between these two interaction regimes results in a discontinuous transition between these two interferometric signal waveforms and two discontinuous phase changes, 33A and 33B, are created in the resulting interferometric signal waveform observed, as shown in Figures 9B and 10A. Figure 10B shows a wavelet scalogram of the interferometric signal waveform comprising sections 300A, 300B and 300C which include the two discontinuous phase changes, 33A and 33B occurring at times ti and t’l respectively. It has been found that significant artefacts are created in the wavelet scalogram by the existence of discontinuous phase changes, 33A and 33B (or elsewhere) in the interferometric signal waveform. Similar artefacts are also found in other forms of frequency spectra (e.g., Fourier spectra, not shown) of the interferometric signal waveform. The artefacts present themselves as a significant change (e.g., drop) in value of the scalogram (or other form of frequency spectrum) as compared to the value that the scalogram would otherwise have in the absence of the discontinuous phase changes. Figure 11A shows a sequence of discontinuous changes in optical phase (tt phase changes) in an optical wave as seen by a particle traversing the optical wave according to a particular particle trajectory and speed. For example, consider the trajectory 7 of a particle 4 passing through a laser beam 2 comprising a sequence of four successive laterally arrayed optical wave sections 200 as schematically shown in the simplified example of Figure 3B. Here the example is simplified by showing the wavefronts within the optical wave as flat wavefronts, as a simplified representation of the curved optical wavefronts of Figure 3A. Upon entering the laser beam 2, the particle 4 is bathed by laser light successively within each one of a succession of four laterally arrayed optical wave sections 200 of equal width. At any given instant in time, the optical phase of the optical wave at a given location along the light propagation direction in any one of the four laterally arrayed sections 200, differs from the optical phase at an equivalent location in any of its immediate neighbouring sections 200, by n radians. This corresponds to a spatial “phase pattern” of discontinuous changes in optical phase. In the example shown, the particle trajectory 7 is perpendicular to the light propagation direction at a uniform speed and experiences discontinuous (practically-speaking) changes in relative optical phase by n radians as it crosses each boundary between any two laterally arrayed optical wave sections 200. For example, consider that it takes lOps for the particle 4 to traverse each one of the four optical wave sections 200 such that a first change in relative optical phase (by it radians) occurs lOps after entering the optical beam 2, followed by a second change in relative optical phase (by n radians) after a further lOps, and then a third change in relative optical phase (by n radians) after a further lOps, and a fourth change in relative optical phase (by n radians) after another lOps. This sequence of relative optical phase changes ‘seen’ by the particle long its trajectory is shown in Figure 11A as phase changes occurring at times lOps, 20ps, 30ps and 40ps after entering the laser beam 2. Figure 11A also considers subsequent phase changes due occurring at times 60ps, 80ps, 90ps, 120ps and 130ps due to additional laterally arrayed optical wave sections 200 not shown in Figure 3B. Although the explanation given above is made with reference to Figure 3B, for ease of explanation, it is to be understood that the same principles apply when the laser bean 2 comprises curved optical wavefronts such as is schematically shown in Figure 3A. Accordingly, Figure 1 IB shows a self-mixing interferometric signal associated with light scattered by a particle from a laser beam comprising an optical wave having curved optical wavefronts such as in Figure 3A, and possessing a spatial “phase pattern” of discontinuous changes in optical phase (n phase changes) equivalent to the spatial “phase pattern” of Figure 3B described above. The presence of curvature in the optical wavefronts of the optical wave results in the ‘chirp’ structure of changing frequency seen in the self-mixing interferometric signal of Figure 1 IB, whereas the presence of optical phase discontinuities in the optical wave results in phase discontinuities seen in the self-mixing interferometric signal at the points in time (1 Ous, 20us, 30ps and 40us, 60us, 80ps, 90us, 120ps and 130ps) after the particle 4 has entered the laser beam 2 and crosses successive boundaries between neighbouring laterally arrayed sections 200 of the optical wave. The sequence of discontinuous changes in optical phase (tt phase changes) in the optical wave, as ‘seen’ by the particle 4 traversing the optical wave, results in a corresponding sequence of discontinuous changes in phase (n phase changes) in the self-mixing interferometric signal at the same time points. Figure 1 IC shows a wavelet scalogram of the self-mixing interferometric signal waveform of Figure 1 IB generated by applying a Morlet wavelet transformation to the self-mixing interferometric signal. At each time point (lOps, 20ps, 30ps and 40ps, 60ps, 80ps, 90ps, 120ps and 130us) corresponding to a discontinuous change in phase (n phase change) in the selfmixing interferometric signal one can see a strong artefact present in the scalogram. The artefact has a structure that is common to each time point, in general terms. That structure has a first structural component comprising a rapid and time-localised (and frequency-localised) fall in the value or magnitude of the scalogram at the given time-point, from a value that is otherwise locally maximal at the time point in question to a value that is much smaller or negligibly small. In addition, the artefact structure has a second structural component comprising a rapid and time-localised rise in the value or magnitude of the scalogram at the given time-point from a value that is otherwise locally minimal or negligible at the time point, to a value that is significantly larger and non-negligible. This second structural component extends continuously along a band of frequencies coinciding with the time point and extending from the frequency value of the first structural component to higher frequency values. Notionally, the first structural component has the appearance of a “hole” in the scalogram, and the second structural component has the appearance of a “spike” extending (in frequency) above the “hole”. Figures 12A shows another example of a sequence of discontinuous changes in optical phase (tt phase change) in the optical wave 2 as seen by a particle 4 traversing the optical wave according to a different particle trajectory 7 as compared to the particle trajectory responsible for the sequence of discontinuous changes in optical phase shown in Figure 11 A. Figure 12B shows the consequential self-mixing interferometric signal associated with light scattered by the particle from the laser beam, and Figure 12C shows the associated wavelet scalogram of the waveform of Figure 12B. At each time point (lOps, 40ps, 50ps and 70ps, 90ps, lOOps, HOps and 120ps) corresponding to a discontinuous change in optical phase (n phase change), a corresponding discontinuous change in phase (tt phase change) occurs in the self-mixing interferometric signal, and a corresponding strong artefact appears in the scalogram. Figure 13A shows an example of a sequence of discontinuous changes in optical phase (n / 2 phase change) in an optical wave as seen by a particle traversing the optical wave according to the same particle trajectory and speed considered for the Figure 12A. The optical phase is n / 2 as opposed to n. Figure 13B shows the resulting self-mixing interferometric signal bearing discontinuous changes in phase of n / 2 at each time point (lOps, 40ps, 50ps and 70ps, 90ps, lOOps, 1 lOps and 120p s) of optical phase change. Figure 13C shows a wavelet scalogram of the waveform of Figure 13B in which a corresponding strong artefact appears in the scalogram. Figure 14A shows an example of a sequence of discontinuous changes in optical phase (n / 2 phase change) in an optical wave as seen by a particle traversing the optical wave according to the same particle trajectory and speed considered for the Figure 11 A. The optical phase is n / 2 as opposed to n. Figure 14B shows the resulting self-mixing interferometric signal bearing discontinuous changes in phase of n / 2 at each time point (lOps, 40ps, 50ps and 70ps, 90ps, lOOps, 1 lOps and 120ps) of optical phase change. Figure 14C shows a wavelet scalogram of the waveform of Figure 14B in which a corresponding strong artefact appears in the scalogram. Figures 15A shows an example of a sequence of discontinuous changes in optical phase (n phase change) in an optical wave as seen by a particle traversing the optical wave according to the same particle trajectory but twice the speed considered for the Figure 11 A. The optical phase is n as in Figure 11 A. Figure 15B shows the resulting self-mixing interferometric signal bearing discontinuous changes in phase of n at each time point (5ps, lOps, 15ps and 20ps, 40ps, 45ps, 60ps and 65ps) of optical phase change. Figure 15C shows a wavelet scalogram of the waveform of Figure 15B in which a corresponding strong artefact appears in the scalogram. Here the particle trajectory comprises a particle speed that is twice the speed of the particle possessing the trajectory associated with Figures 11A to 1 IC. The pattern of artefacts present in the scalogram of Figure 15C corresponds to the pattern of artefacts present in the scalogram of Figure 1 IC, but simply compressed in time (i.e., along the time axis of the scalogram) by a factor of two (2) corresponding to the particle having twice the speed. The processing module is configured to implement two general data processing steps upon recorded data describing the current self-mixing interferometric signal of Figure 11B, 12B, 13B, 14B or 15B. Those general steps are: (a) A step that detects a sequence of phase changes in phase in the recorded current selfmixing interferometric signal waveform data; (b) A step that relates a series of phase changes detected in step (a) with a previously established valid series of such phase changes pre-associated with a known particle trajectory and / or speed. Step (a) preferably comprises of applying a wavelet transform, such as a Morlet wavelet transformation, to the current self-mixing interferometric signal data to generate data describing a scalogram, such as a scalogram shown in Figure 1 IC, 12C, 13C, 14C or 15C, and then detecting a sequence of artefacts within the scalogram that corresponds to the sequence of changes in phase the current self-mixing interferometric signal waveform. The processing module may be configured to time-tag the positions of the artefacts within the scalogram (e.g., the “holes”) using suitable algorithms readily available to the person of ordinary skill in the art, and known in the domain of image processing. For example, algorithms for detecting changes in the wavelet scalogram magnitude may be configured to employ line detection algorithms, such as a Hough Transform algorithm, or a Line Segment Detector algorithm (e.g., Lin et al.: "A Comprehensive Review of Image Line Segment Detection and Description: Taxonomies, Comparisons, and Challenges’", arXiv:2305.00264vl [cs.CV] 29 Apr 2023). Detected lines may be represented in a parametric form, with the x-axis (time) of the wavelet discretized in steps, representing intervals of time. A line has thus the following equations: Given a direction vector, v. v = (vx, vy), and given a starting point on the line, Po: Po = (v0, vof the parametric equations of the line are: x(t): x(t) = x0 + vxt y(tf yW = y0 + vyt For each time point, t, in the in the line, the processing module may be configured filter the pixel neighbourhood convolving it with a kernel of size k (e.g., a gaussian kernel). The size k of the kernel is a parameter that depends by the “thickness” of the line. In particular, for example, for each point of the line the processing module may be configured apply the following process: Given a kernel function, H\ 2(72 Calculate a convolution function G: G(xt,yt) = u=—kv=—k H(u,v)I(xt +u,yt + v) Here, the function / (xf + u, yt + v) represents the intensity / power of the continuous wavelet transform (CWT) where it forms the line on the scalogram. The artefact (“hole”) detection and time-tagging can then be based on the value of the function G(xt, y^. When the value of G(xt,yt) falls below a suitable pre-set threshold value, then the processing module determines that a “hole” artefact is detected. In general, the procedure evaluates time positions of the “hole” artefacts. The procedure identifies the location of a line in the scalogram, and evaluates the power of the CWT along that line, using a kernel to consider a neighbourhood around each point of the line. It evaluates when the CWT power drops under a threshold. This procedure is effective in detecting “hole” artefacts generated by discontinuous changes in phase of n in the self-mixing interferometric signal, and in detecting “hole” artefacts generated by discontinuous changes in phase of tt / 2 in the selfmixing interferometric signal, and phase differences in between those values or similar (more than tt, less than n / 2). The processing module may be configured to time-tag the positions of the artefacts within the scalogram (e.g., the “holes”) identified in this way. Referring to Figures 9A and 9B, suppose a particle 4 travels across the laser beam 2, causing a self-mixing interferometric signal 30 to be created. The wavelet transformation of that self-mixing interferometric signal will appear as a continuous trace with artefacts arrayed along it (e.g., “holes”) e.g., Figure 1 IC etc. The artefacts are time tagged to define a series of time tags. In Figure 1 IC, the time tags are [10, 20,30,40,60,80,120,140] ps. The processing module may be configured to generate from the time tag series of artefacts a binary code. In this operation, the processing module may be configured to: • Use a defined time interval to describe the rate of bits of the binary code; and, • Switch the binary code (from 0 to 1 or from 1 to 0) at the times corresponding to the time tags (i.e., where a phase change occurs). For example, if a lOps time interval is used, the series of time tagged events [10,20,30,40,60,80,120,140] ps will correspond to the binary sequence: 101011001000100 (or 010100110111011, depending how the first transition is labelled). If a time interval of 5ps is used, the binary sequence will be: 110011001111000011000000110000. Each binary sequence may represent unique digital key which is associated to a trajectory 7 and a speed of a particle 4 through the “phase map” of the laser beam 2 in the monitored region. Given a 3D spatial distribution of phases (i.e., a 3D “phase map”) it is possible to pre-calculate the unique digital keys associated to pre-calibrated particle trajectories and speeds. The binary code sequence recorded during a current event is compared, by the processing module, with all the pre-calculated possible digital keys (binary code sequences). The processing module may employ a similarity metric to quantify the degree of similarity between a current binary code sequence and a given pre-calculated binary code sequence, and to determine that a match has been found when the similarity metric reaches a suitable threshold value. This method may be applied to each of the scalograms illustrated in the examples of Figures 1 IC, 12C, 13C, 14C and 15C. Figures 11A-11C, 14A-14C and 15A-15C share a common temporal sequence of phase shifts, albeit with a different phase shift value for figures 14A-14C and a different particle speed for figures 15A-15C. Figures 12A-12C and 13A-13C share a common temporal sequence of phase shifts, albeit with a different phase shift value for figures 13A-13C. This is summarised in Table 1, below. Table 1: Figure Phase Sequence Phase Shift Speed (arb) 11A-11C 1 n lx 12A-12C 2 n lx 13A-13C 2 n / 2 lx 14A-14C 1 n / 2 lx 15A-15C 1 n 2x Figures 11 A-l IC correspond to a first particle trajectory, while figures 12A-12C correspond to a different second particle trajectory, and figures 15A-15C correspond to a third particle trajectory which is the same direction as the first trajectory but at a different, higher speed (x2). Each scalogram uniquely identifies a different particle trajectory and the processing module may be configured to preferably configured to use that uniqueness to identify the different particle trajectories (direction and speed). The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] Lim, Soon Wei Daniel &Park, Joon-Suh &Meretska, Maryna &Dorrah, Ahmed &Capasso, Federico. (2021). “Engineering Phase and Polarization Singularity Sheets’". Nature Communications. 12. 4190. [2] Beijersbergen, M. W.; Coerwinkel, R.P.C.; Kristensen, M., Woerdman, IP (1994). "Helical-wavefront laser beams produced with a spiral phase plate". Optics Communications. 112 (5--6): 321. [3] Soskin, M.; Gorshkov, V.; Vasnetsov, M.; Malos, J.; Heckenberg, N. (1997). " Topological charge and angular momentum of light beams carrying optical vortices". Phys. Rev. A. 56 (5): 4064. [4] Karimi, E.; Piccirillo, Bruno; Nagali, Eleonora; Marrucci, Lorenzo; Santamato, Enrico (2009). "Efficient generation and sorting of orbital angular momentum eigenmodes of light by thermally tunedq-plates". Applied Physics Letters. 94 (23): 231124. [5] Du, L., Man, Z., Zhang, Y. et al. “Manipulating orbital angular momentum of light with tailored in-plane polarization states’’’. Sci Rep 7, 41001 (2017). [6] Lin et al.: “A Comprehensive Review of Image Line Segment Detection and Description: Taxonomies, Comparisons, and Challenges”, arXiv:2305.00264vl [cs.CV] 29 Apr 2023.

Claims

1. A self-mixing interferometer configured to monitor particulate material within a monitored region of space comprising:a laser cavity assembly;an optical assembly configured to bathe the monitored region with laser light of the interferometer possessing one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region;a laser monitoring unit configured to acquire an interferometric signal generated by the interferometer in response to light returned to the laser cavity assembly from said optical wavefronts by said particulate material;a processing module configured to determine a property of the particulate material within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal.

2. A self-mixing interferometer according to claim 1 wherein the optical assembly comprises one or more optical elements configured to receive light from the laser cavity and to create therefrom said laser light possessing said one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region.

3. A self-mixing interferometer according to claim 2 wherein the one or more optical elements comprise a metalens configured to manipulate said light received from the laser cavity so as to form said one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase.

4. A self-mixing interferometer according to any preceding claim wherein the optical assembly is configured to bathe the monitored region with laser light of the interferometer possessingoptical wavefronts having different directions at different respective locations within the monitored region.

5. A self-mixing interferometer according to claim 4 wherein the processing module is configured to determine a property of the particulate material within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal.

6. A self-mixing interferometer according to claim 5 wherein the processing module is configured to determine a property of the particulate material within the monitored region according to a continuous change in the frequency of said waveform.

7. A self-mixing interferometer according to any preceding claim wherein the optical assembly is configured to bathe the monitored region with a static divergent and / or convergent beam of said laser light possessing a curved wavefront defined by an optical wave comprising said one or more rapid or discontinuous changes in optical phase, in which the monitored region comprises regions other than the focal region of said laser light.

8. A self-mixing interferometer according to any preceding claim wherein the optical assembly is configured to bathe the monitored region with a beam of said laser light possessing a substantially flat wavefront defined by an optical wave comprising said one or more rapid or discontinuous changes in optical phase, and to move the flat wavefront across the monitored region to a plurality of different directions.

9. A self-mixing interferometer according to any preceding claim wherein the processing module is configured to determine a property of the particulate material within the monitored region according to a wavelet transformation of the interferometric signal.

10. A self-mixing interferometer according to any preceding claim wherein the interferometer is configured to generate said interferometric signal comprising a voltage signal to be acquired by the laser monitoring unit, wherein the voltage signal corresponds to a voltage across electrical drive terminals of a laser cavity of the laser cavity assembly and comprises a voltage signal waveform in response to movement of said particulate material along a path within said monitored region of space.

11. A self-mixing interferometer according to any preceding claim wherein the interferometer is configured to generate said interferometric signal comprising an optical output power signal to be acquired by the laser monitoring unit, wherein the optical output power signal corresponds to an optical output power of a laser cavity of the laser cavity assembly and comprises an optical output power signal waveform in response to movement of said particulate material along a path within said monitored region of space.

12. A self-mixing interferometer according to any preceding claim wherein the property of the particulate material comprises a property of the path thereof within the monitored region.

13. A self-mixing interferometer according to claim 12 wherein the property of said path comprises one or more of: a distance to said particulate material relative to the interferometer; a speed of said particulate material relative to the interferometer; a direction of said particulate material relative to the interferometer.

14. A portable electronic device comprising the self-mixing interferometer according to any preceding claim.

15. A wearable electronic device comprising the portable electronic device of claim 14.

16. An air purification device comprising the self-mixing interferometer according to any of claims 1 to 13.

17. A method for monitoring particulate material within a monitored region of space using selfmixing interferometry comprising:providing an interferometer comprising a laser cavity assembly and an optical assembly;by the optical assembly, bathing the monitored region with laser light of the interferometer possessing one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region;acquiring an interferometric signal generated by the interferometer in response to light returned to the laser cavity assembly from said optical wavefronts by said particulate material;by a processing module, determining a property of the particulate material within the monitored region according to one or more rapid or discontinuous changes in the phase of a waveform within at least a part of the interferometric signal.

18. A method according to claim 19 including, by the optical assembly, receiving light from the laser cavity and creating therefrom said laser light possessing said one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase within the monitored region.

19. A method according to claim 18 comprising providing the optical assembly with a metalens and therewith manipulating said light received from the laser cavity so as to form said one or more optical wavefronts defined by an optical wave comprising one or more rapid or discontinuous changes in optical phase.

20. A method according to any of claims 18 to 19 comprising, by the optical assembly, bathing the monitored region with laser light of the interferometer possessing optical wavefronts having different directions at different respective locations within the monitored region.

21. A method according to claim 20 comprising, by the processing module, determining a property of the particulate material within the monitored region according to changes in the frequency of a waveform within at least a part of the interferometric signal.

22. A method according to claim 21 comprising, by the processing module, determining a property of the particulate material within the monitored region according to a continuous change in the frequency of said waveform.

23. A method according to any of claims 17 to 22 comprising, by the optical assembly, bathing the monitored region with a static divergent and / or convergent beam of said laser light possessing a curved wavefront defined by an optical wave comprising said one or more rapidor discontinuous changes in optical phase, in which the monitored region comprises regions other than the focal region of said laser light.

24. A method according to any of claims 17 to 23 comprising, by the optical assembly, bathing the monitored region with a beam of said laser light possessing a substantially flat wavefront defined by an optical wave comprising said one or more rapid or discontinuous changes in optical phase, and to move the flat wavefront across the monitored region to a plurality of different directions.

25. A method according to any of claims 17 to 24 comprising, by the processing module, determining a property of the particulate material within the monitored region according to a wavelet transformation of the interferometric signal.

26. A method according to any of claims 17 to 25 comprising, by the interferometer, generating said interferometric signal comprising a voltage signal to be acquired by the laser monitoring unit, wherein the voltage signal corresponds to a voltage across electrical drive terminals of a laser cavity of the laser cavity assembly and comprises a voltage signal waveform in response to movement of said particulate material along a path within said monitored region of space.

27. A method according to any of claims 17 to 26 comprising, by the interferometer, generating said interferometric signal comprising an optical output power signal to be acquired by the laser monitoring unit, wherein the optical output power signal corresponds to an optical output power of a laser cavity of the laser cavity assembly and comprises an optical output power signal waveform in response to movement of said particulate material along a path within said monitored region of space.

28. A method according to any of claims 17 to 28 wherein the property of the particulate material comprises a property of the path thereof within the monitored region.

29. A method according to claim 28 wherein the property of said path comprises one or more of: a distance to said particulate material relative to the interferometer; a speed of said particulate material relative to the interferometer; a direction of said particulate material relative to the interferometer.

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