WO2010060460A1

WO2010060460A1 - Device for multispectral and spatial shaping

Info

Publication number: WO2010060460A1
Application number: PCT/EP2008/066136
Authority: WO
Inventors: Ivan Amat Roldan; Pablo LOZA ÁLVAREZ; David ARTIGAS GARCÍA
Original assignee: Institut De Ciències Fotòniques, Fundació Privada; Universitat Politècnica De Catalunya
Priority date: 2008-11-25
Filing date: 2008-11-25
Publication date: 2010-06-03

Abstract

A device for multispectral and spatial shaping an input pulsed beam, said pulsed beam comprising light of a plurality of wavelengths, said device comprising: a first means for spatially decomposing the said plurality of wavelengths comprised in said pulsed beam into an independent number of beams of the said plurality of wavelengths; a second means for independently spatially modulating each of said spatially decomposed beams of a plurality of wavelengths; and a third means for spatially recombining the said spatially decomposed and independently modulated plurality of wavelengths into a new shaped pulsed beam.

Description

DEVICE FOR MULTISPECTRAL AND SPATIAL SHAPING

FIELD OF THE INVENTION

The present invention relates to optical multispectral beam shaping and, in particular, to optical multiwavelength spatiotemporal shapers .

STATE OF THE ART

In general, cutting edge photonics technologies divide devices into two main categories: (1) Spatial manipulation of light (beam shaping) ; and (2) Temporal manipulation of light (temporal shaping) .

The two categories of cutting edge photonics technologies (beam shaping and temporal shaping) are illustrated in figure 1. Current devices for shaping have been mainly focused to either beam shaping or temporal shaping. Beam shaping devices utilize monochromatic light to produce the desired effects and temporal effects are neglected. In general, beam shaping underperforms when pulsed light is utilized, especially for pulses shorter than one picosecond. Temporal shaping devices manipulate different spectral components (or wavelengths) of pulsed

(or polychromatic) light to produce the desired effects and spatial effects are neglected. In general, temporal shaping has no meaning when monochromatic light is utilized. Examples of application which use beam shaping are: optical tweezers, adaptive optics, holography, projectors, etc. Examples of devices to achieve beam shaping are: Spatial Light Modulators, liquid crystal based devices, DOEs, holograms and diffusers. Examples of devices to perform temporal shaping are based on (1) Acousto-optic programmable dispersive filters (AOPDF) like the DAZZLER or (2) Spatial Light Modulators (SLM) like the device described in patent application US20070268546.

There is a wide range of devices for beam shaping applications: 1) optical tweezers; 2) optical trapping, which is interesting for manipulation of biological samples; 3) optical sorting which uses spatial shaping to classify objects or specimens according to its size and/or shape; 4) projectors; 5) adaptive optics, especially for astronomy and microscopy, or imaging applications in general; 6) quantum information technologies such as quantum computing, quantum optical processing, quantum criptography, 7) to transfer motion energy to micro-machines by exploiting, for example the angular momentum of a wavefront, and 8) spatially shape the output beam of a laser or optical parametric oscillator .

In general, current beam shaping devices work at a single or only a few wavelengths, because they are devices with a design which is problem-oriented and they are highly simplified and limited. For example, 2D or 3D projectors, in which three wavelengths are used (blue, green and red) are combined for displaying images (2D or 3D), or optical tweezers devices, in which a single wavelength is spatially shaped to trap one or many objects . A growing field is that of temporal shaping devices. Examples of devices performing temporal shaping are dazzlers and SLM-based temporal shapers . Probably, the most common applications are in the telecom industry where temporal shaping is exploited to 1) code as much information as possible to increase transmission speeds and 2) perform optical signal processing tasks. Temporal shaping started a new era in laser technologies by including dispersion stages in laser cavities. This importantly produced new types of lasers which generate sub-picosecond pulsed light, which are named "ultrashort pulsed lasers". This also produced new types of optical parametric oscillators which were synchronously pumped.

Another field in which temporal shaping devices are being introduced recently is imaging and detection devices. For instance, multiphoton microscopy has used temporal shaping techniques to gain new sources of contrast to discriminate within different specimens or obtain functional information. As illustrated in figure 2, temporal shapers comprise a dispersing element G followed by a lens L2.

Nevertheless, there are a few cases in which spatiotemporal shaping is reported at the expense of imposing some tight relationship between space and time.

A recent work (E. TaI et al . , "Transformation from an ultrashort pulse to a spatiotemporal speckle by a thin scattering surface", Opt. Lett. 31 3529, 2006) studies the temporal properties of spatiotemporal speckles generated by an ultrashort pulse in the far field after passing through a diffuser (thin scattering plate) . However, in this experiment there is no control on the speckle distribution and dimensions, especially in the longitudinal axis. Therefore, these approaches do not achieve: (1) independent shaping of a discrete number of wavefronts at different wavelengths; or (2) 2-D optical confinement of light.

Additionally, US patent application number US5682262 describes a method and device for generating spatially and temporally shaped optical wavefronts. The device disclosed in US5682262 disperses a pulsed light source in a grid of points to perform optical communications and optical signal processing. It generates arbitrary pulse sequences within multiple regions which are spatially isolated at the output plane. However, the device described in US5682262 does not achieve full independent shaping of a discrete number of wavefronts at different wavelengths or 2-D optical confinement of light. More specifically, the device described in US5682262 is restricted to produce sequences of pulses within a grid of isolated spots in the output plane.

Other works regarding multiple point scanning of pulsed laser light have been carried by exploiting some specifically tailored diffractive optical elements (DOEs) in conjunction with dispersion elements to perform specific tasks. However, these approaches do not achieve independent shaping of a discrete number of wavefronts at different wavelengths (coherent or incoherent) and the differences are equivalent to those explained above, referring to US5682262. One of the most active fields of photonic technologies is imaging. The ability of light to perform accurate imaging of a manifold of objects has found several applications ranging from heavy industry to medical equipment.

The devices most widely used for acquiring high resolution imaging of living specimens is confocal microscopy and multiphoton microscopy, which achieve detailed 2D and 3D images of any specimen by means of "optical sectioning". "Optical sectioning" consists in a focused spot which is scanned throughout a 2D plane to build an intensity image of the imaged object. The name naturally comes from the notion that light from a thin section or slice is imaged, where thin section refers to a thickness dictated by Rayleight limit and depends on wavelength and Numerical Aperture of the imaging lens. Then, 3D images are trivially achieved by stacking up several "optical sections", or in another means, by scanning a 3D volume.

Especially, microscopy for biomedical applications has focused in the latter years in developing new high resolution 3D imaging which overcome current bottlenecks of confocal and multiphoton microscopy such as acquisition time and complex electronic synchronization devices. In this line, several new microscopy techniques have emerged: Dynamic Speckle Imaging (DSI), Temporal focusing, Selective Plane Illumination Microscopy (SPIM), Structured Illumination Microscopy (SIM) . However, these approaches listed above have not successfully solved the current bottlenecks such as acquisition time and complex electronic synchronization devices.

Therefore, the above mentioned techniques show that there is a need to improve conventional optical shapers for the upcoming applications related to new photonic and laser technologies, which appears of special interest for imaging and microscopy applications.

SUMMARY OF THE INVENTION

The present invention is intended to address the above mentioned need.

The multispectral and spatial shaper of the invention allows: (1) a systematic, independent and direct spatial and multispectral manipulation of light, which means that there is no limit in the number of simultaneous wavefronts that can be used and/or (2) 2-D optical confinement of light.

Specifically, the invention provides a device to perform simultaneous, direct and independent spatiotemporal shaping (amplitude, phase, polarization,...) of an input optical multispectral beam (beam composed by multiple wavelengths) to generate a spatiotemporal shaped pulsed-beam (coherent or incoherent) at the output. With respect to other works which exploit some specifically tailored diffractive optical elements (DOEs) in conjunction with dispersion elements, the inventive device independently shapes a discrete number of wavefronts at different wavelengths (coherent or incoherent) in a manner which has never been reported before .

One of the direct applications of the present invention is imaging and microscopy to overcome the prior-art technological bottlenecks such as acquisition time and complex synchronization devices. Overcoming such technological bottlenecks is achieved by the inventive multispectral and spatial shaper, which spatiotemporally shapes an input pulse, confines the multiphoton interaction of light to a bidimensional plane perpendicular to the optical axis of propagation at an output plane and is capable of obtaining optical sectioning with axial resolutions below Rayleigh diffraction limit, using pulsed light of any duration. Of especial interest is its application in pulsed light with pulse duration below 100 ps, sub-picosecond or sub- femtosecond which cannot be shaped by means of electronic devices. In this manner, the effect of optical sectioning is achieved and the current Rayleigh diffraction limit can be overcome in the axial direction.

The inventive device comprises means for achieving beam dispersion, modulation and spectral recombination. Additional means can be added to perform imaging to transfer the spatiotemporally shaped pulsed-beam to a sample plane.

In a first aspect of the present invention there is provided a device for multispectral and spatial shaping an input pulsed beam, said pulsed beam comprising light of a plurality of wavelengths. The device comprises: a first means for spatially decomposing the said plurality of wavelengths comprised in said pulsed beam into an independent number of beams of the said plurality of wavelengths; a second means for independently spatially modulating each of said spatially decomposed beams of a plurality of wavelengths; and a third means for spatially recombining the said spatially decomposed and independently modulated plurality of wavelengths into a new shaped pulsed beam.

Preferably, the device further comprising a fourth means for imaging the said new shaped pulsed beam.

In a particular embodiment, the first means comprises: a first focusing means having a focal length, for focusing said input pulsed beam; a first dispersive element placed at a distance equal to the focal length of said first focusing means, said dispersive element being configured for spatially decomposing said plurality of wavelengths comprised in said input pulse beam; a first collimating means having a focal length for collimating each of said plurality of wavelengths.

In a particular embodiment, the second means comprises: a spatial modulator placed at a distance equal to the focal length of said first collimating means, said spatial modulator being configured for independently shaping at least one spatial characteristic of each of said plurality of wavelengths.

In a particular embodiment, the third means comprises: a second focusing means having a focal length, said second focusing means being placed at a distance equal to said focal length from said spatial modulator, for focusing said spatially modulated beams; and a second dispersive element placed at a distance equal to the focal length of said second focusing means, said second dispersive element being configured for reconstructing the previously spatially-shaped pulsed beam.

Besides, the fourth means preferably comprises: an imaging system in turn comprising at least a lens having a focal length for imaging the said reconstructed spatially shaped beam to any desired plane.

The spatial modulator is preferably built from a kinoform phase plate. More preferably, said said kinoform phase plate has a certain thickness, said thickness being defined according to the following mathematical function:

wherein X₁ is the central wavelength of said pulsed light beam, n is the refractive index of the material forming said phase plate, M is a discrete number of wavelengths in which the pulsed-beam is decomposed to form a region indexed by index i, x and y are the transversal coordinates, in the Cartesian frame, of the said phase plate and φj_(x, y, λ_∑) is a function representing a phase modulation to be caused by said kinoform. The functions φ_± (x, y, X₁) preferably take the following form for each of the respective i wavelength regions, wherein i is the wavelengths index: φ_ι {x, y,λ_t ) = angle{F{s_l (x, y) QXp(Je₁ (x, y))})

wherein function angle () denotes a complex argument, F{} denotes the Fourier transform, O₁ (x, y) denotes a uniformly distributed random variable from 0 to 2π and function S₁ (x, y) denotes any required shape for a specific wavelength in the i-th region.

Preferably, the kinoform phase plate is designed to form concentric rings of speckles at the fourier plane of said second focusing means, each of said concentric rings being generated by a different wavelength of said plurality of wavelengths, said device thus obtaining 2D optical confinement of light and diffraction-limited axial resolution.

Besides, the function S₁ (x, y) is preferably selected to be rings of radius r_lf each of said radius being different for each wavelength i.

In an alternative embodiment, the device comprises a single kinoform phase plate in which the following are integrated: said first means for spatially decomposing the said plurality of wavelengths comprised in said pulsed beam; a second means for independently spatially modulating the said spatially decomposed plurality of wavelengths; and a third means for spatially recombining the said spatially decomposed and modulated plurality of wavelengths into a new shaped pulsed beam; and wherein said single kinoform phase plate has a certain thickness, said thickness being defined, according to the following mathematical function: h(x,y,λ) = ———φ{x, y,λ)

2π n(Λ) - 1 wherein λ is the central wavelength of said pulsed light beam, n is refractive index of the material forming said phase plate, x and y are the transversal coordinates, in the Cartesian frame, of the said phase plate and φ{x, y, λ) is a function representing the phase modulation to be caused by the kinoform, and φ(x, y, λ) is a function representing a phase modulation to be caused by said kinoform, said function taking the following expression: ø{x,y,λ) = angle{F{s(x,y)exp(jθ(x,y))})

wherein function angle () denotes a complex argument, F{} denotes the Fourier transform, θ(x, y) denotes a uniformly distributed random variable from 0 to 2π and function s (x, y) is selected to be a single ring of radius r, said radius being dependent on the central wavelength .

Preferably, the kinoform phase plate is engraved in a dispersive substrate, said dispersive substrate being transparent to the spectral bandwidth of the input pulsed beam. More preferably, the dispersive substrate is selected from a group of highly dispersive glasses or crystals, preferably, from the following group of highly dispersive glasses or crystals: Quartz, calcium, calcium fluoride, SFlO, BK7, LAKL21 and any combination thereof. This kinoform phase plate is specially designed for being used with pulses shorter than around 200 fs .

The kinoform phase plate is preferably engraved in an artificial material with engineered dispersion, preferably in one of the following artificial materials with engineered dispersion: porous materials, photonic crystals, metamaterials, superprisms, virtual imaged phase arrays and any combination thereof, thus allowing using said device with pulses having duration larger than

200 fs.

In a first aspect of the present invention there is provided the use of the device previously mentioned, said device being used as: a multispectral optical tweezer; or a multispectral optical trapping; or a multispectral optical sorting for classifying objects or specimens according to their size and/or shape; or a multispectral projector for 2D and 3D displays; or a multispectral adaptive optics, especially for astronomy and microscopy; or a spatio-temporal optical multiplexer or demultiplexer; or an optical confining and light interaction means to perform optical sectioning for imaging or ablation; or a spatiotemporal dispersion stage to mode-lock a laser cavity or optical parametric oscillators .

BRIEF DESCRIPTION OF THE DRAWINGS

In order to provide for a better understanding of the invention, a set of drawings is provided, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be embodied. The drawings comprise the following figures :

Figure 1 illustrates a scheme with the two categories of cutting edge photonics technologies (beam shaping and temporal shaping) .

Figure 2 illustrates the dispersion stage of a conventional temporal shaper.

Figure 3 shows a multispectral and spatial shaper device according to an embodiment of the present invention .

Figure 4 shows the effect of the beam dispersing means of the multispectral and spatial shaper of figure 3.

Figure 5 shows a multispectral and spatial shaper to perform specific spatiotemporal shaping and achieve 2-D optical confinement of light according to an alternative embodiment of the present invention.

Figure 6 shows a highly simplified multispectral and spatial shaper to perform specific spatiotemporal shaping and achieve 2-D optical confinement of light according to another alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the terms "about" and "around".

Figure 3 shows a multispectral and spatial shaper according to an embodiment of the present invention, which is capable of shaping an input multispectral beam which comprises a light beam of a plurality of wavelengths. The spatiotemporal shaper comprises: means for spatially decomposing the plurality of wavelengths which form a multispectral input beam into an independent number of beams (one per wavelength) ; means for spatially modulating each of those spatially decomposed beams; means for spatially recombining those spatially decomposed and independently modulated plurality of wavelengths into a new shaped multispectral beam or image; and means for imaging the new shaped multispectral beam or image. In particular, the means for spatially decomposing the plurality of wavelengths (also called means for beam dispersion or beam dispersor) comprises: a lens Ll, acting as focusing element, which is located at an optical axis z at a distance fi (being the focal length of lens Ll) from the plane which is considered as the input plane I_p; a dispersing element Gl, which is located in the same optical axis z as lens Ll, and at a distance fi from that lens Ll (that is to say, at a distance 2fi from the input plane I_p) ; and another lens L2 also located in the same optical axis z as lens Ll and dispersing element Gl, and at a distance f₂ (being the focal length of lens L2) from the dispersing element Gl (and therefore at a distance fi+f₂ from the first lens Ll and at a distance 2fi+f₂ from the input plane I_p. The second lens L2 acts as a collimating element and is located at a plane called spectrally resolved plane (SRp) .

The focusing element Ll, which can be implemented by a lens or equivalent imaging element, located prior to a traditional dispersion stage of a temporal shaper, is extremely important because it is responsible for the focusing of the input light beam at the dispersive element Gl. According to this focusing, lens L2 independently collimates the beams corresponding to a different colour (that is to say, wavelength) and a different modulation can be independently assigned at the modulation stage. These lenses Ll and L2 need to be radially symmetric. The means for independently spatially modulating each of those spatially decomposed beams comprises a spatial modulator SM located in the same optical axis z as lenses Ll and L2 and dispersing element Gl, and at a distance f₂ from the second lens L2 (and therefore at a distance 2f₂ from the dispersive element Gl, at a distance fi+2f₂ from the first lens Ll and at a distance 2fi+2f₂ from the input plane I_p) . The spatial modulator SM is located at a plane called plane of spatial modulator SMp. Non-limiting examples of the spatial modulator are an amplitude modulator, a phase modulator (such as a programmable phase modulator (PPM) or a spatial light modulator (SLM) ) or a polarization modulator, built from, for example, liquid crystal or deformable mirror. In this configuration, after lens L2, the beam is arriving to the SM as the conjunction of spatially decomposed collimated beams, each of them with a different wavelength. Therefore, beams with different wavelength reach different region in the SM and thus they can be simultaneous and independently modulated (introducing different modulation characteristics in each region at the same moment) .

The means for spatially recombining those spatially decomposed and independently modulated plurality of wavelengths into a new shaped pulsed beam (also called means for spectral recombination) comprises a lens L3, acting as focusing element, which is located at that optical axis z at a distance f₃ (being the focal length of lens L3) from the spatial modulator SM (that is to say, at a distance f₃+f₂ from lens L2, at a distance f₃+2f₂ from dispersing element Gl, at a distance fi+f₃+2f₂ from lens Ll and at a distance 2f_!+f₃+2f₂ from the plane which is considered as the input plane I_p) ; and a dispersing element G2, which is located in that same optical axis z and at a distance f₃ from lens L3 (as illustrated in figure 3) , for reconstructing the previously spatially-shaped multispectral beam. The plane at which the dispersing element G2 is located is called Fourier plane Fp.

The dispersive elements Gl G2 are preferably gratings. Non-limiting examples of alternative dispersive elements can be: prism, superprisms, virtually imaged phased array (VIPA) , grism, or any other similar device that introduces dispersion or any combination of them.

Optionally, the means for imaging (or imaging system) the new shaped multispectral beam or image can comprise an additional lens L4, located in that same optical axis z and at a distance f₄ (being the focal length of lens L4) from dispersing element G2 (as illustrated in figure 3) for imaging the reconstructed spatially shaped beam to any desired plane (in figure 3, at an output plane Op) .

In comparison to the conventional dispersion stage of a temporal shaper, which is shown in figure 2, the inventive beam dispersor, shown in figure 4, which forms part of the inventive multispectral and spatial shaper of figure 3, requires an extra lens or equivalent imaging element Ll to be located prior to a traditional dispersion stage of a temporal shaper. Additionally, dispersion stages of conventional temporal shapers (figure 2) use cylindrical lenses as lens L2, whereas in the inventive beam dispersor of figure 4 (belonging to the multispectral and spatial shaper of figure 3) this lens L2 has radial symmetry. In summary, with respect to the conventional temporal shaper of figure 2, the inventive multispectral and spatial shaper requires: two extra lenses (Ll and L4) and L2 and L3 to be normal lenses (with radial symmetry) as illustrated in figure 3. The additional lenses, plus the fact that all the lenses need to have radial symmetry, conceptually changes the effect of all subsequent optical elements of the device and swaps real and conjugate planes of the whole optical system with respect to conventional temporal shapers. This is illustrated in figure 4, wherein the effect of the beam dispersing means of figure 3 is detailed. As can be observed, the dispersive stage of figure 4 (of the beam dispersor which comprises Ll, Gl and L2) works differently from the one of the temporal shaper (figure 2) . Here, since the temporal shapers do not have an extra lens Ll, the dispersive stage of temporal shapers focuses individual wavelengths at the plane of the spatial modulator (SMp in figure 2) . In this way all the components comprised within a multispectral beam are mixed and cannot be independently shaped. On the contrary, the beam dispersor of the device of the present invention spectrally resolves individual beams at different spatial locations with different wavelength as shown in figure 4, which is fundamental to achieve independent and simultaneous multispectral beam shaping as described in the present invention and further enables spatiotemporal engineering of the input light, which is not achieved by any other existing device. The multispectral and spatial shaper just described is capable of shaping the incoming light at the input plane of many types of light, ranging from a temporally coherent beam (pulsed beams of any repetition rate or period and any pulse duration) to an incoherent light source. Therefore, this device can be extremely useful in the spatiotemporal manipulation of ultrashort pulses (sub-picosecond) or picosecond pulses (sub-nanosecond) for a manifold of applications.

One of the direct applications of the present invention is imaging and microscopy to overcome the prior-art technological bottlenecks such as acquisition time and complex synchronization devices. Overcoming such technological bottlenecks is achieved by spatiotemporally shaping the pulse to confine the multiphoton interaction of light to a bidimensional plane perpendicular to the optical axis of propagation and obtain optical sectioning with picosecond pulses and ultrashort pulses (sub- picosecond pulses) .

Importantly, it is the first time in which non- scanning optical sectioning with axial resolutions below Rayleigh diffraction limit is achieved. In this manner, the effect of optical sectioning is achieved and the current Rayleigh diffraction limit can be overcome in the axial direction. In opposition, conventional spatiotemporal shapers, like the one in US5682262, are not concerned with the axial resolution because they code the information in a transversal intensity distribution

(x,y) and not in the axial one. Besides, TaI et al . show that there is no 2-D confinement of multiphoton interaction in the reported experiment because the axial confinement is much bigger than the Rayleigh diffraction limit and therefore their proposed scheme cannot produce "optical sectioning".

The multispectral and spatial shaper described herein can be used, for example, to produce arbitrary spatiotemporal patterns at a specific plane or correct broadband optical systems by means of a single device. Non-scanning microscopy and imaging applications for high resolution 2D or 3D imaging, for example multiphoton microscopy, require to eliminate high intensity outside the output plane and perform "optical sectioning", which imposes a nontrivial constraint in the design of the device.

The multispectral and spatial shaper described up to here imposes no limitations on the spatiotemporal patterns produced at the output plane, and therefore, it is not concerned in how the light is propagated outside the output plane Op (figure 3) . However, bidimensional optical confinement to achieve, for example, optical sectioning, can be obtained by specific spatiotemporal shaping by means of the present invention and some additional specific design of the spatial modulator SM, which is detailed below. Therefore, the design of the multispectral and spatial shaper can be further constrained to produce bidimensional optical confinement. The main advantage of optical confinement of light is that it produces negligible light intensity outside the output plane Op and therefore, there is no multiphoton interaction or optical sectioning. This more particular embodiment is described next.

Additionally, the spatial modulator SM can be, in general, of two types: dynamic SM or static SM. The utilized type will depend on the application. For example, dynamic SMs, such as deformable mirrors or liquid crystal-based modulators, are of interest for applications in the field of adaptive optics and projectors. The present invention is of special interest in dynamic applications because the modulation is direct and does not require any type of computation (such as iterative algorithms) and can be performed in real time. Alternatively, static modulations can be engraved in a plate or substrate for many reasons, such as miniaturization or cost.

There is a widespread of optical elements with an engraved modulation (amplitude, phase, polarization, ...) . Typically phase plates are a preferable choice because there is minimal loss of light intensity. However, they require a complex step of engineering.

In this line, several further implementations are detailed:

(1) Preferably, a kinoform phase plate (KPP) is chosen as a SM for devices oriented to specific tasks, such as microscopy, imaging, ablation, among others, which require a static multispectral shaping. (2) In particular, the KPP or any other type of modulating plate can be designed to generate a series of rings with arbitrary radius depending on the incident wavelengths contained within the input multispectral beam at the far field. This specific design produces 2-D optical confinement of light at the output plane because it minimizes the spatiotemporal autocorrelation of the input multispectral beam. This is referred as multispectral and spatial shaper for non-scanning optical confinement. The steps for the proper design of the KPP are described below.

(3) In an even more specific embodiment, the multispectral and spatial shaper for non-scanning confinement of light can be further simplified by designing a KPP which simultaneously disperses and diffracts the incident multispectral beam which is recombined at the output plane. This is possible if the material dispersion properties of (a) a plate, substrate or optical device located after the modulating phase plate, for example, KPP or (b) a plate or substrate in which the modulation is engraved, are considered (as shown in figure 6) .

These alternatives are explained in detail next.

A first alternative embodiment is described. Figure 5 shows a spatiotemporal shaper in which the means for spatially modulating of figure 3 is a means for spatially phase modulating and is implemented by means of a phase plate. Preferably, such phase plate is a kinoform phase plate (KPP) . Alternative implementations of the phase plate are, among others: a diffractive optical element (DOE) , a hologram and a hologram optical element (HOE) .

Kinoforms are computer generated holograms that can be used as a particular case of spatial modulators. They possess different advantages: (1) Many algorithms have been developed to achieve the desired shape. (2) Efficient (minimal power losses) . (3) Scalable fabrication, and cheap for big orders.

Kinoforms possess an engraved pattern, h(x,y), that is transferred to the wavefront, φ(x,y), as follows: φ(x,y)=^{n(λ)-l)h(x,y)

A

In particular, in the embodiment illustrated in figure 5, the spatial modulator (SM) is implemented by a multi-kinoring phase plate. Specifically, for confinement of light interaction and achieve, for example, optical sectioning and perform imaging or ablation, the kinoform phase plate is designed to generate a series of rings with arbitrary radius depending on the optical wavelength .

In figure 5, the dispersive elements Gl G2 are also preferably gratings. Non-limiting examples of alternative dispersive elements can be: prisms, superprisms, virtually imaged phased array (VIPA) , grism, or any other similar device that introduces dispersion or any combination of them. In order to obtain the desired modulation (in particular, phase modulation) , the engraved pattern of the inventive kinoform phase plate, φ(x,y), is divided in regions for each wavelength:

φ_ι (x, y,λ,) = angle{F{s_l (x, y) exp(./6> (x, y))})

wherein function angle () denotes a complex argument, F{} denotes the Fourier transform, G₁ (x, y) denotes a uniformly distributed random variable from 0 to 2π (which introduces Gaussian white noise) and function S₁ (x, y) denotes any required shape for a specific wavelength in the i-th region at the Fourier plane (Fp in figure 5) ; for a kinoform phase plate whose thickness is defined as follows:

where n is the index of refraction of the substrate material in which the kinoform is engraved.

To calculate the multi-kinoring phase plate we have chosen s_x(x,y) to be rings of arbitrary radius, r_x, depending on the wavelength. The sequences of radii can be, for example (with no limiting purpose) , increasing (linear, quadratic.) or decreasing (negative dispersion) with wavelength, or even can be wavelength alternating, etcetera .

Figure 5 shows in detail the multi-kinorings at the spatial modulation plane (SMp) and the concentric rings on top of the dispersive element G2 (such as, for example, a grating) .

It is to be remarked that, while conventionally kinoforms are based on algorithms that remove Gaussian noise in the generated intensity pattern, the inventive kinoform here described generates white Gaussian noise into the created intensity pattern (i.e. the intensity pattern is modulated by speckles) . The use of Gaussian noise minimizes the width of the spatiotemporal autocorrelation of the multispectral beam out of the focal plane, helping to produce optical sectioning.

Conventionally, when focusing the light using a lens there was a physical limit which strongly relates the maximum light intensity to a region called the confocal parameter (equal to twice the Rayleigh range) . In a multiphoton microscope, the axial resolution is tightly related to the beam waist and is proportional to the Rayleigh range as:

Az = cte- ° λ where Az is the axial resolution, O)₀ is the beam waist and cte is a constant that depends on the experimental conditions .

It must be noticed that according to the last equation, the confocal parameter increases quadratically with beam waist. This means that large illumination areas have large confocal parameter and therefore, poor optical confinement. In conventional microscopes, to achieve high resolution, the illumination area is a (diffraction limited) point to ensure a minimum confocal parameter.

In contrast, the multispectral and spatial shaper for non-scanning confinement of light with multi- kinorings allows obtaining diffraction limited axial resolution (minimum confocal parameter, or less) with a large beam waist. Importantly, this advantage allows large illumination areas and simultaneous 2D optical confinement which is ideal for high resolution imaging or microscopy .

Next, the last particular embodiment is described in relation to figure 6. Figure 6 shows a specific multispectral and spatial shaper for non-scanning confinement of light to perform confinement of light interaction. In this implementation, the specific kinoform phase plate (KPP) , which in this embodiment is implemented by a kinoring as shown in figure 6, simultaneously disperses and diffracts light and achieves spatiotemporal shaping for 2-D optical confinement of light, if material dispersion properties of (a) a plate, substrate or optical device located after the modulating phase plate, for example, KPP or (b) a plate or substrate in which the modulation is engraved, are considered.

To calculate the single-kinoring phase plate we chose S₁(X_fV) to be a ring of arbitrary radius, r_x, for the central wavelength (or any other wavelength within the spectral bandwidth of the pulse) only. As mentioned above, this kinoring requires a specific substrate in which the single kinoring phase plate is engraved (preferably on the input facet of the substrate) . This substrate is devised to naturally introduce the necessary dispersion to spectrally resolve the input beam (in a similar manner of a grating, but not equivalent) . As in a grating, dispersion is produced by the relative different size of the engraved pattern when compared with the wavelength for each component X₁. However, the actual wavelength in the media depends on the refractive index as λ/n. Since n in turns depends on X₁, the spectral dispersion is increased, resulting in rings with different radius depending on X₁. In this manner, the spatiotemporal shaping is achieved by the combination of spatial modulation of the material thickness and the spectral modulation of the index of refraction (n) , as described above by: φ(x,y,λ)=—{n(λ)-l)h(x,y),

A which is equivalent to shape the material dispersion of a substrate located after the phase plate. Dispersion is related to the index of refraction by its derivative: dn{λ) dispersion(λ) = - dλ

This allows eliminating the dispersive elements Gl G2 (for example, gratings) and two of the lenses Ll and L2 in the previous devices.

In other words, in the single kinoform phase plate of this embodiment the following means are joint: a first means for spatially decomposing the said plurality of O Q

wavelengths comprised in the pulsed beam; a second means for independently spatially modulating the spatially decomposed plurality of wavelengths; and a third means for spatially recombining the spatially decomposed and modulated plurality of wavelengths into a new shaped multispectral beam.

The single kinoform phase plate has a certain thickness which is defined, according to the following mathematical function:

wherein λ is the central wavelength of the pulsed light beam, n is refractive index of the material forming the phase plate, x and y are the transversal coordinates, in the Cartesian frame, of the phase plate and φ{x, y, A) is a function representing the phase modulation to be caused by the kinoform, this function taking the following expression : φ{x^,y,λ) = an^gle{F{s⁽x^,y)expU0⁽x^,y))})

Contrary to the multiple-kinoring, since h(x,y,λ) is designed for a single (usually central) wavelength, this simplified device does not allow an independent modulation by itself. As mentioned above, independent spatiotemporal can be achieved by combining spatial and material dispersion modulation: φ(x,y,λ)=^{n(λ)-\)h(x,y)

A

If the multispectral beams form non-ovelapping shapes (such as rings of different radius for each wavelength for the single-kinoring) at the far field

(located at Fp in figure 6) , the spatiotemporal autocorrelation at the output plane is minimized and produces 2-D optical confinement of light.

As mentioned above, in this manner, the physical limit, the Rayleigh diffraction limit, which strongly relates the maximum light intensity to a region called the confocal parameter (equal to twice the Rayleigh range) when focusing the light using a lens is overcome in the axial direction.

The kinoform phase plate can be engraved in any material, crystal or glass that is transparent to the bandwidth of the pulse. Non-limiting examples of such materials are Quartz, calcium, calcium fluoride, SFlO, BK7 and LAKL21.

Alternatively, the kinoform phase plate can also be engraved in an artificially material with engineered dispersion, such as: porous materials, photonic crystals, metamaterials, superprisms, virtual imaged phase arrays and any combination thereof. The single-kinoring is strongly dependant on the material dispersion characteristics. Currently, the single-kinoring is designed to be used with pulses shorter than approximately 200 fs . However this can be also used with pulses of any duration (for example for pulses longer than 200fs and shorter than lOps) if specialised materials/devices, subject to the evolution of materials sciences, that provide adequate dispersion, are used as substrate. This is explained by the refractive index dependence on the wavelength in equation : φ(x,y,λ)=^{n(λ)-l)h(x,y) A

In order to overcome current material science limitations whenever they are encountered and/or a substrate or device with effective n(λ) can not be fabricated, it is recommended to implement the embodiment of figure 5 instead of this one.

Importantly, the single-kinoring-based device of figure 6 allows optical confinement of light in a sub- diffraction (which means a much smaller than half the confocal parameter) regime in the axial direction, if, as mentioned above, the multispectral beams form non- ovelapping shapes (such as rings of different radius for each wavelength for the single-kinoring) at the far field (located at Fp in figure 6) , because the axial length of the 2-D optical confinement directly depends on the spatiotemporal autocorrelation function at the output plane, which is minimized by increasing the number and distance among the rings produced by the single kinoring at the far field (Fp) .

A specific application for this device is a high resolution non-scanning system based on kinoforms for confining multiphoton interaction using pulsed laser light within a finite 2D plane transversal to the propagation axis.

Additional benefits from optical sectioning can be obtained in the form of additional applications for the present invention. For instance, applications related to ablation, which is another multiphoton process, such as: Tissue surgery, subcellular surgery, Photo Dynamic Therapy, Material processing.

Moreover, the general multispectral and spatial shaper (figure 3) is of special interest in the adaptive optics field (related to astronomy and imaging) , in which multispectral and/or broadband systems can be fully corrected with the present device and allow direct scaling of bandwidth and/or discrete number of wavelengths .

The invention is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of components, configuration, etc.), within the general scope of the invention as defined in the appended claims .

Claims

1. A device for multispectral and spatial shaping an input pulsed beam, said pulsed beam comprising light of a plurality of wavelengths, said device being characterised in that it comprises:

-a first means for spatially decomposing the said plurality of wavelengths comprised in said pulsed beam into an independent number of beams of the said plurality of wavelengths;

-a second means for independently spatially modulating each of said spatially decomposed beams of a plurality of wavelengths; and

-a third means for spatially recombining the said spatially decomposed and independently modulated plurality of wavelengths into a new shaped pulsed beam.

2. The device of claim 1, further comprising a fourth means for imaging (L4) the said new shaped pulsed beam.

3. The device of either claim 1 or 2, wherein said first means comprises:

-a first focusing means (Ll) having a focal length (fi) , for focusing said input pulsed beam;

-a first dispersive element (Gl) placed at a distance equal to the focal length (fi) of said first focusing means (Ll), said dispersive element (Gl) being configured for spatially decomposing said plurality of wavelengths comprised in said input pulse beam;

-a first collimating means (L2) having a focal length (f₂) for collimating each of said plurality of wavelengths.

4. The device of claim 3, wherein said second means comprises :

-a spatial modulator (SM) placed at a distance equal to the focal length (f₂) of said first collimating means

(L2), said spatial modulator (SM) being configured for independently shaping at least one spatial characteristic of each of said plurality of wavelengths.

5. The device of claim 4, wherein said third means comprises :

-a second focusing means (L3) having a focal length (f₃) , said second focusing means (L3) being placed at a distance equal to said focal length (f₃) from said spatial modulator (SM) , for focusing said spatially modulated beams; and

-a second dispersive element (G2) placed at a distance equal to the focal length (f₃) of said second focusing means (L3) , said second dispersive element (G2) being configured for reconstructing the previously spatially- shaped pulsed beam.

6. The device of any preceding claim when dependent on claim 2, wherein said fourth means comprises: -an imaging system in turn comprising at least a lens (L4) having a focal length (f₄) for imaging the said reconstructed spatially shaped beam to any desired plane.

7. The device of of any preceding claim, wherein said spatial modulator (SM) is built from a kinoform phase plate .

8. The device of claim 7, wherein said kinoform phase plate has a certain thickness, said thickness being defined according to the following mathematical function: φXx,y,λ)_f 1=1,2,3,..., M

wherein A₁ is the central wavelength of said pulsed light beam, n is the refractive index of the material forming said phase plate, M is a discrete number of wavelengths in which the pulsed-beam is decomposed to form a region indexed by index i, x and y are the transversal coordinates, in the Cartesian frame, of the said phase plate and φi(x, y, A₁) is a function representing a phase modulation to be caused by said kinoform.

9. The device of claim 8, wherein said functions φ₁{x, y, A₁) take the following form for each of the respective i wavelength regions, wherein i is the wavelengths index:

φ_ι{x,y,A_ι) = angle{F{s_ι(x,y)Q^{jθ_ι(x,y))})

wherein function angle () denotes a complex argument, F{} denotes the Fourier transform, G₁ (x, y) denotes a uniformly distributed random variable from 0 to 2π and function S₁ (x, y) denotes any required shape for a specific wavelength in the i-th region.

10. The device of any claim 7-9, wherein said kinoform phase plate is designed to form concentric rings of speckles at the fourier plane of said second focusing means (L3) , each of said concentric rings being generated by a different wavelength of said plurality of wavelengths, said device thus obtaining 2D optical confinement of light and diffraction-limited axial resolution .

11. The device of claims 9 and 10, wherein said function S₁ (x, y) is selected to be rings of radius r_lr each of said radius being different for each wavelength i.

12. The device of claim 1, which comprises a single kinoform phase plate (KINORING) in which the following are integrated: said first means for spatially decomposing the said plurality of wavelengths comprised in said pulsed beam; a second means for independently spatially modulating the said spatially decomposed plurality of wavelengths; and a third means for spatially recombining the said spatially decomposed and modulated plurality of wavelengths into a new shaped pulsed beam; and wherein said single kinoform phase plate has a certain thickness, said thickness being defined, according to the following mathematical function:

1 λ h(x,y,λ)=- dx,y,λ)

2π n(λ)-\ wherein λ is the central wavelength of said pulsed light beam, n is refractive index of the material forming said phase plate, x and y are the transversal coordinates, in the Cartesian frame, of the said phase plate and φ{x, y, λ) is a function representing the phase modulation to be caused by the kinoform, and φ(x, y, λ) is a function representing a phase modulation to be caused by said kinoform, said function taking the following expression: φ{x, y,λ) = angle{F{s(x, y)exp(jθ(x,y))})

13. The device of claim 12, wherein said kinoform phase plate is engraved in a dispersive substrate, said dispersive substrate being transparent to the spectral bandwidth of the input pulsed beam.

14. The device of claim 13, wherein said dispersive substrate is selected from a group of highly dispersive glasses or crystals, preferably, from the following group of highly dispersive glasses or crystals: Quartz, calcium, calcium fluoride, SFlO, BK7, LAKL21 and any combination thereof.

15. The device of either claim 13 or 14, wherein said kinoform phase plate is designed for being used with pulses shorter than around 200 fs .

16. The device of claim 13 wherein said kinoform phase plate is engraved in an artificial material with engineered dispersion, preferably in one of the following artificial materials with engineered dispersion: porous materials, photonic crystals, metamaterials, superprisms, virtual imaged phase arrays and any combination thereof, thus allowing using said device with pulses having duration larger than 200 fs .

17. Use of the device according to any preceding claim, said device being used as:

-a multispectral optical tweezer; or

-a multispectral optical trapping; or

-a multispectral optical sorting for classifying objects or specimens according to their size and/or shape; or -a multispectral projector for 2D and 3D displays; or

-a multispectral adaptive optics, especially for astronomy and microscopy; or

-a spatio-temporal optical multiplexer or demultiplexer; or -an optical confining and light interaction means to perform optical sectioning for imaging or ablation; or

-a spatiotemporal dispersion stage to mode-lock a laser cavity or optical parametric oscillators.