NL2025691B1 - Simultaneous Space-Time Resolution Broadband Coherent Raman Microscope with In-Situ Spectral Referencing - Google Patents

Abstract

First and second laser devices irradiate a sample for laser non-linear optical scattering with a time-resolved laser probe pulse of the second laser device and a short pulse impulsive laser excitation via the first laser device. A difference in linear polarization between the incident fields resulting in unique polarization control of the coherently scattered light. Resonant and non-resonant coherent Raman signals are generated from the sample in a snap-shot principle continuously over a spatial extent (X-y). The method and system enabling a new level of trueness obtained in quantitative assessments of species concentration and thermometry in gaseous turbulent flows and flames.

Description

P126758NL00 Title: Simultaneous Space-Time Resolution Broadband Coherent Raman Microscope with In-Situ Spectral Referencing

FIELD OF THE INVENTION The invention relates to a method and system for spatiotemporal detection of thermochemical states by polarization sensitive coherent Raman spectroscopy imaging.

BACKGROUND OF THE INVENTION Advances in multidimensional optical imaging techniques have revolutionized our ability to study thermochemical processes at the microscopic level. Emerging technologies for enabling direct observations are needed in research and would provide a major asset also within a wide variety of industrial applications. The requirement for novel laser diagnostic signal generation and detection schemes, is powered by the search for new thermochemical insight, e.g. at those complex and harsh processes involved in gaseous turbulent reacting and non-reacting flows. In WO2018048306 an instantaneous detection of spatial- (2D — x, y) and spectral information (1D — wavelength “lamda”) is acquired simultaneously by broadband- spectroscopy detection performed in the wide-field. This is an example of a spectral imaging sensor that samples the spectral irradiance I(x,y,A) of a scene and thus collects a three-dimensional dataset. Since it is a wide-field detection, there 1s no sequential scanning in the xy plane, and subsequently acquired snapshots of the 2D scene with spectrum can be repeated over time, typically with refresh rate in the millisecond — microsecond range. This technique results in a fast 4D- i.e. I(x,y,A,t) data acquisition of direct spatially-correlated scenes that have planar interrogation (x-y) acquired with time (t) - and frequency (A) resolved laser detection. Using these methods, successful diagnostic of gaseous turbulent reacting and non-

reacting flows can be realized in dynamic scenes with high kHz video-rates.

This is achieved with synchronizing the repetition rate of the laser and the refresh-rate of the camera.

Rotational-vibrational imaging techniques offer intrinsic chemical specificity, in that different classes of molecules have specific rotational and vibrational frequencies serving as unique fingerprints for their identification.

Temperature is deduced from the relative intensity of the resolved spectral lines following the Boltzmann population distribution.

Laser based diagnostics may in general provide measurements with exceptionally high spatial- and temporal resolution, which is important in producing reliable and accurate experimental data.

Coherent anti-Stokes Raman spectroscopy (CARS) is one such versatile technique, which has had a profound impact on a wide variety of fields.

Its unique strengths in molecular specificity and an intense coherent optical signal have enabled study of various physical-, chemical-, and biological complex systems.

Because many complex systems can be fully characterized in multidimensional space, there is a large motivation for the advancement of multidimensional CARS imaging techniques.

In general, when describing a specific optical spectroscopic measurement technique, one can distinguish between the interrogation side and the detection side for the technique.

In numerous spectroscopy techniques, based on incoherent (linear) methods, emitted light, e.g. from laser induced fluorescence, chemically excited processes (chemiluminescence), is sent out from molecules typically in isotropic fashion and can be detected in a solid angle arrangement.

In other coherent (non-linear) techniques, like CARS for instance, the emission is dictated by a phase-matching condition and is sent out “laser-like” at a specific angle from the sample enhancing the signal throughput.

For the signal generation efficiency and boosting the signal intensity (and also avoiding heat to be deposited on the sample which can easily destroy it), the compilation of the full microscope has the prerequisites of 1. short-pulse impulsive excitation (femtosecond duration laser pulses) and 2. simultaneous time-frequency resolved probing (several hundred femtosecond- to picosecond duration laser pulses) on the interrogation side to generate coherent Raman signals with high intensity.

In N.

Hagen, M.

Kudenov: ‘Review of snapshot spectral imaging technologies’, Optical Engineering 52(9) Sept 2013, an image replicating imaging spectrometer is shown that has polarizers in the form of Wollaston polarizers.

This spectrometer is able to split resonant and non-resonant channels with orthogonal polarizations and image it on a camera plane.

However, in rotational-vibrational CARS spectroscopy spectral lines are very close to each other (about 1-10 cm!) which renders this setup unpractical.

Recent investigations aimed at benchmarking time-resolved CARS thermometry have shown a much reduced relative standard deviation over nanosecond CARS thermometry.

If operating in flames, it is difficult to make perfect statements about the relative standard deviation obtained as consequence of the CARS technique only, the outcome of the temperature might be effected by fluctuations at the experimental boundary conditions which is much greater than those inherent to the technique itself, e.g. originating from factors related to the control of the flow and vibration of the platform.

Gas-phase spectroscopy has a need for a flexible hyperspectral spectrometer that 1s much more generic in design and that can image across a wide spectral window (0-4200 cm-1) while handling broad spectral features (~20-200 cm-1) as well.

The present invention therefore has as an object to perform spectroscopic broadband interrogation and detection in the wide-field mode, detecting over an entire image area, illuminating and monitoring the entire sample without any rastering procedures.

SUMMARY OF THE INVENTION The invention aims to achieve this by a method and system of a novel polarization sensitive wide-field coherent imaging spectrometer, capable of acquiring in-situ spectral referencing; by imaging spatial information of a Raman spectrometer from an object plane to an image plane. A sample is irradiated in the object plane of the coherent Raman spectrometer by a laser excitation pulse beam that is combined with a laser probe pulse beam; an impulsive laser excitation is provided to the sample via first laser pulses, i.e. the pump- and the Stokes pulses in the laser excitation beam. The laser probe pulse is provided by a time resolved laser pulse to the sample via a second laser pulse where first and second laser pulses are time-synchronized to generate a beam of coherent Raman signals. including cross polarized resonant and non-resonant parts from the sample over a wide-field spatial extent (x- y) with high intensity across a wide spectral window ranging between 0 and 4200 cm-1. The beam of coherent Raman signals is received in a spectral detector that images the object plane to an image plane coinciding with a wide-field image sensor. The spectral detector comprises a polarizing beam splitter that splits the beam of coherent Raman signals in two orthogonally polarized beam portions corresponding to the cross polarized resonant and non-resonant parts of the coherent Raman signals; one beam cast into a beam path including a pulse shaper, reflecting non-resonant parts of the coherent Raman signal, and one beam path relaying the resonant parts; and a grating arranged in said in such a way that both beam portions are combined and diffracted via the grating to the image sensor in the image plane. A polarization state detection (a projection of the signal on orthogonal s- and p- axis’s) can thus be detected in addition to the wide-field 5 broadband detection (4D- x, y, “lamda”, s/p), to achieve a 5D probing - x, y, “lamda”, s/p, and t, where “t” denotes time as obtained in cinematography. Here, non-resonant CARS susceptibility is used in-situ to extract information on the effective impulsive efficiency. Concurrent resonant- and non-resonant CARS signals are generated in the measurement sample, and simultaneous detection of both the signals is achieved with a novel polarisation sensitive wide-field coherent imaging spectrometer. By separating the resonant and the non-resonant parts of the CARS signal using a polarization approach, and imaging both signals onto different locations at the detector plane, it can be successfully performed spatially divided detection of the resonant and non-resonant CARS signals and achieved the in-situ referencing of the impulsive excitation efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an illustrative embodiment of a snap-shot hyperspectral coherent Raman wide-field microscope. Figure 2 shows an alternative embodiment of a snap-shot hyperspectral coherent Raman wide-field microscope Figure 3 shows the CARS principles for successful signal generation.

DETAILED DESCRIPTION OF EMBODIMENTS Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs as read in the context of the description and drawings. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. In some instances, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present systems and methods. The term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises" and/or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

In CARS, while the resonant part contains the useful spectral information for extracting temperature and species concentrations in the probed volume, the non-resonant part is often neglected. However this part can serve as an inevitable reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the CARS signal along the detection path. In addition, all short-pulse laser devices employed for impulsive excitation (femtosecond laser pulse) and time-resolved probing (picosecond laser pulse), are created and can be described in analogy with a time-bandwidth product. These light pulses are alternatively termed near transform limited femtosecond or picosecond laser pulses. When such short light pulses are employed for in-situ chemical sensing in microscopy and/or spectroscopy, it is desired to have information about the pulse properties during the light- matter interaction. Ex-situ spectral referencing has been employed to simulate the pulse property, and has been incorporated in spectral modelling to simulate the finite bandwidth of the employed laser excitation pulse, however will not perfectly render the true excitation efficiency which leaves an uncertainty to the measurement.

Numerous of microscopy and/or spectroscopy investigations performed with short pulses, do not even account for the excitation efficiency, it has remained an unknown and ex-situ spectral referencing has still not become part of the protocols and the standard measurement procedures.

For these schemes only qualitative scalar imaging applies.

It also might be that when these short pulses are injected to within the medium, these pulses are subjected to changes which depend on the medium itself.

The optimal procedure, would therefore be to obtain automatic in-situ spectral referencing acquired for the actual — real measurement condition.

By separating the resonant and the non-resonant parts of the CARS signal, and imaging both signals onto different locations at the detector plane, spatially divided detection of the resonant and non-resonant CARS signals can be successfully performed and in-situ referencing of impulsive excitation efficiency can be achieved.

By applying this technique to a series of single-shot femtosecond/picosecond one-dimensional CARS spectra an improved temperature precision and accuracy can be obtained of a measurement sample.

The technique can be used for high-fidelity probing of temperature and species concentrations in dynamic environments with gradients- and changes in the index of refraction and which require instantaneous data to be provided, e.g. such as turbulent flames and flows.

With the capacity of probing the spectral referencing in-situ, the technique is reducing the overall measurement uncertainty, which could have an even more significant impact when employing unstable laser pulses, e.g. supercontinuum based CARS.

The present invention provides a new flexible hyperspectral imager that simultaneously generates resonant- and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy signals

(alternatively degenerate four-wave-mixing, Raman induced Kerr effect spectroscopy, Brilloum-Mandelstam light scattering, Polarisation spectroscopy), by measuring the in-situ properties and referencing the impulsive two-photon excitation efficiency, thereby enabling a new level of trueness obtained in quantitative assessments of species concentration and thermometry in gaseous flows. It is proposed for high-fidelity probing of temperature and species 1n both static and dynamic environments operating in optically dispersive media, with gradients- and changes in the index of refraction and which require instantaneous data to be provided, e.g. such as turbulent flames and flows. The technology can diagnose and optimize processes relevant to propulsion and power industry, hydrogen safety and storage, renewable fuels, synthesis of functional nanomaterials (medical- and electronics industry), and photonics. There is no upper limit on the acquisition-rate employed, however today on the shelf products e.g.

regenerative amplifiers and camera sensors operate with a refresh-rate of -1 kHz-100 MHz.

The method and system enabling a new level of trueness obtained in quantitative assessments of species concentration and thermometry in gaseous turbulent flows and flames The relative strength between the resonant part and the non-resonant part in the CARS susceptibility is not only of importance for thermometry, it has been commonly employed for extracting species concentration, i.e. the relative concentration of different chemical substances in a gas as well, and the technique is particularly interesting for detecting H20 which is a recognized challenge for CARS diagnostics because the relatively low Raman cross-section of H20. These methods can be performed in a multiplexed way, to obtain a thermometry image of identified chemical substances.

With time-resolved CARS probing, we refer not only to the freezing of the energy containing scale in the combustion flow (achieved on a microsecond timescale) but also to that the probing time is well within the characteristic molecular response time (dephasing). This temporal window for probing combustion relevant species is usually on a picosecond timescale, the relatively long dephasing time of hydrogen being an exception. On this short timescale the impact on the CARS spectrum from Raman linewidths is small, and the mode-amplitude- and phase-mode fluctuations on the broadband laser emission profile are significantly reduced with a near transform-limited femtosecond laser pulse as compared to the output from a nanosecond pumped dye-laser.

Concurrent resonant- and non-resonant CARS signals are generated in the measurement sample, and simultaneous detection of both the signals is achieved with a novel polarisation sensitive wide-field coherent imaging spectrometer.

Turning now to Figure 1 an embodiment is depicted wherein the spectrometer principle is illustrated for simultaneous time- and frequency resolved coherent Raman spectroscopy imaging achieved over a spatial field. The polarisation sensitive wide-field coherent imaging spectrometer can be used for spatially divided detection of resonant - and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals. Astigmatic convergence of the sheet forming optics is used to enhance the irradiance of the probe-beam at the measurement location. The indexes v (vertically) and h (horizontally) express the alignment symmetry axis of the cylindrical lenses. Two separate detection channels for P- and S- polarized light (orientation determined with respect to the transmission grating), respectively are relay-imaged with ~1:1 magnification from the signal generation plane to the position of the detector.

In the following description, it is distinguished between 1D-CARS and 2D- CARS (here D-dimension refer to spatial dimension), since both has their own specific advantages. The setup for 1D-CARS can be achieved based on a single ultrafast regenerative amplifier laser system, which provide ease and documented robustness.

The one extra spatial dimension in 2D-CARS, is beneficially achieved with the implementation of a separate high-power picosecond laser (probe beam) which is synchronized with a first femtosecond laser used for excitation of the molecules (pump/Stokes beam). The CARS system can be built by irradiation of a sample in the object plane of the Raman spectrometer by a laser excitation pulse beam that is combined with a laser probe pulse beam, e.g by providing an impulsive laser excitation of the sample via first laser pulses, i.e. the pump- and the Stokes pulses in the laser excitation beam by a ~35 femtosecond duration output of a high-power ultrafast regenerative amplifier, with a pulse energy of ~7.5 md provided at a 1 kHz repetition-rate.

These numbers are merely indicative and can be tuned by a skilled person to a desired setting.

The efficient coherent excitation bandwidth from this pulse duration spans the pure-rotational manifold of most all Raman active species (in the window of ~0-1200 cm-1), however the effective bandwidth can be expanded to extend ~0-4200 cm-1 with pulse compression techniques (super-continuum generation, see explanations below). This will allow the monitoring of all Raman active species.

The laser probe pulse may be a time resolved laser pulse to the sample via a second laser pulse; said first and second laser pulses being time-synchronized; to generate a beam of coherent Raman signals, consisting of cross polarized resonant and non-resonant parts from the sample over a wide-field spatial extent (x- y) with high intensity across a wide spectral window ranging between 0 and 4200 cm-1. In the example a narrowband ~6-10 picosecond duration probe-beam is centered at 400 nm, and can be efficiently produced by the principles of second-harmonic bandwidth compression, which is converted from a ~65% portion of the femtosecond laser output.

The near transform-limited femtosecond pump/Stokes-beam can be produced by an external compressor operating on a ~35% portion split of the uncompressed output from the amplifier, allowing for flexible compensation (pre-chirping) of dispersion terms induced by the optical components and the combustor windows along the optical path. This will ensure impulsive excitation at the measurement location.

Both beams are repetition-wise synchronized and automatically phase- locked at the experiment with an arbitrary time-arrival. The ~1.9 md impulsive excitation beam (femtosecond laser pulse) and the ~1.2 mJ probe beam (picosecond laser pulse) are intersected in a crossed plane geometry forming a one-dimensional spatial coordinate. The two beams are synchronized with an optical delay line obtained with a high-finesse translation stage (sub-picosecond temporal resolution). The levelling- and relative polarization of the laser beams may be controlled with turning periscopes and a 800 nm half-wave plate mounted in the pump/Stokes beam path, the shaping of the laser beams can be performed with low-dispersion sheet-forming optics. The irradiance (mJ/cm2) of the probe beam can be significantly enhanced at the measurement location obtained with astigmatic convergence, e.g. convergent sheet forming optics, here achieved by two cylindrical lenses with focal lengths f=400 mm (v‚h), the indexes v (vertically) and h (horizontally) express the alignment symmetry axis, respectively. The laser excitation beam is oriented in a line focus, shaped by convergent sheet forming optics, coinciding with a line focus of the laser probe beam to generate one dimensional spatially resolved coherent Raman signals. In efforts to match the phase-matching condition homogeneously across the line-image, a combination of focal lengths f=500 mm (v) and {=1000mm (h) is employed in the pump/Stokes beam path, however the dependence on phase-matching is relaxed for the driving pure-rotational CARS transitions with the current beam conditioning. The excitation beam may be dumped e.g. before the first collection lens in the wide-field one-to- one plane coherent imaging spectrometer, and the probe beam is separated from the pure-rotational CARS signal obtained with the angle-tuning of a spectral bandpass filter. The resonant- and non-resonant CARS signals may be separated in two polarization detection channels by a polarizing beam splitter that splits the beam of coherent Raman signals in two orthogonally polarized beam portions corresponding to the cross polarized resonant and non-resonant parts of the coherent Raman signals. One beam cast into a beam path including a pulse shaper, reflecting non-resonant parts of the coherent Raman signal, and one beam path relaying the resonant parts. In the example, the separation is provided by two 400 nm half-wave plates and a polarization cube splitter (PBS). Both channels are, in the example, directed through a transmission grating (TG, ~3000 lines/mm) and relay- imaged onto the same detector plane (e.g. sSCMOS camera); similarly as an alternative, a reflective grating may arranged in said in such a way that both beam portions are combined and diffracted via the grating to the image sensor in the image plane. The first half-wave plate rotates the cross- polarized resonant- and non-resonant CARS signals to fit the orthogonal S- and P-polarization transmission axis’s of the analyser, the second half-wave plate is mounted after the analyser to turn the polarization of the non- resonant CARS signal from P- to S-polarization to achieve the maximum grating transmission efficiency of >90% at 400 nm. The crossing angle (0) of the pump/Stokes and probe beams is shallow, providing an estimated probe volume with dimensions of ~0.1 mm (width) x ~1.5 mm (length) x ~2 mm (height). The total field-of-view will cover ~2 mm and the image quality will be retrieved with a ~30 pm pomt-spread function.

There are two efficient ways to produce ultra-broadband high-energy femtosecond pulses in the visible and the near IR spectral range: either via hollow-core waveguide technology or via the use of laser filamentation. Super-continuum (SC) generation with hollow-core waveguide technology has a lower conversion efficiency on the level of ~30%, which is due to coupling losses at the injection of the pump-pulse into the waveguide. It can also be used laser filamentation to produce the SC in inert gases or crystals, since it has been shown to improve the efficiency and it is insensitivity to alignment. Nevertheless, both types of SC generation suffer from considerable pulse-to-pulse spectral density instability, which inevitably leads to fluctuation in the effective bandwidth employed for coherent excitation, thereby reducing the overall accuracy and precision of the CARS diagnostics. The current spectrometer can be used to cancel this uncertainty with the help of “simultaneous referencing of the impulsive excitation efficiency” which is outlined below.

The idea to assess “shot-to-shot” the effective impulsive excitation employed by the SC pulse, 1s to simultaneously generate non-resonant CARS signals which are cross-polarized with respect to the resonant CARS signals. In CARS, it is known that the occurrence of both a resonant part and a non-resonant part in the third order susceptibility contributes to the total CARS signal. While the resonant part contains the useful spectral information for extracting temperature and species concentrations in the probed volume, the non-resonant part is often neglected, however serving as an inevitable reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the CARS signal along the detection path. The polarisation sensitive detection is possible by the knowledge of the polarisation dependence on the resonant- and non- resonant CARS signals, which is determined as a function of the incident relative angle between the probe beam and the pump/Stokes beams, respectively. The “magical” probe angle of about 67.5° relative the vertically polarized pump/Stokes beam results in orthogonally polarized pure- rotational resonant- and non-resonant CARS signals.

In Figure 2 it is shown that for the 2D-CARS signals (or planar CARS), the “simultaneous referencing of the impulsive excitation efficiency” 1s not intuitive or trivial, the broadband “non-feature characteristics” of the non-resonant four-wave-mixing signal would simply result in a convolution between space- and spectrum which is smeared at the position of the detector plane in the wide-field coherent imaging spectrometer. For a 2D CARS signal, the laser excitation beam is oriented in a plane intersecting the laser probe beam, said plane within the rayleigh range of the convergent sheet forming optics, coinciding at the object focal plane to generate two dimensionally spatially resolved coherent Raman signals. The distance (d) is arbitrarily and can be tuned to obtain a larger field-of-view.

The d=0 represents the 1D CARS embodiment, similar to FIG 1 but with another optics setup as described below. In the upper channel the non-resonant signal can be detected, where the broadband features are spectrally filtered identically to the particular transitions of the resonant CARS signal. In the shown embodiment, a second polarizing beam splitter PBS2 1s provided in the non-resonant part beam portion, which transmits the non-resonant part to a second transmission grating TG2 that diffracts the non-resonant part beam portion.

The thus diffracted non-resonant part beam portion is diffracted on a spatial light modulator (SLM) to selectively reflect diffracted portions of said non- resonant part beam portion back into the second polarizing beam splitter PBS2 previous to combining said both beam portions on the first grating TG. The broadband non-resonant signal may pass through a folded 4f-filter, where a circular polarization is induced for each spectral component. After the reflection from the reflective nematic spatial light modulator (SLM), the predefined spectral lines change their handedness of circular polarization and are “switched out” by the polarisation beam splitter (PBS2) after passing through the quarter-wave plate again. Then, after the dispersive element (transmission diffraction grating TG), both the resonant- and the filtered non-resonant CARS signals are dispersed equidistantly at separate rows onto the detector plane of the coherent imaging spectrometer. The focal plane of the imaging lens is at CARS signal plane whereas the physical light is focused behind it.

In Fig 3 it is shown how the CARS beam is generated. For successful generation, two physical conditions must be satisfied. One condition is that of energy conservation, where three incident photons with energies Opump, OStokes, and prove are mixed with the internal energy levels of the probed molecules to generate a fourth photon at energy wcarg according to WCARS = Opump — OStokes + Wprobe. Another condition is phase-matching (momentum conservation), which constrains how the wave vectors of the incident beams must be arranged to effectively generate the Raman shifted coherently scattered light. The microscope can be implemented utilizing the pulse combination two-beam femtosecond/picosecond CARS, which combines highly efficient impulsive excitation in the time domain (pump/Stokes, femtosecond duration laser pulse) with high resolution detection in the frequency domain (probe, several hundred femtosecond- to picosecond duration laser pulse). In the figures, the two broadband femtosecond pulses, i.e. the pump- and the Stokes pulses, are provided by the same laser beam, achieving automatic overlap in time and in space. This is one strong option to go with for the interrogation side considering the intra-pulse configurations, but it is not mandatory.

By phase-locking to an external radio-frequency the laser devices can be phase-locked allowing for arbitrarily arrival step-finesse in the time- synchronization between the excitation and probe pulse. In this way precise electronic timing between the excitation and probe pulse can be achieved which is convenient and beneficial when synchronizing two separate high-

power lasers and when the fidelity in time steps do not need to be more than ~ 5 — 20 picoseconds (ps) and still be able to delay the relative time arrival between the pulses up to ~ 500 ps, which is applicable working in the gas- phase. In soft condensed phase it is not a critical point, since often the pump/Stokes- and probe pulses are originating from the same laser and the delay in time arrival between the pump/Stokes pulse and probe pulse is on the order of a few ps (~ 0-5 ps). For example, starting from the output of the ultrafast femtosecond laser, a portion in output power is split off and can become readily engineered; bandwidth compressed to picosecond in pulse duration, shifted in wavelength but still be femtosecond in pulse duration, bandwidth broadened (temporally compressed to sub 10fs), etc. It is beneficial to delay the probe pulse from the pump/Stokes (excitation) pulse to suppress the four-wave-mixing signal with no chemical contrast being generated in the electronic clouds of the molecules (and not the resonant vibrational transitions), referred to as a non-resonant signal. In some applications the non-resonant signal may be used to amplify the resonant signal without smearing the chemical contrast; the skilled person is per se knowledgeable how to engineer the time arrival between the pulses optimizing and balancing between chemical contrast and signal strength.

The disclosed simultaneous hyperspectral spectrometer is not dependent on a specific laser pulse configuration.

Throughout the application, any means for carrying out the disclosed methods, in particular, as further clarified below: means imaging, means for splitting, means for relaying can be implemented by optics that are known to the skilled person and may differ in form and structure to arrive at the same function; i.e. the function is physically implemented in optical elements such as mirrors, lenses and prisms. Furthermore, the identified controller functions may be implemented in hardware or software, to provide dedicated processing circuitry that processes input data read from system resources. A server function may e.g. be provided by a connected physical network device, but may also be formed as a virtual device, functioning in a network, and which may be implemented on a hardware resource that can be reached via network communication. These functions may be executed by one or more processors configured to perform operational acts in accordance with the present systems and methods, such as to provide control signals to the various other module components. The controller may comprise a processor that may be a dedicated processor for performing in accordance with the present system or may be a general- purpose processor wherein only one of many functions operates for performing in accordance with the present system. The processor may operate utilizing a program portion, multiple program segments, or may be a hardware device utilizing a dedicated or multi-purpose integrated circuit. Any type of processor may be used such as dedicated or shared one. The processor may include micro-controllers, central processing units (CPUs), digital signal processor s (DSPs), ASICs, or any other processor(s) or controller(s) such as digital optical devices, or analog electrical circuits that perform the same functions, and employ electronic techniques and architecture. The controller or processor may further comprise a memory that may be part of or operationally coupled to the controller. The memory may be any suitable type of memory where data is stored. Any medium known or developed that can store and/or transmit information suitable for use with the present systems and methods may be used as a memory. The memory may also store user preferences and/or application data accessible by the controller for configuring it to perform operational acts in accordance with the present systems and methods.

The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

The description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description.

In the drawings, the size and relative sizes of systems, components, layers, and regions may be exaggerated for clarity.

Embodiments are described with reference to schematic illustrations of possibly idealized and/or intermediate structures of the invention.

The planar coherent Raman signals are received from the sample, split in two separate detection channels dependent on their polarization (s/p). A wide-field coherent imaging spectrometer is applied to both the detection channels, i.e. the planar spatial information is retrieved with a telescope imager obtained through a grating, resulting in spectrally dispersed (A) planar coherent Raman signals at the image sensor.

The speed of acquisition (t) is limited by the repetition-rate of the laser device synchronized with the refresh-rate of the sensor device.

Altogether, this resulting in simultaneous 5D (x, y, A, t, s/p) generation and detection of coherent Raman signals.

The device is generic to handle both narrowband (resonant) and broadband (resonant and non-resonant) planar coherent Raman signals.

The wide-field coherent imaging spectrometer equipped with a pulse shaper before the disperser, has the capacity to operate on the space — spectrum information independently, which in the case of broadband (resonant and non-resonant) coherent Raman signals cancelling the space — spectrum convolution at the position of the sensor plane.

This resulting in unique bijective mapping of the space — spectral information at the sample plane.

The correlated detection of both resonant and non- resonant planar coherent Raman signals can be beneficially employed to perform in-situ spectral referencing which has been a previous unknown in all femtosecond CARS spectroscopies and is expected to push the precision and accuracy (trueness) of gaseous scalar measurements (temperature and species) to the ultimate dream-limit of ~1%.

Claims (8)

Conclusions A method for imaging spatial information from a Raman spectrometer from an object plane to an image plane, comprising: — providing a sample to be irradiated in the object plane of the coherent Raman spectrometer by a laser excitation pulse beam combined with a laser probe pulse beam; providing impulsive laser excitation of the sample via first laser pulses, being the pump and Stokes pulses in the laser excitation beam; - providing the laser probe pulse by a time resolved laser pulse to the sample via a second laser pulse; wherein said first and second laser pulses are time synchronized; to generate a beam of coherent Raman signals containing cross-polarized resonant and non-resonant portions of the sample over a wide spatial range (x — y) with high intensity over a wide spectral range between 0 and 4200 cm -1 ; - receiving the beam of coherent Raman signals in a spectral detector; - mapping the object plane to an image plane coincident with a wide-field image sensor by the spectral detector; the spectral detector further comprising - a polarizing beam splitter that splits the beam of coherent Raman signals into two orthogonally polarized beam portions, corresponding to the cross-polarized resonant and non-resonant portions of the coherent Raman signals; a beam projected to a beam path that includes a pulse shaper that reflects non-resonant portions of the coherent Raman signal, and a beam path that transmits the resonant portions; - wherein the spectral detector further comprises a grating arranged such that both beam parts are combined and deflected via the grating to the image sensor in the image plane. A method according to claim 1, wherein said beam parts are combined on a transmission grid via a reflecting mirror. A method according to claim 1, wherein the laser excitation beam is oriented in a line focus formed by convergent plate forming optics coincident with a line focus of the laser probe beam to generate one-dimensional spatially resolved coherent Raman signals. A method according to claim 1, wherein the laser excitation beam is oriented in a plane intersecting the laser probe beam, said plane being within the Rayleigh range of the convergent plate forming optics, and coincident on the object focal plane to produce two-dimensional spatially resolved coherent Raman signals to generate. A method according to claim 1, wherein a second polarizing beam splitter is provided in the non-resonant portion of the beam portion, said second polarizing beam splitter passing the non-resonant portion to a second transmission grating which transmits the non-resonant portion of the beam portion and wherein said method further comprises imaging said diffracted non-resonant portion of the beam portion onto a spatial light modulator to selectively reflect diffracted portions of said non-resonant portion of the beam portion back to the second polarizing beam splitter prior to to combining said two beam parts on the first grid. The method of any one of the preceding claims, wherein 1D or 2D spatially resolved coherent Raman signals are detected from a flame to detect local rotational vibration energies, according to a thermometry image. The method of any one of the preceding claims, wherein 1D or 2D spatially resolved coherent Raman signals are detected to detect local rotational vibration energies for identifying chemicals. The method of claim 6 or 7, wherein the detection is multiplexed to form a thermometry image with species identification.