Patent Description:
The present invention, in some embodiments thereof, relates to imaging and, more particularly, but not exclusively, to a method and a system for infrared imaging.

Imaging of infrared radiation may in general be considered as advantageous, since infrared radiation may carry information which is not obtainable by merely obtaining the images from other types of optical radiation, e.g., the visible radiation.

<CIT> describes an image apparatus for converting infrared light into visible light. The apparatus includes an object lens with an optical crystal and an infrared object lens. The object lens converts an infrared light spectrum image into a visible light spectrum image.

<CIT> describes a quasi-phase matched non-linear crystal for wavelength conversion. The crystal has an aperiodic poled structure, where each period is tuned in a manner that the tuning varies adiabatically along a length of the crystal from a strong negative mismatch at one end of the crystal to a strong positive mismatch at another end of the crystal.

<NPL>), discloses an upconversion system for two-dimensional, mid-infrared spectral imaging. Mid-infrared light is upconverted in a single pass through a nonlinear crystal that mixes the mid-infrared light with a laser beam, to generate the upconverted light at near-visible wavelengths. The phase-match condition of the nonlinear crystal is varied, and images are individually acquired for each phase-match condition, to obtain a set of images, each containing light from only one specific narrow band of wavelengths. Image reconstruction is then employed to piece together the set of individually acquired images. The bandwidth of the upconversion system varies with the phase-matched wavelength, and reaches a maximum of <NUM> at <NUM>, beyond which it decreases.

Additional background art includes <NPL>); <NPL>); <NPL>); <NPL>); <NPL>); and <NPL>); <NPL>).

According to the embodiment of the present invention there is provided an imaging system according to claims <NUM> to <NUM> and a method of imaging according to claims <NUM> to <NUM>.

The mid-infrared (mid-IR) wavelength regime, spanning the wavelength range of <NUM>-<NUM> (<NUM>-<NUM>,<NUM>-<NUM>), is of particular importance to materials science, chemistry, biology and condensed matter physics, as it covers the fundamental vibrational absorption bands of many gaseous molecules and bio-molecules, thus of tremendous scientific and technological interest. As the characteristic vibrational transitions have line strengths that are several order of magnitude stronger that the near-infrared region, those provide a unique information on a sample's molecular composition, thus the mid-IR is also called the fingerprint region. Moreover, as the mid-IR spectral region contains two atmospheric transmission windows (<NUM>-<NUM> and <NUM>-<NUM>), in which the Earth's atmosphere is relatively transparent, it makes mid-IR laser sources useful for many applications including, without limitation, atmospheric, defense and industrial applications. In addition, imaging in the IR can be useful for thermal analysis, security and materials inspection.

Several types of infrared imaging systems are known. One such type includes infrared cameras which are based on the photo-electric effect. Known infrared cameras include mid-wave IR (MWIR) cameras, long-wave IR (LWIR) cameras and short-wave IR (SWIR) cameras. The choice of materials that compose the detectors makes the various systems operate under different circumstances. Compared to visible and the lower range of near-infrared detectors, which are based mainly on Silicon, the mid-infrared detectors are based on indium antimonide (InSb), and/or Mercury Cadmium Telluride (MCT) making them expensive, low resolution, suffering from poor sensitivity and slow response.

In the mid-infrared, a single detector scanning systems or line scanning systems are conventionally used. This requires very accurate mechanical alignment as well as synchronization algorithms. Two-dimensional array detectors are also being used, yet those still have very small number of pixels (hundreds in each axis) and thus have very low spatial resolution compared to the current state of the art in Silicon-based cameras (thousands of pixels in each axis). The detector of a MWIR camera absorbs all the photons that are above the bandgap and therefore the main problem of thermal imaging is that during the detection process, the spectral components are integrated and cannot be later retrieved.

Known are spectral mid-IR imaging techniques that are carried out by using a discrete set of bandpass filters, each allowing the transmission of a narrow window of infrared radiation. However this technique is slow, since it requires the sequential acquisition of images for each filter and requires a significant signal to noise ratio at each spectral window, making it a very low sensitivity technique.

In a search for an improved spectrally resolved imaging in the infrared spectral region the Inventors found that a processes known as adiabatic sum frequency conversion, can be used to convert broadband infrared images into the visible and/or near-infrared spectral range and therefore allow high resolution, color imaging of broad spectra information using low-cost, high sensitivity and robust visible CCD.

Referring now to the drawings, <FIG> are schematic illustrations of an imaging system <NUM>, according to some embodiments of the present invention. System <NUM> comprises a light source <NUM> generating a pump beam <NUM>, and an optical coupling system <NUM>. Optical coupling system <NUM> receives a coherent or incoherent input beam <NUM> of infrared light from a scene <NUM>, optionally and preferably via a lens system <NUM>, and combines the input beam <NUM> with the pump beam <NUM>. In various exemplary embodiments of the invention the intensity of pump beam <NUM> is higher than the intensity of input beam <NUM>. Optical coupling system <NUM> can include any optical coupler known in the art that can in-couple two or more light beams. For example, coupling system <NUM> can be a lens or lens array, a diffractive element, a fiber optic coupler, a dichroic mirror and the like.

The pump beam <NUM> is optionally and preferably characterized by a wavelength range having a central wavelength that is shorter than the central wavelength that is characteristic to the input beam <NUM>. In some embodiments of the present invention the longest wavelength of the pump beam <NUM> is shorter than the shortest wavelength of the input beam <NUM>. As a representative example, the input beam <NUM> can be a mid-infrared beam (e.g., having a plurality of wavelength within the range of <NUM> to <NUM>, inclusive), and the pump beam <NUM> can be a near-infrared beam (e.g., having at least one wavelength within the range of <NUM> to <NUM>, inclusive). Other wavelengths for beams <NUM> and <NUM> are also contemplated.

In some embodiments of the present invention light source <NUM> generates a pulsed pump beam at a repetition rate. System <NUM> can optionally and preferably comprise a cavity <NUM> characterized by a resonance frequency within a bandwidth of pump beam <NUM>, and having a length selected based on the repetition rate of the pulsed pump beam to synchronize between pulses in the pump beam <NUM>. When light source <NUM> generates a pulsed pump beam, the characteristic pulse duration of the generated pulsed pump beam is less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns or less than <NUM> ns. The advantage of having a cavity is that it can allow very short pulses for the pump beam and reduce the exposure time that is required for imaging. The cavity allows effectively reaching high pump intensity density, and therefore can also be used for frequency conversion with continuous wave pump beam (e.g., <NUM> W or more).

System <NUM> optionally and preferably comprises a crystal <NUM> that receives beams <NUM> and <NUM> from coupling system <NUM> and adiabatically mixes these beams to provide an output beam <NUM> having a frequency which is a sum of frequencies of input beam <NUM> and pump beam <NUM>. Optionally and preferably output beam <NUM> is a visible light beam (e.g., having a plurality of wavelength within the range of <NUM> to <NUM>, inclusive). Alternatively, output beam <NUM> can be a near-infrared light beam (e.g., having a plurality of wavelength within the range of <NUM> to <NUM>, inclusive). In embodiments in which system <NUM> comprises cavity <NUM>, the cavity serves for enhancing the mixing power of beams <NUM> and <NUM> in crystal <NUM>.

In various exemplary embodiments of the invention crystal <NUM> is configured for simultaneously mixing pump beam <NUM> with each of a plurality of wavelengths of input beam <NUM>. The wavelengths of input beam <NUM> that are mixed by crystal <NUM> with pump beam <NUM> optionally and preferably spans over a wide wavelength band, e.g., over one, two, three or more octaves. In some embodiments of the present invention the wavelengths of input beam <NUM> that are mixed by crystal <NUM> with pump beam <NUM> span over at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% or at least <NUM>% of a range of wavelengths defined from about <NUM> to about <NUM>.

The lateral area of crystal <NUM> is optionally and preferably sufficiently large to allow crystal <NUM> to receives light rays from all or at least most of the objects or portions of objects in scene <NUM>. According to some embodiments of the invention, a lateral area of the crystal is selected according to a predetermined Point Spread Function (PSF). The PSF relates to the intensity distribution Ipump(x,y) of the pump beam <NUM> in the transverse plane x-y, via the Fourier transform or fast Fourier transform: <MAT>.

Typically, the lateral area of crystal <NUM> is at least <NUM> by <NUM>, or at least <NUM> by <NUM> or at least <NUM> by <NUM>. For example, for a <NUM> pixel size of a <NUM>/<NUM> inch imager, the lateral area of the crystal can be about <NUM><NUM> for every <NUM> pixels of the imager. In these embodiments, a <NUM> megapixel image (<NUM>×<NUM> pixels) can be achieved using a crystal having lateral dimensions of about <NUM> by <NUM>.

The principles and operations of crystal <NUM> will now be explained.

Crystal <NUM> achieves efficient broadband frequency conversion using sufficiently strong pump intensity relative to the intensity of the input beam (e.g., at least two times or at least five times or at least ten times stronger) and by providing the light beams with gradually changing conditions (e.g., gradual change in the phase mismatch). The gradually changing conditions effect a gradual conversion of beam <NUM>'s frequency as it propagates in crystal <NUM>. During the conversion process each original frequency component of beam <NUM> is converted to a converted frequency in the near-infrared or visible or ultraviolet range. The conversion is gradual in the sense that as the beam propagates in crystal <NUM> the intensity of each original frequency component is gradually decreased, and the intensity of each converted frequency component is gradually increased.

Gradually changing conditions can be accomplished in more than one way.

In some embodiments of the invention crystal <NUM> has a periodic or aperiodic structure, and is under a temperature gradient. For example, crystal <NUM> can be in thermal contact with a heating or cooling source (not shown) that applies a thermal gradient along the propagation direction of beams <NUM> and <NUM> in crystal <NUM>. In these embodiments, the beams are exposed to a gradual change of the temperature during their propagation, thereby achieving the aforementioned gradual frequency conversion.

In some embodiments of the invention crystal <NUM> has a Quasi-phase matching (QPM) structure with adiabatic aperiodic poling.

Quasi-phase matching (QPM) with adiabatic aperiodically poled designs is an efficient crystal structure for achieving adiabatic changes since the structure has periods which can be modified to range over a series of phase mismatches from a negative to a positive mismatch for increasing or decreasing the frequency. Modifying the crystal structure from positive to negative mismatch according to some embodiments of the present invention achieves the desired conversion.

The present embodiments modify the QPM crystal structure through a gradual change in the tuning characteristics along the crystal. The result is efficient frequency conversion over a broad frequency range. A particular advantage of crystal <NUM> is that it can achieve both efficiency and broadband. Typically, crystal <NUM> is works in a regime where a strong narrow-band pump (beam <NUM> in the present embodiments) is introduced into the crystal, along with a weaker broadband beam (beam <NUM> in the present embodiments) which is to be converted.

In contrast with conventional requirement of perfect phase matching along the crystal, crystal <NUM> provides continuous adiabatic variation of phase mismatch. Crystal <NUM> has a length dimension z and tuning conditions along the z dimension of the crystal optionally and preferably vary from negative to positive. That is to say, crystal <NUM> is a quasi-phase matched non-linear crystal, having a longitudinal dimension and having a periodic pole structure comprising a plurality of tuned periods. Tuning of the respective periods varies adiabatically, meaning gradually, along the longitudinal dimension of the crystal.

Herein, the term "adiabatic" or "adiabatically" takes its meaning from quantum physics, and refers to the ability of the crystal to set up a quantum mechanical system with the light beams, in which the crystal structure presents to the light beams gradually changing conditions, allowing the system to change its functional form. That is to say, an adiabatic change comprises a change that is sufficiently gradual as to retain an eigenstate of the optical system. This is in contrast with conventional crystals that provide rapidly varying conditions in which there was no time for the functional form of the state (of the quantum mechanical system) to adapt, so that the system remained in its original state. The gradual change in conditions allows the quantum dynamic state to remain stable and respond to the changing conditions. Rapid change by contrast gets ignored.

The phase mismatch, Δk(z) optionally and preferably changes adiabatically from a big negative value, or vice versa, as an analogy to the way in which a red detuned field interacts with a two level system, to a big positive value, as an analogy to the blue detuned field.

For a wave process to be considered adiabatic, the magnitude value of the phase mismatch parameter Δk, in absolute value, is preferably large (e.g., at least <NUM> times larger) in comparison to the value of the coupling coefficient κ. Additionally Δk optionally and preferably starts with a negative (or positive) value, and ends with a positive (or negative) value. Further, the rate of change of Δk is smaller (e.g., at least <NUM> times smaller) than the internal propagation length of the nonlinear process which can be formally written as (Δk<NUM>+κ<NUM>)<NUM>/<NUM>/κ.

QPM is a technique in nonlinear optics which allows a transfer of energy from pump frequency to signal and idler frequencies. It offers several advantages over other phase matching techniques, such as the fact that all optical frequencies involved are collinear with each other and all the optical frequencies can have the same polarization, which allows for the access to the largest nonlinear coefficient of the crystal, d<NUM>. The value of the phase mismatching achieved by the quasi phase matching technique is optionally and preferably ΔkΛ(z)=2π/Λ(z), where A(z) is the width of crystal <NUM>'s period at position z.

The periods and phase mismatch of crystal <NUM> are preferably selected in accordance with the expected wavelengths of the beams <NUM> and <NUM>. In various exemplary embodiments of the invention crystal <NUM> is designed and constructed for adiabatically mixing mid-infrared light with near-infrared light.

Crystal <NUM> can be made of any material that is transparent to the input beam <NUM>. Representative examples include, without limitation lithium niobate, KTP, cadmium silicon phosphide (CdSiP2), orientation-patterned gallium arsenide (OP-GaAs), and orientation-patterned gallium phosphide (OP-GaP).

Referring again to <FIG>, system <NUM> optionally and preferably comprises a visible, near-infrared or ultraviolet light imager <NUM> that collects output beam <NUM>, optionally and preferably via an additional lens system <NUM>, and spectrally resolves output beam <NUM>. Imager <NUM> can be of any type known in the art, that is capable of spectrally resolving light. For example, imager <NUM> can be a silicon based imager, such as, but not limited to, an imager that comprises a MOS circuit or a CMOS circuit. In some embodiments of the present invention imager <NUM> comprises a CMOS imager. It is expected that during the life of a patent maturing from this application many relevant spectral resolving devices will be developed and the scope of the term imagers is intended to include all such new technologies a priori.

In some embodiments of the present invention crystal <NUM> is optimized such that an extent of Kerr effect and/or two-photon absorption due to interaction of light beams <NUM> and <NUM> with crystal <NUM> is less than a predetermined threshold (e.g., less than <NUM>% or <NUM>% or less than <NUM>% of the image generated by system <NUM> is deformed due to the Kerr effect). This can be ensured by collecting light beam from a calibrating scene and measuring the frequency conversion efficiency at each of a plurality of locations on a cross-section of the output beam <NUM> (e.g., at each of at least a portion of the pixels of imager <NUM>). The parameters of crystal <NUM> (for example, the periods or temperature gradient) can then be varied or adjusted until the conversion efficiency is substantially uniform (e.g., with deviation of less than <NUM>%) across the output beam.

In some embodiments of the present invention system <NUM> comprises more than one crystal for the adiabatic mixing (see <FIG>). These embodiments are useful, for example, for increasing the bandwidth over which system <NUM> can operate. For example, one crystal can be selected for frequency conversion of a first sub-bandwidth and another crystal can be selected for frequency conversion of a second sub-bandwidth, wherein the combination (e.g., concatenation) of all the sub-bandwidths results in the desired bandwidth. When more than one crystal <NUM> is employed, system <NUM> optionally and preferably comprises a beam splitter system <NUM> that splits beam <NUM> into two or more secondary beams 18a and 18b. Each of the secondary beams is then directed to a separate crystal 22a and 22b for frequency conversion as further detailed hereinabove, to provide two or more output beams 24a and 24b. The individual output beams 24a and 24b are then combined by a beam combiner system <NUM> to one output beam <NUM> that is collected by the imager <NUM> as further detailed hereinabove.

Each of the crystals 22a and 22b can be pumped by a separate pump beam, as illustrated in <FIG>, or the same pump beam can be used with two or more of the crystals. In the latter embodiment, the pump beam <NUM> from light source <NUM> can be split, for example, by a beam splitter system (not shown) to two or more secondary pump beams, each directed to a separate crystal. Alternatively, pump beam <NUM> can be fed serially, e.g., by means of suitable optics (not shown), through two or more of the crystals.

While <FIG> illustrates an embodiment in which system <NUM> comprises two crystals 22a and 22b, this need not necessarily be the case, since more than two crystals can be employed. One of ordinary skills in the art, provided with the details described herein would know how to construct a system with any number of crystals.

Beam splitter system <NUM> can be of any type suitable to receive a light beam and output two or more light beams. In the simplest configuration, beam splitter system <NUM> comprises a tilted semi-transparent reflector, but other types of beam splitter systems, such as, but not limited to, beam splitter systems employing fiber optics, are also contemplated. Similarly, beam combiner system <NUM> can be of any type suitable to receive two or more light beams and output one light beam. In the simplest configuration, beam combiner system <NUM> comprises a tilted semi-transparent reflector, but other types of beam combiner systems, such as, but not limited to, beam combiner systems employing fiber optics, are also contemplated.

In some embodiments of the present invention system <NUM> comprises an optical filter <NUM> selected to filter out the frequency or frequencies of pump beam <NUM> so as not to contaminate the imager with pump frequencies. Filter <NUM> can be positioned anywhere along the optical path of output beam <NUM>, such as, but not limited to, at the output side of crystal <NUM> or at the input side of imager <NUM>.

In use of system <NUM>, input beam <NUM> is received of infrared light from a scene, and is combined with a pulsed pump beam, which is typically higher in intensity than the input beam. The beams are then transmitted to a crystal that adiabatically mixes the beams to provide an output beam having a frequency which is a sum of frequencies of the input and pump beams. The output beam is preferably a visible or near-infrared light beam, as further detailed hereinabove. The output beam is collected using a light imager that spectrally resolves the output beam.

In some embodiments of the present invention system <NUM> is used for single shot imaging, without combining image information from several image acquisitions. In this embodiment, the entire image information of the scene to be imaged is captured once by the input beam without the need to collect the input beam several times. This is in distinction from conventional upconversion imaging techniques in which a set of images is acquired, wherein each contains light from a specific narrow band of wavelengths, and image reconstruction techniques are then employed to piece the images together.

The technique of the present embodiments enjoys several advantages over conventional technique. Unlike conventional infrared imaging, the technique of the present embodiments leads to spectrally resolved imaging of scenes constituted by mid-IR beams where each wavelength in the mid-IR is uniquely mapped to a color in the imager. This means that the technique of the present embodiments allows, for example, remote detection of gaseous composition of dust clouds and more. Additionally, the quantum efficiency (and hence the sensitivity) is expected to be significantly boosted since the quantum efficiency scales linearly with the mixing power.

System <NUM> can be used for imaging many types of scenes. For example, in some embodiments of the present invention system <NUM> is used in microscopy, wherein scene <NUM> comprises a sample on a microscope slide; in some embodiments of the present invention system <NUM> is used for outdoor imaging, wherein scene <NUM> is an outdoor scene; in some embodiments of the present invention system <NUM> is used for aerial photography wherein scene <NUM> comprises an aerial view of objects; in some embodiments of the present invention system <NUM> is used in astronomical imaging, wherein scene <NUM> is an astronomical scene; and in some embodiments of the present invention system <NUM> is used in medical imaging wherein scene <NUM> is a living body or an organ thereof. Also contemplated are embodiments in which system <NUM> is mounted on a vehicle such as, but not limited to, an automobile, for example, for capturing a rear-view or front-view images of the scene nearby the vehicle.

Adiabatic frequency generation (AFG) allows efficient, robust and scalable transfer of broadband, visible and near-IR lasers to the mid-IR optical regime and vice versa. The AFG can resolve the bandwidth-efficiency trade-off in nonlinear frequency conversion, allowing to simultaneously achieve broad bandwidth conversion with good efficiency. Yet, heretofore AFG has been used to convert laser light sources with very good beam quality, but was not used for transferring spectrally broad images.

Recently, nonlinear conversion attempts of low frequency (energy) to high frequency (energy) photons, showed high quantum efficiency that can allow a single photon sensitivity. However, unlike the technique of the present embodiments these attempts are bound by the phase matching requirement making the color imaging a serial process where for each desired narrow band of wavelengths the angle or temperature of the crystal has to be separately tuned. The AFC imaging process of the present embodiments frees this approach from these constraints and leads to single shot capture color image of Mid IR information.

<FIG> illustrate the differences between MWIR imaging (<FIG>), MWIR hyperspectral imaging (<FIG>), and the adiabatic sum frequency generation (SFG) of the present embodiments (<FIG>).

<FIG> is a schematic illustration of the AFG process in adiabatically poled non-linear crystal. AFC allows generating broadband pulses in the mid-IR wavelength regime, which outperform the currently available mid-IR ultrashort sources. Unlike other techniques the AFC allows an efficient transfer of broadband, high-energy visible and near-IR lasers to the mid-IR and vice versa. The AFC technique applies robust population transfer by rapid adiabatic passage from atomic physics to nonlinear optical frequency conversion, which effectively avoids two main hurdles of optical frequency generation: limited bandwidth and limited conversion efficiency. The process is based at least partially on the physical mechanism that the coupled system remains in one of its eigenstates during the entire evolution. One of the characteristics of the AFC technique is a conversion efficiency curve that is substantially flat for broad bandwidth, meaning there is a one-to-one mapping of the spectral amplitude and phase of the input signal into the generated new color. From the symmetry of the dynamical equations, both difference frequency generation (DFG) and the sum frequency generation (SFG) can occur. In SFG conversion, a low energy photon (signal) is mixed with a pump photon to yield a photon, idler, of energy higher than both the pump and the signal. This process for example converts VIS-NIR to mid-IR and vice versa with a pump in the NIR. In DFG conversion, the pump photon is the one with the higher energy in the whole process, creating an idler that is of lower energy than the signal. Thus, in adiabatically aperiodic crystal, flat conversion curve can reach unity for a very broadband spectrum, while for conventional phase matched nonlinear processes (where the phase-mismatch is constant), the conversion curve is very efficiency only over a narrow wavelength range.

The Inventors of the present invention found that amplitude and phase modulations enacted in the near-IR can be transferred to the mid-IR which can be used for shaping the temporal structure of the mid-IR pulse for coherent control and nonlinear spectroscopy applications. The inventors thus successfully combine the adiabatic concept with 2D coherent and incoherent image upconversion, and allow spectrally resolved broadband mid-IR images to be captured, for example, by a conventional silicon high resolution camera. The concept is depicted in <FIG>. The adiabatically poled crystal converts with high efficiency, in a single shot, a broad range of mid IR frequencies contained in the object and the generated VIS-NIR frequencies are imaged onto a Silicon camera to form an image with a one to one relationship between the generated VIS-NIR wavelengths and the original mid-IR wavelengths. In <FIG>, the mid IR beam is depicted as an oscillating grayscale wave and the VIS-NIR beam is depicted as an oscillating colored wave. The mid IR beam is gradually converted into a VIS-NIR beam as it propagates along the crystal. Thus the amplitude (hence also the intensity) of mid IR beam is gradually decreased and the amplitude (hence also the intensity) of the VIS-NIR beam is gradually increased.

<FIG> illustrates the spectral characteristics of the imaging technique of the present embodiments in greater detail. An object is imaged with a magnified <NUM>-f system where the Fourier transform of the object is created using a first lens of focal length f<NUM> in the crystal. In the crystal, the spatial frequencies are encoded as angular position across the crystal facets, rendering the crystal aperture as an effective spatial frequency filter. The mid-IR radiation spanning over the range of <NUM>-<NUM> is converted by the crystal to the visible range using a pump laser at, for example, <NUM>, that is focused on the crystal with a sufficiently long Rayleigh range, allowing efficient conversion along the crystal length. Then, the spectrally converted object Fourier transform (optionally and preferably together with some phase to the propagation in the crystal) is transformed back to the image plane with a second lens of focal length f<NUM> where a visible detector renders the object's image with a magnification of f<NUM>/f<NUM>.

The detector is optionally and preferably equipped with suitable filter to filter out the pump laser and to allow only the wavelengths originating from the image wavelengths conversion to be perceived.

Claim 1:
An imaging system (<NUM>), comprising:
a light source (<NUM>) generating a near-infrared pump beam (<NUM>);
an optical coupling system (<NUM>) configured for receiving from a scene (<NUM>) an input beam (<NUM>) of mid-infrared light having a plurality of wavelengths, and combining said input beam (<NUM>) with said pump beam (<NUM>), wherein an intensity of said pump beam (<NUM>) is higher than an intensity of said input beam (<NUM>);
a crystal (<NUM>) configured for simultaneously and adiabatically mixing said pump beam (<NUM>) with each of said plurality of wavelengths of said input beam (<NUM>) and providing an output beam (<NUM>) having a frequency which is a sum of frequencies of said input (<NUM>) and pump (<NUM>) beams; and
a visible, near-infrared or ultraviolet light imager (<NUM>) configured for collecting and for spectrally resolving said output beam (<NUM>).