BACKGROUND

Gated imaging is a class of the Time-Of-Flight imaging technologies where a camera with tightly controlled opening and closing times of the shutter is used in conjunction with a high power pulsed light source. Image contrast is enhanced by gated imaging by limiting the exposure time of the camera to the return time of an emitted light pulse from an object at a defined distance d. If the light source and camera are collocated, the exposure time should occur at a time after light pulse emission given by At = 2d/c where c is the speed of light.

An advantage of gated imaging is that it is possible to take an image timed with the reflection of an object of interest, such as a tree or building, whilst filtering out reflections from dust particles, fog or mist, and the like, that would blur the image.

There are various competing technologies used in gated imaging, such as an ingaas photodiode array. However to date, these have all relied on specially developed light sensors that can be electrically switched on and off in a matter of nano-seconds. This approach requires specific and expensive light sensors.

It would be useful to be able to achieve gated imaging, with short controlled exposure times synchronized with the light source.

Liquid crystal tunable filters (LCTFs) are optical filters that use electronically controlled liquid crystal (LC) elements to transmit a selectable wavelength of light and exclude others. Often, the basic working principle is based on the Lyot filter but many other designs can be used.

The main difference with the original Lyot filter is that the fixed wave plates are replaced by switchable liquid crystal wave plates.

LCTFs are known for enabling very high image quality and allowing relatively easy integration with regard to optical system design and software control but having lower peak transmission values in comparison with conventional fixed-wavelength optical filters due to the use of multiple polarizing elements. This can be mitigated in some instances by using wider bandpass designs, since a wider bandpass results in more light traveling through the filter.

Some LCTFs are designed to tune to a limited number of fixed wavelengths such as the red, green, and blue (RGB) colors while others can be tuned in small increments over a wide range of wavelengths such as the visible or near-infrared spectrum from 400 to the current limit of 2450 nm. The tuning speed of LCTFs varies by manufacturer and design, but is generally several tens of milliseconds, mainly determined by the switching speed of the liquid crystal elements.

Higher temperatures can decrease the transition time for the molecules of the liquid crystal material to align themselves and for the fdter to tune to a particular wavelength. Lower temperatures increase the viscosity of the liquid crystal material and increase the tuning time of the filter from one wavelength to another. LCTFs are sometimes used in multispectral imaging or hyperspectral imaging systems because of their high image quality and rapid tuning over a broad spectral range. Multiple LCTFs in separate imaging paths can be used in optical designs when the required wavelength range exceeds the capabilities of a single filter, such as in astronomy applications.

LCTFs are capable of diffraction-limited imaging onto high resolution imaging sensors. LCTFs have been utilized for aerospace imaging. Their light weight and low power requirements make them good candidates for remote-sensing applications. They can be found integrated into compact but high-performance scientific digital imaging cameras as well as industrial- and military-grade instruments (multispectral and high-resolution color imaging systems). LCTFs can have a long lifespan, usually many years. Environmental factors that can cause degradation of filters are extended exposure to high heat and humidity, thermal and/or mechanical shock (most, but not all, LCTFs utilize glass as the principal base material), and long term exposure to high photonic energy such as ultraviolet light which can photobleach some of the materials used to construct the filters. Recent advances in miniaturized electronic driver circuitry have reduced the size requirement of LCTF enclosures without sacrificing large working aperture sizes. In addition, new materials have allowed the effective wavelength range to be extended to 2450 nm.

Another type of solid-state tunable filter is the Acousto Optic Tunable Filter (AOTF), based on the principles of the acousto-optic modulator. Compared with LCTFs, AOTFs enjoy a much faster tuning speed (microseconds versus milliseconds) and broader wavelength ranges. However, since they rely on the acousto-optic effect of sound waves to diffract and shift the frequency of light, imaging quality is comparatively poor, and the optical design requirements are more stringent.

AOTFs also have smaller apertures and have narrower angle-of acceptance specifications compared with LCTFs that can have working aperture sizes up to 35mm and can be placed into positions where light rays travel through the fdter at angles of over 7 degrees from the normal.

Quite different kinds of fdters are based on FTIR (Frustrated Total Internal Reflection) or on the Fabery-Perot Etalon. In FTIR (Frustrated Total Internal Reflection) and in the Fabry-Perot Etalon, the tunability is achieved by mechanical shifts which change the optical arrangements.

The LASER is a type of coherent light. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation" .

A laser differs from other sources of light in that it emits light coherently, spatially and temporally. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum, i.e., they can emit a single color of light. Alternatively, temporal coherence can be used to produce pulses of light with a broad spectrum but durations as short as a femtosecond ("ultrashort pulses").

Q-switching is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode. Compared to mode locking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. The two techniques are sometimes applied together.

Q-switching was first proposed in 1958 by Gordon Gould, and independently discovered and demonstrated in 1961 or 1962 by R.W. Hellwarth and F.J. McClung using electrically switched Kerr cell shutters in a ruby laser.

Spectral imaging is photography that uses multiple bands across the electromagnetic spectrum. While an ordinary camera captures light across three wavelength bands in the visible spectrum, red, green, and blue (RGB), spectral imaging encompasses a wide variety of techniques that go beyond RGB. Spectral imaging may include the use of filters to capture a specific spectral range in an image. It may also involve illumination from outside the visible range, such as ultraviolet illumination. Capturing images in ultraviolet, infrared, x-rays, and other bands is possible through the use of the proper image sensors. It is also possible to capture multiple spectral bands for each pixel in an image, through the use of specialized hardware.

Various distinctions among techniques are applied, based on criteria including spectral range, spectral resolution, number of bands, width and contiguousness of bands, and application.

Multispectral imaging is a sub-category of spectral imaging in which fdters and illumination are modified. For example, a scene could be illuminated with infrared and photographed in the visible range.

Hyperspectral imaging is another sub-category of spectral imaging. It is a combination of spectroscopy and photography, in which a complete spectrum or some spectral information (such as the Doppler shift or Zeeman splitting of a spectral line) is collected at every pixel in an image plane. Often, the phrase "spectral imaging" is used to denote this acquisition of a complete spectrum for every pixel in an image plane. Hyperspectral images are often represented as an image cube, a type of data cube.

When experimental conditions permit, the thoughtful selection of fluorescent labels, laser multi-tracking strategies, filter set characteristics, and control specimen correction factors can combine to yield excellent results in imaging biological specimens. In the real world, however, situations often arise where the choice of experimental parameters is limited and the use of fluorescent probes lacking in significant spectral overlap is not feasible. Such a scenario is often encountered when attempting to conduct multiple labeling investigations with fluorescent proteins. Experiments are also subject to artifacts arising from natural autofluorescence or fluorescence induced by the use of fixatives or DNA transfection reagents, which can span several detection channels. In these cases, a technique known as spectral imaging coupled with image analysis using linear unmixing can be employed to segregate mixed fluorescent signals and more clearly resolve the spatial contribution of each fluorophore (often referred to as emission fingerprinting).

Microscopes are now available that have been specifically designed to accommodate spectral imaging and, although the technique bestows significant advantages, it also increases the complexity and purchase price of the instrument. Spectral imaging merges the two well-established technologies of spectroscopy and imaging to produce a tool that has proven useful in a variety of disciplines that rely on various forms of optical microscopy. The methodology has been extensively applied to visualize the chemical composition of materials ranging from enzymes involved in bio-molecular interactions to the formation of stars. Unlike a typical image, which is acquired over the entire wavelength response band of the detector, a spectral image requires the creation of a three-dimensional data set that contains a collection of images of the same field of view captured at different wavelengths or wavebands. In effect, the spectral image provides a complete spectrum of the specimen at every pixel location (noted as I(c.n.l) throughout the lateral dimensions. Thus, a spectral image stack can be considered as either a collection of images, each of which is measured at a specific wavelength or over a narrow band of wavelengths, or as a collection of different wavelengths at each pixel location.

Remote sensing analysis has for many years employed spectral imaging techniques to investigate how light waves from the sun are reflected and scattered from the Earth's surface and within the atmosphere. Careful examination of the spectral frequencies present in satellite image datasets allows the different objects, landscapes, and terrains to be identified and characterized. In effect, by measuring intensity variations as a function of wavelength in order to correlate pixels with matching spectral information, a number of details can be revealed that are missing from single images captured when all of the light from the visible spectrum is used. These geological spectra are far more complex than those obtained in biology due to the fact that the number of spectral classes is significantly larger than the number of fluorescent probes that can be practically applied in a biological investigation. However, the overall spectral signature of multiband satellite images is analogous to that obtained from a convolved spectrum of multiple fluorophores in a living cell expressing several fluorescent proteins or a pathological thin section stained with absorbing synthetic dyes. Over the past several years, satellite imaging approaches have been applied with increasing success in the examination of biological samples to solve problems related to spectral karyotyping, resonance energy transfer, colocalization, stained tissue analysis, and the elimination of autofluorescence. The spectral information gathered from these biological specimens is used to determine the location of specific dyes and fluorophores, and often yields information about interactions between them. Since spectral imaging is allows for the collection of more data in each pixel of the image, whether a different material, or a different biophysical condition - the “spectral fingerprint” of the difference between pixels provides more information, more informative images, and can facilitate better automatic classification or recognition.

One of the most convenient techniques to construct a spectral imaging system is to use a tunable filter. Nevertheless, until now, tunable filters suffer from well known drawbacks including limited bandwidth and/or limited spectral range, sensitivity to polarization, complexity, size, and price.

Efforts have been made to develop a tunable Fabry-Perot filter based on MEMS, which changes the gap between the mirrors of the Fabry Perot filter in order to change the resonator and the resultant wavelength response. However, these methods involve complex manufacturing processes, and the total performance is often lacking, and desirable characteristics may by hard to achieve.

SUMMARY

A first aspect of the invention is directed to a system for gated imaging of an object comprising a narrow band light source, a photo-detector and a shutter between the object and the photo-detector, such that the shutter is controllable, independent of the photo-detector.

Typically, the light source is a laser or LED.

Typically the light source comprises at least one of:

• a source of visible light;

• a source of infra-red light; and

• a source of ultra-violet light.

Optionally the shutter is a cube-beam splitter wherein the gap between the two prisms is controlled by a piezoelectric material.

Alternatively, the shutter comprises a cube-beam splitter wherein the gap between the two prisms is controlled by a Micro Electro-Mechanical technology.

Alternatively, the shutter comprises a Fabry-Perot interferometer controlled by a Micro Electro-Mechanical technology such as a piezoelectric material.

Optionally the shutter comprises a plurality of optical elements arranged in series, the optical elements selected from the group comprising cube beam splitters, Fabry-Perot interferometers and Fabry-Perot etalon. Optionally the system is configured to operate at a plurality of wavelengths for constructing a multispectral image.

A second aspect is directed to use of a cube-beam splitter for gated imaging.

A third aspect is directed to use of a Fabry-Perot interferometer for gated imaging.

A fourth aspect is directed to use a Fabry-Perot etalon for gated imaging.

A fifth aspect is directed to imaging a same scene at different depths, thereby enabling the buildup of a three dimensional image of said scene.

A sixth aspect is directed to a method for gated imaging of an object comprising illuminating said object with a narrow band light source and detecting reflected light with a photo-detector proximal to the light source while providing a shutter between the object and the photo-detector, such that the shutter is controllable independently of the photo-detector.

Typically the light source is a laser or LED.

Optionally, the light source comprises at least one of:

• a source of visible light.

• a source of infra-red light.

• a source of ultra-violet light.

Optionally, the shutter is a cube-beam splitter; the gap between the two prisms being controlled by a piezoelectric material.

Optionally, the shutter comprises a cube-beam splitter having, the gap between the two prisms being controlled by a Micro Electro-Mechanical technology.

Optionally, the shutter comprises a Fabry-Perot interferometer controlled by a Micro Electro-Mechanical technology.

Optionally, the shutter comprises a plurality of optical elements arranged in series, the optical elements selected from the group comprising cube beam splitters, Fabry-Perot interferometers and Fabry-Perot etalon.

In some embodiments, the system is configured to operate at a plurality of wavelengths for constructing a multispectral image.

A further aspect is directed to an optical element selected from the group comprising a beam splitter, a Fabry-Perot etalon and a Fabry-Perot interferometer comprising at least one pair of optical sub-elements separated by an intermediate layer, wherein the optical characteristics of the intermediate layer or elements thereof are controllable by application of an electrical signal.

Optionally, the optical sub-elements are prisms of a cubic beam splitter, mirrors of a Fabry-Perot interferometer or surfaces of a Fabry-Perot etalon, and the controllable optical characteristic is selected from the group comprising a phase shift and a refractive index.

Optionally, the controllable optical characteristic is controlled by application of an electromagnetic signal.

Optionally, the controllable optical characteristic is controlled by application of an electrooptic effect.

Optionally, the optical parameter is controlled by application of a change in temperature, rf-field, acoustic field, electromagnetic field, or light / photons.

Optionally, the intermediate layer is selected from the group of birefringent crystals, liquid crystals and photonics crystals.

Optionally, the intermediate layer or optical sub-elements comprises sub wavelength structures of at least two different refractive indices, to create a third effective index of refraction.

A further aspect is directed to a device for Q-Switching a laser system comprising a Fabry-Perot etalon, said Fabry-Perot etalon comprising a space between parallel mirrors wherein at least one of the following limitations is true:

• separation of said mirrors is controllable by MEMS techniques;

• reflectivity of said mirrors is controllable by nano-particles; and

• at least one optical characteristic of the space or medium is controllable.

Optionally, the optical characteristic is selected from the group comprising refraction index, phase shift and phase retardation.

Optionally the switching is achieved by a change in at least one parameter selected from the group consisting of temperature, applied rf-field, applied acoustic field, applied electromagnetic field and photonic illumination.

Optionally, the device is tunable by varying the effective index of refraction.

Optionally the space between the mirrors comprises at least one of sub wavelength structures of different refractive index, birefringence crystals, liquid crystals and photonics crystals. Typically the device for Q-Switching is selectively switchable between an “open” state and a“closed” state.

Typically the device for Q-Switching is selectively switchable between a first wavelength and second wavelength.

Typically the device for Q-Switching is selectively switchable between at least two states selected from the group consisting of reflective, absorptive and

transmissive states.

Typically the device for Q-Switching comprises at least one filter.

Optionally, the device for Q-Switching comprises a beam splitter said beam splitter comprising a space or medium between hypotenuses of prisms that is controllable by MEMS or piezoelectric techniques.

Optionally, the device for Q-Switching comprises a beam-splitter that comprises a space or medium between hypotenuses of prisms wherein an optical characteristic of the space or medium is controllable.

Optionally, the optical parameter is selected from the group comprising refraction index, phase shift and phase retardation.

Optionally, the switching is achieved by a change in at least one parameter selected from the group consisting of temperature, applied rf-field, applied acoustic field, applied electromagnetic field, photonic illumination.

Optionally, the device is tunable by varying the effective index of refraction of the space or medium.

Optionally, the space or medium between the prisms comprises sub wavelength structures of different refractive index.

Optionally, the space or medium between the prisms is selected from the group of birefringence crystals, liquid crystals and photonics crystals.

Optionally, the device is selectively switchable between an“open” state and a “closed” state.

Optionally, the device is selectively switchable between a first wavelength and second wavelength.

Optionally, the device is selectively switchable between at least two states selected from the group including reflective, absorptive and transmissive.

Optionally, the device comprises at least one filter.

A method for Q-Switching a laser system comprising:

providing a Fabry-Perot etalon or FTIR that comprises a space or medium between parallel mirrors or hypoteneuses of prisms,

and controlling said Fabry-Perot etalon by MEMS or piezoelectric techniques.

Optionally, reflectivity of said mirrors is controlled by nano-particles.

Optionally, the method comprises controlling an optical characteristic of the space between the mirrors or prisms.

Optionally, the optical characteristic is selected from the group comprising refraction index, phase shift and phase retardation.

Optionally, the method comprises switching by changing at least one of the parameters selected from the group comprising temperature, applied rf-field, applied acoustic field, applied electromagnetic field, photonic illumination.

Optionally, the method comprises varying the effective index of refraction.

Optionally, the device is sthe space or medium between the mirrors comprises sub-wavelength structures of different refractive index and switching comprises changing the effective refractive index.

Optionally, the space between the mirrors or prisms comprises birefringence crystals and the method comprises stimulating a birefringent effect.

Optionally, the space or medium between the mirrors or prisms comprises liquid crystals and switching comprises rotating said liquid crystals.

Optionally, the space or medium between the mirrors or prisms comprises liquid crystals and switching comprises aligning said liquid crystals.

Optionally, the method comprises activating photonics crystals.

Optionally, the method comprises selectively switching between an“open” state and a“closed” state.

Optionally, the method comprises selectively switching between a first wavelength and second wavelength.

Optionally, the method comprises selectively switching between two states selected from the group including reflective, absorptive and transmissive states.

Optionally, the method comprises selectively switching at least one filter.

A system for spectral imaging comprising a light detector for detecting light from a light source, and a tunable filter which is controllable by a change in its physical or optical characteristics.

Optionally, the tunable filter is positioned in front of the light source.

Alternatively, the tunable filter is positioned in front of the detector. Alternatively the system comprises a plurality of filters positioned in different places along the optical channel.

Optionally the tunable filter comprises a Fabry-Perot etalon, in which tunability is achieved by changing the reflectivity of the mirrors by nano-particles.

Optionally the tunable filter comprises a Fabry-Perot etalon, in which tunability is achieved by changing at least one of the refraction index, phase shift and phase retardation of the medium between the mirrors.

Optionally the tunable filter comprises a Frustrated Total Internal Reflection filter, in which tunability is achieved by changing at least one of refraction index, phase shift and phase retardation of the medium between the prisms or of the prisms themselves.

Optionally the pass band of the tunable filter is controlled by an electro-optic effect.

Optionally the pass band of the tunable filter is controlled by change of at least one of temperature, rf-field, acoustic field, electromagnetic field and illumination.

Optionally the pass band of the tunable filter is selected from the group comprising birefringence crystals, liquid crystals and photonics crystals.

Optionally the system comprises a plurality of said filters.

Optionally said filer is a controllable LVF (linearly variable filter) having continuous or non-continuous wavelengths.

A method for spectral imaging comprising providing the system of claim 63 and varying the band-pass of the tunable filter by changing its physical or optical characteristics.

Optionally the tunable filter is positioned in front of the light source.

Optionally the the tunable filter is positioned in front of the detector.

Optionally a plurality of filters are positioned in different places along the optical channel.

Optionally the tunable filter comprises a Fabry- Perot etalon, wherein the reflectivity of the mirrors is changed by nano-particles to control the filter.

The method of claim 75, wherein the tunable filter comprises a Fabry-Perot etalon, wherein tunability is achieved by changing at least one of refraction index, phase shift and phase retardation of the medium between the mirrors.

Optionally the tunable filter comprises a Frustrated Total Internal Reflection filter, in which tunability is achieved by changing at least one of refraction index, phase shift and phase retardation of the medium between the prisms or of the prisms themselves.

Optionally the tunable filter is controlled by an electro-optic effect.

Optionally the the pass band of the tunable filter is controlled by changing temperature, rf-field characteristics, acoustic field characteristics, electromagnetic field characteristics, or by illumination.

Optionally the pass band of the tunable filter is controlled by at least one of birefringency. alignment of liquid crystals and photonics crystals.

Optionally the method comprises using a plurality of said filters.

Optionally the filter is a controllable LVF (linearly variable filter) having continuous or non-continuous wavelengths.

DESCRIPTION OF FIGURES

For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings.

With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

Fig. 1 is a schematic illustration of a system for imaging an object, filtering out reflections from dust, fog and or smoke between the place of the light source and photo-detector, and the object of interest;

Fig. 2(a) shows a cube beam splitter having total internal reflection;

Fig. 2(b) shows a cube beam splitter having frustrated total internal reflection;

Fig. 2(c) shows the cube beam splitter where the gap is minimized, so light is totally transmitted;

Fig. 3 illustrates the band gap of the cube splitter;

Fig. 4 is a schematic representation of the Fabry-Perot interferometer or etalon; Fig. 5 is a schematic illustration of the setup of a simple optically pumped laser of the prior art;

Fig. 6 is a schematic illustration of a active switched laser of the prior art;

Fig. 7 is a schematic illustration of the effect of active switching;

Fig. 8 is a schematic illustration of a passive switched laser of the prior art;

Fig. 9 is a schematic illustration of the effect of passive switching;

Fig. 10 is a laser of the invention, wherein a Fabry Perot Etalon serves as the Q switch, and

Fig. 11 is a schematic illustration showing the principle of spectral imaging.

DESCRIPTION OF EMBODIMENTS

Optical Gated Imaging System and Method

It is proposed to use a non-gated light detector together with a fast shutter. By separating the shutter from the light detector, the shutter may be used as an optical switch with any light detector and may be optimized for any wavelength and compatible with downstream electronics.

With reference to Fig. 1, in general, the present invention is directed to a system for imaging objects 12. The object is illuminated with a lamp 10 that is typically a light source such as laser or LED (Light Emitting Diode), for example. Light from the lamp 10 will be reflected by the object 12 and may be detected with a light detector 16 such as a photodiode or phototransistor, typically a pixilated array such as a CMOS, CCD or similar.

The light source is typically controllable, and may provide a pulsed signal for example.

Unfortunately, light from the lamp 10 will also be scattered by the cloud 14 that is the totality of air borne water droplets (mist or fog), dust particles, smoke and the like, that may he between the lamp 10 and the target object 12. Thus light detected by the detector 16 may not be light reflected by the object 12, but rather light reflected or scattered by the cloud 14. To prevent this, in the prior art, fast switchable light detectors 16 have been used, such as ingaas photodiode arrays that are synchronized with the light source pulses.

In the present invention it is proposed to use a shutter 18 positioned in front of the detector 16 and set to open at Dΐ past the light emission from the source 10 where Dΐ = 2d/c and d is the separation of the light source and detector from the object 12, and c is the speed of light. With reference to Fig. 2(b), in a first embodiment, the fast shutter is a Tunable Frustrated Total Internal Refraction FTIR prism where the refractive image of the boundary layer between the prisms is switchable.

In quantum mechanics, it is possible for a particle to tunnel through a potential barrier because its wave function has a small but finite value in the classically forbidden region. Here we use FTIR as an optical analog of this quantum mechanical phenomenon.

With reference to Fig. 2(a) a 45°-90° prism will deflect a beam of light by total internal reflection. When two such prisms are sandwiched back-to-back and pressed together, the air-glass interface vanishes and the beam then propagates onward undisturbed 2(c). Indeed, the hypotenuse faces of the two prisms can be positioned so as to transmit and reflect any desired fraction of the incident light. Commercial devices which perform this function are known as cube beam splitters.

This transition, from total to no reflection, occurs gradually as the air film is made to thin out by progressively squeezing the prisms together harder until they make intimate contact. Optically speaking, if the evanescent wave extends with appreciable amplitude across the rare medium (air) into a nearby medium of higher refractive index (the 2nd prism), energy may flow across the gap (FTIR).

With reference to Fig. 2(b), Frustrated Total Reflection occurs when the usual total reflection at a surface to a medium with lower refractive index n becomes less than total because the thickness of the lower n medium becomes comparable to the evanescent wave damping length penetrating the lower n medium.

Pressure may be applied on the face of the prism to close the gap by using the piezoelectric effect.

In one embodiment, the optical prisms are fabricated from a piezoelectric single crystal material such as a single crystal having a perovskite structure, or placed on a perovskite substrate.

Use of two optical switches in series, and switching them simultaneously in different directions enables extra fast switching.

In another embodiment, pressure is applied to the single crystal using an external piezoelectric material that translates applied electrical signal (current or voltage spike) into pressure on the FTIR.

With reference to Fig. 3 and 4, in yet another embodiment the fast shutter is a Tunable Fabry Perot Etalon. In optics, a Fabry-Perot interferometer (FPI) or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors. (Precisely, the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. It is named after Charles Fabry and Alfred Perot, who developed the instrument in 1899. Etalon is from the French etalon, meaning "measuring gauge" or "standard". Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Perot interferometers.

With reference to Fig. 3, by controlling either the thickness of the etalon or the separation of the two mirrors, or the refractive index of the gap n2 between the mirrors or of the etalon, possibly by applying pressure from an adjacent piezoelectric element, it is possible to switch the a Fabry-Perot interferometer (FPI) / etalon from a state wherein it reflects back all incident light and acts as a barrier, to a situation when it becomes transparent to the incident radiation. By placing the Fabry-Perot interferometer (FPI) / etalon between the object to be imaged and the light detector, it can serve as a shutter. It can be synchronized with the returning signal that is transmitted from the light source and bounced off from the object to be imaged.

By using flashes of light 10 to illuminate the target 12 and detecting the light from the target only, filtering out light reflected by the atmospheric cloud 14, dust, water vapor or smoke particles, etc., very little illumination is required. The system can be used with visible wavelengths or with infra red or ultraviolet wavelengths for invisible imaging at night.

Additionally, the combination of a specific coherent light source and either the Frustrated total reflection or the Fabry-Perot interferometer or etalon enables imaging during the day, or where there is other illumination, since other wavelengths and incoherent light are effectively filtered out.

It is anticipated that by applying a signal to a shutter that is separate from the light detector, noise can be decreased significantly facilitating to sharper images.

Control of the cube beam splitter embodiment 18’ or the Fabry-Perot interferometer (FPI) / etalon 18” may be achieved by Micro Electro-Mechanical System MEMS technology. Use of piezoelectric material is but one example of this. It will be appreciated that the gating 18 may be between any two of the optical phenomena of reflection, absorption or transmission.

Although described with reference to monochromatic light, it will be appreciated that the above mentioned techniques could be used with two or more wavebands to achieve accuracy, color or spectral imaging.

Physically Tunable Filters

It is useful to image a same scene at different depths, thereby enabling the build up of a three dimensional image of the scene.

One way of achieving this is by the gated imaging of an object. This requires illuminating the object with a narrow band light source and detecting reflected light with a photo-detector proximal to the light source while providing a shutter between the object and the photo-detector, such that the shutter is controlled by micro- electro mechanical means independently of the photo-detector.

In another application, the series of time windows are opened to enable imaging of the same scene at different depths, thereby enabling the buildup of a three dimensional image.

In optics, a Fabry-Perot interferometer (FPI) or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors. (Precisely, the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon.

Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Perot interferometers.

With reference to Fig. 3, in an embodiment directed to Physical Tunable Filters (PTFs), instead of controlling the distance between the prisms in the cube beam splitter embodiment 18’ the refractive index of the boundary layer between the optical elements (e.g. prisms) is controlled.

In a further embodiment, the boundary layer causes a phase retardation, the magnitude of which may be controlled.

With reference to Fig. 4, in yet a further embodiment, instead of altering the distance between the mirrors / sides of the Fabry- Perot interferometer (FPI) / etalon 18, the refractive index of the gap between the mirrors / surfaces or of the etalon is changed. In yet a further embodiment, the phase of the light transmitted across the gap between the mirrors / surfaces or of the etalon is changed.

The change in the refraction index or phase retardation influences the critical angle conditions or the optical path resonator conditions, in such a way that the filter change states between“open” and“close” for a specific wavelength, or is open to one wavelength and closed to another.

Novel Q-Switch

With reference to Fig. 5, a laser oscillator usually comprises an optical resonator (laser resonator, laser cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip, because it experiences some losses, e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy, i.e. it needs to be“pumped”, e.g. by injecting light (optical pumping) or an electric current (electrical pumping in semiconductor lasers). The principle of laser amplification is stimulated emission.

The laser resonator is made of a highly reflecting curved mirror and a partially transmissive flat mirror, the output coupler, which extracts some of the circulating laser light as the useful output. The gain medium is a laser crystal, which is side- pumped, e.g. with light from a flash lamp.

A laser cannot operate if the gain is smaller than the resonator losses; the device is then below the so-called laser threshold and only emits some luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can exceed the resonator losses.

If the gain is larger than the losses, the power of the light in the laser resonator quickly rises, starting e.g. with low levels of light from fluorescence. As high laser powers saturate the gain, the laser power will in the steady state reach a level so that the saturated gain just equals the resonator losses ( gain clamping). Before reaching this steady state, a laser usually undergoes some relaxation oscillations. The threshold pump power is the pump power where the small signal gain is just sufficient for lasing.

Some fraction of the light power circulating in the resonator is usually transmitted by a partially transparent mirror, the so-called output coupler mirror. The resulting beam constitutes the useful output of the laser. The transmission of the output coupler mirror can be optimized for maximum output power (see also: slope efficiency).

Some lasers are operated in a continuous fashion, whereas others generate pulses, which can be particularly intense. There are various methods for pulse generation with lasers, allowing the generation of pulses with durations of microseconds, nanoseconds, picoseconds, or even down a few femtoseconds ( ultrashort pulses from mode-locked lasers).

The optical bandwidth (or linewidth) of a continuously operating laser may be very small when only a single resonator mode can oscillate ( single -frequency operation). In other cases, particularly for mode-locked lasers, the bandwidth can be very large - in extreme cases, it can span about a full octave. The center frequency of the laser radiation is typically near the frequency of maximum gain, but if the resonator losses are made frequency-dependent, the laser wavelength can be tuned within the range where sufficient gain is available. Some broadband gain media such as Tksapphire and CrZnSeallow wavelength tuning over hundreds of nanometers.

Q-switching is achieved by putting some type of variable attenuator inside the laser's optical resonator. When the attenuator is functioning, light which leaves the gain medium does not return, and lasing cannot begin. This attenuation inside the cavity corresponds to a decrease in the Q factor or quality factor of the optical resonator. A high Q factor corresponds to low resonator losses per roundtrip, and vice versa. The variable attenuator is commonly called a "Q-switch", when used for this purpose.

Initially the laser medium is pumped while the Q-switch is set to prevent feedback of light into the gain medium (producing an optical resonator with low Q). This produces a population inversion, but laser operation cannot yet occur since there is no feedback from the resonator. Since the rate of stimulated emission is dependent on the amount of light entering the medium, the amount of energy stored in the gain medium increases as the medium is pumped.

Due to losses from spontaneous emission and other processes, after a certain time the stored energy will reach some maximum level; the medium is said to be gain saturated. At this point, the Q-switch device is quickly changed from low to high Q, allowing feedback and the process of optical amplification by stimulated emission to begin. Because of the large amount of energy already stored in the gain medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in the medium to be depleted almost as quickly. The net result is a short pulse of light output from the laser, known as a giant pulse, which may have a very high peak intensity.

Advanced Laser systems use Q-Switches to optimize laser performance. There are two main types of Q-switches: passive and active Q-Switches. The active are usually more effective but much more expensive, and, many times limited to polarized light only.

With reference to Fig. 6, an active Q-switch laser is shown. In the Active Q-switch the Q-Switch is an externally controlled variable attenuator. This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity.

More commonly it may be some form of modulator such as an acousto-optic device, a magneto-optic effect device or an electro-optic device— a Pockels cell or Kerr cell. The reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal.

The pulse repetition rate can therefore be externally controlled. Modulators generally allow a faster transition from low to high Q, and provide better control. An additional advantage of modulators is that the rejected light may be coupled out of the cavity and can be used for something else. Alternatively, when the modulator is in its low-Q state, an externally generated beam can be coupled into the cavity through the modulator. This can be used to "seed" the cavity with a beam that has desired characteristics (such as transverse mode or wavelength). When the Q is raised, lasing builds up from the initial seed, producing a Q-switched pulse that has characteristics inherited from the seed.

Fig. 7 shows the typical effect of using an active Q switch in a laser to obtain a pulse of high powered energy is obtained.

With reference to Fig. 8, in the passive Q-switch the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG, which is used for Q-switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device. Initially, the loss of the absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully replenished. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's pump power and the amount of saturable absorber in the cavity. Direct control of the repetition rate can be achieved by using a pulsed pump source as well as passive Q- s witching.

Fig. 9 shows the output pulse of a passive laser. In regenerative amplification, an optical amplifier is placed inside a Q-switched cavity. Pulses of light from another laser (the "master oscillator") are injected into the cavity by lowering the Q to allow the pulse to enter and then increasing the Q to confine the pulse to the cavity where it can be amplified by repeated passes through the gain medium. The pulse is then allowed to leave the cavity via another Q switch.

A typical Q-switched laser (e.g. a Nd:YAG laser) with a resonator length of e.g. 10 cm can produce light pulses of several tens of nanoseconds duration. Even when the average power is well below 1 W, the peak power can be many kilowatts. Large-scale laser systems can produce Q-switched pulses with energies of many joules and peak powers in the gigawatt region. On the other hand, passively Q- switched microchip lasers (with very short resonators) have generated pulses with durations far below one nanosecond and pulse repetition rates from hundreds of Hertz to several mega-Hertz (MHz)

Q-switched lasers are also used to remove tattoos by shattering ink pigments into particles that are cleared by the body's lymphatic system. Full removal can take between six and twenty treatments depending on the amount and color of ink, spaced at least a month apart, using different wavelengths for different colored inks.

Nd:YAG lasers are currently the most favored lasers due to their high peak powers, high repetition rates and relatively low costs. In 2013 a pico-second laser was introduced based on clinical research which appears to show better clearance with 'difficult' colors such as green and light blue.

The Q-switch laser is also used by beauticians around the world to treat skin- related issues like acne, pigmentation, dark spots, and fixes for anti-aging. Control of Fabry-Perot interferometer (FPI) / etalon may be achieved by Micro Electro-Mechanical System MEMS technology. Use of piezoelectric material is one example of this.

With reference to Fig. 10, the present invention proposes using a Fabry-Perot Etalon as a Q-Switch (such a that shown in Fig. 4) in a laser system, and switching between fdter states, by changing one (or more) of the parameters of the Fabry-Perot Etalon such as the size of the space by MEMS techniques, the reflectivity of said mirrors, an optical parameter of the such as its refraction index, phase shift or phase retardation or the effective index of refraction. In some embodiments, the switching is achieved by changing at least one of the following parameters: temperature, applied RF-field, applied acoustic field, applied electromagnetic field, photonic illumination.

The space between the mirrors may be filled with a material that shows birefringence, with liquid crystals or with photonics crystals.

The device may selectively switchable between an“open” state and a“closed” state or between a first wavelength and second wavelength.

In some embodiments, the device is selectively switchable between any two of the following states: reflective, absorptive and transmissive.

Typically, the device for Q-Switching comprises at least one filter.

Novel Spectral Camera

With reference to Fig. 11 spectral imaging merges the two well-established technologies of spectroscopy and imaging to produce a tool that has proven useful in a variety of disciplines that rely on various forms of optical microscopy. The methodology has been extensively applied to visualize the chemical composition of materials ranging from enzymes involved in bio-molecular interactions to the formation of stars. Unlike a typical image, which is acquired over the entire wavelength response band of the detector, a spectral image requires the creation of a three-dimensional data set that contains a collection of images of the same field of view captured at different wavelengths or wavebands. In effect, the spectral image provides a complete spectrum of the specimen at every pixel location (noted as I(c.n.l): see Figure 11) throughout the lateral dimensions. Thus, a spectral image stack can be considered as either a collection of images, each of which is measured at a specific wavelength or over a narrow band of wavelengths, or as a collection of different wavelengths at each pixel location. Referring back to Fig. 2(a) a 45°-90° prism will deflect a beam of light by total internal reflection. When two such prisms are sandwiched back-to-back and pressed together, the airglass interface can be made to vanish and the beam then propagates onward undisturbed 2(c).

Indeed, the hypotenuse faces of the two prisms can be positioned so as to transmit and reflect any desired fraction of the incident light. Commercial devices which perform this function are known as cube beam splitters.

This transition, from total to no reflection, occurs gradually as the air film is made to thin out by progressively squeezing the prisms together harder until they make intimate contact.

Optically speaking, if the evanescent wave extends with appreciable amplitude across the rare medium (air) into a nearby medium of higher refractive index (the 2nd prism), energy may flow across the gap (FTIR).

With reference to Fig. 2(b), Frustrated Total Reflection occurs when the usual total reflection at a surface to a medium with lower refractive index n becomes less than total because the thickness of the lower n medium becomes comparable to the evanescent wave damping length penetrating the lower n medium.

Pressure may be applied on the face of the prism to close the gap by using the piezoelectric effect.

Referring back to Fig. 4 the Tunable Fabry Perot Etalon is shown.

Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Perot interferometers.

By controlling either the thickness of the gap between prisms in the cube beam splitter, the thickness of the etalon or the separation of the two mirrors, possibly by applying pressure from an adjacent piezoelectric element, it is possible to change the frequency of the light transmitted /reflected by the cube beam splitter or the Fabry- Perot interferometer (FPI) / etalon.

Control of the cube beam splitter embodiment 18’ or the Fabry-Perot interferometer (FPI) / etalon 18” may be achieved by Micro Electro-Mechanical System MEMS technology. Use of piezoelectric material is one example of this.

In embodiments of the present invention, it is proposed to use a tunable optical element such as a cube beam splitter or a Fabry-Perot etalon, but to tune it by changing the optical parameters of the material separating the prisms or mirrors, such