WO2013017881A1 - Système laser et procédé permettant de faire fonctionner le système laser - Google Patents

Système laser et procédé permettant de faire fonctionner le système laser Download PDF

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Publication number
WO2013017881A1
WO2013017881A1 PCT/GB2012/051869 GB2012051869W WO2013017881A1 WO 2013017881 A1 WO2013017881 A1 WO 2013017881A1 GB 2012051869 W GB2012051869 W GB 2012051869W WO 2013017881 A1 WO2013017881 A1 WO 2013017881A1
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Prior art keywords
pumping radiation
medium
lasing medium
spatial
lasing
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PCT/GB2012/051869
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English (en)
Inventor
Harry James Coles
Philip J. W. HANDS
Stephen Matthew Morris
Timothy David Wilkinson
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Cambridge Enterprise Limited
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Publication of WO2013017881A1 publication Critical patent/WO2013017881A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H01S3/1022—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2383—Parallel arrangements
    • H01S3/2391—Parallel arrangements emitting at different wavelengths
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094034—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a dye
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094038—End pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094061—Shared pump, i.e. pump light of a single pump source is used to pump plural gain media in parallel
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094076—Pulsed or modulated pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1686—Liquid crystal active layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/20—Liquids
    • H01S3/213—Liquids including an organic dye

Definitions

  • the present invention relates to a laser system and to a method of operating a laser system to generate a light output from the laser system.
  • the invention has particular, but not exclusive, applicability to dye doped liquid crystal lasers.
  • Tuning of the wavelength of the laser output is also possible based on changing the0 spacing of the interference fringes which drive the periodic variation in refractive index in the solvent holding the laser dye.
  • Bor and GIier (1986) [Z. Bor and A. Muller "Picosecond Distributed Feedback Dye Lasers" IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, NO. s, AUGUST 1986 1524-1533] disclose tuning over the range 400-800 nm by suitable variation of the optical set-up to achieve variation in5 the spacing of interference fringes in the laser cavity.
  • the interference fringes in Bor and MCcluder (1986) (and in other published work) are formed using static diffraction gratings. Variation in spacing of the interference fringes is provided in some cases using moveable mirrors.
  • Dye lasers have been produced which use cholesteric liquid crystals (having a self- organised helical structure) to form the required distributed feedback structure of a periodic spatial variation in refractive index.
  • the variation in refractive index arises because the liquid crystals are highly birefringent.
  • the cholesteric liquid crystal is doped with a suitable laser dye.
  • the output from the pump laser is received by a 10 x 10 lensiet array, the lenslets each being arranged to illuminate a different spot in the liquid crystal ceil, in this way, an array of lasing sites is provided in the liquid crystal ceil.
  • This increases the overall energy density of the liquid crystal laser.
  • Also disclosed in Hands et al (2008) is the simultaneous pumping of different domains in the liquid crystal cell, each domain providing a slightly different wavelength of laser output.
  • WO 2004/002165 discloses a method for the laser projection of images. Pump radiation is supplied to a pixeiated LC projection element, acting as a spatial light modulator, the output of which is projected directly onto a lasing medium. WO 2004/002165 explains that this arrangement provides an efficient means for the generation of high contrast images, because the spatial light modulator is able to reduce the light intensity in a required region of the lasing medium to below a threshold level for lasing.
  • organic laser systems including dye lasers, polymer based lasers, liquid crystal lasers, and hybrid variants of a combination of these types of organic laser systems are usually limited to pulsed operation only.
  • Static pump beams with long pulse lengths ⁇ > 50ns) or high repetition rates (including continuous wave (cw)) tend to generate a build-up of unwanted triplet states within the lasing medium, which hinder lasing and dramatically reduce output power.
  • the lasing medium needs time to recover from this "fatigue" before it can be pumped with successive pulses to generate further lasing.
  • some types of lasing medium can suffer from bleaching or other problems associated with pumping for extended periods of time.
  • the present invention has been devised in order to address at least one of the above problems.
  • the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.
  • the present inventors have realised that it may be possible to dynamically control the intensity of the pump beam at in the lasing medium. This can provide a wide range of technical benefits.
  • the present invention is based on this realisation, but can have broader applicability than merely addressing one or more of the specific problems identified above.
  • the present invention provides a laser system comprising:
  • the pump source is operable to generate pumping radiation directed to the lasing medium via the spatial intensity control means
  • the spatial intensity control means is a spatial light modulator (SLM) operable to modulate the amplitude and/or phase of the pumping radiation on a pixei-by-pixel basis in order temporally to change a spatial intensity profile of the pumping radiation incident at the lasing medium to control lasing in the lasing medium.
  • SLM spatial light modulator
  • the present invention provides a method of generating a light output from a laser system, wherein pumping radiation is directed to a lasing medium via a spatial intensity control means, wherein the spatial intensity control means is a spatial light modulator (SLM) which operates to modulate the amplitude and/or phase of the pumping radiation on a pixel-by-pixei basis in order temporally to change a spatial intensity profile of the pumping radiation incident at the lasing medium to control lasing in the lasing medium.
  • SLM spatial light modulator
  • the present invention provides a laser system comprising: a pump source;
  • the pump source is operable to generate pumping radiation directed to the lasing medium via the spatial intensity control means
  • the lasing medium has a spatial extent in at least one dimension to allow the pumping radiation to excite lasing at a first location in the lasing medium, at a second, different location in the lasing medium but not at a third, different location in the lasing medium, spatially interposed between the first and second locations,
  • the spatial intensity control means is operable temporally to change a spatial intensity profile of the pumping radiation incident at the lasing medium to reduce the intensity of the pumping radiation at the first and second locations and thereby cease lasing from the first and second locations but to increase the intensity of the pumping radiation at the third location to cause lasing from the third location.
  • the present invention provides a method of generating a light output from a laser system, wherein pumping radiation is directed to a lasing medium via a spatial intensity control means, and the lasing medium has a spatial extent in at least one dimension so that the pumping radiation excites lasing at a first location in the lasing medium, at a second, different location in the lasing medium but not at a third, different location in the lasing medium, spatially interposed between the first and second locations, and wherein the spatial intensity profile of the pumping radiation incident at the lasing medium is changed to reduce the intensity of the pumping radiation at the first and second locations and thereby to cease lasing from the first and second locations but to increase the intensity of the pumping radiation at the third location to cause lasing from the third location.
  • the present invention provides a laser system comprising:
  • the pump source is operable to generate pumping radiation directed to the lasing medium via the spatial intensity control means, the position of the lasing medium being fixed with respect to the position of the pump source, and the lasing medium having a spatial extent in at least one dimension to allow the pumping radiation to excite lasing at least at a first location in the lasing medium, but not at least at a second, different discrete location in the lasing medium
  • the spatial intensity control means is operable temporally to change a spatial intensity profile of the pumping radiation incident at the lasing medium to reduce the intensity of the pumping radiation at least at the first location and thereby cease lasing from at least the first location but to increase the intensity of the pumping radiation at least at the second location to cause lasing from at least the second location
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the lasing medium by time-varying reflection, refraction and/or interference of the pumping radiation.
  • the present invention provides a method of generating a light output from a laser system, wherein pumping radiation is directed from a pump source to a lasing medium via a spatial intensity control means, the position of the lasing medium being fixed with respect to the position of the pump source, and the lasing medium has a spatial extent in at least one dimension so that the pumping radiation excites lasing at least at a first location in the lasing medium, but not at least at a second, different discrete location in the lasing medium, and wherein the spatial intensity profile of the pumping radiation incident at the lasing medium is changed to reduce the intensity of the pumping radiation at least at the first location and thereby to cease lasing from at least the first location but to increase the intensity of the pumping radiation at least at the second location to cause lasing from at least the second location, wherein the spatial intensity control means operates to vary the intensity profile of the pumping radiation at the lasing medium by time-varying reflection, refraction and/or interference of the pumping radiation
  • adaptive pumping of the lasing medium is provided. This can avoid the need to wait for previously-excited lasing medium to recover before generating a further laser output, by changing the locations in the lasing medium that are subjected to pumping. Fresh regions of the lasing medium are therefore excited sequentially. This enables much higher laser repetition rates to be delivered, and also improved longevity and stability of the laser device.
  • the first, second, third, fourth, fifth and/or sixth aspect of the invention may be combined with each other in any combination. Furthermore, the first, second, third, fourth, fifth and/or sixth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.
  • the lasing medium is an organic lasing medium, such as a liquid crystal lasing medium, a dye lasing medium or a polymer based medium.
  • the lasing medium is a hybrid of different organic lasing medium, such as a liquid crystal lasing medium, a dye lasing medium and/or a polymer based medium.
  • the lasing medium is a liquid crystal lasing medium.
  • Typical liquid crystal lasing media incorporate a laser dye in a liquid crystal.
  • Suitable liquid crystals are discussed above, e.g. cholesteric liquid crystals which provide a self-organised helical structure which in turn, combined with the birefringence of the liquid crystals, provides a periodic variation in refractive index thereby allowing distributed feedback via Bragg reflection.
  • Control of the periodicity of the liquid crystal lasing medium controls the photonic band structure of the lasing medium. Suitable variation of the periodicity can be achieved by controlled addition of chiral dopants to the liquid crystal.
  • the liquid crystal lasing medium has more than one section, the photonic band structure being different in different sections of the lasing medium. This allows different wavelengths of laser output from the different sections of the lasing medium.
  • different laser dyes may be incorporated in the different sections of the lasing medium. Preferably, at least one section corresponds to red output, at least one section corresponds to green output and at least one section corresponds to blue output. In this way, the laser system may provide RGB output.
  • the different sections may be provided as separable segments. Alternatively, the different sections may be provided in the form of a gradient pitch liquid crystal lasing medium.
  • the lasing medium preferably has a spatial extent in two dimensions to allow the pumping radiation to excite lasing not only at the first and second locations mentioned above for the fifth and sixth aspects of the present invention, but also at third, and optionally at fourth, and optionally at fifth, etc., locations in a two dimensional array.
  • the two dimensional array may be ordered. Alternatively, the two dimensional array may be random.
  • the array of first, second, etc., locations may be at least a 2 x 2 array, more preferably a 3 x 3 array, more preferably still at least a 5 x 5 array.
  • a 10 x 10 array may be suitable.
  • the lasing medium preferably has a spatial extent in two dimensions to allow the pumping radiation to excite lasing not only at the first, second and third locations mentioned above for the third and fourth aspects of the present invention, but also at fourth, and optionally at fifth, and optionally at sixth, etc., locations in a two dimensional array.
  • the two dimensional array may be ordered. Alternatively, the two dimensional array may be random.
  • the array of first, second, third, etc., locations may be at least a 3 x 3 array, more preferably at least a 5 x 5 array.
  • a 10 x 10 array may be suitable.
  • the output light from the lasing medium may be received by a collector.
  • a suitable collector is a condenser lens.
  • the collector may direct the combined output light along an optical fibre.
  • the pump source can be any suitable pump source. Typically, a laser pump source is used, but this is not considered to be essential.
  • the pumping radiation is preferably circularly polarized when it reaches the lasing medium. However, it is not necessarily- essential to use solely circularly pola ised light. Therefore any radiation source may be used, including unpolarised sources, e.g. an LED light source, flash lamp or other incoherent light source. For other applications, a coherent laser output may be wanted for the system, in those cases, the pump source is typically a laser pump source.
  • Suitable laser pump sources typically operate in pulsed mode.
  • a suitable pulsed mode may use nanosecond-duration pulses (e.g. pulses between 1 and 10 ns in duration) at a repetition rate of at least 50 Hz (e.g. about 100 Hz). This may be acceptable for some applications, e.g. display applications.
  • the pump source operates in continuous or quasi- continuous mode. This allows the output of the system to be continuous or quasi- continuous.
  • the spatial intensity control means operates to vary the intensity profile of the pumping radiation at the iasing medium relative to the position of the pump source.
  • the iasing medium itself is preferably stationary with respect to the pump source.
  • the intensity profile itself is preferably moved, and the Iasing medium preferably remains substantially stationary with respect to the pump source.
  • the modulation of the intensity profile can be achieved very quickly and accurately, without need for change of the position of the pump source or of the iasing medium.
  • Mechanical movement of the pump source and/or of the iasing medium is typically either too slow, too large in size or requires too high power consumption to achieve a suitable operation of the system.
  • mechanical movement of the spatial intensity control means may be suitable in some embodiments to provide suitable steering of the pumping radiation, e.g. by refraction and/or reflection.
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the Iasing medium by refraction of the pumping radiation.
  • Temporal variation of refraction of the pumping radiation can be achieved using a variable lens arrangement.
  • the variable lens arrangement is operable to vary refractive length. This allows the pumping radiation to be focused and then defocused at the Iasing medium. This change in focus allows the local intensity of the pumping radiation at the iasing medium to be changed. In turn, this allows the intensity of the pumping radiation to be cycled above and below a iasing threshold intensity, thereby turning Iasing on and off from the selected location in the Iasing medium.
  • the spatial intensity control means includes a spinning, moving or vibrating prism (or other refractive element) to achieve time-varying refraction of the pumping radiation.
  • the prism rotates, vibrates or moves relative to the laser source (the pump source) to move the intensity profile with respect to the lasing medium while the pump source and the lasing medium remain stationary with respect to one another.
  • the spatial intensity control means includes a variable lens
  • variable lens arrangement is an array of variable lenses.
  • the array of variable lenses is typically arranged so that the pumping radiation refracted by the variable lenses is directed to match the spatial extent of the lasing medium.
  • a two dimensional array of lasing locations is required in the lasing medium, preferably there is a corresponding array of variable lenses.
  • one variable lens is required per lasing location.
  • each variable lens is an adaptive lens in which the refractive index profile of each lens is varied in response to a control signal.
  • variable lens arrangement may be provided by an adaptive lenslet or an array of adaptive lenslets.
  • the adaptive lenslet or each adaptive lenslet in the array preferably incorporates a liquid crystal layer and an associated electrode arrangement. Operation of the electrode arrangement may therefore provide temporal variation of the spatial distribution of refractive index in the liquid crystal.
  • Suitable lenslets and lenslet arrays are discussed in WO 2008/155583, the content of which is hereby incorporated by reference in its entirety, including the discussion of the background to that invention, it is noted here that other adaptive lens technologies may be used, as will be apparent to the skilled person.
  • the adaptive modal liquid crystal lenses described in Fravai (2010) [N. Fraval and J.L. de Bougrenet de la Tocnaye "Low aberrations symmetrical adaptive modal liquid crystal lens with short focal lengths" Applied Optics, Vol. 49, issue 15, pp. 2778-2783 (2010)].
  • lenslets or lenslet arrays allow the local intensity of the pumping radiation in the lasing medium to be varied in order to provide the desired laser output from the system.
  • the lasing medium has RGB sections
  • suitable control of the lenslet or lens!et array allows the colour of the output of the laser system to be varied arbitrarily. It is to be noted here that since each red, green and blue source can be designed to have a relatively narrow band output, many more colour combinations are possible from such an RGB system, comprising a iensiet array, than are typically available from the use of conventional filtered white light sources which have relatively broad emission spectra.
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the lasing medium by reflection of the pumping radiation.
  • an adaptive mirror or an array of adaptive mirrors may be provided, corresponding in operation to the adaptive lens!et and array of adaptive lenslets discussed above, the operation of the adaptive mirrors changing the focal length and/or the reflection direction of the mirrors with time.
  • the control of the shape of the mirror may allow the direction of reflection of a beam of pumping radiation to be varied with time.
  • An alternative mirror or array of mirrors may be provided by a digital micromirror device, sometimes referred to as a DLP (Digital Light Processing) chip.
  • a suitable digital micromirror device provides an array of mirrors held on deformable supports, each mirror being deflectable by operation of a suitable actuation means.
  • Typical actuation means rely on electrostatic forces.
  • the actuation means are typically individually addressable, allowing individual on/off control of the position of each mirror.
  • “on/off” refers to whether the mirror directs pumping radiation towards the lasing medium or not. In this way, the intensity profile of the pumping radiation at the lasing medium can be controlled and very quickly changed with time.
  • the adaptive mirror or array of adaptive mirrors may be a
  • MEMs deformable mirror microelectromechanicai deformable mirror
  • a suitable MEMs deformable mirror comprises a mirror which can be deformed in three dimensions by suitable electromechanical control means. Use of this type of adaptive mirror arrangement allows the intensity profile of the pumping radiation at the lasing medium to be controlled and very quickly changed with time.
  • the spatial intensity control means includes a spinning or vibrating mirror to achieve time-varying reflection of the pumping radiation, for example a galvo- mirror.
  • the spinning or vibrating mirror is a focusing mirror.
  • the mirror spins around an off-centre and/or an off-normal axis.
  • the mirror rotates or vibrates relative to the laser source (the pump source) to move the intensity profile with respect to the iasing medium while the pump source and the lasing medium remain stationary with respect to one another.
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the lasing medium by pixel-based spatial modulation of the amplitude of the pumping radiation.
  • the iasing medium is located optically far enough from the spatial intensity control means that there is not the simple projection of the spatial intensity control means. Instead, it is preferred that the lasing medium is located in the Fraunhofer regime from the spatial intensity control means, so that interference occurs at the lasing medium between pumping radiation from two or more pixels of the spatial intensity control means.
  • This type of control is preferred in many embodiments because the use of interference at the iasing medium to control the intensity profile allows efficient use of the power of the pump source, because it reduces wastage of photons.
  • the pumping radiation intensity in the iasing medium is controlled via
  • Suitable control of the intensity of the pumping radiation may be provided by a liquid crystal-based device.
  • the pumping radiation may be directed into the spatial intensity control means, the amplitude of the pumping radiation which leaves the spatial intensity control means being dependent on the orientation of the liquid crystal in each pixel compared with the orientation of polarising filters associated with the device.
  • the liquid crystal orientation can be controlled by control of the electric field distribution in each pixel.
  • the spatial intensity control means is a reflection-based liquid crystal-based device.
  • the spatial intensity control means may be a liquid crystal over silicon (LCoS) device, in which a layer of liquid crystal is controlled by electronics built into a backplane of semiconductor (typically Si-based), reflection being provided by a reflective layer over the semiconductor.
  • LCoS devices are preferred to transmission devices in view of the very small pixel size that is possible by incorporating CMOS manufacturing techniques.
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the lasing medium by spatial modulation of the phase of the pumping radiation.
  • This type of control is preferred in many embodiments because it can use interference at the lasing medium to control the intensity profile. This allows efficient use of the power of the pump source, because it further reduces wastage of photons.
  • the pumping radiation intensity in the lasing medium at a spatial location where lasing is required is controlled via interference of pumping radiation from at least two pixels of the SLIVL
  • the spatial intensity control means controls the intensity profile of the pumping radiation at the lasing medium by time-varying
  • the spatial intensity control means is a SLM which operates to modulate the amplitude and/or phase of the pumping radiation on a pixel-by- pixel basis in order to control the pumping radiation intensity at the first and/or second locations in the lasing medium by interference of pumping radiation from at least two pixels of the SL .
  • the spatial intensity control means is referred to as a spatial light modulator (SLM).
  • SLM spatial light modulator
  • the SLM may be optically addressed. However, this is not preferred. More preferred is the use of an electrically addressed SLM.
  • phase modulation SLM devices are known to the skilled person.
  • Devices from Hoioeye may also be used (ht ⁇ :// ww.hoioeye.CQm/spatiai light moduiators- technology.htmO (accessed 20 July 201 1 ), such as the LC 2002 SLM, the LC-R 720 SLM, the LC-R 1080 SLM or the PLUTO SLM.
  • Devices from Forth Dimension Displays may be used ⁇ ht ⁇ ?:/ w w.forthdd,com/products) (accessed 20 July 201 1 ).
  • the SLM utilizes a f!exoelectro-optic effect liquid crystal material in order to control the phase of light transiting the liquid crystal.
  • a f!exoelectro-optic effect liquid crystal material in order to control the phase of light transiting the liquid crystal.
  • Such a device is disclosed in WO 2010/1 19252, the content of which is incorporated herein by reference in its entirety.
  • the use of such a device can provide fast response times (e.g. of the order of 1 -10 kHz).
  • the device is not limited to binary phase modulation but instead allows multi-level phase modulation with fast response times.
  • the SLM device operates so that, for each pixel, there is provided means for applying an electric field to the layer of liquid crystal material so as to deflect the optic axis of the liquid crystal layer, thereby providing a phase shift to light transiting the liquid crystal layer.
  • the phase shift is variable substantially continuously with the electric field applied to the liquid crystal layer in the SLM.
  • the phase shift may be available at least 5 phase shift levels (corresponding to suitable electric field input signals), but preferably significantly more phase shift levels are available, e.g. at least 10, at least 20, at least 30, at least 40 or at least 50 phase shift levels.
  • the phase shift is variable substantially linearly with the electric field applied to the liquid crystal layer.
  • the phase shift provided to the light varies with the amount of deflection of the optic axis, up to a practical limit.
  • the practical limit typically will be determined by the maximum electric field that can be applied to the liquid crystal, or to the behaviour of the liquid crystal above a threshold electric field.
  • the response time (typically defined as the 10%-90% response time) is 100 ms or less. More preferably the response time is 50 ms or less. More preferably, the response time is 1 ms or less, 500 s or less, e.g. about 100 ⁇ or faster.
  • the liquid crystal material is a chiral nematic liquid crystal material.
  • the liquid crystal material has a helical structure.
  • WO 2006/003441 contains a detailed discussion of flexoelectro-optic liquid crystal materials.
  • the content of WO 2006/003441 is hereby incorporated by reference in its entirety, in particular in respect of its disclosure of suitable properties of and suitable materials for the flexoelectro-optic liquid crystal.
  • the helical axis is substantially perpendicular to the direction of the applied electric field. In this way, the application of an electric field allows flexo-eiectric deformation to occur stably.
  • the helical pitch of the fiexoelectro-optic liquid crystal may be shorter than the wavelength of the incident light.
  • the helical pitch of the f!exoelectro-optic liquid crystal may be substantially shorter than the wavelength of the incident light. In this way, rotational dispersion effects may be reduced. Furthermore, the use of a short pitch can reduce the response time of the device.
  • the layer of liquid crystal has a thickness direction corresponding to its smallest dimension.
  • the helical axis is substantially perpendicular to the thickness direction.
  • the helical axis may be parallel to a substrate of the device.
  • the orientation and geometry of the liquid crystal material may be that of a uniform lying helix (ULH) geometry.
  • UH uniform lying helix
  • the helical axis may be non-parallel to the substrate.
  • the helical axis may be perpendicular or substantially perpendicular to the substrate. Such an arrangement is typically referred to as a standing helix arrangement.
  • the layer of liquid crystal is held between a substrate and a cover.
  • the cover is typically substantially transparent to the incident light.
  • the means for applying an electric field typically includes an electrode formed at the substrate. More preferably, the means for applying an electric field includes an array of electrodes formed at the substrate. Each may be selectively addressable. Each may correspond to discrete pixels or sub-pixels of the device. Thus, each pixel or sub-pixel may be selectively operable to provide a phase shift to light transiting the liquid crystal layer at the pixel or sub-pixel.
  • the means for applying an electric field may include an electrode (preferably substantially transparent, e.g. indium tin oxide (ITO) or the like) formed at the cover. This electrode may be a common electrode.
  • ITO indium tin oxide
  • the SLM device includes an array of a large number of pixels or sub-pixels. In a one-dimensional array, there may be at least 100 (more preferably at least 1000) pixels or sub-pixels. In a two-dimensional array, there may be at least 100 x 100 (more preferably at least 100 x 1000 or 1000 x 1000) pixels or sub-pixels, or more.
  • the SLM device may be operable in transmission mode, in this case, the substrate is preferably substantially transparent to the incident light. However, it is preferred that the device operates in reflection mode.
  • the incident light may reflect from the substrate. Preferably, the incident light reflects from a surface of at least one of the electrodes formed on the substrate.
  • the device may be configured to apply a suitable phase shift to the incident light based on a two-way transit through the liquid crystal layer, i.e. from the cover to the substrate and from the substrate to the cover and out of the device.
  • the SLM device may include a quarter wave plate in the light path, in this way, the phase shift applied to the light may be increased.
  • the incorporation of a quarter wave plate may therefore double the response of the device.
  • the phase depth available from the SLM is at least rr, more preferably at least 2 IT.
  • the substrate includes at least a layer of semiconductor material, such as silicon. Most preferably, the substrate is based on a LCoS architecture substrate as discussed above.
  • the thickness of the liquid crystal layer may be 20 ⁇ or less. More preferably, the thickness of the liquid crystal layer may bel O ⁇ or less. For example, a thickness of about 5 pm is considered suitable.
  • liquid crystal devices include at least one polarizing layer. Such devices typically operate to modulate the intensity of light transiting the device.
  • liquid crystal displays typically operate by rotating the polarization direction of light through a layer of liquid crystal held between crossed polarizing layers. Such an operation is discussed above in respect of modulation of the intensity of the pumping radiation.
  • the light entering and/or exiting the device does not pass through a polarizing layer.
  • the present invention preferably does not utilize cross polarizing layers. This is because the present invention aims to utilize phase modulation of the light, and the presence of polarizing layers tends to reduce the overall intensity (and thus efficiency) of the device.
  • focussing means to focus the pumping radiation at the iasing medium.
  • the total pump intensity may be varied by suitable control of the pump source. This allows further control of the power output of the laser system, in addition to the control of the power output which is made possible by suitable control of the pumping radiation intensity profile at the iasing medium.
  • Fig. 1 schematically illustrates a laser system substantially according to a prior art disclosure.
  • Fig. 2 schematically illustrates a laser system according to an embodiment of the invention.
  • Fig. 3 shows a schematic exploded view of a iensiet structure suitable for use with the embodiment of Fig. 2,
  • Fig. 4 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 5 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 6 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 7 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 8 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 9 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 10 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 1 1 schematically illustrates a laser system according to another embodiment of the invention.
  • Fig. 12 schematically illustrates a laser system according to another embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER
  • Prior art laser system 10 has a gradient pitch chiral nematic liquid crystal cell 12 in which a layer of liquid crystal is held between first and second covers.
  • the gradient pitch is provided as explained in more detail below. The effect of this is that the periodicity of the chiral nematic liquid crystal varies across the liquid crystal ceil.
  • the liquid crystal layer includes chiral dopants, whose concentration varies across the cell in order to induce the pitch gradient.
  • the liquid crystal layer also includes a laser dye, typically being selected in order to emit light at a wavelength corresponding to the periodicity at different parts of the liquid crystal cell. Together, the liquid crystal, the chiral dopant and the laser dye make up the lasing medium.
  • a lensiet array 4 is provided in order to focus pumping radiation 16 onto the lasing medium.
  • the lensiet array is formed of moulded transparent optical plastics material.
  • the focal length of each lensiet is the same and is fixed. Therefore, provided that the lensiet array is uniformly illuminated with coliimated pumping radiation and is spaced at a suitable distance from the lasing medium, each lensiet operates to focus a spot of pumping radiation in the lasing medium.
  • the intensity of pumping radiation at each spot formed in the pumping medium is therefore the same.
  • each spot provides a laser output.
  • the laser output wavelength varies across the lasing medium due to the variation in the pitch of the chiral structure of the liquid crystals.
  • the result is therefore a corresponding array 18 of polychromatic laser outputs.
  • the wavelength of the output laser light may increase in the direction shown by arrow A.
  • the system of Fig. 1 differs from previous systems. In previous systems, a single lens is typically used to focus the light from the pump source into a single spot.
  • the focus position is adjusted so that it lies within a cell containing the lasing medium - the mixture of nematic liquid crystal, chiral dopant and laser dye. Careful choice of these three components and their respective concentrations enables ceils to be made which are capable of lasing at virtually any chosen visible wavelength.
  • the present inventors devised the embodiments of the present invention in order to improve the functionality and/or performance of optically-pumped laser systems.
  • the invention was first designed with the specific application of liquid crystal (LC) and organic lasers in mind, and the preferred embodiments describe such systems.
  • LC liquid crystal
  • organic lasers organic lasers
  • the present invention is not necessarily limited to such laser systems and features of the invention can be applied to other optically-pumped laser systems.
  • incident pump beams in optically-pumped laser systems are static in their spatial position (although they may be temporally pulsed).
  • the preferred wavelength of incident pump beams in optically-pumped laser systems are static in their spatial position (although they may be temporally pulsed).
  • embodiments of the invention allow for dynamic modification of the spatial intensity profile of the incident pump beam.
  • Appropriate control of the pump beam therefore can lead to a variety of performance improvements.
  • such laser systems can provide control of, and increases in, the repetition rate of pulsed laser outputs beyond their conventional physical limits.
  • Continuous (and quasi-continuous) wave mode outputs may therefore be provided from laser systems that conventionally operate only with pulsed outputs.
  • Further possible performance improvements include dynamic control of, and increases in, the total available power output; beam-shaping and spatial positioning of the optical output; and wavelength tuneabiiity of the laser output from the system.
  • Adaptive pumping avoids the need to wait for this recovery to occur, by rapidly and continually changing the locations(s) in the iasing medium that are subjected to pumping radiation at or above the iasing threshold intensity. This can be done, for example, at a rate corresponding to the timing of sequential pulses from the pump source. Fresh regions of the lasing medium are therefore constantly excited, which enables much higher repetition rates to be delivered.
  • the overall power output of the laser can also be varied. This can be achieved by adaptive optical control of the incident optical power density, or alternatively by changing the number of simultaneous pump beams that are incident upon the iasing medium.
  • Liquid crystal lasers are known that can deliver laser outputs with any wavelength output across the visible and near-infrared spectrum (450-850 nm).
  • LC laser cells can be fabricated with spatially varying composition (e.g. gradient pitch ceils as discussed above, or segmented cells), and which are therefore capable of different emission wavelengths in different regions of the cell.
  • spatially varying composition e.g. gradient pitch ceils as discussed above, or segmented cells
  • emitted laser light of arbitrary wavelength can be selected.
  • multiple simultaneous polychromatic outputs can be obtained, which when combined can give rise to white light emission, or indeed any colour emission in the RGB colour gamut.
  • the pumping radiation needs to be controlled in a dynamic way, using one or more adaptive optical elements.
  • the mechanisms for achieving this are wide-ranging, but can be categorised in two different ways.
  • the first category is diffractive adaptive pumping, the adaptive optical elements in this case being one or more of gratings, holograms, spatial light modulators (SLMs), etc.
  • the second category is refractive/reflective adaptive pumping, the adaptive optical elements in this case being one or more oi adaptive or moving lenses, spinning prisms, gaivo-mirrors, etc.
  • refractive techniques will be exemplified first, although this should not be taken as an indication that refractive implementations of the invention are more preferred than other implementations. Indeed, it is considered at the time of writing that SLM-implementations of the invention have the greatest flexibility of operation.
  • Fig. 2 shows a schematic view of an embodiment of the invention in which the spatial intensity control means is an adaptive liquid crystal lenslet array.
  • This embodiment allows three (red, green and blue) coherent light sources from a liquid crystal (LC) laser system to be simultaneously emitted and independently modulated in intensity. The three light sources are then recombined into a single output, the colour of which may arbitrarily chosen by the user, depending upon the relative intensity combinations of the three lasers.
  • LC liquid crystal
  • Laser system 100 includes liquid crystal (LC) laser cell 102.
  • Laser cell 102 is divided into three independent compartments 104, 106, 108 of roughly equal angular size (about 120°).
  • the three compartments are each filled with a different LC/dopant/dye mixture that is designed to iase at red, green and blue (RGB) wavelengths respectively.
  • compartment 104 is a blue LC laser
  • compartment 106 is a red LC laser and
  • compartment 108 is a green LC laser. Suitable material combinations for these LC lasers are set out in the following section.
  • a high energy coilimated optical pump beam 109 passes through a lenslet array 10.
  • Lenslet array 10 is a segmented adaptive liquid crystal lenslet array described in more detail below.
  • the lenslet array 1 10 generates an array of spots which are focussed over the entire ceil 102.
  • the result is an array of diverging LC laser sources in the cell 102 consisting of simultaneous emission at three different (RGB) wavelengths, if these three LC lasers are designed to emit at equal intensities, then by recombining them into a single source (through the use of a condensing lens 1 12, or simply by allowing them to diverge and overlap in the far-field) then the resulting recombination of them is white light.
  • RGB red, or simply by allowing them to diverge and overlap in the far-field
  • the recombined RGB output (of arbitrary colour) is shown at 1 4 and may be fibre-coupled for transmission to an output (not shown).
  • an arbitrary colour recombined spot In order ior an arbitrary colour recombined spot to be achieved, independent control of the three laser intensities is required. This is achieved by replacing the static lenslet array of Fig. 1 with a segmented adaptive liquid crystal microlens array 1 10.
  • Adaptive LC microlens arrays have previously been published in the literature, capable of adaptive focus through the application of a variable applied voltage.
  • the lenslet array 1 10 shown in Fig. 2 has three segmented regions 1 16, 1 18, 120 which can be
  • the lenslet array is capable of three different focal lengths in the different segments.
  • the segments 1 16, 1 18 and 120 are patterned so as to match the three compartments 104, 106, 108 within the LC laser cell.
  • variation of the address voltages V1 , V2 and V3 to the lenslet array 1 10 results in adaptive control of the focal spot diameter within each LC laser cell
  • Fig. 12 shows an embodiment of the invention which contains the similar features to Fig. 2 except that laser ceil 02 is divided into three independent compartments 104, 106, 108 of roughly equal area lengthwise rather than being divided angularly as for Fig. 2.
  • the three compartments are each filled with a different LC/ ' dopant/dye mixture that is designed to lase at red, green and blue (RGB) wavelengths respectively.
  • compartment 104 is a blue LC laser
  • compartment 106 is a red LC laser and
  • compartment 108 is a green LC laser. Suitable material combinations for these LC lasers are set out in the following section.
  • the adaptive liquid crystal microlens array 1 10 is controlled in the same way as described above for Fig. 2 to provide adaptive control of the focal spot diameter within each LC laser ceil compartment, in this embodiment a condensing lens 1 12 is provided to recombine the light emitted from the lasing ceil into a single output beam 1 17.
  • a condensing lens 1 12 is provided to recombine the light emitted from the lasing ceil into a single output beam 1 17.
  • Fig. 8 shows a schematic view of an embodiment of the invention in which the spatial intensity control means includes a spinning prism 1 13. This embodiment allows the intensity profile of the pumping radiation at the laser cell 102 to be varied relative to the position of the pump source (not shown).
  • a high energy collimated optical pump 109 beam passes through a lens 1 1 1 , e.g. a static lens, and a spinning prism 1 13.
  • the spinning prism rotates in the direction shown by the arrow 1 15.
  • the lens focuses the pumping radiation into a single spot at a lasing cell 102.
  • the spinning prism allows for dynamic modification of the spatial intensity profile of the pumping radiation at the lasing medium, that is, the focused pumping radiation spot at the lasing medium is caused to move around by refraction. Therefore in this embodiment the location in the lasing medium that is subjected to pumping radiation at or above the lasing threshold intensity is rapidly and continually changed in order to reduce the effect of fatigue of the laser medium described above.
  • the lens 1 1 1 may be an adaptive lens operable to control the focus of the pumping radiation 109 incident at the lasing cell 02 to allow additional intensity control of the laser output 1 17 in addition to steering of the pumping radiation to reduce fatigue in the laser medium. Dynamic modification of the spatial intensity profile of the pumping radiation at the iasing medium can also be used to tune the wavelength of the output radiation 1 17.
  • the LC laser cell 102 may have a uniform pitch or may contain different compartments designed to iase at different wavelengths. When the laser ceil 102 has a uniform pitch a monochromatic output is provided.
  • the provision of a spinning prism can be used to control the pumping radiation intensity profile at different spatial locations in the iasing medium and the wavelength of emitted laser light selected.
  • Fig. 9 shows an embodiment of the invention which combines the features the embodiments of Figs. 2 and 8. Similar features to Fig. 2 are not described again here, but are given similar reference numbers, in Fig. 9 the lensiet array 1 10 generates an array of spots which are focussed over the entire cell 102.
  • This embodiment includes a spinning prism, as described above in Fig. 8.
  • the prism allows for dynamic modification of the spatial intensity profile of the pumping radiation at the iasing medium, that is, the spots incident on the Iasing medium are caused to move around. Therefore in this embodiment the locations in the Iasing medium that are subjected to pumping radiation at or above the Iasing threshold intensity are rapidly and continually changed in order to reduce the effect of fatigue of the laser medium described above.
  • BL093 (Merck KGaA). Each one was doped into the LC host and then capillary filled into anti-parallel rubbed 5 ⁇ cells after mixing in a bake oven.
  • the three most suitable dyes were found to be DCM, Coumarin 540A, and Coumarin 504.
  • the pitch is controlled with the concentration of chirai dopant in the LC mixture.
  • the pitch can also be affected by temperature. Unless stated otherwise, the work here is carried out at a single fixed temperature of 25 °C, but it should be noted that a variation in this temperature of 10 °C, for these mixtures, corresponds to a change in emission wavelength of only approximately 5 nm. Thermal control or compensation can be implemented to reduce this further if necessary.
  • Red, Green, and Blue laser samples Three dye-doped chiral nematic mixtures were prepared. These samples are referred to herein as the Red, Green, and Blue laser samples.
  • the Red sample 5 wt % of the high twisting power chiral additive BDH1305 (Merck KGaA) was dispersed into the commercially available nematogen mixture BL093 (Merck KGaA) to generate a photonic band gap that had a long wavelength edge at 610 nm at 25 °C. To this mixture was added about 1 wt % of the laser dye DCM (Lambda Physik).
  • the Green laser sample consisted of a higher concentration of BDH1305 (about 6 wt %) in BL093 which resulted in a band gap with a long-wavelength edge at 530 nm at 25 °C.
  • BDH1305 about 6 wt %
  • the Blue laser sample comprised of 7 wt % BDH1305 and 1 .7 wt % G504 (Exciton) in BL093.
  • the resulting mixture created a N * LC with a long-wavelength band-edge at about 470 nm at 25 °C.
  • the LC cells were constructed from two pieces of 1 ,1 mm thick glass, coated with antiparaliel rubbed poiyimide alignment layers. Substrates were spaced apart by 10 pm beads and glued together to form an empty cell. This thickness was chosen because it corresponds to the region of maximum slope efficiency. Once ceils were filled, optical microscopy was used to confirm that the LC texture was uniform and the solubility of the dye in the LG host was good.
  • the pump source was a Q-switched solid-state laser (in this case a Nd:YAG laser).
  • Liquid crystals have several properties which are used in the manipulation of light, the most common application of which is the variable transmission of light through a liquid crystal cell depending on its alignment between cross polarisers (for example, in LCD displays). Another less commonly used property of such a system is the refractive index of the LC, which varies with the difference in direction of the incident light and LC alignment.
  • the optical path length at different positions across the material can be altered, if this is done with a suitable profile across an aperture, to give a radially varying optical path length, a lensing effect can be achieved.
  • the change in optical path length across different regions of a LC lens depends solely on the voltage applied to the cell. This provides the ability to alter the focal length quickly of the LC lens as quickly as the LC molecules can align to the applied electric field.
  • Nematic LCs have rod shaped molecules (mesogens) which have no positional order, but some degree of directional order/orientation. That is, their positions are not set and they are free to flow, but due to the shape or charge distribution of the mesogens, it is energetically favourable for one molecule to align with some particular orientation to its neighbours. Due to the anisotropy of the mesogens, the free energy of the liquid crystal depends on the orientation of each given mesogen with respect to their neighbours.
  • Alignment reduces the free energy, leading to a tendency for all mesogens in the ceil to align.
  • the mesogens Upon a voltage being applied to a LC, the mesogens are rotated and aligned to the electric field due to intrinsic or induced dipoies.
  • LCs exhibit birefringence, due to the ordered orientation of the elongated mesogens, leading the LCs to behave as an anisotropic dielectric.
  • the uniaxial direction is referred to as the 'slow' axis, with polarised light travelling at 45 ° to this direction experiencing a retardation ⁇ , hence the birefringent properties.
  • an electrode is provided around the aperture defining the ienslet with a layer of material of set resistance over the aperture itself.
  • the opposing side of the LC cell has a transparent grounded electrode.
  • the set resistance depends on the size of the aperture.
  • the material over the aperture should have a resistance of about 100 ⁇ /sq.
  • the focal length of the lenslet depends on the dimensions of the ienslet and the change in refractive index (hence alteration in optical path length) between the edge and centre of the lens, in a similar manner to an optic lens. Assuming a maximal difference in the refractive index between the centre and edge of the lens, we can conclude that the
  • r is the radius of the aperture
  • d is the thickness of the liquid crystal layer
  • ⁇ is the change between the refractive index along and perpendicular to the slow axis of the liquid crystals.
  • Fig. 3 shows a schematic exploded view of a lenslet structure suitable for use with the embodiment of Fig. 2.
  • a pattern of apertures 130 is generated in an electrode layer 132 formed as a layer of aluminium on a glass substrate 134.
  • a resistive layer 136 of ZTO is formed over the apertures.
  • the device is fabricated as two separate pieces, one with the glass substrate 134, the aluminium pattern 132 and the resistive layer 136.
  • the other piece has another glass substrate 138 with an ITO layer 140 (grounded electrode) having an indium contact 148.
  • the two parts are joined together using spacers 142 formed as thin strips of glue containing set size spacer beads.
  • Alignment layers 144, 146 are formed in the structure as shown in Fig. 3.
  • the LC 150 can then be injected between the two sides.
  • the aluminium pattered layer is created using negative resist photolithography, using a lift-off process to create a pattern in aluminium on the surface of the glass.
  • the resist, AZ5214 is spin coated onto the glass sample (15x20mm), prebaked and exposed to UV behind a mask. In order to create a negative image, the sample is baked once again (inversion bake) and then undergoes a UV flood exposure. The resist is developed in a solution of 20% Microchem AZ315 in Dl water. To remove the areas which were not exposed to the UV source, the sampled is then rinsed in Di water and dried using flowing nitrogen. The resist is then coated with a 5Q-8Qnm layer of aluminium using an evaporation coater.
  • the resistive ZTO (zinc tin oxide) layer is deposited under low stress on top of the aluminium using a reactive sputtering process in a Plasma Quest / WordentechSSOO high-target utilisation sputter coater.
  • the alignment layers are then deposited on each side of the LC cell.
  • the type of alignment layer used, and the method for deposition, depends on the type of ceil to be created. Two type of alignment can be considered: planar alignment (PA) and hybrid aligned nematics (HAN).
  • both the alignment layers are imprinted with microgrooves across their surface.
  • the mesogens in contact with the surface then align themselves with the microgroove direction.
  • the rest of the LC will then also align, in order to reduce the free energy of the system.
  • a layer of 20% A 4276 polyimide in cyciopentane is spun onto the samples ZTO or ITO layer and baked to harden. This layer is then gently brushed to form parallel microgrooves.
  • the two parts of the cell are then glued together anti-parallel with respect to their brushing directions.
  • HANs cells feature one surface similar to planar alignment, and another which induces an orientation normal to the surface.
  • HANs ceils have one AM4276 side, created as described above. The other side is heated, and then spun coated with ZLI-3334. This is a polymer with hydrophilic ends, so must be kept dry throughout its use, and kept in a desiccation box. The sample is baked, and dipped in hot water briefly to cause alignment. It is then be dried and glued to the A 4276 side.
  • UV light curing glue is deposited with either spacer beads or set-thickness plastic sheeting along the edges of the samples, and the layers placed together.
  • the device is then vacuum-bagged to press and hold the two sides together, and exposed to UV until the glue sets.
  • the gap between the spacers is then filled with the LC (BL 006) using capillary action.
  • a typical LC thickness is 20 ⁇ .
  • HAN cells are found to have fewer disclinations in the LC layer than the PA cells when subjected to electric fields to provide similar focal power.
  • Fig, 4 shows a schematic view of laser system 200 according to an embodiment of the invention which provide spatial control of the intensity of the pumping radiation at the lasing medium by adaptive diffraction control.
  • LC laser cell 202 is similar to that shown in Fig. 1 except here the LC laser cell is intended to produce only a single output wavelength. This is of interest, for example, in the generation of a continuous (or quasi-continuous) wave output.
  • Pumping radiation spots 206 are caused to move around the lasing medium (in plan view). In the drawing, the spots are caused to move with a rapid oscillatory motion. However, more randomised movement is also possible, provided that the result is the sequential illumination of fresh lasing medium in order to avoid illuminating a part of the lasing medium with a significant concentration of triplet states. This also assists in avoiding dye fatigue and/or bleaching.
  • An input coliimated pump laser beam 208 is directed into a polarising beamsplitter 210 and onto a computer controlled reflective liquid crystal spatial light modulator (SLM) 212 generating a dynamic holographic array.
  • SLM computer controlled reflective liquid crystal spatial light modulator
  • the pumping radiation from the SLM is then directed back through the beamsplitter 210 and through a focussing lens 214 and a quarter waveplate 216.
  • Suitable control of the SLM provides a sequence of holograms which have the effect to the observer of steering and/or switching on and off focussed pump spots in the lasing medium.
  • the resultant laser output from the lasing medium is collected at condensing lens 218 and recombined to form a single continuous (or quasi-continuous) wave output at 220 for fibre-coupling for onward transmission.
  • Fig. 5 shows an embodiment of the invention which combines features of the
  • Fig. 5 the array of spots of pumping radiation are formed at a segmented LC laser ceil as used in Fig. 2, having a blue segment, a green segment and red segment. Suitable control of the SLM controls the size, number and intensity of the pump spots at each segment of the laser cell This allows the relative proportions of RGB in the output to be controlled to give a required colour output.
  • the segmented LC laser cell can be replaced with a gradient pitch cell as used in Fig. 1 in order to provide full wavelength tuning, if desired.
  • Suitable control of the SLM causes time-varying interference of the pumping radiation at the laser cell.
  • the pumping radiation arrives at the SLM the pumping radiation is diffracted, this diffraction results in interference at the laser ceil of pumping radiation from at least two pixels of the SLM.
  • diffraction at the SLM may preferably result in interference of pumping radiation from large numbers of pixels of the SLM.
  • Preferred embodiments of the invention use a commercially available SLM.
  • phase modulation SLM devices are known to the skilled person.
  • Devices from Holoeye may also be used (ht ⁇ ://www.hoioeye.com/ ' sp8ti8i light modulators- technoioov.htmn (accessed 20 July 201 1 ), such as the LC 2002 SLM, the LC-R 720 SLM, the LC-R 1080 SLM or the PLUTO SLM.
  • Devices from Forth Dimension Displays may be used (ht ⁇ ):/7www.forthdd.com/products) (accessed 20 July 201 1 ).
  • some embodiments use a flexoeiectro-optic effect SLM, described in more detail below.
  • LCOS Liquid crystal over silicon
  • Jepsen (2007) [M. L. Jepsen, "A technology roilercoaster Liquid crystal over silicon,” Nature Photon. 1 , 276 - 277 (2007)].
  • the advanced grating chip in Jepsen (2007) can deliver multi-level phase modulation based on planar aligned nematie liquid crystals (LCs).
  • Ferroelectric LCOS devices can deliver frame rates in excess of 10 kHz, but are typically limited to binary phase modulation due to the two stable states that are available through surface stabilization.
  • the flexoelectro-optic effect in chiral nematic LCs, when in the uniform lying helix (ULH) geometry, is a fast switching, in-plane deflection of the optic axis that is linear with an externally applied electric field.
  • the flexoelectro-optic effect is characterized as explained in WO 2010/1 19252, which demonstrates flexoelectro-optic switching on a silicon device and verifies that a uniform lying helix texture can be obtained using a similar approach to that adopted for conventional glass ceil structures. The switching characteristics are found to be fast, and muiti-ieve! phase modulation is achieved.
  • a suitable SLM is formed as an LCoS device using a standard silicon very large scale integration (VLSI) process to create a silicon backplane including an alignment layer, aluminum pixels and an addressing circuitry for the bottom substrate.
  • VLSI very large scale integration
  • On the top of the LCoS device is a glass substrate coated with a patterned electrode layer of indium tin oxide (ITO) on the inner side of the glass.
  • ITO indium tin oxide
  • a low pre-tilt po!yimide alignment layer (AM4276) is rubbed along the long edge of the aluminum pixels on both substrates. The cell gap of the empty cell is created by using 5 pm spacer bails doped in ultraviolet cured glue seal.
  • Fig. 6 shows a further embodiment of the invention.
  • input col!imated pump laser beam 308 is directed into a polarising beamsplitter 310 and onto a ga!vo-mirror 312 (or DLP mirror array).
  • the pumping radiation from the gaivo-mirror 312 is then directed back through the beamsplitter 310 and through a focussing lens 314 and a quarter waveplate 316.
  • the lasing medium is provided in the form of a segmented RGB laser cell 330.
  • the segmented LC laser ceil can be replaced with a gradient pitch ceil as used in Fig. 1 in order to provide full wavelength tuning, if desired.
  • Suitable control of the ga!vo-mirror 312 provides rapid movement of focus spots on the iasing medium.
  • the resultant laser output from the lasing medium is collected at condensing lens 318 and recombined to form a single continuous (or quasi-continuous) wave output at 320 for fibre-coupling for onward transmission.
  • Fig. 7 shows an embodiment of the invention similar to that of Fig. 6. Similar reference numbers are used for similar features, which are not described again here.
  • a difference between Fig. 6 and Fig. 7 is that in Fig. 7, the LC laser cell 350 has a uniform pitch and therefore provides a monochromatic output, as in Fig. 4.
  • Fig. 10 shows a further embodiment of the invention
  • collimated pump laser beam 308 (pumping radiation) is directed through a lens 31 1 onto a spinning mirror 3 5.
  • the lens focuses the pumping radiation onto the LC laser cell 350 with uniform pitch.
  • the spinning mirror 315 rotates about an off-normal axis, as shown by the arrow 319. As the spinning mirror rotates the single spot of pumping radiation incident on the iasing medium is caused to move around. Therefore in this embodiment the location in the iasing medium that is subjected to pumping radiation at or above the Iasing threshold intensity is rapidly and continually changed in order to reduce the effect of fatigue of the laser medium described above.
  • Fig. 1 1 illustrates a similar embodiment to that shown in Fig. 10.
  • a spinning focusing mirror (in this case a concave mirror) 317 is provided to focus the pumping radiation 308 at the laser cell 350 and to move the spot of pumping radiation around the lasing medium as described above.
  • the laser cell may contain a lasing medium provided in the form of a segmented RGB laser or a varying pitch laser.
  • the spinning mirror may be configured to move the spot of pumping radiation around the lasing medium in order to select the colour of the output radiation 321 .
  • the preferred embodiment of the invention provide a laser system comprising a pump source, a lasing medium and a spatial intensity control means.
  • the pump source is operable to generate pumping radiation directed to the iasing medium via the spatial intensity control means.
  • the position of the lasing medium is fixed with respect to the position of the pump source.
  • the iasing medium has a spatial extent to allow the pumping radiation to excite lasing at different locations in the lasing medium over time.
  • the spatial intensity profile of the pumping radiation incident at the iasing medium is varied over time using the spatial intensity control means.
  • the spatial intensity is controlled by time-varying reflection, refraction and/or interference of the pumping radiation.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

La présente invention a trait à un système laser qui comprend une source de pompe, un milieu d'émission laser et un moyen de commande d'intensité spatiale. La source de pompe a pour fonction de générer un rayonnement de pompage dirigé vers le milieu d'émission laser par l'intermédiaire du moyen de commande d'intensité spatiale. La position du milieu d'émission laser est fixée par rapport à la position de la source de pompe. Le milieu d'émission laser est doté d'une étendue spatiale de manière à permettre au rayonnement de pompage d'exciter l'émission laser à différents emplacements dans le milieu d'émission laser dans le temps. Le profil d'intensité spatiale du rayonnement de pompage incident sur le milieu d'émission laser varie dans le temps à l'aide du moyen de commande d'intensité spatiale. L'intensité spatiale est contrôlée par réflexion variant dans le temps, réfraction et/ou interférence du rayonnement de pompage.
PCT/GB2012/051869 2011-08-02 2012-08-02 Système laser et procédé permettant de faire fonctionner le système laser WO2013017881A1 (fr)

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GBGB1113324.6A GB201113324D0 (en) 2011-08-02 2011-08-02 Laser system and method for operating laser system
GB1113324.6 2011-08-02

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WO2013017881A1 true WO2013017881A1 (fr) 2013-02-07

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EP3734777A1 (fr) * 2019-04-29 2020-11-04 Hitachi High-Tech Analytical Science Finland Oy Agencement de laser
CN113296278A (zh) * 2020-02-24 2021-08-24 宁波激智科技股份有限公司 一种准直膜、一种减干涉准直膜、一种贴合型准直膜、一种图像识别模组及其制备方法

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017060793A1 (fr) * 2015-10-06 2017-04-13 Csir Appareil laser ayant une source d'excitation qui comprend un réseau d'émetteurs de lumière pouvant être commandés, et procédé associé
EP3734777A1 (fr) * 2019-04-29 2020-11-04 Hitachi High-Tech Analytical Science Finland Oy Agencement de laser
US11513342B2 (en) 2019-04-29 2022-11-29 Hitachi High-Tech Analytical Science Finland Oy Laser arrangement
CN113296278A (zh) * 2020-02-24 2021-08-24 宁波激智科技股份有限公司 一种准直膜、一种减干涉准直膜、一种贴合型准直膜、一种图像识别模组及其制备方法
CN113296278B (zh) * 2020-02-24 2023-04-18 宁波激智科技股份有限公司 减干涉准直膜、贴合型准直膜、图像识别模组及制备方法

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