Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/35—Non-linear optics
  • G02F1/355—Non-linear optics characterised by the materials used
  • G02F1/3558—Poled materials, e.g. with periodic poling; Fabrication of domain inverted structures, e.g. for quasi-phase-matching [QPM]
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/14—Phosphates
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure

Definitions

• the present invention relates to a method of preparing flux-grown crystals prior to achieving poling, and an arrangement for monitoring periodic poling of ferroelectric crystals by using the electro-optical effect of the crystal, and to the use of poled crystals as hereinafter described.
• the invention relates in particular to applications of periodically poled crystals partly to generate light or other electromagnetic radiation at new wavelengths by non-linear optical frequency-mixing (frequency doubling , frequency differ ⁇ ence generation, frequency summation generation, optical parametric oscillation, and so on), partly for electro-optical applications such as light beam modulation, and partly for acoustic applications such as the generation of acoustic waves from electric voltages applied across the crystal.
• Periodically poled crystals of non-linear optical, electro-optical and acousto-optical crystals such as potassium titanyl phosphate (KTiOPO4) designated as KTP and other crystals within the same crystal family, such as the isomorphs KTA known as potassium titanyl arsenide, RTA known as rubidium titanyl arsenide, CTA known as cesium titanyl arsenide, RTP known as rubidium titanyl phosphate, and others are preferably grown by the so-called flux method.
• Periodically poled crystals have been used mainly for non-linear optical frequency mixing, and in particular to generate light at new wavelengths on the basis of light at available laser wavelengths. There is at present a desire for novel radiation sources based on this principle.
• periodically poled crystals can also be used in electro-optical applica ⁇ tions and acoustic applications.
• One example of an acoustic optical application is that a Bragg lattice that has a periodically varying diffraction index will occur in a periodically poled crystal in response to a voltage applied across the crystal through the medium of homogenous electrodes. This application can be utilized to modulate or deflect a laser beam, for instance.
• the poled crystals can also be used as so-called acoustic transducers, i.e. used to generate or to detect an acoustic wave with the aid of the piezoelectric effect.
• acoustic transducers i.e. used to generate or to detect an acoustic wave with the aid of the piezoelectric effect.
• the quasi phase matching, periodically poled crystals relevant in the present context are based on the principle of producing in an existing crystal periodically ordered regions of alternating crystal orientation and therewith periodically varying non-linear optical, electro- optical and acousto-optical properties. This is preferably achieved by applying an electric voltage across the crystal with the aid of a periodic electrode structure, and is thus effected without disintegrating the crystal mechanically.
• the published European Patent Application 0 687 941 A2 teaches the formation of inverted or pole reversed ferroelectric domain regions, by applying a ramp voltage across a substrate or crystal, among other things, wherein it has been observed that it is possible to predict an increase in the flow of current through the substrate. Poling can then be considered to have taken place and the voltage is switched-off when the anticipated or predicted current flow has been detected.
• the object of the present invention is to enable periodically poled crystals to be produced from non-periodically poled flux-grown crystals, so-called single domain crystals.
• Another object of the invention is to provide a method of monitoring such fabrication.
• Yet another object of the invention is to point out applications or usages of periodically poled crystals for generating light (electromagnetic radiation) at new wavelengths by non ⁇ linear optical frequency mixing (frequency doubling, difference frequency generation, summation frequency generation, optical parametric oscillation, and so on).
• the present invention is also used for electro-optical applications, such as light beam modulation, and for acoustic applications, such as the generation of acoustic waves from electric voltages applied across the crystal.
• the present invention provides a method for the periodic domain inversion of ferroelectric flux-grown crystals of the kind KTP, KTA, RTP, RTA or CTA, wherein the flux-grown crystals either have a low conductivity inherently or are or made lowly conductive prior to effecting domain inversion with domain inversion means.
• the crystals are doped, wherewith the dopant or dopants lowers/lower the conductivity of the crystal.
• the dopant or dopants may be one or more of the dopants Ga, Sc, Cr and Rb.
• domain inversion is effected with the aid of an ion exchange process, wherein a layer that has low ionic conductivity is generated on the surfaces of the crystal, and wherein the major part of an applied voltage, via a voltage divider, lies across said layer and creates one or more domains that grow through the entire crystal under the influence of the electric field.
• the electrodes will preferably be liquid electrodes or metal electrodes, wherein the insulation between the so-called fingers of the electrodes may be air, some other gas, a liquid, glass, a polymer or a vacuum.
• the ion exchange is carried out solely on one side of the crystal.
• the ion exchange is effected periodically so as to obtain a crystal having spatially varying conductivity, wherein domain inversion is effected solely in the generated region of low conductivity .
• the ion exchange may be effected across the entire surface of the crystal, so as to lower conductivity, and a second ion exchange is effected so as to restore a periodically high conductivity, wherein domain inversion is effected by applying electric pulses that have a voltage such that solely the domains of the regions of low conductivity are reversed.
• the ion exchange is preferably effected with one or more of the ions Rb, Ba, Na, Cs, Tl, H, Li, Ca or Sr.
• the ion exchange is effected with the aid of nitrate salts, for instance.
• electro-optic methods are used for actively monitoring domain reversal.
• the objects and aims of the present invention are also achieved with the aid of an arrangement for monitoring the periodic domain inversion of ferroelectric crystals while using the electro-optic effect of the crystals, wherein the arrangement includes light generating means and the light generated is caused to propagate through the crystal at right angles to the z-axis thereof.
• the arrangement also includes means for applying an electric field across two poles of the crystal in the z-direction, wherein one pole has a periodic electrode structure that enables phase shifting between different polarization directions of light that propagates at right angles to said z-axis.
• phase shift between the z-component of the light and any one of its remaining components changes the polarization state of the light by virtue of the electro-optic coefficients of the crystal and the applied electric field, wherein domain inversion is achieved and determined by observing changes that are caused by the voltage pulse generating means via the electric field and the resultant phase shift in an optical manner via means for receiving the phase-shifted light.
• the light is preferably laser generated.
• the means used to receive phase-shifted light is a photodiode that measures the intensity of the light received, whereafter the diode output signal is analyzed by a measuring instrument and compared with the applied voltage pulse, wherein domain inversion is determined by comparing the shape of the intensity curve from pulse to pulse with the use of short voltage pulses, and wherein changes in die voltage pulses determine the optimal time at which domain inversion shall be stopped.
• the crystal that phase-shifts the light and the means for receiving phase-shifted light are included in an interferometer, wherein domain inversion is stopped when the interference rings generated by the interferometer change direction.
• the present invention also includes the use of aforedescribed prepared domain reversed ferroelectric flux-grown crystals of the type KTP, KTA, RTP, RTA or CTA for the frequency conversion of coherent light, other appropriate electromagnetic radiation, electro-optical modulation or for acoustic applications, wherein the flux-grown crystals either have an inherent low conductivity or are made low conductive prior to effecting domain inversion with domain inversion means.
• Fig. 1 illustrates diagrammatically how the conductive contribution and poling contribution can be separated for a very low conductive crystal, by measuring the current that enters a crystal when a voltage pulse is applied;
• Fig. 2 illustrates schematically an arrangement of apparatus for monitoring poling of flux- grown crystals in accordance with one embodiment of the present invention
• Fig. 3 is a schematic illustration that shows how a Mach-Zehnder interferometer is used to monitor the poling process in accordance with another embodiment of the invention.
• the present invention relates to a method and to an arrangement for periodically poling KTiOPO 4 crystals and other crystals from the same family grown by the flux method among other things, and to its field of use.
• KTP KTiOPO 4
• M ⁇ K, Rb, Tl or Cs ⁇
• M ⁇ K, Rb, Tl or Cs ⁇
• M ⁇ K, Rb, Tl or Cs ⁇
• M ⁇ K, Rb, Tl or Cs ⁇
• Each of these crystals has a slightly different property from the others, such as optical non-linearity, optical transmission range, conductivity, etc.
• QPM quasi phase matching technology
• Quasi phase matching also gives higher efficiencies than type 2 phase-matching, by virtue of the possibility of using the largest non-linear coefficient, d 33 , and also permitting alternation between all wavelengths within the transparent spectrum of the crystal by suitable selection of the period in non-linearity modulation.
• the majority of work on quasi phase matching has been carried out on waveguide structures where standard periodic domain inversion has been achieved with the aid of different diffusion methods combined with heat-treatment processes. These methods are well-suited for waveguides where domain inversion is in outer layers in which the waveguide is also located. Waveguides are of particular interest in low power applications using diode lasers with which the light is concentrated to a small cross-sectional area of high power density.
• waveguides are not as suitable for high power applications, since the waveguide may be damaged thereby, due to the high power density among other things. In this latter case, frequency conversion in bulk crystals is normally used instead. This enables the light to be focused without reaching the damaging threshold and the high power can provide a high efficiency.
• Quasi phase matching in bulk crystals can be achieved in crystals that have been poled (pole inversion, domain inversion) periodically in manufacture, or in crystals that have been poled subsequent to their growth. The latter is preferred, because a periodic pattern of much better quality can be obtained in this way.
• periodic metal electrodes can be used and a photoresist deposited over these electrodes and the area therebetween. This avoids the occurrence of disturbing field lines between the electrodes, such field lines being liable to cause undesirable domain inversion and therewith impair the structure of the poling pattern.
• LiNbO3 is a material well suited for QPM. It has high non-linearity and is produced in large crystals of good uniform quality. It can provide components at a good price and is a reproducible manufacturing process.
• periodic domain inversion in LiNbO3 has some disadvantages in comparison with KTP. Firstly, it has been difficult to scale-up the poling technique in respect of thicker samples. Usable samples having a thickness of 0.5 mm have recently been produced, although KTP has been poled to a thickness of 1 mm already in the first publication.
• the thickness of samples is highly significant in respect of frequency conversion with high power lasers and optical parametric oscillators.
• high power lasers it may be desirable not to focus in the non-linear crystal at all.
• the diameter of the laser beam may exceed 1 mm, a crystal having a thickness of several millimeters is desired so as to avoid edge-effect related problems.
• OPOs optical parametric oscillators
• the voltage can be more than sufficient in some regions and insufficient in others.
• a very high current through the crystal can also result in permanent damage.
• This damage can be both so-called electrochromic damage where the crystal loses in transparency and photoavalanche ionization leading to a break-through between the electrodes, and total destruction of the crystal.
• the dopant was added in the flux-smelt and therewith distributed relatively evenly in the crystal.
• One method of lowering the conductivity of a flux-grown KTP is to effect an ion exchange in the crystal. This method is the same as that used in the fabrication of waveguides. Conductivity can be lowered by one power of ten or more with an Rb exchange in KTP, which is sufficient to obtain a crystal that can be easily poled periodically.
• the method is based on the creation of a layer of lower ionic conductivity at the crystal surfaces, by diffusion of another substance.
• One example of such a method is to bathe the
• a voltage corresponding to the coercive field can be achieved in the ion-exchanged regions at a given applied voltage, so as to obtain nucleation of an inverted domain, this inverted domain is then able to grow through the remainder of the crystal, under the influence of the residual electric field.
• a c-cut disc of flux-grown KTP crystal that cannot be poled with known techniques was subjected to an ion exchange process in RbNO 3 at 350°C for six hours.
• the crystal was not patterned and the ion exchange took place on both sides of the disc.
• Conductivity measured transversely of the disc in the direction of the c-axis, was found to decrease. The extent of this decrease differed in accordance with the quality of the crystal, but typically by one or more powers of ten.
• the piezoelectric signal increases with the ion exchange, said signal being used most often to probe the domain patterns of the crystals. Poling was effected by applying a periodic electrode pattern on one of the two c-surfaces (c+ or c-).
• the electrode may be a metallic electrode or a liquid electrode and the insulation a photoresist.
• the crystal was poled by applying electric voltage pulses, normally of 60 ms duration. An electro-optic measuring method was used to determine when poling took place. Pulses having successively increasing voltages were used, and when poling commenced the same or a slightly higher voltage was used for the following pulses until poling was complete.
• on-line frequency doubling may also be used as a monitoring method, e.g. in respect of crystals patterned for frequency doubling of Nd:YAG lasers.
• a periodic metal mask coated on c+ side or c- side was used for a periodic ion exchange.
• the mask may be comprised of titanium for instance, and produced by photolithography with lift-off.
• RbNO3 in a concentration of 100% was used for the ion exchange, to lower the conductivity in the ion-exchanged region and to obtain a potable crystal.
• the rear side was blocked with a fully covering electrode during the ion exchange.
• the metal mask was removed after the ion exchange. This can be achieved with a short HF etching, in an EDTA etching or some other metal etching.
• the crystal is poled with a fully covering electrode, metal or liquid, i.e. no periodic electrode is used.
• the periodically ion- exchanged region can now be domain inverted while the remaining regions remain uninverted.
• the optical results were equivalent to the above results.
• a flux-grown KTP crystal was subjected to ion exchange in accordance with the first two experiments above.
• the crystal was then patterned periodically with a metal mask, e.g. a titanium mask, on one side. The other side was blocked with a fully covering metal mask.
• the crystal was then subjected to an ion exchange in a mixture of 5 % Ba(NO3)2 and 95% RbNO3 over a period of one hour and at a temperature of 350°C.
• the masks were then removed and the crystal poled with a fully covering electrode. Pole inversion then takes place solely in those regions not subjected to an ion exchange in the second bath with Ba(NO3)2.
• the Ba2+ ion that migrates into the crystal in the ion exchange is divalent and followed by a vacancy, at the same time as two monovalent K- ions diffuse out of the smelt.
• the vacancies in this region result in higher ion conductivity and thus prevent pole inversion from taking place.
• the first ion exchange causes the conductivity to decrease across the whole of the crystal, thereby enabling the crystal to be poled.
• the second ion exchange causes conductivity to increase locally, whereby poling cannot take place in these regions.
• the ion exchange results in a composition gradient in the crystal surface. This composition gradient is also accompanied by strong internal mechanical stress.
• the diffusion profile can be removed and the stress in the crystal reduced by heat-treating the crystal subsequent to the periodic poling. Removal of the diffusion profile and stress reduction can be effected over a period of time ranging from some minutes to several hours at a temperature of from 300 to 500 °C or higher.
• poling can be monitored actively so that the periodic structure will become optimal by virtue of following the current flowing through the crystal in the poling process.
• the current has two contributions.
• the first II of these contributions corresponds to the impedence constituted by the crystal in the outer circuit from the voltage unit.
• the second contribution 12 corresponds to the charge transfer that takes place during poling of the crystal.
• Typical examples are II in the uA range for hydrothermally grown ITP and in the mA range for flux-grown ITP. 12 is normally uA.
• KTP crystal by measuring the current that enters the crystal when a voltage pulse is applied, whereas, in principle, the conductive current conceals the poling current for a high conductive crystal, such as ion-exchanged flux-grown KTP, for instance.
• the transversal electro-optical effect meaning that an electrical field applied across the crystal in the z-direction 20 initiates a phase shift between different polarization directions of light that propagates through the crystal at right angles to the z-axis.
• the beam propagates from an He-Ne laser 36, which is linearly polarized at 45° 22 between the y and z axes, along the x-axis of the crystal; see Fig. 2.
• the phase shift between the y and z components of the light changes the polarization state of the light. For instance, in the case of a successively increasing phase shift from 0-radians to 7r-radians, the polarization state changes from a linearly polarized state, via circular polarization and different types of elliptical polarization, to a linearly polarized state at right angles to the original state.
• Voltage pulses 26 are applied across the crystal during poling, wherein a periodic electrode 28 is defined photolithographically on one side of the crystal.
• the poling process is monitored by studying the polarization state of the light that has passed through the crystal while the voltage pulse is applied. This is effected by measuring with the aid of a photodiode 30, the intensity 32 of the light that passes through an analyzer 24 positioned at -45° between the y and the z axes downstream of the crystal.
• the output signal of the photodiode 30 is taken into an oscilloscope 34 and can be studied simultaneously with the voltage pulse 26 generated by the voltage unit 10.
• the intensity 32 (at a linearly rising voltage) will follow a sinusoidal curve (not shown) until its maximum voltage is reached, wherein the intensity remains at a constant level and then follows the same curve back as the voltage decreases.
• the intensity curve 32 will change as the crystal 14 is poled, since domain inversion of one region will cause the electro-optical coefficient in this region to change sign.
• the domain inverted area equal to the non- domain inverted area (50% duty cycle)
• the continued change of the total electro-optical coefficient that the light experiences during its propagation through the crystal will be much smaller, since any domain propagation is a much slower process than the actual poling process.
• the intensity curve will be almost a straight line, indicating that it is time to terminate the poling process (marked by the black and the white domains in the crystal 14). Because the crystal is normally also double-refractive, which also influences the polarization state, it is difficult to forecast where on the sinusoidal curve the intensity is located prior to applying the voltage. This is not important, however, since it is always endeavoured to obtain a straight line, wherein the intensity curve can be readily followed backwards during the course of the poling process.
• the beam 38 from the He-Ne laser 36 is divided for entering into a Mach-Zehnder interferometer 40.
• the beam 38 is permitted to pass through the crystal in one of the arms 42 and an electric field is applied across the crystal in the z direction 20.
• the interference rings 44 at the interferometer output will move either inwardly or outwardly as the voltage across the sample 14 changes. Similar to the previous embodiment, the poling process will change the total electro-optical effect across the crystal 14. When movement of the interference ring 44 begins to change direction, this can be taken as an indication that it is time to terminate the poling process.
• the voltage generating electrodes may be periodic so- called finger electrodes, wherein the insulation between the electrode fingers may be air, some other gas, a liquid, glass, a polymer or a vacuum.

Landscapes

• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Nonlinear Science (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Optics & Photonics (AREA)
• General Physics & Mathematics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Polarising Elements (AREA)

Priority Applications (6)

Applications Claiming Priority (4)

Related Child Applications (1)

Publications (1)

Family

ID=26662476

Family Applications (1)

Country Status (9)

Cited By (2)

Families Citing this family (20)

Citations (3)

Family Cites Families (8)

• 1997
  • 1997-01-10 EP EP97900836A patent/EP1015936B1/en not_active Expired - Lifetime
  • 1997-01-10 AT AT97900836T patent/ATE327523T1/de not_active IP Right Cessation
  • 1997-01-10 JP JP9525149A patent/JP2000503138A/ja not_active Ceased
  • 1997-01-10 DE DE69735956T patent/DE69735956T2/de not_active Expired - Lifetime
  • 1997-01-10 CN CN97192916A patent/CN1213436A/zh active Pending
  • 1997-01-10 WO PCT/SE1997/000026 patent/WO1997025648A1/en not_active Ceased
  • 1997-01-10 IL IL12530397A patent/IL125303A/en not_active IP Right Cessation
  • 1997-01-10 AU AU13257/97A patent/AU733846B2/en not_active Ceased
• 1998
  • 1998-07-10 US US09/113,835 patent/US5986798A/en not_active Expired - Lifetime

Patent Citations (3)

Also Published As

Similar Documents

Legal Events