Patent Application
20050169590
The present invention relates to optical fibre wave guides with tuneable optical properties, and use thereof in various applications, such as optical switching, dispersion compensation, wavelength conversion, polarization control, polarization delay lines (birefringent elements) sensors, attenuators, filters, amplifiers, and lasers; in particular the invention relates to optical fibres and devices that have optical properties that are tuneable using optical, thermal and/or electrical control.
Optical fibres with tuneable optical properties are desired within a vast number of technical areas, ranging from optical communications, sensor technology, spectroscopy, imaging, lithography, medicine, material processing, micro-machining, and many others. Often it is desired to control light propagation through the optical fibre on a short time scale and preferably over a range of optical wavelengths. This applies to both continuous wave and pulsed operation.
For optical telecommunications, for example, it is desired to transmit light signals at high bit rates over long distances. This requires short light pulses of high intensity and optical transmission fibres wherein little or no pulse distortion occurs. The most common causes of pulse distortion are dispersion and/or non-linear effects. Therefore, as optical transmission systems are being pushed towards higher bit rates, there is a need for developing optical fibres with special waveguiding properties, for example large dispersion for reshaping of pulses, and preferably tuneable dispersion properties.
Within recent years a new type of optical fibre has been demonstrated that is capable of guiding light in a core surrounded by micro-structured elements that are elongated in the longitudinal direction of the fibres. These new fibres are referred to by a number of different names including photonic crystal fibres (PCF), micro-structured fibres, holey fibres, photonic bandgap fibres, and hole-assisted fibres. PCFs and aspects related to simulation, modelling, designing, fabricating and their use are extensively described by Bjarklev et al. in “Photonic crystal fibres”, Kluwer Academic Press, 2003 that is incorporated herein by reference. Optical fibres without micro-structured elements shall be referred to as standard optical fibres (such as traditional optical fibres that have been used and developed over several decades).
Although existing PCFs and devices using such optical fibres have a number of advantageous properties, it is desired to provide fibres and devices with improved properties in terms of tuneable optical properties, for example for switching, preferably with high sensitivity and/or practical solutions. This is desired in order to provide fast temporal response and/or low powers of control signals. In particular, it is desired to provide fibres and device that may be tuned and/or switched using optical means.
Litchinitser et al., “Antiresonant reflecting photonic crystal optical waveguides” Opt. Lett. 27, 1592-1594 (200) disclose a photonic bandgap fibre. It is a disadvantage of the disclosed fibres that the optical properties are not tuneable.
Abeeluck et al., “Analysis of spectral characteristics of photonic bandgap waveguides”, Opt. Express 10, 1320-1333 (2002) disclose other photonic bandgap fibres. It is a disadvantage of the disclosed fibres that the optical properties are not tuneable.
Bise et al., “Tuneable photonic band gap fiber” in OSA Trends in Optics and Photonics (TOPS) Vol. 70, Optical Fiber Communication Conference, Technical Digest, Postconference Edition (Optical Society of America, Washington D.C., 2002), pp. 466-468 disclose a tuneable photonic bandgap fibre. It is a disadvantage of the disclosed fibres that the optical properties are not tuneable without the use of relatively high temperature variations. It is a further disadvantage that the optical properties are not optically or electrically tuneable and that no means for optical or electrical tuning are disclosed.
Jasapara et al., “Effect of Mode Cut-Off on Dispersion in Photonic Bandgap Fibers”, Optical Fiber Communication Conference Thl3 (2003) disclose other photonic bandgap fibres. It is a disadvantage of the disclosed fibres that the optical properties are not tuneable without the use of relatively high temperature variations. It is a further disadvantage that the optical properties are not optically or electrically tuneable and that no means for optical or electrical tuning are disclosed.
Westbrook, et al., “Cladding-mode resonances in hybrid polymer-silica micro-structured optical fiber gratings”, IEEE Photonics Technol. Lett. 12, (2000) disclose micro-structured fibres comprising polymers in the cladding. It is a disadvantage of the disclosed fibres that the optical properties are not tuneable without the use of relatively high temperature variations. It is a disadvantage that the disclosed fibres are not optimised for providing photonic bandgab effect. It is a further disadvantage that the optical properties are not optically or electrically tuneable and that no means for optical or electrical tuning are disclosed.
Eggleton et al., “Micro-structured optical fiber devices”, Optics Express 9, 698-713 (2001) disclose micro-structured fibres comprising polymers in the cladding. It is a disadvantage that the disclosed fibres are not optimised for providing photonic bandgap effect. It is a further disadvantage that the optical properties are not optically or electrically tuneable and that no means for optical or electrically tuning are disclosed.
Eggelton et al., EP 1213594 disclose micro-structured fibres with polymer dispersed liquid crystals disposed in the core and/or cladding and/or exterior of the cladding. It is a disadvantage that the fibres are not optimized for providing photonic bandgap effect. It is a further disadvantage that the optical properties are not optically tuneable and that no means for optical tuning are disclosed. It is a disadvantage that the filling material is an emulsion of two materials, which could make manufacturing more difficult.
Larsen et al., “Dynamic waveguiding in photonic crystal fibers”, MC2 conference, Gothenburg, Sweden, February 2003 disclose PCFs comprising liquid crystals. It is a disadvantage that the fibres are not optimized for providing photonic bandgap effect. It is a further disadvantage that the optical properties are not optically tuneable and that no means for optical tuning are disclosed.
Larsen et al., “A Novel Photonic Crystal Fibre Switch”, Conference on Lasers and Electro Optics CLEO/Europe'03, Post Deadline paper (Munich, June 2003) disclose PCFs comprising liquid crystals. It is a disadvantage that no means for optical tuning are disclosed.
Larsen et al., “Tuneable Photonic BandGaps In a Photonic Crystal Fiber Filled With a Cholesteric Liquid Crystal”, 29th European Conference on Optical Communication ECOC'03 (Rimini, Italy, September 2003) disclose PCFs comprising liquid crystals. It is a disadvantage that no means for optical tuning are disclosed.
Larsen et al., “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11, 2589-2596 (2003) disclose PCFs comprising liquid crystals. It is a disadvantage that no means for optical tuning are disclosed.
Larsen et al., “Thermo-optic switching in liquid crystal infiltrated photonic bandgap fibres”, IEEE Electronics Letters Vol. 39, No. 24 (2003) disclose PCFs comprising liquid crystals. It is a disadvantage that no means for optical tuning are disclosed.
Larsen et al., “Low-Voltage Optical Devices based on Liquid Crystal Photonic BandGap Fibres,” Applied Nanoscience, in press (2004) disclose PCFs comprising liquid crystals. It is a disadvantage that no means for optical tuning are disclosed.
It is an object of the present invention to provide an improved micro-structured optical fibre with tuneable or controllable optical properties.
It is an object of the present invention to provide an improved PBG fibre with tuneable or controllable optical properties.
It is a further object of the present invention to provide an improved PBG fibre with optically tuneable or controllable optical properties.
It is a further object of the present invention to provide tuneable optical fibres of short length, and compact tuneable optical devices.
It is a further object of the present invention to provide tuneable optical fibres with low loss, and tuneable optical devices with low insertion loss.
It is a further object of the present invention to provide optically, tuneable optical fibres, and optically tuneable optical devices.
It is a further object of the present invention to provide optically, tuneable optical fibres, and optically tuneable optical devices that may be connected to other optical components. In particular, such fibres and devices that may be spliced to other optical fibres.
It is a further object of the present invention to provide devices comprising optically, tuneable optical fibres and means for optical control.
It is a further object of the present invention to provide thermally, tuneable optical fibres and devices that require relatively low voltage control.
It is a further object of the present invention to provide optical fibre devices with thermal, electrical and/or optical control in the same device, giving flexible tuneable properties.
It is a further object of the present invention to provide optical fibre with fibre Bragg gratings having tuneable properties.
It is a further object of the present invention to provide a tuneable dispersion compensator based on fibre Bragg grating technology.
It is a further object of the present invention to provide Q-switched lasers.
Further objects appear from the description elsewhere.
1. “Liquid Crystal Infiltrated Photonic Bandgap Fibre”
Objects of the invention are fulfilled by an optical fibre having a longitudinal direction and a cross-section perpendicular thereto, the optical fibre comprises:
Wherever the term liquid crystal “phase state” is used, it covers the mesophase of the liquid crystal and the molecular alignment of the liquid crystal in a specific mesophase and in a specific geometry (for example circular geometry). For example, a change in mesophase (for example caused by a change in temperature) will in most cases cause a change in molecular alignment, but a change in molecular alignment (for example caused by an applied optical or electrical field) will not necessarily cause a change in mesophase, but the phase state has changed. The phase state thereby depicts the molecular orientation of a liquid crystal in a specific mesophase, in a specific geometry, under specific surface anchoring and under specific external applied control signals (including none).
The term ‘arranged in a substantially two dimensional periodic manner’ is in the present context taken to mean that, when viewed in a cross section perpendicular to the longitudinal direction of the optical fibre, the pattern represented by the centres of the micro-structured elements is substantially repetitive when translated in each of two different directions (e.g. represented by a unit cell defined by two non-parallel vectors in the cross-sectional plane). The term ‘substantially repetitive’ is taken to mean that the centres are repetitive in the above sense but allowing for processing tolerances introduced during the manufacturing process.
The term ‘arranged in a substantially Bragg-type of manner’ is in the present context taken to mean e.g. an arrangement of cladding elements that surround the core in a substantially concentric, annular manner (e.g. in circular rings). The term ‘substantially concentric’ is to be understood as allowing for processing tolerances introduced during the manufacturing process.
The arrangement of micro-structured cladding elements may additionally comprise any other pattern of cladding elements that are NOT ‘arranged in a substantially two dimensional periodic manner’ or ‘arranged in a substantially Bragg-type of manner’.
The arrangement of micro-structured cladding elements may alternatively comprise any other pattern of cladding elements that are NOT ‘arranged in a substantially two dimensional periodic manner’ or ‘arranged in a substantially Bragg-type of manner’. In an embodiment, the arrangement of micro-structured cladding elements is substantially non-periodic, i.e. the pattern constituted by the centres of the cladding elements does not posses any symmetry or is not arranged in a substantially Bragg type manner.
The term ‘size’ is in the present context taken to be equal to ‘dimension’ or ‘area’, i.e. referring to a one- or two-dimensional extension in a given plane.
The term ‘said micro-structured cladding elements having cross-sectional sizes which are equal or different’ is in the present context taken to mean that in a given cross section, the cross-sectional dimensions (e.g. maximum dimension or area, the latter possibly including form) of the micro-structured elements are either essentially identical or different (different in the meaning that at least one micro-structured element has a dimension (e.g. a radius of a circular hole or a maximum dimension of a non-circular hole) that is different from another micro-structured element). The term ‘different’ is in the present context—when used to compare two physical parameters x1 and x2—taken to mean more different than what may be ascribed to processing tolerances for parameters that are intended to be equal OR that the numerical value of 2(x2−x1)/(x1+x2) is larger than 1%, such as larger than 5%, such as larger than 10%, such as larger than 20%.
It should be noted that the optical fibre as manufactured without LC-material introduced into a section of at least one cladding element may or may not exhibit light guidance by the PBG effect. In an embodiment, the optical fibre as manufactured without LC-material does NOT exhibit light guidance by the PBG effect at the wavelengths of its intended use.
In the present context, the ‘core region’ is defined—when viewed in a cross section perpendicular to a longitudinal direction of the fibre—as a (typically central) light-propagating part of the fibre. The core region is limited in a radial direction by micro-structural elements of the cladding region or by a cladding region having a background material with a refractive index different from the refractive index of the core region (or different from the refractive index of the background material of the core region, if the region comprises micro-structural elements).
Various examples of optical fibres exhibiting a photonic bandgap are given in the prior art section above. Photonic bandgap guidance in photonic crystal fibres (incl. material and structural prerequisites of the core and cladding regions and micro-structural elements) is also discussed in Bjarklev et al. (cf. Chapter 6, pp. 161-215).
If a change of phase state occurs for the liquid crystal, a change in the optical properties of the LCI optical fibre will occur. For example, the LCI optical fibre may provide high transmission of light at λ1 for a first phase state of the liquid crystal and lower (including none) transmission at λ1 for a second phase state. As another example, the LCI optical fibre may provide one dispersion characteristic at λ1 for one phase state and a different dispersion characteristic at λ1 for another phase state.
As another example, the LCI provide a given birefringence at λ1 for a first phase state of the liquid crystal and another birefringence at λ2 for a second phase state of the liquid crystal.
Hence, the PBG effect—and thereby the optical properties—may be tuned/controlled in the LCI optical fibre by changing the phase state of the liquid crystal.
“Micro-structured Cladding Region”
Generally, the cladding elements can be arranged in any suitable structure that provides index guiding or photonic bandgap guiding of the light of at least one wavelength, λ1, in the core region for at least one phase state of the liquid crystal material.
The size and arrangement of cladding elements can vary within broad limits.
In a preferred embodiment, at least some of the cladding elements are holes. The hole size or sizes is (are) advantageously not too small (to facilitate the introduction and/or mobility of the liquid crystal material). On the other hand, the hole size or sizes is (are) preferably not too large either to negatively influence the anchoring of the liquid crystal material. The actual optimum or acceptable hole dimensions depend on the specific liquid crystal material and the surface conditions in the holes (i.e. the material and hole size (form and or radius of curvature of the inner surfaces of the holes).
In a preferred embodiment, said cladding elements are arranged in a substantially two-dimensional periodic structure, whereby the LCI optical fibre may exhibit PBG effects in a liquid crystal filled section.
In another preferred embodiment, said cladding elements are arranged in concentric rings around said core region (Bragg-type of fibre structure), whereby the LCI optical fibre may exhibit PBG effects in a liquid crystal filled section.
Alternatively, the cladding elements may be arranged in any other pattern suitable for providing an optical fibre for guiding light, such as in a non-symmetric or non-concentric manner. Such a fibre may be characterized by index guiding of light. It is to be understood that the features discussed in the section under the heading “Liquid crystal infiltrated photonic bandgap fibre” may be combined with such a fibre.
In a preferred embodiment, substantially all cladding elements have similar ratio d/Λ (d being a maximum cross-sectional dimension of a cladding element, such as a diameter, and Λ being the minimum centre-centre distance between said cladding elements, also interchangeably termed the ‘pitch’), whereby it is obtained that an optical fibre of substantially uniform micro-structured cladding can be obtained. Such uniformity may, for example, be preferred to optimize photonic bandgap effects.
In a preferred embodiment, substantially all cladding elements have substantially equal d/Λ ratio.
Preferably, the maximum cross-sectional dimensions d of the cladding elements are substantially equal for all elements. Alternatively, they may be different, e.g. in such a way that the pattern of cladding elements—including the size and/or forms of the individual cladding elements—still possesses a two-dimensional periodicity, e.g. in that two different hole sizes (dl, d2) are positioned at two different periodic positions of the pattern.
Preferably, Λ is in the range from 1 μm to 20 μm, d in the range from 0.05 μm to 19 μm, and d/Λ is in the range from 0.05 to 0.95.
In a preferred embodiment, the ratio d/Λ of a maximum cross sectional-dimension of a cladding element to the minimum centre-centre distance between said cladding elements is in the range from 0.25 to 0.5.
In a preferred embodiment, a maximum dimension d of said cladding elements is in the range from 0.5 μm to 7 μm, such as from 1 μm to 5 μm, such as from 1 μm to 2 μm or from 3 μm to 5 μm.
Hole sizes in the range from 1 μm to 2 μm and center-to-center-distances of the holes in the range from 3 μm to 5 μm are preferred to provide first order photonic bandgap operation.
To optimize response times for optical tuning of the liquid crystal material, holes should be kept as small as possible, preferably in the range from 1 μm to 2 μm.
To optimize response times for electrical tuning of the liquid crystal material, holes can be relatively small as well as relatively large. However, losses increase for relatively large holes. Hence, an optimum hole size can be found for a specific application and material system. In a preferred embodiment, hole size is in the range from 3 μm to 5 μm.
In a preferred embodiment, the minimum centre-centre distance between said cladding elements is in the range from 1 μm to 20 μm, such as from 2 μm to 10 μm, such as from 3 μm to 5 μm.
“Materials and Additional Structures”
The LCI optical fibre comprises materials that separately are known in the art of optical fibres and liquid crystals.
The mesophases of liquid crystals and issues related to their theory, production, classification and determination of molecular alignment are described extensively in literature including the study of liquid crystal in cylindrical geometries—see e.g. P. G. de Gennes and J. Prost, J. The Physics of liquid crystals, 2nd edition, (Clarendon Press, Oxford 1993); S. Chandrasekhar, Liquid crystals, (Cambridge University Press, 1977); P. Rudquist, M. Buivydas, L. Komitov, and S. T. Lagerwall, “Linear electro-optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy,” J. Appl. Phys. 76, (1994); H.-S. Kitzerow, B. Liu, F. Xu, and P. P. Crooker, “Effect on chirality on liquid crystals in capillary tubes with parallel and perpendicular anchoring,” Phys. Rev. E 54, 568-575, (1996); S. K. Lo, L. M. Galarneau, D. J. Rogers, and S. R. Flom, “Smectic Liquid Crystal Waveguides with cylindrical Geometry,” Mol. Cryst. Liq. Cryst. 201, 137-145 (1991); J. T. Mang, K. Sakamoto, and S. Kumar, “Smectic Layer Orientation in Confined Geometries,” Mol. Cryst. Liq. Cryst. 223, 133-142 (1992); or S. Kralj and S. Zumer, “Smectic-A structures in submicrometer cylindrical cavities,” Phys. Rev. E, 54(2) 1610-1617 (1996), that are incorporated herein by reference.
In a preferred embodiment, said core region, said micro-structured cladding region, or both, comprises silica whereby a well-known optical fibre material for which production techniques exists can be applied.
The optical fibre (PCF) may be filled with liquid crystal material(s) in one or more post-processing steps, where a standard commercially available PCF is purchased (supplies include Crystal Fibre A/S, Birkeroed, Denmark, for example fibres with product names LMA5, LMA8, LMA10, LMA13 and LMA15. For a specific application, a custom designed PCF may be designed and fabricated optimized to the relevant LC-materials, wavelengths, response times, etc.
The optical fibre may, however, comprise different materials for its components.
Accordingly, in a preferred embodiment, said core region, said micro-structured cladding region, or both, comprises silica and/or silica including one or more co-dopant materials, preferably a material selected from the group consisting of Ge, Al, B, or F (or other materials), or a combination thereof.
Generally, the cladding region may comprise any suitable optical fibre material, however, specific properties may be obtained by selecting specific cladding elements.
Accordingly, in a preferred embodiment, said micro-structure cladding elements are selected from the group consisting of liquid crystals.
For certain applications e.g. devices with optically tuneable optical properties, the liquid crystal can be doped with various materials in order to increase a desired optical response and/or properties of the liquid crystal. Doping materials can for example be selected from a range of azobenzene or anthraquinone dyes for example dyes with trade name Disperse red 1, Disperse blue 1, Disperse orange 1, Methyl red etc. from Sigma-Aldrich (St. Louis, Mo. 63103, USA). Other dopants may be used.
In an embodiment, the optical fibre is adapted to allow a dynamic modification of the optical properties of the fibre. Such modification or switching (the latter referring to a modification choosing one set of optical characteristics between two or more predetermined sets of optical characteristics of the fibre) may be induced by various physical effects, e.g. thermal, electrical or optical (or combinations thereof). In particular it is preferred that said cladding elements are filled with liquid crystals that may be switched between mesophases, such as for example between a smectic and a chiral nematic mesophase. Further, it is preferred that the liquid crystal can be made change molecular alignment. Preferably externally controlled means are used for switching, e.g. thermal, electrical and/or optical means.
The term ‘switched between mesophases’ is in the present context taken to mean a change of mesophase, e.g. from Chiral Smectic A to Cholesteric.
In a particular embodiment, the liquid crystal material comprises material with a positive dielectric anisotropy at frequency f1 and a negative dielectric anisotropy at frequency f2 and the means for controlling the phase state of the liquid crystal material are adapted to apply an electric field at frequencies f1 and f2, respectively. Thereby the provision of a polarization controlling component is facilitated.
It is generally preferred that the liquid crystal material is homogeneously distributed in the cladding elements in a cross section as well as over the section or sections of the fibre containing liquid crystal materials. By ‘homogeneously distributed’ is e.g. understood that the orientation of the liquid crystal material molecules is (or may be induced to be) substantially equal over the volume of the liquid crystal filled section(s). To achieve this, the liquid crystal material (and possible additives), the cross-sectional hole sizes and forms, the length of the section(s) filled with liquid crystal material, the adherence properties of the internal surfaces of the holes, etc. are adapted to each other.
For certain applications, it is advantageous to adapt the internal surfaces (e.g. their cross-sectional form, size, and/or adherence properties) of the micro-structured cladding elements to the liquid crystal material filled into them.
In a particular embodiment, the internal surfaces of at least some of the cladding elements, such as holes, at least over a part of their longitudinal extension, are coated with a layer of a material providing a specific anchoring between the liquid crystal material and the cladding background material.
In a particular embodiment, materials used for coating are selected from the group consisting of polyamides, polyimides or lipids such as phosphatidylcholine and combinations thereof.
For certain application e.g. for lasers and amplifiers, the optical fibre comprises one or more active materials for providing lasing action.
Accordingly, in a preferred embodiment, said core region and/or at least a part of said cladding background material comprises an active material, preferably silica doped with a rare earth element, most preferred silica doped with Erbium, Ytterbium, Neodymium, Holmium, Thulium, Samarium or combinations thereof.
In another embodiment, said core region and/or at least a part of said cladding background material comprises co-dopant materials, preferably a material selected from the group consisting Ge, Al, B, or F, (or other materials) or combinations thereof.
In a particular embodiment, the optical fibre further comprises a fibre Bragg grating adapted to reflect light at a wavelength λ. Thereby reflecting elements for selecting particular wavelengths may conveniently be implemented.
In a particular embodiment, the fibre Bragg grating is located in the longitudinal section of the fibre comprising liquid crystal material. Thereby a scheme for varying the optical characteristics of the fibre Bragg grating by tuning the liquid crystal material is provided.
2. “An Article Comprising an LCI Optical Fibre”
In another aspect, according to the present invention, objects of the invention are fulfilled by an article comprising a LCI optical fibre and means for controlling the phase state of the liquid crystal, whereby the optical properties of the article can be controlled/tuned.
In a preferred embodiment, the LCI optical fibre is an optical fibre according to a preferred embodiment of the present invention as described in section 1 “Liquid crystal infiltrated photonic bandgap fibre” above or an optical fibre manufactured by the method described in section 4 “Method of producing a liquid crystal infiltrated optical fibre” below, in the detailed description and figures and in the claims.
The means for controlling the phase state of the liquid crystal material may be based on one or combinations of a multitude of physical effects, including effects induced by thermal, electrical, magnetic, acoustical and optical signals.
In preferred embodiments of the invention, the control means comprise a generator for generating a physical effect (e.g. a light source, such as a laser for generating light or electrodes and connecting current generating devices for generating an electric field) and optionally any devices for coupling or facilitating the access of the signal from the generator into the optical fibre.
In preferred embodiments of the invention, the means for controlling the phase state of the liquid crystal material include means for controlling the temperature of the liquid crystal material. These means may be based on the same signal (e.g. optical) or on different signals (e.g. electrical for controlling the phase state and optical for controlling the temperature).
In a preferred embodiment, the controlling means is or comprises an optical control signal, which will be referred to as pump or pump light, whereby optical tunability is obtained. Alternatively, or additionally, the controlling means may comprise an electrical control signal. The combination of optical and electrical control may provide especially flexible solutions (e.g. in the combination of a controlled heating with the control of the phase state of the liquid crystal material).
In a preferred embodiment, the controlling means is supplied by pump light being coupled to the LCI optical fibre, the pump light having a wavelength, λp, being different than a wavelength, λs, of light being controlled/tuned in the article, whereby the signal and pump lights may be separated. For example, the pump light may be selected at a wavelength of an available laser source.
In a preferred embodiment, the pump light is coupled to the core of the LCI optical fibre, whereby a well-defined coupling can be obtained.
In a preferred embodiment, the LCI optical fibre comprises an unfilled section having cladding elements being voids/holes and a filled section having liquid crystal filled cladding elements, and said pump light being coupled to an end of said unfilled section, whereby coupling can take place at a distance away from the liquid crystal filled section, where coupling could be preferred, for example, if a coupling component or any means for connecting said end of said coupling component does not allow for liquid crystals to be in close proximity.
In a preferred embodiment, said pump light propagates in the core of said unfilled section, whereby an improved optical control or improved coupling of pump light can be obtained. For example, by selecting λp so that light at λp is guided in said unfilled region, but non-guided in said filled regions so that the power from the pump light may leak into the cladding elements comprising liquid crystal material and provide increased sensitivity, lower threshold, and/or faster tuning of the optical properties.
In a preferred embodiment, the controlling means comprise a second optical fibre being spliced to the liquid crystal infiltrated optical fibre, said second optical fibre transmitting said pump light, whereby well-defined or improved coupling can be obtained. The second optical fibre may be a micro-structured fibre or a non-micro-structured fibre.
In a preferred embodiment, the controlling means further comprises a wavelength combining optical device, such as for example a fibre coupler (also referred to as fibre WDM in literature), that combines light at λp and λs, such that relatively easy access to control and signal lights are obtained through the separate fibre ends of the fibre WDM.
The core region may comprise any suitable material including a single material, a single material doped with dopants, a mixture of materials, a mixture of materials doped with dopants, and micro-structured materials comprising liquid crystal materials. The various preferred embodiments of the present invention relating to the cladding elements may to a large extend be transferred to apply for core elements.
In an embodiment, the optical fibre comprises a first fibre Bragg grating located in the longitudinal section of the fibre comprising liquid crystal material and wherein the characteristics of said first fibre Bragg grating may be varied using the means for controlling the phase state of the liquid crystal material.
In a particular embodiment, the variation in characteristics of the first fibre Bragg grating is used to implement a tuneable dispersion compensation module.
In a particular embodiment, the optical fibre comprises a further fibre Bragg grating adapted to reflect a wavelength λ1 and wherein the characteristics of said first fibre Bragg grating are adapted to reflect a wavelength λ1 for a specific time interval Δt. Thereby a Q-switched laser can be made, where the cavity formed from the two FBGs when they are reflecting at wavelength, λ1, is only present in the time interval Δt (Δt e.g. being a tuneable fraction of a repetitive time period).
In a particular embodiment, the means for controlling the phase state of the liquid crystal material comprise a resulting electric field defining an angle with a longitudinal direction of the optical fibre that is different from 90°, such as a few degrees different, such as one or 2 degrees different. Thereby the formation of defects formed by reverse tilt domains in the liquid crystal material can be minimized or avoided.
In a particular embodiment, the liquid crystal material comprises material with a positive dielectric anisotropy at frequency f1 and a negative dielectric anisotropy at frequency f2 and said means for controlling the phase state of the liquid crystal material are adapted to apply an electric field at frequencies f1 and f2, respectively. Thereby the provision of a polarization controlling component can be facilitated.
3. “Predetermined Wavelength”
In a particular application, the optical fibre used depends on the required wavelength or range of wavelength of operation. An operating wavelength shall be referred to as a signal wavelength, λs (and signal light for light at the operating wavelength).
In a preferred embodiment, λs is in the range from 0.1 μm to 2.0 μm. Specifically, in the range from 0.4 μm to 1.7 μm, preferably in the range from 1.3 μm to 1.7 μm, most preferred in the range from 1.5 μm to 1.6 μm whereby the optical fibre may be used for applications using visible to near-infrared light.
4. “Method of Producing a Liquid Crystal Infiltrated Optical Fibre”
In another aspect, according to the present invention, there is provided a method of preparing an LCI optical fibre having a longitudinal direction and a cross section perpendicular thereto, the method comprising:
providing a micro-structured optical fibre having a core region and a cladding region surrounding the core region, the cladding region comprising holes/voids in the cladding with a predetermined arrangement and size;
providing a liquid crystal material that is capable of being in different phase states; and
introducing said liquid crystal material into at least a part of said number of holes/voids over at least one longitudinal section of the optical fibre,
whereby the optical properties of the LCI optical fibre may be tuned/controlled.
In a particular embodiment, said predetermined arrangement and size of holes/voids, and liquid crystal material being selected to provide photonic bandgap effects over the at least one longitudinal section of the optical fibre. This can e.g. be achieved by arranging that at least a number of said cladding elements are arranged in a substantially two dimensional periodic manner or a Bragg-type of manner, such as concentric rings of cladding elements surrounding the core region. Alternatively, the cladding elements may be arranged in any other pattern suitable for providing an optical fibre for guiding light (by index guiding or the PBG-effect), such as in a non-symmetric or non-concentric manner.
In a particular embodiment, it is arranged that said liquid crystal material comprises a component, e.g. in the form of an additive, that is suitable for or selected to provide the possibility of externally controlling/modifying the temperature of the liquid crystal material and thereby its optical properties. The modification may e.g. be induced by an external electrical or optical signal. In a particular embodiment, the component or additive is a dye that absorbs light at particular wavelengths thereby heating the liquid crystal material and changing its optical properties. The light may be entered from an end of the optical fibre or from the side.
In a particular embodiment, the micro-structured optical fibre of step (a) is an optical fibre according to the invention as described in section 1 “Liquid crystal infiltrated photonic bandgap fibre” above, in the detailed description and figures and in the claims.
5. “Method of Producing an Optically Tuneable Article Comprising a Liquid Crystal Infiltrated Optical Fibre”
In another aspect, according to the present invention, there is provided a method of preparing an optically tuneable article according to the invention as described in section 2 “An article comprising an LCI optical fibre” above, in the detailed description and figures and in the claims, the method comprising:
providing a LCI optical fibre;
connecting one end of a second optical fibre or a fibre WDM component or another optical component by but-coupling, free-space coupling or splicing to said LCI optical fibre.
6. “Use of the Optical Fibre”
In another aspect, according to the present invention, there is provided use of a LCI optical fibre according to the present invention or a LCI optical fibre produced in a method according to the invention in an optical switch, an optical polarization controller, an optical birefringent element, an optical compensator for polarization-mode dispersion, an optical filter, an optical communication system, in an optical fibre laser such as a Q-switched laser, or in an optical fibre amplifier, or in one or more parts thereof.
7. “Use of the Tuneable Optical Article According”
In another aspect, according to the present invention, there is provided use of a tuneable optical article according to the present invention or an optical article produced in a method according to the invention in an optical switch, an optical polarization controller, an optical birefringent element, an optical compensator for polarization-mode dispersion, an optical filter, an optical communication system, in an optical fibre laser such as a Q-switched laser, or in an optical fibre amplifier, or in one or more parts thereof.
Definition of Expressions
In the present context it is intended that the term “light” designates electromagnetic radiation, in particular light having a wavelength in the range from 0.1 μm to 30 μm.
The term “from a to b” is intended to mean the range from a to b including a and b.
The term “substantially” is intended to mean being largely but not necessary wholly that which is specified.
The terms “optical fibre”, “micro-structured fibre”, “holey fibre”, “hole assisted fibre”, “PCF” and “PBG fibre” are in this application often used interchangeably for optical fibres that comprise cladding elements that may be filled with liquid crystal.
For definitions and terms related to fibres comprising micro-structured cladding elements, see afore-mentioned Bjarklev et al. reference.
In the following, by way of examples only, the invention is further disclosed with detailed description of preferred embodiments. Reference is made to the drawings in which
a, 4b show microscope photographs of a LCI PCF after filling of liquid crystals in the holes/voids.
In this example, the cladding elements surround the core region and define a substantially two-dimensional periodic lattice in the cross-section of the fibre. In the longitudinal direction of the fibre, at least a section of the optical fibre comprises cladding elements that are filled with liquid crystals. The liquid crystals may be in isotropic phase or exhibit a specific mesophase, for example nematic, smectic a, smetic c, smectic c*, and/or cholesteric.
The optical fibre may be filled with a desired liquid crystal by a setup comprising a container 34 holding the liquid crystal material 32, as for example shown schematically in
To aid the control of the alignment of the liquid crystal, the surface of the PCF holes can be coated with a thin layer of a material providing a specific anchoring between the liquid crystal and the PCF material i.e. planar, homeotropic or tilted alignment. For example, materials used for coating could be polyamides, polyimides or lipids like phosphatidylcholine. These materials could be dissolved in an appropriate solvent e.g. ionised water. The emulsion could be pressed through the holes of the PCF by the use of high-pressure or/and vacuum. Pressing the emulsion through the holes, will leave a small residue on the surface of the PCF holes, giving surface anchoring to the liquid crystal. The surface coating should be done before infiltration with liquid crystal.
In an embodiment, the PCF (before introducing the LC-material) is a so-called hollow core PCF, airguide PCF, or airguide PBG fibre. The liquid crystal material is placed inside the (large) hole that forms the core. The colouring on microscope pictures of such LCI PCFs taken with polarized light appears as a result of the alignment of the liquid crystal molecules and may be used to determine this alignment and the phase state of the liquid crystals. In an embodiment, the mesophase is nematic aligned in a so-called “escaped radial” formation.
Examples of the various phase states of liquid crystals are shown in
Apart from on-off behaviour, it is also within the present invention to provide LCI optical fibres, where the optical fibre transmits light at a given wavelength in two different phase states, but the waveguiding properties, such as for example dispersion is changed. In this way optical fibre and fibre devices with tuneable optical properties may be provided. Optical fibres that exhibit PBG effects are known to have strongly dispersive properties, the dispersion characteristic being strongly dependent on the spectral position of the PBG transmission bands. Hence, by changing/tuning/controlling the PBG transmission bands in LCI optical fibres, an optical fibre or device with tuneable dispersion properties can be made.
Thermal Tuning:
LCI optical fibres and devices according to various preferred embodiments of the present invention may be used for Q-switched fibre lasers, where a change in phase state can switch the fibre laser properties.
Optical Tuning:
Optical and Thermal Tuning:
Further, the liquid crystal 231 can be doped with an appropriate dye 232, which absorbs at wavelength λp, and, in this way, heat the LC, which changes its refractive index. This changes the bandgaps of the fibre and, thereby, also the transmission properties of the fibre.
For devices utilizing optical induced heating, two operating regimes are identified: a non-local and a local regime. In these two regimes, the dynamic response can vary with more than one order of magnitude, when the light at wavelength λp is amplitude modulated.
In the local regime, where the duration of the pulse is shorter than the thermal diffusion time of the LC infiltrated rod, the silica structure surrounding the LC acts as a thermal reservoir, which absorbs the thermal energy from the LC as soon as the light is switched off. The thermal energy of the LC rods is, therefore, transferred to the silica very quickly and the LC is cooled down. In this regime, of 100 μs have been experimentally obtained.
In the non-local regime, where the duration of the pulse is longer than the thermal diffusion time of the LC infiltrated rod, the thermal energy of the LC rod diffuses into the silica and also heats the silica structure before the light pulse is switched off. When the light pulse is switched off, both the silica and the LC have to cool down, and this slows down the dynamic response of the device. In this regime a response time on the order of 3 ms have been experimentally obtained. This is shown on
Preferably the core comprises an optically active material (such as for example one or more rare earths). The active material may be present in a full length of the optical fibre, or in a part of the optical fibre 214. The FGBs may be adapted to reflect light at certain (e.g. predetermined) wavelengths. Preferably, FBG 212 reflects light at a wavelength, λ1, and the reflection wavelength of FBG 213 may be tuned using the LCI section. The optical fibre comprises holes/voids that have been filled (in at least a section of the optical fibre) with liquid crystal. The holes/voids are not shown for reasons of clarity. As illustrated in
In another preferred embodiment, an optical fibre comprises a LCI section, wherein the core comprises a FBG. The FBG characteristics may be tuned using the LCI section. For example, the optical fibre may be used in a tuneable dispersion compensation module.
Electrical Tuning:
For electrical control of the fibre properties, a tilted electrode configuration could be used in order to avoid defects formed by reverse tilt domains in the liquid crystal. These reverse tilt domains can be formed in non-tilted electrodes, where the electrical field is perpendicular to the LC director axis.
Using the tilted electrode configuration, the LC favors to reorient in one direction when the field is turned on, and no defects is formed. The tilted electrode configuration is illustrated on
Further, dual frequency nematic liquid crystals, i.e. nematic liquid crystal with a sign change of the dielectric anisotropy as function of frequency, could be used to provide faster and/or better control of the LC. For example, a planar aligned dual frequency nematic LC, with a positive dielectric anisotropy at frequency f1 and a negative dielectric anisotropy at frequency f2, is infiltrated into a PCF and this is placed in the tilted electrode configuration. Applying a field with frequency f2 reorients the LC such that the direction is perpendicular to the electrical field, and, therefore, it has an angle to the fibre axis, which is equal to the electrode tilt angle. This induces a well defined anisotropy in the fibre, which could find use in polarization controlling components.
It will be apparent to those skilled in the art that various modifications and variations of the present invention can be made without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2003-01953 | Dec 2003 | DK | national |
The present application claims the benefit of Danish Patent Application No. PA 2003-01953 filed in Denmark on Dec. 31, 2003, and U.S. Provisional Application No. 60/536,718, filed in the United States on Jan. 16, 2004, the entire contents of which are hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60536718 | Jan 2004 | US |
