Patent Application
20070237453
The present invention relates in general to coupling of light from one or more input waveguides to an output waveguide or output section of a waveguide having other physical dimensions and/or optical properties than the input waveguide or waveguides.
The present invention relates to an optical component in the form of a photonic crystal fibre for coupling light from one component/system with a given numerical aperture to another component/system with another numerical aperture. The invention further relates to methods of producing the optical component, and articles comprising the optical component, and to the use of the optical component. The present invention is based on properties of photonic crystal fibres (PCF).
The present invention further relates to an optical component comprising a bundle of input fibres that are tapered and fused together to form an input coupler e.g. for coupling light from several light sources into a single waveguide.
The present invention still further relates to the control of a change of the spatial extension of a guided mode (e.g. a mode-field diameter) of an optical beam in an optical waveguide, such as an optical fibre. The invention relates to a tapered longitudinally extending optical waveguide having a relatively larger cross-section that over a certain longitudinal distance is tapered down to a relatively smaller cross section wherein the spatial extent of the guided mode is substantially constant or expanding from the relatively larger to the relatively smaller waveguide cross section.
The invention may e.g. be useful in applications such as fibre lasers or amplifiers, where light may be coupled efficiently from pump sources to a double clad fibre using the optical component.
Optical fibres are today used in numerous applications that span very diverse fields of optics. These fields include telecommunications, medicine, sensors, lasers, and many others. Photonic crystal fibres (PCFs) have recently emerged as an attractive class of fibres, where various properties may be tailored in new or improved manners compared to conventional (solid, non-micro-structured) optical fibers. PCFs are generally described by Bjarklev, Broeng, and Bjarklev in “Photonic crystal fibres”, Kluwer Academic Press, 2003.
An NA Converting Optical Waveguide Coupler
A common problem in fibre optics is to launch light into a fibre efficiently. Often the source of light and the fibre to couple into have different divergence angles (numerical aperture (NA)) and spot/core sizes. A specific problem is to launch light from a pump-diode-laser with a large spot size and relatively low numerical aperture into a double clad fibre laser with a small area and large numerical aperture.
The traditional method of solving this problem is to use bulk optics. This solution has a number of problems. One problem is related to difficulties in achieving coupling with low loss. Another problem is to achieve good coupling for a wide range of wavelengths. A third problem is mechanical stability. Fabrication of devices using bulk optics is also relatively complicated. Furthermore, reflection from the multiple glass surfaces may degrade performance of the system.
WO-2003/019257 deals with optical waveguides, for which improved coupling into cladding pumped optical fibres may be obtained through optimal designs of micro-structured outer cladding regions that provide high NA for mode(s) of an inner cladding region. This is achieved by the use of low index cladding features with a relatively narrow area between neighbouring low-index features constituting an air-clad surrounding the inner cladding.
An Optical Coupler Comprising a Bundle of Input Waveguides
About 10 years ago, a new family of optical fibres has appeared, called double cladding fibres. They consist of two waveguides imbedded into each other; an inner and an outer guiding region. Typically, the inner guiding region is a single mode core, whereas the outer region typically is a multi mode core, also called inner cladding.
Microstructured optical fibres, also known as Photonic Crystal Fibres (in the following called PCFs), holey fibres, hole-assisted fibres and by other terms, is a relatively new class of fibres where the guiding mechanism is provided by introducing air holes into the fibre. These holes typically run parallel with the fibre and extend all the way along the fibre length. The guiding principle can either be based on Total Internal Reflection (TIR) such as in traditional optical fibres, or the Photonic Bandgap (PBG) principle. For TIR-based fibres the waveguide (core) typically consists of solid glass having a larger refractive index than the effective refractive index of the surrounding cladding material, which includes a number of closely spaced holes.
In recent years, PCFs have been developed to also show double cladding features. Here, a ring of closely spaced air holes (air-clad) 13 will define the multi mode inner cladding (see
A typical use for double cladding fibres is to efficiently convert low quality, low brightness light from e.g. semiconductor lasers to high quality, high brightness light. Brightness is defined as optical power per solid angle per area. For multi mode fibres, conservation of brightness means that the NA multiplied with the waveguide diameter is a constant before and after the coupling/conversion.
The brightness conversion is done by doping the core with a rare earth dopant and pumping this with the multi mode light. The rare earth atoms will absorb the pump light and re-emit the energy at lower photon energies. Since the emission will happen through stimulated emission, this light will be guided in the mode core. Typically single mode operation is preferred, but multi-mode operation is also relevant.
This conversion method can be very efficient (up to around 80%) and the brightness can be improved by more than a factor of 100. Such light sources are often used as popular alternatives to high brightness solid state laser, since they are less bulky and far more efficient.
The limiting factor for the traditional fibres is the refractive index of the cladding material covering the inner cladding/pump guide. A low refractive index will result in a high NA of the pump guide. This, in turn, will allow either a smaller inner cladding diameter or the coupling of higher optical powers into the guide. Smaller inner cladding diameter means that the pump intensity is increased, which will allow higher efficiency and shorter laser cavities. The current state of the art for non-PCFs is to use a Fluorine-containing polymer cladding with a low refractive index. This will result in an NA of about 0.45. The problem with this is that such a polymer cladding often has problems tolerating the high optical powers, and will burn or degrade over time.
The PCFs, on the other hand, can achieve very high NAs and may be fabricated using only glass-based materials. This means that the inner cladding diameter can be reduced and that the thermal problems are alleviated. Also, there are further advantages, which will be outlined in the following.
When coupling light into a double-cladding PCF, there are a few considerations to make. To make full use of the high NA of the PCFs, one can use free space optics, such as lenses to couple the pump light into the inner cladding. An example can be seen in
All-fibre pump multiplexers have been developed by several companies, such as ITF, OFS and Nufern. In such couplers, several pump fibres 40 are bundled together and heated to temperatures near melting and tapered 41 (see
The problem with these traditional fibre couplers is that the high NA (higher than 0.3) at the output presents a challenge which until now have not been solved. The object of the invention is thus to provide a fiber coupler for coupling two or more light sources which is improved with respect to the prior art fiber couplers, and in particular a fiber coupler which is improved with respect to low loss.
A Mode Field Converting Optical Fibre
Tapered optical fibres are used in a wide variety of optical applications including couplers and mode converters.
A tapered micro-structured fibre system is disclosed in US-2002/0114574. The micro-structured fibre has a core region, a cladding region and one or more axially oriented elements (e.g. capillary air holes) in the cladding region. In an embodiment, the axially oriented elements are partially or fully collapsed during heating and stretching, leaving a silica cladding in its place and thereby providing a mode expansion. This has the disadvantage of not providing a controllably confined mode field.
U.S. Pat. No. 6,778,562 deals with a coupler for a multimode pump comprising a photonic crystal fibre with a stretched portion and at least one multimode fibre coupled thereto. A disadvantage of this coupler is that the mode field diameter is smaller at the relatively smaller cross sectional end (the down-tapered end) of the tapered fibre than at the relatively larger cross-sectional end (the un-tapered end).
Double-clad fibres, e.g. optical fibres with an active core for amplifying an optical signal, an inner cladding for guiding multi-mode pump light and an outer cladding, are known. Such fibres receive a large interest due to their potential as high power amplifiers and lasers, see e.g. U.S. Pat. No. 5,907,652 or WO-03/019257. While these fibres have attractive properties, several practical difficulties exist. For example, many of the unique properties are related to fibres with large cores, so-called large-mode area fibres, including providing tapered fibre bundles with signal feed-through having large mode area at the reduced-diameter end. Simultaneously coupling of pump and signal light to such fibres is a problem. A common approach is to attach a tapered fibre bundle including a single mode fibre onto the double-clad fibre, such as disclosed in U.S. Pat. No. 5,864,644 or in U.S. Pat. No. 5,935,288. However, it is in practice difficult to accurately control the mode field diameter of the single mode core at the tapered end. In particular, it becomes increasingly difficult to control the mode field diameter, as it is increased compared to the un-tapered end, as the mode field diameter is highly sensitive with respect to the core size for these prior art optical fibres. As the MFD is expanded, it expands much faster than the simple scaling of the dimensions of the optical fibre. Further, a large variation in MFD is observed when the core size is reduced. In other words, great care is needed during tapering of the fibre to ensure correct dimensions. Therefore there is a need for improved tapered, optical fibres, specifically tapered fibres providing a smaller variation or more controllable expansion of MFD in the down-tapered end, thereby relaxing the tolerances of the dimensions of the resulting tapered fibre.
1. A Mode Field Converting Optical Fibre
The problem of the prior art is to provide a sufficiently precise control of the mode field diameter in a tapered waveguide.
The object of the present invention is to provide an improved optical fibre wherein the radial extension of a guided mode (e.g. a mode field diameter) can be flexibly controlled to provide an optical fibre that is suitable for use as an input or output coupler.
The objects of the invention are achieved by the invention described in the accompanying claims and as described in the following.
An object of the invention is achieved by a tapered optical fibre with a thick end and a reduced-diameter end, said tapered optical fibre comprising an inner core, an outer core and an outer cladding, said inner core, outer core and outer cladding being designed in a suitable manner to provide a first mode field diameter, MFD1, at the reduced-diameter end and a second mode field diameter, MFD2, at the thick end, where MFD2 is substantially equal to or larger than MFD1, such that light that is confined by the inner core with MFD1 at the thick end is redistributed as it propagates through said tapered optical fibre and becomes confined by the outer core at the reduced-diameter end with MFD2.
In an aspect of the invention, an optical fibre having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis is provided, the optical fibre being adapted to guide light at an operating wavelength λ, the optical fibre comprising:
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 term ‘micro-structural elements’ is in the present context taken to mean structural elements enclosed by a background material, the micro-structural elements having a different refractive index than said background material. A micro-structural element may e.g. be a hole or void or any other element enclosed in a background material and having a refractive index different from that of the background material, e.g. of a fluid or solid material.
The refractive index nx is generally the conventional refractive index of a homogeneous material. The effective refractive index neff,x is the index that light at a given wavelength, λ, experiences when propagating through a given material that may be inhomogeneous (meaning that the material complex e.g. comprises two or more sub-materials, typically a background material of one refractive index and one or more types of features (typically termed micro-structural elements in the present application) of different refractive index/indices). For homogeneous materials, the refractive and the effective refractive index will naturally be similar.
For photonic crystal fibres according to the present invention, the most important optical wavelengths are in the ultra-violet to infrared regime (e.g. wavelengths from approximately 150 nm to 11 μm). In this wavelength range the refractive index of most relevant materials for fibre production (e.g. silica) may be considered mainly wavelength independent, or at least not strongly wavelength dependent. However, for non-homogeneous materials, such as fibres comprising micro-structural elements, e.g. voids or air holes, the effective refractive index may be very dependent on the morphology of the material. Furthermore, the effective refractive index of such a fibre may be strongly wavelength dependent. The procedure of determining the effective refractive index at a given wavelength of a given fibre structure having voids or holes is well-known to those skilled in the art (see e.g. Broeng et al, Optical Fibre Technology, Vol. 5, pp. 305-330, 1999).
An object of the invention is achieved by An optical fibre having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the optical fibre being adapted to guide light at an operating wavelength λ, the optical fibre comprising:
This has the advantage of providing a flexible scheme for controlling the spatial extension of the mode field of a guided mode of a tapered optical fibre, whereby light coupled to the first core region of the tapered optical fibre at a first relatively larger cross section has a mode field diameter MFD1 substantially determined by the cross-sectional extension of core region 1 and at a second relatively smaller cross section has a mode field diameter MFD2 substantially determined by the cross-sectional extension of core region 2.
The cross-sectional extension of core region 2 is determined by the innermost holes of the cladding region.
In a particular embodiment, the cross sectional dimensions di (i=1, 2, . . . , N) of the holes are substantially equal (i.e. within processing tolerances). In a particular embodiment, the holes are substantially circular, d being an inner diameter of a hole in the cross section in question (z=z1).
In a particular embodiment, the micro-structural holes are arranged in a substantially periodic pattern when viewed in a cross section of the optical fibre perpendicular to the longitudinal axis, the periodicity being defined by the location of the centres of the micro-structural elements. In a particular embodiment, the nearest neighbour mutual centre to centre distances Λij, (i, j=1, 2, . . . , N)—termed the pitch—is substantially equal within processing tolerances.
In a particular embodiment, an optical fibre having a mode field with a diameter MFD which is substantially constant over the tapered length of the fibre from the first cross section to the second cross section of the optical fibre is provided.
In a particular embodiment, in the second fibre cross section, the cross sectional dimensions of at least the innermost holes of the cladding region are larger than zero, such that they substantially determine the confinement of the guided mode. The cross sectional dimensions being larger than zero is taken to mean that the holes are un-collapsed in the second relatively smaller cross section, so that the holes contribute to the confinement of a guided mode of the optical fibre at the operating wavelength.
In a particular embodiment, at least the innermost holes have substantially similar ratio of cross sectional dimension to mutual centre to centre distance d/Λ at the first and second cross sections. In an embodiment the holes are substantially linearly down-scaled.
An object of the invention is achieved by an optical fibre having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the optical fibre being adapted to guide light at an operating wavelength Λ, the optical fibre comprising:
This has the advantage of providing an alternative solution in the form of an all solid optical fibre (i.e. not necessarily comprising micro-structural features).
In a particular embodiment, the optical fibre further comprises an intermediate region surrounding the first core region and being surrounded by the second core region.
In a particular embodiment, the intermediate region is disposed adjacent to the first and second core regions.
In a particular embodiment, the intermediate region exhibits a predetermined refractive index profile nir and wherein nir<ncore−1 and nir<ncore−2.
In a particular embodiment, the geometrically averaged refractive index ng,core−1,ir of the first core and intermediate regions are substantially equal to the refractive index ncore−2 of the second core region. Thereby an optical fibre having a mode field with a diameter MFD1 disposed in said first core region for said first cross section, and a mode field with a diameter MFD2 disposed in said second core region for said second cross section, and wherein MFD2 is larger than MFD1 can be provided.
In a particular embodiment, the absolute value of the difference between ng,core−1,ir and ncore−2 is smaller than 5·10−3, such as smaller than 1·10−3, such as smaller than 0.8·10−3, such as smaller than 0.5·10−3, such as smaller than 0.3·10−3, such as smaller than 0.1·10−3.
In a particular embodiment, the refractive index profile of the first core region is a step-index-profile with an index-step Δn1 down to the refractive index ncore−2 of the second core region. In a particular embodiment, Δn1 is larger than 1·10−3, such as larger than 5·10−3, such as larger than 6·10−3, such as larger than 10·10−3.
In a particular embodiment, the refractive index profile of the first core region is a step-index-profile with an index-step Δn1−clad down to the refractive index of the cladding material nclad. In a particular embodiment, Δn1−clad is larger than 1·10−3, such as larger than 5·10−3, such as larger than 6·10−3, such as larger than 10·10−3.
In a particular embodiment, Δn1 is identical to Δn1−clad.
In a particular embodiment, the refractive index profile of the intermediate region is a step-index-profile with an index-step Δn2 up to the refractive index ncore−2 of the second core region. In a particular embodiment, Δn2 is larger than 0.1·10−3, such as larger than 0.5·10−3, such as larger than 1·10−3, such as larger than 5·10−3, such as larger than 10·10−3.
In a particular embodiment, the refractive index profile of the intermediate region is a step-index-profile with an index-step Δn2−clad up to the refractive index of the cladding material nclad. In a particular embodiment, Δn2−clad is larger than 0.1·10−3, such as larger than 0.5·10−3, such as larger than 1·10−3, such as larger than 5·10−3, such as larger than 10·10−3.
In a particular embodiment, the refractive index profile of the second core region is a step-index-profile with an index-step Δn3 down to the refractive index of the surrounding cladding region. In a particular embodiment, Δn3 is smaller than 5·10−3, such as smaller than 3·10−3, such as smaller than 1·10−3, such as smaller than 0.8·10−3, such as smaller than 0.5·10−3, such as smaller than 0.3·10−3.
In a particular embodiment, Δn1 is in the range from 1·10−3 to 2·10−2, and Δn2 is in the range from 0.1·10−3 to 2·10−2, and Δn3 is in the range from 0.1·10−3 to 1·10−2.
In a particular embodiment, the first core region has a NAcore−1 and dimension, d2,core−1 in said second fibre cross section, and wherein 2π/λ*d2,core−1/2*NAcore−1 is less than 2.
In a particular embodiment, the first core region has a NAcore−1 and dimension, d1,core−1 in said first fibre cross section, and wherein 2π/λ*d1,core−1/2*NAcore−1 is less than 4.
In a particular embodiment,
In a particular embodiment, MFD2≧1.1*MFD1, such as MFD2≧1.2*MFD1, such as MFD2≧1.3*MFD1, such as MFD2≧1.4*MFD1, such as MFD2≧1.5*MFD1, such as MFD2≧1.8*MFD1, such as MFD2≧2.0*MFD1, such as MFD2≧2.5*MFD1, such as MFD2≧3.0*MFD1.
In a particular embodiment, d1,core−1>1.5*d2,core−1, such as d1,core−1>1.8*d2,core−1, such as d1,core−1>2.0*d2,core−1, such as d1,core−1>2.5*d2,core−1, such as d1,core−1>3.0*d2,core−1, such as d1,core−1>3.5*d2,core−1, such as d1,core−1>4.0*d2,core−1.
In a particular embodiment, d2,core−2≧1.2*d1,core−1, such as d2,core−2≧1.3*d1,core−1, such as d2,core−2÷*1.4 d1,core−1, such as d2,core−2÷1.5*d1,core−1, such as d2,core−2≧1.8*d1,core−1, such as d2,core−2≧2.0*d1,core−1, such as d2,core−2≧2.5*d1,core−1, such as d2,core−2≧3.0*d1,core−1.
In a particular embodiment, 2π/λ*d1,core−1/2*NAcore−1≦3.5, such as 2π/λ*d1,core−1/2*NAcore−1≦3.0, such as 2π/λ*d1,core−1/2*NAcore−1≦2.5.
In a particular embodiment, 2π/λ*d1,core−1/2*NAcore−1≦2.4, such as 2π/λ*d1,core−1/2*NAcore−1≦2.2.
In a particular embodiment, 2π/λ*d2,core−1/2*NAcore−1≦1.8, such as 2π/λ*d2,core−1/2*NAcore−1≦1.6, such as 2π/λ*d2,core−1/2*NAcore−1≦1.4, such as 2π/λ*d2,core−1/2*NAcore−1≦1.2, such as 2π/λ*d2,core−1/2*NAcore−1≦1.0, such as 2π/λ*d2,core−1/2*NAcore−1≦0.8.
In a particular embodiment, 2π/λ*d2,core−2/2*NAcore−2≦3.5, such as 2π/λ*d2,core−2/2*NAcore−2≦3.0, such as 2π/λ*d2,core−2/2*NAcore−2≦2.5.
In a particular embodiment, 2π/λ*d2,core−2/2*NAcore−2≦2.4, such as 2π/λ*d2,core−2/2*NAcore−2≦2.2.
In a particular embodiment, the tapered optical fibre has a mode field that varies less than 20% in its radial extension along said longitudinal, optical axis from said first to said second cross section.
An optical fibre for guiding light at a predetermined wavelength, λ, and having a longitudinal, optical axis is further provided, comprising:
A method of producing a tapered optical fibre is further provided, the method comprising the steps of:
In a particular embodiment, the tapered optical fibre is stretched and optionally cleaved to provide a second cross section in which the inner core has a dimension d2,core−1, and wherein 2π/λ*d2,core−1/2*NAcore−1 is less than 2.
In a particular embodiment, the tapered optical fibre is stretched and optionally cleaved to provide:
In a particular embodiment, the stretching is performed using a fibre tapering rig after production of the optical fibre of claim 26.
In a particular embodiment, the stretching is performed during production of a fused, tapered fibre bundle comprising the optical fibre of claim 26.
A method of producing a tapered optical fibre is furthermore provided, the method comprising the steps of:
A method for combining a first optical device having a light guiding structure with a mode field with diameter MFDf and a second optical device having a light guiding structure with a mode field with diameter MFDs different from MFDf is furthermore provided, the method comprising:
An article comprising a photonic crystal fibre according to an aspect of the invention and as described in section ‘1. A mode field converting optical waveguide fibre’ and in the corresponding claims is moreover provided by the present invention, whereby improved devices performing specific functions such as lasers or amplifiers can be provided.
In a particular embodiment, the article is an input or output coupler.
In an embodiment of the invention, the article is a fibre amplifier.
In an embodiment of the invention, article is a fibre laser.
Use of a Tapered Optical Fibre
Use of an optical fibre according to an aspect of the invention and as described in section ‘1. A mode field converting optical waveguide fibre’ and in the corresponding claims is moreover provided by the present invention, whereby specific functional features can be achieved in a relatively simple and economic way.
In embodiments of the invention, use is made of the optical fibre as an input/output coupler.
2. An NA Converting Photonic Crystal Fibre
It is an object of the present invention to provide improved optical components and methods for coupling light into optical fibres.
It is an object of the present invention to provide an optical component that may transform light from one NA and/or core/spot size to another NA and/or core/spot size.
The numerical aperture (NA) is given by the divergence angle. For standard fibre, NA is given by the refractive index difference between the core and the cladding. For air-clad fibres, the NA is given by the core index and the specific geometry and material choice of the air-clad (as described in WO03019257). The larger the NA, the larger angle of incident light is guided by total internal reflection. Typically, NA of a waveguide is defined by: NA=sin(θ), where θ is given as the FWHM or 1/e2 angle of the a light beam emitted from the waveguide.
This and other objects of the invention are achieved by the invention described in the accompanying claims and as described in the following.
An object of the invention is achieved by a photonic crystal fibre comprising
a) a multi-mode core region for propagating light in a longitudinal direction of said photonic crystal fibre,
b) an air-clad region comprising a multitude of longitudinally extending spaced apart micro-structural elements surrounding said multi-mode core region,
the photonic crystal fibre comprising a first and a second end, wherein cross-sectional dimensions of said multimode core region and said air-clad region are reduced from said first end to said second end so that brightness is essentially conserved.
Brightness is defined as luminous flux emitted from a surface per unit solid angle per unit of area, projected onto a plane normal to the direction of propagation. In other words, the term brightness is in the present context taken to mean B=P/(Ω*A), where P is optical power, Ω is solid angle and A is area of emitted light from a facet. Brightness is also known as luminance and luminous sterance.
The term ‘brightness is essentially conserved’ is in the present context taken to mean that the ratio of the brightness at the first and second ends respectively is in the range from 60% to 100%, such as in the range from, 80% to 99%, 90% to 99%.
It is to be understood that in the present context, a ‘photonic crystal fibre’ may have cross sectional dimensions that are normal for a product to be termed an ‘optical fibre’, i.e. including outer cross sectional dimensions in the range hundred to several hundred μm range as well as larger dimensions such as in the mm range.
In the present context, the ‘multi-mode 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 multi-mode core region is limited in a radial direction by micro-structural elements of the air-clad region. The multi-mode core is typically used to guide pump light from one or more pump light sources to a double-clad fibre (standard or air-clad fibres are both of relevance), the double-clad fibre comprising one or more single- or few mode cores that comprise one or more rare earth dopants (active materials). Optionally, a photonic crystal fibre according to a preferred embodiment of the present invention may comprise a single- or few-mode core being surrounded by the multi-mode core. This may for example be preferred in order to combine a pump coupling fibre component and an active, air-clad optical fibre into one device. In current ‘double clad fibre terminology’ a ‘multi-mode core’ is sometimes termed an ‘inner cladding’ or ‘a pump core’.
Advantages of having both a single- or few-mode core surrounded by a multi-mode core may also be that an optical device (comprising a PCF according to the invention) for example may be used to couple pump light into the multi-mode core, as well as to couple light from a single-mode seed or feed signal to the single- or few mode core, for example for seeding a short pulse air-clad fibre amplifier.
The term ‘micro-structural elements’ is in the present context taken to mean structural elements enclosed by a background material, the micro-structural elements having a different refractive index than said background material. A micro-structural element may e.g. be a hole or void or any other element enclosed in a background material having a refractive index different from that of the background material, e.g. of a fluid or solid material.
In an embodiment, the multitude of micro-structural elements constituting the air-clad comprises at least one ring of longitudinally extending micro-structural elements, a ring of micro-structural elements being interpreted as a group of elements located on an annular curve (e.g. a circular or elliptical curve), the elements being located on the curve being understood as each element of the group constituting a ring of elements having their outer boundaries touching or intersecting the annular curve.
In an embodiment of the invention, the micro-structural elements of the air-clad comprise holes or voids. In an embodiment, a majority or all of the micro-structural elements of the air-clad are constituted by holes.
In an embodiment of the invention, the photonic crystal fibre has numerical aperture NA and maximum cross sectional dimension D of the multimode core region (e.g. the diameter of a substantially circular multimode core region) at said first and second ends NA1, D1 and NA2, D2, respectively.
With brightness generally defined as B=P/(Ω*A), Ω*A may be determined from NA*D. Hence, for a loss-less photonic crystal fibre according to an embodiment of the present invention, brightness conservation from the first end to the second end means NA1*D1=NA2*D2. In practice, loss-free operation is not possible (due to many factors including material absorption, structural variations in raw materials, production-induced structural variations, etc.), and essential brightness conservation may be expressed as NA1*D1/NA2*D2 in the range from 0.5 to 1.5. The ratio may be smaller or larger than 1 depending on the exact structure at two ends of the optical fibre.
In an embodiment, the ratio of the product of the numerical aperture and the maximum cross sectional dimension at said first and second ends, NA1*D1/NA2*D2 is in the range from 0.5 to 1.5 such as from 0.8 to 1.2, such as from 0.9 to 1.1.
In an embodiment of the invention, the cross sectional dimensions of the multimode core region and the air-clad region are smaller at the second end than at the first and whereby it is achieved that the PCF has a larger NA at second (NA2) end than at the first end (NA1), and D2 is smaller than D1. The first end may serve as an input end for coupling light from a pump light source and the second end may be used to couple light to a double-clad fibre—for example an active, double-clad fibre. The second end may alternatively be the end of an active, double-clad fibre being part of the NA converting optical fibre.
In an embodiment, the cross sectional dimensions of the PCF at the first end is larger than or around twice the corresponding dimensions at the second end, such as larger than or around three times, such as larger than or around 4 times, such as larger than or around 5 times the corresponding dimensions at the second end.
In an embodiment of the invention, the minimum boundary to boundary distance between neighbouring micro-structural elements of the air-clad in an annular direction—termed the bridge width—is denoted b. In an embodiment, the micro-structural elements of the air-clad are located on concentric rings and b is substantially constant for all elements of a particular ring. In an embodiment, the ratio of the maximum cross sectional dimension of the multimode core region to the minimum boundary to boundary distance between neighbouring elements of a ring of elements of the air-clad, Di/bi is substantially equal at the cross sections of the first and second ends of the PCF. In an embodiment of the invention, the ratio (D1/b1)/(D2/b2) is in the range from 0.5 to 1.5 such as from 0.7 to 1.2, such as from 0.8 to 1.0, such as from 0.8 to 0.9. In an embodiment, the elements are holes.
As shown in FIG. 30 in WO03019257, NA scales essentially linearly with λ/b for large b of the air-clad fibres disclosed therein. For small b, however, (λ/b>1.5), a deviation from linearity is seen. To compensate for this and to preserve a substantially linear relationship between the product of numerical aperture and core diameter NAi*Di (and essentially conserve of brightness) over a length of the PCF where cross sectional dimensions are changed, the bridge width b may, for example, be controlled by pressure control during production of the PCF (in the case of decreasing cross sectional dimensions from a first (index i=1) end to a second (index i=2) end, and further decrease b2 by increasing pressure in the air holes).
For photonic crystal fibres according to the present invention, the most important optical wavelengths are in the ultra-violet to infrared regime (e.g. wavelengths from approximately 150 nm to 11 μm).
In an embodiment of the invention, said core region is homogeneous and made of a single material with refractive index ncore.
In a preferred embodiment of the invention, the cross sectional form of the micro-structural elements is essentially circular, essentially circular meaning drawn from a preform where the corresponding structural elements have a circular cross section. However, the cross sectional form of the micro-structural elements may take on any appropriate form such as essentially triangular, quadratic, polygonal, elliptical, etc., as implemented by drawing a fibre from a preform having corresponding structural elements of corresponding form(s), possibly modifying the form by proper control of the pressure of capillary preform elements during fabrication cf. the section “A method of manufacturing an NA converting photonic crystal fibre” below. In an embodiment of the invention, the micro-structural elements are holes or voids.
In an embodiment of the invention, ‘essentially equal’ in connection with cross sectional fibre dimensions (including those of micro-structural elements) is taken to mean that the fibre is drawn from a preform where the corresponding structural elements (typically canes or tubes of circular cross section) have equal outer maximum cross sectional dimension (typically diameter) or inner maximum cross sectional dimension (typically diameter).
In an embodiment of the invention, the (multimode) core region is essentially circular in a transversal cross section of the fibre, essentially circular meaning drawn from a preform where the corresponding structural element has a circular cross section, e.g. a circular core cane (hollow or solid) surrounded by a number of circular canes constituting a part of the cladding region.
In an embodiment of the invention, a single- or few-mode inner core is present in the multimode core region. In an embodiment, a PCF having a central core region surrounded by an array of air holes implementing a so-called ‘endlessly single-mode’ core is provided (i.e. a specially designed microstructured region inside at least part of the multi-mode core may form the cladding of an endlessly single mode fibre; see e.g. WO09900685 for details). Such an optical fibre will be single mode both at the first end (having relatively large cross sectional dimensions) AND at the second end (having relatively small cross sectional dimensions), since the holes are just scaled down (so that the fibre retains single mode properties).
In an embodiment of the invention, the PCF is an optically active fibre. In an embodiment, the PCF comprises an active central core region (so that the NA converting fibre and the active fibre are integrated). This has the advantage of eliminating the need for splicing/coupling light from the NA converting fibre to the active fibre thereby proving practically loss less in-coupling to an active fibre with a small inner cladding.
In an embodiment, the multimode core region of the PCF comprises a rare-earth doped region. In an embodiment of the invention, said core region comprises rare earth dopant ions, such as Er, Yb, Nd, Ho, Sm or Tm or combinations thereof.
In an embodiment of the invention, said core region comprises refractive index modifying, photosensitive and/or optically active dopant material(s), whereby gratings may be written in the fibre and/or the fibre may be used for optical amplification/lasing.
In an embodiment, the PCF is adapted for use as an amplifier or laser. In an embodiment, the PCF comprise one or more reflecting elements. In an embodiment, the PCF comprises at least one Bragg grating. In an embodiment, the PCF comprises a Bragg grating in the second section having the relatively smaller cross sectional dimensions.
The advantage is that the coupling optics may be built into the same optical fibre that provides the gain for a laser or an amplifier. Thereby the need for bulky and lossy bulk optics is eliminated. The integration may also be achieved through splicing of separate (passive) NA converting fibre to separate active double-clad fibres. In an embodiment, the active double clad fibre is a non-air-clad double clad fibre, such as a polymer-based double clad fibre.
In a particular embodiment, the photonic crystal fibre further comprises a solid cladding region surrounding said multimode core region, said solid cladding region having a refractive index, nclad-solid smaller than nMM core.
In a particular embodiment, the said air-clad region comprises a background material of refractive index, nclad-solid, wherein nclad-solid is smaller than nMM core, whereby the solid cladding region can contribute to the numerical aperture of the fibre.
In a particular embodiment, the sqrt(n2MMcore−n2clad-solid) is larger than 0.12, such as larger than 0.15, such as larger than 0.22 (‘sqrt’ being short for square root), whereby the solid cladding region can contribute to the numerical aperture of the fibre with values larger than 0.12, etc.
In a particular embodiment, the air-clad region in a portion of said first end is collapsed whereby the optical fibre can be provided with a solid end that has an NA larger than 0.12 or 0.15 or 0.22. Such a solid end has the advantage being more practical to work with e.g. for splicing and/or protecting the fibre from contamination of the air holes. The NA for the collapsed fibre end is given by sqrt(n2MMcore−n2clad-solid).
In a particular embodiment, the air-clad region in a portion of said first end is collapsed.
In a particular embodiment, the singlemode or few mode core region has a predetermined refractive index profile ncore−1 and the photonic crystal fibre further comprises a second core region surrounding the singlemode or few mode core region and having a predetermined refractive index profile ncore−2.
In a particular embodiment, the photonic crystal fibre further comprises a cladding region surrounding the second core region, the cladding region comprising a multitude of longitudinally extending spaced apart micro-structural holes disposed in a cladding material, the cladding material having a refractive index nclad, the holes having cross sectional dimensions di(z) and mutual centre to centre distances Λij(z), z being a coordinate along the longitudinal axis of the optical fibre;
This has the advantage of providing a flexible scheme for controlling the spatial extension of the mode field of a guided mode of a photonic crystal fibre.
In a particular embodiment, the micro-structural holes are arranged in a substantially periodic pattern when viewed in a cross section of the optical fibre perpendicular to the longitudinal axis, the periodicity being defined by the location of the centres of the micro-structural elements.
In a particular embodiment, in the second fibre cross section, the cross sectional dimensions of at least the innermost holes are larger than zero.
In a particular embodiment, at least the innermost holes have substantially similar ratio of cross sectional dimension to mutual centre to centre distance d/Λ at the first and second cross sections.
In a particular embodiment, the photonic crystal fibre further comprises cladding region surrounding the second core region, the cladding region having a refractive index nclad, the singlemode or few mode core region, the second core region and the cladding region being located within the multi-mode core region, wherein the refractive index profiles of the singlemode or few mode and second core regions and the refractive index nclad of the cladding region are adapted to provide a mode field of a guided mode of the optical fibre with a diameter MFD1 at the first end, and a mode field with a diameter MFD2 at the second end, and wherein MFD2 is larger than or equal to MFD1.
In a particular embodiment, the PCF further comprises an intermediate region surrounding the singlemode or few mode core region and being surrounded by the second core region.
In a particular embodiment, the intermediate region is disposed adjacent to the singlemode or few mode and second core regions.
In a particular embodiment, the intermediate region exhibits a predetermined refractive index profile nir and wherein nir<ncore−1 and nir<ncore−2.
In a particular embodiment, the geometrically averaged refractive index ng,core−1,ir of the singlemode or few mode core and intermediate regions is substantially equal to the refractive index ncore−2 of the second core region.
In a particular embodiment, the absolute value of the difference between ng,core−,ir and ncore−2 is smaller than 5·10−3, such as smaller than 1·10−3, such as smaller than 0.8·10−3, such as smaller than 0.5·10−3, such as smaller than 0.3·10−3, such as smaller than 0.1·10−3.
In a particular embodiment, the refractive index profile of the singlemode or few mode core region is a step-index-profile with an index-step Δn1 down to the refractive index ncore−2 of the second core region.
In a particular embodiment, Δn1 is larger than 1·10−3, such as larger than 5·10−3, such as larger than 6·10−3, such as larger than 10·10−3.
In a particular embodiment, the refractive index profile of the singlemode or few mode core region is a step-index-profile with an index-step Δn1−clad down to the refractive index of the cladding material nclad.
In a particular embodiment, Δn1−clad is larger than 1·10−3, such as larger than 5·10−3, such as larger than 6·10−3, such as larger than 10·10−3.
In a particular embodiment, Δn1 is identical to Δn1−clad.
In a particular embodiment, the refractive index profile of the intermediate region is a step-index-profile with an index-step Δn2 up to the refractive index ncore−2 of the second core region.
In a particular embodiment, Δn2 is larger than 0.1·10−3, such as larger than 0.5·10−3, such as larger than 1·10−3, such as larger than 5·10−3, such as larger than 10·10−3.
In a particular embodiment, the refractive index profile of the intermediate region is a step-index-profile with an index-step Δn2−clad up to the refractive index of the cladding material nclad.
In a particular embodiment, Δn2−clad is larger than 0.1·10−3, such as larger than 0.5·10−3, such as larger than 1·10−3, such as larger than 5·10−3, such as larger than 10·10−3.
A Method of Manufacturing an NA Converting Photonic Crystal Fibre
The present invention also provides a method of manufacturing a PCF, the method comprising the steps of:
The fabrication of photonic crystal fibres by drawing from a preform is e.g. discussed by Bjarklev, Broeng and Bjarklev in “Photonic Crystal Fibres”, Kluwer Academic Press, 2003 (referred to in the following as [Bjarklev et al.]), chapter 4, pp. 115-130, which is incorporated herein by reference. In particular, the fabrication of air-clad fibres may be performed as described by Broeng et al. in WO03019257 and DiGiovanni et al. in U.S. Pat. No. 5,907,652.
Methods disclosed in the above mentioned references may be adjusted to provide optical components according to the present invention. One possibility is to adjust the drawing speed during air-clad fibre drawing, such that cross-sectional dimensions of the PCF are varying in the longitudinal direction.
Several optical components according to the invention may be manufactured by drawing PCF from a preform at a first drawing speed vd1 (e.g. in the range 1 m/min to 5 m/min) for a certain amount of time t1 (e.g. 1 to 10 min), and increasing the drawing speed (e.g. a factor of two to four, instantaneously or with a specific rate of change) to a second drawing speed vd2, keeping this speed for a certain amount of time t2 (e.g. 0.5 to 5 min) and then reducing the drawing speed vd2 to the first drawing speed vd1, and then repeating the procedure. By this method several components may be manufactured in a continuous process, thus potentially providing a process suitable for industrial production. The drawn fibre may be cut at appropriate positions to yield a large number of NA converting fibres from a single fibre drawing.
In an embodiment of the invention, the preform design, the drawing temperature Td, drawing speeds vdi, drawing times tdi, rate of change of drawing speeds dvdi/dt, and the optionally applied pressure P to the fixed end of the preform for controlling the hole dimensions are optimized to achieve that the ratio of the maximum cross sectional dimension of the multimode core region to the minimum boundary to boundary distance between neighbouring elements of a ring of elements of the air-clad of the drawn PCF, Di/bi is substantially equal at the cross sections of the first and second ends of the PCF. In an embodiment, the ratio (D1/b1)/(D2/b2) is in the range from 0.5 to 1.5 such as from 0.8 to 1.2, such as from 0.9 to 1.1.
In an embodiment, the tapering of the PCF (i.e. change of the maximum cross sectional dimension of the multimode core region) occurs over a length of the PCF in the range of 0.1 m to 10 m, such as over 0.2 m to 5 m, such as over 0.5 m to 0.1 m.
An Article Comprising an NA Converting Photonic Crystal Fibre
An article comprising a photonic crystal fibre as discussed in section “An NA converting photonic crystal fibre” above, in the detailed description and as defined in the corresponding claims is furthermore provided by the present invention, whereby improved devices performing specific functions such as lasers or amplifiers can be provided.
In an embodiment of the invention, the article is a fibre laser.
In an embodiment of the invention, the article is a fibre amplifier.
In an embodiment, the article is a fibre laser sub-assembly. In an embodiment, the article is a fibre amplifier sub-assembly.
Use of an NA Converting Photonic Crystal Fibre
Use of a photonic crystal fibre as discussed in section “An NA converting photonic crystal fibre” above, in the detailed description and as defined in the corresponding claims is furthermore provided by the present invention, whereby specific functional features can be achieved in a relatively simple and economic way.
In embodiments of the invention, use is made of a PCF according to the invention in a fibre amplifier or in a fibre laser.
3. An Optical Coupler Comprising a Bundle of Input Waveguides
With reference to the section termed “An optical coupler comprising a bundle of input waveguides” in the Background Art-section above, the inventors have found that to guide high NA light, the cladding must be made with a very low refractive index material, which typically means either air or polymer. The polymer has the aforementioned problems with high optical powers. Using air gives a challenge in the mounting, as the fibre must be suspended to let air surround the fibre. The air-surrounded solution requires that the sides of the taper must be kept clean, since any contamination, such as a dust particle, will scatter light away from the waveguide.
The inventors have thus realised a solution that solves all the following demands.
All-fibre solution, preferably fusion spliced.
All-enclosed solution, where a reduced amount and preferably essentially none of the guided light reach the outside edge of the fibre.
A solution with no use of polymer material in contact with pump light.
A solution where several pump fibres can be used.
A solution where low NA can be converted to high NA without significant loss of optical brightness.
This is done by providing a coupler as defined in the claims.
The coupler comprising in a first end region having a plurality of separate input fibres. A plurality of input fibers includes at least two input fibers. The input fibers may be identical or they may be different from each other. In principle any optical fiber can be used as input fiber. In a second region the input fibres are tapered to smaller dimension and the input fibres are fused together. The tapering section and the fused section may are both included in the second region and are defining the second region. In other word the length section including tapering of individual input fibers and fusing of the input fibers constitutes the second region. The input fibres may be fused together partly or totally in the section where they are also tapered. These fusion and tapering sections may thus be more or less overlapping. The fused-together input fibres may be tapered to smaller dimension; at least in the tapered part of the coupler. The second region in at least a part of its length comprises an arrangement of holes surrounding the input fibers. By the term “an arrangement of holes” is meant an annular cladding with a plurality of holes which is preferably essentially parallel with each other and is extending in the length direction of the coupler. The arrangement of holes may be provided by an outer layer surrounding the more or less fused-together input fibres and comprising an arrangement of air holes surrounding the coupler over at least a length section of its second region. The coupler may further comprise a third section where the input fibers are fully fused to each other. Said third section may preferably include an output end having an air-clad.
The arrangement of holes may e.g. be in the form of a plurality of holes which seen in cross section form a ring of holes. Alternatively the arrangement of holes may be a plurality of holes which seen in cross section form two or more rings of holes or an annular pattern of holes. The size of the holes may be identical or the may vary, periodically or non-periodically. In one embodiment the arrangement of holes is in the form of a plurality of holes which seen in cross section form a ring of holes where the space between the holes are less than the diameter of the smallest hole, preferably the hole has identical diameters.
As the input fibres in the second region are tapered down in dimensions, they will leak some of their light away. By providing the arrangement of air holes surrounding the more or less fused-together tapered input fibre in the second region, it has been found, thanks to the invention, at least a part of this leaking light may be collected and guiding to an output end of the coupler. The output end may then be coupled, for example by fusion splicing or but-coupling, to an air-clad fibre. It is preferred that the output end of the coupler and an air-clad fibre that is coupled to have substantially similar NA and dimensions of the air-clad. This is preferred in order to provide lowest possible coupling losses (highest possible coupling efficiency).
A variety of numbers of input fibres are feasible, for example, 2, 3, 6, 7 or more. The input fibres are typically multi-mode pump guiding fibres with a pure silica core and an F-doped cladding. However, one or more of the input fibres may also be a single mode fibre. The input fibres may have a core dimensions ranging from at least 2 μm to 1000 μm. Typically, single mode input fibres have core diameters ranging from 2 μm to 30 μm, whereas multi-mode fibres have cores ranging from 50 μm to 1000 μm. However, the present invention is not restricted by these dimensions of the input fibres. The multi-mode input fibres are typically characterised by an NA in the range from 0.15 to 0.25, but other values are feasible as well.
In a particular embodiment, the optical coupler comprises at least one input fibre constituted by an optical fibre according to the embodiments described above under the heading ‘1. A mode field converting optical waveguide fibre’ and as defined in the corresponding claims or by an photonic crystal fibre according to the embodiments described above under the heading ‘2. An NA converting photonic crystal fibre’ and as defined in the corresponding claims. This has the advantage of providing a flexible scheme for controlling the spatial extension of the mode field of a guided mode of an optical coupler.
In a further particular embodiment, such an input fibre is centrally located in the optical coupler.
The invention also includes methods for producing such couplers, which includes methods for controlling the size of the air holes along the taper.
Further embodiments are described in the dependent claims.
Further objects of the invention are achieved by the embodiments defined in the dependent claims and in the detailed description of the invention.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof.
The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:
a and b show cross-sections of a large end and small end, respectively, of an optical fibre according to the invention.
The figures are schematic and simplified for clarity, and they just show details, which are essential to the understanding of the invention, while other details are left out.
Optical components according to the present invention are typically in the form of optical fibre, i.e. flexible light guiding devices. The optical fibres have a longitudinal direction and a cross-section perpendicular thereto. The optical fibre comprises a number of longitudinally extending features that may vary in cross-sectional size along the fibre (a photonic crystal fibre). The variation is in the form of a tapering, providing larger cross sectional feature dimensions in a first fibre end than in a second fibre end. The optical fibre comprises an air-clad. An air-clad is in the present context taken to mean a cladding region comprising holes or voids that surrounds a multi-mode core. As the dimensions of the air-clad are reduced as the fibre is tapered down, the NA of the optical fibre is increased. This is used to provide coupling of light from a large spot/core size and a low NA to a small spot/core size with a high NA.
Most references to physical fibre parameters—such as dimensions—and figures of fibre designs refer to or illustrate a photonic crystal fibre cross-section.
One preferred embodiment of a photonic crystal fibre according to the present invention is shown schematically in
The fibre in
In another preferred embodiment of the fibre, the core or parts thereof is doped with rare earth ions, such as Er, Yb, Nd, Ho, Sm or Tm.
In the top diagram of
In the lower diagram in
It is important to notice that the increase in NA as the core is decreased cannot be obtained using a conventional (non-micro-structured) double clad fibre. For conventional double clad fibres, NA is determined by absolute refractive index differences between a high-index, multi-mode core and a low-index cladding formed typically using low-index polymer. Tapering down the size of such a conventional fibre would reduce the core/spot size, but leave the NA unchanged (as the polymer does not change its refractive index). Hence the brightness would not be conserved for a conventional fibre, as is possible by preferred embodiments of the present invention.
The diagram in
The solid line represents conservation of luminance, with a fixed point at the small core end (numeral 12 in
It may, however, in some cases be an advantage that a small fraction of the light is lost. Considering a device used to pump a double clad fibre; the advantage is that if light emitted from a pump diode is not well defined, the dumping of energy that is not within a specified NA can be done along the length of the tapered fibre and thus not lose all energy at one point. Hence, physical damage from dumping high excess powers at spatially narrow positions may be avoided. Therefore, preferred embodiments of the present invention may be used to obtain high power devices with well-defined NA and spot sizes (the NA and core size being determined from the small end).
Dcore,1/Dmm core,1=Dcore,2/DMM core,2
The multimode core dimension (DMM core,i) corresponds to the parameter D of
At the pump end (first end) DMM core is relatively large, allowing efficient, easy pump coupling, but absorption is the same as at the second end, while retaining single mode output. One or both of the reflecting elements 82 may e.g. be implemented as fibre Bragg gratings.
a and b show microscope photographs of cross-sections of a large end and small end, respectively, of an optical fibre according to the invention. The photographs are of the optical fibre that the measurements in
As stated before, there is a need for devices and methods of coupling the output from several semiconductor diode pumps into one high NA, double cladding fibre. It is an object of the invention to provide an all-fibre, all-silica, high NA coupler solution without exposed waveguides.
In the following a method of fabrication according to a preferred embodiment of the present invention will be outlined. For illustrative purposes there is chosen a specific example of a coupler with 7 input fibres. The input fibres are all-glass solid MM fibres with an NA of 0.22 and core/cladding diameters of 105 μm and 125 μm, respectively.
The ring element is basically two thin tubes of glass with a large number of holes situated in between (see afore-mentioned reference WO03078338). Typically there are more than 100 holes in such a structure. The ratio between the inner and outer diameter is typically about 0.8. The inner diameter is in this example chosen to allow 7 input fibres to be slid into the tube (see
Cleaving and sealing the ring element: Since the ring element is made of all-glass it is easy to cleave the ring element at a well-controlled length. The cleaving may for example be done by scratching the ring element with a diamond tool and applying a mechanical force to cleave the ring element at the scratched position. A typical length range will be 6-12 cm. A well-controlled, short duration heating of the end of the cleaved ring element will melt the end and thus hermetically seal the holes. It is preferred that the inner diameter of the ring element is substantially maintained.
Stripping and inserting the input fibres: As an example, 7 lengths of standard MM fibres are stripped at a length 3 cm longer than the ring element. The input fibres 70 are slid into the ring element 71 as shown in
It is preferred to apply pressure control (for example vacuum) during fabrication. A silicone tube may be attached to one end of the ring element (typically the opposite end of the end wherein input fibres have been inserted). The silicone tube can be connected to a vacuum pump that will force interstitial holes (marked as black regions in
It is preferred to sealing the input end during fabrication. This is done to fixate the position of the input fibres within the ring element as shown schematically in
Mounting in taper machine: In a preferred embodiment, a commercial splicer like the Vytran LDS-1250 is use for heating and tapering. This machine is specially designed to be able to produce well-controlled tapers. Of course, also other heating/pulling arrangements could be made. The ends of the ring element are attached at the fixtures. Note that since the input fibres are melted into the ring element, this mounts securely all elements.
Heating and tapering: Simultaneous heating and pulling will allow well-controlled tapering, both in terms of reduction ratio, final diameter and taper shape. The setup is sketched in
Cleaving: the taper is preferably cleaved somewhere within region 3 in
Note that mode mixing in an air-clad fibre that pump light is coupled to via the coupler will typically homogenise the pump light distribution.
Splicing to an air-clad, active fibre: Since the output end of the coupler and the air-clad fibre are preferably designed to fit each other, splicing these two together with high quality optical performance is feasible using standard splicing equipment, such as Vytran FFS 2000, using suitable adjustments of splicing parameters, including temperature, heat, exposure time etc.
To make coupling efficient, one must try to conserve the brightness of the light. That is the same as saying that the NA multiplied with the waveguide diameter must be constant throughout the entire taper and that the transmission loss must be low. As an example, if the NA is increased from 0.22 to 0.6, the diameter can be reduced to about one third of the size. Note that this means that the intensity increases by a factor of 9.
It has been found that it is possible to taper a multi-mode (MM) fibre with relatively low loss—this is demonstrated in Danish Patent Application PA 2004 00447 that is incorporated herein by reference. As an example, a passive air-clad fibre (a fibre comprising a large MM silica core and an air-clad) was tapered down while efficiently conserving brightness. Reducing the diameter by a factor of 2 and increasing the NA accordingly through the reduction in wall thickness between holes in the air-clad, transmission was better than 98% and brightness was reduced by less than 2%.
Assume that the NA of the input MM fibre is 0.22. It is desired to make a MM fibre with an NA of 0.6, the core diameter can be reduced by 0.6/0.22=2.7. As stated before such a high NA cannot be supported using polymer cladding (and also polymer cladding is undesirable due to high power reliability). Such a high NA is possible using an air-clad.
In the following further description of the tapering shall be given.
The present inventors have realised that tapering can be made for ring elements such as those described in WO03078338. Also such ring elements comprising a plurality of input fibres inserted into their center hole may be tapered down. The tapering may be performed using commercially available tapering equipment as for example from Vytran. Ring elements and their fabrication are disclosed in WO03078338 that is incorporated herein by reference. See e.g.
The present inventors have further found that down-tapering can be done on ring elements comprising solid material with low transmission loss. For example, a down-tapering of a MM, air-clad fibre can be done while efficiently conserving brightness. Reducing the diameter by a factor of 2 (and increasing the NA accordingly), transmission has been found to be better than 98% and brightness maintained to within 2%.
A Mode Field Converting Optical Fibre
In applications such as pump couplers and pump tapers used as interface to a double clad optical fibre e.g. in a fibre laser or amplifier as disclosed above, there is a need for maintaining or even expanding the absolute size of the mode field (e.g. the mode field diameter, MFD) in the face of a down tapering of the waveguide of e.g. a factor of 2 or 3, cf. the schematic situation of the taper of
In the following reference numerals for corresponding features of the relatively larger and smaller cross sections, respectively, are given together when referring to a structural feature in general.
c schematically illustrates the first core region 2295 and the second core region 2296. The first core region is spatially limited by a refractive index difference to the neighbouring region. The first core region 2295 has a refractive index profile designed for single to few mode operation at its relatively larger cross section (e.g. 221 in
In the limit where the mode is guided by the step-index core 227, it should be ensured that the V-parameter VSIF is larger than approximately 1. For robust guiding, it is generally advantageous that 2<VSIF<2.405, where
a being the core radius and ncore and nclad the refractive indices of the core and cladding regions, respectively.
The theoretical mode field radius w relative to the core radius a is given by the following expression:
For a VSIF-value of 1.4, w/a is around 2, indicating that the mode field diameter (=2w) is approximately twice that of the core diameter 2a. The dependence of mode field diameter MFD on core radius a is shown in
One application of the principle of the present invention is to provide sections of signal feed-through in a pump coupler or pump taper component. Such functionality is advantageous when the components are used in connection with double clad optical fibres in an amplifier or laser configuration.
An example is an optical waveguide taper where the cross section of the waveguide is reduced by a factor of 2.7 going from a multimode waveguide of 400 μm cross-sectional (input) diameter to a scaled down (output) diameter of 150 μm. In such a taper, it would be desirable to have a signal feed-through where the input MFD (e.g. 6 μm) is conserved at the output.
From
It can thus be concluded that a tapered optical fibre according to the present invention can provide a relatively constant mode field diameter (here around 6 μm) over a relatively large pitch range ΔΛ (here from 20 to 4 μm), i.e. the MFD is relatively independent of the amount of down scaling of the fibre cross section over a relatively large scaling range.
It may, however, be advantageous even to increase the cross-sectional size (area) of the mode field while down-scaling the cross section of the optical waveguide, i.e. for an optical waveguide having a substantially circular cross section (such as an optical fibre), it is desirable to have a component wherein the MFD increases when the fibre diameter decreases (e.g. in an input/output coupler, e.g. a fibre taper). Such functionality is e.g. advantageous in cases where pump energy is to be coupled into a double clad, optically active fibre having a relatively large MFD to maximize pump absorption and to reduce non-linearities.
A typical example is a DC-fibre (double clad) with a single mode core having a MFD ˜20 μm to be used in an amplifier configuration. A seed source would typically have a MFD ˜6 μm (at 1060 nm). Therefore a coupler needs to convert the MFD from 6 μm to 20 μm. As feed-through in a coupler, the structural scale should still be reduced corresponding to an NA change from ˜0.22 to ˜0.60. I.e., the MFD should increase by a factor of ˜3, while the cross-sectional dimensions are decreased by a factor of ˜3.
The present inventors have realized that this can more controllably be achieved by an optical fibre having characteristics as indicated on
In all cases d/Λ = 0.48, where d is a maximum outer dimension (here diameter) of the cladding features (here holes).
The dotted line 277 corresponds to a pure micro-structured fibre with MFD=1.24·Λ. It is clear from a comparison of
It can further be concluded that a substantial overlap with a 20 μm MFD fibre can be provided. Further, there is a natural compromise between large MFD and potential loss (represented by increased beat length).
The functionality of the index profile of
This is illustrated by
When the cross-sectional dimensions of the waveguide structure are reduced in scale, the (first) central core stops guiding and the depressed cladding (intermediate region 285 of
From
The Production Process
Various aspects of manufacturing a photonic crystal fibre including the preparation of a preform is discussed in chapter 4 in [Bjarklev et al.].
The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims.
Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004 00447 | Mar 2004 | DK | national |
| 2004 00844 | May 2004 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK05/00192 | 3/21/2005 | WO | 12/21/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60554373 | Mar 2004 | US | |
| 60575091 | May 2004 | US |
