MICROSTRUCTURED FIBER AND SUPERCONTINUUM LIGHT SOURCE

Abstract

A microstructured optical fiber includes a core region and a cladding region which surrounds the core region. The cladding region includes a plurality of cladding features within a cladding background material, wherein the cladding region includes an inner cladding region with at least one inner ring of cladding features and an outer cladding region with at least three outer cladding rings of outer cladding features. The inner cladding features have a first characteristic diameter and the outer cladding region includes a plurality of outer cladding features having a characteristic diameter smaller than the first characteristic diameter. The first characteristic diameter is at least about 10% larger than an average diameter of the outer cladding features and the core region has a diameter of at least about 2 ?m.

Claims

1-54. (canceled)

55. A source of optical supercontinuum generation, the source comprising: a microstructured optical fiber and a pump laser source adapted to generate pump radiation at a pump wavelength and to launch said pump radiation into said microstructured optical fiber at an input end thereof, wherein the microstructured optical fiber has a length and a longitudinal axis along its length and comprises a core region and a cladding region surrounding the core region, said cladding region comprises a cladding background material and a plurality of cladding features within the cladding background material, said cladding features being arranged around the core region, wherein said cladding region in at least a length section of the fiber comprises an inner cladding region comprising at least one inner ring of inner cladding features having a first characteristic diameter (d.sub.1) and an outer cladding region comprising at least one outer cladding ring of outer cladding features, wherein each ring of cladding features comprises bridges of cladding background material separating adjacent features of the ring, wherein the bridges of the at least one inner ring having an average minimum width (w.sub.1), and the bridges of said at least one outer cladding ring having an average minimum width (w.sub.2), where the average minimum width (w.sub.2) of the at least one outer ring bridges is at least about 10% larger than the average minimum width (w.sub.1) of the at least one inner ring bridge, and wherein the core region has a diameter of at least about 2 ?m.

56. The source of optical supercontinuum generation according to claim 55, wherein the core region is substantially circular.

57. The source of optical supercontinuum generation according to claim 55, wherein the inner cladding region is adjacent to the core region and the outer cladding region is adjacent to the inner cladding region.

58. The source of optical supercontinuum generation according to claim 55, wherein the average minimum width (w.sub.1) of the bridges of the at least one inner ring is about 1.2 ?m or less.

59. The source of optical supercontinuum generation according to claim 55, wherein the minimum width (w.sub.1) of the bridges of the at least one inner cladding ring is smaller than a Zero Dispersion Wavelength ZDW of the fundamental core mode.

60. The source of optical supercontinuum generation according to claim 55, wherein the bridges of the at least one inner ring have substantially equal minimum width (w.sub.1).

61. The source of optical supercontinuum generation according to claim 55, wherein the outer cladding region comprises at least three outer cladding rings.

62. The source of optical supercontinuum generation according to claim 61, wherein the average minimum width (w.sub.2) of the at least three outer ring bridges is larger than about 1 ?m.

63. The source of optical supercontinuum generation according to claim 55, wherein a plurality of the outer cladding features have a characteristic diameter smaller than the first characteristic diameter (d.sub.1).

64. The source of optical supercontinuum generation according to claim 55, wherein the core region comprises a core background material which, is doped with dopant material decreasing the refractive index of the core region compared to the core background material in undoped condition.

65. The source of optical supercontinuum generation according to claim 55, wherein said core region has a substantially identical diameter along substantially the entire length of the fiber.

66. The source of optical supercontinuum generation according to claim 55, wherein: said outer cladding features have a characteristic diameter having an average diameter (d.sub.2), the inner cladding features of the inner cladding region are arranged at a first pitch (?.sub.1) and the outer cladding features of the outer cladding region are arranged at a second pitch (?.sub.2), and the inner cladding region features in the inner cladding region have a first relative cladding feature size (d.sub.1/?.sub.1) and the outer cladding features in the outer cladding region have a second relative cladding feature size (d.sub.2/?.sub.2), where the difference (d.sub.1/?.sub.1?d.sub.2/?.sub.2) between the first relative cladding feature size and the second relative cladding feature size is larger than about 0.1 and the second pitch is larger than the first pitch.

67. The source of optical supercontinuum generation according to claim 55, wherein the first characteristic diameter (d.sub.1) is larger than about 1.5 ?m.

68. The source of optical supercontinuum generation according to claim 65, wherein the first characteristic diameter (d.sub.1) of the features of the at least one inner ring of the inner cladding region is smaller than the average diameter (d.sub.2) of the features of the at least one ring of the outer cladding region.

69. The source of optical supercontinuum generation according to claim 55, wherein the cladding features are, gas filled voids, liquid filled voids, or glass having a lower refractive index than the cladding background material.

70. The source of optical supercontinuum generation according to claim 55, wherein the core region of the microstructured fiber comprises a first refractive index and the inner cladding region comprises a second refractive index such that a ?-value between the core region and a maximum refractive index for the cladding region as a whole is smaller than about 0.03.

71. The source of optical supercontinuum generation according to claim 55, wherein the optical fiber has a zero dispersion wavelength of from about 860 nm to about 1400 nm.

72. The source of optical supercontinuum generation according to claim 55, wherein the pump radiation comprises a pump wavelength which is between about 1000 nm and about 1100 nm and is up to about 200 nm above or below the zero dispersion wavelength of the optical fiber.

73. The source of optical supercontinuum generation according to claim 55, comprising a cascade optical fiber comprising (a) a first optical fiber in a form of said microstructured optical fiber, and (b) a second optical fiber comprising a second core region that is capable of guiding light along a longitudinal axis of said second optical fiber and a second cladding region surrounding the second core region, wherein a mode field diameter of the first microstructured optical fiber is larger than a mode field diameter of said second fiber, and wherein the first microstructured optical fiber is optically connected to the second optical fiber.

74. The source of optical supercontinuum generation according to claim 55, wherein the microstructured optical fiber at least at its input end, is single mode at said pump wavelength.

75. A microstructured optical fiber having a length and a longitudinal axis along its length and comprising: a core region capable of guiding light along said longitudinal axis and a cladding region which surrounds the core region, said cladding region comprising a cladding background material and a plurality of cladding features within the cladding background material, said cladding features being arranged around the core region, wherein said cladding region in at least a section of the fiber comprises an inner cladding region comprising at least one inner ring of inner cladding features having a first characteristic diameter (d.sub.1) and an outer cladding region comprising at least three outer cladding rings of outer cladding features, said inner cladding region being adjacent to the core region and said outer cladding region being adjacent to the inner cladding region, wherein each ring of cladding features comprises bridges of cladding background material separating adjacent features of the ring, wherein the bridges of the at least one inner ring having an average minimum width (w.sub.1) of about 1.2 ?m or less and the bridges of at least one of said outer cladding rings having an average minimum width (w.sub.2), which is at least about 10% larger than the average minimum width (w.sub.1) of the at least one inner ring bridges, and wherein the core region has a diameter of at least about 2 ?m.

76. The microstructured optical fiber of claim 75, wherein said core region is substantially circular and has a diameter larger than about 10 ?m.

77. The microstructured optical fiber of claim 75, wherein the inner cladding features of the inner cladding region are arranged at a first pitch (?.sub.1) and the outer cladding features of the outer cladding region are arranged at a second pitch (?.sub.2), wherein the second pitch is at least about 10% larger than the first pitch.

78. An illumination source suitable for use in at least one of endoscopy, surgical microscopy, confocal microscopy, optical coherence tomography (OTC), multi modal illumination, autofluorescence, fluorescence lifetime imaging measurements (FLIM), total internal reflection fluorescence (TIRF) microscopy; fluorescence resonance energy transfer (FRET), broadband spectroscopy, nanophotonics, flow cytometry, industrial inspection, ringdown spectroscopy, analytical spectroscopy, molecular imaging, optogenetics, displays, diffuse component characterization, solar cell characterization, quantum dot characterization, plasmonics, dispersive Fourier-Transformation spectroscopy and/or atomic trapping applications, the illumination source comprising: a source of optical supercontinuum generation comprising a microstructured optical fiber, a pump laser source and an optical filter, wherein the microstructured optical fiber comprises a core region and a cladding region surrounding the core region, said cladding region comprising a cladding background material and a plurality of cladding features within the cladding background material, wherein said cladding region comprises an inner cladding region comprising at least one inner ring of inner cladding features having a first characteristic diameter (d.sub.1) and an outer cladding region comprising at least one outer cladding ring of outer cladding features, wherein each ring of cladding features comprises bridges of cladding background material separating adjacent features of the ring, wherein the bridges of the at least one inner ring having an average minimum width (w.sub.1), and the bridges of said at least one outer cladding rings having an average minimum width (w.sub.2), where the average minimum width (w.sub.2) of the at least one outer ring bridges is at least about 10% larger than the average minimum width (w.sub.1) of the at least one inner ring bridges, and wherein the core region has a diameter of at least about 2 ?m.

79. The illumination source according to claim 78, wherein the illumination source forms part of a microscope.

80. The illumination source according to claim 79, wherein the microscope is one of: a surgical microscope, an optical fluorescence microscope, an optical fluorescence microscope based on fluorescence life time imaging (FLIM), and an optical fluorescence microscope based on Total Internal Reflection Fluorescence (TIRF) microscopy.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0134] The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

[0135] FIG. 1a is a micrograph image of a cross-section of a known microstructured fiber;

[0136] FIG. 1b is a schematic representation of the cross-section of the fiber shown in FIG. 1a;

[0137] FIGS. 1c to 1e are schematic representations of the cross-sections of the microstructured fiber of FIG. 1a, with the first, second and third ring of cladding features indicated, respectively;

[0138] FIG. 2a is a micrograph image of a cross-section of an embodiment of a microstructured optical fiber according to the invention;

[0139] FIG. 2b is a schematic representation of the cross-section of the microstructured fiber shown in FIG. 2a;

[0140] FIG. 3a shows a micrograph image a cross-section of an embodiment of a microstructured optical fiber according to the invention;

[0141] FIG. 3b is a schematic representation of the cross-section of the microstructured fiber shown in FIG. 2a;

[0142] FIG. 3c is a schematic representation of a cross-section of an embodiment of a microstructured fiber;

[0143] FIG. 4 shows graphs of power spectral density for a supercontinuum spectrum obtained by launching pump light at a pump wavelength into a standard microstructured optical fiber having a cladding with equally sized cladding features at different power levels.

[0144] FIG. 5 shows graphs of power spectral density for a supercontinuum spectrum obtained by launching pump light at a pump wavelength into a microstructured optical fiber according to the invention at different power levels.

[0145] FIGS. 6a and 6b show graphs of power spectral density for a standard microstructured optical fiber and for a microstructured fiber according to the invention, for two different pump power levels.

[0146] FIG. 7 is a schematic representation of a source of supercontinuum radiation according to the invention.

[0147] FIG. 8 shows an embodiment of a cascade fiber 50 according to the invention.

[0148] FIG. 9 is a schematic representation of an embodiment of a microstructured optical fiber according to the invention with oval inner cladding features.

[0149] FIG. 10 is a schematic representation of an embodiment of a microstructured optical fiber according to the invention with a smaller inner cladding pitch than outer cladding pitch.

[0150] The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

[0151] FIG. 1a is a micrograph image of a cross-section 10 of a known microstructured fiber, perpendicular to a longitudinal axis of the fiber. The fiber is a microstructured fiber comprising a core region 12 and a cladding region 14, the cladding region surrounding the core region 12. The core area or core region 12 is seen as the area inscribed by the cladding features 11 arranged to immediately surround the core 12.

[0152] The cladding region 14 comprises cladding features 11, here the features are in the form of substantially circular holes or voids extending in the longitudinal direction of the fiber, distributed within a cladding background or base material. The cladding features 11 are shown as arranged in a regular array. The microstructured optical fiber 10 shown in FIGS. 1a-1e has a single cladding comprising cladding features, each having substantially the same size.

[0153] FIG. 1b is a schematic representation of the cross-section of the fiber shown in FIG. 1a. FIG. 1b also shows that the cross-section 10 of the known microstructured fiber comprises a core region 12 surrounded by a cladding 14 having cladding features 11 in a regular array.

[0154] FIGS. 1c to 1e are schematic representations of the cross-sections of the microstructured fiber of FIG. 1a, with the first, second and third ring of cladding features indicated, respectively. In FIG. 1c, the dotted hexagon 14-I represents the innermost ring of cladding features with the between features arranged bridges indicated with W. In this innermost or first ring of cladding features are six cladding features. In FIG. 1d, the dotted hexagon 14-II represents the next or second ring of cladding features. This next or second ring of cladding features contains twelve cladding features. In FIG. 1e, the dotted hexagon 14-III represents the next or third ring of cladding features. This third ring of cladding features contains eighteen cladding features.

[0155] FIG. 1c shows that the first or innermost ring 14-I of cladding features consists of those cladding features being closest to the core region. The next or second ring 14-II of cladding features, counted from the core region, consists of those cladding features that are closest to the cladding features of the innermost ring 14-I, etc. Typically, a ring is not circular, but rather shaped according to the shape of the cladding features, such as in a hexagonal shape. The cross-section of the microstructured fiber 10 shown in FIGS. 1a-1e has seven whole rings of cladding features as well as three times six additional cladding features adjacent to the seventh ring of cladding features.

[0156] In the context of the present application, the phrase ring of cladding features refers to the cladding features typically having substantially equal distance to the core region.

[0157] FIG. 2a is a micrograph image of a cross-section of an embodiment of a microstructured optical fiber 20 according to the invention, and FIG. 2b is a schematic representation of the cross-section of the microstructured fiber 20 shown in FIG. 2a.

[0158] In FIGS. 2a and 2b it may be seen that the fiber 20 comprises a core region or core 22 and a cladding region 24 surrounding the core region. It is moreover clear from FIGS. 2a and 2b that the cladding region 24 comprises a cladding background material and a plurality of cladding features 21, 26 within the cladding background material.

[0159] The cladding region comprises an inner cladding region 27 with two inner rings of inner cladding features 26 and an outer cladding region 28 comprising five whole outer cladding rings of outer cladding features 21 plus additional outer cladding features not constituting a ring adjacent to the outermost ring of outer cladding features. The bridges of background material between the features 26 of the inner cladding region 27 have a width w.sub.1 and the bridges of background material between the features 21 of the outer cladding region 28 have a width w.sub.2. It can be seen that w.sub.2 is much larger than w.sub.1 advantageously as described above.

[0160] It can be seen that the inner cladding region 27 is adjacent to the core region 22 and the outer cladding region 28 is adjacent to the inner cladding region. The inner cladding features have a first characteristic diameter (d.sub.1) and the outer cladding region 28 comprises a plurality of outer cladding features 21 having a characteristic diameter smaller than the first characteristic diameter (d.sub.1). The first characteristic diameter (d.sub.1) is at least about 10% larger than an average diameter (d.sub.2) of the outer cladding features 21.

[0161] It should be noted that only a few of the cladding features 21, 26 have been marked with reference numerals in the FIGS. 2a and 2b, that all 18 features of the two innermost rings are inner cladding features within the inner cladding region, and that the remaining cladding features shown in FIGS. 2a and 2b are outer cladding features.

[0162] FIG. 3a is a micrograph image of a cross-section of an embodiment of a microstructured optical fiber 30 according to the invention, and FIG. 3b is a schematic representation of the cross-section of the microstructured fiber 30 shown in FIG. 3a.

[0163] In FIGS. 3a and 3b it can be seen that the fiber 30 comprises a core region or core 32 and a cladding region 34 surrounding the core region. It is moreover clear from FIGS. 3a and 3b, that the cladding region 34 comprises a cladding background material and a plurality of cladding features 31, 36 within the cladding background material.

[0164] The cladding region comprises an inner cladding region 37 with a single inner ring of inner cladding features 36 and an outer cladding region 38 comprising six whole outer cladding rings of outer cladding features 31 plus additional outer cladding features not constituting a ring adjacent to the outermost ring of outer cladding features.

[0165] The bridges of background material between the features 36 of the inner cladding region 37 have a width w.sub.1 and the bridges of background material between the features 31 of the outer cladding region 38 have a width w.sub.2. It can be seen that w.sub.2 is much larger than w.sub.1 advantageously as described above.

[0166] It can be seen that the inner cladding region 37 is adjacent to the core region 32 and the outer cladding region 38 is adjacent to the inner cladding region 37. The inner cladding features 36 have a first characteristic diameter (d.sub.1) and the outer cladding region 38 comprises a plurality of outer cladding features 31 having a characteristic diameter smaller than the first characteristic diameter (d.sub.1). The first characteristic diameter (d.sub.1) is at least about 10% larger than an average diameter (d.sub.2) of the outer cladding features 31.

[0167] It should be noted that only a few of the cladding features 31, 36 have been marked with reference numerals in the FIGS. 3a and 3b, that all six features of the innermost ring are inner cladding features within the inner cladding region, and that the remaining cladding features shown in FIGS. 3a and 3b are outer cladding features.

[0168] FIG. 3c is a schematic representation of a cross-section of an embodiment of a microstructured fiber 40. In FIG. 3c it may be seen that the fiber 40 comprises a core region or core 42 and a cladding region 44 surrounding the core region. It is moreover clear from FIG. 3c, that the cladding region 44 comprises a cladding background material and a plurality of cladding features 41, 46 within the cladding background material.

[0169] The cladding region comprises an inner cladding region 47 with three inner rings of inner cladding features 46 and an outer cladding region 48 comprising five whole outer cladding rings of outer cladding features 41 plus additional outer cladding features not constituting a ring adjacent to the outermost ring of outer cladding features.

[0170] It can be seen that the inner cladding region 47 is adjacent to the core region 42 and the outer cladding region 48 is adjacent to the inner cladding region. The inner cladding features have a first characteristic diameter (d.sub.1) and the outer cladding region 48 comprises a plurality of outer cladding features 41 having a characteristic diameter smaller than the first characteristic diameter (d.sub.1). The first characteristic diameter (d.sub.1) is at least about 10% larger than an average diameter (d.sub.2) of the outer cladding features 41.

[0171] It should be noted that only a few of the cladding features 41, 46 have been marked with reference numerals in the FIG. 3c, that all 36 features of the three innermost rings are inner cladding features within the inner cladding region 47, and that the remaining cladding features shown in FIG. 3c are outer cladding features.

[0172] FIG. 4 shows four graphs of power spectral density for a supercontinuum spectrum obtained by launching pump light at a pump wavelength into a standard microstructured optical fiber having a cladding with equally sized cladding features, for example a fiber 10 as shown in FIGS. 1a and 1 b. The inset in the upper right corner of FIG. 4 shows a cross-section of the fiber used for generating the graphs of FIG. 4. The graphs of FIG. 4 show that an increased pump power results in an increased power spectral density of the generated supercontinuum. The pump power values shown in FIG. 4 relate to an estimated pump effect from the pump light source, the pump effect being the actual pump effect from the light source without any combiner loss.

[0173] FIG. 5 shows graphs of power spectral density for a supercontinuum spectrum obtained by launching pump light at a pump wavelength of 1064 nm into a microstructured optical fiber 30 according to the invention at four different power levels. The inset in the upper right corner of FIG. 5 shows a cross-section of the fiber used for generating the graphs of FIG. 5, viz. the microstructured optical fiber 30. From FIG. 5 it is clear that an increased pump power results in increased power spectral density of the generated supercontinuum, and that for all four pump power levels the supercontinuum extends up to 1750 nm. However, this upper limit is a measurement limitation of the Optical Spectrum Analyser (OSA) used for the measurements, and the spectra all extend to wavelengths above 1750 nm. For the higher pump powers shown, the spectra extend to wavelengths well above 2000 nm. Moreover, for all pump powers but the lowest one, viz. 3.5 W, the spectrum extends to wavelengths below 400 nm; this is in particular clear for the pump powers 16.5 W and 23 W.

[0174] It should be noted that the pump power levels in FIGS. 4 and 5 are not identical, however they are comparable. When comparing the output power spectral densities shown in FIGS. 4 and 5, it is clear that they are of comparable magnitudes for comparable pump powers. Moreover, it is clear that the power spectral density is more stable for the optical fiber 30 according to the invention than for the standard fiber, in particular within the wavelength range between about 400 nm and about 750 nm and in particular for the higher pump powers.

[0175] FIGS. 6a and 6b show graphs of power spectral density for a standard microstructured optical fiber 10 and for a microstructured fiber 30 according to the invention, for a two different pump power levels. FIGS. 6a and 6b show the power spectral density for only a part of the supercontinuum spectrum, viz. the range from 350 nm to 750 nm. In FIG. 6a the graphs are shown for the pump power level 18.6 W, and in FIG. 6b the graphs are shown for the pump power level 21.3 W for the standard microstructured optical fiber 10 and the pump power level 20.8 W for the microstructured optical fiber 30 according to the invention.

[0176] In FIG. 6a it is seen that the power spectral density for a given wavelength is greater for the microstructured optical fiber 30 according to the invention than for the microstructured optical standard fiber 10 for a wavelength range between 410 nm and 750 nm. Moreover, FIG. 6a shows that the spectrum, at least in the wavelength range between about 450 nm and about 650 nm is flatter for the microstructured optical fiber 30 according to the invention than the standard microstructured optical fiber 10. This effect is more pronounced at the power spectral density graphs of FIG. 6b corresponding to a higher pump power. Even though the pump powers used for the standard microstructured optical fiber 10 and the microstructured optical fiber 30 according to the invention are not identical, they are at least comparable. In FIG. 6b it is seen that the power spectral density from the standard microstructured optical fiber 10 has quite a variation as a function of wavelength, in particular in the wavelength range between 475 nm and 575 nm. The microstructured optical fiber 30 according to the invention has a much flatter spectrum in this wavelength range between 475 nm and 575 nm. Moreover, the power spectral density is higher for standard microstructured optical fiber 30 than for the standard microstructured optical fiber 10, even though the pump power level of the microstructured optical fiber 30 according to the invention is lower than that of the standard microstructured optical fiber 10 (viz. 20.8 W for the microstructured optical fiber 30 according to the invention and 21.3 W for the standard microstructured optical fiber).

[0177] FIG. 7 is a schematic representation of a source 100 of supercontinuum radiation according to the invention. The supercontinuum light source 100 comprises a microstructured optical fiber 4 and a pump light source 2. The microstructured optical fiber has two ends: an input end and an output end. In FIG. 7, the input end of the fiber has an end cap 8, and the output end of the fiber is the other end of the fiber 4, viz. the end of the fiber not shown with the end cap. In FIG. 7, the end cap 8 is shown as if it is larger than the optical fiber 4; however, this is not necessarily the case, in that the end cap could have dimensions similar to those of the optical fiber 4. Even though the output end of the optical fiber 4 is shown as if it is a free end, the output end could have an end cap, or it could be spliced to further equipment.

[0178] The pump light source 2 has an output 3 arranged to feed light into the end cap 8 of the microstructured optical fiber 4. The light is fed into the microstructured optical fiber via the end cap 8, wherein a supercontinuum spectrum is created and output from the opposing end of the microstructured optical fiber as indicated by the arrow.

[0179] FIG. 8 shows an embodiment of a cascade fiber 50 according to the invention.

[0180] The cascade optical fiber (50) comprises two optical fibers 30, 20 spliced together or optical connected to each other by other means. At least one of the fibers is a microstructured optical fiber according to the invention. The other fiber or the second fiber is an optical fiber comprising a second core region that is capable of guiding light along a longitudinal axis of second fiber and a second cladding region surrounding the second core region,

[0181] The dimension of the fibers 30 and 20 are chosen such that a mode field diameter of the microstructured optical fiber 30 is larger than a mode field diameter of the microstructured optical fiber 20.

[0182] In the example shown in FIG. 8, both of the two optical fibers are optical fibers according to the invention, for example the fibers 30 and 20 shown in FIGS. 3a-3b and FIGS. 2a-2b, respectively or the fibers shown in FIG. 9 or 10 respectively. However, one of the fibers of the cascade optical fiber could for example be a multi-mode fiber and/or a microstructured optical fiber wherein the cladding has cladding features each of substantially identical size. The arrow 51 indicates light input into the fiber 50 whilst the arrow 52 indicates light output from the fiber 50.

[0183] The microstructured optical fiber shown in FIG. 9 comprises a core region (or simply referred to as core) 62 and a cladding region surrounding the core. The cladding region comprises a cladding background material and a plurality of cladding features 61, 66 within the cladding background material.

[0184] The cladding region comprises an inner cladding region 67 with a single inner ring of inner cladding features 66 and an outer cladding region 68 comprising 3 or more outer cladding rings of outer cladding features 61. Please observe that for simplifying the drawing only 2 rings of outer cladding features 62 are shown.

[0185] The bridges of background material between the features 66 of the inner cladding region 67 have a width w.sub.1 and the bridges of background material between the features 61 of the outer cladding region 68 have a width w.sub.2. It can be seen that w.sub.2 is much larger than w.sub.1 advantageously as described above.

[0186] The features 66 of the inner ring of the inner cladding region are oval with a larger diameter d.sub.L and a perpendicular smaller diameter d.sub.S, advantageously with an aspect ratio d.sub.S:d.sub.L of from about 1:1.2 to about 1:3 as described above.

[0187] The oval features 66 of the inner cladding region 67 are orientated with their smaller diameter d.sub.S in radial direction relative to the longitudinal axis of the optical fiber. As see the resulting thickness of the inner cladding region 67 is relatively low in the shown embodiment about 30% of the core diameter.

[0188] The microstructured optical fiber shown in FIG. 10 comprises a core region (or simply referred to as core) 72 and a cladding region surrounding the core. The cladding region comprises a cladding background material and a plurality of cladding features 71, 76 within the cladding background material.

[0189] The cladding region comprises an inner cladding region 77 with a single inner ring of inner cladding features 76 and an outer cladding region 78 comprising 3 or more outer cladding rings of outer cladding features 71. Please observe that for simplifying the drawing only 2 rings of outer cladding features 72 are shown.

[0190] The inner cladding features 76 of the inner cladding region 77 are arranged at a first pitch (?.sub.1) and the outer cladding features 71 of the outer cladding 78 are arranged at a second pitch (?.sub.2), wherein the second pitch is much larger than the first pitch. In the shown embodiment the second pitch (?.sub.2) is about twice the first pitch (?.sub.1).

[0191] The bridges of background material between the features 66 of the inner cladding region 67 have a width w.sub.1 and the bridges of background material between the features 61 of the outer cladding 68 region have a width w.sub.2. It can be seen that w.sub.2 is much larger than w.sub.1 advantageously as described above.

[0192] The features 76 of the single inner ring of the inner cladding region 77 have a characteristic diameter which is much smaller than the average diameter of the features 71 of the outer cladding region 78. In the shown embodiment the characteristic diameter of the features 76 of the single inner ring is about half the average diameter of the features 71 of the outer cladding region 78. As it can be seen the axial thickness of the inner cladding region is very narrow, thereby enabling an effective higher order mode stripping off.

[0193] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0194] The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope.

[0195] 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.