SOURCE OF SUPERCONTINUUM RADIATION AND MICROSTRUCTURED FIBER

Abstract

A microstructured optical fiber including 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 μm. A cascade optical fiber with at least one fiber as described, as well as a source of optical supercontinuum generation.

Claims

1. A microstructured optical fiber having a length and a longitudinal axis along its length and comprising: a core region capable of guiding light along the longitudinal axis and a cladding region which surrounds the core region, the cladding region comprising a cladding background material and a plurality of cladding features within the cladding background material, the cladding features being arranged around the core region, wherein the cladding region comprises an inner cladding region comprising an innermost ring of cladding features having a characteristic diameter (d.sub.1) and an outer cladding region comprises outer cladding rings of outer cladding features having an average diameter (d.sub.2), wherein the characteristic diameter (d.sub.1) is smaller than the average diameter (d2), wherein the outer cladding rings comprise at least two outer cladding rings, the innermost ring consisting of those cladding features being closest to the core region, wherein the rings of cladding features each comprise bridges of cladding background material separating adjacent features of the ring, wherein the bridges of the innermost ring have an average minimum width (w.sub.1), the minimum width of a bridge of a ring being the shortest distance between two adjacent features of the ring; and wherein the at least two outer cladding rings have an average minimum width (w.sub.2) of bridges that is larger than the average minimum width (w.sub.1) of the bridges of the innermost ring.

2. The microstructured optical fiber of claim 1, wherein the core region has a diameter of at least about 2 μm.

3. The microstructured optical fiber of claim 1, wherein the core region has a diameter larger than about 10 μm.

4. The microstructured optical fiber of claim 1, wherein the average minimum width (w.sub.1) of the bridges of the innermost ring is about 1.2 μm or less.

5. The microstructured optical fiber of claim 1, wherein the average minimum width (w.sub.2) of the bridges of the at least two outer rings is at least about 10% larger than the average minimum width (w.sub.1) of the bridges of the innermost ring.

6. The microstructured optical fiber of claim 1, wherein the at least two outer cladding rings is at least three outer cladding rings.

7. The microstructured optical fiber of claim 1, wherein the inner cladding features of the inner cladding region are arranged at a first pitch and the outer cladding features of the outer cladding region are arranged at a second pitch, wherein the second pitch is larger than the first pitch.

8. The microstructured optical fiber of claim 7, wherein second pitch is about twice the first pitch.

9. The microstructured optical fiber of claim 1, wherein the core region has a substantially identical diameter along substantially the entire length of the fiber.

10. The microstructured optical fiber of claim 1, wherein the microstructured optical fiber is being configured to provide supercontinuum radiation including light at wavelengths below 500 nm when pumped by pump radiation generated by a pump laser source.

11. The microstructured optical fiber of claim 1, 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.

12. The microstructured optical fiber of claim 1, wherein the characteristic diameter (d.sub.1) is about half the average diameter (d2).

13. The microstructured optical fiber of claim 1, wherein the axial thickness of the inner cladding region is configured to enable higher order modes to be stripped off effectively.

14. A source of optical supercontinuum radiation, the source comprising: the microstructured optical fiber as recited in claim 1, and a pump laser source adapted to generate pump radiation at a pump wavelength and to launch the pump radiation into the microstructured optical fiber at an input end thereof.

15. The source of optical supercontinuum radiation of claim 12, wherein the microstructured optical fiber is configured to provide supercontinuum radiation including light at wavelengths below 500 nm upon launch of the pump radiation into the microstructured optical fiber.

16. The source of optical supercontinuum radiation of claim 12, wherein the core region of the microstructured optical fiber has a diameter of at least about 2 μm.

17. The source of optical supercontinuum radiation of claim 12, wherein the bridges of the innermost ring of the microstructured optical fiber have an average minimum width (w.sub.1) of about 1.2 μm or less.

18. The source of optical supercontinuum radiation of claim 12, wherein the core region of the microstructured optical fiber 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.

19. The source of optical supercontinuum radiation of claim 12, wherein the average minimum width (w.sub.2) of the bridges of the at least three outer rings of the microstructured optical fiber is at least about 10% larger than the average minimum width (w.sub.1) of the bridges of the innermost ring.

20. The source of optical supercontinuum radiation of claim 12, wherein the microstructured optical fiber has a zero dispersion wavelength of from about 860 nm to about 1400 nm.

21. The source of optical supercontinuum radiation of claim 12, 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 microstructured optical fiber.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0135] The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

[0136] FIG. 1a is a micrograph image of a cross-section of a known microstructured fiber;

[0137] FIG. 1b is a schematic representation of the cross-section of the fiber shown in FIG. 1a;

[0138] 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;

[0139] FIG. 2a is a micrograph image of a cross-section of an embodiment of a microstructured optical fiber according to the invention;

[0140] FIG. 2b is a schematic representation of the cross-section of the microstructured fiber shown in FIG. 2a;

[0141] FIG. 3a shows a micrograph image a cross-section of an embodiment of a microstructured optical fiber according to the invention;

[0142] FIG. 3b is a schematic representation of the cross-section of the microstructured fiber shown in FIG. 2a;

[0143] FIG. 3c is a schematic representation of a cross-section of an embodiment of a microstructured fiber;

[0144] 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.

[0145] 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.

[0146] 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.

[0147] FIG. 7 is a schematic representation of a source of supercontinuum radiation according to the invention.

[0148] FIG. 8 shows an embodiment of a cascade fiber 50 according to the invention.

[0149] FIG. 9 is a schematic representation of an embodiment of a microstructured optical fiber according to the invention with oval inner cladding features.

[0150] 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.

[0151] The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] 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.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] 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.

[0172] 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.

[0173] 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 1b. 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.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] 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).

[0178] 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.

[0179] 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.

[0180] FIG. 8 shows an embodiment of a cascade fiber 50 according to the invention.

[0181] 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,

[0182] 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.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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 advantageously as described above.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] 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.

[0191] 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).

[0192] 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.

[0193] 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.

[0194] 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.

EMBODIMENTS

[0195] 1. 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 double clad 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 three outer cladding rings of outer cladding features having an average diameter (d.sub.2), 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), and the bridges of at least one of said outer cladding rings having an average minimum width (w.sub.2), where the average minimum with (w.sub.2) of at the least one outer ring bridges is at least about 10% larger than the average minimum with (w.sub.1) of the at least one inner ring bridges, and wherein the core region is substantially circular with a diameter of at least about 2 μm.

[0196] 2. A microstructured optical fiber according to embodiment 1, wherein the bridges of the at least one inner ring having substantially equal minimum width (w.sub.1), preferably the average minimum with (w.sub.2) of the at least one outer ring bridges is at least about 20%, such as at least about 50% larger than the minimum width (w.sub.1) of the at least one inner cladding ring bridges.

[0197] 3. A microstructured optical fiber according to embodiment 1 or embodiment 2, wherein the average minimum with (w.sub.2) of the at least three outer ring bridges is at least about 10%, such as at least about 20%, such as at least about 50% larger than the minimum width (w.sub.1) of the at least one inner cladding ring bridges.

[0198] 4. A microstructured optical fiber according to any of the embodiments 1 to 3, wherein the minimum with (w.sub.2) of each of the at least three outer ring bridges are substantially equal, preferably the minimum with (w.sub.2) of all of the at least three outer ring bridges are substantially equal.

[0199] 5. A microstructured optical fiber according to any of the embodiments 1 to 4, wherein the minimum width (w.sub.1) of the at least one inner cladding ring bridges is about 1.2 μm or less, such as about 1 μm or less, such as about 0.8 μm or less such as about 0.6 μm or less such as from about 0.4 to about 1.2 μm, advantageously the minimum width (w.sub.1) of the at least one inner cladding ring bridges is smaller than a Zero Dispersion Wavelength ZDW of the fundamental core mode, such as a lowermost Zero Dispersion Wavelength ZDW of the fundamental core mode if more than one core ZDW.

[0200] 6. A microstructured optical fiber according to any of the embodiments 1 to 5, wherein the average minimum with (w.sub.2) of the at least three outer ring bridges is larger than about 1 μm, such as at least about 1.2 μm, such as at least about 1.5 μm, such as at least about 2 μm.

[0201] 7. A microstructured optical fiber according to any of the embodiments 1 to 6, wherein said double clad length section has a length which is sufficient to couple at least a first higher order mode (LP11) out of the core, preferably said double clad length section is at least about 10 cm, such as at least about 25 cm, such as at least about 50 cm, more preferably said double clad length section extends in substantially the entire length of the fiber.

[0202] 8. A microstructured optical fiber according to any of the embodiments 1 to 7, wherein a plurality of outer cladding features having a characteristic diameter smaller than the first characteristic diameter (d.sub.1), wherein the first characteristic diameter (d.sub.1) is at least about 10% larger than the average diameter (d.sub.2) of the outer cladding features.

[0203] 9. A microstructured optical fiber according to any of the embodiments 1 to 8, wherein an outer diameter of the outer cladding region and the average diameter (d.sub.2) of the outer cladding features are dimensioned so as to allow the microstructured optical fiber to guide light at wavelengths above 2000 μm.

[0204] 10. A microstructured optical fiber according to any of the embodiments 1 to 9, wherein said first characteristic diameter (d.sub.1) is at least about 15% larger than said average diameter (d.sub.2) of the outer cladding features, such as at least about 20% larger than said average diameter (d.sub.2) of the outer cladding features, such as at least about 25% larger than said average diameter (d.sub.2) of the outer cladding features, such as at least about 30% larger than said average diameter (d.sub.2) of the outer cladding features, such as at least about 35% larger than said average diameter (d.sub.2) of the outer cladding features.

[0205] 11. A microstructured optical fiber according to any of the embodiments 1 to 10, wherein said microstructured fiber is a non-linear fiber, a fiber for supercontinuum generation or a passive delivery fiber.

[0206] 12. A microstructured optical fiber according to any of the embodiments 1 to 11, wherein the optical fiber is non-tapered.

[0207] 13. A microstructured optical fiber according to any of the embodiments 1 to 12, wherein said core region is substantially circular and has a diameter between about 2 μm and about 6 μm, preferably between about 2.5 μm and about 5 μm, preferably between about 3 and about 4 μm.

[0208] 14. A microstructured optical fiber according to any of the embodiments 1 to 13, wherein said core region is substantially circular and has a diameter larger than about 10 μm, preferably larger than about 12 μm, preferably larger than about 15 μm, preferably larger than about 20 μm, preferably larger than about 30 μm.

[0209] 15. A microstructured optical fiber according to any of the embodiments 1 to 14, wherein said core region has a substantially identical diameter along substantially the entire length of the fiber.

[0210] 16. A microstructured optical fiber according to any of the embodiments 1 to 15, wherein said core region comprises a core background material, wherein said core background material is doped with dopant material decreasing the refractive index of the core region compared to said undoped core background material.

[0211] 17. A microstructured optical fiber according to any of the embodiments 1 to 16, wherein the core region is microstructured.

[0212] 18. A microstructured optical fiber according to any of the embodiments 1 to 17, wherein the inner cladding region comprises one, two or three inner rings of inner cladding features, preferably the minimum width of bridges of said inner rings, where there is more than one, are substantially equal.

[0213] 19. A microstructured optical fiber according to any of the embodiments 1 to 18, wherein the inner cladding region comprises only one ring of inner cladding features.

[0214] 20. A microstructured optical fiber according to any of the embodiments 1 to 19, 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), and wherein 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, preferably larger than about 0.15, preferably larger than about 0.2, preferably larger than about 0.25, preferably larger than about 0.3.

[0215] 21. A microstructured optical fiber according to any of the embodiments 1 to 20, wherein the difference between the first characteristic diameter (d.sub.1) and the average diameter (d.sub.2) of the outer cladding features is larger than about 0.3 μm, preferably larger than about 0.4 μm, preferably larger than about 0.5 μm, preferably larger than about 0.6 μm.

[0216] 22. A microstructured optical fiber according to any of the embodiments 1 to 21, wherein the first characteristic diameter is larger than about 1.5 μm, such as larger than about 1.8 μm, such as larger than about 2.0 μm, such as larger than about 2.2 μm, such as larger than about 2.4 μm, such as larger than about 2.6 μm, such as larger than about 2.8 μm.

[0217] 23. A microstructured optical fiber according to any of the embodiments 1 to 22, wherein the average diameter (d.sub.2) of the outer cladding features is between about 1.1 μm and 1.8 μm, such as between about 1.15 μm and about 1.7 μm, such as between about 1.2 μm and about 1.5 μm, such as about 1.3 μm.

[0218] 24. A microstructured optical fiber according to any of the embodiments 20 to 23, wherein the first pitch (∧.sub.1) and the second pitch (∧.sub.2) each are between about 2.5 and about 3.5 μm.

[0219] 25. A microstructured optical fiber according to any of the embodiments 1 to 24, wherein the features of the at least one inner ring of the inner cladding region are oval with a larger diameter and a perpendicular smaller diameter having an aspect ratio of from about 1:1.2 to about 1:3, preferably from about 1:1.5 to about 1:2.5, preferably the features of the at least one inner ring of the inner cladding region are orientated with their smaller diameter in radial direction relative to the longitudinal axis of the optical fiber.

[0220] 26. A microstructured optical fiber according to any of the embodiments 1-7 and 11-19, 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 larger than the first pitch, such as at least about 10% larger, preferably at least about 25% larger, more preferably at least about 50% larger, preferably the inner cladding region has an redial thickness which is about 80% or less relative to the core diameter, such as about 60% or less, relative to the core diameter.

[0221] 27. A microstructured optical fiber according to embodiment 26, wherein the features of the at least one inner ring of the inner cladding region has a characteristic diameter which is smaller than the average diameter of the features of the at least 3 outer rings of the outer cladding regions, preferably the average diameter of the outer cladding features is at least about 10% larger, such as at least about 20% larger, such as at least about 50% larger than the characteristic diameter of the inner cladding features.

[0222] 28. A microstructured optical fiber according to any of the embodiments 1 to 27, wherein the inner cladding region has a lower effective refractive index than the outer cladding region.

[0223] 29. A microstructured optical fiber according to any of the embodiments 1 to 27, wherein the inner cladding region has an effective refractive index which is equal to or higher than the effective refractive index outer cladding region.

[0224] 30. A microstructured optical fiber according to any of the embodiments 1 to 29, wherein the cladding features of the inner and outer cladding regions are sized so that the highest effective refractive index of the cladding region is larger than the effective index of at least the first higher order core modes at about 1400 nm and above; preferably at about 1200 nm and above, preferably at about 1100 nm and above, preferably at about 1050 nm and above, preferably at about 1030 nm and above.

[0225] 31. A microstructured optical fiber according to any of the embodiments 1 to 30, wherein the outer cladding region has a higher effective refractive index than the effective index of at least the first higher order core modes at wavelengths spanning at least about 100 nm, such as at least about 500 nm in the wavelength range of from about 400 nm to about 2400 nm.

[0226] 32. A microstructured optical fiber according to any of the embodiments 1 to 31, wherein the cladding features has a refractive index which is lower than the refractive index of the cladding background material, preferably the cladding features are air filled, gas filled or liquid filled voids, or glass having a lower refractive index than the base material, e.g., fluor-doped silica glass.

[0227] 33. A microstructured optical fiber according to any of the embodiments 1 to 32, wherein the fiber has a transmission loss below 1 dB/m for the fundamental mode of light at wavelengths between about 400 nm and about 2300 nm when pump light is launched into the fiber.

[0228] 34. A microstructured optical fiber according to any of the embodiments 1 to 33, wherein the optical fiber has a zero dispersion wavelength of from about 860 nm to about 1400 nm, preferably the optical fiber has a zero dispersion wavelength of from about 900 nm to about 1200 nm, and the fiber being single mode at the zero dispersion wavelength.

[0229] 35. A microstructured optical fiber according to any of the embodiments 1 to 34, wherein the core region of said microstructured fiber comprises a first refractive index and the inner cladding region comprises a second refractive index such that the Δ-value between the core region and the maximum refractive index for the cladding region as a whole is smaller than about 0.03, such as smaller than about 0.025.

[0230] 36. A microstructured optical fiber according to any of the embodiments 1 to 35, wherein the fiber is a single mode fiber for any wavelength above about 350 nm, such as for any wavelengths above about 500 nm, such as for any wavelength above about 600 nm, such as for any wavelength above about 700 nm, such as for any wavelength above about 800 nm, such as for any wavelength above about 900 nm, such as for any wavelength above about 1000 nm, such as for any wavelength above about 1100 nm, such as for any wavelength above about 1200 nm.

[0231] 37. A microstructured optical fiber according to any of the embodiments 1 to 36, wherein the outer cladding region comprises at least four outer cladding rings of outer cladding features, such as at least five outer cladding rings of outer cladding features, such as at least six outer cladding rings of outer cladding features, such as seven outer cladding rings of outer cladding features.

[0232] 38. A cascade optical fiber comprising [0233] a. a first microstructured optical fiber according to any of the embodiments 1 to 37, and [0234] 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, [0235] 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, preferably by being spliced thereto.

[0236] 39. A cascade optical fiber according to embodiment 38, wherein the double clad length section of said first microstructured optical fiber extends in substantially the entire length of the fiber and said second optical fiber comprises at least a single clad length section, preferably said second optical fiber is a single cladding fiber in its entire length.

[0237] 40. A source of optical supercontinuum generation, the source comprising a microstructured optical fiber according to any of the embodiments 1 to 39 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 is arranged to provide supercontinuum radiation upon launch of said pump radiation into said microstructured optical fiber.

[0238] 41. A source of optical supercontinuum generation according to embodiment 40, wherein the microstructured optical fiber is a cascade optical fiber according to any of the embodiments 38 or 39 and the pump laser is arranged to launch said pump radiation into the first microstructured optical fiber at an input thereof.

[0239] 42. A source of optical supercontinuum generation according to embodiments 40 or 41, wherein the optical supercontinuum is generated substantially exclusively in the fundamental mode of the fiber, preferably the microstructured optical fiber at least at its input end, is single mode at said pump wavelength.

[0240] 43. A source of optical supercontinuum generation according to embodiments 40 to 42, wherein the pump wavelength is between about 1000 nm and about 1100 nm, preferably between about 1030 nm and about 1070 nm, such as about 1030 nm or about 1064 nm.

[0241] 44. A source of optical supercontinuum generation according to embodiments 40 to 43, wherein the pump wavelength is up to about 200 nm above or below the zero dispersion wavelength of the optical fiber.

[0242] 45. A use of a source of optical supercontinuum generation according to any of the embodiments 40 to 44, preferably for endoscopy, surgical microscopy, confocal microscopy, optical coherence tomography (OTC), multi modal illumination, autofluorescence, fluorescence lifetime imaging measurements (FLIM), molecular imaging, optogenetics, displays, diffuse component characterization, solar cell characterization, quantum dot characterization, plasmonics, dispersive Fourier-Transformation spectroscopy and/or atomic trapping applications.

[0243] 46. A method of illuminating suitably for use in performing of a procedure selected from endoscopy, surgical microscopy, confocal microscopy, optical coherence tomography (OTC), multi modal illumination, autofluorescence, fluorescence lifetime imaging measurements (FLIM), molecular imaging, optogenetics, displays, diffuse component characterization, solar cell characterization, quantum dot characterization, plasmonics, dispersive Fourier-Transformation spectroscopy and/or atomic trapping applications, the method comprising [0244] providing a source of optical supercontinuum generation according to any of the embodiments 40 to 44, [0245] launching said pump radiation into said microstructured optical fiber at its input end, [0246] generating a supercontinuum within the microstructured optical fiber, and emitting at least a fraction of said supercontinuum towards a target to be subjected to said procedure.

[0247] 47. An illumination source suitably 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 according to any of the embodiments 40 to 44.

[0248] 48. An illumination source according to embodiment 47, wherein the source comprises at least one optical filter arranged to filter a supercontinuum beam generated from the source of optical supercontinuum generation, the optical filter is preferably a tunable filter for tuning the output of the illumination source to one or more wavelengths and/or ranges of wavelengths.

[0249] 49. An illumination source according to any of embodiments 47-48, wherein the illumination source comprises a dispersive pulse stretching element for temporal stretching of light pulses, said dispersive pulse stretching element preferably is or forms part of an optical fiber.

[0250] 50. An illumination source according to any of embodiments 47-49, wherein the source of optical supercontinuum generation comprises a fiber output end for generated supercontinuum light, said fiber output end is optically connected to a delivery fiber for delivering generated supercontinuum light to a position of illuminating, preferably said delivery fiber is at least partly incorporated into a probe and/or sensor.

[0251] 51. An illumination source according to any of embodiments 47-50 wherein the illumination source form part of a microscope, such as a surgical microscope, an optical fluorescence microscope, such as an optical fluorescence microscope based on fluorescence lifetime imaging (FLIM), a total Internal Reflection Fluorescence (TIRF) Microscopy.

[0252] 52. An illumination source according to embodiment 50, wherein the illumination source is configured for use in eye surgery, said delivery fiber is at least partly incorporated into a probe for illumination onto or inside an eye prior to and/or during surgery, preferably said probe has an outer diameter of up to about 125 μm.

[0253] 53. An illumination source according to embodiment 52, wherein said probe comprises a multi-spot generator optically coupled receive at least a part of an output beam from said delivery fiber.

[0254] 54. An illumination source according to any of embodiments 52-53, wherein said probe comprises a diffractive optic element for generating a light pattern.

[0255] 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.

[0256] 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.