Photonic crystal fiber, a method of production thereof and a supercontinuum light source

Abstract

A Photonic Crystal Fiber (PCF) a method of its production and a supercontinuum light source comprising such PCF. The PCF has a longitudinal axis and includes a core extending along the length of said longitudinal axis and a cladding region surrounding the core. At least the cladding region includes a plurality of microstructures in the form of inclusions extending along the longitudinal axis of the PCF in at least a microstructured length section. In at least a degradation resistant length section of the microstructured length section the PCF includes hydrogen and/or deuterium. In at least the degradation resistant length section the PCF further includes a main coating surrounding the cladding region, which main coating is hermetic for the hydrogen and/or deuterium at a temperature below T.sub.h, wherein T.sub.h is at least about 50 C., preferably 50 C.<T.sub.h<250 C.

Claims

1. A microstructured fiber (MSF) comprising: a solid core extending along a longitudinal axis of the MSF, a cladding region surrounding the core, wherein at least the cladding region comprises a plurality of inclusions extending along the longitudinal axis of the MSF in at least a length section, wherein the MSF, in at least a selected length section of said length section, comprises at least one of hydrogen and deuterium, wherein the MSF, in at least said selected length section, further comprises a coating surrounding the cladding region, wherein the coating is hermetic for said hydrogen and/or deuterium at a temperature below T.sub.h, wherein T.sub.h is at least about 50 C.

2. The MSF of claim 1, wherein the plurality of inclusions is arranged in a pattern comprising at least two rings of inclusions surrounding the core.

3. The MSF of claim 1, wherein the plurality of inclusions comprise gas inclusions.

4. The MSF of claim 1, wherein said selected length section is the whole length of the MSF with an exception of any closed ends of the MSF, wherein a closed end can have a length along the longitudinal axis of the MSF of up to about 3 mm.

5. The MSF of claim 1, further comprising inclusions located in the outer cladding region (outer inclusions), wherein said inclusions located in the inner cladding region (inner inclusions) are larger than the outer inclusions.

6. The MSF of claim 1, wherein material between the inner cladding region and the coating forms a reservoir for hydrogen and/or deuterium.

7. The MSF of claim 1, wherein the core in at least said selected length section has a core diameter of about 10 m or less.

8. The MSF of claim 1, wherein an innermost inclusion in at least said selected length section has a center-to-center distance to the core of less than about 50 m.

9. The MSF of claim 1, wherein the MSF has anomalous dispersion for at least one wavelength between 1000 nm and 1100 nm.

10. The MSF of claim 1, wherein the core of the MSF is spatially single mode at 1064 nm.

11. The MSF of claim 1, wherein the core of the MSF is single mode at 1030 nm.

12. The MSF of claim 1, wherein at least the core of the MSF is essentially free of Germanium.

13. The MSF of claim 1, wherein at least the core of the MSF is essentially free of active material.

14. The MSF of claim 1, wherein the coating comprises nitride, carbon, aluminum, gold, copper, nickel, metallic glass or a combination or an alloy comprising one or more of the before mentioned.

15. The MSF of claim 1, wherein the coating has a thickness of from about 5 nm to about 25 m.

16. The MSF of claim 1, wherein the coating is diffusion open for hydrogen and/or deuterium at a temperature above T.sub.o, where T.sub.o is larger than T.sub.h.

17. The MSF of claim 1, wherein the MSF comprises at least one additional coating for mechanical protection outside said coating.

18. The MSF of claim 1, wherein the MSF comprises at least one tapered length section wherein said core in a first end of the tapered length section has a core diameter D1 and said core in a second end of the tapered length section has a core diameter D2, wherein D1 is larger than D2.

19. The MSF of claim 18, wherein the first end of the tapered length section is at a launching end of the fiber or up to 5 cm along the length from the launching end of the fiber.

20. The MSF of claim 1, wherein the MSF comprises at least two spliced fiber length sections, wherein at least one spliced fiber length section is or comprises said selected length section.

21. The MSF of claim 1, wherein the MSF comprises: a first length section with a first length L1, wherein the inclusions of said MSF at least at a first cross-section through the first length section perpendicularly to the longitudinal axis have a first pitch 1, a first inclusion diameter d1, and a first relative size d1/1 of inclusions, a second length section with a second length L2, wherein the inclusions of said MSF at least at a second cross-section through the second length section perpendicularly to the longitudinal axis have a second pitch 2, a second inclusion diameter d2, and a second relative size d2/2 of inclusions, and wherein at least one of said first length L1 and said second length L2 comprises said selected length section.

22. The MSF of claim 1, wherein the MSF comprises a mode-adaptor extending along the length of the MSF in at least a mode-field adapting length section extending from a launching end of the MSF up to 5 cm from the launching end of the MSF, wherein said mode-field adapting length section has a length of at least about 5 cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and/or additional objects, features and advantages of the present invention will be further elucidated by the following illustrative and non-limiting description of embodiments of the present invention, with reference to the appended drawings.

(2) FIG. 1 is a cross-sectional view of a PCF of an embodiment of the invention.

(3) FIG. 2 is a cross-sectional view of a PCF of another embodiment of the invention.

(4) FIG. 3 is a cross-sectional view of a PCF of yet another embodiment of the invention.

(5) FIGS. 4a, 4b and 4c show respectively a side view of a PCF according to an embodiment of the invention and cross-sections through a first and second length section thereof.

(6) FIG. 5 is a schematic representation of an embodiment of a supercontinuum light source of radiation according to the invention.

(7) FIG. 6 is a schematic drawing of a drawing tower where the main coating and an additional coating is applied in an in-line process and where the drawing tower comprises a coating station comprising a reactor for application of a carbon coating.

(8) FIG. 7 is a schematic drawing of a drawing tower where the main coating and an additional coating is applied in an in-line process and where the drawing tower comprises a coating station application of a metal coating.

(9) The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

DETAILED DESCRIPTION

(10) Further scope of applicability of the present invention will become apparent from the 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.

(11) The PCF shown in FIG. 1 has a core 1 and a cladding region 2, 3 surrounding the core 1. The PCF has a not shown length and a longitudinal axis which in the shown embodiment is coincident with the center axis of the core. The cladding region comprises an inner cladding region 2 and an outer cladding region 3. The inner cladding region comprises a plurality of microstructures in the form of inclusions extending along the longitudinal axis of the PCF. As described above the inclusions can comprise any material, but are advantageously gas inclusions, such as air inclusions. Advantageously the inclusions are collapsed at one or more positions along the length of the fiber such as at each end of a degradation resistant length section which in one embodiment is substantially the whole length of the PCF as described above.

(12) As it can be seen the cross sectional view of the PCF is a cross sectional view in the degradation resistant length section of the PCF, which as mentioned may comprise the whole length of the PCF or only a part of the length of the PCF.

(13) The PCF is loaded with not shown hydrogen and/or deuterium preferably in the form of hydrogen molecules and/or deuterium molecules (H2/D2). The hydrogen and/or deuterium will usually be in both the core 1 and the cladding region 2, 3. The PCF comprises a main coating 4 which is hermetic for hydrogen and/or deuterium at a temperature below T.sub.h. Different types of preferred main coatings are described above.

(14) The PCF comprises an additional coating 5 for mechanical protection and optionally for providing the PCF with a desired appearance and/or texture.

(15) In use when the PCF is subjected to high peak power of light, such as described above, the light may cause defects in the core material. This effects, which are believed to be caused by different chemical reactions are sometimes called photo induced degradation or photodarkening. The hydrogen and/or deuterium has been found to mitigate the degradation by binding to the material e.g. to terminate free radicals.

(16) As the hydrogen and/or deuterium in the core 1 is/are spent, fresh hydrogen and/or deuterium migrates to the core 1 from the cladding region 2, 3. Due to the main coating 4 which is hermetic for hydrogen and/or deuterium when the PCF is in use or stored prior to use, the required amount of hydrogen and/or deuterium can be relatively low and/or the PCF is protected against excessive degradation for a long time, such as up to several years e.g. 3, 4 or even 5 years or longer.

(17) The PCF is advantageously of silica e.g. doped as described above.

(18) The PCF shown in FIG. 2 has a core 11 and a cladding region 12 surrounding the core 11. The PCF has a not shown length and a longitudinal axis which in the shown embodiment is coincident with the center axis of the core 11. The cladding region 12 comprises a plurality of microstructures 12a in the form of inclusions in the cladding background material 12b. The inclusions 12a extend along the longitudinal axis of the PCF. As described above the inclusions can comprise any material, but are advantageously gas inclusions, such as air inclusions. Advantageously the inclusions are collapsed at one or more positions along the length of the fiber such as at each end of a degradation resistant length section which in one embodiment is substantially the whole length of the PCF as described above.

(19) The plurality of inclusions 12a is arranged in the cladding region in a pattern comprising several rings of inclusions surrounding the core. The innermost ring of inclusions surrounding the core is marked with the dotted ring 12c.

(20) As it can be seen the cross sectional view of the PCF is a cross sectional view in the degradation resistant length section of the PCF, which as mentioned may comprise the whole length of the PCF or only a part of the length of the PCF.

(21) The PCF comprises a main coating 14 which is hermetic for hydrogen and/or deuterium at a temperature below T.sub.h. Different types of preferred main coatings are described above.

(22) The PCF comprises an additional material layer 16 which is sufficiently far from the core 11 to have any effect as a cladding (i.e. the refractive index of the material of the material layer 16 does not influence the light guiding of the core).

(23) The radial distance 17 between an outermost of the inclusions 12a of the cladding region and the main coating 14 is at least about 10 m.

(24) The additional material layer 16 may be of the same or of a different material than the cladding background material 12b. The additional material layer 16 is advantageously selected to have a high capacity for hydrogen and/or deuterium to thereby act as a reservoir for hydrogen and/or deuterium.

(25) The PCF is loaded with not shown hydrogen and/or deuterium as described above. The hydrogen and/or deuterium will usually be in both the core 11 and the cladding region 12 as well as in the additional material layer 16.

(26) In use when the PCF is subjected to high peak power of light, such as described above, and as the hydrogen and/or deuterium in the core 11 is/are spent fresh hydrogen and/or deuterium migrated to the core 11 from the cladding region 12 and the material layer 16. Due to the main coating 14 which is hermetic for hydrogen and/or deuterium when the PCF is in use or stored prior to use, the required amount of hydrogen and/or deuterium can be relatively low and/or the PCF is protected against excessive degradation for long time, such as up to several years e.g. 3, 4 or even 5 years or longer.

(27) The PCF is advantageously of silica e.g. doped as described above.

(28) The PCF shown in FIG. 3 has a core 21 and a cladding region 22, 23 surrounding the core 21. The PCF has a not shown length and a longitudinal axis which in the shown embodiment is coincident with the center axis of the core 21.

(29) The cladding region comprises an inner cladding region 22 and an outer cladding region 23. The inner cladding region 22 comprises inner inclusions 22a in the inner cladding background material 22b. The outer cladding region 23 comprises outer inclusions 23a in the outer cladding background material 22b.

(30) The inner inclusions 22a comprise two rings of inner inclusions and the outer inclusions 23a comprise 5 rings of outer inclusions.

(31) The inclusions 22a, 23a extend along the longitudinal axis of the PCF. As described above the inclusions can comprise any material, but are advantageously gas inclusions, such as air inclusions. Advantageously the inclusions are collapsed at one or more positions along the length of the fiber such as at each end of a degradation resistant length section which in one embodiment is substantially the whole length of the PCF as described above.

(32) The background material 22b of the inner cladding region 22 and the background material 23b of the outer cladding region 23 and optionally the core material are advantageously of the same material such as of silica optionally doped with fluorine.

(33) As it can be seen the cross sectional view of the PCF is a cross sectional view in the degradation resistant length section of the PCF, which as mentioned may comprise the whole length of the PCF or only a part of the length of the PCF.

(34) The PCF comprises a main coating 24 which is hermetic for hydrogen and/or deuterium at a temperature below T.sub.h. Different types of preferred main coatings are described above.

(35) The PCF comprises an additional material layer 26 which is sufficiently far from the core 21 to have any effect as a cladding.

(36) The additional material layer 26 is in this embodiment the same as the cladding background material 23b.

(37) The radial distance 27 between an outermost of the inner inclusions 22a of the inner cladding region and the main coating 24 is at least about 10 m.

(38) The PCF is loaded with not shown hydrogen and/or deuterium as described above. The hydrogen and/or deuterium will usually be in both the core 21 and the cladding region 22, 23 as well as in the additional material layer 26.

(39) In use when the PCF is subjected to high peak power of light, such as described above, and as the hydrogen and/or deuterium in the core 21 is/are spent, fresh hydrogen and/or deuterium migrate to the core 21 from the cladding region 22, 23 and the material layer 26. FIGS. 4a, 4b and 4c show an embodiment of a PCF 30 which comprises two spliced fiber length sections, wherein at least one spliced fiber length section is or comprises a degradation resistant length section as described above. This type of fiber is also called a spliced cascaded optical fiber. FIG. 4b is a cross sectional view of a first length section 31 and FIG. 4c is a cross sectional view of a second 32 length section spliced to the first length section. Preferably at least the first length section 31 of the PCF is a degradation resistant length section as described above.

(40) The PCF 30 is arranged for generating supercontinuum light upon feeding of light having a first wavelength .sub.1 e.g. from about 900 nm to about 1100 nm into the launching end 34 of the PCF 30.

(41) Along its length the optical fiber 30 comprises a first length section 31, a second length section 32 and a splicing 33 between the first and second length sections 32, 33. The optical fiber 30 may optionally include a not shown end cap to close the inclusions.

(42) The first length section 31 has a core 41a with a first core diameter W.sub.1 and a cladding region 32a with a first pitch .sub.1, a first inclusion diameter d.sub.1 and a first relative size of inclusions .sub.1/d.sub.1. The first length section comprises a main coating 44a which is hermetic for hydrogen and/or deuterium at a temperature below T.sub.h. Different types of preferred main coatings are described above. At least the first length section is loaded with hydrogen and/or deuterium.

(43) The second length section 32 has a core 41b with a second core diameter W.sub.2 and a cladding region 42b with a second pitch .sub.2, a second inclusion diameter d.sub.2 and a second relative size of inclusions .sub.2/d.sub.2.

(44) Advantageously at least one of the dimensions the first core diameter W.sub.1, the first pitch .sub.1, the first inclusion diameter d.sub.1 and the first relative size of inclusions .sub.1/d.sub.1 differs from the corresponding dimension the second core diameter W.sub.2, the second pitch .sub.2, the second inclusion diameter d.sub.2 and the second relative size of inclusions .sub.2/d.sub.2 of the second length section 32.

(45) Throughout the first length section 31 the dimensions of the fiber are substantially constant and throughout the second length section 32 dimensions of the fiber are substantially constant.

(46) The respective lengths of the first and the second length section 31, 32 are in this embodiment respectively 1-10 m and 10 m. However, it should be understood that these lengths are only given as example and the fiber length sections may in principle have any other lengths.

(47) FIG. 5 is a schematic representation of a supercontinuum light source. The supercontinuum light source 50 comprises a PCF 54 comprising a degradation resistant length section as described above and a pump source 52. The PCF has two ends: a launching end 55 and an output end 56. In FIG. 5, the launching end 55 of the PCF 54 has or is optically connected to a mode adaptor 58 for adapting the mode of the pump pulses from the pump source 52. In FIG. 5, the mode adaptor 58 is shown as if it is larger than the optical fiber 54; however, this is only for illustrative purpose and in practice the mode adaptor may have any suitable outer dimensions e.g. outer dimensions similar to those of the optical fiber 54. Even though the output end 56 of the optical fiber 54 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.

(48) The pump light source 52 has an output 53 arranged to feed light into the PCF 54 via a delivery fiber 57 and via the mode adaptor 58 and a supercontinuum spectrum is generated in the PCF and output from the output end 56 of the PCF. The delivery fiber 57 may e.g. be omitted or replaced e.g. by an optical element such as a lens.

(49) The drawing tower shown in FIG. 6 is in the process of drawing a PCF 63 from a preform 63a. The preform is enclosed in a pressure control chamber 61 comprising one or more pressure chambers for controlling the pressure of gas inclusions in the PCF. A bottom part extends into a furnace 62, where the bottom part of the preform is heated to enable drawing the PCF 63. The velocity of the PCF and thereby the PCF diameter is controlled by the drawing wheel 69 pulling the PCF through the various stations of the drawing towers. The velocity of the PCF 63 is adjustable and by adjusting the temperature of the furnace 62 and the velocity of the PCF 63 the diameter of the fiber may be adjusted. The PCF is passed through a monitoring station 67a where the diameter of the PCF from the furnace 62 is monitored in-line.

(50) From the monitoring station 67a the PCF 63 is passed to the coating station for application of a main carbon coating.

(51) The PCF 63 is passed through the reactor chamber of the reactor 64 and as indicated with the arrows a reaction is introduced and withdrawn in a continuous flow to keep a substantially constant amount of fresh gas in the reactor.

(52) To ensure that the PCF 63 has a sufficiently high temperature when entering the reactor 64, it is desired that the reactor is positioned relatively close to where the PCF 63 leaves the furnace 62. Alternatively an oven may be positioned prior to the reactor for preheat the PCF 63, however the latter alternative embodiment is not preferred due to the additional cost of the oven.

(53) The thickness of the carbon layer may be adjusted e.g. by adjusting the concentration of the reactive carbonaceous gas in the reaction gas or by changing the PCF velocity.

(54) From the reactor the carbon coated PCF passes to an additional coating station for application of an additional coating, which in the shown embodiment is a polymer coating station 65. From the coating station the coated PCF is passed to a concentricity monitor 67b and further to a curing station 66 where the polymer coating is cured by light.

(55) From the curing station 66 the coated PCF is passed further to an additional monitor 67 c for monitoring the fiber diameter. From the drawing wheel 69 the coated PCF 63 passed to spooling onto a spool 68.

(56) The coated PCF 63 may advantageous be hydrogen or deuterium loaded on the spool by subjection the coated PCF on the spool 68 to the hydrogen and/or deuterium in a loading chamber.

(57) The drawing tower shown in FIG. 7 is in the process of drawing a PCF 73 from a preform 73a. The preform is enclosed in a pressure control chamber 71 comprising one or more pressure chambers for controlling the pressure of gas inclusions in the PCF. A bottom part extends into a furnace 72, where the bottom part of the preform is heated and the fiber 73 is drawn to a desired thickness. The velocity of the fiber is controlled by the drawing wheel 79 pulling the PCF 73 through the various stations of the drawing towers. From the furnace 72 the fiber is passed through a monitoring station 77a where the diameter of the fiber is monitored in-line.

(58) From the monitoring station 77a the PCF 73 is passed to the coating station 74 for application of a main metal coating.

(59) The coating station 74 comprises a liquid metal melt at a relatively high temperature, but to ensure an even coating layer the fiber should have a temperature below the temperature of the melt. A blower or similar cooling means may be applied prior to the coating station 74 to blow cool air 70 to cool the PCF 73.

(60) Suitable melt temperatures are listed in table 1:

(61) TABLE-US-00001 TABLE 1 Metal Melt temperatures ( C.) Aluminum 660 Aluminum Alloy 400-671 Gold, 24K Pure 1063 Cobber 1063 Alloys of Cobber and/or Gold 550-1063

(62) The PCF 73 is passed through the metal melt at a desired velocity equal to the fiber drawing velocity. The thickness of the metal coating may be adjusted e.g. by adjusting the amount of melt in the melt chamber of the coating station 74 or the fiber velocity.

(63) From the coating station 74 the metal coated PCF is passed further to an additional coating station 75 for application of an additional coating, which in the shown embodiment is a polymer coating station 75. From the coating station 75 the coated PCF is passed to a concentricity monitor 77b and further to a curing station 76 where the polymer coating is cured by light.

(64) From the curing station 76 the coated PCF is passed further to an additional monitor 77 c for monitoring the fiber diameter. From the drawing wheel 79 the coated PCF is passed to spooling onto a spool 78.