Optical Fiber with Microstructured Core

Abstract

A micro-structured optical fiber suitable for supercontinuum generation, a preform therefor, and a method of production thereof and a supercontinuum light source. The optical fiber includes a core microstructured length section L.sub.cm, which includes a micro-structured core region having at least a first region forming a central part of the core region and a second region surrounding the first region; and a cladding surrounding the core region; wherein the first and second regions are of a first and second silica material, respectively, which differs with respect to composition, and wherein the core region in at least a length part of the core microstructured length section L.sub.cm has a cross sectional diameter D.sub.cm perpendicular to the longitudinal axis which is about 8 μm or less.

Claims

1-28. (canceled)

29. A micro-structured optical fiber for supercontinuum generation and having a length and a longitudinal axis along its length, said optical fiber comprising a core microstructured length section L.sub.cm, said core microstructured length section L.sub.cm comprises: a micro-structured core region comprising at least a first region forming a central part of the core region and a second region surrounding the first region; and a cladding surrounding the core region; wherein the first and second regions are of a first and second silica material, respectively, and wherein the first silica material has a different concentration of at least one of hydroxyl (OH) and chlorine (Cl) than the second silica material.

30. The micro-structured optical fiber according to claim 29, wherein the difference in concentration of the at least one of OH and Cl is at least 100% based on the concentration in the second silica material.

31. The micro-structured optical fiber according to claim 29, wherein the first silica material has a higher concentration of OH than the second silica material and/or the first silica material has a lower concentration of Cl than the second silica material.

32. The micro-structured optical fiber according to claim 29, and wherein the core region in at least a length part of the core microstructured length section L.sub.cm has a cross sectional diameter D.sub.cm perpendicular to the longitudinal axis which is about 8 μm or less.

33. The micro-structured optical fiber according to claim 29, and wherein at least the first silica material is essentially germanium free.

34. The micro-structured optical fiber according to claim 29, wherein the first region of the micro-structured core region has a higher refractive index than the second region of the micro-structured core region.

35. The micro-structured optical fiber according to claim 29, wherein the concentration of OH in the first silica material is higher than about 100 ppm and the concentration of OH in the second silica material is lower than about 10 ppm.

36. The micro-structured optical fiber according to claim 29, wherein the concentration of Cl in the first silica material is lower than about 1000 ppm.

37. The micro-structured optical fiber according to claim 29, wherein the micro-structured core region is solid.

38. The micro-structured optical fiber according to claim 29, wherein the first region and the second region constitute said micro-structured core region.

39. The micro-structured optical fiber according to claim 29, wherein the micro-structured core region comprises a third region, said third region surrounds at least the first region.

40. The micro-structured optical fiber according to claim 29, wherein the second region has a larger area than an area of the first region of the micro-structured core region seen in cross section perpendicular to the longitudinal axis.

41. The micro-structured optical fiber according to claim 29, wherein said core microstructured length section L.sub.cm has a zero dispersion wavelength of about 2200 nm or less and said core microstructured length section L.sub.cm is single mode at a wavelength of about 1064 nm.

42. The micro-structured optical fiber according to claim 29, wherein the core microstructured length section L.sub.cm is uniform along its length.

43. The micro-structured optical fiber according to claim 29, wherein the core microstructured length section L.sub.cm has a tapered length section longer than about 10 cm.

44. The micro-structured optical fiber according to claim 29, wherein the second silica material is a fluorine-doped silica material.

45. A method for the fabrication of a micro-structured optical fiber for supercontinuum generation wherein the method comprising: manufacturing a core cane for a core microstructured length section L.sub.cm of the fiber by stacking at least one of a first type of solid rods made of a first silica material and a plurality of a second type of solid rods made of second silica material such that; at least one rod of the first type of solids rods is located in the center of the stack and rods of the second type surround the first type of rod at the center; and arranging the stack of rods in a sleeve; and drawing the sleeved stack to a core cane; manufacturing a preform from said core cane by stacking a plurality of layers of silica rods and/or silica tubes to fully surround said core cane; and arranging the stack of core cane, rod and/or tubes in a sleeve; and drawing the sleeved stack to a preform; and manufacturing said core microstructured length section L.sub.cm by drawing, wherein the first silica material has a different concentration of at least one of hydroxyl (OH) and chlorine (Cl) than the second silica material.

46. A supercontinuum light source comprising a pump light source and a nonlinear micro-structured optical fiber, said nonlinear microstructured optical fiber comprises a core microstructured length section L.sub.cm having a microstructured core region and said pump light source being operatively connected to said nonlinear micro-structured optical fiber to feed pulses of light into said nonlinear micro-structured optical fiber said nonlinear micro-structured optical fiber comprises a core microstructured length section L.sub.cm, said core microstructured length section L.sub.cm comprises: a micro-structured core region comprising at least a first region of a first a silica material forming a central part of the core region and a second region of a second silica material surrounding the first region; and a cladding surrounding the core region; wherein the first silica material has a different concentration of at least one of hydroxyl (OH) and chlorine (Cl) than the second silica material.

47. The supercontinuum light source of claim 46, wherein said supercontinuum light source is configured for delivering a supercontinuum signal spanning at least 300 nm with a spectral density of at least about 1 nW/nm, said pump light source is operatively connected to said nonlinear micro-structured optical fiber to feed pulses of light with a pulse length in the range of 1-100 ps and a peak power of at least about 5 kW into said nonlinear micro-structured optical fiber.

48. The supercontinuum light source of claim 46, wherein the concentration of OH in the first silica is higher than about 100 ppm, the concentration of OH in the second silica is lower than about 10 ppm and the second core region fully surrounds the first core region.

49. A preform for a micro-structured optical fiber for supercontinuum generation, the preform comprises a core cane surrounded by a cladding preform structure composed of fused silica rods or silica tubes, wherein the core cane comprises at least a first region forming a central part of the core cane and a second region surrounding the first region; and wherein the first and second regions are of a first and second silica material, respectively, wherein the first silica material has a different concentration of at least one of hydroxyl (OH) and chlorine (Cl) than the second silica material.

50. The preform of claim 49, wherein the cladding preform structure is composed of fused silica rods and silica tubes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0113] The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings.

[0114] FIG. 1A shows a cross-section of a stack of rods for a prior art microstructured optical fiber, where the cladding is microstructured and the core is solid.

[0115] FIG. 1B shows a cross-section of a stack of rods for another prior art microstructured optical fiber of the invention, where the cladding is microstructured and the core is microstructured.

[0116] FIG. 2 shows a cross-section of a stack of rods for an embodiment of a microstructured optical fiber of the invention, where the cladding is microstructured and the core is microstructured.

[0117] FIG. 3 shows a cross-section of a stack of rods for another embodiment of a microstructured optical fiber of the invention, where the cladding is microstructured and the core is microstructured.

[0118] FIG. 4 shows the transmission loss of a prior art optical fiber and the resulting reduction in beam quality.

DETAILED DESCRIPTION

[0119] In the following description, reference is made to the accompanying figure, which shows by way of illustration how the invention may be practiced.

[0120] FIG. 1A shows cross-sections of arrangement of rods for manufacturing a preform which can be used for the manufacture of prior art micro-structured optical fibers.

[0121] In FIG. 1A the stack 101 contains a centrally arranged rod 102, which often is of un-doped silica, surrounded by six tubes 103. When such a stack is drawn to a cane the interstitial spaces between the rod 102 and the tubes often collapse while the air holes of the tubes are kept open by applying a pressure thereto. There will then be an index-step between the part of the drawn cane formed by the silica material and the surrounding region containing the air holes.

[0122] FIG. 1B shows cross-sections of arrangement of rods for manufacturing a preform which can be used for the manufacture of prior art micro-structured optical fibers with microstructured core.

[0123] In FIG. 1B the stack 105 contains six tubes 103 surrounding a central region. The central region is formed by a single germanium-doped silica rod 106 surrounded by six un-doped silica rods 107. The germanium doped part both provides an additional index step in the central region and allows for the inscription of e.g. Bragg gratings in the core using UV exposure techniques. The prior art optical fibers produced from such preform usually have large core as e.g. described in WO WO2007107164. Such prior art optical fibers are not suitable for supercontinuum generation.

[0124] FIG. 2 shows a cross-section of arrangement of rods for manufacturing a preform which can be used for manufacture of a micro-structured optical fiber according to an embodiment of this invention.

[0125] Similar to the rod arrangement of FIG. 1B the stack 210 contains six tubes 203 surrounding a central region. However, in FIG. 2 the central region is formed by a single first rod of a first type 211 surrounded by six rods of a second type 212 where the first and second type of rods are made of a first and second silica material, respectively, which differs with respect to composition and both are passive germanium-free silica materials.

[0126] The first type of rod can advantageously be a Heraeus F110 silica rod having a relatively high concentration of OH and a relatively low concentration of Cl. The second type of rod can advantageously be a rod typically used for telecom applications, such as a Heraeus F500 rod made of silica having a relatively low concentration of OH.sup.− and thus a low transmission-loss.

[0127] Some candidates for the different types of rods are listed here:

TABLE-US-00001 Glass (Hereaus) OH (ppm) Cl (ppm) Core region F300HQ <1 <2500 Second F500HQ <0.1 <2500 Second F320-08HQ <1 <200 Potentially both as first and second F100 Typical 700 Typical 200-300 First F110 Typical 400 Typical 200-300 First

[0128] FIG. 3 shows a cross-section of arrangement of rods for manufacturing a preform which can be used for manufacture of a micro-structured optical fiber according to an embodiment of this invention.

[0129] Similar to the core-cane of FIG. 2 the stack 315 contains six tubes 303 surrounding a central region formed by a single first rod of a first type 211 surrounded by six rods of a second type 212 where the first and second type of rods are made of a first and second silica material, respectively, which differs with respect to composition and both are passive germanium-free silica materials. However, in FIG. 3 the second silica material is fluoride doped silica glass while the first silica material is un-doped silica glass. This provides an index step in the central part of the core of a fiber drawn from this cane. In the case of some photo-induced lowering of the refractive index in the central part of the core region in response to a high-intensity seed light there would still not be a depression in the central part of the core which accordingly would be able to support a Gaussian mode for more hours of operation.

[0130] A super continuum generation system based on a fiber according to this application can for example be realized by using a pump source configured according to a Master Oscillator Power Amplifier (MOPA) system where the seed pulses have a wavelength around 1030 nm or 1064 nm, have a pulse length of 7-8 ps, a peak power of 10-15 kW and a Repetition Rate of 80 MHz. In an embodiment the MOPA comprises means for reducing the repetition rate, such as e.g. a pulse picker.

[0131] In an embodiment a non-linear fiber for a supercontinuum source is a hybrid non-linear fiber comprising at least two sections, where the first section which provides the input length section for the seed beam is the microstructured length section L.sub.cm and the second section is a further fiber length section Lf in form of a conventional fiber. Preferably the length of the microstructured length section L.sub.cm is larger than a length L.sub.deg over which photo induced degradation would have occurred without the microstructured core provided by the optical fiber of the present invention.

[0132] Although embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention.

[0133] In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

[0134] It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0135] FIG. 4 shows the transmission loss of a prior art optical fiber and the resulting reduction in beam quality. From the figure it can be seen that where the transmission loss is high the beam quality becomes lower and where the transmission loss is very high in the 500-600 nm range the beam quality is so poor that it is clear that the core of the prior art of is not capable of supporting a Gaussian mode. Since the invention solves the problem of increase transmission loss, the microstructured optical fiber of the invention is able to support a Gaussian mode for more hours of operation.