EP-4310560-B1 - REDUCING LIGHT-INDUCED LOSS IN OPTICAL FIBRE

Inventors

• LYNGSØ, Jens
• SMITH, CAMERON
• HARPØTH, Anders
• JACKOBSEN, Christian
• ALKESKJOLD, Thomas Tanggard
• KUBAT, Irnis

Dates

Publication Date

20260513

Application Date

20180724

Claims (15)

1. A passive nonlinear microstructured optical fiber (104) having a core region and a cladding region including longitudinally extending air-holes disposed about the cladding region, wherein the microstructured optical fiber is silica-based and at least a part of the core region of the microstructured optical fiber is doped with a dopant selected to reduce light-induced non-bridging oxygen hole centre loss in the microstructured optical fiber, where the dopant is provided in a doped region of the core region, and the number of dopant atoms in the doped region is greater than 0.05% of the number of silicon atoms in the doped region, and where the core region of the microstructured optical fiber further comprises hydrogen and/or deuterium.
2. The microstructured optical fiber of claim 1, wherein the core region has a core diameter of less than 5 µm.
3. The microstructured optical fiber of claim 1 or 2, wherein the microstructured optical fiber is a large mode area fiber.
4. The microstructured optical fiber of claim 1, 2 or 3, wherein the microstructured optical fiber is a supercontinuum optical fiber.
5. The microstructured optical fiber of any one of claims 1 to 4, wherein the number of dopant atoms in the doped region is greater than 1% of the number of silicon atoms in the doped region.
6. The microstructured optical fiber of any one of claims 1 to 4, wherein the number of dopant atoms in the doped region is greater than 3% of the number of silicon atoms in the doped region.
7. The microstructured optical fiber of any one of claims 1 to 6, wherein the core region has a core diameter of less than 4 µm.
8. The microstructured optical fiber of any one of claims 1 to 6, wherein the core region has a core diameter of less than 3 µm.
9. The microstructured optical fiber of any one of claims 1 to 8, wherein the dopant comprises aluminium, cerium, phosphorus or boron.
10. The microstructured optical fiber of any one of claims 1 to 9, wherein at least a part of the doped region is co-doped with a further dopant selected to control the refractive index of the core region to compensate for at least part of the change in refractive index that would otherwise be caused by the presence of the dopant.
11. The microstructured optical fiber of claim 10, wherein the further dopant comprises fluorine or cerium.
12. The microstructured optical fiber of claim 11, wherein the core region is doped with aluminium and fluorine, cerium and fluorine, phosphorus and fluorine, boron and fluorine, or aluminium and cerium.
13. The microstructured optical fiber of any one of claims 1 to 12, wherein the number of dopant atoms in the doped region is greater than 0.1% of the number of silicon atoms in the doped region, such as greater than 1%, such as greater than 3%.
14. The microstructured optical fiber of any one of claims 1 to 13, wherein the microstructured optical fiber contains no gain material providing optical gain when pumped at an appropriate pump wavelength.
15. The microstructured optical fiber of any one of claims 1 to 14, wherein the microstructured optical fiber is free of ytterbium and erbium.

Description

Field This specification relates to reducing light-induced loss in optical fibre. Background At high power levels, photodarkening can occur in optical fibre due to light-induced defects, particularly in the visible region of the electromagnetic spectrum. One mechanism for photodarkening is the creation of non-bridging oxygen hole centre (NBOHC) defects in the optical fibre. NBOHCs create significant loss in the visible wavelength region of the spectrum centred at approximately 615 nm. Since NBOHCs are created by light propagating through the optical fibre, the resulting loss increases the more the optical fibre is used, thus limiting its useful lifetime. US 2010 061415 A1 relates to reduce photo darkening in rare earth doped optical amplifier silica glass material by choice of silica glass material composition. WO 2007/110081 A1 describes a high power amplifier silica glass material is provided wherein photo darkening due to high optical flux is reduced considerably. Wang Chao et al., "Properties of Non-Bridging Oxygen Hole Centers Defects in Yb3+/Al3+ Co-Doped Photonic Crystal Fiber by Using Powder Melting Technology", Journal of Lightwave Technology, Vol 31 No 17 1 September 2013 relates to using powder melting technology to prepare Yb3+/AL3+ co-doped silica glass. WO 03/079074 relates to a nonlinear optical fibre having a small core and special dispersion properties, a method of producing such a fibre, and use of such a fibre. WO 2016/095923 A1 relates to a photonic crystal fiber (PCF), a method of producing the PCF and a supercontinuum light source comprising such PCF, microstructured optical fiber and to a source of optical supercontinuum radiation. Summary This specification provides a supercontinuum source, comprising a pump source and a supercontinuum generator configured for receiving light derived from the pump source and for generating supercontinuum light. The supercontinuum generator may comprise a nonlinear microstructured optical fibre having a core region comprising silica. At least a part of the core region is doped with a dopant selected to reduce light-induced non-bridging oxygen hole centre loss in the nonlinear microstructured optical fibre. The concentration of the dopant is sufficient to provide resistance against degradation of the supercontinuum power in the visible region of the spectrum. Note that in the art, the term "doped" is used to distinguish from "undoped" fibres in which the quantity of dopant is negligible (e.g. at a level below 1000 ppm). The dopant may comprise aluminium. Alternatively, or in addition, the dopant may comprise cerium, phosphorus and/or boron. By reducing degradation of the supercontinuum power in the visible region of the spectrum, the useful lifetime of the supercontinuum source is extended for applications in which significant power in the visible region of the spectrum is desired. The concentration of the dopant may be sufficient to improve the transmission loss at 615 nm after 1200 hours of pumping (compared to the situation in which the fibre is not doped with the dopant and pumped for the same period). The improvement (i.e. reduction) in transmission loss (e.g. after 1200 hours pumping) may be more than 0.1 dB/ km, more than 0.5 dB / km, more than 1 dB/km, more than 5 dB/km, more than 10 dB/km, more than 50 dB/km, more than 100 dB/km, more than 200 dB/km, more than 500 dB/km, or more than 600 dB/km. The dopant may be provided in a doped region of the core region, which may be the whole core region, a single part of the core region (e.g. a single longitudinally-extending part of the core region), or a plurality of parts of the core region (e.g. separate parts which are not contiguous with one another). In various embodiments, the concentration of the dopant in the doped region is greater than 1000 ppm. The number of dopant atoms in the doped region may be greater than 0.05% of the number of silicon atoms in the doped region, greater than 0.1% of the number of silicon atoms in the doped region, greater than 1% of the number of silicon atoms in the doped region, greater than 3% of the number of silicon atoms in the doped region, greater than 8% of the number of silicon atoms in the doped region, or greater than 20% of the number of silicon atoms in the doped region. In some embodiments, there is a "trade-off" for the reduction of degradation of the supercontinuum power that occurs in the visible region of the spectrum; in particular the presence of the dopant may also increase loss in the blue and/or ultraviolet (UV) regions of the spectrum. In some embodiments the transmission loss of the doped optical fibre at 400 nm may be greater than 600 dB/km, greater than 750 dB/km, greater than 1000 dB/km, greater than 1500 dB/km, greater than 2000 dB/km, greater than 2,500 dB/km, or greater than 3000 dB/km. The core region may have a generally circular cross section. The core diameter may be less than 10 µm (e.g. around 8 µm), less than 5 µm, less than 4 µm,