Gain flattening filter, and method for manufacturing gain flattening filter

Patent metadata

Field	Value
Publication number	US-12204140-B2
Application number	US-202117917493-A
Country	US
Kind code	B2
Filing date	Apr 5, 2021
Priority date	Apr 20, 2020
Publication date	Jan 21, 2025
Grant date	Jan 21, 2025

Abstract

A gain flattening filter includes a first optical fiber that has a core, a first cladding, and a second cladding and that has a uniform composition in a length direction; and a pair of second optical fibers fused to both ends of the first optical fiber. The first optical fiber has a first section in which a slanted refractive index grating is formed and a pair of second sections connecting both ends of the first section to the pair of second optical fibers. The first cladding contains a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength. In the core, a tensile stress remains in the first section. An average MFD of the second sections is larger than an average MFD of the second optical fibers and smaller than an average MFD of the first section.

First claim

The invention claimed is:

1. A gain flattening filter comprising:
a first optical fiber that includes a core, a first cladding surrounding the core from outside in a radial direction, and a second cladding surrounding the first cladding from outside in the radial direction and that has a uniform composition in a length direction; and
a pair of second optical fibers fused to both ends of the first optical fiber in the length direction,
wherein the first optical fiber has a first section in which a slanted refractive index grating is formed and a pair of second sections connecting both ends of the first section to the pair of second optical fibers,
the first cladding contains a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength,
in the core, a tensile stress remains in the first section, and
an average mode field diameter of the second sections is larger than an average mode field diameter of the second optical fibers and smaller than an average mode field diameter of the first section.

2. The gain flattening filter according to claim 1 ,
wherein a difference between the average mode field diameter of the second sections and the average mode field diameter of the second optical fibers is 1.3 μm or less.

3. The gain flattening filter according to claim 1 ,
wherein the core comprises silica-based glass and does not contain a dopant in such an amount that a relative refractive index difference of more than 0.01% with respect to pure silica is provided,
the first cladding contains fluorine, and
a fluorine concentration of the first cladding is more than 0.40% and 0.75% or less in terms of a change in a relative refractive index with respect to pure silica.

4. The gain flattening filter according to claim 1 ,
wherein, in the first section, a minimum value of the tensile stress remaining in the core is 1 MPa or more and 200 MPa or less.

5. The gain flattening filter according to claim 4 ,
wherein, in the second sections, a stress remaining in the core is a tensile stress lower than the tensile stress remaining in the core in the first section or a compressive stress.

6. The gain flattening filter according to claim 1 ,
wherein the average mode field diameter of the first section is 11.0 μm or more and 15.0 μm or less, and
the average mode field diameter of the second optical fibers is 9.0 μm or more and 10.9 μm or less.

7. The gain flattening filter according to claim 1 ,
wherein, in the first section,
the core has a diameter of 8.0 μm or more and 10.5 μm or less, and
a relative refractive index difference of the core with respect to the first cladding is 0.16% or more and 0.36% or less.

8. The gain flattening filter according to claim 1 ,
wherein the first cladding has an inner peripheral region in contact with the core, and
the photosensitive material is contained in the inner peripheral region.

9. The gain flattening filter according to claim 1 ,
wherein mode field diameters of the pair of second optical fibers increase toward the both ends of the first optical fiber.

10. A method for manufacturing a gain flattening filter, the method comprising:
fusing a pair of second optical fibers onto both ends of a first optical fiber in a length direction, the first optical fiber being formed with a slanted refractive index grating and including a core, a first cladding surrounding the core from outside in a radial direction and containing a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength, and a second cladding surrounding the first cladding from outside in the radial direction, and the pair of second optical fibers each having a mode field diameter smaller than a mode field diameter of the first optical fiber,
wherein the fusing includes heating the first optical fiber and the pair of second optical fibers such that temperatures of the pair of second optical fibers are higher than temperatures of both the ends of the first optical fiber, and a tensile stress remaining in the core is released at both end portions of the first optical fiber to reduce average mode field diameters at both the end portions of the first optical fiber.

11. The method for manufacturing a gain flattening filter according to claim 10 ,
wherein, in the fusing, a fusing temperature is 4000° C. or more and 6500° C. or less, and a fusing time is 0.1 seconds or more and 100 seconds or less.

12. The method for manufacturing a gain flattening filter according to claim 10 , the method further comprising:
heating an end portion of each of the pair of second optical fibers to be fused to both the ends of the first optical fiber to thereby increase a mode field diameter of the end portion before the fusing, the end portion being disposed on a side to be fused to the first optical fiber.

13. The gain flattening filter according to claim 1 ,
wherein a boundary between the first section and the second sections of the first optical fiber is defined at a position where the average MFD of the first section is reduced by a preset amount.

Assignees

• Sumitomo Electric Industries
• Sumitomo Electric Optifrontier Co Ltd

Inventors

• Nagano Shigehiro
• Hasegawa Takemi
• Kinugasa Jun
• Masui Kazuaki

Classifications

• G02B6/03622

  having 2 layers only · CPC title

• G02B6/02052

  comprising optical elements other than gratings, e.g. filters (comprising gratings G02B6/02057) · CPC title

• G02B6/29317

  Light guides of the optical fibre type · CPC title

• G02B6/255

  Splicing of light guides, e.g. by fusion or bonding · CPC title

• H01S2301/04

  Gain spectral shaping, flattening · CPC title