Polarization-maintaining optical fiber, method of manufacturing polarization-maintaining optical-fiber connecting portion, and polarization-maintaining optical-fiber connecting portion

A method of manufacturing a polarization-maintaining optical-fiber connecting portion comprising:

• forming an elliptic core region at a facet of a polarization-maintaining optical fiber that includes a core region and a cladding region formed around the core region, the cladding region having a refractive index lower than a refractive index of the core region, wherein a refractive index profile of the core region is either one of a step shaped or a concave shaped, and the cladding region includes two holes formed in such a manner that a shortest distance W between the core region and a hole represented by W=L−(a+b)/2 is −0.1 μm to 0.2 μm, where L is a distance between centers of the core region and the hole, a is a core diameter of the core region, and b is a diameter of the hole, at locations in opposite to each other across the core region, by collapsing the holes at the facet,
• wherein a relation a≧−24.221×Δ3+51.718×Δ2−40.786×Δ+16.878 is satisfied and Δ is equal to or larger than 0.32%, where Δ is a relative refractive index difference in percent of the core region with respect to the cladding region, and a is a core diameter in μm of the core region,
• noncircularity of the core region is smaller than 10%
• a leakage loss at a wavelength of 1550 nm is 0.01 dB/km or less, and
• the cladding region, except the two holes, is formed of a uniform material;
• and
• fusion splicing the polarization-maintaining optical fiber and a stress-applying polarization-maintaining optical fiber after aligning polarization axes of the elliptic core region and the stress-applying polarization-maintaining optical fiber.

A method of manufacturing a polarization-maintaining optical-fiber connecting portion comprising:

• fusion splicing a polarization-maintaining optical fiber that includes a core region and a cladding region formed around the core region, the cladding region having a refractive index lower than a refractive index of the core region, wherein a refractive index profile of the core region is either one of a step shaped or a concave shaped, and the cladding region includes two holes formed in such a manner that a shortest distance W between the core region and a hole represented by W=L−(a+b)/2 is −0.1 μm to 0.2 μm, where L is a distance between centers of the core region and the hole, a is a core diameter of the core region, and b is a diameter of the hole, at locations in opposite to each other across the core region, and a stress-applying polarization-maintaining optical fiber after aligning polarization axes of the polarization-maintaining optical fiber and the stress-applying polarization-maintaining optical fiber in such a manner that the holes at a facet of the polarization-maintaining optical fiber are not collapsed,
• wherein a relation a ≧−24.221×Δ3+51.718×Δ2−40.786×Δ+16.878 is satisfied and Δ is equal to or larger than 0.32%, where Δ is a relative refractive index difference in percent of the core region with respect to the cladding region, and a is a core diameter in μm of the core region, and
• noncircularity of the core region is smaller than 10%,
• a leakage loss at a wavelength of 1550 nm is 0.01 dB/km or less, and
• the cladding region, except the two holes, is formed of a uniform material;
• and
• forming an elliptic core region at the facet by heating the facet of the fusion spliced polarization-maintaining optical fiber so that the holes are collapsed.

A polarization-maintaining optical-fiber connecting portion manufactured by a method comprising:

• forming an elliptic core region at a facet of a polarization-maintaining optical fiber that includes a core region and a cladding region formed around the core region, the cladding region having a refractive index lower than a refractive index of the core region, wherein a refractive index profile of the core region is either one of a step shaped or a concave shaped, and the cladding region includes two holes formed in such a manner that a shortest distance W between the core region and a hole represented by W=L−(a+b)/2 is −0.1 μm to 0.2 μm, where L is a distance between centers of the core region and the hole, a is a core diameter of the core region, and b is a diameter of the hole, at locations in opposite to each other across the core region, by collapsing the holes at the facet,
• wherein a relation a ≧−24.221×Δ3+51.718×Δ2−40.786×Δ+16.878 is satisfied and Δ is equal to or larger than 0.32%, where A is a relative refractive index difference in percent of the core region with respect to the cladding region, and a is a core diameter in μm of the core region, and
• noncircularity of the core region is smaller than 10%
• a leakage loss at a wavelength of 1550 nm is 0.01 dB/km or less, and
• the cladding region, except the two holes, is formed of a uniform material;
• and
• fusion splicing the polarization-maintaining optical fiber and a stress-applying polarization-maintaining optical fiber after aligning polarization axes of the elliptic core region and the stress-applying polarization-maintaining optical fiber, wherein
• in a fusion-splicing portion, a polarization crosstalk is equal to or smaller than −30 dB, and a connection loss is equal to or smaller than 1.0 dB.

A polarization-maintaining optical-fiber connecting portion manufactured by a method comprising:

• forming an elliptic core region at a facet of a polarization-maintaining optical fiber that includes a core region and a cladding region formed around the core region, the cladding region having a refractive index lower than a refractive index of the core region, wherein a refractive index profile of the core region is either one of a step shaped or a concave shaped, and the cladding region includes two holes formed in such a manner that a shortest distance W between the core region and a hole represented by W=L−(a+b)/2 is −0.1 μm to 0.2 μm where L is a distance between centers of the core region and the hole, a is a core diameter of the core region, and b is a diameter of the hole, at locations in opposite to each other across the core region, by collapsing the holes at the facet,
• wherein a relation a ≧−24.221×Δ3+51.718×Δ2−40.786×Δ+16.878 is satisfied and A is equal to or larger than 0.32%, where Δ is a relative refractive index difference in percent of the core region with respect to the cladding region, and a is a core diameter in μm of the core region, and
• noncircularity of the core region is smaller than 10%,
• a leakage loss at a wavelength of 1550 nm is 0.01 dB/km or less, and
• the cladding region, except the two holes, is formed of a uniform material;
• and
• fusion splicing the polarization-maintaining optical fiber and a stress-applying polarization-maintaining optical fiber after aligning polarization axes of the elliptic core region and the stress-applying polarization-maintaining optical fiber, wherein
• in a fusion-splicing portion, a polarization crosstalk is equal to or smaller than −30 dB, and a connection loss is equal to or smaller than 0.5 dB.