Few mode optical fiber links for space division multiplexing having trenched fibers with high leak losses for leaky modes and low bend losses
The invention relates to an optical link comprising N optical fibers, with N≥2. Each optical fiber comprises an optical core and an optical cladding surrounding the optical core, the optical core having a single α i graded-index profile with α i ≥1, and the optical core having a radius R 1i , where i E [1; N] is an index designating said optical fiber. Said optical cladding comprises a region of depressed refractive index n trenchi , called a trench, surrounding the optical core. According to embodiments of the invention, for all optical fibers in said link, said optical core radius R 1i and said length L i are chosen such that R 1i ≥13.5 μm and so as to satisfy a criterion C of quality. Thus, the invention provides a few-mode optical fiber link, which allow guiding an increased number of LP modes as compared to prior art FMF links, while reaching low Differential Mode Group Delay.
1. An optical link comprising N optical fibers, with N≥2, N being an integer,
each optical fiber comprising an optical core and an optical cladding surrounding the optical core, the optical core having a single α i graded-index profile with α i ≥1, x i being a non-dimensional parameter defining an index profile shape of the optical core, and the optical core having a radius R 1i , expressed in microns (μm), and a maximal refractive index n 0i , where iϵ[[1;N]] is an index designating said optical fiber,
wherein the optical cladding having a refractive index n Cli at an outer edge of the optical cladding,
wherein said optical cladding comprises a region of depressed refractive index n trenchi being a trench, surrounding the optical core, said trench having an inner radius R 2i , with R 2i ≥R 1i , and an outer radius R 3i , with R 3i >R 2i ,
said optical link having an average optical core radius R 1link satisfying a criterion C of quality of optical communications defined by the following equation:
C
=
10
·
Max
|
DMGD
link
|
(
R
1
link
2
·
Dn
1
link
)
3
where DMGD link is the Differential Mode Group Delay between two guided modes in said optical link,
where Max|DMGD link | is expressed in ps/km and is an absolute maximum value of Differential Mode Group Delay between any combination of guided modes in said optical link,
where
R
1
link
=
∑
i
=
1
N
R
1
i
L
i
∑
i
=
1
N
L
i
with L i a length of optical fiber i in said link,
and where
Dn
1
link
=
∑
i
=
1
N
Dn
1
i
L
i
∑
i
=
1
N
L
i
with Dn 1i =n 0i −n Cli is a core-cladding index difference for optical fiber t, at λ=λ C , where λ c is a central transmission wavelength of an operating band for the optical fiber,
and for at least one optical fiber i in said link, said optical core radius R 1i is chosen such that R 1i ≥13.5 μm and for all optical fibers iϵ[[1;N] in said link, said length L i are chosen such that C≤18, and wherein
at least one of said optical fibers has trench parameters 55≤1000·|(R 3i −R 2i )·Dn 3i ·(R 1i 2 ·Dn 1i )|≤150 with R 1i , R 2i , and R 3i each being expressed in microns (μm), and
where Dn 3i =n trenchi −n Cli is the trench-cladding index difference at λ=λ C .
2. The optical link according to claim 1 , wherein Dn 3 ≤−3·10 3 .
3. The optical link according to claim 1 , wherein at least one of said optical fibers has an optical core radius R 1i and an α i -value of said graded index profile such that:
C
=
10
·
Max
|
DMGD
i
|
(
R
1
i
2
·
Dn
1
i
)
3
≤
18
where DMGD i is the Differential Mode Group Delay between two guided modes in said optical fiber,
where Max|DMGD i | is the absolute maximum value of DMGD between any combination of guided modes in said optical fiber,
and where Dn 1i −n 0i −n Cli is the core-cladding index difference at λ=λ C for said optical fiber.
4. The optical link according to claim 3 , wherein said optical fiber is R 1i ≤20 μm.
5. The optical link according to claim 1 , wherein the optical link guides 4 to 16 LP modes.
6. The optical link according to claim 1 , wherein the optical link guides 6 to 16 LP modes.
7. The optical link according to claim 1 , wherein for all optical fibers iϵ[[1;N]] in said link, said lengths L i are chosen so as to minimize Max|DMGD link | on said link.
8. The optical link according to claim 1 , wherein at least two optical fibers in said link have DMGD i showing opposite signs for at least one mode guided by said optical fibers, where DMGD i is the Differential Mode Group Delay between said one mode and any other guided mode in optical fiber i.
9. The optical link according to claim 1 , wherein, for at least one of said fibers iϵ[[1;N]] in said optical link, said optical core has a minimal refractive index n 1i =n Cli , and said optical cladding also comprises an inner cladding layer directly surrounding said optical core, with an inner radius R 1i and an outer radius R 2i ≥R 1i , said inner cladding layer having a constant refractive index n 2i , such that n 2i ≠n Cl i and n 2i ≥n trenchi .
10. The optical link according to claim 1 , wherein, for at least one of said fibers iϵ[[1;N]] in said optical link, said optical core has a minimal refractive index n 1i ≠n Cli , and said optical cladding also comprises an inner cladding layer directly surrounding said optical core, with an inner radius R 1i and an outer radius R 2i ≥R 1i , said inner cladding layer having a constant refractive index n 2i such that n 2i =n 1i and n 2i >n trenchi .
11. The optical link according to claim 1 , wherein, for at least one of said fibers iϵ[[1;N]] in said optical link, said optical core has a minimal refractive index that equals n Cli , said optical cladding also comprises an inner cladding layer directly surrounding said optical core, with an inner radius R 1i and an outer radius R 2i ≥R 1i , said inner cladding layer being an extension of said single graded-index profile of said optical core, and said inner cladding layer has a minimal refractive index n 1i =n trenchi .
12. An optical system comprising the optical link of claim 1 .