Photosensitive optical waveguide
View Patent ↗An optical waveguide suitable for a slanted Bragg grating (SBG) to be written thereinto has a refractive index profile defining a core and a cladding and a specific photosensitivity profile. This photosensitivity profile W(r) is such that, for a given SBG writing angle θ, the coupling of the fundamental mode into itself as counterpropagation in the waveguide, K(θ), is substantially zero and such that its derivative K′(θ) with respect to the angle θ is also substantially equal to zero. The zero derivative, and also the zero coupling of the fundamental mode as a function of the angle, means that a widened reflection pocket about the grating write angle is guaranteed.
1. A photosensitive optical waveguide, comprising:
a core;
a cladding;
a refractive index defining the core and the cladding;
a slanted Bragg grating, SBG, with a given slant angle, θ, written into the optical waveguide; and
a photosensitivity profile, W(r), such that:
k
(
θ
)
=
∫
E
core
2
(
r
)
·
J
0
(
2
π
·
r
·
sin
(
θ
)
Λ
)
·
W
(
r
)
r
·
ⅆ
r
∂
k
(
θ
)
∂
θ
=
k
(
θ
)
=
-
∫
E
core
2
(
r
)
·
J
1
(
2
π
·
r
·
sin
(
θ
)
Λ
)
·
W
(
r
)
·
2
π
Λ
cos
(
θ
)
·
r
2
·
ⅆ
r
where: r is the radius of the fiber
θ is the slant angle of the grating,
E core is the electric field of the fundamental mode,
W(r) is the photosensitivity profile,
Λ is the period of the Bragg grating, and
J 0 is the 1st order Bessel function; and
wherein k(θ) is a minimum and k′(θ) is a minimum for the given SBG slant angle, θ, and wherein W(r) has at least one photosensitive step, and a radial position of the photosensitive step being independent of a radial position of an index step between the core and the cladding of the waveguide.
2. The waveguide as claimed in claim 1 , wherein the photosensitivity profile W(r) has at least one step located within the cladding of the waveguide.
3. The waveguide as claimed in claim 1 , wherein the photosensitivity profile W(r) has at least one step located within the core of the waveguide.
4. The waveguide as claimed in claim 1 , wherein the photosensitivity profile W(r) is suitable for writing a slanted Bragg grating (SBG) with a slant angle (θ) of between 1° and 4.5°.
5. The waveguide as claimed in claim 1 , wherein the photosensitivity profile W(r) is such that the minimum in the coupling of the fundamental mode into itself as counterpropagation in the wave guide (K(θ)) has an angular width (Δθ) of between 0.30° and 0.60° about the given angle (θ) into which the Bragg grating (SBG) is written.
6. An optical filter comprising at least one slanted Bragg grating (SBG) written into a portion of an optical waveguide as claimed in claim 1 .
7. The optical filter as claimed in claim 6 , wherein the slanted Bragg grating (SBG) has a slant angle of between 10° and 4.5°.
8. The optical filter as claimed in claim 6 , wherein the slanted Bragg grating (SBG) has a variable period (Λ).
9. An optical transmission system that includes a gain flattening filter comprising a concatenation of optical filters as claimed in claim 6 .
10. The photosensitive optical waveguide according to claim 1 , wherein k(θ) is zero or substantially equal to zero and k ′(θ) is zero or substantially equal to zero.
11. The photosensitive optical waveguide according to claim 1 , wherein the refractive index profile is uncorrelated with the photosensitivity profile and wherein there is no step in the photosensitivity profile at a radial position of the waveguide having a refractive index step.
12. The photosensitive optical waveguide according to claim 11 , wherein the photosensitivity profile is independent of the refractive index change between the core and the cladding.