Method of making gratings and phase masks for fiber grating fabrication
View Patent ↗In accordance with the invention, the fabrication of a grating phase mask is improved by providing a multiple-scan exposure which can provide an accumulated exposure that is effectively phase modulated or modulated rapidly in amplitude. Applicants have determined that exposure scans can be chosen so that each is modulated in amplitude and without modulation in phase, but the accumulated exposure of the multiple scans is modulated in phase and/or modulated in amplitude. The improved method can be used to make phase masks for fabrication of sophisticated fiber gratings such as superstructure gratings.
1. A method for fabricating a phase mask comprising the steps of:
supplying a substrate;
coating the substrate with resist;
exposing the resist to a plurality of spatial holographic illumination steps according to the expanded terms of an equation describing an optical grating, the illumination steps comprising a single beam exposure, a double beam exposure through a mask and a double beam exposure with the mask shifted;
developing the resist; and
etching the substrate.
2. The method of claim 1 wherein the plurality of illumination steps produces an accumulated exposure that is modulated in phase and/or modulated in amplitude.
3. The method of fabricating a phase mask according to claim 2 , wherein the three terms of the equation are:
D
(
x
)
=
D
0
+
A
0
[
1
2
-
1
2
cos
(
2
Φ
(
x
)
-
π
4
)
]
+
A
0
cos
(
2
Φ
(
x
)
)
sin
2
(
2
π
x
Λ
0
)
+
A
0
cos
(
2
Φ
(
x
)
-
π
2
)
sin
2
(
2
π
x
Λ
0
+
π
4
)
,
where D is the illumination dosage, A is the groove depth, λ is the grating period, and Φ is the phase.
4. The method of fabricating a phase mask according to claim 1 , wherein the resist is exposed to five holographic illuminations according to five expanded terms of an equation describing the optical grating.
5. The method of fabricating a phase mask according to claim 4 , wherein the five terms of the equation are:
D
(
x
)
=
D
0
-
A
0
[
3
2
+
1
2
cos
(
2
Φ
(
x
)
-
π
4
)
]
+
2
A
0
cos
2
(
Φ
(
x
)
)
sin
2
(
2
π
x
Λ
0
)
+
2
A
0
cos
2
(
Φ
(
x
)
-
π
4
)
sin
2
(
2
π
x
Λ
0
+
π
4
)
+
A
0
sin
2
(
2
π
x
Λ
0
-
π
2
)
+
A
0
sin
2
(
2
π
x
Λ
0
-
π
4
)
,
where D is the illumination dosage, A is the groove depth, A is the grating period, and Φ is the phase.
6. The method of claim 4 wherein the phase shifts π/4,−π/4,−π/2 are achieved by shifting the mask substrate with respect to the holographic pattern.
7. The method of fabricating a phase mask according to claim 1 wherein supplying the substrate comprises supplying a quartz substrate.