Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-08-27
2001-03-13
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06200711
ABSTRACT:
TECHNICAL ART
The present invention relates generally to a diffraction grating-forming phase mask and its fabrication method, and more particularly to a phase shift mask for using ultraviolet laser light to form a diffraction grating in an optical fiber used for optical communications, etc.
BACKGROUND ART
Optical fibers have brought about breakthroughs in the globalization of communications to make high-quality and large-capacity inter-oceanic telecommunications feasible. So far, it has been known that a Bragg diffraction grating is provided in an optical fiber by creating a periodic refractive index profile in an optical fiber core along the optical fiber, and the magnitude of reflectivity and the width of the wavelength characteristics of the diffraction grating are determined by the period and length and the magnitude of refractive index modulation of the diffraction grating, whereby the diffraction grating can be used as a wavelength division multiplexer for optical communications, a narrow-band yet high-reflection mirror used for lasers or sensors, a wavelength selection filter for removing extra laser wavelengths in fiber amplifiers, etc.
However, the wavelength at which the attenuation of a quartz optical fiber is minimized and which is suitable for long-distance communications is 1.55 &mgr;m. It is thus required hat the grating spacing be about 500 nm in order to allow the optical fiber diffraction grating to be used at this wavelength. At the beginning, it was considered difficult to make such a minute structure in an optical fiber core; that is, a Bragg diffraction grating was provided in the optical fiber core by a sophisticated process comprising a number of steps, e.g., side polishing, photoresist coating, holographic exposure, and reactive ion beam etching. For this reason, much fabrication time was needed, resulting in low yields.
In recent years, however, a method of fabricating a diffraction grating by irradiating an optical fiber with ultraviolet radiation to cause a refractive index change directly in an optical fiber core has been developed. This ultraviolet irradiation method has been steadily put to practical use with the advance of peripheral technologies, because of no need of any sophisticated processes.
Since the grating spacing is as fine as about 500 nm as mentioned above, this method using ultraviolet light is now carried out by a two-beam interference process, a writing-per-point process wherein single pulses from an excimer laser are focused to make diffraction grating surfaces one by one, an irradiation process using a phase mask having a grating, etc.
Regarding the two-beam interference process, a problem arises in conjunction with the quality of the beams in the lateral direction, i.e., spatial coherence. A problem with the writing-per-point process is on the other hand that strict step control of the submicron order is needed to focus light on a small point for writing light on many surfaces. Another problem arises in conjunction with processability.
To solve these problems, attention has focused on the irradiation process using a phase mask. According to this process, a phase mask
21
comprising a quartz substrate provided on one surface with grooves of given depth at a given pitch is irradiated with KrF excimer laser light (of 248 nm wavelength)
23
to give a refractive index change to a core
22
A of an optical fiber
22
, thereby producing a grating (diffraction grating), as shown in FIG.
7
(
a
). For a better understanding of an interference pattern
24
on the core
22
A, the pattern
24
is exaggerated in FIG.
7
(
a
). FIG.
7
(
b
) is a sectional view of the phase mask
21
, and FIG.
7
(
c
) is a partial top view corresponding to FIG.
7
(
b
). The phase mask
21
has a binary phase type of diffraction grating structure where the substrate is provided on one surface with grooves
26
having a depth D at a repetition pitch P, with a strip
27
substantially equal in width to each groove being provided between adjacent grooves
26
.
The depth of each groove
26
on the phase mask
21
(the difference in height between strip
27
and groove
26
) D is chosen such that the phase of the excimer laser light (beam)
23
that is exposure light is modulated by &pgr; radian. Thus, zero-order light (beam)
25
A is reduced to 5% or less by the phase mask
21
, and chief light (beam) leaving the mask
21
is divided into + first-order diffracted light
25
B containing at least 35% of diffracted light and − first-order diffracted light
25
C, so that the optical fiber
22
is irradiated with the + first-order diffracted light
25
B and − first-order diffracted light
25
C to produce an interference fringe at a given pitch, thereby providing a refractive index change at this pitch in the optical fiber
22
.
When the diffracting grating is fabricated in the optical fiber
22
by interference of + first-order light
25
B and − first-order light
25
C using such a phase mask
21
, deposition of foreign matters on the surface of the phase mask
21
causes defects in the diffraction grating exposed to light in the optical fiber
22
. This in turn gives rise to noises in the characteristic spectra of the diffraction grating.
When the optical fiber
22
is irradiated with ultraviolet radiation
25
B and
25
C according to such an arrangement as shown in FIG.
7
(
a
), the covering resin of the optical fiber
22
is sublimated due to exposure to ultraviolet radiation
25
B and
25
C, filling up the grooves
26
in the phase mask
21
. This offers a similar defect problem with respect to the diffraction grating exposed to light in the optical fiber.
A prior grating constituting such a diffraction grating-fabricating phase mask
21
has a reduced diffraction efficiency and so shows an about 3% transmittance with respect to zero-order light
25
A because the grooves
26
are of a rectangular wave shape in section, as shown in FIG.
7
(
b
). This zero-order light component
25
A makes noises, which in turn appear in the reflection spectra of the transferred optical waveguide diffraction grating.
DISCLOSURE OF THE INVENTION
In view of such problems with the prior art as mentioned above, one object of the present invention is to provide a diffraction grating-forming phase mask which, even when foreign matters, resins sublimated from an optical fiber, etc. are deposited on the surface thereof, makes it unlikely to introduce defects in the diffraction grating to be formed.
Another object of the invention is to provide a diffraction grating-fabricating phase mask which can reduce as much as possible a zero-order light component transmitting through the phase mask without subjected to diffraction, so that no noise can be introduced in the reflection spectra of an optical waveguide diffraction grating obtained by transfer, and a method of fabricating such a phase mask.
To achieve the aforesaid first object of the invention, the invention provides a diffraction grating-forming phase mask comprising a transparent substrate provided on one side with a grating form of repetitive groove-and-strip pattern for forming a diffraction grating with interference fringes of diffracted light through said repetitive pattern, characterized in that an optically transparent protective layer is applied over said one side with said repetitive groove-and-strip pattern formed thereon.
Preferably, the protective layer should comprise a sheet or film formed of any one of SiO
2
, CaF
2
, MgF
2
, ZrO, HfO, and fluorine resin.
Preferably, the protective layer should be a sheet or film having a thickness of 0.1 mm to 2 mm.
According to this embodiment of the invention, there is also provided a diffraction grating-forming phase mask comprising a transparent substrate provided on one side with a grating form of repetitive groove-and-strip pattern for forming a diffraction grating with interference fringes of diffracted light through said repetitive pattern, characterized in that an optically transparent material layer having a refractive index different from
Kurihara Masa-aki
Segawa Toshikazu
Dai Nippon Printing Co. Ltd.
Dellett and Walters
Rosasco S.
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