Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-04-13
2001-04-10
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C385S124000
Reexamination Certificate
active
06214495
ABSTRACT:
ART FIELD
The present invention relates generally to a phase mask for processing optical fibers and its fabrication method, and more specifically to a phase mask for fabricating a diffraction grating in an optical fiber used for optical communications using ultraviolet laser light, and a method of fabricating the same.
BACKGROUND ART
Optical fibers have achieved global communication-technology breakthroughs, and enabled high-quality yet largecapacity transoceanic telecommunications. So far, it has been known that a Bragg diffraction grating is prepared in an optical fiber by providing a periodic index profile in a core along the optical fiber. By determining the magnitude of reflectance and the width of frequency characteristics of the diffraction grating depending on the period and length, and the magnitude of refractive index modulation thereof, the diffraction grating is used for wavelength division multiplexers for optical communication purposes, narrow-band high-reflecting mirrors used with lasers or sensors, wavelength selective filters for filtering out extra wavelengths in fiber amplifiers, etc.
However, the wavelength where quartz optical fibers show a minimum attenuation and which is suitable for long-haul communication systems is 1.55 &mgr;m. To use an optical fiber diffraction grating at this wavelength, a grating spacing of about 500 nm must be needed. Initially, to make such a fine structure in a core has been considered to be in itself difficult. Accordingly, some complicated process steps comprising side polishing, photoresist step, holography exposure, and reactive ion beam etching are used to make a Bragg diffraction grating in an optical fiber core. Much time is needed for such processes, resulting in limited yields.
In recent years, however, a method of making a diffraction grating by irradiating an optical fiber with ultraviolet radiation for the direct change of a refractive index in a core has been known in the art. This ultraviolet irradiation method has been steadily put to actual use with the progress of peripheral technologies due to no need of complex processes.
This method using ultraviolet light is now carried out by some processes such as an interference process comprising interference of two ray bundles, a writing-per-point process wherein a diffraction grating surface is formed one by one by focusing of a single pulse from an excimer laser), and an irradiation process using a phase mask having a grating, because the grating spacing is as fine as about 500 nm as mentioned above.
The interference process comprising interference of two ray bundles offers a problem in connection with the quality of lateral beams, i.e., spatial coherence, and the writing-per-point process have some operation problems such as the need of submicron careful step control, and the necessity of writing of many surfaces with fine pencils of light.
To address the above problems, an irradiation method using a phase mask has now received attention. As shown in FIG.
7
(
a
), this method uses a phase shift mask
21
obtained by providing grooves on one side of a quartz substrate at a given pitch and a given depth. The phase shift mask
21
is then irradiated with KrF excimer laser light
23
(of 248-nm wavelength) to impart a refractive index change directly to a core
22
A of an optical fiber
22
, thereby forming a grating. It is here to be noted that reference numeral
22
B stands for a cladding of the optical fiber
22
. In FIG.
7
(
a
), an interference pattern
24
in the core
22
A is illustrated on an enlarged scale for a better illustration thereof. FIG.
7
(
b
) is a sectional view of the phase mask
21
, and FIG.
7
(
c
) is a view illustrating a part of the upper surface of the phase mask
21
. The phase mask
21
has a binary phase type diffraction grating structure wherein grooves
26
, each having a depth D, are provided on one surface thereof at a repetitive pitch P, and a strip
27
having substantially the same width as that of each groove is provided between adjacent grooves
26
.
The depth D (a height difference between strip
27
and groove
26
) of each groove
26
on the phase mask
21
is selected such that the phase of the excimer laser light (beam)
23
that is the exposure light is modulated by a &pgr; radian. Zero-order light (beam)
25
A is reduced to 5% or lower by the phase shift ask
21
, and primary light (beam) leaving the mask
21
is divided into plus first-order diffracted light
25
B including 35% or more of diffracted light and minus first-order diffracted light
25
C. By carrying out irradiation using an interference fringe at a given pitch determined by the plus first-order diffracted light
25
B and the minus first-order diffracted light
25
C, the refractive index change at this pitch is imparted to the core of the optical fiber
22
.
The grating in the optical fiber, fabricated using such a phase mask
21
as mentioned above, has a constant pitch, and so the grooves
26
on the phase mask
21
used for grating fabrication, too, have a constant pitch.
Such a phase mask is fabricated by preparing pattern data corresponding to a grating form of groove pitch and carrying out writing with an electron beam writing system to form a grooved grating.
In this regard, a chirped grating wherein the grating pitch increases or decreases linearly or nonlinearly depending on the position of a grating groove in a direction perpendicular to the grating groove (the repetitive direction of grating) is now demanded for the Bragg diffraction grating to be formed in an optical fiber. Such a grating, for instance, is used for high-reflecting mirrors having a widened reflection band, and as delay time control means.
When such a grating having a grating pitch changing linearly or nonlinearly depending on the position of grooves in the lengthwise direction of an optical fiber is fabricated by the interference of plus first-order diffracted light and minus first-order diffracted light using a phase mask, it is required that the pitch of grooves on the phase mask, too, increase or decrease linearly or nonlinearly in a position-dependent manner, as can be seen from the principle shown in FIG.
7
(
a
). The smaller the pitch of grooves on the phase mask, the larger the angle between the plus first-order diffracted light and the minus first-order diffracted light and the smaller the pitch of interference fringes. For the fabrication of such a phase mask with an electron beam writing system, an enormous amount of writing data is needed to write grooves or inter-groove strips all over the range of the mask. This often makes mask fabrication difficult.
DISCLOSURE OF THE INVENTION
In view of such problems with the prior art, an object of the invention is to provide a method of fabricating an optical fiber-processing phase mask which enables a phase mask with a groove pitch changing depending on the position of grooves in a direction perpendicular to the grooves to be easily fabricated by electron beam writing, and an optical fiber-processing phase mask fabricated by this method.
Another object of the invention is to provide an optical fiber-processing phase mask with a groove pitch changing depending on the position of grooves in a groove direction, and a method of fabricating the same by electron beam writing.
According to one aspect of the invention, these objects are achieved by the provision of an optical fiber-processing phase mask comprising on one surface of a transparent substrate a repetitive pattern of grooves and strips located in a grating form, so that an optical fiber is irradiated with diffracted light according to said repetitive pattern to make a diffraction grating in said optical fiber by an interference fringe of diffracted light of different orders, characterized by juxtaposition of a plurality of patterns having a linearly or nonlinearly increasing or decreasing pitch, with a constant width ratio between said grooves and said strips.
In this aspect of the invention, the patterns may be juxtaposed either in a direction perpendicular to the grooves
Kurihara Masa-aki
Segawa Toshikazu
Dai Nippon Printing Co. Ltd.
Dellett and Walters
Rosasco S.
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