Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-04-27
2002-10-15
Henry, Jon (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C359S566000, C430S005000
Reexamination Certificate
active
06466714
ABSTRACT:
TECHNICAL ART
The present invention relates to a method of fabricating a phase mask for processing optical fibers, and an optical fiber with a Bragg diffraction grating, which is manufactured using the optical fiber-processing phase mask. More particularly, the present invention relates to a method of fabricating a phase mask for making a diffraction grating in an optical fiber used for optical communications, etc. using ultraviolet laser light, and an optical fiber with a Bragg diffraction grating, which is manufactured using the mask.
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 silica optical fiber is minimized and which is suitable for long-distance communications is 1.55 &mgr;m. It is thus required that 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 an interference process using the interference of two light beams, 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 interference process using the interference of two light beams, 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 ultraviolet laser light (of 190 to 3.00 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.
5
(
a
). For a better understanding of an interference pattern
24
Qn the core
22
A, the pattern
24
is exaggerated in FIG.
5
(
a
). FIG.
5
(
b
) is a sectional view of the phase mask
21
, and FIG.
5
(
c
) is a top view corresponding to FIG.
5
(
b
). The phase mask
21
has a binary phase type of diffraction grating structure where a 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 ultraviolet laser light (beam) 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 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
.
The grating produced in the optical fiber using such a phase mask
21
as mentioned above has a constant pitch, and so the phase mask
21
used for grating production is provided with grooves
26
at a constant pitch.
Such a phase mask is produced by writing an electron beam on positions, corresponding to grooves
26
, on the quartz substrate coated with an electron-beam resist, using an electron-beam writing system and etching out the written portions.
To achieve a narrow-band optical fiber diffraction grating, however, such a phase mask
21
as mentioned above is required to have a size of the order of 100 mm in the repetition direction of grooves
26
(in the sectional direction in FIG.
5
). In addition, it is not easy to continuously expose a phase mask blank to writing beams in one operation. Thus, such an optical fiber diffraction grating is fabricated by writing the entire region of a phase mask blank with writing beams by a step-and-repeat process wherein the entire region of the phase mask blank is divided into small segments (at an interval of about 7 mm). One segment is first written with writing beams while a writing stage is fixed, and then the writing stage is moved by one segment to write the next segment with writing beams. This operation is repeated while the segments are connected to one another, so that the entire region of the phase mask blank can be sequentially written with the writing beams.
However, a problem with this step-and-repeat process is that there is a phase shift (a stitching error) in the repetition period of grooves
26
in the connection of adjacent segments to each other. In an optical fiber diffraction grating fabricated using a phase mask having such a stitching error, a number of unnecessary peaks other than essential side lobes occur on both sides of the center Bragg peak, as can be seen from the wavelength vs. reflectivity relation shown in FIG.
8
.
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 method of fabricating an optical fiber-processing phase mask which can reduce or substantially eliminate stitching errors ascribable to a deterioration in the wavelength selectivity of the optical fiber diffraction grating to be fabricated. The present invention also includes an optical fiber with a Bragg diffraction grating, which is manufactured using such a phase mask for processing optical fibers.
According to one aspect of the present invention, this object is accomplished by the provision of a method of fabricating an optical fiber-processing phase mask comprising a transparent substrate provided on one surface with a grating form of repetitive groove-and-strip pattern, in which an optical fiber is irradiated with light diffracted by said repetitive pattern to produce an interference fringe by interference of diffracted light of different orders, thereby providing a diffraction g
Komukai Tetsuro
Kurihara Masa-aki
Nakazawa Masataka
Segawa Toshikazu
Dai Nippon Printing Co., Ltd. and Nippon Telegraph and Telephone
Dellett & Walters
Henry Jon
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