Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-10-25
2004-09-21
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S147000, C065S031000, C430S290000
Reexamination Certificate
active
06795614
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of producing a phase mask for processing an optical fiber and also relates to an optical fiber with a Bragg diffraction grating fabricated by using the optical fiber-processing phase mask. More particularly, the present invention relates to a method of producing a phase mask for fabricating a diffraction grating by using an ultraviolet laser beam in an optical fiber used for optical communications or the like. The present invention also relates to an optical fiber with a Bragg diffraction grating fabricated by using the mask.
BACKGROUND ART
Optical fibers have brought about a great revolution in global communications and allowed high-quality and large-capacity transoceanic telecommunications. It has heretofore been known that a Bragg diffraction grating is fabricated in an optical fiber by producing a periodic index profile in the core along the optical fiber, and the level of reflectivity of the diffraction grating and the width of the wavelength characteristics of the diffraction grating are determined by the period and length of the diffraction grating and the magnitude of refractive index modulation thereof, thereby allowing the diffraction grating to be used as a wavelength-division multiplexer for optical communications, a narrow-band high-reflecting mirror for use in a laser or a sensor, a wavelength selection filter for removing extra laser wavelengths in a fiber amplifier, etc.
However, the wavelength at which silica optical fibers exhibit a minimum attenuation and which is suitable for long-haul communication systems is 1.55 &mgr;m. Therefore, it is necessary in order to use an optical fiber diffraction grating at this wavelength that the grating spacing should be about 500 nm. At the beginning, it was deemed to be difficult to make such a fine structure in the core. To make a Bragg diffraction grating in the core of an optical fiber, the conventional practice is to carry out many complicated process steps including side polishing, photoresist process, holography exposure, reactive ion beam etching, etc. For this reason, the conventional practice suffers a long production time and a low yield.
However, there has recently been known a method of making a diffraction grating by irradiating an optical fiber with ultraviolet radiation to produce a refractive index change directly in the core of the optical fiber. The method using irradiation with ultraviolet radiation needs no complicated process. Therefore, increasing use has been made of the method using ultraviolet radiation with the progress of peripheral techniques.
With the method using ultraviolet radiation, because the grating spacing is as fine as about 500 nm, as has been stated above, the following methods are adopted: an interference method wherein two light beams are caused to interfere with each other; a method wherein writing is carried out for each point (single pulses from an excimer laser are focused to form diffraction grating surfaces one by one); a method wherein irradiation is effected by using a phase mask having a grating; and so forth.
The above-described interference method in which two light beams are caused to interfere with each other involves problems in terms of the quality of lateral beams, i.e. spatial coherence. The method wherein writing is carried out for each point needs precise step control of the order of submicrons and requires writing many surfaces with light narrowed down. Therefore, the method suffers from a problem in terms of operability.
Accordingly, the irradiation method using a phase mask has attracted attention as a method capable of coping with the above-described problems. This method uses, as shown in FIG.
5
(
a
), a phase shift mask
21
having grooves with a predetermined depth that are provided on one surface of a quartz substrate at a predetermined pitch. A KrF excimer laser beam (wavelength: 190 to 300 nm)
23
is applied to the mask
21
to produce a refractive index change directly in a core
22
A of an optical fiber
22
, thereby fabricating a grating (reference numeral
22
B denotes a cladding of the optical fiber
22
). It should be noted that in FIG.
5
(
a
) an interference fringe pattern
24
in the core
22
A is shown as enlarged so as to be readily understandable. FIG.
5
(
b
) is a sectional view of the phase shift mask
21
. FIG.
5
(
c
) is a fragmentary top view corresponding to the sectional view. The phase shift mask
21
has a binary phase type diffraction grating structure in which grooves
26
with a depth D are provided on one surface of the phase shift mask
21
at a repeating pitch P, and a strip
27
with approximately the same width as that of each groove
26
is provided between each pair of adjacent grooves
26
.
The depth D of the grooves
26
in the phase shift mask
21
(i.e. the height difference between the strips
27
and the grooves
26
) is selected so that the phase of the excimer laser beam
23
, which is exposure light, is modulated by a &pgr; radian. Zeroth-order light (beam)
25
A is reduced to 5% or less by the phase shift mask
21
. Principal light (beam) emerging from the mask
21
is separated into plus 1st-order diffracted light
25
B, which includes 35% or more of the diffracted light, and minus 1st-order diffracted light
25
C. Thus, the optical fiber
22
is irradiated with interference fringes of predetermined pitch determined by the plus 1st-order diffracted light
25
B and the minus 1st-order diffracted light
25
C, thereby producing a refractive index change at this pitch in the optical fiber
22
.
The grating fabricated in the optical fiber by using the above-described phase mask
21
has a uniform pitch. Therefore, the grooves
26
of the phase mask
21
used to fabricate the grating also have a uniform pitch.
To make such a phase mask, a quartz substrate coated with an electron beam resist is irradiated with an electron beam at regions corresponding to the grooves
26
by an electron beam writing system, and the irradiated regions are removed by etching.
Incidentally, there has recently been a demand for a chirped grating as a Bragg diffraction grating to be formed in optical fibers. The chirped grating is such a diffraction grating that the grating pitch increases or decreases linearly or nonlinearly according to the position in a direction (repeating direction of the grating) perpendicular to the grating grooves. Such a grating is used, for example, as a means for compensating for chromatic dispersion of a high-reflecting mirror having a widened reflection band or that of an optical fiber.
To make a grating in which the grating pitch changes linearly or nonlinearly according to the position in the longitudinal direction of the optical fiber, as stated above, on the basis of the interference of plus 1st-order diffracted light and minus 1st-order diffracted light by using a phase mask, it is necessary that the pitch of the grooves of the phase mask should increase or decrease linearly or nonlinearly according to the position as well, as will be clear from the principle shown in FIG.
5
(
a
) (as the pitch of the grooves of the phase mask decreases, the angle formed between the plus 1st-order diffracted light and the minus 1st-order diffracted light increases, and the pitch of the interference fringes decreases). To make such a phase mask by writing using an electron beam writing system, the conventional practice needs a large amount of writing data for writing grooves or strips between them over the whole area of the mask. Accordingly, there are cases where it is difficult to produce the phase mask. The writing data may have errors occurring in relation to the address grid.
In fabricating a grating in which the grating pitch changes linearly or nonlinearly, the problem of pitch deviation (connection error) may arise at the joint between patterns different in grating pitch from each other. An optical fiber diffraction grating fabricated by using a phase mask containing such connection errors produces a large number of unwanted peaks other than the desire
Komukai Tetsuro
Kurihara Masa-aki
Nakazawa Masataka
Segawa Toshikazu
Dai Nippon Printing Co. Ltd.
Dellett & Walters
Lee John D.
Pak Sunk
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