Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-06-27
2004-02-10
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06690860
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a grating which modulates refractive index in an optical waveguide such as an optical fiber or a planar lightwave circuit and, more particularly, to a method of manufacturing a grating of a grating device such as a band-pass filter or a dispersion equalization device used in an optical communication system which need precise characteristics.
2. Description of the Prior Art
In an expected high density wavelength division multiplex transmission system (hereinafter referred to as DWDM transmission system) having a wavelength interval of 50 GHz (wavelength: 0.4 nm) or 25 GHz (wavelength: 0.2 nm) expected to be realized, a band-pass filter obtained by disposing a grating in an optical waveguide such as an optical fiber or a planar lightwave circuit (hereinafter referred to as PLC) is a necessary device. In a future ultra-high speed transmission system having a bit rate of not less than 10 Gbit/s, or not less than 40 Gbit/s, a dispersion equalization device obtained by disposing a grating in an optical waveguide is a necessary device.
The grating device such as a band-pass filter or a dispersion equalization device can be manufactured such that, for example, the interference fringes of an ultraviolet laser beam are irradiated on an optical waveguide such as an optical fiber or a planar lightwave circuit (PLC) obtained by adding Ge to a core consisting of silica. In addition, more specifically, the grating device is manufactured by the following operations. An optical waveguide is left in high-pressure hydrogen of several 10 to several 100 atms for several days to several weeks to fill hydrogen in the optical waveguide, and a change in refractive index by ultraviolet irradiation is sensitized. The interference fringes of ultraviolet laser beams split into two-beam of light by a phase mask or a half-mirror, and a change in refractive index depending on the interference fringes is formed. The formation of the change in refractive index depending on the interference fringes is called modulation, and the magnitude of the change is called the degree of refractive index modulation. When a grating pitch of the grating formed in the optical waveguide is represented by &Lgr;, and when an equivalent refractive index of the optical waveguide is represented by N
eff
, of light components being incident on the optical waveguide, a light component having a wavelength &lgr;
B
which satisfies the following Equation 1 causes Bragg reflection and is reflected on the incident side.
[Equation 1]
&lgr;
B
=2·
N
eff
·&Lgr; (1)
Note that an equivalent refractive index is an equivalent refractive index which is received by a light component propagated through the optical waveguide, is determined by an interactive operation between a core and a cladding, and is also called an effective refractive index or a valid refractive index.
When a wavelength &lgr;
B
which satisfies the relationship of Equation 1 is set to be constant over the entire grating, only a light component having a specific wavelength can be efficiently reflected. For this reason, a band-pass filter having a band-pass characteristic which is considerably sharp can be obtained. On the other hand, the grating pitch &Lgr; or the equivalent refractive index N
eff
of the grating are changed in a propagation direction to form a chirp grating the Bragg wavelength &lgr;
B
is changed depending on the position of the grating, so that a dispersion equalization device such as a dispersion compensator or a dispersion slope compensator can be obtained.
In order to obtain a characteristic used in an application of the band-pass filter or a dispersion equalization device, high precision is required to manufacture the grating. More specifically, in a band having a wavelength of 1.55 &mgr;m used in an optical communication system, a grating pitch is about 500 nm, and the grating should be uniformly manufactured over a length from about several mm to about 100 mm of the grating. In addition, since the degree of refractive index modulation or the equivalent refractive index N
eff
also changes by an irradiation amount of an ultraviolet laser beam, the ultraviolet laser beam should be irradiated at a high precision over the entire length of the grating. The error of the grating pitch from the design is called a phase error, and an error of the degree of refractive index modulation or the equivalent refractive index N
eff
is called an amplitude error. These errors cause degradation of an amount of out-of-band attenuation in a band-pass filter, and cause a ripple of a group delay time characteristic, i.e., a group delay ripple in an dispersion equalization device. This fact is described in [Richardo Feced, et al., “Effect of Random Phase and Amplitude Errors in Optical Fiber Bragg Gratings”, Journal of Lightwave Technology, Vol. 18, No. 1, pp. 90-101, January, 2000, issued by IEEE].
As methods of manufacturing gratings the errors of which are reduced, several methods are proposed. For example, in a method of manufacturing an optical waveguide diffraction grating disclosed in Japanese Laid-Open Patent Publication No. 8-286066, as shown in the perspective view in
FIG. 17
, fluorescence having a predetermined wavelength (about 240 nm) irradiated to detect fluorescence generated when the grating is formed, and alignment is performed such that an amount of received fluorescence is maximum. More specifically, when an ultraviolet laser beam having a wavelength of about 240 nm is irradiated on an optical fiber, fluorescence having wavelength of 350 to 550 nm is generated by the core of the optical fiber. A part of the generated fluorescence is propagated through the optical fiber to reach a detector
8
. An amount of fluorescence received by the detector
8
is adjusted to maximum, so that a laser beam irradiated on the core of the optical fiber
1
is maximum. As an ultraviolet laser having a wavelength of 240 nm, a KrF excimer laser (wavelength of 248 nm) and a second harmonic (wavelength of 244 nm) of an argon laser are known.
In [Komukai Tetsuro, et al., “Examination of Cause of Generation of Group Delay Ripple in Chirp Fiber Grating”, Technical Report of the Institute of Electronics, Information and Communication Engineers OFT2000-49, pp. 31-35, issued by, a corporation, the Institute of Electronics, Information and Communication Engineers], the following is described. That is, the position of an optical fiber is controlled such that an ultraviolet laser beam is uniformly irradiated by always monitoring fluorescence while scanning an ultraviolet laser beam having a wavelength of 244 nm which is a second harmonic of an argon laser in the direction of the optical axis of the optical fiber. In addition, many causes of group delay ripples generated by chirp gratings used as dispersion equalization device exist in processes in manufacturing the gratings, the following causes will be described:
(1) Fluctuation of the power or mode of an ultraviolet laser beam to be irradiated (amplitude error).
(2) Fluctuation of the composition of the core of an optical waveguide such as an optical fiber in the longitudinal direction.
(3) Incompletion of apodization of a chirp grating.
(4) A positional error between a phase mask and an optical waveguide caused by mechanical vibration (phase error).
(5) Incompletion of positional control of an optical waveguide and laser beam irradiation (amplitude error).
(6) Insufficiency of washing of an optical waveguide (amplitude error).
(7) Incompletion of a phase mask such as a stitching error (amplitude error and phase error).
On the other hand, in a method of manufacturing a grating disclosed in Japanese Laid-Open Patent Publication No. 10-90545, as shown in
FIG. 18
, heat generated when a KrF excimer laser beam having a wavelength of 248 nm and serving as an ultraviolet laser beam is irradiated on an optical waveguide formed in a PLC is radiated through a heat radiator. In this cas
Hoshizaki Junichiro
Matsumoto Sadayuki
Matsuno Shigeru
Ohira Takuya
Takabayashi Masakazu
Stahl Mike
Ullah Akm Enayet
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