Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-07-30
2002-12-31
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S123000, C385S011000
Reexamination Certificate
active
06501881
ABSTRACT:
CLAIM OF PRIORITY
This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled “DEVICE FOR FABRICATING POLARIZATION INSENSITIVE LONG PERIOD FIBER GRATING” filed in the Korean Industrial Property Office on Aug. 4, 2000 and there duly assigned Serial No. 2000-45196.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical communication, and in particular to a device for fabricating a long-period fiber grating used in an optical communication field.
2. Description of the Related Art
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. Long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure, i.e., a cladding mode. The long-period fiber grating has been used as a fiber filter, such as a non-reflecting band rejection filter and band-pass filter, an optical sensor, and a gain flattener for the erbium-doped amplifier.
A typical method for fabricating the long-period fiber grating is based on a well known principle that “germanium-doped optical fiber is photosensitive to ultraviolet light” in pp.647-649 of APPL. PHYS. Lett. Vol. 32, “Photosensitivity in optical fiber wave guides: Application to reflection filter fabrication” suggested by K. O. Hill et. al. and published in 1978.
A representative method for fabricating the long-period fiber grating is ultraviolet irradiation using “side writing” as suggested in pp.823-825 of Opt. Lett. Vol. 14, “Formation of Bragg gratings in optical fibers by transverse holographic method” by G. Meltz et. al. and published in 1989.
FIG. 1
is a structure diagram illustrating a conventional device for fabricating a long-period fiber grating. Referring to
FIG. 1
, the conventional device
100
for fabricating the long-period fiber grating includes a ultraviolet laser
110
for irradiating a ultraviolet laser beam, a lens
120
for adjusting a focus of the ultraviolet laser beam irradiated from the ultraviolet laser
110
, an amplitude mask
130
for selectively passing the ultraviolet laser beam from the lens
120
, and a fixing unit
140
for fixing both ends of a fiber (f). The positioning and orientation of amplitude mask
130
are such that when ultraviolet laser beam is transmitted therethrough, an interference pattern is formed which extends through the fiber (f). Here, the period of the grating can be adjusted by tilting amplitude mask relative to the longitudinal extend of fiber (f).
When the conventional device
100
, as described in the preceding paragraph, is used to fabricate the long-period fiber grating, the ultraviolet laser beam is asymmetrically irradiated on one side of the fiber, thus the refractive index of the fiber core is anisotropically varied due to a polarization status of the ultraviolet laser beam and causes birefringence in the fiber. That is, the core of the non-birefringent fiber is substantially circular-symmetric. The circular symmetry ensures that the refractive index of the core mode is essentially insensitive to the state of optical polarization. In contrast, in hi-birefringent single mode fibers the effective refractive index of the core mode is substantially different between two principal polarization states. For background information, the birefringence due to the polarization of the ultraviolet laser beam has been disclosed in detail in a treatise of T. Erdogan, p.2100 to p.2105 of ‘Journal of the optical society of America B’ in 1994.
In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing which is typically at least 10 times larger than the transmitted wavelength, and thus has serious variations of the refractive index due to the polarization status of the ultraviolet laser beam, namely high polarization dependence. The polarization dependence of the long-period fiber grating results in a undesirable PDL (Polarization Dependent Loss) and PMD (Polarization Mode Dispersion), thereby deteriorating the property of the long-period fiber grating as an optical communication device.
SUMMARY OF THE INVENTION
It is, therefore, the present invention relates to a device for fabricating a polarization insensitive long-period fiber grating, which can reduce the property loss due to birefringence that is resulting from the polarization status of a ultraviolet laser beam.
According to an aspect of the present invention to provide a device for fabricating a polarization insensitive long-period fiber grating, which can reduce a property loss due to birefringence that is generated when ultraviolet light is irradiated merely to one side of a fiber.
Accordingly, there is provided a device for fabricating a polarization insensitive long-period fiber grating. The device includes: an ultraviolet laser for irradiating a ultraviolet laser beam; an ultraviolet polarizer for converting a polarization status of the ultraviolet laser beam into a linear polarization status in parallel along the longitudinal direction of a fiber; a lens for adjusting a focus of the ultraviolet laser beam from the ultraviolet polarizer; an amplitude mask for selectively passing the ultraviolet laser beam from the lens; and a mount for holding both ends of the fiber such that the outer circumferential surface of the fiber can be exposed to the ultraviolet laser beam from the amplitude mask.
In another aspect of the present invention, a device for fabricating a polarization insensitive long period fiber grating includes: an ultraviolet laser for irradiating a ultraviolet laser beam; an ultraviolet polarizer for converting a polarization status of the ultraviolet laser beam into a linear polarization status in parallel along the longitudinal direction of a fiber; a lens for adjusting a focus of the ultraviolet laser beam from the ultraviolet polarizer; an amplitude mask for selectively passing the ultraviolet laser beam from the lens; a mount for holding both ends of the fiber such that the outer circumferential surface of the fiber can be exposed to the ultraviolet laser beam from the amplitude mask; and a mirror for reflecting the ultraviolet laser beam from the amplitude mask to the outer circumferential surface of the fiber.
According to another aspect of the invention, there is provided a method for passing light through an amplitude mask to create interference pattern into a fiber. The method includes the steps of: generating a ultraviolet laser beam; converting a polarization of the ultraviolet laser beam into a linear polarization in a parallel relationship with the fiber; directing the converted ultraviolet laser beam through an amplitude mask and into the fiber, such that when the converted ultraviolet laser beam is transmitted through the amplitude mask and exposed to along the fiber, an interference is formed which extends through the fiber.
REFERENCES:
patent: 5327515 (1994-07-01), Anderson et al.
patent: 5550654 (1996-08-01), Erdogan et al.
patent: 5604829 (1997-02-01), Brusselbach
patent: 5881188 (1999-03-01), Starodubov
patent: 5912999 (1999-06-01), Brennan, III et al.
patent: 6204969 (2001-03-01), Jang
patent: 6370301 (2002-04-01), Kokura
patent: 6384977 (2002-05-01), Laming et al.
patent: 6414764 (2002-07-01), Quellette
patent: 2001/0008466 (2001-07-01), Kim et al.
Cha Steve
Cha & Reiter
Healy Brian
Samsung Electronics Co,. Ltd.
Wood Kevin S
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