Coated optical fiber

Details Coated optical fiber Coated optical fiber

1999-06-09

2001-05-29

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S031000, C385S123000, C385S127000, C385S128000, C427S163200, C359S341430

Reexamination Certificate

active

06240224

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the field of fiber optics and, more particularly, to a coated optical fiber with improved ease of manufacture and reliability.
BACKGROUND OF THE INVENTION
Fiber Bragg gratings are widely used in the fields of optical fiber communication and fiber sensor systems. These Bragg gratings have a number of uses in fiber optics, notably as filters and reflectors. In a known process of fiber grating fabrication, all coatings must be stripped off before the grating can be inscribed or written and, in order to preserve the mechanical strength of the fiber, it must be re-coated soon after the grating is written. This method is time consuming and has the potential of reducing the fiber strength due to exposure of the bare fiber to the air.
To solve this problem in the art of inscribing fiber Bragg gratings, a number of solutions have been proposed. These include using a specially developed UV-transparent polymer coating, writing the grating using near UV light around 330 nm instead of at more conventional wavelengths, writing on-line as the fiber is being pulled, and using a specially developed coating which can be removed thermally, then immediately re-coating in an automated production system.
The polymer used in Espindola et al., Fibre Bragg Grating Written Through a Fiber Coating, although it has a lower absorption than the normal UV-curable polymer coating, still has a strong absorption at 248 nm or 257 nm. This increases the grating writing time and reduces the mechanical strength of the fiber when higher UV exposure power is used. In Starodubovet al., Efficient Bragg grating fabrication in a fibre through its polymer jacket using near-UV light, a specially developed phase mask is needed in order to operate at the non-standard wavelength. In addition, care is also required to control the average laser power in order to reduce damage to the coating. On-line production of fiber gratings, such as that described in Dong et al., Single pulse Bragg gratings written during fibre drawing, is limited to the manufacture of gratings which can be made with a single laser pulse.
The method proposed in Singh et al., Automated In-Line Production of Fibre Bragg Grating Using Special Coatings, although attractive, requires both a special coating and an automatic production system. The thermally strippable coating also has a low heat resistance in many applications.
SUMMARY OF THE INVENTION
The present invention provides the ability to write a fiber grating through standard fiber coating at a wavelength of 248 nm. We have shown that the scheme is not only simple but also has several advantages over other previously demonstrated techniques. The ability to write through the fiber coating is most useful since it will significantly simplify the grating writing process, particularly for long arrays of gratings as required for fiber sensors. At the same time, the longer possible writing time allows more complicated grating structures to be written. This is important for WDM devices and some sensor applications. The better heat resistance not only means that it can be used in many different applications but also allows the grating to be annealed if there is such a requirement, e.g. after hydrogen loading.
An object of the present invention is to improve on known apparatus by writing of a fiber grating through a coating with a method which provides greater reliability and lower cost.
According to a non-limiting embodiment of the present invention, there is provided a coated optical fiber comprising a coating, an optical fiber, at least one waveguiding region and an index grating, in which the waveguiding region contains at least one photosensitive region, and in which the index grating is formed by writing through the coating using UV light, and in which the coating transmits UV light at the wavelength at which the index grating is written.
The index grating may be an optical fiber Bragg grating, a long-period grating, or a blazed grating. The waveguiding region preferably comprises a core and a cladding. The core may be photosensitive, or the cladding may be photosensitive. The cladding may be depressed in that its refractive index is less than the refractive index of the surrounding silica.
The waveguiding region may comprise a core and a plurality of claddings. The index grating may be formed in the core, or in at least one of the claddings, or in both the core and at least one of the claddings. The photosensitive region may be formed by hydrogen loading of the coated optical fiber prior to the writing of the index grating. This is usually achieved by placing the optical fiber into a hydrogen atmosphere at 20° C. to 100° C. and at a pressure of 100 bar to 1000 bar.
Alternatively, the photosensitive region may be formed by doping the optical fiber perform during manufacture with one or more of the dopants germania, tin, boron, or cerium.
The coating should allow the transmission of the UV light which is used to write the grating without the requirement for the coating to be removed. The coating should not contain a photo-initiator, since photo-initiators are often UV sensitive. Nevertheless, if such a photo-initiator is used, then it must be transparent at the wavelength at which the index grating is written.
The coating may have a refractive index less than the refractive index of the outside of the optical fiber, or equal to the refractive index of the outside of the optical fiber, or greater than the refractive index of the optical fiber.
The coated optical fiber may be cured by thermal annealing in which the coated optical fiber is thermally treated after the index grating is formed. This process is important because it stabilizes the long-term properties of the index grating.
The coating is preferably a thermally cured coating because this allows a coating to be selected which does not contain a photo-initiator. Moreover, it allows the coated optical fiber to be more easily thermally cured after the index grating is formed. This coating choice is therefore compatible with the preferred method of forming the photosensitive region, namely hydrogen loading of the coated optical fiber prior to the writing of the index grating. Using such a thermally cured coating which does not require removing the coating prior to writing the index grating increases the reliability of the coated optical fiber.
According to a first embodiment of the invention, there is provided an apparatus and a method for the manufacture of the coated optical fiber in which the optical fiber is drawn with the coating and UV light is written through the coating to form the index grating.
The source of the UV light may be a laser operating in the region 220 nm to 350 nm. The laser may be an excimer laser, a frequency doubled Argon laser, or a frequency doubled dye laser, a frequency quadrupled high-power laser, a nitrogen laser, an optical parametric oscillator, an Argon ion laser, a Krypton ion laser or a copper vapor laser.
The index grating may be written using a phase mask, using an interferometer, or using an amplitude mask. Alternatively, it may be written point by point, i.e., one element of the grating at a time
According to a second embodiment of the invention, there is provided apparatus for sensing physical parameters which apparatus comprises a light source, a receiver, and at least one coated optical fiber.
According to a third embodiment of the invention, there is provided an optical filter comprising at least one coated optical fiber.
According to a fourth embodiment of the invention, there is provided a fiber laser comprising at least one coated optical fiber and in which the waveguiding region contains at least one amplifying region containing at least one rare-earth dopant.
According to a fifth embodiment of the invention, there is provided an optical fiber amplifier comprising at least one coated optical fiber and in which the waveguiding region contains at least one amplifying region containing at least one rare-earth dopant.


REFERENCES:
patent: 5930420 (1999-

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