Apparatus and method of fabricating long-period fiber grating

[0002] This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application APPARATUS AND METHOD OF FABRICATING LONG-PERIOD FIBER GRATING filed with the Korean Industrial Property Office on Oct. 7, 1998 and there duly assigned Serial No. 41861/1998.

[0003] 1. Field of the Invention

[0004] The present invention relates to an apparatus and method of fabricating a long-period fiber grating and more particularly, to an apparatus for fabricating a long-period fiber grating using a string which vibrates with a predetermined period.

[0005] 2. Description of the Related Art

[0006] Typical long-period fiber gratings are elements for coupling a core mode, in which light travels through the core of an optical fiber, to a cladding mode, and thus are well suited to gain-flattening of an erbium doped fiber amplifier (EDFA) since they do not act as a reflective type device.

[0007] Long-period fiber gratings can be fabricated by a methods of using an amplitude mask on which a light transmission/non-transmission portion has a predetermined period; by vibrating an optical fiber at a predetermined period; or by a method of applying stress to the optical fiber at a predetermined period.

[0008] In the method of using an amplitude mask, the refractive index of the core of an optical fiber, which is sensitive to ultraviolet rays, is periodically varied using an amplitude mask having a predetermined period. That is, to obtain an arbitrary coupling peak wavelength λ, the refractive index difference between the core and cladding of an optical fiber is formed by directly projecting an ultraviolet laser onto an amplitude-modulated mask having a grating cycle Λ, thereby fabricating a long-period fiber grating. Here, the coupling peak wavelength value λ is determined by the following Equation 1:

[0009] λ=(n_core−n_cladding)Λ  (1)

[0010] wherein n_coreis the refractive index of the core, n_claddingis the refractive index of the cladding, and Λ is the period of an amplitude mask.

[0011] A conventional apparatus for fabricating a long-period fiber grating using an amplitude mask includes an amplitude mask mounted on a fiber, and a laser source for irradiating laser light onto the amplitude mask. However, in the method of using an amplitude mask, the period of the amplitude mask is determined according to the intended coupling peak wavelength to be obtained. Therefore, the ultraviolet laser may be collimated with various methods or a dual mask may be applied when the period of the amplitude mask is changed.

[0012] The long-period fiber grating can also be fabricated by vibrating an optical fiber or by applying stress using comb-shaped probes. However, these alternative methods have problems in that stability and reproducibility are difficult, and the devices required to fabricate the long-period fiber grating are large.

[0013] It is therefore an object of the present invention to provide an improved method of fabricating a long-period fiber grating.

[0014] It is also an object of the present invention to provide an improved apparatus for fabricating a long-period fiber grating.

[0015] It is a further object of the invention to provide a method of fabricating a long-period fiber grating with excellent stability and reproducibility.

[0016] It is a yet further object of the present invention to provide a method of fabricating a long-period fiber grating in which no collimation of the laser is required when changing the period of the fiber.

[0017] It is a still further object of the present invention not to require a dual mask.

[0018] To achieve the above objectives of the present invention, the present invention provides an apparatus and method of fabricating a long-period fiber grating by locating a string which vibrates at a predetermined cycle over a fiber and illuminating an ultraviolet laser on the string and the fiber.

[0019] Accordingly, to achieve the above objective, the present invention provides a long-period fiber grating fabrication apparatus including: a fiber; a string, one end of which is fixed, that is located over the fiber and vibrates at a predetermined vibration frequency; a vibrating portion, which vibrates the string at the vibration frequency, to which the other end of the string is connected; and a laser source for illuminating laser light on the string and the fiber, wherein the laser light is periodically transmitted by the fiber due to the vibration of the string, and thus the refractive index of the fiber is changed.

[0020] To achieve the above objectives, the present invention also provides a long-period fiber grating fabrication apparatus including: a fiber; a string, one end of which is fixed, that is located over the fiber and vibrates at a predetermined vibration frequency; a vibrating portion, which vibrates the string at the vibration frequency, to which the other end of the string is connected; a period controller connected to the vibrating portion for controlling the period of the string by moving the vibrating portion upward and downward; and a laser source for illuminating laser light onto the string, wherein the laser light is periodically transmitted by the fiber due to the period-controlled vibration of the string, and thus the refractive index of the fiber is changed.

[0021] To achieve the above objectives, the present invention provides a long-period fiber grating fabrication method including the steps of: aligning a string over a fiber in the longitudinal direction of the fiber; forming a standing wave by vibrating the string at a predetermined vibration frequency; and illuminating laser light to the vibrating string and the fiber, wherein the laser light is periodically transmitted by the fiber due to the vibration of the string, and thus the refractive index of the fiber is periodically changed.

[0022] A more complete appreciation of the invention, and may of the attendant advantages, thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

[0023] FIG. 1 is an outline view of a conventional apparatus for fabricating a long-period fiber grating using an amplitude mask;

[0024] FIG. 2 is an outline view of an apparatus for fabricating a long-period fiber grating according to the present invention;

[0025] FIG. 3A illustrates a process for fabricating a long-period fiber grating by illuminating ultraviolet rays on a fiber and a string which vibrates at a predetermined cycle;

[0026] FIG. 3B shows a long-period fiber grating formed by the process of FIG. 3A; and

[0027] FIG. 4 shows an apparatus in which a period controller is attached to the vibrating portion of the apparatus of FIG. 2.

[0028] Turning now to the drawings, FIG. 1 is an outline view of the conventional apparatus for fabricating a long-period fiber grating using an amplitude mask described above. The apparatus of FIG. 1 includes an amplitude mask 102 mounted on a fiber 100, and a laser source (not shown) for irradiating laser light onto the amplitude mask 102.

[0029] The present invention will now be described with reference to the drawings. Referring to FIG. 2, an apparatus for fabricating a long-period fiber grating according to the present invention includes a fiber 200, a string 202 which is oriented in the same direction as the direction of the fiber 200 and vibrates at a predetermined vibration frequency, a fixing portion 204 to which one end of to the string 202 is fixed, a vibrating portion 206 to which the other end of the string 202 is connected, and a laser source (not shown) for illuminating laser light 220 on the string 202.

[0030] The string 202 is located over and along the fiber 200. The thickness of the string 202 is determined according to the characteristics of long-period fiber grating intended to be fabricated. The thickness of the string depends on the desired period of the long-period fiber grating, and for example, may be in the range of approximately several micrometers to tens of micrometers. The string 202 is formed of a material which can vibrate at a desired vibration frequency and which can prevent UV light from being transmitted to the optical fiber. Preferably, the string should be formed of a material which can provide a mechanical vibration in a horizontal or vertical direction by pulling the string in the longitudinal direction of the string. The string may, for example, be made of steel, or the string may be made of materials capable of keeping out the ultraviolet light.

[0031] The string 202, one end of which is fixed to the fixing portion 204 and the other end of which is connected to the vibration portion 206, vibrates at a predetermined vibration frequency due to the horizontal movement (arrow 230) of the vibrating portion 206, and then forms a standing wave. Vibrating portion 206 may contain a piezoelectric component for vibrating the string. Ultraviolet (UV) laser light 220 is projected onto the string 202 below or, more generally, behind which the fiber 200 is located. Here, the fiber 200 is sensitive to the UV light.

[0032] FIG. 3A illustrates a process for fabricating a long-period fiber grating by illuminating ultraviolet rays on a fiber and a string which vibrates with a predetermined period. Referring to FIG. 3A, a string 302, which vibrates at a predetermined vibration frequency, is located over a fiber 300, and laser light 320 is projected onto the resultant structure. The laser light does not illuminate node portions 304 where the fiber 300 is overlapped by the string 302. The refractive index of the core of the fiber 300 is periodically changed in portions 306 exposed to the laser. FIG. 3B shows a long-period fiber grating formed by the process of FIG. 3A.

[0033] FIG. 4 shows an apparatus in which a period controller is attached to the vibrating portion of the apparatus of FIG. 2. The apparatus of FIG. 4 includes a fiber 400, a string 402 located over and along the fiber 400, a fixing portion 404 to which one end of the string 402 is fixed, a vibrating portion 406 to which the other end of the string 402 is connected, a period controller 408 for moving the vibrating portion 406 upward and downward (arrow 440), and a laser source (not shown) for illuminating laser light 420 on the string 402. The period controller moves to adjust the string to a particular length which remains fixed while the vibrating portion vibrates (arrow 430).

[0034] It is preferable that the period controller 408 is a micromotor. The period of the string 402 is determined by a variation in the vibration frequency of the vibrating portion 406 and the position of the vibrating portion determined by the upward and downward movement made by the period controller 408.

[0035] According to the present invention, a long-period fiber grating is easily formed by locating a string which vibrates at a predetermined vibration frequency, over a fiber and illuminating a laser on the resultant structure. Also, the laser is not required to be collnnated, and the period of the long-period grating is easily changed.