Apparatus and method for manufacturing ribbon fiber with fiber bragg grating

Abstract

Disclosed are an apparatus and a method for manufacturing a ribbon fiber having a fiber Bragg grating, in which an apparatus for inscribing a grating includes: a femtosecond (FS) laser that emits FS laser pulses; a phase shift mask adapted to diffract the FS laser pulse according to a diffraction pattern formed that can be projected on the first optical fiber, where the diffracted FS laser pulse is adapted to inscribe a grating on a core of the first optical fiber according to the diffraction pattern.

Description

Technical Field

[0001] The subject matter of the present disclosure relates to the field of optical communication technology and to methods for producing them. More specifically, the subject matter of the present disclosure relates to an apparatus for writing fiber Bragg gratings using a femtosecond (FS) laser and a phase-shift mask and a method for manufacturing a ribbon fiber Bragg grating by a femtosecond laser. Background Art

[0002] Optical fiber is a flexible, transparent optical fiber used to transmit light. The optical fiber acts as a waveguide. Some optical fibers use a reflector called a distributed Bragg reflector. A distributed Bragg reflector consists of multiple layers of alternating materials with different refractive indices. Each layer partially reflects the light waves. Light waves with a specific wavelength combine at the distributed Bragg reflector in constructive interference and are reflected at each layer.

[0003] A fiber Bragg grating (FBG) is a type of distributed Bragg reflector in an optical fiber that reflects light waves with a specific wavelength and transmits all other light waves. In other words, an FBG is a variation of the refractive index in an optical fiber. The periodic variation of the refractive index acts as a wavelength-specific reflector. An FBG blocks or reflects light waves with a specific wavelength and transmits other light waves.

[0004] FBG is an important optical passive component, which plays an important role in optical communication, access network, sensor and signal detection. With the continuous development of technology, the demand for FBG with faster transmission speed is also increasing. Traditional bundled FBG or single FBG may not meet these demands.

[0005] The disclosed subject matter includes apparatus and methods for inscribing FBGs of smaller size in optical fibers and improved inscription techniques. Summary of the invention

[0006] Disclosed herein is an apparatus for writing a grating in, for example, an optical fiber. In one aspect of the disclosure, an apparatus comprises: a femtosecond (FS) laser adapted to emit FS laser pulses; a phase-shifting mask adapted to diffract the FS laser pulses into a diffraction pattern projectable onto a first optical fiber, wherein the diffracted FS laser pulses are adapted to write a grating on the core of the first optical fiber according to the diffraction pattern. In another aspect of the disclosure, the grating is a fiber Bragg grating. In another aspect of the disclosure, the phase-shifting mask comprises a first region having alternating first thin regions and first thick regions, wherein the first thin regions and the first thick regions are adapted to shift the phase of the FS laser pulses. In yet another aspect of the disclosure, the size of the FS laser pulses is designed to illuminate the first thin region and the first thick region so that the phase-shifted FS laser pulses diffract into a diffraction pattern.

[0007] In one aspect of the present disclosure, the phase-shift mask is a multi-phase mask comprising a plurality of phase-shift masks, wherein at least other phase-shift masks of the plurality of phase-shift masks comprise alternating second thin regions and second thick regions, the second thin regions and the second thick regions being adapted to shift the phase of the FS laser pulses by a predetermined phase angle, respectively. In another aspect of the present disclosure, a device further comprises a linear slider adapted to provide a linear relative motion of the FS laser and the multi-phase mask. In one aspect of the present disclosure, the device is adapted to adjustably hold a ribbon optical fiber comprising a first optical fiber and a second optical fiber, and the ribbon optical fiber and the multi-phase mask are linearly adjustable so that the plurality of phase-shift masks are optically aligned with the first optical fiber and the second optical fiber, respectively. In another aspect of the present disclosure, the FS laser is a Nd:YAG laser. In another aspect of the present disclosure, the average wavelength of the FS laser is between 980 nm and 1550 nm, or between 1050 nm and 1060 nm.

[0008] A method for writing a grating is disclosed herein. In one aspect of the method, the method includes: emitting a femtosecond (FS) laser pulse onto a phase-shifting mask; diffracting the FS laser pulse via the phase-shifting mask to form a diffraction pattern; projecting the diffraction pattern onto the core of the first optical fiber through the cladding of the first optical fiber; and writing a grating on the core according to the diffraction pattern. In another aspect of the disclosure, the method also includes focusing the diffraction pattern along the core of the first optical fiber. In another aspect of the disclosure, the grating is a fiber Bragg grating. In yet another aspect of the disclosure, the phase-shifting mask includes a first region having alternating first thin regions and first thick regions, the first thin regions and the first thick regions being suitable for shifting the phase of the FS laser pulse.

[0009] An exemplary method disclosed for writing a grating may include the phase-shift mask being a multi-phase mask having a plurality of phase-shift masks, wherein the method further includes: linearly adjusting the multi-phase mask to align other phase-shift masks in the plurality of phase-shift masks with the FS laser pulse; diffracting the FS laser pulse via other phase-shift masks in the plurality of phase-shift masks to form a second diffraction pattern; projecting the second diffraction pattern onto a second core of the second optical fiber through a second cladding of the second optical fiber; and writing a second grating on the second core according to the second diffraction pattern. In another aspect of the present disclosure, the linearly adjusting step includes adjusting a linear slider. In another aspect of the present disclosure, the writing method is performed on a ribbon optical fiber, wherein the ribbon optical fiber includes a first optical fiber and a second optical fiber. In yet another aspect of the present disclosure, the average wavelength of the disclosed FS laser pulse is between about 980 nm and about 1550 nm, or between about 1050 nm and about 1060 nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A cross-sectional view of an optical fiber according to one or more embodiments is shown.

[0011] Figure 2 A portion of a ribbon optical fiber is shown according to one or more embodiments.

[0012] Figure 3 A schematic diagram of an optical fiber with a FBG is shown according to one or more embodiments.

[0013] Figure 4 A ribbon optical fiber is shown being inscribed by a FS laser according to one or more embodiments.

[0014] Figure 5 An inscribed optical fiber ribbon is shown according to one or more embodiments.

[0015] Fig. 6A and Figure 6B A top view and a side view, respectively, of a multi-phase mask according to one or more embodiments are shown.

[0016] Figure 7 A perspective view of a single phase of a multi-phase mask is shown according to one or more embodiments.

[0017] Figure 8 FS laser pulses are shown being focused on an optical fiber core by a lens, according to one or more embodiments.

[0018] Fig. 9 A schematic diagram of a FBG according to one or more embodiments is shown. Figure 2 ribbon optical fiber.

[0019] Fig.10 A cross-sectional view of a ribbon fiber according to one or more embodiments is shown. DETAILED DESCRIPTION

[0020] Figure 1 A cross-sectional view of an optical fiber 100 is shown. The optical fiber 100 includes a core 104 coated with a cladding 106, which together form an optical waveguide within the core 104. The cladding 106 may be, for example, a UV-cured polyurethane acrylate composite or a polyimide material applied to the core 104. The core 104 and the cladding 106 are dielectric materials, which may have a dielectric constant of 1.3 or higher, for example.

[0021] The optical fiber 100 is a cylindrical dielectric waveguide that transmits light 108 along the axis of the optical fiber 100. The light is confined in the core 104 by total internal reflection: light waves that reach the interface between the optical core 104 and the cladding 106 are not refracted into the cladding 106, but are effectively totally reflected back into the optical core 104. Total internal reflection occurs when the refractive index of the cladding 106 is higher than that of the optical core 104 and the light waves are incident on the interface at a sufficiently oblique angle. Several optical fibers 100 can be bundled into a ribbon fiber, such as Figure 2 shown and described below.

[0022] Figure 2 A portion of an optical fiber ribbon 200 is shown, in which N optical fibers 100 are bundled into the optical fiber ribbon 200. The N optical fibers 100 are numbered 1, 2, . . . , N-1, N.

[0023] In addition, optical fibers can be bundled. Bundled optical fibers that extend parallel to each other in a plane are called ribbon fibers. Ribbon fibers with FBGs have the advantages of dense transmission, simple and convenient splicing, and saving installation time and cost, so they are widely used. Ribbon fibers can include one or more FBGs in the fiber, and such fibers can have similar or different FBGs for different purposes, where the optical fibers are designed to transmit similar or different wavelengths of light. It is complex and expensive to produce each individual optical fiber with a specific FBG, and the performance of each fiber may not be consistent.

[0024] Although not shown, each end of the optical fiber 100 may typically be terminated in a pigtail ( Figure 2 ). A pigtail is a single, typically relatively short (relative to the entire length of the ribbon fiber 200) segment of optical fiber 100 that is separated from the ribbon fiber 200 and may be pre-installed with an optical connector on one end, and has a segment of optical fiber 100 exposed and separated at the other end. The pigtail may have any connector known in the art suitable for the intended purpose of the pigtail, such as a female connector, a male connector, a plug connector, or a receptacle connector.

[0025] Each cladding 106 may include a ribbon segment 108 having a color or marking that is different from the color or marking of the other claddings 106 to distinguish, for example, wavelengths that are blocked by a corresponding FBG (not shown) of that particular optical fiber 100. Figure 3 One or more of the optical fibers 100 may have notches 304 inscribed in the core 104, the core and notches together forming a FBG 302, as will be discussed further below.

[0026] The FBG 302 described herein can be written in an optical fiber by a pulsed laser. For example, a femtosecond laser can write the FBG by photothermal interaction of the laser beam with the core of the optical fiber. The interaction generates heat in the core, which generates cracks (i.e., notches 304) in the core, thereby causing the refractive index of the core to change. The size of the notches 304 generated by the FBG can be adjusted based on the intensity, length, and area of ​​the laser exposure.

[0027] Continue to refer Figure 3 , shows an exemplary optical fiber 100 with a schematically represented exemplary FBG 302. It should be noted that although Figure 3A generally uniform distribution of FBGs 302 is shown, but other distributions may be used depending on the desired design of a particular FBG. FBGs 302 may be semi-encapsulated by cladding 106, for example, after writing a ribbon FBG, there is no specific encapsulation housing, and both ends of the optical fiber are not equipped with any optical fiber plugs, thereby allowing end users to select one or more FBGs in the ribbon FBG according to their specific needs.

[0028] The refractive index of the plurality of exemplary scores 304 is different from the refractive index of the optical core 104' between the scores 304. Thus, the scores 304 produce a periodic variation in the refractive index in the core 104, thereby producing a wavelength-specific dielectric reflector. The scores 304 collectively block light waves having specific wavelengths, which may vary based on the score periodicity and distribution.

[0029] There are several processes for creating the notches 304 in the optical fiber 100. These methods include, but are not limited to, subsurface laser engraving, interference, sequential writing, photomasks, point-by-point writing, and UV lithography. These methods are described below.

[0030] Subsurface laser engraving can be used to engrave notches 304 in the optical core 104. This is accomplished by focusing a laser within the core 104 to act on the core 104 and create regions of different refractive index.

[0031] The interference process can use the interference pattern to produce the score 304 in the optical core 104. In this process, the UV laser is split into two beams that interfere with each other, thereby forming a periodic intensity distribution along the interference pattern. Therefore, the refractive index of the photosensitive core 104 changes according to the intensity of the light to which the core 104 is exposed.

[0032] Sequential writing produces a complex sub-grating profile by sequentially exposing a large number of small, partially overlapping gratings. The sub-gratings are formed by UV pulse exposure. The interfering UV beams are focused onto the core 104, and as the optical fiber 100 moves, the fringes move along the core 104 by translating the mirrors in the interferometer.

[0033] Point-by-point writing uses a single UV laser to write the notches 304 into the optical fiber core 104. For point-by-point writing, the beam of the UV laser is narrow.

[0034] UV lithography is an additional process that forms notches 304 by using intense UV light (such as a UV laser) to write variations in the refractive index into the core 104 of the optical fiber 100, for example, through interference and / or a mask. Extreme ultraviolet lithography (EUV lithography) is an optical lithography process that uses a range of extreme ultraviolet wavelengths to create patterns by exposing a reflective photomask to UV light that is reflected onto a substrate covered by a photoresist.

[0035] It is worth noting that in many processes, the writing light cannot penetrate the cladding of the optical fiber. Therefore, the cladding usually needs to be stripped from the core and cleaned. Stripping the cladding may cause physical damage to the core and may increase fiber pullout (failure due to weak bonding). Once the score is made, the core must be re-coated with the cladding. After the core is coated with the cladding again, the fiber may have to be tested. These steps are manually handled and difficult to automate, resulting in low production efficiency. This results in poor consistency of ribbon fibers.

[0036] To overcome the typical drawbacks of ribbon fibers with inscribed fibers, the scores 304 of the present disclosure are inscribed by a femtosecond laser (FS laser) according to the present disclosure.

[0037] The FS laser process of the present disclosure allows for direct processing of the ribbon fiber 100 in a manner different from typical multi-fiber Bragg grating and ribbon processing techniques, which require individual fiber Bragg gratings to be bonded or bundled together to form a ribbon fiber with FBGs and require additional molding processing, thereby increasing processing time. The individual fibers 100 of the ribbon fiber 200 can be written with gratings that are similar or different from each other for applications with similar or different wavelengths of light, but if necessary, individual fiber Bragg gratings can also be removed from the ribbon fiber (individual fiber tearing) to facilitate single-channel split-line fusion. As for advantages, the FS laser process is an improved and simpler process that does not require stripping of the cladding and then coating because the FS laser is able to penetrate the cladding. The FS laser provides short pulses with high peak power to immediately evaporate the material of the core before the heat can dissipate. Therefore, the FS laser is able to write smaller FBGs in the core compared to the indentations made by microsecond lasers and nanosecond lasers.

[0038] The FS laser process produces ribbon optical fiber 200 with good performance consistency, high tensile performance, easy automation and low cost.

[0039] The core can also be written with an ultra-wideband laser. To write the core with an ultra-wideband laser, the core can be loaded with hydrogen. Hydrogen atoms are injected into the core 104 of the optical fiber 100 to form germanium-oxygen defect centers with germanium as a dopant. Hydrogen loading can include incubating the optical fiber 100 in a high pressure hydrogen environment (1200 to 2000 psi for about 4 to 14 days), which allows the hydrogen atoms to penetrate into the optical core 104. However, writing the core with a FS laser does not require core loading with hydrogen.

[0040] Figure 4FS laser 402 is shown focusing laser beam 404 on optical fiber 100 of ribbon fiber 200 to create point-by-point scores 304 in core 104. FS laser 402 generates laser pulses with femtosecond duration and high peak power to instantly vaporize material of core 104, thereby creating scores 304.

[0041] The FS laser can be an infrared laser. The wavelength of the FS laser can vary between 980nm and 1550nm. An exemplary average wavelength of the FS laser can be 1053nm. In some embodiments, the FS laser is a Nd:YAG laser, which uses a Nd:YAG crystal as a laser medium. The Nd:YAG crystal is doped with triple ionized neodymium Nd (III). Neodymium ions provide laser activity in the Nd:YAG crystal. The Nd:YAG laser produces a rapidly expanding cloud of free electrons and ionized molecules. Therefore, in some embodiments, the pulse duration of the Nd:YAG laser is between nanoseconds (10-9 seconds) and femtoseconds (10-15 seconds). In other embodiments, the FS laser has a fundamental wavelength of 1030nm. However, based on the frequency doubling of the laser, the wavelength used is 515nm. Since FS laser has extremely narrow pulse width and extremely high peak power, spatial length and tens of microns, the converged femtosecond laser has enough energy power at the focus to carve gratings on the fiber core, while the energy penetrating the coating will not destroy the structure of the coating.

[0042] Laser beam 404 writes a first point in the core of the optical fiber. Then, the FS laser stops lasing and the FS laser moves along the optical fiber to be written. After moving the FS laser, the FS laser is turned on again and writes a second point on the core of the same optical fiber. Writing the core point by point is called point by point writing. Precision motion system 400 can move objective lens 402 for precise placement of notch 304 in core 104. Alternatively, ribbon fiber 200 can be moved while the FS laser writes notch 304. In another alternative, ribbon fiber 200 and the FS laser can be moved while the FS laser writes FBG 304.

[0043] To overcome laser beam distortion caused by the inherent curvature of the fiber geometry, the optical fiber 100 can be placed in a square capillary seal (not shown) containing a refractive index matching fluid (e.g., glycerol) so that the surface geometry presented to the path of the incident FS laser beam 404 is flat.

[0044] The focus of the FS laser is inside the core 104 to avoid stripping the cladding 106 from the core 104. In addition, the laser beam 404 of the FS laser passes through the cladding 106 and does not damage the cladding 106.

[0045] To further improve point-by-point FS laser writing and reduce fabrication time and inconsistency, an alternating phase-shift mask can be used to write the core of the fiber without moving the FS laser.

[0046] The phase shift mask can be transparent in some areas and opaque in other areas, or thicker in some areas and thinner in other areas to form a pattern, and can be formed by photolithography. In one example, the phase mask can be photolithographically written to write regular thick and thin stripes on a substrate of fused silica. However, other phase shift mask materials (such as calcium fluoride) can also be used. The stripes are caused by defects carved into the material. General parameters of the phase mask template include period, chirp rate, etc. These parameters ultimately determine the FBG refractive index parameters that can be written into the optical fiber through the phase mask. The phase shift mask can change the phase of the FS laser pulse passing through the phase shift mask to form a diffraction pattern behind the phase shift mask. The diffraction pattern can be used as the basis for writing the FBG in the core.

[0047] The disclosed embodiments may also include multiple phase-shifting masks for writing multiple diffraction patterns in multiple optical fibers.

[0048] Figure 5 An apparatus for producing a ribbon optical fiber 200 with notches 304 in an optical fiber 100 using a FS laser is shown. A laser beam 504 is focused by an objective lens (not shown). The focused laser beam 504 passes through a phase shift mask 502 having a plurality of regions therein for producing a plurality of diffraction patterns. The phase shift mask 502 diffracts the FS laser pulse into a diffraction pattern that is projected onto the core of the optical fiber. The core is then inscribed according to the diffraction pattern. The phase shift mask 502 serves as a template for FS laser inscription.

[0049] and Figure 4 Compared with the point-by-point writing shown, Figure 5 The writing technique shown using a multiphase mask writes the entire length of the core 104 at one time. Figure 4 Compared with the writing technology shown in Figure 5 The writing technology is more efficient and more suitable for industrial production.

[0050] In addition, to further improve productivity, the phase shift mask 502 and / or the ribbon fiber 200 can be moved, for example, via a linear slider to write additional fibers. For example, if other optical fibers 100 of the ribbon fiber 200 are to be written with the same FBG from the first phase of the phase shift mask 502, only the ribbon fiber 200 needs to be moved before additional writing. In addition, if other optical fibers 100 of the ribbon fiber 200 are to be written with different FBGs, the phase shift mask 502 is also shifted to align different areas of the multi-phase shift mask 502 with the laser beam 504 and the new optical fiber 100, so that the FS laser illuminates the second area of ​​the multi-phase mask instead of the first area. Maintaining the laser and associated optical devices stationary prevents the need for additional calibration processes and improves manufacturing productivity. In one example, the size of the phase shift mask 502 can be approximately about 25 mm by about 30 mm, where the size of each individual phase mask for each diffraction pattern is about 25 mm by about 3 mm. However, the overall size of each phase shift mask can vary.

[0051] Fig. 6A and Figure 6B A top view of a multi-phase mask 502 having multiple phases 502A is shown. Fig. 6A ) and side view ( Figure 6B In one example, the multiphase mask 502 can have an outer length B from about 30 mm to about 50 mm, an outer width A from about 25 mm to about 40 mm, and an outer thickness C (orthogonal to the outer length B and the outer width A) from about 2 mm to about 3 mm.

[0052] The multiphase mask 502 is schematically shown as having a plurality of phases 502A. In the example shown, there are four phases 502A, each of which is schematically represented as being different in their respective modes. In one example, each phase 502A can have a phase length D from about 25 mm to about 45 mm and a phase width E from about 8 mm to about 10 mm.

[0053] Figure 7 A single phase 502A (eg Fig. 6A 502A is shown as a perspective view of one of the phases 502A shown as regular narrow groove stripes. Phase 502A has a pattern of thin regions 510 and thick regions 514 of the phase mask. The thickness of the thick regions 514 is approximately equal to the outer thickness C of the multi-phase mask 502, and the thickness 512 of the thin regions 514 is approximately equal to the thickness C minus the depth 516, which can be several nanometers (e.g., between about 100 nm and about 350 nm) and is depicted as grooves in the phase mask 502A.

[0054] Figure 8 Shown according to Figure 5 Equipment for writing FBG. Figure 5In addition to the equipment Figure 8 The apparatus further comprises a lens 604 which focuses the FS laser pulses along the first region of the polyphase mask 502. The lens 604 is implemented as a cylindrical lens 604 by way of example only.

[0055] Fig. 9 Shown according to Figure 2 1. The ribbon fiber 200 has scores 304 written in the fiber core 104 by a FS laser. The markings 704 on the fiber 100 may indicate the location of the written scores 304 or other details related to a particular FBG score (such as the configured wavelength).

[0056] Fig.10 A cross-sectional view of three different ribbon fibers 700A, 700B, 700C having different FBGs is shown. The optical fiber 100 in all three ribbon fibers 700A, 700B, 700C includes a cladding 106. The optical fiber 100 in each of the ribbon fibers 700A, 700B, 700C includes a core 104A, 104B, 104C, respectively, and each ribbon fiber 700A, 700B, 700C may include the same FBG in each ribbon fiber, or include different FBGs in each ribbon fiber (in the optical fiber 100 included in each ribbon fiber).

[0057] The above description of preferred embodiments and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concept conceived by the applicant. With the benefit of this disclosure, it will be understood that the above-described features according to any embodiment or aspect of the disclosed subject matter can be utilized in any other embodiment or aspect of the disclosed subject matter alone or in combination with any other described features.

[0058] In exchange for disclosing the inventive concepts contained herein, the applicants desire all patent rights afforded by the appended claims. Accordingly, the appended claims are intended to cover all modifications and variations that come within the scope of the appended claims or their equivalents.

Claims

1. A device for writing a grating, wherein include: A femtosecond FS laser, wherein the femtosecond laser is suitable for emitting FS laser pulses; A phase-shifting mask adapted to diffract the FS laser pulses into a diffraction pattern capable of being projected onto a first optical fiber, wherein the diffracted FS laser pulses are adapted to write a grating on a core of the first optical fiber according to the diffraction pattern.

2. The apparatus of claim 1, wherein the grating is a fiber Bragg grating.

3. The apparatus of claim 1, wherein the phase-shift mask comprises a first region having alternating first thin regions and first thick regions, the first thin regions and the first thick regions being adapted to shift the phase of the FS laser pulses.

4. The apparatus of claim 3, wherein the size of the FS laser pulses is designed to illuminate the first thin region and the first thick region so that the phase-shifted FS laser pulses diffract into the diffraction pattern.

5. The device of claim 1, wherein The phase-shift mask is a multi-phase mask comprising a plurality of phase-shift masks, wherein at least other phase-shift masks of the plurality of phase-shift masks comprise alternating second thin regions and second thick regions, wherein the second thin regions and the second thick regions are adapted to shift the phase of the FS laser pulse by predetermined third and fourth phase angles, respectively.

6. The apparatus of claim 5, further comprising a linear slide adapted to provide linear relative motion of the FS laser and the polyphase mask.

7. The apparatus of claim 5, wherein the apparatus is adapted to adjustably hold a ribbon optical fiber including the first optical fiber and the second optical fiber, and the ribbon optical fiber and the multi-phase mask are linearly adjustable to optically align the plurality of phase-shifting masks with the first optical fiber and the second optical fiber, respectively.

8. The apparatus of claim 1, wherein the FS laser is a Nd:YAG laser.

9. The apparatus of claim 1, wherein the average wavelength of the FS laser is between 980 nm and 1550 nm, or between 1050 nm and 1060 nm.

10. A method for writing a grating, wherein include: The femtosecond FS laser pulse is emitted onto the phase-shift mask; diffracting the FS laser pulse through the phase-shift mask to form a diffraction pattern; projecting the diffraction pattern through the cladding of the first optical fiber onto the core of the first optical fiber; as well as A grating is written on the core according to the diffraction pattern.

11. The method of claim 10, further comprising focusing the diffraction pattern along the core of the first optical fiber.

12. The method of claim 10, wherein the grating is a fiber Bragg grating.

13. The method of claim 10, wherein the phase-shift mask comprises a first region having alternating first thin regions and first thick regions, the first thin regions and the first thick regions being adapted to shift the phase of the FS laser pulses.

14. The method of claim 10, wherein the phase-shift mask is a multi-phase mask having a plurality of phase-shift masks, the method further comprising: include: linearly adjusting the multi-phase mask to align other phase-shift masks of the plurality of phase-shift masks with the FS laser pulse; diffracting the FS laser pulse through the other phase-shift masks of the plurality of phase-shift masks to form a second diffraction pattern; projecting the second diffraction pattern onto a second core of the second optical fiber through a second cladding of the second optical fiber; as well as A second grating is inscribed on the second core according to the second diffraction pattern.

15. The method of claim 14, wherein the linearly adjusting step comprises adjusting a linear slider.

16. The method of claim 14, further comprising an optical fiber ribbon, wherein the optical fiber ribbon includes the first optical fiber and the second optical fiber.

17. The method of claim 10, wherein the average wavelength of the FS laser pulses is between about 980 nm and about 1550 nm, or between about 1050 nm and about 1060 nm.

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