CN120028904A - Apparatus and method for manufacturing ribbon optical fiber with fiber Bragg gratings - Google Patents

Abstract

Disclosed are an apparatus and method for manufacturing a ribbon optical fiber having a fiber Bragg grating, wherein an apparatus for writing the grating comprises: a femtosecond (FS) laser that emits FS laser pulses; a phase-shifting mask that is suitable for diffracting the FS laser pulses according to forming a diffraction pattern that can be projected onto a first optical fiber, wherein the diffracted FS laser pulses are suitable for writing the grating on the core of the first optical fiber according to the diffraction pattern.

Description

Apparatus and method for manufacturing ribbon fiber with fiber Bragg grating

Technical Field

The subject matter of the present disclosure relates to the field of optical communication technology and to methods for their production. More particularly, the presently disclosed subject matter relates to an apparatus for writing fiber bragg gratings using a Femtosecond (FS) laser and a phase shift mask and a method for fabricating ribbon fiber bragg gratings by the femtosecond laser.

Background

The optical fiber is a flexible transparent optical fiber for transmitting light. The optical fiber acts as a waveguide. Some optical fibers use a reflector known as a distributed bragg reflector. The distributed bragg reflector comprises multiple layers of alternating materials having different refractive indices. Each layer partially reflects light waves. Light waves having a particular wavelength are combined with constructive interference at the distributed bragg reflector and reflected at the layers.

A Fiber Bragg Grating (FBG) is a distributed bragg reflector in an optical fiber that reflects light waves having a specific wavelength and transmits all other light waves. In other words, FBG is a change in refractive index in an optical fiber. The periodic variation of the refractive index acts as a wavelength specific mirror. FBGs block or reflect light waves having a specific wavelength and transmit other light waves.

FBGs are important optical passive components that play an important role in optical communications, access networks, sensors and signal detection. With the continued development of technology, the demand for FBGs having faster transmission speeds is also increasing. Conventional beam FBGs or single FBGs may not meet these requirements.

The presently disclosed subject matter includes apparatus and methods for inscribing smaller sized FBGs in optical fibers and improved inscription techniques.

Disclosure of Invention

Disclosed herein are devices for inscribing gratings in, for example, optical fibers. In one aspect of the disclosure, an apparatus includes a Femtosecond (FS) laser adapted to emit FS laser pulses, a phase shift mask adapted to diffract the FS laser pulses into a diffraction pattern that is projectable on 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. In another aspect of the disclosure, the grating is a fiber bragg grating. In another aspect of the disclosure, a phase shift mask includes a first region having alternating first thin regions and first thick regions adapted to phase shift FS laser pulses. In yet another aspect of the disclosure, the FS laser pulse is sized to illuminate the first thin region and the first thick region such that the phase shifted FS laser pulse diffracts into a diffraction pattern.

In one aspect of the 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 adapted to shift the phases of the FS laser pulses by a predetermined phase angle, respectively. In another aspect of the disclosure, an apparatus further includes a linear slide adapted to provide linear relative movement of the FS laser and the multiphase mask. In one aspect of the disclosure, the apparatus is adapted to adjustably hold a ribbon fiber including a first fiber and a second fiber, and the ribbon fiber and the multi-phase mask are linearly adjustable to optically align the plurality of phase shift masks with the first fiber and the second fiber, respectively. In yet another aspect of the present disclosure, the FS laser is a Nd: YAG laser. In another aspect of the present disclosure, the FS laser has an average wavelength between 980nm and 1550nm, or between 1050nm and 1060 nm.

Methods for inscribing a grating are disclosed herein. In one aspect of the method, the method includes emitting a Femtosecond (FS) laser pulse onto a phase shift mask, diffracting the FS laser pulse via the phase shift mask to form a diffraction pattern, projecting the diffraction pattern onto a core of a first optical fiber through a 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 further comprises 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 present disclosure, a phase shift mask includes a first region having alternating first thin regions and first thick regions adapted to phase shift FS laser pulses.

The disclosed exemplary method 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 ones of the plurality of phase shift masks with the FS laser pulses, diffracting the FS laser pulses via the other ones of the plurality of phase shift masks to form a second diffraction pattern, projecting the second diffraction pattern through a second cladding of a second optical fiber onto a second core of the second optical fiber, and writing the second grating on the second core according to the second diffraction pattern in another aspect of the disclosure, the linearly adjusting step including adjusting the linear slider. In another aspect of the disclosure, a inscription method is performed on a ribbon fiber, wherein the ribbon fiber includes a first fiber and a second fiber. In yet another aspect of the present disclosure, the disclosed FS laser pulses have an average wavelength between about 980nm and about 1550nm, or between about 1050nm and about 1060 nm.

Drawings

FIG. 1 illustrates a cross-sectional view of an optical fiber in accordance with one or more embodiments.

FIG. 2 illustrates a portion of a ribbon fiber in accordance with one or more embodiments.

FIG. 3 illustrates a schematic diagram of an optical fiber with an FBG in accordance with one or more embodiments.

FIG. 4 illustrates a ribbon fiber inscribed by a FS laser in accordance with one or more embodiments.

FIG. 5 illustrates a inscribed ribbon fiber according to one or more embodiments.

Fig. 6A and 6B illustrate top and side views, respectively, of a multi-phase mask in accordance with one or more embodiments.

FIG. 7 illustrates a perspective view of a single phase of a multi-phase mask in accordance with one or more embodiments.

Fig. 8 illustrates FS laser pulses focused by a lens onto a fiber core in accordance with one or more embodiments.

FIG. 9 illustrates the ribbon fiber of FIG. 2 with FBG's in accordance with one or more embodiments.

FIG. 10 illustrates a cross-sectional view of a ribbon fiber in accordance with one or more embodiments.

Detailed Description

Fig. 1 shows a cross-sectional view of an optical fiber 100. The optical fiber 100 includes a core 104 coated with a cladding 106 that together form an optical waveguide within the core 104. The cladding 106 may be, for example, a UV cured urethane acrylate composite or 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.

The optical fiber 100 is a cylindrical dielectric waveguide that transmits light 108 along the axis of the optical fiber 100. Light is confined in the core 104 by total internal reflection-light waves arriving at 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 the optical core 104 and the light waves are incident on the interface at a sufficiently oblique angle. Several optical fibers 100 may be bundled into a ribbon fiber as shown in fig. 2 and described below.

Fig. 2 shows a portion of a ribbon fiber 200 in which N optical fibers 100 are bundled into the ribbon fiber 200.N optical fibers 100 are numbered 1, 2, &..the N-1, N.

Furthermore, the optical fibers may be bundled. Bundled optical fibers extending parallel to each other in a plane are referred to as ribbon fibers. The ribbon fiber with the FBG has the advantages of dense transmission, simple and convenient splicing, installation time and cost saving, and the like, and is widely used. The ribbon fiber may include one or more FBGs in the fiber, and such fiber may have similar or different FBGs for different applications, where the fiber is designed to transmit light of similar or different wavelengths. Producing individual fibers each with a particular FBG is complex and expensive, and the performance of each fiber may not be uniform.

Although not shown, each end of the optical fiber 100 may generally terminate in a pigtail (not shown in FIG. 2). A pigtail is a single, generally relatively short (relative to the entire length of the ribbon fiber 200) section of optical fiber 100 that is separated from the ribbon fiber 200 and may have an optical connector pre-installed on one end and a section of exposed and separated optical fiber 100 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.

Each cladding 106 may include a ribbon section 108 having a color or marking that is different from the color or marking of the other cladding 106 for distinguishing wavelengths that are blocked, for example, by a corresponding FBG (not shown) of that particular optical fiber 100. Referring to FIG. 3, one or more of the optical fibers 100 may have a notch 304 inscribed in the core 104, the core and notch together forming an FBG 302, as will be discussed further below.

The FBG 302 described herein may be inscribed in an optical fiber by a pulsed laser. For example, a femtosecond laser can inscribe an FBG by photo-thermal interaction of a laser beam with the core of an optical fiber. The interaction generates heat in the core that creates a crack (i.e., score 304) in the core, resulting in a change in the refractive index of the core. The size of the generated notch 304 of the FBG can be adjusted based on the intensity, length and area of the laser exposure.

With continued reference to FIG. 3, an exemplary optical fiber 100 having an exemplary FBG 302 schematically represented is shown. It should be noted that while FIG. 3 shows a substantially uniform distribution of FBGs 302, other distributions may be used depending on the desired design of the particular FBG. The FBG 302 may be semi-encapsulated by the cladding 106, e.g., after writing the ribbon FBGs, without a specific encapsulation housing, and without any fiber plugs at both ends of the fiber, thereby allowing the end user to select one or more of the ribbon FBGs according to their specific needs.

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 notch 304 causes a periodic variation in the refractive index in the core 104, thereby creating a wavelength specific dielectric mirror. Together, the scores 304 block light waves having specific wavelengths, which may vary based on score periodicity and distribution.

There are several processes for creating the score 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 will be described below.

Subsurface laser engraving may be used to engrave the score 304 in the optical core 104. This is accomplished by focusing the laser within the core 104 to act upon the core 104 and create regions of different refractive index.

The interference process may use an interference pattern to create the score 304 in the optical core 104. In this process, the UV laser is split into two beams that interfere with each other, forming a periodic intensity distribution along the interference pattern. Accordingly, the refractive index of the photosensitive core 104 varies according to the intensity of the light to which the core 104 is exposed.

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

Spot-wise inscription a single UV laser is used to inscribe the score 304 into the optical core 104. For point-by-point writing, the beam of UV laser is narrower.

UV lithography is an additional process of forming the notch 304 by writing a change in refractive index into the core 104 of the optical fiber 100 using strong UV light (such as a UV laser), for example, by interference and/or masking. Extreme ultraviolet lithography (EUV lithography) is an optical lithography process that uses a range of extreme ultraviolet wavelengths to create a pattern by exposing a reflective photomask to UV light that is reflected onto a substrate covered by a photoresist.

Notably, in many processes, the inscription light cannot penetrate the cladding of the fiber. Thus, the cladding is typically 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 adhesion). Once the score is formed, the core must be recoated with cladding. After coating the core with cladding again, it may be necessary to test the fiber. These steps are manual and difficult to automate, resulting in low production efficiency. This results in poor ribbon fiber consistency.

To overcome typical drawbacks of ribbon fibers with inscribing fibers, the score 304 of the present disclosure is inscribed by a femtosecond laser (FS laser) according to the present disclosure.

The FS laser process of the present disclosure allows for the direct processing of ribbon fiber 100 in a manner different from typical multi-fiber grating and ribbon processing techniques that require bonding or bundling individual fiber gratings together to form a ribbon fiber with FBGs and require additional molding processing, thereby increasing processing time. Individual fibers 100 of ribbon fiber 200 may be inscribed with gratings similar or different from each other for similar or different wavelength light applications, but individual fiber gratings (individual fiber tears) may also be removed from the ribbon fiber if desired to facilitate single channel split-line fusion splicing. As an advantage, the FS laser process is an improved and simpler process that does not require stripping of the cladding and then recoating, as the FS laser is able to penetrate the cladding. FS lasers provide short pulses with high peak power to evaporate the material of the core immediately before the heat can dissipate. Thus, the FS laser is able to scribe smaller FBGs in the core than is possible with micro-and nanosecond laser-made scores.

The FS laser process produces ribbon fiber 200 with good uniformity of performance, high tensile properties, ease of automated production, and low cost.

The core may also be inscribed with ultra-wideband laser. To write the core with an ultra-wideband laser, the core may be loaded with hydrogen. Hydrogen atoms are implanted into the core 104 of the optical fiber 100 to form germanium-oxygen defect centers having germanium as a dopant. Hydrogen loading may include incubating the optical fiber 100 in a high pressure hydrogen environment (1200 to 2000psi maintained for about 4 to 14 days) that causes hydrogen atoms to penetrate into the optical fiber core 104. However, writing the core with FS laser does not require hydrogen loading of the core.

Fig. 4 shows FS laser 402 focusing laser beam 404 on fiber 100 of ribbon fiber 200 to create point-wise score 304 in core 104. FS laser 402 generates laser pulses with a femtosecond duration and high peak power to immediately vaporize the material of core 104, thereby creating score 304.

The FS laser may be an infrared laser. The wavelength of the FS laser may vary between 980nm and 1550 nm. An exemplary average wavelength of the FS laser may be 1053nm. In some embodiments, the FS laser is a Nd: YAG laser that uses Nd: YAG crystals as the lasing medium. YAG crystals are doped with triple ionized neodymium Nd (III). The neodymium ions provide laser activity in the Nd: YAG crystal. YAG laser produces rapidly expanding free electrons and ionized molecular clouds. Thus, 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 1030 nm. However, based on the frequency multiplication of the laser, the wavelength used was 515nm. Because FS lasers have extremely narrow pulse widths and extremely high peak powers, spatial lengths, and tens of microns, the converging femtosecond laser has enough energy power at the focal point to engrave the grating on the core, while the energy penetrating the coating does not destroy the structure of the coating.

The laser beam 404 inscribes a first spot in the core of the fiber. Then, the FS laser stops emitting laser light and moves along the optical fiber to be inscribed. After moving the FS laser, the FS laser is turned on again and writes a second point on the core of the same fiber. The writing of the core point by point is referred to as point by point writing. Precision motion system 400 may move objective lens 402 for precise placement of score 304 in core 104. Alternatively, ribbon fiber 200 may be moved while FS laser inscribes score 304. In another alternative, the ribbon fiber 200 and the FS laser may be moved while the FS laser inscribes the FBG 304.

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

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.

To further improve spot-wise FS laser writing and reduce manufacturing time and non-uniformity, an alternating phase shift mask may be used to write the core of the fiber without moving the FS laser.

The phase shift mask may 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 may be formed by photolithography. In one example, the phase mask may be lithographically etched to write regular thin and thick stripes on the fused silica substrate. However, other phase shift mask materials (e.g., calcium fluoride) may also be used. The streaks are caused by defects engraved in the material. Typical parameters of the phase mask include period, chirp rate, and the like. These parameters ultimately determine the FBG refractive index parameters that can be written into the fiber through the phase mask. The phase shift mask may 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 a basis for inscribing FBGs in the fiber core.

The disclosed embodiments may also include a plurality of phase shift masks for inscribing a plurality of diffraction patterns in a plurality of optical fibers.

Fig. 5 shows an apparatus for producing a ribbon fiber 200 with a score 304 in the fiber 100 using an FS laser. The 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 fiber. The core is then inscribed according to the diffraction pattern. The phase shift mask 502 serves as a template for FS laser writing.

In contrast to the point-by-point inscription shown in fig. 4, the inscription technique using a multi-phase mask shown in fig. 5 inscribes the entire length of the core 104 at one time. Therefore, the writing technique according to fig. 5 is more efficient and more suitable for industrial production than the writing technique shown in fig. 4.

In addition, to further increase productivity, the phase shift mask 502 and/or the ribbon fiber 200 may be moved, for example, via a linear slider, to inscribe additional fibers. For example, if the other fibers 100 of the ribbon fiber 200 are to be inscribed with the same FBG of the first phase from the phase shift mask 502, then only the ribbon fiber 200 needs to be moved before additional inscription can be made. In addition, if other 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 regions of the multi-phase shift mask 502 with the laser beam 504 and the new fiber 100 so that the FS laser irradiates a second region of the multi-phase mask instead of the first region. Keeping the laser and associated optics stationary prevents the need for additional calibration processes and improves manufacturing productivity. In one example, the dimensions of the phase shift mask 502 may be approximately about 25mm by about 30mm, with the size of each individual phase mask for each diffraction pattern being about 25mm by about 3mm. The overall dimensions of each phase shift mask may vary.

Fig. 6A and 6B show top view (fig. 6A) and side view (fig. 6B) of a multi-phase mask 502 having a plurality of phases 502A. In one example, the multi-phase mask 502 may have an outer length B of from about 30mm to about 50mm, an outer width a of from about 25mm to about 40mm, and an outer thickness C of from about 2mm to about 3mm (orthogonal to the outer length B and the outer width a).

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

Fig. 7 shows a perspective view of a single phase 502A (e.g., one of the phases 502A shown as regular slot stripes of fig. 6A). Phase 502A has a pattern of thin regions 510 and thick regions 514 of the phase mask. The thickness of the thick region 514 is approximately equal to the outer thickness C of the multi-phase mask 502, and the thickness 512 of the thin region 514 is approximately equal to the thickness C minus a depth 516, which may be a few nanometers (e.g., between about 100nm and about 350 nm) and is depicted as a groove in the phase mask 502A.

FIG. 8 shows the apparatus for inscribing the FBG according to FIG. 5. In addition to the apparatus of fig. 5, the apparatus of fig. 8 also includes a lens 604 that focuses the FS laser pulses along a first region of the multi-phase mask 502. For example only, the lens 604 is implemented as a cylindrical lens 604.

Fig. 9 shows a ribbon fiber 200 according to fig. 2 with a score 304 inscribed in the core 104 by the FS laser. The markings 704 on the optical fiber 100 may indicate the location of the inscribed notch 304 or other details (such as configured wavelength) related to the particular FBG notch.

FIG. 10 shows a cross-sectional view of three different ribbon fibers 700A, 700B, 700C with different FBGs. The optical fibers 100 in all three ribbon fibers 700A, 700B, 700C include 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 contain the same FBG within each ribbon fiber, or different FBGs within each ribbon fiber (in the optical fiber 100 contained in each ribbon fiber).

The above description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the applicant. It will be appreciated, given the benefit of this disclosure, that features described above in accordance with any embodiment or aspect of the disclosed subject matter may be utilized alone or in combination with any other described features in any other embodiment or aspect of the disclosed subject matter.

In exchange for disclosing the inventive concepts contained herein, the applicant expects to obtain all patent rights afforded by the appended claims. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the scope of the claims or their equivalents.

Claims (17)

1. An apparatus for inscribing a grating, comprising:

A femtosecond FS laser adapted to emit FS laser pulses;

A phase shift mask adapted to diffract the FS laser pulse into a diffraction pattern capable of being projected onto a first optical fiber, wherein the diffracted FS laser pulse is adapted to inscribe 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 adapted to phase shift the FS laser pulses.

4. The apparatus of claim 3, wherein the FS laser pulse is sized to illuminate the first thin region and the first thick region such that the phase shifted FS laser pulse diffracts into the diffraction pattern.

5. The apparatus 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 adapted to shift the phase of the FS laser pulse by a predetermined third phase angle and fourth phase angle, respectively.

6. The apparatus of claim 5, further comprising a linear slider adapted to provide linear relative movement of the FS laser and the multiphase mask.

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

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

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

10. A method for inscribing a grating, comprising:

transmitting the femtosecond FS laser pulse onto the phase shift mask;

Diffracting the FS laser pulses through the phase shift mask to form a diffraction pattern;

projecting the diffraction pattern through the cladding of a first optical fiber onto the core of the first optical fiber, and

And writing a grating 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 adapted to phase shift 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:

Linearly adjusting the multiphase mask to align other phase shift masks of the plurality of phase shift masks with the FS laser pulse;

diffracting the FS laser pulses through the other phase shift mask of the plurality of phase shift masks to form a second diffraction pattern;

Projecting the second diffraction pattern through a second cladding of a second optical fiber onto a second core of the second optical fiber, and

And writing a second grating 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 slide.

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

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