LOW SCATTERING LOSS HIGH TEMPERATURE STABLE FIBER BRAGG GRATING SENSOR BASED ON MICROPORE FORMATION AND METHOD FOR PRODUCING SAME

Abstract

A method and apparatus for inscribing a high-temperature stable Bragg grating in an optical waveguide, comprising the steps of: providing the optical waveguide; providing electromagnetic radiation from an ultrashort pulse duration laser, wherein the wavelength of the electromagnetic radiation has a characteristic wavelength in the wavelength range from 150 nanometers (nm) to 2.0 micrometers (m); providing cylindrical focusing optics; providing a diffractive optical element that when exposed to the focused ultrashort laser pulse, creates an interference pattern on the optical waveguide, wherein the irradiation step comprises irradiating a surface of the diffractive optical element with the focused electromagnetic radiation, the electromagnetic radiation incident on the optical waveguide, from the diffractive optical element, being sufficiently intensive to cause permanent (Type II) change in the index of refraction within multiple Bragg grating planes in the core of the optical waveguide resulting from at least one micropore.

Claims

1. A method for inscribing a Bragg grating in an optical waveguide, comprising the steps of: providing electromagnetic radiation from an ultrashort pulse duration laser; providing a focusing optical element to focus the electromagnetic radiation from an ultrashort pulse duration laser; providing a diffractive optical element that when exposed to the focused electromagnetic radiation generates a beam on the optical waveguide having an interference pattern; and irradiating the optical waveguide with the electromagnetic radiation to form a Bragg grating, the electromagnetic radiation incident on the optical waveguide being sufficiently intensive to cause a permanent (Type II) change in the index of refraction within multiple Bragg grating planes in the core of the optical waveguide resulting from at least one micropore.

2. The method of claim 1 wherein the electromagnetic radiation comprises a single laser pulse.

3. The method of claim 1 wherein the electromagnetic radiation comprises plurality of laser pulses, wherein said plurality is equal or less than ten.

4. The method of claim 1, wherein the electromagnetic radiation has a pulse duration of less than or equal to 1 picosecond.

5. The method of claim 1, wherein the wavelength of the electromagnetic radiation is in a range from 150 nm to 2.0 microns.

6. The method of claim 1, further comprising providing a focusing optical element corrected for spherical aberration for focusing the electromagnetic radiation on the diffractive optical element.

7. The method of claim 1, further comprising providing a cylindrical lens corrected for spherical aberration for focusing the electromagnetic radiation on the diffractive optical element.

8. The method of claim 1, further comprising positioning the optical fiber at a distance with respect to the diffractive optical element where the confocal parameter of a line-shaped laser focus created by the focusing optical element is smallest and the peak intensity in the focus is highest due to substantial or complete cancelation of i) negative spherical aberration and conical diffraction caused by the diffractive optical element and ii) chromatic aberration of the focusing optical element and chromatic dispersion of the diffractive optical element.

9. The method of claim 1, wherein the change in the index of refraction within multiple Bragg grating planes in the core of the optical waveguide results from at least one elongated micropore.

10. The method of claim 1, wherein the change in the index of refraction within multiple Bragg grating planes in the core of the optical waveguide results from a plurality of spherical micropores.

11. The method of claim 1, wherein the scattering loss due to the permanent (Type II) change in the index of refraction in the optical waveguide is less than 10.sup.5 dB per grating period.

12. Apparatus for inscribing a Bragg grating in an optical waveguide, comprising: an ultrashort pulse duration laser for providing electromagnetic radiation; a focusing optical element to focus the electromagnetic radiation from an ultrashort pulse duration laser; and a diffractive optical element that when exposed to the focused electromagnetic radiation from the focusing optical element produces an interference pattern in the optical waveguide, wherein positioning the optical waveguide at a distance with respect to the diffractive optical element along the propagation direction of the electromagnetic radiation where the confocal parameter of line-shaped laser focus is smallest and the peak intensity in the focus is highest causes the of i) negative spherical aberration and conical diffraction caused by the diffractive optical element and ii) chromatic aberration of the focusing optical element and chromatic dispersion of the diffractive optical element to substantially or completely cancel each other out; and wherein irradiating the optical waveguide with the electromagnetic radiation forms a Bragg grating, the electromagnetic radiation incident on the optical waveguide being sufficiently intensive to cause permanent (TypeII) change in the index of refraction within multiple Bragg grating planes in the core of the optical waveguide resulting from at least one micropore when exposed to one of either a single laser pulse or a plurality of laser pulses, wherein said plurality is equal or less than ten.

13. The apparatus of claim 12, wherein the at least one micropore comprises at least one elongated micropore.

14. The apparatus of claim 12, wherein the at least one micropore comprises a plurality of spherical micropores.

15. The apparatus of claim 12, wherein the scattering loss due to the permanent (Type II) change in the index of refraction is less than 10.sup.5 dB per grating period.

16. The apparatus of claim 12, wherein the optical waveguide is an optical fiber.

17. The apparatus of claim 12, wherein the optical waveguide is a polymer-coated optical fiber.

18. The apparatus of claim 12, wherein the optical waveguide is a buried channel waveguide.

19. The apparatus of claim 12, wherein the optical waveguide is a ridge waveguide.

20. The apparatus of claim 12, wherein the electromagnetic radiation has a pulse duration of less than or equal to 1 picosecond.

21. The apparatus of claim 12, wherein the wavelength of the electromagnetic radiation is in a range from 150 nm to 2.0 microns.

22. The apparatus of claim 12, wherein the ultrashort pulse duration laser comprises a Ti-sapphire regeneratively amplified laser system operating at a central wavelength of 800 nm.

23. The apparatus of claim 12, wherein the diffractive optical element comprises a uniformly pitched phase mask.

24. The apparatus of claim 12, wherein the diffractive optical element comprises a chirped phase mask.

25. The apparatus of claim 12, wherein the diffractive optical element comprises a phase-shifted phase mask.

26. The apparatus of claim 12, further comprising providing a focusing optical element corrected for spherical aberration for focusing the electromagnetic radiation on the diffractive optical element.

27. The apparatus of claim 12, further comprising providing a cylindrical lens corrected for spherical aberration for focusing the electromagnetic radiation on the diffractive optical element.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 depicts an interferometric setup based on the phase mask technique for the inscription of a fiber Bragg grating in an optical fiber, according to an exemplary embodiment.

[0028] FIG. 2 denotes the orientation of the electric field vector E of the pulses (i.e., pulse polarization) with respect to the optical fiber of FIG. 1.

[0029] FIG. 3, comprising FIGS. 3(a) and 3(b), shows SEM images of modification in the core of the optical fiber induced by a single laser pulse, where FIG. 3(a) denotes an SEM image in backscattered electrons and FIG. 3(b) denotes an SEM image in secondary electrons.

[0030] FIG. 4, comprising FIGS. 4(a) and 4(b), shows SEM images of modification in the core of the optical fiber induced by five laser pulses, where FIG. 4(a) denotes an SEM image in backscattered electrons and FIG. 4(a) denotes an SEM image in secondary electrons.

DETAILED DESCRIPTION OF THE INVENTION

[0031] FIG. 1 shows a linearly polarized femtosecond beam 10 generated, for example, by a regeneratively amplified Ti:sapphire femtosecond laser system with transform limited 80 fs pulses and operated at an 800 nm wavelength. The beam 10 is expanded 3.5 times in the horizontal plane (i.e., along the x-axis, FIG. 1) and focused through a zeroth-order-nulled holographic phase mask 11 with a pitch =1.07 m (mask grooves are aligned along the y-axis) using a plano-convex cylindrical lens 12 having its curved surface designed to correct spherical aberration in one dimension. The effective numerical aperture of the cylindrical lens 12 in the yz-plane is estimated at 0.25. The beam expansion is used to produce a quasi-flat-top intensity distribution along the x-axis at the cylindrical lens 12. A slit 13 of width 2w15 mm is aligned along they-axis and placed between the plano-convex cylindrical lens 12 and the phase mask 11, as depicted in FIG. 1. Optical fiber 14 (for example SMF-28 fiber manufactured by Corning Incorporated) with its protective coating removed, is placed 300 m away from the phase mask 11 where the peak intensity in the focus is highest. The location of the highest peak intensity may be determined using the technique taught by Abdukerim et at in Opt. Express vol. 27, pp. 32536-32555 (2019), incorporated herein by reference. To inscribe FBGs, the regeneratively amplified Ti:sapphire femtosecond laser system is operated at 1 Hz and the pulses are fired at the optical fiber 14 one at a time using a synchronized shutter. The morphology of the laser-induced modification in the fiber core 21 is revealed using scanning electron microscopy (SEM) on fiber samples cleaved along the yz-plane in the middle of the respective FBGs, as discussed below.

[0032] FIG. 2 denotes the orientation of the electric field vector E of the pulses (i.e., pulse polarization) with respect to the optical fiber, according to an aspect of this specification. The fiber axis is defined as the x-axis, while the yz-plane denotes the cross section of the fiber.

[0033] Abdukerim et al. in Opt. Len., vol. 45, pp 443-446 (2020) show that the peak laser intensity at the focus (i.e., at the center of the two-beam interference pattern produced by the phase mask 11) in the fiber core 21 can be calculated using

[00001]$\begin{array}{cc}{I}_p=\frac{2}{}\frac{E_p}{{}_x{}_y},& \left(1\right)\end{array}$

where E.sub.p is the laser pulse energy, is the pulse duration (full-width half-maximum), and 2.sub.x and 2.sub.y are focal spot sizes (at the 1/e.sup.2-intensity level of the Gaussian intensity profile) along the x-axis and y-axis inside the fiber, respectively. The x-axis is parallel to the fiber axis and the mask grooves are aligned along the y-axis. The scaling coefficient is a product of three factors: =.sub.1.sub.2.sub.3. The constant .sub.1 is related to the pulse shape, being .sub.1=0.88 for sech.sup.2-shaped laser pulses. The coefficient .sub.2 accounts for the polarization-dependent intensity distribution at the focus of the cylindrical lens 12. If there are no losses associated with Fresnel reflection at the front surface of the optical fiber 14 and the phase mask 11 has a 100% diffraction efficiency, the variation of the focal peak intensity along the x-axis at the fiber core 21 can be written as

[00002]$\begin{array}{cc}{I}_y\left(x\right)=I_0\left[1+\cos \left(\frac{4x}{}\right)\right]& \left(2\right)\end{array}$

when the laser pulse polarization E (FIG. 2) is aligned along they-axis (y-polarization) and as

[00003]$\begin{array}{cc}{I}_x\left(x\right)=I_0\left[1+\cos \left(2{}_1\right)\cos \left(\frac{4x}{}\right)\right]& \left(2a\right)\end{array}$

when the laser pulse polarization E is aligned along the x-axis (x-polarization). In Eqs. (2) and (2a), I.sub.0 is the focal peak intensity in the incident beam 10 if it is focused inside the fiber without the phase mask in the beam path, is the mask pitch and .sub.1=arcsin[/(n.sub.1)] is the diffraction angle of the focused 1 orders inside the fiber (with refractive index n.sub.1) if the fiber lies in the diffraction plane and is oriented normal to the bisector between the orders. Hence, .sub.2, which is defined as the ratio of the peak intensity in the interference pattern to I.sub.0, will be equal to 2 for y-polarization and will be given by

[00004]$\begin{array}{cc}{}_2=2\frac{{\left(n_1\right)}^2-{}_2}{{\left(n_1\right)}^2}& \left(3\right)\end{array}$

in the case of x-polarization. For fused silica (n.sub.1=1.453 at =800 nm), i.e. the material from which the cladding of optical fiber 14 is predominantly made of, Eq. (3) gives .sub.2=1.47 for =1.07 m. The coefficient .sub.3 accounts for i) Fresnel reflection losses at the glass-air interfaces of the focusing cylindrical lens 12 and the front surface of the bare (i.e., without coating) optical fiber 14 and ii) polarization-dependent diffraction efficiency of the phase mask 11. In this disclosure, .sub.3 is 0.71 for y-polarization and 0.76 for x-polarization, 2.sub.x is 25 mm and 2.sub.y is 2.4 m.

[0034] In order to investigate how the laser pulse polarization E affects the modification morphology, the fiber core 21 is irradiated with one x-polarized pulse and one y-polarized pulse in two separate spots near the fiber axis. The orientation of the linear pulse polarization is adjusted by means of a polarizer and a half-wave plate. Using the dark-field microscopy technique taught by Mihailov et al. in U.S. Pat. No. 10,520,669, the onset of Type-II structural changes in the fiber core 21 in the single-pulse regime of irradiation are observed at 1.910.sup.3 W/cm.sup.2 for y-polarization and 2.210.sup.13 W/cm.sup.2 for x-polarization. The appearance of Type II modification at the respective intensities is also accompanied by a sharp growth of cladding modes in the transmission spectra of the resultant FBGs, as monitored using a broadband source (spectrally centered at 1550 nm) and an optical spectrum analyzer. The FBGs whose internal morphology is shown in the SEM images in FIG. 3 are written at a peak light intensity I.sub.p of 5.510.sup.13 W/cm.sup.2 for both polarizations, i.e., at 2.5 times the single-pulse intensity threshold for Type II modification. The calculations of I.sub.p are based on the formalism presented by Abdukerim et al. in Opt. Lett., vol. 45, pp 443-446 (2020).

[0035] FIG. 3 shows SEM images of modification in the core of optical fiber 14 induced by a single laser pulse. The orientation of the laser pulse polarization E (in front of the phase mask 11) is shown with double-sided arrows. FIG. 3(a) is an image of the cleaved surface of the cross section of the optical fiber 14 at the plane of a Bragg grating inscribed with a single pulse using the method set forth above. The core 21 of the optical fiber is denoted by the 8 m diameter light gray circle in the image. FIG. 3(b) is a magnified image of FIG. 3(a) showing micropores 31 embedded into smooth modification 32.

[0036] The SEM images in FIG. 3(b) clearly show that for single-pulse irradiation every grating plane of the FBG is built of a highly elongated micropore 31 (0.12 m.sup.2 in the yz-plane) embedded into a region of smooth material modification 32 with a much larger cross-sectional area in the yz-plane, i.e., 0.75 m.sup.2. It is also noteworthy that there is no characteristic difference between the modification produced with x-polarization and y-polarization. In this respect, the resultant structure comprising the index change is neither a nanograting that depends on the laser pulse polarization E and requires multiple pulses to be formed nor a laser-polarization insensitive spherical microvoid produced with a tightly focused single pulse using the point-by-point technique.

[0037] The spectral strength of 15 mm-long FBGs inscribed at I.sub.p5.510.sup.13 W/cm.sup.2 using one pulse is 10 dB in transmission, with the corresponding broadband scattering loss being at the level of 0.02 dB if measured 10 nm away from the Bragg resonance on the long-wavelength side. The spectral strength and loss of the FBGs can be 7-12 dB and 0.01-0.03 dB, respectively, depending on how accurately the laser-induced material modification is positioned with respect to the fiber axis (1.5 m along the y- and z-axis in this disclosure). In this disclosure, the alignment of the line-shaped laser focus with respect to the fiber core 21 is performed using nonlinear photoluminescence as taught by Mihailov et al. in U.S. Pat. No. 10,520,669. The difference in the n.sub.eff along the slow and fast axis of the FBGs produced with x-polarization and y-polarization is 210.sup.5 and 310.sup.5, respectively. The high-temperature behavior (up to 1000 C.) of such micropore-based FBGs is described by Abdukerim et al. in Opt. Lea., vol. 45, pp 443-446 (2020), incorporated herein by reference.

[0038] Low-loss thermally-stable FBGs that are built of elongated micropores 31 can be inscribed through the protective polymer coating (e.g., acrylic or polyimide coating) on the optical fiber 14. As in the case of bare fibers, the alignment of the line-shaped laser focus with respect to the fiber core 21 may be performed using nonlinear photoluminescence as taught by Mihailov et al. in U.S. Pat. No. 10,520,669. If the focusing lens 12 is corrected for spherical aberration and the distance between the mask 11 and the fiber 14 is chosen so as to determine the location of the highest peak intensity in the laser focus, as taught by Abdukerim et al. in Opt. Express vol. 27, pp. 32536-32555 (2019), no visible damage to the coating occurs at focused intensities that are sufficient to produce elongated micropores 31 in the fiber core 21.

[0039] When more than one pulse at I.sub.p5.510.sup.13 W/cm.sup.2, the rest of the laser writing parameters being the same as disclosed above, is used for the inscription, the FBG spectral strength, scattering loss and birefringence increase. As an example, for a five-pulse irradiation the FBG spectral strength becomes more than 20 dB in transmission for both x- and y-polarization, with the respective scattering loss of 0.2 dB for x-polarization and 0.05 dB for y-polarization, and the respective birefringence of 310.sup.5 for x-polarization and 610.sup.5 for y-polarization. As shown in FIG. 4, this is caused by i) growth of micropores and ii) the polarization-sensitive nature of the corresponding light-matter interaction (Hnatovsky et al. Opt. Lett., vol. 42, pp. 399-402, (2017)). Similar to the one-pulse irradiation regime set forth above, to investigate how the laser pulse polarization E affects the modification morphology, the fiber core 21 is irradiated with five x-polarized pulses and five y-polarized pulse in two separate spots near the fiber axis. The elongated micropores 31 that are axially symmetric with respect to the z-axis when the fiber is irradiated with one laser pulse become stretched perpendicular to the electric field vector E of the pulses when several pulses are used, as shown in FIG. 4 where micropores 41 and 42 are produced with five x-polarized and y-polarized pulses, respectively. As in the case of single-pulse irradiation, micropores 41 and 42 produced with five pulses are embedded into smooth material modification 43 with a much larger cross-sectional area in the yz-plane. The high-temperature behavior (up to 1000 C.) of Type II FBGs produced with several pulses is described by Abdukerim et al. in Opt. Lett., vol. 45, pp 443-446 (2020), incorporated herein by reference.

[0040] FIG. 4 shows SEM images of modification in the core of optical fiber 14 induced by five laser pulses. The orientation of the laser pulse polarization E (in front of the phase mask 11) is shown with double-sided arrows. FIG. 4(a) is an image of the cleaved surface of the cross section of optical fiber 14 at the plane of a Bragg grating inscribed with five pulses using the method set forth above. The core 21 of the optical fiber is denoted by the 8 m diameter light gray circle in the image. FIG. 4(b) is a magnified image of FIG. 4(a) showing micropores 41 and 42 embedded into smooth modification 43.

[0041] The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.