HIGH EFFICIENCY OPTICAL FIBER BRAGG GRATING DEVICE BASED ON MICROPORE FORMATION AND METHOD FOR PRODUCING SAME

Abstract

A method and apparatus for inscribing a Bragg grating in the core of an optical waveguide. Electromagnetic radiation at a chosen wavelength passes through a diffractive optical element optimized for the wavelength such that a beam is generated on the waveguide having an interference pattern so as to form a Bragg grating in the core of the optical waveguide, the beam being sufficiently intense to cause a permanent (Type II) change in the index of refraction in the core in the form of at least one elongated micropore. The Bragg grating period can be selected to promote coupling of guided light into a radiation mode for detection by a detector to form a spectrometer. The Bragg grating is characterized by a scattering loss less than 10.sup.5 dB per grating period at the Bragg resonance but outcoupling efficiency at visible wavelengths of 0.03% per grating period.

Claims

1. A method for inscribing a Bragg grating in the core of an optical waveguide for use in an optical waveguide spectrometer system, comprising the steps of: providing electromagnetic radiation at a wavelength chosen for the optical waveguide system; focusing the electromagnetic radiation through a diffractive optical element optimized for the wavelength such that when exposed to the focused electromagnetic radiation a beam is generated on the optical waveguide having an interference pattern; irradiating the optical waveguide with the beam to form a Bragg grating, the beam incident on the optical waveguide being sufficiently intense to cause a permanent (Type II) change in the index of refraction in the core of the optical waveguide in the form of at least one micropore; and wherein the Bragg grating period is selected to promote coupling of guided light into a radiation mode.

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

3. The method of claim 1, wherein the electromagnetic radiation comprises a plurality of ultrashort laser pulses.

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

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

6. The method of claim 1, wherein the optical waveguide system is a spectrometer, and the period of the diffractive optical element is optimized for a wavelength of the electromagnetic radiation to form said Bragg grating having a period such that when about 250 nm-5500 nm light propagates in the waveguide the outcoupling of said light into radiation modes in the form of 1st diffraction orders.

7. The method of claim 6, wherein the outcoupled power in the 1st diffraction orders exceeds one of either 50% or 50% divided by the number of grating planes.

8. The method of claim 1, wherein the optical waveguide system is an inline optical power monitor, and the period of the diffractive optical element is optimized for a wavelength of the electromagnetic radiation to form said Bragg grating having a period such that when about 250 nm-5500 nm light propagates in the waveguide the outcoupling of said light into radiation modes in the form of 1st diffraction orders and the outcoupling of said diffraction orders is at 30 with respect to the normal to the fiber axis.

9. The method of claim 3, wherein the plurality of ultrashort laser pulses is equal to or less than ten.

10. The method of claim 1, wherein the change in the index of refraction within the core of the optical waveguide comprises at least one elongated micropore.

11. The method of claim 1, wherein the change in the index of refraction within the core of the optical waveguide comprises a plurality of spherical micropores.

12. The method of claim 1, wherein the change in the index of refraction of a single plane of the Bragg grating within the core of the optical waveguide comprises a single spherical micropore.

13. The method of claim 1, wherein light propagating in the waveguide is outcoupled by the Bragg grating into a single pair of diffraction orders.

14. The method of claim 13, wherein the period of the Bragg grating is selected to alter the outcoupling angle of the diffraction orders with respect to the normal to the fiber axis.

15. The method of claim 10, wherein the length of micropores or the number thereof are varied to alter the azimuthal directionality of the outcoupled diffraction orders.

16. An optical waveguide system, comprising: an optical waveguide having a core in which a Bragg grating containing micropores is formed; and a detector placed adjacent to the optical waveguide to receive scattered light from the Bragg grating containing micropores.

17. The optical waveguide system of claim 16, wherein the waveguide is an optical fiber comprising at least one light-guiding core and a cladding.

18. The optical fiber system of claim 16, wherein the Bragg grating comprises at least two rows of micropore Bragg gratings forming a two-dimensional grating array across the cross-section of the core in order to promote outcoupling into one of either a +1 or 1 diffraction order.

19. The optical waveguide system of claim 16, wherein the Bragg grating is a chirped Bragg grating.

20. The optical waveguide system of claim 16, wherein the Bragg grating has a period optimized for use as a spectrometer.

21. The optical waveguide system of claim 16, wherein the Bragg grating has a period optimized for use as an in line optical power monitor.

22. A system for inscribing a Bragg grating onto an optical waveguide having a core, comprising: a laser for generating ultrashort pulse duration electromagnetic radiation at a selected wavelength; a focusing optical element for focusing the ultrashort pulse duration electromagnetic radiation from the laser onto the optical waveguide; and a diffractive optical element intermediate the focusing optical element and the optical waveguide optimized for receiving the electromagnetic radiation and generating a beam having an interference pattern for irradiating the optical waveguide so as to form a Bragg grating therein, the electromagnetic radiation incident on the optical waveguide being sufficiently intense to cause a permanent (Type II) change in the index of refraction in the core of the optical waveguide in the form of at least one micropore.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0025] These together with other aspects and advantages, as well as a discussion of the prior art, are more fully set forth below, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

[0026] FIG. 1 depicts a fiber Bragg grating (FBG) spectrometer based on tilted Bragg gratings, according to the prior art.

[0027] FIG. 2 depicts a FBG spectrometer based on point-by-point femtosecond laser inscribed Bragg gratings, according to the prior art.

[0028] FIG. 3 depicts an interferometric setup based on the phase mask technique for the inscription of a FBG with micropore based index change, according to the 148 application.

[0029] FIGS. 4A and 4B depict interferometric setups using near-infrared and violet femtosecond sources to produce FBGs with grating periods .sub.G=536 nm and .sub.G=300 nm, respectively.

[0030] FIG. 5 depicts a self-focusing, all-fiber spectrometer utilizing a micropore-based FBG, according to an embodiment, with the outcoupled spectrum in focus.

[0031] FIG. 6 is a comparison of reconstructed outcoupled visible radiation patterns of the FBG spectrometer in FIG. 5 (dashed line) with that of an optical spectrum analyzer (solid line).

[0032] FIG. 7 depicts the maximum outcoupling efficiency over all diffraction orders for the FBG spectrometer of FIG. 5, with .sub.G=536 nm (NIR written) and .sub.G=300 nm (violet written) at three wavelengths (i.e., 405 nm, 520 nm and 637 nm).

[0033] FIG. 8 depicts a test set up using a motorized fiber polarization controller to measure the polarization dependence of the outcoupling efficiency.

[0034] FIG. 9A shows polarization dependence of the outcoupling efficiency for a FBG with .sub.G=536 nm (NIR written), and FIG. 9B shows a FBG with .sub.G=300 nm (violet written), where the outcoupling efficiency is measured as a function of the rotation angles of the half- and quarter-wave plate paddles of the polarization controller of FIG. 8.

[0035] FIG. 10A depicts how the azimuthal directionality of a pair of outcoupled diffraction orders with a given m depends on the length of micropores that form a FBG, and FIG. 10B depicts how the azimuthal directionality of a single outcoupled diffraction order with a given m depends on the length of micropores that form a FBG.

DETAILED DESCRIPTION OF THE INVENTION

[0036] As discussed above, the inventors teach in the 148 application that FBGs with extremely low scattering loss at wavelengths close to but not at the Bragg resonanceless than 10.sup.5 dB per grating periodcan be fabricated in silica fibers by utilizing the phase mask technique for grating inscription using a fs-NIR laser to induce Type II modification in the fiber core. Although these FBGs have low scattering loss in the infrared, the inventors have discovered that the FBGs strongly scatter guided light that is at visible wavelengths. Outcoupling efficiencies of 0.03% per grating period have been observed. Scanning electron microscopy (SEM) has established that for single-pulse irradiation through the phase mask, every grating plane of the FBG produced under these conditions, as seen by light guided in the fiber, consists of a highly elongated micropore embedded within a narrow region of resolidified glass. Thus, the phase mask technique combined with a fs-NIR laser can be used to fabricate FBGs that possess outcoupling characteristics of highly localized FBGs produced by the point-by-point technique. As set forth below, the high outcoupling efficiency of light from the fiber, i.e., the high efficiency of coupling guided light to radiation modes, is useful for creating a fiber spectrometer.

[0037] FIG. 1 shows a prior art example of an optical fiber spectrometer based on UV laser induced tilted Bragg gratings as taught by Walker et al. U.S. Pat. No. 7,245,795. A tilted FBG 110 is formed within an optical fiber 100 in such a way that, when light at a wavelength propagating along the fiber 100 impinges upon the FBG 110, at least a portion of it, the outcoupled light 120, is coupled out of the fiber 100 through a side surface thereof 170, and dispersed by the FBG 110 azimuthally about an optical axis 150 of the fiber 100 to photodetectors 130 in dependence on the wavelength 2. This is accomplished by selecting the FBG parameters so that, for a predetermined operating wavelength range, the FBG 110 operates substantially away from the Bragg resonance condition.

[0038] FIG. 2 shows a prior art example of an optical fiber spectrometer 200 based on femtosecond laser induced point-by-point gratings as taught by Waltermann in U.S. Pat. No. 11,698,302, wherein the core 160 of fiber 100 has an FBG 110 inscribed therein, as in FIG. 1, however the fiber 100 and inscribed FBG 110 bend with a curvature that leads to constructive interference such that different wavelengths are focused at different locations on photodetectors 130, which largely simplifies the structure of the spectrometer.

[0039] Turning to FIG. 3, apparatus 300 is shown for fabrication of a micropore-based FBG with a femtosecond pulse duration near-infrared laser and a phase mask, according to the 418 application. A linearly polarized femtosecond beam 310 is generated, for example, by a regeneratively amplified Ti:sapphire femtosecond laser system with transform limited 80 fs pulses at a central wavelength of 800 nm. The beam 310 is expanded 3.5 times in the horizontal plane (i.e., along the x-axis, FIG. 3) and focused through a zeroth-order-nulled holographic phase mask 311 with a pitch. .sub.G=1.072 m (mask grooves are aligned along the y-axis) using a plano-convex cylindrical lens 312 (15 mm focal length) having its curved surface corrected for spherical aberration in one dimension. The effective numerical aperture of the cylindrical lens 312 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 312. A slit 313 of width 2w15 mm is aligned along the y-axis and placed between the plano-convex cylindrical lens 312 and the phase mask 311. Optical fiber 314 (for example a fiber transmitting visible light in the single-mode regime such as SMF460HP) with its protective coating removed, is placed 300 m away from the phase mask 311 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 al. 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 314 one at a time using a synchronized shutter.

[0040] The inventors have discovered that finer Bragg grating periods can be created by reducing the period of the phase mask 311 used to inscribe the pattern. The wavelength of the femtosecond laser beam also needs to be reduced, such that it is always less than the mask period. To reduce the femtosecond laser wavelength, the femtosecond beam can, for example, be frequency doubled with a 0.5 mm-thick BBO crystal (-Ba (BO.sub.2).sub.2) resulting in the generation of the beam 310 having 110 fs violet pulses centered at a wavelength of 400 nm. The phase mask 311 can therefore have a pitch >400 nm.

[0041] FIGS. 4A and 4B depict phase mask interferometric setups using near-infrared and violet femtosecond sources to produce FBGs with grating periods .sub.G=536 nm and .sub.G=300 nm, respectively. Specifically, FIG. 4A depicts the setup of FIG. 3, using a mask 311A with a pitch .sub.M=1072 nm irradiated by a focused fs-NIR (800 nm) beam to create a grating structure within fiber 314A having a grating pitch of .sub.G=536 nm, while FIG. 4B depicts the setup, according to an embodiment, using a mask 311B with a pitch .sub.M=600 nm irradiated by a violet (400 nm) femtosecond beam to create a grating structure within fiber 314B having a grating pitch of .sub.G=300 nm. FBGs produced using femtosecond pulses at 400 nm-800 nm can efficiently outcouple guided light within a wavelength range of 250 nm-2000 nm, which covers the wavelengths of light that silica fibers can easily transmit. However, if other single mode fibres are used, such as ZBLAN fluoride-based glass fibers, the wavelength range can expand into the mid infrared (5500 nm from 2000 nm for silica).

[0042] If a FBG with a uniform period .sub.G is probed by light with a wavelength/at which the fiber is single mode, the diffraction orders outcoupled from the fiber core into the fiber cladding obey the following equation:

m=n.sub.G(1+sin .sub.D.sup.(m))(1)

[0043] where m is the order of diffraction, .sub.D.sup.(m) is the internal diffraction angle with respect to the normal to the fiber axis for light outcoupled into diffraction order m, and n is the refractive index of the fiber material (the fiber core and the fiber cladding are assumed to have the same refractive index).

[0044] If the phase mask with a uniform period converts the femtosecond beam into a two-beam interference pattern, with which the FBG inscription is performed, then the relation between .sub.G and .sub.M is .sub.G=.sub.M/2.

[0045] From Equation (1),

sin .sub.D.sup.(m)=[m/(nG)]1(2)

[0046] noting that the absolute value of sin .sub.D.sup.(m) must be less than or equal to 1.

[0047] Using Equation (1) and Snell's law at the air-fiber cladding interface, it can be shown that the angle .sub.DF.sup.(m) at which light in diffraction order m is outcoupled with respect to the normal to the fiber axis is given by

.sub.DF.sup.(m)=arcsin[(m/.sub.G)n]180/p(3)

[0048] The spectral range within which only 1st (m=1) diffraction orders are outcoupled from the fiber to the surrounding space can be found by using Equation (2) and Equation (3). The shortest wavelength .sub.min at which light is not outcoupled from the fiber core into 2nd (m=2) and higher (m>2) diffraction orders is determined by the condition sin .sub.D.sup.(2)=1. In this case, according to Equation (2), .sub.min=n.sub.G. According to Equation (3), the longest wavelength Amax at which light in +1st (m=1) diffraction orders can still be outcoupled from the fiber into the surrounding space, i.e., when total internal reflection of the light does not take place at the air-fiber cladding interface, is determined by the condition (/.sub.G)n=1. In this case, .sub.max=(n+1)/G. Hence, taking into account that =.sub.max.sub.min, is given by

=.sub.G(4)

[0049] Following Equations (1)-(3), the number of diffraction orders into which light with a wavelength is outcoupled from the fiber core to the fiber cladding and from the fiber cladding to the surrounding space can be selected (i.e., optimized for an application) by changing .sub.G. For example, if the light from a laser source with 2=520 nm is coupled into the core of a single-mode fiber and the protective coating is removed from the fiber, a FBG with .sub.G=536 nm (.sub.M=1072 nm) written with NIR femtosecond pulses (800 nm) allows the outcoupling of +1st (m=1) and 2nd (m=2) diffraction orders from the fiber. By reducing the wavelength of the femtosecond pulses, for example by frequency doubling the original NIR femtosecond pulses, phase masks with finer pitches .sub.M can be used and, as result, FBGs with finer pitches produced. By using Equations (2) and (3), it can be shown that .sub.G can be selected such that light is outcoupled into only one diffracted order pair. For example, a FBG with .sub.G=300 nm created using a phase mask with .sub.M=600 nm and violet femtosecond pulses (400 nm) produces outcoupling into only 1st diffraction orders.

[0050] Equations (2)-(4) allow one to design FBGs suitable for different applications that are based on outcoupling of light from the fiber. For instance, Equations (2) and (3) define for a given wavelength what period .sub.G is needed in order to have radiation in +1st diffraction orders outcoupled at 90 to the fiber axis.

[0051] The unique geometry of the micropore enables strong coupling of the guided mode to radiation modes, which is not possible with traditional planar Bragg grating structures, including tilted FBGs. By bending the fiber at the FBG location, the radiation modes can be focused on a flat surface, such as a CCD/CMOS sensor, without the need for additional focusing optics.

[0052] FIG. 5 shows a schematic image of a self-focusing, all-fiber spectrometer 500 according to an embodiment, with the radiated spectrum in focus for two colors (=520 nm and =637 nm). The outcoupled spectrally narrow light is sharply focused onto a paper card 510 at 518 by bending fiber 514 at the FBG location, by means of a fiber rotator 520 and fiber clamp 530, with the bending radius in this instance being approximately 50 mm.

[0053] The efficiency and resolution of the FBG spectrometers were validated experimentally using narrow-band visible laser diode sources. The spectral intensity profiles were captured with a CMOS beam profiler having a 5.5 m pixel pitch located at the focal plane of the bent fiber 514. The bent micropore-based FBG provided a strong linear dispersion of 210 m at the focal plane (i.e., the paper card 518) per 1 nm of guided wavelength.

[0054] FIG. 6 shows an example of reconstructed spectrum (black dashed line) of the visible Fabry-Perot laser source at =637 nm compared to the spectrum (black line) recorded using a high-resolution (30 pm) optical spectrum analyzer (OSA), wherein the exemplary spectrometer 500 resulted in a 200 pm spectral resolution in the visible band. Altering the bend curvature of the fiber 514 by means of the rotator 520 results in easy adjustment of the spectral resolution and working bandwidth of the spectrometer 500, however it is not possible with prior art fiber spectrometers based on chirped FBGs as taught by Waltermann in U.S. Pat. No. 11,698,302 or bulky lenses and dispersive components as disclosed by Qin et al. in Optics Letters 44, 5129-5132 (2019) to provide fiber-optic spectrometers based on Type II FBGs with a 300 nm pitch (smaller pitches are possible) as well as the ability to produce such spectrometers using the phase mask technique, which is robust and reproducible.

[0055] In fact, the exemplary FBG 500 spectrometer is capable of resolving peaks as narrow as 250 pm (at FWHM) across the visible spectrum. A similar resolution was achieved with the FBGs with .sub.G=300 nm. The maximum theoretical spectral resolution achievable with a FBG spectrometer is given by 2/N, where is the wavelength of guided light to be analyzed and N is the number of grating planes in the FBG. For a 15 mm long FBG with .sub.G=536 nm, N is 2.810.sup.4 and consequently the maximum resolution is 23 pm at =637 nm. For a 6 mm long FBG with .sub.G=300 nm, N is 210.sup.4 and its maximum theoretical resolution at =637 nm would be 32 m. In both cases, the maximum achieved resolution (i.e., 250 pm) of the devices based on FBGs with .sub.G=536 nm and .sub.G=300 nm is lower than its theoretical limit, mainly because of optical aberrations originating from the cylindrical shape and bending of the fiber.

[0056] One of the main challenges with fiber-based spectrometers is their low outcoupling efficiency as disclosed by Rahnama et al. Adv. Photon. Res., vol. 1, no. 2, 2020, Art. no. 2000026. However, the exemplary spectrometer 500, which is based on micropore FBGs, has significantly enhanced coupling efficiency to radiation modes and, as a result, enhanced outcoupling efficiency. The outcoupling efficiency is the ratio of the power of the outcoupled light to the power of the input fiber-coupled light. FIG. 7 illustrates the outcoupling efficiency of spectrometer 500 based on FBGs with .sub.G=536 nm and .sub.G=300 nm. The outcoupling efficiency of the FBG with .sub.G=536 nm was 86% at =405 nm, 46% at =520 nm, and 76% at =635 nm, with the average value being 70%. In the case of the FBG with .sub.G=300 nm, the wavelength-dependent variation of the outcoupling efficiency was smaller, namely, 55% at =405 nm, 60% at =520 nm, and 49% at 2=637 nm, with the average value being 54%.

[0057] FIG. 7 reveals that the radiation modes, i.e., the outcoupled light, have a polarization dependence. To measure the polarization dependence of the outcoupling efficiency, a motorized fiber polarization controller 700 was placed before a FBG 710 under test, as shown in the experimental setup of FIG. 8. A Fabry-Perot laser source 720 emitting light at 2=405, 520 and 635 nm, provided a linearly polarized output. By rotating the half-wave and quarter-wave plate paddles of the polarization controller 700, one at a time, the input state of polarization was varied and changes in the power of the outcoupled light were recorded from one side of the FBG 710 using an integrating sphere power meter 730, while the transmitted power was measured using a regular OSA 740.

[0058] The outcoupling efficiency of the spectrometers based on FBGs with .sub.G=536 nm and .sub.G=300 nm is presented in FIG. 8. The outcoupling efficiency of the FBG with .sub.G=536 nm was 86% at 2=405 nm, 46% at 2=520 nm, and 76% at 2=635 nm, with the average value being 70%. In the case of the FBG with .sub.G=300 nm, the wavelength-dependent variation of the outcoupling efficiency was smaller, namely, 55% at =405 nm, 60% at =520 nm, and 49% at =637 nm, with the average value being 54%.

[0059] FIGS. 9A and 9B show the polarization-dependent variation of the outcoupled power for the FBGs with .sub.G=536 nm and .sub.G=300 nm when the quarter-waveplate (solid circles) or half-wave plate (empty circles) of the polarization controller is rotated in 5 increments. FIG. 9A shows that the FBGs written with the NIR femtosecond pulses (.sub.G=536 nm) exhibit a weaker polarization dependence of the outcoupling efficiency compared to the FBGs written with the violet pulses (.sub.G=300 nm). More specifically, the outcoupled power deviates by 18% and 30% from its peak value, respectively, when the quarter-wave and half-wave plates are rotated (FIGS. 9A and 9B). The relatively low polarization dependence of the outcoupled light is believed to be due to the material modification morphology of the devices wherein each grating plane consists of a single elongated micropore produced with a 35 nJ single pulse or 10 nJ double pulses.

[0060] In summary, an all-fiber optical system is set forth herein using the phase mask technique and one or two femtosecond pulses, thus eliminating the need to incorporate additional focusing and collimating optics into the spectrometer's design. The spectral range (Equation (4)) of optical fiber systems incorporating short-period FBGs that consist of micropores is from 400 nm to 700 nm with a sub-nanometer resolution. The unique morphology of the micropore structures within the fiber core facilitates strong outcoupling, paving the way for the fabrication of highly efficient optical fiber systems. High efficiency of the optical fiber systems yields a wide range of applications, from biotechnology to telecommunications.

[0061] Other embodiments of the invention can utilize phase masks with smaller pitches (i.e., smaller .sub.M) to produce fundamental Bragg resonances in the visible portion of the electromagnetic spectrum and ultraviolet femtosecond laser radiation in order to generate single order (i.e., 1st diffraction order) outcoupling from the dispersive FBG element. The phase mask approach to produce micropore-based FBGs using a femtosecond laser is more repeatable and less time consuming than prior art techniques, making fiber spectrometer fabrication more reproducible and in less time than using the point-by-point femtosecond laser technique. Importantly, Type II FBGs with .sub.G=300 nm can be produced with ultraviolet (wavelength from 150 nm to 400 nm) or visible (wavelength shorter than 600 nm) femtosecond pulses. Such ultrashort pulses are characterized by a pulse duration of less than or equal to about 1 picosecond.

[0062] Hnatovsky et al. showed in Optics Express vol. 30 issue 26 pgs. 47361-47374 (2022) that it is possible to inscribe a number of Bragg gratings consisting of micropores that are stacked one on top of the other along the cross-section of the fiber core normal to fiber axis in order to make a 2-dimensional grating array. Rahnama et al. showed in the Proceedings of the SPIE vol. 11676 article 116760Y (2021) that such a 2-D grating array across the fiber cross section can result in preferential outcoupling of diffracted light into a single diffracted order, thus further improving the efficiency of the fiber spectrometer. According to another embodiment, a 2-D grating array can be inscribed consisting of at least 2 rows of micropore gratings to promote coupling into a single diffracted order, e.g., m=+1. The physical length of the micropores and/or variation in the number of them alters the azimuthal directionality of the outcoupled light patterns in a plane orthogonal to the fiber axis, as depicted in FIGS. 10A and 10B. The azimuthal directionality of a pair of outcoupled diffraction orders with a given m (FIG. 10A) or a single outcoupled diffraction order with a given m (FIG. 10B) is defined by the subtended angle j. For a given fiber core diameter, j is larger for shorter micropores and is smaller for longer micropores.

[0063] As discussed above, Equations (1)-(3) may be applied to optimize the grating period .sub.G to produce a radiation mode outcoupling at a wavelength of interest. For example, a further embodiment of the invention relates to the production of an optical tap (i.e., an in-line optical power tapping device) to monitor, for example, network traffic in fiber-optic telecommunications networks, optical power in fiber-optic delivery systems, and output or intra-resonator optical power of fiber-optic lasers. Based on Equation (2), an untilted Bragg grating with a period .sub.G=1.0675 m generates an outcoupled beam that is at 90 with respect to the fiber axis when the light guided by the fiber is at 1550 nm, within the telecom C-band. With a detector placed adjacent to the fiber, this Bragg grating could then be used as an optical tap to monitor network traffic. It is important to note that according to U.S. Pat. No. 6,993,221, the grating with a period .sub.G=1.0675 m will also have a second order retroreflective Bragg resonance at 2=1543 nm which falls within the telecom C-band, thus nullifying the radiative out-tapping/outcoupling at that wavelength. To avoid this situation, a Bragg grating could be fabricated with a 2nd order Bragg resonance that falls outside the telecom S-, C- and L-bands (1460-1530 nm, 1530-1565 nm, and 1565-1625 nm, respectively). For example, a FBG with .sub.G=1.000 m has a 2nd order Bragg resonance at 1450 nm, i.e., below the telecom S-band, but the out-tapped/outcoupled radiation is still at a near-normal angle to the fiber axis (i.e., 86) when the wavelength of the light propagating in the fiber is in the center of the C-band.

[0064] In embodiments of the invention, the outcoupled power in the 1st diffraction orders exceeds one of either 50% or 50% divided by the number of grating planes N (i.e., 0.03% per Bragg grating plane).

[0065] The efficient outcoupling of light demonstrated by micropore-based Bragg gratings according to this specification, is not limited to fiber spectrometers and network monitoring optical taps. Other applications of the invention include free space coupling devices, directional in vivo thermal beam sources for endoscopic medical therapeutics, directional in vivo imaging methods, such as, but not limited to optical coherence tomography or photoacoustic imaging, surface plasmon based sensors for micropore-based Bragg gratings with metallic coating on the fiber to name but a few.

[0066] 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 scope of the claims. For example, a person of skill in the art will understand that an optical fiber is a specific kind of optical waveguide. Accordingly, the principles set forth herein may be applied to other forms of optical waveguides such, for example, an embodiment where FBGs are inscribed in optical planar waveguides made in a microfabrication facility. Numerous other 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.