Hollow core waveguide with optimized contour

Abstract

A waveguide with a hollow core (16) delimited by a closed contour includes a succession of arcs (20) of negative curvature, each arc including a chord (24), characterized in that the contour of the hollow core (16) includes small arcs (PA) and large arcs (GA) arranged alternately, each arc (20) being symmetric with respect to a straight line passing through the center (18) of the hollow core (16) and the middle of the chord (24) thereof, the ratio b=2Ra/C of the large arcs being greater than 0.9 for the large arcs (GA), Ra corresponding to the maximum distance between the chord (24) and the arc (20), C corresponding to the length of the chord (24).

Claims

1. A hollow-core waveguide, comprising: a closed contour that comprises a series of arcs with a negative curvature, each arc comprising a chord, wherein the contour of the hollow core comprises small arcs and large arcs arranged alternately, each arc being symmetrical with respect to a straight line passing through a center of the hollow core and the middle of the chord of the arc, a ratio b=2Ra/C of the large arcs being greater than 0.9 for the large arcs, Ra corresponding to a maximum distance between the chord and the arc, C corresponding to a length of the chord.

2. The waveguide according to claim 1, wherein the contour of the hollow core comprises at least three small arcs and at least three large arcs.

3. The waveguide according to claim 1, wherein all of the large arcs all have the same ratio b.

4. The waveguide according to claim 1, wherein the ratio b′ of the small arcs is less than or equal to 0.8.

5. The waveguide according to claim 1, wherein a thickness of the arcs is less than or equal to one-half of a maximum guided wavelength.

6. The waveguide according to claim 1, wherein a material of the arcs has a refractive index of greater than 1.2 in a range of target wavelengths.

7. The waveguide according to claim 1, wherein the hollow core is filled with a gas that is suitable for the function of the waveguide.

8. A hollow core photonic crystal fiber comprising the hollow-core waveguide according to claim 1, whose contour of the hollow core comprises small arcs and large arcs arranged alternately, each arc being symmetrical with respect to a straight line passing through the center of the hollow core and the middle of the chord, the ratio b=2Ra/C of the large arcs being greater than 0.9 for the large arcs, Ra corresponding to the maximum distance between the chord and the arc, C corresponding to the length of the chord.

9. The fiber according to claim 8, wherein further comprising a sheath with a structure that makes possible to obtain an inhibited coupling guiding.

10. The fiber according to claim 9, wherein the structure of the sheath is a large-pitch Kagome.

11. A device that makes possible offsetting of a laser power that comprises the waveguide according to claim 1.

12. A laser pulse compression device that comprises the waveguide according to claim 1.

13. A gas laser comprising the waveguide according to claim 1.

14. An imaging device that comprises the waveguide according to claim 1.

15. A frequency calibration device that comprises the waveguide according to claim 1.

16. A guiding device that comprises a waveguide according to claim 1.

17. A device that makes possible offsetting of a laser power that comprises the fiber according to claim 8.

18. A laser pulse compression device that comprises the fiber according to claim 8.

19. An imagery device that comprises fiber according to claim 8.

20. A frequency calibration device that comprises the fiber according to claim 8.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) Other characteristics and advantages will emerge from the following description of the invention, a description that is provided only by way of example, relative to the accompanying drawings in which:

(2) FIG. 1 is a cutaway of a first variant of a hollow-core photonic crystal fiber that illustrates the invention with a first sheath structure,

(3) FIG. 2 is a cutaway of another variant of a hollow-core photonic crystal fiber that illustrates the invention with another sheath structure,

(4) FIG. 3 is a cutaway that illustrates in detail the contour of the hollow core of the fiber illustrated in FIG. 2,

(5) FIG. 4 is a cutaway that illustrates in detail the contour of the hollow core of a fiber according to another variant of the invention,

(6) FIG. 5A is a cutaway of a fiber according to a first embodiment of the invention associated with a diagram that illustrates the curve of the transmission losses based on the wavelengths of said fiber, and

(7) FIG. 5B is a cutaway of a fiber according to another embodiment of the invention associated with a diagram that illustrates the curve of the transmission losses based on the wavelengths of said fiber.

DETAILED DESCRIPTION OF THE INVENTION

(8) At 10, FIGS. 1 to 4 show a waveguide in the form of a photonic crystal fiber.

(9) The photonic crystal fiber 10 comprises, from the outside to the inside, a protective envelope 12, a sheath 14, and a hollow core 16 that is delimited by a contour. The hollow core can comprise air or a gas that is suited to the function of the fiber.

(10) The sheath 14 comprises a large number of inclusions that extend over the entire length of the photonic crystal fiber 10.

(11) The center of the core of the fiber is referenced 18.

(12) The structure of the sheath 14 (namely the distribution, the forms of inclusions, and the materials used) makes it possible to confine the electromagnetic waves in the hollow core 16.

(13) According to a first variant that is illustrated in FIG. 1, the sheath 14 has a Kagome-type structure. According to another variant that is illustrated in FIG. 2, the sheath 14 has a triangular structure. Of course, the invention is not limited to these variants.

(14) Thus, the pattern of the structure cannot be the same over the entire cross-section of the sheath. Thus, the sheath can have multiple concentric patterns relative to the center 18 of the fiber.

(15) According to a preferred embodiment, the sheath has a structure that makes it possible to obtain inhibited coupling guiding. For this purpose, the structure of the sheath is called large pitch (in English, “large pitch”). Preferably, the sheath has a large-pitch structure of the Kagome type.

(16) Thus, the structure of the sheath is selected in such a way as to impart the index of the sheath to the effective indices of the guided modes.

(17) Large pitch is defined as the structure that has a pitch that is greater than or equal to five times the wavelength of the guided wave.

(18) The sheath is not described in more detail because it can have different configurations. According to the variants, the sheath can be uniform, can comprise a periodic, quasi-periodic or non-periodic structure.

(19) The hollow core 16 is delimited by a closed contour that comprises a series of arcs 20 with a negative curvature (bent part oriented toward the center 18). Each arc 20 comprises two tips 22.1 and 22.2, two adjacent arcs having a common tip. The sheath 14 plays the role of protecting and maintaining arcs 20.

(20) According to an important point, for the rest of the description, the geometry of the hollow core is described as the finished fiber, or after the drawing phase in the case of a process for production integrating a drawing phase and not before the drawing phase.

(21) Each arc 20 comprises a chord 24 that corresponds to the straight line that passes through the tips 22.1 and 22.2 of the arc.

(22) Preferably, each arc 20 is symmetrical with respect to a straight line that passes through the center 18 and the middle of the chord 24. Thus, the tips 22.1 and 22.2 of all of the arcs 20 are arranged on a circle referenced 26 that has as its center the center 18 of the hollow core of the fiber. Each arc is a quasi-hypocycloid whose director circle is the circle 26 of the tips of the arcs.

(23) For each arc 20, the chord has a length C, and Ra corresponds to the maximum distance between the chord 24 and the arc 20.

(24) According to an important characteristic of the invention, the contour of the hollow core comprises small arcs PA and large arcs GA arranged alternately. Thus, each small arc PA is arranged between two large arcs GA, and each large arc GA is arranged between two small arcs PA. The contour comprises as many small arcs PA as large arcs GA.

(25) According to the invention, the large arcs GA have a ratio b=2Ra/C that is greater than or equal to 0.9.

(26) Preferably, all of the large arcs GA have the same ratio b.

(27) Preferably, all of the small arcs PA have the same ratio b′=2Ra/C.

(28) Advantageously, the ratio b′ of the small arcs PA is less than or equal to 0.8.

(29) According to a preferred embodiment that is illustrated in FIG. 3, the contour of the hollow core comprises six large arcs GA and six small arcs PA.

(30) According to another embodiment that is illustrated in FIG. 4, the contour of the hollow core comprises three large arcs GA and three small arcs PA.

(31) The invention is not limited to these embodiments. Regardless of the embodiment, the hollow contour comprises at least three large arcs GA and at least three small arcs PA.

(32) As illustrated in the figures, the small arcs PA define an underlying circle Rout, and the large arcs define an underlying circle Rint that has a diameter that is smaller than the underlying circle Rint, the two underlying circles Rint and Rout having the same center 18, that of the hollow core of the fiber.

(33) According to an embodiment that is illustrated in FIGS. 1 and 2, the hollow core 16 is delimited by capillaries whose centers are arranged on the circle 26, with the capillaries alternately being in the form of a circle and an ellipse and juxtaposed.

(34) The geometry of the contour of the hollow core that is defined above makes it possible to prevent a spatial recovery between the fields of the mode that is guided into the hollow core and the contour, to increase the length of the contour, and consequently to enhance the inhibition of the coupling between the basic mode of the core with those carried by the contour. The geometry also makes it possible to push the tips 22.1, 22.2 as far back from the center 18 of the core as possible.

(35) According to another characteristic of the invention, the thickness of the arcs 20 denoted t is less than or equal to half of the maximum guided wavelength. By way of example, if the maximum guided wavelength is equal to 2 μm, the thickness of the arcs is to be less than 1 μm.

(36) Relative to the materials that are used, the material of the arcs 20 is selected in such a way as to obtain a refraction index that is greater than 1.2 in the target wavelength range.

(37) According to an embodiment that is suitable for guiding waves in the range of microwave-type wavelengths that are on the order of 1 m to 1 mm, the thickness t of the arcs is on the order of 0.5 mm. The selected material is highly reflective, such as, for example, a reflective metal such as copper, or has a low absorption coefficient like quartz. In this wavelength range, the arcs 20 can be made of metal, glass, borosilicate glass, quartz, a ceramic material, polytetrafluoroethylene . . . .

(38) According to an embodiment that is suitable for guiding waves in the range of THz wavelengths that are on the order of 100 μm to 1 mm, the thickness t of the arcs is on the order of 50 μm. The material that is selected is Teflon or a material that exhibits a low absorption coefficient and a refraction index that is greater than 1.2, such as, for example, polytetrafluoroethylene.

(39) According to an embodiment that is suitable for guiding waves in the range of wavelengths that are on the order of 2 μm to 10 nm, the thickness t of the arcs is on the order of 1 μm. The selected material will be the most transparent possible, such as, for example, pure silica or soft glass.

(40) For all of these wavelength ranges, the fiber can be obtained by stacking and drawing or by machining and drawing.

(41) FIGS. 5A and 5B show hollow-core photonic crystal fibers with a contour that comprises six small arcs PA and six large arcs GA. These fibers comprise a Kagome-type structure. The structure can be produced by using the teachings of documents WO2006077437 and WO2009/044100.

(42) These fibers are produced by stacking and drawing capillaries with circular cross-sections. The profile of the contour of the core will be obtained by optimizing the rheological parameters of the material, the temperatures and the pressure differentials between the capillaries during the implementation of the drawing phase.

(43) In FIG. 5A, the fiber has a contour with b=0.9 and t=800 nm.

(44) In FIG. 5B, the fiber has a contour with b=1 and t=1,400 nm.

(45) The measurements taken in the spectral range extending from 800 nm to 1,200 nm show that the fibers guide by exhibiting low-guide bands that are due to an interaction that resonates between the guided modes in the core and those in the arcs. As can be seen in the diagrams, the transmission losses drop to a level of 20 dB/km for the wavelengths encompassed between 800 nm and 1,200 nm. By way of comparison, the fiber that is illustrated in the publication “Low Loss Broadband Transmission in Hypocycloid-Core Kagome Hollow-Core Photonic Crystal Fiber” Mar. 1, 2011/Vol. 36, No. 5/OPTICS LETTERS″ has, for the largest arcs, a curvature b that is less than or equal to 0.75 and generates a transmission loss that at best is equal to approximately 180 dB/km.

(46) Consequently, the optimization of the contour of the core and more particularly of the value b of the large arcs obtains a significant reduction of the transmission losses.

(47) The optimization of the parameter of curvature b of the contour of the hollow core 18 makes it possible to increase the inhibition of the coupling and consequently to reduce the transmission losses of the waveguide. The optimization of the parameter of curvature b makes it possible to obtain a drastic reduction of the recovery of the optical power in the sheath that makes possible in particular the laser power offsetting.

(48) The photonic crystal fiber according to the invention makes it possible to obtain a guide that combines a low transmission loss, a monomodal guiding, and a very high laser damage threshold.

(49) Although described in a preferred manner applied to a photonic crystal fiber, the invention can be applied in a general manner to a waveguide. However, it is more particularly suitable for fibers with inhibited coupling guiding, which optimizes both the structure of the sheath and the contour of the hollow core.

(50) The waveguide or the hollow-core fiber with inhibited coupling guiding with an optimized contour according to the invention can be used in the following (non-exhaustive) applications: The offsetting of a laser power in the field of laser micro-machining, surgery, treatment of cells (cancerous cells), The compression by non-linear effects of laser pulses, in particular high-flux pulses, Terahertz imagery, Gas lasers, with a core that is filled with an active gas used as an amplification medium, The monomodal guiding and with low wave transmission losses in the range of microwaves or THz, The calibration of frequency, with the core being filled with a gas.