BACKGROUND

The disclosure relates to a hollow-core fibre (HCF, or: hollow-core photonic crystal fibre, HC-PCF) of non-bandgap type (or: hollow-core anti-resonant-reflecting fibre, HC-AF), in particular having an axial hollow core region and an inner cladding region comprising an arrangement of anti-resonant elements (AREs) surrounding the core region. Furthermore, the disclosure relates to an optical device including at least one HC-AF and to a method of manufacturing an HCF of non-bandgap type. Applications of the disclosure are available in the fields of data transmission, in particular low-latency data transmission, high-power beam delivery systems, in particular for material processing, modal filtering, gas-based nonlinear optics, in particular supercontinuum generation from the ultraviolet to the infrared or generation of ultrashort pulses, fibre gyroscopes or chemical sensing.

In the present specification, reference is made to the following, illustrating the technical background of light-guiding fibres, in particular HCF's of bandgap or non-bandgap type:[1] T. A. Birks et al. in “Optics Letters” 1997. 22(13): p. 961-963;[2] N. A. Mortensen et al. in “Optics Letters” 2003. 28 (20): p. 1879-1881;[3] US 2015/0104131 A1;[4] P. J Roberts et al. in “Optics Express” 2005. 13(1): p. 236-244;[5] J. K. Lyngso et al. in “22nd International Conference on Optical Fiber Sensors” Pts 1-3, 2012. 8421;[6] J. M. Fini et al. in “Nature Communications” 2014. 5: article number 5085;[7] F. Couny et al. in “Optics Letters” 2006. 31(24): p. 3574-3576;[8] B. Debord et al. in “Optics Letters” 2014. 39(21): p. 6245-6248;[9] W. Belardi et al. in “Optics Express” 2014. 22(8): p. 10091-10096;[10] W. Belardi et al. in “arXiv: 1501.00586v2” 2015: p. [physics.optics];[11] P. Jaworski et al. in “Optics Express” 2015. 23(7): p. 8498-8506;[12] A. Hartung et al. in “Optics Express” 2014. 22(16);[13] Benabid, A. F., et al., Hollow-core photonic crystal fibre, in U.S. Pat. No. 8,306,379 B2, GLOphotonics SAS;[14] F. Poletti in “Optics Express” 2014. 22(20): p. 23807-23828;[15] T. G. Euser et al. in “Optics Express” 2008. 16(22): p. 17972-17981;[16] E. A. J. Marcatili et al. in “Bell Systems Technical Journal” 1964. 43: p. 1783-1809;[17] J. M. Fini et al. in “Optics Express” 2013. 21(5): p. 6233-6242;[18] C. Wei et al. in “Optics Express” 2015. 23: p. 15824; and[19] A. Hartung et al. in “Optics Letters” 2015. 40(14): p. 3432.

Solid-core fibres are generally known and broadly used e.g. in data communication applications. Solid-core fibres can be designed for low-loss, single-mode transmission in a broadband transmission range of the fibre material, like quartz glass. So-called endlessly single-mode guidance (ESM, i.e. all higher order modes, HOMs, are leaky while the fundamental LP01mode is fully confined) is achieved in solid-core photonic crystal fibres (PCFs) by engineering a cladding structure surrounding the solid-core, as shown inFIG. 7A, such that the diameter d of channels in the cladding structure and their centre-centre spacing (pitch) A fulfils the geometrical condition d/A<0.41 ([1, 2]). However, due to light guiding in the solid fibre material, disadvantages exist in terms of increased latency in data transmission, optically non-linear effects resulting in new light frequencies, and relatively low damage thresholds.

Through their ability to guide light in a non-solid core region, which is evacuated (vacuum core), filled with a gas or filled with a liquid, hollow-core photonic crystal fibres, HC-PCFs, have unique advantages compared to solid-core fibres, resulting in application areas such as low-latency data transmission, high-power beam delivery, gas-based nonlinear optics, light guiding with ultralow nonlinearities and chemical sensing. HC-PCFs are typically divided into two classes depending on the physical guidance mechanism: hollow-core photonic bandgap fibres (HC-PBFs) and hollow-core anti-resonant-reflecting fibres (HC-AFs).

FIGS. 7B to 7Ishow a selection of scanning electron micrographs of different types of conventional HC-PCFs.FIGS. 7B and 7Cshow HC-PBFs that confine modes inside a central hollow core by means of a photonic bandgap in the cladding [4-6]. These types of PCF typically have relatively low loss (ca.<20 dB/km) at telecommunication wavelengths. However, due to the wavelength-specific effect of the photonic bandgap, they guide light over a relatively narrow bandwidth (ca.<15 THz) only. Although in general HC-PBFs support HOMs, Fini et al. ([6], [3]) have shown that bended HC-PBFs can be made effectively single-mode by including “satellite” hollow cores in the cladding (FIG. 7B). These satellites strongly suppress HOMs in the core by phase-matching to them, causing high HOM loss. HC-PBGs can also be made truly single-mode over a narrow spectral range (ca.<7 THz) if a small enough core is used (seeFIG. 7C) [5], but this results in fabrication difficulties and significantly higher loss for the desired fundamental mode.

FIGS. 7D to 7Ishow a selection of HC-AF structures, i.e. fibres having guidance mechanism based mostly on anti-resonant effects.FIGS. 7D and 7Ehave a Kagome-lattice cladding [7, 8, 13] andFIGS. 7F and 7Ghave one ring of single (F) or nested (G) anti-resonant elements (AREs).FIG. 7Hshows an HC-AF with a square core [12] andFIG. 7Idepicts an HC-AF with guiding properties in the ultraviolet [19]. Compared to HC-PBFs, the loss of HC-AFs is in general larger because of the non-ideal confinement, but the transmission window is broader.

F. Poletti showed numerically that by engineering the radial distance between nested AREs, an HC-AF can be made effectively single-mode over a limited wavelength range [14], but this has not been demonstrated experimentally. In particular, mode suppression has been suggested in [14] for a certain centre wavelength and a certain structural parameter z/R only, wherein z is a diameter difference of AREs and nested structures in the AREs and R is the radius of the hollow core. This theoretical finding cannot be extended to a general design and manufacturing of HC-AFs.

Also Wei et al. ([18]) and A. Hartung et al. ([19]) showed that HOM suppression can be enhanced. This was achieved in HC-AFs with touching AREs, but the overall HOM suppression was relatively low. According to [18], the effect of varying the thickness t of the touching AREs on the effective refractive index of the hollow core and the AREs has been investigated. An optimum thickness has been found wherein HOMs are suppressed. As a disadvantage of this approach, HOM suppression has been shown for a certain wavelength only, but not for a broad wavelength range.

Compared to solid-core fibres, one particular drawback of conventional HC-AFs is their inability to be purely single-mode, i.e. higher order modes (HOMs) are supported for relatively long distances. As a consequence the output beam quality is degraded, which is undesirable in many applications as it introduces modal beating, blurring the focal spot and—if the fibre experiences change in stress—causes power fluctuations. Another disadvantage of conventional HC-AFs results from limitation in manufacturing thereof, in particular positioning the AREs at specific azimuthal locations in a sufficient stable and reproducible manner.

The objective of the disclosure is to provide an improved hollow-core fibre of non-bandgap type, which is capable of avoiding disadvantages of conventional HC-AFs. In particular, the HC-AF is to be provided with an extended wavelength range of single-mode transmission, increased loss of HOMs, and/or increased ratio in loss between the highest-index core HOM and the fundamental, e. g. LP01mode. Furthermore, the objective of the disclosure is to provide an improved optical device being equipped with at least one HC-AF and avoiding limitations of conventional optical devices, in particular in terms of low loss single mode guidance, damage-free high power light delivery and targeted creation of optically nonlinear effects. Furthermore, the objective of the disclosure is to provide an improved method of manufacturing a hollow-core fibre of non-bandgap type resulting in the improved inventive HC-AF and avoiding limitations of conventional manufacturing methods.

These objectives are solved with a hollow-core fibre of non-bandgap type, an optical device and a method of manufacturing a hollow-core fibre of non-bandgap type, comprising the features of the independent claims, resp. Advantageous embodiments and applications of the disclosure are defined in the dependent claims.

BRIEF SUMMARY

In some embodiments, a hollow-core anti-resonant-reflecting fibre (HC-AF) includes a hollow-core region, an inner cladding region, and an outer cladding region. The hollow-core region axially extends along the HC-AF. The inner cladding region includes a plurality of anti-resonant elements (AREs) and surrounds the hollow-core region. The outer cladding region surrounds the inner cladding region. The hollow-core region and the plurality of AREs are configured to provide phase matching of higher order hollow-core modes and ARE modes in a broadband wavelength range.

In some embodiments, the hollow-core region and the plurality of AREs are further configured to provide effectively endlessly single-mode (eESM) behavior. In some embodiments, the hollow-core region and the plurality of AREs are further configured to provide at least a frequency of 10 THz in the broadband wavelength range. In some embodiments, the hollow-core region and the plurality of AREs are further configured to provide at least a frequency of 20 THz in the broadband wavelength range. In some embodiments, the hollow-core region and the plurality of AREs are further configured to provide all wavelengths within a hollow-core transparency window of a transverse fundamental hollow-core mode.

In some embodiments, the refractive indices of the hollow-core region and the plurality of AREs are equal. In some embodiments, each of the plurality of AREs includes an elliptical transverse cross-section. In some embodiments, each of the plurality of AREs includes a circular transverse cross-section. In some embodiments, the HC-AF further includes a plurality of longitudinal protrusions disposed between the inner cladding region and the outer cladding region.

In some embodiments, the plurality of AREs includes a first plurality of AREs and a second plurality of AREs. In some embodiments, each of the first plurality of AREs has at least one physical characteristic that is different than each of the second plurality of AREs.

In some embodiments, the plurality of AREs includes a first plurality of AREs and a second plurality of AREs, each of the first plurality of AREs includes a first transverse cross-sectional dimension (d1), each of the second plurality of AREs includes a second transverse cross-sectional dimension (d2), and the second transverse cross-sectional dimension (d2) is no greater than the first transverse cross-sectional dimension (d1). In some embodiments, the HC-AF further includes a support tube disposed between the first plurality of AREs and the second plurality of AREs.

In some embodiments, the hollow-core region includes a first transverse cross-sectional dimension (D), each of the plurality of AREs includes a second transverse cross-sectional dimension (d), and a ratio of the first and second transverse cross-sectional dimensions (d/D) is approximated to a quotient of zeros of Bessel functions of the first kind (ulm,ARE/ulm,core), multiplied with a fitting factor in a range of about 0.9 to about 1.5, with m being the m-th zero of the Bessel functions of the first kind of order l, the zeros of the Bessel functions describing linearly polarized (LPlm) ARE modes and linearly polarized (LPlm) higher order hollow-core modes, respectively.

In some embodiments, an optical device includes a hollow-core anti-resonant-reflecting fibre (HC-AF). The HC-AF includes a hollow-core region axially extending along the HC-AF, an inner cladding region comprising a plurality of anti-resonant elements (AREs) and surrounding the hollow-core region, and an outer cladding region surrounding the inner cladding region. In some embodiments, the hollow-core region includes a first transverse cross-sectional dimension (D) and each of the plurality of AREs includes a second transverse cross-sectional dimension (d). In some embodiments, the hollow-core region and the plurality of AREs are configured to provide phase matching of higher order hollow-core modes and ARE modes in a broadband wavelength range.

In some embodiments, a ratio of the first and second transverse cross-sectional dimensions (d/D) is configured to provide effectively endlessly single-mode (eESM) behavior. In some embodiments, a ratio of the first and second transverse cross-sectional dimensions (d/D) is configured to provide at least a frequency of 10 THz in the broadband wavelength range. In some embodiments, a ratio of the first and second transverse cross-sectional dimensions (d/D) is configured to provide all wavelengths within a hollow-core transparency window of a transverse fundamental hollow-core mode.

In some embodiments, a method of manufacturing a hollow-core anti-resonant-reflecting fibre (HC-AF) includes providing an inner cladding region comprising a plurality of anti-resonant elements (AREs) and an outer cladding region surrounding the inner cladding region. The method further includes coupling the plurality of AREs to an inner surface of the outer cladding region. The method further includes forming a hollow-core region axially extending along the HC-AF and surrounded by the inner cladding region. In some embodiments, the hollow-core region includes a first transverse cross-sectional dimension (D) and each of the plurality of AREs includes a second transverse cross-sectional dimension (d). In some embodiments, the hollow-core region and the AREs are configured to provide phase matching of higher order hollow-core modes and ARE modes in a broadband wavelength range.

In some embodiments, the forming includes forming a ratio of the first and second transverse cross-sectional dimensions (d/D) such that the broadband wavelength range covers at least a frequency of 10 THz. In some embodiments, the method further includes filling the HC-AF with a gas, a liquid, or a material having a non-linear optical response.

Further features and advantages of the disclosure, as well as the structure and operation of various embodiments of the disclosure, are described in detail below with reference to the accompanying drawings. It is noted that the disclosure is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

