阅读说明：本技术 中空芯光纤和激光系统 (Hollow core optical fiber and laser system ) 是由 J·K·朗格塞 C·雅各布森 M·米基耶莱托 于 2016-12-22 设计创作，主要内容包括：本发明公开了中空芯光纤和激光系统。一种中空芯光子晶体光纤(PCF),包括外包层区域和由外包层区域围绕的7个中空管。中空管中的每一个熔合到外包层以形成限定内包层区域和由内包层区域围绕的中空芯区域的环。中空管彼此不接触,但以距相邻中空管的距离布置。中空管各自具有平均外直径d2和平均内直径d1,其中d1/d2等于或大于约0.8,诸如等于或大于约0.85,诸如等于或大于约0.9。还公开了一种激光系统。(The invention discloses a hollow core optical fiber and a laser system. A hollow core Photonic Crystal Fiber (PCF) comprising an outer cladding region and 7 hollow tubes surrounded by the outer cladding region. Each of the hollow tubes is fused to the outer cladding to form a ring defining an inner cladding region and a hollow core region surrounded by the inner cladding region. The hollow tubes do not contact each other but are arranged at a distance from adjacent hollow tubes. The hollow tubes each have an average outer diameter d2 and an average inner diameter d1, wherein d1/d2 is equal to or greater than about 0.8, such as equal to or greater than about 0.85, such as equal to or greater than about 0.9. A laser system is also disclosed.)

1. A hollow core Photonic Crystal Fiber (PCF) comprising an outer cladding region and 7 hollow tubes surrounded by said outer cladding region, wherein each of said hollow tubes is fused to said outer cladding region to form a ring defining an inner cladding region and a hollow core region surrounded by said inner cladding region, wherein said hollow tubes are not in contact with each other, and

wherein each of the hollow tubes comprises a core central facing region having a wall thickness, and at least one of the hollow tubes has a different wall thickness than at least one other of the hollow tubes.

2. The hollow core photonic crystal fiber of claim 1, wherein at least one of the hollow tubes has a wall thickness that is at least 5% greater than a wall thickness of at least one other of the hollow tubes, such as at least 10% greater than a wall thickness of at least one other of the hollow tubes.

3. The hollow core photonic crystal fiber of claim 1, wherein at least one of the hollow tubes is free of nested sub-tubes having an average outer sub-tube diameter less than the average inner diameter d1 of the hollow tube and is disposed in the hollow structure of and fused to the hollow tube.

4. The hollow core photonic crystal fiber of claim 1, wherein at least one of the hollow tubes comprises a nested sub-tube disposed in the hollow structure of and fused to the hollow tube, the sub-tube having an average outer diameter d substantially less than the average inner diameter d1 of the hollow tube_subSuch as wherein the average outer diameter d2_subUp to 0.9 x d2 of the hollow tube.

5. The hollow core photonic crystal fiber of claim 1, wherein the average outer diameter of the hollow tube relative to the core diameter D2/D of the hollow core region is 0.5 to 0.75, such as 0.65 to 0.72.

6. The hollow core photonic crystal fiber of claim 1, wherein the hollow tubes each have an average outer diameter d2 and an average inner diameter d1, wherein d1/d2 is equal to or greater than 0.8, such as equal to or greater than 0.85, such as equal to or greater than 0.9.

7. The hollow core photonic crystal fiber of claim 1, wherein the wall thickness t of each of the hollow tubes is up to 1 μ ι η, such as up to 2.1 μ ι η, such as in the range of 150nm to 350nm, in the range of 650nm to 850nm or in the range of 900nm to 2.1 μ ι η.

8. The hollow core photonic crystal fiber of claim 1, wherein the hollow core region has a core diameter D of 10 to 100 μm.

9. The hollow core photonic crystal fiber of claim 1, wherein the hollow tubes have a center-to-center distance Λ between adjacent hollow tubes, the center-to-center distance Λ being between 1.01 x d2 and 1.5 x d2, such as between 1.05 x d2 and 1.2 x d 2.

10. The hollow core photonic crystal fiber of claim 1, wherein the minimum distance between adjacent hollow tubes is at least 0.1 μ ι η, such as at least 2 μ ι η, such as at least 5 μ ι η.

11. A laser system for delivering laser light to a user device, the laser system comprising a laser light source and a fiber optic delivery cable for delivering light from the laser light source to the user device, wherein the fiber optic delivery cable comprises the hollow core photonic crystal fiber of claim 1.

Technical Field

The present invention relates to hollow core photonic crystal fibers and laser systems comprising such hollow core fibers.

Background

Hollow core Photonic Crystal Fibers (PCFs) have been known for many years and in some applications are very attractive compared to solid core PCFs due to their very low propagation loss, low nonlinear effects and high damage threshold.

A hollow optical fiber is an optical fiber that substantially guides light within the hollow region such that only a small portion of the optical power propagates in the solid optical fiber material surrounding the core. The hollow region may be filled with air or any other gas, or it may be evacuated to have a very low gas pressure.

Several types of hollow core PCFs have been developed over the years, such as hollow core photonic bandgap fibers, for example as described in US6892018, and Kagome design fibers, for example as described in US 8306379.

The Kagome type fiber guides light by means of anti-resonance effects and allows substantially wider spectral transmission than is achieved in photonic bandgap fibers.

Such hollow core anti-resonance fibers (ARFs) have been extensively studied in recent years, particularly with the aim of further improving the fibers with respect to low loss and broad spectral transmission.

To reduce attenuation, Kolyadin et al in optical prompter (Optics Express)2013, volume 21, pages 9514 to 9519, propose a hollow core optical fiber having a hollow core surrounded by 8 identical capillaries. It was concluded that larger core sizes are preferred to reduce losses, and that reduced losses can be achieved by increasing the number of capillaries in the cladding.

Another disadvantage of prior art hollow core anti-resonance fibers is that they are not purely single mode waveguides, since they also support some Higher Order Modes (HOMs). Since these HOMs generally have relatively low losses, it is difficult to emit the pure LP01 mode without HOM contamination, and other HOMs can be excited by bending or external stress.

In an attempt to solve this problem and achieve as high an HOM suppression as possible while maintaining a reasonably low loss of the LP01 modality, library arXiv, kanel university, on 8/27 th month 2015, in Gunedi et al: 1508.06747[ physical optics ] "broadband-band robust single-mode hollow-core PCF by resonance filtering of high-order modes" proposes an improved fiber design. The ARR fiber described in this article comprises a hollow core (inner diameter D) surrounded by six uniformly spaced and non-contacting capillaries (hereinafter also referred to as hollow tubes) having a wall thickness t and an inner diameter D, supported within a thick-walled support capillary. The non-touch glass elements have tailored modes to ensure resonant phase-matched coupling with the higher-order core modes so that they leak at a very high rate into the supporting solid glass sheath. It has been found that very high HOM suppression can be achieved over a broad wavelength band where the fiber is guided with low loss, as long as the ratio D/D ≈ 0.68 and the wall of the tube wall T is sufficiently thin, i.e. T/D ═ 0.01.

WO15185761 describes a similar anti-resonant hollow core optical fibre comprising a first tubular cladding member defining an inner cladding surface, a plurality of second tubular members attached to the cladding surface and together defining a core having an effective radius, the second tubular members being arranged in spaced apart relation with a spacing between adjacent ones of the second tubular members, and a plurality of third tubular members each nested within a respective one of the second tubular members. It was concluded that the optimal number of tubes for suppressing HOMs was 6.

Disclosure of Invention

It is an object of the present invention to provide a hollow core PCF that alleviates at least one of the above disadvantages.

In one embodiment, it is an object to provide a hollow core PCF with high spectral transmission and high HOM suppression.

In one embodiment, it is an object to provide a hollow core PCF with high single mode transmission efficiency and substantially insensitive to bending.

In one embodiment, it is an object to provide a hollow core PCF that in a simple manner can be designed with a desired low loss transmission band having a bandwidth of at least about 50nm and preferably having a transmission loss of less than about 50 dB/Km.

In one embodiment, it is an object to provide a hollow core PCF with a transmission band comprising a wavelength in the range of about 400nm to about 1200 nm.

These and other objects have been solved by the invention or embodiments thereof as defined by the claims and as described hereinafter.

It has been found that the invention or embodiments thereof have many other advantages, which will be apparent to those skilled in the art from the following description.

The phrase "radial distance" refers to a distance determined in a radial direction from the central axis of the hollow core. The phrase "radial direction" is a direction from the core central axis and radially outward.

The term "substantially" herein should be considered to include common product variations and tolerances.

The term "about" is generally used to include content within the measurement uncertainty. The term "about" when used within a range should be considered herein to mean including within the measurement uncertainty within that range.

It should be emphasized that the term "comprises/comprising" when used herein is to be interpreted as an open-ended term, i.e. it should be taken to specify the presence of the stated features (such as elements, units, integers, step components, and combinations thereof), but does not preclude the presence or addition of one or more other stated features.

Throughout the specification or claims, the singular encompasses the plural unless the context otherwise indicates or requires.

Unless otherwise indicated or clear from context, diameters, thicknesses, and other structural values are visible in the cross-sectional view of the hollow core PCF.

The hollow core Photonic Crystal Fiber (PCF) of the present invention comprises an outer cladding region and 7 hollow tubes surrounded by the outer cladding region. Each of the hollow tubes is fused to the outer cladding to form a ring defining an inner cladding region and a hollow core region surrounded by the inner cladding region. The hollow tubes do not contact each other. It has been found that hollow tubes each having an average outer diameter referred to as d2 and an average inner diameter referred to as d1 are advantageous for suppressing undesirable higher order modes. The ratio dl/d2 is equal to or greater than about 0.8.

The average outer diameter d2 and/or the average inner diameter of the respective hollow tubes may be the same or different from hollow tube to hollow tube.

In one embodiment, d1 and/or d2 are substantially the same for at least three of the hollow tubes, such as for all seven hollow tubes.

According to the present invention and in view of all the teachings of the hollow core PCF of the prior art, it was surprisingly found that a hollow core PCF with a non-contacting inner cladding structure with seven non-contacting tubes provides a greater suppression of Higher Order Modes (HOMs) than the above-described hollow core PCF with six non-contacting inner cladding tubes. In particular and as shown in the examples, it has been found that the hollow core PCF of the invention with seven non-contact tubes has an increased suppression of HOMs with azimuthal numbers greater than 1 (such as third order HOMs) while having low losses in the fundamental mode of at least one transmission band below 2 μm, preferably below 1.5 μm.

The term "transmission band" is used herein to refer to a band having a bandwidth of transmission wavelength of at least about 10nm, such as at least about 25nm, and preferably at least about 50nm or even at least about 100 nm.

The term "low loss transmission band" refers to a transmission band of light in its fundamental mode with a transmission loss of less than about 100dB/km, preferably less than about 60dB/km, and more preferably less than about 50 dB/km.

It was further found that by increasing the ratio d1/d2, a low loss transmission band can be achieved at even lower wavelengths, such as a low loss transmission band comprising wavelengths below 1200nm, such as below 1 μm, such as below 800 μm, such as below 600 μm. Advantageously, the ratio d1/d2 is equal to or greater than about 0.85, such as equal to or greater than about 0.9.

The outer cladding comprises a solid cladding material and is preferably entirely solid.

In one embodiment, the hollow tubes are arranged at substantially equal distances from adjacent hollow tubes. So that a very high gaussian beam quality can be obtained. In an alternative embodiment, the hollow tubes are arranged at different distances from adjacent hollow tubes. In the latter embodiment, the distance variation between adjacent hollow tubes is provided by having different tube outer diameters d 2. In the case where the inner diameter d1 and/or the outer diameter d2 of the hollow tube are different, the inner diameter d1 and the outer diameter d2 are calculated as average d1 and d2 values, respectively, unless the hollow tube thickness is calculated or otherwise specified. Thus, the ratio dl/d2 is determined by the average dl and d2 values. It has been found that optimal suppression of HOMs requires that the minimum distance between adjacent hollow tubes be relatively small, and preferably less than about half the outer diameter d2 of the hollow tubes. On the other hand, the distance should not be too narrow, as this may lead to the risk of completely eliminating the distance along part of the entire fiber length or along the entire fiber length during drawing of the fiber due to surface tension and material attraction. To avoid this risk, it is desirable that the minimum distance between adjacent hollow tubes is at least 0.01 times d 2.

Advantageously, the minimum distance between adjacent hollow tubes is at least about 0.1 μm, such as at least about 1 μm, such as at least about 2 μm, such as at least about 5 μm.

In one embodiment, the minimum distance between adjacent hollow tubes is about 5 μm or less, such as about 4 μm or less.

It has been found that it is most preferred that the center-to-center distance Λ between adjacent hollow tubes is between about 1.01 × d2 and about 1.5 × d2, such as between 1.05 × d2 and 1.2 × d 2.

The hollow tubes advantageously have substantially parallel central axes. In practice, the central axis of the hollow tube may deviate slightly from a straight line due to process variations. For example, the hollow tube may be spiraled around the core at a very long pitch (such as a pitch of up to 1km, such as a pitch of 1cm to 100 m).

Due to the structure of the hollow core PCF, the core can in principle be designed with a core region of any diameter. The core region is advantageously substantially circular. In one embodiment, the core region is not circular, but rather is elliptical or angular, such as substantially pentagonal. The core diameter D is defined as the diameter of the largest circle inscribed by the 7 hollow tubes.

Advantageously, the hollow core region has a core diameter D of about 10 μm to about 100 μm, such as about 10 μm to about 60 μm.

It has been found that the optimum core diameter D can be scaled together with the center wavelength of the transmission band of the hollow core PCF.

For a center wavelength of about 1.0 μm, the core diameter D is advantageously from about 20 μm to about 50 μm, such as from about 25 μm to about 40 μm. The preferred core diameter D scales directly proportional to the center wavelength of the transmission band of the hollow core PCF.

It has been found that a hollow core PCF can have a very high beam quality even with a relatively large core. In one embodiment, the beam quality M²Is about 1.75 or less, such as about 1.6 or less, such as about 1.5 or even less.

In one embodiment, the average outer tube diameter is about 0.5 to about 0.75, such as about 0.65 to about 0.72, relative to the core diameter D2/D, thereby ensuring the desired minimum distance between the hollow tubes and very effective suppression of HOMs (particularly the third and fourth HOMs).

Furthermore, the modal quality of the beam obtained with the hollow core PCF average outer tube diameter relative to the core diameter D2/D is about 0.5 to about 0.75, showing to be very high.

It has been found that the wall thickness of the hollow tube significantly affects the low loss transmission band or bands of the hollow core PCF. In fact, it has been found that the relevant wall thickness t is the core center facing region of the hollow tube wall.

Thus, it has been found that the wall thickness t (or average value t) of the core central facing region of the hollow tube primarily affects the position relative to the wavelength of the low loss transmission band. Typically, a hollow core PCF may have one or more low loss transmission bands, such as 1, 2, 3, 4 or even more low loss transmission bands.

Advantageously, the hollow core PCF has at least 3 low loss transmission bands, such as at least 4 low loss transmission bands. To obtain one or more low loss bands below 1.2 μm, it is desirable that the wall thickness t is up to about 2.1 μm, such as up to about 1 μm, such as in the range of about 150nm to about 350nm or in the range of about 650nm to about 850nm or in the range of about 900nm to about 2.1 μm.

For example, it has been found that for a hollow core PCF according to an embodiment of the invention, the following fibers have low loss transmission at 1030-: for a wall thickness t of 150-.

Advantageously, the hollow core PCF is designed such that the hollow core guides light in the fundamental mode and comprises at least one wavelength of wavelengths between 200nm and 4000nm, such as between 400nm and 2000nm, such as between 800nm and 1600nm, such as between 1000nm and 1100nm, with a loss of less than about 1000 dB/km.

Advantageously, the hollow core PCF has a low loss transmission band for the fundamental mode, wherein the transmission loss is less than about 100dB/km, preferably less than about 60dB/km, and more preferably less than 50dB/km, comprising wavelengths in the interval between 1000nm and 1100nm, and preferably comprising at least wavelengths between 1030nm and 1064 nm.

In one embodiment, the hollow core PCF has a transmission loss for the HOM of greater than about 2000dB/km in its low transmission band for the fundamental mode, thereby ensuring efficient single mode transmission.

It has further been found that hollow core PCFs are substantially insensitive to bending, e.g., it has been found that hollow core PCFs of one embodiment of the present invention have low losses of less than about 5%/km at 1030nm-1060nm when rolled into a diameter of about 6 cm.

Advantageously, the wall thickness t of each of the hollow tubes is substantially the same. Preferably, the hollow tubes are substantially identical and are arranged at the same distance from adjacent hollow tubes. Thereby, the hollow core PCF becomes easier to design for the desired wavelength transmission.

In one embodiment, at least one of the tubes has a different wall thickness t than at least one other of the hollow tubes. Preferably, 3 of the hollow tubes have one wall thickness and the remaining 4 hollow tubes have another wall thickness. It has been found that with this arrangement, the hollow core PCF can become birefringent, largely dependent on the difference in wall thickness t and the relative positions of the hollow tubes with greater and lesser wall thicknesses t.

In one embodiment, at least one of the hollow tubes has a wall thickness that is at least about 5% greater than the wall thickness of at least one of the other hollow tubes, preferably at least one of the hollow tubes has a wall thickness that is at least about 10% greater than the wall thickness of at least one of the other hollow tubes.

In one embodiment, each of the hollow tubes is substantially circular. In practice, it is very difficult to obtain perfectly circular hollow tubes, firstly due to slight deformations in which the respective hollow tube is fused to the inside of the outer cladding, and secondly because the hollow tubes may be attracted to the adjacent hollow tubes during fiber drawing.

In one embodiment, each of the hollow tubes has a long inner diameter D_lAnd a long inner diameter D_lPerpendicular short internal diameter Ds, where D_lDetermined in a radial direction along the central axis of the hollow core.

Advantageously, the ratio Ds/D_lFrom about 0.5 to about 0.99, such as from about 0.8 to about 0.99.

In one embodiment, Ds/D_lIs greater than about 0.9, such as greater than about 0.95, which is actually easier to manufacture.

In one embodiment, Ds/D_lIs less than about 0.95, such as less than 0.9, which results in a relatively large core diameter D and may therefore help to reduce the transmission loss of the fundamental mode.

In one embodiment, at least one of the hollow tubes comprises at least one nested sub-tube arranged in the hollow structure of the hollow tube, wherein the sub-tube is fused to the hollow tube, preferably aligned with the fusion to the outer cladding (i.e. furthest from the core region).

A hollow tube with one or more nested tubes may for example be as described in the PCF of WO15185761, comprising a second tubular element, wherein a third tubular element is embedded within the respective second tubular element, with the difference that the hollow core PCF has 7 second tubular elements with nested third tubular tubes.

The sub-tubes advantageously have an average outer diameter d substantially smaller than the average inner diameter d of the hollow tube_sub. Average outer diameter d2_subPreferably up to about 0.9 d2 of the hollow tube, such as up to about 0.9 d2, preferably the inner sub-tube is fused to the hollow tube at its maximum radial distance from the central axis of the core.

In one embodiment, at least one of the hollow tubes comprises one or more nodules arranged at a core central facing region of the one or more hollow tubes, preferably the nodules are arranged at a boundary of the hollow core region, the nodules preferably being arranged to be anti-resonant at the operating wavelength such that light of the fundamental mode is substantially excluded from the nodules. An even stronger confinement of the fundamental mode can thereby be obtained to further reduce the loss of light in the fundamental mode transmitted in the core region of the hollow core PCF.

The nodules may advantageously be in the form of nodular or beaded structures, or locally thicker regions of the wall extending along at least a portion of the length of the fiber. In prior art optical fibers, such nodules are sometimes referred to as nodes or spots, for example as described in US7,321712. The nodules and the rest of the hollow tube are advantageously of the same material.

The core region and/or the hollow tube may in principle comprise any fluid. Preferably, the core region and/or the hollow tube comprise, independently of each other, a gas determined at Standard Ambient Temperature and Pressure (SATP) at a temperature of 25 ℃ and an absolute pressure of exactly 100000 Pa. Suitable gases include air, argon, nitrogen, or mixtures including any of the gases described. Optionally, the hollow core region and/or the hollow tubes are evacuated (evacuated to have a very low gas pressure) or filled with a pressurized gas independently of each other.

In one embodiment, the hollow core region and/or hollow tube is evacuated at standard temperature to a pressure of about 1 mbar or less, such as to a pressure of about 0.1 mbar or less, such as to a pressure of about 0.01 mbar or less.

In one embodiment, the hollow core region and/or the hollow tube is pressurized at standard temperature to a pressure of up to 2 bar, such as up to about 1.5 bar.

In principle, the outer cladding region may have any dimensions as long as it provides sufficient mechanical support for the hollow core PCF for the hollow tube. In one embodiment, the outer cladding region has an outer diameter of at least about 125 μm, 150 μm, such as at least about 200 μm.

Typically, it is desirable for the hollow core PCF to be a single material, preferably glass, more preferably silica, optionally doped with a refractive index changing dopant. In one embodiment, one or more, such as all, of the hollow tubes are doped silica and the outer cladding region is undoped silica. The dopant may, for example, comprise a refractive index changing material such as F, Ge, P, B, or a combination thereof.

It has furthermore been found that confinement losses can be reduced by providing an outer cladding region to comprise a photonic band gap structure surrounding said inner cladding region.

The photonic band gap structure may be provided by any means, for example by providing the outer cladding region with a refractive index grating comprising concentric rings having different refractive indices and/or by comprising microstructures having different refractive indices.

In one embodiment, the outer cladding region comprises a refractive index N_ocAnd a plurality of inclusions having a refractive index different from that of the background material. The inclusions advantageously have a refractive index lower than that of the background material. The inclusions preferably extend substantially parallel to the core region. The inclusions may extend in a length section of the hollow core PCF or substantially in the entire length of the hollow core PCF.

In one embodiment, the plurality of inclusions in the outer background material are arranged in a cross-sectional pattern comprising at least two rings of inclusions, such as at least 3 rings, such as at least 4 rings of inclusions, surrounding the inner cladding region.

The phrase "inclusion ring" refers to inclusions of the outer cladding inclusions having substantially equal radial distances from the core and aligned in a ring configuration around the core. Typically, the inclusion ring is not perfectly circular, but is shaped with many soft angles, such as in a hexagonal shape. Preferably, all inclusions of the ring of inclusions are of substantially the same size, and preferably have the same solid material, voids and/or gas.

The background material may advantageously be silicon dioxide, such as undoped silicon dioxide or doped silicon dioxide. The dopant may, for example, comprise a refractive index changing material such as F, Ge, P, B, or a combination thereof.

In one embodiment, the plurality of inclusions in the outer background material are arranged in a substantially hexagonal pattern.

In one embodiment, the plurality of inclusions is a void or a gas, such as air.

The diameter of the inclusions is advantageously selected to minimize losses at a selected wavelength or range of wavelengths. The diameter can therefore be chosen to be optimized for the desired transmission profile.

In one embodiment, the inclusions have substantially the same diameter.

Advantageously, the plurality of inclusions has such an average diameter (d)_inc) It is up to about 2.5 μm, such as up to about 2 μm, such as between about 1.1 μm and 1.8 μm, such as between about 1.15 μm and about 1.7 μm, such as between about 1.2 μm and about 1.5 μm, such as about 1.3 μm.

Furthermore, the distance between the respective inclusions is shown to be relevant for optimizing (minimizing) the constraint losses. In one embodiment for optimizing confinement loss, the plurality of inclusions is arranged at a pitch (Λ inc) of up to about 6 μm, such as up to about 5 μm, such as up to about 4 μm, such as between about 2 μm and 4 μm.

In one embodiment, the inclusions are at such a pitch (Λ)_inc) An arrangement of up to about 3.5 μm, such as up to about 3 μm, such as up to about 2.5 μm, such as between about 1.1 μm and 2 μm.

The invention also includes a laser system for delivering laser light to user equipment, the laser system comprising a laser light source and a fiber optic delivery cable for delivering light from the laser light source to the user equipment, wherein the fiber optic delivery cable comprises a hollow core PCF as described above.

The fiber optic delivery cable comprising a hollow core PCF may preferably have a length of up to 50m, such as about 0.3m to about 20m, such as about 1m to about 15 m.

Advantageously, the laser light source is configured for generating laser pulses and is optically connected to the fiber optic transmission cable, preferably the laser light source is a femtosecond laser source.

In one embodiment, the laser light source may be arranged to feed light directly to the hollow core PCF, e.g. by fusing to the hollow core PCF. In an embodiment, the laser light source is arranged for feeding light to the hollow core PCF via one or more optical elements and/or via free space.

In one embodiment, the laser light source has a pump duration of about 30fs to about 30ps, such as about 100fs to about 10 ps.

In one embodiment, the laser light source has a peak power determined at the laser light source exit that is at least about 5kW, such as at least about 10kW, such as at least about 30kW, such as at least about 50 kW.

The laser light source is advantageously a mode-locked laser light source. In one embodiment, the laser light source is a actively mode-locked laser. In one embodiment, the laser light source is a passive mode-locked laser. The mode-locked laser preferably includes one or more amplifiers.

In one embodiment, the hollow core PCF is configured for guiding light, preferably single mode light, comprising at least one wavelength in the range of about 200nm to about 4.5 μm, preferably at least one wavelength in the range of 1000nm to about 1100 nm.

In one embodiment, the hollow core PCF is configured to direct a continuous wavelength of light, preferably spanning at least about 0.1 μm, such as at least about 0.3 μm, such as at least about 0.5 μm.

In one embodiment, the hollow core PCF has a first fiber end adapted to be connected to user equipment and a second fiber end optically connected to an output fiber of the laser light source via a fiber coupling structure. The optical fibre coupling structure preferably provides protection for the first optical fibre end and preferably the end face of the first optical fibre end to ensure that the end face and/or hollow core is not contaminated by dust, moisture or the like. Furthermore, to ensure a safe low-loss connection, the coupling structure may comprise a lens, such as a focusing lens, a graded-index element or other elements, such as those known in the art as commonly used for hollow-core optical fibers.

In one embodiment, the fiber coupling structure includes a focusing lens, such as described in US8,854,728, for example.

In one embodiment, the fiber coupling structure includes a graded index element (GRIN) such as described in US 2003/0068150.

In one embodiment, the fiber coupling structure includes a protective element such as described in US7,373,062.

In a preferred embodiment, the first FIBER end is mounted in a ferrule structure, preferably having a ferrule structure entitled "PHOTONIC CRYSTAL FIBER ASSEMBLY" (PHOTONIC CRYSTAL FIBER ASSEMBLY) as described in PA 201570876 DK, and this application is incorporated herein by reference for the present disclosure, provided that the subject matter explicitly disclosed herein prevails in the event of any inconsistency between the subject matter explicitly disclosed herein and the incorporated subject matter.

THE hollow core PCF as described herein may advantageously be produced by drawing FROM a PREFORM, wherein THE drawing is performed while controlling THE pressure within THE hollow tube, e.g. as described in US6,954,574, US8215129, US7,793,521 AND/or PA 201670262 DK, entitled "ring element FOR PREFORM, PREFORM AND OPTICAL FIBER DRAWN FROM PREFORM" (A RING ELEMENT FOR a PREFORM, a PREFORM AND AN OPTICAL FIBER draw FROM PREFORM) ".

The preform is advantageously produced by providing the outer cladding region with one hollow tube and the inner cladding region with 7 hollow tubes, wherein the hollow tubes for the outer cladding region have an inner diameter which is two times larger than the outer diameter of the 7 hollow tubes, such as at least 3 times larger, such as at least 4 times larger, or the largest outer diameter of the 7 hollow tubes for the inner cladding if the outer diameters of the 7 hollow tubes are different. Preferably, the outer cladding region has an inner diameter large enough to arrange 7 hollow tubes inside and in contact with the hollow tubes for the outer cladding region so that the 7 hollow tubes do not contact each other. In order to arrange 7 hollow tubes inside the hollow tube for the outer cladding region, a short piece of support element (e.g. a glass tube or a glass rod) may be arranged at each end of the hollow tube. After fusing 7 hollow tubes to the hollow tubes for the outer cladding region, the ends of the fused hollow tubes including the supporting members may be cut off.

The preforms are then preferably pulled into the fiber draw tower by simultaneously performing individual or collective pressure control of the pressure within the respective hollow tubes.

In one embodiment, pressure control of each of the one or more hollow tubes (also referred to as "elongated holes" or just "holes") of the preform is provided by disposing a pressure tube between the holes and a pressure supply source. The pressure supply source ensures that the pressure within the hollow tube is controlled to a desired level via the pressure tube during drawing of the optical fiber from the preform.

The pressure pipe is advantageously a hollow pressure pipe. The phrase "in the tube" refers to the hollow portion of the pressure tube.

Thus, in one embodiment, the method comprises inserting the first end of the pressure tube into a hole of the preform at the first end of the preform and subjecting the hole of the preform to a controlled pressure via the pressure tube during drawing. Advantageously, at least the length of the pressure tube, including the first end of the pressure tube, is inserted into the elongated hole of the preform. The pressure tube length should preferably have a length of at least about 0.5mm, such as about 1mm to about 20cm, such as about 2mm to about 5cm, such as 0.5cm to 1 cm.

In practice, it is desirable for the pressure tube length to have a length sufficient to provide a seal between the pressure tube length and the elongated bore, but it should not be too long because the length of the preform comprising the pressure tube length may not be drawn into an optical fiber in one embodiment, or if it is drawn into an optical fiber in an alternative embodiment, the resulting optical fiber will have different properties than an optical fiber drawn from the material of the preform without the tube length.

It has been found that a safe gas connection can be obtained by simply inserting the first end of the pressure tube into the bore and optionally providing expansion of the length of the pressure tube such that its outer surface fits the peripheral surface of the elongated bore, and/or applying a sealing material such as glue, epoxy, grease and/or rubber or any other flexible sealing material.

The pressure tube advantageously has an outer diameter and a circumference selected such that it fits into the holes of the preform. The cross-sectional shape of the holes may be circular or oval or any other suitable shape. The surfaces defining the aperture are also referred to as the peripheral surfaces of the elongated aperture. The pressure tube preferably has an outer cross-sectional shape corresponding to the cross-sectional shape of the holes of the preform, however its average diameter is slightly smaller than the average diameter of the holes of the preform, so that the first end of the pressure tube can be inserted into the holes.

In one embodiment, the pressure tube, or at least a length of the pressure tube section of the pressure tube, has an average outer diameter that is about 80% up to 100% of the average inner diameter of the bore, such as about 90% to about 99% of the average diameter of the bore.

Advantageously, the pressure pipe has a supply section outside the bore, i.e. the part of the pressure pipe leading from the bore to the gas connection to the pressure supply source.

In one embodiment, the supply section of the pressure tube has a supply opening that is in gas connection with a pressure source for controlling the pressure in the bore.

In one embodiment, the supply section of the pressure tube has a supply opening located within the pressure regulating chamber. By adjusting the pressure in the pressure regulating chamber, for example by a pressure source, the pressure in the pressure tube is also regulated and thereby the pressure in the elongated bore.

In one embodiment, the supply section of the pressure tube has a supply opening that is directly connected to a pressure source to regulate the pressure within the pressure tube and thereby regulate the pressure within the elongated bore.

The pressure pipe can in principle be of any material. In one embodiment, the pressure tube is a thermoformable material, for example, a material that can be molded or drawn over a fiber draw tower. Advantageously, the pressure pipe is silica optionally comprising a polymer coating. In one embodiment, at least the supply section of the pressure tube has an external polymer coating, and optionally, the length section of the pressure tube is free of polymer coating. The polymer coating increases the flexibility of the pressure pipe and reduces the risk of rupture of the pressure pipe.

By providing a silica pressure tube with a pressure tube length section having an uncoated pressure tube length section and providing the pressure tube supply section with a polymer coating, the desired large hollow section cross-sectional diameter of the pressure tube can be achieved while at the same time the pressure tube supply section can be ideally flexible and rupture resistant.

As mentioned above, several holes of the preform may be pressure controlled with a pressure tube as described above. The pressure pipes may be connected to the same pressure source or to different pressure sources.

All the features of the invention described above and the embodiments of the invention including the ranges and preferred ranges may be combined in various ways within the scope of the invention, unless there is a specific reason for not combining these features.

Drawings

The above and/or additional objects, features and advantages of the present invention will be further clarified by the following illustrative and non-limiting detailed description of embodiments of the present invention with reference to the attached drawings.

FIG. 1 shows a cross-section of one embodiment of the hollow core PCF of the present invention, with one of the hollow tubes enlarged to show the inner diameter d1, outer diameter d2, and wall thickness t.

Fig. 2 shows a cross section of an embodiment of the hollow core PCF of the invention, wherein some of the hollow tubes have a larger wall thickness t than others.

Fig. 3 shows a cross section of one embodiment of the hollow core PCF of the invention with an elliptical hollow tube.

FIG. 4 shows a cross-section of one embodiment of the hollow core PCF of the present invention in which the hollow tube comprises nested sub-tubes disposed in the hollow structure of the hollow tube and fused to the hollow tube.

FIG. 5 shows a cross-section of one embodiment of the hollow core PCF of the present invention in which the hollow tube includes nodules disposed at the central core facing region.

Fig. 6 shows a cross section of one embodiment of the hollow core PCF of the invention, wherein single mode light has been fed to the hollow core PCF.

Fig. 7 is a graph showing several low-loss transmission bands of a hollow core PCF of one embodiment of the invention.

Fig. 8 is a graph showing a wide low-loss transmission band of the hollow core PCF of one embodiment of the present invention.

FIG. 9 is a graph showing HOM suppression for a prior art hollow core PCF having six hollow tubes.

FIG. 10 is a graph showing HOM suppression for a hollow core PCF of one embodiment of the present invention having seven hollow tubes.

Fig. 11 is a schematic diagram of a laser system and user equipment of one embodiment of the present invention.

FIG. 12 shows a first end of a preform for one embodiment of the hollow core PCF of the present invention having 7 hollow tubes forming 7 elongated holes, and wherein the first ends of the respective pressure tubes are inserted into the respective holes for pressure control of the holes during drawing.

Fig. 13a shows a hollow core PCF with an outer cladding region with a photonic band gap structure according to one embodiment of the invention.

Fig. 13b is a graph showing transmission loss of two variations of the hollow core PCF of the embodiment of fig. 13 a.

FIG. 14 is a graph showing the beam quality M of the optical fiber manufactured in example 1²A graph of the measurements of (a).

The figures are schematic and simplified for clarity. The same reference numerals are used throughout the same or corresponding parts.

Detailed Description

The hollow core PCF of the invention shown in fig. 1 comprises an outer cladding region 1 and seven hollow tubes 2 surrounded by said outer cladding region. Each of the hollow tubes 2 is fused to the outer cladding 1 at a fusion point 3 to form a ring defining an inner cladding region and a hollow core region 4 surrounded by the inner cladding region and having a core diameter D.

The hollow tubes do not contact each other and are generally referred to as non-contact hollow tubes. As shown in the enlarged view of the hollow tubes, each of the hollow tubes 2 has an average outer diameter d2 and an average inner diameter d1 and a wall thickness t in the central facing region of the respective hollow tube 2. The outer cladding 1 has an inner diameter ID and an outer diameter OD. In this embodiment, the hollow tubes 2 are identical and substantially circular in cross-section.

In the embodiment shown in fig. 2, the hollow core PCF comprises an outer cladding region 11 and seven non-contacting hollow tubes 12a, 12b surrounded by said outer cladding region, wherein the hollow tubes 12a, 12b are fused to the outer cladding 11 at fusion points 13 to form a ring defining an inner cladding region and a hollow core region 14 surrounded by the inner cladding region. Three of the non-contact hollow tubes 12a have a greater wall thickness t than the remaining four of the non-contact hollow tubes 12 b. As shown, each of the respective seven non-contact hollow tubes 12a, 12b is uniform in thickness, thereby providing simpler manufacture of the hollow core PCF.

In the embodiment shown in fig. 3, the hollow core PCF comprises an outer cladding region 21 and seven identical non-contacting hollow tubes 22 surrounded by said outer cladding region, and the hollow tubes 22 are fused to the outer cladding 21 to form a ring defining an inner cladding region and a hollow core region 24 surrounded by an inner cladding region. The hollow tubes 22 are oval and each of the hollow tubes 22 has a long inside diameter D_lAnd a long inner diameter D_lPerpendicular short internal diameter Ds, where D_lDetermined in the radial direction.

In the embodiment shown in fig. 4, the hollow core PCF comprises an outer cladding region 31 and seven identical non-contacting hollow tubes 32 surrounded by said outer cladding region, and the hollow tubes 32 are fused to the outer cladding 31 at fusion points 33 to form a ring defining an inner cladding region and a hollow core region 34 surrounded by the inner cladding region. The hollow tube 32 comprises nested sub-tubes 35, which nested sub-tubes 35 are arranged in the hollow structure of the hollow tube and are fused to the hollow tube at a respective maximum radial distance from the core central axis at the fusion point 33.

In the embodiment shown in fig. 5, the hollow core PCF comprises an outer cladding region 41 and seven non-contacting hollow tubes 42a, 42b surrounded by said outer cladding region, wherein the hollow tubes 42a, 42b are fused to the outer cladding 41 to form a ring defining an inner cladding region and a hollow core region 44 surrounded by an inner cladding region. Four of the non-contact hollow tubes 12a are uniform in their respective wall thicknesses, while the remaining three of the non-contact hollow tubes 42b include nodules 45 disposed at the core central facing region of the respective hollow tubes. Since the outer diameters d2 of the respective non-contact hollow tubes 42a, 42b are substantially the same, the nubs 45 are disposed at the boundaries of the hollow core regions.

In the embodiment shown in fig. 6, the hollow core PCF mainly has a structure as the hollow core PCF of fig. 1. A single-mode laser source, not shown, is arranged to irradiate the generation laser to the PCF at a wavelength in the PCF's low-loss transmission band. After 5 meters or even 10 meters, the beam sent in the PCF as indicated with reference 6 has gaussian beam quality and is completely monomodal. After 10 meters of transmission, the loss of the fundamental mode is very low, e.g. the transmission efficiency is over 85%, such as over 90%.

Fig. 7 is a graph showing several low-loss transmission bands and a core size D of about 30 μm and t 750nm for an optical fiber having the structure shown in fig. 1. The bands are numbered in increasing numbers from the longest wavelength band to the shorter wavelength band. As can be seen, the hollow core PCF has four low-loss transmission bands, wherein 3 of the low-loss transmission bands comprise wavelengths below 1.2 μm.

The graph of fig. 8 shows a closer view of the transmission loss of the hollow core PCF of fig. 1 with a transmission band II of D-30 μm and t-750 nm. It can be seen that the low loss transmission band is very wide, with a bandwidth of 200nm to 250nm around 1.064 μm.

A hollow core PCF having six hollow tubes (a prior art hollow core PCF) and a hollow core PCF having seven hollow tubes (an embodiment of the present invention) were simulated. The simulation was performed at 1032nm at D-30 μm and t-750 nm.

Fig. 9 and 10 show the HOM extinction ratios with respect to D/D (Dcore) of the prior art hollow core PCF (fig. 9) having six hollow tubes and the hollow core PCF (fig. 10) of the embodiment of the present invention having seven hollow tubes.

As can be seen from fig. 10, an optimal ratio D/D between 0.6 and 0.75 ensures that the Lp 11-type mode and several other HOMs of higher order are resonantly coupled to the cladding mode.

As can be seen in fig. 9, the use of six hollow tubes allows partial suppression of Lp 11-type modes. However, the modes with higher azimuthal numbers are still undisturbed, limiting the overall HOM extinction.

It is thus clearly shown that the inventive hollow core PCF with seven hollow tubes has significantly improved performance for HOM suppression over the prior art hollow core PCF with six hollow tubes.

A further comparison between a Hollow core PCF of one embodiment of the invention and a Hollow core PCF with six Hollow tubes is disclosed in the article "Hollow-core fibers for high power pulse delivery" by Michieletto et al, month 3 optical quick report 7103-. The contents of this article are incorporated by reference into the present disclosure provided that the contents of the subject matter explicitly disclosed herein prevail in the event of any inconsistency between the subject matter explicitly disclosed herein and the incorporated subject matter.

The laser system shown in fig. 11 comprises a laser light source 51 and a fiber optic transmission cable 52 for transmitting light from the laser light source 51 to a user device 54. The fiber optic delivery cable 52 includes a hollow core PCF as described above as its waveguide, with one or more low loss transmission bands associated with the user equipment. As noted, the fiber optic transmission cable 52 may be quite long while still transmitting single mode light to the subscriber equipment 54 in the fundamental mode with high efficiency and low loss. The fiber optic transmission cable 52 has a first end 53a and a second end 53 b. In the illustrated embodiment, each of the first end 53a and the second end 53b is mounted in a ferrule arrangement for connection to the user equipment 54 and the laser light source 5, respectively.

In an alternative, not shown embodiment, the second end of the fiber optic delivery cable 52 is spliced to the fiber output of the laser light source 51.

The preform shown in fig. 12 is a preform for one embodiment of a hollow core optical fiber as described herein.

The preform comprises a preform overcladding region 165 and 7 hollow preform tubes 161a, 161b, the 7 hollow preform tubes 161a, 161b being arranged in a non-contacting ring surrounded by the preform overcladding region 165 and fused to the preform overcladding region 165 (i.e. the tubes do not contact each other).

The pressure tube 164 is arranged to connect each of the three preform tubes 161a (main hollow tubes) to a pressure supply source, not shown, to control the pressure in the main hollow tube 161a during drawing. The length of pressure tube 164a inserted into the bore of each main hollow tube 161a is advantageously uncoated silica, while the remainder of the pressure tube 164 (referred to as the pressure tube supply section) is polymer coated silica. THE pressure in THE secondary hollow tube 161b can be advantageously controlled in a pressure chamber such as that shown in fig. 15 AND 16 of PA 201670262 DK entitled "annular element FOR PREFORM, PREFORM AND OPTICAL FIBER DRAWN FROM PREFORM (A RING ELEMENT FOR a PREFORM AND AN OPTICAL FIBER draw FROM PREFORM)".

In the embodiment shown in fig. 13a, the hollow core PCF comprises an outer cladding region 171 and seven non-contacting hollow tubes 172 forming the inner cladding region and surrounded by said outer cladding region 171. The hollow tube 172 is fused to the inside of the outer cladding 171 at fusion point 173 to form an inner cladding region and a hollow core region 174 surrounded by the inner cladding region. The outer cladding region includes a photonic band gap structure in the form of a microstructure (inclusion) 175a having a different refractive index than the cladding background material 175 b.

The Photonic Band Gap (PBG) structure may be provided in any way, for example by providing the outer cladding region with a refractive index grating comprising concentric rings having different refractive indices and/or by including the structure.

The inclusions 175a in the outer background material 175b are arranged in a cross-sectional pattern including about 5 rings.

As mentioned above, it is preferred that the inclusions are voids or gases and have a relatively small diameter and are arranged short in order to optimize (minimize) the confinement loss for the desired wavelength or wavelength range.

Fig. 13b shows the transmission loss of two variants of the hollow core PCF of embodiment fig. 13 a. Two variants of hollow core PCF with PBG structures in their respective cladding layers have been optimized to reduce confinement losses around the desired center wavelength (1064 nm (left) and 1550nm (right), respectively). As can be seen, the losses are very low. It has further been found that this method of reducing confinement loss can also be used in polarization maintaining anti-resonant fibers.

It has further been found that by increasing the overall thickness of the photonic band gap structure, confinement losses exhibit an arbitrarily reduced behavior.

Example 1

A hollow core PCF having the structure shown in fig. 1 is manufactured using stacking and primitive techniques. The manufactured optical fiber shown has a core diameter of about 30 μm, D2-17 μm, where D2/D-0.57. The mode field diameter measured at 1064nm was 22 μm. The tubes exhibit small dimensional differences but the fibers show significant low loss and bending loss, as well as good modal quality. Using camera-based M²The measurement system (Spiricon M2-2OOS) measures the modal quality factor of the fabricated fiber at a wavelength of 1064nm and a laser of 5M FUT. We performed the measurements: the fiber was wound on a standard 8cm spool and not spooled. The results are summarized in table I and shown in fig. 14. The fiber output beam exhibits negligible astigmatism and asymmetry and an M of 1.2²。

Table 1

Asymmetry of a wave	Light scattering	M2X	M2Y
				1.02	0.01	1.22	1.2