Flexible delivery of Er:YAG radiation at 2.94 µm with negative curvature silica glass fibers: a new solution for minimally invasive surgical procedures

Megawatt peak-power, single-mode, mid-infrared femtosecond pulse delivery at 5-6 μm via a silica-based anti-resonant hollow core fiber

We demonstrate the first, to our knowledge, delivery of megawatt peak power, single-mode mid-infrared (MIR) femtosecond pulses at 5-6 μm using a silica-based anti-resonant hollow core fiber (AR-HCF). Benefiting from the light confinement inside the hollow core, the AR-HCF exhibits high damage thresholds, reliable power stability, efficient spatial beam self-cleaning, and pulse shape preservation. Pumped by a homemade LGS-based two-stage optical parametric amplifier generating high-power ∼200 fs pulses, the fiber achieves a maximum delivered peak power of 4 MW at 5.1 μm and 5 MW at 6.1 μm, with peak intensities reaching 100 GW/cm2, despite fiber losses exceeding 2 dB/m. This flexible, meter-scale delivery system demonstrates exceptional potential for addressing the challenges of high peak power MIR laser delivery in precise, minimally invasive interventional ablation, particularly at resonant peaks such as amide-I (6.1 μm) and cholesterol esters (5.75 μm).

3.1 W mid-infrared fiber laser at 4.16 µm based on HBr-filled hollow-core silica fibers

We present the characteristics of a continuous-wave (CW) mid-infrared fiber laser source based on HBr-filled hollow-core fibers (HCFs) made of silica. The laser source delivers a maximum output power of 3.1 W at 4.16 µm, showing a record value for any reported fiber laser beyond 4 µm. Both ends of the HCF are supported and sealed by especially designed gas cells with water cooling and inclined optical windows, withstanding higher pump power accompanied by accumulated heat. The mid-infrared laser exhibits a near-diffraction-limited beam quality with a measured M² of 1.16. This work paves the way for powerful mid-infrared fiber lasers beyond 4 µm.

Hundred-meter-scale, kilowatt peak-power, near-diffraction-limited, mid-infrared pulse delivery via the low-loss hollow-core fiber

We report a high-power single-mode mid-infrared (MIR) pulse delivery system via anti-resonant hollow-core fiber (HCF) with a record delivery distance of 108 m. Near-diffraction-limited MIR light was transmitted by HCFs at wavelengths of 3.12-3.58 µm using a tunable optical parametric oscillator (OPO) as the light source. The HCFs were purged beforehand with argon in order to remove or reduce loss due to parasitic gas absorption (HCl, CO₂, etc.). The minimum fiber loss values were 0.05 and 0.24 dB/m at 3.4-3.6 µm and 4.5-4.6 µm, respectively, with the 4.5-4.6 µm loss figure representing, to the best of our knowledge, a new low loss record for a HCF in this spectral region. At a coupling efficiency of ∼70%, average powers of 592 mW and 133 mW were delivered through 5 m and 108 m of HCF, respectively. Assuming the 120-ps duration of the MIR pulses remained constant over the low-dispersion HCF (theoretical maximum: 0.4 ps/nm/km), the corresponding calculated peak powers were 4.9 kW and 1.1 kW.

High-energy, mid-IR, picosecond fiber-feedback optical parametric oscillator

A compact, mid-infrared (MIR), synchronously pumped, fiber-feedback optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN) is developed with tunable signal and idler wavelength ranges of 1472.0-1758.2 nm and 2559.1-3562.7 nm, respectively. A solid-core SMF-28 fiber and a hollow-core fiber (HCF) were used as the feedback fibers in order to compare the effect of their substantially different levels of nonlinearity. The OPO generates 1-MHz, 120-ps, MIR pulses with up to 1.50-µJ pulse energy and 11.7-kW peak power.

A Review of Antiresonant Hollow-Core Fiber-Assisted Spectroscopy of Gases

Antiresonant Hollow-Core Fibers (ARHCFs), thanks to the excellent capability of guiding light in an air core with low loss over a very broad spectral range, have attracted significant attention of researchers worldwide who especially focus their work on laser-based spectroscopy of gaseous substances. It was shown that the ARHCFs can be used as low-volume, non-complex, and versatile gas absorption cells forming the sensing path length in the sensor, thus serving as a promising alternative to commonly used bulk optics-based configurations. The ARHCF-aided sensors proved to deliver high sensitivity and long-term stability, which justifies their suitability for this particular application. In this review, the recent progress in laser-based gas sensors aided with ARHCFs combined with various laser-based spectroscopy techniques is discussed and summarized.

Low-loss single-mode modified conjoined tube hollow-core fiber

We explain the effects of cladding geometries on conjoined tube hollow-core negative curvature fibers and offer a modified conjoined tube negative curvature fiber with appropriate positioning of an additional negative curvature D-shaped layer joining the flat bar to reveal attractive performances over existing recent related fibers. The proposed fiber ensures the least loss of 0.003 dB/km at 1.43 µm, a ∼0.04dB/km loss covering the wide bandwidth of approximately 300 nm, the lowest surface scattering loss of ∼0.02dB/km, and the lowest microbending loss of ∼0.04dB/km, thus providing a propagation loss of 0.10 dB/km at the 1.55 µm wavelength and also offering excellent bend loss performance (∼0.015dB/km loss at a 7 cm bend radius). The fiber, with a core diameter of 30.50 µm, also shows a higher-order mode extinction ratio of ∼1600 and maintains greater than 100 over most of the telecom bands; hence, it effectively provides single-mode operation. We show the potential of conjoined tube hollow-core negative curvature fibers in optical communications systems.

Wideband low confinement loss anti-resonant hollow core fiber with nested U-shape tube

We propose a nested U-shape tube anti-resonant hollow core fiber (UARF) that can effectively reduce the confinement loss (CL) as well as the loss oscillations. The key parameters of UARF have been optimized via numerical analysis. Simulation results show that the CL of proposed UARF is lower than 0.01dB/km over a 550 nm operational bandwidth range from 1.3 µm to 1.85 µm. This CL is nearly one order of magnitude lower than the nested anti-resonant nodeless fiber (NANF). Moreover, the loss ratio between higher-order modes to the fundamental mode is verified to be more than 100,000 over a ultrawide bandwidth of 1000 nm, which indicates its excellent single mode performance. The tolerance towards the structure deformation of UARF has been evaluated for the purpose of practical fiber fabrication. Thus, the proposed UARF has potential application in large capacity data transmission, nonlinear optics, gas sensing and so on.

Impact of cladding elements on the loss performance of hollow-core anti-resonant fibers

Understanding the impact of the cladding tube structure on the overall guiding performance is crucial for designing a single-mode, wide-band, and ultra low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly investigate on how the propagation loss is affected by the nested elements when their geometry is realistic (i.e., non-ideal). Interestingly, it was found that the size, rather than the shape, of the nested elements has a dominant role in the final loss performance of the regular nested HC-ARFs. We identify a unique 'V-shape' pattern for suppression of higher-order modes loss by optimizing free design parameters of the HC-ARF. We find that a 5-tube nested HC-ARF has wider transmission window and better single-mode operation than a 6-tube HC-ARF. We show that the propagation loss can be significantly improved by using anisotropic nested anti-resonant tubes elongated in the radial direction. Our simulations indicate that with this novel fiber design, a propagation loss as low as 0.11 dB/km at 1.55 μm can be achieved. Our results provide design insight toward fully exploiting a single-mode, wide-band, and ultra low-loss HC-ARF. In addition, the extraordinary optical properties of the proposed fiber can be beneficial for several applications such as future optical communication system, high energy light transport, extreme non-nonlinear optics and beyond.

Femtosecond laser machining of hollow-core negative curvature fibres

Hollow core negative curvature fibres (NCFs) are a relatively new class of microstructured optical fibre with potential applications in areas such as the delivery of high power laser light and gas sensing. For sensing, it is necessary for the measurand to interact with the guided mode. To facilitate this, a novel femtosecond laser-based machining protocol has been developed that allows the precision sculpting of access slots into the NCF core along the length of the fibre. The process is a direct-write process using a digitally defined scanning strategy with no need for physical masks or additional processing such as wet etchants and/or focussed ion beam machining. Due to the inherent flexibility of the machining strategy and the high level of control over the depth of material removal, it is likely that this new technique will be transferable to a wide range of microstructured fibres.

Mid-infrared transmission by a tellurite hollow core optical fiber

We experimentally demonstrate for the first time a successful fabrication of a tellurite hollow core optical fiber which has a mid-infrared transmission range. The wall thickness of each cladding air-hole is about 2.8 µm and the outer diameter of the full air-hole structure D is approximately 110 µm. The results show that the measured transmission spectrum can expand up to 3.9 µm. In addition, it is expected that the transmission can extend to around 6 µm. When the input light is linearly polarized, it can be maintained after propagating through a 17-cm-long fiber.

Second-Order Vector Mode Propagation in Hollow-Core Antiresonant Fibers

Second-order vector modes, possessing doughnut-shaped intensity distribution with unique polarization, are widely utilized in material micromachining, optical tweezers, and high-resolution microscopy. Since the hollow-core fiber can act as a flexible and robust optical waveguide for ultra-short pulse delivery and manipulation, high-order vector modes guided in hollow-core fibers will have huge potential in many advanced applications. We firstly reveal that a second-order vector mode can be well guided in a hollow-core antiresonant fiber with the suppression of the fundamental mode and other second-order vector modes at the red side of transmission band. We interpret our observation through a phase-matched coupling mechanism between core modes and coupled cladding modes. A single second-order vector mode such as TE₀₁, TM₀₁, or HE₂₁ can be guided with low confinement loss at specific wavelengths with appropriate structure parameters. Our proposed hollow-core fibers have a modal engineering function which will open up a new avenue toward the single second-order vector mode propagation and its fiberized applications.

Multioctave supercontinuum from visible to mid-infrared and bend effects on ultrafast nonlinear dynamics in gas-filled hollow-core fiber

Broadband supercontinuum generation is numerically investigated in a Xe-filled nested hollow-core antiresonant (HC-AR) fiber pumped at 3 μm with pulses of 100 fs duration and 15 μJ energy. For a 25 cm long fiber, under 7 bar pressure, the supercontinuum spectrum spans multiple octaves from 400 nm to 5000 nm. Furthermore, the influence of bending on ultrafast nonlinear pulse propagation dynamics is investigated for two types of HC-AR fibers (nested and non-nested capillaries). Our results predict similar nonlinear dynamics for both fiber types and a significant reduction of the spectral broadening under tight bending conditions.

Approximate model for analyzing band structures of single-ring hollow-core anti-resonant fibers

Precise knowledge of modal behavior is of essential importance for understanding light guidance, particularly in hollow-core fibers. Here we present a semi-analytical model that allows determination of bands formed in revolver-type anti-resonant hollow-core fibers. The approach is independent of the actual arrangement of the anti-resonant elements, does not enforce artificial lattice arrangements and allows determination of the effective indices of modes of preselected order. The simulations show two classes of modes: (i) low-order modes exhibiting effective indices with moderate slopes and (ii) a high number of high-order modes with very strong effective index dispersion, forming a quasi-continuum of modes. It is shown that the mode density scales with the square of the normalized frequency, being to some extent similar to the behavior of multimode fibers.

Single-mode, low loss hollow-core anti-resonant fiber designs

In this paper, we numerically investigate various hollow-core anti-resonant (HC-AR) fibers towards low propagation and bend loss with effectively single-mode operation in the telecommunications window. We demonstrate how the propagation loss and higher-order mode modal contents are strongly influenced by the geometrical structure and the number of the anti-resonant cladding tubes. We found that 5-tube nested HC-AR fiber has a wider anti-resonant band, lower loss, and larger higher-order mode extinction ratio than designs with 6 or more anti-resonant tubes. A loss ratio between the higher-order modes and fundamental mode, as high as 12,000, is obtained in a 5-tube nested HC-AR fiber. To the best of our knowledge, this is the largest higher-order mode extinction ratio demonstrated in a hollow-core fiber at 1.55 μm. In addition, we propose a modified 5-tube nested HC-AR fiber, with propagation loss below 1 dB/km from 1330 to 1660 nm. This fiber also has a small bend loss of ~15 dB/km for a bend radius of 1 cm.

Multi-stage generation of extreme ultraviolet dispersive waves by tapering gas-filled hollow-core anti-resonant fibers

In this work, we numerically investigate an experimentally feasible design of a tapered Ne-filled hollow-core anti-resonant fiber and we report multi-stage generation of dispersive waves (DWs) in the range 90-120 nm, well into the extreme ultraviolet (UV) region. The simulations assume a 800 nm pump pulse with 30 fs 10 µJ pulse energy, launched into a 9 bar Ne-filled fiber with a 34 µm initial core diameter that is then tapered to a 10 µm core diameter. The simulations were performed using a new model that provides a realistic description of both loss and dispersion of the resonant and anti-resonant spectral bands of the fiber, and also importantly includes the material loss of silica in the UV. We show that by first generating solitons that emit DWs in the far-UV region in the pre-taper section, optimization of the following taper structure can allow re-collision with the solitons and further up-conversion of the far-UV DWs to the extreme-UV with energies up to 190 nJ in the 90-120 nm range. This process provides a new way to generate light in the extreme-UV spectral range using relatively low gas pressure.

Deciphering the Microstructure and Energy-Level Splitting of Tm3+-Doped Yttrium Aluminum Garnet

Thulium-doped yttrium aluminum garnet (Tm:YAG) is an important solid-state laser crystal. The energy-level splitting within it is still an unresolved problem. Here, we perform a theoretical study on the microstructure of Tm³⁺-doped YAG using the CALYPSO structure search method in conjunction with first-principles calculations. The calculated results show that the 4.16% doping concentration of Tm³⁺ impurity causes an obvious structural distortion of YAG crystal, forming an orthorhombic phase in C₂₂₂ symmetry. On the basis of our developed WEPMD method, we obtain a new and complete set of free-ion and crystal field parameters by a good fit (with proper irreducible representations) to 69 observed energy levels and determine the exact energy-level splitting of Tm³⁺ in YAG. The calculated Stark levels and electric dipole transitions are in excellent agreement with the measured data and similar theoretical calculations. Some promising emission lines between ³F₃, ³F₂, ¹D₂, and ¹I₆ states are presented. These findings offer fundamental insights and practical tools for further exploration of the structural and electronic properties of other transition-metal-doped YAG crystal.

Flexible single-mode delivery of a high-power 2 μm pulsed laser using an antiresonant hollow-core fiber

We demonstrate flexible single-mode transmission of a high average power 2 µm nanosecond pulse using antiresonant hollow-core fibers (AR-HCFs). 39.1 W average power is delivered using a coiled 1.7 m AR-HCF, which is designed for single-mode guidance and good higher-order mode suppression. The effect of bending on the fiber output power and beam profile is also investigated. The Gaussian-like output beam profile is maintained up to a 7.5 cm bending radius. This is the highest average power delivered by a flexible long HCF in this wavelength without the need for an enclosed controlled environment, to the best of our knowledge.

Mid-infrared 1 W hollow-core fiber gas laser source

We report the characteristics of a 1 W hollow-core fiber gas laser emitting CW in the mid-IR. Our system is based on an acetylene-filled hollow-core optical fiber guiding with low losses at both the pump and laser wavelengths and operating in the single-pass amplified spontaneous emission regime. Through systematic characterization of the pump absorption and output power dependence on gas pressure, fiber length, and pump intensity, we determine that the reduction of pump absorption at high pump flux and the degradation of gain performance at high gas pressure necessitate the use of increased gain fiber length for efficient lasing at higher powers. Low fiber attenuation is therefore key to efficient high-power laser operation. We demonstrate 1.1 W output power at a 3.1 μm wavelength by using a high-power erbium-doped fiber amplifier pump in a single-pass configuration, approximately 400 times higher CW output power than in the ring cavity previously reported.

Function of second cladding layer in hollow core tube lattice fibers

Modes attenuation of the tube lattice fiber (TLF) is characterized by D/λ, where D is the core diameter and λ is the wavelength. Hence, the TLF is structured with a large core to ensure a low attenuation loss. A small core, on the other hand, facilitates the gas-filled TLF applications, but at the expense of the increased mode attenuation. We show that adding a second cladding layer to the conventional one layer TLF (1TLF) can resolve the contradicting requirements. The mode attenuation of TLF with two cladding layers (2TLF) is less influenced by the D/λ value as compared to 1TLF, thus realizing a low loss small core TLF. Furthermore, we found that adding the second layer brings another advantage to a bending performance. With a determined core size, D, a 1TLF with smaller capillary hole size, d, experiences less bending loss. However, the reduced d increases the confinement loss that counteracts the bending loss improvement. This confliction is substantially alleviated in 2TLF thanks to the second cladding layer. Theoretical investigations and experimental demonstrations are presented to evidence the important role of the second cladding ring in the TLF, which has been overlooked in prior studies.

Mid-infrared supercontinuum generation in supercritical xenon-filled hollow-core negative curvature fibers

We present an investigation on the generation of supercontinuum in the mid-infrared (mid-IR) spectral region. Namely, we study a silica-based anti-resonant hollow-core fiber which has good guidance properties in the mid-IR filled with supercritical xenon providing the necessary high nonlinearity. Our numerical study shows that by launching a 200 nJ pump of 100 fs centered at 3.70 μm, a supercontinuum that spans from 1.85 to 5.20 μm can be generated. Such sources are potentially useful for applications, such as the remote sensing of various molecules, medical imaging diagnosis, and surgery.

Bending loss characterization in nodeless hollow-core anti-resonant fiber

We report high performance nodeless hollow-core anti-resonant fibers (HARFs) with broadband guidance from 850 nm to >1700 nm and transmission attenuation of ~100 dB/km. We systematically investigate their bending loss behaviors using both theoretical and experimental approaches. While a low bending loss value of 0.2 dB/m at 5 cm bending radius is attained in the long wavelength side (LWS) of the spectrum, in this paper, we pursue light guidance in the short wavelength side (SWS) under tight bending, which is yet to be explored. We analytically predict and experimentally verify a sub transmission band in the SWS with a broad bandwidth of 110 THz and an acceptable loss of 4.5 dB/m at 2 cm bending radius, indicating that light can be simultaneously guided in LWS and SWS even under tight bending condition. This provides an unprecedented degree of freedom to tailor the transmission spectrum under a tight bending state and opens new opportunities for HARFs to march into practical applications where broadband guidance under small bending radius is a prerequisite.

Bending-induced mode non-degeneracy and coupling in chalcogenide negative curvature fibers

We study bend loss in chalcogenide negative curvature fibers with different polarizations, different tube wall thicknesses, and different bend directions relative to the mode polarization. The coupling between the core mode and tube modes induces bend loss peaks in the two non-degenerate modes at the same bend radius. There is as much as a factor of 28 difference between the losses of the two polarization modes. The fiber with a larger tube wall thickness, corresponding to a smaller inner tube diameter, can sustain a smaller bend radius. The bend loss is sensitive to the bend direction when coupling occurs between the core mode and tube modes. A bend loss of 0.2 dB/m at a bend radius of 16 cm, corresponding to 0.2 dB/turn, can be achieved in a chalcogenide negative curvature fiber.

Empirical formulas for calculating loss in hollow core tube lattice fibers

In this paper scaling laws governing loss in hollow core tube lattice fibers are numerically investigated and discussed. Moreover, by starting from the analysis of the obtained numerical results, empirical formulas for the estimation of the minimum values of confinement loss, absorption loss, and surface scattering loss inside the transmission band are obtained. The proposed formulas show a good accuracy for fibers designed for applications ranging from THz to ultra violet band.

Low-loss single-mode hollow-core fiber with anisotropic anti-resonant elements

A hollow-core fiber using anisotropic anti-resonant tubes in the cladding is proposed for low loss and effectively single-mode guidance. We show that the loss performance and higher-order mode suppression is significantly improved by using symmetrically distributed anisotropic anti-resonant tubes in the cladding, elongated in the radial direction, when compared to using isotropic, i.e. circular, anti-resonant tubes. The effective single-mode guidance of the proposed fiber is achieved by enhancing the coupling between the cladding modes and higher-order-core modes by suitably engineering the anisotropic anti-resonant elements. With a silica-based fiber design aimed at 1.06 µm, we show that the loss extinction ratio between the higher-order core modes and the fundamental core mode can be more than 1000 in the range 1.0-1.65 µm, while the leakage loss of the fundamental core mode is below 15 dB/km in the same range.

Femtosecond pulsed laser ablation to enhance drug delivery across the skin

Laser poration of the skin locally removes its outermost, barrier layer, and thereby provides a route for the diffusion of topically applied drugs. Ideally, no thermal damage would surround the pores created in the skin, as tissue coagulation would be expected to limit drug diffusion. Here, a femtosecond pulsed fiber laser is used to porate mammalian skin ex vivo. This first application of a hollow core negative curvature fiber (HC-NCF) to convey a femtosecond pulsed, visible laser beam results in reproducible skin poration. The effect of applying ink to the skin surface, prior to ultra-short pulsed ablation, has been examined and Raman spectroscopy reveals that the least, collateral thermal damage occurs in inked skin. Pre-application of ink reduces the laser power threshold for poration, an effect attributed to the initiation of plasma formation by thermionic electron emission from the dye in the ink. Poration under these conditions significantly increases the percutaneous permeation of caffeine in vitro. Dye-enhanced, plasma-mediated ablation of the skin is therefore a potentially advantageous approach to enhance topical/transdermal drug absorption. The combination of a fiber laser and a HC-NCF, capable of emitting and delivering femtosecond pulsed, visible light, may permit a compact poration device to be developed.

Low-loss hollow-core silica fibers with adjacent nested anti-resonant tubes

We report on numerical design optimization of hollow-core anti-resonant fibers with the aim of reducing transmission losses. We show that re-arranging the nested anti-resonant tubes in the cladding to be adjacent has the effect of significantly reducing leakage as well as bending losses, and for reaching high loss extinction ratios between the fundamental mode and higher order modes. We investigate two versions of the proposed design, one optimized for the mid-IR and another scaled down version for the near-IR and compare them in detail with previously proposed anti-resonant fiber designs including nested elements. Our proposed design is superior with respect to obtaining the lowest leakage losses and the bend losses are also much lower than for the previous designs. Leakage losses as low as 0.0015 dB/km and bending losses of 0.006 dB/km at 5 cm bending radius are predicted at the ytterbium lasing wavelength 1.06 µm. When optimizing the higher-order-mode extinction ratio, the low leakage loss is sacrificed to get an effective single-mode behavior of the fiber. We show that the higher-order-mode extinction ratio is more than 1500 in the range 1.0-1.1 µm around the ytterbium lasing wavelength, while in the mid-IR it can be over 100 around λ = 2.94 μm. This is higher than the previously considered structures in the literature using nested tubes.

High energy green nanosecond and picosecond pulse delivery through a negative curvature fiber for precision micro-machining

In this paper we present an anti-resonant guiding, low-loss Negative Curvature Fiber (NCF) for the efficient delivery of high energy short (ns) and ultrashort (ps) pulsed laser light in the green spectral region. The fabricated NCF has an attenuation of 0.15 dB/m and 0.18 dB/m at 532 nm and 515 nm respectively, and provided robust transmission of nanosecond and picosecond pulses with energies of 0.57 mJ (10.4 kW peak power) and 30 µJ (5 MW peak power) respectively. It provides single-mode, stable (low bend-sensitivity) output and maintains spectral and temporal properties of the source laser beam. The practical application of fiber-delivered pulses has been demonstrated in precision micro-machining and marking of metals and glass.

Preclinical investigations of articular cartilage ablation with femtosecond and pulsed infrared lasers as an alternative to microfracture surgery

Microfracture surgery is a bone marrow stimulation technique for treating cartilage defects and injuriesin the knee. Current methods rely on surgical skill and instrumentation. This study investigates the potential useof laser technology as an alternate means to create the microfracture holes. Lasers investigated in this study include an erbium:YAG laser (λ = 2.94 μm), titanium:sapphire femtosecond laser system (λ = 1700 nm), and Nd:glass femtosecond laser (λ = 1053 nm). Bovine samples were ablated at fluences of 8 to 18 J∕cm2 with the erbium:YAG laser, at a power of 300 ± 15 mW with the titanium:sapphire femtosecond system, and at an energy of 3 μJ∕pulse with the Nd:glass laser. Samples were digitally photographed and histological sections were taken for analysis. The erbium:YAG laser is capable of fast and efficient ablation; specimen treated with fluences of 12 and 18 J∕cm2 experienced significant amounts of bone removal and minimal carbonization with saline hydration. The femtosecond laser systems successfully removed cartilage but not clinically significant amounts of bone. Precise tissue removal was possible but not to substantial depths due to limitations of the systems. With additional studies and development, the use of femtosecond laser systems to ablate bone may be achieved at clinically valuable ablation rates.

Double antiresonant hollow core fiber--guidance in the deep ultraviolet by modified tunneling leaky modes

Guiding light inside the hollow cores of microstructured optical fibers is a major research field within fiber optics. However, most of current fibers reveal limited spectral operation ranges between the mid-visible and the infrared and rely on complicated microstructures. Here we report on a new type of hollow-core fiber, showing for the first time distinct transmission windows between the deep ultraviolet and the near infrared. The fiber, guiding in a single mode, operates by the central core mode being anti-resonant to adjacent modes, leading to a novel modified tunneling leaky mode. The fiber design is straightforward to implement and reveals beneficial features such as preselecting the lowest loss mode (Gaussian-like or donut-shaped mode). Fibers with such a unique combination of attributes allow accessing the extremely important deep-UV range with Gaussian-like mode quality and may pave the way for new discoveries in biophotonics, multispectral spectroscopy, photo-initiated chemistry or ultrashort pulse delivery.

Hollow antiresonant fibers with low bending loss

We first use numerical simulations to show that bending losses of hollow antiresonant fibers are a strong function of their geometrical structure. We then demonstrate this by fabricating a hollow antiresonant fiber which presents a bending loss as low as 0.25 dB/turn at a wavelength of 3.35 μm and a bend radius of 2.5 cm. This fiber has a relatively low attenuation (<200 dB/km) over 600 nm mid-infrared spectral range.

Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications

We present high average power picosecond and nanosecond pulse delivery at 1030 nm and 1064 nm wavelengths respectively through a novel hollow-core Negative Curvature Fiber (NCF) for high-precision micro-machining applications. Picosecond pulses with an average power above 36 W and energies of 92 µJ, corresponding to a peak power density of 1.5 TWcm⁻² have been transmitted through the fiber without introducing any damage to the input and output fiber end-faces. High-energy nanosecond pulses (>1 mJ), which are ideal for micro-machining have been successfully delivered through the NCF with a coupling efficiency of 92%. Picosecond and nanosecond pulse delivery have been demonstrated in fiber-based laser micro-machining of fused silica, aluminum and titanium.

Effect of core boundary curvature on the confinement losses of hollow antiresonant fibers

We use numerical simulations to investigate how the curvature of the fiber core boundary influences the attenuation of hollow antiresonant fibers. We show the importance of a "negative" curvature core boundary in reducing confinement losses and also how, for certain curvatures, optical power is coupled resonantly to cladding modes. We simulate bending losses and find results in agreement with previously-reported experiments.

Spectral attenuation limits of silica hollow core negative curvature fiber

In this paper we discuss the limits of attenuation of silica hollow core negative curvature fibers in the wavelength range from 800 nm up to 4.5 µm. Both numerical and experimental results are presented and show good agreement. A minimum attenuation of 24.4 dB/km was measured at around 2400 nm wavelength, while 85 dB/km was measured at 4000 nm.

Light transmission in negative curvature hollow core fiber in extremely high material loss region

In this paper we demonstrate the light transmission in a spectral range of 2.5 to 7.9 µm through a silica negative curvature hollow core fiber (NCHCF) with a cladding consisting of eight capillaries. A separation between the cladding capillaries was introduced to remove the additional resonances in the transmission bands. The measured optical loss at 3.39 µm was about 50 dB/km under a few modes waveguide regime.