Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre.

Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 × 106 photon s-1 eV-1 around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

Observation on temperature and strain dependency of Brillouin dynamic grating in a few-mode fiber with a ring-cavity configuration.

We experimentally conduct Brillouin dynamic grating (BDG) operation using a 1-km-long four-mode fiber. By employing a simplified ring-cavity configuration with single-end pumping, the BDG is effectively generated in $ {{\rm LP}_{01}} $LP01 mode within a range of 250 m, and three higher-order modes, namely, $ {{\rm LP}_{11b}} $LP11b, $ {{\rm LP}_{21a}} $LP21a, and $ {{\rm LP}_{02}} $LP02, are chosen as probes to analyze the BDG with a spatial resolution of 1 m. To the best of our knowledge, this is the first time to characterize the responses of BDG frequency to temperature and strain for different modes in a conventional few-mode fiber. By employing the pump-probe pair of $ {{\rm LP}_{01}}{{\rm - LP}_{02}} $LP01-LP02 mode, the highest temperature and strain sensitivities of 3.21 MHz/°C and $ - 0.0384\;{\rm MHz}/{\unicode{x00B5}}{\unicode{x03B5}} $-0.0384MHz/µÎµ have been achieved. Also, the performance of simultaneously distributed temperature and strain sensing based on BDG is evaluated.

Mode-selective few-mode Brillouin fiber lasers based on intramodal and intermodal SBS.

Mode-selective fiber lasers have advantages in a number of applications. Here we propose and experimentally demonstrate a transverse mode-selective few-mode Brillouin fiber laser using the mode-selective photonic lantern. We generated the lowest three orders of linearly polarized (LP) modes based on both intramodal and intermodal stimulated Brillouin scattering (SBS). Their slope efficiencies, optical spectra, mode profiles, and linewidths were measured.

Control over the transverse structure and long-distance fiber propagation of light at the single-photon level.

Quantum entanglement is arguably the cornerstone which differentiates the quantum realm from its classical counterpart. While entanglement can reside in any photonic degree of freedom, polarization permits perhaps the most straightforward manipulation due to the widespread availability of standard optical elements such as waveplates and polarizers. As a step towards a fuller exploitation of entanglement in other degrees of freedom, in this work we demonstrate control over the transverse spatial structure of light at the single-photon level. In particular we integrate in our setup all the technologies required for: (i) fibre-based photon pair generation, (ii) deterministic and broadband single-photon spatial conversion relying on a passive optical device, and (iii) single-photon transmission, while retaining transverse structure, over 400 m of few-mode fibre. In our experiment, we employ a mode selective photonic lantern multiplexer with the help of which we can convert the transverse profile of a single photon from the fundamental mode into any of the supported higher-order modes. We also achieve conversion to an incoherent or coherent addition of two user-selected higher order modes by addressing different combinations of inputs in the photonic lantern multiplexer. The coherent nature of the addition, and extraction of usable orbital angular momentum at the single-photon level, is further demonstrated by far-field diffraction through a triangular aperture. Our work could enable studies of photonic entanglement in the transverse modes of a fibre and could constitute a key resource quantum for key distribution with an alphabet of scalable dimension.

Accelerated nonlinear interactions in graded-index multimode fibers.

Multimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. As shown in recent studies, the complex nature of such heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we study for the first time, accelerated nonlinear intermodal interactions in core-diameter decreasing multimode fibers. We demonstrate that in the anomalous dispersion region, this spatiotemporal acceleration can lead to relatively blue-shifted multimode solitons and blue-drifting dispersive wave combs, while in the normal domain, to a notably flat and uniform supercontinuum, extending over 2.5 octaves. Our results pave the way towards a deeper understanding of the physics and complexity of nonlinear, heavily multimoded optical systems, and could lead to highly tunable optical sources with very high spectral densities.

Nonlinear pulse compression to 43 W GW-class few-cycle pulses at 2 µm wavelength.

High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 µJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 µm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.

Instant and efficient second-harmonic generation and downconversion in unprepared graded-index multimode fibers.

We show that germanium-doped graded-index multimode silica fibers can exhibit relatively high conversion efficiencies (∼6.5%) for second-harmonic generation when excited at 1064 nm. This frequency-doubling behavior is also found to be accompanied by an effective downconversion. As opposed to previous experiments carried out in single- and few-mode fibers where hours of preparation were required, in our system, these χ(2) related processes occur almost instantaneously. The efficiencies observed in our experiments are, to the best of our knowledge, among the highest ever reported in unprepared fibers.

Reduced-symmetry LMA rod-type fiber for enhanced higher-order mode delocalization.

We present a novel design of a micro-structured large-pitch, large-mode-area (LMA) asymmetric rod-type fiber. By reducing the cladding symmetry through six high-refractive index germanium-doped silica inclusions, the fiber features strong higher-order mode (HOM) delocalization, leading to a potentially enhanced preferential gain for the fundamental mode in active fibers. In addition, high resolution spatially and spectrally (S2) resolved mode analysis measurements confirm HOM contributions below 1% and LP1m-like HOM contributions below the detection limit. This proposed fiber design enables single-mode operation, with near-diffraction-limited beam quality of M2=1.3 and an effective mode area of 2560 µm2 at 1064 nm. This design opens new insights into improving the threshold-like onset of modal instabilities in high-power fiber lasers and fiber amplifiers by efficiently suppressing LP11 modes.

Tailoring frequency generation in uniform and concatenated multimode fibers.

We demonstrate that frequency generation in multimode parabolic-index fibers can be precisely engineered through appropriate fiber design. This is accomplished by exploiting the onset of a geometric parametric instability that arises from resonant spatiotemporal compression. By launching the output of an amplified Q-switched microchip laser delivering 400 ps pulses at 1064 nm, we observe a series of intense frequency sidebands that strongly depend on the fiber core size. The nonlinear frequency generation is analyzed in three fiber samples with 50 µm, 60 µm, and 80 µm core diameters. We further demonstrate that by cascading fibers of different core sizes, a desired frequency band can be generated from the frequency lines parametrically produced in each section. The observed frequency shifts are in good agreement with analytical predictions and numerical simulations. Our results suggest that core scaling and fiber concatenation can provide a viable avenue in designing optical sources with tailored output frequencies.

Visible supercontinuum generation in a graded index multimode fiber pumped at 1064 nm.

We observe efficient supercontinuum generation that extends into the visible spectral range by pumping a low differential mode group delay graded index multimode fiber in the normal dispersion regime. For a 28.5 m long fiber, the generated spectrum spans more than two octaves, starting from below 450 nm and extending beyond 2400 nm. The main nonlinear mechanisms contributing to the visible spectrum generation are attributed to multipath four-wave mixing processes and periodic spatio-temporal breathing dynamics. Moreover, by exploiting the highly multimodal nature of this system, we demonstrate versatile generation of visible spectral peaks in shorter fiber spans by altering the launching conditions. A nonlinearly induced mode cleanup was also observed at the pump wavelength. Our results could pave the way for high brightness, high power, and compact, multi-octave continuum sources.

Few-mode erbium-doped fiber amplifier with photonic lantern for pump spatial mode control.

We demonstrate a few-mode erbium-doped fiber amplifier employing a mode-selective photonic lantern for controlling the modal content of the pump light. Amplification of six spatial modes in a 5 m long erbium-doped fiber to ∼6.2 dBm average power is obtained while maintaining high modal fidelity. Through mode-selective forward pumping of the two degenerate LP21 modes operating at 976 nm, differential modal gains of <1 dB between all modes and signal gains of ∼16 dB at 1550 nm are achieved. In addition, low differential modal gain for near-full C-band operation is demonstrated.

Mode-selective amplification in a large mode area Yb-doped fiber using a photonic lantern.

We demonstrate selective spatial mode amplification in a few mode, double-clad Yb-doped large mode area (LMA) fiber, utilizing an all-fiber photonic lantern. Amplification to multi-watt output power is achieved while preserving high spatial mode selectivity. We observe gain values of over 12 dB for all modes: LP01, LP11a, and LP11b, when amplified individually. Additionally, we investigate the simultaneous amplification of LP01+LP11a and LP11a+LP11b, and the resultant mode competition. The proposed architecture allows for the reconfigurable excitation of spatial modes in the LMA fiber amplifiers, and represents a promising method that could enable dynamic spatial mode control in high power fiber lasers.

Compact fiber-optic curvature sensor based on super-mode interference in a seven-core fiber.

A compact, low loss, and highly sensitive optical fiber curvature sensor is presented. The device consists of a few-millimeter-long piece of seven-core fiber spliced between two single-mode fibers. When the optical fiber device is kept straight, a pronounced interference pattern appears in the transmission spectrum. However, when the device is bent, a spectral shift of the interference pattern is produced, and the visibility of the interference notches changes. This allows for using either visibility or spectral shift for sensor interrogation. The dynamic range of the device can be tailored through the proper selection of the length of the seven-core fiber. The effects of temperature and refractive index of the external medium on the response of the curvature sensor are also discussed. Linear sensitivity of about 3000 nm/mm(-1) for bending was observed experimentally.

Six mode selective fiber optic spatial multiplexer.

Low-loss all-fiber photonic lantern (PL) mode multiplexers (MUXs) capable of selectively exciting the first six fiber modes of a multimode fiber (LP01, LP11a, LP11b, LP21a, LP21b, and LP02) are demonstrated. Fabrication of the spatial mode multiplexers was successfully achieved employing a combination of either six step or six graded index fibers of four different core sizes. Insertion losses of 0.2-0.3 dB and mode purities above 9 dB are achieved. Moreover, it is demonstrated that the use of graded index fibers in a PL eases the length requirements of the adiabatic tapered transition and could enable scaling to large numbers.

Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement.

In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of +/- 1.4 microepsilon and +/- 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature.

Control of surface modes in low loss hollow-core photonic bandgap fibers.

We report on the fabrication and characterization of hollow-core photonic bandgap fibers that do not suffer from surface mode coupling within the photonic bandgap of the cladding. This enables low attenuation over the full spectral width of the bandgap--we measured a minimum loss of 15 dB/km and less than 50 dB/km over 300 nm for a fiber operating at 1550 nm. As a result of the increased bandwidth, the fiber has reduced dispersion and dispersion slope--by a factor of almost 2 compared to previous fibers. These features are important for several applications in high-power ultrashort pulse compression and delivery. Realizing these advances has been possible due to development of a modified fabrication process which makes the production of low-loss hollow-core fibers both simpler and quicker than previously.

Design of 7 and 19 cells core air-guiding photonic crystal fibers for low-loss, wide bandwidth and dispersion controlled operation.

We study the modal properties of feasible hollow-core photonic bandgap fibers (HC-PBGFs) with cores formed by omitting either 7 or 19 central unit-cells. Firstly, we analyze fibers with thin core surrounds and demonstrate that even for large cores the proposed structures are optimum for broad-band transmission. We compare these optimized structures with fibers which incorporate antiresonant core surrounds which are known to have low-loss. Trade-offs between loss and useful bandwidth are presented. Finally, we study the effects that small modifications to the core surround have on the fiber's group velocity dispersion, showing the possibility of engineering the dispersion in hollow-core photonic bandgap fibers.