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.

All-fiber few-mode multicore photonic lantern mode multiplexer

The emergence of space division multiplexing (SDM) for ultrahigh capacity networks has heralded pioneering Petabit-class optical transmission systems. In parallel to novel SDM fibers, a new class of components to enable scalable, low-loss schemes for unlocking fiber capacity is being developed. In this work, an all-fiber mode selective photonic lantern mode multiplexer designed for launching into few-mode multicore fibers is demonstrated. This device is capable of selectively exciting LP₀₁, LP_11a and LP_11b modes in a seven-core configuration, resulting in 21 spatial channels, with less than 38 dB core-to-core crosstalk and insertion loss below 0.4 dB. The multicore photonic lantern multiplexer is scalable to larger number of cores and modes per core, and can be easily integrated with emerging ultra-high bandwidth few-mode multicore optical communication systems.

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 LP_1m-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.

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.

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.

White light Bessel-like beams generated by miniature all-fiber device

Micron-sized white light propagation invariant beams generated by a simple and compact fiber device are presented. The all-fiber device is fabricated by splicing a short piece of large-core multimode fiber onto a small-core single mode white light delivery fiber. Because this fiber device offers an inherent spatial coherence, nondiffracting white light beams can be created with a temporally incoherent broadband light source (a halogen bulb) and, most importantly, the surrounding fringes don't fade as the bandwidth of the light source increases because the underlying physics of this fiber device is different from that of the axicon. White light Bessel-like beams have been generated from multimode fibers with core diameters of 50 μm, 105 μm, and 200 μm. The distance of nondiffracting propagation of the white light Bessel beam increases with increasing core size of the multimode fiber. Propagation characteristics of red, green, and blue individual beams are also presented.

Coherent beam transformations using multimode waveguides

Physical insights and characteristics of beam transformations based on multimode interference (MMI) in multimode waveguides are illuminated and analyzed. Our calculations show that, utilizing a short piece of cylindrical multimode waveguide, an input Gaussian beam can be readily transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and Bessel-like beams in the Fresnel or the Fraunhofer diffraction range, or even in both ranges. This is a consequence of diffractive propagation of the field exiting the waveguide. The performance of the beam shaper based on MMI can be controlled via tailoring the dimensions of the multimode waveguide or changing the signal wavelength. This beam shaping technique is investigated experimentally using monolithic fiber devices consisting of a short piece of multimode fiber (approximately 10 mm long) and a single-mode signal delivery fiber.

Detailed investigation of self-imaging in large-core multimode optical fibers for application in fiber lasers and amplifiers

Properties of the self-imaging effect based on multimode interference (MMI) in large-core passive optical fibers are investigated and analyzed in detail, with the purpose of using multimode active fibers for high power single-transverse-mode emission. Although perfect self-imaging of the input field from a standard single-mode fiber (SMF-28) in a multimode fiber becomes practically impossible as its core diameter is larger than 50 microm, a quasi-reproduction of the input field occurs when the phase difference between the excited modes and the peak mode inside the multimode fiber is very small. Our simulation and experimental results indicate that, if the length of the multimode fiber segment can be controlled accurately, reproduction of the input field with a self-imaging quality factor larger than 0.9 can be obtained. In this case, a low-loss hybrid fiber cavity composed of a SMF-28 segment and a very-large-core active multimode fiber segment can be built. It is also found that for the hybrid fiber cavity, increasing the mode-field diameter of the single-mode fiber improves both the self-imaging quality and the tolerance on the required length accuracy of the multimode fiber segment. Moreover, in this paper key parameters for the design of MMI-based fiber devices are defined and their corresponding values are provided for multimode fibers with core diameters of 50 microm and 105 microm.

Single-transverse-mode output from a fiber laser based on multimode interference

An alternative original approach to achieve single-transverse-mode laser emissions from multimode (MM) active fibers is demonstrated. The fiber cavity is constructed by simply splicing a conventional passive single-mode fiber (SMF-28) onto a few centimeters-long active MM fiber section whose length is precisely controlled. Owing to the self-imaging property of multimode interference (MMI) in the MM fiber, diffraction-limited laser output is obtained from the end of the SMF-28, and the MMI fiber laser is nearly as efficient as the corresponding MM fiber laser. Moreover, because of the spectral filtering effect during in-phase MMI, the bandwidth of the MMI fiber laser is below 0.5 nm.

Distributed feedback fiber laser pumped by multimode laser diodes

A distributed feedback fiber laser made of highly Er-Yb codoped phosphate glass fiber has been demonstrated experimentally. Efficient pump absorption allows for multimode pumping into the cladding of the active fiber. Output powers up to 160 mW have been achieved. The 35 mm long fiber laser device emits with >50 dB side mode suppression ratio.

Variation of Bragg condition in low-glass-transition photorefractive polymers when recorded in reflection geometry

Two low-glass transition photorefractive polymer composites were investigated in a symmetric reflection geometry. The holograms recorded in 105 mum thick devices have reached diffraction efficiencies as high as 60%. Unlike the gratings recorded in transmission geometry, holograms recorded in reflection geometry showed high angular selectivity and the Bragg condition was observed to be sensitive to the magnitude of the external bias field. We attribute this effect to poling-induced birefringence and give a theoretical analysis to describe the observed results.

Birefringent in-phase supermode operation of a multicore microstructured fiber laser

We report the first observation of birefringent in-phase supermode operation of a phase-locked multicore fiber laser. The in-phase mode operation of our 12-core rectangular-array microstructured fiber laser was confirmed by the near-field distribution, the far-field diffraction pattern, and the optical spectrum. The birefringence of the in-phase mode in propagation constant Deltay was measured as ~ 4 x 10(-6) 1/mum. The break of the polarization degeneracy indicates the possibility of single polarization operation of phase-locked multicore fiber lasers and amplifiers.

Phase locking and in-phase supermode selection in monolithic multicore fiber lasers

We report a compact multicore fiber laser that utilizes an all-fiber approach for phase locking and in-phase supermode selection. By splicing passive coreless fibers of controlled lengths to both ends of an active 19-core fiber, we demonstrate that the fundamental in-phase supermode can be selectively excited with a completely monolithic fiber device, instead of conventional free-space and bulk optics, to achieve phase-locked operation for a multiemitter laser device.

Single-frequency fiber oscillator with watt-level output power using photonic crystal phosphate glass fiber

Utilizing phosphate glass fiber with photonic crystal cladding and highly doped, large area core a cladding-pumped, single-frequency fiber oscillator is demonstrated. The fiber oscillator contains only 3.8 cm of active fiber in a linear cavity and operates in the 1.5 micron region. Spectrally broad, multimode pump light from semiconductor laser diodes is converted into a single-mode, single-frequency light beam with an efficiency of about 12% and the oscillator output power reached 2.3 W.

Photorefractive polymer device with video-rate response time operating at low voltages

The high-voltage bias required for video-rate compatible, efficient operation of a photorefractive polymer composite is reduced from 6-8 to 1.3 kV. At this low voltage, the device can hold erasable Bragg holograms with 80% efficiency in addition to having a video-rate response time. The transition of the hologram's state from thick to thin is analyzed in detail.

Investigation of modal properties of microstructured optical fibers with large depressed-index cores

We present what is, to the best of our knowledge, the first systematic study on how negative core-cladding index difference influences microstructured optical fiber's modal behavior. Single-mode lasing has been realized for short-length cladding-pumped phosphate glass microstructured fibers with large depressed-index Er(3+)-Yb(3+)-codoped cores.

Generation of watt-level single-longitudinal-mode output from cladding-pumped short fiber lasers

We generate as much as 1.6 W of continuous-wave 1550 nm single-longitudinal-mode output from a cladding pumped Er-Yb codoped phosphate fiber laser. This power is to our knowledge among the highest in single-longitudinal-mode fiber lasers. The narrowband fiber Bragg grating output coupler is demonstrated to be an effective element for providing the single-longitudinal-mode selection.

Short-length microstructured phosphate glass fiber lasers with large mode areas

We report fabrication and testing of the first phosphate glass microstructured fiber lasers with large Er-Yb-codoped cores. For an 11-cm-long cladding-pumped fiber laser, more than 3 W of continuous wave output power is demonstrated, and near single-mode beam quality is obtained for an active core area larger than 400 microm2.

3-Dimensional thermal analysis and active cooling of short-length high-power fiber lasers

A fully 3-dimensional finite element model has been developed that simulates the internal temperature distribution of short-length high-power fiber lasers. We have validated the numerical model by building a short, cladding-pumped, Er-Yb-codoped fiber laser and measuring the core temperature during laser operation. A dual-end-pumped, actively cooled, fiber laser has generated >11 W CW output power at 1535 nm from only 11.9 cm of active fiber. Simulations indicate power-scaling possibilities with improved fiber and cooling designs.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Evidence for intervalence band coherences in semiconductor quantum wells via coherently coupled optical Stark shifts

We report the experimental observation of coherently coupled heavy-hole-light-hole Stark shifts, i.e., light-hole exciton shifts under heavy-hole exciton pumping conditions, in InGaAs quantum wells. The theoretical analysis of the data is based on a full many-body approach (dynamics-controlled truncation formalism) in the third-order nonlinear optical regime. It is shown that the Stark shift data can be interpreted as strong evidence of suitably defined nonradiative intervalence band coherences in a semiconductor quantum well. Hence, the observations establish a semiconductor analog of Raman coherences in three-level atoms.

Two-photon resonant third-harmonic generation in La2CuO4

Combining linear absorption and nonlinear third harmonic generation (THG) experiments, we investigate details of the electronic structure of the highly correlated electronic system in La2CuO4. We demonstrate strong THG mainly due to the charge transfer excitation from O (2p(sigma)) to Cu (3d(x2-y2)). The THG spectrum shows pronounced features due to three-photon and two-photon resonance enhancement as well as quantum interference effects. We obtain excellent agreement with a THG spectrum calculated in terms of the excitonic cluster model and can identify both odd and even symmetry excitation modes.