Fiber-connectorized ultrafast-laser-inscribed K-band integrated optics beam combiner for the CHARA telescope array.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1d B over the 2-2.3 µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Experimental study of the mode instability onset threshold in high-power FA-LPF lasers.

We report here on an experimental investigation of the temporal behavior of transverse mode instabilities into "fully aperiodic large-pitch fibers" (FA-LPFs) operated in high-power continuous-wave laser configuration. To ensure an effective transverse single-mode emission into FA-LPFs, a perfect index matching between the active core and the background cladding materials (Δn=0) is required. The original design of such fibers enables an effective transverse single-mode emission by strengthening the higher-order mode delocalization out of the gain region, even for high heat load levels, consequently leading to the improvement of the beam spatial quality. The study was conducted over fibers of various gain region diameters, from 58 to 100 µm, for a refractive index mismatch Δn of about +8×10-5. The emitted beam is characterized using both M2 measurements and time traces to study the changeover of a stable temporal behavior to an unstable one.

140 µm single-polarization passive fully aperiodic large-pitch fibers operating near 2 µm.

In this paper, we demonstrate a single-polarization feature out of passive very-large-mode-area fully aperiodic large-pitch fibers. It has been previously shown theoretically that one of the two polarizations of the fundamental mode is selectively coupled to a cladding mode. This coupling does not require fiber bending, which permits us to avoid any decrease in mode effective area. The coupling is achieved owing to boron-doped silica inclusions implemented into the microstructured cladding and acting as stress-applying parts. This mechanism has been assessed experimentally in this work using fibers of two different core diameters: 60 µm and 140 µm, providing mode field areas of 3637 µm2 and 14,590 µm2, respectively, at 1942 nm. The tested fibers have a length of 45 cm and an outer diameter exceeding 1 mm, yielding rod-type fibers. Each sample has been characterized using an unpolarized laser source emitting at 1942 nm. This laser, based on a thulium-doped large-mode-area step-index fiber, has a spectral bandwidth of about 0.5 nm. After passing through a piece of the passive fiber, a polarization extinction ratio higher than 16 dB has been achieved.

Spectral-temporal dynamics of high power Raman picosecond pulse using H2-filled Kagome HC-PCF.

We report on the spectral-temporal characterization of a 1.8 µm wavelength and high power picosecond pulse Raman source. It is generated via frequency conversion to the first-order Stokes of a 27 ps chirped pulse Yb-doped fiber laser inside a molecular hydrogen-filled Kagome hollow-core photonic crystal fiber (HC-PCF). Depending on the average power and chirp of the pump laser, the average power of this Raman source can be as high as 9.3 W, and its pulse duration can be as short as ∼17 ps. In agreement with stimulated Raman scattering under the very high gain transient regime, the experimental results show the Stokes spectral structure to change following a three-stage sequence when the average pump power is increased. For a pump with a chirp corresponding to a bandwidth of 200 GHz, we found that for a pump power lower than 7 W, the Stokes spectrum is generated from the blue side of the pump spectrum, and then it exhibits a spectral replica of the pump spectrum for 7-14 W pump power range. Finally,the Stokes spectrum is chiefly generated from the red side of the pump spectrum when the pump power is further increased. Conversely, the Stokes pulse temporal profile shows a strong dependence with the pump power. For a low pump power range, the Stokes pulse exhibits a single peak with a full width at half-maximum of ∼17 ps. For higher pump powers, the Stokes pulse presents a double-peak structure with each peak having a duration of less than 15 ps. The present results can be used to develop compact and efficient frequency down-convertors to the increasingly widespread Yb-based picosecond lasers.

Broadband single-mode single-polarization passive fully aperiodic large-pitch fibers.

Two evolutions of fully aperiodic large-pitch fiber designs employing few stress-applying parts are presented. The induced elasto-optic stress discriminates the two orthogonal polarization modes (LP01x and LP01y) of the fundamental mode, selectively delocalizing one of them into the cladding via a suitable coupling to one or several cladding modes. This ensures the propagation of a single linear polarization mode. For the largest core dimensions, however, the applied stress can strongly influence the intensity distributions of core modes, and a tailored design process must thwart this. The polarization properties are investigated experimentally with core scalability over a large spectral bandwidth into passive structures, leading to the evidencing of a single-mode single polarization over a large span from 1 to 1.6 µm with a core dimension of 80 µm and, notably, at 1400 nm for a core dimension of 140 µm. The polarization extinction ratio is also determined.

Precompensation of the thermal-induced refractive index changes into a fully aperiodic LPF for heat load resilience.

In this paper, a strategy consisting of precompensating the thermal-induced transverse refractive index changes is undertaken to push further the appearance threshold of a multimode regime. First, a standard air-silica large pitch fiber (LPF) and a fully aperiodic large pitch fiber are confronted in regard to their heat load resilience and capabilities for single-mode emission. Thereafter, slight refractive index depressions are judiciously introduced into the active core area. This approach enhances the delocalization of the high-order modes even under severe heat load levels. This combination of aperiodic cladding microstructuration and index-precompensation theoretically increases the multimode regime threshold while preserving large mode field areas. This investigation is performed at 1.03 and 2 µm operating wavelengths.

Demonstration of a homogeneous Yb-doped core fully aperiodic large-pitch fiber laser.

The first demonstration of a 40 µm core homogeneously ytterbium-doped fully aperiodic large-pitch fiber laser, to the best of our knowledge, is reported here. In this concept, the amplification of unwanted high-order modes is prevented by means of an aperiodic inner-cladding structure, while the core and inner-cladding material has a higher refractive index than pure silica. In a laser configuration, up to 252 W of extracted power, together with an optical-to-optical efficiency of 63% with respect to the incident pump power, have been achieved. While an average M2 of 1.4 was measured, the emitted power becomes temporally unstable when exceeding 95 W, owing to the occurrence of modal instabilities.

50.4% slope efficiency thulium-doped large-mode-area fiber laser fabricated by powder technology.

We report on a triple clad large-mode-area Tm-doped fiber laser with 18 µm core diameter manufactured for the first time by an alternative manufacturing process named REPUSIL. This reactive powder sinter material enables similar properties compared to conventional CVD-made fiber lasers, while offering the potential of producing larger and more uniform material. The fiber characterization in a laser configuration provides a slope efficiency of 47.7% at 20°C, and 50.4% at 0°C with 8 W output power, with a laser peak emission at 1970 nm. Finally, a beam quality near the diffraction-limit (M(x,y)2<1.1) is proved.

Over-five octaves wide Raman combs in high-power picosecond-laser pumped H(2)-filled inhibited coupling Kagome fiber.

We report on the generation of over 5 octaves wide Raman combs using inhibited coupling Kagome guiding hollow-core photonic crystal fiber filled with hydrogen and pumped with 22.7 W average power and 27 picosecond pulsed fiber laser. Combs spanning from ~321 nm in the UV to ~12.5 µm in the long-wavelength IR (i.e. from 24 THz to 933 THz) with different spectral content and with an output average power of up to ~10 W were generated. In addition to the clear potential of such a comb as a laser source emitting at spectral ranges, which existing technology poorly addresses like long-wavelength IR and UV, the combination of high Raman net gain and short pump-pulse duration makes these spectra an excellent candidate for intra-pulse waveform synthesis.

Highly efficient higher-order modes filtering into aperiodic very large mode area fibers for single-mode propagation.

We report here on the first experimental demonstration, to the best of our knowledge, of a new generation of very large mode area (VLMA) fibers intended to strengthen single-mode propagation. The originality of this work relies on an aperiodicity of the inner cladding microstructuration exacerbating the spatial rejection of higher-order-modes (HOMs) while preserving a significant confinement of the fundamental mode. The single-mode behavior was demonstrated using an optical low-coherence interferometry measurement based on the group-velocity dispersion. As suggested through a preliminary numerical approach, this outstanding characteristic/behavior is evidenced over a large spectral range spanning from 1 to 2 µm for a core diameter of 60 µm. Core scalability was also investigated.

Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber.

Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 µm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.

Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm.

We report on a high-power narrow-linewidth pulsed laser source emitting at a wavelength of 257 nm. The system is based on a master oscillator power amplifier architecture, with Yb-doped fiber preamplifiers, a Yb:YAG single crystal fiber power amplifier used to overcome the Brillouin limitation in glass fiber and nonlinear frequency conversion stages. This particularly versatile architecture allows the generation of Fourier transform-limited 15 ns pulses at 1030 nm with 22 W of average power and a diffraction-limited beam (M(2)<1.1). At a repetition rate of 30 kHz, 106 µJ UV pulses are generated corresponding to an average power of 3.2 W.