LMA PCF Research Articles

Fibre laser system with wavelength tuning in extended telecom range

We report on a laser system on the base of a fibre telecom source, a chirped pulse amplification system, and low-birefringence large mode area photonic crystal fibre (LMA PCF) generating ∼ 100 fs pulses with the energy ∼ 10 nJ in the 1600–1700 nm wavelength range. The characteristics of the output spectrum corresponding to Raman solitons are studied for different polarisations of the pump pulse at the input of the PCF. We have shown that the wavelength of the output spectrum maximum can be tuned in the whole long (L) and ultra-long (U) telecom bands by adjusting the polarisation state of the pump pulse while the output power is constant.

Highly fluorine and ytterbium doped polarization maintaining large mode area photonic crystal fiber via the sol-gel process

Highly Yb-doped silica glass with low refractive index for fabrication of Yb-doped large-mode-area photonic crystal fiber (LMA PCF) is in favor of decreasing fiber length and thus increasing the threshold of nonlinear effects in pulse laser amplification. Accordingly, fluorine incorporation in highly Yb-doped silica glass is vitally important to compensate the sharp increase in refractive index caused by ytterbium and aluminum ions. In this work, the fluorine doping concentration in Yb/Al/P/F co-doped silica glass was significantly improved by a modified sol-gel method combined with high temperature sintering. The effects of fluorine doping on glass structure have been investigated in details by Raman spectra, nuclear magnetic resonance (NMR) and advanced pulse electron paramagnetic resonance (EPR) measurements. The incorporation of fluorine yields Si-F bonds and Yb-F bonds formation and leads to the mild change in spectroscopic properties. An optimized silica core glass rod with high Yb (0.77 wt.%) and fluorine (0.8 wt.%) doping concentration, low refractive index and acceptable optical quality was prepared. Based on the highly fluorine and ytterbium doped silica core glass, a polarization maintaining (PM) photonic crystal fiber with 40 µm core was prepared and the pump absorption coefficient at 976 nm was ∼6.5 dB/m. An average amplified power of 103 W was achieved from a 2-m-long PCF with the bend diameter of 23 cm. The slope efficiency (with respect to pump power) was 52% with laser beam quality factor M2 of 1.46.

Issue Information

Cover Photograph: Microscopic images of cross sections of the (Upper left) pristine and (Upper right) annealed large‐mode‐area photonic crystal fi bers (LMA PCF). Laser beams of the pristine (Lower left) and annealed (Lower right) LMA PCF. Image credits: Mengting Guo and Lili Hu. [https://doi.org/10.1111/jace.17777] image

Structural origin of thermally induced refractive index changes in Yb3+/Al3+/P5+/F−‐co‐doped silica glass

AbstractIn this work, the relationship among the thermal history, refractive index (RI), and structures of Yb3+/Al3+/P5+/F−‐co‐doped silica glass is studied. We reveal the structural origin of the thermally induced RI change in the Heraeus F300 and Yb3+/Al3+/P5+/F−‐co‐doped silica glasses with different states of thermal history by advanced solid‐state nuclear magnetic resonance and Raman scattering techniques, and the effect of the thermal history on the beam quality of Yb3+‐doped large‐mode‐area photonic crystal fiber (LMA PCF). The structural analysis demonstrates that the RI change in the glass is inextricably linked to the change in the AlPO4 domains, coordination number of the Al and Si–O–Si rings. An intimate and direct connection of the thermally induced physical behaviors is developed with the structural pictures of glasses, and the beam quality of the LMA PCF is greatly improved by proper thermal annealing. The results provide a significant reference for the precise manipulation of the numerical aperture of Yb3+‐doped active fiber.

Yb-doped silica glass and photonic crystal fiber based on laser sintering technology

We demonstrate the fabricating method for Yb3+-doped silica glass and double-cladding large mode area photonic crystal fiber (LMA PCF) based on laser sintering technology combined with a liquid phase doping method. The doped material prepared shows the amorphous property and the hydroxyl content is approximately 40 ppm. The attenuation of the fabricated LMA PCF is 14.2 dB m−1 at 976 nm, and the lowest value is 0.25 dB m−1 at 1200 nm. The laser slope efficiency is up to 70.2%.

Spectroscopic and laser properties of Al-P co-doped Yb silica fiber core-glass rod and large mode area fiber prepared by sol-gel method

A large-size (∅5 mm) Yb3+-doped silica fiber core-glass rod with an Al-P co-doped composition: 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%) was prepared by the sol-gel method combined with high-temperature melting technology. We successfully solved the doping homogeneity problem caused by the volatility of P2O5. The doping homogeneity of the glass rod was very high with the maximum refractive index fluctuation Δn < 2 × 10−4. The refractive index of the glass rod was very low because of the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure. A large-mode-area single cladding fiber (LMA SCF) and photonic crystal fiber (LMA PCF) with core diameters of 35 and 50 µm, respectively, were drawn from this glass rod. Owing to the low refractive index of the core-glass rod and the corresponding low numerical aperture (NA, 0.033) of the core, the LMA SCF exhibited a high laser beam brightness with M2 = 1.3-1.4. The LMA PCF exhibited the maximum output power, which was limited by the available pumping power, of 46 W and the slope efficiency of 61%.

Characteristics and Laser Performance of Yb3+-Doped Silica Large Mode Area Fibers Prepared by Sol–Gel Method

Large-size 0.1 Yb2O3–1.0 Al2O3–98.9 SiO2 (mol%) core glass was prepared by the sol–gel method. Its optical properties were evaluated. Both large mode area double cladding fiber (LMA DCF) with core diameter of 48 µm and large mode area photonic crystal fiber (LMA PCF) with core diameter of 90 µm were prepared from this core glass. Transmission loss at 1200 nm is 0.41 dB/m. Refractive index fluctuation is less than 2 × 10−4. Pumped by 976 nm laser diode LD pigtailed with silica fiber (NA 0.22), the slope efficiency of 54% and “light-to-light” conversion efficiency of 51% were realized in large mode area double cladding fiber, and 81 W laser power with a slope efficiency of 70.8% was achieved in the corresponding large mode area photonic crystal fiber.

Fabrication and laser behaviors of Yb3+ doped silica large mode area photonic crystal fiber prepared by sol–gel method

Yb3+, Al3+ co-doped silica large mode area photonic crystal fiber (LMA PCF) has been prepared by sol–gel method combined with stack-capillary-draw technique. The core glass with composition of 0.3Yb2O3–3Al2O3–96.7SiO2 (mol%) has 927μs fluorescence lifetime, 0.82pm2 stimulated emission cross section at ∼1μm and 1.4ppm residual hydroxyl group. 6.8W continuous-wave fiber laser ranging from 1040 to 1070nm has been obtained from a piece of 98-cm-long PCF with the Yb3+ concentration of 6.49×1019ions/cm3 pumped by a commercial 927-nm laser diode. The laser has a threshold of 1.45W launched power with a slope efficiency of 48%. The maximum output power of 6.8W is limited by the available pumping power, and the optical loss is 1.2dB/m at 1053nm. To the best of our knowledge, this is the highest laser output power report on Yb3+, Al3+ co-doped silica LMA PCF prepared by sol–gel method.

High energy femtosecond supercontinuum light generation in large mode area photonic crystal fiber

A large mode area photonic crystal fiber (LMA PCF) with an effective area of 180 μm 2 is used to generate a high energy, micro-joule range, flat, octave spanning supercontinuum (SC) extending from ~ 600 nm to ~ 1720 nm. A train of femtosecond pulses from a widely-tunable parametric amplifier pumped by a Ti:Sapphire regenerative amplifier system are coupled into a 20 cm length of LMA PCF generating a SC of 1.4 μJ energy. We present an experimental study of the high energy SC as a function of the input power and the pumping wavelength. The spectrum obtained at a pump wavelength of 1260 nm presents spectral flatness variation less than 12 dB over more than 1.1 octave bandwidth. The physical processes behind the SC formation are described in the normal and the anomalous dispersion regions. Our experimental results are successfully compared with the numerical solution of the nonlinear Schrödinger equation.

Yb3+-doped large-mode-area photonic crystal fiber laser with 210 W continuous-wave output power

An Yb3+-doped double-clad large-mode-area photonic crystal fiber (LMA PCF) laser with up to 210 W of continuous-wave output power centered at 1.05 µm is demonstrated. The length of the fiber used is 2 m and the produced laser power per meter can attain 105 W. The PCF is pumped by two diode lasers with central wavelength of 976 nm. The slope efficiency is 76%, and the beam quality factor M2 at x and y axes are measured to be 1.06 and 1.08, respectively. No thermo-optical problems and other roll-over even are observed at the highest output power.

High-Power Coherent Beam Combining of Two Photonic Crystal Fiber Lasers

Coherent beam combining of two large mode area photonic crystal fiber (LMA PCF) lasers is studied experimentally by using a self-imaging technique. Steady interference strips with high visibility of 35% are observed in high-power operation, which demonstrates the obvious advantages of the LMA PCF over the conventional double-clad fiber in coherent beam combining. Experiments also show that the reflectivity of coupling output mirror affects the coherent output power. The maximum coherent output power of 80 W with approximate 90% combining efficiency is achieved when the pump power is 130 W and the reflectivity of coupling output mirror is 5%.

Infrared supercontinuum from a large mode area PCF under extreme picosecond excitation

Evolution of the long-wavelength edge of supercontinuum (SC) in a large mode-area photonic crystal fiber (LMA PCF) under extreme (close to the fiber damage threshold) picosecond excitation is analyzed for the first time to our knowledge. The obtained results are interpreted on the basis of the numerical simulations explaining both spectral and temporal characteristics of SC in dependence on the pulse power and the fiber length. An existence of the minimum LMA PCF length providing the broadest spectrum under the highest picosecond excitations is predicted. The multimode cross-modulation inside the LMA PCF is analyzed, as well.