Photonic crystal fibers design for 2 μm wavelenght operation

New photonic crystal fiber designs are presented and numerically investigated in order to improve the state of art of high power fiber lasers. The focus of this work is targeted on the region of 2 μm laser emission, which is of high interest due to its eye-safe nature and due to the large amount of applications permitted. Thulium doped fiber amplifiers are suitable for emitting in this region. Different fiber designs have been proposed, both flexible and rod-type, with the aim to enlarge mode area while maintaining robust single mode operation. The analysis of thermal effects, caused by the high thulium quantum defect, have been taken in consideration. Solutions to counteract issues derived by detrimental thermal effects have been implemented.

Fiber Bragg grating structures inscribed using 800nm femtosecond laser in single- and multi-core mid-IR glass fibers with their spectral, thermal and strain properties

Single- and multi-core passive and active germanate and tellurite glass fibers represent a new class of fiber host for in-fiber photonics devices and applications in mid-IR wavelength range, which are in increasing demand. Fiber Bragg grating (FBG) structures have been proven as one of the most functional in-fiber devices and have been mass-produced in silicate fibers by UV-inscription for almost countless laser and sensor applications. However, because of the strong UV absorption in germanate and tellurite fibers, FBG structures cannot be produced by UVinscription. In recent years femtosecond (fs) lasers have been developed for laser machining and microstructuring in a variety of glass fibers and planar substrates. A number of papers have been reported on fabrication of FBGs and long-period gratings in optical fibers and also on the photosensitivity mechanism using 800nm fs lasers. In this paper, we demonstrate for the first time the fabrication of FBG structures created in passive and active single- and three-core germanate and tellurite glass fibers by using 800nm fs-inscription and phase mask technique. With a fs peak power intensity in the order of 1011W/cm2, the FBG spectra with 2nd and 3rd order resonances at 1540nm and 1033nm in a single-core germanate glass fiber and 2nd order resonances between ~1694nm and ~1677nm with strengths up to 14dB in all three cores of three-core passive and active tellurite fibers were observed. Thermal and strain properties of the FBGs made in these mid-IR glass fibers were characterized, showing an average temperature responsivity of ~20pm/°C and a strain sensitivity of 1.219±0.003pm/µe.

Continuous-wave ultraviolet light induced fiber Bragg gratings in few- and single-mode microstructured polymer optical fibers

Reported are observations and measurements of the inscription of fibre Bragg gratings in two different types of microstructured polymer optical fibre: few-moded and endlessly single mode. Contrary to FBG inscription in silica microstructured fibre, where high energy laser pulses are a prerequisite, we have successfully used a low power CW laser source operating at 325nm to produce 1-cm long gratings with a reflection peak at 1570 nm. Peak reflectivities of more than 10% have been observed.

Measurements of stress-optic coefficient in polymer optical fibers

We have systematically measured the differential stress-optic coefficient, ?C, in a number of poly(methyl methacrylate) (PMMA) fibers drawn with different stress, ranging from 2 up to 27 MPa. ?C was determined in transverse illumination by measuring the dependence of birefringence on additional axial stress applied to the fiber. Our results show that ?C in PMMA fibers has a negative sign and ranges from -4.5 to -4.5×10-12 Pa-1, depending on the drawing stress. Increase of the drawing stress results in greater initial fiber birefringence and lower ?C.

Thermal response of Bragg gratings in PMMA microstructured optical fibers

We report on the thermal characteristics of Bragg gratings fabricated in polymer optical fibers. We have observed a permanent shift in the grating wavelength at room temperature which occurs when the grating has been heated above a threshold temperature. This threshold temperature is dependent on the thermal history of the grating, and we attribute the effect to a shrinking of the fiber. This effect can be avoided by annealing the fiber before grating inscription, resulting in a linear response with temperature and an increased linear operating temperature range of the grating.

Fabrication of micro-channels in optical fibers using femtosecond laser pulses and selective chemical etching

The fabrication of micro-channels in single-mode optical fibers is demonstrated using focused femtosecond laser processing and chemical etching. Straight line micro-channels are achieved based on a simple technique which overcomes limitations imposed by the fiber curved surface.

Fiber Bragg gratings inscribed using 800nm femtosecond laser and a phase mask in singleand multi-core mid-IR glass fibers

For the first time, Fiber Bragg grating (FBG) structures have been inscribed in single-core passive germanate and three-core passive and active tellurite glass fibers using 800nm femtosecond (fs) laser and phase mask technique. With fs peak power intensity in the order of 1011W/cm2, the FBG spectra with 2nd and 3rd order resonances at 1540 and 1033nm in the germanate glass fiber and 2nd order resonances at ~1694 and ~1677nm with strengths up to 14dB in all three cores in the tellurite fiber were observed. Thermal responsivities of the FBGs made in these mid-IR glass fibers were characterized, showing average temperature responsivity ~20pm/°C. Strain responsivities of the FBGs in germanate glass fiber were measured to be 1.219pm/µe.

New developments in the study of optical parabolic pulses in normally dispersive fibers

We report two recent studies dealing with the evolution of parabolic pulses in normally dispersive fibres. On the one hand, the nonlinear reshaping from a Gaussian intensity profile towards the asymptotic parabolic shape is experimentally investigated in a Raman amplifier. On the other hand, the significant impact of the fourth order dispersion on a passive propagation is theoretically discussed: we numerically demonstrate flat-top, coherent supercontinuum generation in an all-normal dispersion-flattened photonic crystal fiber. This shape is associated to a strong reshaping of the temporal profile what becomes triangular.

Dissipative nonlinear structures in optical fiber systems II : self-similar parabolic pulses in active fibers

In this second talk on dissipative structures in fiber applications, we overview theoretical aspects of the generation, evolution and characterization of self-similar parabolic-shaped pulses in fiber amplifier media. In particular, we present a perturbation analysis that describes the structural changes induced by third-order fiber dispersion on the parabolic pulse solution of the nonlinear Schrödinger equation with gain. Promising applications of parabolic pulses in optical signal post-processing and regeneration in communication systems are also discussed.