Shyamal Kumar Bhadra

Picosecond fiber MOPA pumped supercontinuum source with 39 W output power

We report picosecond fiber MOPA pumped supercontinuum source with 39W output, spanning at least 0.4–1.75µm with high and relatively uniform spectral power density of ∼31.7mW/nm corresponding to peak power density of ∼12.5W/nm in 20ps pulse.

Effects of higher-order dispersion on resonant dispersive waves emitted by solitons

Dispersive waves (DW) are generated owing to perturbation of solitons by higher-order dispersion (HOD) and nonlinearity during supercontinuum (SC) generation. The frequencies of these waves are governed by a phase-matching condition in the form of a polynomial whose coefficients depend on the numerical values of the properly normalized third- and HOD parameters. Our extensive numerical solutions show that all odd HOD terms generate a single peak on the blue or the red side of the carrier frequency, depending on the sign of the corresponding term. In contrast, positive even HOD terms create conjugate DW peaks, in both the blue and red sides. No radiation is observed for negative values of these parameters. The combination of all even and odd HOD coefficients may generate more than two DW peaks for some specific choice of parameters. The results predicted by the phase-matching condition agree well with extensive numerical simulations revealing interesting facts of SC generation.

Wideband EDFA Based on Erbium Doped Crystalline Zirconia Yttria Alumino Silicate Fiber

A wideband erbium-doped fiber amplifier (EDFA) is demonstrated using an Erbium-doped zirconia fiber as the gain medium. With a combination of both Zr and Al, we could achieve a high erbium doping concentration of 4320 ppm in the glass host without any phase separations of rare-earths. The Erbium doped fiber (EDF) is obtained from a fiber preform, which is fabricated in a ternary glass host, zirconia-yttria-aluminum codoped silica fiber using a MCVD process. Doping of Er2O3 into Zirconia yttria-aluminosilicate based glass is done through solution doping process. The maximum gain of 21.8 dB is obtained at 1560 nm with 2 m long of EDF and co-pumped with 1480 nm laser diode. At high input signal of -4 dBm, a flat-gain at average value of 8.6 dB is obtained with a gain variation of less than 4.4 dB within the wavelength region of 1535-1605 nm and using 3 m of EDF and 100 mW pump power. The corresponding noise figure is maintained below 9.6 dB at this wavelength region.

Design of all-solid leakage channel fibers with large mode area and low bending loss

We investigate a novel design for all-solid large mode area (LMA) leakage channel fibers (LCFs) for high-power Yb-doped fiber lasers and amplifiers, based on a single down-doped-silica rod ring surrounding a seven-cell pure-silica core, aiming for effectively single-mode behavior and low bending loss characteristics. Through detailed numerical simulations based on the finite element method (FEM), we find that the proposed all-solid LMA-LCFs, having a seven-cell core and two different sizes of down-doped rods, can achieve sufficient differential mode loss and much lower bending loss, as compared with a previously-reported LCF with a one-cell core and six large down-doped-silica rods.

Performance comparison of Zr-based and Bi-based erbium-doped fiber amplifiers.

In this Letter, we present a comprehensive comparison of the performance of a zirconia-based erbium-doped fiber amplifier (Zr-EDFA) and a bismuth-based erbium-doped fiber amplifier (Bi-EDFA). The experimental results reveal that a Zr-EDFA can achieve comparable performance to the conventional Bi-EDFA for C-band and L-band operations. With a combination of both Zr and Al, we could achieve a high erbium-doping concentration of about 2800 ppm (parts per million) in the glass host without any phase separations of rare earths. The Zr-based erbium-doped fiber (Zr-EDF) was fabricated using in a ternary glass host, zirconia-yttria-aluminum codoped silica fiber through a solution-doping technique along with modified chemical vapor deposition. At a high input signal of 0 dBm, a flat gain at average value of 13 dB is obtained with a gain variation of less than 2 dB within the wavelength region of 1530-1575 nm and using 2 m of Zr-EDF and 120 mW pump power. The noise figures are less than 9.2 at this wavelength region. It was found that a Zr-EDFA can achieve even better flat-gain value and bandwidth as well as lower noise figure than the conventional Bi-EDFA.

Dynamics of Raman soliton during supercontinuum generation near the zero-dispersion wavelength of optical fibers.

We observe unique dynamics of Raman soliton during supercontinuum process when an input pulse experiences initially normal group-velocity dispersion with a negative dispersion slope. In this situation, the blue components of the spectrum form a Raman soliton that moves faster than the input pulse and eventually decelerates because of Raman-induced frequency downshifting. In the time domain, the soliton trajectory bends and becomes vertical when the Raman shift ceases to occur as the spectrum of Raman soliton approaches the zero dispersion point. Parts of the red components of the pulse spectrum are captured by the Raman soliton through cross-phase modulation and they travel with it. The influence of soliton order, input chirp and dispersion slope on the dynamics of Raman soliton is discussed thoroughly.

Limitation on Effective Area of Bent Large-Mode-Area Leakage Channel Fibers

We investigate the bending characteristics of leakage channel fibers (LCFs) to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for compact Yb-doped fiber applications. Through numerical simulations, carried by the full-vectorial finite-element method, we present the limitations on the effective area of LCFs under bent condition and compare their limits with that of conventional step-index LMA fibers. Due to a better controllability of the low numerical aperture and a large value of the differential bending loss (~20 dB/m) between the fundamental and higher order modes in LCFs, the LMA of ~500 μm2 (core diameter of ~36 μm) at 1064 nm can be achieved when the optimized LCF is bent into a 10 cm bending radius.

Diode-pumped 1028 nm Ytterbium-doped fiber laser with near 90% slope efficiency

Highly efficient laser action from an Ytterbium-doped fiber (YDF) is demonstrated using a fiber Bragg grating (FBG) in conjunction with a 4% Fresnel reflection at room temperature. The YDF used is drawn from Yb2O3-doped preform, fabricated through deposition of porous layer of composition SiO2-GeO2 by the MCVD process in conjunction with a solution doping technique. The fabricated YDF has a core composition of 0.2 wt % of Yb2O3, 1.8 wt % of Al2O3 and 23 wt % of GeO2 with a pump absorption of 9.0 dB/m. The fiber laser operates at wavelength of 1028 nm with a slope efficiency of 88% with respect to the launched 976 nm pump power using the YDF length of 7 m.

Blue-Extended Sub-Nanosecond Supercontinuum Generation in Simply Designed Nonlinear Microstructured Optical Fibers

We report experimentally observed extreme blue-enhanced supercontinuum (SC) generation down to 372 nm wavelength in simply designed and easily fabricated nonlinear microstructured optical fibers (MOFs). Three different MOFs of various core sizes and dispersion profiles are fabricated in order to optimize the parameters for achieving deeper blue components by group-index matching. The physical mechanism involved in the generation of extreme blue component is explained along with other nonlinear processes participating in the spectral broadening. We also explore the extent of applicability of the group-index matching technique for obtaining blue-enhanced SC and finally optimize the location of the zero dispersion wavelength (ZDW) with respect to the pump wavelength to achieve the maximum blue shift.

Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier

We report design and subsequent fabrication of an intrinsically gain flattened Erbium doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ≥28 dB (gain excursion below ±2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at ―20 dBm/ channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head.