Atasi Pal

Ytterbium-sensitized Thulium-doped fiber laser in the near-IR with 980 nm pumping

The use of an unidirectional auxiliary pump at approximately 1600 nm in conjunction with a 980 nm primary pump for Ytterbium (Yb(3+))-sensitized-Thulium (Tm(3+))-doped single mode silica fiber (YTDF) is found to be very effective to activate the most significant resonance energy transfer from Yb(3+) to Tm(3+), in order to obtain significant emission in the near-infrared. The resulting laser performance of the YTDF at 1874 nm is reported here. The influence of the Tm(3+)/Yb(3+) concentration, their relative proportions and the host glass composition on the lasing efficiency has also been investigated to optimize the fiber parameters for maximum laser output power.

The mechanism of rare earth incorporation in solution doping process

The mechanism involved during solution doping process has been systematically investigated by correlating the soot characteristics and solution parameters with the amount of rare earth (RE) incorporated in the core of optical fiber. Experiments show that the amount of RE incorporation may be controlled with better precision by adjusting Al ion concentration in the soaking solution. A model has been developed on the basis of cooperative adsorption mechanism correlating different parameters in the overall process. Theoretical estimation shows good agreement with the experimental results and can be used to predict the extent of RE incorporation for any composition if the soot layer characteristics are known.

High power laser fiber fabricated through vapor phase doping of Ytterbium

We present the characteristics of an ytterbium-doped alumino-silicate fiber, fabricated through vapor phase doping of aluminum and ytterbium in the core, along with silica and in conjunction with the modified chemical vapor deposition process. The vapor phase doping of rare-earths provides the opportunity to fabricate large core active fibers with a uniform distribution of dopants. The fibers fabricated exhibited low OH− content, negligible center dip and good optical properties. Lasing performance was tested up to output power of 105 W, with a slope efficiency of 77% with respect to launched pump power. The linear variation of the laser power with a pump shows its potentiality for further power scaling.

Sensitive detection of CO2 implementing tunable thulium-doped all-fiber laser.

In this paper a compact, yet sensitive gas detection system based on a modulated, tunable thulium-doped fiber laser in the 2 μm wavelength region is reported. The laser operating wavelength range centered at a wavelength of 1.995 μm has been selected to access the R(50) transition (ν1+2ν2+ν3) of CO2 based on its line strength and to achieve isolation from interfering high-temperature water absorption features. The laser linewidth and tuning range are optimized accordingly. The modulation of the fiber laser, achieved through pump source modulation and a locking detection mechanism, has been utilized to stabilize the laser system and therefore to create a compact gas sensor with high sensitivity. The absorption spectrum, as well as the line strength and the concentration level of CO2, have been monitored through absorption spectroscopy techniques. The measured minimum detectable concentration of CO2 obtained using the system shows that it is quite capable of detecting trace gas at the ppm (parts in 10(6)) level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact, yet sensitive fiber-laser-based gas sensor systems.

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.

Vapor Phase Doping of Rare-Earth in Optical Fibers for High Power Laser

This letter describes an optimized vapor phase doping technique using modified chemical vapor deposition system to fabricate rare-earth doped optical fibers for a high power laser. The process comprises deposition of aluminium oxide and ytterbium oxide in vapor phase simultaneously in combination with silica during formation of the core layer. The process parameters have been judiciously controlled to deliver aluminium chloride and rare-earth-chelate compounds to the reaction zone without decomposition and/or condensation of the precursor materials prior to the reaction zone. The standardization of the process parameters resulted in a good repeatability with a very low variation of dopant concentrations throughout the length of the preform. The fabricated fibers exhibit good optical properties with lasing efficiency of 76% at 1.06 μm.

Empirical Relations for Design of Linear Edge Filters Using Apodized Linearly Chirped Fiber Bragg Grating

Apodized linearly chirped Bragg grating (CFBG) filters have been studied with a view of determining an optimal set of grating parameters to design and fabricate linear edge filter for Bragg grating sensor demodulation. A rigorous numerical computation towards understanding the relation of all the grating parameters with filter characteristics like the linear bandwidth and slope efficiency produced some simple empirical formula for the design of edge filters of specific desired characteristics. The results are corroborated with experiments.

An Optimized Vapor Phase Doping Process to Fabricate Large Core Yb-Doped Fibers

The paper demonstrates a standardized process of vapor phase doping to fabricate large core Yb-doped preforms with longer useful length in reproducible manner. The optimization of the process led to successful achievement of Yb-doped core thickness of 4.5 mm (in 14.8 mm of preform diameter) by depositing up to 30 number of core layers with controlled amount of generated precursor vapors. The influence of the process parameters was studied rigorously to enhance the useful preform length up to 380 mm. A combination of Yb and Al in different proportions was doped into the core with uniform dopant concentration along the length by adjusting few process parameters efficiently. The Al2O3 concentration up to the level of 17.8 mol% has been achieved successfully which resulted in NA of 0.31. This is the highest ever doping of Al in passive fibers by any modified chemical vapor deposition process. The Yb2O3 content in the active fibers is as high as 0.47 mol%.

Generation of supercontinuum and its theoretical study in three-ring silica microstructured optical fibers.

We report supercontinuum generation in nonlinear microstructured optical fibers (MOFs) especially fabricated in a two-step stack and draw process having three rings of airholes. High air-filling fraction (>0.9) is obtained in a simple and straightforward way during the drawing process which is essential to enhance nonlinearity. Two of the fabricated samples are characterized and zero dispersion wavelength is tailored to achieve efficient pumping in the anomalous group velocity dispersion regime. The characteristics of the supercontinuum band as observed experimentally show good agreement with the predicted numerically simulated results, where soliton mediated dispersive waves are distinctly observed.

Large core Yb-doped optical fiber through vapor phase doping technique

Rare earth (RE) doped optical fibers have shown tremendous progress for producing high power fiber lasers for industrial, medical and strategic applications. However, fabrication of large core, high Yb-doped fiber is still a challenge through conventional process due to poor repeatability and limitation regarding core size. This paper presents successful fabrication of Yb-doped fibers through vapor phase doping technique. Preform fabrication was carried out using a specially constructed MCVD system containing High Temperature Vapor Delivery Unit with sublimators for Al and Yb precursors. The novelty of the present work lies in deposition of Al2O3 and Yb2O3 in vapor phase simultaneously with silica during formation of sintered core layer which result in uniform dopants distribution in the preform. The fibers exhibited lasing efficiency of 76% with low ‘photodarkening effect’.