Design of a polymer-based cladding mode stripper with a distributed temperature profile.

An optimized polymer-based fiber cladding mode stripper (CMS) is presented experimentally. A unique partial fiber stripping method is introduced in CMS fabrication to enable distributed power extraction and temperature along the length of the CMS. The designed CMS based on a 400 µm double-clad fiber (DCF) extracts 300 W of inner cladding power, with an attenuation coefficient of 18.8 dB along with a temperature gradient of 0.17°C/watt. The fabricated CMS is used to develop an all-fiber 150 W thulium fiber laser at 1.94 µm.

Ex vivo testing of air-cooled CW/modulated 30 W thulium fiber laser for lithotripsy.

A diode-pumped, air-cooled, all-fiber, quasi-continuous-wave thulium laser at an operating wavelength of 1.94 µm has been designed to study the performance of the laser parameter on the rate of fragmentation and its dependence on stone composition, fragmented particle size, as well as the retropulsion effect. The optimized laser cavity with an active fiber core/cladding diameter of 10/130 µm under a counter-propagating pump provides a stable laser power of 30 W at a slope efficiency of 50% and wall plug efficiency of 17%. The rate of fragmentation along with the retropulsion effect has been studied with human calcium oxalate monohydrate (COM) urinary stones (N=36) of different composition by using the designed laser and 200-µm-core low OH silica delivery fiber. The thulium fiber laser setting of 2.7 J pulse energy at the pulse rate of 10 Hz, pulse width of 90 ms, and peak power of 30 W is successful in breaking human COM stones in a controlled manner at a fragmentation rate of 0.8±0.4 mg/s, with almost uniform fragments of particle size less than 1.6 mm. During the stone fragmentation, the stone displacement (retropulsion effect) is less than 15 mm, even for the fragmented stone mass of 15±5 mg.

Hybrid pumped gain-switched thulium fiber laser at a high repetition rate.

A gain-switched all-fiber thulium laser at 2 µm with high repetition rate has been demonstrated under a hybrid pumping scheme combined of a pulsed pump at 1.56 µm and CW pump at 793 nm. The in-band pulsed pump at 1.56 µm triggers the gain-switching pulses while the CW pump at 793 nm facilitates the energy storage. Therefore, the seed cavity delivers high energy pulses allowing the elimination of multistage amplification. Such hybrid pump configuration is effective for generating gain-switched pulses of high average power with better slope efficiency and pulse width of a few hundreds of a nanosecond. The optimized cavity under such hybrid pump configuration provides output power of 5.92 W from the gain-switched oscillator with slope efficiency of 60% and pulse width of 300 ns at a repetition rate of 344 kHz. Maximum pulse energy of 17.2 µJ and peak power of 53.9 W has been achieved at this repetition rate. Stable gain-switched pulses at reduced pump pulse energy have been achieved by the use of a CW pump at 793 nm. This novel pump configuration facilitates gain switch at higher repetition rates where enough pump pulse energy may not be available. The gain-switched laser also operates at 520 kHz and 1.3 MHz repetition rate by changing the 1.56 µm pulsed pump and cavity length.

Diverse mode of operation of an all-normal-dispersion mode-locked fiber laser employing two nonlinear loop mirrors.

In this paper, we propose an all-normal-dispersion ytterbium-fiber laser with a novel ring cavity architecture having two nonlinear amplifying loop mirrors (NALM) as saturable absorbers, capable of delivering distinctly different pulses with adjustable features. By optimizing the loop lengths of the individual NALMs, the cavity can be operated to deliver Q-switched mode-locked (Q-ML) pulse bunches with adjustable repetition rates, mode-locked pulses in dissipative soliton resonance (DSR) regime or noise-like pulse (NLP) regime with tunable pulse width. The DSR pulses exhibit characteristic narrowband spectrum, while the NLPs exhibit large broadband spectrum. The operation regime of the laser can be controlled by adjusting the amplifier pump powers and the polarization controllers. To the best of the authors' knowledge, this is the first demonstration of a single mode-locked cavity where narrowband DSR pulses and broadband NLPs alongside Q-ML pulse bunches can be selectively generated by employing two NALMs.

High repetition rate gain-switched 1.94 µm fiber laser pumped by 1.56 µm dissipative soliton resonance fiber laser.

A dissipative soliton resonance (DSR) mode-locked Er:Yb fiber laser has been used to pump a thulium fiber laser to generate gain-switched pulses at high repetition rates. Here 412 ns long DSR pulses with a center wavelength of around 1.56 µm at a repetition rate of 410 kHz have been fed to a thulium fiber laser, resulting in generation of gain-switched pulses at 1.94 µm. The minimum pulse width achieved was 256 ns with an average power of 4.6 W at 66% slope efficiency. Gain-switched pulses at 520 kHz and 750 kHz were generated through changing the pump pulse repetition rate by modifying the DSR cavity. To the best of our knowledge, this is the first demonstration of a high repetition rate gain-switched thulium fiber laser pumped by a DSR mode-locked fiber laser. As DSR pulses can be generated with high seed average power and energy independent of the operating wavelength regime as well as mode-locking technique, the proposed method can be applied to generate gain-switched pulses at high repetition rates and various wavelengths without the need of any optical or electrical modulators.

Continuous-wave and quasi-continuous wave thulium-doped all-fiber laser: implementation on kidney stone fragmentations.

A continuous-wave (CW) as well as quasi-continuous wave (QCW) thulium-doped all-fiber laser at 1.94 µm has been designed for targeting applications in urology. The thulium-doped active fiber with an octagonal-shaped inner cladding is pumped at 793 nm to achieve stable CW laser power of 10 W with 32% lasing efficiency (against launched pump power). The linear variation of laser power with pump offers a scope of further power scaling. A QCW operation with variation of duty cycle from 0.5% to 90%, repetition rate from 0.1 Hz to 1 kHz, and pulse width from 40 µs to 2 s has been presented. Laser power of 9.5 W in CW mode of operation and average power of 5.2 W with energy range of 10.4-104 mJ in QCW mode of operation has been employed to fragment calcium oxalate monohydrate kidney stones (size of 1.5-4 cm) having different colors and composition. Dependence of ablation threshold, ablation rate, and average fragmented particle size on the average power and energy has been studied. One minute of laser exposure results in fragmentation of a stone surface with ablation rate of 8 mg/min having minimum particle size of 6.54 µm with an average size of 20-100 µm ensuring the natural removal of fragmented parts through the urethra.

"All-fiber" tunable laser in the 2 µm region, designed for CO2 detection.

A stable and tunable thulium-doped "all-fiber" laser offering a narrow linewidth has been created specifically to act as a compact and simple laser source for gaseous CO(2) detection. This has been done through a careful design to match the laser output wavelengths to the CO(2) absorption lines at 1.875 and 1.997 µm, respectively. A sustainable output power of 11 mW over a tuning range of 7 nm has been obtained by using a combination of a high-reflective fiber Bragg grating with a low-reflective broadband mirror, fabricated at the end of the fiber through silver film deposition. The tuning was achieved using the relaxation-compression mechanism of the fiber Bragg grating, which formed an integral part of the laser resonant cavity. A fiber Bragg grating at 1.548 µm was utilized as a wavelength reference to monitor the tuning of the laser output over the 2 µm wavelength range with a simple and inexpensive interrogator, to avoid the use of an expensive optical spectrum analyzer and to facilitate "in-the-field" operation. This "all-fiber" laser resonator has been shown to be superior in terms of laser tuning range, output power, and linewidth compared to that created with a fiber Bragg grating pair, which was limited by the nonuniform strain transfer to both fiber Bragg gratings.

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.

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.

RecA-mediated cleavage of lambda cI repressor accepts repressor dimers: probable role of prolyl cis-trans isomerization and catalytic involvement of H163, K177, and K232 of RecA.

The lambda cI repressor is found to be cleaved in the presence of activated RecA in its DNA-bound dimeric form at a rate similar to that in the absence of operator DNA in contrast to previous studies inferring repressor monomer as a preferred substrate. Though activated RecA does not possess any measurable isomerase activity against a standard peptide substrate, prolyl isomerase inhibitors cyclosporin A and rapamycin do inhibit RecA-mediated cleavage. Histidine and lysine to a smaller extent, are shown to cleave cI repressor in a non-enzymatic fashion whereas arginine and glutamate do not. When activated RecA filament is covalently modified by using an excess of diethyl pyrocarbonate or maleic anhydride, RecA-mediated cleavage of cI repressor is inhibited. Combining our chemical modification data with model building and earlier mutagenesis data, it is argued that H163, K177, and K232 in RecA are crucial residues involved in cI repressor cleavage by combining with the catalytic Ser149 and K192 in the repressor. It is suggested by model building that subunits n, n+4, and n+5 in the RecA filament contribute one loop each for holding the C-terminal domain of the repressor during cleavage within the RecA helical groove, explaining why its ADP-form is inactive and its ATP-form is active regarding repressor cleavage.

Digestion of the lambda cI repressor with various serine proteases and correlation with its three dimensional structure.

Partial proteolysis of the lambda cI repressor has been carried out systematically with trypsin, chymotrypsin, elastase, endoproteinase Glu-C, kallikrein, and thrombin. The cleavage sites have been determined by (i) comparison of fragments produced and observed in SDS-polyacrylamide gel with known fragments and plots of distance migrated versus log (molecular weight of fragment), (ii) partial Edman sequencing of the stable C-terminal fragments to identify cleavage points, and (iii) electrospray mass spectrometry of fragments produced. Most cleavage points are found to occur in the region 86-137, saving some in the N-terminal domain observed for trypsin and Glu-C. Region 86-137 can be further subdivided into three regions 86-91, 114-121, and 128-137 prone to cleavage, with intermediate regions resistant to cleavage to all six proteases. These resistant regions show that much of the region 93-131 previously called a 'linker' is actually part of the C-domain as first proposed in all models from our laboratory. Region 92-114 includes the cleavage site Ala-Gly, which must be buried in the intact repressor. The observed cleavage points in region 114-137 can be used to judge the best among three previously proposed models since they differ from each other in the structure of region 93-131. Model 1j5g is adjudged to be better than model 1lwq (which is based on 1kca, a crystal structure) as susceptible residues are more exposed in the former and lack of cleavages at six sites is better explained. Likewise, the models 1j5g and 1lwq are compared with a recent crystal structure of fragment 101-229 in 2ho0 and another low resolution crystal structure in 3bdn.

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.

Improved model of a LexA repressor dimer bound to recA operator.

A complete three dimensional model for the LexA repressor dimer bound to the recA operator site consistent with relevant biochemical and biophysical data for the repressor was proposed from our laboratory when no crystal structure of LexA was available. Subsequently, the crystal structures of four LexA mutants Delta(1-67) S119A, S119A, G85D and Delta(1-67) quadruple mutant in the absence of operator were reported. It is examined in this paper to what extent our previous model was correct and how, using the crystal structure of the operator-free LexA dimer we can predict an improved model of LexA dimer bound to recA operator. In our improved model, the C-domain dimerization observed repeatedly in the mutant operator-free crystals is retained but the relative orientation between the two domains within a LexA molecule changes. The crystal structure of wild type LexA with or without the recA operator cannot be solved as it autocleaves itself. We argue that the 'cleavable' cleavage site region found in the crystal structures is actually the more relevant form of the region in wild-type LexA since it agrees with the value of the pre-exponential Arrhenius factor for its autocleavage, absence of various types of trans-cleavages, difficulty in modifying the catalytic serine by diisopropyl flourophosphate and lack of cleavage at Arg 81 by trypsin; hence the concept of a 'conformational switch' inferred from the crystal structures is meaningless.

pH-dependent autocleavage of lambda repressor occurs in the operator-bound form: characterization of lambda repressor autocleavage.

The first-order rate constants for the RecA-independent, spontaneous, pH-dependent autocleavage of the lambda cI repressor was measured in the present study at pH 10.6 at 27, 37 and 42 degrees C respectively. Autocleavage of the repressor occurs also at pH 9 and 8, although at progressively slower rates. We demonstrate that the spontaneous autocleavage occurs also in the operator-bound state, at a rate either higher than or equal to the rate in solution, depending on the pH value. Owing to the near equality of the rate constant in both operator-free and operator-bound repressors, it can be inferred that the cleavage site has a similar structure and dynamics with respect to the catalytic site in both forms at neutral pH. Covalent modification using PMSF, brought about by a large molar excess of the reagent, inhibits autocleavage of the lambda repressor. The difficulty in obtaining this covalent modification is rationalized using our recent lambda repressor models. Bimolecular type II trans -cleavage was observed previously for mutant LexA repressors lacking a crucial catalytic serine or lysine residue [Kim and Little (1993) Cell (Cambridge, Mass.) 73, 1165-1173], but it could still be cleaved by an 85-202 'enzyme' fragment possessing an improved or hypercleavable character lacking its own cleavage site. Such a type II trans -cleavage was not observed with the covalently modified intact lambda repressor used as substrate and the purified wild-type lambda repressor 112-236 fragment used as the 'enzyme'. All these results show that for the wild-type lambda repressor, the catalytic site is close to the cleavage site in both operator-free and -bound states. In the lytic pathway, the repressor is mainly cleaved via RecA-mediated cleavage, which occurs much faster than the spontaneous autocleavage; the possible biological significance of this slow, spontaneous, but constant, autocleavage is related to the lysogenic state, when RecA-mediated cleavage is absent.