Narrow-linewidth broadly tunable Yb-doped Q-switched fiber laser using multimode interference filter.

A narrow-linewidth broadly tunable Yb-doped Q-switched fiber laser using an acousto-optic modulator and multimode interference filter (MMIF) in the linear bulk cavity resonator and an all-fiber ring cavity resonator has been demonstrated. Insertion of an MMIF in the linear cavity resonator using bulk components decreased the spectral bandwidth of the Q-switched signal by two orders of magnitude from 11 to less than 0.1 nm. Spectral tunability of more than 16 nm in the range from 1057 to 1073 nm has also been achieved by the combination of MMIF and a standard polarization controller (SPC). A decrease in the pulse duration with a decrease in the spectral bandwidth of the output signal has also been recorded. The pulse duration of the Q-switched signal was reduced from ∼305 to ∼240 ns by the introduction of the MMIF in the resonator at the same value of the input pump power. In the case of the all-fiber Q-switched ring cavity resonator, the spectral bandwidth of the Q-switched signal was reduced by two orders of magnitude from ∼17 to less than 0.1 nm due to the introduction of the MMIF in the resonator. The spectral tunability of more than 12 nm in the range from 1038 to 1050 nm was achieved by an MMIF and an SPC.

Modular pump head design of diffused, metal, and hybrid pump geometry for diode-side-pumped high power Nd:YAG laser.

In this paper, we present a comparative study on pump heads for a high power diode-side-pumped Nd:YAG laser. The pump head is a modular type, which is in the form of discs, with each disc holding three pump diodes kept at 120° with respect to each other. Unabsorbed pump light from the active medium is reflected by reflectors mounted adjacent to the pump diodes. The performance of a high power pump head made of modular discs mounted with specular or diffused type reflectors was studied. Hybrid pump geometry was also investigated, where the pump head is made up of discs loaded with metal and diffused reflectors, alternately. The discs are loaded around the active medium in such a way that successive discs are rotated by sixty degrees with respect to each other. Fluorescence profiles, thermal lensing, laser output power, and M2 values were studied for pump heads made up of metal, diffused, and hybrid type reflectors. All of the pump heads were studied for three different resonator lengths to maximize the output power with the best beam quality. The experimental results show that the diffused reflector-based geometry in a sixty degree rotated configuration produced the maximum output power and best beam quality in terms of the M2 value.

Generation of more than 40 W of average output power from a passively Q-switched Yb-doped fiber laser.

We report on the generation of 41.6 W of average output power from a passively Q-switched ytterbium-doped fiber laser using Cr4+:YAG crystal as a saturable absorber (SA). This is the highest average power from passively Q-switched fiber lasers reported so far in the literature, to our knowledge, and it has been achieved by using a specially designed T-type double-end pumping configuration. Variation in average output power, pulse energy, pulse duration, pulse frequency, and pulse-to-pulse stability has also been studied using SAs of different linear transmissions. The effect of an intracavity SA on self-pulsing dynamics was also investigated and it was observed that, at lower input pump power near threshold, the presence of an SA enhances the peak power of relaxation oscillations to trigger the generation of stimulated Raman scattering in the gain fiber. With an increase in pump power, when the passive Q-switching threshold is reached, high peak power random self-pulses regenerate into low amplitude regular Q-switched pulses. The effect of the length of the gain medium on dual-wavelength generation at very low input pump power and broadband generation at sufficiently higher pump power has also been explored.

Modular pump geometry for diode side-pumped high-power Nd:YAG rod laser.

In this paper, we present a modular design configuration to side-pump an Nd:YAG rod of a diode-pumped high-power CW laser. Independent modules in the form of discs are loaded with three-diode linear bar arrays at 120° with respect to each other and are assembled along the length of the rod. By keeping successive discs at three different angular separations (the minimum, intermediate, and maximum), helical, 60° rotated, and linear geometries, respectively, were created. Parameters like thermal lensing, fluorescence profile, laser beam profile, and beam quality factor (M2) were studied to compare the performances of different geometries. The results indicate that the 60° rotated configuration provides the best performance when compared with rest of the schemes. To the best of our knowledge, this is the first time that such novel modular-type pump geometries with a provision for the angular separation of pump diodes have been tried.

Demonstration of CW mode locked Cr:forsterite laser using self-shortening and transverse mode degeneracy driven mode locking.

This paper reports a nearly Fourier transform limited CW mode locked Cr:forsterite laser at 1282 nm, with 131 fs pulse duration, based on self-shortening and transverse mode degeneracy (TMD) driven mode locking, operating near the point of fourth-order TMD. The cavity employs a combination of instantaneous intensity driven self-shortening, and operation on the right side of the fourth-order TMD, to generate the self-amplitude modulation necessary for self-mode locking.

Studies on optical limiting characteristics of silicon nanoparticles synthesized by laser ablation.

We present synthesis of silicon nanoparticles dispersed in toluene by laser ablation and studies on their optical limiting properties with nanosecond laser pulses at 532 nm. Silicon nanoparticles in toluene show better optical limiting compared to standard optical limiter fullerene C60 in toluene. Optical limiting threshold of silicon nanoparticles is about three times less than that of C60. Detailed studies using Z-scan experiments, angle dependent scattering, intensity dependent transmission and temporal profile measurements indicate that apart from non-linear scattering, nonlinear absorption and nonlinear refraction also contribute to the optical limiting behavior of silicon nanoparticles.

Investigation of optical limiting in iron oxide nanoparticles.

We present the study of optical limiting in iron oxide nanoparticles of diameters 31, 44, and 61 nm dispersed in toluene under exposure to nanosecond laser pulses at 532 nm. In the low fluence region smaller size nanoparticles show better optical limiting compared to larger size nanoparticles while in the high fluence region all the three samples show same limiting performance. Experimental results were compared with the well reported limiter fullerene C(60) dissolved in toluene. Iron oxide nanoparticles show better optical limiting compared to C(60) in the intermediate fluence region and comparable performance in the high fluence region. The pico-second Z-scan studies indicate that the contribution of electronic nonlinear refractive index and the two-photon absorption to the optical limiting is negligible. Our observations further indicate that the dominant mechanism for the optical limiting in iron oxide nanoparticles is nonlinear scattering.

A sensitive and high dynamic range cw laser power meter.

We report the design of a cost effective, highly sensitive cw laser power meter with a large dynamic range based on a photodiode. The power meter consists of a photodiode, a current to voltage converter circuit, an offset balancing circuit, a microcontroller, an analog to digital converter, reed relays, and an alphanumeric liquid crystal display. The power meter can record absolute laser power levels as low as 1 pW. The dynamic range measured with a cw laser at a wavelength of 532 nm is 8x10(10). The high sensitivity and large dynamic range are achieved by the implementation of an analog background balancing circuit and autoranging.

Abnormal haemoglobin levels in acute coronary syndromes.

BACKGROUND: Anaemia is an adverse prognostic marker in acute coronary syndromes (ACS), but the epidemiology of abnormal haemoglobin levels in such patients is uncertain. AIMS: To investigate the prevalence, nature and predictors of abnormal haemoglobin levels in ACS patients at admission. DESIGN: Observational study. METHODS: All emergency admissions from January to April 2005 were assessed within 24-48 h of hospital admission. ACS patients (unstable angina, non-ST-elevation or ST-elevation myocardial infarction) were enrolled (n = 320, 190 men). Clinical information was recorded. RESULTS: Overall, 71% had unstable angina; 18% non-ST-elevation myocardial infarction (MI), and 11% ST-elevation MI. Mean +/- SD haemoglobin was 14.3 +/- 1.7 g/dl in men and 13.2 +/- 1.5 g/dl in women. Abnormal haemoglobin was more common in men (65, 34%) than in women (34, 22%) (p = 0.013). Anaemia (haemoglobin <13 g/dl in men, or <12.0 g/dl in women) was recorded in 35 (18%) men and 24 (18%) women. All had admission haemoglobin >8 g/dl, and anaemia was usually normocytic. Multivariate predictors of anaemia (OR, 95%CI) were age (1.07, 1.04-1.1) and serum albumin (0.90, 0.81-1.00). Elevated haemoglobin (>16 g/dl) was recorded in 30 (16%) men and 4 (3%) women (p < 0.01), and was more common in ST-elevation MI patients (26%) than in unstable angina or non-ST elevation MI patients (9%) (p = 0.005). In patients who underwent invasive management with a post-procedure haemoglobin the following day (n = 85), 15 (18%) new cases of anaemia were detected. Admission duration correlated with haemoglobin (p < 0.01), creatinine (p < 0.01), troponin I (p < 0.01) and C-reactive protein (p < 0.01). Anaemia was more common in those who died in hospital (3, 60%) than in those who survived (56, 18%) (adjusted p = 0.0135). DISCUSSION: Abnormal haemoglobin levels were common in our ACS admissions. Anaemia was generally mild. Increasing age and interventional management were associated with anaemia, which in turn was associated with adverse in-hospital outcomes. Interventions to prevent and detect anaemia in this setting merit prospective testing.