Optical Whispering Gallery Mode (WGM) microresonators are an ideal platform for miniature and high sensitive sensors. They exhibit very small mode volumes and extremely high Q factors of the order of 106 - 109 providing a platform with low nonlinearity thresholds and low detection limits to environmental changes. Here we present and discuss some of the WGM-based applications we have been developing in our lab. Specifically, we present a WGM strain sensor based on a silica stretchable microsphere, with a Q factor of 108 . Tension stress is applied along a microsphere with two fiber tails by pulling on one tail. Consequently, the microsphere's WGMs are shifted and the measurement may serve as a strain sensor. Based on this strain sensor, a WGM magnetic sensor is suggested where the mechanical response of a magnetostrictive material to a varying magnetic field applies the tension stress. In addition, a narrow linewidth laser source for these sensors is described here as well, where a multimode diode laser is locked to a single WGM of the microsphere via self-injection feedback from the microsphere. This optical locking is accompanied with a substantial spectral narrowing of the multimode diode laser.
A variety of optical components are fabricated by the so called “Heat and Pull” technique in which optical fibers are fused and tapered. A numerical model simulating the temporal and spatial material distribution in such components is presented and validated by comparison with experimental results.
Over the years, numerous models and tools have been developed to simulate the optical behavior of fused fiber-optic components. While these models are well established, their predictions depend on accurate knowledge of the component’s physical structure and its refractive index distribution. Unfortunately, no such generic simulation tools are readily available. The need of a high fidelity structural simulation tool for such components is further emphasized in complex systems, which are difficult to fabricate and are optically sensitive to small structural variation. In view of the above, we developed a novel numerical methodology based on Immersed Boundary (IB) Method specifically designed to simulate flows in the presence of complex geometries and moving boundaries. In the present formulation pressure and interface curvature are implicitly embedded into the system of incompressible Navier-Stokes equations as distributed Lagrange multipliers. The developed methodology is currently capable to simulate two phase flows in two dimensions and is also adapted to solve quasi-3D evolutions. For validation, the simulation output is compared to the cross-sectional material distribution of a real component fabricated at our lab. The developed model, as well as the experimental results and the comprehensive analysis predicting the structure of symmetric and non-symmetric optic fiber components are presented and discussed.
A study of the various channels of energy transfer from the upper lasing level of Nd:YVO4 following direct pumping at 880nm is presented. The dependency of the heat that was generated in the laser crystals as a function of doping concentrations and different pumping and lasing conditions was measured along with the laser performance. In the absence of a laser resonator, the heat to pump power relation behaved linearly and was strongly dependent upon the Nd+3 doping concentration. During lasing at 1064nm, the heat to pump ratio dropped dramatically but still did increase linearly with growing concentrations. The heat to pump dependency upon different output couplers with the same laser crystal was ~linear too and increased with increasing intra-cavity power density. These experimental results suggest that different mechanisms govern the heat creation before and after laser threshold. Cross relaxation on top of the Stokes shift seems to govern heat creation before threshold, while excited state absorption and crystal impurities may be the mechanisms that govern the excess heat creation above threshold.
We have demonstrated the feasibility of cooling high power solid-state lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than sapphire's, the material conventionally used for this purpose. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. An output power of 200Watt was achieved from a single Nd:YVO4 slab in a zigzag configuration when pumped with 600Watt diodes at 808nm. The maximum output power previously reported in the literature with Nd:YVO4 using conventional cooling schemes is only about 100W. A 2.3x4x24mm3 slab was pumped from its broad side (4x24 mm2) through a 0.3mm thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. The output of a two-head configuration was 295Watt. By using a RTP Q-switch, 124Watt average power was obtained at 15kHz with a pulse width of 17nsec, pumping at 650Watt.
An additional larger head was developed to pump a Nd:YAG slab. The concept of the pumping and cooling is identical to the Nd:YVO4 laser head. An output power of 1000Watt was achieved from a single Nd:YAG slab when pumped with 2500Watt diodes at 808nm. The slab dimensions are 3×12×90mm3.
An overview of the ongoing research taking place in our laboratory comparing direct and traditional pumping is given. It includes both Nd:YAG and Nd:YVO4 pumping with either Ti:Sapphire or diode lasers as the pumping source. Latest results addressing basic quantities connected with the pump-lase cycle in Nd:YAG lasers will be presented in detail. By comparing heat generation and laser performance of Nd:YAG oscillators pumped via two channels - direct pumping to the upper lasing level at 885nm and band pumping around 808nm, it was found that the heat generated during lasing is 27% lower with direct pumping as compared to traditional band pumping. Moreover, the experimental results suggest that the coupling efficiency between the pump band and the upper lasing level is unity, and about 8% of the upper lasing level population decays via non-radiative channels.
Heat generation and laser performance were studied in Nd:YAG oscillators pumped with a Ti:Sapphire laser in two regimes: band pumping at 802nm and direct pumping at 885nm. Slope efficiencies of 52% and 57%, when pumped at 802nm and 885nm, were obtained, respectively. Heat per unit laser output was found to be 27% lower when pumped at 885nm (direct pumping regime) as compared to traditional band pumping around 808nm.
We have demonstrated the feasibility of cooling high power solid-stae lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than sapphire's. An output power of 200Watt was achieved froma single Nd:YVO4 slab in a zigzag configuration when pumped with 600Watt diodes at 808nm. The maximum output power reported in the literature with conventional cooling schemes is about 50W. A 2.3x4x24mm3 slab was pumped from its broad side (4x24 mm2) through a 0.3mm thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. By using a BBO Q-switch, 70Watt average power was obtained at 20kHz with a pulse width of 19msec and with a beam quality of 3 and 12 times diffraction limit in the zigzag and transverse directions respectively. The output of a two-head configuration was 295Watt.
We report a diode pumped 2μm Tm:YAG laser operating in a pulsed mode, with an RTP Pockels cell as the Q-switch generator. A maximum output of 2.4mJ with a pulse width of 57ns was achieved. The crystal was pumped with an input energy of 85mJ at a rate of 20Hz. To the best of our knowledge, this is the first time that RTP was used as the Q-switch generator in a 2μm laser resonator.
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