Raman detection of nitrogen gas is very difficult without a multi-pass arrangement and high laser power. Hollow-core
photonic bandgap fibers (HC-PBF) provide an excellent means of concentrating light energy in a very small volume and
long interaction path between gas and laser. One particular commercial fiber with a core diameter of 4.9 microns offers
losses of about 1dB/m for wavelengths between 510 and 610 nm. If 514nm laser is used for excitation, the entire Raman
spectrum up to above 3000 cm-1 will be contained within the transmission band of the fiber. A standard Raman
microscope launches mW level 514nm laser light into the PBF and collects backscattered Raman signal exiting the fiber.
The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about
several thousand over what is attainable with the objective in air and no fiber. The design and fabrication of a flow-through
cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen.
The enhancement was also found to be a function of fiber length. Due to the high achieved Raman signal, rotational
spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge.
We investigate a self-aligning method used to couple a vertical emitting laser (flip-chip) to a planar single-mode waveguide through a 45° mirror based upon solder self-alignment. The alignment tolerances to achieve targeted coupling loss of 3.5dB or better were determined for all axes by modeling the optical behavior of the vertical waveguide/45° mirror interconnect. Simulation models for optical design are carried out using commercial software "FullWAVE" from Rsoft, Inc. The design and optimization of the joint's parameters are performed using a public-domain software "Surface Evolver" for surface energy minimization in conjunction with GE proprietary Six Sigma regression and optimization tools. The parameters that were considered for the models included the misalignment along all axes, solder volume, the height of the joints, the radius of the metallization pads, the initial placement error, and the solder reflow time. It has been assumed that intermetallic growth at the solder/metallization/air triple line does not couple with the self-alignment process in order to simplify this problem. The study shows that, given good control over noise parameters such as vibration and reflow temperature fluctuations, solder self-alignment can be harnessed to achieve the targeted coupled efficiency.