We present the design and initial investigation of a fibre
optical system which may be used both for intra-cavity and for ring-down
measurements of absorption losses. The system consists of a fibre loop
containing a length of erbium-doped fibre pumped at 980 nm, with gain
adjustment below or above threshold for the two types of operation. The
fibre loop is constructed from standard fibre optical components and
includes a micro-optical gas cell. The intended application is for
measurement of levels of trace gases which possess near-IR absorption lines within
the gain bandwidth of the erbium fibre amplifier. We discuss the key issues
involved in operation of the system and the level of sensitivity
required. Our initial experimental investigations have demonstrated that
ring-down times of several microseconds can be obtained, which can be
altered through adjustment of the attenuation or gain factor of the loop.
Gain control is one of the most important issues and we explain how this
may be achieved.
We study linear wave propagation in the case of equal-mass plasmas. We show that the special symmetry of such plasmas simplifies the problem and causes the disappearance of well-known phenomena such as Faraday rotation and whistler wave modes. We exploit the Symmetry of the problem to derive analytical expressions for wave propagation for various fluid models of the plasma. We also find which distribution functions will retain the symmetry properties of the fluid models.
A new methodology for the development of miniature photoacoustic trace gas sensors using 3D printing is presented. A near-infrared distributed feedback (DFB) laser is used together with a polymer-based gas cell, off-the-shelf fiber optic collimators, and a microelectromechanical system (MEMS) microphone to measure acetylene at 1532.83 nm. The resonance behavior of the miniature gas cell is analyzed using a theoretical and experimental approach, with a measured resonance frequency of 15.25 kHz and a Q-factor of 15. A minimum normalized noise equivalent absorption of 4.5×10(-9) W cm(-1) Hz(-1/2) is shown together with a 3σ detection limit of 750 parts per billion (ppb) for signal averaging times of 35 s. The fiber-coupled delivery and miniature cost-effective cell design allows for use in multipoint and remote detection applications.
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