All the sensors are fabricated in specialized in-line technique. The fundamental of that technique is the fiber with a number of peculiar optical and mechanical characteristics. The fiber maintains its optical guiding ability under high elastic or even plastic deformations. This makes possible the fabrication of various fiber optic components directly on a fiber length by shaping it at high temperatures when quartz glass becomes soft. The sequent fabrication of the ring interferometer components (couplers, polarizer, SLD module) on a single fiber length makes them naturally connected without optical loss. For fiber shaping, an fusion-tapering technique and equipment were developed. During the couplers fabrication process the two fiber leads are installed together and held by two moveable holders. A stabilized high-frequency arc discharge is applied to the fibers so that they melt together. Simultaneously, the two fiber holders are moved apart so that a fused tapered region is formed. To ensure a low-loss coupler (polarizer), it is essential that the holders move apart in a straight line with no sideways motion or vibration. The speed of separation and heating length control the shape of the resulting taper and this also has a significant influence on the resulting loss. The arc-flame is of particular careful consideration. It is necessary to use an optimal arc length and arc current not to disturb the taper. The quality of the single-mode fiber is extremely important. The core and cladding must be highly circular and concentric with one another. Inferior quality fibers can result in high losses in the resulting coupler. It is also possible to monitor the coupler’s power-splitting ratio during fabrication and to make a coupler with any required splitting ratio at a given wavelength. The fabrication of the polarizer begins with a similar tapering process, with bigger elongation to achieve the waist diameter about 5 microns. After that, the fiber waist is placed into melt material from which the birefringent crystal is grown in such a way that the fiber is buried in the middle of the crystal body. The taper length and size of the crystal determine the polarizer extinction ratio. To build SLD-module the soldering process is used. SLD crystal and fiber lead are soldered each to the separate copper blocks. The blocks are soldered one to another at lower temperature after precise mutual alignment. Both major techniques (fusion and soldering) produce temperature and mechanically stable components that bring to the sensor reliable and stable performances in a wide range of various environments. Just after fabricating all optical components are mounted inside the sensor’s case and covered with protecting silicon gel. Electronics is mounted on the top cover of the sensor for a better thermal conductivity to the ambient media.
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