Abstract: An apparatus for sensing a value of a property in a borehole having hydrogen gas penetrating a subsurface formation includes an optical fiber configured to be disposed in the borehole having the hydrogen gas and comprising a core having a fiber Bragg grating that is responsive to the value of the property and a cladding disposed about the core, wherein (i) the core is doped with a first dopant that is photo-sensitive for writing the fiber Bragg grating and that has a concentration in the core of 2 Mole % or less and (ii) the cladding is doped with a second dopant that lowers an index of refraction of the cladding.

Abstract: An apparatus for sensing a value of a property includes: an optical sensor having a single mode optical fiber responsive to the property; an optical interrogator having a tunable laser to transmit polarized light to the optical sensor, a photo-detector to receive sensor light, and a controller configured to process the received light and output the value of the property; and a passive random depolarizer disposed between the tunable laser and the single mode optical fiber and having (i) a first polarization maintaining (PM) optical fiber of length L1 having a first fast optical axis and a first slow optical axis and (ii) a second PM optical fiber of length L2 having a second fast optical axis and a second slow optical axis rotationally spliced to the first PM optical fiber in which the second fast and slow optical axes are offset from the first fast and slow optical axes.

Abstract: An apparatus for sensing pressure and temperature includes: a hollow glass tube; a first optical fiber having an end disposed into a first end of the tube; a second optical fiber being disposed in a second end of the tube, the second optical fiber having a first solid core section followed by a hollow core section followed by a second solid core section, a first gap formed between the first and second optical fibers, a length of the first solid core section forming a second gap, and a length of the hollow core section forming a third gap; an optical interrogator that transmits light at various wavelengths and measures an intensity of reflected light due to the first gap, second gap, and third gap as a function of frequency to provide interrogation data; and a processor that matches the interrogation data to reference data to estimate the pressure and/or temperature.

Abstract: An apparatus for sensing pressure and temperature includes: a hollow glass tube; a first optical fiber having an end disposed into a first end of the tube; a second optical fiber being disposed in a second end of the tube, the second optical fiber having a first solid core section followed by a hollow core section followed by a second solid core section, a first gap formed between the first and second optical fibers, a length of the first solid core section forming a second gap, and a length of the hollow core section forming a third gap; an optical interrogator that transmits light at various wavelengths and measures an intensity of reflected light due to the first gap, second gap, and third gap as a function of frequency to provide interrogation data; and a processor that matches the interrogation data to reference data to estimate the pressure and/or temperature.