Patent ID: 7595865

Claim:
An optical time domain reflectometry apparatus, for sensing a parameter to be measured in a region of interest, the apparatus comprising: an optical fiber; a source operable to launch optical radiation into the optical fiber at a probe wavelength having a preselected intensity, wherein the optical fiber includes a first section, into which the optical radiation at the probe wavelength is launched, and a second section deployed in the region of interest; detection means operable to produce electrical output signals in response to optical radiation backscattered from the optical fiber; and a remote amplifier arranged between the first and second sections which is operable to compensate for attenuation losses in the intensity of the probe wavelength in the first section, wherein the remote amplifier comprises a section of fiber doped with rare-earth ions, the fiber exhibiting gain at the probe wavelength when illuminated by the optical radiation at a pump wavelength and wherein the gain of the remote amplifier is selected to increase the intensity of the optical radiation transmitted into the second section to a value just below an intensity threshold for the onset of non-linear effects of the second section, thereby allowing the intensity of the radiation launched into the first section to be low compared to the intensity threshold for the onset of non-linear effects of the first section.