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Raman effect enhances sensor range

28 Jul 2004

Scientists in Korea sense strain and temperature at the end of 50km of optical fiber.

Korean researchers have built an optical fiber sensor that performs simultaneous measurements of strain and temperature at a distance of up to 50 km. The system, developed by a team from the Korea Institute for Science and Technology (KIST) and Kyung Hee University in Seoul, combines fiber Bragg grating (FBG) technology with Raman amplification (Optics Express, 12 15).

Although FBG sensors are a well-established solution for measuring strain and temperature their maximum read-out distance is typically limited to around 25 km due to fiber loss and scatter. The Korea team has managed to double this distance by using Raman amplification to overcome the loss of the fiber.

In the configuration, light from two laser diodes at 1455 and 1465 nm is combined and pumps a 50 km length of singlemode fiber (SMF) that is connected to an FBG sensor head. The team says that a pump power of 600 mW generates a gain of 12 dB which is sufficient to overcome the system losses.

The Korean team then uses the excess pump power (about 23 mW) exiting the SMF to pump a 10 m length of erbium-doped fiber (EDF) which is located just before the FBG. The resulting broadband amplified spontaneous emission (ASE) around 1550 nm acts as a convenient signal for interrogating a narrowband FBG centred at 1555.7 nm with a reflection of 99.6%. This approach avoids the need for an external laser to probe the FBG sensor.

In experiments, the system was able to measure temperatures between 30 and 100 °C by monitoring the change in the wavelength of the reflection from the FBG. It was also able to measure a strain from 0 to 1700 µε. The changes in reflection were captured on an optical spectrum analyser located at the end of the 50 km of SMF. The sensitivity of the measurements was 8.2 pm/°C and 1.1 pm/µε respectively.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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