Fiber Sensor Uses Raman and Brillouin Scattering

Breck Hitz

Fiber sensors are replacing traditional transducers in many monitoring and measuring applications, a result of fiber sensors' superior reliability, longevity and flexibility to function in multiple sensing applications. In one example, these sensors have measured both strain and temperature when the fiber is illuminated with laser light and the signal reflected by Brillouin scattering is quantified. Such simultaneous measurement could be useful, for example, in real-time monitoring of the strain and temperature in a critical engine part.

Brillouin scattering can be understood as a purely classical effect in which the laser light is reflected from a spontaneous sound wave propagating in the fiber. Because the sound wave is moving, the light reflected from it is Doppler-shifted to a different frequency. The velocity of the sound wave can be inferred from the magnitude of the frequency shift of the reflected light. The velocity depends on both the temperature of the fiber and the strain to which it is subjected, so the measured frequency shift contains information about both parameters.

An immediate problem is that one cannot distinguish the frequency shift due to temperature and that due to strain in the Brillouin-scattered light. To finesse this problem, scientists have developed a Brillouin time-domain reflectometry technique in which pulses of light reflected from portions of the fiber are separated by their return time to the detector. By making independent measurements of the Brillouin frequency shift and the intensity, they can calculate both the strain and temperature along the fiber. Unfortunately, this technique is limited by the accuracy of the intensity measurement.

Recently, researchers at the University of Southampton in the UK demonstrated a means of simultaneously measuring the Brillouin- and Raman-scattered radiation from a fiber -- and, from those measurements, inferring both the temperature and strain with greater accuracy than has been achieved with Brillouin scattering alone. The intensity of Raman-scattered radiation contains information about the temperature alone, and once the temperature is known, the strain can be computed from the frequency shift of the Brillouin signal.

Raman scattering is a quantum-mechanical effect in which a photon is scattered from a molecular vibration. In the type of Raman scattering used in the Southampton sensor (first anti-Stokes), the photon scatters from an excited molecule, gaining the molecule's energy and leaving the molecule in its ground state. The intensity of Raman scattering depends on the thermal population of the excited molecular energy level, and the intensity of Raman-scattered light hence is a function of temperature. Strain, on the other hand, has virtually no effect on the energy levels or their populations, so the Raman signal is unaffected by it.

The scientists used a narrow-linewidth laser at 1533.2 nm, together with a pair of erbium-doped fiber amplifiers and an acousto-optical modulator, as the probe source in their demonstration (Figure 1). A circulator separated the return signal from the fiber sensor and sent it to the detectors.

Figure 1. The scientists directly detected the intensity of the Raman-scattered light, whose wavelength was shifted by ~100 nm from the laser wavelength. They used a coherent-detection scheme to measure the frequency shift of the Brillouin-scattered light. Images ©OSA.

The sensor comprised four sections of fiber and was 1.3 km long, separated from the source/detector by 22 km of spooled fiber, demonstrating that the detector could be located a significant distance from the source/detector. The first section of the sensor (400 m) was in an oven at 60 °C. The second section (600 m) was at room temperature, was subjected to no strain and provided a reference signal. The third section (130 m) was suspended from a system of pulleys and loaded with weights to induce a known strain. The final section (200 m) served as a second reference section.

Figure 2. The Raman-scattered signal shows a clear intensity enhancement from the section of fiber in the oven. The inset shows the signal from the 400 m of fiber in the oven, and the following 600 m of reference fiber, normalized to the Raman signal when the oven was at room temperature.

The acousto-optic modulator chopped the probe light into 100-ns pulses, enabling a spatial resolution of ~10 m in the fiber. The Raman-scattered light, shifted by approximately 100 nm, showed a clear intensity enhancement from the section of fiber inside the oven (Figure 2). The Brillouin-scattered light showed a frequency shift from the section of the fiber in the oven and from the section stretched over the pulleys (Figure 3). From the results, the Southampton scientists calculated that the section of fiber in the oven was heated to 60 °C, consistent with the directly measured temperature.

Figure 3. The frequency shift of the Brillouin-scattered light showed a different frequency shift from the section of the fiber in the oven and from the section stretched over the pulleys. The strain was uneven as a result of friction in the pulley system.

Knowing the temperature profile, they calculated the strain in the section of fiber stretched over the pulleys. As indicated by the irregular profile in Figure 3, the strain was not uniformly distributed because of friction in the pulley system. The resolution of the temperature measurement was ~6 °C, and the resolution of the strain measurement was ~150 µε.