White-light interferometric fiberoptic-sensor systems for measuring strain or temperature usually consist of a broadband light source, a sensing interferometer (such as Bragg gratings along the length of a fiber or fiber Fabry-Perot cavities) that converts the data to be measured into a series of fringes, and a spectrometer to analyze the peaks and valleys of the fringes to demodulate the signal. In these high-speed sensor systems, the spectrometer is often the bottleneck. But by converting the spectrum information into a time-domain waveform (called time-wavelength spectroscopy), researchers in the Center for Photonics Technology (CPT) at Virginia Polytechnic Institute and State University (Blacksburg, VA) have developed a spectrometer that can demodulate interferometric fiberoptic-sensor signals at high speed.1
The spectrometer operates by first performing intensity modulation of an incoming continuous-wave (CW) arbitrary light signal to form light pulses. The pulse bandwidth is narrow compared to the light-signal bandwidth, such that the pulses are conceptually treated as the superposition of light pulses with different spectral components. The light signal then enters a dispersion element (in this case, a dispersive fiber) that delays the light propagation according to its wavelength, spreading the light in the time domain at its output. Measuring this output in the time domain using high-speed data acquisition techniques allows for easy mathematical determination of the optical spectrum.
The researchers built a prototype spectrometer using a low-cost superluminescent light-emitting (SLED) diode operated in CW mode. Light from the SLED is directed into a Fabry-Perot fiberoptic sensor, and then reflected back into the spectrometer for analysis by way of an optical circulator. The spectrometer consists of an erbium-doped fiber amplifier (EDFA) to initially magnify the reflected signal, followed by a pulse generator for modulation, and then a 5 km length of dispersion-compensating fiber (DCF) and an optical-electrical (O/E) converter to convert the dispersed light signal into an electronic signal for further analysis. Total dispersion of the DCF is 500 ps/nm at 1550 nm and width of the generated pulses is 6 ns at a repetition rate of 1 MHz. Suppression of noise from the EDFA and the O/E converter is accomplished by averaging the output signal from the O/E converter 100 times, creating an equivalent spectrum-acquisition rate of 10,000 frames per second.
Analysis of the spectra produced by Fabry-Perot fiberoptic sensors with cavity lengths of 63.3 and 73.8 µm by the experimental spectrometer is comparable to that of a traditional white-light interferometer and optical-spectrum analyzer (OSA; see figure). One exception is some small distortions at both ends of the spectrum when using the experimental spectrometer. The researchers note that these distortions are due to the bandwidth limit of the EDFA used in the setup. Wider measurement range could be achieved by using a Raman amplifier instead. The resolution of the spectrometer—defined as the minimum wavelength difference of two pulses that are distinguishable at the output of the spectrometer—was tested to be 15 nm at a wavelength of around 1550 nm.
High-speed signal-interrogation methods for interferometric fiberoptic-sensor analysis continue to be an active area of research. “The conventional OSA- and CCD-based spectrometers only have maximum speeds of ten to several hundred hertz, which has greatly limited the widespread use of many sensors that are inherently high speed,” says Anbo Wang, director of CPT at Virginia Tech. “Our spectrometer can have a signal-processing speed greater than 1 MHz, which is at least four orders of magnitude faster.” He adds, “Future research will focus on improving the spectral measurement resolution to subnanometer levels by using specialty fibers with extremely high dispersion, such as specially designed photonic-crystal fibers and Bragg fibers.”
1. Y. Wang et al., Optics Lett. 31(16) 2408 (Aug. 15, 2006).