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Sensing & Measurement

Dynamic sensors based on wavelength-swept lasers

Two designs, one based on a wavelength filter and one on Fourier domain mode locking, enable dynamic measurements to be made using fiber-optic sensors.
15 January 2015, SPIE Newsroom. DOI: 10.1117/2.1201412.005653

Due to their high spectral resolution, wavelength-swept lasers (WSLs)—with an emission that sweeps across a range of wavelengths—represent a promising optical source for optical coherence tomography, optical fiber sensors, and optical beat source generation.1–12WSLs are fabricated by employing a narrowband wavelength-scanning filter, such as a fast rotating polygonal scanner filter,1, 2 a diffraction grating on a galvo-scan mirror,3,4 or a fiber Fabry-Pérot tunable filter (FFP-TF).5–10 The speed of conventional WSLs, based on FFP-TFs, is limited by both the tuning speed of the filter and the laser cavity lifetime. Many kinds of WSLs have been developed with an aim to improve the scanning rates and scanning bandwidth.5, 6

Purchase SPIE Field Guide to Interferometric Optical TestingDue to the many advantages they have over conventional techniques, fiber Bragg gratings (FBGs) have been widely implemented in fiber-optic sensors for the purpose of structural health monitoring. Their inherent advantages include electromagnetic immunity, compactness, remote sensing ability, ease of fabrication, and wavelength selectivity. The key technology behind fiber-optic sensing using FBGs is wavelength-shift interrogation. FBG interrogation based on a WSL represents a promising technique for high-speed and high-accuracy fiber-optic sensor systems10, 13–15 due to the linear relationship between the spectral and temporal domains (see Figure 1). A wavelength in the spectral domain therefore exactly corresponds to a pulse in the temporal domain, enabling dynamic measurement in fiber-optic sensors.10–16

Figure 1. Schematic diagram of the fiber Bragg grating (FBG) sensor interrogation based on a wavelength-swept laser (WSL). The FBG interrogation system consists of a WSL and an FBG sensor array. The WSL comprises a gain medium, a tunable filter, and an output coupler. The output from the WSL goes to the FBG sensor array through an optical circulator. Reflected wavelengths from the FBG array then arrive at the photodetector, again through the optical circulator.

We recently developed a dynamic FBG sensor interrogation using two kinds of WSLs.14, 15 Figure 2(a) shows a schematic diagram of one of our experimental setups, based on a WSL with a polygon-scanner-based wavelength filter (∼1550nm).14 The output from this WSL is coupled into the multiplexed FBG array, which is arranged in series with five FBGs. One of the multiplexed FBGs in this array is fixed on the stage of the piezoelectric transducer (PZT) stack, which allows for the application of dynamic periodic strain under voltage. The periodic reflected signals collected by the photodetector are digitized using a data acquisition board. These reflected outputs from the multiplexed FBG array are simultaneously detected as a series of reflected wavelengths in the spectral domain, and as a series of pulses in the temporal domain, by scanning over the spectral range. Since there is a correspondence between the spectral and temporal domains for these reflected signals, the variation of the wavelength in each FBG can easily be converted to account for the sweeping speed of the WSL.

Figure 2. (a) Schematic diagram of the experimental setup based on the WSL with a polygon-scanner-based wavelength filter. (b) Periodic signal output from FBG 3 under dynamic periodic strain in the multiplexed FBG array. (c) The corresponding fast Fourier transform (FFT) spectrum output from the periodic output of (b), showing a peak at 2kHz.14SOA: Semiconductor optical amplifier. PZT: Piezoelectric transducer. DAQ: Data acquisition.

As an example, a sinusoidal waveform with a frequency of 2kHz and a voltage of 50V was applied to the PZT stack. Figure 2(b) shows the periodic signal of the dynamically strained FBG 3 in the multiplexed FBG array, using the WSL at a sweeping rate of 18kHz. The corresponding FFT spectrum from this periodic output shows a peak for the 2kHz frequency component: see Figure 2(c). The signal-to-noise ratio and frequency bandwidth were determined to be more than 40dB and around 10Hz, respectively.

We were also able to demonstrate dynamic FBG sensor interrogation using a Fourier domain mode-locked (FDML) WSL. Figure 3 shows the experimental setup for resonance dynamic FBG sensor interrogation based on the FDML WSL.15 The resonance FDML laser comprises multiple FBGs inside the laser cavity, attached via an optical circulator. The four FBGs are arranged in series and operate at the selective FDML laser wavelength. Each FBG in the array reflects back to the laser cavity via the optical circulator. The resonance FDML WSL is operated by driving the FFP-TF with a sinusoidal waveform at a frequency corresponding to the round-trip time of the laser cavity.

Figure 3. Schematic diagram of the experimental setup for a resonance FBG sensor interrogation based on the Fourier domain mode-locked WSL.15 PC: Polarization controller. FC: Fiber coupler. DMF: Dispersion-managed fiber. FFP-TF: Fiber Fabry-Pérot tunable filter.

In addition, the WSL can be used to measure the dynamic modulation frequency of an applied electric field.16 Figure 4 shows our experimental setup for the dynamic measurement of the modulation frequency under an applied electric field using a nematic liquid crystal (NLC) FP etalon and the WSL. To measure the dynamic variation in the transmitted peaks, we applied the amplitude-modulated electric field to the NLC FP etalon. When an electric field of the proper amplitude and frequency is applied to the etalon, the wavelengths of the transmitted peaks are modulated with the same frequency. The amplitude modulation frequency can be estimated by measuring the peak positions of the modulated pulses using a high-speed photodetector.

Figure 4. (a) Experimental setup for the dynamic measurement of the modulation frequency. (b) Measured dynamic variations in the time difference for two consecutive pulses at an amplitude modulation frequency of 100Hz. (c) Several FFT spectra of the periodic outputs. LC: Liquid crystal.16

Figure 4(b) shows the measured dynamic variations at an amplitude modulation frequency of 100Hz. The frequency closely tallies with the amplitude modulation frequency of the applied electric field. The fast Fourier transform (FFT) spectra of the periodic outputs are shown in Figure 4(c). Up until now, it has not been easy to measure the modulation response of dynamic modulation of the NLC at more than 1kHz. We have, however, successfully measured the FFP spectra of the amplitude modulation at frequencies of 100, 1000, and 1500Hz.

In summary, we have developed two kinds of WSLs. One employs a polygon-scanner-based wavelength filter and the other is based on FDML. Using these WSLs, we have been able to successfully demonstrate dynamic measurements, showing promise for future health monitoring applications. We are currently working to develop new dynamic fiber-optic sensors using these high-speed WSLs.

Several coworkers in our laboratory contributed to this work. This research was financially supported by the Ministry of Education and National Research Foundation of Korea through the Human Resource Training Project for Regional Innovation (2013H1B8A2032213).

Min Yong Jeon, Myeong Ock Ko, Byeong Kwon Choi, Yong Seok Kwon
Chungnam National University
Daejeon, Republic of Korea

Min Yong Jeon is a professor in the Department of Physics and has more than 24 years' experience in optical fiber sensors, photonic devices, and fiber lasers. His current research focuses on fiber-optic sensors, fiber lasers, and THz photonics.

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