A novel approach for ultrafast and temperature-insensitive strain interrogation using a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac loop interferometer (SLI) and linear wavelength-to-time (WTT) mapping is proposed and experimentally demonstrated. The PM-PCF incorporated in the SLI is used as the sensing element to achieve stable strain sensing with ultra-low temperature-dependence due to its intrinsic thermal insensitivity, which can be used to eliminate the cross-sensitivity effect and increase the measurement accuracy. A dispersive element is employed to realize the WTT mapping and real-time strain interrogation is obtained by converting the strain-encoded wavelength shift to time shift in the temporal domain, which can be directly monitored by a real-time oscilloscope. The proposed system offers an ultrafast interrogation speed of 100 MHz and a strain sensitivity of -0.17 ps/με.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber optic sensors have been intensively investigated and developed in the past decades due to their advantages against conventional electrical sensors, such as small size, low cost, high durability, immunity to electromagnetic interference and the ability of distributed sensing . Therefore, they have attracted much attention for the detection of different parameters like strain, temperature, acceleration, or pressure in a harsh environment. In recent years, axial strain sensors based on phase modulation [2,3], wavelength modulation , and Rayleigh scattering  have been widely studied and experimentally demonstrated. Among these various implementations, the wavelength-modulated FBG sensors are particularly useful for strain or temperature sensing since the sensing information is wavelength-dependent and encoded in the wavelength shift. An optical spectrum analyzer (OSA) is usually used to monitor the wavelength shift with high measurement accuracy. However, the scan speed is slow especially when high resolution is needed.
To improve the interrogation speed, several demodulation techniques have been proposed. A static frequency discriminator can be employed to realize relatively high-speed interrogation by converting the FBG wavelength shift to optical power change, like an optical edge filter , a wavelength-division-multiplexed (WDM) fiber coupler , or a holographic grating-based spectroscopic charge-coupled device (CCD) . But the power variations from the laser source would affect the measurement accuracy and the interrogation resolution of this method is poor. To eliminate the impact of light source power fluctuations, techniques based on interferometer scanners, which can be implemented through a Mach-Zehnder interferometer , a Michelson interferometer , or a Fabry-Perot interferometer  are proposed to convert the wavelength shift to optical phase change. Hence, the measurement resolution is improved compared with the static frequency discriminator scheme. However, due to the intrinsic characteristic of optical interferometers, they are sensitive to environmental changes, which would reduce the measurement accuracy. Moreover, the interrogation speed of these techniques mentioned above is usually limited to the kHz regime.
Ultrafast and real-time interrogation with a measurement speed of tens of MHz is highly desirable in studying dynamic events, such as molecular dynamics, and monitoring fast vibrating objects like aircraft engine diagnostics. To this end, a mode-locked laser (MLL) is employed as the light source to obtain ultrafast and single-shot measurements due to its pulsed nature. Photonic time-stretch was initially developed to overcome the speed and resolution limitations of high-speed analog-to-digital converters (ADC) . Recently, many research efforts have been put into this ultrafast data acquisition method to realize fast real-time measurements, including the discovery of optical rogue waves  and soliton explosions , high-speed and single-shot characterization of electronic/optoelectronic devices [15,16] and ultrafast imaging [17,18]. Photonic time-stretch based FBG strain interrogation systems have also been demonstrated [19,20] with ultrafast measurement speed. Meanwhile, a technique for real-time spectral measurement [21,22], which is enabled by wavelength-to-time (WTT) mapping, also known as real-time dispersive Fourier transformation, has been successfully applied to FBG strain interrogation in recent years. In , a linearly chirped FBG strain sensor based on WTT mapping is demonstrated with an ultrahigh interrogation speed of 48.6 MHz. However, FBG is sensitive to both strain and temperature variations, leading to difficulty in distinguishing the Bragg wavelength shift, which would cause a cross-sensitivity effect and limit the practical applications. Therefore, additional compensations for the environmental temperature change are needed in FBG-based strain sensors for more accurate measurements [24,25]. To overcome this drawback, a temperature-insensitive strain sensor based on polarization-maintaining photonic crystal fiber (PM-PCF), which has become commercially available recently, has been proposed and experimentally presented. A stable strain interrogation system based on PM-PCF without the requirement of additional temperature compensation has been reported . However, a simple approach to realize temperature-insensitive strain interrogation in real-time with ultrafast measurement speed is highly expected in practical applications.
In this paper, we propose and experimentally demonstrate a novel and simple strain interrogation system with an ultrafast measurement speed of 100 MHz. The key component is a Sagnac loop interferometer (SLI) incorporating a PM-PCF serving as the strain sensing element. Due to the intrinsic thermal insensitivity of the PM-PCF, the temperature-induced cross-sensitivity effect can thus be neglected, and stable strain measurement is realized with improved measurement accuracy. An ultra-short pulse generated by an MLL is spectrally shaped by the SLI and then mapped into the temporal domain through a dispersive element. Therefore, the strain-encoded wavelength shift is converted to time shift of the temporal pulse, which can be monitored in real-time. A sensing resolution of 294 με is achieved with ultralow thermal dependence and the experimental results show the system has a strain sensitivity of -0.17 ps/με with a measurement range of 0 to 1536 με.
The schematic of the proposed PM-PCF based SLI interrogation system for ultrafast and temperature-insensitive strain measurement is shown in Fig. 1. It consists of an MLL, an optical isolator (ISO), a 3-dB optical coupler, a polarization controller (PC), a segment of PM-PCF and a dispersive element. The inset on the top of Fig. 1 shows the SEM image of the cross-section of the PM-PCF used in the experiment. The PM-PCF is composed of a single material with a periodic arrangement of air holes and therefore it is insensitive to temperature variation. The MLL is used to generate the ultrashort pulse train with a broad optical spectrum. The SLI is formed by splicing the two ends of the PM-PCF to the arms of a 2×2 3-dB coupler. An ISO employed prior to the SLI is to maintain unidirectional transmission of the optical signal. A PC incorporated in the SLI is used to optimize the polarization states. The generated optical pulse is first power divided by a 1×2 optical coupler (OC1), with a small part of power directly sent to a photodetector (PD2) and converted to an electrical signal and then captured by a real-time oscilloscope (OSC) through channel 2, serving as the time reference signal (trigger), and the remaining laser power is injected to the 3-dB coupler and spectrally shaped by the PM-PCF based SLI. A dispersion compensating fiber (DCF) is used to realize linear WTT mapping, which maps the spectrally shaped optical pulse to the temporal domain and enables the interrogation in real-time. The OC2 is also used to split the light power so that the strain information can be demodulated both in the spectrum and time domain. Note that the position of the DCF and OC2 can be changed and it does not affect the results since the interrogation scheme is a linear time-invariant system. The OSA is used to monitor the strain-induced wavelength shift of the optical spectrum as a comparison, while the time shift of the corresponding temporal pulse is captured by an OSC through channel 1. An ultrafast strain interrogation system with a measurement speed identical to the repetition rate of the MLL is realized.
2.1 PM-PCF based Sagnac loop interferometer
In Fig. 1, the input light from the ISO is equally split into two counter-propagating waves by the 2×2 3-dB coupler, and the two waves experience different optical path lengths due to the birefringence property of the PM-PCF. Interference happens when they recombine at the coupler after clockwise and counterclockwise propagating around the loop. Ignoring the insertion loss of the SLI, the transmission spectrum of the fiber loop is approximately a sinusoidal function of the wavelength which can be written as:$\theta$ is the polarization angle deflection of light after propagating through the PC, $\varphi = \pi BL / \lambda$ is the phase difference caused by the group modal birefringence, B is the birefringence of the PM-PCF, L is the total length of the PM-PCF and λ is the operating wavelength. The wavelength spacing (S) between adjacent interference fringes is given by: 2) that the wavelength spacing can be adjusted by changing the length of the PM-PCF to guarantee S is smaller than the spectral range of the pulsed laser and hence transmission dips would fall into the spectrum, which can be monitored by the OSA. The wavelength shift of the transmission minimums caused by the applied axial strain is defined as : 1.
2.2 Wavelength-to-time mapping
It is known that real-time WTT mapping can be achieved by using a dispersive element with linear dispersion. The spectrum of the pulsed laser is first shaped by the SLI and then the spectrally shaped spectrum is mapped to the time domain and a temporal interference pattern is generated. The mapping relationship between the wavelength shift and the corresponding time shift is given by :3) to Eq. (4), we have:
3. Experimental results
A proof-of-concept experiment is conducted based on the setup shown in Fig. 1 to verify the feasibility of the proposed interrogation system. In the experiment, the MLL (ROI, EFLA-B-1560-100) is centered at 1565 nm generating a short ∼100 fs optical pulse with a repetition rate of 100 MHz. Two ends of a 20-cm PM-PCF (YOFC, PC1013-A) are spliced to single-mode fibers (SMF-28) of the arms of the 3-dB coupler. Core diameters of the PM-PCF are 4.8 μm on the X-axis (the direction along dual big air holes) and 7.0 μm on the Y-axis (the vertical direction of Y-axis) respectively, and the cladding diameter of the PM-PCF is 125 μm. The combined loss of the two splicing points in the SLI is measured to be ∼12 dB, which is relatively high due to the mode-field mismatch between PM-PCF and SMF and the air hole collapse of the PM-PCF during splicing. Lower splicing loss can be achieved by choosing a weaker fusion current and a shorter fusion time but note that there is a tradeoff between the splicing loss and the mechanical strength . However, the high insertion loss does not affect the measurement results since the transmission dips are monitored and moreover, the optical power injected into PD1 (-5 dBm) is sufficient for the real-time measurement. The DCF used in the experiment has a total dispersion of -100 ps/nm at 1550 nm, which is to linearly map the wavelength shift to the temporal domain and determine the measurement sensitivity. Figure 2 shows the optical spectrum of the MLL used in the experiment (solid line) with a 3-dB bandwidth of 23 nm, and the transmission spectrum of the SLI with the pulsed laser injected as the light source (dashed line). Based on the length of the PM-PCF used in the SLI, there are several dips appear in the transmission spectrum, which can be used to monitor the strain-induced wavelength shift.
The OC1 (98/2) is used to split the laser power with the first branch (98%) launched into the Sagnac loop, and the second branch (2%) is detected by a 1-GHz PD (PD2, CONQUER, KG-PR-1G) and then digitized by an OSC (KEYSIGHT, DSOX6002A) with a 6-GHz bandwidth and a 20-Gs/s sampling rate through channel 2. Meanwhile, the first branch (10% output) from OC2 (90/10) is monitored by an OSA with a resolution of 0.1 nm, while the remaining (90% output) is detected by one output of the PD1 (Optilab, BPR-23-M) after linear WTT and digitized by the OSC using channel 1. The temporal interference pattern from channel 1 is triggered by channel 2. Since they have the same repetition rate, the applied strain information can be interrogated from the time shift of the temporal pulse in channel 1. The PC is adjusted to obtain temporal pulses with a higher SNR and sharp dips, and Fig. 3 shows the mapped temporal interference pattern under no strain (solid line) and the trigger signal (dashed line), and it can be seen the dispersion value can be further increased as long as no aliasing occurs between temporal pulses.
3.1 Measurement of strain sensitivity
The two ends of the 20-cm PM-PCF are fixed on the translation stages and different axial strains are applied by moving one of the translation stages. In the experiment, the applied strain is varied from 0 to 1536 με with a step of 384 με by increasing the distance between the two stages. The transmission spectra first measured by the OSA under different strains are shown in Fig. 4(a), which are considered as a reference to temporal interrogation. The transmission dip located at 1574.5 nm shows a higher SNR and therefore is monitored. It can be seen the dip shifts towards the longer wavelength with the increase of applied strain for 2.43 nm. Excellent linearity between wavelength shifts and applied strains can be seen from Fig. 4(b) with an R-square value of 0.99856 and a strain sensitivity of 1.58 pm/με is achieved in the spectrum domain.
The mapped temporal interference waveforms are recorded by the OSC using channel 1. Because the DCF has a negative dispersion value, the waveform shifts towards a shorter time delay with the increase of applied strain in Fig. 5(a). Figure 5(b) shows the dip shift of the temporal waveform has a linear relationship against the applied strain with an R-square value of 0.9825. The highest horizontal resolution of the OSC is 50 ps and a strain sensitivity in the temporal domain of -0.17 ps/με is obtained. Therefore, the strain resolution of the system is 294 με. A higher strain resolution can be achieved with a DCF having larger dispersion or an OSC with a higher sampling rate. Note that there is a tradeoff between the repetition rate of the MLL and the maximum value of DCF. A maximum time shift of -250 ps against the applied strain can be seen from Fig. 5(b), which agrees well with the theoretical analysis based on Eq. (5).
3.2 Measurement of temperature sensitivity
It is known that PM-PCF has a very low thermal dependence [26,29] and to study the temperature influence on the proposed strain sensor, the PM-PCF is placed in a temperature-controlled chamber with the temperature increased from 30 ℃ to 80 ℃ with a step of 10 ℃. It can be seen from Fig. 6 that the wavelength shift of the transmission spectra and the corresponding time shift are extremely low with the temperature increased up to 80 ℃. The maximum wavelength shift is 52.4 pm from Fig. 6(a), corresponding to the temperature sensitivity of -1.1 pm/℃, which is much smaller than the conventional FBG sensors of ∼10 pm/℃ . Due to the limited resolution of the OSA, the result can be used as a reference and an OSA with finer resolution should be adopted for better characterization in the future. There is no time shift from Fig. 6(b) with the increase of temperature due to the limited time resolution. A higher temperature resolution can be obtained with a larger dispersion value or an OSC with a higher sampling rate. However, it also indicates the temperature influence on the proposed strain interrogation system can be neglected in practical applications. Meanwhile, Table 1 shows the comparison of different strain interrogation schemes and it can be seen our proposed system has a low cross-sensitivity, which enables real-time strain measurement with improved accuracy.
We have proposed and experimentally demonstrated a PM-PCF based SLI sensor for ultrafast and temperature-insensitive strain measurement in real-time. The key significance of this approach is mapping the spectrally shaped waveform to the temporal domain, converting the strain-induced wavelength shift into the time shift. By using a dispersive element, the linear WTT mapping is realized and enables the strain information to be interrogated by a real-time oscilloscope. Due to the intrinsic thermal insensitivity of the PM-PCF, the cross-sensitivity effect caused by temperature variation can thus be neglected and stable strain interrogation is realized with improved accuracy. The system offers an ultrafast interrogation speed of 100 MHz and a strain sensitivity of -0.17 ps/με. The proposed technique has the advantages of ultrafast strain interrogation and extremely low thermal-dependence with a simple structure, which can find applications where high-speed and stable strain sensing is required.
Fundamental Research Funds for the Central Universities (2018YJS014); National Natural Science Foundation of China (61620106014, 61827818).
The authors would like to thank ROI optoelectronics technology for providing the mode-locked laser.
The authors declare that there are no conflicts of interest related to this article.
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