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We proposed and experimentally demonstrated a highly sensitive optical fiber torsion sensor based on a reflective Lyot filter for the first time to our knowledge. The reflective Lyot filter is constructed by inserting a section of polarization-maintaining fiber (PMF) between a fiber linear polarizer and a 3dB coupler based fiber loop reflector. Based on the intensity modulation, the proposed torsion sensor exhibits a high torsion sensitivity of up to 20.336 dB/rad, one order of magnitude higher than the achieved in state-of-the-art. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to −2.0 × 10−4 rad/°C and −6.39 × 10−6 rad/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, we perform the theoretical simulation of the proposed torsion sensor, and the simulation result obtained agrees well with the experiment results, vividly confirming the viability of the fiber Lyot filter based torsion sensor. Such fiber Lyot filter may find potential applications of highly sensitive torsion sensors in the field of modern smart structure monitoring.
© 2017 Optical Society of America
1. Introduction
The measurement of torsion is an important technique for the in-service evaluation and monitoring of the health condition of engineering structures such as bridges, buildings, trains and so on. To fulfil this task, several types of torsion sensors have been developed in the past years. One type of torsion sensor is based on the electromagnetic-phenomena, which has the advantage of remarkable resolution, good accuracy, and easier installation [1]. However, the complicated manufacturing, bulky heavy structure, and the need for a robust magnetic shield limit the practical application of such type of torsion sensor. The other type of torsion sensor is based on the electrical methods [2]. And yet, the large cross-sensitivity from the electrical noise and temperature cause the measurement of torsion inaccuracy, which should be restrained in practical applications.
In comparison, optical fiber torsion sensors are regarded as another very promising candidates because of their inherent particular characteristics, such as electromagnetic interference- and hazard-free, compact size, light weight, and distributed measurement over a long distance. Recently, many all fiber torsion sensors have been developed [3–21]. In general, the fiber torsion sensors are divisible into two classes: the fiber grating-based torsion sensors and the fiber interferometer-based torsion sensors. For the fiber grating-based torsion sensors, long period gratings [3–7] or fiber Bragg gratings [8–12] are used as the sensor heads and have been widely applied for their absolute response parameter, large dynamic range and high sensitivities. However, a shortcoming of these sensors is that the fabrication of fiber gratings requires the expensive laser, phase masks, etc. On the other hand, the fiber interferometer-based torsion sensors have been extensively investigated, owning to their high sensitivity, fast response and ease of being implemented [13–21]. So far, various fiber interferometers have been developed as torsion sensors, including square no-core fiber (NCF) based multimode interferometer (MMI) [13], bared single mode fiber loop and coated single mode fiber (SMF) based dual polarized Mach-Zehnder interferometer (DPMZI) [14], femtosecond laser-fabricated helical Mach–Zehnder interferometer [15] polarization-maintaining fiber taper based Mach-Zehnder interferometer (MZI) [16], and birefringent fiber based Sagnac interferometers (SI) [17–21]. Among the aforementioned fiber interferometer-based torsion sensors, the SI based torsion sensors are most widely used. Up to now, suspended twin-core fibers [17], highly birefringent photonic crystal fibers (PCF) [18], low birefringent PCFs [19], side-leakage PCFs [20] and polarization-maintaining elliptical core fibers (PM-ECFs) [21] are employed individually to construct SIs for torsion sensing application.
As a very unique polarization interferometer, Lyot filter has been widely studied since the first demonstration. In principle, Lyot filter is based on a birefringence cavity sandwiched by two polarizers. Currently, different Lyot filters including bulk Lyot filters [22,23], bulk-fiber mixed Lyot filters [24,25] and all fiber Lyot filters [26,27] have been reported and widely used in spectral imaging [22,23], laser [24,25] and optical communication [28]. In this paper, a reflective Lyot filter is fabricated by inserting a section of polarization-maintaining (PM) fiber between a fiber linear polarizer and a 3dB coupler based fiber loop reflector for torsion sensing application. Experimentally, we demonstrate a reflective Lyot filter-based torsion sensor that exhibits a high torsion sensitivity of up to 20.336 dB/rad. To our best knowledge, this is the first time that a reflective Lyot filter is proposed for torsion sensing and such torsion sensitivity is the highest one for intensity-modulated torsion sensors ever reported. Compared with the high torsion sensitivity, the temperature sensitivity and strain sensitivity of the proposed torsion sensor are low to −4.07 × 10−3 dB/°C and −1.3 × 10−4 dB/με, thus overcoming the cross-sensitivity problem resulting from temperature and strain. In addition, we perform the theoretical simulation of the reflective all-fiber Lyot filter based torsion sensor. The theoretical simulation results agree well with the experiment results, which confirms the viability of the reflective all-fiber Lyot filter based torsion sensor.
2. Experimental details
The schematic diagram of the proposed reflective all-fiber Lyot filter is shown in Fig. 1. The reflective Lyot filter is constituted by inserting a section of polarization-maintaining (PM) fiber between a fiber linear polarizer and a 3dB coupler based fiber loop reflector. Here, the fiber linear polarizer is used to convert the light launched from the supercontinuum light source into linearly polarized light, and acts as a fiber polarization analyzer for the light reflected by the fiber loop reflector simultaneously. The PM fiber plays the role of the birefringence cavity. With the polarization axes spliced at an angle (normally 45°) to the direction of the fiber linear polarizer, the light is coupled into both polarization axes of the PM fiber. So a relative phase difference between the light traveling along the fast and slow axes of PM fiber is induced resulting from the birefringence existing in the PM fiber. With no fiber birefringence, the light entering into the 3dB coupler based fiber loop reflector is all reflected back into the PM fiber, resolved into two light beams again. And then the two light beams meet at the fiber linear polarizer. In a word, the working principle of this Lyot filter can be summarized as follow: the incident light is converted into linearly polarized light by the fiber linear polarizer, and then the linearly polarized light is resolved into two polarized light beams propagating twice along the polarization axes of the PM fiber with a relative phase differenceof Δφ. In the end, the two polarized light beams combine at the fiber linear polarizer, resulting in the polarization interference. For the reflective spectrum of the proposed Lyot filter, the length of the birefringence PM fiber determines the free spectral range (FSR), and the fringe contrast is only related to the ratio of the two polarized beams propagating along the fast- and slow-axis of the PM fiber. When linearly polarized light passes through a section of SMF under torsion, the light undertakes polarization rotation, which leads to the change of the ratio of the two polarized beams propagating along the fast- and slow-axis of the PM fiber, and then resulting in the variation of the fringe contrast. In this case, the torsion angle applied on the SMF can be deduced by simply monitoring the intensity change of the fringe dip. Thus, based on the proposed reflective all-fiber Lyot filter, we demonstrate a torsion sensor.
The schematic diagram of the experiment setup for the reflective all-fiber Lyot filter-based torsion sensing system is shown in Fig. 2. The light source used in the experiment is a super-continuum light source with wavelength ranging from 450 nm to 2400 nm. The reflective spectrum of the proposed Lyot filter is measured by an optical spectrum analyzer (OSA, Yokogawa AQ6370C, operation wavelength ranges from 600 nm to 1700 nm) with a resolution of 0.02 nm. An optical circulator is employed to connect the super-continuum light source, the reflective all-fiber Lyot filter and the optical spectrum analyzer together. The birefringence cavity of the reflective all-fiber Lyot filter is a section of 50 cm long PM fiber (Corning Panda PM15-U2A) with a birefringence of about 1.95 × 10−4. Since the incident light propagates along the PM fiber twice through the fiber loop reflector, thus the equivalent length of the birefringence cavity is 1 m. As aforementioned, the fringe contrast in the interference pattern depends strongly on the ratio of the two polarized beams propagating along the fast- and slow-axis of the PM fiber, respectively. To get a high original fringe contrast, a fiber polarization controller (PC) inserted between the PM fiber and the fiber loop reflector is employed to carefully adjust the state of polarization (SOP) of the input light. In the practical application, it should be noted that a high original fringe contrast can be get justby precisely pre-setting the angle (normally 45°) between the polarization axis of the PM fiber and the direction of the fiber linear polarizer. Hence, the polarization controller is not essential for this torsion sensor. A section of 20 cm long SMF between the PC and the 3dB coupler based fiber loop reflector is fixed by a fiber holder on one end and mounted at the center of a fiber rotator on the other end. As an engraved dial is embedded into the fiber rotator, precisely torsion angle can be applied on the SMF by the fiber rotator. Moreover, two manual translation stages are employed to fix the fiber holder and the fiber rotator, respectively. By pre-moving one translation stage, the fiber is kept straight to eliminate any bending effects. The original reflection spectrum of the proposed Lyot filter without torsion is measured in the wavelength range from 1540nm to 1600nm, as shown in Fig. 3. From this figure, it is clearly seen that the output light intensity varies periodically with the wavelength, confirming that the interference spectrum is approximately a periodic wavelength-dependent function. The FSR between adjacent two fringe dips is ~12nm, corresponding to 1 m long equivalent birefringence cavity. What’s more, the original fringe contrast is up to ~28 dB near the wavelength of 1570nm, which is significant to the torsion sensing.
Fig. 2 Schematic diagram of the experiment setup for the reflective all-fiber Lyot filter-based torsion sensing system.
3. Experimental results
The torsion response of the proposed Lyot-filter is investigated experimentally by rotating the fiber rotator from 0 to 180° with a step of 10° in the clockwise direction and recording the reflection spectra of the Lyot filter under different torsion angles simultaneously. The reflection spectra of the Lyot filter with the corresponding torsion angle values in a range from 10° to 50° are shown in Fig. 4. It can be found from Fig. 4 that when the value of the applied torsion angle changes, the intensity of the fringe dip changes evidently, the wavelength of the fringe dip, however, nearly remains the same. It is reasonable since the torsion applied on the SMF results in the SOP change of the incident light, while the birefringence of the SMF almost keeps the same, as reported in [12]. The SOP change of the incident light leads to the variation of the ratio between the two polarized beams, thus the intensity of the fringe dip is strongly modulated by the torsion. On the other hand, the birefringence change of the SMF induced by the torsion is too little to shift the wavelength of the fringe dip evidently. Based on this experimental phenomenon, we adopt the intensity of the fringe dip, rather than the wavelength of the fringe dip, to encode this torsion sensor. Among the three fringe dips shown in Fig. 4, the intensities of the fringe dip at the wavelength of around 1570 nm with the corresponding torsion angle from 0° to 180° are recorded and displayed in Fig. 5. It is clearly observed that when the torsion applied on the SMF is increased from 0° to 90°, the intensity of selected fringe dip gets higher; However, the intensity of selected fringe dip becomes smaller as the applied torsion increases from 90° to180°. On the whole, the intensity variation of the selected fringe dip is a sine-like function of the applied torsion angle. Within the torsion angle range from 10° to 50°, the slope of the linearly fitting curve is 0.35486, corresponding to the torsion sensitivity of 20.336 dB/rad, one order of magnitude higher than those achieved (1.911 dB/rad [7], 0.955 dB/rad [10], and −0.687dB/rad [17]) in state-of-the-art. In the regions from 0° to 90° or 90° to 180°, the torsion angle can be detected by simply measuring the intensity variation of the fringe dip in real time.
Fig. 5 The measured intensity of the selected fringe dip under the corresponding torsion angle from 0° to 180°.
For the remainder of the work, we investigate the temperature response and the axial strain response of the proposed torsion sensor under different torsion, respectively. The twist angles pre-applied on the SMF are set to 30°, 60° and 90°. In the experiment, a heating block controlled by a temperature controller is employed to heat the pre-twisted SMF from 20 °C to 70 °C with a step of 10 °C, while recording the intensity of the selected fringe dip (at the wavelength of around 1570 nm) at each set point after 10 min interval. Fig. 6 shows the measured intensity variation of the selected fringe dip with different temperature levels for the 30°, 60° and 90° pre-twisted sensor. From Fig. 6, we can see that the intensity of the selectedfringe dip almost remains stable, with a maximal linear fitting coefficient of only −4.07 × 10−3, corresponding to the maximal temperature cross-sensitivity of −2.0 × 10−4 rad/°C. The experimental results above vividly confirm that the sensor is immune to temperature.
Fig. 6 The measured intensity variation of the selected fringe dip with different temperature levels for 30°, 60° and 90° pre-twisted sensor.
At the last part, we investigate the axial strain response of the proposed torsion sensor. As stated in Part 2, the ends of the twisted SMF are fixed on two manual translation stages. By translating one translation stage manually, the strain from 0 to 500 με with an interval of 50 με are applied on the twisted SMF, and the corresponding intensity variations of the selected fringe dip are shown in Fig. 7. As seen in Fig. 7, the maximal intensity variation is only 0.07 dB for the strain range from 0 to 500 με, indicating that the intensity is almost exactly equal to the twisted fiber without strain applied. The maximal linear fitting coefficient is −1.3 × 10−4, corresponding to the maximal strain cross-sensitivity of −6.39 × 10−6 rad/με, which means that, strain compensation is not required for this torsion sensor.
Fig. 7 The measured intensity variation of the selected fringe dip with different strain levels for 30°, 60° and 90° pre-twisted sensor.
4. Analysis and discussion
For the proposed Lyot filter, the direction of the fiber linear polarizer for the incident light is aligned α to the direction of the slow-axis of the PM fiber. As stated in Part 2, a PC is used to carefully adjust the original SOP of the input light to get a high original fringe contrast, which is equivalent to change the direction of the fiber linear polarizer for the reflective light. So we define the direction of the fiber linear polarizer for the reflective light as β with respect to the direction of the slow-axis of the PM fiber.
By using the transfer matrix method [29], the reflective Lyot filter can be described as:
Where the matrices from left to right in order represent the fiber linear polarizer for the output light, the PM fiber, the 3dB coupler based fiber loop reflector, the PM fiber and the fiber linear polarizer for the input light, respectively.The normalized reflectivity of the light passed through the Lyot filter is:
whereΔφ is the relative phase difference; LPM is the length of PM fiber cavity; Δn is the birefringence of PM fiber and the λ is the working wavelength.The input light transmitted through the fiber linear polarizer is converted into linearly polarized light with a polarization state of α. Then the linearly polarized light passes through the PM fiber cavity with a constant polarization state of α.
When a SMF is under linear twist θ shown in Fig. 8, for the input polarization of α, the output polarization α1 is [12]:
Subsequently, linearly polarized light with a polarization state of α1 travels into the 3dB coupler based fiber loop reflector shown in Fig. 9. As analyzed in [30], the light is all reflected by the fiber loop reflector with a rotated polarization of α2:
Fig. 9 The polarization state evolution of the light passing through the 3dB coupler based fiber loop reflector.
The normalized reflectivity of the light passed through the twisted Lyot filter is:
When Δφ = (2m + 1)π (m = 0, 1, 2, 3…), the min reflectivity occurs. In the case of this twisted Lyot filter, the final expression of the normalized minimum reflectivity is:
Then we replot the experiment data previously shown in Fig. 5 in linear scale by green dot and present the simulation result of the normalized minimum reflectivity vs torsion angle by red line, as shown in Fig. 10. The coupling angles α, β in the simulation are assumed to be 86° and 15°, respectively. The simulation result obtained here agrees well with the experiment results, which vividly confirms the viability of the fiber Lyot filter based torsion sensor.
5. Conclusions
In conclusion, we propose and experimentally demonstrate a high sensitivity torsion sensor based on a reflective fiber Lyot filter for the first time, and then present a theoretical description of the reflective fiber Lyot filter based torsion sensor. The experiment result is very consistent with the theoretical simulation, which verifies the feasibility of the reflective fiber Lyot filter based torsion sensor. Based on the intensity modulation, the proposed torsion sensor exhibits a high torsion sensitivity of up to 20.336 dB/rad, one order of magnitude higher than those achieved in state-of-the-art. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to −2.0 × 10−4 rad/°C and −6.39 × 10−6 rad/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. We believe that this reflective fiber Lyot filter based torsion sensor adds a new application for the Lyot filter and offers a new choice for the field of optical torsion sensing.
Funding
Director Fund of WNLO; National 1000 Young Talents Program, China.
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