A temperature sensor employing the Vernier effect generated from a cascaded fiber rings based microwave photonic filter (MPF) is proposed and experimentally demonstrated. The structure of the fiber ring is used as a sensing element as well as the sampling and delaying component of the MPF in our proposed sensing scheme. The sensing characteristics of both single ring and cascaded fiber rings based sensors have been studied and compared. By employing two cascaded fiber rings of slightly different length, the Vernier effect can be generated in the frequency response of the MPF. The sensing interrogation of the cascaded fiber rings based sensor is conducted by detecting the frequency shift of the upper envelope of the measured frequency response curve. The experimental results show that the sensitivity of the cascaded fiber rings based sensor can be improved about 30 times compared with the single fiber ring based temperature sensor.
© 2017 Optical Society of America
Fiber-optic sensors have been widely studied for their unique advantages such as light weight, compactness, immunity to electromagnetic interference and so on . Traditionally, the interrogation of fiber-optic sensors is usually realized by monitoring the parameters in optical domain such as power, wavelength. During the past few years, fiber-optic sensor systems employing microwave photonic technology have been proposed and demonstrated in order to explore better performance [2, 3], in which the sensor parameters are resolved in microwave domain. Due to huge frequency gap of lightwave and microwave, tiny variations in optical domain will cause significant changes in microwave domain, which enables the MPF based optical sensors with high sensitivity and improved resolution capability. Furthermore, the frequency of microwave is much lower compared with lightwave, thus it is easy to be detected. In [4–8], MPF based FBGs sensor system have been demonstrated. Among those schemes, FBGs based sensing heads are used to form an MPF system, and the environmental variations caused resonance wavelength shift of the FBG is reconstructed by monitoring the change of the frequency response of the MPF. Besides FBG based sensing system, MPF technology also can be used to interrogate the interferometer type sensors, and in [9–12] MPF based on sliced optical source and dispersive medium have been used to interrogate those sensors. The sensor information can be deduced directly from the center frequency of the MPF’s passband without fast Fourier transform (FFT) process of the optical spectrum which is commonly used in traditional interferometer type sensors. Another important kind of MPF based sensor can be called “microwave interferometers”, in which the sensor head is usually the fiber itself [13–17]. In those cases, the interrogation is usually realized by tracing the resonance frequency of the MPF response or applying other signal processing method, such as FFT process to the measured MPF frequency response.
Vernier effect is a useful method to improve the measurement accuracy, and has been widely used in instruments such as Vernier calipers, micrometers and so on. Besides, it has also been applied to various optical sensor system in order to improve the sensitivity . In [19–25], Vernier effect has been applied to interrogate integrated optical sensors on silicon-on-insulator or Complementary Metal Oxide Semiconductor (CMOS) platforms. In addition, Vernier effect has also been demonstrated successfully in fiber interferometer sensors such as Sagnac type interferometers , microfiber resonators  as well as Fabry-Perot type [28–30] sensors in order to improve the sensitivity. The general interrogation principle is by tracking the spectrum envelope of the cascade sensors rather than the spectrum response of single sensor. With the advantage of Vernier effect, the shift of envelope is more distinct comparing with spectrum shift of a single sensor. Both the upper envelope and the lower envelope are available in those cases.
In this paper, a fiber sensor system combining MPF technology with Vernier effect is proposed and demonstrated. A temperature sensor based on a single fiber ring MPF (FR-MPF) is firstly analyzed and experimentally tested. In order to improve the sensitivity, two cascaded fiber rings based temperature sensor taking advantage of the Vernier effect is proposed and experimentally verified. In this proposal, two fiber rings with slightly different length are cascaded, and thus the Vernier effect can be generated in frequency response of the cascaded fiber rings MPF. Unlike other previously reported fiber optic sensor systems using the Vernier effect demonstrated in optical domain, the Vernier effect in our sensing scheme is generated in electronic domain. To the best of our knowledge, this is the first demonstration of MPF based optical fiber sensor combining with the Vernier effect. The interrogation of the cascaded fiber ring sensor is conducted by detecting the envelope shift of the frequency response curve of the MPF. With the advantages of the Vernier effect, the shift of the envelope is more significant and thus the sensitivity is magnified.
2. Principles and characteristics of the FR-MPF based sensor
2.1 The basics of a FR-MPF
A FR-MPF is a typical MPF based on recirculating delay line with periodic frequency response. The schematic diagram of the proposed FR-MPF based sensor is shown in Fig. 1.
The fiber ring is formed by connecting one input port and one output port of a 2 × 2 optical coupler (OC). In this scheme, a section of fiber in the fiber ring structure is used as the sensor head. Lightwave emitted from the broadband optical source (BOS) passes through an optical isolator (ISO) and then feeds into a Mach-Zender opto-electronic modulator (EOM). A polarization controller (PC) is connected between the ISO and the EOM in order to control the polarization state of the light and get the best modulation efficiency. The light is modulated by the microwave signals emitted from the vector network analyzer (VNA) via the EOM. Microwave signal is recovered at the photodiode (PD) and then sent into the VNA for analysis. The VNA is used for measuring the transmission frequency response of the whole system (character of a microwave filter).
Without considering the losses and conversion efficiency of the system, the frequency transmission response function () of the FR-MPF can be expressed as :
Figure 2(a) gives the calculated frequency response employing different coupler ratios under fixed fiber lengths with .The calculation shows that the MPF gets best extinction ratio when . Figure 2(b) depicts the calculated MPF frequency response with fiber length varying from 0.41m to 0.44m and the coupler ratio of 0.7. As shown in Fig. 2, the frequency response of the FR-MPF shows a periodic characteristic. The free spectral range (FSR) of the MPF frequency response is given by:
Equations (1) and (2) indicate that the frequency response of the FR-MPF is determined by . This characteristic makes FR-MPF available for sensing the physical parameters that can affect such as temperature, force, or fiber length. The FR-MPF is used as a temperature sensor in our proposal. The sensor interrogation method used in this case is to track the temperature caused frequency shift of the notch points, which are the points with minimum transmission. The frequency of the notch point can be determined as:
The temperature variation () caused frequency shift of the notch point can be expressed as:Eq. (4), since the temperature caused effective optical path change is far less than the original optical length of the fiber ring, thus the frequency shift of the notch points can be simplified as having a linear relationship with the temperature. According to Eq. (4), has a coefficient of , which means that the sensor sensitivity is related to the frequency detection range. This attributes to the so-called accumulation effect.
2.2 Temperature sensing character of the FR-MPF
A reel of single mode fiber with a length of about 200m is inserted into the fiber ring and used as a sensor head. The optical fiber coupler used in this experiment has a coupling ratio of 0.7 in order to get a MPF frequency response with high extinction ratio. During the test, with a temperature controller, the temperature of the sensing fiber is increased from 36 °Cto 76 °C with a step of 4 °C. The frequency response of MPF is recorded at each temperature. Figure 3 shows the measured frequency response curves in different frequency ranges when the temperature reaches 36 °C, 56 °C and 76 °C. The frequency span of the presented curves is 3 MHz.
From the Fig. 3, one can see that the frequency of the notch points shift to a lower frequency with the rising temperature. Notch points with frequency around 1 GHz, 1.5 GHz and 2.5 GHz (A1, B1, C1, as indicated in shadow area of Fig. 3) are chosen as the reference notch points in this case. Figure 4 depicts the frequency shift of the reference notch points versus the temperature and the fitted results, which show that the frequency shift of the reference notch points have a negative linear relationship with the temperature. As analyzed above, the sensitivity is nearly proportional to the detecting frequency range. From the fitted results, the sensitivity of the single fiber ring based sensor is found to be −7.796 kHz/°C and −11.456 kHz/°C, −19.068 kHz/°C, respectively, in those three different detection ranges, which agrees with analysis.
3. Enhancing fiber ring sensor sensitivity using the Vernier effect
In this experiment, Vernier effect is employed to improve the sensitivity of the fiber ring MPF based temperature sensor. Figure 5 illustrates the experimental setup of the sensing scheme. Two cascade fiber rings with different fiber lengths can generate Vernier effect in the microwave frequency response of the MPF. In order to have the Vernier effect with better performance, it usually needs two interferometers with similar FSRs. In this case, two fiber rings with slightly different lengths (about 200 m) are adopted in the experiment. The OCs used in this test have the same coupling ratios (0.7). In the two fiber rings, one acts as a sensing ring, while the other serves as a reference ring.
Figure 6 shows the measured frequency response with two individual fiber rings as well as the frequency response of the two cascaded fiber rings based MPF. In order to get the upper envelope curve of the microwave frequency response, a third order curve fitting algorithm has been applied to the extreme value points of the measured frequency response curves. The measured frequency response curve of the cascaded fiber rings at initial temperature shows a clear Vernier effect and the FSR of the MPF is significantly amplified. According to the characteristics of cascaded interferometers generated Vernier effect [25, 26, 28], the FSR of the envelope of the two-cascaded fiber rings can be deduced as:
In our experiment, the FSR of the FR-MPF with only the sensor fiber ring is 1.017119 MHz and the FSR of the FR-MPF with only the reference fiber ring is 0.983767 MHz, respectively. The FSR of the envelope of the cascaded fiber rings based MPF is around 30.24 MHz, which agrees well with the theoretical evaluation (about 30 MHz). Thus, by detecting the upper envelope, the FSR of the frequency response of the cascaded fiber ring FR-MPF sensor can be amplified about 30 () times.
In order to have a comparison with the single ring based sensor, the temperature sensing characteristics of the cascaded fiber rings based sensor have been tested. In the experiment, the frequency response is measured when the temperature of the sensor fiber is changed while the temperature of the reference fiber ring is set at a constant. Figure 7 shows the measured frequency response of the cascaded fiber rings based MPF sensor and the upper envelope of the frequency response curve in different frequency ranges when the sensing fiber ring is under three different temperatures. One can see clearly that the frequency response of the envelope exhibits a more significant shift compared with that of the single fiber ring based MPF sensor.
The temperature variation information is resolved by tracking the frequency shift of the upper envelope of the frequency response curve of the cascaded rings based MPF. Figure 8 shows that the measured relationship between the frequency of the reference points and temperature in different detection ranges. Notch points of the upper envelope with frequency around 1 GHz, 1.5 GHz, 2.5 GHz are taken as the detection reference points (A2,B2,C2, as indicated in shadow area of Fig. 8). The fitted results indicate that, by tracing the notch points of A2,B2,C2, the sensitivity of the cascaded fiber rings based sensor is −226.818 kHz/°C, −345.38 kHz/°C, and −556.856 kHz/°C, respectively. Compared with the sensitivity of the single fiber ring based temperature sensor, the sensitivity has been magnified about 30 times in each corresponding detect range, which agrees well with the above analysis. In order to further show an intuitive improvement with the help of Vernier effect, a comparison between the sensitivity of the proposed sensor with other temperature sensors based on tacking the notch point frequencies of MPFs in [13,17] has been made. The single fiber ring temperature sensor has a similar sensitivity with sensor in  and a lower sensitivity than the sensor in . The cascaded fiber rings based sensor has a much higher sensitivity than both sensors in  and , which means the Vernier effect is a useful method to improve the sensitivity of the FR-MPF based sensor.
In this paper, the working principles and characteristics of a FR-MPF based temperature sensor are analyzed and experimentally studied. The Vernier effect introduced by the cascaded fiber rings MPF is used to improve the sensitivity. The sensing characteristics of both single ring and cascaded fiber rings based sensors have been experimentally studied and compared. The results show that, with the help of the Vernier effect, the temperature sensitivity has been improved about 30 times in our experiment. The proposed sensor is easy to be interrogated since the detected signals are in microwave domain which has a much lower frequency than optical domain. The sensitivity magnification factor can be tailored by properly setting the length difference of two fiber rings. What is more, besides temperature, the proposed approach can also be designed for sensing other physical parameters.
WNLO; The National 1000 Young Talents Program of China; National Natural Science Foundation of China (No. 61575166)
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