We propose and experimentally demonstrate a novel simultaneous strain and temperature fiber optic sensor. The sensing head is formed by two concatenated ultra-short distributed Bragg reflector lasers that operate in single longitude mode with two polarization modes. The total length of the sensing head is only 18 mm. The two lasers generate two polarization mode beat notes in the radio-frequency range which show different frequency response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by radio-frequency measurement. This approach has distinctive advantages of ease of interrogation and avoidance of expensive wavelength detection.
©2011 Optical Society of America
There has been considerable interest in developing simultaneous strain and temperature fiber optic sensors. This is not only because cross sensitivity is a key issue for the practical applications of fiber optic sensors, but also because multi-parameter sensors can reduce the complexity of the sensing systems in situations requiring multi-parameter and multi-point measurement. The principle of simultaneous strain and temperature sensors are usually based on the detection of two physical parameters which have different sensitivities to strain and temperature. Fiber Bragg gratings have been of great interest in sensing technology in recent years because of their small size, wavelength-encoding and multiplexing capability. Many techniques based on fiber Bragg gratings have been reported for simultaneous strain and temperature measurement. A simple and straightforward approach is to employ two independent Bragg gratings with the first one subjected to strain and temperature and the second one isolated from strain. The concept of a sensing head formed by two Bragg gratings with different strain and temperature response has been explored. Examples include configurations based on two gratings in different diameter fibers , in different dopant fibers , in different base material fibers , and operating at different wavelengths . Several approaches based on a single Bragg grating for simultaneous strain and temperature measurement was also demonstrated, such as utilization of a single Bragg grating in birefringent fibers , superstructure Bragg grating , a single Bragg grating in tapered fiber , and a single Bragg grating straddling over the junction of two fibers . A number of schemes based on a sensing head formed by Bragg gratings in combination with other fiber optic devices have also been demonstrated. The configurations include the combination of two Bragg gratings and a long period grating , the combination of a Bragg grating and Fabry-Perot interferometer , the combination of a Bragg grating and Mach-Zehnder interferometer , and the combination of a Bragg grating and photonic crystal fiber based inter-modal interferometer . Recently, a sensing head formed by a dual-polarization fiber grating laser was demonstrated , where the mean wavelength and polarization mode beat frequency of the laser were utilized to discriminate strain and temperature. All above approaches can be divided into two categories. The first category is based on the detection of two separate wavelengths which have different response to strain and temperature. The second category is based on the detection of wavelength and intensity (or spectrum bandwidth). The disadvantage of the second category is, the intensity detection undercuts multiplexing capability of fiber Bragg grating sensors. For all above simultaneous strain and temperature sensors, wavelength detection is necessary. However, it is know that, complex and expensive optical systems are required to achieve accurate wavelength measurement. The high cost of the wavelength detection unit impedes further applications of fiber Bragg grating sensors. It will be highly desirable if we can develop a simultaneous strain and temperature sensor which not only shares the advantages of fiber Bragg grating sensors but also avoids expensive wavelength detection.
Polarimetric fiber grating laser sensor converts the measurrand into a corresponding change in the beat frequency between the two polarization modes from the laser [14–16]. Because the beat frequency is in the radio-frequency range, this type of sensor has distinctive advantages of ease of interrogation and avoidance of expensive wavelength detection that is required in the passive fiber Bragg grating sensors. In this paper, we present a novel simultaneous strain and temperature fiber optic sensor based on radio-frequency measurement. The sensing head is formed by two concatenated ultra-short distributed Bragg reflector (DBR) lasers. The total length of the sensing head is only 18 mm. Both lasers operate in robust single longitude mode with two polarization modes. Each laser generates a polarization mode beat note at radio-frequency range. The two lasers have different beat frequencies which exhibit different response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by monitoring the two beat frequencies.
Figure 1 shows the schematic diagram of the proposed simultaneous strain and temperature sensor. The sensor head is formed by two concatenated DBR fiber lasers with the first one fabricated in Er-doped fiber and the second one fabricated in Er/Yb co-doped fiber. Typical DBR fiber lasers are a few cm long, leading to the laser longitude mode spacing much smaller than the grating reflection bandwidth. As a result, there are multiple modes that meet conditions for lasing. The dominant mode oscillates first and other modes are suppressed, so normally the lasers can operate in single longitude mode. However, mode hopping will occur when the laser is subjected to external perturbations that distort the grating spectrum, such as temperature or strain gradient or a localized perturbation to the subsection of the Bragg gratings. This is a key problem limiting the practical applications of DBR fiber lasers. To address this problem, here ultra-short DBR fiber lasers which have longitude mode spacing comparable to the grating reflection bandwidth were employed. Because the ultra-short cavity supports only one longitude mode, it absolutely obviates possibility of mode hopping when the laser is subjected to any external perturbations.
Here the DBR fiber lasers operate in single longitude mode with two polarization states. When the laser output is monitored with a high speed photodetector, a beat note will be generated by the two polarization lines. The beat frequency is given by
When the DBR laser is subjected to strain or temperature perturbation, the birefringence will be changed. As a result, the beat frequency will shift and so can be considered as an effective signal output. The response of the beat frequency to strain and temperature can be expressed as
Because of different dopant and slightly different structure-parameters, the DBR lasers in Er-doped fiber and Er/Yb DBR fiber exhibit different beat-frequency-response to strain and temperature. When temperature and strain change simultaneously, using the Eq. (2), we can get the matrix
The coefficient matrix K can be defined by separately measuring the strain and temperature responses of the polarization beat frequency of the two lasers. Then the strain and temperature can be determined simultaneously by measuring the beat frequencies of the Er-doped fiber laser and the Er/Yb co-doped fiber laser.
3. Experiment and results
The ultra-short DBR fiber lasers were fabricated by directly inscribing two wavelength-matched Bragg gratings in active fibers using the setup described in . A 193 nm excimer laser and phase mask method were used. Because the 193 nm UV light induces index change by two-photon excitation process, it does not require hydrogen loading to photosensitize the fiber. This avoids the laser efficiency degradation due to hydrogen-induced loss at pump wavelength and excited-state lifetime reduction of Er3+ ions. The DBR laser in Er/Yb co-doped fiber consisted of 2.2-mm-long low reflectivity grating, 4-mm-long high reflectivity grating, and 2 mm grating spacing. The total length of the Er/Yb co-doped fiber laser was 8.2 mm. The DBR laser in Er-doped fiber consisted of two 3-mm-long gratings and 2-mm grating spacing. The entire length of the Er-doped DBR fiber laser was only 8 mm. The two DBR lasers were concatenated in a single fiber as the sensing head. The total length of the sensing head was only 18 mm. The 980 nm pump light was launched into the laser array from the Er-doped fiber laser side through a wavelength division multiplexer (WDM). The backward laser output was launched into a high speed photodetector (PD) through a polarization controller (PC) and an in-line polarizer. A radio-frequency spectrum analyzer was used to monitor the beat notes of the lasers.
Figure 2 shows the output spectrum of the laser array with pump power setting to 187 mW. The Er-doped fiber laser operated around 1536.12 nm with signal-to-noise ratio of ~55 dB. The Er/Yb co-doped fiber laser operated around 1539 nm with signal-to-noise ratio of ~60 dB. Figure 3 shows the beat note spectrum of the laser array measured with the radio-frequency spectrum analyzer. The Er-doped fiber laser generated a beat note at 2.664 GHz with signal-to-noise ratio better than 50 dB. The Er/Yb co-doped fiber lasers generated a beat note at 1.336 GHz with signal-to-noise ratio better than 60 dB. The beat frequency of the Er-doped fiber laser is much higher than that of the Er/Yb co-doped fiber. This denotes that the Er-doped fiber has much higher birefringence than the Er/Yb co-doped fiber. In spite of the Er-doped fiber laser in the front, the Er/Yb co-coped fiber laser had higher laser output and stronger beat note. This is because the Yb ions have strong absorption at 980 nm and transfer their energy to the Er ions with high efficiency, significantly increased the laser efficiency.
The strain response was investigated by bonding both sides of the sensing head onto two translation stages with epoxy. While the sensing head was stretched with the translation stage, the laser beat frequencies were monitored with the radio-frequency spectrum analyzer. The environment temperature was kept at 15 °C during the strain response measurement. Applied strain was calculated from the elongation of the stretched fiber divided by the original length. During the strain response characterization, environment temperature was kept constant. Figure 4 shows measured sensor response to strain in the range from 0 to 1200 με. It is clear that the beat frequencies increase with strain, and the strain sensitivity of Er-doped fiber laser is higher than the Er/Yb co-doped fiber laser. The strain coefficients of the Er-doped fiber laser and Er/Yb co-doped fiber laser were estimated, using linear regression fits, as kEr,ε = 8.75 ± 0.104 KHz/με(R2 = 0.9986), and kEr/Yb,ε = 6.42 ± 0.068 KHz/με (R2 = 0.9989), respectively.
The temperature response was investigated by putting the sensing head into a tube oven. A thermocouple was placed near the sensing head for measurement of the temperature. The sensing head was kept unstrained. Figure 5 shows the measured beat frequency shifts as functions of temperature in the range from 15 °C to 100 °C. As shown in Fig. 5, the beat frequencies decrease with temperature, and the temperature sensitivity of the Er/Yb co-doped fiber laser is higher than the Er-doped fiber laser. The temperature coefficients of the Er-doped fiber laser and Er/Yb co-doped fiber laser were estimated, using linear regression fits, as kEε,T = −678 ± 5.52 KHz/ °C (R2 = 0.9995), and kEr/Yb,T = −1142 ± 4.11 KHz/°C (R2 = 0.9999), respectively.
By taking the inverse matrix of K and the measured coefficients in (3), we can get the matrix
We reported a novel fiber-optic sensor for simultaneous strain and temperature measurement based on radio-frequency detection. The sensor head was formed by two concatenated ultra-short DBR fiber lasers with the first one fabricated in Er-doped fiber and the second one fabricated in Er/Yb co-doped fiber. The total length of the sensing head was only 18 mm. The two lasers generate two polarization mode beat notes at radio-frequency range, which show different frequency response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by monitoring the two beat frequencies. The distinctive advantages of the proposed simultaneous strain and temperature sensor are ease of interrogation and avoidance of expensive wavelength detection. Other advantages include absolute frequency encoding and capability to multiplex a number of sensors on a single fiber by use of frequency division multiplexing technique.
This work was supported by the Key Project of National Natural Science Foundation of China (60736039) and the Fundamental Research Funds for the Central Universities (21609102).
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