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Abstract
We report and demonstrate a highly-sensitive refractive index (RI) sensor based on a linear-cavity dual-wavelength erbium-doped fiber laser (DWEDFL). The optical spectrum of the laser varies as the external environmental RI changes from 1.3 to 1.335. The DWEDFL has a linear-cavity configuration with two fiber Bragg gratings (FBGs) with central wavelengths < 1 nm apart. Since both FBGs share the same EDF gain medium, gain competition occurs in the cavity. Optical loss of one wavelength can be introduced by immersing the sensing component, a 15 mm micro-fiber (MF), in a solution under test. Experimental results demonstrate a high sensitivity of −231.1 dB/RIU (refractive index unit) and 42.6 dB/RIU in the range from 1.300 to 1.335. The relative power change at the two FBG wavelengths reveals a higher sensitivity of −273.7 dB/RIU with better stability due to reduced light source jitter and external perturbation. Due to its high sensitivity and simple structure, the dual wavelengths gain competition RI sensor has potential applications in chemical and biochemical sensing fields.
© 2017 Optical Society of America
Corrections
6 July 2017: A typographical correction was made to the title.
1. Introduction
Refractive index sensing has become an important research area and attracted great attention because of its wide applications in many fields of modern industries, such as liquid level sensing [1,2], pH value measurement [3,4]. Optical fiber based RI sensors are now one of the major research directions due to its many advantages: lightweight, compactness, high sensitivity, large bandwidth, ease in manipulating light transportation and immune to electromagnetic interference (EMI) [5]. Based on signal demodulation mode, these technologies for RI sensing can be classified as follows: (1) Wavelength type including FBG [6,7], LPG [8]. They are usually simple and reliable but with relatively low sensitivity. (2) Interference type such as Mach-Zehnder interferometer (MZI) [9], Sagnac [10], Fabry-Perot interferometer (FPI) [11], multi-mode interferometer (MMI) [12]. They have advantages of being highly sensitive, but are susceptible to external environmental perturbation. In addition, the cross-sensitivity of other parameters cannot be ignored. (3) Some research groups investigate the beat frequency interrogation methods based on dual wavelength [2] or polarization [13]. They benefit from their high sensitivity (21.2 MHz/m for liquid level [2], 19 GHz/MPa for hydrophone [13]), ease of interrogation, good reliability and stability. However, for sensors either rely on dual-wavelength beat or polarized beat detection, it is technically challenging to obtain two elaborate designed FBGs in terms of central wavelength separation, narrow bandwidth, and FBG length. (4) Intensity-type RI sensors (tip or taper) [14,15] show advantages of real-time responses and simple construction, although sensitivity has yet to be improved (such as <26 dB/RIU [14], ~0.22 dB/RIU [15]).
In recent years, some novel optical fiber sensing (OFS) mechanisms have been developed rapidly to provide alternative approaches for sensor development. Photonic crystal fiber (PCF) based fiber sensors [16–22] are especially popular due to its unique structure and performance. However, they are rarely utilized in RI sensing applications because of complicated production processes. A variety of techniques are developed in order to make sensors of excellent performance, such as coating [23], micro-machining [24], micro/nano structure [25], multiple interference [26], vernier effect [27]. Fiber laser sensors (FLS) have been investigated extensively for their excellent performance of high optical power and narrow bandwidth [13, 28]. Dual-wavelength erbium-doped fiber laser (DWEDFL) opens new possibilities for fiber based applications in fields of optical fiber communication system [28–30], but is rarely used in optical fiber sensing system [31], especially refractive index sensing.
In this article, we demonstrate a novel highly-sensitive RI sensor based on a dual-wavelength erbium-doped fiber laser (DWEDFL). The DWEDFL has a linear-cavity configuration, which includes two FBGs with similar bandwidth but different central wavelengths < 1 nm apart from each other. Gain competition between the two FBG wavelengths occurs because the same gain medium is shared by two wavelengths components. Optical loss of one wavelength can be introduced by immersing a short section of micro-fiber (MF) with 15 mm length and ~14 µm waist diameter in a surrounding solution. By subtracting the optical power at two lasing wavelengths, the power jitter due to common sources, i.e. the pump power fluctuations, can be canceled out. This achieves a total high sensitivity of −273.7 dB/RIU in the RI range of 1.300-1.335. Owing to its simple structure and extremely high sensitivity, RI sensors based on dual wavelength gain competition mechanism have potential applications for RI measurement in chemical or biochemical sensing fields.
2. Experimental setup and principle
The dual wavelength erbium-doped fiber laser has a simple linear configuration as shown in Fig. 1. The key components for introducing dual wavelength gain competition are two FBGs (FBG1 and FBG2 in Fig. 1) with similar reflection of ~85%, similar 3-dB bandwidth of < 0.15 nm, The central wavelengths are slightly different (λ1 = 1550.04 nm, λ2 = 1550.98 nm), as measured by optical spectrum analyzer before the FBGs were integrated in the laser setup. A 980 nm laser diode (LD) pumps the 5 m EDF (gain factor ~6 dB/m, YOFC. Inc.) through a wavelength division multiplexing (WDM). The laser light is amplified and further reflected by an optical fiber reflector (OR), finally outputs through a 50:50 output coupler (OC). This laser configuration allows the formation of two resonant cavities: 1) FBG1 to OR (denoted as cavity C1), and 2) FBG2 to OR (denoted as cavity C2). Both cavities share the same EDF gain medium. A variable optical attenuator (VOA) is embedded in cavity C1 to realize cavity loss control during the gain competition processes.
Fig. 1 Schematics of the dual wavelength erbium-doped fiber laser (DWEDFL). Inset: Microscope lateral view of the MNF. VOA: variable optical attenuator; OC: fiber output coupler; WDM: wavelength division multiplexer; EDF: Erbium doped fiber; MF: micro-fiber; OR: optical fiber reflector; OSA: optical spectrum analyzer.
In order to achieve stable oscillation of dual wavelengths, the single-path loss is required to match the cavity gain, i.e. the following condition needs to be satisfied [32].
Here, subscript 1 and 2 are denoted for two FBG wavelengths, represents the gain parameter of,denotes the EDF length. , are the single-path threshold gain and single-path loss of, respectively. According to our experimental parameters and data, the single-path threshold gain can be calculated as = = 6 dB/m*5 m = 30 dB. And the cavity loss can be obtained by the following addition = + + + + + = 15 + 0.9 + 3 + 2.9 + 7.1 + 0.9 dB≈30 dB. As we know,≈, so ≈≈ = can be established, that is to say stable oscillation of dual wavelengths can be achieved.Because of the gain competition caused by the homogeneous gain broadening of EDF [33], the optical power at the two lasing wavelengths will compete with each other. In this case, the optical power is very sensitive to loss induced by external parameters, for example, immersing a short section of fiber in a solution. The inset in the dashed box is the microscope side view of our micro-fiber (MF) in Fig. 1 with a diameter of ~14 μm and length of ~1.5 cm. The MF structure is prepared from a common single mode fiber (SMF) by a heating and pulling method with hydrogen flame, also sensitive to external liquid RI surrounding due to its great evanescent field. So a simple DWEDFL for RI sensing scheme based on gain competition mechanism is constructed.
It is worth noting that the VOA is used for tuning output power of by controlling the optical loss of cavity C1. By doing this, the output power of and can be adjusted to almost the same level after MF is embedded in cavity C2. So an intense gain competition between the two wavelengths can be achieved as we expected.
The output spectrum of our DWEDFL is recorded by an optical spectrum analyzer (OSA, YOKOGAVA AQ6370C) with a resolution of 20 pm. Figure 2 shows the output optical spectrum of the DWEDFL at a pump power of 162 mW (200 mA control current). The two dominant wavelength components, ~1550.2nm and ~1551nm, have signal-noise-ratio (SNR) of around 15 dB. The two lasing wavelengths and their full width at half maximum (FWHM) bandwidth agrees with the FBGs spectral from the manufacturer (T&S, Co., Ltd). The output power of two dominant wavelength components is almost the same (λ1 = −59.4 dBm, λ2 = −58.9 dBm), which indicates a strong gain competition process between the two closely spaced wavelengths.
3. Experiment results and discussion
In order to experimentally verify the RI sensing performance of our DWEDFL based on gain competition mechanism, the RI measurement experiment is carried out by utilizing refractive index matching solution (Cargille Laboratories Inc.). In the case where the RI of the MF (~1.45) is larger than that of the external solution, light is confined in the fiber due to total internal reflection. As the refractive index of the external solution increases, leakage of light through the MF wall occurs, leading to an increasing loss in the final laser output power. As shown in Fig. 3, the optical power at λ1 increases with the solution RI from 1.300~1.335, while the peak power for λ2 decreases. This optical power change can be explained by the gain competition mechanism in the EDF caused by the variation of total loss in the laser cavity. The laser is in favor of lasing at λ1 when the total cavity loss of C2 is high, because cavity C1 will get a larger gain relative to C2 in this case. In addition, we discover that the power change is less dramatic for λ1 than that for λ2 as the solution RI changes. The reason can be that the λ2 in cavity C2 is our sensing structure which is sensitive to external RI surrounding, the induced optical loss will lead to a gain contention by cavity C1. When dual wavelengths interval is small, the gain competition and impact to each other will be greater than the case of dual wavelengths interval is large. With the wavelengths of the two FBGs closer to each other, we expect to see power varying improvement in λ1 due to stronger gain competition. This work is still in progress.
To extrapolate the sensitivity of the sensor in terms of refractive index unit (RIU), we perform a linear fit to the peak power of the two separate wavelength components as a function of RI variation, as shown in Fig. 4. The results give a sensitivity of 42.6 dB/RIU at λ1 and −231.1 dB/RIU at λ2, with linearity of 0.984 and 0.943, respectively. That is to say, we achieve a novel and highly sensitive RI sensor based on our DWEDFL applying gain competition mechanism in the range from 1.300 to 1.335 within a step of 0.005.
Besides the sensitivity and sensing range, stability is another important aspect for evaluating the performance of the sensor. There are two major factors that affect the stability of the proposed RI sensor. The first noise source is from optical power fluctuation of the 980 nm pump source. The second source is perturbation from the external environment, such as temperature, humidity, pressure and other parameters, which affects the stability of the laser output. Because these two wavelengths have experienced the same light source jitter and external environment changes, the instability introduced by the above mentioned factors will be effectively removed by measuring the differential power between the two cavities. Figure 5 shows the extrapolated sensitivity of −273.7 dB/RIU using this differential method. Figure 6 shows the power fluctuation of dual wavelength as well as its differential power fluctuation when the MF is exposed in air. An overall differential power fluctuation of ± 0.26 dB indicates a great power stability (0.44%) for reliable sensing application.
Another important technical aspect of fiber sensor is the minimum resolution, which is closely related to system noise and other uncertainties. Here in our scheme, the proposed OSA based intensity measurement for RI implies a minimum RI resolution of 1.5 × 10−3 RIU with the OSA linear accuracy ± 0.4 dB. This value is orders of magnitude larger than that in interferometric and SPR systems (10−5~10−7). We believe this is due to the low precision of our OSA. While this defect can be well-resolved by using an OSA with high precision, and our work aims to verifying the feasibility of the proposed concept.
The comparison of RI sensing performance between our DWEDFL gain competition mechanism based scheme and other recently reported intensity-type schemes are listed in Table 1. The RI sensor in this work has a significant improvement in the sensitivity.
Table 1. Comparison of sensing performance of our RI sensor with recently reported isotype schemes
4. Conclusions
We have report a highly-sensitive refractive index (RI) sensor based on a simple linear-cavity dual-wavelength erbium-doped fiber laser (DWEDFL). Two resonant cavities supporting slightly different wavelengths are sharing the same EDF gain medium, leading to gain competition between the two wavelength components when the cavity loss is varied. The change in cavity loss is introduced by immersing a 15 mm micro-fiber (MF) with ~14 µm waist diameter in a solution under test. Differential power measurement for the two wavelengths is carried out to achieve a total high sensitivity of −273.7 dB/RIU as well as a stability of ± 0.26 dB in the range from 1.300 to 1.335. The proposed sensor shows supreme sensitivity as compared with the recently reported schemes for RI sensing and has great applications for RI measurement in chemical or biochemical sensing fields.
Funding
National Natural Science Foundation of China (61138006, 61308028, and 60925021); Science Foundation of Wuhan Institute of Technology (k201616); Shenzhen Research Foundation (JCYJ20140901003938993).
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