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This paper firstly demonstrated the refractive index (RI) characteristics of a singlemode-claddingless-singlemode fiber structure filter based fiber ring cavity laser sensing system. The experiment shows that the lasing wavelength shifts to red side with the ambient RI increase. Linear and parabolic fitting are both done to the measurements. The linear fitting result shows a good linearity for applications in some areas with the determination coefficient of 0.993. And a sensitivity of ~131.64nm/RIU is experimentally achieved with the aqueous solution RI ranging from 1.333 to 1.3707, which is competitively compared to other existing fiber-optic sensors. While the 2 order polynomial fitting function, which determination relationship is higher than 0.999, can be used to some more rigorous monitoring. The proposed fiber laser has a SNR of ~50dB, and 3dB bandwidth ~0.03nm.
© 2014 Optical Society of America
1. Introduction
Refractive index (RI) is an important parameter in chemical and biotechnological industries. All-fiber RI sensors have been widely used due to their distinct advantages such as high sensitivity, compact size and immunity to electromagnetic interference. Many ways, such as fiber Bragg gratings (FBGs) [1], long period fiber gratings (LPFGs) [2], Fabry-Perot interferometry [3], fiber surface plasma technology [4], fiber taper technology [5] and the combination of two or more techniques above [6], have been demonstrated in RI measurement. However, the fabrication of LPFG and FBG (generally require phase mask and UV laser) is complicated and expensive. While the taper based fiber device is fragile and need special splicing program. Recently, singlemode-multimode-singlemode (SMS) fiber structure based optical devices [7–13] have been investigated for various applications benefits from their unique advantages, ease of fabricated and low cost, for example. The structure is fabricated by using a section of multimode fiber (MMF) sandwiched splicing into two single-mode fibers (SMF) leads. Several applications such as the measurement of axial strain [7], displacement [8], micro-bend [9], temperature [10], RI [11], and chemical vapor concentration [12] have been demonstrated and show a great performance. And some researchers studied the SMS characteristics as fiber lens [13] and band-pass filter [14]. It is worth noting that the ASE C-band light source is used in the conventional SMS fiber structure based sensing systems [7–11], which intensity is low. And thus causes the transmission spectrum of the sensing system relatively low. Another problem existed in the SCS based sensing systems is that the 3dB bandwidth of the transmission spectrum is large, and thus these systems have a lower resolution. All those problems will cause the measurements inaccuracy, which should be restrained in practical applications
Fiber laser sensor [12,] [15], [16] is investigated extensively for their excellent performance of high intensity and narrow 3dB bandwidth. Fiber laser sensors is conventionally divided into two types according to the characteristic parameters monitored in the system, one is based on the varied lasing wavelength, while another monitors the changed beat frequency because of ambient disturbance. Though the fiber laser sensors have been widely demonstrated, its applications in measurement of ambient RI have not been further discussed. Lan [11] demonstrated fiber vapor sensor using an fiber ring laser combined with a SMS fiber structure with coating. The scheme measured the concentration of the chemical vapor using refractive characteristic of the coated zeolite when the film is exposed to chemical vapor, which is complicated in operation. The coating can be considered as the cladding of the MMF for the origin cladding is etched. And thus difficult ensure the consistent of the sensor.
In this paper, we proposed and demonstrated a RI sensor using fiber ring laser interrogated with a singlemode-claddingless-singlemode (SCS) fiber structure. The SCS structure in this laser sensing system is used not only as the filter but as the sensing head. The proposal has the additional advantages of higher OSNR and narrower 3dB bandwidth than the conventional sensing system based on SCS fiber structure. The linear characteristic of the sensor is well enough in small range measurement applications. The measurements has also been fitted using 2 order polynomial fitting, the coefficient of determination is higher than 0.999, shows that the proposal can be applied in measurement of RI after well calibrated, though the calculation is a little complex than that linear result.
2. Principle and Experiment setup
2.1. RI characteristics of the SMS fiber structure
The basis of this paper is the concept of multimode interference (MMI) effect in the MMF section when the light is injected from a SMF leads. Assuming that the spliced point between SMF and MMF is ideally aligned and the MMF has circular cross, thus only linear polarized radial modes will be excited and transmitted in the MMF section. The power distribution in the MMF is determined by the coupling coefficient from LP01 to LP0m mode. The coupling coefficient, according to coupling mode equations, can be expressed by
Where Es(r) and Φn(r) are the eigenmode of the input SMF and the nth eigenmode of the MMF.
The field distribution after propagate a distance z can be expressed as
Where βn is the longitudinal propagate constant of nth mode within the MMF. N is the total mode number supported by the MMF.
When the ambient RI changes, the propagation constant changes, and hence causes the transmission spectrum changes. In the other word, the pass-band of the SCS fiber structure filter is varied when there is a variation of ambient RI, for the reason that the ambient aqueous solution act as the cladding of the claddingless fiber. At the junction between MMF and output SMF leads, the higher order modes are filtered.
According to the MMI theory, the wavelength spacing can be expressed as follows
Where neff is the effective refractive index of the wavelength, a and L are the radius and length of MMF, m and n are the mode order. Therefore the peak wavelength of the filter will change when there is a variation of the ambient solution.
2.2 Experiment setup
The conventional sensing system based on SCS fiber structure is established according to the configuration shown in Fig. 1. The homemade claddingless fiber is spliced between two SMF-28s using a commercial splicer (FSM-60s, Fujikuwa). The diameter of the homemade claddingless fiber is 104μm. The optical spectrum analyzer (OSA, YOKOGAWA AQ6375) is used to record the varied spectra when the ambient RI is changed.
Fig. 1 Configuration of conventional SCS based RI sensing system. Inset: schematic of the SCS fiber structure.
The proposed fiber ring cavity laser is also founded in accordance with the schematic shown in Fig. 2 (a).
Fig. 2 Experiment setup of the proposed laser sensing system (a); and the function relationship between the length of the claddingless fiber and FSR of the filter (b).
The fiber ring cavity is formed by a section of claddingless fiber with length of 23.3cm, an isolator, a wavelength division multiplexer (WDM) of 980nm and 1550nm, and a section of homemade Er-doped fiber (EDF). A 976nm pump light source and OSA are also used in this configuration. From the Eq. (3), it can be easily obtained that the wavelength spacing is inversely proportional to the length of MMF and the function relationship between the length of claddingless fiber and the FSR of the filter is shown is Fig. 2 (b). Thus, the length of the MMF is either too short for large wavelength spacing or too long for complicated operation. As such, in the experiment setup, the length of the MMF is chosen to be ~25cm, at which the wavelength spacing is less than 20nm. While after splicing the claddingless fiber between the two SMFs with low insertion loss (<5dB), the length of claddingless fiber is rested about 23.3cm.
3. Experiment results and discussions
The transmission spectra of the system with the claddingless fiber in the air (black line) and pure water (red line) are recorded as shown in Fig. 3. The ambient temperature is ~21°C. The volume of the water using in this experiment is 120ml. To guarantee the consistence of the experiment as possible, the aqueous solution volume using in the experiment is a constant (120ml).
Fig. 3 Transmission losses of the conventional SCS fiber structure with the claddingless fiber in the air (black line) and pure water (red line).
Using the parameters of the homemade claddingless fiber, the mode coupling coefficient is calculated. The result shows that the 3rd and 4th order modes have a larger coupling coefficient. From Eq. (3), the calculated wavelength spacing of the section claddingless fiber is ~20.6nm, which is approximately equals to the measured wavelength space between the two dips shown in Fig. 3. As the claddingless fiber is immersed into the pure water, the spectrum moves 7.193nm to red side, which brings about the next minima of the transmission spectrum out of the OSA scanning range. Noting that the wavelength space between two dips signed in Fig. 3, which are the interferences between two modes have identical order in the air and pure water, is approximately equal. The result presents that ambient RI causes a small impact to the wavelength space. Therefore, the changed wavelength spacing of the filter cannot be taken into consideration in the experiment.
As shown in Fig. 3, the spectrum of the conventional sensing system induced a red-shift by the ambient RI increase, and the peak wavelength of the spectra shifts 7.193nm to longer wavelength. Noting that there are two pass-bands of the SCS based filter, but only a lasing wavelength is excited. This is because of the peak gain wavelength of the EDF based laser is related to the length of EDF. In other words, one can choose the lasing wavelength by changed the length of EDF when the filter has more than one pass-band. We choose the ~1550nm as the characteristic wavelength for the reason that it is near the central of the ASE spectrum. It is clear that the maximum of the SCS filter immersed into the water is ~1557.5nm, which is consistent with the lasing wavelength measured in the experiment shown in Fig. 4 (black line). The 3dB bandwidth, obtained from the conventional sensing system, is larger than 2nm, which is hardly assured the measurement precise. Some researchers use the dip wavelength as the characteristic to calculate the ambient parameters. However, the optical intensity of the dip is lower than −45dBm, which is mainly resulted from that the lower optical intensity of the ASE light source (the insert loss of the structure of the sensing system is less than 3dB).
Figure 4 shows the lasing spectrum at different RI of the aqueous solution, and shows a red-shift with the RI increase same as the SCS filter. In the experiment, the RI is changed by the variation of glycerol concentration. The claddingless fiber is cleared after every measurement using a piece of lens paper with appreciates alcohol. The OSNR of the laser sensing system, obtained from Fig. 4, is higher than 50dB, while the conventional sensing system has an OSNR of only ~10dB ranging from 1535nm to 1560nm. And the peak optical intensity (~0dBm) is also largely higher than the conventional sensing system which is less than −40dBm. All of the above shows that the proposal possibly can be applied in long-distance operation. Also it is valuable especially applications in harsh environment for the reasons mentioned above. While the conventional sensing system based on the structure cannot because of its lower intensity and OSNR.
The function relationship between the lasing wavelength and the RI of the aqueous solutions is shown in Fig. 5. The red and navy lines shown in Fig. 5 are the linear and parabolic fittings of the measurement.
Fig. 5 Measured central wavelength shift vs. glycerol concentration and the linear (red) and parabolic (navy) fittings. R2: coefficient of determination in the fittings.
The RI range of the aqueous solutions measured in the experiment is from 1.333 to 1.3707. The experiment changes the glycerol concentration at a step of 3%, corresponding to RI varies ~0.13.
From Fig. 5, it can be concluded that the proposed laser system can be used in the measurement of ambient RI after well calibration. The two order polynomial fitting shows that the function relationship between ambient RI and lasing wavelength is not linear, which agrees the Ref [11]. well. While for small range monitor, the relationship between the two parameters can be considered as linear, and thus the sensor can be easily applied in the practical operation. In larger range or rigorous measurement, the 2 order function relationship obtained from this experiment can also be useful, though a little complex in demodulation.
4. Conclusion
This paper proposed and demonstrated a wavelength encoded fiber ring cavity laser sensing system. A SCS fiber structure simultaneously acts as the sensing head and filter of the laser. With the aqueous solution RI varies, the pass-band of the SCS filter changes, and hence changes the lasing wavelength. Linear fitting is done to the measurement and represent a favorable linearity with the R2 of 0.993. This shows that the proposal has a good ability in measurement of the aqueous solution RI. Then we do a 2 order polynomial fitting to the measurement presents R2 of 0.9998, which shows a great RI sensing characteristic of the sensing system and certainly can be applied in some rigorous areas.
Acknowledgments
This work was supported by the Major State Basic Research Development Program of China granted No. 2010CB328206.
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