1. Introduction

Optical microfibers have attracted considerable attention as basic functional elements for physical, chemical or biological sensing [1–16]. Due to the small dimension for light confinement, a waveguiding microfiber leaves a large fraction of the guided field outside the fiber as evanescent waves, making it highly sensitive to the change of ambient medium. In the past years, extensive research and development activities have been devoted to evanescent-field-based microfiber sensing applications, such as refractive index sensor [1–13], hydrogen detection [15] and ultra-sensitive surface absorption spectroscopy [16]. Among the various sensing schemes, refractive index sensing is widely employed since many biological or chemical specimens can be identified by measuring their refractive indices. Microfiber ring resonators, in forms of loops, knots or coils, are promising candidates for refractive index sensing owing to their small footprints, high sensitivity and simple structures, as have attracted much attention recently [9–13]. However, when operating in liquids, freestanding microfiber loops are likely to be disturbed by the flowing liquid, resulting in serious distortion of the resonance. Moreover, in a small loop assembled with sharply bent microfiber, when the index contrast between the fiber material and the surrounding media decreases to a certain degree (usually happens when the concentration of the specimen goes high), the high bending loss of the microfiber loop appears. For example, Fig. 1 shows typical transmission spectra of a copper-rod-supported loop (assembled with a 2.1-µm-diameter microfiber) immersed in a glycerol aqueous solution. With the increasing concentration of the glycerol (from 60 to 76 wt.%), the refractive index of the solution increases (from 1.40 to 1.43 at the wavelength of 1.22 µm [17,18]), and the bending-loss-induced cutoff wavelength shifts from 1400 nm to 900 nm. More recently, polymer embedded microfiber loops are reported for sensing in liquids [10,13], demonstrating a promising solution to the fragility of a microfiber loop sensor. Here we show that, a rod-supported microfiber loop that has been reported elsewhere [19], offers a robust microfiber loop resonator for optical sensing in liquid. In addition, by shifting the critical coupling and resonance peaks to a relatively shorter wavelength, this kind of loops can be used for optical sensing in high-concentration solution with low index contrast.

 

Fig. 1. Transmission spectra of a 480-µm-diameter copper-rod-supported loop (assembled with a 2.1-µm-diameter silica microfiber) immersed in a glycerol aqueous solution with glycerol concentration of 60, 68, 72 and 76 wt.% (corresponding to a refractive index of 1.40, 1.41, 1.42, 1.43), respectively.

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2. Sensor configuration

The refractive index sensor investigated in this work is schematically illustrated in Fig. 2. A microfiber loop resonator, with a tunable coupler formed at the overlap area, is assembled by wrapping the waist of a biconical fiber taper around a 480-µm-diameter copper rod. The uniform waist (microfiber), usually a few micrometers in diameter, is fabricated by taper-drawing of a standard single-mode fiber (Corning SMF28). The two ends of the microfiber connected to the loop are used as input and output ports, respectively. The copper rod and the microfiber loop are immersed in a pool of the solution to be detected. The convex meniscus at the pool edges allows the entrance of the microfiber into the liquid.

The working principle of the sensing element is as following: A change of the surrounding refractive index of the loop, for example caused by adding specimens, will change the effective index of the propagating mode guided along the microfiber, and consequently shift the resonance peak used to retrieve the information of the specimens.

To investigate the resonance properties of the microfiber loop when immersed in solution, light from a tunable laser is launched into the loop and collected at the output port for transmission measurement. Figure 3 shows typical transmission spectra of a copper-rod-supported loop when immersed in pure water, which is assembled using a 2.4-µm-diameter microfiber. The intensity of the transmission is not normalized. The relative low power level (lower than -30 dB) comes from the low power level of the light source and additional losses from copper-rod absorption and scattering of the microfiber at the interface of air and water. The measured Q factor and FSR of the resonator are about 4300 and 1.09 nm, respectively. The red and black spectra under two different coupling conditions are obtained by winding or unwinding the microfiber around the copper rod. The wide-range shift of the resonant spectrum shows flexibility to tune the coupling coefficient precisely to balance the optical losses over a wide range, indicating a promising way to shift the critical-coupling resonance to a shorter wavelength to avoid high bending loss for sensing in high-concentration solution that usually exhibits low index contrast between the fiber and the surrounding.

 

Fig. 2. Schematic side view of a copper-rod-supported microfiber loop used for refractive index sensing. The copper rod and the microfiber loop are immersed in a pool of the solution to be detected. The convex meniscus at the pool edges allows the entrance of the microfiber into the liquid.

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Fig. 3. Transmission spectra of a copper-rod-supported loop when immersed in pure water. The loop is about 480 µm in diameter and is assembled with a 2.4-µm-diameter microfiber. The red and black spectra are obtained under different coupling conditions.

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3. Microfiber loops for high index-contrast sensing

For high index-contrast sensing, we first immerse the loop (assembled with a 2.4-µm-diameter microfiber) into pure water, and then modify the refractive index by adding ethanol using a microlitre syringe. The loop is operated around 1.55-µm wavelength with a measured Q factor and FSR of about 4000 and 1.12 nm, respectively. Shown in Fig. 4(a) are eight resonance peaks obtained by adding a 5-µL ethanol (index~1.34 at the wavelength of 1.55 µm [17,18]) into a 500-µL water (index~1.31 at the wavelength of 1.55 µm [17,18]) in steps, which corresponds to an index increase of about 2.6×10^-4 for each step. Figure 4(b) shows excellent linear dependence of the resonance wavelength shift on the increasing refractive index, with a calculated slope (sensitivity of the sensor) of 17.8 (nm/RIU). Measured long-term drift of the resonance peak is about 2 pm, therefore the detection limit of the sensor estimated from Fig. 4(b) can go down to 1.1×10^-4 in index change of the solution, corresponding to a 0.064 mol/L change in the ethanol concentration.

 

Fig. 4. Refractive-index-dependent resonance shifts of a copper-rod-supported loop in an ethanol solution. The loop is about 480 µm in diameter and is assembled with a 2.4-µm-diameter microfiber. (a) Spectral shifts of a resonant peak caused by index change of the solution. The eight peaks are obtained by adding a 5-µL ethanol into a 500-µL water in steps. (b) Resonant wavelength as a function of the refractive index change. The black dots are resonant wavelengths extracted from (a), and the numerical fitting is obtained with a calculated slope of 17.8 (nm/RIU).

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4. Microfiber loops for low index-contrast sensing

Generally, it is not difficult to obtain good resonance when the refractive index contrast between fiber and surrounding is relatively high. However, with increasing concentration of the specimen, the index contrast goes down, resulting in decreasing confinement of the guided light in the microfiber, which usually leads to higher optical loss that may terminate the resonance (see Fig. 1). In such cases, operating at a shorter wavelength is an effective solution to obtain good resonance with relatively low refractive index contrast. To show this, we retrieve the index change of a high-concentration glycerol aqueous solution by measuring the wavelength shift of a resonance peak centered around 1222 nm. In the experiment, we use a 500-µL 48 wt.% (5.84 mol/L) glycerol aqueous solution as the starting solution, and change its index by adding pure water and 72 wt.% (9.28 mol/L) glycerol aqueous solution. A copper-rod-supported loop with a Q factor of 4300 and an FSR of 0.71, is formed with a 2.2-µm-diameter microfiber. At the wavelength of 1.22 µm, the refractive indices of pure water, 48 wt.% and 72 wt.% glycerol aqueous solution are 1.32, 1.39, 1.42, respectively [17,18]. As shown in Fig. 5(a), the resonant peak shift to the short wavelength as the index of the solution goes down when pure water is added in 5-µL steps (corresponding to an index change of about 5.7×10^-4 in each step). Numerical fitting (red line) of the measured data (black dots) yields a slope of -101 (nm/RIU). When the index of the solution is increased by adding 72 wt.% glycerol aqueous solution in 5-µL steps (corresponding to an index change of 3.3×10^-4 in each step), the resonance peak shift to longer wavelength, as shown in Fig. 5(b). The slope of the numerical fitting is 109.7 (nm/RIU), which is in good agreement with that of decreasing index situation in Fig. 5(a). The sensitivity of the sensor estimated with the long-term drift of the resonance peak (about 2 pm) is about 1.8×10^-5 in index change of the solution. In addition, in this work, the temperature is kept almost constant (around 25°C) during the measurement, and the temperature-induced drift is very small. More details on the sensitivity quantification of resonant refractive index sensors can be found elsewhere [20].

 

Fig. 5. Refractive-index-dependent resonance shifts of a copper-rod-supported loop in a glycerol solution. The loop is about 480 µm in diameter and is assembled with a 2.2-µm-diameter microfiber. A 500-µL 48 wt.% (5.84 mol/L) glycerol aqueous solution with a refractive index of 1.39 is used as the starting solution. (a) The resonant peak shifts to shorter wavelength as the index of the solution goes down when pure water is added in steps (5 µL in each step). (b) The resonance peak shifts to longer wavelength when the index of the solution is increased by adding 72 wt.% glycerol aqueous solution in steps (5 µL in each step).

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5. Conclusion

In conclusion, we have demonstrated refractive-index sensors based on copper-rod-supported microfiber loops. The tightly wrapped loop provides a robust structure for sensing in liquid, and the flexibility of tuning critical coupling within a broad spectral range makes it possible to obtain high sensitivity in low- and high-concentration solutions. By detecting around wavelengths of 1.55 and 1.22 µm, sensitivity of refractive-index measurement of 1.1×10^-4 and 1.8×10^-5 are obtained in a low-concentration ethanol solution and high-concentration glycerol solution, respectively. Our results show that, copper-rod-supported microfiber loops are promising for refractive index sensing in liquid with high stability, high sensitivity and large dynamic range.