1. Introduction

The synergistic integration of photonics and microfluidics has wide potentials for extensive applications [1, 2]. Combination of microfluidics and fiber microstructures has received increasing attentions since the microfluidics can interact with light effectively in a compact geometry [3–8]. Owing to the enhanced interaction between the light and microfluidics, a number of fiber functional devices, such as lab-in-fiber devices [3, 4], tunable optical fiber devices [5–7], and fiber refractive index sensors [8, 9] have been reported. In general, the microfluidic delivery to fiber microstructures was mostly based on the capillary action [4–6] or pressure/vacuum driven methods [7–9]. With these simple and low-cost methods of microfluidic delivery, the microfluidics have been positioned successfully to the desired place in the fiber devices. Along with the development of the fiber functional devices based on the combination of microfluidics and fiber microstructures, a pressing requirement is the further improvement of the microfluidic delivery techniques for the effective control of the microfluidic flow.

Cavitation bubbles based micro-pumps driven by optical breakdown have been reported as efficient microfluidic actuators in the lab-on-chip systems [10, 11]. Owing to the explosive-like growth and collapse processes of the cavitation bubbles, the liquid can be accelerated to very high speeds, which makes the cavitation bubbles suitable for microfluidic mixing and pumping [11]. Since the optical breakdown driven micro-pump has the advantages of superior controllability, flexibility and efficiency [10, 11], it can also be used for the fabrication of fiber functional microfluidic devices without compromising their compactness.

In this paper, we reported a high efficient method to deliver quantitative liquid to a fiber micro-cavity by employing a micro-pump driven by femtosecond laser induced breakdown. The amount of the pumped liquid can be controlled by changing the irradiation time of the femtosecond laser pulses. Through adjusting the volume ratio of the pumped liquid vs. residual air in the fiber micro-cavity, we fabricated a fiber spirit level based on a mobile microbubble in the micro-cavity. Besides, such a pumping method has potential for fabricating the functional optical fiber devices by selective liquid delivery to the specific hole in the photonic crystal fiber (PCF) and efficiently injecting the analytes into a lab-in-fiber for chemical and biological analysis.

2. Experiments and results

To demonstrate such a new method of delivering the microfluidics to the fiber microstructure experimentally, a simple fiber micro-cavity with a length of 146 µm was fabricated. Such a micro-cavity was fabricated by fusion splicing a section of silica capillary tube (SCT) between two SMFs. A microscope image of the micro-cavity is shown in Fig. 1(a). During the process of fusion splicing, the discharge power and position were adjusted manually through a fusion splicer to avoid the collapse of the SCT. Before pumping the microfluidics, a capillary channel functioned as an inlet was drilled by the femtosecond laser-induced water breakdown [12, 13]. The fabrication processes of the microfluidic inlet are as follows: (i), the fabricated fiber micro-cavity with a cover slip on its top was fixed on a slide and immersed in water. (ii), the slide was fastened horizontally on a computer-controlled X–Y–Z translation stage with a step precision of 100 nm. (iii), through operating the 3D stage, the initial focus position (objective, NA = 0.8, Nikon) was chosen, as shown in Fig. 1(a). (iv), the femtosecond laser (800 nm, 120 fs, 1 kHz) with a power of 1 mW was turned on and the 3D stage was translated at a speed of 2 µm/s along the minus Y direction. In the process of translation, the interaction between the femtosecond laser and water induced water breakdown, and the generated shock wave, high-speed jets caused the ablation of the micro-cavity wall. Meanwhile, the introduced fresh water facilitated the debris removal from the ablation region, which enhanced the quality and efficiency of fabricating the capillary channel [14]. Finally, as shown in Fig. 1(b), a capillary channel with a diameter of 5 µm was manufactured to connect the inner space of the micro-cavity with outer environment.

 

Fig. 1 (a) Fabrication of the capillary channel by femtosecond laser-induced water breakdown. (b) Quantitative injection based on an optical breakdown driven micro-pump.

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To pump the liquid to such an fiber micro-cavity, the laser focus was firstly set in the vicinity of the inlet opening by the objective (NA = 0.8, Nikon), as shown in Fig. 1(b), and the femtosecond laser power was adjusted to 2 mW. The illustration of the optical breakdown driven micro-pump is shown in Fig. 2. When a mechanical shutter with a response time of 100 ms was turned on, the cavitation bubbles were generated by optical breakdown in water [15, 16]. The cavitation bubbles grew rapidly as a result of the initial overpressure within the bubbles [10, 17]. When the cavitation bubble gained its maximum expansion in the proximity of rigid boundary, it experienced an asymmetric collapse under the action of the external fluid pressure, giving rise to a high-speed jet [11, 18, 19]. The generated jet induced a high-speed flow ejecting toward the inlet opening [11, 19]. In the presence of the fabricated capillary channel, the liquid were pumped continuously to the micro-cavity in the form of net jetting flow. To illustrate the method of quantitative microfluidics delivery, the microscope images of the pumped liquid were captured by a CCD camera after each switching cycle of the shutter. The initial state of the fiber micro-cavity is shown in Fig. 3(a). Trace liquid was infiltrated into the micro-cavity in advance due to the capillary action. As can be seen in Figs. 3(b)–3(e), accompany with the air escaping, the liquid were delivered to the micro-cavity rapidly during the pumping process. Meanwhile, the amount of the liquid was controlled by changing the irradiation time of the femtosecond laser pulses. Through adjusting the volume ratio of the pumped liquid vs. the residual air, a microbubble with a diameter of 47 µm was formed, as shown in Fig. 3(e). Since the diameter of the microbubble is smaller than the inner diameter of the SCT (50µm), it can drift in the liquid and get equilibrium at the middle of the micro-cavity, as shown in Fig. 3(f). The jet flows caused by the interaction between the cavitation bubbles and the boundary have been studied both experimentally and theoretically in [20]. The generated jets induced high speed jet-flows in the presence of a capillary channel, which made the cavitation bubble function as a micro-pump. The advantages of using this method to pump microfluidics are that no additional manufacturing step for the liquid delivery is necessary, no mechanical or electrical connections are needed for operation, and the position of actuation can suitably be addressed by controlling the laser focus [11]. As an efficient method of microfluidic delivery, the optical breakdown driven micro-pump can be used to equip the fiber microstructures with many functionalities, such as optical tunability, high spectral sensitivity and flexible operation capability, through pumping the quantitative liquid to the fiber microstructure [21, 22]. It is worth noting that the purpose of this paper is to demonstrate the potential of delivering liquid to fiber microstructure based on the micro-pump driven by the femtosecond laser induced breakdown. It is envisioned that other microfluidic channels can be fabricated to facilitate the liquid transport and flow. With such a pumping technique, the biological and chemical properties of the analytes can be analyzed effectively.

 

Fig. 2 Illustration of the optical breakdown driven micro-pump. (a) Position of the laser focus. (b) Maximum bubble size. (c) Collapsing bubble with a high speed jet. (d) Jetting flow through the capillary channel.

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Fig. 3 Process of the quantitative microfluidic pumping with the irradiation time variation.

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3. Application

To keep the microbubble in a stable environment, the capillary channel was sealed by the UV curing adhesive (NOA 61) as soon as the microbubble was formed. The processes were as follows: firstly, the end of the fiber microstructure was immersed into NOA 61 and taken out promptly; secondly, the liquid polymer was cured for 15 minutes by the UV-light to close the inlet of the capillary channel.

The microscope image of the initial fiber micro-cavity with a pretreated distal end is shown in Fig. 4(a). The distal end of SMF was cut at an angle of 10 degrees by the femtosecond laser to reduce the influence of the undesired reflected light. The microbubble was in the middle of the micro-cavity when the fabricated fiber microstructure was horizontal. When the fabricated fiber microstructure was tilted clockwise from the horizontal, the microbubble drifted toward the left side of the micro-cavity due to the buoyancy and finally stayed at the left end, as shown in Fig. 4(b). From Figs. 4(b)–4(f), we can see that the microbubble drifted from the left side to the right side of the micro-cavity on condition that the fabricated fiber microstructure was tilted from clockwise to counterclockwise, and vice versa. As shown in Fig. 4(c), the space between the left end of the micro-cavity and the left wall of the microbubble during the drifting process was denoted as L. Since the microbubble suspends in the liquid, its drifting direction depends on the tilt direction of the fiber microstructure. Such a dynamic microbubble makes the fabricated fiber microstructure function as a fiber spirit level [23]. The spaces L were measured as 0 µm, 45 µm, and 98 µm separately, corresponding to the clockwise tilt state 1, horizontal state 3 and counterclockwise tilt state 5, respectively, as shown in Figs. 4(b), 4(d), and 4(f).

 

Fig. 4 (a) Microscope image of the initial micro-cavity. (b)–(f) Microscope images of the microbubble corresponding to different states of the fiber spirit level. (b) Clockwise tilt state; (d) Horizontal state; (f) Counterclockwise tilt state; (c) and (e) Transition states.

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In consideration of the general position of the microbubble, the schematic diagram of the fabricated fiber device is shown in Fig. 5(a). From Fig. 5(a), we can see that the two interfaces between the SMF and the liquid, together with the two opposite walls of the microbubble act as four reflectors (reflector 1, 2, 3, and 4). The Fabry-Perot cavities I, II and III are constituted by reflectors 1 and 2, 2 and 3, 3 and 4, separately. Additionally, the reflectors 1 and 3, 1 and 4, 2 and 4 constitute other Fabry-Perot cavities. When a light beam I₀ travels from the lead-in fiber, it is reflected by the four reflectors, respectively. Four reflected light beams (namely I₁, I₂, I₃ and I₄) come back to the SMF and interfere with each other. Such a fabricated fiber device can be treated as a four-beam fiber Fabry-Perot interferometer. However, the low reflectivity of the four reflectors and the spherical reflector of the microbubble greatly increase the reflection loss. In addition, since the light from the lead-in SMF end is divergent, the scattering loss and transmission loss are both increased when the light travelled in the liquid region. As a result, only a fraction of light is reflected back to the fiber core by the reflector 2 and interferes with that reflected by reflector 1. The intensities of modes in the microbubble cavity II and those in the liquid cavity III are weak enough to be negligible. Therefore, during the drifting process of the microbubble, the fiber spirit level can be considered as a low-finesse two-beam fiber Fabry-Perot interferometer. The intensity of the reflected light can be expressed as:

 

Fig. 5 (a) Schematic diagram of the fabricated fiber spirit level. (b) Reflection spectra corresponding to different states of the fiber spirit level.

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According to the Eq. (1), the dip wavelength of the spectrum can be deduced by the equation:

The fast-Fourier transform (FFT) was taken to transform the interference spectrum to the spatial frequency spectra and the results are shown in Fig. 6. The FFT has been proven to be a useful method to analyze the interference spectrum as it can provide the accurate information, such as frequency [25] and amplitude [26]. The corresponding spatial frequency of the interference spectrum can be expressed as: $\xi =2nL/{\lambda }_1{\lambda }_2$ _[24]. As shown in Fig. 6, only one main peak exists in every spatial frequency spectrum, which indicates that such a fiber spirit level is based on a low finesse two-beam Fabry-Perot interferometer. The other weak peaks are from the multiple weak reflections in the hybrid cavities. As can be seen from the Fig. 6, every unique frequency value corresponding to different state of the fiber spirit level. When the fiber spirit level was tilted clockwise, the microbubble finally drifted to the left end of the micro-cavity. The corresponding frequency value obtained by taking the FFT of the interference spectrum is 0.039. Similarly, the frequency values of 0.055 and 0.105 are for the horizontal state and the counterclockwise tilt state of the fiber spirit level, respectively. Through monitoring the FFT results, we can distinguish that whether or not the fiber spirit level is horizontal. Meanwhile, we can also distinguish the tilt direction of such a spirit level promptly. According the Eq. (1), the interferometric principle of the fiber spirit level is based on the optical path difference (denoted as 2nL) variation due to the mobility of the microbubble, which is similar to that reported in the previous work [27]. The interference spectrum changed with the optical path difference. Consequently, the corresponding spatial frequency varied corresponding to the tilt state of the fiber spirit level. Compared with the reported interference spectrum-tracked method in [23], the monitoring of the spatial frequency spectrum obtained by the FFT can get the fiber spirit level response to the tilt more easily and accurately.

 

Fig. 6 Spatial frequency spectra corresponding to different states of the fiber spirit level.

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4. Conclusion

In conclusion, we reported a novel method of delivering liquid to a fiber microstructure based on a micro-pump driven by the femtosecond laser induced breakdown. Through changing the irradiation time of the femtosecond laser pulses, liquid can be pumped quantitatively and efficiently. With this method, we fabricated a fiber spirit level based on a mobile microbubble drifting in a liquid filled fiber micro-cavity. The states of the fiber spirit level, such as whether horizontal or not and tilt direction, can be distinguished by just monitoring the corresponding spatial frequency spectrum. Such a method has potential for fabricating optical fiber functional devices, facilitating the injection of analytes into lab-in-fiber for the analysis of the chemical and biological properties.