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Abstract
In this paper, a liquid optical switch is proposed, and the 1550 nm infrared/visible switching function based on hydraulic control can be realized. An infrared light switch cavity, a visible light cavity and a liquid control cavity are stacked to form the main framework of the device. The glycerol, dyed liquid, and transparent liquid are filled in the cavities, respectively. Two elastic films are fabricated between the cavities for controlling the liquid volume of the cavities. With such a structure, in the initial state, the 1550 nm infrared light and visible light are absorbed by the glycerol and dyed liquid, respectively. The device shows infrared light-off and visible light-off states. When the elastic film is actuated by the liquid pressure, the shape of the elastic film can be changed. Once the elastic film touches to the substrate, a light channel can be formed so that the infrared light or visible light can pass through it. It shows infrared light-on or visible light-on states. In this way, the device can be worked as an infrared light and visible light switchable optical switch. The experiments show that the device can obtain the optical attenuation from ∼1.02 dB to ∼18.24 dB for 1550 nm infrared light optical switch and ∼0.66 dB to ∼8.70 dB @ λ=450 nm; ∼0.62 dB to ∼8.74 dB @ λ=532 nm; ∼0.77 dB to ∼9.00 dB @ λ=633 nm for visible light optical switch. The device has potential applications in the fields of optical fiber communications, variable optical attenuators, and light shutters.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optofluidic technology fundamentally aims at manipulating fluids and light at the microscale and exploiting their interaction to create the highly versatile systems. The optofluidic devices have widespread applications such as micro-pumping [1–2], droplet manipulation [3–4], imaging [5–6], sensing [7–8], and biology [9–10]. On account of the important role in communication, imaging and beam control, liquid optical switches have been widely studied in recent years. The most common way to realize the switch function is employing the dyed liquid to absorb the light or blocking the light path directly. In the common liquid devices, the liquids are usually driven by the mechanisms of electrowetting control [11–12], dielectric force control [13–14], pneumatic control [15–16], and hydraulic control [17–18]. The response time of the electrowetting and dielectric force driven devices can be controlled within a few hundred milliseconds. However, long-time driving may cause the conductive liquids to be ionized and would finally shorten the lifetime. In 2019, researchers a zero-insertion-loss optical shutter based on the electrowetting-on-dielectric with a low voltage of 25 V. The mechanical stability of the liquid device is still an important issue to be solved [19]. For the pneumatic driven optical switch, the airtightness is very important, which may increase the processing difficulty to a certain extent. Although all these approaches can realize the optical switch function, they can only be used as a visible light (VL) switch. Few of them are suitable for switching the infrared light (IL).
The IL switch can be used to reduce the optical power at a certain level, and it has been an essential component in optical fiber switching, photonic signal processing, and sensing. At present, the majority of the infrared switches are based on semiconductor material [20–21] or metamaterials [22–24]. The semiconductor material-based device has a fast response time within 10 ms and a relative high extinction ratio of 15 dB [20]. However, most of the designs need to select special materials and fabricated based on micro-nano processing technology, the high cost and complex process cannot be ignored. Thus, liquid crystal (LC) based [25–27] and liquid-based [28–29] IL switches have attracted much attentions of the scholars. The LC-based device can attenuate IL by either scattering or phase modulation without mechanical moving parts. For the scattering type, when the unpolarized light passes through the LC device, the output light is sensitive to polarization due to the anisotropic droplets [25]. For the phase type, the device usually needs to increase the cell gap for enhancing the phase shift, which will enhance the driving voltage dramatically [27]. The liquid-based device employs a special droplet moving to a certain position to absorb the IL. Thus, the mechanical stability of the devices should also be considered [29].
Recently, few studies have reported on IL/VL switchable liquid optical switch. In this paper, we employ the glycerol to absorb the 1550 nm IL and the dyed liquid to absorb the VL. For the sake of simplicity, in this paper, the infrared light is referred to 1550 nm infrared light. By changing the curvature of the elastic films in different cavities, the device can achieve both IL and VL optical switch functions. That is also the key novelty of this work. Compared with our previous works about optical switch [12,18, 30–31] and other optical switches, this work has the advantages of high attenuation ratio: ∼1.02 dB to ∼18.24 dB for IL optical switch and ∼0.66 dB to ∼8.70 dB @ λ=450 nm; ∼0.62 dB to ∼8.74 dB @ λ=532 nm; ∼0.77 dB to ∼9.00 dB @ λ=633 nm in VL optical switch. All the liquids are sealed in the cavities and actuated by the elastics film, the gravity effect can be ignored, and it can be placed vertically or horizontally. Moreover, the device also owns the advantages of simple structure and low cost. It can greatly expand the real applications in optical communications and optical attenuators.
2. Structure, mechanism, and fabrication
2.1 Structure and mechanism of the device
Figure 1 shows the cross-section drawn and mechanism of the device. The substrate, VL switch cavity, liquid control cavities, and IL switch cavity are stacked to form the main framework of the device, as shown in Fig. 1(a). Liquid-1 is the glycerol for absorbing the IL and liquid-2 is the dyed liquid for absorbing the VL. The spectrogram of the glycerol between 375 nm-2000 nm is shown in Fig. 2. We can see from Fig. 2 that the glycerol has a strong absorption of the infrared light from 1450 nm to 1650 nm, dramatically. Therefore, it can be used as a liquid material to absorb the IL.
Fig. 1. Cross-section drawn and mechanism of the device. (a) Infrared light-on and visible light-off state; (b) Infrared light-off and visible light-on state; (c) Infrared light-on and visible light-on state.
In the initial state, the glycerol and dyed liquid are fully filled in each cavity. When the device is irradiated by the IL at 1450 nm to 1650 nm and VL range, it shows the IL-off and VL-off state. When liquid-3 is injected into the liquid control cavity from inlet-1, the shape of the elastic film can be changed. When liquid-3 is injected furtherly, the elastic film touches to the substrate. Then a light channel can be formed and the excess liquid-1 will flow out from outlet-1, simultaneously. In this state, the IL can pass through the device, while VL is absorbed by the dyed liquid, as shown in Fig. 1(b), which shows the IL-on and VL-off state. In a similar way, when liquid-3 is injected into the liquid control cavity from inlet-2, the elastic film can touch to the substrate and the light channel can be formed. In this state, the VL can pass through the whole device, while IL is absorbed by the glycerol, as shown in Fig. 1(c), which shows the IL-off and VL-on state. If liquid-3 is injected into the two liquid control cavities at the same time, and the hydraulic pressure is kept to increase until the two elastic films attach to the substrates, in this state, the light channel can allow the IL and VL pass through the device, as shown in Fig. 1(d). It shows the IL-on and VL-on state.
2.2 Fabrication
Before assembly, the IL switch cavity, VL switch cavity, and liquid control cavity are fabricated by the 3D printer (Type of E3 @ JGAURORA, China). The sizes of the IL switch cavity, VL switch cavity, and liquid control cavity are 20.0 mm×20.0 mm×2.0 mm, 20.0 mm×20.0 mm×4.0 mm, and 20.0 mm×20.0 mm×2.0 mm, respectively. The diameters of the inlet and outlet microchannels are both 0.5 mm. We use two syringe pumps to connect inlet-1 and inlet-2, respectively. The outlet-1 and outlet-2 are connected with the storage reservoirs. The substrate and elastic film are made from polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS), respectively. The size of the PMMA substrate is 20 mm×20 mm×0.5 mm. The thickness of the elastic film is 200 µm (tensile strength: 5.0 Mpa; tearing strength: 10.0 KN/m; elasticity modulus: 2.3 Mpa). The VL/IL switch cavities and liquid control cavity are made from photosensitive resin and the thickness of all the cavities is ∼2 mm. A light hole with the diameter of 12 mm is fabricated in the middle of the switch cavity. The whole size of the device is 20.0 mm×20.0 mm×15.0 mm. The viscosity of glycerol is up to ∼500 mpa·s to 800 mpa·s, it has so poor fluidity that it is difficult to move within the cavity. Thus, in the experiment, water mixed with glycerol at 1:1 is used as liquid-1. Water mixed with ink is used as liquid-2, and the pure water is used as liquid-3. The ink is carbon black ink from HERO Co. Ltd with the type of H/234. The carbon black ink is a mixed solution which mainly consists of carbon, phthalates, and glycol esters. The characteristics of the liquids in the cavities are also listed in Table 1.
Table 1. Characteristics of the materials in the device.
Figure 3 illustrates the fabrication flow of the device. Two liquid control cavities, one substrate, and two elastic film are assembled in sequence by glue UV-331, as shown in Figs. 3(a)–3(b). Then, the IL switch cavity, VL switch cavity, and two substrates are also assembled in sequence by glue UV-331, as shown in Figs. 3(c)–3(d). At last, the three liquids are injected into each cavity respectively, as shown in Fig. 3(e). The real elements of the device are also depicted in Fig. 3(f).
Fig. 3. Fabrication process of the device. (a) Assembling the two liquid control cavities; (b) Fixing the elastic films; (c) Assembling the IL switch cavity and VL switch cavity; (d) Fixing the substrates; (e) Injecting the three liquids; (f) Finishing the device.
3. Experiment and discussion
We had done one principle experiment before stating this work, and the principle video showed that the elastic film can be deformed by different hydraulic pressures, as depicted in Fig. 4. In the principle experiment, the aperture size of the hole is 5 mm, when the injected liquid volume changes from 0 µL to ∼140 µL, the curvature of the elastic film can be changed accordingly. The dynamic response of the principle experiment is also included in Visualization 1.
Fig. 4. Elastic film deformed by different hydraulic pressures. (a) ΔV1=0 µL; (b) ΔV2=20 µL; (c) ΔV3=50 µL; (d) ΔV4=80 µL;(e) ΔV5=110 µL; (f) ΔV6=140 µL.
3.1 IL optical switch function
According to the above process, all the elements are fabricated and assembled to form the device. The setups of the experiments are shown in Fig. 5. In the IL switch experiment, a power adjustable fiber laser λ=1550 nm (from Changchun new industries optoelectronics technology Co., Ltd., China) with a collimating lens is used as the test IL source. The maximum power of the fiber laser is 5.0 mw and the focal length of the collimating lens is 100 mm with a light spot of ∼5 mm, as shown in Fig. 5(a). The IL photodetector (the detection wavelength is 800-1700nm and the active area is 3.14 mm2) connected with an oscilloscope is used to test the IL switch function. Liquid-3 is injected into inlet-1 by using a syringe pump (from Longer Pump with the type of TS-1B, China).
Fig. 5. Experiment setups. (a) Setup of the IL switch experiment; (b) Setup of the VL switch experiment.
As we know, the IL can also be absorbed by the dyed liquid (Liquid-2). We first measure the normalized intensity when the device is only filled with the glycerol. Then the device filled with both liquid-1 and liquid-2 (glycerol + dyed liquid) is also measured for the comparison. The results are shown in Fig. 6. As can be seen from Fig. 6, the fiber laser can be absorbed by the dyed liquid about 19.2%. When the device is irradiated by the fiber laser, the attenuation can be calculated by the following equation:
where A represents the light intensity attenuation, Pi is the input optical power, and P0 is the output power.The input power of the fiber laser is modulated to be 200 µw. In the initial state, liquid-1, liquid-2, and liquid-3 are filled in each cavity. IL and VL is absorbed by the glycerol and dyed liquid, the device shows IL-off and VL-off state. The measured output power is ∼3 µw. In this state, the optical attenuation reaches to the maximum value ∼18.24 dB. When liquid-3 is injected from inlet-1 with a pump speed of 0.30 ml/s, the elastic film is actuated to form a convex surface. When the injected liquid volume changes from 0 ml to 1.50 ml, the elastic film can be attached on the substrate to form a light channel. So, the IL can pass through the device, while VL is absorbed by the dyed liquid. The device shows IL-on and VL-off state. The output power is measured to be ∼158 µw. That is to say, the insert loss is ∼1.02 dB. The normalized intensity during the driving process is measured and depicted in Fig. 7.
In order to test the absorption ability of different concentrations of the glycerol to the IL, a new solution of water and glycerol with the ratios of 0:1, 1:1, 10:1, and 50:1 are prepared and injected into the IL switch cavity. The thickness of the new solution is 4 mm. The attenuation ratio A’ can be calculated as A’=O/I. Where O is the normalized output intensity and I is the normalized input intensity, respectively. The attenuation ratios of different concentrations under different optical power are measured, as depicted in Fig. 8. When the optical power is changed within 0.25 mw, the absorptivity of different concentrations of the glycerol is almost the same. However, when the optical power is increased gradually, the attenuation ratio decreases sharply. We can also conclude that the effect of concentration on the attenuation ratio is limited.
3.2 VL optical switch function
In the VL switch experiment, a power adjustable white LED light with a maximum power of 1.0 mw is used as the light source, as shown in Fig. 5(b). The spectrogram of the white LED is depicted in Fig. 9(a). In the experiment, a CCD camera (YVSion Co. Ltd, type of YS-HU800C: 2/3” CMOS, China) is used to record the images and evaluate the performance of the VL switch function. Before the experiments, we first measured the spectrogram of the dyed liquid for absorbing the visible light, as shown in Fig. 9(b). In the initial state, liquid-1 and liquid-2 are fully filled in each cavity. The device shows the IL-off and VL-off state. The input power of the LED light is modulated to be 200 µw at λ=450 nm. The output power is measured to be 27 µw. The optical attenuation reaches to the maximum value ∼8.70 dB. When liquid-3 is injected from inlet-2 with a pump speed of 0.05 ml/s, the elastic film is actuated to form a convex surface. When the injected liquid volume ΔV changes from 0 ml to 0.8 ml, the elastic film is gradually close to the substrate to form a light channel. So, the VL can pass through the device, while IL is absorbed by the glycerol. The device shows IL-off and VL-on state. When ΔV = 0.8 ml, the output power is measured to be ∼172 µw. The insert loss is ∼0.66 dB. The experiment results of the VL switch function with the volume changing from 0.1 ml to 0.8 ml is shown in Fig. 10. The dynamic response of the VL switch function is also included in Visualization 2.
Fig. 9. (a) Spectrogram of the white LED; (b) Spectrogram of the dyed liquid for absorbing the visible light.
Fig. 10. Experiment results of the VL switch function. (a) ΔV1=0.1 ml; (b) ΔV2=0.2 ml; (c) ΔV3=0.3 ml; (d) ΔV4=0.4 ml; (e) ΔV5=0.5 ml; (f) ΔV6=0.6 ml; (g) ΔV7=0.7 ml; (h) ΔV8=0.8 ml.
The absorption of the dyed liquid is different at the visible light. The experiment measurements of the VL switch at λ=450 nm, 532 nm, and 633 nm are shown in Fig. 11. When the input power of the LED is 200 µw at λ=450 nm, the input power is measured to be ∼157 µw at 532 nm and ∼135 µw at 633 nm. In the VL-on and VL-off states, the output power is 136 µw and 21 µw at λ=532 nm. The insert loss and maximum optical attenuation are ∼0.62 dB and ∼8.74 dB, respectively. In a similar way, the output power is measured to be 113 µw and 17 µw at λ=633 nm, in VL-on and VL-off states. The insert loss and maximum optical attenuation are ∼0.77 dB and ∼9.00 dB, respectively. As for the IL-off and VL-on state, the light channel is fully filled with the water. That is to say, the absorptions for both wavelength ranges of the device are depended on the absorb of the water. Based on the spectrum of water, the IL and the VL absorption are ∼14% and ∼1%, respectively.
3.3 Discussion
In the experiments, the curvature of the elastic film increases when it is actuated by liquid pressure. Thus, the center thicknesses of the dyed liquid/glycerol will be decreased accordingly. In this state, the part VL/IL can pass thought the device gradually. If the elastic film is further actuated by the liquid pressure until it is attached to the top/bottom substrate, the light channels can be formed allowing most of the VL/IL passing thought the device. As for the VL optical switch, the shape of the elastic film will finally affect the size of the optical aperture, as shown in Fig. 10. While, for the IL optical switch the shape of the elastic film will finally affect the optical attenuation.
If we change the dyed liquid to a transparent liquid, the device can be worked as a tunable lens. We also simulate the tunable lens function using Zemax. In the simulation, we replace the dyed liquid to NaCl solution (refractive index is 1.37, abbe number is ∼55.8) as an example and maintain liquid-2 and liquid-3 in the cavities. Figure 12(a) is the diagram of the tunable lens. Figures 12(b)–12(d) are the schematic diagrams of the lens simulation under three limit conditions. The simulation shows that the focal length of the tunable lens can be tuned from -54.52 mm to -145.74 mm.
Fig. 12. Simulation of the tunable lens function. (a) Schematic diagram of the device; (b) State when F1=-85.73 mm; (c) State when F2=-54.52 mm; (d) State when F3=-145.74 mm.
In our experiments, the high refractive index difference of the water and the glycerol will make the IL degraded, which also can be seen from Fig. 12. While, the influence is limited in a period of the actuation process. When the elastic film is attached to the bottom substrate and actuated by the liquid pressure furtherly, the device cannot be worked as a tunable lens. It is just like a parallel plate in the center area of the device. The focal length variation is also simulated by Zemax. In the simulation, we use water to replace the dyed liquid. The focal length can be tuned from negative infinity to -80.73 mm. In our further work, we can change liquid-3 to phenylmethyl silicone oil with the refractive index of 1.48. In this way, the effect of the light divergence can be reduced.
The key novelty of the device is that it can achieve the IL/VL switchable optical switch function with an easy fabrication and simple structure. Some optical properties can also be improved. In our experiment, the maximum optical attenuation of the VL switch is just about ∼8.70 dB to ∼9.00 dB, which are much smaller than that of the IL switch (∼18.24 dB). The main reason is that the thickness of the dyed liquid is just 2 mm and the dyed liquid may stick to the elastic film. We can enhance the thickness of the dyed liquid and make hydrophobic treatment on the elastic film. However, the dyed liquid would also absorb the IL and increase the insert loss of the IL switch. From Fig. 6 and Fig. 11, we can see that the average actuation time of one cycle driving process is up to 20 s. That all depends on the injection speed of the syringe pump. However, the fast injection speed can cause a vibration of the elastic film which has a negative influence on the mechanical stability. So, we should take a tradeoff between the fast speed and mechanical stability.
4. Conclusion
This paper presents a 1550 nm IL/VL switchable liquid optical switch. The glycerol and dyed liquid are used to absorb the 1550 nm IL and VL, respectively. The 1550 nm IL/VL switchable optical switch function can be realized by injecting the liquid from inlet-1 and inlet-2 alternately. The experiments show that the device can obtain the optical attenuation from ∼1.02 dB to ∼18.24 dB for the 150 nm IL optical switch function and the maximum optical attenuation from ∼8.70 dB to ∼9.00 dB for the VL switch. The device has potential applications in optical communications, variable optical attenuators, and light shutters.
Funding
National Natural Science Foundation of China (61927809, 61805169, 61805130); China Postdoctoral Science Foundation (2019M650421, 2019M650422).
Acknowledgments
The authors would thank Prof. Miao Xu from Hefei University of Technology for her technical assistance in spectral measurement.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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