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Abstract
This paper presents a multifunction reflector controlled by liquid piston for optical switch and beam steering. The multifunction reflector consists of two liquid cavities that are designed with microchannels. Two holes covered with elastic membranes are fabricated on the upper surface of the liquid cavities. When the liquid cavity is injected with liquid, the shape of the elastic membrane changes to form a liquid piston in the position of the holes accordingly. The magnetic base covered with a reflector is fixed on the surface. We can adjust the active number and height of the liquid pistons to drive the reflector deflecting to different directions. Our experiments show that the multifunction reflector can realize the function of 2×2 optical switch. It can also deflect the light beam through an angle of 0°∼72° in two directions. The multifunction reflector has potential applications of free-space optical communications, laser detections and variable optical attenuators.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, optofluidic technologies enable an active control of the shapes and positions of the liquids without bulky and complex mechanical moving parts. Due to these features, the optofluidic devices are more adaptable and reconfigurable to improve the optical performances. Numerous optofluidic devices such as liquid lenses [1–4], optical switches [5–11], liquid prisms [12–22], optical slits [23–24], and optical irises [25–27] have been demonstrated by the actuation mechanisms of electrowetting, dielectrophoresis, hydraulic control and pneumatic control.
Among them, liquid optical switches and liquid prisms have been studied intensively for the wide applications of free-space optical communications, laser detections, variable optical attenuators and light shutters. As for the liquid optical switches, researchers are usually employing a dyed liquid to absorb the light [5–6] or changing of the liquid-liquid (L-L) interface to scatter the light [10–11]. Some researchers proposed an optical switch with a reconfigurable dielectric liquid droplet and realized a red color light switch with ∼10:1 contrast ratio and ∼300 ms response time [7]. While the contrast ratio is limited by the concentration of the dyed liquid droplet which would be also attached on the top substrate partly. In 2010, a 2×2 optical switch with 23.0 dB within-channel isolation by controlling two L-L interfaces was proposed [10]. Though the L-L interface is controlled by the external voltage, the stability and repeatability of the optical switch still need to be considered. As for the liquid prisms, the most common designs are based on the mechanisms of electrowetting effect and dielectrophoresis effect to form a tilt L-L interface. Some researchers also proposed an electrowetting-actuated prism model by using four rectangular indium tin oxide (ITO) electrodes in 2011 [12]. This design can reach a beam steering angle of ± 26°. In 2016, the German scholars improved the structure by employing a cylindrical electrode to reach the function of rotating laser scan [17]. However, electrolysis, Joule heating and microbubble formation often occur in electrowetting devices due to the transportation of the free electric charges and the alternating electric fields [27]. Hydraulic and pneumatic controlled prism-like devices are also significant methods for the prism design. These liquid prisms are usually fabricated by the polydimethylsiloxane (PDMS) with liquid or air filling in the hollow structures [20]. These prisms can be highly integrated in the microchips, but the rigid geometry structure has a limitation in expanding the practical applications. Furthermore, these kinds of designs are often affected by mechanical vibrations picked up from the external environment.
In this paper, we demonstrate a multifunction reflector controlled by liquid pistons (LPs). The key novelty of the proposed device is that it can reach the function of 2×2 optical switch with 100% light attenuation. At the same time, it can also realize the function of beam steering with a wide tracking angle of 72° in two-directions. Compared with our previous works about optical switch [8–9] and other optical switches, this work has the advantages of high light attenuation and reliable mechanical stability. Compared with our previous works about beam tracking [21–22] and other liquid prisms, this work focuses on solving two main issues: one is to keep a relative high-quality beam shape in principle and the other is to realize a large beam tracking angle in multi-directions. The proposed multifunctional reflector can greatly expand the real applications in laser scanning [19], ranging lidar system and optical communication.
2. Mechanism and fabrication
Figure 1 shows the schematic structure and the operation mechanism of the proposed multifunction reflector. A liquid cavity is designed with two microchannels on the sidewalls and two holes on the upper surface. The liquid cavity, one elastic film and one substrate are packaged like a sandwich. The two holes can be functioned as LPs when the liquid is pulled in/out from the microchannels. The purpose of the beam window is to divide the two light beams and decrease the crosstalk of the beams. A reflector is fixed on a magnetic base, as shown in Figs. 1(a)–1(b). We can control the injected volume of the LPs in order to keep or change the tilt angles of the reflector. When two light beam (LB) are irradiated on the proposed device and only LP-4 is actuated, LB-1 can pass through, while LB-2 is blocked by reflector-1. In a similar way, when only LP-1 is actuated, LB-1 is blocked, as shown in Fig. 1(c). In this state, the proposed device can work as a 2×2 optical switch. When LP-2 is actuated to keep reflector-1 tilt with an angle of 45°, LP-4 is actuated to make reflector-2 scanning to different directions, as shown in Fig. 1(d). In this state, the proposed device can reach the function of beam steering.
Fig. 1. Schematic structure and operation mechanism of the multifunction reflector. (a) Schematic structure of the multifunction reflector. (b) Structure of the liquid cavity. (c) Mechanism of the optical switch. (d) Mechanism of the beam steering.
Figure 2 illustrates the fabrication flow of the proposed device. Firstly, the framework and the liquid cavity which has microchannels and holes are fabricated with a 3D printer (Type of E3 @ JGAURORA, China), as shown in Figs. 2(a)–2(b). The size of the framework and liquid cavity are 29.0 mm×29.0 mm×15.0 mm and 27.0 mm×14.0 mm×2.0 mm, respectively. In the design of the liquid cavity, the diameters of the microchannels and holes are 0.1 mm and 8.0 mm, respectively. Secondly, a PDMS elastic membrane is prepared for forming the LPs. The thickness of the elastic film is 200 µm (the tensile strength is 5.0 Mpa; the tearing strength is 10.0 KN/m; the elasticity modulus is 2.3 Mpa). The elastic film has a relatively high tensile strength which means it can keep a good shape and a reasonable repeatability. Then, a substrate matched with two holes, the elastic membrane and the liquid cavity are assembled together in a sandwich-like structure, as depicted in Figs. 2(c)–2(d). The substrate is made of polymethyl methacrylate (PMMA) and the spacing between the two holes is 3 mm. At last, a magnetic base and a reflector are fabricated on the middle of the substrate, as shown in Figs. 2(e)–2(f). The height and diameter of the magnetic base are 2.0 mm and 1.5 mm, respectively. As depicted in Fig. 2(e), a quadrate iron foil coated with a silver film is functioned as the reflector whose length of side and thickness are 10 mm and 100 µm, respectively. The weight of the iron foil is ∼2.0 mg.
Fig. 2. Fabrication process of the multifunction reflector. (a) Framework fabrication with a 3D printer. (b) Liquid cavity fabrication with a 3D printer. (c) Bonding elastic membrane. (d) Bonding substrate. (e) Fixing the magnetic base. (f) Fixing the reflector.
3. Calculation, experiments and discussion
3.1 Calculation
According to Fig. 1, when the liquid is injected into the microchannels, the shape of the elastic membrane changes from flat to convex profile. As depicted in Fig. 3(a), if we use a spherical profile to substitute the parabolic profile and suppose they have the same parameters (the same maximum displacement h and the same base radius r0), then the maximum error Δzmax in displacement can be expressed as the following equation [28]:
Fig. 3. Calculation of (a) the curvature of the LP and (b) the sweeping distance with the beam steering.
In our experiments, when the heights of the actuated liquid pistons change within 4.0 mm, the Δzmax < 1.00 mm, the error of injected liquid volume is within ∼15 µL. This kind of approximation is valid. However, when the heights of the liquid pistons change from 4.0 mm to 6.0 mm, the Δzmax < 3.37 mm, and the maximum error of injected liquid volume can reach to ∼35 µL. In this state, the error cannot be ignored. For simplicity, we still take it as a spherical surface in calculation approximately during all the actuation time and draw an error graph in the data processing section.
When the fluid pump works, the liquid volume in the microchannels will force the elastic membrane to bulge outward. We take one LP as an example. The radius of the piston curvature R and liquid volume V have the following relationship:
where r0 is the radius of the hole. The relationship of R and h is shown by the following equation: We can substitute Eq. (3) into Eq. (2) and calculate the relationship between the displacement (h) and V.As we know, when a reflector rotates an angle of θ, the reflected angle is 2θ. The distances swept across the screen can be expressed by the following equations:
where D is the sweeping distance of the beam steering on the screen, L is the distance between the incident beam and screen, h0 is the height of the magnetic base, h1 is the piston height under the injecting volume of V1, θ1 is the corresponding steering angle, h2 is the piston height under the injecting volume of V2, and θ2 is the corresponding steering angle, as depicted in Fig. 3(b). Thus, we can substitute Eqs. (5)–(6) into Eq. (4) and calculate the relationship between the sweeping distance (D) and the volume change (V2- V1).3.2 Experiments
We first fabricate all the elements of the proposed device, and set up the experimental system as depicted in Fig. 4. A He-Ne laser (λ=632.8 nm) is used to irradiate the two beam splitters, and then two laser beams can be obtained. The light power of the laser beam is attenuated to 0.05 mw. One CCD camera is placed at the top view of the proposed device to record the movements of the elements and the other CCD camera is placed in front of the screen to record the beam switch and steering. As shown in Fig. 4, we only demonstrate one CCD camera to make a clear sight of the system.
In the optical switch experiment, we inject the liquid (water with a density of 1.0 g/cm3) into the microchannels using a fluid pump (Longer Pump TS-1B, China). The speed of the pump is 150 µl/s. In the initial state, two laser beams are adjusted to just pass through the proposed device, as shown in Fig. 5(a) and Fig. 6(a). When LP-1 is actuated with the liquid volume of 300 µl, 500 µl, 700 µl, 900 µl, respectively, the height of LP-1 changes accordingly, as shown in Figs. 5(b)–5(e). In this state, LB-1 is blocked by LP-1 completely. While, LB-2 can pass through the proposed device, as shown in Figs. 6(b)–6(c). In a similar way, when LP-3 is actuated with the liquid volume of 200 µl, 400 µl, 600 µl, 800 µl, as depicted in Figs. 5(g)–5(j). In this state, LB-2 is blocked by LP-3, while, LB-1 can pass through the proposed device, as shown in Figs. 6(d)–6(e). Thus, it can realize the function of the 2×2 optical switch and the 100% light intensity attenuation can be achieved both in theory and practical. The reason for different liquid volume change in LB-1/LB-2 switch is that the incident positions of the two laser beams between the sidewalls are not the same. The position of LB-2 is much closer to the sidewall than that of LB-1, thus LP-1 should be actuated to a higher position to block the laser beam, as shown in Fig. 5(e) and Fig. 5(j). The dynamic response video of the actuated pistons and the optical switch are included in Visualization 1 and Visualization 2, respectively.
Fig. 5. Driving process at the top view in the optical switch (see Visualization 1). (a) Initial state. (b) LP-1 actuated with 300 µL liquid. (c) LP-1 actuated with 500 µL liquid. (d) LP-1 actuated with 700 µL liquid. (e) LP-1 actuated with 900 µL liquid. (f) Recovery to the initial state. (g) LP-3 actuated with 200 µL liquid. (h) LP-3 actuated with 400 µL liquid. (i) LP-1 actuated with 600 µL liquid. (j) LP-1 actuated with 800 µL liquid.
Fig. 6. Experiment results of the 2×2 optical switch (see Visualization 2). (a) Initial state. (b) LP-1 actuated with 450 µL liquid. (c) LP-1 actuated with 900 µL liquid. (d) LP-3 actuated with 400 µL liquid. (e) LP-3 actuated with 800 µL liquid. (f) Recovery to the initial state.
Response time is the key parameter to measure the performance of the optical switch. We define the actuation time as the time that the light intensity of LB-1 (LB-2) decreases from 100% to 0% of the total intensity. Figure 7 shows the results of the normalized intensity during actuation and relaxation procedure. The measured actuation times of LB-1 and LB-2 are 280 ms and 260 ms, respectively. We also define the relaxion time as the time that the light intensity of LB-1 (LB-2) increases from 0% to 100% of the total intensity. The measured relaxation times are 230 ms and 220 ms, respectively.
In the second experiment, we only need one laser beam to irradiate the proposed device to check the function of beam steering. In the initial state, LP-2 is actuated by changing the liquid volume and reflector-1 is to be tilt with an angle, accordingly, as depicted in Figs. 8(a)–8(b). When the injected liquid volume increases to 1200 µL, the height of LP-2 is ∼8.5 mm. In this state, the tilt angle of reflector-1 is ∼45° and the laser beam is exactly perpendicular irradiated on reflector-2, as shown in Fig. 8(c). Then, reflector-1 is kept staying a 45° tilt angle and LP-3 is gradually actuated by the liquid pressure, as shown in Figs. 8(d)–8(f). When the injected liquid volumes change to 250 µL, 750 µL and 950 µL, the measured tilt angles of reflector-2 are ∼4°, ∼27°, and ∼36°, respectively. Thus, the maximum steering angle is ∼72°. The dynamic response video of the actuated pistons and the reflectors tilt is included in Visualization 3.
Fig. 8. Top view of the tilt angle changing during the actuation procedure (see Visualization 3). (a) LP-2 actuated with 600 µL liquid. (b) LP-2 actuated with 750 µL liquid. (c) LP-2 actuated with 1200 µL liquid. (d) Keeping LP-2 actuated and LP-3 actuated with 250 µL liquid. (e) Keeping LP-2 actuated and LP-3 actuated with 750 µL liquid. (f) Keeping LP-2 actuated and LP-3 actuated with 950 µL liquid.
As shown in Fig. 3(b), when the laser beam irradiates on the multifunction reflector, the sweeping distance of the beam steering on the screen can be swept from infinity to a certain position in theory. But in fact, the reflected laser beam would be blocked by the framework of the proposed device and the sweeping distance is also determined by the distance (L) between reflector-2 and the screen. In the confirmatory experiment, L is adjusted to be 100 mm, and we record the beam steering when reflector-2 is tilted from different angles. When LP-2 and LP-3 are actuated, the positions of the laser beam can be changed, as shown in Fig. 9(a). When the tilt angle of LP-3 is changed during 27° to 36°, the positions of the laser beam can be seen from Figs. 9(b)–9(c). The dynamic response video of the beam tracking and steering is included in Visualization 4.
Fig. 9. Experiment results of the laser beam steering on the screen (see Visualization 4). (a) Laser beam position when LP-2 and LP-3 are actuated. (b) Positions of the laser beam when the tilt angle of LP-3 is changed from 27° to 36°. (c) Positions of the laser beam when the tilt angle of LP-3 is changed from 36° to 27°.
When the liquid volumes are changed, the heights of the LPs have been also measured. In order to prove the repeatability of the proposed device, the data were measured once a day for the duration of five days. The results with error bars and the simulation curve are shown in Fig. 10(a). The tilt angles of reflector-2 during liquid volume changes are shown in Fig. 10(b). Because the height of the magnetic base is 2 mm, when the injected liquid volume is under ∼200 µL, the height of the LPs is within 2 mm. The tilt angles of reflector-2 are approximate to be ∼0°. When the injected liquid volumes change from 200 µL to 1000 µL, the tilt angle of reflector-2 can be tuned from 0° to 37°.
Fig. 10. (a) Heights of the LPs when the liquid volumes are changed and (b) tilt angles of reflector-2 during changing the liquid volume.
3.3 Discussion
The application of the multifunction reflector can also be expanded. When one light beam irradiates the proposed device from the input, and no LPs are actuated. In this state, the light beam can be detected by output-1. When LP-2 and LP-4 are actuated to make the reflectors tilt with an angle of 45°, in this state, the light beam can be detected by output-2, as shown in Fig. 11(a). That is to say it can be used as a 1×2 optical switch. When LP-3 is actuated, the light beam can be steered to the negative direction compared with the above experiments. Hence, the proposed device can be extended to two-directions of beam steering, as depicted in Fig. 11(b).
Fig. 11. Application expansion of the proposed device. (a) Application of 1×2 optical switch. (b) Application of two-directions of beam steering.
However, the proposed device still has some issues and limitations as follows. As can be seen from Fig. 9. The quality of the laser beam is relatively poor when it is irradiated on the screen. The main reason is that the reflector is made of an iron foil whose surface is not exactly flat. Besides, the laser beam has been reflected twice by the reflector and the reflectivity might be decreased below 80%. All these would have a negative effect on the beam quality. If we choose a lightweight material with a flat surface and coated with a reflecting film, the beam quality can be improved and will be much better than the liquid prism without doubt. Another issue is that the sidewalls of the framework would limit the beam steering angle. As depicted in Fig. 8(f), if the tilt angle of reflector-2 is increased above ∼38°, the laser beam would be blocked by the sidewall. We can also optimize the structure to realize a much larger beam steering angle. Our further work will focus on the improve the properties of the proposed device.
4. Conclusion
In this paper, we report a multifunction reflector actuated by hydraulic control. The two reflectors which are controlled by four LPs can be tilted to different angles and the functions of optical switch and beam steering can be achieved. The experiments show that the proposed device can achieve beam steering within an angle of 0°∼72° in latitude. It can also realize the function of 2×2 optical switch with an actuation (relaxion) time of 280 ms and 260 ms (230 ms and 220 ms), respectively. The proposed device can be expanded to the applications of 1×2 optical switch and two-directions of beam steering.
Funding
National Natural Science Foundation of China (61805169, 61805130, and 61535007); China Postdoctoral Science Foundation (2019M650421 and 2019M650422).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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