1. Introduction

High speed beam deflection devices have important applications, such as in high-speed optical switch [1], optical beam scanning [2], shaping and beam smoothing of a high-power laser [3,4], ultrafast all-optical streak camera [5]. Beam deflectors based on mechanical rotator have been applied [6], but they are bulky, expensive, inconvenient for control, and the moving component limits their applications involving high speed operations. Beam deflection devices with motionless component are more attractive. Beam deflectors based on liquid crystal have been proposed to achieve large angle deflection [7–9]. However, high speed modulation and integration are difficult with liquid crystal, especially when smoothing of high-power laser beam is considered. Acousto-optic and electro-optic deflectors have higher speed and higher reliability than those based on liquid crystal. For the acousto-optic deflector, the deflection angle of light beam is changed by varying the driving frequency [3,10]. The acousto-optic deflector has a typical modulation speed of about 100 MHz, which however is much slower than that of an electro-optic deflector. In addition, the angle resolution of acousto-optic deflector is relatively low [11].

Electro-optic deflectors, based on various materials that permit electro-optic control of local refractive index, can enable high-speed modulation. Thus, the rapid development of electro-optic deflectors has greatly been benefited from the demands of high speed and integration in various fields. Electro-optic Bragg deflectors in periodically poled lithium niobate (LN) waveguides with a drive voltage of ∼5 V (λ = 633 nm) have been demonstrated. However, their practical applications have been limited by the diffraction efficiency [12]. The deflection angle of optical beam from an electro-optic prism can be varied by applying an electric field. Based on this principle, various methods have been proposed to realize beam deflection [11,13–15]. In these methods, reducing the driving power or voltage requires electro-optic materials with large electro-optic coefficients. Hence, the LN material exhibiting a large electro-optic coefficient allows beam deflection with lower voltages, which is useful in integrated-optics applications [16–24]. In our previous work, we have demonstrated the beam deflection based on a LN waveguide with serrated electrodes [25,26]. A refractive index serrated prism array was formed when applying an external voltage, and the refractive index distribution in the waveguide was changed by varying the voltage. The application of voltage to electrodes changes the propagation direction of the light beam passing through the waveguide. A∼1.28-μm beam deflection was achieved by applying a voltage (20 V) to the electrodes. For practical applications, however, it is required to further increase of the deflection angle and reduce the driving voltage. In addition, the output mode patterns from the waveguides were multi-modes in prior works, which can limit its applications such as for high-speed optical switch and beam smoothing of a high-power laser.

In this paper, we demonstrate a high speed mode deflection device for deflection of the quasi-single mode with a low voltage and large deflection angle, based on LN annealed proton exchange (APE) waveguides. The device has isosceles-triangle-shaped array electrodes and a horn-shaped input waveguide. The principle of mode deflection is based on control of local refractive index of the waveguide through varying the voltage applied to the electrodes, which thereby enables the change of the output beam direction. The optimized electrodes enable a larger deflection angle with a lower voltage. To ensure the quasi-single mode operation, the input waveguide is tapered down into a single-mode waveguide. The LN APE waveguide has weak transverse refractive index contrast which is beneficial for transverse beam deflection. The deflection angles are measured by applying different direct voltages. The beam smoothing with modulation is also investigated with alternating voltages. Compared with our previous work [25], this new device has a larger deflection of up to 15.1 μm with an even smaller driving voltage of −50 V.

2. Method and results

Figure 1(a) shows a schematic diagram of our high speed mode deflection device. The fabricated LN APE waveguides with different widths are shown in Fig. 1(b). The isosceles-triangle-shaped array electrodes are deposited on the top of the LN APE waveguide. With electrodes all along the waveguide, a refractive index serrated prism array is formed which can continuously deflect the guided mode. The dimensions of the electrodes are shown in Fig. 1(a). Three LN APE waveguides with widths of 15, 30, 80 μm, respectively, as shown in Fig. 1(a), are investigated. The dimensions of the electrodes and input waveguide, the width and height of the LN APE waveguide are optimized using 3D finite-difference beam propagation method (3DFD-BPM) (BeamPROP, RSoft) and COMSOL Multiphysics [26]. At the input of the device, the waveguides are tapered down into single-mode waveguides with a width of 8 μm, which ensures that only the fundamental mode can be excited. Therefore, it works in the quasi-single mode. The propagation of the fundamental mode in the tapered region was numerical simulated with the 3D finite-difference beam propagation method. The total lengths of these array electrodes are 8 mm. The taper length and the total length of the device are 4 mm and 1.5 cm, respectively. The electric field of the 80 μm waveguide with isosceles-triangle-shaped array electrodes is shown in Fig. 2(a). The isosceles-triangle-shaped array electrodes have a stronger electric field distribution than that of the right-triangle-shaped array electrodes, indicating that larger refractive index change can be achieved. Therefore, the voltage can be accordingly reduced with isosceles-triangle-shaped array electrodes. With triangles all along the waveguide, the refractive index distribution of the waveguide acts as prism array for mode deflection. The index variation is inhomogeneous in the transverse direction of the waveguide. Fig. 2(b) presents the calculated mode deflections of the 80 μm waveguide with an electrode length of 8 mm. As shown in Fig. 2(b), the isosceles-triangle-shaped array electrodes provide larger mode deflections than the right-triangle-shaped array electrodes. The mode deflection depends on the number of triangle electrodes. Figure 2(c) shows the calculated mode deflection as a function of electrode length with a voltage of 20 V for the 80 μm waveguide with isosceles-triangle-shaped array electrodes. As shown in Fig. 2(c), the mode deflection increases with electrode length. Larger mode deflection can be obtained with lager electrode length, but the loss increases with electrode length.

 

Fig. 1 (a) Schematic of the LN APE waveguides with isosceles-triangle-shaped array electrodes, (b) fabricated LN APE waveguides.

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Fig. 2 (a) Electric field of the 80 μm waveguide with isosceles-triangle-shaped array electrodes. (b) Calculated mode deflections and the comparison of the 80 μm waveguide with right-triangle-shaped and isosceles-triangle-shaped array electrodes. (c) Dependence of mode deflection on the electrode length for the 80 μm waveguide with isosceles-triangle-shaped array electrodes.

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The LN waveguide was created by proton exchange, the index contrast is ∼0.005, this can be relatively easier adjusted by the electro-optic effect in lithium niobate. The fabrication process of this device can be divided into four steps [27–31]. Firstly, the SiO₂ mask was formed on the top of a 500-μm thick X-cut LN wafer. In the second step, after the proton exchange (under 180 °C,3.0 h) and annealing step (under 333 °C, 8.5 h), APE waveguides with a thickness of ∼4.5 μm were formed. The third step was to deposit a SiO₂ buffer layer with a thickness of ∼100 nm on the waveguides. Finally, the Au electrodes were fabricated on the SiO₂ buffer layer with a thickness of ~150 nm.

The experimental setup for mode deflection and modulation is shown in Fig. 3. The light source used was a pigtailed laser (SNKOO, SNKFL-D) (λ = 1064 nm), which was coupled into the input port of the waveguide with a width of 8 μm. The fundamental mode was excited by carefully tuning position of the input fiber. Only the TE mode exists in the waveguide, and the polarization extinction ratio is ∼23 dB [21]. The insertion loss of the device was measured to be ∼4.6 dB. The input coupling loss, output coupling loss and propagation loss are estimated (assisted by 3D finite-difference beam propagation method) to be ∼1.0 dB, ∼1.6 dB, and ∼2.0 dB, respectively. Since there is a taper to guide the smaller waveguide to large waveguide and the metal absorption exists even distanced by a buffer layer of SiO₂, the propagation loss is relative large comparing with standard single mode APE waveguide (order of 0.1 dB/cm). The output mode pattern was inspected by a near infrared CMOS camera (Photonfocus, MV1-D1312I-C031-160-CL) with a frame rate of 108 fps. In our experiment, we firstly measured the output mode patterns of the 15, 30 and 80 μm waveguides. Figures 4(a)-4(c) respectively show the microscopic images of the 15, 30 and 80 μm LN APE waveguides with isosceles-triangle-shaped array electrodes. The output mode profiles from the waveguides are shown in Figs. 4(a)-4(c), which clearly exhibit quasi-single modes, suggesting a good beam quality of the deflection beam. Then, the mode deflection was characterized by applying direct voltages to the electrodes. Figures 5(a)-5(c) depict mode profiles taken at the outputs of the 15, 30 and 80 μm waveguides by applying different direct voltages to the electrodes, respectively. As shown in Figs. 5(a) and 5(b), the changes of mode positions for the 15 and 30 μm waveguides are small when direct voltages are applied. For the 80 μm waveguide shown in Fig. 5(c), the mode pattern shifts to the left and right sides when negative and positive voltages are applied to the electrodes, respectively. As expected, the 80 μm waveguide thus has a better performance. The performances of the 15 and 30 μm waveguides, on the other hand, are limited by the lithography of electrode tips. In addition, the modes are well confined in the center of the 15 and 30 μm waveguides. This is the reason why there is no significant deflection in the 15 and 30 μm waveguides.

 

Fig. 3 Experimental setup for monitoring mode deflection and modulation.

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Fig. 4 Microscopic images of the fabricated device and output mode profiles for (a) 15 μm, (b) 30 μm and (c) 80 μm LN APE waveguides.

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Fig. 5 Mode profiles taken at the output of the device at 1064 nm at different direct voltages for (a) 15 μm, (b) 30 μm, and (c) 80 μm APE waveguides. Optical field distributions with different values of the applied voltage for (d) 15 μm, (e) 30 μm, and (f) 80 μm APE waveguides.

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Figures 5(d)-5(f) are the optical field distributions corresponding to Figs. 5(a)-5(c), which show the output modes from the 15, 30, and 80 μm waveguides are quasi-single modes. Compared with the Gaussian shape, the intensity deviations of the output mode profiles under 0 V are 0.8%, 0.4% and 1.4% for the 15, 30 and 80 μm waveguides, respectively. Fig. 5(f) shows apparent shifts of mode positions when direct voltages are varied from −50 V to 50 V. As shown in Fig. 5(f), the mode locates at the center when 0 V voltage is applied. It shifts to the left and right sides when negative and positive voltages are applied, respectively. With higher voltages applied to the electrodes, the side lobes appear. This indicates when the refractive index changes, the fundamental mode can be coupled to the higher-order modes. In addition, the depth of the APE waveguide depends on the width of the waveguide with SiO₂ deposited mask for annealed proton exchange, the larger width the deeper waveguide can be obtained. That means the output of the quasi-single mode was obtained by the taperisation of both width and depth of the waveguide. But the parameters are somehow not so easy to be accurate in fabrication process, this might produce the side lobes. The 3D finite-difference beam propagation method (3DFD-BPM) (BeamPROP, RSoft) was employed to calculate the mode propagation in the waveguide and the mode deflection under different voltages. Both calculated and experimental studies were performed for the 80 μm waveguide, since it has the best performance, as shown in Fig. 6. The 80 μm waveguide has maximum mode shifts of 15.6 and 15.1 μm for the calculated and experimental results, respectively. Under −50 V, the mode deflection is 15.1 µm, which can find applications in beam smoothing of a high-power laser. The mode deflections are 0.267 and 0.265 μm/V for the simulation and experiment results, respectively, giving a difference of 0.7%. In our previous works [26], the mode deflection increases with applied voltage at a rate of 0.06 μm/V. In addition, in [26] the output is multimode without taper at the input. Therefore, the new device has improved results. We investigated the dependence of mode deflection on wavelength. The mode profiles were recorded by the CMOS camera at the output of the 80 μm waveguide with a 980 nm laser source. As shown in Fig. 7, the mode deflection increases with applied voltage at a rate of 0.291 and 0.240 μm/V for the simulation and experiment results, respectively.

 

Fig. 6 Mode deflection as a function of the applied voltage for the 80 μm APE waveguide at 1064 nm.

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Fig. 7 Mode deflection as a function of the applied voltage for the 80 μm APE waveguide at 980 nm.

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Then the modulation characteristics of the 80 μm waveguide were experimentally investigated with a modulation frequency of 50 Hz and a sinusoidally alternating voltage. Figure 8(a) shows the output mode profiles for direct voltages of 0 V, −18 V, 18 V, as well as sinusoidal signal with a voltage of 18 V. As shown in Fig. 8(a), compared to case of 0 V, the mode shifts to the left and right sides for −18 V and 18 V, respectively. The intensity distribution of the output mode profile under alternating voltage in Fig. 8(a) is more uniform, which was obtained by averaging over 69 images taken by the CMOS camera with alternating voltages. The uniform intensity distribution with beam modulation confirms the smoothing of non-uniform density distribution of light beam with high-speed modulation. Figure 8(b) shows the intensity distribution of the mode patterns in Fig. 8(a). With beam modulation, the intensity distribution is broadened and the peak is decreased. The three-dimensional optical mode profiles are shown in Fig. 8(c).

 

Fig. 8 Beam modulation in the 80 μm LN APE waveguide. (a) Output mode profiles for direct and alternating voltages, (b) the corresponding intensity distribution of the modes and (c) the three-dimensional optical mode profiles.

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3. Conclusions

We have proposed an electro-optic mode deflection device based on a LN APE waveguide with isosceles-triangle-shaped array electrodes. With horn-shaped input waveguide, the output of this device is a quasi-single mode. The mode deflections are 0.265 and 0.240 μm/V for 1064 and 980 nm, respectively. By studying the CMOS image of mode profile under alternating voltage, beam smoothing with modulation is achieved. This electro-optic mode deflection device has potential applications in high-speed optical switch, and beam smoothing of a high-power laser.