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Abstract
We propose an electro-optic on-chip beam shifting device based on gradient microstructured electrodes and an optical tapered waveguide fabricated using lithium niobate (LN). The distribution of refractive index variations of the optical waveguide can be electro-optically defined and tailored by the designed gradient microstructured electrodes, which directs the beam propagation and shifting. The length of the beam shifting device is 18 mm and the width of the waveguide is gradually increased from 8 μm to 80 μm. The functionality of the beam shifting device is experimentally demonstrated, and it is observed that it has an electro-optic tunability of 0.41 μm/V, and a high-speed response time of 19 ns (λ=1310 nm). This study can provide potential applications in optical switching and modulation, beam scanning and ranging, optical spatial communications, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Beam shifting or deflection technology can be utilized for precise control of the direction of laser beam propagation, which has important applications in optical switching and modulation [1–3] , light detection and ranging (Lidar) [4–6], and optical spatial communication [7,8], etc. MEMS (micro-electromechanical systems) devices can be used to control the direction of the beam propagation. However, MEMS devices cannot fulfill the requirements of high-speed application and the vibration-sensitive environment. Therefore, they are an unviable alternative [3,9–12]. Conversely, non-mechanical beam shifting can be achieved by electronic control such as electro-optic (EO) and acousto-optic (AO) beam shifting devices, liquid crystal optical beam scanning, and optical phased array (OPA) devices, which are expected to overcome the above shortcomings.
However, AO beam shifting is prone to problems such as limited modulation speed and diffraction limit of grating forming by surface wave [13–15]. EO beam shifting devices incorporated with liquid crystal can achieve a large dynamic range (deflection angle or distance), but the modulation speed is lower than that of the spatial light modulator (SLM) [16–19]. Photonic integrated solution can be used to develop compact OPA devices, as beam scanning is enabled by changing the phase of light transmitted through the OPA [20–22]. Presently, the main beam scanning of the OPA is thermal-optically driven using current electrodes, which limits the speed of beam shifting or steering. EO tuning might be a solution to solve this problem, the waveguide arrays of OPA equipped electrodes arrays, which were utilized for phase distribution control. In addition to the complexity of electronic driving, the crosstalk between the waveguides is difficult to avoid.
Easy integration and high-speed beam shifting of EO deflection have been proposed as an alternative solution. The selection of optical materials with excellent EO coefficient and the design of electrodes are crucial for the implementation of on-chip high-speed beam shifting. Crystals with large EO coefficients, such as lithium niobate (LN), lithium tantalate, and potassium tantalate niobate, are generally used [23]. In the previous study, fork-fingered electrodes were configured in a massive LN proton exchange waveguide, and a MHz order modulated rate beam deflection based on the waveguide leakage mode was realized [24]. Periodically polarized LN chip as grating structure and electrodes on the upper and lower surfaces of the chip were used to deflect the beam with a driving voltage of $\sim$5 V ($\lambda$ = 633 nm). However, it was difficult to integrate and it cannot be modulated at high speed [25]. Based on the secondary EO effect of KTa$_{1-x}$Nb$_x$O$_3$, the device can achieve a deflection angle of 250 mrad at a driving voltage of $\pm$250 V by configuring upper and lower surface electrodes [26]. An octagonal structure LN EO deflector with quadrupole electrodes was reported, which can obtain a deflection sensitivity of 0.312 $\mu$rad/V [27].
LN is a typical EO material in the field of integrated photonics [28], which exhibits a broad transparency window (0.35-4.5 $\mu$m), relatively high optical refractive index, large EO coefficient ($\gamma _{33}$= 33.8 pm/V), and high Curie temperature [29] (1200$^{\circ }$C). These properties ensure that LN is a suitable material for EO beam shifting and deflection. Using an annealed-proton exchange (APE) LN wide waveguide with triangular electrode arrays, beam shifting can be achieved through EO-induced cascaded prism arrays (variation of refraction index) [2,30–32]. However, in a straight waveguide with a width of 80 $\mu$m, clean mode distribution and high-speed tuning were not obtained [32], which is unfavorable for various applications.
In this paper, we propose an on-chip beam shifting device based on microstructured gradient electrodes and a LN optical tapered waveguide, which can direct the beam propagation by tailoring the refractive index variations distribution via microstructure electrodes. The electro-optic definition and modulation of beam shifting can be achieved by applying different DC voltages and high frequency driving signals, respectively. An electro-optic tunability of beam shifting 0.41 $\mu$m/V and the high-speed switching time of 10 ns order are obtained experimentally. This work demonstrates the on-chip EO beam shifting devices constituted simply by a LN tapered waveguide and microstructured gradient electrodes. Comparing to other reported solution, our proposed structure utilized a single tapered waveguide with microstructured gradient electrodes can obtain a larger beam shifting for guided single-mode with relative high-speed modulation, it can be a larger potential way to apply in active and reconfigurable optic devices.
2. Design principles and geometry
The proposed high-speed EO shifting device based on a tapered LN annealed-proton exchange (APE) optic waveguide and gradient microstructured electrodes is shown in Fig. 1. To utilize the maximum electro-optic coefficient $\gamma _{33}$ ($\gamma _{33}$=33.8 pm/V) of the LN crystal, we selected an X-cut LN with Y propagation direction (waveguide). The waveguide fabricated by APE process supports only TE mode polarization [33]. Therefore, the change in refractive index induced by applying an external voltage can be described as
where n$_e$ is the refractive index of extraordinary light of the LN crystal, $\gamma _{33}$ is the electro-optic (EO) coefficient, and E$_z$ is the electric field along the z-axis of the LN crystal. The difference between the refractive index of the waveguide and the LN substrate is $\sim$0.005 (due to the APE process), this contrast is not large but it is favorable for transversally shifting the guide-mode of waveguide. The operating wavelength of the device is 1310 nm. The dimensions of the electrodes with a taper width of 8-80 $\mu$m are optimized using co-simulation of beam propagation and electrostatic analysis on COMSOL Multiphysics (as illustrated in Fig. 1(b) and Fig. 1(d), and the optimization process can be found in Supplement 1). The tapered waveguide with slowly varying width and the designed gradient electrodes can preserve the guided single-mode optical modes. A small number of electrodes will lead to the generation of multi-modes, while a larger number will improve beam shifting. Meanwhile the width of tapered waveguide will determine the dynamic range of beam shifting. A single-mode waveguide with a width of 8 $\mu$m is used as the straight waveguide for the input segment. The modulation waveguide (with gradient microstructured elelctrodes) is set as a gradual increase width from 8 $\mu$m to 80 $\mu$m and a length of 15 mm. The length of the device is 18 mm. The number of triangular electrode arrays with applied voltages can define and tailor the variation of refractive index(as shown in Fig. 2(a) and (b) for electric field distribution which directly relate the applied voltage), which directs the beam propagation. We set the initial base length of the isosceles triangle as 9 $\mu$m. The base length of the triangle increases in steps of 9 $\mu$m with an increase in the width of the waveguide. It is ensured that the electrodes were arranged along the waveguide with a gradually increasing width. The distance between the vertex of the triangular electrodes and the ground electrode is 7 $\mu$m.
Fig. 1. Integrated on-chip lithium niobate electro-optic beam shifting device. (a) Schematic diagram of the tapered LN APE waveguide with gradient microstructured electrodes. (b) Top view of the device and main dimension parameters. (c) The beam shifting device. (d) Microscopic image of gradient microstructured electrodes.
Fig. 2. Simulation of E$_z$ electric field distribution under opposite voltage: (a) for positive voltage (b) for negative voltage. (c)Simulation of the intensity distribution of the normalized optical beam at different voltages.
We simulated the electric field generated in the tapered optical waveguide using the gradient isosceles triangle array electrodes as shown in Fig. 2(a) and Fig. 2(b). The intensity of the transverse electric field distribution was non-uniform and mainly concentrated around the tips of the isosceles triangles when a positive voltage was applied, furtherly the electric field distribution between the signal and the ground leads to the gradient refractive index distribution in the tapered waveguide, or vice-versa for a negative voltage, which implied that the variation of refractive index can be electro-optically tailored using the designed electrodes. Additionally, the normalized light field intensity distribution under different voltages was obtained as shown in Fig. 2(c). The optical beam can be shifted to the left of the waveguide when the positive voltage is applied, and a negative voltage is applied to move in the opposite direction (as shown in Fig. 1(a)), and the beam shifting was increasingly prominent with an increase in the voltage.
An x-cut LN with a thickness of 500 $\mu$m was selected as the substrate, and we fabricated the LN APE waveguide on the surface of the LN substrate and coplanar electrodes on top [30,33,34]. A SiO$_2$ mask to define the preset waveguide shape was initially deposited on the LN substrate. Subsequently, a waveguide according to the simulation width with a diffusion depth of 5 $\mu$m was formed by proton exchange (at 180 $^{\circ }$C, 3.0 h), which was followed by annealing process (at 333 $^{\circ }$C, 8.5 h). This step increases the refractive index of n$_e$ and provides a weak refractive index difference of 0.005 between the LN substrate and the APE waveguide. Then a 200-nm thick SiO$_2$ buffer layer was deposited on the upper surface of the waveguide using plasma-enhanced chemical vapor deposition method. Finally, 180-nm thick Au electrodes were deposited onto the SiO$_2$ buffer layer.
3. Measurement results and discussion
To verify the electro-optic beam shifting characteristics, the experimental setup shown in Fig. 3 was used. The light from a narrow-band laser (LSB-DFB-0-CW-10-SM, OPEAK) with a wavelength $\lambda$= 1310 nm was emitted into the input of the waveguide with a single-mode fiber (SMF). The SMF was carefully aligned with the waveguide through the translation stage to ensure that only the fundamental mode was excited. A 20X objective lens was placed at the output of the waveguide, and a near infrared CCD (WiDySwir640 U-S, NIT) was used to view the guided mode profile. A guided single-mode profile was observed in the absence of a voltage applied to the electrode.The insertion loss of the beam shifting device, which included the coupling loss, the propagation loss of waveguide including optical subtle absorption from the metal electrodes on top, was measured to be 6.02 dB. The performance of the beam shifting was evaluated by applying different DC voltages to the electrode with a DC power supply (HCP03-600, HCP). Fig. 4(a) shows the beam shifting obtained by applying different DC voltages to the electrodes. Fig. 4(b) shows the normalized intensity distribution of the optic field.
When a voltage was applied to the gradient microstructured electrodes, the tapered waveguide with a slowly increasing width induces a change in the size, position, and intensity of the optical mode. The shifting of the mode field is experimentally and theoretically illustrated in Fig. 4(c). The maximum shifting distance obtained in the simulation and experiment with an applied voltage of -60 V were 25.18 $\mu$m and 25.5 $\mu$m, respectively (Description of the limit of applied voltage by the device can be found in Supplement 1). A EO shifting tunability of 0.41 $\mu$m/V was obtained while conducting the experiment. The dynamic range of beam shifting can be enhanced by increasing the width of the waveguide with a suitable electrode design, but it might induce a multi-mode waveguide in this case. The dynamic modulation characteristics of the device were experimentally investigated. The experimental setup is shown in Fig. 5. We placed the multimode optic fiber (MMF) at the output of this beam shifting device to collect the transmitted light. The other end of the MMF was connected to a photodetector (FPD610-FC-NIR, THORLABS). A signal generator (SDG2082X, SIGLMENT) was utilized to generate modulation signals, and an oscilloscope (DS1104 Z, RIGOL) was used to detect the variations in the output signal of the photodetector (Details of modulation performance analysis can be found in Supplement 1).
Fig. 4. (a)Experimental results of mode profile driving by applied voltages. (b) Experimental results of field intensity distribution under voltages ranging from -60 V to 60 V. (c)Simulation and experimental results of the beam shifting.
The normalized electro-optic modulation response at modulation frequencies of 1 kHz, 1 MHz, 20 MHz and 50 MHz are respectively shown in Fig. 6(a)$\sim$(d). It can be observed that the response of electro-optic mode shifting can follow the modulation of driving signal, for 10 V driving voltage (peak to peak) of 1 KHz, 1 MHz, 20MHz, and 5 V driving voltage (peak to peak) of 50 MHz , which means the proposed device can be applied in a relatively high-speed regime.
Fig. 6. Normalized electric responses for electro-optic modulation at (a)1 KHz (b)1 MHz (c) 20 MHz (d) 50 MHz.
To measure the response time of the beam shifting, a 1-MHz square-wave signal was applied to the electrodes. The normalized signal trace detected by the oscilloscope is shown in Fig. 7. A period time was extracted from Fig. 7(a) to evaluate the response time. As shown in Fig. 7(b), the rise time T$_r$ and the fall time T$_f$ were both $\sim$19 ns. The measured characteristic impedance of 20.8 $\Omega$ and the capacitance value of 117 pF were obtained, which limited the modulation performance of this device.
Fig. 7. (a)Electrical response for electro-optical modulation with a square wave peak-to-peak voltage of 20 V and a frequency of 1 MHz (b) Rise time T$_r$ and fall time T$_f$ in one cycle.
4. Conclusion
In summary, we proposed and fabricated an electro-optic beam shifting device with a tapered LN APE optical waveguide and gradient microstructured electrodes. Electro-optic definition of the variation of refractive index induced by shaping gradient microstructured electrodes can modify the beam shifting functionality. The experimental results demonstrated that the device can achieve a beam shifting tunability of 0.41 $\mu$m/V. In addition, the device can be operated at a relatively high-speed modulation, with an electro-optic response time of 19 ns. The device proposed in this study has potential applications in optical switching and modulation, beam scanning and ranging, and optical spatial communications, etc.
Funding
National Natural Science Foundation of China (61705089, 61775084, 62075088); National Safety Academic Fund (U2030103); Natural Science Foundation of Guangdong Province (2020A1515010791, 2021A0505030036); Open Fund of Guangdong Provincial Key Laboratory of Information Photonics Technology of Guangdong University of Technology (GKPT20-03).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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