Open Access
Add to BibSonomy
Abstract
Beam steering devices have wide applications in both military and civil fields. The ultimate goal for such devices is to reduce their size, weight, and power consumption. However, the laser source in these devices is spatially separate from the phase shifter, resulting in large size, complex packaging, and low coupling efficiency. To solve these problems, a novel electrically controlled beam steering chip based on coherently coupled vertical cavity surface emitting laser (VCSEL) array directly integrated with liquid crystal optical phased array (LCOPA) is proposed in this paper. Implant-defined in-phase coherently coupled VCSEL arrays (CCVAs) with uniform near-field are designed and fabricated first to act as the coherent laser source for the chip. Then, taking advantage of the CCVA planar structure, the LCOPA is integrated directly on the CCVA by conventional process. The coherent light generated by the in-phase CCVA is uniformly and normally incident into the LCOPA and is electrically steered by the LCOPA. One-dimensional beam steering is achieved by two proof-of-concept integrated chips. The chips based on a 4 × 4 square CCVA and a 16-element hexagonal CCVA offer a field of view of 2.21° and 6.06°, respectively. Independent control of the CCVA and LCOPA guarantees a relatively high wavelength stability and power stability. Theoretical calculations are also performed, which are consistent with the experiments.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Beam steering devices are essential for a variety of applications in both military and civil fields such as LiDAR, communication, sensing and imaging [1–3]. The ultimate goal for such devices is to reduce their size, weight, and power consumption, so that they can be mounted on, e.g. drones and autonomous cars [3]. Conventional mechanical beam steering devices can obtain a large deflection angle, but they are relatively bulky, slow, high-cost, and of poor reliability.
Electrically controlled non-mechanical techniques have been introduced as an alternative without any moving parts, which have increased reliability at a lower cost. A popular method is to use a liquid crystal optical phased array (LCOPA) as the spatial light modulator to steer an already generated beam through introducing a phase gradient along the beam path [4–8]. In this method, the entire beam steering system is usually composed of an external coherent light source, optical elements, and a spatial light modulator, separated in space from each other, resulting in relatively large size and difficulty in miniaturization.
To obtain miniaturized beam steering devices, nanophotonic OPAs are proposed [9–14]. Large-scale 64 × 64 nanophotonic OPAs built on a silicon photonic platform with small area (576 μm × 576 μm) have been achieved [9]. However, this method still needs an external off-chip laser source to provide a coherent beam, making the packaging of the device complex. In addition, tight alignment and strict coupling technology are required for coupling the external laser source with the nanophotonic OPA through optical fiber to ensure a relatively high coupling efficiency [3,9]. On the other hand, employing a VCSEL-based slow light Bragg reflector waveguide as the beam deflector can also realize miniaturized beam steering device [15–18], but it faces the same problem as that of the nanophotonic OPA method. By utilizing bonding technology, the laser source and the OPA can be integrated on one hybrid silicon platform [19,20]. However, a silicon waveguide bus is needed to guide the laser beam into the OPA, which requires strict design to reduce the optical loss. Besides, the fabrication is relatively complicated due to the hybrid III-V/silicon platform.
To sum up, most of the beam steering devices need an external laser source and a fiber- or waveguide-coupled input. The external laser source cannot be integrated with the OPA directly, leading to low compactness, complex packaging, and relatively low coupling efficiency between the laser source and the OPA. The key to solve these problems is to find a suitable coherent laser source that can be integrated with the OPA directly without any optical elements, fibers, or waveguides.
Vertical cavity surface emitting laser (VCSEL) has the advantages of planar structure and vertical emission [21–23], which makes it easier to be integrated with various transmission OPAs directly. Nevertheless, the aperture of a single VCSEL device is usually limited around 3.5μm to ensure coherent single-mode emission [24,25], which either limits its output power or makes it difficult to integrate an OPA on the VCSEL due to the small aperture. On the contrary, if the aperture is too large, the single VCSEL device will operate in multi-mode with degraded coherence and nonuniform near-field. Therefore, a single VCSEL device has difficulty in being integrated with an OPA directly.
Presently, we have realized large-area implant-defined 5 × 5 and 10 × 10 in-phase coherently coupled VCSEL arrays (CCVAs) that with uniform near-field and coherent emission by means of engineering the implantation and array parameters [26]. The proton implantation technology used to fabricate the CCVA is relatively simple and low-cost. Such kinds of in-phase CCVAs have the characteristics of planar structure, coherent single-mode emission, and uniform near-field profile, making them a suitable candidate for being integrated with an OPA directly.
In this paper, we propose a novel electrically controlled integrated beam steering chip based on in-phase CCVA and LCOPA. Taking advantage of the planar structure of the CCVA, the LCOPA is directly integrated on the CCVA by conventional process. The coherent light generated by the in-phase CCVA is uniformly and normally incident into the OPA without going through any optical elements, fibers, or waveguides, leading to high compactness, small size, simple packaging, and high coupling efficiency. In addition, using the implant-defined CCVA as the laser source and the relatively mature LCOPA as the phase shifter makes the cost low. Such an integrated chip could be transformative for various LiDAR, communication, sensing and imaging, and other applications.
2. Device design and fabrication
Two kinds of one-dimensional integrated beam steering chips based on 4 × 4 square CCVA and 16-element hexagonal CCVA are designed, respectively. A schematic structure of the beam steering chip based on a 4 × 4 square CCVA is shown in Fig. 1. Figure 1(a) is the three-dimensional schematic sketch of the chip, and Fig. 1(b) is the cross-section view of the chip along the A-A' direction in Fig. 1(a). The chip is composed of two parts: CCVA and LCOPA. A SiO2 layer is deposited on the CCVA’s surface to realize electrical isolation between the two parts. On the surface of the SiO2 layer, four ITO strips are situated well atop the four rows of the 4 × 4 CCVA correspondingly, acting as the positive electrodes of the LCOPA. An ITO glass is used as the common negative electrode, and between the ITO glass and the ITO positive electrodes is a liquid crystal (LC) layer, thus forms a LCOPA. An LCOPA’s unit corresponds to a row (four VCSEL elements) of the 4 × 4 CCVA, thus forms a 1 × 4 one-dimensional beam steering chip, as shown in Figs. 1(a) and 1(c). The chip based on the 16-element hexagonal CCVA has a similar structure with the chip based on the 4 × 4 CCVA, as shown in Fig. 1(d). With this structure, the emitting light from the CCVA can be directly and normally incident into the LCOPA without going through any optical elements, fibers, and waveguides, so that the coupling efficiency between the CCVA and the LCOPA is guaranteed. By applying voltage to the ITO positive and negative electrodes of each unit of the LCOPA, the LC molecules in between are rotated and the effective refractive index of the LC is changed, therefore, the phase difference between units can be adjusted, so that electrically controlled beam steering can be realized.
Fig. 1 (a) Schematic structure of the beam steering chip based on a 4 × 4 square CCVA. (b) The cross-section view of the chip along the A-A' direction. (c) The arrangement of the ITO positive electrodes of the chip based on a 4 × 4 square CCVA. (d) The arrangement of the ITO positive electrodes of the chip based on a16-element hexagonal CCVA. (e) Top view of a fabricated 4 × 4 square CCVA. (f) Top view of a fabricated 16-element hexagonal CCVA.
The fabrication of the chip is divided into two parts and the first part is the CCVA. The VCSEL epitaxy structure is the same as that in [26] with a designed wavelength of 850nm. The element dimension of the 4 × 4 square CCVA is 6μm × 6μm and the inter-element spacing is chosen as 8μm to achieve in-phase coupling among elements [26]. The elements of the hexagonal CCVA are designed as circles with a diameter of 6μm and an inter-element spacing of 4μm to achieve in-phase coupling. A proton implantation process is performed to realize electrical insulation between VCSEL elements. The fabrication process of the VCSEL arrays is the same as that in [26]. It should be noted that the VCSEL epitaxy wafer (1cm × 1cm in size) does not undergo a thinning process for the sake of simplifying the process. Figures 1(e) and 1(f) are top views of the fabricated 4 × 4 square VCSEL array and 16-element hexagonal VCSEL array, respectively.
And the second part is the integration of the two parts. The detailed integration processes are as follows. First, a SiO2 layer with a thickness of 800nm is deposited on the whole surface of the VCSEL array wafer to realize electrical isolation between the VCSEL array and the LCOPA. Then, the part of the SiO2 layer covering the bondpad of the VCSEL array is etched away by a lithography and a wet etching process. Next, an ITO layer with a thickness of 85nm is deposited on the SiO2 layer. The part of the ITO layer atop each row of the VCSEL array is protected by a lithography process and the rest is removed by wet etching, forming four ITO strips with a width of 6μm (the same size with the VCSEL array element) to act as the positive electrodes of the LCOPA. The edge-to-edge spacing between the ITO strips is equal to the inter-element spacing of the corresponding VCSEL array. After that, four Au bondpads with a thickness of 200nm are formed at the end of the four ITO strips outside the VCSEL array by a sputtering and a lift-off process. Then the VCSEL wafer is treated as the lower substrate of the LC cell, and a glass with 100nm-thick ITO layer coated on the lower surface is employed as the upper substrate (i.e., the common negative electrode), which is supported by photoresist mixed with silicon sphere spacers. The thickness of the LC cell is 10μm, which is determined by the diameter of the silicon sphere spacers. Finally, sealing glue is coated around the LC cell after injecting liquid crystal (MAT-10-875, from Merck), and the fabrication of the chip is finished.
The VCSEL wafer, which is with fabricated beam steering chips on is packaged on a printed circuit board for easy driving. The package is very simple, as shown in Fig. 2(a). There are eight VCSEL arrays in the 1cm × 1cm VCSEL wafer, including four 4 × 4 square VCSEL arrays and four 16-element hexagonal VCSEL arrays, corresponding to eight beam steering chips, respectively. The chips are electrically isolated and independent from each other. Figure 2(b) is a microscope image of the wafer taken at a 200 × magnification, where the liquid crystal cavity confined by photoresist and Si sphere spacers can be seen clearly. The microscope image of the chip based on 4 × 4 VCSEL array taken at a 500 × magnification is shown in Fig. 2(c), where the ITO positive electrodes are marked with black dot lines. The area of the region where beam steering takes place is only about 95μm × 95μm. It can be seen that one ITO electrode covers a row (four VCSEL elements) of the VCSEL array. By adjusting the voltages applied the ITO electrodes, the phase difference between neighbor LCOPA units can be adjusted, thus one-dimensional beam steering can be realized.
Fig. 2 (a) The VCSEL wafer with fabricated beam steering chips, which is packaged on a printed circuit board for easy driving. (b) Microscope image of the wafer taken at a 200 × magnification. (c) Microscope image of the chip based on 4 × 4 CCVA taken at a 500 × magnification.
3. Results and discussion
3.1. Optical characterization of the VCSEL arrays
Before integrating the LCOPA with the CCVA, the performances including power-current-voltage (P-I-V) characteristics, near-field, far-field, and spectrum of the VCSEL arrays are on-chip measured at room temperature under continuous wave condition. The measured results of the typical 4 × 4 square VCSEL array are shown in Fig. 3. Figure 3(a) is the P-I-V characteristics of the array, which shows a threshold current of 30mA. Because the substrate of the VCSEL array has not been thinned and it lacks effective heat dissipation measures, the output power and slope efficiency of the array is relatively low. Enhanced output power and slope efficiency can be achieved by improving the heat dissipation. The near-field image measured at 34mA shows that all the array elements are lasing with relatively uniform intensity, as shown in the inset of Fig. 3(b). The single-peak spectrum measured at 34mA displayed in Fig. 3(b) indicates that the array is working at single mode [27]. The far-field pattern measured at 34mA is shown in Fig. 3(c), from which the coherence characteristics that the optical energy is mainly concentrated in the on-axis central lobe surrounded by several weaker secondary lobes can be seen clearly, indicating that the array is operating in the in-phase mode [26]. To confirm the in-phase operation measured result, we use FDTD solution software to simulate the far-field pattern of a 4 × 4 square in-phase CCVA. In the simulation, the element size and inter-element spacing of array are set the same as the actual structure of the fabricated VCSEL array, and the light sources in each element are set to be Gaussian distribution with a phase of 0 degree, keeping in-phase with each other [26]. The simulated far-field pattern is shown in Fig. 3(d). It can be seen that the optical intensity is mainly concentrated in the on-axis far-field lobe surrounded by several weaker lobes, which is basically consistent with the measured far-field pattern. The intensity profiles along the vertical direction across the central lobe of the measured and simulated far-field patterns are displayed in Fig. 3(e). The fabricated CCVA shows an FWHM beamwidth of 1.6°, which is about 1.6 times of the diffraction limit of 1.0° (i.e., the calculated beamwidth), indicating a relatively high beam quality. The deviation may be caused by the imperfect coherence between the array elements of the fabricated device, which can be suppressed by improving the fabrication process uniformity and optimizing the array structure.
Fig. 3 (a) Measured P-I-V characteristics of the typical 4 × 4 square VCSEL array. (b) Measured spectrum of the 4 × 4 square VCSEL array at 34 mA. (c) Measured far-field pattern of the 4 × 4 square VCSEL array at 34 mA. (d) Simulated far-field pattern of an in-phase coherently coupled 4 × 4 square VCSEL array. (e) The intensity profiles along the vertical direction across the central lobe of the measured and simulated far-field patterns.
The performances of the 16-element hexagonal VCSEL arrays are also characterized and the results of the typical array are shown in Fig. 4. The P-I-V characteristics displayed in Fig. 4(a) shows a threshold current of 21mA, which is much smaller than that of the 4 × 4 CCVA. The smaller area of the array element and the stronger optical coupling in hexagonal VCSEL array result in the lower threshold [28,29]. Relatively uniform intensity is also found in the near-field image of the array measured at 30mA, as shown in the inset of Fig. 4(b). The spectrum of the array measured at 30mA displayed in Fig. 4(b) also shows a single peak, indicating single mode operation [27]. The measured far-field pattern of the array depicted in Fig. 4(c) shows an on-axis maximum intensity, and the central lobe is surrounded by six weaker secondary lobes, exhibiting obvious coherence characteristics, which is consistent with the calculated far-field pattern of the 16-element hexagonal in-phase coherently coupled VCSEL array shown in Fig. 4(d). The measured FWHM beamwidth of the fabricated array is about 2.0°, which is about 1.7 times of diffraction limit of 1.2°, as shown in Fig. 4(e). These results indicate that the fabricated 16-element hexagonal array is an in-phase CCVA [26,28,29], which can satisfy the requirements of LCOPA on laser source.
Fig. 4 (a) Measured P-I-V characteristics of the typical 16-element hexagonal VCSEL array. (b) Measured spectrum of the 16-element hexagonal VCSEL array at 30 mA. (c) Measured far-field pattern of the 16-element hexagonal VCSEL array at 30 mA. (d) Simulated far-field pattern of an in-phase coherently coupled 16-element hexagonal VCSEL array. (e) The intensity profiles along the vertical direction across the central lobe of the measured and simulated far-field patterns.
3.2. Beam steering characterization of the integrated chip
After characterizing the optical performance of the VCSEL arrays, the LCOPA is integrated on the CCVA using the integration process aforementioned. The experimental setup for measuring the beam steering performance of the integrated chip is shown in Fig. 5. The CCVA in the chip is driven by a current source, while the four LCOPA’s units are driven by four manual-adjusting voltage sources with an accuracy of 0.1 V, independently. The generated light from the CCVA is firstly incident into the LCOPA, and then transmits out and finally reaches to a white screen. The far-field pattern on the white screen is detected and monitored by a CCD camera connected with a beam profiler system. During the measurement, the injection current of the CCVA is fixed at the in-phase operating point, while the applied voltages on the units of the LCOPA are adjusted to steer the generated light from the CCVA. The deflection angles of the far-field are recorded and analyzed by the beam profiler system.
Figure 6(a) shows a microscope image of the chip based on the 4 × 4 square in-phase CCVA, of which the ITO positive electrodes are marked by red lines. The applied voltages to the four ITO positive electrodes from top to bottom are denoted by (V1, V2, V3, V4), as shown in Fig. 6(a). Generally, the voltage applied to each LCOPA unit is required to generate a uniform phase gradient along the beam path to realize ideal beam steering [4–8]. However, the actual liquid crystal material and the structure of the fabricated LCOPA have deviations from the ideal condition, so that repeated adjustments of the applied voltages are needed to obtain the ideal phase shifts. Moreover, it is difficult to obtain ideal phase shifts by manually adjusting the four voltage sources simultaneously. Therefore, in order to verify the beam steering function of chip easily and to reduce the measurement complexity, the values of V1 to V4 are set following the simple equal difference increasing rule as (V1 = V0, V2 = 2•V0, V3 = 3•V0, V4 = 4•V0) and the equal difference decreasing rule as [V1 = V0’, V2 = (3/4)•V0’, V3 = (2/4)•V0’, V4 = (1/4)•V0’], respectively.
Fig. 6 (a) A microscope image of the chip based on a 4 × 4 square in-phase CCVA, of which the ITO positive electrodes are marked by red lines. (b) The measured upward deflection results when V0 equals to 0.0, 0.5, and 0.6 V separately. (c) The simulated upward deflection results when V0 equals to 0.0, 0.5, and 0.6 V separately. (d) The measured downward deflection results when V0’ equals to 0.0, 2.0, and 2.4 V separately. (e) The simulated downward deflection results when V0’ equals to 0.0, 2.0, and 2.4 V separately.
In the case of (V1 = V0, V2 = 2•V0, V3 = 3•V0, V4 = 4•V0), the phase shift caused by the LCOPA unit under V1 is the smallest, while the phase shift caused by the LCOPA unit under V4 is the largest. As a result, the far-field pattern of the chip should be steered upward in this case. Figure 6(b) shows the measured beam steering results when V0 equals to 0.0, 0.5, and 0.6 V separately, corresponding to an upward deflection angle of 0.0°, 0.57°, and 0.93°, respectively. The corresponding calculation results under different V0 are given in Fig. 6(c). Both the deflection angle and the deflection direction of the far-field pattern are shown in the figures.
In the case of [V1 = V0’, V2 = (3/4)•V0’, V3 = (2/4)•V0’, V4 = (1/4)•V0’], the phase shift caused by the LCOPA unit under V1 is the largest, while the phase shift caused by the LCOPA unit under V4 is the smallest. As a result, the far-field pattern of the chip should be steered downward in this case. Figure 6(d) shows the measured beam steering results when V0’ equals to 0.0, 2.0, and 2.4 V separately, corresponding to a downward deflection angle of 0.0°, 0.52°, and 0.85°, respectively. Figure 6(e) are the corresponding calculation results. These results verify the feasibility of the integrated beam steering chip and the validity of the way for applying voltage.
Figures 6(f) and 6(g) show the experimental and calculation results of the upward and downward deflection angles at a larger V0 and V0’, respectively. The purple triangle represents the theoretical calculation deflection angle, and the red circle represents the experimental deflection angle. It can be found that as the applied voltage is increased, the deflection angle of the far-field pattern increases. In the case of the equal difference increasing voltage distribution, the upward deflection angle reaches to a maximum of 1.15° when V0 is increased to 0.9 V. In the case of the equal difference decreasing voltage distribution, the downward deflection angle reaches to a maximum of 1.06° when the V0’ is increased to 3.6 V. A total field of view of 2.21° is achieved by the beam steering chip based on the 4 × 4 in-phase CCVA. It can also be found that the relationship of deflection angle versus V0 (V0’) for the experimental results is consistent with that of the calculation results within the whole range of V0 (V0’). However, there are a few deviations between the measured results and the calculation results. These deviations are attributed to the inhomogeneity of the LCOPA caused by imperfect fabrication.
In all the calculations, the actual parameters and structure of the LCOPA including the LC’s elastic coefficients (K11 = 13.6, K22 = 6.3, K33 = 16.5), the dielectric constant (ε∥ = 11.9, ε⊥ = 3.2), and the thickness of the LC cell (10μm) are used. To calculate the deflection angle of the far-field pattern, the phase shifts under different voltages are firstly calculated by using the method given in [30], and then they are input into the VCSEL array model established by FDTD solution software [26]. By this way, the far-field pattern of the VCSEL array with phase shifts between the array rows can be calculated, from which the deflection angle and the deflection direction can be obtained.
The output power and emission wavelength of the chip during beam steering are monitored and measured. Figures 7(a) and 7(b) show the output power and wavelength versus different deflection angles. Since the laser source and the LCOPA are independently controlled, the output power and the spectrum of the chip can keep relatively stable during beam steering. It can be seen from Fig. 7(b) that the wavelength has no blue shifts or red shifts, exhibiting excellent spectrum stability. The output power decreases slightly with an increased deflection angle, as shown in Fig. 7(a). However, the chip still shows much higher power stability and wavelength stability than that of the addressable beam steering VCSEL array [31], making it more desirable for various applications.
Fig. 7 (a) The output power versus deflection angle during beam steering of the chip based on 4 × 4 square in-phase CCVA. (b)The measured spectra versus deflection angle during beam steering of the chip based on 4 × 4 square in-phase CCVA.
At last, we test the performance of the integrated chip based on the 16-element hexagonal CCVA. Similarly, we define the voltages applied to each unit of the LCOPA as (V1, V2, V3, V4), as shown in Fig. 8(a). In order to test the chip more conveniently, only an LCOPA unit is applied with voltage, while the other three units have no voltages on. In this case, beam steering can also be effectively achieved, as shown in Figs. 8(b) and (d). It can be seen that when V1 = V2 = V3 = 0.0 V and V4 is gradually increased from 0.0 V to 2.5 V, the beam is deflected obliquely upward along the direction perpendicular to the ITO electrode, as indicated by the arrow shown in Fig. 8(a). As V4 is increased to 2.5 V, the deflection angle reaches to a maximum of 3.31°, as shown in Fig. 8(b). On the contrary, when V2 = V3 = V4 = 0.0 V and V1 is gradually increased from 0.0 V to 2.5V, the beam is deflected to an opposite direction, as indicated by the arrow shown in Fig. 8(c). As V1 is increased to 2.5 V, the deflection angle reaches to a maximum of 2.75°, as shown in Fig. 8(d). The measured total field of view is 6.06°. Since the LCOPA’s unit of the chip based on 16-element hexagonal CCVA has a width w = 6μm and a spacing d = 4μm, while the LCOPA’s unit of the chip based on 4 × 4 square CCVA has a width w = 6μm and a spacing d = 8μm, the deflection angle of the chip based on the 16-element hexagonal CCVA is larger than that of the chip based on the 4 × 4 square CCVA according to the relationship of θ = arcsin[λ/(w + d)] (θ is the deflection angle, and λ is the laser wavelength), which is in good agreement with the experimental results. During the process of beam steering, the chip also maintains good power stability and wavelength stability (not shown here). These test results of the chip based on 16-element hexagonal CCVA again verify the feasibility and effectiveness of the integrated beam steering chip.
Fig. 8 (a) A microscope image of the chip based on the 16-element in-phase CCVA, of which the ITO positive electrodes are marked by red lines. The arrow shows the deflection direction. (b) The measured beam steering results when V4 equals to 0.0, 1.5, and 2.5 V, respectively. (c) A microscope image of the chip based on a 16-element in-phase CCVA. The arrow shows the deflection direction. (d) The measured beam steering results when V1 equals to 0.0, 1.5, and 2.5 V, respectively.
All the above test results and calculation results show that the integrated electrically controlled beam steering chip based on in-phase CCVA and LCOPA can achieve effective beam scanning. The laser source is directly integrated with the LCOPA, leading to high coupling efficiency, high compactness, and simple packaging. Besides, the wavelength stability and power stability are relatively good during beam steering. Thus, the chip can meet the trend of integration and miniaturization of photoelectric systems. Moreover, if the ITO positive electrodes are properly designed and arranged, two-dimensional beam steering can be realized by such a kind of chip. In addition, the optical phased array used in the chip is not limited to LCOPA, and the VCSEL array is not limited to implant-defined array. As long as the array is planar and phase locked, it can be used in the chip. By combining with large angle step-steering approaches such as volume holograms [7], large steering range can also be achieved by the chip.
4. Conclusions
A novel integrated electrically controlled beam steering chip based on coherently coupled VCSEL array and liquid crystal optical phased array is proposed, which has successfully realized one-dimensional beam steering. The chip based on the 4 × 4 square in-phase CCVA realizes a field of view of 2.21°, and the chip based on a 16-element hexagonal in-phase CCVA realizes a field of view of 6.06°, which have relatively excellent power stability and wavelength stability during beam steering. The LCOPA is directly integrated on the CCVA without any optical elements, fibers, or waveguides, leading to high compactness, high coupling efficiency, and simple packaging. Such a chip has great application prospects in airborne radar, vehicle radar, communication, and many other applications. To further improve the performance of the chip and realize two-dimensional beam steering is our next work.
Funding
National Key R&D Program of China (2018YFA0209000); National Natural Science Foundation of China (61874145, 61604007, 61774175, 11674016, 61751502); Beijing Natural Science Foundation (4172009, 4182012); Beijing Municipal Commission of Education (KM201810005029).
References
1. D. N. Hutchison, J. Sun, J. K. Doylend, R. Kumar, J. Heck, W. Kim, C. T. Phare, A. Feshali, and H. Rong, “High-resolution aliasing-free optical beam steering,” Optica 3(8), 887–890 (2016). [CrossRef]
2. T. Matsuda, F. Abe, and H. Takahashi, “Laser printer scanning system with a parabolic mirror,” Appl. Opt. 17(6), 878–884 (1978). [CrossRef] [PubMed]
3. M. J. R. Heck, “Highly integrated optical phased arrays: photonic integrated circuits for optical beam shaping and beam steering,” Nanophotonics 6(1), 93–107 (2017). [CrossRef]
4. P. F. McManamon, T. A. Dorschner, D. L. Corkum, L. J. Friedman, D. S. Hobbs, M. Holz, S. Liberman, H. Q. Nguyen, D. P. Resler, R. C. Sharp, and E. A. Watson, “Optical phased array technology,” Proc. IEEE 84(2), 268–298 (1996). [CrossRef]
5. D. Engström, M. J. O’Callaghan, C. Walker, and M. A. Handschy, “Fast beam steering with a ferroelectric-liquid-crystal optical phased array,” Appl. Opt. 48(9), 1721–1726 (2009). [CrossRef] [PubMed]
6. T. Hara, “A liquid crystal spatial light phase modulator and its applications,” Proc. IEEE 5642, 78–89 (2005).
7. P. F. McManamon, P. J. Bos, M. J. Escuti, J. Heikenfeld, S. Serati, H. Xie, and E. A. Watson, “A review of phased array steering for narrow-band electrooptical systems,” Proc. IEEE 97(6), 1078–1096 (2009). [CrossRef]
8. D. P. Resler, D. S. Hobbs, R. C. Sharp, L. J. Friedman, and T. A. Dorschner, “High-efficiency liquid-crystal optical phased-array beam steering,” Opt. Lett. 21(9), 689–691 (1996). [CrossRef] [PubMed]
9. K. Van Acoleyen, W. Bogaerts, and R. Baets, “Two-dimensional dispersive off-chip beam scanner fabricated on silicon-on-insulator,” IEEE Photonics Technol. Lett. 23(17), 1270–1272 (2011). [CrossRef]
10. K. Van Acoleyen, K. Komorowska, W. Bogaerts, and R. Baets, “One-dimensional off-chip beam steering and shaping using optical phased arrays on silicon-on-insulator,” J. Lightwave Technol. 29(23), 3500–3505 (2011). [CrossRef]
11. K. Van Acoleyen, W. Bogaerts, J. Jágerská, N. Le Thomas, R. Houdré, and R. Baets, “Off-chip beam steering with a one-dimensional optical phased array on silicon-on-insulator,” Opt. Lett. 34(9), 1477–1479 (2009). [CrossRef] [PubMed]
12. K. Van Acoleyen, H. Rogier, and R. Baets, “Two-dimensional optical phased array antenna on silicon-on-insulator,” Opt. Express 18(13), 13655–13660 (2010). [CrossRef] [PubMed]
13. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, L. A. Coldren, and J. E. Bowers, “Two-dimensional free-space beam steering with an optical phased array on silicon-on-insulator,” Opt. Express 19(22), 21595–21604 (2011). [CrossRef] [PubMed]
14. J. Sun, E. Timurdogan, A. Yaacobi, E. S. Hosseini, and M. R. Watts, “Large-scale nanophotonic phased array,” Nature 493(7431), 195–199 (2013). [CrossRef] [PubMed]
15. X. Gu, T. Shimada, and F. Koyama, “Giant and high-resolution beam steering using slow-light waveguide amplifier,” Opt. Express 19(23), 22675–22683 (2011). [CrossRef] [PubMed]
16. X. Gu, T. Shimada, A. Fuchida, A. Matsutani, A. Imamura, and F. Koyama, “Beam steering in GaInAs/GaAs slow-light bragg reflector waveguide amplifier,” Appl. Phys. Lett. 99(21), 211107 (2011). [CrossRef]
17. X. Gu, T. Shimada, A. Fuchida, A. Matsutani, A. Imamura, and F. Koyama, “Electro-thermal beam steering using Bragg reflector waveguide amplifier,” Jpn. J. Appl. Phys. 51, 020206 (2012). [CrossRef]
18. X. Gu, T. Shimada, A. Fuchida, A. Matsutani, and F. Koyama, “Ultra-compact beam-steering device based on bragg reflector waveguide amplifier with number of resolution points over 100,” Electron. Lett. 48(6), 336–337 (2012). [CrossRef]
19. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, M. L. Davenport, L. A. Coldren, and J. E. Bowers, “Hybrid III/V silicon photonic source with integrated 1D free-space beam steering,” Opt. Lett. 37(20), 4257–4259 (2012). [CrossRef] [PubMed]
20. J. C. Hulme, J. K. Doylend, M. J. R. Heck, J. D. Peters, M. L. Davenport, J. T. Bovington, L. A. Coldren, and J. E. Bowers, “Fully integrated hybrid silicon two dimensional beam scanner,” Opt. Express 23(5), 5861–5874 (2015). [CrossRef] [PubMed]
21. H. Soda, K. Iga, C. Kitahara, and Y. Suematsu, “GaInAsP-InP surface emitting injection-lasers,” Jpn. J. Appl. Phys. 18(12), 2329–2330 (1979). [CrossRef]
22. J. L. Jewell, J. P. Harbison, A. Scherer, Y. H. Lee, and L. T. Florez, “Vertical-cavity surface-emitting lasers: design, growth, fabrication, characterization,” IEEE J. Quantum Electron. 27(6), 1332–1346 (1991). [CrossRef]
23. W. W. Chow, K. D. Choquette, M. H. Crawford, K. L. Lear, and G. R. Hadley, “Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers,” IEEE J. Quantum Electron. 33(10), 1810–1824 (1997). [CrossRef]
24. C. Jung, R. Jäger, M. Grabherr, P. Schnitzer, R. Michalzik, B. Weigl, S. Müller, and K. J. Ebeling, “4.8 mW single mode oxide confined top-surface emitting vertical-cavity laser diodes,” Electron. Lett. 33(21), 1790–1791 (1997). [CrossRef]
25. D. S. Song, S. H. Kim, H. G. Park, C. K. Kim, and Y. H. Lee, “Single-fundamental-mode photonic-crystal vertical-cavity surface-emitting lasers,” Appl. Phys. Lett. 80(21), 3901–3903 (2002). [CrossRef]
26. G. Pan, Y. Xie, C. Xu, M. Xun, Y. Dong, J. Deng, and J. Sun, “Large-scale proton-implant-defined VCSEL arrays with narrow beamwidth,” IEEE Electron Device Lett. 39(3), 390–393 (2018). [CrossRef]
27. B. J. Thompson, Z. Gao, S. T. M. Fryslie, M. T. Johnson, D. F. Siriani, and K. D. Choquette, “Coherence in multielement-phased vertical-cavity surface-emitting laser arrays using resonance tuning,” IEEE Photonics J. 9(5), 1–8 (2017). [CrossRef]
28. M. Xun, C. Xu, J. Deng, Y. Xie, G. Jiang, J. Wang, K. Xu, and H. Chen, “Wide operation range in-phase coherently coupled vertical cavity surface emitting laser array based on proton implantation,” Opt. Lett. 40(10), 2349–2352 (2015). [CrossRef] [PubMed]
29. G. Pan, Y. Xie, C. Xu, Y. Dong, J. Deng, H. Chen, and J. Sun, “Analysis of optical coupling behavior in two-dimensional implant-defined coherently coupled vertical-cavity surface-emitting laser arrays,” Photon. Res. 6(11), 1048–1055 (2018). [CrossRef]
30. X. Wang, D. Wilson, R. Muller, P. Maker, and D. Psaltis, “Liquid-crystal blazed-grating beam deflector,” Appl. Opt. 39(35), 6545–6555 (2000). [CrossRef] [PubMed]
31. M. Xun, C. Xu, Y. Xie, J. Deng, G. Jiang, G. Pan, Y. Dong, and H. Chen, “Phase tuning in two-dimensional coherently coupled vertical-cavity surface-emitting laser array,” Appl. Opt. 55(20), 5439–5443 (2016). [CrossRef] [PubMed]
