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Abstract
Electronically controlled optical beam steering is sensitive and reliable. It is significant for phased laser radar applications. We reported the experimental characterization of three-element coherent vertical cavity surface emitting laser arrays and demonstrated two-dimensional electronical beam steering. 2-D far-field patterns, near-field patterns, and the spectra of the beam under different current conditions were measured. It is demonstrated that beam steering is continuous and predicted. Above 17.9% of the total power of the array is concentrated in the central lobe, showing high coupling efficiency. Up to a 3.26° shift angle from normal is demonstrated. The beam steering mechanism of frequency detuning between the three cavities was analyzed via both theory and experiment.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Vertical cavity surface emitting lasers (VCSELs) have excellent properties, such as low threshold, high speed modulation, wafer testing, single longitudinal mode and monolithic array fabrication capability [1–3]. They are an attractive optical source for data communication, printing, sensing and optical interconnection. Moreover, VCSELs have been applied and developed in the fields of three-dimensional face recognition, light detection and virtual reality/augmented reality (VR/AR) technology in recent years [4]. As the unique configuration, VCSEL arrays are often as a means to get high optical power to suffice some areas which merely need high power. Generally, these VCSELs in the array are independent. The beam quality cannot be enhanced due to two-dimensional integration. If the optical fields from elements are in-phase with each other, a narrow beam will be obtained. Especially, the beam width will be inverse with array number [5,6]. Thus attractive optical sources can be used in high-brightness pumping, high speed data transmission and laser radar [7–10].
Coherently coupled VCSEL array development was pursued by several research groups. Reflectivity-modulated VCSEL arrays are defined by a metal grid near active region and depositing a dielectric DBR [11]. Interference fringes were obtained from a 3×3 array. Another coherent array consists of air-post VCSELs which are separated by etched gap. Out-of-phase mode was obtained from a large array with less than 100 nm separation [12]. Cavity induced antiguided arrays exhibit low index regions with gain surrounded by high-index areas, which has been introduced to create in-phase mode operation. However, cavity induced antiguided arrays suffer from fabrication challenges, such as epitaxial regrowth on AlxGa1-xAs surfaces [13,14]. Proton implantation provides a simple means to fabricate VCSEL coherent arrays. Different scale in-phase coupled VCSEL arrays based on proton implantation have been successfully achieved [15–17]. Moreover, phase difference between elements can be detuned via current injection independently, which will vary beam propagation direction. Beam steering was firstly demonstrated in a 1×2 implant VCSEL array by Choquette et al. [18]. Subsequent, beam deflection was achieved in 3-element arrays and 2×2 arrays [19,20]. However, unstable mode, unpredictable shift and low deflection angle [20] usually exist in beam steering arrays. In addition, the elements act as both optical source and phase shifting mechanism. It is this complex mechanism that makes our understanding of beam deflection incomplete.
In this paper, we demonstrated continuous and predictable beam steering in a three-element VCSEL array. Measured 2-D far-field patterns illustrated the process of continuous beam steering in detail. Then the beam steering mechanism of frequency detuning between the three cavities in three-element VCSEL coupled arrays was analyzed via both experiment and theory. The analysis is to further understand the steering mechanism, which will aid in optimizing these devices for particular applications.
2. Fabrication and results
The epitaxial structures of 850 nm VCSEL wafer were grown on N-type GaAs substrates by metal organic chemical vapor deposition (MOCVD). It contains a top P-type distributed Bragg reflector (DBR) mirror and N-type bottom DBR mirror. The active region sandwiched between the two mirrors contains three pairs of GaAs-Al0.3Ga0.7As quantum well. The device fabrication consists of standard photolithographic procedures. A thick SiO2 mask is formed by inductively coupled plasma (ICP) etch. Next, a high implant energy of 315kev with a 1×1015cm−2 dose is employed to define array elements. To prevent channeling, the wafer was tilted at 7 degrees from normal. To inhibit current flow between elements, multiple stacked implants with successively decreased energy was also made. Subsequently, an Au nano layer is deposited to combine contact and element region. In the end, the proton-implantation resist masks were removed and P-type contacts (Ti-Au) was deposited on the top of p-DBRs to form anode using magnetic sputtering and a photoresist liftoff process. N-type contacts (AuGeNi-Au) were deposited on the GaAs substrate to form the cathode. The contacts were annealed at 420°C for 15 second in a nitrogen (N2) ambient. Figure 1 shows a schematic diagram of a 3-element VCSEL array for beam steering. The separation (center to center) is designed as 9µm. The diameter of every emitter is 6µm.
The output power and voltage for an injection current to Element 1 only, Element 2 only and Element 3 only are plotted in Figs. 2(a)–2(c). The corresponding near field profiles are shown as insets. The near-field images are taken with a CCD camera and objective lens, as shown in Fig. 3(a). From the emission patterns, the injection current was restricted in each aperture, showing good isolation among elements. The tested resistance between contacts is above 1×109 Ω. The good isolation is the premise for tuning injection current into every element individually. The tested threshold currents of three elements in the array are 4.2 mA, 3.8 mA and 3.7 mA, respectively. The difference in the characteristics of elements is mainly due to non-uniformity in the processes of lithography and etching. The output power and voltage for the whole array is shown in Fig. 2(d). Interference fringes can be found in emission near-field pattern under 13.5 mA as inset in Fig. 2(d). The far-field patterns are obtained from a laser beam profiler with front mounted Closed Circuit Television (CCTV) lens, as shown in Fig. 3(b). To avoid saturation in the beam profiler, some filters were used to attenuate the laser beam intensity. Generally, the peak of far-field is not directly on-axis as a result of fabrication imperfections which cause the phase difference and frequency detuning among elements. It can be corrected by current adjustment in the separate contacts.
Fig. 2. Output power and voltage for an injection current to (a) Element 1 only, (b) Element 2 only, (c) Element 3 only and (d) whole array. Inset shows the corresponding near-field profiles.
Completed far-field patterns under different combinations of current values to three elements were tested as shown in Fig. 4. The corresponding value combinations of injection currents into every contact are also listed above every pattern. Initially, adjusting the three current injections made the central lobe peak in far-field lie along on-axis. At this moment (I1=4.5 mA, I2=4.3 mA and I3=5.0 mA), the array operates in-phase array supermode with 2.8° angular full width at half maximum (FWHM) of far-field, showing high coherence among elements. Then the far-field patterns for fixed currents (I1=4.5 mA, I2=4.3 mA) into up element (Element 1) and left element (Element 2), and varying current injection into the right element (Element 3) were measured as shown in Fig. 4(a). It can be found obviously that as the current into Element 3 alters, the peak in far-field is pulled to two opposite directions. A red line with two arrows roughly describes the shifted direction. As I3 is 5.5 mA, the peak of far-field shifts to the down right side for 2.89°. Figure 4(b) shows far-field profiles versus varied I1 while I2 and I3 are fixed as 4.3 mA and 5.0 mA, respectively. The far-field shift to up/down direction as the current to Element 1 increases/decreases. The maximum shifted angle of 3.26° was found as the current to Element 1 is 5.2 mA. Similar shifting in another direction is illustrated in Fig. 4(c). Tuning the current to Element 2 will pull the far-field to the up-right side or down-left side. From Fig. 4, it is obvious that the beam steering in the three-element coherent array is well controlled and predictable in two-dimensional directions. Some particular direction shifting can also be achieved via tuning two element currents or three element currents.
To analyze characteristics in beam steering in coherent VCSEL array, the power in central lobe versus deflection angle from normal was measured as shown in Fig. 5. The maximum of 21.6% of total power is localized in central lobe when the beam is on-axis. The trend shows the power in central lobe decreases with increased deflection angle. However, above 17.8% proportion is demonstrated even in process of steering, showing highly coherent in-phase characteristics. The stable power in the central lobe is due to strong coupling between the elements in the fabricated array. The divergences of these steering far-fields are between 2.8° and 3.2° for all current combinations.
Fig. 5. Proportion of power concentrated in central lobe in far-field versus deflection angle from normal.
To study the beam steering mechanism in coherent VCSEL arrays, the spatially resolved spectra of Element 1, Element 2 and Element 3 are measured for a constant bias I1=4.5 mA, I2=4.3 mA and varied I3, as shown in Fig. 6. With the increasing of I3, the spectral peak of Element 3 appears red shift with a rate of 0.62 nm/mA. It is due to more heat produced as the current injection increased. According to the known relation between temperature and the shift in resonance wavelength of ∂λ/∂T ≈ 0.065 nm/ °C [21], the temperature rise rate for active region of Element 3 can be calculated to be 9.54°C/mA. Because of heat dissipation, the heat produced in Element 3 will also increase the temperature in both inter-element and other element regions. Thus the spectral peaks of both Element 1 and Element 2 will also appear red shift with the increased I3. As illustrated in Fig. 6, the shift rate for Element 1 and Element 2 is 0.095 nm/mA with the increased I3. The calculated temperature rise rate for both Element 1 and Element 2 is 1.46°C/mA. Thus, it is easy to know the temperature rise of every element while the currents are tuning. The emission spectra of Element 1 and Element 2 are inset in Fig. 6 as I3 is 5 mA, showing Gaussian mode operation. The spatially resolved peaks are 846.51 nm and 846.52 nm, respectively. The spatially resolved peak of Element 3 is 846.5 nm, which indicates the three elements are locked with almost same resonance wavelength. As the I3 is deviated from 5 mA, the spectral detuning will make the far-field shift. The shift angle will enhance with the increased spectral detuning among the elements. This is consistent with the measured far-field patterns in Fig. 4.
Fig. 6. Spatially resolved spectral data wavelength versus current for Elements Right while I1=4.5 mA and I2=4.3 mA.
Element 1 and Element 2 are locked with almost same wavelength as I1=4.5 mA and I2=4.3 mA. The wavelength difference between Element 3 and Element 1 (Element 2) is shown in Fig. 7. It can be seen that with increasing current to Element 3, the wavelength detuning increases linearly. As I3 is above 5 mA, the wavelength detuning, λ3-λ1(λ2), is above zero. As I3 is below 5 mA, the wavelength detuning is below zero. Some typical near-field profiles under different current value of Element 3 are inset in Fig. 7. It is noted that the position of these central lobes will shift with the biased current increases. As I3 is low, the small central lobes in inter-element region are near Element 1 and Element 2. As I3 is 5 mA, the central lobes appear in the middle of inter-element region. Uniform near-field intensity showing good in-phase mode characteristic. The central lobes are pulled to Element 3 as I3 is above 5.0 mA. The optical intensity of Element 3 is brighter than that of other elements. The optical intensity of Element 1 and Element 2 get weak, which means large wavelength detuning not only impact shifted angle but also the output power of the array.
Fig. 7. Wavelength detuning, λ3-λ1(λ2), versus current to Element 3. Insets: measured near-field profiles under some current values.
A fully three-dimensional model was founded to simulate beam propagation in the arrays by finite-difference time-domain (FDTD) simulation. The perfect matched layer (PML) boundary condition was set. In the simulation, the virtual light sources in each element were set to be Gaussian distribution with the same initial phase, emitting from the active region into top DBRs. The monitor from where can observe light field was placed on the upper surface of the P-DBRs. The far-field was calculated via propagation from near-field intensity based on Fraunhofer approximation.
In the simulation, assuming the emitted wavelengths of Element 1 and Element 2 keeps as a constant of 846.5 nm, the wavelength detuning between Element 3 and Element 1(Element 2) is from −0.3 nm to 0.45 nm. Figure 8 shows the simulated 2-D and 3-D far-field patterns with the varied wavelength detuning value. The 1-D cutting line along A-A’ direction is also plotted in the right of Fig. 8. The steering direction towards Element 3 is defined as the positive angle. As the wavelength detuning is 0, the maximum intensity exists on-axis, indicating in-phase mode operation. Once there is wavelength difference between elements, the peak of far-field shifts to the direction of larger wavelength region. As illustrated in Fig. 8, the shift angle increases with the wavelength detuning linearly. As the emitted wavelength detuning is 0.27 nm, the shifted angle reaches about 2.8°. However, the lager side lobe intensity appears, which is bad for practical application. Because of wavelength difference will increase with the current detuning from Fig. 7, so the shift angle will increase with the current detuning to reach a maximum. This model can be used to analyze the mechanism of two-dimensional beam steering in coherent VCSEL arrays.
Fig. 8. Simulated 3-D and 2D far-field patterns along A-A’ direction with the varied wavelength of Element 3 as the wavelength of other elements keeps a constant of 846.5 nm.
3. Conclusion
We demonstrated two-dimensional controlled and predictable beam steering from a three-element in-phase coherent vertical cavity surface emitting laser (VCSEL) array. Frequency detuning tuning among the elements in the array was achieved via properly designed separate contacts. Up to 3.26° deflection angle from normal is demonstrated. A fundamental beam steering mechanism was found to future understand the beam steering behavior, which will aid in optimizing the arrays for particular applications. In future work, we will address larger scale arrays to achieve high resolution beam steering.
Funding
National Natural Science Foundation of China (NSFC) (61804175); ‘Supporting First Action’ Joint Foundation for Outstanding Postdoctoral Program (2017LH046); China Postdoctoral Science Foundation (2017M620943); National Key Basic Research Program (2017YFB0102302).
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