Emitting direction tunable slotted laser array for Lidar applications

Journal Pre-proof Emitting direction tunable slotted laser array for Lidar applications Yanmei Su, Yu Bi, Pengfei Wang, Jie Sun, Xiuyan Sun, Shuai Luo, Jiaoqing Pan, Yejin Zhang

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S0030-4018(20)30024-9 https://doi.org/10.1016/j.optcom.2020.125277 OPTICS 125277

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Optics Communications

Received date : 29 November 2019 Revised date : 6 January 2020 Accepted date : 7 January 2020 Please cite this article as: Y. Su, Y. Bi, P. Wang et al., Emitting direction tunable slotted laser array for Lidar applications, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125277. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Journal Pre-proof Emitting Direction Tunable Slotted Laser Array for Lidar Applications Yanmei Su, Yu Bi, Pengfei Wang, Jie Sun, Xiuyan Sun, Shuai Luo, Jiaoqing Pan, and Yejin Zhang * State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China Center of Materials Science and Optoelectronics Engineering，University of Chinese Academy of

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Sciences, Beijing 100049, China

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*Corresponding author at: State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China E -mail address: [email protected] (Yejin Zhang)

Abstract

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In this work, a new type of emitting direction tunable slotted laser array for Lidar applications is proposed. By designing the structure of periodic slots, the laser array can achieve tunable emitting direction in a wide range with low vertical divergence angle. The laser array can be directly used as

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Lidar emitting chip without complex beam reshaping, coupling or mechanically moving parts, which makes it has the advantages of good stability, system simplification and low cost compared with other Lidars. Experimentally, an 11-channel laser array of C band with periodic slots at the output end is fabricated based on AlGaInAs/InP material system. As the slot period changes from 8 μm to 18 μm, the inclined angles of emission from 66.6° to 20.25° with all the vertical divergence angles less than 3.5° are obtained, and the minimum vertical divergence angle is even 1.3°. Typical optical power is 13.5 mW under 600 mA continuous injection. Keywords: emitting direction tunable laser, laser array, slotted laser, vertical divergence angle

1. Introduction

Artificial intelligence (AI) technology is leading to a new round of scientific and technological

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revolution, and now is deeply changing human lives. It can be widely used in intelligent transportation, intelligent robots, self-driving vehicles, intelligent medical care, etc. Light Detection and Ranging (Lidar) is one of the key technologies of AI technology. Lidar is also needed in other

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Journal Pre-proof important fields such as laser ranging, laser imaging and atmospheric detection. So far, most of the commercially available Lidar systems utilize mechanical beam-steering, which makes the entire system bulky, slow, and unreliable [1]. Optical phased arrays (OPAs) technology provides a solution for non-mechanical optical beam steering Lidar, but it is complex in structure, difficult to integrate with light source and can hardly get high output power [2-5]. There is an urgent demand for a highly integrated, easy to manufacture and low cost Lidar. It is a good choice to use laser array directly as

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Lidar emitting chip. Complex light source coupling is not demanded, and so it is helpful to simplify the process and improve the output power. For this purpose, special lasers with emitting direction

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tunable, wide-range light scanning and low divergence angle are needed. As we know, the vertical divergence angle of traditional edge emitting laser is very large, which is usually about 40° [6]. To reduce the vertical divergence angle, different approaches have been demonstrated, such as asymmetric waveguides [7], low index quantum barriers [8, 9], vertically extended waveguide structure [10], graded refractive index waveguide [11], Bragg reflection waveguide [12] and hybrid

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silicon nitride Bragg grating [13]. While all of these technologies can only reduce the vertical divergence angle to about 10°, and cannot change the emitting direction. Recently our team has demonstrated an inclined emitting slotted laser and a 54.6° inclined angle light beam with a vertical

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divergence angle of 1.7° at Full-Width at Half-Maximum (FWHM) was obtained [6]. In this work, we present a novel slotted laser array in which periodic slots are used to reduce the vertical divergence angle and control the light emitting direction. The 3D schematic structure of the laser array is shown in Fig. 1. There are several channels on the laser array with slots of different period at the output end of each channel which can be used to control the emitting direction of light and reduce the vertical divergence angle. Lasers with different inclined angles and very low vertical divergence angles emit from different channels. Combined with a driving circuit, the array can achieve fast beam scanning in different directions. Compared with other Lidars, emitting technology of this laser array has several advantages. First, no application of mechanically moving parts results in high reliability. Second, no complex fabrication process and only standard i-line photolithography are used which leads to low production costs. Last

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but not least, no complex optical coupling and reshaping is needed, the system is greatly simplified. By carefully optimizing the structure of the slots, very small vertical divergence angle and large range of tunable emitting direction are experimentally realized. An 11-channel emitting direction

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Journal Pre-proof tunable laser array based on periodic slots is presented. The fabricated laser array demonstrates a wide range of light emitting direction angle from 20.25° to 66.6°, the vertical divergence angles of

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all the 11 channels are all less than 3.5° and the minimum divergence angle is even 1.3°.

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Fig. 1. 3D schematic structure of emitting direction tunable slotted laser array. Device structure

The laser epitaxial structure is based on a standard 1550 nm LD design, as shown in table 1.

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The active region consists of five AlGaInAs quantum wells. Above them are 0.19 μm-thick AlGaInAs separate confinement heterostructure (SCH) layer, 1.6 μm-thick p-doped InP cladding layer, and 0.25 μm-thick GaInAs p-contact layer. Below the quantum wells are 0.13 μm-thick ndoped AlGaInAs layer and 0.8 μm-thick n-doped InP contact layer. The whole epitaxial structure is grown on an n-doped InP substrate.

Table 1: Laser Epitaxial Structure Name

Composition

Thickness(µm)

P-contact

p-Ga0.47In0.53As

0.25

Cladding

p-InP

1.6

SCH

[Al0.9Ga]In0.53As

0.19

[Al0.24Ga]In0.71As×5

0.006

[Al0.44Ga]In0.49As×6

0.01

[Al0.9Ga]In0.53As

0.13

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QW

N-layer

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n-InP

Substrate

n-InP

0.8

As shown in Fig.1, the laser array has 11 channels, each of which is a slotted laser. Each laser of the array has a 6.0 μm-wide surface ridge waveguide structure with a group of slots uniformly distributed at the output end. The total length of each laser in the array is designed as 2000 μm. In

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order to effectively isolate the p-electrodes of each laser, the height of the ridge is 2.2 μm, which is 0.07 μm deeper than the quantum wells. The width of the slots is designed as 1.1 μm, and the depth

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of the slots is 1.4 μm. For all of the 11-channel lasers in the array, the periods of the slots are designed as 8, 9, 10,11, 12, 13, 14, 15, 16, 17, and 18 μm.

3. Device fabrication and characteristics

The slotted laser array is fabricated using conventional process of semiconductors. For the 1.1

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μm width of the slots, the standard i-line photolithography is adopted without expensive electron beam exposure, which makes the process easy to manufacture and low cost. Then the 1.4 μm-depth slots are defined by ion beam etching with Ar gas. The SEM picture of etched slots is shown in Fig.

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Fig. 2. SEM picture of etched slots.

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From the SEM picture, some issues with the fabrication are shown: the sidewalls of the microslots are about 70° and not vertical, making the bottom width of the micro-slots about 0.85μm and the top width of the micro-slots about 1.8 μm which may lead to an increase in optical loss. After

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Journal Pre-proof that, the ridges are defined also by standard photolithography and ion beam etching with Ar gas. The depth of the ridges is 2.2 μm which is 0.8 μm deeper than the slots. The whole laser array is then passivated with SiO2 and openings are made on the top of ridges where patterned Ti/Au p-ohmic contacts are formed by magnetron sputtering. All the 11-channel lasers are isolated by high ridges which are deeper than QW layer and discrete p-ohmic contacts, allowing independent current injection. Following thinning of the substrate to 100 μm, an AuGeNi/Au contact is evaporated on to

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the n-type substrate. After a 400°C alloy process, the devices are cleaved to 2000 μm. Finally, the laser array bars are mounted on a copper heat sink and a thermoelectric cooler is used to control the

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chip temperature.

A thorlab PM16-401 power meter is used to measure the output powers of the slotted lasers in the array at room temperature. Typical light output power and voltage versus the continuous injection current is shown in Fig. 3. The threshold current is 160 mA, and the relatively high threshold current is due to the long cavity length of 2000 μm. The typical optical power is 13.5 mW

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under the condition of continuous injection of 600 mA. Voltages at 10 mA and 600 mA are 0.77 V

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and 2.36 V, respectively. The differential resistance is about 2.7Ω.

Fig. 3. Light output power and voltage versus the continuous injection current. The typical optical spectrum is shown in the inset.

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The optical spectra of the slotted lasers in the array are measured by coupling the laser output into an Anritsu MS9710C optical spectrum analyzer through a 9/125 μm fiber. The typical optical

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Journal Pre-proof spectrum is shown in the inset of Fig. 3. The peak wavelength is 1555.6 nm under 300 mA injection current. Although the typical optical spectrum of the slotted lasers is relatively wide, it will not affect its applications in Lidar. In Lidar applications, tunable beam emitting direction and low divergence angle are the most two important aspects, which are exactly the slotted laser array achieves. A scanning method is used to measure the far-field characteristics of the slotted lasers. A 180° vertical detecting scan is executed at room temperature. The far-field pattern and vertical divergence

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angle of the slotted laser array is shown in Fig. 4 (a) and Fig. 4 (b). From the results, we can see that the beam emitting direction varies with the period of slots. Surprisingly, as the period of the slots

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changes from 18 µm to 8 µm, a quite wide range of beam inclined angle from 20.25° to 66.6° is obtained. At the same time, by introducing the periodic slots, the vertical divergence angle of the beam is greatly compressed. The vertical divergence angles at FWHM of the whole array are all less than 3.5°, and the minimum vertical divergence angle is only 1.3° (40° for the ordinary edge emitting laser). The experimental results show that if combined with driving circuit, the slotted laser

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array can realize beam scanning with very small vertical divergence angle, which makes it having great potential in the application of Lidar. As shown in Fig. 4 (a), a small part of the light can be seen at 0 °, which is horizontal emission laser of normal edge emitting laser. By increasing the depth

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of the slots and so enhancing the coupling between the light field and the periodic slots, the ratio of horizontal emitting can be reduced. Besides the main peak, some side lobes also can be seen. But the intensity of side lobes is very small, the intensity ratio of main and side lobes is about 7-8 dB. In the future research, we will reduce the side lobes by some other technologies, such as periodic modulation. For Lidar applications, horizontal divergence angle is also an important parameter. Our next work is to reduce the horizontal divergence angle by changing the waveguide structure, such as waveguide width and other parameters.

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(a)

(b)

Fig. 4. (a) Vertical far-field pattern and

(b) vertical divergence angle of slotted laser array from experiments. 4. Theoretical simulation

On the basis of experimental research, theoretical simulation of the slotted laser array is also carried out. In the simulation, three-dimensional (3D) ridge waveguide is reduced to a two-

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dimensional (2D) structure by effective index approximation. 2D beam propagation method in frequency domain is used to obtain the steady-state field intensity distributions of the slotted lasers. A calculated fundamental mode is used as an input source. The effective refractive index of the

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Journal Pre-proof separate confinement heterostructure (SCH) layer, active region, and InP layer is 3.24, 3.32, and 3.16 [6], respectively. Fig. 5 shows the near-field optical distribution of a slotted laser with 9 μm slot period, 2000 μm cavity length, 1.4 μm slot depth and 20 slots. As shown in Fig. 5, scattering light deflecting at a certain angle is obviously generated by periodic slots with relatively parallel emission

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direction which leads to inclined emitting with small divergence angle.

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Fig. 5. Near-field pattern of slotted laser.

The far-field pattern is the Fourier transform of the near field at the laser output surface and be given by formula (1) [14], where far field intensity.

is the near field at the laser output surface, and I(θ) is relative

(1)

The near-field patterns of all the 11channels (11 slotted lasers with slot period changes from 8µm to 18µm) of the slotted laser array are simulated and then the far-field patterns are obtained. The simulated far-field pattern is shown in Fig. 6, the beam inclined angle changed from 22.25° (slot period of 18 µm) to 68.25° (slot period of 8 µm). The minimum vertical divergence angle is 1.7°

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Fig.6. Vertical far-field pattern from simulation.

Fig. 7 shows the comparison of the simulation and the experiment results of (a) the beam

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inclined angles and (b) the vertical divergence angles of all the 11 channels in the slotted laser array.

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As shown in Fig. 7, the simulation results agree well with the experiment ones.

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(a)

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(b)

Fig.7. Comparison of the simulation and the experiment results of

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(a) the beam inclined angles and (b) vertical divergence angles. 5. Conclusion

We present an 11-channel emitting direction tunable laser array with periodic slots at the output end of the lasers. Periodic slots affect the distribution of near field and generate scattering light with

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relatively parallel emission direction at a certain angle, and so can be used to control the beam emitting direction and vertical divergence angle. The fabricated laser array exhibits a wide range of tunable emitting direction from 20.25° to 66.6° as the period of slots changes from 18µm to 8µm. The vertical divergence angles of the 11 channels are all less than 3.5°, and the minimum one is surprisingly 1.3°. Typical optical power is 13.5 mW under 600 mA continuous injection. Theoretical simulation is carried out based on effective index approximation and 2D beam propagation method. Simulation results agree well with experiment ones. Based on wide range of tunable emitting direction, low divergence angle and conventional manufacturing process, such a slotted laser array has great potential for application in low-cost, stable and simple-process Lidar.

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Acknowledgements

This work is supported by National Key R&D Program of China (2018YFE0203103), National Natural Science Foundation of China (61934007, 61974141), Beijing Municipal Science and

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Journal Pre-proof Technology Project (Z191100004819011), and Beijing Municipal Natural Science Foundation, China (4182064).

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*Credit Author Statement

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Credit Author Statement Yanmei Su: Conceptualization; Methodology; Writing - original draft；WritingReviewing and Editing； Yu Bi：Data curation;

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Pengfei Wang: Validation； Jie Sun：Visualization；

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Xiuyan Sun: Investigation; Shuai Luo: Formal analysis; Jiaoqing Pan: Supervision；

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Yejin Zhang: Project administration；Resources；Software；