Beam steering experiments through an optical phased array with wavelength tuning

. In this paper, we have demonstrated the beam steering experiments with the help of our optical phased array devices. The lateral beam steering has been showed successfully with the wavelength through the tunable laser source. The number of output channels were 512 and the array width was kept more than a millimeter with a 2-μ m pitch. The beam steering angle has been measured as 45° and the beam movement was 1°/nm . As the chip does not collimate the beam vertically, so, a commercial collimating lens has been used in the vertical direction.


Introduction
LiDAR (Light detection and ranging) technology plays a significant role in the future of the autonomous vehicles and free-space optical communication.However, the expensive mechanical parts allowing the beam steering, i.e., the scanning of the space with light, represents a serious takeover in the technology of miniaturized LiDARs [1].The recent developments in the solid-state devices [2] and optical phased array (OPA) [3] have been made to improve the performances and cost criteria for applications such as in the autonomous vehicle industry.In this work, we have demonstrated an OPA-based device on thick silicon-on-insulator (SOI) platform for one dimensional (1D) beam steering horizontally with the help of wavelength tuning.Off-chip cylindrical lenses are necessary to vertically collimate the beam due to the higher beam divergence in the vertical direction of the output from the OPA chips.

Design of the optical phased array
The utilization of OPAs (Optical Phased Arrays) enables beam shaping, as these arrays consist of coherent optical emitters that closely resemble phased array antennas commonly used in radio wave and microwave technology [4].By manipulating the phase and/or amplitude of these emitters, precise control over the electromagnetic field within the near field region can be achieved.The calculation [6] of the steering angle θ can be performed. ( where Δ is phase shift, is the wavelength and is the waveguide pitch.
Phase shift Δ [6] can be defined from a path length difference ΔL in each waveguide of the OPA. (2) The first generation of our OPAs consisted of 37 output channels with 3-μm waveguide spacing [5].This first OPA was designed with = 8.25 μm which corresponds to a of 85 nm centred at 1550 nm.However, in the current study, we demonstrate our latest OPA with a 2-μm pitch.The total width of the output array is around 1 mm and suitable for long-range collimation.The output of the OPA can be seen with the infrared camera as shown in Fig. 1.The bright line is for the central peak at wavelength 1560 nm.The output of the OPA will be steered with the wavelength and the steering is measured to be 45° and the whole wavelength tuning range is 45 nm.

Characterization of optical phased arrays with the collimating lens
The experimental setup used to measure the power spectra involves a semiconductor tunable laser source operating at a wavelength of 1.5 μm and an intensity of 13 dBm.The laser source is connected to the waveguide chip through a polarization maintaining fiber (PMF, 62.5/125 μm core/cladding diameters).The input and output micrometer stages offer movement capabilities in all three x, y, and z directions.The output light emerging from the chip is collected by a multimode fiber (MMF) connected to a Hewlett Packard 81635A detector.The intensity of the signal can be observed and monitored on a computer screen.
The measured vertical amplitude of the OPA is in the order of a few mm.As the chip does not offer vertical collimation, thus, a commercial cylindrical lens (LJ1227L2-C) is used to collimate the beam in this direction.The output of the lens is scanned along the vertically direction and the result can be seen in Fig. 2. The minimum spot size is measured at 10 cm from the chip's output (Fig. 3).The beam waist increases before and after this distance.
Fig. 3 Evolution of the intensity at beam waist (10 cm away from the chip) along the vertical direction.
The calculated beam waist radius from the experiment performed in the laboratory is approximately 0.047 mm in 1/e 2 level [7].Mathematical calculations are performed by substituting the value of beam waist in the respective formulae [6].The Rayleigh range and divergence angle are calculated to be 4.44 mm and 10.56 mrad (0.6°), respectively.

Conclusion
In this paper, we have presented beam steering experiments of the optical phased array devices with 2 μm pitch.The number of output waveguide is 512.As the chip does not provide the collimation in vertical direction, thus, the collimated beams are demonstrated with the help of the commercial cylindrical lens.The output array has been increased up to 1 mm and the steering angle has been recorded as 45°.The beam movement is around 1°/nm.The collimation distance increased by at least an order of magnitude with a wider 512 waveguides array than with 37 waveguides.

Fig. 1
Fig. 1 Near infrared image showing the output of the OPA without lens for vertical collimation.The bright line is the signal at the central operating wavelength 1560 nm.

Fig. 2
Fig. 2 Measured vertical intensity distributions at different distances.