• Chinese Optics Letters
  • Vol. 20, Issue 2, 021301 (2022)
Hongxiang Zhang1, Changpei Liang1, Jian Song1, Chenzhong Fu1, Xiaofei Zang1, Lin Chen1, and Jingya Xie1、2、*
Author Affiliations
  • 1Terahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center, Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093, China
  • 2Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China
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    DOI: 10.3788/COL202220.021301 Cite this Article
    Hongxiang Zhang, Changpei Liang, Jian Song, Chenzhong Fu, Xiaofei Zang, Lin Chen, Jingya Xie. Terahertz out-of-plane coupler based on compact spot-size converter[J]. Chinese Optics Letters, 2022, 20(2): 021301 Copy Citation Text show less


    Low-loss dielectric terahertz (THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 µm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer. The out-of-plane coupling efficiency was found to be ∼5 dB at 194 GHz. The fan-beam steering range was found to be around 40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being non-mechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.

    1. Introduction

    Integrated terahertz (THz) devices are capable of improving system performance in security imaging, high-speed communication, molecular spectroscopy, and other applications[1,2]. Various THz circuits including conventional microstrip lines[3], coplanar waveguides[4,5], substrate-integrated image guides (SIIGs)[6,7], and spoof surface plasmon polariton (SSPP) waveguides[813] have been reported. Metallic waveguides suffer from high ohmic losses at the THz band[1418]. They are costly and difficult to fabricate. Thus, the all-dielectric chips are preferred. As the all-dielectric chips are based on guided-wave optics, they are reasonably efficient. However, it is not possible to independently operate the THz dielectric functional circuits due to the difficulties in the on-chip generation and detection of THz radiation. Usually, the dielectric functional circuits are connected to the conventional THz rectangular metallic waveguides or antennas. Hence, it is necessary to develop the THz couplers with high efficiency. Recently, different types of THz couplers have been reported, including inverse-taper waveguides[19], Luneburg lenses[20], and directly coupled couplers[21]. All of these couplers work on the principle of coupling the THz wave through the chip facet in-plane. However, out-of-plane couplers can be used directly to inject and extract THz waves at arbitrary locations on the chip, which is very important for on-wafer testing. A promising approach is to couple the light to (or from) a chip via a diffraction grating with a spot-size converter formed on the chip surface. This technique is widely used in the optical domain[2226]. Interestingly, the diffraction grating structures have good beam steering properties[27,28]. However, this coupler will be very large in size in the THz frequency range, probably more than 10 cm in length, due to the large wavelength of the THz wave. Thus, it will be difficult to integrate the coupler with other THz functional devices on a single wafer.

    Hongxiang Zhang, Changpei Liang, Jian Song, Chenzhong Fu, Xiaofei Zang, Lin Chen, Jingya Xie. Terahertz out-of-plane coupler based on compact spot-size converter[J]. Chinese Optics Letters, 2022, 20(2): 021301
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