Abstract— In this paper, two categories of 1x4 TTD-OBFNs are implemented within a 4x32 mm 2 ultralow loss silicon nitride chip -- switched delay line (SDL) based OBFN and optical ring resonators (ORRs) based OBFN. The SDL-OBFN is based on switches routing light among multiple physical paths. An improved ripple-free architecture is employed and 6 delay distributions with delay difference in range of 0-22.5ps are achieved. The ORR-OBFNs employ three cascaded optical ring resonators as the tunable delay line for each channel with continuously tuned TTDs. For one delay channel, a dynamic tuning ranges of 209 ps for TTD bandwidths of 6.3 GHz are achieved, which corresponds to a phase shift of 37.5π for a 90-GHz signal. A 55° beam angle equivalent OBFN response for a 90 GHz half-wavelength pitch antenna array is achieved. Using the SDL-OBFN, the mmW beamforming experiment demonstrates 6 beamsteering angles in the range of -51° ~ 32°, and beam radiation patterns agree well with the simulations. To our best knowledge, this is the first TTD based beamforming experiment with Photonic Integrated Circuits for mmW signal. Index Terms— Integrated photonics devices, true time delays, millimeter waves, beam steering, optical ring resonator. I. INTRODUCTION Microwave photonics is the discipline that utilizes photonic components and techniques to assist RF processing or provide alternatives to achieve existing functions. In the same way that microwave photonics has leveraged mature commercial-off-the-shelf component technologies that were developed for the telecommunications industry, the emerging field of integrated microwave photonics (IMWP) is leveraging integrated photonic technologies that have been maturing at an accelerated rate due to the demand of data communications [1]. Broadband optical beamforming networks (OBFNs) are one of the key components for photonic assisted wide band communications. Optical phase shifters can replace traditional RF shifters to perform beam steering of the RF signal, since they are equivalent in the heterodyne process. However, the beam-squint issue due to this and limits the bandwidth [2]. True time delays (TTDs) is an enabling technology for optical signal processing functions in microwave photonics, particularly in OBFNs for photonics-enabled signal generation with squint-free beam steering capability [3, 4]. IMWP is desirable for broadband OBFNs owing to the large available optical bandwidth for realizing broadband TTDs [5]. The compactness, low loss, and precise waveguide length control of integrated photonics J. Klamkin and Y. Liu are with the Electrical and Computer Engineering D epartment, University of California, Santa Barbara, CA 93106 USA (email: [email protected], [email protected]). Brandon Isaac is with the Materials Department, University of California, Santa Barbara, CA 93106 USA. J. Kalkavage, E. Adles and T. Clark are with The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20763 USA. The authors acknowledge funding from the NASA Space Technology Mission Directorate Early Stage Innovations program. devices enhance the potential for use in photonics assisted scalable TTD-based phased array antennas (PAAs). Thus far, several schemes of integrated TTDs have been implemented. One such implementation is based on highly dispersive devices such as photonic crystal waveguides, which modifies the group index and dispersion by carefully designing a lattice structure as the waveguide cladding [6]. Another implementation, namely switched delay line (SDL), is realized by switching between physical paths with varying lengths using Mach-Zehnder (MZ) switches or arrayed waveguide gratings [7-9]. This technique is relatively simple to control but only supports discrete delays and is limited in achievable delay resolution. Another scheme, using all-pass filters such as ORRs, is particularly attractive for small chip footprint and the ability to continuously tune the delay [10-14]. TTD devices can be realized with traditional silicon photonics waveguides based on silicon on insulator (SOI), or with low-loss waveguides such as those based on silicon nitride. Silicon nitride with silicon oxide cladding provides ultra-low-loss waveguide with demonstrated propagation loss below 0.1 dB/cm. Furthermore, this platform has demonstrated very high optical power handling up to 1-Watt [15], which makes it possible to eliminate low noise amplifiers before antennas for small scale PAA applications. In this paper, we demonstrate two types of ultra-low loss silicon nitride 1x4 OBFNs based on MZ-SDLs and ORRs, respectively. The MZ-SDL architecture is modified specifically to achieve a ripple-free TTD. The two OBFNs are precisely tuned, characterized and compared. II. MODIFIED MZ-SDL BASED OBFN A typical architecture of an integrated SDL is shown in Fig. 1. A balanced Mach-Zehnder interferometer (MZI) is employed as a switch for each stage determining to pass through or skip the delay. For a traditional MZ-SDL, a binary-bits delay scheme (i.e. τn= 2τn−1) is used to maximize the flexibility of the SDL. However, this requires a perfect switch for routing the entire optical signal in/out of the delay line. The MZI switch transfer function can be studied using T-matrix method, which is expressed as 11 12 21 22 T T T T T (1) 11 ) (1 j e T (2) 12 2 ( ) 2 1 2 2 cos 2 (1 ) j T e T (3) 22 ) (1 j T e (4) where is the coupling coefficient of directional coupler in the MZI, and is the phase difference between two MZI arms. (1)-(4) show Beamforming with Photonic Integrated Circuits for Millimeter Wave Communications and Phased Arrays Jonathan Klamkin, Yuan Liu, Brandon Isaac, Jean Kalkavage, Eric Adles, Thomas Clark and Jonathan Klamkin
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Abstract— In this paper, two categories of 1x4 TTD-OBFNs are
implemented within a 4x32 mm2 ultralow loss silicon nitride chip -- switched
delay line (SDL) based OBFN and optical ring resonators (ORRs) based
OBFN. The SDL-OBFN is based on switches routing light among multiple
physical paths. An improved ripple-free architecture is employed and 6
delay distributions with delay difference in range of 0-22.5ps are achieved.
The ORR-OBFNs employ three cascaded optical ring resonators as the
tunable delay line for each channel with continuously tuned TTDs. For one
delay channel, a dynamic tuning ranges of 209 ps for TTD bandwidths of 6.3
GHz are achieved, which corresponds to a phase shift of 37.5π for a 90-GHz
signal. A 55° beam angle equivalent OBFN response for a 90 GHz
half-wavelength pitch antenna array is achieved. Using the SDL-OBFN, the
mmW beamforming experiment demonstrates 6 beamsteering angles in the
range of -51° ~ 32°, and beam radiation patterns agree well with the
simulations. To our best knowledge, this is the first TTD based beamforming
experiment with Photonic Integrated Circuits for mmW signal.
Index Terms— Integrated photonics devices, true time delays,
millimeter waves, beam steering, optical ring resonator.
I. INTRODUCTION
Microwave photonics is the discipline that utilizes photonic
components and techniques to assist RF processing or provide
alternatives to achieve existing functions. In the same way that
microwave photonics has leveraged mature commercial-off-the-shelf
component technologies that were developed for the
telecommunications industry, the emerging field of integrated
microwave photonics (IMWP) is leveraging integrated photonic
technologies that have been maturing at an accelerated rate due to the
demand of data communications [1].
Broadband optical beamforming networks (OBFNs) are one of the
key components for photonic assisted wide band communications.
Optical phase shifters can replace traditional RF shifters to perform
beam steering of the RF signal, since they are equivalent in the
heterodyne process. However, the beam-squint issue due to this and
limits the bandwidth [2]. True time delays (TTDs) is an enabling
technology for optical signal processing functions in microwave
photonics, particularly in OBFNs for photonics-enabled signal
generation with squint-free beam steering capability [3, 4]. IMWP is
desirable for broadband OBFNs owing to the large available optical
bandwidth for realizing broadband TTDs [5]. The compactness, low
loss, and precise waveguide length control of integrated photonics
J. Klamkin and Y. Liu are with the Electrical and Computer Engineering D
epartment, University of California, Santa Barbara, CA 93106 USA (email:
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[15] C. G. H. Roeloffzen, L. Zhuang, C. Taddei, A. Leinse, R. G. Heideman, P. W. L. van Dijk, R. M. Oldenbeuving, D. A. I. Marpaung, M. Burla,
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Fig. 12 . 94-GHz beam pattern for beam angle of -51°, -33° , -16° , -2° , 14° and 31°, respectively. (Blue curves refer to the measured data whereas the red