RF beam transmission of X-band PAA system utilizing large ...chen-server.mer.utexas.edu/2016/Submission.pdfcontrol of PAAs. Chip-scale and integrated TTD modules promise a miniaturized,

RF beam transmission of X-band PAA system utilizing large-area, polymer-based true-time-delay module developed using imprinting

and inkjet printing

Zeyu Pana,*, Harish Subbaramanb,*, Cheng Zhangc, Qiaochu Lic, Xiaochuan Xub, Xiangning Chena, Xingyu Zhanga, Yi Zoua, Ashwin Pandayc, L. Jay Guoc, Ray T. Chena,*

aDepartment of Electrical and Computer Engineering, The University of Texas at Austin, 10100

Burnet Rd, PRC/MER 160, Austin, TX 78758, USA bOmega Optics, Inc., 8500 Shoal Creek Blvd, Building 4, Suite 200, Austin, TX 78757, USA

cDepartment of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Ave, Ann Arbor, MI 48109, USA

ABSTRACT

Phased-array antenna (PAA) technology plays a significant role in modern day radar and communication networks. True-time-delay (TTD) enabled beam steering networks provide several advantages over their electronic counterparts, including squint-free beam steering, low RF loss, immunity to electromagnetic interference (EMI), and large bandwidth control of PAAs. Chip-scale and integrated TTD modules promise a miniaturized, light-weight system; however, the modules are still rigid and they require complex packaging solutions. Moreover, the total achievable time delay is still restricted by the wafer size. In this work, we propose a light-weight and large-area, true-time-delay beamforming network that can be fabricated on light-weight and flexible/rigid surfaces utilizing low-cost “printing” techniques. In order to prove the feasibility of the approach, a 2-bit thermo-optic polymer TTD network is developed using a combination of imprinting and ink-jet printing. RF beam steering of a 1x4 X-band PAA up to 60° is demonstrated. The development of such active components on large area, light-weight, and low-cost substrates promises significant improvement in size, weight, and power (SWaP) requirements over the state-of-the-art.

Keywords: Polymer, waveguide, thermo-optic, switch, imprinting, inkjet printing, true time delay, phased array antenna

1. INTRODUCTION Integrated optical switches are important building blocks in optical links and systems [1-6]. Among various optical switches, polymer-based thermo-optic (TO) switches have been found very attractive, owing to advantages of 1) large thermo-optic coefficient (-1~3 x10-4 K-1) [7-9], 2) high transparency in the telecommunication wavelength windows, and 3) fabrication feasibility over large areas on PCBs and other kinds of substrates. With these special features, TO polymer switches have enabled widespread applications in several areas, such as communication and radar, add/drop multiplexing, bypass switching in the event of a network failure or network jam, packet switching, etc. [7-22]. However, until now, the most common methods for polymer optical device fabrication are either using Reactive-ion Etching (RIE) to define the pattern into a resist, and transferring the pattern to the optical polymer via plasma etching [23-25]; or directly writing the pattern in a low-loss UV/Ebeam curable polymer using lithography [19, 20, 26, 27]. Although these methods are straightforward, they are not a cost-effective way due to the complicated fabrication process involved. Moreover, these techniques are not scalable beyond the size of a wafer. Previously, we introduced a novel and an etch-less solution processing technique utilizing a combination of imprinting and ink-jet printing for developing polymer photonic devices [17, 21, 28-32]. The structure of a complete true-time-delay reconfigurable module, comprising of an array of interconnected TO switches and polymer delay lines [10-12, 33-36], was introduced in [22]. In this work, we demonstrate phase delay configuration of a 2-bit TO TTD module and the beam steering of a 1x4 X-band PAA system utilizing the fabricated TTD modules. Owing to the roll-to-roll (R2R) compatibility of the employed solution processing techniques, photonic system development over large areas on either rigid or flexible substrates, at high-throughput, and

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at a low cost, is possible. Moreover, these devices can be integrated with other printed photonic and electronic components, such as light sources, modulators, antennas on the same substrate, thus achieving an integrated system that can be conformably integrated on any platform.

2. RECONFIGURABLE 2-BIT THERMO-OPTIC POLYMER BASED TTD NETWORK In Ref. [22], a single TO-polymer switch device was demonstrated. In this work, we incorporate such TO switches in a reconfigurable delay line architecture, shown in Figure 1, and demonstrate a 2-bit TTD module. The 2-bit TTD module consists of three 2×2 TO polymer switches interconnected via judiciously chosen lengths of polymer waveguide delay lines. The minimum length increment (ΔL) determines the minimum achievable time delay step (Δτ) according to Δτ = neff· (ΔL/c), where neff is the effective index of the mode in the waveguide and c is the speed of light in vacuum. At the first switch (n=0), the optical signal is delivered to either the reference waveguide (length L0) or the delay line (length L0+ΔL), depending on the chosen ON or OFF state of TO polymer switch. Then, the second switch (n=1) couples the optical signal into two more waveguides with lengths L0 and L0+2·ΔL. The last switch (n=2) of the 2-bit delay TTD line is used to control the optical signal to couple into one of the output waveguides.

Output waveguides

Waveguide delay lines

Δ 012

Input waveguides

2ΔReference lines

Figure 1. Schematic of a reconfigurable 4-bit TTD unit comprising of 2×2 TO polymer switches and polymer waveguide delay lines.

Table 1 lists the switch ON and OFF configuration for a 2-bit TTD module providing the time delay 0 ps, 11.54 ps, 23.09 ps, and 34.64 ps, the largest time delay chosen in order to provide a 60° steering of RF signal in an X-band PAA. Note that this largest time delay we chose is not restricted due to the printing technology, but due to our measurement capability in our laboratory setup. When both n=0 and n=1 switches are OFF, the optical signal goes through reference lines. When n=0 switch is ON and n=1 switch is OFF, the optical signal travels additional length ΔL compared to the reference line, thus the TTD module provides 11.54 ps time delay w.r.t the reference line. Similarly, other delay configurations can be understood from Table 1.

Table 1. ON and OFF configuration for each switch in a 2-bit reconfigurable TO-switch based TTD module.

Configuration 0Δτ 1Δτ 2Δτ 3Δτ

Phase delay 0 ps 11.54 ps 23.09 ps 34.64 ps

n=0 ∆ OFF ON OFF ON

n=1 ∆ OFF OFF ON ON

We fabricated the 2-bit TTD module using the fabrication process outlined in our previous publications [21, 22]. In order to measure the time delay from the fabricated 2-bit module, we performed a phased versus frequency measurement and derived the time delay from the plots. The schematic and setup to measure the time delay of the 2-bit reconfigurable TO-switch based TTD module is shown in Figure 2. The optical signal paths are shown as blue lines and the electrical signal





paths are indicated as red lines in Figure 2(a). Light at a wavelength of 1550nm from a tunable laser (Santec ECL200) is passed through a LiNbO3 electro-optical modulator (COVEGA MACH-40), which modulates the RF signal generated by a network analyzer (Agilent 8510C) onto the optical carrier. The modulated light signal is then amplified by an EDFA (Amonics AEDFA-C-30I-B). A lensed polarization maintaining fiber (OZ Optics) and a lensed single mode fiber (OZ Optics), which work as the input and output fiber, respectively, are mounted on an eight-axis positioning stage (Newport XPS-C8). The sample is placed in between and the position stages are controlled using an automation software to efficiently couple light into and out of the TTD module. The output power is amplified by an EDFA (BaySpec Metro-III_AE), which is then detected using a photodiode (Discovery Semiconductors DSC40S). The detected RF signal is then input into the second port of the network analyzer for analyzing the time delay.

Tunable laser Sample

Photodiode8-axis position stages

PMF SMFModulator EDFA EDFA

Network analyzer

White source for camera Tunable

laser EDFA

DC power supply

Optical power meter

Modulator

network analyzer

EDFA

Computer to control stages alignment

8-axis position stages

Camera

Photodiode

Figure 2. (a) Schematic and (b) experimental setup to measure the time delay from the reconfigurable 2-bit TTD module. The blue and red lines in (a) indicate the optical and the electrical signal propagation lines, respectively.

The measured phase versus frequency plots for all the delay configurations indicated in Table 1 are shown in Figure 3(b). The phase versus frequency plot is a straight line for all the configurations, thus confirming true-time-delay behavior. The time delay is determined by calculating the slope of the straight lines using:

(b)

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ACKNOWLEDGEMENTS

The authors would like to acknowledge the Air Force Office of Scientific Research (AFOSR) for supporting this work under the Small Business Technology Transfer Research (STTR) program (Grant No. FA9550-14-C-0001), monitored by Dr. Gernot Pomrenke.

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RF beam transmission of X-band PAA system utilizing large ...chen-server.mer.utexas.edu/2016/Submission.pdfcontrol of PAAs. Chip-scale and integrated TTD modules promise a miniaturized,