Near-field imaging in the microwave regime has many applications from radar to through-the-wall imaging and cancer cell detection. Currently, practical near-field imagers are implemented as benchtop systems and therefore are bulky, expensive, and typically susceptible to electromagnetic interference. Here we introduce and demonstrate the first single-chip nanophotonic near-field imager, where the impinging microwave signals are upconverted to the optical domain and optically delayed and processed to form the near-field image of the target object. The 121-element imager, which is integrated on a silicon chip, is capable of simultaneous processing of ultra-wideband microwave signals and achieves 4.8° spatial resolution for near-field imaging with orders of magnitude smaller size than the benchtop implementations and a fraction of the power consumption.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ultra-wideband (UWB) near- and far-field imagers utilize a large bandwidth in the microwave regime enabling implementation of near-field radar [1,2], through-the-wall imaging [3,4], tracking and positioning , low power communication for Internet-of-things , and high depth resolution imaging . In medicine, these imagers have been used for cancer cell detection [8,9], brain imaging , imaging of heart motions [11,12], and respiration rate monitoring , to name a few.
Conventionally, UWB near-field imagers have been implemented as benchtop systems [1,2], where a train of narrow time domain pulses, often monocycles [14–16], illuminate the target object and the reflected pulses are received using a wideband antenna array and processed to form the image. Despite excellent performance, these benchtop implementations are bulky, expensive, and suffer from high power consumption and are susceptible to electromagnetic interference.
One way to improve the performance of such imagers is to concurrently receive the impinging signals from different directions using multi-beam antenna arrays. Figure 1(a) shows the 1D architecture of a multi-beam antenna array, where fixed delay elements in a delay-sharing architecture are used . Depending on the angle of incidence, two antennas receive the signal with different delays. The received signals are coherently combined at a certain output, where the delay difference is compensated by the delay line network. The delay sharing architecture can be extended to a 2D antenna array, as shown in Fig. 1(b).
In the electrical domain, on-chip electrical delay lines are typically implemented either by setting the length of a transmission line [17,18] or periodically loading the transmission line with series inductors and shunt capacitors to change the propagation velocity . In both methods, the loss of the silicon substrate introduces a large propagation loss for the on-chip delay lines  (Supplementary Note S1 of Supplement 1). For imagers with a large number of delay lines, amplifiers and buffers may be used  to compensate for the loss of the delay line at the cost of increased power consumption and noise, reduced bandwidth, and delay non-uniformity. Also, long transmission lines per delay element (due to the high propagation velocity) in the former method, and the large size of the inductors in the latter method result in a large per-delay element area and high vulnerability to electromagnetic interference.
High optical confinement, low propagation loss offered by nanophotonic waveguides, and large bandwidth available around the optical carrier make complementary metal–oxide–semiconductor (CMOS) compatible silicon photonic platforms good candidates for implementation of integrated microwave photonic systems for different applications from signal generation to phased arrays [21–23].
The large group index in silicon-on-insulator waveguides, corresponding to a lower wave propagation velocity compared to typical electrical transmission lines, results in larger delay per-length, which together with significantly lower propagation loss and immunity to electromagnetic interference make the optical delay lines a good candidate for implementation of UWB near-field imagers with a large pixel count.
Here we report the demonstration of the first integrated nanophotonic near-field imager, where the reflected wideband microwave signals from the target object are received using UWB antennas, upconverted to the optical domain using electro-optic ring modulators, delayed and processed using a network of optical delay lines, and photodetected using a matrix of photodiodes. The photocurrents are further amplified and energy detected to form the near-field image of the target object. The implemented nanophotonic near-field imager utilizes photonic delay lines with about 44 times smaller chip area compared to the equivalent state-of-the-art electrical delay lines [14,17] and achieve more than 16 times lower propagation loss compared to the equivalent delay lines implemented on electronic processes with advanced metal stack (Supplementary Note S1 of Supplement 1). The photonic delay line implementation, processing, distribution, and beamforming enable the scalability of the proposed architecture to an imager with a large number of pixels. Furthermore, unlike electronic implementations, the photonic delay lines and devices are immune to undesired magnetic coupling and electromagnetic interference in the microwave regime. The imager chip is capable of receiving 121 simultaneous beams with a delay resolution of 9.8 ps (with a footprint of ) and is used to form the near-field image of different objects.
For the integrated near-field imager, the spatial resolution and the field-of-view can be calculated as 
2. NANOPHOTONIC NEAR-FIELD IMAGER DESIGN
A key concept in the implementation of the proposed nanophotonic near-field imager is that an electrical pulse can be optically delayed, that is, if an optical carrier is modulated with an electrical pulse, optically delayed by , and demodulated, the recovered electrical pulse is delayed by (Supplementary Note S1 of Supplement 1). Figure 1(c) shows the structure of the nanophotonic waveguides used to implement the photonic delay lines [Fig. 1(d)].
Figure 2(a) shows the structure of the photonic-assisted one-dimensional UWB antenna array as an essential building block of the implemented imager. The impinging wideband microwave signal is received using two antennas, amplified, and used to modulate the input light using ring modulators . The light at the output of each modulator is guided to an array of delay lines. A directional coupler is placed after each delay element to tap-off a part of the light. The coupling length of each directional coupler is adjusted to ensure that all directional couplers have the same output power. An array of Y-junctions is used to combine the outputs of the directional couplers in the top delay line array (processing the light traveling from left to right) with the outputs of the corresponding directional couplers in the bottom delay line array (processing the light traveling from right to left) forming the output waveguide array from out 1 to out 11 in Fig. 2(a). Note that two separate delay line arrays are used to isolate the optical path after each ring modulator, avoiding wave interference at undesired nodes. In this case, depending on the angle of incidence of the impinging pulse, the two optical waves, traveling in opposite directions, will be coherently power combined at one of the outputs (out 1 to out 11). The design of the photonic-assisted 1D UWB delay line array is discussed in more detail in Supplementary Note S2 of Supplement 1.
Figure 2(b) shows the structure of the implemented nanophotonic near-field imager. A laser emitting 30 mW at 1550 nm is coupled into the chip input waveguide using a grating coupler. The coupled light is then split into four branches and guided to the top and bottom 1D UWB antenna arrays, which are identical to the structure in Fig. 2(a) and serve as the top and bottom distribution networks for 11 delay line columns. Delay line columns are also identical to the delay line array in Fig. 2(a) (without the ring modulators) but rotated by 90°. An array of photodiodes, the imager pixels, are used to photodetect the outputs of all columns. The impinging signals are received by an array of UWB antennas, amplified, upconverted to the optical domain, and travel through the on-chip network of delay lines. Depending on the angle of incidence, four UWB pulses will arrive at the same time at a certain pixel for which the relative delays between the received RF pulses are compensated by on-chip optical delay lines and, ideally, are coherently power-combined before photodetection. However, since a coherent light source is coupled to the chip, a path mismatch and/or thermal gradient can affect the phase of these four aligned pulses at the combining point preventing a constructive addition. To address this issue, the laser is frequency chirped and is guided to the four ring modulators using on-chip waveguides with different lengths resulting in different optical carrier frequencies at the point of modulation as well as the combining points at the pixels (see Supplementary Note S4 of Supplement 1 for more details).
Figure 2(c) shows the zoomed-in version of the waveguide connections. Figures 2(d)–2(f) show the microphotographs of the SiGe photodiode, ring modulator, and the integrated nanophotonic near-field imager, respectively.
3. IMAGER CHARACTERIZATION AND NEAR-FIELD IMAGING DEMONSTRATION
Figure 3(a) shows the measurement setup used to characterize the nanophotonic near-field imager chip, where the output of an UWB pulse generator is divided into four branches and fed to the four inputs of the imager chip after per-channel delay adjustment emulating an UWB signal impinging from different directions. Parallel lines are used to differentially read all pixels (column-by-column) to form the image after power detection. The pixel readout scheme and circuitry are discussed in Supplementary Note S5 of Supplement 1. Figure 3(b) shows the formed images for five different delay settings emulating five different angles of incidence.
The measurement setup used for wireless characterization of the imager is shown in Fig. 3(c). A wideband antenna radiates UWB pulses toward the receive antenna array from different directions. The impinging signals are received using a antenna array (with 7 cm spacing), amplified, and fed to the input of the imager chip. Figure 3(d) shows the images formed for five different angles of incidence.
The nanophotonic imager chip was used to demonstrate microwave near-field imaging. Figure 4(a) shows the near-field imaging setup, where the target object is illuminated using an UWB monocycle pulse train with a pulse repetition rate of 1 MHz. The reflected UWB pulses from the target object are received by the antenna array of the imager and the corresponding near-field image of the target object is formed after photonic processing followed by post-processing. To demonstrate the near-field imaging performance, a metallic square, a metallic surface with a square hole at its center, and the UPenn logo with a metallic surface were used as the target objects. Figure 4(b) shows the UWB near-field images of the target objects. The details of the system calibration and image formation are discussed in Supplementary Note S5 of Supplement 1.
Note that for the case that the optical power at the input of one of the ring modulators is set to 3 dBm and the modulator is driven with a 7 V (peak-to-peak) input UWB pulse, a pulse with peak current of 2.5 μA is detected at the output of each photodiode. Also, note that as an important advantage of UWB pulse receivers, the system noise factor can be improved significantly by integrating (averaging) many pulses within the integration time (also referred to as the exposure time) . In this case, since the received pulses are correlated while the noise is uncorrelated, the SNR improves by a factor of , where is the number of received pulses averaged within the integration time. For our measurements, the pulse repetition rate was set to 1 MHz and the integration time was set to 1 ms (corresponding to ). As a result, the total noise figure of the system can be calculated to be 3.8 dB. Also, under this condition and for a minimum required SNR of 10 dB, the system spurious-free dynamic range is estimated to be 64.8 dB.
4. DISCUSSION AND SUMMARY
The pulse width of the UWB signal can be reduced by increasing the signal spectral bandwidth enabling near-field imaging of smaller objects at a higher resolution. The high bandwidth of the ring modulators and SiGe photodiodes enable the implemented nanophotonic imager chip to be used for near-field imaging in millimeter-wave regime without any modifications.
The implemented nanophotonic near-field imager can be scaled to an imager with a very large number of pixels using the tiling scheme, where multiple imagers with smaller pixel counts can be placed next to each other to form an imager with a large pixel count. Figure S7 of Supplement 1 illustrates an example of this idea, where four identical imagers are placed next to each other to form a imager. More details on the scalability of the implemented near-field imager are discussed in Supplementary Note S6 of Supplement 1.
In summary, we have demonstrated the first nanophotonic near-field imager chip, where compared to the state-of-the-art all-electrical designs, the on-chip photonic delay lines are more than 44 times smaller, achieve significantly lower propagation loss, and are immune to the electromagnetic interference, making the proposed nanophotonic imager scalable to an imager with a large number of pixels.
See Supplement 1 for supporting content.
1. A. G. Yarovoy, T. G. Savelyev, P. J. Aubry, P. E. Lys, and L. P. Ligthart, “UWB array-based sensor for near-field imaging,” IEEE Trans. Microwave Theory Tech. 55, 1288–1295 (2007). [CrossRef]
2. A. O. Boryssenko, C. Craeye, and D. H. Schaubert, “Ultra-wide band near-field imaging system,” in IEEE Radar Conference, Boston (2007), pp. 402–407.
3. S. Kidera, T. Sakamoto, and T. Sato, “High-resolution 3-D imaging algorithm with an envelope of modified spheres for UWB through-the-wall radars,” IEEE Trans. Antennas Propag. 57, 3520–3529 (2009). [CrossRef]
4. X. Liang, J. Deng, H. Zhang, and T. A. Gulliver, “Ultra-wideband impulse radar through-wall detection of vital signs,” Sci. Rep. 8, 13367 (2018). [CrossRef]
5. M. R. Mahfouz, C. Zhang, B. C. Merkl, M. J. Kuhn, and A. E. Fathy, “Investigation of high-accuracy indoor 3-D positioning using UWB technology,” IEEE Trans. Microwave Theory Tech. 56, 1316–1330 (2008). [CrossRef]
6. P. A. Catherwood and J. McLaughlin, “Internet of things-enabled hospital wards: ultra-wideband doctor-patient radio channels,” IEEE Antennas Propag. Mag. 60(3), 10–18 (2018). [CrossRef]
7. H. Hashemi, T. S. Chu, and J. Roderick, “Integrated true-time-delay-based ultra-wideband array processing,” IEEE Commun. Mag. 46(9), 162–172 (2008). [CrossRef]
8. H. Song, S. Sasada, T. Kadoya, M. Okada, K. Arihiro, X. Xiao, and T. Kikkawa, “Detectability of breast tumor by a hand-held impulse-radar detector: performance evaluation and pilot clinical study,” Sci. Rep. 7, 16353 (2017). [CrossRef]
9. A. Rahman, M. T. Islam, M. J. Singh, S. Kibria, and Md. Akhtaruzzaman, “Electromagnetic performances analysis of an ultra-wideband and flexible material antenna in microwave breast imaging: to implement a wearable medical bra,” Sci. Rep. 6, 38906 (2016). [CrossRef]
10. A. T. Mobashsher, A. Mahmoud, and A. M. Abbosh, “Portable wideband microwave imaging system for intracranial hemorrhage detection using improved back-projection algorithm with model of effective head permittivity,” Sci. Rep. 6, 20459 (2016). [CrossRef]
11. Y. Lee, J. Y. Park, Y. W. Choi, H. K. Park, S. H. Cho, S. H. Cho, and Y. H. Lim, “A novel non-contact heart rate monitor using impulse-radio ultra-wideband (IR-UWB) radar technology,” Sci. Rep. 8, 13053 (2018). [CrossRef]
12. S. Brovoll, T. Berger, Y. Paichard, O. Aardal, T. S. Lande, and S. E. Hamran, “Time-lapse imaging of human heart motion with switched array UWB radar,” IEEE Trans. Biomed. Circuits Syst. 8, 704–715 (2014). [CrossRef]
13. R. Chavez-Santiago and I. Balasingham, “Ultra-wideband signals in medicine,” IEEE Signal Process. Mag. 31(6), 130–136 (2014). [CrossRef]
14. T. S. Chu and H. Hashemi, “True-time-delay-based multi-beam arrays,” IEEE Trans. Microwave Theory Tech. 61, 3072–3082 (2013). [CrossRef]
15. F. Elbahhar, A. Rivenq, M. Heddebaut, and J. M. Rouvaen, “Using UWB Gaussian pulses for inter-vehicle communications,” IEE Proc.- Commun. 152, 229–234 (2005). [CrossRef]
16. T. Kikkawa, P. K. Saha, N. Sasaki, and K. Kimoto, “Gaussian monocycle pulse transmitter using 0.18 μm CMOS technology with on-chip integrated antennas for inter-chip UWB communication,” IEEE J. Solid-State Circuits 43, 1303–1312 (2008). [CrossRef]
17. T. Chu and H. Hashemi, “A CMOS UWB camera with 7 × 7 simultaneous active pixels,” in IEEE International Solid-State Circuits Conference (2008), pp. 120–600.
18. S. Park and S. Jeon, “A 15-40 GHz CMOS true-time delay circuit for UWB multi-antenna systems,” IEEE Microwave Wireless Compon. Lett. 23, 149–151 (2013). [CrossRef]
19. T. Chu, J. Roderick, and H. Hashemi, “An integrated ultra-wideband timed array receiver in 0.13 μm CMOS using a path-sharing true time delay architecture,” IEEE J. Solid-State Circuits 42, 2834–2850 (2007). [CrossRef]
20. J. Roderick, H. Krishnaswamy, K. Newton, and H. Hashemi, “Silicon-based ultra-wideband beam-forming,” IEEE J. Solid-State Circuits 41, 1726–1739 (2006). [CrossRef]
21. D. Marpaung, J. Yao, and J. Capmany, “Integrated microwave photonics,” Nat. Photonics 13, 80–90 (2019). [CrossRef]
22. W. Zhang and J. Yao, “Silicon photonic integrated optoelectronic oscillator for frequency-tunable microwave generation,” J. Lightwave Technol. 36, 4655–4663 (2018). [CrossRef]
23. V. C. Duarte, J. G. Prata, C. Ribeiro, R. N. Nogueira, G. Winzer, L. Zimmermann, R. Walker, S. Clements, M. Filipowicz, M. Napierała, T. Nasiłowski, J. Crabb, L. Stampoulidis, J. Anzalchi, and M. V. Drummond, “Integrated photonic true-time delay beamformer for a Ka-band phased array antenna receiver,” in Optical Fiber Communication Conference (2018), paper M2G.5.
24. Z. Xuan, Y. Ma, Y. Liu, R. Ding, Y. Li, N. Ophir, A. E. Lim, G. Q. Lo, P. Magill, K. Bergman, T. B. Jones, and M. Hochberg, “Silicon microring modulator for 40 Gb/s NRZ-OOK metro networks in O-band,” Opt. Express 22, 28284–28291 (2014). [CrossRef]
25. Z. N. Low, J. H. Cheong, and C. L. Law, “Low-cost PCB antenna for UWB applications,” IEEE Antennas Wireless Propag. Lett. 4, 237–239 (2005). [CrossRef]
26. T. S. Chu, “Silicon-based broadband short-range radar architectures and implementations,” Doctoral dissertation (University of Southern California, 2010), pp. 29–30.