photonic integrated circuit fmcw lidar on a chip · this document was cleared by darpa on may 14,...
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This document was cleared by DARPA on May 14, 2018.
Distribution Statement "A" (Approved for Public Release, Distribution Unlimited).
CLRC 2018, June 18 – 21 1
Photonic Integrated Circuit FMCW Lidar On A Chip
Paul J.M. Suni (a), James R. Colosimo (a) S.J. Ben Yoo (b),
John Bowers (c), Larry Coldren (c), Jonathan Klamkin (c)
(a) Lockheed Martin Advanced Technology Center, Louisville, CO, USA
(b) University of California Davis, Davis, CA, USA
(c) University of California Santa Barbara, Goleta, CA, USA
Abstract: Photonic integrated circuits (PICs) enable construction of complex optical
systems at a miniaturized scale not possible with bulk elements. All active and passive
components required to fabricate chip-scale coherent lidar systems have been
demonstrated and system demonstrations are currently underway. In this paper we
describe our efforts to demonstrate a chip-scale frequency modulated continuous-
wave (FMCW) lidar. This system incorporates wavelength tuning and phased array
operation to enable 2D non-mechanical beam steering. We further discuss approaches
aimed at scaling aperture dimensions from mm scales to 1 cm and 10 cm scales. These
efforts include development of micron-sized mirror arrays to enable coupling between
optical layers and demonstration of wafer-scale routing of light at high efficiency.
Keywords: FMCW lidar, PIC, Photonic Integrated Circuits
1. Introduction
Chip-scale lidar systems have been sought for many years due to the potential for low size, weight,
and power (SWaP) remote sensing systems. The possibility of fabricating parts at the wafer-level is
also an attractive means to reduce cost for high volume applications. The recent explosion in photonic
integrated circuit (PIC) development has now reached a state where coherent complex active optical
systems can be constructed, including coherent lidar.
Power handling of sub-micron waveguides and other functional elements remains a fundamental
limitation. Depending on the materials used typical upper limits before non-linear optical effects set
in are tens to hundreds of mW in silicon (Si), limited by two-photon absorption (TPA), while silicon
nitride (SiN) can handle up to ~1 W power levels. This means that very high peak power lidar systems
are unlikely to be completely developed in PIC form. However, coherent systems, such as frequency
modulated continuous-wave (FMCW) [1], are entirely feasible. We also note that the power limits
apply to single waveguides. By constructing systems with large numbers of parallel channels with
embedded semiconductor optical amplifiers (SOA), the total power can be scaled up by orders of
magnitude.
The DARPA Modular Optical Aperture Building Blocks (MOABB) program was initiated in 2016
with the goal of developing aperture scalable coherent optical systems with lidar as the demonstrator
application. In Phase 1, which is ending as of this writing, the goal was to construct a 1 mm2 coherent
aperture. The program goals increase to 1 cm2 in Phase 2 and 10 cm x 10 cm in Phase 3. Total power
output goals for the three phases are 5 mW, 0.5 W, and 100 W, respectively.
A critical aspect of the MOABB program is incorporation of non-mechanical beam steering (NMBS).
Previous efforts, including the DARPA SWEEPER program, have developed techniques to do so. To
date the best way [2] to implement NMBS in two dimensions creates diffraction gratings in
waveguides and tunes the laser wavelength to steer the beam in one dimension at a rate of ~0.14
degrees/nm of laser tuning. In the second (lateral) dimension a large number of parallel waveguides
incorporating phase shifters are used to create transverse linear phase gradients on the beam and steer
laterally as an optical phased array (OPA).
Mo8
Paul Suni 19th Coherent Laser Radar Conference
CLRC 2018, June 18 – 21 2
2. Technology Challenges
One challenge in fully integrated PIC-based coherent lidar incorporating NMBS is heterogeneous
integration of a laser and detectors. Silicon is an indirect bandgap semiconductor and hence cannot
be used to make lasers and detectors (in the infrared). To incorporate these elements indium phosphide
(InP) chips need to be bonded to silicon-based structures that contain the remaining functional lidar
elements [2].
A greater challenge is to produce an OPA with low or no sidelobes. This requires ultra-dense
waveguide pitches. Small pitches lead to cross-talk for long parallel waveguides unless measures are
taken to reduce the effect. Figure 1 illustrates the relationship between coupling length and sidelobe-
free steering angle for conventional waveguides. Color coding in the right plot indicates etch depth
between waveguides. Note that to get sidelobe-free steering over ±90º requires a waveguide pitch of
ƛ/2. We have to date demonstrated 1.3 µm waveguide pitches which produce a ±10º sidelobe-free
steering range [3]. The program goals require much greater steering and we are exploring novel
techniques aimed at producing crosstalk-free waveguide pitches of ~0.85 µm to enable ±55º sidelobe-
free steering [4].
Figure 1. Relationship between sidelobe-free OPA steering, cross-talk coupling lengths, and
waveguide pitch
3. Demonstrator System
We have developed a system design that incorporates all elements to demonstrate coherent lidar
operation. Figure 2 illustrates the layout of the PIC. The transceiver section incorporates a widely
tunable single-frequency sampled-grating distributed Bragg reflector (SG-DBR) laser [5], LO splitter,
semiconductor optical amplifiers (SOA), and a balanced detector pair. Light is transmitted into a star
coupler [6] which splits the light into N secondary waveguides. Light is transported into an equal
number of phase shifters which can be independently controlled. From there light propagates into a
grating section where weak gratings diffract light into free space along the length of the 5-10 mm
long grating structure. In the lidar case light is received in the same waveguide structure and
propagates back through the system, is picked off at a transmit/receive 50/50 coupler, and propagates
to the detectors.
Figure 3 left illustrates the physical layout of the transceiver chip. The right figure shows a photo of
a completed chip mounted to a “supercarrier” and connected to two small boards containing laser
control electronics and the receiver front end. As of this writing the transceiver chip is undergoing
final testing and characterization.
Paul Suni 19th Coherent Laser Radar Conference
CLRC 2018, June 18 – 21 3
Figure 2. PIC architecture implementing a coherent FMCW lidar on a chip
Figure 3. Transceiver front end as fabricated.
To demonstrate NMBS we used a 120-channel chip and an external tunable SG-DBR laser from
Freedom Photonics. Figure 4 shows the hardware used for this demonstration. Left – system with
control electronics NMBS demonstration. Center – PIC chip on silicon interposer with fiber input
from external laser. Right – pattern written on wall using wavelength and OPA steering.
Figure 4. Demonstration of 2D NMBS
4. Next Generation PIC Lidar
The emission area of the first chip is approximately 1 mm2, which is useful for demonstrations, but
not for practical applications. We have developed concepts to scale emission apertures to much larger
dimension. Doing so while maintaining high emission area fill factor requires 3D integration of
optical layers as well as electronics for SOA drivers and phase shifters. Extensive work has been
carried out to demonstrate 3D integration aspects and is described in greater detail by Yu et al. [7].
Figure 5 illustrates some of these efforts. From left to right: two-layer optical chip with splitters and
phase shifters in one layer and emitter grating in second layer; sub-micron 45º mirror pairs used to
couple light between layers; ultrafast laser inscription (ULI) used to write 3D optical waveguides of
arbitrary 3D shape for stitching parts together optically.
Paul Suni 19th Coherent Laser Radar Conference
CLRC 2018, June 18 – 21 4
Figure 5. 3D structures developed for future large aperture scaling.
Figure 6 illustrates concepts for scaling these systems to larger aperture dimensions. Left – planar
structure with 1 cm2 grating emission area fed by external laser. Center – 3D structure with parts
folded under grating layer. Right – concept for construction of 10 x 10 cm flat system outputting 100
W optical power.
Figure 6. Concepts for future aperture scaling to larger aperture dimensions.
5. References
[1] B. W. Krause, B. G. Tiemann, and P. Gatt, "Motion compensated frequency modulated continuous wave
3D coherent imaging ladar with scannerless architecture", Applied Optics, vol. 51, pp. 8745-8761, 2012.
[2] J. C. Hulme, J. K. Doylend, M. J. R. Heck, J. D. Peters, M. L. Davenport, J. T. Bovington, L. A. Coldren,
and J. E. Bowers, "Fully integrated hybrid silicon two dimensional beam scanner", Optics Express, vol. 23,
pp. 5861-5874, 2015/03/09 2015.
[3] Yu Zhang, Yi-Chun Ling, Yichi Zhang, Kuanping Shang, and S. J. Ben Yoo, “Sub-wavelength-pitch
Silicon-Photonic Optical Phased Array for Large Field-of-Regard Coherent Optical Beam,” submitted for
presentation at European Conference on Optical Communications, 2018.
[4] T. Komljenovic, R. Helkey, L. Coldren, and J. E. Bowers, “Sparse aperiodic arrays for optical beam
forming and LIDAR”, Optics Express, (25)3, 2511-2528, February 6, 2017
[5] A. Sivanathan, Hyun-chul Park, Mingzhi Lu, John S. Parker, Eli Bloch, Leif Johansson, Mark Rodwell,
and Larry Coldren, “Integrated Linewidth Reduction of a Tunable SG-DBR Laser”, Conf. on Lasers and
Electro-Optics, OSA Technical Digest (Optical Society of America, 2013), paper CTu1L.2.
[6] E. J. Stanton, N. Volet, T. Komljenovic, and J. E. Bowers, "Star coupler for high-etendue LIDAR," in
Conf. on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2017), paper
STh1M.4.
[7] Yu Zhang, Yi-Chun Ling, Yichi Zhang, Kuanping Shang, and S. J. Ben Yoo, "High-Density Wafer-Scale
3D silicon-photonic integrated circuits,” to be published IEEE Journal of Special Topics in Quantum
Electronics, Special Issue on Emerging Areas in Integrated Optics, 2018.
6. Acknowledgement
This research was developed with funding from the Defense Advanced Research Projects Agency
(DARPA) under contract HR0011-16-C-0106. The views, opinions and/or findings expressed are
those of the authors and should not be interpreted as representing the official views or policies of the
Department of Defense or the U.S. Government.