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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

paul.suni@lmco.com

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

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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.