A
n 8x8 Heterodyne Lens
-
less
OPA Camera
Reza
Fatemi, Behrooz Abiri, Ali Hajimiri
California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125
sfatemi@caltech.edu
Abstract:
This paper presents an
8x8
optical phased array (OPA) r
eceiver that operates as
a
lens
-
less
camera
using a heterodyne architecture on
a thin silicon
-
photonics integrated
SOI
substrate
. It
has a receiving beam width of 0.75°
and beam steering range of 8°.
OCIS codes:
(130.0130) Integrat
ed optics; (130.6750) Systems; (110.5100) Phased
-
array imaging systems;
1.
Introduction
The
Integration of photonics components on silicon chips has enabled new applications and novel system designs.
Optical phased array transmitter is an example of such
integrated systems that has attracted interest in recent years
[1] [2]. Proper adjustment of the phase of the light fed to each array element forms a beam of light by adding the
output wave of the elements constructively in a certain direction and destruct
ively in other directions. The beam
direction is steerable electronically by controlling the phases. Reciprocity of the electromagnetic waves makes it
possible to use the same principle for receiving the light coming from a certain direction and reject
ing
others
[3].
This enables a le
n
s
-
less ultra
-
thin synthetic aperture OPA camera by s
weeping the beam in all directions and putting
the measured intensity of the light coming from each direction together
and thereby forming an image
. In this paper
,
we present
a proof of concept two
-
dimensional
OPA
receiver based on heterodyne architecture that operates as a
n
ultra
-
thin
lens
-
less
camera.
2.
Design and Implementation
The schematic diagram of the phased array receiver camera is shown in Figure 1(a) and the fabri
cated chip in
Figure
1
(b). An 8
x
8 array of grating couplers capture the incident light on the chip surface and each grating coupler
guides the li
ght into a waveguide.
In this heterodyne scheme, e
ach wave
guide is then routed to a directional coupler
where t
he received light is combined with the reference light. The output of the directional coupler is fed to a pair of
balanced photodiodes where the signal is mixed down to an
electrical
intermediate frequency (IF)
in the MHz range
.
The output current of all t
he photodiodes associated with the receiving elements are summed up by
placing them in
parallel electrically producing
the output signal of the receiver. The reference light is coupled
in
through a grating
coupler and split into 64 paths. Each path goes th
rough a PIN diode phase shifter and fe
e
d
s a
directional coupler.
A
receive beam is formed by adjusting
the phase shifts of each path
so that the amplitude of the signal arriving from a
certain direction add constructively
, while rejecting the intensity of
incident light from other directions.
This is
tantamount to looking in a certain direction.
Figure
1. (a) Schematic of the design (b) Photograph of the chip.
3.
Experimental setup and measurement results
The chip is mounted on a printed circuit board (
PCB) that includes electronic circuit for controlling the phase
shifters on the chip and processing the output signal of the chip. A microcontroller is used to control the electronics
and communicating with a computer. A 1550nm laser is used as the source
,
whose associated frequency corresponds
to f
0
=194THz
. The output light of the laser is split into two paths for reference and illu
mination. Each path is fed to
a
single
-
sideband (
SSB
)
modulator. The two SSB modulators shift the optical light frequency
in t
he reference and
illumination
by f
1
= 1
.
15MHz and f
2
= 1
.
75MHz
, respectively
.
Due to the
heterodyne mixing
process
, the output
current of the photodiodes on the chip has the carrier frequency of
f
IF
=
f
1
+ f
2
= 2
.
9MHz.
One
advantage of
this
shifted frequen
cy scheme is that random delay fluctuations in
reference and illumination
paths (e.g., thermal)
manifest
themselves
as phase noise at the output
, not affecting the measured amplitude and therefore
not
degrading
the SNR of the detected signal amplitude. Mor
eover, 1
/
f noise of the electronics needed for amplification and
processing the output current falls out of band
by using a high enough intermediate frequency (IF)
. A polarization
maintaining fiber which carries the reference path is fixed
to
the chip with
transparent
adhesive
. This allows moving
and rotating the receiver for characterization and calibration.
In practice, the optical path length of the waveguides
has variations b
ecaus
e of the fabrication mismatches
. An optimization algorithm with random jum
p search is
implemented to calibrate the fabrication mismatches
before the imaging phase
. The result of optimization provides a
lookup table for phase shifter settings to form receiving beams for different
azimuth,
, and elevation
angles
,
.
Figure 2
show
s the measured receiving pattern after calibration
for 0°
. The beam width is 0.75°
and grating lobes are
8°
apart
enabling
an 8
-
pixel by 8
-
pixel
image.
Fig
ure
2. (a)
Normalized r
eceiving
p
attern cross section of yz
-
plan
e
(b)
Normalized
r
eceiving
p
attern cross section of
xy
-
plane
. (c)
Normalized
3D pattern
.
To illustrate the imaging function of the receiver,
a
copper tape with a hole is pasted on the
p
lexiglas
s
to block the
light except at the hole
. The surface of the
p
lexiglas
s
is
sanded
to
enhance
scattering
,
Figure 3(a)
and
(b). The image
of the object is
captured by loading the phase setting for different angles and measuring the optical density at each
pixel. Figure 3(c) shows the gray scale graph of the measurement result. The white bright spot
clearly
shows the
position of the hole.
Fig
ure
3. (a)
Sanded
p
lexiglass
(b)
Copper tape covered
imaging target
.
(c) Captured image in gray scale
4.
Conclusion
s
A
2D
lens
-
less
ultra
-
thin
camera based on optical phased array receiver is demonstrated. The camera has the beam
width of 0.75°and 8°of field of view. The beam is electronic
ally steerable for imaging.
5. References
[1]
B. Abiri, F. Afl
atouni, A. Rekhi, A. Hajimiri, “
Electronic two
-
dimensional beam steering for integrated optical
phased arrays,” in Optical
Fiber Communications Confer
ence and Exhibition (OFC), 2014
, vol., no.
, pp.1
-
3,
9
-
13 March 2014
.
[
2] H. Abediasl and H. Hashemi, “
Monolithic optical phased
-
array transceiver in a standard SOI CMOS process,”
Opt. Express 23, 6509
-
6519
(2015)
.
[3] R. Fate
mi, B. Abiri, and A. Hajimiri, “
A
o
ne
-
d
imensional
h
eterodyne
l
ens
-
Free
OPA
c
amera,” in Conference
on Lasers and Electro
-
Optics,
OSA Technical Digest (2016) (Optical Society of America, 2016), paper
STu3G.3.