Abstract— In this paper, two categories of 1x4 TTD-OBFNs are
implemented within a 4x32 mm2 ultralow loss silicon nitride chip -- switched
delay line (SDL) based OBFN and optical ring resonators (ORRs) based
OBFN. The SDL-OBFN is based on switches routing light among multiple
physical paths. An improved ripple-free architecture is employed and 6
delay distributions with delay difference in range of 0-22.5ps are achieved.
The ORR-OBFNs employ three cascaded optical ring resonators as the
tunable delay line for each channel with continuously tuned TTDs. For one
delay channel, a dynamic tuning ranges of 209 ps for TTD bandwidths of 6.3
GHz are achieved, which corresponds to a phase shift of 37.5π for a 90-GHz
signal. A 55° beam angle equivalent OBFN response for a 90 GHz
half-wavelength pitch antenna array is achieved. Using the SDL-OBFN, the
mmW beamforming experiment demonstrates 6 beamsteering angles in the
range of -51° ~ 32°, and beam radiation patterns agree well with the
simulations. To our best knowledge, this is the first TTD based beamforming
experiment with Photonic Integrated Circuits for mmW signal.
Index Terms— Integrated photonics devices, true time delays,
millimeter waves, beam steering, optical ring resonator.
I. INTRODUCTION
Microwave photonics is the discipline that utilizes photonic
components and techniques to assist RF processing or provide
alternatives to achieve existing functions. In the same way that
microwave photonics has leveraged mature commercial-off-the-shelf
component technologies that were developed for the
telecommunications industry, the emerging field of integrated
microwave photonics (IMWP) is leveraging integrated photonic
technologies that have been maturing at an accelerated rate due to the
demand of data communications [1].
Broadband optical beamforming networks (OBFNs) are one of the
key components for photonic assisted wide band communications.
Optical phase shifters can replace traditional RF shifters to perform
beam steering of the RF signal, since they are equivalent in the
heterodyne process. However, the beam-squint issue due to this and
limits the bandwidth [2]. True time delays (TTDs) is an enabling
technology for optical signal processing functions in microwave
photonics, particularly in OBFNs for photonics-enabled signal
generation with squint-free beam steering capability [3, 4]. IMWP is
desirable for broadband OBFNs owing to the large available optical
bandwidth for realizing broadband TTDs [5]. The compactness, low
loss, and precise waveguide length control of integrated photonics
J. Klamkin and Y. Liu are with the Electrical and Computer Engineering D
epartment, University of California, Santa Barbara, CA 93106 USA (email:
[email protected], [email protected]). Brandon Isaac is with the
Materials Department, University of California, Santa Barbara, CA 93106
USA. J. Kalkavage, E. Adles and T. Clark are with The Johns Hopkins
University Applied Physics Laboratory, Laurel, MD 20763 USA.
The authors acknowledge funding from the NASA Space Technology
Mission Directorate Early Stage Innovations program.
devices enhance the potential for use in photonics assisted scalable
TTD-based phased array antennas (PAAs).
Thus far, several schemes of integrated TTDs have been
implemented. One such implementation is based on highly dispersive
devices such as photonic crystal waveguides, which modifies the
group index and dispersion by carefully designing a lattice structure as
the waveguide cladding [6]. Another implementation, namely
switched delay line (SDL), is realized by switching between physical
paths with varying lengths using Mach-Zehnder (MZ) switches or
arrayed waveguide gratings [7-9]. This technique is relatively simple
to control but only supports discrete delays and is limited in achievable
delay resolution. Another scheme, using all-pass filters such as ORRs,
is particularly attractive for small chip footprint and the ability to
continuously tune the delay [10-14]. TTD devices can be realized with
traditional silicon photonics waveguides based on silicon on insulator
(SOI), or with low-loss waveguides such as those based on silicon
nitride. Silicon nitride with silicon oxide cladding provides
ultra-low-loss waveguide with demonstrated propagation loss below
0.1 dB/cm. Furthermore, this platform has demonstrated very high
optical power handling up to 1-Watt [15], which makes it possible to
eliminate low noise amplifiers before antennas for small scale PAA
applications.
In this paper, we demonstrate two types of ultra-low loss silicon
nitride 1x4 OBFNs based on MZ-SDLs and ORRs, respectively. The
MZ-SDL architecture is modified specifically to achieve a ripple-free
TTD. The two OBFNs are precisely tuned, characterized and
compared.
II. MODIFIED MZ-SDL BASED OBFN
A typical architecture of an integrated SDL is shown in Fig. 1. A
balanced Mach-Zehnder interferometer (MZI) is employed as a switch
for each stage determining to pass through or skip the delay. For a
traditional MZ-SDL, a binary-bits delay scheme (i.e. τn= 2τn−1) is used
to maximize the flexibility of the SDL. However, this requires a
perfect switch for routing the entire optical signal in/out of the delay
line. The MZI switch transfer function can be studied using T-matrix
method, which is expressed as
11 12
21 22
T TT
T T
(1)
11 )(1 jeT (2)
12 2
( )2
122 cos
2(1 )
j
T eT
(3)
22 )(1 jT e (4)
where is the coupling coefficient of directional coupler in the MZI,
and
is the phase difference between two MZI arms. (1)-(4) show
Beamforming with Photonic Integrated Circuits for Millimeter Wave
Communications and Phased Arrays
Jonathan Klamkin, Yuan Liu, Brandon Isaac, Jean Kalkavage, Eric Adles, Thomas Clark and
Jonathan Klamkin
that a perfect switch can be interpreted as maintaining a coupling ratio, , of the MZI precisely at 0.5, where all the matrix elements can be
tuned from 0~1 by tuning the phase shifter of the MZI. However, this
is difficult to achieve due to fabrication variation, wavelength
dependence and operating temperature instability. If is detuned from
0.5, T12 and T21 cannot reach 1, which means the signal is not entirely
routed in the cross state of the MZI. Consequently, two signals with
differing delays will intermix at the output and cause delay ripple and
suboptimal beam steering. Figure 2(a) and 2(b) show the simulation
results for a normalized delay and power spectra for various for a
3-stage binary-bits SDL (τ3 = 2τ 2 = 4τ1) within 1-FSR in the
worst-case scenario. When = 0.35, the peak-to-peak ripple is twice
the desired delay. However, T11 and T22 can reach 1 for any .
Therefore, in the pass-state, the MZI switch can route 100% the optical
signal for any κ.
Building on the analysis, we proposed a new modified ripple-free
topology whereby all the delay elements have equal length (τ1 = τ2 =
…= τn =τ). Only one of the switches is in cross-state, and all others are
in pass-state. As a result, there will be no intermixing of differently
delayed optical signals and can provide a very large bandwidth both
for TDD response and power uniformity, so that it is suitable for high
frequency RF signal heterodyne process. The switch set to cross-state
determines the delay of the SDL. We implemented 4 five-stage
ripple-free MZI-SDLs to form a 1×4 OBFN. The delay elements are
designed as τ =4.5, 3, 1.5, and 0 ps for each path, respectively. The last
path is the reference path and can be replaced with a length matched
waveguide to simplify the OBFN. Fig. 3. (a) shows the measured
delay response for path-1 tuned at the 6 delay values. We believe that
the ripple in the delay curve comes from the measurement itself or
some unknown resonance on the chip. A 10 Gbps data was sent to the
delay path very the delay measurement. As shown in Fig. 3. (b), the
signal was delayed by 22.5 ps as expected.
By simultaneously changing location of cross-state switches in the
four delay lines, the 1x4 OBFN can achieve a delay response for
beamsteering angle of 0o, 7.06o, 14.2o, 21.7o, 29.5o and 40.0o for 41
GHz signal or 0o, 15.7o, 32.7o, and 54.1o for 90 GHz signal using half
wavelength dipole antenna, respectively. Figure 4 shows the delay
response MZ-SDL OBFN for the maximum beamsteering angle with
Fig. 1. Schematic of a typical MZI-SDL. Inset: photograph of the fabricated chip (8×32 mm2) and IR image of a delay element.
(a)
(b)
Fig. 2. (a) and (b) Simulation results of the worst case normalized delay
and power spectra for a traditional 3-stage binary-bits MZ-SDL.
(a)
22.5ps
(b)
Fig. 3. (a) Measured delay response for improved 5-stage MZI-SDL. (b)
Eye diagrams for a 10 Gbps OOK NRZ signal delayed by the SDL.
delay increment of 7.5 ps over 8 nm (1 THz).
III. ORR BASED OBFN
An ORR is a kind of all pass filter that tunes the delay by changing
its resonances. A single ORR exhibits a bell-shaped group delay
response. The maximum delay and delay bandwidth with the coupling , as depicted in the inset in Fig. 5. However, the delay-bandwidth
product of a single ORR is constant [13], which implies that there is a
trade-off between the delay value and bandwidth. Cascading multiple
ORRs can increase this product. In multi-ORR OBFN, two adjacent
delay paths can share rings to reduce the system complexity. Figure 5
shows the simulated relation between the ripple (delay flatness),
bandwidth, number of rings and sharing strategy. As shown, more
rings can provide improved TTD bandwidth and delay flatness but at
the expense of increased system complexity [12].
Figure 6 shows the schematic of a 1x4 OBFN with a two-stage
binary tree topology based on 3-ORRs with two rings shared by two
adjacent paths. The chip utilizes low-loss silicon nitride waveguides
and has a dimension of 8 x 32 mm2. The ORRs employ symmetric
Mach-Zehnder interferometers (MZIs) as the tunable coupler to the
bus waveguide, as illustrated in the inset of Fig. 6. Chromium heaters
are placed on the MZI coupler and feedback waveguide of the ring for
control of the coupling coefficient and the resonance frequency,
respectively.
After careful optimization of the coupling coefficient and resonance
of each ORR using a genetic algorithm [11], the 3-ORR delay line
demonstrates a flattened delay response with 209 ps continuous tuning
range and 6.3 GHz bandwidth as shown in Fig. 7. This range
corresponds to a phase shift of 37.1π for a 90-GHz signal and could
feed a large scale PAA up to 1x37 (1-D) or 18x18 (2-D) for a 180o
beam steering angle for half wavelength dipole antenna arrays.
Figure 8 shows the optimized delay response of all the four paths of
the OBFN with a TTD bandwidth of 6.3 GHz. Each path was tuned to
have 4.6 ps more delay than its previous path, which is equivalent to a
beamsteering angle of 22o for a 41 GHz, or 55.3o for a 90 GHz
half-wavelength dipole antenna array.
A system test was performed for millimeter wave generation using
up-conversion method [16, 17]. The schematic of the experimental
setup is as depicted in Fig. 10. The laser source is modulated by
driving a null biased MZM modulator with a 20.5 GHz local oscillator
(LO) which generates two dominant optical sidebands spaced by 41
GHz. A wavelength division multiplexer (WDM) separates the two
sidebands, and one sideband is encoded with 3 Gbps
non-return-to-zero (NRZ) data. The two optical tones are then
combined by another WDM, and both tones are coupled to the OBFN
chip. One path of the OBFN chip is used and optimized to have a delay
of 147 ps. As a result, a delayed microwave signal with a 41 GHz
carrier and 3 Gbps NRZ data was generated and its electrical spectrum
is shown in Fig. 9.
Fig. 7. Ripple optimized group delay spectra for 3-ORR delay line at the
bandwidth of 6.3GHz. The dots denote the measured delays, whereas the
solid curves denote the simulation result.
Fig. 6. Schematic of an ORR based 1x4 OBFN. (Insets: Chip photograph
and detailed ORR layout).
Fig. 4. The delay response of the MZ-SDL OBFN for maximum beam
steering angle.
Fig. 5. Simulation demonstrating trade-offs between ripple, bandwidth,
and number of cascaded rings for delay line with 3-ORR, 5-ORR, 3-ORR
sharing 1-ORR, and 3-ORR sharing 2-ORR. The FSR is 23 GHz and target
delay is 147.8 ps. The ripple is characterized as the maximum delay
deviation from the target delay over a given bandwidth. (Inset: single ORR
bell-shaped group delay spectra for different values of κ.)
IV. W-BAND BEAM STEERING EXPERIMENT
The W-band signal was generated using a frequency up-conversion
heterodyne process. The schematic of the system is as depicted in Fig.
11. A narrow linewidth external cavity laser is set to 1551.745 nm. The
laser light is modulated by driving a null biased MZ modulator with a
local oscillator at 47 GHz, which suppresses the central optical carrier
but generates two dominant optical sidebands spaced by 94 GHz. A
wavelength division multiplexer (WDM) with 50 GHz channel grids
separates the two sidebands. One sideband is modulated and serves as
the data tone, and the other sideband serves as the reference tone. The
two optical tones are combined by another WDM and then amplified
by an erbium-doped fiber amplifier (EDFA). The signal is coupled to
the integrated OBFN chip and splits into four paths and is
appropriately delayed for the desired beamsteering angle. The output
signals are sent to an ultra-high-speed photodiode array to generate the
94 GHz signal and emit from a 1×4 antenna array. Since discrete
components are used for the signal radiation, four tunable delay lines
were employed to equalize the optical path length with Conf. 3 of the
OBFN. If the photodiode and antenna array were integrated with the
OBFN chip, these tunable delay lines could be eliminated. Figure 12
shows the measured and theoretical radiation patterns of the 94-GHz
signal for multiple OBFN chip configurations and beam angles of
-51°, -33° , -16° , -2° , 14° and 31°, respectively. The measured data
agrees with the theory well, except for some distortion in the
side-lobes. This could be due to the slightly mismatched optical length
and slight variations of output intensity of each channel, which could
be relieved if the photodiode and antenna array were integrated. The
linewidth of the signal is less than 1 kHz and the 3-dB beam width is
28°. A narrower beam width can be achieved for a larger array size.
High bit-rate data transmission experiments will be performed in
future work.
Fig. 8. The optimized delay response of the 1x4 3-ORR OBFN.
Fig. 9. Normalized electrical spectrum of 41 GHz millimeter signal with
3 Gbps NRZ data modulation.
Fig. 10. mmW generation and delay experiment test setup. (ESA: electrical spectrum analyzer; OSA: optical spectrum analyzer; DE/MUX: de/multiplexer.)
Fig. 11 . Schematic of 93-GHz signal generation and beamsteering system. Inset: picture of two-tone generation and data encoding testbed, packaged
integrated OBFN chip, 94-GHz signal emission testbed. (DE/MUX: de/multiplexer.)
V. CONCLUSIONS
In this paper, we demonstrate two different integrated 1x4 OBFNs
using ultra-low loss silicon nitride waveguides: one is based on
ripple-free 5-stage MZ-SDLs and the other is based on 3-ORR delay
lines. The MZ-SDL OBFN demonstrated a maximum delay of 22.5 ps
with a 4.5 ps tuning step and could provide 6 discrete delay
configurations for beam steering over the bandwidth of 8 nm (1 THz).
The 3-ORR delay line and the corresponding OBFN achieved a 209 ps
of continuously TTD tuning with 6.3 GHz of bandwidth, as well as a
55.3o beam steering configuration for 90 GHz signal. A 94 GHz beam
steering experiment based on a 1×4 PAA was also demonstrated based
on the MZ-SDL OBFN. Beam steering angles of −51°, ±32°, ±15° and
0° were achieved, demonstrating the potential for using integrated
OBFNs in W-band PAA applications. Future experiments will
demonstrate high bit-rate data transmission with beam steering
capability. A 41 GHz millimeter wave generation experiment with 3
Gbps data was demonstrated and a 90 GHz beam steering experiment
utilizing these OBFNs will be performed in the future.
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