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:

klamkin@ece.ucsb.edu, yuanliu@umail.ucsb.edu). 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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