Abstract
The research described in this thesis was focused on the design, fabrication, and characterization
of InP-based photonic integrated true-time-delay beamformers. A photonic beamformer is
an optical circuit that controls the direction of the signal transmitted by a phased-array antenna
(PAA). Unlike regular antennas, a PAA can transmit a signal only in the direction of the intended
receiver without wasting any energy towards undesired directions. Furthermore, with a
PAA, the direction of the transmission can be switched electronically without having to physically
rotate the PAA itself. PAAs have been intensively studied in RADAR applications, where
they are used to enable tracking of potential targets and to reduce the interference from different
obstacles. Recently, PAAs are becoming increasingly popular in other applications, such
as wireless communication networks where they can be used to increase the system capacity.
A PAA consists of several closely-spaced regular antennas called the PAA elements. The
direction of the transmitted signal is set by applying different phase delays to the signals of
each PAA element. The circuit that controls the phase of each PAA-element signal is called a
beamformer. Traditionally, the phase delays in an electronic beamformer have been realized by
using a phase modulator on each PAA-element signal, or via digital signal processors (DSP).
However, as the operating frequencies increase to enable higher transmission bandwidths, it
becomes more and more difficult to realize high-frequency electronic phase modulators and
DSPs. Photonic beamformers, that can introduce the required phase delays in the optical domain,
offer many advantages over electronic beamformers, such as immunity to electromagnetic
interference, lower propagation loss, and remoting of the beamformer with respect to the
PAA.
For the research reported in this thesis, we have designed a total of three photonic integrated
beamformers. These beamformers have been fabricated in InP technology and characterized.
The InP technology offers several advantages over other technologies such as compactness
due to a high refractive index, fast electro-optic switching capabilities, and the possibility for
amplification of the optical signals by integrating semiconductor optical amplifiers.
The firstly fabricated beamformer contained an arrayed-waveguide-grating (AWG) (de)-
multiplexer integrated with three switched-delay-line (SDL) section. Each SDL section consists
of four fast electro-optic Mach-Zehnder-interferometer (MZI) switches cascaded by a
bypass line and a delay line on each output. The MZI switches are used to direct the optical
signal either through the delay line or the bypass line. This way, we can introduce a so-called
true-time phase delay between the different optical wavelengths of the SDL sections. True-time
phase-delay beamforming offers higher bandwidth than phase delay introduced by an optical
phase modulator, because it does not suffer from beam squint. The size of the firstly fabricated
beamformer was 8×11mm2, and this device had the capability to switch between eight different
transmission directions of a four-element PAA operating at 40GHz. The on-chip optical
losses of this beamformer were around 20dB. Although, we have shown the beamforming
functionality of this chip, the performance was limited in terms of the polarization dispersion,
i.e. the wavelength shift of the AWG (de)-multiplexer channels for different polarizations
(1.1nm difference between TE- and TM polarized output), and the switching voltages of the
MZI switches (> 20V).
Therefore, we subsequently realized a re-design of this beamformer with optimized components.
The size of this second beamformer was similar to that of the first beamformer. In
this re-design, the length of the phase shifters in the MZI switches was increased from 0.8mm
to 3mm, which resulted in a decrease in switching voltage down to around -5V. Also, the
waveguide width of the AWG arms was changed from 1.7µm to 1.6µm, and this resulted in a
decrease in the polarization dispersion of the AWG down to 0.4nm. The switching speed of
the MZI switches was measured to be lower than 5ns. The on-chip losses were increased to
27dB.
The third beamformer consisted of a single SDL section with InP-based electro-optic
switches and off-chip dispersive-fiber delay elements. As this device requires a total of 14
fiber-chip connections, we have developed spot-size converters (SSCs) to lower the losses associated
with the large mode-mismatch between the InP waveguides and the fibers. The SSC
developed here consist of a vertical taper where the optical signal is coupled from the regular
shallow waveguide (mode size = 2.7µm × 0.9µm) to a much larger fiber-matched waveguide
(6.5-9µm × 5.5µm) underneath. We have developed a fabrication process to realize vertical
tapers using standard lithography and dry-etching steps. Measurements on a test chip containing
SSCs showed fiber-chip losses as low as 0.8dB/SSC.
These SSCs were integrated with the MZI switches to realize the third beamformer. The
size of the InP switch array containing the four MZI switches was 5.8mm × 6.5mm and the
total fiber-to-fiber losses of the complete beamformer, including the spot-size converters, is
38.6dB, which was close to specs. A future outlook to this work will be to integrate semiconductor
optical amplifiers in the beamformers to compensate for the optical losses.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 12 Sep 2006 |
Place of Publication | Eindhoven |
Publisher | |
Print ISBNs | 90-386-1833-6 |
DOIs | |
Publication status | Published - 2006 |