Photonic integrated true-time-delay beamformers in InP technology

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.