Electrical mode-locked oscillators generate a periodic train of electrical pulses. In the frequency domain, the pulse output corresponds to multiple harmonic modes with a certain phase relationship, or 'locked' to each other. Because of their short duration and inherent repetition-rate stability, these mode-locked pulses can be useful in high speed sampling, ultrafast time-domain metrology, and injection-lock based frequecny synthesis.
I achieved the fastest ever electrical mode locking by integrating an oscillator circuit that combines a coplanar waveguide and a special amplifier in an advanced GaAs technology. In this circuit, the amplifier is responsible for the pulse formation, loss compensation, and oscillation stability. Measurement results show that the oscillator self generates and sustains a periodic train of pulses with a pulse width of 16 ps at a pulse repetition rate of 18.7 GHz.
Soliton and Nonlinear Wave Electronics
Solitons are spatially localized nonlinear waves that propagate maintaining their pulse shape and at constant speed. The balancing act of nonlinearity and dispersion is responsible for generaton of solitons, and this exists in several places in nature, such as shallow water waves, optical fibers, plasma, mechanical lattices, and superconducting Josephson junctions.
In the electrical domain, the nonlinear transmission line (NLTL) serves as a dispersive and nonlinear medium for generation and propagation of electrical solitons. The NLTL has been extensively investigated since 1960s, and it is now well known for its pulse sharpening capability down to subpicoseconds. Just before I joined my research group at Harvard, David S. Ricketts et al., a former member of my group, reported the first electrical soliton oscillator, which self-generates a periodic train of electrical solitons. In this circuit, a pulse propagagates in a circular loop, consisting of an NLTL and a special two-port amplifier.
I developed the reflection soliton oscillator, where, instead of circling, a soliton pulse travels back and forth on a straight NLTL. One side of the NLTL is connected to the reflection amplifier, which provides gain to the soliton and ensures oscillation stability by suppressing undesired perturbations via its level-dependent gain. The other end of the NLTL is an electrical open, where the soliton reflects back. Measurement results confirm that the oscillation self-starts from noise when dc power is turned on, and the circuit self-generates a stable, periodic train of sharp soliton pulses at the steady state. This reflection structure has certain advantages on the circular one, such as its capability to generate sharper pulses, its significantly smaller area, its more efficient DC-RF energy conversion dynamics, and its enhanced stability.
Chaos Generation From Solitons
& Chaotic Communications
Chaos is aperiodic long-term behavior in a deterministic system that exhibits extremely sensitive dependence on initial conditions. By encouraging the unruly soliton dynamics, it is possible to make an elecrical chaotic soliton oscillator. This circuit can be useful in physical random number generation and broadband/secure communications, where the message signal is modulated/masked into a chaotic carrier signal. In such a system, although it is very difficult for an outsider to extract the message from the chaotic carrier signal, it is possible to extract it with a matched/synchronized receiver circuit. In other words, for an outsider the carrier signal is random, but for the receiver, which knows the system dynamics, the carrier signal is deterministic. This is why it is possible to define chaos as the randomness generated by deterministic systems.
Because of NLTL's superb pulse sharpening capability, a chaotic soliton oscillator can potentially generate an extremely broadband signal, corresponding to a very high data rate in secure communications, making the chaotic soliton oscillator a candidate circuit for future secure communication applications. Thus far, I attained self generation of chaos from electrical solitons. Currently, I am working on devising a suitable chaotic communications scheme for it. Its implementation in the microwave and millimeter-wave regimes will be my next goal.
CMOS Readout ICs for Micro-Electro-Mechanical Systems (MEMS)
(Master's Degree at METU, Ankara, Turkey)
The main two detector types of infrared imaging technology are cooled photonic and uncooled thermal detectors. On one hand, photonic detectors provide higher performance but need to be cooled down to cryogenic temperatures; on the other hand, the thermal detectors can work in the room temperature but their performance is moderate. Uncooled infrared imaging technology has gained wide attention for civilian application, for its several advantages such as low cost, low weight, and low power consumption.
Microbolometers are most widely used uncooled infrared detectors due to their compatibility with CMOS technology. They can be monolithically integrated with CMOS chips which allows their low cost. At METU, the MEMS Research Group, which I was working with, produces microbolometer-type uncooled infrared detectors and their integrated readout electronics in standard CMOS technologies.
In the figure, a suspended microbolometer pixel is shown. This structure was fabricated at METU via post-processing a CMOS chip. Upon the incident of infrared radiation, the temperature of the suspended part of the pixel rises, as the suspended bridge is thermally isolated from the substrate. The rise in the detector temperature causes a change in an electrical parameter of the detector (i.e. resistance, diode voltage, capacitance, etc.) which is sensed by the readout electronics, which is integrated in the same chip. The readout circuit includes an amplification circuit for the detector signal before any signal processing step as well as a pixel addressing circuit.
I worked on the physical design and design verification of readout circuits for resistive bolometers, specifically analog channels (preamplifiers, operational amplifiers, sample-and-hold circuits, digital-to-analog concverters (DACs)) and digital timing and pixel selection circuitry. In addition to my regular responsibilities, I came up with a totally new idea and developed the first dynamic nonuniformity compensation circuit for microbolometers. The proposed circuit eliminates the need for DACs, and hence, reduces the chip area and simplifies the external electronics.