The design for a photonic microwave filter tunable in both bandwidth and operating frequency is proposed and experimentally demonstrated. The circuit is based on a single sideband modulator used in conjunction with two or more transmission fiber Bragg gratings (FBGs) cascaded in series. It is demonstrated that the optical filtering characteristics of the FBGs are instrumental in defining the shape of the microwave filter, and the numerical modeling was used to optimize these characteristics. A multiphase-shift transmission FBG design is used to increase the dynamic range of the filter, control the filter ripple, and maximize the slope of the filter skirts. Initial measurements confirmed the design theory and demonstrated a working microwave filter with a bandwidth tunable from approximately 2 to 3.5 GHz and an 18 GHz operating frequency tuning range. Further work is required to refine the FBG manufacturing process and reduce the impact of fabrication errors.
The top cladding layer in planar lightwave circuits (PLC) is more than an optical buffer. By variously doping, adjusting
the thickness of, etching patterns in and annealing the cladding layers in waveguide devices, a wide range of sensors and
photonic devices can be realized. The material properties of the cladding determine, for instance, the modal
birefringence of the waveguides; knowledge and control of these properties can be harnessed to produce polarization-independent
components. The fabrication of thermo-optically controlled switches and interferometers for tunable
filtering and optical signal processing is possible through the creation of micro heaters on top of the cladding. The
optimization of such components can benefit from engineering of the cladding, ranging from better planarization and
thickness control, to selective etching to better confine the heat distribution and provide stress relief. In addition, the
thermal properties of a given device can be radically enhanced by using a polymer layer as top cladding, which yields an
order of magnitude increase in the temperature sensitivity, an invaluable enhancement that can be harnessed for phase-tunable
waveguides or sensor structures. Long period gratings (LPGs) can be etched in the lower cladding to provide
filtering, signal processing, or sensor functions. In a borophosphosilicate cladding, typically used in silica-on-silicon
PLCs, control of the reflow properties through composition can be exploited to manufacture fillable microchannels that
are monolithically integrated with solid-core devices, enabling a unique platform for sensing, signal processing, or
The optimization of a 2×2 silica-on-silicon Mach-Zehnder interferometer (MZI) thermo-optic switch is presented. The
device consists of 2 multimode interference (MMI) couplers as splitter and combiner with metal heater strips for phase
control. The switching characteristics of the devices have been examined in detail as a function of several parameters.
The electrical power consumption of the switch has been reduced by a factor of 2 by etching trenches alongside the
waveguide heaters located on the arms of the MZI, and the polarization dependent loss has been controlled and reduced
through adjustment of top cladding properties. The effect on the response time of the switch of these design changes has
been investigated. Detailed characterization of the devices will be presented, and trade-offs in optimization discussed.
Incorporation of these device elements into increasingly complex components for new applications in optical signal
processing will be demonstrated.
Optical waveguide crossings based on silica-on-silicon technology are investigated. The effect of crossing angle
(θ) on light transmitted at through and cross-port on a sequence of waveguide crossings with angle varying from
7 to 28° is modeled and experimentally validated. Results demonstrate that structures with small footprint
(θ≈9°) can achieve low crosstalk of -32 dB with high throughput, insensitivity to wavelength of operation, low
polarization dependent loss of 0.6 dB, and low sensitivity to fabrication tolerances. As a result, waveguide
crossings with small crossing angle present an attractive approach to reducing the overall component footprint
without compromising the performance.
There are many applications where a very wideband phase shifter is required. Analog pre-distorters to linearize Ka-band
amplifiers require a frequency-independent phase shift over at least 1 GHz. The same requirement applies to phased-array
antennas or antenna feeds, as well as direct radiating array antennas. Most electrical phase shifters have a fixed
operating frequency, discrete phase shift steps (e.g., 5-bit control) and some frequency and temperature dependent
responses which result in sub-optimum system performance. The requirements in the 50/40 GHz band will be even more
demanding where the bandwidth to be covered could extend up to 5 GHz.
The use of photonic technology mitigates the limitations of electrical phase shifters. Operation over a wide range of
frequencies (e.g., 4 to 50 GHz) using a single design is possible, and a flat phase response over many GHz's can be
achieved. This paper discusses the use of novel microwave photonic technologies to enhance the performance of a
broadband phase shifter with respect to power, mass, volume, electromagnetic interference and compatibility of future
on-board satellite subsystems. The targeted phase shifter is equally applicable to analog linearizers, phased-array
antennas/feeds or other smart antenna schemes where relative phase shifts are required.
The results of a prototype phase shifter are presented showing a broadband response over several GHz. Limitations of
this device and justification for an integrated version will be discussed. Finally, preliminary results for an integrated
device are presented.
A novel approach of a first order optimization technique applicable to design process of photonic sensing devices and waveguide geometries is presented. The application of the optical field sensitivity mapping technique enables first order optimization of geometrical parameters with the final goal of enhancing the overall device sensitivity. The technique is simple, requiring only two simulations of optical field propagation and the extraction of a sensitivity map. The method is demonstrated in a design optimization process of a realistic multi mode interference sensing device. As a result, optimization of the MMI active section length, sensing region width, and the output location was accomplished. A comparison between the optimized device and two other ones of different length showed a 10 dB higher dynamic range of the output power ratio characteristic and better linearity, demonstrating enhanced sensitivity of the final device.
This paper presents a thermo-mechanical analysis of an optoelectronic system including a Mach-Zehnder optical modulator integrated with a broad-band GaAs driver amplifier, forming a module which then is placed into a low temperature co-fired ceramic (LTCC) substrate. All module connections such as voltage supply, RF signals and fiber optic input/output are realized through the LTCC. Thermal analysis of this
integrated system shows elevated temperatures in the optical component caused by the heat generated in the power amplifier and dissipated into the substrate-carrier and from there into the LTCC. Temperature profiles along the MZ modulator reveal a strong non-uniformity, reaching a 26C temperature difference between the optical component input and output. A stress-strain analysis is also performed. Preliminary results show significant physical distortion of the optical component, which could cause optical misalignments and additional coupling losses. These findings indicate a need for thermal consideration in early design stages.
A compact Y-junction waveguide switch with electrically reconfigurable output waveguide arms is demonstrated in InGaAsP/InP. Simulations indicate that the plasma effect or the thermo-optic effect can be used as the active switching mechanism, as corroborated by experimental tests. For the plasma effect the induced index change under the electrode, Δn, is negative. The Y-junction device has a measured switch contrast ratio ~ 20 dB, with a response time of ~ 5 ns. Using the thermo-optic effect Δn is positive and the observed contrast ratio is better than 10 dB. The highly localized nature of the thermal gradient in these devices yields thermo-optic switching into the hundred of nanoseconds range, several orders of magnitude faster than the overall thermal response time. This is the fastest thermo-optic switch reported to date. Fabrication of these switches, and in particular the use of O+-ion implantation to provide electrical isolation of the waveguide branches, is described.