Optical resonators have micrometer size dimensions and come mostly in two flavors, namely circular and racetrack
shaped microrings (MR), and microdisks (MD), although microsphere (MS) and photonic crystal microring (PCMR)
resonators are also expected to gain prominence. Highly advanced fabrication techniques in recent years resulted in the
reduction of propagation losses and in a remarkable increase of resonator Q factor and finesse. Newly developed
microresonators are therefore ideally suited for applications in highly selective communication filters, delay lines,
distributed and localized sensing, industrial measurements, microlaser mirrors and high-resolution spectroscopy. Since
the optical signal recirculates and spends a relatively long time trapped in a high Q cavity, microresonators enhance
light-light and light-particle interactions and are for this reason most promising to exploit nonlinear effects.
The talk will focus on advances in multiring photonic devices such as the coupled resonator optical waveguide (CROW)
and the side-coupled integrated space sequenced optical resonator (SCISSOR), on the link between photonic and
microwave filter design, on the effect of polarization on filter response and its control, on schemes and efficiency of
tuning and modulation and on MR composites used as reflectors and laser mirrors. The talk will also cover issues related
to design trends and technological advances, such as vertically stacked MRs, coiled optical resonators and resonators not
based on propagating waves, as well as techniques to extend the free spectral range (FSR) of periodic filters through the
Vernier principle and the use of polymer materials and two-dimensional photonic crystals to fabricate optical resonators.
The spectrum of resonance frequencies in so called stand-alone resonators, built with coupled ring or disk resonators is analytically and numerically investigated. These composite resonators which constitute the core of numerous photonic circuits used in channel dropping filters, dispersion compensators, laser mirrors, etc., determine the width of their passband and their free spectral range (FSR). The spectral characteristics of the resonances are determined by the dimensions of the resonators and the strength of coupling between them. Novel relationships between these parameters are described that ensure invariance of the splitting ratios and as a consequence maintain the passband characteristics of the associated devices. Waveguide attenuation is found to have no effect on the spectral characteristics of the composite resonators.
Phase matching of the signal paths between input and drop ports significantly improves filter characteristics and results in a box-like passband and a deeply suppressed, flat stopband. As a result add/drop multiplexers built with only two filter elements, each consisting of a chain of one, two, or three series coupled microrings exhibit satisfactory filter characteristics and very moderate degradation due to resonator loss. Accurate phase matching however requires thermooptic or electrooptic tuning, usually embedded in one arm of the multiplexer. Analytical and simulated results are presented covering transmission and group delay characteristics, sensitivity to waveguide loss and free spectral range (FSR) extension techniques. A temperature/stress sensor application is also proposed.
Coherent, passive, linear optical signal processing networks, built with 2 by 2 couplers, gratings, mirrors and waveguides are discussed. The signal processing function is accomplished by the interference of coherent signals. Through the artifice of a generalized coupler complex network topologies are reduced to cascaded four-port and feedback-assisted cascaded four-port circuits, which are then treated by standard network analysis techniques. A wide range of configurations can thus be evaluated through a single numerical code. Several applications relevant to telecommunication, measurement and sensing are enumerated.
Numerically simulated performance of a new type of bandpass filter, based on discontinuity-assisted ring resonator (DARR)
loaded couplers is presented. The operating characteristics of the device are similar to those of a long grating with a centered
cavity or a photonic crystal with a defect. The filter is tuned by varying the length or the refractive index of the cavity.
Selectivity and tuning range is controlled by the coupling coefficient of the couplers and by the reflection coefficient of the
discontinuities. A design procedure and the effect of loss are also presented.
We report on simulated performance results obtained for resonant and lattice circuits, comprised of 2 X 2 couplers, waveguides, Bragg gratings and wavelength independent reflective elements. The range of operating wavelength depends on the type of components used and on the adequacy of the circuit model. A unified analysis allows the investigation of a large variety of network configurations by the same computer code. The method of analysis and the numerical results should be useful for the wavelength engineering of spectral filters, add/drop combiners, sensors, etc.
This paper deals with the spectral properties of optical networks that consist of a single, distributed 3x3 coupler embedded in a feedback circuit. We first specify a set of coupled linear differential equations that characterize the coupler. The solution of this set of equations yields the scattering transfer matrix of the coupler from which, through linear transformation, we obtain its scattering matrix. Subsequently, we determine the scattering matrix of the composite network, i.e., the coupler, embedded in the external circuitry. Numerical experiments illustrate the properties of composite networks of various configuration.
An outline is presented for a program to teach the modelling of optical components such as couplers, filters, resonators, interferometers, etc. to senior electrical engineering students. The program exploits the student’s background in linear algebra, circuit analysis and transmission line theory. It is designed to complement an elective course in microwave engineering. Numerous simulation examples associated with optical communication, measurement and sensing provide the experimental environment.
A two-step proton-exchange process is employed to produce waveguides and gratings in lithium niobate. In both steps mixtures of lithium benzoate and benzoic acid are used. The fabricated components are characterized at 0.633mm and 0.442/mm. In particular, we have observed that the light intensity diffracted out into the air by the grating is about 0.5% of that in the waveguide.
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