The communications industry’s main challenge is managing the exponential increase in data traffic while ensuring that energy consumption (and its associated costs) do not grow exponentially. This challenge demands new generations of optical components that prioritize energy efficiency, as measured in femtojoules of energy per bit (fJ/bit). Minimizing this metric is critical for meeting future scalability demands.
This trend also includes electro-optical modulators. These components encode electrical signals into optical signals by altering the properties of light, such as amplitude, phase, or polarization, through changes in an active electro-optical material.
Current modulator components are reliable but have some limitations. Typical modulators are relatively “large” components (in this industry, large means centimeter or millimeter sizes) and higher driving voltages. These large footprints make the dense integration of components difficult, while the higher driving voltages increase power consumption, operational costs, and environmental impact.
Ring resonator modulators (RRMs) offer a more compact and efficient approach to optical modulation. These devices use optical resonances to achieve efficient modulation at low driving voltages in an ultra-compact footprint. This white paper will briefly review how RRMs work and explore some of their advantages and application spaces.
Conceptual representation of a plasmonic phase shifter with two metal electrodes that serve as photonic waveguides and also to apply the electrical field.
Let’s quickly review how a ring resonator works and can be used as a modulator.
In its simplest form, a ring resonator consists of a waveguide that forms a closed loop, and a regular bus waveguide used to couple in and out of the loop. While many simplified diagrams will represent this loop waveguide as a circular ring, most real-life ring resonators have a loop that looks more like a racetrack.
We describe the distance that light travels inside the ring not just as a function of the physical length of the ring but also as a function of how the waveguide material slows down the light (i.e., the refractive index of the material). This distance is known as the optical path length. When the optical path length of the ring matches a whole number of the light’s wavelength, resonance occurs, and light constructively interferes as it circulates within the ring. This resonance amplifies the interaction between light and the modulator, enabling more efficient control over light’s properties.
A voltage-controlled phase shifter can be integrated into the ring to turn it into a light modulator. This phase shifter alters the refractive index of the waveguide material, which shifts the optical path length and, thus, the resonant wavelength of the ring. Driving the phase shifter with an electrical signal will change the optical response and encode the information into the light passing through the ring resonator.
Key features of an RRM with its coupler structure, a ring often implemented as a race track and a phase shifter to modulate the optical length of the track.
When evaluating an RRM, the most important parameters and specifications typically depend on its intended application. However, from a general perspective, here are some key parameters and intrinsic, design-driven variables of the RRM that govern its performance.
— Waveguide Loss: Waveguide loss originates from material absorption, scattering, and imperfections within the waveguides forming the ring resonator. This loss reduces the light circulating in the resonator, diminishing the device’s ability to modulate light effectively.
— Coupling Ratio: This is the fraction of optical power transferred from the bus waveguide to the ring waveguide at the coupling region. It is influenced by factors such as the physical gap between the waveguides, the overlap of their optical fields, the coupling section length, and the waveguide material’s refractive index. This parameter controls the balance between light that resonates within the ring and light that bypasses it.
— Quality Factor (Q-factor): The Q-factor measures the sharpness of the resonance, indicating how effectively the resonator confines light. It is influenced by waveguide losses and the coupling ratio. A higher Q-factor improves energy confinement and energy-efficient modulation but typically reduces bandwidth.
— Tuning Range: The total wavelength or frequency shift achievable through tuning. Resonator tuning is often achieved by altering the refractive index of the waveguide material, either via temperature changes (thermal tuning) or applied electric fields (electro-optic tuning).
— Tuning Sensitivity: This reflects how effectively the resonator’s resonance wavelength or frequency responds to external control parameters, such as temperature, voltage, or current. It depends on the material’s tuning properties (e.g., thermo-optic or electro-optic coefficients) and the resonator’s geometry.
The specifications below are some of the measurable, application-driven outputs of the RRM that result from the parameters we described earlier.
— Insertion loss: The total optical power loss as light passes through the RRM. It is influenced by waveguide losses and the coupling ratio. Lower insertion loss is vital for maintaining signal strength over long distances.
— Drive Voltage: It represents the electrical power required for effective modulation. It depends on the tuning sensitivity and the resonator’s quality factor. This specification directly affects the modulator’s power consumption and compatibility with other electronic components, such as digital signal processors.
— Modulation Bandwidth: The frequency range over which the RRM can modulate signals effectively. It is influenced by the resonator’s quality factor as well as the tuning range and sensitivity. The bandwidth determines the data rate that the modulator can handle.
— Extinction Ratio: This measures the difference in transmitted optical power between the modulator’s “on” and “off” states. It depends on the coupling ratio and the quality factor. A high extinction ratio is crucial for clear signal encoding and decoding.
— Resonant Spectrum: The resonant spectrum describes the shape and width of the resonant peak. A narrower resonance, resulting from a higher Q-factor, enables more efficient modulation but requires tighter tuning tolerances. The spectrum must remain stable under varying environmental conditions to ensure reliable performance.
Optical response of a RRM with its characteristic multiple resonance dips. The distance between each dip is the free spectral range (FSR). The static insertion loss (IL) is the difference between the point with maximal transmission and zero attenuation (0 dB). The static extinction ratio (ER) is the difference between the maximal and the minimal transmission.
The way RRMs work has a few notable advantages compared to other modulators.
— Compactness: One of the most compelling advantages of RRMs is their small size. Since light passes through the same loop waveguide multiple times, RRMs can perform similar functions to other modulators but over much shorter physical waveguide lengths. They are a hundred times smaller than typical modulators. Such compactness makes them an excellent choice for dense integration in photonic integrated circuits, where space is at a premium. These size advantages multiply when building multi-channel communication devices on a chip.
— Low Insertion Loss: RRMs offer lower insertion loss compared to other modulator setups due to their simple and compact structure. Their smaller size means light travels shorter distances and accumulates lower losses. RRMs have just one coupling point, where the ring waveguide meets the bus waveguide. Other modulators with more complex structures have more coupling points, introducing additional coupling and mismatch losses.
— Energy Efficiency: The resonant behavior of RRMs amplifies the interaction between light and the modulator, which allows for modulation at lower driving voltages. This translates to reduced energy consumption per bit, which is crucial in data centers and large-scale telecommunications networks.
— Wavelength Selectivity: Tuning the ring’s parameters can make it resonant to specific wavelengths while filtering out others. This makes them a natural choice for wavelength-division multiplexing (WDM) systems, where multiple data streams are transmitted simultaneously on different wavelengths.
In a practical implementation the two electrodes are connected to a modulating signal and to ground, or to the inverse of the modulating signal.