Ring Resonators Flashcards
What are the key Performace Parameters for a ring resonator?
FSR, Extinction Ratio, Q Factor and Finesse
Describe the basic Operation Of a ring resonator
A waveguide in the form of a closed-loop, normally a ring or racetrack. Light from an input waveguide can be coupled into this ring via evanescent field coupling. The input waveguide must be in close proximity to the ring and is thus known as the input bus. Ring acts as interferometer, the ring will resonate with light with a phase change of m2π, where m is an integer. Other Light is transmitted through the bus.
What RIng Resonator Config is suitable for OADM,
How is it used?
The waveguide coupled ring resonator is suitable. Here a second waveguide, the output bus is added to the opposite side of the ring resonator.
The Phase matching condition must still be satisfied, the propagation constant of the waveguide should match that of the ring.
To use, as OADM, wavelengths can be added via the add port, coupled through the ring and exit via the through port of the input bus.
What are the requirements of a good ring resonator?
Low Optical Losses
Single Mode
Polarization Independent design
Low Transmission Loss
Low Bend Loss
High FSR
High Extinction Ratio
High Q Factor
High Finesse
How is flexibility introduced to ring and racetrack resonators?
To modify a ring resonator design, you can adjust the waveguide width, bend radius and bus-ring separation.
For the racetrack resonator, you can do the same as well as adjust the length of the straight section for added flexibility
What Are Microdisk Resonators?
Microdisk Resonators are similar to ring resonators however, the centre of the ring is not etched out in mircodisk resonators. This allows them to act as much larger waveguide. This allows them to be suitable for hybrid 3-5 lasers. However, means multimode operation.
Why are Microdisk Resonators useful?
Can support much more wavelengths as the centre is filled. Means suitable for hybrid (III-V) lasers. However, the multimode operation can lead to problems
Describe a notch filter
A notch filter is a device that blocks the transmission of frequencies within a certain range or ranges and permits the transmission of frequencies outside of those ranges.
How can ring resonators act as notch filters?
The ring resonator can act as an interferometer, certain frequencies of light are resonant with the ring. The resonant light is coupled into the ring and isolated from the output waveguide via the waveguide isolation trench. Frequencies that are not resonant pass through unimpeded. This is in essence a notch filter as frequencies in certain ranges are restricted. Important to note is the resonator will couple light at multiple frequencies as they correspond with different orders of m.
Why are there multiple dips in the spectrum of the notch filter?
The multiple dips correspond with multiple solutions to the equation
where N is waveguide propagation constant.
Define FSR
FSR or free spectral range is the distance between two adjacent maxima/minima in the spectrum response.
What is an optical add-drop multiplexer?
An OADM is a device used to route different channels of light into a single optical fibre. A typical OADM consists of three stages, the optical multiplexer, the optical multiplexer and a method of adding and dropping the paths between the multiplexer.
Benefits of using higher-order resonators for multiplexers
The higher order configurations lead to an increased extinction ratio.
Define extinction ratio
Difference between maximum and minimum in the spectral response measured in dBs
Define Q Factor
Dimensionless parameter used to evaluate circuits. It is
The number of oscillations of the field before the circulating energy is reduced to 1/e of its original value.
Finesse
A dimensionless parameter which can be used to measure the field enhancement or damping, like the Q factor.
It is defined as the number of round trips made by light around the ring before its energy is reduced to 1/e of it’s original value.
Describe the operation of a 1xN resonator-based MUX/DEMUX
If the ring is resonant with the incident light, it is coupled from the input bus to the drop port. This happens for each ring in the MUX/DEMUX configuration.
How does radius affect ring resonators?
A small radius means large FSR, however, the Q factor shrinks whilst the FWHM increases.
In addition, measured loss increases.
Equations relating to energy loss in ring resonator
F is the finesse.
M is the number of round trips required to reduce optical energy to 1/e of its original value.
T is the time taken to male a round trip around the ring.
What are the Equations for energy loss rate in a ring resonator?
PL = - dU/dt
- dU/dt = ω0 U/Q → U(t) = U0 e-ω0t/Q
U0 e-ω0t/Q = U0 e-t/τ
What is the cavity lifetime?
Represents the rate of decay/growth of a photon within an optical cavity.
Calculated as τ = Q/ω0
where τ represents tau, i.e. time.
Gives the time when cavity energy drops to 1/e of the original energy Uo
What is cavity energy proportional to?
Cavity Energy is Proportional to the square of the electric field amplitude, represented by E.
What applications are there for ring resonators?
Ring Resonator Modulator, Biosensing,
CROW is a method of light filtering via coupled-resonator optical waveguides.
