Calculator Inputs
Example Data Table
| Case | λ₀ (nm) | Radius (µm) | nₑff | nᵍ | ΔT (°C) | Expected Use |
|---|---|---|---|---|---|---|
| Silicon ring trim | 1550 | 10 | 2.40 | 4.20 | 10 | Filter alignment |
| Low power heater | 1310 | 15 | 2.35 | 4.00 | 6 | Sensor biasing |
| Large FSR device | 1550 | 5 | 2.45 | 4.35 | 15 | Dense channel tuning |
Formula Used
Resonance condition: mλ = nₑffL, where L = 2πR.
Thermal wavelength shift: Δλ = (λ₀ / nᵍ) × (dnₑff/dT + nₑffα) × ΔT.
Heater based temperature rise: ΔT = P × Rth × η.
Free spectral range: FSR = λ₀² / (nᵍ × 2πR).
Heater voltage and current: V = √(PR) and I = √(P/R).
How to Use This Calculator
Enter the cold resonance wavelength and ring radius first. Add effective index and group index from your waveguide model. Then enter the thermo optic coefficient and thermal expansion coefficient. Choose direct temperature, heater power, or combined mode. Add heater resistance and power for electrical estimates. Press calculate to see the tuned wavelength, shift, FSR, heater values, and target power.
Thermal Ring Resonator Tuning Overview
Ring resonators shift their resonance when temperature changes. The guided mode sees a new effective index. The physical ring length also expands. Both effects move the wavelength. In many silicon photonic designs, the thermo optic effect is dominant. A small heater can therefore tune a resonance by several picometers or nanometers.
Why Thermal Shift Matters
Thermal tuning is useful in filters, modulators, sensors, and wavelength division systems. Fabrication errors can move a resonance away from its target. Controlled heating pulls it back. Designers also use heat to align many rings on one chip. The electrical side matters because heater power, resistance, and pulse time set energy use and response limits.
Main Design Inputs
This calculator uses the cold resonance wavelength, ring radius, effective index, group index, and thermo optic coefficient. It can accept a direct temperature rise. It can also estimate temperature from heater power and thermal resistance. The heater efficiency field lets you include losses into substrate, metal, or package layers. A larger efficiency means more of the electrical heat reaches the optical mode.
Interpreting The Result
The wavelength shift shows how far the resonance moves from the starting value. The tuned wavelength is the expected new resonance. The free spectral range estimate shows the spacing between nearby resonances. The FSR fraction tells whether the tuning crosses a large part of one resonance period. A value near one means the resonance has moved about one full FSR.
Engineering Notes
Use measured coefficients when possible. Published values are only a starting point. Group index can vary with waveguide width and wavelength. Thermal resistance depends on cladding, substrate, heater shape, and heat sinking. Very high temperature can change loss, stress, or reliability. For fast pulses, steady state thermal resistance may overestimate the real shift. Use transient simulation or measurement for final hardware.
Practical Use
Start with nominal optical parameters from layout simulation. Enter realistic heater resistance and power. Compare a direct temperature case with a power based case. Then export the table. Keep it with lab notes, test data, and mask revision history. This makes tuning choices easier to explain. It also helps compare rings with different radii, heaters, and waveguide stacks during later design reviews.
FAQs
What does thermal tuning mean?
Thermal tuning means shifting the ring resonance by changing temperature. Heat changes the waveguide index and ring length. The resonance wavelength then moves.
Why is group index used?
Group index improves the wavelength shift estimate. It accounts for dispersion near the operating wavelength. This is often better than using effective index alone.
Can I use heater power instead of temperature?
Yes. Select the power based mode. The calculator estimates temperature rise from heater power, thermal resistance, and heater efficiency.
What is the FSR fraction?
It compares the wavelength shift with the free spectral range. A value of 0.5 means the resonance moved about half of one spacing.
What coefficient should I use for silicon?
Many silicon designs use a thermo optic coefficient near 1.86e-4 per degree Celsius. Use your measured or simulated value when available.
Does thermal expansion matter?
It can matter, but it is often smaller than the thermo optic effect. It is still included for better advanced estimates.
Why enter loaded Q?
Loaded Q gives an estimated resonance linewidth. The calculator compares thermal shift against this linewidth to show tuning strength.
Is this enough for final chip design?
No single calculator replaces measurement or simulation. Use it for planning, review, and early estimates. Verify final heaters with thermal and optical tests.