Thermally Tunable Laser with Vernier Rings

PICWave performs 3D time evolving simulations of a tunable laser created with thermally tunable ring resonators. This example shows how power is maximised at each target wavelength and acts as an easy to adapt template for this design.

Also mentioned: 

• FIMMPROP - rigorous optical propagation simulations for ring couplers / entire ring resonators. Uses passive frequency domain EME.
• Harold - thorough simulations of MQW (or quantum dot) cross sections to calculate gain spectra

The device consists of two epitaxy structures - an InGaAsP ridge waveguide is used as a gain section and transitions to a Si3N4 core in silica waveguide where Vernier rings can be made compactly.

Combining rings of different radii in series produces a reflectivity spectrum with a prominent single wavelength. PICWave allows users to investigate changing the radii and ring coupler strengths to optimize the vernier rings for a target wavelength. 

Combined with the gain section and a reflective facet, this produces a lasing cavity with a single lasing wavelength.

• Ring to bus coupling depends on the waveguide’s separation which can be calculated rigorously in Photon Design’s EME FIMMPROP and imported into PICWave (through UI or Python scripting).

Top: Spectral response of rings connected in series with prominent central wavelength and side-modes.
Bottom: Plot of spectral response of each ring (100 um and 102 um). Peaks in their combined spectra are found where resonances align (left) and light is not reflected where resonances are not aligned (right).

Heating the Si3N4 waveguide changes the effective index of the waveguide mode resulting in a change in the resonant wavelengths; this allows us to tune the central wavelength of the reflectivity spectrum produced by the rings and tune the lasing wavelength of the cavity.

• In this example, the user can set the target wavelength of the Vernier rings and calculations are done to find the appropriate applied heating voltage.
   
• The relationship between applied power, material heating, and resultant change in effective index can be found in Photon Design’s mode solver FIMMWAVE with Poisson solvers included with thermo-optic model.

Thermal profile due to applied power at contact in example ridge waveguide.

The tuning range of a vernier ring depends on the difference in radius between the two rings. The smaller the difference in ring radii the more susceptible the design is to fabrication errors.

An additional modulator is included in this example outside of the Vernier rings. This allows us to change the relative heights of the Verinier ring’s central wavelength and its adjacent side-modes. Sweeping this phase shift (though sweeping applied voltage) will therefore alter the lasing power and eventually (as the side mode becomes larger than the target wavelength) induce a mode hop to a different lasing wavelength. 

Left: Output power changes as phase shifter is linearly tuned. Includes instantaneous changes in power.
Right: Lasing wavelength changing in instantaneous ‘mode-hops’ causing changes in power.

By tuning this phase shifter we can ensure lasing at the target wavelength and maximise the laser’s output power.

This effect is due to the relative heights of the central wavelength and the adjacent sidemodes of the Vernier rings. By tuning this additional phase shifter we can find the maximum output power and ensure lasing operation at the target wavelength.

1. Create your waveguide cross section, copy and paste this to each element
2. Import relationship between effective index and heating power (can be found with FIMMWAVE).
3. Select appropriate ring radii and coupling strengths
4. Use FIMMPROP EME to create coupler with desired coupling strength
5. Select target wavelength
6. Tune phase shifter to maximise power and ensure 

Additionally:

• Thoroughly simulate gain spectra with Harold and import to gain section.
• Construct full layout of device in MT-FIMMPROP where rigorous EME simulations can be performed on the full device.

Intensity Plot - Rigorous 3D optical simulation from MT-FIMMPROP (600umx400um, runtime ~13s on standard desktop)