# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 3 Comparison of Classical Widely-Tunable Architectures
### Discussion and Conclusions  
Having analyzed, fabricated, and measured four different laser architectures, we conclude that the best design to realize a widely-tunable ring-resonator based laser is the RBR. The RBR laser has very predictable tuning, stable lasing due to the standing-wave nature of the resonator, and high passive side-mode suppression ratio (SMSR), resulting in high lasing SMSR. Added benefits are largely fixed mirror reflectivities due to the broadband Sagnac loop mirrors used. The biggest disadvantage of this architecture is a somewhat lower output power, although that can be corrected by placing a booster semiconductor optical amplifier (BSOA) after the laser. In a heterogeneously integrated design, integration of a BSOA is straightforward. The reason for lower output power is the tuning section, where light goes through each ring twice, increasing the insertion loss of the structure, and consequently, the cavity loss, but at the same time, this results in improved filtering, and higher passive SMSR. We show tuning of the RBR laser in more detail in Figure 9a, where directions of fine (dashed red line) and coarse (dotted blue line) tuning are drawn. The black line shows continuous tuning without mode hops, where phase section corrects for the cavity length, as wavelength is tuned. To guarantee sufficient phase shift, the phase tuning section has to be optimized, and in general, up to a few nanometers of continuous tuning is possible, in practice, before temperatures at phase and ring tuners become excessive, and added heat starts to influence the laser performance significantly. Figure 8: Measured linewidths for (**a**) RBR laser; (**b**) CRR 1\(\times\) laser; and (**c**) Vernier racetrack laser. The linewidth of the CRR 2\(\times\) laser was 196 kHz at the one wavelength where it was measured. Figure 7: Plot of peak wavelength vs. ring tuning power for (**a**) RBR laser; (**b**) CRR 1\(\times\) laser; and (**c**) Vernier racetrack laser. We do not show the CRR 2\(\times\) laser tuning performance due to unpredictability. The CRR \(1\times\) laser has higher output power than RBR, primarily due to lower losses at the CRR reflector, compared to RBR filter realization. The biggest problem with CRR is the lower passive SMSR, translating to smaller tuning ranges, and/or lower laser SMSR. As it is very challenging to design a single CRR structure with high SMSR, a logical idea would be to replace the Sagnac loop mirrors on both sides of the cavity with CRR structures. This further reduces the cavity losses, as shown, with the highest output powers obtained with CRR \(2\times\) laser. The laser also features high passive SMSR, but the tuning is extremely challenging, for several reasons. First is that the laser now has four controls for tuning, as each ring inside the CRR has its own tuner, and the fifth control would be the phase to align the longitudinal mode. A way to simplify the control would be using a single heater shared per CRR structure. A problem with this approach is that the rings have to be exactly the same circumference to act as a reflector, and any non-symmetry or thermal crosstalk will lead to resonance splitting, as shown in Figure 9d,e. The problem is further complicated due to a need to have distinctly different coupling values between the two rings and the bus waveguide [23], which makes full symmetry impossible. For our laser, we have designed independent heaters for each ring, and have also provided the ability to use the two heaters as one big heater covering both rings for each CRR structure, but in all cases, the tuning was very challenging. Due to such complicated controls, it is hard to recommend the CRR \(2\times\) structure, despite it having the highest output power. The last considered structure, Vernier racetrack laser, has high output power, is straightforward to control, and has decent SMSR, but as it has a ring type of cavity, it inherently has bidirectional lasing, where two travelling waves compete for gain.