# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 4 Advanced Designs for Linewidth Reduction
### Control of Widely-Tunable Laser with High-\(Q\) Resonator  
A three ring resonator widely-tunable laser is a more complicated device to control than a standard widely-tunable laser, like the one shown in Figure 2. For a standard design using the Vernier effect, either with Bragg gratings or ring resonators, generation of a look-up table (LUT), which will map the laser temperature and grating/ring resonator controls to the output wavelength, is required. The procedure usually involves an optical spectrum analyzer or a wavemeter to monitor the output wavelength, as a function of Vernier element control signals. This process inevitably takes time, as the number of points for measurements tends to be relatively large. For tuning just the two Vernier rings/gratings, the result is a 2D tuning map that can also have laser temperature corrections. To optimize the side-mode suppression-ratio (SMSR), a third variable, control of the phase section, should be introduced. This results in 3D tuning maps and significantly slower LUT generation. The high-\(Q\) laser increases the complexity by one more dimension, resulting in a 4D tuning map if brute-force generation is attempted. The problem is further complicated by narrow resonances of the high-\(Q\) resonator, resulting in an increase in granularity of the control signals. While all this is theoretically possible, a simpler and faster laser control mechanism is possible, provided that the laser includes on-chip monitor photodetectors. In the case of a fully-integrated laser design, the monitor photodetectors are usually straightforward to realize, with no extra processing steps, which highlights the benefits of integrated designs. We have designed and fabricated a laser using the architecture shown in Figure 13a, but have included two monitor photodetectors to control the laser operation, as shown in Figure 14a [39]. A photograph of the fabricated chip on a temperature controlled stage is shown in Figure 14b. By use of on-chip monitor photodetectors, it is straightforward to control both the high-\(Q\) resonator, as well as cavity phase. In Figure 15, we plot the currents from two monitor photodetectors when the laser is biased. From the periodicity, we can easily determine the tuning efficiencies; in this case \(\sim\)14 mW/\(\pi\) for phase section, and \(\sim\)26 mW/\(\pi\) for the high-\(Q\) ring. Also, it is straightforward to position both elements at a particular position, without use of any external elements. An illustrative alignment strategy would be to position the high-\(Q\) ring and phase to maximize the output power (output monitor photocurrent), and then use the high-\(Q\) monitor to control and detune the high-\(Q\) ring for maximum linewidth reduction factor, as illustrated in Section 2. It is interesting to point out that the position of maximum output power in Figure 15a corresponds to a local minimum of the high-\(Q\) ring monitor photodetector shown in Figure 15b. In other words, maximum output power is obtained when cavity losses are minimized, which is when all three rings are aligned. The power at the high-\(Q\) ring monitor photodetector is locally minimized due to the extinction ratio of the high-\(Q\) ring at the pass port. Ideally, if the high-\(Q\) ring was critically coupled, that power would be zero, but the ring is intentionally over-coupled to reduce the insertion loss and consequently cavity loss. Figure 14: (**a**) Schematic of laser with high-\(Q\) ring inside the cavity that includes two monitor photodetectors and a booster SOA (BSOA); (**b**) A probed laser bar on a stage comprising of a number of laser structures. A single device with high-\(Q\) ring inside the cavity occupies an area of \(\sim\)4 mm\({}^{2}\). By using on-chip photodetectors, fast alignment of the high-\(Q\) ring and phase section is possible, making control of this laser very similar in complexity to typical laser designs utilizing only two Vernier rings. The laser bias tuning and wavelength tuning are both very predictable.