# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 4 Advanced Designs for Linewidth Reduction
### Utilizing Delayed On-Chip Optical Feedback
Output power is around 10 mW, which can easily be increased by the use of a booster SOA. The on-chip MPD is a very useful tool to monitor the on-chip performance of a laser with an external cavity. It has been shown that lasers can be influenced by feedback levels as low as \(-90\) dB [34], and a straightforward way to measure the feedback is to change the phase of the returned signal, while monitoring the laser output power. We show such a measurement in Figure 11a, where the external phase heater is tuned for various gain section biases, and output power is recorded by the on-chip MPD placed at the laser output. The resulting effect is an almost sinusoidal modulation of output power, with the phase of the returning signal as predicted for lower levels of feedback [33]. This signal can be used to phase align the two cavities, and improve the laser linewidth. We measured the linewidth of the laser with optimized feedback from the external cavity. The feedback was manually tuned for best performance, and results are shown in Figure 11b. The linewidths were below 100 kHz across the entire tuning range, and the best single-mode linewidth is 50 kHz. The quoted 3 dB linewidths are calculated from \(-20\) dB points by assuming a Lorentzian lineshape using a delayed self-heterodyne method. Without the external cavity, the linewidths were in the 300 kHz range. It should be noted that the lasers were not packaged, so there were a number of sources of technical noise present. We were able to measure even lower linewidths (below 20 kHz) at higher currents supplied to the external SOA, but the laser was multimode, with mode spacing determined by the external cavity length. In this case, both the relative intensity noise (RIN) and frequency noise spectrums have peaks at this mode separation. This shows that the external cavity can provide even better performance if the filtering section is optimized. We also believe that the external SOA increased the noise in the feedback signal, due to random spontaneous emission events, thus limiting the linewidth improvement performance over the theoretically predicted one, as for the measurements shown in Figure 11b, it was operated below transparency. Driving the external SOA more strongly would result in multimode behavior, as the SWF cannot filter out a single longitudinal mode of the external cavity in such a design. The \(Q\) of tuning rings is in the 10,000 range. As an improvement of this work, we suggest use of a variable optical attenuator (VOA) in place of an SOA. VOAs could provide similar functionality in controlling the feedback level, with some  
Figure 10: A schematic view of a tunable laser design with integrated external cavity. Tuners are yellow (two phase sections and two rings for wide tuning), and SOAs are dark orange (gain section inside the laser cavity, and external SOA that is used to control the level of feedback). The output monitor photodiode (MPD) is used to measure the laser output power for adjustment of laser parameters. differences. They only attenuate the signal, and so, depending on the configuration, potentially higher levels of feedback (regime V) would not be achieved, and the extinction ratio can be limited, at least for single stage structures. By cascading the VOAs, higher extinction ratios can be achieved, but with an increase in the insertion loss.