# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 5 A Performance Review of Widely-Tunable Semiconductor Lasers
The early SG-DBR lasers had linewidths in the MHz range, as the injection of carriers resulted in increased cavity loss, and consequently wider linewidth. For different lasing wavelengths, due to different carrier concentrations, the linewidth varies between several 100s kHz to several MHz, as showed by the error bar in Figure 17[40; 41]. It should be noted that such lasers can be tuned extremely rapidly which can be beneficial for some application. Thermal tuning does not suffer from tuning induced loss, and has previously enabled linewidths as low as 400 kHz in a super-structure grating (SSG) DBR laser using on-chip microheaters[42]. Thermal tuning is generally much slower, in the 10s of kHz range. The SG-DBR lasers' performance can further be optimized by thermal engineering and careful design of sampled gratings, so (instantaneous) laser linewidths down to 50-70 kHz were demonstrated, with +17 dBm of fiber coupled power, and SMSR \(>\)50 dB over 41 nm of tuning range in C-band[43]. A DFB laser array is another technique for widely tunable lasers commonly realized in monolithic III/V platform. DFB lasers are simpler than SG-DBR type lasers, and narrow linewidth performance can be achieved by increasing cavity length \(L_{\text{DFB}}\) while maintaining the optimum \(\kappa\)-\(L_{\text{DFB}}\) product, where \(\kappa\) is the grating coupler coefficient[44]. A narrow (integrated) linewidth in the range from 88 kHz to 160 kHz was achieved by a tunable DFB laser array with a long cavity (1500 \(\upmu\)m) DFB structure[45]. By integrating a booster SOA, waveguide coupler, and 12 DFB lasers which were temperature controlled in the range from 15 to 50 \({}^{\circ}\)C, laser tuning range covered 40 nm in the L-band, with a fiber output power of 13 dBm, and a high SMSR of \(>\)50 dB. The heterogeneous silicon photonics platform opens up a new possibility in improving the coherence of semiconductor lasers, by providing a mechanism to separate the photon resonator and highly-absorbing active medium[59]. Individual DFB lasers with sub-kHz instantaneous linewidths were demonstrated[60], and it would be relatively straightforward to integrate them in a DFB array to form a widely-tunable chip-scale semiconductor laser. Extremely impressive results were achieved by MRR based hybrid lasers, where reflective semiconductor optical amplifiers (RSOAs) are butt-coupled to external waveguide circuits with ring resonators forming the SWF. The SWF can be realized in various passive waveguide platforms, such as Si and SiN. The RSOA and the SWG can be fabricated separately, and hand-picked for best performance. With Si based SWF, over 20 dBm fiber coupled output power, and (integrated) linewidth narrower than 15 kHz along the whole C-band, was achieved[50]. The authors point at least two reasons for such an impressive performance: the optimized Si-wire waveguides with losses lower than 0.5 dB/cm in C-band, and the passive alignment technology for precisely mounting a Si-waveguide tunable filter,  
Figure 17: Widely-tunable integrated lasers linewidth versus time. We make a distinction between III/V based monolithic lasers, hybrid lasers where III/V gain chips are butt coupled to Si or SiN chips, and heterogeneous integrated lasers where the III/V gain material is bonded to silicon. (sampled grating distributed bragg reflector (SG-DBR) lasers, distributed feedback (DFB) lasers and micro-ring resonator (MRR) lasers). gain chip, and booster SOA on a common platform on silicon, with minimum excess loss. A similar structure, but using the SWF made in SiN, has shown a record low (instantaneous) linewidth of 290 Hz, thanks to the 0.1 dB/cm low-loss Si\({}_{3}\)N\({}_{4}\) waveguide, and the use of a three ring structure inside the cavity, as analyzed in Section 4.2[51]. The effective on-chip cavity length approaches 0.5 m, resulting in extremely narrow instantaneous linewidth. Tuning range is equal to about 81 nm, with maximum fiber-coupled output power of 11 dBm.