# Widely-Tunable Ring-Resonator Semiconductor Lasers
## 5 A Performance Review of Widely-Tunable Semiconductor Lasers  
Chip-scale widely-tunable semiconductor lasers have been developed on monolithic III/V, hybrid (where III/V gain region is butt-coupled to Si or SiN chip providing single mode filtering and tuning), and heterogeneous (where III/V material is bonded to Si) platforms. To cover a broad tuning range and provide single longitudinal mode lasing, a Vernier effect is typically used, either by using sampled grating distributed Bragg reflectors (SG-DBR), or micro-ring resonator (MRR) structures. An alternative approach for wide tuning utilizes an array of distributed-feedback (DFB) lasers with different design central wavelengths, where each laser covers a few nm of tuning. The output of this array is typically coupled to a single waveguide and amplified by an SOA, to compensate coupling losses and increase output power. Most tunable lasers generally utilize a booster SOA, which serves at least two functions. First is the amplification and control of output power without the need to control the laser bias current that could influence tuning. Second is the ability to quickly switch-off the laser output when the wavelength is tuned, which can be very important in, e.g., live WDM networks, to prevent crosstalk or disruption of neighboring channels. An additional benefit of using a booster SOA is the lower power inside the laser cavity, which reduces the nonlinear effects that can negatively impact laser performance, such as for example, two-photon absorption. The tuning range basically depends on the net gain bandwidth of gain material, and the design of the single wavelength filter (SWF). For designs having arrays of lasers, the tuning range is determined by the net gain bandwidth, number of parallel laser diodes, and the tuning range of each diode, which is usually designed in such way that there are no blind spots inside the tuning range. Our original idea was to compare tunable laser performance in terms of output power, tuning range, and linewidth, across years, platforms, and tuning mechanisms, but as tuning range and output powers did not show any clear trends, we decided to plot only the linewidth performance. This actually makes sense, as tuning range is mostly determined by the gain medium, if tuning filters are properly designed, while output power can easily be increased by optimizing booster SOA. Laser linewidth is extremely important for a number of applications, such as coherent communications, LIDAR, other types of optical sensing, etc. It has been extensively researched, and there has been a growing need for chip-scale lasers with narrow linewidth. In Figure 17, we compare the linewidth of widely-tunable lasers [23; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58], and a few clear trends can be noted. First is that the linewidth has steadily been improving from MHz range to kHz range in the past 20 years, and the second is that ring based tunable lasers (MRR) offer superior linewidth performance, as shown with circles. This is to be expected, keeping in mind the effective cavity length enhancement and negative optical feedback effects introduced in Section 2. We should point out that some numbers in the plot correspond to integrated linewidth as measured with the delayed self-heterodyne technique, while some correspond to the white noise limit or so-called instantaneous linewidth determined from phase noise measurements, which are not directly comparable. Nevertheless, the trends are clear. Next, we give descriptions of some representative laser designs. SG-DBR type designs are widely studied and used in monolithic III/V tunable lasers, where the wider tuning range filter is provided by the product of the two differently spaced and independently tuned reflection combs of the SG-DBRs at each end of the cavity. Wide tuning can be realized by carrier injection or thermal effect, both of which change the effective index of refraction, but do so at different speeds and different excess losses.