Ring resonator enhanced mode-hop-free wavelength tuning of an integrated extended-cavity laser

Abstract

Extending the cavity length of diode lasers with feedback from Bragg structures and ring resonators is highly effective for obtaining ultra-narrow laser linewidths. However, cavity length extension also decreases the free-spectral range of the cavity. This reduces the wavelength range of continuous laser tuning that can be achieved with a given phase shift of an intracavity phase tuning element. We present a method that increases the range of continuous tuning to that of a short equivalent laser cavity, while maintaining the ultra-narrow linewidth of a long cavity. Using a single-frequency hybrid integrated InP-Si3N4 diode laser with 120 nm coverage around 1540 nm, with a maximum output of 24 mW and lowest intrinsic linewidth of 2.2 kHz, we demonstrate a six-fold increased continuous and mode-hop-free tuning range of 0.22 nm (28 GHz) as compared to the free-spectral range of the laser cavity. arXiv:1912.09455v1 [physics.optics]

Thermally Tuned High-Performance III-V/Si3N4 External Cavity Laser

“…It should be noted that the sweeping range (or modulation speed) should be evaluated in combination with the laser longitudinal mode spacing. As for a fixed phase shift (or driving signal amplitude), the laser frequency shift is proportional to the longitudinal mode spacing [31]. In this study, the measured longitudinal mode spacing is 4.5 GHz, slightly smaller than the simulation.…”

“…It should be noted that since the longitudinal mode shifts within the passband of the Vernier filter, there is slight power variation during the sweep. If the electrical signals are applied to both the MRRs and the phase shifter, a wider frequency tuning range without power variation can be obtained [31]. Table 1 compares the performance of this work with other Si3N4 and silicon ECLs with a similar configuration reported in the literature.…”

“…The power consumption is 240 mW to shift one Vernier FSR (equal to 2π phase shift). The power required for thermal tuning is reduced by more than half compared with the thin Si3N4 waveguides [31]. Fig.…”

“…For the hybrid lasers, various external cavity structures including Fabry-Perot (FP) cavities [24], distributed feedback (DFB) gratings [25], Bragg gratings [26,27], sampled grating distributed Bragg reflectors (SG-DBR) [28], interferometric structures [29], and Vernier microring resonators (MRRs) have been demonstrated on the silicon or Si3N4 platforms [21,[30][31][32]. Among them, the Vernier MRRs-based hybrid external cavity lasers (ECLs) have attracted lots of attention due to the merits of narrow passband response with a large free spectral range (FSR) and a small footprint.…”

Hybrid integrated silicon nitride lasers

“…Note, that this wavelength tuning is discrete and that the mode-hop-free tuning range is far smaller. Typically our hybrid integrated laser can be tuned a few 10s of picometers before a mode-hop occurs, however, it has recently been shown that the mode-hop-free tuning range can be increased by simultaneously tuning the phase section (see Figure 3) of the laser and a mode-hop-tuning range of 0.22 nm has been reported [18]. For many application the noise characteristics of the laser modules are critical.…”