Towards Tunable Lasing in Thin-Film Lithium Niobate/III-V Cavities

Thin-film lithium niobate (LN) is a promising platform for high performance chip-scale optical systems, owing to its large electro-optic effect, low-loss waveguides, and wafer scalability. An outstanding challenge for thin-film LN photonics, and many photonic platforms in general, is integration of tunable high-power, low-noise, and narrow-linewidth lasers. Integrated lasers have previously been demonstrated on LN through butt-coupling of high performance DFB lasers and heterogeneous integration of III-V gain material. Further improvements can be made through injection locking with high-<i>Q </i>rings and external cavity laser development.<br/>Here, we demonstrate lasing action from a InP reflective semiconductor optical amplifier (RSOA) using thin-film LN as a wavelength-selective reflector. Our scheme is composed of several integrated components on 600 nm thick x-cut LN. First, we designed low-loss horn couplers to efficiency collect light from the RSOA to the LN chip. Next, we use two cascaded ring resonators with different free spectral ranges and integrated heaters to act as an active thermal filter. We purposefully overcoupled the rings (Q factors ~50,000) to reduce on-chip losses. Finally, we included a narrow-band photonic crystal to act as a partial reflector in the spectral region with the largest optical gain. Our RSOA (Freedom Photonics) emits maximum spontaneous emission at 1590 nm and small-signal gain of 25 dB. The nanofin photonic crystal has a period of 410 nm to match the stop band bandwidth with the maximum gain of the amplifier. We make the mirror partially reflective (R = 70%) by using 150 unit cells.<br/>Our initial demonstration of lasing relies on tunability of the Vernier filter using thermo-optic shifting. We chose ring radii of 120 and 130 microns to set the Vernier FSR similar to the bandwidth of the DBR mirror. After aligning the ring resonances, we estimate the roundtrip reflection to be -17.5 dB, substantially lower than the gain provided by the amplifier. We couple the amplifier to the LN chip, forming a cavity with high reflection only at the optical mode filtered by the Vernier filters within the band gap of the DBR. With proper thermal tuning to manage the mode profiles, we find single-mode lasing over a range of RSOA injection currents from 0.1 A to 0.4 A. We measured both the LI curve and laser linewidth to demonstrate the lasing action of our device. The LI curve shows the clear onset of lasing around 0.1A and saturation above 0.3A. The maximum lasing power we achieved is -9.5 dBm, limited by the distance between the RSOA and input facet of the LN chip. Finally, we measured single mode lasing with sideband suppression ratios greater than 40 dB, and linewidths as low as 250 kHz. Finally, we will discuss progress towards active control and high power, narrow-linewidth lasing using the electro-optic effect in LN. Our results represent progress towards a tunable, robust, and low-cost integrated light source with narrow linewidth and high power, amenable for long haul telecommunication networks, data center optical interconnects, and microwave photonic systems.