Lithium niobate thin films (TFLN), with the significant advantages of wide transparency window, high refractive index, and large acousto-optic/electro-optic/non-linear optical coefficients, have emerged as a promising material platform for the fabrication of high-performance photonic integrated devices for both classical and quantum applications, where photonics devices, such as low-loss waveguides, high-quality microresonators, high-speed modulators, and high-efficiency optical frequency converters, have now been realized. However, single-crystal lithium niobate lacks effective luminescence and detection capabilities.
Recently, lithium niobate crystals doped with rare-earth ions have started to be a practical solution for realizing the optical gain function of TFLN platforms, and rare-earth-ion doped optically pumped microlasers have been experimentally demonstrated. Rare earth ion doped lasers have the advantages of wide bandwidth, polarization insensitivity, high applicable temperature and good compatibility. In addition, rare-earth ion-doped lasers are more promising in achieving high power output, mode-locked operation, and coherent beam combinations. However, currently, all rare-earth ion-doped TFLN lasers are optically pumped by external lasers connected using optical fibers, which hinders the development of integrated photonics on TFLNs.
Recently, a compact hybrid lithium niobate micro-ring laser was demonstrated by Prof. Cheng Ya's team at East China Normal University. Figure 1 shows a schematic diagram of the proposed compact hybrid lithium niobate micro-ring laser pumped by a semiconductor laser, which consists of a commercially available CoS-packaged semiconductor laser and a high-Q Er3+-doped micro-ring. The alignment between the input port of the micro-ring and the output port of the CoS-encapsulated semiconductor laser tube is achieved by a 6-axis alignment system with an adjustable accuracy of 10 nm, which in turn enables efficient optical coupling. To achieve stable and tight bonding, UV adhesive is applied by a dispenser and the two chips are fixed by UV irradiation. The key to this work is that the Er3+-doped micro-ring has a high Q value at the pump wavelength of the semiconductor laser. As shown in Fig. 2, the center wavelength of the pump laser of the semiconductor is 976.24 nm, and at this wavelength, the Q value of the micro-ring is 7.3×105. The center wavelength of the laser produced by the Er3+-doped micro-ring under the pump light is 1531.27 nm, and the linewidth of the laser is 0.05 nm, and the single-mode laser emission is attributable to the mode-dependent loss and gain competition. In addition, the linewidth of the emitted laser is two orders of magnitude narrower than the linewidth of the pump light generated by the CoS-packaged semiconductor laser.
Fig. 1. (a) Schematic diagram of a compact hybrid lithium niobate micro-ring laser consisting of a CoS-packaged semiconductor laser and a high-Q Er:TFLN micro-ring laser. (b) Top view of the compact hybrid lithium niobate micro-ring laser. (c) Close-up photo under optical microscope of the interface between the CoS-encapsulated semiconductor laser and the Er:TFLN micro-ring.
Fig. 2. (a) Q of the micro-ring at 976 nm is 7.3 × 105. (b) Q of the micro-ring at 1531 nm is 1.85 × 105. (c) Spectrum near the center wavelength of the semiconductor laser. (d) Spectrum near the wavelength of the Er3+-doped micro-ring emitting a single-frequency laser at 1531.27 nm with a linewidth of 0.05 nm.
This work explores a powerful hybrid lithium niobate micro-ring laser source that has potential applications in coherent optical communications and precision metrology, and the results are published online in OpticsLetters.
