A light source is an indispensable component in on-chip systems. Compared with hybrid or heterogeneous integrated laser, monolithically integrated laser is more suitable for high-density photonic integrated circuits because of the capability of large-scale manufacturing, lower active-passive coupling loss, and less test complexity. Recent years have seen the spark of research on the rare-earth ion-doped thin film lithium niobate, and demonstrations have been made in both classical and quantum chips. However, low output power and limited quantum emitting efficiency hinder the application of the chip-scale laser source based on this platform. Here a highly efficient integrated laser assisted by an amplifier is proposed and experimentally prepared on Erbium-doped thin film lithium niobate. A slope efficiency of 0.43% and a linewidth of 47.86 kHz are obtained. The maximum integrated laser power is 7.989 μW. Our results show a viable solution to improve efficiency without changing the intrinsic quantum emitting efficiency of the material, and our design has potential applications in being incorporated with functional devices such as optical communications, integrated quantum memory, and quantum emission.

Figure 1. Concept and structure of the IAL. (a) Schematic of the IAL. (b) Microscopic images of the IAL with pump. (c−e) Scanning electron microscopy images of the IAL; zoomed-in figures are the edge coupler and waveguide of amplifier.

The structure of IAL is shown in Figure 1a. When the pump laser is injected in the waveguide through an edge coupler and coupled into the microdisk resonator, the Er ions in the whispering-gallery-mode (WGM) cavity would be excited to higher states (4I11/2 with 980 nm pump, 4I13/2 with 1480 nm pump), thanks to the enhanced light−matter interaction. After a rapid nonradiation decay, they would emit C-band laser signals. As the signal laser and residual pump laser couple back and propagate along the spiral waveguide amplifier, the pump laser excites the Er ions and builds up population inversion. The signal laser from the microdisk would be amplified, and the energy of the pump laser is further transferred into the signal laser. The energy level diagrams included in Figure 1a plot the lasing and amplification processes. It is worth noting that the cooperation upconversion of Er ions under a pump would also generate a green light. This process is detrimental to the quantum emitting efficiency of the C-band laser, while it could help us to estimate the power density of the pump laser. Figure 1b shows the microscopic images of the device with the pump on. The green emission highlights the microdisk cavity and the spiral waveguide.

Figure 2. Performance under a nontunable pump. Emission power at 1531.6 nm (a) and spectra (b) of the IAL under a nontunable pump. (c) Emission power comparison between the microdisk laser and IAL; the inset shows the details about the laser threshold. (d) Laser peaks comparison between the microdisk laser and IAL with the same nontunable pump at 17.98 mW. (e) Emission power at 1531.6 nm of the IAL at 50 °C. (f) Wavelength drift of laser peaks versus different temperatures (from 40 to 70 °C). Evolution of laser mode around 1530 nm (g) and 1562 nm (h) with temperature increasing from 20 to 70 °C.

Figure 3. Performance under tunable pump. (a) Relationship between laser power and tunable pump power. (b) The collected spectrum of the IAL under tunable pump. (c) The linewidth of the signal laser. (d) Lorentzian fitting of a measured mode around 1530.56 nm exhibiting a loaded Q factor of 5.59 × 10^5.

The result is published in ACS photonics, named as "Efficient Integrated Amplifier-Assisted Laser on Erbium-Doped Lithium Niobate".

Link: DOI: 10.1021/acsphotonics.4c00391