Crystalline lithium niobate (LN) is considered the “silicon of photonics” because of its outstanding optical properties, including a broad transparency window and high piezoelectric, acousto-optic, second-order nonlinear, and electro-optic coefficients, which are critical for photonic integrated circuit (PIC) applications. Recent breakthroughs in nano­fabrication technology facilitate a variety of high-performance integrated photonic devices on thin-film LN, such as ultrafast electro-optic modulators, broadband optical frequency combs, and high-effi­ciency optical frequency convertors.

As an indispensable component for PICs, on-chip microlasers have recently been realized on a rare-earth-doped LN chip at various wavelength bands (~1550 and 1030 nanometers). To enable many appli­cations ranging from lidar to metrology the LN micro­lasers should operate with ultranarrow linewidths and high wavelength tunability. A high Q factor is a key parameter. According to the Schawlow–Townes theory, increasing the Q factor will lead to quadratic reduction of a microlaser linewidth. The highest Q factors demonstrated to date are those of whispering gallery mode (WGM) micro­cavities where light confinement is achieved by the continuous total internal reflection around the smooth circular periphery.

However, the dense WGMs within the optical gain bandwidth usually give rise to multimode lasing in the micro­cavity. In principle, single-mode lasing can be achieved by reducing the size of the WGM micro­cavity, owing to the broadening of the free spectral range (FSR). Unfor­tunately, such a strategy inevitably leads to increased radiation loss, which is unfavorable for laser generation. So it remains a challenge to achieve single-mode lasing on a single-microdisk reso­nator.

To meet this challenge, researchers from Shanghai Institute of Optics and Fine Mechanics, East China Normal University, University of Victoria, Zhejiang University, and Zhejiang Lab recently demons­trated a unique single-frequency ultranarrow linewidth erbium-doped LN microdisk laser. They achieved this through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. They used photolithography-assisted chemo­mechanical etching (PLACE) to fabricate the LN micro­cavities integrated with microelectrodes in a controllable and cost-effective way. The microcavities provide an ultra­smooth surface, which enables ultrahigh Q factors for the cavity WGMs.

The polygon modes were coherently combined by multiple WGMs triggered by a weak pertur­bation from a tapered fiber. The polygon modes are sparse within the optical gain bandwidth compared to the WGM counterpart, while their Q factors remain ultrahigh (e.g., ~10 million), resulting in single frequency lasing with a linewidth as narrow as 322 Hertz. Plus, the system offers real-time electro-optical tuning of the microlaser wavelength, thanks to the strong linear electro-optic coefficient of LN; the research team demonstrated a high tuning effi­ciency of ∼50 pm/100V.

The formation of coherent polygon modes with ultrahigh Q factors ensures the realization of single-mode narrow-linewidth microlasers in single LN microdisks, which has signi­ficant impli­cations for miniaturized optical systems that must incorporate highly coherent laser sources. Further exploration of the strong piezo­electric, acousto-optic, and second order nonlinear properties of the LN substrate promises to advance the performance and func­tionality of the single-mode microdisk laser, to bypass the need for hetero­geneous inte­grations. (Source: SPIE)

Reference: J. Lin et al.: Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser, Adv. Phot. 4, 036001 (2022); DOI: 10.1117/1.AP.4.3.036001

Link: Shanghai Institute of Optics and Fine Mechanics, Shanghai, China