Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser

Jintian Lin; Saeed Farajollahi; Zhiwei Fang; Ni Yao; Renhong Gao; Jianglin Guan; Li Deng; Tao Lu; Min Wang; Haisu Zhang; Wei Fang; Lingling Qiao; Ya Cheng

doi:10.1117/1.AP.4.3.036001

Abstract

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.

Broad transparency window and high piezoelectric, acousto-optic, second-order nonlinear, and electro-optic (EO) coefficients characterize crystalline lithium niobate (LN) as the “silicon in photonics.”1^–7 Recent breakthroughs in the nanofabrication technology on thin-film LN platforms4 gave birth to a variety of integrated photonic devices such as high-performance EO modulators,8^,9 broadband optical frequency combs,10^–12 and high efficiency frequency converters.13^–15 To build a monolithic integrated photonic system on an LN chip, the capacity of microlaser fabrication on this platform is essential. The high EO coefficient of LN ensures that such lasers outperform all other on-chip counterparts in broad range and real-time wavelength tunability. However, similar to silicon, LN itself does not provide optical gain for lasing. Therefore, to functionalize LN, doping of the gain medium is necessary. In particular, to make lasers operating at widely needed telecom wavelengths, the erbium ion ( ${Er}^{3 +}$ ) is a favorable choice of dopant.16^–22 More importantly, to enable many applications and outperform silicon counterparts, such lasers should operate at a single frequency with an ultranarrow linewidth and fast wavelength tunability.23^–27 According to the Schawlow–Townes theory, the laser fundamental linewidth is quadratic inversely proportional to the undoped cavity $Q$ factor.28^–30 Therefore, increasing the $Q$ factor will quadratically reduce the linewidth of a microlaser. The highest $Q$ factors demonstrated to date are those of whispering gallery mode (WGM) microcavities where light confinement is achieved by continuous total internal reflection at the smooth resonator periphery.31 However, the dense WGMs within the optical gain bandwidth would easily lead to multifrequency lasing in a microcavity. To reduce the number of modes, attempts have been made to reduce the microcavity size,23 which inevitably increases the radiation loss and decreases the mode volume, leading to a reduction of both the $Q$ factor and optical gain in the microcavity. Consequently, the pump threshold increases while the laser power remains low.