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• Vol. 4, Issue 3, 036001 (2022)
Jintian Lin1、2、†, Saeed Farajollahi3, Zhiwei Fang4, Ni Yao5、6, Renhong Gao1、2, Jianglin Guan4、7, Li Deng4、7, Tao Lu3、*, Min Wang4、7, Haisu Zhang4、7, Wei Fang6、8、*, Lingling Qiao1、2, and Ya Cheng1、2、4、7、9、10、11、*
Author Affiliations
• 1Chinese Academy of Sciences (CAS), Shanghai Institute of Optics and Fine Mechanics (SIOM), State Key Laboratory of High Field Laser Physics and CAS Center for Excellence in Ultra-Intense Laser Science, Shanghai, China
• 2University of Chinese Academy of Sciences, Center of Materials Science and Optoelectronics Engineering, Beijing, China
• 3University of Victoria, Department of Electrical and Computer Engineering, Victoria, British Columbia, Canada
• 4East China Normal University, School of Physics and Electronic Science, XXL—The Extreme Optoelectromechanics Laboratory, Shanghai, China
• 5Research Center for Intelligent Sensing, Zhejiang Lab, Hangzhou, China
• 6Zhejiang University, College of Optical Science and Engineering, The Interdisciplinary Center for Quantum Information, State Key Laboratory of Modern Optical Instrumentation, Hangzhou, China
• 7East China Normal University, State Key Laboratory of Precision Spectroscopy, Shanghai, China
• 8Jiaxing Institute of Zhejiang University, Intelligent Optics & Photonics Research Center, Jiaxing, China
• 9Shanxi University, Collaborative Innovation Center of Extreme Optics, Taiyuan, China
• 10Shandong Normal University, Collaborative Innovation Center of Light Manipulations and Applications, Jinan, China
• 11Shanghai Research Center for Quantum Sciences, Shanghai, China
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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. Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser[J]. Advanced Photonics, 2022, 4(3): 036001 Copy Citation Text show less

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.

1 Introduction

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.”17 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,1012 and high efficiency frequency converters.1315 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 ($Er3+$) is a favorable choice of dopant.1622 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.2327 According to the Schawlow–Townes theory, the laser fundamental linewidth is quadratic inversely proportional to the undoped cavity $Q$ factor.2830 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.

Copy Citation Text
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. Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser[J]. Advanced Photonics, 2022, 4(3): 036001