Visible Kerr Comb Generation in a High-Q Silica Microdisk Resonator with a Large Wedge Angle

Research Article Vol. 7, No. 5 / May 2019 / Photonics Research 573 Visible Kerr comb generation in a high-Q silica microdisk resonator with a large wedge angle 1,† 1,† 1,† 1 1 2 1, JIYANG MA, LONGFU XIAO, JIAXIN GU, HAO LI, XINYU CHENG, GUANGQIANG HE, XIAOSHUN JIANG, * 1,3 AND MIN XIAO 1National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and School of Physics, Nanjing University, Nanjing 210093, China 2State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA *Corresponding author: [email protected] Received 4 January 2019; revised 14 March 2019; accepted 14 March 2019; posted 14 March 2019 (Doc. ID 356704); published 25 April 2019 This paper describes the specially designed geometry of a dry-etched large-wedge-angle silica microdisk resonator that enables anomalous dispersion in the 780 nm wavelength regime. This anomalous dispersion occurs naturally without the use of a mode-hybridization technique to control the geometrical dispersion. By fabricating a 1-μm-thick silica microdisk with a wedge angle as large as 56° and an optical Q-factor larger than 107, we achieve a visible Kerr comb that covers the wavelength interval of 700–897 nm. The wide optical frequency range and the closeness to the clock transition at 698 nm of 87Sr atoms make our visible comb a potentially useful tool in optical atomic clock applications. © 2019 Chinese Laser Press https://doi.org/10.1364/PRJ.7.000573 1. INTRODUCTION Recently, several approaches have been employed to – Optical frequency combs are coherent light sources with a large generate visible Kerr combs [29 38], such as using second- number of evenly spaced frequency lines, which can be harmonic/third-harmonic generation [29–33] to translate a exploited in many scientific applications [1–4]. Compared near-infrared comb to the visible range or designing a with traditional optical frequency combs based on large-size peculiar geometry possessing large anomalous dispersion to mode-locked lasers that cannot be integrated on chip [1], overcome the inherent normal dispersion of the material the up-to-date Kerr-type microcombs [5,6] based on cascaded [34–38]. However, all those methods suffer from certain four-wave-mixing processes possess many significant advan- disadvantages, such as a narrow visible bandwidth [34,35], tages. These include a compact size, low optical loss, and fea- large pump power needed [31,33,37,38], limited con- sibility of on-chip integration [5–8]. Since their invention, this version efficiency [29–32], non-integrated platforms [34,35], field has garnered tremendous attention and enjoyed rapid limited visible comb line power [37], and rigidities in progress. Kerr-type microcombs have been realized in many fabrication [36]. platforms [9–22], some of which are already integrated on chip. In this work, we present a new method of visible comb gen- After successfully generating optical frequency combs in differ- eration. By employing a large-wedge-angle silica microdisk res- ent material systems, many studies have been geared up toward onator with a thickness of 1 μm, the normal dispersion of silica potential applications, such as arbitrary waveform generation material in the 780 nm regime can be completely compensated [19], coherent optical communication [23], optical synthesizer by its geometric dispersion. As a result, we obtain a resonator [24], ultrafast optical ranging [25,26], and astronomical spec- that produces anomalous dispersion in the visible wavelength trum calibration [27,28]. So far, most of the Kerr frequency regime without using mode hybridization, so that a visible microcombs and their corresponding applications have been comb can be generated directly without using any frequency realized in the wavelength near 1550 nm [9–22]. The gener- conversion technique. As is shown in the next section, the mi- ation of the visible microcombs that are desirable for optical crodisk possesses anomalous dispersion over the thicknesses of atomic clocks and optical coherence tomography is still 800–1200 nm, which makes the fabrication process less strin- challenging due to the strong normal dispersion induced by gent than the method by Lee et al. [36]. Furthermore, the the material and the lower optical quality factor of the visible comb obtained in our experiment covers the wave- microcavity in the visible wavelength region. lengths ranging from 700 nm to 897 nm. To the best of our 2327-9125/19/050573-06 Journal © 2019 Chinese Laser Press 574 Vol. 7, No. 5 / May 2019 / Photonics Research Research Article knowledge, this is the broadest wavelength range among the illustrates the mode profile of the fundamental TM10 mode experimentally demonstrated visible Kerr combs. (we define the first mode number according to the maximum intensity in the radial direction, while the second mode number l − m m l 2. DEVICE FABRICATION AND DISPERSION is defined as for and as defined in Ref. [43]), which SIMULATION we harness as the pump mode in the subsequent experiment inside our cavity. Figure 2(b) shows the dispersion relationship The experiment conducted in this study uses the silica micro- with respect to the wavelength of the cavity used in the experi- disk resonator shown in Fig. 1. The fabrication process is sim- ment. As one can see, the TM10 mode shows the anomalous ilar to that described in our previous work [39,40]. The disk is dispersion characteristics (D>0) at 780 nm. Figure 2(c) μ μ 1 m thick with a diameter of 80 m. The most intriguing presents the dispersion characteristics of the TM10 mode at feature of our disk is the large wedge angle of 56° (the definition the 780 nm wavelength for different disk thicknesses and differ- of the wedge angle is illustrated in Fig. 2(a), and it is indicated ent wedge angles. It is clear that a wedge angle of 56° provides α by angle ), which is usually hard to attain if the silica micro- an anomalous dispersion over a large thickness range between disk is fabricated by wet etching [41]. It is this special geometry 0.8 and 1.2 μm; additionally, the anomalous dispersion is that allows our disk to overcome the large normal dispersion insensitive to the wedge angle. These characteristics greatly re- induced by the silica material and naturally exhibit anomalous duce the constraints put on the disk thickness and the wedge dispersion in the 780 nm wavelength regime. In previous stud- angle, thus making fabrication significantly easier than the ies where buffered hydrofluoric acid (HF) was used to etch a et al. ∼1 5 μm method by Lee [36], in which the disk thickness has a thicker silica disk ( . thickness), the wedge angle could tolerance of only 50 nm. Figure 2(d) depicts the effective index not reach this level, and mode hybridization was employed to at different thicknesses for different mode families at a wedge enable the small-wedge-angle disks to provide anomalous angle of 56° and a wavelength of 780 nm. The smooth and dispersion at ∼780 nm [36]. However, this produced extra ’ continuous variation behaviors of the dispersion and effective problems in the fabrication process, namely that the disk s refractive index with the disk’s thickness in Figs. 2(c) and 2(d) thickness should not deviate by more than 50 nm, as mode indicate that our disk does not exhibit the characteristics of hybridization is very sensitive to the thickness of the disk. mode hybridization [36]. On the contrary, our disk displays the feature of a large wedge Figure 2(e) shows the variation of zero dispersion wave- angle, and it naturally exhibits anomalous dispersion in the lengths with different wedge angles for the silica microdisk res- 780 nm wavelength region, independent of mode hybridization onators that are 1 μm in thickness and with a 80 μm diameter. [see Figs. 2(c) and 2(d)]. It is clear that when increasing the wedge angle, the zero Before conducting the experiment, we first simulate the dispersion wavelength will shift to a shorter wavelength [39]. dispersion characteristics of our disk using the standard Figure 2(f) illustrates the dispersion character of the TM10 finite-element method (FEM) [42]. Figure 2(a) schematically mode for a 56° wedge angle silica disk with a diameter of 2 mm. The zero dispersion wavelength can be shifted to ∼680 nm, which is beneficial for visible comb generation with a shorter wavelength, as well as a lower repetition rate. 3. Q-FACTOR AND THRESHOLD MEASUREMENT We now describe an experiment using the setup shown in Fig. 3(c). First, we test the quality factor (Q) of our cavity by sending a scanning laser into the cavity and measuring the linewidth of the transmitted Lorentz peak, as illustrated in Fig. 3(a). The doublet resonance dip is due to the coupling between the clockwise and counter-clockwise traveling modes induced by surface roughness [44]. The microdisk resonator has the intrinsic and loaded Q factors of 1.08 × 107 and 0.98 × 107, respectively. Next, we measure the threshold power of the optical parametric oscillation (OPO). We find the OPO process occurs at the threshold power of 1.62 mW (launched power in fiber). The formula of the OPO threshold power can be written as [14,45] 2 V 0n0ω Pth 2 , 8n2cηQ Fig. 1. SEM images of the large-wedge-angle microdisk resonator V n used in our experiment. (a), (b) Full-scale view of the microresonator. where 0 is the effective mode volume, 0 is the refractive in- (c) Close-up of the microresonator to show the detailed characteristics dex of silica, ω∕2π denotes the resonance frequency, n2 repre- of the large wedge angle.

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