Kerr Frequency Comb and Stimulated Raman Comb Covering S+C+L+U Band Based on a Packaged Silica Spherical Microcavity

Optical microresonators supported whispering gallery modes (WGMs) are one of the most cost-effective platforms for optical frequency comb generated due to their advantages of ultra-high quality (Q) factor and ultra-low mode volume. In this paper, we report a portable and robust packaged silica spherical microcavity by melting the end of a standard single-mode fiber with ultra-high Q factor up to ${10}^8$, and theoretically and experimentally demonstrate a broadband optical frequency comb (OFC) generation including Kerr OFC and stimulated Raman comb. Kerr OFCs are separated by one, two, eight and twelve free spectral ranges corresponding to 1.4, 2.8, 11.2, and 16.8 nm, respectively. The transition between Kerr OFC and stimulated Raman comb is achieved by changing the pump laser power and the detuning frequency resulting from gain competition between modulation instability and Raman gain. Benefitting from the dispersion control and ultra-high Q factor persistence in our packaged microcavity, OFC can be adjusted continuously covering from S-band to C-band, L-band and U-band. We also explain these results by numerical simulations using a model framework combined with the Lugiato-Lefever equation and Raman response function.

I. Introduction

Optical frequency comb (OFC) sources have been proven be useful in numerous applications including molecular spectroscopy [1], RF photonics [2], millimeter wave generation [3], frequency metrology [4], atomic clock [5] and dense/ultra-dense wavelength division multiplexed high speed optical communications [6]. These applications require highly stable, coherent and cost OFC sources. Typical OFC sources generate multiple carriers offering low power consumption and complexity by replacing multiple independent lasers with a single subsystem [7]. OFCs in whispering gallery modes (WGMs) microcavities [7], or ‘microcombs’, are coherent light sources generated by a continuous-wave laser pump and have attracted extensive attention [9]–[10]. The WGM microcavity-based OFC is first realized in the silica microtoroid cavity [11]. Up to now, Kerr frequency combs are successfully generated in spherical, ring, toroid, disk, and bottle microcavities made of different materials with third-order optical nonlinearity or second-order nonlinearity [12], [13] [14], [15], [16], [17], [18], [19], [20], [21], [22]. Compared with traditional OFCs built on mode-locked lasers, microcomb is considered a new type of coherent light source that shows unique and promising advantages of lower power consumption as well as whole system integratability. Further, microcombs are also capable of generating ultrashort pulses with gigahertz to terahertz repetition rates [23]. As shown in Table I, typical microcombs are briefed. OFC generation with different spectral coverages have been demonstrated various material platforms, including the micro-cavity types, materials, Q factor values, wavelength ranges, etc. Among them, the silica spherical microcavity is one of the simplest ones because it is easy to be engineered to sphere shapes by melting the end of a standard single-mode fiber using CO₂ laser [21]. The microsphere cavity has become an ideal candidate for OFC generation at low threshold powers [17], [18], [19], [20]. TABLE I Typical Parameters of Microcavities Made of Different Materials

Material	Structure	Q factor	FSR (GHz)	Wavelength range (nm)	Refs
Si3N4	microring	0.6×106	1000	850 to 2000	[12]
	microring	1.5×106	∼230	1500 to 1650	[13]
LiNbO3	microring	1.1×106	∼200	1830 to 2130	[14]
SiO2	microbubble	5×107	∼464	1530 to 1560	[15]
	microbottle	1×107	∼400	1520 to 1600	[16]
	microsphere	3.7×107	∼260	1500 to 1600	[17]
		5×107	∼408	1400 to 1700	[21]
		2×107	∼357	1300 to 1700	[22]